Manganese (Mn) is highly redox sensitive and commonly exists as Mn(II), Mn(III) and Mn(IV) in natural environment. The oxidation states of Mn in geological records have been used as indicators for redox conditions of paleoenvironment. Recently, Opportunity and Curiosity rovers found a wide occurrence of Mn oxides in veins or fractures on Mars, suggesting a highly oxidizing environment. Mn oxides on Earth are believed to form through biotic oxidation of aqueous Mn(II) by O2. However, the contemporary Mars is scarce in atmospheric O2 and microbial activity, and how Mn oxides occur on Mars remain an enigma. Here we report that mineral-surface catalyzed oxidation of Mn(II) by bromate, a halogen species occurring in the Martian regolith, can lead to rapid formation of Mn oxides in high saline solution relevant to Martian soils. In 0.5 M MgSO4 solution, hematite, a common type of Fe oxide in Martian soils, catalyzed rapid oxidation of Mn(II) by bromate to form Mn(III) or Mn(IV) oxides. The oxidation was favored at higher pH, hematite concentrations and bromate to Mn(II) molar ratios, and in the presence of low concentrations of MgSO4. Hematite was less reactive than goethite but more than bentonite for catalyzing the oxidation. Modeling the kinetic data suggests that homogenous oxidation of Mn(II) by bromate in solution concurs with the surface-mediated reaction. Our results suggest that the presence of Mn oxides does not necessarily indicate an O2-rich environment on Mars.

Mineral-surface catalyzed oxidation of Mn(II) by bromate: Implications for the occurrence of Mn oxide on Mars

ACS Spring 2022

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.