Understanding the mechanism of geogenic arsenic mobilization from sediments to groundwater is important for safe and sustainable drinking water supply and water quality management in floodplain aquifers. Laboratory-scale microcosm incubation experiments and seasonal biogeochemical monitoring were conducted to elaborate the effects of microbial sulfate reduction on As mobilization in the shallow aquifer systems from the Jianghan Plain, central Yangtze River Basin. Dissolved As species (including thio-As), Fe(II), dsrB/arrA functional gene abundance, 16S rRNA gene sequences and iron mineral phase transformation (by sequential extraction) were analyzed during the incubation and field monitoring study.
Sulfate reduction promoted arsenic mobilization at the initial stage of microcosm incubation with the increase of dsrB gene and arrA gene abundance, since sulfidogenesis significantly contributed to the mobilization of As via sulfide-induced reduction of both As-bearing Fe(III) oxide minerals and As(V), while a part of the arsenic was converted to soluble thio-arsenates (H2AsS4-). 16S rRNA gene sequences identified as Desulfomicrobium could be responsible for the microbially mediated sulfate reduction process. A decrease of dissolved As and Fe(II) was observed at 14th day after incubation, the results of sequential extraction of sediments indicated the increase of Fe(II) sulfide mineral phases, suggesting the precipitation of Fe(II) sulfides can restrict the build-up of dissolved Fe(II) and sequester As from the solution. Then after transient arsenic removal, As re-released into the solution without corresponding increase of dissolved Fe(II), probably due to the competitive adsorption for the surface sites of iron-sulfide minerals between As and other ions (Fe(II), bicarbonate,etc) and newly formed crystalline iron (hydro)oxides.
Our lab-scale microcosm observation was consistent with the results from our long-term field monitoring. Microbially mediated sulfate reduction could promote seasonal increase of As and Fe(II) through abiotic reduction of iron (hydro)oxides minerals in aquifer sediments by bacterially generated HS-, which was evidenced by corresponding increase between Fe(II) and δ34SSO4 in groundwater. Moreover, Fe(II) sulfide formation could scavenge the dissolved Fe(II), which leads to the decoupled seasonal variation of As and Fe(II) under sulfidogenesis conditions. While sulfate reduction process only resulted in transient and incomplete arsenic immobilization due to competitive adsorption between As and phosphate in groundwater. Our results provide new insights into the coupled As-Fe-S biogeochemical processes accounting for arsenic mobilization and seasonal variation in alluvial aquifer systems.