Relating Silica Scaling in Reverse Osmosis to ... - ACS Publications

Subscriber access provided by University of Newcastle, Australia

Article

Relating Silica Scaling in Reverse Osmosis to Membrane Surface Properties Tiezheng Tong, Song Zhao, Chanhee Boo, Sara M. Hashmi, and Menachem Elimelech Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b06411 • Publication Date (Web): 28 Mar 2017 Downloaded from http://pubs.acs.org on March 31, 2017

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 30

Environmental Science & Technology

1 2 3 4

Relating Silica Scaling in Reverse Osmosis to Membrane Surface Properties

5 6 7 8 9 10 11 12 13 14 15 16


Revised: February 20, 2017 ‡

Tiezheng Tong

17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36

§∥

†‡

, Song Zhao

∥,

‡

‡

Chanhee Boo , Sara M. Hashmi , and ‡

Menachem Elimelech

§*

†

School of Chemical Engineering and Technology, Tianjin Key Laboratory of Membrane Science and Desalination Technology, Tianjin University, Tianjin 300072, P. R. China

‡

Department of Chemical and Environmental Engineering, Yale University, New Haven, Connecticut 06520-8286 §

∥

Nanosystems Engineering Research Center for Nanotechnology-Enabled Water Treatment (NEWT), Yale University

These authors contribute equally

* Corresponding author: email: [email protected]; Tel. +1 (203) 432-2789

1 ACS Paragon Plus Environment


37

ABSTRACT

38

We investigated the relationship between membrane surface properties and silica scaling in

39

reverse osmosis (RO). The effects of membrane hydrophilicity, free energy for heterogeneous

40

nucleation, and surface charge on silica scaling were examined by comparing thin-film

41

composite polyamide membranes grafted with a variety of polymers. Results show that the rate

42

of silica scaling was independent of both membrane hydrophilicity and free energy for

43

heterogeneous nucleation. In contrast, membrane surface charge demonstrated a strong

44

correlation with the extent of silica scaling (R2 > 0.95, p < 0.001). Positively charged membranes

45

significantly facilitated silica scaling, whereas a more negative membrane surface charge led to

46

reduced scaling. This observation suggests that deposition of negatively charged silica species on

47

the membrane surface plays a critical role in silica scale formation. Our findings provide

48

fundamental insights into the mechanisms governing silica scaling in reverse osmosis and

49

highlight the potential of membrane surface modification as a strategy to reduce silica scaling.

50 51

TOC Art

52 53 54


Page 2 of 30

Page 3 of 30


55

INTRODUCTION

56

Silica is one of the most common inorganic scalants in membrane desalination systems such as

57

reverse osmosis (RO). Silica is ubiquitous in natural waters, with concentrations normally in the

58

range of 1-60 mg/L 1-3. Once feedwater silica concentration exceeds its solubility limit (typically

59

100-150 mg/L at near neutral pH

60

membrane surface. Silica scaling causes severe flux decline and requires chemical and

61

mechanical cleaning operations

62

desalination. In particular, silica scaling is a major barrier for efficient operation of inland

63

brackish water RO desalination where high water recovery is critical for brine management 4, 6, 8.

64

The chemistry and mechanisms of silica scaling are complex and not well understood 4.

65

Silica scaling involves the polymerization of monomeric silicic acids forming Si-O-Si bonds via

66

dehydration

67

ranging from dimers and trimers to polymers and particles 10. Although heterogeneous nucleation,

68

in which silicic acids deposit and polymerize on the membrane surface, has been proposed as the

69

major mechanism of silica scaling

70

important role 4, 7.

4, 9

2, 4, 5

2-4, 6, 7

), insoluble silica precipitates and forms scale on the

, which limits the efficiency and water recovery of RO

. Polymerization of the weakly acid silicic acids produces various silica species

2, 11

, bulk deposition of colloidal silica may also play an

71

Current strategies for silica scaling control in membrane desalination rely heavily on the use

72

of scale inhibitors (or anti-scalants). Scale inhibitors stabilize silica species in solution and

73

subsequently prevent the formation of scale on the membrane surface

74

scale inhibitors increase the operation cost of desalination and can result in organic and

75

biological fouling

76

pH > 10) has been used as an alternative strategy to scale inhibitor addition in RO desalination of

77

silica-rich feedwater at high water recoveries

78

precipitation of calcium and magnesium silicates 19. As a result, extensive pretreatments, such as

79

chemical softening and cationic exchange, are required to remove hardness from the feedwater 20.

80

Therefore, silica scaling remains a challenging problem facing membrane desalination, making

81

development of new strategies for scaling control highly desired.

15, 16

12-14

. However, the use of

. Since the solubility of silica increases with pH, high operation pH (i.e., 17, 18

. But high operation pH leads to the

82

Membrane surface modification may be a promising approach to reduce silica scaling.

83

Previous studies have demonstrated that surface chemistry influences silica scaling. For example,

84

by using an organic silica precursor, Wallace et al.

21

have shown that silica nucleation occurred



85

on model surfaces with carboxyl or hybrid amine/carboxyl functional groups, but not on amine-

86

terminated surfaces. When compared to a cellulose acetate membrane with surface hydroxyl

87

groups, polyamide membranes with native carboxyl groups showed more irreversible silica

88

scaling in both RO and forward osmosis operation 2.

89

To date, membrane surface modification has been used extensively for reducing organic and 22-28

90

biological fouling in membrane desalination systems

91

focused on surface modification for inorganic scaling control. For example, a hydrophilic brush

92

layer of poly(methacrylic acid) or poly(acrylamide) was shown to mitigate gypsum scaling on

93

polyamide RO membranes

94

underlying membrane surface, and their partial local motility reduced the attachment rate of

95

gypsum nuclei and/or crystallites

96

membrane surface modification in reducing silica scaling have not been reported in the literature.

97

Elucidating how membrane surface properties influence silica scaling is the prerequisite for

98

developing scaling-resistant membrane surfaces.

29, 30

. In contrast, very few studies have

. These brush layers provided effective screening of the

29, 30

. To the best of our knowledge, however, applications of

99

In this work, we investigated the relationship between membrane surface properties and

100

silica scaling in reverse osmosis. A commercial thin-film composite (TFC) polyamide membrane

101

was modified with diverse polymer coatings, which provided different surface hydrophilicity,

102

free energy for heterogeneous nucleation, and surface charge. The performance of the modified

103

membranes with a silica-saturated feed solution was tested in a bench-scale cross-flow RO

104

system. The water flux decline rate obtained from the scaling experiments was used to examine

105

the effect of membrane surface properties on the extent of silica scaling. We demonstrate that

106

membrane surface chemistry significantly influences silica scaling, with membrane surface

107

charge identified as the primary regulating factor. Our findings promote a mechanistic

108

understanding of silica scaling of RO membranes, which may guide the design and development

109

of effective scaling-resistant membranes.

110 111

MATERIALS AND METHODS

112

Materials and Chemicals. Acrylamide (>99.0%), [2-(methacryloyloxy)-ethyl] dimethyl-(3-

113

sulfopropyl)ammonium hydroxide (also known as sulfobetaine methacrylate, SBMA), [2-

114

(methacryloyloxy)ethyl] trimethylammonium chloride solution (MTAC, 80 wt% in H2O), 4 ACS Paragon Plus Environment

Page 4 of 30

Page 5 of 30


115

polyethylenimine (PEI, branched, Mw of ~800 g/mol and ~1,300 g/mol), 1H,1H,2H,2H-

116

perfluorodecanethiol (PFDT, 97%), acrylic acid (99%), N,N-dimethylformamide (DMF),

117

triethylamine (TEA, ≥99%), α-bromoisobutyryl bromide (BiBB, 98%), dopamine hydrochloride,

118

copper(II) chloride (99%), copper(II) bromide (99%), tris(2-pyridylmethyl)amine (TPMA), L-

119

ascorbic acid, potassium persulfate (K2S2O8, ≥99%), sodium metabisulfite (Na2S2O5, ≥99%), and

120

sodium metasilicate pentahydrate (Na2SiO3·5H2O, >95.0%) were purchased from Sigma-Aldrich.

121

Calcium chloride dihydrate (CaCl2·2H2O) and magnesium chloride hexahydrate (MgCl2·6H2O)

122

were purchased from Alfa Aesar and J.T. Baker, respectively. Commercial TFC RO membranes

123

(SW30 XLE) were provided by Dow Chemical. Deionized (DI) water was obtained from a Milli-

124

Q ultrapure water purification system (Millipore).

125

Membrane Surface Modification Approaches. Commercial RO membranes were

126

immersed in 25% isopropanol for 30 minutes, after which the membranes were washed

127

thoroughly with DI water and stored at 4 °C until use. Three modification approaches were

128

employed to tailor the surface chemistry of RO membranes with various polymer coatings: (i)

129

activators regenerated by electron transfer−atom transfer radical polymerization (ARGET-

130

ATRP), (ii) redox radical initiation, and (iii) dopamine-assisted direct grafting (Figure 1). The

131

selected polymers (as described below) have different functional groups, creating diverse

132

membrane surface properties in terms of surface hydrophilicity, free energy for heterogeneous

133

nucleation, and surface charge.

134

[FIGURE 1]

135

ARGET-ATRP is a robust and versatile approach to produce polymer brushes with narrow

136

polydispersity and controllable thickness and architecture

31, 32

137

following literature protocols with slight modification

31,

138

poly(acrylamide)

139

(methacryloyloxy)ethyl] trimethylammonium chloride) (PMTAC) on the membrane surface.

(PAM),

poly(sulfobetaine

. This approach, performed

33

methacrylate)

, was used for grafting

(PSBMA),

and

poly([2-

140

Briefly, dopamine hydrochloride (400 mg, ~2.10 mmol) was dissolved in 20 mL of DMF in

141

an amber bottle with a PTFE/red rubber septum, followed by adding TEA (0.15 mL, 1.05 mmol)

142

and Bibb (0.13 mL, 1.05 mmol). After three hours of stirring under N2 at room temperature, the

143

mixture

containing

dopamine-Bibb

was

added


to

100

mL

aqueous


144

tris(hydroxymethyl)aminomethane buffer (pH 8.5), which was then immediately poured onto the

145

membrane active layer. Dopamine-Bibb self-polymerized and formed poly(dopamine-Bibb)

146

(PDA-Bibb) on the membrane surface, which served as the initiator for polymer growth. After

147

three hours, the PDA-Bibb deposited membrane was thoroughly rinsed with DI water.

148

Acrylamide (14.2 g, ~0.2 mol), SBMA (15.64 g, ~56 mmol), or MTAC (14.52 g, ~56 mmol)

149

monomers were dissolved in 200 mL of 1:1 isopropanol:DI water mixture (v/v) in a bottle with a

150

septa lid (covered with aluminum foil). After degassing with N2 for 10 minutes, the PDA-Bibb

151

deposited membrane was placed into the bottle. After another 10 minutes degassing with N2, a

152

solution of copper(II) salt and TPMA in 8 mL 1:1 isopropanol:DI water mixture (v/v) was

153

injected into the bottle. Cu(II) bromide (0.008 g, ~35.8 μmol) and TPMA (0.065 g, ~0.225 mmol)

154

were used for grafting PAM, whereas Cu(II) chloride (0.004 g, ~29.8 μmol) and TPMA (0.056 g,

155

~0.193 mmol) were used for grafting PSBMA and PMTAC. After an additional 10 min

156

degassing with N2, 8 mL of ascorbic acid (0.8 g, ~4.5 mmol) in 1:1 isopropanol:DI water mixture

157

(v/v) were injected into the bottle to initiate the polymerization. The reaction lasted for 1 h, 3 d,

158

and 7 d for PSBMA, PMTAC, and PAM, respectively. The longer polymerization duration for

159

PMTAC and PAM was due to their slower polymerization rate than SBMA. Finally, the bottle

160

was opened to air to terminate the reaction. The modified membranes were washed thoroughly

161

with DI water and stored at 4 °C until use.

162

Redox radical initiation was used to create poly(acrylic acid) (PAA)-modified membranes, 34

163

following the protocol reported by Belfer et al

. In this approach, oxygen-centered radicals

164

formed by the action of redox initiators (i.e., K2S2O8 and Na2S2O5) are effective to graft vinyl

165

monomers with subsequent polymerization 34. In brief, 1 M solution of acrylic acid was prepared,

166

followed by adding 0.01 M K2S2O8 and 0.01 M Na2S2O5 to the monomer solution. The mixture

167

was then immediately poured onto the membrane active layer. After one hour, the PAA-modified

168

membrane was rinsed thoroughly with DI water and stored at 4 °C until use.

169

Dopamine-assisted direct grafting modification was used to create PEI- and PFDT-modified

170

membranes. The catechol groups of polydopamine (PDA) (after oxidation to quinone) react with

171

amine or thiol groups of the polymers through the Michael-type addition or Schiff base reaction

172

35-37

173

hydrochloride (30 mg, ~0.157 mmol) was dissolved in 30 mL tris(hydroxymethyl)

, thereby grafting the polymers directly onto the membrane surface. In brief, dopamine


Page 6 of 30

Page 7 of 30


174

aminomethane buffer (10 mM, pH 8.5). The mixed solution was then poured onto the membrane

175

active layer. Dopamine polymerization to PDA lasted for one hour at room temperature. After

176

rinsing with DI water, the PDA-deposited membrane was immersed in 10 g/L PEI aqueous

177

solution or 0.05% (v/v) PFDT ethanol:DI water solution (1:1, v/v) for four hours at room

178

temperature. After the reaction, the solution was removed and the PEI- or PFDT-modified

179

membrane was rinsed thoroughly with DI water and stored at 4 °C until use.

180

Membrane Surface Characterization. Membrane surface morphology was investigated

181

by scanning electron microscopy (SEM, Hitachi SU-70). Before taking the images, membrane

182

samples were dried and sputter-coated with a thin layer of chromium or iridium. Membrane

183

surface roughness was evaluated by atomic force microscopy (AFM, Bruker Dimension Fastscan)

184

in tapping mode with a silicon nitride probe (ScanAsyst-air, Burker). Micrographs were captured

185

from six different locations with an area of 10 μm × 10 μm. Attenuated total reflectance-Fourier

186

transform infrared (ATR-FTIR) spectra were collected using a Thermo Nicolet 6700

187

spectrometer with 32 scans for each sample. Membrane surface hydrophilicity was analyzed by

188

measuring the water contact angle using the sessile drop method

189

membrane surface was determined using a streaming potential analyzer with an asymmetric

190

clamping cell (EKA, Brookhaven Instruments). The measurements were conducted with a

191

solution containing 1 mM KCl and 0.1 mM KHCO3. Details on the procedure used to calculate

192

the zeta potential from the measured streaming potential are described elsewhere 39.

38

. The zeta potential of the

193

Free Energy for Heterogeneous Silica Nucleation. The thermodynamic barrier to

194

silica nucleation is determined by the free energy of forming a silica nucleus of a critical size

195

( DGcri )

196

het hom heterogeneous nucleation ( DGcri ) is smaller than that for homogeneous nucleation ( DGcri ). The

197

critical free energy for heterogeneous nucleation can be calculated from 21, 40, 41

40

. Because the membrane surface promotes nucleation, the critical free energy for

198

het hom DGcri = f (q )DGcri

199

f (q ) =

(2 + cosq )(1- cosq )2 4

(1) (2)

200

The correction factor f(θ), or the wetting function, describes the geometry of the nucleus-

201

surface interaction (Figure S1) 21. The value of f(θ) ranges from zero to one, and is a function of 7 ACS Paragon Plus Environment


202

the contact angle (θ) between the silica nucleus and the membrane surface. θ is calculated using

203

Young’s equation that involves the interfacial free energy among silica, water, and the

204

membrane surface 41:

cosq =

205

g mem-wat - g mem-SiO g SiO -wat

(3)

2

2

206

where g mem-wat , g mem-SiO , and g SiO -wat denote the interfacial free energies associated with the 2

2

207

membrane-water, membrane-silica, and silica-water boundaries at thermodynamic equilibrium,

208

respectively.

209

hom Since DGcri for silica is constant at a fixed temperature and saturation index (eqn S1 in

210

het Supporting Information), DGcri is proportional to the value of f(θ), which is a characteristic

211

property of the membrane surface. The value of f(θ) was calculated for each membrane, with a

212

larger f(θ) corresponding to a higher thermodynamic barrier (or lower membrane scaling

213

propensity) to heterogeneous silica nucleation. The procedure for calculating f(θ), g mem-wat ,

214

g mem-SiO , and g SiO -wat is detailed in the Supporting Information, following methods and 2

215

2

equations described in the literature 23, 40, 42.

216

Measurement of Membrane Transport Properties. The transport properties of the

217

membranes were determined using a bench-scale crossflow RO system. Details of the

218

experimental setup have been described in our previous work

219

an effective area of 20.02 cm2) was compacted overnight using DI water under a pressure of 31.0

220

bar (450 psi). The water permeability coefficient (A) was calculated from the pure water flux

221

measured under 27.6 bar (400 psi) at 22±1 °C. The salt permeability coefficient (B) was

222

calculated from the salt rejection measured under 27.6 bar (400 psi) at 22±1 °C with 50 mM

223

NaCl as the feed solution and a cross-flow velocity of 21.3 cm/s 45.

43, 44

. The membrane coupon (with

224

RO Membrane Silica Scaling Tests. Scaling tests with the various membranes were

225

conducted using a bench-scale RO system and a silica-saturated feed solution. The configuration

226

of the RO system was identical to that used in measuring the membrane transport properties 43, 44.


Page 8 of 30

Page 9 of 30


227

The silica-saturated feed solution was composed of 2.8 mM Na2SiO3·5H2O, 7.0 mM CaCl2,

228

3.5 mM MgCl2, and 35 mM NaCl, simulating brackish groundwater after 70-80% recovery 46. A

229

solution containing Na2SiO3·5H2O and NaCl was prepared first with the pH of ~11. Then, CaCl2

230

and MgCl2 were added after the pH was adjusted to less than 7 in order to avoid precipitation of

231

calcium and magnesium silicate salts

232

Thermodynamic calculations using PHREEQC 48 and the database MINTEQ (version 4) showed

233

that silica was the only precipitate formed in the feed solution, with a saturation index (defined

234

as the ratio of the ion activity product to the solubility product) of 1.5 for amorphous silica.

13, 47

. The pH was further adjusted to 6.50 ± 0.05.

235

Prior to silica scaling, membranes were compacted overnight using DI water under 31.0 bar

236

(450 psi), after which pure water flux of the membrane was recorded under 27.6 bar (400 psi) at

237

a crossflow velocity of 8.5 cm/s. Then, the silica scaling experiment was initiated by adding the

238

silica-saturated solution to the RO feed reservoir. The applied pressure (27.6 ± 2.8 bar) was

239

adjusted to create an initial water flux of 56±2 L m-2 h-1. The water flux was continuously

240

monitored for 1400 minutes at a constant crossflow velocity of 8.5 cm/s. A recycling mode was

241

employed, with the permeate recycled back to the feed solution. The feed solution temperature

242

during the silica scaling tests was maintained constant at 22±1 °C.

243

After silica scaling, membrane cleaning was performed by rinsing the membrane with DI

244

water at a high crossflow velocity of 21.3 cm/s for 30 minutes. After the cleaning step, pure

245

water flux of the membrane was measured at 27.6 bar (400 psi) and a crossflow velocity of 8.5

246

cm/s to determine the flux recovery ratio. The membranes before and after silica scaling were

247

analyzed by SEM, energy-dispersive X-ray (EDX) spectroscopy, and ATR-FTIR spectroscopy,

248

to examine the morphology and chemical composition of silica scale formed on the membrane

249

surface.

250 251

RESULTS AND DISCUSSION

252

Modified Membranes Exhibit Diverse Surface Properties. The membranes tested in this

253

study were analyzed by a combination of different techniques to characterize key surface

254

properties, namely membrane morphology, surface functionality, surface hydrophilicity, and

255

surface charge. 9 ACS Paragon Plus Environment


Page 10 of 30

256

SEM was employed to observe the morphology of membrane surfaces (Figure S2). Except

257

for the membrane grafted with PEI of 1,300 g/mol (PEI-1300), all modified membranes

258

exhibited a ridge-and-valley surface structure similar to the control commercial RO membrane,

259

indicating that the polymer films formed on the membrane surfaces were ultra-thin. The surface

260

roughness of each membrane was further quantified by AFM (Figure S3). Results show that

261

surface modification did not change the membrane surface roughness. The lack of alteration in

262

surface roughness after modification by PEI-1300 was likely due to the low thickness of the

263

polymer layer relative to the surface roughness of the unmodified (control) membrane.

264

Membrane surface functionality was characterized by ATR-FTIR spectroscopy. As shown in

265

Figure S4, ATR-FTIR spectra of the tested membranes represented a combination of spectra

266

from both the polyamide active layer and polysulfone support layer. For example, the absorbance

267

at ~1670 cm-1 and ~1540 cm-1 corresponds to N-C=O and C-N-H vibrations of the amide groups

268

49

269

was observed at 1726 cm-1 for both PSBMA- and PMTAC-modified membranes, which is

270

attributed to the carbonyl in the ester group of SBMA and MTAC molecules

271

modified membrane also showed an additional peak at 1039 cm-1 that arises from the symmetric

272

stretch of sulfonate group in SBMA

273

polyamide active layer due to the hydrolysis of unreacted acyl chloride groups

274

(approximately 1-30 charges nm-2

275

unmodified control membrane 49. The PAA-modified membrane exhibited an increased signal at

276

~1720 cm-1 due to C=O stretching 52, 53, suggesting an increase of carboxyl group density on the

277

membrane surface. For the other modified membranes, the characteristic peaks of the grafted

278

polymers overlapped with those of the TFC polyamide membrane. Therefore, no difference in

279

the ATR-FTIR spectra was observed as compared to the control membrane.

, while the peak at 1294 cm-1 originates from the S=O stretching in polysulfone 31. A new peak

51

31

31

. The PSBMA-

. Although native carboxyl groups are present in the 50

, their density

) was too low to be detected by ATR-FTIR for the

280

Membrane surface hydrophilicity was determined by measuring the water contact angle of

281

each membrane. As shown in Figure 2A, the control membrane was relatively hydrophilic with a

282

water contact angle of ~30°, which was lower than reported values for pristine TFC polyamide

283

membranes (> 50°

284

commercial membrane surface

285

polymers, the membrane surface became more hydrophilic, with water contact angles lower than

38, 54

). We attribute this result to proprietary polymer coating on the 55

. After modification with PAM, PSBMA, PEI, and PAA


Page 11 of 30


286

20°. The reduction in water contact angle was attributed to the introduction of polar functional

287

groups (-O=C-NH2, -NH2/-NH, and -COO- for PAM, PEI, and PAA, respectively) or

288

zwitterionic brush layer (for SBMA), which increased the affinity of water molecules to the

289

membrane surface

290

surface (water contact angle > 110°) due to the abundance of fluorine groups

291

membrane modifications with PMTAC, PDA, and PDA-Bibb resulted in a slight, but statistically

292

insignificant, increase of membrane hydrophilicity as compared to the control membrane.

56-58

. Conversely, grafting of the PFDT polymer created a more hydrophobic 36

. In addition,

293

[FIGURE 2]

294

Zeta potential indicates the surface charge properties of the membrane 39, which may impact

295

the adsorption of charged foulants due to electrostatic interactions. The zeta potential of each

296

membrane was calculated from streaming potential measurements as a function of pH

297

shown in Figure 2B, the control membrane had negative surface charge over the range of

298

investigated solution pH (i.e., pH of 3-9). The negative surface charge is attributed to the

299

deprotonation of carboxylic groups formed in the interfacial polymerization process 59.

39

. As

300

Membrane surface modification with different polymers altered the membrane zeta potential,

301

providing additional evidence for the success of polymer grafting. PMTAC and PEI increased

302

the membrane zeta potential dramatically, with the membrane surface charge shifting from

303

negative to positive at almost every investigated solution pH. The positive charge was derived

304

from the quaternary ammonium of MTAC 31, 60 or abundant amine groups of PEI molecules 61, 62.

305

In contrast, the PAA-modified membrane exhibited a more negatively charged surface than the

306

control membrane, consistent with its higher surface density of carboxyl groups as revealed by

307

ATR-FTIR spectra. The other polymers used in this study generally diminished the negative

308

surface charge as compared to the unmodified control membrane. For example, the zeta potential

309

at near neutral pH changed from ~-12 mV for the control membrane to ~-6 mV for the PSBMA-

310

and PAM-modified membranes. The grafting of these net zero charge polymers covered the

311

underlying negatively charged functional groups 29, 31.

312

Membrane Transport Properties. As shown in Figure 2C, most modified membranes

313

exhibited slightly higher water permeability coefficients (A between 2.6±0.0 and 3.0±0.2 L m-1 h-

314

1

bar-1) than the control membrane (A of 2.5±0.0 L m-1 h-1 bar-1), probably due to their improved 11 ACS Paragon Plus Environment


Page 12 of 30

315

membrane hydrophilicity. However, both PEI-modified membranes showed reduced water

316

permeability (2.3±0.1 L m-1 h-1 bar-1). We noticed that the grafting of hydrophobic PFDT

317

polymer did not decrease membrane water permeability. This counterintuitive phenomenon was

318

attributed to the use of ethanol to dissolve PFDT during the membrane modification process.

319

Ethanol could swell the polyamide active layer by enhancing chain flexibility and chain-chain

320

distance, resulting in a larger free volume that facilitates water transport 63. After immersing the

321

control membrane in 1:1 ethanol:DI water solution for four hours, the membrane water

322

permeability increased by 40% and became significantly higher than that of the PFDT-modified

323

membrane (Figure S5).

324

Salt permeability coefficients (B) of the membranes were calculated from the measured 45

325

water flux and salt rejection after accounting for concentration polarization (Figure 2C)

326

average salt rejection of the control membrane was 99.0% with 50 mM NaCl in the feed solution,

327

while the modified membranes exhibited similar or slightly lower salt rejection of 98.4%99.1%.

328

Except for the membrane modified with PEI-1300, the calculated salt permeability coefficients

329

moderately increased after membrane modification, reflecting the water permeability−salt

330

permeability trade-off of TFC polyamide membranes

331

transport properties were not greatly affected by surface modification in our study.

64

. The

. Therefore, the intrinsic membrane

332

Membrane Surface Modification Influences Silica Scaling. The silica scaling tests

333

were conducted with a silica-saturated feed solution (saturation index of 1.5) in a bench-scale

334

cross-flow RO system. As shown in Figure S6A, the water flux of the control membrane

335

decreased gradually with a total flux decline of ~15% after 1400 minutes. No flux recovery was

336

achieved after membrane cleaning (Figure S6B), consistent with our previous finding that silica

337

scaling of polyamide membranes in RO mode was irreversible 2. SEM images revealed that the

338

scaled membrane surface was fully covered by a layer of particles (Figures S6C and S7), with

339

the ridge-and-valley surface structure no longer visible. EDX spectra clearly showed the energy

340

peak of Si at 1.74 keV (Figure S6C); no signal of Ca or Mg was detected, thus excluding the

341

formation of calcium and magnesium scale. Compared to the pristine membrane, increased

342

signals associated with Si-OH (1654 cm-1) 65 and Si-O-Si bonds (between 1050 and 1100 cm-1) 66,

343

67

344

unambiguously confirmed the formation of silica scale on the membrane surface.

were detected in the ATR-FTIR spectra after membrane scaling (Figure S6D). These analyses


Page 13 of 30


345

Water flux decline curves of all modified membranes were compared with that of the control

346

membrane (Figure 3A). RO membranes modified with different polymers showed varied extents

347

of flux decline due to silica scaling. Since other factors potentially influencing the rate of silica

348

scaling (e.g., permeate flux, trans-membrane pressure, cross-flow velocity, and feedwater

349

chemistry) were kept the same during the scaling tests for all membranes, the observed variation

350

in water flux decline is attributed to the difference in membrane surface properties. Membranes

351

modified with PMTAC and two PEI polymers experienced the most severe flux decline (~30%

352

after 1400 minutes), significantly higher than that for the control membrane (~15%, p < 0.015 in

353

paired t test). In contrast, the PAA-modified membrane showed the lowest flux decline (~12%).

354

All other modified membranes displayed a larger water flux decline than the control membrane,

355

suggesting that the grafted polymer layers favored silica scale formation. In addition, except for

356

the PAA-modified membrane, no recovery of water flux was observed for all other modified

357

membranes after membrane cleaning (Figure 3B), underscoring the irreversibility of silica

358

scaling in RO operation.

359

[FIGURE 3]

360

Silica scaling on the membrane surface is governed by both silica-membrane and silica-

361

silica interactions 2. The membrane surface was directly exposed to silica scalants at the initial

362

stage of the scaling tests, when membrane surface properties influenced silica deposition and

363

nucleation by controlling silica-membrane interactions. However, the influence of membrane

364

surface properties diminished as the membrane surface was progressively covered by a silica

365

layer, after which further accumulation of silica scale was controlled by silica-silica interactions.

366

This transition was notably observed for the MTAC- and PEI-modified membranes that have

367

high scaling propensity. Specifically, these membranes experienced a rapid flux decline at the

368

beginning of the scaling tests, reducing the water flux by more than 10% within six hours. The

369

flux decline rate was then decelerated and became comparable to the other membranes after

370

~800 minutes, indicating that the membrane surfaces were covered by silica scale. In contrast,

371

the PAA-modified membrane maintained 99% of the initial water flux during the first six hours,

372

suggesting that the initial deposition of silica species on the membrane surface was effectively

373

reduced with PAA polymers.



Page 14 of 30

374

Silica Scaling is Independent of Membrane Hydrophilicity and Free Energy for

375

Heterogeneous Nucleation. In order to delineate the relationship between membrane

376

hydrophilicity and silica scaling, the water flux decline ratio of each membrane was plotted

377

against the corresponding water contact angle (Figure 4A). No correlation was observed (R2 =

378

0.03, p = 0.61), indicating that silica scaling is independent of membrane hydrophilicity. For

379

example, although membranes modified with PFDT, PDA, and PSBMA exhibited distinct water

380

contact angles (119±7°, 26±3°, and

Relating Silica Scaling in Reverse Osmosis to ... - ACS Publications

Recommend Documents