Occurrence and Environmental Weathering of Agricultural Microplastics on Beaches across Japan: Insights into Surface Mineralization and Metal Binding

We investigated the coastal occurrence and environmental weathering of microplastics derived from polymer-coated fertilizers (PCFs) across Japan. A field survey along the Japanese coastline revealed that PCF-derived microplastics are widely distributed on beaches throughout the country, indicating that agricultural microplastics have dispersed far beyond agricultural landscapes. Compositional analysis showed that these particles consist predominantly of polyolefins (∼70%) and polyurethanes (∼30%) and commonly exhibit oxidative degradation accompanied by iron enrichment. Bulk elemental analysis demonstrated positive correlations between iron and multiple heavy metals. Micro-X-ray fluorescence mapping using a focused 20-μm beam showed that chromium is heterogeneously distributed on the plastic surface and spatially associated with iron, while X-ray absorption spectroscopy indicated its occurrence in ferrihydrite-like phases or as chromite. Although detailed spectroscopic evidence was obtained from a single selected particle, the widespread occurrence of PCF-derived microplastics and their observed metal enrichment suggest that similar geochemical processes may occur at other coastal sites. By linking nationwide field observations with process-level characterization, this study shows that microplastics have the potential to transition from inert debris to geochemically reactive substrates after coastal deposition.