Polymer-specific transformation of microplastics under soil freeze–thaw versus UV aging: Multiscale insights into atrazine interaction mechanisms

Microplastics (MPs) and herbicides like atrazine (ATZ) are widely detected in cold-region agricultural soils, raising environmental concerns. However, the effects of freeze-thaw cycles (FTCs), a typical climatic stress, on the modification of the surface properties of MPs and the subsequent interactions between FTC-aged (FT-) MPs and coexisting ATZ remain unclear. Here, Polystyrene (PS) and polyethylene (PE) MPs prevalent in cold regions were aged via soil FTCs, with UV aging as a reference, to assess physicochemical alterations and ATZ interactions using experiments and simulations. FT-PS exhibited marked surface oxidation (O/C ratio increased 3.2-fold) and structural degradation (specific surface area increased 1.8-fold), resulting in the highest ATZ adsorption capacity (441.920 μg/g), which was 120% and 109% greater than that of unaged and UV-aged PS, respectively. In contrast, PE exhibited minimal physicochemical changes under either aging condition, and its adsorption was mainly governed by hydrophobic interactions. Density functional theory and molecular dynamics simulations revealed that FTCs introduced oxygen-containing groups and surface defects on PS, promoting hydrogen bonding and stronger van der Waals interactions with ATZ. These results highlight the overlooked role of FTCs in shaping MP-pollutant interactions and call for greater attention to co-contamination risks in the agricultural ecosystems of cold regions.