Transformation characteristics of microplastics derived from sludge and their Cd(II) adsorption behavior during bioleaching

Although sludge bioleaching has been widely applied for dewatering and heavy metal removal, its effects on the occurrence characteristics and property changes of microplastics remain largely unreported. This study aimed to evaluate the effects of bioleaching treatment on the occurrence characteristics, physicochemical properties, and Cd adsorption capacity of microplastics in sludge from two wastewater treatment plants. Microplastic abundance decreased significantly after bioleaching treatment from 1557 to 997 items/L and from 4117 to 1371 items/L in the two plants. The proportion of microplastics sized 50-200 μm decreased from 72.9 % to 47.4 % and from 40.0 % to 34.6 %. Fibrous microplastics remained the dominant form, with polypropylene and polyethylene as the main polymers. Energy dispersive X-ray spectroscopy analysis demonstrated surface aging, with the oxygen content increasing from 2.06 % to 22.14 % and the oxygen-carbon ratio increasing from 0.087 to 0.248. The isolated microplastics demonstrated enhanced Cd adsorption capacity, ranging from 1236 to 1797 μg/g, indicating that bioleaching-induced fragmentation creates additional adsorption sites. Overall, bioleaching alters microplastics occurrence and accelerates aging, thereby enhancing heavy-metal adsorption and offering insights into the environmental risks of sludge-borne MPs.