Mesoporous SiO₂–Prussian blue composite for high-efficiency cesium ion removal

Abstract

This research investigates the development of a mesoporous silica–Prussian blue (HOM-SiO₂-PB) composite adsorbent, created using a one-pot in situ sol–gel method. This technique integrates Prussian blue (PB) into the SiO₂ framework, enhancing its ability to capture cesium ions (Cs⁺) from contaminated water. The composite was thoroughly analyzed using various methods, including XRD, XPS, HR-TEM, SEM, and N₂ adsorption–desorption measurements. Results showed that HOM-SiO₂-PB has a high adsorption capacity of 92.5 mg g⁻¹ for Cs⁺ ions, with the adsorption process governed by ion exchange between Cs⁺ and K⁺ ions. Adsorption isotherms fit models such as the Langmuir model with R² values greater than 0.984, while kinetic data followed a pseudo-first-order model. The adsorbent demonstrated 98.3% removal efficiency, reducing Cs⁺ concentrations from 2.02 mg L⁻¹ to below 20 μg L⁻¹. The material also showed excellent selectivity for Cs⁺ over other ions like Na⁺, K⁺, Mg²⁺, and Ca²⁺, with a distribution coefficient (K_d) of 3.9 × 10³ L g⁻¹, and maintained high stability and reproducibility after seven regeneration cycles. This study underscores the potential of HOM-SiO₂-PB as an effective and sustainable solution for removing radioactive Cs⁺ ions from contaminated water.

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Water impact

This research advances water science by creating a highly efficient HOM-SiO₂-PB adsorbent for radioactive cesium removal. It demonstrates a 98.3% removal rate and excellent selectivity, enabling effective water decontamination. This offers a sustainable, regenerative solution for mitigating nuclear contamination's environmental impact, directly improving water security and ecosystem protection.

Introduction

Radioactive cesium ions (Cs⁺), especially Cs-137, are major environmental pollutants resulting from nuclear accidents, improper waste disposal, and the use of radioactive materials in medicine and industry.^1–5 Exposure to Cs⁺ ions can cause serious health problems, including radiation sickness, cancer, and long-term ecological damage, due to their long half-life and tendency to accumulate in living organisms.^6–9 Consequently, removing Cs⁺ ions from contaminated water and soil is crucial for protecting human health and the environment. Effective remediation strategies are essential, and adsorption methods have become an efficient approach for this purpose.^10–15 One promising method involves using SiO₂ materials intercalated with Prussian blue (PB), which helps mitigate the harmful effects of radioactive Cs while aiding in environmental cleanup.

Recent advancements in materials science have focused on mesoporous SiO₂ combined with PB nanoparticles.^16–20 This hybrid material takes advantage of Prussian blue's strong affinity for Cs⁺ ions, while the mesoporous SiO₂ framework enhances its stability and structural integrity. The use of nanostructures and their orientations is critical for improving material performance, with lyotropic liquid crystals (LLCs) being proven effective in creating oriented nanostructures at a macroscopic scale. LLCs are useful templates for synthesizing mesoporous SiO₂ through the sol–gel method, transferring their structure to the silicon oxide.^21–24 Since the 1990s, efforts have been made to combine mesoporous SiO₂ with metal species and liquid-crystal structures, though environmental factors often reduce the effectiveness of LLCs when integrated with inorganic materials.^25–27 Recent research has focused on methods to control nanostructure orientation after hybridization, with catalyst-triggered synthesis techniques showing promise for enhancing LLC systems.²⁸

In this study, we introduce a new method for synthesizing mesoporous SiO₂ intercalated with PB using a one-pot in situ sol–gel lyotropic liquid crystal process. This approach simplifies synthesis while ensuring a uniform distribution of PB within the SiO₂ matrix, maximizing its adsorption capacity. The study aims to evaluate the performance of this composite material in efficiently removing Cs⁺ ions, contributing to environmental remediation and advancing wastewater treatment technologies.

Experimental

Materials

The materials used in this study included tetramethylortho-silicate (TMOS), FeCl₃·6H₂O, K₃[Fe(CN)₆], and the triblock copolymer Pluronic F108, all sourced from Aldrich. CsCl, KCl, LiCl, NaCl, CaCl₂, MgSO₄, RbCl, and SrCl₂, were obtained from Merck, and the concentration was adjusted using deionized water. All materials were of analytical reagent grade and did not require further purification.

Synthesis of the HOM-SiO₂-PB nanocomposite

The synthesis of the HOM-SiO₂-PB nanocomposite followed these successive procedures: (i) mix 20 mL of an ethanol/water mixture (3 : 1) with 1.5 g of FeCl₃·6H₂O (0.2 M) in a 50 mL beaker at 25 °C. In a separate balloon flask, mix 20 mL of ethanol, 5.7 g of Pluronic F108, and 6 mL of tetramethylorthosilicate (TMOS). (ii) Transfer the mixture in the balloon flask to an EYELA NVC-2100 rotary evaporator set at 45 °C and 1023 hPa to form a homogeneous sol–gel, and then add the K-ferrocyanide solution to the balloon flask containing the FeCl₃ solution. (iii) After 5 min, add 5 mL of 0.1 M HCl (pH = 1.3) to stop the reaction and evacuate the mixture for 15 min to form a blue sol–gel-like SiO₂-PB monolith. (iv) Gently dry the product at room temperature for 6 h, then seal and leave it at 25 °C for 24 h. Perform solvent extraction with ethanol to remove Pluronic F108, completing the preparation of the HOM-SiO₂-PB material.¹⁹

Batch study for Cs⁺ ions adsorption

In the batch study for Cs⁺ ion adsorption, a solution containing Cs⁺ ions at an initial concentration of 120 mg L⁻¹ was mixed with 20 mg of the HOM-SiO₂-PB composite, achieving a sorbent dosage of 1 g L⁻¹. The pH of the mixture was adjusted between 1 and 8, and the mixture was shaken at 300 rpm for 120 h at temperatures of 25 °C, 30 °C, and 35 °C. Residual Cs⁺ ion concentrations (C_e, mg L⁻¹) were measured using ICP-OES, and the sorption capacity (q_e, mg g⁻¹) was calculated using the formula:^21,23

q_e = [(C₀ − C_e) × V]/W (1)

Further experiments investigated the impact of co-existing ions and adsorption kinetics. The initial Cs⁺ ion concentration was increased to 120 mg L⁻¹ at pH 6.0 to measure sorption isotherms and uptake times.

Results and discussion

Characterization of the synthesized sorbent

The mesoporous high ordered monolith SiO₂ decorated with PB (HOM-SiO₂-PB) composite was synthesized using a direct templating method, resulting in a cage-mesostructured carrier with uniform pores. The porous SiO₂ monoliths provided enhanced mechanical strength and improved Cs⁺ ion adsorption capacity. The synthesis employed an efficient, rapid direct-templating technique in microemulsion systems, which allowed for the quick formation of stable monoliths with ordered cubic structures. PB nanoparticles were then precisely precipitated into the SiO₂ pores, forming the nanocomposite.¹⁹ The composite was created in two stages: first, SiO₂ was polymerized in situ, and then PB was formed through a reaction within SiO₂ pores through both physical confinement and chemical bonding. Primarily, the mesoporous silica structure acts as a rigid host, physically entrapping PB nanoparticles within its accessible pore network, which prevents aggregation and enhances stability. Furthermore, the use of TMOS precursors creates a functionalized silica template that act as nucleation sites. These functional groups chemically coordinate with the iron ions of Prussian blue, effectively anchoring the PB crystals to the silica matrix. To fully analyze the composition and morphologies of HOM-SiO₂-PB, SEM and EDS analytical techniques were implemented. The HOM-SiO₂-PB sorbent revealed a variety of particles attached to the SiO₂ framework of varying sizes, as demonstrated by the SEM images in Fig. 1(a and b). Furthermore, EDS spectroscopy confirmed the presence of signals for elemental silicon (Si), potassium (K), iron (Fe), oxygen (O), nitrogen (N) and carbon (C) in HOM-SiO₂-PB particles. In addition, all the presented elements (Si, Fe, K, O, N, and C) are distributed homogeneously in the whole EDS analysis area, implying the successful formation of the HOM-SiO₂-PB sorbent (Fig. S1(a–f)). The elemental distribution shows that the atomic percentages are 5.79% (Si), 2.33% (K), 23.01% (Fe), 19.6% (O), and 3.19% (N). The unique potassium signal (2.33 at%) definitively confirms successful Prussian blue formation, constituting 2.33% of the composite's atomic composition.

Fig. 1 (a and b) SEM images, (c and d) HR-TEM images, (e) selected area electron diffraction (SAED) analysis, and (f) XRD pattern of the monolithic particles of the HOM-SiO₂-PB composite.

In addition, HR-TEM images confirmed that PB particles were uniformly distributed within the composite, and revealed the presence of ordered cages, voids, and pore openings that effectively trap large quantities of Cs⁺ ions Fig. 1(c and d). SAED analysis showed that SiO₂ superstructures were amorphous, as indicated by diffuse ring patterns typical of amorphous materials (Fig. 1(e)). The incorporation of PB contributed to these amorphous characteristics, which, along with the hierarchical structure, enhanced the composite's ability to retain Cs⁺ ions.

XRD analysis. Fig. 1(f) presents the XRD analysis of the synthesized composite, revealing diffraction peaks at 17.469°, 24.754°, 35.322°, and 39.542°. The observed peaks confirm the face-centered cubic structure of PB, with shifts in the positions indicating that SiO₂ has been incorporated into the composite. The pronounced peaks suggest that PB was deposited at the nanoscale on the SiO₂ surface, while the absence of distinct SiO₂ peaks points to its amorphous character. Additional weak peaks further support the presence of PB crystals in the composite. The successful synthesis of the HOM-SiO₂-PB sorbent is thus confirmed. The average crystallite size was calculated using the Debye–Scherrer equation, yielding a value of 7.36 nm.²⁹

D = kλ/β cos θ (2)

The crystallite size (D) is measured in nanometres, with the Scherrer constant (k) typically set to 0.94, which varies depending on the shape of the crystallites. The wavelength of the X-ray radiation (λ) is usually that of Cu Kα radiation. The Bragg angle (θ), given in radians, is half of the diffraction peak angle, and the full width at half maximum (FWHM, β) of the diffraction peak is also expressed in radians.

Textural properties. The specific surface area and pore configuration of the synthesized sorbent greatly influence its characteristics. Nitrogen adsorption–desorption isotherms were used to assess these properties, as shown in Fig. 2, which illustrates a type IV isotherm with an H2-type hysteresis loop.¹⁹ The measured pore volume of 0.283 cm³ g⁻¹ and a pore width of 20 nm confirm the mesoporous nature of the synthesized materials, aligning with IUPAC classifications. The specific surface area (S_BET) was calculated to be 203 m² g⁻¹ using the BET equation, roughly half that of the pristine SiO₂ monolith produced by the same method, which had a surface area of 376 m² g⁻¹ (Fig. 2 (inset)).

Fig. 2 (a) N₂ adsorption/desorption isotherms of HOM-SiO₂-PB composite, pore volume distribution (inset), and (b) TGA analysis of the SiO₂–Prussian blue composite.

Thermal analysis. The TGA curve of the HOM-SiO₂-PB nanocomposite shows a gradual mass loss of 37.5% up to 600 °C, followed by a stable mass plateau up to 900 °C. The initial mass loss up to 200 °C is attributed to the evolution of physisorbed and coordinated water. The subsequent continuous weight loss is assigned to the removal of the residual Pluronic F108 surfactant and solvents, which were not fully eliminated during the ethanol extraction process. The sharp mass loss event at approximately 600 °C corresponds to the final combustion of these organic residues. The subsequent mass stabilization indicates that the inorganic framework—comprising the Prussian blue nanoparticles supported by the porous silica matrix—is itself thermally stable. This high thermal stability, up to 900 °C, is a key advantage for high-temperature applications and is a direct result of the robust silica structure that supports and protects the Prussian blue nanoparticles.

X-ray photoelectron spectroscopy (XPS) was employed to determine the surface elemental composition and chemical states of the synthesized HOM-SiO₂-PB nanocomposite. The survey spectrum (Fig. 3a) confirms the presence of silicon (Si 2p), iron (Fe 2p), oxygen (O 1s), nitrogen (N 1s), and carbon (C 1s), with atomic percentages of 14.04%, 11.16%, 17.13%, 9.9%, and 47.45%, respectively. This composition supports the successful formation of the SiO₂-PB composite. In the Fe 2p spectrum (Fig. 3b), the doublet separation for Fe 2p_3/2 and Fe 2p_1/2 was constrained to 13.5 eV. The deconvolution reveals characteristic peaks of Prussian blue, where the Fe 2p_3/2 peak is fitted with components at 708.2 eV and 709.8 eV, assigned to Fe²⁺ in [Fe(CN)₆]⁴⁻ and Fe³⁺ in [Fe(H₂O)₆]³⁺ species, respectively. The corresponding Fe 2p_1/2 spin–orbit components are observed at 721.4 eV and 724.2 eV. The N 1s spectrum (Fig. 3c) was fitted with a single dominant component at 398.2 eV, which is exclusively assigned to the nitrogen atoms in the cyanide bridging ligands (C≡N–Fe). The C 1s spectrum (Fig. 3d) shows the main adventitious carbon peak at 284.8 eV and a distinct component at 286.2 eV, attributed to the carbon atom in the cyanide group (C≡N). The consistent binding energies of the N 1s and C 1s peaks provide further confirmation of the cyanometallate structure. The O 1s and Si 2p signals (Fig. 3e and f) are present at binding energies of 532.25 eV and 102.16 eV, respectively, which are like the reported value of silica. Furthermore, the C 1s peak at 286.2 eV and the O 1s peak at 531.62 eV approach the values of C–O and Fe–O. Furthermore, the Si 2p spectrum (Fig. 3e) exhibits a single peak at 103.5 eV, characteristic of silicon in its fully oxidized silicate (SiO₂) environment, confirming the integrity of the silica matrix. All peak positions, full width at half maximum (FWHM) values, and area ratios were consistent with the literature reports for Prussian blue and silica composites.^19,30–34 Furthermore, the Cs 3d spectrum (Fig. 3g) offered important insights into the oxidation state and local environment of adsorbed cesium ions. A well-defined spin-orbit doublet was observed, with the Cs 3d_5/2 and Cs 3d_3/2 peaks located at 724.2 eV and 738.2 eV, respectively, showing a characteristic splitting energy of 14.0 eV. This doublet is a clear indicator of cesium in the +1 oxidation state (Cs⁺).²³

Fig. 3 (a) Survey XPS spectrum, and (b–g) spectra for the Fe 2p, C 1s, N 1s, O 1s, Si 2p, and Cs 3d states of the monolithic particles of the HOM-SiO₂-PB composite.

Cs⁺ ion adsorption

Effect of pH. In adsorption systems, pH plays a crucial role in determining the composition of adsorbate species and the surface charge of the adsorbent. A study on Cs⁺ ions adsorption onto the HOM-SiO₂-PB composite at 25 °C revealed that the maximum adsorption capacity of 45.6 mg g⁻¹ was achieved at pH 6 with a Cs⁺ concentration of 50 mg L⁻¹ (Fig. 4). Fig. 4a reveals that the HOM-SiO₂-PB composite provides a more stable and reliable adsorption performance compared to pristine PB, which, despite a slight uptake advantage at neutral pH, exhibited significantly lower stability and uptake under acidic conditions due to its susceptibility to decomposition. The adsorption mechanism for Cs⁺ on PB is understood by comparing it to other monovalent Cs⁺ ion adsorption reactions. The surface zeta potential of HOM-SiO₂-PB was found to be −8.3 mV (Fig. 4b), indicating a negative surface charge. PB's cubic lattice structure, which features mixed-valence Fe²⁺/Fe³⁺ cations and cyanide (CN⁻) bridges, makes it a highly effective adsorbent for Cs⁺ removal.³⁵ The high adsorption efficiency is due to the compatibility between the hydrated Cs⁺ cation and the pore size of the PB lattice. PB contains two types of pores: interstitial sites occupied by counter ions (such as Na⁺, K⁺, Ca²⁺, Mg²⁺) and vacancy sites, both of which enhance its adsorption capacity. Further investigation using X-ray photoelectron spectroscopy (XPS) showed a shift in the N 1s peak after Cs⁺ adsorption, indicating that Cs⁺ ions are captured by the cyano groups in the PB structure. Overall, these findings suggest that Cs⁺ ions are primarily adsorbed onto PB through electrostatic interactions and are stabilized by the cyano-groups, with a significant amount of Cs⁺ being incorporated into the PB crystal lattice. The pH of the solution plays a key role in influencing the ionization of the surface groups on the HOM-SiO₂-PB sorbent (such as silanol groups on SiO₂) and the ionic forms of the adsorbate (Cs⁺). A slightly acidic to neutral pH generally promotes ion exchange by maintaining the stability of PB structure and supporting the ion exchange process. The adsorption of Cs⁺ ions onto the mesoporous HOM-SiO₂-PB sorbent is a complex mechanism involving both physical and chemical interactions between Cs⁺ ions and HOM-SiO₂-PB. Mesoporous SiO₂, with its high surface area and large number of adsorption sites, acts as a key component. When combined with PB, the material's adsorption capacity of Cs⁺ ions is enhanced due to the presence of Fe centers in the PB structure. Iron centers (Fe²⁺/Fe³⁺) in the PB structure can form ionic bonds with Cs⁺, and the mesoporous nature of the SiO₂ allows Cs⁺ to diffuse into the bulk, enhancing the ion exchange.^19,35 The bonds within PB are mainly electrostatic, with potential coordination bonding to Fe, facilitating strong and selective adsorption of Cs⁺ ions.¹⁰ Prussian blue has a strong ability to selectively adsorb Cs⁺ ions, driven by electrostatic attraction between the negatively charged sites in the structure and the positively charged Cs⁺ ions. This ion exchange can be represented as follows:^10,35

KFe[Fe(CN)₆]_(s) + Cs⁺_(aq) → CsFe[Fe(CN)₆]_(s) + K⁺_(aq) (3)

where K⁺ is replaced by Cs⁺, depending on the initial cationic composition of the PB. After ion exchange, the Cs⁺ ions become part of the PB lattice, forming a stable Cs–PB complex.³⁶

Fig. 4 (a) Effect of pH on the adsorption of Cs⁺ ions on HOM-SiO₂-PB composite; initial Cs⁺, conc. 50 mg L⁻¹, eq. time 120 min, temp. 25 °C. (b) The effective zeta potential of the HOM-silica-PB suspension over time.

Effect of contact time

The adsorption equilibrium of Cs⁺ ions onto the HOM-SiO₂-PB composite was reached in approximately 75 min, for an initial Cs⁺ concentration of 60 mg L⁻¹ at pH 6.0. During the first 15 min, the uptake efficiency reached 58%, indicating a moderate rate of adsorption (Fig. 5). The slow diffusion of Cs⁺ ions through the composite's pores, rather than rapid surface adsorption, was identified as the key factor governing the adsorption process. Additionally, Cs cations can penetrate the electric double layer (EDL) on the composite surface, further contributing to the slower kinetics. Despite this, the overall adsorption capacity remained high. The adsorption kinetics of Cs⁺ onto the HOM-SiO₂-PB sorbent was comprehensively evaluated using various kinetic models, including pseudo-first-order, pseudo-second-order, liquid film diffusion, intraparticle diffusion, Bangham, and Elovich models (Table 1).^37–41 The adsorption kinetics of Cs⁺ ions onto HOM-SiO₂-PB sorbent firstly was analyzed using both pseudo-first-order and pseudo-second-order models (Fig. 5(a and b)). Apparent rate constants (k₁ for pseudo-first-order and k₂ for pseudo-second-order) were derived from experimental data, and adsorption capacity (q_t) versus time (t) plots were used to estimate theoretical capacities and rate constants. The pseudo-second-order model demonstrated a higher correlation coefficient (R² = 0.986) than the pseudo-first-order model (R² = 0.992), suggesting that the second-order model provides a more accurate description of the adsorption kinetics of Cs⁺ ions. The equilibrium adsorption capacity (q_e = 51.45 mg g⁻¹) was in close agreement with the predicted value from the pseudo-second-order model, further supporting the conclusion that the adsorption process is chemically controlled and occurs in multiple stages, involving chemical reactions, pore diffusion, film diffusion, and bulk diffusion. The pseudo-second-order model better describes chemical adsorption where stronger bonds like ionic or covalent interactions, and the adsorption rate depends on the amount of adsorbate already present on the surface. This highlights the importance of understanding the nature of the [HOM-SiO₂-PB→Cs] interaction when choosing between these models.

Fig. 5 Adsorption kinetics of Cs⁺ on the HOM-SiO₂-PB composite. (a) Effect of contact time (conditions: 60 mg L⁻¹ Cs⁺, 20 mg adsorbent, 20 mL, 25 °C). Corresponding nonlinear and linear fits of the (b) Pseudo-first-order, (c) Weber–Morris intraparticle diffusion, (d) Elovich, (e) Liquid film diffusion, and (f) Bangham kinetic models.

Table 1 Kinetic data for the adsorption of Cs⁺ ions on HOM-SiO₂-PB composite

Kinetic models	Equation	Parameters	Values
Pseudo first order	q t = qe (1 – e^−kt)	q 1 mg g^−1	60.9
		K 1 min^−1	0.947
		R ^2	0.986
Pseudo second order		q 2 mg g^−1	51.45
		K 2 g mg^−1 min^−1	1.0 × 10^−3
		R ^2	0.992
Weber–Morris diffusion	q e = x + Kit^1/2	K i [mg (g^−1 min^−0.5)]	4.78
		x	7.13
		R ^2	0.873
Elovich kinetic		β g mg^−1	0.072
		α [mg (g^−1 min^−1)]	6.24
		R ^2	0.960
Liquid film diffusion	log(1 – F) = −KF/2.303 t	K f	0.023
		R ^2	0.995
Bangham	log log(Ci/Ci − mqt) = log(mKB/2.303 V) + α log(t)	K B(L g^−1 min^−1)	6.02
		m	0.53
		R ^2	0.954

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Further analysis was conducted using the McKay model, which incorporates both film and particle diffusion (Table 1). Non-linear regression was used for the intraparticle diffusion model, while linear regression was applied to the liquid film diffusion model to characterize the adsorption kinetics of Cs⁺ ions onto the HOM-SiO₂-PB sorbent (Fig. 5(c and d)). These models focus on different stages of the adsorption process. The liquid film diffusion model, which emphasizes resistance to mass transfer in the liquid phase around the adsorbent (HOM-SiO₂-PB), suggests that the rate-limiting step is the diffusion of Cs⁺ ions through the liquid film surrounding the HOM-SiO₂-PB particles, rather than diffusion into the particle pores. The film diffusion rate constant (K_f) was calculated to be 0.023 min⁻¹, with the liquid film thickness influencing the rate of Cs⁺ ion adsorption. However, the plot showed a non-zero intercept, indicating that film diffusion alone does not control the adsorption rate. The Weber–Morris model, which focuses on intra-particle diffusion, suggested that the process is mainly controlled by diffusion within the mesopores of the HOM-SiO₂-PB composite material. The rate constant of the adsorption of Cs⁺ ions into HOM-SiO₂-PB mesopores for intraparticle diffusion (K_i) can be directly obtained from the non-linear regression model, with the intercept x helping distinguish between boundary layer resistance and pore diffusion (Fig. 5(c)).

The Bangham model, which assumes that boundary layer diffusion is the rate-limiting step, was also applied and predicts that the adsorption of Cs⁺ ions increases logarithmically over time, initially occurring quickly before slowing down as the HOM-SiO₂-PB sorbent approaches saturation (Fig. 5(f)). The rate constant (k_B) was determined by fitting experimental data to the Bangham model by plotting log log(C₀/C₀ − mq_t) versus log(t) (Table 1), indicating that the adsorption of Cs⁺ ions into HOM-SiO₂-PB surfaces is controlled by diffusion through the liquid boundary layer and is particularly relevant in systems where physisorption occurs, and it may not be suitable for Cs⁺ adsorption process. Furthermore, the Elovich model, alternatively used to describe chemisorption processes, was applied to analyze the adsorption of Cs⁺ ions on HOM-SiO₂-PB surfaces (Fig. 5(e)). This model is especially useful for systems with heterogeneous adsorption sites, where adsorption rates vary with surface coverage. The initial adsorption rate (α) and the desorption constant (β) were determined. The results in Table 1 indicate that the Elovich model provided a better fit to the experimental data, showing a higher goodness of fit (R² = 0.960) compared to the Bangham model (R² = 0.954). This suggests that the adsorption of Cs on HOM-SiO₂-PB follows a heterogeneous diffusion process. Therefore, the pseudo-second-order and Elovich models confirm that the process of Cs adsorption into HOM-SiO₂-PB surfaces is chemically controlled, with ion-exchange playing a significant role, while the liquid film diffusion and intraparticle diffusion models highlight the importance of mass transfer limitations in both the liquid phase and the internal pores of the adsorbent.

Effect of Cs⁺ concentration. In order to evaluate the maximum sorption capacity of Cs⁺ ions into the mesoporous HOM-SiO₂-PB, adsorption isotherms were constructed using the data gathered at room temperature and pH values of 6. As can be seen from Fig. 6, the uptake of the Cs⁺ ions on the mesoporous HOM-SiO₂-PB sharply increases before reaching saturation at a maximum adsorption capacity of 92.4 mg g⁻¹. Higher concentrations of Cs⁺ in solution can increase the driving force for ion exchange, leading to more efficient replacement of other cations by Cs⁺ ions. However, excessive Cs⁺ ions concentrations could lead to saturation of HOM-SiO₂-PB adsorption sites. Isotherm modeling is important in order to observe the relationship between HOM-SiO₂-PB and Cs⁺ ions under equilibrium conditions. A good understanding of this would significantly enhance the design of the adsorption system, and the pattern describing the [HOM-SiO₂-PB→Cs] interaction. The adsorption of Cs⁺ ions on HOM-SiO₂-PB was anticipated using four isotherm models: Langmuir, Freundlich, Temkin, and Dubinin–Radushkevich (D–R),^40–43 all of which were analyzed through nonlinear regression, with the equations and parameters listed in Table 2. The Langmuir model assumes monolayer adsorption on a surface with a fixed number of identical, non-interacting sites. It predicts that once saturation is reached, no further adsorption occurs. The equation defines q_e as the amount of Cs⁺ ions adsorbed per gram of HOM-SiO₂-PB (mg g⁻¹), C_e as the equilibrium concentration of Cs⁺ ions in solution (mg L⁻¹), and K_L as the adsorption energy constant. The maximum adsorption capacity (q_L) was found to be 101.2 mg g⁻¹, closely matching the experimental plateau value, confirming the model's applicability. The R² value for the Langmuir model was 0.996, showing an excellent fit to the data (Fig. 6(b)). Furthermore, the Freundlich model describes adsorption on heterogeneous surfaces with multilayer adsorption, where adsorption sites are not uniform. The model yielded an R² value of 0.874, which was lower than that of the Langmuir model, but still indicates a reasonable fit. Parameters such as K_F = 12.54 and n_F = 1.83 suggest a favorable adsorption process, with n_F > 1 indicating normal adsorption intensity (Fig. 6(c)). Moreover, the Temkin model considers the interactions between adsorbate molecules and assumes that the heat of adsorption decreases with increasing coverage. It showed an R² of 0.938, indicating a good fit, with positive values for the adsorption constant K_T (11.8) and heat of adsorption B (14.4 J mol⁻¹), which suggests that the adsorption is favorable and endothermic in nature (Fig. 6(d)). Additionally, the D–R model is based on a pore-filling mechanism and is applicable to both homogeneous and heterogeneous surfaces. It provided a relatively good fit with an R² of 0.931, but was less accurate than the Langmuir model (Fig. 6(e)). The model parameters suggest pore-filling adsorption, with the mean free energy of adsorption lower than in the Langmuir model, indicating a less favorable fit. In general, the Langmuir model provided the best fit for the experimental data, with a high R² value (0.996) and a maximum adsorption capacity of 101.2 mg g⁻¹, consistent with the observed saturation. This suggests that the adsorption of Cs⁺ occurred by monolayer adsorption on uniform, non-interacting sites. The D–R models also showed reasonable fits but was less accurate than the Langmuir model. The Temkin model, although a good fit, indicated lower adsorption energy compared to the Langmuir and D–R models. In terms of fit accuracy, the order of preference for the isotherm models is: Langmuir > D–R > Temkin > Freundlich. This indicates that the adsorption of Cs⁺ on HOM-SiO₂-PB is primarily monolayer adsorption, with favorable interactions and some contributions from pore filling. The EDS analysis provides definitive and multi-faceted evidence for the successful and efficient adsorption of Cs⁺ onto the HOM-SiO₂-PB sorbent. The homogeneous co-distribution of Cs with the sorbent's fundamental elements (Si, O, Fe, C, N) confirms a uniform and widespread adsorption process across the entire composite matrix. Most critically, the pronounced decrease or complete disappearance of the potassium signal serves as unambiguous proof of the ion-exchange mechanism, directly demonstrating that Cs⁺ ions have replaced K⁺ within the Prussian blue crystal lattice. This synergistic data not only verifies adsorption but also elucidates the primary chemisorption pathway responsible for the high cesium uptake (Fig. 7).

Fig. 6 (a) Adsorption of Cs⁺ ions on the HOM-SiO₂-PB surfaces as a function of initial concentrations, (b) Freundlich, (b) Langmuir, (c) Temkin, (d) and D–R, isotherms of Cs⁺ ions (pH 6.0, eq. time 75 min) on HOM-SiO₂-PB from a single ion solution; sorbent weight 20 mg, solution vol. 20 mL.

Table 2 Parameters of different models for Cs⁺ adsorption isotherms

Isotherm	Equation	Parameters	Value
Langmuir		Q L (mg g^−1)	101.2
		K L (L g^−1)	0.273
		R ^2	0.996
Dubinin–Radushkevich	q e = qmax exp(−KDRε^2)	q max (mg g^−1)	87.58
		K DR (mol^2 kJ^−2)	6.40
		R ^2	0.931
Temkin		B (KJ mol^−1)	14.20
		K T (L mg^−1)	11.80
		R ^2	0.938

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Fig. 7 (a) STEM micrograph and (b–i) associated EDS elemental mapping of the Cs-loaded HOM-SiO₂-PB sorbent, revealing the spatial distribution of its constituent elements (Si, O, K, Fe, C, N) and the adsorbed Cs. (j) Quantitative EDS analysis presenting the atomic percentages of all detected elements.

Thermodynamic studies. Evaluating the thermodynamic parameters is crucial for optimizing the adsorption process and demonstrating its feasibility. To estimate the thermodynamic parameters for the adsorption of Cs⁺ ions onto the HOM-SiO₂-PB composite, the distribution coefficient K_d (K_d(L g⁻¹) = q_e/C_e) was determined at an initial concentration of 120 mg L⁻¹ at different temperatures (25, 30, 35, and 40 °C) to be 3780, 5490, 10 320, and 15 000 mL g⁻¹. These values of ln(K_d) were then substituted into the van't Hoff equation (eqn (4)) to calculate the thermodynamic parameters.^19,21

ln(K_d) = (−ΔH°)/RT + (ΔS°)/R (4)

where q_e is the amount of Cs⁺ ions adsorbed per unit mass of the sorbent, and C_e is the equilibrium concentration of Cs⁺ ions in the solution at temperature T. By plotting ln(K_d) versus 1/T using the van't Hoff equation, a straight line was obtained (Fig. 8). The slope and intercept of this line were used to calculate the values of the enthalpy change (ΔH°) and entropy change (ΔS°);²¹

ΔG° = −ΔH° + TΔS° (5)

Fig. 8 van't Hoff plot of adsorption of Cs⁺ ions on the HOM-SiO₂-PB composite.

The values of ΔH°, ΔS°, and ΔG° were calculated to assess the thermodynamic nature of the adsorption process. The positive value of the enthalpy change (ΔH° = 42.6 kJ mol⁻¹) confirms that the adsorption process is endothermic. The positive entropy change (ΔS° = 0.147 J mol⁻¹) indicates a preference of the adsorbent for Cs⁺ ions, reflecting the increasing randomness or disorder in the system as the adsorption occurs. The negative values of Gibbs free energy (ΔG°) at different temperatures (298, 303, 308, and 308 K), with values of −1.2, −2.1, −2.45, and −2.67 kJ mol⁻¹, respectively, suggest that the process is thermodynamically favorable and spontaneous. The change in ΔG° with temperature further indicates that the process becomes more favorable at higher temperatures. Additionally, the relation ∣ΔH°∣<∣−TΔS°∣ implies that the adsorption process is primarily driven by entropic rather than enthalpic changes. This supports the conclusion that the adsorption of Cs⁺ ions becomes faster at higher temperatures, as the process is more influenced by the increase in entropy with temperature. In summary, the thermodynamic analysis confirms that the adsorption of Cs⁺ onto the HOM-SiO₂-PB composite is a spontaneous, endothermic process that becomes more efficient at elevated temperatures.

Selectivity and Cs⁺ removal from contaminated seawater

To prove the real applicability of HOM-SiO₂-PB on the removal of radioactive Cs⁺ ions from environmental samples, we used Mediterranean seawater samples as the carrying solution. The Cs⁺ concentration in the seawater was chosen to be 5 mg L⁻¹, and the pH value was kept natural during the adsorption test. The presence of other cations such as Na⁺, K⁺, Ca²⁺, Sr²⁺ and Rb⁺ ions can compete with Cs⁺ for the same adsorption sites, reducing the efficiency of the ion exchange process. The ion concentration in the seawater samples is summarized in Table 3. Even from such a complex system, the adsorption capacity of Cs⁺ using the HOM-SiO₂-PB sorbent is 94.6%, with a distribution coefficient (K_d) of 3.9 × 10³ mL g⁻¹. This interesting finding suggests the usefulness of the designed HOM-SiO₂-PB for the selective removal of radioactive Cs⁺ from the environmental seawater samples containing Na⁺, K⁺, Ca²⁺, Sr²⁺, and Rb⁺ ions in an efficient way. Nevertheless, the ion exchange mechanism for Cs⁺ adsorption onto mesoporous HOM-SiO₂-PB involves the selective replacement of other cations in the PB lattice or on the SiO₂ surface by Cs⁺ ions. The high selectivity for Cs⁺, coupled with the material's regenerative capacity, makes this ion exchange process highly effective for removing Cs⁺ ions from aqueous solutions.¹⁰

Table 3 Adsorption of Cs⁺ ions from a simultaneous seawater sample by 50 mg HOM-SiO₂-PB in 50 mL solution volume

Ions conc. in seawater, mg L^−1	Spiked sample, mg L^−1	Amount adsorbed, mg L^−1	Adsorption efficiency, %
Cs^+: 0.02, Sr^2+: 6.5, Na^+: 12274.8, K^+: 419.5, Ca^2+: 501.8, Mg^2+: 1141.8, B^3+: 3.83, Fe^3+: 2.09, Cu^2+: 1.50, Ba^2+: 71.1, Pb^2+: 1.80, Cr^6+: 17.5, Cd^2+: 10.20, Mn^2+: 22.50, Ni^2+: 58.50, U^6+: 0.002, Cl^−: 22200, SO4^−2: 2250, HCO^−3: 132.5	2.02	1.99	99.2

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Conclusions

This study successfully demonstrated the development of a mesoporous HOM-SiO₂-PB adsorbent, synthesized through a one-pot in situ sol–gel method. The integration of PB into the SiO₂ matrix significantly enhanced the composite's ability to remove Cs⁺ ions from contaminated seawater. The HOM-SiO₂-PB composite exhibited a high adsorption capacity of 92.4 mg g⁻¹ for Cs⁺, with the process predominantly driven by ion exchange between Cs⁺ and K⁺ ions. The adsorption behavior was well-described by Langmuir > Dubinin-Radushkevich > Temkin > Freundlich models, and kinetic data followed a pseudo-first-order model, indicating fast and efficient ion removal. The composite demonstrated an impressive removal efficiency of 98.3%, reducing Cs⁺ concentrations from 2.02 mg L⁻¹ to below 20 μg L⁻¹. Additionally, the material exhibited excellent selectivity for Cs⁺ over other common ions such as Na⁺, K⁺, Mg²⁺, and Ca²⁺, with a high distribution coefficient (K_d) of 3.9 × 10³ mL g⁻¹. The composite also maintained high stability and reproducibility after seven regeneration cycles, indicating its potential for long-term use in water purification. These findings highlight HOM-SiO₂-PB as a promising, effective, and sustainable adsorbent for the removal of radioactive Cs⁺ ions from seawater, with significant implications for environmental remediation.

Author contributions

N. Abdelmageed, S. F. Hegazi, and M. Abdelrahman carried out the experiments. E. Elshehy wrote the manuscript with support from M. A. Eldoma, N. Zouli, and M. Abdu. M. Hassan and S. F. Hegazi fabricated and characterized the adsorbent material. M. A. Mahmoud and A. F. Abouatiaa helped supervise the project. M. S. Alomar and M. Abdu conceived the original idea. E. A. Elshehy and M. A. Mahmoud supervised the project.

Conflicts of interest

There are no conflicts to declare.

Abbreviations

PB	Prussian blue
HOM-SiO2-PB	High ordered monoliths silica-Prussian blue composite
XRD	X-ray diffraction
SEM	Scanning electron microscopy
EDS	Energy dispersive spectroscopy
HR-TEM	High resolution transmission electron microscopy
XPS	X-ray photoelectron spectroscopy
S BET	Specific surface area
K d	Distribution coefficient
LLCs	Lyotropic liquid crystals
TMOS	Tetramethylortho-silicate
Pluronic F108	Poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol)
FWHM	Full width at half maximum
EDL	Electric double layer
D–R	Dubinin–Radushkevich

Data availability

All data supporting the findings of this study are included in the manuscript and its supplementary information (SI). Additional raw or processed data can be made available from the corresponding author upon reasonable request. Supplementary information is available. See DOI: https://doi.org/10.1039/d5ew00411j.

Acknowledgements

The authors gratefully acknowledge the funding of the Deanship of Graduate Studies and Scientific Research, Jazan University, Saudi Arabia, through project number: JU-202503209-DGSSR-RP-2025.

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