Amino-Functionalized UreaâFormaldehyde Framework Mesoporous

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Amino-functionalized urea–formaldehyde framework mesoporous silica for U(VI) adsorption in wastewater treatment Kegang Wei, Qingliang Wang, Long Huang, and Lei Sun Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b03653 • Publication Date (Web): 13 Nov 2016 Downloaded from http://pubs.acs.org on November 16, 2016

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Industrial & Engineering Chemistry Research

1

Amino-functionalized urea–formaldehyde framework

2

mesoporous silica for U(VI) adsorption in wastewater treatment

3

Wei, Kegang*. Wang, Qingliang. Huang, Long. Sun, Lei

4 5 6

Institute of Nuclear Resource Engineering, University of South China, Hengyang

7

421001, Hunan, China

8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

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Abstract

25 26

A mesoporous silica prototype (MSP) was produced by attaching SiO2 colloid to

27

urea formaldehyde resin. Treatment of the MSP using different grafting methods

28

yielded three different amino-functionalized mesoporous silica samples. The materials

29

were characterized by scanning electron microscopy, energy-dispersive spectroscopy,

30

transmission electron microscopy, X-ray photoelectron spectroscopy,

31

magnetic

32

sorption/desorption. For a 1:2000 solid-to-liquid ratio, all adsorbents could almost

33

completely remove uranyl from sulfate and carbonate solutions of 3.6 mg L-1 U(VI) at

34

pH 3.5 to 5.5 and pH 6.5 to 9.5, respectively. The adsorption equilibrium time was

35

less than 30 min. The adsorption equilibrium curve showed that the maximum

36

sorption capacity was 117 mg L-1 at pH 4.0. Through the experimental adsorption, it

37

was found that the most effective sample could reduce the U(VI) concentration from

38

3.6 mg L-1 to 0.79 µg L-1 with 99.98% removed. In desorption experiments, 0.1 mol

39

L-1 nitric acid could desorb U(VI) almost completely from the adsorbent. However,

40

this kind of adsorbent is unsuitable for use in high acid or alkaline environments.

41

Keywords: mesoporous silica; adsorption; uranium; amino-functionalized; silica

resonance,

zeta

potential

measurements,

42 43 44 45 46 47 48 49 50 51 52 53


and

13

C nuclear

isothermal

N2

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54

1. Introduction

55

Uranium is an important nuclear fuel, but, regardless of its advantages, it can

56

cause serious environmental and health damage1-2. Several methods have been

57

developed to separate uranium from wastewater, such as ion-exchange3, liquid

58

extraction4, ultra-filtration5, chemical precipitation, and adsorption6-7. Adsorption is a

59

high-efficiency process for easy removal of uranium from wastewater8. Adsorbents

60

such as clay minerals8, metal oxides10, carbon11, colloids, and biomass 12-13 are used to

61

adsorb uranium from aqueous solution.

62

Amino-functionalized mesoporous silica is a high-efficiency absorbent for U(VI)

63

from aqueous solution and has been introduced widely recently. A mass of 100 mg

64

mesoporous silica SBA-1514 can remove U(VI) almost completely from 4 L of

65

aqueous solution with a 4.2 ppb U(VI) concentration without any significant change

66

in the amount of adsorbed U(VI) ions with increase in ionic strength after

67

amino-functionalization.

68

mesoporous-structured silica has a similar adsorption ability. Mesoporous silica

69

MCM-48 has been shown to be more efficient than mesoporous silica MCM-41

70

because of its better three-dimensional mesoporous structure15.

Some

studies

have

shown

that

different

71

In this study, urea–formaldehyde resin was used as a framework to build

72

mesoporous silica16. This type of urea–formaldehyde framework mesoporous silica

73

has complex pore structure. To maintain the surface area17 and a high surface

74

activity18, urea–formaldehyde polymer/SiO2 composite microspheres were heated

75

gradually after attaching SiO2 colloid onto the framework under strict temperature and

76

pH conditions. Super-pure high activity silica was obtained19. Results were similar to

77

that mesoporous silicas SBA-15 14, which use costly P123 as framework, respectively.

78

Because mesoporous materials have a significant effect on the removal of heavy metal

79

in aqueous solution14-29-31, the adsorbents used in this study were place in a complex

80

solution environment. According to the experimental data, the removal ability of

81

heavy metal in aqueous solution is not only related to electrostatic attraction, surface

82

precipitation and chemical interaction between the metal ions and the surface 3 ACS Paragon Plus Environment


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83

functional groups32, but also related to its surface free energy22-28, and its pore system.

84

Therefore, a new theory of heavy metal removal was developing and need further

85

discuss.

86

2. Experimental

87

2.1 Material synthesis

88

2.1.1 Reagents and materials

89

(3-Aminopropyl)trimethoxysilane

was

purchased

from

Aladdin,

USA.

90

3-[2-(2-Aminoethylamino)ethylamino]propyl-trimethoxysilane was purchased from

91

Acros, Belgium. Tetraethyl orthosilicate, thionyl chloride, toluene, ethanol, carbamide,

92

formaldehyde, tetrahydrofuran, ethylenediamine, hydrochloric acid, ammonium

93

hydroxide, sulfuric acid, sodium bicarbonate, acrylamide, and ceric ammonium nitrate

94

were purchased from SCRC, China. Uranyl sulfate (1.029 g L-1) and uranyl carbonate

95

(1.022 g L-1) standard solutions were obtained from CNNC 272 Uranium Industry

96

Limited Liability Company, China. All chemicals were of analytical grade and were

97

used without further purification. Deionized water was used in all experiments and

98

was obtained from the ELGA lab water purification system (PURELAB Option-S,

99

High Wycombe, England).

100

2.1.2 MSP synthesis

101

MSP was prepared by the following steps using the modified protocol of Yang

102

and Zhang19. In a 20°C thermostatic water bath and with stirring, 40 mL ethanol was

103

added slowly into 45 mL tetraethyl orthosilicate, and 150 mL deionized water was

104

slowly added into the mixture. The solution pH was adjusted to 2 using 1:1

105

hydrochloric acid and ammonia after water addition. The pH was readjusted to 2 after

106

stirring for 10 h, and 16 g carbamide was added into the mixture. To this solution,

107

26.6 mL of 40% formaldehyde was added and stirred quickly for 1 min after the

108

carbamide had dissolved. The suspension was placed in a 15°C thermostatted water

109

bath for 48 h. The jelly-like mixture was placed into 2 L deionized water and stirred

110

rapidly. Finally, a white solid precipitate was obtained. After extracting the white solid

111

by filtering, 100 mL ethanol was added to the solid and the mixture was heated at 4 ACS Paragon Plus Environment

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112

75°C until the white solid had dried. The urea–formaldehyde polymer/SiO2 composite

113

microspheres were heated at 120°C for 12 h, 180°C for 1 h, 240°C for 2 h, 260°C for

114

2 h, 300°C for 2 h, and 550°C for 4 h to remove the urea–formaldehyde framework

115

and to produce a white power (termed MSP).

116

2.1.3 Adsorbent synthesis

117

Under refluxing condition, 3 g MSP were added into 150 mL toluene. APS (1.5

118

mL) was added slowly into the mixture and the reaction mixture was stirred at 120°C

119

for 12 h. After filtration, washing with toluene and drying, a functionalized product

120

was obtained (termed APTES).

121

Under refluxed condition, 3 g MSP was added into 150 mL toluene. AEPS (0.9

122

mL) was added slowly into the mixture and the reaction mixture was stirred at 120°C

123

for 12 h. After filtration, washing with toluene and drying, a functionalized product

124

was obtained (termed TPDA).

125

In a sealed environment, 3 g MSP was added into 200 mL thionyl chloride and

126

was stirred at 70°C for 12 h. The sediment was filtered, washed with tetrahydrofuran,

127

and dried in vacuo. The residue was added into 200 mL ethylenediamine and was

128

stirred at 120°C under refluxed conditions for 24 h. The yellow sediment was isolated

129

and washed with ethanol. To this sediment, 100 mL ethanol, 50 mL 1 mol L-1

130

acrylamide solution and 5 mL 0.2 mol L-1 ceric ammonium nitrate were added

131

gradually. The reaction mixture was stirred at 30°C under nitrogen for 12 h. After

132

filtration, washing with ethanol, and drying, a functionalized product was obtained

133

(termed NNSO). A scheme depicting the above synthesis is shown in Figure 1.



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134 135

Figure 1: Scheme depicting synthesis of amino-functionalized urea–formaldehyde

136

framework mesoporous silica for U(VI) adsorption

137

Table 1 Substance abbreviation list of this study

138

Abbreviation

Substance

MSP

Urea–formaldehyde framework mesoporous silica

APTES

(3-Aminopropyl)trimethoxysilane functionalized MSP

TPDA

3-[2-(2-Aminoethylamino)ethylamino]propyl-trimetho xysilane functionalized MSP

NNSO

Chloride hydroxide and acrylamide functionalized MSP

SBA-15

Regular pore system mesoporous silica synthesis by using Pluronic P123 (EO20PO70EO20) as templates

D201

Field application ion-exchange resin, resemble to that Amberlite IRA-900 resin

139 140

2.2. Material characterization

141

Samples obtained were studied by scanning electron microscopy (SEM)

142

energy-dispersive spectroscopy (EDS) (JSM-6490LV Neptune Texs, JEOL, Tokyo,

143

Japan) and transmission electron microscopy (TEM) (JEM-3010, JEOL, Tokyo,

144

Japan). Sample surface area and pore parameters were measured by the 6 ACS Paragon Plus Environment

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145

Brunauer–Emmett–Teller method using a surface area analyzer (Autosorbi/monosorb,

146

Quantachrome, Boynton Beach, Florida, America) with N2 absorption at 77 K. The

147

form and characteristics of the surface elements were analyzed by X-ray

148

photoelectron spectroscopy (XPS, ESCALAB 250Xi, Thermo Fisher Scientific,

149

Boston, Massachusetts, America).

150

The sample molecular structure was analyzed by CP/MAS 13C nuclear magnetic

151

resonance (NMR) spectra (AVANCE III 400 MHz, Bruker, Switzerland) using 1H

152

MAS and 13C CP MAS. Because the 13C signal is weak and difficult to detect directly,

153

1

154

13

155

H MAS was performed first. The 1H signal was transferred to adjacent 13C so that the C signal could be detected. Zeta potential profiles were obtained using a Nano ZS90 (Zetasizer,

156

Worcestershire, England).

157

2.3 Uranyl adsorption experiments

158

2.3.1 Adsorption experiments

159

Adsorption experiments were carried out using a parallel batch method. All

160

experiments were carried out at room temperature of 25°C. Solutions with different

161

initial U(VI) concentrations were prepared from diluting uranyl sulfate (1.029 g L-1)

162

and uranyl carbonate (1.022 g L-1) standard solutions (obtained from CNNC 272). The

163

solution pH was adjusted after standard solution was diluted, and then the U(VI)

164

concentration of diluted solution was checked. Because the solution pH will change

165

during the experimental process, two micro syringes were used to add 10% sulfuric

166

acid or 10% ammonium carbonate to the solution to maintain a stable pH (pH error

167

range ≤ 0.1) until the adsorption equilibrium had been reached. In a typical

168

experiment, 250 mL triangular flasks that contained 100 mL solution of different

169

uranium concentration were prepared. Adsorbent (50 mg) was added to these flasks.

170

Each flask was shaken for a certain period in a water bath shaker at 298 K. When the

171

adsorption equilibrium was reached, the suspension was separated by centrifugation

172

(15 min at 8000 rpm), and the uranium concentration in the filtrate was analyzed

173

using an ultraviolet–visible spectrophotometer (UV-Vis) (T6, Persee, Beijing, China) 7 ACS Paragon Plus Environment


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174

and inductively coupled plasma-mass spectrometer (ICP-MS) (7700 Series, Agilent

175

Technologies, Santa Clara County, California, America).

176

UV-Vis analysis is more accurate when the U(VI) concentration exceeds 0.5-1

177

mg L-1, because, if the ion concentration is too low, the spectrum will be weak and it

178

is difficult to obtain accurate data. ICP-MS analysis is more accurate when the U(VI)

179

concentration is less than 1 mg L-1 because too many ions in the solution can cause

180

blockages and burnout the quartz tube. Therefore, we first analyzed the samples by

181

chemical titration. After determining the concentration range of the uranium solution,

182

a more accurate analysis was carried out by UV-Vis or ICP-MS based on the chemical

183

titration data. A parallel sample model was used to prepare experimental data. The adsorption

184 185

capacity of different samples in different solutions was calculated from:

186

qe =

(Co − Cea )Va m

Removal precent(%) = 187

(1) (Co − Ceq ) Co

×100% (2)

188

where qe is the adsorption capacity of the adsorbent (mg g-1), m is the adsorbent mass

189

(mg), Va is the uranium solution volume (L), Co is the initial uranium concentration

190

(mg L-1), and Cea is the equilibrium uranium concentration after adsorption (mg L-1).

191

The effect of solid-to-liquid ratio was tested by adding adsorbent (50 mg) into an

192

aqueous solution that contained 0.5 mg U(VI) with different volumes. The solution

193

volume was increased from 10 to 2000 mL and the solid-to-liquid ratio (mg mL-1) was

194

decreased from 5×10-3:1 to 2.5×10-5:1 When the experimental design of the

195

solid-liquid ratio is too low, in order to guarantee the dispersion of the adsorbent in

196

the solution, 50 mg adsorbent was divided into ten parts (5 mg each), and then added

197

into certain solution.

198

The effect of contact time was studied using a separating funnel with filter

199

membrane on the outlet as contact vessel. Adsorbent was added into an aqueous U(VI)

200

solution and stirred. As soon as the designated contact time had been reached, the

201

suspension was filtered using a vacuum filtration connection to the outlet of the 8 ACS Paragon Plus Environment

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202

separating funnel.

203

D201 resin is used for U(VI) removal and recovery from wastewater in the

204

Chinese nuclear industry, and it resembles Amberlite IRA-900 resin. The adsorption

205

ability of D201 was compared with that of the synthesized materials. An initial 0.5-5

206

mg L-1 U(VI) concentration and 1:2000 solid-to-liquid ratio was used to investigate

207

uranyl adsorption from a uranyl sulfate solution at pH 4.0 and from a uranyl carbonate

208

solution at pH 8.0.

209

A 1:1000 solid-to-liquid ratio was used to investigate adsorption in a complex

210

solution environment. Artificial wastewater at pH 4.5, 5.5, 7.0 and 8.0 contained 2 mg

211

L-1 U(VI), Pb(IV), Mn(II), Cu(II), Co(II), V(III), Mo(IV), Zn(II), and Ni(II). Two

212

different actual nuclear industry process wastewater samples were introduced in this

213

part of the experiment. These samples were analyzed before adsorption. Due to the

214

fact that the concentration of various ions in uranium mine wastewater is influenced

215

by season, weather and the production status in real time, the use of a certain uranium

216

mine wastewater as a practical object does not represent the actual situation23-24.

217

Therefore, wastewater used in this experiment from a hydrometallurgy factory, which

218

collect the yellow cake from uranium mines and process it. This selection ensures that

219

the concentration of the ions in the wastewater of this experiment is more stable and

220

reliable.

221

2.3.2 Desorption experiments

222

The desorption of uranium adsorbed in the SiO2 materials involved elution of 50

223

mg adsorbent of a certain qe (mg g-1) by 10 mL addition of 0.1 mol L-1 HNO3,

224

adsorbent wetting, and washing with deionized water. After elution agent had been

225

added, the mixture was stirred to complete suspension (approximately 15 min) and

226

was then separated by centrifugation (15 min at 8000 rpm). This process was carried

227

out many times so that a total of 130 ml HNO3 was used. The uranium concentration

228

in the washing solution was analyzed using an ultraviolet-visible spectrophotometer

229

and ICP-MS as described previously.



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Desorption precent(%) = 230

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Ced × Vd × 100% qe m

(3)

231

where Ced is the uranium concentration in the washing solution after adsorption and

232

Vd is the volume of washing solution (L).

233

3. Results and discussion

234

3.1 Material characterization

235

The Brunauer-Emmett-Teller (BET) sample surface area and pore parameters are

236

given in Table 2. Supporting information Figure 1 is the N2 adsorption isotherm and

237

Barrett-Joyner-Halenda (BJH) pore size distribution of MSP. The data show that MSP

238

is a mesoporous material. The specific surface area, average pore diameter, and total

239

MSP pore volume decreased after grafting treatment.

240

Table 2 Brunauer-Emmett-Teller (BET) sample surface area and pore parameters Adsorbent sample

MSP

APTES

TPDA

NNSO

BET area (m2 g-1)

683.64

265.27

109.06

238.86

Average pore diameter (nm)

6.03

3.95

2.51

3.83

Total pore volume (cm3 g-1)

0.85

0.47

0.24

0.44

241 242

The SEM, TEM, and EDS analyses of the MSP samples are shown in Figure 2. It

243

shows that MSP is a super-pure microspherical SiO2 material and that the MSP was

244

formed from nanoscale silicon oxide. After the urea–formaldehyde framework was

245

removed during the heating process, the remaining space formed a complex MSP pore

246

system. SEM images of APTES, TPDA, and NNSO samples are shown in the

247

supporting information Figures 2 to 4. The MSP aggregated partly after

248

amino-functionalized treatment.


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249



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250

251 252

Figure 2: SEM (a), TEM (b) images and EDS (c) analysis of MSP 12 ACS Paragon Plus Environment

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253

Table 3 shows the form and characteristics of the surface elements analyzed by

254

XPS; additional details are provided in the Supporting Information, Figures 5 to 7,

255

this data provide information of the number of activated sites and atomic bond

256

information. The peak at 284.6 suggests the existence of C-C chemical bonding and

257

those at 400.1 and 402.4 suggest the existence of C-N chemical bonding. The atomic

258

percentage content of nitrogen on the TPDA sample surface is higher than in the other

259

two samples (Supporting Information, Figure 6). The peak at 286.39 suggests the

260

existence of C=O chemical bonding (Supporting Information, Figure 7). Data in Table

261

3 suggest that each sample has a different elemental surface area composition. The

262

increased number of -NH- and -NH2 groups provides more adsorption free energy22,

263

which is a key factor for adsorption. Silane coupling agents such as APS and AEPS

264

could aggregate MSP. Using AEPS instead of APS can increase the number of -NH-

265

and -NH2 groups significantly without losing too much surface area and functioning

266

pore system. The relationship between the number of activated sites and the number

267

of ions that were removed from aqueous solution can be evaluated by those

268

information. The result shows that the number of ions that was removed from the

269

aqueous solution was greater that the number of activated sites.

270

Table 3 Adsorbent surface area element composition (XPS analysis)

271

Adsorbent sample

APTES

TPDA

NNSO

Atomic Si (%)

26.48

24.06

25.96

Atomic C (%)

17.49

23.3

15.04

Atomic N (%)

3.71

6.88

3.31

Atomic O (%)

52.31

45.76

55.69

272 273

Supporting Information Figure 8 is the sample molecular structure characterise by 13

274

CP/MAS

C NMR analysis, the NMR analysis data and the XPS analysis data

275

(Supporting

276

amino-functionalized mesoporous materials were prepared.

Information

Figures

5

to

7)

confirm


that

three

different


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277

Figure 3 shows the sample zeta potential profile. The isoelectric point of MSP

278

(affected by -Si-OH only) exists at a pH of around 2.2. An increased number of -NH-

279

and -NH2 groups were grafted onto the MSP, which resulted in a higher isoelectric

280

point. The overall zeta potential of sample particulates was affected by -Si-OH, -NH-,

281

and -NH2, which means that the -NH- and -NH2 groups may still provide positive

282

potential at a negative overall zeta potential. The results indicate that -NH- and -NH2

283

groups grafted onto MSP in different reaction environments could behave in the same

284

way. High zeta potential reflect high adsorption free energy22, which is the key to high

285

adsorption ability.

286 287 288

Figure 3: Zeta potential profiles of MSP, APTES, TPDA and NNSO. 3.2 Effect of pH

289

Aqueous solution pH may play an important role in U(VI) adsorption. Uranium

290

mining tailings pond wastewater has a pH that ranges from 4 to 8.5 23. Chinese in-situ

291

and heap leaching uranium mining industries use quicklime to neutralize their

292

wastewater, which contains more than 0.3 mg L-1 U(VI) at pH 8.5 24. Increasing the

293

wastewater pH by using quicklime is also not environmentally friendly. Therefore,

294

adsorption of U(VI) by APTES, TPDA, and NNSO in aqueous solution with a pH that

295

ranges from 2.5 to 8.5 was studied. Uranium wastewater from nuclear power plants 14 ACS Paragon Plus Environment

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296

and in-situ and heap-leach solutions from uranium mining contain large amounts of

297

SO42- and CO32-, which makes uranyl perform mainly as a coordination of hexavalent

298

uranium sulfate or uranyl carbonate complex in solution

299

sulfate in aqueous solution begins to precipitate at a pH above 5, uranyl carbonate can

300

only exist in aqueous solution for a pH range from 6.5 to 10. For an initial 3.6 mg L-1

301

U(VI) concentration, this study used uranyl sulfate to evaluate the effect of pH from 2

302

to 5 on sample adsorption, and uranyl carbonate to evaluate that at a pH from 6.5 to

303

10. The data are shown in Figure 4. Adsorption of U(VI) by the adsorbents is strongly

304

pH-dependent, and the adsorption increases rapidly as the solution pH increases from

305

3 to 7. At pH 7 to 10, the percentage U(VI) removed decreases slightly and U(VI)

306

adsorption is almost complete (> 97.5%) from pH 3.5 to 9.5. The highest percentage

307

of APTES, TPDA, and NNSO removed was 99.7%, 99.98%, and 99.5%, and the

308

largest adsorption capacity resulted at pH 5.5 (pH error range ≤ 0.1).

25-27

309


. Because most uranyl


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310

311 312

Figure 4 Percentage U(VI) removed for different pH (initial U(VI) concentrations of 16 ACS Paragon Plus Environment

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313

3.6 mg L-1, solid-to-liquid ratio of 1:2000, contact time of 2 h) a: APTES, b:TPDA,

314

c:NNSO.

315

3.3 Adsorption isotherms

316

The Langmuir adsorption equation is based on the assumption of a homogeneous

317

adsorbent surface with identical adsorption sites. The mathematical expression of the

318

Langmuir adsorption equation is given by:

319

Ce 1 C = + e qe b qmax qmax

320

The Freundlich model describes the non-ideal and reversible adsorption, the

321

adsorption describe in this model represent a non-uniform distribution of adsorption

322

heat and affinities over a heterogeneous surface. Also, the adsorption is not limited to

323

monolayer formation. It can be applied to multilayer adsorption.

324

The equation is expressed as follows:

(4)

1

(5)

325

qe = K f Cen

326

The data in Figure 5 and 6 show that the equilibrium adsorption capacity

327

obtained from the Langmuir and Freundlich model approximates the experimental

328

values closely, which suggests that physisorption plays an important role in this

329

experiment.

330



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331

332 333

Figure 5 Langmuir isotherms for adsorbents (APTES, TPDA and NNSO) in water 18 ACS Paragon Plus Environment

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334

solution at pH 4.0 (a) and pH 8.0 (b) (solid-to-liquid ratio of 1:2000, contact time of 2

335

h)

336

337



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338 339

Figure 6 Freundlich isotherms for adsorbents (APTES, TPDA and NNSO) in water

340

solution at pH 4.0 (a) and pH 8.0 (b) (solid-to-liquid ratio of 1:2000, contact time of 2

341

h)

342 343 344 345

3.4 Effect of solid-to-liquid ratio The data shown in Figure 7 suggest that the difference in solid-to-liquid ratio affects the results slightly, and this ratio can be decreased further.


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346

347 348

Figure 7 Solid-to-liquid ratio effect on U(VI) removal rate (%) of adsorbents (APTES,

349

TPDA and NNSO) in aqueous solution at pH 4.0 (a) and pH 8.0 (b) for a contact time

350

of 2 h 21 ACS Paragon Plus Environment


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351

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3.5 Effect of contact time

352

The data in Figure 8 suggest that the adsorption equilibrium time is short and that

353

most U(VI) was captured onto the adsorbent within 5 min. By fitting the experimental

354

data with the first and the second order kinetic models, it shows that the adsorption

355

model of this study is more conform to Pseudo-first order, the model assume that the

356

adsorption rate is proportional to the number of free sites.

357

Pseudo-first order equation:

358

dqt = k1 (qe − qt ) dt

359

The data shown in Figure 9

(6)

360


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361 362

Figure 8 Effect of contact time on adsorption capacity of adsorbents (APTES, TPDA

363

and NNSO) in aqueous solution at pH 4.0 (a) and pH 8.0 (b) for an initial U(VI)

364

concentration of 3.6 mg L-1

365



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366

367 368

Figure 9 Pseudo-first order kinetic model analysis (APTES, TPDA and NNSO) in 24 ACS Paragon Plus Environment

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369

aqueous solution at pH 4.0 (a) and pH 8.0 (b) for an initial U(VI) concentration of 3.6

370

mg L-1

371 372

3.6 Comparison of adsorption ability

373

After processing by using an ion-exchange resin, wastewater from industrial

374

uranium production contains 0.5-2 mg L-1 of uranium. It is difficult to treat this

375

low-concentration aqueous uranium solution by using an ion-exchange resin. Using a

376

newly transformed ion-exchange resin, or using liquid concentration by evaporation

377

and then an ion-exchange resin, to treat this aqueous solution could alleviate the

378

situation. The efficiency of this method is not sufficiently high and requires

379

significant manpower and resources. Therefore, the ability to exploit a low-cost and

380

efficient material, which can be used in the treatment of low-concentration uranium

381

aqueous solutions has become a topic of great interest in the uranium industry.

382

Ion-exchange resins, such as D201, are used in U(VI) removal from aqueous

383

solution. The MSP sample resembles SiO2 colloids (which also used in industrial

384

low-concentration aqueous uranium removal), and its adsorption ability is stronger

385

than that of ordinary SiO2 colloids. The data shown in Figure 10 suggest that the

386

percentage U(VI) removed by the amino-functionalized mesoporous materials is

387

much better than that of D201 resin in aqueous solutions of low U(VI) concentration

388

and has a much larger capacity than MSP. Considering the low U(VI) concentration,

389

the ion-exchange resin does not perform well on the nuclear industry wastewater24.

390

Instead of continuously sending newly regenerated ion-exchange resin to treat

391

wastewater and achieve a poor result, some mining corporations collect their

392

wastewater and condense it by natural evaporation, and then use an ion-exchange

393

resin to recover uranium from the condensed wastewater. This uranium recovery

394

process requires considerable wastewater storage space and is inefficient. Using

395

amino-functionalized mesoporous materials, this wastewater could be treating

396

immediately and with a high efficiency. The effect of solid-to-liquid ratios mentioned

397

in Section 3.4 indicates that fluctuations in U(VI) concentration have little or no 25 ACS Paragon Plus Environment


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398

influence on the adsorbent ability, which is a critical factor in wastewater treatment by

399

ion-exchange resins.

400


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401 402

Figure 10 Comparison of percentage U(VI) removed for different samples (D201

403

ion-exchange resin; adsorbents MSP, APTES, TPDA and NNSO) in aqueous solution

404

at pH 4.0 (a) and pH 8.0 (b) for a solid-to-liquid ratio of 1:2000

405 406 407

3.7 Adsorption in a complex solution environment Using the equation: Removal precent(%) =

408

(Co − Ceq )

Co

×100% (2)

409

Supporting Information Tables 1 to 6 is the experimental data of adsorption in

410

several complex solution environments. The data shown that U(VI), Pb(IV), Cu(II),

411

and V(III) are more easily to be removed in a complex solution environment and the

412

adsorption ability of the amino-functionalized mesoporous material is stronger when

413

the pH is close to 7.

414

A double layer (DL, also termed an electrical double layer, EDL) is a structure

415

that appears on the surface of an object when it is exposed to a fluid. The object may 27 ACS Paragon Plus Environment


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

416

be a solid particle, a gas bubble, a liquid droplet, or a porous body. The DL refers to

417

two parallel layers of charge that surround the object. The first layer, the surface

418

charge (either positive or negative), comprises ions that are adsorbed onto the object

419

because of chemical interactions. The second layer is composed of ions that are

420

attracted to the surface charge via a coulomb force, and result by electrically

421

screening the first layer. This second layer is loosely associated with the object. It is

422

made of free ions that move in the fluid under the influence of an electric attraction

423

and thermal motion, rather than being firmly anchored. It is thus called the "diffuse

424

layer".

425

The data here and in Section 3.3 suggest that the removal of ions is caused not

426

only by adsorption, but also by the pore system of the mesoporous material. At a

427

certain pH, the amino groups of the material prefer to be protonated and are positively

428

charged. Therefore, the UO22+ ions are not favored by the positively charged binding

429

groups because of the electrostatic repulsion, which leads to lower adsorption

430

capacities. As the pH increases, the amino groups tend to be deprotonated gradually.

431

The electrostatic interaction between N in the amino moiety and the U(VI) ions leads

432

to an increase in adsorption capacity. At the higher pH, the U(VI) species will

433

transform from free UO22+ to multi-nuclear hydroxide complexes. Although these

434

hydroxide complexes may be more favored by the adsorbent obtained, the U(VI) ions

435

undergo severe hydrolysis and precipitate from the higher pH solution32. Adsorption

436

will begin when an ionic solution runs through the pore system of the adsorbent.

437

Adsorption will change the solution pH of the inner pore system, and the solution

438

volume of the inner pore system is rather small, so it is inevitable that the pH of the

439

inner pore system will change. Because the pH of the inner pore system changes,

440

some ions become unstable and begin to coagulate. Furthermore, the complex pore

441

system of the mesoporous material makes liquid movement inside the pore system

442

irregular and slow, which means that there may be a pH difference between the inner

443

and outer pore systems, especially when adjusting the outer solution pH when

444

conducting the experiment. If the pH-sensitive ions coagulate to form larger particles, 28 ACS Paragon Plus Environment

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445

they could remain inside the pore system (be removed from the outer solution). This

446

assumption explains the data in Section 3.4, where the solution volume of the inner

447

pore system shows no change for the different solid-to-liquid ratios. When the

448

solution saturates the pore system of the adsorbent, the -NH- and -NH2 groups create

449

an inner pH environment that is different from that of the outer pore system; the inner

450

pH environment helps the adsorbent attract and capture U(VI) to form a solution.

451

When amino-functional mesoporous silica or 5-formyl-8-hydroxyquinoline functional

452

mesoporous silica is used, few metal ions are obtained from adsorption in solution at a

453

low pH. Because H+ exhibits a strong competitive adsorption ability and the N of the

454

amino groups or the N of the quinoline groups of the material prefers to be protonated

455

and positively charged, with an increase in pH, the electrostatic interaction between N

456

in the amino or quinoline moiety and the metal ions, such as Cu(II) and Co(II), also

457

begin to hydrolyse and transform to [Cu(H2O)]2+ and [Co(H2O)6]2+, which leads to an

458

increase in adsorption capacity. At a higher pH, the metal ions transform to

459

multi-nuclear hydroxide complexes, so the adsorption capacity decreases. The

460

adsorption mechanism is equal. The ionic radius, covalent bond index, and the

461

coupling between metal ions will affect the adsorption of metal ions by the adsorbent.

462

This phenomenon cannot explain completely at this study, and further research is

463

needed.

464

3.8 Desorption experiments

465

Supporting Information, Figure 9 is the data of desorption experiments. The data

466

show that most of the U(VI) adsorbed on the material can be eluted rapidly by using

467

0.1 mol L-1 HNO3, but some residual U(VI) remained because the adsorbents have a

468

complex pore system. The U(VI) concentration of the elution agent after desorption is

469

much higher than that of the absorbent. The U(VI) concentration sometimes exceeds

470

200 mg L-1 (e.g., 0-20 ml in Supporting Information Figure 9(a) and 20-30 ml in

471

Supporting Information, Figure 9(b)) and the highest U(VI) concentration is 293.4 mg

472

L-1, which makes it easy for subsequent treatment, such as uranium recovery using an

473

ion-exchange resin. 29 ACS Paragon Plus Environment


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474 475

In a continuity experiment, adsorption-desorption cycles were performed 5 times, and the adsorbent still remained at 60% of its full capacity.

476

Because of the chemical properties of the silica materials, these adsorbents will

477

dissolve if they are used in a strong acid or strong alkali environment. Each time the

478

material is regenerated, it will lose some of its surface layer, which leads to a

479

reduction in functional groups and a decrease in adsorption capacity.

480 481

5. Conclusions

482

This method is low-cost, easy to execute, and has a high yield. In a typical

483

synthesis, more than 80% silicon from ethylsilicate is converted to mesoporous silica

484

material. These advantages and research results are key to industrial mass production.

485

It can function over a wide pH range (from 3.0 to 9.0) with a short required contact

486

time. The optimal adsorption conditions appear at pH 5.5 (pH error range ≤ 0.1). A

487

high concentration of uranium solution can be obtained by elution of the saturated

488

adsorbent, this solution can be return to ion-exchange process and recover uranium.

489

Expire adsorbent can be dissolved in sodium hydroxide, generate solution sodium

490

silicate and serve as concrete coagulant.

491

The number of amino groups on the adsorbent surface is a determining factor to

492

create a higher adsorption free energy. The adsorption free energy and the complex

493

pore system are key factors in U(VI) removal from wastewater. Therefore, using

494

silane coupling agents that possess more -NH- and -NH2 groups in a single molecule

495

to create amino-functionalized mesoporous silica could yield a more powerful

496

adsorbent.

497

For the nanoscale mesoporous structure, the movement of water molecules is

498

subject to surface tension. Filling of the pore system with water molecules will be

499

time-consuming and irregular.According the double-layer theory mentioned above

500

(Section3.7), and the fact that the adsorption process changes the solution pH, it is

501

likely that adsorption in the pore system will result in a pH difference between the

502

inner and outer pore systems. 30 ACS Paragon Plus Environment

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503

However, in complex ionic solution environment, the removal of metal includes

504

electrostatic attraction, surface precipitation, interaction between precipitate metals

505

and chemical interaction between the metal ions and the surface functional groups.

506

This study draw the conclusion that the complex material pore system has a

507

significant influence on the experimental results. Follow-up studies need to be further

508

expanded and determining the decisive adsorption factor of this kind of material.

509

There may even be an effect that is produced by a particular combination. Even

510

similar materials, because of differences in their structure, may have different effects.

511

Further study is required.

512 513 514 515 516 517 518 519 520 521 522 523 524 525 526 527 528 529 530 531



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532

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assessment of environmental and health impacts of electricity-generating systems.

535

Appl. Energy. 2000, 65, 211-229.

536

(2) Venkatesan, KA.; Sukumaran,V.; Antony, MP,; Rao, PRV. Extraction of uranium

537

by amine, amide and benzamide grafted covalently on silica gel. J. Radio. Nucl. Chem.

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2004, 260, 443-450.

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(3) Akyil, S.; Aslani, MAA.; Eral, M. Adsorption characteristic of uranium onto

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composite ion exchangers. Radio. Nucl. Chem. 2003, 256, 45-51.

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(4) Sodaye, H.; Nisan, S.; Poletiko, C.; Prabhakar, S.; Tewari, PK. Extraction of

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(5) Kornilovich, BY.; Kovalchuk, IA.; Pshinko, GN.; Tsapyuk, EA.; Krivoruchko, AP.

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Demineralization Technology-Water purification of uranium by the method of

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ultrafiltration. J. Water Chem. and Tech. 2000, 22, 43-47.

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(6) Ganesh, R.; Robinson, KG.; Chu, L.; Dan, K.; Reed, GD. Reductive precipitation

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of uranium by Desulfovibrio desulfuricans: evaluation of contaminant effects and

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selective removal. Water Res. 1999, 33, 3447-3458.

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(7) Feng, Y.; Fa-Cheng, YI. Adsorptive property of rice husk for uranium. Atomic

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Energy Sci. and Tech. 2011, 45, 161-167.

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(8) Liu, YH.; Wang, YQ.; Zhang, ZB.; Cao, XH.; Nie, WB.; Li, Q.; Hua, Rong.

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Removal of uranium from aqueous solution by a low cost and high-efficient adsorbent.

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(9) Song, J. Study on the performance of adsorbing uranium by attapulgite clay and its

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(10) Peng, GW.; Ding, DX.; Nan, HU.; Yang, YS.; Wang, XL. Adsorption properties

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and mechanism of sacharomyces cerevisiae loaded by nano-Fe3O4 on uranium.

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(13) Zou, W.; Zhao, L.; L Zhu. Efficient uranium (VI) biosorption on grapefruit peel:

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(14) Liu, Y.; Yuan, Y.; Lan, Z.; Li, Y.; Feng, Y.; Zhao, Z.; Shi, W. A high efﬁcient

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Radio Nucl. Chem. 2012, 292, 803-810.

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beta-MCM-48 for diesel hydrodesulfurization. J. of Porous Materials. 2013, 20,

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amino-functionalized mesoporous silica thin films with highly ordered large pore

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Inorg. Chem. 2008, 47, 1634-1638.

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Complexes of Uranyl and Carbonate with Alkaline Earth Metals (Mg2+, Ca2+, Sr2+,

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