Reduction of HgII by MnII

Abstract

The reduction of Hg^II to Hg^I or Hg⁰ can lead to significant changes in Hg toxicity and mobility in the environment. Photochemical reduction is the primary process for the reduction of Hg^II to Hg⁰ in sunlit environments; however, dark reduction of Hg^II can occur via microbial metabolic processes and/or reduction by reduced natural organic matter, Fe^II mineral phases, Fe^II sorbed to minerals, or aqueous Fe^II. Here, we demonstrate a novel Hg^II reduction pathway involving another environmentally relevant reductant, Mn^II. Abiotic reduction of Hg^IIO by Mn^II was studied as a function of pH and anion environment (perchlorate, sulfate, chloride) using X-ray absorption spectroscopy to characterize the solid-phase Hg and Mn species. At circumneutral pH of 7.5, about 70% of Hg^II was reduced to elemental Hg⁰ within 2 h. In contrast, 12 h were needed to achieve the same extent of reduction at pH 6.9. In the presence of sulfate and chloride, Hg^I species were formed. Hg^II reduction was initially rapid and coupled with the oxidation of soluble Mn^II-oxides to insoluble Mn^IV-oxides, followed by a significantly slower reduction of Hg^II during the Mn^II-catalyzed transformation of the Mn^IV-oxides to hydroxide and oxyhydroxide minerals. The observed reduction of Hg^II by Mn^II at circumneutral pH could be an important transformation pathway for environmental Hg, affecting its bioavailability and mobility under mildly reducing conditions.