Effect of Mn(II) photochemical oxidation on Cd immobilization in hematite

Hematite, a commonly stable iron oxide in the environment, which can not only adsorb Cd in the environment, but also catalyze the photochemical oxidation of Mn(II) in the environment. However, the impact of Mn(II) on the structure of hematite and the adsorption of Cd during the surface oxidation of hematite remains unknown. In this study, we investigated the surface and structural changes of hematite after the photochemical oxidation of Mn(II), as well as the geochemical behavior of Cd during this process. The results demonstrate that Mn(II) was oxidized to Mn(III/IV) on the hematite surface, with some Mn(III) being incorporated into the hematite structure. Simulations using XRD data showed that higher Mn(II) concentrations resulted in increased levels of Mn doping, leading to significant variations in the hematite unit cell. This was further confirmed through FTIR and Raman spectroscopy characterization. The oxidation of Mn(II) on the hematite surface resulted in a shift in surface charge from positive to negative, enhancing the adsorption capacity of Cd. However, when Mn(II) exceeded 0.4 mM, the immobilization of Cd within the system decreased. This was attributed to the competitive adsorption of Mn(II) and a reduction in the relative abundance of Mn(IV) oxides.