Radium uptake during barite recrystallization at 23 ± 2 °C as a function of solution composition: An experimental 133Ba and 226Ra tracer study

High-purity synthetic barite powder was added to pure water or aqueous solutions of soluble salts (BaCl2, Na2SO4, NaCl and NaHCO3) at 23 +/- 2 degrees C and atmospheric pressure. After a short pre-equilibration time (4 h) the suspensions were spiked either with Ba-133 or Ra-226 and reacted under constant agitation during 120-406 days. The pH values ranged from 4 to 8 and solid to liquid (S/L) ratios varied from 0.01 to 5 g/l. The uptake of the radiotracers by barite was monitored through repeated sampling of the aqueous solutions and radiometric analysis. For both Ba-133 and Ra-226, our data consistently showed a continuous, slow decrease of radioactivity in the aqueous phase. Mass balance calculations indicated that the removal of Ba-133 activity from aqueous solution cannot be explained by surface adsorption only, as it largely exceeded the 100% monolayer coverage limit. This result was a strong argument in favor of recrystallization (driven by a dissolution precipitation mechanism) as the main uptake mechanism. Because complete isotopic equilibration between aqueous solution and barite was approached or even reached in some experiments, we concluded that during the reaction all or substantial fractions of the initial solid had been replaced by newly formed barite. The Ba-133 data could be successfully fitted assuming constant recrystallization rates and homogeneous distribution of the tracer into the newly formed barite. An alternative model based on partial equilibrium of Ba-133 with the mineral surface (without internal isotopic equilibration of the solid) could not reproduce the measured activity data, unless multistage recrystallization kinetics was assumed. Calculated recrystallization rates in the salt solutions ranged from 2.8 x 10(-11) to 1.9 x 10(-10) mol m(-2) s(-1) (2.4-16 mu mol m(-2) d(-1)), with no specific trend related to solution composition. For the suspensions prepared in pure water, significantly higher rates (similar to 5.7 x 10-10 mol m(-2) s(-1) or similar to 49 mu mol m(-2) d(-1)) were determined. Radium uptake by barite was determined by monitoring the decrease of Ra-226 activity in the aqueous solution with alpha spectrometry, after filtration of the suspensions and sintering. The evaluation of the Ra uptake experiments, in conjunction with the recrystallization data, consistently indicated formation of non-ideal solid solutions, with moderately high Margules parameters (W-AB = 3720-6200 J/mol, a(0) = 1.5-2.5). These parameters are significantly larger than an estimated value from the literature (W-AB = 1240 J/mol, a(0) = 0.5). In conclusion, our results confirm that radium forms solid solutions with barite at fast kinetic rates and in complete thermodynamic equilibrium with the aqueous solutions. Moreover, this study provides quantitative thermodynamic data that can be used for the calculation of radium concentration limits in environmentally relevant systems, such as radioactive waste repositories and uranium mill tailings. (C) 2010 Elsevier Ltd. All rights reserved.