Soil-phase immobilization of hexavalent chromium using L-ascorbic acid - kinetics, process optimization, and...

l-ascorbic acid is found to be an effective and environmentally friendly reagent for remediation of Cr(VI)-contaminated soil. Soil-phase batch kinetics experiments elucidated the effects of chromium(VI) and l-ascorbic acid dosing, pH, soil-to-water ratio, and temperature on Cr(VI) reduction. An extended reductive environment was observed at a lower pH range, possibly due to ligand oxidation of several reactive intermediates. The kinetic data were fitted into a second-order rate model, and the rate constant was evaluated. A reaction mechanism was proposed. Reduction was substantially complete within about 3 h at natural pH conditions at an appropriate dosing of the reagent, and any residual Cr(VI) is reduced by the remaining ascorbic acid during aging of the soil. The reagent worked much better than other organic reductants reported in the literature. The rate constant correlated linearly with the soil-to-water ratio, while its logarithm correlated linearly with pH within the specified range. The reduction was primarily attributed to the presence of carboxylic, hydroxyl, and carbonyl groups in the ascorbic acid molecule, as confirmed by FTIR and XRD analysis. Medium-term stability experiments suggest that treatment with l-ascorbic acid significantly decreased leaching concentrations of Cr(VI) and total Cr which remained stable for 75 d. Ecological studies have shown that remediated soil promotes plant growth and increases earthworm survival rates, thus negating the emergence of any secondary pollutants. Response surface methodology (RSM) was used to investigate the relationship between Cr(VI) reduction and experimental parameters and to determine the optimum values. About 98.8 % reduction was achieved, which was consistent with the calculated optimal value. The study provides new insights into the use of l-ascorbic acid for sustainable remediation of Cr(VI)-contaminated soil. The optimum process conditions and the rate data obtained in this study are expected to be useful for the process design of a remediation facility for Cr(VI)-contaminated soil.