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- 2019
Abiotic Reduction of Chlorate by Fe(II) Minerals: Implications for Occurrence and Transformation of Oxy-Chlorine Species on Earth and MarsDOI: https://doi.org/10.1021/acsearthspacechem.8b00206 Abstract: Recent investigations have reported a widespread occurrence of chlorate (ClO3–) and perchlorate (ClO4–) throughout the solar system, including terrestrial arid environments. ClO3– and ClO4– are deposited/accumulated at an approximate equal molar ratio, with some exceptions, such as the Antarctica Dry Valley soils (MDV) and perhaps Martian surface material, where ClO4– is the dominate ClOx– species. All known ClO4– production mechanisms produce molar ratios of ClO3–/ClO4– equal to or much greater than 1, suggesting that reduced ratios may be due to post-depositional mechanism(s). The objective of this study was to investigate potential iron-mediated abiotic reduction of ClO3–, similar to transformation mechanisms reported for nitrate (NO3–) by Fe(II) minerals. Three types of Fe(II)-containing minerals, wüstite (FeO), siderite (FeCO3), and sulfate green rust (GRSO42–), were investigated in completely mixed batch reactors as potential ClO3– reductants at a range of pH (4–9) and iron mineral concentrations (1–10 g/L). ClO3– was stoichiometrically reduced to chloride (Cl–) by wüstite, siderite, and green rust, but no transformation occurred by dissolved Fe(II). Wüstite and green rust reduced NO3– but not by siderite. When both NO3– and ClO3– are reduced simultaneously, ClO3– is reduced preferentially to NO3–, although the effect is somewhat concentration-dependent. An increased background salt concentration (NaCl) increased ClO3– reduction but decreased NO3–. The stability of ClO3– and subsequent impacts on the ratio of ClO3–/ClO4– in the environment have implications for understanding the cycling of oxyanions and stability of iron minerals, and related to this, the ratio of ClO4– and ClO3– may be an indicator of the past availability of free water. On Mars, these reactions may help to explain the unusually high concentrations of ClO4– compared to ClO3– and NO3–
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