Different Weathering Strategies Result in Different Surface Properties and Adsorption Behavior in Aged Microplastics

Several approaches have been used to reproduce the environmental aging of microplastics (MPs) in the laboratory; however, because they are based on different oxidizing species, they may lead to MPs with distinct surface properties and environmental behavior. This study compares the impact of ultraviolet (UV), ozone, and activated persulfate aging on the surface chemistry and contaminant adsorption of high-density polyethylene MPs aged to identical carbonyl index (CI) values. Surface characterization of aged MPs revealed crucial structural differences between the aging protocols: at the same CI of 0.1, the surface-oxidized carbon content was higher for persulfate-aged MPs (2.74%) than for UV (2.18%) or ozone (2.23%). Persulfate aging also resulted in higher specific surface area, going from 0.22 m²/g (pristine MPs) to 0.79 m²/g at a CI of 0.1, compared with 0.67 and 0.56 m²/g for ozone and UV aging. Isotherm experiments with phenanthrene showed a decrease in adsorption with surface oxidation, while, for 9-phenanthrol, adsorption was higher for UV- and ozone-aged MPs than pristine MPs and declined for the highly oxidized persulfate-aged MPs. These results demonstrate that different oxidation strategies produce structurally and functionally distinct MPs, highlighting the importance of the aging protocol in laboratory studies.