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Co-precipitation of radium in high ionic strength systems: 2. Kinetic and ionic strength effects
Authors:YO Rosenberg  V Metz  Y Volkman
Institution:a Department of Geological and Environmental Sciences, Ben-Gurion University of the Negev, P.O. Box 653, Beer Sheva 84105, Israel
b Institute for Nuclear Waste Disposal (INE), Karlsruhe Institute of Technology, P.O. Box 3640, 76021 Karlsruhe, Germany
c Department of Desalination and Water Treatment, Zuckerberg Institute for Water Research, Ben-Gurion University of the Negev, Sede Boqer 84190, Israel
Abstract:High concentrations of naturally occurring radium pose environmental and health concerns in natural and industrial systems. The adsorption of Ra2+ in saline water is limited compared to its adsorption in fresh water, but the process of co-precipitation may be effective in decreasing its concentration. However, despite its importance, Ra co-precipitation has rarely been studied in high ionic strength environments such as those in evaporitic systems.The fate of Ra in the reject brine of a desalination plant was studied via evaporation batch experiments at ionic strengths (I) ranging from 0.7 to 7.0 mol kg−1. Precipitation sequences revealed that Ra co-precipitated with barite, even though the latter was a trace mineral compared to the precipitated gypsum. The concentration-based effective partition coefficient, View the MathML source, for the co-precipitation reaction was 1.04 ± 0.01. This value of View the MathML source is significantly lower than the value for relatively diluted solutions (1.8 ± 0.1). This low value of View the MathML source is mainly the result of a kinetic effect but is also slightly affected by the ionic strength.Both effects are quantitatively examined in the present paper. It is suggested that a kinetic effect influences the nucleation of (Ra,Ba)SO4, reducing the value of the partition coefficient. This kinetic effect is caused by the favorable nucleation of a more soluble phase (i.e., a phase with a higher BaSO4 fraction). An additional decrease in the partition coefficient results from the ionic strength effect. Considering the activity of Ra2+ and Ba2+ in the solution (rather than their concentration) makes it possible to determine the activity-based partition coefficient (View the MathML source), which accounts for the ionic strength effect. View the MathML source was calculated empirically from the experiments and theoretically via a kinetic model. The two derived values are consistent with one another and indicate the combined effect of ionic strength and precipitation kinetics.Finally, the common assumption that γRa2+/γBa2+=1 was re-examined using a numerical model to predict the experimental results. As the ionic strength increases, this assumption becomes less appropriate for predicting the change in View the MathML source as calculated in the experiments. Understanding the co-precipitation of Ra in such systems is crucial for risk assessments in which both Ra concentration and ionic strength are relatively high.
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