New insights into the uranium adsorption behavior of mesoporous SBA-15 silicas decorated with alkylphosphine oxide ligands

Abstract

Alkylphosphine oxides functionalized mesoporous silicas (SBAVx-P(O)Pr₂) were prepared by a two-step process involving: (1) co-condensation synthesis of vinyl-containing mesoporous silica (SBAVx), and (2) addition reaction with secondary n-propylphosphine oxide to anchor the alkylphosphine oxide ligands. The functionalized mesoporous silicas exhibited ordered mesoporosity, high specific surface area, and narrow pore size distribution. Due to the strong binding ability of the alkylphosphine oxide ligands and the surface silanol groups, the mesoporous silicas showed effective adsorption toward uranyl ions in HNO₃ solutions with a wide-range of concentrations. The existence of numerous ordered meso-pores in the silica matrix facilitated the mass transfer, resulting in fast adsorption kinetics with an equilibrium obtained in ∼30 minutes. The relationship between structural parameters (ligand density, pore and network architecture) and uranyl adsorption performance was studied. Meanwhile, the adsorption mechanism associated with the variable role of the alkylphosphine oxide ligands and silanol groups in different acidic solutions was discussed. Besides, cycling experiments by column operation revealed that the mesoporous silicas had a reliable performance and could be conveniently recovered by elution with K₂CO₃ solutions. This paper highlights the potential application of mesoporous silicas functionalized with alkylphosphine oxide ligands as promising candidates for the preconcentration and adsorption of uranium from acidic or neutral aqueous solutions.

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Introduction

With the fast development of nuclear energy, large quantities of radioactive contaminants are being accumulated at a high speed.^1,2 Uranium, due to its radioactive toxicity, has been recognized as one of the major contaminants threatening environmental safety and human health. Meanwhile, uranium is the primary fuel of nuclear reactors. In recent years, the general awareness of the limited natural uranium resources has led to widespread attention to uranium reclamation from unconventional sources, e.g., to enrich uranium from seawater.^3–6 Therefore, either due to environmental concerns or for the sustainable utilization of nuclear resources, uranium separation is an imperative issue to be addressed.

Various methods have been developed during the past decades for efficiently separating uranium from different media, such as precipitation,⁷ solvent extraction,^8,9 volatilization,¹⁰ inorganic ion exchange,¹¹ etc. In comparison, adsorption technology is considered as a more promising option for uranium separation on a commercial scale, because of its advantages of simple operation and low energy consumption.^6,12–14 Moreover, it has a strong adaptability to the uranium solution with the characteristic of large volume but low concentration.

Recently, the advance of functional material research has provided new candidate composite materials, including nano-particles,^3,15,16 magnetic Fe₃O₄,^6,13 mesoporous carbon,^17–19 graphene oxides,²⁰ MOFs,^21–23 for the adsorption of uranium. Especially, organic–inorganic hybrid materials based on the organic functionalization of mesoporous silicas have shown significant advantages due to their high specific surface area, chemical and irradiation durability. Besides, the abundant silanol groups on the inside and outside surfaces of the mesoporous silicas can provide anchoring sites for further organic modification. Various kinds of organic ligands, such as amidoxime groups,^24,25 dihydroimidazole groups, phosphonate groups,^19,26 iminodiacetic acid derivative,¹³ have been introduced to the mesoporous silica matrix. These functionalized mesoporous silicas showed great affinity toward uranium, which could be potentially applied for uranium recognition and separation.

When concerning the uranium reclamation from radioactive liquid wastes (RLWs), a large obstacle is the highly acidic condition, because a mass of inorganic acids are utilized in several unit processes of nuclear fuel cycle, such as leaching of uranium ore²⁷ and dissolving of spent fuel.²⁸ The acids in RLWs can dramatically influence the binding ability and the structural stability of the materials, which should be seriously taken into account when developing adsorption materials of uranium.

Trialkylphosphine oxides (TRPO, where R represents alkyl chain) is an neutral organophosphorus extracting agent, which plays a key role in the TRPO processes proposed by Chinese researchers for the removal of actinides from high-level liquid waste (HLLW).^29–32 The trialkylphosphine oxide groups possess good affinity to actinides in aqueous solutions, and high stability in strong acidic and radiation environment. In our previous communication,³³ we proposed a new protocol for developing uranium adsorption materials by covalently anchoring varieties of alkylphosphine oxide functionalities on the surface and inner channels of mesoporous silicas. The mesoporous silicas showed highly-efficient adsorption toward uranium even in strong nitric acid solutions, which is of great significance for the recovery of uranium from radioactive liquid wastes. In this contribution, a series of new mesoporous silicas with different anchoring amounts of alkylphosphine oxide groups were synthesized for a comprehensive study on the adsorption behavior toward uranium in HNO₃ solutions with a wide-range of acidity. New insights into the relationship between structural parameters (ligand density, pore and network architecture) and uranium adsorption performance were obtained. Binding mechanism associated with the role of the alkylphosphine oxide ligands and silanol groups was discussed. Moreover, from a practical perspective, column separation and elution of uranium was performed, and the reusability of the functionalized mesoporous silicas was evaluated by adsorption–elution cycle operations.

Experimental

Materials

Tetraethoxysilanes (TEOS) and vinyltrialkoxysilanes (TEVS) were commercially purchased from TCI and used as supplied. Poly(ethylene glycol)-b-poly(propylene glycol)-b-poly(ethylene glycol) (P123, MW = 5800) was supplied by Alfa Aesar. All the other reagents and solvents were of AR grade and used without further purification.

Synthetic procedure

Mesoporous silicas functionalized with vinyl groups (SBAV) were synthesized using P123 as the surfactant by a similar method with Nie et al.³⁴ The TEOS was added to an aqueous HCl-P123 solution under stirring, followed by the addition of the TEVS 4 h later. The molar ratio was (1 − x)TEOS : xTEVS : 0.017P123 : 162H₂O : 5.86HCl, where x = 0.1, 0.2, 0.3 or 0.4. The resulting viscous mixture was stirred at 35 °C for 20 h, followed by an aging step at 100 °C for 24 h. After filtered and thoroughly washed with a large quantity of deionized water, the surfactant was removed using a solution of 30 mL of aqueous HCl (37%) in 250 mL of ethanol per 5 g of sample. This mixture was refluxed for 12 h and filtered under reduced pressure, and then repeated twice. The resulting powder was washed with ethanol and water three times, and then dried in vacuum at 60 °C for 24 h. The vinyl groups functionalized mesoporous silicas were labelled as SBAVx, where x is the mole ratio of TEVS/(TEOS + TEVS).

Dipropylphosphine oxides were prepared according to the method reported by Hays.³⁵ Then, propylphosphine oxide modified mesoporous silicas SBA-P were synthesized by the P–H addition reaction.³³ Specifically, 1 g dry SBAVx, 0.1 g AIBN and 0.5x mol Pr₂PH(O) were refluxed in toluene at 125 °C for 12 h under Ar atmosphere. Toluene were distilled from Na/benzophenone. After filtration, the functionalized products were obtained by washing with ethanol and water for several times and drying in vacuum at 60 °C for 24 h.

Characterization

Small angle X-ray diffraction (XRD) patterns were obtained on a Rigaku D/max-2400 X-ray powder diffractometer using Cu-Kα radiation. Solid-state magic-angle spinning nuclear magnetic resonance (MAS NMR) spectra were recorded on a Bruker AV300 NMR spectrometer. ³¹P spectra were measured at 121.5 MHz using 4 mm rotors spinning at 7 kHz. ¹³C CP-MAS spectra were measured at 75.5 MHz using a 7 mm rotor spinning at 6.5 kHz. And ²⁹Si MAS NMR spectra were measured at 59.6 MHz using 7 mm rotors spinning at 5 kHz. The chemical shifts of ²⁹Si and ¹³C are relative to tetramethylsilane (TMS), and that of ³¹P is relative to phosphoric acid. The N₂ adsorption/desorption isotherms were measured at 77 K using a Surface Area and Porosity Analyzer (Nava 3200e). Prior to N₂ sorption experiments, the samples were outgassed at 373 K for 4 h. Brunauer–Emmett–Teller model was used to calculate the specific surface areas. And pore size distributions were calculated using Barrett–Joyner–Halenda methods. Elemental analyses was performed on a Flash EA 1112 Element Analyzer.

Transmission electron microscopy (TEM) was performed using a JEOL JEM1230 at an accelerating voltage of 100 kV with a LaB6 filament. The powders were dispersed in ethanol and sonicated for 15 min in a sonic bath. Then, 5 μL of the suspension was placed onto a nickel/formvar grid and allowed to dry before the observation.

Adsorption experiments

Solutions of U(VI) were prepared by dissolving uranium oxide in HNO₃ and further diluted with deionized water. The pH of solutions was adjusted by adding diluted nitric acid or sodium hydroxide solutions. The batch experiments were carried out in a thermostatic water bath at 25 °C.

In a typical experiment, 10 mg of functionalized mesoporous silicas was added into 10 mL U(VI) solution in a flask and stirred for 24 h. The filtration was obtained with 0.22 μm filter membranes.

The initial and final concentrations of the U(VI) in solutions were determined by Arsenazo III Spectrophotometric Method. The analysis was carried out at the wavelength of 652 nm with a quartz cuvette of 1 cm path length, and the adsorption capacity (q, mg g⁻¹) were calculated by the following formula:

in which C_i and C_f are the initial and final concentration. V and M designate the volume of the solution and weight of the mesoporous silicas used in the adsorption experiments, respectively. Batch experiments were performed in triplicates, with the uncertainty within 5%.

The kinetic study was performed in the same fashion except that the weight of adsorbent and the volume of test solution were increased to 30 mg and 60 mL, respectively. This could minimize the change (<3%) in liquid-to-solid ratio due to sampling at different times.

For column experiment, 200 mg sample was weighed and uniformly packed in a column with an internal diameter of 8.9 mm. The packing length was about 5 mm. The column was preconditioned by nitric acid or pH solution optimized through batch tests prior to use. 6 mL solution containing 1.0 g L⁻¹ U(VI) was pumped through the column at the flow rate of 0.5 mL min⁻¹.

5 wt% K₂CO₃ solution was used as eluent for unloading the U(VI) from the adsorbents. The extraction capacity of the adsorbents (q) can be defined as the amount of extracted uranium ion (mg) divided by the net weight of the engaged adsorbents (g), which can be easily obtained based on the principle of mass conservation.

Results and discussion

Materials characterization

The structure of the alkylphosphine oxide modified mesoporous silicas was studied by MAS NMR analysis. Fig. 1 shows the ¹³C MAS NMR spectra of SBAVx (left) and SBAVx-P(O)Pr₂ (right). Firstly, the SBAVx results confirm the presence of vinyl functionalities introduced into the silica frameworks. The intense peaks at 129 ppm (1) and 134 ppm (2) are assigned to the vinyl carbon atoms, and the peaks at 59 ppm (3) and 17 ppm (4) correspond to the residual ethoxy groups. The signal intensity of vinyl carbon peaks increases with the added molar ratio of TEVS (x). After the reaction with HP(O)Pr₂, the signals of vinyl carbons are almost disappeared in the NMR spectra of the functionalized products SBAVx-P(O)Pr₂. The appearance of signal peaks at 4 ppm (5), 30 ppm (6, 7), 17 ppm (8, 9) are attributed to the carbon atoms in the n-propyl chain and the saturated vinyl groups between Si and P atoms. These results confirm that the phosphine oxide ligands were covalently and integrally attached into mesoporous silicas.

Fig. 1 ¹³C MAS NMR spectra of SBAVx (left) and SBAVx-P(O)Pr₂ (right).

Fig. 2 shows the ²⁹Si MAS NMR spectra of SBAVx (left) and ³¹P MAS NMR spectra of SBAVx-P(O)Pr₂ (right). For SBAVx, the signals are associated to the T_m groups (vinyl-Si(OSi)_m(OH)_3−m) and Q_n groups (Si(OSi)_n(OH)_4−n) of the silica framework. The peaks at −70 ppm and −78 ppm represent T₂ and T₃ signals, respectively. The three peaks within the range from −94 ppm to −110 ppm are assigned to the Q₂, Q₃, and Q₄ silicon species. With the increasing of x, the ratio of signal intensity is increased for T_m and decreased for Q_n. This is in accordance with the change of carbon signals in SBAVx. With the increase of vinyl group contents in SBAVx, the alkylphosphine oxide ligand contents obtained by adding reaction is increased. The existence of the alkylphosphine oxide ligands in SBAVx-P(O)Pr₂ can be further confirmed by ³¹P MAS NMR with an intense peak at about 50 ppm, which is ascribed to the P atom in the alkylphosphine oxide groups.

Fig. 2 ²⁹Si MAS NMR spectra of SBAVx (left) and ³¹P MAS NMR spectra of SBAVx-P(O)Pr₂ (right).

The quantitative amount of the alkylphosphine oxide ligands (P=O) anchored to the mesoporous silicas was determined by element analysis. The results were summarized in Table 1. Apparently, with more addition of TEVS monomers in the synthesis, the carbon content in SBAVx show an increase because more vinyl groups were introduced to the mesoporous silica framework. Then, the carbon content in SBAVx-P(O)Pr₂ further increased after the reaction with HP(O)Pr₂. Based on a comparison of the carbon content data before and after the adding reaction, the alkylphosphine oxide ligands content was obtained which also showed an increase with the molar ratio of TEVS (x).

Table 1 Specific surface area, average pore size, C content and P=O content of the mesoporous silicas

x	SBAVx			SBAVx-P(O)Pr2
	C content (wt%)	Specific surface area (m^2 g^−1)	Average pore size (nm)	C content (wt%)	P=O content (mmol g^−1)	Specific surface area (m^2 g^−1)	Average pore size (nm)	Availability of P=O ligands (%) in 2 mol L^−1 HNO3
0.1	7.5	667	5.92	10.3	0.451	589	5.14	6.3
0.2	10.6	673	5.51	15.1	0.779	328	4.51	7.1
0.3	12.7	688	4.32	18.5	1.041	409	3.86	9.0
0.4	15.7	637	4.51	21.7	1.179	365	3.85	9.2

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The ordered mesoporous structure of SBAVx and SBAVx-P(O)Pr₂ was examined by small angle X-ray diffraction (SAXD). It can be seen in Fig. 3, both the precursor SBAV0.1 and the alkylphosphine oxide functionalized mesoporous silica SBAVx-P(O)Pr₂ show three well-resolved peaks, which can be indexed as (100), (110) and (200) diffractions, respectively. When more vinyl functionalities were incorporated in the co-condensation synthesis, the ordering degree of the mesoporous framework is decreased. As a result, the (110) and (200) diffractions are not obvious in the other samples. However, the intense (100) peaks suggest a two-dimensional hexagonal P6mm symmetry in all the samples. After the adding reaction with HP(O)Pr₂, the XRD patterns in SBAVx-P(O)Pr₂ (red circles) are almost the same with SBAVx. This result indicates that the adding reaction has little effect to the structural ordering of the mesoporous silicas.

Fig. 3 SAXD patterns of the mesoporous silicas SBAVx and SBAVx-P(O)Pr₂.

The 2-D hexagonal nature of the pore structure of mesoporous samples was also confirmed by transmission electron microscopy, as shown in Fig. 4. Both the precursors and the alkylphosphine oxide functionalized mesoporous silicas display ordering arrays of uniformly sized ‘pores’ or mesoporous ‘tubes’ separated by cylindrical organosilica rods. The 2-D P6mm hexagonal symmetry can be clearly observed.

Fig. 4 Transmission electron microscopy photographs of SBAVx and SBAVx-P(O)Pr₂.

The nitrogen adsorption–desorption isotherms of the mesoporous silicas are shown in Fig. 5. All the samples show a type IV isotherm with clear H1-type hysteresis loops at high relative pressure (0.4–0.8), which corresponds to the mechanism of monolayer–multilayer adsorption, followed by capillary condensation.³⁴ Compared with SBAVx, the nitrogen adsorption quantity of SBAVx-P(O)Pr₂ shift to lower dramatically, reflecting a decrease in specific surface areas due to the introduction of alkylphosphine oxides.

Fig. 5 The N₂ adsorption–desorption isotherms of SBAVx and SBAVx-P(O)Pr₂.

The pore size distribution patterns of the samples are shown in Fig. 6. All the patterns have a sharp and narrow peaks, implying a uniform pore size. Compared with SBAVx, the peaks of SBAVx-P(O)Pr₂ shift to relatively lower pore diameter, reflecting a decrease in pore size because of the introduction of alkylphosphine oxides. The specific surface area and average pore diameter of all the samples are listed in Table 1. Evidently, for the alkylphosphine oxide functionalized mesoporous silicas SBAVx-P(O)Pr₂ samples, the specific surface areas as well as the average pore diameters reduced compared to that of the precursors SBAVx.

Fig. 6 The pore size distribution curves of SBAVx and SBAVx-P(O)Pr₂.

The adsorption performance of the mesoporous silicas towards U(VI) in HNO₃ media was investigated by the batch operation. Aqueous phases with a wide concentration range of HNO₃ (1 × 10⁻⁶ mol L⁻¹ to 2 mol L⁻¹) were employed. The results are shown in Fig. 7. It can be seen that the all the alkylphosphine oxide functionalized mesoporous silicas SBAVx-P(O)Pr₂ showed adsorption ability in strong HNO₃ solutions. When the pH value of aqueous phase was less than 3, the adsorption capacity increased with the molar ratio (x) of the alkylphosphine oxide ligands anchored to the mesoporous silica framework. For each sample, varying the HNO₃ concentration did not result in a significant change of their adsorption capacity of U(VI) ions. But interestingly, in weak acidic solutions with pH value >3, the adsorption ability of SBAVx-P(O)Pr₂ showed a negative correlation with x.

Fig. 7 Adsorption of SBAVx-P(O)Pr₂ to uranium in HNO₃ solutions with different concentrations.

This phenomenon could be explained by different binding mechanisms. Under strong acidic conditions, the alkylphosphine oxide ligands were responsible for the binding of U(VI) ions. However, the silanol groups, which also possessed good affinity to U(VI), played a major role in weak acidic to neutral solutions.^18,33,36 For the SBAVx-P(O)Pr₂ samples, more introduction of the organic functionalities led to a decrease in density of the silanol groups. Meanwhile, a large number of alkylphosphine oxide ligands with steric hindrance had a screen effect, preventing the contact between silanol groups and U(VI) ions. So, the SBAVx-P(O)Pr₂ samples with higher x showed poor adsorption ability to U(VI) in weak acidic solutions. These results are consistent with our previous study,³³ in which we had a detailed comparison for the U(VI) adsorption by mesoporous silica framework with or without alkylphosphine oxide ligands. Overall, it can be concluded that, the acidity of aqueous phase influenced the U(VI) competition between the alkylphosphine oxide ligands and the silanol groups. And, under different acidic conditions, density of the ligands or silanol groups, as well as their accessibility, contributed to the adsorption ability of the mesoporous silicas.

Besides, based on the adsorption capacity in 2 mol L⁻¹ HNO₃ and the alkylphosphine oxide ligands (P=O) content in the SBAVx-P(O)Pr₂ samples, the availability of P=O, defined as percentage of the coordinated alkylphosphine oxide ligands, was calculated according to a 2 : 1 P(O)Pr₂ to U(VI) coordination.³² The results in Table 1 show that the availability of P=O increases with the increase of addition of TEVS monomers. However, it is noteworthy that the highest availability of P=O is no more than 10%. For most of the alkylphosphine oxide ligands, it seems to be hard to coordinate with U(VI) by a bidentate manner due to steric factors.

Adsorption isotherms

As discussed above, the alkylphosphine oxide ligands or the silanol groups are the main contribution for the coordination with U(VI) ions, which is dependent on the acidity of the aqueous phase. So, the functionalized mesoporous silicas are expected to show different adsorption behaviors toward U(VI) ions according to different coordination mechanisms. To verify this assumption, we further investigated the adsorption ability of a selected sample SBAV0.3-P(O)Pr₂ to U(VI) ions in 2 mol L⁻¹ and pH = 5 HNO₃ solutions, respectively.

Equilibrium studies were carried out by using a fixed mass of SBAV0.3-P(O)Pr₂ (0.0100 g) contact with 10 mL HNO₃ solutions with varying initial U(VI) concentrations from 50 to 500 mg L⁻¹. The results in Fig. 8 show that the U(VI) adsorption capacity increased rapidly at low concentration range of uranium concentrations, followed by a saturation trend. Due to the affinity of a large number of silanol groups, the mesoporous silicas exhibited better adsorption ability to U(VI) in pH = 5 solution. While in 2 mol L⁻¹ HNO₃ solution, an adsorption capacity more than 10 mg g⁻¹ was obtained. Therefore, the alkylphosphine oxide functionalized mesoporous silicas should have potential for uranium adsorption at different circumstances.

Fig. 8 Adsorption isotherms of U(VI) ions in 2 mol L⁻¹ and pH = 5 HNO₃ solutions by SBA0.3P(O)Pr₂.

Adsorption isotherms were studied by using the Langmuir and Freundlich models to fit the experimental data. The fitted parameters are summarized in Table 2. With better correlation coefficients R², the Langmuir model was more suitable to describe the adsorption behavior of SBAV0.3-P(O)Pr₂ to U(VI) in both 2 mol L⁻¹ and pH = 5 HNO₃ solutions. This implies that the adsorption of U(VI) ions to the functionalized mesoporous silicas is more likely to be a monolayer adsorption process via the coordination with the alkylphosphine oxide ligands or the silanol groups in the surface of the silica framework.

Table 2 Parameters of the thermodynamic and kinetic models for the SBAv0.3-P(O)Pr₂ adsorption behaviors to U(VI)

HNO3 media	Thermodynamic models						Kinetic models
	Langmuir			Freundlich			Pseudo first order model			Pseudo second order model
	Kl^a	Qm^b	R^2	Kf^c	n^d	R^2	K1st^e	Qe^f	R^2	K2nd^g	Qe	R^2
a Qm is the theoretical maximum adsorption capacity, the unit is mg g^−1.b Kl is a binding constant in the Langmuir isotherm, the unit is L g^−1.c Kf is the Freundlich constant with the unit of L g^−1.d n is the Freundlich constant.e K1st is the constant of the pseudo first order kinetic models, the unit is min^−1.f Qe refers to the theoretical value of the adsorption to U(VI), the unit is mg g^−1.g K2nd is the constant of the pseudo second order kinetic models, the unit is g mg^−1 min^−1.
pH = 5	0.075	56.0	0.998	35.24	13.6	0.830	0.042	9.8	0.936	0.006	12.5	0.996
2 mol L^−1	0.029	13.4	0.970	6.31	9.0	0.505	0.049	32.6	0.956	0.003	49.9	0.999

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Adsorption kinetics

The adsorption kinetics of the SBAV0.3-P(O)Pr₂ to U(VI) were investigated by batch experiments also in 2 mol L⁻¹ and pH = 5 HNO₃ solutions. The adsorption capacity of U(VI), as a function of the contact time, is plotted in Fig. 9. The adsorption behaviors in HNO₃ solutions with different acidity are similar to each other. Immediately after the contact with the aqueous phase, the loading of the U(VI) ions grows rapidly. Adsorption equilibrium was obtained in tens of minutes, indicating that the mesoporous silicas functionalized with alkylphosphine oxide ligands have fast adsorption kinetics.

Fig. 9 U(VI) adsorption kinetics in 2 mol L⁻¹ and pH = 5 HNO₃ solutions by SBA0.3P(O)Pr₂.

Several kinetic models were employed to fit the experimental data in order to describe the adsorption process. The kinetic parameters are summarized in Table 2. The pseudo second order model gave the best fitting results with correlation coefficient R² > 0.99, implying that rate limiting step may be not the diffusion process but the coordination between U(VI) and the alkylphosphine oxide ligands or the silanol groups dependent on the acidity of the aqueous phase.

Elution and reusability

The column operation of SBAV0.3-P(O)Pr₂ toward U(VI) ions was carried out, and elution conditions and reusability of the mesoporous silicas were investigated. Carbonate is a typical eluent used in TRPO process for the elution of U(VI).³² Desorption of the loaded U(VI) from the mesoporous silicas packed in the column was performed by using 5% K₂CO₃ solutions at a flow rate of 1.0 mL min⁻¹. Fig. 10 and 11 show the adsorption capacity of U(VI) in 2 mol L⁻¹ and pH = 5 HNO₃ solutions, as well as the elution rate in three cyclic operations. It can be seen that the adsorption capacity only showed a slight decrease in the cyclic experiments, while the elution rate maintained as constant, which was in agreement with that in the first run. All the loaded U(VI) ions could be effectively eluted by using carbonate solutions, and the column had favorable regeneration efficiency. These results suggest that the alkylphosphine oxide ligands and/or the silanol groups were well-preserved during the adsorption in strong acidic or pH solutions, and that the alkylphosphine oxide ligands functionalized mesoporous silicas have an excellent reusability for U(VI) adsorption.

Fig. 10 Column adsorption of U(VI) in 2 mol L⁻¹ HNO₃ (left) and elution of U(VI) by 5% K₂CO₃ solution (right) with cyclic operations.

Fig. 11 Column adsorption of U(VI) in pH = 5 HNO₃ solution (left) and elution of U(VI) by 5% K₂CO₃ solution (right) with cyclic operations.

Conclusions

In this study, new mesoporous silicas functionalized with high density of alkylphosphine oxide ligands were developed. The adsorption behavior toward U(VI) ions associated with different binding mechanism was comprehensively studied in HNO₃ solutions with different acidity. The mesoporous silicas showed effective adsorption ability toward U(VI) ions in strong HNO₃ media (pH < 3), which is of importance for the uranium recovery in acidic radioactive liquid waste (RLW). Meanwhile, in pH > 3 HNO₃ solutions, higher U(VI) adsorption capacity was obtained due to the great affinity of the silanol groups with easy accessibility. The fast adsorption rate was another distinctive feature of the alkylphosphine oxide functionalized mesoporous silicas. Column experiments showed the loaded U(VI) could be efficiently eluted and the mesoporous silicas had a reliable performance during cyclic operations. This class of alkylphosphine oxide functionalized mesoporous silicas may be of potential for the preconcentration and adsorption of uranium from acidic aqueous solutions.

Acknowledgements

The study was supported by Program for Changjiang Scholars and Innovative Research Team in University (IRT13026), National Science Fund for Distinguished Young Scholars (51425403) and National Natural Science Foundation of China under Project No. 51473087, 91226110 and U1430234.

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