Polysulfide has been frequently used in remediation of chromate-contaminated soil and groundwater systems. This reagent has been reported capable to establish a prolonged reducing environment in the subsurface long after its injection during site remediation, but the associated mechanism is yet to be understood. In this study, magnetite (Fe3O4), a common iron oxide in sediment was selected as a representative iron oxides and its redox interaction with polysulfide was examined in pH 9-11. The resulted mineral (R-Fe3O4) was then evaluated in its capability to reduce chromate in pH 6-9 under the anaerobic condition. Batch experiments revealed that the total Fe(II) produced from Fe3O4 reduction by polysulfide increased with decreasing solution pH. The newly formed Fe(II) was found mainly distributed in the solid/sorbed forms, with much less fraction (less than 20%) existing in the dissolved form. The reduced magnetite was found capable in reducing chromate to Cr(III)  in the tested pH range, with higher reducing efficiency generally observed at lower pH. Characterization by XPS, XRD and Mössbauer spectroscopy of R-Fe3O4 and its reaction product with chromate suggested that, both the newly formed Fe(II) and iron sulfide from Fe3O4–polysulfide reaction would be responsible for the additional reducing power. A mechanism was proposed for the possible reaction pathways, in accordance with both spectral characterization and the experimental results.

Reduction of Magnetite by Polysulfide and its effect on reductive immobilization of Chromate

2019 Spring National Meeting

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