Reductive Immobilization of Hexavalent Chromium by Polysulfide-Reduced Lepidocrocite

The anoxic reduction of lepidocrocite by polysulfide was studied under alkaline conditions (pH 9.5), and the reducing capacity of the resultant reduced lepidocrocite (RL) for hexavalent chromium (Cr(VI)) reduction was examined in the pH range from 6 to 10. The results indicated that polysulfide had a high reactivity in reducing lepidocrocite at pH 9.5. Elemental sulfur was the main product from polysulfide oxidation, and the produced ferrous iron was mainly distributed in solid and adsorbed forms, with much less observed in the dissolved state. RL maintained a good reducing activity in the pH range from 6 to 9. X-ray photoelectron spectroscopy analysis suggests that both Fe(II) and the reduced sulfur species in RL participated in Cr(VI) reduction. The possible reaction paths of lepidocrocite reduction by polysulfide were proposed and consistent with the observed changes in reactants and products, and the reaction process may involve the adsorption of polysulfide, the electron transfer, and the detachment of produced Fe(II). This work also explained why polysulfide could maintain a prolonged reducing condition in field remediation of Cr(VI)-contaminated soil and groundwater.