CHALLENGES OF ACCURATELY DETERMINING THE CR(VI) CONTENT OF SOILS CONTAMINATED WITH CHROMIUM ORE PROCESSING RESIDUE

Speciation of Cr is important as Cr(VI) has known toxicity while Cr(III) is relatively non-toxic. Extraction of Cr(VI) from solid phase materials is difficult because acidic conditions promote its rapid reduction to Cr(III). It has previously been shown that a standard alkaline method for extraction of Cr(VI) from solid matrices (EPA Method 3060A) does not adequately extract Cr(VI) from solids contaminated by chromium ore processing residue (COPR). This waste product is a result of the high lime process for isolating Cr from chromite ore and has historically been used as fill material in residential and commercial areas. It contains weight percent concentrations of total Cr with a substantial fraction in the Cr(VI) oxidation state. Previous studies have shown that Cr(VI) extracted from COPR-contaminated materials using alkaline methods is only a fraction (~20 to 60%) of the total Cr(VI) determined by X-ray absorption near edge structure (XANES) spectroscopy.

We systematically tested modifications to prescribed parameters of EPA 3060A towards improving extraction efficiency of Cr(VI) from NIST SRM 2701, a standard COPR-contaminated soil from NJ. The alkaline extraction fluid leached Al, Si, and B from the prescribed glass vessels which interfered with Cr(VI) extraction from the material. The use of polytetrafluoroethylene vessels increased the Cr(VI) extraction efficiency. Intensive grinding of NIST 2701 resulted in the extraction of 730±30 mg kg^-1 Cr(VI), which is substantially greater than the certified Cr(VI) value of 551±35 mg kg^-1 but still less than the Cr(VI) value of ~3000 mg kg^-1 previously determined by XANES. Increasing the extraction fluid to sample ratio also increased the efficiency of Cr(VI) extraction from NIST 2701. Ratios similar to those prescribed by 3060A resulted in low and highly variable extraction efficiencies. Ratios of 900 mL g^-1 or greater resulted in relatively consistent extraction, yielding as much as ~900 mg kg^-1 Cr(VI) from intensively ground NIST 2701. Increasing the extraction time to 48 hours resulted in yet more Cr(VI) yield from NIST 2701 (up to 1250 mg kg^-1). In addition, we have performed numerous XANES analyses on several different treatments of NIST 2701 and have determined that the Cr(VI) content is substantially less than the ~3000 mg kg^-1 reported by a previous study.