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

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 7.0molkg^-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, KD,barite', for the co-precipitation reaction was 1.04±0.01. This value of KD' is significantly lower than the value for relatively diluted solutions (1.8±0.1). This low value of KD,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)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 (KD,barite″), which accounts for the ionic strength effect. KD,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 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 KD,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.