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IN SITU CR(VI) TREATMENT USING A FERROUS IRON-BASED REDUCTANT
Citation:
Ludwig*, R, C. Su, F P. Beck*, P J. Clark*, S Acree*, R R. Ross*, AND P K. Jones*. IN SITU CR(VI) TREATMENT USING A FERROUS IRON-BASED REDUCTANT. Presented at 4th Annual Int'l. Conf. on Remediation of Chlorinated & Recalcitrant Compounds, Monterey, CA, May 24, 2004.
Impact/Purpose:
To inform the public.
Description:
Laboratory and field studies were conducted to evaluate the performance of a ferrous sulfate/ sodium hydrosulfite (dithionite) reductant blend in treating a hexavalent chromium (Cr(VI)) source area and Cr(VI) dissolved phase plume at a former industrial site in Charleston, South Carolina. The Cr(VI) source area, consisting largely of chromite ore processing residue (COPR) generated in the production of ferrochrome alloy, is highly alkaline with groundwater pH values measured as high as 11.5, dissolved phase Cr(VI) concentrations as high as 57 mg/L, and solid phase Cr(VI) levels as high as 550 mg/kg. Laboratory tests indicated ferrous iron in the form of ferrous sulfate or ferrous chloride was most effective in treating the solid phase Cr(VI). The direct injection of ferrous iron salts into the saturated zone, however, was considered impractical due to the known tendency of ferrous iron to rapidly precipitate out of solution during injection, particularly under high pH conditions such as those in effect at the site. To address this concern, laboratory tests were conducted using ferrous sulfate in combination with sodium hydrosulfite. Although sodium hydrosulfite itself was observed to be ineffective in treating the Cr(VI) source material (based on laboratory studies conducted), it was theorized that sodium hydrosulfite could serve as a stabilizing agent to maintain the ferrous iron in solution and thereby allow for its effective dissemination in the subsurface.
Two in situ field pilot studies were subsequently conducted at the site. One was conducted in the source area and involved injection of 18,000 liters of a 0.25M ferrous sulfate/0.25M sodium hydrosulfite solution blend directly into the saturated zone source material. The second was conducted outside the source area in the path of a dissolved phase Cr(VI) plume and involved injection of 5000 liters of the same reductant blend into each of two injection wells in the saturated zone. The objective of the second pilot study was to determine whether a ferrous iron treatment zone could be established to passively treat dissolved phase Cr(VI). Groundwater samples were collected before and after injection from a system of monitoring wells installed in each pilot test area. Groundwater samples were monitored in the field for Cr(VI), pH, ORP, conductivity, temperature, dissolved oxygen, ferrous iron, alkalinity, and sulfide. Samples were analyzed in the laboratory for cations and anions.