Oxidation of V(IV) by Birnessite: Kinetics and Surface Complexation

Vanadium is a redox-active metal that has been added to the EPA's Contaminant Candidate List with a notification level of 50 μg L^-1 due to mounting evidence that V^V exposure can lead to adverse health outcomes. Groundwater V concentration exceeds the notification level in many locations, yet geochemical controls on its mobility are poorly understood. Here, we examined the redox interaction between V^IV and birnessite (MnO₂), a well-characterized oxidant and a scavenger of many trace metals. In our findings, birnessite quickly oxidized sparingly soluble V^IV species such as häggite [V₂O₃(OH)₂] into highly mobile and toxic vanadate (HVO₄^(3-) in continuously stirred batch reactors under neutral pH conditions. Synchrotron X-ray absorption spectroscopic (XAS) analysis of in situ and ex situ experiments showed that oxidation of V^IV occurs in two stages, which are both rapid relative to the measured dissolution rate of the V^IV solid. Concomitantly, the reduction of birnessite during V^IV oxidation generated soluble Mn^II, which led to the formation of the Mn^III oxyhydroxide feitknechtite (β-MnOOH) upon back-reaction with birnessite. XAS analysis confirmed a bidentate-mononuclear edge-sharing complex formed between V^V and birnessite, although retention of V^V was minimal relative to the aqueous quantities generated. In summary, we demonstrate that Mn oxides are effective oxidants of V^IV in the environment with the potential to increase dissolved V concentrations in aquifers subject to redox oscillations.