Coupling of tartaric acid-promoted soil dissolution and Cr(VI) reduction in an Oxisol

Environmental risk posed by Cr(VI)-contaminated soils could be effectively alleviated through the reduction of Cr(VI) to Cr(III) by organic compounds, which was usually limited in practical application due to the slow reaction rate. Soil surfaces have been found to catalyze many redox reactions. In this study, an Oxisol containing abundant Fe and Mn-oxides was selected to investigate the soil surfaces-catalyzed Cr(VI) reduction by tartaric acid with the aim to elucidate the coupling of soil Fe/Mn oxides dissolution and Cr(VI) reduction. In the absence of tartaric acid, the reduction of Cr(VI) by soil itself could be excluded, and only Cr(VI) adsorption onto the soil was observed, which proceeded very rapidly at the initial stage, followed by a slow progress. No measurable Cr(VI) reduction by tartaric acid was found without the addition of the soil during 24 h, while Cr(VI) reduction would occur after the soil was introduced to the system. It was believed that Cr(VI) reduction mainly occurred in solutions not the solid surfaces since Cr(VI) adsorption was significantly inhibited due to the competitive effect of excessive tartaric acid and the background electrolyte. Aqueous Fe(III) and Mn(II) released from tartaric acid-promoted soil dissolution accounted for the acceleration of Cr(VI) reduction, which was also confirmed by the results from the experiments conducted in aqueous solutions. Addition of tartaric acid to soil suspensions pre-equilibrated with Cr(VI) for 24 h would lead to more Cr(VI) reduced to Cr(III). This result was mainly explained as the enhancement of H+ adsorption caused by the increase in negative charge due to the Cr(VI) adsorption, which would promote the release of Fe(III) and Mn(II) from soil surfaces to solutions in turn. Moreover, desorption of the adsorbed Cr(VI) to solutions was the necessary step for the reduction of Cr(VI) adsorbed onto the soil. Part of Cr(III) generated in solutions would be adsorbed to soil surfaces, evidenced by the decrease in total aqueous Cr concentration. The present study gave some insight into the mechanisms of soil surfaces catalyzed Cr(VI) reduction by organic compounds, and were helpful for formulating the remediation strategy of Cr(VI)-contaminated soils.

► Cr(VI) reduction by excessive tartaric acid in the Oxisol mainly occurred in solution.
► Aqueous Fe(III) and Mn(II) released from the soil surfaces accelerated the reaction.
► Desorption of Cr(VI) was necessary for reduction of adsorbed Cr(VI) by tartaric acid.
► Part of the Cr(III) generated in solutions would be adsorbed onto the soil surfaces.