Study on Adsorption Mechanism of Polypropylene Microplastics for Lead Ion Under Synergistic Effects of Multiple Aging and Biofilms

Polypropylene (PP) microplastics are one of the most common types of plastics in the environment, which are widely derived from disposable packaging materials, daily necessities, and medical protective equipment, etc. They undergo aging due to various external forces such as light exposure, temperature changes, and hydrodynamic disturbances, which in turn affects their interfacial adsorption of other pollutants. In this study, medical masks were selected as the experimental PP materials, and typical aging processes were simulated through freeze-thaw cycles, artificial wave action, and ultraviolet irradiation. Natural seawater was used to cultivate biofilm growth on the microplastic surface. The adsorption characteristics and mechanisms of Pb(Ⅱ) on PP microplastics under different aging modes and biofilm loading were systematically evaluated. The results showed that all three aging methods promoted the formation of oxygen-containing functional groups and increased the number of active sites on the microplastic surface. Among them, UV aging and wave-induced abrasion with mixed sand produced the highest degree of oxidation and the most significant enhancement in Pb(Ⅱ) adsorption capacity. Biofilm loading further altered the surface energy-level structure and types of adsorption sites, markedly strengthening the binding capacity for Pb(Ⅱ). Kinetic analysis indicated that Pb(Ⅱ) adsorption followed a pseudo-second-order model dominated by chemisorption. Statistical physics modeling revealed that biofilm introduction transformed the adsorption system from a single-energy-site to a dual-energy-site configuration and induced evident multi-ion aggregation behavior. This study demonstrated a synergistic effect between aging-induced surface oxidation and biofilm-driven functional group proliferation, jointly regulating the interfacial adsorption process of Pb(Ⅱ) on microplastics.