11 research outputs found
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Evaluation of chromium speciation and transport characteristics in the Hanford Site 100D and 100H areas
Field and laboratory investigations have been conducted to define the fate and transport characteristics of chromium contamination present in the 100D/H Areas of the Hanford Site. This information is relevant to assessing the impact of the release of hexavalent chromium to the Columbia River. Included in this study was the determination of the concentration and aqueous speciation of chromium in the unconfined acquifer and an assessment of potential changes in speciation as groundwater passes through the river/acquifer transition zone and mixes with the Columbia River. The results of this study indicate that chromium present within the Hanford acquifer is predominantly in the oxidized hexavalent state. Chromium is apparently stable in the oxidized form owing to its lack of organic matter within the acquifer. A portion of the chromium is removed as groundwater passes through the transition zone due to reduction and precipitation associated with sediment/water interaction processes. Chemical data collected from seep water samples, however, suggests, that most of the hexavalent chromium ultimately discharges into the Columbia River. Dilution of hexavalent chromium subsequently occurs during the mixing of groundwater and river water, with relatively little change taking place in speciation
In Situ Redox Manipulation by Dithionite Injection: Intermediate-Scale Laboratory Experiments
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Gas treatment of Cr(VI)-contaminated sediment samples from the North 60`s pits of the chemical waste landfill
Twenty sediment samples were collected at depths ranging from 5 to 100 ft (1.5 to 30 m) beneath a metal-contaminated plating-waste site and extensively characterized for Cr(VI) content and environmental availability. Three samples were selected for treatment with diluted gas mixtures with the objective of converting Cr(VI) to Cr(III), which is relatively nontoxic and immobile. These tests were designed to provide information needed to evaluate the potential application of gas injection as an in situ remediation technique. Gas treatment was performed in small columns (4.9-cm ID, 6.4- to 13.9-cm long) using 100 ppm ({mu}L L{sup -1}) H{sub 2}S or ethylene mixtures in N{sub 2}. Treatment progress during the tests involving H{sub 2}S was assessed by monitoring the breakthrough of H{sub 2}S. Evaluation of H{sub 2}S treatment efficacy included (1) water-leaching of treated and untreated columns for ten 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{sub 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 suggest that this residual Cr(VI) is present in low solubility phases such as PbCrO{sub 4} or sequestered in unreacted grain interiors under impermeable coatings formed during H{sub 2}S treatment. However, this fraction is essentially immobile and thus unavailable to the environment