Modifications to EPA Method 3060A to Improve Extraction of Cr(VI) from Chromium Ore Processing Residue-Contaminated Soils

It
has been shown that EPA Method 3060A does not adequately extract Cr­(VI)
from chromium ore processing residue (COPR). We modified various parameters
of EPA 3060A toward understanding the transformation of COPR minerals
in the alkaline extraction and improving extraction of Cr­(VI) from
NIST SRM 2701, a standard COPR-contaminated soil. Aluminum and Si
were the major elements dissolved from NIST 2701, and their concentrations
in solution were correlated with Cr­(VI). The extraction fluid leached
additional Al and Si from the method-prescribed borosilicate glass
vessels which appeared to suppress the release of Cr­(VI). Use of polytetrafluoroethylene
vessels and intensive grinding of NIST 2701 increased the amount of
Cr­(VI) extracted. These modifications, combined with an increased
extraction fluid to sample ratio of ≥900 mL g–1 and 48-h extraction time resulted in a maximum release of 1274 ±
7 mg kg–1 Cr­(VI). This is greater than the NIST
2701 certified value of 551 ± 35 mg kg–1 but
less than 3050 mg kg–1 Cr­(VI) previously estimated
by X-ray absorption near edge structure spectroscopy. Some of the
increased Cr­(VI) may have resulted from oxidation of Cr­(III) released
from brownmillerite which rapidly transformed during the extractions.
Layered-double hydroxides remained stable during extractions and represent
a potential residence for unextracted Cr­(VI).