Weathering effect triggers the sorption enhancement of microplastics against oxybenzone

Microplastics are a growing concern due to their potential negative environmental and human health impacts. This study investigated the physiochemical changes in polyethylene terephthalate (PET) microplastics upon aging in air, seawater, and freshwater through exposure to Xenon light (artificial aging) and sun (natural aging). We used SEM to examine the surface morphology of natural and aged PET microplastics and found that aged particles exhibited mechanical grinding, irregular wrinkles, cracks, and a small subset of mesopores. FTIR analysis revealed that microplastics were degraded through an oxidation process, resulting in the appearance of carboxyl and hydroxyl groups. BET analysis showed that freshwater-aged microplastics had the highest specific surface area (S_BET) value. We also studied the sorption behavior of oxybenzone, a UV filter compound, onto microplastic particles. Our results indicated that the sorption behavior followed the pseudo-second-order model, with hydrophobic interactions and hydrogen bonding formation as the main mechanisms. The equilibrium adsorption values for aged microplastics increased from 6.62 to 9.10 mg/g for air, seawater, and freshwater, respectively, compared to 6.44 mg/g for virgin microplastics. The sorption isotherm experiments showed that the Langmuir model could represent the sorption mechanism of the virgin materials, while air- and seawater-aged microplastics were more accurately described using the Freundlich isotherm model. The initial concentrations of oxybenzone in the background solutions exhibited an increasing trend with increasing concentrations, and pH values showed different effects on microplastic sorption capacity depending on the age of the microplastics and the solution pH. Our results suggest that microplastics exhibit an increasing trend in enhanced sorption capacity from virgin to aged conditions, with uneven distribution of active groups on each surface area due to uneven crack formation. Further research is needed to explore desorption experiments using aged microplastics from natural resources.