PNNL

Abstract

This report describes our continuing studies directed at enhancing the dissolution of Cr(III) from Hanford tank sludges by treatment with oxidants under alkaline conditions. This study evaluates the use of ferrate, FeO42-, permanganate, MnO4-, and persulfate, S2O82-, at selectively removing chromium from washed Hanford Tank 241-S-110 sludge. Variables examined include the initial hydroxide concentration, time, and temperature. It was found that all oxidants enhanced both the rate and extent of chromium dissolution, with > 90% of the total chromium being dissolved under optimum conditions after 48-h contact times. The dissolved chromium was determined to be present as chromate, CrO42-. Elevated transuranic (TRU) element concentrations in the leach solutions were observed, attributed to enhanced Pu dissolution, but in all cases an immobilized form of the leach solutions would be considered a low-level waste, not a TRU waste. Evaluation of the immobilized high level waste (IHLW) that would be generated following oxidative alkaline leaching indicates that both extensive Al and Cr removal are needed to maximize the benefit of pretreatment. The amount of IHLW waste would be reduced by 20% by the almost quantitative Al removal documented to be possible through an extended caustic leach. Oxidative leaching of chromium sufficient to remove 95% of the remaining Cr provides for a further reduction of almost 50% in the amount of glass produced for a total of roughly 70% volume reduction in glass over that produced from untreated waste.