Engineered Slippery Surface to Mitigate Gypsum Scaling in

Subscriber access provided by Kaohsiung Medical University

Remediation and Control Technologies

Engineered Slippery Surface to Mitigate Gypsum Scaling in Membrane Distillation for Treatment of Hypersaline Industrial Wastewaters vasiliki Karanikola, Chanhee Boo, Julianne Rolf, and Menachem Elimelech Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b04836 • Publication Date (Web): 14 Nov 2018 Downloaded from http://pubs.acs.org on November 15, 2018

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 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 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.

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 24

Environmental Science & Technology

1 2 3 4 5

Engineered Slippery Surface to Mitigate Gypsum Scaling in Membrane Distillation for Treatment of Hypersaline Industrial Wastewaters

6 7

Submitted to 8


9 10

Revised: November 6, 2018 11 12

Vasiliki Karanikola,† Chanhee Boo,† Julianne Rolf,

13

and Menachem Elimelech*

14

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

15 16 17 18 19 20 21 22 23 24

†

V.K. and C.B. contributed equally to this work. *Corresponding author: e-mail: [email protected]; Tel: +1 (203) 432-2789

1

ACS Paragon Plus Environment


25

ABSTRACT

26

Membrane distillation (MD) is an emerging thermal desalination process, which can potentially

27

treat high salinity industrial wastewaters, such as shale gas produced water and power plant

28

blowdown. The performance of MD systems is hampered by inorganic scaling, particularly when

29

treating hypersaline industrial wastewaters with high-scaling potential. In this study, we

30

developed a scaling-resistant MD membrane with an engineered “slippery” surface for

31

desalination of high-salinity industrial wastewaters at high water recovery. A polyvinylidene

32

fluoride (PVDF) membrane was grafted with silica nanoparticles, followed by coating with

33

fluoroalkylsilane to lower the membrane surface energy. Contact angle measurements revealed

34

the “slippery” nature of the modified PVDF membrane. We evaluated the desalination

35

performance of the surface-engineered PVDF membrane in direct contact membrane distillation

36

using a synthetic wastewater with high gypsum scaling potential as well as a brine from a power

37

plant blowdown. Results show that gypsum scaling is substantially delayed on the developed

38

slippery surface. Compared to the pristine PVDF membrane, the modified PVDF membranes

39

exhibited a stable MD performance with reduced scaling potential, demonstrating its potential to

40

achieve high water recovery in treatment of high-salinity industrial wastewaters. We conclude

41

with a discussion of the mechanism for gypsum scaling inhibition by the engineered slippery

42

surface.

43 44 45 46

47

TOC Art

2


Page 2 of 24

Page 3 of 24


48

INTRODUCTION

49

Thermoelectric power plants use a substantial amount of water, which accounts for

50

approximately 40% of freshwater withdrawals in the United States. 1-3 There is a critical need to

51

impose stringent discharge regulations on power plants to reduce their freshwater consumption,

52

wastewater discharge, and environmental impact. The U.S. Environmental Protection Agency

53

(EPA) established a regulatory limit of wastewater discharge from power plants to the

54

environment to be reduced by ~57 billion gallons per year. 4 Brine discharge from thermoelectric

55

power plants are strictly prohibited in regions with limited freshwater supplies. In such

56

conditions, the facilities need to achieve zero liquid discharge (ZLD) for waste (brine) streams.

57

Sustainable management of wastewaters from steam electric power generation is of critical

58

importance at the water-energy nexus.

59

Steam-driven electric power plants require water in two processes: one as the means to

60

generate steam to drive a turbine and one as the cooling water to remove heat from the

61

condensed steam and the system components; 5 the latter accounts for the majority of the water

62

consumption. During cooling tower operation, the process water is continuously concentrated up

63

to eight times due to evaporation, environmental losses, and infrastructure leakage, 6 which

64

causes scaling and corrosion of the system. To prevent scaling and corrosion, many chemicals,

65

including biocides, biodispersants, corrosion inhibitors, pH adjusters, and anti-scalants are added

66

to the cooling water.5 In addition, many power plants are designed to use reclaimed municipal

67

wastewater effluent or surface water as cooling process water, which contain diverse inorganic

68

and organic matter as well as microorganisms.6 The concentrated cooling water contains high

69

concentrations of total dissolved solids (TDS) and chemical additives. Hence, wastewater from a

70

power plant is challenging to treat by conventional water treatment practices.

71

Membrane distillation (MD) is a thermal separation process driven by a vapor gradient

72

resulting from a temperature gradient created across a hydrophobic microporous membrane. 7

73

Theoretically, MD can achieve complete salt rejection, owing to its phase-change based

74

desalination mechanism. Furthermore, performance of MD is only slightly sensitive to the feed

75

salinity, because the vapor pressure of feed solution does not change much with salt

76

concentration.8 Nonetheless, MD is energetically intensive as any thermal separation process. 9 3



77

Energy-efficient MD operation is possible if abundant waste or low-grade heat is readily

78

available10. The waste heat produced from a power plant cooling tower can be exploited to drive

79

the MD process. With its effective incorporation into the thermoelectric power plant scheme,

80

MD can provide sustainable process water management by achieving ZLD for waste streams. 11

81

Membrane scaling is of critical concern when treatment of high-salinity industrial wastewater

82

at high water recovery is desired, such as desalination of power plant blowdown. 12, 13 At higher

83

MD system recovery, TDS concentrations in the feed increases to supersaturated conditions. 14

84

Nucleation of the crystals in the bulk solution and subsequent crystal deposition on the

85

membrane surface, as well as heterogeneous nucleation of the crystals on the membrane surface,

86

take place under such supersaturated conditions.15, 16 The scaling mechanisms in MD are closely

87

related to the membrane surface properties, feed solution chemistry, and the system

88

hydrodynamic conditions.15,

89

desalination performance of MD membranes.18 In addition, the scaling layer aggravates

90

temperature and concentration polarization by reducing the effectiveness of heat and mass

91

transfer near the membrane surface.19

17

Scaling leads to pore wetting and subsequent decrease in the

92

Recent studies have shown that engineering the surface with special wettability can enhance

93

MD membrane performance.20-22 Polyvinylidene fluoride (PVDF) is one of the commercially

94

available hydrophobic membranes widely used for MD applications. 23-26 However, in many

95

cases, conventional PVDF membranes are susceptible to scaling. 27-29 The non-adhesive surface

96

nature adapted from a lotus leaf can potentially be leveraged to fabricate anti-scaling MD

97

membranes.28,30-32 Coating a hydrophobic substrate with ultralow interfacial energy material can

98

create a surface with unique properties, including a significantly high water contact angle, low

99

contact angle hysteresis, and non-adhesive slipperiness. Such unique surface properties are

100

expected to reduce scaling potential of MD membranes by limiting energetically favorable

101

regions for crystal growth.33-36

102

In this study, we demonstrate the potential application of MD membranes with

103

superhydrophobic, slippery surface properties in treatment of high-salinity industrial wastewaters

104

at high water recovery. MD membranes with a slippery surface were prepared via facile and

105

scalable surface engineering of hydrophobic PVDF membranes. Water flux and salt rejection 4


Page 4 of 24

Page 5 of 24


106

performance of the slippery membrane were evaluated and compared to membranes with

107

hydrophobic and hydrophilic surfaces in direct contact membrane distillation, using

108

supersaturated gypsum feed water. We further evaluated MD performance of the slippery-surface

109

membrane in the desalination of power plant blowdown wastewater. Based on the observed

110

results, we discuss possible mechanisms for reduced scaling potential of the engineered slippery

111

surface.

112 113

MATERIALS AND METHODS

114

Materials and Chemicals. ACS grade sodium hydroxide (NaOH, J.T. Baker), (3-

115

Aminopropyl)triethoxysilane (99%, APTES) (Sigma-Aldrich), anhydrous ethanol (100%, Decon

116

Laboratories, Inc., PA), silica nanoparticles (SiNPs, Ludox SM, avg. diameter 8 nm, 30%,

117

Sigma−Aldrich), and (heptadecafluoro-1,1, 2 ,2- tetrahydrodecyl)trichlorosilane (FDTS, Gelest

118

Inc., PA) were used for the surface modification of the polyvinylidene fluoride substrate.

119

Supersaturated gypsum (CaSO4·2H2O) solution was prepared by mixing calcium chloride

120

dihydrate (CaCl2·2H2O, Sigma-Aldrich) and sodium sulfate (Na2SO4, Sigma-Aldrich) in

121

deionized (DI) water.

122

Surface Modification of PVDF Membrane. A four-step protocol was used to modify a

123

flat sheet polyvinylidene fluoride (PVDF) hydrophobic membrane with a nominal pore size of

124

0.45 μm and an average thickness of 125 μm (HVHP, Millipore). 20 The first step was to immerse

125

the PVDF substrate into a 7.5 M NaOH solution at ∼70 °C for 3 h to functionalize the surface

126

with hydroxyl groups. The alkaline-treated PVDF membrane was then rinsed with DI water and

127

oven-dried at 80 ºC for 1 h. The dried membranes were subsequently placed in 1% v/v APTES in

128

ethanol under continuous stirring for 1 h. APTES covalently binds with the hydroxyl groups to

129

produce amine terminal groups, rendering the membrane surface positively charged. Then, the

130

amine-functionalized PVDF substrate was immersed in an aqueous SiNP suspension for 1 h

131

under gentle mixing. SiNPs are negatively charged and bound electrostatically to the positively

132

charged PVDF membrane surface. The aqueous SiNP suspension was prepared by adding 1

133

wt % SiNPs in acetate buffer with an ionic strength of ~1 mM. The pH of the SiNP suspension

134

was adjusted to 4 to promote effective electrostatic attraction between the negatively charged 5



135

SiNPs and the positively charged amine terminal groups on the PVDF substrate. Before use, the

136

SiNP suspension was bath sonicated for 30 minutes to minimize particle aggregation. Finally, the

137

SiNP-coated PVDF substrate was functionalized with fluoroalkylsilane via covalent bonding (i.e.,

138

(heptadecafluorotetrahydrodecyl)trichlorosilane, hereafter denoted as 17–FAS) to lower the

139

membrane surface energy via vapor phase silanization for 12 h under vacuum and heating at 70

140

ºC.

141

Membrane Characterization. Surface morphology of the pristine and modified PVDF

142

membranes was examined by scanning electron microscopy (SEM, Hitachi SU-70). Before

143

imaging, membrane samples were sputter-coated with a 4-nm iridium layer (BTT-IV, Denton

144

Vacuum, LLC, Moorestown, NJ). The SEM images were obtained to confirm SiNP coating after

145

surface modification and to observe scalants formed on the membrane surface after MD scaling

146

experiments. Scaling of MD membranes on the surface and inside the pores was further analyzed

147

by elemental mapping using SEM equipped with an energy dispersive X-ray spectroscopy (EDS,

148

Bruker XFlash 5060FQ Annular detector). The scaled membrane samples were sputter-coated

149

with a 4-nm iridium layer (BTT-IV, Denton Vacuum, LLC, Moorestown, NJ) to eliminate

150

charging during EDX analysis. We note that EDX elemental analysis is not affected by the

151

iridium coating as it has binding energy far from the elements expected on our samples.

152

The elemental composition of the pristine and surface modified PVDF membranes was

153

analyzed by X-ray photoelectron spectroscopy (XPS, PHI VersaProbe II, Physical Electronics

154

Inc., MN). Membrane samples were oven dried at 70 ºC overnight prior to performing XPS. The

155

XPS spectra were collected using monochromatic 1486.7 eV Al Kα X-ray source with a 0.47 eV

156

system resolution. The energy scale was calibrated using Cu 2p3/2 (932.67 eV) and Au 4f7/2

157

(84.00 eV) peaks on a clean copper plate and a clean gold foil.

158

Static contact angle and dynamic surface wettability of the PVDF membranes with

159

hydrophobic, hydrophilic, and slippery surface were measured by a contact angle goniometer

160

(OneAttension, Biolin scientific instrument) using the sessile drop method. Static water contact

161

angles were measured by placing a 5-μL droplet on the membrane surface. The shape of water

162

droplet was photographed for 30 s using a digital camera. The digital images obtained were

163

analyzed by a postprocessing software (OneAttension software), and the data were averaged. 6


Page 6 of 24

Page 7 of 24


164

Dynamic surface wettability was tested by dropping a 10-μL water droplet from a 4-cm height

165

and monitoring the movement of the droplet on the surface over time.

166

Synthetic and Blowdown Feed Waters. DI water (1 L) at 60 and 20 °C was used as

167

feed and permeate streams, respectively, for initial one hour direct contact membrane distillation

168

(DCMD) runs. Calcium chloride (1 M) and sodium sulfate (1 M) stock solutions were prepared

169

in advance and filtered with a 0.45 m cellulose acetate membrane filter (Corning, Tewksbury,

170

MA). After 1 h stabilization, stock solutions were added to the feed reservoir to obtain a scaling

171

solution comprising 20 mM CaCl2 and 20 mM Na2SO4 without pH adjustment. The gypsum

172

(CaSO4·2H2O) saturation index (SI) of this solution at 60 °C was calculated using OLI Stream

173

Analyzer (OLI Systems, Inc., Morris Plans, NJ) at 1.1. The gypsum scaling experiments were

174

conducted until the permeate flux nearly reached to zero and the conductivity of the permeate

175

begun to increase.

176

Blowdown water from the cooling towers at Redhawk Power Station in Arlington, Arizona,

177

was prefiltered through 11-μm filter (Ashless grade 44, GE Whatman, PA) to remove particulate

178

and suspended organic matter prior to DCMD experiments. The major composition and key

179

properties of the prefiltered blowdown water were analyzed by a third-party environmental

180

laboratory (Environmental Service Laboratories, Inc., PA) (Table S1). The blowdown water was

181

treated with chlorine and stored at 4 °C to prevent biological growth. The DCMD experiments

182

with blowdown water were conducted until the maximum amount of feed solution (initial

183

volume of 1 L) was recovered before the conductivity of the permeate started to increase  an

184

indication of pore wetting.

185

Membrane Distillation Scaling Experiments. We evaluated the scaling behavior of the

186

pristine and surface-modified PVDF membranes using a laboratory-scale direct contact

187

membrane distillation (DCMD) unit. A custom-made acrylic cell with channel dimensions of 77

188

mm in length, 26 mm in width, and 3 mm in depth, translating to an effective membrane area of

189

20.0 cm2, was used for testing. The temperatures of feed and permeate solutions were maintained

190

at 60 and 20 °C, respectively. A low (8.5 cm/s) and high (17 cm/s) feed cross-flow velocities

191

were employed to investigate the effect of hydrodynamics on the membrane scaling behavior.

192

The water vapor flux, Jw, across the membrane was determined by measuring the increase in 7



193

permeate weight over time. Concentration of salt in the permeate was measured using a

194

calibrated conductivity meter (Oakton Instruments, Vernon Hills, IL). The electrical conductivity

195

of the permeate solution was monitored to calculate the salt rejection.

196 197 198

RESULTS AND DISCUSSION

199

Properties of Slippery Surface Membrane. Figure 1a schematically illustrates the steps to

200

synthesize a slippery surface on a hydrophobic PVDF substrate. The PVDF substrate is first

201

treated with an NaOH (7.5 M) solution at 70 ºC to functionalize the surface with hydroxyl groups.

202

During this process, simultaneous defluorination and oxygenation take place on PVDF polymer

203

chains by substitution of fluorine into hydroxyl moieties.37 The NaOH-treated PVDF membrane

204

remained mechanically robust and flexible as shown in Figure S1-A, and no morphological

205

differences were observed in SEM images of the PVDF membranes before and after alkaline

206

treatment (Figures S1-B and S1-C). The alkaline treated PVDF substrate is then aminosilanized

207

(APTES 1% v/v in ethanol) to render the surface positively charged. APTES binds covalently

208

with the hydroxyl groups on the membrane and serves as a substrate for SiNP deposition onto the

209

membrane surface. The negatively charged SiNPs are electrostatically bound with the amine-

210

functionalized substrate surface. Finally, the PVDF membrane with attached SiNPs is vapor

211

silanized with 17–FAS through a hydrolysis-condensation reaction to produce a surface with

212

ultralow interfacial energy.33

213

FIGURE 1

214

Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) were

215

used to confirm SiNP coating on the PVDF substrate. The top-down view of the SiNP-coated

216

membrane clearly shows a dense layer of the spherical nanoparticles on the PVDF substrate

217

(Figure 1b). The cross-section image of SEM-EDS elemental mapping for the SiNP-coated

218

membrane indicates that SiNP modification creates a several micrometer thick coating layer on

219

the porous substrate (Figure 1c). We also observed SiNP coating inside the substrate pores,

220

consistent with our previous studies.33 The internal pore functionalization is expected to increase 8


Page 8 of 24

Page 9 of 24


221

the resistance to pore wetting and potentially reduce scaling potential, as we discuss later in this

222

paper.

223

XPS spectra for the pristine PVDF membrane (denoted as hydrophobic), SiNP-coated PVDF

224

membrane (denoted as hydrophilic), and SiNP and 17–FAS functionalized PVDF membrane

225

(denoted as slippery) verify the substrate coating with SiNPs and surface fluorination with 17–

226

FAS (Figure 1d). The pristine hydrophobic PVDF membrane exhibits two major peaks at 289

227

and 285 eV, which are assigned to the binding energies of carbon to fluorine (CF 2) and carbon to

228

hydrogen (CH2), respectively.38 The peak intensities for CH2 and CF2 are nearly equal, consistent

229

with the chemistry of PVDF. The hydrophilic surface membrane exhibits binding energy peaks

230

identical to those of the pristine hydrophobic membrane; however, their intensities are lower

231

because the SiNP coating covers the surface elemental composition of the native PVDF. Instead,

232

we observed strong XPS spectra relevant to oxygen (O) and silicon (Si) for the SiNP-coated

233

PVDF membrane (Figure S2). After functionalization with 17–FAS, the slippery membrane

234

exhibits a slightly different spectrum, which displays an energy peak at 293 eV ascribed to the

235

terminal CF3 group of 17–FAS chain. We also observed a shift of the CF2 peak to 291 eV due to

236

the formation of CF2-CF2 after surface 17–FAS functionalization. The obtained XPS spectrum

237

corroborates successful surface modification of the pristine hydrophobic substrate to produce

238

membranes with an engineered slippery surface that provides a high water repellency (Figure 1e).

239

Surface wettability and the non-stick slippery nature of the pristine and surface-modified

240

PVDF membranes were evaluated using a contact angle goniometer with DI water as the testing

241

liquid (Figure 2). The pristine hydrophobic surface membrane showed a relatively high static

242

contact angle of ~120º. As expected, the hydrophilic surface membrane exhibited the lowest

243

static water contact angle of ~70º, while the slippery surface membrane showed the highest

244

hydrophobicity with a water contact angle of > 150º (Figure 2a). When the pristine hydrophobic

245

membrane was subjected to dynamic surface wettability testing, the water droplet remained in

246

place with no bouncing (Figure 2b-1). This observation indicates that water and the surface do

247

not strongly repel each other despite the inherent surface hydrophobicity. The hydrophilic

248

surface membrane exhibited the lowest tendency to repel water as demonstrated by stable droplet

249

settling on the surface during the dynamic surface wettability test (Figure 2b-2). This observation

250

indicates that the attraction between the hydrophilic surface and water is higher than the kinetic 9



251

energy exerted by the bouncing droplet. In contrast, the slippery surface membrane illustrates

252

immediate bouncing of water droplet upon contact with the engineered slippery surface (Figure

253

2b-3). This observation suggests that the modification produced a low energy surface where the

254

kinetic energy of the bouncing droplet can overcome the attractive forces between the droplet

255

and membrane surface. FIGURE 2

256 257

Gypsum Scaling Behavior. The gypsum scaling behavior of the hydrophobic,

258

hydrophilic, and slippery surface membranes was studied through DCMD experiments. Permeate

259

flux was measured over time as the membranes were exposed to a supersaturated gypsum feed

260

solution (SI = 1.1) prepared by mixing 20 mM of CaCl2 and Na2SO4. We assessed the gypsum

261

scaling potential of these membranes by monitoring the flux and electric conductivity of the

262

permeate simultaneously in a batch DCMD mode (i.e., feed solution was concentrated over time).

263

The effect of cross-flow rate was examined by applying two cross-flow velocities (i.e., 8.5 and

264

17 cm/s), both in the laminar flow regime.

265

Figure 3a illustrates the water flux decline (expressed as normalized permeate flux) and

266

permeate conductivity of the hydrophobic, hydrophilic, and slippery surface membranes obtained

267

during MD scaling experiments at a low feed cross-flow velocity (8.5 cm/s). The hydrophobic

268

and hydrophilic surface membranes exhibited similar flux decline behavior, with the hydrophilic

269

membrane allowing for a slightly higher water recovery (~12.5%) before a rapid flux decline.

270

With the sudden drop of permeate flux, we observed a sharp increase of permeate conductivity,

271

which is attributable to the onset of scaling-induced pore wetting. 39 The slippery surface

272

membrane outperformed both the hydrophobic and hydrophilic surface membranes by delaying

273

both the decline in permeate water flux and increase of permeate conductivity. This observation

274

clearly indicates that the MD membrane with an engineered slippery surface resists gypsum

275

scaling better than those with hydrophobic and hydrophilic surfaces.

276

FIGURE 3

277

We further performed the gypsum scaling experiments at a higher feed cross-flow velocity

278

(17 cm/s) to evaluate the effect of hydrodynamics on the scaling behavior of membranes with

279

different degrees of surface wettability (Figure 3b). The high cross-flow velocity reduced the 10


Page 10 of 24

Page 11 of 24


280

gypsum scaling potential of the hydrophobic and slippery surface membranes but not the

281

hydrophilic surface membrane. The reduced gypsum scaling potential at a higher feed cross-flow

282

velocity is attributed to better mixing, which in turn reduces the buildup of scale near the

283

membrane surface.40 Interestingly, the gypsum scaling potential of the hydrophilic surface

284

membrane is likely independent of the flow hydrodynamics as this membrane exhibits similar

285

flux decline rate at the two different feed cross-flow velocities. The hydrophilic surface is

286

expected to strongly attract the hydrated gypsum scalants to form a strong hydration layer by

287

increasing hydrogen bonding interactions with water molecules.41 Once the gypsum scalants are

288

deposited on the hydrophilic surface they are harder to remove by shear force (i.e., higher cross-

289

flow) compared with the hydrophobic and slippery surfaces that provide lower interaction with

290

the scalants.42 Thus, cross-flow velocity has less influence on sweeping away the deposited

291

gypsum scalants from the hydrophilic surface membrane.

292

To better understand the effect of surface wettability on gypsum scaling behaviors of MD

293

membranes, we investigated the gypsum scalants formed on the membrane surface at two stages

294

of scaling. The first is identified as the early stage of scaling, which occurs just before the flux

295

started to decline (points b-1 and b-3 in Figure 4a), at approximately 150 mL cumulative

296

permeate volume. The second is identified as the long-term stage which occurs when the

297

permeate flux reached nearly zero and simultaneously the conductivity of the permeate began to

298

rapidly increase (points b-2 and b-4 in Figure 4a). The membranes were taken out of the cell as is

299

and examined under SEM. Figure 4b shows the SEM images of the membrane surface after each

300

scaling experiment. The slippery surface membrane taken at the earlier scaling stage exhibited a

301

clean surface without discernible nucleation of gypsum crystals (Figure 4b-3), while rosette-like

302

embryo gypsum crystals were found on the hydrophobic surface membrane (Figure 4b-1), a

303

manifestation of heterogeneous surface nucleation. 43, 44 SEM images of both the hydrophobic and

304

slippery surface membranes obtained after long-term scaling experiments showed that the

305

surfaces were substantially covered with needle-like crystals resulting from bulk deposition of

306

gypsum scalants (Figures 4b-2 and 4b-4).45 The slippery surface membrane, however, achieved a

307

higher water recovery compared to the hydrophobic surface membrane before a significant water

308

flux decline appeared, suggesting a retardation of bulk crystal deposition on this membrane.

309

Such an enhanced resistance to deposition of gypsum crystals further reduced the potential of 11



310

pore wetting of the slippery surface membrane as demonstrated by the delay of conductivity

311

increase in the permeate (Figure 4a). FIGURE 4

312 313

Desalination Performance with a Blowdown Wastewater. We conducted DCMD

314

experiments with a brine collected from the cooling tower at the Redhawk Power Station

315

(Phoenix, AZ) to evaluate the desalination performance of the pristine hydrophobic and modified

316

slippery surface membranes. Major composition and key properties of the prefiltered blowdown

317

water are given in the Supporting Information (Table S1). Redhawk’s blowdown has a neutral

318

pH (~7.1) and a high conductivity (23.9 mS/cm) attributed to the high concentrations of various

319

inorganic salts. In particular, relatively high levels of calcium (620 mg/L) and sulfate (6390

320

mg/L) were measured, implying that the blowdown water has a significant gypsum scaling

321

potential.

322

Flux decline curves expressed as normalized water flux and permeate conductivity as a

323

function of water recovery were obtained from DCMD experiments with the Redhawk’s

324

blowdown as the feed solution (Figure 5a). A rapid increase in permeate conductivity, which

325

indicates a failure of MD desalination capacity by pore wetting, was observed at ~ 40% of water

326

recovery for the hydrophobic surface membrane. At this point, the hydrophobic surface

327

membrane experienced ~ 85% reduction in water flux compared to the initial performance

328

(Figure 5a). In contrast, no substantial increase of permeate conductivity was observed for the

329

slippery surface membrane until water recovery reached ~ 60%. Note that we were not able to

330

continue DCMD experiments beyond this water recovery due to limitations of the experimental

331

DCMC bench-scale setup. Nonetheless, the results demonstrate that membranes with an

332

engineered slippery surface increase water recovery significantly compared to the conventional

333

hydrophobic surface membrane in treatment of saline power plant blowdown with high-scaling

334

potential.

335

FIGURE 5

336

To explain why the slippery surface membrane outperformed the hydrophobic surface

337

membrane, SEM imaging (Figure S3) and SEM-EDS elemental mapping (Figure 5b) were

338

conducted for the cross-section and top surface of the membranes after long-term scaling 12


Page 12 of 24

Page 13 of 24


339

experiments with the blowdown wastewater. Only representative crystal elements expected to

340

form on the membrane were examined. We also note that the green signal represents fluorine (F),

341

which is the main elemental composition of the PVDF substrate. The SEM-EDS analysis showed

342

high intensity of Na and Ca on the surface of the hydrophobic membrane (Figure 5b-2) possibly

343

due to accumulation of Na2SO4 and CaSO4 crystals. We also detected a wide distribution of Na

344

inside the pores of the hydrophobic surface membrane, which suggests membrane pore wetting.

345

In contrast, the slippery surface membrane showed less crystal accumulation on the surface

346

(Figure 5b-4) and very little presence of Na and Ca inside the pores (Figure 5b-3) compared to

347

the hydrophobic surface membrane. This observation clearly demonstrates that the slippery

348

surface membrane provides a higher resistance to scaling than the hydrophobic surface

349

membrane, thereby allowing less crystal deposition on the surface as well as inside the pores

350

and, consequently, a lower potential for pore wetting.

351

Mechanism for Reduced Scaling with Slippery Surface. Classical nucleation theory

352

(CNT) explains the influence of surface properties on heterogeneous surface nucleation of

353

gypsum crystal based on the thermodynamic expression that describes the Gibbs free energy

354

required for the formation of spherical particles. 46 The Gibbs free energy, ∆G, of the forming

355

phase, assuming spherical nuclei of radius r and molecular volume υ, was calculated by an

356

energy balance between the sum of surface excess Gibbs free energy (positive) and volume

357

* excess Gibbs free energy (negative).47 The maximum Gibbs free energy, Ghomogeneous ,

358

corresponding to the energy barrier that must be overcome by the nuclei to form a crystal in the

359

bulk solution (also referred to as homogeneous nucleation) can be calculated using 48  G h*o m o geneo us 

  3 2 ( kT )

2

1 ln 2 S

(1)

360

where β is the surface geometrical factor (16π/3 for spheres), γ is the precipitate surface energy

361

(here CaSO4), υ is the molecular volume of the CaSO4 crystallizing phase, k is the Boltzmann

362

constant, T is the temperature, and S is the supersaturation index.49 The potential of

363

* by incorporating membrane heterogeneous surface nucleation can be deduced from Ghomogeneous

364

properties, including surface hydrophobicity and porosity, using the following equation: 50

13



G

* heterogeneous

 G

* homogeneous

2  1  cos θ    2 1  2  4  2  cos θ 1  cos θ   1      1  cos θ  

Page 14 of 24

3

(2)

365

where θ is the apparent contact angle between the solution and membrane and ε is the membrane

366

surface porosity.

367

Figure 6a presents the calculated maximum Gibbs free energy of heterogeneous nucleation,

368

* Gheterogeneous , indicating that a membrane with higher hydrophobicity and lower surface porosity

369

provides a higher energetic barrier, which translates to lower scaling potential. We previously

370

observed a relatively clean surface with no discernible gypsum crystals on the slippery surface

371

membrane while the hydrophobic surface membrane showed sparse growth of rosette-like

372

embryo crystals at the earlier stage of scaling experiments (Figures 4b-1 and 4b-3). Provided that

373

reduction of the membrane surface porosity after a thin layer coating with relatively small SiNPs

374

(~ 8 nm) is insignificant, the obtained lower heterogeneous scaling potential of the slippery

375

surface membrane compared to the hydrophobic surface membrane is attributed to its higher

376

surface hydrophobicity (or lower surface energy shown in Figure 2a), which develops a larger

377

energetic barrier for surface gypsum crystallization.

378

FIGURE 6

379

Given the use of supersaturated gypsum feed solution (SI = 1.1) during DCMD experiments,

380

deposition of scalant crystals on the membrane surface (bulk deposition) is expected to be an

381

important mechanism governing MD membrane scaling behavior.51 Previous studies claim that a

382

rapid flux decline with concomitant increase in permeate conductivity during MD scaling

383

experiments are mainly attributed to bulk crystal deposition on the membrane surface followed

384

by pore wetting.52 Figure 6b illustrates the impact of surface wettability on the contact between

385

the gypsum crystals and the membrane surface. MD membranes with an engineered slippery

386

surface make minimal contact with the scaling solution due to their high surface hydrophobicity,

387

and thus allowing limited gypsum crystal deposition on the surface. The non-stick, slippery

388

surface nature (Figure 2b-3) further reduces the chance of crystal accumulation on these

389

membranes. In contrast, a moderately hydrophilic surface (i.e., water contact angle, θ, ~ 70º) and

390

a conventional hydrophobic surface (i.e., θ ~ 120º) form much more intimate contact with the

391

scaling solution compared to the slippery surface (i.e., water contact angle, θ is ~ 150º) (Figure 14


Page 15 of 24


392

6b), resulting in an increased probability of gypsum crystals deposition or heterogeneous

393

nucleation on the membrane surface.

394

In summary, we demonstrated the fabrication of MD membranes with an engineered slippery

395

surface that provides improved resistance to scaling. The modified slippery surface membrane

396

exhibited a stable MD performance with high water recovery of a supersaturated gypsum scaling

397

solution as well as brine from a power plant blowdown. Based on the results obtained, we

398

discussed the mechanisms responsible for the reduced scaling potential of the slippery surface

399

for further improvement of anti-scaling properties. MD membranes that are highly resistant to

400

scaling will be a critical component of a high recovery MD system for sustainable treatment of

401

hypersaline industrial wastewaters with minimal production of waste (brine) streams.

402

ASSOCIATED CONTENT

403

The Supporting Information is available free of charge on the ACS Publication website at DOI:

404

Composition and properties of power plant blowdown wastewater after pretreatment through an

405

11-μm filter (Table S1); Mechanical stability and SEM imaging of the PVDF and NaOH-treated

406

PVDF membrane (Figure S1); XPS survey spectra of the hydrophilic, hydrophobic, and slippery

407

surface membranes (Figure S2); Cross-section and top view SEM images of scaled membranes

408

taken after an experiment with an industrial blowdown wastewater (Figure S3);

409

ACKNOWLEGMENTS

410

This research was made possible by the postdoctoral fellowship (to Vasiliki Karanikola)

411

provided from the Agnese Nelms Haury Program in Environment and Social Justice and the

412

University of Arizona. We acknowledge the National Science Foundation (NSF) Graduate

413

Research Fellowship Program (GRFP) for the support to Julianne Rolf (Fellowship 2016227750).

414

Facilities used were supported by the Yale Institute of Nanoscale and Quantum Engineering

415

(YINQE). The author also thanks the assistance of Dr. Min Li (Yale West Campus Materials

416

Characterization Core) with the XPS measurements and SEM imaging. The characterization

417

facilities were supported by the Yale Institute for Yale West Campus Materials Characterization

418

Core (MCC).

419 15



420

REFERENCES

421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461 462 463 464 465 466

1. Averyt, K.; Fisher, J.; Huber-Lee, A.; Lewis, A.; Macknick, J.; Madden, N.; Rogers, J.; Tellinghuisen, S. Freshwater Use by U.S. Power Plants: Electricity's Thirst for a Precious Resource; Cambridge, MA: Union of Concerned Scientists, 2011; pp 1-62. 2. Maupin, M. A.; Kenny, J. F.; Hutson, S. S.; Lovelace, J. K.; Barber, N. L.; Linsey, K. S. Estimated Use of Water in the United States in 2010; U.S. Geological Survey: November 5, 2014, pp 1-64. 3. Mekonnen, M. M.; Hoekstra, A. Y., Four billion people facing severe water scarcity. Science advances 2016, 2, (2), e1500323. 4. Postponement of Certain Compliance Dates for the Effluent Limitations Guidelines and Standards for the Steam Electric Power Generating Point Source Category. In 179 ed.; (EPA), E. P. A., Ed. Federal Register: September 18, 2017, 2017; Vol. 82, pp 43494-43500. 5. Koeman-Stein, N.; Creusen, R.; Zijlstra, M.; Groot, C.; van den Broek, W., Membrane distillation of industrial cooling tower blowdown water. Water Resources and Industry 2016, 14, 11-17. 6. Yu, X.; Yang, H.; Lei, H.; Shapiro, A., Experimental evaluation on concentrating cooling tower blowdown water by direct contact membrane distillation. Desalination 2013, 323, 134-141. 7. Khayet, M.; Matsuura, T., Membrane Distillation: Principles and applications. Elsevier: Oxford, 2011. 8. Lawson, K. W.; Lloyd, D. R., Membrane distillation. Journal of membrane Science 1997, 124, (1), 1-25. 9. Boo, C.; Elimelech, M., Carbon nanotubes keep up the heat. Nat Nanotechnol 2017, 12, 501. 10. Deshmukh, A.; Boo, C.; Karanikola, V.; Lin, S.; Straub, A. P.; Tong, T.; Warsinger, D. M.; Elimelech, M., Membrane distillation at the water-energy nexus: limits, opportunities, and challenges. Energy & Environmental Science 2018, 11, (5), 1177-1196. 11. Xie, M.; Shon, H. K.; Gray, S. R.; Elimelech, M., Membrane-based processes for wastewater nutrient recovery: Technology, challenges, and future direction. Water Research 2016, 89, 210-221. 12. Kim, J.; Kwon, H.; Lee, S.; Lee, S.; Hong, S., Membrane distillation (MD) integrated with crystallization (MDC) for shale gas produced water (SGPW) treatment. Desalination 2017, 403, 172-178. 13. Davenport, D. M.; Deshmukh, A.; Werber, J. R.; Elimelech, M., High-Pressure Reverse Osmosis for Energy-Efficient Hypersaline Brine Desalination: Current Status, Design Considerations, and Research Needs. Environmental Science & Technology Letters 2018, 5, (8), 467-475. 14. Kim, J.; Kim, J.; Hong, S., Recovery of water and minerals from shale gas produced water by membrane distillation crystallization. Water Research 2018, 129, 447-459. 15. Shaffer, D. L.; Tousley, M. E.; Elimelech, M., Influence of polyamide membrane surface chemistry on gypsum scaling behavior. Journal of Membrane Science 2017, 525, 249-256. 16. Tijing, L. D.; Woo, Y. C.; Choi, J.-S.; Lee, S.; Kim, S.-H.; Shon, H. K., Fouling and its control in membrane distillation—A review. Journal of Membrane Science 2015, 475, 215-244. 17. Tong, T.; Zhao, S.; Boo, C.; Hashmi, S. M.; Elimelech, M., Relating Silica Scaling in Reverse Osmosis to Membrane Surface Properties. Environmental Science & Technology 2017, 51, (8), 43964406. 18. Guillen-Burrieza, E.; Thomas, R.; Mansoor, B.; Johnson, D.; Hilal, N.; Arafat, H., Effect of dryout on the fouling of PVDF and PTFE membranes under conditions simulating intermittent seawater membrane distillation (SWMD). Journal of membrane science 2013, 438, 126-139. 19. Gryta, M., Fouling in direct contact membrane distillation process. Journal of membrane science 2008, 325, (1), 383-394. 20. Boo, C.; Lee, J.; Elimelech, M., Omniphobic polyvinylidene fluoride (PVDF) membrane for desalination of shale gas produced water by membrane distillation. Environmental science & technology 2016, 50, (22), 12275-12282.

16


Page 16 of 24

Page 17 of 24

467 468 469 470 471 472 473 474 475 476 477 478 479 480 481 482 483 484 485 486 487 488 489 490 491 492 493 494 495 496 497 498 499 500 501 502 503 504 505 506 507 508 509 510 511 512 513 514


21. Wang, Z.; Elimelech, M.; Lin, S., Environmental Applications of Interfacial Materials with Special Wettability. Environmental Science & Technology 2016, 50, (5), 2132-2150. 22. Ma, M.; Hill, R. M., Superhydrophobic surfaces. Current Opinion in Colloid & Interface Science 2006, 11, (4), 193-202. 23. Nejati, S.; Boo, C.; Osuji, C. O.; Elimelech, M., Engineering flat sheet microporous PVDF films for membrane distillation. Journal of Membrane Science 2015, 492, 355-363. 24. Khayet, M.; Matsuura, T., Preparation and Characterization of Polyvinylidene Fluoride Membranes for Membrane Distillation. Industrial & Engineering Chemistry Research 2001, 40, (24), 5710-5718. 25. Woo, Y. C.; Kim, Y.; Shim, W.-G.; Tijing, L. D.; Yao, M.; Nghiem, L. D.; Choi, J.-S.; Kim, S.H.; Shon, H. K., Graphene/PVDF flat-sheet membrane for the treatment of RO brine from coal seam gas produced water by air gap membrane distillation. Journal of Membrane Science 2016, 513, 74-84. 26. Mavukkandy, M. O.; Bilad, M. R.; Kujawa, J.; Al-Gharabli, S.; Arafat, H. A., On the effect of fumed silica particles on the structure, properties and application of PVDF membranes. Separation and Purification Technology 2017, 187, 365-373. 27. Li, X.; García-Payo, M.; Khayet, M.; Wang, M.; Wang, X., Superhydrophobic polysulfone/polydimethylsiloxane electrospun nanofibrous membranes for water desalination by direct contact membrane distillation. Journal of Membrane Science 2017, 542, 308-319. 28. Liu, K.; Tian, Y.; Jiang, L., Bio-inspired superoleophobic and smart materials: design, fabrication, and application. Progress in Materials Science 2013, 58, (4), 503-564. 29. Gryta, M.; Karakulski, K., The application of membrane distillation for the concentration of oilwater emulsions. Desalination 1999, 121, (1), 23-29. 30. Lee, Y.; Park, S. H.; Kim, K. B.; Lee, J. K., Fabrication of hierarchical structures on a polymer surface to mimic natural superhydrophobic surfaces. Advanced Materials 2007, 19, (17), 2330-2335. 31. Nosonovsky, M.; Bhushan, B., Biomimetic superhydrophobic surfaces: multiscale approach. Nano letters 2007, 7, (9), 2633-2637. 32. Wong, T.-S.; Kang, S. H.; Tang, S. K. Y.; Smythe, E. J.; Hatton, B. D.; Grinthal, A.; Aizenberg, J., Bioinspired self-repairing slippery surfaces with pressure-stable omniphobicity. Nature 2011, 477, 443. 33. Lin, S.; Nejati, S.; Boo, C.; Hu, Y.; Osuji, C. O.; Elimelech, M., Omniphobic membrane for robust membrane distillation. Environmental Science & Technology Letters 2014, 1, (11), 443-447. 34. Shillingford, C.; MacCallum, N.; Wong, T.-S.; Kim, P.; Aizenberg, J., Fabrics coated with lubricated nanostructures display robust omniphobicity. Nanotechnology 2013, 25, (1), 014019. 35. Dalvi, V. H.; Rossky, P. J., Molecular origins of fluorocarbon hydrophobicity. Proceedings of the National Academy of Sciences 2010, 107, (31), 13603-13607. 36. Sunny, S.; Vogel, N.; Howell, C.; Vu, T. L.; Aizenberg, J., Lubricant-Infused Nanoparticulate Coatings Assembled by Layer-by-Layer Deposition. Advanced Functional Materials 2014, 24, (42), 6658-6667. 37. Ross, G.; Watts, J.; Hill, M.; Morrissey, P., Surface modification of poly (vinylidene fluoride) by alkaline treatment1. The degradation mechanism. Polymer 2000, 41, (5), 1685-1696. 38. Shaulsky, E.; Nejati, S.; Boo, C.; Perreault, F.; Osuji, C. O.; Elimelech, M., Post-fabrication modification of electrospun nanofiber mats with polymer coating for membrane distillation applications. Journal of Membrane Science 2017, 530, 158-165. 39. Warsinger, D. M.; Swaminathan, J.; Guillen-Burrieza, E.; Arafat, H. A.; Lienhard V, J. H., Scaling and fouling in membrane distillation for desalination applications: A review. Desalination 2015, 356, 294-313. 40. Seidel, A.; Elimelech, M., Coupling between chemical and physical interactions in natural organic matter (NOM) fouling of nanofiltration membranes: implications for fouling control. Journal of Membrane Science 2002, 203, (1), 245-255.

17



515 516 517 518 519 520 521 522 523 524 525 526 527 528 529 530 531 532 533 534 535 536 537 538 539 540 541 542 543

41. 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. 42. Chen, L.; Thérien-Aubin, H.; Wong, M. C. Y.; Hoek, E. M. V.; Ober, C. K., Improved antifouling properties of polymer membranes using a ‘layer-by-layer’ mediated method. Journal of Materials Chemistry B 2013, 1, (41), 5651-5658. 43. Lee, S.; Lee, C.-H., Effect of operating conditions on CaSO4 scale formation mechanism in nanofiltration for water softening. Water Research 2000, 34, (15), 3854-3866. 44. Jawor, A.; Hoek, E. M. V., Effects of feed water temperature on inorganic fouling of brackish water RO membranes. Desalination 2009, 235, (1), 44-57. 45. Benecke, J.; Haas, M.; Baur, F.; Ernst, M., Investigating the development and reproducibility of heterogeneous gypsum scaling on reverse osmosis membranes using real-time membrane surface imaging. Desalination 2018, 428, 161-171. 46. Volmer, M.; Weber, Α., Keimbildung in übersättigten Gebilden. Zeitschrift für physikalische Chemie 1926, 119, (1), 277-301. 47. Clouet, E., Modeling of nucleation processes. arXiv preprint arXiv:1001.4131 2010. 48. Becker, R.; Döring, W., Kinetische behandlung der keimbildung in übersättigten dämpfen. Annalen der Physik 1935, 416, (8), 719-752. 49. Tröger, J.; Lunkwitz, K.; Bürger, W., Determination of the surface tension of microporous membranes using contact angle measurements. Journal of colloid and interface science 1997, 194, (2), 281-286. 50. Curcio, E.; Fontananova, E.; Di Profio, G.; Drioli, E., Influence of the structural properties of poly (vinylidene fluoride) membranes on the heterogeneous nucleation rate of protein crystals. The Journal of Physical Chemistry B 2006, 110, (25), 12438-12445. 51. Di Profio, G.; Curcio, E.; Drioli, E., Supersaturation Control and Heterogeneous Nucleation in Membrane Crystallizers: Facts and Perspectives. Industrial & Engineering Chemistry Research 2010, 49, (23), 11878-11889. 52. Gryta, M., Calcium sulphate scaling in membrane distillation process. Chemical Papers 2009, 63, (2), 146-151.

544 545

18


Page 18 of 24

Page 19 of 24


(b)

(c)

(a) F

F

NH3+ NH3+ NH3+ NH3+

OH OH OH OH OH

OH OH OH OH OH

PVDF

PVDF

(i) Alkaline Treatment

(ii) APTES Grafting

Si O

Si O O Si O O

Si O

PVDF

(iii) SiNP Coating

PVDF

(vi) Slippery, NonAdhesive Surface

546 547 548 549 550 551 552 553 554 555 556 557 558

F FF FF FF FF F FF FF FF

F F F F F F F F Si

O

O

F

Si

F F F F FF F F F F F F F F

F F

O

F F

Membrane Cross-Section

F F F F

O

(d)

O

Tape

30 m

(e)

Hydrophobic Hydrophilic Slippery CF2CH2 CH2 CF2CF2

PVDF

(iv) 17 - FAS Coating

Si F

0.5 m

Si

CPS (a.u.)

SiNP

F F

CF3

(v) 17 – FAS Coating 295

290

285

280

Binding Energy (ev)

Figure 1. (a) Schematic illustrating the protocol of engineering a slippery surface of PVDF membrane. (i) The surface of a hydrophobic PVDF membrane is hydrolyzed by alkaline treatment with a 7.5 M NaOH solution. (ii) (3-Aminopropyl)triethoxysilane (APTES) is grafted on the hydrolyzed PVDF membrane surface. (iii) Negatively charged silica nanoparticles (SiNPs) bind electrostatically on the APTES functionalized PVDF membrane. (iv-v) SiNPs are covalently bound with (heptadecafluorotetrahydrodecyl)trichlorosilane (17–FAS) via vapor-phase silanization, creating (vi) a non-adhesive, slippery surface. (b) SEM top-down image of the SiNP-coated PVDF membrane. (c) SEM-EDS cross-section image of the SiNP-coated PVDF membrane, revealing a thin-layer SiNP surface coating. (d) XPS analysis of the hydrophobic PVDF membrane, SiNP-coated PVDF membrane (denoted as hydrophilic membrane), and modified slippery PVDF membrane. (e) Photograph of water droplet on the modified slippery PVDF membrane.

559 560

19



0.05 s

Page 20 of 24

0.15 s

0.25 s

Water Contact Angle ()

(b-1) Hydrophobic

200

(a)

150

(b-2) Hydrophilic

100 50 0

(b-3) Slippery Hydrophobic Hydrophilic

Slippery

561 562 563 564 565

Figure 2. (a) Static contact angle of hydrophobic, hydrophilic, and slippery surface PVDF membranes measured with a 5-μL DI water droplet placed on the sample surface. (b) Dynamic surface wettability of the membranes evaluated by dispensing a 10-L DI water droplet from a syringe at a 4-cm height and recording the motion of the water droplet with time.

566

20


Page 21 of 24


Permeate Normalized Conductivity (S) Water Flux (J /J0)

(a) Low Cross-Flow Velocity 1.0 0.8 0.6

Hydrophobic Hydrophilic Slippery

0.4 0.2 0.0 100 80 60 40 20 0 0

567

50

100

150

200

250

300

350

Cumulative Permeate Volume (mL)

Normalized Permeate Conductivity (S) Water Flux (J /J0)

(b) High Cross-Flow Velocity 1.0 0.8 0.6

Hydrophobic Hydrophilic Slippery

0.4 0.2 0.0 100 80 60 40 20 0 0

568 569 570 571 572 573 574 575 576 577

50

100

150

200

250

300

350


Figure 3. Normalized water flux and permeate conductivity of the hydrophobic, hydrophilic, and slippery surface PVDF membranes obtained during MD scaling experiments at (a) low feed cross-flow velocity (8.5 cm/s) and (b) high feed cross-flow velocity (17 cm/s). The feed solution contained 20 mM CaCl2 and 20 mM Na2SO4, with a gypsum (CaSO4·2H2O) saturation index (SI) of 1.1, pH 7.08, and temperature of 60 °C. An initial volume of 1 L was employed for both feed and permeate solutions. Temperature of permeate solution was maintained at 20 °C. The average initial water flux was 18.0, 18.5, and 14.0 L m−2 h−1 at a low feed cross-flow velocity and 21.0, 22.6, and 15.5 L m−2 h−1 at a high feed cross-flow velocity for the hydrophobic, hydrophilic, and the slippery surface membranes, respectively.

578

21



(b-1) Hydrophobic at 150 mL (b-2) Hydrophobic ‒ Long-term

Permeate Normalized Conductivity (S) Water Flux (J /J0)

(a) Low Cross-Flow Velocity

50 m

0.8

(b-3)

0.6 0.4 0.2 0.0

580 581 582 583 584 585 586 587 588 589

500 m

(b-4)

(b-2)

(b-3) Slippery at 150 mL

Hydrophobic - Long-term Hydrophobic - Early Terminated Slippery - Long-term Slippery - Early Terminated

100 80 60

500 m

(b-4) Slippery ‒ Long-term High 50 m

40 20 0 0

579

High

(b-1)

1.0

Page 22 of 24

50

100

150

200

250

300

350

500 m

500 m


Figure 4. (a) Normalized water flux and permeate conductivity of hydrophobic and slippery surface PVDF membranes obtained during scaling experiments at low feed cross-flow velocity (8.5 cm/s). The feed solution contained 20 mM CaCl2 and 20 mM Na2SO4, with a gypsum (CaSO4·2H2O) saturation index (SI) of 1.1, pH 7.08, and temperature of 60 °C. The average initial water flux under these conditions was 18.0 and 14.0 L m −2 h−1 for the hydrophobic and the slippery surface membranes, respectively. Long-term scaling experiments were conducted until a significant decline in permeate flux was observed. Independent scaling experiments were conducted until the cumulative permeate volume reached 150 mL (indicated as ‘Early Terminated’). (b) SEM images of the membrane surface after long-term and early terminated gypsum scaling experiments.

590

22


Page 23 of 24


(b-1) Hydrophobic – Cross-Section (b-2) Hydrophobic – Top View

Permeate Normalized Conductivity (S) Water Flux (J / J0)

(a) Blowdown Wastewater 1.0 0.8 0.6 0.4

PVDF Membrane

0.2

Na

0.0 80

592 593 594 595 596 597 598 599

F

40 m

Na

Ca

F

100 m

(b-4) Slippery – Top View

60 40 20 0 0

591

Si

(b-3) Slippery – Cross-Section

Hydrophobic Slippery

100

Ca

10

20

30

40

50

Water Recovery (%)

60

PVDF Membrane Na

Ca

Si

F

40 m

Na

Ca

F

100 m

Figure 5. (a) Normalized water flux and permeate conductivity of the hydrophobic and slippery surface PVDF membranes as a function of water recovery obtained during DCMD experiments with an industrial blowdown wastewater. Feed cross-flow velocity of 8.5 cm/s and temperature of 60 °C were employed. The average initial water flux under these conditions was 18.0 and 10.7 L m−2 h−1 for the hydrophobic and the slippery surface membranes, respectively. (b) Images of SEM-EDS elemental mapping for cross-section and top surface of the scaled membranes after DCMD experiments with the blowdown wastewater. Chemical composition of the blowdown wastewater is presented in Table S1.

600 601 602 603 604 605 606 607 608 609 610 611 23



(a) Heterogeneous Nucleation

Contact Angle ()

160

Gibbs Free Energy (G, mJ mol-1)

150 140 130 120 110 100

0.2

0.4

0.6

0.8

1.0

35 31 28 25 21 18 14 11 7 4 0

Porosity 612 613 614 615 616 617

Figure 6. (a) Gibbs free energy of gypsum crystals (CaSO 4·2H2O) as a function of surface porosity and contact angle which corresponds to the energy that the gypsum crystal must overcome to start forming heterogeneous crystallization on a surface (calculated based on classical nucleation theory). (b) A diagram illustrating the effect of surface wettability on gypsum crystal deposition on the MD membrane. θ1, θ2, and θ3 indicate the static water contact angles of the hydrophilic, hydrophobic, and slippery surface membranes measured in Figure 2a.

618 619 620 621 622 623 624 625 626 627 628 629

24


Page 24 of 24

Engineered Slippery Surface to Mitigate Gypsum Scaling in

Recommend Documents