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Thermo-responsive Acidic Microgels as Functional Draw Agents for Forward Osmosis Desalination Yusak Hartanto, Masoumeh Zargar, Haihui Wang, Bo Jin, and Sheng Dai Environ. Sci. Technol., Just Accepted Manuscript • Publication Date (Web): 30 Mar 2016 Downloaded from http://pubs.acs.org on March 30, 2016

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Environmental Science & Technology

1

Thermo-responsive Acidic Microgels as Functional

2

Draw Agents for Forward Osmosis Desalination

3 4

Yusak Hartanto1, Masoumeh Zargar1, Haihui Wang2, Bo Jin1, Sheng Dai*1

5

1

School of Chemical Engineering, The University of Adelaide, SA 5005, Australia

6

2

School of Chemistry & Chemical Engineering, South China University of Technology,

7

Guangzhou, 510640, PR China

8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26

* To whom corresponding should be addressed [email protected]

ACS Paragon Plus Environment

1


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Abstract

28

Thermo-responsive microgels with carboxylic acid functionalization have been recently

29

introduced as an attractive draw agent for forward osmosis (FO) desalination, where the

30

microgels showed promising water flux and water recovery performance. In this study, various

31

co-monomers containing different carboxylic acid and sulfonic acid functional groups were

32

copolymerized with N-isopropylacrylamide (NP) to yield a series of functionalized thermo-

33

responsive microgels possessing different acidic groups and hydrophobicities. The purified

34

microgels were examined as the draw agents for FO application, and the results show the

35

response of water flux and water recovery was significantly affected by various acidic co-

36

monomers. The thermo-responsive microgel with itaconic acid shows the best overall

37

performance with an initial water flux of 44.8 LMH, water recovery up to 47.2 % and apparent

38

water flux of 3.1 LMH. This study shows that the incorporation of hydrophilic dicarboxylic acid

39

functional groups into MCG-NP microgels leads to the enhancement on water adsorption and

40

overall performance. Our work elucidates in detail on the structure-property relationship of

41

thermo-responsive microgels in their applications as FO draw agents and would be beneficial for

42

future design and development of high performance FO desalination.

43

Keywords: microgels; forward osmosis; desalination; water flux; water recovery; N-

44

isopropylacrylamide.


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Introduction

46

Membrane-based desalination process offers a more viable option to provide fresh water for

47

global water scarcity than thermal-based desalination process in terms of energy consumption 1.

48

Reverse osmosis (RO) process is currently the most common membrane desalination approach

49

where hydraulic pressure is applied to overcome the osmotic pressure of saline water to produce

50

fresh water. Although this technology consumes less energy than thermal desalination methods,

51

there is a growing demand to develop more energy-efficient desalination process.

52

Forward osmosis (FO) desalination is an attractive membrane-based desalination process

53

which consists of a two-stage process of water drawing and water recovery. The former relies on

54

the osmotic gradient between saline feed solution and draw solution to drive water adsorption

55

and the latter is an additional separation unit to recover the adsorbed water. To date, the adsorbed

56

water can be recovered by means of some simple separation methods such as pressure-driven

57

membrane, thermal separation, precipitation, electro-dialysis and stimuli-response and the

58

recycled draw solute can be concentrated and reused for further water drawing 2. For the water

59

drawing process, both membrane and draw solute affect the performance of overall FO

60

desalination. Although there is a rapid development on FO desalination membranes, research on

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exploring high performance draw solutes is still limited. This is especially important if fresh

62

water has to be recovered as the final product or if the major FO desalination energy

63

consumption comes from the dewatering stage 3.

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Ideally, suitable draw solute materials have to meet three key criteria, a high osmotic pressure,

65

economical regeneration process and low impact on internal concentration polarization 4.

66

Ammonium bicarbonate (NH4HCO3)

67

have been proposed as the draw solutes for FO desalination due to their ease of recovery via

5

and and trimethylamine-carbon dioxide (TMA-CO2)


3

6


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68

distillation process. However, trace residuals left behind in unacceptable level during the

69

separation process makes these compounds unsuitable for producing fresh water for potable use

70

7

71

divalent salts of Na2SO4 or MgSO4

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their higher osmotic pressures 9. The process was hybridized with nanofiltration process to

73

produce potable water and recover the draw solutes. The nanofiltration process used to produce

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potable water and recover draw solutes has some drawbacks such as the possibility of membrane

75

scaling 9 and high energy consumption 10.

. Other attempts have been developed to use divalent inorganic salts as the draw solutes. The 8

produced higher water flux than monovalent salts due to

76

Instead of using inorganic salts for FO desalination, synthetic materials are proposed as next

77

generation draw solutes. The large molecular size of these materials makes the recovery process

78

easier and cheaper to operate 10. Hydrophilic magnetic nanoparticles have been proposed as the

79

draw solute for FO desalination, where the surface of nanoparticles were coated with organic

80

compounds such as citrate

81

Isopropylacrylamide)

82

styrene 4-sulfonate)

83

aggregation in the recovery process limit its end-use applications. Meanwhile, a variety of

84

synthetic organic materials, such as polymers

85

switchable solvents 33, have also been employed as draw solutes for FO process. Among them,

86

polymer-based draw solute shows comparable performance and recovery cost as inorganic salts.

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Polyacrylic sodium salt was able to generate high water flux as the seawater draw solute and had

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lower salt leakage 21. However, the use of polyacrylic sodium salt might release sodium ions to

89

final water product. In addition, the energy cost for separation using ultrafiltration membrane

90

might not be lower than the RO desalination 3.

15

17

11

, dextran

12

, triethylene glycol

, polyacrylic acid 13

13-14

, 2-pyrrolidone

, poly(ethylene glycol) diacid

16

14

, poly(N-

and poly(sodium

. Low water flux during the water drawing course and nanoparticle

18-29

, hydroacid complex


4

30-31

, carbon dots

32

and

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91

Recently, stimuli-responsive hydrogels were introduced as an alternative draw solute for low

92

energy desalination technology and to overcome the reverse draw solute problem 26, 34. However,

93

the low water fluxes of these hydrogels restrict them for the FO desalination application. Later,

94

functionalized thermo-responsive microgels were proposed to improve the FO desalination

95

performance

96

recovery and overall water production when compared to bulk hydrogels due to the large surface

97

area of microgels. Thus, it is necessary to carry out in depth study on functionality of these

98

thermo-responsive microgels. In this study, we systematically design and investigate the effect

99

of incorporation of various functional acidic co-monomers containing carboxylic groups and

100

sulfonic acid group on N-isopropylacrylamide-based thermo-responsive microgels. Our results

101

show that different functional co-monomers have significant impact on both water flux and water

102

recovery ability in FO desalination process and the significant enhancement on water flux using

103

dicarboxylic group co-monomers has been observed.

35

and these microgels have showed significantly enhanced water flux, water

104 105

Materials and Methods

106

Materials

107

N-isopropylacrylamide (NP, > 98%), purchased from Tokyo Chemical Industry, was purified

108

by recrystallization in n-hexane and dried overnight. N-N’-methylenebisacrylamide (MBA,

109

>98%), acrylic acid (AA, 99%), methacrylic acid (MAA, 99%), itaconic acid (IA, >99%), maleic

110

acid (MA, >99%), 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS, 99%) and ammonium

111

persulfate (APS) were purchased from Sigma-Aldrich. Cellulose triacetate forward osmosis

112

(CTA-FO) membranes were purchased from Hydration Technologies Inc. (HTI, USA).

113


5


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Synthesis of thermoresponsive microgels

115

The N-isopropylacrylamide-based microgels with different functional co-monomers were

116

synthesized using surfactant-free semi-batch emulsion polymerization process. In a typical

117

experiment, 0.475 g of NIPAM, 0.025 g of functional co-monomers and 0.005 g of MBA

118

(crosslinkers) were dissolved in 47 mL of Millipore or deionized water. The solution was then

119

transferred to a 250 mL three-necked flask fitted with a condenser, a mechanical stirrer and gas

120

inlet/outlet. The semi-batch feeding solution was prepared by dissolving 3.325 g of NIPAM,

121

0.1725 g of functional co-monomers and 0.035 g of MBA in 50 mL of Millipore water. The

122

detailed synthesis recipes for each microgel sample can be found in Table S1.

123

After degassing batch and semi-batch feeding solutions for 45 minutes, the batch solution was

124

heated to 70 °C under nitrogen atmosphere. 3.0 mL of APS solution (0.4 g) was injected to

125

initiate the polymerization. Semi-batch feeding solution was injected at a rate of 3 mL/hour one

126

hour after the batch solution turned into cloudy. The polymerization was carried out overnight

127

under continuous stirring and nitrogen blanket. After cooling, the microgels were purified using

128

membrane dialysis (MWCO 12–14 kDa) against Millipore water for three days to remove any

129

unreacted residuals. Finally, the purified microgels were dried and grounded into fine powder.

130 131

Fourier Transform Infrared Spectroscopy (FTIR) measurement

132

Attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) was used to

133

confirm the incorporation of functional co-monomers into the microgels. FTIR bands were

134

recorded using a Thermos Scientific NICOLET 6700 spectrometer equipped with a diamond

135

ATR with wavenumber resolution of 4 cm-1 in the range of 400 – 4000 cm-1.

136


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Dynamic light scattering

138

Dynamic light scattering was used to measure the hydrodynamic diameters (dh) of the

139

microgels at different temperatures using a Zetasizer (Malvern, Nano-ZS). The swelling ratio

140

(SR) of microgel is calculated using the following equation:

141

SR =

142

where SR is the swelling ratio of the microgels, dh,25 (nm) is the hydrodynamic diameter of the

143

microgels at 25 oC and dh,40 (nm) is the hydrodynamic diameter of the microgels at 40 oC.

, ,

(1)

144 145

Conductometric and potentiometric titration

146

The exact amount of acidic co-monomers in the microgels was determined using

147

conductometric and potentiometric titrations. Typically, the pH of a 100 mL MCG-NP-MAA

148

microgel dispersion (~ 1 mg/mL) was adjusted to 3.5 using concentrated hydrochloric acid. The

149

solution was then back titrated using a 0.1 M NaOH solution. After each addition of NaOH,

150

solution conductivity and pH were measured using a pre-calibrated Aqua-CP/A pH and

151

conductivity meter.

152 153 154

Water flux evaluation Grounded microgel powders (100 mg) were placed in our customized FO membrane setup 35

155

equipped with an on-line conductivity monitoring system

156

adopted in this experiment was the active layer facing draw solute (AL-DS) with an effective

157

area of 3.16 cm2. The powder area density for all experiments is 0.1 g/3.16 cm2. Calibrated

158

conductivity probe was immersed in the feed solution of 2000 ppm NaCl to continuously

159

monitor the conductivity against time for two hours. The volume of feed solution was 190 mL.


7

. The membrane configuration


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160

Before any measurement, the FO membrane was soaked overnight in the feed solution and the

161

system was conditioned by contacting the membrane surface area in 2000 ppm NaCl solution for

162

30 minutes and monitoring the conductivity until constant prior to placing the microgels on the

163

membrane. The conductivity data can be converted into the concentration of sodium chloride in

164

solution through a calibration curve.

165 166

The water flux was calculated using the conductivity data based on the mass balance equation which is described below: V =

C V C

(2)

167

J =

168

where Vt (mL) is the volume of feed at time t, Vi (mL) is the initial volume of feed, Ci (ppm) is

169

the initial feed concentration, Ct (ppm) is the feed concentration at time t, Jw (LMH) is the water

170

flux, A (m2) is the effective membrane surface area and ∆t (h) is the time interval where the

171

conductivity of the feed solution changes.

(3)

∆

172 173

Water recovery

174

The swelled microgels were transferred to centrifuge tubes and weighed after the two-hour

175

water adsorption period. The microgels were then centrifuged at 40 oC and 10,000 rpm (9,300 g)

176

for 10 minutes to separate adsorbed water from polymer microgels. The water recovered from

177

deswelled microgels was calculated using the following equations:

178

C =

(4)

W = W! (1 − C ) 179

&

R =

'

(5)

x 100%

(6)


8

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180

where CP (g microgels/g H2O) is the concentration of microgels in the centrifuge tube, WP (g) is

181

the weight of dry microgel powders, WW (g) is the weight of water adsorbed by the microgels

182

determined from water flux measurement, WWG (g) is the weight of water in the microgels, WH

183

(g) is the weight of microgels in the centrifuge tube, WR (g) is the weight of adsorbed water

184

recovered from the tube and R (%) is the percentage of water recovered from the deswelled

185

microgels.

186 187 188 189

Apparent Water Flux Apparent water flux is defined as the amount of water that can be released from the microgels per unit area per unit cycling time and it is written as follows 36: J+,, =

-.

(7)

/012 + 04 56

190

where Japp (LMH) is the apparent water flux, mW (L) is the volume of water that can be

191

released during dewatering process, Teq (h) is the estimated time to reach equilibrium water

192

content of microgel from its dried state, TR (h) is the estimated time needed to dewater the

193

microgels from centrifugation process and A (m2) is the effective membrane area.

194 195

Microgels recycling evaluation

196

The microgels after the first cycle were taken from the membrane setup and dried in oven until

197

constant weight. The dried microgels were grounded into fine powders and placed in our setup

198

for the second cycle measurement. The same approach was repeated for the third cycle. The

199

water recovery cycle measurement was conducted using similar approach by gravimetric

200

method.

201


9


202

Results and Discussion

203

Synthesis and characterization of thermo-responsive acidic microgels

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Surfactant-free emulsion polymerization process was employed to synthesize the microgels to

205

avoid surfactant contamination on the final microgels. In this study, various carboxylic and

206

sulfonic acid co-monomers are chosen to represent the different degrees of deprotonation of

207

acidic co-monomers. The chemical structures of different acidic co-monomers selected in this

208

study are shown in Figure 1. The co-monomers with carboxylic acid groups can be further

209

divided into two main categories. The co-monomers with monocarboxylic acids are represented

210

by acrylic acid and methacrylic acid while co-monomers with dicarboxylic acids are represented

211

by maleic acid and itaconic acid. The co-monomer with sulfonic acid group is represented by 2-

212

acrylamido-2-methyl-1-propanesulfonic acid.

213 214

Figure 1 Different acidic co-monomers used to functionalize thermo-responsive N-

215

isopropylacrylamide microgels.

216 217

In order to confirm the successful copolymerization of these microgels, Fourier transform

218

infrared spectroscopy (FTIR) analysis was carried out to identify functional groups within

219

microgels. The FTIR spectra in Figure 2 show typical peaks corresponding to N-

220

isopropylacrylamide at the bands around 1640 and 1550 cm−1 which are attributed to the C=O


10

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221

asymmetric stretching and the bending of the C-N. The peak at around 1450 cm−1 is also

222

assigned to the stretching of the C-N bond and asymmetric bending of the C-H bond in the

223

methyl groups of N-isopropylacrylamide

224

except for MCG-NP-AMPS is attributed to the C=O bond in carboxylic acids which present in

225

the microgels containing AA, MAA, IA and MA 37. The appearance of these peaks confirms the

226

successful incorporation of co-monomers into microgels. Likewise, appearance of the band at

227

1038 cm−1 for the MCG-NP-AMPS spectrum which is assigned to the symmetric stretching of

228

the S=O in the sulfonic acid group verifies the successful synthesis of the microgel with sulfonic

229

acid functional group 39.

37-38

. The band at 1711 cm−1 in all microgel spectra

230 231

Figure 2 FTIR spectra for different thermo-responsive acidic microgels

232 233

The volume phase transition temperature (VPTT) and swelling ratio (SR) determined from

234

dynamic light scattering are related to the required temperature in dewatering process and the

235

amount of water can be adsorbed and recovered during the FO process. The hydrodynamic

236

diameters of these microgels vs. temperatures are shown in Figure 3a. The incorporation of

237

functional co-monomers with different acid dissociation constants shifts the VPTT depending on


11


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238

the degree of ionization and the hydrophobicities of the co-monomers 40,41. The dicarboxylic acid

239

copolymer microgels have slightly higher volume phase transition temperatures than the

240

monocarboxylic acid microgels. The swelling ratios of the copolymer microgels are shown in

241

Figure 3b. The MCG-NP-MAA microgel has the highest swelling ratio among other microgels

242

due to the presence of methyl group which contributes to its more hydrophobic character. On the

243

other hand, MCG-NP-AMPS microgel has less responsive properties to temperature change due

244

to existence of strong electrostatic repulsion from the strongly ionized sulfonic acid groups.

245 246

Figure 3 (a) Hydrodynamic diameters of thermo-responsive microgels with various functional

247

acidic co-monomers measured between 25 oC and 50 oC at pH 6.8. (b) Swelling ratios of the

248

above thermo-responsive acidic microgels.

249 250

The percentages of incorporated acidic co-monomers into the microgels were determined by

251

potentiometric and conductometric titration. The average amount of incorporated functional co-

252

monomer is summarized in Table S1.

253 254


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Water flux profile for each functional co-monomers

256

Equilibrium swelling time (EST) is the time needed to fully saturate the microgels with water

257

and it is an important parameter in microgel-driven FO desalination. It is desirable that these

258

microgels have relatively short equilibrium swelling time to maximize the amount of water that

259

can be adsorbed during each cycle.

260

Water flux and water content profiles for various microgels over a two-hour water adsorption

261

period are shown in Figure 4. MCG-NP-MAA microgel reaches equilibrium in 110 minutes,

262

longer than MCG-NP-AA microgel which is only 60 minutes. For comparison, MCG-NP

263

microgel has an equilibrium swelling time of 100 minutes. The longest equilibrium time

264

observed in MCG-NP-MAA can be explained by presence of hydrophobic methyl functional

265

groups in MAA. These groups adopt compact structure during polymerization resulting that most

266

of methacrylic acid moieties are located in the core after polymerization

267

adsorption occurs due to the main contribution of poly(N-isopropylacrylamide) chains, resulting

268

in a long period of time to saturate the microgels. Conversely, acrylic acid co-monomer is more

269

evenly distributed throughout the microgel, resulting in shorter time to reach equilibrium 42.


13

42

. This causes water


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270 271

Figure 4 Water flux profiles for different thermo-responsive microgels: (a) MCG-NP, (b) MCG-

272

NP-MAA, (c) MCG-NP-AA, (d) MCG-NP-MA, (e) MCG-NP-IA ,and (f) MCG-NP-AMPS over

273

a two-hour water adsorption period. The vertical green line represents the approximate

274

equilibrium swelling time determined when the water content of the copolymer microgels

275

reaches plateau. Membrane orientation is AL-DS. Feed solution is 2000 ppm NaCl. pH of the

276

feed solution is 6.8. Temperature of the feed solution is 25 oC.

277 278

Shorter EST was observed in dicarboxylic acid copolymer microgels and sulfonic acid

279

copolymer microgel due to stronger polymer solvation behavior of dicarboxylic copolymer

280

microgels than monocarboxylic copolymer microgels. The strong solvation behavior originates

281

from the presence of two acidic dissociation constant (pKa) in dicarboxylic acid co-monomers.

282

MA has the pKa of 2.0 and 6.23 while IA has the pKa of 3.85 and 5.45 which imply two-stage

283

ionization process of carboxylic acids and the higher amount of deprotonated carboxylic acid

284

moieties in dicarboxylic acid copolymer microgels than monocarboxylic acid copolymer


14

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41

285

microgels

. MCG-NP-MA and MCG-NP-IA microgels requires 50 minutes to reaching their

286

equilibrium conditions while MCG-NP-AMPS needs around 40 minutes to reach its equilibrium

287

condition due to the presence of strongly ionized sulfonic acid 39.

288 289

Initial water flux, water recovery, apparent water flux and recyclability of various

290

copolymer microgels

291

The effect of different functional co-monomers on initial water flux and water recovery is

292

shown in Figure 5. MCG-NP shows the lowest initial water flux among other copolymer

293

microgels. The initial water flux and water recovery for the MCG-NP employed as draw agent

294

are 7.5 LMH and 72.1%.

295

Microgels functionalized with monocarboxylic acids, MCG-NP-MAA and MCG-NP-AA,

296

show enhanced initial water fluxes compared to MCG-NP microgel. The initial water fluxes for

297

MCG-NP-MAA and MCG-NP-AA are 10.9 LMH and 16.7 LMH, respectively. The major

298

improvement in water adsorption originates from the osmotic pressure generated within the

299

microgels due to the presence of deprotonated carboxylic acids

300

contains hydrophobic methyl groups which contribute to the slight hydrophobic character of this

301

microgel. As a result, relatively lower water flux and higher water recovery than MCG-NP-AA

302

microgel were observed in MCG-NP-MAA. MCG-NP-MAA has water recovery of 76.7% while

303

MCG-NP-AA has water recovery of 55.8%.


15

43

. However, MCG-NP-MAA


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304 305

Figure 5 Initial water flux and water recovery for different thermo-responsive microgels with

306

various functional co-monomers. Membrane orientation is AL-DS. Feed solution is 2000 ppm

307

NaCl. pH of the feed solution is 6.8. Temperature of the feed solution is 25 oC.

308 309

Copolymer microgels with dicarboxylic acid and sulfonic acid show better initial water fluxes

310

than monocarboxylic acid coppolymer microgels. The water fluxes of MCG-NP-MA and MCG-

311

NP-IA are 39.1 LMH and 44.8 LMH, respectively. These high water fluxes are caused by the

312

presence of more hydrophilic acidic co-monomers that allows the microgels have more

313

deprotonated carboxylic acid groups. The water flux for MCG-NP-MA is slightly lower than

314

MCG-NP-IA due to the higher value of second acid dissociation constant (pKa2) of maleic acid

315

(6.24) than the pKa2 of itaconic acid (5.45). Lower pKa2 of itaconic acid implies there are more

316

carboxylic acid groups in their deprotonated state than the ones present in MCG-NP-MA

317

microgels at the same pH value. As a result, MCG-NP-IA has higher charge density than MCG-

318

NP-MA which increases its osmotic pressure and its ability to draw water from saline solution in

319

a faster rate than MCG-NP-MA. Although dicarboxylic acids copolymer microgels have superior


16

Page 17 of 28


320

performance in water adsorption abililty, the water recovery is compromised due to high charge

321

repulsions from deprotonated carboxylic acid groups. The water recovered from MCG-NP-MA

322

and MCG-NP-IA are 39.1% and 47.2%. These recovery rates agree well with the swelling ratio

323

data from Figure 3b where the swelling ratio of MCG-NP-IA is higher than the swelling ratio of

324

MCG-NP-MA. MCG-NP-AMPS microgel shows comparable water flux to dicarboxylic acid

325

copolymer microgels. The water flux for MCG-NP-AMPS is 42.9 LMH. However, no water can

326

be released during recovery process due to strong electrostatic repulsion created by complete

327

dissociation of sulfonic acid functional group within the microgel. This prevents the microgels to

328

collapse during the dewatering process

329

traditional draw agents i.e. inorganic salts and polymer-based materials in Table 1.

35

. In this study, we also presents a comparison on

330


17


331

Page 18 of 28

Table 1 Comparison of FO Desalination Performance of Various Draw Agents

Draw Solution

NH4HCO3

MgSO4

Polysodium acrylate

N-Isopropylacrylamide-cosodium acrylate hydrogels N-Isopropylacrylamide-coacrylic acid microgels Tributylhexylphosphonium p-styrenesulfonate hydrogels N-Isopropylacrylamide-coitaconic acid microgels

Experimental Condtions Feed Concentration: 0.5 M Draw Concentration: 6 M Temperature: 50 oC PRO Mode Feed Concentration: 3,970 ppm brackish water Draw Concentration: 2 g/L PRO Mode Molecular weight: 1,800 Feed: DI Water Draw Concentration: 0.72 g/mL Temperature: 22 oC PRO Mode Feed: 2000 ppm NaCl Draw Mass: 1 g PRO Mode Feed: 2000 ppm NaCl Draw Mass: 0.1 g PRO Mode Feed: 2000 ppm NaCl Draw Mass: 0.6 g PRO Mode Feed: 2000 ppm NaCl Draw Mass: 0.1 g

Flux

FO Performance Water Apparent Recovery Flux

Ref.

23 LMH

-

-

5

13 LMH

15 %

-

8

19 LMH

-

-

21

0.6 LMH

62 %

-

26

23.8 LMH

52 %

-

35

2 LMH

50%

0.94 LMH

36

44.8 LMH

47.2 %

3.1 LMH

This Study

332 333

As can be seen from Table 1, the water flux generated by MCG-NP-IA microgel is relatively

334

high compared to other selected draw agents. The water recovery is also comparable with other

335

types of thermo-responsive hydrogels. In addition, our microgel has an apparent flux of 3.1 LMH

336

which is higher than the recent result of thermo-responsive polyionic liquid hydrogels 36.

337

In practical application, these microgels will not start from their dried state to adsorp water

338

from saline solution. As a result, the performance of microgel-based system needs to be

339

evaluated simultaneously in terms of their water adsorption and water recovery. Recently, a

340

concept of apparent water flux has been proposed to assess the overall performance of hydrogel-

341

driven FO desalination 36. The apparent water fluxes of these microgels are shown in Figure 6.


18

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Apparent Water Flux (LMH)

3.5 3 2.5 2 1.5 1 0.5 0

342

Functional Microgels

343

Figure 6 Apparent water flux per 100 mg dried microgels for different thermo-responsive

344

microgels with various functional co-monomers.

345 346

MCG-NP has the lowest apparent water flux of 0.7 LMH among other copolymer microgels.

347

MCG-NP-MAA and MCG-NP-AA microgels have higher apparent fluxes than MCG-NP

348

microgel, 1.3 LMH and 2.2 LMH, respectively. On the other hand, MCG-NP-MA and MCG-NP-

349

IA have apparent fluxes of 2.5 LMH and 3.1 LMH, respectively while the apparent flux of

350

MCG-NP-AMPS microgel is close to zero as there is no water can be recovered from these

351

microgels. The higher apparent water fluxes in dicarboxylic acid copolymer microgels are due to

352

their relatively fast equilibrium time and higher water recovery. This means designing thermo-

353

responsive microgels as functional draw agents by incorporating the right co-monomers is

354

crucial to achieve high overall performance of microgel-driven FO desalination.

355

Lab-scale FO experiments are usually operated in semi-batch mode due to the requirement of

356

recovering the water as final product and reconcentrating the draw agents. In addition, operating

357

microgel-based FO desalination in semi-batch cascade configuration allows reusing the draw

358

agent after recovery process and achieving more sustainable water production


19

44

. In this study,


359

we studied the recyclability of our thermo-responsive microgels in three cycles. We select MCG-

360

NP-MAA as a model for monocarboxylic acid functionalized microgels and MCG-NP-IA as a

361

model for dicarboxylic acid functionalized microgels to be used in recycling process. As can be

362

seen from Figure 7, both microgels show good recyclability in terms of water flux and water

363

recovery in three cycles.

Page 20 of 28

364 365

Figure 7 Recyclability study of thermo-responsive microgels in three cycles: (a) Initial water

366

flux recyclability and (b) Water recovery recyclability

367 368

In summary, thermo-responsive microgels with varied functional acidic co-monomers were

369

synthesized and were employed as draw agents for FO desalination. Their performance in terms

370

of water flux, water recovery, apparent flux and recyclability were evaluated systematically.

371

Thermo-responsive microgels with dicarboxylic acid functionalization show the best apparent

372

water fluxes for microgel-based FO desalination. These microgels also hold a potential to be a

373

viable alternative for low-energy desalination technology based on the theoretical energy

374

requirement and cost analysis.

375


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Page 21 of 28


376

Supporting Information Detailed synthesis recipe, pH and conductivity titration curves, cost

377

comparison table of draw agents are available in Supporting Information file. This material is

378

available free of charge via the Internet at http://pubs.acs.org.

379 380

Author Contributions

381 382

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

383 384

Acknowledgement

385

The authors would like to thank the financial support from the Australian Research Council

386

(DP110102877). YH would like to appreciate the support of Adelaide Graduate Research

387

Scholarships (AGRS).

388 389

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Thermoresponsive Acidic Microgels as Functional ... - ACS Publications

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