Abiotic Reduction Of Nitrate And Chlorate By Green Rust

Fe -rich minerals are ubiquitous on the earth's surface and on Mars, Fe3' and interactions of iron minerals with chlorate (CI03) and/or nitrate (NO3-) may I l tiv Fe2" influence the redox cycling of chlorine and nitrogen in the terrestrial and Martian S042- or CI' environments. The objective of this study was to investigate the reactivity of two OH' different types of synthesized green rusts (GRs), chloride (CI-GR) and sulfate (SO4-GR), for C103- and NO3- reduction. C1-GR and SO4-GR were synthesized by co -precipitation from an Fe"" Fein (ferrous iron and ferric iron) solution with the addition of NaOH. Both Cl-GR and SO4-GR degraded CI03 much faster than NO3-. There were no significant competitive effects on the reduction of CI03- or NO3- when both species were present simultaneously. The Fell/FeIII ratio in GR had a strong influence on the rates and the extents of oxyanion transformation. ClGR with an Fell/Fell" ratio of 2:1 and 3:1 showed higher reactivity than 1:1 CI-GR for both CI03 and NO3- reduction. In comparison, SO4-GR with an Fen/Fen" ratio of 1:1 transformed C103- at a higher rate than at Fen/Fein ratios of 2:1 and 3:1. The pH effect was studied within a pH range of 4.5-8.5. More complete CI03 and NO3 transformation was achieved at pH 4.5, and the apparent reaction rate constants decreased with increasing pH. In all experiments, nearly 100% of the initial CI03 (10 mg/L) was degraded by GR except for C1-GR with an FeII/Fein ratio of 1:1 at pH 8.5. However, for NO3-, there was an initial rapid transformation followed by a period of slow or no transformation, implying the formation of passivating products that hindered continuous NO3 reduction. Feu did not appear to degrade NO3 or CI03- at detectable rates over the time period of interest (20 days), except at the highest concentration studied (1000 mg/L). The reactions between GR and CI03- or NO3- have implications for the cycling of chlorine and nitrogen and the stability of iron minerals. On Mars, these reactions may help to understand the occurrence and distribution of CI04, CI03-, and NO3.