Simultaneous Removal of Cesium and Strontium Ions with Advanced Co-Precipitation Methods and Its Process Intensification

137Cs and 90Sr are known as hard-to-remove radionuclides using a single conventional treatment process due to their differences in charge density, and solubility. Also, they may cause a biological hazard and increase the challenge in spent nuclear fuel management due to strong gamma/beta radiation, relatively long half-life (30.17 years and 28.8 years for 137Cs and 90Sr, respectively), and the heat generation in the waste composition. In case of elevated contamination, they can easily migrate between ecosystems and be taken as essential nutrients by organisms, which can eventually lead to various diseases and even death. Therefore, the separation of these fission products from the waste solution and their efficient immobilisation for long-term storage is significantly important.
In literature, several technologies have been proposed to remove Cs+ and Sr2+ from waste effluents and some of them have been efficiently employed in industrial-scale operations. Indeed, especially after the accident at the Fukushima Daichi Nuclear Power Plant in 2011, interest in developing technologies for the treatment of wastewater contaminated with Cs+ and Sr2+ has increased. However, most of proposed technologies focused on single metal removal and the simultaneous removal of Cs+ and Sr2 is limited. Among these technologies, clinoptilolite ion exchange is a widely used technique to selectively remove Cs+ and Sr2+ ions from waste solution and has been used industrially in the treatment of cooling pond water of a legacy nuclear reprocessing plant in the UK. It is noteworthy that the electrostatic affinity of Cs+ towards sorbent materials is higher than Sr2+ ions and hence more Cs+ ions can be extracted by clinoptilolite ion exchange. Furthermore, co-precipitation is another widely utilised technology allowing industrial-scale operation for Cs+ and Sr2+ ions removal. In particular, co-precipitation with barite (BaSO4) is a proven crystal to efficiently remove Sr2+ ions along with other radioactive divalent ions and is part of the Fukushima purification system. The high density and very low solubility of Ba-SO4 make it a suitable precipitate for metal removal and high-degree dewatering.
In this thesis, the composite coagulants were synthesised by combining clinoptilolite ion exchange and co-precipitation with BaSO4 to remove Cs+ and Sr2+ simultaneously in a single process and to enhance solid/liquid separation degree. Firstly, the batch adsorption kinetics of natural clinoptilolite was investigated and fitted by the Pseudo-Second Order (PSO) rate model, giving higher Cs+ removal than Sr2+ ions. Then, the combined process was studied in the batch system using natural and NaCl-activated clinoptilolite and the maximum simultaneous removal was recorded as >95% for Cs+ and >99.9% for Sr2+. The morphology, particle size and surface charge analysis were also conducted to characterise the composite coagulants. Moreover, the sedimentation and compressive yield stress analysis was performed to provide insight into the dewatering degree and faster sedimentation was observed in compo-sites compared to BaSO4 dispersions due to denser aggregates, leading to a high de-watering degree.
In addition, the process intensification (PI) of the combined batch study was then studied by operating an agitated tubular reactor (ATR) at 3 Hz and 5 Hz agitation. The plug flow behaviour of ATR was firstly characterised with a tracer dye and the optimum plug flow was recorded at 3 Hz compared to 5 Hz at which better sinusoidal motion was achieved. Regarding metal removal, the adsorption performance of clinoptilolite in ATR gave slightly higher removal efficiency than the batch system in 15 min residence time. In the combined system, the composites synthesised in the ATR led to a higher removal (95.76% for Cs+ and 99.96% for Sr2+) at 5 Hz agitation. This was further increased by postponing the BaCl injection, and maximum removal efficiencies of 96.33% for Cs+ and >99.98% for Sr2+ were achieved. Importantly, the physical characterisation, sedimentation and pressure filtration analyses were conducted and smaller and denser flocs were obtained from ATR, highlighting enhanced compression and dewatering degree.
Finally, this thesis explored nuclear magnetic resonance (NMR) spectrometry for the determination of surface area for commercial and precipitated BaSO4 dispersions based on solvent relaxation measurements. The spin-lattice (T1) relaxation was performed to calibrate NMR using silica particles with known surface area. A good agreement between NMR and the BET analyses was recorded for precipitated BaSO4 surface area based on the relative relaxation enhancement from spherical equivalent surface area of silica particles giving surface area of 6.28 m2/g and 5.88 m2/g from NMR and the BET, respectively. The electrolyte effect was also studied and the negative effect was recorded for the relaxation enhancement.