Freezing Enhances Leaching of Ferrous Ions but Hinders Reductive Dissolution of Ferric Ions from Iron Oxides

Abstract

Iron governs biological activities in the natural environment as an essential element. Although the concentrations of dissolved iron species are primarily regulated by the solubility of iron minerals, there are additional pathways of the supply of dissolved iron species from solids. Redox processes play important roles in the dissolution of iron species from minerals because ferrous ions are more soluble than ferric ions. In addition, recent studies have indicated that freezing enhances the dissolution of iron species from oxides in the presence of acids or ligands. Thus, although freeze-specific phenomena may play essential roles in the determination of this process of fundamental and environmental importance, the details have not been elucidated. In the present paper, the redox behaviors of major iron oxides, that is, magnetite, maghemite, and hematite, are studied using cyclic voltammetry in frozen acidic media. The results indicate that freezing facilitates the leaching of Fe2+ from magnetite but hinders the reduction of ferric ions in the oxide crystals. The latter is commonly observed for all iron oxides studied here. This hindrance is caused by the freeze enrichment of Fe2+ at the interface between the iron oxide and freeze-concentrated solution and by the surface adsorption of Fe2+.