Fig. 1: Biological management and valorisation of poly(ethylene terephthalate) (PET) plastic waste.

PET plastic waste in the environment undergoes fragmentation and weathering, generating smaller particles and microplastics. PET is shown at the polymer-chain level in both crystalline and amorphous states, and crystalline PET can undergo physical or chemical pre-treatment to increase amorphous content and enhance enzymatic accessibility. Purified PET-depolymerising enzymes, or enzymes secreted during whole-cell biocatalysis, cleave the ester bonds linking the monomers, yielding terephthalic acid (TA) and ethylene glycol (EG). These monomers can be purified and repolymerised to produce recycled PET (rPET) with properties comparable to virgin PET, enabling closed-loop recycling. Alternatively, TA and EG can be biotransformed by engineered microorganisms into value-added compounds, including vanillin, violacein, paracetamol, rhamnolipids, adipic acid and hydroxyalkanoyloxyalkanoates (HAAs), or converted via chemical upgrading routes into precursors for biopolymers such as polyhydroxyalkanoates (PHAs) and other useful materials, including bio-based polyurethane (Bio-PU) derived from HAAs, supporting open-loop upcycling pathways. Note that several value-added products shown represent niche, high-value outlets and do not constitute PET-scale end markets; in most scenarios, most recovered monomers would return to polymer production, with only a fraction diverted to higher-margin chemicals.