Abstract
Iron and manganese oxides are ubiquitous in soils and sediments, and their formation and conversion processes affect the migration and transformation of heavy metals and organic pollutants. Mn2+ was found to affect the formation of iron oxides from Fe2+ oxidation. However, little attention has been paid to the oxidation process at the initial stage and generation of manganese oxides in the reaction system of Fe2+aq and Mn2+aq. This work investigated the formation process and mechanism of iron and manganese oxides in a mixed solution system of Fe2+aq and Mn2+aq under oxic condition. The effect of pH (5.0–9.0) on the reaction process and products was further studied. The results indicated that Fe2+aq was oxidized to goethite and lepidocrocite, and Mn2+ adsorbed on the surface of iron oxides was catalytically oxidized to poorly crystalline manganese oxides by dissolved O2 in a mixed solution of Fe2+aq and Mn2+aq. Compared with the absence of Mn2+aq, the presence of Mn2+aq caused no obvious changes in the species of iron oxides, but significantly increased the oxidation rate of Fe2+aq. Formation of manganese oxides on the surface decreased the crystallinity of the iron oxides. The increase of pH enhanced the oxidation of Fe2+aq and Mn2+aq and formation of lepidocrocite. This work expands our understanding of the interactions and geochemical processes of Fe2+aq and Mn2+aq.
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