Improvement of a Cr(VI) Extraction Method for Chromite Ore Processing Residue-Contaminated Materials

Oral Presentation

Prepared by C. Mills1, A. Foster2, R. Wolf1, B. Carleton1, J. Morrison1, H. Miner1
1 - U.S. Geological Survey, Building 20, MS964D, Denver Federal Center, Denver, CO, 80225, United States
2 - U.S. Geological Survey, MS 901, 345 Middlefield Road, Menlo Park, CA, 94025, United States


Contact Information: cmills@usgs.gov; 303-236-5529


ABSTRACT

Method EPA 3060A is the standard for extraction of Cr(VI) from a solid matrix such as soil. However, several studies have shown that this alkaline digestion method does not adequately extract Cr(VI) from solids contaminated by chromite ore processing residue (COPR). This waste product is a result of the high lime process for isolating chromium from chromite ore and has historically been used as fill material in residential and commercial areas. It contains weight % concentrations of total Cr and a substantial fraction (25 to 30%) of this total Cr is often in the toxic Cr(VI) oxidation state. It has been shown that Cr(VI) extracted from COPR-contaminated materials is only a fraction (~20-60%) of the total Cr(VI) determined by X-ray absorption near edge structure spectroscopy (XANES). It is likely that the residence of Cr(VI) in COPR limits the efficiency of the 3060A extraction. 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.

We present several potential modifications to EPA3060A that improve extraction of Cr(VI) from NIST SRM 2701, a standard COPR-contaminated soil from NJ. Intensive grinding of NIST 2701 resulted in the extraction of 73030 mg kg-1 Cr(VI) which is substantially greater than the certified Cr(VI) value of 55135 mg kg-1 but still less than the Cr(VI) value of ~3000 mg kg-1 determined by a XANES study1. We found that increasing the extraction fluid to sample ratio also increased the efficiency of Cr(VI) extraction from NIST 2701. Extraction fluid to sample 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 efficiencies yielding as much as ~900 mg kg-1 Cr(VI) from NIST 2701 for intensively ground samples. Increasing the extraction time to 48 hours resulted in yet more Cr(VI) yield from NIST 2701 (up to 1250 mg kg-1) but we are still testing for the potential of significant Cr oxidation over this time period. We are analyzing pre- and post-extraction solids by XANES to assess the mass balance of Cr(VI) and to provide additional data on the amount of Cr(VI) in NIST 2701.

1Mahlerbe et al. (2011) Environmental Science and Technology 45, 10492-10500.