Mobilization mechanisms and toxicity risk of sediment trace metals (Cu, Zn, Ni, and Pb) based on diffusive gradients in thin films: A case study in the Xizhi River basin, South China.

Identifying the mobilization mechanisms and predicting the potential toxicity risk of metals in sediment are essential to contamination remediation in river basins. In this study, a sequential extraction procedure and diffusive gradients in thin film (DGT) were employed to investigate the mobilization mechanisms, release characteristics, and potential toxicity of sediment metals (Cu, Zn, Ni, and Pb). Acid-soluble and reducible fractions were the dominant geochemical species of Cu, Zn, Ni, and Pb in sediments, indicating high mobility potentials for these metals under reducing conditions. In summer, the sediment acted as a source of water-column metals due to mineralization of organic matter and reductive dissolution of iron/manganese oxides in surface sediments, and the formation of metal sulfide precipitates markedly lowered DGT-labile metal concentrations with depth, while localized sulfide oxidation was responsible for fluctuating labile metal concentrations. Stable distribution patterns of labile metals resulted from the weak reducing conditions of sediment in winter, when the sediment shifted to a metal sink. The interstitial water criteria toxicity unit (IWCTU), calculated from DGT measurements, indicated no and low-to-moderate toxic risk of sediments in summer and winter seasons, respectively, and Pb was the major contributor to the predicted toxic effects in the soft interstitial water.