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Thermodynamic Model for the Solubility of Thorium Dioxide in The

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Thermodynamic Model for the Solubility of Thorium Dioxide in The Radiochim. Acta 88, 2972306 (2000) by Oldenbourg Wissenschaftsverlag, München Thermodynamic model for the solubility of thorium dioxide + – – ° ° in the Na -Cl -OH -H2O system at 23 C and 90 C By Dhanpat Rai1,*, Dean A. Moore1, Charles S. Oakes1 and Mikazu Yui2 1 Pacific Northwest National Laboratory, Richland, Washington 99352, USA 2 Japan Nuclear Cycle Development Institute, Tokai Works, Tokai, Japan (Received August 30, 1999; accepted in revised form March 8, 2000) Solubility / Thermodynamics / Thorium dioxide / ThO2(c) / at room temperature. Therefore, potential thorium concen- ThO2(am) / Temperature effects on ThO2 crystallinity / trations in aqueous solutions contacting failed repositories Solubility product / Solubility products at different may be many orders of magnitude lower than indicated by temperatures measurements of ThO2(am) solubility at 25 °C. At 25 °C, the precipitation/dissolution kinetics of Summary. Data are extremely limited on the effects of tem- ThO2(c) are not known, which makes it difficult to accu- perature on crystallinity and the resulting changes in solubility rately ascertain the solubility product of ThO2(c) at this products of thermally transformed thorium oxide phases. Such temperature. Fortunately, in addition to being relevant to data are required to reliably predict thorium behavior in high- expected repository conditions, a moderate increase in tem- level waste repositories where higher than ambient tempera- perature accelerates the precipitation/dissolution kinetics tures are expected. Solubility studies were conducted as a func- such that the solubility of ThO2(c) can be approached from tion of pH and time and at 0.1 M NaCl for 1) ThO2(am) at both the oversaturation and undersaturation directions. ° 23 C, 2) ThO2(am c), a thermally transformed amorphous ! ° ° Therefore, it is possible to study the effect of temperature thorium hydrous oxide at 90 C, and 3) ThO2(c) at 23 C and ° ° on the crystallinity of thorium hydrous oxides as well as 90 C. Results show that when ThO2(am) is heated to 90 C, it transforms to a relatively insoluble and crystalline solid the solubility products of these solid phases at elevated temperatures. While the solubility product of ThO2(c) can [ThO2(am c)]. At a fixed pH, the observed solubility of ! ° be calculated (see Appendix A), experimental verification ThO2(am) at 23 C is more than 11 orders of magnitude greater ° 1 than those for ThO2(c) at 23 C or of ThO2(am c) and is required due to uncertainties in the thermodynamic ° ! 0 41 0 ThO2(c) at 90 C. Solubility data were interpreted using the quantities for ThO2(c), Cp for Th , and the absence of Cp Pitzer ion-interaction model. The log of the solubility product values for aqueous Th41 at temperatures .55 °C. Thus, the for the thorium dioxide dissolution reaction [ThO2(s) 1 2H2O primary objectives of this study were to demonstrate the 41 2 <Th 1 4OH ] was determined to be 244.9 for ThO2(am) effects of elevated temperature on the degree of crystal- ° ° at 23 C, $256.9 for ThO2(c) at 23 C, and 251.4 for ThO2(c) linity of ThO and the consequent effects on the solubilities at 90°C. At 90°C, a relatively less crystalline phase, 2 of the thermally transformed phases. ThO (am c), showed slightly higher solubility (log K 5 2 ! sp 249.2) than crystalline ThO2(c). Experimental procedure Introduction Preparation of materials Thorium hydrous oxide [ThO2(am)] is generally used to Stock solutions were prepared from reagent grade Th(NO3)4 estimate leachable thorium concentrations under waste re- ·4H2O(c), concentrated NaOH(aq) (Anachemia), concen- pository conditions (e.g., JNC [1]). The temperatures in trated reagent grade HCl(aq) (GFS), and reagent grade high-level nuclear waste repositories are expected to be NaCl(c) (Alfa Products). considerably higher than ambient and may reach 00°C. 1 A 100 g per liter solution of thorium was prepared in Under these conditions, predictions of leachable thorium 0.1 M HNO (aq) using the reagent grade Th(NO ) ·4HO. require a solubility model of the appropriate thorium oxide 3 3 4 2 Stock solutions of 5.0 M NaOH and 1.0 M NaCl were pre- phase as a function of temperature. Data are extremely lim- pared using the reagents mentioned above. Deionized dis- ited on the effects of temperature on crystallinity and re- tilled water was used in all solution preparations. sulting changes in the solubility products of thermally 1 0 41 transformed phases. Prasad et al. [2] showed that the trans- For example, the reported Cp values for Th vary considerably. ° Values at 298 K are 260 J · mol21 ·K21 based on Morss and McCue’s formation of ThO2(am) to crystalline ThO2 is slow at 25 C [5] data as recalculated by Hovey [6] and 2224 J · mol21 ·K21 report- (about 270 days) and much more rapid (about 12 days) 21 21 ° ed by Hovey [6]; 111 and 2228 J · mol ·K at 303 K reported by at 100 C. The solubility product (Ksp) of crystalline ThO2 Apelblat and Sahar [7] and Hovey [6], respectively. Hovey’s data are [ThO2(c)] has been shown to be approximately eight orders most recent and appear more reliable and thus were selected for calcu- of magnitude lower than that for the hydrous oxide [3, 4] lations reported here. However, Hovey’s heat capacity measurements extend to only 55°C; thus, extrapolation to higher temperatures is de- * Author for correspondence (E-mail: [email protected]). pendent upon the reliability of the extrapolation method. 298 D. Rai, D. A. Moore, C. S. Oakes et al. Table 1. Comparison of filtered and unfiltered aqueous thorium con- Aliquots of suspensions equilibrated at 23 or 90°C were ° centrations from solutions maintained at 90 C. centrifuged at 5000 rpm for 10to15 minutes filtered 21 a through Amicon Centricon-30 filters (30,000 MW cutoff), pH log (CTh/mol · kg ) acidified, and then analyzed for thorium. Thorium concen- filtered unfiltered trations in unfiltered but centrifuged and filtered samples were similar, indicating that a significant amount of col- 2.51 25.52 25.50 2.57 25.25 25.29 loidal material was absent and that the results from the un- 2.69 25.46 25.48 filtered samples are reliable (Table 1). This is particularly 2.78 25.48 25.48 significant to the experiments at 90 °C where available fil- ters may not be stable. a: All solutions were centrifuged at 5000 rpm for 10to15 minutes. For the 90°C experiments, all implements (e.g., pipette Amicon Centricon-30 filters (30,000 MW cutoff) were used. tips, sampling tubes, rinse water, pH buffers, centrifuge and centrifuge containers) that contacted or could change sample temperatures prior to pH measurement or sampling ThO (am) was precipitated from the nitrate solution by 2 were maintained at 90°C. For pH measurements, the elec- adding an aliquot to 10 mL of water and adjusting the pH trode was rinsed with 90 °C water before being placed in to about 10.5 using the NaOH(aq) stock solution. The re- samples. Maximum temperature decrease during pH sulting suspensions were centrifuged and the supernatant measurements was about 5 °C. Subsequently, suspensions discarded. Soluble nitrates, chlorides, and sodium were re- were centrifuged and small aliquots 1) from all suspensions moved by washing twice with 10-mL aliquots of water. were filtered through Amicon filters, equilibrated at 90 °C ThO (c) was prepared by initially dissolving a quantity 2 for 1 hour, and 2) a few selected samples were withdrawn of Th(NO ) ·4HO(c) in hot 0.3 M HNO (aq). Oxalic acid 3 4 2 3 for analysis. The filtered and unfiltered aliquots were then dihydrate was then vigorously stirred into the solution at analyzed by ICP-MS. The analytical data obtained in this 75 °C. The resulting precipitate and solution were then al- study are listed in Appendix Tables B.1 through B.4. lowed to rest for 30 minutes between 50 and 75 °C and then Three sets of experiments ranging in pH and amount of filtered through a coarse sintered glass filter. ThO (c) was 2 ThO (am) were conducted at 23 °C (Table 2). All three then produced by firing the oxalate in air for 2 hours at 2 sample sets were subsequently heated to 90 °C. Heating of 750 °C. X-ray diffraction (XRD) analysis confirmed the ThO2(am) suspensions was expected [2] to result in increas- solid to be crystalline ThO2. ing crystallinity and decreasing solubility [3] due to the formation of crystalline ThO2. For example, heating 41 Equilibration, sampling, and analysis ThO2(am) suspensions in equilibrium with soluble Th or the samples in which the ThO2(am) dissolves completely, All experiments were conducted on a benchtop at room will result in the formation of crystalline ThO either temperature (236 2°C) 2 or at 90°C. A heating block was 2 ° through solid-solid transformation (Eq. (1)) or by precipi- used for the experiments at 90 C. For each experiment, tation (Eq. (2)). The precipitation mechanism (Eq. (2)) will several milligrams of ThO2(am) or ThO2(c) were loaded also be accompanied by an increase in H1 concentration in into a number of 50-mL polypropylene centrifuge tubes proportion to the amount of precipitated thorium. containing approximately 38 mL of 0.1 M NaCl(aq). The pH of the suspensions was adjusted using HCl, and the ThO2(am) < ThO2(c) (1) tubes were then sealed with Nalgene caps. The amount of 41 1 Th 1 2H2O<ThO2(c) 1 4H (2) HCl used in the pH adjustments was recorded and used in conjunction with the added NaCl to calculate the final Therefore, to obtain a large range in pH values of heated chloride molality.
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