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Hexavalent Chromium Solubility and Reduction in Alkaline Soils Enriched with Chromite Ore Processing Residue

Soil Chemistry Laboratory, Agronomy Dep., Univ. of Maryland, College Park, MD 20742.

^* Corresponding author.

ABSTRACT

Health-based regulatory limits and remediation strategies for chromate [Cr(VI)] in alkaline soils enriched with chromite ore processing residue (COPR) should be based on oxidation-reduction and solubility interactions of Cr. A laboratory study characterized the solubility of Cr(VI) by sequential batch equilibrations and teachings from two COPR-enriched soils using dilute salt and acid solutions. Batch equilibrations quantified reduction of soluble and insoluble Cr(VI) in the whole soils and extracted solutions. The soils, termed Low Hex and High Hex, contained 105 and 460 mg Cr(VI) kg^–1, 1.8 and 10.4 g total Cr kg^–1, and had pH values of 8.5 and 10.4, respectively. Total cumulative Cr(VI) removed by the extractions was 49 and 386 mg kg^–1 by both the batch equilibrations and leaching methods in the Low and High Hex soils, respectively, and no differences were found among the extractants. Addition of Mn²⁺ resulted in 50 to 100% reduction of soluble Cr(VI) in both soils without pH adjustment, a new finding that is thermodynamically predictable at pH > 7.2, and it is in sharp contrast to the ability of Mn(III,IV) oxides to oxidize Cr(III) to Cr(VI) in more acidic soils and Mn oxide suspensions. Additions of Fe²⁺, steel wool (Fe), leaf litter, and lactic acid (a component of human sweat) also reduced various quantities of soluble and insoluble Cr(VI) in the soils, depending on pH, reducing agent, and soil. Soluble Cr(VI) in these COPR-enriched soils appeared to be controlled by dissolution of a moderately soluble chromate salt, such as CaCrO₄, and not by sorption or oxidation-reduction equilibria.