Field application of calcium polysulphide for ex situ treatment of soils contaminated with chromite ore processing residue
Richard J.F Bewley, Stuart Clarke

A field trial was undertaken to evaluate the application of calcium polysulphide (CaSx) for the treatment of hexavalent chromium (Cr(VI)) associated with chromite ore processing residue (COPR) deposited in south-east Glasgow, Scotland.

Various concentrations of 29% CaSx solution based upon 0.1x, 0.3x, 0.5x and 0.8x of the dosage that had achieved over 99% reduction in a laboratory trial were applied to skips containing approximately 4000 kg of material in conditions close to, or at, soil saturation.

Both the 0.5x and 0.8x dosages resulted in a reduction of Cr(VI) from mean starting concentrations of 742 and 822 mg/kg to below 2 mg/kg after 24 hours, with mean concentrations of 0.6–1.3 mg/kg remaining after 50 days. Leachable Cr(VI) (accounting for almost all the total Cr) was reduced from initial concentrations of 62–80 mg/L to 0.3 mg/L or below, following treatment. Dosages of 0.1x and 0.3x, or the 0.5x treatment under unsaturated conditions achieved a lesser degree of Cr(VI) reduction, although leachable Cr was still reduced to around 3 mg/L for the 0.3x application, and 0.6 mg/L for the 0.5x application in unsaturated soil.

Addition of CaSx did not generate detectable hydrogen sulphide and had little effect on concentrations of calcium or sulphide, although sulphate concentrations increased two- to three-fold.

In most treatments there were no significant differences in the mobility of other heavy metals following application. Increased leachability was observed in the case of selenium and vanadium, and to a lesser degree with mercury. However, such increases tended to be transient, at least for dosages up to and including the 0.5x application.

The findings demonstrate that CaSx represents a potentially viable treatment for Cr(VI) associated with the COPR deposited in the Glasgow conurbation.

Key words: calcium polysulphide, chromite, chromium, COPR, leachable, reduction

Land Contamination & Reclamation, 18 (1), 1-12 (2010)

DOI 10.2462/09670513.984

© 2010 EPP Publications Ltd

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