Effects of Mn2+, Ni2+, and Cu2+ on the Formation and Transformation of Hydrosulfate Green Rust: Reaction Processes and Underlying Mechanisms

Green rusts (GRs), which are important intermediate phases during Fe2+ oxidation, are commonly associated with various metal cations during their crystallization in soils and sediments, but the effects of these foreign metal cations on the formation of GRs and on their subsequent transformation to Fe (hydr)oxides remain unclear. In the present study, the effects of Mn2+, Ni2+, and Cu2+ on the evolution processes of hydrosulfate green rust (GR2) are documented under various conditions and the mechanisms leading to cation incorporation in the reaction products are determined. The rates of GR2 formation and of its transformation to Fe (hydr)oxides both decrease in the order of Cu2+ > Ni2+ > Mn2+ and increase with increasing metal cation concentration. During GR2 crystallization, a small fraction of foreign metal cations is structurally incorporated in GR2 by replacing Fe2+, and their amount in the mineral follows the order of Cu2+ > Ni2+ > Mn2+. Under all conditions, the final reaction products are a mixture of lepidocrocite and goethite; a slow oxidation rate of mineral Fe2+ and a strong catalytic effect of surface Fe2+ both facilitate the goethite formation from GR2, reversely, favorable to lepidocrocite formation. Additionally, the three cations possess different speciation and distribution in lepidocrocite and goethite: Mn exists mainly as Mn(III) and probably minor Mn(II)–Mn(III) molecular clusters and occurs mainly in the mineral interior by isomorphic substitution or coated by the Fe (hydr)oxides crystals; Ni is present as Ni(II) and uniformly distributed in the newly formed minerals by either isomorphic substitution or surface adsorption; finally, Cu is mainly sorbed at the mineral surface as Cu(II) with minor Cu(I). These cations may thus be structurally incorporated in Fe oxides in the order of Mn(III) > Ni(II) > Cu(II). These new insights into the interaction between GR2 and trace metal cations improve our understanding of Fe oxide crystallization processes and of the environmental geochemical behavior of associated metal cations in redox alternating soils and sediments