Thermodynamics of the solid solution - Aqueous solution system (Ba,Sr,Ra)SO4 + H2O: I. The effect of strontium content on radium uptake by barite

Abstract

Thermodynamic properties of mixing in the ternary (Ba,Sr,Ra)SO 4 solid solution are determined using first principles based total energy calculations and Monte Carlo simulations. Two levels of theory, which correspond to the regular mixing and the generalized Ising model, are considered. The results show that the regular mixing parameters increase along the row of Ba-Ra, Ba-Sr and Sr-Ra binary systems proportionally to the squared difference of molar volumes of the end-members. The magnitudes of pairwise interactions similarly increase along the same row, manifesting a tendency to short-range ordering (SRO). In the (Ba,Sr)SO 4 system the SRO effect is approximately equivalent to a 40% decrease in the value of the regular mixing parameter. The ternary solid solution is well described as a regular mixture with the binary parameters W BaRa = 2.47 ± 0.22, W BaSr = 4.95 ± 0.75 and W SrRa = 17.50 ± 1.40 kJ/mol. These values imply that admixing RaSO 4 to the (Ba,Sr)SO 4 solid solution stabilizes Ba-rich and destabilizes Sr-rich compositions. Consequently, an addition of a small amount of RaSO 4 to a Sr-rich solid solution leads to a nucleation of a Ba-and Ra rich phase. This phenomenon, predicted in our thermodynamic modelling study, is directly confirmed by our experiments on recrystallizing a powder of celestite with traces of Ba in the presence of an aqueous Ra-bearing solution. At a measurably high content of Ra in the system Ra-uptake by celestite occurs via a formation of a Ra-rich phase. The aqueous concentration of Ra in such systems would be governed mainly by the common anion effect caused by the relatively high solubility of Sr-rich sulphates. At lower Ra contents the retention of Ra would be enhanced both by the common anion and the dilution effects. Our simulations with the GEM-Selektor code predict that the optimum condition for Ra uptake is achieved when the barite solid solution contains 10 ± 5 mol % of SrSO 4 .

Structural, elastic and vibrational properties of celestite, SrSO4, from synchrotron x-ray diffraction, thermal diffuse scattering and Raman scattering

“…In an earlier study [8] we noticed that in large super-cell calculations a full geometry optimisation led to slight deviations from the orthorhombic symmetry, but this was not explored further. Similar calculations carried out here reproduced the result and the structure shown in Vinograd et al [8], where the SO 4 -groups are slightly rotated in comparison to the orthorhombic structure ( Fig. 5).…”

Organic–mineral interfacial chemistry drives heterogeneous nucleation of Sr-rich (Ba x , Sr 1− x )SO 4 from undersaturated solution

“…Studies on the thermodynamics of (Ba x , Sr 1−x )SO 4 solid solutions are still under active development (37), and one of the uncertainties is the interaction parameter of Ba-Sr ions in (Ba x , Sr 1−x )SO 4 solid solutions, which determines the ideality (SI Appendix, section 1.3) and the free energy of mixing (SI Appendix, section 1.4) of the solid solution (35)(36)(37)(38)(39). Using the interaction parameters developed by different models (35,36), the calculated compositions (i.e., x) varied to some extent but were all in general consistent with the measured compositions of (Ba x , Sr 1−x )SO 4 .…”

Interaction of Nonideal, Multicomponent Solid Solutions With Water: A Simple Algorithm to Estimate Final Equilibrium States

“…Inspection of Table and Figure shows that the higher the Sr‐content in the host solid, the lower the equilibrium concentration of Pb in the aqueous solution. Vinograd et al () have also observed that trace amounts of SrSO 4 in the barite solid solution improves Ra‐uptake. In practice, reaching equilibrium requires the parent solid to be completely dissolved and recrystallized, which maybe kinetically hindered.…”