Iron (Hydr)oxides and Oxalic Acid Drive Vanadium Transformation in Mine Tailings under Repeated Redox Fluctuations

Vanadium(V) is a primary pollutant in vanadium-titanium magnetite tailings, with its geochemical behavior strongly shaped by redox dynamics. To date, how Fe transformations and organic acids regulate V mobilization and sequestration in tailings under repeated redox fluctuations has not yet been sufficiently studied. Here, we examined the coupled effects of Fe (hydr)oxides and oxalic acid (OA) on V transformation and (im)mobilization in vanadium-titanium magnetite tailings over 56 days across four redox cycles. Under anoxic conditions, OA promoted the dissolution of Fe (hydr)oxides, releasing substantial dissolved Fe(II) and comobilizing V. During subsequent redox cycles, progressive crystallization of secondary Fe phases (e.g., hematite) markedly immobilized V. Oxalic acid not only mediated Fe transformation but also facilitated redox-driven V(V) reduction and redistribution among acid-soluble, reducible, oxidizable, and residual fractions. In moderate (0.5 mM) and high (5.0 mM) V treatments, OA significantly promoted V immobilization by shifting acid-soluble V into the reducible fraction, particularly under anoxic conditions in the moderate case at the end of the experiment. These findings highlight the dual role of OA in promoting short-term V mobilization while enhancing its long-term sequestration through Fe (hydr)oxides transformations over repeated redox cycling, providing new insights into redox-driven stabilization strategies for V-bearing tailings.