Zeolite for Cs/Sr Removal from Nuclear Decommissioning Liquid Waste

Q: Why is clinoptilolite used to remove ¹³⁷Cs and ⁹⁰Sr?

¹³⁷Cs dissolves as Cs⁺ and ⁹⁰Sr as Sr²⁺, so clinoptilolite—with its negatively charged framework and cation exchange capacity (CEC 1.6–2.0 meq/g)—captures them by ion exchange without any modification. High-silica zeolites, in particular, favor large monovalent cations according to Eisenman theory, so clinoptilolite's selectivity series runs Cs>K>Sr=Ba>Ca≫Na (Dyer et al. 2018). As a result, trace Cs⁺ and Sr²⁺ can be selectively concentrated even in effluents rich in Na⁺ and Ca²⁺. Unlike anion adsorption, this is a representative cation-exchange application that requires no modification.

Q: Has clinoptilolite actually been operated at a real nuclear facility?

Yes. The SIXEP (Site Ion Exchange Effluent Plant) at the UK Sellafield reprocessing site has used a packed bed of Mud Hills (California) clinoptilolite to remove ¹³⁴/¹³⁷Cs and ⁹⁰Sr from alkaline storage-pond effluent for about 30 years (Dyer et al. 2018, J. Radioanal. Nucl. Chem.). However, because clinoptilolite degrades structurally at high pH, SIXEP places a carbonation tower upstream of the columns to lower the pH from about 11 to 7 before feeding the water through. This is an academic case provided for information only; actual adoption is contingent on regulatory approval and on-site verification.

Nuclear Decommissioning Liquid Waste: What Is the Problem?

During the permanent shutdown and decommissioning of nuclear power plants and the operation of spent-fuel storage ponds, large volumes of low-level liquid waste (LLLW) are generated. Spent-fuel pond water, decontamination wash solutions, and system flush water contain trace amounts of the fission products cesium-137 (¹³⁷Cs) and strontium-90 (⁹⁰Sr) in dissolved form. Because ¹³⁷Cs (half-life about 30 years) and ⁹⁰Sr (half-life about 29 years) are highly mobile in the environment and bioaccumulative, lowering their concentrations below regulatory limits before disposal or discharge is a core task of the decommissioning process.

In water, these radionuclides exist as the monovalent cation Cs⁺ and the divalent cation Sr²⁺, respectively. They are therefore difficult to remove by precipitation and filtration alone, requiring ion-exchange/adsorption processes that selectively capture dissolved cations. The challenge is that the effluent contains non-radioactive competing cations such as Na⁺, Ca²⁺, Mg²⁺, and K⁺ at concentrations tens of thousands to hundreds of thousands of times higher than the radionuclides. To selectively concentrate only the trace radioactive cations requires an exchanger with high Cs/Sr selectivity even in the presence of competing ions.

Why Clinoptilolite Is Considered for Cs/Sr Removal

Natural clinoptilolite is a cation exchanger whose aluminosilicate framework carries a negative charge balanced by exchangeable cations. Because ¹³⁷Cs and ⁹⁰Sr are cations, they are captured by ion exchange at the framework's negatively charged sites without any modification. The key is not simple exchange but selectivity. According to Eisenman theory, high-silica zeolites favor large monovalent cations with low charge density, and as a result clinoptilolite exhibits the following selectivity series.

Cs⁺ > K⁺ > Sr²⁺ = Ba²⁺ > Ca²⁺ ≫ Na⁺ > Li⁺

This series means that Cs⁺ and Sr²⁺ can be selectively concentrated even against a large Na⁺ background. The clinoptilolite for Sellafield SIXEP reported by Dyer et al. (2018) was able to selectively extract 1 mol of Sr²⁺ and 20 mol of Cs⁺ even in the presence of 7.5×10⁵ mol of Na⁺. Structurally, clinoptilolite's 3.5–3.9 Å channels are similar in size to the hydrated Cs⁺ ion, which is proposed as one reason for the high Cs⁺ selectivity (DOI:10.1007/s10967-018-6329-8).

Unlike the removal of anions (phosphate, fluoride, arsenic, etc.), the target here is a cation, so the framework's intrinsic cation-exchange mechanism operates as-is. In other words, this is a representative unmodified cation-exchange application that does not require modifications such as surfactant modification (SMZ) or metal loading. KMIZEOLITE's clinoptilolite has a purity of 97% and a CEC of 1.6–2.0 meq/g, and is mined and processed at the Amargosa Valley mine in Nevada, USA.

KMIZEOLITE Key Properties

Property	Value
Clinoptilolite purity	97%
Cation exchange capacity (CEC)	1.6–2.0 meq/g
Specific surface area	40.0 m²/g
Pore diameter	4.0–7.0 Å
pH stability range	3.0–10.0
Hardness	4.0–5.0 Mohs
Thermal stability	700°C
Specific gravity	1.89
Bulk density	45–54 lbs/ft³
Certifications	OMRI KMI-10365, FDA GRAS (21 CFR 182.2729), TSCA, EN-71-3

Application Case — Sellafield SIXEP Demonstration

The case that best demonstrates the application of clinoptilolite to nuclear liquid-waste treatment is the SIXEP (Site Ion Exchange Effluent Plant) at the UK's Sellafield reprocessing site. SIXEP has used a packed bed of clinoptilolite mined at Mud Hills, California (Calico Hills formation) to remove ¹³⁴/¹³⁷Cs and ⁹⁰Sr from alkaline spent-fuel pond effluent for about 30 years. Dyer et al. (2018) compiled and reported column and modeling data obtained between 1978 and 2012 (DOI:10.1007/s10967-018-6329-8). The main operating characteristics are as follows.

Configuration: operated in the sequence settling tank → sand filtration → carbonation tower → two clinoptilolite columns (in series, lead/lag)
pH correction: because clinoptilolite degrades structurally at high pH, the upstream carbonation tower lowers the effluent pH from about 11 to 7 before flow-through
Contact time: owing to the very high flow rate, the column contact time is about 8 minutes (the short residence time means adsorption kinetics, not equilibrium, govern performance)
Treatment capacity: a fresh packed bed was observed to operate without significant breakthrough up to about 20,000–25,000 BV (bed volumes)
Replacement method: when the lead column becomes saturated it is replaced, and the lag column is promoted to lead in this series operation

This case shows that clinoptilolite can be operated long-term in full-scale nuclear effluent treatment, but it is strictly an academic/operational case provided for information only; for domestic application, the Nuclear Safety Act framework, regulatory approval, and verification tailored to on-site effluent characteristics must come first.

Competing Ions and Operating Variables — Effect on Breakthrough

Actual effluent contains non-radioactive cations at far higher concentrations than the radionuclides, so they compete for exchange sites and advance breakthrough. The column experiments of Dyer et al. (2018) quantitatively reported the effects of competition strength and operating variables.

Variable	Observed effect
Competing-ion order	advances breakthrough in the order Ca²⁺ > Mg²⁺ > K⁺ > Na⁺
Sr²⁺ sensitivity	particularly sensitive to Ca and Mg because its binding site is similar to Ca²⁺
Cs⁺ sensitivity	increasing K⁺ (1→5 ppm) reduces the 5% breakthrough BV from 23 to 17.5 kBV
Na⁺ pulse	transient Sr²⁺ leaching increases in high-Na intervals; performance recovers quickly once the Na concentration returns to normal
pH	high pH causes framework degradation → pretreatment (carbonation) is needed to feed water in the neutral range

There are two key takeaways. First, Cs⁺ and Sr²⁺ behave differently. Cs⁺ is more sensitive to K⁺ and Sr²⁺ to Ca²⁺ and Mg²⁺, so managing both radionuclides simultaneously requires looking at both the hardness and the K concentration of the effluent. Second, at short residence times (SIXEP's 8 minutes), ion exchange does not reach equilibrium, so adsorption kinetics govern performance; the bed replacement interval must therefore be determined by breakthrough testing under the actual matrix and flow velocity, not by the catalog equilibrium adsorption capacity.

Recommended Particle Size and Product Specifications

Nuclear liquid waste is typically treated by packed-bed (column) flow-through. A Fine–Medium Granule (14×40–30×50 mesh) that balances permeability and contact area is considered, and SIXEP's laboratory columns also used 420–500 μm particles. Powder causes pressure loss and channeling and is therefore unsuitable for packed beds.

Product group	Mesh	Particle size	Typical use
Powder	100 mesh or finer	<150μm	pozzolan, feed, powder adsorption
Fine Granule	30×50 mesh	0.3–0.6mm	water treatment, filtration, soil
Medium Granule	14×40 mesh	0.4–1.4mm	filter beds, bedding, litter
Coarse Granule	8×14 mesh	1.4–2.4mm	swimming pools, de-icing, large filtration
Extra Coarse	4×8 mesh	2.4–4.8mm	packed beds, air scrubbers

→ View products by mesh size · Product selection guide by application

Pilot Testing and On-Site Review Points

When evaluating a clinoptilolite packed bed for nuclear decommissioning liquid waste, the following items must always be checked together.

Effluent chemistry diagnosis: accurately determine the target radionuclide (¹³⁷Cs·⁹⁰Sr) concentrations and radioactivity levels together with the competing-cation concentrations and hardness (Na, Ca, Mg, K, etc.) and pH
Breakthrough testing (column): plot the breakthrough curve versus BV under the actual matrix and operating flow velocity to determine treatable capacity and replacement interval (do not substitute catalog equilibrium values)
pH pretreatment: for alkaline effluent, place a pH-correction process such as carbonation or neutralization upstream of the bed to prevent framework degradation (SIXEP uses pH 11→7)
Pretreatment filtration: design preceding treatment such as sand filtration and coagulation so that suspended solids and sludge do not clog the bed
Radionuclide-specific optimization: configure the series-column and replacement strategy to reflect the behavioral differences between Cs⁺ (K-sensitive) and Sr²⁺ (Ca·Mg-sensitive)
Post-use disposal route: because the saturated medium becomes radioactive waste, design the solidification/immobilization, classification, and disposal route in advance, and always precede implementation with specialized engineering and licensing review to ensure regulatory compliance

For academic references, Jimenez-Reyes et al. (2021) reviewed the overall treatment of radioactive waste using zeolites (DOI:10.1016/j.jenvrad.2021.106610), and Faghihian et al. (1999) reported that clinoptilolite can remove radioactive Cs and Sr as well as Pb, Ni, Cd, and Ba, and the reversibility of that exchange (DOI:10.1016/S0969-8043(98)00134-1).

→ View TDS (Product Data Sheet) · View MSDS (Material Safety Data Sheet)

Cs/Sr Removal from Nuclear Decommissioning Liquid Waste — FAQ

Why is clinoptilolite used to remove ¹³⁷Cs and ⁹⁰Sr?

¹³⁷Cs dissolves as Cs⁺ and ⁹⁰Sr as Sr²⁺, so clinoptilolite—with its negatively charged framework and cation exchange capacity (CEC 1.6–2.0 meq/g)—captures them by ion exchange without any modification. High-silica zeolites, in particular, favor large monovalent cations according to Eisenman theory, so clinoptilolite's selectivity series runs Cs>K>Sr=Ba>Ca≫Na (Dyer et al. 2018). As a result, trace Cs⁺ and Sr²⁺ can be selectively concentrated even in effluents rich in Na⁺ and Ca²⁺. Unlike anion adsorption, this is a representative cation-exchange application that requires no modification.

Has clinoptilolite actually been operated at a real nuclear facility?

Yes. The SIXEP (Site Ion Exchange Effluent Plant) at the UK Sellafield reprocessing site has used a packed bed of Mud Hills (California) clinoptilolite to remove ¹³⁴/¹³⁷Cs and ⁹⁰Sr from alkaline storage-pond effluent for about 30 years (Dyer et al. 2018, J. Radioanal. Nucl. Chem.). However, because clinoptilolite degrades structurally at high pH, SIXEP places a carbonation tower upstream of the columns to lower the pH from about 11 to 7 before feeding the water through. This is an academic case provided for information only; actual adoption is contingent on regulatory approval and on-site verification.

Do co-existing Na, Ca, and Mg ions affect removal performance?

Yes. Competing cations contend for the ion-exchange sites, and in the column experiments of Dyer et al. (2018) Ca²⁺>Mg²⁺>K⁺ acted as stronger competitors than Na⁺, advancing breakthrough. Sr²⁺ in particular is sensitive to Ca and Mg because its binding site is similar to that of Ca²⁺, while Cs⁺ is more sensitive to K⁺. The bed replacement interval (treatable BV) should therefore be determined through breakthrough testing that reflects the actual hardness and Na concentration of the effluent matrix.

Which particle size (mesh) and form is suitable for a packed bed?

Because nuclear liquid waste is typically treated by column (packed-bed) flow-through, a Fine–Medium Granule (14×40–30×50 mesh, 0.3–1.4 mm) that balances permeability and contact is commonly considered. SIXEP's laboratory column studies also used 420–500 μm particles. Powder causes pressure loss and channeling and is therefore unsuitable for packed beds. Please refer to the product selection guide by application.

How is the saturated, spent zeolite handled?

Clinoptilolite that has adsorbed Cs and Sr becomes radioactive waste. As an aluminosilicate mineral, it is highly compatible with solidification/immobilization matrices such as cement and vitrification, making it favorable for encapsulation into long-term storage and disposal forms (Dyer et al. 2018). Waste classification, the solidification method, and the disposal route must be determined in accordance with the Nuclear Safety Act and regulatory guidance, and specialized engineering and licensing review must always precede implementation.

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Notice

Applicability may vary depending on site conditions, regulations, and test results. Treatment of nuclear decommissioning liquid waste is a high-risk field contingent on the Nuclear Safety Act framework and regulatory approval, so before any actual application, testing tailored to on-site conditions and specialized engineering and licensing review must always come first. Zeolite should be understood not as a cure-all for this field but as a material that supports existing processes.