Abiotic photochemically-induced oxidation of aqueous Mn2+ to Mn(IV) oxide solids in the presence of cations

Abstract:

Manganese (Mn) (III/IV) oxides are ubiquitous in the aquatic and terrestrial environments with diverse polymorphic structures. These minerals have excellent oxidation and adsorption capability that affect numerous biogeochemical cycles of trace elements, metal ions, and organics in the environment. Among all potential mechanisms, biotic oxidation of Mn²⁺ (aq) is believed to be the dominant pathway from Mn²⁺ (aq) to Mn(III/IV) solid (s) formation because fungi or bacteria can accelerate the oxidative process by several orders of magnitude. On the other hand, abiotic inorganic Mn oxidation has been considered a minor contribution owing to its slow kinetics. However, we recently have discovered fast, inorganic photochemically-induced oxidation of Mn²⁺(aq) to δ-MnO₂ (s) by superoxide radicals generated from nitrate photolysis, at a comparable oxidation rate to that of biotic processes. In this study, we further examine how cations affect the photochemically-induced Mn²⁺ (aq) oxidation process at 0.1 mM Mn²⁺ (aq), 1 mM NaNO₃, and circumneutral pH. Specifically, we investigated how species such as Na⁺, Mg²⁺, Ca²⁺, and Co³⁺ are incorporated into newly formed Mn (IV) oxides. Using X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, and scanning electron microscopy with energy dispersive X-Ray analysis, we found that the crystalline structures of Mn oxides formed within 6 hours were greatly affected by different oxidation states and the ionic radius of the cations, and their incorporation mechanisms. This study provides new insights into the effects of cation incorporation into photochemically-induced Mn (IV) oxide formation. It also suggests new environmentally-friendly synthesis pathways (under neutral pH and ambient pressure and temperature conditions with sunlight exposure) for Mn oxide materials with desired crystalline phases.