Hydrophilic and Compressible Aerogel: A Novel Draw Agent in

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Hydrophilic and compressible aerogel: a novel draw agent in forward osmosis Mingchuan Yu, Hanmin Zhang, and Fenglin Yang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b10229 • Publication Date (Web): 11 Sep 2017 Downloaded from http://pubs.acs.org on September 12, 2017

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ACS Applied Materials & Interfaces

Hydrophilic and compressible aerogel: a novel draw agent in forward osmosis Mingchuan Yu, Hanmin Zhang*, and Fenglin Yang

Key Laboratory of Industrial Ecology and Environmental Engineering (MOE), School of Environmental Science and Technology, Dalian University of Technology, Dalian 116011, P. R. China.

Keywords: Graphene oxide, Alginate aerogel, Forward osmosis, Draw agent, Wastewater treatment

Corresponding author. E-mail address: [email protected] (HM. Zhang). 1 Environment ACS Paragon Plus


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ABSTRACT Forward osmosis (FO) technology is an efficient route to obtain purity water for drinking from wastewater or seawater. However, there are some challenges in draw solution to limit its application. We firstly introduce a novel sodium alginate-graphene oxide (SA-GO) aerogel as draw agent for highly efficient FO process. The GO nanosheets covalently cross-linked to SA matrix to form a three-dimensional and highly porous aerogel to provide excellent water flux and operation stability, together with the property of compressibility served by SA-GO aerogel resulting in easy water production and regeneration process. When the deionized water used as the feed solution, the SA-GO aerogel exhibited a high water flux (15.25±0.65 L m-2 h-1, abbreviated as LMH) than that of 1 mol L-1 NaCl and there was no non-reverse osmosis phenomenon. The water fluxes were stabilized in the range of 5-6.5 LMH during recycle process of absorbing and releasing water as high as 100 times. It also had a great desalination capacity (water flux was 7.49±0.61 LMH) with the seawater (Huanghai coast) as the feed solution. Moreover, the water production and regeneration process of the SA-GO aerogel can be rapidly and cost-effectively accomplished with low-strength mechanical compression (merely 1 kPa). The results present that the SA-GO aerogels as a promising innovation draw agent can make the FO process simpler, more efficient and lower energy consumption. It can be a potential material for hydration bags to fast and repeatable product fresh water from saline water or wastewater.

2 Environment ACS Paragon Plus

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1

1. INTRODUCTION

2

With population growth and economic development, water scarcity and pollution are

3

becoming increasingly serious.1, 2 Since the middle of the 20th century, membrane filtration

4

technology including microfiltration,3 ultrafiltration,4 nanofiltration5 and reverse osmosis6 has

5

been widely used in drinking water treatment, groundwater replenishment and industrial

6

wastewater treatment to change the unsustainable use of water resources. Forward osmosis

7

(FO) is a membrane separation process that relies solely on osmotic pressure, which means

8

that water molecules spontaneously penetrate through the semipermeable membrane from the

9

feed solution side (the high water chemistry potential) to draw solution side (the low water

10

chemistry potential). FO technology with lower energy consumption of draw water process 1,

11

higher water recovery rate, and smaller membrane fouling tendency is superior to other

12

membrane technology and is widely investigated. There have been many successful attempts

13

in desalination, power generation, industrial wastewater treatment, drug release and food

14

processing. Particularly, the hydrated bag is one of FO commercial applications used in

15

drinking water shortage conditions of the military, recreational and emergency rescue fields.7

16

But the FO development is still facing a challenge from draw solutions. In previous researches,

17

many

18

macromolecules,10 magnetic nanoparticles,11 hydrophilic carbon quantum dots12 and so on13,

19

are developed successively. However, these draw solutions can’t fully meet the FO

20

technology requirements resulting from high energy consumption of the regeneration process,

21

heavy reverse osmosis, low water flux or other problems.7 Recent years, hydrogels, as an

22

emerging draw agent, can completely avoid the reverse osmosis, attributing to the structure of

23

the three dimensional (3D) networks of polymer chains.14-16 Nevertheless, the water flux

24

(driving force) and water recovery rate of hydrogels are much low owing to large volumes of

25

water entrapping into the 3D networks structure and high water retention capacity.14, 17 It

26

severely restricts its development and application. In the comparison with hydrogel, aerogel,

materials,

such

as

inorganic

salts,8

natural


macromolecules,9

synthetic


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as a kind of functional material with high porosity, low density and large specific surface area,

28

has diverse physical and chemical properties according to its composition.18-28 Sun et al

29

directly synthesized the ultra-flyweight aerogel by cryodesiccation and reduction reaction

30

with graphene oxide and carbon nanotube. It was called “super sponge” which can still kept

31

its original scale and morphology after the fatigue test of 1000 cycles.29 Therefore, we select a

32

hydrophilic and resistant compressed aerogel as a new type of draw agent referring to

33

hydrogel characteristics and similar draw water behavior of beverage powder used in the FO

34

hydration bags.30, 31 It can maintain the property of no reverse osmosis relying on its 3D

35

networks structure. Simultaneously, a higher FO driving force can be produced because its

36

structure contains amount of hydrophilic functional groups on the chains and no water

37

molecule inside its structure. Compared with most of draw agents, the compressible property

38

of aerogel draw agent can greatly simplify its regeneration process, instantly complete the

39

regeneration and water recovery to reduce the energy consumption.

40

Herein, a hydrophilic and compressible sodium alginate-graphene oxide (SA-GO) aerogel was

41

obtained through esterification, ionic crosslinking and lyophilization process in Figure 1 and

42

employed as a draw agent in the FO process for the first time. As expected, the as-prepared

43

SA-GO aerogels exhibited many favorable properties: (i) high water flux, (ii) non-reverse

44

osmosis, (iii) convenient and efficient regeneration process (merely rely on 1 KPa

45

compression can simply and rapidly complete the water production), (iv) non-toxic and (v)

46

stable performance in continuous reuse process.

47

2. MATERIALS AND METHODS

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2.1. Materials and chemicals. All chemicals and solvents were purchased from Aladdin and

49

used without purification. Sodium alginate (C6H7O6Na)n (AR), Calcium chloride anhydrous

50

(CaCl2) (AR), Ethanol absolute (CH3CH2OH) (AR) and Sodium chloride (NaCl) (AR) were

51

purchased from Aladdin. Graphite flake (~150 µm flakes) was purchased from Sigma-Aldrich.

52

Potassium permanganate (KMnO4) (AR), Sulfuric acid (H2SO4) (AR), Phosphoric acid 4 Environment ACS Paragon Plus

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(H3PO4) (85%), Hydrogen chloride (HCl) (36% aqueous solution) and Hydrogen peroxide

54

(H2O2) (30% aqueous solution) were purchased from Sinopharm Chemical Reagent. DI water

55

from a Milli-Q (Millipore, USA) system was used in all experiments.

56

2.2. Preparation of graphene oxide (GO). GO was prepared according to the previously

57

reported method with slight modifications.32 In brief, 3.0 g natural scale graphene (1 wt equiv)

58

was added into the 1000 mL round-bottomed flask and mixed in a 9:1 mixture of concentrated

59

H2SO4/H3PO4 (360:40 mL) solution under magnetic stirring. After that, 18.0 g KMnO4 (6 wt

60

equiv) was slowly added to the flask under ice-cooling. The water bath was then heated to

61

50 °C and stirred for 12 h. The reaction was cooled to room temperature and then the

62

suspension was poured onto ice water (about 500 mL). The reaction solution needed to be

63

protected from light and waited until the temperature drops to room temperature. Then, H2O2

64

was slowly added dropwise to the reaction solution until the solution became golden yellow.

65

For workup, the reaction solution was washed in succession with 300 mL of 0.1 M HCl

66

aqueous solution (multiple times) and 300 mL of distilled water (multiple times). The treated

67

solution was centrifuged (10000 rpm for 10 min) until the pH of the supernatant was closed to

68

5. The GO solid was obtained by vacuum drying (room temperature).

69

2.3. Synthesis of sodium alginate-graphene oxide (SA-GO) aerogel. Typically, 0.6 g

70

sodium alginate (SA) was dissolved in 29.4 g of distilled water under magnetic stirring about

71

12 h to form a homogeneous aqueous solution. Then, a uniform dispersion was formed after

72

mixing the GO solid and the SA solution through ultrasonication for 2 h to form. The mass

73

ratio of the SA and GO were 8:1, 16:1 and 80:1, respectively. After that, 0.24 M CaCl2

74

aqueous solution was sprayed to the surface of the reaction solutions, and reaction solutions

75

stood for 24 h at room temperature to form the hydrogels. The hydrogels were freeze-dried (-

76

90 °C for 24 h) to obtain aerogels after washed with the distilled water until the aqueous

77

solution had no Ca2+.



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2.4. Physical Characterization. Attenuated total reflection-fourier transform infrared

79

intensity (ATR-FTIR) measurements were carried out by using a ThermoFisher 6700

80

Spectrometer Spectrum (Thermo Fisher, USA) to determine the functional groups in the SA-

81

GO aerogels among 4000 and 400 cm-1. Nitrogen adsorption-desorption isotherms were

82

measured by Quantachrome Autosorb-1MP sorption analyzer (Quantachrome, USA). The

83

specific surface areas were calculated using the BET method. Crystallographic pattern of the

84

SA-GO aerogels were obtained by Empyrean X-ray diffraction (XRD) (PANalytical,

85

Netherlands) using Cu-Kα radiation. Raman spectra were recorded using a DXR Smart

86

Raman spectrometer (Thermo Fisher, USA). Surface morphology of the aerogels was studied

87

using NOVA Nano SEM 450 scanning electron microscopy (SEM) (FEI, USA) at an

88

acceleration voltage of 15 kV.

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The compressive test was performed on the home-made equipment shown in Figure S6.33

90

The Dip coater (CZ-4200, HTLAB Co., Ltd.) drives the compression head up and down at a

91

preset speed. The load on the sample was recorded using a pressure sensor.

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samples with a diameter of 20mm were cut into 20mm length (L/D=1) and tested at room

93

temperature (25 °C) using a compression rate of 500 µm min-1 up to 50% compressive strain.

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In FO process, the ability of aerogels to absorb water was measured as draw agents in FO

95

device. The water absorption capacity (Wa) (g g-1) of SA-GO aerogels was measured and

96

calculated as:

97

() =

33

Cylindrical

(1)

98

where mt and m0 are quality of wet (running time was t) and dry aerogel, respectively.

99

2.5. FO performance. FO experiments were carried out through a lab-scale rig, as shown in

100

Figure S7. Commercial Cellulose Triacetate (CTA-ES) FO membranes from Hydration

101

Technologies Inc. (HTI, USA) were used, which involve a dense selective active layer onto a

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phase inversed polysulfone supporting layer. The FO membranes are immersed in DI water 2

103

h before use. All FO experiments used the FO mode (the membrane orientation of the active 6 Environment ACS Paragon Plus

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layer facing the feed solution). During the FO tests, the feed solution side of the FO

105

membrane (effective membrane area of 12.6 cm2) was circulated by one peristaltic pump

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(LongerPump, LongerBT100-2J). The temperatures of the feed was maintained at 25±0.5 °C.

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In FO process, the aerogel draw agents were all 0.2 g dry cylinders according with the FO

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module and DI water feed solution without special instructions. And the initial volume of the

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feed solution was 1 L. A feed container was placed on a weighing scale (MSE2203, Sartorius,

110

Germany) that was connected to a computer data logging system to record weight change at

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regular time intervals (interval time is 1 min). And the used FO membrane was washed by DI

112

water and then were stored in DI water.

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The water flux, Jw, (LMH) was calculated from the weight change of the feed solution using

114

Equation (2). ∆

= ∆

115

(2)

116

where ∆V (L) is the volume change of the feed solution over a predetermined time ∆t (h) and

117

A is the effective membrane surface area (m2).

118

This FO process doesn’t need to test the reverse salt flux due to structural characteristics of

119

the aerogels without reverse osmosis.

120

Recovery of SA-GO aerogels from their drawing water state: After an FO process, the draw

121

agent needed to produce water and regenerate by a mechanical compression device. A

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commercial injection pump (LongerPump, LSP01-2A) with a computer controller was used

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for draw agent regeneration. (

124

Water recovery rate (Wr) =

125

where mt, m0 and mw are quality of wet, dry aerogel and water recovery, respectively.

126

3. RESULTS AND DISCUSSION

127

3.1. Structure and Properties of Aerogels.


(3)


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In this paper, SA-GO aerogels were successfully synthesized by esterification, ionic

129

crosslinking and lyophilization with SA, GO and CaCl2, as shown in Figure 1. During the

130

synthetic route, SA was a strong hydrophilic and non-toxic raw material, and easy to form a

131

gel. CaCl2 as an ion crosslinking agent can accelerate the formation of SA hydrogel in mild

132

conditions. Meanwhile, GO as an auxiliary crosslinking agent (chemical crosslinking) was

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introduced to enhance the crosslinking strength, compressibility and hydrophilicity of the

134

aerogel because it possessed a considerable amount of hydroxyl and epoxide functional

135

groups on both surfaces of each sheet and carboxyl groups mostly at the sheet edges.34 In

136

order to validate the crosslinking reaction between SA and GO and the effect of physical and

137

chemical properties, a series SA-GO aerogels with various volume ratios of SA and GO

138

(Table 1) were synthesized, characterized and tested.

139 140

Figure 1. Synthetic route of the SA-GO aerogels.

141 142

Table 1. The feed amount of raw materials in the reaction. SAa) (g) GO (mg)

CaCl2b) (mL)

aqueous solution pure SA

30

0

10

SA-GO-1

30

7.5

10

SA-GO-2

30

37.5

10

SA-GO-3

30

75

10


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143

a)

The concentration of SA aqueous solution is 2 wt%

144

b)

The concentration of CaCl2 aqueous solution is 0.24 mol L-1

145

The structure information of aerogels was confirmed by ATR-FTIR (Figure 2a). The

146

characteristic bands of SA aerogel at 3422 cm-1 (O-H stretching vibration) and 1615 cm-1 (-

147

C=O stretching vibration of carboxylate) indicated the appearance of hydroxyl and

148

carboxylate groups. The absorption bonds of GO (Figure S1) present characteristic bands at

149

3312, 1720, and 1610 cm-1 attributing to stretching vibration of hydroxyl, carboxyl and

150

carboxylate group, respectively.35 In comparison with the raw materials of SA and GO, a new

151

band at 1740 cm-1 (-C=O stretching vibration of ester) appears after condensation reaction of

152

SA and GO, inferring that a part of carboxyl groups (SA) and epoxy groups (GO) might

153

convert into esters. It means crosslinking between SA and GO. The X-ray diffraction (XRD)

154

measurement further shows that GO is attached into the SA-GO aerogels (Figure S2). The

155

typical diffraction peak for GO appeared at around 11 ° belonging to (001) crystallographic

156

plane.33 And the diffraction peak at 23.5 ° is assigned to the amorphous morphology of pure

157

SA (002). By contrast, the XRD pattern of the SA-GO aerogels gives the peak at 18.3 ° and

158

its shoulder peak at 21.5 ° revealing greater amorphous morphology than pure SA (002).36

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The consequent increase of amorphous phase confirmed the successful conjugation between

160

SA and GO. Otherwise, the Raman spectra of SA-GO aerogels all show two prominent peaks

161

at the same range with that of GO (D band and G band), manifesting the existence of GO in

162

SA-GO aerogels (Figure 2b). XPS was conducted to obtain more detailed information about

163

elemental analysis and chemical structure of the aerogels. Here, the SA-GO-1 (the optimum

164

condition) was chose to make a comparative analysis with SA. As the Figure S3a shown, the

165

peaks in the full spectra indicated the existence of carbon, oxygen and calcium elements in the

166

SA and SA-GO-1 aerogel. In the C1s spectrum (Figure S3b) of the SA-GO-1 aerogel, peak

167

fitting showed four different peaks: C-C/C-H/C=C (284.5 eV), C-O (286.1 eV), C=O (287.3

168

eV) and O-C=O (288.3 eV). For the SA aerogel, the C1s spectrum was deconvoluted into 9 Environment ACS Paragon Plus


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three main bands appearing at 284.6 (C-C/C-H), 286.2 (C-O) and 287.8 (O-C=O) eV. After

170

introducing of GO, SA-GO-1 had more oxygen-containing groups and interacted (chemical

171

crosslinking and hydrogen bonding) with SA. In addition, the peaks at 347 and 350.6 eV in

172

Figure S3d and e were due to Ca2p3/2 and Ca2p1/2, respectively, indicating that the calcium

173

ion was bivalent 28, 33. All the results verify the structure of the SA-GO aerogels.

174 175

Figure 2. (a) ATR-FTIR and (b) Raman spectra of SA-GO and pure SA aerogels.

176

The morphology and microstructure of as-prepared aerogels were examined by SEM (Figure

177

3a). In the pure SA aerogel, the wall thickness of the structure is about 3.6 µm and a large

178

number of agglomerations are on its surface. It is attributed to the fact that the pure SA

179

aerogel only replied on ionic crosslinking (Ca2+) to form gel. But the wall thickness of the

180

SA-GO aerogels is thinner and a mass of deep-submicron pores (pore diameter: 100-500nm)

181

are observed on the surface due to the addition of GO. The wall thickness of the SA-GO

182

aerogel became thicker with the increasing of GO content (from 0.52 to 2.3µm), and the

183

numbers of deep-submicron pores on the wall layers decreased distinctly which is consistent

184

with the variation tendency of the specific surface area data (Table 2). Among SA-GO

185

aerogels, the SA-GO-1 aerogel has more deep-submicron pores on the surface of the wall

186

which in favor to increasing the specific surface area. 37 Furthermore, it also reveals that a

187

mass of hydrophilic groups (SA) were encapsulated by the excess GO nanosheets inducing

188

the compressibility and hydrophilic to be decreased. It is consistent with the water contact

189

angle and compressibility results (Table 2, Figure S4 and Figure S5). 28 10 Environment ACS Paragon Plus

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190

The specific surface area (SBET) and pore properties, the important affecting factors on the FO

191

performance, were obtained from the nitrogen sorption measurement (Table 2). For pure SA

192

aerogel without the cross-linking of GO, its SBET is 33.42 m2 g-1, which might be induced by

193

the severe stacking and intertwining of the aerogel structure during the formation process.

194

And, as the increasing of the GO content in SA-GO aerogels, the SBET had an obvious

195

decreasing tendency. Combined with the SEM images analysis, when the GO content was

196

appropriate (at lower state), it only might be used as a crosslinking agent to crosslink the SA

197

fragments while enhancing the hydrophilic and compressibility of the aerogel. However, with

198

the increasing of GO content (more than the amount required for the crosslinking agent), only

199

part of GO took effect as the crosslinking agent, the residual GO might crosslink with each

200

other or induce the unformed hydrogel fragment structures to be wrapped and stacked. The

201

SEM images exhibits that the layer structure of the SA-GO-3 has serious stacking

202

phenomenon than the other two SA-GO aerogels leading to lower SBET. Moreover, as the

203

Table 2 shown, the SA-GO aerogels possess a low density in the range of 0.031-0.041 g cm-3,

204

which is comparable to those of monolithic graphene based materials.19, 29, 38 It is due to the

205

rich open-pore structures interpenetrating the skeleton of aerogel (Figure 3a). And, it also can

206

be seen that pore volume (VP) is in the range of 0.118-0.364 cm3 g-1.

207

The hydrophilicity of the aerogels can be directly proved by the water contact angle

208

measurement. As demonstrated in Figure 3b, a water droplet drops onto the SA-GO aerogel

209

surface is absorbed immediately (less than 100 ms). In other words, there was no initial

210

nonzero contact angle for the aerogels. In order to investigate the different hydrophilicity

211

between the inherent materials and aerogels, the sheets of SA-GO and SA were obtained

212

through grinding and pressing of the aerogel materials. The water contact angles of SA-GO

213

and pure SA sheets followed the sequence of SA-GO-3 > SA-GO-2 > pure SA > SA-GO-1

214

(Table 2 and Figure 3c). It might be due to the excessive GO encapsulated the unformed

215

segments of SA-GO hydrogel structures. In other words, a mass of hydrophilic groups (SA) 11 Environment ACS Paragon Plus


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216

were encapsulated inducing the reducing tendency of the SA-GO aerogels.39,40 In addition, the

217

hydrophilicity of aerogel status is more superior than sheet status attributed to the 3D porous

218

network structure of aerogel.

219 220

Table 2. Porous and hydrophilic properties of SA-GO and pure SA aerogels. Specific

surface Pore

area (SBET)(m2 g-1)

volume Density

Water contact

(VP)(cm3 g-1)

(g cm-3)

anglea (°) a)

pure SA

33.42

0.18

0.033

27.7

SA-GO-1

311

0.364

0.031

24.6

SA-GO-2

167.73

0.217

0.033

28.0

SA-GO-3

76.55

0.118

0.041

31.8

221

a)

222

of SA-GO aerogel materials

The water contact angles were the SA-GO sheets which were made by grinding and pressing

223


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224 225

Figure 3. (a) SEM images of the dry SA-GO and pure SA aerogels. (b) The water contact

226

angles of dry aerogel and (c) sheet states with the SA-GO and pure SA materials.

227 228

3.2. FO performances testing.

229

The water fluxes of these dry aerogels, as shown in Figure 4a, followed the sequence of SA-

230

GO-1>SA-GO-2>SA-GO-3>pure SA, during 20 min. After introducing GO into SA aerogel,

231

the water fluxes of SA-GO aerogels are higher than that of pure SA aerogel. At an appropriate

232

GO content (SA-GO-1), the water flux was superior to others referring to the GO content had

233

an effect on the SBET, water contact angle and microstructures. Compared with traditional

234

draw agent-NaCl (1 mol L-1, abbreviated as 1 M, 11.02±0.34 L m-2 h-1, abbreviated as LMH),

235

the water flux of dry SA-GO-1 aerogel, upped to 15.25±0.65 LMH, is higher, when the

236

running time is 20 min. However, with the running time exceeding 20 min, the water fluxes of

237

aerogels were observed a significant decay trend. This phenomenon might be affected by the

238

saturated water absorption capacity of the aerogels themselves (Table S1). Combined with

239

Table S1 data, the water absorption quality of aerogel draw agents had gradually approached

240

saturation state with the extension of running time. Similar to the hydrogel, when the water

241

absorption quality of aerogel was close to the saturated state in FO process, the driving force

242

also gradually weakened.17, 41 The water flux of the SA-GO-1 presented especially obvious

243

attenuation tendency than that of other aerogels which was induced by the SA-GO-1 13 Environment ACS Paragon Plus


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244

absorbing the massive water in short time to reach to its saturated water absorption capacity.

245

And water absorption capacities of SA-GO aerogels were all larger than pure SA aerogel

246

consisting with the results of the water flux in FO process. The Wa of SA-GO-1 aerogel

247

(17.95 g g-1) is 6.4 times, 2.99 times and 5.5 times than that of SA, SA-GO-2 and SA-GO-3

248

aerogel during 20 min, respectively. Besides the reverse osmosis of the draw agents is an

249

important indicator for the FO performance evaluation. It exists in most of the draw agents (in

250

addition to hydrogel) in previous reports inducing the wastage of the draw agent, the

251

reduction of the water flux and the secondary pollution of the feed solution.42 But the SA-GO

252

aerogel can completely avoid reverse osmosis because of its 3D network structure similar

253

with hydrogel.41

254 255

Figure 4. (a) Water fluxes of SA-GO, pure SA aerogels and 1 M NaCl. (b)Effect of the

256

different water recovery rates on water flux recovery.

257 258

In regeneration process, the Wr of the wet aerogel is determined by the degree of compression

259

which can directly affect its water flux. In order to optimize the water flux of the regenerated

260

wet aerogels, the different degrees of compression (water recovery rate) were investigated in

261

FO process. Figure 4b exhibits the relationship of Wr with water flux in a series of

262

regenerated wet SA-GO aerogels. As the Wr increases, the water fluxes of SA-GO aerogels

263

showed an increasing tendency. And, in this process, it indicated that the increasing trend of


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264

the water flux on the Wr from 30% to 50% were more remarkable than that from 50% to 70%.

265

Furthermore, when Wr reached 70%, the SA-GO aerogel structure was irreversible damaged,

266

affecting its continuous use performance. Therefore the Wr (50%) was selected to dewater

267

from the wet aerogel in recovery process.

268

Water recovery process can be completed by mechanical compression, as shown in Figure 5,

269

Video 1 and Video 2. The SA-GO-1 aerogel, as the test sample, was subjected to compression

270

for this measurement. When the pressure reached 1 kPa, the aerogel can complete the

271

compression and production water process in short-time. By contrast, the regeneration method

272

of NaCl (traditional draw agent) is RO technology which requires high running pressure (~1.5

273

MPa).41 And Razmjou et al reported a bilayer polymer hydrogels draw agent which can

274

continuously product fresh water, but it also consumed larger energy (input energy from 0.5

275

to 2 kW m-2) and longer regeneration time (dewatering rate from 10 to 25 LMH) than that of

276

SA-GO-1 aerogel.31,

277

commercialized in hydration bags.7, 46 But the production water in hydration bags contains

278

glucide because of glucose or fructose as the draw agent, which leading to the limited target

279

users. And the draw agents are disposable and need to be replaced frequently. If SA-GO-1

280

aerogel as draw agent is used for the FO hydrated bags, the water recovery and regeneration

281

process can be easily accomplished in short-time by a compression of an adult palms (2 MPa

282

pressure) and the scope application of production water could increase substantially. In a

283

word, the SA-GO-1 aerogel can facile and quickly accomplish this process which

284

demonstrated significantly superiority than the aforementioned draw agents.

44, 45

To the best of our knowledge, FO technology has been



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285 286

Figure 5. Different state of SA-GO-1 aerogel in compression process. a) the dry state of

287

aerogel. b) the wet state of aerogel. c) the compressed wet aerogel. d) the released wet aerogel.

288

As a draw agent of FO technology, the re-use performance of aerogel is also an important

289

index to balance the FO performance of draw agent. Therefore, we evaluate the re-use

290

performance of SA-GO-1 with deionized water as feed solution in the FO continuous cycle

291

test. According to the water flux results, 20 min was selected as the running time. As seen of

292

Figure 6a, the initial water flux of the dry aerogel is 15.5 LMH. After the compression and

293

regeneration, the wet aerogel was re-used to measure the draw-release water capacity in FO

294

process. In the course of continuous cycle tests (100 cycles), the water fluxes of wet SA-GO-1

295

generally kept high and stabilized in the range of 5-6.5 LMH, indicating SA-GO-1 aerogel

296

can be recycled and maintain a high FO performance. The obvious attenuation of water flux

297

between dry aerogel and wet aerogel is attributed to the above-mentioned water absorption

298

capacity in FO process. The SA-GO-1 aerogel showed high structure stability and repeatable

299

draw-release water capacity. Furthermore, the water fluxes of wet SA-GO-1 aerogel draw

300

agent are also greater than that of all most hydrogels using as draw agents.14-16, 41


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301 302

Figure 6. (a) The water fluxes of SA-GO-1 in FO continuous cycle test. (b) Desalination test

303

of SA-GO-1 aerogel.

304 305

3.3. Application of SA-GO aerogel in FO. The SA-GO-1 aerogel was selected and applied

306

to seawater desalination, owing to its remarkable pure water flux (high hydrophilic), great

307

mechanical strength and large specific surface area, among this series aerogels. In the

308

desalination test, the seawater (Huanghai coast, conductivity is 26.8 mS cm-1 and total

309

dissolved solids (TDS) is about 24800 ppm) and SA-GO-1 aerogel were used as feed solution

310

and draw agent, respectively. As shown in Figure 6b, it presented a similar trend but lower

311

water flux compared with the deionized water because that the seawater contains numbers of

312

inorganic salts leading to a higher osmotic pressure

313

water. Eventually, the high water flux and water quality were obtained in FO process. The

314

water flux of dry SA-GO-1 aerogel is as high as 7.49±0.61 LMH (running time: 20 min).

315

After regeneration with 50 % Wr, the water flux of the wet aerogel can still carry out

316

3.18±0.39 LMH, which is higher than the pure water flux of most hydrogel draw agents.46

317

Meanwhile, the conductivity and TDS of production water were 10-40 µS cm-1 and 4.5-19

318

ppm, respectively, suggesting that there are no leaching ions and organics of aerogel in

319

production water. Considering the above-mentioned properties, this kind of aerogel is an

320

innovative and alternative draw agent in the FO process.

47

(25 atm) than that of the deionized

321



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322

4. CONCLUSIONS

323

In summary, we have demonstrated, for the first time, the hydrophilic and compressible SA-

324

GO aerogels as a novel kind of draw agent in FO process. The structure and properties of SA-

325

GO aerogels were evaluated through the ATR-FTIR, Raman, XRD, water contact angle

326

analyzer and sorption analyzer. The SA-GO aerogels with lightweight (0.031-0.041 g cm-3),

327

large specific surface area (76-311 m2 g-1) and 3D hierarchically porous structure were

328

constructed through SA and GO via esterification, ionic crosslinking and lyophilization.

329

Particularly, the SA-GO-1 aerogel showed an excellent hydrophilicity and compressibility,

330

offering suitable properties for FO process. The water flux of dry SA-GO-1 aerogel

331

(15.25±0.65 LMH) was 1.46 times of 1 M NaCl (11.02±0.34 LMH), and higher than most

332

hydrogel draw agents when the feed solution was DI water. For SA-GO aerogel, the reverse

333

osmosis can be completely avoided and a facile recovery process can be handled owing to its

334

monolith 3D framework structure. In a continuous cycle test (100 times), its water fluxes were

335

basically kept high and steady (5-6.5 LMH). In the practical application of seawater

336

desalination, the water fluxes of dry and wet SA-GO-1 aerogel were as high as 7.49±0.61 and

337

3.18±0.39 LMH, respectively. Meanwhile, only 1kPa mechanical pressure (far less than the

338

compression of an adult palms) can rapidly and cost-effectively realize the SA-GO aerogel.

339

Therefore, SA-GO aerogel as FO draw agent can break through the bottleneck of reverse

340

osmosis and complicated regeneration to realize production water with lower cost and higher

341

efficiency from saline water or wastewater. If applied in FO hydration bags, the aerogels

342

would draw purity water from wastewater through a compression of an adult palms in many

343

times, which can effectively simplify operation conditions and obviously increase the scope

344

application of FO hydration bags.

345

ACKNOWLEDGEMENTS

346

Financial support of Natural Science Foundation of China (No. 51278079) is highly

347

appreciated. 18 Environment ACS Paragon Plus

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ASSOCIATED CONTENT Supporting Information This supporting information is available free of charge via the Internet at http://pubs.acs.org.

The FT-IR spectrum of GO. The XRD spectra of GO and SA-GO aerogels. The compressibility of the dry aerogels. Compression tests of SA-GO aerogels. Schematic illustration of equipment for the compressive test. The Lab-scale FO-aerogel recovery process. Water absorption test of aerogel draw agents in FO process. The measurement of the antibacterial property of SA-GO aerogels by the bacteriostasis rate. The compression/release process of the SA-GO aerogel.

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