Introduction

Part 1 Background

Plastic pollution

In our daily lives, plastic has become an indispensable material due to its light weight, durability, and high cost-effectiveness. From plastic bags for food packaging to essential mobile phones for modern people, plastics and their derivatives are ubiquitous.

Jyoti Mathur Filipp, Executive Secretary of the Secretariat of the Intergovernmental Negotiating Committee (INC), pointed out that humans produce about 430 million tons of plastic every year 1 , of which two-thirds have a short lifespan and will soon become waste. Moreover, according to data published by the United Nations Environment Program (UNEP) 2 and the Organization for Economic Cooperation and Development (OECD), the total annual production of plastics continues to rise exponentially.

Global palstic production and accumulation

Figure 1 Global palstic production and accumulation

Global plastic production with projection,1950-2060

Figure 2 Global plastic production with projection, 1950-2060

However, the generation of large-scale plastic waste and improper disposal methods have made plastic waste (white pollution) one of the most serious environmental problems at present. Discarded plastic products cannot be automatically degraded, resulting in long-term accumulation of plastic waste that is difficult to improve through physical, chemical, and biological methods, ultimately causing not only landscape damage but also serious damage to ecosystems.

Plastic pollution harms soil and water sources 3, poses a risk of suffocation, poisoning, and other deaths to marine organisms that ingest it 4, exacerbates the greenhouse effect 5 and poses a threat to human health through food chain transmission, respiratory intake, and other means 6. Despite the increasing emphasis on restricting the use and recycling of plastic products in countries around the world, a large amount of plastic is still directly released into the environment. These exposed plastic products form microplastics under the influence of physicalaction, photodegradation, and other processes, ultimately entering the ecosystem and food chain, bringing potential impacts to the entire ecosystem.

Microplastic harm

Microplastics are small fragments of plastic debris accumulated in the environment, which can be divided into two categories: primary microplastics, with a particle size of less than 5 mm that directly enter the environment, and secondary microplastics, formed by physical, chemical, and biological processes that cause fragmentation and volume reduction from larger plastic fragments 7.

In 2004, Richard C. Thompson and others from Plymouth University in the UK first proposed the concept of “microplastics”. Over the past decade, numerous studies have revealed that microplastics can be absorbed by crops, fish, earthworms, chickens, bees, and other marine and terrestrial animals as well as humans,affecting their growth, development, and reproductive capacity 8. Microplastics can enter the human body through various pathways, including the respiratory system, digestive system,and cardiovascular system 9. If it enters the human body through the respiratory tract and accumulates in the lungs and respiratory tract, it may affect the lungs and lead to respiratory related health problems, with a long-term risk of cancer.

In addition, microplastics are easily absorbed by organisms in food and water 10, and then enter the human body through the digestive system along the food chain 11. They may accumulate in the intestines, causing damage to the liver and intestines, interfering with normal hormone levels, and affecting reproductive and developmental functions. The pathogenic bacteria and antibiotic-resistant bacteria attached to microplastics exacerbate their harmfulness, which may lead to disease transmission and the spread of antibiotic resistance.

Human expousure to microplastic and nanoplastic particles

Figure 3 Human expousure to microplastic and nanoplastic particles

Introdution of PET

PET (polyethylene terephthalate) is awidely used thermoplastic polyester. It is composed of terephthalic acid (TPA) and ethylene glycol (EG) through esterification or trans-esterification.

In the global plastic market, PET occupies an important position. According to statistics, PET accounts for about 20% of all plastic types. The global packaging industry is the largest end-user industry for PET 12. The global plastic packaging production is expected to increase from 140 million tons in 2023 to around 180 million tons in 2029. The demand for PET and the production of PET continue to expand and grow 13.

Value of Polyethylene Terephthalate Consumed by end user industry

Figure 4 Value of Polyethylene Terephthalate Consumed by end user industry

However, despite the recyclability of PET 14, there are some problems with the current treatment of PET plastic waste, and the collection rate of PET is not high 15. Many PET products are not effectively recycled after use, but are landfilled or discarded in the natural environment, which leads to resource waste and environmental pollution. In addition, the recycling process of PET is more complicated, requiring multiple steps such as cleaning, separation, melting, and granulation, which consume a lot of energy and may produce secondary pollution 16. Therefore, finding effective and environmentally friendly PET plastic degradation and recycling methods has become a challenge, waiting for us to overcome.

Part 2 Current Methods

PET has a dense structure and high degree of crystallinity, which makes its natural degradation process slow. At present, discarded plastics are mainly treated through landfilling, incineration, and recycling, with 77% of plastics being buried, 13% being incinerated, and only 10% of discarded plastics being recycled 17.

Among them, the landfill method for treating discarded PET is less effective, takes a long time, and can affect groundwater and soil. When incineration is used to treat discarded PET, it can achieve rapid decomposition of PET plastic, but it also leads to numerous environmental issues, such as generating a large amount of emissions that exacerbate global warming, and the burning of PET can produce toxic smoke,polluting the atmosphere 18 .

At present, the recycling and utilization of PET plastic has become one of the most vibrant sectors within the industry, with a range of solutions including physical, chemical, and biological methods.Nevertheless, there are significant challenges, such as low conversion rates, high costs, and complex procedures 19 .

Physics methods

The primary physical degradation methods for PET include photodegradation and thermal decomposition. The photodegradation process is highly susceptible to environmental factors. Without light, or when plastics are stacked or buried by soil, the degradation process cannot proceed, significantly limiting the amount of plastic that can be degraded.

The recycling and reuse of PET through thermal decomposition also have certain disadvantages, such as the requirement for high-temperature reactions in the thermal decomposition recycling process, which involves substantial equipment investment and high recycling costs. Furthermore, the mechanical properties, including impact strength, of the recycled PET secondary products are diminished, greatly restricting the application potential of the plastic 20 .

Chemistry methods

The principal chemical degradation techniques for PET encompass hydrolysis, alcoholysis, and glycolysis.Nonetheless, these approaches have several inherent shortcomings:

Ⅰ. The high energyconsumption and the need for high-temperature, high-pressure conditions escalate energy expenditure and production costs.

Ⅱ. The selectivity and stability of catalysts, along with the challenges associated with their recovery and reuse post-reaction, present significant hurdles.

Ⅲ. The purification of high-purity monomersor valuable compounds from the degradation by-products can be both costly and intricate.

Ⅳ. Despite the intent of chemical recycling to mitigate the environmental footprint of PET waste, certain chemical processes may produce noxious by-products ornecessitate the utilization of harmful chemicals 21.

Biology methods

Compared with physical and chemical methods, biodegradation is more energy-saving, time-saving and environmentally friendly, and more in line with the trend of social development 22. Biological methods include microbial degradation methods and enzymatic digestion methods, and despite their many advantages, they still face some challenges, such as:

Ⅰ. The degradation conditions are strict, requiring degradation at high temperature and more alkaline conditions. The glass transition temperature of PET is 70~80℃, and at high temperature, PET in the amorphous state is more likely to bind to the enzyme and degrade.In addition, the pH during the degradation process of PET is generally more alkaline (7-11), which makes the reaction conditions of the enzyme more stringent.

Ⅱ. The degradation efficiency is not high. The natural expression of PET hydrolase is generally low, limiting their efficiency and cost-effectiveness in industrial applications.

Ⅲ. Intermediates will inhibit the degradationof PET. PET degrading enzymes release the Mono (2-hydroxyethyl) terephthalate (MHET), Bis (2-hydroxyethyl) terephthalate (BHET), Terephthalic acid (TPA)and Ethylene glycol (EG) during the degradation of PET, and the large accumulation of the released intermediates BHET and MHET will inhibit the growth of degrading enzymes and hinder the further degradation of PET.

Ⅳ. The recycling method of plastic degradation products is limited and the utilization rate is low. At present,most plastic degradation products can be transformed into Protocatechuic Acid (PCA), Vanillin and other small molecule intermediate substances through transgenic and whole-cell catalysis. In some studies, TPA and EG wereeven directly metabolic mineralized and were not used for the production of high-value compounds, causing carbon loss, which did not conform to the PETcycle economy.

Part 3 Our Project

This project, SUPERB, using machine learning from the known performance of PET hydrolase sequence database, predicts potential efficient mutant types, and the O-glycosylation site mutation, in order to solve the problem of plastic degradation enzyme thermal stability and acid base tolerance.

Furthermore, molecular docking is performed with the mutated enzyme and PET, MHET, and BHET to identify the most optimal mutant variant. At the same time, by optimizing the chassis cell Pichi pastoris and expression elements, a strain with high expression of plastic degrading enzyme was constructed to improve the efficiency of plastic degrading enzyme. Combined with the way of biological transformation, the plastic degradation products of Terephthalic Acid (TPA) and Ethylene glycol (EG) are synthesized into bacterial cellulose to realize therecycling of plastics.

Instead of our experiments, we know that individuals are weak, but we also know that the whole is power. So, in 2024, the team of Hubei University of Technology devoted a lot of energy to human practice through six months, with the hope of building evidence-based arguments to support their technology-science-engineering decisions through human practice. In order to make our project design more reflective, responsible, responsive, we also designed some targeted activities and get feedback from them. This feedback helps us to design the event, advance the project, and make the future outlook and self-reflection.(see more in Human practices , Sustainable development , Inclusivity,Education)

References

Footnotes

  1. A historic treaty to curb plastic pollution is expected to be reached | | 1 United Nations News ↩

  2. Accumulation ofplastic waste during COVID-19. Science 369(6509), 1314-1315. ↩

  3. Coleby, A.M.and Grist, E.P.M. (2019). Prioritized area mapping for multiple stakeholdersthrough geospatial modelling: A focus on marine plastics pollution in HongKong. Ocean and Coastal Management 171, 131-141. ↩

  4. Alomar, C.,Sureda, A., Capó, X., Guijarro, B., Tejada, S. and Deudero, S. (2017).Microplastic ingestion by Mullus surmuletus Linnaeus, 1758 fish and itspotential for causing oxidative stress. Environmental Research 159, 135- 142. ↩

  5. Amaral-Zettler,L.A., Zettler, E.R., Slikas, B., Boyd, G.D., Melvin, D.W., Morrall, C.E. et al.(2015). The biogeography of the plastisphere: Implications for policy.Frontiers in Ecology and the Environment 13(10), 541-546. ↩

  6. Biancamaria,S., Lettenmaier, D.P. and Pavelsky, T.M. (2016). The SWOT Mission and itscapabilities for land hydrology. ↩

  7. Shim, W.J.,Hong, S.H. and Eo, S. (2017). Identification methods in microplastic analysis:A review. Analytical Methods 9, 1384-1391. ↩

  8. Redondo-Hasselerharm,P.E., Gort, G., Peeters, E.T.H.M. and Koelmans, A.A. (2020). Nano- andmicroplastics affect the composition of freshwater benthic communities in thelong term. Science Advances 6(5), eaay4054. ↩

  9. Vethaak, A.D.and Legler, J. (2021). Microplastics and human health. Science371, 672-674. ↩

  10. Wang, J.,Coffin, S., Sun, C., Schlenk, D. and Gan, J. (2019b). Negligible effects ofmicroplastics on animal fitness and HOC bioaccumulation in earthworm Eiseniafetida in soil. Environmental Pollution 249, 776-784. ↩

  11. Porter, A.,Lyons, B.P., Galloway, T.S. and Lewis, C. (2018). Role of marine snows inmicroplastics fate and bioavailability. Environmental Science and Technology52(12), 7111-7119. ↩

  12. Niaounoukis,M., Kontou, E., Pispas, S., Kafetzi, M. and Giaouzi, D. (2019). Aging ofpackaging films in the marine environment. Polymer Engineering Science 59(issues2), E432-441 ↩

  13. Polyethylene Terephthalate (PET) Market Size and Share Analysis - Industry Research Report - Growth Trends ↩

  14. Vázquez-Morillas,A., Beltrán-Villavicencio, M., Alvarez-Zeferino, J.C., Osada Velázquez, M.H.,Moreno, A., Martínez, L. et al. (2016). Biodegradation and ecotoxicity ofpolyethylene films containing pro-oxidant additive. Journal of Polymers and theEnvironment 24, 221-229. ↩

  15. HIRAGA K,TANIGUCHI I, YOSHIDA S, KIMURA Y, ODA K. Biodegradation of waste pet: asustainable solution for dealing with plastic pollution[J]. EMBO Reports, 2019,20(11): e49365. ↩

  16. SAGONG HY, SONFH, SEO H, HONG H, LEE D, KIM KJ. Implications for the PET decompositionmechanism through similarity and dissimilarity between PETases from Rhizobactergummiphilus and Ideonella sakaiensis[J]. Journal ofHazardous Materials, 2021, 416: 126075. ↩

  17. Jinchang. Cross-blending pretreatment strategy to improve the enzymatic hydrolysis efficiency and harmless treatment of PET plastics [D]. Jiangnan University, 2023. ↩

  18. Nannan JING, Wenhong LIU, Qiang LI,Qingqing LI, Xia WANG, Jianzhuang YAO. Research Progress of PET PlasticDegradation and Modification Methods of Degrading Enzymes[J]. Journal ofPetrochemical Universities, 2024, 37(1): 16-24. ↩

  19. Muszyński, Marcin., Muszyński, Marcin., Nowicki, Janusz., Zygadło,Mateusz.,  & Dudek, Gabiela.. (2023).Comparsion of Catalyst Effectiveness in Different Chemical DepolymerizationMethods of Poly(ethylene terephthalate). Molecules (Basel, Switzerland). ↩

  20. Zheng Chenni. Semirrational transformation of PET-degrading enzymes and its molecular mechanism [D]. Hangzhou Normal University, 2021. ↩

  21. JIN Yufeng, QIU Jiarong, ZHANG Liangqing, ZHU Menglei. Biodegradation ofpolyethylene terephthalate: a review[J]. Chinese Journal of Biotechnology,2023, 39(11): 4445-4462. ↩

  22. LIU Pan. Biodepolymerization and resource utilization of polyethylene terephthalate (PET) [D]. Shandong University, 2023. ↩