Symbiosis mechanism of iron and manganese oxides in oxic aqueous systems

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 Fe-aq(2+) and Mn-aq(2+). This work investigated the formation process and mechanism of iron and manganese oxides in a mixed solution system of Fe-aq(2+) and Mn-aq(2+) under oxic condition. The effect of pH (5.0-9.0) on the reaction process and products was further studied. The results indicated that Fe-aq(2+) 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 O-2 in a mixed solution of Fe-aq(2+) and Mn-aq(2+). Compared with the absence of Mn-aq(2+), the presence of Mn-aq(2+) caused no obvious changes in the species of iron oxides, but significantly increased the oxidation rate of Fe-aq(2+). Formation of manganese oxides on the surface decreased the crystallinity of the iron oxides. The increase of pH enhanced the oxidation of Fe-aq(2+) and Mn-aq(2+) and formation of lepidocrocite. This work expands our understanding of the interactions and geochemical processes of Fe-aq(2+) and Mn-2+(aq)