Biodegradability of Conventional Plastics: Opportunities, Challenges, and Misconceptions 0323898580, 9780323898584

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Biodegradability of Conventional Plastics: Opportunities, Challenges, and Misconceptions
 0323898580, 9780323898584

Table of contents :
Biodegradability of Conventional Plastics:Opportunities, Challenges, and Misconceptions
List of contributors
1 . Life cycle assessment and environmental impact of plastic waste
Life cycle assessment
Definition of life cycle assessment
Fate of plastics in the environment
Movement of plastic trash: from land to aquatic ecosystem
Effect of plastic dumping on aquatic ecosystem
Environmental impact of various plastic products
Major repercussions of plastic waste
Global production of plastics and generation of waste
Management of plastic wastes
Recommendations to reduce and control plastic wastes
Future directions and recommendations
2 . Composition, properties and other factors influencing plastics biodegradability
Microbial degradation of plastic materials
Mechanisms of biodegradation
Microorganisms involved in biodegradation
The plastisphere and future possibilities
Plastic-degrading enzymes
Influence of plastic properties on biodegradation
Polymeric composition
Molecular properties
Molecular weight
Hydrophobicity and morphology
Additive chemicals
Influence of environmental and external parameters on plastic biodegradation
Salinity and pH
Dissolved oxygen
Sunlight and UV exposure
Moisture and humidity
Fragmentation and transport of polymers in the marine environment
Adsorption of pollutants to plastic debris
Biodegradation potentials in marine environments
Marine micro- and macrofauna involved in plastic degradation
Ghost fishing
Challenges and misconceptions
Conclusions, knowledge gaps, and future research
3 . Bioplastics, biodegradable plastics, and degradation in natural environments
Types of plastics
Synthetic plastics
Bioplastics classification
Biobased nonbiodegradable plastics
Fossil-based biodegradable plastic
Biobased biodegradable plastic
Bioplastics synthesized from bioderived monomers
Bioplastics from microorganisms
Bioplastics from biomass product
Cellulosic-based bioplastics
Bioplastics based on plant and animal proteins
Starch-based bioplastics
Challenges of starch-based bioplastics
Mechanical properties
Processability of starch-based materials
Solutions for starch-based bioplastic challenges
Improvement in hydrophilic property
Improvement in mechanical property
Improved processability
Thermal stability
Problem statement
Preparation of starch biobased plastic from banana peels
Preparation of filler
Types of degradation
Photo (radiation) degradation
Thermal degradation
Chemical degradation
Biological degradation (biodegradation)
4 . Bioplastics overview: are bioplastics the panacea for our environmental woes?
What are bioplastics?
Could bioplastics tackle the issue of natural plastic accumulation?
What are the likely environmental dangers of using bioplastics?
What is the capacity for bioplastics to tackle pollution caused by conventional petroleum-based plastics?
Disposable plastic items
Agricultural application (biodegradable plastic mulch film)
High-end market
What are the opportunities and difficulties of using bioplastics?
5 . Generation and impact of microplastics and nanoplastics from bioplastic sources
Bioplastics: sources and sinks
Bioplastics market
Biobased polyethylene
Biobased polypropylene
Biobased polyethylene terephthalate
Biobased polyvinyl chloride
Polylactic acid
Cellulose acetate
Microplastics: sizes, forms, and manufacturing
Distribution of microplastics
Aquatic environment
Terrestrial environment
Fate of microplastics and nanoplastics
Quantification of microplastics and nanoplastics
Visual sorting
Spectroscopic techniques
Fourier-transform infrared spectroscopy
Raman spectroscopy
Thermal degradation
Mitigation of microplastics and nanoplastics
Health impacts of microplastics and nanoplastics
Knowledge gaps and key directions
6 . Biodegradability of synthetic plastics: effective degradation mechanisms
Market growth of synthetic polymers and challenges in degradation
Synthetic polymers and biodegradation by microbial species
Biodegradation of polyethylene
Biodegradation of polystyrene
Biodegradation of polyvinyl chloride
Factors affecting the rate of biodegradation
Role of enzymes in biodegradation
Tests for assay of biodegradation of synthetic polymers
Further reading
7 . Biodegradability of polyolefins: Processes and procedures
Oxo-biodegradation mechanism of polyolefins
What is biodegradation?
Biodegradation mechanism of polyolefins
Enhanced polyolefin biodegradation
Blending and mixing
Blending polyolefins with natural biodegradable polymers
Blending polyolefins with synthetic biodegradable polymers
Additives and prooxidants
Pretreating with external conditions
Ultraviolet light
High energy radiation
Thermal treatment
Chemical treatment
Incorporation with polymer nanocomposites
Introduction to microbes and microbial products
Genetically modified microorganisms
Programmed biodegradation and its consequences
Future trends
8 . Biodegradability and current status of polyethylene terephthalate
Synthesis and properties of polyethylene terephthalate
Polyethylene terephthalate: applications and environmental impact
Mechanism of polyethylene terephthalate biodegradation
Polyethylene terephthalate-degrading microorganisms
Ideonella sakaiensis—the polyethylene terephthalate specialist
Polyethylene terephthalate-hydrolyzing enzymes
Bioeconomy of polyethylene terephthalate biodegradation and bioproduction
Recent advances in polyethylene terephthalate production and degradation
Future prospects
9 . Biodegradability and bioremediation of polystyrene-based pollutants: An overview of biological degradation of polystyrene and modified polystyrene for future studies
What are the forms of polystyrene?
Why are polystyrene-based pollutants so hard to biodegrade?
Biodegradability of polystyrene-a review of known methods
Microorganisms (fungi, bacteria, and archaea) in biodegradation
Larvae in biodegradation
Invertebrates in biodegradation
Plastic depolymerizing enzymes
Advantages and implications of PS biodegradation
Future perspectives
10 . Biodegradability of Polyvinyl chloride
Types and properties of PVC
PVC waste and environmental challenges
PVC disposal methods
Physical treatment
Chemical treatment
Biological treatment
Stages of PVC biodegradation
Assimilation and mineralization
Factors affecting PVC biodegradability
PVC-degrading insects
PVC-degrading microorganisms
PVC-degrading enzymes
Conclusion and prospects
11 . Biodegradability of automotive plastics and composites
Plastic pollution, an environmental health concern
Plastic biodegradation challenges
Remediation methods for plastics
Plastics biodegradation
Polyvinyl chloride
Best practices for plastics biodegradation
Biodegradation of plastics: steps and mechanism
Factors affecting plastics biodegradation by microbes
Polymer composites
Biodegradation of polymer composites
Future prospects
12 . Biodegradability of agricultural plastic waste
Issues and consequences of agricultural plastic waste
Misunderstanding (Rujnić-Sokele & Pilipović, 2017)
Indistinguishable nature (Razza & Innocenti, 2012)
Uncontrolled burning of agricultural plastic waste (Briassoulis et al., 2013)
High cost (Zheng et al., 2005)
Analysis of agricultural plastic wastes
Mapping of agricultural plastic wastes
Management of plastic waste
Source reduction
Reduced packaging
Product reuse
Improved product durability
Product safety associated with aesthetic pollution
Inferior quality of life
Physical and mental health issues
Loss of the regional natural beauty
Monetary losses
Significance of managing plastic wastes
Legal requirements
Environmental impact
Improved human health
Customized commercial waste management services
Advantages of biodegradable plastic
Less waste sent to landfills or incinerators
Reduced energy to manufacture
Fewer harmful substances released during breakdown
13 . Utilization of chemical additives to enhance biodegradability of plastics
Biodegradable plastic
Chemical additives
Biodegradation-promoting additives
Accelerating degradation
Inorganic oxo-degradation agents
Organic prodegradation agents
Blending with natural polymers
Reinforcement with natural fibers
Scheme for allocating degradation agents
Commercially available degradation-promoting additives
14 . The role of nanomaterials in plastics biodegradability
Environmental concerns for microplastics and nanoplastics
Remediation techniques for plastic pollution
Bioplastics: a new generation of polymers
Types of bioplastics
Polylactic acid
Limitations of bioplastics
Biodegradation mechanism
Biodegradation of plastics via microorganisms
Biodegradation of plastics via nanomaterials
Biodegradation of various polymers
Degradation of polypropylene
Photo- and thermal degradation of polypropylene
Biodegradation of polypropylene
Photothermal degradation of polyethylene terephthalate
Conclusion and future scope
15 . Microbial attachment studies on “plastic-specific” microorganisms
Background of plastics within the environment
Microbial bioremediation of plastic
Mechanisms of microbial biodegradation
Bioassimilation and mineralization
Microbial colonization on the plastic surface
The fate of microbial carbon biomass resolution
Plastic forms and microbial attachment bioremediation
Starch-based bioplastics
Polylactic acid bioplastics
Aliphatic-based bioplastics
Synthetic plastics
Bioremediation of marine-specific microplastics
Characterization for biodegradation analysis
16 . Plastic waste to plastic value: Role of industrial biotechnology
Impact of plastic waste on the environment and human health
Recycling plastic waste
Primary recycling
Secondary recycling
Tertiary recycling
Biological recycling
Quaternary recycling
Role of industrial biotechnology in plastic waste management
Use of plastic waste as substrate for value-added products
Challenges and prospects of industrial biotechnology
Concluding remarks and future perspectives
17 . Future prospects for the biodegradability of conventional plastics
Scope of biodegradation
Biodegradation parameters
Properties of degradation of polymers
Impact of biopolymers
Research on plastics degradation
Industrial biodegradable polymers
Biodegradable polymers as biosensors
Outlook for plastics degradation
Prospects for biopolymers

Citation preview

Biodegradability of Conventional Plastics Opportunities, Challenges, and Misconceptions

Edited by Anjana Sarkar Department of Chemistry, Netaji Subhas University of Technology, Delhi, India

Bhasha Sharma Department of Chemistry, Shivaji College, University of Delhi, India

Shashank Shekhar Department of Applied Science and Humanities, Faculty of Technology, University of Delhi, Delhi, India

Elsevier Radarweg 29, PO Box 211, 1000 AE Amsterdam, Netherlands The Boulevard, Langford Lane, Kidlington, Oxford OX5 1GB, United Kingdom 50 Hampshire Street, 5th Floor, Cambridge, MA 02139, United States Copyright © 2023 Elsevier Inc. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Details on how to seek permission, further information about the Publisher’s permissions policies and our arrangements with organizations such as the Copyright Clearance Center and the Copyright Licensing Agency, can be found at our website: This book and the individual contributions contained in it are protected under copyright by the Publisher (other than as may be noted herein). Notices Knowledge and best practice in this field are constantly changing. As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary. Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein. In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility. To the fullest extent of the law, neither the Publisher nor the authors, contributors, or editors, assume any liability for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions, or ideas contained in the material herein. ISBN: 978-0-323-89858-4 For information on all Elsevier publications visit our website at Publisher: Matthew Deans Acquisitions Editor: Ana Claudia A. Garcia Editorial Project Manager: Sara Greco Production Project Manager: Kamesh Ramajogi Cover Designer: Matthew Limbert Typeset by TNQ Technologies

List of contributors Muhammad Afzal Department of Bioinformatics and Biotechnology, Government College University Faisalabad, Faisalabad, Punjab, Pakistan Omar Amin Mechanical Engineering, Ain Shams University, Cairo, Egypt Ayodeji Emmanuel Amobonye Department of Biotechnology and Food Science, Faculty of Applied Sciences, Durban University of Technology, Durban, South Africa Christiana Eleojo Aruwa Department of Biotechnology and Food Science, Faculty of Applied Sciences, Durban University of Technology, Durban, South Africa; Department of Microbiology, School of Sciences, Federal University of Technology, Akure, Nigeria Farrukh Azeem Department of Bioinformatics and Biotechnology, Government College University Faisalabad, Faisalabad, Punjab, Pakistan Prashant Bhagwat Department of Biotechnology and Food Science, Faculty of Applied Sciences, Durban University of Technology, Durban, South Africa Andy M. Booth SINTEF Ocean, Trondheim, Norway Odd Gunnar Brakstad SINTEF Ocean, Trondheim, Norway Vijay Chaudhary Department of Mechanical Engineering, Amity School of Engineering and Technology, Amity University, Noida, Uttar Pradesh, India Partha Pratim Das Department of Mechanical Engineering, Amity School of Engineering and Technology, Amity University, Noida, Uttar Pradesh, India; Department of Materials Science and Metallurgical Engineering, Indian Institute of Technology Hyderabad, Sangareddy, Telangana, India Sherifa ElHady Industrial Engineering, SESC Centre, Nile University, Cairo, Egypt Amal Elhussieny Industrial Engineering, SESC Centre, Nile University, Cairo, Egypt Irene Samy Fahim Industrial Engineering, SESC Centre, Nile University, Cairo, Egypt



List of contributors

Sanjeev Gautam Advanced Centre for Polymer Science, Department of Chemistry, Netaji Subhas University of Technology, Delhi, India Pallav Gupta Department of Mechanical Engineering, Amity School of Engineering and Technology, Amity University, Noida, Uttar Pradesh, India Sumit Gupta Department of Mechanical Engineering, Amity School of Engineering and Technology, Amity University, Noida, Uttar Pradesh, India Sigrid Hakva˚g SINTEF Ocean, Trondheim, Norway Muhammad H. Hasan Department of Mechanical and Industrial Engineering, Ryerson University, Toronto, Canada Md. Enamul Hoque Department of Biomedical Engineering, Military Institute of Science and Technology (MIST), Dhaka, Bangladesh Muhammad Imran Department of Environmental Sciences, COMSATS Institute of Information Technology, Islamabad, Pakistan Habibul Islam Department of Biomedical Engineering, Military Institute of Science and Technology (MIST), Dhaka, Bangladesh Hira Kanwal Department of Bioinformatics and Biotechnology, Government College University Faisalabad, Faisalabad, Punjab, Pakistan M. Mahfuza Khatun Deptartment of Genetic Engineering and Biotechnology, Bangabandhu Sheikh Mujibur Rahman Maritime University, Dhaka, Bangladesh Khushbu Department of Applied Chemistry, Delhi Technological University, Delhi, India Stephan Kubowicz SINTEF Industry, Oslo, Norway Amit Kumar Department of Chemistry, Dayal Singh College, University of Delhi, India Lakhan Kumar Department of Biotechnology, Delhi Technological University, Delhi, India Meenu Malaviya National Institute of Technology JLN Marg, Jaipur, Rajasthan, India

List of contributors


Mankeshwar Kumar Mishra Department of Mechanical Engineering, Amity School of Engineering and Technology, Amity University, Noida, Uttar Pradesh, India Saima Muzammil Department of Microbiology, Government College University Faisalabad, Faisalabad, Punjab, Pakistan Habibullah Nadeem Department of Bioinformatics and Biotechnology, Government College University Faisalabad, Faisalabad, Punjab, Pakistan Shubham Pant Electrochemical Process Engineering Division, CSIR-Central Electrochemical Research Institute (CECRI), Karaikudi, Tamil Nadu, India; Academy of Scientific and Innovative Research (AcSIR) e CSIR, Ghaziabad, Uttar Pradesh, India Santhosh Pillai Department of Biotechnology and Food Science, Faculty of Applied Sciences, Durban University of Technology, Durban, South Africa Md. Zillur Rahman Department of Mechanical Engineering, Ahsanullah University of Science and Technology, Dhaka, Bangladesh Manviri Rani Malaviya National Institute of Technology JLN Marg, Jaipur, Rajasthan, India Ijaz Rasul Department of Bioinformatics and Biotechnology, Government College University Faisalabad, Faisalabad, Punjab, Pakistan Justyna Rybak Wrocław University of Science and Technology, Faculty of Environmental Engineering, Wroclaw, Poland Anuradha Saha Department of Applied Sciences, Galgotias College of Engineering & Technology, Greater Noida, Uttar Pradesh, India Anjana Sarkar Department of Chemistry, Netaji Subhas University of Technology, New Delhi, India Uma Shanker Department of Chemistry, Dr. B R Ambedkar National Institute of Technology Jalandhar, Jalandhar, Punjab, India Amit Kumar Sharma Department of Chemistry, Ramjas College, University of Delhi, University Enclave, Delhi, India Bhasha Sharma Department of Chemistry, Shivaji College, University of Delhi, India


List of contributors

Reetu Sharma Department of Chemistry, Netaji Subhas University of Technology, New Delhi, India Shreya Sharma Department of Chemistry, Netaji Subhas University of Technology, New Delhi, India Shashank Shekhar Department of Chemistry, Netaji Subhas University of Technology, New Delhi, India Muhammad Hussnain Siddique Department of Bioinformatics and Biotechnology, Government College University Faisalabad, Faisalabad, Punjab, Pakistan Ashok Singh Department of Mechanical Engineering, Amity School of Engineering and Technology, Amity University, Noida, Uttar Pradesh, India Harjeet Singh Guru Nanak Dev Institute of Technology, Delhi Skill and Entrepreneur University, Delhi, India Suren Singh Department of Biotechnology and Food Science, Faculty of Applied Sciences, Durban University of Technology, Durban, South Africa Agnieszka Stojanowska Wrocław University of Science and Technology, Faculty of Environmental Engineering, Wroclaw, Poland Prakash Chander Thapliyal Advanced Structural Composites and Durability Group, CSIR-Central Building Research Institute, Roorkee, Uttarakhand, India Ravi Babu Valapa Electrochemical Process Engineering Division, CSIR-Central Electrochemical Research Institute (CECRI), Karaikudi, Tamil Nadu, India; Academy of Scientific and Innovative Research (AcSIR) e CSIR, Ghaziabad, Uttar Pradesh, India Sudhir G. Warkar Department of Applied Chemistry, Delhi Technological University, Delhi, India Farhad Zeynalli Wrocław University of Science and Technology, Faculty of Environmental Engineering, Wroclaw, Poland Muhammad Zubair Department of Bioinformatics and Biotechnology, Government College University Faisalabad, Faisalabad, Punjab, Pakistan


Life cycle assessment and environmental impact of plastic waste


Partha Pratim Das1,2, Ashok Singh2, Mankeshwar Kumar Mishra2, Vijay Chaudhary2, Sumit Gupta2 and Pallav Gupta2 1

Department of Materials Science and Metallurgical Engineering, Indian Institute of Technology Hyderabad, Sangareddy, Telangana, India; 2Department of Mechanical Engineering, Amity School of Engineering and Technology, Amity University, Noida, Uttar Pradesh, India

Introduction Expanding environmental consciousness to achieve product sustainability has encouraged impressive efforts to use more environmentally friendly products in product designs (Ahmad et al., 2020; Chaudhary et al., 2018; Das & Chaudhary, 2020; Das et al., 2021). Crude oil is recognized globally as a key unsustainable source of carbon dioxide (CO2) and methane (CH4) concentrations worldwide that has largely surpassed natural consumption levels (Huber, 2004; Pretty & Bharucha, 2018). Most plastics used today are made from crude oil and other fossil fuels, including natural gas and coal. Moreover, their predicted lives, and thus their environmental perseverance, are far from certain. Of the problems of resource depletion, CO2 from fossil fuel combustion should also be considered. This undoubtedly and significantly affects global warming, which could have future societal, economic, and environmental effects if it is not addressed. A list of properties should be considered for engineers to design products according to rigidity, strength, density, and working temperature to ensure that the material chosen best fits the intent and respective production technology (Manral et al., 2020). Such a list must also resolve possible environmental concerns concerning energy efficiency, pollution emissions, and recycling (Chaudhary & Ahmad, 2020; Das & Chaudhary, 2021a). A product design focusing on environmental issues will use less environmentally damaging products and choose cleaner manufacturing processes (Knight & Jenkins, 2009; Roy, 2000; Tsoulfas & Pappis, 2006). As a result of such designs, dangerous and harmful products would be avoided, and energy efficiency in processing would be simultaneously maximized. Design considerations also include the use, management, and recycling of the product (Das & Chaudhary, 2021c). Life cycle assessment (LCA), as shown in Fig. 1.1, is a valuable tool for creating products to solve environmental problems. It is a comprehensive method to determine the overall environmental impact and a clear structure for minimizing them (Arena et al., 2003; Gu et al., 2017; Zhao et al., 2009).

Biodegradability of Conventional Plastics. Copyright © 2023 Elsevier Inc. All rights reserved.



Chapter 1 Life cycle assessment

FIGURE 1.1 Steps of life cycle assessment.

Life cycle assessment Definition of life cycle assessment ISO 14040:2006 describes LCA as “compiling and assessment, over the entire life cycle, of the inputs, outputs and possible impacts on the environment generated by a productive system,” as shown in Fig. 1.2. LCA can be introduced to estimate the effect of a final product by considering the impacts of resource production over its entire life cycle. LCA supports the design and evaluation of technological solutions used in the manufacturing phase to mitigate impacts from manufacturing, use, and end-oflife periods (Pryshlakivsky & Searcy, 2013; ISO, 2006). LCA is a method assessing the environmental impact of a commodity over its entire life span, beginning with the removal of the Earth’s raw materials and ending with the return of the commodity’s waste products to the Earth. LCA gathers information and translates it into environmental effects (using impact assessment methodologies), such as climate change contribution, smog generation, eutrophication, acidification, and human and ecosystem toxicities. It also covers inputs and outputs such as waste and process resources (life cycle inventory) (Finnveden et al., 2009; Yarramsetty et al., 2018).

Movement of plastic trash: from land to aquatic ecosystem


FIGURE 1.2 Life cycle assessment procedure according to ISO 14040.

Fate of plastics in the environment Determining the environmental fate of micro/nanoplastics is inherently difficult, mostly due to the multiplicity of sources and entry routes into the environment and the timescales necessary to determine their degradation pathways (Wang et al., 2021). Environmental analyses of smaller particles are made difficult by their size. Quantifying these materials is rather difficultdparticularly for smaller-sized plastics, standardized methods for sampling, unit normalization, data expression, quantification, and identification are lacking (Bergmann et al., 2016). In addition, a unified definition is absent for these materials, especially for nanoplastics. Microplastics have been identified across the globe, including in remote locations, from the Arctic to the Antarctic, throughout the water column, and from the surface to the depths (benthos). Microplastics are also found in rivers and lakes, agricultural soils, sediments, and the atmosphere in both indoor and outdoor environments (Boots et al., 2019; Hurley et al., 2018; Vianello et al., 2019; Woodall et al., 2014). Plastics reach the environment through various routes, particularly the marine environment. Abiotic or biotic processes may cause the environmental degradation of plastics. Such biodegradation requires abiotic degradation as the vital first step. Abiotic degradation results in materials with reduced structural and mechanical integrity and particles with higher surface-area-to-volume ratios, thus making them more susceptible to microbial action (Alshehrei, 2017; da Costa et al., 2020). However, including plastics in various environmental matrices almost certainly leads to expanded physical, chemical, and biological interactions with potential environmental and ecotoxicological consequences (Paluselli et al., 2018).

Movement of plastic trash: from land to aquatic ecosystem Plastic is a synthetic material made from hydrocarbons that can be molded into solid objects of nearly any shape or sizedcracking crude oil results in various petrochemicals as the bases for plastics.


Chapter 1 Life cycle assessment

Plastics include polyethylene and polypropylene (PP), synthesized from olefins, and materials synthesized from aromatic hydrocarbonsdfor example, polyamide (PA) and polystyrene (PS, or nylon). Plastics are typically synthesized in 0.5 to 5 mm nurdles or spherical pellets; these preproduction materials are transported to factories where they are heated, blow-molded, or extruded into the required shape for the intended purpose. Currently, the plastics sectors are divided into packaging, transportation, building, textiles, electronics, safety, and leisure. In 2017, plastic production approximated 348 million tons worldwide. Plastics have replaced heavier and more expensive materials such as glass, steel, and aluminum. Plastics come in various configurations, depending on the chemical building blocks used. The primary polymers currently produced are PP, high-density polyethylene (HDPE), low-density polyethylene, PS, polyethylene terephthalate (PET), PA, and polyvinyl chloride (PVC) (Barnes, 2019; Laskar & Kumar, 2019; Verma et al., 2016; Wagner et al., 2014). Presently, only 9% of all plastics manufactured are ever recycled (d’Ambrie`res, 2019), which is characteristically accomplished through mechanical recycling, or closed-loop recycling, which retains the chemical structure of recycled materials, and open-loop recycling, wherein recycled plastics are used for different purposes than those from which they were recovered. Mechanical recycling methods produce a slightly lower-quality product than virgin plasticdthis is due to the degradation processes, which result in decreased material quality. New recycling techniques are being developed to improve recyclability in closed-loop recycling; these methods include chemical recycling (by dissolving the plastics in solvents) and thermochemical recycling (pyrolysis). Twelve percent of plastic waste is incinerated. A few countries obtain energy from this burning process to heat houses and produce electricity. Globally, most plastic is disposed of in landfills. In addition, a fraction of plastic is lost to the environment directly through littering, estimated at 2% of total plastic production (Geyer et al., 2017; Letcher, 2020; Thiounn & Smith, 2020). The movement of plastic trash from land to aquatic environments causes marine debrisdlitter that ends up in oceans, seas, or other large bodies of water. This manufactured waste enters the water in various ways. Humans often leave trash on beaches or throw it from boats or offshore facilities into the water. Occasionally, litter makes its way into the ocean from land, carried by storms, drains, canals, or rivers. The wind can also blow trash from landfills and other areas into the water. Plastic products are very harmful to marine life. For example, loggerhead sea turtles often mistake plastic bags for jellyfish, their favorite food. Also, many birds and sea animals have been choked by the plastic rings often used to hold six-packs of soda together (Isangedighi et al., 2018; Provencher et al., 2019; Schwarz et al., 2019).

Effect of plastic dumping on aquatic ecosystem The plastic revolution that profoundly and rapidly influenced the maritime industry has also produced telling impacts on the marine environment. One study determined that 86% of manufactured debris in the northern Pacific Ocean is plastic material. Several varieties of marine life are perniciously affected by plastic pollution. The plastic threat is now a global phenomenon that afflicts crustaceans, fish, turtles, marine birds, and mammals. The scope and severity of this threat vary by species and the type of plastic involved. Plastics threaten the marine environment in various ways but principally through the entanglement of marine animals. For example, birds that dive beneath the water for their prey can become ensnared in the nearly invisible plastic monofilament line discarded by recreational fishermen. When these birds return to their nests, the line may snag in tree branches and may even entangle the birds’ young in some cases. Entanglement in fishing lines has replaced DDT poisoning as the primary

Movement of plastic trash: from land to aquatic ecosystem


cause of mortality for the endangered brown pelican. Seabirds may become entangled in other forms of plasticware as well. Plastic six-pack rings can ensnare birds, impairing their ability to fly or breathe. Other birds drown in discarded pieces of fishing nets that have been left in the same locations as the birds’ prey. Entanglement in fishing nets was estimated in 1987 to cause nearly one-third of the deaths of North Sea gannets. Pieces of plastic fishing nets also present a serious threat to fur seals and sea lions in the northern Pacific Ocean. It is the nature of young seal pups to play in drifting natural debris, such as kelp. For this reason, seal pups are attracted to discarded gill net pieces, where they can become entrapped. The curiosity of young seals may also prompt them to insert their heads through plastic strapping bands. As the seal grows, the band tightens and slowly strangles the creature. A recent study suggests that entanglement of northern fur seal pups may well be the chief reason for their declining population (Chae & An, 2018; Derraik, 2002; Prat et al., 1999; Wilber, 1987). Plastic debris further threatens marine life when it is accidentally ingested; once eaten, the durable properties of plastic prevent it from being easily digested. Ingested plastic particles often remain inside a creature, producing several harmful effects. Plastic debris can be accidentally ingested in several ways. Some species of whales, for example, may unintentionally eat plastic debris while feeding on schools of fish. Others are thought to confuse translucent plastic bags for the squid on which they feed. Autopsies recently performed on whales have revealed abdominal infections caused by irritating plastic wastes. Raw plastic pellets are consumed by a variety of sea life. So tiny that they often escape notice, these particles are among the most collected plastic items in the world’s oceans. One survey discovered that 90% of Hawaiian albatross chicks had plastic pellets in their digestive tracts. The chicks were likely fed the pellets after their parents mistook them for flying fish eggs. Another study concluded that several Alaskan seabird species consumed pellets because they resembled their typical crustacean prey. Ingested plastic items create a variety of problems for sea life. Besides damaging an animal’s stomach lining, plastics may inhibit the sensation of hunger, thereby depressing the animal’s feeding drive. Plastics also may furnish a base to which toxic chemicals such as polychlorinated biphenyls (PCBs) and DDT can attach. The plastic material may also be a source of chemicals that could damage tissue or cause eggshell thinning in seabirds or turtles. Plastic pollution poses a serious threat largely because it tends to concentrate in regions where abundant marine life exists. Like other floating debris, plastic litter concentrates along “ocean fronts” located at the margins of ocean streams and continental shelves. The same process concentrates plankton and other forms of life seeking shelter within the debris. The drifting debris acts as a kind of oasis, attracting marine animals in search of prey. Sea turtle hatchlings, which spend their juvenile stage along ocean fronts, can unintentionally consume plastic pellets. In later life, sea turtles sometimes frequent ocean fronts to search for food. These turtles are known to ingest plastic bags, which they mistake for jellyfish. Because plastic cannot be digested, the result is often fatal (Guzzetti et al., 2018; van Truong & BeiPing, 2019; Vince & Hardesty, 2017). Plastic pollution has severely affected the marine environment. Scientists can only observe the surface effects of the problem since afflicted animals may be eaten or sink to the seabed. What is important to realize is that the durable properties of plastic enable it to kill repeatedly. Marine animals already threatened by other manufacturing processes must now confront the threat of plastic debris. Some victimized species have commercial value; plastic pollution thus carries economic costs as well. The considerable efforts involved in untangling fouled boat propellers and cleaning up littered beaches represent additional costs of plastic pollution (Duckett & Repaci, 2015; LI et al., 2016; Ronkay et al., 2021; Worm et al., 2017).


Chapter 1 Life cycle assessment

Environmental impact of various plastic products Plastics are made up of synthetic polymers and are commonly used to manufacture bottles, food packaging, electronic goods, clothing, construction materials, medical supplies, etc. Various uses and manufacturing of plastics and plastic products started in 1839 after PS and vulcanized rubber were discovered. Nowadays, environmental pollution by plastic waste is a major environmental burden, especially for wildlife. Phthalates, polyfluorinated chemicals, brominated flame retardants, and antimony trioxide are some plastic constituents with contrary impacts on the environment and public health. From 1950 to 2018, about 6.3 billion tons of plastics were produced worldwide, of which only 9% and 12% were recycled and burned, respectively (Barnes, 2019). PET is a smooth, transparent, and relatively thin plastic. PET is commonly used in bottles for juice, mouthwash, soft drinks, cosmetics, and water because of its antiinflammatory and fully liquid properties. PET exposed to high temperatures can spread toxic additives like acetaldehyde, antimony, and phthalates. HDPE, a heat-resistant plastic made from petroleum, is commonly assumed to be safe for food and drink. PVC, a heat-resistant polymer used in packaging cooking oil, fruit juice, etc., is assumed to be highly toxic because of chemical ingredients that include heavy metals, phthalates, and dioxins. PVC has been ascribed to birth defects, cancers, ulcers, genetic changes, deafness, liver dysfunction, skin diseases, indigestion, and vision failure. PS, used in the manufacturing of insulators and covering media, is a petroleum plastic that contains benzene, which has carcinogenic, cytogenetic, and hematological effects on humans. Microplastics (plastics with a diameter less than 5 mm), either manufactured by design or resulting from plastic deterioration, are the prime pollutants reported in ecosystem degradation. The environmental report says that most used microplastics are from cosmetic products and cleaning additives like toothpaste and microbeads in face washes (Agarski et al., 2019; Hahladakis et al., 2018; Rajendran et al., 2012; ISO, 2006). Approximately 10% of household waste is plastic and is mostly disposed of on land, resulting in the release of toxic chemicals at the disposal site. Incineration is an alternative to land disposal of plastic, but it releases hazardous chemicals. For example, plastic incineration releases PCBs, dioxins, and furans, and plastic waste gas releases halogenated ingredients and PVC (Alabi et al., 2019). Table 1.1 shows the compounds generated during PVC incineration and their harmful effects. Plastics incineration creates soot, ashes, and various powders that settle on plants and the soil and can prospectively resettle within the aquatic environment. Some of these toxic composites permeate the soil with the help of rainfall, which may result in groundwater contamination or uptake by plants, enabling these toxins to enter the food chain. Some plastic incineration outputs react with water, resulting in altered pH and thus the altered functioning of aquatic ecosystems. Regained energy from plastics is an advantage of plastic incineration (da Costa et al., 2016; Peng et al., 2020). Various impact categories of plastic waste are analyzed through LCA, as presented in Fig. 1.3.

Major repercussions of plastic waste Plastics are made up of synthetic organic polymers that are widely used in a wide range of applications: water bottles, clothing, food packaging, medical supplies, electronic goods, construction materials, etc. Synthetic plastics that accumulate in the environment to the point of creating problems for wildlife, wildlife habitats, and human populations are considered plastic pollution. In 1907, the

Major repercussions of plastic waste

Table 1.1 Compounds generated during polyvinyl chloride incineration and their harmful effects. Compound

Health effect(s)

Acetaldehyde Acetone Benzaldehyde Benzol Formaldehyde Phosgene Polychlorinated dibenzo-dioxin Polychlorinated dibenzofuran Hydrochloric acid Salicylic aldehyde

Nervous system damage, lesions Eye and respiratory tract irritation Eye, skin, and respiratory system irritation; limiting of brain function Carcinogenic; adversely affects bone marrow, the liver, and the immune system Serious eye damage, carcinogenic Gas used in WWI; corrosive to eyes, skin, and respiratory organs Carcinogenic; irritates the skin, eyes, and respiratory system; damages the circulatory, digestive, and nervous system as well as the liver and bone marrow Irritates the eyes and respiratory system; causes asthma

Toluene Xylene Propylene Vinyl chloride

Corrosive to the eyes, skin, and respiratory tract Irritates the eyes, skin, and respiratory tract; can affect the central nervous system Irritates the eyes and the respiratory tract; can cause depression Irritates the eyes; can also affect the central nervous system, reduce consciousness, and impair learning ability Damages central nervous system by reducing consciousness Carcinogenic; irritates eyes, skin, and respiratory system; affects the central nervous system, liver, spleen, and blood-forming organs

FIGURE 1.3 Impact categories resulting from plastic waste.



Chapter 1 Life cycle assessment

invention of Bakelite brought about a materials revolution by introducing truly synthetic plastic resins into world commerce. By the end of the 20th century, however, plastics had become persistent polluters of many environmental niches, from Mount Everest to the bottom of the sea. Whether being mistaken for food by animals, flooding low-lying areas by clogging drainage systems, or simply causing significant aesthetic blight, plastics have increasingly attracted attention as large-scale pollutants (Singh et al., 2020). Plastics are polymeric materialsdthat is, materials with very large molecules that often resemble long chains made up of a seemingly endless series of interconnected links. Natural polymers such as rubber and silk exist in abundance, but nature’s “plastics” have not been implicated in environmental pollution because they do not persist in the environment. Today, the average consumer comes into daily contact with a huge array of plastic materials developed specifically to defeat natural decay processesdmaterials derived mainly from petroleum that can be molded, cast, spun, or applied as coatings. Since synthetic plastics are largely nonbiodegradable, they persist in natural environments. Moreover, many lightweight, single-use plastic products and packaging materials, which account for approximately 50% of all plastics produced, are not deposited in containers for subsequent removal to landfills, recycling centers, or incinerators. Instead, they are improperly disposed of at or near the location where they end their usefulness to the consumer. Dropped on the ground or inadvertently carried off by a gust of wind, they immediately pollute the environment. Indeed, landscapes littered with plastic packaging have become common in many locations worldwide. Global studies have not attributed this to any specific countries or demographic groups, although population centers generate the most litter. The causes and effects of plastic pollution are truly global (Rajkumar, 2015; Yousefi et al., 2021).

Global production of plastics and generation of waste In modern life, plastics are ubiquitous. Its early use dates back to 1600 BCE, when human hands shaped natural rubber and polymerized it into different useful objects in prehistoric Mesoamerica. The diverse use and manufacturing of plastics and plastic products began in 1839 when PS and vulcanized rubber were discovered. Production of Bakelite, the first truly synthetic polymer, began in 1907 in Belgium; by 1930, Bakelite was everywhere, especially in fashion, communication, and electrical and automotive industries. It took a decade after this for mass production of plastics to begin, and it has constantly expanded ever since. As of 2008, annual plastic production was estimated to be 245 million tons globally. At present, single-use packaging is the largest sector, accounting for nearly 40% of overall plastic use in Europe, followed by consumer goods and materials for construction, automotive, electrical, and agricultural applications at 22%, 20%, 9%, 6%, and 3%, respectively. It has been estimated that in 2015, the highest rate of production was in Asia, with 49% of total global output, with China as the largest global producer (28%), followed by North America and Europe at 19% each. The remaining regions are less important for production though not necessarily for consumption.

Management of plastic wastes LandfillingdApproximately 10% of household waste is plastics, and most of it ends up in landfills. Even though landfilling is the conventional approach for waste management in many countries,

Management of plastic wastes


dwindling space for landfills is becoming a major problem. Environmental pollution and risks to public health can be reduced if landfills are well managed, although soil and groundwater contamination by disintegrated plastic by-products and additives has the potential for long-term environmental persistence. Plastic incinerationdAn alternative to landfilling plastic waste is incineration, but concerns continue to grow about the potential atmospheric release of hazardous chemicals during incineration. For instance, plastic waste fumes release halogenated additives and PVC, while furans, dioxins, and PCBs from plastics incineration are released. The disadvantage of plastics combustion is air pollution caused by the noxious fumes released into the atmosphere. The combustion heater of flue systems is permanently damaged by plastics during plastics incineration, and the by-products of this plastics combustion are detrimental to both humans and the environment. Compounds of low molecular weight can vaporize directly into the air, thereby polluting the air. Depending on their varieties, they may form a combustible mixture or oxidize in solid form. Plastics incineration is usually accompanied by chalk formation, and the extent of coking depends on the incineration conditions. Gaseous releases during plastic and plastic composite products incineration are very dangerous. For example, Table 1.1 shows the compounds released during PVC incineration and the health effects of those compounds. Plastic incineration produces soot, ash, and various powders that eventually settle on plants and soil and can potentially migrate to the aquatic environment. Rainfall can enable some of these toxic compounds to permeate the soil, contaminate the groundwater, or be absorbed by plants, thus becoming incorporated into the food chain. Some of these plastic incineration products can chemically react with water, and the resulting compounds can alter the pH, thereby changing the functioning of aquatic ecosystems. Due to the potential environmental pollution impact, plastic incineration is not employed for waste management as frequently as recycling and landfilling. Plastics recyclingdReprocessing of recovered plastic scraps or wastes into useable products is called plastic recycling. Most plastics are nonbiodegradable in nature; hence, the fundamental work is the reduction of waste emissions, effective management, and recycling of resulting wastes. Recycling of plastics is a major aspect of the worldwide efforts in minimizing the yearly eight million tonnes of plastics in the waste stream entering the Earth’s oceans. According to Hopewell et al. plastic recycling terminology is complex due to various recovery activities and recycling. There are four main categories of recycling which are: primary (which involves the mechanical reprocessing of plastics into a new product with equivalent properties), secondary (which involves the mechanical reprocessing of plastics into a product with lower properties), tertiary (which consists of the recovery of the chemical constituents of the plastics) and quaternary (which requires energy recovery from the plastics). In contrast to the lucrative metal recycling but like the low value of glass recycling, recycling plastics is often more challenging because of low density and low value. Also, there are several technical issues to deal with when recycling plastic. Melting together of different plastic types often cause phase separation like oil and water, and they are set in these layers. The resulting phase boundaries are responsible for structural weakness in the final product(s), which has limited the application of these polymer blends. This is the case with polyethylene and PP, the two commonly manufactured plastics, thus limiting their recyclability. Recently, block copolymers have been proposed as a form of macromolecular welding flux or molecular stitches to overcome the challenge of phase separation during plastic recycling. There can be an increase in the percentage of plastics with the possibility of full recycling instead of the large quantity generated as wastes if packaged goods manufacturers reduce their mixing of packaging materials and eliminate contaminants.


Chapter 1 Life cycle assessment

Table 1.2 Disadvantages of plastic waste management technologies. Sl. No.





2. 3.

Plastic incineration Mechanical recycling


Chemical recycling

Landfills are responsible for climate change, soil and water contamination, and cancers, respiratory disorders, and other ailments linked to prolonged landfill exposure. Emission of toxic pollutants; high costs The efforts surrounding collection, sorting, washing, and recycling and the high percentage of material loss involve enormous costs and produce a raw material of limited quality. This makes it commercially unattractive to reuse the recycled raw materials from these streams in new products. The waste flows required for chemical recycling must be cleaner than previously thought. In addition, techniques with high CO2 reduction place higher demands on waste quality.

Environmental pollution by plastic wastesdThe distribution of plastic waste is associated with human populations. An increase in the human population has led to increasing demand for plastics and plastic products. Indiscriminate disposal of wastes from plastics and plastic products can lead to environmental pollution, which is evident in several ways, including deterioration of the beauty of the natural environment, entanglement and death of aquatic organisms, sewage system blockage in towns and cities, especially in developing countries, that creates a conducive environment for mosquitoes and other disease-causing vectors, production of foul smells, and reductions in water percolation and normal agricultural soil aeration, thus causing the reduced productivity of such lands. The disadvantages of plastic waste management technologies are highlighted in Table 1.2 (Das & Chaudhary, 2021c; Letcher, 2020).

Recommendations to reduce and control plastic wastes Many countries are laboring to control environmental pollution from plastic wastes by reducing the production of plastics and plastic products, prohibiting excessive packaging, capturing litter, and recycling. In the struggle against plastic pollution, the following recommendations might be helpful: PolicymakingdTo combat and curb persistent environmental pollution by plastics, there is a need for realistic policies that are properly followed and enforced. This should include the need for a global convention on environmental pollution by plastics to mandate plastic producers to declare all ingredients in their plastic products and put a warning on the products for consumers about the potential health effects of such constituents. Policies to classify some of the harmful ingredients in plastic products should be enacted. It is also important for the government to enforce and implement regulations that will check the production, consumption, use, and eventual disposal of plastics irrespective of their hazardous status. The 3 Rsdreduce, reuse, and recycledmust be employed at all stages to prevent zero diversion to landfills and indiscriminate disposal to the environment. Plastic waste management and recyclingdIn reducing the toxic effects of plastic wastes on the environment and public health, waste management plays a major role. For the global reduction of

Future directions and recommendations


plastic litter and ocean pollution, there is a need to improve proper plastic waste collection, treatment, and disposal. Inadequate landfill management will make way for harmful chemicals in plastic wastes to leach into the environment, polluting the soil, air, and underground water. Proper wastewater management will prevent microplastics from entering the environment from landfills. Education and public awarenessdEfforts must be made to educate the general populace on the potential environmental and public health effect of pollution by plastic wastes. This will go a long way toward reducing the pollution rate and preserving environmental quality. People should be made aware of the chemical constituents of plastic products and their health effects. Educational curricula at various levels must include plastic pollution reduction methods and waste management systems as information resources. Bioplastics as an alternativedBioplastics was first produced from cellulose, made of wood pulp by a British chemist in the 1850s. Now, bioplastics can be produced from different biodegradable and nonbiodegradable materials, including weeds, hemp, plant oils, potato starch, cellulose, and corn starch. Sugar-based bioplastics can biodegrade under normal conditions for composting. Bioplastics are environmentally friendly since they require fewer fossil fuels during production compared with other plastics. We believe that the problem of plastic waste generation and the accompanying environmental and public health effects can be handled if, globally, manufacturers can embrace the use of bioplastics. The biodegradability with little or no toxic products left behind will go a long way to protect our natural environment from the menace of conventional plastic wastes, protect our world’s organisms, and make the world safer for humans.

Future directions and recommendations Plastic waste management refers to a group effort to recycle plastic or produce electricity by incineration. Recycling has several advantages, including the ability to save energy and protect the environment. It can also assist businesses in lowering their production costs by recycling products, resulting in increased revenue generation. These advantages improve the market’s growth rate by offering a major boost to demand. The bulk of the industry’s players serve the commercial, residential, and industrial sectors. Consolidation is increasing the market concentration as developed economies and corporations outsource waste management and disposal services (Kamaruddin et al., 2017; Lebreton & Andrady, 2019; Wong et al., 2015). Over the forecast era, this is projected to drive the plastic waste management industry. Various factors such as rising environmental issues, growing industrialization, rapid urbanization, and rising waste management process innovation are expected to drive market growth over the forecasted era. The global plastic waste management market is anticipated to reach USD 42.2 billion by 2027, expanding at a CAGR of 3.1%. Economic growth, rising industrialization, increasing urbanization, and growing health awareness are the major factors likely to boost market growth over the forecast period. People’s increasing environmental consciousness, as well as tougher regulations implemented by many end-use industries and various specifications pertaining to plastic waste management, would boost consumer demand. In addition, because of rapid urbanization and industrialization, there has been a surge in the adoption of sustainable waste management efforts and processes, paving the way for plastic waste management services. Advanced technologies and frameworks for reprocessing, organizing, and sorting recyclable plastics are opening


Chapter 1 Life cycle assessment

new possibilities within the plastic waste management market. Furthermore, laws and guidelines enforced by the government and related agencies across the countries for plastic waste disposal management are anticipated to drive the market over the forecast period (Das & Chaudhary, 2021b). The cost at which recycled plastic can be created is stable, while the cost of virgin plastic fluctuates based on the performance of the oil and gas industry and the demand for crude oil-based products across the end-use industries. This fluctuation in prices promotes the incentive to make long-term investments in recycling technology and infrastructure. Various recommended approaches to plastics focusing on innovation are shown in Fig. 1.4. Therefore, a new plastic paradigm is needed. Achieving a long-term sustainable future for plastics will require integration along the entire value chain, from design to reuse, together with the transition to a truly circular economy. A circular economy is characterized by a value chain approach in which a

FIGURE 1.4 Recommended approach for plastics focusing on innovation.



product’s end-of-life is considered from the moment it is developed, and resources are reused rather than being continuously added. In terms of innovation, this will require an increased focus on composition, in terms of both smarter materials design to improve recyclability and the development of bio-based alternatives.

Conclusions Science has yet to develop consistent and reliable baseline data on plastic’s existence, fluxes, pathways, fates, and effects within various environmental compartments. Despite the immediate interest arising from the clearly measurable impact of pollutants on plants and animals and evidence of the transboundary and far-reaching nature of plastics, far more attention has thus far been paid to marine pollution. Some estimates, however, indicate that pollution levels in freshwater systems and soils may surpass those recorded in the marine environment, especially for microplastics. Despite all the wellfounded assumptions, uncertainty and unknowns abound. These complexities and information gaps make it difficult to thoroughly evaluate the health effects, thus limiting informed decisions by customers, societies, and policymakers. At all levels of the life cycle of plastic products, inadequate and incomplete information leads to possible long-term environmental and health consequences. As a result, more focused science and policy attention must be paid to these environmental compartments, not in place of but in addition to current marine (micro)plastic emission studies. Over the forecast period, various factors such as increasing environmental concerns, growing industrialization, rapid urbanization, and rising waste management process innovation are expected to drive market growth. The global plastic waste management market is expected to expand at a CAGR of 3.1% to USD 42.2 billion by 2027. Economic development, industrial expansion, rising urbanization, and increasing health awareness are the major drivers of market growth over the forecast period.

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Chapter 1 Life cycle assessment

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Composition, properties and other factors influencing plastics biodegradability


Sigrid Hakva˚g1, Odd Gunnar Brakstad1, Stephan Kubowicz2 and Andy M. Booth1 1

SINTEF Ocean, Trondheim, Norway; 2SINTEF Industry, Oslo, Norway

Introduction There is no universal definition of biodegradation (biological degradation). In this chapter, we use the United Nations definition: a “process by which organic substances are decomposed by microorganisms (mainly aerobic bacteria) into simpler substances such as carbon dioxide, water and ammonia” (United Nations, 1997). Several other definitions of biodegradability exist, depending on whether the biodegradation solely alters the chemical structure of a material or the material is completely mineralized by microorganisms (Harrison et al., 2018). The term “biodegradable” also does not yield any useful information regarding the timescale and extent of the decomposition process (Harrison et al., 2018; Kubowicz & Booth, 2017a; Montazer et al., 2020). In many cases, the complete mineralization of a compound/ultimate biodegradability is used as a definition, where the process produces new biomass plus carbon dioxide and water (aerobic conditions) or methane (anaerobic conditions) (Gu, 2003; Jacquin et al., 2019). Plastics are typically biodegraded aerobically (Gu, 2003; Jacquin et al., 2019; Shah et al., 2008). Thermodynamically, oxygen is a more efficient electron acceptor than either sulfate or CO2, and therefore, aerobic respiration is more energy-rewarding than anaerobic respiration. Aerobic respiration can consequently support a greater microorganism population than anaerobic respiration. Anaerobic biodegradation occurs in landfills and sediments, while partially aerobic conditions are characteristic of compost and soil. It should be noted that even under optimized laboratory conditions, the biodegradation rates of conventional thermoplastics are very low (Krueger et al., 2015). In marine and terrestrial environments, the lifetimes of conventional plastics can extend into centuries (Chamas et al., 2020). A range of individual standards and test methods have been developed to assess polymeric material biodegradability in various aerobic and anaerobic environments, including both managed and open environments (Harrison et al., 2018; Montazer et al., 2020). While most of these available test procedures are sufficiently reliable for assessing biodegradability, different tests can significantly overestimate or underestimate the duration required for polymer biodegradation and mineralization within natural ecosystems. These differences result from the complex interaction of multiple factors that influence the biodegradability of a specific plastic item under a particular set of environmental conditions. Biodegradability of Conventional Plastics. Copyright © 2023 Elsevier Inc. All rights reserved.



Chapter 2 Composition, properties and other factors

Table 2.1 Factors affecting plastics biodegradation. Factors affecting biodegradation Plastics (intrinsic)

Chemical properties Physical properties

Exposure conditions (extrinsic)

Microbial community (biotic) Environmental factors (abiotic)

Polymeric composition (backbone) Additive chemicals Molecular properties: crystallinity, molecular weight, hydrophobicity, morphology Extracellular enzymes Hydrophobicity Biosurfactants Temperature Salinity and pH Dissolved oxygen Pressure Sunlight and UV exposure Moisture and humidity Fragmentation and transport of polymers Adsorption of pollutants

So what factors affect plastic biodegradation? Broadly speaking, biodegradation factors can be divided according to the intrinsic physical and chemical properties of the polymer and extrinsic exposure conditions; see Table 2.1). The latter is a function of the type of organism (biotic) and the environmental parameters that externally influence the degradation process (abiotic) (Ahmed et al., 2018; Kijchavengkul & Auras, 2008; Kumar et al., 2019). The intrinsic physical and chemical characteristics of a particular polymer material are highly important factors in biodegradability. The microbial and enzymatic accessibility of polymers is determined by molecular weight, melting temperature, additives, crystallinity, flexibility, and functional group (Kale et al., 2015; Tokiwa et al., 2009). For example, enzymes mainly attack the loosely packed amorphous domains of polymer materials, whereas crystalline regions typically exhibit greater resistance to biodegradation. The kinetics of polymer biodegradation also depend on the environmental conditions surrounding the material. Furthermore, a synergistic relationship exists between abiotic and biotic exposure parameters. A microbial community’s composition, growth rates, and metabolic activity are affected by abiotic environmental conditions; hence, these conditions also play a significant role in the biodegradation of (bio)plastics (Emadian et al., 2017). Abiotic factors include temperature, moisture, pH, and UV radiation. In addition to directly influencing microbial parameters, the same abiotic factors can affect the rate of hydrolysis and thereby affect polymer bioavailability to microorganisms. The biotic factors involve the production of extracellular enzymes by microorganisms as depolymerases and biosurfactants. Consequently, plastic biodegradation is a complex interplay of multiple biotic and abiotic factors and proceeds at various rates for different polymer types and environmental compartments and matrices.

Microbial degradation of plastic materials


Microbial degradation of plastic materials Mechanisms of biodegradation The biodegradation of plastics can be schematically divided into stepwise processes comprising (1) microbial film formation, (2) biodeterioration, (3) biofragmentation, (4) assimilation, and (5) mineralization (Jacquin et al., 2019; Kumar et al., 2019; Lucas et al., 2008). Importantly, biodegradation occurs after or simultaneously with physical and chemical polymer degradation. These processes increase the available surface area and change the surface chemistry, making it easier for microorganisms to metabolize the polymer. As a result, biodegradation proceeds at an increasingly higher rate as physical and chemical degradation processes progress. Biofilm formation is initiated by the adsorption of naturally occurring dissolved organic molecules. Once these molecules have formed a base layer, the attachment of microbial cells proceeds. Primary colonizers rapidly attach to the plastic surfaces and produce biofilms without necessarily including plastic-degrading microorganisms (Lobelle & Cunliffe, 2011). Further development usually involves colonization by unicellular eukaryotes and attachment of multicellular eukaryotes (de Carvalho, 2018). Bacterial colonization starts rapidly on the plastic. It has been shown that biofilm development on polyethylene (PE) in seawater coincides with reduced hydrophobicity and more neutral buoyancy of the polymer (Lobelle & Cunliffe, 2011). Biofilms are associated with the production of extracellular polymeric substances (EPSs), which comprise polysaccharides, proteins, extracellular DNA, and lipids (Di Martino, 2018). Biofilm formation on plastic surfaces follows successional states, separating between early and late colonizers (Dang & Lovell, 2000; Lee et al., 2008; Pinto et al., 2019; Salta et al., 2013). A typical biofilm consists of dead and active cells inside a matrix of extracellular polymers formed by EPS-producing microbes). Oxygen may become limited in the interior of the biofilm as it evolves, making conditions favorable for anaerobic microbes, which in the marine environment may include sulfate-reducing prokaryotes. Biodeterioration relates to both the physical and chemical deterioration of the polymer due to the development of the biofilm (e.g., due to the production of enzymes, acids, and peroxides). While microorganisms play an essential role in the deterioration and degradation of both synthetic polymers and those that occur naturally, even minor differences in the chemical structures of the polymers can result in significant variation in the material’s biodegradability (Gu, 2003). Once attached, extracellular enzymes produced by microorganisms in the established biofilm begin to act on the polymer surface. As plastics can represent a source of carbon as well as energy, microorganisms have evolved a range of mechanisms to successfully degrade complex polymers. This includes direct degradation by using plastic fragments as a source of nutrition or indirect degradation through microbial enzymes (Gu, 2003; Kumar et al., 2019). Bacteria require the substrate to be assimilated through the cellular membrane into the cells, so an increase in the molecular weight of a polymer will result in a decrease in its biodegradability. Monomers, dimers, and oligomers of the repeating units of a polymer, on the other hand, are more easily degraded and mineralized, as they are small enough to pass through the outer membranes of the bacteria. This is a primary reason for biodegradation being considered more effective after physical and chemical degradation processes have already acted upon a polymer material. Biofragmentation of plastic polymers is primarily performed by extracellular (exoenzymes) and intracellular (endoenzymes) depolymerase, eventually leading to an increase in microbial biomass due to the assimilation of smaller constituents (monomers


Chapter 2 Composition, properties and other factors

FIGURE 2.1 Schematic diagram of polymer degradation under aerobic and anaerobic conditions. From Gu, J.-D. (2003). Microbial deterioration and degradation of synthetic polymeric materials: recent research advances. In International Biodeterioration and Biodegradation (Vol. 52, pp. 69e91). 10.1016/S0964-8305(02)00177-4.

and others) by the microorganism. Complete oxidation of the materials (mineralization) is the last step in the biodegradation of plastics and results in the production and release of CO2, H2O, and CH4 depending on conditions. Anaerobic and aerobic biodegradation pathways are shown in Fig. 2.1.

Microorganisms involved in biodegradation Microorganisms capable of degrading various plastics have been isolated from numerous open environments, including marine water, sewage sludge, landfills, soil from plastic-dumping sites, mulch film waste, and crude oil-contaminated soil. These microorganisms have been summarized in several reviews over the last few years (Catania et al., 2020; Jacquin et al., 2019; Kale et al., 2015; Montazer et al., 2020; Ru et al., 2020; Shah et al., 2008). Among the microorganisms listed, the bacteria and fungi associated with the biodegradation of polyethylene (PE) (Ru et al., 2020), bioplastics (Emadian et al., 2017), and various types of polymers under laboratory conditions (Haider et al., 2019; Jacquin et al., 2019) are the most studied, including Pseudomonas, Streptomyces, Bacillus, and Aspergillus spp. Fungi and actinobacteria are considered significant taxa because of their vast metabolic potential (Catania et al., 2020). Larger organisms such as waxworms and mealworms are also reported as eating and degrading PE films; however, it is speculated that microbial symbionts promote this ability (Danso et al., 2019; Ru et al., 2020). Microbial inhabitants of plastic biofilms have been variously reported to be both “common for different plastic types” and “plastic specific” (Pinto et al., 2019). Examples of plastic-specific microbial taxa include (1) members of the Alcanivoracaceae and Cryomorphaceae families, having been observed to undergo specific enrichment on polyethylene terephthalate (PET) surfaces compared with glass surfaces and (2) members of the Hyphomonadaceae and Erythrobacteraceae families, dominating PE and PS surfaces compared with the surface of wood pellets (Oberbeckmann et al., 2018). Several bacterial groups associated with plastic biofilms are also associated with hydrocarbon and petroleum degradation, including members of the families Alteromonadaceae, Oceanospirillaceae, and Erythrobacteraceae (Bacosa et al., 2018; Golyshin et al., 2002; Lofthus et al., 2018; Ro¨ling et al., 2002).

Microbial degradation of plastic materials


PE, the plastic with the highest global production volume, has been typically considered “not biodegradable” because its high molecular weight impedes microbial degradation (Ru et al., 2020; Yang et al., 2014). Physiochemical pretreatment of PE leads to depolymerization of long-chains and the formation of carbonyl groups and low-molecular-weight products that permit further degradation by microorganisms (Montazer et al., 2020; Ru et al., 2020; Yang et al., 2014). Biodegradation of PE that has not been pretreated has, however, been reported for several strains. Loss of 30% of the initial weight of the PE was reported after biodegradation by Serratia marcescens (70 days) and two cyanobacteria, Phormidium lucidum and Oscillatoria subbrevis (42 days). However, it was not determined whether the weight loss was due to degradation of the long-chain PE or of the low-molecular-weight components. More elaborate data indicating biodegradation were reported for Enterobacter asburiae YT1 and Bacillus sp. YP1 that had been previously isolated from the gut of the waxworm Plodia interpunctella (Yang et al., 2014). After a 60-day incubation period, the average molecular weight of the polymer films was significantly lowered, and degradation products could be detected. However, a definition of true biodegradation of plastics such as PE is needed, supported by standardized protocols for assessing the microbial degradation of these compounds (Harrison et al., 2018; Montazer et al., 2020).

The plastisphere and future possibilities The microbial community inhabiting plastic debris is also referred to as the “plastisphere” (AmaralZettler et al., 2020; Kirstein et al., 2019). In recent years, the use of molecular methods such as high-throughput DNA sequencing has increased our understanding of the diversity of this community. It has been shown that the microbial community on plastic can differ from the surrounding environment and from that on other particles in the same environment (Kirstein et al., 2019; Ogonowski et al., 2018). Furthermore, the microbial community has been demonstrated to vary between polymer types, possibly related to the additive chemical content (Rosato et al., 2020). However, it is still unknown to what extent this microbiome is unique to plastic. Improved knowledge of the plastisphere will allow a better understanding of the substrate specificity of the plastic microbiome (Amaral-Zettler et al., 2020; Kirstein et al., 2019). Highly active enzymes for the biodegradation of most types of plastic have currently not been identified using cultivation techniques. As a result, the future application of multiomics approaches for analyzing the plastisphere might play an important role in further understanding and developing the topic of plastic biodegradation.

Plastic-degrading enzymes Extensive work has been conducted to identify both plastic-degrading enzymes and the microorganisms that produce them for use in bioremediation and industrial applications. Plastic polymers, including PE, PET, polypropylene (PP), polystyrene (PS), and polyvinyl chloride (PVC), are typically considered recalcitrant to biodegradation (Kale et al., 2015). This group of polymers is characterized by having high molecular weights, complex three-dimensional structures, and hydrophobic nature. These are all critical factors that reduce the availability of polymers to microorganisms. Many of the frequently used plastics contain additional chemicals that are more readily broken down than the actual polymer backbone. As a result, the main difficulty for microbes is in the initial breakdown of high molecular-weight and highly robust polymers and their crystalline structures (Danso et al., 2019).


Chapter 2 Composition, properties and other factors

PE, PP, and expanded PS polymers contain stable backbones that are typically challenging for microorganisms to biodegrade (Amaral-Zettler et al., 2020). In contrast, polymers such as PET, polyurethane (PUR), and polycarbonate contain heteroatoms in their backbone, which makes them more susceptible to hydrolysis and the enzymes that catalyze these reactions. Different enzymes possess unique active sites, potentially enabling the biodegradation of various types of polymers. The most important enzyme-derived polymer degradation pathways are enzymatic hydrolysis and enzymatic oxidation. In contrast to polymer hydrolysis, which is limited to specific polymer types, it is worth noting that oxidative degradation can transform both hydrolyzable and nonhydrolyzable polymers (Yuan et al., 2020). An overview of the pathways for polymer degradation is shown in Fig. 2.2. The plastic-degrading enzymes identified to date have been found to act mostly on the highmolecular-weight polymers PET and ester-based PUR, although with moderate turnover rates at best. For the most widely used plastics, such as the high-molecular-weight polymers PS, polyamides

FIGURE 2.2 Overview of main polymer biodegradation pathways. Polymer hydrolysis is contrasted with oxidative degradation, which can transform both hydrolyzable and nonhydrolyzable polymers. Green lines indicate functional groups, including hydrolyzable bonds inside hydrolyzable polymers. Question marks indicate enzymes that have yet to be identified. Abbreviations: Cut, cutinase; Nyl, nylon hydrolase; AlkB, alkane hydroxylase; Lac, laccase; MnP, manganese peroxidase. From Yuan, J., Ma, J., Sun, Y., Zhou, T., Zhao, Y., & Yu, F. (2020). Microbial degradation and other environmental aspects of microplastics/plastics. Science of the Total Environment, 715, 136968.

Influence of plastic properties on biodegradation


(e.g., nylon), PVC, PP, ether-based PUR, and PE, few or no enzymes have been identified to date. Among the enzymes demonstrated to degrade plastic, depolymerases appear to play a significant role in polymer biodegradation (Emadian et al., 2017; Gu, 2003; Siracusa, 2019). Proteases, lipases, ureases, and cutinases, however, represent other important microbial hydrolytic enzymes responsible for polymer biodegradation (Gu, 2003; Kumar et al., 2019; Siracusa, 2019). For more complex polymers like poly-(ethylene adipate) and polycaprolactone (PCL), enzymes like esterases and lipases have been shown to be active. PET hydrolases have been isolated from fungi and bacteria, however, all with relatively low turnover rates (Catania et al., 2020; Danso et al., 2019). Despite this, the recently discovered bacterium Ideonella sakaiensis 201-F6 has been demonstrated to grow on PET, utilizing the polymer as a major carbon and energy source. Polymers with stable carbonecarbon (CeC) bonds need to be oxidized prior to their further depolymerization. Polymer degrading enzymes in the alkB family alkane hydroxylases are reported to degrade low molecular-weight PE (up to 27,000 Da) (Ru et al., 2020; Yuan et al., 2020). Reports also describe the involvement of enzymes like manganese peroxidases and laccases in the biodegradation of PE (Wei & Zimmermann, 2017). Detailed lists of potential plastic-degrading enzymes have been published in several reviews, including a summary of enzymes able to degrade a range of the most widely used synthetic plastics (PE, PS, PP, PVC, PUR, and PET) (Ru et al., 2020). Enzymes currently involved in high-molecularweight polymer plastic degradation have also been extensively reviewed by Danso et al. (2019). However, Danso et al. (2019) highlighted that several of the reported enzymes capable of degrading plastic possibly contained biodegradable chemical additives that were likely preferentially degraded instead of the bulk polymer materials. This issue limits many current standard methods for assessing the biodegradability of plastics, and such studies require further verification using more advanced technologies and analytical tools.

Influence of plastic properties on biodegradation Plastics biodegradation and biodegradability are governed by a range of physical and chemical properties that vary between different types of plastic. A wide range of polymer characteristics plays an important role in the potential for biodegradation, including mobility, tacticity, crystallinity, molecular weight, the type of functional groups and substituents present in the polymer structure, and the type and concentration of plasticizers and other plastic additives that are added to the polymer (Artham & Doble, 2008; Gu, 2003; Gu et al., 2000). Table 2.2 summarizes the main characteristics, influences on plastics, and primary intrinsic properties of plastics in biodegradation.

Polymeric composition The polymeric composition of an individual plastic material plays a critical role in the potential for it to degrade, particularly in biodegradation. When considering the degradation of plastic polymers, it is typical to group them into two main categories: (1) polymers exhibiting a CeC backbone (e.g., PE, PP, PS, and PVC) and (2) polymers that contain heteroatoms (oxygen, nitrogen, sulfur) in the main backbonede.g., PET, polylactic acid (PLA), and PUR (Gewert et al., 2015). This is also one of the primary drivers for whether a polymer is considered hydrolyzable or nonhydrolyzable Fig. 2.2.


Chapter 2 Composition, properties and other factors

Table 2.2 Overview of the main intrinsic properties of plastics that influence biodegradability. Intrinsic parameter

Influences on biodegradation

Polymeric composition

Polymeric composition plays a critical role in biodegradation potential. Polymers comprised of a CeC backbone (e.g., polyolefins such as PE) are not generally susceptible to biodegradation without first undergoing abiotic degradation processes. The presence of aromatic rings (PS) and halogen atoms (PVC) decreases biodegradability. Biodegradation can proceed directly for polymers containing heteroatoms in the main backbone; however, those containing aromatic rings (e.g., PET) are resistant. Plastics exhibiting a more crystalline polymer structure will prevent enzymes from penetrating and initiating the degradation process, while an amorphous structure makes this process possible. In a semicrystalline material, biodegradation can occur at various rates in different regions. Larger, higher-molecular-weight polymers typically exhibit slower rates of biodegradation. The smallest polymer units, such as the monomers, dimers, and oligomers, are more easily degraded and mineralized. Biodegradation decreases as the hydrophobicity of a polymer increases. Polymers containing many side chains/branches are less readily assimilated by microorganisms and are more resistant to biodegradation. At larger scales, plastic items with rough surfaces have a higher surface area and more secure contact points for microbes to colonize the plastic surface. Plastic additive chemicals can lead to a reduction or an increase in the rate of different plastic degradation processes, including biodegradation. The additive chemicals themselves exhibit a broad range of susceptibilities to biodegradation.


Molecular weight

Hydrophobicity Morphology

Additive chemicals

Thermoplastic polyolefins, such as PE, have long polymer chains comprised in a CeC backbone. These polymers typically exhibit high resistance to chemical-characterized alteration, are not easily modified by strong acids or bases (oxidizing or reducing agents), and are recalcitrant to hydrolysis and microbial degradation. To be biodegraded, these polymers must first undergo abiotic degradation (e.g., photoinitiated or oxidative degradation), which leads to chain scission and the formation of lowmolecular-weight fragments that are more available to microorganisms. The primary step in microbial oxidation of these low-molecular-weight fragments is hydroxylation, which introduces an eOH group to the structure. This process results in the formation of the corresponding primary or secondary alcohols. These alcohol groups can be further oxidized to aldehydes (-CHO) or ketones (-C]O), with the process completed with the formation of the corresponding acid (-COOH), which can then be readily biodegraded (Eubeler et al., 2010). However, abiotic and biotic degradation susceptibility also varies within the group of polymers with a CeC backbone; see Fig. 2.3. For example, microorganisms can attack any terminal methyl group present along the backbone of PE. However, biodegradation is faster when the molecular weight of the PE chain is smaller than 500 Da (Gewert et al., 2015). PP is not only less stable than PE but also more susceptible to photoinitiated or photocatalyzed degradation. For PP, chain scission leads to the formation of smaller-molecular-weight fragments, increasing the resistance of PP to aerobic biodegradation. As a result, PP is significantly more resistant to microbial degradation than PE. PS has a much lower biodegradation rate relative to PE and PP. This is primarily

Influence of plastic properties on biodegradation


FIGURE 2.3 Enzymatic degradation of plastic materials. Biological pathways of polymer degradation include the mechanical action of organisms that grow in cracks and crevices of the polymer surface (not shown), as well as enzymatic processes that can hydrolyze the polymer into oligomers and ultimately monomers. Polyethylene, polypropylene, and expanded polystyrene contain very stable backbones and are difficult to degrade, whereas polyethylene terephthalate (PET), polyurethane (PUR), and polycarbonate are more susceptible to hydrolysis and to enzymes that catalyze these reactions. Enzymes that can hydrolyze polypropylene and polycarbonate have not yet been reported to the best of the author’s knowledge (indicated by question marks). From Amaral-Zettler, L. A., Zettler, E. R., & Mincer, T. J. (2020). Ecology of the plastisphere. Nature Reviews Microbiology, 18(3), 139e151.

due to PS containing an aromatic ring attached to the main polymer backbone, which means PS is considered one of the least susceptible thermoplastic polymers toward biodegradation. The presence of halogen atoms in PVC increases its resistance to aerobic biodegradation. For PVC to be biodegraded, it is generally accepted that the polymer must first undergo a dechlorination step driven by abiotic processes. In contrast to PE, PP, and PS, the hydrophilic polymer PVA, which also has a CeC backbone, is the only known thermoplastic to be mineralized by microorganisms (Shimao, 2001). Degradation of polymers that contain heteroatoms in the main backbone (e.g., the oxygencontaining polyester family of polymers) can occur independently through photooxidation, hydrolysis, and biodegradation, with the potential for all three to occur concurrently (Gewert et al., 2015; Singh & Sharma, 2008). The component monomers in polyester polymers are bonded via ester linkages that are easy to hydrolyze (Shimao, 2001). As many kinds of esters occur in nature, the enzymes that have evolved to degrade them (esterases) are found in most living organisms. While many polyesters composed of aliphatic monomers appear to be degradable by lipases, most aromatic polyesters are considered biologically inert (e.g., PET). In aliphaticearomatic copolyesters, biodegradability appears to decrease with increasing aromatic constituent content (Witt et al., 1995). The ester linkages in the biodegradable polymer PLA are sensitive to chemical hydrolysis and enzymatic chain cleavage, where several enzymes are known to degrade the polymer: proteinase K, pronase, and


Chapter 2 Composition, properties and other factors

bromelain (Williams, 1981). Despite this apparent ready biodegradability, only a few PLA-degrading microorganisms have been identified to date, and they are not considered widespread within the natural environment (Shimao, 2001). Comparison of the degradation of conventional thermoplastics, e.g., polyamide/PP films, PET, and materials designed/assumed/expected to be more biodegradable (e.g., PLA) in seawater and marine sediments, measured by weight loss, indicated that the conventional polymers were unaffected over a period of 365 days in a natural seawater/sediment system at 16 C. The PLA was particularly susceptible to degradation in sediment versus seawater, with five times faster weight loss in the water than for the synthetic polymers (Beltra´n-Sanahuja et al., 2020). While it is well established that PUR is susceptible to biodegradation from fungi, bacterial and enzymatic degradation are also possible, where it is the urethane bonds or polyol segments that are the principal areas that are degraded/cleaved (Morton & Surman, 1994; Nakajima-Kambe et al., 1999; Zheng et al., 2005). Polyester segments are degraded more easily by microorganisms than by polyether segments present in PU. Although enzymes can cleave the polymer chain, it is considered unlikely that they can diffuse into the bulk polymer due to their size. As a result, the degradation process mainly occurs on the surface of the polymer material and results in the formation of cracks.

Molecular properties Crystallinity Most plastics have a semicrystalline structure. This means they exhibit a combination of distinct regions with highly ordered and oriented polymer chains (crystalline) and other regions with randomly ordered/oriented polymer chains (amorphous) (Fig. 2.4). Across different polymer types, the degree of crystallinity can range from as little as 10% and up to 80% (Ehrenstein, 2001). More crystallinity results in stronger plastic material but also in more brittleness. The amorphous regions of a plastic material provide flexibility that can be useful in certain consumer products. The degree of crystallinity also influences the biodegradability of specific plastics, where plastics exhibiting a more rigid and compact crystalline polymer structure prevent enzymes from penetrating and initiating the degradation process. Conversely, enzymes can enter plastics exhibiting a more amorphous polymer structure much more readily than in plastics with a crystalline structure, penetrating more deeply and in greater volume. As a result, it is the amorphous regions of a plastic that are the most susceptible to biodegradation. In a semicrystalline material, this can result in degradation occurring at various rates in FIGURE 2.4 Schematic of the differences between crystalline and amorphous regions of a polymer. From Kubowicz, S. (n.d.).

Influence of plastic properties on biodegradation


different regions of the material (Tokiwa et al., 2009). Although there is no established relationship between the glass transition temperature (Tg) of synthetic plastics and their susceptibility to biodegradation, it has been suggested that the structural changes that occur at the Tg of specific plastic material may enhance microbial attack (Lucas et al., 2008). Finally, the degradation of plastic with heteroatoms was slower for polymers with glass transition temperatures (Tg) higher than ocean temperature (e.g., polylactic acid and PET) than those polymers with a Tg lower than ocean temperature (e.g., polybutylene adipate terephthalate) (Min et al., 2020).

Molecular weight The molecular weight of a specific polymer can also significantly affect its biodegradation rate. Larger, higher-molecular-weight polymers typically exhibit slower rates of biodegradation. For example, PCL with a high molecular weight (i.e., >4000) has been observed to degrade more slowly by a lipase from the mold Rhizopus delemar relative to PCL with a lower molecular weight (Tokiwa et al., 2009). It is therefore widely acknowledged that the smallest polymer units, such as the monomers, dimers, and oligomers, are more easily degraded and mineralized (Amobonye et al., 2021, p. 1435). As biodegradation increases as the size of the molecules in a polymeric material decreases (Singh & Sharma, 2008), it is recognized that biodegradation will proceed more rapidly once the process has already been initiated and has begun to generate shorter polymer fragments and small molecules products. The active site of hydrolases is often located in a deep cavity, meaning there is a need for long polymer chains to penetrate the enzyme to reach the active site. This makes the mobility of the polymer chains one of the most important factors controlling the degradability of polymers such as polyester. In principle, mobility is correlated with the melting point of a specific plastic material. As a result, it seems unlikely that high melting polyesters, such as PET, will degrade at a practical rate (Mueller, 2006). Polymer biodegradability is generally considered to decrease as molecular weight increases. In contrast, the monomers, dimers, and oligomers comprised in the small, low-molecular-weight repeating units of a polymer are much more readily biodegraded and mineralized by microorganisms. Furthermore, polymers with high molecular weights are characterized by an acute decrease in aqueous solubility that makes them difficult for microorganisms to attack. This is because bacteria require the material to be transported through the cellular membrane before it can be further degraded by intracellular enzymes (Shah et al., 2008).

Hydrophobicity and morphology The hydrophobic/hydrophilic properties of an individual polymer impact the potential for biodegradation to occur. In general, biodegradation decreases as the hydrophobicity of a polymer increases. Firstly, microbial colonization of a plastic surface is typically enhanced with increasing hydrophilicity due to their characteristic wettability, greater surface energies, and lower contact angles, leading to higher degradation rates (Chamas et al., 2020). In addition, the activity of extracellular enzymes is also considered constrained as the hydrophobicity of a polymer increases (Amobonye et al., 2021, p. 1435). The influence of polymer hydrophilicity/hydrophobicity has been shown using molecular dynamic simulations (Min et al., 2020). When comparing a highly hydrophobic plastic (PP) with a relatively highly hydrophilic (nylon), the hydrophobic PP was significantly less susceptible to biodegradation. The presence of polar functional groups in plastic polymers increases their hydrophilicity, but this can be a dynamic polymer property as the degree of polar groups can change over time due to other factors. For example, a plastic material impacted by environmental weathering processes such as UV exposure


Chapter 2 Composition, properties and other factors

will gradually be modified by forming new oxygen-containing polar functional groups. As such oxidation processes proceed, they increase the number of polar functional groups on the material surface over time. This leads to a gradual decrease in the contact angle with water and a corresponding increase in the hydrophilicity of the material. The morphology of a polymer can also significantly influence the rate at which it undergoes degradation. Key morphological properties include the degree of branching (side chains) and the physical form of the polymer material. Plastic polymers containing many side chains/branches are less readily assimilated by microorganisms and more resistant to biodegradation (Amobonye et al., 2021, p. 1435). In terms of the physical form, surface roughness is an important parameter. Plastic items with rough surfaces have a higher surface area and more secure contact points for microbes to attach and colonize the plastic surface. This increased microbial colonization has the corresponding impact of increasing the potential for biodegradation to occur. Studies have also been made to link the biotic and abiotic degradation behavior of plastic polymers in seawater. For instance, the octanol-water coefficients (LogP) of polymers seem to be more important for degradation than molecular weight, while the presence of functional groups that lower the LogP (e.g., carbonates, esters, and amides) facilitate both abiotic and biotic degradation pathways. Biotic polymer degradation is faster in seawater than abiotic degradation (hydrolysis), with the latter also being more sensitive to LogP and polymer crystallinity than biodegradation.

Additive chemicals A wide range of organic and metal-based compounds can be added to individual plastic to impart particular physical or chemical properties to the material and products it is used in. Individual additive chemicals are typically added to alter an individual parameter or property of plastic material, allowing the overall material properties to be tailored according to the planned application. Plastic properties that additive chemicals can be used to modify include the material’s appearance (e.g., color, design) and mechanical, thermal, electrical, and optical performance. In addition, certain chemicals are added to improve the processability during the molding and extrusion processes of plastic production (Kyrikou & Briassoulis, 2007). Additive chemicals can also alter a plastic material’s long-term behavior by protecting against specific degradation mechanisms (e.g., sunlight, heat, hydrolysis, weathering) or by preventing (or reducing) processes such as creep, relaxation, and fatigue. A large proportion of plastic additive chemicals are simple molecules, which typically makes them inexpensive to produce and allows them to be widely used in the production of plastic materials. In addition, cheap fillers are also added to some plastic materials and polymer formulations to help reduce the overall cost of the final product. Plastic additives used to slow or prevent specific degradation processes from occurring help to ensure a long service life for plastic products. This class of additive chemicals is often referred to by the generic term “stabilizers.” It includes UV stabilizers, antioxidants, and antimicrobial agents that have been specifically designed to impart protection against these degradative processes. While antimicrobials are most often added to prevent microbial contamination of a product and any subsequent transfer to humans, they also protect the material from natural microbial degradation. The inclusion of additive chemicals plastic materials can affect the type and quantity of functional groups associated with the polymeric structure. This changes the hydrophilicity/hydrophobicity of the material, which can lead to activation, inhibition, or catalyzation of the biodegradative process (Fotopoulou & Karapanagioti, 2019). Therefore, plastic additive chemicals can lead to either a

Influence of environmental and external parameters on plastic biodegradation


reduction or an increase in the rate of different plastic degradation processes, including biodegradation, contributing directly to the degree of persistence when the plastic material enters the natural environment. It is important to highlight that many additive chemicals employed as stabilizers to protect plastic materials from degradation processes are consumed or destroyed during the process. As the stabilizers are gradually consumed, a process that might take decades, the protective effect reduces, and the plastic material degrades more rapidly over time. It is, however, worth considering that additive chemicals themselves will exhibit a broad range of susceptibilities to biodegradation. While this may impact the efficacy in cases where they are readily biodegradable, it can also lead to misleading results in biodegradation studies of plastic when they are preferentially degraded over the polymer material (Gayta´n et al., 2020; Harrison et al., 2018). Although additives have been proposed that specifically increase the biodegradation potential of conventional polymers (e.g., PE, PET) (Amobonye et al., 2021, p. 1435), there is limited evidence to suggest they are particularly effective (Selke et al., 2015). Despite having the potential to significantly influence plastic through a range of degradation mechanisms, additive chemicals are typically neglected as a potentially key parameter in understanding and assessing the biodegradability of plastic materials within the natural environment.

Influence of environmental and external parameters on plastic biodegradation Plastics and microplastics are widely distributed contaminants within the natural environment, particularly in marine compartments (Wu et al., 2019). Although some plastic and microplastic in the marine environment derive from offshore use and activities, the greatest proportion originates from land-based activities and emissions. Plastic litter released into the environment is exposed to various environmental processes that may affect the fate and behavior of the plastic, including biodegradation. Microbial biodegradation largely depends on the initial action (individual or synergistic) on the polymers by different environmental processes (Amobonye et al., 2021, p. 1435), and the kinetics of biodegradation are heavily influenced by complex surrounding environments. Consequently, when the conditions and parameters of one environment vary significantly from another’s, the rate of microbial degradation in these environments will also vary. Knowledge regarding the distribution and fate of microplastics in the terrestrial environment is limited (Wang et al., 2020; Zhu et al., 2019). Therefore, this section primarily focuses on the influence of various environmental parameters on biodegradation, using the marine environment as an example. Where relevant and available, information for the terrestrial environment, including industrial composting, is presented. The potential pathways of microplastic transportation and biological interactions in the marine environment are summarized in Figs. 2.5 and 2.6. Fig. 2.5 also highlights how much conditions can vary in the various compartments of the marine environment. Factors such as pH, humidity, availability of oxygen and nutrients, the composition of the microbial community, and temperature differ from compartment to compartment and from season to season, and so too do the rates of biodegradation (Folino et al., 2020). The effect of a specific environmental factor on the biodegradation rate is influenced strongly by the type and shape of an individual piece or particle of plastic (Chamas et al., 2020). The degradation rates of various plastics within the natural environment have been reviewed by Chamas et al. (2020), describing how different types of plastic will degrade at varying rates in terrestrial and marine environments (Chamas et al., 2020). Table 2.3 summarizes the main environmental parameters that have the potential to influence plastic degradation.


Chapter 2 Composition, properties and other factors

FIGURE 2.5 Transport, accumulation, and fate of microplastics within the environment. From Wu, P., Huang, J., Zheng, Y., Yang, Y., Zhang, Y., He, F., Chen, H., Quan, G., Yan, J., Li, T., & Gao, B. (2019). Environmental occurrences, fate, and impacts of microplastics. In Ecotoxicology and Environmental Safety, 184, 109612e109612). Academic Press.

In the marine environment, seawater temperature, salinity, and dissolved oxygen (DO) concentrations may differ significantly, both temporally and spatially, in the water column and marine sediments. These environmental factors also vary with geolocality, from surface waters in tropical and subtropical areas to the Arctic Ocean and the meso- and bathypelagic ocean zones. At ambient temperatures within the natural environment, the chemical degradation of polymers usually involves either hydrolysis (requiring H2O) or oxidation (requiring O2) (Meereboer et al., 2020). Both degradation mechanisms can be accelerated by the simultaneous combination of microbial action, light, and heat. Since plastic persistence is highly influenced by exposure to UV light (UV-catalyzed oxidation), water depth and light penetration are important parameters. In addition, pressure increases by depth, and in the deep oceans, organisms are specifically adapted to the pressure and constant darkness. Biogeochemical processes like aggregation and marine snow events may be important transport mechanisms, particularly for micro- and nanoplastic particles incorporated into such aggregates. Plastic in the sediments of the literal zone is exposed to continuous mechanical abrasion and fragmentation by sediment and wave actions. Biodegradation of plastic relies mainly on primary colonization of the particles by microbes, followed by biofilm formation. As with other particles in the environment, the degree of colonization may rely on the particle surface, with decreasing particle size

Influence of environmental and external parameters on plastic biodegradation


FIGURE 2.6 Potential interactions between marine microorganisms and plastic/microplastic in marine environments.  czuk, A. M. (2018). Degradation of plastics and plastic-degrading bacteria in cold From Urbanek, A. K., Rymowicz, W., & Miron marine habitats. In Applied Microbiology and Biotechnology, 102(18), 7669e7678. Springer Verlag. s00253-018-9195-y (

resulting in larger surfaces relative to particle volume. It is important to note that environmental parameters can indirectly increase biodegradation through enhancement of other degradation mechanisms that result in the formation of new structural homogeneities and functional groups, as well as chain scission processes that produce small polymer fragments that are more bioavailable to microbes. While biodegradation processes are mainly associated with bacterial processes, larger organisms, such as worms, shrimps, and seabirds, have been observed to ingest, digest, and degrade plastic.

Temperature The seawater temperatures in the world’s oceans vary from >30 C in surface water and littoral and sublittoral zones in tropical and subtropical regions to CoPal3 > CoLau3, indicating that the chain length of carboxyl functionality also plays a critical function in PE degradation (Roy et al., 2006, 2007). Inorganic fillers are frequently used with polyolefins to form composites where the filler augments the mechanical properties. Montmorillonite (MMT) boosts the mechanical behavior of polyolefins and speeds up the photooxidation (Mailhot et al., 2003; Morlat et al., 2004; Therias et al., 2005). Qin et al. (2003, 2004) have deliberated on the photooxidative degradation of PE/MMT nanocomposites and observed no noteworthy rise in PE degradation in specimens by means of nanodispersion or microdispersion of MMT, save for Fe3þ modified MMT, which hastens the degradation of PE. Kumanayaka et al. (2010) report that the decomposition of alkylammonium ions in MMT created olefins along with acidic sites resting on clay faces leads to faster radical development in the PE matrix. MMT was used as a filler in the photooxidation of PP (Mailhot et al., 2003; Therias, Mailhot, Gonzalez, et al., 2005), PE (Qin et al., 2003), and EPDM (Therias, Mailhot, Gardette, et al., 2005), and these nanocomposites degrade faster. Iron (Fe3þ) formed may catalyze the decomposition of primary

Accelerating degradation


hydroperoxide (Mailhot et al., 2003; Osawa et al., 1988; Therias, Mailhot, Gardette, et al., 2005). Apart from nanocomposite formation, the speed of photooxidative degradation of PE/MMT nanocomposite and PE/M n þMMT microcomposites is much faster than that of pure PE. It was also suggested by Qin et al. (2004) that the decomposition of ammonium ions might generate acidic sites on layered silicates. Thus, the reversible photoredox reaction of transition metal cations has a catalytic outcome on degradation of the polymer matrix. Reddy, Deighton, Bhattacharya et al. (2009), Reddy, Deighton, Gupta et al. (2009) studied the outcome of organically modified montmorillonite (OMMT) on PE biodegradation and observed thermal oxidation (TO) of PE was influenced considerably through prooxidant only but not OMMT. Dintcheva et al. (2009) investigated the influence of developed compatibilizer PE-grafted maleic anhydride (PE-g-MA) on photooxidation of straight-chain LLDPE/MMT and established that PE-gMA speeds up the photooxidation of PE nanocomposites compared with a clean polymer and PE/ OMMT. The main cause was thermal degradation products of alkyl ammonium surfactant, at last resulting in extra polymeric peroxidation while providing prooxidant effects. Photodegradation of commercial and newly formulated cyclic olefin copolymers films amid different metallic stearates (Fe, Co, and Mn) was investigated by means of a UV lamp (340 nm) for 1 month as per ASTM D5208-01, and it revealed that copolymers demonstrated higher sensitivity to photodegradation in the presence of metal stearates, Fe salt showing the maximum oxidative action (Villarreal et al., 2017). Trang et al. (2018) studied the degradation of HDPE films containing CaCO3 and 10 wt% prooxidant additives (manganese (II) stearate/ferric stearate/cobalt (II) stearate, 18:4:1) for 96 h UV exposure and demonstrated that HDPE samples with more than 5% CaCO3 were more quickly degraded than those with less than 5% CaCO3. CoSt-filled PP film layer biodegradation was deliberated under restricted composting circumstances, and the degradation products were evaluated for their ecological and toxicological effects. As CoSt was increased in PP films, the tensile strength and thermal permanence decreased. The compounding of CoSt in PP reduced its crystalline nature and was established by DSC and XRD, leading to improved degradation. Subsequent to biodegradation, SEM of customized PP films clearly shows rougher morphology than prior to subjecting to biodegradation. The highest biodegradation (19.78%) was revealed, with the film having two parts per hundred of resin CoSt. The ecotoxicity tests of degraded PP films, specifically plant growth, microbial, and earthworm acute toxicity tests, verified the nontoxic nature of the biodegradation intermediates. Hence, CoSt packed PP has more manufacturing prospective to build biodegradable flexible wrapping (Sable et al., 2020). Plastic bags containing an unrevealed prooxidant filler (Inorganic additive) and acquired from local Kuwait dealers and were formulated with either LLDPE or HDPE. Calcium carbonate (CaCO3) was added for strength and density. The tested plastic showed slight fragmentation and degradation in accelerated weathering (AW) and soil burial tests. Venkataramana et al. (2020) synthesized NiAl2O4 spinels by coprecipitation and hydrothermal routes and used them for the photodegradation of PP plastics. They observed 12.5% wt loss in 5 h, and hydrothermally prepared NiAl2O4 spinels showed a faster degradation pace. Green solvent methyl lactate was obtained from chemical degradation of end-of-life poly(lactic acid) catalyzed with Zn(II) ethylenediamine composite (Ramirez et al., 2020).


Chapter 13 Utilization of chemical additives to enhance

Organic prodegradation agents A few degradant additives do not include transition metals and have ketone copolymers, unsaturated alcohols or esters, 1,2-oxohydroxyl groups, a-pyrones, benzophenones, b-diketones, polyisobutylene, and chosen amines and peroxides (Ammala et al., 2011). Benzyl and CoSt additives with LDPE films were studied (Roy et al., 2005, 2011) for tensile properties, CI and apparent density just before degradation. They observed that benzyl’s efficacy was inferior to transition-metal complexes. When LDPE films were mixed with CoSt, the pace of degradation was improved and depended on the CoSt amount. One more way to kick off the degradation of polyolefins is to bring in definite frail sites, such as dithiocarbamates, carbonyls, and carbon monoxide on the hydrocarbon backbone/side chain. Adding olefinic bonds all through polymerization has been used previously but with a high price tag (Corti et al., 2010; Roy et al., 2011). Chelliah et al. (2017) analyzed the effect of different loading amounts of cobalt stearate (CoSt) as a prodegradant additive in PET and revealed that CoSt influences the maximum in blends consisting of 0.25 CoSt for thermal degradation of PET. Stloukal and Kucharczyk (2017) developed an additive based on carboxyl functionalized comblike poly(lactic acid) copolymer and prepared PLA films with 5 %e15% of this additive. The outcome showed that the developed hydrolysis additive proficiently promoted biodegradation even at a lower quantity of 5% w/w. Maryudi et al. (2017) integrated HDPE with manganese palmitate, manganese laurate, and manganese stearate as prooxidant additives (0e1.0 wt%) and studied NW conditions of Gambang, Malaysia. The number of prooxidant additives enhanced the degradation of specimens, and manganese stearate behaved better than either manganese laurate or manganese palmitate. Xu et al. (2018) used poly(ethylene glycol) modified nano-TiO2 particles as prooxidant additives for LDPE films. The hydrophilic alteration of TiO2 by PEG helped the photooxidation of LDPE by synergistic effect into small molecular weight residues resulting in enhanced biodegradation. Cesur (2018) considered the degradation of the polycaprolactone and found that a minute volume of inorganic additives (OMMT clay, Aldrich, 0.1 and 0.4 wt%) delayed the biodegradation. However, adding a clay additive in one or higher wt %resulted in the development of defective crystals and thereby accelerated the biodegradation with the aid of the organic additives. As soon as the organoclay and glycerol monooleate additive was used together at 3 wt% or further, the biodegradation moment was drastically reduced from 2 years to around 6 months. Meereboer et al. (2020) reviewed the recent progress of PHAs as a biodegradable substitute wherever nonbiodegradable and petro-derived plastics are presently in use. PHAs are a familiar group of biodegradable plastics that provide a path to carbon neutrality and thus support an additional sustainable trade. Several PHAs poly(3-hydroxybutyrate) and poly(3-hydroxybutyrate-co-3hydroxyvalerate) demonstrate biodegradable behavior inside environments as per various ASTM standards and can be used to construct entirely compostable biodegradable products. However, PHAs are comparatively more costly than petro-based alternatives. To cut the price tag, PHAs can be used in biocomposites, especially where biobased agro residues are being integrated and maintaining performance. Cellulosic organic fillers and threads can also improve polymer properties. When mixed in biocomposites along with PHAs, they enhance rates of biodegradation in all conditions. Besides cellulose, further biobased fillers such as proteins and starch have been reported in the direction of improving soil and oceanic biodegradability. Meereboer et al. (2020) further found that additional constituents also affect biodegradability and include additives (i.e., chain extenders) and

Accelerating degradation


compatibilizers (e.g., maleic anhydride). These are put in place to optimize service life characteristics, although they slow biodegradation by changing polymer hydrophilicity. A large number of feasible combinations of polymers with fillers and fibers, and their effect on biodegradation of PHA-based composites is still an uncharted area on the cutting edge. The prospective benefits of PHA-based composites compose a tough case for an advanced investigation into this field.

Blending with natural polymers Several researchers have used direct integration of natural polymers to improve biodegradability. The microbial adaptation of polymers in these types of mixes was found to enhance the surface area of manufactured materials and make them more vulnerable to biodegradation (Liu et al., 2013; Pang et al., 2013; Scott and Wiles, 2001; Vieyra et al., 2013; Yu et al., 2006).

Starch By adding prooxidants or organometallic additives to the starcheplastic blend, degradation of Naturegrad Plus (NP) was achieved via a combination of microbial and chemical processes (Imam et al., 1992). Control films subjected to thermal pretreatment but without soil burial did not show noteworthy amendment of the measured material parameters. It is apparent from the outcome that HDPE and LDPE degradation in the natural environment poses a grave environmental worry due to their sluggish degradation pace. A new group of environmentally friendly PEs was tested, such as NP, which contains 9% starch and a prooxidant additive (a mixture of catalytic agents, autoxidizable fatty acid ester, and transition metals), are highly recommended since the degradation in environmental situations is much faster. After 5 months of soil burial, considerable NP degradation was achieved as indicated by weight loss (36%) and decreased tensile strength (59%), more CO2 creation, and changes in functional groups in the infrared spectrum. Moreover, the degradation of NP was further sustained up to 15 months of monitoring (Orhan et al., 2004). Starch is one such renewable polymer that has the capability to substitute petroleum-derived plastics (Yu et al., 2006). The biodegradation of PE and its joining through starch have been extensively deliberated (Inceoglu & Menceloglu, 2013; Pang et al., 2013; Pushpadass et al., 2010; Rehim et al., 2004; Li et al., 2011; Schlemmer et al., 2009; Vieyra et al., 2013; Zuchowska et al., 1998) and it is evidently implicit that the initiation of the biodegradation of PE blends is largely owing to starch part and extra is the uninterrupted starch segment content more is the biodegradation. UV degradation of oxo-degradable PE/thermoplastic pea starch (TPPS) blends has shown that TPPS acts as a prooxidant to hasten the photooxidative decomposition of PE (Muthukumar et al., 2010). Exposure to direct daylight showed maximum weight loss along PE specimens covered in soil showed minimum weight decrease. Prooxidant blended HDPE had a high weight decrease compared with new starch blend specimens. Different starches have been used to get better compatibility of PE (Kang et al., 1996; Torres et al., 2008), and their biodegradation behaviors have been studied. Acetylated starch accelerated biodegradation of LDPE, whereas oxidized starch slowed biodegradation compared with indigenous starch. This can be explained due to the detail that oxidized starch shattered amorphous zones in addition to amplification of the starch crystalline region, whereas acetate damaged the structured crystalline phase (Torres et al., 2008). Further biodegradability of populated (Garg & Jana, 2011) starch/LDPE films amplified through mounting starch concentration, although decreased amid growing extent of replacement (Ammala et al., 2011).


Chapter 13 Utilization of chemical additives to enhance

Chitosan Chitosan is a naturally occurring high molecular weight polymer produced by N-acetyl-D-glucosamine units with b (1e4) glycosidic bonds. Chitosan is positively charged owing to unbonded amino groups of chitin and generates crystal clear films to extend the storage life of food products (Camacho et al., 2013; Vasile et al., 2013; Yu et al., 2006). Chitosan lowered the tensile strength of PP composites but enlarged Young’s modulus. On the other hand, impact strength increased with the adding up of chitosan. The modified PP/chitosan composites were established to have better structural properties, for instance, tensile strength, superior Young’s modulus, and enhanced impact strength compared with raw specimens owing to the superior distribution of the chitosan among PP composites and hence the superior interfacial bonding (Amri et al., 2013). The upgrading of LDPE biodegradability by adding palm oil plasticizer and chitosan as filler was deliberated by Sunilkumar et al. (2012) amid Aspergillus niger on a potato dextrose agar media and incubation next to normal temperature meant for 21 days. Biodegradation tempo was established to boost amid rising chitosan loading in the polymer matrix. The plasticized specimens showed superior biodegradability pace and hydrophilicity compared with the unplasticized specimens. Chitosan and palm oil cross-toughened LDPE has been verified as a new permutation amid the amplified biodegradation tempo of LDPE and has invoked potential applications within bioseparation and food packaging, for example.

Protein The biodegradation performance of soy protein grafted PE deliberated amid soil burial technique by Kaur et al. (2009) which concluded that the weight loss augmented with the coverage phase. Further microanalysis of the samples showed a boost in the microbial counts with a mounting number of days. In addition, this study supported that the degradation products were safe to the escalation of the vegetation.

Reinforcement with natural fibers Plant-based natural fibers boast many advantages, including biodegradation, renewability, low cost, high strength, low density, ease of partition and recyclability, carbon dioxide seizure, and biodegradability (Nakatani et al., 2011). Fiber-toughened polymeric composites have seen extensive interest in recent years due to their towering precise strength and modulus, along with matched economic viability and better biodegradability. For lignocellulosic materials, photodegradation leads to the degradation of fiber, matrix, or the interface and reduces the capability of the composite to successfully shift stress between components resulting in poor mechanical properties (Lundin et al., 2004). It has been established that PE/natural fiber mixes are not as steadier as either virgin polymers or natural fiber. The mechanical performance decreased considerably with mounting contact time and the degradation deepness enlarged with rising contact moment in time. Tajeddin et al. (2010) established poly(ethylene glycol) (PEG) plasticized kenaf cellulose (KC)/LDPE composite had enhanced fibermatrix adhesion. The speed of biodegradation of LDPE and KC blends was little but was enough for its collapse into natural surroundings with adding up of PEG inflated biodegradation marginally.

Scheme for allocating degradation agents The consequence of prodegradant (Fe- and Co-based) circulation within hybrid PE/starch mixes was initially deliberated by Yu et al. (2013) via twin step procedure with part supply on the reactive

Commercially available degradation-promoting additives


extruder. Evaluations of carbonyl functionality by FTIR spectroscopy were performed to look into the degradation outcome immediately past photooxidative (UV) exposure. The disparity in the structural characteristics was found to be elevated once the prodegradants were scattered in the PE matrix, with the number of carbonyl groups improving with UV exposure. Microcracking was seen on the border stuck between starch and PE following the addition of prodegradants. Prodegradants prefer to continue with starch while having mutual polar surfaces. When prodegradants were scattered in the HDPE stage, minute cracks chiefly appeared in the HDPE matrix, and the concentration was higher. Vieyra et al. (2013) reported that a starch PE mix containing 40% starch is anticipated toward the end for 11.98 years prior to undergoing total degradation. The involvement of natural polymers in speeding up of degradation of polyolefins requires to be deliberated extra expansively in the upcoming time.

Commercially available degradation-promoting additives There are several biodegradation-promoting additives available in the market, to name a few, i.e., d2w (Symphony, UK), Reverte (Wells Plastics, UK), Renatura (Green Ready Plastic, Norway), Addiflex (Add-X, Sweden), TDPA and Enviro (EPI, Canada), OxoElite (EcoPoly, Canada), OBD (Enter Plastics, UAE), P-Life SMC2360 and SMC2522 (Programmed Life, Japan), BDA, PDQ-H, and PDQM (Willow Ridge, USA). Envirocare additives are specialty chemicals put into usual thermoplastics and commodity plastics to get degradable agricultural plastic articles allowing the fabrication of degradable plastic products processable with typical machinery, without affecting the properties of the plastic product and with apparent costs, advantages, e.g., films, small subway films, banana bags, twines, covers, ropes, and pots are handy agricultural applications meant for this expertise. Envirocare provokes degradation in two steps: initially, the plastic is thermo- and photooxidized by outside exposure, where the degradation scheme has been activated by light or heat, Envirocare acts by escalating the degradation pace and keeps degrading until the object is entirely degraded. Since degradation is both photolytically and thermally triggered, it occurs both outside and in the soil. Ground exposure in diverse environments integrates the trial outcome and allows for the devising additives tailored for particular requirements (Bonora & Corte, 2003). EPI TDPA and Envirocare formulations are commercially available additives and can be compounded with usual polymers at suitable levels to manage the formation and decomposition of hydroperoxides, thereby allowing control of the life span of plastic articles. These additives maintain the solidity of plastic during handling, storage, and instant end use. After use, when the material is not needed, oxidative degradation is accelerated. The resulting oxidized molecular fragments are hydrophilic in nature and make up 1/10th or even less of the initial molar mass, and are eventually biodegradable (Arnaud et al., 1994; Scott, 1997, 2000). Polyolefines and other polymers destabilized with TDPA and Envirocareadditives can be used in agricultural mulching films and for litter and landfill management (Billingham et al., 2002). Chiellini et al. (2006a, 2006b) studied PE film specimens containing proprietary TDPA thermal prooxidant additives of EPI, Canada and found that these proprietary prodegradant systems were valuable in facilitating the oxidative degradation of PE backbone. Further, it was also found that in PE film containing thermal prooxidant additives (FCBZSK15, LDPE-DCP540 and FCB-ZSK10), PE samples were regioselectively oxidized at 1,3position and hence biodegradation observed (Chiellini et al., 2007).


Chapter 13 Utilization of chemical additives to enhance

Oxo-biodegradation-promoting agent Ciba Envirocare AG1000 works with the same resin as a drop-in additive and meets composting requirements (>58 C). In 2009, Ciba Geigy was merged into BASF. BASF continued with biodegradable additive Envirocare till 2010 and then discontinued to focus on Ecoflex biopolymers with additive Ecovio which support circular economy contributing to sustainability ((Ciba, 2010) Vogt and Kleppe (2009) put PE with PP samples and 2% Renatura prooxidant to a different period of weathering and then switched to shadowy thermal exposure (at 70 C) degradation for 45 days’ time. The outcome demonstrated that oxidative degradation, following preliminary light exposure, continued speedily in dim thermal circumstances and better exposure to light raises thermal-oxidative degradation. Lo´pez et al. (2017) studied the outcome of AW, NW, and TO on plastics, i.e., HDPE; oxodegradable HDPE, HDPEoxo; compostable plastic, Ecovio; PP and oxo-degradable PP, PPoxo. Plastic films subjected to AW and NW demonstrated a wide-ranging decline in structural properties. Oxoplastics showed elevated degradation than the usual counterpart, and compostable plastic was unwilling to degrade in studied abiotic circumstances. This study proved that the degradation of plastics in real-life situations will fluctuate depending on the composition and environment. Yashchuk et al. (2012) collected oxo-biodegradable additive d2w from the Argentine market and studied the degradation behavior of PE films. They found that the additive promoted degradation amplified microbial action in the early stage of biodegradation. Nevertheless, the additive is not adequate in the direction of creating full mineralization of PE. Actually, after 90 days of incubation at 55  1 C, only 24% of PE or PE þ AD biodegraded. The oxo-degradable HDPE plastic bags with additive d2w was studied by Villamizar and Morillas (2018) and found that oxo-degradable plastics with and without preceding oxidation demonstrated more rapid degradation than usual HDPE, meaning the prooxidant additives did support abiotic degradation.

Conclusions Contemporary trends in biodegradable polymers show notable progress in design strategies and manufacturing and can make available superior polymers with comparatively superior performance. Nevertheless, there are many shortfalls in either technology or cost, particularly in applications in terms of environmental pollution. Therefore, there is a call for a new perception of the design, properties, and utilities of polymers for potential future developments. The biodegradable polymers can be prepared from petroleum resources, but the chief ingredients are attained from renewable assets (Saini, 2017; Song et al., 2009). The discussion about the biodegradability of plastic is far from completion. However, it must forge ahead with the contention that plastics splinter toward confirming whether the time experienced for entire biodegradation is adequate on or after an ecological concern and whether this is to occur in normal environments. Most of the formulations are still proprietary and classified. Without any sort of directive means, there is no warranty that plastic will achieve suitably in markets and with no toxicity in end environments. Researchers are continuously building bioplastics as the main element of sustainable plastics trade via improvement in handing out techniques and addressing the complete fabrication lifecycle. Better perception of polymer constitution and utility results in bioplastics with enhanced bulk characteristics



at lower costs. Society, industry, and government have interactive roles to play in developing sustainable plastics, which can prove positive for all of us and planet earth. Several newer plastics technologies have integrated biodegradable additives into their chemistry to compose plastics that will not become eternal pollutants. These additives are designed to permit plastics to break down physically, whether they are inside a landfill or wayside. While they degrade, these plastics break down into carbon dioxide, humus or biomass, and methane gas. That is a huge upgrade over practically imperishable detergent jugs and soda bottles. Much work remains to make biodegradable additives a part of most plastics, especially with regard to government legislation, recycling standards, and consumer public relations.

Acknowledgments The author wishes to show gratitude to Director, CSIR-Central Building Research Institute, Roorkee for his constant encouragement.

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The role of nanomaterials in plastics biodegradability


Manviri Rani1, Meenu1 and Uma Shanker2 1

Malaviya National Institute of Technology JLN Marg, Jaipur, Rajasthan, India; 2Department of Chemistry, Dr. B R Ambedkar National Institute of Technology Jalandhar, Jalandhar, Punjab, India

Introduction About nine billion tons of plastics have been produced since the 1950s worldwide (Barnes et al., 2009). Nine million tons are thrown into our oceans annually, and only 9% of plastic gets recycled, with the rest of it dumped in landfills (Nithin & Goel, 2017). Plastics take 500 years to degrade and leak poisonous chemicals from the plastic into the environment (Shimao, 2001). Therefore, there is an urgent need for awareness about and management of waste plastics debris. Developing countries such as India are suffering from abandoned plastic debris manufactured in metropolitan and community areas due to a lack of negative impact from the waste as well as the environmental protection schemes run by the government (Bhattacharya et al., 2018; Johannes et al., 2021; Rani & Shanker, 2021). Furthermore, due to the inert and nonbiodegradable nature of plastic, its treatment and disposal is the main problem in urban solid waste management (Gurjar et al., 2021). Polymers in plastic release various contaminants and metals once left unprotected in environmental circumstances (Ali et al., 2021; Karapanagioti et al., 2011; Tanaka et al., 2013). Plastics have various applications, including construction, packaging, pipes, furniture materials, automobile parts, and containers for drinks (Teuten et al., 2009; Wilcox et al., 2015). The life cycle of plastic is described in Fig. 14.1. Various synthetic plastics, including polyethylene terephthalate (PET), polyvinyl chloride (PVC), polystyrene (PS), polyethylene (PE), and polypropylene (PP), are manufactured using petrochemicals as raw materials (Fred-Ahmadu et al., 2020; Leo´n et al., 2018; Avio et al., 2017). These plastics cover major parts of polymer industries throughout the world. The consumption of plastics is 13,000, 8000, 5000, and 3000 kt for PE, PP, PVC, and PET, respectively, in European industries. The application of various plastics is discussed in Table 14.1. As mentioned, nonbiodegradability and leaching of toxic chemicals from plastics cause a negative impact on the environment (Koelmans et al., 2014; Wright & Kelly, 2017). There are several well-known traditional strategies for plastic degradation, such as mechanical, thermal, photo-oxidation, and biodegradation processes (Jenkins et al., 2019; Singh & Sharma, 2008). However, these traditional techniques are of further concern to the environment because they produce many small-sized plastic particles (micro- and nanoplastics). However, pollution by micro- and nanoplastics leads to further adverse effects on the ecosystem (Chae & An, 2018; Rhodes, 2018; Wilcox et al., 2015). Therefore, the need for advanced techniques which is eco-friendly, cost-effective, Biodegradability of Conventional Plastics. Copyright © 2023 Elsevier Inc. All rights reserved.



Chapter 14 nanomaterials in plastics biodegradability

FIGURE 14.1 Manufacturing, application, and recycling of plastic.

require less energy consumption, and produce a lower volume of toxic by-products. Among all degradation techniques, biodegradation is widely used, as it is a more economical method (Kyrikou & Briassoulis, 2007; Sivan, 2011; Zheng et al., 2005). The potential interest in biodegradation of plastic debris involves environmentally friendly eradication via microorganisms and nanomaterials, resulting in the mineralization of debris for the protection of the environment from these pollutants (Alshehrei, 2017; Kumar & Maiti, 2016). Meanwhile, the Assimilation of nanoparticles in a polymer matrix or by manufacturing nanocomposites chokes the limitations of the biodegradable plastics. One of the great advantages of nanoparticles is to adjust the rate of biodegradation both increase and decrease as paralleled to that of the rate of pure polymer accordingly to requirements (Luo et al., 2019; Pandey et al., 2015). Therefore, the physicochemical characteristics of the plastics can be altered and controlled to enhance their biodegradability for desirable application in various fields (biomedical, fishing, and other areas). Therefore, bioplastic is a sustainable alternative to nondegradable plastic and has overcome the problem of disposal of plastic waste in terms of environmental protection aspects. Therefore, the problem of plastics is resolved by using either biodegradable plastics (bionanocomposites as matric) or some nanomaterials/biocatalysts for biodegradation of plastic debris.

Environmental concerns for microplastics and nanoplastics Pollution by plastic waste has been a great concern for the environment and aquatic life. The ocean has been bearing a huge volume of plastic debris in various size ranges, including large-sized debris from fishing and nanosized particles from various single-use plastics (Rillig et al., 2021; Rochman, 2015). Although the negative effect of macro plastics debris (large plastic) in oceanic environments has been well known, the probable damage caused by microplastics as well as nanoplastics is yet to be not as

Environmental concerns for microplastics and nanoplastics


Table 14.1 Application of plastics in everyday life in various fields. Polymer/plastic types


Ureaeformaldehyde (UF)

UF is used as phenolics to improve the surface hardness and moisture resistance of the coatings in wood products. It is mostly used as an adhesive for the bonding of particleboard, chipboard, electrical switch housings, plywood, hardboard, and other wood products. PTFE used coating in nonsticky surfaces frying pans, watersides, insulating electrical cables, and plumber’s tape because of its low friction or heatresistance. It is more commonly known as teflon. PMMA is a biodegradable thermoplastic used as an alternative to glass due to its low cost, ease of shape, and synthesis by various agricultural products. It is used in various commercial products such as contact lenses, fluorescent light diffusers, car windows, aquariums, and rear light covers for cars as artistic color. A high-temperature, chemically stable polymer that does not crystallize Strong, chemical, and heat-resistant thermoplastic; biocompatibility allows for use in implant applications and aerospace moldings. One of the most expensive commercial polymers PEEK is a thermosetting plastic with the familiar trade name bakelite. It has a high heat resistance and a modulus constant that make it feasible for application in electronic and other commercial products. Biodegradable and heat-resistant thermoplastic composed of modified corn starch

Polytetrafluoroethylene (PTFE) Polymethyl methacrylate (PMMA)

Polylactic acid Polyetherimide (Ultem)

Polyetheretherketone (PEEK) Starch material phenolics (phenol formaldehyde) Melamine formaldehyde

Polyurethanes Polycarbonate/ acrylonitrile butadiene styrene Polycarbonate Acrylonitrile butadiene styrene Polyamides (nylons) High-impact polystyrene Polystyrene Polypropylene Low-density polyethylene (LDPE) Nylon

It is used as a substitute for phenolics of various colors, having aminoplast segments. It has various applications in the ceramic industry, such as ceramic cups, bowls, and plates for children. It is the most frequently used polymer in printing rollers, thermal insulation foams, surface coatings, and cushioning foams. Used in car interior and exterior parts and mobile phone bodies

It is used to make stronger plastic for various applications. It has applications in making riot shields, traffic lights, compact discs, security windows, and eyeglasses. It has applications in electronic apparatuses such as drainage pipes, printers, computer monitors, and keyboards. Fibers, toothbrush bristles, fishing line, under-the-hood car engine moldings Refrigerator liners, food packaging, vending cups It is frequently used in packaging, disposable cups, food containers, cassettes, cutlery, plastic tableware, CDs, and plastic boxes. LDPE is used for making yogurt containers, bottle caps, car bumpers, and drinking straws. Nylon is a polyamide used in water hose nozzles, racehorse shoes, small bearings, windshield wipers, inks, speedometer gears, cellophane, football helmets, rainwear, and clothing parachute fabrics. Continued


Chapter 14 nanomaterials in plastics biodegradability

Table 14.1 Application of plastics in everyday life in various fields.dcont’d Polymer/plastic types


Polyvinyl chloride

Plumbing pipes and guttering, shower curtains, window frames, flooring, food packaging Detergent bottles and milk jugs

High-density polyethylene Polyethylene Polyethylene terephthalate Polyester

It is mostly used in the packaging of food and other daily use application of plastic bags. Carbonated drink bottles, peanut butter jars, plastic films, microwavable packaging Fibers, textiles

clear (Hwang et al., 2020; Zimmermann et al., 2020). Several reports have concluded that plastics debris is ubiquitous and is accumulated by marine organisms, furthering the transfer into the food chain and having a negative impact on aquatic and human life (Meeker et al., 2009; Rani et al., 2017; Zarfl & Matthies, 2010). Additionally, plastic particles in the ocean comprise fairly high levels of organic contaminants; e.g., PS has a large number of hexabromocyclododecanes, a flame retardant, and bisphenol A (BPA) in polycarbonate plastics (Mato et al., 2001; Rios et al., 2007). In addition, BPA is a plastic additive suspected to damage the male reproductive system and disturb hormonal regulation (Liu et al., 2018; Vom Saal et al., 2007). The leaching of BPA residues (1e100 ng) from the everyday life appliance made from plastic has been observed in human plasma, amniotic fluid, urine, follicular fluid, and breast milk by various reports (Ikezuki et al., 2002; Wolff et al., 2008; Yamada et al., 2002). Subsequently, the quality of marine and beaches decreases via ingestion of plastic debris by nearly 395 marine organisms (marine mammals, birds, sea turtles, crustaceans, bivalves, etc.) (Besseling et al., 2014; Engler, 2012). Consequently, an estimated $13 billion financial loss to fisheries and tourism companies every year (da Costa et al., 2016). Oceans have been polluted by plastic debris, amounting to nearly eight million tons per year and approximately 580,000 plastic pieces per square kilometer worldwide (Willis et al., 2018). The discharge of plastic debris into the ocean is expected to continue increasing in the future (Cincinelli et al., 2021; Eriksen et al., 2014; Jambeck et al., 2015). In support of that, several controlled studies have been reported that highlight the transfer of organic chemicals from microplastics and resulted in worsening of physiological functions and health of the targeted organisms (Endo et al., 2013; Van den Oever et al., 2017; Yamashita et al., 2011). Concentration levels/abundance and chemicals profiles are compared in organisms (fish, whales, and birds) with that of microplastics in nearby waters (or gut matters) (Hao et al., 2005; Kunioka et al., 2006; Rigamonti et al., 2014). Despite this, limited information is available on statements that may say that plastic marine debris is a vector to transfer dangerous compounds into marine organisms. However, control methods for small microplastics and nanoplastics are yet to be in the developing stage, which concluded that their particular amount in the oceans is still unknown. Therefore, there is a need for plenty of data on this issue to further prove the field evidence. The toxicity, negative impact, and exact concentration of small plastic (micro and nanosized) particles of virgin and debris plastic to date are unknown (Kunioka et al., 2009; Tachibana et al., 2010). Therefore, there should be an increased focus

Remediation techniques for plastic pollution


on the assessment and biodegradation of plastic either by fabricating bioplastic or degrading by any advanced method to avoid continuous contaminations in the environment and for environmental protection.

Remediation techniques for plastic pollution Plastics mainly have a long or branched chain of organic and inorganic chemicals (additives such as plasticizers, UV stabilizers, and fillers) in their manufacturing process (Zheng et al., 2005; Shah, Hasan, Akhter, et al., 2008). The majority of polymers, such as ethylene and propylene, use fossil hydrocarbons as raw materials (Geyer et al., 2017). It is imperative to have basic knowledge about the interaction between polymers and the environment under natural circumstances for the degradation of plastic (including micro- and nanoplastics). The interaction between the surface of polymer and catalyst changes their surface properties which form the new chemical morphology of polymer and further degrade them. The degradation of plastics with nanoparticles is discussed in Fig. 14.2. Various physiochemical properties of the polymer, such as hydrophobicity, high molecular weight, high water repellency, and lack of functional groups attribute to its nonbiodegradability (Gawande et al., 2012; Sivan, 2011). In this direction, various methods have been used, such as thermal degradation, mechanochemical degradation, photooxidative degradation, catalytic degradation, photooxidative degradation, and biodegradation, for the treatment of plastic debris (Karapanagioti et al., 2011; O’Brine & Thompson, 2010). All conventional techniques break the chemical bonds present in polymers corresponding to their physical and chemical natures. Biodegradation is superior to all others because of its eco-friendly and cost-effective approach. However, all other approaches are less significant, consume high energy, have a high cost of maintenance, and produce toxic by-products (Urbanek et al., 2018; Williams & Simmons, 1996). Due to the amphiphilic nature of biocatalysts

FIGURE 14.2 Degradation of plastic with nanoparticles under environmental conditions.


Chapter 14 nanomaterials in plastics biodegradability

or biosurfactants, they increased the surface area of hydrophobic water-insoluble substances, increasing the water bioavailability of such substances, consequently degrading the polymer. Recently, several manufacturing companies have fabricated bioplastics items such as bags, cups, etc., using special additives. These plastic items are biodegradable via microorganisms under UV irritation (Mohee & Unmar, 2007).

Bioplastics: a new generation of polymers Bioplastics are made from biomass and have almost the same properties as ordinary plastics. European Bioplastics Association designed the two main definitions of biopolymers either manufactured by fossil raw materials or renewable sources (Leal Filho et al., 2021; Dilkes-Hoffman et al., 2019). The biodegradability of polymer is approved by EN 13432, which states that bioplastics are maybe completely biodegradable or nonbiodegradable (break down into small plastic particles, either microor nanoparticle). Bioplastics have many applications for basic purposes, but their primary contribution is in packing industries (bags, medical implants, straws, bottles, 3-D printing, plastic piping, containers, nondisposable carpet, and car insulation). The bioplastics market is growing globally and will have grown to approximately $44 billion in 2022, according to various survey reports (Arikan & Ozsoy, 2015; Sinan, 2020). Application of bioplastics in the various field described in Fig. 14.3. Synthetic plastics have various limitations, such as being manufactured from nonrenewable sources (oil or other fossil fuel), nonbiodegradability, and increasing the level of carbon dioxide in the

FIGURE 14.3 Application of bioplastics in everyday life.

Bioplastics: a new generation of polymers


environment due to burning after disposal (Philp et al., 2013; Vea et al., 2021). This shortcoming of synthetic or traditional plastics makes them less favorable even though they have light weight, durability, and chemical inertness. In this direction, Bioplastics have excellent import properties such as low heat distortion temperature, high brittleness, and less resistance to continued procedures (Emadian et al., 2017; Kalia et al., 2000; Peelman et al., 2013). Polylactic acid (PLA), polyhydroxyalkanoates (PHA), and other liner polyester-derived bioplastics are derived from the different carbohydrates sources (corn starch, chip starch, sugarcane, and tapioca roots source) in different regions across the world (United States, Asia, and Canada). Bioplastics are assumed to be less harmful compared to traditional plastics, as they do not contain toxic chemicals such as BPA, which is suspected to be an endocrine disrupter (Pilla, 2011; Thakur et al., 2018). However, there is a much need for improvements in bioplastics yet because they are not completely removing the plastic debris problem. Recently, several nanofillers (mainly biofillers) reduced the number of conventional additives and enhanced the characteristics of polymers (surface appearance, density process, capability, flexibility). Application of bioplastics in the various area depicted in Table 14.2.

Types of bioplastics Based on the source of raw materials, fabricated bioplastics are classified into two categories based on renewable sources, e.g., PLA and living organisms such as bacteria, e.g., polyhydroxyalkanoates (PHAs). These two main types of bioplastics, PLA and PHA, are discussed here. Table 14.2 Some biodegradable plastics and their uses. Polymer/plastic


Polyglycolic acid

It is biocompatible, nontoxic, biodegradable, and easy to fabricate. It has applications in dental equipment, drug delivery, nails, and screw fabrication. Wrapping and paper coatings, other feasible merchandise include comfort release systems for pesticides and fertilizers, mulch films, and compost sack Long-lived objects, compost and other agricultural films, herbicides (fiber-containing) to inhibit the growth of aquatic weeds, seedling containers, moderate delivery systems for drugs Products like containers, bags, wrapping films, and single-use napkins, as a substance for tissue engineering moieties and optimized drug release carriers Films and paper coverings, biomedical applications, curative delivery of worm medicine for cattle, and smooth transfer systems for pharmaceutical drugs and fungicides, germicides, and toxicants Wrapping and sacking use, when dissolved in water to excrete the substance like washing detergent, pesticides, and health care washables Adhesives (binder, thinner), the wrapping applications involve boxboard production, paper sacks, paper coverings, tube winding, and moistenable labels

Polylactic acid




Polyvinyl alcohol

Polyvinyl acetate


Chapter 14 nanomaterials in plastics biodegradability

Polylactic acid PLA is a polymer unit of lactic acid mainly derived from carbohydrates (corn starch, lactobacillus, sugarcane, and cassava) by the fermentation process. PLA is derived from lactic acid, although a few citric acids are mixed in to enhance the chain length of repeating unit consequence, long-chain PLA bioplastics (Gupta et al., 2007; Kulkarni et al., 1971). Similar to other polymers (PET, PS), PLA has great properties like good flexibility and high mechanical and thermal strength. PLA has two meso forms, D and L. The L isomer of PLA is more brittle and crystalline and has a higher melting point. PLA has various applications in packaging, such as food containers, blow molders, drink bottles, and film ware like other polymers (PE, PS, and PP). PLA is a well-thought-out renewable as well as biodegradable material. PLA is biodegradable via hydrolysis as well as by microbial degradation process under environmental conditions. Employing these conditions, PLA can be degraded into small molecules like carbon dioxide and water within 50e90 days under 40e50 C (Anderson & Shenkar, 2021; Williams, 1981). PLA does not produce toxic dioxins while burning as it has not contained chlorine atoms or other heavy metals (additives). However, other conventional plastics released toxic chemicals when they were dumped into the environment, consequently causing concern.

Polyhydroxyalkanoate PHA is manufactured by hydroxy alkanoates as a repeating unit obtained from microorganisms (as an energy or carbon source) under optimum conditions. The microbes contain high levels of carbon; however, they are low in other nutrients like elements (phosphorus, oxygen, and nitrogen). PHA acts as a carbon asset, is stored in granules, and has physicochemical properties similar to other conventional polymers (de Donno et al., 2021). Approximately 300 different types of microorganisms were used for the fabrication of PHAs. Out of all types of polyhydroxyalkanoates, polyhydroxy-butyrate (PHB) and polyhydroxy butyrate valerate (PHBV) are significantly used biopolymers (van der Zee, 2021). PHBV is a copolymer of hydroxyl-valerate and hydroxyl-butyrate units. PHAs are biodegradable, so they are not toxic to living tissue and environments. They have various applications in the area of biomedicine (slings, skin substitutes, sutures, and bone plates) and are used for single-use food packaging.

Limitations of bioplastics Bioplastics have advantages over conventional plastics, but plastic pollution is a matter of concern for environmentalists, scientific groups, and law agencies. Some research groups have concluded that bioplastics cause more pollution because of their frequent use in agriculture, packing, and other areas. Bioplastics cause greater ozone depletion than traditional plastics due to the excess release of carbon dioxide when they are degraded after use (manufactured from plants that absorb more carbon dioxide, which grows in the polyhouse under limited conditions). The various survey investigated that use of bioplastics limit the use of renewable energy, as they manufacture via microorganisms but increase the emissions of greenhouse gas (Lagaron & Lopez-Rubio, 2011; Mbotho et al., 2021). Also, bioplastics need more facilities for their biodegradation into small or less toxic molecules. The lack of facilities for the biodegradability of bioplastics causes more landfills in many cities, which may release many toxic gases, including methane, carbon dioxide, and other greenhouse gases. These disadvantages of bioplastics make them not a paradigm shift from traditional or conventional plastics (van der Zee, 2021).

Biodegradation mechanism


Biodegradation mechanism During the biodegradation of recyclable plastics under aerobic conditions, microbes gain carbon and energy. The mechanism that occurred during this process can be shown in Fig. 14.4 (Van den Oever et al., 2017). Microorganisms integrated the carbon of the polymer (Cpolymer) into biomass (Cbiomass), followed by fast mineralization into CO2 and H2O. This biomass can also be used for development and reproduction (more Cbiomass), which is further mineralized in the long term because of succeeding attacks of the soil microbial community. Hence, it was observed for a bi-phasic pattern that includes a rapid phase for CO2 production after a slower second phase for CO2 evolution, as seen by the final conversion into mineralization of organic matter. Overall, there are two reactions with different kinetics: (1) conversion of Cpolymer into Cbiomass and (2) conversion of obtained Cbiomass into CO2. The first reaction is biodegradation, which is indicated by either intake of reagents or the appearance of the yields, and the other one belongs to mineralization. Biodegradation could be monitored and quantified by measuring reagents of reactions (O2) or the end-product (CO2) of energy metabolism. The percentage of biodegradation can be defined as the ratio of evolved CO2 over theoretical CO2 (ThCO2), i.e., the quantities of CO2 predictable for total oxidation of the carbon existent in the plastic sample (Cpolymer) acquainted within the reactor. Instead, oxygen intake can be measured with a similar method utilizing the theoretic oxygen demand to designate the maximum oxygen uptake for the entire oxidation of the plastics. Measurement of biomass accurately by a consistent method is still not offered. Accordingly, the assessment of biodegradation has been achieved by mineralization as presented in Eq. (3) in Fig. 14.4. In this chapter, biodegradation and mineralization have been used as synonyms. As per guidelines from the OECD (1980), “readily biodegradable” chemical is presumed to undergo quick and final biodegradation in the environment exposure and no supplementary examination of the biodegradability of the polymer, or the probable environmental effects of conversion products, is usually essential. Biodegradable plastics cannot be regarded as “readily biodegradable” as they do not show such fast rates of degradation. The OECD Procedures have been established for

FIGURE 14.4 Degradation mechanism of polymer.


Chapter 14 nanomaterials in plastics biodegradability

water-soluble or water-dispersible chemicals, while plastics are based on polymers that are solid at room temperature and usually immiscible in aqueous media. The heterogeneous reaction is believed to involve the biodegradation of solid substances since it occurs on a solid/liquid interface, where the microbial enzymes present in the liquid phase interact with the macromolecules accessible at the surface of the solid plastic samples subjected to biodegradation (Albertsson et al., 1987). The polymers in the internal portions of the plastic sample are not involved in the reaction, as they are not available. “Available C” stands for carbon present in the exterior part of plastics, effectively available to the enzymes in the liquid phase and biodegradation, while the rest is buried in the core of the plastic. Therefore, Eq. (3), when applied to macromolecules rather than chemicals, should be rewritten as a variation in the biodegradation rate of polymer with alternation of the surface area of polymeric materials under controlled conditions or laboratory levels (Edo et al., 2020; Wang et al., 2018). In the initial phase of the degradation study, surface area and biodegradation are positively related to each other. For instance, a high surface area results in faster degradation of the polymer. At the later stages of degradation, the rate of easily biodegradable plastics became closely independent of the form of the samples, while the effect was constant for slowly degrading plastics. The various degradation process of plastic detailed in Table 14.3.

Biodegradation of plastics via microorganisms The conversion of biochemical into compounds using microorganisms is called biodegradation (Zheng et al., 2005). Various parameters affect the degradation of plastics, such as molecular weight, crystallinity, different functional groups, additives, and their mobility. Biodegradation of plastics by microorganisms first reduces their molecular weight, and then large molecules are broken down into smaller monomers, which are further degraded into mineralized products (CO2, H2O, and methane) (Shah, Hasan, Akhter, et al., 2008; Shah, Hasan, Hameed, et al., 2008; Shah et al., 2014). The giant size of some polymers creates hindrance in the cellular membrane, so for the prevention of this, they are depolymerized into mineralized products and readily absorbed by microorganisms. Enzymatic hydrolysis of plastics is one of the highly used pathways for their degradation in which hydrolytic cleavage occurs with the attacking of enzymes to the polymer. Following the hydrolysis process, the Table 14.3 Various polymer degradation routes. Factors (requirement/ activity)


Energetic agent Requirement of heat

High-energy radiation Nonmandatory

Rate of degradation

Initiation is slow, but propagation is fast Eco-friendly nevertheless highenergy not applied Acceptable but costly

Other considerations

Overall acceptance

Thermooxidative degradation


Heat and oxygen Higher than optimum temperature Fast

Microbial agents Nonmandatory

Environmentally not suitable


Not suitable

Cheap and very acceptable


Biodegradation mechanism


extracellular enzymes get concealed and get degraded into oligomers, dimmers, and monomers by microorganisms. Dehydrogenase, lipase, and proteinase k are the enzymes excreted by microorganisms (bacteria and fungi) in the biodegradation process of plastics (natural or synthetic) (Mohee & Unmar, 2007; Shimao, 2001). Clear zone formation is another way for the degradation of plastics where polymer penetrates the synthetic medium agar (emulsified polymers) and develops halo zones around the plastic surface. Poly (caprolactone); PCL, poly (propiolactone); PPL, and poly (hydroxybutyrate); PHB are different polymers that can be reduced using the abovementioned approach using 39 bacterial strains of class Firmicutes and Proteobacteria and gram-positive and gram-negative bacteria streptomyces and fungi, respectively (Shah et al., 2014; Shimao, 2001). Categories of biodegradable plastics are compiled in Fig. 14.5. Gu et al. described the damage of natural polymers such as starch, proteins, and cellulose in the microbiological process with the revelation to soil such as lifetimes of films (Gu, 2003). The mechanism pathways, acceleration, sunlight, water, stress, living organisms, oxygen, temperature, and pollutants of the process may affect the rate of degradation of the polymer. The biodegradability of plastics is determined in soil microbiology using microorganisms in different soil concentrations. The amount of surface functional groups (especially the carbonyl group) gets reduced due to the enhancement of microorganisms (Arkatkar et al., 2010; Hadad et al., 2005; Holmes et al., 2014). The key drawback of the colonization method is the interaction of microorganisms (hydrophilic behavior) with PE (hydrophobic surface) (Harshvardhan & Jha, 2013). The higher hydrophobic behavior of polymers plays a vital role in starting phase of the colonization method. Therefore, the hydrophobic behavior of bacterial cells increases the interaction between polymer and miro-organisms. Surfactants


• Plants • Bacteria


Biodegradable plastic


FIGURE 14.5 Categories of plastics with their syntheses.

• • • • • •

• • • •

Starch blends PLA blends PBS PBAT



Chapter 14 nanomaterials in plastics biodegradability

may also affect the interaction of hydrophilic and hydrophobic surfaces in the polymer colonization process. Initially, the size of the polymer gets reduced, and on further b-oxidation, hydrocarbon is converted into a carboxylic acid (Krebs cycle). The biodegradation process occurs at acidic pH due to the emission of organic acids and gases (Ramos et al., 1994). At minimum pH, a large amount of CO2 gas evolved, and this is a feasible condition for the growth of fungi. Gu et al. suggested that in the degradation process of NP and LDPE samples by fungi, PE acts susceptibly (Gu, 2003) (Gu et al., 2003). The susceptibility behavior decreases (loss in physical properties) to NP, then LDPE, followed by high-density polyethylene (HDPE). The surface distortion of PE is described in Table 14.4. The microbial enzymatic systems can be repelled by the hydrophobicity of PE. Similar behavior is observed when starch in NP films provides optimized conditions in the colonization process, and the reduction of esters and starch results in the disintegration of NP. The attack of microorganisms may decline the mechanical properties, and it controls microbiological activity and produces CO2 in large amounts. The infrared (IR) spectrum stayed unaffected for HDPE samples. In IR spectra, the absorption band of the carbonyl group of LDPE and NP gets shifted from 1625 to 1850/cm in the biodegradation of PE. The carbonyl peak was observed at 1720 and C¼C peak at 1650 cm for NP sample in the degradation pathway of PE (Chiellini & Solaro, 2003; Gu, 2003). Absorbance between 1400 and 1800/cm was observed due to proteins and enzymes in retained starch granules. The removal of starch from plastic was confirmed by the loss of the CeO peak (960-1290/cm), leaving the residue of the PE network. The slow degradation rate of HDPE and LDPE may cause serious environmental issues and affects human health. Despite this, an eco-friendly PE has been generated, having only 9% of starch, and can quickly degrade in the environment. The 5-month exposure of NP in soil may lose 36% of the weight and 59% of the strength with the production of CO2 and total viable bacterial count activity. Furthermore, the degradation of NP was observed for 15 months. The most abundant polluting source of microplastic is low-density polyethylene (LDPE). A new study found that LDPE decay was enhanced, and the size of the plastic was reduced after passing through the gut of the earthworm Lumbricus terrestris (Oligochaeta). Earthworm gut bacteria, i.e., gram-positive (Actinobacteria and Firmicutes), also affect microplastic decay. Various by-product molecules of bacterial LDPE-MP decay are listed in Table 14.5.

Biodegradation of plastics via nanomaterials Advancements in industries and social networking pollute the environment and cause concern for human and aquatic life. Numerous techniques have been used to remediate pollution created by dumped plastic debris. Various studies have investigated the degradation of waste plastics into fuel oils or other smaller products by conventional methods (Shimao, 2001; Liu et al., 2018; Mustafa et al., 2021, pp. 781e803). Conversely, this conventional technique has some limitations, such as high energy consumption and the cost of maintenance. Recently, plastic-eating organisms have also been used for plastics removal; however, they did not break down these highly recalcitrant polymers because of their high molecular inaccessibility and low rates of degradability. Therefore, there is an urgent need for more advancement in this field and to find and engineer efficient plastic-degrading enzymes or catalysts (nanomaterials) as well as associated processes. Also, research is needed to characterize plastic degradation via nanomaterials of a wide variety of plastic monomers. Some studies have investigated conventional plastics such as PET and PE to be degraded and metabolized by numerous catalysts (microbes and nanomaterials). Polymers are broken into their monomer units and further into

Biodegradation mechanism


Table 14.4 Main changes in the surface functional groups for polyethylene after exposure to different biodegradation conditions. Type of PE

Degradation mode


Marine bacteria Monitor environment after 10 years in soils Exposed to Bacillus sphaericus for 1 year Marine bacteria Monitor environment after 10 years in soils LDPE mixed with natural soils for 10 years

LDPE LDPE LDPE LDPE after UV irradiation PE with TDPA

Rhodococcus rhodochrous, cladosporium cladosporioides, and Nocardia asteroides

LDPE with 60 marines bacteria from the Arabian sea LDPE mixed with different natural soils

Monitor environment

Monitor environment

LDPE with 12% starch

P. chrysosporium in soils with LDPE with 12% starch


Pseudomonas sp. AKS2

PE plastic food bags HDPE and LDPE

Monitor environment (PE in 2 m depth in the sea) Monitor environment


Rhodococcus, Rhodococcus ATCC29672 after UV irradiation

Environment parameter and changes 1733e1743/cm; ketone 1733e1743/cm; ester 1733e1743/cm; ester decrease 1712e1723/cm; acids 1712e1723/cm; ketone UV irradiation increased carbonyl index, peak at 905e915/cm due to biodegradation Surface physically weak, readily disintegrated under mild pressure band at 1088/cm due to polysaccharides increased carbonyl index, decreased molecular weight reduction in molecular weight The peak at 905e915/cm reduction in crystallinity Peaks at 1448e1470/cm, 2800e300/ cm; weight loss due to biodegradation, tensile strength decreased, elongation at break decreased 1650e1860/cm; carbonyl compounds. 900e1200/cm peaks due to biodegradation Rough surface with cracks and grooves time-dependent weight loss, reduction in tensile strength Decreased hydrophobicity Decreased hydrophobicity, increased surface roughness, decreased carbonyl index weight loss with higher rates for LDPE Reduction in molecular weight increased at 1712/cm for pre-photooxidized samples, HPDE film behaves differently than LDPE and LLDPE and was not favorable for microbial metabolism


Chapter 14 nanomaterials in plastics biodegradability

Table 14.5 List of different microorganisms reported degrading different types of plastics. Synthetic plastics





Brevibacillus borstelensis Rhodococcus rubber Penicillium simplicissimum YK Comamonas acidovorans TB-35 Curvularia senegalensis, Fusarium solani, Aureobasidium pullulans, Cladosporium sp. Pseudomonas chlororaphis Pseudomonas putida AJ, Ochrobactrum TD Pseudomonas fluorescens B-22 Aureobasidium pullulans

(Hadad et al., 2005) (Chen, 2010) (Mehnaz & Javaid, 2020) (Akutsu et al., 1998)

(Shalini & Sasikumar, 2015) (Webb et al., 2000)

Thermomonospora fusca Schlegelella thermodepolymerans Pseudomonas lemoignei Pseudomonas indica K2

(Singh et al., 2021) (Shah, Hasan, Akhter, et al., 2008) (Tomasi et al., 1996) (Tiwari et al., 2018)

Streptomyces sp. SNG9

(Mabrouk & Sabry, 2001)

Ralstonia pickettii T1, Acidovorax sp. TP4

(Wang et al., 2002)

Alcaligenes faecalis, Pseudomonas stutzeri, Comamonas acidovorans

(Shah, Hasan, Akhter, et al., 2008)

Alcaligenes faecalis S. thermodepolymerans, Caenibacterium thermophilum Clostridium botulinum, clostridium acetobutylicum C. botulinum, C. acetobutylicum Fusarium solani

(Kita et al., 1997) (Romen et al., 2004)


Polyvinyl chloride

Natural plastics

Plasticized polyvinyl chloride BTA-copolyester Poly(3-hydroxybutyrate-co3-mercaptopropionate) Poly(3-hydroxybutyrate) Poly(3-hydroxybutyrate-co3-mercaptopropionate) Poly(3-hydroxybutyrate) poly(3-hydroxybutyrate-co3-hydroxyvalerate) Poly(3-hydroxy-butyrateco-3-hydroxypropionate) poly(3-hydroxy-butyrateco-3-hydroxypropionate) Poly(3-hydroxybutyrate) poly(3-hydroxypropionate) poly(4-hydroxybutyrate) poly (ethylene succinate) poly (ethylene adipate) Poly(3-hydroxybutyrate) Poly(3-hydroxybutyrate)

Poly(3-hydroxybutyrate-co3-hydroxyvalerate) Polycaprolactone Polycaprolactone

(Shah, Hasan, Akhter, et al., 2008)

(Liu et al., 2007) (Danko et al., 2004)

(Ba´tori et al., 2018) (Ba´tori et al., 2018) (Murphy et al., 1996)

Biodegradation mechanism


Table 14.5 List of different microorganisms reported degrading different types of plastics.dcont’d Synthetic plastics

Polymer blends




Polylactic acid

Fusarium moniliforme, Penicillium roquefort, Amycolatopsis sp., Bacillus brevis, Rhizopus delemar Aspergillus niger, Penicillium funiculosum, Phanerochaete chrysosporium Streptomyces, P. chrysosporium

(Chen, 2010)



(Shah, Hasan, Akhter, et al., 2008)

(Shah, Hasan, Akhter, et al., 2008)

simple molecules up to mineralization by different nanomaterials. This constitutes a paradigm shift in what can be considered biodegradable plastic. In this view, heterogeneous catalysis of polymers can occur via nanomaterials under optimum conditions and sunlight irradiation (Harmaen et al., 2016). Owing to their excellent physicochemical properties such as low cost, high surface activity, small size with low bandgap, and good photostability, nanomaterials act as great photocatalysts for treating plastic debris and other wastes for environmental protection. Nowadays, various nanocatalysts such as metal oxide and carbon-based nanoparticles are available for the biodegradation of plastic waste. Recently, most investigations have been concerned with plastics management processes, such as recycling and degradation with the help of nanomaterials. In this direction, TiO2 nanoparticles are potential catalysts or semiconductors for remediating various contaminants due to their chemical stability, inexpensiveness, nontoxicity, and high photocatalytic activity (David et al., 2020). High surface area and more pour size make them the best catalysts for plastic degradation (Pereira de Abreu et al., 2012). TiO2 nanoparticles are fabricated via various methods such as solegel, hydrothermal, micelle and inverse micelle, solvothermal, direct oxidation, chemical vapor deposition, electrodeposition, sonochemical, and microwave (Moharir & Kumar, 2019). On applying the TiO2 nanoparticles on the polymer surface, it was seen that an effective decomposition starts compared with normal photolysis of polymers under natural conditions. Some interactions start between the chemical bonds of polymers and catalysts (nanoparticles) at the initial stage. After that, cleavage of bond results in a monomer polymer unit that further degrades into simple molecules such as carbon dioxide and water molecules under sunlight irritation. Shang and a coworker investigated plastic degradation via polymer-TiO2 composite (Shang et al., 2003). They observed the photocatalytic cleavage of polymer units by the huge number of reactive oxygen species from polymer-TiO2 composite on the surface of the polymer and converted it into degradable by-products that are less toxic. Another report suggests the degradation of pure PE by nanocomposite film, which includes the various process of degradation such as cross-linking, scission, and photolytic degradation (An et al., 2014). Zan and the company fabricated the LDPE-TiO2 nanocomposite film for investigation of its photo-degradability under natural sunlight irritation (Zan et al., 2006). Further, it has been observed in various studies that dopants may enhance the photocatalytic activity of metal nanocomposite films which are used for the


Chapter 14 nanomaterials in plastics biodegradability

photo-degradability of polymers (Fig. 14.3). In this direction, the various heterogeneous metal nanocomposite is fabricated in which metal or nonmetal acts as dopant to enhance the surface activity of metal oxide-based polymer nanocomposites, which become easily biodegradable after disposal (Fig. 14.6). For instance, TiO2-PS nanocomposite films use iron (II) phthalocyanine as a dopant for enhancing photocatalytic activity under sunlight irradiation (Fa et al., 2008). Similarly, iron oxide nanoparticles have been used to degrade polymer materials. They effectively degrade the polymer compared with various bacterial groups. The iron oxide nanoparticles incorporated with bacterial groups enhance the degradation rate of LDPE with the tremendous growth of bacteria (Kapri, Zaidi, & Goel, 2010). Furthermore, a ferrite-based nanostructure with its electrical polarity interacted with bacteria and helped in its growth (Kapri, Zaidi, & Satlewal, 2010). Nano barium titanate (BaTiO2) is a combination of barium and titanium oxides. Nano barium titanate potentially degrades low-density polymers with an exponential inclination of bacterial growth. The FTIR and TG-DTG-DTA data show the significant change in polymeric structure observed by the shift in l-max to 224.11 nm compared to the consortium without NBT (Table 14.6).

Biodegradation of various polymers Biodegradation is a kinetics study to examine various factors like the origin and nature of degradation products or additives, as well as the quantitative and qualitative assessment of leachable or degradation

FIGURE 14.6 Scheme of degradation for polystyrene by TiO2 nanoparticles.

Biodegradation of various polymers


Table 14.6 List of different nanoparticles reported degrading various plastics. Nanomaterials


Reaction condition



TiO2-rGO nanocomposite TiO2 nanoparticles


Solar irradiation for 130 Solar irradiation for 200 h Solar irradiation for 240 h UV light irradiation for 400 h Under sunlight irradiation Under sunlight irradiation Under sunlight irradiation


(Verma et al., 2017)

68% 35.4%

(Thomas & Sandhyarani, 2013) (Li et al., 2010)


(Zan et al., 2006)


(Zan et al., 2006)

Polypyrrole/TiO2 nanocomposite TiO2 nanocomposites TiO2 nanocomposites Iron oxide nanoparticles Fullerene-60 nanoparticles

Polypropylene Polyethylene Low-density polyethylene Polystyrene Low-density polyethylene Low-density polyethylene

e 12.5%

(Kapri, Zaidi, & Goel, 2010) (Sah et al., 2010)

products in adjacent distant organs during a given period. The main reason for the nonbiodegradability of plastics is their chemical inertness, and for some reason, many degradation processes only work on the surface of the plastic. The inner or bulk part of plastic is ineffective or not readily available for biodegradation resulting in plastic waste causing serious cancer. Moreover, the biodegradation rate of the tested sample is directly related to its surface area at a laboratory level, with all other conditions identical. However, organic photosensitizers cannot solve the problem of this manufactured polymer in photo or biodegradation because stabilizers that are added during production cause their long-term degradation or other environmental problems (Ahmed et al., 2008). Therefore, urgent need for a new advanced technique for the removal of polymer waste for environmental protection. Herein, we comprise the detailed degradation of various polymers.

Degradation of polypropylene PP is a thermoplastic material used in numerous applications that includes household items, labeling, packaging, textiles, etc. Because of its low cost and high processability, PP is broadly manufactured polymers, particularly for the auto industry. Pristine PP is impervious to photo-oxidation and thermal oxidation at modest temperatures. Furthermore, PP is sensitive to several external stimuli like temperature, light, and irradiation. Under exposure to high energy conditions, oxygen may attack the tertiary hydrogen of the PP unit (Murphy et al., 1996). Oxidation PP depends on both light irradiation and temperature in outdoor aging circumstances. Furthermore, In the wavelength range from 310 to 350 nm, various molecular chains of PP are suspected to be photodegraded.

Photo- and thermal degradation of polypropylene At optimum temperatures, PP shows resistance to photo-oxidation while sensitive to many external conditions of environments like heat, light, and radiation and hence has a comparatively lower service


Chapter 14 nanomaterials in plastics biodegradability

temperature. When PP is irradiated at a high temperature, the tertiary hydrogen atoms present in PP chains are susceptible to being attacked by oxygen. The wavelengths of over 290 nm are sufficient to start the degradation and create discoloration, chalking, and embrittlement of PP. Therefore, assessment of the service life of PP in a natural environment is a well-established practice. Several reports show fast degradation situations for PP. The degradation rate of PP is significant under UV irradiation compared to that of natural sunlight. Numerous efforts have been made to examine the outcomes of photo-oxidation of PP to deliver a good estimation of its continuing service performances. Outdoor weathering test and accelerated weathering test are two methods usually employed for this purpose (Restrepo-Flo´rez et al., 2014).

Biodegradation of polypropylene Polyolefins are highly stable plastic and, therefore, have a sluggish degradation mechanism that involves the role of environmental factors, including microorganisms. An example of this category is a PP that is usually reluctant to biodegradation because of its high molecular weight along with the repetitive chain of methylene units and hydrophobic nature. There is a very deprived generation of biofilm or add-on of microorganisms on PP. For degradation of polyolefins in environmental implications, there should be either improvisation of hydrophilic characters or plummeting chain length of the polymer, more favorable conditions to microbial degradation. Scientists are working in this direction to progress these possessions as well as to understand the role of additives in biodegradation (Singh et al., 2021). Actions counting are chemical methods, thermal procedures, and UV exposure that may be responsible for oxidative degradation led to the generation of active functional groups (>C¼O, eCOOH, and ester) on the surface of the polymer. These groups are polar, increasing the hydrophilicity, especially on the surface of the polymer, as a result of the fast biodegradation of the polymer.

Polyesters PET (aromatic polyesters) is a significant polymer class that has prevalent commercial uses in several fields in the form of fibers, films, food packaging, and beverage containers due to its brilliant material characteristics. PET has very high resistance to atmospheric and biological agents and is frequently observed as substantial waste items on land and beaches. Therefore, limited studies of degradation or microbial degradation are reported in the literature on PET till dated. PET can be formed in the laboratory by reaction of dimethyl terephthalate (terephthalic acid) with ethylene glycol, followed by two or three- polymerization steps dependent on the desired molecular weight. In general, repeated units of PET have an average molecular weight of 200 with a 1.09 nm unit length (Rani et al., 2015). Nevertheless, the ester-bond can be simply broken; still, the presence of an aromatic ring with a short aliphatic chain makes it stiffer in strength if we compare it with other aliphatic polymers (polyolefin and polyamide). In addition, moderately high thermal stability is credited to the segmental flexibility of polymer chains. A textile-grade polymer (100 repeating units/molecule; 100 nm length; molecular weight of w20,000) needs upper stages of polymerization to yield higher strength fibers. On the other hand, small moisture can affect melt viscosity along with melt stability and may lead to hydrolytic degradation. The addition of heteroatoms in the polymer backbone (e.g., polyesters and polyamines) also improve the susceptibility to microbial or oxidative degradation (Jang et al., 2014). Meanwhile, the inclusion of secondary qualifiers in aromatic polymers can make them further tougher to degradation.

Conclusion and future scope


Photothermal degradation of polyethylene terephthalate On exposure to near-UV-light, PET undergoes photodegradation involving scission of the polymeric chain via Norrish I and II reactions. It may also show further cross-linking if the reaction is carried out in the presence of linkers and gelling agents. A yellow or brown polyene is formed when PET is thermally degraded, which is indicated by discolored final polymer after that increase of more carboxyl terminated species. Carboxyl groups catalyzed the formation of vastly conjugated species and more discoloration of polymer and hence, quick degradation. Thermo-oxidative stability also decreased in the presence of rich carboxyl content (Thompson et al., 2009). Studies suggested that the preliminary phase of thermal-degradation generates the carboxyl and vinyl-ester molecules by random ester chain or linkage scissoring of vinyl-ester groups. Transesterification of the vinyl ester yields the vinyl alcohol that is converted instantaneously to acetaldehyde. Environmental degradation of PET has been scarcely reported to now. During the study of degradation of PET by irradiation by MeVHeþ, the white solution turns brittle and yellow color which reveals the function of irradiation. Hydrolytic degradation of PET was also reported for its copolymer-containing nitrated units (Rani et al., 2015; Thompson et al., 2009). It was observed that degradation of copolymers PET is faster as compared to amorphous and semicrystalline PET, and nitrate units in copolymers PET further enhance the rate of degradation. During the study of hydrolytic degradation, a significant change occurred in the v(CeH) bond of PET. This might be due to the influence of environmental conditions on the functional groups, extensive hydrogen bonding along with aliphatic methyl groups; all factors are also responsible for increasing the hydrophilic character as well as degradation of PET (Arikan & Ozsoy, 2015).

Conclusion and future scope Today, plastic waste management is a major concern for scientific groups and law agencies for the smooth flow of society development along with a healthy environment. However, the regularity of accumulation is much higher than that of the frequency of biodegradation, and we should be more concerned about this for the sustainability of the ecosystem. Meanwhile, traditional plastics consume renewable sources for their manufacturing process, which is another factor for the urgent need for developments in the field of polymer. For this reason, bioplastics signify a worthy opportunity to decline plastic pollution as they are usually biodegradable. However, these materials are not able to fulfill all the requirements of the plastic industry yet, particularly because of their poor thermomechanical and barrier properties. Therefore, advancement in the field of the bioplastics industry is still progressing and refined by using nanocomposites technologies which have great promise in terms of opportunities to cover the range of usability of bioplastics. However, the physical degradation of plastics yields microplastic upon photodegradation, and further new methodologies bring about change in microplastics and form nanoplastics. In this direction, advanced techniques have been developed for the biodegradation of plastic, such as microorganisms and nanoparticles. To solve the problem related to the pollution created by plastics debris and their remediation from the environment, biodegradation via nanomaterials is the most innovative way in this direction. Many nanoparticles have great potential for breaking down plastics into smaller and safer products. This chapter gave knowledge about the biodegradation of plastic by some microorganisms and nanoparticles that break the plastics into their respective smaller units (monomers, oligomers) and then finally degrade up to mineralization into carbon dioxide and water. Meanwhile, the use of biodegradable plastics should be a


Chapter 14 nanomaterials in plastics biodegradability

proliferation in social lifestyles to have a safer aquatic as well as human life. The future generation should enhance the utilization of nanoparticles for biodegradable plastics in certain applications for sustainable environmental safety. Investigations on the effects of bioplastics in terms of microplastics are still limited and largely unknown. The problem of plastics is an existing and ongoing phenomenon that needs more attention on degradation.

Acknowledgments One of the authors Dr. Manviri Rani is grateful for the financial assistance from DST-SERB, New Delhi (Sanction order no. SRG/2019/000114) and TEQIP-III, MNIT Jaipur, Rajasthan, India. The authors are also thankful toTEQIP-phase eIII NIT Jalandhar for financial support.

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Chapter 14 nanomaterials in plastics biodegradability

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Shalini, R., & Sasikumar, C. (2015). Biodegradation of low-density polythene materials using microbial consortiumean overview. International Journal of Pharmaceutical and Chemical Science, 4(4), 507e514. Shang, J., Chai, M., & Zhu, Y. (2003). Solid-phase photocatalytic degradation of polystyrene plastic with TiO2 as photocatalyst. Journal of Solid State Chemistry, 174(1), 104e110. Shimao, M. (2001). Biodegradation of plastics. Current Opinion in Biotechnology, 12(3), 242e247. Sinan, M. (2020). Bioplastics for sustainable development: General scenario in India. Current World Environment, 15(1), 24. Singh, R., Mehrotra, T., Bisaria, K., & Sinha, S. (2021). Environmental hazards and biodegradation of plastic waste: Challenges and future prospects. In Bioremediation for environmental sustainability (pp. 193e214). Elsevier. Singh, B., & Sharma, N. (2008). Mechanistic implications of plastic degradation. Polymer Degradation Stability, 93(3), 561e584. Sivan, A. (2011). New perspectives in plastic biodegradation. Current Opinion in Biotechnology, 22(3), 422e426. Tachibana, Y., Giang, N. T. T., Ninomiya, F., Funabashi, M., & Kunioka, M. (2010). Cellulose acetate butyrate as multifunctional additive for poly (butylene succinate) by melt blending: Mechanical properties, biomass carbon ratio, and control of biodegradability. Polymer Degradation Stability, 95(8), 1406e1413. Tanaka, K., Takada, H., Yamashita, R., Mizukawa, K., Fukuwaka, M. A., & Watanuki, Y. (2013). Accumulation of plastic-derived chemicals in tissues of seabirds ingesting marine plastics. Marine Pollution Bulletin, 69(1e2), 219e222. Teuten, E. L., Saquing, J. M., Knappe, D. R., Barlaz, M. A., Jonsson, S., Bjo¨rn, A., Rowland, S. J., Thompson, R. C., Galloway, T. S., Yamashita, R., & Ochi, D. (2009). Transport and release of chemicals from plastics to the environment and to wildlife. Philosophical Transactions of the Royal Society London, B, 364(1526), 2027e2045. Thakur, S., Chaudhary, J., Sharma, B., Verma, A., Tamulevicius, S., & Thakur, V. K. (2018). Sustainability of bioplastics: Opportunities and challenges. Current Opinion in Green and Sustainable Chemistry, 13, 68e75. Thomas, R. T., & Sandhyarani, N. (2013). Enhancement in the photocatalytic degradation of low -density polyethyleneeTiO2 nanocomposite films under solar irradiation. RSC Advances, 3(33), 14080e14087. Thompson, R. C., Moore, C. J., Vom Saal, F. S., & Swan, S. H. (2009). Plastics, the environment and human health: Current consensus and future trends. Philosophical Transactions of the Royal Society London, B, 364(1526), 2153e2166. Tiwari, A. K., Gautam, M., & Maurya, H. K. (2018). Recent development of biodegradation techniques of polymer. International Journal of Research Granthaalayah, 6(6), 414e452. Tomasi, G., Scandola, M., Briese, B. H., & Jendrossek, D. (1996). Enzymatic degradation of bacterial poly (3hydroxybutyrate) by a depolymerase from Pseudomonas lemoignei. Macromolecules, 29(2), 507e513. Urbanek, A. K., Rymowicz, W., & Mironczuk, A. M. (2018). Degradation of plastics and plastic-degrading bacteria in cold marine habitats. Applied Microbiology and Biotechnology, 102(18), 7669e7678. Van den Oever, M., Molenveld, K., van der Zee, M., & Bos, H. (2017). Bio-based and biodegradable plastics: Facts and figures: Focus on food packaging in The Netherlands (No. 1722). Wageningen Food & Biobased Research. Vea, E. B., Fabbri, S., Spierling, S., & Owsianiak, M. (2021). Inclusion of multiple climate tipping as a new impact category in life cycle assessment of polyhydroxyalkanoate (PHA)-based plastics. Science the Total Environment, 147544. Verma, R., Singh, S., Dalai, M. K., Saravanan, M., Agrawal, V. V., & Srivastava, A. K. (2017). Photocatalytic degradation of polypropylene film using TiO2-based nanomaterials under solar irradiation. Materials and Design, 133, 10e18. Vom Saal, F. S., Akingbemi, B. T., Belcher, S. M., Birnbaum, L. S., Crain, D. A., Eriksen, M., Farabollini, F., Guillette, L. J., Jr., Hauser, R., Heindel, J. J., & Ho, S. M. (2007). Chapel hill bisphenol A expert panel


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Microbial attachment studies on “plastic-specific” microorganisms


Shubham Pant1,2 and Ravi Babu Valapa1, 2 1

Electrochemical Process Engineering Division, CSIR-Central Electrochemical Research Institute (CECRI), Karaikudi, Tamil Nadu, India; 2Academy of Scientific and Innovative Research (AcSIR) e CSIR, Ghaziabad, Uttar Pradesh, India

Introduction Plastics are combinations of repeating molecular monomers bonded to form macromolecules with endless applications in human society. More than 20 different major forms of plastic are in use worldwide. Plastics are low-cost, lightweight, sturdy, robust, corrosion-resistant, and thermally and electrically insulative. Polymer’s flexible and diverse properties are applied to produce a wide range of goods that offer clinical and technical innovations and other societal benefits (Andrady, 2011; Andrady & Neal, 2009). Plastics are beneficial because they are solid, light, and long-lasting; however, they have the disadvantage of being immune to biodegradation, resulting in contamination and damage to the ecosystem. The effective manufacturing and promotion of biodegradable plastics would aid in reducing environmental emissions. Over the last 10 years, various non-petroleum-based biodegradable plastics have been introduced to the market. However, effective degradation in landfills or other dumping areas is difficult for all biodegradable polymers. Polyethylene (PE), polyethylene terephthalate (PET), polybutylene terephthalate, nylons, polypropylene (PP), polystyrene (PS), polyvinyl chloride (PVC), and polyurethane (PUR) are the most widely used plastics, with PE and PP accounting for more than half of total consumption. Discarded plastic substances accumulate as plastic debris in our ecosystem. Plastic waste production is increasing worldwide in parallel with plastic consumption. Around 26 billion tons of plastic waste are estimated to be produced by 2050. Nearly half of that will be dumped into landfills, eventually entering ecospheres, including oceans and lakes, and causing significant environmental pollution (Jambeck et al., 2015). The debate about plastic waste accumulation in our environment has grown into a challenge to global biodiversity due to plastic’s resistance to degradation and its widespread use in industry. Plastic pollution may originate on land or in the sea. Plastic dumping can continue in two ways: intentionally, i.e., through unauthorized or improper channels for industrial and domestic waste dumping, and via dumping by households and small industries (Webb et al., 2013). Nowadays, only 9% to 12% of global plastic waste is recycled and incinerated. In comparison, 79% of waste is dumped into landfills, suggesting that an effective strategy must be investigated for novel recycling methods to dispose of these wastes (Garcia & Robertson, 2017). Recycling plastic waste may seem better than disposal since it recovers both the material and the energy consumed during manufacturing. However, due to contamination and the introduction of Biodegradability of Conventional Plastics. Copyright © 2023 Elsevier Inc. All rights reserved.



Chapter 15 Microbial attachment studies

impurities, plastic quality deteriorates with each recycling cycle, making it unsuitable for high-value applications such as those in the healthcare sector (Urtuvia et al., 2014). As a result, researchers have been motivated to look for degradative mechanisms that minimize harm to the environment. Landfilling, incineration, recycling, and biodegradation are the four primary means of treating disposed plastics. Plastics may be incinerated or disposed of in landfills. Plastic bags, bottles, cans, and other items have polluted the world’s oceans, posing a serious environmental threat. Meanwhile, landfills consume a significant amount of space while not recovering the chemical components and conserved energy of the plastic. Some energy used in plastic processing can be recovered through incineration, but the process generates negative environmental and health complications. Several plastics and the materials constituting them can be recycled. Due to difficulties in collecting and processing plastic waste, this approach is not fully utilized. At least two factors make it important to improve recycling: (1) nearly 88% energy savings can be achieved by manufacturing plastics goods from recycled rather than raw materials, reducing fossil fuel consumption, and (2) recycling plastics diverts them from the waste stream, repurposing these persistent products and keeping them out of landfills and the environment. Therefore, plastic recycling is critical in today’s efforts to conserve the environment. Plastic, being nonbiodegradable, takes several decades to degrade to the point where it can be reused. Therefore, cultivating effective microorganisms and their metabolic products to address the global issue is viewed as an effective method (North & Halden, 2013). In this case, there are two ways to alleviate irregular circulation. The first is to enhance the latent ability of microbes to decompose traditional plastic or develop synthetic polymers susceptible to degradation (Hosoya et al., 1978). In addition, some polymers are biodegradable, thus preventing pollution-related long-term harm to the environment (Hopewell et al., 2009). It would be safer for the planet if more people were willing to use recycled plastic items. Several studies have reported microorganisms that secrete extracellular enzymes capable of decomposing synthetic plastics like PE, PS, PP, PVC, PUR, and PET. Biodeterioration is an initial step in this degradation process that affects the plastic’s physicochemical properties. Frequently, abiotic parameters lead to a weakening of the macromolecule structure (Helbling et al., 2006). Microbial biofilm formation within and on plastic surfaces appears to cause biodeterioration. Biofilm formation is influenced by the composition and structural properties of the plastic, as well as environmental factors (Lugauskas et al., 2003). Microbial biofilm can cause significant physicochemical damage to the polymer surface in two ways. First, microbial biofilm growth is facilitated by the secretion of extracellular polymeric substances (EPSs) that adhere the biofilm to the plastic surface. This leads to microorganism growth, pore size expansion, and cracks, weakening the plastic’s mechanical and physical properties (Bonhomme et al., 2003). Biochemical deterioration introduces acid-releasing (nitrous, nitric, or sulfuric acid) bacteria such as chemolithotrophic bacteria (e.g., Nitrosomonas spp., Nitrobacter spp., Thiobacillus spp.). Acid alters the pH within the pores, resulting in a gradual deterioration of the plastic matrix’s microstructure (Zettler et al., 2013). Environmental factors such as pH, thermal stability, molecular weight, and cross-linkage determine the optimum enzyme efficacy of microbial degradation. Understanding the microbial activity and pathway involved in the enzymatic degradation of traditional/biodegradable plastics is crucial. Inorganic spherical nanoparticles must be incorporated into the polymer matrix with care because they can alter physicochemical properties and impart new features to the resulting polymer (Jacob et al., 2020, pp. 97e115). While producing a biodegradable polymer, the incorporation of these fillers can be tuned. Microbial activity for plastic waste decomposition has been extensively researched and reported for the marine environment. Each plastic type has its own bacterial community corresponding to

Background of plastics within the environment


FIGURE 15.1 Overview of studies on microbial attachment to the plastic surface.

differences revealed by physicochemical analyses (Puglisi et al., 2019). PE is one of the most popular and widely used synthetic polymers. The processing of LDPE accounts for 60% of all plastics is the most common solid waste. Microbial enzymes efficiently accelerate the biodegradation of LDPE without harming the ecosystem. Because of its chemical composition, PE is quite hindered for biodegradation (Contat-Rodrigo & Ribes Greus, 2002). Various microbial strains, including actinomycetes, bacteria, and fungi, have been identified for plastic degradation (Swift, 1997). Extracellular enzymes are generated outside the cell and large enough to deeply pierce the polymer content. As a result, they only work on the polymer surface. As such, plastic biodegradation is normally a surface erosion (Gajendiran et al., 2016). Another method for determining hydrolytic degradation is to examine the surface erosion of polymer films (Roohi et al., 2017). The following measures to reduce the use of plastics are linked to preventing adverse health effects and shifting to recyclable products. The long-term solution may lie in deciding which products genuinely need to be used and which provide only short-term benefits, as well as developing biobased plastics for manufacturing disposable items with predetermined life spans (Urtuvia et al., 2014). A brief description of microbial attachment and the biodegradation process is shown in Fig. 15.1.

Background of plastics within the environment Plastics have replaced traditional materials, such as wooden items, metals, and glass, in a wide range of items because of their resilience, barrier properties, and higher stability under abiotic conditions. Therefore, plastics have become an unavoidable part of human life. Plastics pose significant environmental risks due to their slow degradation rate or nonbiodegradability in natural environments (Webb et al., 2013). Around 300 different types of plastics are manufactured, with 60 of them being especially common. Plastics are classified as general or engineering plastics, depending on their intended use (Yuan, 2009). Plastic is made from oil, and it is estimated that the United States uses 1.6 billion liters of oil annually to manufacture 380 billion plastic bags and wraps (Garcia & Robertson, 2017). Approximately 380 million tons are produced annually. The chemical bonds of plastics are strong and made to last, so plastics take 400 years to decompose. The accumulation of poorly handled plastic waste in the atmosphere is becoming a global problem. It is critical to know precisely where


Chapter 15 Microbial attachment studies

litter is produced to target priority areas for mitigation policies (Lebreton & Andrady, 2019). Most plastic monomers, such as ethylene and propylene, are made from fossil hydrocarbons. Biodegradability is not a function of commonly used plastics. Consequently, rather than decomposing, they collect in dumping grounds, landfills, and the natural ecosystem (Barnes et al., 2009). Plastics are typically classified according to their physical properties. The three types of plastics are thermosets, elastomers, and thermoplastics. The key distinction between these groups is the complexity of their molecular structures. Most plastics are thermoplastic, which means they can be heated and reformed several times after manufacture. Remelting thermosets, on the other hand, is impossible. Synthetic plastics are commonly used in packaging, such as containers for pharmaceuticals, food, cosmetics, chemicals, and detergents. Plastic waste is currently treated in three ways: dumping in landfills, direct incineration, and recycling (Zhang et al., 2004). In landfills, due to the lack of oxygen, plastic waste components have been found to last more than 20 years. For effective degradation, the surrounding environment needs to be aerobic, which allows for hastening of the decomposition process (Massardier-Nageotte et al., 2006; Tansel & Yildiz, 2011). In addition, bisphenol A produced by landfill plastics has been linked to increased H2S production by sulfatereducing bacteria in the soil rhizosphere. Hydrogen sulfide in high concentrations can be fatal (Tsuchida et al., 2011). However, plastic incineration can overcome some limitations of landfilling. It minimizes the area required for decomposition and conserves energy as heat (Sinha et al., 2010). As plastics are incinerated at significant volumes, toxins and greenhouse gases are released, such as polycyclic aromatic hydrocarbons, polychlorinated biphenyls, heavy metals, and free radicals (Astrup et al., 2009; Khoo & Tan, 2010; Valavanidis et al., 2008). Chemical and mechanical processing are the two most common methods for recycling plastics. Chemolysis is used to chemically treat plastics, which results in polymerization of the plastic (Awaja & Pavel, 2005). Mechanical recycling of plastics refers to transforming plastic goods into raw materials. This material is melted into pellets, which are then used to produce other products (Awaja & Pavel, 2005). Currently, four primary plasticdecomposing techniques are used: UV irradiation, thermal degradation, oxidative/hydrolytic degradation, and microbial biodegradation (Andrady, 2011). The most effective and environmentally friendly technique for biodegrading polymer surfaces is microbial consortium. Various microbial strains have been identified in the last few decades. Studies have shown complete degradation or assimilation inside the microbial cell. Microbial colony attachment is facilitated by the release of EPSs to the plastic surface. The proliferation and metabolism of microbial cells release extracellular enzymes over the polymeric surface. Depolymerase or hydrolase activity cleaves the polymeric chains and forms oligomers or monomers. The fragmented products are then assimilated into the microbial cell. This process supports microbial energy consumption in the form of carbon as the sole energy source. Finally, bioproducts are released into the environment. as CO2, CH4, and H2S

Biodegradation Various living organisms use biodegradation to decompose polymeric compounds and assimilate them inside the microbial cell. It works as an ecosystem of biotic stress complemented by abiotic stress. By the end of the process, by-products such as water, carbon dioxide, methane, and biomass are produced (Kesti, 2019). Microorganism use in plastic bioremediation has reached a critical juncture. Plastics can be used to create microbial communities that rely solely on polymeric molecules for carbon. Microbes are responsible for forming biofilms and cling to the surface of pollutants, forming the plastisphere;

Mechanisms of microbial biodegradation


this facilitates the production of acid products and extracellular enzymes. However, this enzymatic activity depends on the type of polymer, its morphological properties and molecular weight, and other biotic and abiotic factors (John & Salim, 2020). Since microorganisms secrete enzymes that enable them to consume plastic as a substrate, they are ideal for reducing plastic contamination. Microorganisms can catalyze oxidation-reduction reactions in plastic polymers to break down chemical bonds (Moussa & Young, 2014). Another important factor influencing biodegradability is crystallinity. Plastic polymers are less accessible to microbial enzymatic action due to their crystalline form. As a result, as the temperature rises, degradation efficacy decreases (Slor et al., 2018). Biosurfactants and additives affect microbe behavior. Microbes’ ability to decompose pollutants corresponds with their remarkable ability to consume plastic materials as a food source or through indirect enzymatic metabolism. Pseudomonas fluorescens, P. aeruginosa, and Penicillium are the most popular strains for plastic biodegradation (Ahmed et al., 2018; Raziyafathima et al., 2016).

Microbial bioremediation of plastic Microbial degradation is influenced by several environmental conditions, polymer properties, and pretreatment methods. Properties include ductility and friability, glass transition, degradation temperature, elastic modulus, and functional group existence. The types of additives used and their substituents affect microbial degradation. Depending on their properties, microorganisms involved in microbial degradation have various modes of action and optimum growth conditions in soil (Shah et al., 2008). Microorganisms have several methods for breaking down a good range of molecular weight plastics and microplastics to meet their energy needs or indirectly through microbial enzymes (Ahmed et al., 2018). Enzymatic biodegradation uses many enzymes found in the environment, such as those in compost. Most of the time, environmental fungi and bacteria serve as enzyme sources. Depolymerization is aided by the extracellular enzymes of exoenzymes. Intracellular enzymes, also known as endoenzymes, function within cells and contribute to depolymerization. Many experiments have shown that microbes, such as actinomycetes, bacteria, and fungi, can develop extracellular enzymes. In addition, under certain environments, their products easily degrade biomass-based polymers (Ahmed et al., 2018). Bacteria residing primarily either in soil or marine environments can degrade plastic. Simone et al. (2020) created a fungal library that includes polyesters, which led to the discovery of new strains that contain enzymes that hydrolyze polyester. Fusarium oxysporum and F. solani fungi can eat PET; bacteria, on the other hand, are better for large-scale exploitation (Carrington, 2018; Yoshida et al., 2016). Petroleum-based plastics have been shown to degrade in vitro in response to more than 90 microorganisms, including bacteria and fungi (Jumaah, 2017).

Mechanisms of microbial biodegradation The initial colonization of microbes and formation of microbial biofilms on the polymer surface are the first steps in polymer biodegradation. The plastisphere is home to several microbial species, including microbes that degrade plastic (Kirstein et al., 2019). During degradation, the microbe attaches to the polymer surface first before colonies surface nearby (Tokiwa et al., 2009). Microbial biofilms form quickly on plastics, causing a substantial increase in hydrophilicity (Lobelle & Cunliffe, 2011). With repulsive and attractive interactions among the surfaces, the medium and their cell wall microbes come


Chapter 15 Microbial attachment studies

in contact and alter the surface properties of the material (Dang et al., 2008; Jones et al., 2007). After bacterial inoculation, the topography of the plastic surface may be altered, and pits and cavities have been observed on the plastic surface. Over a period, microbes can colonize the surface area of exposed plastic by altering the molecular weight, buoyancy, and density and thereby escalating biodegradation. Furthermore, to achieve biodegradation, successive fragmentation of the polymeric chain into smaller fragments with a high surface/volume ratio is needed (Lambert & Wagner, 2016). Depolymerization is a crucial step in the degradation process because polymers must first be depolymerized into small molecules. Depolymerization enzymes are known to be secreted by some eukaryotic microorganisms (yeasts, fungi, and so-called mushrooms) (Kawai & Hu, 2009). The various stages of biodegradation are described below:

Biodeterioration Biodeterioration is degradation that affects characteristic polymer properties using various physical and chemical action modes of microorganisms. The visualized degradation marks can also be associated with long-term exposure to different abiotic factors. Microbial colonization and attachment begin as a way to initial weaken the polymeric structures (Helbling et al., 2006). The formation of a hydrophilic surface or introduction of a functional group such as a hydroxyl and carbonyl group on the surface is the primary need for this activity. Therefore, biofilm formation becomes necessary in the case of a hydrophobic group-containing polymer. For example, in the case of low-density PE degradation, the biofilm formation by Pseudomonas species showed strong attachment results. In a report by (Tribedi & Sil, 2013), mineral oil acted as a growth promoter that enhanced colonial attachment; on the other hand, surfactants diminished the biodegradation process. However, the fungal cell could grow on almost all types of plastic surfaces, and mostly the attachment was seen through their hyphae. This step was followed by an affected swelling rate and decreased physical and mechanical properties. In addition, the deterioration was accelerated by incorporating plasticizers and enzymatic additives in the polymer matrix (Ru et al., 2020). Studies have exposed the role of exopolysaccharides for stronger, effective bioadhesion against the deterioration process. The composition and structure of the polymer shall be provided to be an appropriate adhesion-promoting agent; it can thereby enhance biofilm growth (Lugauskas et al., 2003).

Biofragmentation These steps follow the catalytic degradation of products formed after the biodeterioration step. It usually occurs with the secretion of extracellular enzymes and the generation of oxygen-based free radicals (Singh & Sharma, 2008). The process follows in two ways, reducing high-molecular-weight polymeric chains and hydrolyzing low-molecular-weight chain fragments. This hydrolyzed product is susceptible to extracellular enzymatic action. Glycoside, peptide, and ester bonds are primarily susceptible to cleaving by these enzymes, which typically attack the carbonyl group on the polymer backbone (Lugauskas et al., 2003). This process follows in two ways. The first is an exo attack, where the resulting product is either an oligomer or a monomer. The other action, an endo attack, is not very effective since the products cannot be assimilated into the microbial cell for complete degradation. The role of different organic and inorganic compounds secreted from microbes is also very important, as they can interact to form a stable complex with polymer surface cations (n.d.).

Microbial colonization on the plastic surface


FIGURE 15.2 Stages of microbial degradation mechanism with advancement in techniques.

Bioassimilation and mineralization The process begins with the supply of polymer fragments into the cytoplasm of the microbes for bioassimilation. For this purpose, two-channel transportation, i.e., active and passive, is followed by the microbes. For example, a polymer fragmented product, octadecane, that was observed to have transportation through two different channels facilitated diffusion and active transport with respect to ion concentration across the membrane of Pseudomonas spp. DG17 (Hua et al., 2013). Another example is the inward transport of TPA formed as a result of PET degradation; a very specific transporter is reported in Comamonas. With the complete conversion of carbon content to CO2, water, and CH4 as a result of aerobic or anaerobic degradation, mineralization takes place. Different techniques have been employed to identify the mineralization process; these include Strum’s method for CO2 release quantification and isotopic tracing (Yang et al., 2013). However, some monomeric products, such as recalcitrant styrene and PET by-products, would enter the Krebs cycle through a series of enzymatic reactions. Mineralization can occur aerobically or anaerobically, and accordingly, this decides the by-product formation. However, the role of lipase, esterase, laccases, and cutinase is very crucial. Fig. 15.2 illustrates the steps involved in microbial degradation.

Microbial colonization on the plastic surface The first work that employed contemporary large DNA sequencing techniques to offer a thorough picture of microbial life on plastic coined the term “plastisphere” Microbial eukaryotes have been found in various investigations, beginning with landfill areas, waste treatment plants, and plastics collected at sea or fresh plastics incubated under marine circumstances. PE, PP, and PS are the most


Chapter 15 Microbial attachment studies

common plastic trash at sea and on land (Auta et al., 2017). Aspects of the plastisphere that live in the water column other than the surface layer have received far less study. Most studies have been confined to collecting surface waters using manta trawls due to methodological restrictions; this accounts for less than 1% of the total amount of plastic in the open ocean (Cozar et al., 2014). Other research has focused on the various colonization processes of new plastics incubated in marine settings, in addition to studies on plastic directly sampled at sea and land levels. Plastics are quickly coated by a “conditioning film” or biofilm consisting of inorganic and organic materials at sea and on land, which is then rapidly colonized by bacteria. In the early phases of colonization, hydrophobicity and other substratum characteristics such as crystallinity, wettability, glass transition temperature (Tg), melting temperature (Tm), and modulus of elasticity may play a role in bacterial community selection (Pompilio et al., 2008). The production of EPSs supports the formation of microbial biofilms and their growth. EPSs are microbially produced biopolymers in which biofilm bacteria are embedded. Biopolymers are produced by archaeal, bacterial, and eukaryotic microorganisms. EPSs are much more than polysaccharides, contrary to popular assumptions. They also include a diverse range of proteins, glycoproteins, and glycolipids, as well as unexpected quantities of extracellular DNA under rare circumstances (Flemming & Wingender, 2002). Polysaccharides are typically just a small component in environmental biofilms. A few EPS components require special attention. The best-studied component of mucoid Pseudomonas aeruginosa biofilms is alginate, a polyanion polymer. Other polysaccharides, however, have been found to contribute to biofilm formation by nonmucoid P. aeruginosa strains, which are thought to be the first to colonize cystic fibrosis patients, according to many recent studies. The expression of the PSL operon, which is necessary to maintain the biofilm structure following attachment, is a recent example. Overproduction of the Psl polysaccharide increased P. aeruginosa cell surface and intercellular adhesion, resulting in substantial alterations in biofilm architecture (Ma et al., 2006). The growth and maturation phases of biofilm development, which were previously seen for surfaces such as glass, acrylic, steel, or rocks and algae, were also identified for plastics of various compositions. Biofilm formation on PET-based plastic bottles and PEbased plastic bags was seen in saltwater for many weeks (Lobelle & Cunliffe, 2011). Compared with nonbiodegradable PE polymers, poly(3-hydroxybutyrate-co-3-hydroxyvalerate) and preoxidized PEbased oxodegradable polymers (OXO) had higher colonization by active and specific bacteria after 6 weeks (Eich et al., 2015). Biofilm development and maturation varied according to the polymer type, i.e., PE, PP, PET, or polycarbonate, in longer-term experiments conducted over a 6-month to 1-year timeframe (PC) (Webb et al., 2013). Only one investigation compared the heterotrophic production of bacteria growing on plastic with that of bacteria living in saltwater. Heterotrophic bacteria living on plastics were very active. Unfortunately, these findings were acquired as part of a study on the colonization of new plastics that had been cultured at sea for a brief length of time (45 days). Surprisingly, Cyanobacteria were found to be overrepresented on plastics compared with the surrounding free-living and organic particle-attached fractions by most studies aimed at characterizing the plastisphere. The proportional relevance of photosynthetic activities carried out by Cyanobacteria living on plastic in terms of worldwide pelagic primary production is unclear. Obtaining a greater understanding of the influence of the plastisphere on carbon cycling in the seas will require the combining of primary and heterotrophic production observations over broad temporal and geographical dimensions. Microorganisms are engaged in all biogeochemical cycles, including nitrogen, sulfur, iron, manganese, chromium, phosphorus, calcium, and silicate cycles, which may be influenced by the plastic in the ecosystem (Hutchins & Fu, 2017). The plastisphere composition is controlled by a variety of factors,

The fate of microbial carbon biomass resolution


most notably geographical and seasonal factors, but also by polymer type, surface characteristics, and size. Bacterial colonies colonizing plastics along an environmental gradient came to similar findings. The freshwater to marine environmental circumstances forms these communities are first and foremost and then by the type of plastic used (PS and PE). In contrast, research based on a significant number of microplastics collected in the western Mediterranean Sea found no influence of geographical location (both coastal and open ocean samples) or plastic type (mostly PE, PP, and PS) on bacterial community composition. Although an increasing volume of research on the plastisphere provides a better understanding of the microbial biofilm community on plastics in the seas, the complex network of effects remains a source of dispute. More comprehensive research using a wide range of samples, as well as fuller descriptions of the physical and chemical characteristics of the polymers, should provide a clearer picture (Jacquin et al., 2019). Even under ideal laboratory settings, rates of microbial breakdown of common plastics are quite low. In both marine and terrestrial settings, most common plastics are resistant to biodegradation, resulting in lives of decades or even centuries. Plastics have a limited bioavailability because they are typically solid and comprise tightly cross-linked polymers that restrict bacteria and enzymes to the outermost layer of the objects. Plastics are biodegraded in the pelagic environment by microorganisms’ aerobic metabolism, which results in microbial biomass, CO2, and H2O as process end products. The anaerobic biodegradation route would be more common in sediment, and it is expected to be slower than in the pelagic zone (Ishigaki et al., 2004). Biodegradation of other hydrocarbon-based products in the seas is hampered by an unfavorable C/N ratio and may result in limiting plastic biodegradation (Sauret et al., 2016). So far, there are only limited data on the rate of plastic mineralization in the seas. There are several congruent descriptions of the plastisphere, which produces an abundant biofilm defined by a wide range of bacteria with active plastic-specific properties (Debroas et al., 2017).

The fate of microbial carbon biomass resolution To meet the demands of hundreds of end products, the chemical composition (e.g., polyesters, polyolefins) and physicochemical properties of many plastic types, including those used in this study, are highly varied. Plastic foils were selected as the study’s substrate since they are often used in packaging and construction. Because the differences in mature biofilms across plastic types were generally small, it was hypothesized that “plastic-specific” bacteria are firmly attached to the polymeric surface and may be represented by uncommon but active species. The microbial absorption and use of monomers and oligomers produced from the polymers by enzymatic hydrolysis is the final stage in mulch film biodegradation. Polymer biodegradation is a complex process regulated by a variety of parameters such as polymer properties, microbe type, and chemical structure. Mobility, tacticity, crystallinity, molecular weight, the sort of functional groups and substituents present in the polymer’s structure, and the plasticizers or additives added to the polymer all have a part in its breakdown. Polymers are reduced to monomers during degradation, and subsequently, these monomers are mineralized. Since microplastics are too big to enter through cellular membranes, they must first be depolymerized into smaller monomers before being ingested and biodegraded by bacteria. Physical, chemical, and biological factors can all contribute to the early disintegration of a polymer. Mechanical damage to polymeric materials can be caused by physical forces such as heating/cooling, freezing/ thawing, or wetting/drying. Because microorganisms may permeate the polymer solids, their


Chapter 15 Microbial attachment studies

proliferation can induce small-scale swelling and rupture. Microbial enzymes may also depolymerize polymers through a hydrolysis process; after that, the monomer biodegrades inside the microbial cell. Enzymatic hydrolysis of microparticles is a two-step process in which the enzyme first binds to the polymer substrate and then catalyzes a hydrolytic cleavage. Depolymerases may break down polymer in both intracellular and extracellular environments. Intracellular degradation refers to the hydrolysis of an endogenous carbon reservoir by accumulating bacteria, whereas extracellular degradation refers to using an external carbon source by accumulating microbes, although not always. Extracellular enzymes from microbes break down complicated polymers into short chains or smaller molecules, such as oligomers, dimers, and monomers, which may pass across semipermeable membranes. Depolymerization is the name of the procedure. The breakdown of these short-chain molecules into end products, such as CO2, H2O, or CH4, is known as mineralization, and these end products are used as carbon and energy sources. The release of enzymes kicks off the microbial breakdown of polymers, causing a chain cleavage of the polymer into monomers. Split sections metabolism leads to enzymatic dissimilation of macromolecules from the chain ends, finally resulting in chain fragments that may be digested by microbes. Microbial enzymes play a significant role in the breakdown of polymers by microbes. Much research, including the isolation and identification of enzymes in microorganisms, has been undertaken in recent years to explore the processes related to the microbially mediated polymer breakdown (Lou et al., 2020). Ideonella Sakaiensis 201-F-6, a new bacterium identified by (Yoshida et al., 2016), can utilize PET as its primary energy and carbon source. This strain generates two enzymes (ISF6-4831 and ISF6-0224) capable of hydrolysis when cultivated on PET. However, most recombinant plastic-degrading enzymes have been found to be cloned from Pseudomonas species. P. fluorescens ST was used as a source of PS catabolism genes. Gene expression in the host system revealed monooxygenase, epoxystyrene isomerase, and epoxystyrene activity in gene products (Gautam et al., 2007). A polyester PUR-degrading enzyme was cloned from the same species, as well as another species of the same genus, P. chlororaphis, and showed exceptional activity against Impranil. A polyester hydrolase, designated as a type IIa PET hydrolytic enzyme, was cloned and expressed in E. coli from P. aestusnigri (Bollinger et al., 2020). PE, bis(2-hydroxyethyl) terephthalate, and amorphous PET were all degraded by the recombinant enzyme; however, commercial PET bottles could not be degraded. Site-directed mutagenesis, on the other hand, enhanced the enzyme’s ability to break down films from PET bottles. Much research has been conducted on Ideonella sakaiensis extraordinary capacity to significantly break down PET. In this context, the PETase gene from the bacterium has been the subject of a large volume of genetic manipulation research. The PETase gene from I. sakaiensis has been cloned and expressed in a number of different host systems, including E. coli (Joo et al., 2018) and Phaeodactylum tricornutum (Moog et al., 2019).

Plastic forms and microbial attachment bioremediation Bioplastics Interest has been sparked in biobased plastics as a sustainable alternative. Biopolymers are differentiated from traditional polymers by being comprised in natural biomass (“Environmental Benefits of Recycling: An International Review of Life Cycle Comparisons for Key Materials in the UK Recycling Sector,” 2006; WRAP, 2006). Biobased plastics can reduce CO2 emissions and minimize energy consumption compared with synthetic plastics. Bioplastic is biobased polymers that meet two criteria

Plastic forms and microbial attachment bioremediation


for sustainability: biodegradability and renewability. They generally do not have superior barrier properties, but that can be improved with additives incorporation. Thermoplastic plant and starchderived polymers such as PLA and polyhydroxyalkanoate (PHA) are all-important bioplastics that contain at least some renewable carbon (Lackner, 2015, pp. 1e41). The increased content of polysaccharides in the polymers would lead to a quicker degradation process. However, such blends do not degrade completely; the backbone polymer still persists in the environment in minute forms that are no longer visible to the eyes (Vilpoux, 2004). Nowadays, bioplastics are reinforced with some antioxidants, vitamins, etc., that impart additional health-related benefits while used in food packing materials (Jacob et al., 2020, pp. 97e115). Since 1997, Pseudonocardiaceae has been described as the most important family for PLA degradation by Actinobacteria. Furthermore, esterases and proteases in a variety of Actinobacteria have potential use in the future regulation of bioplastic waste. Biobased biodegradable plastics degrade fully biologically, making them useful in many industries. Secretion of these extracellular enzymes polymers degrades over time. Starch and proteins are commonly used in the food industry to manufacture biodegradable plastics for edible and non-edible-based packaging (Kale et al., 2007). Table 15.1 shows various microbial species and their substrate degradation under optimum enzyme conditions.

Starch-based bioplastics Bioplastic made from corn starch is known as starch-based bioplastic. Starch-based polymers are easily biodegradable, as they can be achieved through amylase and other hydrolases. B. subtilis is well known for this action. Frequently, they are combined with biodegradable polyesters. Polycaprolactone (PCL)/starch blend is made by combining starch-based bioplastics with biodegradable polyesters. The degradation of PCL films was obtained with Pseudozyma japonica-Y7-09 hydrolytic enzyme activity and showed 93.33% degradation on the 15th day (Abdel-Motaal et al., 2014). Other reports confirm the PCL/starch blend biodegradation of films under aerobic and anaerobic conditions, suggesting that landfilling and composting degradation can opt for PCL/starch blend plastics (Cho et al., 2011).

Polylactic acid bioplastics Polylactic acid (PLA) is translucent and made from corn and dextrose. It has the same properties as petrochemical-based plastics and can be manufactured using the same methods as conventional plastics. PLA is widely used to make films, fiber, cups, and bottles. PLA degradation in the environment is a very slow process. It takes almost 80 years to completely degrade. However, in the last few decades, attempts have been made to isolate particular enzymes through their corresponding microbes. Based on 16S rRNA sequences, Actinomycete PLA degraders were found to be most prominent. PLA microbial degradation starts with extracellular depolymerase enzyme secretion on the polymer surface. Rapid production of depolymerase enzymes can be stimulated with gelatin, silk fibroin, amino acids, and some peptides. Depolymerization results in cleavage of the PLA ester linkage to form oligomers and monomers. After this, the low molecular compound enters into the microbial cell for assimilation to emit CO2, CH4 and water by successor enzymes. The appropriate environmental conditions and microbial colonization is a must for this process (Qi et al., 2017). Fig. 15.3 shows the biochemical pathway for PLA biodegradation.


Chapter 15 Microbial attachment studies

Table 15.1 Microbes, related enzymes, and their substrates in bioplastic degradation. Enzyme involved

Optimum temperature

Polycaprolactone (PCL)




Polylactic acid (PLA)




Polybutylene succinate (PBS) PCL




Catalase/ Protease


PCL, PES, PLA, P(3HB), P(3HB-co3HV) Polyhydroxybutyrate (PHB)

PCL depolymerase












P. Antarctica













Rhizopus delemar





Penicillium, Rhizopus arrhizus Firmicutes
















Aspergillus flavus Amycolatopsis sp. K104-1 Aspergillus oryzae Aspergillus niger Brevundimonas sp. MRLAN1 Penicillium funiculosum Amycolatopsis orientalis ssp. orientalis Cryptococcus sp. S-2CsCLE Cryptococcus flavus GB- 1 CfCLE Cryptococcus magnus Paenibacillus amylolyticus TB-13 Fusarium

Optimum pH





References Tokiwa and Calabia (2009) Emiko et al. (2005) Hiroshi et al. (2005) Tokiwa and Calabia (2009) Nawaz et al. (2015) Tokiwa and Calabia (2009) Li et al. (2008) Kamini et al. (2000) Watanabe et al. (2015) Suzuki et al. (2013) Yukie et al. (2003) Shimao (2001) Tokiwa and Calabia (2009) Tokiwa and Calabia (2009) Tokiwa and Calabia (2009) Tokiwa and Calabia (2009)

Plastic forms and microbial attachment bioremediation


Table 15.1 Microbes, related enzymes, and their substrates in bioplastic degradation.dcont’d Microorganism


Pseudomonas stutzeri Ralstonia sp. MRL-TL

Polyhydroxyalkanoate (PHA) PCL, PES, PLA, P(3HB), P(3HB-co3HV) PBS, PCL, PET

Thielavia terretris CAU709 (TtcutA)

Enzyme involved

Optimum temperature

Optimum pH

Serine hydrolase PCL depolymerase







TtcutA (cutinase)

References Shimao (2001) Shah et al. (2008) Yang et al. (2013)

FIGURE 15.3 PLA degradation biochemical process involved. From Qi, X., Ren, Y., & Wang, X. (2017). New advances in the biodegradation of Poly(lactic) acid. International Biodeterioration and Biodegradation, 117, 215e223.


Chapter 15 Microbial attachment studies

Aliphatic-based bioplastics PHB (poly-3-hydroxybutyrate), PHA (polyhydroxyalkanoates), PHV (polyhydroxyvalerate), polyhydroxyhexanoate PHH, PLA, and polyamide 11 (PA11) are examples of biobased polyesters. They are all susceptible to hydrolytic degradation to varying degrees and can be mixed with other substances. A wide variety of PHA degrading fungal species Penicillium, Aspergillus, Paecilomyces, Acremonium, Verticillium, Cephalosporium, Trichoderma, Chaetomium, and Aureobasidium. A total of 81.5% of degradation was achieved with P. lilacinus, a fungus isolated from soil of the larch rhizosphere (Boyandin et al., 2012). Fig. 15.4 shows the trend of plastic production till 2020.

Synthetic plastics Synthetic plastics are a new type of environmental contaminant that has been discovered in marine waters all over the world. Under laboratory conditions, trials have reported several microbes capable of decomposing a range of this traditional plastic in recent years (Wierckx et al., 2018, pp. 1e29). Polyolefins such as PE, PS, PP, and PVC are products of monomers combined with strong CeC bonds. These bonds can be easily cleavable with various identified enzymes in the last few decades. The resulted fragments are then transported into the microbial cells for assimilation and mineralization (Goldberg, 1995). However, other synthetic plastics with hydrolyzable ester and urethane linkage are more susceptible to depolymerization. Pseudomonas species that have been isolated are identified to depolymerize and metabolize synthetic plastics. The bioremediation of petroleum-based polymers has been credited to Pseudomonas species, which are found in both marine and terrestrial environments (Timmis, 2002; Tribedi & Sil, 2013). The oxidation or hydrolysis of high-molecular-weight polymers

FIGURE 15.4 Plot showing the Bioplastic production percentage till 2020 year.

Synthetic plastics


by enzymes to produce functional groups that increase hydrophobicity is the main mechanism for the biodegradation of synthetic plastics (Dang et al., 2008). Sometimes biodegradation conjugates with UV irradiation, mechanical forces, and oxidizing agents for optimum results. This pretreatment results in exposure of carbonyl or olefin functional group to the plastic surface. This helps most of the depolymerase and hydrolase for their enzymatic activity. Table 15.2 lists various enzymes identified from microbes for synthetic plastic biodegradability. Ligninolytic enzymes were observed to break the CeC bond in almost all the polyolefins. The inconsistency of corresponding enzymes can be improved with different optimized pretreatment and operating conditions. Isolated laccases from different PE degrading white-rot fungus (Actinomyces and ligninolytic) increase the carbonyl group on the plastic surface and thereby accelerate biodegradation. However, for PE degradation, the CeC bond is harder

Table 15.2 Summary of various microbial enzymes identified and working optimum conditions for synthetic plastic biodegradability. Microorganism


Trichoderma sp.

Polyurethane (PUR) PUR

Pestalotiopsis microspora Phanerochaete chrysosporium Penicillium, Rhizopus arrhizus Pseudomonas sp. E4 Pseudomonas putida AJ Pseudomonas aeruginosa Pseudomonas sp. Pseudomonas stutzeri Pseudomonas vesicularis PD White-rot fungus IZU-154 Agromyces sp. Trametes versicolor

Polyethylene (PE) Polyethylene adipate Lowmolecularweight PE Polyvinyl chloride & polystyrene Polyester (a type of PUR) Polyethylene terephthalate Polyethylene glycol (PEG) Polyvinyl alcohol Nylon Nylon-6 (oligomers) Nylon, PE

Enzyme involved

Optimum temperature

Optimum pH




Serine hydrolase Manganese peroxidase Lipase







Tokiwa and Calabia (2009)

Alkane hydroxylase



Yoon et al. (2012)

Alkane hydroxylase









PEG dehydrogenase Esterase





Verce et al. (2000), Danko et al. (2004) Mukherjee (2011) Cameron et al. (2020) Obradors & Aguilar (1991) Kawai (2009)







Manganese peroxidase Nylon hydrolase Laccase

References Trevino et al. (2011) Russell et al. (2017) Shimao (2001)

Deguchi et al. (1998) Negoro (2012) Fujisawa et al. (2001)


Chapter 15 Microbial attachment studies

to break than CeC in lignin and CeO bonds; therefore, substantial degradation is possible only with some biotic and abiotic stimuli (Chen et al., 2020). PS biodegradation is also possible with certain microbes, but identification of operating enzymes is limited. Hydroquinone peroxidase isolated from Azotobacter beijerinckii HM121 (lignin-decolorizing bacteria) with hydrogen peroxidase performs the PS degradation in an organic-aqueous biphasic reaction (Nakamiya et al., 1997). Hydrolyzable ester and urethane linkages plastics (i.e., PUR and PET) are more susceptible to biodegradation since urethane and ester linkages are well known for esterase and cutinase activity. PUR undergoes protease and esterase mediated cleaves at the polyester site. A bacterial enzyme combines both enzyme activity and the hydrolyzing of urethane compounds. For PET degradation, esterase, cutinase, and lipases are the responsible enzymes. However, these esterases lack the optimum degradability, as they are restricted with the oligomer substrates. Because of substrate binding pocket hydrophobic lid hindrance, the lipases’ enzyme activity comes as inferior for the operation (Eberl et al., 2009). Therefore, attempts to have been made to improve these enzymes’ efficacy. Ideonella sakaiensis 201-F6 strain expresses two different enzymes for complete PET decomposition. It starts with the secretion of cutinase kind of enzymes for PET ester bond hydrolyses to produce three different monomers (mono(2-hydroxyethyl) terephthalate (MHET), bis(2-hydroxyethyl) terephthalate, and TPA). Following that, secretion of (MHETase) feruloyl esterase helps it assimilate into TPA and ethylene glycol (Yoshida et al., 2016). The enzyme’s efficacy and extent of depolymerization were also reported with an additive and surfactants. Fig. 15.5 shows the consolidated synthetic plastic degradation mechanisms and by-products.

Microplastics Microplastics primarily result from minute polymeric particles formed after abiotic and biotic stress over some period and the slow degradation of traditional plastic; these particles are spread throughout the ecosystem with the help of ocean wind circulations (Thevenon et al., 2014). Microplastics are polymeric particles having sizes less than 5 mm. Therefore, because of their size and reachability, they impart a serious threat to different components of the ecosystem and can cause recalcitrant growth and oxidation stress. Microplastics in large amounts can be found in wastewater treatment plant (WWTP) discharges, which are then released into rivers. WWTP results in an uncontrollable release of microplastics into the aquatic environment (Gies et al., 2018). Heavy metal adherence to microplastics is also observed, including Cr (VI), Cu, Hg, and Au, and their effects on certain algal species (Turner & Holmes, 2015). Microplastics that float or are submerged in the water have a very slow degradation rate by being transported into the food chain.

Bioremediation of marine-specific microplastics Land-based sources and wind circulation contribute around 80% of the plastic in the aquatic environment. Due to improper waste management and insufficient sewage disposal, plastics from landfills accumulate in the ocean (Mattsson et al., 2015). Plastic contamination in the ocean has several dangerous and environmentally harmful consequences. Marine animals may become entangled in plastic items (such as the plastic rings that hold drinks cans together), ingest the plastic, or be exposed to plastic chemicals, all of which may cause physiologic changes over time. The most serious risk

Bioremediation of marine-specific microplastics


FIGURE 15.5 Synthetic plastic biodegradation pathways and by-product formation. From Ru, J., Huo, Y., & Yang, Y. (2020). Microbial degradation and valorization of plastic wastes. Frontiers in Microbiology, 11.


Chapter 15 Microbial attachment studies

associated with marine plastic is that plastic eaten by animals remains with them for a long time, resulting in reduced feeding stimuli, gastrointestinal blockage, and reduced appetite. High concentrations of chemical contaminants have been found in the ocean from plastic particles. Alcanivorax borkumensis was found to be a key player in the formation of thick biofilms and the degradation of LDPE in studies on the marine microbial ecosystem (Delacuvellerie et al., 2019). Whales and dolphins have been observed to eat large amounts of microplastic debris. Whales’ consumption of microplastic debris allows them to consume substantial numbers of plankton while traveling, increasing overall plankton consumption. Nylon, polyester, PET, PE, PP, phenoxy resin, polyester, rayon, polyamide resin, and LDPE are common examples of microplastics found in the gut or intestine (Nelms et al., 2019). Microorganisms such as Bacillus cereus and Bacillus gottheilii, as well as Bacillus sp. strains, Rhodococcus sp. strains 36 and 27, the JASK1 strain of Aspergillus clavatus, Bacillus pumilus M27, Kocuria palustris M16, Bacillus pumilus M16, and B. subtilis H1584 have been found to degrade microplastics in the marine environment (Auta et al., 2018; Gajendiran et al., 2016). Most studies, such as those using 16S rRNA sequences, have revealed the presence of major unidentified organisms in the lower temperature range (4 C) in deep-sea sediments. In Toyama Bay, two distinct types of PCLdegrading bacteria corresponding to the Pseudomonas genus with an operating temperature of 4 C were identified from deep seawater. Furthermore, bacteria Shewanella, Moritella, Psychrobacter, and Pseudomonas genera were isolated from a depth of 5000e7000 m (Sekiguchi et al., 2009). In seawater, plastic releases dissolved organic carbon into seawater, which stimulates the activity of heterotrophic microbes. Microorganisms can develop new characteristics as a result of their adaptation to new carbon sources, especially lower temperature range active enzymes, and give rise to plastic degradation (Romera-Castillo et al., 2018). The identification of lipase activity from bacterial strains with lower temperature operation and lipase activity, both psychrophilic and psychrotolerant strains, could be significant for biodegradation because some lipases can hydrolyze polyesters like PCL (Yu et al., 2009). Biodegradation of hydrocarbon-rich plastics, i.e., PCV or PET, is facilitated by the PETase enzyme isolated from Ideonella sakaiensis, which has been identified in a recent study in the polar environment (Austin et al., 2018). As microplastic persists in the environment in mixed form, the degradation by microbes must be facilitated by utilizing mixed microbial cultures or a combination of two or more enzymes. Table 15.3 shows various methods used with marine-specific microplastics.

Characterization for biodegradation analysis One of the first characterizations for biodegradation is to measure the molecular weight of any polymer. Size exclusion chromatography is most suited for this purpose. To be more precise, gel permeation chromatography is the most accurate in terms of polymers. The degradation should depict the reduced molecular weight. This technique distinguishes the molecules in the solvent according to their effective size. Other entities, including atoms of the polymeric chain and functional groups present, must also be analyzed. For qualitative identification of these functional groups, Fourier transform infrared spectroscopy is among the primary techniques. The overall functional group can be identified with a single spectrum. Ion chromatography and ion-selective electrodes are being used for getting helpful information on chemical composition and elemental composition, i.e., inductively coupled plasma or atomic absorption spectroscopy. However, these techniques are mostly not used, as they are not suited to halogen and lightweight elements such as C, N, S, H, and O. For this purpose,


Characterization for biodegradation analysis

Table 15.3 Summary of various methods used and characterization methods for accessing degree of biodegradibility for marine-specific microplastics biodegradation. Type of plastic

Material and method

Characterization methods

Results achieved

Phanerochaete chrysosporium PV1

Polyvinyl chloride (cellulose blended)

Soil burial and shake flask treatment

Fourier transform infrared (FTIR), CO2 analyzer

Fusarium spp. AF4

Polyethylene (PE)

Sewage collected from power plant followed by shake flask treatment

Scanning electron microscopy and CO2 evolution

Pseudomonas aeruginosa, Bacillus subtilis, Staphylococcus aureus, Streptococcus pyogenes, and Aspergillus niger Bacillus spp. and Pseudomonas spp.

Polystyrene (disposable plates)

Percentage weight loss calculated for all five strains

Sample weighing

Decoloring and cracks on polymeric surface and less intense peak in FTIR confirmed by CO2 concentration after 30 days SEM images show further roughening of the surface with pit formation and increased CO2 evolution after 2 months B. subtilis shows maximum percentage weight loss

High-impact polystyrene (HIPS)

Sequencing 16S rRNA to identify bacterial strains with HIPS as a single carbon Trypticase soy broth media for 72 h at 37 C Woods Hole media with dechlorinated water (incubation of samples in

FTIR, nuclear magnetic resonance, highperformance liquid chromatography, thermogravimetric analysis FTIR, weight loss, SEM, and tensile strength

Microbial strain

Escherichia coli


Pirellulaceae, Phycisphaerales, Cyclobacteriaceae, and Roseococcus (biofilm formation)

PE and polypropylene

DNA isolation, amplification, and further sequencing

References Muhammad et al. (2013)

Shah et al. (2009)

Kamble et al. (2015)

Up to 23% weight loss (for Bacillus spp.)

Arya et al. (2016)

A 0.4% reduction in polymeric content Not reported

Usca´tegui et al. (2016)

Miao et al. (2019)



Chapter 15 Microbial attachment studies

Table 15.3 Summary of various methods used and characterization methods for accessing degree of biodegradibility for marine-specific microplastics biodegradation.dcont’d Microbial strain

Amycolatopsis orientalis

Type of plastic

Polylactic acid

Material and method

Characterization methods

Results achieved

greenhouse tanks exposed to natural light) Incubation for 8 h at 140 rpm and 30 C

FTIR, change in pH, and enzyme activity

Complete polymeric degradation was observed


Jarerat et al. (2006)

a CHNS analyzer should be used. Other techniques for elemental analysis include X-ray fluorescence spectroscopy, a technique that is rapid and nondestructive. Especially with organic compounds, i.e., polymers, nuclear magnetic resonance (NMR) is another spectrometric technique. The NMR-suited nuclei 1H and 13C are best as abundant elements in polymers. Morphological visualization of the polymeric surface can be conducted with scanning electron microscopy (SEM). The magnification below a micron or up to a few nanometers is possible with SEM. The only drawback is that SEM requires electrically conductive samples, and polymers are mostly insulators. However, the advantage includes the use of energy-dispersive X-ray analysis along with SEM to acquire elemental distribution information. Thermal characterization, such as thermogravimetric analysis and differential scanning calorimetry (DSC), is the most important available technique. The curve is obtained for temperature change versus the measured property. These techniques work on the principle of temperature applied versus a mass or enthalpy change. Reductions in mass after biodegradation can be analyzed easily with the obtained curve. The presence of different chemical groups and their decomposition and dissociation also can be accessed through thermal analysis. With DSC, changes in different properties like glass transition temperature (Tg), cold crystallization temperature (Tcc), and melt crystallization temperature (Tmc) give indications of temperature shifts in the polymeric chain arrangement and biodegradation examination. In addition, reductions in molecular weight can be accessed with this technique. The melting peak can provide evidence of heterogeneous entities or impurities. That would be beneficial for analyzing the extent of biodegradation with or without additives. Apart from these, rheological studies of materials can be analyzed with a rheometer. This technique can help us determine average molecular weight (zero shear viscosity), damping properties (Tan delta), crosslinker length and viscosity (plateau modulus), and complex viscosity (Kaykhaii & Linford, 2020).

Conclusion Plastic persists in the environment for a long time in various sites, affecting various components of the ecosystem. The operating limitations of physical methods can be overcome by microbial bioremediation techniques. Bioplastics come with excellent biodegradability and allow sustainable use. Additionally, bioplastic can be useful for delivering healthy micronutrients with reinforcement by



vitamins and antioxidants to the polymer. The degradation of synthetic plastics is a major research concern because plastics have been accumulating in our environment since the Industrial Revolution. Plastic deterioration is generated through various physical, mechanical, and chemical treatments; however, these methods are not eco-sustainable. These methods impose threats on ecosystem components. For plastics degradation consistent with Sustainable Development Goals, many microbial strains have been isolated and tested for their degradation ability. These microbes consume carbon sources present in polymers as their energy source. Microbes achieve the consumption of plastic sources by forming biofilm over plastic surfaces. EPSs are responsible for biofilm formation and growth. Data are available for a significant number of microbial strains and enzymes, as well as the optimum conditions of their operation. Most have shown promising effects but are limited by degradation. The inclusion of enzyme engineering could be effective for integrating the capabilities of two different polymerases. Additionally, scientific community must use genetic engineering and enzyme engineering techniques in order to enhance efficiency of the microbes employed for biodegradtion with existing conventional techniques. Incorporating the gene of interest into the microbe genome and then culturing the microbes can have desirable enzyme production that can be used to degrade long molecular chains (>1000 KDa) of synthetic plastic. It can increase the efficiency of enzyme secretion and provide flexible optimum conditions or enzymatic operation. Many nanoparticles incorporated with enzymes have been used to further catalyze enzyme activity. However, many microbial potentials of degradation have yet to be studied; this work could be accelerated using 16S rRNA studies and metagenomics. Microplastics impose serious threats to the environment; for the same mixed microbial culture or combination, engineering enzymes can be employed. In that way, we can expect significantly more information about the attached microbial degradation. In-depth study of enzymes and their interactions with various metabolic components and the polymeric chain can be studied by modeling all the participating nodes in the mathematical model and further optimizing. Integrating these approaches and extensive research in this respect should significantly reduce the accumulation of plastic in our ecosystem and proceed with steps for sustainable development. A greater understanding of the various mechanisms of plastic breakdown by microorganisms may help improve biodegradation. Several factors can be investigated for better biodegradation of polymer plastics, including using surface active ingredients or surfactant-producing microorganisms to aid microbe adhesion to the surface layer, blending polymeric materials with biodegradable polymers like PLA or PCL, and pretreating the plastics, which includes chemical and oxidative treatments. As a result, more study is needed to expand the variety of enzymes and microbes that operate on these polymers. This can be accomplished with noncultivated microorganisms’ worldwide metagenomic data. Achieving these factors alone can result in innovative biocatalysts and organisms and allow the great majority of human-made polymers to be rapidly degraded, recycled, or used for value-added purposes.

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Chapter 15 Microbial attachment studies

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Plastic waste to plastic value: Role of industrial biotechnology


Md. Zillur Rahman1, M. Mahfuza Khatun2 and Md. Enamul Hoque3 1

Department of Mechanical Engineering, Ahsanullah University of Science and Technology, Dhaka, Bangladesh; Deptartment of Genetic Engineering and Biotechnology, Bangabandhu Sheikh Mujibur Rahman Maritime University, Dhaka, Bangladesh; 3Department of Biomedical Engineering, Military Institute of Science and Technology (MIST), Dhaka, Bangladesh


Introduction Plastic use has increased significantly over the past few decades, as plastics are cheap, lightweight, versatile, corrosion-resistant, water-resistant, and durable. The annual growth in plastic production has been 9% (Thompson et al., 2009). Plastics are made from scarce petroleum resources (around 8%) and used in an enormous range of products in transport, health, telecommunications, agriculture, textiles, footwear, construction/building, and packaging. The same characteristics, such as durability and resistance to degradation, that make plastics so valuable for many applications, also make them difficult or impossible for nature to assimilate. Many uses for plastic products have a life span of less than 1 year, and most of these plastics are then discarded. Plastic wastes are also long-lasting substances that can withstand extreme environmental situations while staying relatively unchanged over time. Plastic waste of around 6300 million metric tons (MMT) was produced worldwide in 2015, with about 9% recycled, 12% incinerated, and 79% ending up in landfills or the environment. If these manufacturing and waste disposal trends persist, plastic waste of almost 12,000 MMT will be in landfills or the environment by 2050 (Geyer et al., 2017). The historical global production, consumption, and disposal of polymers, synthetic fibers, and additives are shown in Fig. 16.1. Plastics can be broken into microplastics (