Microplastic Degradation in Sewage Sludge by Hydrothermal Carbonization: Efficiency and Mechanisms

Sewage sludge is an important vehicle for the diffusion of microplastics (MPs) into the environment, and thus, efficient removal of MPs from sludge is in urgent need. In this study, hydrothermal carbonization (HTC) is proposed and its potential for the removal of MPs from sewage sludge is assessed. Optical microscopy and micro-FTIR analysis showed that the concentrations of MPs in sewage sludge decreased significantly, exhibiting a 79% reduction with a HTC temperature of 260 °C. The potential decomposition mechanism of condensation polymers and addition polymers were investigated through HTC experiments, using polyethylene terephthalate-microplastics (PET-MPs) and polypropylene-microplastics (PP-MPs). During the HTC process, the disintegration efficiency of PET-MPs was significantly higher than PP-MPs, due to the PET ester bond being easily monomerized by hydrolysis. Furthermore, analyses of physicochemical properties of the residual PP-MPs indicated that exposure to heat cause PP to undergo pyrolysis reaction, resulting in the random rupture of polymer molecular chains. Overall, these results provide the first insight into the critical role of HTC in the removal of MPs from sewage sludge, providing a novel solution for reducing the risk posed by MPs in sewage sludge in the future.