Effects of manganese, iron and sulfur geochemistry on arsenic migration in the estuarine sediment of a small river in Xiamen, Southeast China

The geochemistry of iron (Fe), manganese (Mn) and sulfur (S) and their effects on arsenic (As) mobility in the mudflats of small river estuaries remain unclear. Here, diffusive gradient in thin films (DGT) and high-resolution dialysis (HR-Peeper) techniques combined with a sequential extraction procedure (BCR) were employed to investigate As, Fe, Mn and S geochemistry in the mudflat of the Jiuxi River estuary, Southeast China. Grain size analysis indicated that fine-grained particles were likely to be deposited in the estuarine intertidal zone and coastal area. DGT and HR-Peeper results revealed that in the estuary and coastal area, the dissolved As in sediment in summer was controlled by Mn geochemistry, which includes not only the release of As through Mn/Fe reduction but also the stabilization of dissolved As in pore water. This stabilization of dissolved As may due to the formation of As–Mn-OM complexes. In winter, the significant positive correlations between DGT-Fe, DGT-Mn, DGT-As and DGT-S indicated that sulfate reduction was the start of As mobilization in sediment in winter. In both the estuary and the coastal area, the easily reducible Fe, Mn and As contents in intertidal sediment were higher than those in the subtidal zone. Combined with the As flux across the sediment-overlying water interface (SWI), these phenomena suggested that As in subtidal sediment diffused into overlying water and that As in overlying water tended to accumulate in the intertidal sediment. The total organic carbon content (TOC) and DGT results in the lower reach, estuary and coastal areas indicated that organic matter is the controlling factor of Fe/Mn reduction, sulfate reduction and As mobilization. The BCR test results showed higher reactive fraction contents of Fe, Mn and As in winter sediment, which threaten the overlying water quality.