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Two-step synthesis of a macroporous LTA zeolite
sorbent by supercritical CO₂ coating and
hydrothermal conversion: implementation in fixed-
bed nuclear wastewater treatment

Angel Manuel Escamilla-Pérez, Yves Barré, Agnès Grandjean, Audrey Hertz

<figure><img src="image_1.png" /></figure>

PII: S0896-8446(23)00104-3

DOI: https://doi.org/10.1016/j.supflu.2023.105940

Reference: SUPFLU105940

To appear in: *The Journal of Supercritical Fluids*

Received date: 18 October 2022

Revised date: 4 April 2023

Accepted date: 5 April 2023

Please cite this article as: Angel Manuel Escamilla-Pérez, Yves Barré, Agnès Grandjean and Audrey Hertz, Two-step synthesis of a macroporous LTA zeolite sorbent by supercritical CO₂ coating and hydrothermal conversion: implementation in fixed-bed nuclear wastewater treatment, *The Journal of Supercritical Fluids*, (2023) doi:https://doi.org/10.1016/j.supflu.2023.105940

This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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# Two-step synthesis of a macroporous LTA zeolite sorbent by supercritical CO₂ coating and hydrothermal conversion: implementation in fixed-bed nuclear wastewater treatment

Angel Manuel Escamilla-Pérez, Yves Barré, Agnès Grandjean, Audrey Hertz*

CEA, DES, ISEC, DMRC, Univ Montpellier, Bagnols-sur-Cèze F-30207, France.

* Corresponding author: audrey.hertz@cea.fr

Present address: LEITAT Technological Center, Applied Chemistry and Materials Department, C/Pallars, 185, 08005 Barcelona, Spain

## ABSTRACT

Fixed-bed sorption on solid substrate is a common approach to safely extract cationic radionuclides (such as ⁹⁰Sr²⁺) from nuclear wastewater and minimize final waste volumes. This article describes the preparation and characterization of a new hierarchical zeolite-glass bead composite for this application. A homogeneous layer of microporous amorphous silica is deposited on the beads in supercritical CO₂ and then transformed into a 1 µm-thick LTA (Linde Type A) zeolite film (40 wt.% of the composite) by hydrothermal treatment. Microstructural data show that this design improves the accessibility of the sorbent sites for Sr²⁺ cations. Batch sorption data in different types of simulated wastewater quantify the effects of competing cations and high salinity. Dynamic breakthrough experiments with simulated saline wastewater show that the proposed composite has delayed leakage and faster exchange kinetics than an existing commercial LTA-zeolite sorbent, which should translate into greater waste volume reduction in industrial fixed-bed extraction systems.
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**Keywords:**

Supercritical CO₂ coating, hydrothermal transformation, LTA zeolite sorbent, hierarchical porosity, strontium removal, wastewater treatment

# 1 INTRODUCTION

One of the major requirements for sustainable nuclear energy generation is efficient treatment and management of the contaminated solid and liquid waste from the different stages of the fuel cycle and from the decommissioning of nuclear and pilot plants. This study focuses on extraction of radionuclides from contaminated wastewaters. Two of the most abundant radionuclides in nuclear wastewater are strontium and cesium, typically present in cationic form (i.e. Sr²⁺ and Cs⁺). Beta-emitting ⁹⁰Sr, with a half-life of 29 years, is particularly radiotoxic, accumulating in bones because of its similarity with calcium and also being readily assimilated by aquatic organisms. ⁹⁰Sr²⁺ ions must therefore be transferred from wastewater to a stable solid material to reduce the final waste volume and facilitate safe storage [1, 2]. Various processes have been developed to extract radionuclides from contaminated wastewaters, the main ones being precipitation, filtration, evaporation, solvent extraction, and organic/polymer ion exchange. However, these methods are often inefficient (low decontamination factors or concentration factors), costly, or generate secondary waste that is difficult to treat and store.

Radionuclide extraction using mineral sorbents (ionic exchangers for instance) is an attractive alternative as they are more resistant to radiolysis and more selective than classic organic ion exchange resins [2-4]. A wide range of mineral sorbents [5, 6], including clays [4, 7, 8], zeolites [9-11], titanates [12-15] and silicotitanates [16-19], have been investigated for strontium removal from aqueous media. Zeolites are particularly interesting in this context as they are simple and inexpensive to produce. Zeolites are crystalline aluminosilicate-based
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materials with a three-dimensional structure based on SiO₄ and AlO₄ tetrahedra with charge-
compensating ions such as Na⁺ in their channels (microporous network). These ions can be
exchanged to capture Sr²⁺ in the zeolite framework. The small micropores (< 7 Å) in Na-LTA
(Linde Type A) zeolite make it particularly well-suited to Sr²⁺ sorption, with high selectivity
at low calcium (competing cation) concentrations [9, 20-22].

However, the small particle size of LTA zeolites (ranging from a few nanometers to few
micrometers) is a limitation for large scale processes because they are difficult to separate
from the treated wastewater. So, zeolite sorbents have to be processed so that they can be
collected, but without altering their structure to avoid reduced performance. The typical
approach for on-site wastewater treatment is therefore to granulate zeolites so that they can be
used in fixed-bed processes. The efficiency of such a process then depends on the kinetics and
selectivity of the sorbent for the targeted elements [23, 24]. Just as in catalysis, granular
sorbents must allow efficient transport into the column (at the macrostructural level) and fast
ion diffusion to the sorption sites (at the microstructural level) [25-29]. However, producing
the hierarchical porous networks required for efficient ⁹⁰Sr²⁺ removal with these materials
remains challenging [30-32]. One option is to incorporate LTA zeolites in composite
materials with either organic or inorganic binders such as polyacrylonitrile [33, 34], alginate
foam [35], or carbon substrates [36, 37]. Another approach that has been developed for
membranes is to deposit zeolite layers on ceramic or metal templates, typically by seeded
growth techniques such as dip-coating, rubbing and vacuum seeding [38-44]. LTA zeolites
can also be deposited using *in-situ* methods (without seeding) combined with microwave
heating [45, 46] or substrate functionalization with a covalent linker [47-50]. However, these
coating methods are not well suited for the functionalization of substrates with complex 3D
geometries.
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The objective of this study was to develop porous composite sorbents based on macroporous beads coated with LTA zeolite film. Based on previous results, supercritical CO₂ (SC-CO₂) was used to obtain a homogeneous, tightly attached micro/meso-porous SiO₂ film [51-58], which was then transformed by hydrothermal treatment into Na-LTA zeolite [59-61]. The developed sorbents were characterized by X-ray diffraction, N₂-physisorption, Hg porosimetry and scanning electron microscopy (SEM). Sr²⁺ sorption properties were studied by batch and column (fixed-bed) experiments and results were compared with those of Na-LTA zeolite powders and commercial granules, outlining the general scope of these composite materials for industrial applications.

# 2 MATERIALS AND METHODS

## 2.1 Substrates and chemicals

Macroporous glass beads (denoted G in abbreviations hereafter; bead size, 0.25–0.5 mm; bulk density, 340 kg·m⁻³) with a bimodal pore size distribution (about 0.1–10 µm and 50–150 µm) were obtained from Dennert Poraver GmbH (Schlüsselfeld, Germany). Such substrate is well adapted for radionuclide extraction in column treatment since the mean macropore size between the beads after column bed-packing is optimal (200 µm) for effluent transport (Fig. S1) and it is radiochemically resistant (for considered medium term storage).

Tetraethyl orthosilicate (TEOS) (> 99 % purity, ACROS Organics, Geel, Belgium) was used as silica precursor without further purification, and CO₂ was used as received from Air Liquide, Paris, France (99.998 % purity, water amount = 3 ppmv). Nitric acid (HNO₃) (69 %, Sigma-Aldrich, Munich, Germany) was used as sol stabilizer and isopropanol (iPrOH) (anhydrous, 99.5 % purity%, Sigma-Aldrich, Munich, Germany) as co-solvent. Deionized water, sodium aluminate (NaAlO₂) (VWR Chemicals, technical grade, Pennsylvania, PA, USA) and sodium hydroxide (NaOH) (Sigma-Aldrich, pellets, > 98.0 % purity, Munich, Germany) were used for the hydrothermal transformation of silica into zeolite.
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Sorption experiments were performed using sodium nitrate (NaNO₃) (VWR Chemicals, ≥ 99 % purity, Pennsylvania, PA, USA), strontium nitrate (Sr(NO₃)) (Sigma-Aldrich, 99 % purity, Munich, Germany), cesium nitrate (CsNO₃) (Sigma-Aldrich, 99 % purity, Munich, Germany), calcium nitrate (Ca(NO₃)₂-4H₂O) (Sigma-Aldrich, 99 % purity, Munich, Germany), deionized water and mineral water ([Ca²⁺] = 80 mg·L⁻¹, [Mg²⁺] = 26 mg·L⁻¹, [Na⁺] = 6.5 mg·L⁻¹, [K⁺] = 1 mg·L⁻¹, [SiO₂] = 15 mg·L⁻¹, [HCO₃⁻] = 360 mg·L⁻¹, [SO₄²⁻] = 14 mg·L⁻¹, [Cl⁻] = 10 mg·L⁻¹, [NO₃⁻] = 3.8 mg·L⁻¹, pH = 7.2).

## 2.2 Two-step coating procedure

A TEOS-based sol (50 mL of ¹PrOH + 10 mL of TEOS + 5 mL of HNO₃) was treated in SC-CO₂ inside a stirred reactor (0.5 L), to obtain silica by in-situ hydrolysis and thermal decomposition [62]. Nitric acid was included in the precursor to stabilize the sol, control silica cluster nucleation-growth kinetics and active the glass bead substrate to anchor the silica film. The TEOS sol was poured with the glass beads (2.5 g) in the reactor. The reactor was closed and CO₂ was injected up to a pressure of 5 MPa at ambient temperature (~ 20 °C). The temperature was then increased up to 150°C, leading to a pressure increase up to 25 MPa. The system was stirred mechanically and the reaction was maintained for 1 h. After depressurizing and cooling the reactor, the product (Silica coated glass beads named G_Si + surplus silica powder named SC-CO₂ SiO₂) was recovered and sieved (160 µm mesh) to separate the silica coated glass beads from the uncoated silica powder.

SC-CO₂ synthesis at higher temperature (210 °C – 25 MPa) and lower CO₂ pressures (100 and 18 MPa – 150 °C) were evaluated, given lower silica specific surface area with more aggregation and lower macroporous bead infiltrations. Such G_Si composites are less adapted for complete silica transformation into LTA zeolite and results are not presented here.

In addition, glass bead substrates were also functionalized with SBA-15 mesoporous silica using classical sol-gel synthesis, without supercritical CO₂. The amount of silica incorporated
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in the composite (< 5 wt%) was too low to consider the transformation into LTA-zeolite.

Actually, glass bead substrates directly contacted with TEOS solutions are not well immersed, and tend to float due to air blocking in the macropores (even under vacuum). The use of pressurized CO2 forces the solution infiltration into the beads and favor TEOS diffusion.

The silica-coated beads were then hydrothermally treated in stainless steel digestion vessels (V=125 mL, Parr Instrument Company, Moline, Illinois, USA) with polytetrafluoroethylene (PTFE) lining using classical conditions for the transformation of silica into LTA-zeolite, adapted from Galarneau et al. [59-61]. The hydrothermal treatment involved local SiO₂ dissolution and aluminosilicate gel precipitation with preservation of the morphology of the SiO₂ films, followed by gel crystallization. First, 5.75 g of NaAlO₂ were dissolved in 35 mL of deionized water and simultaneously, 2.65 g of NaOH were dissolved in 26.25 g of deionized water. The solutions were poured together into the digestion vessel and 2.0 g of silica-coated beads (G_Si) were added. The digestion vessel was sealed and heated for 20 h at 40 °C and then for 24 h at 70 °C, under autogenous pressure. The product was washed with deionized water (1 L), dried at 80 °C overnight and sieved (160 µm mesh) to separate the LTA-zeolite-coated glass beads (G_Zeo(Na)) from LTA-zeolite powder (Zeo(Na)). Silica transformation was also performed with 2 h ageing time at 40 °C and hydrothermal treatment of 48 h at 80 °C. The LTA zeolite was formed but crystal size was higher (>> 1 µm) and zeolite film was less adherent to the glass bead surface.

Series of LTA-zeolite-coated beads were thus prepared and mixed together to have a homogeneous 15g batch of composite material, in order to realize all the characterization and sorption tests.

## 2.3 Sorbent characterizations

The silica and zeolite-coated materials were characterized by X-ray diffraction (XRD), N₂ adsorption/desorption experiments and Scanning electron microscopy (SEM). X-ray
diffraction patterns were obtained using an X'Pert PRO-PANalytical diffractometer (Malvern,
Worcestershire, UK), with Cu Kα radiation (λ=1.54056 Å) and a step size of 0.033° (2θ scale)
from 5 to 70°. N₂ physisorption experiments were performed at -196 °C on a Micromeretics
ASAP 2020 device (Norcross, Georgia, USA). Samples were out-gassed under vacuum at 90
°C for 16h and specific surface areas, pore volumes and pore sizes were calculated using the
BET (Brunauer, Emmett, Teller) and BJH (Barrett, Joyner, Halenda) methods, respectively.
Hg porosimetry experiments were performed on a Micromeretics AutoPore IV 9500 device
(Norcross, Georgia, USA). Scanning electron microscopy images were acquired using either a
FEI Inspect S50 electron microscope (Hillsboro, Oregon, USA) equipped with Bruker
(Billerica, Massachusetts, USA) Nano Quantax energy dispersive X-ray (EDS) detector or a
ZEISS Ultra+ FESEM (Oberkochen, Germany) equipped with Bruker-SDD EDS system
(Billerica, Massachusetts, USA). Samples were deposited on carbon tape or embedded in
epoxy resin and polished. All samples were then coated with a thin layer of platinum before
observation.

**2.4 Batch (stirred reactor) and dynamic (column) sorption experiments**

Three types of simulated wastewater containing Sr²⁺ and competitive cations (Na⁺, Ca²⁺) were prepared: (i) a *simplified wastewater solution* (for ion exchange studies, to understand the behavior of the sorbent) consisting of 0.01 M Na⁺ (10 meq·g⁻¹ of sorbent) and 250 mg·L⁻¹ Sr²⁺ (5.7 meq·g⁻¹) in deionized water, (ii) a *saline wastewater solution* (with a composition similar to that of salted wastewater from the nuclear industry) containing 0.25 M Na⁺ (250 meq·g⁻¹), 90 mg·L⁻¹ Sr²⁺ (2.05 meq·g⁻¹) and 50 mg·L⁻¹ Ca²⁺ (2.5 meq·g⁻¹) in deionized water, and (iii) a *contaminated groundwater solution* (with low salinity but a complex composition, similar to that of natural groundwater) consisting of 30 mg·L⁻¹ Sr²⁺ (0.7 meq·g⁻¹) in mineral water containing various cations (principally ~7 mg·L⁻¹ Na⁺ (0.25 meq·L⁻¹) and ~60 mg·L⁻¹ Ca²⁺ (3 meq·g⁻¹)) and anionic counter-ions (see section 2.1).
The Sr²⁺ sorption properties of the zeolite powder and the zeolite-glass bead composite were evaluated and compared to those of a commercial Na-LTA-zeolite sorbent (~80 wt% granulated LTA zeolite with clay binder). Sorption kinetics and isotherms were measured in batch mode in a stirred closed system, and breakthrough curves were measured in dynamic flow experiments performed on an open-ended lab-scale column.

For the batch kinetic experiments (sorption capacities, $Q_t$, versus time, $t$), 20 mg of sorbent were placed in 20 mL of solution (sorbent concentration, 1 g·L⁻¹) in closed flasks. The suspensions were stirred at room temperature and supernatants were collected at fixed times (5 min to 144 h), filtered (0.2 µm ACRODISC PSF GHP syringe filter, Pall corporation, New-York, NY, USA) and analyzed by inductively coupled plasma atomic emission spectrometry (ICP-AES; Thermo Scientific, iCAP 6000 series, Waltham, Massachusetts, USA) to measure the concentrations of strontium, sodium and calcium in the solution. The strontium and calcium sorption capacities, and the amounts of released sodium (in meq·g⁻¹) were calculated using $Q_t(\text{Sr or Ca}) = (C_0 - C_t)\frac{V}{m}$ and $Q_t(\text{Na}) = (C_t - C_0)\frac{V}{m}$, with $C_0$ and $C_t$ (meq·L⁻¹) the concentrations of the respective elements initially and at time $t$, $V$ (L) the volume of the solution, and $m$ (g) the mass of sorbent.

The batch sorption kinetics and isotherm experiments (equilibrium capacities, $Q_{eq}$, or distribution coefficients, $K_d$, versus Sr²⁺ concentration in the solution at equilibrium, $C_{eq}$) were conducted in closed flasks containing 20 mg of sorbent and 20 mL of simulated wastewater with different Sr²⁺ concentrations (0.5, 1, 3, 5, 7, 10, 15, 25, 50, 100, 250, 500, 750 and 1000 mg·L⁻¹). The suspensions were stirred at room temperature for 48 h and the supernatants were collected, filtered and analyzed by ICP-AES to quantify the concentrations of the different species at equilibrium. The Sr²⁺ equilibrium capacity ($Q_{eq}$ in meq·g⁻¹) and distribution coefficient ($K_d = Qt/C_t$, in mL·g⁻¹) were calculated for each Sr²⁺
concentration. The associated kinetic curves were then fitted by a pseudo-second order kinetic model (Ho and Mc Kay equation [63]) and the isotherms by a Langmuir model [64]).

For the dynamic flow experiments, a glass column (Ø 1.0 cm) was filled to different heights (5, 8, or 11 cm, corresponding to packed-bed volumes of about 3.9, 6.3 and 8.6 mL, respectively) with the G_Zeo composite or commercial LTA sorbent and conditioned with 10⁻² M NaNO₃ solution for 72 h (66 h in static mode, then 6 h at a linear flow rate of 20 mL·h⁻¹). The column was then integrated into an open-ended system. The simulated wastewater was pumped through the column (top to bottom) at either 20 or 80 mL·h⁻¹.

Samples were collected at different times at the column outlet with an automatic collector (Labomatic, Labocol Vario-4000, Allschwil, Switzerland). Strontium concentrations were analyzed by ICP-AES to produce breakthrough curves (outlet Sr²⁺ concentration versus volume of treated solution). Normalized breakthrough curves were obtained by plotting C/C₀ (the ratio of initial to output Sr²⁺ concentrations) versus V/V₁/₂ (the volume of treated solution divided by the volume of treated solution at C=C₀/2). Characteristic values, namely Kd(V₁/₂); Kd(leak) the distribution coefficient at the leak concentration of Sr²⁺ at the column outlet; the value of Kd(leak) expected for real wastewater containing only trace amounts of Sr²⁺; the column saturation (Sat%), and the concentration factor (CF, the percentage decrease in waste volume) were evaluated and compared for the different sorbents and column configurations (method outlined in Fig. S2 and the accompanying paragraph in the supplementary material).

For such sorption experiments, variability on the sorbent mass and solution volume measurements is lower than 0.5% and 5% respectively. In addition, ICP-AES elemental analyses are performed three times with a concentration variability lower than 1% for all the elements. Sorption capacities are thus calculated with an average uncertainty of about 10% (this value was validated on a wide range of sorbents and solution compositions in our lab).

# 3 RESULTS AND DISCUSSION
## 3.1 Silica coating in SC CO₂

The SEM images (Fig. 1) show that the silica films obtained by thermal decomposition of TEOS in SC-CO₂ are homogeneous, amorphous and well attached to the entire surface of the glass bead substrate, with few submicronic aggregates visible on the film surface. Silica film is also homogeneously deposited inside the glass beads, on the surface of the macropores (Fig. 1c-d).

The BET specific surface area of the glass beads increases from ~1 to 134 m²·g⁻¹ after silica coating in SC-CO₂ (Table 1), due to incorporation of a large amount of silica with high specific surface area in the glass bead substrate. The BET specific surface area of 481 m²·g⁻¹ measured for the SiO₂ powder suggests that the silica nanoparticles are about 4.7 nm in diameter (assuming the spherical particles are disconnected and uniformly distributed). The type IV isotherm measured for the SC-CO₂ SiO₂ powder (Fig. 2a) with H3/H4 hysteresis, typical of aggregated materials with non-rigid texture [65], indicates the presence of interparticle micropores and a few mesopores. The isotherm measured for the G_Si sample is also Type IV, but with very little H3/H4 hysteresis (Fig. 2b). This material is therefore also microporous, with interparticle pores mostly less than 3 nm in diameter between the well-arranged nanoparticles in the silica coating. The poorly defined mesoporous structure confirms the presence of a few residual silica aggregates on the surface of the silica film.
<figure><img src="image_2.png" /><figcaption>**Fig. 1.** Scanning electron micrographs of silica-coated glass beads prepared by thermal decomposition of tetraethyl orthosilicate in supercritical CO₂: (a, b) outer surface, (c) cross section, and (d) inner surface of a macropore.</figcaption></figure>

**Table 1.** Specific surface area ($S_{BET}$) and average pore diameter ($Dp$) for reference powders, the glass bead substrate (G), and the studied composites.

<table><thead><tr><th>Sample</th><th></th><th>S<sub>BET</sub> (m<sup>2</sup>·g<sup>-1</sup>)</th><th>Dp (nm)</th></tr></thead><tbody><tr><td>SC-CO<sub>2</sub> SiO<sub>2</sub></td><td>Powder</td><td>481</td><td>8.41</td></tr><tr><td>Zeo(Na)</td><td>Powder</td><td>4</td><td>20.1</td></tr><tr><td>Zeo(Sr)</td><td>Powder</td><td>266</td><td>10.4</td></tr><tr><td>G</td><td>Substrate</td><td>1</td><td>6.8</td></tr><tr><td>G_Si</td><td>Composite</td><td>134</td><td>5.3</td></tr><tr><td>G_Zeo(Na)</td><td>Composite</td><td>4</td><td>14.1</td></tr><tr><td>G_Zeo(Sr)</td><td>Composite</td><td>112</td><td>7.5</td></tr></tbody></table>

SC-CO₂, SiO₂, SiO₂ synthesized in supercritical CO₂; Zeo(Na) and Zeo(Sr), as-synthesized and Sr-exchanged LTA zeolite powder; G_Si, glass beads coated with silica in supercritical CO₂; G_Zeo(Na) and G_Zeo(Sr), as-synthesized and Sr-exchanged zeolite-glass bead composites.
<figure><img src="image_3.png" /><figcaption>**Fig. 2.** Nitrogen physisorption isotherms of (a) powder samples (SC-CO₂ SiO₂, SiO₂ synthesized in supercritical CO₂; Zeo(Na) and Zeo(Sr), as-synthesized and Sr-exchanged LTA zeolite powder) and (b) bare (G) and coated glass beads (G_Si, glass beads coated with silica in supercritical CO₂; G_Zeo(Na) and G_Zeo(Sr), as-synthesized and Sr-exchanged zeolite-glass bead composites).</figcaption></figure>

Since the microstructures of the SiO₂ film and powder are similar, the amount of coating in the G_Si composite can be estimated at about 30 wt.% from the ratio of the specific surface areas. This is probably a slight underestimate given the slightly higher level of aggregation evidenced in the SiO₂ film compared with the powders.

These results indicate that this method of coating using supercritical CO₂ promotes the infiltration and diffusion of the TEOS precursor into the complex macroporous structure of the glass beads, which is difficult to access with classical precursor sols in aqueous or alcoholic media. By limiting the growth of nanosized silica clusters, supercritical CO₂ also contributes to increasing the specific surface area and porosity of the resulting film, making it easier to transform into zeolite by hydrothermal treatment.

## 3.2 Zeolite-based sorbent characterization
Fig. 3 shows that after hydrothermal treatment, the glass beads are homogenously covered with cubic particles 1–2 µm in size, the typical morphology of Na-LTA zeolite crystals (Fig. S3) [66, 67]. Al, Si and Na SEM-EDX maps of the G_Zeo(Na) composite show the presence of aluminosilicate coating both on the outer surface of the beads (Fig. 3a,b) and on the surface of the internal macropores (Fig. 3c–e). The monolayer of cubic particles inside the macropores is about 800 nm thick.

<figure><img src="image_4.png" /><figcaption>**Fig. 3.** Scanning electron micrographs and energy-dispersive X-ray maps of zeolite-coated glass beads: (a, b) external surface, (c–e) cross sections and pore interior.</figcaption></figure>

The XRD data in Fig. 4 show that the composite contains two crystalline phases, predominantly Na-LTA zeolite hydrate and a smaller amount of Na-LTA zeolite. The average crystallite size in the Na-LTA zeolite hydrate phase (estimated using the Scherrer formula) is 52 ± 4 nm. The XRD patterns also show evidence of a residual amorphous phase corresponding to the glass bead substrate.
Mercury porosimetry data (Fig S1 and Fig. S4) show that the porous structure of the glass beads does not change significantly after hydrothermal treatment, with the same bimodal pore size distribution in the zeolite coated composite. The macropores around 200 µm in size correspond to gaps between the particles in the packed-bed arrangement (the coating process does not affect the arrangement of the beads). The few macropores 50–150 µm in size correspond to the larger intraparticle pores, but the majority are around 2 µm in size. The fact that they are more widely and homogenously distributed after coating suggest that they may be interconnected (with the hydrothermal treatment increasing the level of interconnection).

<figure><img src="image_5.png" /><figcaption>**Fig. 4.** Powder X-ray diffraction patterns of ground glass bead substrate (G), LTA zeolite powder before and after $Sr^{2+}$ exchange (Zeo(Na) and Zeo(Sr)), and LTA zeolite-coated glass beads before and after $Sr^{2+}$ exchange (G_Zeo(Na) and G_Zeo(Sr)).</figcaption></figure>

The BET specific surface area of the G_Zeo composite (Table 1, Fig. 2b) is only $4 \cdot m^2 \cdot g^{-1}$ because the $Na^+$ ions in the Na-LTA zeolite nanocages prevent the diffusion of the $N_2$ probe molecules into the material by steric hindrance. This value therefore only corresponds to the
external surface of the LTA zeolite particles. The near-zero specific surface area suggests that all the silica coating is transformed during the hydrothermal treatment. After Na⁺ exchange with Sr²⁺, the N₂ molecules can then diffuse into the zeolite nanocages and probe the microporous network. The type IV isotherm obtained (Fig. 2b) confirms the microporous nature of the composite, with residual particle agglomeration on the surface of the zeolite film (reduced H3/H4 hysteresis). The BET specific surface area of the G_Zeo(Sr) (Sr-LTA-zeolite-coated beads) is 112 m²·g⁻¹ whereas that of the corresponding Zeo(Sr) powder is about 266 m²·g⁻¹. The ratio of these values suggests that the composite contains about 40 wt.% LTA zeolite.

### 3.3 Strontium sorption properties of the zeolite-based sorbent in batch mode

Strontium sorption kinetics and isotherms were measured with the G_Zeo composite in three types of simulated wastewater (Fig. 5, Table 2) and compared with data obtained with G untreated glass beads (0 wt.% zeolite), Zeo(Na) pure LTA zeolite powder (100 wt.% zeolite) and a commercial LTA zeolite sorbent.

The kinetics curves highlight the good liquid transport and ion diffusion properties of the LTA zeolite-coated beads. The sorption capacity of the G_Zeo composite equilibrates after less than 24 h in the simplified solution (with 95% of the equilibrium sorption capacity reached in less than 5 h), similar to the results obtained with the LTA zeolite powder. In comparison, the equilibration time of the LTA commercial grains is 80 h under the same conditions (with more than 24 h required to reach 95% of the equilibrium sorption capacity).

<caption>**Table 2** Comparison of the Sr²⁺ sorption properties of zeolite–glass bead composite with those of other sorbents in different types of simulated wastewater</caption>

Kinetics (1 g·L⁻¹)

Isotherms (1 g·L⁻¹, 48 h)
<table><thead><tr><th></th><th>[Sr<sup>2+</sup>]<sub>initial</sub><br>mg·L<sup>-1</sup><br>meq·g<sup>-1</sup></th><th>Equilibration<br>time</th><th>Q<sub>eq</sub> (Sr<sup>2+</sup>)<br>meq·g<sup>-1</sup></th><th>Q<sub>max</sub><br>meq·g<sup>-1</sup></th><th>K<sub>d</sub> (Sr<sup>2+</sup>)<br>mL·g<sup>-1</sup></th></tr></thead><tbody><tr><td colspan="6"><strong>Simplified wastewater solution</strong></td></tr><tr><td>G</td><td>250</td><td>5.7</td><td>-</td><td>0</td><td>0</td><td>0</td></tr><tr><td>Zeo</td><td>250</td><td>5.7</td><td>&lt; 24 h</td><td>4.8</td><td>198</td><td>229</td></tr><tr><td>Commercial</td><td>250</td><td>5.7</td><td>~ 80 h</td><td>4.0</td><td>176</td><td>-</td></tr><tr><td>G_Zeo</td><td>250</td><td>5.7</td><td>&lt; 24 h</td><td>2.2</td><td>97</td><td>92</td></tr><tr><td colspan="6"><strong>Contaminated groundwater solution</strong></td></tr><tr><td>G_Zeo</td><td>30</td><td>0.7</td><td>~ 50 h</td><td>0.5</td><td>22</td><td>97</td></tr><tr><td colspan="6"><strong>Saline wastewater solution</strong></td></tr><tr><td>G_Zeo</td><td>90</td><td>2.05</td><td>&lt; 24 h</td><td>0.8</td><td>37</td><td>62</td></tr><tr><td>Commercial</td><td>-</td><td>-</td><td>-</td><td>-</td><td>2.0</td><td>88</td></tr></tbody></table>

G, untreated glass beads; Zeo, pure LTA zeolite powder; Commercial, commercial LTA-zeolite granulated sorbent; G_Zeo, zeolite-coated glass bead composite

<figure><img src="image_6.png" /><figcaption>**Fig. 5** Sr²⁺ batch sorption isotherms for several sorbent materials in simplified wastewater ([Sr²⁺] = 250 mg·L⁻¹ and [Na⁺] = 0.01 mol·L⁻¹). The dotted lines are fits using Ho and Mc Kay's pseudo-second order reaction model [63]. G, untreated glass beads; Zeo, pure LTA zeolite powder; Commercial, commercial LTA-zeolite sorbent; G_Zeo, zeolite-coated glass beads.</figcaption></figure>
The absence of sorption with the untreated glass beads (Fig. 5) shows that the sorption properties of the G_Zeo come exclusively from the LTA zeolite coating. The sorption curves were well fit by a pseudo-second order kinetic model (Ho and Mc Kay equation [63]), corresponding to a chemisorption mechanism. The fact that the curves for Sr²⁺ sorption and Na⁺ release overlap (Fig. 5) confirms that the extraction mechanism is ion exchange. Comparing the maximum sorption capacities of the G_Zeo coated beads and the pure LTA zeolite powder (2.1 and 5.2 meq·g⁻¹, respectively; Table 2, Supplementary Material 5) suggests that LTA-zeolite content of the composite material is about 40 wt.%, in agreement with the value estimated from the ratio of BET specific surface areas.

Strontium and calcium sorption kinetics were also measured for the G_Zeo composite in the saline wastewater solution with competing Ca²⁺ cations and a high Na⁺ concentration (Fig. 6a), as well as in the contaminated groundwater solution with several types of potentially competing cations (Fig. 6b). In both cases, the sorption curves are well fit by the Ho and Mc Kay pseudo-second order reaction model, corresponding to Na⁺/Sr²⁺ and Na⁺/Ca²⁺ ion exchange. The sorption kinetics in the saline solution were as fast as in the simplified solution (equilibration in ~24 h) but the Sr²⁺ equilibrium sorption capacity was lower (0.8 meq·g⁻¹ instead of 1.9 meq·g⁻¹). The presence of Ca²⁺ was only marginally detrimental under these conditions, since only 0.3 meq·g⁻¹ was sorbed. The higher affinity of LTA zeolite for Sr²⁺ over Ca²⁺ (due to better adaptation of Sr²⁺ cation size to zeolite pore size, and stronger cation-zeolite framework interactions) is confirmed since from similar initial concentrations (2.0 meq·g⁻¹ and 2.5 meq·g⁻¹, respectively) the sorption capacity of the composite was almost three times higher for Sr²⁺ than it was for Ca²⁺. Hence, the total cation (Sr²⁺ + Ca²⁺) sorption capacity of the composite reached 1.1 meq·g⁻¹, compared with 1.9 meq·g⁻¹ in the simplified solution, indicating that the main sorption-limiting factor was the high Na⁺ concentration.
<figure><img src="image_7.png" /><figcaption>**Fig. 6.** Sr²⁺ and Ca²⁺ sorption kinetics for the zeolite–glass bead composite in (a) the saline wastewater solution ([Sr²⁺] = 90 mg·L⁻¹, [Ca²⁺] = 50 mg·L⁻¹ and [Na⁺] = 0.25 mg·L⁻¹) and (b) the simulated contaminated groundwater solution (mineral water with [Sr²⁺] = 30 mg·L⁻¹).

The dotted lines are fits using Ho and Mc Kay's pseudo-second order reaction model [63].</figcaption></figure>

In the simulated groundwater with a more complex composition, slower sorption kinetics were observed (Fig. 6b), with equilibration times of more than 60 h for the main competing cations (Sr²⁺ and Ca²⁺). The presence of several competing cations probably hinders ion diffusion in the zeolite channel. Furthermore, the presence of carbonates may lead to precipitation (with Na⁺, Ca²⁺ or Sr²⁺ ions) in the confined environment of the LTA cages, potentially limiting access to some sorption sites. The presence of calcium also lowers the percentage of active sites available for Sr²⁺ exchange. The calcium sorption capacity at equilibrium (Qeq(Ca) ~ 1.1 meq·g⁻¹) was double that of strontium (Qeq(Sr) ~ 0.5 meq·g⁻¹)—though note that the initial Ca²⁺ concentration (3 meq·g⁻¹) was four times higher than the initial Sr²⁺ concentration (0.7 meq·g⁻¹)—giving a total cation sorption capacity (Sr²⁺ + Ca²⁺) of about 1.6 meq·g⁻¹, compared with 1.9 meq·g⁻¹ in the simplified medium. This lower
equilibrium sorption capacity is possibly due to the presence of several other competing cations ($Mg^{2+}$, $K^{+}$, $Na^{+}$ etc.) in the simulated groundwater solution.

Similar maximum $Sr^{2+}$ sorption capacities were measured for the G_Zeo composite in the simplified wastewater and groundwater solutions (~ 2 meq·g⁻¹, Table 2), indicating that the presence of $Ca^{2+}$ has little effect at high $Sr^{2+}$ concentrations. However the maximum sorption capacity was significantly lower (1.4 meq·g⁻¹) in the saline wastewater solution, presumably because of the high concentration of competing $Na^{+}$ ions. Furthermore, compared with the simplified solution without competing cations, the presence of $Ca^{2+}$ ions (groundwater solution) and a high $Na^{+}$ concentration (saline solution) were each associated with a drastic decrease in the $K_d$ measured for trace $Sr^{2+}$ concentrations (8000, 1900 and 270 000 mL·g⁻¹, respectively, Fig. 7a). The data for the groundwater and saline solutions are well fit by a Langmuir isotherm, consistent with an ion exchange mechanism [64]. In contrast, the data measured in the simplified solution diverge from the Langmuir model at low initial $Sr^{2+}$ concentrations (Fig. 7a), suggesting that the selectivity coefficient varies significantly between the different sorption sites in the zeolite, violating the assumptions of the Langmuir model (single sorption site, with no variation in sorption energy during the ion exchange process). Furthermore, the amounts of sorbed $Sr^{2+}$ and released $Na^{+}$ do not correspond at low ionic strengths (Fig. 7b) suggesting the zeolite may have been partially hydrolyzed, and some $Na^{+}$ are released due to exchange with protons ($H^+$). When LTA zeolite structure is stabilized with large amount of $Sr$ in their cages, the proton sorption is not favored anymore.
<figure><img src="image_8.png" /><figcaption>**Fig. 7.** (a) Batch $Sr^{2+}$ sorption isotherms of G_Zeo zeolite–glass bead composite in various types of simulated wastewater. The isotherm measured for the commercial LTA-zeolite sorbent in the saline wastewater solution (empty red circles) is shown for comparison. The dotted lines are fits using the Langmuir model [64]. (b) Amounts of sorbed $Sr^{2+}$ versus released $Na^+$ in simplified wastewater solution during the isotherm experiments.</figcaption></figure>

**3.4 Strontium sorption properties of zeolite-based sorbent in dynamic process**

The good solution transport and $Sr^{2+}$ ion diffusion afforded by the G_Zeo composite in a fixed-bed system are evidenced by the breakthrough curves measured in simulated saline wastewater (Fig. 8). $Sr^{2+}$ leakage occurs much later than with the commercial LTA-zeolite sorbent (Fig. 8a), and the volume of treated solution is thus much higher. Furthermore, the steeper slope of the breakthrough curves suggests that the ion exchange process is faster in the G_Zeo composite than in the commercial sorbent.
<figure><img src="image_9.png" /><figcaption>**Fig. 8.** Sr²⁺ sorption breakthrough curves measured in simulated saline wastewater for (a) 5 cm beds of the zeolite–glass composite (G_Zeo) at flow rates of 20 or 80 mL·h⁻¹ and of a commercial LTA-zeolite sorbent at 20 mL·h⁻¹ and (b) different bed heights (5, 8 or 11 cm) of the G_Zeo composite at 20 mL·h⁻¹.</figcaption></figure>

The Kd estimated from V₁/₂ of the breakthrough curve obtained at 80 mL·h⁻¹ (340 mL·g⁻¹, Fig. 8a) is lower than the value measured in the equivalent batch experiment (450 mL·g⁻¹, Table 3) indicating that equilibrium is not reached at this higher flow rate. The higher Kd measured at 20 mL·h⁻¹ (390 mL·g⁻¹) is consistent with batch sorption at equilibrium, the small difference being attributable to a small amount of zeolite loss during rinsing and column conditioning. Similar batch and dynamic Kds were likewise obtained for the LTA commercial grains (Table 3).

<table><caption>**Table 3.** Column operational parameters and dynamic sorption characteristics with simulated saline wastewater compared with those of equivalent batch experiments.</caption><thead><tr><th>Column/flow parameters</th><th>Batch</th><th>Dynamic</th></tr></thead></table>
<table><thead><tr><th>Sorbent</th><th>Bed height (cm)</th><th>Sorbent mass (g)</th><th>Flow rate (mL·h<sup>-1</sup>)</th><th>Kd<sup>a</sup> (mL·g<sup>-1</sup>)</th><th>Kd<sup>b</sup> (mL·g<sup>-1</sup>)</th><th>Sorbed Sr<sup>2+</sup> (meq·g<sup>-1</sup>)</th><th>V<sub>1/2</sub> (mL)</th><th>Kd<sup>c</sup> (mL·g<sup>-1</sup>)</th></tr></thead><tbody><tr><td>G_Zeo</td><td>5</td><td>1.661</td><td>80</td><td>1900</td><td>450</td><td>0.65</td><td>570</td><td>340</td></tr><tr><td>G_Zeo</td><td>5</td><td>1.698</td><td>20</td><td>1900</td><td>450</td><td>0.83</td><td>670</td><td>390</td></tr><tr><td>G_Zeo</td><td>8</td><td>2.689</td><td>20</td><td>1900</td><td>450</td><td>0.81</td><td>1060</td><td>390</td></tr><tr><td>G_Zeo</td><td>11</td><td>3.51</td><td>20</td><td>1900</td><td>450</td><td>0.73</td><td>1320</td><td>380</td></tr><tr><td>Commercial</td><td>5</td><td>3.388</td><td>20</td><td>1200</td><td>550</td><td>-</td><td>1680</td><td>500</td></tr></tbody></table>

Fig. 8b shows that increasing the bed height from 5 cm up to 11 cm shifts the breakthrough curves to higher treated volumes but does not alter their overall shape. This suggests that the values measured in this laboratory-scale column can be extrapolated up to higher scale (Table 4). At a decontamination factor of 20 with the G_Zeo composite (i.e. at an Sr<sup>2+</sup> outlet concentration of 5% of the initial Sr<sup>2+</sup> concentration) and a flow rate of 20 mL·h<sup>-1</sup>, the efficiency of the decontamination process increases with the bed height, the dynamic Kd, the treated volume, and the column saturation rate (percentage of occupied adsorption sites) all increasing substantially (Table 4). Under these conditions, 1 g of G-Zeo composite could treat more than 1 L of saline wastewater containing trace concentrations of Sr<sup>2+</sup> with a concentration factor (waste volume reduction) of 500, whereas 1 g of the commercial LTA-zeolite sorbent would treat four time less saline wastewater with a concentration factor of just 220.

<caption>**Table 4.** Column extraction efficiency parameters measured in simulated saline wastewater containing Sr<sup>2+</sup> at 90 mg·L<sup>-1</sup> and extrapolated for a real saline wastewater containing trace concentrations of Sr<sup>2+</sup>.</caption>

<footnote><sup>a</sup> Measured at trace concentrations of Sr<sup>2+</sup></footnote>

<footnote><sup>b</sup> Measured at an initial Sr<sup>2+</sup> concentration of 90 mg·L<sup>-1</sup></footnote>

<footnote><sup>c</sup> Estimated from the point at V<sub>1/2</sub> of the breakthrough curve, details of the calculations in Fig. S2</footnote>
<table><thead><tr><th>Sorbent and bed height</th><th>V<sub>leak</sub><br>(mL)</th><th>Kd<sub>leak</sub><sup>a</sup><br>(mL·g<sup>-1</sup>)</th><th>Column<br>saturation*</th><th>Expected Kd<sub>leak</sub><sup>a</sup><br>for trace Sr<sup>2+</sup> (mL·g<sup>-1</sup>)</th><th>Expected<br>CF</th></tr></thead><tbody><tr><td colspan="6">Decontamination factor = 20 (C/Co = 5%)</td></tr><tr><td>G_Zeo – 11 cm</td><td>970</td><td>280</td><td>62 %</td><td>1180</td><td>500</td></tr><tr><td>G_Zeo – 8 cm</td><td>730</td><td>270</td><td>60 %</td><td>1140</td><td>480</td></tr><tr><td>G_Zeo – 5 cm</td><td>430</td><td>250</td><td>56 %</td><td>1060</td><td>450</td></tr><tr><td>Commercial – 5 cm</td><td>390</td><td>120</td><td>22 %</td><td>260</td><td>220</td></tr><tr><td colspan="6">Decontamination factor = 100 (C/Co = 1%)</td></tr><tr><td>G_Zeo – 11 cm</td><td>770</td><td>220</td><td>49 %</td><td>930</td><td>390</td></tr><tr><td>G_Zeo – 8 cm</td><td>560</td><td>210</td><td>47 %</td><td>890</td><td>370</td></tr><tr><td>G_Zeo – 5 cm</td><td>360</td><td>210</td><td>47 %</td><td>890</td><td>370</td></tr><tr><td>Commercial – 5 cm</td><td>160</td><td>50</td><td>9 %</td><td>110</td><td>90</td></tr></tbody></table>

CF, concentration factor = initial volume of liquid waste/final volume of solid waste

The G_Zeo composite performs even better relative to the commercial sorbent if a higher decontamination factor is chosen. If the treatment process is stopped at a leakage concentration 1% of the initial concentration (a decontamination factor of 100), the column saturation rate is still 50% for the G_Zeo composite but less than 10% for the commercial sorbent (Table 4). These values suggest that 1 g of G_Zeo composite would be able to treat about 0.90 L of saline wastewater with trace Sr²⁺—a concentration factor of 390— while 1 g of the commercial sorbent would only treat 0.11 L of saline wastewater, corresponding to a concentration factor of just 90.

## 4 CONCLUSION AND PROSPECTS

This article describes a method of preparing macro- and micro-structurally optimized LTA-zeolite-based sorbents for Sr²⁺ extraction in fixed-bed systems. The two-step method involves coating simple glass beads with silica in SC-CO₂ and then converting the silica into LTA-zeolite by hydrothermal treatment. The use of SC-CO₂ is crucial, as its diffusion properties allow the silica precursor to penetrate into the macroporous substrate and form homogeneous layers inside the pores. The high surface area of the silica layer obtained thanks to SC-CO₂

<footnote>aDetails of the calculations in the supplementary material (Fig. S2)</footnote>
facilitates its subsequent hydrothermal transformation into LTA-zeolite (governed by the kinetic balance of SiO₂ dissolution and aluminosilicate gel precipitation).

The macroporous composite obtained in this way contains around 40 wt.% LTA-zeolite homogenously coated in a micron-thick layer throughout the substrate. The fast kinetics and sharp breakthrough curves measured here indicate that this composite is well-suited to fixed-bed systems, outperforming a commercial LTA-zeolite sorbent in terms of treated volumes and concentration factors with representative simulated wastewater solutions.

The hydrothermal treatment process (ageing, temperature, duration) could nevertheless still be optimized to ensure more of the silica is converted into LTA zeolite, thereby increasing the concentration of sorption sites in the material. The composite could also perhaps be improved by integrating intercrystal mesopores into the zeolite film or by using the same synthesis method with different selective sorbents or substrates with more homogeneous macroporous structures.

## 5 Acknowledgements

The authors would like to thank Anh-Hoang Le and Virginie Fremy (STDC, CEA Marcoule) for the sorption experiments, Adrien Gerenton and Pascal Antonucci (SEAD, CEA Marcoule) for help with N₂ physisorption and XRD measurements, respectively, Abdeslam El Mansouri (IEM, Montpellier) for the mercury porosimetry experiments, and Sébastien Pairis (Neel Institute, Grenoble) for FESEM and cartography mapping. The authors also thank ORANO, for financial support.

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Graphical abstract

<figure><img src="image_10.png" /></figure>
**Declaration of interests**

☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

**Highlights**

* Supercritical CO₂ enhances precursor infiltration and silica film adherence

* Complex macroporous substrates are homogeneously coated with LTA zeolite

* Macroporous beads coated with LTA zeolite have fast Sr²⁺ sorption kinetics

* The composite sorbent is well suited to fixed-bed wastewater treatment