Adsorption-desorption behavior of sulfamethazine on biodegradable polyester microplastics: Effects of sunlight vs. sewage microbial exposure

The co-occurrence of biodegradable microplastics (BMPs) and antibiotics in wastewater poses environmental concern. However, the effects of different environmental ageing processes on antibiotic-BMPs interfacial interactions remain inadequately characterized. This study systematically compared the adsorption-desorption behaviour of sulfamethazine (SM2) onto biodegradable poly(butylene succinate) microplastics (PBS-MPs) subjected to sewage microbial exposure (B-PBS-MPs) and sunlight exposure (P-PBS-MPs). Kinetics of SM2 adsorbed on all MPs conformed to the Elovich model, indicating that the adsorption rates synergistic controlled by liquid-film and intra-particle diffusion. The maximum adsorption capacities predicted by Langmuir model increased from 76.52 μg/g for PBS-MPs to 316.58 μg/g for B-PBS-MPs and 351.49 μg/g for P-PBS-MPs, indicating that sunlight exposure exerted more pronounced adsorption enhancement than microbial exposure. Mechanistic divergences were observed: pristine PBS-MPs exhibited Langmuir-type monolayer adsorption dominated by non-bonding interactions, whereas sunlight-exposed P-PBS-MPs followed Freundlich-type multilayer adsorption driven by hydrogen bonding and van der Waals forces. For microbial-exposed B-PBS-MPs, adsorption obeyed the Langmuir-Freundlich model—micropore filling may dominated at low SM2 concentrations, while complexation of SM2 and biofilm-associated extracellular polymeric substances (EPS) prevailed at higher concentrations. The results of desorption hysteresis indicated the most stable SM2 retention in microbial-exposed MPs via SM2-EPS chemical bonds and the least stable SM2 adsorption in sunlight-exposed MPs due to the weak binding force. These findings demonstrate that ageing differentiates biodegradable MPs into distinct antibiotic vectors: microbial exposure enhanced adsorption capacity and stability, whereas sunlight exposure elevated remobilization risks. This work contributes to the assessment of the compounded environmental risks of BMPs and antibiotics.