Microplastic as a Vector of Metformin: Influence of Aging on Adsorption and Desorption Properties

This study investigated how ultraviolet aging alters the physicochemical properties of polyethylene microplastics and affects the adsorption and desorption of metformin in water. Virgin polyethylene microspheres (PMP) were irradiated for 30 (MP 30 ) and 120 (MP 120 ) days to simulate environmental weathering. Aging caused evident surface cracking, increased specific surface area by up to 77%, and promoted the formation of oxygen‐containing surface functionalities, indicating progressive surface oxidation. These changes also shifted the surface properties of the particles, with lower zeta potential magnitude and more acidic pH at the point of zero charge values (pHZPC). Metformin (MET) adsorption was strongly affected by pH, adsorbent dosage, and temperature. The maximum equilibrium adsorption capacity increased from 3.54 mg g − 1 for virgin polyethylene to 6.78 mg g − 1 after 30 days of aging and 7.79 mg g − 1 after 120 days, corresponding to an increase of about 120% after prolonged aging. Adsorption was rapid in the initial stage, with substantial uptake occurring within the first 5 min. Equilibrium data were better described by the Freundlich model, indicating adsorption on energetically heterogeneous surfaces. Desorption decreased markedly with aging, from 2.670 mg g − 1 for virgin polyethylene to 1.613 mg g − 1 after 30 days and 0.367 mg g − 1 after 120 days, corresponding to an 86% reduction relative to virgin polyethylene. Overall, ultraviolet aging substantially increased metformin retention on polyethylene microplastics, highlighting the greater environmental relevance of weathered particles as persistent sorbent phases in aquatic systems.