Effect of sulfidogenesis on ferrihydrite geochemistry and associated arsenic fate

The behavior of arsenic (As) in groundwater is closely related to the sulfidation of ferrihydrite. In the ternary ferrihydrite-As-sulfide system, ferrihydrite can either initially adsorb As before sulfide reduction or first encounter sulfide and then interact with the aqueous As, altering As fate. However, their relative contributions to the mineralogical transformation of ferrihydrite and subsequently associated As mobilization/redistribution remain poorly understood. Therefore, batch experiments combined with chemical, microscopic, and spectroscopic analyses were conducted to clarify the geochemistry of ferrihydrite and its influence on As behavior. Results indicated that in the pre-sorption groups, the secondary minerals were predominantly presented in amorphous phase due to the retardative effect of As. At low sulfide concentrations (S/Fe=0.04), the content of residual ferrihydrite was large, which favored As immobilization. At high sulfide concentrations (S/Fe=0.8), however, As was initially released into the solution and subsequently re-immobilized by secondary minerals. The adsorption capacity of the secondary minerals for As decreased with the increase in amorphous mackinawite formation. In the pre-sulfidation groups, rapid ferrihydrite reduction promoted the formation of crystalline minerals, significantly reducing their adsorption capacity. At low sulfide concentrations, the released As was partially adsorbed on the surface of crystalline goethite and lepidocrocite. At high sulfide concentrations, magnetite formed and favored As immobilization through its incorporation into magnetite particles. These results provide important insights into the geochemistry of Fe, S, and As in groundwater systems.