Recent Advances in Biological Recycling of Polyethylene Terephthalate (PET) Plastic Wastes

Abstract

Polyethylene terephthalate (PET) is one of the most commonly used polyester plastics worldwide but is extremely difficult to be hydrolyzed in a natural environment. PET plastic is an inexpensive, lightweight, and durable material, which can readily be molded into an assortment of products that are used in a broad range of applications. Most PET is used for single-use packaging materials, such as disposable consumer items and packaging. Although PET plastics are a valuable resource in many aspects, the proliferation of plastic products in the last several decades have resulted in a negative environmental footprint. The long-term risk of released PET waste in the environment poses a serious threat to ecosystems, food safety, and even human health in modern society. Recycling is one of the most important actions currently available to reduce these impacts. Current clean-up strategies have attempted to alleviate the adverse impacts of PET pollution but are unable to compete with the increasing quantities of PET waste exposed to the environment. In this review paper, current PET recycling methods to improve life cycle and waste management are discussed, which can be further implemented to reduce plastics pollution and its impacts on health and environment. Compared with conventional mechanical and chemical recycling processes, the biotechnological recycling of PET involves enzymatic degradation of the waste PET and the followed bioconversion of degraded PET monomers into value-added chemicals. This approach creates a circular PET economy by recycling waste PET or upcycling it into more valuable products with minimal environmental footprint.

Keywords: PET; PET hydrolase; bioconversion; biodegradation; cutinase; plastic recycling.

Figure 1

Global plastic production. The most plastic is destined for single use packaging. More than 240 million tons of plastic waste are generated every year. About 40% of plastic waste has accumulated in landfills and 25% has been incinerated [7,8].

Figure 2

Plastic waste recycling and recovery routes [6,45,46].

Figure 3

Physical, chemical, and biological approaches for PET recycling [2,54,55].

Figure 4

Mechanism of microbial degradation of plastics.

Figure 5

Microbial degradation of PET. PETase, PET hydrolase; MHETase, MHET hydrolase; BHET, bis(2-hydroxyethyl) terephthalic acid; MHET, mono(2-hydroxyethyl) terephthalic acid; TPA, terephthalic acid; EG, ethylene glycol; PCA, protocatechuic acid.

Figure 6

PET metabolic pathway by Ideonella sakaiensis [128].