The circular economy aspects of PET (polyethylene terephthalate) waste conversion into value-added products are discussed concerning different governmental policies and industrial protocol for plastic degradation.
The use of microbial enzymes such as PET hydrolase is discussed regarding PET (polyethylene terephthalate) degradation.
The primary purpose of this perspective is a critical analysis of the correlation of the current state-of-the-art rising circular economy platform enacted across the world with close looping of PET (polyethylene terephthalate)-based plastic polymer disposal and sustainable recycling and upcycling technology. The goal of the upcycling process is to get the low-cost value-added monomer than those obtained from the hydrocarbon industry from the sustainability prospect. A summary of the circular bio-economic opportunities has also been described. Next, how the PET hydrolase enzyme degrades the PET plastic is discussed. It is followed by an additional overview of the effect of the mutant enzyme for converting 90% of plastics into the terephthalate monomer. A site-directed mutagenesis procedure obtains this particular mutant enzyme. The diversity of different microbial organism for producing PET hydrolase enzyme is finally discussed with a suggested outlook of the circular economy goal from the viewpoint of plastic degradation objectives soon.
The environmental impact of ongoing plastic waste in the form of PET (Polyethylene terephthalate) bottles or other items is causing a grave concern to both humans and animals as a whole biota. For instance, recently, it is evident that most of the plastic is finally ends up killing birds and other aquatic animals by the ingestion process. A case study of catsharks reveals how plastic alters the gene expression process here and subsequently affects the immune system.1 Another concern recently emerges with the contamination of drinking water by microplastic, and it is predicted to be increased by twofold by 2030. The plastic polymer composition and fragmentation process need to be analyzed very well to understand the formation of such microplastic. Microplastic can easily penetrate via cell membrane in the various organism and affects the food chain.2 The effect of microplastic on soil properties and related microbiota has also been studied. It was found that the microplastic evolves from biodegradable plastic has no severe effect at all.3 This microplastic also poses a grave threat to human health. The study related to the impact of polystyrene-based nano plastic has recently been carried out on human colon cells, as most ingested food has direct interaction with colons.4 Not only plastic, but other fashion industry waste create serious problem to the environment. For instance, the consumption of around 100 million ton textiles by collective human beings leads to different types of waste, where 15% only recycle, and rest need to be landfilled.5 It also created another environmental impact concerning disposal.
The recycling process is invented two decades back to deal with the growing quantum of the plastic disposal problem, and it is coming up with another problem rather than a solution in the form of increasing carbon footprint to the atmosphere. A recent review dealing with various processes of plastic recycling is discussed broadly regarding a circular economy perspective.6 On the other hand, landfilling with plastics is done in most cases due to the nonavailability of the conversion process into the desired product or value-added chain. It is mostly due to the use of multiplayer polymer composite used to make a particular plastic product. An innovative process of plastic upcycling, therefore, is needed to cope up with this growing pollution problem from plastics. If it does not resist now, it will affect all spheres of life on a log run with a no point of return. However, recently a green chemistry-based solution provides a technological hallmark. It helps to convert the polymer (polyethylene terephthalate) used in PET bottle into terephthalic acid. The green chemistry methodology also plays an essential role in increasing resource efficiency by creating green and sustainable growth.7 It finally helps with the proper use of waste to make valuable products via a circular rather than linear processing technology.
The concept of “circular economy” (CE) commonly refers to the “take-make-use-break-make” concept, where the main goal is to create economic impetus from the waste. It also enables the close-looping of product – waste – building block cycle. The commercial aspect comes here with the reduction of associated cost to get the starting materials such as monomer to make a polymer. By carefully going to the current state-of-the-art about the circular economy, it is gaining very much importance in both scientific and techno-economic aspects. For the same, many stakeholders are already created and developed the process needed for plastic breakdown into monomers, which is cost-effective rather than obtaining from the hydrocarbon industry. According to IUPAC, the next 10-year chemistry priority research mandate, this is one of the techniques that is going to have lots of influence in the technological world.8 For example, the conversion of polylactic acid (PLA) into monomers by heat treatment can be easily accomplished with circular economy steering. The mechanical recycling and chemical one gaining importance to get rid of the associated problem of plastic disposal, but here the cost of carbon dioxide emission needs to be taken into account. However, with the recent discovery of biodegradable plastic, chemical waste disposal and degradation will cease, but this will also not help mitigate the rising problem of plastic disposal.
Nevertheless, there are many reviews available elsewhere in biodegradable plastic, but a correlation of plastic upconversion technology with a clear focus on the circular economy concept is missing. The purpose of this perspective is to give a precise relationship between the circular economy and plastic degradation, followed by a discussion of various types of biochemical enzymes from the standpoint of bioengineering. Regarding this, how microbial chemistry and other organism helps for plastic degradation will also be discussed in this perspective.
What are the circular economy and bio-economy approaches for plastic and bio-waste recycling?
Circular economy of plastics
According to the Ellen MacArthur Foundation report, the critical building blocks for a successful circular setup are divided into four pillars. They are design for circularity, new business models, reverse cycles, and lastly, enablers and favorable system conditions.9 Economically in the USA alone, the annual production of plastic production is 78 million tones via linear way. Out of this, an $80–120 bn loss happens due to the non-utilization of waste materials and leakage occurs besides landfilling and incineration energy production processes. Concerning this report, the city of Phoenix, Arizona, put efforts in the circular manufacturing of plastic according to published report elsewhere.10 Similarly, due to strict governmental regulation on plastic recycling and recently, Chinese law (National Sword Policy) on the rejection of highly contaminated plastics leads to another burden on the efficient use of plastic recycling. Therefore, circular economy effort concerning policy analysis to make a close loop has recently been carried out in thesis research.11 Here, two critical factors are taken into account. Command-and-Control, – where it is emphasized that both private and public sector must follow the governments establish regulations and market based, – the government will give financial incentive to customers, and private industry depends upon their action on plastic disposal and recycling. Here, market-based systems imply the incentive offered by the government to customers or the public for following the associated protocol for plastic disposal or recycling. Whereas the market will provide some incentive for customers in the form of a subsidized product. At Duke University's Fuqua School of Business, thesis research is carried out on the implementation of the small-scale, closed-loop system at a university level. It is based on the successful business model start-up such as Premirr Plastics, which deals with the recycling of PET plastic into monomers allowing the continuous reuse of material.12 The research deals with both consumer and institutional interest. Despite the support of both benefits, it is challenging to implement the business model due to the specific barrier.
However, by launching a pilot-scale system as a marketing plant, revenue can be generated by this kind of circular economy. European Commission realizes the growing need for a plastic-based circular economy in 2015 with the implementation of the EU action plan. Here, with a particular focus on the analysis of the value chain and life cycle assessment, a technical strategy is undertaken for plastic to deal with the challenges posed by it.13 It sets out a plan to use only recyclable plastic in all plastic packaging by 2030. The vision for Europe's new plastic economy is the commissioning smart and sustainable plastic industry. It will deal with design and production driven by the need for reuse, repair, and recycle process. It will finally help in the creation of jobs and cutting greenhouse gas emissions. It is estimated that the EU uses around 50 million plastic per year, which is predicted to continue to grow. So, to design and produce valuable products from used plastic will be a bright element to realize the circular economy. The SusChem report14 states that 27 million tons of plastic collected in Europe in 2016, and 27% left for landfilled besides recycling and energy recovery. So the loss of waste provides an apparent demand for the stakeholders to go for circularity. Stakeholders behind the report also make conditions such as materials design with a particular focus on separation and recycling and to take feedstock from other sources such as bio-waste (to be discussed in the next section) to achieve this circularity. In this regard, Estonia plays an essential role in the treatment of municipal solid waste (MSW) into the value-added chain rather than the energy recovery process (incineration) besides going to landfill.
However, to fill the EU target of MSW recycling, the collection of plastic and segregation measure need to be taken into account to fulfill the circular economy goal.15 In Norway also, 2019 was considered as a year of circular economy as the government insisted on forming the Norwegian circular economy platform and making a strategy for CE.16 In Germany, PET bottles are continuously recycled for the last two decades, but the PET-based Tray used for food packaging finds an interesting aspect which is usually not recycled.17 The recycling of plastic food packaging materials is considered as a matter of food safety and consumer protection besides the circular economy. Here, the recycled product from plastic waste with direct contact with food needs to follow the European Food Safety Authority (EFSA) guidelines. If a proper protocol can be developed to make the safe recycled without genotoxic compounds, this type of plastic tray waste upcycling can lead to another circular economic perspective from the food industry. Besides, from the airing of TV shows Blue Planet II by BBC, which shows David Attenborough's documentary regarding the effect of the plastic material on the marine environment, the UK government takes a significant interest in the plastic recycling for circular economy development with businesses, research institutions, and the public.18 The application of the eco-industrial initiative is taken seriously by China, making it a law for plastic recycling to have a circular economy. The eco-industrial effort is defined as a platform of close-looping of industrial cycles where at one end, industrial waste is turned into a value-added chain, the later used in another point of utilization.19
Nevertheless, for the successful implementation of the circular economy concept, good policy is required as a mandate to be followed. A current state-of-the-art of different circular economy practices theories reveals that manufacturing and redistribution factors are rarely included in the circular economy.20 On the other hand, the waste management strategies for the circular plastic economy lies currently in the mechanical and chemical recycling processes. A particular example here is the recycling of PLA-based recycling process is reviewed by Payne et al.21 Recently, a techno-economic analysis of the recycling of polyethylene terephthalate from PET and polyolefins are carried out to reuse the material optimization processes.22 The implementation of green chemistry principles, as mentioned above in the circular economy system, is recently gaining importance in the various sector with a clear vision to balance economic growth, resource sustainability, and environmental protection.23
The circular economy is not restricted to plastic upscaling only. A recent review discusses how textile waste valorization by applying the sustainable chemistry approach can contribute to the circular economy. For example, few of the obtained products from textile waste are glucose and polyester, which are considered as value-added products.24 Besides this, paper waste materials are reviewed for recyclability to fulfill the circular economy demand.25 Here, different composite materials, cellulose nanofibers, and nanocrystals, and films of biopolymers can be produced. For the successful implementation of a circular plastic economy, the energy market in parallel needs to be adjusted in terms of tariff hikes. Therefore, the use of renewable energy can help here to run the plastic upcycling process in terms of lower tax tariffs from energy usage. It is worthwhile to mention that the circular economy focus will be on resource optimization, energy conservation, and proper business model.26 Finally, another aspect of the circular economy is encountered in the carbon foot reduction by adequate selection of materials. A recent review discussed the design of building architecture with a particular focus on using materials with zero carbon dioxide emissions. It is advised that engineers should be better aware of global climate change and make a design compatible with the environmental perspective (both inside and outside).27
Circular bio-economy for bio-waste recycling
Here, different resources of bio-waste-based circular economy will be discussed regarding waste biomass valorization. It is evident that municipal-based solid waste needs a considerable effort for disposal, and it generated mostly in large quantities from various stages of food supply chains, open markets, and catering services.28 In the EU, high priority is made to make this biomass disposal as a resource to realize the circular bio-economy to have sustainable and resource-efficient policies. The biorefinery concept evolves from this prospect can be applied in developing countries as a solution for waste biomass disposal. Also, with the help of the incineration process, it can be utilized further to produce fuel, power, heat, and value-added products.29 The technique involves in the biorefinery process are fermentation, pyrolysis, anaerobic digestion, and gasification. This waste biorefinery could also provide energy generation and land saving in developing countries via a circular bio-economic pathway. In one review on the Biorefinery concept,30 it is stated that it also helps to get rid of energy-environment nexus, whereby it considered harmful waste materials as a potential renewable feedstock. It is further processed via Hybrid thermochemical and biochemical conversion in a biorefinery.31 Here, municipal food waste is hydro mechanically treated.
According to the EU action plan on bio-economy,32 opportunities arise from it leads to a competitive, circular, and sustainable economy driven by industries with zero carbon emission. It will also mitigate the ongoing climate change issue. Bio-economy also defined as an area where creation, development, and revitalization of economic systems are made from the sustainable use of biological resources.33 The challenges associated with the bio-economic manufacturing process are the utilization of resources and produce the product at a significant scale. Due to this, high-tech bioengineering industries are diverse in developed countries. For instance, in Zurich city, bio-waste is collected to make electricity in a resource-efficient way by the incineration process. Besides biological waste, microorganisms such as bacteria and fungi also play a role in the circular bio-economy contribution. Fungi used to possess the capability of converting organic materials into a rich and diverse set of essential products such as PET degradation.34 From a review of the comparison between bacteria and fungi, it is found that most enzymes’ biodegradation is higher in fungi than the bacteria. It can also transform the petroleum-based economy into a bio-based circular economy. Not only bio-waste, whether biotechnology can be useful in plastic upconversion technology, but a recent opinion has also been made.35 Metabolic engineering is considered an option to make efficient use of biotechnology. In summary, both the circular economy and circular bio-economy platforms are depicted in Fig. 1.
Different processes of plastic upcycling technology
The PET plastic upcycling technology development is at the cornerstone of the current state-of-the-art research. The PET waste is considered as a renewable resource36 for both the recycled and upcycled plastic industry. For the same, a catalytic glycolytic process is developed, known as the “VolCat” process, which uses a volatile catalyst. The upcycling process of plastic or PET plastic is recently broadly reviewed beyond mechanical recycling with different types of methods such as thermal process, solvation, and to name a few.6 Basic energy science roundtable37 on upcycling of plastic polymers suggested that (i) How to master the polymer deconstruction, re-construction mechanisms, and cross-functionalization strategies? (ii) How to make an integrated process to make upcycled mixed plastic, which is challenging in the current mandate? and (iii) How to make chemical circularity for next-generation polymers?
Regarding the plastic upcycling processes, recently reclaimed PET bottles are upconverted into fiberglass-reinforced plastics38 by the reaction of biomass-derived diols, which leads to deconstructed PET as mentioned above. De-constructed PET further reacts with bio-reusable olefinic acid to result in an unsaturated polyester which reacts with bio-referable reactive diluent to make the fiberglass-reinforced plastics finally. It is a clear example of a green chemistry approach to achieve a sustainable circular economy.
Role of biotechnology for plastic upcycling
The circular economy's biotechnological perspective lies in the proper upcycling of plastic waste into value-added products by using microbial enzymes. The microbial biotechnology helps with the advent of the new gene-editing technique,39 where microbes can be manipulated to do the task of plastic decomposition and reduce the growing concern of plastic recycling with zero carbon footprint. Here, microbial fermentation can help the production of biodegradable plastic from thermochemically depolymerized plastic. It will further add value to the plastic upcycling chain.
How microbes make the plastic biodegradable?
Here, the role of microbes in biodegradation will be discussed and how it will contribute to the circular economy perspective. The first question to be asked how bacteria do this task? A recent study stated that microbes form thick biofilms on the plastic surface, and due to the production of active site catalytic enzymes, the polymer degradation happens.40 Not only microbes but also wax moth Galleria mellonella is found to degrade plastic such as polyethylene into ethylene glycol.41 Apart from this, biotechnology tools effectively help to get value-added products from plastic biodegradation. It will help further to achieve the goal of circular economy42
Biologically motivated plastic upcycling to value-added products
Role of enzymes
The biocatalytic degradation of plastic with the help of naturally available enzymes is another interesting aspect. It can be called as a conglomeration of both enzyme and polymer chemistry. The plastic obtained this route is high in activity and stability in aqueous media.43 The enzymatic degradation of polymethyl methacrylate with α-chymotrypsin and subtilisin is an example. Besides these, protease incorporation inside plastic matrices yields different types of peptides and nucleoside esters. Microbes’ interface with electrochemistry can be an alternative approach for green energy generation in the form of microbial fuel cells. The bioelectrocatalysis concept is discussed broadly elsewhere.44 It can also be utilized for making biodegradable materials. Biotechnology helps close the carbon cycle of the polymer industry by opening new vistas such as surface functionalization of biodegradable polymers to get new renewable products.45 The first direct evidence of microbial degradation of polymer is seen with the bacterial strain (Ideonella sakaiensis 201-F6)46 by screening natural microbial communities exposed to PET. This particular strain produces enzyme PET hydrolase. It converts PET into environmentally friendly monomers terephthalic acid and ethylene glycol. The interplay of the enzyme with PET and associated degradation mechanisms is shown in Fig. 2.
Followed by this, advances in metagenomics, gene sequencing, and tools help to improve the microbial properties. The microbial properties mentioned here usually refers to the production of enzymes with a specific purpose of plastic degradation abilities. In this context, metagenomics will not directly improve these microbial properties. Metagenomic is defined as a technique for directly accessing the DNA from microorganisms found in nature which also refers to a strategy for the discovery of different enzymes.48 Advances with metagenomic science or metagenomes reveal the cascade of genomic data from microorganism DNA. It will be with a particular emphasis for producing specific PET hydrolase enzyme. It is usually encoded in the genome of microorganisms. Here, metagenomic helps to get direct access to these specific data sets to screen microorganism. It is the theme of the next discussion. The metagenomic methods have limitation for the discovery of complete genes from environmental samples due to the inherent complexity associated with the microbial species. Recently, a hidden Markov model motif-based search engine is used to overcome this. It is used to search for existing genomes and metagenomes databases to locate the presence of PET hydrolase enzymes. With this approach, 800 potential PET hydrolase enzymes have been isolated.49 The purpose in this regard is to tune it to the specific use of plastic degradation and make value-added products. This particular problem will be tackled from the microorganism already dominated in a plastic waste concentrated area, and their effect of habitat profoundly influences the ecological symbiosis of microbes with plastic.50 With the development of new generation genetic engineering methodology, recently, mutant enzymes are created, which profoundly increases the efficiency of plastic recycling with 90% conversion to monomers.51 A demonstration plant is set to be open to scale up the technology by next year. This particular mutant PET hydrolase converts 90% of polymer into monomer for 10 h with a yield of 16.7 g of terephthalate per liter per hour.52 This enzymatic depolymerization of PET is almost as same efficient as synthetic plastic recycling. Here, the enzyme is modified by a sire-directed mutagenesis procedure. The procedure involves the cutting of amino acid chain of from peptide linkage of the enzyme. It leaves finally a more catalytically active site.
The PET hydrolase enzyme is also expressed on the surface of yeast known as Pichia pastoris, which acts as a whole-cell catalyst to improve its degradation ability.53 It increases both pH and thermal tolerance of PET hydrolase, and turnover yield is 36-fold than a purified PET hydrolase for a highly crystallized PET. In another work, the glycolysis reaction is used besides hydrolysis for the depolymerization of PET. The enzyme used here is Humicola insolens cutinase and Candida antarctica lipase, respectively.54 Here, the glycolysis reaction favors the more accumulation of esterifies end products. A recent review of enzymatic PET degradation is made elsewhere.55 Besides this, a mini-review focuses on advanced bioinformatics tools to design metagenomic databases of enzymes found in microbial genomes. From here, it is evident that microbial enzymes will play a critical role in the circular economy's realization, followed by the utilization of green chemistry principles.56
Diversity of microbes and PET degradation
A different group of bacterial species produces laccase, cutinase, and hydrolase enzymes for plastic degradation. These species are Pseudomonas, Bacillus, Streptococcus, and so on.57 The hydrolysis of PET by microbial polyester hydrolase leads to the products used for PET re-synthesis and helps microbes for carbon assimilation. This combination of biodegradation and biosynthesis enables a complete circular bio-PET economy besides the usual recycling methodology of PET.58
The discussion is so far on PET-related microorganisms and enzymes. Some specific examples of the bacterium will be considered to shed light on other types of polymer degradation and product formation. The bacterium Pseudozyma antarctica JCM 10317 degrades poly(butylene succinate) and poly(butylene succinate-co-adipate).59 An enzyme named PaE is responsible for this degradation process, and it is found that this has a specific activity of 54.8 ± 6.3 U/mg. Not only poly (butylene succinate), other polymers PLA and Caprolactam can also be degraded with this enzyme. Another type of bacterium Pseudomonas species is recently isolated from soil rich in brittle plastic waste, which can grow on polymers such as polyurethane used for the fabrication of foam and building insulation materials.60 This particular polymer is mostly used as a shock absorber in the packaging of almost all industrial items, and it is thrown away after the delivery of sensitive electronic and sophisticated analytical equipment. The degradation of the same is of utmost importance. Mycobacterium neoaurum B5-4 is another species able to degrade polymer intermediate 2,6-dimethyl phenol from polyphenylene oxide.61
Conclusion and outlook
Finally, a detailed analysis of the correlation of PET-based plastic degradation and associated circular economy aspects of the process is accomplished here with the help of results and discussion from the available literature. From the critical analysis of all information, it can be concluded that the grave concern of rising plastic concentration in the seawater can be overcome by the minimization of waste generated with a close-looping of the whole recycling process. The circular economy approach will help here to get low-cost monomer to make the PET plastic again in comparison to the monomer obtained from the hydrocarbon industry. The application of different government policy in strict sense will also definitely help to accomplish this goal, which is now undergone in many different developed countries of the world. If developing countries also follow the same protocol, such as incineration technique to generate electricity from the municipal and household wastes, will be a game-changer for the Asian and African continents.
On the other hand, the application of biotechnology also helps here as the available microbial enzymes PET hydrolase and the microbe's family for producing the same can be grown on a mass scale. However, the scalability of such enzymes needs to be a push from the angle of the existing biotech industry to make synergy with the circular plastic economy. Nevertheless, the biotechnological aspect of plastic upcycling is crucial in developing more stable enzymes, which will help in plastic degradation for the extensive scale industrial application. Also, it should be extended to other polymers besides PET for degradation and upcycling. The bio-based plastic degradation needs to be accomplished with the green chemistry methodology to reach the final goal of complete closure of the value-added chain. The upcycling of plastic waste and bio-waste will open up new vistas for the industrial sector soon with close-looping of carbon cycle followed upon the foundation of the circular economy. The future outlook will be to apply this methodology cost-effectively from the viewpoint of energy consumption and techno-economic perspective.
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The author acknowledges the funding received from JU management start-up grant: 11(39)/17/004/2017SG to carry out this manuscript work.
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Bora, D.K. Rise of the sustainable circular economy platform from waste plastics: A biotechnological perspective. MRS Energy & Sustainability 7, 28 (2020). https://doi.org/10.1557/mre.2020.28