# Prussian blue functionalized MIL-101(Cr)-SO3H for Cs+ ion capture for the management of contaminated water

Koffi Jean Baptiste Alloko, Laurent Cantrel, Jean Denis, Thierry Loiseau, Xavier Deschanels, Jérémy Causse, Alain Moissette, Christophe Volkringer, A-C Grégoire

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Koffi Jean Baptiste Alloko, Laurent Cantrel, Jean Denis, Thierry Loiseau, Xavier Deschanels, et al. Prussian blue functionalized MIL-101(Cr)-SO3H for Cs+ ion capture for the management of contaminated water. Microporous and Mesoporous Materials, 2026, Microporous and Mesoporous Materials, 400, pp.113902. 10.1016/j.micromeso.2025.113902. hal-05334590

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# Prussian blue functionalized MIL-101(Cr)-SO₃H for Cs⁺ ion capture for the management of contaminated water

K.J.B. Alloko<sup>a, *, b</sup>, L. Cantrel<sup>a, b</sup>, J. Denis<sup>a, b</sup>, T. Loiseau<sup>b</sup>, X. Deschanels<sup>c</sup>, J. Causse<sup>c</sup>, A. Moissette<sup>d, e</sup>, C. Volkringer<sup>b</sup>, A. Grégoire<sup>a, e</sup>

## ARTICLE INFO

Keywords:

Prussian blue analogs

Cs⁺ removal

Metal-organic frameworks

## ABSTRACT

This study investigates the property of Metal-Organic Frameworks (MOFs) grafted with Prussian blue analogs (PBA) for the efficient removal of Cs⁺ ions from aqueous solutions. MIL-101(Cr)-SO₃H-PBA material was synthesized using a two-step process: hydrothermal reaction between monosodium 2-sulfoterephthalic acid (BDC-SO₃Na) and chromium oxide (CrO₃) to produce MIL-101(Cr)-SO₃H, followed by grafting with CuK₂[Fe(CN)₆] (PBA). Both materials were characterized using XRD, IR, N₂ sorption and Raman spectroscopy. The results confirmed the conservation of MOF porosity after functionalization and the successful incorporation of PBA. Batch adsorption experiments were conducted to evaluate Cs⁺ removal performance of the adsorbents. The study examined contact time (up to 1440 min), pH (2, 7 and 11), initial Cs⁺ concentration (from 9 ppb to 1800 ppm) and competing ions. The resistance of the porous solids under drastic conditions involving gamma radiation (from 0.5 MGy up to 10 MGy) was also investigated.

MI-101(Cr)-SO₃H-PBA exhibited a maximum Cs⁺ adsorption capacity of 242 mg·g<sup>-1</sup>g<sub>MOF</sub>, compared to 161 mg·g<sup>-1</sup>g<sub>MOF</sub> for pristine MIL-101(Cr)-SO₃H and showed high selectivity toward Cs⁺ in the presence of competing ions (Na⁺, K⁺, Mg²⁺, Ca²⁺, Sr²⁺). MI-101(Cr)-SO₃H-PBA also demonstrates remarkable stability under radiation and alkaline conditions, maintaining 95 % efficiency, whereas the performance of pristine MIL-101(Cr)-SO₃H material decreased significantly (7.5-fold decrease).

## 1. Introduction

The release of radioactive cesium isotopes (<sup>137</sup>Cs, <sup>135</sup>Cs and <sup>134</sup>Cs) from nuclear accidents, such as those at Fukushima and Chernobyl, poses a significant long-term threat to the environment and public health [1,2]. This concern stems from their high solubility in water, which facilitates environmental mobility, and their long half-lives, particularly for <sup>137</sup>Cs (T<sub>1/2</sub> = 30.2 years) and <sup>135</sup>Cs (T<sub>1/2</sub> = 2.3 × 10<sup>6</sup> years) [2,3]. The development of efficient and selective adsorbents for cesium capture is therefore crucial for mitigating radioactive contamination and managing contaminated water streams.

A wide range of materials has been investigated for the removal of cesium from aqueous solutions. Clay minerals, such as montmorillonite and bentonite, are commonly used due to their negative surface charge

and ion-exchange capabilities, though their performance is highly dependent on their structure and swelling properties [4–9].

Zeolites are another major class of adsorbents, valued for their high ion-exchange capacity, rapid kinetics, and the ability to be sintered into stable waste forms after use [10–23].

Inorganic adsorbents like metal hexacyanoferrates (MHCFs) and Prussian blue analogs (PBAs) are particularly notable for their exceptional selectivity and high affinity for Cs⁺ ions, even in the presence of competing cations [24–34].

More recently, metal-organic frameworks (MOFs) have emerged as promising candidates for Cs⁺ epuration due to their high surface areas, tunable porosity, and the ability to functionalize their structures for enhanced performance [35–42].

However, for application in nuclear waste remediation, the radiation

<footnote><sup>a</sup> Autorité de Sûreté Nucléaire et de Radioprotection (ASNR), PSN-RES/SEREX/L2EC, Saint-Paul-Lez-Durance, F - 13115, France</footnote>

<footnote><sup>b</sup> UCCS UMR 8181, Univ. Lille, CNRS, Centrale Lille Institut, Univ. Artois, Cité Scientifique - CS20048, Villeneuve d'Ascq Cedex, 59651, France</footnote>

<footnote><sup>c</sup> ICSM, Univ Montpellier, CEA, CNRS, ENSCM, Bagnols-sur-Cèze cedex, 30207, France</footnote>

<footnote><sup>d</sup> Laboratoire de Spectroscopie pour les Interactions, la Réactivité et l'Environnement (LASIRE), Université de Lille – Sciences et Technologies, Villeneuve d'Ascq, 59655, France</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: koffi-jean-baptiste.alloko@asnr.fr (K.J.B. Alloko).</footnote>
stability of adsorbents is a critical factor. While MOFs generally exhibit good chemical stability, their resistance to ionizing radiation is variable and must be carefully evaluated [43]. Studies have shown that the radiation tolerance of MOFs depends on factors such as metal-ligand bond strength and the metal ion's radiation absorption cross-section, with aluminum-based MOFs like MIL-53(Al) and MIL-100(Al) demonstrating exceptional stability (Volkringer et al., 2016). In contrast, frameworks containing metals with higher cross-sections, such as zirconium or copper, can undergo structural degradation, often initiated by the radiolysis of organic linkers ([44,45]; Volkringer et al., 2016). The functionalization of MOF linkers can also influence radiation resistance, with some multi-functional MOFs showing enhanced stability [44].

Given the high selectivity of PBAs for $Cs^+$ and the structural versatility of MOFs, this work aims to combine these advantages by functionalizing the sulfonated MIL-101(Cr)-SO$_3$H with Prussian blue analogs. The objective is to improve the sorption capacity of this material for the capture of $Cs^+$ from aqueous solution. The synthesis route for MIL-101(Cr)-SO$_3$H in this study is based on a one-step hydrothermal method, ensuring the optimal incorporation of sulfonic groups during the synthesis process. The functionalization of the MOF with PBA is achieved through direct impregnation, which is a method analogous to that employed for CoFe PBA-silica nanocomposites [46]. In order to mimic the conditions occurring during a nuclear accident, the MOFs will be also submitted to $\gamma$-radiations and their affinity for $Cs^+$ we are also analyzed in artificial sea water, with given amounts of alkaline and alkaline earth cations.

The synthesized MIL-101(Cr)-SO$_3$H-PBA composites underwent comprehensive characterization to assess their structural integrity and investigate PBA integration for effective $Cs^+$ adsorption. SEM and TEM analyses revealed the surface morphology and nanoscale structure, corroborating PBA distribution. BET analysis provided insights into specific surface area, pore size, and porosity, crucial for adsorption potential. XRD analysis investigate the crystalline structure and PBA incorporation, while TGA evaluated thermal stability and compositional robustness under temperature variation. Collectively, these characterizations verify the material's structural stability and adsorption capabilities, supporting its suitability for efficient cesium capture.

The property of the MIL-101(Cr)-SO$_3$H-PBA composites for cesium removal from aqueous environments was systematically evaluated under various conditions, including pH levels, initial $Cs^+$ ion concentration, the presence of competing ions (such as $Na^+$, $K^+$, $Ca^{2+}$, and $Sr^{2+}$) and contact time between the adsorbent material and the solution through batch adsorption experiments. Additionally, similar tests were conducted with MIL-101(Cr)-SO$_3$H alone for comparison with MIL-101(Cr)-SO$_3$H-PBA. Finally, specific evaluation of $Cs^+$ adsorption capacity after $\gamma$-radiation allowed verification of the materials' effectiveness following radiation exposure. These variables provide a comprehensive understanding of the adsorption efficiency of these materials in environments that simulate real-world conditions, such as natural waters and contaminated effluents. Adsorption kinetics studies were conducted to determine the rate of cesium uptake and to elucidate the adsorption mechanism. The impact of competing ions on cesium adsorption was investigated to assess the selectivity of the materials in complex aqueous matrices.

## 2. Section 1: Experimental section

All chemicals used are listed in the "Materials" section of the Supporting Information (SI). The synthesis procedures for the pristine materials MIL-101(Cr) and MIL-101(Cr)-SO$_3$H are detailed in the SI section. Section 1 provides a comprehensive analysis of the synthesis steps, highlighting key parameters, the reagents used, reaction conditions, and the characterization techniques employed.

### 1. Synthesis MIL-101(Cr)-SO$_3$H-PBA

Two impregnation methods were implemented to investigate the effect of each precursor on the incorporation of PBA into MIL-101(Cr)-SO$_3$H. Each method consists of two sequential steps.

**Method 1.** 100 mg of dried MIL-101(Cr)-SO$_3$H was suspended in a 0.5 M aqueous solution (20 mL) of $Cu(NO_3)_2 \cdot 2.5H_2O$ and stirred for 3 h. The resulting blue mixture was centrifuged, and the precipitate was washed three times with water (60 mL, 25 °C) to remove any excess copper species adsorbed on the surface of the MOF. The resulting product was then dried at 70 °C for 3 h. This solid was re-suspended in a yellow 0.5 M aqueous solution (20 mL) of potassium ferrocyanide trihydrate $K_4Fe(CN)_6 \cdot 3H_2O$ and stirred for another 3 h. After centrifugation at 4000 rpm for 20 min, the green precipitate was successively washed with water (3 x 60 mL, 25 °C) and acetone (1 x 20 mL, 25 °C), then treated with ethanol (40 mL, 50 °C, 1 h) to remove residual ferrocyanides prior to drying at 70 °C for 3 h. The resulting solid is labeled as MIL-101(Cr)-SO$_3$H-PBA.

**Method 2.** This method followed the reverse sequence of Method 1. Initially, 100 mg of dried MIL-101(Cr)-SO$_3$H was suspended in a 0.5 M aqueous solution (20 mL) of $K_4Fe(CN)_6 \cdot 3H_2O$ and stirred for 3 h. The resulting yellow mixture was centrifuged, and the green precipitate was washed three times with water (60 mL, 25 °C) to remove any residual $K_4Fe(CN)_6 \cdot 3H_2O$ adsorbed on the MOF surface. The resulting product was then dried at 70 °C for 3 h. Subsequently, the resulting product was re-dispersed in a 0.5 M aqueous solution (20 mL) of $Cu(NO_3)_2 \cdot 2.5H_2O$ and stirred for additional 3 h. After centrifugation at 4000 rpm for 20 min, the green precipitate was washed successively with water (3 x 60 mL, 25 °C) and acetone (1 x 20 mL, 25 °C), followed by drying at 70 °C for 3 h.

### 3. Irradiation

The irradiation experiments were performed using the EPICUR facility (hosted by ASNR), which is equipped with six sealed cylindrical cobalt-60 ($^{60}$Co) sources of $\gamma$-radiation. At the time of the experiments, the dose rate was approximately 30 kGy/h at a reference distance of 10 cm from the plot containing the sources. A closed UV glass vessel containing dried MOF powder was placed on a planar sample holder positioned at a distance of 13.6 cm from the $^{60}$Co sources.

The samples were irradiated at five different total absorbed doses: 0.5 M Gy, 1 M Gy, 2 M Gy, 5 M Gy, and 10 M Gy. These doses corresponded to irradiation durations of 16, 34, 68, 170, and 340 h, respectively, based on the calibrated dose rate at the sample position.

Following irradiation, the irradiated MOFs were immediately subjected to subsequent characterization to minimize post-irradiation effects.

### 4. Batch adsorption experiments

Batch adsorption experiments were conducted to evaluate the adsorption capacity of the materials for stable cesium ($^{133}Cs^+$) using $CsNO_3$ (Alfa Aesar) as the precursor. A stock solution of 1000 ppm $Cs^+$ was initially prepared, with subsequent dilutions performed as required for specific experimental conditions. In each test, 20 mg of as-synthesized (non-thermally activated) MOF powder was introduced into 40 mL of $Cs^+$ aqueous solutions under varying experimental conditions and stirred using a Fisherbrand rotary shaker. The solution was then subjected to centrifugation at 4000 rpm for 20 min. Subsequently, the supernatant was filtered using 0.2 µm pore-size polyethersulfone syringe filters (Whatman™, 25 mm diameter) under manual pressure and subsequently collected in sterile tubes for further analysis. The entire study was conducted at room temperature (21 ± 3 °C). The cesium concentrations in aqueous solution before and after adsorption were quantified using inductively coupled plasma mass spectrometry (ICP-MS). The specific experimental parameters are summarized in Table 1. The adsorption capacity $q_e$, the removal rate $R^{Cs}(\%)$ of samples and distribution coefficient ($K_d$) of adsorbed $Cs^+$ were determined using
<caption>**Table 1**</caption>

Summary of experimental parameters investigated for $Cs^+$ adsorption studies: effects of contact time, pH, initial concentration, competing ions, and gamma irradiation.

<table><thead><tr><th>Effect Studied</th><th>Variable Parameter</th><th>Fixed Parameter</th><th>Specific Conditions</th></tr></thead><tbody><tr><td>Contact time</td><td>t = 5, 10, 20, 60, 120, 360 and 1440</td><td>pH = 5.2, [Cs<sup>+</sup>] = 0.2 ppm, adsorbent mass = 20 mg</td><td>-</td></tr><tr><td>pH</td><td>pH = 2, 7, 11</td><td>t = 1440 min (24 h), [Cs<sup>+</sup>] = 0.2 ppm, adsorbent mass = 20 mg</td><td>-</td></tr><tr><td>Initial Cs<sup>+</sup><br>concentration</td><td>[Cs<sup>+</sup>] = 0.009, 0.195, 3, 10, 30 et 100, 271, 536, 762 and 1828</td><td>pH = 5.2, t = 1440 min (24 h), adsorbent mass = 20 mg</td><td>-</td></tr><tr><td>Competing ions</td><td>[Cs<sup>+</sup>] = 0.2 and 3 ppm in simulated seawater (35 g/L)</td><td>pH = 7.8, t = 1440 min (24 h), adsorbent mass = 20 mg</td><td>Simulated seawater composition: Na<sup>+</sup>, Ca<sup>2+</sup>, K<sup>+</sup>, Cs<sup>+</sup>, Sr<sup>2+</sup>.</td></tr><tr><td>γ-irradiation</td><td>Dose = 0.5, 1, 2, 5 and 10 MGy</td><td>pH = 5.2, t = 1440 min (24 h), [Cs<sup>+</sup>] = 0.2 and 3 ppm, adsorbent mass = 20 mg</td><td>-</td></tr></tbody></table>

equations (1)–(3), respectively, as outlined below [47–49]:

$$q_e = \frac{(C_0 - C_e)V}{m} \qquad (1)$$

$$R^{Cs} = \frac{100(C_0 - C_e)}{C_0} \qquad (2)$$

$$K_d = \frac{C_0 - C_e}{C_0} \times \frac{V}{m} \qquad (3)$$

where $R^{Cs}$ represents the % of $Cs^+$ removal, $C_0$ corresponds to the initial concentration of cesium (ppm), $C_e$ is the equilibrium concentration (ppm), V is the volume of the solution (L), m the mass of the adsorbent used (g).

This investigation systematically examined [Cs$^+$]concentrations spanning from 0.009 to 1828 ppm, encompassing the full range of values documented in the scientific literature ([50]; Suresh et al., 2024b), thereby ensuring the robustness and applicability of our findings across diverse contamination scenarios.

# 5. Section 2: Results and discussion

## 5.1. Characterization of the functionalized MOF materials

A systematic comparison of the two synthesis protocols reveals fundamentally distinct reaction mechanisms, highlighting the critical influence of metal precursor addition sequence on the controlled

formation of Prussian blue analogue (PBA) within the MIL-101(Cr)-SO$_3$H porous matrix.

Method 1 (Fig. 1a–d) relies on monitoring a characteristic chromatic transition during synthesis. A marked color change (from green to brown) occurs upon addition of the $K_4Fe(CN)_6 \cdot 3H_2O$ solution to the green precipitate obtained from initial treatment with the blue Cu $(NO_3)_2 \cdot 2.5H_2O$ solution (Fig. 1c). This characteristic chromatic transition, resulting in a stable brown coloration of the final powder (Fig. 1d), serves as a reliable visual indicator for the successful formation of Prussian blue analogue (PBA) within the MIL-101(Cr)-SO$_3$H matrix. The reaction mechanism proceeds through two key steps: (1) initial coordination of $Cu^{2+}$ ions to sulfonate sites in MIL-101(Cr)-SO$_3$H accompanied by simultaneous proton ($H^+$) release [51], followed by (2) generation of an intermediate complex capable of cyanide ligand binding. This process confirms the remarkable ability of sulfonate ligands to construct extended 2D/3D supramolecular architectures through direct Cu(II) coordination [52,53].

Using the second method (Fig. 1e–h), no color change was observed when the precipitate, initially treated in a yellow solution of $K_4Fe(CN)_6 \cdot 3H_2O$, was brought into contact with the precursor solution of Cu $(NO_3)_2 \cdot 2.5H_2O$ (Fig. 1g). The final observed color corresponds solely to the presence of copper from the MOF, with no detectable formation of the Prussian Blue Analogue. This observation suggests that this synthetic approach does not lead to the production of PBA within the framework. Several mechanistic insights, supported by recent studies, explain the failure of Prussian Blue analogue (PBA) formation on MIL-101(Cr)-SO$_3$H. The sulfonic acid groups (-SO$_3$H) in MIL-101(Cr)-SO$_3$H act as

<figure><img src="image_3.png" /><figcaption>**Fig. 1.** Synthesis of MIL-101(Cr)-SO$_3$H-PBA illustrated by steps above: (a, e) initial MIL-101(Cr)-SO$_3$H; (b) addition of blue Cu$(NO_3)_2 \cdot 2.5H_2O$ solution to MIL-101(Cr)-SO$_3$H; (c) addition of yellow $K_4Fe(CN)_6 \cdot 3H_2O$ solution to the resulting product; (d) final powder of MIL-101(Cr)-SO$_3$H-PBA (method 1); (f) addition of $K_4Fe(CN)_6 \cdot 3H_2O$ to green MIL-101(Cr)-SO$_3$H powder; (g) addition of Cu$(NO_3)_2 \cdot 2.5H_2O$ to the green precipitate from the same step; and (h) final unmodified MIL-101(Cr)-SO$_3$H powder (method 2). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)</figcaption></figure>
strong Brønsted acid sites that electrostatically bind and immobilize [Fe(CN)₆]⁴⁻ anions, leading to pre-complexation that saturates coordination centers and inhibits subsequent interaction with Cu²⁺ ions [54,55]. Since PBA formation relies on the availability of free [Fe(CN)₆]⁴⁻ to bridge with Cu²⁺ via cyanide ligands, this strong host–guest interaction effectively disrupts Fe–CN–Cu coordination, as clearly evidenced by the absence of the characteristic brown coloration arising from Fe³⁺–CN–Cu²⁺ charge transfer transitions [56,57]. Furthermore, sulfonate functionalities likely compete with Cu²⁺ for [Fe(CN)₆]⁴⁻ binding, further suppressing the framework assembly [54,58]. Recent studies on non-cubic PBA phases (e.g., hexagonal CuCo analogs) corroborate this ligand-accessible templating mechanism. [59]. However, in the context of our study, the most plausible explanation is that the persistently negative charge of the MIL-101(Cr)-SO₃H particles across the entire pH range due to the presence of sulfonate groups (SO₃) [60] and confirmed by the zeta potential study (Fig. S1b), induces an electrostatic repulsion upon the addition of the K₄Fe(CN)₆·3H₂O solution. Consequently, the [Fe(CN)₆]⁴⁻ anions, being unable to anchor to the matrix surface, are removed during the subsequent washing step prior to the addition of the Cu(NO₃)₂·2.5H₂O solution, which corroborates the lack of Prussian blue analogue formation via method 2. UV–visible analyses, consistent with the zeta potential data and detailed in Supporting Information Section S2, confirmed the formation of the Prussian blue analogue on MIL-101(Cr)-SO₃H via method 1 and its absence in method 2.

X-ray diffraction (XRD) analysis provides precise characterization of the structural modifications occurring in MIL-101(Cr)-SO₃H during successive functionalization steps. The reference powder XRD pattern (a), simulated from CIF file data, displays the characteristic Bragg peaks of MIL-101(Cr) at 2θ = 5.22°, 8.52°, 9.13°, 10.4° and 16.6°, corresponding to the crystallographic planes of its three-dimensional framework. These theoretical data establish the baseline for evaluating the quality of synthesized samples (Fig. 2).

The experimental powder XRD pattern (b) of synthesized MIL-101(Cr) faithfully reproduces both the positions and relative intensities of simulated peaks, confirming the excellent crystallinity as well as phase

purity of the obtained material.

The XRD pattern of MIL-101(Cr)-SO₃H (pattern c) shows preserved diffraction peak positions at characteristic 2θ values of MIL-101(Cr), confirming the maintained structural integrity (phase purity and high crystallinity) [61–63].

Powder XRD pattern (d) displays the distinctive diffraction signatures of the Prussian blue analogue, with sharp peaks at 2θ = 18°, 24.7°, 25.5°, 34.1° and 36.4° corresponding to its crystalline planes. These reference data are essential for identifying potential incorporation of this species within the MOF’s porous structure.

Analysis of the MIL-101(Cr)-SO₃H-PBA composite prepared by **method 1** (pattern e) reveals two significant phenomena: a marked decrease in host MOF peak intensities accompanied by the simultaneous appearance of Prussian blue analogue peaks. These structural modifications provide conclusive evidence for successful PBA nanoparticle incorporation within the MIL-101(Cr)-SO₃H porous matrix, forming a hybrid material [64] with slightly altered yet preserved crystallinity.

In striking contrast, the pattern (f) of the sample prepared by **method 2** exhibits exclusively MIL-101(Cr) peaks without any detectable signals from the Prussian blue analogue. This definitive result establishes **method 2's** inability to incorporate the target species into the MOF structure. These XRD observations thus confirm the successful **method 1** for preparing MIL-101(Cr)-SO₃H-PBA composite, which will consequently be employed for subsequent studies.

XRD analysis has systematically elucidated the evolution of crystallinity and phase composition during the functionalization steps of MIL-101(Cr)-SO₃H-PBA, establishing a robust structural framework. In order to complement these structural insights and gain molecular-level understanding of the chemical modifications, FTIR spectroscopy has been employed to characterize the chemical environment and unambiguously confirm the successful incorporation of PBA species within the MOF matrix.

The FTIR spectrum of MIL-101(Cr)-SO₃H is presented in Fig. 3a. The band at 1404 cm⁻¹ was attributed to the symmetric stretching of carboxylic groups in the BDC ligand [65]. The absorption bands observed at

<figure><img src="image_4.png" /><figcaption>**Fig. 2.** Powder XRD patterns of (a) simulated MIL-101(Cr) (b) pristine MIL-101(Cr) (c) pristine MIL-101(Cr)-SO₃H (d) Prussian blue analogue (e–f) MIL-101(Cr)-SO₃H-PBA (method 1 and method 2, respectively) Wavelength copper. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>Fig. 3. FT-IR spectra of: (a) MIL-101(Cr)-SO₃H, (b) PBA and (c–d) MIL-101(Cr)-SO₃H-PBA (respectively method 1 and method 2).</figcaption></figure>

1228 cm⁻¹ and 1176 cm⁻¹ correspond respectively to the asymmetric (ν₃₃₃) and symmetric (ν₃₃₄) stretching modes of the O=S=O moiety, consistent with literature reports [66–68]. Additionally, the peak appearing at 1079 cm⁻¹ can be assigned to in-plane skeletal vibrations involving the SO₃ group attached to the aromatic rings of the terephthalate linker [69]. Furthermore, the distinct band at 1023 cm⁻¹ arises from the S-O stretching vibration, completing the vibrational signature of the sulfonated aromatic system [51,62,63]. The vibrational bands at 1630 cm⁻¹ and 599 cm⁻¹ correspond to the asymmetric stretching of carboxylates (COO⁻) coordinated to chromium (Cr³⁺) and Cr-O stretching, respectively, which are characteristic features of the MIL-101(Cr)-SO₃H structure. Additionally, the peak at 623 cm⁻¹ was assigned to the carbon-sulfur (C–S) stretching vibration [62,63,70,71]. The broad absorption band at 3380 cm⁻¹, attributed to the O–H stretching mode, indicates the presence of water molecules within the structure of MIL-101(Cr)-SO₃H [72], which aligns well with the thermogravimetric analysis (TGA) results further discussed in this section.

The FTIR spectrum of the Prussian blue analogue (CuK₂[Fe(CN)₆]I) is shown in Fig. 3b. The spectrum displays a single characteristic C ≡ N vibration band at 2073 cm⁻¹, which is consistent with the typical CN vibrations observed for PBA structures.

The FTIR spectrum of MIL-101(Cr)-SO₃H-PBA (method 1), presented in Fig. 3c, shows vibrational bands corresponding to the SO₃ groups at 1080 cm⁻¹ and 1180 cm⁻¹, similar to those observed in MIL-101(Cr)-SO₃H [73]. This result clearly confirms that the sulfonic group remains unaltered during the modification process [74]. Additionally, the spectrum exhibits a new vibrational band at 2082 cm⁻¹, attributed to the C ≡ N stretching mode, which is shifted by +9 cm⁻¹ compared to the C ≡ N vibration observed in pure PBA (Fig. 3b). This blue shift of the C ≡ N band provides direct evidence of Cu²⁺ coordination to CN groups, resulting in the formation of characteristic Cu-N ≡ C-Fe linkages. This observation has been previously reported in the literature for similar systems involving metal-cyanide coordination, thereby confirming the nature of the metal-ligand interactions in this hybrid material [75]. The sample prepared via method 2 (Fig. 3d) exhibits vibrational signatures similar to method 1 yet demonstrates a greater than 16-fold intensity

reduction for the characteristic PBA C ≡ N stretching band, accompanied by a +25 cm⁻¹ blue shift relative to the reference PBA CN band. This spectral shift unequivocally indicates that the cyanide groups from both PBA grafting methods onto MIL-101(Cr)-SO₃H occupy distinct chemical environments. Specifically, the 2098 cm⁻¹ band (Fig. 3d) corresponds to cyanides coordinated to Fe³⁺ centers showing slight perturbation of the C ≡ N triple bond [76], while the 2082 cm⁻¹ band (Fig. 3c) arises from Cu²⁺-C ≡ N-Fe³⁺ bridging cyanides where copper-carbon interaction modifies the inductive effect, consequently lowering the vibrational frequency. This FTIR evidence conclusively establishes method 1 as the successful route for fabricating a well-defined hybrid composite. Furthermore, the preservation of characteristic framework vibrations (Cr³⁺-coordinated COO⁻ at 1630 cm⁻¹ and Cr-O at 599 cm⁻¹) along with maintained sulfonic group signals (1080-1180 cm⁻¹) confirms that the hybrid formation process compromises neither the host matrix’s chemical functionality nor its crystallinity, as further corroborated by XRD data showing retained peak positions with modified intensities. To further validate these findings and gain additional structural insights, Raman spectroscopy analysis was performed to probe vibrational modes that are either inactive or weakly detectable by FTIR.

The analysis of the Raman spectra (Fig. 4) reveals that MIL-101(Cr)-SO₃H exhibits six main vibrational peaks at 762, 904, 1030, 1170, 1437, and 1605 cm⁻¹, corresponding respectively to C-S, Cr-O, C=C, S-O, and symmetric/antisymmetric C-O stretching modes, in perfect agreement with literature data [60,77]. Simultaneously, the spectra of PBA and the MIL-101(Cr)-SO₃H-PBA (method 1) composite show characteristic vibrational signatures of C ≡ N groups in two distinct regions: (i) 300-540 cm⁻¹ for bending modes and (ii) 2070-2170 cm⁻¹ for stretching modes, consistent with typical cyanometallate behavior. As described by Alexandrov et al. [78] in their work on Prussian blue analogs, these C ≡ N vibrations are sensitive markers of metal-ligand coordination environments. The systematic blue shift observed for the composite, correlated with FTIR data, provides incontrovertible evidence of specific chemical interactions between the nitrogen atoms of PBA’s C ≡ N groups and the Cr³⁺ metal centers of MIL-101(Cr)-SO₃H, thereby confirming
<figure><img src="image_6.png" /><figcaption>Fig. 4. RAMAN spectra of: (a) MIL-101(Cr), (b) MIL-101(Cr)-SO₃H, (c) PBA and (d) MIL-101(Cr)-SO₃H-PBA (method 1), left-handed overall and right-handed zoom of the CN bending in the 400 cm⁻¹ and of the CN stretching in the 2100 cm⁻¹ spectral range.</figcaption></figure>

the successful chemical incorporation of PBA within the MOF matrix. This observation shows perfect consistency with phenomena reported for analogous systems [79], reinforcing the validity of our interpretation.

The vibrational spectroscopy (IRTF/Raman) revealed molecular-level interactions between PBA and MIL-101(Cr)-SO₃H through characteristic C ≡ N shifts and preserved functional groups. The combined analysis using scanning electron microscopy (SEM) and transmission electron microscopy (TEM) coupled with EDX spectroscopy provides complementary insights into the integration of Prussian blue analogue (PBA) within the MIL-101(Cr)-SO₃H framework.

MIL-101(Cr) crystals exhibit well-defined octahedral morphology (Fig. 5d and Fig. S4a), consistent with literature data [80–82]. While this morphology is maintained in MIL-101(Cr)-SO₃H material (Fig. 5e), despite the presence of sulfonic acid groups. This result is consistent with observations reported in the literature. [83,84], the MIL-101 (Cr)-SO₃H-PBA composite (method 1) displays significant crystal edge alteration (Ren et al., 2023) and surface defects [78] observable by both SEM and TEM (Fig. 5b and g and Fig. S5).

Elemental mapping through SEM-EDS (Fig. 5c) and TEM-EDX (Fig. 5g and Fig. S5) reveal homogeneous PBA (Fig. 5f and Fig. S4b–c) dispersion within the MOF matrix, evidenced by spatial overlap of Cr (MOF framework), Cu, and Fe (PBA) signals in identical crystalline regions. The derived molar ratios (Cr/Cu ≈ 6 and Cr/Fe ≈ 1.3) indicate a MOF/PBA stoichiometry of ~1.25, matching the proposed structural formula Cu<sub>0.8</sub>K<sub>1.6</sub>Fe<sub>0.8</sub>(CN)<sub>4.8</sub>Cr<sub>3</sub>O(H<sub>2</sub>O)<sub>3</sub>(BDC-SO<sub>3</sub>H)<sub>2</sub>(BDC-SO<sub>3</sub>) Fe<sub>3</sub>K<sub>0.7</sub>Cr<sub>1.8</sub>S. These findings, supported by morphological alterations (increased roughness, irregular edges) and structural perturbations observed in XRD (reduced peak intensities) [85], confirm deep PBA incorporation in MIL-101(Cr)-SO₃H matrix rather than superficial adsorption [83,86].

The Raman and IR blue shifts of C ≡ N groups along with partial retention of octahedral morphology despite high PBA loading suggest primarily intraporous distribution with preferential coordination to Cr³⁺ sites via cyanide groups. This intimate PBA-MOF association, demonstrated by nanoscale elemental uniformity and crystalline co-localization, likely results from Cr³⁺/CN⁻ coordination interactions rather than simple physical mixing [83,84,86].

The set of analyses performed unequivocally demonstrates that method 1 enables the controlled formation of a homogeneous hybrid material, in which PBA is chemically incorporated into the MIL-101(Cr)-

SO₃H framework through structured coordination interactions (IRTF, Raman characterization), while preserving the chemical and crystalline integrity of the host MOF (XRD and TEM characterization). In contrast, method 2, which induces neither the characteristic colorimetric evolution nor the appearance of the structural and spectroscopic signatures of PBA, fails to achieve true hybridization. These observations highlight the critical role of the precursor introduction sequence in precisely controlling the formation of the MIL-101(Cr)-SO₃H-PBA composite.

To further evaluate the impact of PBA incorporation on the host matrix, textural properties were investigated through nitrogen physisorption analysis. The N₂ adsorption-desorption isotherms at 77 K (Fig. S3a) exhibit a characteristic Type I profile, typical of microporous materials, featuring rapid adsorption at low relative pressure (P/P₀) followed by a saturation plateau [36,87]. Textural analysis reveals a significant decrease in porosity properties upon functionalization, as shown in Table 2: the BET surface area drops from 2678 m² g⁻¹ for pristine MIL-101(Cr) to 616 m² g⁻¹ for MIL-101(Cr)-SO₃H-PBA prepared via method 1, while the total pore volume concurrently decreases from 1.37 to 0.28 cm³ g⁻¹. This textural evolution is strongly correlated with DFT analysis results (Fig. S2a) demonstrating profound changes in pore size distribution: the parent material exhibits four pore families at 12, 14, 22 and 27 Å, whereas the functionalized derivative retains only three pore populations at 12, 20, and 25 Å, with notable disappearance of the 14 and a reduction in the size of the largest pore. The drastic reduction in pore accessibility results from partial occupation of the MOF framework cages by two species (sulfonate groups (-SO₃H) and Prussian blue analog (PBA) nanoparticles) inserted within the MIL-101(Cr)-SO₃H pores [88–90]. This result is consistent with the chemical modification of the host framework structure [90].

BET and DFT analyses effectively characterize porosity modifications; however, they cannot directly identify the chemical nature of the inserted species or assess their thermal stability. This is precisely where thermogravimetric analysis (TGA) provides crucial complementary information by enabling quantitative assessment of pore-hosted species through their thermal decomposition profiles and confirming effective PBA anchoring through its distinctive thermal signature.

The thermogravimetric curve (Fig. 6 blue) of the functionalized MIL-101(Cr)-SO₃H-PBA material exhibits significant differences compared to its unmodified counterpart. The initial decomposition stage between 0 and 150 °C shows an increased mass loss of 22 % versus 19 % in the unmodified MIL-101(Cr)-SO₃H, indicating enhanced adsorption of polar
<figure><img src="image_7.png" /><figcaption>**Fig. 5.** SEM images of (a) MIL-101(Cr), (b) MIL-101(Cr)-SO₃H, (c) MIL-101(Cr)-SO₃H-PBA (method 1), (d) corresponding EDS mapping of Cr, Fe and Cu and TEM images of (e) MIL-101(Cr), (f) MIL-101(Cr)-SO₃H, (g) PBA and (h) MIL-101(Cr)-SO₃-PBA (method 1).</figcaption></figure>

molecules such as water, ethanol, and methanol, attributed to the increased hydrophilicity induced by the presence of PBA nanoparticles [91,92]. The most pronounced distinction appears in the 150–400 °C range, where the mass loss dramatically increases from 11 % to 31 %,

reflecting not only the decomposition of sulfonic acid groups but also the thermal degradation of the embedded PBA ([92]; Ren et al., 2023b). A third degradation stage, observed between 400 and 900 °C and absent in the reference material, is associated with the decomposition of residual
<table><caption>**Table 2**
Surface area, pore volume (P/P₀ = 0.95) and average pore width of the samples.</caption><thead><tr><th>Sample</th><th>S<sub>BET</sub> (m<sup>2</sup> g<sup>-1</sup>)</th><th>V<sub>total</sub> (cm<sup>3</sup> g<sup>-1</sup>)</th><th>Pores/cages width (Å)</th></tr></thead><tbody><tr><td>MIL-101(Cr)</td><td>2678</td><td>1.37</td><td>12; 14; 22; 27</td></tr><tr><td>MIL-101(Cr)-SO<sub>3</sub>H</td><td>1518</td><td>0.49</td><td>12; 20; 25</td></tr><tr><td>MIL-101(Cr)-SO<sub>3</sub>H-PBA</td><td>616</td><td>0.28</td><td>12; 20; 25</td></tr></tbody></table>

metal oxides originating from the PBA structure [86]. These thermogravimetric changes, when analyzed alongside the substantial reductions in BET surface area and pore volume (from 2678 m² g⁻¹ to 616 m² g⁻¹ and from 1.37 to 0.28 cm³ g⁻¹, respectively), provide unequivocal evidence of successful PBA incorporation within the MOF’s porous matrix [93].

<figure><img src="image_8.png" /><figcaption>**Fig. 6.** Thermogrametric curves of: (blue) MIL-101(Cr)-SO₃H and (red) MIL-101(Cr)-SO₃H-PBA (under air; heating rate: 5 °C.min⁻¹). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>**Fig. 7.** TEM image of (a) MIL-101(Cr)-SO₃H-Cs and (b) MIL-101(Cr)-SO₃-PBA-Cs (method 1).</figcaption></figure>
## 5.2. Adsorption performance of MIL-101(Cr)-SO₃H-PBA

This comprehensive study of batch adsorption parameters for the MIL-101(Cr)-SO₃H-PBA material (method 1) was conducted to evaluate its potential for selective cesium ion ($Cs⁺$) capture. The systematic approach enables elucidation of the interaction mechanisms between the material's active sites - including both sulfonic groups and PBA nanoparticles - and $Cs⁺$ ions, while establishing precise correlations between the material's structural properties and its adsorption performance. Fig. 7 illustrates a homogeneous distribution of cesium ions ($Cs⁺$) across both materials studied after the adsorption process. This analysis reveals that cesium ($Cs$) is distributed both on the surface and within the MOF structure for both MIL-101(Cr)-SO₃H and MIL-101(Cr)-SO₃H-PBA materials. Notably, in the case of MIL-101(Cr)-SO₃H-PBA, cesium is not only incorporated into the MOF matrix but also specifically bound to the Prussian blue analogue (PBA) particles, demonstrating the dual adsorption sites of this hybrid material.

### 5.2.1. Adsorption kinetics of cesium

The $Cs⁺$ adsorption kinetics for MIL-101(Cr)-SO₃H and MIL-101(Cr)-SO₃H-PBA (Fig. S7a) involves a two-stage process. During the first stage, the rapid adsorption, reaching a high uptake within the first 20 min, is attributed to the abundance of accessible binding sites. According to Thallapally et al., a cation exchange may occur between the proton of the -SO₃H groups and $Cs⁺$ ions in aqueous solution during the adsorption process of $Cs⁺$ on MIL-101(Cr)-SO₃H [35]. This exchange mechanism involves the substitution of $Cs⁺$ ions for protons in the sulfonic acid sites, facilitating effective adsorption without disrupting the structural integrity of the MOF framework [94]. In MIL-101(Cr)-SO₃H-PBA, in addition to the SO₃H sites, the embedded Prussian blue analogue (PBA) introduces additional binding sites. The presence of $K⁺$ ions within the PBA nanoparticles is conducive to ion exchange between $Cs⁺$ and $K⁺$, thereby enhancing the efficiency of cesium ion adsorption [95]. The dual presence of sulfonate and PBA sites in MIL-101(Cr)-SO₃H-PBA enhances its adsorption efficiency in the initial stage. In the second stage, as the binding sites on both materials become saturated with $Cs⁺$ ions, the adsorption rate slows until equilibrium is achieved. This behavior is consistent with the filling of all available high-affinity sites, after which only limited adsorption occurs due to the lower availability of unoccupied sites and reduced driving force. The adsorption mechanism was further investigated using kinetic models

<caption>**Table 3**</caption>

Kinetic parameters of $Cs⁺$ adsorption on pristine MIL-101(Cr)-SO₃H and MIL-101(Cr)-SO₃H-PBA.

<table><thead><tr><th rowspan="2">Kinetic model</th><th rowspan="2">Equation</th><th colspan="2">MIL-101(Cr)-SO₃H</th><th colspan="2">MIL-101(Cr)-SO₃H-PBA</th></tr><tr><th>Parameters</th><th>R²</th><th>Parameters</th><th>R²</th></tr></thead><tbody><tr><td rowspan="5">Pseudo-first-order</td><td rowspan="5">ln(q<sub>e</sub>·q<sub>f</sub>) = lnq<sub>e</sub> - k<sub>f</sub>t</td><td>k<sub>1</sub> = 0.03458 mg·g<sup>-1</sup>·min<sup>-1</sup>, q<sub>e</sub> = 0.050497 mg·g<sup>-1</sup></td><td rowspan="5">0.84301</td><td>k<sub>1</sub> = 0.02404 mg·g<sup>-1</sup>·min<sup>-1</sup>, q<sub>e</sub> = 0.00821 mg·g<sup>-1</sup></td><td rowspan="5">0.99485</td></tr><tr><td>k<sub>2</sub> = 23.596 mg·g<sup>-1</sup>·min<sup>-1</sup>, q<sub>e</sub> = 0.37362 mg·g<sup>-1</sup></td><td>k<sub>2</sub> = 25.354 mg·g<sup>-1</sup>·min<sup>-1</sup>, q<sub>e</sub> = 0.3823 mg·g<sup>-1</sup></td></tr><tr><td>k<sub>d1</sub> = 0.02358 mg·g<sup>-1</sup>·min<sup>-1/2</sup></td><td>k<sub>d1</sub> = 7.95 × 10<sup>-4</sup> mg·g<sup>-1</sup>·min<sup>-1/2</sup>, k<sub>d2</sub> = 7.57 × 10<sup>-6</sup> mg·g<sup>-1</sup>·min<sup>-1/2</sup></td><td>0.99998</td></tr><tr><td>q<sub>f</sub> = k<sub>d</sub>t<sup>1/2</sup> + C<sub>t</sub></td><td>0.99397</td><td>0.98159 and 0.97371</td></tr></tbody></table>

[96–98] (Fig. S7b, S7c and S7d). The results, summarized in Table 3.

The optimal fit to the pseudo-second-order model ($R² = 0.99998$ for MIL-101(Cr)-SO₃H-PBA) indicates that the adsorption is controlled by chemisorption. The intraparticle diffusion model reveals a two-stage process: rapid diffusion to the surface ($7.95 × 10⁻⁴$ mg·g⁻¹·min⁻¹/²), followed by slow diffusion into the internal porosity ($7.57 × 10⁻⁶$ mg·g⁻¹·min⁻¹/²). This analysis is consistent with the morphology observed by microscopy, confirming the occupation of coordination sites on the surface and within the pores.

where $q_t$ and $q_e$ represent respectively the amount of $Cs⁺$ adsorbed at contact time t (min) and the amount of $Cs⁺$ adsorbed at equilibrium, $k_1$ (min⁻¹) and $k_2$ (g·mg⁻¹·min⁻¹) represent the rate constants of the pseudo-first order and pseudo-second order kinetic models, respectively. $k_d$ (mg·g⁻¹·min⁻¹/²) and $C_i$ (mg·g⁻¹) represent respectively the intraparticle diffusion rate constant and the thickness of the boundary layer at stage i.

### 5.2.2. Adsorption isotherms of cesium

The study of $Cs⁺$ adsorption in MIL-101(Cr)-SO₃H-PBA was conducted at varying $Cs⁺$ concentrations (ranging from 9 ppb to 1828 ppm) and at pH 5.2 (Fig. 8). The nonlinear Langmuir adsorption isotherm model was employed to analyze the adsorption behavior and underlying mechanisms [99]. This model is particularly suitable as it assumes monolayer adsorption on homogeneous, non-interactive sites, with a maximum adsorption capacity when all sites are occupied. The parameters for this model are listed in Table 4.

$$ \text{Langmuir adsorption isotherm } q_e = \frac{Q_m K_L C_e}{1 + K_L C_e} \quad (7) $$

where $q_e$ and $Q_m$ are respectively adsorption capacity and maximum adsorption capacity (mg·g⁻¹), $K_L$: Langmuir coefficient (L·mg⁻¹) and $C_e$: Concentration of $Cs⁺$ at equilibrium (mg·L⁻¹).

The Langmuir isotherm analysis demonstrates that MIL-101(Cr)-SO₃H-PBA, with its maximum adsorption capacity of 242 mg·g⁻¹, outperforms MIL-101(Cr)-SO₃H (161 mg·g⁻¹) by 50%, highlighting the crucial role of Prussian blue analogs (PBA) in enhancing $Cs⁺$ ion adsorption performance (see Table 4). The Langmuir constant ($K_L$) of the hybrid material is 3.3 times higher (0.0906 versus 0.0275 L·mg⁻¹), confirming stronger affinity for $Cs⁺$ ions, which aligns perfectly with the pseudo-second-order kinetic model ($R² = 0.99998$). Table 5 compares the $Cs⁺$ adsorption capacities of various materials, highlighting the particularly remarkable performance of MIL-101-SO₃H-PBA. The reported capacities vary considerably, ranging from 13.60 mg·g⁻¹ for kaolinite to 432.9 mg·g⁻¹ for the MOF [Zr₆(µ₃-O)₄(µ₃-OH)₄(O-BA)₄(OH)₃(H₂O)₃(Me₂NH₂)]ₙ. With an adsorption capacity of 242 mg·g⁻¹, MIL-101-SO₃H-PBA significantly surpasses several other adsorbents, such as SS-SO₃H-MIL-101(Cr)-3 (141 mg·g⁻¹) and HKUST-1 impregnated with potassium nickel hexacyanoferrate (153 mg·g⁻¹). It is noteworthy that this high performance is maintained despite a significant reduction in the specific BET surface area and pore volume following PBA functionalization. This preserved efficiency is attributed to the introduction of additional active sites, including the sulfonate groups (-SO₃H) [100] from the monosodium 2-sulfoterephthalic acid ligand and the cyanometalate (Fe-C ≡ N-Cu) sites from PBA, which confer a strong specific affinity for $Cs⁺$ through electrostatic interactions and ion-exchange mechanisms. These combined characteristics establish MIL-101-SO₃H-PBA as a highly promising adsorbent for cesium removal from aqueous environments.

### 5.2.3. Effect of pH

The effect of pH (Fig. 9) on the adsorption of $Cs⁺$ was studied according to the parameters described in Table 1. The adsorption amount of $Cs⁺$ in MIL-101(Cr)-SO₃H is weak (52%) when the pH is 2 after 24 h. Indeed, at low pH, the $H⁺$ ions in the solution compete with the metal ions, which reduces the adsorption in MIL-101(Cr)-SO₃H. This
<figure><img src="image_10.png" /><figcaption>**Fig. 8.** Equilibrium adsorption isotherms of $Cs^+$ in MIL-101(Cr)-$SO_3H$ and MIL-101(Cr)-$SO_3H$-PBA, fitted with Langmuir isotherm model (adsorbent dose = 0.5 g L$^{-1}$, t = 24 h, T = 298 K, pH = 5.2).</figcaption></figure>

<caption>**Table 4**</caption>

Langmuir isotherm parameters for $Cs^+$ adsorption in MIL-101(Cr)-$SO_3H$ and MIL-101(Cr)-$SO_3H$-PBA.

<table><thead><tr><th rowspan="2">Sample</th><th colspan="3">Langmuir model</th></tr><tr><th>Q<sub>m</sub> (mg·g<sup>-1</sup>)</th><th>K<sub>L</sub> (L·mg<sup>-1</sup>)</th><th>R<sup>2</sup></th></tr></thead><tbody><tr><td>MIL-101(Cr)-SO<sub>3</sub>H</td><td>161</td><td>0.0275</td><td>0.9662</td></tr><tr><td>MIL-101(Cr)-SO<sub>3</sub>H-PBA</td><td>242</td><td>0.0906</td><td>0.9856</td></tr></tbody></table>

phenomenon was also observed by Suresh et al. [104] in the case of other mesoporous MOFs [104]. However, the adsorption of $Cs^+$ remains high (>95 %) in MIL-101(Cr)-$SO_3H$-PBA across the entire pH range studied, which can be attributed to the central role of PBA sites in ion exchange. The PBA sites, which are specific for $Cs^+$ exchange, remain active even in the presence of $H^+$ ions. This enables efficient $Cs^+$ adsorption despite Ph variations. This observation is in agreement with the work of Riya (Riyal et al., 2024b) who showed that on nanoparticles of $Fe_3O_4$/graphene oxide/potassium copper hexacyanoferrate, the rate of adsorption of $Cs^+$ would decrease only towards pH = 1. Under neutral conditions (pH 7), MIL-101(Cr)-$SO_3H$ exhibits maximum adsorption efficiency with a 92 % $Cs^+$ removal rate, attributed to the partial deprotonation of sulfonic acid groups (-$SO_3H$), which enhances electrostatic interactions between the cationic metal ions ($Cs^+$) and the negatively charge surface of MIL-101(Cr)-$SO_3H$. Ren et al. reported similar findings for their SS-$SO_3H$-MIL-101(Cr)-3 material (Ren et al., 2023b). In contrast, under alkaline conditions (pH 11), a significant decrease in adsorption capacity is observed (78 %), resulting from the complete neutralization of sulfonic groups by hydroxide ions ($OH^-$) according to the reaction $-SO_3H + OH^- \rightarrow -SO_3 + H_2O$ [105,106]. This chemical transformation drastically alters the surface properties of the material by reducing the number of available active adsorption sites, as clearly demonstrated in various studies [107,108]. The decrease in the material's efficiency can also be explained by the competition between $Na^+$ and $Cs^+$ ions (Fig. 10).

<caption>**Table 5**</caption>

Comparison of adsorption capacities of various materials for $Cs^+$ ions. Adsorption capacities are expressed in mg of $Cs^+$ per gram of adsorbent.

<table><thead><tr><th>No.</th><th>Materials</th><th>Adsorption capacity (mg·g<sup>-1</sup>)</th><th>Ref.</th></tr></thead><tbody><tr><td>1</td><td>Kaolinite</td><td>13.60</td><td>[101]</td></tr><tr><td>2</td><td>Bentonite</td><td>49.40</td><td>[9]</td></tr><tr><td>3</td><td>Montmorillonite (MMT)</td><td>204.7</td><td>[102]</td></tr><tr><td>4</td><td>Zeolite 4A</td><td>222.9</td><td>[13]</td></tr><tr><td>5</td><td>Magnetic zeolite composite (MZC)</td><td>207.4</td><td>[13]</td></tr><tr><td>6</td><td>NaY Faujasite</td><td>241.9–509.0</td><td>[23]</td></tr><tr><td>7</td><td>vermiculite/zinc hexacyanoferrate (VMT/ZnHCF)</td><td>379.8</td><td>[25]</td></tr><tr><td>8</td><td>ZnHCF-MCM-41</td><td>103.1</td><td>[25]</td></tr><tr><td>9</td><td>Prussian blue analog@γ-alumina (PBA@Al<sub>2</sub>O<sub>3</sub>)</td><td>15.65</td><td>[26]</td></tr><tr><td>10</td><td>Prussian blue/maghemite (NiPB/γ-Fe<sub>2</sub>O<sub>3</sub>)</td><td>102.0</td><td>[27]</td></tr><tr><td>11</td><td>Graphene oxide aerogels (RGOA) doped with Triphenylborane TPBA (RGOA/TPBAs)</td><td>226.9</td><td>[103]</td></tr><tr><td>12</td><td>[Zr<sub>6</sub>(μ<sub>3</sub>-O)<sub>4</sub>(μ<sub>3</sub>-OH)<sub>4</sub>(OBA)<sub>4</sub>(OH<sub>3</sub>)(H<sub>2</sub>O)<sub>3</sub>(Me<sub>2</sub>NH<sub>2</sub>)]<sub>n</sub> (OBA = 4,4-oxy bis(benzoic acid))</td><td>432.9</td><td>[42]</td></tr><tr><td>13</td><td>SS-$SO_3H$-MIL-101(Cr)-3</td><td>141.0</td><td>[41]</td></tr><tr><td>14</td><td>HKUST-1 impregnated with potassium nickel hexacyanoferrate (MOF/KNiFC)</td><td>153.0</td><td>[39]</td></tr><tr><td>15</td><td>MIL-101(Cr)-$SO_3H$</td><td>161.0</td><td>This work</td></tr><tr><td>16</td><td>MIL-101(Cr)-$SO_3H$-PBA (method 1)</td><td>242.0</td><td>This work</td></tr></tbody></table>

### 5.2.4. Study of the selectivity of MIL-101(Cr)-$SO_3H$ and MIL-101(Cr)-$SO_3H$-PBA

To assess the adsorption efficiency of MIL-101(Cr)-$SO_3H$-PBA for $Cs^+$ removal under marine condition, batch experiments were conducted using two simulated seawater solutions (40 g L$^{-1}$ total salt content, pH 7.8) contained the following major ion concentrations: $[Na^+] = 11039$ ppm, $[Mg^{2+}] = 1330$ ppm, $[K^+] = 400$ ppm, $[Ca^{2+}] = 424$ ppm and $[Sr^{2+}] = 15.6$ ppm designed to replicate natural oceanic conditions. The
<figure><img src="image_11.png" /><figcaption>**Fig. 9.** Effect of solution pH on adsorption of $Cs^+$ in MIL-101(Cr)-$SO_3H$ and MIL-101(Cr)-$SO_3H$-PBA ([Ci (mg/L) = 0.2, adsorbent dose = 0.5 g L$^{-1}$, t = 24 h, T = 298 K).</figcaption></figure>

<figure><img src="image_12.png" /><figcaption>**Fig. 10.** Effect of coexisting cations (a) MIL-101(Cr)-$SO_3H$ and anions (b) MIL-101(Cr)-$SO_3H$-PBA ($C_i$ (mg/L) = 0.2, 3, adsorbent dose = 0.5 g L$^{-1}$, t = 24 h, T = 298 K, pH = 7.8).</figcaption></figure>

first solution contained $Cs^+$ at 3 ppm while the second solution featured a lower $Cs^+$ concentration of 200 ppb. These experimental conditions were carefully selected to simulate realistic seawater composition where trace-level $Cs^+$ (ppb to ppm range) coexists with major cations present at several orders of magnitude higher concentrations, thereby testing the material's selective adsorption capability under competitive conditions.

The $Cs^+$ removal rate in MIL-101(Cr)-$SO_3H$ (Fig. 10a) was found to be very low (3.36 % in the first solution and 5.97 % in the second), due

to competition between $Cs^+$ and the other ions in the solutions for available adsorption sites. MIL-101(Cr)-$SO_3H$ exhibited preferential adsorption for two divalent cations such as $Ca^{2+}$ and $Sr^{2+}$ (74 % and 76 %, respectively), whose adsorption was favored over monovalent ions. This preference occurs because divalent ions, due to their larger ionic radius and higher reactivity, tend to be preferentially adsorbed [42]. The remarkable $Cs^+$ selectivity ($K_d$ = 43,086.4 mL g$^{-1}$ at 200 ppb and 10,046.2 mL g$^{-1}$ at 3 ppm; Table S2) exhibited by MIL-101
(Cr)-SO₃H-PBA (Fig. 10b), as evidenced by its complex composition (see section 1), stems from three well-documented synergistic adsorption mechanisms. First, the abundance of K⁺ ions within the PBA framework enables highly efficient ion exchange with Cs⁺, facilitated by their comparable ionic radii (1.38 Å for K⁺ versus 1.67 Å for Cs⁺) [109]. Second, Cs⁺ cations permeate through vacant sites from the surface into the crystalline network [110]. Third, the acidic sulfonic groups (BDC-SO₃H) provide complementary proton-Cs⁺ exchange as observed in MIL-101(Cr)-SO₃H.

MIL-101(Cr)-SO₃H-PBA also demonstrates significant removal efficiency for Sr²⁺ (over 74 %). These findings highlight the material's ability to effectively adsorb not only Cs⁺ but also Sr²⁺ ions present in simulated solutions. Notably, the concentrations of Ca²⁺, Sr²⁺ and Mg²⁺ in the simulated seawater solutions are substantially higher than that of Cs⁺. Despite this disparity, the material's performance indicates that lower concentrations of these divalent cations could potentially be extracted while maintaining high efficiency to Cs⁺. The corresponding adsorption capacity (qₐ, mg·g⁻¹) and distribution coefficient (Kd, L·g⁻¹) values for various ions adsorbed on both MIL-101(Cr)-SO₃H and MIL-101(Cr)-SO₃H-PBA are compiled in Table S2.

## 5.3. Effect of γ-radiation

γ-radiation can compromise the stability of MOF type materials by inducing bond cleavage or structural rearrangements. To evaluate the radiation resistance of MIL-101(Cr)-SO₃H-PBA, comparative studies were conducted on three materials: pristine MIL-101(Cr), its sulfonated derivative MIL-101(Cr)-SO₃H, and the functionalized material MIL-101(Cr)-SO₃H-PBA.

### 5.3.1. Characterization

The powder X-ray diffraction patterns of the MIL-101(Cr), MIL-101(Cr)-SO₃H, and MIL-101(Cr)-SO₃H-PBA samples irradiated at 0.5, 1, 5, and 10 MGy are presented in Fig. S8a–S8c. The XRD pattern for MIL-101(Cr) (Fig. S8a) exhibits sharp peaks with high intensities, indicative of the high crystallinity of these samples for every integrated dose. This crystalline stability is further supported by nitrogen adsorption measurements at 77 K, which show that the BET surface area and pore volume remain nearly unchanged (Fig. S3b). In contrast, the X-ray diffraction patterns for MIL-101(Cr)-SO₃H and MIL-101(Cr)-SO₃H-PBA (Fig. S8b and S8c) reveal a broadening of characteristic peaks beyond 1 MGy (width at half maximum increases from 0.13° (2θ) to 0.23° (2θ)). This broadening indicates a loss of crystallinity in the samples. This loss of crystallinity is further confirmed by nitrogen adsorption measurements at 77 K, which reveal a significant reduction in BET surface area and pore volume. The corresponding values are summarized in Table 6.

The impact of gamma ionizing radiation on MOFs such as MIL-101(Cr) can be attributed to several chemical mechanisms. High-energy gamma radiation can induce bond cleavage, particularly affecting the Cr-BDC (benzene-1,4-dicarboxylate) linkages, which are crucial for the structural integrity of the MOF. The breaking of carboxylate (—OCO)

<caption>**Table 6**</caption>

Evolution of total pore volume (P/P₀ = 0.95) and BET surface area for MIL-101(Cr), MIL-101(Cr)-SO₃H and MIL-101(Cr)-SO₃H-PBA materials after γ-irradiation (0–10 MGy).

<table><thead><tr><th rowspan="2">γ-irradiation dose (MGy)</th><th colspan="2">MIL-101(Cr)</th><th colspan="2">MIL-101(Cr)-SO₃H</th><th colspan="2">MIL-101(Cr)-SO₃H-PBA</th></tr><tr><th>S<sub>BET</sub> (m²/g)</th><th>V<sub>total</sub> (cm³/g)</th><th>S<sub>BET</sub> (m²/g)</th><th>V<sub>total</sub> (cm³/g)</th><th>S<sub>BET</sub> (m²/g)</th><th>V<sub>total</sub> (cm³/g)</th></tr></thead><tbody><tr><td>0</td><td>2678</td><td>1.37</td><td>1518</td><td>0.49</td><td>616.0</td><td>0.28</td></tr><tr><td>0.5</td><td>2536</td><td>1.15</td><td>—</td><td>—</td><td>—</td><td>—</td></tr><tr><td>1</td><td>2430</td><td>1.22</td><td>53</td><td>0.04</td><td>483</td><td>0.30</td></tr><tr><td>5</td><td>2558</td><td>1.31</td><td>21</td><td>0.03</td><td>28</td><td>0.05</td></tr><tr><td>10</td><td>2610</td><td>1.30</td><td>—</td><td>—</td><td>29</td><td>0.07</td></tr></tbody></table>

bonds in the ligand and associated vibrational modes of the µ₃-OH ion have been observed in several studies, indicating that the structure of the MOF may be compromised under radiation exposure [45,111]. Furthermore, studies have shown that post-irradiation, Cr–O–C bond energies shift to higher binding energies, reflecting reduced electron density at the junction between the metal clusters and organic linkers. This shift suggests partial cleavage of bonds between Cr–O clusters and BTC carboxylate linkers under irradiation, indicating possible defect formation at the metal-organic interface [112]. Compared to other MOFs such as ZIF-8 or UiO-66 [113], MIL-101(Cr) exhibits superior stability under radiation due to its robust Cr-O coordination bonds. However, the introduction of sulfonic acid groups (–SO₃H) can weaken the structure, accelerating degradation when exposed to ionizing radiation.

The incorporation of Prussian blue analogs (PBA) into the MIL-101(Cr)-SO₃H matrix serves as a stabilizing agent up to 1 MGy as evidenced by the XRD analyses (Fig. S8c). The high electron density of PBA and its effective radiation shielding capabilities contribute to mitigating the structural degradation of the MOF. The electron-rich nature of Prussian Blue analogs (PBAs) enables them to effectively absorb and dissipate radiation energy, thereby providing protection to the underlying MOF structure. This characteristic enhances the structural stability of MOFs exposed to radiation. Previous studies have demonstrated that materials with high electron density, possess the ability to mitigate damage from radiation by acting as energy-absorbing shields [114,115]. The homogeneous incorporation of PBAs into MOF matrices (Fig. S6) has been shown to improve the overall resilience of the composites under intense radiation conditions, reducing the likelihood of structural degradation [116].

Given the structural modifications induced by gamma irradiation, we next evaluated the cesium (Cs⁺) adsorption capacity of MIL-101(Cr)-SO₃H and MIL-101(Cr)-SO₃H-PBA to correlate radiation-induced defects with functional performance.

### 5.3.2. Cs⁺ adsorption experiment on irradiated MIL-101(Cr)-SO₃H and MIL-101(Cr)-SO₃H-PBA

Cs⁺ ion adsorption tests were conducted on MIL-101(Cr)-SO₃H and MIL-101(Cr)-SO₃H-PBA materials allowing the protocol described in Section 5 and Table 1. Fig. 11a demonstrates a significant decrease in the Cs⁺ removal efficiency in MIL-101(Cr)-SO₃H as the irradiation dose increases. At 3 ppm Cs⁺, the adsorption rate drops from 87 % to 12 %, and at 200 ppb, it decreases from 91 % to 17 %. This marked reduction in adsorption efficiency can be attributed to a loss of porosity of the material induced by irradiation.

In contrast, Fig. 11b demonstrates that the Cs⁺ adsorption capacity of MIL-101(Cr)-SO₃H-PBA remains virtually unchanged following irradiation, indicating that the material's adsorption performance is unaffected by radiation exposure. This observation is supported by XRD analyses (Fig. S7c), which confirm the structural stability of MIL-101(Cr)-SO₃H-PBA even after exposure to high radiation doses up to 1 MGy of ionizing radiation. Indeed, structural data reveal that the PBA component remains intact within the material at these high irradiation doses, thereby maintaining its Cs⁺ capture capability.

## 6. Conclusion

This study investigated two synthesis methods for MIL-101(Cr)-SO₃H-PBA, revealing significant differences in their outcomes. The first method successfully incorporated a Prussian Blue Analogue (PBA) into MIL-101(Cr)-SO₃H structure, as confirmed by comprehensive characterization including X-ray diffraction (XRD), IRTF and Raman spectroscopy, along with scanning (SEM) and transmission electron microscopy (TEM). In contrast, the second approach only achieved copper incorporation without PBA formation. While PBA functionalization reduced the specific surface area (from 1518 to 616 m²·g⁻¹) and pore volume due to partial pore blocking, it simultaneously enhanced
<figure><img src="image_13.png" /><figcaption>**Fig. 11.** Adsorption capacity of (a) MIL-101(Cr)-SO₃H and anions (b) MIL-101(Cr)-SO₃H-PBA (Cᵢ (mg/L) = 0.2, 3, adsorbent dose = 0.5 g L⁻¹, t = 24 h, T = 298 K, pH = 5.2).</figcaption></figure>

the material's Cs⁺ adsorption properties.

Thermogravimetric analysis revealed substantial hydration (~20 wt %) and limited thermal stability above 300 °C. Despite these limitations, the composite demonstrated exceptional performance in simulated seawater conditions, achieving >98 % Cs⁺ removal efficiency while maintaining strong affinity for divalent cations like Sr²⁺ and Ca²⁺ (>74 % adsorption). These remarkable properties stem from the synergistic combination of the remaining porous MIL-101(Cr) framework, sulfonic acid groups (-SO₃H) for enhanced electrostatic interactions and ion-exchange mechanism facilitated by PBA.

The PBA grafting strategy proved particularly advantageous, yielding an adsorbent with 242 mg g⁻¹ Cs⁺ capacity, fast kinetics (20 min equilibrium) and excellent selectivity even in highly competitive ionic media (Kd = 43,086.4 mL g⁻¹ and 10,046.2 mL g⁻¹ respectively at 200 ppb and 3 ppm of Cs⁺). Furthermore, the material exhibited notable γ-radiation resistance (>1 MGy) while retaining 95 % of its initial efficiency, attributable to PBA nanoaggregates that dissipate radiative energy while preserving MOF structural integrity and active site accessibility.

The unique combination of high capacity, selectivity, thermal stability and γ-radiation resistance establishes MIL-101(Cr)-SO₃H-PBA as a next-generation hybrid material with outstanding potential for radioactive effluent treatment. These findings open new avenues for developing advanced materials tailored for nuclear waste management and radiological emergency response applications, demonstrating how strategic functionalization can overcome inherent material limitations to achieve superior performance in challenging environments.

### CRediT authorship contribution statement

**K.J.B. Alloko:** Writing – review & editing, Writing – original draft, Resources, Investigation. **L. Cantrel:** Supervision, Project administration, Funding acquisition, Conceptualization. **J. Denis:** Supervision, Project administration. **T. Loiseau:** Visualization, Supervision, Methodology. **X. Deschanel:** Visualization, Supervision, Methodology. **J. Causse:** Supervision, Methodology. **A. Moissette:** Visualization, Investigation, Formal analysis. **C. Volkringer:** Visualization, Supervision, Project administration, Methodology, Conceptualization. **A. Grégoire:** Visualization, Supervision, Project administration, Methodology, Conceptualization.

### Declaration of competing interest

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.

### Acknowledgements

A special thank you goes to the Laboratory of Adaptive Nano-materials for Energy (LNAR) at the Institute of Separation Chemistry of Marcoule (ICSM) in Bagnols-sur-Cèze for their ongoing collaboration and unwavering support.

We would also like to warmly thank Béatrice Baus-Lagarde for her meticulous work in performing certain ICP-MS and ICP-OES analyses.

Additionally, our appreciation goes to the Director of the Environmental Experimentation and Chemistry Laboratory (L2EC) at ASNR in Cadarache, as well as to all of his team of researchers and technicians, for their valuable support and technical expertise.

A big thank you is also extended to Xavier Le Goff from the Laboratory for the Study of Matter in Environmental Mode (L2ME) for his active collaboration and continued involvement in our MET analyses. We would also like to thank all the PhD students of L2EC in Cadarache, particularly Keymoon Abdallah and Fouzia Djeriouat, for their dedication.

Finally, we express our gratitude to the ICP-MS manager, Tornabene Calogero, as well as the EPICUR team from L2EC in Cadarache, particularly Coralie Le Maout Alvarez and Florent Friboulet, for their constant support and involvement throughout our research.

## Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.micromeso.2025.113902.

### Data availability

The authors do not have permission to share data.

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