Inhibition of Chromium(III) Oxidation through Manganese(IV) Oxide Passivation and Iron(II) Abiotic Reduction
Journal Article
·
· ACS Earth and Space Chemistry
- Chapman Univ., Orange, CA (United States); SLAC
- SLAC National Accelerator Laboratory (SLAC), Menlo Park, CA (United States)
- New Mexico Institute of Mining and Technology, Socorro, NM (United States)
- Univ. of California, Riverside, CA (United States)
- Chapman Univ., Orange, CA (United States); Univ. of California, Riverside, CA (United States)
Manganese (Mn) oxides are strong oxidants that are ubiquitous in soils and can oxidize redox-active metals, including chromium (Cr). In soil environments, trivalent chromium (Cr(III)) is a benign, immobile micronutrient, whereas the hexavalent Cr(VI) form is present as a highly mobile, toxic chromate oxyanion. Although many studies have characterized the capacity of Mn(III/IV) oxides to oxidize Cr(III) to toxic Cr(VI), the oxidative capacity of Mn oxides in the presence of potentially passivating soil constituents, specifically reduced soluble iron (Fe(II)aq), remains unresolved. We hypothesized that chemical processes at redox interfaces, such as diffusion-limited environments within soil aggregates, can lead to decreased Cr(VI) production from Mn oxide-driven oxidation due to passivation by Fe(II)aq. A multichamber diffusion-limited reactor was used to simulate transport at soil redox interfaces and investigate the capacity of poorly crystalline and crystalline Mn oxides to oxidize solid Cr(III) minerals to Cr(VI) in the presence of Fe(II)aq. As predicted, Cr(VI) was produced through the Mn oxide-catalyzed oxidation of Cr(III) at a rate controlled by the solubility of Cr(OH)3. However, in the presence of Fe(II)aq, the concentration of aqueous Cr(VI) decreased as a function of the Fe(II)aq concentration, where high concentrations of Fe(II)aq completely inhibited Cr(VI) production, likely through both the passivation of the Mn oxide and the direct reduction of Cr(VI) by Fe(II). At both low (14 μM) and high (100 μM) Fe(II)aq concentrations, the iron oxide minerals hematite (Fe2O3) and goethite (α-FeOOH) were associated with the Mn oxides, which can cause surface passivation, a likely role that decreases Cr(III) oxidation. Additionally, the Cr(III) oxidation rate decreased with increasing crystallinity of the Mn oxides whether or not Fe(II) was present.
- Research Organization:
- SLAC National Accelerator Laboratory (SLAC), Menlo Park, CA (United States)
- Sponsoring Organization:
- USDOE Office of Science (SC), Basic Energy Sciences (BES)
- Grant/Contract Number:
- AC02-76SF00515
- OSTI ID:
- 2404890
- Journal Information:
- ACS Earth and Space Chemistry, Journal Name: ACS Earth and Space Chemistry Journal Issue: 12 Vol. 7; ISSN 2472-3452
- Publisher:
- American Chemical Society (ACS)Copyright Statement
- Country of Publication:
- United States
- Language:
- English
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