Modifications to EPA Method 3060A that improve the Cr(VI) extraction efficiency from chromium ore processing residue-contaminated soils

Metals and Metals Speciation Analysis in Environmental Samples
Oral Presentation

Presented by R. Wolf
Prepared by C. Mills1, A. Foster2, C. Bern1, J. Morrison1, W. Benzel1
1 - U.S. Geological Survey, Building 20 MS964D, Denver Federal Center, Denver, CO, 80225, United States
2 - U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA, 94025, United States


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


ABSTRACT

The Environmental Protection Agency (EPA) method 3060A is the standard for extraction of Cr(VI) from solid matrices. 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 generated waste product of the high lime process for isolating chromium from chromite ore 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 the total Cr is often in the toxic Cr(VI) oxidation state. It has been shown that method 3060A extracts only of fraction (~20-60%) of total Cr(VI) in COPR-contaminated materials as independently determined by X-ray absorption near edge structure spectroscopy (XANES).

We present potential modifications to EPA 3060A that improve the extraction of Cr(VI) from NIST SRM 2701, a standard COPR-contaminated soil from NJ. Intensive grinding of NIST 2701 and the use of PTFE instead of glass extraction vessels resulted in the extraction of 85140 mg/kg Cr(VI). This is substantially greater than the certified Cr(VI) value of 55135 mg/kg but still less than the Cr(VI) value of ~3000 mg/kg previously estimated by XANES [1]. 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 variable extraction efficiencies. Ratios of 900 mL/g or greater resulted in relatively consistent extraction efficiencies yielding ~940 mg/kg Cr(VI) from intensively ground NIST 2701. Additionally, increasing the extraction time to 48 hours resulted in even greater extraction efficiencies (up to 1270 mg/kg). The combination of our modifications to EPA 3060A resulted in XANES-based estimate for total Cr(VI) content of NIST 2701 to be closer to 1300 mg/kg than the previously estimated 3000 mg/kg.

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