Cr(vi) soil analytical method: A reliable analytical method for extracting and quantifying Cr(vi) in soils

Abstract

Chromium has been used in the industrialized world in many applications for more than a century. Chromium is a trace metallic element found in the Earth's crust, and when it is found in concentrated ore deposits it is principally as FeCr₂O₄ (chromite ore). In the environment, chromium is typically found in the trivalent and hexavalent states. These two oxidation states have differing toxicities and mobilities. Hexavalent chromium [Cr(VI)] is classified as a known human carcinogen (via inhalation) and is rather mobile, whereas trivalent chromium [Cr(III)] is comparatively benign (it is an essential dietary element for humans) and relatively immobile. This significant toxicological and geochemical disparity between the two valence states necessitates that environmental investigators be able to quantitatively distinguish between these two forms in solid environmental media. Several regulatory‐approved analytical techniques exist for the quantitative differentiation between Cr(VI) and Cr(III) in aqueous solutions and for the measurement of total Cr in the solid phases. However, until recently, a regulatory‐approved analytical technique for the extraction of total Cr(VI) from soils and solid phases did not exist. Previous research was evaluated in order to develop an accurate and precise method for extracting and analyzing total Cr(VI) from solid media without causing Cr(III) oxidation or Cr(VI) reduction during the extraction and analytical process. Incomplete Cr(VI) spike recoveries [viz., reduction of Cr(VI) to Cr(III)] and possible oxidation of Cr(III) to Cr(VI) during the Cr(VI) extraction and analysis process have been suggested as insurmountable issues that precluded the development of a defensible extraction technique. Extensive research and an elaborate method evaluation study using a hot alkaline extractant combined with routine redox characterization of test samples resulted in the development of a reliable extraction method for the analysis of total Cr(VI). Based on that research, in June 1997 the U.S. EPA promulgated SW‐846 Method 3060A for inclusion in the Third Update to the Test Methods for Evaluating Solid Wastes, SW‐846, 3rd ed. The proposed SW‐846 Method 3060A provides procedural detail for method utilization with significant guidance to environmental investigators on sample redox characterization, method limitations, and the interpretation of matrix spike data.