Co-precipitation of radium in high–salinity environments: Implications from laboratory experiments and field surveys

Abstract

The co-precipitation of radium (Ra) with minerals is prevalent in high-salinity environmental systems, with significant implications for geochemical cycling and radiation risk management. This study extensively investigated Ra co-precipitation through both indoor lake experiments and field investigations of saline lakes. 1) In the indoor experiments, calcium ions (Ca2+) concentration remained stable under high-salinity conditions, while barium ions (Ba2+) showed a marked and continuous decline. Ra is less likely to co-precipitate with Ca minerals but has a higher tendency to co-precipitate with Ba sulfates. However, field investigations provided limited support for co-precipitation based on water chemistry. Variations in Ca2+ and Ba2+ with total dissolved solids (TDS) in saline lakes showed no significant correlation, and both calcite and Ba sulfates may precipitate from solution. Thus, water chemistry profiles can provide an initial assessment of potential co-precipitation occurrences. 2) Our study revealed the responses of four Ra species in high-salinity solutions. Within the selected salinity range, the activity of long-lived Ra significantly decreased, and the calculated precipitation rates indicated their co-precipitation with minerals. Although the co-precipitation signals of short-lived Ra may be obscured by desorption and rapid decay, reasonable calculations confirm that they also underwent co-precipitation. The co-precipitation of all Ra species may be attributed to the compression of the anti-ionic diffusion layer around particles under high-salinity conditions. The molar ratio of Ra to Ba in Ba sulfates is significantly higher than that in gypsum and calcite (Ra/Ca), indicating the probably dominant role of Ba sulfates in co-precipitation. Additionally, variations in Ra/Ba ratios and concentrations of Ba and SO₄2− across these systems further elucidate the control exerted by Ba sulfates on Ra co-precipitation. 3) Previous studies have focused primarily on Ra co-precipitation mechanisms in groundwater and controlled experimental systems, while research on other high-salinity environments, such as saline lakes, remains limited. Findings from our saline lake systems further confirm the prevalence of Ra co-precipitation and provide important insights for other high-salinity natural systems (e.g., the Dead Sea) and polluted environments (e.g., mining sites) where Ra co-precipitation constraints may differ from those in saline lakes. In saline lake systems, salinity/TDS regulate mineral saturation indices (SI) by modulating Ra desorption and SO₄2− levels, thereby controlling (Ra, Ba)SO₄ formation, while the effects of pH and temperature are relatively minor. A limitation of this study is the lack of investigation into the influence of fine colloids and potential complexes on Ra species, a discussion that could offer preliminary insights into Ra transport and applications in other high-salinity systems.