Abstract
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 (MnO(2)), 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 [V(2)O(3)(OH)(2)] into highly mobile and toxic vanadate (H(n)VO(4)((3-n)-)) 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.