Solid–aqueous equilibrium in the BaSO4–RaSO4–H2O system: First-principles calculations and a thermodynamic assessment
Solid–aqueous equilibrium in the BaSO4–RaSO4–H2O system: First-principles calculations and a thermodynamic assessment
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DOI：
10.1016/j.gca.2013.08.028
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发表时间：
2013-12
影响因子：
5
通讯作者：
V. Vinograd;F. Brandt;K. Rozov;M. Klinkenberg;K. Refson;B. Winkler;D. Bosbach
中科院分区：
文献类型：
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作者：
V. Vinograd;F. Brandt;K. Rozov;M. Klinkenberg;K. Refson;B. Winkler;D. Bosbach
BaSO4-RaSO4-H2O 系统中的相关系对于理解重晶石类矿物在控制天然水库中 Ra2+ 浓度方面的作用非常重要。这些关系对端元溶度积的差异和固溶体相的非理想程度极其敏感。 RaSO4 标准熵和重晶石-RaSO4 固溶体的规则相互作用参数的实验约束是不明确的。本研究的重点是根据第一原理确定这些参数。借助密度泛函微扰理论计算了RaSO4的声子态密度。 BaSO4-RaSO4 固溶体中的常规相互作用参数 WBaRa 被解释为无限稀释极限 (xRa=0) 下混合焓的斜率，并根据由于插入 Ra 的单个替代缺陷而导致的 BaSO4 2 × 2 × 2 超晶胞总能量的变化来计算。通过计算具有重晶石和文石结构的更广泛的二元固溶体的 W 值来验证该方法。 WBaRa= 2.50 ± 1.00 kJ/mol 的计算值意味着 BaSO4-RaSO4-H2O 体系中的固水平衡可能在靠近 BaSO4 端元的位置有一个均质点。对重晶石在含 Ra 水溶液中重结晶的现有数据的评估表明，重晶石晶体可能在百天的时间尺度内完全平衡。
Phase relations in the BaSO4–RaSO4–H2O system are important for understanding the role of barite-type minerals in controlling the concentration of Ra2+in natural water reservoirs. These relations are extremely sensitive to the difference in the solubility products of the end-members and to the degree of non-ideality of the solid solution phase. Experimental constraints to the standard entropy of RaSO4and the regular interaction parameter of the barite–RaSO4solid solution are ambiguous. This study is focused on determination of these parameters from first principles. The phonon density of states of RaSO4is computed with the aid of the density functional perturbation theory. The regular interaction parameter in the BaSO4–RaSO4solid solution,WBaRa, is interpreted as the slope of the enthalpy of mixing in the limit of infinite dilution (xRa= 0) and is calculated from the change in the total energy of a 2 × 2 × 2 supercell of BaSO4due to the insertion of a single substitutional defect of Ra. The method is validated by computingWvalues for a wider range of binary solid solutions with barite and aragonite structures. The computed value ofWBaRa= 2.50 ± 1.00 kJ/mol implies that the solid–aqueous equilibrium in the BaSO4–RaSO4–H2O system may have an alyotropic point in close proximity to the BaSO4end-member. The assessment of available data on re-crystallization of barite in Ra-bearing aqueous solutions suggests that the barite crystals may fully equilibrate on the time scale of hundred days.