In Situ Source Treatment of Cr(VI) Using a Fe(II)-Based Reductant Blend: Long-Term Monitoring and Evaluation

The long-term effectiveness of an FeSO4+Na2S2O4 reductant solution blend for in situ saturated zone treatment of dissolved and solid phase Cr(VI) in a high pH chromite ore processing solid waste fill material was investigated. Two field pilot injection studies were conducted that showed sustained treatment of Cr(VI) over evaluation periods of more than 1,000 days. No well or aquifer formation clogging was observed during injection although treatment was limited to the pore volume displacement radius of the injected reductant. Analysis of posttreatment core samples suggested >85% treatment effectiveness of solid phase Cr(VI), whereas lab tests suggested treatment of the solid phase Cr(VI) can be complete provided the chromite ore processing solid waste sediments are sufficiently dosed with the reductant. The sustained treatment of dissolved phase Cr(VI) migrating through the treatment zones for more than 1,000 days following injection provided strong evidence of a residual treatment capacity having been imparted to the formation solids. Scanning electron microscopy-energy dispersive x-ray spectroscopy analyses of posttreatment core samples indicated that much of the Cr(VI) may be removed through the formation of a Cr-bearing precipitate, possibly a complex carbonate, characterized by an Fe:Cr molar ratio of roughly 1:1.