Adsorption and desorption behaviors of polyamide 66 microplastics toward methyl orange and ciprofloxacin: Opposing adsorption changes induced by UV aging

As microplastics (MPs) have received growing attention as vectors of environmental contaminants, their aging processes and the associated changes in adsorption and desorption behavior still require further investigation. In this work, we systematically examined how UV weathering influences the adsorption and release of methyl orange (MO) and ciprofloxacin (CIP) on polyamide 66 (PA66) MPs. The results showed that UV exposure caused chain cleavage and surface oxidation of PA66, leading to a more negatively charged surface, more oxygen-containing functionalities, and an enlarged specific surface area. Adsorption experiments demonstrated that the adsorption of MO and CIP on both virgin and aged PA66 followed the Langmuir model, and the processes were spontaneous and exothermic. After aging, the adsorption capacity for MO decreased significantly from 3.41 to 0.52 mg/g, while that for CIP increased from 1.37 to 1.97 mg/g. This contrasting behavior was attributed to enhanced electrostatic repulsion between aged PA66 and the anionic form of MO, inhibiting hydrogen bonding (HB), whereas new hydroxyl groups on aged PA66 promoted additional HB with the zwitterionic form of CIP. Further adsorption and desorption experiments under varying pH and salinity conditions showed that increased pH and salinity weakened HB, enhanced electrostatic repulsion, and promoted competition for active sites. Desorption experiments in simulated gastrointestinal fluids indicated significantly higher release rates of MO and CIP than in freshwater, especially under warm-blooded conditions. This work elucidates how pollutants behave on PA66 before and after aging, providing new perspectives on the environmental migration and transformation of MPs carrying organic contaminants.