Abiotic Reduction of Chlorate by Fe(II) Minerals: Implications for Occurrence and Transformation of Oxy-Chlorine Species on Earth and Mars

Recent investigations have reported a wide-spread 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, wiistite (FeO), siderite (FeCO3), and sulfate green rust (GRsO(4)(2-)), 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 (Cr) by wiistite, siderite, and green rust, but no transformation occurred by dissolved Fe(II). Wiistite 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-.