Immobilization and release behaviors of uranium mediated by the redox processes between manganese oxides and dissolved organic matter: Effects of pH and goethite

Both manganese dioxide (MnO₂) and dissolved organic matter (DOM) exert a significant influence on the chemical species of uranium in the contaminated soils, yet the impacts of the interactions between MnO₂ and DOM, particularly in the presence of iron oxyhydroxides, on the environmental behaviors of uranium have not been elucidated. In this study, the dynamic behaviors of uranium were investigated during the reactions of DOM with δ-MnO₂ in the presence of goethite at different pH values, by employing a combination of kinetic experiments, spectrophotometric titration, X-ray photoelectron spectroscopy, and electrochemical analysis. Our results indicated that the presence of DOM decreased uranium adsorption on MnO₂ and promoted the release of uranium bound to DOM and MnO₂ through the oxidation of DOM and the reduction of MnO₂, respectively. Goethite increased uranium adsorption on its surface and hindered the direct oxidation of DOM by MnO₂, but the indirect oxidation of goethite-adsorbed DOM by MnO₂ provided an additional route for uranium release. We found that uranium concentration in solution was positively correlated with Mn(II) concentration at pH 4.5, whereas it was positively correlated with the concentration of dissolved organic carbon and negatively correlated with the aromaticity and molecular weight of DOM at pH 6.5. Above results highlighted the significance of the redox process between MnO₂ and DOM in regulating the dynamic behaviors of uranium, which contributed to a better understanding of the sequestration and stability of uranium in the contaminated soils around the uranium tailings ponds.