Laser direct infrared (LDIR) spectroscopy reveals microplastic sorting and risk evolution in a subtropical...

Microplastic (MP) pollution threatens subtropical estuarine wetlands, critical transition zones along the river-estuary-coastal continuum. While these ecosystems serve as sinks and secondary sources of MPs, distribution and burial patterns under varying sedimentary conditions remain poorly understood. Here, this study used high-resolution laser direct infrared (LDIR) imaging to systematically quantify MPs (10-500 μm) in sediment cores from the Zhangjiang Estuary, a subtropical mountain river-estuary system in China. Results show that the average MP abundance ranged from 10,463 to 399,481 items/kg, with corresponding surface areas of 0.61-7.39 cm²/kg. MP concentrations decreased downstream, with enrichment in the surface layer (0-20 cm) and accumulation at depth (up to 80 cm), indicating potential historical deposition based on sedimentation rates. Polymer assemblages, dominated by polyethylene (PE), polyamide (PA), polyurethane (PU), polyethylene terephthalate (PET), and rubber, reflected agricultural, industrial, and domestic sources. MP abundance correlated positively with silt and organic matter content and negatively with salinity and pH, suggesting that fine, organic-rich sediments enhance MP retention, while water-salt dynamics influence vertical long-term burial and downstream transport. Critically, salinity and sediment texture were associated with variations in MP size and polymer composition, which may affect site-specific environmental risk. Estuary exhibited elevated risk due to higher proportions of hazardous polymers such as polyvinyl chloride (PVC) and acrylonitrile butadiene (AB). These findings indicate that MP size and polymer type are important factors for assessing estuarine MP risk, with implications for management across estuary systems.