Andrew R. Felmy

The Solubility of Hydrous Thorium(IV) Oxide in Chloride Media: Development of an Aqueous Ion-Interaction Model

The solubility of hydrous Th(IV) oxide was measured in NaCl solutions ranging in concentration from 0.6 to 3.0 Μ and in KCl at 0.6 M, over a wide range of hydrogen ion concentration (pcH* 3 to 11), and over equilibration times extending to more than one year. Our results show solubilities higher (by three to four orders of magnitude) than have been reported by other investigators in NaC104 media. Our thermodynamic modeling calculations indicate that these differences in solubility are a result of differences in the ionic media and the ionic strength of the solutions. We have used the thermodynamic model of Pitzer and coworkers, which is valid to high ionic strengths, to analyze our data for solubility in both chloride and Perchlorate media. The analysis required the use of specific ion-interaction parameters only for the bare Th ion with the bulk anion Cl~. The final thermodynamic model gives a good representation of all of our solubility data in NaCl and KCl solutions below pcof ~ 7 as well as the solubility data in NaC104 media and the osmotic data for ThCl4 solutions.

The solubility of (Ba,Sr)SO4 precipitates: Thermodynamic equilibrium and reaction path analysis

The solubility of (Ba,Sr)SO4 precipitates, varying in SrSO4 mole fraction from 0.05–0.90, was investigated at room temperature with an equilibration period extending to almost three years. The data show that on or before 315 days of equilibration the precipitates reach a reversible equilibrium with the aqueous solution. The reversibility of this equilibrium was verified both by the attainment of steady-state concentrations with time and by heating the samples to perturb the equilibrium and then observing the slow return to the initial equilibrium state. The dissolution of the (Ba,Sr)SO4 precipitates does not, in general, follow limiting reaction paths as defined by the Lippmann solutus or stoichiometric dissolution curves. In addition, activity coefficient calculations for the BaSO4 and SrSO4 components of the solid phase, using either total bulk analysis or near-surface analysis of the component mole fractions, do not satisfy the Gibbs-Duhem equation, demonstrating that a single solid-solution phase does not control both the aqueous Ba and Sr concentrations. Instead, our long-term equilibration data can be explained by the unavoidable formation of small amounts of barite and substitution of Sr into a solid-solution phase with the BaSO4 component of the solid-solution phase never reaching thermodynamic equilibrium with the aqueous phase.

The solubility of barite and celestite in sodium sulfate: Evaluation of thermodynamic data

The solubilities of barite [BaSO4(c)] and celestite [SrSO4(c)] in Na2SO4 were studied and found to be significantly lower than the experimental values reported in the literature. Our new solubility data are in excellent agreement with the predictions of ion interaction models, which have previously been parameterized primarily from solubility data obtained in chloride media. Our solubility data were analyzed both in terms of aqueous thermodynamic models that included ion association species and in terms of ion interaction models that did not require the explicit recognition of such species. In the case of SrSO4, although both ion association and ion interaction models can accurately model our solubility data, the ion interaction approach is preferred because it is easier to extend to higher concentrations. In the case of BaSO4, the aqueous ion interactions appear to be stronger than those for SrSO4, and so the explicit recognition of a BaSO4(aq) ion association species is preferred. The logarithms of the thermodynamic solubility products (log Ksp) for celestite and barite were −6.62±0.02 and −10.05±0.05, respectively. When the data were analyzed using models that include ion association species, the logarithms of the thermodynamic equilibrium constants for the SrSO4(aq) and BaSO4(aq) association reactions were 1.86±0.03 and 2.72±0.09, respectively.

Thermodynamic model for the solubility of Cr(OH)3(am) in concentrated NaOH and NaOH-NaNO3 solutions

The main objective of this study was to develop a thermodynamic model for predicting Cr(III) behavior in concentrated NaOH and in mixed NaOH–NaNO3 solutions for application to developing effective caustic leaching strategies for high-level nuclear waste sludges. To meet this objective, the solubility of Cr(OH)3(am) was measured in 0.003 to 10.5 m NaOH, 3.0 m NaOH with NaNO3 varying from 0.1 to 7.5 m, and 4.6 m NaNO3 with NaOH varying from 0.1 to 3.5 m at room temperature (22 ± 2°C). A combination of techniques, X-ray absorption spectroscopy (XAS) and absorptive stripping voltammetry analyses, were used to determine the oxidation state and nature of aqueous Cr. A thermodynamic model, based on the Pitzer equations, was developed from the solubility measurements to account for dramatic increases in aqueous Cr with increases in NaOH concentration. The model includes only two aqueous Cr species, Cr(OH)4− and Cr2O2(OH)4− (although the possible presence of a small percentage of higher oligomers at >5.0 m NaOH cannot be discounted) and their ion–interaction parameters with Na+. The logarithms of the equilibrium constants for the reactions involving Cr(OH)4− [Cr(OH)3(am) + OH− ⇌ Cr(OH)4−] and Cr2O2(OH)42− [2Cr(OH)3(am) + 2OH− ⇌ Cr2O2(OH)42− + 2H2O] were determined to be −4.36 ± 0.24 and −5.24 ± 0.24, respectively. This model was further tested and provided close agreement between the observed Cr concentrations in equilibrium with Cr(OH)3(am) in mixed NaOH–NaNO3 solutions and with high-level tank sludges leached with and primarily containing NaOH as the major electrolyte.

Thermodynamics of the U(VI)-Ca2+-Cl- -OH- -H2O System: Solubility Product of Becquerelite

Summary The solubility of synthetic becquerelite (Ca(UO2)6O4(OH)6·8H2O) was determined in 0.02, 0.1, and 0.5 M CaCl2 solutions and at pCH+ values ranging from approximately 4 to 11. The presence of becquerelite in equilibrated samples was confirmed by a combination of techniques involving X-ray diffraction, total chemical composition, and analyses of solubility data. The solubility data were interpreted using Pitzers aqueous thermodynamic model and the thermodynamic data for U(VI) species available in the literature. The log of the solubility product for becquerelite [Ca(UO2)6O4(OH)6·8H2O + 14H+ ⇌ Ca2+ + 6UO22+ + 18H2O] was determined to be 41.4 ± 0.2. This value is similar to the values previously reported for other synthetic becquerelites, but is drastically different from a value reported for a natural sample.

Hydrolysis constants and ion-interaction parameters for Cd(II) in zero to high concentrations of NaOH−KOH, and the solubility product of crystalline Cd(OH)2

AbstractThe solubility of Cd(OH)2(c) was studied in 0.01M NaClO4 solutions, from both the over- and the undersaturation directions, with OH− ion concentration ranging from 10−6 to 1.0 mol-L−1, and the equilibration period ranging from 2 to 28 days. Equilibrium Cd concentrations were reached in less than 2 days. The Cd(OH)2(c) solubility showed an amphoteric behavior. In the entire range of OH−/H+ investigated, the only dominant aqueous Cd(II) species required to explain the solubility of Cd(OH)2(c) are Cd2+, Cd(OH)20, and Cd(OH)42−. The logarithms of the thermodynamic equilibrium constants of the Cd(OH)2(c) solubility reactions involving these species, that is, the reactions

An aqueous thermodynamic model for a high valence 4∶2 electrolyte Th4+−SO 4 2− in the system Na+−K+−Li+−NH 4 + −Th4+−SO 4 2− −HSO 4 − −H2O to high concentration

\begin{gathered} {\text{ }}Cd(OH)_2 (c) \rightleftarrows Cd^{2 + } + 20H^ - ,{\text{ }}Cd(OH)_2 (c) \rightleftarrows Cd(OH)_2^0 , \hfill \\ and Cd(OH)_2 (c) + 20H^ - \rightleftarrows Cd(OH)_4^{2 - } \hfill \\ \end{gathered}

An aqueous thermodynamic model for Ca2+−SO 3 2− ion interactions and the solubility product of crystalline CaSO3·1/2H2O

were found to be −14.14±0.21, −7.04±0.21, and −5.62±0.32, respectively. The ion-interaction parameters reported in the literature, in conjunction with the values for Cd(OH)20−Na+(−0.20), Cd(OH)42−−Na+ (β0 = 0.41, β1 = 0.7), and Cd(OH)42−−K+ (β0 = 0.44, β1 = 1.44) obtained in this study, show that our low-ionic strength solubility data are also consistent with Cd(OH)2(c) solubility data obtained in solutions as concentrated as 10M in NaOH or KOH and 7M in Na(OH, ClO4).