Co-precipitation of radium in high ionic strength systems: 2. Kinetic and ionic strength effects

Abstract

High concentrations of naturally occurring radium pose environmental and health concerns in natural and industrial systems. The adsorption of Ra 2+ 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, K D,barite ′ , for the co-precipitation reaction was 1.04 ± 0.01. This value of K D ′ is significantly lower than the value for relatively diluted solutions (1.8 ± 0.1). This low value of K D,barite ′ 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)SO 4, 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 BaSO 4 fraction). An additional decrease in the partition coefficient results from the ionic strength effect. Considering the activity of Ra 2+ and Ba 2+ in the solution (rather than their concentration) makes it possible to determine the activity-based partition coefficient ( K D,barite ″ ), which accounts for the ionic strength effect. K D,barite ′ 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 γ Ra 2 + / γ Ba 2 + = 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 K D,barite ′ 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.