Identification of key As mobilization processes in estuarine sediments is challenging due to the transitional hydrodynamic condition and the technical restriction of obtaining fine-scale results. Herein, high-resolution (μm to mm) and in situ profiling of As with associated elements (Fe, Mn, and S) by the diffusive gradients in thin-film (DGT) technique were applied and coupled with pore water and solid phase analysis as well as microbial high-throughput sequencing, to ascertain the driving mechanisms of As mobilization in the sediments of Pearl River Estuary (PRE). Significant diffusion fluxes of As from sediment to water were observed, particularly in the upper estuary. With the seaward increase of salinity, the driving mechanism of As mobilization gradually shifted from microbial-induced dissimilatory Fe reduction to saltwater-induced ion exchange. Correspondingly, the dominant Fe-reducing bacteria (FeRB) in sediments changed from the genera Clostridium_sensu_stricto_1 and Bacillus to Ferrimonas and Deferribacter. The presence of dissolved sulfide in deeper sediments contributes to As removal through the formation of As-S precipitates as supported by theoretical calculations. Fine-scale findings revealed seaward changes of As mobilization mechanism in the sediments of a human-impacted estuary and may benefit the understanding of As biogeochemical behavior in estuaries worldwide.
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