Reductive transformation processes of ferrihydrite and U(VI) triggered by S^2- under an anaerobic environment

Uranium (U) contamination poses a significant environmental threat due to its mobility, radiotoxicity, and persistence in water bodies near mining and ore-processing sites. In sulfidic and anoxic conditions common to these settings, the sulfidation of iron (hydr)oxides may play a key role in the transformation and immobilization of U(VI) and iron (hydr)oxides, yet the mechanisms remain poorly understood. In this study, we systematically examined the reaction mechanisms between U(VI) and ferrihydrite in the presence of S^2- under anaerobic conditions. The sulfidation can indeed induce pronounced structural and chemical alterations of ferrihydrite, including reductive dissolution and the release of Fe²⁺, leading to the formation of reactive mineral phases (goethite, hematite, and mackinawite) and surface sites. Notably, the mineral transformation pathways were pH-dependent, at pH 7, the system favored the formation of more crystalline and thermodynamically stable phases such as hematite and goethite, whereas poorly crystalline phases and higher concentrations of dissolved Fe²⁺ dominated the transformation products under pH 5 conditions. These formation processes of newly formed iron mineral phases promoted the reductive conversion of aqueous U(VI) to insoluble U(IV), with Fe²⁺ acting as an effective secondary reductant. The co-evolution of iron and U species highlights a tightly coupled redox and sorption process that governs U(VI) immobilization and ferrihydrite transformation. Furthermore, the dynamic transformation of ferrihydrite under S^2- influence reveals key mineralogical pathways that contribute to U retention in reducing environments. It is noted that the oxidation of S^2- was a stepwise process experiencing S⁰, SO₃^2-, and finally SO₄^2-. Our findings provide novel mechanistic insights into how sulfur-driven processes modulate U behavior through interlinked mineral and redox dynamics. This work offers a scientific basis for developing passive remediation strategies that leverage natural biogeochemical processes to stabilize U in contaminated waters.