Fast Depolymerization of PET Bottle Mediated by Microwave Pre-Treatment and An Engineered PETase

Designing Biobased Poly(ethylene-co-isosorbide terephthalate) Copolyesters with Tunable Properties and Degradability

Production of high-performance polyesters with tailored degradability remains a challenge. Here, a series of poly(ethylene-co-isosorbide terephthalate) (PEIT) copolyesters were synthesized by varying the isosorbide (IS) content (0-20 mol %) using tetrabutyl titanate (TBT) as the catalyst. By variation of the IS content, the thermal, mechanical, and optical properties of the copolyesters were effectively tailored. As the IS content increased, the Tg was raised from 80 to 101 °C, and the tensile strength from 58.8 to 68.7 MPa. Moreover, excellent transparency was maintained (up to 90% light transmittance). Interestingly, the susceptibility to hydrolytic degradation was significantly enhanced by the incorporation of IS, with PEIT-20 showing approximately 3.5 times higher weight loss compared to PET after 50 days of alkaline degradation in 0.1 M NaOH solution. This outlines an attractive approach for developing high-performance copolyesters with tunable properties and degradation rates, suitable for applications in transparent thermal packaging materials.

Impact of Ball Milling on the Microstructure of Polyethylene Terephthalate

Polyethylene terephthalate (PET) is a semi-crystalline polymer that finds broad use. Consequently, it contributes to the accumulation of plastics in the environment, warranting PET recycling technologies. Ball milling is a commonly used technique for the micronization of plastics before transformation. It has also recently been reported as an efficient mixing strategy for the enzymatic hydrolysis of plastics in moist-solid mixtures. However, the effect of milling on the microstructure of PET has not been systematically investigated. Thus, the primary objective of this study is to characterize the changes to the PET microstructure caused by various ball milling conditions. PET of different forms was examined, including pre- and post-consumer PET, as well as textiles. The material was treated to a range of milling frequencies and duration, before analysis of particle size, crystallinity by differential scanning calorimetry and powder X-ray diffraction, and morphology by scanning electron microscopy. Interestingly, our results suggest the convergence of crystallinity to ~30 % within 15 minutes of milling at 30 Hz. These results are consistent with an equilibrium between amorphous and crystalline regions of the polymer being established during ball milling. The combined data constitutes a reference guide for PET milling and recycling research.

Conformational Selection in Enzyme-Catalyzed Depolymerization of Bio-based Polyesters

Enzymatic degradation of polymers holds promise for advancing towards a bio-based economy. However, the bulky nature of polymers presents challenges in accessibility for biocatalysts, hindering depolymerization reactions. Beyond the impact of crystallinity, polymer chains can reside in different conformations affecting binding efficiency to the enzyme active site. We previously showed that the gauche and trans chain conformers associated with crystalline and amorphous regions of the synthetic polyethylene terephthalate (PET) display different affinity to PETase, thus affecting the depolymerization rate. However, structural-function relationships for biopolymers remain poorly understood in biocatalysis. In this study, we explored the biodegradation of previously synthesized bio-polyesters made from a rigid bicyclic chiral terpene-based diol and copolymerized with various renewable diesters. Herein, four of those polyesters spanning from semi-aromatic to aliphatic were subjected to enzymatic degradations in concert with induced-fit docking (IFD) analyses. The monomer yield following enzymatic depolymerization by IsPETase S238 A, Dura and LCC ranged from 2 % to 17 % without any further pre-treatment step. The degradation efficiency was found to correlate with the extent of matched substrate and enzyme conformations revealed by IFD, regardless of the actual reaction temperature employed. Our findings demonstrate the importance of conformational selection in enzymatic depolymerization of biopolymers. A straight or twisted conformation of the polymer chain is crucial in biocatalytic degradation by showing different affinities to enzyme ground-state conformers. This work highlights the importance of considering the conformational match between the polymer and the enzyme to optimize the biocatalytic degradation efficiency of biopolymers, providing valuable insights for the development of sustainable bioprocesses.

Enhancing Microplastic Degradation through Synergistic Photocatalytic and Pretreatment Approaches

Microplastics (MPs) pollution has emerged as a pressing environmental concern in recent years. Owing to their minute dimensions, conventional plastic remediation approaches are inadequate for addressing the challenges posed by MPs. Herein, spherical (BOC-S) and nanosheet (BOC-N) BiOCl photocatalysts were prepared and applied to the degradation of poly(ethylene terephthalate) (PET) MPs after hydrothermal pretreatment. The results indicated that the degradation efficiency of pretreated PET MPs using BOC-S and BOC-N photocatalysts was 8.8 and 6.9 times that of the unpretreated MPs under the same conditions. Comparative experiments confirmed the excellent performance of the photocatalysis-pretreatment system. The creation of pores on the surface of pretreated PET MPs facilitates the entry of active substances into the interior to cause damage, while the enhancement of hydrophilicity and specific surface area facilitates the contact between the catalyst and PET MPs, thus increasing the degradation efficiency. Free radical trapping experiments revealed that hydroxyl radicals (·OH) produced by photocatalysis had the greatest influence on the degradation performance of pretreated PET MPs. Finally, a possible photocatalytic degradation mechanism for PET MPs was proposed. This research offers a novel perspective on MPs degradation, providing valuable insights for advancing the efficacy of the process.

Degradation of PET microplastic particles to monomers in human serum by PETase

More than 8 billion tons of plastic waste has been generated, posing severe environmental consequences and health risks. Due to prolonged exposure, microplastic particles are found in human blood and other bodily fluids. Despite a lack of toxicity studies regarding microplastics, harmful effects for humans seem plausible and cannot be excluded. As small plastic particles readily translocate from the gut to body fluids, enzyme-based treatment of serum could constitute a promising future avenue to clear synthetic polymers and their corresponding oligomers via their degradation into monomers of lower toxicity than the material they originate from. Still, whereas it is known that the enzymatic depolymerization rate of synthetic polymers varies by orders of magnitude depending on the buffer and media composition, the activity of plastic-degrading enzymes in serum was unknown. Here, we report how an engineered PETase, which we show to be generally trans-selective via induced fit docking, can depolymerize two different microplastic-like substrates of the commodity polymer polyethylene terephthalate (PET) into its non-toxic monomer terephthalic acid (TPA) alongside mono(2-hydroxyethyl)terephthalate (MHET) in human serum at 37 °C. We show that the application of PETase does not influence cell viability in vitro. Our work highlights the potential of applying biocatalysis in biomedicine and represents a first step towards finding a future solution to the problem that microplastics in the bloodstream may pose.

Factorial Analysis and Thermal Kinetics of Chemical Recycling of Poly(ethylene terephthalate) Aided by Neoteric Imidazolium-Based Ionic Liquids

Poly(ethylene terephthalate) (PET) waste accumulation poses significant environmental challenges due to its persistent nature and current management limitations. This study explores the effectiveness of imidazolium-based neoteric solvents [Emim][OAc] and [Bmim][OAc] as catalytic co-solvents in the glycolysis of PET with ethylene glycol (EG). Reaction thermal kinetics showed that both ionic liquids (ILs) significantly enhanced the depolymerization rate of PET compared to traditional methods. The use of [Emim][OAc] offered a lower activation energy of 88.69 kJ·mol-1, thus making the process more energy-efficient. The contribution of key process parameters, including temperature (T), plastic-to-ionic liquid (P/IL) mass ratio, and plastic-to-solvent (P/S) mass ratio, were evaluated by means of a factorial analysis and optimized to achieve the maximum PET conversion for both neoteric solvents. The relevance sequence for both ionic liquids involved the linear factors T and P/S, followed by the interaction factors T×P/S and T×P/IL, with P/IL being the less significant parameter. The optimal conditions, with a predicted conversion of 100%, involved a temperature of 190 °C, with a P/IL of 1:1 and a P/S of 1:2.5, regardless of the IL used as the catalytic co-solvent.

Designed to Degrade: Tailoring Polyesters for Circularity

A powerful toolbox is needed to turn the linear plastic economy into circular. Development of materials designed for mechanical recycling, chemical recycling, and/or biodegradation in targeted end-of-life environment are all necessary puzzle pieces in this process. Polyesters, with reversible ester bonds, are already forerunners in plastic circularity: poly(ethylene terephthalate) (PET) is the most recycled plastic material suitable for mechanical and chemical recycling, while common aliphatic polyesters are biodegradable under favorable conditions, such as industrial compost. However, this circular design needs to be further tailored for different end-of-life options to enable chemical recycling under greener conditions and/or rapid enough biodegradation even under less favorable environmental conditions. Here, we discuss molecular design of the polyester chain targeting enhancement of circularity by incorporation of more easily hydrolyzable ester bonds, additional dynamic bonds, or degradation catalyzing functional groups as part of the polyester chain. The utilization of polyester circularity to design replacement materials for current volume plastics is also reviewed as well as embedment of green catalysts, such as enzymes in biodegradable polyester matrices to facilitate the degradation process.

Bottlenecks in biobased approaches to plastic degradation

Plastic waste is an environmental challenge, but also presents a biotechnological opportunity as a unique carbon substrate. With modern biotechnological tools, it is possible to enable both recycling and upcycling. To realize a plastics bioeconomy, significant intrinsic barriers must be overcome using a combination of enzyme, strain, and process engineering. This article highlights advances, challenges, and opportunities for a variety of common plastics.

Computational redesign of a hydrolase for nearly complete PET depolymerization at industrially relevant high-solids loading

Biotechnological plastic recycling has emerged as a suitable option for addressing the pollution crisis. A major breakthrough in the biodegradation of poly(ethylene terephthalate) (PET) is achieved by using a LCC variant, which permits 90% conversion at an industrial level. Despite the achievements, its applications have been hampered by the remaining 10% of nonbiodegradable PET. Herein, we address current challenges by employing a computational strategy to engineer a hydrolase from the bacterium HR29. The redesigned variant, TurboPETase, outperforms other well-known PET hydrolases. Nearly complete depolymerization is accomplished in 8 h at a solids loading of 200 g kg-1. Kinetic and structural analysis suggest that the improved performance may be attributed to a more flexible PET-binding groove that facilitates the targeting of more specific attack sites. Collectively, our results constitute a significant advance in understanding and engineering of industrially applicable polyester hydrolases, and provide guidance for further efforts on other polymer types.