The mechanism of polystyrene microplastics to affect arsenic volatilization in arsenic-contaminated paddy soils

Different concentrations and sizes of polystyrene microplastics (PS-MP) were added to low (25.9) and high (56.8) mg kg^-1 As-contaminated soil to investigate the effects of PS-MP on soil As volatilization. Either S₁ (10 μm) or S₂ (0.1-1 μm) PS-MP was added to As-contaminated soil at 0.8%, which increased As volatilization by 13.7% and 7.4% in low As-contaminated soil; and 21.8% and 16.5% in high As-contaminated soil, respectively. The addition of PS-MP reduced the water-soluble (WS) As content, increased the non-specifically-sorbed (NSS), specifically-sorbed (SS) As content, soil catalase (CAT) and urease (UE) activities. The abundance of Proteobacteria and Firmicutes showed opposite trends to As volatilization, while the abundance of Bacteroidetes and arsM gene expression exhibited similar variability to As volatilization over the 7-week experiment. Therefore, we postulate that in As-contaminated soil, As volatilization was enhanced in the presence of PS-MP due to two possible mechanisms: 1) PS-MP affects the abundance of Proteobacteria, Firmicutes, Bacteroidetes, and arsM gene in soil; 2) PS-MP increases As volatilization via reducing soil nutrient and increasing the content of SS As. Our results highlighted the importance of investigating impacts of microplastics on the volatility of specific contaminants to implement effective environmental remediation strategies in polluted farmlands.