Large-Pore 3D Cubic Mesoporous (KIT-6) Hybrid Bearing a HardâSoft

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Large Pore 3-D Cubic Mesoporous (KIT-6) Hybrid Bearing Hard-soft Donor Combined Ligand for Enhancing U(VI) Capture: An Experimental and Theoretical Investigation Li-Yong Yuan, Lin Zhu, Chengliang Xiao, Qun-yan Wu, Nan Zhang, Ji-Pan Yu, Zhifang Chai, and Wei-Qun Shi ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b15642 • Publication Date (Web): 09 Jan 2017 Downloaded from http://pubs.acs.org on January 10, 2017

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ACS Applied Materials & Interfaces

Large Pore 3-D Cubic Mesoporous (KIT-6) Hybrid Bearing Hard-soft Donor Combined Ligand for Enhancing U(VI) Capture: An Experimental and Theoretical Investigation Li-Yong Yuan,†‡ Lin Zhu, †§‡ Cheng-Liang Xiao,*§ Qun-Yan Wu,† Nan Zhang, † Ji-Pan Yu, † Zhi-Fang Chai,†§ and Wei-Qun Shi*†

†

Laboratory of Nuclear Energy Chemistry, Institute of High Energy Physics, Chinese Academy

of Sciences, Beijing 100049, China.

1

ACS Paragon Plus Environment


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ABSTRACT The preorganized tetradentate phenanthrolineamide (DAPhen) ligand with hard-soft donors combined in the same molecule has been found to possess high extraction ability towards actinides over lanthanides from acidic aqueous solution in our previous work. Herein, we grafted phenanthrolineamide groups onto a large pore three-dimensional (3-D) cubic silica support by the reaction of DAPhen siloxane with KIT-6 substrate to prepare a novel uranium-selective sorbent, KIT-6-DAPhen. The as-synthesized sorbent was well characterized by SEM, HRTEM, N2 adsorption/desorption, XRD, FT-IR,

13

C CP/MAS NMR, and TGA techniques, which

confirmed the consummation of the functionalization. Subsequently, the effects of contact time, solution pH, initial U(VI) concentration, and the presence of competing metal ions on the U(VI) sorption onto KIT-6-DAPhen sorbent were investigated in detail. It was found that KIT-6-DAPhen showed largely enhanced sorption capacity and excellent selectivity towards U(VI). The maximum sorption capacity of KIT-6-DAPhen at pH=5.0 reaches 328 mg U/g sorbent, which is superior to most of functionalized mesoporous silica materials. Density functional theory (DFT) coupled with quasi-relativistic small-core pseudopotentials method was used to explore the sorption interaction between U(VI) and KIT-6-DAPhen, which gives a sorption reaction of KIT-6-DAPhen + [UO2(H2O)5]2+ + NO3- = [UO2(KIT-6-DAPhen)(NO3)]+ + 5H2O. Finding of the present work provides new clues for developing new actinides sorbents by combining new ligands with various mesoporous matrixes.

Keywords: Phenanthroline, mesoporous silica, KIT-6, uranium, DFT

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INTRODUCTION Although Fukushima nuclear accident in 2011 had a negative effect on the development of nuclear power, after three years, the situation shows signs of a turnaround. The consequent nuclear wastes with long-term radiotoxicity, especially resulting from actinide ions, still cause major social and environmental problems, which would restrict further expanding of nuclear power. From the viewpoint of environmental protection and sustainable nuclear fuel cycle, it is highly desired to develop efficient and selective sorbents for the removal of actinides from waste water. Mesoporous silica materials are defined in term of their pore diameters of 2-50 nm, such as MCM-41, SBA-15, and KIT-6, which exhibit distinct advantages in large surface area, well-defined pore size, chemical stability, and easy to modify1. Such excellent characteristics make mesoporous silica an ideal support to synthesize functional sorbents for the separation of radiotoxic actinides from waste water2, 3. A recent review contributed by Florek and Kleitz et al.4 on the topic of mesoporous materials for actinides extraction has been published, in which a series of examples of successfully applying this kind of materials on actinides separation were summarized. Fryxell et al.5-9, for example, pioneered the synthesis of functional ordered mesoporous silica adsorbents entitled self-assembled monolayers on mesoporous supports (SAMMS) for actinide sequestration. After modified with glycinyl-urea, salicylamide, acetamide- phosphonate, and hydroxypyridinone ligands (Figure 1) onto mesoporous MCM-41 supports, these organic-inorganic hybrid materials showed fast and selective capturing of 3



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actinides. In addition, Yousefi et al.10 grafted a 5-nitro-2-furaldehyde (fural) group (Figure 1) onto MCM-41 substrates for U(VI) sorption. Chen et al.11, 12 prepared mesoporous SBA-V based materials bearing phosphine oxide ligands for binding uranium in HNO3 media. Recently, we developed several efficient mesoporous silica materials for solid phase extraction of actinides from aqueous solution. Phosphonate, dihydroimidazole, and amino groups (Fig. 1) were successfully modified onto mesoporous MCM-41 or SBA-15 supports and the resulting materials exhibited extremely high sorption capacity towards U(VI) ions13-17. Compared to mesoporous SBA-15 and MCM-41 support, KIT-6 support holds interconnected large pore 3-D cubic mesoporous nature, which is beneficial for reducing risks of pore blocking during functionalization with large ligands. In addition, the three dimensional mesopore is highly accessible for the solution and metal ions during sorption process. Larivière and Kleitz et al.18, 19 opened

up

the

post-synthesis

modification

of

mesoporous

KIT-6

silicas

with

(2-diethylphosphatoethyl)triethoxysilane (DTPS) and found this sorbent showed obvious superiority over its 2-D hexagonal analogue (SBA-15-DTPS) in the sorption of U(VI) ions. Furthermore, the same group reported mesoporous KIT-6 hybrid materials functionalized with diglycolylamide (DGA) groups (Fig. 1) for efficient recovery and enrichment of rare earth elements and some actinides20. In these novel sorbents, there may be an adverse effect on the sorption performance because of the rotation of σ-σ bonds in DGA groups. If one can reduce the flexibility of diglycolyamide groups, the sorption properties would improve. From a chemical point of view, further modifications in the design and synthesis of new preorganized ligands for 4


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selective capturing actinides should be made. A so-called preorganized ligand is one that requires lowest energy to complex a target guest ion with less structure change. CH2COOH

O

N N

CH2COOH

CH2COOH

ED3A

CH2COOH N

CH2COOH

N

N

CH2COOH

CH2COOH

DT4A

P

OC2H5 OC2H5 O

diethylphosphate

P

phosphine oxide

R1

R2 N

N

CH2COOH

N

CyD3A

O

O

O N H O

O OH

acetamide-phosphonate

OH

N

1,2-HOPO

OH O

2,3-HOPO

N H N O

O

O

N H

O P

OH N

salicylamide

N H

OH

R

OH

N H

O

3,4-HOPO

N

COOH glycinyl-urea

N H

3-aminopropyl

NH2

CH2COOH CH2COOH O N H

dihydroimidazol

N

NO2

5-nitro-2-furaldehyde

O O

N H

N H diglycolylamide

mesoporous SBA-15/V, MCM-41, KIT-6 silica support

Fig. 1 Functional organic ligands used in the mesoporous silica sorbents for sequestering actinides.

Recently,

we

have

reported

a

novel

tetradentate

preorganized

2,9-diamide-1,10-

phenanthroline (DAPhen) ligand, Et-Tol-DAPhen, which exhibited high extractability and excellent selectivity towards actinides21-23. The “ligand pre-organization” here takes place in two nitrogen atoms of the phenanthroline moiety in view of its rigid coplanar structure. Unlike

5



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dipyridyl derivatives in which there is a certain flexibility provided by the single bond between two pyridine, rigid coplanar structure of phenanthroline make two nitrogen donors occur on the same side, which is thus preorganized for forming complexes with U(VI) ions. Following our previous success with actinide extraction, we herein propose to graft DAPhen groups onto the surface of large pore 3-D cubic mesoporous KIT-6 support (named as KIT-6-DAPhen), aiming to recover and enrich of actinides from waste water. In this work, the synthesis and characterization as well as sorption properties of this mesoporous KIT-6-DAPhen sorbent towards U(VI) were investigated in detail, and the sorption mechanism was explored by density functional theoretical (DFT) method.

EXPERIMENTAL Chemicals and Materials Tetraethoxysilane (TEOS, 98%) and Pluronic P123 EO-PO-EO triblock copolymer were purchased from Sigma-Aldrich, USA. 3-aminoprpoyl trimethoxysilane (APTS, 97%) was purchased from Meryer, China. Uranyl nitrate hexahydrate (UO2(NO3)2·6H2O, ACS grade) was purchased from Merck, Germany. All the other chemical materials (thionyl chloride, HCl, butanol, etc.) were of analytical grade and used without further purification. The stock U(VI) solution was prepared by dissolving appropriate amounts of UO2(NO3)2·6H2O in deionized water. The type and purity of the metal ions used in the selectivity experiments are listed in Table S4 in Supporting Information. The concentration of each metal ion is around 0.5 mmol

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L−1. Deionized water used in all experiments was obtained from the Milli-Q water purification system. Synthesis of KIT-6-DAPhen sorbent Mesoporous KIT-6 support was prepared according to the method reported by Kleitz et al20 and used after thermally activated at 150 oC overnight. 2,9-Dicarboxylic acid-1,10-phenanthroline was synthesized based on our previous procedures21. The KIT-6-DAPhen sorbent was synthesized as depicted in Fig. 2.

Step 1

DAPhen siloxane

DAPhen siloxane

KIT-6

Step 2

KIT-6-DAPhen

Fig. 2 Schematic representation of the synthesis of phenanthrolineamide functionalized KIT-6.

DAPhen siloxane. Firstly, 2,9-dicarboxylic acid-1,10-phenanthroline (0.509 g, 1.9 mmol) was refluxed with thionyl chloride (20 mL) in an inert atmosphere. After 3 h, the mixture was cooled and thionyl chloride was removed by reduced pressure distillation. When the solvent was completely evaporated, the grey solid was dissolved in dry toluene (25 mL) and cooled in ice water. Then, 1 mL (4.0 mmol) of (3-aminopropyl)-triethoxysilane and 0.6 mL of triethylamine

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were added into the above solution and the resulting mixture was stirred for 24 h under reflux and N2 protection. Then the DAPhen siloxane solution was obtained. To confirm that the reactant, i.e. 2,9-dicarboxylic acid-1,10-phenanthroline, was fully converted into its silanized version, the thin layer chromatography analysis was performed using petroleum ether/ethyl acetate (1:1) as eluent. Only two components besides toluene and triethylamine were obtained. One component was confirmed to be unreacted APTS since a 5% overdose of APTS was used, while another component was characterized by 1H NMR (see Fig. S1 in Supporting Information). All the resonance signals in the NMR spectra can be assigned to appropriate H atoms of N2,N9-bis(3-(triethoxysilyl)propyl)-1,10-phenanthroline-2,9-dicarboxamide,

which

evidences

that the DAPhen siloxane has been successfully prepared. KIT-6-DAPhen. 0.5 g of activated KIT-6 support and 0.5 mL of triethylamine were added into 25 mL of dry toluene. The DAPhen siloxane solution was transferred into the above suspension and the resulting mixture was left stirring for 24 h under reflux and N2 protection. After cooling to room temperature, the mixture was filtered and washed with large amounts of toluene, ethanol, and dichloromethane for six times. The white solid product named KIT-DAPhen was obtained after dried at 70 oC overnight under vacuum. Characterization Methods The morphologies and microstructures of the samples were characterized with a field emission scanning electron microscopy (SEM, HITACHI S-4800). High resolution transmission electron microscopy (HRTEM) was performed with a JEOL JEM-2100 microscope operating at 200 kV. 8


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Powder X-ray diffraction (PXRD) patterns were obtained on a Bruker D8-Advance X-ray Diffractometer with a Cu Kα radiation. Thermogravimetric curves were recorded on a thermal gravimetric analyzer (TGA, TA Instruments, Q500) from 20-900 oC with a heating rate of 5 oC min-1 under an air flow. The N2 sorption experiments were measured on a micromeritics ASAP 2020 HD88 instrument at a liquid nitrogen temperature (-196 oC). The samples were degassed under vacuum at 120 oC before measurements. The specific surface area was calculated by the Brunauer–Emmett–Teller (BET) method. The pore size was obtained from the max 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. Fourier transform infrared (FT-IR) spectra were recorded on a Bruker Tensor 27 spectrometer with a potassium bromide pellet 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. Inductively coupled plasma optical emission spectrometer (ICP-OES, Horiba JY2000-2) was used to determine the residual concentration of tested ion(s) in supernatants in the selectivity test experiment. The UV absorbance of Arsenazo III–U(VI) complex was recorded in a photometry mode on HITACHI UV-3900 spectrophotometer with a quartz cuvette of 1 cm path length. Sorption Experiments All sorption experiments were carried out using the batch method in air at room temperature with initial concentrations of U(VI) ranging from 5 to 200 mg L-1. The solution pH was adjusted by adding negligible volumes of diluted nitric acid or sodium hydroxide. In a typical experiment, 4 9



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mg of sorbent was added into either 10 mL of U(VI) solution or 10 mL of multi-ion test solution in a flask (the solid-liquid ratio is 0.4 g/L). The control experiment was performed simultaneously using the identical U(VI) solution in the absence of the sorbent. After stirred for a desired time, the solid phase was separated from the aqueous solution using a 0.22 µm nylon membrane filter, and then the the concentration of U(VI) and other metal ions in the aqueous phase was determined by the Arsenazo III spectrophotometric method or ICP-OES. For Arsenazo III spectrophotometric method (the detection limit is below 0.1 ppm), the supernatant was diluted 2.5~100 times to make sure that the U(VI) concentration in the dilution is 0.1~5 µg/mL, corresponding to the UV absorbance of 0.05~1.0 at 656 nm. For ICP-OES method (the detection limit is below 0.01ppm), the supernatant was diluted 25~100 times to make sure that the concentration of metal ions in the dilution is 1~5 µg/mL. The sorption capacity (q) of metal ions were defined as q=(C0-Ce) × Vsolution/msorbent, and where C0 and Ce represent the concentrations of metal ions in the aqueous phase for the control experiment and the sorption experiment after 2 h stirring, respectively; msorbent and Vsolution designate the weight of the sorbent and the solution volume used in the sorption experiment, respectively. All values were measured in duplicate with the uncertainty within 5%. Theoretical Methods The geometries were optimized using hybrid functional, B3LYP, performed in Gaussian 0924. The two-component small-core quasi-relativistic effective core potentials (RECP)25, 26, which replace 60 core electrons for uranium atom, have been adopted here in combination with the 10


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corresponding basis set with a segmented contraction scheme. For light atoms (H, C, N, O and Si), the polarized all-electron 6-31G(d) basis set was used for geometry optimizations and energy calculations. Mayer bond order and electron density of the bond critical point were carried out using Amsterdam density functional (ADF 2012) package27. B3LYP method and Slater type orbital (STO) basis set with the quality of triple-zeta plus polarization (TZP) basis set were used without frozen core28. The scalar relativistic effects were taken into account using the zero-order regular approximation (ZORA) approach29.

RESULTS AND DISCUSSION Characterization of KIT-6-DAPhen Sorbent (a)

(b)

(c)

(d)

Fig. 3 SEM/TEM images of KIT-6(a, b) and KIT-6-DAPhen (c, d)

Fig. 3 and Fig. 4 show the characterization results of KIT-6 and KIT-6-DAPhen sorbents. Irregular morphology of KIT-6 and KIT-6-DAPhen is seen from SEM images and 3-D cubic 11



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pore structures are determined by TEM (Fig. 3). The small angle XRD pattern (Fig. 4a) indicates that the mesoporous structure is commensurate with the Ia3d symmetry. In addition, the diffraction peak of KIT-6-DAPhen is not largely shifted compared to that of KIT-6, which may be due to the large pore diameter and pore volume of KIT-6. The N2 sorption/desorption isotherms of KIT-6 and KIT-6-DAPhen are shown in Fig. 4b. Both isotherms curves belong to typical type IV with H1 hysteresis loop according to the IUPAC classification. The surface areas of KIT-6 and KIT-6-DAPhen are calculated to be 746.5 m2/g and 395.6 m2/g, respectively, using the Brunauer-Emmett-Teller (BET) method. The total pore volumes are estimated to be 0.95 cm3/g and 0.46 cm3/g for KIT-6 and KIT-6-DAPhen, respectively. After grafted with the phenanthrolineamide groups, the average pore size is reduced from 8.9 to 5.6 nm (Fig. 4c). To confirm the covalent attachment and the chemical stability of the functional groups in the prepared hybrid materials, FT-IR, solid state NMR, and TGA measurements were performed. As shown in Fig. 4d, compared to that of KIT-6, two new characteristic bands in the FT-IR spectrum of KIT-6-DAPhen at 1551 cm-1 and 1501 cm-1 can be assigned to the stretching vibration of C=C groups of aromatic ring from the grafted phenanthrolineamide groups, while the new bands at 3051 and 2943 cm-1 represents the stretching vibration of C-H groups. When enlarged the spectra (shown as inset in Fig.4d), another new peak that for KIT-6-DAPhen was revealed at 1655 cm-1, which overlaid with the band assigned to H–O–H deformation vibrations of absorbed H2O molecules at 1630 cm-1. These peaks clearly indicate the successful introduction of phenanthrolineamide groups into the mesoporous silica. The solid state

13

C

12


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CP/MAS NMR spectrum of KIT-6-DAPhen is shown in Fig. 4e. Three intense resonance peaks were observed at chemical shifts δ = 12, 24, and 45 ppm, which can be definitely assigned, in turn, to methylene carbon atoms from silicon end to nitrogen end in (−Si−CH2−CH2−CH2−N−) (No. 1-3, respectively). A small peak that represent the amide carbon atoms was revealed at δ = 167 ppm (No.4). Several peaks at δ range of 120-160 ppm overlaid each other, but can be reasonably assigned to aromatic carbon atoms of phenanthroline moiety (No. 5-10, respectively). Besides, it is noted that a small peak assigned to ethoxy groups in (−Si−O−CH2CH3) appears at δ = 60 ppm, and the wide peak at δ = 24 ppm is believed to mask the signal of the remaining methyl carbon species, which is an indication that a little part of ethoxysilane keep unreacted during the functionalization. From the NMR data, however, it is possible to conclude that covalent

attachment

of

the

proper

extraction

agent

analogue

functionality

(i.e.,

phenanthrolineamide groups) on the KIT-6 pore surface was successful. From TGA curve of KIT-6-DAPhen (Fig. 4f), we can see that four distinct stages of weight loss occurs with an increase of temperature. The first stage arises from the volatilization of physically adsorbed water and organic solvent, while the stage II, III and IV can be mainly attributed to the pyrolysis of the grafted phenanthrolineamide on the surface of KIT-6 since the pyrolysis temperature of DAPhen ligand is between 200 to 600oC as shown in Fig. S5. The total weight loss is 22.9% except the volatilization of water and toluene, from which the grafting ratio of the targeted functional groups can be calculated to be ~ 0.66 mmol g-1 based on the assumption that all the weight losses are assigned to phenanthrolineamide degradation (the molecular mass of 13



phenanthrolineamide functional groups is 348u). The grafting rate, however, is slightly overestimated because the unreacted amino groups also contribute the weight loss in this temperature range. 750

2

3

4

5

450 300 150

3051

1655

3200

2400

5

10

15

20

25

30

35

100 3

O C O

N H

Si

2

(e)

1

2.1%

95

O

KIT-6 KIT-6-DAPhen

4.6%

O

KIT-6-DAPhen

1501

1600

0

Pore diameter (nm)

8,9 4,5 7 6 10

1800

0.05

0.0 0.2 0.4 0.6 0.8 1.0

9 78 6 3 1 O N 10 N O Si 2 NH 4C O5 O

2943

1551

0.10

Relative press (p/p0)

KIT-6

KIT-6 KIT-6-DAPhen

0.00

0

2 Theta (degree)

(d)

(c)

0.15

Weight (%)

1

KIT-6 KIT-6-DAPhen

600

3

KIT-6 KIT-6-DAPhen

0.20

(b)

3

(a)

dV/dD (cm /g nm)

Volume adsorbed (cm /g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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90

(f) 11.6%

85 80

6.7%

75

1400

1600

800 -1

240

Wavenumber /cm

160

80

0

Chemical shift (ppm)

70

200

400

600

800

o

Temperature ( C)

Fig. 4 Characterizations of KIT-6 and KIT-6-DAPhen sorbents. (a) XRD patterns, (b) N2 sorption/desorption isotherms, (c) Pore size distribution, (d) FT-IR spectra, (e)

13

C CP/MAS

NMR spectra, and (f) TGA profiles.

U(VI) Sorption into KIT-6-DAPhen sorbent To further explore the sorption behavior of U(VI) on the KIT-6 and KIT-6-DAPhen sorbent, we conducted the batch sorption of U(VI) from aqueous solution under various conditions of pH,

14


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contact time, and initial U(VI) concentration. The selectivity towards U(VI) and reusability of the sorbent were also assessed for the aim of practical applications. 300 250

q (mg U/g sorbent)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


200 150 100

KIT-6 KIT-6-DAPhen

50 0 0

50

100

150

200

250

300

350

Time (min)

Fig. 5 Effect of the contact time on the U(VI) sorption by KIT-6 and KIT-6-DAPhen. pH=5.5, msorbent/Vsolution =0.4 mg mL-1, [U]initial=100 mg/L. Solid line shows pseudo-first-order model fitting results.

Sorption kinetics. To determine the sorption kinetics of U(VI) ions onto KIT-6 and KIT-6-DAPhen, the sorption at a time range of 1-360 min were performed with an initial U(VI) concentration of 100 mg/L. As shown in Fig. 5, the sorption of U(VI) ions by both KIT-6 and KIT-6-DAPhen is ultra-fast in the initial 10 min, indicating a high affinity between U(VI) ions and the surface of sorbents. With the decrease in the number of active sites, the uptake rate of U(VI) ions slowly declines and then reaches an equilibrium. The final sorption equilibrium time is about 60 min and 120 min for KIT-6 and KIT-6-DAPhen sorbent, respectively. That is, U(VI)

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sorption onto KIT-6-DAPhen shows a slower kinetics. In subsequent experiments, the mixture of the sorbent and the solution were all stirred for at least 240 min to ensure equilibrium. In order to clarify the sorption process of U(VI) onto KIT-6 and KIT-6-DAPhen sorbents, the pseudo-first-order kinetic model and pseudo-second-order kinetic model were applied to analyze the sorption kinetic data (Fig. S3). The detailed description of the two models and the fitting results are presented in SI-3 and Table S2 in Supporting Information. It is found that the pseudo-first-order model poorly matches (R2=0.8) with the experimental kinetics data, whereas the pseudo-second order model gives much better correlation coefficient (R2>0.99). In addition, the calculated qe values from the pseudo-second-order kinetic model for KIT-6 and KIT-6-DAPhen are in good agreement with the experiment data. Obviously, the pseudo-second order model is more appropriate for explaining the kinetics of U(VI) sorption by both KIT-6 and KIT-6-DAPhen, implying that the sorption process is mainly dominated by chemical sorption. Besides, to further assess the effect of intraparticle diffusion to the entire sorption process, the sorption kinetics data were also analyzed by intraparticle diffusion model given by Weber and Morris (Table S2 and Fig. S3). It is clear that before the sorption reaches equilibrium, the experimental data reasonably fit with the intraparticle diffusion model, suggesting that intraparticle diffusion is one of the key points for the rate determination in the U(IV) sorption by both KIT-6 and KIT-6-DAPhen. A better correlation coefficient (R2= 0.98) and a lower sorption rate constant (kid) for KIT-6-DAPhen imply that the intraparticle diffusion played more important roles for U(VI) sorption by KIT-6-DAPhen than that by KIT-6, corresponding to a 16


Page 17 of 38

slower sorption kinetics for KIT-6-DAPhen than that for KIT-6. This is actually understandable from the porous structure of the sorbents. That is, the introduction of the bulky organic ligand, i.e. phenanthrolineamide group, into KIT-6 caused an obvious decrease of the surface area, pore volume, and pore size of the sorbent as denoted by the BET measurement, and the connectivity of pores was also partly reduced. In such a case, the intraparticle diffusion of U(VI) in KIT-6-DAPhen was limited, thus leading to a slower sorption kinetics. Effect of pH. The pH value of aqueous solution is one of the important factors in the sorption process, since H+ ion can not only affect the surface charge of the sorbent, but also change the speciation of metal ions in the solution. Additionally, H+ ions can compete with metal ions in the complexation with functional groups. Herein we investigated the effect of pH ranging from 2.0 to 6.0 on the U(VI) sorption onto KIT-6 and KIT-6-DAPhen sorbents.

KIT-6-DAPhen

200

20

150 0 100

KIT-6

50

-20

Zeta potential(mv)

40

250

q (mg U/g sorbent)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


blank control 0 2

3

4

5

6

-40

pH

Fig. 6 Effect of pH on the U(VI) sorption by KIT-6( ▲ ) and KIT-6-DAPhen( ■ ). msorbent/Vsolution=0.4 mg/mL, [U]initial =100 mg/L. The blank control data were also given as (●); 17



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 38

Two dash dot lines represent zeta potentials of KIT-6-DAPhen (top) and KIT-6 (bottom) as a function of pH.

As shown in Fig. 6, the U(VI) sorption by the two sorbents rapidly increases with the augmentation of the solution pH. As reported previously30, the U(VI) speciation in aqueous solution largely depends on the solution pH. We thus here calculated the aqueous and solid speciation of U(VI) as a function of pH in the absence of sorbent according to the thermodynamic data reported previously,31 as shown in Supporting Information SI-2. It is clear that higher pH arouse the formation of multi-nuclear hydroxide complexes32, and the multi-nuclear hydroxide complexes are believed to be more favored by the sorbents, thus leading to an enhancement of the U(VI) uptakes with increase of the solution pH. On the other hand, pH-dependent surface charge of the sorbent may also be responsible for the pH-dependent U(VI) sorption. Also shown in Fig. 6 are zeta potentials of KIT-6-DAPhen and KIT-6 as a function of pH. As can be seen that at a lower pH (here pH=2), the surface of both KIT-6 and KIT-6-DAPhen is positive charged, and now almost no U(VI) sorption occurred for KIT-6, whereas for KIT-6-DAPhen a U(VI) sorption of ca. 28 mg/g was accomplished, which is of practical value for the most applications. This result clearly reveals the important roles of the grafted ligand in KIT-6-DAPhen for binding U(VI).21 As the solution pH increased, the surface charge for KIT-6 becomes negative due to the deprotonation of the surface –OH, thus the U(VI) sorption enhanced as the result of the electrostatic interaction.33,

34

For KIT-6-DAPhen,

18


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however, the surface charge keeps positive in the whole test pH range, and the U(VI) sorption enhanced with the increasing of pH probably because of the improved complexation ability between phenanthrolineamide and U(VI) ions by weak protonation of phenanthrolineamide groups. Such a result further confirm the binding effect of the grafted ligand in KIT-6-DAPhen.21 Besides, both the blank control data as shown in Fig. 6 and the calculation (SI-2) show that a precipitate such as schoepite occurs at pH ≥ 6. To avoid the interference of the U(VI) precipitation and achieve higher sorption capacity, a pH range of 4~ 5 was selected for further sorption experiments. Sorption isotherms. The sorption isotherms of KIT-6 and KIT-6-DAPhen sorbents towards U(VI) were determined at a constant pH value of 5.0±0.1 by varying the initial U(VI) concentration ranging from 5 to 200 mg/L. As shown in Fig. 7, the U(VI) sorption onto KIT-6-DAPhen is significantly higher than that of unmodified KIT-6. The maximum sorption capacity of U(VI) for KIT-6-DAPhen, for example, is larger than 300 mg/g at Ce > 60 mg L-1, while that for KIT-6 is only ca. 125 mg/g at Ce = 30 mg/L. The result clearly reveals the important role of the grafted phenanthrolineamide groups for binding U(VI) ions. Besides, it is noted that the U(VI) sorption onto KIT-6-DAPhen has a sharp peak at a Ce of < 10 ppm, corresponding to ca. 180 mg/g of sorption, and beyond this value, the Ce greatly exceeds the maximum concentration limit of WHO set for U(VI) in drinking water (15 ppb). That is, the effective sorption capacity of KIT-6-DAPhen for U(VI) during the purification of water was deemed as less than 180 mg/g, while the saturation sorption capacity is larger than 300 mg/g. 19



350

KIT-6 KIT-6-DAPhen

300

q (mg U/g sorbent)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 38

250 200 150 100 50 0 0

20

40

60

80

100

Ce (mg/L)

Fig. 7 Sorption isotherms of U(VI) by KIT-6 and KIT-6-DAPhen sorbent, pH = 5.0±0.1, msorbent/Vsolution = 0.4 mg mL-1. solid line: Langmuir model; dotted line: Freundlich model.

In order to understand the sorption mode of U(VI) onto KIT-6 and KIT-6-DAPhen, the sorption data were applied to Langmuir, Freundlich and Dubinin–Radusckevich (D-R) models. The detailed descriptions of the three models are presented in the supporting information (SI-4) and the fitting plots as well as the parameters are given in Fig. S4 and Table S3. It can be seen that the Langmuir model gives better correlation coefficient (> 0.99) and much closer saturated capacity (143 mg/g for KIT-6, 328 mg/g for KIT-6-DAPhen ) to the experimental value (125 mg/g for KIT-6, 310 mg/g for KIT-6-DAPhen), suggesting that Langmuir model, i.e. monolayer and uniform sorption mode35, is more appropriate to explain the sorption of U(VI) in both KIT-6 and KIT-6-DAPhen. Besides, from the D-R fitting, the mean free energy (E) of 11.3 kJ mol-1 for KIT-6 and 10.7 kJ mol-1 for KIT-6-DAPhen were obtained. Since the numerical value of E in the range of 1–8 and 9–16 kJ/mol forecasts the physical sorption and chemical sorption, 20


Page 21 of 38

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respectively, the E values obtained in this work clearly suggests that the sorption of U(VI) in both KIT-6 and KIT-6-DAPhen 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 Langmuir model. Table 1 compared the maximum sorption capacity of KIT-6-DAPhen toward U(VI) with other functionalized mesoporous silica materials reported in previous literatures. From the point of sorption capacity view, KIT-6-DAPhen is superior to most of mesoporous silica materials in U(VI) sorption. It may be due to the interconnected large pore 3-D cubic mesopore structure and the strong complexation ability of phenanthrolineamide functional groups towards U(VI). Thus, it is believed that the KIT-6-DAPhen sorbent is quite promising in uranium removal and recovery from aqueous solution. Table 1 Comparison of U(VI) sorption onto various functionalized mesoporous silica sorbents. sorbents

experimental conditions

qmax(mg/g)

ref

MSU-H

Ambient temperature, pH =8.3±0.1

81

36

MSA-III

Ambient temperature, pH=8.3±0.1

68

36

MSPh-III


185

36

DIMS


268

16

NP10

T=298 K, pH=6.9±0.2

303

14

PA-SBA-15

T=284 K, pH=5.5

373

15

SBA-15-DT4A

T=298±1 K, pH=4.00±0.02

122

37

KIT-6-DAPhen

T=298 K, pH=5

310

this work 21



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Thermodynamic study.

Page 22 of 38

To obtain thermodynamic data of the U(VI) sorption onto

KIT-6-DAPhen and further understand the nature of the sorption process, the sorption of U(VI) by KIT-6-DAPhen sorbent at different temperatures was performed, and the results were shown in Fig. 8. It can be seen that the U(VI) uptake onto KIT-6-DAPhen increased with increasing of temperature, indicating that a higher temperature would be more favorable for the U(VI) sorption. It is well known that the temperature dependence of a sorption process is associated with changes in thermodynamic parameters. Herein, three basic parameters, standard free energy (△Go), standard enthalpy (△Ho), and standard entropy (△So) were derived from the equations as:

△Go= △Ho-T△So, LnKd = -△Ho/(RT)+△So/R, where Kd is the distribution coefficient (mL g-1), R is the gas constant (8.314 J mol-1 K-1) and T is the absolute temperature (K). The results were shown in inset in Fig. 8 and Table 2. The positive value of △H° (11.2 kJ mol-1) indicates the endothermic nature of the U(VI) sorption onto the KIT-6-DAPhen sorbent, while the negative values of △Go suggests the spontaneous nature of the sorption process. Besides, the more negative values of △Go at higher temperature further confirm that a higher temperature is more favorable for the sorption process. Table 2 Thermodynamic parameters for U(VI) sorption onto KIT-6-DAPhen Temp. (K) ∆Ho (kJ mol-1) 298 308 328

11.2

∆So (J (mol K)-1)

∆Go (kJ mol-1)

108.7

-21.2 -22.3 -24.5

22


Page 23 of 38

300

250 9.0 -1

Ln Kd (mL g )

q (mg U/g sorbent)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


200

150

8.8 8.6

2

R =0.99

8.4 0.0030

100

0.0032

-1

0.0034

1/T (K ) 300

310

320

330

Temp.(K)

Fig. 8 U(VI) sorption onto KIT-6-DAPhen at different temperatures. pH=5.0, msorbent/Vsolution = 0.4 mg mL−1, [U]initial=150 mg L−1. Inset shows plot of lnKd vs.1/T. Selectivity test. The sorption of U(VI) by KIT-6-DAPhen from the aqueous solution containing Co(II), Ni(II), Zn(II), Sr(II), La(III), Nd(III), Sm(III), Gd(III), and Yb(III) was investigated at pH 4.0 and 5.0, respectively. The results are shown in Fig. 9. The U(VI) uptake by KIT-6-DAPhen is ~40 mg/g at pH=4.0 while the value at pH = 5.0 reaches ~120 mg/g. In addition, the U(VI) uptake decreases compared to that in the absence of competing ions, which means there is a negative effect of the competing ions on the U(VI) sorption. However, the KIT-6-DAPhen sorbent exhibit superior U(VI) sorption over other metal ions. The selectivity coefficient is commonly used to assess selectivity of a sorbent for a certain metal ion. Herein, the selectivity coefficient (SU/M) for U(VI) relative to competing ions is defined as38:

SU / M =

KdU KdM

23



where and K dM are the distribution coefficients of U(VI) and competing ions, respectively. It is found that the SU/M value for U(VI) relative to all the competing ions is larger than 11 at pH = 5.0, revealing that the KIT-6-DAPhen sorbent shows a desirable selectivity for U(VI) over a range of competing metal ions at this pH value. This result is clearly understandable since there are several fundamental differences between uranyl ion and other test metal ions, e.g. different sizes, different effective charges, and different ionic potentials. Next work we will compare the selectivity of other actinides with lanthanides by KIT-6-DAPhen to confirm our previous conclusion that the preorganized tetradentate phenanthrolineamide (DAPhen) ligand with combined hard-soft donors shows selective extraction ability towards actinides over lanthanides21. 140 120

q (mg/g) e

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 38

pH=4, KIT-6-DAPhen pH=5, KIT-6-DAPhen

100 80 60 40 20 0

Co Gd

La

Nd Ni

Sm

Sr

U

Yb

Zn

Fig. 9 Competitive sorption of coexistent ions by KIT-6-DAPhen, C(Metal ions) = 0.5 mmol/L. Stability assessment, desorption, and reusability of the sorbent. XRD and BET techniques were employed to get the knowledge about the structural and textural evolvements of the sorbent upon its interaction with U(VI), as shown in Fig. S6 in Supporting Information. It is observed 24


Page 25 of 38

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that the small angle XRD pattern of KIT-6-DAPhen shows no largely changes following U(VI) sorption, suggesting that the ordered mesostructure of the sorbent can be maintained during the sorption. The decrease in intensity is reasonably attributed to disturbance of the sorbed U(VI) ions. The N2 adsorption-desorption measurements show that after U(VI) sorption, both the surface area and the pore volume of KIT-6-DAPhen was slightly reduced, and the reductions is a certain indication that the sorbed U(VI) ions enter into the sorbent pores. To desorb the U(VI) ions from KIT-6-DAPhen, the U(VI)-loaded sorbent was treated using HNO3 solution. Quantitative desorption of U(VI) using various concentrations of HNO3 is shown in Table S5. It was found that most of U(VI) (> 88%) ions were desorbed from KIT-6-DAPhen by using 0.05 mol L-1 HNO3 solution, while a complete desorption (~100%) of U(VI) was achieved by using 0.5 mol L−1 or more concentrated HNO3 solution. To further evaluate the stability of the sorbent following U(VI) desorption, the reusability of the KIT-6-DAPhen sorbent was tested by equilibrating 4 mg of reclaimed KIT-6-DAPhen with 10 mL of U(VI) solution (100 mg L−1) at pH 5. After 4 h sorption and determination of solution U(VI) concentration, the desorption of U(VI) was performed as described above, thus forms a sorption–desorption cycle. For comparison, a reference sorption experiment was also conducted using fresh KIT-6-DAPhen at the same conditions during each sorption–desorption cycle. The results are listed in Fig. 10. It can be seen that the U(VI) sorption by the regenerated KIT-6-DAPhen was not obviously decreased even after 5 cycles, which suggest an excellent reusability of the sorbent. During each sorption–desorption cycle, 0.2 mol L−1 HNO3 solution 25



was used as the eluent and the sorbent was redried at 100 °C. The repeated acid-treated process and heat-treated process did not raise discernible changes of the sorbent, which give further support that the KIT-6-DAPhen sorbent shows a good stability. 100 Reusability %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 38

50

0

1

2 3 4 Cycle number

5

Fig. 10 The reusability of KIT-6-DAPhen for U(VI) uptake. [U]initial=100 mg L−1, pH=5, msorbent/Vsolution = 0.4 mg mL−1

Test with simulated seawater. As denoted in Fig. 6, much better performance of KIT-6-DAPhen for U(VI) sorption was achieved in a higher pH, which gives an indication that this kind of sorbent perhaps could be used to extraction U(VI) from seawater. Keep this in mind, analysis of simulated seawater samples by KIT-6-DAPhen was performed. The simulated seawater contains extremely high concentration of other ions such as 20335.8 mg/L NaCl, 10693.2 mg/L Mg(NO3)2·6H2O, 677 mg/L KCl and 1515 mg/L CaCl2, which was spiked with around 4, 16 and 40 µg/L U(VI), respectively. The prepared simulated seawater was kept at room temperature for ~ 7 days in order to fully absorb CO2. The extraction results were shown in Table 3. It is clear that KIT-6-DAPhen sorbent shows high performance for U(VI) extraction from simulated

26


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seawater. For the three test U(VI) concentrations, the extraction percent is all over 98%. At C0=3.8 ppb (close to the U(VI) concentration in real seawater), the U(VI) extraction was almost

complete (> 99%). Such a result clearly reveals the vast opportunity of this kind of sorbent applied in U(VI) uptake from seawater. Further detail works, however, should be done before its real application. Table 3 U(VI) extraction by KIT-6-DAPhen towards simulated seawater sorbent

pH

KIT-6-DAPhen 4 mg/10 mL

8.30 8.27 8.33

U concentration(ug/L) C0 Ce 3.8 0.0055 16.5 0.026 42.0 0.042

U extraction (%) > 99 98.3 98.0

Interaction between Uranyl and KIT-6-DAPhen

Fig. 11 Optimized structures of the uranyl complexes with the ligand. H, C, N, O and U atoms are represented by grey, green, blue, red and pink spheres, respectively.

To elucidate the binding modes of the U(VI) ion onto KIT-6-DAPhen, we have simulated uranyl complexes theoretically. To simplify the computation, DAPhen siloxane (L) was selected as a molecular model, but the Si atoms in the model ligand were fixed during the modeling process to simulate the case that DAPhen ligand was anchored to the silica surface via the silanol groups. Three structures of the uranyl complexes with DAPhen siloxane ligand were optimized at the 27



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Page 28 of 38

B3LYP/6-31G(d) level of theory (Fig. 11). In these structures, the ligand is coordinated in a tetradentate chelating mode through two nitrogen donors of the phenanthroline moiety and two oxygen donors of the amide moieties. Unlike [UO2L]2+ complex, in [UO2L(H2O)2]2+ and [UO2L(NO3)]+ complexes, one uranyl ion is also coordinated with two water molecules and one nitrate anion, respectively. The selected bond lengths are listed in Table 4. It can be seen that the U=O bond length in [UO2L(NO3)]+ complex is about 1.766 Å, the longest bond length in the three complexes. The distance of the uranium atom and the nitrogen atom of phenanthroline moiety (U-NL) are all longer than that of the uranium atom and the oxygen of the amide moieties (U-OL) for each the complex. In [UO2L(NO3)]+ complex, for example, the distance of U-NL bond is about 2.74 Å, which is 0.29 Å longer than that of the U-OL bond. The U-NL and U-OL bond distances increase when the water molecules and nitrate anion are considered. In addition, the distance between the uranium atom and the oxygen atom of nitrate anion (U-Onitrate) are shorter than that of the uranium atom and the oxygen atom of water molecule (U-Owater), which indicates that U(VI) ion binds more strongly with the oxygen atom of the nitrate anion than that of the water molecules. Table 4 Selected bond lengths (B, Å), Mayer bond orders (MBO) and electron density (ρ, a.u.) at bond critical point for the uranyl complexes

B

Species

U=O

U-NL

U-OL

U-Onitrate

[UO2L (NO3)]+

1.767

2.740

2.451

2.445

[UO2L (H2O)2]2+

1.758

2.669

2.454

U-Owater

2.582 28


Page 29 of 38

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MBO

ρ

[UO2L]2+

1.755

2.541

2.324

[UO2L (NO3)]+

2.002

0.205

0.298

[UO2L (H2O)2]2+

2.027

0.269

0.331

[UO2L]2+

2.060

0.326

0.440

[UO2L (NO3)]+

0.317

0.036

0.056

[UO2L (H2O)2]2+

0.324

0.043

0.057

[UO2L]2+

0.326

0.060

0.078

0.402 0.157

0.064 0.043

To obtain the bonding nature between the uranyl ion and DAPhen siloxane ligand, we have investigated the Mayer bond order and the electron density at bond critical point (BCP) (Table 4). It is important to point out that the Mayer bond order for the U=O bond is about 2.00, an indication of keeping double bond character. For the U-OL bonds, the largest value is only about 0.44 in [UO2L]2+ complex. As to the U-Onitrate bond, the Mayer bond order is about 0.40, a value higher than that of U-Owater bond, which suggests that the affinity of U(VI) ion with nitrate anion is stronger than with water molecules. These results are in accordance with the analysis of the corresponding bond length. The bonding nature of the U(VI) ion and DAPhen siloxane ligand was also revealed by the topological analysis of the electron density. As we known, the bonding interactions can be characterized and classified according to the properties of the electron density (ρ). In general, the electron density at BCP ρ(r) > 0.20 au can be described as a covalent bond while ρ(r) < 0.10 au is for an ionic bond. The electron density of U=O are over about 0.30, indicating a covalent bond. As for the other bonds connected with uranium atom, their electron

29



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Page 30 of 38

densities are lower 0.10, which reveals that the interactions between U(VI) ion and DAPhen siloxane ligand are predominately ionic.

Fig. 12 The diagrams of canonical valence MOs and the corresponding energies (eV) of the complex [UO2L(NO3)]+. H, C, N, O and U atoms are represented by grey, green, blue, red and pink spheres, respectively. The negative and positive MOs are denoted by yellow and blue color, respectively. In order to further understand the interaction between the uranyl cation and U(VI) ion, the canonical valence molecular orbitals (MOs) relevant to the uranyl cation and ligands are provided. Because the MOs of three complexes are similar, here, we just showed the MOs of the complex [UO2L(NO3)]+ as displayed in Fig. 12 and the atomic orbital compositions of these MOs are also listed in Table S6. It is worth mentioning the MOs possess σ characters and are mainly contributed by the p atomic orbital of the oxygen/nitrogen atom of the ligand and the f and d orbitals of uranium atom. In addition, the percentage of the f orbital is predominant to the MOs, which indicate that the f orbital has significant contribution to all MOs. 30


Page 31 of 38

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In order to explore the complexation mechanism of the uranyl cation and KIT-6-DAPhen ligand, three thermodynamic reactions were considered. As provided in Table 5, the changes of the Gibbs free energies are negative, which indicates that these three reactions seem to be favorable species in thermodynamics. However, the change of the Gibbs free energies for the reaction L + [UO2(H2O)5]2+ + NO3- = [UO2L(NO3)]+ + 5H2O is the lowest (-390.11 kcal/mol) among the three reactions, revealing a most possible sorption mechanism between U(VI) and KIT-6-DAPhen described by this reaction. This result consists with the reported DAPhen-U(VI) complex in our previous works.21 Four of the six coordination sites in the equatorial plane of UO22+ are occupied by the tetradentate DAPhen ligand thus the left two are insufficient to bind with another tetradentate ligand. Moreover, the steric-hindrance of the bulky DAPhen also limits water molecules and nitrate ions to approach the first coordination sphere of UO22+, thus one of the two nitrate ions act as both counter ions and chelating agent in inner-sphere, while another one is only a counter ion in outer-sphere. Table 5 Calculated changes of the Gibbs free energies (∆Gg, kcal/mol) for the uranyl cations with L in the gas phase. Reaction

∆Gg

L + [UO2(H2O)5]2+ + NO3- = [UO2L(NO3)]+ + 5H2O

-390.11

L + [UO2(H2O)5]2+ = [UO2L(H2O)2]2+ + 3H2O

-245.85

L + [UO2(H2O)5]2+ = [UO2L]2+ + 5H2O

-219.71

31



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Page 32 of 38

CONCLUSION In summary, we reported here a new efficient and feasible U(VI) sorbent, phenanthrolineamide functionalized mesoporous KIT-6 (KIT-6-DAPhen). The efficiency is clearly demonstrated by the fast sorption kinetics of ca. 10 min, large sorption capacity of more than 300 mg/g at a relatively low pH, and desirable selectivity for U(VI) over a range of competing metal ions. The practicability is from the fact that the sorbent shows good potential of reusability, thus reduces the cost for the practical application. With regard to the detail sorption mode, the pre-organized phenanthroline moiety of KIT-6-DAPhen provides rigidity, while the lateral carbon chains provide certain flexibility. When U(VI) ions were incorporated, the flexibility of the lateral carbon chains make oxygen donors of the amide moieties rotate towards nitrogen side of phenanthroline, finally forming most favorable complex geometry as denoted by the DFT calculation. Besides, the thermodynamic analysis further indicated that the sorption might be described

by

the

reaction

of

KIT-6-DAPhen

+

[UO2(H2O)5]2+

+

NO3-

=

[UO2(KIT-6-DAPhen)(NO3)]+ + 5H2O. This work provides new clues for assessing the feasibility of mesoporous silica as supporting material for recovery of uranium from waste water and/or preconcentration of uranium from seawater. Further works are in progress to test the adsorbability of KIT-6-DAPhen towards americium, thorium, and europium, and to assess radiation stability of this new sorbent for assessing its feasibility applied in group separation of actinides over lanthanides.

32


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ASSOCIATED CONTENT Supporting Information (SI): 1H NMR spectra of DAPhen siloxane; The calculated aqueous speciation of U(VI); The U(VI) sorption data fitting by Kinetics models and Isotherm models; TGA profile of DAPhen ligand; Compositions of the coexistent ions solution in selectivity test; XRD patterns and N2 sorption/desorption isotherms of KIT-6-DAPhen before and after U(VI) sorption; The desorption of U(VI) from KIT-6-DAPhen; Contribution (%) of uranium (U) and the nitrogen (NL) and oxygen (OL) atoms of ligand, and the oxygen atom of nitrate anion (ON) to the delocalized canonical MOs for the complex [UO2L(NO3)]+. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author Prof. Dr. Wei-Qun Shi. Tel:86-10-88233968; E-mail: [email protected]; Dr. Cheng-Liang Xiao. E-mail: [email protected]

Present Addresses §

School for Radiological and Interdisciplinary Sciences (RAD-X) and Collaborative Innovation

Center of Radiation Medicine of Jiangsu Higher Education Institutions, Soochow University, Suzhou 215123, China

Author Contributions ‡

The first two authors contributed equally to this work. 33



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Page 34 of 38

ACKNOWLEDGMENT

This work was supported by the National Natural Science Foundation of China (Grant No. 21471153, 21477130 and U1432103), the Natural Science Foundation of Jiangsu Province (BK20150313), the “Strategic Priority Research Program” of the Chinese Academy of Sciences (Grant No. XDA030104). This work is also supported by a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). The results described in this work were obtained on the ScGrid of Supercomputing Center, Computer Network Information Center of Chinese Academy of Sciences. REFERENCES (1) Li, W.; Zhao, D. An Overview of the Synthesis of Ordered Mesoporous Materials. Chem. Commun. 2013, 49 (10), 943-946. (2) Hoffmann, F.; Cornelius, M.; Morell, J.; Froba, M. Silica-based Mesoporous Organic-inorganic Hybrid Materials. Angew. Chem. Int. Ed. Engl. 2006, 45 (20), 3216-3251. (3) Makowski, P.; Deschanels, X.; Grandjean, A.; Meyer, D.; Toquer, G.; Goettmann, F. Mesoporous Materials in the Field of Nuclear Industry: Applications and Perspectives. New J. Chem. 2012, 36 (3), 531-541. (4) Florek, J.; Giret, S.; Juère, E.; Larivièrea, D.; Kleitz, F. Functionalization of Mesoporous Materials for Lanthanide and Actinide Extraction. Dalton Trans. 2016, 45(38), 14832-14854. (5) Birnbaum, J. C.; Busche, B.; Lin, Y. H.; Shaw, W. J.; Fryxell, G. E. Synthesis of Carbamoylphosphonate Silanes for the Selective Sequestration of Actinides. Chem. Commun. 2002, (13), 1374-1375. (6) Yantasee, W.; Lin, Y.; Fryxell, G. E.; Wang, Z. Carbon Paste Electrode Modified with Carbamoylphosphonic Acid Functionalized Mesoporous Silica: A New Mercury-Free Sensor for Uranium Detection. Electroanalysis 2004, 16 (10), 870-873. (7) Fryxell, G. E.; Mattigod, S. V.; Lin, Y. H.; Wu, H.; Fiskum, S.; Parker, K.; Zheng, F.; Yantasee, W.; Zemanian, T. S.; Addleman, R. S.; Liu, J.; Kemner, K.; Kelly, S.; Feng, X. D. Design and Synthesis of Self-assembled Monolayers on Mesoporous Supports (SAMMS): The Importance of Ligand Posture in Functional Nanomaterials. J. Mater. Chem. 2007, 17 (28), 2863-2874. 34


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(8) Feng, X. Functionalized Monolayers on Ordered Mesoporous Supports. Science 1997, 276 (5314), 923-926. (9) Davidson, J. D.; Wiacek, R. J.; Burton, S.; Li, X. S.; Fryxell, G. E.; Addleman, R. S.; Yantasee, W.; Sangvanich, T.; Pattamakomsan, K. Improved Deposition and Deprotection of Silane Tethered 3,4 hydroxypyridinone (HOPO) Ligands on Functionalized Nanoporous Silica. Inorg. Chem. Commun. 2012, 18, 92-96. (10) Yousefi, S. R.; Ahmadi, S. J.; Shemirani, F.; Jamali, M. R.; Salavati-Niasari, M. Simultaneous Extraction and Preconcentration of Uranium and Thorium in Aqueous Samples by New Modified Mesoporous Silica prior to Inductively Coupled Plasma Optical Emission Spectrometry Determination. Talanta 2009, 80 (1), 212-217. (11) Zhang, W.; Ye, G.; Chen, J. Novel Mesoporous Silicas Bearing Phosphine Oxide Ligands with Different Alkyl Chains for the Binding of Uranium in Strong HNO3 Media. J. Mater. Chem. A 2013, 1 (41), 12706-12709. (12) Zhang, W.; He, X.; Ye, G.; Yi, R.; Chen, J. Americium(III) Capture Using Phosphonic Acid-functionalized Silicas with Different Mesoporous Morphologies: Adsorption Behavior Study and Mechanism Investigation by EXAFS/XPS. Environ. Sci. Technol. 2014, 48 (12), 6874-6881. (13) Liu, Y.; Yuan, L.; Yuan, Y.; Lan, J.; Li, Z.; Feng, Y.; Zhao, Y.; Chai, Z.; Shi, W. A High Efficient Sorption of U(VI) from Aqueous Solution using Amino-functionalized SBA-15. J. Radioanal. Nucl. Chem. 2011, 292 (2), 803-810. (14) Yuan, L. Y.; Liu, Y. L.; Shi, W. Q.; Lv, Y. L.; Lan, J. H.; Zhao, Y. L.; Chai, Z. F. High Performance of Phosphonate-functionalized Mesoporous Silica for U(VI) Sorption from Aqueous Solution. Dalton Trans. 2011, 40 (28), 7446-7453. (15) Wang, X.; Yuan, L.; Wang, Y.; Li, Z.; Lan, J.; Liu, Y.; Feng, Y.; Zhao, Y.; Chai, Z.; Shi, W. Mesoporous Silica SBA-15 Functionalized with Phosphonate and Amino Groups for Uranium Uptake. Sci. China Chem. 2012, 55 (9), 1705-1711. (16) Yuan, L. Y.; Liu, Y. L.; Shi, W. Q.; Li, Z. J.; Lan, J. H.; Feng, Y. X.; Zhao, Y. L.; Yuan, Y. L.; Chai, Z. F. A novel Mesoporous Material for Uranium Extraction, Dihydroimidazole Functionalized SBA-15. J. Mater. Chem. 2012, 22 (33), 17019-17026. (17) Yuan, L. Y.; Bai, Z. Q.; Zhao, R.; Liu, Y. L.; Li, Z. J.; Chu, S. Q.; Zheng, L. R.; Zhang, J.; Zhao, Y. L.; Chai, Z. F.; Shi, W. Q. Introduction of Bifunctional Groups into Mesoporous Silica for Enhancing Uptake of Thorium(IV) from Aqueous Solution. ACS Appl. Mater. interfaces 2014, 6 (7), 4786-4796. (18) Lebed, P. J.; de Souza, K.; Bilodeau, F.; Lariviere, D.; Kleitz, F. Phosphonate-functionalized Large Pore 3-D Cubic Mesoporous (KIT-6) Hybrid as Highly Efficient Actinide Extracting Agent. Chem. Commun. 2011, 47 (41), 11525-11527. (19) Lebed, P. J.; Savoie, J.-D.; Florek, J.; Bilodeau, F.; Larivière, D.; Kleitz, F. Large Pore Mesostructured Organosilica-Phosphonate Hybrids as Highly Efficient and Regenerable Sorbents for Uranium Sequestration. Chem. Mater. 2012, 24 (21), 4166-4176. 35



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37



TOC art

140 120 100

qe(mg/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 38 of 38

80 60 O O Si O

40

N

N

N C O H

C O

U

N H

pH=4 pH=5

O Si O O

20 0

Co

Gd

La

Nd

Ni

Sm

Sr

U

Yb

Zn

38


Large-Pore 3D Cubic Mesoporous (KIT-6) Hybrid Bearing a HardâSoft

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