Behavior of Uranium(VI) during HEDPA Leaching for Aluminum Dissolution in Tank Waste Sludges
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0893-JJ07-02.1
Behavior of Uranium(VI) during HEDPA Leaching for Aluminum Dissolution in Tank Waste Sludges Brian A. Powell1, Linfeng Rao1, Kenneth L. Nash2, Leigh Martin2 1 2
Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA Department of Chemistry, Washington State University, Pullman, WA
ABSTRACT Batch adsorption/dissolution experiments were conducted to examine the interactions between 233U(VI) and a synthetic aluminum oxyhydroxide (boehmite, γ-AlOOH) in 1.0M NaCl suspensions containing 1-hydroxyethane-1,1-diphosphonic acid (HEDPA). In the pH range 4 to 9, complexation of Al(III) by HEDPA significantly enhanced dissolution of boehmite. This phenomenon was especially pronounced in the neutral pH region where the solubility of aluminum, in the absence of complexants, is limited by the formation of sparsely soluble aluminum hydroxides. At high pH levels, dissolution of synthetic boehmite was inhibited by HEDPA, likely due to sorption of Al(III)/HEDPA complexes. Addition of HEDPA to equilibrated U(VI)-synthetic boehmite suspensions yielded an increase in the aqueous phase uranium concentration. The concentration of uranium continually increased over 59 days. Partitioning of uranium between the solid and aqueous phase was found to correlate well with HEDPA partitioning. INTRODUCTION The underground storage tanks at the Hanford site contain the byproducts from chemical separations processes for plutonium production. Over time, the material in the tanks has stratified to produce a clay-like sludge layer, a slurry phase, and an overlying salt cake. The transuranium elements are believed to be concentrated in the sludge phase, which is primarily composed of metal (oxyhydr)oxides. The current strategy for treatment of the tank waste calls for vitrification of the sludge followed by emplacement in a geological repository. The high concentration of aluminum and other non-radioactive elements increases the volume of the sludge and will in turn increase the volume of the waste package. Pretreatment of the sludge to remove non-radioactive constituents may reduce the volume of sludge. This is desirable as any reduction in volume will significantly reduce the cost of vitrification. However, the aluminum oxides and/or aluminosilicates have proven to be difficult to remove through baseline leaching processes. Previous experiments examining aluminum dissolution by treating sludge simulants with increasingly aggressive leaching procedures (0.01M NaNO2 + 0.01M NaOH, 3M NaOH, 0.05M glycolic acid + 0.10M NaOH, 0.10M HNO3, 2.0M HNO3, 0.5M HEDPA) found that no single treatment achieved complete dissolution [1]. Additionally, interactions of actinides with aluminum oxides have significant implications in designing strategies for sludge leaching. The objective of this study was to investigate the ability of HEDPA to accomplish dissolution of synthetic boehmite and the behavior of actinides during dissolution. HEDPA is a diphosphonate complexant that forms strong complexes with actinides and aluminum in acid
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