Introduction
According to the National Bureau of Statistics, China's plastic product output surged from 58.306 million tons in 2010 to 80.4 million tons in 2021. The production, subsequent incineration and degradation process of plastic materials contribute significantly to greenhouse gas emissions. However, the adoption of 100% recycled polyethylene terephthalate (PET), as opposed to 100% pure PET, can curtail carbon dioxide emissions throughout the lifecycle of a bottle, reducing them from 446 grams to 327 grams, thereby effecting a 27% reduction. Consequently, the efficient recycling of PET will play a pivotal role in achieving sustainable development.
Our wet experiments have successfully facilitated the degradation of the PET, yielding the terephthalic acid (TPA) and ethylene glycol (EG). Drawing from literature study and experimental verification, we had explored a combined chemical-biological approach to realize closed-loop recycling of PET. To mitigate the potential environmental impact of our experimental outputs and align with the principles of sustainable development, our NNU-CHINA team members had initiated a brainstorming process. We imagined some ways to accomplish open-loop up cycling of PET. Our vision is to offer innovative concepts for maximizing the utilization of PET degradation byproducts across multiple pathways.
Furthermore, we have noted the presence of persistent microplastics, which pose substantial risks to ecosystems and biological systems. Considering the implications of microplastics on human health, we envisioned the application of synthetic biology approaches to aid in the degradation of microplastics within the human body. Our aspiration is to contribute to the resolution of the pressing issue of microplastic pollution (Fig. 1).
1. The Open-loop up Cycling of PET
New polymer
PETG Copolyester Chips
Our vision involves synthesizing environmentally friendly polyethylene terephthalate glycol (PETG) copolyester chips through substitution reactions, esterification reactions and catalysis of TPA and EG. PETG boasts commendable chemical resistance and holds promise in packaging applications. Ultimately, these packaging products will be converted into water and carbon dioxide in the environment, aligning with environmental standards.
Polyester and Supercotton-like fiber
We aspired to employ PET degradation products for esterification and polycondensation reactions to yield polyester fiber. Subsequent alkali treatment will result in super-cotton-like fibers characterized by improved thermal properties and a closer resemblance to cotton fibers. This fiber can be harnessed in the clothing sector to craft high-quality apparel, catering to the demand for superior clothing.
Polyester Paint
Our ambition lies in utilizing PET degradation products to manufacture polyester paint distinguished by excellent acid and alkali resistance and comprehensive attributes. This type of paint finds utility in coatings, excelling in corrosion resistance.
Polyurethane Materials
Our goal is to synthesize polyurethane materials with desirable shape memory, adhesion, and compatibility using TPA. This material can find application in the adhesive domain, generating high-performance adhesive products.
Chemical Additives
Flame Retardant DDP-EG
Our strategy involves combining a flame retardant intermediate (DOPO) and methylene succinic acid with ethylene glycol to synthesize a less toxic and less irritant flame retardant, DDP-EG. This compound finds utility in manufacturing automobiles, ships, defense equipment, specialized protective gear, enhancing material flame retardancy and safeguarding lives and property.
Epoxy Resin Curing Agent
We aspired to synthesize epoxy curing agents that enhance or regulate curing reactions by treating PET degradation products. This curing agent facilitates the formation of stable epoxy polymer structures, enhancing product performance and longevity.
Energy Source
Clean Fuel and Potassium Source
Our concept entails upcycling of PET degraded TPA and EG into commercial chemical potassium dicarboxylate (KDF) via electrocatalytic reactions in alkaline electrolytes. The process also yields clean fuel, H2. Potassium dicarboxylate serves as a valuable potassium source, supplementing potassium in animals and effectively reduce intestinal dehydration. This environmentally conscious energy conversion method holds promise for addressing conventional energy limitations.
New Material
Graphite
Considering PET's carbon-rich composition and lack of inorganic components, it serves as a promising carbon material source. We propose high-temperature carbonization of PET degradation products, followed by boron-assisted catalysis to achieve TPA and EG graphitization. Graphite can find versatile applications in energy and material science as secondary battery anode material and carbon composite filler. Moreover, graphite synthesis can provide an alternative source for graphene production.
Polymer Blends
Recognizing the challenge of effectively enhancing the value of mixed plastic waste, we aim to transform TPA and EG into blend compatibilizers through treatment. This approach enhances mixed plastic waste, yielding polymer blends with superior properties. Blending and capacity addition methodologies can elevate material properties and increase the value of mixed plastic waste (Fig. 2).
2. The Degradation of Microplastics
Researchers have calculated that individuals consume between 39,000 and 52,000 microplastics annually through their food and beverages. When factoring in inhaled microplastics, this range escalates to 74,000 to 121,000 particles each year. During the European Society of Gastroenterology's conference, researchers disclosed groundbreaking findings: human feces contain up to nine diverse types of microplastics, ranging from 50 to 500 microns in diameter. This study indicates that plastic materials end up within the gastrointestinal tract. Even more concerning is the capability of the smallest microplastics to infiltrate the blood, lymphatic system, and even the liver.
Additionally, microplastics in the digestive tract could potentially influence the immune response of the digestive system. It's evident that microplastics, once ingested by organisms, cannot be digested or absorbed within the body. This inability to be processed can lead to malnutrition, impaired immune functions, growth inhibition, or even mortality in organisms. Furthermore, the migration and transformation of microplastics can release organic monomers and toxic additives. These additives can affect the endocrine function of organisms, induce genetic abnormalities, and cause harm to reproductive and developmental processes. Therefore, the presence of microplastics in the human body evidently poses a significant threat to human health and well-being.
Considering the context of this project's objectives, we propose employing synthetic biology and gene editing techniques to introduce plastic degradation genes into recipient bacteria. This alteration would enable these recipient bacteria to secrete plastic-degrading enzymes within the human body, which would then circulate through the bloodstream, effectively working to break down microplastics.
While oral administration of live bacteria is a common practice, it necessitates careful consideration of the recipient bacteria's tolerance to the stomach's acidic environment and the stability of enzyme proteins in low pH conditions. Furthermore, biomacromolecules such as enzymes face challenges in terms of absorption through the gastrointestinal wall. To address these concerns, we contemplate the intravenous injection of live bacteria. This approach would allow the bacteria to travel through the bloodstream and exert their degrading effects.
The potential pathogenicity of bacteria is a significant safety concern when utilizing them as carriers. Bacteria can release toxins that inhibit the production and release of antibodies and cytokines, leading to immune suppression. These toxins, acting as enzymes, are highly specific to cell substrates and function as signaling molecules. They can disrupt cell cycles, inhibit mitosis, and hinder lymphocyte clonal expansion, thereby impairing immunity. To address this challenge, we suggest employing a reported attenuated bacterium, Salmonella typhimurium VNP-20009. The virulence gene mshB in Salmonella is responsible for the myristoylation of lipid A, a crucial element of lipopolysaccharides (LPS) found in the outer membrane of Gram-negative bacteria. Deleting mshB can significantly reduce the expression of TNF-α, thereby minimizing the toxicity of Salmonella. Furthermore, to ensure the bacteria automatically degrade plastic after completing their mission, we propose incorporating a "kill switch". This switch is an engineered system that triggers lethal gene expression under specific conditions, causing the bacteria to self-destruct. Introducing a suicide switch into the bacteria would ensure they initiate autophagy under predefined circumstances, preventing their accumulation in the human body and the potential for adverse effects (Fig. 3).
Conclusion
In conclusion, our approach seeks to achieve versatile PET transformation and utilization using a combined chemical-biological method for sustainable development. We also intend to utilize synthetic biology methods to introduce plastic-degrading enzyme genes and conditional response suicide genes into attenuated Salmonella Typhimurium VNP-20009. This would enable the bacteria to produce target proteases within the human body, facilitating microplastic degradation, and to initiate autophagy upon task completion.
Moving forward, we will continue to brainstorm and devise innovative solutions to contribute positively to plastic recycling and promote sustainable development.
Reference
1. Hopewell, J.; Dvorak, R.; Kosior, E., Plastics recycling: challenges and opportunities. Philosophical Transactions of the Royal Society B-Biological Sciences 2009, 364 (1526), 2115-2126.
2. Li, A.; Sheng, Y.; Cui, H.; Wang, M.; Wu, L.; Song, Y.; Yang, R.; Li, X.; Huang, H., Discovery and mechanism-guided engineering of BHET hydrolases for improved PET recycling and upcycling. Nature Communications 2023, 14 (1), 4169.
3. Jintang, J. X. W. Y. X. F. S. X. W. Study on synthsis and properties of PETG. Synthetic Technology and Application 2023, 38 (02), 39-43.
4. Qingqing, J. F. L. M. W. Progress in Reuse of Waste Polyester Chemical Methods. Plastics 2022, 51 (04), 136-145.
5. Tengteng, L. R. Z. Z. Z. Synthesis of a Novel Flame Retardant DDP-EG. 2013; Huangshan, Anhui, China.
6. Zhou, H.; Ren, Y.; Li, Z.; Xu, M.; Wang, Y.; Ge, R.; Kong, X.; Zheng, L.; Duan, H., Electrocatalytic upcycling of polyethylene terephthalate to commodity chemicals and H2 fuel. Nature Communications 2021, 12 (1), 4679.
7. Ko, S.; Kwon, Y. J.; Lee, J. U.; Jeon, Y.-P., Preparation of synthetic graphite from waste PET plastic. Journal of Industrial and Engineering Chemistry 2020, 83, 449-458.
8. Jehanno, C.; Alty, J. W.; Roosen, M.; De Meester, S.; Dove, A. P.; Chen, E. Y. X.; Leibfarth, F. A.; Sardon, H., Critical advances and future opportunities in upcycling commodity polymers. Nature 2022, 603 (7903), 803-814.
9. Cox, K. D.; Covernton, G. A.; Davies, H. L.; Dower, J. F.; Juanes, F.; Dudas, S. E., Human Consumption of Microplastics. Environmental Science & Technology 2019, 53 (12), 7068-7074.
10. Schwabl, P.; Koeppel, S.; Koenigshofer, P.; Bucsics, T.; Trauner, M.; Reiberger, T.; Liebmann, B., Detection of Various Microplastics in Human Stool A Prospective Case Series. Annals of Internal Medicine 2019, 171 (7), 453-457.
11. Leslie, H. A.; van Velzen, M. J. M.; Brandsma, S. H.; Vethaak, A. D.; Garcia-Vallejo, J. J.; Lamoree, M. H., Discovery and quantification of plastic particle pollution in human blood. Environment International 2022, 163，107199-107199.
12. Yang, X.; Man, Y. B.; Wong, M. H.; Owen, R. B.; Chow, K. L., Environmental health impacts of microplastics exposure on structural organization levels in the human body. Science of the Total Environment 2022, 825，154025-154025
13. Sawant, S. S.; Patil, S. M.; Gupta, V.; Kunda, N. K., Microbes as Medicines: Harnessing the Power of Bacteria in Advancing Cancer Treatment. International Journal of Molecular Sciences 2020, 21 (20), 7575.
14. Chien, T.; Doshi, A.; Danino, T., Advances in bacterial cancer therapies using synthetic biology. Current opinion in systems biology 2017, 5, 1-8.
15. Low, K. B.; Ittensohn, M.; Le, T.; Platt, J.; Sodi, S.; Amoss, M.; Ash, O.; Carmichael, E.; Chakraborty, A.; Fischer, J.; Lin, S. L.; Luo, X.; Miller, S. I.; Zheng, L.; King, I.; Pawelek, J. M.; Bermudes, D., Lipid A mutant Salmonella with suppressed virulence and TNFalpha induction retain tumor-targeting in vivo. Nature biotechnology 1999, 17 (1), 37-41.
