Solution Chemistry for Actinide Borate Species to High Ionic Strengths: Equilibrium Constants for AmHB 4 O 7 2+ And AmB
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Solution Chemistry for Actinide Borate Species to High Ionic Strengths: Equilibrium Constants for AmHB4O72+ And AmB9O13(OH)4(cr) and Their Importance to Nuclear Waste Management Yongliang Xiong 1* 1 Sandia National Laboratories (SNL), Carlsbad Programs Group, 4100 National Parks Highway, Carlsbad, NM 88220, USA, e-mail: [email protected] ABSTRACT Borate is present in natural groundwaters and borate is also released into groundwaters when borosilicate glass, waste form for high level nuclear waste, is corroded. Borate can form an aqueous complex, AmHB4O72+, with actinides in +III oxidation state. In this work, we present our evaluation of the equilibrium constant for formation of AmHB4O72+ and the associated Pitzer interaction parameters at 25oC. Using Nd(III) as an analog to Am(III), solubility data of Nd(OH)3(s) in NaCl solutions in the presence of borate ion from the literature, is used to determine Am(III) interactions with borate. The log10K for the formation reaction is 37.34. This evaluation is in accordance with the Waste Isolation Pilot Plant (WIPP) thermodynamic model in which the borate species include B(OH)3(aq), B(OH)4–, B3O3(OH)4–, B4O5(OH)42–, and NaB(OH)4(aq). The WIPP thermodynamic database uses the Pitzer model to calculate activity coefficients of aqueous species. In addition, the equilibrium constant for dissolution of AmB9O13(OH)4(cr) at 25oC is evaluated from the solubility data on NdB9O13(OH)4(cr) in NaCl solutions, again using Nd(III) as an analog to Am(III). The log10K for the dissolution reaction is –79.30. In the evaluation for log10K for the dissolution reaction, AmHB4O72+ is also considered. The equilibrium constant and Pitzer parameters evaluated by this study will be important to describe the chemical behavior of Am(III) in the presence of borate in geological repositories. INTRODUCTION In natural groundwaters such as brines associated with salt formations, there may be relatively high concentrations of borate [1]. In addition, when borosilicate glass for high level nuclear waste (HLW) is corroded, borate is also released into the groundwater [2-3]. Borate can impact nuclear waste management in two aspects. First of all, borate can form an aqueous complex with Nd(III) and Eu(III) [4-6], analogs to actinides in +III oxidation state, such as Am(III). Therefore, borate could become a potential transport agent for actinides if the Am(III)borate complex contributes significantly to the total Am(III) concentrations. In the second aspect, numerous actinide borates have been recently successfully synthesized [7-8], including a Pu(III) borate, Pu2[B12O18(OH)4Br2(H2O)3]•0.5H2O, in addition to the well known lanthanide borates such as NdB9O13(OH)4 [9]. This implies that actinides in waste could be transformed into, or be sequestrated as, actinide borates, if their solubility limits are reached. Actinides in +III state represented by Am(III) are present in nuclear waste in geological repositories. Therefore, the interactions between Am(III) and borate are important for performance assessment (PA) f
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