Nucleation and growth kinetics of Ra_xBa_1-xSO₄ solid solution in NaCl aqueous solutions

Co-precipitation of Ra and Ba in barite (i.e., the formation of a Ra_xBa_1-xSO₄ solid solution) has long been established as an important process that has the potential to control Ra concentration. This process is commonly described by a distribution model. Ample studies have shown that the key parameter of this model, the partition coefficient, varies in the range of 1-2 as a function of temperature, salinity and precipitation kinetics of the Ra_xBa_1-xSO₄ solid solution. This roughly twofold change in the partition coefficient may lead to large differences in the concentration of dissolved Ra.The present study systematically investigated the co-precipitation kinetics of the Ra_xBa_1-xSO₄ solid solution from aqueous solutions up to 5.9 molkgH2O-1 NaCl, circum-neutral pH and at ambient temperature. Laboratory batch experiments designed to follow the nucleation of the Ra_xBa_1-xSO₄ solid solution and the co-precipitation kinetics of Ba and Ra from aqueous solutions which were initially supersaturated with respect to barite (degree of supersaturation, β_barite=20±2).The following empirical law describes the dependence of the activity-based partition coefficient, KD,barite″, on the degree of supersaturation, β_barite:KD,barite″=(1.99±0.05)-(0.58±0.06)·log(βbarite).This empirical law is in good agreement with other literature data. The outcomes of the empirical law are compared to the prediction of a model for the nucleation of two-dimensional islands.