Quaternary phosphonium-grafted porous aromatic framework

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Quaternary phosphonium-grafted porous aromatic framework for preferential uranium adsorption in alkaline solution Yinglin Shen, nini chu, suliang yang, xiaomin li, hong cao, and guoxin tian Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b03580 • Publication Date (Web): 06 Sep 2019 Downloaded from pubs.acs.org on September 6, 2019

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Industrial & Engineering Chemistry Research

1 2

Quaternary phosphonium-grafted porous aromatic framework for preferential uranium adsorption in alkaline solution

3 4 5 6

Yinglin Shena*, Nini Chua, Suliang Yangb*, Xiaomin Lia , Hong Caoa, and Guoxin Tianb* a School of Nuclear Science and Technology, Lanzhou University,730000, P. R. China b Radiochemistry Department, China Institute of Atomic Energy, Beijing 102413,China.

7

ABSTRACT: Uranium, as an energy source and radioactive waste, is very

8

important in the nuclear fuel cycle. Recovery of uranium from nuclear waste solution

9

is essential for further treatment and disposal. Herein an ionic liquid functionalized

10

porous aromatic framework materials P-C4 (PPN-6-CH2P+(C4H9)3Cl-) for uranium

11

adsorption from alkaline solution was synthesized by grafting PPN-6 with the

12

quaternary phosphonium for the first time. The U-exchange kinetics perfectly

13

conforms to pseudo-second-order dynamic model which reveals the chemical

14

adsorption process. P-C4 exhibits a record high uranium exchange capacity of over

15

670 mg∙g-1 and can efficiently capture [UO2(CO3)3]4- ions in the presence of the high

16

concentrations of HCO3-, CO32-, F-, SO42-, Cl-, and NO3-. In addition, the uranyl

17

tricarbonate in the loaded meterial could be easily eluted with a diluted hydrochloric

18

acid. These advantages make P-C4 a new potential material for separating uranium

19

from alkaline solution.

20

KEYWORDS: Porous aromatic framework, Quaternary phosphonium, Uranium

21

separation, Anion exchange materials

22

1. Introduction

23

Radioactive wastewater is produced in every step of the nuclear fuel cycle. Only by

24

separating and treating the radionuclides can it meet the emission standards and be

25

discharged. Uranium is one of the most important radioactive element, different

26

materials have been developed to separate uranium, such as organic polymers,1

27

biopolymers,2 silicon-based functionalized materials,3 metal-organic frameworks4 and

28

so on. However, these materials suffer from their own shortcomings so it is very

29

important to study separation materials and methods of uranium.

ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 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

30

The highly porous aromatic framework (PAF) material has an extremely robust

31

allcarbon scaffold, high surface areas and high water and chemical stability, which

32

enable them to be used in harsh conditions.5-10 A supported ionic liquids (SILs) can be

33

prepared by immobilization of ionic liquid on PAF by chemical reaction. The

34

immobilization process of ILs can transfer their desired properties to substrates.

35

Combination of the advantages of ILs with those of support materials will derive

36

novel performances while retaining properties of both moieties. Once the

37

ion-exchange groups are grafted to the hydrophobic backbones of the highly porous

38

robust framework, a high density of readily accessible ion-exchange sites that are

39

arranged into threedimensional nanospace will be achieved (Scheme 1). It is expected

40

to provide a new ion exchange material with strong ion exchange ability, fast ion

41

exchange kinetics, controllable swelling and high chemical stability. At present, only

42

Ma Shengqiang grafted quaternary ammonium salt onto PPN-6 to remove precious

43

metals from electroplating wastewater. 11

44

In the process of uranium enrichment, yellow cake is converted into UO2 in

45

fluidized bed furnace, then into uranium tetrafluoride (UF4), and then into uranium

46

hexafluoride (UF6). UF6 is isotopically enriched, and the waste gas is absorbed by

47

sodium carbonate or ammonium carbonate solution. This process will produce

48

alkaline uranium-rich wastewater. In the alkaline medium of the industrial effluent,

49

uranium is capable of forming anionic species, mainly as uranyl carbonate complexes,

50

[UO2(CO3)2]2- and [UO2(CO3)3]4- .12,13

51

Herein, we focus on functionalizing PPN-6 for efficient uranium adsorption from

52

the alkaline aqueous solutions by grafting the quaternary phosphonium. This work

53

originated from the discovery that uranyl tricarbonate can be extracted efficiently

54

from water to quaternary phosphonium ionic liquids (P66614Cl ) through anion

55

exchange reaction.14 This SILs grafting of ionic liquids on PPN-6 will have many

56

advantages over free ILs, including avoiding the leaching of ILs, reducing their

57

dosage, and improving the recoverability and reusability of themselves.

58

In this contribution, we demonstrate, for the first time, a new type of ion


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59

exchange material capable of rapidly exchanging uranyl tricarbonate with an

60

exceptional uptake capacity of over 670 mg/g, excellent selectivity and high chemical

61

stability without swelling or entrainment. Moreover, the PAF-based meterial can be

62

readily regenerated and reuse without significant loss of uranium adsorption capacity.

63

2. Experimental

64

2.1 Reagents

65

The reagents purchased were analytical and not further purified. PPN-6 and

66

PPN-6-CH2Cl were synthesized according to previously reported procedures. 15,16

67

2.2 Adsorbent Synthesis

68

Synthesis of P-C4 (PPN-6-CH2P+(C4H9)3Cl-), P-C2 (PPN-6-CH2P+(C2H5)3Cl-), and

69

N-C2 (PPN-6-CH2N+ (C2H5)3Cl-)

70

The quaternary phosphonium IL functionalized P-C4 was synthesized by

71

PPN-6-CH2Cl(200 mg) reacting with tributylphosphine (0.5 mL) in 30 mL EtOH at

72

80 °C for 3 days under N2. The reacting mixture was filtered, washed with water and

73

methanol, and then dried under vacuum to yield P-C4 (PPN-6-CH2P+(C4H9)3Cl-). The

74

synthesis of P-C2 and N-C2 is similar to P-C4, except that tributylphosphine was

75

replaced by triethylphosphine or triethylamine.

76

2.3 Batch adsorption experiments

77

Sorption kinetic tests

78

Uranyl tricarbonate solution was prepared according to the reference17 by dissolving

79

UO2(NO3)2·6H2O and Na2CO3 in distilled water with an 1:5 molar ratio (pH∼10).

80

Each 14 mL of uranyl tricarbonate solution (50 ppm) and 2 mg of P-C4 were added to

81

a 50 mL centrifuge tube, respectively, and shaken on the oscillator for 5, 45, 60, 90,

82

180, 210, 240, 270, and 300 min at 25 °C and the adsorbents were separated by

83

centrifugal.

84

spectrophotometer and for extra low concentration using trace uranium analyzer. The

85

exchange capacity qt (mg∙g-1) of uranium was calculated with the following equation

The

supernatant

concentrations

were


determined

using

UV


86

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(1): qt  (

87

c0  ct )V m

(1)

88 89

where qt (mg g-1) is the amount of adsorbed uranium at time t, Co and Ct are the

90

concentration of uranium initially and at time t, respectively. V is the volume of

91

solution, and m is the mass of sorbent used.

92

Sorption isotherm tests

93

The sample of 14 ml uranyl tricarbonate solution was prepared. The solution

94

concentration ranged from 20 ppm to 175 ppm and the pH was about 10. P-C4 (2 mg)

95

was added to each sample, and the mixture was separated by centrifugation after

96

shaking for 4 hours at 25 °C on the oscillator. The supernatant was analyzed by trace

97

uranium analyzer. Uranyl tricarbonate solution without adsorbent was used as

98

negative control and analyzed in each adsorption experiment. The exchange capacity

99

qe (mg∙g-1) of uranium was calculated with the following equation:

100

qe  （

c0 - ce ）V m

(2)

101

where C0 and Ce are the concentration of uranium initially and at equilibrium,

102

respectively. V is the volume of solution, and m is the mass of sorbent used.

103

Mixed solutions uranium adsorption test

104

The mixed solution was prepared as follows: sodium chloride (6.4 g, 0.44 M), sodium

105

carbonate (0.0612 g, 2.3×10-3 M), potassium sulphate (0.1317 g, 2.96×10-3 M), sodium

106

fluoride (0.0311 g, 2.96×10-3 M ), sodium nitrate ( 0.0629 g, 2.96×10-3 M) and uranyl

107

nitrate hexahydrate (0.0037 g, 7.05 ppm ) were dissolved in distilled water and fixed

108

volume to 250 mL (pH >8 adjusted with NaOH solution). The mixed solution of 200

109

ml and adsorbent (5 mg) were added to 250 ml plastic trial, and 5 ml sample was

110

taken out after 12 h oscillation at 25 °C. The adsorbent was filtered and separated by


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111

0.45 um membrane. The concentration of uranium in the filtrate was determined by

112

trace uranium analyzer.

113

Mixed solutions trace uranium adsorption test

114

The mixed solution was prepared as follows: sodium chloride (6.4 g, 0.44 M), sodium

115

carbonate (0.0612 g, 2.3×10-3 M), potassium sulphate (0.130007 g, 2.96×10-3 M),

116

sodium fluoride (0.0311 g, 2.96×10-3 M), sodium nitrate (0.0629 g, 2.96×10-3 M) and

117

uranium nitrate hexahydrate (0.0005 g, 1.02 ppm) were dissolved in distilled water

118

and fixed volume to 250 mL (pH >8 adjusted with NaOH solution). 200 ml of the

119

mixed solution and P-C4 (5 mg) were added into 250 ml plastics trial, then 5 ml

120

sample was taken out after 12 h oscillation at 25 °C. The adsorbent was filtered and

121

separated by 0.45 um membrane. The concentration of uranium in the filtrate was

122

determined by trace uranium analyzer.

123

Reuse Test

124

After one adsorption, loaded P-C4 was desorbed and regenerated with dilute

125

hydrochloric acid and washed with water. After vacuum drying, the material was

126

used to adsorb again. The experimental conditions are as follows: Uranium aqueous

127

solution of 25 ml (80 ppm) and P-C4 of 15 mg were added to 50 ml centrifugal tube

128

and oscillated at 25 °C. After 4 h, the mixture was separated by centrifugal and the

129

supernatant concentration was determined using trace uranium analyzer. Then 100

130

mL 0.5 M dilute hydrochloric acid was added to the material loaded uranium and

131

shaken for 4 hours on the oscillator, after centrifugal separation it was washed with

132

20 mL distilled water.

133

3. Results and discution

134

3.1 Synthesis and characterization

135

Tri(hexyl)tetradecylphosphonium

136

chloride

(P66614Cl)

that

is

one

of

the

nonfluorinated and commercially available ILs was found to be obvious effective in



137

extracting uranium by ion exchange reaction.18 To avoid the leaching, reduce dosage,

138

and improve the recoverability and reusability of ILs, the quaternary phosphonium IL

139

is immobilized on supports PPN-6 (scheme 1) by treating PPN-6-CH2Cl with

140

tributylphosphine.

141

organic polymers (PPN-6) that exhibit high surface areas and high water/chemical

142

stabilities, via chloromethylation of PPN-6 followed by the treatment with P(C4H9)3

143

(Scheme1).

144 145 146 147 148 149

Scheme 1 Chemical reaction process for the preparation of P-C4 : (a)AcOH, H3PO4, HCl, Paraformaldehyde;(b) tributyl phosphine, tetrahydrofuran.

150

The successful grafting of quaternary phosphonium salts onto PPN-6 was confirmed

151

by Fourier transform infrared spectroscopy (FT-IR) and energy-dispersive X-ray

152

spectroscopy (EDS) studies. The FT-IR spectra of P-C4 (PPN-6-CH2P+(C4H9)3Cl-)

153

show the aliphatic C-H stretching bands at 2960 cm-1 and 2870 cm-1 as well as the

154

characteristic band of C-P stretching bands at 1310 cm-1 compared with the pristine

155

PPN-6 which without these peaks (Fig. 1). The FT-IR spectra of P-C4 also show the

156

same peaks of C-C stretching bands of benzene ring as those of PPN-6 at 1740 cm-1,

157

1630 cm-1 and 1490 cm-1 and C-H bending bands at 908 cm-1 and 809 cm-1. In spectra

158

of P-C4 loaded uranium, the new peak at 1558 cm-1 appears, which was attributed to

159

carbonate stretching bands. Moreover, the U=O bond (at 948 cm-1) asymmetric

160

stretching band ν3 and the U=O bond (at 835 cm-1) symmetric stretching band ν1 can

161

be seen at the same time in the spectra of the P-C4 loaded uranium, suggesting uranyl

162

tricarbonate went into P-C4 .

3.1.1 FT-IR spectra


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163 164 165

Fig. 1 FT-IR spectra of PPN-6, P-C4 and U-loaded P-C4 .

166 167

3.1.2 EDS

168

The graphted quaternary phosphonium was also confirmed by energy dispersive

169

spectroscopy (EDS). Elemental analysis reveals a phosphorus content of 8 wt.%

170

corresponding to 76 mmol g-1 -P(C4H9)3Cl groups in P-C4, which indicates 50 % of

171

phenyl rings are grafted with one -P(C4H9)3Cl group on per benzene ring (Fig. 2a).

172

Elemental mapping of the pristine P-C4, U-exchanged products and regenerated P-C4

173

are shown in Fig. 2. Elemental mapping of the P-C4 confirmed the presence of

174

phosphorus and its homogeneous distribution in the sample (Fig. 2d).

175



176

177

178 179 180

Fig. 2 The energy-dispersive X-ray spectra of (a) P-C4 (b) P-C4 loaded U (c)

181

regenrated P-C4 and its elemental distribution maps of P in P-C4 (d); U in P-C4

182

loaded U (e) and Cl in regenrated P-C4 (f).

183

3.1.3 Nitrogen adsorption and desorption experiments

184

Nitrogen gas sorption and desorption isotherms collected at 77 K as shown in

185

Fig. 3a. It indicates that the grafting of quaternary phosphonium salts leads to a

186

decrease in the Brunauer–Emmett–Teller (BET) surface area from 2726 m2. g-1 for

187

PPN-6 to 110 m2. g-1 for P-C4 (Fig. 3a) and an increase in pore size about 6 nm (Fig.

188

3b).


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189

190

191 192 193 194

Fig. 3 (a) N2 adsorption(closed)/desorption(open)isotherms at 77 K; (b) Pore size distribution curves.



195

3.2 Uranium sorption studies

196

To evaluate the effectiveness of P-C4 for removing uranium from alkaline

197

carbonate solution, a sample of P-C4 (5 mg) was placed in 20 mL dilute solution

198

(pH~10) containing 1ppm of uranium. After 12 h the uranium concentration in the

199

filtrates was less than 0.04 ppb, that is, almost 99.9% of the uranium was removed by

200

P-C4 under such condition. The EDS analyses of P-C4 loaded-U show that uranyl

201

entered into the materials and chlorides came out (Fig. 2b). Elemental mapping of the

202

exchanged products confirmed the presence of captured uranium and its

203

homogeneous distribution in the sample (Fig. 2e). Under similar conditions, the

204

adsorption capacity of PPN-6 is negligible, indicating that the capture of almost all

205

uranium species is related to the functional groups of the grafted quaternary

206

ammonium chloride. Kd value reflects the affinity and selectivity of adsorbents, and is

207

an important parameter reflecting the adsorptive performance of adsorbents.

208

Calculated with the equation (3), P-C4 affords an extremely high Kd value of 2.2×107

209

mL/g (solid-liquid ratio of 5:20 mg/mL), which is one of the largest Kd value in the

210

reported sorbents. These results therefore highlight the superior ffectiveness of P-C4

211

for removing uranium from alkaline carbonate aqueous solutions.

212 213

214

kd  （

ci - ce V ） ce m

(3)

3.2.1 Sorption kinetic tests:

215

In order to further evaluate the adsorption efficiency of P-C4 for removing

216

uranium from alkaline carbonate aqueous solutions, the uranium adsorption kinetics

217

of P-C4 has been examined (shown as Fig. 4). Time-dependent adsorption

218

measurements show that the exchange capacity reached 80% in 30 minutes and

219

slowly increased to 99.9% after 4 hours. Therefore, all the subsequent experiments

220

were performed for 4h at 298 K except selectivity tests. The experimental data were

221

fitted with the pseudo-second-order kinetic model using the equation (4): where k2 (g.

222

mg-1. min-1) is the rate constant of pseudo-second order adsorption, qt (mg . g-1) is the


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223

amount of uranium adsorbed at time t, and qe (mg. g-1) is the amount of uranium

224

adsorbed at equilibrium.

225

t qt



1 t  2 qe k 2qe

(4)

226

227 228 229 230 231

Fig. 4 (a) Change of adsorption capacity with contact time (50 ppm U in aqueous solution using P-C4 ). (b)The linear regression by fitting thethe kinetic plot data with the seudo-second-order dynamic model.

232

The t/qt curve of kinetic data shows a perfect linear relationship (Fig. 4b), and the

233

correlation coefficient R2 is 0.993, which indicates that the rate limiting step of



Page 12 of 22

234

adsorption process is chemical adsorption. From the changes of chlorine and uranium

235

content detected by EDS (Figs. 2a and 2b), it can be seen that the adsorption process

236

is uranyl tricarbonate exchange chloride. In addition, we found that P-C4 does not

237

adsorb uranium when the pH value of uranium solution is less than 8, because there is

238

almost no uranyl tricarbonate in the solution when the pH value of uranium solution

239

is less than 8. This also indicates that uranium is exchanged in the form of uranyl

240

tricarbonate.

241

3.2.2 Sorption isotherm tests

242

In order to evaluate the overall exchange capacity, the adsorption isotherm curve

243

was graphed in Fig. 5a by the uranium equilibrium concentration (20.0 -175.1 ppm,

244

pH∼10) against the capacity of U-exchange. As can be seen from Fig. 5a, the

245

maximum uranium exchange capacity of P-C4 is about 670 mg.g−1, which is one of

246

the

247

amidoxime-functionalized PPN-627 containing uranyl specific chelate group has a

248

uranium uptake capacity of over 300 mg/g. But the quaternary phosphonium

249

functionalized PPN-6(PPN-6-CH2P+(C4H9)3Cl-) affords more superior adsorption

250

performance than the amidoxime-functionalized meterial. It indicates that the

251

quaternary phosphonium functionalized PPN-6 has more accessible exchange sites

252

and stronger affinity for uranyl tricarbonate.

highest

reported

values

of

uranium

adsorption


materials.19-26

The

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253

254 255 256

Fig. 5 (a) Adsorption isotherm for P-C4 (b) The linear regression by fitting the equilibrium adsorption data with Langmuir adsorption model.

257 258

The isotherm was well fitted with the Langmuir model, giving rise to a correlation

259

coefficient R2=0.9945(Fig. 5b). The Langmuir isotherm model describes adsorption

260

on a homogenous surface and presumes that a maximum uptake exists. From Fig. 5a,



261

the adsorption capacity of P-C2 (PPN-6-CH2P+(C2H5)3Cl-) is similar to P-C4 . It is

262

found that shortening the chain length of quaternary phosphonium salts has little

263

effect on the adsorption capacity. It is also found from Fig. 5a the adsorption capacity

264

of N-C2 (PPN-6-CH2N+(C2H5)3Cl-) is lower than that of P-C4 and P-C2 under the

265

same conditions.

266

3.2.3 Selectivity tests

267

Uranium coexists with a large number of ions in alkaline low-level radioactive

268

wastewater in nuclear industry, it is required that the adsorption material selectively

269

adsorb uranium. To illustrate the adsorption selectivity of P-C4 for [UO2(CO3)3]4−,

270

the adsorptions of uranyl tricarbonate by P-C4 were studied in the presence of

271

different concentration of Cl-, NO3-, SO42-, CO32-, HCO3-, and F-, respectively, in

272

aqueous solution at a pH value of about 8-10. The whole experiment was repeated

273

three times. The results are listed in the table 1.

274

Table 1 shows the adsorption properties of P-C4 for uranium with increasing

275

concentration of coexisting anions in turn. It is apparent that P-C4 has a desirable

276

selectivity for U in the presence of a large excess of Cl-, NO3-, SO42-, CO32-, HCO3-

277

and F-, respectively. In particular, the presence of NO3- (NaNO3:U molar ratio 102

278

~104), SO42- ( K2SO4:U molar ratio 102 ~103) and Cl- ( NaCl:U molar ratio 102 ~104)

279

has little effect on the adsorption of uranium, and the removal rate of P-C4 for

280

uranium is more than 93%, and the value of U selectivity coefficient Kd is more than

281

105ml/g.The presence of CO32- (Na2CO3:U molar ratio 102~103) and HCO3-

282

(NaHCO3:U molar ratio 102~103) has some influence on the adsorption properties of

283

P-C4 for U, but Kd values for U are still more than 103 mL/g. When the molar ratio of

284

F- to uranium is less than 102, it has little effect on uranium adsorption and Kd reaches

285

9.73 × 105 mL/g, when NaF:U molar ratio is 102~6 × 103, the adsorption ability of

286

P-C4 for U decreases, but Kd values for U is still more than 104 mL/g. These data

287

indicates that P-C4 has a higher selectivity and stronger affinity for U than other

288

anions. The reason is that uranyl tricarbonate has larger size and higher negative

289

charge than other anions, so it has stronger electrostatic interaction with quaternary


Page 14 of 22

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290


phosphonium cations

291 292

Table 1 Adsorption properties of P-C4 for uranium under single salt coexisting

293

with uranyl tricarbonate (20 ppm), solid-liquid ratio of 1:7 mg/mL, pH > 8, at

294

25 °C.

295

Compounds

Concentration of compounds（M）

Molar ratio of coexisting anions to U

Adsorption percentage (%)

Kd (mL/g)

NaCl

8.4×10-3

102

95.00%

1.33×105

NaCl

8.4×10-2

103

95.00%

1.33×105

298

NaCl

8.4×10-1

104

95.00%

1.33×105

299

NaHCO3

8.4×10-3

102

60.00%

1.05×104

NaHCO3

4.2×10-2

5×102

40.71%

4.81×103

300

NaHCO3

8.4×10-1

103

25.71%

2.42×103

301

Na2CO3

8.4×10-3

102

52.14%

7.63×103

302

Na2CO3

4.2×10-2

5×102

45.00%

5.73×103

Na2CO3

8.4×10-1

103

40.71%

4.81×103

NaNO3

8.4×10-3

102

94.28%

1.16×105

NaNO3

8.4×10-2

103

94.28%

1.16×105

NaNO3

8.4×10-1

104

94.28%

1.16×105

K2SO4

8.4×10-3

102

98.57%

4.84×105

K2SO4

4.2×10-2

5×102

93.57%

1.02×105

K2SO4

8.4×10-1

103

93.57%

1.02×105

NaF

8.4×10-3

102

99.29%

9.73×105

NaF

8.4×10-2

103

71.43%

1.75×104

NaF

5.1×10-1

6×103

62.86%

1.18×104

296 297

303 304 305 306 307 308 309 310

When a tremendous excess of Cl-, NO3-, SO42-, CO32-, and F- coexisted, the

311

performances of P-C4 for U-exchange were also tested and compared with that of the

312

quaternary ammonium functionalized N-C2 (PPN-6-CH2N+(C2H5)3Cl-). The results

313

were listed in table 2. From table 2, we find that the relative amount of U removed

314

and Kd value of P-C4 are 88.6 % and 3.13 ×105 mL/g in a competitive exchange

315

experiment containing 0.44 M NaCl, 2.3×10-3 M Na2CO3, 2.96×10-3 M K2SO4,

316

2.96×10-3 M NaF, 2.96×10-3 M NaNO3, and 7.05 ppm U in aqueous solution. Under

317

the same conditions, the relative amount of U removed and Kd value are 9.6%,

318

4.24×102 mL/g, respectively, for N-C2. It is worth noting that, P-C4, quaternary

319

phosphonium functionalized materials has a higher selectivity for uranyl tricarbonate



Page 16 of 22

320

than N-C2, quaternary ammonium functionalized materials, although the raw

321

materials PPN-6-CH2Cl for N-C2 synthesis are the same as those for P-C4

322

synthesis and the percentage of grafted functional groups is close to each other. This

323

phenomenon indicates that besides static electricity interaction between anions and

324

cations there may be a strong interaction between the uranyl and the phosphorus

325

atom, which plays an important role in large ion exchange capacity, high removal

326

efficiency and excellent selectivity for uranium. The difference between P atom and

327

N atom is that P atom has a hollow 3d orbital, the axial oxygen atom of uranyl ion

328

may interact with the hollow 3d orbital of P atom, which makes uranyl combine

329

stronger with P-C4 than with N-C2.

330 331

Table 2 Adsorption properties of P-C4 for uranium under various salts

332

coexisting with uranyl tricarbonate, solid-liquid ratio of 1:40 mg/mL, pH > 8, at

333

25 °C

334

Compounds

Molar ratio of coexisting ions to U

NaCl

1.48×104

Na2CO3 K2SO4 NaF NaNO3

78 100 100 100

NaCl

1.02×105

Na2CO3 K2SO4 NaF

538 691 691

NaNO3

691

344

NaCl

1.48×104

345

Na2CO3

78

346

K2SO4

100

NaF

100

NaNO3

100

335 336 337 338 339 340 341 342 343

347

Material

Concentration of U

Adsorption percentage (%)

Ce

Kd

P-C4

7.05 ppm

88.6 %

800 ppb

3.13×105

P-C4

1.02 ppm

99.6 %

4.36 ppb

9.32×106

N-C2

7.05 ppm

9.6 %

6.38 ppm

4.24×102

348 349

We also examined the U-exchang performance of P-C4 towards low


Page 17 of 22 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


350

concentration uranium in the presence of a tremendous excess of Cl-, NO3-, SO42-,

351

CO32-, and F-. In the exchange experiments containing 0.44 M NaCl, 2.3×10-3 M

352

Na2CO3, 2.96×10-3 M K2SO4, 2.96×10-3 M NaF, 2.96×10-3 M NaNO3, and 1.02 ppm

353

U in aqueous solution, P-C4 affords an outstanding Kd value of 9.32 × 106 mL/g and

354

remarkably the removal rate of 99.6 % and effectively reduces the uranium

355

concentration from 1.02 ppm to 4.36 ppb.

356

3.2.4 Reuse Test

357

After one adsorption, P-C4 was desorbed and regenerated with dilute

358

hydrochloric acid and washed with water. After vacuum drying, the material was

359

used to adsorb again. The results indicate that the loaded meterial can be easily eluted

360

by treating with a 0.5 M HCl solution, which was confirmed by EDS. The EDS

361

analyses of the regenerated P-C4 show that chlorides entered the materials and

362

uranium disappeared (Fig. 2c and Stable 1). Elemental map of the regenerated P-C4

363

confirms the presence of chlorine and its homogeneous distribution in the sample (Fig.

364

2f). This high elution efficiency highlights the great potential of the material for

365

separation of uranium from nuclear waste solution. N-C2 loaded uranium was

366

desorbed by diluted hydrochloric acid more easily than P-C4, which indicates that

367

there is only electrostatic interaction between quaternary ammonium salts and uranyl

368

tricarbonate.

369

Conclusion

370

In summary, a new supported ionic liquid by porous aromatic framework is

371

provided with higher adsorption capacity and better selectivity for uranyl tricarbonate

372

than that of chelating material. The maximum uranium exchange capacity of P-C4

373

is 670 mg/g, which is one of the highest reported values for uranium adsorbent

374

materials. The kinetics perfectly conforms to pseudo-second order reaction in the ion

375

exchange process. This reveals the chemical adsorption process and its ion exchange

376

mechanism. In addition, P-C4 can efficiently capture [UO2(CO3)3]4- ions in the



377

presence of the high concentrations of Na+, K+ or HCO3-, CO32-, F-, SO42-, Cl-, and

378

NO3-. P-C4 has excellent adsorption properties for [UO2(CO3)3]4-, which can be

379

attributed to three reasons: the three-dimensional nano-space and pore size

380

distribution of the matrix are favorable for the ions access the exchange site; the high

381

negative charge and large size of uranyl tricarbonate determine the stronger

382

electrostatic interaction with quaternary phosphonium cations than other anions; there

383

may be a strong interaction between the axial oxygen atom of uranyl and the

384

phosphorus atom on quaternary phosphonium, which determines the strong affinity.

385

Moreover, the loaded-U meterial can be easily eluted with a diluted hydrochloric acid.

386

This kind of material is all-carbon skeleton, which can be incinerated after multiple

387

use without generating secondary waste. These advantages render P-C4 a promising

388

adsorption material of radioactive U from low level radioactive waste water.

389

Acnowledgments

390 391

We gratefully acknowledge the financial support from the National Natural Science Foundation (Grant No. 21976075 and 21571179 ).

392 393 394 395 396

ASSOCIATED CONTENT Supporting Information Available: **[ Element content of EDS in P-C4 , P-C4 loaded U and P-C4 after desorption, SEM images of P-C4， P-C4 loaded U and P-C4 after desorption, and synthetic method of PPN-6 and PPN-6-CH2Cl ]**

397 398

The contact information for the corresponding authors:

399

Yinglin Shen, Donggang West Road 199, Lanzhou city, Gansu province, 730000,

400

P. R. China, Tel: 8615095370178, Email: [email protected]

401 402

References：

403

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Copolymer Initiator by Atom-Transfer Radical Polymerization. Angew. Chem.,

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Int. Ed. 2013, 52, 13458-13462


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144x93mm (300 x 300 DPI)


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Quaternary phosphonium-grafted porous aromatic framework

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