Solar-Powered Hydrothermal Recycling of Polyethylene Terephthalate Waste to Terephthalic Acid: Process Performance and Life Cycle Assessment

Decarbonizing the process heat required for chemical recycling would improve the environmental performance of plastic-waste valorization. This study presents an evaluation of the use of concentrated solar thermal energy as the reaction heat source for the hydrothermal depolymerization of post-consumer polyethylene terephthalate (PET). The solar-driven hydrothermal process (HTP-S) maintained an internal reactor temperature of approximately 200 °C for 4 h under favorable irradiance conditions. During the single experimental run, the system received 14.95 MJ of incident solar energy, whereas the conventional hydrothermal process (HTP-C) consumed 47.52 MJ of electricity per run. Starting from 1.2 g of PET, HTP-C and HTP-S produced 0.862 and 0.895 g of dry recovered solid, corresponding to recovered-solid yields of 71.7% and 74.5%, respectively. Fourier-transform infrared (FT-IR) spectroscopy, thermogravimetric analysis and derivative thermogravimetry (TGA/DTG), transmission electron microscopy (TEM), and X-ray diffraction (XRD) showed that the recovered solids exhibit physicochemical characteristics consistent with the formation of a crystalline terephthalic acid (TPA)-rich product. A cradle-to-gate life cycle assessment normalized to 1 kg of treated PET showed that cumulative energy demand decreased from 44,383.7 MJ for HTP-C to 7342.5 MJ for HTP-S, while global warming impacts decreased from 9717 to 1607 kg CO2-eq. These results demonstrate the technical feasibility of coupling concentrated solar heating with hydrothermal PET depolymerization and identify reaction heating as the principal opportunity for reducing external electricity demand. These outcomes indicate that solar-powered hydrothermal processing provides a feasible pathway for PET recycling, reducing the environmental footprint.