Nucleation and growth kinetics of RaxBa1-xSO4 solid solution in NaCl aqueous solutions

Co-precipitation of Ra and Ba in barite (i.e., the formation of a RaxBa1-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 RaxBa1-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 RaxBa1-xSO4 solid solution from aqueous solutions up to 5.9 mol kg(H2O)(-1) NaCl, circum-neutral pH and at ambient temperature. Laboratory batch experiments designed to follow the nucleation of the RaxBa1-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, beta(barite) = 20 +/- 2). The following empirical law describes the dependence of the activity-based partition coefficient, K-D,K-barite '', on the degree of supersaturation, beta(barite): K-D,K-barite '' = (1.99 +/- 0.05) - (0.58 +/- 0.06) . log(beta(barite)). 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. (C) 2013 Elsevier Ltd. All rights reserved.