Emerging Carbon-Based Nanocomposites for Environmental Applications [1 ed.] 9781119554851

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Emerging Carbon-Based Nanocomposites for Environmental Applications

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

Emerging Carbon-Based Nanocomposites for Environmental Applications

Edited by

Ajay Kumar Mishra, Chaudhery Mustansar Hussain and Shivani Bhardwaj Mishra

This edition first published 2020 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 © 2020 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 rep­ resentations 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 merchant-­ ability or fitness for a particular purpose. No warranty may be created or extended by sales representa­ tives, 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 informa­ tion does not mean that the publisher and authors endorse the information or services the organiza­ tion, 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 9781119554851 Cover image: Pixabay.Com Cover design by Russell Richardson Set in size of 11pt and Minion Pro by Manila Typesetting Company, Makati, Philippines Printed in the USA 10 9 8 7 6 5 4 3 2 1

Contents Preface xi 1 Emerging Carbon-Based Nanocomposites for Remediation of Heavy Metals and Organic Pollutants from Wastewater 1 Prasenjit Kar, Pratyush Jain, Raju Kumar Gupta and Kumud Malika Tripathi 1.1 Introduction 2 1.2 Graphene Oxide 5 1.2.1 GO and GO-Nanocomposite for Water Remediation via Adsorption 6 1.3 Carbon Nanotube 15 1.3.1 CNTs as Adsorbent 16 1.4 Conclusion 19 Acknowledgements 20 References 20 2 Functional Green Carbon Nanocomposites for Heavy Metal Treatment in Water: Advance Removal Techniques and Practices Sandip Mandal, Sangeeta Adhikari, Pu Shengyan, Ajay Kumar Mishra and R.K Patel 2.1 Introduction 2.2 Water Contamination by Heavy Metals 2.3 Functional Green Carbon Nanocomposites 2.4 Advanced Removal Techniques in Water 2.4.1 Sedimentation 2.4.2 Chemical Coagulation/Flocculation 2.4.3 Chemical Oxidation/Reduction 2.4.4 Ion-Exchange Process 2.4.5 Adsorption 2.5 Conclusion and Future Directions References

31 32 33 34 36 36 38 39 40 42 48 48 v

vi  Contents 3 Green Nanocomposites: Advances and Applications in Environmentally Friendly Carbon Nanomaterials Naveen Bunekar and Tsung Yen Tsai 3.1 Introduction 3.2 Nanocomposites and their Processing Methods 3.3 Structures of Carbon Materials 3.4 Polymer/Carbon-Based Nanocomposite 3.5 Removal of Chemical Contaminants 3.6 Energy Sector 3.7 Gas Sensors 3.8 Conclusion and Outlook Acknowledgment References 4 Carbon-Based Nanocomposites as Heterogeneous Catalysts for Organic Reactions in Environment Friendly Solvents Priyanka Choudhary, Ajay Kumar, Ashish Bahuguna and Venkata Krishnan 4.1 Introduction 4.2 Carbon-Based Nanocomposites for Coupling Reactions 4.2.1 C-C Coupling 4.2.2 C-N Coupling 4.3 Carbon-Based Nanocomposites for Oxidation Reactions 4.3.1 Oxidation of Alcohols to Aldehydes/Ketones/Acids 4.3.2 Oxidation of Amines to Imines 4.3.3 Oxidation of Other Functional Groups 4.4 Carbon-Based Nanocomposites for Reduction Reactions 4.4.1 Reduction of Nitro Compounds 4.4.2 CO2 Reduction 4.4.3 Hydrogenation Reactions 4.5 Carbon-Based Nanocomposites for Other Organic Transformation Reactions 4.5.1 Aza-Michael Addition 4.5.2 Tandem Reaction 4.5.3 Esterification Reaction 4.5.4 Synthesis of Amides From Alcohols 4.6 Conclusion and Perspectives References

55 55 57 58 58 60 63 64 65 65 65 71 72 74 74 77 80 80 85 85 90 90 94 97 100 100 108 108 110 113 114

Contents  vii 5 Carbon-Based Polymer Nanocomposite and Environmental Perspective 121 Sukanchan Palit and Chaudhery Mustansar Hussain 5.1 Introduction 122 5.2 The Vision of the Study 122 5.3 The Vast Scientific Doctrine of Carbon-Based Polymer Nanocomposites 123 5.4 Environmental Sustainability and the Vision for the Future 124 5.5 Environmental Protection, the Scientific Ingenuity, 124 and the Visionary Future 5.6 Recent Advances in the Field of Nanocomposites 125 5.7 Recent Advances in the Field of Carbon-Based Polymer 129 Nanocomposites and Environmental Pollution Control 5.8 Carbon-Based Polymer Nano-Composites 133 for Adsorbent Applications 5.9 Carbon-Based Polymer Nano-Composites 135 as Anti-Microbial Agents and Membranes 5.10 Applications of Carbon Nanocomposites in Removal of Hazardous Organic Substances 136 5.11 Water Purification, Groundwater Remediation, 137 and the Future of Science 5.12 Arsenic and Heavy Metal Groundwater Remediation and Composite Science 138 5.13 Integrated Water Resource Management, Human Factor 138 Engineering, and Nanotechnology—A Definite Vision 5.14 Technology Management, Environmental Protection, 139 and Water Resource Management 5.15 Future of Nanocomposite Applications 140 and Future Research Trends 5.16 Conclusion, Summary, and Vast Scientific Perspectives 141 References 142 Important Websites for Reference 144 6 Biochar-Based Adsorbents for the Removal of Organic Pollutants from Aqueous Systems Nhamo Chaukura, Thato M Masilompane, Willis Gwenzi and Ajay K. Mishra 6.1 Introduction 6.2 Biosorbents 6.2.1 Raw Biomass 6.2.2 Activated/Synthetic Biomaterials

147 148 149 150 152

viii  Contents 6.3 Biochar Production Techniques 6.4 Application of Biosorbents for the Sequestration of Selected Organic Pollutants 6.4.1 Sequestration of Endocrine Disrupting Compounds and Pharmaceuticals 6.4.2 Removal of Dyes 6.4.3 Removal of Polycyclic Aromatic Hydrocarbons 6.5 Removal Mechanisms 6.6 Challenges Associated With Biochar Technology 6.7 Conclusion 6.8 Future Scenario References

155 156 156 157 157 163 164 164 165 165

7 Advances in Carbon Nanomaterial-Based Green Nanocomposites 175 Ambika and Pradeep Pratap Singh 175 7.1 Introduction 176 7.2 Carbon Nanomaterial-Based Green Nanocomposites 7.2.1 CNT-Filled Green Nanocomposites 176 7.2.2 Graphene and Its Derivative Filler-Based Nanocomposites 177 7.2.3 Nanodiamond-Filled Green Nanocomposite 177 7.3 Methods of Processing for Carbon-Based Nanocomposites 178 7.3.1 Melt Intercalation 178 7.3.2 Exfoliation Adsorption 178 7.3.3 Emulsion Polymerization 178 7.3.4 In Situ Polymerization 178 178 7.3.5 Template Synthesis (Sol-Gel Technology) 179 7.3.6 Green Methods 7.4 Unique Properties of Carbon-Based Green Nanocomposites 179 7.4.1 Size and Structure 179 7.4.2 Thermal and Mechanical Properties 180 7.4.3 Electrical Properties 182 7.5 Applications of Carbon-Based Green Nanocomposites 182 7.5.1 Wastewater Treatment 183 184 7.5.2 Packaging and Coating 7.5.3 Sensing and Detection 185 186 7.5.4 As Catalyst 7.5.5 Biomedical Applications 187 7.5.6 Miscellaneous 188

Contents  ix 7.6 Future Prospects 7.7 Conclusions References

188 189 190

8 Removal of Trihalomethanes from Water Using Nanofiltration Membranes 203 Feyisayo Victoria Adams and Peter Apata Olubambi 204 8.1 Introduction 8.2 Factors Influencing the Removal of THMs From Water 204 8.2.1 Effects of Other Contaminants on Formation 205 and Removal of THMs 8.2.2 Effects of Transmembrane Pressures, Fluxes, and Feed Concentrations 206 8.2.3 Effect of THMs Adsorption on Rejection 206 8.2.4 Effect of Membrane Materials 207 8.3 Summary and Outlook 208 References 209 9 Nanocomposite Materials as Electrode Materials in Microbial Fuel Cells for the Removal of Water Pollutants Akil Ahmad, Asma Khatoon, Mohammad Faisal Umar, Syed Zaghum Abbas and Mohd Rafatullah 9.1 Introduction 9.2 Microbial Fuel Cells: An Emerging Wastewater Treatment and Power Technology 9.3 Pollutants Removal Using MFCs System 9.3.1 Metal Removal Using MFCs System 9.3.2 Organic Pollutants Using MFCs System 9.4 Conclusion and Outlook Acknowledgement References 10 Plasmonic Smart Nanosensors for the Determination of Environmental Pollutants Yeşeren Saylan, Fatma Yılmaz, Erdoğan Özgür, Ali Derazshamshir and Adil Denizli 10.1 Introduction 10.2 Principle of Plasmonic Nanosensors 10.3 Applications of Plasmonic Nanomaterials in Sensing 10.3.1 Recognition Molecules 10.3.1.1 Enzymes 10.3.1.2 Antibodies

213 213 215 215 216 224 227 228 228 237 238 239 241 242 242 243

x  Contents 243 10.3.1.3 Aptamers 10.3.1.4 DNAzymes 246 10.3.1.5 Whole Cells 246 10.3.2 Quantum Dots 248 10.3.3 Gold Nanoparticles 249 10.3.4 Graphene and Graphene Oxide 253 10.4 Plasmonic Nanosensors 254 254 10.4.1 Evanescent Wave Fiber Nanosensors 10.4.2 SPR Nanosensors 255 10.4.3 SERS and LSPR-Based Optical Nanosensors 261 10.5 Plasmonic Nanosensors for Pollution Control and Early Warning 264 265 10.6 Conclusion, Key Trends and Perspectives References 266

Index 281

Preface The advances in science and technology is tackling a vital concern to both humans and the environment: Water contamination is one of the foremost and extreme alarming problems that demands effective treatment. Environmental contamination has emerged as a most exigent problem. Hybrid materials possess higher specific stiffness and strength, toughness, corrosion resistance, low density, and thermal insulation. Carbon-based nanocomposites have been widely studied due to the small size of fillers increases the interfacial area as compared to conventional composites. Carbon is an excellent host material for the nanoparticles and semiconductor. Carbon nanostructures such as carbon nanotubes, fullerene, and graphene with prominent electrical and structural characteristics have drawn much attention, as they contribute to the development of composites with improved catalytic performance. Carbon-based material such as graphene-composite have been synthesized and explored as it has great potential for the removal of noxious pollutants from wastewater. Much attention has been focussed recently using carbon-based nanocomposites due to the application of magnetic separation technology to solve the various environmental problems. Carbon-based nanocomposites have unique magnetic separability and potential adsorption for pollutants. Carbon matrices avoid the agglomeration of iron oxide nanoparticles, and magnetic separation is a substitute to filtration or centrifugation as it avoids the loss of materials. Magnetic separation has been considered to be a quick and operative technique. Functionalized magnetic nanocomposites have distinct advantages over conventional materials due to their selective absorptivity, strong magnetic responsiveness, favorable water dispensability, and benign biocompatibility. Carbon-based nanocomposites have therefore revealed high ability for the catalytic degradation of contaminants from aqueous segment. The application of photocatalysis using heterogeneous semiconductor is inexpensive, non-toxic, broad absorption spectra with higher absorption coefficients and capability for multi-electron transfer. xi

xii  Preface The current book provides the comprehensive summary of the development and advancements based on emerging carbon-based nanocomposites for wastewater applications. It consists of 10 chapters. Chapter 1 provides details about emerging carbon-based nanocomposites for remediation of heavy metals and organic pollutants from wastewater, whereas Chapter 2 is focused on functional green carbon nanocomposites for heavy-metal treatment in water: advance removal techniques and practices. Chapter 3 summarizes green nanocomposites: advances and applications in ­environmentally-friendly carbon nanomaterials, whereas Chapter 4 discusses the carbon-based nanocomposites as heterogeneous catalysts for organic reactions in environment-friendly solvents. Chapter 5 consists of carbon-based polymer nanocomposite and environmental perspective. Chapter 6 presents biochar-based adsorbents for the removal of organic pollutants from aqueous systems, whereas Chapter 7 details advances in carbon nanomaterial-based green nanocomposites. Chapter  8 describes the removal of trihalomethanes from water using nanofiltration membranes. Chapter 9 details the nanocomposite materials as electrode materials in microbial fuel cells for the removal of water pollutants, whereas Chapter 10 describes the plasmonic smart nanosensors for the determination of environmental pollutants. The book elucidates the scientific advancements and recent scientific development in the field of emerging carbon-based nanocomposites. Researchers involved in nanomaterials, environmental science, and water research will be the major beneficiaries of the book. The book will be highly beneficial to students who are working for their graduate and postgraduate degrees in this area. Ajay Kumar Mishra, Chaudhery Mustansar Hussain and Shivani Bhardwaj Mishra Editors July 2020

1 Emerging Carbon-Based Nanocomposites for Remediation of Heavy Metals and Organic Pollutants from Wastewater Prasenjit Kar1, Pratyush Jain1, Raju Kumar Gupta1,2* and Kumud Malika Tripathi3† Department of Chemical Engineering, Indian Institute of Technology Kanpur, Uttar Pradesh, India 2 Center for Environmental Science and Engineering, Indian Institute of Technology Kanpur, Uttar Pradesh, India 3 Department of Bionanotechnology, Gachon University, Seongnam-daero, Sujeong-gu, Seongnam-si, Gyeonggi-do, Republic of Korea 1

Abstract

Nano-carbons are emerging as promising materials for environmental remediation applications. Unique and extraordinary optical, electrical, and surface properties of nano-carbons in all of their forms have substantially and successfully been investigated widely for water remediation for the removal of diverse range of contaminants. Furthermore, revolutions in synthesis of nano-carbons–based composites led to the facile and fast adsorbent technologies for remediation applications. Ease in synthesis along with a wide scope to engineer the surface structure, porosity, electronic and magnetic properties of nano-carbons is the most significant criteria to the rational design of diverse adsorbent. In this chapter, carbon-based nanomaterials, especially graphene oxide (GO) and carbon nanotubes (CNTs) have been explored for their water remediation capabilities via adsorption process. Mechanistic insights and the interactions responsible for the adsorption are highlighted. Structural engineering of these materials into more suitable form for handling purposes has shown their practical application for treatment of water resources. Dyes, toxic metals, and other organic pollutants have been described for their removal via adsorption through GO and CNTs. Amphiphilic nature of GO helps it to remove pollutants from water easily where most of the adsorption occurs due to the electrostatic interactions between functional *Corresponding author: [email protected] † Corresponding author: [email protected] Ajay Kumar Mishra, Chaudhery Mustansar Hussain and Shivani Bhardwaj Mishra (eds.) Emerging Carbon-Based Nanocomposites for Environmental Applications, (1–30) © 2020 Scrivener Publishing LLC

1

2  Emerging Carbon-Based Nanocomposites group on GO and pollutants present. CNTs have shown very high specific area with tunable functionality, which can be used for removing polar and non-polar pollutants. Keywords:  Graphene oxide, carbon nanotube, organic pollutants, heavy metals, adsorption, water remediation

1.1 Introduction Water soluble toxic pollutants are one of the vital concerns to both human and environment health worldwide in recent past few decades. Ever increasing world population, expanded use of chemical products and their demands toward the exploitation of natural resources has made it inevitable to develop unique methods and materials for the environmental remediation [1–4]. Hazardous industrial wastes, unethical agricultural practices, and unplanned human waste management have caused intoxication of water resources almost worldwide [5–7]. Although, stronger regulations by governments of various countries have been implemented to curb the unruly contamination of water bodies but recalcitrant nature of pollutants towards treatment in conventional wastewater treatment plants (WWTPs) has necessitated us to approach this grave problem from a new perspective [8, 9]. Widespread occurrence of water soluble pollutants, such as heavy metal ions [Cr(VI), Al(III), Hg(II), Ag(I), Pb(II), Fe(III), As(III) and Co(II)], synthetic organic contaminants like textile dyes, new emerging contaminants such as pharmaceutical and personal care products (PPCPs), microplastic pesticides, endocrine disruptors, nanomaterials textile dyes, and synthetic and natural organic contaminants, have threatens the balance of nature and causing concerns and research interests worldwide [10, 11]. Even these compounds are present at trace levels but could result in adverse effects over human health and aquatic life because of high toxicity and carcinogenic nature. Their resistant nature towards the bio-decomposition led to their gradual accumulation over a period of time in water bodies and their concentration may rise well above the safer limits [12]. Conventional techniques including primary processes like sedimentation, chemical precipitation filtration along with biological treatment as secondary process have been used for the removal of these contaminants. Advance processes for tertiary treatment like adsorption, micro and ultra-filtrations, catalytic wet air oxidation, photocatalysis, and electrocatalysis can be used to improve the efficacy of the treatment processes [13] (Figure 1.1). Researchers’ concern is to investigate and develop new class of materials for the advanced wastewater treatment while addressing other important aspects like preventing secondary pollution, reusability, chemical stability, physical integrity, and the cost of the materials [14].

Nanocomposites for Wastewater Remediation   3 Water Treatment and Recycling Technologies

Primary Water Treatment Technologies

Screening, Filtration and Centrifugal Separation

Sedimentation and Gravity Separation

Coagulation

Secondary Water Treatment Technologies

Aerobic Process

Anaerobic Process

Tertiary Water Treatment Technologies

Distillation

Crystallization

Evaporation

Solvent Extraction

Oxidation

Precipitation

Ion Exchange

Micro- and Ultra Filtration

Reverse Osmosis

Adsorption

Electrolysis

Electrodialysis

Flotation

Figure 1.1  A schematic representation for the different process of water remediation technologies. Reprinted with permission [13].

Carbon being one of the most versatile elements on the earth provides an opportunity to fabricate materials with unconventional architectures. The narrow energy band gap and the electron transfer between 2s and 2p orbitals of carbon allows it to exist in different hybridization states. These stable states facilitate formation of diverse organic species with carbon as well as different bulk configuration of itself (Figure 1.2) [15]. Formation of flexible configurations results in varying strength of carbonaceous materials even at stable structures at nanoscale. Formation of nanomaterials enhances the surface area due to shrink in the diameter with bulk being maintained at the same density. Smaller size provides higher mobility which enables fast movement of nanoparticles (NPs) in the solution helping it to increase the coverage over a larger surface with relatively lesser amount of the material. The enhancement in aspect ratio also creates exposure of high number of low-coordinated atoms at the surface, edges, and

4  Emerging Carbon-Based Nanocomposites

Nanodiamond

Fullerene C60

Fullerene C540

Carbon Onion

SWNT

MWNT

sp3

Graphene

sp2 + π

Figure 1.2  Hybridization states of carbon-based nanomaterials. Reprinted with permission [15].

vortices which increases the reactivity of these nanomaterials. These phenomenal properties enhance the ability of these nanomaterials to scavenge pollutants in aqueous medium [16]. Tunable physico-chemical properties of carbon have inspired an innovative approach towards fabrication of carbonbased nanocomposites. Low-dimensional carbon-based nanomaterials like graphene, graphene oxide, and carbon nanotubes (CNTs) have gained significant attention for various applications over the past three decades [17]. Owing to extraordinary physiochemical properties nano-carbons have been extensively studies for application in energy, electronics, sensors, biomedical, thermal devices, and environmental remediation [15, 16, 18–20]. Graphene exhibits fascinating physical properties like tunable band gap, high electron mobility, high surface area, and enhanced active sites, which render it suitable candidate for applications related to catalysis [21, 22]. Graphene oxide (GO), derived from graphite via strong oxidation, can be thought as functionalized graphene with various oxygenated groups attached to its surface [23, 24]. GO has excellent surface functionality, aqueous processability, and amphiphilicity, providing an advantage for the removal of pollutants compare to conventional adsorbent materials [25, 26]. Due to difficult production protocols of pristine graphene via top down process, it is generally derived from reduction of GO, termed as reduced graphene oxide (rGO). Since graphene suffers from low dispersibility, hydrophobicity, and tendency to from agglomerate, arising from their low entropy gain for mixing, rGO can offset this disadvantage by stable aqueous colloids formation through electrostatic stabilization [27, 28]. CNTs are cylindrical form of graphite sheets. Recent developments have increased the utilization of CNTs in environmental remediation applications due to their highly available active surface area, catalytic sites, easily modifiable surface, enhanced chemical reactivity, and less impact on the environment [29–31]. Other carbon-based nanomaterials like carbon nanofibers,

Nanocomposites for Wastewater Remediation   5 carbon beads, and nano-porous carbon has also been explored for their application in the field of wastewater treatment [32–34]. The efficiency of carbon-based nanomaterials towards application perspective can be further enhanced by various modifications. Component manipulation can be done by doping foreign element, whereas morphological control can be gained by encapsulation facilitating NPs to grow in a desired fashion, structural engineering and functionalization can also enhance various physical properties useful for adsorption activity towards wastewater treatment [35]. In this chapter, low-dimensional materials like GO and CNTs are focused due to their extensive contribution in the wastewater remediation.

1.2 Graphene Oxide GO is one of the most fascinating macroscopic forms of graphene and exists in a non-stoichiometric form of molecules. It is a two-dimensional (2D) layered structure with oxygen functionalities including hydroxyls (OH), carboxylic (COOH) carbonyl (C=O), and alkoxy groups (C–O–C) [36, 37]. As shown in Figure 1.3, these functional groups make GO amphiphilic and enhancing its mono-dispersibility in contrary to graphene. The structural characteristic of GO helps it to interact with other molecules and ions via electrostatic forces, π-π interactions, hydrogen bonding, and hydrophilic interactions [38]. GO can be easily synthesized in both dry and wet medium. The former approach involves oxidation of graphene layers in ultra-high vacuum and subsequent treatment with ozone in ultra violet light; while, the later approach involves strong oxidation of graphite in an aqueous medium and subsequent exfoliation. Wet method is preferred due to abundance of graphite and low cost associated with the synthesis [24, 39, 40]. OH HOOC

OH

HOOC O O HO

OH

HO

COOH

HO

O HO

O

O O

HOOC COOH

Figure 1.3  Functional groups attached on basal plane of GO. Reprinted with permission [41].

6  Emerging Carbon-Based Nanocomposites

1.2.1 GO and GO-Nanocomposite for Water Remediation via Adsorption GO and its composite have attracted tremendous attention as a most potential adsorbent for water remediation due to highly exposed surface structure, abundant surface functionalities and active surface sites [42]. Nanocomposites of GO are promising because of their versatile properties and diverse variables available for fine-tuning. The GO-based nanocomposites have been widely explored for the adsorbative removal of various contaminants from water including heavy metal ions and synthetic dyes [43]. The adsorption can be controlled by various factors including physiochemical properties of adsorbent, adsorbent doses, as well as the background of contaminants chemistry. The fate, transformation, and toxicity of GO-based nanocomposite towards aquatic life have also been explored. One recent review by Asghar et al. documented the fate of magnetic GO nanocomposite and adsorption mechanisms for the adsorptive removal of heavy metal ions [44]. Dyes have shown adverse effects on human health, aquatic environment and lethal to plants. Since they have been proven to be mutagenic, they should be treated prior to discharge from industrial sources [45]. Since most of the dyes exhibit organic backbone and cationic groups, they are favorable for adsorption on GO because of π−π stacking and ionic interactions [46]. A number of dyes, like methylene blue (MB), crystal violet (CV), malachite green (MG), basic green (BG), Rhodamine B (RhB), have been chosen for this purpose to show removal efficiency of GO synthesized via different routes. Table 1.1 comprises the removal efficiency of dyes with their maximum adsorption capacity on GO as suspension and sponge form. Most of the time, GO is prepared via Hummer’s method and its modification [24, 47–49]. In general, oxidized graphite is subjected to sonication to exfoliate quasi-stacked GO sheets, which afterwards are differentially centrifuged and recovered with the supernatant. MB is one of the most studied dyes for adsorption as a model compound. Most of the studies had shown GO to follow Langmuir adsorption isotherm and well fitted with pseudo second order kinetics. Bradder et al. [50] have shown that adsorption capacity of GO does not depends upon the surface area, rather than surface chemistry of GO plays an important role where electrostatic attractions are responsible for increased adsorption capacity. He et al. [51] found that properties of GO can be significantly altered by the method of its synthesis. GO prepared from Hummers and modified Hummers method (HGO and HmGO, respectively), GO obtained from exfoliation of HGO and HmGO (HGeO and HmGeO, respectively) were compared for adsorption of MB.

GO

–

–

GO sponge

GO

48.4

GO sponge

Acridine Orange

–

HGeO[51]

124

–

HmGeO

GO

–

GO

Methylene Blue

Congo Red

28

Catalyst

Dye

BET surface area (m2 g−1)

Table 1.1  Dye adsorption on GO-based materials.

1,328

12.56

286.9

396.9

389.8

549.5

597.0

351.1

Adsorption capacity (mg g−1)

–

Pseudo second order

Pseudo second order

Pseudo second order

–

Pseudo second order

Pseudo second order

Pseudo second order

Adsorption kinetics

Langmuir

[38]

[55]

[54]

Sips Langmuir

[52]

[53]

[51]

[51]

[50]

(Continued)

Reference

–

–

Freundlich

Freundlich

Langmuir

Adsorption isotherm

Nanocomposites for Wastewater Remediation   7

133 133 17 48.4

SRGO

GO

GO

GO

GO

GO

GO Sponge

Congo Red

Acid Orange 8

Direct Red 23

Basic Red 12

Methylene Orange

Malachite Green

Methyl Violet

–

–

–

Catalyst

Dye

BET surface area (m2 g−1)

402.7

248.1

16.8

63.7

14.0

25.6

2,158

Adsorption capacity (mg g−1)

Table 1.1  Dye adsorption on GO-based materials. (Continued)

Pseudo second order

Pseudo second order

Pseudo second order

Pseudo second order

Pseudo second order

Pseudo second order

–

Adsorption kinetics

–

Langmuir

Langmuir

Langmuir

Langmuir

Langmuir

Langmuir

Adsorption isotherm

[52]

[50]

[57]

[57]

[56]

[56]

[55]

Reference

8  Emerging Carbon-Based Nanocomposites

Nanocomposites for Wastewater Remediation   9 A surprising trend as HmGO>HGO>HmGeO>HGeO was obtained for the adsorption. The reason for adsorption trend favoring high adsorption of MB on graphite oxide was attributed to smaller sheets of single or few layered GO. This would decrease the number of MB molecules intercalated in the lamellar space of GO, which is not favorable for flocculation. The increased adsorption efficiency for HmGO and HmGeO when compared to HGO and HGeO, respectively, was attributed to modification in the Hummers method causing higher oxidation degree which induces more negative charge favorable for adsorption of cation MB. Liu et al. [52] prepared a lightweight GO sponge using centrifugal vacuum evaporation technique. Due to hydrophilic nature and condensed phase of GO the sponge was dispersed in the aqueous phase and adsorbed dyes efficiently. Dispersed GO was in millimeter sized which is easy to recover by vacuum filtration, contrary to nanosized GO sheets, which requires ultrahigh centrifugation and can also act as secondary pollutant. Adsorption was accredited to π–π stacking and ionic interaction between GO and dyes rich in aromatic rings and cationic atoms. Xiao et al. showed that rGO synthesized by using L-Cysteine have the ability to remove a large number of both cationic and anionic dyes [58]. In another study by Guo et al., hierarchical sandwiched nanocomposites of GO and Fe3O4 NPs synthesized by layer-by-layer assembly was utilized for the effective removal of dyes from wastewater as shown in Figure 1.4 [59]. In an interesting study, the dimensions and geometries of Fe3O4 NPs was tuned by taking the advantage of physical interactions attributed to surficial functionalities of GO as sulphonated GO to decrease the contact area of NPs. Such fabricated nanocomposites exhibited high regeneration capability [60]. Although GO and hybrid of GO-based material demonstrated high adsorptive removal of dyes but facile separation of adsorbent from aqueous environment is still a major issue due to the contamination of water with adsorbent as secondary pollutant and toxicity issues. To overcome the issues related with separation of adsorbent after water remediation magnetic composite of GO and hydrophobic GO, aerogels have been explored [61, 62]. Cheng et al. reported a facile synthesis of composite magnetic gel using GO and poly(vinyl alcohol)(PVA) (mGO/PVA CGs) showing high adsorption capacity of cationic dyes as MB and MV along with convenient magnetic separation capability [63]. The synthetic process of composite aerogel and magnetic capability for convenient separation is shown in Figure 1.5. A bioinspired nanocomposite material of PVA/poly(acrylic acid)/GO@polydopamine (PVA/PAA/GO-COOH@PDA) with high regeneration ability was also utilized for the removal of CR, RhB, and MB [64]. Polyacrylamide composite

10  Emerging Carbon-Based Nanocomposites

RhB

MB

GO sheet Sandwiched nanocomposites

Fe3O4 nanoparticles Pure water

Figure 1.4  Removal of organic dyes from water using hierarchical sandwiched nanocomposites of GO and Fe3O4. Reprinted with permission [59].

NH3·H2O

Iron ion GO PVA Fe3O4

Figure 1.5  Schematic illustration of the synthetic route to mGO/PVA CG. Reprinted with permission [63].

hydrogels of GO with self-healing capacity was utilized for the acceleration of adsorption of toxic dyes. The most significant capacity of such hydrogels was to rapid removal of dyes from hydrogels just by simple heating, which could be beneficial for regeneration [65]. Myung et al. reported a green synthesis of highly efficient aerogel composed of graphene nanosheets using pear as a raw material [68]. Such synthesized

Nanocomposites for Wastewater Remediation   11 aerogels showed a significant potential for the the removal of a number of cationic and anionic dyes as shown in Figure 1.6, theses GO nanosheets exhibits metal ion removal capacity comparable to earlier reported metal organic framework (MOF), copper terephthalate and zeolite. Metal removal from water bodies has always been a crucial task due to the toxicity issues associated with heavy metal ions induce various harmful effects on both human and animals like neurological and immunological disorders, carcinogenicity, bio-­ accumulation, renal dysfunction, bone degradation, and liver and blood damage [66, 67]. Zhao et al. [69] synthesized few layered GO nanosheets (FGO) for the pH dependent and ionic strength independent removal of Pb(II). At low pH values dominating species is Pb(OH)+ which is easy to get adsorbed on FGO surface, whereas at higher pH values, Pb2+ exists in Pb(OH)3− ions, which cannot be adsorbed due to electrostatic repulsion with negatively charged FGO. (a)

3.0

MB After 12 h adsorption

(b)

RhB After 12 h adsorption

3.5 3.0

2.0

Absorbance

Absorbance

2.5

1.5 1.0

2.5 2.0 1.5 1.0

0.5

0.5

0.0 450

(c)

0.0 500

650 700 600 Wavelength (nm)

550

750

(d)

MO After 12 h adsorption

1.4

450

800

500

550 600 Wavelength (nm)

650

BG 1 After 12 h adsorption

0.8

1.0

Absorbance

Absorbance

1.2

0.8 0.6 0.4

0.6 0.4 0.2

0.2 0.0

0.0 350

500 550 450 Wavelength (nm)

400

500

600

(e)

(f) 4

MO

2

MB

Dyes

3

1

BG 1 0

1

2

3

4

5

6

Adsorption capacity/ mg*mg–1

7

8

600 650 Wavelength (nm)

700

750 MB MO BG 1 Rh B

10 Adsorption capacity/ mg*mg–1

Rh B

550

8 6 4 2 0

1

2

3

4

5 6 Cycles

7

8

9

10

Figure 1.6  Adsorption behavior of GAs toward different dyes; adsorption spectra of (a) MB, (b) Rh B, (c) MO, and (d) BG1 before and after 12 hof adsorption; inset show corresponding digital photographs. (e) Adsorption efficiency of GAs toward MB, Rh B, MO, and BG1 after 12 h ofadsorption. (f) Demonstration of the reusability of GAs toward adsorption of MB, Rh B, MO, and BG1. Reprinted with permission [68].

12  Emerging Carbon-Based Nanocomposites Removal efficiency was reported to be increased with increase in temperature from 842 mg/g (293 K), 1,150 mg/g (313 K), and 1,850 mg/g (333K), denoting the adsorption process to be endothermic for Pb2+ ions. Sun et al. [70] studied the highly efficient adsorption of Eu3+ using GO nanosheets, which was reported due to the formation of surface complex between Eu3+ and GO. Li et al. [71] used GO single layered NS for adsorption of U6+. High adsorption of on GO NS was attributed to formation of inner complexes of U6+ on GO. Tan et al. [72] studied adsorption of Cu2+, Cd2+, and Ni2+ on GO membrane and found high adsorption attributed to Lewis base and Lewis acid interaction between surface functionalized GO and metal ions. The adsorption reached quickly at equilibrium due to large inter layer spacing of GO sheets and strong attraction forces between substrate and metal ions. Henriques et al. [73] synthesized GO foam and carried out Hg2+ adsorption. GO foam was functionalized with nitrogen and sulfur, where an increase in removal efficiency with nitrogen functionalization was observed. Composite fabrication and surface functionalization are well known to introduce selectivity towards particular metal ions. For instance, chitosan/ Sulfydryl-functionalized of GO was done for selective removal of Cu2+, Pb2+, and Cd2+ from single and multi-metal ions system [76]. MOF functionalized GO (IRMOF-3/GO) exhibits high selectivity towards Cu2+ with an absorption capacity of ~254.14 mg/g (Figure 1.7). Further, the IRMOF-3/

DMF Zn2+

100ºC, 24h graphene oxide (GO)

2-aminoterephthalic acid

IRMOF-3/GO

n

rptio

adso

O O Zn O Zn O

O Zn O O

Zn O O H2N

O O Zn

Cu2+

O Zn O

O Zn O O

NH2 Cu2+

Cu2+ Cu2+

H2N O O Zn

2+

Cu

O Zn O

Zn O O

O Zn O O

Zn O O

NH2

O O Zn O Zn O

O

O Zn O Zn O O

Figure 1.7  Schematic diagram of the preparation of IRMOF-3/GO and the adsorption of Cu2+ on IRMOF-3/GO. Reprinted with permission [77].

Nanocomposites for Wastewater Remediation   13 GO membrane was fabricated using nano-filtration, which showed up to ~90% rejection of Cu2+ and high stability up to 2,000 min [77]. Alginate/rGO hydrogels is also reported for adsorptive removal of Cu2+ [78]. GO-chitosan and PVA composite hydrogel was fabricated via a freeze-draw physical cross-­ linking process to reduce the synthesis cost. This hydrogel was further applied for the removal of Cd2+ and Ni2+ [79]. Removal of Hg2+ was achieved by the functionalization of GO with 2-pyridinecarboxaldehyde thiosemicarbazone [80]. GO-MnO2 nanocomposite was explored for the simultaneous adsorptive removal of Th4+ and U6+ with the adsorption capacity of 497.5 mg/g and 185.2 mg/g, respectively [81]. Adsorption capacity and model kinetics of different metal ions on Go based materials is tabulated at Table 1.2. Other organic pollutants can also be removed from water bodies using GO and their composites as adsorbent material. Increasing contamination of PPCPs in water bodies has been a major concern as next-generation pollutants due to their significant activity at even lower concentrations [82]. Agricultural and industrial waste also contains pollutant compounds with notably unsaturated aromatic rings and oxygen rich functionalities that are difficult to degrade. Jiang et al. [83] carried out adsorption of 17β-estradiol (E2) on GO NS. Table 1.2  Metal ion adsorption on GO materials. Adsorption capacity (mg/g)

Model used

Reference

Pb

842 (293 K) 1,150 (313 K) 1,850 (333 K)

Langmuir

[69]

GO NSs

Eu3+

175

Langmuir

[70]

GO single layered NS

Th4+

411

Langmuir

[74]

GO single layered NS

U6+

299

Langmuir

[71]

GO aerogel

Cu2+

17.7 (283 K) 19.6 (298 K) 29.6 (313 K)

Langmuir

[75]

GO foam

Hg2+

35

Langmuir

[73]

GO membrane

Cd2+ Cu2+ Ni2+

83.8 72.6 62.3

Langmuir

[72]

Catalyst

Metal ion

Few layered GO

2+

Langmuir

Freundlich

43.9 1.19 2.62 5.90 6.12 174.6 59.0

Atenolol Propranolol

Diclofenac Sulfamethoxazole

Naphthalene Phenanthrene Pyrene

Phenanthrene Biphenyl

GO

GO NS

GO

67 116 Freundlich

Langmuir– Freundlich

Langmuir

Few layered GO

149.4

17β-estradiol

GONS

Model used

Organic pollutant

Catalyst

Adsorption capacity (mg/g)

Table 1.3  Organic pollutants’ adsorption on GO base materials.

–

Pseudo second order

–

Pseudo second order

Pseudo second order

Rate kinetics

[87]

[86]

[85]

[84]

[83]

Reference

14  Emerging Carbon-Based Nanocomposites

Nanocomposites for Wastewater Remediation   15 Kyzas et al. [84] adsorbed Atenolol (ATL) and Propranolol (PRO). Adsorption was found to be of endothermic nature and high adsorption was attributed to the electrostatic interactions between GO and pollutants. Nam et al. [85] studied adsorption of diclofenac (DCF) and sulfamethoxazole (SMX) on GO. Wang et  al. [86] tested naphthalene, phenanthrene, and pyrene adsorption on GONS. Unusual low adsorption of the pollutants on GO may result from interaction between hydrophobic pollutants and hydrophilic GO partially neutralizing the negative charge, increasing the aggregation tendencies, which may reduce adsorption affinity for high concentration of pollutants. Apul et al. [87] selected Phenanthrene (PNT) and biphenyl (BP) for adsorption over GO. GO has good adsorption capacity but when compared to graphene NSs uptake of the pollutants was lesser due to possibility of functional group present on GO surface making water clusters on surface, which could decrease the number of adsorption sites. Table 1.3 shows adsorption capacity and rate kinetics of different organic pollutants on GO base materials. Highly porous GO-MOF composite was applied to remove antiinflammatory drugs from water [88]. Yang et al. targeted the adsorption of a series of emerging contaminants including di-n-butyl phthalate (DnBP), cephalexin (CLX), di(2-ethylhexyl) phthalate (DEHP), caffeine (CAF), and sulfamethoxazole (SMX) by using graphene ceremics composite [89].

1.3 Carbon Nanotube CNT is an attractive member of carbon family, which gained a huge interest in nanotechnology due to its unique structural and optoelectrical properties [90–94]. CNTs was first discovered by Iijima in 1991 [95]. CNTs are considered a tubular form of single graphite sheet and are classified as single walled CNTs (SWCNTs) and multi-walled CNTs (MWCNTs) depending on the number of layers sheet of graphite that has been rolled into a tube with single, double, or multiple walls (Figure 1.8) [96]. CNTs usually have a diameter varied from 0.1 nm to 10 nm and lenght can be varied upto few hundreds centimeters. CNT network generally consists of a hexagonal network of sp2 hybridized carbons. CNTs can be classified into three varieties of tubule structures (armchair, zigzag, and chiral) depending on the rolling up direction of the graphene sheet [97]. The difference in orientation controls electrical properties of the CNTs have direct impact on their electrical properties [90]. CNTs are highly studied for their excellent adsorption properties owing to high surface area, chemical interness and excellent stability [98]. Adsorption on CNT is dependent on its various physical and chemical properties like surface area, hydrophobic interactions, functionalization of

0 .2

–5 µ

m

16  Emerging Carbon-Based Nanocomposites

0.38 nm

0.4–2 nm

2–100 nm

(a)

(b)

Figure 1.8  Diagrams of (a) single-walled CNTs (SWCNTs) and (b) multi-walled CNTs (MWCNTs). Reprinted with permission [90].

basal structure, π−π bonds, electrostatic interactions, surface, and capillary condensation. Physio-chemical properties of adsorbing species like polarity, size and structure of the molecule, charge, pH, and ionization strength of the solution may also affect their extent and affinity for adsorption on CNTs. Though pristine CNT exhibits a hydrophobic nature, it can well adsorb non-polar molecule. Tuning the hydrophilicity by functionalizing the CNTs can increase the adsorption capacity for polar molecules [99]. Figure 1.9 shows change in affinity of CNT towards adsorption of polar and non-­polar molecules on the basis of functionality. CNTs can be engineered into different structures with controlled orientation and configuration, which gives them further advantage to be used as a versatile adsorbent [99]. Adsorption of polar molecules increases with the functionalization of CNT, whereas for non-polar molecules pristine CNTs were best adsorbents [100].

1.3.1 CNTs as Adsorbent CNTs showed considerable potential for the removal of diverse metal ions and a range of toxic dyes from water owing to unique surface properties and tubular structure [101–104]. Mohammadi et al. observed the adsorption of toxic metal ions especially divalent heavy metal ions (Pb2+, Cd2+, Co2+, Cu2+, Zn2+) from water using CNT sheets [105]. The adsorption capability of CNT sheets were observed in the order of Pb2+ > Cd2+ > Co2+ > Zn2+ > Cu2+ due to the electrostatic interactions between negatively charged acidic functional groups on CNTs surface and posively charged metal ions. Another report by Pyrzynska et al. demonstrated the different affinity for the adsorptive removal

Adsorption increased

Adsorption increased

Oxidized catalyst

Catalyst removed

Acid treated

Amorphous carbon removed; metal catalyst oxidized

Heat treated (350ºC)

Amorphous carbon

For nonpolar and/or planar chemicals: adsorption decreased; for polar chemicals: adsorption increased

Functional groups added

Functionalized

For polar chemicals: adsorption decreased; for nonpolar chemicals: adsorption increased.

Functional groups removed

Graphitized (2200ºC)

Figure 1.9  Effect of functionalization of CNTs on adsorption of polar and non-polar molecules [99]. Reprinted with permission [99].

Catalyst

Inner pores blocked

As-grown CNT

Nanocomposites for Wastewater Remediation   17

18  Emerging Carbon-Based Nanocomposites of toxic metal ions using CNTs and it follow the order of Cu2+ > Pb2+ > Co2+ at pH 9 [106]. Mao et al. observed adsorption of MB onto CNTs because of attribute to interactions of MB with SWNTs [107]. Gong et al. studied the adsorption of cationic dyes including brilliant CB, MB and neutral red by magnetic MMWCNTs nanocomposite and adsorption capacity follow the order of MB > neutral red > brilliant CB [108]. Yao et al. reported a high adsorption capacity up to 35 mg/g at 298 K for the removal of MB onto CNTs [109]. Adsorption capacity by CNTs for different dyes was tabulated in Table 1.4. It is observed that oxidation or surface functionalization of CNTs significantly improved the adsorption capacity [115]. Several factors, such as pH of solution, ionic strength, dose of catalyst, temperature, concentration of toxic metal ions in wastewater, and other factors have considerable impact towards the removal of contaminants from wastewater. The adsorption of cobalt over CNTs were varied with pH from 3 to 9; sharp increase in adsorption was observed with increase in pH [106]. Low adsorption at low pH due to competition between H+ and Co2+, while pH attributed to the surface negative charge which facilitates more adsorption of metal ions. M. Ghaedi et al. observed that adsorption capacity of both morin and ARS have inversely depended on pH over range 1 to 5 [116]. Increased adsorption at low pH due to enhanced electrostatic interaction between anionic dye and protonated surface functional groups. Presence of other ions adversely affected the adsorption of targeted metal ions [115]. Decrease in adsorption was reported for Ni2+, Cd2+, Cu2+, and Pb2+ by various research groups Table 1.4  Maximum adsorption capacities for different dyes with CNTs. Adsorption capacity, Q (mg/g)

Model used

Reference

Adsorbates

Adsorbents

MWCNTs

Procion red MX-5B

44.68

Langmuir

[110]

MWCNTs

Sufranine O

43.48

Langmuir

[111]

MWCNTs

Acid red 18

166.67

Langmuir

[112]

Oxidized MWCNTs

Bromothymol blue

55

Langmuir

[113]

MWCNTs

Methyl violet

71.76

Langmuir

[114]

CNTs

Methylene blue

64.7

Langmuir

[109]

Nanocomposites for Wastewater Remediation   19 observed in presence of high ionic strength [117, 118]. Amount of adsorption for the metal ions like Pb2+, Ni2+, Cd2+, and Cu2+ were observed with increase the loading of CNTs due to increase in surface active sites, which facilitates better interaction between surface functional groups with toxic metal ions [114, 119]. Similar phenomenon were observed for dyes where enhancement of adsorption capacity with increasing adsorbent dosage was attributed to increase in surface adsorption sites along with enhanced surface area of CNTs [116]. From the available literature, it was observed that the adsorption efficiency of CNTs increases with temperature because of efficient mobility of the metal ions, dyes towards catalytic surface, and easily overcome lower activation energy barrier during adsorption processes [110, 111, 116]. Generally, adsorption of metals, dyes over CNTs were preferably follow Langmuir or the Freundlich model. Langmuir isotherm preferably useful for uniform surface coverage, where there is no interaction between adsorbate molecule [111, 115]. On the other hand, Freundlich isotherm for the heterogeneous surfaces where interaction between molecules takes place. Adsorption of toxic Cr6+ via MWNTs-poly(acrylic acid) (PAA)– poly(4-amino diphenyl amine) (PADPA) preferably follow by Freundlich isotherm as reported by Kim et al. [120]. In another report, adsorption of Ni(II) over MWCNTs preferably follow Langmuir adsorption isotherm because of no observed interactions between Ni2+ and MWCNTs along with high active surface area [121]. Wu et al. reported adsorption efficiency of CNTs for Procion Red and MX-5B follow Freundlich isotherms [110]. SWCNT and MWCNT composite membrane was fabricated for the removal of PPCPs including triclosan (TCS), ibuprofen (IBU), and acetaminophen (AAP) having the removal efficiency from 10%–95% in the order of AAP≈IBU Cr > Ni with 15 min settling time as optimized. In another study, gum-ghatti(Gg)-grafted poly(acrylamide-co-methacrylic acid) (AAm-co-MAA) hydrogels were designed in the assistance of microwave to be used a biodegradable flocculants and adsorbents. The thermal stability of the hydrogels was quite good in comparison to pristine gum-ghatti under neutral conditions. The hydrogels were employed for saline water removal from petroleum industries. The removal of lead and copper was about 94% and 75% in aqueous solutions. These hydrogels are biodegraded completely in 50 days. Such biodegradable polymers can be effectively utilized for heavy metals removal from effluents of industries [50]. Green flocculant composed of silicon-aluminum-ferric-starch was developed by grafting process. This flocculant has not been utilized for heavy metal treatment but has been utilized for sludge dewatering effectively. It could be believed that such biodegradable materials would be promising and cost effective leading as examples for developing green flocculants with higher efficiency in future [51]. This process cannot totally remove heavy metals from waste­ water; thus, alternative treatment approaches such as co-precipitation, ion exchangers and bio-adsorption must be studied.

2.4.3 Chemical Oxidation/Reduction In situ reduction involves the removal of heavy metals form water on the remediation sites directly. This method is a novel approach and widely being used in treatment of ground water source and similar to in-situ chemical oxidation. The technique is usually utilized in the environment, by adding reducing additives into the contaminated area of the liquid phase or by applying a chemical reduction medium in the direction of the contaminant plume [52]. These may be used to treat a number of heavy metals, some of which are resistant to natural deterioration. Chemical reduction is half the redox reaction that leads to the acquiring of electrons. One reaction is oxidized in the reaction, or the electrons are lost, while the other reaction decreases or obtains electrons. In in situ reaction process, that is, compounds that accept electrons given in the reaction of other compounds are used to convert contaminants into compounds with lower hazards. Highly reactive zero valent metals are widely used for the purpose; however, the

40  Emerging Carbon-Based Nanocomposites only limitation is corrosion over reaction time and decrease in the reactivity of the metal. In this background, the ability of iron as Fe(0) and Fe(II) to inhibit redox-sensitive elements has been demonstrated both on a laboratory scale and in field studies [53]. Chitosan beads with entrapped nZVI has been effectively used for reduction of hexavalent chromium from wastewater. The mechanism revealed that chromium removal took place via physisorption of hexavalent chromium on the surface of chitosan-nZVI beads which subsequently reduced chromium into trivalent form from its hexavalent form. There has been very little contribution from the amino and hydroxyl groups in chitosan, but nZVI in the composite helps in non-segregation and oxidation of particles. The reduction of hexavalent chromium is caused by zero-valent chitosan iron particles. The increase in temperature and amount of nZVI enhances the reduction of hexavalent chromium, whereas initial chromium concentration and pH significantly decreases the efficiency [54]. In another study, zero-valent iron was deposited in beads of graphene oxide@ alginate (Fe@GA beads) by immobilizing Fe0 nanoparticles on graphene oxide adjusted alginate gel. There has been a systematic distribution of Fe0 nanoparticles on GA beads. Although, removal process is governed by adsorption of the Cr(VI) species on the beads but the removal mechanism is via reduction of hexavalent chromium species using Fe0 present in the system (Figure 2.3) [55].

2.4.4 Ion-Exchange Process Ion-exchange is a separation process that exchange the less toxic and mobile ions with the toxic ions. This method is widely implemented in heavy metal wastewater treatment and industrial wastewater outlets. Today, ion-exchangers are widely being used in conventional water filters in heavy metal ions removal from drinking water. Compared to the coagulation process, the yield of the sludge in the ion exchange phase is very small [56]. Effectively, the process removes 99% of heavy metals and the ion exchange resin formulations are used to substantially restore or recycle metals. The shielding with various metal ions can be carried out on the basis of the chemical properties of the resins. The few limitation of the method is the stability, regeneration, and heavy metals centric-based removal. The green composite not only enhances the stability of the ion exchangers but also provides the unique opportunity to degrade in efficient way and recover the heavy metals. Basically, ion-exchange is designed to prevent high pressure and natural degradation effectively and could suitably present the substrate-ligand bindings [57]. This further stimulates

Nanocomposites for Water Treatment Application  41 (b)

i

ii

iii

iv

Cr(III)

Fe(II)/Fe(III)

(a)

Cr(VI) Removal Efficiency (%)

Cr(VI) Fe@GA beads

90

(c)

Fe0

80

A beads

70 GOA beads

60

Fe@GOA beads

50 40

Fe@GA beads

30 20 10 0

0

20

40

60

80 100 120 140 160 180 Time (min)

Figure 2.3  (a) Mechanism of chemical reduction; (b) Macroscopic photos of (i) A beads, (ii) GOA beads, (iii) Fe@GOA beads, and (iv) Fe@GA beads; (c) Comparative removal efficiencies using all the fabricated beads [55].

polymer support for bridging. The resin consists of a networked polymer matrix, whereby functional groups are combined with resins by covalent bond. The available vacancies and structure allow corresponding charge transfer. These resins are divided into mainly synthetic resins and natural resins. Synthetic resins are widely popular as compared to the natural resins to separate heavy metals efficiently; however, the challenge is to recover the metals for further re-utilization of the ion-exchanger. The natural carbon-based green nanocomposites are biodegradable and provide easy recovery of the metal ions. The efficiency of green nanocomposites is also found higher. A cation exchanger, composed of a strong acidic resin and a weak alkaline resin is the most popular ion exchanger. Sulfonic acid and carboxylic acid groups are found in acidic and simple resins, respectively. Hydrogen ions may provide convertible ions to metal cationic ions [58]. A study reports on designing of an ion-exchange resin by covalently bonding the single-walled carbon nanotubes (SWNCTs) with short chain polyelectrolytes. The developed material was incorporated into the membrane system for analysis of the analytes. In this direction, SWNCTs/ carbon polymer ion exchange membranes were developed. The material

42  Emerging Carbon-Based Nanocomposites was observed very stable and relatively exhibited a higher adsorption capacity. Ten different compounds (drugs, pesticides, disinfection by-products, and fluoroalkylated materials) were measured with elimination efficiencies as high as 95%–100%. It was also witnessed that the material has a regenerative capacity of 20 cycles and can be used for sustainable purification of water. This material exhibits electrostatic binding mechanisms for several classes of generalized compounds. The usefulness of such films is to test them against various substances and the the concentration of the analytes by 95%–100%. Analytical techniques such as dynamic light dispersion (DLS), zeta potential, and adsorption capability of many synthetic batches have uniform hydrodynamic diameter, large surface charge, and adsorption properties, suggesting that the carbon of the material nano-structures has not been screened or no escape from the membranes. Thus, it is marked safe for environmental and drinking water applications [59]. Similarly, via solgel method a Zr(IV) phosphate complex (GT/ZPC) based gelatin material was developed for evaluation of ion-exchange property [60]. The GT/ZPC IEC was found to be better (1.04 meq g-1) to its inorganic counterpart (0.64 meq g-1). The pH studies showed GT/ZPC with its single-function nature. Distribution studies have shown that GT/ZPC has a high Cd2+ ion selectivity relative to other metal ions. The environmental suitability of the ion exchanger was tested and a binary separation method was used to separate metal ions using a column device. Cd2+ is entirely isolated from the synthetic mixture of Zn2+, Pb2+, Ni2+, Co2+ and Cu2+ metal ions. Another study puts forward a strategy to coat the ion-exchange resins using chitosan [61]. The study revealed that chitosan coated ion-exchange resin was used to successfully remove heavy metals from the lean amine solution. Experimental work has been done to understand the efficacy of the ionexchange resin at equilibrium conditions. Continuous adsorption studies were performed to establish the breakthrough times of the ion exchange process and to promote the design of the device by designing the operating conditions as useful for extracting heavy metals from lean amine samples. Initial research based on heavy metal ions such as copper, chromium and lead collected in lean (methyldiethanolamine) MDEA solution in the cycle of gas sweetening.

2.4.5 Adsorption The process of adsorption is widely used in the heavy metals treatment in aqueous solution. This is simply a mechanism through which the movement of mass takes place from the liquid to the surface of the solid by physical or chemical interactions. The key method of adsorption is based on the direct binding of cations/anions of metals to the surface of the adsorbent through

Nanocomposites for Water Treatment Application  43 electrostatic attraction which leads to the elimination of more than 50% of heavy metals. Heavy metals are adsorbed to form flocs on the adsorbate sheet. The process is quite economic and widely utilized in water treatment systems. Most of the heavy metals removal using carbon-based green nanocomposites takes place via adsorption process. In this context, polyacrylonitrile and ferric nitrate is used to produce fibrous and granular magnetic structural carbons (MCFs and MCPs) and showed major morphological dependence on the treatment of wastewater. An understanding of the rational designing of the adsorbent for removal purpose was understood. The experimental results advocated that the activity of fibrous nano-adsorbent increased from 12.6% to 51.4%. The hexavalent chromium removal was dependent on the initial Cr(VI) concentration which further depends on the increased content of Fe(NO3)3·9H2O in the precursor from 10% to 40%. The enhanced removal of 4 mg/L is mainly due to the higher specific surface area of the fibril sample, resulting in a more active Cr(VI) adsorption site and the generated Cr(III) ions. Additionally, the stability of the fibril sample (MCFs-40) showed enhanced removal ability (43.17 mg/g), which was 3 times higher than the particulate nanosorbent (MCPs-40) which removed only 15.88 mg/g of hexavalent chromium. The removal mechanism of hexavalent chromium from the aqueous solution has been demonstrated in Figure 2.4. The fibrous samples can be found to be more suitable than other samples used for wastewater treatment. On treated MCPs-40 and MCFs-40, Cr(VI) and Cr(III) were identified, which suggested Cr(VI) and produced Cr(III) ions were adsorbed on the sample surface. The Cr(VI) removal Fe Fe2+/Fe3+

Totally removed

Cr6+

Cr(VI)

Cr3+

10~20 min

Fe3+ Fe2+

MCFs Before

Cr6+ Cr3+

Removing Fe2+

Cr(VI)

Fe3+ Fe2+/Fe3+

MCPs

Fe

Fe/Fe2+

After Partly removed

Cr6+ Cr3+ 10~20 min Cr6+ Cr3+

Fe2+/Fe3+

Cr(VI)

Cr(III)

Figure 2.4  Removal mechanism of Cr(VI) over fibrillar and particulate magnetic carbon adsorbents (Obtained with permission from ACS) [62].

44  Emerging Carbon-Based Nanocomposites performance of MCFs-40 is nearly three times greater for the neutral solution than that for MCPs-40, which is due to the more specific surface area of the MCFs-40. The cyclic stability of the designed adsorbent was also good till five consecutive experimental runs with Cr(VI) removal efficiency of 1.4 mg/g from MCFs-40 and 0.41 mg/g from MCPs-40 in neutral solution containing the initial Cr (VI) concentration at 4 mg/L [62]. One of recent study documented by utilizing activated carbon from spent coffee grounds and natural clay as novel green carbon nanocomposite. The composite was functionalized by activation and magnetization process, simultaneously. The developed magnetic nanocomposite was employed for utilization and removal of Cu(II), Ni(II), and Pb(II) ions from aqueous solution. The optimal conditions for the treatment of heavy metals were; contact time-60 min, temperature- 25°C, pH ~5.5 and an ideal adsorbent dosage of 2 g/L. Mechanism of adsorption was understood by relaying the adsorption data to Langmuir, Freundlich, Temkin, and other kinetic models. Although, authors reported fair recyclability and regeneration of the adsorbent, but the biodegradability and feasibility studies was not reported. The research stated that the Langmuir adsorption process is suitable and obtained higher adsorption capacity of 143.56, 96.16 and 84.86 mg/g for Pb(II), Cu(II) and Ni(II) respectively. The research also indicated that adsorbent material is effective for the removal of toxic heavy metals from wastewater [63]. In another strategy to the production of carbon-based green nanocomposites, graphene oxide embedded calcium alginate (GOCA) beads were synthesized and further functionalized with poly(ethylenimine) to improve the adsorption potential towards heavy metal ions. The reported material has been used for removal for Pb(II), Hg(II), and Cd(II) from aqueous solution under different experimental conditions, where the functionalized beads were found to have high potential for adsorption relative to the non-functionalized beads. The study also reports the value of uptake of metal ions to be 602, 374, and 181 mg/g for Pb(II), Hg(II), and Cd(II) ions, respectively. Moreover, through model fitting, it was understood that the removal of these heavy metals followed second order kinetics and monolayer adsorption took place as the adsorption data well fits the Langmuir adsorption theory. The as-synthesized green substance demonstrates a combination between adsorption and desorption up to five periods in sequence. This newly synthesized green adsorption beads demonstrated superior removal of metal ions in contrast to conventional materials and had a synergistic impact. The preparation mechanism, green materials functionalization, and efficient utilization as adsorbent for heavy metals removal are presented in Figure 2.5 [64].

Nanocomposites for Water Treatment Application  45

Bead

SA + GO solution

CaCl2 solution

preparation

PEI functionalization & in situ reduction

PEI solution

SA : sodium alginate GO : graphene oxide Polyethylenimine

Graphene oxide

=

Graphene oxide

=

Alginate matrix

Alginate matrix

(a) O H 2N OH

1.

O COOH

HC

OH

COOH O

OH

N

NH2 N H H2N

N

NH2

N H

PEI

a

N

N H

NH2 OH

NH2

NH2

N

NH

H N

N N

OH

N N

H2N

O

...2

N H

H N

NH2

NH2

a

COOH

HC

OH

2. M+

O

OH

O

HN

COO HN M

N

N

H2N HN

Polyethylenimine

H2N

Metal ions

N

Alginate matrix

N HN NH2

NH2

N NH NH2

N

H2N

HN

HN

N

Graphene oxide

NH2

HN

N

NH2

NH H2N

(b)

Figure 2.5  (a) Beads preparation (i) SA and GO completely dispersed in water, (ii) SA-GO mixture made as beads in CaCl2 solution, and (iii) functionalization and reduction of GOCA beads in PEI at 40 °C; (b) The schematic representation and the possible chemical complexation of metal ion in the fGOCA beads. (Insert: Pictorial representation of fGOCA) [64].

The trend of utilizing polymer matrix and carbon nanocomposite simultaneously provides a unique opportunity to develop biodegradable green carbon polymeric nanocomposites. In this aspect, Zare et al. have made effort towards development of biodegradable polyaniline/dextrin conductive nanocomposites. Dextrin-based conducting nano-composites were synthesized in the presence of dextrin by polymerisation of aniline. The analysis finds that the conductivity of green composites improves by

46  Emerging Carbon-Based Nanocomposites rising the amount of polyaniline [65]. Furthermore, they report increase in the antioxidant capability with increment in the presence of aniline content in nanocomposite. The nano-composite PANI/Dextrin showed a high activity of antioxidants of up to 70% which efficiently scavenged free radicals of 2, 2-Diphenyl-1-picrylhydrazyl (DPPH). The green material is used to remove heavy metal ions from the standard water solution including Cu(II), Cd(II) and Pb(II). Additionally, in vitro biodegradability of polyaniline/dextrin nanocomposites with different weight ratios was studied by soil burial tests. The result established that the nanocomposites are biodegradable under soil environment by degradation range between 30.18% and 74.52%. This presents a good strategy and a significant observation in the direction of green carbon materials. The decomposition of the material under natural conditions is presented in Figure 2.6 [65]. However, all of the above reports with carbon-based green composites arises few significant inquiries, which is, the stability of the heavy metals in the environmental composite and its bioavailability. III

15

30

45

60

45

60

Time (day) (a) IV

15

30 Time (day) (b)

Figure 2.6  Decomposition of synthetic nanocomposites (a) III [Ani:Dex (1:3)] and (b) IV [Ani:Dex (2:1)] that were buried in the soil with natural microorganisms. (Ani-aniline and Dex-dextrin) [65].

Nanocomposites for Water Treatment Application  47 An experimental attempt was made to synthesize magnetic nanocage (Mag@CNC) with pine resin and ferric nitrate salt as a source of carbon and iron. In order to prepare magnetic carbon nanocage (Mag@CNCs), it is treated at an elevated temperature under an inert atmosphere through carbonization. Microporosity and medium porosity demonstrate the higher surface area of green carbon nanocage through surface properties analysis. The material exhibits a uniform distribution of core-shell magnetic nanoparticles in a carbon matrix, forming iron carbide (Fe3C) and metallic iron (α-Fe), having a size range of 20 to 100 nm, surrounded by a small amount of graphite layer wall. Mag@CNCs material was used to study adsorption of arsenic (III) species. The resultant analysis suggested that As(III) is combined at two types of surface positions of Mag@CNCs, namely, on the carbon surface material of the nanoparticles (≡CxOH2) and the Fe-oxide layer (≡FeOH2). This indicates that the advanced morphology and surface-driven synergistic properties of Mag @CNC materials are the basis for their As(III)-absorption properties [66]. New resources such as hazelnut shell, rice husk, pecan shells, jackfruit, maize cob or husk, fibers, and biomass contain higher carbon content which can be used as a green adsorbent for heavy metal uptake after chemical modification or conversion by heating into activated carbon/bio-carbon. Introduction of NM/metal oxide and ion exchange component not only enhances the binding capability but also provides superior stability during treatment. These materials are considered as green material, since they are eco-friendly and does not affect the normal phenomena of ecosystem. Adsorption is popular and easy to use application with lower maintenance. Generally, there are three main processes involved in toxic sorption of green adsorbents: (i) the movement of the pollutant from the bulk solution to the surface of the sorbent; (ii) the adsorption of the surface of the sample; and (iii) transport within the sorbent pores. Technological applicability and efficiency are essential factors in the selection of the most suitable adsorbent for the treatment of heavy metals in wastewater systems. In the technology strategy of green carbon nanocomposites and wide applicability of the process “adsorption”, Kanmani et al. [67] have elaborated applications of chitosan-based biopolymers and green materials for heavy metals treatment from wastewater systems. Similarly, there are other research materials reported which describes materials in terms as eco-friendliness but are yet to be exploited for heavy metals remediation purpose. The term of green composite is still unclear in some context which creates the need of its further studies with high novelty keeping in mind the sustainability of the environment.

48  Emerging Carbon-Based Nanocomposites

2.5 Conclusion and Future Directions Since the beginning, water treatment technology has come a long way. Several advances have been explored, ranging from simple ex-situ physical separation techniques to advanced in-situ technologies. Rapid solutions are possible but do not necessarily provide a permanent solution to the problem. As a result, suitability and use depend primarily on material, performance, regeneration and operating costs. Different technologies such as coagulation/flocculation, ion exchange, flotation, membrane filtration, chemical precipitation, electrochemical treatment and adsorption for the removal of heavy metals have been introduced in order to comply with the current environmental regulation. This chapter deals with developing technologies for the removal of heavy metals from wastewater by using carbon nanocomposite. Clear explanations for the ability to handle single and multivalent heavy metals ions are presented. This further illustrates the benefits, drawbacks and weaknesses of the method used for heavy metal removal techniques. Studies have shown that the adsorption process has become a successful alternative to many conventional heavy metal removal techniques. This chapter examines the different green materials and their applicability through the adsorption process. Overall, availability and cost effectiveness are the two main parameters required to find the most likely heavy metal treatment sorbent in wastewater. Other important parameters, including pH, initial metal concentration and regeneration, can play a key role in the selection of the appropriate inorganic effluent treatment process. All the above criteria should be taken into account when choosing the most efficient and economical environmental treatment. A detailed study of the biodegradability and consequences of secondary pollution of such green materials is required. The applicability and implementation of biodegradable carbon-based green nanocomposites need to be studied in depth and should be studied according to practical applications.

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52  Emerging Carbon-Based Nanocomposites 47. Khaydarov, R.A., Khaydarov, R.R., Gapurova, O., Water purification from metal ions using carbon nanoparticle-conjugated polymer nanocomposites. Water Res., 44, 1927–1933, 2010. 48. Teh, C.Y., Budiman, P.M., Shak, K.P.Y., Wu, T.Y., Recent Advancement of Coagulation–Flocculation and Its Application in Wastewater Treatment. Ind. Eng. Chem. Res., 55, 4363–4389, 2016. 49. Fosso-Kankeu, E., Mittal, H., Waanders, F., Ntwampe, I.O., Ray, S.S., Preparation and characterization of gum karaya hydrogel nanocomposite flocculant for metal ions removal from mine effluents. Int. J. Environ. Sci. Technol., 13, 711–724, 2016. 50. Mittal, H., Jindal, R., Kaith, B.S., Maity, A., Ray, S.S., Flocculation and adsorption properties of biodegradable gum-ghatti-grafted poly(acrylamide-co-methacrylic acid) hydrogels. Carbohydr. Polym., 115, 617–628, 2015. 51. Lin, Q., Peng, H., Zhong, S., Xiang, J., Synthesis, characterization, and secondary sludge dewatering performance of a novel combined silicon– aluminum–iron–starch flocculant. J. Hazard. Mater., 285, 199–206, 2015. 52. Liu, H., Bruton, T.A., Doyle, F.M., Sedlak, D.L., In situ chemical oxidation of contaminated groundwater by persulfate: Decomposition by Fe(III)- and Mn(IV)-containing oxides and aquifer materials. Environ. Sci. Technol., 48, 10330–10336, 2014. 53. Liu, J., Wang, C., Shi, J., Liu, H., Tong, Y., Aqueous Cr(VI) reduction by electrodeposited zero-valent iron at neutral pH: Acceleration by organic matters. J. Hazard. Mater., 163, 1, 370–375, 2008. 54. Liu, T., Zhao, L., Sun, D., Tan, X., Entrapment of nanoscale zero-valent iron in chitosan beads for hexavalent chromium removal from wastewater. J. Hazard. Mater., 184, 724–730, 2010. 55. Lv, X., Zhang, Y., Fu, W., Cao, J., Zhang, J., Ma, H., Jiang, G., Zero-valent iron nanoparticles embedded into reduced graphene oxide-alginate beads for efficient chromium (VI) removal. J. Colloid Interface Sci., 506, 633–643, 2017. 56. Dabrowski, A., Hubicki, Z., Podkościelny, P., Robens, E., Selective removal of the heavy metal ions from waters and industrial wastewaters by ionexchange method. Chemosphere, 56, 91–106, 2004. 57. Zewail, T.M. and Yousef, N.S., Kinetic study of heavy metal ions removal by ion exchange in batch conical air spouted bed. Alexandria Eng. J., 54, 83–90, 2015. 58. Song, W., Gao, B., Guo, Y., Xu, X., Yue, Q., Ren, Z., Effective adsorption/ desorption of perchlorate from water using corn stalk based modified magnetic biopolymer ion exchange resin. Microporous Mesoporous Mater., 252, 59–68, 2017. 59. Sahu, A., Blackburn, K., Durkin, K., Eldred, T.B., Johnson, B.R., Sheikh, R., Amburgey, J.E., Poler, J.C., Green synthesis of nanoscale anion exchange

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3 Green Nanocomposites: Advances and Applications in Environmentally Friendly Carbon Nanomaterials Naveen Bunekar* and Tsung Yen Tsai† Department of Chemistry, Chung Yuan Christian University, Chung-Li, Taiwan, ROC

Abstract

This chapter is a study of green nanocomposites and their advance applications, through this research we show the use of carbon nanomaterial composites in energy storage devices, automotive, construction, packaging, defense system, renewable energy (wind blades), sustainable energy (solar/fuel cell) environmental pollution prevention and medical applications. Green polymer nanocomposites are very important because they posses unique properties of combining the advantages of natural or synthetic nanofiller and organic polymers. These green nanocomposites relatively high strength and stiffness, low cost of acquisition, and low density. This book chapter summarizes the recent advances applications of green nanocomposites; efforts, techniques of production, trends, and prospects in this green nanocomposites field. Keywords:  Green nanocomposites, carbon nanomaterial, natural fillers, polymers

3.1 Introduction In recent times, research attempts have been made towards developing environment friendly and sustainable polymer composites for use in different industrial applications such as aeronautical, automotive, *Corresponding author: [email protected] † Corresponding author: [email protected] Ajay Kumar Mishra, Chaudhery Mustansar Hussain and Shivani Bhardwaj Mishra (eds.) Emerging Carbon-Based Nanocomposites for Environmental Applications, (55–70) © 2020 Scrivener Publishing LLC

55

56  Emerging Carbon-Based Nanocomposites construction, packaging, farming, and medical fields. Polymeric materials with enhanced advanced properties and functionalities attract much attention for the new materials world. The mechanical properties such as elasticity, strength, dimensional stability, thermo-mechanical properties and permeability have been observed to improve significantly. The other properties are thermal stability, flame retardancy, chemical resistance, surface appearance, heat distortion temperature, smoke emissions, electrical conductivity and physical weight. The properties of polymers have been improved with the addition of nano particles to the polymers which made them useful in various industries medical, automotive and construction [1, 2]. Nanocomposite consists of the reinforcing materials which are in order of nanometers in dimensions. They predominant properties than the conventional composites due to this nanometric size and because the maximization of inter-facial adhesion. Moreover, the different synthetic composite materials prepared very high strength values making them ideal structural elements for various industrial applications. Earlier, nanocomposites were produced using synthetic materials such as nano-clays, carbon fibers, glass, polystyrene, polyvinyl alcohol, polyesters, polyester amides, aliphatic polyesters, aliphatic aromatic polyesters, carbon nano-tubes etc. But, there have been great challenges posed during the use of these materials [3–5]. Some of the challenges were scarcity of the organic compounds due to crumbling oil and gas resources and rising oil and gas prices leading to uneconomical costs, environmental concerns to degrade them, global warming, consumer toxicity and cross contaminations during recycling [6–10]. These concerns led to search for materials that can conquer these challenges and sustain the required properties for various applications [11]. The environment problems arising due to use of synthetic polymers can be reduced by using polymer composites from renewable resources [12]. These help in developing economically sustainable and ecologically attractive technology. In recent times, due to increasing need of biodegradable polymers, preservation of fossil based raw materials, complete biodegradability and also reduction in volume of carbon dioxide released into atmosphere green composites have been most looked for materials (Figure 3.1). The research interests towards green materials have been increasing as these are produced using agricultural resources (wastes and products). With the exponential growth of use of these nanocomposites in green and advanced applications, it is anticipated to improve manufacturing speed and its recycling would possess enhanced environmental compatibility [13]. This chapter aims to explore nanocomposites development covering in wastewater treatment and energy applications.

Green Nanocomposites: Advances and Applications   57

Environmental

Biomedical

Energy Nanocarbon/ Nanocomposites Health and safety

Space

Automobiles

Figure 3.1  Application of carbon-based nanocomposites [23].

3.2 Nanocomposites and their Processing Methods Nanocomposites can be produced using various techniques. The polymer composite formation can be categorized into two types i.e., direct compounding and in situ synthesis. In situ polymerization is economic and is made up of solvent, oligomer and nanoparticles of three-dimensional matrix. In compounding method, the solvents and monomers are not used environmental and economic advantage due to this it is the preferred method in polymer nanocomposites preparation. For preparation of homogeneous and welldispersed inorganic material in the polymer, blending polymers and nanoparticles are to be provided and which pose significant challenges. In both the methods, to increase the interaction among the polymers and nanofillers or to get a good dispersion in the polymer matrix, usually nanofillers are modified by functional groups which can help good compatibility in polymer mixing. The ease of handling and better performance of the final products are some of the advantages reported by this method [14–16]. The above techniques use polymer mixed with other plastics resulting in formation of blends, mixed with nanofillers such as clay, talc and carbon-based nanomaterials form filled systems to form nanocomposites. The plastics engineer by this “mix and match” Recently, carbon-based nanofillers are being used in various engineering fields. Carbon nanotubes (cylindrical graphene tubes with 1D sp2 hybridized

58  Emerging Carbon-Based Nanocomposites structure), graphene (single atomic layer of graphite with 2D sp2 hybridized structure) are spotted as the reinforcements in polymer matrices. Carbonbased nanocomposites are considered to be the prospective materials due to their specific characteristics in applications such as environment, sustainable energy and green technology. Similarly, several other forms of carbon such as fullerene, nanodiamond, nanofibers, carbon black, etc. have been discovered and explored. The distinctive and adjustable properties of carbon provide potential future progress in energy and ecological systems towards energy efficiency, pollutant transformation and toxicity control [17, 18]. Green nanocomposites have been designed for automotive, aerospace sectors, solar cell and fuel cell parts, wastewater treatment, and air pollution monitoring. Carbon has been employed to enhance the properties of green engineered nanocomposites considering environment and safety concerns [19–23].

3.3 Structures of Carbon Materials Carbon nanostructures and its offshoots gained significant attention because of their exceptional properties such as high aspect ratio and high surface to volume ratio due to which most of the atoms are located on surface and they can be exposed directly to the surrounding environment. In addition to this, different bonding order of carbon atom in various nano structures contributes to different tunable properties. Hence these materials are being used not only in a number of existing applications but also will be used in emerging ones. Few important forms of carbon i.e., fullerenes (0D), graphene sheets (2D) and diamond-like carbon (3D) [24–27]. Due to the unique properties (electrical conductivity, supreme mechanical strength, high thermal conductivity, extraordinarily high surface area, excellent photo luminescent properties [28], high transparency, and structural stability), it makes carbon nano architectures promising for applications stretching from thin film transistors [29], transparent conducting electrodes [30], photovoltaics [31], super capacitors [32], to biosensors [33], drug delivery [34], tissue engineering [35], and photothermal therapy [36]. These forms of carbon have frequent applications in automotive/ aircraft nanocomposite, solar cell, fuel cell and membrane materials.

3.4 Polymer/Carbon-Based Nanocomposite Nanofillers can be classified based on the their dimensions into one dimensional such as nanowires, nanotubes; two dimensional like graphene,

Green Nanocomposites: Advances and Applications   59 nanoclays and three dimensional such as cubical and spherical nanoparticles [37–40]. Carbonaceous nanofillers like graphene and nanotubes exhibit excellent properties as mentioned above (Figure 3.2). Carbon based nanocomposites have been developed consisting of polymers and nanostructures as filler for new dimensions for novel applications in fuel cells, solar cells, nanodevices, chemical sensors, biosensors, aerospace, automotive, etc. The first the Ajayan group carbon-based polymer nanocomposite was reported [41]. Thereafter, various carbon based material systems were built and studied. Different green synthesis routes have been adopted to fabricate the carbon nanocomposites. In order to enhance the properties of polymeric nanocomposites different forms of carbon nanostructures, high cost carbon nanotubes, low cost carbon black have been used as fillers [42]. Graphene and carbon nanotubes have been most favorable carbon based nanofillers till date. Carbon nanofillers possess specific characteristics which up on addition help in improving electrical conductivity of polymeric materials. There are number of publication on carbon based nanocomposites such as polystyrene (PS)/CNT composite, and poly(vinyl alcohol) (PVA)/modified graphene for improve the mechanical properties. Apart from improving the properties of nanocomposites, the combination of these materials also lead to many new and intriguing applications (Figure 3.3). In addition to this, nanostructure carbon and carbon-based polymer nanocomposites have been most reliable absorbents of environmental contaminants and latest developments have formed one nanohybrid material which is a combination of features of both polymer and carbon-based materials [43]. Nanohybrid materials have exhibit significant improvements in the properties which are difficult to be achieved with use of pure polymers or conventional composites. These composites can be used for conventional drinking water treatment, in which organic and inorganic contaminants are removed using physicochemical sorption processes. Continuous research on wastewater treatment has improved the understanding of sorption mechanisms which helped in enhancing sorbent properties. Despite nanomaterials

Fullerene (0D)

Nanotube (1D)

Graphene (2D)

Graphite (3D)

Figure 3.2  Different forms of carbon nanostructure [27].

Diamond (3D)

60  Emerging Carbon-Based Nanocomposites

polymerization

Nanocomposites

Nanomaterials

Carbon nanomaterials/ polymer nanocomposites

Wastewater

Nanocomposites

Metal ion/organic molecules Functionalized nanomaterials

Clean water

Insitu polymerization Initiator

Figure 3.3  General reaction mechanism for wastewater treatment.

being economical in environmental applications, the indemnification of common contaminants was not limited by their sorptive ability [44].

3.5 Removal of Chemical Contaminants Nanocomposites and carbon based nanomaterials like graphene, carbon nanotubes etc., are available in either functionalized or non-­functionalized forms. For example, functionalized carbon nanomaterials are highly dispersed in water when compared to its pure counter parts [45, 46]. With the use of metal or metal oxides, nanomaterials can be functionalized. The disinfection and adsorption properties of carbon based nanomaterials have been enhanced by the incorporation of carbon materials to the polymers of nanocomposites as shown in Table 3.1. The addition of organic functional groups through the process of surface functionalization to the carbon based nanomaterials helps in improving their adsorption capacities. This functionalization of groups on their surfaces is performed by either noncovalent or covalent methods [47–51]. The functionalization of carbon-based nanomaterials and nanocomposites not only enhances the hydrophilicity of carbon surface but also improves their dispersion in solution medium.

Green Nanocomposites: Advances and Applications   61 Table 3.1  Commonly used carbon-based nanomaterials, nanocomposites for treating common water contaminants. Carbon based nanomaterials/ nanocomposites

Chemical and biological removal

GO

Biomolecules, metals, bacteria, organic contaminants [54–57]

MWNT

Organic contaminants, metals [58, 59]

G

Bacteria, metals, organic contaminants [54, 60]

Activated carbon

Metals and organic contaminants and bacteria [61]

NOM

Metals, bacteria, viruses [62–63]

Polyaniline/GO

Metals [64]

CS-GO

Metals [65]

Poly(vinyl alcohol)-MWNT (PVA-MWNT)

Organic contaminants [66]

Polysulphone-MWNT

Metals [67]

PVA/PAA/TiO2/GO

Organic contaminants [68]

Polyacrylic acid modified magnetic mesoporous carbon

Metals

As stated earlier, extreme aspect ratio implies high surface area to volume ratio for carbon nanocomposites and their nanomaterials due to which they have an advantage compared to other macro or micro-scale materials for water treatment applications [52]. Because of these specific properties they can be used as adsorbents or scaffolding materials [52, 53]. The adsorption is affected by the nanomaterials aggregation state its highly depended on the solution chemistry. The aggregation state of nanomaterials affects the adsorption and is very much dependent chemistry of the solution [54]. The selfinteractions among nanomaterials and hydrophobic, electrostatic and Vander waals interactions between nanocomposites/nanomaterials and other compounds in the solution affect the aggregate behavior. pH and ionic strength are two key parameters in solution chemistry. pH and concentration are the two main factors which influence the dispersion stability of nanomaterials in aqueous solution. The pH of the solution not only effects the ionization state but also effects the nanomaterials’ aggregation states specifically the

62  Emerging Carbon-Based Nanocomposites non-functionalized ones for example: graphene, SWNT and MWNT which holds a negative charge value at pH6 and above obstructing the formation of aggregates. At pH3, the surface charge is greatly reduced, more readily allowing to aggregation [54]. Therefore, the efficiency of contaminants and ionization state of functional groups are influenced by change in pH level. The available surface of each nanoparticle and therefore their exposure to chemical contaminants are increased with increase dispersion [54]. Based on the surface charge properties of target contaminant, sorption capacity can be enhanced by maximizing the electrostatic interactions between sorbent and sorbate which can be achieved by altering the nanomaterial surfaces. It has been determined that the actual driving force behind adsorption of positively charged compounds was electrostatic interaction. Till now, the experiments and studies have been performed under stipulated laboratory conditions and the combination of complex chemistries like pH, organic matter and salts in the water has not been explored. In future, studies need to be performed to investigate the treatment of real water contaminants with complex water chemistries using carbon based nanomaterials [52]. Most of the researchers synthesize carbon-based nanomaterials with polymer nanocomposites during the study of water treatment. The polymers used are either bio degradable or non-bio degradable and as these carbon-based nanomaterials possess enriched ability to remove heavy metals, the carbon-based nanomaterials to polymer matrices fixation boosts their accessibility to heavy metals. At pH 5 to 11, Pb2+ can be removed at rate of 887.98 mg/g with addition of only 10 wt% of GO to poly-N-vinyl carbazole-GO (PVK-GO) nanocomposite. This nanocomposite material can used for commercial applications due to its low use of GO load and can be formulated easily [69, 70]. Polyacrylic acid modified magnetic mesophorous carbon (PAA/MMC), the estimated Cd(II) ion, the modified adsorbent’s optimum adsorption capacity has been higher than that of pristine materials and has increased from 140.8% to 406.6 mg/g. Similarly, for heavy metal removal polysulphone or functionalized MWNT (1wt%) can be used. In comparison with pristine polysulphone, the functionalized MWNT improves the metal adsorption capacity of Cd2+ by 78% by enhancing the hydrophilicity of the composite. In situ polymerized polyaniline/GO nanocomposite has been prepared using low cost aniline monomer with GO [71]. In contrast to pure polyaniline, this nanocomposite has been observed to possess large surface area and is highly porous nanostructure. It has showed a 94% of increase in adsorption for Hg(II) with addition of about 15 wt% GO or polyaniline in aqueous solution over pristine polyaniline. The use of carbon-based nanomaterials and their nanocomposites can eliminate the limitations of traditional water treatment techniques and

Green Nanocomposites: Advances and Applications   63 their application hold significant role in water treatment. This is largely dependent on the chemistry of water to treated (like pH), contact time, mixing rate (to some extent) and also the presence of functional groups on nanocomposites and nanomaterials. By the recent study, it is inferenced that the chemical contaminants which possess weak affinity towards traditional sorbents can be strongly binded from emerging using various carbon, carbon-based nanomaterials & nanocomposites and their high adsorption capabilities. These materials are highly efficient in removing heavy metals during water treatment when compared to the current adsorbent materials being used. Nevertheless, in order to productively commercialize this, more enhancements in terms of price and large scale production of materials would be required. Apart from this, the most uncharted question is whether would have similar removal capacities when treated on complex water chemistries in real world. Therefore, its important that research should also target in attesting the productiveness of the nanomaterials and nanocomposites when tested on natural water samples taking into consideration of various environmental factors that would effect the same. Also, its essential to explore novel methods which can aid in large-scale production of carbon-based nanomaterials in an economic way. Ultimately, the design of nanomaterials and nanocomposites should be such that their affect on humans and ecological system should be minimum along with viable solution for water treatment.

3.6 Energy Sector In energy sector, carbon and carbon-based nanocomposites have been successfully considering the green technology. Application in the various kinds of devices shows major innovation of carbon materials and nanocomposites in energy sector. Because of nanocarbon provides nanoscale active surface area for massive photon absorption. Owing to nano-size and delocalized π-electrons, charge transfer mobility has been enhanced several times [72] and also improves the stability and efficiency of energy devise [73]. Various conducting organic polymers used with nanocarbon include poly(3-hexylthiophene), poly(3-octylthiophene), [6,6]-phenyl-C61-butyric acid methyl ester, etc., usually use for energy storage devises. Nanocarbon serves to improve the thermal stability, dimensional resistance, and proton exchange performance by providing efficient charge transport at polymer/ nanocarbon interface. In addition, for other kind of devices, it is required to form percolating network which dependent on the nanomaterials. polymers’ physical properties, concentration of the carbon nanomaterials and

64  Emerging Carbon-Based Nanocomposites also composite preparation method. In polymer matrix of insulating type, the conduction level is dependent on the amount of conductive filler at low and high concentrations. The gaps between the conductive elements progressively decreases to tunnelling distance with increase in the concentration of the nanocarbons. In order to improve the number of conducting nanofiller networks in the polymer matrix, the synergistic effect i.e., the use of two or more carbon fillers evenly has been applied. At higher concentration of nanocarbons, the composites’ conductivity reaches an optimum constant value which is almost equal to the nanofiller conductivity. Various studies have been reported on carbon-based photovoltaic applications because carbon based materials and its composites possess large specific surface area, very good electronic conductivity and optical transparency. Some of the applications are transparent electrodes, electron acceptor, light absorber etc. The combination of carbon based materials with polymeric materials are regarded as the extremely good sensing elements for many sensing applications like gas sensors, mechanical sensors, electrochemical sensors, humidity sensors, temperature sensors and bio sensors. These materials are being used widely for sensing applications because of they possess capability of achieving high sensitivity with low cost, low noise, facile fabrication and are biocompatible in nature. Gradually, the traditional expensive sensing materials are being replaced by the carbon nanocomposites. The urge to develop new and smart sensing devices are the main factors driving the nanocomposite materials’ research.

3.7 Gas Sensors In recent years, there have been remarkable improvements in electronic and water treatment application fields. Here in this chapter we summarized the environmentally friendly application of carbon and carbon based nanocomposites. In the field of electronics, sensors play most significant role in many applications starting from food safety, human health, environmental monitoring to industrial, space applications. Furthemore, use of carbon nanocomposites and nanomaterials as sensing elements can help in improving the device functioning for development of advanced sensing applications. The combination of carbon based materials with polymeric materials are regarded as the extremely good sensing elements for many sensing applications like gas sensors, mechanical sensors, electrochemical sensors, humidity sensors, temperature sensors and bio sensors. Apart from this, due to the presence of catalytic degradation and adsorption mechanisms, these have been used in alleviation of contaminants from ground water, industrial effluents, gases, soils etc.

Green Nanocomposites: Advances and Applications   65 In recent times, gas sensors have gained more interest compared to other sensing applications as they play a major role in not only industrial and security applications but also in hindering the consequences of harmful gas exposure on environment and human health. With expansion of industries, the harmful emissions are also increasing due to which it has become necessary for the development of gas sensors that detect these emissions which in turn have to be treated to minimize the potential risks.

3.8 Conclusion and Outlook Carbon nanomaterials and their carbon-based nanocomposites have been widely developed functional materials in the water treatment and energy field. These nanomaterials and nanocomposites can firmly bind the emerging chemical contaminants which have very weak affinity towards traditional sorbents as they possess high adsorption capacities at specific condition. This abilities are advantageous in development of novel carbonbased nanocomposites for use in water treatment methods. Yet, there is a need of conspicuous advancements of these hybrids in terms of mass production and economic affordability so that they can be commercialized. Moreover, future research should mainly base on upholding the efficacy of the nanomaterials and nanocomposites when treating water samples in real world conditions and also the affects of surrounding environmental factors should also be taken into consideration. The researches are also concentrated on the implementation of composite materials in the fields of energy like super capacitors, solar cells, energy devices and in sensor applications.

Acknowledgment This work was financially supported by the ministry of science and technologytaiwan (MOST 107-2113-M-033-003).

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4 Carbon-Based Nanocomposites as Heterogeneous Catalysts for Organic Reactions in Environment Friendly Solvents Priyanka Choudhary, Ajay Kumar, Ashish Bahuguna and Venkata Krishnan* School of Basic Sciences and Advanced Materials Research Center, Indian Institute of Technology Mandi, Kamand, Himachal Pradesh, India

Abstract

Efforts toward protection of the environment from toxic and non-biodegradable materials are an important topic of discussion. The development of chemical processes that use environmentally benign solvents and non-toxic catalytic materials is of utmost importance. Water, ionic liquids, glycerol, polyethylene glycol, and supercritical fluids are some representative examples of environment friendly Green solvents that have demonstrated their promising activity for various reactions. Recently, carbon-based nanocomposites have emerged as low cost, nontoxic, highly efficient, and recyclable heterogeneous catalysts for several reactions, due to their easy availability and facile synthesis strategies. In this book chapter, we would like to focus on the utilization of some of the carbon-based nanocomposites as heterogeneous catalysts for organic reactions in environment friendly solvents. The carbon-based materials include graphene, carbon nanotubes, graphitic carbon nitride, and activated carbon, and some of the representative organic reactions include hydrogenation, oxidation, cyclo-addition, esterification, and C-C coupling. In the first part, a detailed note on the basic concept of organic reactions in environment friendly solvents has been presented followed by the recent advancements in the use of carbon-based nanocomposites as heterogeneous catalysts. Subsequently, an outlook into the future perspectives is provided.

*Corresponding author: [email protected] Ajay Kumar Mishra, Chaudhery Mustansar Hussain and Shivani Bhardwaj Mishra (eds.) Emerging Carbon-Based Nanocomposites for Environmental Applications, (71–120) © 2020 Scrivener Publishing LLC

71

72  Emerging Carbon-Based Nanocomposites Keywords:  Carbon, nanocomposites, heterogeneous catalysts, organic reactions, Green solvents

4.1 Introduction The process of catalysis can be categorized mainly as homogeneous or heterogeneous. Homogeneous catalysis is referred to the process in which catalyst is in the same phase as the reaction mixture, and in heterogeneous catalysis, the phase of the catalyst is different from that of the reaction mixture. As the catalyst is in a different phase, it has the advantage of easy separation and recyclability. The use of heterogeneous catalysts has gained a lot more importance than homogenous catalysts industrially as well as economically [1–3]. Over the years, heterogeneous catalysis has been explored for many prominent applications, including organic transformation reactions, such as olefin hydrogenation, C-H activation, C-C coupling, and many more [4–9]. Carbon-based nanocomposites have gained huge interest in catalysis due to their small size and shape-dependent physicochemical properties. Particularly, carbon materials (mainly graphene, graphitic carbon nitride, carbon nanotubes, and activated carbon) based nanocomposites have demonstrated extraordinary catalytic activity in organic reactions [10]. The products prepared by using carbon nanocomposites as catalysts have been found to be highly valuable in various fields, including pharmaceutical, biomedical, agricultural, and other industrial fields. The carbon-based materials generally consist of sp2 hybridization of carbon atoms. Due to sp2 hybridization of carbon, it generally possesses a two-dimensional (2D) order where it exists as layers of fused aromatic rings stacked through weak van der Waal interactions [11]. The degree of graphitization of carbon materials depends upon the planar nature of these layers [12, 13]. Thus, it can be concluded that the ordered arrangement of the layers comes under graphitic category while the disorder arrangement lead to the formation of carbon material with activated centers. For the production of fine chemicals industrially, carbon-based materials are of major importance [14–16]. The carbon-based materials used consists of graphitic inner structure surrounded by the π-electron density, which is mainly responsible for its activity. Using surface functionalization techniques, the carbon-based materials can be modified by various heteroatoms, such as with O, N, S, etc., for tuning its activity [17, 18]. However, the oxygen group functionalized carbon-based materials are the most studied because of the formation of various organic acids or bases

Nanocomposites as Heterogeneous Catalysts  73 on its exposure to atmosphere. The physical adsorption by carbon-based materials is also its important feature due to its porous nature [3, 19, 20]. In addition, there are several properties of carbon-based materials that are beneficial, such as high thermal and chemical stability, high surface area, excellent recyclability, and cost effectiveness [21]. High surface area and porosity are also some of the main parameters to be optimized before using it as a catalyst or catalyst support. Conventional heterogeneous catalysis generally suffers from the limitation that the most catalytically active sites are not properly exposed to the reactants. The selection of the appropriate support is of utmost importance to expose many of the catalytically active sites of the catalyst [21]. Thus, the carbon support-based materials are one of the emerging research areas for the rational development of the new generation heterogeneous catalysts. However, the complete experimental and theoretical understanding of the surface functionalities, defects, and tenability with other materials is also important [22]. In this chapter, the carbon-based materials are mainly divided into these categories: graphitic carbon nitride (GCN), graphene (GR),

NO2 R H H3C C CH3 OH O

HO

O H N R2 X

R1 Carbon nanotubes

Graphene Oxide

O OH

O Carbon based nanocomposites Graphitic carbon nitride

HO O Activated carbon

O

Graphene

O

NH2 O

O Ph

R

OH

O

O O

O

Figure 4.1  Schematic representation of various carbon-based nanocomposites along with some of the representative products formed using them as heterogeneous catalysts.

74  Emerging Carbon-Based Nanocomposites carbon nanotubes (CNTs), and activated carbon (AC) as shown in Figure 4.1. The use of the carbon-based materials for sustainable organic transformation reactions is a rapidly growing field in the recent decades and it has been discussed in detail in this chapter. Green chemistry is the need for the sustainable civilization since heterogeneous catalysis in aqueous solvent is considered a sustainable approach to reduce the environmental factor (E-factor) value in a reaction. The use of the heterogeneous catalysis for the organic transformation reactions in environmentally benign conditions is the first step towards the sustainable chemistry [23]. From the past few years, many industries have started using heterogeneous catalysts for the synthesis of fine chemicals and value added products. The key principle for the sustainable Green chemistry is the replacement of the hazardous solvents with environmentally benign solvents. Solvents used in chemical processes have crucial impact on health and the environment. Thus, the development of solvent-free processes or use of solvents which are environmental friendly is of paramount importance.

4.2 Carbon-Based Nanocomposites for Coupling Reactions Carbon-based nanocomposites are in particular very interesting materials showing potential applications as catalyst supports, adsorbents, electrodes, etc. Coupling reactions are usually exhibited by the metals supported on the carbon-based materials. For example, carbon supported palladium is used as heterogeneous catalyst in various hydrogenation and coupling reactions. These nanocomposites exhibit beneficial properties, such as negligible metal leaching, higher catalytic efficiency, and higher recyclability. In general, carbon supported palladium is used as heterogeneous catalysts in various hydrogenation and coupling reactions.

4.2.1 C-C Coupling Carbon-carbon (C-C) coupling reactions, such as Ullmann, Suzuki, and Heck reactions, have profound importance for applications in chemical, pharmaceutical, and biochemical industries [24–27]. Coupling reactions were earlier preferred to be performed using homogeneous catalysis. But, it was difficult to achieve high catalytic activity and selectivity of products. Also, the recyclability and the potential contamination of the products were also challenging tasks. Hence, significant efforts have been devoted in

Nanocomposites as Heterogeneous Catalysts  75 order to address this issue, which includes the use of carbon support-based heterogeneous catalysts. For example, Sun and coworkers have synthesized a nanocomposite in which Pd nanoparticles were supported on 2D sheet of graphitic carbon nitride (g-C3N4), and further, the prepared nanocomposite was utilized for the well-known Suzuki-Miyaura reaction which involves the C-C coupling between an organohalide and boronic acid. The nanocomposite was successfully synthesized via using a facile one-step photo-deposition method. The catalyst exhibited high performance for Suzuki-Miyaura coupling reactions at room temperature in environmentally benign solvent system without any use of phase transfer agents or toxic solvents achieving Green catalysis. The high catalytic activity of the nanocomposite was attributed to the specific characteristics of the unique nanostructure of Pd/g-C3N4. The porous g-C3N4 contains large number of nitrogen-containing sites, which helps to disperse and stabilize the supported Pd nanoparticles. The π-π stacking interaction between halobenzene molecules and g-C3N4 helps in accelerating the reaction. With the help of the prepared nanocomposites, a wide substrate scope has been achieved, which demonstrated high catalytic activity resulting in higher product yields as shown in Table 4.1 [28]. In addition to g-C3N4, graphene/graphene oxide/reduced graphene oxide have also been widely explored as carbon-based support materials for C-C coupling reactions. These heterogeneous catalysts synthesized using graphene support are of low cost, exhibiting high recyclability and promote environmental friendly Green synthesis of desired products. Li and co-workers [29] reported a facile method to synthesize a nanocomposite of graphene uniformly decorated with small sized Pd nanoparticles. Generally, the C-C coupling reaction needs a mixture of organic solvent and an aqueous inorganic base and inert atmosphere. But, the synthesized nanohybrids (Pd–graphene hybrids) with controllable size of palladium nanoparticles were found to be very effective catalysts for the Suzuki coupling reaction in aqueous medium under aerobic conditions. The excellent activity of the Pd-graphene catalyst can be attributed to the stabilization of Pd nanoparticles on the 2D graphene sheets, which prevents aggregation of nanoparticles. Also, the smaller sized Pd nanoparticles decorated on graphene sheets were found to be catalytically more active than that with the bigger ones due to the availability of large number of catalytic sites. Further, in another study in situ reduction of Pd precursor was performed on graphene oxide (GO) and chemically derived graphene (CDG) and the prepared nanocomposite has been utilized for the Suzuki-Miyaura reaction in ethanol-water solvent system. The catalyst showed extraordinarily high activity with turnover frequencies (TOFs) of up to 39,000 h−1 [30].

76  Emerging Carbon-Based Nanocomposites Table 4.1  Effect of aryl halides on the catalytic conversions in Suzuki-Miyaura coupling reaction performed using Pd/g-C3N4 nanocomposite as heterogeneous catalyst [28]. B

Br +

Entry

Aryl halide

OH

Pd/g-C3N4

OH

KOH, 25ºC EtOH/H2O

Product

Conversion (%)

1

I

100.0

2

Cl

38.6

3

Br

93.6

4

CH3

CH3

89.1

Br

5

Br

H3C

6

98.1

H3C CHO

CHO

89.4

Br

7

OHC

OHC

95.3

Br

8

OHC

Br

OHC

98.8

9

O2N

Br

O2N

98.2

10

HO

Br

HO

93.9

11

NC

Br

NC

100.0

12

COOH

COOH

90.7

Br

Notes: Aryl halides (0.2 mmol, 1 equiv), Phenylboronic acid (0.24 mmol, 1.2 equiv), KOH (0.6 mmol, 3 equiv, 4 ml; EtOH(1:1) and Pd0.10/g-C3N4, 25oC, 30 min.

Nanocomposites as Heterogeneous Catalysts  77 Another important coupling reaction is Heck coupling reaction. This reaction involves the Pd catalyzed substitution of vinylic hydrogen with a vinyl, aryl, or benzyl group. However, the reactions are generally conducted under harsh conditions (high temperature) using polar aprotic solvents like MeCN, DMSO, or DMAC, which are not environmentally benign. So, there was a need of an effective strategy which can accelerate the reaction under mild conditions using environment friendly solvents. In this regard, Elazab et al. [31] have reported a carbon support-based metal nanocomposite for Heck coupling reactions under mild reaction conditions using Green solvents. The highly active catalyst consists of Pd/Fe3O4 nanoparticles supported on graphene which was prepared by using microwave assisted chemical reduction method. Furthermore, a broad range of highly functionalized molecules were synthesized in high yields and are summarized in Table 4.2. The catalyst showed high recyclability up to 10 cycles without any major loss in its catalytic activity [31]. Moreover, there was a need of multifunctional catalyst which can single handedly do multiple types of C-C coupling reactions (Suzuki, Heck, Sonoghasira, and Ulmann) with high catalytic efficiency and selectivity in environmentally benign solvents. The use of carbon-based support has resolved this issue to an extent but still the high efficiency is a challenging aspect. However, the defect-based engineering and large number of catalytically active sites can improve the efficacy of the reactions. One such catalyst was designed by Moussa and coworkers [32] exhibiting high efficiency and turnover number (TON). Pd nanoparticles were supported on partially reduced graphene nanosheets (Pd/PRGO) by using pulsed laser irradiation. The laser irradiations of the PRGO lead to the formation of the defects on the surface of the catalyst which improves the catalyst-support interactions which consequently enhances the recyclability of the catalyst. The Pd/PRGO catalyst can catalyze multiple coupling reactions, such as Suzuki, Heck, and Sonogashira under mild reaction conditions. Overall, the carbon support-based catalysts are highly efficient and selective and can be recycled multiple times without any loss in catalytic activity.

4.2.2 C-N Coupling Nitrogen-based compounds are of much significance because of their biological, industrial, and pharmaceutical importance. The formation of C-N bond paves the way for introducing nitrogen into the organic compounds. C-N coupling reactions, such as Click reaction, Buchwald-Hartwig cross coupling reaction are very important for the synthesis of various chemicals in industries [33]. The formation of C-N bond under mild conditions

78  Emerging Carbon-Based Nanocomposites Table 4.2  Diversity of the Heck coupling reactions using the catalyst (Pd/Fe3O4 supported on graphene) [31]. R2 Br R1

+

Compound

Cat. (0.3 mol%) K2CO3 3 eq. 150ºC uw / 10 min H2O:EtOH (1:1)

R2

Alkyl halide

A

Alkene

R1

Product

Br NC NC

90% B

Br O O

94% C

Cl

Br Cl

NC

NC

95% D

Br HO O

90% E

Br O

F F O

82% (Continued)

Nanocomposites as Heterogeneous Catalysts  79 Table 4.2  Diversity of the Heck coupling reactions using the catalyst (Pd/Fe3O4 supported on graphene) [31]a. (Continued) R2 Br R1

+

Compound

Cat. (0.3 mol%) K2CO3 3 eq. 150ºC uw / 10 min H2O:EtOH (1:1)

R2

Alkyl halide

F

Alkene

R1

Product

Br H O

H O

92% Aryl bromide (0.32 mmol), alkene (0.64 mmol), potassium carbonate (0.96 mmol), Pd/ Fe3O4/G (0.3 mol%) in 4 ml (H2O:EtOH) (1:1) was heated under microwave at 150°C for 10 min. Isolated yields are also given. a

and cost-effective manner is a challenging task. However, the construction of C-N bond under mild reaction conditions is possible by using carbon support-based materials. For example, a Green synthesis route has been adopted for the synthesis of pristine graphene-CuO nanocomposite (Figure 4.2) [34]. Further, the prepared nanocomposite has been utilized Cu(CO2CH3)2• H2O CH3COOH, NaOH (90ºC) MW

CuO NPs

Figure 4.2  Schematic representation of microwave assisted hydrothermal synthesis of graphene-CuO nanocomposites. Reprinted with permission from ref. [34] Copyright 2017 American Chemical Society Publishers.

80  Emerging Carbon-Based Nanocomposites Table 4.3  Comparison of catalytic activity of different catalysts for CuAACReaction [34]a. N3 O2N

+

Catalyst (5.1 mol%) Sod. Ascorbate (40 mol%) H2O, 30ºC, 1 h

N O2N

Catalyst

Yield (%)b

CuOnano

50

RGOCuO

86

GCuO

99

N N

1-Azido-4-nitrobenzene (1 mmol), phenyl acetylene (1.2 mmol), and sodium ascorbate (40 mol %), in water (5 ml) stirred for 1 h at 30°C. bIsolated product yield. a

for the C-N coupling reactions. Pristine graphene has a very similar structure as that of graphite, and thus due to its large surface area, high thermal stability, electric conductivity, and absence of defects, it can behave as a perfect support material for nanoparticles [35]. The catalyst showed excellent activity due to the π−π interactions of the reactants with the catalyst (Table 4.3). The prepared catalyst was used for Cu-catalyzed azide-alkyne cycloaddition (CuAAC) reaction.

4.3 Carbon-Based Nanocomposites for Oxidation Reactions For the synthesis of various natural products, bioactive compounds, industrial chemicals and other valuable products, oxidation reactions are of paramount importance. Several organic transformation reactions, such as alcohols to aldehyde, aldehyde to acid, amines to imines, etc., can take place using conventional catalyst, but it requires drastic conditions and use of some harmful chemicals. However, by using carbon support-based heterogeneous catalysts, these reactions can be made more environment friendly.

4.3.1 Oxidation of Alcohols to Aldehydes/Ketones/Acids Use of metal-free catalysts is one of the cost-effective strategies for the selective aerobic oxidation of alcohols to aldehydes or ketones. Non-metal functionalization of the graphene sheet is done by using a high temperature

Nanocomposites as Heterogeneous Catalysts  81 nitration method to synthesize a series of catalysts for oxidation reactions (Figure 4.3) [36]. For the aerobic oxidation of alcohols, the graphitic nitrogen was found to be more active as compared to pyridinic and pyrrolic nitrogen atoms. The nitrogen doping on the multilayer graphene sheets provides the active site to activate dioxygen, which can further oxidize alcohols directly to aldehydes. Thus, metal free, N-doped graphene nanocomposite can be used for the Green and efficient aerobic oxidation reactions under mild reaction conditions (Table 4.4) [36]. Furthermore, the graphene-based catalyst has also been modified with metal nanoparticles decorated on β-cyclodextrin (rGO@Ru-RMβ-CD) by simultaneous one pot synthesis in water (Figure 4.4) [37]. The prepared catalyst showed excellent efficiency and good selectivity for the aerobic oxidation of alcohols under aqueous conditions. The reaction conditions are favorable for the oxidation of alcohols to synthesize aldehydes in good yield, high selectivity, and without any side products. A plausible mechanism has also been proposed for the oxidation of alcohols. In the presence of air, a thin film of RuO2 has been formed on the surface of Ru. While in aqueous environment ruthenium hydroxide has been formed on RuO2 film. Further, the formation of Ru-alcoholate takes place in the presence of alcohol substrate. Finally, the β-elimination takes place to generate corresponding carbonyl compound. The prepared catalyst has been successfully utilized for the total synthesis of natural product Brittonin A under Green reaction conditions (Figure 4.5).

Benzaldehyde Benzyl alcohol Water Oxygen

Figure 4.3  Nitrogen-doped graphene nanosheets as metal free catalysts catalyzing aerobic selective oxidation of benzylic alcohols. Reprinted with permission from ref. [36] Copyright 2012 American Chemical Society Publishers.

H

H

H

H

H

H

NO2

NO2

F

F

2b

3

4

5

6

7

8

9

10

1.0

H

H

1.0

1.0

1.0

1.0

1.0

1.0

1.0

1.0

O2 (105 Pa)

H

H

Cat.

343

313

343

313

343

313

343

313

343

313

T(K)

Reaction conditions

1/2O2

1.0

+

H

H

H

H

H

H

1b

R2

R1

Entry

R1

R2

OH

R1

Table 4.4  Catalytic oxidation of various types of alcohols over the catalysts [36]a. R2 +

H 2O

15.9

4.4

13.4

4.2

12.8

3.5

0.3

0.4

Conversion (%)

O

100

100

>95

100

100

100

100

100

Selectivity (%)

>98

>99

>98

>99

>98

>99

>99

>99

(Continued)

Yield (%)

82  Emerging Carbon-Based Nanocomposites

Cat.

313 343

1.0 1.0 1.0 1.0

H

H

CH3

CH3

H

H

H

H

H

13

14

15

16

17

18

19

1.0

1.0

1.0

343

343

343

313

343

R1

R2 +

H2O

4.0

6.5

6.1

1.8

0.5

10.6

3.0

14.8

3.2

Conversion (%)

O

100

100

100

>98

100

>98

100

100

100

Selectivity (%)

>98

>98

>98

>98

>99

>98

>99

>98

>99

Yield (%)

a

Reaction conditions: 30 mg of the catalyst; 0.1 mmol of alcohol dissolved in 80 ml of H2O; stirring speed, 1,300 rpm; reaction time, 10 h. bNo catalyst.

H

H

H

CH3

CH3

343

1.0

H

CH3O

313

12

1.0

H

CH3O

T(K)

11

O2 (105 Pa)

R2

Reaction conditions

1/2O2

R1

+

Entry

R1

R2

OH

Table 4.4  Catalytic oxidation of various types of alcohols over the catalysts [36]a. (Continued)

Nanocomposites as Heterogeneous Catalysts  83

84  Emerging Carbon-Based Nanocomposites –

Br + Ph3P

OH R1

O

R2

R1

R1: Aryl, alkyl R2: H, Aryl, alkyl

R3

R1 R2

R2

29 examples 93-75% yield

O2

R3

R2

Dehydrobrittonin

Brittonin A

H2 H2

O2 O2

Ru

R1

R3

O2

H2 H2 H2

O2

OH HOOC

O

OH

O

HO

O

OH OH OH

OH O

HO

= Ru nanoparticles

= Cyclodextrin

Figure 4.4  Schematic representation of rGO@Ru-RM β-CD catalyst for on-water aerobic oxidation of alcohols. Reprinted with permission from ref. [37] Copyright 2018 American Chemical Society Publishers.

O

1) O2 ballon rGO@Ru-RMβ-CD, K2CO3, H2O, 85ºC, 10-12h

OH

O O

2)

O

O O Br– PPh3+

O O KOt-Bu, 10-45ºC

O

O

O O

78% Brittonin A

3) H2 Balloon, 60ºC, 6h

Figure 4.5  One-pot total synthesis of brittonin A in aqueous medium using rGO@ Ru-RMβ-CD as catalyst.

The oxidation of carbonyl compounds to acids is one of the most fundamental organic transformation reactions and plays a vital role in various organic syntheses. Generally, the conventional oxidizing agents (KMnO4 and K2Cr2O7) are used for this type of oxidation reactions which is highly toxic and also produce several unwanted side products. But, the use of the

Nanocomposites as Heterogeneous Catalysts  85 HO

O

O

Au-Pd/CNT Water, O2

H

O H

Figure 4.6  Au-Pd/CNT catalyzed aerobic oxidation of 5-hydroxymethylfurfural (HMF).

carbon-based materials makes it feasible to perform this reaction under mild reaction conditions using Green solvents. For example, CNTs, another class of carbon-based materials, have also been used for the efficient oxidation reaction under mild reaction conditions. The oxidation of the 5-hydroxymethylfurfural (HMF) to 2,5-furandicarboxylic acid (FDCA) in water has been explored by using a bimetallic Au-Pd catalyst supported on CNTs (Figure 4.6) [38]. The CNTs having more carbonyl and quinone groups as compared to carboxylic groups helps in more adsorption of the reactants. The aqueous phase oxidation of HMF to FDCA becomes more convenient by the synergistic effect of CNT and Au-Pd nanoparticles.

4.3.2 Oxidation of Amines to Imines Imine-based compounds are of paramount importance as they can act as versatile intermediates for the synthesis of chemical, pharmaceutical, and biological molecules. Imines exhibit significant biological properties like antibacterial, antituberculosis, antitumor, and antimicrobial. Due to the presence of >C=N moiety in imines, they can easily interact with biological entities, like RNA, DNA, protein, lipid, etc. [39]. The conventional synthesis of imines involve condensation of amines with carbonyl compounds under drastic reaction conditions [40]. But, in the past few years, many efforts have been made for the direct synthesis of imines through oxidation process under mild reaction conditions. In this regard, Huang and coworkers [41] have reported one pot synthesis of imines from amines by using heterogeneous catalysts. This method is quite easy and eco-friendly for the synthesis of various derivatives of imines. It involves the use of 2D carbon-based material (graphene oxide) which is highly efficient, recyclable, and cost-effective catalyst for the transformation of amines to corresponding imines. Direct synthesis of symmetrical, unsymmetrical, and cyclic imines was possible by using this method as shown in Table 4.5.

4.3.3 Oxidation of Other Functional Groups The oxidation of C-H group can lead to the formation of various industrially important products, such as terephthalic acid which is core of

86  Emerging Carbon-Based Nanocomposites Table 4.5  Oxidation of amines to imines catalyzed by GO [41]a. NH2 Graphite oxide, O (5atm) 2

N

100ºC

Entry

Substrate

1

NH2

2

NH2

3

NH2

Time [h]

Con.b [%]

Yieldc [%]

4

99

98(91)

3.5

99

97

3

99

98(92)

3

91

88

3

84

80

5

95

89

6

96

88

2

97

96(90)

2

93

88

4

n.r.

–

8

98

88(81)

8

95

83

O

4

O

NH2

5

NH2 O

6

NH2 Cl

7

NH2 F

8

N

9

O

10

11 12

NH2

NH2 NH2

NH

NH

(Continued)

Nanocomposites as Heterogeneous Catalysts  87 Table 4.5  Oxidation of amines to imines catalyzed by GO [41]a. (Continued) NH2 Graphite oxide, O (5atm) 2

N

100ºC

Entry

Substrate

13

Time [h]

Con.b [%]

Yieldc [%]

8

97

75

5

97

95(90)

4

n.r.

–

4

n.r.

–

N H

14 NH

15 NH

16

NH2

Reaction conditions: amine (5 mmol), 50 wt% loading GO, 5 atm O2, 100°C, n.r. = no reaction. bConversion of amine to the corresponding imine, as determined by GC with anisole as external reference. cNumbers in parenthesis refer to yields of isolated products. a

various polyester fibers. The oxidation of p-xylene to terephthalic acid is of profound interest. Generally, this oxidation reaction was carried out by using AMOCO process [42] which requires highly acidic medium and was catalyzed by magnesium, cobalt, and bromide compounds. This method still has some drawbacks, such as aggressive nature of its components and requirement of highly specialized equipments. Therefore, it was highly required to development an environment friendly method with similar capabilities to synthesize value added products. For example, nanoscale graphene oxide (NGO) sheets were used for chemo selective oxidation of benzylic C-H group to corresponding COOH group in aqueous medium [43]. The prepared metal-free catalysts showed conversion of toxic, cheap, and easily available materials to non-toxic and more valuable oxygenated chemical feed stocks. The nanoscale GO sheets are smaller in size and possess high density of defects due to the large number of edge sites and thus results in high catalytic performance. The effect of various co-solvents is shown in Table 4.6 which demonstrates that on moving from one solvent to other there is significant change in the selectivity and product yield. Overall, the carbon-based heterogeneous catalysts are very important

H2O2(eq)

7

7

7

7

7

7

7

7

7

Entry

1

2

3

4

5

6

7

8

9

50

200

100

100

100

0

50

100

200

NGO (wt%)

100

Ethanol

Acetone

Acetone

100

100

100

100

100

100

100

100

Acetonitrile

Acetone

HO

O

Temperature (oC)

Ambient Pressure H2O as Solvent

H2O2 Oxidant NGO Carbocatalyst

Co-solventa

CH3

CH3

24

24

24

24

24

24

24

24

24

Time (h)

O

OH

Table 4.6  Optimization of the oxidative conversion of p-xylene by NGO carbo-catalysts [43]a.

40

70

65