HEXAVALENT CHROMIUM EXTRACTION FROM SOILS - A COMPARISON OF 5 METHODS

A quantitative extraction method for total Cr(VI) (soluble and insoluble forms) in soils is needed to assess potential hazards from this heavy metal to humans and ecological systems. A laboratory study was conducted to measure native levels of Cr(VI) and recovery percentages of soluble and insoluble CrO42- matrix spikes in four diverse soil materials following equilibration with commonly used extractants: distilled water (pH 5.7), phosphate buffer (5 mM K2HPO4/5 mM KH2PO4, pH 7.0), carbonate-hydroxide extractant (0.28 M Na2CO3/0.5 M NaOH: pH approximate to 11.8) with and without heating, and hydroxide solution (0.1 M NaOH; pH 13) with sonication. The heated carbonate-hydroxide solution was the most effective extractant for operationally defining total Cr(VI) in soils that contained native Cr(VI) or in soils that had a sufficiently high redox status to maintain Cr as Cr(VI). More than 90% of a soluble K2CrO4 spike [1.0 g of Cr(VI) kg(-1)] was recovered using the heated carbonate-hydroxide extraction method on a redox-inert quartz sand, loamy-textured A horizon soil (Ultic Hapludalf), and chromite ore processing residue-enriched soil (HighHex COPR) containing 29% of its total Cr(VI) (1.4 g kg(-1)) in insoluble forms. No Cr(VI) was recovered by any of the methods after spiking an anoxic sediment, as predicted by the reducing conditions of this soil for Cr(VI). Lead chromate (K,, 1.8 x 10(-14)) was completely recovered by the heated carbonate-hydroxide extractant in the absence of a soil matrix, after being spiked into the loam soil containing no native Cr(VI), and after being spiked into another COPR sail (LowHex COPR) with 61% of its' native Cr(VI) in insoluble forms. In contrast, BaCrO4 (K-sp 1.6 x 10(-10)) was completely recovered by the carbonate-hydroxide extractant in the absence of a soil matrix, but 63 and 78% recoveries were measured after spiking into the loam and LowHex COPR soils, respectively. Method-induced reduction of soluble Cr(VI) was not observed under the conditions of the heated alkaline extraction.