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The Solubility of Uranophane Under Oxidizing Conditions in Caci2 and Sio2(Aq)

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The Solubility of Uranophane Under Oxidizing Conditions in Caci2 and Sio2(Aq) L The Solubility of Uranophane under Oxidizing Conditions in CaClz and Si02(aq) Test Solutions James D. Prikryl’.’ and William M. Murphy’ 1 Center for Nuclear Waste Regulatory Analyses, Southwest Research Institute, 6220 Culebra Road, San Antonio, Texas 78238-5 166 2Department of Geosciences, California State University, Chico, California 95929-0205 Uranophane /Solubility /Synthesis / Oxidizing conditions / Yucca Mountain Summary. Uranophane [Ca(UOz)2Siz07 6H20] is a corrosion product of long-term leaching of spent fuel under oxidizing conditions and is a weathering product of uraninite in uranium ore deposits hosted by siliceous volcanic rocks. The solubility of uranophane may, therefore, play an important role in radionuclide release at the proposed nuclear waste repository at Yucca Mountain, Nevada. In this study, the solubility of uranophane in Ca- and Si-rich test solutions was investigated. Batch solubility experiments were designed to approach uranophane equilibrium in both undersaturated and supersaturated solutions that had initial U concentrations of 10” to lo-’ mol . L-I in matrices of lo-’ mol . L-’ CaCI2 and 10” mol . L-I SiO,(aq). Experimental solutions were reacted with synthetic uranophane (confumed by XRD and chemical analyses) and analyzed at 1-week intervals over 7 weeks. A drop in solution pH and strong Ca precipitation in solutions with higher initial U concentrations characterized initial reaction of uranophane with experimental solutions. After the initial effect, reaction paths were dominated by uranophane dissolution coupled to uranyl mineral precipitation. Calculated reaction quotients (log Qs) for uranophane dissolution were related to solution pH and ranged from 10.89 to 14.62. Based on higher solution pH, test solutions with lower initial U contents ended closer to the solubility limit of uranophane. However, continued uranophane dissolution indicated that test solutions at the ends of the experiments were undersaturated with respect to uranophane. Introduction Under oxidizing aqueous conditions, uraninite (U02)and spent nuclear fuel (generally greater than 95% UOz) are thermodynamically unstable and will undergo dissolution leading to the formation of secondary uranyl phases. Crystallographic theory and experimental data indicate that spent fuel waste species (e.g., Np) may be incorporated in the structures of relatively stable secondary uranyl phases [ 1,2]. The solubilities of secondary uranyl minerals may, therefore, play an important role in constraining the source term for radionuclide release from high-level nuclear waste (HLW) repositories. For example, in recent performance assessments conducted by the Center for Nuclear Waste Regulatory Analyses for the proposed * Author for correspondence (E-mail: [email protected]) 1 HLW geologic repository at Yucca Mountain (YM), Nevada, a source term model is included based on the assumption that radionuclides from the spent fuel matrix become incorporated in the oxidized secondary uranium mineral schoepite. Radionuclide releases then occur in proportion to the solubility-limited dissolution of schoepite [3-51. Studies of uranium (U) deposits that are natural analogs to the proposed high-level nuclear waste repository at YM (e.g., deposits in the Peiia Blanca Uranium District of Chihuahua, Mexico) indicate that the calcium uranyl silicate, uranophane [Ca(U02)2Siz07 6H20] is the end product of U mineralization hosted by siliceous volcanic rocks [6]. Similarly, long-term leaching studies of synthetic U02and spent U02fuel designed to mimic conditions in a YM repository indicate that uranophane is a probable end product of the alteration of spent nuclear fuel [7, 81. Natural uranophane has been noted to incorporate Th [9]. Therefore, uranophane that is secondary after spent fuel could incorporate part of the nuclear waste inventory and control its release fiom the engineered barrier system. However, the lack of, and often conflicting, thermodynamic data for uranyl minerals hamper predictive modeling of uranyl mineral formation and dissolution. In this study, the solubility of uranophane under oxidizing conditions was studied by reacting uranophane with Ca- and Si-rich solutions calculated to bracket uranophane solubility. Reasonable interpretation of solubility data requires experimental samples to be relatively pure, single-phase minerals of known composition and structure. Although natural uranophane samples are available, they are typically of insufficient quantity or purity for use in solubility experiments [lo]. Moreover, crystal defects in natural samples, which may be present due to radiation damage, could adversely affect the results of the experiments. Thus, experiments were performed using synthesized uranophane, whose synthesis and chemistry are described in this paper. Experimental Uranophane synthesis and characterization Uranophane was synthesized based on the method of Cesbron et al. [l 11 using reagent grade uranyl acetate [U02(CH3C00)2. 2H20], sodium metasilicate pa2Si03. 9H20], and calcium acetate [Ca(CH3C00)2. H20]. About 105 g ofthe reagents in the stoichiometric ratio Ca:U:Si = 1:2:2 were placed in a teflon-lined 2 stainless steel reaction vessel. Then about 1,350 g of deionized, degassed water (with pH lowered to -1 .O by addition of HCI) were added to the vessel. The vessel was sealed and evacuated to remove air. The temperature was raised to 150 "C and the reaction was allowed to proceed for 10 days. After cooling, the supernatant liquid was decanted. The solid product was washed several times with deionized water, dried in an oven at about 60 "C, and stored in a dessicator at room temperature (20.5 f 2.0 "C). An X-ray powder diffraction (XRD) pattern of the synthetic uranophane is shown in Fig. 1, together with a reference pattern for natural uranophane taken from the International Centre for Diffraction Data (ICDD) database [12]. Correspondence between the sample and reference patterns is good. Two small peaks (at 26.9" and 28.2" 20) appear in the pattern for the synthetic sample and are absent in the reference pattern. However, these peaks are reported in a reference pattern for natural uranophane (Powder Diffraction File Number 8-442) from the ICDD Minerals Data Book [ 131 and appear in uranophane synthesized by Nyugen et al. [ 141. The magnitude and sharpness of the peaks in the XRD pattern indicate that the synthesized material is well crystallized. Fig. 1 also includes a scanning electron photomicrograph showing the fine-grained character and acicular morphology of the synthesized uranophane. Chemical analyses of synthetic uranophane were performed using a whole rock procedure that entails a complete dissolution of the sample in 0.1 M HCI followed by inductively coupled plasma mass spectrometry (ICP-MS). Using measured analytical contents, stoichiometric coefficients for Ca:U:Si in the synthesized uranophane were calculated to be 0.99(~0.01):2.01(~0.01):2.01(f0.01).The Ca, U, and Si stoichiometry of the synthesized uranophane corresponds closely to the values expected for ideal uranophane (1:2:2, respectively). The Na content of the uranophane was 0.1 l(10.02) ppm and indicates that only minor Na is incorporated in the uranophane. Solubility experiments Solubility experiments were designed to approach uranophane equilibrium in both undersaturated and supersaturated solutions. Estimation of the standard state Gibbs free energy of formation of uranophane by a theoretical method of prediction [ 151, together with data for aqueous species from the EQ3/6 DataO.com.R2 database [ 161, was used to estimate the equilibrium solution chemistry for uranophane. The DataO.com.R2 database adopts properties of aqueous uranium species from Grenthe et al. [ 171 and, together 3 with the EQ3NR geochemical code (version 7) [ 161, was used in all aqueous speciation calculations in this study. Based on the above estimate, EQ3NR was used to determine starting solution compositions for the solubility experiments. Experimental solutions had initial U concentrations of to lo-’ mol . L-’ in matrices of IOm2mol . L-’ CaClz and IO” mol . L-’ SiOZ(aq). Before addition of uranophane, the pH of experimental solutions were adjusted to -6.0 by addition of CaC03and allowed to equilibrate with atmospheric C02(g) for 2 weeks. The U concentration and pH of the experimental solutions before reaction with uranophane are shown in Table 1. The solubility experiments were carried out by reacting measured volumes of these solutions (1 00 ml) with measured amounts of synthetic uranophane (0.5 g) in 250 ml polycarbonate bottles. Experiments were conducted at room temperature (20.5 f 2.0 “C) under atmospheric PCO, conditions. Solutions were continuously mixed during the experiments using a gyratory shaker. Aliquots (5 ml) of the experimental solutions were taken at 1 week intervals over 7 weeks. The aliquots were passed through 0.45 pm syringe filters during the sampling procedure. Experimental solution weights were measured before and after each sampling to track loss of solution due to sampling and evaporation. The pH of sample aliquots were measured immediately upon sampling using a glass combination pH electrode (Orion) previously calibrated using commercial buffer solutions (Fisher Scientific). After pH measurement, sample aliquots were acidified to pH < 2.0 by addition of concentrated HN03. Concentrations of major cations in starting solutions and sample aliquots were determined by ICP; U concentrations were measured by ICP-MS. Res u Its Experimental Data
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