Abiotic Reduction of Nitrate and Chlorate by Green Rust

Zhen Zhang, Weile Yan, Ophelie Messan, Jian Fang, W. Andrew Jackson

Research output: Contribution to journalArticlepeer-review

Abstract

Fe-rich minerals are ubiquitous on the earth's surface and on Mars, and interactions of iron minerals with chlorate (ClO3-) and/or nitrate (NO3-) may influence the redox cycling of chlorine and nitrogen in the terrestrial and Martian environments. The objective of this study was to investigate the reactivity of two different types of synthesized green rusts (GRs), chloride (Cl-GR) and sulfate (SO4-GR), for ClO3- and NO3- reduction. Cl-GR and SO4-GR were synthesized by co-precipitation from an FeII-FeIII (ferrous iron and ferric iron) solution with the addition of NaOH. Both Cl-GR and SO4-GR degraded ClO3- much faster than NO3-. There were no significant competitive effects on the reduction of ClO3- or NO3- when both species were present simultaneously. The FeII/FeIII ratio in GR had a strong influence on the rates and the extents of oxyanion transformation. Cl-GR with an FeII/FeIII ratio of 2:1 and 3:1 showed higher reactivity than 1:1 Cl-GR for both ClO3- and NO3- reduction. In comparison, SO4-GR with an FeII/FeIII ratio of 1:1 transformed ClO3- at a higher rate than at FeII/FeIII ratios of 2:1 and 3:1. The pH effect was studied within a pH range of 4.5-8.5. More complete ClO3- 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 ClO3- (10 mg/L) was degraded by GR except for Cl-GR with an FeII/FeIII 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. FeII did not appear to degrade NO3- or ClO3- 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 ClO3- 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 ClO4-, ClO3-, and NO3-.

Original languageEnglish
Pages (from-to)2042-2051
Number of pages10
JournalACS Earth and Space Chemistry
Volume5
Issue number8
DOIs
StatePublished - Aug 19 2021

Keywords

  • ClO
  • NO
  • green rust
  • oxyanions
  • reduction

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