Adsorption Pathways and Differential Mechanisms of Typical Organic/Inorganic Pollutants on Microplastics: A Case Study of Sulfamethoxazole and Cr(VI) on Aged Polypropylene
Microplastics (MPs) act as vectors for co-migrating antibiotics and heavy metals, forming complex pollution systems with potential joint toxicity. However, the differential adsorption behaviors and underlying mechanisms of MPs toward organic versus inorganic pollutants remain insufficiently understood. This study selected polypropylene (PP) microplastics, a major component of agricultural plastic films, and investigated the adsorption of sulfamethoxazole (SMX) and Cr(VI) onto aged PP under varying environmental conditions. Results demonstrated that aging increased the maximum adsorption capacity by 2–3 times for both pollutants. Notably, aged PP exhibited approximately 30 times higher adsorption capacity for SMX than for Cr(VI). Characterization revealed that aging introduced oxygen-containing functional groups (e.g., carbonyl) on the MP surface, enhancing adsorption. Mechanistic analysis indicated that hydrogen bonding and electrostatic interactions dominated SMX adsorption, while Cr(VI) adsorption was primarily governed by electrostatic interactions and pore-filling. The stronger intermolecular forces for SMX compared to reversible pore-filling for Cr(VI) explained the observed differences. Increasing pH induced electrostatic repulsion, reducing adsorption of both pollutants. High concentrations of Na+ and Mg2+ caused charge shielding, potentially enhancing Cr(VI) adsorption but inhibiting SMX adsorption due to competition for active sites. The presence of organic matter (humic acid) had negligible effects on Cr(VI) adsorption but reduced SMX adsorption, likely due to complexation. These findings elucidate distinct molecular-level pathways for organic versus inorganic pollutant adsorption on aged MPs, highlighting the roles of hydrogen bonding and pore-filling in driving differential behaviors.