| Abstract
| - A method has been developed for removing chromiumfrom alkaline high-level radioactive tank waste. Removingchromium from these wastes is critical in reducing thevolume of waste requiring expensive immobilization anddeep geologic disposition. The method developed is basedon the oxidation of insoluble chromium(III) compounds tosoluble chromate using ferrate. This method could be generallyapplicable to removing chromium from chromium-contaminated solids, when coupled with a subsequentreduction of the separated chromate back to chromium(III).The tests conducted with a simulated Hanford tanksludge indicate that the chromium removal with ferrate ismore efficient at 5 M NaOH than at 3 M NaOH. Chromiumremoval increases with increasing Fe(VI)/Cr(III) molar ratio,but the chromium removal tends to level out for Fe(VI)/Cr(III) greater than 10. Increasing temperature leads to betterchromium removal, but higher temperatures also led tomore rapid ferrate decomposition. Tests with radioactiveHanford tank waste generally confirmed the simulant results.In all cases examined, ferrate enhanced the chromiumremoval, with a typical removal of around 60−70% of thetotal chromium present in the washed sludge solids.The ferrate leachate solutions did not contain significantconcentrations of transuranic elements, so these solutionscould be disposed as low-activity waste.
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