Influence of Active Layer and Support Layer Surface Structures on

Subscriber access provided by Oregon State University

Article

Influence of Active Layer and Support Layer Surface Structures on Organic Fouling Propensity of Thin-Film Composite Forward Osmosis Membranes Xinglin Lu, Laura H Arias Chavez, Santiago Romero-Vargas Castrillón, Jun Ma, and Menachem Elimelech Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/es5044062 • Publication Date (Web): 07 Jan 2015 Downloaded from http://pubs.acs.org on January 21, 2015

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Environmental Science & Technology is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 26

Environmental Science & Technology

Influence of Active Layer and Support Layer Surface Structures on Organic Fouling Propensity of Thin-Film Composite Forward Osmosis Membranes

Environmental Science & Technology Revised: November 12, 2014

Xinglin Lu1, Laura H. Arias Chavez2, Santiago Romero-Vargas Castrillón2, Jun Ma*,1, and Menachem Elimelech*,2 1

State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology, Harbin 150090, China. 2

Department of Chemical and Environmental Engineering, Yale University, New Haven, Connecticut 06520-8286, USA *Corresponding Authors * E-mail: [email protected] (J.M.), [email protected] (M.E.). Tel: +86 451 86283010 (J.M.), +1 203 432 2789 (M.E.). Fax: +86 451 86283010 (J.M.), +1 203 432 4387 (M.E.).

-1ACS Paragon Plus Environment


Page 2 of 26

1

Abstract

2

In this study, we investigate the influence of surface structure on the fouling propensity of thin-

3

film composite (TFC) forward osmosis (FO) membranes. Specifically, we compare membranes

4

fabricated

5

(dimethylformamide, DMF and n-methyl-2-pyrrolidinone, NMP) during phase separation. FO

6

fouling experiments were carried out with a feed solution containing a model organic foulant.

7

The TFC membranes fabricated using NMP (NMP-TFC) had significantly less flux decline (7.47

8

± 0.15%) when compared to the membranes fabricated using DMF (DMF-TFC, 12.70 ± 2.62%

9

flux decline). Water flux was also more easily recovered through physical cleaning for the NMP-

10

TFC membrane. To determine the fundamental cause of these differences in fouling propensity,

11

the active and support layers of the membranes were extensively characterized for physical and

12

chemical characteristics relevant to fouling behavior. Polyamide surface roughness was found to

13

dominate all other investigated factors in determining the fouling propensities of our membranes

14

relative to each other. The high roughness polyamide surface of the DMF-TFC membrane was

15

also rich in larger leaf-like structures, while the lower roughness NMP-TFC membrane

16

polyamide layer contained more nodular and smaller features. The support layers of the two

17

membrane types were also characterized for their morphological properties, and the relation

18

between support layer surface structure and polyamide active layer formation was discussed.

19

Taken together, our findings indicate that support layer structure has a significant impact on the

20

fouling propensity of the active layer, and this impact should be considered in the design of

21

support layer structures for TFC membranes.

through

identical

procedures

except

for

the

22


use

of

different

solvents

Page 3 of 26


TOC Art



23

INTRODUCTION

24

Membrane processes hold significant promise for addressing the global challenge of water

25

scarcity and the need for greater sustainability.1-3 Highly permeable and selective polyamide

26

thin-film composite (TFC) membranes have facilitated the rise of reverse osmosis (RO) as the

27

dominant technology for desalination.2, 4 The ability to separately optimize the two membrane

28

layers (i.e., active and support layers) has also allowed the development of TFC membranes for

29

forward osmosis (FO) and pressure retarded osmosis (PRO).5-7 These technologies have wide

30

potential applications in the water, energy, waste reclamation, agricultural, and biomedical

31

industries.8

32

Despite the high water permeability and solute selectivity of TFC polyamide membranes,

33

their performance is significantly hampered by fouling.9-13 Intrinsic hydrophobicity and native

34

carboxyl groups, which interact with foulants, make the aromatic polyamide active layer

35

intrinsically prone to fouling.2 Consequently, extensive efforts in surface modification of the

36

polyamide active layer have sought to mitigate fouling14-17. Other studies have shown that the

37

characteristic nanoscale ridge-and-valley structure of the polyamide layer can also exacerbate

38

membrane fouling.18-20

39

The fabrication of a TFC membrane generally comprises two steps4: (i) fabrication of a

40

support layer through non-solvent induced phase separation and (ii) formation of a thin selective

41

layer on the support layer via interfacial polymerization. Previous publications21-24 suggest that

42

the support layer plays a significant role in determining active layer perm-selectivity through its

43

presence during active layer formation. Singh et al.21 studied the effect of support layer surface

44

pore size on TFC RO membrane salt rejection and water permeability. Kim et al.22 evaluated the

45

influence of support layer hydrophilicity on membrane perm-selectivity. Ghosh et al.23

46

investigated the impacts of support membrane structure and chemistry on RO membrane active

47

layer morphology. In addition, Tiraferri et al.24 fabricated different polysulfone supports by

48

varying casting conditions in the phase separation step and evaluated the perm-selectivity of

49

formed TFC FO membranes. Their results also indicated a significant influence of the

50

polysulfone support on the FO membrane active layer transport properties.

51

The above studies suggest a critical role for the support layer in determining active layer

52

perm-selectivity and morphology, with transport properties having received particular attention. -4ACS Paragon Plus Environment

Page 4 of 26

Page 5 of 26


53

It is also known that surface structure, like the intrinsic ridge-and-valley structure of a polyamide

54

thin film, strongly influences membrane fouling propensity25,

55

importance of support layer structure in determining polyamide fouling propensity, through its

56

effect on polyamide structure, has not been evaluated. Only Ramon et al.26, through modeling

57

work, have considered the possibility that the underlying support layer might influence active

58

layer fouling in a significant way. Given the intensity of the current research focus on re-

59

designing support layer structures to maximize transport in FO and PRO, it is of paramount

60

importance to investigate whether these support layer changes might also be contributing to

61

different fouling propensities, which will be highly important for real-world implementation.

62

Knowing the extent to which the support layer surface structure might affect polyamide fouling

63

would also inform efforts to compare polyamide fouling of TFC membranes across RO, FO, and

64

PRO platforms, as these technologies have differently structured support layers.24

26

. However, the relative

65

In this work, we seek to investigate the influence of support layer surface structure on the

66

fouling propensity of TFC FO membranes. Two types of TFC membranes were fabricated with

67

two different support layers, formed using different solvents for the polymer solution used in the

68

phase separation step. Fouling experiments with a model organic foulant were carried out to

69

compare the fouling propensity of the two TFC FO membranes. Our results indicate that support

70

layer structure significantly impacts the polyamide active layer structure and the organic fouling

71

behavior of the TFC FO membranes. The underlying mechanisms for the difference in the

72

fouling behaviors are elucidated and implications for the design of TFC membrane with anti-

73

fouling properties are discussed.

74 75

MATERIALS AND METHODS

76

Materials and Chemicals. Polysulfone beads (Mn: 22,000 Da), 1-methyl-2-pyrrolidinone

77

(NMP, anhydrous, 99.5%), N-N-dimethylformamide (DMF, anhydrous, 99.8%), 1,3-

78

phenylenediamine (MPD, >99%), 1,3,5-benzenetricarbonyl trichloride (TMC, 98%), sodium

79

hypochlorite (NaOCl, reagent grade), and sodium bisulfite (NaHSO3, >99%) were used as

80

received (Sigma-Aldrich, St. Louis, MO). The polysulfone support was cast on a commercial

81

poly(ethylene terephthalate) nonwoven fabric (PET, grade 3249, Ahlstrom, Helsinki, Finland)

82

with a thickness of ~40 µm. During interfacial polymerization, TMC was dissolved in Isopar-G,

83

a proprietary non-polar organic solvent (Univar, Redmond, WA). -5ACS Paragon Plus Environment


84

Sodium chloride (NaCl, crystals, ACS reagent) and magnesium chloride (MgCl2·6H2O,

85

crystals, ACS reagent) from J.T. Baker (Phillipsburg, NJ) were used for the membrane

86

performance tests and fouling experiments. Unless otherwise specified, all chemicals were

87

dissolved in deionized (DI) water obtained from a Milli-Q ultrapure water purification system

88

(Millipore, Billerica, MA). Sodium alginate (12 to 80 kDa, Sigma-Aldrich, St. Louis, MO), a

89

polysaccharide, was chosen as a model organic foulant. Toluidine blue O (TBO, technical grade,

90

Sigma-Aldrich) was used to characterize carboxyl group density on the TFC membrane surface.

91

Thin-Film Composite Forward Osmosis Membrane Fabrication. Two types

92

of TFC membranes were fabricated. The fabrication protocols were identical except for the

93

choice of solvent (NMP vs. DMF) in the phase separation step, which produces the polysulfone

94

support layer. Fabrication details are presented below.

95

The polysulfone support layer was prepared by non-solvent induced phase separation in

96

accordance with the method described in previous publications.15, 24 First, polysulfone beads (12

97

wt%) were dissolved in a solvent (NMP or DMF), stirred for 8 h, and deaerated in a desiccator

98

for at least 15 h prior to casting. Low-density PET fabric was attached to a clean glass plate (16

99

cm × 24 cm) using waterproof adhesive tape. NMP was applied to pre-wet the PET fabric, and

100

the excess NMP was removed using Kimwipes (Kimberly-Clark, Roswell, GA). A casting knife

101

(Gardco, Pompano Beach, FL), set at a gate height of 10 mils (~250 µm), was used to spread the

102

polysulfone solution over the wetted PET fabric. The whole composite was immediately

103

immersed in a precipitation bath containing 3 wt% solvent in DI water at room temperature

104

(23 °C) to initiate phase separation. The polysulfone support remained in the precipitation bath

105

for 10 min before being transferred to a DI water bath for storage until polyamide fabrication.

106

The polyamide active layer was formed through interfacial polymerization on the

107

polysulfone support. Supports were first immersed in MPD solution (3.4 wt% in DI water) for 2

108

min. After removing the excess MPD from the membrane surface using an air knife, the

109

membrane was immersed in the TMC solution (0.15 wt% in Isopar-G) for 1 min to form the

110

polyamide selective layer on the polysulfone support, followed by vertical draining of excess

111

TMC solution for 2 min. Then, the membrane was cured in a DI water bath at 95 °C for 2 min,

112

immersed in NaOCl solution (0.2 g/L, 2 min) followed by soaking in NaHSO3 solution (1 g/L,

113

30 seconds), and cured again in DI water at 95 °C for 2 min. The fabricated TFC membranes -6ACS Paragon Plus Environment

Page 6 of 26

Page 7 of 26

114


were rinsed thoroughly and stored in DI water at 4°C.

115

Characterization of Membrane Transport Properties. The water permeability,

116

A, salt permeability, B, and salt rejection, R, of pristine TFC membranes were determined in a

117

bench-scale crossflow RO unit. After evaluation under RO conditions, each membrane coupon

118

was characterized in a bench-scale crossflow FO unit for the support layer structural parameter, S.

119

The water permeability of the polysulfone support was also characterized using a dead-end

120

filtration system. The details of these characterization methods can be found in the Supporting

121

Information (SI).

122

FO Fouling Experiments. A bench-scale crossflow FO unit, with channel dimensions

123

of 77 mm × 26 mm × 3 mm, was used in the FO fouling experiments. The experiments were

124

conducted without spacers and under co-current crossflow velocities of 8.5 cm/s in both the draw

125

solution and feed solution channels. Solution temperatures were maintained at 25 ± 0.5 °C. All

126

fouling experiments were conducted as follows. First, a clean membrane sample was loaded into

127

the FO cell with the active layer facing the feed solution (FO mode). The coupon was screened

128

for the presence of defects through measurement of water flux and solute flux at a known draw

129

solution concentration with DI water as feed solution. These fluxes were compared with

130

expected values, and the experiment proceeded if membrane performance was as expected,

131

indicating a lack of defects. The system was stabilized with DI water on both feed and draw sides.

132

Next, aliquots of inorganic stock solutions were added to the feed solution to obtain a synthetic

133

wastewater of pH 7.4 and calculated ionic strength of 14.9 mM (Visual MINTEQ 3.0).27 The

134

final concentrations of feed solutions were 0.45 mM KH2PO4, 9.20 mM NaCl, 0.61 mM MgSO4,

135

0.5 mM NaHCO3, 0.5 mM CaCl2, and 0.93 mM NH4Cl. The draw solution was supplemented

136

with concentrated draw solution stock to obtain a bulk draw solution concentration (2–4 M NaCl

137

or 2–3.5 M MgCl2) that produced an initial water flux of ~21 L m-2 h-1.

138

After the water flux became stable, sterile sodium alginate stock solution (10 g/L) was

139

introduced to obtain a feed solution with 250 mg/L of alginate for the start of the accelerated

140

fouling experiment. The experiments were conducted for 15–20 h, until a cumulative permeate

141

volume of 500 mL was collected. Water flux throughout the experiment was monitored by a

142

computer, which recorded the mass of the draw solution at one minute intervals. Water flux data

143

were corrected to eliminate the contribution of draw solution dilution to flux decline;15 data -7ACS Paragon Plus Environment


144

below reflect the flux decrease due solely to membrane fouling. Physical cleaning was also

145

performed at the end of fouling experiments to evaluate fouling reversibility. The details of the

146

physical cleaning methodology can be found in the SI.

147

AFM Adhesion Force Measurements. Adhesion force measurements were

148

performed in a multimode AFM (Bruker, Santa Barbara, CA) operating in contact mode using

149

SiN cantilevers (Bruker NP-O10, Santa Barbara, CA; spring constant 0.06 N/m). Cantilevers

150

were inspected under an optical microscope for breaks and cracks before use, and cleaned in a

151

UV/Ozone cleaner for 20 minutes (BioForce Nanosciences, Ames, IA). A 4.0-µm carboxyl-

152

modified latex particle (CML, carboxyl content 19.5 µeq/g, Life Technologies, Eugene, OR) was

153

glued to the tip of the cantilever using UV-curable adhesive (Norland Optical Adhesive 68,

154

Norland Products, Cranbury, NJ), and subsequently cured for 20 minutes in the UV/Ozone

155

cleaner. In order to investigate foulant−membrane interactions, the functionalized cantilever was

156

immersed in a 4.0 g/L alginate solution for at least 16 hours prior to the measurements. During

157

this period of time, alginate molecules adsorbed on the surface of the colloidal particle. Force

158

measurements were collected at a rate of 0.5 Hz and a resolution of 512 samples per line. The

159

deflection sensitivity, in nm/V, was determined from the slope of the compliance region. All

160

force curves were collected in relative trigger mode at a deflection of 100 nm. To avoid large

161

variations between cantilever spring constants, all measurements were performed with the same

162

cantilever, immersing the tip in fresh 4.0 g/L alginate for at least 16 hours between

163

measurements. Adhesion forces were determined by converting curves of cantilever deflection vs.

164

piezoelectric stage retraction to force vs. particle−membrane separation.28 All measurements

165

were performed in a liquid cell filled with ~2 mL of synthetic wastewater with the same solution

166

composition as indicated above. Adequate sampling was ensured by collecting measurements in

167

5 randomly chosen locations on each surface. The total number of measurements for each

168

membrane type was ~200.

169

Characterization of Membrane Structural and Material Properties. The

170

surface hydrophilicity of the TFC membranes was evaluated by measuring the contact angle for

171

DI water using the sessile drop method. A 1-µL droplet was placed on the air-dried membrane

172

surface, and contact angles were measured after 10 s (VCA, Optima XE, AST Products, Billerica,

173

MA). For each type of membrane to be characterized, we performed twelve droplet -8ACS Paragon Plus Environment

Page 8 of 26

Page 9 of 26

174


measurements on each of three independently cast membrane coupons.

175

The membrane surface roughness was characterized by a multimode atomic force

176

microscope (AFM, Bruker, Santa Barbara, CA). Imaging of the air-dried samples was performed

177

in tapping mode with silicon probes coated with 30-nm-thick back side aluminum (Tap300A,

178

Bruker Nano, Inc., Camarillo, CA). The probe had a spring constant of 40 N/m, resonance

179

frequency of 300 kHz, tip radius of 8 ± 4 nm, and cantilever length of 125 ± 10 µm.

180

Membrane surface morphology was observed through a field emission scanning electron

181

microscope (FE-SEM, SU-70, Hitachi, Japan). Membrane samples were air-dried overnight prior

182

to the measurements and sputter-coated (DESK V, Denton Vacuum, LLC, Moorestown, NJ) with

183

a 10-nm-thick layer of chromium. Micrographs of support layer surfaces were quantitatively

184

analyzed for porosity and selected pore metrics using ImageJ 1.46r software (National Institutes

185

of Health, Bethesda, Maryland, USA).

186

The surface carboxyl group density of TFC membrane polyamide surfaces was quantified

187

via the TBO technique developed by Tiraferri et al.29 Briefly, the support surface of the TFC

188

membrane was sealed with waterproof tape to leave only the active layer exposed. Then, the

189

active layer of the TFC membrane was contacted with a freshly-prepared solution of TBO (2

190

mM) and NaOH (pH 11) to bind positively-charged TBO molecules to deprotonated carboxylic

191

acid groups on the polyamide surface. After thorough rinsing with a dye-free NaOH solution (pH

192

11) to remove unbound dye molecules, the membrane coupon was immersed into a NaCl

193

solution at pH 2 to elute the bonded TBO dye from the polyamide surface. The absorbance of the

194

eluent was measured at a 630 nm wavelength to determine the surface carboxyl group density.

195 196

RESULTS AND DISCUSSION

197

Membrane Intrinsic Transport Properties. The hand-cast TFC FO membranes were

198

first characterized for intrinsic transport properties (Table 1). The water permeability and salt

199

permeability are 1.73 ± 0.33 L m-2 h-1 bar-1 and 0.50 ± 0.11 L m-2 h-1, respectively, for the TFC

200

membrane cast on the polysulfone support for which NMP was the solvent used during phase

201

separation (NMP-TFC membrane). The TFC membrane cast on polysulfone support formed

202

using DMF as the solvent (DMF-TFC membrane) has higher values of water permeability and

203

salt permeability (3.14 ± 0.02 L m-2 h-1 bar-1 and 0.60 ± 0.31 L m-2 h-1, respectively), indicating a -9ACS Paragon Plus Environment


204

more permeable active layer for the DMF-TFC membrane. The structural parameter, S, which is

205

an intrinsic property of the polysulfone support, was comparable for the NMP-TFC membrane

206

and the DMF-TFC membrane (817 ± 146 µm vs. 953 ± 2 µm), indicating the severity of internal

207

concentration polarization is similar.

Page 10 of 26

TABLE 1

208 209

Fouling Behavior of FO-TFC Membranes. Alginate, a model polysaccharide,

210

was chosen to evaluate the fouling propensity of the membranes in order to mimic extracellular

211

polymeric substances commonly present in wastewater effluents. Representative fouling curves

212

for the NMP-TFC and DMF-TFC membranes are presented in Figure 1A, and the summarized

213

flux decline data with duplicate experiments can be found in the SI (Figure S1 and Table S1).

214

FIGURE 1

215

FIGURE S1

216

TABLE S1

217

Generally, the loss of water flux due to fouling was relatively small for both membranes

218

(Figure 1A), considering the extremely high foulant concentrations (250 mg/L) used. This

219

observation is consistent with previous FO fouling studies, which demonstrated low fouling

220

propensity of FO membranes.30, 31 The NMP-TFC membranes exhibited significantly smaller

221

water flux decline (7.47 ± 0.15%) than the DMF-TFC membranes (12.7 ± 2.62%). Physical

222

cleaning also led to greater recovery of water flux for the NMP-TFC membrane (Figure S1 and

223

Table S1). Both of these results indicate a lower fouling propensity for the NMP-TFC membrane

224

than for the DMF-TFC membrane.

225

Foulant–Membrane Interaction Forces. The observed fouling propensities of the

226

membranes were in agreement with the corresponding foulant–membrane interaction forces.15, 32

227

The distribution of probe–membrane adhesion forces for the NMP-TFC membrane (Figure 2A)

228

was shifted toward the right (i.e., lower adhesion forces) relative to that of the DMF-TFC

229

membrane (Figure 2B). This resulted in a relative lower mean adhesion force of −0.38 ± 0.31

230

mN m-1 for the NMP-TFC membrane compared to that of −0.60 ± 0.34 mN m-1 for the DMF-

231

TFC membrane. In addition, a significantly higher fraction of the measurements performed on

232

the NMP-TFC membrane (22.6% vs. 3.3% for the DMF-TFC membrane) exhibited no detectable - 10 ACS Paragon Plus Environment

Page 11 of 26


233

adhesion force (counted in the “NO” column), implying the absence of adhesion probe–

234

membrane interactions. As shown in Figure 2C, D, the DMF-TFC membrane had more “sticky”

235

sites, indicated by more frequent measurement (50%) of rupture distances larger than 100 nm

236

(compared to 20% for the NMP-TFC membrane). Accordingly, the average rupture distance for

237

the DMF-TFC membrane (90.8 nm) was nearly twice the value observed for the NMP-TFC

238

membrane (54.9 nm). These AFM characterizations all indicate that the NMP-TFC membrane

239

exhibits a lower fouling propensity than the DMF-TFC membrane, in agreement with our

240

observed fouling trend.

241

FIGURE 2

242

Determining the Causes for Different Fouling Propensities. In the above

243

sections, we observed a significant difference in the fouling propensity of the two TFC

244

membranes. Possible causes for the observed difference are systematically analyzed in this

245

section. Our analysis is organized according to general mechanisms through which the properties

246

of our membranes and experiments might have influenced the overall fouling result. For each of

247

these mechanisms, we considered how specific characteristics of the membranes would be

248

involved and selected methods to capture those possibilities. The particular circumstances of our

249

fouling experiments were also used to rule out certain factors. In this way, we were able to

250

identify a set of techniques that would achieve a comprehensive evaluation of fouling propensity

251

in an efficient manner. An example of our selection process appears in Supporting Information

252

(S6).

253

(a) Is it reverse solute diffusion? In FO, reverse diffusion of draw solute from the

254

draw solution to the feed solution causes an accumulation of salts at the active layer–feed

255

solution interface. This phenomenon occurs in addition to the concentration of feed solutes due

256

to external concentration polarization. The increase in salt concentration reduces the effective

257

osmotic pressure driving force, resulting in water flux decline. Foulants that accumulate on the

258

active layer surface exacerbate these effects through cake-enhanced osmotic pressure (CEOP).30

259

If the NMP-TFC membrane permitted less reverse diffusion of draw solutes than the DMF-TFC

260

membrane, this could explain why its decline in water flux was less than that of the DMF-TFC

261

membrane.

262

Because the two TFC membranes have different transport properties, different initial draw - 11 ACS Paragon Plus Environment


263

solution concentrations were required to reach the same initial water flux (~21 L m-2 h-1). To

264

analyze the effect of reverse solute diffusion on fouling, we determined the reverse solute flux

265

selectivity33:

266

Jw A = nRgT Js B

Page 12 of 26

(1)

267

where Jw is water flux, Js is the reverse solute flux, A and B are water and salt permeability

268

coefficients, respectively, n is the number of dissolved species created by the draw solute (2 for

269

NaCl), Rg is the ideal gas constant, and T is the absolute temperature. The reverse solute flux

270

selectivity is independent of draw solution concentration, and can be obtained simply from the

271

ratio A/B. The calculated A/B values (Table 1) are 3.47 ± 0.43 bar-1 for the NMP-TFC and 6.16 ±

272

2.18 bar-1 for the DMF-TFC membrane. Given the same initial water flux (~21 L m-2 h-1) for

273

both types of membranes in the fouling experiments, a lower A/B value implies greater reverse

274

solute diffusion, Js, for the NMP-TFC membrane, which would cause greater water flux decline.

275

However, the decline in water flux was less severe for the NMP-TFC membrane in our fouling

276

experiments, which rules out this mechanism as the cause for the different fouling propensities.

277

(b) Is it disruption of calcium–alginate complexes by sodium? The calcium ions

278

in our simulated wastewater feed solution facilitate bridging and complexation between carboxyl

279

groups on the polyamide surface and alginate molecules.34-36 The gel network that consequently

280

forms on the active layer could impede water flux. At the same time, sodium ions could disrupt

281

calcium bridges between foulant molecules through cation exchange.37-39 Given the higher

282

reverse solute diffusion for the NMP-TFC membrane, and our use of NaCl as the draw solute,

283

the lower fouling propensity of this membrane could be attributed to greater disruption of

284

calcium bridging and complexation by the sodium ions. To further evaluate this possibility, we

285

repeated our fouling experiments with MgCl2 as the draw solute, which does not have the cation

286

exchange capability of NaCl. The fouling behaviors of the TFC membranes evaluated with

287

MgCl2 draw solution are presented in the Figure 1B. Even without the influx of sodium ions

288

from the draw solution to the foulant layer, the NMP-TFC membrane still exhibited a lower flux

289

decline (78.8 ± 5.7%) than the DMF-TFC membrane (66.5 ± 4.5%). Therefore, the lower fouling

290

propensity of the NMP-TFC membrane cannot be attributed to differences in the degree of cation

291

exchange occurring between sodium and calcium ions. - 12 ACS Paragon Plus Environment

Page 13 of 26


292

(c) Is it membrane surface hydrophilicity or chemistry? Surface energy and

293

surface chemistry strongly influence the interaction of organic foulants with the membrane

294

surface. The layer of strongly-bound water molecules that forms on a hydrophilic surface

295

through hydrogen bonding provides an enthalpic penalty for foulant adhesion that lowers fouling

296

propensity.40,

297

through the availability of sites for calcium–alginate complexation with the surface.42,

298

determine whether differences in surface hydrophilicity or chemistry might explain the observed

299

difference in fouling propensity, we measured water contact angle and the density of native

300

carboxyl groups on the polyamide surfaces (Table 1). Although identical interfacial

301

polymerization procedures were performed on both membrane types, the DMF-TFC membrane

302

had lower value (P-value < 0.001) in water contact angle (55.7 ± 6.6° vs. 66.4 ± 2.3°,

303

respectively) and comparable carboxyl group density (18.96 ± 3.66 nm-2 vs. 20.24 ± 6.17 nm-2)

304

compared to the NMP-TFC membrane. This would suggest less propensity for fouling due to

305

greater hydrophilicity and fewer sites for carboxyl-group-facilitated foulant attachment. However,

306

the observed overall fouling propensity is higher for the DMF-TFC membrane, thereby ruling

307

out both surface hydrophilicity and surface chemistry explanations for the observed fouling

308

behavior.

41

The density of carboxyl groups on a surface also affects fouling propensity 43

To

309

(d) Is it membrane surface structure? During interfacial polymerization, MPD

310

diffuses into the organic phase, where it reacts with TMC to form the polyamide layer with its

311

characteristic ridge-and-valley structure.4, 44 The inherent roughness of this polyamide structure

312

can enhance membrane fouling through (i) greater surface area for foulant attachment and (ii)

313

accumulation of foulants in valley features that can hinder their removal during physical

314

cleaning18. We used AFM to quantify membrane surface roughness (Figure 3) and SEM to

315

inspect surface morphology (Figure 4).

316

FIGURE 3

317

FIGURE 4

318

The NMP-TFC membrane had a root-mean-square roughness (Rrms) of 94.41 ± 3.59 nm, an

319

average roughness (Ra) of 74.64 ± 3.22 nm, and a maximum roughness (Rmax) of 696.42 ± 34.84

320

nm. These values are comparable to those of membranes fabricated under similar conditions.31, 45

321

The surface roughness of the DMF-TFC membrane was higher (P-value < 0.001) than that of the - 13 ACS Paragon Plus Environment


322

NMP-TFC membrane (Figure 3A), with Rrms, of 124.39 ± 5.68 nm, Ra of 98.19 ± 3.79 nm, and

323

Rmax of 969.64 ± 144.53 nm. This is the first comparison between the membranes for which the

324

observed fouling trend agrees with what we would predict from a specific surface characteristic.

Page 14 of 26

325

SEM micrographs of the NMP-TFC and DMF-TFC membrane polyamide surfaces show

326

the expected ridge-and-valley structure (Figure 4A, B, additional replicate in SI, Figure S3).

327

These micrographs were obtained with a 45° stage tilt in order to supplement the top-down

328

perspective provided by SEM. The surface of the NMP-TFC membrane contained many small,

329

nodular features, while the DMF-TFC membrane surface included some larger leaf-like

330

structures (these unique features are indicated by yellow arrows in Figure 4A, B). These leaf-like

331

structures overhang the membrane surface, potentially providing regions where foulants can

332

accumulate and be sheltered from the shear forces produced during physical cleaning.18 The

333

difficulty of quantifying the morphology of the polyamide layer prevents us from conclusively

334

tying this factor to our observed fouling propensity, as we did for roughness. However, we can

335

recognize the possibility that the larger overhanging features of the DMF-TFC polyamide layer

336

might make the build-up of foulants on the DMF-TFC membranes more rapid and less reversible,

337

as observed in our fouling experiments. If this mechanism is at work as we propose, its effects on

338

fouling are combined with those associated with roughness differences to determine the overall

339

fouling propensity.

340

Relating Polyamide Surface Properties to Support Layer Structure. Our

341

finding that subtle changes to the support layer affect polyamide structure in a way that

342

significantly alters fouling propensity warrants a closer look at the support layer itself. Below we

343

employ further characterization of the support layers to examine the current understanding of the

344

role of support layer characteristics on polyamide formation and foulant accumulation on the

345

membrane surface.

346

(a) Origin of polyamide layer structure. Figure 3B depicts surface roughness of the

347

polysulfone supports, which is much lower than the roughness of the polyamide surfaces (Figure

348

3A). Slightly lower values in Rrms, Ra, and Rmax of the DMF-polysulfone indicate that this support

349

is smoother than the NMP-polysulfone support. We note that the converse is observed for

350

polyamide roughness, i.e., the DMF-TFC polyamide surface is rougher than the NMP-TFC

351

polyamide surface. These results indicate that the high roughness features of the polyamide - 14 ACS Paragon Plus Environment

Page 15 of 26

352 353


active layers are neither a translation nor simple amplification of the support layer roughness. High magnification SEM micrographs of the polyamide and polysulfone surfaces were also

354

compared (Figure 4, middle and bottom rows). Previous studies23,

46

355

individual ridge (or nodule) in the polyamide layer arises from one underlying pore in the

356

support layer; we referred to it herein as the “one pore – one ridge” model. This proposed

357

mechanism for polyamide roughness formation was based on an observed trend between pore

358

size and polyamide feature size. While we note the same trend — that the NMP-TFC membrane

359

has both smaller polysulfone surface pores and smaller polyamide features — when micrographs

360

of the polyamide and polysulfone surfaces are compared side-by-side at equal magnification, it

361

becomes very clear that the one pore – one ridge proposed model is not consistent with our SEM

362

micrographs. The polyamide features (Figure 4C, D) are at least an order of magnitude larger

363

than the pores on the polysulfone support (Figure 4E, F), and the base of each polyamide ridge

364

covers an area occupied by many polysulfone pores ( >10 ).

speculated that each

365

(b) Importance for active layer fouling behavior. A recent modeling effort26

366

proposed that a more permeable support might contribute to a more uniform spatial distribution

367

of water permeation across the TFC membrane surface, resulting in a surface with lower fouling

368

propensity. Conversely, a less permeable support would constrain water transport to occur over a

369

reduced effective area, through scattered points with relatively high local water fluxes, referred

370

to as hot spots. Such a membrane would have a higher fouling propensity.

371

In our experiments, we found that the support layer of the lower fouling propensity

372

membrane (NMP-TFC) did have a higher water permeability (Table 1, 1107 ± 193 vs. 290 ± 40

373

L m-2 h-1 bar-1 for the DMF-TFC support), in basic agreement with the hot-spots theory. However,

374

further analysis (SI, section S4) showed that overall water permeability of the support layers was

375

primarily determined by the bulk structure rather than by the dense skin layer. This indicated that

376

the overall support permeability might not accurately reflect water transport through the dense

377

skin layer and adjacent polyamide layer. We therefore turned to more direct characterizations of

378

the support layer surface to inform the comparison of our fouling observations with the proposed

379

hot-spots mechanism.

380

The surfaces of the support layers had comparable pore density (Table 1), but the DMF-

381

TFC membrane support surface had a larger pore size (as shown in the Figure 4E, F and Figure - 15 ACS Paragon Plus Environment


Page 16 of 26

382

S2) and higher surface porosity (1.99 ± 0.60% vs. 1.01 ± 0.60%, Table 1). Therefore, the DMF-

383

TFC membrane would have a greater amount of area available for water permeation at the

384

polysulfone–polyamide interface. Fouling propensity would consequently be expected to be

385

lower for the DMF-TFC membrane, while we observed the opposite. This finding does not

386

negate the plausibility of the ‘hot-spots' theory. Rather, it shows that, if the theory is correct, the

387

effect of polysulfone surface pore size and porosity on polyamide layer fouling in our

388

membranes was overcome by more influential differences in the structure of the polyamide

389

layers.

390

Implications for TFC Membrane Fabrication. Much research effort over the last

391

few years has focused on obtaining a support layer with a structure that minimizes internal

392

concentration polarization in forward osmosis. Our work demonstrates that changes made to the

393

support layer may have unintended and unexamined effects on fouling propensity for TFC

394

membranes. Our comparison of just two slightly different membranes does not support

395

development of a comprehensive model to quantitatively relate support layer structure or

396

fabrication choices to the fouling propensity ultimately observed on an overlying active layer.

397

However, it does highlight the importance of evaluating supposed improvements to support

398

layers for their impact on fouling propensity or on factors known to influence fouling propensity,

399

like those we have presented here.

400 401

ACKNOWLEDGMENTS

402

Financial support from the Department of Defense through the Strategic Environmental

403

Research and Development Program (SERDP, Project No. 12 ER01-054/ER-2217) and the

404

National Science and Technology Pillar Program of China (Project No. 2012BAC05B02) is

405

gratefully acknowledged. We also acknowledge the use of SEM and AFM facilities supported by

406

the Yale Institute for Nanoscience and Quantum Engineering (YINQE) under NSF MRSEC

407

DMR 1119826. This publication was developed under a graduate fellowship awarded by the

408

China Scholarship Council (CSC) to Xinglin Lu and STAR Fellowship (Agreement No. FP-

409

91733801-0) awarded by the U.S. Environmental Protection Agency (EPA) to L.H. Arias

410

Chavez. It has not been reviewed or endorsed by EPA.

- 16 ACS Paragon Plus Environment

Page 17 of 26


411 412

SUPPORTING INFORMATION

413

Details on the evaluation of TFC membrane intrinsic properties (S1); evaluation of polysulfone

414

support properties (S2); physical cleaning methodology (S3); polysulfone support thickness

415

measurements (S4); analysis on support layer structure and resistance to water transport (S5);

416

Selection or Exclusion of Potential Techniques for Evaluating Fouling Propensity (S6); FO

417

fouling and cleaning experiment results (Figure S1 and Table S1); pore diameter distribution of

418

support surfaces (Figure S2); and replicate SEM micrographs of polyamide layers (Figure S3).

419

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

420 421



Page 18 of 26

422

REFERENCES

423 424 425

(1) Shannon, M. A.; Bohn, P. W.; Elimelech, M.; Georgiadis, J. G.; Marinas, B. J.; Mayes, A. M., Science and technology for water purification in the coming decades. Nature 2008, 452, (7185), 301-310.

426 427

(2) Elimelech, M.; Phillip, W. A., The Future of Seawater Desalination: Energy, Technology, and the Environment. Science 2011, 333, (6043), 712-717.

428 429 430

(3) Hoover, L. A.; Phillip, W. A.; Tiraferri, A.; Yip, N. Y.; Elimelech, M., Forward with osmosis: emerging applications for greater sustainability. Environmental Science & Technology 2011, 45, (23), 9824-30.

431 432

(4) Petersen, R. J., Composite Reverse-Osmosis and Nanofiltration Membranes. Journal of Membrane Science 1993, 83, (1), 81-150.

433 434 435

(5) Yip, N. Y.; Tiraferri, A.; Phillip, W. A.; Schiffman, J. D.; Elimelech, M., High Performance Thin-Film Composite Forward Osmosis Membrane. Environmental Science & Technology 2010, 44, (10), 3812-3818.

436 437 438

(6) Wei, J.; Qiu, C.; Tang, C. Y.; Wang, R.; Fane, A. G., Synthesis and characterization of flat-sheet thin film composite forward osmosis membranes. Journal of Membrane Science 2011, 372, (1–2), 292-302.

439 440 441

(7) Wang, R.; Shi, L.; Tang, C. Y.; Chou, S.; Qiu, C.; Fane, A. G., Characterization of novel forward osmosis hollow fiber membranes. Journal of Membrane Science 2010, 355, (1-2), 158167.

442 443

(8) Cath, T. Y.; Childress, A. E.; Elimelech, M., Forward osmosis: Principles, applications, and recent developments. Journal of Membrane Science 2006, 281, (1-2), 70-87.

444 445 446

(9) Kumar, M.; Adham, S. S.; Pearce, W. R., Investigation of Seawater Reverse Osmosis Fouling and Its Relationship To Pretreatment Type. Environmental Science & Technology 2006, 40, (6), 2037-2044.

447 448 449

(10) Tang, C. Y.; Kwon, Y.-N.; Leckie, J. O., Fouling of reverse osmosis and nanofiltration membranes by humic acid—Effects of solution composition and hydrodynamic conditions. Journal of Membrane Science 2007, 290, (1–2), 86-94.

450 451 452

(11) Xu, P.; Bellona, C.; Drewes, J. E., Fouling of nanofiltration and reverse osmosis membranes during municipal wastewater reclamation: Membrane autopsy results from pilotscale investigations. Journal of Membrane Science 2010, 353, (1–2), 111-121.

453 454

(12) Ang, W. S.; Elimelech, M., Protein (BSA) fouling of reverse osmosis membranes: Implications for wastewater reclamation. Journal of Membrane Science 2007, 296, (1-2), 83-92.

455 456 457

(13) Li, Q.; Xu, Z.; Pinnau, I., Fouling of reverse osmosis membranes by biopolymers in wastewater secondary effluent: Role of membrane surface properties and initial permeate flux. Journal of Membrane Science 2007, 290, (1–2), 173-181.

458 459

(14) Rana, D.; Matsuura, T., Surface Modifications for Antifouling Membranes. Chemical Reviews 2010, 110, (4), 2448-2471.

460

(15)

Lu, X.; Romero-Vargas Castrillón, S.; Shaffer, D. L.; Ma, J.; Elimelech, M., In Situ - 18 ACS Paragon Plus Environment

Page 19 of 26


461 462 463

Surface Chemical Modification of Thin-Film Composite Forward Osmosis Membranes for Enhanced Organic Fouling Resistance. Environmental Science & Technology 2013, 47, (21), 12219-12228.

464 465 466

(16) Yu, H.-Y.; Kang, Y.; Liu, Y.; Mi, B., Grafting polyzwitterions onto polyamide by click chemistry and nucleophilic substitution on nitrogen: A novel approach to enhance membrane fouling resistance. Journal of Membrane Science 2014, 449, (0), 50-57.

467 468 469

(17) Romero-Vargas Castrillón, S.; Lu, X.; Shaffer, D. L.; Elimelech, M., Amine enrichment and poly(ethylene glycol) (PEG) surface modification of thin-film composite forward osmosis membranes for organic fouling control. Journal of Membrane Science 2014, 450, 331-339.

470 471 472

(18) Vrijenhoek, E. M.; Hong, S.; Elimelech, M., Influence of membrane surface properties on initial rate of colloidal fouling of reverse osmosis and nanofiltration membranes. Journal of Membrane Science 2001, 188, (1), 115-128.

473 474 475

(19) Hobbs, C.; Hong, S. K.; Taylor, J., Effect of surface roughness on fouling of RO and NF membranes during filtration of a high organic surficial groundwater. Journal of Water Supply Research and Technology-Aqua 2006, 55, (7-8), 559-570.

476 477 478

(20) Gu, Y.; Wang, Y.-N.; Wei, J.; Tang, C. Y., Organic fouling of thin-film composite polyamide and cellulose triacetate forward osmosis membranes by oppositely charged macromolecules. Water Research 2013, 47, (5), 1867-1874.

479 480 481 482

(21) Singh, P. S.; Joshi, S. V.; Trivedi, J. J.; Devmurari, C. V.; Rao, A. P.; Ghosh, P. K., Probing the structural variations of thin film composite RO membranes obtained by coating polyamide over polysulfone membranes of different pore dimensions. Journal of Membrane Science 2006, 278, (1-2), 19-25.

483 484 485

(22) Il Kim, H.; Kim, S. S., Plasma treatment of polypropylene and polysulfone supports for thin film composite reverse osmosis membrane. Journal of Membrane Science 2006, 286, (1-2), 193-201.

486 487 488

(23) Ghosh, A. K.; Hoek, E. M. V., Impacts of support membrane structure and chemistry on polyamide-polysulfone interfacial composite membranes. Journal of Membrane Science 2009, 336, (1-2), 140-148.

489 490 491

(24) Tiraferri, A.; Yip, N. Y.; Phillip, W. A.; Schiffman, J. D.; Elimelech, M., Relating performance of thin-film composite forward osmosis membranes to support layer formation and structure. Journal of Membrane Science 2011, 367, (1-2), 340-352.

492 493

(25) Hoek, E. M. V.; Bhattacharjee, S.; Elimelech, M., Effect of membrane surface roughness on colloid-membrane DLVO interactions. Langmuir 2003, 19, (11), 4836-4847.

494 495 496

(26) Ramon, G. Z.; Hoek, E. M. V., Transport through composite membranes, part 2: Impacts of roughness on permeability and fouling. Journal of Membrane Science 2013, 425–426, 141148.

497

(27)

498 499

(28) Senden, T. J., Force microscopy and surface interactions. Current Opinion in Colloid & Interface Science 2001, 6, (2), 95-101.

500

(29)

Ben-Asher, J., Irrigation with saline water. GeoJournal 1987, 15, (3), 267-272.

Tiraferri, A.; Elimelech, M., Direct quantification of negatively charged functional - 19 ACS Paragon Plus Environment


Page 20 of 26

501

groups on membrane surfaces. Journal of Membrane Science 2012, 389, 499-508.

502 503

(30) Lee, S.; Boo, C.; Elimelech, M.; Hong, S., Comparison of fouling behavior in forward osmosis (FO) and reverse osmosis (RO). Journal of Membrane Science 2010, 365, (1-2), 34-39.

504 505 506

(31) Tiraferri, A.; Kang, Y.; Giannelis, E. P.; Elimelech, M., Highly Hydrophilic Thin-Film Composite Forward Osmosis Membranes Functionalized with Surface-Tailored Nanoparticles. Acs Applied Materials & Interfaces 2012, 4, (9), 5044-5053.

507 508 509

(32) Li, Q.; Elimelech, M., Organic Fouling and Chemical Cleaning of Nanofiltration Membranes: Measurements and Mechanisms. Environmental Science & Technology 2004, 38, (17), 4683-4693.

510 511 512

(33) Phillip, W. A.; Yong, J. S.; Elimelech, M., Reverse Draw Solute Permeation in Forward Osmosis: Modeling and Experiments. Environmental Science & Technology 2010, 44, (13), 5170-5176.

513 514

(34) Lee, S.; Elimelech, M., Relating organic fouling of reverse osmosis membranes to intermolecular adhesion forces. Environmental Science & Technology 2006, 40, (3), 980-987.

515 516 517

(35) Liu, Y.; Mi, B., Combined fouling of forward osmosis membranes: Synergistic foulant interaction and direct observation of fouling layer formation. Journal of Membrane Science 2012, 407–408, 136-144.

518 519 520

(36) Xiang, Y.; Liu, Y.; Mi, B.; Leng, Y., Molecular Dynamics Simulations of Polyamide Membrane, Calcium Alginate Gel, and Their Interactions in Aqueous Solution. Langmuir 2014, 30, (30), 9098-9106.

521 522

(37) Lee, S.; Elimelech, M., Salt cleaning of organic-fouled reverse osmosis membranes. Water Research 2007, 41, (5), 1134-1142.

523 524

(38) Skjåk-Bræk, G.; Grasdalen, H.; Smidsrød, O., Inhomogeneous polysaccharide ionic gels. Carbohydrate Polymers 1989, 10, (1), 31-54.

525 526

(39) Matsumoto, T.; Mashiko, K., Viscoelastic properties of alginate aqueous solutions in the presence of salts. Biopolymers 1990, 29, (14), 1707-1713.

527 528

(40) van Oss, C. J.; Chaudhury, M. K.; Good, R. J., Monopolar Surfaces. Advances in Colloid and Interface Science 1987, 28, (1), 35-64.

529 530 531

(41) van Oss, C. J.; Wu, W.; Docoslis, A.; Giese, R. F., The interfacial tensions with water and the Lewis acid-base surface tension parameters of polar organic liquids derived from their aqueous solubilities. Colloids and Surfaces B-Biointerfaces 2001, 20, (1), 87-91.

532 533 534

(42) Jin, X.; Huang, X. F.; Hoek, E. M. V., Role of Specific Ion Interactions in Seawater RO Membrane Fouling by Alginic Acid. Environmental Science & Technology 2009, 43, (10), 35803587.

535 536 537

(43) Wu, J.; Contreras, A. E.; Li, Q., Studying the impact of RO membrane surface functional groups on alginate fouling in seawater desalination. Journal of Membrane Science 2014, 458, 120-127.

538 539

(44) Freger, V., Kinetics of film formation by interfacial polycondensation. Langmuir 2005, 21, (5), 1884-1894.

540

(45)

Tang, C. Y. Y.; Kwon, Y. N.; Leckie, J. O., Effect of membrane chemistry and coating - 20 ACS Paragon Plus Environment

Page 21 of 26


541 542 543

layer on physiochemical properties of thin film composite polyamide RO and NF membranes II. Membrane physiochemical properties and their dependence on polyamide and coating layers. Desalination 2009, 242, (1-3), 168-182.

544 545 546 547

(46) Klaysom, C.; Hermans, S.; Gahlaut, A.; Van Craenenbroeck, S.; Vankelecom, I. F. J., Polyamide/Polyacrylonitrile (PA/PAN) thin film composite osmosis membranes: Film optimization, characterization and performance evaluation. Journal of Membrane Science 2013, 445, 25-33.

548 549



Page 22 of 26

550

Table 1. Intrinsic properties, salt rejection, and surface characteristics of the fabricated TFC FO

551

membranes.

NMP

DMF

1.73 ± 0.33 0.50 ± 0.11 3.47 ± 0.43 817 ± 146 95.94 ± 0.60 98.07 ± 1.56

3.14 ± 0.02 0.60 ± 0.31 6.16 ± 2.68 953 ± 2 98.18 ± 0.53 98.84 ± 0.89

TFC Membrane A a (L m-2 h-1 bar-1) B b (L m-2 h-1) A/B (bar-1) S c (µm) R (NaCl) d (%) R (MgCl2) e (%)

Active Layer Contact Angle f (°) Carboxyl group density g (nm-2)

66.4 ± 2.3 20.24 ± 6.17

55.7 ± 6.6 18.96 ± 3.66

Support Layer Water permeability h (L m-2 h-1 bar-1) Surface pore area i (nm2) Nominal surface pore diameter i (nm) Surface porosity i (%) Surface pore number density i (103 µm-2) t j (µm) ε/τ k (-)

1107 ± 193 37.22 ± 53.39 6.0 1.01 ± 0.40 26 ± 8 130.0 ± 7.4 0.159 ± 0.009

290 ± 40 70.55 ± 108.71 8.0 1.99 ± 0.60 28 ± 6 90.9 ± 4.5 0.095 ± 0.005

Values are presented as mean ± one standard deviation. a Water permeability, determined by RO experiment with DI water at 25 °C and 27.6 bar with three coupons cut from three independently cast membranes. b Salt permeability, measured by RO experiment with NaCl (50 mM) at 25 °C and 27.6 bar. c Structural parameter, determined in the FO experiment with 1 M NaCl as draw solution and DI water as feed solution. d Salt rejection, measured by RO experiment with 50 mM NaCl at 25 °C and 27.6 bar. e Salt rejection, measured by RO experiment with 50 mM MgCl2 at 25 °C and 27.6 bar. f Contact angle measurements performed with DI water at room temperature (23 °C). g Measured by the TBO method. h Measured by ultrafiltration with DI water at 25 °C and 0.35 bar. i Determined through SEM images analysis by ImageJ software. j Micrometermeasured thickness. k Porosity/tortuosity, obtained by dividing S by t.


Page 23 of 26


Normarlized Water Flux Jw/Jw,0(%)

NaCl Draw Solution NMP DMF

100 95 90 85

A 0

100

200

300

400

500

Cumulative Permeate Volume (mL) MgCl2 Draw Solution Normarlized Water Flux Jw/Jw,0(%)

100

NMP DMF

95 90 85 80 75 70

B

65 0

100

200

300

400

500

Cumulative Permeate Volume (mL)

552

Figure 1. FO alginate fouling results with (A) NaCl and (B) MgCl2 draw solutions. Fouling

553

conditions were as follows: feed solution chemistry simulating domestic wastewater (pH 7.4) as

554

described in Materials and Methods, supplemented with 250 mg/L alginate as model organic

555

foulant; draw solutions of (A) 2–4 M NaCl and (B) 2–3.5 M MgCl2, resulting in an initial

556

permeate water flux of ~21 L m-2 h-1. The system temperature was maintained at 25 °C, and the

557

crossflow velocity of the feed and draw streams was set to 8.5 cm/s. To reduce experimental

558

noise, data were smoothed using a 5-point window moving average. Note that y-axis scales differ.



A

-1 40 Mean Force = -0.38 ± 0.31 mN m

B

Frequency Count

Frequency Count

NMP

30 20 10 0

-2.5 -2.0 -1.5 -1.0 -0.5

0.0

NMP 25

Mean Rupture Distance = 54.9 ± 54.9 nm

20 15 10

0

NO

D

DMF 40 Mean Force = -0.60 ± 0.34 mN m

-1

Frequency Count

Frequency Count

30

5

50

30 20 10 0

559

C

50

Page 24 of 26

0

50

100

150

200

250

300

250

300

30

DMF 25

Mean Rupture Distance = 90.8 ± 54.5 nm

20 15 10 5

-2.5 -2.0 -1.5 -1.0 -0.5

0.0

0

NO

0

50

100

150

200

Rupture Distance (nm)

Adhesion Force (mN/m)

560 561

Figure 2. Distribution of foulant–membrane adhesion forces and rupture distance for active

562

layers formed on polysulfone support using different solvents: NMP (upper row) and DMF

563

(lower row). Measurements were performed by contact mode AFM at room temperature in a

564

liquid cell filled with synthetic wastewater (pH 7.4). At least 125 retraction force measurements

565

distributed over five randomly chosen locations were performed for each sample. The columns

566

labeled ‘NO’ indicate the fraction of the population of force measurements for which no

567

adhesion was observed.

568


Page 25 of 26


140 130 120 110 100 90 80 70 10 0

A

NMP-TFC DMF-TFC

Rrms

Ra

30

Roughness (nm)

Roughness (nm)

569

25

NMP-PSf DMF-PSf

20 15 10 5 0

Rmax/10

B

Rrms

Ra

Rmax/10

570 571 572

Figure 3. Roughness parameters measured by AFM tapping mode analysis. (A) Polyamide

573

surface and (B) polysulfone surface. Rrms is the root mean square of roughness, Ra is the average

574

roughness, and Rmax/10 is the maximum roughness divided by a factor of 10. Roughness values

575

shown averaged together from a total of 6 random spots on three separately cast membrane

576

samples. Error bars indicate one standard deviation. Note that in y-axis scales differ.

577



578

579 580 581

Figure 4. SEM micrographs displaying the structure of the TFC FO membranes at the (top row)

582

surface of the polyamide layer imaged at 45 degree angle of incidence and at lower

583

magnification, with yellow arrows indicating the nodular (NMP-TFC) and leaf-like (DMF-TFC)

584

structures of the polyamide surfaces; (middle row) surface of the polyamide layers at equal

585

magnification; and (bottom row) the top surface of the bare polysulfone support layers (yellow

586

arrows indicate pores). Membranes formed using NMP are shown on the left and those formed

587

using DMF are shown on the right. - 26 ACS Paragon Plus Environment

Page 26 of 26

Influence of Active Layer and Support Layer Surface Structures on

Recommend Documents