Amidoxime-functionalized hydrothermal carbon materials for uranium removal from aqueous solution

Abstract

A novel solid-phase extractant, i.e., amidoxime-functionalized hydrothermal carbon (AO-HTC), has been synthesized, with acrylonitrile grafted starch serving as the precursor, via a simple, economic and green hydrothermal reaction to separate uranium from nuclear industrial effluents selectively. The characterization and analysis indicated that abundant amidoxime groups (approximately 6.56 mmol g⁻¹) were introduced onto the surface of hydrothermal carbon. The AO-HTC was applied to adsorb U(VI) from aqueous solutions and exhibited a high sorption capacity towards U(VI) due to the strong chelation between amidoxime groups and U(VI). The main results are as follows: (1) AO-HTC could reach its limiting saturation capacity of 724.6 mg g⁻¹ at pH = 5.0 and T = 298.15 K in pure-U(VI) solution, which was an improvement of approximately 18.2 times relative to the raw hydrothermal carbon; (2) the selectivity was above 60% for a wide pH range from 1.0 to 5.0, which has not been reported to date, and reached a maximum of 68.92% in a weak acidic multi-cation solution (pH = 2.0); (3) the uranium sorption on the AO-HTC was a pH-dependent, endothermic and spontaneous process. Furthermore, possible mechanisms for the selective recognition of uranyl ions onto the AO-HTC surface were explored based on experimental characterization and chemical rationales.

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Introduction

Adsorption, due to its high efficiency and ease of handling, has been developed for the removal and recovery of uranium from radioactive wastewater in consideration of the dual significance of eliminating potential environmental health threats and recovering the nonrenewable resource of nuclear energy.¹ Carbon-family materials, such as activated carbon,² carbon nanotubes,³ carbon fibers,⁴ mesoporous carbon,⁵ etc., have been gradually used as effective adsorbents because of their higher thermal and radiation resistance, compared with organic exchanger resins, and better acid–base stability and simpler reprocessing, compared with the familiar inorganic sorbents.²

Moreover, as a new carbon-family material, hydrothermal carbon (HTC) is a type of semi-carbonized material⁶ that can be prepared from mono- and polysaccharides, such as glucose,⁷ sucrose,⁸ fructose,⁹ starch,¹⁰ and cellulose¹¹ because of its low cost, mild reaction conditions and it being absolutely “green”, i.e., it excludes organic solvents, catalysts or surfactants.¹² However, there are only a few oxygen-containing functional groups (such as –OH and –COOH) acting as the adsorption activity sites on the surface of these materials,¹³ which causes low adsorption capacity and weak selectivity of uranium. Therefore, the surface modification of HTC by introducing various functional groups is often required to enhance the enrichment capacity and selectivity of uranium.

Generally, surface functionalized HTC can be obtained using two main approaches. One is incorporating a specific polymer¹⁴ or choosing a typical precursor^15,16 with exclusive functional groups during the hydrothermal process, but this requires critical selectivity of the substrate materials and strict control of the synthesis conditions. The other is surface grafting modification, in which functional components, such as amidoxime, 5-azacytosine and salicylideneimine, can be grafted onto the surface of a carbon matrix via covalent bonds, hydrogen-bonding interactions and electrostatic effects.^17,18 Nevertheless, these approaches require choosing or synthesizing specific compounds containing active functional groups with high affinity for the target uranyl ion. Subsequently, the fixation of the functional components onto the HTC matrix also requires a complicated and multi-step synthesis procedure.

Amidoxime (AO) group, one of the most effective chelating functional groups, has attracted special attention for the removal of uranium from various aqueous solutions because of its high selectivity and affinity for uranium.^19,20 Inclusion of an amidoxime ligand on carbon materials has been achieved via functionalization during synthesis¹⁴ or post-synthesis surface grafting,²¹ where a covalent bond is formed between carbon materials and functional components generated in situ.²² However, the condition of post-synthesis surface grafting is usually harsh; thus, the grafting ratio is low and the sorption capacity is not high enough. Therefore, further investigation to improve the loading capacity of HTC towards uranium is of great importance.

Starch, a natural macromolecular compound and an important green chemical raw material, can be easily grafted by using vinyl monomers such as acrylonitrile.²³ Because the nitrile group on the surface of starch can be reserved during the hydrothermal process, we propose that a novel amidoxime-functionalized hydrothermal carbon could be prepared by employing nitrile group grafted starch as the precursor for the first time in this work. Our goal is to prepare hydrothermal carbon with high enrichment capacity and selectivity and good stability at a low cost, which could serve as a promising sorbent in uranium pollution clean-up and uranium enrichment.

Experimental

Preparation of amidoxime-functionalized hydrothermal carbon (AO-HTC)

The procedure for the preparation of AO-HTC is illustrated in Scheme 1. Typically, 5 g of soluble starch and 0.15 g of ceric ammonium nitrate were dissolved in 100 mL of distilled water and evenly stirred to obtain a homogenous dispersion; then, 2 mL of acrylonitrile was added to it slowly. Subsequently, the reaction system was heated at 35 °C for 2 h with stirring, filtered and washed using DMF and anhydrous ethanol, respectively, and then dried at 50 °C in a vacuum oven for 12 h. The resulting material was acrylonitrile grafted starch (AN-starch).

Scheme 1 Schematic illustration of the full synthesis processes for AO-HTC.

Then, 2.0 g of AN-starch was dissolved in 15 mL of distilled water, evenly stirred for 30 min and then transferred to a 20 mL Teflon-lined stainless steel autoclave, which was then heated at 180 °C for 16 h. The black product was washed thoroughly using deionized water, ethanol and acetone alternately and then dried in a vacuum oven at 50 °C for 12 h (denoted as CN-HTC).

Finally, 1.0 g of CN-HTC was added to 30 mL of 3% hydroxylamine hydrochloride solution (pH = 7.0) and stirred at 75 °C for 3 h. Then, the mixture was filtered; the resultant was extensively washed using deionized water and ethanol and then dried at 60 °C in vacuum overnight (denoted as AO-HTC).

Results and discussion

Characterization

SEM. The typical SEM images (Fig. 1) of the as-prepared materials showed large amounts of spherical nanoparticles after the hydrothermal carbonization; moreover, the surface was smooth and regular. The HTC (Fig. 1a) had a diameter of 3–8 μm, but that of AO-HTC (Fig. 1b) reduced to 2–4 μm because the acrylonitrile groups on the surface hindered the starch from hydrolyzing to corresponding monosaccharides during the first step of the hydrothermal process.²⁴

Fig. 1 SEM images of (a) HTC and (b) AO-HTC.

FT-IR. To investigate the type of functional groups and the relevant changes after nitrile and amidoxime functionalization, FT-IR analysis was conducted; the results are shown in Fig. 2. The spectrum of HTC (Fig. 2a) show the characteristic peaks at 1604 and 3248 cm⁻¹, which are related to both C=O carboxylate and O–H groups, respectively.²⁵ A comparison of the FT-IR spectra of HTC and AN-HTC (Fig. 2a and b) shows that a new sharp band appeared at 2242 cm⁻¹, corresponding to the characteristic vibration absorption band of C≡N¹⁷ indicating that the nitrile group can be reserved during the hydrothermal process of AN-starch. Meanwhile, when comparing the spectra of AN-HTC and AO-HTC (Fig. 2b and c), the stretching band of C≡N at 2242 cm⁻¹ disappeared; herein, three new bands at 1631 cm⁻¹, 1274 cm⁻¹ and 927 cm⁻¹, belonging to C=N, C–N and N–O stretching vibrations, respectively, were observed,^17,22 clearly indicating that the nitrile groups have been converted to amidoxime groups in the presence of hydroxylamine hydrochloride.

Fig. 2 FT-IR spectra of (a) HTC, (b) AN-HTC, and (c) AO-HTC.

XPS. XPS analysis was used to investigate the surface chemical composition and bonding properties of HTC and AO-HTC. As shown in Fig. 3a, the peaks of C 1s and O 1s were seen for the expected components in AO-HTC, and the N 1s level was also detected. The C 1s core-level spectrum of HTC (Fig. 3b) can be curve-fitted using three peak components, with BEs at 284.45 eV, 285.85 eV and 288.5 eV for the C=C, C–O and O–C=O species, respectively;¹⁵ compared with the HTC, there is a new peak with BE at 286.8 eV, assigned to H₂N–C=N–OH in AO-HTC.²⁶ Moreover, the N 1s spectrum of AO-HTC (Fig. 3c) could be separated into two peaks at 399.19 eV and 400.21 eV, corresponding to the nitrogen of the NH₂ species (C–N̲H₂) and oxime nitrogen (C=N̲OH) in the amidoxime group.³ The successful functionalization of the amidoxime group in the AO-HTC can be deduced from the appearance of the C 1s core-level signal at a BE of 286.8 eV and N 1s signal at BEs of 399.5 eV and 400.21 eV.

Fig. 3 (a) XPS survey spectrum of HTC and AO-HTC, (b) C 1s for HTC and AO-HTC, and (c) N 1s for AO-HTC.

N₂-BET. The N₂ adsorption–desorption isotherm at 77 K and pore size distribution of HTC and AO-HTC are shown in Fig. S1† and Table 1. It can be observed that the specific surface and pore volume of HTC and AO-HTC were very small due to their smooth surfaces, which also confirmed by the results from SEM. Herein, only the amidoxime group on the external surface could bind uranyl ions.

Table 1 Specific surface, pore volume and average pore diameter of HTC and AO-HTC

	HTC	AO-HTC
Surface area SBET (m^2 g^−1)	1.66	2.75
Pore volume (cm^3 g^−1)	0.02	0.02
Average pore diameter (nm)	5.53	7.93

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Elemental analysis

Elemental analysis was further performed to calculate the exact amount of amidoxime groups in AO-HTC; the results are shown in Table 2. The nitrogen content of AO-HTC (9.45%) increased more than that of pristine HTC (0.26%), indicating that the amidoxime groups were grafted onto the surface of HTC, as supported by the previous FT-IR and XPS analyses. The amount of amidoxime groups was calculated based on the increment of the content of nitrogen to be approximately 6.56 mmol g⁻¹.

Table 2 Element analysis of HTC and AO-HTC

Samples	C%	H%	N%
HTC	78.19	5.78	0.26
AO-HTC	76.63	4.92	9.45

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Zeta potential

Fig. 4 presents the zeta potential curves of HTC and AO-HTC and indicates that the zeta potential of AO-HTC was positive in a wider pH range (pH < 4.69) due to the protonation of N atoms of –NH₂ in the amidoxime groups.²⁷ Moreover, the zeta potential of AO-HTC decreased gradually with increasing pH value due to the synergism of the decrease of the protonation degree of –NH₂ in the amidoxime groups of AO-HTC and the increase of the dissociation degree of the oxime hydroxyl groups (the amidoxime groups were amphoteric,²⁸ and the dissociation of oxime hydroxyl groups produced negative ions). After pH > 4.69, the zeta potential of AO-HTC became negative due to the amidoxime groups deprotonating severely and the dissociation of oxime hydroxyl groups increasing, which was favorable for binding with the positive radionuclides ions.²⁹ Moreover, the point of zero charge of AO-HTC (pH_PZC = 4.69) was much lower than that of HTC (pH_PZC = 3.16), which was directly associated with the greater amount of amidoxime groups on the surface of AO-HTC.

Fig. 4 Zeta potentials of HTC and AO-HTC as a function of pH.

Batch sorption experiments and interaction mechanism

Effect of pH. The pH value of an aqueous solution is an important parameter for U(VI) sorption due to its effect on the surface charge of the sorbent and the speciation of the metal ions.³⁰ As shown in Fig. 5, the sorption amount of U(VI) on the two sorbents increased with increasing pH value; the sorption amount of AO-HTC (455.6 mg g⁻¹) was remarkably higher than that of HTC (26.7 mg g⁻¹) at pH 5.0, indicating that the sorption process is clearly pH-dependent. The pH value investigated was limited to less than 5.0 because uranyl ions in the designed system would precipitate at higher pH values according to species distribution for U(VI) hydrolysis.³ The low sorption at lower pH could be due to the protonation of the oxime and imino groups of amidoxime on the AO-HTC and the competition of H⁺ with the active sites.³¹ At low pH values, the hydroxyl groups of oxime and imino group of amidoxime on AO-HTC will be highly protonized, resulting in decreasing the oxime's nucleophilicity towards U(VI) and limiting the U(VI) sorption onto the sorbent.³ As the pH values increase, the protonation degree of the oxime groups will be weakened and the hydroxyl proton in the oxime group will be easy to strip off,¹⁹ allowing the lone-pair electron on the negatively charged oxygen to be more prone to occupy the empty orbits of the uranium atom and consequently favoring the formation of the complex.^19,32 Apart from the adsorption process from the bulk solution due to concentration gradient into fabricated adsorption sites, there was also a parallel formation of polymeric species such as UO₂(OH)⁺, (UO₂)₂(OH)₂⁺ and (UO₂)₃(OH)₅⁺ which precipitated out of solution.³³ Therefore, an optimum pH value for effective separation was regarded as 5.0 for further studies.

Fig. 5 Effect of pH on U(VI) sorption on HTC and AO-HTC (w = 10 mg, C₀ = 50 mg L⁻¹, t = 240 min, T = 298.15 K, and V = 100 mL).

Effect of contact time and kinetic studies. The effect of contact time on U(VI) adsorption onto HTC and AO-HTC is shown in Fig. 6. It was evident that the sorption amount of U(VI) increased rapidly during the first 60 min due to the higher active site availability for U(VI) sorption, which occupied over 85% of the total sorption capacity during this stage and then gradually tended toward equilibrium.

Fig. 6 Effect of contact time on U(VI) adsorption onto HTC and AO-HTC (pH = 5.0, C₀ = 50 mg L⁻¹, V = 100 mL, m = 10 mg, and T = 298.15 K).

Three kinetic models, i.e., the pseudo-first-order model, pseudo-second-order model and intraparticle diffusion model, were employed to investigate the controlling mechanism of the adsorption process. The linear forms of the three models can be expressed as eqn (1)–(3), respectively.^34,35

ln(q_e − q_t) = ln q_e − k₁t (1)

q_t = k_intt^0.5 + C (3)

where k₁ (min⁻¹) and k₂ (g mmol⁻¹ min⁻¹) refer to the kinetic constants for the pseudo-first-order model and pseudo-second-order model, respectively, q_t is the amount of U(VI) sorbed (mg g⁻¹) at any time t, k_int (mmol g⁻¹ min^−0.5) is the intraparticle diffusion rate constant, and C (mmol g⁻¹) is the constant proportional to the extent of the boundary layer thickness.

The linear plots of the aforementioned three models are given in Fig. S2† and the values of constants obtained from the slopes and intercepts of the fitted curves are shown in Table S1.†

Azizian point out the theoretical basis for the pseudo-first-order and pseudo-second-order kinetics models, which predict the sorption process obeys pseudo-second-order kinetics at lower initial concentration of solute.³⁶ In the present work, the U(VI) concentration in aqueous solution was kept as 50 ppm and 10 ppm located on the same number order, which is significantly lower than the concentration of U(VI) (184 ppm).³⁷ Additionally, the higher correlation coefficient value (R² = 0.99) with q_e,cal (467.3 mg g⁻¹) closer to q_e,exp (455.6 mg g⁻¹) suggested the second-order nature of the present sorption process, implying that the process of U(VI) adsorbing onto the AO-HTC might be regarded as chemisorption involving valance forces through the sharing or exchanging of electrons between the adsorbent and adsorbate.¹⁵

The results in Fig. S2c† show that the plot of q_t vs. t^0.5 is not linear for the whole range of reaction time, implying that the intra-particle diffusion was not the only rate-controlling step. More specifically, the process could be divided into three straight parts corresponding to three mechanisms.³⁸ The initial rapid uptake rate due to immobilizing uranyl ions to active sites on the external surface was controlled via surface adsorption. In the case of reaching saturation, the uranyl ions began to migrate into AO-HTC particles via the pores and were adsorbed by the interior surface, where intra-particle diffusion was the rate-determining step. Then, the adsorption equilibrium was reached as the intra-particle diffusion started to slow down. Therefore, the intraparticle diffusion negligibly occurred during the process of U(VI) uptake by AO-HTC due to its poor porosity.

Effect of initial concentration and isotherm studies. The adsorption isotherm of U(VI) is presented in Fig. 7; the sorption amount of U(VI) increased with increasing equilibrium U(VI) concentration. The maximum amount of U(VI) that adsorbed on the AO-HTC was approximately 724.6 mg g⁻¹ under this system, which is much higher than that of the other carbon materials, except for the amidoxime-functionalized hydrothermal carbon, prepared by using the simple small molecule carbon source strategy,¹⁵ listed in Table 3. The higher U(VI) sorption capacity of AO-HTC could be explained by there being more amidoxime groups on the AO-HTC with nitrogen and oxygen-containing active sites chelating the uranyl species. The AO-HTC with such a high sorption ability towards U(VI) exhibited great potential for applications regarding the removal and recovery of U(VI) from large volumes of aqueous solutions.

Fig. 7 Effect of initial uranium concentration on the adsorption of uranium onto HTC and AO-HTC (pH = 5.0, V = 100 mL, m = 10 mg and T = 298.15 K).

Table 3 Comparison of sorption capacity of U(VI) on various carbon materials

Sorbents	Experimental conditions	Capacity (mg g^−1)	Ref.
Amidoxime-functionalized GONRs	pH = 4.5, T = 298.15 K	502.6	30
Amidoximated magnetite/graphene oxide	pH = 5.0, T = 298.15 K	284.9	22
AO-g-MWCNTs	pH = 4.5, T = 298.15 K	145.0	3
AO/CMK-3	pH = 5.0, T = 298.15 K	238.7	39
HCC produced with chitosan	pH = 7.92, T = 338.15 K	264.55	40
CSs synthesized with glucose	pH = 6.0, T = 298 K	68.71	13
HTC-PO4 synthesized in the presence of phosphoric acid	pH = 5.0, T = 298.15 K	285.7	41
HTC produced with pine needles	pH = 6.0, T = 298 K	62.7	42
HTC-AO	pH = 4.5, T = 298.15 K	1021.6	14
HCSs through low-temperature heat treatment	pH = 7.0, T = 298.15 K	179.95	43
Salicylideneimine-functionalized HTC (HTC-sal)	pH = 4.3, T = 288.15 K	261	18
Phenolic ligand-functionalized HTC (HTC-btg)	pH = 4.5, T = 298.15 K	307.3	16
Amidoxime-grafted HTC (AO-HTC-DAMN)	pH = 4.5, T = 293.15 K	466	17
5-Azacytosine-functionalized HTC (HTC-Acy)	pH = 4.5, T = 333.15 K	408.36	44
HTC with phosphate group (HCS-PO4)	pH = 6.0, T = 298 K	434.78	45
Amidoxime-functionalized HTC (HTC-AO)	pH = 5.0, T = 298.15 K	724.6	This work

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Furthermore, the Langmuir isotherm, Freundlich isotherm and Dubinin–Radushkevich (D–R) models were used to evaluate the experimental data to describe the specific sorption characteristics more adequately.

The Langmuir isotherm model assumes that the sorption occurs on a homogeneous surface via monolayer sorption according to linear eqn (4).⁴⁶

where q_m is the maximum sorption capacity (mg g⁻¹) and b_L is the Langmuir constant, which is related to the energy of sorption (L mg⁻¹).

The essential characteristics of the Langmuir isotherm can be explained in terms of separation factor R_L, which is defined by eqn (5).²

The Freundlich isotherm model presumes a multilayer sorption with a heterogeneous energetic distribution of active sites; its linear form is eqn (6).⁴⁷

where K_F [mg g⁻¹ (L mg)^1/n] is the Freundlich constant, which is related to the sorption capacity, and n_F is the Freundlich exponent, which is associated with the sorption intensity.

The D–R isotherm can provide more important information regarding chemical and physical properties and is a semi-empirical equation; its linear form is given in eqn (7).^47,48

ln q_e = ln q_DR − βε² (7)

where q_e and q_DR are the sorption amount per mass of sorbent (mol g⁻¹) and the theoretical sorption capacity (mol g⁻¹), respectively. β is a constant related to the sorption energy (mol² kJ⁻²) and ε is the Polanyi potential, calculated using eqn (8).where R is the gas constant (8.314 × 10⁻³ kJ mol⁻¹ K⁻¹) and T is the temperature (K). C_e is the equilibrium concentration of uranium (mol L⁻¹).

The mean sorption energy (E_DR, kJ mol⁻¹) can be obtained from the β value of the D–R isotherm using the following eqn (9).

The correlation coefficients and corresponding parameters of the three models obtained from the slopes and intercepts of the fitted straight lines (Fig. S3†) are listed in Table S2.† The experimental data fit the Langmuir model better than the Freundlich, suggesting that U(VI) that absorbed on the AO-HTC forms a monolayer coverage. The value of R_L (Table S3†) demonstrates that the U(VI) that adsorbed on AO-HTC was favorable and irreversible due to the R_L being between 0 and 1.² Moreover, the U(VI) sorption was more favorable at lower concentration due to the higher R_L value. The experimental data of AO-HTC was also described by the D–R adsorption isotherm model, with a high correlation coefficient of R² = 0.98, from which the E_DR (12.5 kJ mol⁻¹) was within the range of 8–16 kJ mol⁻¹, indicating a chemical adsorption,⁴⁸ which is in agreement with the kinetic studies.

Effect of temperature and thermodynamic studies. The effect of temperature on U(VI) sorption onto HTC and AO-HTC was investigated; the results are shown in Fig. 8. The sorption capacity of U(VI) increased gradually with increasing temperature, indicating that the present process was endothermic.

Fig. 8 Effect of temperature on the U(VI) uptake by HTC and AO-HTC (pH = 5.0, C₀ = 50 mg L⁻¹, V = 100 mL, m = 10 mg and t = 120 min).

To evaluate the thermodynamic feasibility and the nature of the sorption process, several thermodynamic parameters, such as standard free energy (ΔG), standard enthalpy (ΔH) and standard entropy (ΔS), were calculated using eqn (10) and (11).⁴⁹

ΔG = ΔH − TΔS (11)

where K_d is the distribution coefficient (mL g⁻¹) and T and R are the absolute temperature (K) and the gas constant (8.314 J mol⁻¹ K⁻¹), respectively.

The values of ΔH and ΔS, calculated from the slope and intercept of the plots of ln K_d vs. T⁻¹ (Fig. S4†), are listed in Table S4; † the values of ΔG were calculated by using eqn (11). The positive value of ΔH revealed the endothermic nature of the sorption process on AO-HTC, and the positive values of ΔS indicated an increasing randomness at the solid–liquid interface during the sorption. The negative values of ΔG suggested the spontaneous nature of the adsorption process; furthermore, the values of ΔG shifted to a lower negative value as the temperature increased from 283.15 K to 303.15 K, revealing that the spontaneous adsorption of uranium(VI) on AO-HTC was more efficient at higher temperature.⁵⁰ Moreover, the value of ΔG of AO-HTC (−25.48 kJ mol⁻¹ at 283.15 K) was lower than that of HTC (−11.76 kJ mol⁻¹ at 283.15 K), suggesting that the sorption of U(VI) on the AO-HTC was more favorable than that on the HTC.⁵¹

Effect of ionic strength. The effect of ionic strength on the U(VI) sorption capacity of AO-HTC at a NaClO₄ concentration between 0 to 3 mol L⁻¹ was investigated to simulate the practical nuclear industrial effluent with high salinity. As shown in Fig. 9, the uranium sorption decreased with increasing NaClO₄ concentration and then increased as the concentration exceeded 0.05 mol L⁻¹. Similar results for the ionic strength effect have been reported in some previous studies.¹⁵ The decrease of sorption may be due to the formation of outer-sphere complexes between uranyl ions and the active sites on the surface.⁵² Therefore, the competitive adsorption of Na⁺ diminished the further sorption of uranium. However, when the NaClO₄ concentration surpassed 0.05 mol L⁻¹, the electrical double layer was compressed;⁵³ consequently, the uranyl ions could approach the sorbent surface more easily. Therefore, the high concentration of Na⁺ was also more favorable to U(VI) adsorption for the solvent molecules because of the salting-out effect.¹⁶

Fig. 9 Effect of ionic strength on the sorption of U(VI) onto AO-HTC (pH = 5.0, C₀ = 50 mg L⁻¹, V = 100 mL, m = 10 mg, t = 120 min and T = 298.15 K).

Effect of competitive ions. The effect of competitive ions on the selectivity sorption of U(VI) by AO-HTC was investigated in a specially designed multi-ion solution containing 12 co-existing cations at pH values between 1.0 and 5.0; the results are shown in Fig. 10. Both q_e-total and q_e-U increased with increasing pH value, and the U(VI) sorption capacity reached a maximum value of 129.5 mg g⁻¹ at pH 5.0, accounting for approximately 67.7% of the total adsorption amount. Moreover, of note were the trends in S_U, which always remained above 60% over the wide pH range from 1.0 to 5.0 and reached a maximum of 68.92% at 2.0.

Fig. 10 Effect of pH on the selectivity sorption of U(VI) onto AO-HTC in a multi-ion system (m = 0.01 g, V = 25 mL, C₀ = 0.5 mmol L⁻¹ and T = 298 K).

Moreover, the selective separation performances for uranium of both HTC and AO-HTC were compared in the same multi-ion sorption system at pH 5.0. The corresponding results in Fig. 11 and Table S5† indicate that the selectivity coefficients (S_U(VI)/M(x)) of AO-HTC improved significantly for all competing ions compared with that of HTC, especially for Sr(II), Cs(I) and Mn(II). Therefore, AO-HTC presented a desirable selectivity for uranyl ion over a range of competing metal ions.

Fig. 11 Competitive sorption capacities of coexistent ions on HTC and AO-HTC (m = 0.01 g, V = 25 mL, C₀ = 0.5 mmol L⁻¹, T = 298 K and pH = 5.0).

Desorption and reusability study

The regeneration–reusability property is also very important for practical application due to its enhancement of the economic value. As illustrated in Fig. 5, the U(VI) sorption process of AO-HTC was clearly pH-dependent, with a smaller sorption amount for a lower pH range, which implied that adsorbed uranyl ions can be desorbed from the spent sorbent by an acid medium. In this work, desorption experiments of U(VI) were performed with HCl solutions in the concentration range from 0.1 to 1.5 mol L⁻¹; the results are shown in Fig. 12a. More than 98% of uranyl ions could be desorbed using 0.5–1.5 mol L⁻¹ HCl solution; consequently, 0.5 mol L⁻¹ HCl solution was selected as the eluent for AO-HTC. The regenerated AO-HTC was reused for five consecutive sorption/desorption cycles to evaluate its reusability. As shown in Fig. 12b, the sorption capacity of U(VI) decreased slightly from 438.52 mg g⁻¹ to 406.21 mg g⁻¹, with a decrement of 7.37%, revealing that the AO-HTC presented excellent reusability and can be used as a superior sorbent for the environment remediation of U(VI).

Fig. 12 (a) Effect of HCl concentration on U(VI) desorption; (b) regenerated use of AO-HTC.

Possible sorption mechanism

To further investigate the interaction mechanism between U(VI) and AO-HTC at the molecular level, the XPS survey spectrum before and after U(VI) adsorption (named AO-HTC-U) was measured; the results are shown in Fig. 13a. The peaks of C 1s, O 1s and N 1s appeared for the expected components of AO-HTC, and the U 4f level was also detected, revealing that uranium was adsorbed onto the AO-HTC surface. Fig. 13b shows the presence of the characteristic doublets of U 4f_5/2 and U 4f_7/2 at 392.39 and 381.48 eV,^54,55 in which the speak at 381.48 eV is attributed to a covalent bond of AO-U(VI).⁵⁶ The N 1s spectrum in AO-HTC, shown in Fig. 13c, could be separated into two peaks, 399.19 eV and 400.21 eV, corresponding to the nitrogen of the NH₂ species (C–N̲H₂) and the oxime nitrogen (C=N̲OH) in the amidoxime group,^3,57 respectively. Compared to AO-HTC, the relative intensity of the C–N̲H₂ speaks of AO-HTC-U greatly decreased, indicating the increase in electron density for the nitrogen atoms.³ This demonstrate that the U(VI) mainly interacted with C–N̲H₂ of the amidoxime group. Fig. 13d shows the O 1s spectra of AO-HTC and AO-HTC-U. For AO-HTC, the peak could be decomposed into two peaks at 533.1 eV and 531.5 eV, assigned to C=O and bridging –OH, respectively.³⁰ Compared to AO-HTC, upon coordination with uranyl ions, the bridging –OH exhibited a higher binding energy. The great variation of the O 1s peak before and after U(VI) sorption revealed that uranyl ions can form strong complexes with bridging –OH in the amidoxime group.⁵⁸ The finding indicates that the sorption of U(VI) on AO-HTC is relevant to both nitrogen and oxygen atoms of amidoxime groups.²⁶ Additionally, the maximum uranium saturation capacity is 724.6 mg g⁻¹ or 3.08 mmol g⁻¹. And, AO functional group density is approximately 6.56 mmol g⁻¹, which is nearly twice that of maximum uranium saturation capacity. The ratio of uranyl ions to amidoxime group is 1 : 2 in the complex. Therefore, the sorption mechanism of U(VI) on AO-HTC is suggested in Scheme 2.

Fig. 13 (a) XPS survey spectrum of AO-HTC and AO-HTC-U, (b) U 4f XPS spectrum of AO-HTC-U, (c) N 1s XPS spectrum of AO-HTC and AO-HTC-U, and (d) O 1s XPS spectrum of AO-HTC and AO-HTC-U.

Scheme 2 Probable sorption mechanism of U(VI) on AO-HTC.

Conclusions

A novel solid-phase extractant, i.e., amidoxime-functionalized HTC, has been successfully synthesized using ubiquitous, commercially available, cheap raw materials and an easy, mild preparation procedure. AO-HTC not only has strong affinity but also demonstrates high selectivity toward uranium(VI), even in a multi-ion system. The U(VI) sorption on AO-HTC was a pH-dependent, endothermic, spontaneous and pseudo-second-order process. Repeated sorption-desorption experiments indicated that AO-HTC can be effectively regenerated and reused for U(VI) sorption without an obvious loss in the sorption amount. The results suggested that the new HTC-based sorbent may be a promising candidate for applications regarding the selective separation of uranium from nuclear fuel effluents, as well as other related water sources.

Acknowledgements

This work was financially supported by the National Basic Research Program of China (No. 2014CB460604), the National Natural Science Foundation of China (Grant No. 21301028, 11475044, 41461070, 21561002, 21401022), the Program for Changjiang Scholars and Innovative Research Team in University (Grant No. IRT13054), the Science & Technology Support Program of Jiangxi Province (Grant No. 20141BBG70001, 20151BBG70010), the Advanced Science & Technology Innovation Team Program of Jiangxi Province (Grant No. 20142BCB24006), and the Innovation Team Program of Jiangxi Provincial Department of Science and Technology (Grant No. 2014BCB24006).

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Footnote

† Electronic supplementary information (ESI) available: Reagents, characterization, N₂ adsorption–desorption isotherm (Fig. S1), batch experiments, three kinetic models (Fig. S2 and Table S1), isotherm models (Fig. S3, Tables S2 and S3) thermodynamic model (Fig. S4 and Table S4), selectivity coefficients (Table S5). See DOI: 10.1039/c6ra21986a

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