Influence of Carbonate on Uranium Solubility in the WIPP
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Influence of Carbonate on Uranium Solubility in the WIPP Jean-Francois Lucchini1, Sally Ballard2, and Hnin Khaing3 1 Los Alamos National Laboratory, Earth and Environmental Sciences Division, 115 N. Main, Carlsbad, NM 88220, U.S.A 2 Carlsbad Environmental Monitoring and Research Center, New Mexico State University, 1400 University Drive, Carlsbad, NM 88220, U.S.A. 3 URS, Washington Safety Management Solutions, 1400 University Drive, Carlsbad, NM 88220, U.S.A. ABSTRACT In the performance assessment (PA) for the Waste Isolation Pilot Plant (WIPP), the solubility of uranium (VI) was conservatively set at 10-3 M for all expected WIPP conditions, including the potential and likely effects of carbonate complexation [1]. Under WIPP-relevant conditions, long-term experiments were performed to establish the uranium (VI) solubility limits in WIPP-simulated brine over a broad range of pCH+ values [7.5-12.5] and to evaluate the contribution of carbonate complexation and hydrolysis to uranium (VI) speciation. Data obtained in carbonate-free ERDA-6 brine, a simulated WIPP brine, were reported earlier [2]. In the absence of carbonate, uranium solubility approached 10-7 M at the expected pCH+ in the WIPP (~ 9.5). In the presence of a significant amount of carbonate (millimole levels), recent experimental results showed that uranium (VI) concentrations will not exceed 10-4M. This measured solubility limit is an order of magnitude lower than the uranium solubility value currently used in the WIPP PA [3]. A small effect of borate complexation was found in the pCH+ range [7.5-10]. At pCH+ ≥ 10, hydrolysis overwhelmed carbonate effects, and no amphoteric effect was observed. INTRODUCTION In the anoxic and strongly-reducing environment expected in the WIPP, tetravalent uranium (IV) is predicted to be the dominant oxidation state and will establish uranium solubility at a very low value (about 10-8M) [4]. However, some uranium (VI) phases and aqueous species, although not expected to predominate in the WIPP, could be present due to localized oxidation effects. Consequently, in WIPP PA, both IV and VI oxidation states of uranium are considered. Uranium is assumed to be uranium (VI) in 50% of the PA vectors and uranium (IV) in 50% of the PA vectors [2]. The solubility of uranium (VI) was conservatively set at 10-3 M in WIPP PA for all expected WIPP conditions, including the potential and likely effects of carbonate complexation [1]. The solubility of uranium (VI) in the WIPP is expected to be defined by the combined contribution of hydrolysis and carbonate complexation. Experimental data published in previous papers [2, 5, 6] were obtained under carbonate-free conditions, so they provided a baseline for carbonate effects on uranium (VI) solubility. In this paper, some results of long-term
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experiments carried out in the presence of a significant amount of carbonate (up to millimole levels) are given. EXPERIMENT Long-term experiments to establish the solubility of uranium (VI) in the presence of carbonate were performed in ERDA-6 and G
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