Hexavalent Chromium Extraction from Soils: A Comparison of Five Methods

摘要

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 CrO_4~(2-) matrix spikes in four diverse soil materials following equilibration with commonly used extractants: distilled water (pH 5.7), phosphate buffer (5 mM K_2HPO_4/5 mM KH_2PO_4, pH 7.0), carbonate-hydroxide extractant (0.28 M Na_2CO_3/0.5 M NaOH; pH ≈ 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 K_2CrO_4 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. Mo Cr(Ⅵ) was recovered by any of the methods after spiking an anoxic sediment, as predicted by the reducing sonditions of this soil for Cr(Ⅵ). Lead chromate [K_(sp) 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 soil (LowHex COPR) with 61% of its lative Cr(Ⅵ) in insoluble forms. In contrast, BaCrO_4 (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.