Catalytic oxidation and adsorption of Cr(III) on iron-manganese nodules under oxic conditions.

The speciation, toxicity and mobility of chromium (Cr) are significantly affected by natural iron-manganese nodules due to the adsorption and redox reactions in soils. However, the redox processes in oxic environments have received little attention. In this work, the interaction mechanism between Cr(III) and natural iron-manganese nodules was studied under oxic conditions, and the effects of chemical composition, dissolved oxygen concentration, pH, ionic strength and coexisting ions were further investigated. The results showed that iron-manganese nodules could effectively oxidize dissolved Cr(III), and most of the newly formed Cr(VI) was adsorbed on the surface of nodules. In iron-manganese nodules, manganese oxides mainly contributed to Cr(III) oxidation, and iron oxides facilitated the adsorption and immobilization of Cr(VI). In addition, Cr(III) could be catalytically oxidized to Cr(VI) on the surface of manganese oxides through the generation of Mn(III) intermediate or Mn(IV) oxides from released Mn(II) under oxic conditions. The oxidation rate of Cr(III) by the nodules decreased with increasing pH from 2.0 to 8.0, and increased with increasing ionic strength. This work reveals the adsorption and catalytic oxidation mechanism of Cr(III) by iron-manganese nodules in a simulated open system, and improves the understanding of the geochemical behavior of chromium in soils.