Organic Fouling of Graphene Oxide Membranes and Its Implications

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Organic Fouling of Graphene Oxide Membranes and Its Implications for Membrane Fouling Control in Engineered Osmosis Meng Hu, Sunxiang Zheng, and Baoxia Mi Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.5b03916 • Publication Date (Web): 21 Dec 2015 Downloaded from http://pubs.acs.org on December 23, 2015

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Environmental Science & Technology

MANUSCRIPT

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Organic Fouling of Graphene Oxide Membranes and Its Implications for Membrane Fouling Control in Engineered Osmosis

8 9 10 11 12

Submitted to


13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34

Revision submitted on October 30, 2015

Meng Hu 1, Sunxiang Zheng 2, and Baoxia Mi 2*

*

1

Department of Civil and Environmental Engineering University of Maryland College Park, MD 20742, USA

2

Department of Civil and Environmental Engineering University of California Berkeley, CA 94720, USA

Author to whom correspondence should be addressed. +1-510-643-5264

e-mail: [email protected]; tel.: +1-510-664-7446; Fax:

ACS Paragon Plus Environment


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Abstract

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This study provides experimental evidence to mechanistically understand some contradicting

37

effects of the characteristic properties of graphene oxide (GO), such as the high hydrophilicity,

38

negative charge, strong adsorption capability, and large surface area, on the antifouling properties

39

of GO membranes.

40

GO barrier layer on the back (i.e., porous) side of an asymmetric membrane for fouling control

41

in pressure retarded osmosis (PRO), an emerging engineered osmosis process that can be

42

advantageously exploited to extract the sustainable osmotic energy yet its advancement has been

43

much hindered due to the severe irreversible fouling that occurs as foulants accumulate inside the

44

porous membrane support.

45

operated in forward osmosis mode, the GO membrane exhibited fouling performance

46

comparable with that of a polyamide (PA) membrane. Analysis of the membrane adsorption

47

capacity showed that, likely due to the presence of hydrophobic regions in the GO basal plane,

48

the GO membrane has an affinity towards organic foulants 4 to 5 times higher than the PA

49

membrane.

50

noticeably aggravate the fouling problem.

51

foulants are adsorbed mainly on the basal plane of GO nanosheets while water enters the GO

52

membrane primarily around the oxidized edges of GO, making foulant adsorption not create

53

much hindrance to water flux. When operated in PRO mode, the GO membrane exhibited

54

much better antifouling performance than the PA membrane.

55

membrane for which foulants can be easily trapped inside the porous support and hence incur

56

severe irreversible fouling, the GO membrane causes the foulants to accumulate primarily on its

57

surface due to the sealing effect of the GO layer assembled on the porous side of the asymmetric

Furthermore, this study demonstrates the effectiveness of forming a dense

Protein and alginate were used as model organic foulants.

When

Such a high adsorption capacity along with a large surface area, however, did not Our explanation for this phenomenon is that organic

1


This is because unlike the PA

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membrane support.

Results from physical cleaning experiments further showed that the water

59

flux of GO membranes operated in PRO mode can be sufficiently restored towards its initial pre-

60

fouling level.

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Introduction

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Graphene oxide (GO), a single-layer carbon sheet that has a unique two-dimensional (2D) shape

64

and carries abundant oxygenated functional groups, holds great promise as a very useful

65

antifouling material due to its high hydrophilicity, charge properties, and antimicrobial

66

properties.1-6

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thus weaken the adhesion forces between organic foulants and membrane surfaces.7 Besides, by

68

introducing more negative charges to a membrane, GO creates electrostatic repulsion against

69

microorganism deposition and thus inhibit membrane biofouling.8

70

properties, GO can generate the oxidative stress by producing reactive oxygen species, create the

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membrane stress via the direct contact with sharp nanosheets during the initial cell deposition on

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GO, and enable the cell wrapping that reduces the metabolic activities and eventually kills

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bacterial cells.9-13 Taking advantages of these antibacterial properties, polyamide (PA)

74

membranes can be grafted with GO for biofouling control.14

For example, GO is able to effectively enhance the membrane hydrophilicity and

Regarding its antimicrobial

75

Despite the above fruitful research on the use of GO to control membrane fouling, a

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fundamental question remains to be answered: Do the characteristic properties of GO, especially

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the strong adsorption capability towards organic molecules

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(around 2630 m2/g),17 adversely affect the antifouling properties of GO membranes?

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a strong adsorption capacity along with a large surface area would tend to attract more foulants

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to the membrane. If the absorbed foulants do block the water channels within a GO membrane,

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the fouling problem can be aggravated.

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individual and overall effects of the different properties (e.g., the hydrophilicity and adsorption)

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of GO on membrane fouling will offer key information for the development of highly fouling-

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resistant GO membranes.

15, 16

and the large surface area Intuitively,

Therefore, a fundamental understanding of both

3


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In most of the GO-related membrane fouling studies, GO has been blended into the

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polymer matrix to form various nanocomposite membranes, including polyvinylidene fluoride

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(PVDF) microfiltration,2 PVDF ultrafiltation,1,

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nanofiltration membranes.18

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GO is not directly exposed to foulants, thereby making the interactions between GO and foulants

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very limited.

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for membrane fouling control, have unfortunately not been fully exploited.

3

polysulfone,8 and polyether sulfone

One of the major disadvantages of the embedding approach is that

As a result, many of the excellent GO properties, which can be especially useful

92

A more effective GO-based fouling control approach is to deposit GO nanosheets

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directly on the membrane surface so that the interactions between GO and foulants can be

94

maximized.

95

conveniently interact with foulants.

96

modified by grafting GO

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interactions 19 to enhance the membrane fouling resistance to bacteria and bovine serum albumin

98

(BSA). However, these studies mainly focus on the biocidal effect of GO.

99

research is needed to mechanically identify the physicochemical properties of GO that most

100

significantly contribute to the membrane fouling resistance and investigate their overall effects

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on membrane antifouling performance.

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The unique 2D structure of GO offers a significantly large contact area for GO to

14

For this reason, the surface of a PA membrane can be

or by stacking GO and amine-functionalized GO via electrostatic

Clearly, more

To our best knowledge, the potential benefit of the unique 2D shape of GO has not been

103

explicitly exploited in the literature to control membrane fouling.

104

it extremely convenient to assemble a thin film on practically any surface20, 21, including the

105

rough surface of the more porous (i.e., back) side of an asymmetric membrane.

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convenience can be very useful for mitigating fouling that occurs on the back side of an

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asymmetric membrane used in a pressure retarded osmosis (PRO) process.

4


The 2D shape of GO makes

Such


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PRO is a membrane process that uses the osmotic pressure difference between two

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streams as a driving force to create water flow from the low concentration stream to the high

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concentration stream through a semipermeable membrane, and the water flow can drive a

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mechanical turbine to generate electricity.22-25 PRO has been proposed to harvest the natural

112

salinity energy from the mixing of river water and high-salinity water,26-28 hence a new

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renewable energy source becomes available to help meet the grand energy challenges.29-31

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One of the bottleneck technical problems that have severely impeded the advancement

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of the promising PRO technology is the membrane fouling, which occurs as foulants enter the

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porous support layer on the back side (facing the feed solution) of a typically asymmetric,

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semipermeable membrane and accumulate inside the pore structure of the membrane.26,

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Control of such fouling poses a significant challenge.

119

membranes used in PRO mode all have relatively large pores and rough surfaces on the back side.

120

Such a structure is highly susceptible to foulant trapping.

121

increases, water flux declines, and fouling enhanced concentration polarization occurs,

122

eventually lowering the power generation efficiency of a PRO system.26, 32-37

30

This is because the currently available

As a result, the transport resistance

123

Cleaning of membrane fouling in PRO is another formidable task because foulants are

124

trapped inside the support layer.26 Note that for membranes operated in forward osmosis (FO)

125

mode, the back porous side of the support faces the draw solution.

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almost entirely on membrane front dense surface only and can be nearly completely removed

127

using simple physical cleaning (by increasing the cross-flow velocity),38

128

because membranes in PRO suffer from irreversible fouling, physical cleaning (or even chemical

129

cleaning) is not very effective due to the shielding effects of support layer.26

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backwash effort (by switching the feed and draw solutions) can at best recover ~60% water flux

5


As a result, fouling occurs

In comparison,

Even an osmotic

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or equivalently ~44% project power density.26

132

An intuitive and also most effective way to control membrane fouling in PRO mode is to

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create an antifouling “barrier” layer on the back side of the asymmetric membrane such that the

134

entry of foulants to the support layer can be blocked while water and desired ions/molecules can

135

still pass through the barrier.39, 40

136

such as phase inversion and interfacial polymerization, pose huge technical challenges to

137

creating such a dense layer on the back side.

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membrane support using soft polymers (e.g., hyperbranched polyglycerol) to alleviate fouling in

139

PRO.40 However, it is nearly impossible for the large pores (with sizes ranging from hundreds of

140

nm to a few micrometer) on the back side of the membrane to be fully covered with soft

141

polymers using any currently available surface modification technique.

142

modification approach is not sufficient to satisfactorily prevent the accumulation of foulants

143

inside the porous support, thereby causing severe long-term fouling concerns.

144

However, the traditional membrane synthesis approaches,

So far, researchers have attempted to modify the

Therefore, the surface

The present study aims at thoroughly understanding the role of GO in controlling the

145

organic fouling of membranes.

The GO membrane was prepared by the layer-by-layer (LbL)

146

assembly of GO nanosheets and cationic polyelectrolyte on both the front (i.e., dense) and back

147

(i.e., coarse pored) sides of a membrane support.

148

GO membrane and a control PA membrane was systematically tested in FO and PRO modes,

149

respectively.

150

fouling in PRO mode by assembling a dense GO barrier layer on the back side of the porous

151

support are emphasized.

152

characterize the membrane-foulant interactions in order to fundamentally explain the membrane

153

fouling mechanisms.

The fouling and cleaning behavior of both the

In particular, the salient advantages of using GO to effectively control membrane

Quartz crystal microbalance with dissipation (QCM-D) was used to

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Materials and Methods

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Chemicals and Materials

157

All chemicals were used as received from Sigma-Aldrich (St. Louis, MO) unless noted otherwise.

158

The following chemicals were used for membrane synthesis: polyacrylonitril (PAN) (Mw ≈

159

150,000 kDa), N,N-Dimethylformamide (DMF), sodium hydroxide (NaOH), poly(allylamine

160

hydrochloride) (PAH), polysulfone (PSf) pellets (Udel P3500, Solvay Specialty Polymers USA,

161

Douglasville, GA), 1-methyl-2pyrrolidinone (NMP), 1,3,5-benzenetricarbonyl trichloride (TMC),

162

1,3-phenylenediamine (MPD), and Isopar-G (Univar, Redmond, WA). Sucrose was used as draw

163

solutes in the fouling experiments. Sodium alginate (Mw 12–80 kDa) and BSA (lyophilized

164

powder, Mw ~ 66 kDa) were selected as model organic foulants in the study.

165

were prepared by dissolving 10 g/L of each type of foulant in deionized (DI) water and then

166

stored in sterilized bottles at 4 °C.

Stock solutions

167 168

Preparation of GO and PA Membranes

169

GO was prepared by the modified Hummer’s method as described in our previous studies.21, 41

170

The prepared GO has a carbon-to-oxygen ratio of about 2.21 The GO membrane was synthesized

171

by the LbL assembly of GO and PAH.41

172

a membrane support.42 First, a hand-cast PAN membrane was hydrolyzed in 1.5 M NaOH

173

solution for 90 min to make a hydrolyzed PAN (hPAN) support.

174

alternately soaked in 1 g/L PAH and 0.5 g/L GO solutions (both at pH~4) for five times to

175

assemble a five GO-PAH double-layer film on both sides of the hPAN support.

176

study showed that GO-PAH membranes with two to ten double layers demonstrate similar

177

performance in forward osmosis,41 but we chose to synthesize five double-layer membrane in

LbL is an effective approach to create dense layers on

7


Then, the hPAN support was

Our previous

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this study as a conservative minimum in order to minimize potential membrane imperfections

179

due to fewer cycles of LbL assembly.

180

The PA membrane was prepared by interfacial polymerization.43, 44

Briefly speaking, a

181

PSf membrane support was prepared by casting a PSf film (12 wt% dissolved in NMP) onto a

182

polyester nonwoven fabric (PET, grade 3249, Ahlstrom, Helsinki, Finland), which was then

183

immediately transferred into a 3 wt% NMP/DI coagulation bath to allow complete phase

184

separation.

185

by interfacial polymerization between a MPD solution (3.4 wt% in DI) and a TMC solution (0.15

186

wt% in Isopar-G).

187

NaOCl aqueous solution, and NaHSO3 aqueous solution.

188

can be found in the SI.

189

DI water and stored in 4 °C ready for testing and characterization.

190

Membrane Performance Tests

191

The membrane performance was tested in a custom-built FO cross-flow membrane system as

192

described in our previous study.45 The system consists of a membrane cell with an effective

193

membrane area of 20 cm2, two magnetic pumps (Cole-Parmer, Vernon Hills, IL) that circulate

194

the feed and draw solutions through the membrane cell from their respective reservoirs, a water

195

bath (NESLAB RTE 10, Thermo Scientific, Newington, NH) that keeps the system temperature

196

at 25 °C throughout an experiment, a digital balance that sends the weight of the draw solution to

197

a personal computer for recording every 5 min, and a stir plate that constantly mixes the feed

198

solution.

The prepared PSf membrane was then used as support to prepare the PA membrane

The PA membrane was subsequently post-treated using hot water, 200 ppm A more detailed synthesis procedure

The synthesized GO and PA membranes were thoroughly rinsed with

199

The water permeability coefficient A and solute permeability coefficient B of the

200

membrane active layer and the structural parameter S of the membrane support layer were

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characterized using an available protocol 46 described as follows.

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characterization experiment consists of four stages, for which the sucrose concentration of the

203

draw solution was increased incrementally from 200 to 900 mM.

204

draw solution was monitored to calculate the water flux Jw and the concentration of sucrose in

205

the feed solution was measured by the total organic carbon (TOC) to calculate the solute flux Js

206

using the following equations:46

Conducted in FO mode, each

The weight change of the

207

Jw =

∆m ρAm ∆t

(1)

208

Js =

C (V0 − J w Am ∆t ) − C 0V0 Am ∆t

(2)

209

where ∆m is the weight change of the draw solution at a given time period ∆t, ρ is the density of

210

water, Am is the effective membrane area, C0 and C are the sucrose concentrations before and

211

after time ∆t, respectively, and V0 and V are the volumes of the feed solution before and after

212

time ∆t, respectively.

213

following transport governing equations were fitted with Jw, Js, and the corresponding solution

214

concentration and osmotic pressure of both the feed and draw solutions.46

215

216

To obtain the membrane transport/structural parameters A, B, and S, the

= =

(3)

(4)

( )

( )

217

where CD and CF are the concentrations of the draw and feed solutions, respectively, πD and πF

218

are the osmotic pressures of the draw and feed solutions, respectively, D is the bulk diffusion

219

coefficient of sucrose (4.8 × 10-10 m2/s),47 and k is the external mass transfer coefficient, which

220

approaches infinity when the external concentration polarization is negligible.46

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In order to characterize the membrane fouling, fouling experiments were performed with

222

200 mg/L foulant and 50 mM NaCl in the feed solution, and an initial water flux of ~4 µm/s,

223

which was achieved by adjusting the sucrose concentration in the draw solution.

The feed and

224

draw solutions flowed concurrently at a cross-flow velocity of 11 cm/s.

The fouling

225

experiments were started with 2 L feed and 2 L draw solutions and stopped when the cumulative

226

volume of water that permeated from feed to draw solution reached ~400 mL (after 16 to 20 h

227

depending on membranes).

228

water through the system concurrently for 15 min at a cross-flow velocity of 21 cm/s, after which

229

the water flux was measured again to determine the flux recovery.

230

tests, the same draw solution was used to yield the initial water flux of 4 µm/s, and the same feed

231

solution was used but without the adding of foulants. Baseline experiments were performed

232

under the same operating conditions as in the fouling experiments except that foulants were not

233

added in the feed solution so that

234

flux decline in fouling experiments was corrected using the baseline flux profile.

Physical cleaning was then immediately performed by flushing DI

To perform the cleaning

the flux drop due to dilution effects was accounted for. The

235 236

Characterization of Membrane Surface Properties

237

QCM-D (E-4, Biolin Scientific, Linthicum Heights, MD) was used to quantify the LbL assembly

238

of GO membrane (Figure S1), charge density, and foulants adsorption onto a membrane.

239

order to perform the QCM-D measurement, GO and PA membranes were coated, respectively,

240

onto a gold sensor (Biolin Scientific, Linthicum Heights, MD).

241

( ~ 0.2 mL) of MPD solution (3.4 wt% in DI) was spread on a thoroughly cleaned gold sensor,

242

the extra solution was absorbed with a Kimwipe along the edge of the sensor, and then a drop of

243

TMC solution (0.15 wt% in Isopar-G) was pipetted onto the MPD-treated sensor to form a PA

10


In

For the PA coating, one drop


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layer on the sensor. Both the GO and PA-coated sensors were then exposed to a series of 1 mM

245

CsCl solutions at pH of 4, 7, and10, respectively, to quantify the charge density.48

246

the CsCl solution was adjusted with 1 mM CsOH solution.

247

rinsed with DI water, a fouling solution containing 200 mg/L foulant (BSA or alginate) and 50

248

mM NaCl, was pumped through the QCM-D module to measure the adsorption of foulants onto

249

the membrane surface. The changes in frequency f and dissipation D were recorded upon the

250

introducing of the foulants and were modeled with a viscoelastic representation to obtain the

251

mass of foulants adsorbed onto the GO and PA-coated sensors, respectively.

The pH of

After the sensor was thoroughly

252

To compare the surface hydrophilicity of the GO and PA membranes, a sessile drop

253

method was used to measure the water contact angle using a Kruss goniometer (G10, Kruss USA,

254

Charlotte, NC). To determine the location of fouling occurrence, the clean and fouled (in PRO

255

mode) PA and GO membranes were characterized using Fourier transform infrared (FTIR)

256

spectroscopy (Nicolet 6700, Thermo Scientific, Marietta, OH).

257 258

Results and Discussion

259

Comparison of GO and PA membrane properties

260

Figure 1 illustrates the major structural differences between the GO and PA membranes.

261

the GO membrane was synthesized by coating five GO-PAH double layers on both front and

262

back sides of a porous hPAN support, both surfaces of the GO membrane exhibit a relatively

263

rough but non-porous surface morphology (Figure 1(a)).

264

study,41 the thickness of a 5 double-layer GO membrane was ~80 nm, and the equivalent

265

molecular size cutoff was around 1 nm.41 In comparison, the synthesized PA membrane has a

266

typical asymmetric structure, with a rough, dense PA layer on the front side and a porous surface

11

Since

As characterized in our previous


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267

on the back side (Figure 1(b)).

The double-sided coating enables the GO membrane to have

268

some unique transport and antifouling properties, as discussed subsequently.

269

Figure 1

270

In order to compare the transport properties of GO and PA membranes, water and solute

271

fluxes were measured experimentally using sucrose as the draw solute, and then they were

272

modeled to obtain water and solute transport parameters A and B as well as membrane structural

273

parameter S for both membranes.

274

higher water flux and a lower solute flux than the PA membrane.

275

the GO and PA membranes have similar solute permeability coefficient B, while the GO

276

membrane has a much higher water permeability coefficient A, indicating a much lower

277

permeation resistance to water.

278

parameter S than the PA membrane, most likely due to the longer diffusion distance generated by

279

the double-sided coating on the support of the GO membrane.49 It should be noted that coating a

280

GO layer on the back side of a support could also help mitigate the internal concentration

281

polarization (ICP) of an FO membrane. This is because GO may serve as an additional selective

282

layer that prevents the draw solutes from moving into the porous support.50

283

As shown in Figure 2(a-b), the GO membrane demonstrates a As summarized in Figure 2(c),

In the meantime the GO membrane has a higher structural

Figure 2

284

Membrane fouling and cleaning in FO mode

285

BSA and alginate fouling. As shown in Figure 3(a), negligible BSA fouling was observed with

286

both PA and GO membranes when they were tested in FO mode, where the front (dense) side of

287

the support faced the feed solution while the back (porous) side faced the draw solution.

288

calcium ions were present, there was ~17% flux decline for the PA membrane but a negligible

289

decline for the GO membrane (Figure 3(b)).

When

It is believed that the slightly aggravated BSA

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fouling for the PA membrane was caused by the bridging effects of calcium ions that formed

291

complexes with carboxylate groups from both BSA molecules and PA membrane surfaces.51, 52

292

Compared with BSA fouling, alginate fouling caused more flux decline especially for

293

the PA membrane under the tested conditions.

As shown in Figure 3(c-d), the water flux of the

294

PA membrane decreases to ~80% and ~60% of the initial flux for the cases without and with

295

calcium ions, respectively.

296

presence of calcium is known to be the bridging effects of calcium ions that cause higher

297

intermolecular interaction forces and form a gel-like alginate fouling layer on the PA

298

membrane.51, 52

299

even when calcium ions were present.

300

GO membrane is comparable with or slightly better than the PA membrane when operated in FO

301

mode.

302

Flux recovery by cleaning. Immediately after fouling, a membrane was physically cleaned by

303

flushing the membrane surface with DI water for 15 min at an increased cross-flow rate of 21

304

cm/s.

305

restored to the initial level by physical rinsing.

306

membrane after cleaning stays between ~80 to ~90% of the initial water flux, even when there is

307

no obvious flux decline during the fouling cycle.

308

hypothetically attributed to the re-arrangement of the PAH/GO double-layers during cleaning, as

309

reported on self-assembled polymeric multi-layers.41 To verify this hypothesis, a GO membrane

310

was tested with the same fouling and cleaning protocols but without the adding of any foulants in

311

the fouling solution, as described in the Supporting Information.

312

water flux was ~90% (Figure S2), suggesting that the decreased water flux was not due to

The mechanism underlying the aggravated alginate fouling in the

In contrast, the alginate fouling of the GO membrane was still relatively mild Therefore, the overall antifouling performance of the

As shown in Figure 3, the water flux of the fouled PA membranes can be effectively

13

Interestingly, the water flux of the fouled GO

Such a water flux decrease after cleaning is


The resulting final recovered

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membrane fouling.

Then, a second fouling and cleaning cycle was conducted (as described in

314

the Supporting Information).

315

same level, further confirming that the structure of PAH/GO double-layers changes under the

316

increased cross-flow shear force during the cleaning cycle but stayed stable afterwards.

It is found that the GO membrane water flux remained at the

317

Figure 3

318

Contradicting effects of GO properties on membrane fouling

319

Considering the GO membrane has a few properties (e.g., simultaneously being hydrophilic and

320

hydrophobic, negatively charged, and adsorptive) that may have opposing effects on membrane

321

fouling behavior, we characterized the charge, hydrophilicity, and adsorption properties of GO

322

and PA membrane surfaces in order to identify the dominating mechanisms for antifouling.

323

pH increases from 4 to 10, the GO membrane exhibits no more than 2 negatively charged

324

functional sites/nm2, while the charge density of the PA membrane increases from very low (~0)

325

to 22 sites/nm2 (Figure 4(a)).

326

relatively low density of carboxylate functional groups in the GO nanosheets and possibly their

327

partial neutralization by positively charged PAH during the electrostatic layer-by-layer assembly

328

of GO and PAH layers.41

329

foulant-surface interactions and thus reduces the fouling potential of the membrane surface,51-53

330

most likely plays an important role in improving the antifouling property of the GO membrane

331

especially in the presence of calcium ions.

As

The low charge density of GO membrane can be attributed to the

Because the low carboxylate group density, which weakens the

332

As compared in Figure 4(b), the GO membrane surface has a the water contact angle of

333

less than 20° and thus is much more hydrophilic than the PA membrane surface, which has a

334

water contact angle of ~60°.

335

decrease in the interactions between foulants and the GO membrane surface, thereby lowering

Such high hydrophilicity of the GO membrane contributes to a

14



336

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the membrane fouling propensity.

337

On the other hand, other GO properties, such as large surface area (around 2630 m2/g) 17

338

and the hydrophobicity of the un-oxidized region of the GO carbon basal plane, may provide

339

high adsorption capability towards organic foulants, thereby potentially undermining the

340

antifouling properties of the GO membrane.

341

used QCM-D to evaluate the adsorption of organic foulants onto the GO and PA membrane

342

surfaces.

343

four times more for BSA and five times more for alginate) than the PA membrane.

344

the insignificant flux decline observed in Fig. 3 for the GO membrane, apparently the higher

345

adsorption capacity of GO does not appreciably deteriorate its antifouling property.

346

explanation for such phenomena is that organic foulants are adsorbed mainly on the basal plane

347

of GO nanosheets while water enters the GO membrane primarily around the oxidized edges of

348

GO nanosheets (as illustrated in Figure 4(d)).

349

nanosheets does not create much hindrance to the water flux of a GO membrane.

In order to understand such negative effects, we

As depicted in Figure 4(c), the GO membrane adsorbed much more foulants (nearly

350

Considering

Our

As a result, foulant adsorption onto GO

Figure 4

351

Membrane fouling and cleaning in PRO mode

352

Figure 5 demonstrates the fouling behavior of GO and PA membranes in PRO mode, where the

353

front (dense) side of the support faced the draw solution and the back (porous) side now faced

354

the feed solution.

355

much less flux decline and hence lower organic fouling than the PA membrane, which has a

356

porous surface on the back side.

357

to the fact that foulants accumulated within the porous support of the PA membrane while the

358

GO layer coated on the back side successfully prevented the foulants from blocking the water

The GO membrane, with its back surface sealed with a GO layer, exhibited

Such a huge difference in fouling behavior is apparently due

15


Page 17 of 29


359

channels of the GO membrane.

360

apparent discrepancy in the cleaning efficiencies for the two membranes — the flux recovery for

361

the PA membrane by physical cleaning was in general much lower than that of the GO

362

membrane.

363

This statement is further reinforced by the existence of an

Figure 5

364

The observations that the GO membrane exhibited significantly better antifouling

365

behavior than the PA membrane in PRO mode can be schematically explained by Figure 6(a).

366

For the PA membrane, foulants are easily trapped inside the porous support, thereby causing

367

severe irreversible fouling that practically cannot be removed by the shear force of physical

368

cleaning.

369

barrier that sufficiently prevents the foulants from entering the pores of the support, making the

370

foulants only possibly accumulate on the surface of the GO membrane.

371

force of physical cleaning can readily flush off the foulants deposited on the membrane surface

372

and thus the membrane permeability can be satisfactorily restored.

In contrast, the GO layer created on the back surface of the GO membrane acts as a

As a result, the shear

373

To provide more evidence on the effectiveness of the GO layer in preventing the

374

occurrence of irreversible fouling, the FTIR spectra of the active layers of both clean and fouled

375

PA and GO membranes in PRO mode were recorded (Figure 6(b)).

376

membrane active layers show obvious peaks that are indicative of BSA (amide I at 1652 cm-1,

377

amide II at 1531 cm-1) and alginate (asymmetric –COO- stretching vibration at 1603 cm-1, CH-

378

OH stretching vibration at 1030 cm-1), respectively.

379

(front side) of the fouled GO membrane are essentially identical to that of the clean GO

380

membrane (Figure 6(c)), while the spectra of the support (back side) of the corresponding fouled

381

GO membranes do show additional peaks attributable to foulants (Figure S3). Such comparison

16

It is seen that the fouled PA

In contrast, the spectra of the active layer



Page 18 of 29

382

further proves that, when operated in PRO mode, the GO membrane is able to keep the foulants

383

only on its surface due to the blocking effect of the GO layer on the back side of the support,

384

whereas PA membrane suffers the accumulation of foulants inside the unblocked porous support.

385

Figure 6

386

Promise of using GO-coated membranes for fouling control in engineered osmosis

387

The large lateral dimensions of GO nanosheets enable the convenient assembly of

388

semipermeable thin films on both sides of a membrane support with relatively large open pores

389

on the bottom.

390

traditional membrane synthesis approaches or surface modification techniques with soft

391

polymers.

392

a porous electron spun support,54 but it still remains a challenge to seal the back side of the

393

support.

394

the resulting GO membrane almost immune to irreversible fouling and thus ideal for use in

395

engineered osmosis such as PRO.

396

process can be best improved, leading to a more sustainable energy supply.

Such a dense coverage of a porous support is normally not achievable using

It has been recently reported that a dense layer can be successfully created on top of

In comparison, coating a GO layer on the back side of an asymmetric support makes

Consequently, the energy-generating efficiency of the PRO

397 398

Supporting Information

399

Complete information about materials and methods, QCM-D monitoring of the GO membrane

400

assembly (Figure S1), effects of rinsing on membrane flux (Figure S2), and FTIR spectra of the

401

membrane support layers before and after PRO fouling (Figure S3). This material is available

402

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

403 404

Acknowledgements

17


Page 19 of 29


405

This material is based upon work supported by the National Science Foundation under Grant nos.

406

CBET-1351430 and CBET-1154572. We thank Yan Kang and Yoontaek Oh for their

407

experimental assistance. The opinions expressed herein are those of the authors and do not

408

necessarily reflect those of the sponsors.

409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 444 445 446

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539 540 541 542 543 544 545 546 547 548 549 550 551 552 553 554 555 556

49. Duong, P. H. H.; Chung, T. S.; Wei, S.; Irish, L., Highly permeable double-skinned forward osmosis membranes for anti-fouling in the emulsified oil-water separation process. Environ Sci Technol 2014, 48, (8), 4537-4545. 50. Wang, K. Y.; Ong, R. C.; Chung, T. S., Double-skinned forward osmosis membranes for reducing internal concentration polarization within the porous sublayer. Ind Eng Chem Res 2010, 49, (10), 4824-4831. 51. Mi, B.; Elimelech, M., Chemical and physical aspects of organic fouling of forward osmosis membranes. Journal of Membrane Science 2008, 320, (1–2), 292-302. 52. Li, Q.; Elimelech, M., Organic fouling and chemical cleaning of nanofiltration membranes: Measurements and mechanisms. Environmental Science & Technology 2004, 38, (17), 4683-4693. 53. Mo, Y. H.; Tiraferri, A.; Yip, N. Y.; Adout, A.; Huang, X.; Elimelech, M., Improved antifouling properties of polyamide nanofiltration membranes by reducing the density of surface carboxyl groups. Environ Sci Technol 2012, 46, (24), 13253-13261. 54. Bui, N.-N.; McCutcheon, J. R., Nanofiber supported thin-film composite membrane for pressure-retarded osmosis. Environmental Science & Technology 2014, 48, (7), 4129-4136.

557 558 559

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Page 23 of 29

560


Table of Contents (TOC) Art

561 562

22



563 564 565

Figure 1. The structure and surface morphology of (a) GO and (b) PA membranes.

23


Page 24 of 29

Page 25 of 29


566

0.4

GO

2

8

Solute Flux (mole/m /h)

(a)

2

Water Flux (L/m /h)

10

6

PA 4 2 0 0

4

8

12

Osmotic pressure (bar)

567

(c)

Membrane PA GO

(b) PA

0.3 0.2 0.1

GO

0.0 0

4

8

12

Osmotic pressure (bar)

A (LMH/bar) 0.82 5.4

B (LMH) 2.8 2.1

S (µm) 218 354

568 569

Figure 2. The experimental measurements and corresponding model fitting curves of (a) water

570

flux and (b) solute flux for GO and PA membranes. The fitted model parameters are

571

summarized in a table (c).

572

11 cm/s, with a feed solution of DI water and a draw solution of sucrose. In order to vary the

573

osmotic pressure of the draw solution, the sucrose concentration was adjusted in the range of 0.2

574

to 0.9 M.

The experiments were conducted in FO mode at a crossflow rate of

24



Page 26 of 29

575 200 mg/L BSA, no Ca2+

(b)

200 mg/L BSA, 0.5 mM Ca2+

PA

GO 1.0

0.4 0.2 100

200

300

0.8 0.6 0.4 0.2 0.0 0

500 520

Cumulative Volume (mL)

Normalized Flux

1.0 0.8

200 mg/L Alginate, no Ca2+ GO

1.0

PA

0.6 0.4 0.2 0.0 0

100 200 300 500 520 Cumulative Volume (mL)

(d) 200 mg/L Alginate, 0.5 mM Ca2+

Normalized Flux

(c)

After Cleaning

576

PA After Cleaning

0.6

0.0 0

Normalized Flux

GO

0.8

After Cleaning

Normalized Flux

1.0

0.8 0.6

PA

0.4 0.2 0.0 0


GO After Cleaning

(a)


577 578

Figure 3. Comparison of the fouling and cleaning behavior of GO and PA membranes in FO

579

mode with different fouling conditions.

580

an overall ionic strength of 50 mM.

581

water flux of ~ 4 µm/s (14.4 L/m2/h), a cross-flow rate of 11 cm/s, and a draw solution of sucrose.

582

The fouling experiment was stopped when the cumulative volume reached ~400 mL, and then

583

the membrane was cleaned with DI water for 15 min at a cross-flow velocity of 21 cm/s. The

584

cleaned membrane was then tested for flux recovery with the same initial conditions as used in

585

the fouling experiments but without the adding of any foulants.

The fouling solution was dosed with NaCl to maintain

Each fouling experiment was performed with an initial

586 587

25



2

Negative Charge Density (sites/nm )

Page 27 of 29

588

25 20

(a)

15

PA

10 5

GO

0 4

7

10

pH

(c)

2

Foulant Adsorption (µg/cm )

3 GO

2

1

GO PA PA

0 BSA

Alginate

589 590

Figure 4. Comparison of (a) negative charge density, (b) water contact angle, and (c) foulant

591

adsorption property of the GO and PA membranes; and (d) the schematic illustration of foulant

592

deposition on the surface of the GO membrane.

593

foulant adsorption capacity were measured by QCM-D. The negative charge density of the

594

membranes was probed by 1 mM CsCl solution at different pHs using GO- and PA-coated

595

sensors. The foulant adsorption capability of the two membranes was evaluated by exposing the

596

sensors to model foulant solutions containing 200 mg/L foulants and 50 mM NaCl.

Both the negative charge density and the

597

26



2+ (a) 200 mg/L BSA, no Ca

2+ (b) 200 mg/L BSA, 0.5 mM Ca

1.0

0.6

PA

0.4 0.2 0.0 0

598

Normalized Flux

After Cleaning

Normalized Flux

0.6 PA

0.2 0.0 0

0.2

100 200 300 500 520 Cumulative Volume (mL) 200 mg/L Alginate, 0.5 mM Ca2+

1.0

0.8

0.4

0.4

(d)

GO

PA

0.6

0.0 0


2+ (c) 200 mg/L Alginate, no Ca

1.0

0.8

GO

0.8 0.6 0.4

PA

0.2 0.0 0


After Cleaning

0.8

GO

After Cleaning

Normalized Flux

GO

After Cleaning

Normalized Flux

1.0

Page 28 of 29


599 600

Figure 5. Comparison of the fouling and cleaning behavior of GO and PA membranes in PRO

601

mode under different fouling conditions.

602

as those described in Figure 3.

The fouling and cleaning experiments were the same

603 604

27


Page 29 of 29


605

606

(b)

(c)

Transmittance

1652

1531

800

GO BSA-fouled GO Alginate-fouled GO

1603

Alginatefouled PA

1385

BSAfouled PA

1030

Transmittance

PA

1000 1200 1400 1600 1800 -1 Wavenumber (cm )

800

1000 1200 1400 1600 Wavenumber (cm-1)

1800

607 608

Figure 6. (a) Mechanisms for the physical cleaning of PA and GO membranes, (b) FTIR spectra

609

of the active layers of the clean and fouled PA membranes, and (c) FTIR spectra of the active

610

layers of the clean and fouled GO membranes, all in PRO mode.

611 612

28


Organic Fouling of Graphene Oxide Membranes and Its Implications

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