Thermodynamic properties of the (Ba,Pb)SO4 solid solution under ambient conditions: Implications for the behavior of Pb and Ra in the environment

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Despite numerous studies, the degree of non-ideality of the (Pb,Ba)SO4 solid solution has remained uncertain. Although there has been a tendency to interpret the linear variation of the unit cell parameters as a sign of ideal behavior, the lack of intermediate compositions in natural samples has led many authors to consider this solid solution to be non-ideal, with a positive enthalpy of mixing (ΔHM>0) and a symmetric miscibility gap. Paradoxically, some classical experimental studies on trace Pb2+ partitioning in barite point towards a non-ideal solid solution with ΔHM<0. In this work, we use ambient-temperature solution calorimetry and powder-XRD to clarify the non-ideal character of the anglesite–barite series. The obtained mixing model is examined in the light of existing data on the dissolution–crystallization behavior of this solid solution. Finally, the implications on the behavior of lead and radium in the environment are discussed.Our experimental data indicate that there is a significant positive enthalpy of mixing for the majority of compositions, which implies a tendency to develop a miscibility gap. However, the gap is asymmetric due to the existence of a slightly negative enthalpy of mixing in the Ba-rich miscibility range. Furthermore, there are a number of anomalies that suggest the need for a separate treatment of the Ba-rich miscibility range (PbSO4 molar fraction=XAng<∼0.14), which is in fact the most relevant from an environmental point of view. The equilibrium distribution coefficient, DPb(eq), increases dramatically in the Ba-rich side, from values less than 0.001 for XAng=0.14 to approximately 0.056 for ultra-trace concentrations of Pb. These values are significantly greater than the nearly constant value of approximately 0.0005 given by the mixing model based on natural occurrences. Even so, the preferential tendency of barium to incorporate into the solid phase precludes considering barite as a definitive host phase for lead sequestration. More interesting is the effectiveness of the anglesite–barite solid solution to incorporate radium substituting for barium and lead in the lattice positions. Although the interaction of barite with a Ra-bearing aqueous solutions leads to a significant decrease in the concentration of dissolved Ra2+, our calculations demonstrate that such a reduction is dramatically greater when the interaction occurs with stable Pb-bearing barite solids.