Preamble

Efficient removal of U(VI) by self-assembly organic-embedded mesoporous silica spheres Yaorui Li¹,², Yanan Ren¹, Nanting Qiu¹, Zhihao Wei¹, Heyang Sun¹,², Mengxun Liu¹

Abstract

In this research, an in-situ encapsulation strategy was employed to construct two organic-embedded mesoporous silica spheres (OMSS-1 and OMSS-2), achieving efficient U(VI) extraction from solution. By confining L-aspartic acid or citric acid within mesochannels (2.12–2.14 nm) during self-assembly, the composites attained exceptional U(VI) affinity, exhibiting high adsorption capacities of 172.4 and 188.3 mg/g for OMSS-1 and OMSS-2, respectively. Adsorption isothermal and kinetic models were applied to investigate U(VI) adsorption, revealing that the processes conform to the Langmuir model and pseudo-second-order kinetics. Crucially, the aspartate-embedded spheres (OMSS-1) demonstrated excellent selectivity in multicomponent systems containing 13 competing ions. The confinement-stabilized ligands enabled outstanding regenerability, retaining over 93% capacity across five adsorption-desorption cycles with minimal leaching (<0.8 wt%/cycle). This work redefines design principles for advanced adsorption materials by reconciling the traditionally conflicting objectives of high capacity, specificity, and stability in radionuclide management.

Keywords: Radioactive wastewater treatment, Mesoporous silica spheres, Organic functionalization, Uranium, Adsorption

Introduction

The rapid expansion of nuclear energy as a pivotal low-carbon power solution has substantially mitigated energy security concerns and carbon emissions worldwide. Nevertheless, uranium-containing wastewater from mining, fuel fabrication, and spent fuel reprocessing has emerged as an escalating ecological hazard, primarily attributed to uranium's combined chemical nephrotoxicity and protracted radiological risks [1, 2]. Of particular concern are soluble U(VI) species, which exhibit exceptional migration capacity in aqueous systems and trophic accumulation tendencies, ultimately threatening human health via groundwater contamination and agricultural irrigation pathways [3]. Adsorption technology has gained prominence among remediation approaches for its operational simplicity, cost-effectiveness, and adaptability to complex matrices [4, 5]. While conventional adsorbents like activated carbon and zeolites, as well as advanced porous materials including MOFs/COFs, have demonstrated U(VI) capture capabilities [6, 7, 8, 9], their practical applications are frequently constrained by limited selectivity, pH-dependent performance, and insufficient binding sites. This technological gap underscores the critical need for adsorbent innovation through rational structural design and performance optimization.

Silica-based adsorbents have emerged as particularly promising candidates due to their exceptional chemical inertness, mechanical robustness, and cost-effective synthesis protocols [10, 11]. The inherent advantages of porous silica architectures, particularly their ultrahigh specific surface areas and tunable mesoporous structure, create abundant accessible binding sites for U(VI) coordination [12, 13]. Moreover, the silica composition enables versatile functionalization through silane coupling reactions, polymer grafting, or heteroatom doping [14, 15], which is a critical feature for tailoring uranium adsorption and selectivity. For instance, dopamine-modified SBA-15 demonstrates 196 mg/g adsorption capacity at pH 6.0 [16].

In this work, we propose an in-situ encapsulation strategy to construct organic-embedded mesoporous silica spheres (OMSS). Unlike conventional post-synthetic grafting approaches, derivatives of L-aspartic acid and citric acid are strategically incorporated into the silica matrix during the self-assembly process, achieving molecular-level dispersion of chelating motifs within the mesoporous framework. The L-aspartic acid and citric acid functionalized composites are denoted as OMSS-1 and OMSS-2, respectively. Systematic characterization confirms the successful integration of organic components throughout the spherical architecture, rather than mere surface decoration. Batch adsorption studies reveal that the OMSS-1 and OMSS-2 composites exhibit exceptional U(VI) uptake capacities, with OMSS-1 showing superior selectivity for U(VI) in competitive adsorption experiments with various metal ions.

2.1 Materials and reagents

The reagents used in this study were purchased from Sinopharm Chemical Reagent Company (Beijing, China) and utilized as received without purification. The deionized water had a specific resistance of 18.25 MΩ·cm, ensuring high purity for experimental use.

2.2 Synthesis of OMSS

Prior to silica condensation, citric acid and L-aspartic acid underwent molecular pre-organization through ethylenediamine-mediated crosslinking, designed to establish covalent bridges between organic molecules and the developing silica network. Specifically, 2.10 g of L-aspartic acid or citric acid was co-dissolved with 2.68 mL ethylenediamine (EDA) in 50 mL deionized water under controlled conditions (60°C, 800 rpm magnetic stirring for 30 min). The homogenized precursor was subsequently transferred to a 100 mL Teflon-lined autoclave for hydrothermal treatment at 150°C under autogenous pressure for 5 hours, followed by natural cooling to ambient temperature.

The solid SiO₂ spheres were first synthesized via a precisely controlled Stöber process. In a typical procedure, 6 mL tetraethyl orthosilicate (TEOS) was hydrolyzed in a ternary solvent system comprising 74 mL anhydrous ethanol, 10 mL deionized water, and 5 mL ammonium hydroxide under continuous magnetic stirring (500 rpm) for 1 h. The resultant monodisperse silica colloids were collected through vacuum filtration (0.45 μm) and rigorously washed with ethanol/water (3:1 v/v) to remove residual reactants.

For the encapsulation process, 50 mg pristine SiO₂ spheres were ultrasonically dispersed in 10 mL deionized water to form a stable suspension. This dispersion was then introduced into a CTAB-templating system containing 200 mg hexadecyltrimethylammonium bromide (CTAB), 43 mL H₂O, 15 mL ethanol, and 0.8 mL 2 M NaOH. After 30 min equilibration, 0.125 mL TEOS was rapidly injected, followed by immediate addition of 3 mL pre-synthesized ligand solution, initiating simultaneous silica condensation and organic encapsulation. The reaction proceeded under vigorous stirring for 6 h to ensure complete framework growth. The products were collected by centrifugation and redispersed in 10 mL deionized water. Subsequently, 200 mg Na₂CO₃ was introduced as a mineralization accelerator along with 2 mL ligand solution. After stirring at 50°C for 10 h, the final products were isolated through vacuum filtration (0.45 μm) and subjected to three cycles of ethanol/water washing to eliminate CTAB templates.

2.3 Characterizations

The structural and functional properties of OMSS-1 and OMSS-2 composites were systematically investigated using a series of characterization techniques. Mesoscale ordering was verified through small-angle X-ray diffraction (SAXRD) using a PANalytical X'Pert PRO MRD system with parallel-beam optics (Cu Kα radiation, λ = 1.5406 Å), with 2θ scans performed from 1° to 5° (0.02° step, 10 s/step). Surface topography was analyzed using a scanning electron microscope (SEM, S-4800, Hitachi) operated at 20 kV, while transmission electron microscopy (TEM) images were acquired on an H-7650 TEM (Hitachi) at 100 kV. Textural parameters were quantified via nitrogen physisorption at 77 K using a surface area and pore size analyzer (Autosorb-iQ, Quantachrome). Chemical functionality analysis was conducted via Fourier-transform infrared spectroscopy (FT-IR, VERTEX80, Bruker) using the KBr pellet methodology. Thermal stability was assessed through thermogravimetric analysis (TGA, STA449F5, NETZSCH) under synthetic air flow (50 mL/min) with a ramp rate of 10°C/min from 25°C to 900°C.

2.4 U(VI) Adsorption experiments

Batch adsorption experiments were conducted by dispersing either OMSS-1 or OMSS-2 in uranyl nitrate solutions with an initial U(VI) concentration of 400 mg/L at different pH values. The pH was precisely regulated with 1.0 M HNO₃/NaOH, and the suspensions were maintained under continuous agitation (200 rpm) in 25°C water baths for 360 min. Residual uranium concentrations were quantified via inductively coupled plasma optical emission spectrometry (ICP-OES, 7500 A-X-II, Hongkong Sincere). The U(VI) adsorption capacity (qₑ, mg/g) was calculated as:

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

where C₀ and Cₑ represent the initial and equilibrium concentrations (mg/L) of U(VI), respectively, V is the solution volume (L), and m is the adsorbent mass (g).

Adsorption kinetics experiments were performed by exposing OMSS-1 or OMSS-2 composites to uranyl nitrate solutions containing 400 mg/L U(VI) at pH 4 for 360 min. Samples were periodically collected and filtered through a 0.45 μm membrane for ICP-OES analysis to determine temporal U(VI) concentration variations. The experimental kinetic data were modeled using pseudo-first-order and pseudo-second-order kinetic models.

The pseudo-first-order kinetic model [17] is expressed as:

$$q_t = q_e(1-e^{-K_1t})$$

where qₑ represents the equilibrium adsorption capacity (mg/g), qₜ is the adsorption amount (mg/g) at time t (min), and K₁ is the pseudo-first-order rate constant (min⁻¹).

The pseudo-second-order kinetic model [18, 19] is expressed as:

$$q_t = \frac{K_2q_e^2t}{1 + K_2q_et}$$

where K₂ signifies the pseudo-second-order rate constant (g·mg⁻¹·min⁻¹).

For the adsorption isotherm investigation, the Langmuir and Freundlich models were used for fitting. The Freundlich model equation [20] is:

$$q_e = K_fC_e^{1/n}$$

where K_f (L/mg) and n are parameters indicating adsorption capacity and strength, respectively, and Cₑ is the equilibrium adsorbate concentration (mg/L).

The Langmuir model equation [21] is:

$$q_e = \frac{q_mbC_e}{1 + bC_e}$$

where b is the constant (L/mg) related to adsorption energy, and q_m is the maximum adsorption capacity (mg/g) obtained by fitting.

The U(VI) specificity of OMSS-1 and OMSS-2 composites was evaluated through competitive sorption experiments against various metal ions (Na⁺, K⁺, Mg²⁺, Ca²⁺, Fe³⁺, Ni²⁺, Cu²⁺, Zn²⁺, Sr²⁺, Ag⁺, Cd²⁺, Cs⁺ and Pb²⁺) typically coexisting in nuclear effluents. A standardized challenge solution containing equimolar concentrations (400 mg/L each) of target and competing ions was prepared, and composite materials (2 g/L dosage) were exposed to the multimetallic solution under optimal sorption conditions for 360 min. The concentration of metal ions after adsorption was determined by ICP-OES.

3.1 Characterization of OMSS

The successful fabrication of hierarchically structured organic-silica hybrids is substantiated by comprehensive electron microscopy analysis. As shown in Figs. 1a–c [FIGURE:1], both OMSS-1 and OMSS-2 exhibit monodisperse spherical morphology with diameters of 400–800 nm as quantified by SEM and TEM statistical analysis. Structural integrity is corroborated by small-angle XRD patterns (Fig. 1d), which show characteristic Bragg reflections at 2θ = 2.12°. Nitrogen physisorption isotherms (Fig. 1e) display representative Type IV curves with H1 hysteresis loops, affirming uniform mesopores consistent with TEM and XRD results. The calculated pore size distributions center at 2.1 nm for both composites (Fig. 1f). BET surface areas of OMSS-1 and OMSS-2 are 690.41 m²/g and 729.35 m²/g, respectively (Table 1 [TABLE:1]), and the composites have similar pore volumes (OMSS-1: 0.98 cm³/g; OMSS-2: 0.85 cm³/g).

[FIGURE:1] Fig. 1 Characterization results of materials: (a) SEM image of unmodified mesoporous silica sphere; SEM image of (b) OMSS-1 and (c) OMSS-2; (d) XRD patterns of composites; (e) N₂ adsorption–desorption isotherms of composites; (f) Pore size distribution of composites.

[TABLE:1] Table 1. BET surface area, pore volume and pore diameter of OMSS-1 and OMSS-2

TG results (Fig. 2a [FIGURE:2]) provide definitive evidence for successful organic encapsulation within the silica matrix. Pristine silica spheres exhibit minimal mass loss (3.6%) over the 30–900°C range under oxidative atmosphere, primarily attributed to evaporation of adsorbed water and polycondensation of residual silanol groups [22, 23]. In stark contrast, both OMSS composites demonstrate three-stage decomposition behavior: an initial dehydration step below 150°C accounts for <3.0% mass reduction, followed by a pronounced weight loss plateau between 150–300°C corresponding to oxidative decomposition of encapsulated organic ligands (35.9% for OMSS-1 and 35.67% for OMSS-2). Subsequent gradual mass reductions (4.8% and 3.5% for OMSS-1 and OMSS-2, respectively, from 300–500°C) originate from combustion of carbonaceous residues formed during primary decomposition.

Comparative FT-IR spectra (Fig. 2b [FIGURE:2]) reveal critical vibrational signatures of OMSS samples. All materials show persistent siliceous framework peaks at 1080 cm⁻¹ (vibration of Si-O-Si) [24, 25, 26]. OMSS-1 and OMSS-2 samples present emergent carbonaceous functionalities at 1640 cm⁻¹ (C=C aromatic stretching) (Al-Meer et al., 2024; Karunanithy et al., 2021). Furthermore, OMSS-1 shows more pronounced carbonaceous functionalities at 1467 cm⁻¹ (N-H from amine moieties) [27, 28, 29]. Additionally, covalent linkage Si-O-C vibration at 960 cm⁻¹ and aliphatic C-H stretching at 2840 and 2920 cm⁻¹ also indicate successful organic loading on mesoporous silica spheres [30, 31].

[FIGURE:2] Fig. 2 (a) TG and (b) FT-IR of unmodified and modified mesoporous silica spheres.

3.2 U(VI) adsorption on composites

Fig. 3 [FIGURE:3] delineates the pronounced pH-modulated U(VI) uptake profiles of the composites, revealing fundamental ligand-coordination mechanisms. Both OMSS-1 and OMSS-2 exhibit progressively enhanced adsorption capacities with increasing pH, escalating from marginal values of 20.3 mg/g (OMSS-1) and 12.3 mg/g (OMSS-2) at pH 1.0 to maxima of 172.4 mg/g (OMSS-1) and 188.3 mg/g (OMSS-2) at pH 6.0, respectively. This behavior originates from competitive protonation equilibria. Under highly acidic conditions (pH < 4), excessive H⁺ concentrations protonate critical binding sites, thereby impeding uranyl ion complexation. Notably, OMSS-1 demonstrates superior performance below pH 4.0, attributable to the bifunctional nature of encapsulated L-aspartate ligands. Here, protonated amine groups (–NH₃⁺) engage in intramolecular charge-assisted hydrogen bonding with adjacent deprotonated carboxylates (–COO⁻), forming stabilized chelation pockets that preferentially sequester UO₂²⁺ through electrostatic attraction and coordination. Conversely, OMSS-2 dominates beyond pH 4.0 due to stepwise deprotonation of citrate tricarboxylic acid moieties.

[FIGURE:3] Fig. 3 Influence of pH values on the adsorption capacities of OMSS-1 and OMSS-2

The uranium sorption isotherms (Fig. 4a [FIGURE:4]) reveal fundamental differences in saturation behavior between composites, with OMSS-2 exhibiting consistently higher uptake than OMSS-1 across all equilibrium concentrations. Fitting with classical adsorption models (Table 2 [TABLE:2]) demonstrates superior correlation with the Langmuir equation (R² > 0.996) versus Freundlich, indicating monolayer coverage on homogeneous binding sites. The Langmuir-derived maximum adsorption capacities are 172.9 mg/g (OMSS-1) and 181.4 mg/g (OMSS-2).

[FIGURE:4] Fig. 4 Adsorption (a) isotherms and (b) kinetics models fitting of OMSS-1 and OMSS-2

[TABLE:2] Table 2 Adsorption isotherms fitting parameters of OMSS-1 and OMSS-2

Kinetic profiles (Fig. 4b) exhibit rapid uranium capture, achieving >80% saturation within 60 min. As shown in Table 3 [TABLE:3], the pseudo-second-order model (R² > 0.998) outperforms pseudo-first-order fitting (R² < 0.980), signifying rate-limiting chemisorption involving electron sharing. The calculated equilibrium capacities (163.1 mg/g for OMSS-1; 165.3 mg/g for OMSS-2) deviate by <4.5% from experimental data (156.3–158.2 mg/g). The intrinsic rate constants (K₂ = 3.636×10⁻⁴ and 4.028×10⁻⁴ g·mg⁻¹·min⁻¹ for OMSS-1 and OMSS-2) correlate with ligand accessibility: citrate's flexible aliphatic chain facilitates faster U(VI) coordination than aspartate's constrained β-carboxylate geometry.

[TABLE:3] Table 3 Adsorption kinetics fitting parameters of OMSS-1 and OMSS-2

The composites demonstrate exceptional U(VI) specificity within multicomponent systems containing 13 competing cations spanning alkali metals (Na⁺, K⁺, Cs⁺), alkaline earths (Mg²⁺, Ca²⁺, Sr²⁺), transition metals (Fe³⁺, Ni²⁺, Cu²⁺, Zn²⁺, Ag⁺), and heavy metals (Cd²⁺, Pb²⁺) at equimolar concentrations. As quantified in Fig. 5 [FIGURE:5], both materials exhibit remarkably higher adsorption capacities (131.3 mg/g on OMSS-1, 110.8 mg/g on OMSS-2) for U(VI) than for other metals. Crucially, OMSS-1 achieves 20.5 mg/g higher U(VI) uptake than OMSS-2 in this competitive environment, demonstrating superior selectivity originating from aspartate's dual functionality. This may be caused by the mechanism: the borderline Lewis acidity of UO₂²⁺ (ionic index: 3.32) preferentially coordinates with aspartate's intermediate-hard amine ligands (pKa ≈ 9.8) over citrate's purely carboxylate donors, as confirmed by Pearson's principle [32, 33].

[FIGURE:5] Fig. 5 Selectivity adsorption capacity of OMSS-1 and OMSS-2 in mixed metal ion solutions.

[FIGURE:6] Fig. 6 Recyclable adsorption experiment of OMSS-1 and OMSS-2

The engineered composites demonstrate exceptional reusability, a critical metric for practical uranium recovery applications. As quantified in Fig. 6, both composites retain >93% of initial adsorption capacities after five adsorption-desorption cycles. Even following ten cycles, sustained efficiencies of 84.2% (OMSS-1) and 86.1% (OMSS-2) are achieved, indicating that highly stable encapsulation limits organic leaching to <0.8 wt%/cycle.

4 Conclusions

This study demonstrates a groundbreaking in-situ confinement strategy for fabricating organics-embedded mesoporous silica spheres, achieving simultaneous optimization of U(VI) adsorption capacity, selectivity, and regenerability. Materials characterization confirms that L-aspartate and citric acid ligands are successfully loaded into mesoporous silica spheres. Through coordination with these ligands, the OMSS-1 and OMSS-2 composites exhibit high U(VI) adsorption capacities of 172.4 and 188.3 mg/g, respectively. The composites, especially OMSS-1, demonstrate outstanding adsorption selectivity for U(VI) from multicomponent systems containing various competing cations. The adsorption processes of U(VI) on OMSS-1 and OMSS-2 conform to Langmuir adsorption isotherms and the pseudo-second-order kinetic equation. Additionally, the composites exhibit excellent recyclability in adsorption-desorption cycle experiments. This study provides new insights for the synthesis of adsorption materials and offers a theoretical basis for the application of composite materials in radioactive wastewater treatment.

Author contributions: All authors contributed to the study conception and design. Material preparation, data collection, and analysis were performed by Yaorui Li, Yanan Ren and Nanting Qiu. The first draft of the manuscript was written by Yaorui Li, and all authors commented on previous versions of the manuscript. All authors read and approved the final manuscript.

Declarations: The authors declare that they have no conflict of interest.

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