Fe(II) reduction of pyrolusite (β-MnO2) and secondary mineral evolution

• Published in: Geochemical transactions GT Vol. 18; no. 1; pp. 7 - 11
• Main Authors: Schaefer, Michael V., Handler, Robert M., Scherer, Michelle M.
• Format: Journal Article
• Language: English
• Published: Cham Springer International Publishing 05-12-2017; Springer Nature B.V; BMC
• Subjects: Analytical methods; Chemical precipitation; Contaminants; Earth; Earth and Environmental Science; Earth crust; Earth Sciences; Electron microscopy; Electron transfer; Environmental Chemistry; Fe(II); Geochemistry; Heavy metals; Iron; Iron 57; Iron isotopes; Isotopes; Magnetite; Manganese; Manganese dioxide; Manganese oxide; Mossbauer spectroscopy; Mössbauer spectroscopy; Organic chemistry; Oxidation; Oxide coatings; Oxides; Oxidoreductions; Precipitates; Prof. Donald Sparks: “Frontiers and Advances in Environmental Soil Chemistry”; Pyrolusite; Redox; Redox reactions; Research Article; Sediment pollution; Sediments; Soil; Soil contamination; Solids; Spectroscopy; Trace metals; X-ray diffraction; Redox; Electron transfer; Fe(II); Manganese oxide; Mössbauer spectroscopy
• ISSN: 1467-4866; 1467-4866
• DOI: 10.1186/s12932-017-0045-0

Iron (Fe) and manganese (Mn) are the two most common redox-active elements in the Earth’s crust and are well known to influence mineral formation and dissolution, trace metal sequestration, and contaminant transformations in soils and sediments. Here, we characterized the reaction of aqueous Fe(II) with pyrolusite (β-MnO
2
) using electron microscopy, X-ray diffraction, aqueous Fe and Mn analyses, and
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Fe Mössbauer spectroscopy. We reacted pyrolusite solids repeatedly with 3 mM Fe(II) at pH 7.5 to evaluate whether electron transfer occurs and to track the evolving reactivity of the Mn/Fe solids. We used Fe isotopes (56 and 57) in conjunction with
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Fe Mössbauer spectroscopy to isolate oxidation of Fe(II) by Fe(III) precipitates or pyrolusite. Using these complementary techniques, we determined that Fe(II) is initially oxidized by pyrolusite and that lepidocrocite is the dominant Fe oxidation product. Additional Fe(II) exposures result in an increasing proportion of magnetite on the pyrolusite surface. Over a series of nine 3 mM Fe(II) additions, Fe(II) continued to be oxidized by the Mn/Fe particles suggesting that Mn/Fe phases are not fully passivated and remain redox active even after extensive surface coverage by Fe(III) oxides. Interestingly, the initial Fe(III) oxide precipitates became further reduced as Fe(II) was added and additional Mn was released into solution suggesting that both the Fe oxide coating and underlying Mn phase continue to participate in redox reactions when freshly exposed to Fe(II). Our findings indicate that Fe and Mn chemistry is influenced by sustained reactions of Fe(II) with Mn/Fe oxides.