Impact of δ-MnO2 on the chemical speciation and fractionation of Cr(III) in contaminated soils

Oxidation of Cr(III) by birnessite (δ-MnO2) was an important geochemical reaction determining the toxicity and mobility of dissolved Cr(III) in soils. Herein, changes of Cr speciation and fractionation were systematically studied in Cr(III)-contaminated soils with δ-MnO2 in soil aging process. The results showed that Cr(III) could be rapidly oxidized to Cr (VI) by δ-MnO2, and the coating of Fe and Al oxides on δ-MnO2 had a strong hindering effect on the oxidation of Cr(III). The Cr(III) oxidation process by δ-MnO2 followed a two-phase model of pseudo first-order kinetics. The rapid decrease of oxidation rate constant in second phase was due to the coverage of adsorbed Cr(III) and newly generated Cr(VI) and Mn(II) on the active sites of δ-MnO2. X-ray photoelectron spectroscopy analysis further confirmed that the diffusion and adsorption of Cr(III) on the electron-accepting sites were important factors affecting the Cr(III) oxidation by δ-MnO2. Compared with the soils without δ-MnO2, high contents of Cr(VI) were generated in silt (22.30 mg/kg) and sandy soil (70.95 mg/kg) with 2 wt% δ-MnO2 after the addition of Cr(III) wastewater within 2 days, and the Cr(VI) contents were above 1 mg/kg in these two soils during the whole incubation process. Moreover, the total Cr proportion of the exchangeable fraction increased by 12.8% in silt and 5.2% in sandy soil with 2 wt% δ-MnO2 after soil aging for 120 days. The presence of δ-MnO2 markedly increased the oxidation potential and mobility of exogenous Cr(III) in soils.Graphical abstract