Influence of sludge treatment methods on behaviors of microplastics adsorbed cadmium and its driving factors

The complicated contamination of microplastics (MPs) and heavy metals in sludge has garnered substantial attention in recent years; however, research on the behavior of MPs loading of heavy metals in sludge after sludge treatment methods is limited. Four representative sludge treatment methods were selected herein: anaerobic digestion, thermal drying, thermal hydrolysis (TH), and aerobic composting. Before and after sludge treatment, the chemical bonding of MPs, cadmium (Cd) adsorption properties, and metabolic changes in the microbial community succession was analyzed, and the factors influencing differences in Cd sorption by sludge MPs were explored. The results revealed that Cd adsorption by MPs occurs as multilayer physical adsorption that can be well fitted by Freundlich isotherms. Compared with the other three treatments, TH led to the most significant effect on the chemical bonding properties of the MPs, with a more than two-fold increase in C-O single bonds and CO double bonds, as well as adsorption of the highest amount of Cd at 767 μg/g. In addition, sludge conductivity and water content also affected Cd sorption capacity, with correlation coefficients of 0.405 and -0.384. Pedobacter, Flavobacterium, Lysobacter, and Sphingobacterium in the sludge presented a high degree of coupling with adsorption capacity, it was inferred that the above dominant species of bacteria may affect the adsorption of Cd by microplastics through the production of extracellular enzyme forms.