Adsorption of Alachlor, Lindane, And Methomyl onto Polystyrene Microplastics: Effects of Aging Treatments

We investigated the adsorption of organic pesticides on environmentally relevant polystyrene (PS) microplastics. In this study, the PS microplastics underwent distinct laboratory aging treatments to stimulate the aged microplastic waste found in natural environments: i) ultraviolet (UV) light exposure-only treatment (UV-aged group), ii) UV light exposure combined with hydrogen peroxide oxidation treatment (UV + H2O2 group), and iii) a control group without artificial aging treatments (nonaged group). Both aging treatments led to surface defects and generated carbonyl and hydroxyl functional groups on the microplastics' surface, increasing the hydrophilicity and decreasing the zeta potential of PS microplastics. Three hazardous organic pesticides—alachlor, lindane, and methomyl—with different physicochemical characteristics were selected as adsorbates. The adsorption experiments revealed diminished capacities for alachlor and lindane on PS microplastics after both aging treatments. The adsorption of methomyl was below the limit of detection across all the PS microplastics groups. Within the same PS microplastics adsorbent group, the adsorption affinity of the three pesticides to the MPs was ranked from highest to lowest as lindane, alachlor, methomyl, corresponding with their octanol/water partition coefficient (log Kow) values, indicating that hydrophobic interactions were the primary adsorption mechanism for the PS microplastics. The better correlation using the Freundlich model implies heterogeneous adsorption sites on the PS microplastics' surface. These findings emphasize that aging processes can increase the hydrophilicity of PS microplastics and highlight the importance of considering the aging effects on PS microplastics when evaluating their potential risks as adsorbents and carriers of organic contaminants in the environment.