Polysulfide is a promising reductant for in-situ geochemical fixation of Cr(VI) in soil and groundwater systems due to its high stability and capability in maintaining an effective reducing environment for a prolonged period of time. Formation of Fe(II) on ferric-(hydro)oxide-coated sediment reduced by polysulfide has been speculated to help create an effective reducing environment, but the redox kinetics of Fe(II) formation, its speciation and the associated mechanisms have not been clearly understood, and the capability of newly-formed polysulfide-reduced iron minerals for Cr(VI) reduction is yet to be examined. In this study, the redox reactions between three common ferric-(hydro)oxide minerals (lepidocrocite, hematite and goethite) and polysulfide were studied at pH 9.5 under anoxic condition. The polysulfide-reduced minerals (RM) were then examined in their capabilities for further reducing Cr(VI) under a wide range of pH from 6-10. Experimental results showed that, for all the three polysulfide-reduced minerals, the produced Fe(II) mainly distributed in the solid and sorbed forms, with solid form Fe(II) accounting for 55% (lepidocrocite), 70% (hematite), and 85% (goethite) of the total Fe(II) after 6 h of reaction. Further experiments indicate all the polysulfide-reduced minerals could reduce Cr(VI) under the pHs tested, with generally higher reaction rates observed at lower pH values. Cr(VI) reducing capabilities of the polysulfide-reduced minerals were found to follow the sequence: lepidocrocite &gt; hematite &gt; goethite. XPS analysis revealed that both Fe(II) and the reduced sulfur species on polysulfide-reduced minerals’ surface participated in the reaction with Cr(VI). Mechanisms for ferric-(hydro)oxides reduction by polysulfide, and Cr(VI) reduction by the reduced minerals were also proposed in consistent with the observed reaction kinetics and product formation.

Reductive immobilization of hexavalent chromium by polysulfide-reduced iron (hydro)oxides

253rd National Meeting (2017)

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