Hydrothermal pretreatment reduced microplastics in sewage sludge as revealed by the combined micro-Fourier transform infrared (FTIR) and Raman imaging analysis

The massive amount of sludge was considered one of the significant sinks of microplastics (MPs). Low-temperature hydrothermal treatment applied as a pretreatment process (e.g., 170 degrees C & 30 min) of sludge received growing attention since it could improve the dewaterability and increase the methane yield of sludge. However, the changes of sludge-based MPs during the hydrothermal pretreatment (HTP) were not studied. The present study combined micro-Fourier transform infrared and Raman imaging to explore the removal effect of sludge-based MPs (minimum test-limitation is 1 um) via the HTP process. When the conditions of HTP changed from 170 degrees C & 30 min to 220 degrees C & 30 min, the removal rate of prominent MPs (with size >50 mu m) increased from 11.5 +/- 6.1% to 85.01 +/- 5.2% and the removal rate of tiny MPs (with size <50 mu m) increased from 50.21 +/- 5.6% to 87.35 +/- 1.11%, indicating that MPs' degradation was strongly associated with the treatment temperature and particle size. Physiochemical property changes (morphology, functional groups, surface elements) and dissolved organic carbon (DOC) releasing of three HTP-treated typical MPs were further characterized. The results proved that sludge-based MPs were degraded in two stages via HTP, including particle cracking by harsher conditions and polymer backbone scissions by the dihydroxylation and carbonylation effects.