Hydrogen Sulfide Gas Treatment of Cr(VI)-Contaminated Sediment Samples from a Plating-Waste Disposal SiteImplications for in-Situ Remediation

Twenty sediment samples were collected at depths ranging from 5 to 100 feet beneath a chromate-contaminated plating-waste site and analyzed for Cr(VI), total chromium, and related constituents. Three of the samples were selected for treatment with dilute hydrogen sulfide (H 2 S) gas to evaluate this approach as a possible in-situ remediation technique. Gas treatment was performed in soil-packed columns using 100 ppm (μL L - 1 ) H 2 S mixtures, and treatment progress was assessed by monitoring the breakthrough of H 2 S. Evaluation of treatment efficacy included (1) water-leaching of the treated and untreated columns for 10 days, (2) repetitive extraction of treated and untreated subsamples by water, 0.01 M phosphate (pH 7) or 6 M HCl solutions, and (3) Cr K-edge X-ray absorption near-edge structure (XANES) spectroscopy of treated and untreated subsamples. Results of the water-leaching studies showed that the H 2 S treatment decreased Cr(VI) levels in the column effluent by 90% to nearly 100%. Repetitive extractions by water and phosphate solutions echoed these results, and the extraction by HCl released only 35−40% as much Cr in the treated as in the untreated samples. Analysis by XANES spectroscopy showed that a substantial portion of the Cr in the samples remained as Cr(VI) after treatment, even though it was not available to the water and phosphate extracting solutions. These results indicate that the residual Cr(VI) was sequestered in unreacted grain interiors under impermeable coatings formed during H 2 S treatment. However, this fraction is immobile and thus unavailable to the environment.