Mineralogical Changes during Ferrihydrite Sulfidization and Its Impact on Metal Behavior

            Iron sulfides are common minerals that have important roles in permeable reactive barriers (PRB) and anoxic soils. Previous investigations have examined the dissolution of iron oxides in H₂S solutions, but these studies focused on sulfide oxidation and iron oxide dissolution kinetics. Mineral phase transformation in the Fe-O-S system has not been explored in detail. This study investigates mineralogical changes during sulfidization of ferrihydrite by hydrogen sulfide and its impact on heavy metal behavior in reducing environments. Preliminary results show that FeS and S₀ formation occurs rapidly through reductive dissolution of ferrihydrite by H₂S. FeS transformation to FeS₂ is slow at 25°C. However, transformation of FeS to FeS₂ is accelerated at 45°C. Sulfur partitioning results indicate that FeS and FeS₂ are inversely correlated within 21d. XRD shows a decrease in S₀ over time, and S₀ begins to disappear after 14d, which is in agreement with the stabilization of FeS. This indicates that transformation of FeS to FeS₂ is halted due to exhaustion of S₀. The results suggest that FeS₂ is formed through FeS transformation, instead of direct pyritization of ferrihydrite. Nickel (Ni) is used as an example in this study to demonstrate how sulfidization of iron oxides affects metal partitioning in reducing environments.