Integrating Green Chemistry and Sustainable Engineering 9781119509837, 1281291331, 1119509831

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Integrating Green Chemistry and Sustainable Engineering

Scrivener Publishing 100 Cummings Center, Suite 541J Beverly, MA 01915-6106 Publishers at Scrivener Martin Scrivener ([email protected]) Phillip Carmical ([email protected])

Integrating Green Chemistry and Sustainable Engineering

Shahid-ul-Islam Department of Textile Technology, Indian Institute of Technology, Delhi, India

This edition first published 2019 by John Wiley & Sons, Inc., 111 River Street, Hoboken, NJ 07030, USA and Scrivener Publishing LLC, 100 Cummings Center, Suite 541J, Beverly, MA 01915, USA © 2019 Scrivener Publishing LLC For more information about Scrivener publications please visit www.scrivenerpublishing.com. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, except as permitted by law. Advice on how to obtain permission to reuse material from this title is available at http://www.wiley.com/go/permissions. Wiley Global Headquarters 111 River Street, Hoboken, NJ 07030, USA For details of our global editorial offices, customer services, and more information about Wiley products visit us at www.wiley.com. Limit of Liability/Disclaimer of Warranty While the publisher and authors have used their best efforts in preparing this work, they make no representations or warranties with respect to the accuracy or completeness of the contents of this work and specifically disclaim all warranties, including without limitation any implied warranties of merchantability or fitness for a particular purpose. No warranty may be created or extended by sales representatives, written sales materials, or promotional statements for this work. The fact that an organization, website, or product is referred to in this work as a citation and/or potential source of further information does not mean that the publisher and authors endorse the information or services the organization, website, or product may provide or recommendations it may make. This work is sold with the understanding that the publisher is not engaged in rendering professional services. The advice and strategies contained herein may not be suitable for your situation. You should consult with a specialist where appropriate. Neither the publisher nor authors shall be liable for any loss of profit or any other commercial damages, including but not limited to special, incidental, consequential, or other damages. Further, readers should be aware that websites listed in this work may have changed or disappeared between when this work was written and when it is read. Library of Congress Cataloging-in-Publication Data ISBN 978-1-119-50983-7 Cover image: Pixabay.Com Cover design by Russell Richardson

Set in size of 11pt and Minion Pro by Exeter Premedia Services Private Ltd., Chennai, India Printed in the USA

Contents Preface

xix

1 Third Generation Biofuels: A Promising Alternate Energy Source Mushtaq Ahmad Rather and Parveena Bano 1.1 Introduction 1.2 Biofuel Types 1.3 Advantages of Third Generation Biofuels 1.4 Technology of Third Generation Biofuel Production 1.5 Transformation Potential of Algae Into Third Generation Biofuels 1.6 Recent Developments in Biomass Transformation Into Third Generation Biofuels by Hydrothermal Conversion (HTC) 1.7 Conclusion References 2

Recent Progress in Photocatalytic Water Splitting by Nanostructured TiO2-Carbon Photocatalysts – Influence of Interfaces, Morphological Structures and Experimental Parameters V. Preethi, M. Mamatha Kumari, N. Ramesh Reddy, U. Bhargav, K. K. Cheralathan, C. H. Shilpa Chakra and M. V. Shankar 2.1 Photocatalysis 2.2 Carbon Nanotubes-TiO2 and Other Nanocomposite for Photocatalytic Water Splitting 2.3 Factors Influencing Liquid-Phase Hydrogen Production 2.3.1 Direct Photolysis and Its Limitations 2.3.2 Need for Reducing Polysulphide Ions Formation

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24 26 31 33 33

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Contents 2.3.3 Role of Sulphite Ions in Conversion of Photo Sulphides to Thiosulphate 2.3.4 Influence of Catalyst Dosage 2.3.5 Effect of pH 2.3.6 Effect of Recycle Flow Rates and Reactor Design on H2 Generation 2.3.7 Dependence of Hydrogen Production on Volume and Depth of Photolytic Solution 2.3.8 Influence of Light Irradiation on Hydrogen Yield 2.3.9 Sulphur Recovery 2.3.10 Reusability of the Nanophotocatalysts 2.4 Factors Influencing Gas-Phase Photocatalytic Hydrogen Production 2.4.1 Effect of H2S Gas Concentration 2.4.2 Effect of Gas Flow Rate 2.4.3 Effect of Catalyst Dosage 2.4.4 Effect of Light Irradiation 2.5 Future Prospects References

3 Heterogeneous Catalytic Conversion of Greenhouse Gas CO2 to Fuels Kaisar Ahmad, Firdaus Parveen, Anushree and Sreedevi Upadhyayula 3.1 Introduction 3.1.1 Greenhouse Gas CO2 3.1.2 Mitigation of CO2 Concentration 3.1.3 Reducing CO2 Emissions 3.1.4 Zero Emissions 3.1.5 Carbon Capture and Storage or Sequestration (CCS) 3.2 Thermodynamics of CO2 Hydrogenation to Methanol, DME, and Hydrocarbons 3.3 Catalytic Conversion of CO2 to Methanol, DME, and Hydrocarbons 3.3.1 Effect of Alkali Promotors 3.3.2 Effect of Metal Particle Crystal Phase in CO2 Hydrogenation 3.3.3 Effect of Support 3.4 Mechanism of CO2 Hydrogenation to Methanol, DME, and Hydrocarbons

34 37 37 41 41 41 45 45 45 46 46 47 47 48 49 57

58 58 59 59 59 59 60 64 65 67 68 69

Contents vii 3.4.1 CO2 Hydrogenation to Methanol 3.4.1.1 Formate Route 3.4.1.2 Carboxylate Route 3.4.1.3 RWGS Route 3.4.2 CO2 Hydrogenation to Dimethyl Ether 3.4.3 CO2 Hydrogenation to Hydrocarbons 3.4.3.1 Indirect Conversion of CO2 Into Hydrocarbons 3.4.3.2 Direct Conversion of CO2 Into Hydrocarbons 3.5 Challenges and Opportunities in CO2 Hydrogenation Process References 4 Energy Harvesting: Role of Plasmonic Nanocomopsites for Energy Efficient Devices Jaspal Singh, Subhavna Juneja and Anujit Ghosal 4.1 Introduction 4.2 Plasmonic Nanostructures 4.3 Plasmonic Nanocomposites 4.4 Plasmonic Nanocomposites for Energy Harvesting 4.4.1 Plasmonic Nanocomposites for Photovoltaic Applications 4.4.2 Plasmonic Nanocomposites for Water Purification 4.4.3 Plasmonic Nanocomposites for Hydrogen Production 4.5 Conclusions References 5 Catalytic Conversion of Biomass Derived Cellulose to 5-Hydromethyl Furfural Firdaus Parveen, Kaiser Ahmad and Sreedevi Upadhyayula 5.1 General Overview 5.2 Biomass Conversion Processes 5.3 HMF as a Platform Chemical 5.4 Hydrolysis of Cellulose to Glucose 5.4.1 Hydrolysis of Cellulose to Glucose Using Liquid Acid 5.4.2 Hydrolysis of Cellulose to Glucose Using Solid Acid

69 70 70 71 71 71 73 74 75 75

81 81 83 85 88 88 92 98 102 103

113 113 119 121 123 123 126

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Contents 5.4.3 Hydrolysis of Cellulose to Glucose Using Ionic Liquids 5.4.3.1 Ionic Liquids 5.4.3.2 Cellulose Hydrolysis in Ionic Liquids Using Mineral Acid as Catalyst 5.4.3.3 Cellulose Hydrolysis in Ionic Liquids Using Metal Salts as Catalyst 5.4.3.4 Cellulose Hydrolysis in Ionic Liquids Using Heterogenous Catalyst 5.4.3.5 Cellulose Hydrolysis in Ionic Liquids Using Ionic Liquids as Catalyst 5.5 Glucose Conversion to 5-Hydroxymethyl Furfural 5.6 Conclusion and Future Prospects References

6 Raman “Green” Spectroscopy for Ultrasensitive Analyte Detection Subhavna Juneja, Anujit Ghosal and Jaydeep Bhattacharya 6.1 Introduction 6.2 Application of Nanotechnology in Medicine 6.2.1 Bio-Imaging 6.2.2 Bio-Sensing and Diagnosis 6.2.3 Targeted Drug Delivery 6.2.4 Food Technology 6.2.5 Regenerative Medicine 6.2.6 Nanomedicine with Emphasis on Early Disease Detection 6.2.7 Raman and Surface Enhanced Raman Spectroscopy (SERS) 6.2.7.1 Raman “Green” Spectroscopy 6.3 Conclusion and Future Outlook References 7 Microwave Synthesized Conducting Polymer-Based Green Nanocomposites as Smart Promising Materials Neha Kanwar Rawat and P.K Panda 7.1 Introduction 7.2 Brief Introduction of Conducting Polymers 7.2.1 CPs 7.2.2 Synthesis of Conducting Polymers

128 129 133 136 138 141 143 150 150 165 166 167 167 167 168 169 169 171 172 173 184 184 191 192 194 194 195

Contents ix 7.3 Microwave Synthesis 7.3.1 Principle of MW Heating 7.3.2 Dielectric Properties 7.3.3 Significance of Tan δ 7.3.4 Advantages of Microwave Over Conventional Heating 7.4 Literature/Research Present 7.4.1 PANI and Derivatives 7.4.2 PTh and Their Derivatives 7.5 Application of MW synthesized CPs in varying Arena 7.6 Conclusion and outlook References 8

Biobased Biodegradable Polymers for Ecological Applications: A Move Towards Manufacturing Sustainable Biodegradable Plastic Products Sudhakar Muniyasamy, Kulanthaisamy Mohanrasu, Abongile Gada, Teboho Clement Mokhena,Asanda Mtibe, Thulasinathan Boobalan, Vimla Paul and Alagarsamy Arun 8.1 Introduction 8.2 Biodegradable and Compostable Polymer Materials 8.2.1 Defining Biodegradability 8.2.2 Biodegradable Polymers (Fossil or Renewable) 8.3 Biopolymer From Microbial Synthesis and Its Applications 8.3.1 Intracellular Biological Polymers 8.3.1.1 Application of PHAs 8.3.2 Extracellular Polymeric Substances (EPS) 8.3.2.1 Important Properties of EPS 8.4 Chitin 8.4.1 Chitin Recovery 8.4.2 Characterization of Chitin 8.4.3 Applications of Chitin 8.5 Conventional Synthesis of Biopolymers and Its Application 8.5.1 Biorenewable Biopolymers 8.6 End-of-Life of Biopolymer Based Materials and Composites and Its Applications 8.6.1 Biopolymer Blend 8.6.2 Biocomposites 8.6.2.1 PLA-Natural Fibre Composite 8.6.2.2 PLA Based Composites From Petro Based Biodegradable Polymer

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Contents 8.6.2.3 PLA-Non-Biodegradable Polymer from Renewable and Non-Renewable Sources 8.6.2.4 PLA Based Composites from Renewable Biodegradable Polymer PLA/ microbial Polyester 8.7 Concluding Remarks References

9 Cashew Nut Shell Liquid (Phenolic Lipid) Based Coatings: Polymers to Nanocomposites Fahmina Zafar, Anujit Ghosal, Eram Sharmin and Nahid Nishat 9.1 Introduction 9.2 CNSL (Col) 9.3 CNSL (Col) Based Polymeric Coatings 9.3.1 CNSL (Col)-Epoxy Coatings 9.3.2 CNSL (Col) Polyamides (CPAs) Coatings 9.3.3 CNSL (Col)-Formaldehyde/ Furfuraldehyde Coatings 9.3.4 CNSL (Col) phenalkamines coatings 9.3.5 CNSL (Col) Benzoxazine (Bnz) Coatings 9.3.6 Col-Polyol Coatings 9.3.7 CNSL(Col)-Polyurethane (PU) Coatings 9.4 CNSL (Col) Non Isocyanate Polyurethanes (NIPUs) or Green Coatings 9.5 CNSL (Col) Waterborne and UV Cured Coatings 9.6 CNSL(Col) Based Antifouling/antibacterial Coatings 9.7 CNSL (Col) Based Nanostructured Coatings and Nanocomposites Coatings 9.8 Conclusions References 10 Ionic Liquids as Potential Green Solvents Their Interactions with Surfactants and Antidepressant Drugs Nisar Ahmad Malik and Ummer Farooq 10.1 Introduction 10.2 Basic Properties of ILs 10.3 Applications of ILs 10.4 Antidepressant Drugs 10.5 Ionic Liquid – Surfactant and Ionic Liquid-Antidepressant Drug Interaction

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256 257 258 259 264 267 268 271 273 274 274 277 280 283 284 285 291 292 293 297 300 303

Contents xi 10.6 Conclusions and Perspectives References 11 Role of Green and Integrated Chemistry in Sustainable Metallurgy Sadia Ilyas, Muhammad Farhan and Haq Nawaz Bhatti 11.1 Introduction 11.2 Role of Green and Integrated Chemistry in Sustainbale Metallurgy of Primary Resources 11.2.1 Role of Integrated Chemistry in Processing of Sulfide Minerals 11.2.2 Role of Integrated Chemistry in Processing of Oxide Minerals 11.3 Role of Green and Integrated Chemistry in Sustainable Metallurgy of Secondrey Resources 11.3.1 Role of Integrated Chemistry in Processing of Smelter Dust 11.3.2 Role of Integrated Chemistry in Processing of Converter/Smelter Slags 11.3.3 Role of Integrated Chemistry in Processing of Spent Catalyst/Lithium Ion Batteries (SC/LIBs) 11.3.4 Role of Integrated Chemistry in Processing of Waste Electric and Electronic Equipment (WEEEs) 11.3.4.1 Processing of Precious Metals From WEEEs with Integrated Routes 11.3.4.2 Processing of Rare Earth Elements from WEEEs with Integrated Routes 11.4 Perspectives on Integrated Chemical and Biological Routes for Mineral/Material Processing References 12 Biological Nitrogen Fixation and Biofertilizers as Ideal Potential Solutions for Sustainable Agriculture Shymaa Ryhan Bashandy, Mohamed Hemida Abd-Alla and Magdy Mohamed Khalil Bagy 12.1 Introduction 12.2 Non-Symbiotic Biological Nitrogen Fixation 12.2.1 Factors Affecting Non-Symbiotic N2 Fixation 12.2.1.1 Soil and Environmental Factors 12.3 Symbiotic Biological Nitrogen Fixation 12.3.1 Rhizobia

311 312 325 325 326 327 329 331 331 332 334 335 336 337 338 340 343

344 346 346 346 348 348

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Contents 12.3.2 Legumes 12.3.3 Legume - Rhizobium Symbiosis 12.3.4 The Signals From the Host Plants (Flavonoids) 12.3.5 Nod Factors 12.3.6 Molecular Genetics of Nodulation (The Nod Genes) 12.3.7 Nodule Formation 12.3.8 Nitrogen Fixation 12.3.9 Ecological Factors Affecting Signal Exchange 12.4 Plant Growth Promoting Rhizobacteria 12.4.1 Direct Mechanisms of PGPR 12.4.1.1 The Biological N2 Fixation 12.4.1.2 Phosphate Solubilization 12.4.1.3 Potassium Solubilization 12.4.1.4 Iron Chelation (Siderophores Production) 12.4.1.5 Phytohormone Production 12.4.2 Indirect Mechanics of PGPR 12.5 Conclusions and Future Research References

349 352 352 354 355 356 356 357 360 361 361 362 363 364 365 366 367 368

13 Natural Products in Adsorption Technology Ahmet Gürses 13.1 Introduction 13.2 Adsorption and Surface Chemistry 13.3 Characteristics of Adsorbents and Selection of Adsorbent 13.4 Common Processes in Adsorption Technology 13.5 Adsorpbents Used in Adsorption Technology References

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14 Role of Microbes in the Bioremediation of Toxic Dyes Tanvir Arfin, Kamini Sonawane, Piyush Saidankar and Shraddha Sharma 14.1 Introduction 14.2 Dye 14.3 Classification of Dye 14.4 Dye Colour 14.4.1 Factor 14.5 Techniques for the Removal of Dye 14.6 Decolouration Mechanisms of Microbial 14.7 Biosorption 14.7.1 Merits of Biosorption Process

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443 444 444 444 445 446 446 448 448

Contents xiii 14.7.2 Factors Influences Metal Biosorption 14.8 Consortia of Microorganisms 14.9 Decolourization by Fungi 14.9.1 Factors Affecting Dye Decolourisation by Fungal Biomass 14.10 Dye Removal by Bacteria 14.10.1 Gram-Positive 14.10.2 Gram-Negative 14.10.3 Bacteria Classification by Shape 14.10.4 Decolourization by Bacteria 14.10.5 Bacterial Strain Used for Dye Removal 14.10.6 Different Bacteria Removes Different Dye 14.11 Algae 14.11.1 Classification of Algae 14.11.2 Microalgae 14.11.3 Effect of Algal Photosynthesis 14.11.4 Algae for Dye Removal 14.12 Conclusion References 15 Valorization of Wastes for the Remediation of Toxicants from Industrial Wastewater Shumaila Kiran, Tahsin Gulzar, Sarosh Iqbal, Noman Habib, Atya Hassan and Saba Naz 15.1 Introduction 15.2 Toxicants Present in Industrial Waste Water 15.2.1 Heavy Metals 15.2.1.1 Chromium (Cr) 15.2.1.2 Cadmium (Cd) 15.2.1.3 Iron (Fe) 15.2.1.4 Nickel (Ni) 15.2.1.5 Lead (Pb) 15.2.1.6 Copper (Cu) 15.2.1.7 Zinc (Zn) 15.2.1.8 Mercury (Hg) 15.2.1.9 Selenium (Se) 15.2.1.10 Arsenic (As) 15.2.2 Dyes 15.3 Waste Volarization 15.3.1 What is Adsorption?

449 450 451 453 457 458 458 459 460 461 462 462 462 465 466 467 467 467 473

474 475 475 475 476 476 476 477 477 477 477 478 478 478 479 480

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Contents 15.3.1.1 Mechanism of Adsorption 15.3.1.2 Types of Adsorption 15.3.2 Types of Wastes 15.3.2.1 Natural Materials 15.3.2.2 Agricultural Wastes 15.3.2.3 Biomass 15.3.2.4 Industrial Wastes 15.6 Conclusion References

16 Wound Healing Potential of Natural Polymer: Chitosan “A Wonder Molecule” Tara Chand Yadav, Amit Kumar Srivastava, Navdeep Raghuwanshi, Naresh Kumar, Ramasare Prasad and Vikas Pruthi 16.1 Introduction 16.2 Wound Healing 16.3 Need for Advance Dressing Material 16.4 Chitosan 16.5 Physicochemical Properties of Chitosan 16.5.1 Crystalline Nature 16.5.2 Molecular Weight 16.5.3 Degree of N-Acetylation (DA) 16.5.4 Solubility and Charge Density 16.5.5 Chemical Reactivity 16.5.6 Film-Forming Properties 16.5.7 Ion Binding 16.5.8 Gelling Behavior 16.5.9 Porosity 16.5.10 Biodegradability 16.5.11 Non-Immunogenic Nature 16.5.12 Hemostatic Property 16.6 Wound Healing Applications of Chitosan 16.6.1 Chitosan Hydrogels 16.6.1.1 Physical Hydrogels 16.6.1.2 Chemical Hydrogels 16.6.2 Chitosan as Antimicrobial Agent 16.6.3 Chitosan-Based Natural/ Synthetic Polymer Scaffolds for Wound Healing

480 480 482 482 488 499 503 507 509

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528 529 530 532 534 534 535 536 537 538 539 539 540 541 542 543 543 544 544 544 545 546 547

Contents 16.6.4 Chitosan-Based Composite Scaffolds for Wound Healing 16.6.5 Chitosan-Based Oil Restrained Scaffolds for Wound Healing 16.6.6 Chitosan-Plant Extract Based Scaffolds for Wound Healing 16.6.7 Chitosan Derivatives for Wound Healing 16.6.7.1 Trimethyl Chitosan 16.6.7.2 Toxicity Assessment of Tri-Methyl Chitosan 16.6.7.3 Role of Trimethyl Chitosan in Wound Healing 16.6.7.4 Role of Carboxymethyl Chitosan and Carboxymethyl-Trimethyl Chitosan 16.6.8 Chitosan/Derivatives-Peptides Conjugates for Wound Healing 16.6.9 Chitosan-Based Commercial Wound Dressing Bandages 16.7 Conclusion and Future Perspectives References 17 Nanobiotechnology: Applications of Nanomaterials in Biological Research Muhammad Irfan Majeed, Haq Nawaz Bhatti, Haq Nawaz and Muhammad Kashif 17.1 Introduction 17.2 Classification of Nanomaterials 17.2.1 Liposomes 17.2.2 Superparamagnetic Nanoparticles 17.2.3 Fullerenes: Bucky Balls and Carbon Nanotubes 17.2.4 Dendrimers 17.2.5 Quantum Dots 17.3 Bio-Inspired Green Synthesis of Nanomaterials 17.4 Green Synthesis of Nanomaterials 17.5 Applications of Nanomaterials in Biology Research 17.5.1 Imaging and Labelling 17.5.2 Nano Biosensors 17.5.3 Tissue Engineering 17.5.4 Dentistry 17.5.5 Antimicrobial Therapy 17.5.6 Wound Healing

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548 550 551 552 553 554 555 555 556 557 559 560 581

582 583 584 584 585 585 585 586 588 589 590 593 596 598 599 602

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Contents 17.5.7 Drug Delivery 17.5.8 Gene Delivery 17.6 Summary References

18 Biotechnology: Past-to-Future Tanvir Arfin and Kamini Sonawane 18.1 Introduction 18.2 History 18.3 Global Forum 18.4 Functions of the Global Forum 18.5 Objectives of Biotechnology Development 18.5.1 So What Should be the Objectives While Considering the Biotechnological Area? 18.6 Categorization of Biotechnology 18.6.1 Ancient Biotechnology 18.6.2 Classical Biotechnology 18.6.3 Modern Biotechnology 18.7 Biotechnology Categories 18.7.1 Green Biotechnology 18.7.2 White Biotechnology 18.7.2.1 Industrial Sustainability 18.7.2.2 Applications 18.7.3 Blue Biotechnology 18.7.3.1 Applications 18.7.4 Red Biotechnology 18.7.5 Environmental Biotechnology 18.7.6 Genetic Engineering 18.8 Need of Biotechnology 18.9 Heal the World 18.10 Fuel The World 18.11 Feed the World 18.12 Health and Medicines 18.13 Future of Biotechnology 18.14 Advantages of Biotechnology 18.15 Opportunities and Risks 18.16 Biotechnology Industry 18.17 Innovation 18.18 Future of Biotechnology 18.19 Conclusion References

603 606 610 611 617 618 619 619 620 621 621 622 622 623 623 624 624 627 628 629 629 630 632 635 635 636 638 638 638 638 639 639 640 641 641 643 644 644

Contents xvii 19 Biogenic Nanoparticles as Theranostic Agents: Prospects and Challenges Navdeep Raghuwanshi, Amit Kumar Srivastava, Tara Chand Yadav, Sonam Gupta and Vikas Pruthi 19.1 Introduction 19.2 Phytochemicals Stabilized Biogenic Nanoparticles as Theranostic Agents 19.2.1 Antimicrobial Applications of Biogenic Nanoparticles 19.2.2 Antioxidants and Anti-Inflammatory Applications of Biogenic Nanoparticles 19.2.3 Antineoplastic Applications of Biogenic Nanoparticles 19.2.4 Dermatological Applications of Biogenic Nanoparticles 19.2.5 Catalytic Applications of Biogenic Nanoparticles 19.2.6 Antidiabetic Applications of Biogenic Nanoparticles 19.2.7 Biosensing Applications of Biogenic Nanoparticles 19.3 Biosurfactants Stabilized Biogenic Nanoparticles as Theranostic Agent 19.3.1 Antimicrobial Activity of Biosurfactant Stabilized Metallic Nanoparticles 19.3.1.1 Glycolipid based Nanoparticles 19.3.1.2 Lipopeptide Stabilized Nanoparticles 19.4 Applications of Nanoparticles in Tissue Engineering 19.5 Toxicological Effects of Nanoparticles 19.6 Prospects and Challenges References Index

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648 650 652 657 658 661 667 667 670 670 671 671 671 672 673 674 675 685

Preface Green chemistry and engineering plays a growing role in the chemical processing industries. Green chemistry and engineering are relatively new areas focused on minimizing generations of pollution by utilizing alternative feedstocks, developing, selecting, and using less environmentally harmful solvents, finding new synthesis pathways, improving selectivity in reactions, generating less waste, avoiding the use of highly toxic compounds, and much more. In an effort to advance the discussion of green chemistry and engineering, this book contains 19 chapters describing greener approaches to the design and development of processes and products. The contributors describe the production of third generation biofuels, sustainable and economic production of hydrogen by water splitting using solar energy, efficient energy harvesting, mechanisms involved in the conversion of biomass, green nanocomposites, bio-based polymers, ionic liquids as green solvents, sustainable nitrogen fixation, bioremediation, and much more. The book aims at motivating chemists and engineers, and also undergraduate, postgraduate, Ph.D students and postdocs to pay attention to an accute need for the implementation of green chemistry principles in the field of chemical engineering, biomedical engineering, agriculture, enviromental enginnering, chemical processing and material sciences. In conclusion, it is my pleasant duty to thank all the authors for contributing their time and expertise in preparing the informative and in-depth chapters related to areas of green chemistry and engineering which has made this book a reality. I would like to express my sincere appreciation to Martin Scrivener (Scrivener Publishing) for inviting me to put together a textbook on integrating green chemistry and engineering. Shahid-ul-Islam Indian Institute of Technology Delhi (IITD), Hauz Khas, New Delhi, India November 14, 2018 xix

1 Third Generation Biofuels: A Promising Alternate Energy Source Mushtaq Ahmad Rather1,* and Parveena Bano2 1

Associate Professor, Chemical Engineering Department, National Institute of Technology (NIT) Srinagar Kashmir 2 Assistant Professor, SKUAST-K, Srinagar Kashmir, Shalimar, India

Abstract Global energy demand is projected to increase by at least 50 % by 2030. Fossil fuels available at present cannot catch-up the current demand. Continuous use of fossil fuels for energy purposes has caused devastating effects to our environment due to greenhouse gas emissions. Thus search for ‘clean energy’ or green and sustainable renewable energy as an alternative to fossil fuels is the need of hour. Green and sustainable renewable energy sources are important to foster a transition towards more sustainable energy availability. Among the several alternatives available at present, biofuels have attracted huge attention. Use of biofuels provides environmental benefits by decreasing the harmful emissions of gases like CO2, SOx etc. Biofuels have attracted intense debate from a variety of perspectives, including societal, economic, and environmental. First-generation biofuels are made from sugars, starch and vegetable oils, so have an impact on the food security. Second generation biofuels generated from plant material may lead to felling of trees and shrubs. Biofuels generated from non-food crops like microalgae and macroalgae are referred to as third generation biofuels, have great potential to meet part of future global energy demand without making any compromise with human food security. In the present chapter, we present a review of recent research interests in the different aspects of production of third generation biofuels by hydrothermal conversion, one of the thermal conversion routes. Keywords: Hydrothermal conversion, biofuels, energy, biomass

*Corresponding author: [email protected] Shahid-ul-Islam (ed.) Integrating Green Chemistry and Sustainable Engineering, (1–22) © 2019 Scrivener Publishing LLC

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Integrating Green Chemistry and Sustainable Engineering

Introduction

Fossil fuels have been primary source of energy for centuries now. However their fast depletion and associated harmful effects on the climate (by increasing the atmospheric green house gas emissions) , has led to search for ‘clean energy’. To tackle the problem, environmentally friendly alternate sources of energy are being harnessed. Some of the options of clean energy sources available are solar, wind, tidal and biomass. Utilization of biomass in various transformed forms has proved to be an adequate way-out for meeting the part of global energy demand [1]. Various forms of biomass in nature are wood, vegetation, crops, aquatic plants and algae etc. Biomass can be transformed into a versatile fuel referred to as biofuel, being studied and implemented universally nowadays. Study of biofuels has transformed into an area of complex interest and debate from various reasons including societal, economic, and environmental. Biofuels are renewable fuels, having potential to decrease several harmful emissions such as soot, carbon monoxide, and carbon dioxide [2]. Among the main biofuel producing countries in world for transportation, USA and UE have taken lead and already set specific targets for future.USA has planned to substitute 20% of road transport fuel with biofuel by 2022, while UE has adopted 10% as a goal of biofuel for transport energy by 2020 [3]. Green and sustainable renewable energy sources are important to foster a transition towards more sustainable energy availability. Biomass can be used to produce and substitute fossil fuels in many choices, to replace the petrochemical compounds. As oil is processed in a refinery to fuels, and chemicals; the “biorefinery” concept is equivalent to an oil refinery because biomass is transformed into various products, ranging from chemicals to biofuels [4]. Biofuels are made through the conversion of biomass in three different ways; thermal conversion, chemical conversion and biochemical conversion. The resulting biofuel can be produced in solid, liquid or a gaseous form. Microalgae can be converted to biofuels mainly by two routes viz. biochemical fermentation (anaerobic digestion) and thermo chemical processes. Thermochemical processes include gasification, pyrolysis and hydrothermal processing. Gasification involves the partial controlled oxidation of organic material to syngas. Syngas contains CO, H2 and CO2. Apart from direct combustion, the other promising application of syngas is its conversion to synthetic gas which later can be converted to liquid fuel (GTL), by the Fisher-Tropsch process. Above converts H2 and CO

Third Generation Biofuels

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to straight chain liquid hydrocarbons which are suitable renewable substitute to petroleum derived diesel fuel. Pyrolysis refers to the thermal decomposition of feedstocks in the absence of air. The process drives off the moisture and volatiles, improves the handling properties and increases the carbon content of a fuel. At temperatures up to 300 °C the process is known as torrefaction which is being increasingly used to process biomass to a more suitable solid fuel for co-combustion with coal. At higher temperatures the product distribution favours the production of liquid bio oil, a highly oxygenated, acidic liquid resembling crude oil. Pyrolysis in the presence of subcritical liquid water is called hydrothermal conversion (HT) [5]. The process generates both solid and liquid biofuels. The process may be referred to as hydrothermal carbonization (HTC) or hydrothermal liquefaction (HTL) based up on whether solid or liquid product respectively, has been emphasized in the generation process. The solid fuel generated in HT conversion is referred to as hydrochar (biochar) and liquid product as bio-crude or bio oil. Hydrothermal conversion involves the reaction of biomass in water at high temperature and pressure with or without the catalyst. The hydrothermal processing of biomass was investigated by Shell research in the 1980s [6]. Hydrothermal processing of lignocellulosic biomass has received extensive attention over the last two decades for production of solid fuels, liquid fuels (subcritical conditions) and for gaseous fuels (supercritical conditions) [7, 8]. A non hydrothermal energy conversion pyrolysis process of biomass requires its prior drying. Prior drying of biomass necessitates expensive and energy intensive dewatering and drying steps for processing of aquatic weeds and microalgae, which have enormous amounts of water accompanied with them. An alternative route is to convert the aquatic biomass into biofuels in the aqueous phase itself, thereby obviating biomass drying. A simple comparison of the enthalpies of liquid water at 350 °C and water vapor at 50 °C (i.e., drying the biomass) indicates that processing in liquid water saves 921 kJ/kg. Hot compressed liquid water near its thermodynamic critical point (Tc = 373.95 °C, Pc = 22.064 MPa) behaves very differently from liquid water at room temperature. As water is heated along its vapor–liquid saturation curve, its dielectric constant decreases due to the hydrogen bonds between water molecules being fewer and less persistent. The reduced dielectric constant enables hot compressed water to solvate small organic molecules, allowing organic reactions to occur in a single fluid phase. Additionally, the ion product of water increases with temperature up to about 280 °C, but then decreases as the critical point is approached. This higher ion product leads

Integrating Green Chemistry and Sustainable Engineering

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Inser gas out

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5 6 Inser gas ir

3 1. Reactor 2. Band heater 3. Thermocouple 4. Controller 5. Pressure gauge 6. Strirrer

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2

Figure 1.1 Schematic diagram of a hydrothermal conversion reactor.

to higher natural levels of hydronium ions in hot compressed water, which can accelerate the rates of acid-catalyzed hydrolytic decomposition reactions [9]. Hydrothermally processing wet biomass can produce a hydrochar that retains a large proportion of the chemical energy and lipids in the original biomass. These char-bound lipids can be reacted with alcohol to produce biodiesel. At same time processing of wet aquatic biomass also produces crude bio-oil. Figure  1.1 given above presents a general schematic diagram of a hydrothermal conversion reactor used to produce the third generation biofuels. Biomass consists primarily of proteins, carbohydrates, and lipids; the principal role of hydrothermal conversion is to decompose the biomacromolecules into smaller molecules that can then be further treated, if desired, to produce specific fuels. The hydrothermal environment promotes the hydrolytic cleavage of ester linkages in lipids, peptide linkages in proteins, and glycosidic ether linkages in carbohydrates. These cleavage reactions can be accelerated by catalysts [9]. Cellulose is not soluble in water at standard conditions, but starts dissolving at 180 °C and completely dissolves around 330 °C. Due to amorphous structure, hemicellulose is easily hydrolyzed in waters at temperatures above 160 °C to monomers, which could be, at acid water conditions further transformed into chemicals. Lignin is chemically most resistant component of lignocelluloses. Dissolution and hydrolysis to monomers starts in near and supercritical water [10]. Homogenous catalysts like Na2CO3, K2CO3, NaOH, KOH, HCOOH, CH3COOH, zeolite have received attention for hydrothermal conversion. Recently some common heterogeneous catalysts have also been studied [9].

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Present chapter gives an exhaustive overview of different biofuel types, advantages of third generation biofuels and the technology involved in their production. Further transformation of algae into third generation biofuels with recent developments in the field of hydrothermal conversion processing has also been focused.

1.2

Biofuel Types

Green energy is renewable energy that is generated from sources that are considered environmentally friendly. The term biofuel is referred to as liquid, solid or gaseous fuels predominantly produced from biomass by green pathways. Biofuel is produced through contemporary biological processes, such as agriculture and anaerobic digestion, rather than a fuel produced by geological processes such as those involved in the formation of fossil fuels, such as coal and petroleum. The biomass can be converted to convenient energy-containing substances in three different ways: thermal conversion, chemical conversion, and biochemical conversion. First generation biofuels are made from the sugars and vegetable oils, which are found in arable crops. Second generation biofuels also known as advanced biofuels are made from lignocellulosic biomass or woody crops, agricultural residues or waste, which makes it harder to extract the required fuel. Advanced biofuels or biofuels produced from lignocellulosic materials made-up only 0.2% of total biofuel production (Year 2010). Third generation biofuels are the fuels derived from aquatic originated biomass, predominantly in form of algae, which are being largely focused nowadays. Biofuels can be produced from non-toxic and biodegradable renewable resources such as starch, vegetable oils, animal fats, waste biomass and algal biomasses [11]. According to the recent classification of biofuels by the European Parliament [12], they are classified as: first-generation biofuels, as those obtained from crops and animal fats, and based on mature and well-established technologies; second-generation biofuels, as those mainly obtained from ligno-cellulose biomass (i.e., wood); and third-generation biofuels, “The most accepted definition of third-generation biofuels is ‘fuels that are produced from algae-derived biomass’” [12]. First-generation biofuels include ethanol and biodiesel and are directly related to a biomass that is more than often edible. Ethanol is generally produced from the fermentation of C6 sugars (mostly glucose). Only a few different feedstocks, mostly sugarcane or corn, are actually used for the production of first-generation bioethanol. Other more marginal feedstock used to produce first-generation bioethanol

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Integrating Green Chemistry and Sustainable Engineering

includes but are not limited to whey, barley, potato wastes, and sugar beets. Sugar cane is a common feedstock for biofuel production, Brazil being one of the leading countries for its use. Second-generation biofuels are defined as fuels produced from a wide array of different feedstocks, especially non-edible lignocellulosic biomass. The conversion process for production of second-generation biofuels is usually done by two different approaches, “thermo” and “bio” pathways. The “thermo” approach covers specific processes where biomass is heated with a minimal amount of oxidizing agent, if any. All processes in that category lead to conversion of biomass into three fractions: solid known as biochar, liquid referred to as pyrolytic oil or bio-oil, and gas known as syngas, which is usually composed of carbon monoxide, hydrogen, short chain alkanes, and carbon dioxide. The “bio” pathway on other hand is somewhat comparable with a pulping process because, in most cases, cellulose is first isolated from the lignocellulosic biomass. Many processes have been considered, including classical pulping processes, steam explosion, and organosolv processes. Isolation of cellulose is a technological challenge because it has to produce the highest purity of cellulose to remove most inhibitors without consuming too much energy or too many chemicals. The most accepted definition for thirdgeneration biofuels is fuels that would be produced from algal biomass, which has a very distinctive growth yield as compared with classical lignocellulosic biomass. Production of biofuels from algae usually relies on the lipid content of the microorganisms. Usually, species are targeted having high lipid content (around 60% to 70%) and their high productivity (7.4 g/L/d for Chlorella protothecoides). Lipids obtained from algae can be processed via transesterification or can be submitted to hydrogenolysis to produce kerosene grade alkane suitable for use as drop-in aviation fuels [13]. Biofuel generations from algae is promising as algae can be grown quickly, are non toxic and biodegradable, and during their growth green house gas fixation takes place. Also growing algae does not need arable land, so there is no competition with food or feed crops [14] Algae (both macro and micro) have been suggested as potential future sources of renewable energy in transport in Europe [15].

1.3

Advantages of Third Generation Biofuels

The first generation biofuels possess notable economic, environmental and political concern as the mass production of biofuel requires more arable

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agricultural lands resulting in reduced lands for human and animal food production. Moreover, production process of first generation biofuels is also responsible for environmental degradation. Therefore, enthusiasms about first generation biofuels have been demised. As first generation biofuels are not viable, researchers focused on second generation biofuels. Because of the reason that second generation biofuels production process requires expensive and sophisticated technologies; the biofuel production from the second generation is not profitable for commercial production. Therefore, the researchers focused on third generation biofuels. It is currently considered to be a feasible alternative renewable energy resource for biofuel production overcoming the disadvantages of first and second generation biofuels. Microalgae can provide several different types of renewable biofuels. This includes methane, biodiesel and bio-hydrogen. There are many advantages for producing biofuel from algae as microalgae can produce 15 to 300 times more biodiesel than traditional crop on area basis. The harvesting cycle of microalgae is very short and growth rate is very high. Moreover, high quality agricultural land is not required for microalgae biomass production [16]. The basic composition of biofuels is more complex than the composition of fossil fuels. While fossil fuels consist only of carbon and hydrogen atoms, or hydrocarbons, biofuels contain oxygen atoms, and their chemical composition may include acids, alcohols and esters. Biofuel as fuels adds less carbon to the environment than fossil fuels as the carbon released by burning biofuel already existed as part of the carbon cycle [17]. Biofuel generation from non-food crops is highly desirable as no compromise is made with food security. Aquatic weeds (microalgae and macroalgae) are non-food crops, can serve as potential sources of biofuel generation without risking the food security [18]. They can be grown on saline water as well [16]. Micro-algae are becoming popular candidate for biofuel production due to their high lipid contents, ease of cultivation and rapid growth rate. Micro-algae seem to be a promising source since algae are able to efficiently convert sunlight, water, and CO2 into suitable products for renewable energy applications [11].

1.4

Technology of Third Generation Biofuel Production

Microalgae are thallophytes (plants lacking roots, stems, and leaves) having chlorophyll-a as their primary photosynthetic pigment .They lack a sterile covering of cells around the reproductive cells. Microalgae are naturally

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Integrating Green Chemistry and Sustainable Engineering

found in fresh water and marine environment. There are more than 300,000 species of micro algae, diversity of which is much greater than plants. They are generally more efficient converters of solar energy than higher plants because of their simple cellular structure. They have better access to water, CO2, and other nutrients because the microalgae cells grow in aqueous medium .Microalgae can convert CO2 directly into energy compounds, such as fatty acids and starch. The microalgal biodiesel production process involves three main phases: (i) microalgal biomass cultivation; (ii) microalgal oil extraction; and (iii) microalgal oil conversion (transesterification).Microalgae can be cultivated in two systems, as either open ponds or photo-bioreactors (PBRs). Open ponds consist of large shallow ponds in which the microalgae are supplied with water and nutrients, while the PBRs are closed systems that can be designed according to different configurations (i.e., tubular, flat-plate [19]). Third generation biofuels are produced from microalgae biomass by number of ways which includes: (a) biochemical conversion, (b) chemical reaction, (c) direct combustion, and (d) thermo chemical conversion [16]. In recent time thermochemical conversion has been carried in water medium under elevated temperature and pressure conditions referred to as “hydrothermal conversion (HTC)”. This conversion process is carried out with or without catalyst at subcritical water conditions (below the water critical point temperature of 374 °C and pressure of 22 MPa). Merit of HTC lies in that it obviates the need for drying of the feed stocks and is thus highly suitable for aquatic biomass. Also HTC as renewable energy process is less damaging to the environment when compared to some other competitive nonrenewable conversion processes. HTC biofuels generated are hydrochar (solid biofuel) and bio-oil (liquid biofuel) [20]. Figure 1.2 given at below shows the different stages of production processes of biodiesel and bioethanol from microalgae. Light

Nutrients

CO2

Microalgae cultivation

Harvesting

Drying

Cell disruption & oil extraction

Lipids & free fatty acids

Transesterification

Biodiesel

Starch and proteins Water Recycle

Starch hydrolysis

Formentation

Distillation

Bioethanol

Figure 1.2 Biodiesel and bioethanol production processes from microalgae.

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Table 1.1 Bio-Oil Yields From Different Biomass Resources (Adapted From [16]). Oil yields Litre/Hectare/Year

Barrels/Hectare/Year

Soybeans

400

2.5

Sunflower

800

5

Canola

1,600

10

Jathropha

2,000

12

Palm Oil

6,000

36

60,000–240,000

360–1,500

Microalgae

1.5

Transformation Potential of Algae Into Third Generation Biofuels

Microalgae form an ideal third generation biofuel feedstock (green and sustainable) due to their rapid growth rate, greenhouse gas fixation ability and high production capacity of lipids (fat). Biofuels are generally in solid, liquid or gaseous forms derived from organic matter. Production process of first generation biofuels is responsible for environmental degradation with associated economical and political issues. As second generation biofuels production process requires expensive and sophisticated technologies, so the biofuel production is not profitable for commercial production. Therefore, the researchers had to focus upon third generation biofuels. The main component of third generation biofuels is microalgae. Table  1.1 at below presents the different sources with their biooil production potential. As is clear microalgae have great prospect for this process.

1.6

Recent Developments in Biomass Transformation Into Third Generation Biofuels by Hydrothermal Conversion (HTC)

Hydrothermal conversion which originated from the charcoal formation is a 100 year old technique now in the synthesis of materials. The Bergius

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process, firstly developed by German Chemist, Friedrich Bergius in 1913 is a method of production of liquid hydrocarbons for use as synthetic fuel by hydrogenation of high-volatile bituminous coal at high temperature and pressure [21]. He was awarded the Nobel Prize in Chemistry in 1931 for his development of high pressure chemistry [22]. Friedrich appears not to have been so much interested in the generation of coal-like end product, but instead in liquid or gaseous hydrocarbon fuel [23]. HTC came into existence as mankind needed the availability of sustainable and greener energy resources. Green engineering can be broadly defined as transforming existing engineering disciplines and practices to those that lead to sustainability. Green Engineering incorporates development and implementation of products, processes, and systems that meet technical and cost objectives while protecting human health and welfare and elevating the protection of the biosphere as a criterion in engineering solutions [24]. The section below presents the recent review of HTC processes in which the primary objective has been the production of liquid product (bio crude). However other byproduct viz hydrochar, a solid biofuel generated in same process is also discussed wherever possible. The summary additionally includes HTC of some terrestrial biomass sources. Dote et al., [25] performed liquefaction of Bofryococcus braunii, a colony-forming microalga, with high moisture content, with or without sodium carbonate as a catalyst for conversion into liquid fuel and recovery of hydrocarbons. A greater amount of oil than the content of hydrocarbons in B. bruunii (50 wt. % db) was obtained, in a yield of 57–64 wt. % at 300 °C. The oil was equivalent in quality to petroleum oil. The recovery of hydrocarbons was maximum (>95%) at 300 °C. Catallo et al., [26] studied to examine the hydrothermal treatment of three invasive aquatic plants (i.e., Lemna sp., Hydrilla sp., and Eichhornia sp.) with respect to the generation of semi-volatile hydrocarbon product mixtures. Identical HT treatments yielded similar semi-volatile product mixtures for Hydrilla sp. and Eichhornia sp. versus a significantly different mixture for Lemna sp. Pre-treatment (i.e., control) extracts of the plant substrates showed no semi-volatile hydrocarbons. Post-HT treatment product mixtures were comprised of complex mixtures of compounds including branched and unbranched alkanes and alkenes as well as light aromatics including substituted benzenes and phenols. All three plant HT product mixtures were dominated by phenol, C1 alkyl phenols, and oxygenated cycloalkenes. Lemna sp. Products showed much more diverse distributions of C2-C5 alkyl benzenes, alkyl indanes, and alkyl naphthalenes at higher relative levels. Other products from the Lemna sp. HT treatment included C2-C4 phenols, and alkyl indole and indanol compounds.

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Results of wet chemical analyses showed that a major difference between Lemna sp. and the other two plants was significantly higher extractives levels in the former. It was found that this fraction accounted for much of the complexity in HT product mixture of the Lemna sp. biomass. For all HT treatments the substrate mass was reduced by 95% or more. Xiu et al., [27] treated duckweed over a temperature range 250–374 °C, a retention time of 5–90 min, and a catalyst loading of 0–10 wt. % by a high-pressure reactor. Operating temperature, retention time, and catalyst loading were all found to affect oil yield. The lower limit of reaction temperature for the production of heavy oil was found to be 260 °C. The highest oil yields were obtained at 30% on an organic basis under the conditions: reaction temperature of 340 °C, retention time of 60 min, and operation without catalyst. The average heating value of the bio-oil product was estimated at 34 MJ/kg. Zhou et al., [28] converted marine macroalgae Enteromorpha prolifera, to bio-oil by HTC in a batch reactor at temperatures of 220–320 °C. The liquefaction products were separated into a dichloromethane-soluble fraction (bio-oil), water-soluble fraction, solid residue, and gaseous fraction. Effects of the temperature, reaction time, and Na2CO3 catalyst on the yields of liquefaction products were investigated. A moderate temperature of 300 °C with 5 wt. % Na2CO3 and reaction time of 30 min led to the highest bio-oil yield of 23.0 wt. %. The raw algae and liquefaction products were analyzed using elemental analysis, FT-IR spectroscopy, GC-MS, and 1 hr nuclear magnetic resonance (NMR). The higher heating values (HHVs) of bio-oils obtained at 300 °C were around 28–30 MJ/kg. The biooil was a complex mixture of ketones, aldehydes, phenols, alkenes, fatty acids, esters, aromatics, and nitrogen containing heterocyclic compounds. Acetic acid was the main component of the water-soluble products. Brown et al., [29] converted the marine microalga Nannochloropsis sp. into a crude bio-oil product and a gaseous product via hydrothermal processing from 200 °C to 500 °C and a batch holding time of 60 min. A moderate temperature of 350 °C led to the highest bio-oil yield of 43 wt. %. They estimated the heating value of the bio-oil to be about 39 MJ /kg, which was comparable to that of a petroleum crude oil. The H/C and O/C ratios for the bio-oil decreased from 1.73 and 0.12, respectively, for the 200 °C product to 1.04 and 0.05, respectively, for the 500 °C product. Major bio-oil constituents included phenol and its alkylated derivatives, heterocyclic N-containing compounds, long-chain fatty acids, alkanes and alkenes, and derivatives of phytol and cholesterol. Nearly 80% of the carbon and up to 90% of the chemical energy originally present in the microalga was recovered as either bio-oil or gas products.

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Integrating Green Chemistry and Sustainable Engineering

Eberhardt et al., [30] examined HT conversion of seeds from Chinese privet (Ligustrum sinense) from a lipid-rich biomass resource. Aqueous seed slurries were transformed into biphasic liquid systems comprised of a milky aqueous phase overlain by a black organic layer. Headspace contained elevated levels of CO2 and acetic acid. Analysis of the semi-volatiles by GC-MS showed the formation of alkyl substituted benzenes, oxygenated cyclicalkenes, phenol, substituted phenolics, and alkyl substituted pyridines. Biller and Ross [31] liquefied a range of model biochemical components, microalgae and cyanobacteria with different biochemical contents under hydrothermal conditions at 350 °C, 200 bar in water, 1 M Na2CO3 and 1 M formic acid. The model compounds included albumin and a soya protein, starch and glucose, the triglyceride from sunflower oil and two amino acids. Microalgae included Chlorella vulgaris, Nannochloropsis occulata and Porphyridium cruentum and the cyanobacteria Spirulina. The yields and product distribution obtained for each model compound was used to predict the behaviour of microalgae with different biochemical composition and was validated using microalgae and cyanobacteria. Broad agreement was reached between predictive yields and actual yields for the microalgae based on their biochemical composition. The yields of bio-crude were 5–25 wt. % higher than the lipid content of the algae depending upon biochemical composition. The yields of bio-crude followed the trend, lipids > proteins > carbohydrates. Duan and Savage [32] determined the influence of a Pt/C catalyst, highpressure H2, and pH on the upgrading of a crude algal bio-oil in supercritical water (SCW). The SCW treatment led to a product oil with a higher heating value (42 MJ/kg) and lower acid number than the crude bio-oil. The product oil was also lower in O and N and essentially free of sulfur. Including the Pt/C catalyst in the reactor led to freely flowing liquid product oil with a high abundance of hydrocarbons. Overall, many of the properties of the upgraded oil obtained from catalytic treatment in SCW were similar to those of hydrocarbon fuels derived from fossil fuel resources. The work showed that the crude bio-oil from HTC of a microalga could be effectively upgraded in supercritical water in the presence of a Pt/C catalyst. Carrier et al., [33] converted uncontaminated and As-contaminated fronds of Pteris vittata L. and As-hyperaccumulator fern (used to phytoextract As from contaminated soils and water) by subcritical water (300 °C, 25 Pa) and supercritical water (400 °C, 25 Pa) treatments. Frond biomass was reduced between 70% and 77%. Compared to sub-critical conditions, supercritical conditions decreased C and inorganic contents in both the

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solid and liquid phases for uncontaminated and contaminated fronds and promoted CH4 formation. Higher As, Fe and Zn contents in contaminated fronds promoted decreasing C contents and the formations of cyclopentenones and benzenediols in the liquid phase. Anastasakis and Ross [34] converted the brown macro-alga Laminaria saccharina into bio-crude by HTC in a batch reactor. Experiments were performed in a batch bomb type stainless steel reactor (75 ml). The heating rate of the reactor was 25 °C min–1. The reactor was charged with the appropriate amounts of seaweed biomass and water. In the catalytic runs the appropriate amount of KOH was added to the reactants. Liquefaction experiments were performed for a range of different biomass/water ratios (5–20%), residence times (15–120 min), temperatures (250–370 °C) and catalyst/ biomass loadings (0–100%). on completion, the reactor was cooled using compressed air. The gases were vented, and the rest of the products were separated using DCM solvent. The influences of variables such as reactor loading, reaction time, and temperature were investigated with and without the addition of an alkali catalyst (KOH). Mass balance was determined for the different product phases. The bio-crude composition was analyzed for its elemental content, molecular weight distribution, and chemical composition. A maximum bio-crude yield of 19.3 wt. % was obtained with a 1:10 biomass: water ratio at 350 °C and a residence time of 15 min without the presence of the catalyst. The bio-crude had an HHV of 36.5 MJ/kg and was similar in nature to a heavy crude oil or bitumen. Yields of bio-crude were low and contained large amounts of high molecular weight material and nitrogen. The HHV of the bio-crude was high compared to the original seaweed and the oxygen content was significantly lowered. Julia et al., [35] investigated the fast HTC of green marine algae Nannochloropsis sp. at batch reaction times of 1, 3, and 5 min and set-point temperatures of 300–600 °C. They also performed conventional liquefaction for 60 min at the same temperatures. The biocrude yield obtained for 1 min reaction time, which was only long enough to heat the reactor from room temperature to about half of the set-point temperature (in °C), increased with an increasing set-point temperature to 66 ± 11 wt. % (dry and ash free basis) at a set-point temperature of 600 °C. The biocrude obtained at this condition contained 84% of the carbon and 91 ± 14% of the heating value present in the dry algae feedstock. For a reaction time of 1 min, as the set-point temperature increased, light biocrude (i.e., hexane solubles) made up less of the total biocrude. The biocrudes produced by fast liquefaction had carbon contents and higher heating values similar to biocrudes from the traditional isothermal liquefaction process, which involves treatment for tens of minutes.

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Integrating Green Chemistry and Sustainable Engineering

Singh et al., [36] performed catalytic hydrothermal upgradation of wheat husk at 280 °C for 15 min in the presence of alkaline catalysts (KOH and K2CO3). Total bio-oil yield (31%) comprising of bio-oil1 (ether fraction) and bio-oil2 (acetone fraction) was maximum with K2CO3 solution. Powder XRD analysis of wheat husk as well as bio-residue samples showed that the peaks due to cellulose, hemicellulose and lignin become weak in bio-residue samples which suggested that these components had undergone hydrolytic cleavage/decomposition. The FT-IR spectra of bio-oils indicated that the lignin in the wheat husk samples was decomposed to low molecular weight phenolic compounds. Duan et al., [37] studied to treat hydrothermally duckweed (Lemna sp.) and to determine the influence of four different variables (temperature, time, reactor loading and K2CO3 loading) on products (bio-oil, solid residue, gas, and water soluble) distribution and composition. The ranges for each of the independent variables: temperature between 270 °C and 380 °C, batch holding time between 10 and120 min, reactor loading between 0.5 and 5.5 g, and K2CO3 loading between 5 and 50 wt. % were considered. The reactor with total internal volume of 25 ml was custom made high-pressure/corrosion-resistant mini autoclave that allowed for the recovery and analysis of both the liquid- and gas phase products in a single run. The selection of the most appropriate operating variable ranges was based on the yield of bio-oil so that the highest yield can be achieved in the range of each variable. The bulk properties (i.e., elemental composition, and heating value) and physico-chemical characteristics (i.e., molecular constituents, functional group allocation, and carbon speciation) of bio-oils were characterized in detail. Of the four variables, temperature and K2CO3 loading were always the most influential factors to the relative amount of each component. The presence of K2CO3 was unfavorable for the production of bio-oil produced. Bio-oil was enriched in carbon and hydrogen and had reduced levels of O compared with the original duckweed feedstock. The higher heating values of the bio-oil were estimated within the range of 32–36 MJ/kg. Major bio-oil constituents included ketones and their alkylated derivatives, alcohols, heterocyclic nitrogencontaining compounds, saturated fatty acids and hydrocarbons. Analysis of the bio-oil showed that each biomacromolecule-lipid, protein and carbohydrate was converted to intermediate products via hydrolysis, decarboxylation and hydrogenation. The study demonstrated that duckweed biomass could serve as a good source of alternative energy. Eboibi et al., [38] studied the pretreatment of microalgae biomass for protein extraction prior to HTC for biocrude production. The impact of operating conditions on both the pretreatment and HTC steps was

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investigated. Following HTC using the pretreated algae with an initial solid content of 16% w/w for 30 min at 310 °C, the biocrude yield was 65 wt. %, which was more than a 50% improvement in yield as compared to HTC of untreated algae under the same reaction conditions. Similar biocrude yield using the untreated algae required a much higher reaction temperature of 350 °C. Using recycled process water as reaction media led to a 25 wt. % higher biocrude yield. HTC of pretreated algae led to 32–46% nitrogen reduction in resultant biocrude. The biocrude had a HHV of 28 MJ /kg to 34 MJ/ kg. Singh et al., [39] performed thermal and catalytic HTC of water hyacinth at temperatures from 250 °C to 300 °C under various water: hyacinth ratios of 1:3, 1:6 and 1:12. Reactions were also carried out under various residence times (15–60 min) as well as catalytic conditions (KOH and K2CO3). The use of alkaline catalysts significantly increased the biooil yield. Maximum bio-oil yield (23 wt. %) comprising of bio-oil1 and bio-oil2 as well as conversion (89%) were observed with 1 N KOH solution. 1 hr NMR and 13C NMR data showed that both bio-oil1 and bio-oil2 had high aliphatic carbon content. FT-IR of bio-residue indicated that the usage of alkaline catalyst resulted in bio-residue samples with lesser oxygen functionality indicating that catalyst has a marked effect on nature of the bio-residue and helps to decompose biomass to a greater extent compared to thermal case. Barreiro et al., [40] studied HTC while recovering the nutrients, and simultaneously treating the wastewater generated in production of liquid biofuel. Cultivation trials of Nannochloropsis gaditana, Phaeodactylum tricornutum, Chlorella vulgaris and Scenedes musalmeriensis was carried out by reusing nutrients recovered from the aqueous by-products obtained during biofuel production via HTC. The algal biomass was subjected to HTC at 350 °C for 15 min in micro autoclaves with a volume of 10 mL. 70% of their volume was filled with water containing 10 wt. % of algal dry matter. The headspace was flushed with nitrogen, and subsequently pressurized up to 2 MPa. Nutrients were recovered, while simultaneously treating the wastewater generated during the production of biofuel. Direct recycling of the aqueous phase was compared to the use of an intermediate step (supercritical water gasification) to purify this stream. Also, two growth parameters (pH and percentage of substitution of nutrients from the standard medium) were studied. The results showed that the response of microalgae species to the recycling of nutrients was strain-dependent. Vo et al., [41] investigated the HTC of microalga Aurantiochytrium sp. at various reaction temperatures (250–400 °C) and reaction times (10–60 min). They observed that the product distributions of bio-oil,

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Integrating Green Chemistry and Sustainable Engineering

aqueous-phase, and gaseous products were strongly affected by reaction temperature and time. The highest bio-oil yield of 51.22 wt. % was obtained at 400 °C for 10 min. The kinetics rate constants indicated that the formations of bio-oil and aqueous-phase products from decomposition of proteins, lipids, and carbohydrates were the dominant reaction pathways. Huang et al., [42] examined decomposition behavior of two kinds of high-protein high-ash microalgae, including wild Cyanobacteria sp. and cultivated Bacillariophyta sp., via HTC. A maximum bio-oil yield of 21.10 wt. % (Cyanobacteria sp.) and 18.21 wt% (Bacillariophyta sp.) on dry basis was achieved at 325 °C for 45–60 min, accompanying with the maximum (51.38% and 48.76%) of energy recovered in bio-oil. Although high HHV (33.87–36.51 MJ/ kg) and high H/C ratio (1.37–1.62) were estimated for bio-oil products, high N/C ratio (0.06–0.09) and nitrogen content (5.31–7.50%) negatively affected its quality. GC-MS analysis combining with FT-IR and NMR detection revealed that components and functional structure of bio-oils were greatly distinguished from other algae bio-oil. Their study demonstrated the feasibility of applying HTC to produce biooil from high-protein high-ash microalgae, and the findings on bio-oil properties and transfer behavior of carbon and nitrogen supplied useful information for downstream utilization. Deniel et al., [43] reported bio-oil production by HTC of black currant pomace (Ribes nigrumL.), a fruit residue obtained after berry pressing. The bio-oil had a higher heating value of 35.9 MJ/ kg and low ash content, which made it suitable for energy applications. They reported the influence of process parameters on yields and carbon distribution between products: temperature (563–608 K), holding time (0–240 min), mass fraction of dry biomass in the slurry (0.05–0.29), and initial pH (3.1–12.8) by adding sodium hydroxide (NaOH). The bio-oil accounted for at least 24% of mass fraction of the initial dry biomass, while char yield ranged from 24% to 40%. A temperature of 583 K enhanced the bio-oil yield, up to 30%, while holding time did not have a significant influence on the results. Increasing biomass concentrations decreased bio-oil yields from 29% to 24%. Adding sodium hydroxide decreased the char yield from 35% at pH = 3.1 (without NaOH) to 24% at pH = 12.8. It also increased the bio-oil yield and carbon transfer to the aqueous phase. Cao et al., (2016) [44] examined the effect of glycerol used as a co-solvent on yields of bio-oil derived from rice straw through HTC. The reaction was conducted in a high-pressure batch reactor with different volume ratios of glycerol to water. The quality of the derived bio-oil was analyzed in terms of its elemental composition, heating value, water content, ash

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content, and acid number. FT-IR and GC-MS were conducted to analyze the chemical composition of the derived bio-oils. The following optimal conditions were obtained: 1:1 vol ratio of glycerol to water with 5 wt. % of Na2CO3 at 260 °C for 1 hr. Under these conditions, 50.31 wt. % of biooil and 26.65 wt. % of solid residue were produced. It was concluded that glycerol could be used as a co-solvent in HTC of rice straw at moderate temperatures to obtain bio-oil with high yield and quality. Muppaneni et al., [45] investigated the HTC of Cyanidioschyzon merolae algal species under various reaction temperatures and catalysts. Liquefaction of microalgae was performed with 10% solid loading for 30 min at temperatures of 180–300 °C to study the influences of two base and two acid catalysts on HTC product fractions. Maximum biocrude oil yield of 16.98% was obtained at 300 °C with no catalyst. The biocrude oil yield increased to 22.67% when KOH was introduced into the reaction mixture as a catalyst. The algal biocrude and biochar had higher heating values (HHV) of 32.22 MJ/ kg and 20.78 MJ/ kg respectively when no catalyst was used. Rather et al., [46] carried out lab-scale application of hydrothermal carbonization to convert aquatic weed Ceratophyllum demersum into biofuels, bio-oil and hydrochar. Hydrothermal carbonization has been carried out with and without catalysts in a high-pressure reactor under subcritical temperatures 240–320 °C, residence times 10–40 min and biomass to water dilution ratios varying from 1:4 to 1:12. The research is thought to pay a way forward in the direction of meeting part of global energy demand and mitigate the problem of secondary pollution caused by piling up and decay of aquatic weeds. Rather et al., [20] also carried out HTC of macrophyte Potamogeton lucens under same conditions as above. Both bio-oil and hydrochar were obtained as product biofuels.

1.7

Conclusion

Biomass can be converted into convenient energy-containing substances in three different ways: thermal conversion, chemical conversion, and biochemical conversion. As first-generation biofuels are made from sugars, starch and vegetable oils, they have an impact on human food security. Thus biofuels in form of biodiesel and bioethanol produced from variety of food crops are thought to be a compromise with the human food security. Second generation biofuels are generated from plant material. However the transformation technology of second generation biofuels at present is not

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commercially viable. Biofuels generated from non-food crops like aquatic weeds (microalgae and macroalgae) are referred to as third generation biofuels, have great potential to meet part of our future energy demand. Above is achieved without compromising food security and the technology also seems to be viable.

References 1. Ben-Iwo, J., Manovic, V., Longhurst, P., Biomass resources and biofuels potential for the production of transportation fuels in Nigeria. Renewable and Sustainable Energy Reviews, 63, 172–192, 2016. 2. Hayes, C.J., Burgess, D.R Jr., Manion, J.A., Combustion Pathways of Biofuel Model Compounds: A Review of Recent Research and Current Challenges Pertaining to First-, Second-, and Third-Generation Biofuels. Advances in Physical Organic Chemistry, 49, 2018. 3. Saladini, F., Patrizi, N., Pulselli, F.M., Marchettini, N., Bastianoni, S., Guidelines for emergy evaluation of first, second and third generation biofuels. Renewable and Sustainable Energy Reviews, 66, 221–227, 2016. 4. Kamm, B., Gruber, P.R., Kamm, M., Biorefineries-industrial Processes and Products. Status Quo and Future Directions. vol. 1. Weinheim, Germany, Wiley-VCH Verlag GmbH, 2006. 5. Tekin, K., Karagöz, S., Bektaş, S., A review of hydrothermal biomass processing. Renewable and Sustainable Energy Reviews, 40, 673–687, 2014. 6. Ruyter et al., Process for producing hydrocarbon-containing liquids from biomass. US Patent to Shell International Research, 4(670), 613, 1987. 7. Peterson, A.A., Vogel, F., Lachance, R.P., Fröling, M., Antal, Jr., M.J., Tester, J.W., Thermochemical biofuel production in hydrothermal media: A review of sub- and supercritical water technologies. Energy Environ. Sci., 1(1), 32–65, 2008. 8. Ross, A.B., Biller, P., Kubacki, M.L., Li, H., Lea-Langton, A., Jones, J.M., Hydrothermal processing of microalgae using alkali and organic acids. Fuel, 89(9), 2234–2243, 2010. 9. Yeh, T.M., Dickinson, J.G., Franck, A., Linic, S., Thompson, L.T., Savage, P.E., Hydrothermal catalytic production of fuels and chemicals from aquatic biomass. J. Chem. Technol. Biotechnol., 88(1), 13–24, 2013. 10. Pavlovič, I., Knez, Ž., Škerget, M., Hydrothermal reactions of agricultural and food processing wastes in sub- and supercritical water: a review of fundamentals, mechanisms, and state of research. J. Agric. Food Chem., 61(34), 8003–8025, 2013. 11. Shuba, E. S., Kifle, D., Microalgae to biofuels: ‘Promising’ alternative and renewable energy, review. Renewable and Sustainable Energy Reviews, 81, 743–755, 2018.

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12. European Commission, The impact of biofuels on transport and the environment, and their connection with agricultural development in Europe. Directorate General for the Internal Policies Policy Department B: Structural and Cohesion Policies -Transport and Tourism, Brussells. (http://www.europarl.europa.eu/RegData/etudes/STUD/2015/513991/IPOL_STU, 2015. 13. Lee, R.A., Lavoie, J.-M., from first- to third-generation biofuels: Challenges of producing a commodity from a biomass of increasing complexity. Animal Frontiers, 3(2), 2–11, 2013. 14. Alaswad, A., Dassisti, M., Prescott, T., Olabi, A.G., Technologies and developments of third generation biofuel production. Renewable and Sustainable Energy Reviews, 51, 1446–1460, 2015. 15. Allen, E., Wall, D.M., Herrmann, C., Xia, A., Murphy, J.D., What is the gross energy yield of third generation gaseous biofuel sourced from seaweed? Energy, 81, 352–360, 2015. 16. Alam, F., Mobin, S., Chowdhury, H., Third generation biofuel from Algae. 6th BSME International Conference on Thermal Engineering (ICTE 2014. Procedia Engineering, 105, 763–768, 2015. 17. Popp, J., Lakner, Z., Harangi-Rákos, M., Fári, M., The effect of bioenergy expansion: Food, energy, and environment. Renewable and Sustainable Energy Reviews, 32, 559–578, 2014. 18. Rather, M.A., Khan, N.S., Gupta, R., Comparative Kinetic Studies on NonIsothermal Pyrolysis of Ceratophyllum demersum. Macrophyte under Inert Conditions. Energy Environ. Focus, 5, 316–322, 2016b. 19. Gambelli, D., Alberti, F., Solfanelli, F., Vairo, D., Zanoli, R., Third generation algae biofuels in Italy by 2030: A scenario analysis using Bayesian networks. Energy Policy, 103, 165–178, 2017. 20. Rather, M.A., Khan, N.S., Gupta, R., Hydrothermal carbonization of macrophyte Potamogeton lucens for solid biofuel production. Engineering Science and Technology, an International Journal, 20(1), 168–174, 2017. 21. Hu, B., Wang, K., Wu, L., Yu, S.H., Antonietti, M., Titirici, M.M., Engineering carbon materials from the hydrothermal carbonization process of biomass. Adv. Mater. Weinheim., 22(7), 813–828, 2010. 22. Bergius, F., Chemical reactions under high pressure. Nobel Lecture, 1932. 23. Bergius, F., Chemical reactions under high pressure. Nobel Lectures, Chemistry 1922-1941. Amersterdam, Elsevier. pp. 244–276, 1966. 24. Ritter, S.K., A green agenda for engineering: new set of principles provides guidance to improve designs for sustainability needs. Chemical & Engineering News, 81(29), 30–32, 2003. 25. Dote, Y., Sawayama, S., Inoue, S., Minowa, T., Yokoyama, S.-ya., Recovery of liquid fuel from hydrocarbon-rich microalgae by thermochemical liquefaction. Fuel, 73(12), 1855–1857, 1994. 26. Catallo, W.J., Shupe, T.F., Eberhardt, T.L., Hydrothermal processing of biomass from invasive aquatic plants. Biomass and Bioenergy, 32(2), 140–145, 2008.

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27. Xiu, S.N., Shahbazi, A., Croonenberghs, J., Wang, L.J., Oil production from duckweed by thermochemical liquefaction. Energy Sources Part A Recovery Utilization &. Env. Effects, 32(14), 1293–1300, 2010. 28. Zhou, D., Zhang, L., Zhang, S., Fu, H., Chen, J., HTC of Macroalgae Enteromorpha prolifera to Bio-oil. Energy and Fuels, 24, 4054–4061, 2010. 29. Brown, T.M., Duan, P., Savage, P.E., HTC and Gasification of Nanochloropsis sp. Energy Fuels, 24, 3639–3646, 2010. 30. Eberhardt, T.L., Catallo, W.J., Shupe, T.F., Hydrothermal transformation of Chinese privet seed biomass to gas-phase and semi-volatile products. Bioresour. Technol., 101(11), 4198–4204, 2010. 31. Biller, P., Ross, A.B., Potential yields and properties of oil from the hydrothermal liquefaction of microalgae with different biochemical content. Bioresour. Technol., 102(1), 215–225, 2011. 32. Duan, P., Savage, P.E., Upgrading of crude algal bio-oil in supercritical water. Bioresour. Technol., 102(2), 1899–1906, 2011. 33. Carrier, M., Loppinet-Serani, A., Absalon, C., Marias, F., Aymonier, C., Mench, M., Conversion of fern (Pteris vittata L.) biomass from a phytoremediation trial in sub- and supercritical water conditions. Biomass and Bioenergy, 35(2), 872–883, 2011. 34. Anastasakis, K., Ross, A.B., Hydrothermal liquefaction of the brown macroalga Laminaria saccharina: effect of reaction conditions on product distribution and composition. Bioresour. Technol., 102(7), 4876–4883, 2011. 35. Julia, L.F., Valdez, P.J., Savage, P.E., Fast hydrothermal liquefaction of Nannochloropsis sp. to produce biocrude. Energy Fuels, 27, 1391–1398, 2013. 36. Singh, R., Bhaskar, T., Dora, S., Balagurumurthy, B., Catalytic hydrothermal upgradation of wheat husk. Bioresour. Technol., 149, 446–451, 2013. 37. Duan, P., Chang, Z., Xu, Y., Bai, X., Wang, F., Zhang, L., Hydrothermal processing of duckweed: effect of reaction conditions on product distribution and composition. Bioresour. Technol., 135, 710–719, 2013. 38. Eboibi, B.E., Lewis, D.M., Ashman, P.J., Chinnasamy, S., Influence of process conditions on pretreatment of microalgae for protein extraction and production of biocrude during hydrothermal liquefaction of pretreated Tetraselmis sp. RSC Adv., 5(26), 20193–20207, 2015. 39. Singh, R., Balagurumurthy, B., Prakash, A., Bhaskar, T., Catalytic HTC of water hyacinth. Bioresour. Technol., 178, 157–165, 2015. 40. Barreiro, D.L., Bauer, M., Hornung, U., Posten, C., Kruse, A., Prins, W., Cultivation of microalgae with recovered nutrients after HTC. Algal Research, 9, 99–106, 2015. 41. Vo, T.K., Lee, O.K., Lee, E.Y., Kim, C.H., Seo, J.-W., Kim, J., et  al., Kinetics study of the hydrothermal liquefaction of the microalga Aurantiochytrium sp. KRS101. Chemical Engineering Journal, 306, 763–771, 2016. 42. Huang, Y., Chen, Y., Xie, J., Liu, H., Yin, X., Wu, C., Bio-oil production from hydrothermal liquefaction of high-protein high-ash microalgae including wild Cyanobacteria sp. and cultivated Bacillariophyta sp. Fuel, 183, 9–19, 2016.

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43. Déniel, M., Haarlemmer, G., Roubaud, A., Weiss-Hortala, E., Fages, J., Optimisation of bio-oil production by hydrothermal liquefaction of agroindustrial residues: Blackcurrant pomace (Ribes nigrum L.) as an example. Biomass and Bioenergy, 95, 273–285, 2016. 44. Cai, J., Li, B., Chen, C., Wang, J., Zhao, M., Zhang, K., HTC of tobacco stalk for fuel application. Bioresour. Technol., 220, 305–311, 2016. 45. Muppaneni, T., Reddy, H.K., Selvaratnam, T., Dandamudi, K.P.R., Dungan, B., Nirmalakhandan, N., et al., Hydrothermal liquefaction of Cyanidioschyzon merolae and the influence of catalysts on products. Bioresour. Technol., 223, 91–97, 2017. 46. Rather, M.A., Khan, N.S., Gupta, R., Catalytic hydrothermal carbonizationof invasive macrophyte Hornwort (Ceratophyllum demersum) for productionof hydrochar: a potential biofuel. Int. J. Environ. Sci. Technol., 2016a.

2 Recent Progress in Photocatalytic Water Splitting by Nanostructured TiO2Carbon Photocatalysts – Influence of Interfaces, Morphological Structures and Experimental Parameters V. Preethi1, M. Mamatha Kumari2,*, N. Ramesh Reddy2, U. Bhargav2, K. K. Cheralathan3, C. H. Shilpa Chakra4 and M. V. Shankar2 1

Department of Civil Engineering, Hindustan Institute of Technology and Science, Tamil Nadu, India 2 Nanocatalysis and Solar Fuels Research Laboratory, Department of Materials Science & Nanotechnology, Yogi Vemana University Kadapa, Andhra Pradesh, India 3 Department of Chemistry, School of Advanced Sciences, Vellore Institute of Technology, Tamil Nadu, India 4 Centre for Nano Science and Technology, Institute of Science and Technology, Jawaharlal Nehru Technological University, Hyderabad, India

Abstract Carbon-based nanostructures such as quantum dots, fullerenes, carbon nanotubes (CNTs) and graphene have been manifested to be useful materials for variety of applications. Especially for photocatalytic hydrogen production, the unique optical, electrical and morphological properties of these materials are highly favorable. Splitting water using solar energy to generate hydrogen gas is the most suitable method for sustainable energy production and it has attracted the whole world because of both economic and environmental benefits. Titania is one of the most common semiconductor photocatalyst due to its ready availability, inexpensiveness, chemical stability and facile photocatalytic activity under UV light. So far, several lab-scale experimental results have been reported with an aim to increase the quantity of hydrogen produced. *Corresponding author: [email protected] Shahid-ul-Islam (ed.) Integrating Green Chemistry and Sustainable Engineering, (23–56) © 2019 Scrivener Publishing LLC

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This chapter is focused on recent progress in enhanced photocatalytic hydrogen production from water by photocatalytic water splitting with TiO 2 - carbon nanocomposites having different morphological structures. Emphasis is also given on the unique interface formed between TiO2 and carbon nanostructures. This chapter further highlights the latest findings on the effect of additives and operating parameters viz., sacrificial agents, catalyst dosage, pH, type of reactor etc., on hydrogen production from water. The perspectives on sustainable and economic production of hydrogen by water splitting using solar energy for large-scale applications are discussed at the end of the chapter. Keywords: Photocatalysis, water splitting, hydrogen production, TiO2 , nanostructures , carbon nanostructures, nanocomposites, sacrificial agents

2.1

Photocatalysis

Over the last few decades, there is a dramatic increase in the need for fossil fuels to be used in automobiles, industries and power plants. Fossil fuel combustion produces hazardous gases which cause environmental degradation and mainly, air pollution. In this regard, researchers around the world are investigating on a new and potential fulcrum through photocatalysis process to use the renewable sources such as hydrogen (H2) fuel [1, 2]. Hydrogen is a clean, eco-friendly, and non-hazardous fuel because combustion of it produces only water [3, 4]. However photocatalysis process itself mainly suffers from fast recombination of e– and h+ (hole) pairs and because of this photocatalytic hydrogen production rate is limited. This limitation can be overcome by introduction of heterojunctions between catalyst and with co-catalyst with surface interaction [5] which will hinder the e– and h+ recombination and also help in effective utilization of the photogenerated electrons for redox reactions. Among the many photocatalysts experimented, tailor-made TiO2 nanoparticles and thin-films were widely used for industrial-scale applications because of its ideal semiconductor characteristics, earth abundant, efficiency, stability and relatively low cost [6–8]. Different hydrogen production methods and their related applications are shown in Figure 2.1. Semiconductors are characterized by having an energy gap between their filled valence band and the empty conduction band [9]. To promote an electron (e-) from valence band to conduction band, absorption of light with energy greater than energy gap should be provided. In the semiconductor photocatalysis, electrons absorb energy from photons and jump to conduction band leaving behind holes (h+) in valance band. Later, some

Recent Progress in Photocatalytic Water Splitting

25

Solar energy Photocatalytic water spliting

Thermal splitting of water

Electrolysis of water

Stationary / domestic / Electricity/ heat generation

Biomass Gasification

Fermentation

Transport applications

Balancing of renewable Electricity production

Pyrolysis

Natural gas Steam reforming with CO2 capture Plasma reforming

Locally stored energy Pyrolysis

Portable electronics

Figure 2.1 Schematic representation of major hydrogen production methods and their applications; hydrogen is mainly used for transportation, electricity generation and fuel cell applications.

of these electrons and holes can move onto the surface active sites to carryout redox reactions while the remaining of the electrons may encounter the left out holes dissipating energy as heat due to recombination. Hence in the semiconductor photocatalysis the following three main steps are involved: § Step 1: Photocatalyst absorbs photons with energy above 1.23 eV (λ Eg

e– (ii)

(i) O2/H2O (V=1.23 eV)

2 VB

h+ (iii)

3 2H2O + 4H+

O2 + 4H+

Figure 2.2 Schematic representation of mechanism of photocatalytic water splitting; semiconductor creates photogenerated electron-hole pair with help of photon energy and then electron migrates to the surface of the semiconductor and reacts with H+ and produce hydrogen (Reprinted with permission from Reddy et al., [10], Copyright 2018, Springer).

2.2

Carbon Nanotubes-TiO2 and Other Nanocomposite for Photocatalytic Water Splitting

One dimensional (1-D) carbon nanostructures such as SWCNTs, MWCNTs and CNHs possess unique structural, chemical, thermal, electrical, optical properties and hence they are attractive for variety of scientific applications [11–13]. These nanostructures, when combined with other nanomaterials, have shown more beneficial effects and express better properties compared to the individual materials. For example, TiO2 suffers from fast recombination of photogenerated electron-hole pairs. But addition of 1-D carbon nanostructures to TiO2, can minimize the recombination and its mechanism is well established [14, 15]. This section, describes the influence of different morphologies of TiO2-carbon nanocomposites for hydrogen production. CNT-TiO2 nanocomposites have been prepared by many physical and chemical routes such as hydrothermal, sol-gel, wet impregnation and template should be template instead of templated based methodologies

Recent Progress in Photocatalytic Water Splitting

27

[16–18]. Preparation method plays a crucial role as it affects morphology, crystallinity, light absorption and formation of interface between carbon nanostructure and TiO2 which may cause some synergetic effects and lead to enhanced photocatalytic water splitting [19, 20]. Numerous TiO2 nanostructures such as nanoparticles, rods, tubes, wires and hierarchical structures have been reported for enhanced hydrogen production [21]. All these TiO2 nanostructures are widely employed along with the base metals, non-metals, noble metals etc. [22, 23] for improvement of hydrogen production. However comparatively limited reports are available in the literature on the use of nanoTiO2-CNT composites for hydrogen production. CNT-TiO2 nanotubes composites were investigated scoring on unique properties of TiO2 nanotubes such as one directional electron flow and high surface area. CNTs were used in the nanocomposite photocatalysts to enhance the electron transfer efficiency and minimize electron-hole recombination. The nanocomposite of CNTs and TiO2 nanotubes (TiNT) synthesized even by simple impregnation method had showed better photocatalytic H2 production than CNTs-TiO2 nanoparticle nanohybrid and TiO2-P25 under solar light irradiation with five vol.% glycerol aqueous solution [24]. Even though both these nanocomposites showed extension of the absorption edge to the visible region, CNT-TiNT nanocomposite displayed higher efficacy under similar experimental conditions. This observation leads to an inference that the morphology of TiO2, whether it is tubular morphology or particle structure decides photocatalytic activity enhancement [24, 25]. X-ray photoelectron spectroscopy, XPS is a general tool used to confirm Ti-O-C bond formation in the nanocomposites. Nature of bonding between CNT and TiO2 affects charge transfer between TiO2 and CNT, which further influences the photocatalytic activity. It is has been evidenced that formation of Ti-O-C bond leads to efficient electron transfer, effective utilization of electron-hole pair. In these composites, CNT works as both sensitizer and co-catalyst with a possible photocatalytic hydrogen production mechanism shown in Figure 2.3. When combined with Pt, a novel CNT-TiNT nanocomposite (Pt loaded carbon/TiNT/CNT) showed enormous improvement in H2 production under full spectrum of solar light [26]. This photocatalyst showed a hydrogen production rate of 37.6 mmol g−1 with continuous light irradiation for up to 100 h which is 156 folds higher than the Pt loaded P-25 and 10 folds higher than TiNT. The possible reason for this enormous activity has been explained as follows: upon illumination of light, excited electrons from TiO2 are immediately attracted by the carbon layer and it acts as a super highway for electron transport. CNT acts as a good conductor and it helps in conducting electrons between carbon layer and TiNT. TEM images showing the uniform coating of carbon on TiNT and the related photocatalytic water

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Integrating Green Chemistry and Sustainable Engineering Solar light

CNT CNH

TiNT

e– e–

e–

Solar UV light

Solar 2H+ visible light

FCNTs

hυ

hυ

H2

Glycerol

h

+

TiNT

FCNTs as co-catalyst

e

hυ Glycerol

CO2 + 14H+

e– –

CB

e–

e

–

2H+ H2

VB h+ FCNTs

CO2+ 14H+

TiNT

FCNTs as sensitizer

Figure 2.3 Hydrogen production mechanism of FCNT-TiNT composite; FCNT acts as a co-catalyst, photoexcited electron of TiNT transfers to the surface of FCNTs; FCNT acts as a sensitizer, photon (light energy) directly irradiates FCNT and the excited FCNT electron transfers itself to CB of TiNT and migrate to the surface then react with H+ and produce hydrogen gas (Reprinted with permission from Kumari et al., [24], Copyright 2017, Royal Society of Chemistry).

splitting mechanism are as presented in the Figure 2.4. It can be seen from the TEM images that, CNTs covered the upper and lower surface of TiO2 with uniform distribution. Increased surface area of the nanocomposites due to smaller size and morphology was also helpful to diminish photogenerated charge recombination. Alteration of the size and diameter of TiNTs is possible by simple means. For example, TiNTs calcined at higher temperatures (near to 700 °C) showed the shorter length and the larger diameter in comparison to the calcined ones at the lower temperatures (near 350 °C) due to dehydration of interlayered OH groups [27]. Bao et al., [28] reported CdS nanostructures for hydrogen production under visible light irradiation. They have prepared the nanoporous CdS nanostructures through precipitation and template based methods. For the enhanced hydrogen production, they have used Pt as co-catalyst. TEM and SEM images of the CdS nanostructures and Pt added CdS nanostructures are shown in Figure 2.5.

Recent Progress in Photocatalytic Water Splitting (a)

29

(b) 0.352 nm

0.341 nm

5 nm

2 nm e– Pt hv

e– h+ + e– e– e–

Pt

H2 (c)

TiO2

H+

Figure 2.4 High Resolution Transmission Electron Microscopy images of CNT-TNT composite (a-b); Photocatalytic hydrogen production mechanism of Pt/CNT/TNT (c), TNT’s surface is covered with CNT and Pt acts as co-catalyst to enhance hydrogen production (Reprinted with permission from Zhao et al., [26], Copyright 2011, Royal Society of Chemistry).

Reduced graphene oxide at the interface of copper doped TiO2 p-n hetero-junction photocatalyst can import electron storing, transferring and shuttling properties to the catalyst and improve hydrogen production [29]. The addition of Cu2O and rGO (Reduced Graphene Oxide) to TiO2 extended absorption of TiO2 towards visible region. This extended absorption region indicates narrowing of bandgap energy of TiO2 and it improves the photoabsorption and photocatalytic performance of TiO2 nanoparticles (TiO2 NPs). The absorption regions and bandgap energies (Tauc plot) of TiO2 NPs, Cu2O-TiO2 and Cu2O-TiO2/rGO are shown in the Figure 2.6.

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Figure 2.5 CdS nanosheets and hollow nanorods: (c1) TEM image of typical Pt-loaded CdS nanostructures; (c2, c3) magnified TEM images of CdS nanosheets showing monodisperse Pt nanocrystals on the surface; (c4) HRTEM image of an individual Pt-loaded CdS nanosheet, showing crystal lattices of both CdS and Pt; (c5, c6) SEM images of Pt-loaded CdS nanostructures. Some Pt nanocrystals have been marked with blue arrows in parts c1, c2, and c6. (Reprinted with permission from Bao et al., [28], Copyright 2008, American Chemical Society).

Addition of bimetallic quantum dots to TiO2 could also improve hydrogen production. A composite photocatalyst, Cu/Ag quantum dotsTiO2 nanotubes have been reported to be a better catalyst for hydrogen production under sunlight irradiation. Photodeposited elements and

Recent Progress in Photocatalytic Water Splitting (b)

4 Cu2O-TiO2/rGO

2 –1 2 (αhν) (eV cm )

3 Absorbance (a.u.)

3.00E+013

TiO2/NPs Cu2O/TiO2

2 m 6n 38 1 nm 43

1

0 200

300

400 500 600 Wavelength (nm)

Cu2O-TiO2/rGO TiO2/NPs Cu2O/TiO2

2.00E+013 2.87 eV

(a)

31

1.00E+013

V

1e

3.2

700

800

0.00E+000 2.0

2.5

3.0

3.5

4.0

Bandgap energy, Eg (eV)

Figure 2.6 (a) UV-vis spectra and (b) Tauc plots of TiO2 NPs, Cu2O-TiO2 and Cu2OTiO2/rGO; TiO2 alone showed absorption edge is at 386 nm and bandgap is 3.12 eV; In Cu2O-TiO2/rGO catalyst, absorption edge is at 431 nm and bandgap is 2.87 eV. (Reprinted with permission from Babu et al., [29], Copyright 2015, Royal Society of Chemistry).

their relevant oxidation states were studied using XPS survey spectrum shown in Figure 2.7 (b–e), the presence of Cu, Ag, Ti, O, and C was confirmed. Figure 2.7 (b–e) gives the binding energy values of Ag 3d3/2 , Ag 3d5/2, Cu 2 p1/2, Cu 2 p 3/2, O1s, Ti 2 p3/2 and Ti 2 p1/2 as 374.4, 368.4, 952.8, 932.9, 530.2, 457.2 and 463.1 eV respectively. Binding energy values of Ag and Cu confirm their reduced forms (Ago), (Cuo) as well as lack of Cu2+ contribution. The oxidation state of Ti as Ti4+ is also confirmed on the basis of binding energy values depicted in Figure 2.7 (e). Glycerol (by-product of bio-diesel) can be used as a sacrificial agents to promote hydrogen production in photocatalytic process. Recently, Reddy et al., [10] reported enhanced hydrogen production through a sustainable approach with Ni(OH)2-TiO2 nanotubes catalyst under natural sunlight. In this report, they have used glycerol and crude glycerol from biodiesel industry for hydrogen production and also explained the cost-effective benefits. The optimized Ni loaded TiO2 nanotubes showed 12 times higher hydrogen production than pristine one. The higher hydrogen production may be due to Ni(OH)2 quantum dots which facilitate effective shuttling of photoexcitations and also due to reduced electron-hole recombination in TiO2 during photocatalysis. The hydrogen production mechanism of Ni(OH)2 decorated TiO2 nanotubes composite is shown in Figure 2.8.

2.3

Factors Influencing Liquid-Phase Hydrogen Production

Several parameters have been found to affect liquid-phase photocatalytic hydrogen production. In this review, effect of sulphide ion concentration,

Integrating Green Chemistry and Sustainable Engineering

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0 1s

500K

Ti 2p 300K

Ag 3d

200K

100K

C 1s

Cu 2p

Intensity (cps)

400K

0K 1200

1000

800 600 400 Binding energy (eV)

(a) 22K 20K

Cu 2p

Ag 3d3/2

16K

368.4 eV

14K

Cu 2p3/2

50K Intensity (cps)

Intensity (cps)

18K

0

55K

Ag 3d

Ag 3d5/2

200

374.4 eV

12K 10K 8K

45K

932.9 eV Cu 2p1/2

40K 952.8 eV 35K

6K 4K

30K 376

(b)

374

372 370 368 Binding energy (eV)

180K

366

935

930

Ti 2p

250k

457.2 eV Intensity (cps)

140K Intensity (cps)

950 945 940 Binding energy (eV)

Ti 2p3/2

0.1s

160K 530.2 eV

120K 100K 80K

200k

Ti 2p1/2 150k

463.1eV 100k

60K 50k

40K 20K 535

(d)

955

(c)

534

531 530 533 532 Binding energy (eV)

529

(e)

0 450 452 454 456 458 460 462 464 466 468 470 Binding energy (eV)

Figure 2.7 XPS spectra of Cu/Ag quantum dots - TiO2 nanotubes survey spectrum (a) and high-resolution spectra of (b) Ag3d, (c) Cu2p, (d) O1s, and (e) Ti2p. (Reprinted with permission from Reddy et al., [30], Copyright 2017, Elsevier).

Recent Progress in Photocatalytic Water Splitting

33

Solar light H2

–2

2H+

CB – – – – –0.26 eV e e e e

0

3

Gl yc ero l+

2H

O

2

e– – e

–0.23 eV

e– 3.2eV

1

+

Ni(OH)2 e–

2H+ H2 2+ Ni /Ni O+/H2 (V=0 eV)

O2/H2O (V=1.23 eV) h+ h+ h+ h+ VB

2

H2

Potential vs (NHE) eV

TiO2 –1

H

hν

+ H + oxidized intermediates

TiO2 nanotubes

ts

m do

antu

qu H)2 Ni(O

Figure 2.8 Pictorial representation of clean energy generation through photocatalytic water Splitting; the photoexcited electrons from conduction band (CB) of TiO2 migrate to CB of Ni(OH)2 quantum dots, and reduce them to Ni0. It is plausible because the CB potential of (−0.23 V vs NHE, pH = 0) of Ni2+/Ni is slightly lower than the CB level (about −0.26V) of anatase TiO2. The glycerol consists of primary, secondary and tertiary alcohols and it acts as hole scavenger to get oxidized into H+ ions and oxidized intermediates, the H+ ions in-turn get reduced with photogenerated electrons at Ni(OH)2 surface to generate H2 gas. (Reprinted with permission from Reddy et al., [10], Copyright 2018, American Chemical Society).

sulphite ion concentration, pH, catalyst dosage, liquid depth, light irradiation and reuse of the catalyst were discussed. Some works have been reported in literature on these effects for photocatalytic reactions [7, 31, 32] .

2.3.1

Direct Photolysis and Its Limitations

Some pollutants are feasible to be degraded in the presences of light. Generally, pollutants can absorb a wide range of wavelength in the absence of catalyst and undergo photolysis. Usually this kind of photo-degradation is slow. So, it cannot be opted for large scale degradation applications. It is known that Na2S absorption starts at 290 nm [33]. So, in the case of hydrogen recovery from sulphide wastewater through photolysis, it also leads to excitation of sulphide ions.

2.3.2 Need for Reducing Polysulphide Ions Formation The concentration of sulphide ions has significant influence on hydrogen generation. An increase in concentration of sulphide ions increases

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Integrating Green Chemistry and Sustainable Engineering

the generation of hydrogen upto a certain value. Further increase in the concentration of sulphide ions causes decrease in hydrogen generation. This is due to the formation of polysulphide ions which abstracts maximum quantity of light photons [34–40]. So, in order to achieve increased hydrogen production, there need to be a balance between sulphide ion and polysulphide ion concentrations. One alternative method to enhance the hydrogen recovery is either to reduce the formation of polysulphide ions or convert them into the structures which are more transparent to light.

2.3.3 Role of Sulphite Ions in Conversion of Photo Sulphides to Thiosulphate Addition of sulphite ions to sulphide ions, suppresses the photocorrosion. The added sulphite ions combine with polysulphide ions and produce thiosulphate. Optimum sulphite ion concentration is a must to control photocorrosion [40]. Further increase in sulphite ion concentration decreases hydrogen production. This is caused by adsorption of sulphite and thiosulphate ions over the active sites of the photocatalyst [35, 36, 41, 42, 43, 44]. The effect of sulphide and sulphite ions on photocatalytic hydrogen production is presented in Table 2.1. Yabo et al., [48] revealed that higher concentration of Na2S leads to a higher pH value, due to which the flat band potential of metal sulphide becomes less negative leading to the decrease of photocatalytic activity. In the absence of sulphite ions (Na2SO3), hydrogen evolution rate is reduced (Table 2.1) as observed by Lu et al., [49], confirming the importance of sulphite ions for the conversion of polysulphides to thiosulphates. In the similar manner, some researchers (Gomathi et al., 2013, Lu Wang et al., 2010 and Wang et al., 2012) [46] have also reported on the effect of sulphite and sulphide ions concentration on different photocatalysts. In all the reports, it was shown that only in one of the optimized concentrations, the highest hydrogen production was obtained. It may be because, these ions could be key players as photo-generated charge carriers, hole scavengers and further the polysulphide ions may abstract maximum quantity of light photons. After the optimized concentration, further increasing the concentration of these ions decreases hydrogen production because probably the formed SO32–, SO42– and S2O32– ions, may block the active sites of the catalyst. Further, at higher concentrations, penetration of sacrificial reagents onto the active sites of the catalyst may not also be ruled out. From the above studies, it is clear that sulphide and sulphite ion concentrations play a prominent role in hydrogen recovery. In order to get maximum

Wang et al., 2010

Wang et al., 2012

Linkous et al., 2004

Macias-Sanchez 0.35 M Na2SO3/0.25 M Na2S and Na2SO3 ion concentrations were Na2S. Cd0.44Zn0.56S optimized and the hydrogen yield was found to et al., 2013 photocatalyst. be high at 0.25 and 0.35 M respectively.

3.

4.

5.

Different concentrations of sodium sulphide solutions.

0.7 M Na2S and 0.1 M Na2SO3.

H2 evolution rates of Cd0.44Zn0.56S photocatalyst with Na2S / Na2SO3.

1400

2640

1600

2640

(Continued)

Hydrogen production(μmol/h)

Hydrogen from sulphide ion solution was found to 11295 increaseat 0.5 M concentration.

Maximum hydrogen evolution was observed at 0.7 M sulphide ion concentration. Similarly 0.1M Na2SO32- was observed to give higher quantity of H2.

Maximum hydrogen recovery was found at 0.25 M Na2S/0.35 M Na2SO3.

Hydrogen recovery increased with increase in sulphide ion concentration. Optimized value of sulphide ion concentration was found as 0.4 M.

2.

Na2SO3 + Na2S solution.

Sankar et al., 2013

1.

Important findings

Reference

S. No

Ions and their concentrations

Table 2.1 Effect of sulphide and sulphite ions on photocatalytic hydrogen production.

Recent Progress in Photocatalytic Water Splitting 35

Preethi et al., 2017

8.

Sulphide and sulphite ion concentrations were varied from 0 to 0.5 M respectively.

Priya et al., 2014 Na2S and Na2SO3 Maximum hydrogen production achieved at concentrations were 0.05 M Na2S. Beyond this concentration, H2 varied from 0 to production reduced. The maximum H2 was 0.125 M and 0 to observed at 0.2 M of sulphite ion concentration. 0.25 M respectively Above this concentration, there will be formation of SO32-, SO42- and S2O32- ions, which may block the active sites of the catalyst.

7.

Maximum hydrogen production occurred at optimized sulphide and sulphite concentrations.

Recovery rate of hydrogen was influenced by changing the concentration.

Sahu et al., 2009 Na2S concentration of 2.5 and 5 mmol g cat−1.

6.

Important findings

Reference

S. No

Ions and their concentrations

Cont.

Table 2.1

1689

2169

536

Hydrogen production(μmol/h)

36 Integrating Green Chemistry and Sustainable Engineering

Recent Progress in Photocatalytic Water Splitting

37

hydrogen recovery, it is imperative to optimize sulphide and sulphite ions which act as sacrificial reagents (Gomathi Sankar et al., 2013) [28].

2.3.4

Influence of Catalyst Dosage

The quantity of photocatalyst added plays an important role on hydrogen generation. It has been observed that with an increase in amount of photocatalyst, hydrogen generation increases upto an optimum value. This may be because, maximum numbers of active sites are on the surface of the catalyst and increasing the amount of catalyst increases the number of active sites available for catalysis. However, above a certain quantity, further addition of catalyst decreases the recovery of hydrogen. It may be because of overlapping of the catalyst particles at higher concentrations and resulted opaqueness which limits the absorption of light by the photocatalyst [46, 50–53]. The effects of catalyst dosage on photocatalytic hydrogen production as reported by various researchers are given in Table 2.2. Hence it is clear that above the optimum catalyst dosage, agglomeration and light blocking of catalyst particles may decrease the effective absorption of light photons over the catalyst surface. Below the optimum catalyst dosage, the number of active sites in the catalyst is not sufficient to reduce the protons to hydrogen.

2.3.5

Effect of pH

Based on the literature, maximum hydrogen recovery was achieved at pH 9 to 10 (Reber & Meier 1984). Above pH 10 [37, 39], due to high concentration of NaOH, hydrogen production decreases. When it is too alkaline, the protons may interact with hydroxide ions to produce water. Further the higher pH may affect the catalyst and sometimes impact the nature of the dissolved species. Table 2.3 represents the effect of pH on photocatalytic hydrogen production reported by different researchers. However, better hydrogen production could also be achieved in acidic pH. Nada et al., [60] used Ru coated TiO2 photocatalyst for hydrogen production and showed the highest activity at pH 1. The conditions and additives used are as follows: 4% methanol, 1% activated carbon, UV lamp 125W, and time of photolysis 6 hr. As per the authors, the better activity at the lower pH might be because of dissolution of H2S in the solution at highly alkaline pH. Hence from the evidences it is clear that pH plays an important role in hydrogen recovery and maximum hydrogen production could be achieved between the pH 9 to 12.

Reference

Zheng et al., 2010

Liu et al., 2012

Dang et al., 2013

Preethi et al., 2012

Priya et al., 2011

S. No

1.

2.

3.

4.

5.

Maximum H2 generation was noted at the catalyst 10,045 weight, 0.2 g. Above this, from 0.2 g to 0.5 g, the generation of hydrogen decreased due to the screening and shadowing effect

CuGa1.6Fe0.4O4; 0.1–0.5 g/500 mL.

11161

14940

Optimum point of catalyst dosage was 0.1 g L−1. Beyond or below 0.1 g L−1, hydrogen production decreased.

Cu/TiNTs; 0.05 to 0.60 g L−1

CdS-ZnS/TiO2: 100 Hydrogen recovery increased from 250 to 500 mg of to 500 mg/L catalyst. Beyond 500 mg it caused agglomeration and shielding.

1170

The production of hydrogen inclined from 5 to 20 mg. Above 20 mg, the production declined.

MWCNTs/Cd0.8 Zn0.2S; 5 to 200 mg/L.

8035

(Continued)

Hydrogen production (μmol/h)

Hydrogen production increased from 0.047 to 0.233 g/L. But further increasing the catalyst dosage from 0.233 to 0.535 g/L, decreased hydrogen production. This is because, when the catalyst amount is above optimal level, the suspension becomes turbid. This will block the light penetration inside the photolytic solution.

Important findings

CuO/SnO2; 0.1 to 0.5 g/L

Catalyst & dosage studied

Table 2.2 Effect of catalyst dosage on liquid-phase photocatalytic hydrogen production.

38 Integrating Green Chemistry and Sustainable Engineering

Maximum hydrogen recovery was achieved at 0.1 g of photocatalytic dosage. Beyond this light penetration through the photolytic solution is affected.

Ce3+-TiO2

Bharatwaj et al., 2018

9.

H2 production increased with increase in concentration from 0.1 to 0.5 g/L. Further increase in concentration decreases hydrogen production..

CdS +ZnS/Fe2O3

Preethi et al., 2018

8.

0.5 g/L gives maximum H2 production. The optimum photocatalyst concentration also based on the photoreactor design, the effect of mixing and light intensity.

Priya et al., 2014

7.

The optimum value for catalyst dosage −2.7 g/L. At lower concentrations the quantity of catalyst was not sufficient and at higher concentrations it leads to agglomeration.

Important findings

CdS–ZnS/TiO2 core–shell NPs 0 to 1 g/L

Estahbanati Pt/TiO2; 0.05 to 5 et al., 2017 g/L

6.

Catalyst & dosage studied

Reference

S. No

Table 2.2 Cont.

5300

8370

2619

81

Hydrogen production (μmol/h)

Recent Progress in Photocatalytic Water Splitting 39

Matsumur et al., Pt/CdS , glass flask reactor and Maximum recovery of H2 at pH 9. Below 9, hydrogen production 1983 500W Hg lamp. decreased.

Preethi et al., 2012

Nada et al., 2005 CdS/HY

Priya et al., 2014 CdS–ZnS/TiO2

3.

4.

5.

RuO2/CuGa1.6Fe0.4O4; H2S in 0.5 M KOH; cylindrical photoreactor; visible light.

pH 11.3 gives maximum H2 production. Beyond which the production declined.

Maximum hydrogen recovery achieved at pH 12. H2S is dissolved into alkaline solution at pH 11–12.

Hydrogen recovery increased with pH from 9 to 12. Dissociation of HS- to S2- and the pKa2 of H2S was close to 12. Beyond pH 12, the recovery decreased.

Hydrogen recovered reduce at pH four and production was high at pH 13.

2.

Pt/CdS scrubber unit with photoreactor and sunlight.

Linkous et al., 1994

1.

Important findings

Reference

S. No

Reactor type/experimental condition

Table 2.3 Effect of pH on liquid-phase photocatalytic hydrogen production.

40 Integrating Green Chemistry and Sustainable Engineering

Recent Progress in Photocatalytic Water Splitting

2.3.6

41

Effect of Recycle Flow Rates and Reactor Design on H2 Generation

The recycle flow rate of the solution undergoing photolysis has an influence on the residence time and mixing of the feed solution. Up to a certain level, the increase in recycle flow rate increases the hydrogen generation. In general, increased recycle flow rate, shortens the residence time [61]. Table 2.4 represents the effect of photolytic solution recycling on photocatalytic hydrogen production. The design and shape of the reactors are important because the exposure of the photolysis solution and catalyst mixture to the light source depends on design and shape of the reactor. In this connection, Preethi et al., [44] studied splitting of sulphide containing waste water into hydrogen with help of CdS-ZnS/Fe2O3 photocatalyst using reactors of different shapes such as cylindrical inner, cylindrical outer, tubular outer and trapezoidal outer irradiated reactors. Among these different reactors, cylindrical inner reactor showed the highest hydrogen production due to effective absorption of the incident light photons. The different reactor setups and hydrogen production details are shown in Figure 2.9. In general, the performance of the catalyst in the batch reactor is affected by the settling of catalytic particles at the bottom of the reactor. In the batch type recycle reactor, uniform mixing of catalyst without settling was achieved by recycling of photolytic solution [62, 64].

2.3.7

Dependence of Hydrogen Production on Volume and Depth of Photolytic Solution

Increment in volume of photolytic solution decreases the hydrogen yield. This is due to the reduced penetration of light into the solution [65, 63]. Huang et al., [63] studied the effect of liquid depth in photocatalytic hydrogen production and found that hydrogen yield depends on depth of the photolytic solution. The quantity of light penetration inside the photolytic solution is affected also by depth of the solution. The hydrogen yield decreases with reduced depth of photolyte. Reduced depth of photolyte will increase the distance between the light source and photolyte, which may not be effective to produce hydrogen.

2.3.8

Influence of Light Irradiation on Hydrogen Yield

Light irradiation, which controls the generation of e-/h+ pairs has significant influence on photocatalytic generation of hydrogen [46, 50, 52, 66].

Reference

Priya et al., 2009

Huang et al., 2011

Priya et al., 2014

S.No

1.

2.

3.

Important findings

Bench scale Tubular Photocatalytic Reactor operated in Batch Recycle Mode. Recycle flowrate 5 to 33 L/h studied.

Batch type and passive selfmixing of photolyte.

Hydrogen recovery was low at lower flow rates (18 L/h, due to lower residence time the hydrogen yield reduced.

Passive mixing yields higher rate of hydrogen production than simple batch type reactor. Since passive mixing promotes the catalyst to be in suspension, it gets maximum exposure to light photons.

Batch reactor (BR) and a batch Batch reactor was not found to be effective due to catalyst settling recycle reactor with continuous at the bottom of the reactor. In BRRwCG, uniform mixing of supply of inert gas (BRRwCG) catalyst was achieved which resulted in maximum hydrogen recovery.

Reactor type

Table 2.4 Effect of photolyte recycling on photocatalytic hydrogen production.

42 Integrating Green Chemistry and Sustainable Engineering

Recent Progress in Photocatalytic Water Splitting

(c)

9000

7857

(d)

Hydrogen production

8000

% Conversion

4.5 4

7000

3.5

6000

3

5000

2.5

4000

2

3000 2000

% Conversion

(b) Hydrogen production (μmol/0.5g/L//h)

(a)

43

1.5 1004.5

848.2

1116.1

1000

1 0.5 0

0 Cylindrical Cylindrical (inner) (outer)

Tubular Trapezoidal

(e)

Figure 2.9 Different photoreactors (a) Cylindrical inner irradiated; (b) Cylindrical outer irradiated; (c) Tubular (d) Trapezoidal reactors and (e) Hydrogen production results of the different reactors; Cylindrical inner irradiated reactor showed the highest hydrogen production due to the effective absorption of incident light photons. (Reprinted with permission from preethi et al., [44], Copyright 2008, Elsevier).

Table 5 presents the effect of light irradiance on hydrogen production observed by different researchers in the presence of different catalysts and conditions. It is clear from the reports that hydrogen production increases while increasing light irradiation. Increment in irradiation of light produces large number of electron and hole pairs, which leads to maximum generation of hydrogen [66]. Recently, the impact of natural sunlight and artificial light on hydrogen production was studied [67] using Ag2O/TiO2 as photocatalyst. The optimized catalyst, 1 wt% Ag added TiO2 showed 76.6 times higher hydrogen production than P-25 (TiO2). It was found out that there is only a slight variation in hydrogen production when the different light sources are used (Figure 2.10). During irradiation of the photocatalytic system, due to the inability of the catalysts to absorb some of the wavelengths and incapability of absorption of all the photons irradiated, there will be loss of energy. In order to

44

Integrating Green Chemistry and Sustainable Engineering Natural light Artificial light

20

15 10 5 0

(a)

Natural light Artificial light

H2 yield (mmol g–1)

H2 yield (mmol g–1)

20

P25 0.5 1

2

3

4

5 6

10 5 0

8 10

Silver loading (wt%)

15

(b)

2 4 6 8 10 12 14 16 18 20 22 24 Time on stream (h)

Figure 2.10 (a) Comparison of photocatalytic hydrogen production activity under natural and artificial solar light over P-25 (TiO2) and Ag loaded TiO2 catalysts using H2O + glycerol. Catalyst 5 mg, Reaction mixture – 5% (v/v) aqueous glycerol. (b) Time on stream analysis in the photocatalytic hydrogen production of Ag 1.0 Ti samples using H2O + Glycerol mixture under natural and artificial solar light using H2O + glycerol. Catalyst 5 mg, Reaction mixture – 5% (v/v) aqueous glycerol. (Reprinted with permission from Kumar et al., [67], Copyright 2017, Elsevier). Light penetration λ ≤ 560 + λ ≤ 310

–

Light

ZnS

CdS

Hydrogen production

PV panel Spectrum energy recovery

Figure 2.11 Hybridization method for photo-catalytic hydrogen production; zinc sulphide catalyst is irradiated as the first receiver of solar light. This photo-catalyst absorbs the UV light within the range of ≤ 310 nm and the penetrated light is supplied to the second reactor where cadmium sulphide suspensions can absorb up to 2.2 eV (560 nm) energy photons. Photons which are not absorbed go to the PV panel and produce electricity. (Reprinted with permission from Baniasadi et al., [68], Copyright 2013, Elsevier).

overcome this issue, a hybridization method incorporating both photoreactor and photovoltaic system has been proposed by Baniasadi et al., [68] as shown in Figure  2.11. The maximum hydrogen production was achieved with 1000 W/m2 light irradiation in this system.

Recent Progress in Photocatalytic Water Splitting

2.3.9

45

Sulphur Recovery

H2S can be split into hydrogen and sulphur under visible light irradiation using sulphide and sulphite reaction media at room temperature. Normally, sodium sulphide (Na2S) is used as source for sulphide and sodium sulphite (Na2SO3) is used as a sacrificial agent. Generally, additional sacrificial agents, such as hypophosphite (H2PO2–), is also used together with the sulphide ions to maintain the catalytic activity while producing various by-products (i.e., S2O32–, SO42–, S2O36–) in photolytic solution instead of sulphur. The sulphur compounds i.e., S2O32–, SO42–, S2O36– were found to be a major source of pollutants and require more cost to treat than decomposing H2S by the conventional Claus process [69]. Hence splitting of H2S to produce H2 and S using solar energy is the most sustainable way to recover hydrogen. Linkous [46] studied about splitting of H2S to produce hydrogen and sulphur using photocatalytic process. He reported that in the closed cycle experiments, the disulphide solution can be introduced with hydrogen sulphide overnight, which will reduce the pH from 13 to 7. Thus the sulphur was precipitated and separated [69]. When HCl (3 M) was added to the solution (taken after the experiment), sulphur was separated as yellow precipitate at pH below 4. In the above experiments, CdS was used as the catalyst and both hydrogen and sulphur were recovered in the ratio of 1:1.

2.3.10

Reusability of the Nanophotocatalysts

The stability of the photocatalysts is vital for industrial applications. But Chalcogenide semiconductors are prone to photocorrosion. Bai et al., [70] studied the stability of CdS/TiO2 nanotubes nanocomposite for the degradation of Rhodamine B and found that TiO2 nanotubes coupled with CdS could partly control the photocorrosion of CdS. It could be reused five times for the removal of Rhodamine B.

2.4

Factors Influencing Gas-Phase Photocatalytic Hydrogen Production

As in liquid phase hydrogen production, several factors have been found to affect gas-phase photocatalytic hydrogen production. The main operating parameters that affect the gas-phase photocatalytic hydrogen production are hydrogen sulphide gas concentration, flow rate, catalyst quantity and light irradiation.

46

Integrating Green Chemistry and Sustainable Engineering

2.4.1

Effect of H2S Gas Concentration

The hydrogen sulphide splitting (i.e., hydrogen conversion rate) will be higher in lower hydrogen sulphide concentrations. This concept could be explained as follows: When the pollutant concentration is higher, only partial degradation will occur due to limited surface area of the photocatalyst and several publications report the same [71–73]. When the pollutant concentration increases, the adsorption and reaction rate get closer which are important for the overall removal rate. At higher pollutant concentrations the adsorption is much faster than the photooxidation and the photoactivity should be the rate controlling factor. At lower concentration of pollutants in the sub ppb to ppb levels, the effect of adsorption is therefore not negligible. The photocatalyst surface consists of active sites, which adsorbs the pollutant at a particular concentration. When the pollutant concentration exceeds the optimal concentration, then the interaction between the pollutants and these sites will tend to increase, thereby it corrodes or cause catalyst poisoning. Hence, the adsorption of pollutants at lower pollutant concentrations is much more sensitive than adsorption at relatively high concentrations. Nonetheless, an optimum concentration level is effective in both adsorption and degradation process.

2.4.2

Effect of Gas Flow Rate

The flow rate of pollutant gas affects the pollutant degradation rate by influencing the residence time. Improvement in the photocatalytic reaction was observed at an optimal flow rate leading to the rising of diffusion between pollutant molecules and the photocatalyst. Nonetheless, if the flow rate was higher than optimum, the splitting of H2S would drop. This is because, at maximum gas flow rate, lower residence time occurs, which would drastically lower the degradation or splitting amount of H2S [74]. So gas flow rate is one of the vital process parameters affecting the rate of photocatalytic degradation. Mehradad et al., [75] worked with various flow rates i.e., 0.1–0.8 m3/h on the degradation of trichloroethylene. The rate of degradation increased with flow rate up to about 0.65 m3/h. While increasing the flow rate further, the mass transfer becomes limited which decreases the degradation rate. Angela et al., [76] observed that catalytic positioning occurs more with increasing the flow rate. When the flow rate is low, the degradation rate is also reduced and the conversion was less influenced by sulphate deposition. At high flow rates, adsorption of H2S could saturate the available

Recent Progress in Photocatalytic Water Splitting

47

active sites, thus causing stronger deactivation of catalyst. Vahid et al., [77] worked with photocatalytic decomposition of H2S and recorded the similar observation.

2.4.3

Effect of Catalyst Dosage

The quantity of photocatalyst plays a vital role in effecting photocatalytic H2 production rate in gas phase photocatalytic reactions. When the quantity of photocatalyst increases, it hinders penetration of the light photons on the surface of the catalyst. Hence the increments in catalyst dosage may reduce the H2 production rate [78, 79]. Several studies reported that initially with increasing the catalyst amount, the photocatalytic H2 production increases but further increase in catalyst amount, decreases the H2 production rate. Kumari et al., [25] worked with different TiO2 dosage (viz.,3 to 100 mg) for photocatalytic H2 production under solar irradiation. Up to 5 mg of the catalyst, the H2 production rate was increased. Further increase in the catalyst dosage decreases the hydrogen generation rate due to agglomeration of the particles which accounts reduction in surface area and increase in opaqueness and hence decreased light harvesting nature of the catalyst. Praveen et al., [80] found the effect of TNT dosage on the H2 production by varying TNT dosage from 3 to 100 mg. This work reported the highest H2 production at 5 mg of TNT. The lower H2 production rate with increase in catalyst dosage can be attributed to reduction in light penetration and increased light scattering. When the catalyst amount increases it leads to agglomeration, which accounts in reduction in surface area of catalyst available for effective light absorption and thereby affects the photocatalytic H2 production. An effective light absorption is achieved at optimized catalyst loading (Pimsuda et al., 2008). Beyond this optimum dosage, the light irradiation could unevenly distribute and may reduce the H2 production.

2.4.4

Effect of Light Irradiation

In photocatalytic process, the light photons are the means to produce electrons and holes for Production of H2 and degradation of pollutants. Hence with increasing the light irradiance on the catalyst it is possible to generate large amount of electrons and holes for effective H2 production and efficient degradation of pollutants (Devhasdin et al., 2003, Husken et al., 2009, Melo & Triches, 2012 and Yu & Brouwers 2009). The electron production is explained by the Beer-Lambert law, according to equation (2.1). This

48

Integrating Green Chemistry and Sustainable Engineering

law correlates the absorption of light photons to the properties of material, through which light is travelling. Ge- = ηinjα(λ)I0e-α(λ)x

(2.1)

Where, ηinj - injection efficiency, α(λ) - wavelength-dependent absorption coefficient of the material, I0 - incident photon flux and L - semiconductor thickness. When the energy greater than the bandgap energy comes in contact with a semiconductor material, electrons from valence band jumps to the conduction band creating holes in the valence band. This absorption process is wavelength-dependent and is described by absorption coefficient α of the semiconducting material, which increases with the decrease of the wavelength of light incident over its surface [81]. When the light with specific wavelength is absorbed by a semiconducting material, the electron and hole formation occurs, which initiates the photocatalytic process. This electron– hole formation strongly dependents on the light intensity. Several studies have been conducted to understand the correlation between amount of H2 production and light irradiation. Jeong et al., [82] reported the visible light effect beneficial for the H2 production with GO-TiO2 catalyst. Reddy et al., [83] showed efficient H2 generation using TiO2 nanostructured photocatalysts under LED light irradiation. Reddy et al., [10] used 250 W mercury vapor light for the efficient H2 production using CNT-TiO2 photocatalyst. Zhigang et al., [84] have used 150 W Xenon light for the H2 production using TiO2 nanoparticles-functionalized N-doped graphene. They have concluded that enhanced H2 production could be obtained by enhancing the light absorption region.

2.5

Future Prospects

Increase in solar energy conversion efficiency is possible only when the photocatalyst has suitable band gap to absorb majority of visible and infrared spectrum of solar radiation. This requires novel materials which have suitable narrow band gap for the effective solar light absorption as well as band edge positions for redox reaction. Surface active sites which can trap the charge carriers are also equally important so that the desired photocatalyst should be prepared in such a way that they have higher surface active sites. In addition, it should utilize all the photogenerated charge carriers effectively without any recombination to produce hydrogen. There is dearth of study to understand the morphology effect and the influence of interface junction charge transfer on the photocatalytic hydrogen production. Since most of the reports states that only 6% of incident photons are

Recent Progress in Photocatalytic Water Splitting

49

involving in the photocatalysis, there is a need to find out ways to prepare narrow band gap photocatalyst with effective electron transfer to the surface active sites for redox reactions without recombination, as this is potential method to replace the fossil fuel by efficient production of hydrogen.

References 1. Sadanandam, G., Lalitha, K., Kumari, V.D., Shankar, M.V., Subrahmanyam, M., Cobalt doped TiO2: A stable and efficient photocatalyst for continuous hydrogen production from glycerol: Water mixtures under solar light irradiation. Int. J. Hydrogen Energy, 38(23), 9655–9664, 2013. 2. Praveen Kumar, D., Lakshmana Reddy, N., Srinivas, B., Durgakumari, V., Roddatis, V., Bondarchuk, O., et al., Stable and active CuxO/TiO2 nanostructured catalyst for proficient hydrogen production under solar light irradiation. Sol. Energy Mater. Sol. Cells, 146, 63–71, 2016. 3. Asahi, R., Morikawa, T., Ohwaki, T., Aoki, K., Taga, Y., Visible-Light Photocatalysis in Nitrogen-Doped Titanium Oxides. Science, 293(5528), 269–271, 2001. 4. Kudo, A., Miseki, Y., Heterogeneous photocatalyst materials for water splitting. Chem. Soc. Rev., 38(1), 253–278, 2009. 5. Chen, X., Mao, S.S., Titanium dioxide nanomaterials: synthesis, properties, modifications, and applications. Chem. Rev., 107(7), 2891–2959, 2007. 6. Fujishima, A., Honda, K., Electrochemical Photolysis of Water at a Semiconductor Electrode. Nature, 238(5358), 37–38, 1972. 7. Bhatkhande, D.S., Pangarkar, V.G., Beenackers, A.A., Photocatalytic degradation for environmental applications - a review. J. Chem. Technol. Biotechnol., 77(1), 102–116, 2002. 8. Leung.Y.C, D., Xianliang, F., Wang, C., Ni, M., Leung.K.H, M., Wang, X., et  al., Hydrogen Production over Titania-Based Photocatalysts. Mini Rev. ChemSusChem, 3, 681–694, 2010. 9. Patsoura, A., Kondarides, D.I., Verykios, X.E., Photocatalytic degradation of organic pollutants with simultaneous production of hydrogen. Catal. Today, 124(3-4), 94–102, 2007. 10. Lakshmana Reddy, N., Cheralathan, K.K., Durga Kumari, V., Neppolian, B., Muthukonda Venkatakrishnan, S, Kumari, D, V, Shankar, M.V., Photocatalytic Reforming of Biomass Derived Crude Glycerol in Water: A Sustainable Approach for Improved Hydrogen Generation Using Ni(OH) 2 Decorated TiO Nanotubes under Solar Light Irradiation. ACS Sustainable Chem. Eng., 6(3), 2 3754–3764, 2018. 11. Lin, Y., Cui, X., Yen, C., Wai, C.M., Platinum/carbon nanotube nanocomposite synthesized in supercritical fluid as electrocatalysts for low-temperature fuel cells. J. Phys. Chem. B, 109(30), 14410–14415, 2005.

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12. Peining, Z., Nair, A.S., Shengyuan, Y., Shengjie, P., Elumalai, N.K., Ramakrishna, S., Rice grain-shaped TiO2-CNT composite—A functional material with a novel morphology for dye-sensitized solar cells. J. Photochem. Photobiol. A Chem., 231(1), 9–18, 2012. 13. Pajootan, E., Rahimdokht, M., Arami, M., Carbon and CNT fabricated carbon substrates for TiO2 nanoparticles immobilization with industrial perspective of continuous photocatalytic elimination of dye molecules. J. Ind. Eng. Chem., 55, 149–163, 2017. 14. Dai, K., Peng, T., Ke, D., Wei, B., Tianyou, P., Dingning, K., Photocatalytic hydrogen generation using a nanocomposite of multi-walled carbon nanotubes and TiO2 nanoparticles under visible light irradiation. Nanotechnology, 20(12), 125603–125608, 2009. 15. Yuvaraj, H., Arunkumar, R., Jin-Bae, L., Suk, Y., H.,Young-Kyu, H., Highly efficient hydrogen production via water splitting using Pt@MWNT/TiO2 ternary hybrid composite as a catalyst under UV–visible light. Synth. Met., 199, 345–352, 2015. 16. Oh, W., Chen, M., Synthesis and Characterization of CNT/TiO2 Composites Thermally Derived from MWCNT and Titanium (IV) n –Butoxide. Bull. Korean Chem. Soc., 29, 159–164, 2008. 17. Kuzmany, H., Kukovecz, A., Simon, F., Holzweber, M., Kramberger, C., Pichler, T., Functionalization of carbon nanotubes. Synth. Met., 141(1-2), 113–122, 2004. 18. Peng, T., Zeng, P., Ke, D., Liu, X., Zhang, X., Hydrothermal Preparation of Multiwalled Carbon Nanotubes (MWCNTs)/CdS Nanocomposite and Its Efficient Photocatalytic Hydrogen Production under Visible Light Irradiation. Energy Fuels, 25(5), 2203–2210, 2011. 19. Orin, A., Orin, R., Recent applications of carbon nanotubes in hydrogen production and storage. Fuel, 90, 3123–3140, 2011. 20. Mauter, M.S., Elimelech, M., Environmental applications of carbon-based nanomaterials. Environ. Sci. Technol., 42(16), 5843–5859, 2008. 21. Veluru, J.B., Manippady, K.K., Rajendiren, M., Mya, K., Rayavarapu, P.R., Appukuttan, S.N., et  al., Photocatalytic hydrogen generation by splitting of water from electrospun hybrid nanostructures. Int. J. Hydrogen Energy, 38(11), 4324–4333, 2013. 22. El-Bery, H.M., Matsushita, Y., Abdel-moneim, A., Fabrication of efficient TiO2-RGO heterojunction composites for hydrogen generation via watersplitting: Comparison between RGO, Au and Pt reduction sites. Appl. Surf. Sci., 423, 185–196, 2017. 23. Lu, Z., Xiang, X., Zou, L., Xie, J., Fluffy-ball-shaped carbon nanotube–TiO2 nanorod nanocomposites for photocatalytic degradation of methylene blue. RSC Adv., 5(53), 42580–42586, 2015. 24. Kumari, M.M., Priyanka, A., Marenna, B., Haridoss, P., Kumar, D.P., Shankar, M.V., Benefits of tubular morphologies on electron transfer properties in CNT/TiNT nanohybrid photocatalyst for enhanced H2 production. RSC Adv., 7(12), 7203–7209, 2017.

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25. MamathaKumari, M., Praveen Kumar, D., Haridoss, P., DurgaKumari, V., Shankar, M.V., Nanohybrid of titania/carbon nanotubes – nanohorns: A promising photocatalyst for enhanced hydrogen production under solar irradiation. Int. J. Hydrogen Energy, 40(4), 1665–1674, 2015. 26. Zhao, C., Luo, H., Chen, F., Zhang, P., Yi, L., You, K., A novel composite of TiO2 nanotubes with remarkably high efficiency for hydrogen production in solar-driven water splitting. Energy Environ. Sci., 7(5), 1700–1707, 2014. 27. Lin, X., Rong, F., Ji, X., Fu, D., Yuan, C., Preparation and enhanced visible light photocatalytic activity of N-doped titanate nanotubes by loaded with Ag for the degradation of X-3B. Solid State Sci., 13(7), 1424–1428, 2011. 28. Bao, N., Shen, L., Takata, T., Domen, K., Self-Templated synthesis of nanoporous CdS nanostructures for highly efficient photocatalytic hydrogen production under visible light. Chem. Mater., 20(1), 110–117, 2008. 29. Ganesh Babu, S., Ramalingam, V., Praveen Kumar, D., Shankar, M.V., HungLung, C., Vinodgopal, K., Influence of Electron Storing, Transferring and Shuttling Assets of Reduced Graphene Oxide at the Interfacial Copper Doped TiO2 p-n Hetero-junction for the Increased Hydrogen Production. Nanoscale, 17, 7849–7857, 2015. 30. Reddy, N.L., Kumar, S., Krishnan, V., Sathish, M., Shankar, M.V., Multifunctional Cu/Ag quantum dots on TiO 2 nanotubes as highly efficient photocatalysts for enhanced solar hydrogen evolution. J. Catal., 350, 226–239, 2017. 31. Gogate, P.R., Pandit, A.B, Aniruddha, B.P., Hydrodynamic cavitation reactors: A state of the art review. Rev. Chem. Eng., 17(1), 1–85, 2001. 32. Gaya, U.I., Abdullah, A.H, Ibrahim., A.H., Heterogeneous photocatalytic degradation of organic contaminants over titanium dioxide: A review of fundamentals, progress and problems. J. Photochem. Photobiol., 9(1), 1–12, 2008. 33. Hara, K., Sayama, K., Arakawa, H., UV photoinduced reduction of water to hydrogen in Na2S, Na2SO3 and Na2S2O4 aqueous solutions. J. Photochem. Photobiol. A Chem., 128(1-3), 27–31, 1999. 34. Furlong, D.N., Grieser, F., Hayes, D., Sasse, W., Wells, D., Kinetics of hydrogen production from illuminated CdS/Pt/Na2S dispersions. J. Phy. Chem, 90, 2388, 1986. 35. Grzyll, L., Thomas, J., Barile, R., Photoelectrochemical conversion of hydrogen sulfide to hydrogen using artificial light and solar radiation. Int. J. Hydrogen Energy, 14(9), 647–651, 1989. 36. Sabaté, J., Cervera-March, S., Simarro, R., Giménez, J., Photocatalytic production of hydrogen from sulfide and sulfite waste streams: a kinetic model for reactions occurring in illuminating suspensions of CdS. Chem. Eng. Sci., 45(10), 3089–3096, 1990. 37. Linkous, C., Muradov, N.Z., Ramser, N., Consideration of reactor design for solar hydrogen production from hydrogen sulfide using semiconductor particulates. Int. J. Hydrogen Energy, 20(9), 701–709, 1995.

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38. Sahu, N., Upadhyay, S., Sinha, A., Kinetics of reduction of water to hydrogen by visible light on alumina supported Pt–CdS photocatalysts. Int. J. Hydrogen Energy, 34(1), 130–137, 2009. 39. Strataki, N., Antoniadou, M., Dracopoulos, V., Lianos, P., Visible-light photocatalytic hydrogen production from ethanol–water mixtures using a Pt-CdSTiO2 photocatalyst. Catal. Today, 151(1-2), 53–57, 2010. 40. Priya, R., Sellappa, K., Concurrent hydrogen production and hydrogen sulphide degradation by solar photocatalysis. Clean Soil Air Water, 44, 1, 2016. 41. Muradov, N.Z., Rustamov, M.I., Guseinova, A.D., Bazhutin, Y.V., Photocatalytic production of hydrogen from H2S solutions over CdS/Pt colloids. React. Kinet. Catal. Lett., 33(2), 279–283, 1987. 42. Cui, W., Feng, L., Xu, C., Lu, S., Qiu, F., Hydrogen production by photocatalytic decomposition of methanol gas on Pt/TiO2 nano-film. Catal. Commun., 5(9), 533–536, 2004. 43. Li, C., Yuan, J., Han, B., Jiang, L., Shangguan, W., TiO2 nanotubes incorporated with CdS for photocatalytic hydrogen production from splitting water under visible light irradiation. Int. J. Hydrogen Energy, 35(13), 7073–7079, 2010. 44. Preethi, V., Kanmani, S., Performance of four various shapes of photocatalytic reactors with respect to hydrogen and sulphur recovery from sulphide containing wastestreams. J. Clean. Prod., 133, 1218–1226, 2016. 45. Sankar, G.P., Hachisuka, K., Katsumata, H., Suzuki, T., Funasaka, K., Kaneco, S., Photocatalytic hydrogen production with CuS/ZnO from aqueous Na2S + Na2SO3 solution. Int. J. Hydrogen Energy, 38, 8625–8630, 2013. 46. Linkous, C.A., Huang, C., Fowler, J.R., UV photochemical oxidation of aqueous sodium sulfide to produce hydrogen and sulfur. J. Photochem. Photobiol. A Chem., 168(3), 153–160, 2004. 47. Macías-Sánchez, S.A., Nava, R., Hernández-Morales, V., Acosta-Silva, Y.J., Pawelec, B., Al-Zahrani, S.M., et al., Cd1-xZnxS supported on SBA-16 as photocatalysts for water splitting under visible light: Influence of Zn concentration. Int. J. Hydrogen Energy, 38(27), 11799–11810, 2013. 48. Yabo, W., Jianchun, Wu., Jianwei, Z., Rongrong, J., Rong, X., Ni2+-doped ZnxCd1_xS photocatalysts from single-source precursors for efficient solar hydrogen production under visible light irradiation. Catal. Sci. Technol., 2, 581, 2012. 49. Lu, W., Wenzhong, W., Meng, S., Wenzong, Y., Songmei, S., Ling, Z., Enhanced photocatalytic hydrogen evolution under visible light over Cd1LxZnxS solid solution with cubic zinc blend phase. Int. J. Hydrogen Energy, 35, 19, 2010. 50. Ueno, A., Kakuta, N., Park, K.H., Finlayson, M.F., Bard, A.J., Campion, A., et al., Silica-supported ZnS/CdS mixed semiconductor catalysts for photogeneration of hydrogen. J. Phys. Chem., 89(18), 3828–3833, 1985. 51. Bangun, J., Adesina, A.A., The photodegradation kinetics of aqueous sodium oxalate solution using TiO2 catalyst. Appl. Catal. A gen., 175(1-2), 221–235, 1998.

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52. Pareek, V.K., Brungs, M.P., Adesina, A.A., Continuous process for photodegradation of industrial bayer liquor. Ind. Eng. Chem. Res., 40(23), 5120–5125, 2001. 53. Sreethawong, T., Puangpetch, T., Chavadej, S., Yoshikawa, S., Quantifying influence of operational parameters on photocatalytic H2 evolution over Ptloaded nanocrystalline mesoporous TiO2 prepared by single-step sol–gel process with surfactant template. J. Power Sources, 165(2), 861–869, 2007. 54. Preethi, V., Kanmani, S., Photocatalytic hydrogen production over CuGa2FexO4 spinel. Int. J. Hydrogen Energy, 37(24), 18740–18746, 2012. x 55. Priya, R., Kanmani, S., Optimization of photocatalytic production of hydrogen from hydrogen sulfide in alkaline solution using response surface methodology. Desalination, 276(1-3), 222–227, 2011. 56. Preethi, V., Kanmani, S., Performance of nano photocatalysts for the recovery of hydrogen and sulphur from sulphide containing wastewater. Int. J. Hydrogen Energy, 43(8), 3920–3934, 2018. 57. Bharatvaj, J., Preethi, V., Kanmani, S., Hydrogen production from sulphide wastewater using Ce3+ –TiO2 photocatalysis. Int. J. Hydrogen Energy, 43(8), 3935–3945, 2018. 58. Linkous, C., Mingo, T., Muradov, N., Aspects of solar hydrogen production from hydrogen sulfide using semiconductor particulates. Int. J. Hydrogen Energy, 19(3), 203–208, 1994. 59. Matsumura, M., Saho, Y., Tsubomura, H., Photocatalytic hydrogen production from solutions of sulfite using platinized cadmium sulfide powder. J. Phys. Chem., 87(20), 3807–3808, 1983. 60. Nada, A., Barakat, M., Hamed, H., Mohamed, N., Veziroglu, T., Studies on the photocatalytic hydrogen production using suspended modified TiO2 photocatalysts. Int. J. Hydrogen Energy, 30(7), 687–691, 2005. 61. Zhou, S., Ray, A.K., Kinetic Studies for Photocatalytic Degradation of Eosin B on a Thin Film of Titanium Dioxide. Ind. Eng. Chem. Res., 42(24), 6020–6033, 2003. 62. Priya, R., Kanmani, S., Batch slurry photocatalytic reactors for the generation of hydrogen from sulfide and sulfite waste streams under solar irradiation. Sol. Energy, 83(1), 1802–1805, 2009. 63. Huang, C., Yao, W., Raissi, T.A., Muradov, N., Development of efficient photoreactors for solar hydrogen production. Sol. Energy, 85, 19–27, 2011. 64. Escudero, J.C., Cervera-March, S., Giménez, J., Simarro, R., Preparation and characterization of Pt(RuO2)/TiO2 catalysts: Test in a continuous water photolysis system. J. Catal., 123(2), 319–332, 1990. 65. Faramarzpour, M., Vossoughi, M., Borghei, M., Photocatalytic degradation of furfural by titania nanoparticles in a floating-bed photoreactor. Chem. Eng. J., 146(1), 79–85, 2009. 66. Mau, A.W.H., Huang, C.B., Kakuta, N., Bard, A.J., Campion, A., Fox, M.A., et al., Hydrogen photoproduction by Nafion/cadmium sulfide/platinum films in water/sulfide ion solutions. J. Am. Chem. Soc., 106(22), 6537–6542, 1984.

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67. Kotesh Kumar, M., Byeong, S.K., Anil Kumar Reddy, P., Misook, K., In-situ photo-reduction of silver particles and their SPR effect in enhancing the photocatalytic water splitting of Ag2O/TiO2 photocatalysts under solar light irradiation: A case study. Mater. Res. Bull., 95, 515–524, 2017. 68. Baniasadi, E., Dincer, I., Naterer, G.F., Hybrid photocatalytic water splitting for an expanded range of the solar spectrum with cadmium sulfide and zinc sulfide catalysts. Appl. Catal. A gen., 455, 25–31, 2013. 69. Guijun, Ma., Hongjian, Y., Zong, X., Ma, B.J., Jiang, H.F., Wen, F.Y., Photocatalytic splitting of H2S to produce hydrogen by gas-solid phase reaction. Chinese J. Catal, 29, 313–315, 2008. 70. Bai, S., Li, H., Guan, Y., Jiang, S., The enhanced photocatalytic activity of CdS/ TiO2 nanocomposites by controlling CdS dispersion on TiO2 nanotubes. Appl. Surf. Sci., 257(15), 6406–6409, 2011. 71. Wang, W., Chiang, L.-W., Ku, Y., Decomposition of benzene in air streams by UV/TiO2 process. J. Hazard. Mater., 101(2), 133–146, 2003. 72. Wang, J.H., Ray, M.B., Application of ultraviolet photooxidation to remove organic pollutants in the gas phase. Sep. Purif. Technol., 19(1-2), 11–20, 2000. 73. Fu, X., Zeltner, W.A., Anderson, M.A., The gas-phase photocatalytic mineralization of benzene on porous titania-based catalysts. Appl. Catal. B, 6(3), 209–224, 1995. 74. Weradeach, S., Laksana, L., Visanu, T., Pummarin, K., Nurak, G., Photocatalytic degradation of BETX using W-doped TiO2 immobilized on fiber glass cloth under visible light. Superlattices Microstruct., 52, 632–642, 2012. 75. Mehrdad, K., Tom, T., Madjid, M., Oxidation of gas phase trichloroethylene and toluene using composite sol–gel TiO2 photocatalytic coatings. J. Hazard. Mater.B, 128, 130–137, 2006. 76. Angela, A.T., Didier, R., Nicolas, K., Valerie, K., A parametric study of the UV-A photocatalytic oxidation of H2S over TiO2. Appl. Catal., B., 115, 209– 218, 2012. 77. Vahid, A., Alex, D.V., Mechanistic model for ultraviolet degradation of H2S and NOx in waste gas. Chem. Eng. J., 244, 597–603, 2014. 78. Puangpetch, T., Sreethawong, T., Yoshikawa, S., Chavadej, S., Hydrogen production from photocatalytic water splitting over mesoporous-assembled SrTiO3 nanocrystal-based photocatalysts. Journal of Molecular Catalysis A Chemical, 312(1-2), 97–106, 2009. 79. Haifeng, D., Xinfa, D., Yingchao, D., Yan, Z., Stuart, H., TiO2 nanotubes coupled with nano-Cu(OH)2 for highly efficient photocatalytic hydrogen production. Int. J. Hydrogen Energy, 38, 2126–2135, 2013. 80. Praveen Kumar, D., Shankar, M.V., Kumari, M.M., Sadanandam, G., Srinivas, B., Durgakumari, V., Nano-size effects on CuO/TiO2 catalysts for highly efficient H2 production under solar light irradiation. Chem. Commun. (Camb.)., 49(82), 9443–9445, 2013.

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3 Heterogeneous Catalytic Conversion of Greenhouse Gas CO2 to Fuels Kaisar Ahmad, Firdaus Parveen, Anushree and Sreedevi Upadhyayula* Department of Chemical engineering, Indian Institute of Technology Delhi, India

Abstract Global warming, ocean acidification, and need for an alternative source of energy are the major challenges of 21st century. CO2 hydrogenation to fuels and starting chemicals for various processes will not only provide an alternative source of energy but also aids in minimizing the effect of global warming. The CO2 as renewable, intoxicant, and rich carbon source, its chemical conversion to fuels and valued chemicals is one of the most concrete ways for reducing CO2 emissions. The inert nature of CO2 molecule, developing a cost-effective, and efficient catalyst for commercial production of fuels and chemicals from CO2 is still a challenge. CO2 hydrogenation leads to the formation of three main products: methanol, dimethyl ether (DME) via methanol dehydrogenation, and hydrocarbons through CO2-FT (Fischer-Tropsch) reaction. Besides introducing the main motivation for the development of such processes, we summarise the thermodynamic aspects, recent developments with reaction mechanism for the direct hydrogenation of CO2 to methanol, DME, and hydrocarbons in this study. Finally, in order to outline the future research and development directions, we summarise the challenges and opportunities involved with all the above processes. Keywords: CO2 conversion, greenhouse gas, methanol, dimethyl ether, hydrocarbons, thermodynamics

*Corresponding author: [email protected] Shahid-ul-Islam (ed.) Integrating Green Chemistry and Sustainable Engineering, (57–80) © 2019 Scrivener Publishing LLC

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58

120 40

60

20

40 10 20 0

Preinductrial value: 280 ppm 1980

1990

0 2000

PERCENT

30

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February 2018

PARTS PER MILLION

100

2010

Figure 3.1 Increase in CO2 concentration from preindustrial era to present. (Source: US NOAA Feb 2016).

3.1

Introduction

3.1.1

Greenhouse Gas CO2

There is no doubt that anthropogenic activities are responsible for changing the atmospheric concentrations and distribution of greenhouse gases [1]. The ever increasing consumption of fossil fuels since the industrial revolution together with some specific industrial activities such as cement production and deforestation of Earth’s rain forests has caused a very marked increase in the global concentration of the major greenhouse gas: carbon dioxide. The CO2 concentration has increased from about 280ppm in pre-industrial times to 402 ppm in 2016 as depicted in Figure 3.1. This represents a net increase of 43% as shown in Figure 3.1. As per the recent data, the level of CO2 has gone beyond CO2 equilibrium concentration which is governed by the photo synthesis and respiration of earth’s biosphere and physical and chemical interaction with oceans. All these interaction operate on different time-scales (ranging from year to centuries). CO2 being subsequently stored in various reservoirs, some of which immediately return CO2 to the atmosphere(uptake into vegetation and surface layer of ocean) and some of have a less immediate, but no less important effect(transfer to soils and to deep ocean). Although CO2 is a rather inert molecule that doesn’t participate in chemistry of atmosphere, it is principle anthropogenic gas.

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The drastic changes in climate and acidification of oceans are considered as the most difficult challenges of present time [2]. According to the UN’s IPCC, around 80% of known fossil fuel reserves would need to stay in the ground for humanity to limit the concentration of CO2 in the atmosphere to 450ppm.

3.1.2

Mitigation of CO2 Concentration

International Energy Agency (IEA) in 2012 pointed out to achieve the ±2 °C goal corresponding to a stable CO2 concentration at 450 ppm, 43 Gt CO2 should be reduced to reach 14 billion tons. As a result, several key technologies and strategies are required including the improvement of energy efficiency (43%), renewable energy (28%), CO2 capture and storage (CCS) technology (22%), etc. [3]. The technological options that may contribute to reduce carbon dioxide emissions can be categorized as follow:

3.1.3

Reducing CO2 Emissions

This is already being done by introducing efficient equipment. One can produce energy with high efficiency. Increase efficiencies of conversion and end use, conserve energy and recycle products.

3.1.4 Zero Emissions One can use energy sources that are CO2 free such as hydro, nuclear, solar, wind or geothermal. One could also burn biomass and practice reforestation and thus gain power with no net emission of CO2.

3.1.5

Carbon Capture and Storage or Sequestration (CCS)

The approach concerns recovering CO2 from sources that contain it in much higher concentrations than the atmosphere. The technology is available. The question then arises what can one do with recovered CO2. One option is to dispose it in natural fields (elimination in aquifers, confinement in depleted gas or oil wells, injection for enhanced oil recovery, disposal in ocean). This is now done in many parts of world like in Norway (sliepner gas field) where 2740 t.day−1 of CO2 are removed from natural gas. This CO2 is then compressed and injected into a water filled sand stone reservoir (deep aquifer) [2]. Another option is to use directly (without conversion) as solvent and for many other commercial uses in any phase. And the last option which deserves our attention is to use it as a source in the

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synthesis of chemicals (synthesis of fuels, intermediates and/or fine chemicals) like the catalytic CO2 hydrogenation to methanol, DME, and hydrocarbons. Carbon dioxide can be converted to methanol by combining it with hydrogenation and using right kind of catalyst. According to the 1994 Nobel Prize in Chemistry winner, George Andrew Olah [4], methanol can replace fossil fuels as a means of energy storage, in what is now referred to as the methanol economy. Of course, the hydrogen must be available on a large scale and its production must be achieved without any emission of CO2 to the atmosphere. This implies that hydrogen must be produced by using renewable energies like solar, hydro or biomass fuel. DME on the other hand is highly expensive and useful chemical. The hydrocarbons have enormous applications depending on their physical state (gas or liquid) and chain length.

3.2

Thermodynamics of CO2 Hydrogenation to Methanol, DME and Hydrocarbons

The knowledge of thermodynamics is vital for the design of a catalytic process involving reduction of CO2 to chemicals or fuels. The inert nature of CO2 molecule is the main challenge for its chemical conversion to value added chemicals. Highly active and selective in combination with the favourable reaction parameters are expected to give good experimental results. Here we have studied the effect of wide range temperature and pressure on the direct hydrogenation of CO2 to methanol, dimethyl ether (DME), and hydrocarbons (HC). The results are depicted in terms of the CO2 conversion and selectivity of methanol, DME, and hydrocarbons. As the variables are pressure and temperature in our system, Gibbs free energy minimization approach is selected. This technique doesn’t require any stoichiometric knowledge of the reactants [5]. Aspen Plus software was used for this thermodynamic analysis. RGIBBS block of Aspen Plus was used to perform the total Gibbs energy minimization of the reaction mixture. Non-ideal behavior in Gibbs energy values was introduced by using the Soave-Redlich-Kwong model in Aspen plus as the equation of state [6]. In case of methanol synthesis from CO2 hydrogenation, reactions R1, R2, and R3 were considered for analysis. Similarly R4, R5, R6, and R7 reaction were considered for DME synthesis. Whereas reaction R8 to R13 were considered for the thermodynamic analysis of hydrocarbon synthesis. All the possible reactions are shown in Table 3.1.

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Table 3.1 Various possible reactions for the synthesis of methanol, DME and hydrocarbons from CO2 hydrogenation [7].

Reaction number

ΔH298 [kJ.mol−1]

Reaction

Methanol synthesis R1

CO2 +3H2

CH3OH + H2O

−49.51

R2

CO2 + H2

CO + H2O

41.19

R3

CO +2H2

CH3OH

−90.70

R4

CO2 +3H2

CH3OH + H2O

−49.10

R5

CO2 + H2

CO + H2O

41.19

R6

2CH3OH

CH3OCH3 + H2O

−37.00

R7

CO +2H2

CH3OH

−90.60

CO + H2O

41.19

DME synthesis

Hydrocarbon synthesis R8

CO2 + H2

R9

nCO2 + (3n + 1)H2 → CnH2n+2 + 2nH2O

-

R10

nCO2 +3nH2 → CnH2n + 2nH2O

-

R11

nCO2 +3nH2 → CnH2n+2O + (2 n-1)H2O

-

R12

nCO + (2n + 1)H2 → CnH2n+2 + nH2O

-

R13

nCO +2nH2 → CnH2n + nH2O

-

From the Figure  3.2(a), it is observed conversion of CO2 increases with increase in temperature for methanol and DME synthesis where as it decreases with increase in temperature for hydrocarbon synthesis up to 560 °C. From the Figure  3.2(b), selectivity of the three main products in their respective reactions decreases with increase in temperature. However, the effect on HC selectivity is negligible up to 400 °C. Reverse water gas shift reaction (R2) plays an important role in CO2 hydrogenation process. Since the reaction R2 is and endothermic reaction unlike methanol, DME and HC synthesis. Hence R2 dominates the process at higher temperatures thereby resulting in the increase in conversion and decrease

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Selectivity %

80

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Figure 3.2 Effect of temperature on (a) CO2 conversion and (b) selectivity of DME, CH3OH, and hydrocarbons in their respective reactions at Pressure = 1 bar and H2/CO2 = 3/1.

in selectivity of the three main products. Methanol and DME synthesis are closely related process as the DME is believed to be formed from the dehydrogenation of methanol [8–10]. This indirect synthesis of DME from methanol dehydration is the reason for their similar values of conversion and selectivity as depicted in Figures 3.2 and 3.3. Since methanol and DME synthesis proceeds with the decrease in number of moles from reactants to products. According to Le Chatelier's

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Hydrocarbon synthesis DME synthesis CH3OH synthesis

CO2 conversion %

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CH3OH DME Hydrocarbon

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50

60

Pressure (bar)

Figure 3.3 Effect of pressure on (a) CO2 conversion and (b) selectivity of DME, CH3OH, and hydrocarbons in their respective reactions.

principle, when a stress is applied to a chemical system at equilibrium, the equilibrium will shift to relieve the stress. Therefore, on increasing the pressure of the system, the process proceeds in the forward direction. This gives the rise in CO2 conversion and selectivity of methanol and DME. HC synthesis is least affected by the pressure which is also supported by the recent experimental studies on atmospheric pressure [11–13]. Effect of pressure on CO2 conversion and selectivity of methanol, DME, and HC is shown in Figure 3.3 (a), (b).

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Temperature (ºC) 100

200

300

400

Temperature (ºC) 500

600 100

200

300

400

500

–60

600 –200

–80

ΔH (kJ)

–250

Hydrocarbon Methanol DME

–300

–120

ΔH (kJ)

Methanol DME Hydrocarbon

–100

–350

–140 –160

–400

–180 –450 15

250 DME Methanol Hydrocarbon

ΔG (kJ)

150

10 5

Hydrocarbon Methanol DME

100

0

50

K

200

–5

0 -50

–10

-100 100

200

300

400

Temperature (ºC)

500

600 100

200

300

400

500

–15 600

Temperature (ºC)

Figure 3.4 Comparison of thermodynamic parameters for the three reactions.

The thermodynamic feasibility of a reaction is decided by its Gibbs free energy change (ΔG). The exothermic and endothermic nature can be analysed from the enthalpy change (ΔH) value. Various thermodynamic parameters calculated for the reactions under consideration are shown in Figure 3.4. The resulting thermodynamic parameters suggest that the CO2 hydrogenation to hydrocarbons is the most feasible reaction fallowed by dimethyl ether synthesis. ΔH values indicate that the CO2 hydrogenation reactions are exothermic in nature. Hence, these reactions are often termed as low temperature reactions.

3.3

Catalytic Conversion of CO2 to Methanol, DME, and Hydrocarbons

Currently the CAMERE (carbon dioxide hydrogenation to form methanol via RWGS) process produces methanol from CO2 and H2 at a capacity of ≈ 75 metric tonnes per year. The process passes through two stages, RWGS reaction over ZnAl2O4 followed by water removal and MeOH synthesis over

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Cu/ZnO/ZrO2/Ga2O3. The disadvantage of this process is that it requires two different catalysts and reactors [14]. Mostly, the catalysts for CO/CO2 hydrogenation contained Cu as a main component together with different promoters such as Zn, Zr, B, Cr, Ce, Ga, V, and Ti [15–17]. Along with main component, an appropriate support is crucial to improve the catalytic activity. Cu/ZnO based Al2O3 prepared by co-precipitation [18] has been extensively used and studied for hydrogenation of CO2 to methanol over the past few decades. Cu is generally selected due to abundant availability coupled with lower cost compared to precious metals. In addition, Cu+ and Cu0 species are the crucial catalytic sites leading to high activity and selectivity. The methanol synthesis over Cu has been found to be a structure-sensitive reaction, and the key to high performance is to provide a large accessible Cu surface area [19]. Although the commercial catalyst Cu/ZnO/Al2O3 exhibits promising performance (with a space-time yield up to 7729gMeOH kgcat−1 h−1) under certain conditions (36 MPa and 10 : 1 H2 :CO2 ratio) [20] the pressure is likely too high for economic conversion of CO2. Cu is an important metal for promoting MeOH synthesis, but the reducibility of Cu and the nature of the support material can also have a significant effect on the catalytic performance. For example, deactivation over Cu/ZnO/Al2O3 can be caused by several factors, excess surface hydroxyls, Cu sintering, and decreasing catalyst reducibility from fixation of Cu in the monovalent oxidation state [9]. To improve the catalytic activity and selectivity, Graciani et al., supported a reducible oxide, CeOx on Cu(111) [10]. The DME formation from CO2 hydrogenation involves methanol synthesis over Cu catalysts fallowed by methanol dehydration over acidic support like HZSM-5. Recently some literature reports have suggested new noble metal catalyst active for DME synthesis. Studt et al., showed the formation of DME from CO2 hydrogenation over Ni5Ga3/SiO2 catalyst under atmospheric pressure [21]. Pd, Cu, Li-Ce and Ga have shown significant activity for DME synthesis [10, 22, 23]. The formation of hydrocarbons from direct hydrogenation of CO2 is similar as Fischer-Tropsch reaction. The Co and Fe loaded catalyst are mostly used for HC synthesis. However, due to their difference in RWGS activity and hydrocarbon chain length, application of these catalysts vary. In addition to the active metals, other factors which influence the catalytic hydrogenation of CO2 are elaborated as under.

3.3.1

Effect of Alkali Promotors

There are two types of promoters employed in heterogeneous catalysis: The structural promoters and electronic promoters. The former enhance and stabilize the dispersion of the active phase on the catalyst support. The

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latter enhance the catalytic properties of the catalytic phase itself. Alkali doping of alumina causes the formation of bulk nickel aluminate in a much larger proportion as in the non-doped Al/Ni catalyst. These effects may be originated from either electronic interactions, i.e., the modification of electron density of alkali-promoted metal surfaces, and/or from electrostatic interactions associated with alkali-metal ions, and/or from site blockage [24–26]. Promotional effect of alkali metals in heterogeneous catalysis often depends on the size of the dopant atom, is maximized for a certain alkali content on the catalytic surface [27, 28]. Electropositive alkali promotors (Na+, K+) decrease the work function (ϕ eV/atom, the energy to bring an electron from the metal Fermi level to a distance of a few micrometres outside the metal surface so that image force interactions are negligible) and thereby changing the chemisorptive properties of the metal surface [28]. Thus, upon adding electronic promoters (i.e., alkali ions) on the catalyst surface, the chemisorptive bond strength of reactants and intermediates is modified and the catalytic kinetics are usually changed dramatically. Alkali promotors do fall in the class of electronic promotors. The effect of these promoters is based on their ability to modify chemisorption properties of the catalyst and hence to significantly alter the chemisorptive bond strength of reactants and intermediates. In case of alumina supported copper catalyst [29], K covers Cu, weakens its acidity and hence increases the stability of reaction intermediates thereby enhances the selectivity towards methanol. Whereas Ba promotes the reducibility of Cu [29] and increases the Cu accessibility by covering alumina leading to the better synthesis of methanol. La on the other hand prevents the dissociation of CO2 into CO by stimulating the hydrogenation of oxygenates (formate) to methanol [30]. Song et al., reported the promotion effect of CaO which increases the adsorption strength of CO2, higher adsorption of CO2 leads to higher conversion and selectivity towards the methanol [31]. The alkali-doped catalysts show a much lower NiO content than the unmodified Al2O3 carrier. A synergistic effect between the carbide phase and the small amount of alkali promotor (Na) seems responsible for the selective formation of heavier hydrocarbons [32]. It is reported at the intermediate coverage, Na acts as a promotor in CO2 activation for dissociation into CO and O [33]. Na in hydrocarbon synthesis from CO2 hydrogenation was found to have superior promotional effect as compared to that of Pd [11, 13]. Promoters improve adsorption strength and product selectivity. The strong promotional effects of K can be explained by the predominant coverage of both Cu and alumina surface sites, creating specific active sites stabilizing surface intermediate species and preferring the RWGS pathway. Potassium, K stabilizes surface intermediates and enhance formate dissociation [34,

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35]. Ba promoter covers the alumina surface exclusively and renders Cu accessible and more easily reducible. Ba inhibits formate dissociation and promote methanol synthesis [29]. Lanthanum, La doping on Cu/ZrO2 promotes formate hydrogenation to MeOH and inhibits its dissociation into CO [36]. ZnO acts as a physical spacer between Cu particles and helps the dispersion of Cu with an optimal Cu:Zn ratio as 70:30 [37, 38]. However, the functions of ZnO to promote catalytic activity are still in dispute. Although the role of ZnO is still in dispute to date, the addition of ZnO certainly promotes the dispersion and stability of Cu on the composite, thus leading to high activity. The presence of oxygen vacancies in ZnO are also reported to be beneficial for CO2 activation process [39, 40]. ZrO2 is a promising support and promoter for catalyst due to its high stability under reducing or reactive atmospheres [13, 41, 42]. CeO2 have proved to be a promising support and promotor for CO2 hydrogenation process [43, 44]. The oxygen storage function of CeO2 is the most notable property to affect the valence state of metal oxides and results in the promotion of catalyst [45, 46]. Ga2O3, as a promising promoter according to its redox property (storage and release of oxygen), can induce the strong metal-support interaction between metal and Ga2O3 surfaces with facilitated electron transfer, giving a higher activity for methanol production from CO2 hydrogenation [47]. It was found that the promoting effect of Ga2O3 in Cu/ZnO catalyst is associated with Ga2O3 particle size. Small Ga2O3 particles favour the formation of an intermediate state of copper between Cu0 and Cu2+, probably Cu+, which lead to the higher catalytic activity [14].

3.3.2

Effect of Metal Particle Crystal Phase in CO2 Hydrogenation

Aside from the particle size effect, it has been found that CO2 hydrogenation activity and selectivity depend on the crystal phase of the catalysts [48, 49]. Catalysts with different crystal phases have different symmetries, and could expose very different facets with distinct electronic and geometrical properties, which would have significant influential on the activity and selectivity of the active sites as well as the site density. Superior activity and selectivity from dense active sites can be achieved with the knowledge of relationship between structure and activity of structure sensitive active sites. Under the realistic reaction conditions, industrial catalysts always consist of small nanoparticles with high surface area exhibiting lots of step edges, kinks, crystal facets, interface, etc. It is therefore difficult to clarify the structure sensitivity unambiguously because of the complexity of surface structures and non-uniformity of the realistic catalysts [50]. Thus,

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Table 3.2 Various catalyst supports based on surface chemical properties [18]. Basic

Acidic

Neutral

Amphoteric

MgO

γ –Al2O3

MgAl2O4

α –Al2O3

CaO

SiO2

MgCr2O4

TiO2

BaO

Al2O3/SiO2 (zeolite)

ZrCrO4

CrO2

--

--

ZnAl2O4

ZrO2

designing efficient catalysts can be realized by changing the composition, morphology, particle size, and supports, formation of interface, addition of promoter, etc [49, 51–53]. Besides as the stated above effects, it has been found that the catalytic activity and selectivity can be controlled further by the crystal phase of the catalysts [21, 53–55]. The CO2 hydrogenation process is structure sensitive [19, 56–58].

3.3.3

Effect of Support

The role of various metal oxides or their compounds in catalysts is not limited to support only but they do act sometimes reaction promotors as well. Based on surface chemical properties fallowing are the metallic supports used in CO2 hydrogenation reactions(Table 3.2). Al2O3 acts as a refractory oxide and structural promoter to increase the total surface area of catalyst, the distribution of Cu on surface, and the mechanical stability of composite, which can stabilize the Cu/ZnO structure and retard the sintering of Cu particles [59, 60]. However, a negative effect of water was observed due to the hydrophilic nature of the support Al2O3[42]. CeO2 acted as support or promoter can also accelerate CO2 hydrogenation. The oxygen storage function of CeO2 is the most notable property to affect the valence state of metal oxides and results in the promotion of catalyst [44]. ZnO is a wurtzite, n-type semiconductor and possesses lattice oxygen vacancies, consisting of an electron pair in the lattice, which may serve as an active site for methanol synthesis [39, 40]. ZrO2, Zirconia can also be considered as a promising catalyst support or promoter due to its high stability under reducing or oxidising atmospheres [13, 61]. However, the activity of methanol formation using zirconia as a support is slightly lower than that of ZnO. SiO2, Silica as catalyst support possesses predominant properties, such as acid base nature, porosity texture and thermal stability. In general, the metal functions as the catalyst,

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and the function of the support is to keep the metal particles highly dispersed. The advantage of highly dispersed metal particles is that a large fraction of the metal is located at the surface of the particle, and therefore it is accessible for the reactants to adsorb. In some instances both the supported metal and the support itself function as a catalyst. However, in majority cases, the reactions are metal catalysed only. Nevertheless, even in these metal-catalyzed reactions the support can have a large influence on the performance of the catalyst. For example, platinum particles deposited on acidic supports show higher turnover frequencies and higher stability against sulphur poisoning in the hydrogenation of aromatics compared to platinum on basic support materials. Surface area of the support and the metal-support interaction have a key role determining the metal (cobalt) crystallite size and consequently the activity of the system.

3.4

Mechanism of CO2 Hydrogenation to Methanol, DME, and Hydrocarbons

3.4.1 CO2 Hydrogenation to Methanol Methanol synthesis from CO2 hydrogenation proceeds through various possible pathways including different intermediates [37, 43, 62, 63].(Figure  3.5) Literature mainly suggests the propagation of methanol synthesis by three routes, viz. formate, carboxylate and RWGS. The catalysts for this process mainly contain Cu as an active component, along with different promoters like Zr, Si, Zn, Al, Ti, Cr, Ga, Ce, etc [64, o

o

c

H

o

H RWGS

H

o

c

Formate

c

o

Methanol synthesis

C H

O H

H

H

Carboxyl

o

c

o

H

Figure 3.5 Various reaction pathways for methanol synthesis from CO2 hydrogenation.

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65]. Figure 3.1 depicts the graphical representation of various reaction intermediates constituting different pathways.

3.4.1.1 Formate Route Majority of researchers suggest that formation of formate intermediate is the first step during CO2 hydrogenation to MeOH [66, 67]. Various experimental studies suggested the presence of HCOO as a most copious surface intermediate during CO2 hydrogenation over Cu catalysts. In formate route, CO2 is first hydrogenated to HCOO with surface atomic H. In next step, hydrogenation of HCOO leads to formation of dioxymethylene (H2COO), followed by its subsequent hydrogenation to methoxy (H3CO), and then to methanol (H3COH), as a final product. Where, hydrogenation of HCOO to H3CO was found to be the rate-limiting step. Behrens et al., affirmed the formate pathway by considering Cu (111), Cu (211), and Zn-doped Cu (211) surfaces during CO2 hydrogenation [68]. Nakatsuji and Hu also found the formate pathway as primary wellspring of methanol synthesis by contemplating the response on Cu (100), and Zn/Cu (100) surfaces [69].

3.4.1.2 Carboxylate Route Theoretical study on formate pathway for CO2 hydrogenation to methanol using DFT calculations indicated that high activation barriers for some elementary steps limits the direct hydrogenation of formate on Cu(111) surface [70]. Cheng et al., observed the existence of a unique hydrogen transfer mechanism in presence of H2O, where kinetics was found to favor the CO2 hydrogenation to hydrocarboxyl (trans-COOH) than formate. Hydrogenation of trans-COOH converts it into hydroxymethylidyne (COH), further hydrogenation forms hydroxymethylene (HCOH), hydroxymethyl (H2COH), and finally methanol [71]. The proposed route for methanol synthesis on Cu(111) not only gives a new insights into methanol synthesis chemistry, but also indicates that spectroscopically observed surface species are not necessarily the critical reaction intermediates but rather spectator species. DFT study by Cheng et al., suggested that under industrially relevant conditions, the carboxylate route is energetically more favorable in CO2 hydrogenation over ceria (110) surface [72].

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3.4.1.3 RWGS Route Graciani et al., [43] suggested that RWGS followed by CO hydrogenation is the overall mechanism for MeOH synthesis, as CO hydrogenation is thermodynamically more favourable than that of CO 2. The nanoparticles supported on metal-oxides are particularly attractive materials for RWGS. The interface between metal and support is important in RWGS, as hydrogen is relatively easily dissociated on the dispersed metal catalytic sites [73], which facilitate the spill-over of reactive atomic hydrogen onto the oxide support, for easy hydrogenation of adsorbed CO2 molecule. Two reaction mechanisms have been suggested for CO formation from RWGS. First is the redox mechanism in presence of Cu-based catalysts, where CO2 oxidizes Cu0 to generate CO and Cu+, and H2 reduces Cu+ to form H2O. Here, H2 is involved as a reducing reagent, without any direct participation in intermediates formation [74, 75]. In second mechanism, CO is formed from decomposition of the formate intermediate. This is evidenced by FTIR spectroscopy studies with Cu/ZnO catalyst, which indicated the CO2 dissociation to CO, along with the formation of formate over Cu0. Jiang et al., also reported that CO produced through RWGS, contributes toward MeOH synthesis over Pd-Cu/SiO2 catalyst [76]. Gacieni et al., also proposed the CO2 hydrogenation to methanol via RWGS reaction on Cu-Ce interface [43].

3.4.2 CO2 Hydrogenation to Dimethyl Ether In case of DME synthesis from CO2 hydrogenation on an acid site, methanol gets converted into DME, instead of leaving the catalyst surface. In DME synthesis, the methanol adsorbed at zeolite forms methoxonium ion (H3COH2+) by the transfer of zeolitic proton to methanol. Further dehydration of this ion leaves a methyl group on zeolitic surface, which forms dimethyl ether by its reaction with another methanol molecule. Further investigations indicated that the reaction of methanol takes place at the Brønsted acid and its adjacent Lewis basic sites with the formation of two surface species, i.e., [CH3‚OH2]+ and [CH3O]-, condensation of these species lead to the formation of DME and water [77] (Figure 3.6).

3.4.3 CO2 Hydrogenation to Hydrocarbons The CO2 hydrogenation to hydrocarbons can be carried out via two routes, including indirect and direct route. The two main indirect routes for CO2

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+H*

Carboxyl RWGS COOH*

H2O splitting (renewable energy)

CO2*

Formate

–H2O

+H*

+HO*

–H2O

HCO*

COH* +H*

–H2O H2CO*

+H*

H2COO*

HCOOH*

+H*

CHOH*

+H*

HCOO*

CO*

–H2O

+H*

–H2O

H3CO*

CH2OH* +H*

+H* CH3OH* Acid site

CH3+X–*

–H2O

methanol

+HX

HXCH3OH*

HXCH3OH* +CH3+X–*

+HXCH3OH* HXCH3OHCH3+* +X–

–HX

HXCH3OCH3* –HX CH3OCH3*

CH3OCH3

DME

Figure 3.6 Schematic diagram of DME synthesis from CO2 hydrogenation.

hydrogenation into hydrocarbons involve: (i) synthesis of methanol from CO2 (equation 3.1) and its subsequent transformation into hydrocarbons, (ii) conversion of CO2 into CO through reverse water-gas shift (RWGS) reaction (equation 3.2), and hydrocarbons production by using the modified Fischer-Tropsch Synthesis (FTS) process. Direct routes include the reaction between CO2 with H2, which leads to the generation of a mixture of syngas, CO2 and H2O. The main difference between two routes is that the indirect route employs multiple reaction zones for reaction, which allows the easy optimisation of conditions to give the maximised yields, even with the simple catalyst design. Direct routes employ the optimised reaction conditions in a single reactor, and a complex but effective catalyst system with different functionalities to facilitate the propagation of every

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reaction stage to completion. The possible routes for hydrocarbons production from CO2 hydrogenation are depicted in Figure 3.1.

 

Δ

− −

− (3.1) − (3.2)

3.4.3.1 Indirect Conversion of CO2 Into Hydrocarbons The process of indirect synthesis of hydrocarbons from CO2 via methanol involves first the formation of methanol followed by its dehydration to hydrocarbons. 3.4.3.1.1 CO2 Hydrogenation to Methanol The catalysts for methanol synthesis from CO2 hydrogenation are categorized under three classes: (i) Cu-based catalysts, mainly the Cu/ZnO and modified catalysts are focus of CO2 hydrogenation to methanol research. Here, Cu species acts as an active component, and the catalytic activity is consistent with Cu+ /Cu0 ratio and its exposed surface area. ZnO exhibits the promotional effect in CO2 hydrogenation by improving the copper dispersion. In-depth studies on formation and properties of Cu-ZnOx active sites and Cu-ZnO interface suggested that the amount of Zn migrated into Cu governs the methanol formation rate [63, 78, 79]. Ga2O3 Ga2O3 and ZrO2 promoters has also been explored for increased activity and stability of catalyst for methanol synthesis. (ii) Noble metal catalysts, mainly includes the Pd, Pt and Au based catalysts. (iii) Other catalysts with oxygen deficient active sites, mainly including In2O3/ZrO2 system. Here, oxygen defects on In2O3 surface activates CO2 and its hydrogenation, also these oxygen vacancies stabilize the intermediates involved in methanol formation [80–83]. 3.4.3.1.2 Methanol Conversion to Hydrocarbons MTH process is the conversion of methanol to hydrocarbons in presence of zeolite-catalyst. Both, methanol and dimethyl ether (dehydration product of methaol) can be used as a feed for MTH. Depending on final products, the abbreviations MTG (methanol to gasoline) and MTO (methanol to olefins) are frequently used. Different pore sized zeolites have been studied for MTH, and their selectivity to MTO or MTG was found to be dependent on zeolite topology and operating conditions. The small pore zeolites (SAPO-34) are specifically active in MTO process (C2-C4), as their small pores enable the sorption of linear hydrocarbons only, while excluding the branched and aromatic hydrocarbons. The medium-pore zeolites (ZSM-5) are specifically

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active in MTG process, leading to C5-C11 hydrocarbon products. Regarding MTH, two aspects of the chemistry has been in debate: (1) the origin of first C−C bond and (2) the mechanism for MTH propagation. In the past decade, broad consensus came into existence, revealing the inability of methanol adsorbed within the zeolite pores to directly couple at the appropriate rates for steady-state MTH catalysis. Among all the possible mechanisms for MTH, the hydrocarbon pool mechanism is most widely accepted [29, 68, 84, 85].

3.4.3.2

Direct Conversion of CO2 Into Hydrocarbons

Direct hydrogenation of CO2 is more economic and environmentally benign in comparison to the indirect route. Here reverse water-gas shift (RWGS) reduces the CO2 to CO firstly, followed by the hydrocarbons synthesis via Fischer-Tropsch synthesis (FTS) using CO and unconverted H2. The hydrogenation of carbon dioxide to hydrocarbon, in presence of Fe base FT catalyst surface takes place through CO2 reduction by iron (II), followed by H radical abstraction. When carbonyl C is attacked by residual H, OH formic acid, and CO are formed (Figure 3.2). By the same manner Fe-CH2 radical is formed, which favours the carbon-carbon bond propagation. Cobalt catalysts exhibit high synthesis of heavier hydrocarbons from syngas in comparison to the iron based catalysts owing to their higher chain growth potential (Figure 3.7).

Renewable energy

H2O splitting

CO + H2O

CO2 + H2

CO + H2O

2O

–H

w Ne anol th st me ataly c

New FTS catalyst (Via CO)

New Bi-func. catalyst (Via CH3OH)

+H2

+H2

Methanol catalyst

CH3OH (DME)

–H2O –H2O

Removal

Modified FTS or multi-functional catalyst

Acid catalyst MTO/MTG (Zeolite)

Hydrocarbons Olefin/LPG/Gasoline/Aromatic

–H2O

Figure 3.7 Different routes for hydrocarbons synthesis from CO2 hydrogenation.

Heterogeneous Catalytic Conversion of Greenhouse Gas CO2

3.5

75

Challenges and Opportunities in CO2 Hydrogenation Process

Controlling the selectivity of CO2 conversion by H2 requires thorough understanding of the thermodynamics, kinetics and key reaction intermediates of the aforementioned pathways. CO2-FT and MeOH synthesis are both exothermic processes, but RWGS is endothermic. Therefore, the temperature regime should be carefully chosen depending on the reaction of interest, as shown in Figures 3.2–3.4. Furthermore, for methanol synthesis and CO2-FT, higher reaction pressures can help drive the reaction forward. Clearly, low temperature operation would result in significant energy and economic benefits; however, CO2-FT and methanol synthesis are kinetically limited while RWGS is thermodynamic limited under these low-temperature conditions.

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4 Energy Harvesting: Role of Plasmonic Nanocomposites for Energy Efficient Devices Jaspal Singh1, Subhavna Juneja2 and Anujit Ghosal2,3,* 1

Department of Physics, Indian Institute of Technology Delhi, New Delhi, India 2 School of Biotechnology, Jawaharlal Nehru University, New Delhi, India 3 School of Life-Sciences, Beijing Institute of Technology, Beijing, PRC

Abstract Energy harvesting is sorted as one of the important green energy endeavor of sustainable future. Increasingly high energy demand as a sequestered response to the expanding population and its dependence on energy sources for various daily needs has directed efforts towards advances for better energy production, usage and storage. Contribution of nanotechnology towards enhanced light harvesting has been a major resource. During the course of this chapter, we discuss the role of nanostructures and its characteristics (morphology, size, phase etc.) for efficient energy harvest, underlying principle of charge separation and photo charge generation with due emphasis on ways to improve energy garnering and subsequently energy efficient systems. Keywords: Plasmonic nanocomposites, localized surface plasmon ersonance (LSPR), photovoltaic application, photocatalytic activity, hydrogen production

4.1

Introduction

In the recent era, developing world has been suffering from severe energy crisis and are in a continuous race to generate new ways to harvest sustainable energy resource. As we all know energy is one of the essential requirement to sustain the society and its economical development. Various *Corresponding author: [email protected], [email protected] Shahid-ul-Islam (ed.) Integrating Green Chemistry and Sustainable Engineering, (81–112) © 2019 Scrivener Publishing LLC

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developing countries invest huge amounts to produce energy through conventional methods such as thermal plants, water plants, biomass and oil plants. According to the BP statistical review of world energy 2016, oil, coal and natural gas are the three most used energy resources all around the world [1]. Among them the consumption of natural oil was found to be 31.8% while the coal and natural gas were 28.6% and 21.2% respectively. It has been predicted that less availability take place due to the high consumption rates of natural resources such as oil and natural gas consequently leading to the increase production cost. Apart from this, association for the study of peak oil and gases also proposed the less availability of natural oil and gas by the year 2050 [2]. Nashawi et al., [3], studied the natural oil supply and demand worldwide and proposed a model which indicates the production of natural oil has been maximum in the year at 2014. Moreover, several other projections point out that by the year 2050 the energy consumption rate will drastically increase while the production rate will reduce due to frequent usage of renewable energy resources. Although fossil fuels have proven to be the back bone of each developed and developing countries but enormous usage of these energy resources generates CO2 and several other toxic pollutants which majorly responsible for the global warming and environment pollution. The increment in the CO2 production by the year from 2014 to 2019 around the world has been in continuous rise and evident by the data collected by Mauna Loa Observatory [4]. To overcome these problems scientific community is under a continuous exploration for alternative energy resources which emits less polluting products. A large number of natural resource such as wind energy, ocean waves, mechanical (vibration) energy, thermal energy and solar energy are available. Among them solar light is one of the promising, non-polluting, and noisefree, long lasting source of energy. Moreover, sun not only provides energy but it also takes care of environment as it does not generate any toxic products. Sun light enabled process which can convert the solar light in to the electrical energy or chemical energy is the best possible alternate candidate to the fossil energy resource [5]. Energy harvesting has been attained significant attention over the past few decades due to the threat of energy cries all around the world. Due to energy harvesting process several devices which are use in the field of environment monitoring, health and infrastructures were operated without external electrical system. Moreover, use of various Nanostructures in the field of energy harvesting make it more effective. Nanostructures not only harvested the energy from the natural resources but also scavenging the energy from the other resources like domestic light, vibration of

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human body, mechanical motion and waste heat from the different electronic devices [6]. Therefore nanotechnology based devices with highly sensing capability towards the minute motion in the environment and tiny sources have been proven more effective for the energy harvesting process. A large number of research groups reported several techniques to harvesting the energy from sun light [5–8], piezoelectric systems [9–12], thermoelectric system [13–15] and sensors [16–19]. In this chapter we will emphasis on the applicability of plasmonic nanostructures for the application in light harvesting process through piezoelectric systems, photocatalysis and photovoltaic. In this chapter, several energy-harvesting methods such as photocatalysis and photovoltaic are reviewed, and their improved performance by using plasmonic nanocomposites.

4.2

Plasmonic Nanostructures

The emergence of plasmonic nanostructure revolutionized the optics due to its significant contribution in optoelectronic industries. Plasmonic nanostructures are the particular class of nanomaterials in which the electron density on the surface can interact with the specific wavelength (larger than size of metal nanoparticles) of electromagnetic radiation. Noble metal nanoparticles such as Ag, Au, Pt, Pd and Cu are considered as plasmonic nanomaterial [17]. Plasmonic nanostructures consist electron enrich surface due to the size in nano dimension which lead to its interaction with equal or higher wavelength of incident radiation and surface electrons were vibrate to and fro with their mean position and formed an imaginary positive dielectric which lead to the electromagnetic filed around the plasmonic nanostructures. This effect is known as surface plasmon resonance (SPR) effect [18]. Upon excitation with the incident light, the electrons in the conduction band travel along the phase and polarized the plasmonic nanoparticles. Therefore, a restoring force was acted on the disturbed electrons from the equilibrium positions and makes them vibrate in a certain frequency. These specific vibrations creates a dipolar field outside the nanoparticles, which significantly enhanced the absorption capability of plasmonic nanoparticles. The generation of surface plasmon resonance effect has been depicted in Figure  4.1. The most fascinating property of plasmonic nanostructures is surface plasmon resonance (SPR) effect which was driven through the size and shape of the noble metal nanoparticles [19]. Due to the existence of SPR effect, noble metal nanoparticles were able to enhance the electromagnetic field around its surroundings.

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Figure 4.1 Schematic revealing the surface plasmon resonance effect in noble metal nanoparticles.

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Figure 4.2 (a-b) Images of Ag nanoparticles solution showing the variation in the optical absorption peak with the Ag solution colors. (Copyright permission) [27]

Among various plasmonic nanomaterials Ag and Au are known to be more popular due to their applications starts from ancient times in for make the glasses colorful. Several colors of glasses can be achieved by dyeing with Ag or Au nanoparticles of different sizes. The optical behavior of the noble metal nanoparticles are strongly dependent on the sizes, shape and inter particles separation between the nanoparticles. Several research groups reported their contributions to prepare various types of plasmonic nanostructures [20–26]. Haung et al., [27], demonstrated the variation in the optical properties of Ag nanoparticles by varying their size and shape (Figure 4.2). They varied the optical absorbance peak of Ag nanoparticles from 393 nm to 798 nm by changes their size from 2 nm to 39 nm respectively. In another study Gonzaliz et al., [28], tuned the morphology of Ag nanoparticles and studied the variation in their optical properties. They also investigated the effect of temperature dependence on the shape and optical properties of Ag nanoparticles. Due to the existence of these extraordinary properties, various plasmonic nanostructures were considered as the valuable topic of interest and applied in large variety of application such as PL

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enhancement [29], photovoltaics [30], energy production [31], sensors [32], SERS [33] and biomedical applications [34].

4.3

Plasmonic Nanocomposites

As the name suggested, a plasmonic nanocomposites is combination of noble metal nanoparticles with a semiconductor or organic material or ceramic material. A large variety of work has been reported by several research groups in the field of plasmonic nanocomposites [35–40].The existence of plasmonic nanostructures in the different dielectric medium significantly modifies their optical absorption properties due to SPR effect. Moreover, it can control the recombination rate and provides the efficient charge transformation and increase the performance of nanocomposites in various applications such as solar cells [41], photocatalysis [42], H2 production [43], sensors [44], antimicrobial activity [45] and environment remediation [46]. It has been reported that noble metal nanoparticles create a Schottky junction with the other dielectric medium, which efficiently minimize the recombination rate in other dielectric material [47]. Apart from this the electronic magnetic field around the plasmonic nanostructure further maintains the separation between electron and holes [48]. Li et al., [49] prepared Ag-TiO2 nanocomposites by using three different chemical methods and proposed that the Schottky junction at the interface of Ag nanoparticles and TiO2 was control the charge recombination in TiO2. Jinag et al., [50] reported the fabrication of Au-Cu2O plasmonic nanocomposites by using chemical method and explored their photocatalytic activity by the decomposition of methyl orange. In their study they concluded that two effects surface plasmon resonance (SPR) and Schottky junction were responsible for the enhanced photo active behavior of Au-Cu2O nanocomposites. Moreover, Schottky junction helps to inject the electron in to the Fermi level while SPR enhance the charge separation as well as improve the optical absorption. For energy harvesting application, plasmonic nanocomposites have been proven a suitable candidate due to its efficient charge separation and wide range of optical absorption ability [51–55]. A large variety of physical methods (magnetron sputtering, anodization method, pulsed laser deposition, thermal evaporation and atomic layer deposition) [56–60] and chemical methods (Photo deposition, chemical precipitation, hydrothermal method, linker strategy and dielectrophoretic assembly method) [61–65] have been reported for the fabrication of plasmonic nanocomposites by various research groups.

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For the fabrication of plasmonic nanocomposites by physical methods several parameters such as vacuum in the chamber, substrate temperature, rate of gas flow, filling factor of the target materials and thickness of the nanostructured thin film should be computed and optimized. Ritmi et al., [66] fabricated Ag-TiON thin films on the polyester substrates by using Pulsed magnetron sputtering and used these nanostructured thin films to explore the antibacterial activity under artificial light. They optimized the sputtering parameter such as current, power and gas flow rate for Ti target as well as Ag target. They have used 128 W power and 280 mA current for the sputtering from Ti target while for Ag sputtering 50 kHz with 15% reversed voltage and power of 140 W was applied. Tan et al., [67] adopted two step strategy for the preparation of Ag decorated ZnO tetrapod using thermal evaporation and RF sputtering method. They have studied their photocatalytic behavior by the degradation of methyl orange dye in water under mercury lamp exposure. Firstly, ZnO tetrapod structures of ZnO were deposited under O2 flow and temperature 930 °C onto silicon substrates by using thermal evaporation method. In second step, by using RF sputtering technique Ag nanoparticles loading was varied on to the ZnO tetrapod structures by the power from 50W to 150W and sputtering time from 1 minutes to 2.5 minutes. Recently Chen et al., [68], fabricated nanostructured Au-TiO2 plasmonic thin films by varying the sequence of Au and TiO2 layers. Au-TiO2 plasmonic nanocomposites thin films fabricated on the FTO substrates through sputtering method and used for the photo electrochemical application. The thickness of TiO2 film and Au films were 10 nm and 5 nm respectively for each case. The sputtering was carried out at 1 × 10−6 bar. A scheme of sequential changes in the preparation of Au-TiO2 plasmonic thin films were depicted in Figure 4.3. On the other hand chemically, routs were very effective for the fabrication of plasmonic nanocomposites. In chemical methods several Sputtering 10 nm Ti

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Figure 4.3 Scheme for the preparation of nanostructured Au/TiO2 thin film showing (a) FTO/TiO2/Au and (b) FTO/Au/TiO2. (Copyright permission) [68]

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parameters such as pH, temperature, precursors ion concentration, reducing agents and solvents can be tuned the size, crystallite size, phase and morphology of the plasmonic nanocomposites. Chen et al., [69] reported the formation of MOS2/graphene nanocomposites through hydrothermal method and decorated with Ag and Au metal nanoparticles. Plasmonic nanocomposites Ag-Au/MOS2/RG showed enhanced electrochemical activity as compared to pure MOS2 nanostructures. The enhanced electrochemical sensing and catalytic activity can be attributed to the effective high surface are due to existence of graphene and LSPR effect of Ag and Au nanoparticles. Elshafey et al., [70] reported the preparation of self assembled Au nanoparticles decoration on to the reduced graphene oxide. They demonstrated that Au-RGO nanocomposites combined Au nanoparticles prepared through citrate ion reduction and reduced graphene oxide fabricated through hummers method efficiently detected the p53 antibodies in serum. Muszynski et al., [71], synthesized Au decorated graphene sheets by using wet chemical method. Initially, graphene sheets were fabricated through the refluxing method and subsequently functionalized with the Octadecylamine (ODA). The Au nanoparticles were chemically attached on the surface of graphene sheets using through the NaBH4 reduction method. This novel strategy for the attachment of Au nanoparticles on to graphene sheets is depicted in Figure 4.4. Dai et al., [72], prepared TiO2 nanowires with cuboid shape by hydrothermal method on the FTO substrates. After that Ag nanoparticles of different sizes were deposited by using magnetron sputtering with vacuum pressure 5 × 10−1 Pa. Ag-TiO2 nanoscomposite on the FTO substrates was used for to study the surface enhanced Raman spectroscopy (SERS).

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Figure 4.4 Schematic diagram reveal the strategy for the preparation of Au decorated graphene sheets. (Copyright permission) [71]

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Plasmonic Nanocomposites for Energy Harvesting

Plasmonic nanocomposites has been proven outstanding candidate to scavenging the energy from Sun. Plasmonic nanocomposites can absorb sun light more efficiently due to the existence of semiconductor material and noble metal nanoparticles. Firstly, semiconductor material creates the electron hole pair then noble nanoparticles efficiently improve its charge separation and minimize the recombination rate. Due to the presence of noble metal nanoparticles the optical absorption capability of plasmonic nanocomposites are significantly improve consequently energy harvesting efficiency further enhance. In this section, several energy harvesting applications of the plasmonic nanocomposites are reviewed.

4.4.1 Plasmonic Nanocomposites for Photovoltaic Applications Sun light is abundantly available energy resource which enables the several alternative energy generation processes. Among them the creation of electricity through sun light has been aroused great attention last few decades. Photovoltaics or solar cell technology convert the sun light in to the electric energy through the absorption of photon followed by the generation of charge carriers. Semiconductors considered as suitable candidate for the solar cell application due to the existence of photovoltaic (PV) effect. Solar cell technology attained significant attention and growing rapidly. In the year 2016, the computed total global energy generated by solar cell technology was 303 GW [73]. The solar cell efficiency depends on the light absorption capability, recombination rate in semiconductor and energy dissipation in the external circuit. Semiconductor nanostructures exhibit unique electronic structures, charge carrier mobility and high light absorption ability. The electron-hole movements in the semiconductor can be governed by tuning the size and shapes of the semiconductor. The quantum size effect in the semiconductor confine the charge dynamics of electrons and give rise to new optical and electronic properties. With the decrease in the size of the semiconductors the effective surface area and surface to volume ratio enhances which is beneficial for the interaction between the semiconductor based device and particular media. Different metal oxide semiconductor such as ZnO, TiO2 and SnO2 have been used to make the photoanode of dye sensitized solar cells [74–76]. Semiconductors based photoanodes were encountered from the high recombination rate, low energy conversion

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Figure 4.5 Schematic diagram reveals the (a) Incident light trapping and scattering of light scattering by surface noble metal nanoparticles, (b) Enhancement in the light absorption due to SPR in metal nanoparticles exists in the semiconductor matrix, (c) Tapping of light by the noble metal back plane. (Copyright permission)[85]

and lower conduction rate of electrons [77]. For further improve the efficiency of solar cells, surface modification with noble metal nanoparticles are quite new. It has been found that attachment of noble metal nanoparticles can improve the photocurrent density consequently efficiency of solar cells enhanced [78–81]. The existence of noble metal nanoparticles on to surface of semiconductor improve the light absorption ability due to SPR effect and provides the efficient charge separation through Schottky junction creation [82, 83]. Metal nanoparticles also help to increase the light absorption capability through the scattering of light and minimize the reflectance of light by semiconductors [84]. Moreover, metal nanoparticles or patterning on the back side generates the photonic mode due to the SPR effect at the interface of semiconductor and metal back layer, which travels through the semiconductor layer and improve the photocurrent density [85, 86]. Figure  4.5 reveals the three different proposed strategies which have significantly improve the light utilization of semiconductors by the use of noble metal nanoparticles [87]. Ferry et al., [88], fabricated the Ag deposited amorphous SiO2 layer based plasmonic solar cell and observed the significant improvement in the photocurrent density. They concluded that due to the SiO2 imprinted lithography nanopatterens incident light is scattered strongly and increase the photocurrent of cell. Moreover, embedded Ag deposition on the amorphous SiO2 layer was also contributed and increases the photo current of Ag-SiO2 patterned cell by 50% as compare with cell with flat pattern. In another study, Lu et al., [89] reported the plasmonic enhancement due to the incorporation Ag and Au nanoparticles in the polymer solar cells. The plasmonic nanoparticles (Ag and Au) in to the PEDOT:PSS polymer improve photocurrent significantly. The photocurrent efficiency

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2 0 –2 –4 –6 –8 –10 –12 –14 –16 –18

AI/Ca cathode

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Light

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Figure 4.6 (a) Schematic representation of fabrication of Ag and Au modified PEDOT:PSS based solar cell, (b) photo-current density characteristic raveling the improvement in the current density using Ag and Au nanoparticles simultaneously. (Copyright permission from ACS) [89]

(a)

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Figure 4.7 (a) N-TiO2 and (b) N-TiO2–Ag (10 wt% Ag) (c) Corresponding SAED pattern (d) lattice resolved TEM image of N-TiO2–Ag (e) photo-current density characteristic of photo anode prepared by (a) pure TiO2 (2.19%), (b) N-TiO2 (2.93%), (c) TiO2–Ag (4.86%) and (d) N-TiO2–Ag (8.15%) photoanode-based DSSCs. [87]

of bare polymer based is 7.25% while the photocurrent with plasmonic nanoparticles is 8.67%. They showed that with the incorporation of metal nanoparticles efficiently improve the charge separation and increase the photocurrent of 20% (Figure 4.6). Figure 4.7 (a) revealed the assembly of Au/Ag/ PEDOT:PSS based solar cell while Figure 4.7 (b) showed the enhancement in the current density using Ag and Au nanoparticles [87]. Tan et al., [90] designed back reflector silicon solar cell and studied the effect of random texture and incorporation of Ag nanoparticles in them. They showed that with the use of Ag nanoparticles the photo conversion efficiency is comparable with the solar cell prepared through the random

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texture. The plasmonic back reflector efficiently provides the broad range and exhibits the reflection above the 80%. They concluded that Ag nanoparticles effectively scattered the incident light which increases the light tapping process consequently photoconversion efficiency of solar cell significantly enhanced. As we have discussed above due to the SPR effects and creation of Schottky junction with the semiconductors plasmonic nanostructures improve their performance in solar cell application. Various research groups improve the efficiency of dye sensitized solar cells (DSSC) through the modification with the noble metal nanoparticles [91–96]. Nahm et al., [97] prepared Au-TiO2 plasmonic nanocomposites by facile chemical method. They showed the concentration effect of Au nanoparticles with 100 nm on the performance of dye sensitized solar cells. The sample contain Au/Ti molar ratio 0.05 shows maximum enhancement in the photocurrent density and efficiency. The Au-TiO2 plasmonic nanocomposites based photoanode of DSSC, exhibited 20% higher photo conversion efficiency of DSSC. They also proposed that higher enhancement in the Au-TiO2 plasmonic nanocomposites due to the unique SPR and optimum scattering due to path length of Au metal nanoparticles. In the earlier study Lim et al., [87] synthesized Ag decorated nitrogen doped TiO2 nanostructures by reduction method and used investigated their application in DSSC. They tuned silver content from 2.5 wt % to 20 wt % on the nitrogen doped TiO2 and found the maximum photoconversion efficiency (8.15 %). They demonstrated that with the increase in the Ag content the efficiency of DSSC enhanced significantly. Figure  4.5 (a–d) depicts the TEM studies of Ag decorated N doped TiO2 nanostructures while Figure 4.5 (e) reveals the Photocurrent properties of photo anodes fabricated through synthesized samples. Bora et al., [98] reported the fabrication of Au-ZnO based DSSCs and optimized the Au nanoparticles loading on the ZnO nanorods for efficient photo current density. In their study they showed the Au-ZnO based DSSC is 5.36% efficient than the bare ZnO nanorod base DSSC. They also reported the 130% enhancement in the photo conversion efficiency due to the modification with Au nanoparticles on ZnO nanorods. Yan et al., [99] prepared g-C3N4/Ag/TiO2 composite by the wet chemical method and designed the photoandoe for DSSC by using this nanocomposite. In their study they demonstrated that Ag modified optimal concentration of g-C3N4 sheet with TiO2 exhibits highest photo conversion efficiency. The PCE for Ag modified g-C3N4/TiO2 nanocomposites is 6.22% while the PCE foe bare TiO2 and g-C3N4/TiO2 nanocomposites are 3.72% and 5.34% respectively. The of g-C3N4/Ag/TiO2 plasmonic.

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5 nm

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CT5 CT-5/Ag TiO2 (P25)

4 2 0 0.0

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Figure 4.8 (a) HRTEM images of g-C3N4/Ag/TiO2 plasmonic nanocomposites (Inset shows the SAED patterens), (b) photo-current density showing that g-C3N4/ Ag/TiO2 plasmonic nanocomposites exhibits higher photocurrent than g-C3N4//TiO2 nanocomposites and P25. (Copyright permission from RSC) [99]

Nanocomposites and photocurrent density were presented in Figure 4.8. Chen et al., [100] designed the Au-SiO2/BHJ-PSC based solar cell and explored the plasmonic enhancement in the photo current density. They showed that with the incorporation of Au-SiO2 core shell nanoparticles the photo conversion efficiency enhanced by 16% from 3.29% to 3.80%. The observed enhancement attributed to the LSPR and scattering effect of Au nanoparticles. By using different plasmonic nanostructures the photoconversion efficiency can be improved. The noble metal nanoparticles loading efficiently improve charge separation and enhance the light utilization which is beneficial for the photo conversion efficiency or performances of the solar cells. In addition, for the more advancement in the solar cell technologies basic aspects of the plasmonic nanostructures such as size, shape, loading and inter particles distance can be significantly improve the solar-cell performance which should be addressed more precisely.

4.4.2 Plasmonic Nanocomposites for Water Purification Water pollution originating from industrial toxic organic pollutants is one of the major threat for the society and each living body. Several organic contaminants like azo dyes, nutrients, bacteria and pathogen present in the water, which are main constitutes of water pollution. Organic contaminations present in wastewaters discard from the industries are the major threat for human health. Approximately more than 7 × 105 tons of different types (100000) of dyes were produced in a year [101]. The organic pollutant may produce critical effects the on the unprotected entity which

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is depending on the illumination time and concentration of dye. Several effluent treatment methods have been developed which can be classified in three different categories; physical, chemical and biological. Due to the cost effectiveness, physical and chemical methods are frequently used for the organic dye treatment [102]. Photocatalysis oxidation process is one of the advance methods as compared to the conventional methods for water detoxification. In the photocatalytic process the organic dye molecules can be efficiently degrades in to inoffensive species [103]. Moreover, sun light induced photocatalysis process is more promising than the UV-light induced photcatalysis. It totally eliminates the harmful effects of UV radiations. Sun light driven photocataysis used 3% of solar spectrum for the completely degradation of organic dye molecules. Use of sun light in the photocatalysis process, not only protect our energy resources but also provide the cost effectiveness contribution in reducing the environmental pollution. In this process solar light is converted in to the chemical energy which is used for the purification of the water. Although various semiconductors have been used by several research groups for the application in the photocatalytic degradation of organic pollutant but their efficiency could not be increase beyond a certain limit. A semiconductor suffers from the inefficient light utilization and fast rate of recombination which limits its efficiency [104]. To further improve the photocatalytic efficiency attachment of plasmonic nanostructures on to semiconductors is most promising rout. Attached plasmonic nanostructures on the surface of semiconductors improve the absorption of sun light consequently photocatalytic activity efficiently enabled and detoxified the water. The fundamental aspects of the plasmonic nanocomposites under sun light can be understood as follows. Under sun light exposure semiconductor material creates the electron-hole pair in the respective conduction and valance band. The plasmonic nanostructures attached with the semiconductor attract the electron due to the existence of Schottky junction between the interface of noble metal nanoparticles and semiconductor [105]. Under sun light plasmonic nanoparticles also get excited and inject electron in to the conduction band of semiconductors due to the SPR effect. But Schottky junction continuously maintains the flow of electron from the conduction band of semiconductor towards the metal nanoparticles [106]. In addition the electromagnetic field around the plasmonic nanoparticles further helps to separate the electron and hole in semiconductor [107]. Under sun light above mentioned process occurs which efficiently minimize the recombination rate and enhance the visible light absorption which significantly Improve the photocatalytic process. Recently Varma et al., [108] reported the preparation of Ag

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Integrating Green Chemistry and Sustainable Engineering O2

e

e–

O2

e– e–

–

e

e–

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–

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EF Ag

Brookite Ag4d h+ h+ OH–

Anatase

h+

h+ h+

h+ +

h

h+

OH

Figure 4.9 Schematic representation of photocatalytic activity of Ag modified mixed phase (Brookite/Anatase) TiO2 under sunlight. (Copyright permission from RSC) [108]

modified mixed phase TiO2 and studied their photocatalytic behavior by the decomposition of methylene blue and p-nitrophenol under sun light. They explained the basic charge injection process during the photocatalytic activity of Ag-TiO2 system. Figure 4.9 depicts the schematic representation of Ag modified mixed phase TiO2 nanostructures. Pirhashemi et al., [109] reported the synthesis of ZnO/Ag/Ag2WO4 plasmonic nanocomposites and used them as visible light photocatalyst. In their photocatalytic experiment 10 μM of MB dye, 13 μM of RhB dye and 10 μM of MO dye solutions were separately decomposed by using 100 mg photocatalyst material. They concluded that the enhanced photocatalytic activity attributed to the enhanced visible light absorption ability and efficiently control the recombination rate due to the modification with Ag nanoparticles. Mondal et al., [110] designed Au-ZnO plasmonic nanocomposites by hydrothermal method and applied these material for the photocatalytic decomposition of five organic dye namely RhB, MB, CR, MO and RB dyes solution in water under sun light illumination. Xiang et al., [111] synthesized Ag decorated TiO2 hollow spheres photocatalyst by combing the hydrothermal method with microwave assisted method. Ag RhB decorated TiO2 hollow spheres showed superior photocatalytic activity for the degradation of dye as compared to p25 TiO2 photocatalyst. The improvement in the photocatalytic behavior ascribed to the enhanced optical absorption originated due to the SPR effect in silver nanoparticles under visible-light irradiation. Luo et al., [112] fabricated Ag/Bi3TaO7

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A2 Bi2TaO2

20 nm

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Figure 4.10 (a-b) TEM images of g Ag/Bi3TaO7 plasmonic nanocomposites, (c) Rate kinetics of tetracycline under visible light exposure using pristine sample BTO and Ag decorated BTO sample, (d) Bar graph reveals the efficiency of pristine sample BTO and Ag decorated BTO samples. (Copyright permission from ACS) [112]

plasmonic nanocomposites through hydrothermal method and used them in to explore the photocatalyst behavior. The photocatalytic activity was test by the degradation of tetracycline under visible light exposure. They concluded that the improvement in the photocatalytic activity attributed to the synergistic effect of SPR effect generated by Ag nanoparticles. The microstructures of Ag modified BTO and corresponding photocatalytic efficiency were revealed in Figure 4.10. Wu et al., [113]. reported the synthesis of Ag2S/Ag@MoS2 plasmonic nanocomposites by using hydrothermal method. The as-synthesized plasmonic nanocomposite was used for the application in photocatalytic decomposition of MB dye under Xenon lamp. The enhancement in photocatalytic activity attributed to the formation in Schottky junction and LSPR effect created by the Ag nanoparticles. Roy et al., [114] prepared Au nanorod decorated Graphene/ ZnO nanocomposites by facile wet chemical method and used these material for the photocatalytic decomposition of nitrobenzene

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Figure 4.11 TEM images of sample (A-C) sample ZA2, (D-F) sample ZG2, (G-I) sample AZG, (J) Optical absorption spectra revealing the degradation of MB dye using sample AZG as photocatalyst (K) Rate kinetics of sample ZP, ZA2, ZG2 and AZG. (Copyright permission from Elsevier.) [115]

under UV lamp exposure. Recently Juneja et al., [115] reported the fabrication of Graphene wrapped Au/ZnO plasmonic nanocomposites using hydrothermal method and explores their photocatalytic activity and antimicrobial activity under sun light illumination. Figure 4.11 (A–C) (Figure 11) reveals the Au decoration of ZnO flower like structures with different magnification while Figure 4.11 (D–F) , and (G-I) shows the Graphen-ZnO nanocomposite and Au decorated Graphen/ZnO nanocomposites respectively at different scales. Figure 4.11 (J) depicts the reduction in the intensity of MB blue by using Graphene-Au-ZnO nanocomposites as a photocatalyst under sun light. Figure 4.11 (K) reveals that Graphene-Au-ZnO nanocomposites (AZG) is found to be most efficient photocatalyst as compare Au/ZnO (ZA2) and Graphene/ZnO (ZG2) under sun light exposure. In their study, they concluded that Au metal nanoparticles significantly minimize the recombination rate by creating the Schottky junction with Graphene/ZnO nanocomposite consequently the photocatalytic activity significantly increased. In another study Li et al., [116] fabricated Au-TiO2 mesoporous nanocomposites by wet chemical method. They demonstrated that different concentration (0.1% to 5%) of Au metal nanoparticles with TiO2 effects the photocatalytic properties significantly and maximum at 0.5% Au-TiO2. In photodegradation experiment, 50 mg Mesoporous Au-TiO2 nanocomposite decomposed 600 μM and 400 μM aqueous solution of phenol and chromium respectively under UV light. They also concluded that visible light absorption responsible for the enhanced

Energy Harvesting: Role of Plasmonic Nanocomopsites Phenol Cr(VI)

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Figure 4.12 TEM image along (a)100 plane (b) 110 plane of Au-TiO2 (0.05% mol Au) nanocomposites, (c) TEM image of Au-TiO2 (1% mol Au) reveling the attached Au nanoparticles, (d) TEM image of Au-TiO2 (2% mol Au) showing the decoration of Au nanoparticles, (e) Photocatalytic degradation of phenol and Chromium using pure TiO2 and Au-TiO2 sample respectively. (Copyright permission from ACS) [116]

photocatalytic activity of mesoporous Au-TiO2. Figure 4.12 (a–d) reveals the TEM images of Au-TiO2 sample prepared through 0.05%, 1% and 2% mol of Au respectively while the photocatalytic behavior for the degradation of phenol and chroumium were depicted in Figure 4.12 (e). Bian et al., [117] prepared Au-TiO2 plasmonic nanostructures by impregnation method and increase the visible light induced photocatalytic efficiency. In photocatalytic test, two organic dyes solution methylene blue, rhodamine b and four chlorophenol were decomposed under the presence of visible light emitted by the Xenon source lamp with intensity 500 W/m2. The used amount of MB, RhB and 4-CP for photodegradation studies were 10 μM /L and these solutions were decomposed in just 20 min, 2 min and 4 hr respectively with the help of 1 gm/L photocatalyst material. Recently Ghasemi et al., [118] designed Au nanoparticles modified Grahene-TiO2 nanocomposites by using photoreduction method and applied these plasmonic nanocomposites to explore the photo degradation of Acid blue −92 dye solution under UV and visible light exposure separately. The enhancement in the photocatalytic activity in the visible region attributed to the SPR effect created by the Au nanoparticles. The plasmonic nanocomposites have great ability to improve the photocatalytic activity which efficiently produces the chemical energy from the solar energy. Moreover, plasmonic nanocomposites dynamically increase the dye degradation rate and enhanced the photocatalytic activity up to certain extent. With the use of plasmonic nanocomposites in the

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water purification the sun light harvesting is significantly improves consequently plasmonic nanocomposites enable the cost effectiveness in water purification systems. In addition photocatalytic process decomposes the organic pollutant without damage to the environment. The low photocatalytic activity and manufacturing the photocatalyst material in the large scale need to be further improvement.

4.4.3 Plasmonic Nanocomposites for Hydrogen Production As we have discussed in the introduction energy production is one of the major challenge for the scientific community. It has been computed that by the year 2050, 30TW energy was consumed by the 9 billion peoples worldwide [119]. The hydrogen production through the sun light harvesting is on the environment friendly, cost effective and more convenient green method, which was not required any large-scale machinery. The scientists are more interested towards the production of hydrogen over the last few decades by using several semiconductors, since it is not harmful for the environment. The basic principle of the H2 production is based up on the water splitting; It is well known that water molecules can be break in to H2 and O2 molecules. Under the presence of light semiconductor material created hole and electrons in the valance band and conduction band respectively. The electrons in the holes in the valance band converted water molecules in to OH- and H+ ions. The electrons in the conduction band react with H+ and converted them in to H2 molecules. The semiconductor encountered by the high rate of recombination consequently the H2 production rate retarded [120]. The surface modification with the plasmonic nanoparticles significantly increase the life time of the photogenerated electron and holes and minimize the recombination rate by forming the Schottky junction as we have discussed earlier above. Moreover, plasmonic nanoparticles enhance the visible light absorption due to the LSPR effect. Thus plasmonic nanocomposites effectively enhance the rate of the H2 production. Recently Li et al., [121] used hydrothermal method to synthesized Au modified TiO2/MoS2 plasmonic nanocomposites and used them for the generation of H2. They initially prepared TiO2 nanorods on FTO then MoS2 layers were attached on the top of TiO2 naorods. After that MoS2/ TiO2 nanocomposites functionzed with Au metal nanorods. Figure  4.13 depicts the FESEM images of Au decorated MoS2/TiO2 plasmonic nanocomposites. Figure 4.13(a) shows the top view of the TiO2 nanorod while Figure 4.13 reveals the MoS2 attached TiO2 nanorods. Figure (c-d) clearly

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(a)

H2 evolution rate (umol.h–1.g–1)

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Figure 4.13 (a) Top view of the TiO2 nanorods, (b) Top view of the TiO2-MoS2 (c-d) Au modified TiO2-MoS2 plasmonic nanocomposites (e) Bar graph representation of H2evolution rate using TiO2, TiO2-MoS2 and Au- TiO2-MoS2. (Copyright permission[121])

Amount of evolved H2 gas (gmol)

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Figure 4.14 (a) Schematic depiction of Au/g-C3N4 nanocomposite used as aphotocatalyst for photocatalytic generation of hydrogen, (b) H2 production evolutionusing pristine g-C3N4 sample and Au modified g-C3N4 samples. (Copyright permission[124])

reveals the modification with the Au nanorods on the surface of MoS2/ TiO2 nanocompsites. They computed that the H2 production rate increased by 52% due to the attachment of Au nanorods on to the surface of TiO2-MoS2 nanocomposites. The observed enhancement in the rate of H2 production ascribed to the LSPR effect and hot electron injection. Zeng et al., [122] used combination of solid state reaction method with chemical reduction method for the synthesis of Au/g-C3N4/NiFe2O4 plasmonic nanocomposite and applied them for the visible light induced photocatalytic generation of H2

100 Integrating Green Chemistry and Sustainable Engineering Graphene

Au

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Figure 4.15 (a) Bright TEM image of Au-graphene-TiO2 plasmonic nanocomposites with 0.25 wt%, (b) HRTEM image of Au-graphene-TiO2 plasmonic nanocomposites with 0.25 wt%, (c) Bar graph depiction of H2. production by using different photocatalyst. (Copyright permission[122])

production. In their study they highlighted that with the incorporation of Au nanoparticles H2 production rate enhanced by 4.3%.The superiority of the Au/g-C3N4/NiFe2O4 photocatalyst as compared to -C3N4/NiFe2O4 photocatalyst ascribed to the LSPR effect and efficient charge separation due to modification with Au nanoparticles. Samanta et al., [123] designed Au decorated g-C3N4 nanostructures by wet chemical method. In their study, the concentration of Au nanoparticles was varied from 0.5wt% to 2.0wt% and optimized that g-C3N4 nanostructure decorated with 1 wt% Au nanoparticles exhibits highest rate for H2 production as compared to other synthesized sample (Figure 14). The amount of H2 production using pristine g-C3N4 and 0.5 Au g-C3N4, Au1.5 g-C3N4 and Au 2.0 g-C3N4 were

Energy Harvesting: Role of Plasmonic Nanocomopsites 101 found to be 23, 379.5, 446, and 312.2 μmol, respectively while for amount generated through sample 1Au g-C3N4,is 532.2 μmol. The H2 production test was carried out under Hg lamp exposure by using 20 mg photocatalyst material. In another study, Chen et al., [125]. reported Au-TiO2 plasmonic nanocomposites on quartz substrate by spin coat followed by photodeposition method and explored their photocatalytic H2 production capability. They concluded that Au nanoparticles due to the LSPR properties effectively increase the light absorption ability and also minimize the recombination rate in TiO2. Apart from this electromagnetic field around the Au nanoparticles further improve the recombination rate in TiO2 consequently the production rate of sun light induced photocatalytic H2 production is significantly enhanced. Liu et al., [126] applied combined approach of microwave assisted and chemical reduction method for the preparation of Ag-TiO2 nanocomposites. In their study Ag nanoparticles loading were varied to optimize the Ag-TiO2 photocatalyst with highest amount of H2 production. The Ag nanoparticles absorb more light from the solar spectrum. They showed that with optimum loading of Ag nanoparticles H2 production yield enhanced 8.5 times as compared to pure sheet like TiO2 nanostructures. Khalid et al., [127] reported the preparation of Ag-TiO2/graphene nanocomposites using microwave assisted hydrothermal method. They systematically varied the weight % of Ag nanoparticles on the graphene/TiO2 prepared by hydrothermal method. At 0.09 wt% of Ag nanoparticles on TiO2/ graphene highest evolution of H2 was achieved. Due to the SPR effect and synergetic effect under sun light Ag nanoparticles increase the production rate of H2 production. Recently Wang et al., [122] designed graphene-Au-TiO2 nanocomposites by using hydrothermal method with microwave assisted method and produced H2 under visible light exposure. The weight ratio of Au nanoparticles was varied from 0.05% to 0.5% on the graphene-TiO2 nanocomposites. They found that graphene/TiO2 nanocomposite loaded with 0.25 wt% Au was most efficiently evolved the H2 as compared to other photocatalyst sample. Figure 4.13 revealed the TEM and HRTEM images of Au modified graphene/TiO2 nanocomposite while Figure 4.13 represents the bar graph representation of H2 production rate using different as-synthesized photocatalyst sample. Recently Lou et al., [128] prepared Au prism/Pt frame decorated on the reduced graphene oxide through wet chemical method. They showed the Au prism/Pt frame decorated on the reduced graphene exhibits significant improvement in the H2 evolution as compared Au nanoprism/RGO and Pt nanoframe/RGO. The enhancement in the H2

102 Integrating Green Chemistry and Sustainable Engineering evolution ascribed due to the dipole and multipole surface plasmon resonance Au nanoprism which separate the charge efficiently and effectively enhanced the generate rate of hydrogen. Yu et al., [129] prepared Pt functionalized TiO2 nanosheets by using combination of hydrothermal method and photoreduction method. The Pt-TiO2 plasmonic nanocomposites with different loading of Pt applied for the generation of H2 production. They showed TiO2 nanosheets decorated with 2% wt Pt exhibits better H2 production rate as compared to commercial available photocatalyst p25 and other as-synthesized sample of TiO2 nanosheets and Pt loaded TiO2 nanosheets. Nadeem et al., [130], fabricated Ag-Pd/ TiO2 nanostructures and explored the effectiveness of Schottky junction and SPR for the photocatalytic production of hydrogen. They concluded that the electron transfer from the conduction band of TiO2 to metal nanoparticles control the H2 production rate while SPR effect is not so effective. Apart from this they showed that mono metallic (Ag-TiO2) photocatalyst is more efficient but it suffers from the oxidation problem. On the other hand bimetallic (Ag-Pd/TiO2) is highly stable but exhibited lower photocatalytic H2 production efficiency. In this section we elaborated the various aspects of different type of plasmonic nanocomposite that have used for H2 production application. For the development of plasmonic nanocomposites various parameters need to be explored. As we mentioned above various parameters such as shapes, sizes and inter particles separation significantly influence the H2 production rate. Plasmonic nanocomposites can be proven a superior candidate for the future application in the field for energy production and environment applications.

4.5

Conclusions

The chapter highlights the importance and need of energy conservation through out the world for a sustainable human life from medicine to technology. Effective harvesting of solar energy, generation of hydrogen gas, detoxifying the environment using sustainable energy resource has been presented citing experimental reported work presented by different authors. Importance of nanotechnology in accessing the best way to convert energy to the useful work and mechanism involved is also discussed in brief. So, it can be concluded the nanotechnology is an integral part in the path of green chemistry and engineering for a sustainable eco system.

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Energy Harvesting: Role of Plasmonic Nanocomopsites 111 110. Mondal, C., Pal, J., Ganguly, M., Sinha, A.K., Jana, J., Pal, T., A one pot synthesis of Au–ZnO nanocomposites for plasmon-enhanced sunlight driven photocatalytic activity. New J. Chem., 38(7), 2999–3005, 2014. 111. Xiang, Q., Yu, J., Cheng, B., Ong, H.C., Preparation, Microwave‐ Hydrothermal., and Visible‐Light Photoactivity of Plasmonic Photocatalyst Ag‐TiO2 Nanocomposite Hollow Spheres. Chemistry–An Asian Journal, 5(6), 1466–1474, 2010. 112. Luo, B., Xu, D., Li, D., Wu, G., Wu, M., Shi, W., et al., Fabrication of a Ag/ Bi3TaO7 plasmonic photocatalyst with enhanced photocatalytic activity for degradation of tetracycline. ACS Appl. Mater. Interfaces, 7(31), 17061–17069, 2015. 113. Wu, J., Zhou, Y., Nie, W., Chen, P., One-step synthesis of Ag2S/Ag@ MoS2 nanocomposites for SERS and photocatalytic applications. J. Nanopart. Res., 20(1), 7, 2018. 114. Roy, P., Periasamy, A.P., Liang, C.T., Chang, H.T., Synthesis of grapheneZnO-Au nanocomposites for efficient photocatalytic reduction of nitrobenzene. Environ. Sci. Technol., 47(12), 6688–6695, 2013. 115. Juneja, S., Madhavan, A.A., Ghosal, A., Ghosh Moulick, R., Bhattacharya, J., Synthesis of graphenized Au/ZnO plasmonic nanocomposites for simultaneous sunlight mediated photo-catalysis and anti-microbial activity. J. Hazard. Mater., 347, 378–389, 2018. 116. Li, H., Bian, Z., Zhu, J., Huo, Y., Li, H., Lu, Y., Mesoporous Au/TiO2 nanocomposites with enhanced photocatalytic activity. J. Am. Chem. Soc., 129(15), 4538–4539, 2007. 117. Bian, Z., Tachikawa, T., Zhang, P., Fujitsuka, M., Majima, T., Au/TiO2 superstructure-based plasmonic photocatalysts exhibiting efficient charge separation and unprecedented activity. J. Am. Chem. Soc., 136(1), 458–465, 2014. 118. Ghasemi, S., Hashemian, S.J., Alamolhoda, A.A., Gocheva, I., Rahman Setayesh, S., Setayesh, S.R., Plasmon enhanced photocatalytic activity of Au@TiO2-graphene nanocomposite under visible light for degradation of pollutants. Mater. Res. Bull., 87, 40–47, 2017. 119. Barney, G.O., The Global 2000 Report to the President of the US: Entering the 21st Century: The Technical Report. Elsevier, 2013. 120. Abe, R., Recent progress on photocatalytic and photoelectrochemical water splitting under visible light irradiation. Journal of Photochemistry and Photobiology C Photochemistry Reviews, 11(4), 179–209, 2010. 121. Li, Y.Y., Wang, J.H., Luo, Z.J., Chen, K., Cheng, Z.Q., Ma, L., et al., PlasmonEnhanced Photoelectrochemical Current and Hydrogen Production of (MoS2-TiO2)/Au Hybrids. Sci. Rep., 7(1), 7178, 2017. 122. Wang, Y., Yu, J., Xiao, W., Li, Q., Microwave-assisted hydrothermal synthesis of graphene based Au–TiO2 photocatalysts for efficient visible-light hydrogen production. J. Mater. Chem. A, 2(11), 3847–3855, 2014.

112 Integrating Green Chemistry and Sustainable Engineering 123. Zeng, J., Song, T., Lv, M., Wang, T., Qin, J., Zeng, H., Plasmonic photocatalyst Au/gC3N4/NiFe2O4 nanocomposites for enhanced visible-light-driven photocatalytic hydrogen evolution. RSC Adv., 6(60), 54964–54975, 2016. 124. Samanta, S., Martha, S., Parida, K., Facile Synthesis of Au/g‐C3N4 Nanocomposites: An Inorganic/Organic Hybrid Plasmonic Photocatalyst with Enhanced Hydrogen Gas Evolution Under Visible‐Light Irradiation. ChemCatChem, 6(5), 1453–1462, 2014. 125. Chen, J.-J., Wu, J.C.S., Wu, P.C., Tsai, D.P., Plasmonic Photocatalyst for H2 Evolution in Photocatalytic Water Splitting. J. Phys. Chem. C, 115(1), 210– 216, 2011. 126. Liu, E., Kang, L., Yang, Y., Sun, T., Hu, X., Zhu, C., et al., Plasmonic Ag deposited TiO2 nano-sheet film for enhanced photocatalytic hydrogen production by water splitting. Nanotechnology, 25(16), 165401, 2014. 127. Khalid, N.R., Ahmed, E., Ahmad, M., Niaz, N.A., Ramzan, M., Shakil, M., et al., Microwave-assisted synthesis of Ag–TiO2/graphene composite for hydrogen production under visible light irradiation. Ceramics International, 42(16), 18257–18263, 2016. 128. Lou, Z., Fujitsuka, M., Majima, T., Two-dimensional Au-nanoprism/reduced graphene oxide/Pt-nanoframe as plasmonic photocatalysts with multiplasmon modes boosting hot electron transfer for hydrogen generation. J. Phys. Chem. Lett., 8(4), 844–849, 2017. 129. Yu, J., Qi, L., Jaroniec, M., Hydrogenproduction by photocatalytic water splitting over Pt/TiO2 nanosheets with exposed (001) facets. J. Phys. Chem. C, 114(30), 13118–13125, 2010. 130. Nadeem, M.A., Al-Oufi, M., Wahab, A.K., Anjum, D., Idriss, H., Hydrogen Production on Ag-Pd/TiO2 Bimetallic Catalysts: Is there a Combined Effect of Surface Plasmon Resonance with Schottky Mechanism on the PhotoCatalytic Activity? ChemistrySelect, 2(9), 2754–2762, 2017.

5 Catalytic Conversion of Biomass Derived Cellulose to 5-Hydromethyl Furfural Firdaus Parveen*, Kaiser Ahmad and Sreedevi Upadhyayula Department of Chemical Engineering, IIT, Delhi, India

Abstract In this chapter, different routes for the conversion of biomass derived cellulose to 5-hydroxymethyl furfural has been discussed. It discusses the different technological routes for biomass conversion to different fuels and their precursor. 5-hydroxymethyl furfural is an important precursor to fuel, commodity chemicals, bulk chemicals and polymers. This chapter focuses on the different catalytic routes for biomass derived cellulose conversion to 5-HMF. Cellulose conversion to 5-HMF is majorly divided into two reactions cellulose hydrolysis to glucose and glucose dehydration to 5-HMF. The detailed studies on cellulose hydrolysis to glucose and its dehydration are presented using variety of catalyst mineral acid, solid acid, ionic liquids and metal chlorides. The last section discusses the conclusion and future prospects. Keywords: Hydrolysis, microcrystalline cellulose, 5-hydroxymethyl furfural, glucose, fructose, ionic liquids, solid acids, mineral acid

5.1

General Overview

Human civilization has indeed achieved tremendous development in the areas of science and technology. However, the irony still remains that despite all the development, the issues like global warming and depletion of fossil fuels persist in the world. The primary reasons are growing population, development and modernization of the world that cause irrational utilization of fossil fuels, leading to global warming [1]. The fuel demand

*Corresponding author: [email protected] Shahid-ul-Islam (ed.) Integrating Green Chemistry and Sustainable Engineering, (113–164) © 2019 Scrivener Publishing LLC

113

114 Integrating Green Chemistry and Sustainable Engineering 200

20 18

Reserves R/P

17550 EJ

160

Reserves *10^3 (EJ)

16 14 12

120

109 9530 EJ

10 8

80

7060 EJ

6

56

53

40

4 2 0

Coal

Oil

Natural gas

0

Figure 5.1 Global reserves and reserves to production ratio.

in the last few decades has tremendously increased. The reserve to production (R/P) ratio for various fossil fuels such as coal, oil and natural gas is 109 years, 53 years and 56 years respectively as depicted in Figure 5.1. The predicted time for depletion of fossil fuels may be varied depending on two factors, (i) the demand for fuel and (ii) the new discoveries of alternative fuels can increase the time for fossil fuel depletion. Over the last 20 years, the R/P ratio for the coal decreased from 229 to 109 year, increased from 43 to 53 years for oil and remained constant for natural gas. The combustion of fossil fuels releases various gases like carbon monoxide, carbon dioxide, nitrogen oxides and sulfur dioxide that are responsible for global warming and deterioration of human health. The atmosphere traps these gases that don't allow solar radiation to escape from the Earth’s surface and increase the surface temperature resulting in global warming. Carbon dioxide concentration is continuously increasing at a very fast pace from 2 billion tonnes to 19 billion tonnes from 1900 to 1980 further it is increased to 31 billion tonnes till 2010 [2]. The countries with high energy consumption have higher CO2 emission rate such as China has highest energy consumption of 106.4 EJ/year, and emit CO2 at a rate of 7,997 Mtonnes (megatonnes), second highest energy consumption and CO2 emitting country is the United States 10.3 EJ/year, 5,637 Mtonnes respectively as shown in Figure 5.2. India consumes energy at a rate of 23.1 EJ/year and emits CO2 1,601 Mton/year [3]. To overcome the problems of fossil fuel depletion and global warming, alternate energy sources are required such as biomass, hydropower, solar, tidal and wind energy [4]. These alternate renewable energy resources

Catalytic Conversion of Biomass Derived Cellulose 115 31,502 538.7

Population (million)

6,863.2

Energy consumption (EJ/year) CO2 emission (Mton/year)

7,997 106.4 1,330.1

1,173.1 23.1 1,601

US CAN MEX GER FRA UK RUS BRA ARG CHN IND JPN BNG ARB AFR EGY ETP World

Country

Figure 5.2 Energy statistics for various countries.

correspond to the 13.3% nearly 73EJ of the total primary energy supply by the end of 2011. Biomass is the well known renewable energy constitute 79.1% among all the renewable energy resources as shown in Figure 5.3. Biomass is majorly used for the production of heat, chemicals, fuels, and electricity. The second largest share comes from hydropower which generates 17.8% of the all renewable energy resources to generate electricity, the rest of the 3% share of renewable energy comes from the geothermal, solar, wind and ocean to produce heat and electricity [3]. Biomass feedstock is used as an alternative to produce carbon neutral renewable fuels, chemically important intermediates and energy storing molecules. There are three classes of biomass feedstock that are appropriate for the production of renewable fuels: starchy feedstock, triglyceride feedstock, and lignocellulosic feedstock. Starchy feedstock consists of a polymer of glucose linked by α-glycosidic linkages, such as amylase and amylopectin which can easily hydrolyze into constituent sugar monomers, making them readily available for the formation of 1st generation bioethanol. Triglyceride feedstock which produce biodiesel, consist of fatty acids and glycerols derived from plant and animal sources. The fuel generated from triglyceride and starchy feedstock

116 Integrating Green Chemistry and Sustainable Engineering

0.02% 0.8% 0.8% 1.6%

Biomass Hydropower Wind Direct solar 17.8%

Geothermal 79.1% Ocean

Figure 5.3 Share of different renewable resources in primary energy supply.

constitute first generation biofuel, whereas, lignocellulosic feedstock based fuel is called second generation biofuel. Feedstock for the first generation biofuel is limited and competes with the food supply chain, whereas, lignocellulosic biomass (nonfood crop) is the most readily available class of biomass as it contributes to the structural integrity of plant, hence always present. As a result, second generation biofuel is considered as better fuel as compared to first generation biofuel as represented in Figure  5.4 [5]. Lignocellulose consists of lignin (15–20 %), hemicellulose (25–35 %) and cellulose (40–50 %), found in the cell wall of plants. The major lignocellulosic material comes from the agricultural residues, municipal waste, forest residues and paper waste [1].

Lignin Lignin is composed of sinapyl, coniferyl, and p-coumaryl alcohols having syringyl, guaiacyl, and p-hydroxyphenyl subunits respectively as shown in Figure 5.5. These subunits are linked through majorly with β-O-4 ether linkages that are responsible for the elongation of the polymer chain. The polymer chain branching is enhanced by the presence C-C and C-O linkages during higher degree of lignification. Lignin provides hydrophobicity, structural integrity to the plants and makes them resistant

Catalytic Conversion of Biomass Derived Cellulose 117 Disadvantage Depletion/declining of petroleum reserve Environmental pollution

Cr

Fuel

n io at er el n en fu ck or st g io sto il c B 1 ed e o Fe abl gar t u ge s ve

Pe tro F le ud ee um e dst pe o tro ck le um

Economics and ecological problems

2nd generation Biofuel Feedstock Non food abundant plant waste biomass

Petroleum

Advantage Environment friendly, Economic & social security Disadvantages Limited feedstock ( food Vs fuel) Blended partly with conventional fuel

1st generation biofuel

2nd generation Biofuel Advantage Not competing with food Advance technology still under development to reduce the cost of conversion Environment friendly

Figure 5.4 Comparison of conventional feedstock with 1st and 2nd generation biofuel.

towards any physical and bacterial attack [6]. Lignin can be modified either by destruction of β-O-4 ether bonds or by removal of lignin by chemical solvation [2].

Hemicellulose Hemicellulose is a polymer of hexoses (glucose, mannose, and galactose) and pentoses sugars (xylose and arabinose). The subunits found in the main chain of hemicellulose, galactoglucomannan are β-Dglucopyranosyl and (1→4)-linked β-D-mannopyranosyl units. In a branched hemicellulose the mannosyl subunits are replaced with the (1→6)-linked α-D-galactopyranosyl units and acetyl groups at C-2 and C-3 position of the galactoglucomannan chain as shown in Figure 5.6 [7]. The hemicellulose chain is bound to cellulose fibrils with non-covalent interactions providing structural integrity to the cellulose fibrils and keep them in place. The introduction of hydrophobic groups such as acetyl and methyl groups enhance the cohesion between the three subunits of lignocellulose. However, the amorphous nature of hemicellulose make it susceptible to depolymerization reactions [8].

Cellulose It is the largest component of lignocellulose, made up of linear chains of glucose units linked together by β-1, 4- glycosidic bonds with β- hydroxyl

118 Integrating Green Chemistry and Sustainable Engineering OMe

OMe OH

OH

O OMe Sinapyl alcohol

OMe Syringyl subunit

OH

OH

O OMe

OMe Guaiacyl subunit

Coniferiyl alcohol

OH O OH p-Coumaryl alcohol

p-Hydroxyphenyl subunit

Figure 5.5 Alcoholic components as building blocks of Lignin.

group at the anomeric carbon [9]. The linear chains together form β- flat sheets in contrast to a starch molecule wherein α- helical shaped polymer is formed because of the α-anomer. The linear flat sheets form the perfectly crystalline structure of cellulose [10]. The two sheets possess inter and intra molecular hydrogen bonding, each glucopyronysl unit have two intra molecular H-bonding within the linear chain and one intermolecular H-bonding between the two linear chains as depicted in Figure 5.7 [11]. The sheets are further stabilized by the presence of van der Waals interactions. The cellulose has the highest degree of polymerization with 10,000 units of glucose in a strand of cellulose [12]. Cellulose is water insoluble, although the monomer, glucose is water soluble, due to the presence of H- bonding, high molecular weight and van der Waals interactions. The perfectly crystalline structure of cellulose makes the it insolubile in water as well as most of the organic solvents avalaible.

Catalytic Conversion of Biomass Derived Cellulose 119 Monomers present in hemicellulose

OH

OH HO

O

HO HO

OH HO HO

OH

Glucose

OH O

OH

OH O

O

HO HO

HO

OH

HO

OH

OH OH Mannose

OH

O OH

OH

Galactose

Xylose

Arabinose

Galactoglucomannan chain OH O

HO O

OH

OH

OH O O OH

OH O

OH

O OAc

O

OH OH

OH O O OH

O OH

OH

O

OH O

O OAc

O

OH

OH O OH

OH

Figure 5.6 (a) Hexoses and pentoses found in Hemicellulose. (b) Galactoglucomannan, a branched Hemicellulose.

5.2

Biomass Conversion Processes

Biomass conversion can be carried out by a variety of processes because of diverse availability of feedstock. The main conversion routes for biomass conversion include thermochemical, biochemical and chemical methods as shown in Figure 5.8. Biomass has to undergo upgradation such as densification, size reduction, torrefaction and drying before undergoing the conversion processes because of its unusual properties. Thermochemical methods include gasification, pyrolysis, and combustion that lead to the production of chemical, heat, electricity, bio-oils and fuels. Thermochemical conversion doesn’t require any specific feedstock since every type of biomass can undergo combustion and gasification to produce heat and gasses. Thermochemical conversion processes are carried out at a high temperature (450–1200 °C). Combustion of biomass is a complete oxidation that produces carbon dioxide, water and heat. Gasification is a partial oxidation of biomass using oxidizing agents such as oxygen andwater in the form of steam and carbon monoxide to produce methane, carbon dioxide, and hydrogen that constitute syn-gas and utilized in the F-T process [13]. Pyrolysis is carried out in the absence of oxygen at various temperatures from 300–1000 °C to produce three major fractions namely solid, liquid and gasses in the form of char and bio-oil [14, 15]. Bio-oils are present in highly unstable oxygenated form leading

120 Integrating Green Chemistry and Sustainable Engineering OH

OH

HO

O O

O

OH

H O

OH

O

O

O HO

O HO

O H

O OH H

H O

O O HO

O

O

HO

HO

O

O H OH HO O

O

O OH

OH

H O

OH

HO

O OH

O O

OH

OH

OH

O

O

H O

OH

O H HO

O

O

HO

OH

H O

OH

HO

OH

OH HO

O

O HO

O H

O

O

OH

OH

H O

OH

HO

OH

HO

O

O

O HO

OH

OH

Figure 5.7 Structure of cellulose depicting H-bonding.

Biomass conversion technology Thermochemical

Gasification Syn gas Pyrolysis Char, Bio-oil, Gas

Biochemical

Digestion Bio-gas Fermentation Ethanol

Combustion Heat

Figure 5.8 Different biomass conversion routes.

Chemical

Esterification Esters Hydrolysis Sugars

Catalytic Conversion of Biomass Derived Cellulose 121 to the corrosion of turbines, engines, and boilers. Hence, upgradation of bio-oils is required for the safe use in industry [16]. Biochemical conversion processes use micro-organism for the decomposition of biomass to methane, bio-gases and ethanol through the two processes, digestion and fermentation [17]. The micro-organisms used for biochemical conversion are variety of yeast and bacteria. Most commonly used yeast and bacteria are Saccharomyces cerevisiae and Zymomonas mobilis respectively. Biochemical conversion is carried-out at lower temperature with lower reaction rate, the main advantage of biochemical conversion is that it is non-polluting and enzyme specific reaction with no side reactions [18]. Chemical conversion processes are mainly divided into esterification and hydrolysis of biomass that is composed of many chemical components such as carbohydrates, lignin, lipids, proteins and many others organics, and alkaloids. Esterification of triglycerides takes place in the presence of acid or alkali with alcohol to yield biodiesel and glycerol as the by-product. Biodiesel can be blended with the conventional fuels or can be directly used as fuel with some engine modification [19]. Hydrolysis of cellulose and hemicellulose under acidic or basic conditions results in the production of simple sugars that can be further upgraded to fuels or chemically important intermediates [20]. Hydrolysis of hemicellulose is much easier as compared to cellulose as hemicellulose is amorphous in nature and can be easily degraded, whereas cellulose is perfectly crystalline.

5.3

HMF as a Platform Chemical

Hydrolysis of cellulose to glucose which on further dehydration leads to the formation of 5-hydroxymethyl furfural (HMF). U.S. department of energy listed the HMF and their derivatives in the top ten list of platform chemicals [21] that can be converted to value added chemical and fuel components as shown in Figure 5.9. The first review on HMF was first reported by Newth et al., [22] and Moye et al., [23] on the industrial application and synthesis of HMF. HMF is a very important platform chemical that can be converted into fuel components like dimethyl furan (DMF), bulk chemicals formic acid (FA) and levulinic acid (LA), solvents 2, 5-dimethyl tetrahydrofuran, and monomer for polymer production and 2, 5-furandicarboxylic acid [24–27]. Among the fuels obtained from the hydrogenation of cellulosic biomass, HMF, 2, five dimethyl furan (DMF) is the new-fashioned liquid transportation biofuel. It is found to be a better fuel than bioethanol because of its higher energy density, 31.5 MJ/L, higher octane number (119), higher boiling

122 Integrating Green Chemistry and Sustainable Engineering

O HO 2-Hydroxymethyl furan (Platform chemical)

HO

O

O

O

2,5-furandicarboxylic acid (monomer for polymerproduction)

2-methylfuran (Biofuel)

HO

OH O

O

OH

OH

O

O 2,5-Bis(hydroxymethyl) furan (monomer for polymer production)

O

2,5-dimethyl furan (Biofuel)

5 hydroxymethyl furfural

O OH O

2,5-dimethyl tetrahydofuran (solvent)

O

O Levulinic acid (Bulk chemical)

OH

H

Formi acid (Bulk chemical)

Figure 5.9 HMF as a platform chemical.

point (92–94 °C) and its immiscible nature with water. Selective hydrogenation of HMF to DMF is an emerging field of Bioenergy [28, 29]. Lewoski [25] and Moreau studied [30] the synthesis, chemistry, and application of HMF in the pharmaceutical, dye, and fine chemical industry. The conversion of carbohydrates to chemicals, 2,5-dicarboxylic acid and HMF was studied by Woodley et al., [31]. Tong et al., reported the synthesis of HMF and the reaction of HMF to yield the chemically valuable product [32]. Recently, some reviews on the HMF synthesis from simple sugars utilizing IL as solvent and catalyst has been published[33–35]. Conversion of cellulose is a multistep reaction carried out by catalyst with multiple functionalities as shown in Figure 2.9. Initially, cellulose is hydrolysed to glucose, cellobiose or oligosaccharides in the presence of acidic catalyst. The glucose obtained can undergo dehydration to HMF via two pathways glucose isomerization to fructose under basic or Lewis acidic conditions

Catalytic Conversion of Biomass Derived Cellulose 123 and fructose dehydration to HMF losing three molecules of water in the presence of Bronsted acid as represented in Scheme 1 [36].

5.4

Hydrolysis of Cellulose to Glucose

Cellulose is one of the largest components of lignocellulose, varying between 35–50 wt% for different biomass feedstock. Cellulose is a straight chain polymer of glucose units linked by 1–4-β glycosidic bonds to form flat β-sheets [37]. Van der Waals interactions along with inter and intramolecular hydrogen bonding between the two cellulose fibrils stabilize the cellulose polymer and make the cellulose recalcitrant towards hydrolysis reaction. Hydrolysis of cellulose can be carried out using any acid, like mineral acid, solid acid and hetero polyacid and ionic liquids.

5.4.1 Hydrolysis of Cellulose to Glucose Using Liquid Acid Cellulose, in the presence of acids, is depolymerized to glucose and xylose reducing sugars, cellobiose or oligosaccharides. Mineral acids, for example sulphuric acid, hydrochloric acid, phosphoric acid, organic carboxylic acid can be used for the hydrolysis of cellulose as shown in Table 5.1. Cellulose can undergo hydrolysis with sulphuric acid and carbonic acid in the temperature range of 90–260 °C under atmospheric or high pressure [42, 44]. Hydrolysis with dilute acid condition requires high temperature and longer time of reaction, whereas, with concentrated acid, hydrolysis reaction requires shorter time and low temperature [45]. Hydrolysis of cellulose with concentrated acid increases the rate of reaction but decreases the selectivity to glucose due to further dehydration of glucose. Furthermore, Liu et al., concluded that dilute acid, atmospheric pressure, long reaction time favors the hydrolysis reaction with minimum dehydration product [40]. Lee et al., studied a kinetic model for different acid concentrations and reaction temperatures and concluded that at low temperature and low acid concentration the cellulose structure remain stable and glucose was found to be the main product. Whereas, at high temperature and high acid concentration, the acid enters the cellulose chains, disrupts the hydrogen bonding abruptly leading to dehydrating products [46]. Shonnard et al., developed a kinetic model and studied kinetic parameters for the hydrolysis of timber and switchgrass using dilute sulphuric acid. The hydrolysis reaction strongly depends on the acid concentration and reaction temperature and was found to be particle size independent. Activation energies and the pre-exponential factors for the hydrolysis of timber and switch

Catalyst

H2SO4 (0.5 wt %)

H2SO4 (0.17 wt %)

HCl (5 wt %)

H3PO4 (10 wt %)

CO2 + H2O (100% saturation)

SO2 impregnation

Authors/Year

Parajo et al.,/2002

Emmel et al.,/2003

Liu et al.,/2006

Lehnihan et al.,/2010

Rogalinski et al.,/2008

De Bari et al.,/2007 Aspen chips

Cellulose

Potato peel

Walnut shells

Wood chips

Corn cobs

Feedstock

Table 5.1 Hydrolysis of cellulose using mineral acid.

205 °C , 3 min 16.2 bar(H2O)

135 °C,~2 min 200–250 bar CO2

135 °C, 8 min

250 °C , 430 min 15–86 bar (H2O)

210 °C, 2 min

125 °C, 165 min

Reaction conditions

Glucose Xylose

Glucose

Reducing sugar

Levulinic acid

Xylose

Xylose

Typical products

37.0 10.3

11.0

82.5

12.0

47.0–54.0

25.0

Yield (%)

[43]

[42]

[41]

[40]

[39]

[38]

Ref

124 Integrating Green Chemistry and Sustainable Engineering

Catalytic Conversion of Biomass Derived Cellulose 125 Hydrolysis

Isomerization CH2OH O

OH OH

OH

OH O

HO

O O HO

Cellulose

OH

O HO

O n

HO

Dehydration OH

O O

CH2OH

OH

Glucose

OH OH

OH

HO

Fructose

HMF

Scheme 5.1 Step wise schematics for Cellulose conversion to HMF.

grass to xylose were reported to be in the range of 49–180 kJ/mol and from 7.5 *104 to 2.6*1020 min−1 respectively. It can be concluded that hydrolysis of a mixture of species doesn’t affect hydrolysis higher yields of monomers can be achieved [47]. Heeres et al., developed a model for cellulose conversion to levulinic acid at a temperature range of 150–200 °C and reported the maximum yield of 76 mol% levulinic acid using continuous back mixing reactor [48]. Parajo et al., explained the concept of auto and post hydrolysis that was found to increase the overall yield of hydrolysis reaction by 25% [38]. Bari et al., also found that SO2 can reduce the degree of polymerization up to 50% and moreover, it is less corrosive than the sulphuric acid [43]. Besides temperature, pressure, and catalyst loading the type of acid used also affects the hydrolysis reaction. The product distribution depends on the type of the acid used. Hydrolysis with HCl, results in a further degraded product such as levulinic acid (LA) and formic acid (FA) as reported by Shen et al., [49] Lin et al., reported the degradation of cotton cellulose in water-ethanol using hydrochloric acid as the catalyst and showed that ethanol has a profound effect on the degradation of cellulose [50]. Phosphoric acid also shows excellent catalytic activities towards hydrolysis reaction. Phosphoric acid, being less active, hydrolyzes the biomass to fermentable sugars with minimum dehydrated products such as LA and FA as reported by Vazquez et al., [51] Later Walker et al., reported hydrolysis of potato peel using dilute phosphoric acid in a high-pressure reactor to produce 82.5% sugar yield at an optimum temperature of 135 °C [41]. Later, Ramos et al., studied the effect of pre-treatment at various conditions (200–210 °C, 2–5 min) with 0.087% and 0.175% (w/w) H2SO4. The maximum yield of hydrolytic products 47–54 % was achieved at 0.175% H2SO4 (210 °C, 2 min) which is 70% of the corresponding xylose theoretical yield. The longer residence time showed negative effect on hydrolytic products and high temperature and pressure conditions are required for the pretreatment of higher lignin content of the biomass [39]. Pretreatment with SO2 was found to be more desirable than the sulphuric acid as even better glucose yields were obtained. Cellulose can be hydrolysed

126 Integrating Green Chemistry and Sustainable Engineering using water without any acid under subcritical conditions as reported by Asghari et al. The decomposition of cellulose under subcritical conditions results in the generation of soluble oligomers and monomers such as cellohexaose, cellopentaose, cellotetraose, cellotriose, cellobiose and glucose by destruction of H-bonding and these oligomers do not require any acid for their hydrolysis [52].

5.4.2 Hydrolysis of Cellulose to Glucose Using Solid Acid Solid acid catalysts have some advantages over liquid acid-catalyzed hydrolysis of cellulose such as ease of separation of the catalyst after the reaction, reusability of the catalyst and minimization of the reactor corrosion [53–55]. Various solid acid catalysts have been studied for the hydrolysis of biomass such as sulphonic acid functionalized carbonaceous and siliceous catalyst, Bronsted acidic zeolites, metal oxides and hetero polyacids as listed in Table  5.2 [65, 66]. Bekkum et al., reported the hydrolysis of polysaccharide using ion exchange resin and zeolite such as H-modernite, H-beta, and MCM-41 with good selectivity to glucose. It was confirmed by their investigations that the active species from the catalyst comes into the solution phase and actual catalysis takes place under homogenous condition [65]. Fukuoka reported the hydrolysis of starch and sucrose using water tolerant silica modified with sulphonic acid groups. This showed better catalytic activity in terms of conversion and turnover frequency as compared to conventional resin Amberlyst −15 and Nafion [66]. Domen et al., reported water tolerant hetero polyacid for various reactions such as Friedel-crafts reaction, acetalization, hydrolysis, and esterification reactions. Heteropoly acids showed better catalytic activity than zeolites and ion exchange resins. This was attributed to their high hydrothermal stability of the catalyst and the presence of strong Brønsted acid sites which enhanced the catalytic properties [56, 60, 67]. Onda et al., reported the efficient conversion of cellulose to glucose with high selectivity employing H-zeolites. The catalyst with high Si/Al ratio increases the hydrophobicity and reduces catalyst deactivation. Sulfated zirconia showed even better catalytic activity than zeolites, but sulfated groups leached out after the reaction and led to numerous by-products [58]. Hara et al., synthesized carbonaceous catalyst with multiple functional group modification such as SO3H, COOH, and OH groups as an efficient and reusable catalyst for the conversion of cellulose to glucose. The catalyst showed good catalytic activity apart from its small surface area and less effective acid sites. The good catalytic performance of the carbonaceous catalyst depends on its good adsorption capability of β−1,4 glucan, which was found to be less in

150 °C, 24 hr 100 °C, 3 hr 180 °C, 2 hr

AC–SO3H

AC–SO3H + COOH + OH

H3PW12O40

Sulfonated silica/carbon nanocomposites

AC–SO3H (Sulfonation T = 250C)

BC–SO3H

Nafion/silica

Onda et al.,/2008

Yamaguchi et al.,/2009

Tian et al.,/2010

Van de Vyver et al.,/2010

Pang et al.,/2010

Wu et al.,/2010

Hegner et al.,/2010

463 °C, 24 hr

MW (350 W), 100 °C, 1 hr

150 °C, 24 hr

150 °C, 24 hr

100 °C, 3 hr

AC–SO3H + COOH + OH

Suganuma et al.,/2008

Glucose Levulinic acid

Glucose

Glucose

Glucose

Glucose LA HMF

Glucose β−1,4-glucan

Glucose

Glucose β−1,4-glucan

Glucose

130 °C, 12 hr

HNbMoO6

Takagaki et al.,/2008

Typical products

Reaction conditions

Catalyst

Authors/ Year

Table 5.2 Cellulose hydrolysis using solid acids.

9.0 2.0

19.8

61.0

50

[64]

[63]

[62]

[61]

[60]

[59]

~8.1b 0.93 50.5 0.14 0.6

[58]

[57]

4.0a 64.0 40.5

[56]

Ref

21

Yield (%)

Catalytic Conversion of Biomass Derived Cellulose 127

128 Integrating Green Chemistry and Sustainable Engineering the case of other solid acids [57]. The hydrolysis occurs with the splitting of a water molecule into H+ (hydronium ion) and –OH (hydroxide ion) known as deprotonation. Shimizu et al., explored that catalyst that reduces the deprotonation enthalpy of a solid acid catalyst will show better catalytic activity. It was reported that higher Bronsted acidity favors the breaking of β-1,4 glycosidic bonds present in cellulose [68]. Yamaguchi et al., showed that the concentration of water controls the hydrolysis reaction. The amount of water comparable to the weight of solid catalyst produces maximum glucose yield. Kinetics and equilibrium suggested that an amount of water less than the optimum produces β−1,4 glucan, whenever decreases the hydrolysis of β−1,4 glucan to glucose [59]. Sels et al., synthesized novel silica/carbon nanocomposites for the hydrolysis of cellulose to glucose in high yield. The high yield of glucose was attributed to Bronsted acidic sites and interpenetrating structure of hybrid catalyst containing carbon and silica that enhances the adsorption of β−1,4 glucan [61]. Zhang et al., studied the degradation of cellulose using water and sulfonated amorphous carbon. The sulphonation temperature affects the catalyst behavior. It was found that at 250 °C, sulfonated catalyst showed better catalytic activity as it has a high acid density with a minimum reduction in the specific surface area of the catalyst [62]. Biomass can itself undergo carbonization to form activated charcoal and bio-char. Yin et al., used bio-char acid as a catalyst for the cellulose hydrolysis which showed better turn-over numbers (1.33–1.73) as compared to the conventionally used dilute sulphuric acid solution (0.02). The higher turn-over numbers were attributed to the affinity of β−1,4 glycosidic bonds on the surface of the catalyst and also on the microwave reaction condition that increases the collision frequency enhancing the breaking of glycosidic bonds [63]. Hegner et al., employed Nafion SAC 13 and FeCl3/silica catalyst for cellulose hydrolysis and its conversion to levulinic acid at an optimum temperature of 190 °C. Nafion showed good reusability during the reaction and can be used in continuous flow reactors for the synthesis of biofuel precursors [64]. Recently, Hu et al., developed novel magnetic carbonaceous catalyst with chloro and sulphonic acid groups for cellulose hydrolysis to glucose [69].

5.4.3 Hydrolysis of Cellulose to Glucose Using Ionic Liquids Cellulose is a major component of lignocellulose and is made up of linear chains of glucose units linked via β- 1, four glycosidic bonds [70]. Inter and intramolecular hydrogen bonding between the two chains of cellulose fibrils is the bottleneck for cost effective hydrolysis of cellulose [71, 72]. Hence, harsh reaction conditions, such as concentrated acid at low

Catalytic Conversion of Biomass Derived Cellulose 129

O

R1

R N

R3

F3C

P R3

Cations

N

Anions

CI

R1

R3

PF6

N R2

O O HO

N R2

s O

BF4

N

R3

O O H3C

O

R2

R2

R1

s

s

O

O MC Ix M= Tra me nsiti tal on

R1

Scheme 5.2 Cations and anions for ionic liquid.

temperatures or dilute acid at high temperatures, are required for cellulose hydrolysis. The major drawbacks of these routes are reactor corrosion, large quantities of neutralization wastes such as gypsum and low yields of sugars [73]. The hydrolysis of cellulose using solid acid catalysts and enzymes also has disadvantages such as slow reaction rate requiring high reaction temperatures, and the high cost of the enzymes [1]. Therefore, there is an immediate requirement for more efficient and greener catalytic system for the hydrolysis of cellulose. Ionic liquids were found to be effective solvents as well as a catalysts for the biomass conversion to sugars, fuels, and chemicals [74].

5.4.3.1

Ionic Liquids

ILs are new class of compounds having both organic and inorganic cations and anions bound together to have a melting point below 100 °C. Ionic liquids with a melting point above 100 °C have also been reported as room temperature ILs [75]. The common cations and an anion comprising IL are shown in Scheme 2. ILs showed wide application in, electrochemical applications, bio-catalysis, chromatography and chemical catalysis as good solvents as well as catalysts. Industrially important catalytic reactions such as Friedel-crafts alkylation, biomass pre-treatment and conversion to chemically important reactions, and organic reactions are possible using ILs, because of their good physicochemical properties such as high thermal stability, high electrical conductivity, large electrochemical window and good solubilizing power [76–82]. Biomass conversion to fuel is a multistep reaction catalyzed by both acidic and basic functionalities of the ILs. ILs can have Bronsted acidity in

130 Integrating Green Chemistry and Sustainable Engineering terms of ionizable proton, and Lewis acidity in terms of electron deficiency or both the acidities. 5.4.3.1.1 Ionic Liquids with Bronsted Acidity ILs are classified as Bronsted acidic based on Bronsted Lowry theory given by Johannes Nicolaus Bronsted and Thomas Martin Lowry in 1923. According to this theory Bronsted acidic IL donate a proton (H+) attached to N or O atom in the molecule of the IL. The acidic proton can be present on cation, anion, both on cation and anion, functional groups in the cation or functional groups both on cation and anion as reported in Figure 10 [83]. ILs with proton present on the cation, are synthesized by reaction of a Bronsted acid and Bronsted base in a stoichiometric amount [84]. Gabriel in 1888 discovered ethanol ammonium nitrate as the first protic acidic IL [85]. There are chances of in complete proton transfer, aggregation, and association of the ions. This can be measured by evaluating the ionic character or ionicity of the IL [86–89]. The ionicity of the IL can be measured by Walden plots. Walden plots compare the fluidity with the molar conductivity [90–95]. The majority of the ILs showed less conductivity as per prediction of viscosity that can be explained by the aggregation of protic ionic molecules or non-Newtonian fluid having hydrogen bonded networks [96] (Figure 5.10). IL with an acidic proton in the anion of the IL is prepared by polybasic acid; examples are dialkylimidalium hydrogen sulphate [97–99] and dialkylimidalium dihydrogen phosphate [97]. Watanabe et al., showed the use of diethyl-methyl ammonium hydrogensulfate as fuel cell electrolyte having protons both on the cation and anion [100]. Davis and Forbes [101] reported the acidic IL–SO3H having acidic proton in the cationic moiety. Later on Yoshizawa et al., reported the preparation of in two consecutive steps first synthesis of zwitterion then its subsequent neutralization [102]. The –SO3H group attach to the imidazolium core of the IL with ring or carbon chain. These types of IL can be synthesized by reaction of imidazole with sultone to afford zwitter ion followed by neutralization with the mineral acid. Similarly, ILs having multiple acidic protons either on cation or anion can be synthesized [103]. 5.4.3.1.2 Ionic Liquids with Lewis Acidity Ionic liquid with Lewis acidity is electron deficienct either in cation or anion as shown in Figure  5.11. IL with Lewis acidity in the anion can be synthesized by the reaction of metal halide with the IL under anhydrous conditions. Hurley and Wier reported the synthesis of ethyl pyridinium chloroaluminate for the aluminium electrodeposition [104]. Later

Catalytic Conversion of Biomass Derived Cellulose 131

H

N

N

N

H

N

Cl

X

N

H

–

N Br

H

X – –

X=Cl , Br , I , BF4–, CF3SO3–

X=Br–,NTf2–

Protic acidic ionic liquid with acidic hydrogenon cation

N OH HOOC

N

R1

COO

N

N

R1

R2

HSO4

N

H2PO4

N R2

OH COO HOOC

Protic acidic ionic liquid with acidic hydrogen on anion

N N

H

H H3C

HSO4

HSO4

N

N

H

HSO4 Protic ionic liquid with acidic hydrogen on cation and anion SO3H

N

N

SO3H

N

N

n

HSO4

HSO4

N

SO3H

N

N

n HSO4

COOH n

HSO4

Bronsted acidic ionic liquids with acidic hydrogens on functional groups as on cation/anion.

Figure 5.10 Ionic liquids with Bronsted acidity.

Osteryoung et al., reported these salts as ILs for electrochemical applications [105]. Wilkes et al., synthesized dialkyl imidazolium chloroaluminate IL for electrochemistry and spectroscopy [106]. The anionic species can be more than one depending on the molar ratio of aluminium chloride to the IL. In the case of aluminium chloride, AlCl4–, Al2Cl7–, and Al3Cl10– are the various anionic species formed. Chlorometallate ILs are sensitive to moisture but have electrical conductivities and low viscosities. A number

132 Integrating Green Chemistry and Sustainable Engineering

R1

N

N

N

N

R2

R1

AlCl4–

FeCl4–

N

R

AlCl4–

R1

R1 R2

N

R2

R2

R4

N

N

R4 –

–

FeCl4

AlCl4

R –

FeCl4

R3

R3

Lewisacidityinanion

R

N

N

B(OH)2

Br– Lewisacidityincation

Figure 5.11 Ionic liquids with Lewis acidity.

of papers have been published on the study of acidity [107, 108], electrodeposition [109], conductivity [110, 111], electrochemical reduction [112], density, and viscosity [113, 114] of IL. ILs with zinc, iron and tin chloride were found to be moisture resistant, but have higher melting point than the chloroaluminate. Abott et al., reported quaternary ammonium based IL with Zn and Sn as anionic species for the electrochemical applications [115–117]. Zinc chloride containing choline based ILs are moisture, and air insensitive found various application such as zinc alloy deposition [118], catalysts in organic reactions and battery electrolytes [115, 119]. FAB mass spectra, Raman spectra, and NMR spectroscopic techniques have been widely used for the detection of various hallometallate species found in the metal containing IL [120]. The species formed in case zinc and sin chlorides are ZnCl3– Zn2Cl5–, Zn3Cl7– and SnCl3ˉ, Sn2Cl5ˉ [121]. Santini et al., confirmed the presence of ZnCl3– at a mole ratio of ZnCl2 of 0.5, whereas, at a molar ratio greater than 0.5, a mixture of anionic species is found [122]. ILs with iron chloride showed the various application in glycosidation [123], Michael addition [124], conversions of carbon dioxide to cyclic carbonates [125], benzylation [126], desulphurizations [127], rechargeable batteries [128] and glycolysis [129]. Yoshido et al., studied the effect of structural parameters such as effect of carbon chain and halides of anion on the physical parameters such as thermal stability, density,

Catalytic Conversion of Biomass Derived Cellulose 133

CH3 SO3H

N

N

N

H

FeCl4

H AlCl4

ZnCl3

R

N

N

[(1/2.Zn)SO4)]

R

N

N

[(1/2.Fe)SO4)] SO3H

SO3H

R

N

N

ZnCl3 SO3H

Figure 5.12 Ionic liquids with Bronsted-Lewis acidity.

viscosity, magnetic properties, IR, UV-Vis spectra and ionic conductivity. The longer chain length and replacement of chloro groups with bromo in IL reduce the fluidity and ionic conductivity [129]. 5.4.3.1.3 Ionic Liquids with Bronsted and Lewis Acidity IL with both Bronsted and Lewis acid functionalities are commonly known as Dual functionalized IL as reported in Figure 5.12. These types of ILs are synthesized by addition of metal chloride or oxides in the desired ratio to the IL. They found to be a promising catalyst for the catalytic reactions occurring in multistep, each step catalyzed by different functionalities. Commonly found dual-functionalized ILs are [C1HPyrr][ZnCl3],[130] [(HSO3)3C3C1im][(0.5Zn)SO4],[131][(HSO3)4C4C1im][(0.5Fe)SO4] [132], [(HSO3)3C3C1im][ZnCl3] [133] and [ (C2)3N][AlCl4] [134]. The Lewis acidity of these types of IL can be determined by spectroscopic techniques. Li et al., employed acetonitrile and pyridine as a probe for FTIR spectroscopic Lewis acidity determination [135].

5.4.3.2 Cellulose Hydrolysis in Ionic Liquids Using Mineral Acid as Catalyst Inter and intra molecular hydrogen bonding in cellulose make it recalcitrant towards hydrolysis [37]. IL showed good solubility of cellulose among all

134 Integrating Green Chemistry and Sustainable Engineering available organic and inorganic solvents [136]. The transformation of cellulose to glucose using various mineral acid in IL has been depicted in Table 5.3. Butyl methyl imidazolium chloride [BMIM]Cl found to be the promising solvent for the dissolution of cellulose [72]. Hydrolysis of cellulose in IL as solvent using sulphuric acid is much better than the without solvent where hydrolysis occurs at the surface of the cellulose. Whereas, BMIMCl form a homogenous solution of IL and cellulose by breaking the hydrogen bonding and making it more accessible towards proton from an acid to break β−1,4 glycosidic bonds between the glucose units [142]. Li et al., studied the various mineral acid such as H2SO4, H3PO4, HNO3 and HCl as a catalyst using BMIMCl as a solvent for the cellulose degradation. Among all acid H2SO4, showed the highest activity due to its highest acidity [137]. Apart from the role of solvent BMIMCl also enhance the acidity of the catalyst and thus kinetic of the acid catalyzed breaking of β−1,4 glycosidic bonds in cellulose to yield highest 73% total reducing sugar and 39% glucose at 100 °C [142]. Binder et al., used HCl instead of H2SO4 in EMIMCl as a solvent for cellulose hydrolysis to 90% glucose yield at 105 °C. He also studied the effect of addition of water during the hydrolysis of cellulose, adding water gradually increased the yield of glucose [138]. He has also used IL with acetate, nitrate, triflate, bromide, chloride and tetrafluoro borate anion as a solvent for cellulose hydrolysis. The results obtained confirmed that cellulose dissolution is favored by chloride containing ILs. Morales et al., employed various acids such as H2SO4, H3PO4, HCl, acetic acid, trifluoroacetic acid, p-toluene sulphonic acid and trifluoromethane sulfonic acid as catalyst and BMIMCl as solvent for cellulose hydrolysis [139]. They found that hydrolytic reaction is a function of acidity of the catalyst. Acids with the lowest pKa values being more acidic will be highly active towards hydrolysis reaction and vice versa. The maximum glucose selectivity about 89%was achieved with HCl as catalyst and BMIMCl as a solvent. Whereas, [BMIM(OAc)] as a solvent instead of BMIMCl showed cellulose hydrolysis up to 10% only. This can be explained based on the reaction, resulting in the production of less acidic acetic acid that is not able to hydrolysis cellulose [143]. Later on he has studied the cellulose hydrolysis in two step first dissolution of cellulose then its regeneration after adding the antisolvent. The regenerated cellulose then hydrolysed using phosphotungstic acid as catalyst to yield 87% glucose yield at 140 °C. These results showed that ILs apart from functioning as solvent also increases the acidity of the catalyst which enhance the hydrolysis of the cellulose [142]. Zhu et al., also reported two-step method for cellulose hydrolysis to glucose in first step cellulose hydrolysed to oligomers using phosphoric acid at 50 °C then oligomers separated out using ethanol as

Dissolution 135 °C Hydrolysis 103 °C 100 °C(MW) 15 min

[EMIM]Ac/HCl

[TMPA][NTf2]+HCl + LiCl

Phosphoric acid/ [BMIM]Cl

Morales et al.,/2012

Kamimura et.al/ 2012

Zhu et al.,/2013

50 °C

Glucose HMF

105 °C, 2–4 hr

HCl (20 wt %) / + [EMIM]Cl

Binder et al.,/2010

Glucose TRS

Glucose

Cellobiose

Glucose TRS

175 °C, 35 min

[BMIM]Cl/ H2SO4

Li et.al.,/2007

Typical products

Reaction conditions

Catalyst

Authors/ Year

Table 5.3 Cellulose hydrolysis using IL as solvent and mineral acid as catalyst.

57 87

51

10

90.0 5–7.0

39 66

Yield (%)

[141]

[140]

[139]

[138]

[137]

Ref

Catalytic Conversion of Biomass Derived Cellulose 135

136 Integrating Green Chemistry and Sustainable Engineering solvent. In the second step oligomers hydrolysed to TRS 87% and glucose 57% yield respectively. The two step method for cellulose hydrolysis showed better effect as degree of polymerization of cellulose decreased in the first step [144]. Recently Kamimura et al., investigated the combined effect of LiCl and HCl in a hydrophobic IL trimethylpropylammonium bis-(trifluoromethanesulfonyl) amide ([TMPA]NTf2) on the hydrolysis of cellulose under microwave conditions to yield maximum of 51% glucose yield. The glucose produced is separated from the reaction mixture using acetone-isopropanol mixture [140].

5.4.3.3 Cellulose Hydrolysis in Ionic Liquids Using Metal Salts as Catalyst Metal chlorides in ionic liquid catalyze the hydrolysis of cellulose faster than the sulphuric acid under similar reaction conditions by breaking the hydrogen bonding. [141] Table 5.4 listed the metal effect on cellulose hydrolysis. Su et al., reported the effect of CuCl2 paired with another metal chloride (CrCl3, FeCl3, CrCl2, or PdCl2) in trace amount on the hydrolysis of cellulose [141]. Hydrolysis using paired metal chlorides results in better glucose yield as compared to the single catalyst. CuCl2 coupled with PdCl2 results in total product and glucose yield of 73% and 42% respectively at a catalyst loading of 37μmol/g [EMIM]Cl with a mole fraction of Cu at 0.83. The mechanistic studies revealed that Cu2+ helps in the generation of a proton from the hydrolysis of water, and Pd2+ catalyzes the depolymerisation by interacting with the catalytic protons. Hence, Cu2+ and Pd2+ showed synergistic effect in breaking of H-bonding and increasing the metal mobility in an IL solvent to enhance the cellulose hydrolysis which was further proved by differential scanning calorimetry (DSC) and electron paramagnetic resonance (EPR) techniques. DSC and EPR measurements show that the addition of paired metal chlorides to ionic liquid containing cellulose decreases the onset temperature from 76 °C to 64 °C indicating the disruption of H-bonding, and broadening in the spectra of the EPR signals with addition of paired metal chlorides shows increase mobility of the metal ions in IL. Amarasekara et al., investigated the effect of metal ions Cr3+, Mn2+, Fe3+, Co2+, Ni2+, Cu2+, Zn2+ and La3+ in their chlorides form as a co-catalyst in the saccharification reaction in a temperature range of 140–170 °C in the presence of an acidic IL 1-(1- propylsulfonic)−3-methylimidazolium chloride [145]. Metal chlorides showed enhancement in the catalytic saccharification producing higher total reducing sugar (TRS) yield. Mn2+ produced highest TRS yield 90.2% at 160 °C. Mn2+, Fe2+, Co2+ and La2+ showed

160 °C, 3 hr

CuCl2+PdCl2 /[BMIM]Cl

Mn2+/1-(1- propylsulfonic)−3methylimidazolium chloride

Quaternary ammonium perrhenates /AMIMCl

Methyl trioxorhenium (MTO) /1-allyl-3-methyl imidazolium chloride

Su et al.,/2011

Amarasekara et al.,/ 2015

Wang et al.,/2015

Yuan et al.,/ 2016

TRS

Glucose TRS

Typical products

150 °C, 30 min Glucose MW TRS

150 °C, 30 min Glucose MW TRS

80–100 °C, 30 min

Catalyst

Authors/ Year

Reaction conditions

Table 5.4 Cellulose hydrolysis using IL as solvent and metal salt as catalyst.

24 51

42.0 92.0

90.2

42.0 73.0

Yield (%)

[147]

[146]

[145]

[141]

Ref

Catalytic Conversion of Biomass Derived Cellulose 137

138 Integrating Green Chemistry and Sustainable Engineering maximum enhancement in the TRS yield whereas Cr3+ showed no effect. Yuan et al., employed methyl trioxorhenium (MTO) as a catalyst and 1-allyl-3-methyl imidazolium chloride as a solvent for cellulose hydrolysis to yield TRS and glucose in 51% and 24% yield respectively under microwave irradiation at 150 °C. The reaction is catalyzed by the nucleophilic attack of oxygen of β−1, four glucan on the electron deficient Re atom of the catalyst to disrupt the β−1,4 glycosidic bond [147]. Later, Wang et al., studied conversion of cellulose to TRS and glucose with 92% and 42% yield respectively using quaternary ammonium perrhenates as catalyst in AMIMCl as a solvent. They suggested that hydrolysis reaction is catalysed by the formation of H-bonding between the ReO4 and hydroxyl group of the cellulose which allows the interaction of water molecule with the β−1, four glycosidic bonds. The catalytic activity of perrhenates is decreased with the increase in the alkyl chain length in quaternary ammonium perrhenates as bulky alkyl chains resist the perrhenates to form H-bonding. Whereas, addition of acidic moieties such as COOH and SO3H promote the catalytic activity of perrhenates [146].

5.4.3.4 Cellulose Hydrolysis in Ionic Liquids Using Heterogenous Catalyst Solid acid catalysts show tremendous application in the hydrolysis of cellulose, also solve the problem of separation and reusability associated with the homogenous catalyst. Hydrolysis of cellulose has been conducted using various solid acid such as zeolite, magnetic solid acid, polymeric resin and carbonaceous catalyst as listed in Table 5 [148–150]. Cai et al., investigated the effect of H-type zeolites such as HY, H-beta, HZSM-5 and SAPO-34 on the hydrolysis of cellulose using [BMIM]Cl as solvent. Among all catalyst used HY showed highest catalytic activity due to its large pore diameter which allows the large kinetic diameter of the reactant and product molecules to pass without diffusional constraints. The pore diameter was further enhanced by the dilation due to the [BMIM]Cl confirmed with the XRD studies. HY release the proton by an ion exchange mechanism with the [BMIM]Cl which is confirmed by FTIR spectroscopy and elemental analysis. IL cation BMIM+ enters into the channel of HY and gets attached to it releasing the Bronsted acidic proton which catalyzes the hydrolysis of the β−1,4 glycosidic bond by the gradual addition of water to yield 50% of glucose [151]. Rinaldi et al., employed polymeric resin with acidic –SO3H sites, Amberlyst-15 (styrene-divinylbenzene based polymer) with high surface area for the cellulose hydrolysis in IL. Cellulose hydrolysed to oligomers with 90% yield in 1.5 hr, which further degrades to reducing sugars

110 °C 240 min 150 °C, 3 hr

HY /[BMIM]Cl

Sulfonated carbonized material/ [BMIM]Cl

CaFe2O4/[AMIM]Cl

SPS-DVB/[BMIM]Br

NSAC/[BMIM]Cl

Superparamagnetic carbonaceous catalyst/[BMIM]Cl

Fe3O4@SiO2–SO3H/[BMIM]Cl

MCF-SO3H/[BMIM]Cl

Fe3O4@C /[BMIM]Cl

Cai et al.,/2012

Guo et al.,/2012

Zhang et al.,/2012

Fan et al.,/2013

Liu et al.,/2013

Guo et al.,/2013

Xiong et al.,/2014

Xu et al.,/2015

Liu et al.,/2016

110 °C, 3 hr

110 °C, 1 hr

130 °C, 10 hr

130 °C, 3 hr

120 °C, 4 hr

120 °C, 10 hr

130 °C, 2 hr

100 °C, 1.5 hr

Amberlyst-15/[BMIM]Cl

Rinaldi et al.,/2008

Reaction conditions

Catalyst

Authors/ Year

Table 5.5 Cellulose hydrolysis using IL as a solvent and solid acid as catalyst.

TRS

Glucose

TRS

TRS

Glucose TRS

Glucose TRS

Glucose

TRS

Glucose

Oligomers

Typical products

72

88

73

69

59 80

48 60

37

72

50

90

Yield (%)

[161]

[160]

[159]

[157]

[156]

[154]

[158]

[155]

[151]

[152]

Ref

Catalytic Conversion of Biomass Derived Cellulose 139

140 Integrating Green Chemistry and Sustainable Engineering and glucose by increasing the time of reaction [152]. Later, in his studies he figured out that cellulose chain length has an effect on the hydrolysis reaction and product distribution. Cellulose with long chain fibril preferred to produce oligomers rather than reducing sugars and glucose. The reaction is catalyzed by the acidic proton released after ion exchange between [BMIM]Cl and Amberlyst-15 [153]. Fan et al., synthesized sulfonated poly styrene-co-divinylbenzene (SPS-DVB) which catalyzes the hydrolysis of cellulose in [BMIM]Br yielding 60% TRS and 48% glucose at 120 °C and 10 hr reaction time. After the reaction, the catalyst looses its activity, if not regenerated after every run, due to ion exchange between Aberlyst-15 and [BMIM]Br [154]. Guo et al., synthesized sulfonated carbonaceous catalyst by incomplete carbonization of glucose, and then its sulfonation with sulphuric acid for the hydrolysis of cellulose to yield the maximum TRS 72% yield at 110 °C in [BMIM]Cl. Carbonaceous catalyst found to have –SO3H, -COOH, and -OH functional groups as confirmed from the FTIR spectroscopic data. These functional groups show a synergistic effect in the hydrolysis reaction as -COOH and -OH groups help in the adsorption of the cellulose through the hydroxyl group of the cellulose through H-bonding while –SO3H group catalyses the breaking of the glycosidic bond. The addition of water amount also affects the hydrolysis reaction, water less than 2% favors the hydrolysis. Amount of water whereas greater than 2% restrains the hydrolysis reaction by decreasing the solubility of cellulose in [BMIM]Cl [155]. Liu et al., investigated the effect of glucose, sucrose and nut shell activated carbon (NSAC) derived sulfonated catalyst in the hydrolysis of cellulose. The higher concentration of –SO3H in the catalyst derived from glucose and sucrose leads to better catalytic activity as compared to sulfonated NSAC catalyst. The sucrose derived sulfonated catalyst showed maximum activity yielding 59% glucose and 80% total product yield at 120 °C. The reduction in catalytic activity after the repeated runs was attributed to the leaching of the –SO3H group which was confirmed by spectroscopic studies [156]. Guo et al., synthesized magnetic carbonaceous catalyst by grafting Fe3O4 and –SO3H groups after incomplete hydrothermal carbonization of glucose for the hydrolysis of cellulose in [BMIM]Cl as a solvent. This magnetically separable catalyst shows good catalytic activity yielding 70% TRS at 130 °C. The activity of the catalyst was reduced after repeated use due to adsorption of humins on the active site of the catalyst [157]. Liu et al., synthesized the magnetic catalyst by sulfonation of core structure Fe3O4@C for the hydrolysis of cellulose to yield 72% TRS with 82% glucose selectivity [161]. Similarly, Xiong et al., synthesized the sulfonated siliceous magnetic catalyst, Fe3O4@SiO2-SO3H for cellulose hydrolysis in

Catalytic Conversion of Biomass Derived Cellulose 141 [BMIM]Cl to yield 73% TRS at 130 °C [159]. Zhang et al., prepared spinel based solid catalyst CaFe2O4 by calcination of hydroxides obtained after precipitation of an aqueous solution of Ca(NO3)2.6H2O and Fe(NO3)3.9H2O in the presence of urea. The hydrolysis of cellulose using CaFe2O4 as a catalyst and [AMIM]Cl as a solvent to yield 50% hydrolysis yield 74% glucose selectivity at 150 °C. The control experiment for detecting the actual active species of the catalyst, suggested that CaFe2O4 in solid form catalyzes the hydrolysis reaction rather than the Fe2O3 and Ca2+ ion form. Further, the recovered catalyst shows properties similar to the fresh one, resulting in high yields and glucose selectivity [158]. Recently, Xu et al., prepared sulfonated mesocellular silicon foam supported poly(chloromethylstyrene) (MCF-SO3H) to hydrolyse the cellulose to 88% TRS in [BMIM]Cl as solvent at 110 °C. The large pore size of the catalyst is responsible for its high catalytic activity as it allows faster diffusion of the bulkier molecules like cellulose [160].

5.4.3.5 Cellulose Hydrolysis in Ionic Liquids Using Ionic Liquids as Catalyst Besides functioning as solvents, ILs can also be used as sustainable and greener catalyst in the hydrolysis reaction of cellulose to reducing sugars as reported in Table 5.6. Acidic –SO3H functionalized ILs have been extensively studied for the hydrolysis reaction [74]. Amarasekara and Owereh prepared imidazolium, pyridinium, and triethanolium based –SO3H functionalized ILs and tested their activity in cellulose hydrolysis. Methyl imidazolium based IL showed highest catalytic activity among pyridinium and triethanolium based ILs due to better solubility of cellulose. Methylimidazolium IL with shorter chain length 1-(3-propylsulfonic acid)−3-methylimidazolium chloride ([C3SO3HMIM]Cl) showed better activity than IL with longer chain length 1-(1-butylsulfonic acid)−3-methylimidazolium chloride ([C4SO3HMIM]Cl). Imidazolium-based –SO3H functionalized IL with shorter chain length ([C3SO3HMIM]Cl yield maximum TRS yield of 62% and glucose yield of 14% at 70 °C [162]. Liu et al., synthesized imidazolium and triethyl ammonium based –SO3H functionalized IL for the cellulose hydrolysis in [BMIM]Cl as a solvent. Among all ILs, triethyl-(3-sulfopropyl)-ammonium hydrogen sulfate yielded maximum TRS yield of 99% at 100 °C. The authors also showed the adverse effect of water present in the [BMIM]Cl, as it reduced the solubility of cellulose leading to loss in catalytic activity [164]. In 2014, Zhou et al., prepared 2-phenyl-2-imidazoline based –SO3H functionalized ILs with HSO4ˉ, Clˉ and H2PO4ˉ as anion for the hydrolysis of cellulose in [BMIM]Cl. ILs with HSO4ˉ and Clˉ showed

IL immobilized on silica/[BMIM]Cl 70 °C, 6 hr

100 °C,90min 100 °C, 1 hr 37 °C , 4 hr

Triethyl-(3-sulfo-propyl)ammonium hydrogen sulfate

1-propylsulfonic acid −2-phenyl imidazoline hydrogen sulfate

NMPCl/ FeCl3 in [BMIM]Cl

ZnCl21.74H2O-1-(1propylsulfonic)−3methylimidazolium chloride

Amarasekara et al.,/2010

Liu et.al.,/2012

Zhou et al.,/2015

Zhao et al.,/ 2016

Amarasekara et al.,/2016

100 °C, 2.5 hr

70 °C, 30 min

[C3SO3HMIM]Cl

Amarasekara and Owereh/ 2009

Reaction conditions

Catalyst

Authors/ Year

Table 5.6 Cellulose hydrolysis using IL as solvent as well as a catalyst.

Glucose TRS

TRS

TRS

TRS

Glucose TRS

Glucose TRS

Typical products

19 78

99

85

99

67 26

14 62

Yield (%)

[167]

[166]

[165]

[164]

[163]

[162]

Ref

142 Integrating Green Chemistry and Sustainable Engineering

Catalytic Conversion of Biomass Derived Cellulose 143 better catalytic activity than the IL with H2PO4ˉ as anion because of their higher acidity that catalyzes faster hydrolysis of cellulose. IL (1-propylsulfonic acid −2-phenyl imidazoline hydrogen sulfate) yielded maximum 85% TRS at 100 °C in [BMIM]Cl as a solvent containing optimum water [165]. Zhao et al., worked on pyrrolidinium-based Bronsted-Lewis acidic ILs containing metal chlorides for cellulose hydrolysis. These ILs were prepared using N-methyl-2-pyrrolidinium chloride (NMPCl) and MClx (M = Fe, Zn, Al, and Cu) in 1:1 molar ratio. A TRS yield of 99% was achieved using NMPCl/ FeCl3 in [BMIM]Cl. The authors studied the interaction of metal chlorides and cellulose using Density functional theory. The result showed that [NMP]Cl/FeCl3 have a stronger interaction with the cellulose than the other metal chlorides [166]. Amarasekara et al., also reported the use of ILs with metal salt, ZnCl2.1.74H2O-1-(1-propylsulfonic)−3-methylimidazolium chloride in cellulose hydrolysis which gave 78% TRS yield and 19% glucose yield at 37 °C in 4 hr. The authors suggested that the reaction is catalyzed by changing the conformation of polysaccharides resulting from the ZnCl2 interaction with the carbohydrate [167]. Further, Amarasekara et al., immobilized the IL on the silica support by condensation reaction to hydrolyze the microcrystalline cellulose into 67% TRS and 26% glucose yields at 70 °C using [BMIM]Cl as the solvent [163]. Liu et al., synthesized nanoporous polymer functionalized with acidic IL (PDVB-SO3H[C3vim]-[SO3CF3] for cellulose hydrolysis to achieve TRS and glucose yield better than solid acid such as Amberlyst-15, homogenous acidic IL, and mineral acids [168].

5.5

Glucose Conversion to 5-Hydroxymethyl Furfural

Glucose obtained from the hydrolysis of cellulose undergoes isomerization and dehydration to 5-Hydroxymethyl furfural (HMF), a platform and valueadded chemical. Isomerization step is catalyzed under basic or Lewis acid conditions where, dehydration reaction is catalyzed by the Bronsted acidity present in the catalyst [169]. The transformation of glucose to HMF using different catalysts have been listed in Table 5.7 Zhao et al., studied glucose transformation to HMF using 1-alkyl-3-methyl imidazolium chloride and CrCl2 as catalyst to yield 70% HMF at 100 °C and 3 hr of reaction time. The author suggested that the active species is CrCl2 which helps in mutarotation of α-anomer to the β-anomer which is an intermediate that undergoes isomerization to fructose [170]. Yong et al., reported the transformation

100 °C, 6 hr 100 °C, 1 hr 100 °C, 3 hr

100 °C, 3 hr 120 °C, 6 hr

CrCl3/[BMIM]Cl

CrCl3.6H2O /[BMIM]Cl

SnCl4·5H2O/[BMIM]Cl

Yb(OTf)3 /[BMIM]Cl

Cr-HAP[BMIM]Cl

Boric acid/[EMIM]Cl

GeCl4/[BMIM]Cl

AlEt3 /[EMIM]Cl

12-TPA/H3BO3/[BMIM]Cl

CrCl3/H3BO3

CrCl3/TEAC

Dowex 50W × 8–200[EMIM]Cl

Cr0-NPs[BMIM]Cl

Yong et al.,/2008

Li et al.,/2009

Hu et al.,/2009

Stahlberg et al.,/2010

Zhang et al.,/2011

Stahlberg et al.,/2011

Zhang et al.,/2011

Liu et al.,/2012

Hu et al.,/2012

Hu et al.,/2012

Hu et al.,/2012

Moyna et al.,/2012

Chen et al.,/2013

130 °C,10min

120 °C,30min

140 °C,40min

120 °C, 6 hr

100 °C, 2 hr

120 °C, 3 hr

MW, 2.5 min

140 °C, 6 hr

100 °C, 3 hr

CrCl2/[BMIM]Cl

Zhao et al.,/2007

Reaction conditions

Catalyst

Authors/ Year

Table 5.7 Dehydration of glucose to HMF.

50

53

71

79

52

51

48

41

40

24

62

17

81

68

[183]

[182]

[181]

[180]

[179]

[178]

[177]

[176]

[175]

[174]

[173]

[172]

[171]

[170]

Ref

(Continued)

Yield (%)

144 Integrating Green Chemistry and Sustainable Engineering

150 °C,50min 110 °C,30min 110 °C, 2 hr

Hβ-zeolite (Si/Al = 25) [BMIM]Cl

Cr3+-modified ion-exchange resins

C10(EPy)2]2Br, CrCl2

Choline dihydrogen citrate/ glycolic acid/H3BO3 100 °C, 2 hr

Hu et al.,/2014

Liu et al.,/2015

Chinnappan et al., /2015

Matsumiya et al.,/2015

Reaction conditions

Catalyst

Cont.

Authors/ Year

Table 5.7

60

47

61

50

Yield (%)

[187]

[186]

[185]

[184]

Ref

Catalytic Conversion of Biomass Derived Cellulose 145

146 Integrating Green Chemistry and Sustainable Engineering of glucose to HMF using CrCl2 and CrCl3 with N-heterocyclic carbenes (NHC) in [BMIM]Cl as a solvent. The authors described the function of NHC as a ligand and its steric effect influenced the catalytic conversion of glucose to HMF. Glucose conversion to HMF favored by more crowded NHC that protect CrCl2 to react with [BMIM]Cl, Cr center remains unaffected which catalyses the glucose isomerization to fructose. A maximum of 81% HMF yield was obtained using bulkier NHC such as 1,3-bis(2,6diisopropyl) phenyl imidazolinylidene and 1,3-bis(2,6-diisopropylphenyl) imidazolinylidene [171]. Microwave irradiation reduced the time of reaction yielding 91% of HMF yield from glucose using CrCl3/[BMIM]Cl catalytic system as reported by Li et al.,[172]. Hu et al., catalyzed the dehydration reaction of glucose to HMF using SnCl4/[EMIM]BF4 (1-ethyl-3-methylimidazolium tetrafluoroborate) yielding 62% of HMF. The authors suggested the formation of five membered chelate complex from two hydroxyl groups of glucose and Sn atom to form enol intermediate which is an important intermediate in the glucose dehydration reaction confirmed by NMR spectroscopic studies. These observations were confirmed by performing the control experiments using the alcohols such as ethanol, ethylene glycol and 1,3 dipropanediol. [173] Stahlberg et al., reported the glucose transformation to HMF using lanthanide salts in various ILs like [EMIM]Cl, [BMIM]Cl, 1-hexyl-3-methyl imidazolium chloride ([HeMIM]Cl) and 1-octyl-3-methyl imidazolium chloride ([OMIM]Cl). [OMIM]Cl with ytterbium chloride showed higher catalytic activity because of its higher hydrophobicity. The trend is opposite in the case of chromium chloride as ytterbium chloride doesn’t form complexes with the IL. However, the yields of HMF obtained were low, about 24% only [174]. Later, Stahlberg developed metal-free catalytic system for glucose conversion to fuel. The efficient conversion of glucose to 41% HMF was achieved using [EMIM]Cl and boric acid as a promoter. The authors confirmed the formation of glucose borate complex that facilitates glucose isomerization to fructose using DFT [176]. Glucose conversion to HMF using germanium chloride GeCl4/[BMIM]Cl to achieve 48% HMF was studied by Zhang et al., [177] To enhance the activity of the catalyst, water was removed by adsorption over 5 Å molecular sieves. The authors confirmed the interaction of glucose and GeCl4 and the formation of fructose during the reaction using [13] C NMR spectroscopic data. Liu et al., reported aluminium alkyl and alkoxy Lewis acidic catalyst in glucose conversion to HMF. AlEt3/[EMIM]Cl showed better catalytic activity yielding around 51% HMF as compared to AlCl3/[EMIM]Cl which yielded only 1.6% HMF [178]. Similarly, a number of catalyst have been reported in literature for glucose conversion to HMF such as ScCl3/ [BMIM]Cl

Catalytic Conversion of Biomass Derived Cellulose 147 [188] and HfCl4/[BMIM]Cl.[178] Hu et al., developed a catalytic system with two active components such as 12-tungstophosphoric acid and boric acid (HPA/B(OH)3) showing synergistic effect to catalyze the glucose conversion to HMF to yield 52% HMF in [BMIM]Cl as solvent. Boric acid catalyzes the transformation of glucose to 1,2-enediol intermediate which further can be dehydrated to HMF by 12-tungstophosphoric acid and boric acid [179]. They have also used CrCl3 in place of 12-tungstophosphoric acid with boric acid for glucose transformation to HMF 79% yield [180]. Zhang et al., developed a heterogenous system hydroxyapatite supported chromium chloride (Cr-HAP) for the dehydration of glucose to HMF using [BMIM]Cl as solvent system under microwave irradiation. The maximum HMF yield of 40% was achieved in 2.5 min and the catalyst remained active even after five experiments [175]. The ion exchange resin, Dowex 50W, catalyzed the glucose dehydration to 53% HMF as reported by Moyna et al.,[182] Chen et al., synthesized Cr0 nanoparticles in-situ from hexacarbonyl chromium for glucose dehydration to HMF in [EMIM]Cl [183]. Hu et al., studied the glucose dehydration to HMF using zeolites in [BMIM]Cl [184]. The maximum yield of 50% HMF was achieved over H-beta zeolite with BEA structure and Si/Al ratio of 25 having both Lewis and Bronsted acidic sites. Liu et al., synthesized Cr3+ modified ion exchange resin for glucose conversion to HMF using [BMIM]Cl as a solvent. The highest HMF yield of 61% was achieved at 110 °C but they reported loss of catalytic activity after six cycles [185]. Hu et al., synthesized tetraethyl ammonium chloride as a solvent for the glucose conversion to HMF using CrCl3 as the catalyst. This catalyst system was found to be resistant towards water resulting in good yields of HMF about 71% [181]. Han et al., synthesized DBU (1,8-diazabicyclo[5.4.0]undec-7-ene) based solvent system for glucose conversion to HMF [189] and reported a HMF yield of 83% using CrCl3/Et-DBUBS (Ethyl-DBU benzenesulfonate) as catalytic system due to the slow rate of side reactions. Chinnappan et al., synthesized 1, 1 -decane-1, 10-diylbis (3-ethylpyridinium) dibromide ([C10(EPy)2]2Br−) to function as a solvent for CrCl2 catalytic system and reported 47% HMF yield [186]. Matsumiya et al., catalyzed the conversion of glucose to HMF using boric acid as promoter in deep eutectic solvent (DES) comprising of choline dihydrogen citrate and glycolic acid yielding 60% HMF at 140 °C [187]. Zeolites are the well-known heterogenous catalyst for the dehydration of glucose derived from cellulose to the 5-HMF. They possess both Bronsted acidity and Lewis acidity for the tandem reactions such as hydrolysis, isomerization, and dehydration involved in the conversion of cellulose to HMF [190]. Bokade et al., reported the conversion the dehydration of cellulose in water using desilicated H-ZSM-5.

Reaction conditions 100 °C, 3 hr 100 °C, 6 hr 100 °C, 3 hr 190 °C , 4 hr 140 °C, 5 hr

Catalyst

CrCl2/[BMIM]Cl

CrCl3/[BMIM]Cl

SnCl4·5H2O/[BMIM]Cl

H-ZSM-5

HNTs–PSt–PDVB–SO3H

Authors/ Year

Zhao et al.,/2007

Yong et al.,/2008

Hu et al.,/2009

Bokade et al.,/2014

Zhang et al.,/2014

25

46

62

81

68

High temperature and Low yields

Low temperature and high yields

HMF yield (%) Remarks

Table 5.8 Table of comparison for homogenous and heterogeneous methods.

148 Integrating Green Chemistry and Sustainable Engineering

Glucose (59) TRS (80) HMF (68)

[BMIM]Cl/ H2SO4

HCl (20 wt %) / + [EMIM]Cl

[EMIM]Ac/HCl

Amberlyst-15/[BMIM] Cl

SPS-DVB/[BMIM]Br

NSAC/[BMIM]Cl

CrCl2/[BMIM]Cl

CrCl3/[BMIM]Cl

CrCl3.6H2O /[BMIM]Cl HMF (17)

Li et.al.,/ 2007

Binder et al.,/2010

Morales et al.,/2012

Rinaldi et al.,/2008

Fan et al.,/2013

Liu et al.,/2013

Zhao et al.,/2007

Yong et al.,/2008

Li et al.,/2009

HMF (81)

Glucose (48) TRS (60)

Oligomers (90)

Cellobiose (10)

Glucose (90) HMF (5–7)

Glucose (39) TRS (66)

Catalyst

Authors/ Year

Typical products(%yield)

Table 5.9 Table of comparison between different methods.

Mineral acids used

Disadvantages of the process

Direct conversion to HMF

Leaching of metal ions

Catalysts are recoverable Low yields of glucose

Cellulose hydrolyzed with good yields

Advantages of the process

Catalytic Conversion of Biomass Derived Cellulose 149

150 Integrating Green Chemistry and Sustainable Engineering The maximum conversion 67% was achieved at 190 °C and 4 hr with loss in activity for four catalytic cycles [191]. Later, Zhang et al., synthesized polymer based heterogenous catalyst for the cellulose conversion to HMF through precipitation polymerization [192]. The main shortcomings of using heterogenous catalyst is the requirement of high temperature. Ionic liquid use as catalyst as well solvent found to be the better than zeolites as it requires lower temperature and shorter time of reaction (Table 5.8, 5.9).

5.6

Conclusion and Future Prospects

Depletion of fossil fuel and global warming requires immediate conversion of abundant and renewable lignocellulosic biomass into value-added chemicals and fuel products using environmentally benign routes. The Conversion of Cellulosic biomass to fuel has been carried out utilizing various catalysts such as liquid acid, solid acid, ionic liquids and metal chlorides under homogenous and heterogenous conditions. There is no doubt that using ILs as catalyst and solvent for the chemical conversion of lignocellulosic biomass is very promising and encouraging. It also reflects that this field is still in its infancy, and there are many challenges.

Future Prospects HMF production from whole lignocellulosic biomass can be addressed with the special attention to the development of robust catalyst having high activity, selectivity, stability and reusability. The hydrolysis of MCC using heterogenised ILs can be scaled-up for the industrial level. Among the three components of lignocellulosic biomass, lignin destruction is least explored because of its varied structure and recalcitrant amorphous nature. Upgradation of HMF to fuel components and value added chemicals can be a promising area of research. Hydrogenation of HMF to higher alkanes that can be utilized as fuels is an important aspect of research for renewable fuels. Oxidation of HMF to intermediates important in medical applications has scarce studies available.

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Catalytic Conversion of Biomass Derived Cellulose 161 152. Rinaldi, R., Palkovits, R., Schüth, F., Depolymerization of cellulose using solid catalysts in ionic liquids. Angew. Chem. Int. Ed. Engl., 47(42), 8047–8050, 2008. 153. Rinaldi, R., Meine, N., vom Stein, J., Palkovits, R., Schüth, F., Which controls the depolymerization of cellulose in ionic liquids: the solid acid catalyst or cellulose? ChemSusChem, 3(2), 266–276, 2010. 154. Fan, G., Liao, C., Fang, T., Wang, M., Song, G., Hydrolysis of cellulose catalyzed by sulfonated poly(styrene-co-divinylbenzene) in the ionic liquid 1-n-butyl-3-methylimidazolium bromide. Fuel Processing Technology, 116, 142–148, 2013. 155. Guo, H., Qi, X., Li, L., Smith, R.L., Hydrolysis of cellulose over functionalized glucose-derived carbon catalyst in ionic liquid. Bioresour. Technol., 116, 355–359, 2012. 156. Liu, M., Jia, S., Gong, Y., Song, C., Guo, X., Effective Hydrolysis of Cellulose into Glucose over Sulfonated Sugar-Derived Carbon in an Ionic Liquid. Ind. Eng. Chem. Res., 52(24), 8167–8173, 2013. 157. Guo, H., Lian, Y., Yan, L., Qi, X., Smith, R.L., Cellulose-derived superparamagnetic carbonaceous solid acid catalyst for cellulose hydrolysis in an ionic liquid or aqueous reaction system. Green Chem., 15(8), 2167, 2013. 158. Zhang, F., Fang, Z., Hydrolysis of cellulose to glucose at the low temperature of 423 K with CaFe2O4-based solid catalyst. Bioresour. Technol., 124, 440– 445, 2012. 159. Xiong, Y., Zhang, Z., Wang, X., Liu, B., Lin, J., Hydrolysis of cellulose in ionic liquids catalyzed by a magnetically-recoverable solid acid catalyst. Chemical Engineering Journal, 235, 349–355, 2014. 160. Xu, S., Tan, Z., Cai, G., Xiong, C., Tan, W., Zhang, Y., Sulfonic acid catalyst based on silica foam supported copolymer for hydrolysis of cellulose. Catal. Commun., 71, 56–60, 2015. 161. Liu, X., Xu, Q., Liu, J., Yin, D., Su, S., Ding, H., Hydrolysis of cellulose into reducing sugars in ionic liquids. Fuel, 164, 46–50, 2016. 162. Amarasekara, A.S., Owereh, O.S., Hydrolysis and Decomposition of Cellulose in Brönsted Acidic Ionic Liquids Under Mild Conditions. Ind. Eng. Chem. Res., 48(22), 10152–10155, 2009. 163. Amarasekara, A.S., Owereh, O.S., Synthesis of a sulfonic acid functionalized acidic ionic liquid modified silica catalyst and applications in the hydrolysis of cellulose. Catal. Commun., 11(13), 1072–1075, 2010. 164. Liu, Y., Xiao, W., Xia, S., Ma, P., SO3H-functionalized acidic ionic liquids as catalysts for the hydrolysis of cellulose. Carbohydr. Polym., 92(1), 218–222, 2013. 165. Zhuo, K., Du, Q., Bai, G., Wang, C., Chen, Y., Wang, J., Hydrolysis of cellulose catalyzed by novel acidic ionic liquids. Carbohydr. Polym., 115, 49–53, 2015. 166. Zhao, Z., Li, N., Bhutto, A.W., Abdeltawab, A.A., Al-Deyab, S.S., Liu, G., et al., N-methyl-2-pyrrolidonium-based Brönsted-Lewis acidic ionic liquids

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6 Raman “Green” Spectroscopy for Ultrasensitive Analyte Detection Subhavna Juneja, Anujit Ghosal* and Jaydeep Bhattacharya* Nanobiotechnology Lab, School of Biotechnology, Jawaharlal Nehru University, New Delhi, India

Abstract Nanobiotechnology emerges at the intersection of interdisciplinary fields of biotechnology and nanotechnology. In comparison to conventional systems, nanoscale devising for medical technology holds myriad of advantages such as sensitivity, selectivity, flexibility, multiplexing and miniaturization. Raman spectroscopy or Surface Enhanced Raman Spectroscopy (SERS) more appropriately, over the years has seen immense improvement as a medical diagnostic and sensing tool. SERS is non-destructive, label free (requires no additional tags such as dyes for identification), fast (cuts down operational time and cost), accurate, specific and sensitive (sometimes to the level of a single molecule). As a scientific progression another avenue that has recently caught the eye of the researchers across the globe working on Raman spectroscopy or related fields is its sustainability. The technique per say, involves less chemical usage with increase in biosynthetic routes for synthesis of nanoparticles, is an energy efficient system as seen in hand held Raman systems, it is capable of multiplexing catalytic degradation along with detection, is apt for in-situ separation using magnetic nanoparticles and one can also fabricate re-useable SERS substrates which would reduce operational costs and improve product shelf-life. Keywords: Surface enhanced raman spectroscopy (SERS), surface plasmon, biotechnology, energy efficient devices, green label free detection, ultra-sensitive

*Corresponding authors: [email protected]; [email protected], jaydpb@gmail. com Shahid-ul-Islam (ed.) Integrating Green Chemistry and Sustainable Engineering, (165–190) © 2019 Scrivener Publishing LLC

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6.1

Introduction

Biotechnological advances brought highly effective diagnostic and treatment approaches for chronic disorders notably molecular diagnosis (PCR/ELISA), gene therapy and drug delivery [1–6]. All these current technologies depend majorly on micrometer dynamics of the biological moiety. Although it has sought major breakthroughs, it still lacks ultimate sensitivity, which is paramount in view of present medical scenario, where we need to fight multidrug resistance (MDR), cancer and bioterrorism [5, 7–11]. Biomedical aims can thus be reframed to develop novel techniques which are target specific, sensitive and most importantly noninvasive. Organ transplantation for instance, is a boon but is often accompanied with several restrictions such as availability and quality of organs, regular usage of immunosuppressive drugs etc. Current regenerative medicine, thus, aims to develop techniques which can omit the need of organ transplant altogether and allow accurate cell health and function monitoring [12–14]. In another aspect, different pharmaceutical and biotechnology companies have produced valuable drugs which could potentially treat complex diseases like cancer; however, they are comparatively costly and can add only few days to the lives of the patient [15]. These drugs could garner improved value if made available at an early disease stage and possibly at lower dosage level to subdue any chances of drug side effects. All the above concerns and limitations, partially or even completely in certain cases, could be addressed by making use of nanotechnology and its tool. High surface area to volume ratio of the nanoparticles allows greater drug carrying capacity, this alone can help in achieving: 1. selective drug localization by nanoparticle functionalization 2. increase time profile to achieve drug response due to longer penetration residence of the nanosystem 3. effective delivery due to limited restriction from blood brain barrier and 4. improved drug efficacy and reduced toxicity due to localized drug accumulation. This interdisciplinary scientific avenue which utilizes nanomaterials for biotechnological interventions with an aim to improve contemporary healthcare situation across the world is referred to as “Nanobiotechnology” or “Nanomedicine”, more commonly [3, 10].

“Green” Spectroscopy for Ultrasensitive Analyte Detection 167 It finds branches expanding in different medical subgroups such as Bioimaging, Early Diagnosis, Biosensing, Targeted Drug Delivery, Food Technology and Regenerative medicine. A brief descriptive of each of these have been discussed in the following section.

6.2 6.2.1

Application of Nanotechnology in Medicine Bio-Imaging

Bio-imaging serves as an excellent tool to gain insight into biological processes. Different imaging modalities, Computed Tomography (CT), Ultrasound, Magnetic Resonance, Positron Emission etc. have developed over time with an aim to achieve better resolved images. Constant need for improved imaging techniques arises from the fact that each technique has its own benefits and limitations over the other. For instance, MRI is one of the most well-established imaging modality, with excellent contrasting ability however its sensitivity is poor. Fluorescence imaging allows molecular level visualization and is non-destructive but is limited by photo-bleaching and high cost. Nanoparticles as bio-labels are becoming efficiently important tools for improving medical diagnosis. Compared to conventional available contrast agents, nanomaterials improve the quality of in-vivo imaging because of biocompatibility, photo-stability, longer residence time and multiplexing [16–18]. Clinical trials have also affirmed their superiority to conventional contrasting agents. Imaging with nanoparticles also opens up avenue for targeted killing, a field commonly referred to as “theranostics” where along with diagnosis, nanoparticles are used to treat the diseased cells. Gold nanoparticles are excellent fluorophores, but they are an equally good source for hyperthermia based localized heat generation. Upon excitation with laser, gold nanoparticles heat up, in response to the incoming energy. The excess energy they gain is released in the form of extra heat which is transferred to cells where they have been targeted. Not only does this kill the desired cell population, it also avoids multiple surgeries and reduces treatment costs.

6.2.2

Bio-Sensing and Diagnosis

Any analytical device that converts a biological response into electrical signal is referred to as a biosensor. Efficient signal transduction from biological reaction to a detectable electrochemical, magnetic or optical signal is a major challenge for biosensor development. Inherently

168 Integrating Green Chemistry and Sustainable Engineering rich in large surface areas nano-materials have influenced bio-sensing research by allowing ultralow detection limit and reduced response time [11, 13, 19, 20]. Increased analyte immobilization at reduced volumes drives the efficiency of the sensor. Among different nano-materials available, graphene [21], gold nanoparticles [22], metal oxides [23], quantum dots [24] etc. have been extensively studied. In addition to the enhanced surface areas each of the material mentioned above have unique properties of their own which make them an excellent biosensing resource. CNT or graphene materials for example, have biocompatibility, wire like morphology and electrical properties that make them a superior electrical transducing material. Additionally, CNT has enhanced interface formation probability owing to the presence of functional groups which can be used to bind bioreceptors or these can even act as docking sites for biomolecules mediating bio-electrochemical reactions. Porous 3D networks of CNT are naturally electro-active which acts as a suitable anchor for bioreceptor binding [25].

6.2.3 Targeted Drug Delivery The process of drug administration followed, to achieve therapeutic relief defines what we call drug delivery. Although used widely, conventional drug delivery practices face major shortcomings. Most of the drugs used for therapy fail to reach the target tissue and get distributed to normal cells as well reducing the treatment efficiency. Cytostatic drugs administered during chemotherapy for cancer treatment, fail to distinguish between healthy and diseased cells, leading to severe side effects and killing of normal cells [26]. A selective strategy targeting malignant cells only would be an indistinguishable progress. Drug instability and premature drug release and loss are driving parameters for the pharmaceutical industry. Nanotechnology contributes towards drug delivery in two major ways: Drug reformulation and development of novel drug carriers. Drug discovery is an enriching however an expensive process whilst redevelopment of existing drugs are comparatively easier, have increased lifetimes and allows faster market availability [27]. Right reformulation restricts drug aggregation in the blood plasma, leading to enhanced performance, higher solubility, instant dissolution and improved bioavailability since nanoparticles can easily cross brain-blood barrier. In addition, nanoparticles inherently rich with high surface area to volume ratios can be employed as a suitable drug carrier, lesser nanoparticle concentration can carry more drug molecules. Polymers [28], Ceramics [29], Micelles [30], Dendrimers [31], Liposomes [32] and Gold [33] are some examples of nanomaterials that

“Green” Spectroscopy for Ultrasensitive Analyte Detection 169 have been extensively worked upon as efficient nano drug carriers. These new drug delivery vehicles are biocompatible, illicit minimal immunological agitations, improve pharmacokinetic properties increasing treatment effectiveness, safety, whilst reducing drug dosage and production cost.

6.2.4

Food Technology

Application of nanoscience to the field of agriculture and food technology is relatively new and finds use in nutrient delivery [34], gene therapy [35], food packaging [36, 37] and detection of chemical and biological contaminants [38, 39]. Nanotechnology contribution in food industry can be classified under the subheadings of Food packaging and Food applications. Traditional food packaging materials are majorly non-biodegradable and derived from technologies using non-renewable energy resources. Contemporary biomaterial inspired active food packaging films exhibit poor mechanical properties and limited protection against moisture and contamination. Use of nanobiocomposites has proven to not only protect food from contamination but also increases its shelf life, is environmentally benign when compared to conventionally used plastic. ZnO-PLA nanocomposite films were developed as an alternative packaging films by Marra et al., [40]. Derivable from natural resource, PLA might be costly compared to polyolefin used presently, but is compostable, benign, biodegradable and shows improved tensile strength [41]. Food applications constitute use of nanomaterials for controlled release of encapsulated nutrients, delivery of vitamins and flavours, detection of pollutants and pathogens for food safety and quality analysis. As an example, dry milling has been used to reduce the size of green tea leaves powder below 1000 nm. By physically reducing the scale, the new green tea leads to better nutrient digestion and absorption and improved enzyme activity associated to oxygen removal [42]. Food preservation is also a field of critical importance for the food industry. Food spoilage detection has been achieved using an array of nanosensors, a schematic of which is presented in scheme 1. The aim of these nanosensors is to reduce the detection time and improve sensitivity for pathogen presence.

6.2.5

Regenerative Medicine

Regenerative medicine essentially deals with developing methodologies to repair, replace or regrow damaged or diseased organs, tissues or cells in view of restoring normal organ functioning [43]. It broadly includes stem cell studies, development of artificial organs, tissue engineering and

170 Integrating Green Chemistry and Sustainable Engineering Detection of food borne contaminants Array biosensing

Wine discrimination

Lab on a chip

Electronic nose

Nano sensors

NEMS

Detection of pathogens

Microfluidic devices

Carbon nanotube based sensors Detection of capsaicinoids in chilli peppers

Scheme 6.1 Nanosensors and their wide application in food technology [41].

bone reformation. Success of any engineered tissue is heavily dependent on the development of an efficient scaffold system. Porous 3D scaffolds acts as template for tissue regeneration by ensuring appropriate growth environment. These scaffolds are seeded with cells, growth factors and certain biophysical stimuli, if need be, to ensure healthy cell growth. Over time, the smooth surface of the growing cells leads to loosening of the scaffold carrying implants, which triggers immunological reactions leading to graft rejection by the recipient. Second most important aspect of tissue engineering is bone regeneration however, innate high mechanical strength and anatomical complexities of natural bones make it difficult to replicate the system. Since nanoparticles are of the same order as constituents of the natural bone, it has made bone regeneration a possibility. Nanoparticles can be used to fortify the scaffolds and provide it with improved properties such as superior mechanical strength, osteo-induction, conduction and integration. Role of gold nanoparticles as an osteogenic agent has been studied and reported by Heo and his group [44]. They fabricated a gelatin hydrogel fortified with gold nanoparticles and established the role as a promoter of cellular differentiation and proliferation of stem cells derived from adipose tissues. Nanoparticles have also been used to coat the implants

“Green” Spectroscopy for Ultrasensitive Analyte Detection 171 with the aim to reverse graft rejection and improve adhesion properties. These scaffolds can also be used as delivery vehicles for administration of drugs globally or locally. Nanoparticles loaded with drugs can be functionalized within the scaffold. Additionally, nanoparticles with intrinsic optical properties can also be used as imaging sources to monitor tissue growth and health.

6.2.6

Nanomedicine with Emphasis on Early Disease Detection

Nanomedicine instantly found flourishing acceptance within the scientific community, medical and nonmedical alike. It has the ability to address broad spectrum of medical issues and at different levels as seen in the previous section, however we feel early and accurate diagnosis holds the key. In a 2005 article published in Biotechnology healthcare, John Mack mentions how nanotechnology could help cure cancer. He says “Nanotechnology could help with early detection of some cancers, which depends on the detection of very small quantities of protein markers in the blood or on cell surfaces. This is the case with ovarian cancer, for which Inhibin A, a hormone, is possibly a marker. Current technology is not sensitive enough to detect this protein until the disease has progressed to an advanced stage, by which time the 5 year survival rate is less than 30 percent.”[6] Timely detection of a possible disease state or presence of potent contaminant would allow effective prognosis design, subsequently improving the chances of disease prevention, effective treatment and reduced drug toxicity. The World Health Organization (WHO) defines diagnostic techniques as all investigatory tests and examinations that are carried out to identify the cause of any disease or illness and constitutes laboratory tests and imaging techniques. A vast variety of imaging modalities have been developed over the years to image and identify cell health such as Fluorescence, Ultrasound, Magnetic resonance imaging (MRI) etc. All the aforementioned techniques used for imaging are excellent source of information about the physical being of the cell but assessing the chemical or functional state is still a challenge [45]. Raman spectroscopy offers to allow non-invasive biochemical cell health assessment that too label free. It can also be used to determine tissue compositions, cellular alterations, strain level pathogen detection [46], study mitosis [47], apoptosis [48] and even post-transplant organ regeneration efficacy [49]. Raman spectroscopy holds great promise as an indispensible diagnostic tool, which is specific as well as sensitive upto single molecule level. This chapter summarizes the physical principle, different applications, present research and technological advances of Raman

172 Integrating Green Chemistry and Sustainable Engineering Light (h o)

Molecular vibration Virtual states

E = h( oE=h

o

abs)

E = h( o+

abs)

Vibrational states Rayleigh scattering

Stokes scattering

Anti-stokes scattering

Figure 6.1 Energy diagram describing the scattering processes.

Spectroscopy and how it is a benign technique by rules of both, green chemistry and green engineering.

6.2.7 Raman and Surface Enhanced Raman Spectroscopy (SERS) Photons interacting with matter undergo scattering either elastically or inelastically. In the prior case, where the photon-matter interaction is elastic, incident and scattered photons have same energy, a process commonly referred to as Rayleigh Scattering. While in the latter, where the interaction is in-elastic, scattered photons possess lower or higher frequencies on account of energy loss or gain when compared to the incident photon, both these processes are referred to as Raman scattering: Stokes and Anti-Stokes, respectively [50]. Figure 6.1 describes these scattering processes through molecular energy diagram for easy comprehension. Frequency or energy shift in an inelastic collision is a function of the molecules’ structure. Interacting analyte molecule channelizes the fractional energy loss or gain through re-emittance of the light at a different frequency which depends on the vibrational

“Green” Spectroscopy for Ultrasensitive Analyte Detection 173 mode or polarizability of the bond to be precise. Polarizability is a function of interatomic bond distance which changes when a molecule losses or gains energy upon light interaction [51]. Inelastically scattered photon can thus, be said to contain fingerprint information of the molecules’ vibrational modes explaining the peculiarity of a Raman spectrum. Despite its various advantages Raman scattering is an inefficient process due to extremely small cross section (10–29 cm2/molecule) for most analytes leading to innate poor Raman signal intensities [52]. Surface Enhanced Raman Spectroscopy (SERS), involves augmentation of Raman scattering signals from analytes in close proximity of metal surfaces. Enhanced signal is an outcome of Electromagnetic Enhancement (EM) originating from the collective oscillations of the electrons in the conduction band (Surface Plasmons) of the metallic nanoparticles. When the incoming electromagnetic field (light) interacts with analyte-metal, metal nanoparticles undergo charge separation. Since the surface electron density of nanoparticle is very high, the magnitude of plasmon generated is notable and so is the generated electric field. This additional electromagnetic field (light +nanoparticle), produces a pronounced dipole in the analyte molecule leading to square times increment in Raman intensity (IRaman~ Efield2) [53]. Another pathway found to be responsible for incremental Raman signal gain, is referred to as Chemical Transfer (CT) which originates from charge transfer mechanism between analyte and nanoparticle surface. However, CT increase is largely confined to the order of 102 or below, while for metal nanostructures it alone can be as high as 1012. Though the role of CT has been unanimously agreed upon, in order to further increase the magnitude of SERS enhancement on non-metal materials, they are strategically functionalized using different noble metals [54]. Noble metals increase the SERS effect by generating pronounced local dielectric fields on account of SPR. For instance, in a noble metal-metal oxide system we can attain Raman signal augmentation in synergism from Chemical transfers and Electromagnetic enhancement.

6.2.7.1 Raman “Green” Spectroscopy Advent of nanoscience and technology leaves us in no doubt that it will dominate the future global market, however, if development is unidirectional it will have restricted uses and applications. With application of nanotechnologybased techniques and tools in diverse fields as medicine, agriculture, automobile, energy and environment, it is bound to create havoc if not managed appropriately. One of the easiest alternative, we feel would be generating a

174 Integrating Green Chemistry and Sustainable Engineering sustainable approach to generation, utilization and disposal of nanomaterials being used. Sustainability, per say is a subjective term with multiple definitions and perspectives, which if conventionally defined, can be the capability of a system to maintain itself over a long period of time causing minimal adverse consequence to the environment. Re-useable SERS substrates are one such examples that comply with the principles of sustainable engineering, which of course is a subset of green chemistry. Initially experimental explorations for SERS majorly emphasized on metal nanostructures due to its excellent LSPR, as noted in previous section. However, the non-uniformity, stability and to certain extent high cost pushed the researchers to look for alternatives which could overcome these problems. The necessity and also knowledge revelations about the chemical transfer phenomenon initiated the use of non-metal based nanomaterials for SERS study of which metal-oxide semiconductors have been more promising and prominent. Metal oxide nanostructures such as TiO2, ZnO, Fe3O4 etc are known to have exceptionally good photocatalytic activities, i.e. in the presence of light they can catalyse redox reactions which is commonly used to destruct contaminants. For example consider, vertically aligned Au/ZnO nanorods grown by Sinha et al., to be used as a cost effective and efficient SERS active substrate. These nanorods were fabricated by combining low temperature hydrothermal route with sputtering. These Au/ ZnO nanorods, based on the principle of photocatalysis act as self-cleaning substrates in UV light. Upon UV irradiation it gets photoexcited, generating photocarriers and thus reactive radicals such as hydroxyl and super oxides leading to MB dye dissociation, their analyte. Since these structure stay put permanently, these substrates can have a long shelf life and a single substrate can be used multiple times. These substrates are greener not only in their reusability but also since it can help remediate environmental pollution allowing simultaneous detection [55]. Physical growth methods are one alternative, off lately researchers have also been able to grow such permanent self-cleaning substrates chemically or using soft chemical methods. Consider for example, Kandjani et al., were able to grow a uniform ZnO thin film by sol gel process using zinc acetate and monoethanolamine. Following thermal annealing these thin films were subjected to hydrothermal synthesis to allow nanorod array formation. Chemicals used were zinc nitrate and hexamine. After successfully growing the nanorod array, these substrates were deposited with silver nanoparticles electrolessly. Their ZnO array serves two basic purposes. Firstly, allows reproducible spectra generation due to uniformity of grown structures and secondly, its high surface area maximizes photolytic cleaning. This soft chemical route mediated SERS susbtrate demonstrated good reproducibility and recyclability for reuse [56].

“Green” Spectroscopy for Ultrasensitive Analyte Detection 175 In another scenario, non-chemical techniques such as femtolaser, lithography and atomic layer deposition has been used to grow permanent nanostructures onto solid surfaces to be used as re-usable SERS substrates. Though the initial fabrication cost must be a little higher since these techniques have high operational costs, but their multiple cycle use favours its utilization as a greener alternative. Gill et al., grew nanospiked structures on silicon wafer using a FS Ti: sapphire laser. These nanospikes were later functionalized with silver nanoparticles of 50 nm and used for detecting R6G and MB upto picomolar level. This is a dip cleaning surface which can be cleaned by simply immersing in water to remove the adsorbed analytes and re-used [57]. Hatab et al., followed a rather unconventional nanofabrication approach to grow nanopatterns of silver disks for efficient SERS measurments. Periodic arrays of square, triangle and elliptical pillars were created via electron beam lithography. These arrays served as reusable stamps. These nanopatterns were transferred from the stamps onto PDMS to develop as nanocomposites substrates with periodic morphologies. Silver is then vapor deposited physically onto the stamp using nanotransfer printing. Transferred silver nanodisk-PDMS susbtrates were used as SERS susbstrates for probing Rhodamine 6G, Crystal Violet and Mitoxantrone. The standout feature of this work was stamp and repeat methodology [58]. In continuation to their previous work in SERS sensing, Kandjani et al., developed a reusable Hg+2 SERS sensing platform which could be easily regenerated for re-use upon heating, which otherwise is irreversible and difficult. The SERS carried nanofabricated Ag-ZnO arrays that were grown soft chemically. Upon UV irradiation, ZnO gets sensitized yielding an electron hole pair and converting Hg+2 into water insoulable Hg0 allowing its easy removal. This simultaneous detection and removal adds to the multiplexing ability of a good Raman system. These SERS substrates were reusable and multifunctional. They could also be used to detect and remove other analytes. Such reusable susbtrates certainly define the future of sustainable engineering [56]. Thus, Raman spectroscopy can go a long way as a green chemistry technique, which can conduct scientifically important research without contributing much to ecological footprint. More elaborately, by the virtue of green chemistry and engineering principles [2], Raman spectroscopy classifies to be considered as a green technique under the following heads: 1. Prevention (label free detection and imaging) 2. Less Hazardous Chemical Synthesis (Biosynthetic routes for producing plasmonic/non plasmonic nanoparticles)

176 Integrating Green Chemistry and Sustainable Engineering 3. Energy Efficient Designs (Room temperature synthetic processes) 4. Catalysis (in-situ catalysis and identification) 5. Design for Degradation (Simultaneous degradation and identification of analyte) Engineering 1. Inherent rather than circumstantial (Reusable SERS substrates) 2. Design for separation (Magnetic nanoparticles based detection) 3. Maximize efficiency 4. Renewable rather than depleting (Photocatalytic Substrate) 5. Design for Commercial Afterlife All these topics are dealt in detail in the following section of the chapter, with emphasis on citing current research articles to ensure better understanding and allow translational insight into the variety of areas of application. 6.2.7.1.1 Prevention The best way to manage waste generation is to not produce it. Raman mediated spectroscopic analysis contributes to prevention by omitting the use of any labels, which are inevitable in other imaging techniques such as Fluorescence microscopy or MRI. These not only add to the toxicity but are also expensive. Additionally, Raman also offers to provide biochemical identification of analyte completely avoiding the use of any additional technique and its constituents (infrastructure, chemicals or waste generation) for complete identification. Raman as a rapid label free imaging tool was used to develop a diagnostic approach to detect malarial infection [59]. Mouse spleen tissues, normal and infected with malarial parasite served as the substrate. The distinguishing feature as analyzed through acquired Raman spectra relied on the biochemical changes observed within the infected and normal tissues. The infected cells showed an increase in heme-based Raman vibrations, postulated to be contributed by hemozin, a malaria pigment. The result summary is presented as Figure 6.2. In another study, Raman mediated in-situ DNA-mettalization study was conducted to detect DNA, free of any labels or dyes [60]. A peptide nuclei acid (PNA) recognition probe was designed and immobilized on the glass

“Green” Spectroscopy for Ultrasensitive Analyte Detection 177 Non-infected spleen Infected spleen

1439

1636

1561

1396 1370

1307

1130 998

1087

0.2

1225

0.4

1170

0.6

1618

1338

0.8

745

Normalised raman intensity (a.u.)

1585

1.0

0.0 900 1000 1100 1200 1300 1400 1500 1600 1700 Raman shift (cm–1)

1130 1170

1.30

1225

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0.00

1439

–0.05 1087

Normalised raman intensity (a.u.)

800

745

700

–0.30 700

800

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1000 1100 1200 1300 1400 1500 1600 1700 Raman shift (cm–1) I1130/I1307

I1170/I1307

Control

I745/I1307

Infected

10 μm

10 μm 1.68

0.26

2.40

0.10

1.40

0.36

Figure 6.2 Analysis of Raman peak intensity for control and P. berghei-infected tissue sections. (a) Averaged, normalised spectra from a mapped area of control and infected tissue. Spectra were then normalised against the peak at 1585 cm−1. (b) Subtracted Raman spectrum. The average, normalised control spectrum was subtracted from the average, normalised infected spectrum, both shown in (a), to highlight any potential biological components that were being altered within the malaria infected tissue. (c) Image shows white light images of both tissue sections, with the grey box indicating the area that was mapped, along with peak intensity ratio maps for selected Raman peaks. These ratio maps visually show relative changes in the intensity of these Raman peaks within the area of P. berghei-infected tissue that was mapped compared to the uninfected area. These maps suggest an increase or alteration to some key biological components within the infected tissue including, multiple hemoglobin vibrations, lipids, and proteins. (Reprinted with Copyright permission [59])

178 Integrating Green Chemistry and Sustainable Engineering

Intensity

736

PNA Target DNA

Silver deposition

SERS Raman shift

Scheme 6.2 Label free Raman based SERS strategy for DNA detection. (Presented with Copyright Permissions [60]).

slide. PNA immobilization was followed by DNA hybridization step, were anayte DNA was allowed to bond with complementary PNA sequence. Following hybridization, silver enhancement step was performed, where the negatively charged backbone of the DNA allows positive Ag+ ion absorption. The silver ions were reduced to Ag nanoparticles (10 nm) by hydroquinone. Silver nanoparticles grown in-situ on the DNA allows enough vibrational interactions between DNA bases to attain improved Raman signals. Adenine spectral feature at 736 cm–1 was marked as the endogenous marker for DNA detection. The limit of detection achieved with this method was 34 PM. Similar studies have been conducted to study determination and localization of breast cancer-colonized bone alterations [61], 2-Aminopurine substitutions in probe DNA [62], localization of cyanogenic glucoside dhurrin in sorghum cells [63], differential effects of polyphenols on MCD-7 cancer cells [64] and tetramolecular i-motif detection amongst others [65]. 6.2.7.1.2 Less Hazardous Chemical Synthesis and Energy Efficient Designs Present biotechnological diagnostic techniques majorly rely upon immunoassays and nucleic acid measurements such as ELISA or PCR respectively. Accurate diagnostic approaches have also been developed using Recombinant DNA technology and laboratory designed pure antibodies. These techniques bestow the system with accuracy and precision however adversely also requires time and cost besides highly skilled human resource and infrastructure [66]. PCR, for example depends on previously available knowledge about the analyte to be detected, say bacteria. Complete analysis of the bacterial strain would require elaborate laboratory procedures like culturing, primer designing, cell lysis for DNA amplification and post processing, while for Raman analysis all we need would be bacteria infected food/drink or any other substrate which

“Green” Spectroscopy for Ultrasensitive Analyte Detection 179 (a)

SERS signal (b)

Laser

Bacterium Van AAO

Ag

Ag

Ag

Ag

SERS substrate

Figure 6.3 Diagram showing the cross-sectional view of a bacterium on a Van-coated substrate. (b) SEM image of bacteria on the substrate (scale bar, 500 nm) [66].

needs to be analyzed. Raman analysis is a few step process, thus requires lesser resources subsequently has skimmed chances of waste generation. As an example, consider the following literature where pathogen detection has been achieved culture free [67]. Sub-10-nm inter-particle gaps, in an anodic aluminium oxide (AAO) were used as a template to grow silver nanoparticles. The nanoparticle carrying AAO were then coated with a layer of vancomycin (van), which serves as an anchor for bacterial cells. Bacterial cells bind to vancomycin by developing hydrogen bonds between cell wall peptidoglycan and the carbonyl and amine groups of vancomycin. Increased bacteria capture probability of vancomycin coated Ag substrate allows efficient bacterial detection. This model was further used to selectively distinguish between van-resistant and susceptible strains of Enterococcus [68]. This culture free detection has been realized for a number of different microbial systems such as E.coli, Staphylococcus epidermidis, Pseudomonas aeruginosa amongst others [69, 70] and have been shown in Figure 6.3. Apart from sensitivity, Raman analysis also has specificity. As an example, consider ultra-sensitive detection of 17ß-estradiol (E2) [71]. E2 is one of the most active estrogens and is typically found associated with deleterious consequences for human and aquatic organisms. Since it is present in a very small concentration in the environment, its detection is commonly difficult. In a contemporary research conducted by Zhao and his colleagues, a highly sensitive and specific aptamer-based SERS biosensing system was develop which could successfully detect E2 presence from water. Plasmonic core-shell nanoparticles tagged with 4-MBA were used as probe molecules to achieve selective E2 detection upto the 0.005 pM. The designed E2 aptamer showed high binding affinity to E2 allowing significant Raman signals acquisition, even in presence of interferents.

180 Integrating Green Chemistry and Sustainable Engineering Antibiotics (e.g. Polymyxin B)

Dead or alive?

Raman intensity / a.u.

E. coli

Wild type K-12 E. coli + Polumyxin B 0 min 5 min 20 min 60 min 240 min

Ag nanoparticles

2 μm

Raman analysis

400

800

1200 1600 Raman shift / cm–1

2000

Figure 6.4 Detection of dead or alive microorganisms with the help of nanotechnology and Raman analysis. (Reprinted with Copyright permission [72])

Considering another prospect, development of biosynthetic and in-situ routes for synthesis of SERS enhancers (metal and non-metal) also subdues ecological toxicity making the overall process cleaner. Alula et al., identified and quantified Mycobacterium smegmatis via Raman spectroscopy by growing silver nanoparticles in-situ within the cell wall [6]. Cell suspensions of different mycobacterial cells were used to chemically nucleate silver nanoparticles directly onto the cell envelope allowing reproducible, repeatable and label free bacterial detection. This method was used to identify different strains as well. In another study, Ag nanoparticle coating bacterial cell walls were used as a model to not only detect but also distinguish live and dead bacteria. The Ag nanoparticles which serve as SERS enhancers were synthesized in-situ within the bacterial cell wall without the use of any additional chemicals. Raman signals acquired from cell suspension of live and dead bacteria were considerably different. Live culture gave strong Raman signals while from dead almost no signals were obtained. A plausible reason would be that dead bacteria might not be able to reduce Ag ions to form Ag nanoparticles while live could. Different chemicals studies were conducted to understand the involved mechanism as shown in Figure 6.4 [68]. Recent few years have seen an upsurge in noble-metal/semiconductor nanomaterials being used for efficient SERS activity in view of synergistic EM and CT enhancement attainable. For example, 4-MBA presence was detected from a recyclable Au-TiO2 nanocomposite substrate where charge transfer from TiO2 to analyte molecule

“Green” Spectroscopy for Ultrasensitive Analyte Detection 181 and CT from LSPR of Au contributed towards improved Raman signals. Enhancement factor (EF) was estimated to be of the order of 104. Utilizing the superior photocatalytic ability of metal oxide systems, Au-TiO2 system was irradiated with UV light to degrade 4-MBA molecules attached on the substrate surface. Following light treatment the system was renewed for subsequent reactions or detections, offering advantages such as recyclability, long service lifetimes and economical benefits [73]. An inexpensive recyclable SERS active substrate was fabricated by growing vertically aligned ZnO nanorods using low temperature hydrothermal method. The grown nanorods were then spot decorated with plasmonic gold nanoparticles to induce EM effect. With good reproducibility, Methylene Blue dye with different concentrations (10–4–10–12 M) was detected. Further, UV light source was used for cleaning the substrate by allowing photocatalytic degradation of the adsorbed dye. The development of such substrates gives them longer shelf life, allows multiple detection and multiplexing subsequently reducing limited manufacturing and ecological footprint [55]. Different metal oxide systems having variety of morphological variations have been used [56, 74–79]. Development of recyclable SERS substrates is not restricted to metal-oxide based systems only. Different classes of metals as-well have been employed to synthesize susbtrate which are re-useable. Gold nanoparticle (Au) decorated silicon nanowire heterostructures were grown as arrays, to be used for SERS measurements. The surface plasmons contributed by Au nanoparticles, enhance optical and optoelectronic properties of the silicon nanowires allowing improved Raman signals. For the optimized Au-Si nanowire substrate an enhancement factor as high as 109 was achieved at analyte (RhodamineB) concentration of 10–11 M. These substrates were selfcleaned using UV light (Figure 6.5) [80]. SERS substrates thus can also find application efficient catalytic resources. Ag2S is comparatively a rare nanomaterial which has been used for SERS based detection of organic dyes, as model pollutants. Ag2S microrod arrays were fabricated by solvothermal method using elemental sulfur and methanol. The developed substrate was used for identification of different types of dyes namely, Methylene blue, Crystal Violet, Rhodamine 6G, Methyl Orange and Sudan I, emphasizing its wide scale applicability [81]. As a further improved system a ZnO-RGO-Au bi-functional nanocomposite system was fabricated, which was used both as a photocatalyst and a SERS substrate for real time quantitative monitoring of the R6G degradation (Figure 6.6) [82].

182 Integrating Green Chemistry and Sustainable Engineering No SERS

SERS

UV photolysis Au-decorated Si nanowire arrays CO2 H2O

Figure 6.5 Recycling behavior of the Au-SiNWA SERS substrate. (Reprinted with Copyright permission [80])

SI wafer

(1)

(2)

ZnO NPs layer

ZnO seed

ZnO nanorod array

Growing ZnO Covering RGO

Calcined Au/RGO/ZnO

HAuCl4 solution

(5)

UV irradiation (4)

ZnO seed ZnO rod

(3)

RGO

RGO/ZnO

UV light

Au NPs

Figure 6.6 Fabrication and monitoring of R6G degradation using ZnO-RGO-Au bi-functional nanocomposite. (Reprinted with Copyright permission [82])

6.2.7.1.3 Design for Separation Another class of materials that have emerged as SERS enhancers are magnetic nanoparticles. In addition to the inherent properties associated with nanoparticles, magnetic nanomaterial has another advantage i.e. separation capabilities. With application of finite electromagnetic field, the particles can easily be separated. This is particularly beneficial for target analyte enrichment allowing easier detection. As example,

“Green” Spectroscopy for Ultrasensitive Analyte Detection 183 (a) Target sequence Capture oligonucleotide probe Reporter oligonucleotide probe Raman reporter molecule (DSNB) Blocking agent (MCH)

Au MNP

GNP

Reporter-GNPs

Capture-Au@MNPs

(b)

785 nm laser

λlaser

(c) LSPR GNP

Au

High SERS

MNP

Au-Au junction “hot spot” (d) S

PR

LS

N Flow

GNP

MNP

Low SERS

Au-iron oxide junction in MNP-based SERS assay

Figure 6.7 Utilization of MNP and GNP based SERS for microRNA biomarkers for cancer diagnosis. (Reprinted with Copyright permission [85]).

consider the following articles [83–85]. Multifunctional popcorn shaped core shell nanoparticles were synthesized and used for magnetic separation of MDR Salmonella DT104 and subsequently its destruction by photothermal therapy. The popcorn core was magnetic while the shell was made of gold and conjugated with M3038 antibody. This nanoconjugate could selectively bind to the Salmonella allowing excellent SERS imaging limit of 102 CFU/ml. Further, these nanopopcorns were irradiated with 1.5 Wcm–2 light source, which resulted in localized and almost 100% killing of the microbe. This bioassay was rapid and directed towards selective separation and killing of pathogenic strains. In a recent report, magnetic nanoparticle-based SERS assay for detection of miR-141, a cancer biomarker has been developed. In the designed scheme two set of nanoparticles were used, where one was gold coated magnetic nanoparticles conjugated with probe sequence while the other was bare gold nanoparticles conjugated with reporter probe sequence and a Raman tag. In presence of miR141, the probe and reporter sequence undergo hybridization, forming a complex with paramagnetic nanoparticles. Resultant hybridization complexes were

184 Integrating Green Chemistry and Sustainable Engineering separated from the reaction mix by magnetic pull down and struck with laser excitation for SERS detection. The scheme viability was achieved in a microfluidic based system with an estimated LOD of 100 fM [85] (Figure 6.7).

6.3

Conclusion and Future Outlook

Raman Spectroscopy, as a bio-analytical technique has found significant acceptance in different aspects of research fields. Un-parallel sensitivity, specificity and multiplexing ability of Raman spectroscopy makes it an excellent tool for medical studies. Major limitation of Raman which is poor cross-sections has been resolved to considerable extent by the advent of nanoscale materials and development of SERS. Present SERS substrates or enhancers have the capability to enrich, separate and detect analytes at ultralow concentration. Last decade has seen an upsurge in the advances accompanying Raman instrumentation as well, contributing significantly to establishment of several proof of principles, carrying out single molecule detections and even imaging. Although in its infancy, Raman Spectroscopy and imaging has potential to influence future medicine, that too sustainably. Green approaches towards development of nanoparticles or fabrication of re-useable substrates adds to reduced environmental footprint up-regulating its sustainability quotient. However, significant global acceptance of SERS for medical diagnosis and characterization is also restricted by effectiveness of innovation on larger scale, scalability of the tool in itself, financial assistance and less resources.

References 1. Emerich, D.F., Thanos, C.G., Nanotechnology and medicine. Expert Opin. Biol. Ther., 3(4), 655–663, 2003. 2. Ghosal, A., Mishra, A., Tiwari, S., Polymers and Nanocomposites for Biomedical Applications. Nova Science Publisher, 2017. 3. Ghosal, A., Tiwari, S., Mishra, A., Vashist, A., Rawat, N.K., Ahmad, S., Design and Engineering of Nanogels. Nanogels for Biomedical Applications. pp. 9–28, 2017. 4. Mishra, A., Ghosal, A., Synthesis and Applications of Biopolymeric Nanoparticles. Nova Science Publisher, 2017. 5. Vashist, A., Kaushik, A., Jayant, R.D., Vashist, A., Ghosal, A., Nair, M., Hydrogels: Stimuli Responsive to on-Demand Drug Delivery Systems. Kaushik,

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6.

7. 8. 9. 10.

11.

12. 13. 14. 15. 16.

17. 18. 19. 20. 21.

22.

23.

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186 Integrating Green Chemistry and Sustainable Engineering 24. Mukherjee, A., Shim, Y., Myong Song, J., Quantum dot as probe for disease diagnosis and monitoring. Biotechnol. J., 11(1), 31–42, 2016. 25. Chamorro-Garcia, A., Merkoçi, A., Nanobiosensors in diagnostics. Nanobiomedicine, 3, 2016. https://doi.org.10.1177/1849543516663574. 26. Sahoo, S.K., Labhasetwar, V., Nanotech approaches to drug delivery and imaging. Drug Discov. Today, 8(24), 1112–1120, 2003. 27. Bamrungsap, S., Zhao, Z., Chen, T., Wang, L., Li, C., Fu, T., et  al., Nanotechnology in therapeutics: a focus on nanoparticles as a drug delivery system. Nanomedicine, 7(8), 1253–1271, 2012. 28. Shi, J., Votruba, A.R., Farokhzad, O.C., Langer, R., Nanotechnology in drug delivery and tissue engineering: from discovery to applications. Nano Lett., 10(9), 3223–3230, 2010. 29. Ceramics for Drug Delivery. Bio‐Ceramics with Clinical Applications. 30. Torchilin, V.P., Lipid-core micelles for targeted drug delivery. Curr. Drug Deliv., 2(4), 319–327, 2005. 31. Madaan, K., Kumar, S., Poonia, N., Lather, V., Pandita, D., Dendrimers in drug delivery and targeting: Drug-dendrimer interactions and toxicity issues. J. Pharm. Bioallied Sci., 6(3), 139, 2014. 32. Medina, O.P., Zhu, Y., Kairemo, K., Targeted liposomal drug delivery in cancer. Curr. Pharm. Des., 10(24), 2981–2989, 2004. 33. Dreaden, E.C., Austin, L.A., Mackey, M.A., El-Sayed, M.A., Size matters: gold nanoparticles in targeted cancer drug delivery. Ther. Deliv., 3(4), 457–478, 2012. 34. Huang, Q., Yu, H., Ru, Q., Qingrong, H., Hailong, Y., Qiaomei, R., Bioavailability and delivery of nutraceuticals using nanotechnology. J. Food Sci., 75(1), R50–R57, 2010. 35. Torney, F., Trewyn, B.G., Lin, V.S., Wang, K., Mesoporous silica nanoparticles deliver DNA and chemicals into plants. Nat. Nanotechnol., 2(5), 295–300, 2007. 36. Duncan, T.V., Applications of nanotechnology in food packaging and food safety: barrier materials, antimicrobials and sensors. J. Colloid Interface Sci., 363(1), 1–24, 2011. 37. Tankhiwale, R., Bajpai, S.K., Preparation, characterization and antibacterial applications of ZnO-nanoparticles coated polyethylene films for food packaging. Colloids Surf. B Biointerfaces, 90, 16–20, 2012. 38. Sun, Z., Du, J., Yan, L., Chen, S., Yang, Z., Jing, C., Multifunctional Fe3O4@ SiO2-Au Satellite Structured SERS Probe for Charge Selective Detection of Food Dyes. ACS Appl. Mater. Interfaces, 8(5), 3056–3062, 2016. 39. Wang, Y., Ravindranath, S., Irudayaraj, J., Separation and detection of multiple pathogens in a food matrix by magnetic SERS nanoprobes. Anal. Bioanal. Chem., 399(3), 1271–1278, 2011. 40. Marra, A., Silvestre, C., Duraccio, D., Cimmino, S., Polylactic acid/zinc oxide biocomposite films for food packaging application. Int. J. Biol. Macromol., 88, 254–262, 2016.

“Green” Spectroscopy for Ultrasensitive Analyte Detection 187 41. Sozer, N., Kokini, J.L., Nanotechnology and its applications in the food sector. Trends Biotechnol., 27(2), 82–89, 2009. 42. Shibata, T., Method for producing green tea in microfine powder. Google Patents, 2002. 43. Zhang, L., Webster, T.J., Nanotechnology and nanomaterials: Promises for improved tissue regeneration. Nano Today, 4(1), 66–80, 2009. 44. Thota, S., Crans, D.C., Metal Nanoparticles: Synthesis and Applications. Pharmaceutical Sciences. John Wiley & Sons, 2018. 45. Zhang, Y., Hong, H., Cai, W., Imaging with Raman spectroscopy. Curr. Pharm. Biotechnol., 11(6), 654–661, 2010. 46. Walter, A., März, A., Schumacher, W., Rösch, P., Popp, J., Towards a fast, high specific and reliable discrimination of bacteria on strain level by means of SERS in a microfluidic device. Lab Chip, 11(6), 1013–1021, 2011. 47. Yu‐San, H., Takeshi, K., Masayuki, Y., Hiro‐o, H., Molecular‐level pursuit of yeast mitosis by time‐ and space‐resolved Raman spectroscopy. Journal of Raman Spectroscopy, 34, 1–3, 2003. 48. Yu, K.N., Lee, S.M., Han, J.Y., Park, H., Woo, M.A., Noh, M.S., et  al., Multiplex targeting, tracking, and imaging of apoptosis by fluorescent surface enhanced Raman spectroscopic dots. Bioconjug. Chem., 18(4), 1155– 1162, 2007. 49. Pudlas, M., Koch, S., Bolwien, C., Walles, H., Raman spectroscopy as a tool for quality and sterility analysis for tissue engineering applications like cartilage transplants. Int. J. Artif. Organs, 33(4), 228–237, 2010. 50. Colthup, N., Introduction to infrared and Raman spectroscopy. Elsevier, 2012. 51. Hong, S., Li, X., Optimal size of gold nanoparticles for surface-enhanced Raman spectroscopy under different conditions. Journal of Nanomaterials, 2013(2), 49–9, 2013. 52. Liu, H., Zhang, L., Lang, X., Yamaguchi, Y., Iwasaki, H., Inouye, Y., et  al., Single molecule detection from a large-scale SERS-active Au79Ag21 substrate. Sci. Rep., 1, 112, 2011. 53. Cao, Q., Liu, X., Yuan, K., Yu, J., Liu, Q., Delaunay, J.-J., et al., Gold nanoparticles decorated Ag(Cl,Br) micro-necklaces for efficient and stable SERS detection and visible-light photocatalytic degradation of Sudan I. Applied Catalysis B Environmental, 201, 607–616, 2017. 54. Kochuveedu, S.T., Jang, Y.H., Kim, D.H., A study on the mechanism for the interaction of light with noble metal-metal oxide semiconductor nanostructures for various photophysical applications. Chem. Soc. Rev., 42(21), 8467–8493, 2013. 55. Sinha, G., Depero, L.E., Alessandri, I., Recyclable SERS substrates based on Au-coated ZnO nanorods. ACS Appl. Mater. Interfaces, 3(7), 2557–2563, 2011. 56. Kandjani, A.E., Mohammadtaheri, M., Thakkar, A., Bhargava, S.K., Bansal, V., Zinc oxide/silver nanoarrays as reusable SERS substrates with controllable 'hot-spots' for highly reproducible molecular sensing. J. Colloid Interface Sci., 436, 251–257, 2014.

188 Integrating Green Chemistry and Sustainable Engineering 57. Gill, H.S., Thota, S., Li, L., Ren, H., Mosurkal, R., Kumar, J., Reusable SERS active substrates for ultrasensitive molecular detection. Sensors and Actuators B: Chemical, 220, 794–798, 2015. 58. Abu Hatab, N.A., Oran, J.M., Sepaniak, M.J., Surface-enhanced Raman spectroscopy substrates created via electron beam lithography and nanotransfer printing. ACS Nano, 2(2), 377–385, 2008. 59. Frame, L., Brewer, J., Lee, R., Faulds, K., Graham, D., Development of a labelfree Raman imaging technique for differentiation of malaria parasite infected from non-infected tissue. Analyst (Lond)., 143(1), 157–163, 2018. 60. Qian, Y., Fan, T., Yao, Y., Shi, X., Liao, X., Zhou, F., et al., Label-free and Raman dyes-free surface-enhanced Raman spectroscopy for detection of DNA. Sensors and Actuators B Chemical, 254, 483–489, 2018. 61. Zhang, C., Winnard, P.T., Dasari, S., Kominsky, S.L., Doucet, M., Jayaraman, S., et  al., Label-free Raman spectroscopy provides early determination and precise localization of breast cancer-colonized bone alterations. Chem. Sci., 9(3), 743–753, 2018. 62. Barhoumi, A., Halas, N.J., Label-free detection of DNA hybridization using surface enhanced Raman spectroscopy. J. Am. Chem. Soc., 132(37), 12792– 12793, 2010. 63. Heraud, P., Cowan, M.F., Marzec, K.M., Møller, B.L., Blomstedt, C.K., Gleadow, R., Label-free Raman hyperspectral imaging analysis localizes the cyanogenic glucoside dhurrin to the cytoplasm in sorghum cells. Sci. Rep., 8(1), 2691, 2018. 64. Mignolet, A., Wood, B.R., Goormaghtigh, E., Intracellular investigation on the differential effects of 4 polyphenols on MCF-7 breast cancer cells by Raman imaging. Analyst (Lond)., 143(1), 258–269, 2018. 65. Li, Y., Han, X., Yan, Y., Cao, Y., Xiang, X., Wang, S., et al., Label-Free Detection of Tetramolecular i-Motifs by Surface-Enhanced Raman Spectroscopy. Anal. Chem., 90(5), 2996–3000, 2018. 66. Chen, X., Liu, Y., Huang, J., Liu, W., Huang, J., Zhang, Y., et al., Label-free techniques for laboratory medicine applications. Frontiers in Laboratory Medicine, 1(2), 82–85, 2017. 67. Shanmukh, S., Jones, L., Driskell, J., Zhao, Y., Dluhy, R., Tripp, R.A., Rapid and sensitive detection of respiratory virus molecular signatures using a silver nanorod array SERS substrate. Nano Lett., 6(11), 2630–2636, 2006. 68. Liu, T.Y., Tsai, K.T., Wang, H.H., Chen, Y., Chen, Y.H., Chao, Y.C., et  al., Functionalized arrays of Raman-enhancing nanoparticles for capture and culture-free analysis of bacteria in human blood. Nat. Commun., 2, 538, 2011. 69. Wu, X., Chen, J., Li, X., Zhao, Y., Zughaier, S.M., Culture-free diagnostics of Pseudomonas aeruginosa infection by silver nanorod array based SERS from clinical sputum samples, Nanomedicine: Nanotechnology. Biology and Medicine, 10, 1863–1870, 2014.

“Green” Spectroscopy for Ultrasensitive Analyte Detection 189 70. Zhou, H., Yang, D., Ivleva, N.P., Mircescu, N.E., Niessner, R., Haisch, C., SERS detection of bacteria in water by in situ coating with Ag nanoparticles. Anal. Chem., 86(3), 1525–1533, 2014. 71. Liu, S., Cheng, R., Chen, Y., Shi, H., Zhao, G., A simple one-step pretreatment, highly sensitive and selective sensing of 17β-estradiol in environmental water samples using surface-enhanced Raman spectroscopy. Sensors and Actuators B Chemical, 254, 1157–1164, 2018. 72. Zhou, H., Yang, D., Ivleva, N.P., Mircescu, N.E., Schubert, S., Niessner, R., et al., Label-Free in Situ Discrimination of Live and Dead Bacteria by SurfaceEnhanced Raman Scattering. Anal. Chem., 87(13), 6553–6561, 2015. 73. Tian, F., Zhang, Y., Zhang, J., Pan, C., Raman Spectroscopy: A New Approach to Measure the Percentage of Anatase TiO2 Exposed (001) Facets. J. Phys. Chem. C, 116(13), 7515–7519, 2012. 74. Ying, Z., Jin, C., Li, Z., Liangbao, Y., Multifunctional TiO2‐Coated Ag Nanowire Arrays as Recyclable SERS Substrates for the Detection of Organic Pollutants. Eur. J. Inorg. Chem., 2012, 3176–3182, 2012. 75. Liangbao, Y., Zhiyong, B., Yucheng, W., Jinhuai, L., Clean and reproducible SERS substrates for high sensitive detection by solid phase synthesis and fabrication of Ag‐coated Fe3O4 microspheres. Journal of Raman Spectroscopy, 43, 848–856, 2012. 76. Xie, Y., Meng, Y., Wu, M., Visible-light-driven self-cleaning SERS substrate of silver nanoparticles and graphene oxide decorated nitrogen-doped titania nanotube array. Surf. Interface Anal., 48(6), 334–340, 2016. 77. Ding, Q., Zhang, L., Yang, L., A simple approach for the synthesis of Ag-coated Ni@TiO2 nanocomposites as recyclable photocatalysts and SERS substrate to monitor catalytic degradation of dye molecules. Mater. Res. Bull., 53, 205–210, 2014. 78. Hu, H., Wang, Z., Wang, S., Zhang, F., Zhao, S., Zhu, S., ZnO/Ag heterogeneous structure nanoarrays: Photocatalytic synthesis and used as substrate for surface-enhanced Raman scattering detection. J. Alloys Compd., 509(5), 2016–2020, 2011. 79. Tao, Q., Li, S., Zhang, Q.Y., Kang, D.W., Yang, J.S., Qiu, W.W., et al., Controlled growth of ZnO nanorods on textured silicon wafer and the application for highly effective and recyclable SERS substrate by decorating Ag nanoparticles. Mater. Res. Bull., 54, 6–12, 2014. 80. Yang, X., Zhong, H., Zhu, Y., Shen, J., Li, C., Ultrasensitive and recyclable SERS substrate based on Au-decorated Si nanowire arrays. Dalton Trans., 42(39), 14324–14330, 2013. 81. Cao, Q., Che, R., Chen, N., Facile and rapid growth of Ag2S microrod arrays as efficient substrates for both SERS detection and photocatalytic degradation of organic dyes. Chem. Commun. (Camb.)., 50(38), 4931–4933, 2014. 82. Wen, C., Liao, F., Liu, S., Zhao, Y., Kang, Z., Zhang, X., et al., Bi-functional ZnO-RGO-Au substrate: photocatalysts for degrading pollutants and SERS

190 Integrating Green Chemistry and Sustainable Engineering substrates for real-time monitoring. Chem. Commun. (Camb.)., 49(29), 3049–3051, 2013. 83. Fan, Z., Senapati, D., Khan, S.A., Singh, A.K., Hamme, A., Yust, B., et  al., Popcorn-shaped magnetic core-plasmonic shell multifunctional nanoparticles for the targeted magnetic separation and enrichment, label-free SERS imaging, and photothermal destruction of multidrug-resistant bacteria. Chemistry, 19(8), 2839–2847, 2013. 84. Chang, Z.-min., Wang, Z., Shao, D., Yue, J., Lu, M.-meng., Li, L., et  al., Fluorescent-magnetic Janus nanorods for selective capture and rapid identification of foodborne bacteria. Sensors and Actuators B Chemical, 260, 1004–1011, 2018. 85. Zhang, H., Yi, Y., Zhou, C., Ying, G., Zhou, X., Fu, C., et al., SERS detection of microRNA biomarkers for cancer diagnosis using gold-coated paramagnetic nanoparticles to capture SERS-active gold nanoparticles. RSC Adv., 7(83), 52782–52793, 2017.

7 Microwave Synthesized Conducting Polymer-Based Green Nanocomposites as Smart Promising Materials Neha Kanwar Rawat* and P.K Panda Materials Science Division, CSIR-National Aerospace Laboratory, Bengaluru, India

Abstract: The Green Chemistry and Engineering focusses to improve the present globe today and for future generations, by imparting sufficient pioneering solutions and practices to accomplish sustainability challenges, while simultaneously fulfilling social, economic, and environmental goals. Midst varying polymers, conducting polymers (CPs) are considered as green polymers, due to their exhilarating creative properties, they have developed as one of the most eminent members in the field of green chemistry and engineering. The new promising technology of using Microwave (MW) radiations for synthesis and processing using MW-assisted heating, under controlled conditions has been shown to be a valuable technology for CPs as well. The significant reduction in their reaction times – typically from days or hours to minutes or even seconds. They have prominent applications in varying arenas of super capacitors, sensors, electronic materials, drug release, biosensors, radar absorbing materials, aerospace applications, tissue engineering, as well as stimuli responsive and bio mimetic polymeric materials. The chapter discusses role of conducting polymers and nanocomposites, with their wide applications as nanocomposites synthesized by green approach viz. microwave irradiation and their broad contribution in materials science and engineering with their emerging prospects. Keywords: Conducting polymers, microwave synthesis, green chemistry, nanocomposites, material applications

*Corresponding author: [email protected];[email protected] Shahid-ul-Islam (ed.) Integrating Green Chemistry and Sustainable Engineering, (191–214) © 2019 Scrivener Publishing LLC

191

192 Integrating Green Chemistry and Sustainable Engineering

7.1

Introduction

Green chemistry or sustainable chemistry, is a part of chemistry and chemical engineering dedicated to the design of products and practices that reduces the usage and generation of hazardous matter (solid, liquid or gas). The main focus relies, in reduction of pollution by declining consumption of non-renewable resources via development of suitable advancement in technology. The overarching objectives of green chemistry comprises design and development of efficient technology for the design of materials their processing via safe, green and economical pathways. Almost all branches of Chemistry are following these principals, prominently polymer chemistry which plays a substantial role for producing green and safe products via sustainable route. Conducting polymers (CPs) have become eminent part of the polymer Science and Engineering since their emergence three decades back. These family of polymers, have also been influenced by green and sustainable technology, considered as green polymers. CPs has generated new projections towards design of polymer composites having potential for a wide spectrum of applications [1–4]. Copious noteworthy advances, have been found till date, and more technological challenges await the optimization of a system to fill the gap between prospects and practical performance. Despite this tremendous progress, challenging issues related to their processibility and behavior phenomenon within a polymer matrix have been researched and many are still in progress. Therefore, the formation of CPs/ functional polymer composites would be a meaningful way to improve or extend the properties of these polymers. Some shortcomings like poor processibility can be modified by combining these carbon materials as discussed above, with other things, in form of their composites, hybrids or IPNs etc [1, 5–7]. Conducting polymers since long has been considered as prominent carbon fillers in almost all material science areas, owing to their easy synthesis [1, 8]. They have been given promising name as green fillers, as the amount used in a material is of very less quantity and fillers dispersed/ doped belongs to nano range, hence they are known as nanoCPs. The features like processibility and numerous marvelous properties, which make them strong prospective candidate. Conducting polymers (CPs), find vast applications in the field of fuel cells, computer displays, super capacitors, corrosion resistant coatings and many other applications [9–12]. These versatile polymers has many ways to be synthesized, which

Green Nanocomposites as Smart Promising Materials 193 includes alone as polymers, interpenetrating polymer networks, composites, hybrids, and as nanostructured materials [4]. Among the newly developed synthesis techniques, MW technique has been found to be highly chemical/energy efficient, time saving and ecofriendly in nature. A scant literature is available on the MW assisted synthesis of conducting polymers. Some of them includes, in case of substituted Polythiophne (PTh) as reported by some researchers in case of CPs like PANI, PTh and their derivatives polyterthiophenes. Melluci et al., reported synthesis of PTh oligomers via Suzuki coupling reactions [13]. Moreover, Rawat et al., reported, the MW synthesis of unsubstituted PTh in aqueous medium, which is renewable and green in nature as during the synthesis no harmful VOCs are produced and synthesis was done in aqueous medium [14]. Our group have been working on their MW synthesis, more than half a decade, recent work depicting synthesis and application studies of conducting Poly(o-phenyldiamine) (PDA), following seven principles of “green chemistry” approach. This includes less hazardous chemical synthesis, atom economy, prevention, safer solvents, design as energy efficient process, reduction of side products and remarkably, eco-friendly nature. The synthesis using aqueous medium proved to be green, renewable and free from harmful VOCs. We had also demonstrated the influence of MW irradiation on various properties of Poly(o-phenyldiamine)(mPDA) and its comparison with conventionally polymerized PDA(cPDA) [15]. Mrija et al., successfully synthesized, PANI nanoparticles, with the usage of MW waves. They have been involved in MW synthesis of CPs via MW reactor since a decade [16–18] (Figure 7.1). In this chapter, we focus on the preparation, characterization of fascinating conducting polymer nanocomposites and their novel properties and applications. This presents the development of existing approaches in

Figure 7.1 For reproduction of material from ACS-JPC-C: [17] - Reproduced by permission ACS.

194 Integrating Green Chemistry and Sustainable Engineering R H N

*

*

*

*

*

n

N

n

n Polyaniline

Polypyrrole

*

S Polythiphenes

H R1

O

O * *

*

S

*

R2

n

Polyethylenedioxythiophene

n Poly (p-Phenylene vinylene)s

Figure 7.2 Various conducting polymers with their structures.

the arena of CPs and their composites where fillers are conducting polymers via sustainable and green routes viz. MW irradiation, with emphasis being located on the recent progress, exciting advances and active pursuit in this growing field of green chemistry and engineering.

7.2

Brief Introduction of Conducting Polymers

7.2.1

CPs

Conducting polymers, are considered as “synthetic metals”, having highly alternate double bond and conjugated polymeric chain [1], having delocalization of electrons. Owing to such a great discovery, 2000 Noble prize in Chemistry, was given to, Alan J. Heeger, Alan G. MacDiarmid and Hideki Shirakawa [2–4]. They have classified in varying families which includes polyacetylene (PA), polyaniline (PANI), polypyrrole (PPy), polythiophene (PTh), poly(para-phenylene) (PPP), poly(phenylenevinylene) (PPV), polyfuran (PF), etc. They have varying chemical structures of that contributes in giving them different names, illustrated in Figure 7.2. According to electronic theory, they can be considered as electrical insulators, semiconductors or conductors, depending on the level of doping from external source or during synthesis and nature of the dopants. Addition of some optimized and selected dopants and/or subjecting to chemical or electrochemical redox reactions, can increase the electrical conductivity of these conjugated

Green Nanocomposites as Smart Promising Materials 195 polymers by some orders of magnitude. For example, the emeraldine base PANI (undoped form) is an insulator having conductivity, of only 10-10 to 10-8 S/cm, on comparing after doping with some agents like acids, carbon fillers etc., increase to 102 to 103 S/cm or higher. The secret behind their such a versatile use is their phenomenon, known as doping which creates these promising materials with high conductivity, producing their wide range applications in electro-optic, molecular and nano electronic devices to microwave absorption and corrosion protection coatings. Additionally, they have the ability of the reversible doping–de doping which provides them advantage in applications areas like sensors, actuators and separation membranes [19], [20] Table 7.1, Figure 7.2 and Figure 7.3.

7.2.2 Synthesis of Conducting Polymers The synthesis of CPs is performed generally by two main methods: chemical and electrochemical polymerization phenomenon’ as depicted in (Figure 7.4). Out of the two, the most common method is electrochemical polymerization in which their monomers are polymerized at the electrode surface and electrolyte interface in the presence of a catalyst, (i.e., an oxidant such as FeCl3, BaCl2, oxides and inorganic chlorides). The key requirement for CPs to conduct electricity is their oxidation/ reduction phenomenon by ionizing their polymer backbone. This provides the extra ions commonly known as dopant ions, which can be introduced either during the synthesis reaction of CPs (in situ) or after it (ex situ). The doping process mainly occurs by either of the two approaches comprising simple mixing or chemical immobilization of these external ions, i.e., dopants on the backbone of the polymer. In biomedical field, doping has a significant role, which can work by two modes, firstly when dopant is low molecular weight (small ion) and the non-covalent interactions are present, it leaches out of the CP matrix into the biological milieu resulting in reduction of conductivity of polymer. This phenomenon is used for CP-based drug delivery devices which are based on the principal of expulsion of the biologically active dopant from the material on electrical stimulation. Secondly, in case of high molecular weight dopants like paratoluene sulphonic acid(PTSA) [30], dodecyl benzene sulfonic acid (DBSA) and when covalent bonds are present the polymer is known to function by self-doping process. Above all, other highly efficient synthesis technique, which are widely used is Microwave synthesis. Microwave chemistry is the science of performing synthesis in which microwave radiation is only used to carry out chemical reactions. Microwaves creates very high frequency electric fields inside the reaction process and produces heat any material

Abbreviation

PANI [1]

PPY [20]

PTh [21, 22]

PoA [23–25]

PEDOT [26–28]

PPV [29]

PFu [1, 11]

Polymer

Polyaniline

Polypyrrole

Polythiophene

Poly(o-ansidine)

Poly(3,4ethylenedioxythiophene)

Polyparavinylene

Polyfuran

102

0.1–10

Hetero 1–102 aromatic

Amine

Hetero 102-–103 aromatic

Amine

green

brown, brick

yellow, blue

dark green

brown, brick

Hetero 102–103 aromatic

green, black

Color

green, blue

102

Conductivity(in order)

Hetero 10–103 aromatic

Amine

Family

small chains, easy fusible

easy synthesis, processibility

Highly conducting

easy synthesis, processibility

cost effective, bio-degradable

varying forms, cost effective, easy modification

cost effective, easy synthesis, processibility

Advantages

Applications

neural probes

bio-electronics sensors FETs

biosensors

Biocompatibility Sensors super capacitors

Small ring size easy bio degradability to modify limited application

Easy fusible rings

prohibitive cost, nonbiodegradable

less processible, bulky side group

prohibitive cost, nonbiodegradable

health hazards, low Hydrogels processibility Corrosion EMI shielding

less Biosensors biodegradability Corrosion EMI shielding

ShortComings

Table 7.1 CPs and heir properties including prominent works in varying field [9] (Reproduced with permission from NOVA science publishers).

196 Integrating Green Chemistry and Sustainable Engineering

Green Nanocomposites as Smart Promising Materials 197 Multiple redox states

Electrochromic devices

Tunable properties

Conducting polymerCNT nanocomposites

Tissue engineering

Actuators

Processibility Biocompatibility

Corrosion protection

Biosensors High surface to volume ratio

Super capacitors

High aspect ratio

Solar cells

Dopingdoping behavior

Figure 7.3 Various properties and applications of MW synthesized CPs and nanocomposites (Rawat et al., reproduced with permission from Apple Science publishers book series on Nanotechnology-In Press).

Green solvent methanol No surfactant/template Polymerization MWCNTs Incident wave (E1, H1) Reflected wave (E1, H1)

Aniline MWCNTs-CPs-BaTiO3 based nano composites

SHIELD

Transmitted wave (E1, H1)

MWCNT-CP-BaTiO3 nano composite coated Fabric

Scheme 7.1 Showing Aniline as their application as EMI shielding materials, In press Neha Rawat et al.

198 Integrating Green Chemistry and Sustainable Engineering Synthesis

Electrochemical polymerization Chemical polymerization

WE SS

Emulsion polymerization

Microwave synthesis

RE

CE Pt

Aqueous phase

Organic phase GO Aniline

Template synthesis

Figure 7.4 Classification of CPs.

containing electric charges due to presence of polarity in a solvent or conducting ions present inside a solid. This chapter purely discusses how, MW synthesis have made polymer synthesis (in general) and conducting polymers (here), so easy now with details and discussions about present prospects in this branch of chemistry (Figure 7.4).

7.3

Microwave Synthesis

Microwave synthesis is aptly known as Chemistry at a speed of light. Since 1980s. MW synthesis started by the groups of Gedye and Giguere/Majetich in 1986, has become an important part and integral of scientific community [31]. With a passage of time, microwave irradiation to carry out chemical transformations increased, till now more than 3500 articles have been published in this fast moving and rousing field. The significance of microwave heating relies on reduce reaction times, rise in product yields and enhance product purities by subsiding unwanted side reactions compared to conventional heating methods. The benefits of this enabling technology have, more recently, also been exploited in the perspective of multistep total synthesis and bio-medical engineering, drug discovery [32], and new innovative fields such as polymer synthesis, material science, nanotechnology and biological technology/engineering This rapid usage of MW irradiation in chemistry has thus become such a widespread procedure in the scientific community that it might be assumed, in a few years, most chemists will probably use microwave energy to perform/heat chemical reactions on a laboratory scale. Above

Green Nanocomposites as Smart Promising Materials 199 all, any chemical reaction that requires heat can be performed under microwave conditions, has today been generally accepted, as a fact by the scientific community. Earlier, the motive of using MW synthesis (Figure  7.1 showing Polyaniline synthesis using MW), was only solution for researchers where some reactions needs less completion time, they had failed. In other words, origination of MW synthesis was need of the hour at that time. In present time, MW synthesis has become an integral part of any routine synthetic transformations in every laboratory, all over the world. Due to discussed advantages, this innovative technology has an unbeatable future asset, the current situation is bound to alter over the next coming years and less expensive equipment would become available to promote its growing availability. The major drawbacks of this fascinating innovative technology, are the cost of microwave reactors but this current situation is bound to change over the next several years and less expensive equipment should be accessible.

Conduction mechanism +

Array of spherical globular particles (30±5 nm)

s +

s

s

s

Polarons (radical cations)

s

s

s

s

Bipolarons (dication)

Conventional heating (Hours to days)

n + n

Well dispersed spherical particles (10+1 nm) Microwave heating (Seconds to minutes)

Conventional methods Bulk to atomic level

Microware irradiation Atomic level to bulk

Monomers

Polymerization

Micelle formation

mPTH/epoxy coating

200 x

Current density/ A cm

–2

cPTh/epoxy coating

0.01

pH7

1E-3 1E-4 1E-5 1E-6 1E-7 1E-8

CS epoxy-PA cPTh/epoxy-PA mPTh/epoxy-PA

1E-9

200 x –0.55 –0.50 –0.45 –0.40 –0.35 –0.30 –0.25 Potential/ (V) vs. (Ag/AgCI)

Figure 7.5 MW synthesized PTh, reported for first time and their epoxy based coatings, for reproduction of material from RSC Advances: [14] - Reproduced with permission of The Royal Society of Chemistry (RSC).

200 Integrating Green Chemistry and Sustainable Engineering By then, microwave reactors will have truly become the Bunsen burners of present times and will be significantly abundant equipment in every chemical laboratory. Fast advanced techniques in microwave reactors has made heating of reaction mixtures, to such a high speed with maintaining high pressures as well as elevated temperatures – in other words relative 10 times the factor of far above the boiling point of the solvents used during course of reactions. This method significantly reduces reaction times from several hours down to only a few minutes. This rate acceleration is particularly based on the well-known chemical law by Arrhenius known by his name, which states as a rule of thumb that the reaction rate is doubled as the reaction temperature increases by 10 °C [33].

7.3.1 Principle of MW Heating (Figure 7.5) Microwave chemistry has been a very interesting topic to study its mechanism by researchers across world. The basis of MW heating is efficient heating of materials (in most cases solvents) by known as significant method of dielectric heating and its causing effects. Dielectric heating works via two major mechanisms: Dipolar polarization [34] The requisite condition to generate heat or to be irradiated is its dipole moment property, i.e. its molecular structure must be partly negatively and partly positively charged or both. Since the microwave field is oscillating, the dipoles in the field align to the oscillating field. This alignment originates necessary rotation, which leads to friction and eventually in heat energy. Ionic conduction During ionic conduction, dissolved charged particles i.e. ions, they oscillate back and forth under the influence of microwave irradiation. This oscillation causes collisions of the charged particles with neighboring molecules or atoms, which are ultimately responsible for creating heat energy. As an example: if two samples are taken one having dipole moment and other having not, one with dipole moment will be irradiated fast by MW.

Green Nanocomposites as Smart Promising Materials 201 10m

10cm

1mm Microwaves

Radio

10μm

100μm

IR

UV

1nm

10fm Hard X-rays

Soft X-rays

Y-rays

Intermolecular vibration Intramolecular vibration Molecular rotation THz Gap

Electronics 106 (MHz)

108

1010 (GHz)

1012 (THz)

Photonics 1014

1016 (PHz)

1018 (EHz)

1020

Figure 7.6 showing frequency bands for various electromagnetic waves.

7.3.2 Dielectric Properties The MW efficient materials (Figure 7.6) are synonym with what we called dielectric materials. As the term “dielectric heating” signifies, a material having certain dielectric [35, 36] properties in order to be efficiently heated in the microwave range of frequency or field. The heating characteristics of a particular material under microwave irradiation conditions are purely reliant on the ability of a specific material to change electromagnetic energy into heat. This ability is determined by the so-called loss tangent, known as tan δ [37].

7.3.3

Significance of Tan δ

Literally, dielectric constant is a measure of the charge retention capacity of a medium. The dielectric loss tangent is defined by the angle between the capacitor's impedance vector and the negative reactive axis, as described in terms of angle notations or geometrical view. It determines of the conditions based on medium. Like dielectric constant, low loss tangents result in a "fast" substrate while large loss tangents result in a "slow" substrate. The dielectric constant can be calculated using: ε = Cs/ Cv , where Cs is the capacitance with the specimen as the dielectric, and CV is the capacitance with a vacuum as the dielectric. The dissipation factor can be calculated using: D = tan δ = cot θ = 1/(2π f RpCp), where δ is the loss angle, θ is the phase angle, f is the frequency, Rp is the equivalent parallel resistance, and Cp is the equivalent parallel capacitance.

202 Integrating Green Chemistry and Sustainable Engineering

(a)

(d)

(b)

(e)

(c)

(f)

Figure 7.7 SEM micrographs of 2ABA-PANI after 20 min under MW (a) 2:1, (b) 1:1, (c) 1:2; and CS (d) 2:1, (e) 1:1, and (f) 1:2 mole ratios. Reproduced with permission from ACS Physical Chemistry [17].

7.3.4

Advantages of Microwave Over Conventional Heating

The organic synthesis is a process in which a reaction is performed by refluxing a reaction mixture using a heat source from a hot oil bath. This method has been most favorite among researchers worldwide, although its slow and energy inefficient, transferring heat to vessel and then to contents of vessel on heating. This method is not good for sensitive materials, which needs no overheating phenomenon. In contrast, microwave irradiation effects in energy efficient internal heating via direct coupling of microwave energy with dipoles and/or ions which forms constituent in the reaction mixture (Figure 7.7). Microwaves permits through the vessel wall and heat the reaction mixture on a

Green Nanocomposites as Smart Promising Materials 203 molecular level by direct interaction with the molecules (reactants: monomers, solvents, reagents, catalysts, etc. Table 7.2, Figure 7.8) . Due to the inside heating or so-called core heating of vessel during reaction, MW makes it at such a level, that there is no occurrence of any temperature gradient, as compared to a conventionally heated system. Hence, the conversion of electromagnetic energy into heat energy works highly proficiently and outcomes in awfully fast heating rates-not reproducible with conventional heating. As the speed of overall process is extremely high, so formation of by-products is suppressed in this case. On comparison of both conventional and MW processes, another huge advantage of microwave heating, is higher product yields with a simple work-up.

7.4

Literature/Research Present

7.4.1 PANI and Derivatives Marija et al., successfully synthesized nanofibers of PANI for the first time under microwave radiation in a relatively high yield (76.2%) as compared to conventional synthesis. The morphology was confirmed by SEM and the PANI structure by FTIR and Raman. This method provides a convenient and environmentally friendly method for PANI nanostructures. Because of the high penetration depth of microwave radiation and specific heating mechanisms the effects resulted in enhanced reaction kinetics. The research group hypothesized about how both non-thermal and thermal effects could be responsible for the fast kinetics and effects on morphology. The effect of microwave irradiation on chemical processes is still debatable [17]. Marija et al., via facile and fast microwave- (MW) assisted synthesized copolymers of aniline and 2-aminobenzoic acid (2ABA) or 2-aminosulfonic acid (2SULFO) by chemical polymerization of several mole ratios of aniline to functionalized aniline (FA). MW synthesized PANI and derivatives, CPs were found to be promising radical scavengers [16]. The same research group efficiently again performed a series of energyand time-efficient enhanced MW syntheses (EMS) of polyaniline (PANI). The syntheses were performed at different MW levels keeping reaction system at a constant temperature of 24 ± 1°C, with the samples extracted after 10 min of start of reaction. Their studies revealed that the molecular weight of the microwave-generated materials is proportional on applied power: the higher the power level, the greater the molecular weight [17].

Synthesis methods

Microwave

Microwave

Microwave

Microwave

Microwave

Microwave

Polymer

Polyaniline

Polypyrrole

Polythiophene

Polyaniline Derivatives like Poly(o-ansidine), Poly(o-phenyldiammine),

Poly(3,4-ethylenedioxythiophene)

Polyfuran

bio degradability limited application

neural probes

easy synthesis, processibility

small chains, easy fusible

bio-electronics sensors FETs

Biosensors Corrosion MW sheiding

Biocompatibility Sensors super capacitors

Hydrogels Corrosion EMI shielding

Biosensors Corrosion EMI shielding Tissue engineering

Applications

Highly conducting

easy synthesis, processibility

cost effective, bio-degradable

varying forms, cost effective, easy modification

Easy Time-efficient cost effective, scalable approach

Advantages as per application studies in varying areas

Table 7.2 CPs and Their Synthesis Methods and their Fields of Application.

[87, 88]

[26–28]

[15, 24, 25]

[10, 14, 65 21, 22, 42, 63, 66–86]

[19, 20, 42, 50–64]

[1, 17, 30, 38–49]

References

204 Integrating Green Chemistry and Sustainable Engineering

Green Nanocomposites as Smart Promising Materials 205 MW irradiation

Nanospheres formation

oPDA, Tween80 and methanol

Microwave synthesis of nanoPoPDA and dispersed epoxy nanocomposite coatings Photographs of formulated coatings

Epoxy

mPDA-epoxy-PA coating

Ultrasonication Brush tecnique

Carbon steel

Nanocomposite

Figure 7.8 Showing MW synthesized PDA and corrosion protection studies, permission for reproduction of material from Materials Chemistry and Physics (Elsevier) [15].

Rawat et al., synthesized microwave synthesis (MW) of conducting Poly(o-phenyldiamine) (PDA), following seven principles of “green chemistry” approach. This includes less hazardous chemical synthesis, atom economy, prevention, safer solvents, design as energy efficient process, reduction of side products and remarkably, eco-friendly nature. They reported that use of aqueous medium during synthesis process proved to be green, renewable and free from harmful VOCs. They have also analyzed the effect of MW irradiation, on various properties of Poly(o-phenyldiamine)(mPDA) and its comparison with conventionally polymerised PDA(cPDA). The mPDA dispersed epoxy-polyamide (mPDA-epoxy) formulated coatings, showed superior coating properties than cPDA-epoxy nanocomposite coatings [15].

7.4.2 PTh and Their Derivatives Escobar et al., found a detailed study of P3HT synthesis in CH2Cl2 solvent by oxidative method with MW assistance and comparison was done has been conducted. The influence how MW synthesis effects the physical properties of P3HT products along with other parameters and their application in hybrid CdS/P3HT photovoltaic devices were studied. It is

206 Integrating Green Chemistry and Sustainable Engineering observed that the use of MW as well as the reaction time affected the reaction yield and properties of the polymer products, in case of conventional method, the maximum yield and the properties of the polymer products were similar after 2 h or 24 h of synthesis [89]. Rawat et al., studied effect of microwave (MW) irradiation on various properties of polythiophene (PTh) and compared it with conventionally polymerized nano PTh (cPTh). The effect of MW irradiation on the morphology, size, solubility and electrical properties were also investigated with the help of TEM, SEM, CV and four probe conductivity measurements. The synthesis has been carried out in aqueous medium, advantageously in green, renewable, environmentally friendly and free from harmful VOCs. The nano PTh dispersed epoxy-polyamide (PA) nanocomposite coatings were developed on carbon steel (CS) [14].

7.5

Application of MW synthesized CPs in Varying Arenas (Figure 7.5)

Due to easiness of MW synthesized CPs, their range of applications are vast. It is not even left any field in materials science and engineering. It has

H H

H N

N

H

me th od ion al

Conventional heating (Hours to days)

Microwave heating (Seconds to minutes)

is es th yn es av ow icr

Co nv en t

Well dispersed spherical particles (10±1)

Array of spherical globular particles (30±5 nm)

M

s

o-phenyl diammine

Microware irradiation atomic level to bulk

Conventional methods bulk to atomic level

Monomers

Polymerization

Figure 7.9 Showing mechanism how MW and conventional methods varies and advantage of MW synthesis, reproduced with permission from Rawat et al., [15] Elsevier-Material Chemistry and Physics.

Green Nanocomposites as Smart Promising Materials 207 (a)

(b)

0.4 μm

100 nm

(c)

(d)

20 nm (e)

100 nm

0.4 μm (f)

200 nm

Figure 7.10 TEM images of PANI synthesized with microwave assistance in HCl solutions with different concentrations. (a–c) 0.1 mol/L, (d, e) 0.3 mol/L, and (f) 1 mol/L [Colorfigure can be viewed at wileyonlinelibrary.com] [46].

wide nature of being used in super capacitors as conducting electrodes, sensors with their efficient redox properties, electronic materials with their semi-conducting behavior, drug release with their high adsorption–release nature, biosensors, radar absorbing materials with aerospace applications as they possess high conductivity as well as efficient material properties, tissue engineering, as well as stimuli responsive and bio mimetic polymeric materials. They shows temperature controlled cell adhesion for the development and design of 3D scaffolds providing a suitable and proper environment for easy attachment, proliferation, differentiation and detachment of cells [58, 90–92].

208 Integrating Green Chemistry and Sustainable Engineering

7.6

Conclusion and outlook

Novel functional polymer composites of CPs show promising performance and applications, but more work is still necessary in this regard. One of the most crucial factors, to be assessed for their application, is to enhance their properties, together with the ability to use them in controllable mode, which would be complimentary for varying area of applications [93]. Therefore, future improvements should focus on refining synthetic methods and developing novel assembly ways for better structure, morphology, control of the size, composition, of these nanocomposites (Figure 7.9, Figure 7.10). Developing improved synthesis procedures like MW matching chemistries that enhance functional properties not making any compromises in their processing, hereby conducting polymers scaleup are all areas that continue to require a significant research focus.

Acknowledgements Dr. Neha Rawat is thankful to the Government of India, Science & Engineering Research Board (SERB) for financial support in the form of National Post-Doctoral Fellowship (PDF/2017/002907). The authors are also thankful to Materials Science Division, CSIR-NAL Bengaluru-560017, for implementation of this fellowship.

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Green Nanocomposites as Smart Promising Materials 213 65. Jaymand, M., Hatamzadeh, M., Omidi, Y., Modification of polythiophene by the incorporation of processable polymeric chains: Recent progress in synthesis and applications. Prog. Polym. Sci., 47, 26–69, 2015. 66. Abou-Elenien, G.M., El-Maghraby, A.A., El-Abdallah, G.M., Electrochemical relaxation study of polythiophene as conducting polymer. Synth. Met., 146(2), 109–119, 2004. 67. Cardenas, L., Gutzler, R., Lipton-Duffin, J., Fu, C., Brusso, J.L., Dinca, L.E.. Chemical Science, 2013. 68. de Leon, A.C., Pernites, R.B., Advincula, R.C., Superhydrophobic colloidally textured polythiophene film as superior anticorrosion coating. ACS Appl. Mater. Interfaces, 4(6), 3169–3176, 2012. 69. El-Maghraby, A.A., Abou-Elenien, G.M., El-Abdallah, G.M., Electrochemical relaxation study of polythiophene as a conducting polymer. Synth. Met., 160(11-12), 1335–1342, 2010. 70. Fagerström, J., Stafström, S., Inter- and intrachain electron-hole recombination in polythiophene. Synth. Met., 85(1-3), 1065–1068, 1997. 71. Fréchette, M., Belletete, M., Bergeron, J.-Y., Durocher, G., Leclerc, M., Monomer reactivity vs regioregularity in polythiophene derivatives: A joint synthetic and theoretical investigation. Synth. Met., 84(1-3), 223–224, 1997. 72. Grana, E., Katsigiannopoulos, D., Karantzalis, A.E., Baikousi, M., Avgeropoulos, A., Synthesis and molecular characterization of polythiophene and polystyrene copolymers: Simultaneous preparation of diblock and miktoarm copolymers. Eur. Polym. J., 49(5), 1089–1097, 2013. 73. Kousik, G., Pitchumani, S., Renganathan, N.G., Electrochemical characterization of polythiophene-coated steel. Progress in Organic Coatings, 43(4), 286– 291, 2001. 74. Lee, S.J., Lee, J.M., Cheong, I.W., Lee, H., Kim, J.H., A facile route of polythiophene nanoparticles via Fe3+-catalyzed oxidative polymerization in aqueous medium. J. Polym. Sci. A Polym. Chem., 46(6), 2097–2107, 2008. 75. Lim, I., Yoon, S.J., Lee, W., Nah, Y.C., Shrestha, N.K., Ahn, H., et al., Interfacially treated dye-sensitized solar cell with in situ photopolymerized iodine doped polythiophene. ACS Appl. Mater. Interfaces, 4(2), 838–841, 2012. 76. Liu, R., Liu, Z.. Chinese Science Bulletin, 54, 2028–2032, 2009. 77. Palraj, S., Selvaraj, M., Vidhya, M., Rajagopal, G., Synthesis and characterization of epoxy–silicone–polythiophene interpenetrating polymer network for corrosion protection of steel. Progress in Organic Coatings, 75(4), 356–363, 2012. 78. Park, D.H., Kim, B.H., Jang, M.K., Bae, K.Y., Lee, S.J., Joo, J., Synthesis and Characterization of Polythiophene and Poly (3-methylthiophene) Nanotubes and Nanowires. Synth. Met., 153(1-3), 341–344, 2005. 79. Reedijk, J.A., Martens, H.C.F., van Bohemen, S.M.C., Hilt, O., Brom, H.B., Michels, M.A.J., Charge transport in doped polythiophene. Synth. Met., 101(1-3), 475–476, 1999. 80. Sakamoto, K., Nakabayashi, K., Fuchigami, T., Atobe, M., Electrochemical and Photoelectrochemical Behaviors of Polythiophene Nanowires Prepared by

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8 Biobased Biodegradable Polymers for Ecological Applications: A Move Towards Manufacturing Sustainable Biodegradable Plastic Products Sudhakar Muniyasamy1,2,*, Kulanthaisamy Mohanrasu3,5, Abongile Gada1,2, Teboho Clement Mokhena1,2, Asanda Mtibe2, Thulasinathan Boobalan3, Vimla Paul4 and Alagarsamy Arun3 1

Polymers and Composites, Materials Science and Manufacturing Unit, Council for Scientific and Industrial Research (CSIR), Port Elizabeth, South Africa 2 Department of Chemistry, Faculty of Science, Nelson Mandela University, Port Elizabeth, South Africa 3 Bioenergy and Bioremediation Laboratory, Department of Microbiology, Alagappa University, Tamil Nadu, India 4 Department of Chemistry, Faculty of Science, Durban University of Technology, Durban, South Africa 5 Department of Energy Science, Alagappa University, Tamil Nadu, India

Abstract In recent years, the emerging environmental concern for traditional plastic materials has posed a challenge to academia and industries to come up with an alternative eco-friendly material. This is because the post-consumer plastic items are non-biodegradable when disposed in natural environments such as landfill and marine sites. However, these plastics accumulate in these natural environments and create serious pollution that persists to cause environmental damage for decades. In order to address these issues, an innovative global circular economic concept in manufacturing new sustainable green products is currently underway to develop sustainable bioplastic products that will have economic, environment and social benefits. In this chapter the development of biopolymers directly extracted from biomass, monomer production from fermentation and microbial synthesis of biopolymer and their current potential applications are discussed. *Corresponding author: [email protected] Shahid-ul-Islam (ed.) Integrating Green Chemistry and Sustainable Engineering, (215–254) © 2019 Scrivener Publishing LLC

215

216 Integrating Green Chemistry and Sustainable Engineering Special attention on the ecological impact of these post consumers bio-based plastic materials when they enter into waste streams such as landfill, compost and marine water have been addressed. The biodegradation (mineralization processes) of biopolymers are also reviewed. Various analytical techniques for evaluating the potential biodegradability and its toxicity level of polymeric materials in different environments are discussed in accordance with international standards and regulations. Keywords: Biopolymers, biodegradation processes, natural environments

8.1

Introduction

Advances in oil based fuels and polymers have profited humanity from multiple points of view. However, oil reserves are depleting and the prices are probably going to keep on rising in the future which are the driving forces to develop their replacements. Moreover, during their production and transportation, large amount of CO2 is discharged into atmosphere which contributes to environmental pollution. In addition, the recent environmental global legislations restrict the use of conventional plastics from oil-based due to the serious environmental pollution they create. This necessitates the development of eco-friendly materials as alternatives to non-biodegradable materials. In recent years, much attention has been given towards the use of sustainable and renewable natural resources to mitigate these previously mentioned drawbacks [1]. Over the past years, biopolymers from sustainable and renewable resources have been considered as potential substitutes for the existing petroleum-based synthetic polymers because of their unique attributes such as, eco-friendliness, abundant natural availability and biodegradability. Biopolymers derived from sustainable and renewable resources are important in reducing overdependence on fossil fuels, with positive environmental impacts responsible for reducing greenhouse gas (GHG) emissions. These biopolymers are naturally occurring high molecular weight polymeric materials made up of monomers derived from natural resources which are polymerized via chemical and/or biological methods. Not all biopolymers are biodegradable and not all biodegradable polymers are biobased. For instance, polylactic acid (PLA) is biopolymer and is non- degradable in aqueous medium and soil, on the other hand polybutylene adipate-co-terephthalate (PBAT) is derived from fossil fuels but it is biodegradable. The first generation of biopolymers were derived from agricultural feedstock such as potatoes, and corn by fermentation processes. Biopolymers have been used in biomedical, packaging and

Biobased Biodegradable Polymers for Ecological Applications 217 Biodegradable polymers

Biopolymers

Polymers

(renewable resources)

(fossil resources)

Biomonomer synthesis

Extracted from biomass

Produced by microorganisms

PCL PGA

Proteins

Polysaccharides

PLA

PHA PBSA Pullulan

Whey protein

Starch

Casein

Carrageenan

Gluten

Cellulose

FucoPol

Figure 8.1 Biodegradable polymer classification and biopolymer synthesis.

agricultural applications since they endow distinctive properties which include low cost, renewable nature and biocompatibility [2]. The biopolymers can be categorized into three: (i) natural-occurring polymers, (ii) fermentation of monomers followed by polymerization, and (iii) biosynthesis using bacteria as depicted in Figure 8.1.

8.2

Biodegradable and Compostable Polymer Materials

8.2.1 Defining Biodegradability The term "biodegradation" is becoming more popular with regard to the environmental protection and production of green materials [3, 4]. In the biomedical area, "biodegradation" refers to hydrolyses and oxidation processes of polymer into smaller compounds. According to material science, biodegradation occurs in three different stages: (i) deposition of microorganism on the surface of biopolymers resulting in the loss of their properties; (ii) biofragmentation of biopolymers into oligomers and monomers; (iii) bioassimilation of oligomers and monomers into carbon dioxide, water and humus. Thus many different degradation modes, either abiotic degradation (i.e., degradation due to agents such as oxygen, water and sunlight)

218 Integrating Green Chemistry and Sustainable Engineering Polymer

Polymerase

Oligomers & Monomers

Aerobic

CO2, H2O and humus

Anaerobic

CH4/H2S, CO2, H2O and humus

Figure 8.2 Polymer biodegradation process summary.

or biotic degradation (i.e., degradation due to microorganisms), can synergistically combine in natural conditions to degrade polymers, leading to different degrees of degradability (i.e., disintegration, fragmentation and solubilisation) and each specific action is difficult to isolate. A diagrammatic summary of the biodegradation process (Figure 8.2).

8.2.2 Biodegradable Polymers (Fossil or Renewable) In addition, a number of standard authorities have sought to produce definitions for biodegradable plastics. The term “biopolymers” refers to a plastic that is derived (wholly or partly) from renewable biomass (plant)/biogenic sources. The term biopolymers can be applied to the following materials: (i) non-biodegradable biopolymers-obtained from renewable resources but is not biodegradable e.g. biobased polyethylene (PE) derived from sugarcane has similar properties as like PE derived from fossil resource (ii) biodegradable biopolymers-are referred to polymers derived from renewable resources and are biodegradable.

8.3

Biopolymer From Microbial Synthesis and Its Applications

The naturally synthesized polymers found in microorganisms as intracellular and extracellular macromolecules (i.e., polyester, polysaccharides, polyamides etc.) are chemically metabolized from biological building blocks

Biobased Biodegradable Polymers for Ecological Applications 219 and are known as biopolymers [5]. In nature, microbial biopolymer plays a critical role in cell protection, energy reserving material and cell functioning [6]. These biopolymers are easily degraded by micro-organisms and transformed into simple molecular compounds (CO2, H2O and humus) by series of enzymatic reactions. The properties and application of microbial synthesized biopolymers have been investigated in several fields such as bioadhesives, probiotics, stabilizers, food and cosmetics [7, 8]. It is also used as biosorbent, bioflocculant and emulsifier in environmental applications [9]. Alginate, xanthan, levan and scleroglucan bacterial polysaccharides are used in food and medicinal industries. Polyhydroxyalkanoates (PHAs) and polylactides are used as alternative to the synthetic plastics for reduction of plastic pollution [10]. The living microbial can synthesize wide range of biopolymers which are classified according to their chemical structures, namely polyhydroxyalkanoates (PHAs), polysaccharides, nucleic acids, polyamides, poly (malicacid) and cutin, polythioesters polyisoprenoides polyphenols [11].

8.3.1 Intracellular Biological Polymers Microbial-produced biopolymers are synthesized in cytoplasm by enzymatic process as illustrated in Figure 8.3. They are produced in cytoplasmic Carbon source

Gluconeogenesis

KDPG

Pentose phosphate cycle

Glycolysis

PGI Alginate

Fructose-6-P

NDP sugars and derivatives

PGM Glycogen

+ ATP

+ATP Poly-γ-glutamate and ε-poly-L-lysine

PhaA and PhaB FAB

PHAscl PhaG

ATP FAD

+ATP Cyanophycin

Gellan, curdlan and cellulose

Glucose-6-P

Acetyl-CoA TCA cycle

Xanthan, colanic acid and K30 antigen

Glucose-6-P

PHAmcl PhaJ

Polyphosphate

Fatty acids

Sucrose

Dextran

Figure 8.3 Bacterial polymer biosynthesis pathways Adapted from reference [12].

220 Integrating Green Chemistry and Sustainable Engineering membrane in different compartments. Cyanophycin [13], glycogen [14], PHAs [15], polyphosphate [16], starch [17] are biopolymers that accumulate inside the cytoplasm and the amount of accumulated polymer depends on the available space in the cytoplasm. Amongst these biopolymers, PHAs garnered much attention due to their unique properties in various applications hence the development to improve its production into industrial scale has been a major subject in research and industrial communities. PHA is a family of biopolymers produced by 300 different species of grampositive and gram negative bacteria. It is primarily functions as a storage compound [18]. PHAs are synthesized as intracellular granules in excess of carbon available as energy source and limited nitrogen. The PHAs are piezoelectric and isotactic thermoplastic or elastomeric polymers that are insoluble in water, highly hydrophobic, non-toxic and good UV degradation resistance compared to synthetic polymers [11]. These properties of PHAs depend on their chemical structure. In addition, they are completely biodegradable in soil, marine, and freshwater habitat. The PHAs are classified by the number of carbon atoms present along the polymer chain and it can be either short or medium chain length. The short chain length PHAs (PHASCL) have 3 to 5 carbon atoms along the chain and the polymeric units are either homo or heteropolymer [19, 20], whereas medium chain length PHAs (PHAMCL) have six to 14 (or above) carbon atoms [21]. The synthesized PHAs is directly dependent on the substrate specificity of PHA synthases, for example Alcaligenes eutrophus can polymerise 3–5 carbon atoms of 3HAs, whereas Pseudomonas oleovorans can produce 6–14 carbon atoms of 3HAs [21]. There are 150 different types of PHAs monomers that have been synthesized biologically recorded to date [11]. PHA biosynthesis are mainly carried out by eight different pathways using different bacteria secreting various enzymes such as NADPH-dependent acetoacetyl-CoA reductase, b-ketothiolase, PHA synthase. Some bacteria have the ability to produce PHAs to about 90% of cell dry weight [22]. Recent investigations have proved that PHA is beneficial to bacteria in several ways such as cell survivability under stressful low nutrient conditions [11, 21, 22]. It is also useful in cell motility, biological nitrogen fixation, aerotaxis, chemotaxis, cyst formation, sporulation, germination, controlled exopolysaccharide production, environment stress toleration such as osmotic and solvent stress, heat, cold, UV, osmotic shock, irradiation, desiccation and ethanol. Over the past decades, PHAs as one of ‘green materials’ received much interest because of their properties comparable to fossil fuel based materials. Different parameters such as carbon sources (arabinose, glucose, glycerol, lactose, lactic acid, mannitol, sodium acetate, starch and sucrose), nitrogen sources (ammonium

Biobased Biodegradable Polymers for Ecological Applications 221 chloride, ammonium sulphate, glycine, potassium nitrate and protease peptone) and pH are essential in order to reduce existing gaps between small scale and industrial scale production [23]. However, the cost associated with the production of PHAs limit their success. In order to reduce the production cost, PHAs are currently synthesized from various agricultural by-products Agricultural by-products are undoubtedly one of the potential carbon source substrate for production of PHAs which are inexpensive and abundantly available. These include molasses from sugar industry, alcohol, starch, lignocellulosic feedstock and waste lipids (sesame, olive, and palm oils, meat and bone) [21, 24, 25]. Many research efforts resulted in synthesis of various PHA copolymers, however only four of them are commercially produced in industrial scale [22].

8.3.1.1 Application of PHAs Primarily PHAs were developed mainly for the daily routine purposes as packaging materials (i.e., dairy containers, meat wraps, and shampoo bottles). Subsequently, they gained popularity and now they are used in several fields such as paper coatings, shopping bags, disposable items (i.e., feminine hygiene products, cosmetic, razors, utensils, diapers, cups), carpet, upholstery, medical surgical garments, sutures stents, implants, cardiovascular patches, nerve guides, tendon repair devices, slings, wound dressing materials, articular cartilage substitutes, bone fraction fixation system, bone marrow scaffolds and cell growth supporting materials as depicted figure below (Figure 8.4) [26–28]. Over the past two decades PHAs and other copolymers (PHB, PHO, P4HB, PHBV and PHBHHx) have been utilized to develop controlled drug release and in tissue engineering [22]. The potential properties of biocompatibility, biodegradability and degradation by surface erosion of PHA became fully used to control drug delivery carriers [29]. PHB and PLA microspheres was used in vitro and in vivo release of drug targeting, however PHB microspheres are faster compared with PLA microspheres and incorporation with ethyl esters or butyl esters of fatty acids with PHB has attained increased drug releasing rate [30, 31]. Zhang et al., [32] proposed to use mcl 3-hydroxyalkanoate methyl ester (3HAME) and 3-hydroxybutyrate methyl ester (3HBME) used as a biofuels via esterification process. The combustion heats of 3HAME, 3HBME, n-propanol, ethanol, diesel, gasoline and blended fuels of 3HBME and 3HAME was investigated. The combustion heats of blended fuels such as 3HBME/gasoline or 3HBME/diesel and 3HAME/gasoline or 3HAME/diesel was lower

222 Integrating Green Chemistry and Sustainable Engineering

Implants and scaffolds Drug delivery

Energy Polyhydroxya lkanoates (PHAs) Applications Tissue engineering

Agriculture

Packaging material

Figure 8.4 Various applications of polyhydroxyalkanoates (PHAs).

than the pure gasoline and diesel [32]. In agriculture, bacterial inoculants are used, with seed or soil during planting as a carrier, which can withstand environmental stress like acidity and temperature. These inoculums also show enhanced plant growth due to the presence of PHB. Pesticides encapsulated in PHAs are best known to release the chemical compound in controlled manner in presence of PHAs degrading bacteria and leave less pesticides in the environment [33, 34]. Successful PHA biosynthesis is grown in trial and error method types. Hence, value added PHA applications are a key for development of bio based polymer productions, still now 19 biopolymer producing companies (Ex. Biopol, Nodax, Degra pol and Biogreen) producing tons of polymer every year.

8.3.2 Extracellular Polymeric Substances (EPS) The extracellular polymeric substances (EPS) are biosynthetic polymers produced by both prokaryotic and eukaryotic microorganism. These viscous polymers are synthesized and released by both natural and manmade means into the environment; they are composed of high molecular weight organic polymeric macromolecules (proteins, nucleic acid, polysaccharides and phospholipids) and low molecular weight non-polymeric

Biobased Biodegradable Polymers for Ecological Applications 223 materials [35]. The microbe’s aggregates produced by adaptation to protect themselves against extreme conditions like temperature variation, pH and also act as reserve food material [36, 37]. The exogenous secretion of EPS contain neutral carbohydrates as a major component (mainly hexose and rarely pentose), uronic acids (galacturonic, glucuronic and mannuronic acid), protein, nucleic acids, lipids, polysaccharides, glycoproteins and humic-like substances at various propositions [38–41]. EPS matrix is composed of substantial amounts of several proteins (enzymes) and most of that are involved in EPS degradation when bacteria face nutrient deficit conditions, these extracellular enzymes act as substrates like organic particles. The water soluble (proteins, nucleic acids, some polysaccharides) and insoluble polymers (lipids, chitin and cellulose) act as sources of carbon and energy for bacteria [42]. The chemical natures (anionic or cationic and neutral) of EPS macromolecules are mainly based on uronic acids and common substitutes such as pyruvate ketals, acetate ester, succinates, formats and inorganic (sulphate and phosphate) [38]. The viscous biofilm matrix (EPS) can be differentiated based on structural bound, defined as slime (S-EPS), capsular (CEPS), loosely bound (LB-EPS) and tightly bound (TBEPS) [38]. Microbes producing biofilms contain E-DNA (eg., extracellular deoxyribonucleic acids) and they have been largely found in wastewater biofilms. The quantity of EPS production differs depending on substrate where they grow, particularly in wastewater [42].The presence of E-DNA in the EPS matrix of Streptococcus mutans is associated with the export of a large quantity of competence-signalling peptides to the medium that helps in horizontal gene transfer [43]. EPS is mostly hydrophilic due to the hydrated polysaccharides (DNA molecules and protein) whereas some EPS are having hydrophobic properties (Rhodococcus spp) because of lipids and lipids derivatives in it [38]. Surface active EPS (Biosurfactants) are an important factor for oil droplets attachment and detachment as well as help to colonization during struggle for competition resources [39]. EPS matrix contain humic substances as an integral part, however they are not produced by microbes themselves, they are mostly absorbed from natural environment like sludge, soils, wastewater and the presence of humic substances are an important factor to determine biodegradability and adsorption of EPS [35]. Production of EPS by bacteria composed of homopolymer (single type of sugar like glucose, xylose, etc.) and heteropolymer (more than one type) [37]. The homo polysaccharides (EPS) are classified into three different groups based on the linkage and carbohydrate it contains. The

224 Integrating Green Chemistry and Sustainable Engineering first classified group is composed of α-(1,6)-linked D-glucosyl units with degree of branch α-(1,3)-linkage, α-(1,2)-linkage and α-(1,4)-linkage macromolecules is produced by Leuconostoc mesenteroides, second group is composed of β-(1,3)-linked D-glucosyl units (β-(1,2)-linkage) produced by Pediococcus sp and Streptococcus sp bacteria and the third group is β-(2,6)- linked fructosyl units synthesized by Streptococcus salivarius [43]. The homo polysaccharides EPS such as cellulose is synthesized by Acetobacter xylinum, Curdlan synthesized by Alcaligenes faecalis and Dextran formed by Leuconostoc mesenteroides, Hetero polysaccharide such as alginate by Azotobacter vinelandii, gellan by Sphingomonas elodea, gellan gum by Sphingomonas elodea, xanthan gum by Xanthomonas campestris and welan – Alcaligenes faecalis / Sphingomonas sp. bacteria [37, 38]. EPS are synthesized by microbes for various physiological functions such as adhesion, aggregation of bacterial cells, binding of enzymes, energy, cohesion of biofilms and retention of water. It is also used as a transport carrier for export of cell components, electron donor or acceptor, protective barrier, sorption of organic compounds, exchange of genetic information, as a nutrient reserve, sorption of inorganic ions and enzyme activity [42].

8.3.2.1 Important Properties of EPS The global production of microbial exopolysaccharides is rapidly increasing due to its potential properties of biodegradation, adsorption (metals and organic/inorganic compounds) and hydrophobicity/hydrophilicity. Four hundred types of EPS have been reported in which some biofilm matrixes are receiving technical and commercial interest [44]. Industrial scale production of microbial biopolymers are enhanced using efficient native and genetically improved strain, cheaper fermentation feedstock (lignocellulosic biomass, cheese whey, olive mill wastewater, vegetable and fruit pomace) under optimized temperature and pH [45]. In food industry, EPS producing cultures have been utilized in various food processing industries with products like Yogurt, Mexican Pulque, Kefir (For viscosity incensement and texture improvement), Cheeses (For increased moisture, improved cheese yield, flavor and Pleasant to taste) and Fermented Breads (For enhanced texture, quality and improved viscoelasticity) [46]. Several types of microbial polysaccharides have been discovered but certain EPS are drawing commercial applications such as dextran, xanthan, alginate, gellan, pullulan and hyaluronic acid. Xanthan, a bacterial extracellular polysaccharide is a polyanionic heteropolysaccharide composed of repeating unit of

Biobased Biodegradable Polymers for Ecological Applications 225 five sugars that includes mannose, glucose, glucuronic acid and molecular weight ranges from 2 × 106 to 20 × 106 Da mol−1. Xanthan produced by Xanthomonas malvacearum, Xanthomonas campestris and Xanthomonas axonopodis [10]. Xanthanis is widely used in food industries due to its properties of pseudo plasticity, viscosity and gellation properties [10]. Exocellular kefiran is by Lactobacillus kefiranofaciens which possess commercial applications as emulsifier, stabilizer, thickener, gelling agent, edible film in food, packaging industries, thickeners and stabilizers in food industries [10]. Microbial cellulose is extracellularly produced by Aerobacter, Acetobacter, Agrobacterium, Rhizobium. Cellulose nanofibers are prepared as arranged in ribbon like structure of 100 μm length and 100 nm in diameters, such type of cellulose nanofibre are chemically stable, bio-adaptable and biodegradable. Thus these types of cellulose nanofibres are applicable in food packaging, diet food, as regenerative medicines, in skin tissue repair, paper additive and vascular grafts [47, 48]. The impressive microbial exopolysaccharides has potential applications in medical fields. Dextran is utilized as blood plasma volume expander, xanthan as a stabilizer in pharmaceutical creams and as controlled drug carriers. Similarly alginates are employed as thickener, stabilizer in syrup, dental impressions cast, antacid and stomach protector, gellan in nasal drug formulations (tablet disintegration). Hyaluronic acid/hyaluronan is used in treating wounds that are difficult to heal, chronic osteoarthritis treatment, in vaccines, gene delivery, tissue engineering and cell encapsulation. Pullulan consist of maltotriose units, are used in tablet granulation, tablet coating, oxygen impermeable film forming and binder, non-animal capsules and wound care products [49]. Extracellular polymeric substances are predominantly used in biological wastewater treatments, Ma et al., [50] used Bacillus sp for EPS production, EPS was used in removal of kaolin turbidity in jar with different sorbent materials. They have observed following efficiency of turbidity removal such as EPS (86%), Fe2(SO4)3 (96%) and Al2(SO4)3 (95%) and compared with EPS removal, whereas the combination of EPS and Fe2(SO4)3 showed effective for raw water treatment. EPS are considered for drinking water treatment (combined with CaCl2) by using several bacterial strains like Pseudomonas plecoglossicida, Pseudomonas pseudoalcaligenes, Bacillus subtilis, Klebsiellaterrigena and Exiguobacterium acetylicum was removed river water turbidity 84.1% to 93.6% at 10 mg/L concentration of EPS [51]. Chemical flocculation method is most commonly used one to remove suspended solids in wastewater treatment. In recent days EPS is used as an alternative one to the chemical

226 Integrating Green Chemistry and Sustainable Engineering based polymers treatment. Gong et al., [52] effectively used Serratia ficaria EPS for treating various industrial wastes such as meat processing, brewery, pulp, soy sauce brewing and paper. S. ficaria EPS reduced the hazardous effect of effluents considerably. The biosorption capacity of EPS (mostly contain heteropolysaccharides and lipids) in cell wall is due to the presence of carboxyl, amino, phosphate, hydroxyl groups as an attracting force between cell wall and dye [53]. The role of EPS with the metal binding capability greeting influence the removal of heavy metals in soil and aquatic systems. Puyen et al., [54] reported Micrococcus luteus DE2008 with copper (408 mg/g) and lead (1965 mg/g) specific removal capacity. The EPS has the ability to degrade polycyclic aromatic hydrocarbons (PAHs) in soil, sludge and different wastewater. Mangwani et al., [55, 56] observed that Stenotrophomonas acidaminihila NCW-702 had degraded phenanthrene and pyrene in planktonic and biofilm cultures. The findings conclude biofilm culture phenanthrene (71.1 ± 3.1%), pyrene (40.2 ± 2.4%) degradation was observed and in planktonic culture phenanthrene (38.7 ± 2.5%) and pyrene (29.7 ± 1%) was observed respectively. Mangwani et al., [55] reported the use of Pseudomonas aeruginosa N6P6 and Stenotrophomona sacidaminiphila NCW-702 for enhanced bioremediation under different physicochemical parameters such as carbon source, difference in temperature, pH and salt concentration. In both type of cultures phenanthrene and pyrene degradation, diffusion distance, total biomass and thickness of biofilms was observed, however planktonic cells mediated degradation was lower compare to the biofilm facilitated degradation. Mangwani et al., [56] studied effect of increased biofilm growth using Ca2+ and, Mg2+ for phenanthrene degradation and rate of phenanthrene degradation was increased more than 15%. However, enhanced secretion of exopolysaccharides increases the solubility and binding of hydrophobic substrates (eg., PAHs and hydrocarbons) by hydrophobic interactions [56, 57]. EPS can be used as an bioagent to control pollution with various applications like bioflocculant, biosorbent, bioleaching, biodegradation and bioaugmentation.

8.4

Chitin

Chitin (C8H13O5)n is the second most prevalently available polysaccharide in the Earth, naturally next to cellulose, which are polymerized as microfibrils and are composed of 2-acetamido 2-deoxy-β-D-glucose units through β (1,4) linkage. Plants create cellulose in their cell walls and insects and

Biobased Biodegradable Polymers for Ecological Applications 227 crustaceans create chitin in their shells [58]. Cellulose and chitin are, in this manner, two essential and fundamentally related polysaccharides that give basic respectability and security to plants and creatures, individually [58, 59]. Chitin is a light, fine, flaky item that is white or yellowish in color. Common chitin exhibits a variable level of crystallinity, depending on its structure, fluctuating amounts of deacetylation and cross-linking with other molecules [60]. According to an estimate, long-chain polymeric polysaccharide was obtained from many sea creatures with exoskeleton producing arthropods (i.e., crab, shrimp, lobsters, crayfish, krill, barnacles), molluscs or invertebrate animals (i., chitons, oysters, clams, geoducks, scallops, fossils, snails, mussels), insects (i.e., scorpions, ants, beetles, cockroaches, brachiopods, spiders), certain fungi [60] and amount of chitin is different from species and seasonal variations (Figure 8.5). Similarly the seafood industry produce about 10 gigatons of waste annually in the biosphere which includes waste product of some plants, jellyfish, krill, insects, oysters, crayfish, clams, algae and fungi which Crustacean shells

Chitin (%)

Cancer (crab) Carcinus (crab) Callinectes (blue crab) Paralithodes (king crab) Crangon & pandlus (shrimp) Shrimp Alaska Homarus (lobster) Lepas (Goose barnacles) Nephros (lobster)

72.1a 64.2b 14.0c 35b 17–40 28d 60–75a 58.3a 69.8a

Insects

Chitin (%)

Blatella (cockroach) Bombyx (silk worm) Calleria (wax worm) Cleoptera (ladybug) Diptera Periplanta (cockroach) Pieris (butterfly)

18.4a 44.2a 33.7a 27–35a 54.8a 2d 64a

Chitin

Mollusks Clam Krill (deproteinized shells) Shell Oysters Squid pen

Chitin (%) 6.1 40.2 3.6 41

Fungi Aspergillus niger LactariusVellereus Mucor Rouxii Penicillium crysogenum Penecelium notatum Saccharomyces cervisiae

Chitin (%) 42e 19 44.5 20.1e 18.5e 2.9e

Figure 8.5 Chitin (%) present in different organisms. (superscript: aweight of the organic cuticle; bDWof the body; cfresh weight of the body; dtotal weight of the cuticle; eDW of the cell wall. DW: dry weight) [61].

i i i i i S (with depolymerization) s s s s s

Water

Dilute acids

Dilute and concentrated alkalies

Alcohol

Organic solvents

Concentrated HCl, H2SO4 or H3PO4, anhydrous HCOOH

N,N-Dimethylacetamide (DMAc)/5% LiCl

Dinitrogen tetroxide/N,N-dimethylformamide (DMF)

Fluoroisopropanol/hexafluoroacetone

N,N-Dimethylacetamide/N-methyl-2-pyrrolidone/ LiCl

N-Methyl-2-pyrrolidone/5% LiCl

I, represents insoluble and S, represent soluble, respectively

Solubility

Solvent/solvent mixture

Table 8.1 α-Chitin solubility, at room temperature using different solvents and solvent mixtures (Daraghmeh et al., 2011).

228 Integrating Green Chemistry and Sustainable Engineering

Biobased Biodegradable Polymers for Ecological Applications 229 are also used to produce polysaccharides [62, 63]. Currently most of the chitin production depends on marine food processing industrial wastes (50–60% of total weight) such as shrimp, crab shells (CSs) and crustacean shells [64]. The shells are composed of chitin (serves as an exoskeleton), minerals (mostly inorganic carbonate salts) and proteins (serves shells as a living tissue). Chitin is a high molecular weight polymer, chemically similar to cellulose, it is insoluble in most organic solvents, inelastic, semi-transparent material, low chemical reactivity, nitrogenous polysaccharide, white, hard and easy to be made as powders, sheets and granules [65–67]. Chitin exists in three diverse polymorphic structures, with shifting properties also, the distinctive structures are α, β and γ [68]. The origins of α-chitin are crabs and shrimps; β-chitin is squids; and γ-chitin is loligo. The diverse polymorphic frames vary in their arrangements of polymeric chains like, antiparallel adaptation to each other in α-chitin, polymeric chains are orchestrated in parallel setup in β-chitin, and polymeric chains are organized in two parallel chains taken after by one antiparallel chain in γ-chitin [69]. The solubility of chitin in different medium is shown in Table 8.1. The food processing industry facing a major problem due to the accumulation of higher volume of wastes that are low biodegradable in nature and are dumped in sea water causing coastal area pollution [70]. The seafood processing industry wastes mostly includes chitin (20–30 %), protein (20–40 %), minerals (30–60 %) and lipids (0–14 %). The amount of chitin varies from one species to another and seasonal variations also influence the quantity of chitin. Sea food industry waste contains some economically important sources like chitin, proteins, carotenoids and mineral salts. Through proper extraction methods they can be converted into environmentally beneficial products by which the environmental problems due to the disposal of sea food industry wastes into the sea floor can be minimized. There are number of promising advantages of using chitin biopolymer in various fields such as waste water treatment (removal of dyes, pigments, metal ions), membrane purification processes, coating of seeds, chitin coated fertilizer and agro chemicals for controlled release in the field. Similarly chitin has wide applications such as packaging material, for fat binding, as anti-cholesterol, food additive, preservative, in paper industry (as photographic paper and surface treatment), cosmetics and toiletries (lotions and moisturizer body creams) and biomedical applications (drug delivery, tissue engineering, cancer diagnosis and wound healing) [71].

230 Integrating Green Chemistry and Sustainable Engineering

8.4.1 Chitin Recovery The most share of chitin is gotten from crustacean shell squanders, essentially in view of their voluminous accessibility at a low cost. In these crude materials, chitin is joined with different mixes, for example, proteins, colors, and minerals. In dried and deproteinized shell waste, minerals and chitin are display in about equivalent amounts. Forceful treatment is important to separate the chitin. Generally, traditional isolation process for chitin consists of various steps, such as demineralization (DM), deproteination (DP), bleaching/decoloration and deacetylation. Usually, demineralization can be achieved utilizing shell waste with diluted hydrochloric acid (1% to 8 %) for 1–3 hr, the use of harder extractants like 22% HCl or 90% formic acid or 6 NHCl, or 37% HCl is also been suggested for extraction process [72, 73]. During the demineralization step significant amount of minerals, particularly calcium chloride are produced from crustacean shell that are used in paper and pulp industry [74]. Subsequently, the demineralized material was subjected to deproteinization with 4–5M sodium or potassium hydroxide at 65–100 °C for 1 to 6 hr. The demineralized shells are filtered and washed with distilled water and dried for 24 hr. Bleaching of pigment residue (melanin and carotenoids) will give an almost colorless material, decolonization is mostly carried out using solvent extraction such as sodium hypochlorite or hydrogen peroxide solutions [72, 75]. In chitin extraction, conditions are based on the requirement of applications. In chitin extraction salt removal using drastic demineralization with acid can cause deacetylation of chitin and protein removal. Using harsh alkali conditions can cause depolymerization and deacetylation of chitin. Further, protein extraction by alkali could be used in limited applications, because of stopping of undesirable reactions among amino acids in strongly alkaline medium, in addition to racemization of amino acids. Nevertheless, the hydrolyzates used extracted protein can be used as supplement in fish-based foods or aquaculture feed and as flavoring agents [76]. However, chemical extraction methods using strong acid can cause negative implications such as changing the physico-chemical properties of chitin, chemicals containing effluent and high cost associated purification processes. Because of this problem, the traditional extraction methods are used nowadays to overcome the problems with chemical methods. The ordinary cruel conditions utilized for extraction could affect the nature of the chitin. Removal of salt by demineralization with acids can result in some deacetylation of chitin. Harsh alkaline conditions for protein evacuation can cause depolymerization, cleavage of glycosidic

Biobased Biodegradable Polymers for Ecological Applications 231 linkages and deacetylation of the chitin [60]. Moreover, alkali-extracted protein could be of restricted use, as unwanted reactions between amino acids happen in strongly alkaline media and also racemization of the amino acids. Likewise, treating the alkali wash effluent is fundamental to maintain ecological contamination. To conquer these restrictions, novel techniques are being created to supplant ordinary demineralization and deproteinization to extricate chitin [60]. Using proteolytic enzymes such as pronase or alkalase, trypsin, pepsin and papain have been used, this type of enzyme treatments can remove upto 90% of protein and carotenoids from shrimp wastes and the quality yield of chitin is undisturbed [77]. While most procedures for the creation of chitin and derivatives from shellfish squanders involve a batch creation utilizing chemical, enzymatic or fermentative strategies, some exploration on novel systems has been performed. Mahlous et al., [78] examined the impact of gamma irradiation on the extraction of chitin and chitosan from prawn shells. Heads and shells from Algerian coas prawn (Aristens antennatus) were gathered, dried at 60 °C and cut into little pieces which were then illuminated at a measurement of 75 Gy/min to a measurement of 25 kGy. Light decreased the time required for deproteinization from 3 to 1 hr utilizing 1 N NaOH and a reaction temperature of 85 °C. Other variables or the DD for chitosan were not affected.

8.4.2

Characterization of Chitin

Chitin has some important properties like Molecular weights (Mw), degree of deacetylation (DD) and degree of acetylation (DA). The structural elucidation and proper characterization of chitin have significantly enhanced the material properties like free from protein, glucan contamination and as pure as possible. Therefore, many methods have been proposed and investigated to determine the DD/DA which are UV–Vis spectrometry, IR16 spectroscopy, both liquid and solid states NMR. The proposed methods are used in the determination of DD/DA by interaction with the applied radiation source and chemicals groups [79]. The chemical structure of chitin DA was determined by different type of NMR such as 1H NMR, 13C NMR and 15N NMR by ratio of Ap/AR: where Ap is height or area of the probe signal; and AR is the height or area of reference signal. In NMR, chitin samples were prepared in 2% deuterated acetic acid in D2O solution and recorded between 0 and 10 ppm using proton NMR. Heux et al., [80] has compared both liquid and solid-state NMR for chitin samples in studying the range of acetylation degree. Fourier transform infrared spectroscopy (FTIR) was measured by KBr supported chitin samples at the range of 4000–400 cm−1. Sagheer et al., [81] have validated both α and β form of chitin respectively. In α form

232 Integrating Green Chemistry and Sustainable Engineering chitin amide I band is split in 1650 and 1620 cm−1 but β form of chitin have a sharp band at 1657 cm−1. The amide II band for α and β form chitin appear in 1555 and 1559 cm−1 respectively. The bands at 3100–3285 cm−1 are corresponds to the N–H group in both type of polymers. The absorption bands in 2840–2960 cm−1 region are due to CH, CH2 and CH3 in both type of chitin [60]. Scattering is an effective analytical method for structure elucidation and dynamics of soft materials like polymers, proteins, colloids by using electrons, neutrons or visible light [82]. X-ray crystallography (also known as X-ray diffraction) is the most predominant method used to find the structure of any macromolecule to atomic resolution from a crystal, when the X-ray hits the crystal it scatters in all direction with the information of atoms position. Carlström, [83] proposed the structure of α-chitin from a blend of X-ray and optical investigations. Right now most accepted crystal structure of α-chitin has been proposed by Minke and Blackwell [84]. The crystal structure of α-chitin by high-determination synchrotron X-beam fiber diffraction information was redetermined in [83]. The primary highlights of the crystal structure are when all is said in done concurrence with those proposed by [84]. X-ray diffraction information of β-chitin were first filed for a unit cell containing one chitin deposit by [85]. The X-ray diffraction analysis (XRD) was applied to detect the crystallinity of the isolated chitin. Sagheer et al., [81] studied the crystallinity of extracted chitin using XRD, different XRD patterns were observed in both α and β form chitin. In its substance structure, chitin is like cellulose, however it is diverse in that it has an acetamide group rather than a hydroxyl group at the C-2 position inside the glucose unit. Samples of the local chitin may contain comprehensively different amounts of N-acetyl groups, depending upon their starting point and isolation strategy. The α form of chitin showed some sharp crystalline reflections at 9.6°, 19.6°, 21.1°, 23.7°, 27.7°, 29.3°, 32.1°, 36° and β form chitin more clearly found at 9.6° and 19.6°, both of XRD patterns of α form chitin shows well-resolved and intense peaks and β form shows broad diffuse scattering and less intense peaks. This data concludes that α form chitin was more crystalline polymorph compare to β and it is due to its anti parallel compact chitin structure.

8.4.3

Applications of Chitin

Chitin is a unique material and its applications are vast and diverse. Chitin is used in various fields, such as food and nutrition, cosmetics, toiletries, paper finishing, agriculture, solid-state batteries, tissue engineering, water engineering, feed additives and as waste materials absorbents. In recent years,

Biobased Biodegradable Polymers for Ecological Applications 233 chitin has been extensively used in biomedical applications. In biomedical, chitin is utilized in field of tissue engineering for cell growth, attachment, proliferation, as drug delivery carrier, for wound dressing and in cancer diagnosis. Chitin has been widely investigated in biomedical application due to its unique properties like biodegradability, biocompatibility and non-toxicity. Chitin can be easily processed into membranes, nanofibrils, gel, nanoparticles, microparticles, beads, scaffolds and sponge-like forms [87]. Nowadays chitin nanomaterial biopolymers have been extensively used in wound healing, wound dressing, drug delivery and cancer diagnosis. Tissue engineering involves recuperate/replace, maintain or enhance function of particular tissue or organ and this replaces therapies like allografts, autografts for organ or tissue replacement; the gold standard therapies have some issues like risk of disease transmission, limited availability, lack of enough fusion, cost and morbidity at the donor site [69]. In recent years, bone tissue engineering captures remarkable consideration after the tissue engineering to regenerate natural human bones. Like any other tissue engineering approach, bone tissue engineering also entails a scaffold matrix with/without cells and biological cues for a successful outcome, i.e., bone regeneration. The biomaterials used for developing a scaffold matrix include polymers (natural or synthetic), natural–synthetic polymeric blends, ceramics or polymer–ceramic composites [88]. Chitins are bio-compactable, biodegradable, flexible, prose and ease of processing but they are mechanically unstable and weak. Li et al., [89] studied to enhance the scaffold matrix mechanical strength by chitinalginate hybrid scaffold matrix and obtained an uncompromised mechanical strength of porous scaffold. Kumar et al., [91] have developed composite scaffolds such as novel β chitin/nanosilver for wound healing applications. They also developed a blood-coagulation studies with β-chitin/nanosilver scaffold with and without silver, the blood clotting scaffold with nanosilver result showed increased coagulation compared to in absence of nanosilver. They have studied invitro antibacterial activities of β-chitin/nanosilver against Escherichia coli and Staphylococcus aureus were β-chitin/nanosilver play signifying inhibiting effect and cytotoxicity studies showed β-chitin/nanosilver are non-toxic, so β-chitin/nanosilver composite is having a promising application as wound dressing material. In a similar type of research, Jayakumar et al., [90] have studied chitin nanogels (CNGs), with prepared doxorubicin loaded CNGs (130–160 nm), showed toxic to all tested cancer cells and are used for liver, breast, lung and prostate cancer. Yet another extension of chitin application is chelating of heavy metals like copper, mercury, nickel, iron, lead, chromium, cadmium, zinc, cobalt and silver found in industrial wastewater and it is does not cause any environment problems.

234 Integrating Green Chemistry and Sustainable Engineering Chitin can be used in several value-added food products such as making of biodegradable films, in food preservation, purification of water, de-acidification of fruit juice and also used as food additives includes; thickening agent, color stabilization agent, natural flavor extender, emulsifying agent [87]. One of the perennial areas for chitin application is in agriculture fields as bactericidal and fungicidal agent. Chitintreated seeds (wheat) have accelerating growth as chitin is decreasing the penetration of pathogenic fungi and insects in mixture of soil [91]. Chitin has profusely of applications in cosmetic such as maintaining skin moisture, suppleness of hair, permanent waving lotions, chewing gums, toothpastes, eye shadow, cleansing materials, lipstick, bath agents and cleansing materials [92]. Chitins is extensive amino polysaccharide after cellulose and enormous availability can create environmental issues, so these types of application attempts have been reduce environmental problems.

8.5

Conventional Synthesis of Biopolymers and Its Application

Biopolymers (also called renewable polymers) are produced from biomass for use in the packaging industry. Biomass comes from crops such as sugar beet, potatoes or wheat: when used to produce biopolymers, these are classified as non food crops. These can be converted in the following pathways: Sugar beet >Glyconic acid>Polyglonic acid Starch > (fermentation) >Lactic acid>Polylactic acid (PLA) Biomass > (fermentation) >Bioethanol > Ethene >Polyethylene. Many types of packaging can be made from biopolymers: food trays, blown starch pellets for shipping fragile goods, thin films for wrapping (Figure 8.1).

8.5.1 Biorenewable Biopolymers Polymers of biological origin such as carbohydrates (starch), Proteins (haemoglobin), nucleic acids (DNA) and Lipids. Carbohydrates are organic compounds, it can be source of energy like sugars, store of energy like starch, structural materials like polysaccharides and components of other molecules (i.e., DNA, RNA, glycolipids, glycoproteins) (Figure 8.1).

Biobased Biodegradable Polymers for Ecological Applications 235

8.6

End-of-Life of Biopolymer Based Materials and Composites and Its Applications

Some microorganisms secrete extracellular enzymes to catalyze degradation of chain length of the polymer that becomes available as substrate for the microorganisms. This is the primary phase of biodegradation. This phase can also be simulated by pretreating the polymer with other abiotic chemicals and physical treatment to induce biodegradation and its rate. These treatments include photodegradation, thermal polymer degradation and water/abiotic hydrolytic degradation. The exposure of polymer into these treatments cause physical and chemical changes (reduce ductility, increases embrittlement/erosion, discoloration etc.) due to change in properties resulting in deterioration in functionality caused by formation of bond scissions and subsequently chemical transformation. Pretreatment can speed up the biopolymer degradation increasing the efficiency . The backbone of the polymer determines the process that can be used as a pretreatment. Hetero-chain backbone polymers undergo hydrolysis. Hydrolysis is another way by which polymers can undergo chemical degradation. Hydrolysis is dependent on parameters as water activity, temperature, pH and time. In the presence of water, the main chain of the polymer cleaves and bond with H cation and hydroxide anion. H CH3

O

O

O

H O O

H

O n-m-1

CH3

OH m

O CH3

CH3

O

OH

O O

H

CH3 O

O

CH3

O

H CH3

O n-m-1 O O H H

OH m CH3

O n-m-1

O

+ O H

O OH m CH3

Figure 8.6 PLA schematic diagram of polylactic acid hydrolytic degradation pathway (Redrawn after reference from [86]).

236 Integrating Green Chemistry and Sustainable Engineering Synthetic heterochain polymers that are manufactured worldwide account for only roughly 10% [93]. Poly lactic acid (PLA), a biobased renewable polymer that is synthesized from starch and is primarily degraded through hydrolysis in the presence of water that initiate the breakdown of ester bonds. The water temperature that is above the glass temperature of PLA (56–58 °C) speeds up the process of hydrolysis. The process in aliphatic polyesters begins by absorption of water that facilitate the depolymerisation of PLA. This increase alkalinity of water through continuous release of dimers. This is followed by an electrophilic attack of the hydroxyl-end group on second carbonyl group forming a ring (intramolecular transesterification) resulting to a lactide. The free lactide is further hydrolyzed to form lactic acid molecules that become available for bioassimilation and ultimate mineralization by microorganismsto endproducts such as CO2, H2O and biomass with no toxic residues, under aerobic conditions or methane in anaerobic conditions. In acidic conditions, the hydroxyl end-group transfer a hydrogen atom which result in intramolecular hydrogen bond. In particular, abiotic hydrolysis has been suggested as a major degradation mechanism and the rate limiting step of PLA biodegradation under composting conditions. Literature reveals that PLA is the least susceptible to microbial attack of all polyesters of natural or synthetic origin. The distribution and population of PLA degrading microorganisms as well as the percentage in the soil environment is the smallest compared to those degrading other biodegradable polyesters. The process of abiotic hydrolysis has been reported to be the dominating mechanism of degradation in PLA that initiates and also acts as rate limiting agent in PLA biodegradation when composted and in environment [94]. In a study carried by Stloukal et al., [94] which was carried to confirm importance of abiotic hydrolysis in PLA biodegradation in a duration of 90 days. The PLA and PLA-montmorillonite nanocomposite samples were incubated under composting conditions and another batch of the same sample was immersed in aqueous conditions (in presence of NaN3, a microbial growth-inhibiting substance), both experiments set at temperature of 58 °C. Abiotic hydrolysis was later concluded to be a rate-limiting initial step of PLA biodegradation due to shorter lag phase which represent the onset of hydrolysis, a step preceding microbial mineralization of carbon in biodegradation [94]. Other synthetic polymers consist of carbon backbone into which prolonged exposure to UV light and irradiation of polymers generate radicals and/or ions (oxidation) that cleave to form lower molecular weight compounds/monomers [95]. This is also influenced by incorporation of

Biobased Biodegradable Polymers for Ecological Applications 237 light sensitive chemical additives such as dike tones, carbonyl-containing species that weaken the bond on presence of UV light. Once the initial photodegradation stage is complete, biodegradation with the help of microorganisms take over. Therefore, the exposure to UV radiation triggers biodegradation by initiating the first phase of biodegradation which is oxidation or photolysis. The duration of photo degradation is reduced by increasing radiation dosage exposed to a polymer [96]. Oxygen is among the powerful and rather a major chemical that facilitates degradation which result in free radicals by randomly attacking covalent bonds and in the environment exists as atmospheric form of oxygen (i.e., O2 or O3). The oxidative degradation depends on the polymer structure (i.e., unsaturated links and branched chains) [97]. Generally, UV radiations (290–400 nm) in sunlight are used in polymeric materials with outdoor application to determine their lifespan and majority of synthetic polymers is sensitive to UV and visible light. It was reported that UV infiltrates into a PLLA film with trivial reduction in UV intensity regardless of PLLA structure or crystallinity but amorphous region of the polyester photodegrade at higher degree than crystalline [98]. In a study carried by Zhang et al., [99] revealed that the pretreating of biopolymer with UV can shorten the lengthy compost degradation of biopolymers by using Polylactic acid. This was to come with an alternative method that is more effective that consume less energy than the current dominating method of hydrolysis for composting PLA since it does not degrade in landfills. Chopped and unchopped PLA samples were exposed to UV light for different time lengthstime (i.e., 0, 30, 60 and 90 min). During the exposure period physical change on polymer samples were observed which include among others, discoloration and brittleness and they were becoming more prominent as the duration increases. In their study they found out that the mass loss of the films increased with the duration of treatment. Elsewhere, it was reported that PLA immersed in water result in 93% molecular weight reduction in 9 days whereas its exposure to UV only takes a single hours to reduce by 96% [99]. They further concluded that an exposure to UV treatment before disposal to composting facility in the future can reduce the current PLA degradation time from days, even weeks, to hours as compared to widely used hydrolysis method and it may also aid in reducing the need for heating at high temperature for long periods which also diminish energy consumption, thus reduce cost [99]. Thermal degradation of polymers is molecular deterioration as a result of overheating. At high temperatures the components of the long chain backbone of the polymer can begin to separate (molecular scission) and

238 Integrating Green Chemistry and Sustainable Engineering react with one another to change the properties of the polymer. The chemical reactions involved in thermal degradation lead to physical and optical property changes relative to the initially specified properties. Thermal degradation proceed by oxidation which is accelerated by Infrared radiation (760–2500 nm) on the polymer material. The carbonyl and ester groups are generated when high energy is radiated on polymeric material causing oxidation, cleavage and other forms of degradation. The exposure to thermal environment promotes the oxidation of the carbon backbone of the polymer. PLA has been reported to be easily thermally degraded at temperature that range from 159–178 °C and the temperature varies with the molecular weight and crystallinity of the PLA. Thermal degradation of PLA occur via transesterification of oligolmeric PLA to lactide [100]. The thermal stability of biopolyesters is not significantly high, a fact that inevitably limits their range of applications. The PLA decomposition temperature lies between 230 °C and 260 °C. Gupta et al., [86] concluded that the carbonyl carbon–oxygen linkage is the most likely bond to split under isothermal heating, as suggested by the fact that a significantly larger amount of carboxylic acid end-groups were found compared with hydroxyl end-groups. The reactions involved in the thermal degradation of lactic acid-based polymers can follow different mechanisms, such as thermohydrolysis, zipper-like depolymerization in the presence of catalyst residues, thermo-oxidative degradation and transesterification reactions which give simultaneous bond breaking and bond making [101]. Polylactide (PLA) is unquestionably one of the most promising candidates for more developments because it is not only biodegradable but it is derived from renewable resources (for e.g. sugar beet and corn starch). Somehow, it is important to note that not all biodegradable polymers are derived from renewable resources and vice versa. A great deal of improvement has been made in the progression of biodegradable polymers that will have functional properties that are comparable to those of petroleum-based polymers. Different polymers are blended with bio-based polymers to attain desired properties and preserve their biodegradability. The use of renewable and sustainable resources has potential to circumvent issues of scarcity of energy and environment pollution when compared to other finite resources [1]. It is important to assess the biodegradability of polymers in their natural environments, to meet various commercial and environmental needs for their sustainable growth [102].

Biobased Biodegradable Polymers for Ecological Applications 239

8.6.1

Biopolymer Blend

The process of mixing biodegradable polymers with other polymers is another way that can help reduce the cost of the material and change polymer’s properties to obtain novel materials and change degradation rate. Blending process of biodegradable polymer with a non-biodegradable polymer can lower or even hinder the degradation of a biodegradable polymer. The biodegradable polymers offer a possible alternative to traditional non-biodegradable polymers to circumvent the waste-disposal issues that are related to use of traditional non-biodegradable polymers [1]. Most of the biodegradable polymers were intended to be used in packaging industries, in farming and also in specialized bio-medical applications. There have been plenty of studies done on blending of various biodegradable polymeric materials [103]. In spite the fact that PLA has a good balance of rheological and physical properties, a lot of additives have been combined with it to obtain various properties and rearrange to improve efficiency of material for specific end-use applications [104]. PLA has been blended with various polymers for different purposes, specifically for improving its stiffness and toughness balance [105].

8.6.2

Biocomposites

8.6.2.1 PLA-Natural Fibre Composite The PLA-untreated soy and PLA-wheat straw composites have confirmed to be biodegradable in compost with 90% biodegradation in 70 days, with untreated soy straw and wheat straw biomass achieving 90% biodegradation in 45 days and neat PLA reaching 90% biodegradability in a period of 100 days. As a result, this led to conclusion that natural biomass presence in PLA blend enhanced the degradability of PLA components. These obtained results lead to suggestion that these biocomposite with degradable components helps with maintaining the ASTM D6400 standard requirements of getting 90% degradation in 180 days under composting conditions. This was uncovered in the study conducted by Pradhan et al. [106] investigating the aerobic biodegradability of injection molded PLA-wheat straw and PLA-soy straw biocomposites with 70 – 30% ratio of polymer to filler, as well as their constituent materials. The investigation was done under composting conditions, according to the standard ASTM D5338, by using an acid-base titration procedure [106]. Films of pure or untreated PLA were reported to breakdown faster than natural fiber reinforced biocomposites. The investigation was carried out in the study that tested biodegradability of flax fiber reinforced

240 Integrating Green Chemistry and Sustainable Engineering PLA based biocomposites in the presence of amphiphilic additives by soil burial test. In the case where amphiphilic additives were used, with the presence of mandelic acid in flax reinforced PLA biocomposites, there was higher loss in weight that was observed, while in the presence of dicumyl peroxide there was less weight loss that was obtained for flax reinforced PLA biocomposites. This suggests that based on the intended use of the biocomposites, appropriate amphiphilic additives can be used to trigger biodegradation of composite [106].

8.6.2.2 PLA Based Composites from Petro Based Biodegradable Polymer Polybutyrate (PBAT) and Poly (lactic acid) (PLA) are prominent biodegradable polymers with several studies being conducted on their blending process and properties. PLA and PBAT are reported to possess different biodegradation mechanisms in accordance with the analysis that was conducted by Scanning electron microscope (SEM), Differential scanning calorimetry (DSC), Thermo gravimetric analysis (TGA), IR spectroscopy and elemental analysis. The analysis was conducted after PLA and PBAT samples were exposed to real soil environmental conditions for certain period of time. In addition, similar trends between individual polymer and their blend were observed in melting temperature and the melting point changes before and after biodegradation. The molecular structure of PBAT, PLA, and PBAT/PLA samples showed a decrease in carbon atom content and an increase in oxygen atom content which serve as evidence for degradation of the samples at the end of the process. The biodegradation rates of PBAT and PLA in the PBAT/PLA blend were not the same as those for the individual materials [107].

8.6.2.3 PLA-Non-Biodegradable Polymer from Renewable and Non-Renewable Sources Polypropylene PLA feature of being prone to hydrolysis has led to many efforts in attempt to improve their resistance to hydrolysis and biodegradation. PLA has been blended with different synthetic and bio based polymers in an effort to improve properties of PLA that limit its wide use. Polypropylene (PP) is one of the most cost effective and widely used synthetic polymers and it is widely used in variety of applications including packaging, labeling and textiles. However, this thermoplastic polymer has poor degradability with highly hydrophobic in nature. A blend of polylactic acid and polypropylene

Biobased Biodegradable Polymers for Ecological Applications 241 has incompatible systems with inferior physical properties in contrast to pure PLA or PP. However, it is reported that blending process of PLA and PP manage to improve PLA resistance to hydrolytic degradation. PP and PLA blend might prove to be an easy and effective method to develop a novel material that is not susceptible to hydrolysis, with sensible cost than PLA and faster degradation rate than PP [108].

8.6.2.4 PLA Based Composites from Renewable Biodegradable Polymer PLA/microbial Polyester Polyhydroxybutyrate (PHB) Disintegration of poly (lactic acid) and Polyhydroxybutyrate (PLA-PHB) blends were investigated under composting, where in less than a month the plasticized PLA-PHB blends were effectively disintegrated under composting conditions. In the newly formed blend of PLA/PHB, PHB hinder faster disintegration, while plasticizers enhance disintegration and speed it up. Considerable losses in mechanical properties for all blends were observed as TGA results reveal that plasticizers were lost in the primary stage of disintegration. The swift loss in mechanical properties is proved to be the presence of plasticizers that makes the blend prone to hydrolysis which leads to easy disintegration of the material. PLA and PHB blends with a suitable plasticizer can offer us a good biodegradable product that can find good use in food packaging application [107, 112, 113]. Polyhydroxybutarate (PHB) Blend Composite The two biopolymers poly (3-hydroxybutyrate-co-4-hydroxybutyrate) (Poly (3HB, 4HB)) and poly (lactic acid) are both effectively biodegradable, blending the two polymers can help us obtain biodegradable material with good novel properties. The biodegradability of P (3HB,4HB) and P(3HB,4HB)/PLA blends under real soil conditions was reported after investigation in real soil environments by the analysis of SEM, FTIR and elemental analysis of their degraded remains. In the study different biodegradation rates where obtain that associated with PLA content in the blend, with lesser biodegradation rate where there is higher PLA content [107]. Due to the different degradation mechanisms, P(3HB, 4HB) and PLA have the fastest degradation rate in soil depth, respectively. Their blends also have different degradation rates in different depths of soil with their blends also showing. In soil degradation testing results showed that soil depth affect degradation rates of samples that could be due to various mechanisms involved in degradation process. Poly(3HB, 4HB) and PLA

242 Integrating Green Chemistry and Sustainable Engineering showered faster degradation in soil depth of 20 and 40 cm with their blends also showing different degradation rates with different depths beneath [107]. Starch Composite Starch is one of the most important bio-based materials that can be used in process blending with other polymers. Several efforts have been made to prepare PLA and starch blend to reduce total raw materials cost and improves a material's biodegradability. Muniyasamy et al., [102] and Iovino et al., [109] studied the aerobic biodegradability of composite made of poly (lactic acid)/thermoplastic starch/natural fiber (coir) with the presence and the absence of maleic anhydride functioning as a coupling agent, under compost environmental conditions. After samples were exposed to compost environment conditions for period of 90 days, thermoplastic starch prove to have achieved a higher biodegradation rate than neat PLA in 90 days. Thermoplastic starch is favorably to immediate attack of microorganism that could have resulted to higher production of carbon dioxide. It is reported in the study that natural fibers seem to perform a secondary role in biodegradation of PLA by looking at the small difference in evolution of carbon dioxide [109]. The compatabilised PLA/thermoplastic starch/coir/maleic/anhydride composites show lower biodegradation with low CO2 production than the ones that are not compatibilised [109]. Furthermore, damages in the structure of samples after degradation and presence of microorganisms in surface of biodegraded materials were confirmed by analyzing the changes of degraded material at different incubation times. The investigation was done by thermal analysis, scanning electron microscopy and observing through light microscope to analyze compost degraded samples. The results obtained in this study conclude that PLA/cornstarch/ natural fiber-based composite materials are susceptible to biological degradation as a result they are suitable materials for disposal in landfills [109]. Clay Nano-Composite The biodegradation mechanism of PLA nanocomposite was investigated with different organo-modified montmorillonites (MMT) during thermophilic phase under composting condition. Outcomes of the study revealed that PLA nanocomposites treated with nanoclay have improved biodegradability properties when compared to neat PLA under compost conditions. Addition of nanoclay in PLA nanocomposites enhances biodegradability of PLA, mainly by reducing the lag phase of biodegradation at the beginning of the process. While in the case

Biobased Biodegradable Polymers for Ecological Applications 243 of neat PLA, it was reported that lag phase was witnessed on 27th day of compost environmental condition exposure, with biodegradation of PLA with native MMT was witnessed after 20th day and from 13 to 16th day for PLA with organo-modified MMT [94]. The critical molecular weight of PLA material hydrolysis was lower than the critical molecular weight of PLA material at the beginning of mineralization, which can be alluded to the need of material to be fragmented into tiny fractions so material can be easily assimilated by microbes. The Arthur concluded that to achieve full degradation of PLA under compost conditions, the critical molecular weight of PLA must be reached by making favorable condition by incubating PLA in conditions of an average temperature of 58 °C for period of 20 days [94]. Nanocomposite The polymer/layered silicate nanocomposite technology is said not to be limited to the improvement of mechanical properties of the polymeric material but it can similarly play a major role in improving the biodegradability properties of PLA. The biodegradability of polylactide in nanocomposites can be governed by thoughtful choice of using different organically modified layered silicate since it depends on nature of pristine layered silicates and the use of surfactants for the modification of layered silicate. Researchers investigated usefulness of polymer/layered silicate nanocomposite technology for the nanoscale enhancement of PLA biodegradability, as it is well know that the degradation rate of PLA is inadequate to meet the waste accumulation rate [94, 110]. Poly Aspartic Acid-Co-Lactide (PAL) Poly aspartic acid-co-lactide (PAL) is as an additive have consequence on degradability and thermal stability of PLA in soil, compost and water. A blend of Poly (lactic acid) and Poly (aspartic acid-co-lactide) (PAL) was reported to produce a homogeneous blend that show good mechanical properties, that are similar to that of the pure unblended PLA film. The good transparency of PLA was sustained by this new blend of Poly(lactic acid) and Poly(aspartic acid-co-lactide) preserving the precious values of PLA polymer. The degradability of the blend film was studied in phosphate buffered solution, in compost and soil, by analyzing the properties of the material where PLA hydrolysis is found to be accelerated by the presence of additives (PAL and PAL-Sodium). PAL being incorporated into a blend with PLA can advance thermal stability of PLA which was proven in the study by the melting test of PLA/PAL blend and it is reported to successfully improve the degradability of PLA in water, soil and compost. The

244 Integrating Green Chemistry and Sustainable Engineering blend of PLA and PAL have proven to suitable for variety of application, as it does not easily hydrolysis up until it come in contact with water giving it longer shelf life [97]. Polyvinyl Acetate Polyvinyl acetate (PVA) is a polymer that is prepared from ethylene that is known to be miscible with PLA. The PVA and PLA blend showed an increase in tensile strength as well as elongation at break, while low weight loss was observed in the course of biodegradation. It was also reported that the PVA and PLA blend became immiscible around 10% vinyl alcohol content when PVA was to some extent hydrolyzed to polyvinyl acetate-covinyl alcohol [104]. Thermoplastic Polyurethane (TPU) Thermoplastic polyurethane (TPU) is an essential commercial thermoplastic elastomer that is used for various biomedical applications [111]. Researchers conducted study on the biodegradability of polylactic acid and thermoplastic polyurethane blends, and the investigation was carried out with different blend ratios and morphologies of PLA/TPU blend. Results showed that unblended PLA starts to biodegrade just after 20 days, with the signs of full biodegradation of unblended PLA being observed after 70th days. PLA/TPU blend with globular morphology degraded at a lower initial rate when compared with the co-continuous morphology that degraded at higher initial rate.

8.7

Concluding Remarks

The rapid advancement in science and technology which mankind has achieved eventuate from rampant exploitation of natural resources for meaner economic gain resulted in expeditious depletion of energy source and raw materials. The main crisis today’s society facing is management of waste arises as an undesirable artefact of globalization and so called development. Biodegradable polymers are the need of the hour to tackle this colossal challenge. In this book chapter we have discussed various techniques employed for fabrication of biopolymer by natural means that are comparable superior to the tradition synthetic plastics which are hazardous to the environment. In future, we foresee the successful utilization of biopolymers in diverse fields. Moreover, their novel modifications to overcome their shortcomings in producing

Biobased Biodegradable Polymers for Ecological Applications 245 sustainable polymers will afford their opportunity towards different industrial applications.

Acknowledgements The authors are thankful to the NRF- DST Waste Research, Development and Innovation (RDI) funding 2017/18, IRG - Egypt / South Africa Research Cooperation Programme 2017/19; and CSIR – Parliamentary grant 2017/18.

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Biobased Biodegradable Polymers for Ecological Applications 249 59. Dumitriu, S., Polysaccharides in Medicinal Applications. Boca Raton, FL, CRC Press, 1996. 60. Daraghmeh, N.H., Chowdhry., et al., chitin. In Profiles of Drug Substances. In: G.B, ed. Excipients and Related Methodology Harry, . Waltham, MA, USA, Academic Press. pp. 35–102, 2011. 61. Kaur, S., Dhillon, G.S., Recent trends in biological extraction of chitin from marine shell wastes: a review. Crit. Rev. Biotechnol., 35(1), 44–61, 2015. 62. Harish Prashanth, K.V., Tharanathan, R.N., Chitin/chitosan: modifications and their unlimited application potential—an overview. Trends in Food Science & Technology, 18(3), 117–131, 2007. 63. Srinivasan, H., Kanayairam, V., Ravichandran, R., Chitin and chitosan preparation from shrimp shells Penaeus monodon and its human ovarian cancer cell line, PA-1. Int. J. Biol. Macromol., 107, 662–667, 2018. 64. Xu, Y., Gallert, C., Winter, J., Chitin purification from shrimp wastes by microbial deproteination and decalcification. Appl. Microbiol. Biotechnol., 79(4), 687–697, 2008. 65. Li, H., Greene, L.H., Sequence and structural analysis of the chitinase insertion domain reveals two conserved motifs involved in chitin-binding. PLoS ONE, 5(1), 8654, 2010. 66. Arbia, W., Arbia, L., Adour, L., Amrane, A., Chitin extraction from crustacean shells using biological methods–a review. Food Technol. Biotechnol., 51, 12, 2013. 67. Camci-Unal, G., Pohl, N.L.B., Quantitative determination of heavy metal contaminant complexation by the carbohydrate polymer chitin. J. Chem. Eng. Data, 55(3), 1117–1121, 2010. 68. Jang, M.-K., Kong, B.-G., Jeong, Y.-I., Lee, C.H., Nah, J.-W., Physicochemical characterization of α‐chitin, β‐chitin, and γ‐chitin separated from natural resources. J. Polym. Sci. A Polym. Chem., 42(14), 3423–3432, 2004. 69. Anitha, A., Sowmya, S., Kumar, P.T.S., Deepthi, S., Chennazhi, K.P., Ehrlich, H., et  al., Chitin and chitosan in selected biomedical applications. Prog. Polym. Sci., 39(9), 1644–1667, 2014. 70. Gimeno, M., Ramírez-Hernández, J.Y., Mártinez-Ibarra, C., Pacheco, N., García-Arrazola, R., Bárzana, E., et  al., One-solvent extraction of astaxanthin from lactic acid fermented shrimp wastes. J. Agric. Food Chem., 55(25), 10345–10350, 2007. 71. Jayakumar, R., Menon, D., Manzoor, K., Nair, S.V., Tamura, H., Biomedical applications of chitin and chitosan based nanomaterials—A short review. Carbohydr. Polym., 82(2), 227–232, 2010. 72. Tharanathan, R.N., Kittur, F.S., Chitin — The Undisputed Biomolecule of Great Potential. Crit. Rev. Food Sci. Nutr., 43(1), 61–87, 2003. 73. Muffler, K., Ulber, R., Downstream processing in marine biotechnology. Marine Biotechnology II. Berlin, Heidelberg, Springer. pp. 63–103, 2005.

250 Integrating Green Chemistry and Sustainable Engineering 74. Shahidi, F., Shellfish discard utilization. Shahidi, F., Jones,Y., Kitts, D. D., (Eds.). Seafood Safety, Processing and Biotechnology. Lancaster, Technomic Publishing Co. pp. 131–138, 1997. 75. No, H.K., Meyers, S.P., Preparation and Characterization of Chitin and Chitosan—A Review. Aquat.Food. Prod.Technol., 4(2), 27–52, 1995. 76. Venugopal, V., Marine products for healthcare : functional and bioactive nutraceutical compounds from the ocean. CRC press, 2008. 77. Venugopal, V., Marine polysaccharides: Food applications. CRC Press, 2016. 78. Mahlous, M., Tahtat, D., Benamer, S., Nacer Khodja, A., Nuclear Instruments and Methods in Physics Research Section B. Beam Interactions with Materials and Atoms, 265(1), 414–417, 2007. 79. Khor, E., Chitin: fulfilling a biomaterials promise. Elsevier, 2014. 80. Heux, L., Brugnerotto, J., Desbrières, J., Versali, M.-F., Rinaudo, M., Solid state NMR for determination of degree of acetylation of chitin and chitosan. Biomacromolecules, 1(4), 746–751, 2000. 81. Sagheer, F.A.A., Al-Sughayer, M.A., Muslim, S., Elsabee, M.Z., Extraction and characterization of chitin and chitosan from marine sources in Arabian Gulf. Carbohydr. Polym., 77(2), 410–419, 2009. 82. Maniukiewicz, W., X-ray diffraction studies of chitin, chitosan, and their derivatives. Chitin, Chitosan, Oligosaccharides and Their Derivatives Biological Activities and Applications. USA, CRC Press, Taylor & Francis Group. pp. 83–94, 2011. 83. Carlstrom, D., The crystal structure of α-chitin (poly-N-acetyl-d-glucosamine). J. Biophys Biochem Cytol, 3(5), 683–683, 1957. 84. Minke, R.A.M., Blackwell, J., The structure of α-chitin. J. Mol. Biol., 120(2), 181–181, 1978. 85. Dweltz, N.E., The structure of ß chitin. Biophysica Acta, 51, 289, 1961. 86. Gupta, B., Revagade, N., Hilborn,J P., Poly(lactic acid) fiber: An overview. Prog. Polym. Sci., 32(4), 455–482, 2007. 87. Barikani, M., Oliaei, E., Seddiqi, H., Honarkar, H., Preparation and application of chitin and its derivatives: a review. Iran. Polym. J., 23(4), 307–326, 2014. 88. Kim, B.-S., Park, I.-K., Hoshiba, T., Jiang, H.-L., Choi, Y.-J., Akaike, T., et al., Design of artificial extracellular matrices for tissue engineering. Prog. Polym. Sci., 36(2), 238–268, 2011. 89. Li, Z., Ramay, H.R., Hauch, K.D., Xiao, D., Zhang, M., Chitosan–alginate hybrid scaffolds for bone tissue engineering. Biomaterials, 26(18), 3919–3928, 2005. 90. Jayakumar, R., Nair, A., Rejinold, N.S., Maya, S., Nair, S.V., Doxorubicinloaded pH-responsive chitin nanogels for drug delivery to cancer cells. Carbohydr. Polym., 87(3), 2352–2356, 2012. 91. Ravi Kumar, M.N.V, Kumar, M.N.R., A review of chitin and chitosan applications. Reactive and Functional Polymers, 46(1), 1–27, 2000.

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252 Integrating Green Chemistry and Sustainable Engineering 107. Weng, Y.-X., Jin, Y.-J., Meng, Q.-Y., Wang, L., Zhang, M., Wang, Y.-Z., Biodegradation behavior of poly(butylene adipate-co-terephthalate) (PBAT), poly(lactic acid) (PLA), and their blend under soil conditions. Polym. Test., 32(5), 918–926, 2013. 108. Reddy, N., Nama, D., Yang, Y., Polylactic acid/polypropylene polyblend fibers for better resistance to degradation. Polym. Degrad. Stab., 93(1), 233–241, 2008. 109. Iovino, R., Zullo, R., Rao, M.A., Cassar, L., Gianfreda, L., Biodegradation of poly(lactic acid)/starch/coir biocomposites under controlled composting conditions. Polym. Degrad. Stab., 93(1), 147–157, 2008. 110. Ray, S.S., Yamada, K., Okamoto, M., Ueda, K., Control of biodegradability of polylactide via nanocomposite technology. Macromol. Mater. Eng., 288(3), 203–208, 2003. 111. Lelah, M.D., Cooper, S.L., Polyurethanes in medicine. CRC press, 1986. 112. Arrieta, M.P., López, J., Hernández, A., Rayón, E., Ternary PLA–PHB– Limonene blends intended for biodegradable food packaging applications. Eur. Polym. J., 50, 255–270, 2014. 113. Arrieta, M.P., López, J., Rayón, E., Jiménez, A., Disintegrability under composting conditions of plasticized PLA–PHB blends. Polym. Degrad. Stab., 108, 307–318, 2014. 114. Castro-Sowinski, S., Burdman, S., et al., Natural functions of bacterial polyhydroxyalkanoates. Plastics from bacteria. Berlin, Heidelberg, Springer. pp. 39–61, 2010. 115. Cheng, S., Wu, Q., Zhao, Y., Zou, B., Chen, G.-Q., Effect of poly(hydroxybutyrate-co-hydroxyhexanoate) microparticles on growth of murine fibroblast L929 cells. Polym. Degrad. Stab., 91(12), 3191–3196, 2006. 116. Derraik, J.G.B., The pollution of the marine environment by plastic debris: a review. Mar. Pollut. Bull., 44(9), 842–852, 2002. 117. Gall, S.C., Thompson, R.C., The impact of debris on marine life. Mar. Pollut. Bull., 92(1-2), 179–179, 2015. 118. Koutny, M., Lemaire, J., Delort, A.-M., Biodegradation of polyethylene films with prooxidant additives. Chemosphere, 64(8), 1243–1252, 2006. 119. Kumar, P.T.S., Abhilash, S., Manzoor, K., Nair, S.V., Tamura, H., Jayakumar, R., Preparation and characterization of novel β-chitin/nanosilver composite scaffolds for wound dressing applications. Carbohydr. Polym., 80(3), 761– 767, 2010. 120. Kumar, R., Yakubu, M.K., Anandjiwala, R.D., Biodegradation of flax fiber reinforced poly lactic acid. Express Polym. Lett., 4(7), 423–430, 2010. 121. Lütke-Eversloh, T., Steinbüchel, A., Luftmann, H., Bergander, K., Identification of a new class of biopolymer: bacterial synthesis of a sulfurcontaining polymer with thioester linkages. Microbiology, 147(1), 11–19, 2001. 122. Lütke-Eversloh, T., Fischer, A., Remminghorst, U., Kawada, J., Marchessault, R.H., Bögershausen, A., et  al., Biosynthesis of novel thermoplastic

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9 Cashew Nut Shell Liquid (Phenolic Lipid) Based Coatings: Polymers to Nanocomposites Fahmina Zafar1,*, Anujit Ghosal2,3, Eram Sharmin4,* and Nahid Nishat1 1

Inorganic Materials Research Laboratory, Department of Chemistry, Jamia Millia Islamia, New Delhi, India 2 Nanobiotechnology lab, School of Biotechnology, Jawaharlal Nehru University, New Delhi, India 3 School of Life-sciences, Beijing Institute of Technology, Beijing, PRC 4 Department of Pharmaceutical Chemistry, College of Pharmacy, Umm Al-Qura University, Makkah Al-Mukarramah, Saudi Arabia

Abstract In coatings industry, the substitution of petroleum derived materials by bio-based materials has been in trend because the latter are renewable, abundantly available, inexpensive, non-toxic and biodegradable. Many bio-based materials such as seed oils, chitin/chitosan, protein, starch and cashew nut shell liquid (CNSL) are used as coating materials as an alternative to petroleum derived coating materials with comparable properties. Among them CNSL is regarded as a versatile and valuable raw material for production of thermally stable polymeric coatings. CNSL contains phenolic hydroxyl, aromatic ring, along with long alkyl chain with different unsaturation and inherent fluidity behavior. These features collectively enable it to form low molecular weight polymeric coating materials via various chemical transformations. The present chapter aims to give a brief overview on the development of CNSL based polymeric and nanocomposite coating materials along with their applications. Keywords: CNSL, cardanol, coatings, antibacterial, antifouling, anticorrosive

*Corresponding authors: [email protected]; [email protected] Shahid-ul-Islam (ed.) Integrating Green Chemistry and Sustainable Engineering, (255–290) © 2019 Scrivener Publishing LLC

255

256 Integrating Green Chemistry and Sustainable Engineering

9.1

Introduction

During the last few decades, polymers and their nanocomposites have acquired a central position among various advanced materials with their wide applications. These materials exhibit versatile properties due to their low density, high specific strength, good physico-mechanical, electronic and optical properties. They find promising applications in domestic as well as high-tech industries in form of adhesives, inks, lubricating materials, packaging materials, paints and coatings [1–3]. Globally, coating industry uses huge amount of polymers and has a massive turnover of more than 110 billion USD and would exceed USD 227.70 billion by the year 2025 (Grand View Research, Inc, July 25, 2017) [Global paint and coatings industry market analysis]. Apart from that, more than 4 trillion USD have been spent on corrosion worldwide, whereas 310 USD is only spent by China [4]. Coating may be defined as the process of covering the surface of an object with a thin or thick layer of organic, inorganic or organic -inorganic polymers for their protection and aesthetic appeal. These coating materials can be casted on surfaces such as paper, fabric, metallic or non-metallic surfaces, foils, devices, or sheet stocks, using various techniques [5]. They can be applied in solid state (powder coatings), gaseous suspension state (aerosol) and liquid emulsion state (paint), depending on their practical or aesthetic requirements. Various types of binders are used in the formulation of coatings and can be broadly classified as inorganic or organic in nature. Depending on the nature of binder, coatings are broadly classified as organic and inorganic coatings. Inorganic coatings are brittle in nature, have poor adhesion, non-suitable for various designs and substrates, which limits their wide application while polymeric organic coatings generally comprise of binders, fillers, pigments, solvents and other additives. The carbon based polymeric materials comprise of binder (matrix or vehicle) which is considered to be the main film-forming component, imbibing all other components of organic coatings such as fillers, pigments, solvents, additives like driers, cross-linkers and antifoaming agents [6]. The pigments provide aesthetic sense, sealing and healing effect to coatings for better anti-corrosive performance. Fillers can impart special properties such as high toughness, good texture, UV resistance, conductivity, corrosion protection, abrasion and wear resistance, improved flow properties, anti-freezing and control foaming properties [7, 8]. Organic surface coatings are successfully used for the purpose of protection (in more than 80% areas) because they are flexible, impact resistant, with economical application. Their ease of application, effective protection and suitability

Cashew Nut Shell Liquid (Phenolic Lipid) Based Coatings 257 to almost all structural designs and environments provides and extra edge to them [9–11]. Various prominent polymers are used in industries, however, they lack desirable physico-mechanical and chemical properties like toughness, impact resistance, scratch hardness, acid and alkali resistance. Furthermore, the concerns pertaining to depletion of fossil fuels, increase in cost of petro-based products and environment pollution have put forward the need for development of alternative eco-friendly sustainable resource based polymeric coating materials. Sustainable polymers being low-molecular weight, find extensive applications in field of adhesives, inks, lubricants, packaging materials, paints and coatings [12, 13]. The drawbacks of organic and inorganic coatings can be overcome by application of sustainable green precursors and nanotechnology. The use of nanomaterials and renewable precursors leads to the development of new generation coating materials like bio-based nanostructured coatings [14–18]. Bio-based polymers and nanocomposite coatings provide promising and superior protective performance under different environments for materials under service conditions and have great academic/industrial scope. The objective of the chapter is to discuss briefly CNSL (Col) based polymeric and nanocomposite coating materials along with their applications. The salient features of nanocomposite materials, significance of functional attributes of CNSL constituent and their effect on coatings performance are also covered in this chapter.

9.2

CNSL (Col)

CNSL is an aromatic renewable resource obtained from cashew plant Anacardium occidentale L and its production approaches 4,50,000 tons, annually. CNSL and its derivatives have gained much attention in field of coatings, along with other applications by scientists, and industrialists due to their sustainability, non-toxicity, low cost, non-edible nature, biodegradability characteristics along with abundant availability worldwide. CNSL is available as natural and technical on the basis of method of extraction. The natural CNSL contains mainly Anacardic acid (64–65%) whereas technical CNSL contains Cardol, 2-Methyl cardol and Cardanol (Col, 62–63%). (Figure 9.1). The industrial grade Col contains mainly 90% phenol along with long alkyl chain. Col known as phenolic lipid is obtained from the agricultural waste of CNSL. Col contains phenolic aromatic ring along with long unsaturated alkyl chain (C15, 48–49% pentadecyl and 29–30% pentadecatrienyl) (Figure 9.1) that makes it good platform for chemical transformation to obtain tailor

258 Integrating Green Chemistry and Sustainable Engineering

Cashew apple

Nut

OH

OH

Shell

HO

OH

HO

OH

CNSL COOH R

R

Anacardic acid

R=

Cardanol

R

Cardol

R

2-Methyl cardanol Pentadecyl (5–8%) Pentadecenyl (48–49%) Pentadecadienyl (16–17%) Pentadecatrienyl (29–30%)

Figure 9.1 The main constituents and chemical structure of CNSL.

-made materials. These functional attributes give perfect balance of hardness and flexibility to the coatings. Its side chain olefinic bonds are susceptible to air assisted autoxidation and phenolic hydroxyl along with aromatic ring provide functional sites that are collectively able to facilitate the formation of 3D self/photo or UV -cross-linked robust coatings for various fields of applications.

9.3

CNSL (Col) Based Polymeric Coatings

Nowadays, development of polymers and nanocomposites from renewable resources based feed-stocks has gained much attraction as an alternative of petroleum-derived counterparts due to their day by day increasing cost, toxicity and most importantly depletion of fossil resources in near future that reflect a global requirement of sustainability. Renewable materials such

Cashew Nut Shell Liquid (Phenolic Lipid) Based Coatings 259 as vegetable seed oils, fatty acids, glycerol, rosin gum, sorbitol, chitin and chitosan, starch, protein, CNSL (Col) have been used in the development of polymeric coating materials. These polymeric materials such as alkyds, polyesters, polyesteramides, polyetheramides, epoxies, polyurethanes, and others have been used in paints and coatings [13, 19]. In addition to this, the tremendous increasing demand of environmentally friendly coatings such as high solids, waterborne, UV-cured and powder coatings has been observed since past few decades. This demand is due to environmental concerns such as hazardous volatile organic contents (VOCs) emission and recycling or waste disposal problem of polymers at the end of their economic lifetime along with government regulations. CNSL(Col)-based polymer resins like resole, novolac, epoxies, phenalkamine, benzoxazines, polyurethanes and others have been developed that find applications in environmental friendly surface coatings along with other industrial applications. The utilization of CNSL (Col), a renewable resource, may contribute to environmental friendly and green polymeric paints and coating materials [20]. CNSL is associated with combination of functional groups like phenolic hydroxyl, aromatic ring and unsaturated long alkyl chain. It shows similar kinds of reactions as those of phenol due to presence of phenolic hydroxyl and the presence of long alkyl chain provides additional sites for reaction. CNSL has transformed into different high performance polymers through these functional sites via different chemical reactions (Figure 9.2) [20, 21]. These polymers have found applications in the field of high performance paints and coatings (Figure 9.3) because of important characteristic feature of CNSL and its polymers that are suitable for paints and coatings. These characteristic features are solubility in different solvents, inherent hydrophobicity, weatherability, chemical (acid and alkali) resistance, film forming ability and good adhesion.

9.3.1 CNSL (Col)-Epoxy Coatings A polymer containing epoxy group is most commercially used versatile class of thermoset polymer that possesses excellent adhesion, chemical resistance, mechanical properties, anticorrosive and thermal stability characteristics. These properties make it suitable candidate for coating applications. Generally, epoxy resin does not cure by itself, so, it is mixed with other compounds/monomers/polymeric resins as a curing agent/hardener to obtain cross-linked structure. Amines, polyamides, anhydride, carboxyl and phenolic compounds are the most commonly used epoxy curing agents.

260 Integrating Green Chemistry and Sustainable Engineering Sulphonation

–

OR SO3

M+

C15H31 H2SO4, Zn, Cu

–H2O

Es

n

OR

at

io

O C15H31

O NaCI

RCI

tio

n

+ HCI C15H31

–HCI

EC H OH

OH

C

Na

6H 5C

OC

I

C15H31

ca

Al ka li

+

rifi

OCOC6H5

Ep

ox id

te

–H

HN 4 SO

2

–H

C15H31

rifi

OH

OH O2N

te

3

2O

OR

Es

O

+ C15H31

C15H31 NO2

ca

tio

Ni

n

tra tio n

–R

2 SO 4

C15H31

Figure 9.2 Typical chemical reactions of CNSL with some acidic and basic moieties [20].

Col-epoxy (CEO) resin was developed for partial substitution of commercial epoxy. It was synthesized from epichlorohydrin, bisphenol A and Col that exhibited better properties such as tensile strength, elongation, and lower water vapor transmission of films, as compared to epoxy resin [22]. Paints were developed with this resin, in presence of zinc powder, zinc phosphate, micaceous iron oxide and synthetic iron oxide as pigments, fillers, additives and hardeners (aromatic polyamine adduct) and coatings of the paints showed superior performance than unmodified paints. The films exhibit higher tensile strength (25%) and elongation (about 15%) while value of water vapor transmission lowered down to about 20% than paints made with unmodified resins. The coatings showed good scratch hardness, adhesion, flexibility and abrasion resistance.

Cashew Nut Shell Liquid (Phenolic Lipid) Based Coatings 261 OH OCH2CHCH2OH Epoxy coating

Po ad lyam du in ct e

MDI O

O

is ys ol dr Hy

CH15H31 DBTDL

Polyurethane coatings

Modified polyol

C15H31 HO

OH

ECH NaOH OH

Epoxy, 180ºC

OH

Epoxy-Phenolic coating

High Mol.Wt C15H31

n e io in at liz tam ra ut no OH HO Ne tha e O O Di

C15H31 HCHO

Modified phenolic resin

Pd. Co Driers

Water borne coatings

180–200ºC C15H31

H+ C15H31

OH Malenization

Water soluble binder

NaOh, Reflux

Chlorohydrin

O

O CH HO

OCH2CHCH2OH

IP DL H m yd C15CH31 eth rox ac yet ry hy ta l te

P Lin htha se lic ed an ce oil, hyd ro rid l

Gl

OH

e Pd, Co

Modified alkyd resin UV-Curable Photoinitiator coating material Diluent

Driers

Modified alkyd based coatings

Modified acrylic coating

Figure 9.3 Coatings based on CNSL and its derivatives, obtained through two or three step chemical reactions [20].

The chemical resistance performance of paints was found superior than unmodified epoxy paints. Different CEO based films/coatings systems were prepared by other groups by blending them with diglycidyl ether of bisphenol-A (DGEBA) in different weight percentages and the coatings showed better performance (pencil hardness, impact resistance and chemical resistance) with 40–60% DGEBA as compared to system with virgin DGEBA coating systems [23]. Bio-based epoxy blends that were used as coatings were synthesized by Emilie Darroman et al.,. Isophorone diamine and Jeffamine T403 cured commercial epoxidized-Col (from CNSL) was developed to produce materials exhibiting good coating properties. Blending of CEO-derived materials with sorbitol and isosorbide (sucrose epoxy derivatives) enhanced the coating performance. Enzymes catalyzed (lipase and peroxidase) epoxide-polycardanol was developed by two different routes: (i) synthesis of epoxide containing Col in presence of lipase followed by polymerization of phenolic hydroxyl group using peroxidase, and (ii) peroxidase catalyzed synthesis of polymerized Col from Col, subsequently lipase catalyzed synthesis of epoxide-polycardanol from polycardanol. The thermally cured (at 150 °C) films were found to be transparent with improved pencil hardness as compared to polycardanol coatings. Phenalkamine cured films showed relatively shorter curing time with higher hardness value that can be due to the presence of epoxide contents in polymerized Col [24]. Phenalkamines were synthesized from Col, formaldehyde and polyamines (Mannich base reaction product) and characterized and used as curing agent to cure DGEBA at room temperature. The cured system showed good adhesion with different metal surfaces, generally high adhesion values were observed with copper surface

262 Integrating Green Chemistry and Sustainable Engineering O OH

OH

O O

O

S

and R

R Cardanol

OH

NaOH, PTC

S

O

CI

H2N R

and R

S O

O

NH2 and

O

R

R

O

Cardol O

S

Figure 9.4 Synthesis of Col/Cardol-epoxy and episulfide by [26].

because of high surface energy [25]. DGEBA was blended with episulphide and epoxy groups containing Col/Cardol mixture to form bio-based materials CCES and CCEO and cured with polyamide, for anticorrosive coating applications and compared with CEO and episulphide containing CEO (CES) system (Figure  9.4). Episulphide group exhibited faster curing than epoxide. CCES-DGEBA blend system displayed better adhesion to the metal surface as compared to DGEBA that was tested by water absorption and peel strength test. CCES-DGEBA blend system with 20 wt % of CCES performed optimum corrosion resistance performance. The corrosion test was performed by electrochemical analysis such as corrosion current (Icorr), corrosion potential (Ecorr), corrosion rate (CR) and corrosion resistance efficiency of all the coatings were analyzed. Tafel plot was constructed by measuring Potential vs I and Tafel extrapolation of these curves was done in their linear region to the point of intersection that provided both Ecorr and Icorr of the coatings. It was observed that more negative Ecorr and larger Icorr corresponds to faster corrosion while more positive Ecorr and smaller Icorr corresponds to slower corrosion process [27]. This system possessed higher modulus at low frequencies in EIS bode plots, lowest corrosion current and high corrosion voltage in Tafel test (Figure 9.5) [26]. Col-benzoxazine surfactant was prepared from epoxy-col that can be used as a stabilizer for epoxy aqueous emulsion in the field of coating applications (Figure 9.6). The coatings were prepared by mixing stoichiometric ratio of curing agent with respect to epoxy groups along with 35% water content. Two epoxies such as diglycidyl ether type epoxy resin and epoxidized soybean oil (ESO) were used as dispersed phase and it was found that the surfactant showed excellent compatibility with both epoxies and effectively copolymerized with both of them. The coatings were formed by drying the emulsion and subsequently curing at 150 °C. The incorporation of polybenzoxazine into epoxy improved the thermo-mechanical performance like high value of storage modulus and higher cross-linked density of virgin epoxy resins [28].

Cashew Nut Shell Liquid (Phenolic Lipid) Based Coatings 263 7.5 –5

7.0

–6

6.5 6.0 5.5

Log IZI (Ω)

2

Log (A/cm )

–7 –8 –9 DGEBA CEO-20% CCEO-20% CES-20% CCES-20%

–10 –11 0.0

–0.2

–0.4

–0.6

–0.8

–1.0

–1.2

5.0 4.5 4.0 3.5 3.0 2.5 2.0

–12

–1

–1.4

E (V vs SCE)

(a)

DGEBA CEO-20% CCEO-20% CES-20% CCES-20%

0

1

2 3 Logf (Hz)

(b)

System

Corrosion voltage (V)

Corrosion current (A)

Resistance (Ω)

DGEBA CEO-20% CCEO-20% CES-20% CCES-20%

–0.73 –1.00 –0.82 –0.70 –0.39

3.47 × 10–9 1.32 × 10–8 6.35 × 10–9 8.81 × 10–10 1.43 × 10–10

1.24 × 107 2.66 × 106 5.11 × 106 4.90 × 107 1.85 × 108

4

5

6

(c)

Figure 9.5 Tafel plot of modified system and DGEBA (a), Bode plots of EIS measurement, samples were immersed for 2 hr prior to measurement(The frequency range was set from 100 mHz to 1MHz and amplitude of the alternating current was set as 10 MV) (b), and data comes from Tafel curve by [26]. OH

OH

H2N

O

H2O2/H3CCOOH R1 Cardanol

R2

O

O 3

19

N O

2 (HCHO)n

O 3

19

R2

Epoxidized cardanol

BOX

O R1=

R2=

O O

O O

O

Figure 9.6 Synthesis of Epoxy-Col based benzoxazine surfactant (BOX). The unsaturated long chains substitution in Col may be in cis and /or trans conformation by [28].

Fabrication of CNSL based resins such as phenolic, epoxy, polyurethane, vinyl ester and benzoxazines used formaldehyde that is hazardous to human health (causes sick house syndrome). An alternative research for formaldehydefree resin preparation has been carried out. The reaction of CNSL (Col) with epichlorohydrin in presence of a base formed epoxy-col that on thermal treatment gave CNSL-epoxy prepolymer (ECP) via auto-oxidation of unsaturated alkyl chain (first reaction part). ECP was synthesized by thermal technique

264 Integrating Green Chemistry and Sustainable Engineering without the use of formaldehyde and catalyst as an alternative to commercial CNSL coatings that used organic solvent and formaldehyde. Yellow colored self-standing coatings of ECP prepared by mixing of ethylenediamine (EDA), diethylenetriamine (DETA) and tetraethylenepentaamine (TEPA) (5–15%) at room temperature took about 2.5 hr to harden dry, that is faster than that of commercial cashew coatings. This can be due to crosslinking reaction between epoxy and amine group. The coatings were rubbery at room temperature and showed good chemical stability [29]. Significant improvement in mechanical, chemical and thermal properties along with excellent corrosion resistance properties of commercial epoxy resin were observed by using Col based anhydride as curing agent [30]. Col based phosphorous-sulphur containing di and tetra functional carboxyl curing agents were synthesized for their use in high performance epoxy coatings in the field of anticorrosive and flame resistance applications [31]. The coatings exhibited excellent mechanical and chemical properties along with high thermal stability. It was found that flame retardant and anticorrosive properties of commercial epoxy coatings enhanced with flame-retardant curing agents that can be related to the synergistic effect of phosphorus-sulphur along with chemical structure of the curing agent.

9.3.2

CNSL (Col) Polyamides (CPAs) Coatings

Col-based reactive polyamides (CPAs) with different amine values were prepared for epoxy coating application. The synthesis of amides was carried out by two step reaction: carboxyl functionalization of Col (Figure 9.7) followed by its condensation with polyamine (Figure 9.8). The synthesized CPAs were designated as CPA270, CPA370 and CPA470 (last numeral represent their respective amine number). The synthesized resins were used as curing agents for DGEBA to form coatings. The gloss values were observed in the range of 94–99o at 60o. All the coatings showed excellent adhesion (5B) and flexibility due to the presence of amide linkages that help in strong hydrogen bonding, improving adhesive force at metal-coating interface and long alkyl chains of polyamide impart excellent flexibility. The coatings showed excellent surface hardness. The scratch hardness and pencil hardness increased with amine value of CPAs that can be due to increased cross-linked density. The coating with higher amine value was unaffected after 48 hr of exposure under 5 % HCl while other systems showed dense blistering after 24 hr of immersion. In alkali and water media, all the coatings system showed no blistering even after 48 hr of immersion. The chemical resistance test performed by solvent rub test showed no effect on physical properties even after 2000 cycles with xylene. All coating systems showed slight loss in gloss after 150 rub cycle

Cashew Nut Shell Liquid (Phenolic Lipid) Based Coatings 265 OH

CNSL O O Maleic anhydride

180–200ºC

O OH OH

O

O

O

Diels-Alder mechanism after conjugate double bond formation O

O

O

O OH

Grafting via Ene-type addition O O

n HO

O

O

O

Addition reaction via phenolic hydroxyl

Free radical polymerization of double bond

Figure 9.7 Functionalization of Col to make it a versatile starting material [32].

in methyl ethyl ketone. After 250 hr of exposure, small blisters appeared in all the coating systems under salt spray test (5% NaCl). CPA470 coatings showed comparatively lesser corrosion across the scribed area even after 500 hr of exposure. CPAs showed excellent humidity resistance after 500 hr of exposure (Figure 9.9). Overall, the CPAs system showed good balance between flexibility and hardness performance as well as excellent chemical resistance, good thermal stability (10 % weight loss in the range of 240–400 °C) and anticorrosive properties [32]. The performance of CPAs

266 Integrating Green Chemistry and Sustainable Engineering COOH

HOOC

HO H2 C

C H

C H

H2 C

C H

C H

C H

H2 C

+

CH2

H N

H2N

Diethylenetriamine

Hydrolyzed maleinized cardanol

–2H2O

160-170ºC

H2N H2C HO

NH2

NH2 CH2 H2 C

HN

H2 C T

C H

C H

2

H N

H2 C

O

O

C

C

C H

C H

H N

C H

H2 C

H2 C

H2C 2

CH2

NH

CH3

Cardanol based reactive polyamide

Figure 9.8 Synthesis of reactive polyamide from functionalized Col [32].

(a)

(d)

(b) (c)

Figure 9.9 Coatings cured with polyamide (a) acid (5% HCl) resistance after 48 hr, (b) alkali resistance (5% NaOH) resistance after 48 hr, (c) Salt spray images (a) at 0 hr, (b) at 250 hr, (c) at 500 hr and (d) at 750 hr, and (d) humidity resistance (a) at 500 hr and at 750 hr by [32].

coatings may be due to influence of increased cross-linked density and polar moieties like amide linkages and secondary –OH groups along with long alkyl chain, shielded beneath the cross-linked network, leading to good bonding with metal substrate and inhibiting the penetration of corrosive chemicals through coatings.

Cashew Nut Shell Liquid (Phenolic Lipid) Based Coatings 267

9.3.3

CNSL (Col)-Formaldehyde/ Furfuraldehyde Coatings

CNSL (Col) reactions are similar to phenols. It reacts with formaldehyde via condensation reaction in molar excess under the influence of base to form resoles while reaction with formaldehyde in lower molar ratio forms novolac in presence of different acids. The former resin self-cured but latter needed some cross-linker to form cured coatings. Col based novolac resins have been used as modifier and curing agent for commercial epoxy resins that improved toughness and other mechanical properties. The curing reaction involved two step reaction such as reaction between epoxy, hydroxyl and carboxyl groups of CNSL’s components in the first steps while in the second step, the esterification reaction between the epoxy group and the hydroxyl group that generated from 1st step of the reaction. The activation energy was found to be 45 kJ/mol and 60 kJ/mol for these two aforementioned steps of curing of epoxy/CNSL (60/40), respectively [33]. Styrenated CNSL-phenolic resin has shown improved paint performance. Phosphorylated CNSL-formaldehyde gives coatings with improved heat resistance, adhesion and flexibility. Coatings with inherent insecticidal properties were obtained by addition of dichlorodiphenyltrichloroethane or gamexane to CNSL-formaldehyde or chlorinated CNSL-formaldehyde. Water-based coating material was developed by reaction of chloroacetic acid or bromoacetic acid with condensation product of CNSL, phenol and formaldehyde, in presence of base, followed by neutralization with ammonium hydroxide or amine. The coating material showed good hot water and chemical resistance properties. The incorporation of copper aceto-arsenite or linseed oil rosin condensate into CNSL-aldehyde produced anti-fouling paints with improved flexibility and drying characteristics that can be due to the presence of unsaturated drying oil. Furfuraldehyde (Fur, aromatic aldehyde obtained from agricultural waste) was used instead of formaldehyde for preparation of novolac (ratio of Col:Fur 1:0.6, 1:0.7 and 1:0.8) for green coating formulation (Figure 9.10). HMTA (15 %) was used as curing agent to cure furfuraldehyde novolac resin. The coatings showed better mechanical (except hardness), chemical resistance (except acid resistance) and thermal properties as compared to Col-Formaldehyde resin [34]. Both epoxy and hardener were prepared to develop room temperature cured “green” bio-based organic coatings for petroleum and gas steel in marine environment. The epoxy resin (CNE) was prepared by epoxidation of hydroxyl phenol and unsaturated double bonds of Col novolac pre-polymer, whereas hardener (CPA) was based on polyamine Col prepared by triethylenetetramine (TETA) linking with methyloyl derivatives

268 Integrating Green Chemistry and Sustainable Engineering OH

OH + + CHR-OH

Slow

C15H29

OH

H

Fast

CHR-OH

+

CHR OH + +H

C15H29

OH

C15H29

OH + + CHR-OH C15H29

Slow

OH Fast

+ H

+H

C15H29 CHR-OH

+

C15H29 CHR-OH O

OH

OH

OH + CH-R

OH

CH

+ + H

+ C15H29

C15H29

C15H29

C15H29

CH

CH

R O

R

CH

CH

OH

OH

CH R

R CH

HC

+ + H C15H29

C15H29

CH

R

+

R-CH 4

O

OH

OH

OH

C15H29

O

OH

R O

HC

R CH

O

OH

O

CH

CH OH

O

OH

R-CH C15H29

OH

R CH OH

OH + +H

C15H29

R + CH OH2

OH CHR + H2O

C15H29

C15H29 OR

OH

OH

OH

+ +H C15H29 CHR-OH

C15H29 + H2OH2C R

C15H29

+ H2O

CH-R +

Figure 9.10 Reactions involved during the synthesis of Col-Fur novolac resin by [34].

(Figure 9.11). CNE and CPA were blended in different ratio, 1:1, 2:1 and 3:1, respectively, for coating preparation, coatings were obtained in cured form at room temperature after 7 days, as clear, glossy, and hydrophobic, without any defects, with moderate flexibility. All the coatings passed methyl ethyl ketone solvent rub test. CNE/CPA 2:1 showed best coating performance in terms of wetting characteristics, adhesion strength, hardness, impact resistance and corrosion resistance (by salt spray test up to 21 days) as compared to others in their series and CNSL based coatings (Figure 9.12) [35].

9.3.4

CNSL (Col) Phenalkamines Coatings

CNSL (Col) phenalkamines have been designed in such a way that they satisfy all the requirements of high performance coating industries. A new development towards this approach is the use of solvent free, low viscosity and fast curing product such as M/s Cardolite. Phenalkamine curing

CH2OCH2CH2OCH2CH2NH(CH2CH2NH)2CH2CH2NH2 OH

H2N(CH2CH2NH)2CH2CH2NH2

(b) O O

H3CCOOH

H2O2

(a)

OH

O

O

O

C H2

O

C H2

O NaOH

OH

C H2

n

CI

Succenic acid 120 ºC, 4h

+ HCHO

O

OH

HOH2C

OH

H(NHCH2CH2)3NHCH2CH2OCH2CH2OCH2

CH2OH

NaOH

CI(CH2CH2OCH2CH)CI

+ HCHO

Cashew Nut Shell Liquid (Phenolic Lipid) Based Coatings 269

Figure 9.11 Synthesis of epoxy (CNE) resin (a) and amine (CPA) hardener (b) from Col for “Green” coatings by [35].

agents based coatings provide outstanding corrosion resistance with surface tolerance and can be applied on non-well prepared metallic surfaces under humid conditions. It can cure epoxy resin along a broad range of temperature and these coatings are suitable for heavy duty industrial and marine applications [36]. High performance polyurea coatings were prepared from Col (one of the constituent of CNSL) based phenalkamines. They act as curing agents for blocked isocyanates of hexamethylene diisocyanate (HDI) and isophorone

270 Integrating Green Chemistry and Sustainable Engineering Blank

CNE/CPA (1:1)

CNE/CPA (2:1)

CNE/CPA (3:1)

(a)

(b)

(c)

(d)

Figure 9.12 Salt spray test results of cured epoxy blank and CNE/CPA film having different wt % mixing ratios at different time of exposure (a) before exposure, (b) 21, (c) 90 and (d) 180 days by [35].

diisocyanate (IPDI) to obtain polyurea coatings with improved anticorrosive performance as compared to commercial phenalkamine (AG-141). The coating system showed great balance of rigidity and flexibility on the basis of pencil hardness, scratch hardness along with good impact resistance and flexibility values. Solvent and chemical resistance of coatings were found superior respective to isocyanate used. IPDI based coatings showed higher thermal stability than HDI based coatings. EIS, Tafel and salt spray tests defined the anticorrosive performance of coatings which suggested that all polyurea coatings provided good corrosion resistance except D400 and HMDA coating systems. The performance of coatings can be correlated to the presence of Col chain, inherent amine structure,

Cashew Nut Shell Liquid (Phenolic Lipid) Based Coatings 271 OH

R 2

O +

4H

Cardanol NC-700

H N

C

H

NH2

1 H2N

N H

N, N’-Bis (2-aminoethyl)ethane-1,2-diamine

Paraformaldehyde

4h

R

+

95–100ºC

O H N

N N H

N

O

R

Amine functional benzoxazine resin

Where R= H2C

CH2

Figure 9.13 Synthesis of benxoxazine resin by [38].

nature and structure of isocyanates and polyurea linkages that is collectively responsible for coating performance [37].

9.3.5

CNSL (Col) Benzoxazine (Bnz) Coatings

Col was utilized to develop amine functional benzoxazine (Bnz) resin (Figure  9.13) that on copolymerization with conventional epoxy resins (of various epoxy equivalent weight, EEW) formed poly (Bnz-co-epoxy) coatings (Figure 9.14) for anticorrosive applications [38]. The synthesized coatings were designated as Bnz180, Bnz310 and Bnz500, last numeral indicating EEW of epoxy resin in gm/mol. Bnz resin was synthesized by reaction of hydroxyl group of Col, N, N’-Bis (2-aminoethyl) ethane-1, 2-diamine and paraformaldehyde via Mannich condensation reaction. Bnz-180 coatings showed highest scratch hardness (3.4 kg) and gloss value at 60o (102.67o) as compared to other systems that can be due to greater number of epoxy rings leading to more cross-linked structure. Pencil hardness, cross-hatch adhesion, flexibility and impact resistance (intrusion and extrusion) of all copolymer coatings were found as 4 H-6H, 5B, 0 mm and 120 cm lb, respectively. These results were found to be much improved as compared to polybenzoxazine (PBnz) coating performance. All the copolymer coatings showed no softening or adhesion loss but some changes appeared in their gloss performance in acid and alkali medium

272 Integrating Green Chemistry and Sustainable Engineering R

O

O

H N

N

R

R

+

N

N

O

O

H 3 h 200º C

R R HO R

HO

OH N

N

N

R N HO

R

OH

HO R

Figure 9.14 Curing reaction of amine functional benzoxazine and epoxy resin by [38].

after immersion for 24 hr due to dual cross-linked structure formation in copolymer coatings. All copolymer coatings showed excellent resistance to solvent (xylene, methanol and acetone) scrub (200 cycles) as compared to PBnz coatings. The chemical and solvent resistance of copolymer coatings can be due to 3D structure that hindered diffusion of solvents into coatings. The copolymer coatings showed good thermal stability upto 400°C as compared to PBnz coatings. The % of water absorption of copolymer coatings was found in the range of 0.87–1.02 whereas in PBnz coating as 1.45. The decreased water absorption can be due to highly packed 3D cross-linked coating structure that causes difficulty in the absorption of water. The water absorption generally depends on the nature of films and cross-linked density of the coated surface. Bnz coatings showed lower Icorr value (0.3616 to 0.011 microA) and higher corrosion resistance efficiency (E%) (98.15–99.94%) as compared to PBnz coatings (Icorr: 19.60 microA and E%: 0). Bnz180 showed best corrosion resistance performance [Icorr value (0.011 microA), corrosion rate (1.803 × 10-5) mm/y) and E% (99. 94 %)] as compared to other copolymers based coating systems. PBnz coating showed poor performance in salt spray test after 750 hr as compared to Bnz copolymer coatings (Figure 9.15). Bnz copolymer coatings showed no more spreading of corrosion along the crosscut. These results revealed that coatings of copolymer showed good mechanical, chemical and solvent

Cashew Nut Shell Liquid (Phenolic Lipid) Based Coatings 273

(a)

(b)

Figure 9.15 Salt spray test (a) coating surface before salt spray test in 3.5% NaCl solution; (a) PBnz, (b) Bnz180, (c) Bnz310, (d) Bnz500. (b) coating surface after 750 hr salt spray test in 3.5% NaCl solution; (a) polybenoxazine, (b) Bnz180, (c) Bnz310, (d) Bnz500 [38].

resistance along with good corrosion resistance and thermal properties as compared to cross-linked PBnz coatings. In copolymer coatings, the amine group of Bnz resin reacts with oxirane ring of epoxy resin leading to the formation of 3D cross-linked structure along with the involvement of hydrogen bonds (from secondary hydroxyl groups generated along the backbone by epoxy-amine reaction). The coatings sustained enough hardness and flexibility due to excellent balance maintained between softer segment (long aliphatic chains of epoxy resin) and hardness offered from high cross-linked structure of PBnz coating.

9.3.6 Col-Polyol Coatings Col-polyol was prepared the reaction of diglycidylether of Col and tartaric acid that on curing with melamine formaldehyde (MF) formed coatings for corrosion protection with activation energy in the range of 116.13 to 179.55 kJ/mol. The optical and mechanical performance such as gloss at

274 Integrating Green Chemistry and Sustainable Engineering 60o was found in the range of 95–98, pencil and scratch hardness increased with MF whereas impact resistance decreased with MF. The coating system showed excellent resistance to acid (5% HCl) and alkali (5%NaOH) for 24 hr immersion along with very good hydrolytic stability and solvent resistance. Tafel plot and EIS showed excellent corrosion resistance and good thermal stability (10 wt% loss observed at 238.62–271.56 °C). The excellent performance was related to the presence of heterocyclic ring of MF resin and long alkyl chain of Col that collectively influenced the coating performance. MF acts as a hard segment while Col-polyol acts as a soft segment for the system [39].

9.3.7 CNSL(Col)-Polyurethane (PU) Coatings Polyurethanes (PUs) are important class of polymeric materials that have versatile properties suitable for coating applications in addition to others. PUs are generally prepared from di- or poly-ol (soft segment) and diisocyanate (hard segment) via addition polymerization reaction. ColPUs coatings were developed from CNSL (Col)-based polyols and di- or poly- isocyanates. Col-polyols with different number of functionalities were prepared by simple one step ring opening reaction of diglycidyl ether derived from Col. The synthesized polyols on reaction with polyisocyanates formed cross-linked polymeric coatings with excellent performance with respect to hardness, impact and tensile strength, flexibility, thermal stability, chemical resistance and anticorrosive properties [40]. By using water based Col-polyols and isocyanates (commercial), aqueous 2K PU coatings were developed. The selection of proper polyols and polyisocyanates resulted in coatings with balance between flexibility and hardness characteristics [41]. These coatings can be used for high end applications such as paints and coatings.

9.4

CNSL (Col) Non Isocyanate Polyurethanes (NIPUs) or Green Coatings

Towards the development of ecofriendly and green materials, non isocyanate polyurethanes (NIPUS) from renewable resources have been developed as best possible alternative to petroleum based materials. Firstly, CNSL based bis-cyclic carbonate (CC) was synthesized followed by the reaction of diamines, hexamethylene diamine (HMDA) and isophorone diamine (IPDA) to produce nonisocyanate polyurethane (NIPU) coatings via eco-friendly route (Figure 9.16). Table 9.1 shows the mechanical

Cashew Nut Shell Liquid (Phenolic Lipid) Based Coatings 275 performance of coatings produced therefrom. Chemical resistance of coatings immersed for 24 hrs under 5% HCl and NaOH solution showed no damage in acid whereas slight reduction in gloss was observed in alkali. All coatings passed solvent (xylene and methyl ethyl ketone) rub test except HMDA based coating that showed loss in gloss after about 110 rub cycles in methyl ethyl ketone. HMDA based coatings exhibited highest water absorption among the series whereas IPDA based coatings showed least water absorption. These results can be related to intermolecular hydrogen bonding formed in the cross-linked structure. In addition to this, highly amorphous nature of HMDA cured backbone was formed whereas in IPDA cured system, cycloaliphatic nature provided greater compactness that helps in shielding the hydroxyl group as compared to long alkyl aliphatic amine. The coatings showed thermal stability upto 250 °C. The coating properties can be enhanced by appropriate selection of amine cross linker such as cycloaliphatic or aromatic diamine/polyamine in combination with aliphatic amine as compared to those of epoxy based coatings. This cured system can act as component for coating formulation [42]. NIPU coatings were formed at 150 °C for 30 min. IPDA based coatings were found to be hard and brittle whereas HMDA based coatings were obtained as soft and rubbery with inadequate adhesion to metal substrate. In other words the combination of both these hardness (scratch and pencil) resulted in better adhesion to the substrate. The gloss value increased with O O

O

O

O

O (CH2)6

(CH2)6

CO2. Catalyst (CH2)7

(CH2)7 O

O

O

O

O

NC-514 NH2-R-NH2

CC

O

Triethylamine O R N H

N H

O

O OH

(CH2)4 O

(CH2)7

R O NIPU

O OH

N H

N H

n

Figure 9.16 The reaction involved in the formation of CC followed by the formation of NIPU from reaction of five membered cyclic ring and amine [42].

93±3

92±3

87±2

79±3

3I1H

2I2H

1I3H

HMDA

105±4

102±3

IPDA

HMDA

Epoxy

103±2

Gloss (60°)

IPDA

NIPU

Coating

0

0

0

0

0

0

3.2 cm

Flexibility

4H

5H

2B

2H

+2H

3H

4H

Pencil hardness

1800

2600

900

2100

2400

2400

2800

Scratch hardness (gm)

70

70

70

70

70

70

59

70

70

70

70

70

70

53

Extrusion

Impact (lbs/inch) Intrusion

Table 9.1 Physical and mechanical properties of all the cured coating systems.

5.8

9.1

1.1

3.4

6.3

6.6

11.3

Abrasion resistance (wt loss in mg after 1000 cycles)

276 Integrating Green Chemistry and Sustainable Engineering

Cashew Nut Shell Liquid (Phenolic Lipid) Based Coatings 277 IPDA. The pencil hardners of IPDA based coatings was found best among them.

9.5

CNSL (Col) Waterborne and UV Cured Coatings

In view of the inherent advantages of UV cured waterborne coatings (low energy consumption, high curing speed, environmentally friendly, cost efficient, excellent adhesion, block resistance, low temperature resistance and enhanced performance), water borne UV/oxidative dual curing polyurethane dispersion was developed from Col based polyol (Col-polyol) for high performance environmental friendly coatings. The system has ability to rapid cure under UV irradiation along with post oxidative curing of unsaturated sites of long alkyl chain of Col. Col-polyol was synthesized from the reaction of epoxidized Col (EC) with itaconic acid in presence of catalyst triphenyl phosphine (Figure  9.17). By reaction of Col-polyol, IPDI, dimethyl propionic acid and 2-hydroxyl methylmethacrylate in acetone solvent, was prepared an isocyanate terminated prepolymer, followed by neutralization with triethyl amine and subsequently dispersed in deionized water to form waterborne polyurethane oligomer after removal of acetone [43]. Coatings were prepared by using driers (Co, Zr, and Ca), in oxidative (air) curing, and photoinitiator blend along with amine synergist in UV-cured system and photoinitiator blend along with mixed metal driers along with amine synergist in UV/oxidative dual curing system (Figure 9.18). Oxidative curing was performed at 65 °C in oven for 30 min followed by curing at room temperature for 7 days. While UV- and UV/ oxidative curing systems were performed at 65 °C in oven for 30 min followed by UV curing along with further curing of latter system for 7 days at room temperature. The UV cured polyurethane dispersion coatings show very good performance. The systems show 100% adhesion to metal substrate. Scratch and pencil hardness increased in dual curing systems that can be related to increased cross-linking density. The degree of cure of coatings was checked by solvent resistance. The dual system showed higher solvent resistance than other systems that is also correlated to higher cross-linked density due to both UV and oxidative curing. The systems showed satisfactory chemical resistance performance in different chemical environments. The increased crosslink density in case of dual curing system improved thermal, mechanical, chemical, water and solvent resistance, and overall coating performance properties. UV/oxidative cured polyurethane dispersion

278 Integrating Green Chemistry and Sustainable Engineering OH Monoene Diene Triene

Cardanol

6 mol Epichlorohydrin (ECH) 40% NaOH solutution, 2 gm TBAB 95–105ºC

O CI ECH O O

Epoxidized cardanol (Epicard) O HO

OH O

0.5 mol Itaconic acid (IA, Triethylamine (TEA) 150–160ºC

1A

O

O O OH

O

O O OH

Cardanol based polyol

HO

OH

Figure 9.17 Synthesis of EC and Col based polyol [43].

coating system provide a “greener” solution to coating industry as an alternative of petro-based coatings. Thiol-ene chemistry was used to prepare photo cross-linked aromatic bio-based coatings by using Col methacrylate (CAMA) monomer that was prepared by aqueous emulsion radical polymerizations (Figure 9.19). CAMA prepared by epoxidation, methacrylation and polymerization was carried out both in toluene solution and in aqueous medium. In latter medium, sodium

Cashew Nut Shell Liquid (Phenolic Lipid) Based Coatings 279

O

C

N

N

C

O

HO

OH HO

IPDI

HO

OH

O DMPA

Polyol DBTDL 80 ºC

O

C

N

O

O C

N H

O

O

H N

C O

C

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N H

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O

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Isocyanate terminated Pre-polymer HEMA 80 ºC O O

H O C N

N H

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N H

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N H

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O

Acrylate terminated pre-polymer 2 wt % , it behaves like polar cosolvent. Fletcher and Pandey [1] have investigated the surfactant aggregation within room-temperature ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide. They used solvatochromic probes (Reichardt’s betaine dye, pyrene, and 1,3-bis- (1-pyrenyl)propane) to see the aggregation behaviour of common anionic, cationic, and nonionic

308 Integrating Green Chemistry and Sustainable Engineering surfactants when solubilized within a low-viscosity room-temperature ionic liquid 1-ethyl-3- methylimidazolium bis(trifluoromethylsulfonyl) imide [C2mim][Tf2N]. There results showed that all the studied nonionic surfactants form aggregates, while no aggregation behaviour was observed for cationic surfactant cetyltrimethylammonium bromide and the anionic surfactant, sodium dodecyl sulphate which do not appear to solubilize within [C2mim][Tf2N] at ambient conditions. Inoue et al., [127] studied aggregation behaviour of nonionic surfactants in ionic liquid mixtures by using 1H NMR chemical shift analysis and dynamic light-scattering measurements. They showed that the polyoxyethylene (POE)-type nonionic surfactants do not aggregate in [C6mim] [BF4], while they are immiscible with [C2mim][BF4], means the surfactants are highly solvophilic to [C6mim][BF4] and highly solvophobic to [C2mim] [BF4]. However to their surprise, micellization was observed in mixtures of [C2mim][BF4] and [C6mim][BF4]. It was also observed that the cmc value decreases, while the mean hydrodynamic radii of micelles, and thus micellar aggregation number increase as the mole fraction of [C2mim][BF4] increases in the ionic liquid mixture. These results were further supported by the 1H NMR chemical shift analysis which revealed possible interaction between hexyl group of IL with the hydrocarbon chain of surfactant in the ionic liquid mixtures. Galgano and Seoud [162] have studied the micellar properties of 1-(1-alkyl)−3-methylimidazolium chlorides and compared the results with structurally related surfactants. In addition to micellar properties, these researchers have compared the solution properties of the imidazolium-based surfactants with 1-(1-alkyl)pyridinium chlorides, and benzyl (2-acylaminoethyl)dimethylammonium chlorides in the temperature range from 15°C to 75°C. Conductivity, isothermal titration calorimetry, and static light scattering methods were used to study the interactions in the interfacial region on the micellar properties over the temperature range mentioned. Zhang and coworkers [6] studied interaction between long-chain ionic liquid 1-dodecyl-3-methylimidazolium tetrafluoroborate and Triton X-100 in aqueous solutions by using surface tension, electrical conductivity, 1H NMR and FF-TEM measurements. Surface tension measurements showed that the cmc of TX-100 increases dramatically in presence of [C12mim][BF4]. 1H NMR data revealed hydrogen bonding between imidazolium cation of [C12mim]+ of IL and the hydrophilic PEO groups of Triton X-100, hydrophobic interactions between the dodecyl chain of the imidazolium cation and the hydrophobic chain of Triton X-100 in the mixed micelles. Javadian and co-workers [130]. observed the aggregation behaviour of cetyltrimethylammonium bromide (CTAB) in presence

Ionic Liquids as Potential Green Solvents 309 of three imidazolium-based ionic liquids, N-butyl imidazolium chloride [N-C4mim][Cl], 1-butyl-3-methyl imidazolium chloride [C4mim][Cl] and 1-hexyl-3-methyl imidazolium bromide [C6mim][Br] by using tensiometry, fluorescence, 1H NMR spectroscopy and dynamic light scattering (DLS) studies. From these techniques different parameters such as cmc, interfacial parameters, aggregation number, and polydispersity were obtained. The cmc values in presence of IL was found to be higher than in water, and the surface activity was found to be lower in presence of IL than in pure water. Further it was reported that [C6mim][Br] can be incorporated in CTAB micelles and forms mixed micelles, but [N-C4mim] [Cl] behaved as a solvent toward alerting the physicochemical properties of CTAB. Pal and Chaudhary [13] investigated the effect of varying concentration of hydrophobic ionic liquid 3- methy-1-pentylimidazolium hexafluorophosphate [C5mim][PF6] on the micellar properties of anionic surfactant sodium dodecylsulfate (SDS), by employing conductometry, densiometry and speed of sound at a temperature range of 298.15–318.15 K. They reported that cmc of SDS increased in presence of [C5mim][PF6]. Bhatt and co-workers [4] studied the mixed systems of ionic liquid tetraethyl ammonium tetrafluoroborate [TEA][BF4] and numerous ethylene oxide based non-ionic surfactants by employing surface tension, viscosity and dynamic light scattering (DLS) measurements. Various adsorption and thermodynamic parameters were determined and from these data it was observed that the micelle formation process is enthalpy driven at low temperature and entropy driven at higher temperature. Sharma and coworkers [3] investigated the effect of 1-tetradecyl-3-methylimidazolium bromide [C14mim][Br] on the physicochemical properties of cationic surfactants such as tetradecyltrimethylammonium bromide (TTAB), dimethylditetradecylammonium bromide (DTDAB), alkane-bis (tetradecyldimethylammonium bromide) (14-2-14 and 14-4-14 gemini) by employing conductivity, surface tension, fluorescence and 1 hr NMR techniques. Different thermodynamic parameters, adsorption properties, and miceellar aggregation were calculated. The interaction between IL and surfactants were found to be antagonistic and non-ideal. 1H NMR analysis suggested that the binary mixtures of IL +14–2–14 with more hydrophilic spacer promote micellization more easily than IL +14–4–14. Khan et al., [163] investigated the mixed micellization behaviour of amitriptyline hydrochloride (AMT) with ionic liquid 1-methyl-3-octylimidazolium chloride, [C8mim][Cl], by using conductivity measurements at different temperatures. Rubingh’s regular solution theory was used to find the extent of interaction between different mole fraction mixtures.

310 Integrating Green Chemistry and Sustainable Engineering Non-ideal behaviour (i.e.,,synergistic interaction) of AMT– [C8mim][Cl] binary mixtures was observed at the studied temperatures. The calculated thermodynamic parameters (viz., the standard Gibbs energy change, Δ , the standard enthalpy change, Δ , the standard entropy change, Δ ), suggest the dehydration of hydrophobic part of the drug at higher temperatures (>313K). Mahajan et al., [164] studied the interaction of surface active ionic liquid (SAIL), 1,8-diazabicyclo[5.4.0]undec- 7-ene (DBU) with amitriptyline hydrochloride (AMT) by employing different theoretical models proposed by Rubingh, Motomura and Gibbs. The micellar interaction parameter (βm) was reported to be negative for most of the mole fractions of AMT (αAMT) in DBU-C14/DBU-C16 +AMT systems, indicating attractive interactions. Various interfacial parameters such as surface tension reduction efficiency (pC20), effectiveness of surface tension reduction (Пcmc), maximum surface excess concentration at the air-water interface (Γmax) and minimum area per surfactant molecule (Amin) were also evaluated for the mixed systems of DBU based SAILs with AMT. The negative values of standard Gibbs free energy of micellization ) and the standard free energy of adsorption at the air/water inter(Δ ) confirm the presence of drug–SAIL interactions. Mahajan face (Δ and co-workers [165] have also investigated the drug binding ability of a surface active ionic liquid, such as 1-tetradecyl-3-methylimidazolium bromide [C14mim][Br] by employing conductivity, surface tension, cyclic voltammetry (CV) and 1H NMR measurements and the drugs used were dopamine hydrochloride (DH) and acetylcholine chloride (AC). The results obtained from these techniques were compared with structurally related conventional cationic surfactant tetradecyltrimethylammonium bromide (TTAB). The micellar and adsorption parameters obtained for these drug−surfactant systems (DH/AC + [C14mim][Br] / TTAB) show favourable interactions between them. In addition to these interactions the existence of cation − π as well as π − π interactions between drugs and surfactant molecules have been confirmed from these techniques. 1H NMR studies along with the conductivity and surface tension measurements help in predicting the possible location of adsorption of these drug molecules in [C14mim][Br] and TTAB micelles. Kabir-ud-Din and colleagues [166] investigated two antidepressant drugs (nortriptyline hydrochloride and clomipramine hydrochloride) with four cationic surfactants (monomeric: cetyltrimethylammonium bromide, tetradecylammonium bromide; dimeric: 1,5-pentanediyl-α- ωbis(hexadecyldimethylammonium bromide), 1,4-butanediyl-α- ω bis (hexadecyldimethylammonium bromide) as well as in presence of sodium chloride by using conductivity

Ionic Liquids as Potential Green Solvents 311 measurements. The results obtained by these workers showed formation of mixed micelles. The cmc of drug decreased both in the presence of surindicated attractive interactions in factant and NaCl, negative β and Δ the mixed micelles. Taboada et al., [167] investigated the self-aggregation of antidepressant drug, clomipramine hydrochloride in different aqueous medium by employing ultrasonic speed and density measurements in buffered aqueous solution of pH 3.0 and 5.5 in the temperature range 288.15–313.15 K. Different parameters such as cmc, Gibbs free energy, enthalpy and entropy of aggregate formation were calculated. Sharma and Mahajan [149] studied the interaction of trifluoperazine dihydrochloride (TFP) with ionic surfactants by employing electronic absorption, surface tension and fluorescence measurements. From these techniques various thermodynamic, interfacial, micellar and spectroscopic parameters were calculated. The results indicated that cationic surfactants exhibit less synergism than anionic surfactants. Anionic surfactants in comparison to cationic surfactants have been found to bind strongly with TFP, due to the presence of cationic charge on the head group of drug. This supports the fact that anionic surfactants can act as better drug carrying agents than cationic surfactants even at very low concentration. Fluorescence studies indicate the formation of new complex between interacting species and showed that electrostatic interactions between TFP and ionic surfactants predominate over hydrophobic and van der Waals forces.

10.6

Conclusions and Perspectives

As “green” solvents ILs find an important place in applications owing to their high thermal stability and extremely low vapor pressure. It is observed and investigated by many workers that improvements can be brought into the various known chemical processes by substituting a solvent with an IL. These green solvents have found use in separation science, gas chromatography, liquid chromatography, capillary electrophoresis, liquid–liquid extraction, immunoassays, lubricants, and tissue preservation. Moreover, the properties of ILs can be tuned by changing the cation and/or anion part. As solubilizing agents, SAILs have also found a place. A lot of research is devoted to use these green solvents as solubilizing agents for poorly soluble drugs and other biomolecules. It is thus anticipated that commercialization of methods using these green solvents will pave a way for their continued development and integration into the chemical industry. However, the large scale industrial production of ILs with negligible toxicity and economically viable routes are still to be explored thoroughly.

312 Integrating Green Chemistry and Sustainable Engineering

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Ionic Liquids as Potential Green Solvents 323 173. Burke, S.E., Andrecyk, S.L., Palepu, R., Thermodynamic and aggregation properties of sodium dodecyl sulfate in aqueous binary mixtures of isomeric butanediols. Colloid Polym. Sci., 279(2), 131–138, 2001. 174. Turro, N.J., Yekta, A., Luminescent probes for detergent solutions. A simple procedure for determination of the mean aggregation number of micelles. J. Am. Chem. Soc., 100(18), 5951–5952, 1978. 175. Pandey, S., Acree, W.E., Fetzer, J.C., Cetylpyridinium chloride micelles as a selective fluorescence quenching solvent media for discriminating between alternant versus nonalternant polycyclic aromatic hydrocarbons. Talanta, 45(1), 39–45, 1997. 176. Tringali, A.E., Kim, S.K., Brenner, H.C., ODMR and fluorescence studies of pyrene solubilized in anionic and cationic micelles. J. Lumin., 81(2), 85–100, 1999. 177. Shiraishi, Y., Sumiya, S., Kohno, Y., Hirai, T., A rhodamine-cyclen conjugate as a highly sensitive and selective fluorescent chemosensor for Hg(II. J. Org. Chem., 73(21), 8571–8574, 2008. 178. Paul, B.K., Samanta, A., Guchhait, N., Exploring hydrophobic subdomain IIA of the protein bovine serum albumin in the native, intermediate, unfolded, and refolded states by a small fluorescence molecular reporter. J. Phys. Chem. B, 114(18), 6183, 6183–6196, 2010.

11 Role of Green and Integrated Chemistry in Sustainable Metallurgy Sadia Ilyas*, Muhammad Farhan and Haq Nawaz Bhatti* Department of Chemistry, University of Agriculture, Faisalabad, Pakistan

Abstract Microbial processing for the recovery of metal from divergent resources has emerged as a green technology in metallurgical operations. Bio-hydrometallurgy offers many attractive features such as operational flexibility, low cost and lowenergy consumption, besides being environmentally clean technology. The metal contents in complex resources/secondary wastes are in the form of inorganic sulfides/oxides, pure metallic forms and sometimes as organo-metallic complexes. Microbes interact with metals and metalloids indifferently either for their benefit or detriment. Possible routes of metal interaction from these divergent resources are discussed in detailed in this chapter. Furthermore, present chapter also focused on bio-chemical process integration using eco-friendly design tools for treating complex resources and spent materials as well. Keywords: Bioprocessing, divergent resources, mechanism, process integration

11.1

Introduction

Bio-hydro-metallurgy is a combination of green biotechnology, aqueous chemistry and metallurgy. Hybrid processing integrate both microbial and chemical processing routes to treat various ore/ mineral/ waste & byproduct. Due to different properties of some possibly critical biotopes and dynamic standards of interactions between microbial digestion systems and materials, effective coordinated bio-metallurgical procedures have been developed to extract metals [1]. While the modern application of bio-hydro-metallurgy became reality in 1950s, as the processing of copper *Corresponding authors: [email protected]; [email protected] Shahid-ul-Islam (ed.) Integrating Green Chemistry and Sustainable Engineering, (325–342) © 2019 Scrivener Publishing LLC

325

326 Integrating Green Chemistry and Sustainable Engineering through bio-leaching starts at Kennecott Copper Bingham Mine. Later the processing of (high grade) gold ore initiated by using tank bio-oxidation followed by Cyanide leaching to extract gold from the bio-treated ore [2]. Significant efforts have been made to create an efficient bio-metallurgical approach for processing of wastes and by-products produced from the metallurgical, industrial and man-made resources, in order to the rapid periodical growth and access to the new age materials. Different alternatives have been tried as of not long ago for the administration management of these waste materials in perspective of effective metal recovery and environmental protection [2]. The secondary resource contain metal in the form of pure, inorganic oxides/sulfides and sometime organo-metallic complexes. The metal of interest can be leached out from these resources by using autotrophic or heterotrophic microorganisms supplemented with appropriate energy sources. Autotrophic microorganisms such as bacteria (i.e., Acidithiobacilli sp.) and Archaea (i.e., Ferroplasma sp.); heterotrophic microorganism such as bacteria (i.e., Pseudomonas sp., Bacillus sp.) and fungi (i.e., Aspergillus sp., Penicillium sp.) are the major group of microbes involved in the bio-leaching of metals [3]. The ability of microorganism to solubilize metals from primary and secondary raw materials has accelerated the growth of integrated green chemical technology. Metals present in ores (sulfide, oxide), converter/smelter slags, smelter dusts, metallurgical sludge, electronic scraps and spent catalysts/ lithium ion batteries can be leached by green integrated biohydro-metallurgical processing routes with low environmental degradation and resource reclamation as in Figure 11.1.

11.2

Role of Green and Integrated Chemistry in Sustainbale Metallurgy of Primary Resources

All of these integrated bio-chemical routes mainly based on three fundamental approaches, namely complexolysis, acidolysis and redoxolysis. Microorganisms are able to mobilize metals by biogenic inorganic or organic acids, redox reactions and metal complexing agents. Mineralogical composition of various materials consisted of following phases; Cu–Fe–Sn–Sb, FeO(OH), Mn3O3(OH)6, M2SiO4 (M; Fe, Al, Zn), Сu9S5, Сu5FeS4, FeSiO4, ZnFe2O4, FeCr2O4, Fe3O4, Al2O3, MnO, Ni/Mo/P/Al2O3 FeS2, MoS2, and WS2), As2S3, CuFeS2, Cu2S, FeS, Fe7S8, PbS, MnS2, ZnS, Cu0, Al0, Zn0, Ni0, Fe0, Au0, Pt0, Ag0, Pd0 etc. Metal oxides present in these materials can be leached out by ligand induced and proton induced reactions under acidic conditions.

Green and Integrated Chemistry in Sustainable Metallurgy 327 Bio processing

Bio-oxidation and reduction of Fe, Cu and S compound using Acidithiobacillus, Acidiphillium, sulphobaccillus, Chromobacterium, Pseudomonas, Aspergillus sp. etc

Green integrated processing

Chemical processing

Pre-oxidation of sulfide/carbonaceous material and permeability of metals

Mechano-chemical /heat treatment, roasting use of oxidizing agent/bot digestion

Use of biogenic lexiviant viz., mineral and organic acid thio/cyanide etc. use Aspergillus, Penicillum, Desulfovibrio, Pseudomonas, chromobacterium, Alicyclobacillus sp. etc

Liberation, dissolution and complexation of metals in aqueous solution

Leaching with metal solubilizing media viz mineral or organic acids/alkalies/balides/thios/cynaide

Bioaccumilation/Biosorbtion/Biological using Streptomyces, Escherichia, Rhizopus, Bacillus Sp, Saccharomyces etc.

Separation, purification and enrichment of target metal values

Solvent extraction, ion exchange, Chemica/carbon adsorption etc.

Metal recovery via reduction/precipitation

Cementation, hydrogen reduction, electro-winning, precipitation with organic reagents etc.

Bio-reduction using Geobacter, Desulfovibrio, Rhizopus, Aspergillus, Penicillum, Saccharomyces, Bacillus, Acetobacter Sp, etc

Metal product/s

Figure 11.1 Role of green and integrated chemistry in sustainable metallurgy. Adapted from Ilyae et al., after modification [1].

11.2.1 Role of Integrated Chemistry in Processing of Sulfide Minerals Metals can be extracted from sulfide materials or sulfur supplemented material with direct or indirect leaching, contact or non contact leaching, cooperative leaching, thiosulphate or polysulfide leaching included chemical and microbially mediated reactions [4–8]. Direct leaching involves a transfer of electron from the metal sulfide to the cell attached with the material surface. The reactions may be given as [4, 5]:

→

(11.1)

→

(11.2)

While the indirect leaching involve an oxidizing agent, Fe3+, which is generated from Fe2+ by mean of bacteria either planktonic or attached to the material surface in order to obtained metal from metal sulfide. The reaction can be given as

→

(11.3)

→ →

(11.4) (11.5)

328 Integrating Green Chemistry and Sustainable Engineering Here M can be Cu0, Zn0, Al0, Ni0, Fe0, Co0 etc. Since a direct electron transfer between the metal sulfide and the attached cell through nano wires, enzymes, etc., has not been confirmed, instead, attached cells deliver an efficient extracellular polymeric substance (EPS) filled reaction compartment for indirect leaching with Fe3+ ions [6]. Thus the terms contact and non-contact leaching have been suggested for bioleaching by attached and planktonic cells. A third term, cooperative leaching, has also suggested for dissolution of mineral fragments, sulfur colloids and sulfur intermediates by planktonic cells [7]. Generally, metal dissolution occur by a combination of proton attack and oxidation processes [8]. However, reactivity of metal sulfides with protons contributes mainly for metal extraction that depends upon electronic configuration and bonding of material. The metal sulfides such as, MoS2, FeS2 and WS2 contain pairs of sulfur atom [9] which form nonbonding orbitals. Therefore, the valence bonds of these metal-sulfides (FeS2, MoS2 and WS2) do not contribute to the bonding between the metal and the sulfur moiety of metal-sulfide and hence show resistance against a proton attack. The bonds can only be broken via multistep electron transfers with an oxidant such as Fe3+. After initial attack of Fe3+ (oxidant) the sulfur moiety of pyrite can be oxidized to soluble sulfur intermediates. Here, the main sulfur intermediate is thiosulfate. The mechanism of thiosulfate simplified by the following equations [8]: −

→

(11.6)

→

(11.7)

While the valence bond of As2S3, CuFeS2, FeS, Fe7S8, MnS2, PbS, ZnS etc. are derived from both metal and sulfur orbitals. So, in addition to an oxidant like Fe3+, protons can remove electrons from the valence bond, causing a cleavage of the bonds between the metal and the sulfur moiety of the metal sulfide. Consequently, these metal sulfides are relatively soluble in acid. In this case, the main sulfur intermediate is polysulfide and a series of reactions inherently explain the formation of elemental sulfur via polysulfides. The polysulfide mechanism can be simplified by the following equations.



→ →

≥



(11.8) (11.9)

Green and Integrated Chemistry in Sustainable Metallurgy 329 NO2

NH4

CO2

H2

O2

C3H7O7P

H2O

PH = 6.5

Fe2+ Chemical

Microbial Fe3+

M2+ + S2O32– Bacteria

M2+ + H2S+ + (H2S2)

3+

Fe , O2

Fe3+, O2

SnO62–, S8 Bacteria Fe3+, O2

Bacteria

H2Sn, S8 Fe3+, O2

Bacteria

H+ + SO42–

SO42–+ H+

Figure 11.2 Biological mechanisms (thiosulphate and polysulphide) of bacteria integrated with chemical reactions for processing of minerals and secondrey wastes. Adapted from Ilyas et al., after modification [1].

→

−

(11.10)

Although elemental sulfur is chemically inert in natural environment, it can be biologically oxidized to sulfuric acid. Overall mechanism is depicted in Figure 11.2.

11.2.2 Role of Integrated Chemistry in Processing of Oxide Minerals Existing bio-hydrometallurgical processes use acidophilic prokaryotes (bacteria and archaea) to degrade reduced (sulfide) minerals/materials with oxidative dissolution by direct attach, indirect attach, thiosulfate and polysulfide pathway (Eqs.11.1-10) depending upon the properties of solid to be leached. The metals contained in most of the oxide minerals are generally not present as discrete minerals but entrapped within the structures of host minerals, most often ferric iron oxides or manganese oxyhydroxides (FeO(OH), Mn3O3(OH)6 etc.). In order to solubilize metals from this

330 Integrating Green Chemistry and Sustainable Engineering type of host mineral, the strong bond between oxygen and ferric iron has to be broken. The economics of metal processing from this type of material can be improved significantly by breakage of iron/ manganese and oxygen bond by reductive dissolution. These biocatalytic process may enable potential bio-hydrometallurgical opportunities because of the fact that FeO(OH) is one of the major host minerals of various metals (Ni, Co, Cr etc) and its reduction can be feasible by utilizing a low cost electron donor. All the fundamental process steps, apart from the metal removal step, can be biologically catalyzed. The overall process can make use of low-intensity integrated processing enabled by biocatalyzed reactions to, firstly leach the oxide material, subsequently removing iron from solution, and finally regenerating a significant portion of the acid requirement for reuse in the leaching step. Theoretical mineral composition, dissolution reactions, resulting acid and sulphur consumptions and indicative reaction extents have been summarized in Table 11.2. The overall process involved (Eq. 11.11) catalytic transfer of electrons from sulphur (Eq. 11.12), electron donor half-reaction to ferric iron contained in mineral form (Eq. (11.13); the electron acceptor half-reaction is therefore, an important step in the development of such processing options [10].







−

→

−

−

→



−

(11.11) (11.12)

→

(11.13)

Similarly, cobalt is often associated with manganese oxyhydroxides of the asbolane-lithiophorite group. Since the manganese in this mineral group is in an oxidized form, acid dissolution of asbolane is enhanced under reducing conditions. Bacteria use a similar energy metabolism for dissimilatory iron and manganese reduction Using sulphur electron donor, the overall reductive dissolution of asbolane-lithiophorite group minerals under reductive conditions can be described by Eqs. (11.14-15) [11].









−

→ →

(11.14) (11.15)

This reaction may occur directly (Eq. 11.14) or through the reduction of Mn4+ by Fe2+ (Eq. 11.15). If the secondary resources containing both oxide and sulfidic minerals (spent catalysts, ocean manganese nodules, slags and

Green and Integrated Chemistry in Sustainable Metallurgy 331 contaminated sediments), a multistage stage bio-processing unit can be established with alternative reductive or oxidative dissolutions. The redox/aerobic-anaerobic couple, of particular interest can be used in bioleaching in order to extract metals from waste material such as dusts, slags, fly ashes etc. After oxidative dissolution of metals (sulfur and iron) with acidophilic oxidizing microbes. These metals could be successively precipitated out from the leach solution using biogenic H2S (Hydrogen sulfide) produced by sulfur reducing (SBR) microbes as follow, − −

→ → →

−

(11.16) (11.17) (11.18)

Precipitation of metals as sulfides using biogenic Hydrogen sulfide (H2S) offers numerous advantages such as better condensing characteristics of metal sludge, lower effluent concentrations of metals and the opportunity to recover valued metals over chemical hydroxide precipitation [12].

11.3

Role of Green and Integrated Chemistry in Sustainable Metallurgy of Secondrey Resources

Secondary resources mostly have heterogeneous and complex nature, so integrated bioleaching (including alternative chemical-biological and successive oxidative–reductive leaching stages) of these resources not only reduce leaching time, volume of wastes, increase metals extraction and can also be eco-friendly. Now The integrated bio-hydrometallurgical approach provide an opportunity to conduct advanced research in order to process a number of secondary resources, such as smelter dusts, sludges, slags, electronic wastes, spent catalyst.

11.3.1

Role of Integrated Chemistry in Processing of Smelter Dust

Dusts mainly from the reverberatory/flash and converter furnaces are often recycled in the plant after blending with the concentrate to recover copper at the cost of plant productivity and causing environmental pollution, besides damaging the refractory bricks. The main copper sulphide in a typical smelting dust have been identified as chalcocite (16%),

332 Integrating Green Chemistry and Sustainable Engineering chalcopyrite (2–3 %), bornite (2–3 %) and covellite (1.0%), and 13% copper oxide [13]. The high leaching of Cu from the dust may be associated with the presence of secondary sulphides such as bornite and chalcocite in significant amount which can be leached out by bacteria easily [14]. The bioleaching of copper from the Iranian (Sarcheshmeh) smelter clean was inspected by utilizing coordinated GeoCoat technology [15]. In order to improve the leaching process a (two-stage) stirred tank bioreactors and (two-stage) airlift reactors [15] were applied in a continuous mode. Aside from the Acidithiobacillus ferrooxidans and Acidithiobacillus thiooxidans, the mixed culture also had Leptospirillum ferrooxidans and was injected after variation over the dust feed. The laboratory (2.0 L) scale operation indicate that about 86.8% of Cu has been recovered in 6 days at 32 °C with a pulp density of 70 g/L by arranging the bioreactors (twostage) in a series for 180 days in continuous mode. The airlift reactor operated for 150 days to leach the flue dust using the adapted mixed culture, confirmed the increased rate of dissolution (86% Cu) in 5 days at 70 g/L pulp density and 34 °C. These results indicated that the bioleaching of Cu from the (copper smelter) flue dusts can tested and implemented on industrial scale [16].

11.3.2 Role of Integrated Chemistry in Processing of Converter/Smelter Slags Slags are among the fastest growing category of solid waste streams generated from the metallurgical industries. These wastes are harmful if released to the environment but can be potentially valuable sources of metals if processed efficiently. Slag from copper smelting industry is considered one of the critical metals containing waste materials. Almost 1850000 tons of slag is produced every year and only 30% is recycled for production of cement and the remaining is transferred to landfills. If the slag were treated effectively almost 1.5% of annual copper production can be obtained. Recently an integrated chemical-biological approach was proposed by Ilyas et al., [5] for processing of slag as in Figure 11.3.

 

 

 

 

→



(11.19)

The dissolution of silicates in H2SO4 was observed below 1.5 pH value (as in Eqs.11.20-22) intensified the solution and resulted in the formationof un-filterable gels.

Green and Integrated Chemistry in Sustainable Metallurgy 333 Wild culture, (modified 9K medium) Biogenic acid

Bioleaching of slag (pH 1.5, temp 50 ºC, PD15%)

Solution

9 K medium cell Residue

Biogenic metabolite (Fe3+ , H2SO4)

Metal recovery Solution

Chemical leaching (pH 1.5, temp 65 ºC) Metal salt / compounds

residue

Mineral salt, cell Bio-oxidation at 45 ºC Residue for processing

Figure 11.3 Integrated bio-chemical processing of smelter slag for metal recovery. Adapted from Ilyas et al., after modification [5].

→

 →



(11.20) (11.21)

→

(11.22)

It was observed that the high concentration of Fe3+ ions decrease the leaching of ferrous and zinc but increase the leaching of copper and nickel as in Eqs.11.23–27.

→ →

(11.23) (11.24)

→ (11.25)

→ →

(11.26) (11.27)

334 Integrating Green Chemistry and Sustainable Engineering Recently, Carranza et al., [17] investigated BRISA process, this process deals in chemical leaching of copper converter slag (9.0% Cu) to obtained metals by using ferric sulfate (ferric leaching stage) integrated with the biological generation (biooxidation stage) Acidithiobacillus ferrooxidans in order to regenerated ferric iron used in the leaching step. About 93% of Cu dissolution was carried out in 4 hr at 1.67 pH, 20 g/L pulp density and 60 °C temp with the fine size particles (D80–47.03) in the presence of 11.5 g/L ferric sulphate.

11.3.3 Role of Integrated Chemistry in Processing of Spent Catalyst/Lithium Ion Batteries (SC/LIBs) The use of LIBs has rapidly increased worldwide due to their wide use as electrochemical power sources such as mobile telephones, videocameras, personal computers, other modern-life appliances and also electric-powered auto-mobiles in future. Oil refineries generated spent hydro-processing catalysts which are an important source of valuable metals such as Al, Mo, Ni, V and Fe. Almost 150.000–170.000 tons of solid waste (Spent-catalysts) produced by chemical and petrochemical industries per year. In catalyst metals are usually found in oxide form but in some cases reduce active metals also used depend on the nature of reaction needed to be catalyzed. Normally, the catalysts become inactive after extensive and multiple use due to the deposition of organic materials, e.g toluene, benzene and other impurities. The US Environmental Protection Agency declared it a hazardous material because of the difficulty of disposal due to the presence of Mo, V, Ni, Co, Pd and Pt [18]. Integrated bio-chemical routes facilitate the extraction of lithium and cobalt from spent-LIBs containing LiCoO2, using acidophilic chemo-lithotrophic bacteria such as Acidithiobacillus ferrooxidans for production of ferric iron and sulfuric acid as leaching agents. The growth of bacteria increase in a medium containg iron and elemental sulphur as energy sources. The use of Fe(II) during leaching experiment helps to enhance the growth of (Acidithiobacillus ferrooxidans) bacteria in medium which produces sulfuric acid that can be reused to leach out metals from spent-LIBs. The rate of dissolution of metals decreases by increasing the concentration of Fe(II) because the bacteria convert the Fe(II) ion to Fe(III) ion which precipitated along with the metals in leach residues and hence prevented bacterial activity in leaching process as higher metal concentrations are toxic to the organism. LIBs entertained with bio-leaching method yielded favorable results [19]. Bio-leaching with autotrophic Acidithiobacillus thiooxidans and Acidithiobacillus ferrooxidans using protons and oxidant (such as Fe(III))

Green and Integrated Chemistry in Sustainable Metallurgy 335 in order to solubilize the metals. Organic acid produced by the fungi can be used as leaching reagent during bio-leaching with Penicillium simplicissimum and Aspergillus niger. Analytical-grade organic acids for example citric, gluconic and oxalic acids were used to compare the results of heterotrophic leaching. Wheres ferric chloride and sulfuric acid were used as reagents in order to compare the result of autotrophic leaching. Results show that the surface area of fresh catalyst was 23% higher than the spent catalyst, but during the bioleaching of spent catalyst by Acidithiobacillus thiooxidans and Acidithiobacillus ferrooxidans the surface area increased by 17% and 33 %, respectively. Toxicity characteristic leaching procedure (TCLP) tests were conducted on the residues obtained from bioleaching and chemical leaching to determine if the residue obeyed the toxic limit set by United States-Environmental Protection Agency. The tests conducted on spent catalyst (feed material before bioleaching/chemical leaching) showed that the Ni content exceeded environmental limit set by US-EPA, but was found to be within the toxic limit after bioleaching. Autotrophic bioleaching conducted with Acidithiobacillus thiooxidans was found to be promising with highest recovery of 29.3% Al, 64.5% Mo and 99.8% Ni. More importantly, bacterial leaching was much faster (15–20 days) compared with fungal leaching (45–50 days). Compared with abiotic controls, bioleaching was better in metal extraction from the spent catalyst. An attempt to mobilize Al Co, Mo and Ni from the spent catalyst obtained from an Iranian oil refinery in batch bioleaching using Acidithiobacillus ferrooxidans showed that addition of Fe(II) source into the growth medium at pH 1.8–2.0 resulted in maximum extraction of 63% Al, 96% Co, 84% Mo and 99% Ni in 30 days. On the other hand bioleaching carried out using Acidithiobacillus thiooxidans with addition of sulphur source to the growth medium at pH 3.9–4.4 resulted in maximum extraction of 2.4% Al, 83% Co, 95% Mo and 16% Ni in 30 days [20].

11.3.4 Role of Integrated Chemistry in Processing of Waste Electric and Electronic Equipment (WEEEs) In order to extract metals from WEEEs by bioleaching, much attention was focused on the printed circuit boards (PCBs). The metal contents of the PCBs are quite variable depending on the origin of material, and to some extent on the sampling procedure followed and analytical methods used. The composition is generally in the range of 10–27% Cu, 8–38%

336 Integrating Green Chemistry and Sustainable Engineering Fe, 2–19% Al, 0.3–2% Ni, 1–3% Pb, 200–3000 ppm Ag, 20–500 ppm Au, 10–200 ppm Pd etc [21, 22]. In general, the values of precious metals are the prime economic diver to recycle the e-wastes which is followed by the recovery of rare earths and heavy metals. With the presence of metals like Cu, Fe, Ni, and precious metals such as Ag, Au, Pd and rare earths such as Eu, Tb, Y, Ce, La, the use of acidophillic autotrophic, acidophillic heterotrophic or cyanogenic heterotrophic bacteria or fungi is reported.

11.3.4.1

Processing of Precious Metals From WEEEs with Integrated Routes

Precious metals from those resources are extracted by biogenic cyanide that acts as lixiviant. The well-known cyanogenic bacteria, for gold metallurgy, include Chromobacterium violaceum, Bacillus megaterium and Pseudomonas fluorescens and numerous strains of Marasmius, Clitocybe, Pholiota, Tricholoma and Polyporus [23, 24].

→

−

(11.28)

This biogenic cyanide can form complex with different metals such as Cu, Ni, Ag, Pt, Au, Pd, Rh etc. In-direct approach deals with heterotrophic organisms (bacteria and fungi) for the production of organic acid (i.e., oxalic acid, citric acid, gluconic acid and lactic acid), amino acids, and other metabolites utilized in complexolysis, acidolysis and alkalolysis techniques used for the extraction of different metals (Cu, Zn, Pb, Sn, Ni, Al, Ce, La, Pr, Nd, Y, Eu, Gd, U etc.) from WEEE. Acidolysis is one of the most important leaching mechanism which comprises the protonation of O2 (oxygen atoms) in metal compound. The protonated oxygen atom combines with water, resulted in separation of metal oxide from the surface of solid and is being solubilized [25]. The oxalic, gluconic, acetic, lactic, citric, succinic, formic and pyruvic acids commonly secreted by heterotrophs [26, 27] support in creating an environment having low pH, in order to increase the bio-leaching of metals. Organic acids produced and the proton-translocating ATPase of the plasma membrane are the sources of protons, which decrease the availability of anions to the cations in metal compounds, thus causing the solubilization of metal ions [28]. While organic acid produced from heterotrophs during acidolysis, form complex with metals, bioleached from material and hence complexolysis mechanism can also occur because the organic acids are strong chelating

Green and Integrated Chemistry in Sustainable Metallurgy 337 agents [29]. The metals ion solubilization depends on the complexing capacity of a molecule. The metal will be easily leached out if a strong bond is formed between the metal ion and ligand as compare to the bond present between the metal ion and solid particle. In most cases metals can be directly solubilize because the organic acid act as leaching reagent during bioleaching of solid (electronic scrap) wastes through heterotrophic microorganisms [5]. The ammonia which has ability to leach metals through alkalolysis approach can be produced by de-amination of amino acids or enzymatic hydrolysis of urea by microbes. Alkalolysis is found an effective approach in mobilizing metals from alumino-silicates or silicates and also make possible to perform bio-leaching at high ph value [30].

11.3.4.2

Processing of Rare Earth Elements from WEEEs with Integrated Routes

Rare earth elements (REE) are fairly present in the earth’s crust. Lanthanides along with scandium and yttrium known as REE elements of periodic table, These metals are not often be recycled from secondary waste by using physicochemical recovering processes, because of their low price (than precious metals) and concentration in many commercial products. The use of bacteria in order to mineralize and mobilize of metals is an alternative of old physicochemical recovering processes of REE. Limited literature available on database pertaining to bioleaching of REEs from WEEE but bioleaching can have a potential to replace chemical leaching as a cost effective green technology. Certain microbes can interact with metals and/or produce suitable lixiviant for bioleaching of REE from WEEE. Autotrophic microbes are not suitable for bioleaching REEs from WEEE as these microbes produce inorganic metabolites. In contrast, heterotrophic microbes excrete metabolites such as organic acids, amino acids and proteins to form complexes with the metals contained in WEEE, thus reducing the damage to the metabolic activity of the microbes [25]. Although, bioleaching of heavy metals from electronic computer scrap by heterotrophic microbes has been previously documented [27] but insufficient literature exists regarding leaching of REE from secondary resources. However, a recent study involving bioleaching of REE from red mud is reported by Qu and Lian [31].They investigated that Penicillium tricolor, at 2% (w/v) pulp density and with one step and two step leaching process, showed the

338 Integrating Green Chemistry and Sustainable Engineering Organic phosphate 2+

Po 4

Comp le

2+

e at ph os h p ic at tion ym ita z En ecip pr

M(Po 4) L M+ e

M–L

R ea

xation ctio n wit h ba

cter

n tio cre Se

Up

Dissolution ial m

etab

olite

s

Chemic al intera with bio ct film com ion ponent

tak

Comp lexati on

Po 4

Biochemical uptake

Energy dependent uptake

Bio

m

ine

ral

Adsorption

iza

tio

n

Electron donor Mixed mineral uptake

Electron donor CO2

CO2

M

Fe (II) Direct enzymatic reduction

M (O2)

Indirect reduction

Fe (II) + M(VI) Fe (II) M(VI)

Figure 11.4 Schematic sketch for processing of various minerals/material by integrated bio-chemical routes. Adapted from Ilyas et al., [1].

maximum extraction of the REEs and radioactive elements. However the highest extraction yields, at 10% pulp density, were achieved under two-step bioleaching process. This study provide basis for the possibility of bioleaching of REEs from WEEE. Table 11.3 provides an overview of current level of research of the bioleaching of metals from electronic wastes. A mechanistic process scheme can be proposed as in Figure 11.4

11.4

Perspectives on Integrated Chemical and Biological Routes for Mineral/Material Processing

Integrated biometallurgical processing will continue to play a vital role in metallurgical sector due to certain merits over conventional techniques because bioleaching is economically and technically attractive for processing of low grade ores and complex material.

Green and Integrated Chemistry in Sustainable Metallurgy 339 The advantages of microbial processes include specificity, energetics, and minimal creation of new waste. Increasingly stringent environmental regulations and constraints will drive demand for new methods to recycle and recover scarce and critical metals from the waste streams, providing an ideal niche for bio-processing approach. Some technical challenges and their possible solutions can be as follows for the bioprocessing of metals from ores and various waste streams;1) Heterotrophic bioleaching using indigenous fungal species could be a feasible way to recover rare earth elements (here heavy metals) from the solid wastes. Organic acids in conjunction with biogenic chelators could be efficient lixiviants in liberating rare earth elements from various solid waste streams 2) Widespread industrial use of microbes for leaching precious metals has been constrained by the limited cyanogenic capabilities of lixiviant-producing microorganisms such as Chromobacterium violaceum. Therefore, the construction of a metabolically-engineered strain of Chromobacterium violaceum or related cultures can produce more cyanide lixiviant that may recover more than twice the amount of gold from the electronic wastes compared to that of the wild culture. With further enhancement in cyanogenesis through subsequent metabolic engineering, production of biogenic acids can be enhanced and bioleaching efficiency of metals can be improved 3) For efficient metal recovery from solution, bacterial cells can be immobilized on support such as polyurethane foam and integrated-flow through column reactors can be designed. Metal precipitation and crystallization can be achieved through the production of high concentrations of phosphate ligand locally, which exceeds the solubility product of the metal phosphate in the vicinity of nucleation sites on the cell surface, most probably the lipid, a component of cell-surface lipopolysaccharide. All such metals forming insoluble phosphates can be amenable to bio-recovery in this way 4) Biomass may be derived from the activated sludge or fermentation wastes from the food industries. Microorganisms like, bacteria, fungi, yeast, and algae from their natural habitats are excellent sources of biosorbent. Fast growing organisms e.g., crab shells and seaweeds can also be used as biosorbent. In addition to the microbial sources, the agricultural products such as wool, rice, straw, coconut husks, peat moss, exhausted coffee, waste tea, walnut skin, coconut fiber, cork biomass, rice hulls, soybean hulls and cotton seed hulls, wheat bran, hardwood sawdust, pea pod, cotton and mustard seeds cake can serve as the source of the low cost biosorbent. Similarly, as an alternative approach to the economic provision of phosphate ligand into the crystalline metal phosphate, biological phosphate removing bacteria (i.e., Acinetobacter spp), mostly used in the wastewater

340 Integrating Green Chemistry and Sustainable Engineering treatment, grow aerobically and deposit intracellular reserves of polyphosphate (polyP). Upon transfer to the anaerobic conditions (which can be spatial or temporal) the polyP can be mobilized and inorganic phosphate can be released from the cells. This can harness the deposition of uranium and lanthanum phosphates in a similar way to those bio-manufactured using the phosphatase route. As a second approach, the use of phytic acid, inositol phosphate, a component of plant wastes and a byproduct from biodiesel production, as the phosphate donor molecule can have good economic potential for the metal biorecovery. Practical liquid wastes are important for the bioprocessing of metals which often have extreme pH, high total dissolved solids, ionic strength, organics, solvents and undesired toxic metals, but most of the recovery processes are developed under ideal conditions in which the targeted metal is present as a free solvated ion. But critical/ scarce metals in the waste streams occur as a trace component of nano-sized or micro-sized colloids, polymers, complexes with inorganic and organic ligands, or sorbed on the suspended material in the waste. The studies show how even the seemingly simple waste stream properties such as solution pH and Pd concentration, significantly affects the chemical speciation, and as a result determines the electrostatic interactions, ligand substitution and Pd reductions. Similarly, the effects of essential biological parameters including pH, ionic strength and temperature on the REE recovery can result from the competition between ions, temperature, changes in the activity of REEs and/or microbial functional groups.

References 1. Ilyas, S., Kim, M.-Seuk., Lee, J.-chun., Kim, M.-S., Lee, J. C., Integration of microbial and chemical processing for a sustainable metallurgy. J. Chem. Technol. Biotechnol., 93(2), 320–332, 2018. 2. Brierley, J.A., Brierley, C.L., Present and future commercial applications of biohydrometallurgy. Amils, R., Ballester, A., (Eds.). Biohydrometallurgy and the environment toward the mining of the 21st century. Amsterdam, Elsevier. pp. 81–89, 1999. 3. Schinner, F., Burgstaller, W., Extraction of Zinc from Industrial Waste by a Penicillium sp. Appl. Environ. Microbiol., 55(5), 1153–1156, 1989. 4. Bosecker, K., Bioleaching: metal solubilization by microorganisms. FEMS Microbiol. Rev., 20(3–4), 591–604, 1997. 5. Ilyas, S., Lee, Jc., Shin, D., Kim, B.S., Biohydrometallurgical processing of nonferrous metals from copper smelter slag. Adv. Mat. Res., 825, 250–253, 2013.

Green and Integrated Chemistry in Sustainable Metallurgy 341 6. Sand, W., Gehrke, T., Hallmann, R., Schippers, A., Sulfur chemistry, biofilm, and the (in)direct attack mechanism ? a critical evaluation of bacterial leaching. Appl. Microbiol. Biotechnol., 43(6), 961–966, 1995. 7. Rawlings, D.E., Heavy metal mining using microbes. Annu. Rev. Microbiol., 56(1), 65–91, 2002. 8. Schippers, A., Sand, W., Bacterial leaching of metal sulfides proceeds by two indirect mechanisms via thiosulfate or via polysulfides and sulfur. Appl. Environ. Microbiol., 65(1), 319–321, 1999. 9. Vaughan, D.J., Craig, J.R., Mineral chemistry of metal sulfides. Cambridge University Press, 1978. 10. Hallberg, K.B., Grail, B.M., Plessis, C.Adu., Johnson, D.B., Reductive dissolution of ferric iron minerals: A new approach for bio-processing nickel laterites. Minerals Engineering, 24(7), 620–624, 2011. 11. Lovley, D.R., Phillips, E.J.P., Novel mode of microbial energy metabolism: organic carbon oxidation coupled to dissimilatory reduction of iron or manganese. Appl. Environ. Microbiol., 54(6), 1472–1480, 1988. 12. Kaksonen, A.H., Puhakka, J.A., Sulfate reduction based bioprocesses for the treatment of acid mine drainage and the recovery of metals. Eng. Life Sci., 7(6), 541–564, 2007. 13. Oliazadeh, M., Massinaie, M., Bagheri, A.S., Shahverdi, A.R., Recovery of copper from melting furnaces dust by microorganisms. Minerals Engineering, 19(2), 209–210, 2006. 14. Watling, H.R., The bioleaching of sulphide minerals with emphasis on copper sulphides — A review. Hydrometallurgy, 84(1–2), 81–108, 2006. 15. Bakhtiari, F., Atashi, H., Zivdar, M., Bagheri, S.A.S., Continuous copper recovery from a smelter's dust in stirred tank reactors. International Journal of Mineral Processing, 86(1–4), 50–57, 2008. 16. Lee, J.C., Pandey, B.D., Bio-processing of solid wastes and secondary resources for metal extraction - A review. Waste Manag., 32(1), 3–18, 2012. 17. Carranza, F., Romero, R., Mazuelos, A., Iglesias, N., Forcat, O., Biorecovery of copper from converter slags: Slags characterization and exploratory ferric leaching tests. Hydrometallurgy, 97(1–2), 39–45, 2009. 18. Xu, J., Thomas, H.R., Francis, R.W., Lum, K.R., Wang, J., Liang, B., A review of processes and technologies for the recycling of lithium-ion secondary batteries. J. Power Sources, 177(2), 512–527, 2008. 19. Mishra, D., Kim, D.J., Ralph, D.E., Ahn, J.G., Rhee, Y.H., Bioleaching of metals from spent lithium ion secondary batteries using Acidithiobacillus ferrooxidans. Waste Manag., 28(2), 333–338, 2008. 20. Gholami, R.M., Borghei, S.M., Mousavi, S.M., Bacterial leaching of a spent Mo–Co–Ni refinery catalyst using Acidithiobacillus ferrooxidans and Acidithiobacillus thiooxidans. Hydrometallurgy, 106(1–2), 26–31, 2011. 21. Ilyas, S., Anwar, M.A., Niazi, S.B., Afzal Ghauri, M., Ghauri, A., Bioleaching of metals from electronic scrap by moderately thermophilic acidophilic bacteria. Hydrometallurgy, 88(1–4), 180–188, 2007.

342 Integrating Green Chemistry and Sustainable Engineering 22. Cui, J., Forssberg, E., Characterization of shredded television scrap and implications for materials recovery. Waste Manag., 27(3), 415–424, 2007. 23. Brandl, H., Faramarzi, M.A., Microbe-metal-interactions for the biotechnological treatment of metal-containing solid waste. China Particuology, 4(2), 93–97, 2006. 24. Faramarzi, M.A., Stagars, M., Pensini, E., Krebs, W., Brandl, H., Metal solubilization from metal-containing solid materials by cyanogenic Chromobacterium violaceum. J. Biotechnol., 113(1–3), 321–326, 2004. 25. Burgstaller, W., Schinner, F., Leaching of metals with fungi. J. Biotechnol., 27(2), 91–116, 1993. 26. Silverman, M.P., Ehrlich, H.L., Microbial formation and degradation of minerals. In Advances in applied microbiology. Academic Press, 6, 153–206, 1964. 27. Brandl, H., Bosshard, R., Wegmann, M., Computer-munching microbes: metal leaching from electronic scrap by bacteria and fungi. Hydrometallurgy, 59(2-3), 319–326, 2001. 28. Gadd, G.M., Sayer, J.A., Influence of fungi on the environmental mobility of metals and metalloids. Environmental microbe-metal interactionsAmerican Society of Microbiology. pp. 237–256, 2000. 29. White, L.E., Andrews, T.J., Hulette, C., Richards, A., Groelle, M., Paydarfar, J., et al., Structure of the human sensorimotor system. I: Morphology and cytoarchitecture of the central sulcus. Cereb. Cortex, 7(1), 18–30, 1997. 30. Ehrlich, P.R., Population biology of checkerspot butterflies and the preservation of global biodiversity. Oikos, 63(1), 6–12, 1992. 31. Qu, Y., Lian, B., Bioleaching of rare earth and radioactive elements from red mud using Penicillium tricolor RM-10. Bioresour. Technol., 136, 16–23, 2013.

12 Biological Nitrogen Fixation and Biofertilizers as Ideal Potential Solutions for Sustainable Agriculture Shymaa Ryhan Bashandy*, Mohamed Hemida Abd-Alla and Magdy Mohamed Khalil Bagy Botany and Microbiology Department, Faculty of Science, Assiut University, Assiut, Egypt

Abstract The occurrence of nitrogen sources in soil is probably the most limiting factor in agricultural production. Industrial nitrogen fertilizer is cost-effective and now accounts for more than 50 percent of the total energy going into agricultural production. Given the high cost of fertilizer and the limited market infrastructure for farm inputs, considerable effort and more attention have been directed to biofertilizer research. Biofertilizers can reduce soil problems with increasing crop yield. The increasing demand for using safe, economic, agricultural practices are driving the utilization of fertilizers based on beneficial microorganisms. This chapter will focus on the biological nitrogen fixation (BNF), such as molecular signals between host and microsymbionts that involved in the process of nodule development and nitrogen fixation for increase agronomically potential and significant processes to non-legumes. The solubilization and mobilization of nutrients such as phosphorus, potassium, and zinc by bacteria and their role as bioinoculants have been addressed. In addition, Biofertilizers have a role in stimulating plant growth through the synthesis of growth promoting substances. Biofertilizers play vital roles in the fertility of soil and protect the agro-ecosystem as ecofriendly and cost-effective inputs for the agriculture. Keywords: Biological nitrogen fixation, plant growth promoting rhizobacteria, phosphorus solubilization, potassium solubilization, siderophores production, and phytohormone production

*Corresponding author: [email protected], [email protected] Shahid-ul-Islam (ed.) Integrating Green Chemistry and Sustainable Engineering, (343–396) © 2019 Scrivener Publishing LLC

343

344 Integrating Green Chemistry and Sustainable Engineering

12.1

Introduction

Biofertilizers are synthetically multiplied cultures of beneficial soil microorganisms that can increase soil fertility and crop yield. Biofertilizers are products contain live microbial inoculants which are able to fix atmospheric nitrogen symbiotically or non symbiotically, solubilize soil phosphorus, potassium, and zinc, decompose organic material or oxidize sulfur in the soil. The main sources of biofertilizers are bacteria, fungi and cyanobacteria and therefore would be ideal for sustainable agriculture. As a result of increasing need for using safe economic agricultural practices is driving the use of fertilizers based on beneficial microorganisms. Nitrogen is the vital element for plant growth and crop production. Agriculture is in more acquisition of the nitrogen supplementary compound. Nitrogen enters in the strucrure of nucleic acids, enzymes and proteins. Nitrogen presents naturally approximately 78% of the atmosphere in an inert gas form. To be in available form, the triple bond in the N2 atoms must be broken down, which requires a large amount of energy. The processes, which convert inert gas N2 into the available form such as NH3, NO2, and NO3 called nitrogen fixation (NF). Nitrogen can naturally fix through abiotic processes such as volcanic activity, forest fires and lightening producing different nitrogen oxides in the atmosphere that dissolved in the rain and leaching to the soil as NH3 molecules. Nitrogen fixation is fixed in this way about 12% of annual global [1]. The second method of the nitrogen fixation is industrial (the Haber-Bosch process) which requires high temperatures and pressures. The industrial nitrogen fixation is used in fertilizer production. The annual world production of nitrogen fixation by industrial method estimated about 25% [2]. However, this process utilizes 3–5% of the world’s energy annually [3]. The inefficient use of nitrogenous fertilizer lead to nitrate contamination of soils and ground water, resulting health hazards and compromising agricultural sustainability [2]. The third method of the nitrogen fixation is biologically which a result of normal metabolic activity of only some bacteria and cyanobacteria, known as diazotrophs. Diazotrophs organisms possess enzyme complex called nitrogenase that has the ability to reduce gaseous N2 into NH3 [4]. Biological nitrogen fixation (BNF) is the main source of the nitrogen content of terrestrial ecosystems [1, 4]. The advantage of BNF is environmental friendly and economic cost, therefore, would be ideal for sustainable agriculture. It does not depend on external sources of energy, except for free and renewable sunlight, and has few detrimental ecological effects [5]. Therefore, uses BNF and biofertilzer have the same stratiges

Biological Nitrogen Fixation and Biofertilizers 345 140000 120000 100000 80000

Nitrogen

60000

Phosphate

40000

Potash

20000 0 2015

2016

2017

2018

2019

Figure 12.1 Chemical fertilizer consumed from 2015 to 2019 estimated by thousand tones [8]. 180000 160000 140000 120000

Africa

100000

Americas

80000 60000

Asia

40000

Europe

20000 0 2015

2016

2017

2018

2019

Figure 12.2 Regional nitrogen fertilizer consumed in the world from 2015 to 2019 estimated by thousand tones [8].

of green engineering. Green engineering uses the processes and products that reduce pollution, promote sustainability, and save for human health and economic. Terrestrial BNF occurs either non-symbiotically by freeliving diazotrophs or symbiotically by those associated with plants. Crop rotation with legumes has been known to enrich soil fertility and agricultural production since ancient history [6]. Research in this field is pivotal importance and would be significantly beneficial. The world’s population is estimated to be double by the end of 2033 [7]. The consumption of nitrogenous fertilizers is increasing (Figures 12.1 and 12.2). Total fertilizer nutrient (N+P2O5 +K2O) consumption is estimated at 184.67 million tonnes in 2014 and is forecast to reach 186.6 million tonnes in 2015 (Figures 12.1 and 12.2). With a consecutive growth of 1.6 percent per year, it is anticipated to arrive at 199 million metric tons by the end of

346 Integrating Green Chemistry and Sustainable Engineering 2019 [8]. Biologically fixed nitrogen range in soils estimates from approximately 33 million tonnes N per year [9] to 50–70 million tonnes N per year [10]. This requires great efforts to develop farming systems to increase the food requirement and better yield. Many studies have revealed that nitrogen fertilizers could resolve these challenges of the world, enough and better yield for feeding the increased population. Biological nitrogen fixation is known to be an important to sustain agriculture and to prevent the decrease of soil fertility. It is important to focus research on nitrogen fixing microorganisms and plants participate largely to the production of biofertilizers.

12.2

Non-Symbiotic Biological Nitrogen Fixation

Non-symbiotic (NS) N2 fixing microorganisms referrer to autotrophic and heterotrophic free-living soil prokaryotes that are not involved in a symbiotic relationship with host plants, and associative N2-fixation. Free-living N2 fixing microorganisms can found aggregates on organic matter and decomposing plant materiales [11]. About 50 different culturable diazotrophic bacteria have been recorded [12–14]. Diazotrophic bacteria have a broad range of diversity. Therefore, N2-fixing bacteria have the capacity to survive under different environmental conditions. Activities of the nitrogenase enzyme of non-symbiotic N2 fixing microorganisms have been determined by indirect methods such as C2H2 reduction assay or calculation from nitrogen balance. Rates of non-symbiotic N2 fixation are considerably less than estimated nitrogen inputs from symbiotic N2 fixation which range from 2 to 284 kg N ha−1 year−1 in legume pastures [15] and 0–271 kg N ha−1year−1 in grain legumes [16, 17].

12.2.1 Factors Affecting Non-Symbiotic N2 Fixation 12.2.1.1 Soil and Environmental Factors Diazotrophic communities and their function potential varied significantly according to management factors, environment and edaphic [18–20]. Most diazotrophic bacteria fix N2 under low oxygen or anaerobiosis [13]. Nitrogenase enzymes are sensitive to O2 and rapidly inactivated because of exposition to air [21]. In environment, saturation of soil by moistures creates anaerobic conditions and substantial amounts of N2 fixation have been estimated under these conditions [22], soil aggregation in aerated soils allows microaerophilic and aerobic conditions [11]. Non-symbiotic

Biological Nitrogen Fixation and Biofertilizers 347 N2-fixing bacteria, such as Azotobacter and Azomonas can eliminate O2 through rapid respiration or the formation of extracellular polysaccharide [13, 23]. Carbon (C) availability considers the second major condition required for non-symbiotic N2 fixation. Free-living N2-fixing bacteria obtain their nutrient from decomposition of plant material. Associative N2-fixing bacteria utilise their nutrients from the exudates of the roots in rhizosphere. In both environments, other microbial groups compete for limited energy resources. On the other hand, endophytic N2-fixing bacteria obtain C and their nutrients from the plant [24]. Heterotrophic N2 -fixing bacteria can utilize carbohydrates, alcohols and some organic acids resulted as end products of cellulose decomposition [25, 26]. High rates of N2 fixation occurred when the amount of crop residue and rates of decomposition increased [27]. The crop residues retention can change the composition of nitrogen fixing community structure, and increase their abundance [28, 29]. Jones et al., [30] predicted that the composition and function of nifH communities in the rhizosphere were influenced by quorum-sensing compounds and carbon containing compounds which present in root exudates. Inorganic mineral in soil such as nitrogen can obstruct N2 fixation by NS microorganisms [31]. Increases of ammonium concentrations inhibit N2 fixing to occur. Notwithstanding, when C is abundant, excess ammonium can be utilized by other microbial populations, contributing to an increase of rate N2 fixation [32]. In the presence of immense quantities of crop residue with wide C: N ratios, decomposition can be tedious. But the addition of N when C is low increases the rate of decay by heterotrophic bacteria and the carbon become available for use by N2 -fixing bacteria [33, 34]. In nutrient poor soils, P addition can significantly increase N2 fixation in crops [35, 36] and in grasslands [37, 38]. Mo and Fe also influence on non-symbiotic N2 fixation. Mo and Fe enter into the structure of the nitrogenase enzyme [39]. N2 fixation in soils enhanced in high soil water content as a result of reducing O2 at the sites of fixation [22]. Macro aggregate formations of clays [40] reduce the sites of O2 [41] and become in low concentration thus N2 fixation increases. Roper [42] found that at least 50% water holding capacity was required for nitrogenase activity in disturbed soils in the laboratory, whereas nitrogenase activity can be occurred at soil water contents below 30% water holding capacity in undisturbed soils in the field [42]. N2 fixation can be occurred in harsh semi-arid environments by cyanobacteria or other free-living bacteria in association with a fungus (i.e., lichens) [43, 44]. Temperature plays an important role in N2 fixation process, it has been shown to occur in different ranges of temperature and variable

348 Integrating Green Chemistry and Sustainable Engineering environments such as near 0 °C in Antarctica [45, 46] and in the Arctic [47] to desert environments [44]. Jensen [39] recorded that N2 -fixing bacteria must survive during intervening hot dry conditions up to 60 °C. In Laboratory experiments, Roper [42] indicated that temperature between 30 °C and 35 °C is the most optimum temperatures for N2 fixation, with a range from 4–45 °C. Soil composition can significantly change the capacity of NS N2 fixation. Roper and Smith [48] observed that the optimum pH for nitrogenase activity of free-living bacteria extracted from soil was at pH 7–7.5 regardless of the pH of the original soil. Nelson and Mele [49] cleared that in acidic soils, lime application can promote the abundance of nifH-containing rhizobacteria. There is a little information on the effect of salinity stress on growth and N2 fixation of diazotrophic bacteria [50]. Argandoña et al., [51] able to isolate N2 fixing halophilic bacteria (Halomonas maura) from saline soils. Suliasih [52] recorded that Azospirillum and Azotobacter as non-symbiotic N2 fixing bacteria can survive up to the level of salinity 12.43 dS−1m. Bacillus subtilis strain AS-4 is non-symbiotic N2 fixing bacteria can used for nitrogen fixation in soil containing high concentration of salt [53]. Contaminations of soil with different heavy metal (Zn, Cu, Ni, Cd, Cr, Pb, Hg, As) have a negative effect of abundance and population of free living N2 -fixing bacteria [54, 55] and activity of nitrogenases enzymes [56]. Kamnev et al., [57] revealed that some strains of Azospirillum brasilense have the capacity to survive in the presence of heavy metals contamination such as Co, Cu and Zn. With an increasing of Cd2+ concentration (5 mg kg−1 soil), significant reduction of abundance of Azotobacter spp has been recorded by Prasad et al.,[58].

12.3

Symbiotic Biological Nitrogen Fixation

Some microorganisms can fix nitrogen symbiotically by partnering with a host plant. There are several examples of symbiotic nitrogen fixation such as the water fern Azolla’s symbiosis with a cyanobacterium, the symbiosis between actinorhizal trees and shrubs, such as Alder (Alnus sp.), with the actinomycete Frankia. The most significant nitrogen-fixing symbiotic associations are the relationships between legumes and Rhizobium or Bradyrhizobium bacteria [59].

12.3.1

Rhizobia

Rhizobia are groups of small, rod-shaped, Gram-negative bacteria have the capacity to form nodules on the roots of leguminous plants and

Biological Nitrogen Fixation and Biofertilizers 349 belong to the family Rhizobiaceae, which are part of the α-proteobacteria. Rhizobia present within soil in two forms: as a free-living saprophyte in the soil and in a symbiotic relationship with leguminous plants. In early studies, the taxonomy of rhizobia was based on the rate of growth of isolates on laboratory media and their selective interaction with their plant hosts. Rhizobia can not nodulate all plants, but each rhizobia could nodulate some legumes though not others [60]. As a consequence of developments in molecular biology and advances in bacterial taxonomy, a rhizobial taxonomy was depended on a wide range of characteristics and to the distinction of new genera and species [61]. Currently the rhizobia consist of approximately 50 species in about 12 genera, some of which are listed in Table 12.1 Genera and some species of the Rhizobium [62].

12.3.2

Legumes

Legumes are second only to the Graminiae in their importance to humans [63]. It is the third largest family of flowering plants incluld 800 genera and 20,000 species [64]. Legumes can found as grain crops and known as grain legumes, or found as weeds of cereal crops [65]. Legumes are an important source of nitrogen-rich edible seeds. Grains of legume provide high protein products. In addition, legumes are the main constitute dietary protein in the diets of the poor parts of the world [66]. Legumes are the main source for human and animal consumption, as a source of plant proteins and improving human health [67]. Legumes have the capacity to fix atmospheric nitrogen as a result of their symbiotic relationship with rhizobia. Legume crops improve soil fertility, such as soil organic carbon and humus content, N and P availability [68, 69]. Nitrogen content accumulation in aboveground biomass of field pea and faba bean about 130 and 153 kg N ha−1, respectively, and about 30–60% of the accumulated total N in belowground biomass [70]. Legumes are enhancing farmland biodiversity, particularly reducing greenhouse gas emissions [71, 72], breaking the cycles of pests and diseases [73, 74] and protecting soil and water resources. Thus, legumes increase food security of subsistence farmers, reduced cost of food for poor consumers and enhanced rural incomes. Many studies recently have focused on the sustainable re-introduction of legume plants into crop rotations depending on their alleviation effects on yield and quality characteristics on subsequent crops [72, 75, 76]. The high productivity of wheat-legume rotations has long been recognized by wheat farmers, and for as far back as 2 000 years [77]. A faba bean-wheat

350 Integrating Green Chemistry and Sustainable Engineering Table 12.1 Genera and Some Species of the Rhizobium [62]. Genera

Species

Hosts

Allorhizobium

A. undicola

Neptunia natans, Acacia, Faidherbia, Lotus

Azorhizobium

A. caulinodans A. doebereinerae

Sesbania rostrata Sesbania virgata

Blastobacter

A. denitrificans

Aeschynomene indica

Bradyrhizobium

B. canariense B. elkanii B. japonicum B. liaoningense B. yuanmingense

Chamaecytisus, Lupinus Glycine max Glycine max Glycine max Lespedeza, Medicago, Melilotus

Burkholderia

B. caribensis B. cepacia B. phymatum B. tuberum

Mimosa diplotricha, Mimosa pudica Alysicarpus glumaceus Machaerium lunatum, Mimosa Aspalathus spp.

Devosia

D. neptuniae

Neptunia natans

Ensifer

E. adhaerens

(not reported)

Mesorhizobium

M. amorphae M. chacoense M. ciceri M. huakuii M. loti M. mediterraneum M. plurifarium M. septentrionale M. temperatum M. tianshanense

Ralstonia (Cupriavidus)

R. taiwanensis

Amorpha fruticosa Prosopis alba Cicer arietinum Astragalus sinicus, Acacia Lotus corniculatus Cicer arietinum Acacia Senegal, Prosopis juriflora, Leucaena Astragalus adsurgens Astragalus adsurgens Glycyrrhiza pallidifloria, Glycine, Caragana, Sophora Mimosa (Continued)

Biological Nitrogen Fixation and Biofertilizers 351 Table 12.1 Cont. Genera

Species

Hosts

Rhizobium

R. etli R. galegae R. gallicum R. giardinii R. hainanense R. huautlense R. indigofera R. leguminosarum bv trifolii bv viciae bv phaseoli R. loessense R. mongolense R. sullae R. tropici R. yanglingense

Phaseolus vulgaris, Mimosa affinis Galega orientalis, G. officinalis Leucaena, Onabrychis, Macroptilium, Phaseolus vulgaris Macroptilium, Phaseolus vulgaris, Desmanthus, Leucaena Desmanthus sinuatum, Vigan, Arachis, Centrosema, stylosanthes Sesbania herbacea Indigofera Trifolium, Lathyrus, Lens, Pisum, Vicia, Phaseolus vulgaris Astragalus, Lespedeza Medicago ruthenica, Phaseolus vulgaris Hedysarum coronilla, Phaseolus vulgaris, Dalea, Macroptilium, Leucaena, Onabrychis Amphicarpaea, Cornnilla, Gueldenstaedtia

Sinorhizobium

S. abri S. americanus S. arboris S. fredii S. indiaense S. kostiense S. kummerowiae S. medicae S. meliloti S. morelense S. saheli S. terangae

Abrus precatorius Acacia spp. Acacia Senegal, Prosopis chilensis Glycine max Sesbania rostrata Acacia Senegal, Prosopis chilensis Kummerowia stipulacea Medicago truncatula, Medicago polymorpha, Medicago orbicular Medicago, Melilotus, Trigonella Leucaena leucocephala Acacia, Sesbania Acacia, Sesbania

352 Integrating Green Chemistry and Sustainable Engineering rotation system increases wheat yield up to 77 percent, while reducing the demand of nitrogen fertilizer [77].

12.3.3 Legume - Rhizobium Symbiosis Symbiosis is an interaction between two or more species exchange mutual benefits. One of the well-known symbioses occurs between two partners, legume plants and rhizobia. Rhizobia form symbiotic structure known as root nodules on the roots of their host, and obtain their nutrients from the host plant. In turn, they provide their host plants with nitrogen resources in available form such as ammonia produced by nitrogen gas fixation. The reason of stability this mutualistic relationship is the beneficial effect should be ‘‘partner fidelity feedback’’ [78–81]. The nitrogen fixation process requires much energy (or costs), rhizobia are therefore undergoing two opposite actions that at the same time promote (by providing benefit) and destabilize (by bearing cost) the mutualistic relationship [82]. The development of this symbiotic process is complex, the interaction begins with a specific diffusible molecular signal exchange between the legume and the free-living Rhizobium [83]. Plant roots secrete many different organic compounds into the soil, some of which allow microorganisms to grow in the rhizosphere and include carbohydrates, amino acids, organic acids, vitamins and phenolic derivatives [83, 84]. The main kind of signal from the plant to the bacterial partner is species-specific flavonoid type molecules, either in root exudates or released from the seed coat during germination, act as specific inducers or inhibitors of the nodulation genes in compatible rhizobia [85–88]. In terms of symbiosis, flavonoids are the most important of these compounds, as they trigger the induction of bacterial nodulation (nod) genes [89]. lipochitooligosaccharide is produced as a result of expression of bacterial nodulation genes. A bacterial signal molecule (Nod factor) is essential for induction root hair deformation and nodule formation [90]. In addition to these key regulatory molecules, a variety of other compounds, including exopolysaccharides, plant hormones, and vitamins, have been implicated as regulators of the nodulation process [91–93].

12.3.4 The Signals From the Host Plants (Flavonoids) The roots of the legume plants exude signal compound species-specific flavonoid-type molecules to the bacterial partner [60, 94, 95]. Flavonoids compounds are components used in the chemical communication between legume plants and symbiotic nitrogen-fixing bacteria to form nodules

Biological Nitrogen Fixation and Biofertilizers 353 the site of nitrogen fixation process [96, 97]. Flavonoids can stimulate or prevent rhizobial nod gene expression, induce chemoostatic reactions by rhizobia, prevent root pathogens, promote mycorrhizal spore germination, and chelate soil nutrients [98, 99]. These flavonoids are variable among legume hosts. Therefore, production of certain flavonoid by host legume to induce nod gene of certain rhizobia is one of the first levels of specificity between hosts and nitrogen-fixing rhizobia [100, 101]. Luteolin, the first flavonoid identified as a nod gene inducer activate the nodulation gene in Rhizobium meliloti [102]. Induction of the nodA promoter of Rhizobium leguminosarum by plant flavanones and flavones were recorded by Zaat et al., [103]. Alfalfa seeds exude three flavonoid nod inducers during seed imbibition, even before germination [104]. While the Glycine max releases three root-seedling extract flavonoids to induce nod gene of Bradyrhizobium japonicum and nine flavonoids exuded from seed or root of Phaseolus vulgaris to activate nod gene of Rhizobium leguminosarum bv. phaseoli [105]. Eight flavonoids plus two non-flavonoid signals are librated from seed or root to initiation the Medicago sativa - Sinorhizobium meliloti symbiosis [106]. Soybean (Glycine max L. Merr.) release three root-seedling extract flavonoids by nanomolar concentrations of daidzein, genistein, and coumestrol induce expression of nodC and nolX in Sinorhizobium fredii strain USDA191 [107]. Moreover, Begum et al., [108] found that the optimum concentration of luteolin for maximum expression of nod genes of R. leguminosarum was at 20 μM. Peck et al., [109] reported that luteolin has the ability to activate nod gene expression of Sinorhizobium meliloti. Zhang and Cheng [110] found that 17 out of 23 alfalfa root exudatesinducible genes were flavonoid (apigenin) -inducible S. meliloti nod genes expression released in the early root infection stage of symbiosis. Cooper [111, 112] concluded that nod genes from different rhizobia may respond to different sets of flavonoids. Nutrient solution supplemented with 10 μmol of daidzein, genistein, coumestrol, glycitein and their combination increased the nodule numbers of common bean cv. Adzuki significantly [85]. Combination of hesperetin and apigenin increase growth, nodulation, and nitrogen fixation of fenugreek [87, 113]. Roles for luteolin as a rhizobial chemoattractant have been proposed by Spini et al., [114]. For symbiosis relationship, it may be more appropriate to talk about a “chemical zone” close to the seedling that would have different concentrations of a series of different flavonoids exuded from both roots and seeds [115], which together affect the rhizobial partner [116]. Under these circumstances, the seeds, during imbibition, start to exude flavonoids. Such as luteolin, exuded by alfalfa seeds [117], which act as chemoattractants [118] and growth inducers [119] for the rhizobial partner, and later, with

354 Integrating Green Chemistry and Sustainable Engineering the initiation of root growth, and the concomitant production and exudation of new signal molecules [115]. By this time, there would already have been some bacterial growth, and probably, some exudation of Nod factors by them [120, 121]. Nod factors would increase the production and excretion of flavonoids by the legume root [122] as well as initiate the expression of the early nodulin genes by the plants [123]. Flavonoids, and more generally phenolic compounds, also contribute to defence against biotic stresses [124]. Other organic molecules, especially phenolics act as potent inducers of nodABC genes [125, 126]. Abd-Alla [127] reported that providing various phenolic compounds to soybean plants increased nodulation, as well as NADH-GOGAT and NADH-GDH activities. Certain phenolic compound induced the nodD genes in different Rhizobium species [109, 128]. The phenylalanine- ammonia-lyase (PAL) produces cinnamate from phenylalanine, which is a precursor of a large number of phenolics, phenylpropanoids and flavonoids [129, 130]. Understanding the specificity between plant genotypes for specific rhizobial strains is important to solve agronomic problems for the enhancement of symbiotic N2 fixation, may allow modifications of this interaction to improve symbiotic performance [131–133] and, to solve the nitrogen fixation problem of non fixing agriculturally important food crops [134].

12.3.5

Nod Factors

Nod factors are the main rhizobia nodulation signal back to the plant. When applied in a purified form, they are able to induce most of the plant early nodulation responses [135, 136]. The generic name Nod factor is applied to molecules of a lipochitooligosaccharide nature, consisting of a backbone of tetramers and pentamers of ß1, 4-linked glucosamine residues, N-acylated on the terminal non-reducing sugar and O-acetylated on the other residues [137, 138]. This is the generic structure of a Nod factor backbone, which is constructed by the common nod genes [139, 140]. Each rhizobial species has specific Nod factors for its plant host that has specifies length of the backbone and the type of substitutions at both ends of the molecule [120, 140–142]. These Nod factors, at very low concentrations (on the order of −12 10 M); induce the curling of the root hairs [121, 143]. At slightly higher concentrations (10−9 M range) they induce initiation of mitotic activity in the root cortex [84, 144], which, after some time, may result in formation of a nodule. Herrbach et al., [145] found synergistic interactions between Nod factors and auxin signaling and biosynthesis to initiate

Biological Nitrogen Fixation and Biofertilizers 355 lateral root formation. There is a link between the flavonoid-chitin molecule system of signal exchange during symbiosis establishment and the pathogen-plant interaction system, is widely accepted [146, 147] and it is supposed that this symbiotic system has evolved from a pathogenic relationship.

12.3.6

Molecular Genetics of Nodulation (The Nod Genes)

Some nod genes involved in nodulation are constitutively expressed by nodD [60, 109]. The protein encodes for, nodD, is a member of the LysR family of transcriptional activators [148, 149] and causes the transcription of the other nod genes, when activated in response to specific plant stimuli. As well as activating the transcription of other nod genes, it also regulates its own expression in R. leguminosarum [150]. The N-terminus of nodD is highly conserved, indicating a role in DNA binding. The nod genes/operons induced by nodD all contain a highly conserved sequence termed the ‘nod box’ where it is believed the N-terminus of nodD binds and initiates transcription of the genes/operons [151]. Each rhizobial species/strain interacts with only a specific group of legumes [152]. nodD’s C-terminus is more variable and it may have a function involving flavonoid binding [109, 153]. Different Rhizobium species have different nodD proteins, which respond to different flavonoids specific for different legume types. The ability of nodD to react with specific flavones is a corner stone that determines the specificity of host plant. Rhizobium can nodulate either many host plant species or specific host plants. R. leguminosarum bv. viciae responds to hesperitin [154], which is released by pea and vetch roots, whereas S. meliloti contains three nodD genes, allowing it to respond to a wider array of flavonoids and hence leguminous plants [155]. The common nod genes are nodABC and a mutation in any of these prevents the formation of nodules on inoculated plant roots (nod- phenotype) [156]. The encoded proteins by nodABC, nodA (acyl-transferase), nodB (deacetylase) and nodC (N-acetylglucosaminyltransferase or chitin synthase) function together to catalyze the synthesis of the monoacylated tetrameric or pentameric chitin core structure required in nodule formation [157]. The nodIJ genes are considered to be common nod genes as they are found in many rhizobial species, including R. leguminosarum bv. viciae, bv. trifolii, R. etli and S. meliloti. Their products, nodI and nodJ, are involved in the transport of Nod factors and are believed to be part of an ATP-binding cassette (ABC) transporter [158, 159]. The organization of nod genes differs between rhizobial species, although nodDABCIJ are normally clustered into one organizational

356 Integrating Green Chemistry and Sustainable Engineering unit. Together these nod genes synthesize molecules known as Nod factors, which initiate nodule formation in the host plant.

12.3.7

Nodule Formation

As a result of exudation root legume signial chemoattractant and rhizobial response by production of Nod factors, the bacteria surround and attach to the legume root, causing root hair curling [160]. Rhizobia enclosed in a curled hair, proliferate and infect the outer plant cells, then stimulate plant cells to produce infection threads [161–163]. Bacteria liberated from infection threads into the cytoplasm of plant cells [164]. Ultimately, the presence of bacteroids (endosymbiotic forms of the bacteria) induce the formation of a new plant organ on the root known as root nodule and the nitrogen fixation process begin by nitrogenase enzyme [165].

12.3.8

Nitrogen Fixation

Once the rhizobia are in the nodule most nod genes expression stop [166, 167], due to large quantities of Nod factors may stimulate plant defence reactions [168], and the bacteroids begin the expression of nitrogen fixing genes instead. Many species of the family Rhizobiaceae possess the ability to fix atmospheric nitrogen, a mechanism that is exclusive to prokaryotes [60]. Nitrogenase is an enzyme complex responsible for nitrogen fixation process [169]. When rhizobial genes expressed in roots as part of symbiosis, plant genes are expressed in these conditions; these are called nodulins, which responsible for the production of leghaemoglobin [170]. Leghaemoglobin able to bind oxygen and releases it for bacteroid respiration when the concentration of O2 decreases at a certain level, low free oxygen that is also required, as nitrogenase is inactivated by oxygen [171] Presence of pigment in leghaemoglobin gives healthy nodules their pink/ red colour. Nitrogenase in bacteroids forms NH3 by reduction of dinitrogen gas using protons [172]: 8H+ + N2 + 8e– → 2NH3 + H2 This reaction requires about 16 ATP molecules and estimated about 42 ATP under certain condition [173]. ATP required is generated by respiration using plant provided carbon (dicarboxylates form) at a high rate to fix nitrogen [174, 175]. The plant provides the bacteria with an environment with controlled amounts of oxygen, dicarboxylates (taken in by rhizobia via the dicarboxylate transport (DCT) system) and glutamate (or glutamine), which is then utilized for respiration in the Rhizobium via the

Biological Nitrogen Fixation and Biofertilizers 357 tricarboxylic acid resulting in ATP generation that required for biological nitrogen fixation. The bacteroids provide the legume plant with ammonia and some amino acid such as aspartate and alanine [176, 177]. Plant produces the required amino acid like asparagine form, aspartate and glutamate and glutamine from ammonia. To stop ammonium assimilation by microsymbiont, plant supplies it by amino acids such as glutamate [176]. Essential nutrients are exchange between endosymbiont and legume plant cells through a simplistic and apoplastic pathway [178]. Recently, Yang et al., [179] noticed that the plant could participate by using plant origin electron transport chain to push the nitrogen fixation process. Rhizobiumlegume relationship is one of the most sources for nitrogen fixation in agro-ecosystems [180].

12.3.9 Ecological Factors Affecting Signal Exchange The biological nitrogen fixing symbiosis is complex and sensitive to environmental conditions [181]. When studying any living organism, it is important to know how each species grows and responds to certain conditions that can be found in their natural environment [182]. Understanding the bacterial response to stress conditions, or environmental signals, is an important step to know how the microbes live, thrive and survive [177]. Some of the most important environmental inhibitors of nitrogen fixation are sub-optimal soil conditions, such as temperature extremes, salt stress, high or low soil pH, low water content, pesticide application and nutrient deficiency [180, 181, 183]. Nodule formation is negatively affected in the presence of some environmental condition such as nitrogen availability, soil acidity or salinity and low soil temperature [184–188]. Rhizobial strains vary in their infectivity and competitiveness to nodulate host plant, N2 fixation capacity, compatibility with different plant species, and adaptations to harsh environmental conditions [189]. Thilakarathna and Raizada [190] reported that environmental factor, soil composition, agricultural practices can all impact survival of rhizobia in soil. Higher symbiotic nitrogen fixation and grain yield can be obtained by selection of adapted rhizobial strains for various abiotic stress [181]. Locally adapted rhizobial strains are capable of fixing nitrogen under variable environmental stress conditions such as low or high temperature in comparison with introduced rhizobia [191, 192]. The best temperature range for rhizobial growth is 28–31 °C, and many rhizobia recorded are unable to grow at 37 °C [180]. Some microsymbiont strains isolated from root nodules of different legumes are able to grow at 40 °C have been reported in many published studies [193– 198]. Some rhizobial strains isolated from Erythrina velutina able to grow up to 45 °C [199]. Baldani and Weaver [200] reported that heat tolerance of Rhizobium

358 Integrating Green Chemistry and Sustainable Engineering leguminosarum biovar trifolii strains due to the presence of cryptic plasmids. These plasmids have a role in production of heat shock proteins when the bacteria exposure above normal growth temperatures [201]. Temeprature effects on the molecular signals exchange between endosymbiont and their host, thus reducing nodule formation [182]. Low temperature inhibits the exudation and biosynthesis of plant signal molecules [85, 133, 185]. Although, it is documented that the best legume nodulation in the Mediterranean Cape region of South Africa occurs during the winter rains when temperatures are low, around 10–15 °C [202]. Salinity stress is an important factor limiting the productivity of leguminous crops. Soil salinity has negative effect on survival and growth of rhizobia in the soil and prevents rhizobia-legume symbiosis, resulting in lower yield production of legumes [203, 204]. Under salt stress, selection of salt-tolerant rhizobia from saline soils is the best solution to achieve successful Rhizobium-legume symbioses [205]. In High salinity levels, deformations, curling of root hairs and nodulation, were reduced [206, 207]. Graham [208] and Slattery et al., [209] detected the reduction of number salt-tolerant strains and their ability to fix nitrogen were also decreased. Several studies were indicated the ability of rhizobia and bradyrhizobia to tolerate high level of salinity [195, 210–213]. Guasch-Vidal et al., [214] reported that high salinity level (300 mM NaCl) activate the nod genes of Rhizobium tropici ciat899. However, nod gene expression of R. tibeticum inhibited significantly as the salinity level increased [87]. Under salinity stress, rhizabia change their osmotic potential by accumulation of different solutions to overcome the osmotic stress, also bacterial lipolysaccharides was changed in response to salt stress [215]. Soil pH is one of the most important factors that influence the efficiency of symbiosis between rhizobia and plants legume [216]. Rhizobial growth, survival and their ability to fix nitrogen are affected by soil pH either alkaline or acidic pH. Human activities and agricultural practices cause changing the soil acidity, and thus limit legume crop productivity. The optimum pH and the degrees of pH resistance are dependent on the rhizobial species. Although, neutral soil pH is generally favorable for the most rhizobial species [216]. Soil pH affect on the distribution of soil rhizobia, which is highly dependent on the species [217–219]. Rhizobia can be more sensitive to acidic conditions than their legume host. However, mutants R. leguminosarum was documented the ability of growth at pH 4.5 [220], S. meliloti can grow only down to pH 5.5 [221]. Acidic stress can lead to metal stress and nutrient stress. High pH can inhibit the growth of Rhizobium and effect on the nodulation process, although, Evans et al.,

Biological Nitrogen Fixation and Biofertilizers 359 [222] reported high soil colonization of R. leguminosarum bv. trifolii and high frequency of nodules in alkaline conditions. Only, 3 isolates from 20 rhizobial isolates collected from root nodules of faba bean plants were resistant to the alkaline conditions [223]. However, seven fast growing isolates from twenty isolates collected from root nodules of soybean tolerated high acidic conditions (pH 4) [224]. Pesticide application is considered the accepted solution for protecting plants from pest [225], but bad use of pesticide resulted in negative side effects to natural biological system [226]. Pesticides contaminate the soil resulting in disruption of symbiotic nitrogen fixation and subsequently reduce plant yields [227–231] indicated that root nodule formation was inhibited in cowpea, common bean and lupin by tested pesticides. Reduction of N content in plant tissue as a result of applying pesticide and reduction of nodules formation [230]. Pesticides have the maximum toxicity to plant growth promoting traits of Rhizobium. sp. strain MRL3 and Bradyrhizobium sp. strain MRM6 isolated from lentil-nodules and greengram plants, respectively [232, 233. Heavy metals are most important inorganic pollutants such as Cu, Ni, Cd, Zn, Cr, Pb [234]. Heavy metals enter soil from an industrial operation; animal manures and sewage sludge application after these elements enter into the soil they remain for several thousands years. The presence of heavy metals in different ecological systems leads to harmful effects in the varied agroecosystems [235–238]. Reusing drainage water without suitable treatment may cause adverse effects on soil, crop, animal, and human health [239, 240]. Many of the transitional elements function as essential cofactors in metabolic pathways and are required for microbial growth. Excess of these metal ions have negative adverse effects in bacteria, prevent enzymes, and uncontrolled redox reactions within the cell [241]. The negative effect of environmental stresses on N2 fixation may be divided into: (a) effects on plant infection by rhizobia; (b) effects on nodule growth and development; (c) direct effects on nodule function [242]. There is little knowledge, explain how rhizobia counter oxidative stress induced by heavy metals [243]. It has been proven that there are genes that are expressed under general metal stress [241]. The production of high intercellular carbohydrates and thiols is a response to some of these metals [244, 245]. High metal concentrations in contaminated soils can cause reduction soil microbial population and soil fertility [246]. Alteration of symbiosis genes explains the reason of survival of the Rhizobium in polluted soils [247]. The inhibitory effects of heavy metals on the growth and activity of both symbionts explain the reduction of nodulation and N2 fixation of Rhizobium–fab bean symbiosis [248]. Pal and Bhattacharya [249] reported that at higher concentration of Ni (two ppm) significantly inhibited the growth, total protein,

360 Integrating Green Chemistry and Sustainable Engineering free amino acid, total hexose, inorganic phosphate acid phosphatase and alkaline phosphatase of Rhizobium. Deleterious effects of high concentration of Ni on nodules formation of some leguminous species was recorded by Vijayarengan [250]. Abd-Alla et al., [113] found that the inhibitory effect of Ni on rhizobial growth and nod gene expression appeared at the lowest Ni concentartion (25 ppm) and became toxic with the increase in Ni concentration. Miličić et al., [251] found that the minimum inhibitory concentration of Ni for R. leguminosarum bv. trifolii strains tested was 10 μg ml−1. The nickel tolerant Rhizobium strain RP5 and strain RL9 were isolated from nodules of pea and lentil, grown in metal-contaminated soils, which displayed a high level of tolerance to nickel at 350 and 500 μg ml−1, respectively [252, 253]. High concentration of Al3+ has negative adverse effect on N2-fixing bacteria whether in soil or culture medium [254–257]. Data available in the literature refer that high Al concentration reduce or inhibit nodulation and nitrogen fixtion in several legumes, including Phaseolus vulgaris, Trifolium repens, Stylosanthes species and other tropical species [258, 259]. In chromium-amended soil, Mesorhizobium strain RC3 increased the dry matter accumulation, nodule formation, seed yield and grain protein compared to non-inoculated chickpea plants [260]. Mesorhizobium sp. (UFLA 01–765) showed tolerance and an efficient symbiosis on Zn- and Cd-contaminated mining soils [261]. The harmful effect of Co on rhizobial growth can be diminished by the addition of a mixture of hesperetin and apigenin [88]. Rhizobia have the ability to reduce soil contaminats such as organic pollutants and heavy metals. In addition, rhizobia able to stimulate plant growth making them useful for reclaiming contaminated soils [237, 262, 263]. Introduction of Rhizobium strains with high tolerance to stress has been used to improve symbiotic efficiency and crop productivity in agricultural systems. Improving legume inoculation efficiency is extremely important to improve legume production under drastic conditions.

12.4

Plant Growth Promoting Rhizobacteria

Plant growth promoting rhizobacteria (PGPR) enhance plant growth resulted in a positive effect on crop yield. Great efforts involved to increase crop productivity as a result of increasing requisitions of agriculture products don’t contain any synthetic chemical fertilizers and pesticides. The use of PGPR is considered an environmentally sound way of enhancing plant growth and increasing agriculture products [264]. PGPR is known as bacteria found in the rhizosphere have ability to colonize plant roots and enhance plant growth directly or indirectly [265, 266]. Rhizosphere is

Biological Nitrogen Fixation and Biofertilizers 361 a soil zone around plant root rich by microorganisms (10–100 fold than the bulk soil) that depend on the nutrient exuded by the plant roots [267, 268]. Rhizosphere is habitat for numerous microorganisms such as bacteria (highest population), fungi, actinomycetes, protozoa, and algae [269]. Soil bacterial populations grow rapidly and able to utilize a different substances as nutrient sources. Bacterial biota widely dispersed in the soil, have many interactions forms with the roots of plants [270]. The composition of microbial soil depends on various factors such as plant type [271], stage and plant development [271–273], soil structure [274, 275] abiotic and biotic factors [276]. Plant root exudates (quantity and quality) are able to limit and/or increase microbial populations in the rhizosphere [277] but also to microbial interactions [278]. According to the interactions of PGPR with plant, it can be divided into two groups, symbiotic bacteria (live inside plants), and free-living rhizobacteria (live outside plant cells) [279]. Majority of credible group of PGPR belongs to genera Acinetobacter, Agrobacterium, Arthobacter, Azotobacter, Azospirillum, Burkholderia, Bradyrhizobium, Rhizobium, Frankia, Serratia, Thiobacillus, Pseudomonads, and Bacillus [265, 280, 281]. The mechanisms of PGPR can be divided traditionally into direct and indirect ones. A direct mechanism takes place inside the plant and affects on metabolism of plant by releasing growth regulators (hormones) leading to an improvement of plant growth, while indirect mechanisms, occurs outside the plant [265, 280, 282–285]. On the other hand, in indirect mechanism, microbial communities participate the plants defense mechanisms through induction of systemic resistance to plant pathogens, it can also help plant to resistant the sever condition (abiotic stress) [285–288].

12.4.1 Direct Mechanisms of PGPR The direct mechanisms include biofertilization (fixing atmospheric nitrogen, solubilizing insoluble phosphates and potassium), stimulation of root growth (secreting hormones such as indole acetic acid (IAA), gibberellins (GAs) and cytokines besides 1-Aminocycloprapane-1-carboxylic acid (ACC) deaminase production), rhizoremediation, and plant stress control (antibiosis, competition for nutrients, parasitism, production of metabolites (hydrogen cyanide, siderophores) suppressive to deleterious rhizobacteria).

12.4.1.1 Biological N2 Fixation Symbiotic and non-symbiotic N2 Fixation were previously described in details above.

362 Integrating Green Chemistry and Sustainable Engineering

12.4.1.2 Phosphate Solubilization Phosphorus is the second element after nitrogen required in plants nutrition [289]. It enters in the structure of DNA, phytin and plasma memberane. Phosphorus plays a major role in photosynthesis, respiration, storage and transfer of energy and cell division and elongation [290]. The high content of plant phosphorus enters in seed formation. Phosphorus is not usually found in available form (tricalcium phosphate, rock phosphate, aluminum phosphate) for plant uptake [270]. In acidic soils, inorganic phosphates are present in Al and Fe compounds, while calcium phosphates are found alkaline soils [270]. Plants can only use mono- and dibasic phosphate as useable forms of phosphate [291, 292]. The reason of presence of P in low levels is the high reactivity of soluble P with soil element such as calcium (Ca), iron (Fe) or aluminium resulting P precipitation [291]. It is necessary for maintenance P available in the soil for plant to avoid low plant yield. Therefore, it required to release P in useable form in soil solution from the solid phase. An application of phosphatic fertilizers is considered a solution to overcome the P deficiency in soils. However, after plants adsorb its requirement of phosphorus, the rest of phosphatic fertilizers is rapidly converted into insoluble complexes [293]. Application of phosphate fertilizers regularly is unfavorable because it is expensive and pollutes environments. So, it is required to search a substitute to chemical phosphate fertilizer. Phosphate solubilizing microorganisms are a hopeful solution to increase P content in soil which characterized by environmentally safe, cost next to nothing, improving crop production and increase crop yield. Manzoor et al., [294] indicated that using phosphate solubilizing combined with organic amendments and insoluble rock phosphate is one of the most alternatives to enhance P availability in the soil and improve plant growth. Adnan et al., [295] found that inoculation of phosphate solubilizing microorganisms with organic manure application improves P nutrition and increase antagonistic activity of the soil. Rhizospheric bacteria are able to solubilize insoluble forms of the phosphate using different mechanism(s). Using mineral-dissolving compounds such as organic acids, protons, siderophores, and carbon dioxide is a principal mechanism for solubilization inorganic phosphate [296]. The organic acid production as a result of sugar metabolism is present in root exudates [297]. The released acids by the microorganisms such as acetic, lactic, malic, succinic, tartaric, gluconic, 2-ketogluconic, oxalic and citric acids act as good chelators of divalent Ca2+ cations resulting in the release of phosphates from insoluble phosphatic compounds [296, 298, 299]. Production of CO2 and proton/bicarbonate

Biological Nitrogen Fixation and Biofertilizers 363 release lowering pH and resulting solubilization inorganic phosphate [300]. Solubilization of inorganic phosphates by microorganisms was early known in 1903 [301]. Soil contains a numerous phosphorous organic substrates unavailable for plant growth. To obtain plant on organic P in available form such as inorganic P, it must be hydrolyzed. Microorganisms are able to decompose unavailable organic phosphorus by several phosphohydrolases such as phosphatase, phosphonoacetate hydrolase, phytase, D-α-glycerophosphatase [302]. Among the soil bacterial communities, Bacillus sp [284, 297, 301, 303], Pseudomonas sp [296, 304–307] and Rhizobium sp [308–310] have been described as effective mineral phosphate solubilizers by releasing acid. Other genera like Achromobacter, Agrobacterium, Enterobacter, Erwinia, Escherichia, Flavobacterium, Mycobacterium, and Serratia reported as effective phosphate solubilizers [311]. Chen et al., [312] reported that several species of bacteria have ability to solublize phosphate by secreting organic acids. Also, genera Citrobacter, Proteus , Klebsiella [313] and Bacillus [314] are able to produce significant level of acid phosphatases to mineralize organic phosphorous.

12.4.1.3

Potassium Solubilization

For plant nutrient, several important elements are play important role in limiting plant growth and development such as nitrogen, phosphorus and potassium (K). Potassium is required for increasing plant defense against pathogen and pests. K is important to resist abiotic stresses; activate different enzymes involved in energy metabolism, photosynthesis, starch synthesis, sugar degradation and nitrate reduction [315–320]. K is considering the seventh abundant element in Earth's crust [321]. Although, soil content of potassium is abundant about 0.04% and 3%, potassium availability to plant is 1% to 2% [322]. Potassium present in unusable form (90–98%) to plant due to its bounding with other minerals in soil [322]. Decrease of potassium content in the soil resulting in a negative effect on most crop plants such as cotton [323], sorghum [324], and ryegrass [325]. As a result of bad effects of K-fertilizers on the environment and increasing the price of K-fertilizers every year [326], it is needful to search about an alternative source of K to maintain K level in soils. A wide range of microbial soil communities is able to increase soil fertility by mineralization, decomposition, and solubilization insoluble elements such as K to soluble forms [327, 328]. Solubilization insoluble K can be achieved by different mechanisms including production of acids (inorganic and organic), chelation, and exchange reactions

364 Integrating Green Chemistry and Sustainable Engineering [328–331]. The major mechanism for solubilization rock K is organic acid production resulting in acidification of surrounded soil environment and makes K in useable form [332]. In addition to K solubilization, rhizospheric bacteria can promote plant growth by ACC deaminase production which increase nodulation in mung bean [333] and enhance potassium uptake in mung bean under salinity stress [334]. Liberation of potassium from insoluble sources depends on the amount and type of clay minerals presence in the parent material, oxygen and pH [335]. The efficiency of microbial strains depends on the nature of minerals and environmental factors [329]. Agrobacterium tumefaciens, Agrobacterium tumefaciens, Acidothiobacillus ferrooxidans, Burkholderia cepacia, Bacillus edaphicus, Bacillus mucilaginosus, Enterobacter aerogenes, Microbacterium foliorum, Myroides odoratimimus Pantoea agglomerans, Paenibacillus sp. and Pseudomonas have been documented as potassium solubilizing rhizobacteria [336]. K solubilizing bacteria can use in agriculture purpose as soil inoculums to enhance plant growth and crop yield [331] to avoid soil contamination by synthesized K-fertilizers.

12.4.1.4 Iron Chelation (Siderophores Production) Iron is an essential element for different metabolic process of living organisms. It is involved in the electron and oxygen transfer and the deactivation of radical oxygen [337]. Iron is arranged among element as the fourth element available in ground rock. Unfortunately, abundant amount of iron that present in the trivalent form (Fe 3+) is not useable by most bacteria, plants [338]. To release iron in accessible form to plant, rhizobacteria have the capability to provide iron by various uptake strategies such as production of siderophores. Siderophores are low molecular weight of small peptide molecules produced by bacteria which having the ability to ligands (chelate) with ferric ion in the soil [339–341], after reduction into ferrous ions is taken up. The siderophores can also chelate with other metals present in soil such as zinc and lead [342]. Siderophores can be produced by some plants and a wide range of bacterial and fungal genera [343]. Therefore, these genera such as Agrobacterium, Bacillus, Escherichia, Pseudomonas, Rhizobium and many fungi [244] can be used as biofertilizer to regulate iron uptake by plants [344]. Extracellular production of iron binding compounds is depending on bacterial species. Hider and Kong [341] determined 270 chemical structures of siderophores. Several studies have been published showing the positive effects of bacterial siderophores on stimulating

Biological Nitrogen Fixation and Biofertilizers 365 plant growth [345–347]. Grobelak and Hiller [348] revealed that the produced siderophores by bacteria decrease the negative effect of heavy metals, improved the phytoremediation and increased plant growth. P. fluorescens C7 liberates the ironpyoverdin siderophores which increased iron uptake and enhance the growth of Arabidopsis thaliana plants [349]. Mahmoud and abd- Alla [350] found that production of siderophores by Pseudomonas aeruginosa and Penicillium chrysogenum have positive effect in the nodulation and N2 fixation of mung bean plant inoculated with Bradyrhizobium.

12.4.1.5 Phytohormone Production Production of phytohormones by rhizobacteria meaning that synthetic chemical substances similar to natural plant hormones produced by rhizobacteria. Phytohormones is considered as a chemical messenger with low concentration influence on plant growth, development, cell proliferation and physiological processes [351]. Phytohormones are plant growth regulators can facilitate or prevent the plant growth produced by PGPR [352]. A broad range of microbes in the rhizosphere has ability to produce plant growth promoting hormones [353]. 12.4.1.5.1 Indole Acetic Acid Indole acetic acid can increase cell division and extension of root and shoot; arrange the responses to light, gravity and florescence; affects photosynthesis, formation of pigment, helps in biosynthesis of different metabolites, and increases the resistance to harsh conditions [354]. IAA is a phytohormone can induce root elongation when present in low concentration, while high concentrations of IAA levels induce lateral roots formation and reduce primary root length. Therefore, absorption area of the root increased and plant nutrients are more available [264]. Plant growth promotion by the free living PGPR has been documented in several studies [355–357]. Purified IAA by a silica gel column which produced by Pseudomonas putida UB1 show a positive effect on mustard (Brassica nigra) plants in vitro [358]. Malik and Sindhu [359] demonstrated that nodule number and nodule biomass increased by Mesorhizobium sp when chickpea co-inoculated with IAA-producing Pseudomonas strains. Bacillus amyloliquefaciens S-134, IAA producing bacteria stimulate wheat growth under drought stress [360]. The IAA producing bacteria can used as efficient biofertilizer inoculants to enhance plant growth.

366 Integrating Green Chemistry and Sustainable Engineering 12.4.1.5.2 Gibberellins Gibberellins are plant growth hormones controlled in plant growth, development, seed germination, stem elongation, flowering, fruit setting [361, 362]. Microbial production gibberellins as secondary metabolites improve soil fertility [363]. Gibberellins production by Pseudomonas sp. were detected in several published data [364–369]. Gibberellins production were detected by Acetobacter diazotrophicus and Herbaspirillum seropedicae [370], Azospirillum lipoferum [371], Azospirillum brasilense [372], Azotobacter beijerinckii [373], Bacillus pumilis and Bacillus licheniformis [374], Burkholderia sp [366] and Rhizobium phaseoli [375]. 12.4.1.5.3 Cytokinins Cytokinins are a plant growth regulator and effect on different physiological processes of plants such as cell division, accumulation of chlorophyll, leaf expansion, seed germination, delay of senescence, stimulate vascular differentiation and induce the proliferation of root hairs, but inhibit lateral root formation [376, 377]. Some rhizospheric microorganisms have the ability to produce cytokinins which influence plant growth and development [378]. Cytokinins influence endosperm cells and grain weight [379, 380]. Cytokinins increase the host defense to some viral infection [381]. Phytpathogenic bacteria were detected to produce high concentration of cytokinins [382] while PGPR produce cytokinins in low concentration such as Azotobacter, Rhizobium, Pseudomonas spp [378, 383, 384]. High concentrations of cytokinins have an inhibitory effect on plant [385]. Different bacterial genera including Bacillus, Klebsiella, Proteus, Escherichia and Pseudomonas were reported as producer of cytokinins [386, 387]. The impact of cytokinins produced by B. licheniformis Am2, B. subtilis BC1, and P. aeruginosa E2 cultures under green light on growth and cell division, chlorophyll contents and fresh weight in cucumber plants have been reported [388].

12.4.2

Indirect Mechanisms of PGPR

Phytopathogen microorganisms reduce crop yield resulting in cause a great loss of billions of dollars each year. In addition of decrease of crop quantity, the quality of crop product also decreased and containing toxic chemicals compound [389]. Preventing of plant pathogen damage using the pesticides is expensive and cause environmental pollution. A great offers to reduce negative effect of pesticides using PGPR as biocontrol agents [390, 391]. PGPR can prevent or decrease plant pathogen by several indirect mechanisms by producing antibiotics; hydrolytic enzymes degrade the cell wall of pathogen (fungi, insect and nematodes), ACC deaminase production

Biological Nitrogen Fixation and Biofertilizers 367 [392], competition either for nutrients or for binding sites on the plant root [393, 394], competition for iron through siderophores production, hydrogen cyanide (preventing the proliferation of most fungal phytopathogens) [266, 395–397], induced systemic resistance and quorum quenching [398, 399]. Environmental stresses such as salinity, temperature, drought, heavy metal toxicity and others are a limiting factor for plant growth and agricultural productivity. Plant growth-promoting rhizobacteria have the ability to enhance plant growth and increase crop yield; and promote plant tolerance to different drastic conditions. Different mechanisms can use by PGPRs to induce plant resistant including exopolysaccharide production [400, 401], production of osmolytes for osmotic adjustments [402, 403] induction of stress-responsive genes and modulation of physiological responses [404]. There is a wide range of PGPR, possess different activities in various environmental stresses and soil conditions. Actually, single microorganism has not the ability to make use of all the available mechanisms, which promote plant growth and resist harsh environmental stresses and plant diseases [405]. In addition, several PGPR inoculants can be applied by different agriculture strategies as potential tools to facilitate plant growth and increased crop productivity.

12.5

Conclusions and Future Research

Global demand of agriculture products is in decrease as a result of harsh environmental conditions and other factors. Applying the chemical fertilizer increases the crop productivity, but it’s expensive, energy consumed and contaminate the environment (soil, ground water and plant product) by heavy metals and other toxic chemical compound. The use of different microorganisms as an alternative solution for increasing crop productivity is the best choice. Several studies have been published to use PGPRs as a biofertilizers. Wide ranges of microorganisms have different action on plant growth through increasing the nutrients availability (N, P, K and Fe) and facilitate their uptake by plants. Plant growth promoting rhizobacteria can also protect plants from plant pathogen and make plant able to survive in different harsh conditions. Owing appropriate microorganisms (symbiotic or free living) is economical and safe to environment for increasing agriculture yield. Better crop yield production can be achieved by selecting resistant microorganism to harsh conditions of soil subjected to environmental stress.

368 Integrating Green Chemistry and Sustainable Engineering

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13 Natural Products in Adsorption Technology Ahmet Gürses Ataturk University, K.K. Education Faculty, Department of Chemistry Education, Erzurum, Turkey

Abstract Adsorption that is one of the most important physicochemical processes occurring at the solid-liquid and solid-gas interfaces is known as a fast, inexpensive and universal method among various water treatment and recycling technologies The development of low-cost adsorbents has led to the rapid growth of research interests in this field. The adsorbents used for adsorption are mostly of natural origin, but they can be in waste or by-products from an industrial or agricultural production, or in processed products subjected to a certain activation process. Charcoal, clay, clay minerals, natural zeolites, ores, mineral oxides and biopolymers are the most common natural adsorbents whose common characteristics are relatively inexpensive and are abundant in supply and also that the adsorption capacities can be increased in high ratios by physical and chemical processes. On the other hand, some materials predominantly derived from bio-waste and agricultural byproducts are also called green adsorbents based on their advantages such as sustainability, low cost and minimal waste generation. Most of the natural adsorbents are commonly used as processed or engineered products, such as carbonaceous adsorbents, polymeric adsorbents, oxide adsorbents, and zeolite molecular sieves. Active carbons produced from carbonaceous material by chemical or physical activation are the most commonly used adsorbents. Each adsorbent has its own characteristics such as porosity, pore structure and nature of its adsorbing surfaces. In general, engineered adsorbents exhibit the highest adsorption capacities. This chapter will cover the basic physicochemical principles of the adsorption process and the types and specific applications of the natural and processed adsorbents used in the adsorption technology.

Corresponding author: [email protected] Shahid-ul-Islam (ed.) Integrating Green Chemistry and Sustainable Engineering, (397–442) © 2019 Scrivener Publishing LLC

397

398 Integrating Green Chemistry and Sustainable Engineering Keywords: Natural adsorbents, absorption, desorption, activation, electrical double layer, charcoal, clay minerals, zeolites

13.1

Introduction

In recent years, sustainability and green engineering approaches have gained increasing popularity, and so adsorption processes and technologies have begun to be reassessed in this context. Moreover, more economical, cleaner, or greener technological raw materials that produce minimal waste when used in the adsorption process have started to be called green adsorbents. The green adsorbents are low-cost biomaterials from agricultural residues and wastes, animal bone wastes, or complex adsorbents. However, some of the green adsorbents are not as effective as the conventional adsorbents in practice [1–4]. Various adsorbents such as alumina, bauxite, silica gel, bone char, silicates, fuller's earth, magnesia, activated charcoal were used commonly for gas and vapors drying, refining of sugar, refining of some oils, fats and waxes, recycling of solvents and the deodorization and purification of air and industrial wastes, as well as water treatment and the recovery of certain precious metals [5]. Since the beginnings of the twentieth century, natural clays have been used to refine oils and fats in the oil industry [6]. Clay minerals were also widely used to remove grease from woolen materials. Also, natural layered clays such as kaolin and bentonite were also used for bleaching oils and petroleum spirits [7]. Commonly, the decolorizating of oil and petroleum products was carried out by percolating the oil through a granular clay bed and by agitating it directly in contact with the clay. The refined bauxite which consists of hydrated aluminium oxide was widely used as drying agent and for decolorization of residual oil stocks. Fluoride mineral, an aluminium oxide compound that can absorb water quickly but does not swell in water, is generally used to dry gasses and organic liquids [8]. On the other hand, nowadays, carbonaceous adsorbents produced by physical and/or chemical activation of a wide variety of plantal precursors as well as agricultural and industrial wastes and sewage wastes are being used more effectively in the technological applications of adsorption (i.e., color and odor removal and treatment of wastewater and water resources) as engineering or processed adsorbents [9, 10]. The decolorization of liquids, including the refining of sugar melts, was accomplished by mixing the carboneous adsorbents with the liquid to be bleached and then filtering. In water treatment, the adsorption process was carried out either

Natural Products in Adsorption Technology 399 by passing of water through the beds of carbonaceous adsorbent or adding carbonaceous material to the water in mixing tanks [11]. Afterwards, the effluent was treated with chlorine for disinfection. Alternatively, the contaminated water is first treated with excess chlorine and then passed through a carbonaceous bed. The carbonaceous adsorbents used are usually regenerated or re-activated by suitable techniques and reused [12]. Carbonaceous materials activated by hot air, carbon dioxide and steam were used even in the 1930 s for air purification and solvent recycling [13]. These first applications of air cleaning and solvent recovery can be regarded as the first examples of systems with multi-bed as well as cyclic operations, which can allow for regeneration of carbonaceous adsorbents unlike the bed systems that are operated as adsorber. During the same period, the first industrial application of the process using silica gel as the adsorbent for the dehumidification of the blast furnace gases in steel production was also realized [14]. However, later on, thermal swing adsorption (TSA) processes have gradually become dominant for a wide range of applications [15, 16]. In the pressure swing process (PSA) developed differently from the thermal swing processes (TSA), the adsorbent can be regenerated following the adsorption of the most strongly adsorbed component of a gaseous or liquid mixture [17]. While the increase in the temperature of the adsorbent bed is the driving force for desorption in TSA processes, the decrease in total pressure allows desorption in PSA processes [18]. Natural and synthetic alumina-silica minerals, also called zeolites, are considered to be excellent crystalline microporous structures in terms of their separation properties [19, 20]. Oxides and zeolites, which have strong hydrophilic surface properties, are preferred particularly for the removal of polar or ionic compounds. On the other hand, the excelent adsorptive properties of microporous carbons led to the development of molecular sieve carbons, which are less hydrophilic than zeolites, and can therefore effectively used for purification of wet gaseous streams [21, 22]. The most prominent difference between the two techniques in practice is that the PSA technique needs a much shorter cycle time than the TSA technique [23]. TSA processes are suitable for an adsorptive operation where the interactions between adsorbent and adsorbate are strong and desorption is more sensitive to temperature [24]. However, PSA processes are generally preferred when the interactions between the adsorbent and the adsorbate are relatively weak and a higher purity separation is required. Although both processes are carried out by bed use and continuous product flow, they are not actually considered as a continuous process, such as a moving bed process [25].

400 Integrating Green Chemistry and Sustainable Engineering Although adsorption-based chromatographic processes are predominantly used for the analysis of gas and liquid mixtures on a laboratory scale, processes for preparing batches of pharmaceutical products in the pharmaceutical industry today are predominantly based on the principles of chromatography. In addition, a wide variety of materials such as fruit wastes, coconut shells, scrap tires, bark, tannin-rich materials, sawdust, rice husks, petroleum wastes, fertilizer wastes, fly ash, sugar industry wastes and blast furnace slag as well as chitosan, seaweed, algae and peat moss are widely used in industrial and laboratory scales as adsorbents. This chapter will cover the basic physicochemical principles and technological applications of the adsorption process and the types, characteristics and specific applications of the natural and processed adsorbents used in the adsorption technology.

13.2

Adsorption and Surface Chemistry

Adsorption is a commonly used interfacial process for removing various components from fluid phases such as gas or liquid, and for modifying the interface. Adsorption is defined as an increase in the amount of a particular component of a fluid phase such as gas or liquid at an interface where the solid phase is involved, or as a relative increase in the amount of fluid at the interface where the fluid phase and another phase coexist [26]. The energetic and morphologically heterogeneous surfaces of the solid phase (adsorbent) are characterized by active sites that interact with the atoms, molecules and ions (adsorbate) present in the adjacent phase [27]. The process involving many phenomenes that can alter the distribution of the adsorbate among the phases in contact or in the interface occurs due to adsorbate accumulation at the interface parallel to the transport of the adsorbate from one phase to another [28]. On the other hand, by changing some parameters such as pressure or concentration, temperature and pH, It is possible to transfer the components adsorbed at the interface from there to the fluid phase, and such a process is called desorption. Considering the nature of interactions between the adsorbate and the adsorbent, as well as the values of adsorption enthalpy, the adsorption can be categorized as physical adsorption and chemical adsorption [29, 30]. Generally, physical adsorption or physisorption occurs through weak interactions such as van der Waals, London dispersion and hydrogen bonding and through mechanisms such as ion exchange, ion pairing, hydrophobic bonding and polarization of π-electrons, and the adsorption

Natural Products in Adsorption Technology 401 enthalpy is around 10–20 kJ / mol [31]. In contrast, chemical adsorption or chemisorption occurs through interactions similar to chemical reactions between the adsorbate and the adsorbent, and therefore the enthalpies are almost in the magnitude of the reaction enthalpies (>50–100 kJ/mol) [32]. However, it is assumed that physical adsorption occurs as a preliminary step in chemical adsorption. Generally, another process that occurs in parallel with the adsorption process, but with a different interaction mechanism, is absorption, which is often considered to be a process similar to adsorption [33]. Moreover, the absorption takes place by transferring a substance from a bulk phase to another phase, and the substance is enriched not only in the interface, but also in the entire transferred phase. The uptake of organic materials by soil and sediments, and the fact that many different gases completely fill the pores of microporous materials are typical examples of absorption [34]. In most cases, it is difficult to differentiate between adsorption and absorption and therefore term "sorption" for many systems is commonly used to describe the transport of a certain substance between liquid and solid phases. Thus, it can be argued that the term “sorption” covers both adsorption and absorption [35]. Sorption is a process that is effective in the majority of physical, biological and chemical systems. Many solids, when brought into contact with a polar medium (i.e., aqueous), can acquire a surface electric charge with mechanisms such as ionization, ion adsorption and ion dissolution or isomorphic substitution [36]. The surface charge affects the distribution of ions near the surface of the solid in the polar medium; so that similarly charged ions (co-ions) travel away from the surface when oppositely charged ions (counter ions) are attracted toward the surface [37]. This distribution, together with the mixing tendency of the thermal motion, leads to the formation of an electrical double layer consisting of the charged surface and the diffused layer of ions [38]. The electric double layer can be regarded as consisting of two regions, an inner region, which may include adsorbed ions, and a diffuse region, in which ions are distributed according to the influence of electrical forces and random thermal motion [39]. The requirement of overall electro neutrality is required that for any dividing surface, if the charge per unit area is +σ on one side of the surface, it must be –σ on the other side. It follows, therefore, that the magnitude of σ will depend on the location of the surface [40]. The surface position is generally not easily identifiable due to the geometric and chemical heterogeneity present and the ambiguity of the electrical double layer parameters (potentials, surface charge densities and distances) [41]. The surface potential ψo, therefore, depends on both the

402 Integrating Green Chemistry and Sustainable Engineering surface charge density σo and on the ionic composition of the medium. If the double layer is compressed, then either σo must increase or ψo must decrease or both. The specifically adsorbed ions are strongly attached to the surface by electrostatically and / or van der Waals forces, so that they can overcome the thermal motion. They may be dehydrated, at least in the direction of the surface. The centers of any specifically adsorbed ions are located in the Stern layer, which is between the surface and the Stern plane. Ions located beyond the Stern plane form the diffuse part of the double layer with ψo replaced by ψd [42]. The potential changes from ψo (the surface or wall potential) to ψd (the Stern potential) in the Stern layer and decays from ψd to zero in the diffuse double layer. In the absence of specific ion adsorption, the charge densities at the surface and at the Stern plane are equal. When specific adsorption takes place, counter ion adsorption usually predominates over co-ion adsorption [43]. In particular, it is possible to reverse the surface charge signs by the adsorption of multivalent or surface active ions in the Stern layer (having opposite signs of ψo and ψd). Adsorption of surface-active counter-ions or multivalent ions could create a situation in which ψd has the same sign as ψo and is greater in magnitude. Overall electrical neutrality throughout the whole of the double layer can be expressed as follows (Eq. 13.1): σo + σ1 + σ2 = 0

(13.1)

where σo, σ1, and σ2 are the charge density of surface, the charge density of the Stern layer, and the charge density of the diffuse part of the double layer, respectively [44]. Ψd can be estimated from electro kinetic measurements. Electro kinetic behavior depends on the potential at the surface of shear between the charged surface and the electrolyte solution. This potential is called the electro kinetic or ζ (zeta) potential [38, 45, 46]. The exact location of the shear plane (which, in reality, is a region of rapidly changing viscosity) is another unknown feature of the electric double layer. In addition to ions in the Stern layer, a certain amount of solvent will probably be bound to the charged surface and form a part of the electro kinetic unit [47]. It is, therefore, reasonable to suppose that the shear plane is usually located at a small distance further out from the surface than the Stern plane and that ζ is, in general, marginally smaller in magnitude than ψd [38]. Some solid materials used as adsorbents gain a certain charge by the ionization of carboxyl and amino groups, which can essentially form COOand NH+ ions. The net charge owing to the ionization of these groups depends strongly on the pH of the solution. At low pH, such a molecule

Natural Products in Adsorption Technology 403 will be positively charged and negatively charged at high pH [48]. The pH at which the net charge (and electrophoretic mobility) is zero is called the isoelectric point. A net surface charge can be acquired by the unequal adsorption of oppositely charged ions [49]. Ion adsorption may involve positive or negative surface excess concentrations. Generally, surfaces that come into contact with aqueous media are negatively charged rather than positively charged. This is a consequence of the fact that cations are usually more hydrated than anions and so have a greater tendency to reside in the bulk aqueous medium, whereas the smaller, less hydrated, and more polarizing anions have a greater tendency to be specifically adsorbed [50]. The currently charged surfaces (i.e., by ionisation or isomorphic substitution) generally exhibit a preferential tendency for the adsorption of counterions, especially those with a high charge number. It is possible for counter ion adsorption to cause a reversal of charge [51]. Ionic solids, minerals and oxides can gain a certain surface charge due to the unequal dissolution of oppositely charged ions and the isomorphic substitution of similarly charged ions in the mineral or oxide lattices. In a similar way, hydrogen and hydroxyl ions for hydrous metal oxide sols are potentialdetermining ions, since their concentrations determine the electric potential at the particle surface. The surface chemistry of the adsorbent can directly affect the interactions between the adsorbate and the adsorbent, depending on their nature. This is particularly evident in the adsorption of ionic species by oxidic adsorbents, but can also be observed in some carbonaceous materials such as activated carbon. Oxidic adsorbents such as aluminum oxide or ferric hydroxide are characterized by crystalline structures where positively charged ions and negatively charged oxygen or hydroxide ions are arranged in such a manner that the different charges compensate each other [52]. In aqueous dispersions, negatively charged oxy groups are neutralized by protons, while positively charged sites are neutralized by hydroxide ions. For this reason, the surface of oxidic or oxidized adsorbents can be covered with OH groups. These groups may be exposed to protonation or deprotonation depending on the pH of the medium [53]. The surface is positively charged at low pH’s and negatively charged at high pH’s. On the other hand, there is a pH value at which the sum of the negative charges is equal to the sum of the positive charges and the net charge at the surface is zero, and it is called zero charge point (PZC) [54]. The pH corresponding to PZC is a determining parameter for the adsorption of charged species and is therefore considered an important parameter for adsorbents. In general, the adsorption of charged species to charged surfaces is expected to be strongly influenced by electrostatic

404 Integrating Green Chemistry and Sustainable Engineering attraction or repulsion forces. It is well known that oxygen-containing functional groups exist at the surface of carbonaceous adsorbents. These groups, also referred to as surface oxides, show acidic or basic character [55, 56]. Typical functional groups are carboxyl groups, carboxylic anhydrides, lactone and lactol groups as well as phenolic hydroxyl groups. At activated carbon surfaces, the number of sites that can be protonated and deprotonated is about one to two orders of magnitude lower than that at oxidic surfaces. For this reason, it can be said that these groups are less suitable for ionic adsorbates and thus their adsorptions on activated carbon might be extremely low [34]. On the other hand, the physical adsorption of organic ions, mainly based on their dispersion forces, can be disturbed by the attractive or repulsive interactions between acidic or basic surface groups and the adsorbate, and thus may be reduced compared to adsorption of neutral species.

13.3

Characteristics of Adsorbents and Selection of Adsorbent

For an effective bulk separation or purification, an adsorbent material must possess some basic characteristics, such as large pore volume, wide surface area, high porosity and suitable pore structure and distribution, as well as having intensive and diverse active surface sites [57]. The selectivity of an adsorbent can be defined by its ability to separate a particular component from a two- or three-phase mixture [8]. Since the adsorbents are porous solids, the different densities such as material density, apparent density (particle density) and bulk (bed) density can be considered, depending on the volume. The material density (skeletal density), which is the true density of the solid material, is defined as the ratio of the mass of the adsorbent to the volume of the solid material without the pores. The particle density, also known as the apparent density, is defined as the ratio of the mass of the adsorbent to the volume of the adsorbent, including the pores [58]. Bulk density, an important parameter that can be used to determine the appropriate mass-to-volume ratio for adsorbent systems, is defined as the ratio of mass of adsorbent to total reactor volume filled with liquid and solid [59]. The total reactor volume includes the volume of the adsorbent and the volume of liquid that fills the voids between the adsorbent particles. The bulk density has the character of the mass concentration because in batch reactors, the small quantities of adsorbent are usually

Natural Products in Adsorption Technology 405 dispersed in large quantities of the liquid. In fixed bed adsorbent systems, the adsorbent particles are located in an adsorbent bed and therefore the void volume is much lower when compared to that in the batch reactors, such that in fixed bed systems, the term bed density is often used instead of bulk density. Generally, the porosity specifies the fraction of void space on the total volume [60]. Depending on the total volume considered, a distinction can be made between particle porosity and bulk (bed) porosity. Both porosities can be derived from the densities. The particle porosity (also referred to as internal porosity) gives the void volume fraction of the adsorbent particle [61]. It is therefore defined as the ratio of the pore volume and the volume of the adsorbent particle. Pore sizes are generally classified in three ranges, based on their diameters, as micropores (smaller than 2 nm), mesopores (between 2 and 50 nm), and macropores (greater than 50 nm). The bulk porosity (external void fraction), which can also be used to express different volume-volume or solid-volume ratios, which is characteristic for the conditions given in an adsorber system, is defined as the ratio of the fluid filled void volume between the adsorbent particles to the rector volume. Generally, the term bed porosity is used instead of bulk porosity in the fixed-bed adsorption sytems. The external surface area has a significant effect on the mass transfer rate in the adsorption process [62]. For this reason, the difference between the external and internal mass transfers must be taken into consideration for the porous adsorbents. The external mass transfer is the mass transfer through the hydrodynamic boundary layer around the adsorbent particle [63]. As the boundary layer is very thin, the area available for external mass transfer can be approximatelly estimated with the external adsorbent surface area, while the internal mass transfer takes place through intraparticle diffusion processes [64]. Porous adsorbents generally have much larger internal surface areas than external surface areas. Particularly, in processed or engineered adsorbents having very large internal surface areas, the internal surface area is responsible for almost all the adsorption capacity, and therefore, it can be said that the internal surface area is a very important quality parameter for the adsorbents [65, 66]. However, it is clear that the internal surface area alone would not be sufficient to predict the adsorptive capacity of the adsorbent, since the efficiency and effectiveness of adsorption depends on a number of properties associated with both the adsorbent and the adsorbate. The most commonly used method for determining the internal surface area is based on the application of the Brunauer-Emmett-Teller (BET)

406 Integrating Green Chemistry and Sustainable Engineering adsorption model to gas adsorption at low temperatures (typically nitrogen adsorption at 77 K) [67, 68]. Mercury porosimetry is also a common technique used to determine pore size distribution. In this technique, initially all gases are evacuated from the adsorbent, followed by application of high pressure to allow the mercury to enter the pores, and then the pore size distribution is determined using the pressure-volume curves obtained [69]. Most processed adsorbents, which can have a very large internal surface area, exhibit a high porosity due to the large number of pores of different shapes and sizes. Macro pores and mesopores are predominantly responsible for mass transfer to the micropores of the adsorbent particles. In contrast, microporosity predominantly determines the size of the total surface area and hence the adsorbent capacity. In principle, it can be said that as the micropore volume increases, the adsorption capacity of an adsorbent increases. However, in the case of very fine pores and large adsorbate molecules (for example, high molecular weight natural organic substances), depending on size exclusion, a limitation in efficiency and effectiveness of adsorption may occur [70, 71]. Engineered adsorbents are typically highly porous materials with surface areas in the range between 102.0 and 103.0 m2 /g and their external surface areas are typically below 1.0 m2/g. As a result, the contribution of the external surface area to the total surface area is much lower than the internal surface area. For example, the external surface area of powdered activated carbon with a particle density of 0.6 g/cm3 and a particle radius of 0.02 mm is only 0.25 m2/ g. The high internal surface area of an adsorbent is absolutely necessary for an effective separation or purification process. The total surface area of the adsorbents ranges from about 100.0 m2/g to 3000.0 m2/g, although practical values are generally limited to about 300.0–1200.0 m2/ g. In most applications, since the adsorbents are regenerated after use, the adsorbents should also be selected from natural or synthetic materials which may have good mechanical properties and good kinetic properties so that mechanical and adsorptive properties are not impaired during these processes [72]. Many adsorbent materials, such as carbons, silica gels and aluminas, are amorphous and contain complex networks of interconnected micropores, mesopores and macropores. Differently, the size of the pores or channels in the zeolitic adsorbents is distributed regularly and precisely, but with the addition of a binder, a macroporous structure can form in the pellets produced from the zeolite crystals. Apart from some specific applications, there is a wide range of natural, synthetic and processed adsorbent materials used for purification and separation applications [73]. It can be said that each adsorbent material has its own specific porosity, pore size and distribution and certain surface functional

Natural Products in Adsorption Technology 407 groups. Each or all of these properties have an active role in the separation process. For a separation process, the characteristics related to equilibrium, kinetics, molecular sieving and desorption behavior have significant effects on process performance. Equilibrium effect refers to possible differences in the thermodynamic equilibrium in terms of adsorbate-adsorbent interactions. Differences in the diffusion rates of the different adsorbates to the pores of the adsorbent can be evaluated as kinetic effect [74]. In particular, the pore sizes of the microporous adsorbents may not be sufficiently large to allow the adsorbate molecules to penetrate into the pores of the adsorbent, and this limitation may appear as the molecular sieving effect in conjunction with the kinetic factor. Depending on the pore shape and volume, probable differences in desorption behavior of different adsorbates from the same adsorbent may also be considered as the desorption effect in terms of separation performance [8]. The equilibrium separation depends on the nature of the adsorbent (polar, nonpolar, hydrophilic, hydrophobic etc.) and on the processing conditions such as temperature, pressure and concentration. Kineticbased separation is only possible with molecular sieve adsorbents such as zeolites and carbon sieves, and the separation performance is greatly controlled by the ratio of micro pore diffusivities of the components being separated [16]. For a successful kinetic-based separation, the size of the micropores of the adsorbent should be comparable to the dimensions of the diffused adsorbate molecules [26]. In separation or purification processes, equilibrium adsorption isotherms are practically extremely important, and therefore experimental equilibrium adsorption data obtained for specific temperature and pressure ranges are needed for process design and optimization as well as the selection of the appropriate adsorbent [75]. Even if the appropriate adsorbent can be selected based on the equilibrium data, kinetic data may also need to be taken into account to ensure that the adsorbate uptake rate is adequate and that sufficient purity can be achieved. In addition, the strength of the adsorbent, its chemical resistance and activation performance, as well as its availability and cost, are other factors that have a technologically critical importance in the selection of the adsorbent.

13.4

Common Processes in Adsorption Technology

In the technological applications of adsorption, various processes such as batch (slurry) process, fixed bed process, moving bed process, fluidized

408 Integrating Green Chemistry and Sustainable Engineering

Main processes used in adsorption technology

Batch (slurry) process

Fixed bed process

Moving bed process

Fluidized bed process

Rotary bed process

Figure 13.1 Schematic presentation of the main technological processes used in adsorption applications.

bed process and rotary bed process, and various types of reactors or adsorbers are widely used (Figure  13.1) [76]. Technological options are largely dependent on the particle size of the adsorbent used. For example, batch reactors for powdered adsorbents and fixed bed reactors for granular adsorbents are suitable [77].Kinetically, powder adsorbents such as powdered activated carbon have some advantages, such as higher adsorption rate and thus shorter time to reach equilibrium. However, due to the large increase in flow resistance as parallel to the small particle size, the powdered adsorbents are not suitable for fixed bed reactors [78]. For this reason, batch reactors for powdered adsorbents are much more convenient, but the process requires an additional step to remove the adsorbent after the process. This leads to a significant increase in the cost of the process compared to fixed bed processes. Granular adsorbents such as granular activated carbon are generally used in fixed bed adsorber systems where the adsorbent is fixed to the reactor, and therefore no additional separation step is required. Compared to the batch processes, adsorbent consumption is lower, but the adsorption process is slower due to larger particle size [79]. In batch processes, the adsorbent moves relative to the walls of the reactor and an adsorbent batch is mixed with a liquid batch, usually water [80]. After a sufficient time to reach the adsorption equilibrium, the adsorbent and solution are separated using processes such as precipitation, filtration, decantation or centrifugation [81]. When large quantities of adsorbent are used, a multi-batch or cross-flow system may be required where the adsorbate is first contacted with a fresh adsorbent and the adsorbate is separated from the adsorbent and then re-contacted with a fresh adsorbent

Natural Products in Adsorption Technology 409 batch [82, 83]. The slurry adsorber systems can be run as discontinuous batch adsorber system, but in practice the continuous process is more feasible. For example, in water treatment processes, adsorption in the slurry adsorber is mostly performed as a one-stage process, but a two-stage process is often preferred to reduce adsorbent consumption. In fixed bed processes, separation in a fixed adsorbent bed is considered as an unsteady rate controlled process, in which in most cases the conditions at any point in the fixed bed change with time [84, 85]. Adsorption therefore occurs only in a particular region of the bed, known as the mass transfer region, which moves through the bed with time. The column configurations of the process are usually designed as a multi-stage batch adsorption process. The uppermost regions of the bed are continuously contacted with the fresh adsorbate solution and the lower regions of the bed are brought into contact with the adsorbate solution which is not adsorbed on the upper part. Thus, the bed is first loaded with the upper part of the column and then downwards, resulting in the formation of an adsorption region moving downwards in the column. The following are some of the disadvantages of the process: While the fluid is passed through a fixed bed of adsorbent, the mass transfer of the adsorbate molecules occurs initially at the bed entrance. When the adsorbent in this region becomes saturated with the adsorbate molecules, the region where the mass transfer takes place moves through the bed to the exit. When the adsorbate is removed, the bed should be taken offline so that the adsorbent can be regenerated [8, 86]. Because of the need to disable the adsorbent bed at any time while the process is running, multiple adsorbent beds are needed which can provide a continuous stream of product in the adsorber. Since adsorption usually has an exothermic nature, the desorption tendency can be increased by increasing the temperature of the adsorbent. Heating and cooling of large beds of highly porous adsorbents are relatively difficult in thermal swing processes. Due to the low heat transfer, both very long heating and cooling times and large beds are required. As the temperature of the adsorbate increases, the adsorbing efficiency of the adsorbate decreases [82]. Although the design of the fixed bed processes may seem somewhat simple, the presence of the progressive mass

410 Integrating Green Chemistry and Sustainable Engineering transfer region can make the design relatively complicated [83]. In moving bed processes, two alternatives are generally considered for the adsorbent movement. In the first, the adsorbent particles move relative to the walls of the reactor so that adsorption, regeneration and backwash can occur simultaneously, but at different locations. In the second, the particles remain in a fixed position relative to the reactor walls and during the process, the effective contact of the liquid with the adsorbent continues throughout the running. Moving bed processes have the advantage of not requiring the use of multiple adsorbers since the process is indeed continuous and provides a steady state of operation [84]. However, the cost of operation is higher than that of fixed bed systems, and adsorbent attrition and loss of adsorbent may occur due to the motion of the bed [87, 88]. In fluidized bed processes, in general, applications are directed to processes such as removal of organic compounds from air and vent streams and drying of air with silica gel [87–90]. Fluidized bed processes are not affected by changes in pressure and flow rate due to full fluidization and heat and mass transfer to the adsorbent particles is extremely easy [91]. However, similar to the moving bed adsorber systems, fluidity can reduce the mechanical strength of the adsorbent particles. It is known that the process is also suitable for the use of microspherical activated carbons, known as bead activated carbon activated carbons, as well as polymeric adsorbents in the removal of volatile organic compounds from air streams.

13.5

Adsorpbents Used in Adsorption Technology

The adsorbents used in adsorption technology can be divided into two main groups, essentially as natural and engineered or processed adsorbents. The precursors of engineered adsorbents can also be used as adsorbent in unprocessed form. The classification is schematically illustrated in Figure 13.2. Oxides are common and abundant natural substances as adsorbents. Naturally occurring iron oxide and manganese oxide have been used for the removal of heavy metals like Cr 5+, As 3+, As 5+, Ni2+, Co 2+and Mg 2+in ground water and waste water [93, 94]. Fe-Al oxides have been used for the adsorption of Cu2+, Co 2+, Pb2+ and Zn2+ [95] and Fe (III)/Cr (III) hydroxide for removal of Pb 2+, Ni 2+ and Cd 2+ [96]. Polymetallic sea nodules have been used for adsorption of As 3+and As 5+ [97, 98]. Iron oxide-coated sand (IOCS) was used for removal of both As3+ and As 5+ [99]. Hydrous

Natural Products in Adsorption Technology 411 Adsorbents

Natural adsorbents

Natural materials

Agricultural wastes/byproducts

Industrial wastes/byproducts

Engineered adsorbents

Oxidic adsorbents

Activated carbon

Polymeric adsorbents

Synthetic zeolites

Figure 13.2 Schematic representation of a practical classification of adsorbents used in adsorption technology.

titanium oxide has been exploited for treatment of industrial effluents contaminated with Cr 5+ [100]. In natural conditions, bed sediments of river water can be used to reduce metal concentrations of water by adsorption. Generally, the adsorptive qualities of sediments with particle size 75 μm. Usually clay and silt fractions adsorb metal ions much better than the coarser fractions of sediment. Therefore, in river systems with high sand content and low clay and silt content, the overall contribution of sand to the adsorption of metal ions could be higher than that of the clay and silt fractions [101]. Zeolites are intrinsically microporous aluminosilicates of the general formula [(AlO2)x (SiO2)y].mH2O and can be considered as open structures of silica in which aluminium has been substituted in a fraction x / (x + y) of the tetrahedral sites [102, 103]. Cavities or cages are contained within the framework of zeolites and are connected by pores. These pores are of molecular dimensions and the adsorbate molecules can infiltrate into these pores. They can be considered as derivatives of silicates where Si is partially substituted by Al. As a consequence of the different number of valence electrons of Si (4) and Al (3), the zeolite framework carries negative charges, which are compensated by metal cations. Depending on the molar SiO2/Al2O3 ratio (modulus n), different classes can be distinguished (i.e., the well-known types A (n = 1.5…2.5), X (n = 2.2…3.0), and Y (n = 3.0…6.0). Most zeolites with hydrophilic character are particularly suitable for ion exchange processes (i.e., softening) but are not suitable for adsorption of most organic materials. The hydrophobicity of the zeolites increases with the increase of the n modulus, for example, high silica containing zeolites with n > 10 are more hydrophobic and therefore more suitable adsorbents for organic materials [104]. The process of adsorption and desorption of molecules in zeolites are based on differences in molecular size, shape and other properties such

412 Integrating Green Chemistry and Sustainable Engineering as polarity. Removal of NOx and SOx from gasses, purification of silanes, drying of refrigerants and organic liquids and separation of solvent systems are typical applications of zeolites in adsorption technology. Different types of natural and modified zeolites are well known for their superior adsorptive properties towards metals. Natural zeolites for Zn (II), Cu (II) and Pb (II), natural and cysteamine hydrochloride and cystamine dihydrochloride-zeolite for Hg (II) and amine-modified zeolites for Pb (II) and Cd (II) are typical examples [105–107]. Zeolites occur in nature in high diversity, but for practical applications, mostly synthetic zeolites are used. Synthetic Zeolites can be produced from alkaline aqueous solutions of silicium and aluminum compounds under hydrothermal conditions. There are more than 150 types of synthetic zeolites, the most commercially important of which are types A and X (chabazite, faujasite and mordenite). Among the natural adsorbents, clay minerals have a special position. The application of natural clay minerals as adsorbents has been studied for a relatively long time. The superior adsorptive properties of clay minerals or mineral mixtures such as bentonite (main component montmorillonite) or Fuller's earth (attapulgite and montmorillonite varieties) are due to their structural negative charges [108]. The negative net charge makes them potential adsorbents for heavy metal cations such as Cu2+, Zn2+, and Cd2+ and other positively charged species (eg., cationic dyes in textile waste water). The capacity to adsorb organic materials of clay minerals, which are made organophilic by using organic modifiers increases [37]. Fly Ash which is a coal combustion residue of thermal power plants, is one of the cheapest adsorbents having excellent removal capabilities for heavy metals, fluoride and phenol and substituted phenols from wastewater. Adsorption capacity of fly ash has been found to increase with increase in temperature. The fly ash could be easily solidified after the heavy metals are adsorbed [109–111]. Bagasse Fly Ash, which is a waste material from sugar industry, has been used for removal of pesticides, lindane and Malathion [112] and heavy metals, Cu 2+, Zn 2+, Pb 2+ and Cr 6+ [113, 114]. The bottom ash from thermal power plants [115, 116], steel-plant granulated slag [117], oil-palm fibre and coconut husk [118], coconut- shell carbon [119], peanut hull carbon [120], waste Fe3+/Cr 3+ hydroxide [121] have also been tested for lead removal from wastewater. The bagasse pith has also been used in removing of Hg 2+ and dyes such as basic red 22 and acid red 114 [122, 123]. Blast furnace sludge, which is a by-product of steel industry, was tested for the adsorption of some heavy metals such as, Cu 2+, Ni 2+, Zn2+, Pb 2+, Cd 2+ and Cr 3+ [124].

Natural Products in Adsorption Technology 413 Red mud, which is a waste material from the aluminium industries has been used for the adsorption of Cu 2+, Zn 2+, Ni 2+ and Cd 2+ as well as for Pb 2+ and Cr 6+. Coal and coal based adsorbents have an important place in the applications of pollution control due to the advantages such as abundant availability and low cost. The lignite for Cr 6 + and Pb 2+ removal and the bituminous coal for Pb 2+ and Hg 2+ removal have been used as adsorbents [94, 125–127]. Waste Slurry, which is one of by-products of fertilizer plants, exhibits good sorptive property for removal of Cu 2+, Cr 6+, Hg 2+ and Pb 2+. Another low cost adsorbent showing the ability to adsorb heavy metals is the blast furnace slag from steel plants. The various materials from agricultural wastes are well known natural adsorbents because of their low cost, abundance and eco-friendly nature. For example, rice husk has been tested for adsorption of Ni 2+ [128] and Cd 2+ and Pb 2+ [129]. Sawdust is an example of a natural adsorbent tested for the adsorption of Pb 2+ Cu 2+, Cd 2+ and Pb 2+ [130–132]. Peat Moss, which is a complex soil material containing lignin and cellulose, is a natural substance, widely available and abundant. It has a large surface area (>200 m2/g) and is highly porous so that it can be used for the adsorption of heavy metals. Use of peat for removal of Cu 2+, Ni 2+ and Pb 2+ has been investigated [133].Peat exhibits also a high adsorption capacity for dyes such as basic blue 69 and acid blue 25 [134]. Bio-Adsorbent Materials have many advantages such as versatility, metal selectivity, high affinity for organics and ease of regeneration. Biosorption is a process related to the adsorption of heavy metals, dyes or other contaminants by metabolic-mediated or physicochemical pathways by biological materials.Algae, bacteria and fungi and yeasts have proved to be potential metal biosorbents [135]. Chitin, which is a hydrophilic, natural cationic polymer found in fungi, insects and crustaceans, is the second most abundant natural biopolymer after cellulose, which is produced by alkaline N-deacetylation. It is an effective ion exchanger with a large number of amino groups [136]. The binding ability of chitosan for metal cations is mainly due to the amine groups on the chitosan chain which can serve as coordination sites for many metals. The utilization of chitosan for removal of Cd 2+ Hg2+, Cu 2+, Ni 2+, Cr 6+ and Zn 2+ was intensively investigated [94, 137, 138]. Most natural adsorbent materials or precursors are often used untreated, but in most cases they are subjected to physical and chemical

414 Integrating Green Chemistry and Sustainable Engineering pretreatments, such as heating or activating and hydrolysis, using various chemicals to increase the adsorptive capacities. Carbon molecular sieves (CMS) were obtained from a coal-based material with the modification of the existing carbon pore structure by depositing carbon in the pore mouths by means of cracking of an organic material in the 1970 s [8]. A special production process is used to obtain amorphous carbons in the form of carbon molecular sieves having a very narrow distribution of pore sizes with effective diameters ranging from 0.4 to 0.9 nm. Raw materials may be chemicals such as polyvinylidene dichloride and phenolic resins, or naturally occurring materials such as anthracite or hard coal. The pore structure of activated carbons can be modified to produce a carbon molecular sieve by coating the pore mouths with a carbonized or coked thermosetting polymer giving good kinetic properties with desired selectivity. The surface of CMS is essentially non-polar and the main application area is the production of high purity nitrogen from the air mixture using pressure swing adsorption process [139]. Animal Bones can be carbonized to produce adsorbent materials which have only meso- and macropores and a surface area of about 100 m2/g. The surface contains roughly equal proportions of carbon and hydroxy apatite, which means that bone charcoal can be used for adsorption of organic chemicals as well as metals from aqueous systems. Silica gel is a partially dehydrated polymeric form of colloidal silicic acid with the formula SiO2.nH20 This amorphous material comprises spherical particles 2–20 nm in size which aggregate to form the adsorbent with pore sizes in the range 6–25 nm. The surface areas can range from 100 to 850 m2/g depending on gel density. Silica gel, which has a polar surface due to the presence of SiOH and SiOSi groups, can be used for the adsorption of water, alcohol, phenol and amines over the hydrogen bonding mechanism [140]. The separation of aromatics from paraffins and the chromatographic separation of organic molecules are another important commercial applications of silica gel. The water absorption capacity of silica gel at lower temperatures is higher than that of alumina or zeolites. Activated Carbons can be produced from a wide variety of carbonaceous precursors, using various activation procedures [141]. The most common precursors are wood, wood charcoal, peat, lignite and lignite coke, hard coal and coke, bituminous coal, petrol coke as well as residual materials, such as coconut shells, sawdust, or plastic residuals. A pre-carbonization process is required to convert cellulose structures into carbonaceous material with organic precursors such as wood, sawdust, peat, or coconut shells. Such cellulose structures contain a number of oxygen- and hydrogen-containing functional groups, which can

Natural Products in Adsorption Technology 415 be removed by dehydrating chemicals. The dehydration is typically carried out at elevated temperatures under pyrolytic conditions and leads to a destruction of the cellulose structures with the result that the carbon skeleton is left. This process, referred to as chemical activation, combines carbonization and activation processes. Typical dehydrating chemicals are zinc chloride and phosphoric acid [142]. After cooling the product, the activation agent has to be extracted. Since the extraction is often not complete, residuals of the activation chemicals remain in the activated carbon and might be leached during the application. This is in particular critical for drinking water treatment. Furthermore, the application of chemicals in the activation process requires an expensive recycling, and the products of chemical activation are typically low microporous powders with low densities. Therefore, most of the activated carbons used in drinking water treatment are produced by an alternative process termed physical, thermal, or gas activation. In gas activation, carbonized materials such as coals or cokes are used as precursors. These carbon-rich materials already have a certain porosity. For activation, the precursor is heated by passing an activating gas such as steam, carbon dioxide, air at elevated temperatures (800–1000 ° C) [143]. During the activation, the activation gas reacts with the solid carbon to form gaseous products. In this case, closed pores are opened and existing pores are enlarged. The reactions cause a mass loss of the solid material. Gas activation products often form in granular form and the particle sizes can be changed by grinding and screening. Gas activation processes can also be used for the further activation of chemically activated carbons. Activated carbons are applied in two different forms, as granular activated carbon (GAC) with particle sizes in the range of 0.5 to 4 mm and powdered activated carbon (PAC) with particle sizes < 40 μm [144]. Particle sizes have a critical importance in terms of efficiency and effectiveness of application techniques. For example, powdered activated carbon (PAC) for slurry reactors and granular activated carbon (GAC) for fixed bed reactors are suitable. Activated carbons show a broad variety of internal surface areas ranging from some hundreds m2/g to more than a thousand m2/g depending on the precursors and the activation process used [145]. Activated carbon for water treatment should not have pores that are too fine so that larger molecules are also allowed to enter the pore system and to adsorb onto the inner surface. Internal surface areas of activated carbons utulized for water treatment are typically in the range of 800–1000 m2/g [146]. Activated carbons are able to adsorb a multiplicity of organic substances mainly by weak intermolecular interactions (van der Waals forces), in

416 Integrating Green Chemistry and Sustainable Engineering particular dispersion forces. These attraction forces can be superimposed by π-π interactions in the case of aromatic adsorbates or by electrostatic interactions between surface oxide groups and ionic adsorbates. Their high adsorption capacities make activated carbons to the preferred adsorbents in all water treatment processes where organic impurities should be removed. Besides trace pollutants (micropollutants), natural organic matter (NOM) can also be efficiently removed by activated carbon [147]. Polymeric Adsorbents, also referred to as adsorbent resins, are porous solids with large surface areas and distinctive adsorption capacities for organic molecules. They are produced by copolymerization of styrene, or sometimes also acrylic acid esters, with divinylbenzene as a crosslinking agent. Their structure is comparable to that of ion exchangers, but in contrast to ion exchangers, the adsorbent resins have no or only few functional groups and are nonpolar or only weakly polar. A high porosity can be achieved by carrying out the polymerization in the presence of an inert medium which is miscible with the monomer as well as not significantly influencing chain growth. After polymerization, the inert medium is removed from the product by extraction or evaporation [148]. Polymeric adsorbent materials prepared for special purposes can be produced by changing type and concentration of inert compound, the monomer concentration, the fraction of divinylbenzene, the concentration of polar monomers, and the reaction conditions. Conventional polymeric adsorbents having surface areas up to 800 m 2/g typically exhibit a narrow pore size distribution and their surfaces are relatively homogeneous With increasing degree of cross-linking, the pore size becomes smaller and the surface area increases [34]. Highly cross-linked polymeric adsorbents have adsorption capacities comparable to those of activated carbons. Desorption of the adsorbed organic compounds is possible by extraction with solvents, in particular alcohols, such as methanol or isopropanol. Bununla birlikte, aktif karbonlara kıyasla daha yüksek maliyet gerektiren ve çözücü ekstraksiyonuna dayanan rejenerasyon ile tekrar kullanılabilen polimerik malzemeler, içme suyu veya belediye atıksuları gibi kompleks bileşimlere sahip büyük miktarlarda atıkların arıtılması için sınırlı bir kullanım potansiyeline sahiptir [148]. For this reason, polymeric adsorbents are often preferred for the recycling of valuable chemicals from process wastewaters. The term oxidic adsorbents comprises solid hydroxides, hydrated oxides, and oxides. Among the processed oxide adsorbents, aluminum and iron oxides are the most important ones [149]. The general production procedure is based on precipitation of the hydroxides following partial dehydration at high temperatures. Since hydroxides are thermodynamically

Natural materials

Removal of heavy metal Removal of metalloids Removal of organic pollutants Treatment of wastewaters Oil Spills Phosphate Removal Colour Removal Treating dairy waste-water, agriculture, food processing, medicine, cosmetics, wastewater treatment, and biotechnology Adsorption of dyes Treatment, heavy metal and ammonia removal from wastewater Gas separation processes Organic compounds removal from wastewater Odor control, industrial absorbents, industrial fillers, gas absorption, soil remediation, wastewater filtration, flocculating agent, animal feeds, hydroponics, molecular sieves, catalysts, desiccants, lightweight concrete, acoustics, ceramics,

Coal [154, 155]

Peat [156]

Chitin/chitosan [94]

Clays [157–159]

Natural zeolites [160, 161] Clinoptilolite [162, 163]Mordenite [164, 165]Phillipsite [166]Chabazite [167]Stilbite [168]Analcime [169]Laumontite [170]

(Continued)

Gas adsorption (air purification, motorcycle carbon cans, automotive carbon cans, oily gas recycle in gas stations and storage depots, organic waste gas treatment)

Applications

Wood [153]

Natural adsorbents materials

Adsorbents

Table 13.1 Natural and Processed Adsorbents and Application Areas Commonly Used in Adsorption Technology.

Natural Products in Adsorption Technology 417

Soybean Oil Cake [189]

Tobacco Stem [188]

Rice Husks [185–187]

Coconut Husk [184]

Bagasse [175, 183]

Cotton stalk [182]

Straw [178] corn straw [179] wheat straw [179] rice straw [180, 181]

Sunflower stalks [177]

Corncob waste [175, 176]

Sawdust [173, 174]

Removal of heavy metal ions from water Removing some pollutants

Agricultural wastes/ by-products

Shells, hulls, stones from fruits and nuts [171, 172]

Applications

Cont.

Adsorbents

Table 13.1

(Continued)

418 Integrating Green Chemistry and Sustainable Engineering

Removal of various pollutants from wastewater

Industrial wastes/by- Fly Ash [111, 190] products Blast Furnace Slug and Sludge [124, 191, 192]

Remove heavy metal ions from aqueous solutions Treatment of acidic mine drainage by both neutralizing it and removing anionic and cationic pollutants Removal of Lead from Wastewater Removal of zinc from aqueous solutions Removal of Cu (II) from aqueous solution Red mud has also been explored for gas cleaning and wastewater treatment. Removal of manganese from water

Waste Iron [194]

Iron Slags [195]

Bagasse Fly Ash [196]

Palm Oil Ash [197]

Shale Oil Ash [198]

Red Mud [199–201]

Hydrous Titanium Oxide [100, 202]

(Continued)

The Removal of Pb(II) from aqueous solution and radiator manufacturing industry wastewater

Waste Slurry [193]

Phosphorus removal from wastewater

Applications

Cont.

Adsorbents

Table 13.1

Natural Products in Adsorption Technology 419

Cont.

Water defluoridation Remove anionic uranium and selenium species Removal of arsenic from drinking water Adsorption of phosphates from aqueous medium remove arsenate Removal of arsenic from water

Ferrous hydroxide [204]

Aluminum oxides [149]

Iron oxides [205]

Applications

Aluminum hydroxide [203]

Engineered Adsorbents

Oxidic Adsorbents

Adsorbents

Table 13.1

(Continued)

420 Integrating Green Chemistry and Sustainable Engineering

(Continued)

Removal of heavy metal from wastewater In solution purification and for the removal of taste, color, odors and other objectionable impurities from liquids, water supplies and vegetable and animal oils. Dye removal

Activated Carbon

Activated Carbon [206] Peanut Husk [207, 208] Rice Husk [209] Pecan Shells [210] Wheat [179] Wheat Straw [211] Corn Straw [212] Olive [213] Olive Stones [142, 214] Straw [215] Birch [216] Bagasse [217] Miscanthus [218] Sunflower Shell [219] Pinecone [220] Rapeseed [221] Cotton Refuse [222] Olive Refuse [223] Radiata Pine [224] Eycalyptus [225]

Applications

Cont.

Adsorbents

Table 13.1

Natural Products in Adsorption Technology 421

Adsorbents

Table 13.1

Cont.

Sugarcane Bagasse [226] Apricot Stones [227] Cherry Stones [228] Grape Seeds [229] Nut Shells [230] Pistachio-Nut Shells [231] Macadamia Nutshell [232] Hazelnut Shell [233] Peanut Hulls [234] Almond Shells [235] Oat Hulls [236] Corn Cob [237] Cotton Stalk [238] Straw/Char [239] Corn Stover [240] Olive-Waste Cakes [241] Rice-Straw [242] Cassava Peel [243] Rosa Canina sp. Seeds [244] Cornel seed [245]

Applications

(Continued)

422 Integrating Green Chemistry and Sustainable Engineering

Cont.

Synthetic Zeolites

Polymeric Adsorbents

Adsorbents

Table 13.1

Gas adsorption and separation, energy storage, air and water purification processes, and catalysis. Detergent builder, industrial gas dryer Industrial gas drying, O2 enrichment of air Fluid catalytic cracking catalyst Packed bed reactors to remove Methyl tert-butyl ether (MTBE) from groundwater Xylene isomerization Cation exchanger İon exchange Purification of gas streams for removal of water and volatile organic species, and in the separation of different isomers and gas mixtures Purification of petroleum hydrocarbons contaminated with hydrogen sulfide

Linde Type A (LTA) [248]

Linde Types X and Y (Al-rich and Si-rich FAU) [249]

Silicalite-1 [250]

ZSM-5 (MFI) [251]

Linde Type B (zeolite P) (GIS) [252]

Linde Type F (EDI) [253]

Linde Type L (LTL) [254]

Linde Type W (MER) [255]

Phenol adsorption from aqueous solution

Hypercrosslinked Copolymers (Polystrene) [247]

Macroporous Copolymers (poly(glycidyl metha- crylate-coethylene glycol dimethacrylate) [246]

Applications

Natural Products in Adsorption Technology 423

424 Integrating Green Chemistry and Sustainable Engineering traceable. overheating can only lead to the formation of stable oxides with small surface areas. Activated Aluminum Oxide (γ-alumina, γ-Al2O3) is generally used to remove arsenate and fluoride from drinking water and phosphate from wastewaters [150]. The surface areas are in the range of 150–350 m2/g. Activated aluminum oxide is produced in different particle sizes, ranging from about 0.1 to 10 mm. Also, granular ferric hydroxide is an effective adsorbent for arsenate and phosphate [151, 152]. The surface area is comparable to aluminum oxide (150–350 m2/g) and typical particle sizes range from 0.3 to 3 mm. The ion adsorption of oxide adsorbents is strongly dependent on the pH of the medium. A detailed overview of the natural and processed adsorbents commonly used in adsorption technology including the application areas is given in Table 13.1.

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14 Role of Microbes in the Bioremediation of Toxic Dyes Tanvir Arfin*, Kamini Sonawane, Piyush Saidankar and Shraddha Sharma Environmental Materials Division, CSIR-National Environmental Engineering Research Institute (CSIR-NEERI), Nehru-Marg, India

Abstract Dyes are the important aspect of transmitting colour to the material. The waste substance liberated out at the time of dying mostly has organic or inorganic contamination present in it that may reflect the severe hazardous effect on the environment. For the treatment of wastewater, the microbial process is considered as the suitable one due to cost-effective feature, eco-friendly nature and ability to produce less amount of sludge. The method of microbial decoloration is due to the adsorption process. The main objective of the microbial treatment is for decolorisation and detoxification of the effluents that are contaminated by the dye. In the present book chapter emphasis is levied on summarising the methods applied for evaluating the toxic nature of the dye as well as the product generated for the future development. Keywords: Dye, microbial, biosorption, decoloration, toxicity

14.1

Introduction

The urbanisation and industrialisation cause environmental pollution posing the severe effect on health and the ecosystem where the tremendous amount of pollution is due to textile dyes. In India, textile industries involve a large percentage in term of dyes. It has also been found that textile leads to donate around 14% of industrial production. The groups of dyes experiencing unique chemical features and dyeing process depends on each other for proper functioning.

*Corresponding author: [email protected] Shahid-ul-Islam (ed.) Integrating Green Chemistry and Sustainable Engineering, (443–472) © 2019 Scrivener Publishing LLC

443

444 Integrating Green Chemistry and Sustainable Engineering To remove the dyes, various methods are applied such as adsorption, oxidation, reduction, precipitation and so on. It is noticed that at the time of disposal, some of the toxic substances are liberated and displaced by effluents coming out from the process within the nature. Most of the dyes and pigments may be either toxic or harmful to the human being and aquatic organism as well mainly at the time of discharging for attaining the water to survive. If the concentration of the dyes is more, it may lead to severe health issues such as dermatitis, irritation of the respiratory tract, ulceration of skin etc. The ingestion of dyes may even cause vomiting, haemorrhage, diarrhoea and so on. It has been noticed that in the past decades, the demand for the dyes from the textile industries leads to the high potential of the pollution. It is observed that approximately 10–15% of the dyes are liberated by the wastewater during the process of dyeing.

14.2

Dye

Dyes are the chemical compound that gets bonded to the material and then transmits its colour to that specific material. The colour of the dyes is because of the availability of the chromophore group. Various types of dyes are available, but all the dyes have at least one N = N bond in it. There is a specific adsorption spectrum for each dye where they are linked through the electronic transition in between the molecular orbital. The mechanism is shown in Figure 14.1.

14.3

Classification of Dye

Various type of classification is shown in Figure 14.2

14.4

Dye Color

The colour of the dyes are visible mainly in the low concentration of water of 1 mg/L, but the wastewater of textile industries contains dye concentration between 10–200 mg/L which leads to the problem of aesthetic hindering the process of photosynthesis in aquatic life. It also has hazardous, toxic as well as carcinogenic effects on the living organism of the land. It is noticed that 10 – 15% of the dyes of the textile are not put into use. It is

Role of Microbes in the Bioremediation of Toxic Dyes 445 605–750 Absorbed radiation (nm)

595–605 580–595 560–580 500–560 490–500 480–490 435–480 350–435 Violet

Blue

Greenblue

(a)

blueGreen

Green

YellowGreen

Yellow Organge

Red

Color absorbed 605–750

Absorbed radiation (nm)

595–605 580–595 560–580 500–560 490–500 480–490 435–480 350–435

(b)

YellowGreen

Yellow Organge

Red

Purple

Violet

Color observed

Blue

Greenblue

blueGreen

Figure 14.1 Relationship between absorbed radiation and Color (a) Absorbed and (b) Observed.

found that the least amount of dye is visibly diminishing the penetration of light in the water leading to hurting the process of photosynthesis [1].

14.4.1

Factor

There are various factors for determining the economic as well as the technical feasibility of a single unit of dye removing method such as type and concentration, expenses, outcomes etc. The microbial bioremediation has

446 Integrating Green Chemistry and Sustainable Engineering Cationic Acid Ionic Anionic

Reactive

Direct

Dye

Vat

Non-ionic

Disperse

Figure 14.2 Classification of Dye.

gathered interest for the decolorisation and degradation [2]. If the dyes are available in the aqueous ecosystem, it may reduce the photosynthetic activity as it blocks the penetration of light in the deep water bodies hindering the quality of water and reducing the solubility of the gas. Therefore either the dyes or the degraded substance may cause a toxic effect on the plants and animals [3].

14.5

Techniques for the Removal of Dye

Different range of techniques is innovated for wastewater treatment to diminish the contaminants present in the municipal, potable water which is given in Figure 14.3.

14.6

Decoloration Mechanisms of Microbial

The microbial communities are significant for degrading the dyes from the soil and water because the microbes change the chemistry of dye and mobility by various processes such as accumulation, immobilisation, and reduction. In the upcoming era, it has been found that biodegradation

Role of Microbes in the Bioremediation of Toxic Dyes 447 Effluents

Biological method

Physical method

Chemical method

Reserve osmosis

Oxidation

Filtration Ozonation

Adsorption

Electrolysis

Coagulation/ Floculation

Microorganism

Bacteria

Fungi

Enzymes

Yeast

Figure 14.3 Removal technique for Dye.

has been served as the best alternative to fulfilling all the technological aspect. The microorganism is used as the sources for the process of bioremediation [4]. The bioremediation is proved to be of great importance as it uses the activity of microbes for reducing the toxic substances. There are two mechanisms for decoloration namely biosorption and enzymatic degradation.

448 Integrating Green Chemistry and Sustainable Engineering

14.7

Biosorption

Biosorption has become a replacement for all the processes used for the removal of dyes from wastewater [5]. The process of biosorption includes binding of impurities to the cell membrane surface or the cell wall by the process of physical adsorption, ion exchange, chemical precipitation, electrostatic interaction and chelation. The capacity of biosorption of microorganism is dependent on heteropolysaccharide and lipid substances attached to cell wall which causes a very powerful attractive force between the cell wall and molecule of azo dye [6]. Some study reveals that the ‘Aspergillus foetidus’ is also used for maximum colour removal along with a carbon source at the time of exponential growth [7]. This method can be explained as the separation of organic and inorganic components which can have metals, dye molecule and odour causing substance with the help of live/dead biomass. These biomasses include bacteria, algae, fungus, sludge produced by the biological treatment of wastewater, and by-products of fermentation industries. In the biosorption process, adsorbents are only biological material, and the method mainly involves sorption. Biosorption, when compared with any other method such as ion exchange, adsorption, reverse osmosis, precipitation results in comparable work which is very inexpensive [8]. The efficiency of biomass depends on different factors such as the type of biosorbate, biomass type, additional ion presence and also on some physio-chemical factors. The process by dead biomass is more efficient as compared to live biomass because of no effect of the harmful nature of ions on the whole procedure. Dead biomass is much more active in adsorbing organic impurities as compared to live biomass [9]. In this process, microorganisms are used for removal of heavy metals and impurities from the aqueous solution. This method is inexpensive. Removal of heavy metals takes place by physio-chemical interactions among metal ions [10]. Biosorption process involves two phases, i.e. solid phase and liquid phase. Solid phase includes biosorbent or biological materials whereas the liquid phase includes solvent or water. This process also involves sorbate or metal ion [11].

14.7.1

Merits of Biosorption Process In such a process, naturally available renewable biomaterials are used which makes it cheaper.

Role of Microbes in the Bioremediation of Toxic Dyes 449 Minimum requirements for additional costly reagents. Low investment of capital and even operation cost is low. Because of rapid kinetics, large volumes effluent can be treated. High selectivity Capable of handling various heavy metals. High affinity.

14.7.2 Factors Influences Metal Biosorption The factors which affect the biosorption of metals are as follows: Temperature Biosorption process is not affected by the temperature in the range of 20°C to 35 °C. In a few cases, the high temperature of about 50°C may enhance the biosorption, and such high temperature may damage the microbial living cells which result in a decrease of metals uptake. Adsorption is an exothermic process which tends to increase with a decrease in temperature [12]. Characteristics of Biomass Nature of any biomass is an essential factor along with its applications such as living biofilms, freely-suspended cells and immobilised preparations. Some physical treatments like boiling, mechanical disruption, drying and autoclaving also influence the binding properties while some chemical treatments like alkali treatment increase the biosorption. Nutrition & growth of biomass affects the biosorption process because of some changes in the size of the cell, the composition of the cell wall and product formation etc [13]. The Ratio of Surface Area and Volume This ratio is considered as an important factor for individual particles and cells. It is also essential for the surface area of biofilms which is immobilised. The concentration of biomass influences the biosorption capacity as the increasing concentration of biomass results in decreasing sorption per unit weight [14]. Acidity The pH is an essential factor in the biosorption process. This process is similar to an ion-exchange procedure, i.e. biomass may be referred to as a natural ion exchanger with a weak

450 Integrating Green Chemistry and Sustainable Engineering acid and base groups. Hence, the pH can affect the nature of binding sites available on biomass as well as the solubility of metals. It also influences the metal solution chemistry, action of functional groups within the biomass and also the activity of metal ions [15]. Concentration of Biomass Biomass concentration within the solution influences the uptake of metals and impurities [16]. At equilibrium concentration, adsorption of metal ions by this process is more at low densities of the cell when compared to adsorption at high densities [17]. Hence, the electrostatic attraction is an important factor in the cells. At low concentration of biomass, metal uptake increases because enhancement of concentration of biosorbents results in interaction with binding sites [18]. The Initial Concentration of Metal Ions To overcome the resistance towards metallic mass transfer within the aqueous and solid phases, initial concentration gives a driving force. Metal amount adsorbed by biomass increases with an increase in initial metal concentration [19]. Metal Affinity Physical or chemical treatments influence the permeability and surface charge of any biomass and also makes a binding group of metals more accessible. It may be changed by pre-processing the biomass with heat, alkali and acid detergents which results in increasing the biosorption capacity [20].

14.8

Consortia of Microorganisms

The availability of various chemical compounds in the effluents of dye may lead to hinder the microbial degradation of one species. Therefore, the use of groups of microorganism in the form of the consortium will be a good option. The microbial consortia are mainly the varied group of microorganism including various mechanism at a time along with the types of enzymes to remove the components of dye completely from the material [21].

Role of Microbes in the Bioremediation of Toxic Dyes 451

14.9

Decolorization by Fungi

Various fungi can also be used for decolorization. A fungus named ‘Neurospora crassa’ is used to remove diazo dyes. The result showed about 90% removal of color within a day. Another fungi ‘Schizophyllum’ commune is also helpful in color removal. ‘Trichoderma spp.’ is used to remove color from effluent containing lignin. About 85% of color gets removed within three days. Another fungus, ‘Pycnoporus cinnabarinus’ (white rot fungi) is used to remove the colour of dyes of high concentration. For this procedure, a packed-bed reactor is used. Results were achieved within 48–72 hr incubation [22]. White rot fungi are used for biodegradation of recalcitrant xenobiotic substances such as azo dyes with the use of an oxidative enzyme. In this process, pH should be maintained as 4–5, optimum enzyme activity for proper decolorisation. This process involves an manganese peroxide & lignin peroxide. Some other dyes are also analysed, and their color removal takes place with the help of ‘Phanerochaete chrysosporium’ at different concentrations. This takes place only if veratryl alcohol is available in the medium. It is known that veratryl alcohol increases ligninase activity, resulting in decolorisation. Some wood-rotting fungi are also known for colour removal of various dyes. These are found more capable to remove color in comparison to ‘Phanerochaete chrysosporium’. Dyes which emerged from paper industries can be decolorised by marine fungi which is unidentified. This fungus also formed by manganese peroxide, laccase enzyme and lignin peroxide. Adsorption study reveals that acid dyes such as congo red & fast red and basic dyes such as basic blue and malachite green takes place with the help of ‘Aspergillus niger’, a fungus helpful in forming citric acid. Congo red show 85% of color removal while all other show 98% of color removal [23]. ‘Rhizomucor pusillus’ which is waste biomass of fungus named ‘Mucoralean’ is used as an adsorbent for effluent of bleach plant. Adsorption takes place up to 45% within a few hours, and full color gets removed after 8 hr of treatment [24]. The dye can be removed by biomass of ‘Trametes Versicolor’ through adsorption process and when compared with enzymatic degradation, it is found that dye removal takes place up to 90% by both the processes [25]. Wastewater emerged from Oliver mill contain phenolic compounds which are decolorised by ‘Geotrichum candidum’. 75% removal takes place in three days. Removal takes place as ‘Geotrichum candidum’ remove simple phenolic compounds and hydrolyse phenolic compounds with higher

452 Integrating Green Chemistry and Sustainable Engineering molecular weight. Similarly, the astrozon red dye is decolorised using ‘Funalia trogii’ pellet. In this case, a total of 55% of removal is observed in a day. More the amount of pellets, more the decolorisation in such case. ‘Thelephora sp.’ (white rot fungi) is used to remove the colour of different azo dyes. Some of them are amido black 10B, congo red and orange G. For amido black 10B, about 98% color removal take place. For congo red, about 97% color removal takes place, and for orange G, about 33% color removal takes place [26]. Fungus excretes some extracellular enzymes which increase their capabilities of degradation of those structures which are tough to degrade by bacterial biomass. Congo red is degraded by ‘Gliocladium virens’. Another fungi ‘Trichoderma harzianum’ is useful for degradation of toxic dyes such as acid red, bromophenol blue, basic blue, congo red and direct green. Also, the degradation of waste materials of plants takes place by using fungi. One study also revealed about the degradation of azo dyes such as Congo red, Tropaeolin O and Orange II are by a fungus named ‘Phaenerocheate chrysosporium’. For bioremediation of toxic dyes, some fungus named ‘Aspergillus niger’ and ‘Aspergillus oryzae’ is used as important components. ‘Aspergillus niger’ give great colour removal after 16 days of incubation [7]. Some additional aromatic compounds and phenols in dye molecule can be more toxic than the dye itself. So their degradation can be done by the use of some efficient fungus. Some of them which are useful for degradation of the aromatic substituent of dye are ‘Thamnidium elegans’, ‘Yarrowia lipolytica’ and ‘Zygorhynchus moelleri’. Fungi are proved as an efficient system for dye removal because of its large area and very easy solid-fluid separation. This can be helpful for removal of both organic and inorganic pollutants. Textile effluent contains dye molecule attached with metal as they are in the paper, tannery, pulp and dyeing industries. For the bio-remediation of these dyes, different kinds of microbial strain are the first need which is helpful in the formation of consortia. This results in great bio-remediation. Immobilization of fungi results is the more good result of bio-remediation. For this purpose, several techniques of immobilisation are also developed. Entrapment is the most general method used for immobilisation of some micro-organisms having a single cell. These type of micro-organisms include yeast which is helpful in avoiding loss of micro-organism [27]. The fungus is a suitable organism which is helpful in the treatment of textile wastewater. A large number of different fungal organisms are also known for removal of different dyes. There is a different kind of fungi named ‘Rhizopus arrhizus’ and ‘Rhizopus oryzae’ have also been used for

Role of Microbes in the Bioremediation of Toxic Dyes 453 dye removal treatment of diverse dyes. In case of living cells, the main procedure is biodegradation because they are capable of producing ligninmodifying enzymes, lignin peroxidase, manganese peroxidase and laccase. It is the initial step of the mineralisation process of dyes. The work of lignin peroxidase, manganese peroxidase and laccase has proved to be different for different fungus in dye removal treatment. Biosorption by living fungi is also very helpful in dye removal. In case of dead cells, biosorption process involves some interactions which include adsorption, ion exchange and some other processes. Some studies has revealed about the interaction between different type of dyes having a complex structure and dead biomass of fungus [28]. Some functional groups such as amino, phosphate and carboxyl along with lipid fractions play an important role in fungal biomass. With the help of ‘Aspergillus niger’ biosorption of a dye named basic blue nine gives about main attaching sites. These main sites are carboxyl and amino substituents. In the case of Acid Blue 29, the main binding site is an amino group, and important process is an electrostatic attraction. In removal process of Congo red, some binding sites are found to be important such as amino, phosphate groups and carboxylic acid. Other than electrostatic attraction, other processes also take place in biosorption. In the removal process of Disperse red 1, electrostatic attraction is a primary process and further chemical & physical adsorption takes place. In such process, main binding sites are lipid fraction along with amino group [29]. Table 14.1 shows the dye decolorisation by live fungi and Table 14.2 shows dye decolorisation by dead fungi.

14.9.1

Factors Affecting Dye Decolorisation by Fungal Biomass

Nutrients Improving growth conditions initially results in increasing biodegradation of dye. Some carbon source such as glucose, maltose, cellobiose and starch etc. are efficient in colour removal of cotton bleaching wastewater with the use of white rot fungi [31]. ‘P. chrysosporium’ give good performance at 5 g/l concentration of glucose and 0.05 g/l concentration of ammonium chloride in colour removal of methyl violet [32]. Nutrient nitrogen’s high dose is helpful for colour removal of Congo red [33]. According to another study, nitrogen is not effective on dyes for colour removal by a fungus named ‘Cyathus bulleri’ [34].

454 Integrating Green Chemistry and Sustainable Engineering Table 14.1 Dye Decolorisation by the Use of Live Fungi [30]. Fungi

Name of dye

Contact time

% removal

Aspergillus niger

Polar red

8d

94

Penicillium oxalicum

Reactive Blue 19

80 min

91

Rhizopus oryzae

Rhodamine B

5 hr

90

Aspergillus niger

Acid Blue 29

30 hr

80

Funalia trogii

Astrazon Red

24 hr

92–98

Dichomitus squalens

Orange G

14 d

95

Aspergillus niger

Basic Blue 9

2d

10

Irpex lacteus

Methyl Red

14 d

56

Phanerochaete chrysosporium

Reactive Red 22

30 hr

92–100

Trametes trogii

Anthraquinone Blue

4 hr

88

Phanerochaete chrysosporium

Direct Dyes

15 d

100

Caribena versicolor

Indigo Carmine

1 hr

98

Trametes versicolor

Direct Blue 1

9d

100

Under metabolic conditions, dye decolorisation by ‘Phanerochaete chrysosporium’ results in the release of lignin peroxidase by fungal cells. Fungal growth is dependent on pH values also. Fungi grows at low pH ranging from 4 to 5 and growth temperature is considered to be 25-35 °C [29].

Characteristics of Effluent Containing Dye Decolorization of a dye is fully dependent on the molecular structure of the dye. One study reveals about the dye decolorisation by ‘Trametes versicolor’ is dependent on the molecular structure of the dye. Anthraquinone

Role of Microbes in the Bioremediation of Toxic Dyes 455 Table 14.2 Dye Decolorisation by the Use of Dead Fungi [30]. Fungi

Name of dye

Contact time

% removal

Aspergillus niger

Congo Red

42 hr

89.6

Neurospora crassa

Acid Red 57

40 min

98.78

Thuja orientalis

Acid Blue 40

90 min

48.5

Rhizopus arrihizus

Germazol turquoise Blue-G

24 hr

47.5

Trametes versicolor

Direct Blue 1

6 hr

95.2

Aspergillus niger

Synazol

18 hr

88

Phanerochaete chrysosporium

Astrazone Blue FGRL

2 hr

60

Rhizopus arrhizus

Reactive Black 5

24 hr

62.5

90 min

72.8

Agaricus bisporus mixed with Thuja orientalis Reactive Blue 49 Rhizopus arrhizus

Gryfalan Black RL

400 min

59

Funalia trogii

Astrazone Blue FGRL

2 hr

48

dyes come under the category of laccase substrate, but azo dyes and indigo dyes do not fall under this category [35]. In case of dye removal by ‘P. chrysosporium’, aromatic rings having any substituent (amino, hydroxyl, nitro, acetamido) get mineralised very efficiently to high extent as compared to the unsubstituted aromatic ring. The concentration of dye is also responsible for colour removal. If the concentration of dye is increased to 700–1200 mg/l from 100 to 500 mg/l, then the efficiency of dye decolorisation with the use of ‘Coriolus versicolor’ get reduced by 20% [36]. In the solution, the ionic form of dye depends on the pH of the solution. Similarly, a surface electrical charge also depends on it. Hence, binding sites for dye in fungal biomass and chemistry of dye in the medium is affected by pH values. For Basic Blue 9, effective initial pH value of dye solution is found to be 6, and for Acid Blue 29 it is found to be 4. At pH

456 Integrating Green Chemistry and Sustainable Engineering value 2, no biosorption takes place because proton has a high concentration in case of Basic Blue 9. For Acid Blue 29, no biosorption takes place at pH 12 [37]. Removal of Reactive Red 120 by ‘Lentinus sajor-caju’ increases with a decrease in pH value. Mostly textile effluents are formed at high temperature. Therefore, the temperature can be an important factor for the biosorption process. Biosorption by ‘Lentinus sajor-caju’ increases with an increase in temperature at 5 °C to 35 °C [38]. Biosorption of dyes by ‘Rhizopus arrhizus’ increases with an increase in temperature (45 °C) which give an endothermic character. The process of dyeing uses a salt of the highest amount and therefore dye effluent consist of a very high concentration of salt. Because of this fact, ionic strength becomes an essential factor [39]. One study reveals that the high ionic strength results in high biosorption in case of humic acid by fungal biomass named ‘Rhizopus arrhizus’ [40].

Pre-Treatment Some pre-treatments can enhance the biosorption capacity of fungal biomass. The pre-treatment process involves autoclaving and contacting with inorganic chemicals (H2SO4, CaCl2, NaOH, NaHCO3) or organic chemicals [36]. By heating fungal biomass named ‘Lentinus sajor-caju’ for 10 min at 100 °C results in increasing the biosorption capacity but base-treatment for removal of Reactive Red 120 with the use of 0.1 M NaOH results in decreasing the biosorption capacity of fungal biomass [38]. Immobilization of fungus also influences dye decolorisation. They are used as an alternative to immobilised enzymes. Mostly, the immobilised bacterial cell is used for work, but nowadays fungal cells are also grabbing the attention [41]. Table 14.3 shows decolorisation of dye by immobilised fungi. Fungal biomass is excellent metal for biosorption. So, fungi are nowadays main choice of attention. Fungal biomass is efficient in removal of heavy metals from dilute aqueous solution by biosorption process because of some reasons: It provides high amount cell-wall material for the procedure.It provides good binding properties of the metal.Gives an eco-friendly atmosphere. Available in a high amount from food industries and antibiotics Fungi are available easily and is very important for industrial applications [54].

Role of Microbes in the Bioremediation of Toxic Dyes 457 Table 14.3 Decolorization of Dyes by Immobilised Fungi. Immobilized support

Micro-organism

Dye

References

Plastic discs

Coriolus versicolor

Everzol Turquoise Blue G

[36]

PUF, pine wood

Irpex lacteus

Remazol Brilliant Blue R

[42]

Jute twine

Tinea versicolor

Carpet Dye

[43]

Barley bran

Tinea versicolor

Congo Red

[44]

Stainless steel sponge

Tinea hirsute

Lanaset Marine

[45]

Plastic net

Bjerkandera adusta

Reactive Black 5

[46]

Barley bran

Tinea Versicolor

Indigo Carmine

[47]

Stainless steel sponge

Tinea hirsute

Indigo Carmine

[48]

Sugarcane bagasse

Pleurotus sajor-caju

Textile effluent

[49]

Alginate beads

Tinea Versicolor

Amaranth

[50]

Corn cob

Pleurotus pulmonarius

Ethyl Violet, Methylene Blue, Amido Black, Methyl Green, Remazol Brilliant, Congo Red, Poly R-478, Brilliant Cresyl Blue

[51]

Corn cob, PUF, nylon sponge

Phanerochaete chrysosporium

Poly R-478

[52]

Phanerochaete chrysosporium

Direct Green 6, Direct Brown 2, Direct Orange 26, Congo Red, Direct Brown 2, Tartrazine and Direct Black 38

[53]

ZrOCl2-activated pumice

14.10

Dye Removal by Bacteria

Bacteria are just a biological cell which varies in the structure according to the type. They are very small approximately few micrometres in length. The first living thing to be developed on the surface of the earth

458 Integrating Green Chemistry and Sustainable Engineering Gram positive bacteria

Cocci

Rod shaped micrococcus

Bacilli

Rod shaped micrococcus

Oval shaped enterococcus

Red shaped micrococcus

Red shaped micrococcus

Figure 14.4 Classification of gram-positive bacteria by the shape.

is bacteria. According to the composition of cell wall, bacteria is of two types such as Gram-positive and Gram-negative.

14.10.1 Gram-Positive They consist of thick layer of peptidoglycan. It can be identified by the test of gram stain. Firstly, it appears as crystal-violet and after staining it changes into a purple colour which is then turned into blue. It can be observed through a microscope. The colour is retained after the washing. The classification of gram-positive bacteria based on the shape are shown in Figure 14.4.

14.10.2 Gram-Negative It is opposite to the gram-positive. After decolorisation, it does not retain violet stain. During the degradation stage, the outer layer of gram-negative bacteria can be degraded. The peptidoglycan layer is thinner. In between the outer membrane and inner cell membrane the peptidoglycan layer is present. The colour appears pink or red. Bacteria have large potential and application in different fields. Among them, Decolorization is one of them [55]. For biodegradation and biosorption bacteria serves the good option with no harm. Bacteria can be

Role of Microbes in the Bioremediation of Toxic Dyes 459 Gram negative bacteria

Cocci

Bacilli

Neisseria

Multiple sized moraxella

Brucella

Pasteurella

Nucleatum

Spirillum

Alkaligenes

Streptobacillus

Figure 14.5 Classification of gram-positive bacteria by the shape.

used in two form either live or dead. But more focus is towards live one instead of dead bacteria. The classification of gram-negative bacteria based on the shape are shown in Figure 14.5. Corynebacterium glutamicum was used as a biosorbent, and it was done by Won et al., [56]

14.10.3 Bacteria Classification by Shape A single-celled organism is called as bacteria. They are observed in various shape and size depending on the source, appearance and atmosphere where they are present. Bacteria can be classified mainly as rod-shaped, comma-shaped, circular, curved form etc. Different bacteria cause diseases but not all the bacteria are responsible for causing diseases.

460 Integrating Green Chemistry and Sustainable Engineering a. Coccus They appear as circular, spherical or round shaped bacteria. It can be present as single or in the chain also. Common problems occur due to cocci illness. For example. The bacterium Staphylococcus aureus appears in the clustered form. Pneumonia, Bacterial meningitis and skin problems are the basic causes of such bacteria. b. Bacillus The rod-shaped bacteria can look like sausages and observed as single-celled or in the chain. For example, Escherichia coli is the bacterium which is not harmful. This is present in the intestine of human but diseases related to the urinary tract can occur. By such a type of rod-shaped bacteria, there are the chances of causing diseases through inhalation, ingestion or damaged skin. c. Vibrio The spiral shape bacteria called as vibrio. It can appear as a curved rod or comma-shaped bacteria. The serious disease can be caused by such bacteria like cholera. d. Spirillum Another type of bacteria is called as Spirillum. This is also spiral but more rigid. Diarrhoea is the disease caused by bacteria Campylobacter jejuni.

14.10.4 Decolorization by Bacteria Decolorization process of dyes by bacteria involves the adsorption process. In colour removal of azo dyes, reduction of the chromophore of azo dye takes place by introducing redox mediators [57]. Bacterial biomass can accumulate metal ions because of charged sites present on the cell wall of bacteria. Various bacteria are used for colour removal of dyes from wastewater. Bacterial mixed cultures also decolorise chromophore attached to diazo of molecule dye just in half month, i.e. 15 days. One of the studies has also revealed about decolorisation of dyes by biofilms available on a support material or growing cells in 24–30 hr. ‘P-aminoazobenzene’ is a kind of azo dye which get degraded by ‘Bacillus Subtitles’ has also been reported. Mesophilic microbes are also used for colour removal of dyes. Similarly, thermophilic microbes are also used for decolorisation. A bacterial species named as ‘Pseudomonas’ have been greatly used for dye decolorisation. Some dyes such as brilliant green, malachite green, methylene blue, fast green and congo red etc.

Role of Microbes in the Bioremediation of Toxic Dyes 461 under some aerobic conditions get 30–70 % decolorise. For this particular treatment, pH is maintained 6–8, and 30–40 degree temperature is taken. Colour of congo red, as well as direct black 38, are removed with the help of Pseudomonas. As a result, 98% colour removal of congo red and 72% colour removal of direct black 38 is observed under anaerobic incubation but not under aerobic incubation. Halophiles used for dye decolorisation has also been studied. A halophile named ‘Bacillus sp.’ is used for removal of an azo dye named Red 2G which resulted in 64.8% colour removal. Halophiles are extremophilic because of their high metabolic diversity, and this is the main factor which influences the rate of removal of dye colour. A bacterial species ‘Brevibacterium sp strain VN-15’ is used for reduction of toxicity of the solution contain dye [58].

14.10.5 Bacterial Strain Used for Dye Removal The wastewater or effluent discharged from industries like textile or printing has different types of dyes, acids, bases, detergents and very complex. This discharge can pollute the natural resources, river, lake etc. thereby dissolved oxygen concentration is decreased which results in harming the aquatic life. For such clearance of pollutants and treating the different wastewater technologies are available. The best option for the available techniques is the biological treatment which can treat the wastewater by micro-organisms. Biological processes are more efficient than the other techniques, as they don’t produce a large amount of sludge, environmentally friendly and also cost less than another one. Among such microorganisms, bacteria have the greater effect of treating the wastewater. Rather can fungi, bacteria decolorise and mineralise very fast. Instead of using a single culture mixed culture gives the better result of decolorisation. The bacterial strain can be used for dye removal, and it can be shown with percentage as, 1. Mixed culture 82 [59] 2. Mixed microbial culture 100 [60] 3. Activated sludge obtained from domestic and industrial effluent treatment plants 100 4. Anaerobic packed bed reactor followed by an aerobic stirred tank reactor 90–93 5. Original seed sludge collected from a municipal wastewater treatment plant [61]

462 Integrating Green Chemistry and Sustainable Engineering

14.10.6 Different Bacteria Removes Different Dye During the incubation period of seven days, the degradation percentage is observed by four bacteria as shown in Table 14.4

14.11

Algae

Algae are simply the eukaryotic photosynthetic organisms like plants. The water sources like sea, lakes, ponds and rivers are the common areas where they can be found in the living state. It can be found on the surfaces such as on rock surface and in moist soil. Instead of leaves, roots and stem algae possess the special form of tissues. Microalgae and Macroalgae are the two different types of algae. Microalgae are phytoplankton and are very small. They can be observed under the microscope only. These are a unicellular organism, floating and are minute. Classification of microalgae can be done by colour, shape and size. Micro flagellates and Diatoms are the two types of microalgae. Macroalgae are also called as seaweeds. The word macroalgae itself suggest that macro indicates the larger plants but live in an aquatic environment like the sea and the sand. These are multicellular. It consists of different parts like air bladder, thallus, stipe, mid rib etc. Algae got the colour because of the presence of different pigment. It can be classified by colour as, (a) Red algae, (b) Green algae, and (c) Brown algae

14.11.1

Classification of Algae

Algae are classified into following seven types as given below: Brown Algae (Phaeophyta) It can always be seen in the marine environment. The cell wall of algae shows the material like polysaccharides and cellulose. Cool water is the zone of appearance. These are present with the special type of cell called as reproductive cells. These cells take help of flagella for locomotion. It can store food as carbohydrates. It has the pigment known as ‘fucoxanthin’ which is mainly responsible for its brown colour. Example of brown algae is Sargassum spp.

70.27

80.98

83.38

85.13

Pseudomonas monteilii

Aeromonas hydrophila

Comamonas testosterone

Methyl orange

Name of dye

Pseudomonas putida

Degradation Percentage (%)

Bacteria

44.13

66.01

55.34

85.47

Acid orange II

Table 14.4 Role of Bacteria in Dye with Degradation Percentage (%).

14.3

86.95

18.1

91.01

Malachite green

39.66

40.19

46.83

69.29

Methylene blue

55.93

49.46

56.27

54.51

Rhodamine B

Role of Microbes in the Bioremediation of Toxic Dyes 463

464 Integrating Green Chemistry and Sustainable Engineering Green Algae (Chlorophyta) They can grow in freshwater but sometimes some species grow in the marine environment. Sometimes it occurs in the brackish water. The structure of Green algae is multicellular, and it is macroscopic. Cellulose is the main substance of cell wall which has a maximum concentration of calcium carbonate and hemicellulose. The food is stored in the form of starch. The colour of such algae is due to xanthophyll and carotenoids. The example of green algae is Chlorella, Gonium etc. There are some types of algae like Charophytes which are not like the higher plants but have the potential to develop the sex organs. Yellow- Green Algae (Xanthophyta) The presence of such yellow-green algae is mostly in fresh water. The cell wall of this algae contains pectin compounds and cellulose and also silica for sometimes. The food material carbohydrate is stored by such algae in the form of leucosin. For locomotion or movement, these types of algae have two to three or more flagella. Important pigments for yellow-green algae are the same as chlorophyll a, and the unnecessary pigments are xanthophyll and carotenoids. Red Algae (Rhodophyta) These are mostly the marine type of organism. The range of size can be from microscopic to macroscopic. The red algae need support to grow like on a hard surface or with the help of other algae. Polysaccharides and cellulose are the components of the cell wall of red algae like carrageenan and agar. Flagella is absent in red algae. A special form of a polysaccharide called floridean starch is the food. Chlorophyll a and d are the important photosynthetic pigments whereas the non-essential pigments are phycobilins, xanthophyll and carotenoids. Example of red algae is Plenosporum spp. Euglenoids (Euglenophyta) These are just like protozoa and are unicellular. The cell wall is absent in case of such algae. Thus the outer layer is proteinaceous. It can be named as a pellicle. The food can be stored as carbohydrates and named as paramylon. The flagella are there for their locomotion. This alga has feature same as green algae that is xanthophyll and carotenoids are the important secondary pigments while main pigments are

Role of Microbes in the Bioremediation of Toxic Dyes 465 chlorophyll a and b. Euglenoids are not always photosynthetic. Sometimes they are exceptional, and thus the water serves them food in the form of organic matter. One good thing about such algae is that the photosynthetic algae can able to survive under dark also for some period if they are provided with some food. Pyrrophyta (Fire Algae) It covers unicellular algae of about 1100 species. The main area of fire algae is the marine one, but it is growing in fresh water also. The cell wall of Pyrrophyta consist of cellulose, and for movement two flagella are there, known as dinoflagellates. Similarly, like green algae, it stores food as starch. Here chlorophylls a and c are the photosynthetic pigments and fucoxanthin is for the colour of algae. If the species of two flagella are present in the large amount and it can have said to be ‘red tides’. This name ‘red tides’ is just because of its colour. Such algae can synthesise the chemical which is toxic, and this is dangerous to the aquatic life. It also shows the property of bioluminescence which can lighten the algae at night so that water surface or ocean surface looks bright at night. Golden Brown Algae (Chrysophyta) These can grow in fresh water as well as in marine water. Pectic materials and Cellulose are the main constituents of the cell wall. The cell wall can be made rigid with the help of silica. But in some cases, species of golden brown algae doesn’t have the cell wall. The energy is stored in the form of carbohydrates, and it can be called as leucosin. The other way to store energy is in the form of oil droplets also. Algae can be used for the commercial applications. It serves as the raw material for crude oil. From an open water source, such golden brown algae are really important. They can reduce down or dominate the phytoplankton biomass and productivity. Such phytoplanktons consist of very small particle or fraction, and it can be called as nano-plankton.

14.11.2

Microalgae

The macroalgae are widely used from the effluents of industrial water of the textile industry for removing dyes. This dye can be used again after the removal process also. The usefulness behind using the brown macroalgae

466 Integrating Green Chemistry and Sustainable Engineering CO2

Organic matter

Microalgal photosynthesis

Bacterial oxidation

O2

Biomass

Light

Figure 14.6 The principle of photosynthetic oxygenation. Phytoremediation is the mechanism where algae are used to improve the quality of the water. The microalgae are considered as the most suitable algae for the remediation method because a large amount of toxic and other hazardous substances get treated with the help of such algae which are non-pathogenic. Algae is capable of fixing the CO2 by the process of photosynthesis and are capable of discarding the extra nutrients with less costing.

species Sargassum to remove dye from the water effluents of textile either in the batch or continuous adsorption columns is most to give energy efficient and cheap solution for wastewater treatment. The kinetic study had given a suitable prediction for demonstrating the features of azo dye decolorisation within the immobilised cell systems by applying Aeromonas hydrophila. Such kinetic modelling on interconnection to the graphical analysis gave a reasonable evaluation to investigate about the practical bio-stimulation strategy for the uneven growth of biodegradation technique in the system to enhance the chance of industrial uses [62].

14.11.3

Effect of Algal Photosynthesis

Photosynthesis is the important approach applied by the living organism to attain energy and basic nutrients either as a direct mode or indirect mode. Hence it is foremost that photosynthetic aeration plays a significant role in minimising the cost of operation and also to limit the pollution volatilisation risk as per the mechanical aeration technique. The mechanism is shown in the Figure 14.6.

Role of Microbes in the Bioremediation of Toxic Dyes 467

14.11.4 Algae for Dye Removal Diazo dye is the mostly released dye from textile and printing industries. After many approaches like chemical processes, the biological approach serves the best from all point of view. In this biological way, algae are one of them. Karthikeyan et al., [63] have explained the study of spirogyra species for dye removal. Spirogyra species is commonly observed and can remove the dye. The processes involve namely bio-coagulation, bio-sorption and bio-conversion for the removal of dye. This species can remove up to 88% dye from the industrial wastewater.

14.12

Conclusion

The microbially based approaches have proved to be a significant method for the basic process related to dye such as transformation, degradation and decolorisation. In the present book chapter emphasis is paid on the different biochemical techniques applied to the degradation of dye including the microbial degradation approach employing the biosorption method. Various researchers have presented their reports about the important aspect of handling the transformation of dye towards the minute eco-friendly molecules. In the current book chapter, the application of microbes was undertaken to treat the dye of the textile industry. It is still found that the study related to the use of microbial for removing dye is progressing in the research field. Continuous efforts are required to push forward the research in this field by simulating the various aspect such as choosing the potential microbial biosorption by market strategy posing economic expenses, large-scale study with real wastewater and complete demonstration techniques.

References 1. Vandevivere, P.C., Bianchi, R., Verstraete, W., Review: Treatment and reuse of wastewater from the textile wet-processing industry: review of emerging technologies. J. Chem. Technol. Biotechnol., 72(4), 289, 289–302, 1998. 2. Azmi, W., Sani, R.K., Banerjee, U.C., Biodegradation of triphenylmethane dyes. Enzyme Microb. Technol., 22(3), 185–191, 1998. 3. Thanikaivelan, P., Rao, J.R., Nair, B.U., Ramasami, T., Progress and recent trends in biotechnological methods for leather processing. Trends Biotechnol., 22(4), 181–188, 2004.

468 Integrating Green Chemistry and Sustainable Engineering 4. Khan, A.A., Husain, Q., Ali, K.A., Qayyum, H., Potential of plant polyphenol oxidases in the decolorization and removal of textile and non-textile dyes. J. Environ. Sci., 19(4), 396, 2007. 5. Fu, Y., Viraraghavan, T., Column studies for biosorption of dyes from aqueous solutions on immobilised Aspergillus niger fungal biomass. Water SA, 29, 465, 2003. 6. Solís, M., Solís, A., Pérez, H.I., Manjarrez, N., Flores, M., Microbial decoloration of azo dyes: A review. Process Biochemistry, 47(12), 1723–1748, 2012. 7. Lavanya, C., Dhankar, R., Chhikara, S., Sheoran, S., Degradation of toxic dyes: a review. Int. J. Curr. Microbiol. App. Sci., 3, 189, 2014. 8. Maurya, N.S., Mittal, A.K., Cornel, P., Rother, E., Biosorption of dyes using dead macro fungi: effect of dye structure, ionic strength and pH. Bioresour. Technol., 97(3), 512–521, 2006. 9. Tsezos, M., Bell, J.P., Comparison of the biosorption and desorption of hazardous organic pollutants by live and dead biomass. Water Res., 23(5), 561– 568, 1989. 10. Kapoor, A., Viraraghavan, T., Biosorption of heavy metals on Aspergillus niger: Effect of pretreatment. Bioresour. Technol., 63(2), 109–113, 1998. 11. Ashkenazy, R., Gottlieb, L., Yannai, S., Characterization of acetone-washed yeast biomass functional groups involved in lead biosorption. Biotechnol. Bioeng., 55(1), 1–10, 1997. 12. Goyal, N., Jain, S.C., Banerjee, U.C., Comparative studies on the microbial adsorption of heavy metals. Environ. Res., 7(2), 311–319, 2003. 13. Wang, J., Chen, C., Biosorption of heavy metals by Saccharomyces cerevisiae: a review. Biotechnol. Adv., 24(5), 427–451, 2006. 14. Gadd, G.M., White, C., Copper uptake by Penicillium ochro-chloron: influence of pH on toxicity and demonstration of energy-dependent copper influx using protoplasts. Microbiology, 131(8), 1875, 1875–1879, 1985. 15. Greene, B., Darnall, D.W., Microbial oxygenic photoautotrophs (cyanobacteria and algae) for metal-ion binding. Ehrlich, H. L., Brierley, C. L., (Eds.). Microbial Mineral Recovery. New York, McGraw-Hill. pp. 227–302, 1990. 16. Modak, J.M., Natarajan, K.A., Biosorption of metals using nonliving biomassA review. Miner. Metall. Process, 12, 189, 1995. 17. Gourdon, Rémy., Bhende, S., Rus, E., Sofer, S.S., Comparison of cadmium biosorption by Gram-positive and Gram-negative bacteria from activated sludge. Biotechnol. Lett., 12(11), 839–842, 1990. 18. Fourest, E., Roux, J.-C., Heavy metal biosorption by fungal mycelial by-products: mechanisms and influence of pH. Appl. Microbiol. Biotechnol., 37(3), 399–403, 1992. 19. Zouboulis, A.I., Matis, K.A., Hancock, I.C., Biosorption of metals from dilute aqueous solutions. Sep. Purif. Meth., 26(2), 255–295, 1997.

Role of Microbes in the Bioremediation of Toxic Dyes 469 20. Ahalya, N., Ramachandra, T.V., Kanamadi, R.D., Biosorption of heavy metals. Res. J. Chem. Environ., 7, 71, 2003. 21. Joshi, S.M., Inamdar, S.A., Telke, A.A., Tamboli, D.P., Govindwar, S.P., Exploring the potential of natural bacterial consortium to degrade mixture of dyes and textile effluent. Int. Biodeterior. Biodegradation, 64(7), 622–628, 2010. 22. Banat, I.M., Nigam, P., Singh, D., Marchant, R., Microbial decolorization of textile-dyecontaining effluents: A review. Bioresour. Technol., 58(3), 217–227, 1996. 23. Anjaneyulu, Y., Bindu, HV., Removal of colour from industrial effluents by using wastebiomass and reagent loaded polyurethane foam. Anjaneyulu, Y., (Eds). Proceedings of International Conference on Industrial Pollution and Control Technologies. Hyderabad, Allied Publishers. pp. 457–462, 1997. 24. Christov, L.P., van Driessel, B., du Plessis, C.A., Fungal biomass from Rhizomucor pusillus as adsorbent of chromophores from a bleach plant effluent. Process Biochemistry, 35(1-2), 91–95, 1999. 25. Aretxaga, A., Romero, S., Sarrà, M., Vicent, T., Adsorption step in the biological degradation of a textile dye. Biotechnol. Prog., 17(4), 664–668, 2001. 26. Anjaneyulu, Y., Sreedhara Chary, N., Samuel Suman Raj, D., Chary, N.S., Raj, D.S.S., Decolorization of Industrial Effluents – Available Methods and Emerging Technologies – A Review. Rev. Environ. Sci. Biotechnol., 4(4), 245– 273, 2005. 27. Sen, S.K., Raut, S., Bandyopadhyay, P., Raut, S., Fungal decoloration and degradation of azo dyes: A review. Fungal Biol. Rev., 30(3), 112–133, 2016. 28. Srinivasan, A., Viraraghavan, T., Decolorization of dye wastewaters by biosorbents: A review. J. Environ. Manage., 91(10), 1915–1929, 2010. 29. Fu, Y.Z., Viraraghavan, T., Removal of CI Acid Blue 29 from an aqueous solution by Aspergillus niger. Aatcc Rev., 1, 36, 2001. 30. Srinivasan, A., Viraraghavan, T., Decolorization of dye wastewaters by biosorbents: a review. J. Environ. Manage., 91(10), 1915–1929, 2010. 31. Zhang, F.M., Knapp, J.S., Tapley, K.N., Development of bioreactor systems for decolorization of Orange II using white rot fungus. Enzyme Microb. Technol., 24(1-2), 48–53, 1999. 32. Radha, K.V., Regupathi, I., Arunagiri, A., Murugesan, T., Decolorization studies of synthetic dyes using Phanerochaete chrysosporium and their kinetics. Process Biochemistry, 40(10), 3337–3345, 2005. 33. Tatarko, M., Bumpus, J.A., Biodegradation of Congo Red by Phanerochaete chrysosporium. Water Res., 32(5), 1713–1717, 1998. 34. Vasdev, K., Kuhad, R.C., Saxena, R.K., Decolorization of triphenylmethane dyes by the bird’s nest fungus Cyathus bulleri. Curr. Microbiol., 30(5), 269– 272, 1995.

470 Integrating Green Chemistry and Sustainable Engineering 35. Wong, Y., Yu, J., Laccase-catalyzed decolorization of synthetic dyes. Water Res., 33(16), 3512–3520, 1999. 36. Kapdan, I.K., Kargia, F., McMullan, G., Marchant, R., Effect of environmental conditions on biological decolorization of textile dyestuff by C. versicolor. Enzyme Microb. Technol., 26(5-6), 381–387, 2000. 37. Fu, Y., Viraraghavan, T., Removal of a dye from an aqueous solution by the fungus Aspergillus niger. Water Qual. Res. J. Canada, 35, 95, 2000. 38. Arıca, M.Y., Bayramoğlu, G., Biosorption of Reactive Red-120 dye from aqueous solution by native and modified fungus biomass preparations of Lentinus sajor-caju. J. Hazard. Mater., 149(2), 499–507, 2007. 39. Aksu, Z., Çağatay, Şeyda Şen., Çağatay, Ş.Ş., Investigation of biosorption of Gemazol Turquise Blue-G reactive dye by dried Rhizopus arrhizus in batch and continuous systems. Sep. Purif. Technol., 48(1), 24–35, 2006. 40. Zhou, J.L., Banks, C.J., Mechanism of humic acid colour removal from natural waters by fungal biomass biosorption. Chemosphere, 27(4), 607–620, 1993. 41. Anderson, J.G., Immobilized cell and film reactor system for filamentous fungi in Fungal technology. Smith, J. E., Berry, D. R., Kristiansen, B., (Eds.). The filamentous fungi. London, Edward Arnold. pp. 146–170, 1975. 42. Kasinath, A., Novotný, Čeněk., Svobodová, K., Patel, K.C., Šašek, V., Decolorization of synthetic dyes by Irpex lacteus in liquid cultures and packed-bed bioreactor. Enzyme Microb. Technol., 32(1), 167–173, 2003. 43. Ramsay, J.A., Goode, C., Decoloration of a carpet dye effluent using Trametes versicolor. Biotechnol. Lett., 26(3), 197–201, 2004. 44. Rodríguez Couto, S., Domínguez, A., Sanromán, M.A., Production of manganese-dependent peroxidase in a new solid-state bioreactor by Phanerochaete chrysosporium grown on wood shavings. Application to the decolorization of synthetic dyes. Folia Microbiol. (Praha)., 47(4), 417– 421, 2002. 45. Rodríguez Couto, S., Sanromán, M.A., Hofer, D., Gübitz, G.M., Production of laccase by Trametes hirsuta grown in an immersion bioreactor and its application in the decolorization of dyes from a leather factory. Eng. Life Sci., 233, 2004, 2004. 46. Mohorčič, M., Friedrich, J. and Pavkob, A., Decoloration of the diazo dye reactive black 5 by immobilised Bjerkandera adusta in a stirred tank bioreactor. Acta Chim. Slov., 51, 619, 2004. 47. Couto, S.M.R., Braga, M.Á.S., Continuous decolorisation of a leather azo dye by Trametes hirsuta. Afinidad: Revista de química teórica y aplicada, 61, 460, 2004. 48. Rodríguez Couto, S., Sanromán, M.A., Hofer, D., Gübitz, G.M., Stainless steel sponge: a novel carrier for the immobilisation of the white-rot fungus Trametes hirsuta for decolorization of textile dyes. Bioresour. Technol., 95(1), 67–72, 2004.

Role of Microbes in the Bioremediation of Toxic Dyes 471 49. Kamida, H.M., Durrant, L.R., Monteiro, R.T.R., Armas, E.D.D., Biodegradação de efluente têxtil por Pleurotus sajor-caju. Quím. Nova, 28(4), 629–632, 2005. 50. Ramsay, J.A., Mok, W.H.W., Luu, Y.S., Savage, M., Decoloration of textile dyes by alginate-immobilized Trametes versicolor. Chemosphere, 61(7), 956–964, 2005. 51. Tychanowicz, G.K., Zilly, A., de Souza, C.G.M., Peralta, R.M., Decolorisation of industrial dyes by solid-state cultures of Pleurotus pulmonarius. Process Biochemistry, 39(7), 855–859, 2004. 52. Rodríguez Couto, S., Domínguez, A., Sanromán, A., Utilisation of lignocellulosic wastes for lignin peroxidase production by semi-solid-state cultures of Phanerochaete chrysosporium. Biodegradation, 12(5), 283–289, 2001. 53. Pazarlioglu, N.K., Urek, R.O., Ergun, F., Biodecolorization of Direct Blue 15 by immobilized Phanerochaete chrysosporium. Process Biochemistry, 40(5), 1923–1929, 2005. 54. Abbas, S.H., Ismail, I.M., Mostafa, T.M., Sulaymon, A.H., Biosorption of heavy metals: a review. J. Chem. Sci. Technol., 3, 74, 2014. 55. Forgacs, E., Cserháti, T., Oros, G., Removal of synthetic dyes from wastewaters: a review. Environ. Int., 30(7), 953–971, 2004. 56. Won, S.W., Yun, H.J., Yun, Y.S., Effect of pH on the binding mechanisms in biosorption of Reactive Orange 16 by Corynebacterium glutamicum. J. Colloid Interface Sci., 331(1), 83–89, 2009. 57. Singh, K., Arora, S., Removal of synthetic textile dyes from wastewaters: a critical review on present treatment technologies. Crit. Rev. Environ. Sci. Technol., 41(9), 807–878, 2011. 58. Fulekar, M.H., Wadgaonkar, S.L., Singh, A., Decolorization of dye compounds by selected bacterial strains isolated from dyestuff industrial area. Int. J. Adv. Res. Technol., 2, 182, 2013. 59. Adedayo, O., Javadpour, S., Taylor, C., Anderson, W.A., Moo-Young, M., Decolorization and detoxification of methyl red by aerobic bacteria from a wastewater treatment plant. World J.Microbiol. Biotechnol., 20(6), 545–550, 2004. 60. Imperi, F., Caneva, G., Cancellieri, L., Ricci, M.A., Sodo, A., Visca, P., The bacterial aetiology of rosy discoloration of ancient wall paintings. Environ. Microbiol., 9(11), 2894–2902, 2007. 61. Brás, R., Ferra, M.I.A., Pinheiro, H.M., Cabral Gonçalves, I., Batch tests for assessing decolorisation of azo dyes by methanogenic and mixed cultures. J. Biotechnol., 89(2–3), 155–162, 2001. 62. Chen, B.-Y., Wang, Y.-M., Yen, C.-Y., Lin, S.-H., Deciphering cost-effective biostimulation for dye-laden wastewater treatment using immobilized cell system. J. Taiwan Inst. Chem. Eng., 42(2), 334–340, 2011. 63. Mohan, S.V., Karthikeyan, J., Removal of Diazo dye from aqueous phase by algae Spirogyra species. Toxicol. Environ. Chem., 74(3-4), 147–154, 2000.

15 Valorization of Wastes for the Remediation of Toxicants from Industrial Wastewater Shumaila Kiran1,*, Tahsin Gulzar1, Sarosh Iqbal1, Noman Habib2, Atya Hassan3 and Saba Naz1 1

Department of Applied Chemistry, Government College University, Faisalabad, Pakistan 2 Department of Botany, Government College University, Faisalabad, Pakistan 3 Department of Chemistry, Federal Urdu University of Arts, Science and Technology, Gulshan-e-Iqbal, Karachi, Pakistan

Abstract Swift urbanization and industrial development across the world has modified the nature of solid, liquid and gaseous wastes which is leading to the adulteration of soils, all types of water and air with harmful and noxious chemicals. The increased wastewater generation and its disposal into main water bodies is mainly responsible for environmental disturbance. In general, the technologies for safe discarding of wastes are either less efficient or more energy exhaustive. In the 20th century, the major task for chemical and allied industries is develop greener and more sustainable methods which can proficiently use raw materials for the elimination of wastes, avoiding the usage of lethal and harmful materials. It needs a paradigm modification from old-style ideas of process competence to one that counts economic value, swapping non-renewable resources with renewable ones. Valorization involves recycling of waste into products having economic value and non-toxic nature. It is one of the recent research domains attracting a lot of attention as a possible substitute to the dumping of or converting an extensive variety of remainders into useful products. This chapter emphasizes the use of a variety of natural waste materials for environmental remediation especially focusing on removal of heavy metals and dyes. Keywords: Wastewater, dyes, heavy metals, remediation, valorization, natural materials, green approach

*Corresponding author: [email protected] Shahid-ul-Islam (ed.) Integrating Green Chemistry and Sustainable Engineering, (473–526) © 2019 Scrivener Publishing LLC

473

474 Integrating Green Chemistry and Sustainable Engineering

15.1

Introduction

Wastewater includes water coming from baths, showers, toilets, laundries, kitchen sinks, and industrial processes. On the average, 200–300 L of wastewater per person produces at domestic level every day. 99% of this wastewater consists of water while the other 1% is the polluting waste. During the past decades, the disposal of extremely contaminated wastewater is increasing due to different activities such as agricultural, industrialization and urbanization practices [1]. The industrial wastewater analysis characterized high rate of biological oxygen demand (BOD), chemical oxygen demand (COD), phenolics, total dissolved solids (TDS), suspended solids (SS), colour etc. [2]. Much of the wastewater has been changed so that it cannot be used again unless it is treated. The changes that turn the water into wastewater are: 1. 2. 3. 4. 5.

Cooling or warming it Adding human discharge Adding organic stuff such as food wastes Adding oil, fat or grease Adding toxins such as pesticides, heavy metals, certain organic compounds and synthetic chemicals

Installation of heavy industries has significantly increased toxic materials in the water bodies which has affected our environment adversely. Wastewaters coming from various industries having notorious materials must be treated keeping in view the instructions laid down by the government [3]. Adsorption is an active treatment in industrial practices. In the latest times, it has come to know the use of various adsorbents for the removal of toxicants and metalloids from industrial wastes. The exploitation of cheaper adsorbents licenses the reuse of waste things, thus counting a conservational benefit to this method [4]. Civic apprehension to the protection of the environment urges for novel globally competent affordable tactics for cleaning polluted spots [5]. Many environmental friendly methods have originated owing to the shortcomings of the chemical methods. Phytoremediation skill has grown consideration as a natural remediation mean for removing contaminants [6]. Many literatures have shown that an extensive variety of normally rejected leftover e.g. eggshells, bones, peat, fungi, seaweed, fruit peels, yeast etc. can proficiently eradicate lethal heavy metals from polluted water [7]. Adsorbing biomass can also take away many detrimental metals like arsenic (As), lead (Pb), cadmium (Cd)

Remediation of Toxicants from Industrial Wastewater 475 cobalt (Co), chromium (Cr) etc. [8]. Biosorption may be used as an environmentally friendly filtering technique. In 1970, scientists observed the chelating properties in dead biomass. It turned the attention of researchers from bioaccumulation to biosorption [9]. Treatment of industrial wastewater using the low-cost waste material would be a green line. It may keep the surrounding clean by minimizing the land pollution by using the waste generated by anthropogenic sources [10].

15.2

Toxicants Present in Industrial Waste Water

15.2.1

Heavy Metals

The “heavy metals” are those metals generally, which has relatively higher atomic mass or relatively higher density. This term is mutually referred to a group of metals having density more than 5gcm-3 and atomic number more than 20 [11]. The electroplating, battery productions, paper, painting, pigments, photographic materials, development of explosive, fuels and metal using industries liberate huge quantities of heavy metals [12]. These are the main contaminants in environment because of their noxiousness and hazardous nature to all creatures including humans at higher concentrations as they are non-biodegradable. These characteristics make them highly persistent in the environment. The heavy metals which primarily release from industries are cadmium (Cd), iron (Fe), chromium (Cr), copper (Cu), zinc (Zn), nickel (Ni), cobalt (Co), mercury (Hg), lead (Pb), selenium (Se), arsenic (As) and vanadium (V). These can cause detrimental health effects, such as gastrointestinal disorders, diarrhea, vomiting, stomatitis, hemoglobinuria, tumors, ataxia, paralysis, and convulsions. As every heavy metal reveals a specific toxicity profile, the operative method for heavy metals exclusion from wastewater is essential [13]. A brief description of these heavy metals is given below:

15.2.1.1 Chromium (Cr) Chromium is the seventh most abundant component found in the earth crust. It occurs in the form of ore, which made up of crocoite (PbCrO4), ferric chromite (FeCr2O4) and chrome ochre (Cr2O3). The main industrial sources for this heavy metal are tanning industries, leather industries, textile industry, electroplating industries etc. The waste products coming from these industries contain chromium in the form of Cr (III) and Cr

476 Integrating Green Chemistry and Sustainable Engineering (VI) [3]. It causes kidney and liver damage, skin inflammation, nausea, pulmonary congestion, and ulcer. It must be removed from the wastewater, or essential to transform into the less harmful forms due to its detrimental health effects [14].

15.2.1.2

Cadmium (Cd)

It originates from natural sources, industrial operations, anthropogenic sources, mining, and domestic wastes into the water. It is also one of the highly toxic heavy metals present in industrial waste stream. Its role is very important in many industries such as cadmium−nickel battery, plating, stabilizers, alloys and fertilizers. Its presence even in lower amount is very unfavorable and gets concentrated in the ecosystem. The world health organization (WHO) has set its maximum extent of 0.003 mg/L in waste stream [15]. “Itai-Itai” is a disease spread due to cadmium accumulation in the water bodies which leads the fractures and bone softening to human beings. Furthermore, it also brings out the negative effects such as lung cancer, hepatic toxicity and detrimental to the kidney, respiratory system, reproductive organs and liver [16]. Hence, the efficient and economically effective treatment is vital to eradicate the cadmium from wastewater.

15.2.1.3

Iron (Fe)

Iron is found in the earth‘s crust abundantly. It exists in soluble form in water naturally. It also comes from many industries like iron and steel industry, mining etc. Its presence in drinking water supplies creates issues like odor and reddish color. Its major issues include clogging, taste, corrosion and visual effects [17]. Hence, the efficient and economically effective treatment is vital to eradicate iron from wastewater.

15.2.1.4 Nickel (Ni) It is a hard metal having atomic number 28. It is non-biodegradable in waste stream. Its industrial sources are silver refineries, battery development electroplating, alloy industries etc. [18]. It has found its various uses like coins, catalysis, alloys, jewelry, resistance wires, batteries, parts of machinery etc. The massive utilization of nickel is detrimental to environment. It can cause diseases like cancer and insomnia [19].

Remediation of Toxicants from Industrial Wastewater 477

15.2.1.5 Lead (Pb) Lead is a detrimental metal that gets accumulated in the human body. It is soft and heavy metal that occurs in the form of cerussite (PbCl2), galena and sulfide. The lead-acid battery wastewater is the major cause of its association in the industrial discharge. Its sources in wastewaters include electrical industries, explosive manufacturers, electroplating industries, steel manufacturing etc. [20]. Lead can cause kidney and nervous system disorders, mental retardation and cancer to human body [21].

15.2.1.6

Copper (Cu)

Copper is usually considered a highly detrimental metal when present in higher concentration. It is one of the vital element required by humans and has an essential role in tissues, synthesis of enzyme and bone development [22]. It exists in different forms like elemental form (Cu0), cuprous ion (Cu+) and cupric ion (Cu2+). Cupric ion is highly lethal element in environment [23]. The main sources of copper are metallurgy, mining, electroplating, printing circuit, chemical manufacturing and steel industries, fertilizers, paints etc. [24]. The effects of copper in wastewater include anemia, hair loss, headache and kidney damage [25].

15.2.1.7

Zinc (Zn)

Zinc is an essential metal that maintains the biochemical mechanisms and physiological processes occurring in living tissues. It behaves as a decorative and protective film for the other metals [26]. For example, steel corrosion is avoided by adding zinc in to it. It is used in different industrial operations such as mining, combustion of coal and processing of steel [27]. Though it is required in trace amount to human body but its excess may cause health issues, for example, vomiting, fever, pain, skin inflammation, anemia [28].

15.2.1.8

Mercury (Hg)

It is extremely poisonous heavy metal found in wastewater stream. Mercury poisoning incident took place in Japan, raising awareness around the worldwide about its pollution [29]. It occurs in different forms such as Hg0 (elemental mercury), Hg22+ (mercurous ion) and Hg2+ (mercuric ion) [30]. The mercury metal transfers in water bodies and assembles in ecosystem [31]. Because of its toxicity, it causes numerous environmental issues.

478 Integrating Green Chemistry and Sustainable Engineering To reserve environment and human health from mercury, Minamata convention was designed in 2013 [32]. This convention regulated the mercury containing products and carries out stronger emission standards. For example, methyl mercury, a mercury compound harms the enzyme sites and disturbs the production of protein [33].

15.2.1.9

Selenium (Se)

Selenium is a fascinating element because of its requirement to maintain human health but it is highly toxic at higher concentrations. Though it exists in earth’s crust in traces, and it enters into the environment by natural as well as by anthropogenic means [34]. Excessive selenium can be the reason of eyes and skin irritation, monstrous deformities and neurological damage, pulmonary edema, liver cirrhosis, and death. It is also categorized as teratogenic and mutagenic because of its toxicity [35]. It also causes harmful health issues like other heavy metals [36].

15.2.1.10

Arsenic (As)

Arsenic is a relatively plentiful element in the earth’s crust and sea water. It exists in metalloid, organic and inorganic forms. Generally, the concentrations of arsenic compounds depend on redox potential and pH of water. Arsenic (III) is a major specie found under reducing situation whereas Arsenic (V) in oxidizing condition [37]. Inorganic arsenic compounds include oxides, hydrides, sulfides, halides and acids. It is a lethal poison accounts numerous types of cancers, if present in water. Moreover, other hurtful effects comprise discoloration and thickening of skin, blindness, nausea, diarrhea, stomach pain, paralysis, numbness in hands and feet [38]. The above discussed heavy metals assemble at any time and flocked faster as compare to their emission and extremely lethal to human beings. A small amount of heavy metals is toxic to water bodies and they are reluctant to degradation naturally. Hereafter, researchers are greatly concentrated on the removal of heavy metals from the waste stream.

15.2.2

Dyes

Dyes are the synthetic chemicals which are used extensively for dyeing and printing operations in textile industry. The overall production of dyes in worldwide is about 7 × 105 metric tons annually, of which 5 - 10 % is discharged in sewages. The textile and dyeing sectors apply large quantity of different dyes, hence release the unused dyestuffs into the environment

Remediation of Toxicants from Industrial Wastewater 479 without any prior treatment. Being lethal and notorious in nature for biota, it is an eager need for their removal before getting disposed off [39]. Synthetic dyes are low biodegradable because of their obstinate nature, is an actual threat to the water bodies [40]. Their collection in environment causes numerous effects like severe and long-lasting toxicity for aquatic biota. Moreover, their mutagenic and carcinogenic characteristics are responsible for serious damages to human health [41, 42]. There are different methods available for the removal of dyes, which one is effective, it depends on the nature of the wastewater [43].

15.3

Waste Valorization

Waste volarization is the process in which we take waste materials and change in into something that can be utilized and have value beyond the cost and energy needed to process the transformation. Various activities including in valorization of wastes are 1. 2. 3. 4. 5.

Processing of by-products or residues into raw materials Use of waste in manufacturing process stages Discarding of finished or semi-finished products Use as raw materials or as sources of energy Addition of waste stuff into finished products [44]

Industrial wastewater can be reused after proper treatment. There are number of possible ways to detoxify the industrial wastewater such as; floatation or filtration, centrifugation, sedimentation, coagulation/flocculation [45], chemical precipitation, membrane processes such as reverse osmosis, ultrafiltration, Electrodialysis, ion exchange and adsorption [46]. These processes have drawbacks like incomplete removal, require high energy, less cost effective and toxic sludge production at the end [47]. Here our focus is to discuss about the use of natural wastes for the treatment of waste water comprising of heavy metals and dyes via adsorption.

480 Integrating Green Chemistry and Sustainable Engineering

15.3.1

What Is Adsorption?

Adsorption is a key method involved in physiochemical treatment of the waste stream. It is one of the separation technique in which constituents of the liquefied phase move toward the surface of solid adsorbents. The particles from the bulk of the liquefied phase have ability to assemble at the solid surface, when a solid surface is interacting to liquid phase. Separation happens due to the difference between molecular weight, polarity and shape, which cause some particles or molecules to be attached very firmly on the adsorbent’s surface as compared to others. The pores found on adsorbent’s surface are too little to hold the bigger particles [48].

15.3.1.1

Mechanism of Adsorption

Overall, the main stages in adsorption of contaminants on solid surface of adsorbent are following: 1. Metal ions transfer from bulk solution to the outer solid surface of adsorbent 2. Transfer of internal mass by pore diffusion from external solid surface of adsorbent to the internal surface of porous structure 3. Adsorbate adsorb onto the pores of adsorbent through active sites 4. Generally, the overall adsorption rate is monitored either by intra particle diffusion or film development. Both last stages of adsorption are extremely fast than the other two [49]

15.3.1.2 Types of Adsorption At molecular level, adsorption occurs because of the attractive chemical forces in adsorbent’s solid surface and in particles being adsorbed. There are two types of adsorption which are given below: 1. Physical adsorption: It is due to the intermolecular attractive forces in between adsorbate and adsorbent’s molecules. The attractive forces in molecules that hold the adsorbent on surface are quite physical, known as Vander walls forces. This is a reverse process. The order of magnitude of bonding energy between the adsorbent and adsorbate has the same as, but is mostly larger as compared to the condensation

Remediation of Toxicants from Industrial Wastewater 481 Classification according to the location where biosorption occur

Biosorption mechanism

Intra-cellular accumulation

Cell Surface Adsorption/precipitation

Extra-cellular accumulation/precipitation

Transport across cell membrane

Precipitation

Ion Physical Complexation exchange adosrption

Figure 15.1 General mechanism of Bio-sorption.

energy of the adsorptive. So, no need of activation energy is there [50]. 2. Chemical adsorption: It is because of the chemical bonding solid surface and adsorbed constituent. It is known as activated adsorption. It may be endothermic or exothermic process extending from minor to a greater magnitude. The basic period in chemisorption often comprises greater activation energy [51]. 3. Biosorption: The heavy metals biosorption is a complicated process. Different metal-binding mechanisms for example, physical adsorption, chemisorption (includes ion exchange, complexation, chelation or coordination), micro precipitation and reduction/oxidation may exist throughout the process (Figure  15.1). Many of these mechanisms within the process are instantaneously involve up to a certain limit. To understand the complete biosorption process, the physical and chemical properties of the used biosorbent must be acknowledged [52].

Following are the Advantages of Biosorption: Low cost Simple design Greater efficiency Reduction of biochemical and chemical sludge No need of additional nutrient Biosorbent regeneration [53]

482 Integrating Green Chemistry and Sustainable Engineering Table 15.1 Types of Wastes involved in environmental remediation. Natural materials Egg shells Fish scale Peat Coal Clay

15.3.2

Agriculture waste

Biomass

Industrial waste

Tea Waste Coffee Grounds Coconut Shell Sawdust Activated Carbon Rice Husk Neem Leaves and Bark Fruit Peels Sugarcane bagasse

Lignin Bacteria Fungi

Fly ash Red Mud Dust

Types of Wastes

There are different types of low cost wastes which can be used for the remediation of wastewater. These include (Table 15.1): 15.3.2.1 Natural materials 15.3.2.2 Agriculture waste materials [54, 55] 15.3.2.3 Biomass 15.3.2.4 Industrial waste

15.3.2.1 Natural Materials Egg Shells Egg shells are waste materials which can be freely collected in plenty from households, hatcheries and fast food industries. It is about 11% mass of the total egg weight and comprises of about 95% calcium carbonate [56] and 5% of other constituents like SiO3, Al2O3, Cr2O3, S, Cl, and MnO3 [Table 15.2]. The by-product eggshell is, thus, certainly made up by the combination of calcium carbonate (egg shell) and egg shell membrane (ESM). There are two kinds of shell membranes present around the egg; a thinner one (an internal layer) and a thicker one (an external film) adjacent to the shell. Both membranes made up of protein fibers that organized for the formation of a semi-permeable membrane, making a complex lattice web that is stable and water insoluble. It has greater surface area consequential in numerous applications like adsorbent and support immobilized

Remediation of Toxicants from Industrial Wastewater 483 Table 15.2 Chemical Composition of Egg Shells. Contents

White eggshell powder

Brown eggshell powder

Moisture (%)

0.46

0.20

Protein (%)

3.92

5.04

Ash (%)

94.61

94.28

Fat (%)

0.35

0.08

Calcium (%)

34.12

33.13

Magnesium (%)

0.29

0.36

Phosphorous (%)

0.04

0.07

Potassium (%)

0.03

0.04

Sodium (%)

0.05

0.04

Copper (ppm)