Reactive mineral phases and residual reagents govern vanadium mobilization and transformation in V–Ti magnetite tailings

Vanadium (V)-bearing tailings can act as long-term sources of mobile V, yet the roles of tailings-associated minerals and residual beneficiation reagents in regulating V (im)mobilization remain unclear. Here, binary reactions between dissolved V(V) and representative minerals of V–Ti magnetite tailings, alongside tailings suspensions amended with oxalic acid or carboxymethyl cellulose (CMC), were used to isolate mineral- and reagent-specific controls on V transformation. Poorly crystalline Fe oxides, particularly ferrihydrite and lepidocrocite, showed the strongest V immobilization, decreasing dissolved total V from 25.5 mg/L to 6.16–10.5 and 6.21–8.41 mg/L, respectively. FTIR evidence for Fe–O–V surface complexation on ferrihydrite, together with limited structural changes in crystalline Fe and Fe–Ti phases, indicated that reactive surface sites rather than bulk mineral transformation dominated V immobilization. Birnessite retained V under oxidizing conditions while maintaining the aqueous V pool as V(V), whereas FeS induced reducing conditions and promoted the formation of dissolved non-V(V) species. In whole-tailings systems, oxalic acid mobilized V from the tailings, yielding 26.4 mg/L dissolved total V, and facilitated Fe(III)-to-Fe(II) transformation, with Fe(II) accounting for 95.0% of dissolved Fe by day 30 of incubation. However, XRD and FTIR showed no detectable transformation of the bulk tailings assemblage, indicating selective dissolution and redox transformation of reactive minor or poorly crystalline phases. In contrast, CMC had limited effects on V release. These findings show that reactive minor phases and residual oxalate can strongly influence V (im)mobilization, highlighting the need to incorporate reagent residues and Fe/S redox processes into risk assessments of V–Ti magnetite tailings.