Removal of cesium and strontium ions with enhanced solid-liquid separation by combined ion exchange and BaSO4 co-precipitation

Abstract

Treatment of cesium and strontium is critical in radioactive liquid waste management, where their ions are difficult to remove in single operations, owing to differences in valence state. Here, the efficacy of composite coagulants synthesised by combining fine clinoptilolite with co-precipitated barite (BaSO4) were investigated for the simultaneous removal of Cs+ and Sr2+ ions, producing aggregates with enhanced dewatering properties. Co-precipitated BaSO4 without clinoptilolite was found to be very effective in the removal of Sr2+ (>99 %) while only giving low-level Cs+ removal (~14 %) for solutions containing 25 ppm of Cs+ and Sr2. Conversely, pure clinoptilolite gave high Cs+ removal (>98 %) with rapid adsorption (<1 h) fitted to a Pseudo-Second Order (PSO) rate model. Composite coagulants were then produced using natural clinoptilolite combined with BaSO4 co-precipitation. Higher Sr2+ removal was obtained in all cases (>99.9 %), whereas Cs+ removal was reduced to <90 %, owing to exchange interactions with free Ba2+ ions. However, NaCl-preactivated clinoptilolite overcame low Cs+ removal efficiency, achieving >95 % removal. Their physical properties, sedimentation rates, and compressional yield stress were also studied to characterise the aggregates solid-liquid separation behaviour. The combined coagulates obtained settling rates almost twice that of pure BaSO4, and produced much greater consolidation, owing to increased aggregate density. Also, the combined systems had a higher gel point and lower specific compressive yield stress, suggesting less resistance to compression under centrifugal forces for dewatering. Overall, this study highlights that the use of composite coagulants can improve the removal efficiency of Cs+ and Sr2+ while also accelerating solid-liquid dewatering.