Contrasting effects of microplastic aging upon the adsorption of sulfonamides and its mechanism

The aging behavior of microplastics (MPs) is unavoidable in the natural environment, but the mechanisms underlying the differences in MP aging affecting the adsorption performance of different MPs remain unclear. In this work, the adsorption characteristics of sulfonamides (SAs) on virgin MPs (thermoplastic polyurethanes (TPUs), polyamides (PAs)) and aged MPs were determined under simulated conditions (ultraviolet (UV), ultraviolet combined hydrogen peroxide (UV + H2O2)). Contrasting effects of MP aging on SA adsorption were observed. That is, the adsorption capacities of aged PAs were decreased from 14.45% to 58.53%, whereas those of aged TPUs were increased from 13.93% to 171.30%. To interpret this interesting phenomenon, the structural characteristics of the tested MPs were determined, and the results indicated that the surface structures of MPs changed with aging, which resulted in differences in hydrophobicity and surface charge and a significant difference in the concentrations of oxygen-containing functional groups (P < 0.05). Furthermore, density functional theory (DFT) results revealed that the equilibrium configurations and the interactions (H bond, electrostatic effect, van der Waals (vdW) forces were significantly influenced by the change in the concentration of oxygen-containing functional groups of MPs (P < 0.05), with the strength of H bonds increasing for aged TPUs-SAs and the strength of electrostatic interactions decreasing for aged PA-SAs. Our findings are the first to provide a theoretical basis for an improved understanding of the role of aging in SAs adsorption onto MPs.