Aqueous Vanadate Removal by Iron(II)-Bearing Phases under Anoxic Conditions

Vanadium contamination is a growing environmental hazard worldwide. Aqueous vanadate (HV^VO₄^(3-x)-_(aq)) concentrations are often controlled by surface complexation with metal (oxyhydr)oxides in oxic environments. However, the geochemical behavior of this toxic redox-sensitive oxyanion in anoxic environments is poorly constrained. Here, we describe results of batch experiments to determine kinetics and mechanisms of aqueous H₂V^VO₄^- (100 μM) removal under anoxic conditions in suspensions (2.0 g L^-1) of magnetite, siderite, pyrite, and mackinawite. We present results of parallel experiments using ferrihydrite (2.0 g L^-1) and Fe²⁺_(aq) (200 μM) for comparison. Siderite and mackinawite reached near complete removal (46 μmol g^-1) of aqueous vanadate after 3 h and rates were generally consistent with ferrihydrite, whereas magnetite removed 18 μmol g^-1 of aqueous vanadate after 48 h and uptake by pyrite was limited. Removal during reaction with Fe²⁺_(aq) was observed after 8 h, concomitant with precipitation of secondary Fe phases. X-ray absorption spectroscopy revealed V(V) reduction to V(IV) and formation of bidentate corner-sharing surface complexes on magnetite and siderite, and with Fe²⁺_(aq) reaction products. These data also suggest that V(IV) is incorporated into the mackinawite structure. Overall, we demonstrate that Fe(II)-bearing phases can promote aqueous vanadate attenuation and, therefore, limit dissolved V concentrations in anoxic environments.