Contrasting kinetics of arsenic release from As-bearing ferrihydrite coprecipitates by distinct sulfate-reducing bacteria

Microbial sulfate reduction plays key roles in arsenic (As) biotransformation and release in groundwater aquifers; however, influences by distinct sulfate-reducing strains on the transformation of As-bearing iron (oxyhydr)oxide under different sulfate availability and associated As mobilization are not clear. In this study, anaerobic batch experiments were conducted to clarify the transformation processes of As-bearing ferrihydrite mediated by two distinct sulfate-reducing strains. Results showed that both the purchased model strain Desulfovibrio vulgaris (D-1) and the Citrobacter sp. C-1 isolated from high As groundwater in this study were capable of sulfate, iron and arsenate reduction. However, they presented different patterns of sulfate/iron reduction and As release under As-bearing ferrihydrite conditions attributed to their preferential functions. Sulfate input accelerated the bioreduction of sulfate and ferrihydrite simultaneously by D-1, thereby facilitated arsenic release and formation of vivianite, mackinawite and goethite. The As release was accelerated by 20.7 % and 165.5 % under 1 mM and 10 mM sulfate amendment, respectively. In contrast, sulfate reduction occurred in the early stage (0-7 days) and iron reduction began afterwards in strain C-1 treatment, thus the increased sulfate hindered initial ferrihydrite reduction and suppress As release by 27.9 % and 37.9 % in 1 and 10 mM sulfate systems, respectively. The final secondary minerals were predominantly presented as amorphous goethite by C-1 due to low ferrihydrite reduction. The results highlight the necessary consideration of preferred functions of SRB when exploring the sulfate-mediated As release processes.