Extracellular polymeric substances altered ferrihydrite trans(formation) and induced arsenic mobilization

The behavior of As is closely related to trans(formation) of ferrihydrite, which often coprecipitates with extracellular polymeric substances (EPS), forming EPS-mineral aggregates in natural environments. While the effect of EPS on ferrihydrite properity, mineralogy reductive transformation, and associated As fate in sulfate-reducing bacteria (SRB)-rich environments remains unclear. In this research, ferrihydrite-EPS aggregates were synthesized and batch experiments combined with spectroscopic, microscopic, and geochemical analyses were conducted to address these knowledge gaps. Results indicated that EPS blocked micropores in ferrihydrite, and altered mineral surface area and susceptibility. Although EPS enhanced Fe(III) reduction, it retarded ferrihydrite transformation to magnetite by inhibiting Fe atom exchange in systems with low SO42-. As a result, 16% of the ferrihydrite was converted into magnetite in the Fh-0.3 treatment, and no ferrihydrite transformation occurred in the Fh-EPS-0.3 treatment. In systems with high SO42-, however, EPS promoted mackinawite formation and increased As mobilization into the solution. Additionally, the coprecipitated EPS facilitated As(V) reduction to more mobilized As(III) and decreased conversion of As into the residual phase, enhancing the potential risk of As contamination. These findings advance our understanding on biogeochemistry of elements Fe, S, and As and are helpful for accurate prediction of As behavior. Environmental Implication In natural environments, the behavior of As is closely related to the trans(formation) of ferrihydrite, which often coprecipitates with extracellular polymeric substances (EPS). This research indicated that EPS blocked the micropores in ferrihydrite and retarded ferrihydrite transformation to magnetite. In systems with high SO42-, EPS promoted mackinawite formation and increased As(V) reduction to more mobilized As(III). Besides, EPS inhibited the conversion of As into the residual phase, which increased the potential risk of As contamination. These findings advance our understanding on biogeochemistry of elements Fe, S, and As, and are helpful for prediction of As behavior.