Dynamics of arsenic and its interaction with Fe and S at the sediment-water interface of the seasonal hypoxic Changjiang Estuary.

The dynamics of As at the sediment-water interface (SWI) is an important link in driving As release into the upper water column to affect biological activities. At present, it is unclear about the As cycling at the SWI, especially under the influence of seasonal hypoxia in the Changjiang Estuary. In this study, the profile variations of As in porewater and solid phases from the Changjiang Estuary were studied. Results suggested that As profiles presented the "removal-release-removal" pattern with depth. Arsenic profiles were typically characterized by subsurface maxima in porewater and surficial enrichment in solid phase, which were closely related to Fe and S cycling. The occurrence of porewater As peaks and its obvious speciation transition at subsurface layers, companied by the conversion of dominant phases of solid As from reductive to oxidative fractions at the same layers indicated the reductive release of As at redoxcline. Arsenic species, saturation indexes (SI) and net reaction rates (R) suggested that the removal of As at surface and deep layers were attributed to the adsorption of As(V) by Fe oxides and the formation of orpiment and/or adsorption by Fe sulfides, respectively. The benthic hypoxic environment in late-summer was more favorable for upward migration and diffusion of As. This process was considered as an important contribution to the elevated aquatic As in the Changjiang Estuary.