Hydrothermal carbonization of PET and PS microplastics under conditions relevant to sewage sludge treatment

Hydrothermal carbonization (HTC) has been proposed as a promising technology for the treatment of sewage sludge containing microplastics; however, the fate and transformation of persistent polymers during the process are not sufficiently understood. This study investigates the hydrothermal transformation of polyethene terephthalate (PET) and polystyrene (PS) microplastics at concentrations representative of sewage sludge processing, using distilled water as the sole reaction medium at 220 °C for 1, 2, and 3 h. A comprehensive multi-analytical approach was employed, including thermogravimetric analysis, scanning electron microscopy, Fourier-transform infrared spectroscopy, and Raman microspectroscopy for solid products. In addition, spectroscopic analysis of process liquid for pH, conductivity, total organic carbon, chemical oxygen demand, and phenol concentration was performed to evaluate the release of soluble organic polymers' degradation products. The results revealed different transformation pathways for PS and PET under hydrothermal conditions relevant to sewage sludge processing. PS retained its aromatic structure, showing moderate band broadening and a stable single-step thermal degradation profile (Tmax ∼415–417 °C) with negligible residue (<1%). In contrast, PET exhibited pronounced ester and aromatic band modifications, reduced thermal stability, a shift in the degradation onset from ∼402 °C to 246–284 °C, multi-step decomposition, and a decrease in the residue from 12.7% to ∼1.3% after hydrothermal conversion. Process water analyses further highlighted polymer-specific behaviour in organic load with increasing HTC temperature.