Innovative separation of Caesium and Strontium using flotation and magnetic particles, to convert large waste volumes into small waste packages

Lead Research Organisation: University of Leeds
Department Name: Chemical and Process Engineering


Both the UK and Republic of Korea governments have a strategy to maintain (in the UK) or increase (in the ROK where capacity is planned to increase by 59% by 2022) the level of nuclear power generation in order to provide "low-carbon" energy. The two countries have a long and established civil nuclear industry; producing legacy sites and materials which will need to be managed. There is a clear imperative, given the needs of both new build reactors and legacy wastes, to develop innovative approaches to waste management and decommissioning for safe and cost effective answers to storage and disposal issues.
The treatment of radioactive effluent streams is one of the common yet challenging tasks facing almost all nuclear facilities, whether active plants or decommissioning sites. Various liquid streams from cooling ponds to fuel reprocessing require immediate processing to reduce the activity for safe discharge. While low efficiency removal routes in on-going plant operations can lead to long term environmental accumulation in land and sea, the sudden release of mobile radioactive ions can lead to severe environmental contamination. The acute difficulties of treating large volumes of effluent following major nuclear incidents such as Fukushima, highlights the requirements for new flexible technology. The release of cesium-137 (137Cs) and strontium-90 (90Sr) in particular pose substantial safety and environmental concern due to their high fission yield and significant half-lives (t1/2 ~ 29 years). Once released into the biosphere there is a high potential for the fission products to undergo a series of chemical and geochemical processes that ultimately impact human health. Methods to assist in remediation face substantial scientific and engineering challenges associated with: i) complex interactions between the fission products and the environment, and ii) the requirement to process large volumes of waste.
In this UK-Republic of Korea project, a new concept based on dual-bubble flotation and magnetic separation will be studied to treat radioactively (Cs and Sr) contaminated environments. The 3-stage process: i) Cs+ and Sr2+ adsorption, ii) heterogeneous-bubble nucleation and flotation, and iii) magnetic separation and volume reduction will proceed in series to transform large waste volumes into small waste packages suitable for interim storage.
Surface wettability-based flotation is chosen as the most appropriate method for remediation due to its versatility and applicability for processing large waste volumes. In combination with hydrodynamic cavitation, the process will be capable of separating contaminated particles (fine clays and magnetic carriers) from the unwanted gangue to produce a froth that can be further treated by applying a magnetic field to control the rate of froth destabilization, but also and quite importantly control the overall volume of the waste package.
The research will lay scientific foundation for developing revolutionary technology to treat environmentally contaminated land (aqueous and soils). The scientific/engineering approach has the potential to develop a technological step-forward for global decommissioning programmes and the remediation of legacy nuclear sites, such that the footprint of nuclear power has no long lasting impact on the environment.

Planned Impact

The longer term impact will be realized through the development of a revolutionary technology for the clean-up of environmentally contaminated effluent and soils resulting from the unwanted release of radioactive material. The capability to treat environmental contamination, in a flexible semi-continuous system as the one proposed (dual-bubble flotation and magnetic volume reduction), is extremely pertinent when considering the recent incident at the Fukushima Daiichi nuclear plant in Japan, as well as the large scale national decommissioning programme active within the United Kingdom (decommissioning Sellafield ~£70 B over the next +100 years) and soon to be active in the Republic of Korea. It is envisaged through a technological step-forward, that the footprint of nuclear power will have no long lasting impact on the environment.
The ambitious research programme will lay scientific foundation for developing revolutionary technology to treat various contaminated streams. The scientific leadership will feed directly into the nuclear industry (Sellafield Ltd - please see the Letter of Support) and the remediation supply chain (SMEs) eventually facilitating technology demonstration at the pilot plant and plant scale. Scale-up to produce magnetic carrier particles suitable for flotation and possible technology transfer will be conducted in partnership with Nano Technology (please see the Letter of Support).
Through ingenious design of experiments the research knowledge generated will have impact in the wider scientific (academic and industrial) community, in particular colloids and surface chemistry, and DISTINCTIVE, an EPSRC funded collaboration considering the long-term management strategy of the UK waste inventories. At a higher level, knowledge generated in fine particle flotation would support the ongoing challenges in mining where, depletion of easy processing minerals to meet the ever-growing world population and improved life standards has resulted in the exploration of low grade mineral deposits that require fine grinding to liberate the valuables (production of fine particles).
The scientific and technological impact will be delivered through several routes. Firstly, academic engagement with the nuclear industry will initially be directed through established links such as the UofL-Sellafield Ltd Sludge Centre of Expertise, and the recently established Centre of Nuclear Security and Non-proliferation for research and outreach to public and governmental bodies (KAIST). Secondly, the collaborative programme between the UofL (UK) and KAIST (Republic of Korea) will be used as a springboard to establish a pan Asian-UK nexus. It is hoped that connections can be further grown, enabling a wide base of academic knowledge transfer (research and education) between partners for the mutual benefit for all countries in the Asia-Pacific region linked with the UK. Thirdly, the programme investigators will share their findings with the nuclear supply chain, in particular SMEs. Partnering with an SME and securing further development funding (for example, Innovate UK) will advance the TRL and create opportunity for pilot plant trails (technology demonstration). Finally, dissemination of research findings to the wider scientific community will be achieved by publishing in high impact journals such as Langmuir and Journal of Colloid and Interface Science, with an aim to publish the process research in industry relevant journals such as Nuclear Future and The Chemical Engineer.


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Description The first manuscript titled "Organically modified clay with potassium copper hexacyanoferrate for enhanced Cs+ adsorption capacity and selective recovery by flotation" was published in the Journal of Materials Chemistry A (I.F. 10.733) in July 2017. The selective capture of mobile radioactive nuclides, such as 137Cs+, is crucial to the clean-up and remediation of contaminated environments. While remediation remains a challenging task, the study considers novel organo-clay composites containing potassium copper hexacyanoferrate (KCuHCF) as a viable option for large-scale clean-up. A three-step synthesis has been demonstrated whereby pristine montmorillonite clay was readily modified to incorporate KCuHCF nanoparticles for enhanced and selective Cs+ removal from aqueous environments. Alkyldiamine (DT) was used as an organic modifier to intercalate the clay and provided chelating sites to anchor copper onto the clay matrix, from which KCuHCF nanoparticles were subsequently grown in situ via the coordination of hexacyanoferrate precursors with the immobilized copper ions. The organo-clay-HCF composite particles exhibited a superior Cs+ adsorption capacity (qm = 206 mg g-1), twice that of the pristine clay. The enhanced performance also extended to high Cs+ selectivity in seawater, with the organo-clay-HCF composites demonstrating Cs+ selectivity values in excess of 105 mL g-1, two orders of magnitude greater than the pristine clay. Organo modification of the clay particles reduced the particle wettability, thus facilitating the separation of Cs-loaded composite particles from aqueous environments by collector-less flotation. Batch flotation experiments showed recovery efficiencies of the Cs-loaded composite particles of up to 90%, which was in great contrast to the low recovery of less than 15% for the Cs-loaded pristine montmorillonite. The study provides a new concept for the treatment of contaminated aqueous environments.

The second manuscript titled "Selective separation of caesium contaminated clays from pristine clays by flotation" was published in Chemical Engineering Journal (I.F. = 8.355) in January 2019. The ongoing boom of industrialization is conflicted by concerns regarding increased levels of environmental contamination, in particular the uncontrolled release of heavy metal ions and radionuclides into soils and groundwater systems. The extent of contamination can be substantial, hence ways to remediate and reduce the volume of waste for further treatment and ultimate disposal are highly desired. In the current study, flotation has been considered as an engineering solution to rapidly separate cesium contaminated clays from low-level contaminated and pristine clays. Cesium (Cs+) sorption by montmorillonite clay particles was considered over a range of ionic concentrations (0.01-500 mM), showing a multistage sorption isotherm that can be interpreted using a two-site model, which considers both interlayer ion-exchange and specific ion sorption on the clay basal planes at higher cesium concentrations. Assessment by X-ray photoelectron spectroscopy (XPS) and zeta potential confirmed the increased surface contamination with increasing Cs+ concentration, with the surface enrichment sufficiently altering the surface chemistry of the contaminated clays for them to favourably interact with the flotation collector, ethylhexadecyldimethyl-ammonium-bromide (EDAB). Within a critical concentration range of EDAB, the cesium contaminated clays were separated from pristine clays using flotation, with recovery efficiencies of ~75% for the contaminated clays, compared to <25% for the pristine clays. When contaminated and pristine clays were blended, separation by flotation once again demonstrated excellent selectivity for the contaminated clays. The current study highlights the potential for flotation to rapidly treat contaminated clay rich soils and significantly reduce the volume of contaminated solids for further treatment or ultimate disposal.

The third manuscript titled "Bio-inspired preparation of clay-hexacyanoferrate composite hydrogels as super adsorbents for Cs+" is to be submitted to the Journal of Materials Chemistry A in April 2020. A facile, low-cost, bio-inspired route has been utilized for the first time to prepare a clay-based functional hydrogels containing hexacyanoferrate (HCF) nanoparticles for selective Cs+ removal from aqueous media. Initially montmorillonite was exfoliated into single layers and coated with a thin layer of polydopamine (PDOPA) via self-polymerization of dopamine, named D-Clay. Mixing with hexacyanoferrate precursor followed by addition of copper ions in steps, the D-clay self-assembled into three-dimensional networks with HCF nanoparticles in-situ grown and embedded within the hydrogel. The analytical characterizations confirmed that the network structure formation and the HCF immobilization was realized via a copper-ligand complexation mechanism. Rheological measurements revealed that the composite hydrogels exhibited fairly elastic response in low deformation and have a self-healing capability upon removal of applied large stress, assuring good stability of HCF nanoparticles in the hydrogel. Additionally, a superior Cs+ adsorption capacity (~173 mg/g) and selectivity for Cs+ was identified even at extremely low concentration (0.2 ppm) of Cs+ in presence of seawater. Furthermore, the designed materials (composite hydrogels) have superior mechanical performance and enhanced removal capacity for Cs+ which encourage to utilize it as adsorbent bed in column filtration device to remediate water. In addition, composite hydrogels prepared from different routes and that cross-linked via Fe3+ have also been presented for comparisons with regard to the mechanical and adsorption performance.

Other papers in partnership with our collaborators include:
1) Enhanced adsorption capacity and selectivity towards strontium ions in aqueous systems by sulfonation of CO2 derived porous carbon. Oxygen-enriched carbon materials derived from carbon dioxide were functionalized using sulfonic acid to remove Sr2+ ions from aqueous solutions. Synthesized sulfonated porous carbon materials (PC-SO3H) showed higher adsorption capacity and selectivity towards Sr2+ than non-functionalized porous carbons (PC). The formation of the C-SO3H functional group in PC-SO3H and its ability to proton exchange with Sr2+ was the main contributor to the enhanced performance. The maximum uptake capacity of Sr2+ by PC-SO3H was 18.97 mg g-1, which was 1.74 times greater than PC. PC-SO3H removed 99.9% and 97.6% of Sr2+ from aqueous solutions with initial Sr2+ concentrations of 5 mg L-1 and 10 mg L-1, respectively. Sr2+ adsorption showed rapid kinetics, reaching the adsorption equilibrium within 1 h with high adsorption capacity at equilibrium which is 3.52 times greater than that of PC. Additionally, PC-SO3H selectively adsorbed Sr2+ even in the presence of excess amounts of competing ions. Sulfonation of oxygen-enriched carbon had a significant effect on enhancing the affinity towards Sr2+ and suppressing adsorption towards other competing ions.
2) Highly effective Cs+ removal by turbidity-free potassium copper hexacyanoferrate-immobilized magnetic hydrogels. Potassium copper hexacyanoferrate-immobilized magnetic hydrogel (MHPVA) has been synthesized via a facile freeze/thaw crosslinking method. The citric acid coated Fe3O4 is embedded into the hydrogel matrix to facilitate the dispersion of nano-sized KCuHCF particles for Cs+ removal, followed by the rapid recovery of the composite in a magnetic field. The Cs+ adsorption behavior of the MHPVA is fitted well with the Langmuir isotherm and the pseudo-second-order kinetic model. The MHPVA exhibits both high Cs+ adsorption capacity (82.8 mg/g) and distribution coefficient (Kd) of 1.18 × 106mL/g (8.3 ppm Cs+, V/m = 1000 mL/g). Sorption of above 90% Cs+ to the MHPVA is achieved in less than 3 h of contact time. Moreover, the MHPVA reveals stable and high Cs+ removal efficiency across a wide pH range from 4 to 10. In terms of Cs+ selectivity, the MHPVA shows above 96% removal efficiency in the presence of 0.01 M competing cations such as Mg2+, Ca2+, Na+, and K+ with 1 ppm of Cs+. From a practical perspective, the MHPVA still exhibits stable and promising selective properties even in groundwater and seawater conditions and after 5 days of contact time the used adsorbent is rapidly recovered leaving a turbidity-free aqueous environment.
3) Nanostructured potassium copper hexacyanoferrate-cellulose hydrogel for selective and rapid cesium adsorption. Potassium copper hexacyanoferrate (KCuHCF) was synthesized and immobilized in a cellulose-based hydrogel made of carboxymethyl cellulose (CMC) and hydroxyethyl cellulose (HEC) for the adsorption of cesium ions in aqueous solutions. The immobilization with the cellulose-based hydrogel facilitated the dispersion of nano-sized KCuHCF particles, showing the unprecedented adsorption capacity of the composite. In Cs+ removal experiments, KCuHCF-cellulose hydrogel composites (HCF-gels) exhibited exceptional Cs+ adsorption capacities (2.06-2.32 mmol g-1) which was attributed to the presence of ion-exchangeable sites (COO-Na+) in the cellulose hydrogel. The HCF-gels also exhibited a rapid Cs+ removal (90.1% removal for 0.15 mmol L-1 of Cs+ in 1 h) with the uptake reaction kinetics expressed by a pseudo-second order kinetics model. Notably, the HCF-gels could adsorb Cs+ selectively (>90%) in seawater containing 0.11 mmol L-1 Cs+. Such specificity with fast kinetics is due to the high ion accessibility from the inherent nature of hydrogels and the highly dispersed KCuHCF nanoparticles in the composites.
4) Solvent-assisted synthesis of potassium copper hexacyanoferrate embedded 3D-interconnected porous hydrogel for highly selective and rapid cesium ion removal. Potassium copper hexacyanoferrate-embedded poly(vinyl alcohol)-citric acid hydrogel film (HPC) was prepared via a two-step method of Cu immobilization, followed by the diffusion of potassium hexacyanoferrate accelerated by acetone evaporation. The diffusion-derived KCuHCF formation in the preformed hydrogel facilitated the preservation of the 3D-interconnected hydrogel structure and dispersion of the KCuHCF nanoparticles. Using acetone as a non-solvent, reverse diffusion of the incorporated Cu in the hydrogel matrix was hindered; hence a large amount of KCuHCF was loaded in the matrix. The HPC exhibited substantially enhanced Cs+ removal properties in terms of adsorption capacity, kinetics and selectivity. From the adsorption isotherm, the HPC showed a very high Cs+ uptake of 667 mg/g KCuHCF. Moreover, the adsorbent revealed stable and high Cs+ removal efficiency of 99.9% across a wide pH range from 2 to 10. The kinetics of Cs+ removal was remarkably rapid with 99.5% removal achieved within 30 min from a dilute Cs+ solution (9.18 ppm). When using seawater, the HPC exhibited almost unaltered Cs+ removal efficiency above 99.5%, and high distribution coefficient Kd value of 7.7 x 105 ml/g at an extremely low Cs+ concentration (0.67 ppm, V/m = 1000 ml/g), which highlighted the tremendous affinity for Cs+.
5) Synthesis of functionalized porous montmorillonite via solid-state NaOH treatment for efficient removal of cesium and strontium ions. Solid-state NaOH treatment of montmorillonite clay was used to enhance the removal of Cs+ and Sr2+. Through this facile and low-cost modification, montmorillonite with a large BET surface area (117.1 m2 g-1) and many surface functional groups (SiONa), demonstrated enhanced sorption kinetics (89% removal for 40 mg L-1 Cs+ and 23 mg L-1 Sr2+ in 1 h) with a sorption capacity of 290.7 mg g-1 for Cs+ and 184.8 mg g-1 for Sr2+, greatly exceeding the low sorption capacity (137.0 mg g-1 for Cs+ and 15.6 mg g-1 for Sr2+) of pristine montmorillonite. SEM-EDS and XPS analyses revealed that Cs+ and Sr2+ were ion-exchanged with Na+ on the surface functional groups formed following NaOH treatment. The performance of NaOH-treated montmorillonite was stable following gamma-ray irradiation (at 6 Gy h-1 for 30 min) and across a broad range of pHs (3 to 11), exhibiting a high distribution coefficient (Kd) of 1.5 × 103 mL g-1 for Cs+ (1.58 mg L-1) and 3.7 × 103 mL g-1 for Sr2+ (1.64 mg L-1) under groundwater conditions where various cations including Na+, K+, and Ca2+ (V/m = 1 L/g) were present. The proposed method demonstrated great improvement of the sorption capacity of an abundant and inexpensive montmorillonite.
6) Immobilization of potassium copper hexacyanoferrate in doubly crosslinked magnetic polymer bead for highly effective Cs+ removal and facile recovery. A potassium copper hexacyanoferrate (KCuHCF) embedded magnetic hydrogel bead (HCF-Mbead) was synthesized via a facile double crosslinking methods of Fe3+ ionic binding and freeze-thaw for effective Cs+ removal. The HCF-Mbead had a hierarchical porous structure facilitating fast access of Cs+ ions to embedded active sites. The adsorbent showed enhanced Cs+ removal properties in terms of capacity (69.2 mg/g), selectivity (Kd = 4 × 104 mL/g, 1 ppm Cs+ in seawater) and stability (>99.5% removal in pH 3 ~ 11) with rapid magnetic separation. This study further opens the possibility to develop an efficient material that links the integration of adsorption and recovery.
7) A high-strength polyvinyl alcohol hydrogel membrane crosslinked by sulfosuccinic acid for strontium removal via filtration. This study considered the removal of strontium (Sr2+) from contaminated water using a filtration membrane that exhibits good mechanical strength, high adsorption capacity, and the ability to be regenerated and reused. Polyvinyl alcohol hydrogel membranes were prepared by crosslinking with sulfosuccinic acid in different ratios (2.5, 5, 10 and 20 mol% relative to the PVA monomer), named as PSA2.5, PSA5, PSA10 and PSA20. All PSA membranes showed good Sr2+ adsorption over a wide pH range (pH 2-12), and maintained rapid removal kinetics (> 95% Sr2+ recovered from 5 ppm Sr2+ within 4 h). Furthermore, the Sr2+ adsorption capacities of PSA2.5, PSA5, PSA10 and PSA20 were 27.6, 45.8, 56.3, and 55.3 mg/g, respectively, based on the Langmuir adsorption isotherm. From the four PSA membranes, PSA5 was selected for further filtration studies due to its favorable mechanical and adsorption properties. When filtering 5 ppm Sr2+ and 250 ppm Ca2+, corresponding to the Ca2+ concentration in the wastewater at the Fukushima nuclear plant, 87% Sr2+ was removed using the PSA5 membrane following multiple cycles of regeneration and reuse. Moreover, the tensile strength of the PSA5 membrane remained high (> 100 MPa) following five consecutive uses.
8) Amino-functionalized magnetic chitosan beads to enhance immobilization of potassium copper hexacyanoferrate for selective Cs+ removal and facile recovery. Potassium copper hexacyanoferrate (KCuHCF)-incorporated magnetic chitosan beads (HMC) were synthesized for both selective Cs+ removal in aqueous solutions and facile recovery of the spent adsorbent. To disperse and immobilize large amounts of the KCuHCF, methyl acrylate and diethylenetriamine were sequentially grafted onto the one-step synthesized magnetic chitosan beads. The additional introduction of amino functionality led to the enriched Cu2+ ions on the bead surface to incorporate KCuHCF into the grafting matrix. Consequently, the HMC exhibited a high Cs+ capacity calculated to be 136.47 mg g-1 from the Langmuir model, and the equilibrium was established within 4 h. Moreover, the HMC exhibited excellent stability in a wide pH range from 4 to 11 and an outstanding Cs+ selectivity (>97%) in seawater (1.11 mg L-1 Cs+). From a practical point of view, the HMC was stable during five successive adsorption cycles and easily recovered by magnets, enabling continuous operation to decontaminate a large volume of wastewater.
Exploitation Route Expertise and techniques developed during the project has recently (2018) resulted in the funding of 3-month research supported by Imerys to test the performance of new adsorbent materials for Cs+ and Sr2+ removal. Atkins supported a summer placement project (2016) to explore the potential of flotation to recover contaminants from sludge pond waste. Discussions are currently ongoing with the KAIST collaborators to explore funding opportunities for continuation of the research. Developing collaborations with the National Nuclear Laboratory, Sellafield Ltd. and SNC Lavalin Atkins in the area of ground/groundwater remediation.
Sectors Energy,Environment,Other

Description Nuclear Science and Engineering - Undergraduate teaching, University of Leeds
Geographic Reach Local/Municipal/Regional 
Policy Influence Type Influenced training of practitioners or researchers
Description Batch measurements of a novel ion-exchange resin
Amount £32,700 (GBP)
Organisation Imerys Talc 
Sector Private
Country France
Start 04/2018 
End 06/2018
Description Electrokinetic Separation for Enhanced Decontamination of Soils and Groundwater Systems
Amount £378,287 (GBP)
Funding ID EP/S032797/1 
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Public
Country United Kingdom
Start 08/2019 
End 07/2022
Description Industrial Funding
Amount £2,500 (GBP)
Organisation WS Atkins 
Department Atkins Nuclear
Sector Private
Country United Kingdom
Start 06/2016 
End 08/2016
Description KAIST 
Organisation Korea Advanced Institute of Science and Technology (KAIST)
Country Korea, Republic of 
Sector Academic/University 
PI Contribution Hosted a collaboration meeting at the University of Leeds in September 2015. The meeting was attended by Prof. Jae Lee (KAIST) and 4 of his PhD researchers who are working on the particle synthesis and separations problem. All project collaborators visited the Sellafield site to discuss challenges associated with nuclear legacy wastes and the processing routes.
Collaborator Contribution Financially supported a 1 year sabbatical of Seoyeon Baik (KAIST PhD student) to the University of Leeds. Seoyeon will begin March 2016 and study the synthesis of metal-carbon composites for enhancing strontium adsorption capacity and selectivity. Hosted the 1st International Symposium on Materials and Interfacial Engineering for Nuclear Waste Removal, 29th August, 2016, Daejeon, South Korea. Organized an academic visit to KORI nuclear power plant, August 2016. Financially supported a 1 year sabbatical of Dr Ji Young Yoon (KAIST PhD student) to the University of Leeds.
Impact Kim, Y.K., T. Kim, D. Harbottle, and J.W. Lee, Highly effective Cs+ removal by turbidity-free potassium copper hexacyanoferrate-immobilized magnetic hydrogels - submitted Journal of Hazardous Materials Kim, Y. K., Y. Kim, S. Kim, D. Harbottle and J. W. Lee, Solvent-assisted synthesis of potassium copper hexacyanoferrate embedded 3D-interconnected porous hydrogel for highly selective and rapid cesium ion removal, Journal of Environmental Chemical Engineering, 5(1), 2017, 975-986, DOI: 10.1016/j.jece.2017.01.026 Kim, Y., Y. K. Kim, S. Kim, D. Harbottle and J. W. Lee, Nanostructured potassium copper hexacyanoferrate-cellulose hydrogel for selective and rapid cesium adsorption, Chemical Engineering Journal, 313, 2017, 1042-1050, (2016)
Start Year 2015
Description Nuclear Research Frontiers in Decommissioning and Radioactive Waste Management Conference 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Industry/Business
Results and Impact Presentation and poster on the "Selective separation of cesium contaminated clays from pristine clays by froth flotation". Research Frontiers 2017 is a showcase of R&D across the broad spectrum of technological challenges associated with the management and decommissioning of the UK radioactive waste legacy. Spanning Technology Readiness Levels (TRLs) 1 to 9, speakers are invited to cover subjects from academic research to industrial application and deployment. Attended by leading figures within the UK nuclear industry; Research Frontiers 2017 is an opportunity to build contacts and learn more about the innovative solutions and unique challenges within the nuclear industry.
Year(s) Of Engagement Activity 2012,2017,2020