Environmental Stability of Schwertmannite: A Review
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REVIEW
Environmental Stability of Schwertmannite: A Review Susanta Paikaray1 Received: 24 September 2019 / Accepted: 29 October 2020 © Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract Schwertmannite is sensitive to changes in geochemical, thermal, and microbial conditions. Changes in aqueous pH beyond its stability, i.e. pH 2.5–4.5, triggers its transformation to jarosite or goethite in highly acidic environments (pH ≤ 2.5), depending on the availability of jarosite-directing cations ( Na+, NH4+, K+, etc.), while goethite is the common stable end product at pH > 7.5. Schwertmannite with degraded morphology can stably exist for years in oxic intermediate pH environments (pH 5.5–6.5), but the presence of trace amounts of Fe(II)aq yields goethite/lepidocrocite within a few hours, especially at pH ≥ 6.5. Hematite is the sole end product at ≥ 600 °C dry heating, with goethite and ferrihydrite as intermediate phases. Siderite, maghemite, and mackinawite form in anoxic microbial conditions due to dissimilatory reduction of Fe(III) and SO42− to Fe(II) and HS−, while orpiment forms from As(V)-rich schwertmannites. Sorbed contaminants enhance schwertmannite stability by restricting Fe(II)–Fe(III) electron transfer and microbial degradation by occupying surface sites. Although Fe(III) and sorbed ion mobilization typically has negligible effects on schwertmannite transformation, complete schwertmanniteSO4 release is likely in extreme conditions, and in microbial Fe(II)aq-rich media. Dissolution–reprecipitation and solid state transformation mechanisms broadly govern schwertmannite transformation. Keywords Fe(III)–Fe(II) electron transfer · Acid mine drainage · Dissolution–reprecipitation · Thermal transformation · Acid tolerant bacteria
Introduction Oxidative dissolution of sulfide minerals, such as pyrite (FeS2), arsenopyrite (FeAsS), and chalcopyrite (CuFeS2) on exposure to atmospheric O2 (Eq. 1) and/or dissolved Fe3+ (Eq. 2) can generate the large flux of iron-sulfate-rich acidic effluents known as acid mine drainage (AMD) (Blodau 2006; Jönsson et al. 2006; Langmuir 1997; Murad and Rojik 2003, 2004; Nordstrom 1991). The Fe3+ driven dissolution is ≈ 2–3 times faster and generates excess acidity per unit mass of FeS2 (Eq. 1 vs. Eq. 2), with the required Fe3+ being produced by oxidation of liberated F e2+ (Eqs. 1 and 2), either by atmospheric O 2 (Eq. 3) or by acidophilic bacteria such as Acidithiobacillus ferrooxidans. Ferric oxyhydroxides Electronic supplementary material The online version of this article (https://doi.org/10.1007/s10230-020-00734-2) contains supplementary material, which is available to authorized users. * Susanta Paikaray [email protected] 1
Department of Geology, Panjab University, Chandigarh 160014, India
and/or oxyhydroxysulfates like schwertmannite are produced by Fe3+ hydrolysis (Eq. 4). Rapid schwertmannite formation by a microbial pathway has been documented worldwide, which lowers both Fe2+(aq) and Fe3+(aq) significantly within a few meters downstr
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