Weathering Process and Characteristics of Microplastics in Coastal Wetlands: A 24-Month In Situ Study

Coastal wetlands function as critical retention zones for environmental microplastics, potentially accelerating their degradation through unique hydrological conditions. This study conducted a comprehensive 24-month in situ experiment at the Chongming Dongtan National Nature Reserve, examining the weathering processes of five morphologically distinct polyethylene (PE), polypropylene (PP), and polystyrene (PS) microplastics. Quarterly analyses revealed progressive surface deterioration in all microplastics after initial exposure, followed by polymer-specific fragmentation patterns and environmental pollutant adherence. Surface elemental analysis showed rising O/C ratios, with intertidal zones exhibiting higher variance (0.0014-0.0096 vs 0.0006-0.0028 supratidal). Carbonyl index (CI) displayed fluctuating increases, with PS showing the highest CI rise (75.75%/year intertidal vs 61.77%/year supratidal). Systematic comparisons identified three weathering determinants: enhanced intertidal degradation from mechanical-photochemical synergy; spherical particles degrading faster than films via larger surface area; and polymer vulnerabilities dictating PS > PP > PE degradation rates. These findings demonstrate that microplastic weathering in coastal wetlands is collectively governed by hydrological conditions, particle morphology, and polymer composition, providing crucial quantitative parameters for assessing environmental persistence and ecological risks in these sensitive transition ecosystems.