Influence of calcium carbonate on ferrihydrite bio-transformation and associated arsenic mobilization/redistribution

The sulfate-reducing bacteria (SRB)-induced ferrihydrite transformation is an important cause for arsenic (As) contamination in the aquifer near mining area. Calcium carbonate (CaCO₃) is widespread and has the potential of regulating As fate directly or indirectly. However, the influence of CaCO₃ on ferrihydrite transformation and the associated As mobilization/redistribution in SRB-containing environments remains unclear. Therefore, in this research, batch experiments coupled with a series geochemical, spectroscopic, and microscopic technologies were conducted collectively to address the research gap above. The results suggested that under low CaCO₃ loading conditions, the reductive transformation of ferrihydrite to Fe-S minerals resulted in a significant release of adsorbed As into the solution and As(V) reduction to more mobile and toxic As(III). Inside the cell of SRB, there existed S and As zone (no Fe), where As(V) might be reduced by S^2- without interference of Fe(III). Although the high CaCO₃ loadings exerted little effect on Fe(III) reduction, they promoted ferrihydrite transformation into dufrenite and vivianite, which sequestered As effectively via structural incorporation process, retarding As mobilization. Besides, the coprecipitation of As with dufrenite and vivianite enhanced the stability of solid phase As. Overall, the high CaCO₃ loadings altered the mineralogical transformation of ferrihydrite and the associated As biogeochemistry significantly in SRB-containing environments. Regulating ferrihydrite transformation and the associated As fate via CaCO₃ addition is an effective approach for mitigation of As mobilization in the aquifer near mining area.