Total Hexavalent Chromium Quantification in High Organic Carbon Soils: Minimizing Reduction During Extraction and Analysis.

The current, standard extraction method used to quantify total Cr(VI) in soils and solid wastes is an aggressive, hot alkaline extraction that can falsely indicate a low Cr(VI) concentration in matrices containing elevated levels of organic C. This is due to method-induced reduction of Cr(VI) to Cr(III) by solubilized humic and fulvic acids. To assess and correct for this interference, A and B soil horizons from Maryland, USA, along with chromite-ore processing residue (COPR), were incubated for 1 week with a range of added C as leaf compost. The soils were then subjected to three Cr(VI) spiking treatments: 0 mg Cr(VI)/kg spike, 100 mg Cr(VI)/kg pre-digestion spike and 1 mg Cr(VI)/L post-digestion spike. The solubilized Cr(VI) from the heated extraction was analyzed by the 1,5-diphenylcarbazide method (DPC) method. We compared the results of this combination of EPA-approved extraction and analysis methods to our proposed method with the following modifications: the heating (95^oC) during the one-hour extraction was replaced with shaking at 22^oC, followed by Cr(VI) analysis with ion chromatography using anion separation and conductivity detection (IC). Hexavalent Cr extracted from the heated Maryland soils with > 10 g C/kg could not be analyzed by DPC due to flocculation of humic acids by the acid reagent in the reaction test tubes. The solubilized C also interfered with the IC analysis. The unheated, alkaline extraction, combined with IC analysis, minimized method-induced reduction of Cr(VI) in soils containing organic C. However, COPR soils with no compost added performed best with the standard heated extraction and DPC analysis. The proposed, modified method should be used when working with soils with higher than a 10 g/kg organic C content.