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Synthesis and Application of Organic Phosphonate Salts as Draw Solutes in Forward Osmosis for Oil-water Separation Qingwu Long, Liang Shen, Rongbiao Chen, Jiaqi Huang, Shu Xiong, and Yan Wang Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b02953 • Publication Date (Web): 30 Sep 2016 Downloaded from http://pubs.acs.org on October 2, 2016

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Environmental Science & Technology

1

Synthesis and Application of Organic Phosphonate Salts as Draw Solutes in

2

Forward Osmosis for Oil-water Separation

3 4

Qingwu Longa,b, Liang Shena,c, Rongbiao Chena, Jiaqi Huanga,c, Shu Xionga,c, Yan

5

Wanga,c,*

6 a

7

Key Laboratory of Material Chemistry for Energy Conversion and Storage

8

(Huazhong University of Science and Technology), Ministry of Education, Wuhan,

9

430074, China

10

b

11 12

School of Chemistry and Chemical Engineering, Lingnan Normal University, Zhanjiang 524048, China

c

Hubei Key Laboratory of Material Chemistry and Service Failure, School of

13

Chemistry and Chemical Engineering, Huazhong University of Science & Technology,

14

Wuhan, 430074, P.R. China

15 16 17 18 19 20 21

* Corresponding author. Tel.: 86 027 87543032; fax: 86 027-87543632. E-mail

22

address: [email protected] (Yan Wang)

23

1

ACS Paragon Plus Environment


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24

Abstract

25

The development of suitable draw solution in forward osmosis (FO) process has

26

attracted the growing attention for water treatment purpose. In this study, a series of

27

organic phosphonate salts (OPSs) are synthesized by one-step Mannich-like reaction,

28

confirmed by FTIR and NMR characterizations, and applied as novel draw solutes in

29

FO applications. Their solution properties including osmotic pressures and viscosities,

30

as well as their FO performance as a function of the solution concentration are

31

investigated systematically. In FO process, a higher water flux of 47~54 LMH and a

32

negligible reverse solute flux can be achieved in the PRO (AL-DS) mode (skin layer

33

faces the draw solution) using a home-made thin-film composite membrane

34

(PSF-TFC) and deionized water as the feed solution. Among all OPS draw solutes, the

35

tetraethylenepentamine

36

exhibits the best FO flux at 0.5 mol/kg concentration, which is further applied for the

37

separation of emulsified oil-water mixture. The recovery of diluted OPS solutions is

38

carried out via a nanofiltration (NF) system with a rejection above 92%. The

39

aforementioned features show the great potential of OPS compounds as a novel class

40

of draw solutes for FO applications.

heptakis(methylphosphonic)

sodium

salt

(TPHMP-Na)

41 42

Keywords：forward osmosis, draw solution, membrane, organic phosphonate salts,

43

oil-water separation, nanofiltration recovery

2


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44 45 46

TOC Art

47

INTRODUCTION

48

Over the past few decades, a series of water treatment technologies have been

49

developed to meet the world’s growing water demand, by protecting existing fresh

50

water resources and improving water quality. Membrane-based separation

51

technologies have been extensively used for wastewater reclamation and seawater

52

desalination due to their high separation efficiency and wide applications.1-2

53

Especially, the high-quality clean water produced by reverse osmosis is popular with

54

every household all over the world.3 Recently, forward osmosis (FO) process as an

55

alternative solution for water treatment attracts growing interest from the world.4 In

56

FO process, water molecules in one solution spontaneously transport into another

57

solution which is separated by a semi-permeable membrane. Unlike the typical

58

pressure-driven membrane processes, the spontaneous occurrence of FO possess great

59

potential advantages, such as low system energy consumption, high water recovery,

60

and low fouling tendency.5 However, FO still faces various challenges including the

61

severe solute leakage and energy-intensive recovery of the draw solution. Therefore, it

62

is always imperative to develop suitable draw solutes to break through the

63

development bottle-neck. A desirable draw solute should own the following desirable

3



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64

physicochemical properties, i.e., suitable molecular weight, high osmotic pressure and

65

low boiling point, in order to enhance its FO performance and recovery after FO.5-7

66 67

Numerous researches have been contributed to develop different high-performance

68

draw solutes for FO process.8-9 Representatively, thermolytic ammonium carbonate

69

salts10,11 have exhibited their feasibility as draw solutes because of the high osmotic

70

pressure of the solution and easy recovery by heating distillation with available

71

industrial waste heat, but the reverse salt leakage constrains their practical

72

applications. Later, a series of surface-modified magnetic nanoparticles12-13 were

73

reported as draw solutes, which can produce high osmotic pressure and subsequent

74

high water flux, and be renewed by magnetic field conveniently. However, they face

75

severe aggregation problem after recovery by the magnetic field, leading to a declined

76

FO performance. Therefore, next-generation draw solutions should be explored to

77

address these issues. Very recently, synthetic draw solutes have drawn more attention

78

due to their superior performance, including upper critical solution temperature

79

(UCST) ionic liquids14, Na+-functionalized carbon quantum dots (Na_CQDs)15,

80

micellar solutions16, thermo-sensitive polyelectrolytes17, and CO2 switchable dual

81

responsive polymers18, et al. For example, Ge et al.19-20 explored a series of novel

82

draw solutes based on poly(acrylic acid) sodium (PAA-Na) with minimized reverse

83

salt flux due to their expanded structure and easy recovery by low-pressure

84

ultrafiltration. Besides, hydracids complexes21-23 as draw solutes not only possess the

85

expanded structure to minimize the salt leakage, but also can release partial ions in

86

water to enhance the osmotic pressure and the consequent water flux. In our previous

87

works, a series of carboxyl-containing draw solutes, including carboxyethyl amine

88

sodium salts (CASS)24-25 and sodium tetraethylenepentamine heptaacetate26, are 4


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89

developed as novel draw solutes and exhibit considerable osmotic pressure, superior

90

water solubility, and therefore excellent FO performance. Besides, those

91

carboxyl-contained draw solutes with expanded structure may also result in an

92

extreme low reverse solute flux. Those inspiring works encourage us to explore more

93

organic draw solutes containing acid radicals for FO applications.

94 95

In this work, a series of organic phosphonate salts (OPSs) are synthesized and applied

96

as novel draw solutes for FO applications. Organophosphorus compounds are

97

generally nontoxic, widespread in nature and highly soluble in water. They also play

98

important roles in metabolism, exhibiting good compatibility with the environment

99

and ecosystem.27-29 Organic phosphonate salt can be used as a fertilizer in modern

100

agriculture, and also a versatile agent for water treatment. For example,

101

diethylenetriamine pentakis (methylenephosphonic) sodium salt (DTPMP-Na), an

102

organic phosphonate salt with five phosphonate groups, is widely used as a good

103

peroxide stabilizer, oxidizing bactericide stabilizer and superior corrosion inhibitor for

104

anti-scaling of boiler water.30 With aforementioned advantages, organic phosphonate

105

salts are expected to be good candidates as draw solutes for FO applications. To the

106

best of our knowledge, however, no study has been reported on OPS as draw solutes

107

in FO process yet.

108 109

Therefore, a series of OPSs with various molecular sizes are explored as novel draw

110

solutes in this study, including diethylenetriamine pentakis(methylphosphonic)

111

sodium salt (DTPMP-Na), tetraethylenepentamine heptakis(methylphosphonic)

112

sodium salt (TPHMP-Na), polyethyleneimine (methylenephosphonic) sodium salt

113

(PEI-600P-Na)

and

polyethyleneimine

(methylenephosphonic)

sodium

salt 5



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114

(PEI-1800P-Na). The synthesis, structure characterization, effects of OPS draw solute

115

and membrane types on their FO performance, are investigated systematically. In

116

addition, the potential application of TPHMP-Na draw solution for oil-water

117

separation via FO, is evaluated for the first time. Finally, the regeneration of OPS

118

draw solutions after FO through NF process is studied.

119 120

Scheme 1. Reaction mechanism of the synthesis of OPS compounds

121 122

EXPERIMENTAL SECTION

123

Synthesis and Structure Characterization of OPSs. A series of OPSs were

124

synthesized by one-step Mannich-like reaction according to the previous work with

125

slight modifications.31 The reaction mechanism is shown in Scheme 1. With the

126

synthesis of TPHMP-Na as an example, the reactant TEPA (37.8 g, 0.2 mol) was

127

added into a 1000 mL three-neck flask under ambient temperature, followed by the

128

addition of 250mL DI water to get a homogeneous aqueous solution. The mixture was

129

stirred and warmed into 40 oC, and then formaldehyde (121 g, 1.45 mol) was added

130

dropwisely for about 1 h. During this process, the resulted mixture became more

131

viscous, and changed from colorless to light yellow. After that, the mixture was

132

continually warmed to 60 oC for 1 h, and then cooled below 5 oC in an ice bath. 6


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133

Afterward, PCl3 (243.6 g, 1.45 mol) was further added into the mixture slowly over 2

134

h, and warmed to 120 oC again for 1 h to remove the generated hydrogen chloride

135

completely. Finally, the pH of the obtained raw OPS acid solution was adjusted to 7

136

by the addition of NaOH to get the phosphonate sodium salt. Then most solvent was

137

removed by a Büchi R215 rotary evaporator. The concentrated solution was purified

138

through a dialysis bag with a MWCO of 500 Da for at least four times to eliminate

139

impurity thoroughly. The purification solution was evaporated and dried in a vacuum

140

oven at 60 oC for 5 h to yield a yellow solid product (TPHMP-Na). Other OPSs

141

(PEI-600P-Na and PEI-1800P-Na) were synthesized with a similar process as

142

described above. Since the amine reactants (TEPA, PEI-600, and PEI-1800) are of

143

different numbers of amino groups, the reactant ratio is slightly modified to guarantee

144

a

145

(mutil-amine/formaldehyde/PCl3)

146

PEI-600P-Na and PEI-1800P-Na as the amine, respectively. In addition, DTPMP-Na

147

was obtained directly from commercial available diethylenetriamine pentakis

148

(methylenephosphonic) acid by adjusting the solution pH to 7.

full

conversion

of

amino

groups. were

The

molar

1:9.25:9.25

and

ratios

of

reactants

1:19.5:19.5

with

149 150

Analytical Methods. Chemical structures of synthesized OPS compounds were

151

examined by Nuclear Magnetic Resonance spectroscopy (1H NMR and

152

(Bruker AVANCE III 400 MHz Instrument) and Fourier Transform Infrared

153

Spectroscopy (FTIR) (Brucker VERTEX-70 spectrophotometer) with a wavenumber

154

range of 4000-400 cm-1. The osmotic pressures (π) and relative viscosities (𝜂𝑅 ) of

155

draw solutions with different concentrations were measured using a lab-scale set-up24

156

based on the freezing point depression method and Ubbelohde viscometer,

157

respectively. The FO performance (the water flux, Jw and the reverse solute flux, JR)

31

P NMR)

7



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158

with OPS draw solutions was evaluated using a custom-built FO system (Suzhou

159

Faith Hope Membrane Technology).26 Two types of thin film composite (TFC)

160

membranes were used in the FO test, i.e., the commercial HTI membrane (HTI-TFC)

161

and the home-made membrane on the polysulfone substrate (PSF-TFC). Table S1 also

162

lists the fundamental parameters of the PSF substrate for the PSF-TFC membrane.

163

The size of oil particles was measured by dynamic light scattering (DLS, HORIBA

164

LB-550). The NF recovery of the diluted OPS draw solutions were concentrated

165

through a lab-scale NF system (Suzhou Faith Hope Membrane Technology). All

166

above information is detailed in the Supporting Information.

167 168

RESULTS AND DISCUSSION

169

Synthesis and structure characterization of OPS draw solutes. As illustrated in

170

Scheme 1,a series of OPS compounds (TPHMP-Na, PEI-600P-Na, and PEI-1800P-Na)

171

are synthesized by one-step Mannich-like reaction. The chemical structures of the

172

as-synthesized OPS draw solutes are examined by FTIR, 1H NMR, and

173

Figure 1 shows FTIR curves of four OPS compounds and corresponding raw amines.

174

The strong double peaks at 3281 cm−1, ascribed to the absorption signal of the -NH2

175

from raw amines, become more obtuse for all OPSs, suggesting successful partial

176

substitution of the amino group by –CH2PO3H2 moieties. The sharp characteristic

177

bands of -NH- groups at 1464 and 1303 cm−1 in the curves of raw amines almost all

178

disappear in the spectra of OPSs, which may again be ascribed to the fully reacted

179

-NH- groups in raw amines. Besides, the band at 1579 cm−1, which is the typical

180

signal of tri-substituted =N- groups, becomes stronger, indicating again that the -N-H

181

groups have been substituted. Another new peak at 1672 cm−1 is the characteristic

182

band of P=O groups, suggesting the successful introduced–CH2PO3H2 structure. The

31

P NMR.

8


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183

new bands appearing at 1093 and 973 cm−1 are attributed to the stretching vibrations

184

of P–CH2N- and P–OH groups, in agreement with the previous literature.32 In

185

comparison with other three OPSs, the strong absorption peak of PEI-1800P-Na at

186

1579 cm−1 implies amine groups of PEI-1800 cannot react fully due to its big steric

187

hindrance and its adverse effect on the viscosity. The spectrum of commercial

188

DTPMP compound is also included for a comparison. Typical characteristic peaks of

189

NH, P=O, =N-, P-C, and P-O groups at 3281 (1464-1303), 1672, 1579, 1093, 973

190

cm-1 respectively, are also observed, indicating the same functional groups of =N-, –

191

NCH2 and -PO3H2 contained in the chemical structure.

192

193 194

Figure 1. FTIR Characterization of DTPMP-Na, raw amines, and corresponding

195

synthesized OPS compounds

196 9



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197

NMR spectra further confirm the chemical structures of the synthesized OPSs

198

(DTPMP-Na, TPHMP-Na, PEI-600P-Na, and PEI-1800P-Na). As clearly shown in

199

Figure S1a, the chemical shift of -NH2 of TEPA shows a stronger peak at about 1.3

200

ppm but disappears in the spectra of DTPMP-Na and TPHMP-Na, illustrating that

201

these amino groups in raw amines are fully substituted. Similar phenomenon also

202

occurs in the spectra of PEI-600P and PEI-1800P (shown in Figure S1b in the

203

Supporting Information), where almost all characteristic peaks of amino groups

204

appear at 2.4~2.6 ppm, but they disappear in the spectra of PEI-600P-Na and

205

PEI-1800P-Na. Besides, as for PEI-600P-Na and PEI-1800P-Na, all peaks of

206

corresponding methylene groups locate at positions of a, b, and c (Figure S1b). In

207

addition, the

208

another powerful proof to confirm the existence of –CH2PO3H2 moiety in the

209

synthesized OPS compounds. Chemical shifts of phosphorus at two different positions

210

(15~16 and 16.5~17 ppm in the curve of TPHMP-Na, 5.5~8 and 16.5~19.5 ppm in

211

that of PEI-600P-Na) can be observed, which indicate the presence of phosphorus

212

elements at two different chemical environments. The above results confirm the

213

successful synthesis of OPS compounds.

31

P NMR spectrum (Figure S2 in the Supporting Information) presents

214 215

In addition, Mw of all OPSs can be deduced and calculated by the curve area

216

integration (Table S2 in the Supporting Information). From the 31P NMR spectrum of

217

TPHMP-Na, two peak groups at 17 ppm and 15~16 ppm show the occurrence of

218

seven phosphorus atoms in the chemical structure of TPHMP-Na, which is consistent

219

to the proposed chemical structure of TPHMP in Scheme 1. Likewisely, the 31P NMR

220

spectrum shows that there are nine phosphorus atoms in the chemical structure of

221

PEI-600P-Na. 10


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222 223

Osmotic pressure and relative viscosity of various OPS draw solutions. Generally,

224

the viscosity and the osmotic pressure of the draw solution are important parameters

225

to determine their resultant FO performance. Figure 2a shows the viscosity of OPS

226

solutions at various concentrations. It can be seen that, with the concentration increase

227

of the OPS solution, the viscosity shows an increasing trend, which is consistent with

228

previous studies.23,33-34 Except for the PEI-1800P-Na solution, viscosities of all OPS

229

draw solutions are basically very low (less than 5) and exhibit minor difference (less

230

than 1) when the concentration is below 0.5 mol/kg, demonstrating that the existence

231

of phosphate in OPSs is in favor of their water solubility. Although the viscosity

232

increases with the concentration increase, relative viscosities of DTPMP-Na and

233

TPHMP-Na solutions are still very low (13 and 22, respectively) as compared to most

234

other reported draw solutions.34-42 However, when the concentration is higher than 0.5

235

mol/kg, the viscosity of PEI-1800P-Na draw solution increases sharply. This abnormal

236

phenomenon may be attributed to the relative less phosphate groups in the

237

PEI-1800P-Na structure, as discussed in previous section.

238 239

The osmotic pressures of OPS draw solutions with various concentrations are shown

240

in Figure 2b. We can observe that, except for PEI-1800P-Na, the osmotic pressures of

241

other three OPS solutions show a linear increasing trend, and roughly follow an order

242

of TPHMP-Na > PEI-600P-Na > DTPMP-Na for solutions of the same concentration.

243

Among them, 0.55 mol/kg TPHMP-Na and PEI-600P-Na solutions exhibit a high

244

osmotic pressure of 120 bar, superior to many other reported draw solutions.34-43 In

245

addition, the osmotic pressure of PEI-1800P-Na shows exponentially increasing trend,

246

implying that 0.2 mol/kg PEI-1800P-Na solution is not an ideal solution already due 11



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to its high concentration. Since the viscosity of 0.2 mol/kg PEI-1800P-Na solution is

248

also extremely high, its osmotic pressure cannot be calculated by the conventional

249

van’t Hoff experience formula, based on which the freezing point depression method

250

was employed for the determination of osmotic pressure. Therefore, the osmotic

251

pressure of 165 bar measured for 0.2 mol/kg PEI-1800P-Na may not be reliable.

(a) DTPMP-Na TPHMP-Na PEI-600P-Na PEI-1800P-Na

Relative viscosity

80 60 40 20 0

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

Conc. (mol/kg)

252

Osmotic pressure (bar)

(b) 180

DTPMP-Na TPHMP-Na PEI-600P-Na PEI-1800P-Na NaCl

160 140 120 100 80 60 40 20 0

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

253

Conc. (mol/kg)

254

Figure 2. (a) Relative viscosities and (b) osmotic pressures of OPS solutions with

255

various concentrations at 25 oC 12


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256 257

FO performance with different OPS draw solutions. Because of the good water

258

solubility, high osmotic pressure and low viscosity of OPS solutions, OPS compounds

259

exhibit great potential as draw solutes for FO applications. Figure 3a,b show their FO

260

performance under PRO mode as a function of the molality concentration using

261

HTI-TFC membranes. From Figure 3a, it can be seen that, for all OPS draw solutions,

262

the water flux increases with the OPS concentration increase, because of the greater

263

osmotic pressure generated at a higher solute concentration. The water flux with OPS

264

draw solutions far outperforms that with the conventional NaCl solution because of

265

their higher osmotic pressure. Basically, the water flux with OPS draw solutions

266

follows the order of TPHMP-Na > PEI-600P-Na > DTPMP-Na > PEI-1800P-Na >

267

NaCl, consistent with the order of their corresponding osmotic pressures. Unlike the

268

linear increasing trend of the water flux with the concentration increase of NaCl draw

269

solution, the water flux with all OPS draw solutions shows a linear increasing trend

270

only when the solute concentration is low (below 0.5 mol/kg), but a slow increase

271

with the further increase in concentration. This may be attributed to the external

272

concentration polarization under PRO mode, which significantly reduces the net

273

osmotic pressure in an asymmetric membrane.

274 275

The increase in the reverse solute flux with the concentration increase is also observed

276

in Figure 3b. With a concentration range of 0-0.85 mol/kg, the highest reverse solute

277

flux with all OPS draw solutions is still below 1 gMH, which is far lower than that of

278

NaCl of the same condition, because of the much larger molecule size of OPSs. In

279

addition, it is also found that the corresponding reverse solute flux decreases slightly

280

with the increase in the molecule size of OPS draw solutes. The ratio of reverse solute 13



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281

flux (JR) to water flux (JW), JR/JW, presents an important index of the FO performance,

282

is shown in Figure S3a (in the Supporting Information). It can be observed that the

283

highest JR/JW ratio with all OPS draw solutions is lower than 0.04 g/L, indicating that

284

the insignificant solute leakage in FO process and their superior FO performance to

285

that of typical inorganic salt draw solutes (i.e., NaCl, MgCl2, etc).

286

287 288

Figure 3. (a) The water flux and (b) reverse solute flux with OPS and NaCl draw

289

solutions of different concentrations, and (c) water flux and (d) reverse solute flux

290

using different membranes.

291 292

In addition, as shown in benchmarking Table 1, with 0.5 mol/kg TPHMP-Na draw

293

solution, an excellent FO water flux of 53.4LMH and a low reverse solute flux of

294

0.83gMH is achieved, which outperforms most other reported synthetic draw solutes

295

(i.e., organic salts11,14, 43-44, nanoparticles15,45-46, etc). Meanwhile, the FO performance 14


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296

of TPHMP-Na draw solution also surpasses that of the previous carboxyethyl amine

297

sodium salts draw solutions under similar conditions.26

298 299

Table 1. The FO performance benchmarking with different draw solutions

Draw solute

Feed

Membrane

Water flux (LMH)

DI water, 25 oC

PSF-TFC

54 PRO

0.83

TPHMP-Na, 0.5M

HTI-TFC

30.6 PRO

0.64

HTI-CTA

23.07 PRO

0.75

26

HTI-TFC

~ 11 FO

9.4

11

TFC-PES

~40 PRO

~0.11

23

HTI-TFC

29.7 PRO

8.86

43

HTI-TFC

2.25 FO

2.17

14

HTI-TFC

~ 5.7 FO

0.13

44

HTI-TFC

29.8 PRO

PEI-600P-Na > NaCl, which is consistent with the

324

performance order for different draw solutes using HTI-TFC membrane. A highest

325

water flux of 54 LMH can be achieved using TPHMP-Na draw solution, which is 2

326

times higher than that of NaCl. And the reverse solute flux also follows the order of

327

their molecular sizes, i.e., PEI-600P-Na > TPHMP-Na > NaCl. The average reverse

328

solute fluxes less than 1 gMH are obtained with TPHMP-Na and PEI-600P-Na draw

329

solution, which are 7~10 times lower than that with NaCl solution (6.4 gMH) as

330

shown in Figure 3d. Similarly, the superior performance of TPHMP-Na and

331

PEI-600P-Na over that of NaCl is also reflected in terms of their JR/JW ratios (Figure

332

S3b in the Supporting Information).

333 334

Recovery by NF. The diluted draw solution after FO may be recovered through a

335

regeneration method. Plenty of previous studies have reported the successful 16


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336

regeneration of draw solution by NF, Ultrafiltration, or Membrane Distillation

337

techniques. In this work, NF process is also employed to recover the diluted OPS

338

draw solution (1000 ppm) after FO with three types of NF membranes (MWCO of

339

150 ~ 800 Da) (Table S4 in the Supporting Information). From Figure S5 (in the

340

Supporting Information), it can be observed that the rejection rates for all OPS

341

solutions are all higher than 92%, and increase with the increase in the Mw of OPS

342

compounds due to the size exclusion effect. For PEI-1800P-Na, PEI-600P-Na, and

343

TPHMP-Na draw solutions, high rejection rates than 97.8% are obtained.

344 345

It is found that there are some minor differences in the rejection rates measured by the

346

TOC and Conductometry. Taking the recovery of DTPMP-Na solution as an example,

347

the rejection rate tested by TOC (99.3%) is higher than that by the Conductometry

348

(98.8%), which is probably resulted from the unbalanced transfer of charged ions. In

349

the DTPMP-Na solution ion pairs (Na+ and multi-monomer DTPMP-) in its solution

350

cannot reach an equilibrium state, since the uncoupled Na+ ions can across the NF

351

membrane freely leading to a high conductivity in the permeate, while the

352

multi-charged DTPMP- ions are retained in the solution resulting in a relative low

353

TOC. 26

354 355

Besides, because the water flux generated in NF process is depended on the osmotic

356

pressure of the OPS feed solution, the lab-scale NF system with a trans-membrane

357

pressure of 4 bar places a limit on the maximum concentration of the treated OPS

358

solution, which can be extrapolated from the osmotic pressure of TPHMP-Na solution

359

to be about 0.082 mol/kg (calculated with the simulated equation π = 133.8c - 6.97,

360

from the data in Figure 2b). The result is also confirmed by the draw solution 17



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361

recovery by NF. As shown in Figure S6, the water flux declines with time, while the

362

diluted TPMHP-Na solution is re-concentrated from 0.065 to 0.08 mol/kg, indicating

363

the feasibility of NF process for the recovery of diluted TPHMP-Na solution. When

364

0.5 mol/kg TPHMP-Na solution is diluted to 0.13 mol/kg, the NF process has to be

365

operated with an external pressure higher than 10 bar. This result, however, indicates

366

that the pressure-driven NF process is actually an energy-intensive process for the

367

recovery of the concentrated draw solution. Exploration of other energy-efficient

368

techniques which are not depended on the osmotic pressure of solution, such as

369

membrane distillation supported by solar energy, should be devoted for the practical

370

applications of draw solutes in FO process.

371 372

In conclusion, a series of OPS draw solutes are successfully synthesized through a

373

one-step Mannich-like reaction and verified by IR and NMR characterizations. The

374

introduction of phosphonate groups can greatly improve their solution properties;

375

bring about high osmotic pressure and relative low viscosity, and therefore superior

376

FO performance against the other reported draw solutes. Experiment results show that

377

draw solution concentrations may significantly impact on its viscosity and osmotic

378

pressure. FO tests indicate that OPS draw solutes, especially TPHMP-Na and

379

PEI-600P-Na, may produce comparative higher water flux and lower reverse solute

380

flux in PRO mode against NaCl, contributed by their higher osmotic pressure, lower

381

viscosity, and suitable molecule sizes. The FO performance with OPS draw solutions

382

can be further improved when a more hydrophilicity FO membrane is available. A

383

high water recovery is obtained in the oil-water separation process. In addition, the

384

diluted OPS solution can be easily recovered with a high rejection rates by NF. This

18


Page 19 of 28


385

encouraging outcome demonstrates the suitability of OPS draw solutes for FO

386

processes and accelerates future exploration of next generation draw solutes.

387 388

ASSOCIATED CONTENT

389

Supporting Information

390

Information addressing the information of materials and chemicals; the solution

391

property test of OPS solutions; FO performance evaluation with OPS draw solutions;

392

oil–water separation test; NF recovery of draw solutions; the fabrication of PSF-TFC

393

membrane and fundamental parameters of the PSF substrate for the home-made

394

PSF-TFC membrane (Table S1); MWs of OPS draw solutes (Table S2); contact angle

395

of the PSF-TFC membrane (Table S3); fundamental parameters of NF membranes

396

(Table S4); 1H NMR characterizations of OPS compounds (Figure S1); 31P NMR

397

characterizations of TPHMP-Na and PEI-600P-Na (Figure S2); the ratio of JR to JW

398

(Figure S3); SEM morphologies of PSF-TFC and HTI-TFC membranes (Figure S4)

399

NF rejection rates (Figure S5); water flux and feed concentration in NF (Figure S6);

400

and oil/water separation test (Figure S7).

401 402

AUTHOR INFORMATION

403

Corresponding Author

404

* E-mail address: [email protected] (Yan Wang). Tel.: 86 027 87543032; fax: 86

405

027-87543632.

406

Notes

407

The authors declare no competing financial interest.

408 409

ACKNOWLEDGEMENT 19



Page 20 of 28

410

This work was financially supported by the National Natural Science Foundation of

411

China (no. 21306058), Huazhong University of Science and Technology, China, and

412

“Thousand Youth Talent Plan” Program. We thank the Analysis and Testing Center,

413

the Analysis and Testing Center in School of Chemistry and Chemical Engineering, as

414

well as the State Key Laboratory of Materials Processing and Die & Mould

415

Technology, in Huazhong University of Science and Technology for their help with

416

material measurements.

417

20


Page 21 of 28


418

ABBREVIATIONS

419

DI

= deionized water

420

MW

= weight-average molecular weight (g/mol)

421

FO

= forward osmosis

422

FO mode

= skin layer faces the feed solution

423

FTIR

= Fourier Transform Infrared Spectroscopy

424

JR

= reverse solute flux (gMH)

425

JR/JW

= the ratio of reverse solute flux (JR) to water flux (JW)

426

JW

= water flux (LMH)

427

Mw

= molecular weight

428

MWCO

= molecular weight cut off, Da

429

NF

= nanofiltration

430

NMR

= Nuclear Magnetic Resonance Spectroscopy

431

PEI

= polyethyleneimine

432

PRO

= pressure retarded osmosis

433

PRO mode = skin layer faces the draw solution

434

PSF

= polysulfone

435

TFC

= thin-film composite membrane

436

TOC

= total organic carbon.

437

21



438

Page 22 of 28

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