IMPROVEMENT OF A CR(VI) EXTRACTION METHOD FOR CHROMITE ORE PROCESSING RESIDUE-CONTAMINATED MATERIALS

Chromite ore processing residue (COPR) produced by the high lime process contains high concentrations of total Cr (typically near 5 weight %). A substantial fraction (25 to 30%) of total Cr is often in the toxic Cr(VI) oxidation state. The mineralogy of COPR is well-documented and is dominated by calcite [CaCO₃], periclase [MgO], brucite [Mg(OH)₂], hydrated calcium aluminate minerals, brownmillerite (Ca₄Al₂Fe₂O₁₀), unreacted chromite [(Mg,Al,Fe)Cr₂O₄], and amorphous material. Cr(III) is present mainly in chromite while Cr(VI), which forms an oxyanion, is found substituting for other anions in the hydrated calcium aluminate minerals hydrocalumite, ettringite, and hydrogarnet. Historically, COPR has been used as fill material in residential and commercial areas in the US states of New Jersey and Maryland and in other countries. The mineralogy of COPR typically results in alkaline pH of COPR-contaminated soils. Cr(VI) is redox stable and quite mobile at alkaline pH so that leaching of Cr(VI) poses a serious environmental issue. The accurate determination of total Cr(VI) concentrations in COPR-contaminated soils is essential to address this issue, yet current methods fail to quantitatively extract Cr(VI) from COPR or COPR-contaminated materials. For example, Mahlerbe et al. (2011) showed that the certified Cr(VI) concentration of NIST SRM 2701 (551±35 mg kg^-1) determined by EPA Method 3060A is <20% of the total Cr(VI) concentration of SRM 2701 determined by X-ray absorption near edge structure spectroscopy (XANES). We found that intensive grinding of COPR-contaminated soils prior to extraction resulted in significant improvement in Cr(VI) extraction efficiency. Grinding was performed by micronization with both water and methanol as lubricants. Up to 1.6 times as much Cr(VI) was extracted from ground versus unground samples of SRM 2701. A similar improvement in extraction efficiency was observed for additional COPR-contaminated soil from New Jersey. We are analyzing pre- and post-extraction materials by SEM-EDX, XRD, and XANES as well as extraction solution chemistry to understand whether grinding enhances the dissolution of a specific mineral residence of Cr(VI).

Mahlerbe et al. (2011) Environmental Science and Technology 45, 10492-10500.