Catalytic oxidation of Mn(II) on ferrihydrite and goethite surfaces and the subsequent oxidation and immobilization of coexisting Cr(III)

Semiconductor iron oxides, which are distributed in soils, always catalyze Mn(II) oxidation to produce various Fe–Mn binary oxides. They affect the migration and transformation of heavy metals, i.e., Cr(III). However, the specific effect mechanisms of different catalytic oxidation pathways (i.e., electrochemical or interfacial catalysis) of Mn(II) catalyzed by minerals possessing different characteristics on the oxidation of coexisting Cr(III) remains elusive. Therefore, this study aims to explore different Mn(II) oxidation processes on ferrihydrite and goethite surfaces as well as the subsequent oxidation of coexisting Cr(III) and Cr(VI) immobilization. Herein, long-time aging oxidation tests were performed combined with solution chemical analysis and various spectroscopic techniques such as X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS), to explore the Mn(II) catalytic oxidation behaviors on ferrihydrite and goethite surfaces under different pHs and initial Mn(II) reaction concentrations, as well as the oxidation products of Fe–Mn binary oxides. Subsequently, the oxidation mechanisms of Cr(III) by these different generated Fe–Mn binary oxides were studied. Results indicated that higher pH and initial reaction concentration of Mn(II) were more favorable for Mn(II) oxidation yielding more Mn oxides containing higher valence Mn. Additionally, goethite, which has better conductivity, showed stronger electrochemical catalysis effect for Mn(II) oxidation than ferrihydrite. Thus, more Mn(III) oxides were generated in goethite systems than in ferrihydrite systems. Fe–Mn binary oxides formed from higher Mn(II) oxidation rates and degrees exhibited more improved oxidative properties for Cr(III) and higher Cr(VI) fixation efficiencies than those obtained from lower reactions. These phenomena depended on the stronger oxidation and fixation effect of Mn(II) oxidation products with higher Mn valence states of Mn(III/IV) on Cr(III) and Cr(VI), respectively. Moreover, Mn(III) in Fe–Mn binary oxides exhibited considerably higher oxidation efficiency for Cr(III) than Mn(IV). In summary, high pH, higher initial Mn(II) concentration, and iron oxides with stronger electrochemical catalytic effect are more conducive to the oxidation of Mn(II) as well as the subsequent oxidation of coexisting Cr(III) and the immobilization of formed Cr(IV).