Adsorption Pathways and Differential Mechanisms of Typical Organic/Inorganic Pollutants on Microplastics: A Case Study of Sulfamethoxazole and Cr(VI) on Aged Polypropylene

• • Aging of PP microplastics increased maximum adsorption capacity for both SMX and Cr(VI) by 2–3 times, with aged PP showing ~30-fold higher adsorption for SMX than Cr(VI), indicating a strong affinity difference that must be considered in risk assessments. • • Mechanistic analysis revealed that SMX adsorption on aged PP is governed by hydrogen bonding and electrostatic interactions (intermolecular forces), whereas Cr(VI) adsorption relies on electrostatic interactions and pore-filling; the stronger intermolecular forces for SMX explain its higher adsorption capacity. • • Adsorption of both pollutants decreased with increasing pH due to electrostatic repulsion; high concentrations of Na+ and Mg2+ enhanced Cr(VI) adsorption via charge shielding but inhibited SMX adsorption by competing for active sites, demonstrating ion-specific effects. • • The presence of humic acid (organic matter) did not significantly affect Cr(VI) adsorption but reduced SMX adsorption, likely due to complexation between SMX and organic matter, highlighting the need to consider natural organic matter in predicting MP-facilitated transport.