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High performance of phosphonate-functionalized mesoporous silica for U(VI) sorption from aqueous solution Li-Yong Yuan,a,b Ya-Lan Liu,a Wei-Qun Shi,*a,b Yu-Long Lv,a Jian-Hui Lan,a,b Yu-Liang Zhaoa,b and Zhi-Fang Chaia,b Received 17th January 2011, Accepted 12th May 2011 DOI: 10.1039/c1dt10085h The renaissance of nuclear energy promotes increasing basic research on the separation and enrichment of nuclear fuel associated radionuclides. Herein, we report the first study for developing mesoporous silica functionalized with phosphonate (NP10) as a sorbent for U(VI) sorption from aqueous solution. The mesoporous silica was synthesized by co-condensation of diethylphosphatoethyltriethoxysilane (DPTS) and tetraethoxysilane (TEOS), using cationic surfactant cetyltrimethylammonium bromide (CTAB) as the template. The synthesized silica nanoparticles were observed to possess a mesoporous structure with a uniform pore diameter of 2.7 nm, and to have good stability and high efficiency for U(VI) sorption from aqueous solution. A maximum sorption capacity of 303 mg g-1 and fast equilibrium time of 30 min were achieved under near neutral conditions at room temperature. The adsorbed U(VI) can be easily desorbed by using 0.1 mol L-1 HNO3 , and the reclaimed mesoporous silica can be reused with no decrease of sorption capacity. In addition, the preconcentration of U(VI) from a 100 mL aqueous solution using the functionalized mesoporous silica was also studied. The preconcentration factor was found to be as high as 100, suggesting the vast opportunities of this kind of mesoporous silica for the solid-phase extraction and enrichment of U(VI).

1.

Introduction

With the fast development of nuclear power industry, much attention has been paid to the highly efficient and selective separation, removal and recovery of nuclear fuel associated radionuclides from aqueous wastes. Among these radionuclides, uranium as the predominant fuel for nuclear reactors, can pose a serious threat to the environment through mining and spent nuclear fuel reprocessing activities in nuclear fuel cycles. The separation and enrichment of uranium are thus of great significance from the point of the view of both reasonable utilization of uranium resources and environmental protection. Uranium is well known as a long half-life radionuclide and has a complicated coordination chemistry consisting of several stable oxidation states in solid and aqueous forms. Under ordinary environmental conditions, however, uranium typically occurs in the hexavalent form U(VI) as the mobile, aqueous uranyl ion (UO2 2+ ). The entrapment of the U(VI) ions onto solid materials can thus provide effective clues for the separation and enrichment of uranium.

a Key Laboratory of Nuclear Analytical Techniques, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing, 100049, China. E-mail: [email protected]; Fax: 86-10-88235294; Tel: 86-10-88233968 b Key Laboratory for Biomedical Effects of Nanomaterials & Nanosafety, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing, 100049, China

7446 | Dalton Trans., 2011, 40, 7446–7453

Recently, a series of studies have focused on the sorption of U(VI) from aqueous solution using various sorbents. Kadous et al.,1 for example, prepared a chelating polymeric material, that is a polystyrene resin grafted with ethylenediamino tris(methylenephosphonic) acid, as a sorbent for entrapment of U(VI). The total sorption capacity of U(VI) reached 41.76 mg g-1 under optimum conditions, and the adsorbed U(VI) could be easily desorbed by using 0.1 mol L-1 ammonium carbonate. Sureshkumar et al.2 reported chitosan-tripolyphosphate (CTPP) as the sorbent for U(VI) uptake from aqueous solution. A maximum sorption capacity of 236.9 mg g-1 was estimated based on Langmuir sorption fitting after 72 h of equilibration. The FTIR spectroscopic characterization suggests that the phosphate groups are probably more responsible for the U(VI) sorption on CTPP beads. Sabale et al.3 developed a selective and effective column chromatographic separation method for U(VI), using poly[dibenzo-18-crown-6] as the sorbent. U(VI) ions were partial-selectively adsorbed on the column with a capacity of ca. 60 mg g-1 , and quantitatively and selectively eluted with 0.2 mol L-1 ammonium carbonate. Liu et al.4 synthesized an interpenetration network (IPN) ionimprinting hydrogel (IIH) using UO2 2+ as the template for UO2 2+ removal from aqueous solutions. The maximum sorption capacity calculated from the Langmuir equation was 156 mg g-1 , and equilibrium was achieved within 2 h. In addition, the sorption and complexation of U(VI) with natural or modified minerals,5–9 microorganisms10–13 and biopolymers14,15 were also studied. These works have dealt with the separations and enrichment of uranium This journal is © The Royal Society of Chemistry 2011


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by sorption. The sorption capacities, however, are somewhat low, with slow sorption kinetics. Mesoporous materials, i.e., porous nanomaterials with pore diameters of 2–50 nm such as SBA-15, are receiving an everincreasing interest from various scientific areas including physics, chemistry and material science, owing to the advantages of large surface area, well-defined pore size, excellent mechanical resistance, non-swelling, excellent chemical stability and easy to modify. These advantages also make the ordered mesoporous materials an ideal supporting material in solid extraction. Vidya et al.16,17 reported the entrapment of UO2 2+ in MCM-41 and MCM48 molecular sieves based on direct template-ion-exchange. The entrapment of UO2 2+ is facilitated by the large pore size and the high surfactant content in the as-synthesized host materials, a higher loading of UO2 2+ was thus achieved compared to the corresponding microporous materials. The poor selectivity and slow kinetics for the UO2 2+ entrapment in the mesoporous materials, however, demonstrate the necessity and importance of functionalization on the surface of mesopores. Lee et al.18 reported a kind of surface-modified mesoporous silica (MSUH) as a sorbent for U(VI) uptake, in which carboxymethylated polyethyleneimine (CMPEI) with strong complexing properties was grafted onto the surface of the MSU-H substrate. A high sorption capacity of 153 mg g-1 and an ultra-fast equilibrium time of 10 min were achieved at pH 4.0. Yousefi et al.19 studied the solid phase extraction of U(VI) using 5-nitro-2-furaldehyde (fural) modified mesoporous silica (MCM-41). The sorbent exhibited good stability, reusability, high sorption capacity and fast rate of equilibrium for sorption/desorption of U(VI). Mesoporous silica phases containing heteroatoms such as aluminium and boron were synthesized by Dyer et al. and were used to scavenge radioisotopes from various solutions.20–24 These works highlight the vast opportunities of mesoporous materials for radioisotopes entrapment, especially for U(VI) sorption from aqueous solution. Tributyl phosphate (TBP) is a well known extractant commonly used in PUREX process due to the strong complexation of the phosphoryl group with U(VI) and plutonium. Recently, homometallic UO2 2+ diphosphonates were assembled under ambient and hydrothermal conditions,25 which also suggest the strong affinity of the phosphoryl group and U(VI). In this study, a novel mesoporous silica sorbent functionalized with phosphonate was synthesized by the co-condensation method, and the synthesized sorbent was used to adsorb U(VI) from aqueous solution. The influences of pH, solution volume and ionic strength on the sorption were investigated in detail. The sorption kinetic and isothermal studies were carried out to understand the mechanism of the U(VI) sorption in the sorbent. Furthermore, the desorption of U(VI) from the sorbent was also performed, and the reusability of the sorbent as well as the potential application of the sorbent for preconcentration of U(VI) were evaluated.

2. 2.1.

Experimental Materials

Diethylphosphatoethyltriethoxysilane (DPTS) was purchased from Meryer, China. Tetraethoxysilane (TEOS) and cetyltrimethylammonium bromide (CTAB) were obtained from SCRC, China. Uranyl nitrate hexahydrate (UO2 (NO3 )2 ·6H2 O, ACS grade) was This journal is © The Royal Society of Chemistry 2011

purchased from Merck, Germany. All these materials were used as received. Standard stock solution (1.45 g L-1 ) of U(VI) was prepared by dissolving the appropriate amounts of UO2 (NO3 )2 ·6H2 O in deionized water. All other chemicals were of analytical grade and used without further purification. Deionized water used in all experiments was obtained from the Milli-Q water purification system. 2.2. Preparation of phosphonate-functionalized mesoporous silica (NP10) In a typical synthesis, 0.2 g of CTAB was added to a mixture of 100 g of H2 O and 6 mL of NaOH (0.01 mol L-1 ) aqueous solution, which was stirred at 80 ◦ C. And then 1.2 g of TEOS and 0.2 g of DPTS were added to the solution under vigorous stirring. The reactant composition was 0.9 : 9 : 1 : 2.5 : 9000 CTAB : TEOS : DPTS : NaOH : H2 O. After 2 h, the solid products were collected by centrifugation, washed with deionized water, and dried at 50 ◦ C overnight. The surfactant was removed by refluxing 0.5 g of as-synthesized material in 100 mL of ethanolic solution with 1 mol L-1 HCl for 24 h. 2.3. Sorption experiments The sorption experiments were carried out using the batch method. The initial concentrations of U(VI) varied from 2.8 ¥ 10-5 to 9 ¥ 10-4 mol L-1 . The solution pH was adjusted by adding negligible volumes of diluted nitric acid or sodium hydroxide. In a typical sorption experiment, 4 mg of NP10 sorbent was kept in contact with 10 mL of U(VI) solution in a flask. After stirring for 3 h at room temperature, the two phases were separated by centrifugation for 30 min with a speed of 4000 rpm. The control experiment was performed at the same time using the identical U(VI) solution in the absence of the sorbent. The concentrations of U(VI) in the aqueous solution were determined by the Arsenazo III Spectrophotometric Method at a wavelength of 656 nm. 2.4. Analytical techniques The morphology of the sample was observed with SEM (JEOL JSM-7401F) at an accelerating voltage of 1.0 kV. High resolution transmission electron microscopy (HRTEM) was performed with a JEOL JEM-2100 microscope operating at 200 kV (Cs = 1.4 mm, ˚ ). The image was recorded using a CCD point resolution 2.2 A camera (Keenview, 1376 ¥ 1096 pixels, pixel size 6.4 ¥ 6.4 mm) under low-dose conditions. Powder XRD pattern was recorded on a Rigaku X-ray diffractometer D/MAX-2200/PC with Cu-Ka radiation (40 kV, 20 mA) at a rate of 1.0◦ min-1 over the range of 1–6◦ (2q). The N2 sorption–desorption isotherm was measured at -196 ◦ C with a Quantachrome Nova 4200E. The sample was pretreated at 120 ◦ C for 2 h. The surface area was calculated by the Brunauer–Emmett–Teller (BET) method. The pore size was obtained from the maximum of the pore size distribution curve calculated by the Barrett–Joyner–Halenda (BJH) method using the sorption branch of the isotherm. The total pore volume was evaluated by the single point method. 13 C CP/MAS NMR spectra were measured on a Mercury plus 400 spectrometer at 100 MHz and a sample spinning frequency of 3 kHz. The UV absorbance of Arzenazo III–U(VI) complex was recorded in a photometry mode Dalton Trans., 2011, 40, 7446–7453 | 7447


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Fig. 1

Characterization of NP10 a) SEM image; b) TEM image; c) XRD pattern; d) N2 sorption–desorption isotherm; e) pore size distribution.

on TU-1901 spectrophotometer with a quartz cuvette of 1 cm path length.

3.

Results and discussion

3.1.

Characterization of NP10

Fig. 1 shows the characterization results of the NP10 sample. As shown in Fig. 1, the SEM image indicates that the sample is composed of spherical nanoparticles, and the average particle size is 100 nm, while the HRTEM image reveals a very ordered porous structure in the nanoparticles. The XRD pattern shows that NP10 exhibits an intense diffraction peak in the region of 2q close to 2.1◦ and a weak and broad peak in the region of 2q = 3.5–4.5◦ , which also indicates the presence of an ordered mesostructure. The surface area, primary mesopore volume and pore size are 920 m2 g-1 , 0.73 cm3 g-1 and 2.7 nm, respectively, determined by nitrogen sorption. The nitrogen sorption–desorption isotherm is type IV, typical for a porous material. Fig. 2 shows the solid-state 13 C MAS NMR of NP10. The resonance peaks around 30 ppm produced from the surfactant disappeared after refluxing with an ethanol–HCl solution (see Section 2.2), suggesting that the surfactant has been completely removed during refluxing. The resonance signals at 7.0, 17.5, 67.5 and 19.4 ppm, as denoted in Fig. 2, can be assigned to CI , CII , CIII and CIV of DPTS, respectively. These results clearly confirm that the phosphate group functionalized mesoporous material has been successfully prepared. 3.2.

Fig. 2

13

C CP/MAS NMR spectra of NP10.

conditions. The sorption percent (E) and the sorption capacity (q) of U(VI) were defined as follows:

E (%) =

C 0 - C eq C0

×100%

(1)

Sorption of U(VI) in NP10

To evaluate the sorption property of NP10, the sorptions of U(VI) from aqueous solution into NP10 were performed at different 7448 | Dalton Trans., 2011, 40, 7446–7453

q=

C 0 - C eq m sorbent

×V solution

(2)

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Table 1 Kinetics model constants and correlation coefficients for U(VI) sorption by NP10 Kinetics model


Pseudo-first-order

Pseudo-second-order

qe (mg g-1 )

k1 (min-1 )

R2

qe (mg g-1 )

k2 (g mg-1 min-1 )

R2

Saturation capacity from experiment (mg g-1 )

64

0.069

0.975

282

3.0 ¥ 10-3

0.9995

277

where C 0 and C eq represent the concentrations of U(VI) in the aqueous phase for the control experiment and the sorption experiment after 3 h stirring, respectively; msorbent and V solution designate the weight of NP10 sorbent and the volume of U(VI) solution used in the sorption experiment, respectively. All values were measured in duplicate with the uncertainty within 5%. Sorption kinetics The sorption rate of U(VI) in NP10 was studied at an initial U concentration ([U]initial ) of 131 mg L-1 for a contact time of 1 to 240 min. As shown in Fig. 3, the sorption of U(VI) in NP10 is ultra-fast especially in the initial 5 min, and the sorption process reaches equilibrium at around 30 min. Such a equilibrium time is much faster compared to that of some other porous sorbents, such as 20 h in MCM-41,16 24 h in nanoporous carbon26 and 24 h in multiwalled carbon nanotubes grafted with carboxymethyl cellulose.27

Fig. 3 Effect of stirring time on the U(VI) sorption in NP10. pH = 6.9 ± 0.2; msorbent /V solution = 0.4 mg mL-1 ; [U]initial = 131 mg L-1 .

In order to clarify the sorption process of U(VI) in NP10, the pseudo-first-order kinetic model and the pseudo-second-order kinetic model were applied to analyse the experimentally observed kinetic data. The linearized form of the pseudo-first-order rate equation by Lagergren is given as:28

log(qe − qt ) = log qe −

k1 ×t 2.303

(3)

where qe (mg g-1 ) and qt (mg g-1 ) are the amounts of U(VI) ions adsorbed in NP10 (mg g-1 ) at equilibrium and at time of t (min), respectively, and k1 is the sorption rate constant (min-1 ). The plot of log(qe - qt ) versus t gives a straight line and k1 can be calculated from the slope. The pseudo-second-order kinetic model is given with the equation below:2 This journal is © The Royal Society of Chemistry 2011

1 t t = + qt k 2qe2 qe

(4)

where qe and qt have the same meaning as earlier and k2 is the rate constant (g mg-1 min-1 ) of the pseudo-second-order kinetic model. The plot of t/qt versus t shows linearity, and qe and k2 can be calculated from the slope and intercept. The model parameters and the correlation coefficient obtained by both the models are shown in Table 1. From the results it can be seen that both pseudo-first and pseudo-second-order models reasonably match with the experimental kinetics data. However, the pseudosecond-order model gives a much better correlation coefficient (more than 0.999) and a much closer equilibrium capacity to the experimental value, suggesting that the pseudo-second-order model is more appropriate to explain the kinetics of U(VI) sorption in NP10. This result can be expected because the ordinary type of exchange processes are more rapid and controlled mainly by diffusion,29 whereas, those in a chelating exchanger are slower and controlled either by a particle diffusion mechanism or by a secondorder chemical reaction.30 The NP10 sorbent with its mesoporous structure and functional groups present on its surface most probably behaves like a chelating exchanger. Therefore secondorder chemical reaction kinetics is expected to be followed in the sorption processes. It is known that the sorption process on porous sorbents is generally described by four stages, i.e. bulk diffusion, film diffusion, intraparticle diffusion and sorption of the adsorbate on the surface.31 One or more of these stages may determine the rate of sorption and the amount of sorption on the solid surface. Since NP10 has porous structures, further investigation is needed to find out the effect of intraparticle diffusion to the entire sorption process. Intraparticle diffusion model is expressed with the equation given by Weber and Morris:32 qt = kid t1/2

(5)

where qt has the same meaning as earlier and kid is the intraparticle diffusion constant (mg g-1 h-1 ). The plot of qt as a function of t1/2 gives a straight line, from which kid can be obtained. The experimental kinetic data were tentatively applied to eqn (5), as shown in Fig. 4. It can be seen that the points are not linearly distributed but give three straight lines with three different slopes. Similar kinetics were observed on the sorption of humic acid by cross-linked chitosan beads33 and on the sorption of U(VI) by chitosan-tripolyphosphate beads,2 which indicated that the intraparticle diffusion is not applicable to the entire time scale of the sorption process. Specifically, the initial steep-sloped portion represents the bulk diffusion or exterior sorption rate which is very high, the subsequent linear portion can be attributed to the intraparticle diffusion and the last straight line corresponds to the chemical equilibration of U(VI) in NP10. Such kinetics suggests Dalton Trans., 2011, 40, 7446–7453 | 7449

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Table 2 Isotherm model constants and correlation coefficients for U(VI) sorption by NP10 Isotherm model


Langmuir isotherm

D–R isotherm

Q (mg g-1 )

b (mL mg-1 )

R2

Qm (mg g-1 )

b (mol2 kJ-2 )

E (kJ mol-1 )

R2

Saturation capacity from experiment (mg g-1 )

306

683

0.998

2380

3.5 ¥ 10-3

11.9

0.953

303

the distance from the surface increase. The linear form of the Langmuir isotherm can be expressed as:34

Ce C 1 = e + qe Q Qb

Fig. 4 Intraparticle diffusion kinetics of the sorption of U(VI) in NP10.

that intraparticle diffusion may play an important role in the rate determination in the sorption process but not the sole rate determining factor because of the deviation of the curves from the origin and non-linear distribution of the plots.

(6)

where C e is the equilibrium concentration of adsorbate (mg L-1 ), qe represents the amount of adsorbed adsorbate at equilibrium (mg g-1 ), and Q and b are Langmuir constants related to sorption capacity (mg g-1 ) and affinity of the binding site on sorbent (mL mg-1 ), respectively. Q and b can be obtained by plotting C e /qe versus C e . Table 2 lists the parameters of the Langmuir model for the U(VI) sorption in NP10. From the good correlation coefficient of 0.998 and the fact that the equilibrium sorption capacity (Q) obtained from Langmuir model (306 mg g-1 ) is very close to the experimentally observed saturation capacity (303 mg g-1 ), it can be clearly concluded that the sorption of U(VI) in NP10 follows the Langmuir sorption model. According to the Langmuir model, the favorability of NP10 as a U(VI) sorbent, related to the separation factor RL , can be obtained from the Langmuir sorption constant (b):

Sorption isotherm A sorption isotherm is fundamental in understanding the sorption mode of an adsorbate on sorbent surface once the equilibrium is attained. Herein, the amount of U(VI) adsorbed in NP10 as a function of U(VI) concentration in supernatant at the equilibrium state (C e ), i.e. sorption isotherm, was determined at near neutral conditions (Fig. 5), and the data obtained were applied to the Langmuir isotherm. The Langmuir isotherm is based on the assumption that the sorption occurs in a monolayer, uniform and finite mode on the sorbent and the sorption energy decreases as

RL =

1 1 + bC 0

(7)

where C 0 is the initial metal ion concentration. 0 < RL < 1 designates that the sorbent is a favorable medium for the sorption of the given metal ion. Table 3 lists the calculated RL values at several initial U(VI) concentrations. For all the tested U(VI) concentrations, RL values are observed between 0 and 1, which proves that NP10 is a favorable sorbent for U(VI). Whatever, the applicability of the Langmuir isotherm suggests that the surface of the sorbent is uniform and homogeneous, and the sorption process results in the formation of a monolayer coverage of U(VI) in NP10. Although the mono-layered and uniform sorption has been indicated, the Langmuir isothem does not give enough information about the sorption mechanism. In order to better understand the sorption type of U(VI) in NP10, the sorption data were applied to another commonly used model, Dubinin–Radusckevich (D–R) isotherm, which describes sorption on a single type of uniform pores. Its linear expression can be defined as:35 lnqe = lnQm - be2

(8)

where Qm (mol g-1 ) represents theoretical monolayer saturation capacity, b (mol2 kJ-2 ) is a constant correlated to sorption energy, Table 3 RL values for U(VI) sorption obtained from Langmuir equation

Fig. 5 The sorption isotherm for U(VI) in NP10. pH = 6.9 ± 0.2; msorbent /V solution = 0.4 mg mL-1 .

7450 | Dalton Trans., 2011, 40, 7446–7453

[U]initial (mg L-1 ) RL

6.7

13

34

57

79

120

137

183

217

0.179 0.099 0.042 0.025 0.018 0.012 0.011 0.008 0.007


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and e is the Polanyi potential (kJ mol-1 ) related to the equilibrium concentration, illustrated as:


e = RT ln(1 +

1 ) Ce

(9)

where R is the universal gas constant (kJ mol-1 K-1 ) and T is the absolute T (K). The mean free energy E (kJ mol-1 ) which is used to estimate the sorption type can be calculated from constant b: E = (-2b)-0.5

(10)

Qm and b for U(VI) sorption in NP10 were obtained from the linear plot of lnqe against e2 , as given in Table 2. Although the correlation coefficient (R2 ) is lower than that for the Langmuir isotherm, the experimental data is regarded to reasonably fit the D–R model. The mean free energy (E) was estimated to be 11.9 kJ mol-1 according to eqn (10). Since the numerical value of E in the range of 1–8 and 9–16 kJ mol-1 forecasts the physical sorption and chemical sorption, respectively,36 the E value obtained in this work clearly suggests that the sorption of U(VI) in NP10 is chemical sorption, i.e., chemisorption. The chemical nature of the sorption is actually in good agreement with mono-layered and uniform sorption indicated by the Langmuir model, as well as our expectation that U(VI) ions are adsorbed through complexation with the phosphoryl groups. Effect of pH The pH of the aqueous solution is an important parameter for the metal ion sorption, because it remarkably affects the metal speciation and the surface charge and surface binding sites of the sorbent. The U(VI) sorption in NP10 as a function of pH ranging from 1.5 to 8.0 is shown in Fig. 6. As can be seen, the U(VI) sorption in NP10 is low in acidic solutions and increases with the augmentation of pH, followed by a plateau at pH 6–8. That is, the maximum sorption of U(VI) occurs between pH 6 and 8. Such a pH-dependent sorption can be rationalized based on the U(VI) speciation and surface charge of NP10. At low pH, U(VI) is known to exist as UO2 2+ in the solution, whereas the phosphonate groups, acting as binding sites on NP10, are protonated and positively charged. Owing to the electrostatic repulsion, the positively charged UO2 2+ is not favored by the positively charged binding

groups, resulting in a lower sorption capacity. As the pH increases, the phosphonate group is deprotonated, whereas U(VI) still exists in a positively charged form. The electrostatic interaction and complexation between the O in the phosphonate moiety and U(VI) leads to the increase of the sorption capacity. On the other hand, the species distribution of U(VI) is greatly dependent on the solution pH.27 At pH 6–8, for example, the multi-nuclear hydroxide complexes such as (UO2 2+ )3 (OH)5 + are the predominant species. These species may be more favored by the sorbent, since the maximum sorption of U(VI) in many different sorbents occurred at pH 6–8. Furthermore, the complexation strength between the binding group and metal ion increases with increasing pH, which is another reason for the pH-dependent U(VI) sorption in NP10. To achieve a higher sorption capacity, the optimum pH 7 was selected for further sorption experiments unless otherwise stated. Effect of ionic strength The effect of ionic strength on the sorption of U(VI) in NP10 was studied in the presence of NaClO4 with concentrations varying from 5 ¥ 10-4 to 0.4 mol L-1 . The results are shown in Fig. 7. It is observed that the sorption percent of U(VI) in NP10 increases from 86.3% to 90% with increasing concentration of NaClO4 ([NaClO4 ]) from 5 ¥ 10-4 to 0.01 mol L-1 , and then rapidly decreased with further increasing [NaClO4 ], followed by a plateau at [NaClO4 ] > 0.3 mol L-1 . The decrease of the sorption percent at high [NaClO4 ] can be attributed to the competition between Na+ and U(VI) adsorbed in the NP10 sorbent, while the increase of the sorption percent at low [NaClO4 ] is not as easy to rationalize. Singer et al.37 reported the U(VI) sorption by chlorite in the presence of NaCl and concluded that U(VI) sorbs dominantly as inner-sphere complexes based on the fact that U(VI) sorption is independent of the ionic strength. This seems to give a hint that U(VI) does not sorb as inner-sphere complexes in NP10, since the U(VI) sorption in NP10 is dependent on the ionic strength.

Fig. 7 Effect of ionic strength on the sorption of U(VI) in NP10. msorbent /V solution = 0.4 mg mL-1 ; [U]initial = 45 mg L-1 ; pH = 4.8 ± 0.1.

Desorption of adsorbed U(VI) and reusability of the sorbent Fig. 6 Effect of pH on the sorption of U(VI) in NP10. msorbent /V solution = 1 mg mL-1 ; [U]initial = 138 mg L-1 .


As mentioned above, the U(VI) sorption in NP10 is greatly dependent on the solution pH due to the pH-dependent U(VI) Dalton Trans., 2011, 40, 7446–7453 | 7451

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Table 4 The desorption of U(VI) from NP10 and the reusability of the reclaimed NP10 Desorption

Reusability


-1

[HNO3 ] (mol L ) 0.1

0.05

0.038

Fresh NP10

Reclaimed NP10

Efficiency (%)

77.7

70.4

86.7

86.1

100

speciation and the surface charge of the sorbent. Accordingly, the desorption of U(VI) from NP10 was performed using HNO3 as the eluent. In a typical experiment, 4 mg of NP10 was contacted with 10 mL of U(VI) solution (1.6 ¥ 10-4 mol L-1 ) in the optimum conditions of pH (6.9 ± 0.2) and stirring time (3 h). Then appropriate amount of eluting agent, i.e. HNO3 , was added, followed by 30 min of stirring at room temperature. Quantitative desorption of U(VI) from NP10 using various concentrations of HNO3 is shown in Table 3. A complete recovery can be achieved using a 0.1 mol L-1 HNO3 solution. In order to evaluate the stability of the NP10 sorbent during U(VI) desorption, the reusability of reclaimed NP10 was tested by equilibrating 4 mg of reclaimed NP10 with 10 mL of U(VI) solution (4.7 ¥ 10-4 mol L-1 ) at pH 6.9 ± 0.2 for 3 h stirring. For comparison, a reference sorption experiment was also conducted using fresh NP10 at the same conditions. The results are listed in Table 4. As can be seen, a sorption efficiency of 86.1% for the reclaimed NP10 was achieved, which is comparable to 86.7% for the fresh NP10. It is clear that the NP10 sorbent is reusable with no remarkable decrease in sorption capacity after the desorption, suggesting that the functional groups in NP10 were not destroyed by the HNO3 during desorption, and the desorption can thus be explained as a cation exchange between protons and the U(VI) ions.24

Preconcentration of U(VI) In the sorption of U(VI) from aqueous solutions, the amount of sorbent used in the process is crucial for the economic consideration. Under the effective sorption percent uncertainties, the lower the amount of sorbent is used, the lower the cost. Keeping this in mind, the sorption capacity for U(VI) as a function of the ratio of msorbent to V solution (m/V ) was determined by varying the volume of the U(VI) solutions in the range of 10–100 mL, while the weight of the sorbent (msorbent ) and the total amount of U(VI) in the solutions were kept constant (Fig. 8). The results show that the effect of the ratio of msorbent to V solution (m/V ) on the U(VI) sorption in NP10 is insignificant. Compared to the sorption capacity of 288 mg g-1 at m/V = 0.4, the sorption capacity is still as high as 276 mg g-1 at m/V = 0.04, which reveals the high efficiency of NP10 for U(VI) sorption from aqueous solution. Beside the high efficiency, however, a high preconcentration factor is anticipated for the preconcentration of U(VI). Here the preconcentration factor can be defined as the ratio of the sample volume before sorption to the eluent volume during the desorption process. To obtain a higher preconcentration factor, the adsorbed U(VI) in NP10 were eluted by a small amount of 1 mol L-1 HNO3 . It was found that more than 90% of the adsorbed U(VI) can be desorbed successfully by 1 mL of 1 mol L-1 HNO3 , which produces a preconcentration factor of 100 for 100 mL of U(VI) solution with m/V as low as 0.04. The 7452 | Dalton Trans., 2011, 40, 7446–7453

Fig. 8 Effect of ratio of msorbent and V solution (m/V ) on the sorption of U(VI) in NP10. pH = 6.9 ± 0.2, The total amount of U(VI) is 1.6 mg in all the solutions.

result confirms the vast potential application of NP10 for U(VI) preconcentration from aqueous solution.

4. Conclusions A novel sorbent of phosphonate functionalized mesoporous silica (NP10) was synthesized by co-condensation method and used as support material for U(VI) sorption in batch processes. The sorption of U(VI) in NP10 was studied as a function of various parameters such as time, pH, U(VI) concentration and ionic strength. The kinetics of U(VI) sorption in NP10 is ultra-fast with an equilibrium time of 30 min, and the sorption process was found to follow pseudo-second-order type sorption kinetics. Intraparticle diffusion plays an important role in the sorption processes but it could not be accepted as the sole rate-determining step. The measured maximum sorption capacity is as high as 303 mg g-1 . The sorption isotherm has been successfully modeled by the Langmuir isotherm and Dubinin–Radusckevich isotherm, which reveals a monolayer chemical sorption of U(VI) in NP10. The sorption efficiency of U(VI) in NP10 markedly increases with increasing solution pH ranging from 1.5 to 8 due to the pH-induced change of U(VI) speciation and surface charge of the sorbent, whereas the sorption efficiency first increases and then decreases with the increase of the ionic strength. The desorption of U(VI) from NP10 sorbent using various concentrations of HNO3 and the sorption of U(VI) using the reclaimed sorbent by the same experimental process were performed. The results show that the adsorbed U(VI) can be completely desorbed by 0.1 mol L-1 HNO3 solution and NP10 sorbent is stable and reusable with no decrease of sorption capacity after elution of HNO3 . Additionally, the preconcentration potential of NP10 sorbent for U(VI) was also evaluated. A preconcentration factor of 100 for 100 mL U(VI) solution was obtained at a m/V as low as 0.04, suggesting the high performance of the NP10 sorbent on separation and preconcentration of U(VI) from aqueous solution. This work promises to provide basic data for assessing the feasibility of this new nanomaterial applied in the separation of uranium from waste water and enrichment of uranium from sea water. This journal is © The Royal Society of Chemistry 2011

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Acknowledgements


The authors are grateful to Shun-Ai Che’ group in Shanghai Jiao Tong University for assistance in synthesis and characterization of the sorbent material. This work was supported by the National Natural Science Foundation of China (No. 91026007).

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