Aging of polylactic acid microplastics during hydrothermal treatment of sewage sludge and its effects on heavy metals adsorption

Microplastics' (MPs) aging process and environmental behavior have attracted extensive attention due to the potential long-term ecological impact. MPs enriched in sludge may accelerate aging during sludge treatment and the affecting environmental behavior, i.e., adsorption performance for pollutants. However, the related studies have not been well researched, especially for the biodegradable MPs. This study revealed the influences of hydrothermal treatment on the characteristics of polylactic acid microplastics (PLA-MPs) and the consequences on heavy metals adsorption. The changes in PLA-MPs' physiochemical properties were characterized and compared. PLA-MPs' surface became irregular, and the oxygen-containing functional groups increased through FTIR and XPS analysis. Meanwhile, the molecular weight and crystallinity of PLA-MPs decreased significantly with the rising in hydrothermal temperature. Accordingly, the adsorption capacity of PLA-MPs for Pb2+ increased from 93.97 μg g-1 for the raw PLA-MPs to 1058.03 μg g-1 for the aged PLA-MPs. Multiple adsorption kinetics and isotherms were discussed for the Pb2+ adsorption onto PLA-MPs with different aging of the PLA-MPs. The adsorption mechanisms of Pb2+ relate to electrostatic interaction and complexation. The main difference is that the adsorption for raw PLA-MPs is dominated by physical and chemical adsorption, whereas the adsorption for the aged PLA-MPs prefers chemical adsorption. In addition, we carefully evaluated the influences of pH, dissolved organic matter, and ionic strength on the PLA-MPs adsorption. The present study highlighted the significance of hydrothermal treatment on the MPs aging and the adsorption performance.