Characterization of Chromium(III) Hydroxide Solids and their Oxidation by Hydrogen Peroxide
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Characterization of Chromium(III) Hydroxide Solids and Their Oxidation by Hydrogen Peroxide Zhicheng Zhang1,3, Linfeng Rao1,*, Dhanpat Rai2, Sue B. Clark3 1
Lawrence Berkeley National Laboratory, One Cyclotron Road, Berkeley, CA 94720, U.S.A. Pacific Northwest National Laboratory, Richland, WA 99352, U.S.A. 3 Washington State University, Pullman, WA 99165, U.S.A. 2
ABSTRACT Chromium is of great concern in the vitrification of high-level nuclear waste sludges because it forms separate crystallites in the molten glass. Inadequate removal of chromium from sludges could result in the production of an unacceptably large volume of HLW glass. Alkaline oxidative leaching is considered one of the pretreatment strategies to remove chromium before the vitrification. In this study, Cr(III) hydroxide solids were prepared under different conditions and characterized by EXAFS and IR. The rate of oxidation of the solids by hydrogen peroxide in alkaline solutions was studied by UV absorption spectroscopy. EXAFS and IR experiments indicate that the degree of oligomerization in the Cr(III) hydroxide solids increases with the increase in the concentration of NaOH in solution, the aging temperature and the aging time. The rate of oxidation of the solids follows the same order previously observed for Cr(III) oligomers in solution, i.e., species with higher degree of oligomerization are oxidized more slowly. INTRODUCTION Chromium exists in the high-level nuclear waste (HLW) sludges that are eventually vitrified and made into glass waste forms for geological disposal. Though the total concentration of chromium in most sludges is less than 1.3 wt% [1], it is a problematic element in the vitrification process and the most important constituent in defining the total volume of HLW glass that will be produced from the Hanford tank wastes. Under the proposed operating temperature of the glass melter (1050-1150oC), even small amounts of chromium can result in the formation of a refractory spinel-type crystalline solid phase, adversely affecting the process operation. As a result, the allowable limits for chromium in the melter feed must be as low as 0.34 wt% based on the original design of the vitrification plant at Hanford [2]. Therefore, it is critical to develop pretreatment strategies that can effectively remove chromium from HLW or reduce its concentration to the level that can be tolerated in the vitrification process. The chromium in the sludge is mainly Cr(III) and the majority of the HLW is under alkaline conditions. The baseline strategies of sludge pretreatment at Hanford include caustic washing and oxidative leaching. It is expected that chromium could be removed from the sludge by forming anionic species such as Cr(OH)n(n-3)- (n ≥ 4) in caustic washing, or by oxidation of Cr(III) to Cr(VI) in oxidative leaching. However, the actual removal of chromium from the sludge was inadequate and lower than expected. This is not surprising because of the complexity of chromium chemistry in alkaline solutions and the absence of data f
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