Tandem chemical hydrolysis and bioelectrochemical upcycling of waste polyethylene terephthalate (PET) for sustainable biobutanol and ethanol production ensuring plastics circularity†

Abstract

To establish a sustainable plastic system, it is crucial to implement effective recycling and upcycling strategies that circulate the materials within the market and prevent them from entering the ecosystems. Polyethylene terephthalate (PET), which is the most widely used fossil-derived synthetic polyester, is usually disposed as waste. Development of novel chemical upcycling technologies that can transform plastic wastes into economically viable chemicals is crucial to establish a circular plastics economy. This study delineated a methodology to combine mild chemical pretreatment and biocatalysts via bio-electrofermentation for the conversion of waste PET to sustainable biofuel blendstocks. Initially, PET was depolymerised to its monomers using an alkali catalyst (>98% conversion efficiency), and their structural characteristics were confirmed using FT-IR, NMR (¹H and ¹³C), TGA, FESEM, XRD and XPS techniques. Furthermore, co-culturing with Klebsiella sp. and Clostridium sp. showed positive result towards TPA degradation (55%–74%) with various applied poised potentials to yield high-value bio-fuels. The electrochemical analysis highlighted the role of applied potential in the bioelectrochemical system (BES), where the +0.8 V condition consistently demonstrated a better performance across all metrics, including electron flux, substrate conversion, and product yield. The maximum yield were found to be 0.31 g L⁻¹  for butanol and 0.23 g L⁻¹ for ethanol at +0.8 V in the BES. On the other hand, the life cycle assessment (LCA) methodology was employed to understand the environmental footprints of the studied upcycling process, and it showed a global warming potential of 1.13 ton CO₂ eq. per ton biofuel. While, recycling PET accounted for 1.96 ton CO₂ eq. and 3.06 ton CO₂ eq. from the ideal PET production process. Alternatively, other chemical and enzymatic PET upcycling processes had 3–7 times higher impacts. Therefore, the present work paves a new way for the upcycling of PET and makes a significant contribution to the development of a circular plastics economy.