The enzymatic degradation of post-consumer plastic waste is emerging as a novel and environmental benign and sustainable alternative to conventional recycling processes. Recently, it has been shown that synthetic polyesters such as PET can be completely hydrolysed by microbial enzymes at mild reaction conditions in aqueous media within short reaction times. The resulting monomers can be recovered and reused. By the generation of more powerful biocatalysts employing protein engineering techniques and an optimization of the bioprocess parameters, biocatalytic recycling of PET can be further developed towards an industrial application.
Only a small percentage of plastic waste is currently collected for a closed-loop recycling into the same value applications [01]. Novel and innovative recycling technologies are required to cope with the environmental impact created by ever increasing amounts of plastic waste produced globally and to reduce the consumption of non-renewable feedstock required for plastic manufacture.
In contrast to conventional energy-consuming chemical recycling processes, green chemistry is developing environmental benign products and processes making use of renewable resources instead of fossil-based chemicals to ensure a sustainable economy. Industrial enzymes are proteins mainly produced by bacteria or fungi and can be used as biocatalysts in green chemistry processes. With their defined three-dimensional structures, they accelerate chemical reactions with a high specificity. In contrast to chemical catalysts, enzymes work at mild reaction conditions. Their employment as catalysts typically results in energy-efficient processes performing at ambient temperatures and pressure with lower chemical consumption and less or no formation of toxic by-products.
Plastic waste composed of polyethylene terephthalate (PET) is very resistant against biological degradation. Its accumulation in the environment, especially in marine ecosystems, is of growing concern [02].
A number of microbial biocatalysts with activity against the ester bonds in the PET polymer have already been identified [03, 04]. These polyester hydrolases were isolated from the environment or have been further developed by protein engineering techniques. Some of them have already been successfully employed for the functionalization of polyester films and fibres by a partial hydrolysis of their surface with applications in the textile and electronic industries.
Recently, highly active polyester hydrolases have been obtained able to completely convert low crystalline PET films to terephthalic acid and ethylene glycol at 70°C within a few hours of reaction time. These monomers could be recovered and used to produce virgin PET in a novel closed-loop recycling process.
Fig. 1: Surface of a polyester hydrolase with a PET model compound docked within the substrate groove. Green, carbon atoms; red, oxygen atoms; green and pink surface, lipophilic and hydrophilic surface patches, respectively. The enzyme catalyses the hydrolysis of ester bonds in the PET polymer (R. Wei, Leipzig University).
The terephthalic acid produced by biocatalysis from PET materials could also find alternative applications for the synthesis of products with high purity requirements and significantly added value such as metal-organic frameworks.
Furthermore, a biocatalytic process for the removal of PET layers in composite materials could be of interest for the recycling of PET-PE or metallized PET films and barrier foils. Furthermore, applications of polyester hydrolases for the recycling of blended fabrics containing PET could also be envisaged.
Fig. 2: Scanning electron microscopy images of low crystalline PET films degraded by a polyester hydrolase. (a) Untreated PET, (b) with 10.4% weight loss, (c) with 29.2% weight loss, and (d) with 97.5% weight loss. Arrows indicate holes in the PET film (R. Wei, Leipzig University).
PET materials of high crystallinity or biaxial orientation are presently still difficult to hydrolyse completely by biocatalysis within short reaction times. The construction of more powerful enzymes by protein engineering and the development of suitable pre-treatment methods may remove these limitations.
Fig. 3: Hydrolysis of low crystalline PET in an enzyme bioreactor. The monomers can be separated and reused (Barth M et al., J Mem Sci 494, 182-187, 2015).
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