Enhancing natural clinoptilolite for cesium and strontium removal, using activation, flotation and process intensification

Clinoptilolite is a common zeolite adsorbent for heavy metals removal. Its ores are widely available, it can be simply utilised in exchange columns, while it has good ability to treat liquid radioactive wastes (LRW) such as cesium and strontium. It is also radiation tolerant, as its structure does not degrade or alter due to cesium and strontium radiation. However, only high grade clinoptilolite can be used in the nuclear industry, which will place significant challenges for ongoing effluent treatment operations worldwide, due to future critical supply issues. Therefore, this thesis investigates a number of industrially relevant techniques to enhance the quality of relatively low-grade clinoptilolite for application in the nuclear industry for cesium and strontium removal. In particular, studies focused on chemical pre-activation and milling, flotation and process intensification, as outlined herein.
Firstly, natural clinoptilolite, of relatively low-grade (as highlighted by impurities shown by electron microscopy) was investigated for its capability to remove cesium and strontium ions from fresh water and simulated seawater, by using a batch adsorption technique. Seawater is an important solvent to study, due to the reduction in metal adsorption performance from potassium interaction, and enhancements are critical for treating effluents from Fukushima, for example. To improve its capacity, the material was pre-activated with concentrated NaCl and HCl solutions. Additionally, it was milled to a number of < 300 μm size fractions, to expose exchange sites. The adsorption kinetics and equilibrium for natural and activated results were fitted with the theoretical models, where the adsorption capacity for both cesium and strontium were significantly improved by both pre-activation and milling. In simulated seawater solutions, all materials gave considerably reduced performance due to K+ ion competition, with Sr2+ uptake decreased more extensively compared to Cs+.
Secondly, flotation was investigated as a rapid separation technique to de-water fine, powdered clinoptilolite ion exchange resins by utilising cationic surfactant collectors, for the decontamination of radioactive cesium ions from nuclear waste effluent streams. Initial kinetics and equilibrium adsorption studies of cesium, suggested the large surface area to volume ratio of the fine clinoptilolite contributed to fast adsorption kinetics and high capacities. Measurements of particle sizes confirmed that adsorption of surfactant monolayers did not lead to significant aggregation of the clinoptilolite. Importantly for flotation, both the recovery efficiency and dewatering ratios were measured across various surfactant concentrations. Optimum conditions were found with 0.5 mM of cetylpyridinium chloride (CPC) and addition of 30 µL of methyl isobutyl carbinol (MIBC) frother, giving a recovery of ~90% and a water reduction ratio > 4, highlighting the great viability of flotation.
Lastly, static elution column studies were performed with the natural clinoptilolite, to compare performance to the described batch tests, where process intensification was also performed using a novel agitated tubular reactor (ATR). Here, kinetic breakthrough curves were fitted using two models; Thomas and Modified Dose Response (MDR) models. During static column ion exchange, the maximum adsorption capacity (qe) for ion concentrations of 200 ppm were ~171 mg/g and 16 mg/g for cesium and strontium respectively (correlating to 50% breakthrough times of 700 and 70 bed volumes). Reducing the concentration of strontium down to 100 ppm could lead to higher both qe and bed volume, ~48 mg/g and 400 bed volumes, respectively. In contrast, reducing the residence time from 30 minutes to 15 minutes of 100 ppm strontium could decrease the qe around 13-14 mg/g. By increasing the inner column diameter to 2 cm gave similar results to 1 cm column tests. Meanwhile, for process intensification of ATR, two-column lengths (25 and 34 cm) with 100 ppm strontium concentration and 15 minutes residence time were investigated. The 34 cm length significantly increased qe and breakthrough times around 30%, in comparison to static columns, while process flowrates were increased ~three times for the same performance. Such considerable performance increases highlight the suitability of the ATR to deliver intensified ion exchange for a range of systems.
In conclusion, the enhancement of clinoptilolite using pre-activation and milling could considerably extend the range of natural clinoptilolite ores suitable for cesium and strontium treatment processing of relatively fresh water effluents, but improvements are required for treating cesium and strontium in saltwater in future work. Meanwhile, during flotation, it was observed that cesium contaminated clinoptilolite could be recovered up to ~90% with a water reduction ratio > 4. However, while highlighting its applicability, some aspects should be studied for future work, such as the optimum particle size of clinoptilolite, the solid/liquid ratio and influence of mixed metal ions (incorporating strontium). Also, the use of fine particles from pre-activated clinoptilolite could be investigated further during flotation. In the static column and ATR studies, the ATR led to clear enhancements in the removal the strontium at a high rate. However, there are areas for improvement for future study, such as modifying the size of tubular reactor, its frequency or using pre-activated clinoptilolite.