Microplastic leachates in farmland: impact of acid rain on DOM characteristics and metal release†

Microplastics (MPs) in farmland soil may leach dissolved organic matter (DOM) and metal-based additives during rainfall and irrigation processes, potentially impacting agroecosystems. This study investigated the leaching characteristics of MPs commonly found in agricultural soils and irrigation water under varying acid rain conditions. The MP leachates were analyzed for their physicochemical properties, three-dimensional fluorescence characteristics, and heavy metal release. The results revealed that most MP leachates exhibited neutral to alkaline pH, likely due to the dissolution of inorganic fillers such as CaCO₃. Dissolved organic carbon (DOC) leaching varied by polymer types, with biodegradable MPs and PET-based MPs exhibiting significantly higher DOC concentrations than other MPs. Heavy metal analysis identified antimony (Sb) and zinc (Zn) as the dominant leached metals, particularly in Gr-carpet, C-curtain, and G-cover. Evidence shows that Sb concentrations in these MP leachates exceed China's drinking water safety thresholds by 5.76–26.7 times. Additionally, DOC release was pH-dependent, with neutral conditions enhancing organic matter release, whereas acidic conditions may promoted metal leaching. Correlation analysis suggested that Sb and arsenic (As) interacted with MP-derived organic additives and amide/phenol-like substances, indicating potential metal–organic complexation. This study systematically investigates the leaching characteristics of MPs under simulated acid rain conditions, which helps better assess the environmental impact and potential risks of MPs.