Reactive, Self-Cleaning Ultrafiltration Membrane Functionalized with

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Remediation and Control Technologies

Reactive, Self-cleaning Ultrafiltration Membrane Functionalized with Iron Oxychloride (FeOCl) Nanocatalysts Meng Sun, Ines Zucker, Douglas M. Davenport, Xuechen Zhou, Jiuhui Qu, and Menachem Elimelech Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b01916 • Publication Date (Web): 13 Jul 2018 Downloaded from http://pubs.acs.org on July 14, 2018

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Environmental Science & Technology

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Reactive, Self-cleaning Ultrafiltration Membrane Functionalized with Iron Oxychloride (FeOCl) Nanocatalysts

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Revised: July 11, 2018

12 13 14 15

Meng Sun a, Ines Zucker a, Douglas M. Davenport a, Xuechen Zhou a, Jiuhui Qu b,*, Menachem Elimelech a,*

16 17 18 19 20

a

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

School of Environment, Tsinghua University, Beijing 100084, China.

21 22 23 24

Corresponding author

25 26

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

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1 ACS Paragon Plus Environment


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ABSTRACT

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Self-cleaning, antifouling ultrafiltration membranes are critically needed to mitigate organic

30

fouling in water and wastewater treatment. In this study, we fabricated a novel polyvinylidene

31

fluoride (PVDF) composite ultrafiltration membrane coated with FeOCl nanocatalysts

32

(FeOCl/PVDF) via a facile, scalable thermal-treatment method, for the synergetic separation and

33

degradation of organic pollutants. The structure, composition, and morphology of the

34

FeOCl/PVDF membrane were extensively characterized. Results showed that the as-prepared

35

FeOCl/PVDF membrane was uniformly covered with FeOCl nanoparticles with an average

36

diameter of 1-5 nm, which greatly enhanced membrane hydrophilicity. The catalytic self-

37

cleaning and antifouling properties of the FeOCl/PVDF membrane were evaluated in the

38

presence of H2O2 at neutral pH. Using a facile H2O2 cleaning process, we showed that the

39

FeOCl/PVDF membrane can achieve an excellent water flux recovery rate of ~ 100%, following

40

organic fouling with a model organic foulant (bovine serum albumin). Moreover, the in situ

41

catalytic production of active hydroxyl radicals by the FeOCl/PVDF membrane was elucidated

42

by electron spin resonance (ESR) and UV analysis. The catalytic performance of the

43

FeOCl/PVDF membrane was further demonstrated by the complete degradation of bisphenol A

44

when H2O2 was dosed in the feed solution at neutral pH. Our results demonstrate the promise of

45

utilizing this novel membrane for the treatment of waters with complex organic pollutants.

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TOC Art

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INTRODUCTION

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Low-cost, highly efficient membrane-based technologies have become increasingly widespread

54

for water and wastewater treatment applications.1 Ultrafiltration (UF) is one of the most widely

55

implemented membrane technologies owing to its ability to remove large molecular weight

56

species at low applied pressures. However, the exposure of membranes to a broad range of

57

foulants, including natural organic matter (NOM) and colloidal materials, causes a substantial

58

decrease in water permeability and requires frequent hydraulic cleaning. Therefore, the

59

development of effective strategies for fouling control in UF is of paramount importance.

60

Surface modification of UF membranes is a promising approach to improve fouling

61

resistance. Previous studies have demonstrated a wide variety of surface-modification strategies

62

to mitigate fouling, including grafting anti-adhesive polymeric brushes,2-5 introducing

63

amphiphilic or hydrophilic functional groups,6,

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nanomaterials.8-10 Among the antifouling surfaces suggested in the literature, the incorporation of

65

reactive nanomaterials has been shown to improve antifouling and self-cleaning properties

66

through in-situ generation of reactive oxygen species (ROS).1, 11

7

and binding contact-mediated antifouling

67

Membrane modification with photocatalysts, which generate ROS (e.g., O2•−, •OH, H2O2,

68

and •OOH−) when exposed to light,12-14 has the potential to oxidize pollutants on the membrane

69

surface, destroy recalcitrant foulant cake layers, and enhance self-cleaning properties.14, 15 This

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performance was demonstrated for TiO2 nanoparticles (NPs) which were incorporated onto

71

poly(aryl ether sulfone) UF membranes through a polymeric grafting method.16 Specifically, the

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photocatalytic TiO2 NPs enabled over 50% water flux recovery when fouled membranes were

73

exposed to a simulated solar light. In another study, a polyvinylidene fluoride (PVDF)/TiO2

74

composite membrane was synthesized via phase inversion and exhibited antimicrobial, oxidative,

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and antifouling properties.17 Similarly, a GO/TiO2-PVDF UF membrane fabricated via phase

76

inversion demonstrated both antifouling and self-cleaning properties under UV irradiation.18

77

Although the potential of photocatalytic membranes to control fouling has been demonstrated,

78

practical application of such membranes is greatly hindered by the need for extensive UV light

79

exposure, which is energy-intensive and requires novel module design to deliver light effectively.

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Alternative (non-photo) catalysts were also suggested for membrane modification, including

81

Fe2O3 NPs,19 single metal

20

or multi-metal alloy NPs,21 and metal–organic frameworks 4 ACS Paragon Plus Environment

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(MOFs).22 However, the direct blending, coating, and wrapping of NPs or MOFs with polymer

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matrices may result in particle aggregation, deactivation, and detachment.23-25 More importantly,

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the long-term stability of these modified membranes is challenged by harsh conditions required

85

for catalytic reactions, such as strong acidity, ultrasonication, or prolonged exposure to heat.

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Therefore, there is a growing need to modify UF membranes with state-of-the-art catalysts that

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can achieve effective self-cleaning performance under facile conditions.

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Iron oxychloride (FeOCl) is a novel nanocatalyst, decomposing hydrogen peroxide (H2O2)

89

to form non-selective hydroxyl radicals (•OH).26 The unique structural configuration of iron

90

atoms and the reducible electronic properties of FeOCl allows for efficient •OH generation in

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acidic solutions compared to other Fe-based heterogeneous catalysts. Further, the manipulation

92

of iron coordination environments in FeOCl via a facile annealing method can improve the

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performance of FeOCl by decreasing its pH sensitivity.27 Specifically, the combined chemical

94

state of iron (Fe3+ and Fe2+) and the ordered peripheral coordination of FeOCl impart an effective

95

iron redox cycle, which triggers a homolytic decomposition of iron-hydroperoxy complexes over

96

a broad pH range.28, 29 Therefore, FeOCl is a promising Fenton-based catalyst for antifouling

97

membrane modification due to its ability to perform well over a wide pH range and its excellent

98

regeneration properties.27, 30

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In this paper, we report the fabrication and performance evaluation of a novel self-cleaning

100

FeOCl/PVDF UF membrane. FeOCl nanocatalysts were synthesized via a facile and scalable

101

thermal-treatment method and used to coat a commercial PVDF membrane. The composition,

102

structure, and morphology of the FeOCl/PVDF membrane were comprehensively characterized.

103

The self-cleaning and antifouling performance of the composite membrane was compared to

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pristine PVDF membranes in the presence of H2O2 at neutral pH using bovine serum albumin

105

(BSA) as a model organic foulant. We also demonstrated the catalytic degradation of bisphenol

106

A (BPA) with the FeOCl/PVDF membrane. The findings of this study highlight the potential

107

application of FeOCl nanocatalysts for membrane-based treatment of contaminated waters.

108

MATERIALS AND METHODS

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Materials and Chemicals. Ferric chloride hexahydrate (FeCl3·6H2O, >98.0%), bisphenol A

110

(BPA, ≥99.0%), acetonitrile (CH3CN, 99.8%), potassium dihydrogen phosphate (KH2PO4,

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≥99.0%), dibasic potassium phosphate (K2HPO4, ≥98.0%), 5,5-Dimethyl-1-pyrroline N-oxide 5 ACS Paragon Plus Environment


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(DMPO, ≥97.0%), hydrogen peroxide (H2O2, 30%) and bovine serum albumin (BSA, molecule

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weight ~ 66 kDa, pH = 7, ≥99%) were purchased from Sigma-Aldrich. Hexane, methanol, and

114

isopropanol were purchased from J.T. Baker (Phillipsburg, NJ). Ethanol (100% absolute, USP-

115

grade) was purchased from Decon Labs (King of Prussia, PA). The Micro BCA™ protein assay

116

kit for BSA concentration analysis was purchased from Thermo Fisher Scientific Inc. All

117

chemicals were used as received. Deionized (DI) water was obtained from a Milli-Q ultrapure

118

water purification system (Millipore, Billerica, MA). A commercial polyvinylidene fluoride

119

(PVDF) ultrafiltration membrane (BX CF016) was purchased from Sterlitech and used as a

120

support for further fabrication of composite membranes. Prior to use, the PVDF supports were

121

wetted by immersion in 25% isopropanol for 30 min. To remove the isopropanol, supports were

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then soaked and washed twice for 1 h each in DI water.

123

Fabrication of FeOCl/PVDF Composite UF Membrane. Prior to fabrication,

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different concentrations of FeCl3 ethanol solutions (0.1 to 5 g L-1) were prepared by dissolving

125

FeCl3·6H2O powder in ethanol followed by sonication for 10 min. The preparation of the

126

FeOCl/PVDF composite ultrafiltration membranes is illustrated in Figure S1. Briefly, the pre-

127

washed PVDF membrane was placed at the center of a Petri dish and infiltrated by the as-

128

prepared 0.1 g L-1 FeCl3 ethanol solution dropwise until fully immersed. The infiltrated

129

membrane was then desiccated to produce the FeCl3/PVDF precursor membrane. The obtained

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precursor membrane was placed in a sealed crucible and subsequently heated in an oven at

131

220oC for 1 h. The final membrane (denoted 0.05% FeOCl/PVDF) was obtained by washing the

132

resulting membrane in 25% isopropanol solution to remove impurities and stored in DI water.

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FeOCl/PVDF composite membranes with varying FeOCl loadings were obtained following a

134

similar procedure (Figure S2).

135

Characterization and Analysis Methods. The crystalline composition of pristine and

136

composite membranes was characterized by thin film X-ray diffraction (XRD) (Rigaku, Smart

137

lab) between 5o and 80o with a scan step of 10o min−1 operating at 40 kV and 30 mA using Cu Ka

138

radiation. Scanning electron microscopy (SEM) (Hitachi SU-70 FE-SEM, Hitachi High

139

Technologies America, Inc.) was used to characterize the surface properties of pristine and

140

composite membranes. Energy-dispersive X-ray spectra (EDX) were obtained using an energy

141

dispersive spectroscopy analyzer (XFlash 5060FQ Annular EDX detector, Bruker, Germany)


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attached to the SEM. Before SEM imaging, the membrane surface was dried and sputter coated

143

with 8 nm of chromium to increase sample conductivity. Attenuated total reflectance-Fourier

144

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

145

spectrometer (Thermo Fisher, Waltham, MA) with 32 scans for each sample. Surface

146

hydrophilicity of the membranes was analyzed by measuring the water contact angle using the

147

sessile drop method with a contact angle goniometer (OneAttension, Biolin Scientific, NJ), with

148

a digital camera recording the shape of each liquid droplet tested. The static contact angle was

149

determined 10 seconds after dispensing 2.0 µL of the testing liquid (water) on the membrane

150

surface.

151

scanning XPS microprobe (PHI, VersaProbe II, Japan) using monochromatic Al Kα radiation

152

with a 0.47 eV system resolution. The energy scale has been calibrated using Cu 2p3/2 (932.67

153

eV) and Au 4f7/2 (84.00 eV) peaks on a clean copper plate and a clean gold foil.

X-ray

photoelectron

spectroscopy

(XPS)

data

were

obtained

with

a

154

The concentration of BPA was analyzed by an Agilent high performance liquid

155

chromatography (HPLC) coupled to a photodiode array detector (PDA; Agilent 1100). A

156

solution aliquot (100 µL) containing BPA was added to an HPLC vial with a glass insert.

157

Subsequently, 50 µL of each sample was injected into a C18 column at 20°C. The mobile phase

158

was an isocratic mixture with 45% phosphoric buffer (pH 2.3) and 55% acetonitrile. The flow

159

rate was 2 mL/min. BPA had a retention time of 2.54 min and was detected by UV absorption at

160

230 nm.

161

Analysis of •OH. Electron spin resonance (ESR) spectra were utilized to indirectly

162

determine the concentration of generated •OH using DMPO as a scavenger. The operating

163

parameters of the electron paramagnetic resonance spectrometer (ESP 300E, Bruker) were center

164

field, 3480.00 G; microwave frequency, 9.79 GHz; and power, 5.05 mW. The •OH yield from

165

catalytic oxidation was determined by the tert-butanol assay.31 Tertiary butanol was added in

166

excess (7412 mg L-1) to an 8-mL solution (pH 7) with 340 mg L-1 H2O2 in the presence of the

167

FeOCl membrane (3.14 cm2), to readily react with OH-radicals (6 × 108 M-1 s-1). The main

168

product formed by this reaction is formaldehyde, which was quantified by the Hantzsch

169

method.32 A reagent (2 M ammonium acetate, 0.05 M of acetic acid, and 0.02 M acetylacetone in

170

water) was mixed with a sample at a ratio of 1:1. The mixture was heated for 10 min at 50 °C,

171

and the change in color was measured spectrophotometrically at 412 nm. Finally, the •OH yield

172

can be calculated using 7 ACS Paragon Plus Environment


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=

174

where Fe is the absorbance of the sample, F0 is the absorbance of a blank (i.e., without FeOCl

175

membrane), ε is the formaldehyde extinction coefficient (8000 M-1 cm-1), D is the dilution factor

176

with reagent (i.e., 2), and

∙ ∙

(1)

is the correction factor of •OH to formaldehyde yields (~ 2).33

Analysis of Structural Properties. Membrane porosity (ϕ) was calculated using

177

178

% =

179

where Mw and Md are the weights of the wetted and dry membrane, respectively, A is the

180

membrane area, d is the average membrane thickness, and ρ is the density of water. The pore size

181

distributions of pristine and composite membranes were determined using SEM images. In brief,

182

SEM images of the membrane top surfaces were obtained at random locations at 6-50k

183

magnification. Images were collected and the radii of the membrane pores were measured for

184

each sample (Figure S3).

× 100

(2)

185

Ultrafiltration Performance Tests. The separation, self-cleaning, and anti-fouling

186

properties of the pristine PVDF and FeOCl/PVDF composite membranes were evaluated by

187

measuring the pure water flux (Jw), water flux in the presence of BSA organic foulant (Jp),

188

retention rate of organic foulant (R), total fouling ratio (Rt), flux recovery rate (Fr), and flux

189

growth rate (Fg). These experiments were performed with an effective membrane area (A) of 4.1

190

cm2 using a dead-end ultrafiltration cell (Model 8010, Millipore Sigma Corporation, USA), and

191

the corresponding parameters were collected and determined in the presence and absence of

192

solutions containing the model organic pollutants. Prior to filtration tests, membranes were first

193

compacted using DI water at a pressure of 0.7 bar (10 psi) for 360 min.

194

Membrane water flux was determined by measuring the volume of the permeate water

195

collected over time at an applied pressure of 0.7 bar. Pure water flux (Jw) and water flux in the

196

presence of BSA organic foulants (Jp) were calculated from

197 198 199

=

!

(3)

"

where V is the permeate volume, A is the active membrane area, and t is the interval time. The total fouling ratio (Rt) was calculated using 8 ACS Paragon Plus Environment

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$

%$200

#" % =

201

where Jw is the pure water flux prior to fouling and Jp is the water flux in the presence of BSA

202

foulant in the feed solution.

203

$

× 100

(4)

The retention rate of organic foulants (R) was obtained by using a Micro BCATM protein

204

assay kit to measure the BSA concentration in the permeate solution:

205

# % = 1 −

206

where Cp is the BSA concentration in permeate solution and C0 is the initial BSA concentration

207

(500 mg L-1) in the feed solution.

208

'% '

× 100

(5)

The flux recovery ratios were obtained after H2O2 (Fr,H) and hydraulic (physical) (Fr,w)

209

cleaning for 5 min and calculated using

210

(), % =

$,+

× 100

(6)

211

(),, % =

$,

× 100

(7)

212

where Jw,H is the pure water flux after H2O2 cleaning, Jw,w is the pure water flux after hydraulic

213

(physical) cleaning, and Jw is the pure water flux prior to ultrafiltration membrane fouling.

214

$

$

The flux growth rate was obtained after H2O2 dosing in the feed solution in the presence of

215

BSA foulants:

216

(- % = .

217

where Jp,H is the water flux after H2O2 dosing in the BSA-containing feed solution and Jp is the

218

water flux with the BSA foulant.

219

RESULTS AND DISCUSSION

220

Structural Properties of FeOCl/PVDF Composite Membranes. XRD measurements

221

were performed to characterize the crystalline structure of the reference, pristine, and composite

222

membranes. As shown in Figure 1A, the FeCl3/PVDF precursor membrane presented complex

223

diffraction peaks, which matched well with those of the standard FeCl3 particles and the PVDF

224

membrane, indicating a combined crystalline structure. The FeOCl/PVDF membrane displayed

225

strong XRD peaks at 10.7°, 26.0°, 35.5°, and 38.1° (noted with pink rhombus symbols). These

$%,+/ $%

%$0 × 100

(8)



226

peaks correspond to the standard diffraction of FeOCl particles (JCPDS No. 00-001-0081),

227

suggesting successful formation of FeOCl nanocatalysts. The mean grain size of the

228

nanocatalysts was calculated from the main peak at 10.7° (010) to be 5.12 nm based on the

229

Scherrer equation (details in Supporting Information). The observed extra diffraction peaks are

230

ascribed to the pristine PVDF polymer crystalline structure. No other interference patterns were

231

observed. Therefore, these results confirm that the FeOCl/PVDF membrane is composed of

232

highly crystalline FeOCl catalysts and the PVDF polymer substrate.

233

FIGURE 1

234

High resolution XPS of Fe 2p, O 1s, and Cl 2p of the FeOCl/PVDF membrane reveals the

235

chemical bonding state of the FeOCl coating (Figure 1B). The orbital of Fe 2p exhibited distinct

236

peaks at 710.4 and 724.2 eV, assigned to the 2p3/2 and 2p1/2 of Fe3+, as well as two corresponding

237

satellite peaks located at bonding energies of 714.2 and 726.7 eV, demonstrating the dominance

238

of ferric iron species in FeOCl. In addition, the binding energies of O 1s varied from 530.4 to

239

532.1 eV, divided into representative interactions of metal-oxygen (M-O) and metal-hydroxide

240

(M-OH), respectively. We can infer that the M-O bonding mainly resided in the inherent

241

molecular structure, while the M-OH may contribute to the surface hydrophilicity. In addition,

242

the results of Cl 2p indicate the presence of two kinds of metal-chloride (M-Cl) coordination

243

with slight differences in atomic orbitals on FeOCl, which is consistent with the two different

244

orientations between Fe and Cl in the crystalline structure (Figure S4).

245

FTIR spectroscopy was subsequently carried out to further identify the chemical groups and

246

functional structures of the pristine PVDF and FeOCl/PVDF composite membranes. As shown in

247

Figure 1C, the typical peaks located at 840, 877, 1033, 1178, 1280, and 1404 cm-1 correspond to

248

the β crystalline phase, C-C skeletal vibration, C-F stretching vibration, -CF2- stretching

249

vibration, another β crystalline phase, and -CH2- deformation vibration, respectively. Notably,

250

new peaks appeared on the composite membranes at 1070 and 1230 cm-1, indicating the surface

251

distribution of C-C backbone stretching vibration and -C-O- bonds derived from the hydrophilic

252

FeOCl nanoparticles, which is consistent with the observation from XPS.

253

SEM images shown in Figure 1D depict the morphology of the FeOCl/PVDF composite

254

membrane. The top surface image of the 0.5% FeOCl/PVDF membrane at low magnification

255

showed a rough surface morphology with randomly dispersed pores (diameters ranging from 10 ACS Paragon Plus Environment

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about 30 to 150 nm). Both the top surface and cross-section images at high magnifications

257

showed FeOCl nanoparticles, 1-5 nm in diameter, distributed both on the external and internal

258

membrane surface. The corresponding insets show the EDX analysis of selected areas of the low

259

magnification images, illustrating the elemental composition of FeOCl nanocatalysts on the

260

PVDF membrane. These observations coincide with those obtained by XRD and XPS. We note

261

that the unique morphology and size of the FeOCl observed here were quite different than

262

previous reports on FeOCl catalysts with slice- or plate-like layered structures. It is possible that

263

the precoating of FeCl3 on the membrane might be the decisive factor for achieving such

264

granular FeOCl nanoparticles.

265

The pore size distribution and porosity of the pristine and composite membranes were

266

evaluated as the loading of FeOCl nanocatalysts was increased. As shown in Table 1, the mean

267

pore size of the FeOCl/PVDF membranes was smaller than that of the pristine PVDF membrane,

268

showing a trend of a decreasing mean pore size with increasing FeOCl content. Analyzing the

269

surface pore size distribution also indicated that the proportion of large pores gradually

270

decreased as FeOCl increased. In addition, the porosities of the FeOCl/PVDF composite

271

membranes increased at low nanocatalyst loadings up to 0.25% FeOCl. This observation

272

indicates that a proper decoration of hydrophilic FeOCl nanocatalysts could create the

273

"accessible volume" (internal spaces available to water) of the membrane. However, increasing

274

FeOCl content from 0.25 to 2.5% resulted in a decrease of the effective internal volume

275

throughout the membrane, with porosity decreasing from 69.1 to 48.8%. This observation

276

suggests that excessive FeOCl nanocatalysts blocked some internal membrane pores.

277

TABLE 1

278

Hydrophilicity and Permeability of FeOCl/PVDF Composite Membranes.

279

Membrane hydrophilicity was evaluated by contact angle measurements for PVDF membranes

280

with increased loading of FeOCl nanocatalysts (Figure 2A). The water contact angle decreased

281

from 86.5° for the pristine membrane to 58.2° for the membrane with 2.5% FeOCl loading. This

282

increase in hydrophilicity is attributed to the increase in the surface density of hydrophilic

283

hydroxyl groups introduced by the FeOCl nanocatalysts, which was also confirmed by FTIR

284

analysis. Increasing membrane hydrophilicity is expected to reduce fouling caused by adsorption

285

of hydrophobic organic foulants on the membrane surface. 11 ACS Paragon Plus Environment


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FIGURE 2

286 287

To investigate membrane water permeability, the pure water flux of pristine PVDF and

288

FeOCl/PVDF composite membranes was measured using a dead-end ultrafiltration apparatus at

289

an applied pressure of 0.7 bar (10 psi) for 480 min (Figure 2B). Pure water flux through the 0.05%

290

FeOCl/PVDF composite membrane was slightly higher than the flux through the pristine PVDF

291

membrane, which can be attributed to the enhanced surface hydrophilicity and increased porosity

292

(Table 1) of the 0.05% FeOCl/PVDF membrane. However, a gradual decline in water

293

permeability was observed for composite membranes with increasing nanoparticle loading,

294

possibly due to the decline in porosity from 69.1 to 48.8% (Table 1). Optimizing the loading of

295

FeOCl on PVDF membranes is therefore important in order to achieve hydrophilicity and high

296

porosity which are beneficial for water transport, while avoiding loss of membrane water flux

297

due to pore blockage.

298

Separation Performance of FeOCl/PVDF Composite Membrane. Water flux of

299

the pristine and composite membranes was measured in the presence of 500 mg L-1 BSA at

300

neutral pH and an applied pressure of 0.7 bar. Water flux decreased dramatically due to the

301

deposition of BSA on the membrane surface (Figure 3A). At the end of BSA fouling runs, the

302

pristine PVDF membrane maintained the highest water flux as well as lowest fouling rate and

303

BSA retention rate (65%) compared to the composite membranes (Figure 3B and 3C).

304

Increasing the amount of FeOCl in the composite membranes resulted in reduced water flux,

305

increased BSA fouling rate, and a significant enhancement of BSA retention. For instance, while

306

the 2.5% FeOCl/PVDF membrane had the highest BSA retention rate (~ 91%) and fouling rate

307

(100%), it also exhibited the lowest water flux (~ 0 L m-2 h-1). This observation suggests that

308

decreased porosity, rather than increased hydrophilicity, dominates water permeation for the

309

composite membrane in the presence of organic foulants. In other words, although the

310

incorporation of hydrophilic FeOCl on the PVDF membrane surface improved membrane

311

hydrophilicity and BSA rejection, the decrease in membrane permeability should not be

312

overlooked.

313

FIGURE 3

314

Membrane Self-cleaning with H2O2. The self-cleaning performance of the pristine and

315

composite membranes was evaluated by calculating the water flux recovery ratios (Fr) according 12 ACS Paragon Plus Environment

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316

to eq 6 and 7. The recovered water flux of the BSA-fouled membrane is measured following

317

either hydraulic cleaning (immersing the membrane and stirring for 5 min in DI water) or H2O2

318

cleaning (in 340 mg L-1 H2O2 solution for 5 min). Water flux recovery rates of the pristine PVDF

319

membrane were similar for both hydraulic cleaning and H2O2 cleaning (Figure 4A). However,

320

FeOCl/PVDF membranes exhibited a significant increase in flux recovery rate following H2O2

321

cleaning compared to hydraulic cleaning. The increase in recovery rate was correlated with the

322

amount of FeOCl on the membrane surface, with the highest flux recovery of ~ 40% for

323

hydraulic cleaning and ~ 70% for H2O2 cleaning observed for the 2.5% FeOCl/PVDF composite

324

membrane. We note that although the 0.5% FeOCl/PVDF membrane exhibited severe flux

325

decline compared to the pristine membrane (Figure 3A), a 3-fold higher flux recovery was

326

obtained after H2O2 cleaning.

327

FIGURE 4

328

The unique self-cleaning property of the FeOCl/PVDF membrane is attributed to the

329

heterogeneous catalytic reaction, where hydroxyl radicals are effectively produced by FeOCl

330

nanocatalysts in the presence of H2O2 at neutral or low pH (Figure S5). The organic foulants

331

adsorbed on the membrane surface or trapped within the membrane pores were catalytically

332

degraded by the produced hydroxyl radicals, demonstrating an H2O2-assisted self-cleaning

333

performance of the composite membrane. More evidence for the effective H2O2 cleaning of the

334

FeOCl/PVDF composite membranes compared to the pristine membrane is shown in SEM

335

images (Figure 4B), where adsorbed BSA proteins on the top surface or within the interior pores

336

of the FeOCl/PVDF membrane were mostly degraded after an effective H2O2 cleaning.

337

The practicality of membrane cleaning was evaluated through multiple filtration cycles with

338

the 0.5% FeOCl/PVDF composite membrane as well as the pristine membrane as a control

339

(Figures 4C and 4D). Each cycle includes filtration of BSA solution (500 mg L-1) for 20 minutes

340

followed by H2O2 cleaning (340 mg L-1) for 10 minutes. A sharp drop in water flux is observed

341

for each cycle during BSA filtration where a fouling layer is developed, as shown earlier in

342

Figure 3A. Following H2O2 cleaning, the water flux of the 0.5% FeOCl/PVDF composite

343

membrane recovered considerably, nearly to the initial flux at the beginning of the cycle,

344

allowing high water flux at the beginning of the next filtration cycle (Figure 4C). Repeated

345

cycles showed a similar pattern of efficient water flux recovery after H2O2 cleaning. This 13 ACS Paragon Plus Environment


346

observation is very different from the PVDF membrane with H2O2 cleaning (Figure 4D), and the

347

pristine PVDF and FeOCl/PVDF membranes with traditional hydraulic cleaning (Figure S6),

348

where the membranes exhibit poor water flux recovery and only perform for a few filtration

349

cycles with a recoverable water flux. In addition, BSA retention rates and total organic carbon

350

(TOC) concentration in the permeate solution were measured for membranes treated with

351

different cleaning methods after each filtration cycle. The results demonstrate that the composite

352

membrane not only possessed good self-cleaning performance in terms of water flux recovery

353

but also maintained an acceptable BSA retention rate (Figures S7 and S8). The constant BSA

354

retention rate suggests that the catalytic H2O2 cleaning process did not alter the structure and

355

porosity of FeOCl/PVDF composite membranes. Overall, the FeOCl/PVDF membrane exhibits

356

excellent H2O2-driven self-cleaning performance to degrade organic foulants of high molecular

357

weight and facilitates desirable water flux recovery.

Page 14 of 30

358

Antifouling Performance with H2O2 Dosing. The continuous antifouling performance

359

of the pristine PVDF and 0.5% FeOCl/PVDF composite membrane was evaluated by comparing

360

the membrane flux in the presence and absence of H2O2 (340 mg L-1). Experiments were carried

361

out using a dead-end filtration cell with a feed solution containing 500 mg L-1 BSA at neutral pH

362

(Figure 5A). In the absence of H2O2, the normalized BSA-fouled water flux of pristine and

363

FeOCl/PVDF membranes dropped rapidly to about 0.21 and ~0 respectively. This observation

364

suggests that organic fouling on the composite membrane was more severe than that on the

365

PVDF membrane. However, in the presence of both BSA and H2O2 in the feed solution, the two

366

membranes exhibited quite different filtration behavior. The normalized flux of the PVDF

367

membrane increased from 0.21 to 0.30 when H2O2 was present in the feed solution. Remarkably,

368

a significant increase in water flux was shown for the FeOCl/PVDF composite membrane with

369

H2O2 dosing, with the flux being stable with time after the first 20 minutes. Additionally, in the

370

presence of H2O2, the corresponding BSA retention rates increased from 64 to 74% and 82 to 96%

371

for the PVDF and FeOCl/PVDF membranes, respectively (Figure 5B).

372

FIGURE 5

373

Figure 5C shows the flux growth rates for the pristine and composite membranes calculated

374

from eq 8. The results indicate that the PVDF membrane had only a 30% water flux recovery

375

with H2O2-containing feed solution, but the FeOCl/PVDF composite membrane exhibited a 14 ACS Paragon Plus Environment

Page 15 of 30


376

marked recovery rate, almost 20 times higher than that performed without H2O2. Based on the

377

self-cleaning and antifouling experimental findings, we surmise that for the PVDF membrane,

378

some reversible fouling can be removed to some extent in the presence of H2O2; however, the

379

irreversible fouling, which is strongly bound on the membrane surface and within the membrane

380

pores, is too robust to remove. In contrast, with H2O2 dosing in the feed solution, the strongly

381

bound foulants were disrupted and removed by the catalytic degradation of the FeOCl/PVDF

382

membrane.

383

Catalytic Removal of Organic Pollutants with H2O2 Dosing. The catalytic removal

384

of BPA, a model low-molecular weight organic pollutant, by PVDF and FeOCl/PVDF composite

385

membranes was investigated in the presence of H2O2 at neutral pH. Batch experiments were

386

carried out by submerging the membrane in a solution containing 1.0 mg L-1 BPA and H2O2

387

(Figure 6A). Only a slight decline of BPA concentration can be observed for the PVDF

388

membrane in the presence and absence of H2O2, as well as for the 0.5% FeOCl/PVDF

389

membranes without H2O2 dosing. In contrast, there is a sharp decrease in BPA concentration for

390

the 0.5% FeOCl/PVDF membrane with either 340 or 34 mg L-1 H2O2 dosing at neutral pH. These

391

results indicate that the composite membrane exhibited catalytic performance in removing BPA

392

only in the presence of H2O2. As the inset in Figure 6A depicts, BPA was decomposed into

393

identified intermediates (mainly low-molecular weight organic acids) with a high H2O2 dosing of

394

340 mg L-1 by the 0.5% FeOCl/PVDF membrane. Obviously, the composite membrane acted as

395

a “heterogeneous catalyst” and activated H2O2 to produce active radicals, which contributed to

396

the catalytic degradation of organic pollutants.34, 35

397

FIGURE 6

398

Figure 6B shows a quantitative and qualitative analysis of •OH radicals generated during

399

the batch experiments. There were no •OH radicals detected for the PVDF membrane in the

400

presence and absence of H2O2, and for the 0.5% FeOCl/PVDF membrane without H2O2 dosing.

401

In sharp contrast, the FeOCl/PVDF membrane exhibited significant generation of •OH radicals

402

which reached about 10 µM within 5 min in the presence of 340 mg L-1 H2O2. This observation

403

is in good agreement with the above results of BPA degradation, implying the role of active

404

radicals in the degradation of pollutants. Moreover, the inset in Figure 6B presents the ESR

405

signals of the •OH radicals trapped by DMPO (radical scavenger) for the 0.5% FeOCl/PVDF 15 ACS Paragon Plus Environment


406

membrane. The typical 1:2:2:1 fingerprints of the DMPO-HO adduct validated the existence of

407

•

408

mg L-1, indicating the catalytic performance of the FeOCl/PVDF composite membrane was

409

highly dependent on H2O2 dosing.

OH radicals.31, 36 Notably, the signals intensified with increasing H2O2 dosing from 34 to 340

410

The continuous catalytic degradation of BPA by the FeOCl/PVDF composite membrane

411

was evaluated via dead-end filtration with a continuous dosing of H2O2. Figure 6C shows the

412

change in water flux and BPA concentration in the permeate solution for the 0.5% FeOCl/PVDF

413

composite membrane with a feed solution containing 1 mg L-1 BPA at neutral pH. A sharp

414

decline of BPA concentration (from 1 to < 0.001 mg L-1) in the permeate solution was observed

415

when 34 mg L-1 H2O2 were added in the feed solution starting at 100 min. This observation

416

clearly indicates that the FeOCl/PVDF membrane enabled an H2O2-induced catalytic removal of

417

BPA during ultrafiltration. Compared with the significant variation of BPA, the membrane water

418

flux was relatively stable for the duration of the experiment, suggesting that the catalytic activity

419

of the composite membrane had no impact on its structural stability. Moreover, the relatively

420

high stability of the catalysts (Figure S9 and Table S1) also enabled the continuous catalytic

421

performance of the FeOCl/PVDF membrane.

422

Implications. The development of reactive membranes with superior self-cleaning

423

performance is of great significance for membrane-based water treatment in order to mitigate

424

organic fouling, increase treatment efficiency, and reduce operational costs. We demonstrate that

425

integrating FeOCl nanocatalysts within PVDF ultrafiltration membranes provides long-term

426

antifouling and self-cleaning properties due to in-situ generation of •OH radicals, particularly in

427

the presence of H2O2 at neutral pH. After organic fouling, water flux was effectively recovered

428

(~ 100% recovery rate) by a facile H2O2 cleaning process which was significantly more effective

429

than traditional hydraulic cleaning (~ 20% recovery rate). Further, the H2O2-driven ultrafiltration

430

process enabled the FeOCl/PVDF reactive membrane to effectively remove a typical low

431

molecular weight organic pollutant (BPA) from the feed solution. As such, the developed

432

catalytic ultrafiltration membrane offers great promise for the degradation of trace organic

433

contaminants which are typically not rejected by ultrafiltration (e.g., pharmaceuticals, personal

434

care products, pesticides, and antibiotics).

435

Overall, the FeOCl-based reactive membrane provides alternative fouling mitigation

436

methods by either intermittent or continuous dosing of H2O2. Moreover, this composite 16 ACS Paragon Plus Environment

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Page 17 of 30


437

membrane may offer greater applicability than other reactive membranes due to its ability to

438

form reactive radicals at environmental pH with minimal catalyst leaching. Therefore, the

439

feasibility of such reactive membranes to operate in large-scale systems under realistic

440

environmental conditions must be evaluated. For example, catalyst loading and hydrogen

441

peroxide dosing should be optimized for performance in complex water matrices (i.e., in the

442

presence of foulants, at typical pollutant concentrations, and in the presence of competing

443

compounds in the feed water). Other important factors, such as permeate quality and catalyst

444

regeneration, should also be considered to justify the sustainability and cost-effective application

445

of FeOCl-based membranes.

446

ASSOCIATED CONTENT

447

Schematic illustration for the preparation of the FeOCl/PVDF composite ultrafiltration

448

membrane (Figure S1); mass fraction of FeOCl on different FeOCl/PVDF composite membranes

449

(Figure S2); pore size distributions of the PVDF and FeOCl/PVDF composite membranes

450

(Figure S3); molecular structure of the orthorhombic FeOCl phase along the [010] zone axis

451

(Figure S4); effect of initial pH on membrane water flux recovery rate and BPA removal for the

452

0.5% FeOCl/PVDF membrane (Figure S5); filtration cycles of the PVDF and 0.5%

453

FeOCl/PVDF composite membranes with hydraulic and H2O2 cleaning (Figure S6); BSA

454

retention rates for each filtration cycle of the PVDF and 0.5% FeOCl/PVDF composite

455

membranes with hydraulic and H2O2 cleaning (Figure S7); TOC concentration of the permeate

456

solution for the PVDF and composite membranes for fouling and cleaning experiments (Figure

457

S8); leaching of iron and chloride ions from the composite membrane during batch BPA

458

degradation experiments (Figure S9); percentage of the iron and chloride leaching at different

459

solution pH (Table S1). This material is available free of charge via the Internet at

460

http://pubs.acs.org.

461

AUTHOR INFORMATION

462

Corresponding Author

463

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

464

*E-mail: [email protected]

465

ORCID 17 ACS Paragon Plus Environment


466

Menachem. Elimelech: 0000-0003-4186-1563

467

Jiuhui Qu: 0000-0001-9177-093X

468

Notes

469

The authors declare no competing financial interest

470

ACKNOWLEGMENT

471

We acknowledge the support received from the National Science Foundation (NSF) through the

472

Engineering Research Center for Nanotechnology-Enabled Water Treatment (EEC-1449500).

473

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

474

Engineering (YINQE) under NSF MRSEC DMR 1119826. We acknowledge the YIBS

475

Postdoctoral Fellowship and Tel Aviv University Presidential Postdoctoral Fellowship awarded

476

to I.Z. The characterization facilities were supported by the Yale Institute for Yale West Campus

477

Materials Characterization Core (MCC) and the Yale Institute of Nanoscale and Quantum

478

Engineering (YINQE). We also thank the assistance of Dr. Min Li (Yale West Campus MCC)

479

for the XPS measurement.

480


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REFERENCES

482 483 484 485 486 487 488 489 490 491 492 493 494 495 496 497 498 499 500 501 502 503 504 505 506 507 508 509 510 511 512 513 514 515 516 517 518 519 520 521 522 523 524 525

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(33) Flyunt, R.; Leitzke, A.; Mark, G.; Mvula, E.; Reisz, E.; Schick, R.; von Sonntag, C. Determination of •OH, O2•-, and hydroperoxide yields in ozone reactions in aqueous solution. J. Phys. Chem. B 2003, 107, (30), 7242-7253. (34) Hou, X.; Huang, X.; Jia, F.; Ai, Z.; Zhao, J.; Zhang, L. Hydroxylamine promoted goethite surface Fenton degradation of organic pollutants. Environ. Sci. Technol. 2017, 51 (9), 5118–5126. (35) Georgi, A.; Velasco Polo, M.; Crincoli, K.; Mackenzie, K.; Kopinke, F.-D. Accelerated catalytic Fenton reaction with traces of iron: an Fe–Pd-multicatalysis approach. Environ. Sci. Technol. 2016, 50, (11), 5882-5891. (36) Yuan, S.; Fan, Y.; Zhang, Y.; Tong, M.; Liao, P. Pd-catalytic in situ generation of H2O2 from H2 and O2 produced by water electrolysis for the efficient electro-fenton degradation of rhodamine B. Environ. Sci. Technol. 2011, 45, (19), 8514-8520.



Figure 1. (A) XRD patterns of FeOCl standard reference, FeOCl nanoparticles, FeCl3, PVDF, FeCl3/PVDF precursor, and FeOCl/PVDF composite membrane. (B) High resolution XPS of Fe 2p, O 1s, and Cl 2p of the FeOCl/PVDF composite membrane. (C) ATR-FTIR spectra of the PVDF and FeOCl/PVDF composite membranes. (D) SEM images depicting the top surface and cross-section of the 0.5% FeOCl/PVDF composite membranes in low and high magnification. Insets show corresponding EDX analysis of selected areas outlined in low magnification images.


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A

B

Figure 2. (A) Water contact angles of the PVDF and FeOCl/PVDF composite membranes measured by the sessile drop method. The contact angle was measured 10 s after the droplet (DI water, ~ 2 µL) was equilibrated with the membrane surface. Each measurement was conducted on five random locations of tested membrane. (B) Pure water flux of the PVDF and FeOCl/PVDF composite membranes conducted at a pressure of 0.7 bar (10 psi) after 480 min. Error bars represent standard deviations of two water flux measurements taken for each one of three different batch experiments.



Page 24 of 30

Normalized Water Flux

A PVDF 0.05% FeOCl/PVDF 0.25% FeOCl/PVDF 0.5% FeOCl/PVDF 1.25% FeOCl/PVDF 2.5% FeOCl/PVDF

1.0 0.8 0.6 0.4 0.2 0.0 0

10

20

30

40

50

Time (min) C

B

100

BSA Retention Rate (%)

Fouling Rate (%)

100 90 80 70 60 20 0

F DF DF D F DF DF PVD Cl/PV Cl/PV Cl/PV Cl/PV Cl/PV eO FeO FeO FeO FeO F 5% 0.25% 0.5% .25% 2.5% 0.0 1

90 80 70 60 20 0

F DF DF DF DF DF PVD Cl/PV Cl/PV Cl/PV Cl/PV Cl/PV eO %FeO FeO FeO FeO F 5% 0.25 0.5% 1.25% 2.5% 0.0

Figure 3. (A) Variation of water flux with time for PVDF and FeOCl/PVDF composite membranes at an applied pressure of 0.7 bar (10 psi). Fouling experiments were conducted with 500 mg L-1 BSA solution at neutral pH. Flux performance is expressed as normalized water flux, J/J0, where the initial water flux, J0, is determined for each membrane by averaging its pure water flux. (B) Fouling rates of the PVDF and different FeOCl/PVDF composite membranes after BSA fouling. The fouling rate is obtained by eq 3. Error bars represent standard deviations of two fouling rates from three different fouling experiments. The water flux for each fouled membrane is determined by averaging the permeate flux when the change in flux in 1 min is < 10 L m-2 h-1 (i.e., after 30 min). (C) Retention rates of BSA for the PVDF and different FeOCl/PVDF composite membranes. Error bars represent standard deviations of two BSA retention rates from three different permeate solution samples.


Page 25 of 30


B 80

Hydraulic Cleaning H2O2 Cleaning

60

40

20

0

F F F F DF DF VD VD VD VD PV PV Cl/P eOCl/P C l /P Cl/P eOCl/ O O O e e e F F F %F %F .25% 5% 5% 2.5 0.5 1 0 .2 0 .0

DI Water


C

5-Minute Membrane Cleaning

20-Minute Membrane Organic Fouling

1.0

1.0

0.8

0.8

0.6

0.6

0.4

0.4

0.2

0.2

0.0

0

10

30

55

80

105


Water Flux Recovery Rate (%)

A

0.0 150

130


D

1.0

1.0

0.8

0.8

0.6

0.6

0.4

0.4

0.2

0.2

0.0

0

10

30

55

80


Filtration Time (min)

0.0 105

Filtration Time (min)

Figure 4. (A) Water flux recovery rates of PVDF and FeOCl/PVDF composite membranes after hydraulic (physical) and H2O2 cleaning. The water flux of fouled membranes was determined after fouling with 500 mg L-1 BSA solution at an applied pressure of 0.7 bar (10 psi) for 20 min. Cleaning was performed by immersing the BSA-fouled membranes in DI water (hydraulic cleaning) or 340 mg L-1 H2O2 solution (H2O2 cleaning) with stirring for 5 min. Error bars represent standard deviations of two water flux recovery rates obtained for each one of three



different batch experiments. (B) SEM images depicting top-down and cross-section views of the PVDF and 0.5% FeOCl/PVDF composite membranes after one-time H2O2 cleaning for 5 min. (C, D) Filtration cycles with multiple water flux recoveries for (C) the 0.5% FeOCl/PVDF composite membrane and (D) PVDF membrane after 340 mg L-1 H2O2 cleaning. Each filtration cycle comprised membrane fouling with 500 mg L-1 BSA solution for 20 min followed by 340 mg L-1 H2O2 cleaning for 5 min.


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B FeOCl/PVDF with BSA and H2O2 PVDF with BSA and H2O2 PVDF with BSA FeOCl/PVDF with BSA

0.8 0.6 0.4 0.2 0.0 0

20

40

60

80

100

C 100

10000

Flux Recovery Rate (%)

Normalized Flux

1.0

BSA Retention Rate (%)

A

90 80 70 60 50 20 0

120

DF PV

V Cl/P FeO % 5 0.

Time (min)

DF

1000

100

10 PV

DF 0 .5

l/P eOC %F

VD

F

Figure 5. (A) Changes of normalized water flux with time for PVDF and 0.5% FeOCl/PVDF composite membranes with 500 mg L-1 BSA only, and with both 500 mg L-1 BSA and 340 mg L-1 H2O2 at an applied pressure of 0.7 bar (10 psi) at neutral pH. (B) BSA retention rates of PVDF and 0.5% FeOCl/PVDF composite membranes in the presence of BSA only (green bar), and both BSA and H2O2 (red bar). Error bars represent standard deviations of two BSA retention rates taken for each one of three different permeate solution samples. (C) Water flux recovery rates of the PVDF and 0.5% FeOCl/PVDF composite membranes in the presence of BSA and H2O2 compared to BSA only.



A

B

C

Figure 6. (A) Changes of bisphenol A (BPA) concentration with time for batch experiments with submerged PVDF and 0.5% FeOCl/PVDF membranes with and without H2O2. The inset shows the corresponding high-performance liquid chromatography (HPLC) analysis of BPA and corresponding intermediates for the 0.5% FeOCl/PVDF membrane in the presence of 340 mg L1 H2O2. Transformation products A, C, D, and F were identified as acetic acid, formaldehyde, methyl phenol, and 4-isopropanol phenol, respectively, using chemical standards. Experimental conditions: 1 mg L-1 BPA, 340 or 34 mg L-1 H2O2 (as indicated), pH 6.7, 20 mL solution, and room temperature. (B) Changes in the concentration of hydroxyl radicals with time for batch experiments with PVDF and 0.5% FeOCl/PVDF membranes (shown in panel A). The inset presents the dimethyl pyridine N-oxide (DMPO) trapped electron spin resonance spectroscopy (ESR) characterization at 5 min for the 0.5% FeOCl/PVDF membranes with 340 and 34 mg L-1 H2O2, and without H2O2. (C) Variation of water flux and BPA concentration in permeate solution


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for the 0.5% FeOCl/PVDF composite membrane. The experiment was conducted with an initial BPA concentration of 1 mg L-1, applied pressure of 0.7 bar (10 psi), and H2O2 dosing of 34 mg L1 for 100 min.



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Table 1. The mean pore size (MPS) and porosity of PVDF and FeOCl/PVDF membranes. Membrane

MPS (nm)

Porosity (%)

PVDF

33.5

63.5

0.05% FeOCl/PVDF

29.6

68.0

0.25% FeOCl/PVDF

26.5

69.1

0.5% FeOCl/PVDF

26.3

55.5

1.25% FeOCl/PVDF

25.5

50.1

2.5% FeOCl/PVDF

22.5

48.8


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