Ion-Imprinted Mesoporous Silica for Selective Removal of Uranium

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Ion-Imprinted Mesoporous Silica for Selective Removal of Uranium from Highly Acidic and Radioactive Effluent Sen Yang, Jun Qian, Liangju Kuang, and Daoben Hua ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b09419 • Publication Date (Web): 07 Aug 2017 Downloaded from http://pubs.acs.org on August 11, 2017

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ACS Applied Materials & Interfaces

Ion-Imprinted Mesoporous Silica for Selective Removal of Uranium from Highly Acidic and Radioactive Effluent Sen Yang a, Jun Qian a, Liangju Kuang b, Daoben Hua a,c *

a

School for Radiological and Interdisciplinary Sciences (RAD–X) & College of Chemistry,

Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, China b

Department of Agricultural and Biological Engineering, Purdue University, West Lafayette,

Indiana 47907, USA c

Collaborative Innovation Center of Radiological Medicine of Jiangsu Higher Education Institutions,

Suzhou 215123, China

AUTHOR EMAIL ADDRESS: [email protected] (S. Yang); [email protected] (J. Qian); [email protected] (L. Kuang); [email protected] (D. Hua)

CORRESPONDING AUTHOR FOOTNOTE. Dr. D. Hua Tel & Fax: (+) 86–512–65883261; E–mail: [email protected]

KEYWORDS: uranium sorption; ion-imprinting; mesoporous silica; radioactive; strong acid

1

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ABSTRACT

It is strategically important to recycle uranium from radioactive liquid wastes for future uranium supply of nuclear energy. However, it is still a challenge to adsorb uranium selectively from highly acidic and radioactive waste. In this paper, we report a novel strategy for effective uranium removal from highly acidic and radioactive media by surface ion-imprinted mesoporous silica sorbent. The sorbent was successfully synthesized by a co-condensation method with uranyl as the template ion and diethylphosphatoethyltriethoxysilane (DPTS) as the functional ligands. The pseudo-second-order model and Langmuir model showed better correlation with the sorption kinetic and isotherm data, and the sorption equilibrium could be reached within 40 minutes, the maximum adsorption capacity from Langmuir model was 80 mg/g in 1 mol/L nitric acid (HNO3) solution at 298.15 K. The sorbent showed faster kinetics and higher selectivity towards uranium over other ions compared with non-imprinted mesoporous and other previous sorbents. Furthermore, the ion-imprinted materials exhibited remarkable radioresistance stability and could be regenerated efficiently after five cycles. This work may provide a new approach for highly efficient sorption of uranium from strong HNO3 and radioactive media.

2


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1. INTRODUCTION A large number of radioactive wastes are being produced with the development of nuclear industry. Uranium is considered one of the main contaminants and a hazard to environment and human due to its radioactive and chemical toxicity

1, 2

. Meanwhile, uranium is also the crucial raw material of the

nuclear industry. Therefore, it is important to extract uranium from radioactive liquid wastes for sustainable development of the nuclear industry 3. In the past few decades, liquid-liquid extraction has been the most common technique employed for separation of radionuclide from nuclear waste 4. In comparison with liquid-liquid extraction, solid phase sorption has received much attention recently due to its flexibility, simplicity, economy, and lower environmental pollution 5, 6. To date, a variety of sorbents have been developed for exaction of uranium from highly acidic and/or radioactive medium, such as synthetic resins framework

13

. For example, Subramanian et al.

8

7-9

, mesoporous silica

10-12

and covalent organic

reported the extraction of thorium and uranium

using a polymeric resin from simulated nuclear spent fuel mixtures; Chen et al.

12

synthesized a

category of mesoporous silicas functionalized with different phosphine oxide ligands for adsorbing uranium from strong HNO3 solutions; Li et al. 13 prepared a covalent organic framework material for removal of uranium in high acidic media (1 mol/L HNO3). Although the great progress has been achieved, some challenges are still subsistent for uranium sorption from radioactive liquid wastes, such as strong acidity, high radioactivity and variety of competing metal ions

12, 14, 15

. Therefore, it

still has great signiﬁcance to develop new sorbents for uranium sorption with good radioresistance, high selectivity and stability in strong acidic media. To improve the selectivity of sorbents, an ion-imprinting technique is introduced into the sorption field. The affinity between organic ligand and templated ions as well as the size of the generated 3



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cavities contribute to the high selectivity

16, 17

. For instance, Meng et al.

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18

synthesized a surface

ion-imprinted polymer for the selective uranium sorption at pH 5.0 by graft polymerization with uranium (VI)-methacrylic acid complex; Qian et al.

19

prepared uranyl ion-imprinted

polymer-functionalized composites by locating polymerization for efficient uranium removal from aqueous solution of pH 5.0. However, to our best knowledge, there are few reports on ion-imprinted materials for uranium removal in highly acidic and radioactive media. It is acknowledged that mesoporous silica is a promising matrix owing to its high specific surface area, mechanical and thermal stability, chemical and irradiation durability, and easy organic modification20-23. In addition, organophosphorus compounds have the remarkable coordination ability to fission products and high chemical and mechanical stability in strong acidic and radioactive media

24-28

. Motivated by these findings, we report herein novel ion-imprinted mesoporous silica

functionalized by phosphorous ligands for selective removal of uranium from highly acidic and radioactive media. Specifically, the sorbent was prepared by a co-condensation method with UO22+ as template ion, tetraethoxysilane (TEOS) as silica source, triblock copolymers (P123) as porous template, and diethylphosphatoethyltriethoxysilane (DPTS) as functional ligands (Scheme 1). The ion-imprinted mesoporous silica can be obtained after the elution of the uranyl ion and P123 using ethanol-hydrochloric acid (37 wt%) mixture (9/1, v/v). The recognition cavities for UO22+ in size, shape and chemical functionality were formed on the surface of mesoporous silica. Therefore, it is expected that the obtained uranyl ion-imprinted mesoporous silica (UIMS) could effectively adsorb uranyl ions from strongly acidic and radioactive media.

4


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Scheme 1. The schematic for synthesis of uranyl ion-imprinted mesoporous silica (UIMS).

2. EXPERIMENTAL SECTION 2.1 Materials and reagents Diethylphosphatoethyltriethoxysilane (DPTS, 92%) was purchased from J&K Scientific Co., Ltd. Poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) copolymer (P123) was purchased from Sigma-Aldrich Co., Ltd. Hydrochloric acid (37 wt%, GR),

tetraethoxysilane

(TEOS, AR), ethanol (AR) and nitric acid (AR) were from Sinopharm Chemical Reagent Co., Ltd. UO2(NO3)2·6H2O was purchased from Sigma Aldrich Fluka. Characterization methods for all the samples were shown in Supporting Information. 2.2 Synthesis of uranyl ion-imprinted mesoporous silica (UIMS) UIMS was synthesized according to the modified literature method.

29

The typical procedure was

described as below: UO2(NO3)2·6H2O (0.25 g, 5.0×10-4 mol) and a certain amount of DPTS were added into ultrapure water (10 mL), and stirred magnetically at 35 oC for 16 h to obtain a pre-assembly solution. In the meantime, P123 (4.0 g, 6.9×10-4 mol) was dissolved in 30 mL of ultrapure water and 120 mL of HCl solution (2 mol/L) under stirring for 16 h at 35 oC. The two solutions were mixed together dropwise for 1 h and continue to be stirred for 2 h. TEOS (8.5 g, 4.0× 10-2 mol) was then added into the mixture dropwise under stirring. After stirring for 24 h, the 5



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solution was transferred into an autoclave and heated to 100 oC for another 24 h. The mixture was then filtered and washed with ultrapure water (500 mL × 3), and desiccated in a vacuum oven at 40 °C. P123 and UO22+ were removed by Soxhlet extraction with a solution of ethanol-hydrochloric acid (v/v, 9/1, 50 mL per 1 g sample) at 80 oC for 24 h. FT-IR spectra of UIMS-4 verified diethylphosphonate ester was not hydrolyzed after Soxhlet extraction (Figure S1, Supporting Information). Energy-dispersive X-ray (EDX) analysis was used to confirm that uranyl ions were removed completely. Finally, the UIMS was obtained and desiccated in the vacuum oven at 40 °C. Analogous samples were prepared by the same procedure using the different TEOS/DPTS molar ratios (Table 1). For comparison, the non-imprinted mesoporous silica (NIMS-1 and NIMS-4) was prepared in the same way in the absence of uranyl ions, and mesoporous silica (SBA-15) was synthesized according to the literature methods (Supporting Information) 30. Table 1. The feeding ratio, elemental content and organic content of functional silica. Feeding ratio Sorbents

a

TEOS

DPTS

(mmol)

(mmol)

UIMS-1

40.0

NIMS-1

Organic

UO2

2+

Content

(mmol)

(%)

2.00

0.500

2.80

40.0

2.00

---

2.92

UIMS-2

40.0

5.00

0.500

5.10

UIMS-3

40.0

10.0

0.500

10.4

UIMS-4

40.0

20.0

0.500

12.3

NIMS-4

40.0

20.0

---

13.6

a

Determined by thermogravimetric analysis (TGA).

2.3 Uranium sorption experiments All sorption experiments were executed in polyethylene plastic tubes. Solutions of uranium (VI) 6


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were obtained by dissolving UO2(NO3)2·6H2O in HNO3 and further diluting with ultrapure water. The pH values of the solutions of uranium were adjusted by HNO3 (2 mol/L) and sodium carbonate solution (2 mol/L). A typical sorption procedure was as follows: the sorbent (5.0 mg) was added into uranyl ion solution (5.0 mL) in polyethylene tube and shaken for 12 h until the sorption equilibrium was achieved. The concentration of uranyl ion before and after sorption was tested by ICP-MS. The equilibrium sorption amount (qe, mg/g), distribution ratio (Kd) and sorption efficiency (SE) were calculated according to Equations (1), (2) and (3), respectively: q e = (C 0 − C e )

Kd =

V M

C0 − Ce V × Ce M

SE (%) =

C 0 − Ce × 100 C0

(1) (2)

(3)

where C0 and Ce (mg/L) are the initial and residual concentration of uranium(VI) in solution, respectively. M (g) is the dry sorbent weight, and V (L) is the volume of aqueous solution. The effects of sorbent dose, acidity/pH, competing ions and contact time on the sorption were studied. The isotherm experiments were carried out with the different initial uranium concentrations of uranium in the solution of 1 mol/L HNO3 at 298.15 K. Kinetic studies were investigated using 1× 10-4 mol/L of uranium in the solution of 1 mol/L HNO3 at 298.15 K. The competing sorption was performed in the solution of 1 mol/L HNO3 with 5.0 mg of sorbent and a mixture of UO2(NO3)2·6H2O, ZnCl2, CoCl2, Ba(NO3)2, MnCl2·4H2O, CsNO3, Ni(NO3)2·6H2O, CrCl3·6H2O, SrCl2·6H2O, NaNO3, Nd(NO3)3, EuCl3·6H2O, Sm(NO3)3·6H2O, ZrCl4, Gd(NO3)3·6H2O and La(NO3)2·6H2O. 2.4 Regeneration studies 7



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The reusability of UIMS was evaluated by five sorption-desorption cycles with the same concentration of sorbent (1.0 mg /mL) in the solution of 1 mol/L HNO3 (5.0 mL, 1×10-4 mol/L) at 298.15 K for 1 h until the sorption equilibrium was achieved. After saturated sorption, UIMS-uranium complex was eluted with a solution of ethanol-hydrochloric acid (9/1, v/v) by Soxhlet extraction at 80 oC under stirring for 24 h. 2.5 Irradiation stability study The radioresistance of modified mesoporous silica was evaluated by

60

Co γ-ray irradiation at a 15

Gy/min of dose rate. A typical irradiation process was depicted as follows: 50.0 mg of UIMS-4 was irradiated with different irradiation doses (such as 100, 300, and 500 kGy) under ambient atmosphere or HNO3 solution (1 mol/L).

3. RESULTS AND DISCUSSION 3.1 Characterization of UIMS The morphological features of ion-imprinted mesoporous silicas were characterized by transmission electron microscopy (TEM) (Figure 1). Ordered hexagonal lattice arrangement was observed clearly for SBA-15, UIMS-1 and NIMS-1 (Figure 1A, B and Figure S2, Supporting Information). However, the ordered structures were observably degraded for UIMS samples with an increasing amount of organic ligand (Figure 1, from B to E). It was noticed that UIMS-4 and NIMS-4 didn’t show ordered mesoporous structure, which may be ascribed to the larger amount of organic ligands in their preparation. The small angle XRD patterns further demonstrate this point. For SBA-15, there were three well-resolved peaks that were indexed to (100), (110) and (200), indicating the ordering degree of the mesoporous structure; whereas, the diffraction peaks are not obvious for all modified 8


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mesoporous silica because of the introduction of organic ligands (Figure S3, Supporting Information). The porosity analysis of all the sorbents indicated that the BET specific surface areas and average pore diameters decreased when the amount of organic ligands was increased (Table 2; Figure S4, Supporting Information). What's more, the pore sizes of uranyl ion-imprinted mesoporous silica are smaller than those of non-imprinted ones, because they are the average values of the mesopore of the silica and the uranyl ion-imprinted cavities. It is also noticed that BET surface area and pore volumes for ion-imprinted mesoporous silica are larger than those of non-imprinted ones, which should be attributed to the sum of the above-mentioned two kinds of pores 19, 31.

Figure 1. TEM images of (A) SBA-15, (B) UIMS-1, (C) UIMS-2, (D) UIMS-3, (E) UIMS-4, and (F) NIMS-4 (scale bar: 50 nm). (G) FT-IR spectra and (H) TGA curves of (a) SBA-15, (b) UIMS-1, (c) NIMS-1, (d) UIMS-2, (e) UIMS-3, (f) UIMS-4, and (g) NIMS-4.

9



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Table 2. Parameters of the porous structure for the obtained materials. Specific surface areas Material

Pore volume 3

Pore diameter

2

(m /g)

(cm /g)

(nm)

SBA-15

681

1.00

9.55

UIMS-1

588

0.716

7.61

NIMS-1

561

0.709

7.78

UIMS-2

454

0.580

6.70

UIMS-3

382

0.506

6.20

UIMS-4

286

0.146

1.06

NIMS-4

224

0.125

1.17

The chemical structures of the sorbents were characterized by FT-IR spectra and EDX. Compared with bare SBA-15 (Figure 1G, trace a), the vibration band of P=O (1412 cm-1) cm-1)

33

32

and Si-CH2 (673

(Figure 1G, trace b, c, d, e, f and g) suggested the successful synthesis of UIMS-1~4 and

NIMS-1, 4. The appearance of P elements in EDX spectra further verified DTPS was introduced into the sorbents successfully (Figure S5, Supporting Information). Thermogravimetric analysis (TGA) was conducted to study the chemical content of the samples, and three stages of mass loss was shown in Figure 1H: stage I was the desorption of crystal water; stage II was the process of decomposition of the organic groups in the range of temperatures between 200 oC and 450 oC, which was proportional to the amount of DPTS (Table 1); and stage III can be attributed to the condensation of the neighboring silanol groups to form siloxane bonds at 500 o

C-700 oC 34.

3.2 Effects of sorbent dose and acidity/pH on uranium sorption The sorption performance of sorbent toward uranium is always affected by different experimental conditions. First, we studied the effects of sorbent dose and acidity/pH on uranium sorption. The sorption efficiency increases quickly with the increase of sorbent dose, and reaches an equilibrium at 10


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1.0 g/L sorbent content at pH 5.9 (Figure 2A). Therefore, 1.0 g/L was chosen as the optimum dose for the next sorption experiments. As shown in Figure 2B, the sorption efficiency of uranium in strong acidic environment (such as 1, 2, and 3 mol/L) was calculated as 66.33%, 34.54% and 14.63% respectively. This sorption behavior of sorbent in strong acidic solution was due to the enhanced protonation of phosphonate ligands with the increase of HNO3 concentration. It is known that UO22+ is the main species of uranium (VI) in strong acid solution, and is not favored by the positively charged ligands owing to the electrostatic repulsion, resulting in a lower sorption capacity

28, 35

. In addition, the sorption efficiency of uranyl

ions onto the UIMS-4 exceeded 90% at pH 1~7, which may be ascribed to the generated cavities and surface silanol groups towards uranium (VI). The dominant uranium (VI) species was UO22+ at pH＜ 5,

36

which can match with the imprinted cavities. However, the main uranium (VI) species was not

UO22+ at pH＞5. The good sorption behavior may be attributed to the surface silanol groups

12, 37

.

The results showed that it could be used to recover uranium from strong acidic media.

Figure 2. (A) Effects of sorbent dose on the sorption of uranium by UIMS-4; (B) The effect of acidity/pH on the sorption of uranium (VI) onto UIMS-4. (Experimental conditions: 5.0 mg sorbent dose, 1×10-4 mol/L uranium (VI), contact time = 12 h, 298.15 K, and acidity/pH: A) pH = 5.9, and B) from 3 M to pH = 7). 3.3 Effects of competing ions on uranium sorption

11



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The most advantage of ion-imprinted technique is the ability to enhance the selectivity for the imprinting metal ion. To evaluate the selectivity of ion-imprinted materials, competitive experiments were carried out with the common ions in simulated high-level liquid waste (like Nd, Eu, Sm, La, Gd, Zr, Na, U, Sr, Cs, Ni, Mn, Co, Cr, Ba and Zn)

38, 39

at pH=5.2 and 1 mol/L HNO3 solution,

respectively. Table 3. Selective sorption of uranium on UIMS-4 and NIMS-4. (Experiment condition: 5.0 mg sorbent dose, 5.0 mL HNO3 solution (1 mol/L), and 298.15 K, contact time = 12 h).

Competing Ions

a

Initial Concentration

βa

Kd (mL/g)

βr b

mg/L

UIMS

NIMS

UIMS

NIMS

U (VI)

93.5

412

184

--

--

--

Na(I)

136

12.6

62.6

32.7

2.94

11.1

Cs (I)

167

18.9

86.4

21.8

2.13

10.2

Sr (II)

126

35.0

69.9

11.8

2.64

4.47

Ni (II)

138

23.3

71.3

17.7

2.58

6.85

Zn (II)

209

10.3

77.0

40.0

2.39

16.7

Co(II)

125

16.9

60.8

24.4

3.03

8.06

Mn(II)

131

31.0

68.0

13.3

2.71

4.91

Ba(II)

129

12.4

67.9

33.4

2.71

12.3

Cr (III)

117

15.6

83.3

26.4

2.21

11.9

Zr(IV)

209

14.4

56.4

28.7

3.27

8.76

Nd(III)

58.5

128

154

3.23

1.20

2.70

Sm(III)

37.2

34.6

72.1

11.9

2.55

4.67

La(III)

102

38.8

111

10.6

1.67

6.38

Eu(III)

73.9

92.3

129

4.47

1.43

3.13

Gd(III)

24.4

62.4

120

6.62

1.53

4.32

β = Kd(U)/Kd(M), Kd(U) and Kd(M) are the dispersion coefficient of uranium and competing ions,

respectively; b βr = βimprinted/βnon-imprinted, βimprinted and βnon-imprinted represent the selectivity coefficients of UIMS-4 and NIMS-4, respectively. 12


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The values of Kd and selectivity coefficients are listed in Table 3. The Kd of UIMS-4 for uranyl ions was remarkably larger than other ions, suggesting a much higher selectivity for uranium. Furthermore, the relative selectivity coefficient (βr) was much larger than 1, which indicated much better selectivity for imprinted mesoporous silica to uranium than that for non-imprinted ones. This result may be ascribed to the specific recognition cavities towards uranium generated through surface ion-imprinted technique, which were close to the diameter of uranyl ion of 0.36 nm 16, 40. Noticeably, there was relatively low Kd of UIMS for Na (I), Cs (I), Co (II), Ba (II), Ni (II), Zn (II), Cr (III) and Zr (IV), which may be physically embedded into the cavities of imprinted mesoporous silica. Whereas, the Kd values of UIMS for most lanthanides, such as Nd (III), Sm (III), Gd (III), La (III) and Eu (III), were much larger than the other ions except uranium (VI). The results may be related to the average pore diameters of mesoporous silica materials besides the affinity between P=O group and lanthanide ions12, 41. The hydrated lanthanide ions with larger sizes are relatively closer to the average pore diameters of UIMS-4 (1.06 nm), which are relatively difficult for the ions to go out of pore channel, thereby leading to be easily captured by the materials. In contrast, the ions with smaller size could go in and out of the framework more freely 13, 40. The selectivity of UIMS-4 at pH of 5.2 was also studied (Table S1, Supporting Information). The relative selectivity coefficients (βr) at pH 5.2 were larger than 1, which was consistent with that in 1 mol/L HNO3 solution. This result reveals that imprinted mesoporous silica possess better selecatively both in strong and weak acidic solutions because of the specific recognition cavities towards uranium. In addition, we notice that the βr in 1 mol/L HNO3 solution was larger than those at pH 5.2, which indicated that UIMS-4 possessed better selectivity compared with non-imprinted ones in a higher acidic solution. In other words, the imprinted cavities could greatly improve the selectivity for 13



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uranium in strong acidic condition compared with non-imprinted materials. Therefore, the ion-imprinted technique plays an important role in the selectivity towards uranium in strong acidic solution. The Kd and selectivity coefficients of UIMS-1 and NIMS-1 are listed in Table S2 (Supporting Information). The relative selectivity coefficient (βr) was much larger than 1, which was consistent with UIMS-4 and NIMS-4. The result further verified the better selectivity of ion-imprinted mesoporous silica. Moreover, the selectivity coefficient (β) of UIMS-1 was larger than UIMS-4 (Table 3), which indicated better selectivity to uranium. This result may be ascribed that most of the diethylphosphonate ester in UIMS-1 was imprinted to form the cavities due to a relatively small ratio of DPTS/uranyl ion.

3.4 Sorption kinetics In this study, the effect of initial uranium concentration on sorption efficiency was first investigated (Figure S6, Supporting Information). The sorption efficiency increases with initial uranium concentration from 1×10-6 mol/L to 1×10-4 mol/L. The similar phenomenon was also observed in the previous literatures

42, 43

. The sorption was associated with the relatively weak affinity between

uranyl ions and imprinted cavities. The driving force of sorption may depend on the diffusion of uranyl ions. The higher initial concentration leads to larger concentration gradient and diffusion between solution and sorbent, which will promote the sorption of larger portion of uranyl ions. However, when the initial concentration is over 1×10-4 mol/L, sorption efficiency began to decrease, which may be ascribed to the presence of excessive uranyl ions in solution compared to the available active sites on the UIMS-4. Therefore, we choose 1×10-4 mol/L as the initial uranium concentration to do the effect of contact time on the uranium sorption. 14


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Sorption kinetics of imprinted sorbents and non-imprinted mesoporous silica were studied in HNO3 solution (1 mol/L) and 298.15 K. Figure 3 shows the relationship between sorption amount and contact time in uranium sorption. The sorption equilibrium could be reached within 40 min for all imprinted sorbents and within 3 h for the non-imprinted sorbents. And we can find that the equilibrium time would change when the initial concentration of uranium increased (Figure S7, Supporting Information). Furthermore, it can be seen that the sorption on UIMS is faster than most of other previous porous sorbents in 1 mol/L HNO3 solution (Table S3, Supporting Information). The pseudo-first-order and pseudo-second-order models were used to simulate kinetic profiles, respectively (Figure S8, Supporting Information).

Figure 3. Effect of contact time on the uranium sorption by (a) UIMS-2, (b) UIMS-3, (c) UIMS-4 and (d) NIMS-4，respectively. (Experimental conditions: 5.0 mg sorbent dose, 5.0 mL HNO3 solution (1 mol/L), 1×10-4 mol/L uranium (VI), and 298.15 K.) As shown in Table 4, the relatively larger correlation coefficients (R2) and the more accurate calculated qe indicated that pseudo-second-order model was more suitable for the description of kinetic processes. The k2 for imprinted sorbents was much larger than that for NIMS-4, implying the faster sorption, which may be ascribed to the strong complexation and geometric shape memory between UO22+ and corresponding cavities 44. Meanwhile, the k2 for UIMS-2 was larger than those for UIMS-3 and UIMS-4, which may be ascribed to the larger specific surface area and pore diameter 15



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Page 16 of 37

(Table 2).

Table 4. Kinetic parameters for uranium sorption by UIMS-2, UIMS-3, UIMS-4, and NIMS-4. (Experimental conditions: 5.0 mg sorbent dose, 5.0 mL HNO3 solution (1 mol/L), 1×10-4 mol/L uranium (VI), and 298.15 K.) Pseudo-first order

Pseudo-second order

Samples

qe, exp (mg/g)

k1(h-1)

qe, cal (mg/g)

R2

k2 (g/mg/h)

qe, cal (mg/g)

R2

UIMS-2

4.97

1.60

0.742

0.277

2.76

5.09

0.999

UIMS-3

9.02

3.18

1.14

0.184

2.02

9.28

0.998

UIMS-4

16.2

1.05

1.19

0.120

0.801

16.5

0.998

NIMS-4

16.7

0.815

10.6

0.875

0.108

18.4

0.995

3.5 Sorption isotherms To understand the sorbent capacity, sorption isotherms studies were carried out with the initial concentrations of uranium from 8.40×10-5 mol/L to 1.68×10-3 mol/L in 1 mol/L HNO3 solution at 298.15 K. The relationship between equilibrium sorption amount qe and residual uranyl concentration Ce was shown in Figure 4. The sorption profiles were simulated with Langmuir and Freundlich models, respectively (Figure S9, Supporting Information).

16


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Figure 4. Sorption isotherm plots for uranium sorption by (a) UIMS-1, (b) UIMS-2, (c) UIMS-3, (d) UIMS-4 and (e) NIMS-4, respectively. (Experimental conditions: 5.0 mg sorbent dose, 5.0 mL HNO3 solution (1 mol/L), and 298.15 K).

Table 5. Parameters of Langmuir and Freundlich models for uranium sorption by UIMS-1, UIMS-2, UIMS-3, UIMS-4 and NIMS-4. (Experimental conditions: 5.0 mg sorbent dose, 5.0 mL HNO3 solution (1 mol/L), and 298.15 K). Freundlich

Langmuir Sorbent qmax (mg/g)

b (L/g)

R2

KF (mol1-nLn/g)

1/n

R2

UIMS-1

12.3

1.87

0.990

0.0480

0.799

0.987

UIMS-2

24.3

8.90

0.999

1.20

0.475

0.971

UIMS-3

44.0

7.90

0.997

2.06

0.479

0.983

UIMS-4

79.5

8.47

0.995

4.25

0.459

0.957

NIMS-4

79.2

12.4

0.999

7.38

0.379

0.971

As shown in Table 5, the relatively larger R2 indicated that the adsorption process could be well described by Langmuir isotherm model, implying monolayer sorption because of the homogenous distribution of active sites on the surface of mesoporous silica. The sorption capacity increased with the content of P element on the surface of sorbents (Figure S5, Supporting Information). Furthermore, 17



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the sorption capacities of UIMS-4 and NIMS-4 are similar due to the similar content of organic functionalities and BET surface area (Table1 and Table 2). What’s more, it can be seen that the sorption capacity of UIMS in 1 mol/L HNO3 solution is larger than most of other previous porous sorbents in 1 mol/L HNO3 solution (Table S3, Supporting Information). In addition, there was a smaller equilibrium constant b value of 8.47 for UIMS-4 than that for NIMS-4, which may be attributed to the weaker affinity of imprinted cavities for UIMS-4 than that for NIMS-4, which was consistent with the finding in our previous work

45

. The results indicated that the imprinted

mesoporous silica may be an ideal sorbent because of its large capacity in strongly acidic solution.

3.6 Effect of irradiation stability on uranium sorption Irradiation durability of sorbents is an important issue when used in radioactive waste liquids. Herein, γ-ray irradiation tests of ion-imprinted mesoporous silicas were performed with different irradiation doses under air atmosphere and in HNO3 solution, respectively. Solid-state 31P NMR spectroscopy was conducted to examine the chemical structure of UIMS-4 before and after γ-ray irradiation. As shown in Figure 5A, all the samples showed an intense peak at about 33 ppm, suggesting that structural integrity of the sorbents was well preserved after irradiation in air and HNO3 solution. The sorption capacity of UIMS-4 was further checked before and after γ-ray irradiation toward uranium. As shown in Figure 5B, the sorption capacity of uranyl ions remained almost unchanged after irradiation up to a dose of 500 kGy. The results indicated that the UIMS-4 had good radioresistance and could be a potential sorbent in an irradiation environment.

18


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31

Figure 5. (A) Solid-state P NMR spectra of UIMS-4 before (a) and after irradiation in air (b, 100 kGy; d, 300 kGy; and f, 500 kGy) and 1 mol/L HNO3 solution (c, 100 kGy; e, 300 kGy; and g, 500 kGy), respectively. Star (*) indicates spinning side band in the 31P NMR spectra. (B) Sorption of uranium (VI) by UIMS-4 before and after irradiation. (Experimental condition: 5.0 mg sorbent, 5.0 mL HNO3 solution (1 mol/L), 1×10-4 mol/L uranium (VI), and 298.15 K).

3.7 Regeneration studies Regenerability is of significance for an effective and economical adsorbent. In this research, the reusability of the sorbents was evaluated by five cycles of sorption-desorption with ethanol-hydrochloric acid (37 wt%) solution (9/1, v/v) as the desorbing agent. As shown in Figure 6A, the sorption efficiency almost remained unchanged after five cycles. Furthermore, there was no obvious change in FT-IR spectra of UIMS-4 before and after five cycles (Figure 6B), suggesting that the chemical structure of imprinted materials was well preserved during the sorption-desorption cycles. These results indicated that the sorbents have an excellent reusability for uranium (VI) sorption.

19



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Figure 6. (A) Recycling of UIMS-4 for uranium sorption and (B) FT-IR spectra of UIMS-4 (a) before and (b) after five cycles. (Experimental condition: 5.0 mg sorbent, 5.0 mL 1 mol/L HNO3 solution, 1×10-4 mol/L uranium(VI), and 298.15 K).

4. CONCLUSION In summary, we demonstrate a novel strategy for selective uranium removal from highly acidic and radioactive media by surface ion-imprinted mesoporous silica sorbent. The sorbent was prepared by co-condensation method with uranyl as the template ion. The effects of sorbent dose, acidity/pH, competing ions, contact time, initial concentration, and irradiation stability on uranium sorption were investigated. The sorption kinetic and isotherm data fit well with the pseudo-second-order model and Langmuir model, respectively, and the sorption equilibrium could be reached within 40 minutes, the maximum adsorption capacity from Langmuir model was 80 mg/g in 1 mol/L HNO3 solution at 298.15 K. The UIMS-4 showed faster sorption kinetics and higher selectivity towards uranium over other ions compared with non-imprinted mesoporous silicas. Furthermore, the surface ion-imprinted mesoporous silica sorbent can be used in strong acidic media compared with other ion-imprinted materials

19

,

and

possesses

better

selectivity

towards

uranium

in

comparison

with

organophosphorus-functionalized mesoporous silica in strong HNO3 media12. In addition, the UIMS-4 exhibited remarkable radioresistance stability and could be regenerated efficiently after five 20


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cycles. As far as we know, this is the first report on surface ion-imprinted mesoporous silica which can efficiently separate uranium from highly acidic and radioactive media.

ASSOCIATED CONTENT

Supporting Information: Characterization methods, synthesis of SBA-15, Sorption kinetics and isotherms, comparison of sorption kinetics sorption capacity, EDX spectrum, N2 sorption-desorption isotherms and pore size distribution, effects of competing ions on uranium sorption, FT-IR spectra of UIMS-4 before and after Soxhlet extraction, TEM image of NIMS-1, Small angle X-ray diffraction patterns, Effect of initial uranium concentration. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *Email: [email protected], and Tel & Fax: (+) 86–512–65883261 (D. H.)

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work is supported by Natural Science Foundation of China (91326202, U1532111), Key Project of Natural Science Foundation of the Higher Education Institutions of Jiangsu Province (16KJA310001), a Project Funded by the Priority Academic Program Development of Jiangsu

21



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Page 22 of 37

Higher Education Institutions (PAPD), the Qing-Lan Project of Jiangsu Province, and Jiangsu Key Laboratory of Radiation Medicine and Protection. ABBREVIATIONS DPTS, diethylphosphatoethyltriethoxysilane; UIMS, uranyl ion-imprinted mesoporous silica; NIMS, non-imprinted

mesoporous

silica;

P123,

poly(ethylene

glycol)-block-poly(propylene

glycol)-block-poly(ethylene glycol) copolymer; TEOS, tetraethoxysilane.

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For Table of Contents use only.

Ion-Imprinted Mesoporous Silica for Selective Removal of Uranium from Highly Acidic and Radioactive Effluent Sen Yang, Jun Qian, Liangju Kuang, Daoben Hua

29



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Scheme 1. The schematic for synthesis of uranyl ion-imprinted mesoporous silica (UIMS). 53x22mm (600 x 600 DPI)


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Figure 1. TEM images of (A) SBA-15, (B) UIMS-1, (C) UIMS-2, (D) UIMS-3, (E) UIMS-4, and (F) NIMS-4 (scale bar: 50 nm). (G) FT-IR spectra and (H) TGA curves of (a) SBA-15, (b) UIMS-1, (c) NIMS-1, (d) UIMS2, (e) UIMS-3, (f) UIMS-4, and (g) NIMS-4. 96x113mm (600 x 600 DPI)



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Figure 2. (A) Effects of sorbent dose on the sorption of uranium by UIMS-4; (B) The effect of acidity/pH on the sorption of uranium (VI) onto UIMS-4. (Experimental conditions: 5.0 mg sorbent dose, 1×10-4 mol/L uranium (VI), contact time = 12 h, 298.15 K, and acidity/pH: A) pH = 5.9, and B) from 3 M to pH = 7). 35x15mm (600 x 600 DPI)


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Figure 3. Effect of contact time on the uranium sorption by (a) UIMS-2, (b) UIMS-3, (c) UIMS-4 and (d) NIMS-4，respectively. (Experimental conditions: 5.0 mg sorbent dose, 5.0 mL HNO3 solution (1 mol/L), 1×10-4 mol/L uranium (VI), and 298.15 K.) 43x32mm (600 x 600 DPI)



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Figure 4. Sorption isotherm plots for uranium sorption by (a) UIMS-1, (b) UIMS-2, (c) UIMS-3, (d) UIMS-4 and (e) NIMS-4, respectively. (Experimental conditions: 5.0 mg sorbent dose, 5.0 mL HNO3 solution (1 mol/L), and 298.15 K). 44x33mm (600 x 600 DPI)


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Figure 5. (A) Solid-state 31P NMR spectra of UIMS-4 before (a) and after irradiation in air (b, 100 kGy; d, 300 kGy; and f, 500 kGy) and 1 mol/L HNO3 solution (c, 100 kGy; e, 300 kGy; and g, 500 kGy), respectively. Star (*) indicates spinning side band in the 31P NMR spectra. (B) Sorption of uranium (VI) by UIMS-4 before and after irradiation. (Experimental condition: 5.0 mg sorbent, 5.0 mL HNO3 solution (1 mol/L), 1×10-4 mol/L uranium (VI), and 298.15 K). 42x22mm (600 x 600 DPI)



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Figure 6. (A) Recycling of UIMS-4 for uranium sorption and (B) FT-IR spectra of UIMS-4 (a) before and (b) after five cycles. (Experimental condition: 5.0 mg sorbent, 5.0 mL 1 mol/L HNO3 solution, 1×10-4 mol/L uranium(VI), and 298.15 K). 39x19mm (600 x 600 DPI)


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For Table of Contents use only. 35x17mm (600 x 600 DPI)


Ion-Imprinted Mesoporous Silica for Selective Removal of Uranium

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