Journal
ENVIRONMENTAL SCIENCE & TECHNOLOGY
Volume 55, Issue 17, Pages 11703-11712Publisher
AMER CHEMICAL SOC
DOI: 10.1021/acs.est.1c02464
Keywords
X-ray spectroscopy; EXAFS; redox; groundwater; vanadium; manganese; feitknechtite; haggite
Categories
Funding
- Stanford University [DE-AC02-76SF00515]
- USDA NIFA Hatch Project [CA-R-ENS-5151-H]
- UCR Regents Faculty Fellowship
- T32 Training Grant from the National Institutes of Health [T32 ES018827]
- John D. Montgomery Postdoctoral Fellowship from the Pacific Basin Research Center, Soka University of America
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Vanadium is a redox-active metal found in groundwater at levels exceeding the notification level, and the redox interaction between V-IV and birnessite can accelerate the dissolution of V-IV and increase dissolved V concentrations.
Vanadium is a redox-active metal that has been added to the EPA's Contaminant Candidate List with a notification level of 50 mu g 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 haggite [V2O3(OH)(2)] into highly mobile and toxic vanadate (HnVO4(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 (beta-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.
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