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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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Reactive, Self-cleaning Ultrafiltration Membrane Functionalized with Iron Oxychloride (FeOCl) Nanocatalysts
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Revised: July 11, 2018
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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
27
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ABSTRACT
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Self-cleaning, antifouling ultrafiltration membranes are critically needed to mitigate organic
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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
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when H2O2 was dosed in the feed solution at neutral pH. Our results demonstrate the promise of
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utilizing this novel membrane for the treatment of waters with complex organic pollutants.
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INTRODUCTION
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Low-cost, highly efficient membrane-based technologies have become increasingly widespread
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for water and wastewater treatment applications.1 Ultrafiltration (UF) is one of the most widely
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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
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foulants, including natural organic matter (NOM) and colloidal materials, causes a substantial
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decrease in water permeability and requires frequent hydraulic cleaning. Therefore, the
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development of effective strategies for fouling control in UF is of paramount importance.
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Surface modification of UF membranes is a promising approach to improve fouling
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resistance. Previous studies have demonstrated a wide variety of surface-modification strategies
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to mitigate fouling, including grafting anti-adhesive polymeric brushes,2-5 introducing
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amphiphilic or hydrophilic functional groups,6,
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nanomaterials.8-10 Among the antifouling surfaces suggested in the literature, the incorporation of
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reactive nanomaterials has been shown to improve antifouling and self-cleaning properties
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through in-situ generation of reactive oxygen species (ROS).1, 11
7
and binding contact-mediated antifouling
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Membrane modification with photocatalysts, which generate ROS (e.g., O2•−, •OH, H2O2,
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and •OOH−) when exposed to light,12-14 has the potential to oxidize pollutants on the membrane
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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
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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
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exposed to a simulated solar light. In another study, a polyvinylidene fluoride (PVDF)/TiO2
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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
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inversion demonstrated both antifouling and self-cleaning properties under UV irradiation.18
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Although the potential of photocatalytic membranes to control fouling has been demonstrated,
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practical application of such membranes is greatly hindered by the need for extensive UV light
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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
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Fe2O3 NPs,19 single metal
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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
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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)
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to form non-selective hydroxyl radicals (•OH).26 The unique structural configuration of iron
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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
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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
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state of iron (Fe3+ and Fe2+) and the ordered peripheral coordination of FeOCl impart an effective
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iron redox cycle, which triggers a homolytic decomposition of iron-hydroperoxy complexes over
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a broad pH range.28, 29 Therefore, FeOCl is a promising Fenton-based catalyst for antifouling
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membrane modification due to its ability to perform well over a wide pH range and its excellent
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regeneration properties.27, 30
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In this paper, we report the fabrication and performance evaluation of a novel self-cleaning
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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,
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structure, and morphology of the FeOCl/PVDF membrane were comprehensively characterized.
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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
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(BSA) as a model organic foulant. We also demonstrated the catalytic degradation of bisphenol
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A (BPA) with the FeOCl/PVDF membrane. The findings of this study highlight the potential
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application of FeOCl nanocatalysts for membrane-based treatment of contaminated waters.
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MATERIALS AND METHODS
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Materials and Chemicals. Ferric chloride hexahydrate (FeCl3·6H2O, >98.0%), bisphenol A
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(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
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isopropanol were purchased from J.T. Baker (Phillipsburg, NJ). Ethanol (100% absolute, USP-
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grade) was purchased from Decon Labs (King of Prussia, PA). The Micro BCA™ protein assay
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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
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water purification system (Millipore, Billerica, MA). A commercial polyvinylidene fluoride
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(PVDF) ultrafiltration membrane (BX CF016) was purchased from Sterlitech and used as a
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support for further fabrication of composite membranes. Prior to use, the PVDF supports were
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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.
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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
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FeCl3·6H2O powder in ethanol followed by sonication for 10 min. The preparation of the
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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
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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
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220oC for 1 h. The final membrane (denoted 0.05% FeOCl/PVDF) was obtained by washing the
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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).
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Characterization and Analysis Methods. The crystalline composition of pristine and
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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
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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
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with 8 nm of chromium to increase sample conductivity. Attenuated total reflectance-Fourier
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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.
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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
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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)
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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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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
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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.
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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
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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
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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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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
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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
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(1) Werber, J. R.; Osuji, C. O.; Elimelech, M. Materials for next-generation desalination and water purification membranes. Nat. Rev. Mater. 2016, 1, (5), 16018. (2) Krishnamoorthy, M.; Hakobyan, S.; Ramstedt, M.; Gautrot, J. E. Surface-initiated polymer brushes in the biomedical field: applications in membrane science, biosensing, cell culture, regenerative medicine and antibacterial coatings. Chem. Rev. 2014, 114, (21), 1097611026. (3) Keating, J. J.; Imbrogno, J.; Belfort, G. Polymer brushes for membrane separations: a review. ACS Appl. Mater. Interfaces 2016, 8, (42), 28383-28399. (4) Liu, C.; Faria, A. F.; Ma, J.; Elimelech, M. Mitigation of biofilm development on thinfilm composite membranes functionalized with zwitterionic polymers and silver nanoparticles. Environ. Sci. Technol. 2017, 51, (1), 182-191. (5) Liu, C.; Lee, J.; Ma, J.; Elimelech, M. Antifouling thin-film composite membranes by controlled architecture of zwitterionic polymer brush layer. Environ. Sci. Technol. 2017, 51, (4), 2161-2169. (6) Chen, W.; Su, Y.; Peng, J.; Zhao, X.; Jiang, Z.; Dong, Y.; Zhang, Y.; Liang, Y.; Liu, J. Efficient wastewater treatment by membranes through cnstructing tunable antifouling membrane surfaces. Environ. Sci. Technol. 2011, 45, (15), 6545-6552. (7) Asatekin, A.; Kang, S.; Elimelech, M.; Mayes, A. M. Anti-fouling ultrafiltration membranes containing polyacrylonitrile-graft-poly(ethylene oxide) comb copolymer additives. J. Membrane Sci. 2007, 298, (1), 136-146. (8) Chu, K. H.; Huang, Y.; Yu, M.; Her, N.; Flora, J. R. V.; Park, C. M.; Kim, S.; Cho, J.; Yoon, Y. Evaluation of humic acid and tannic acid fouling in graphene oxide-coated ultrafiltration membranes. ACS Appl. Mater. Interfaces 2016, 8, (34), 22270-22279. (9) Omi, F. R.; Choudhury, M. R.; Anwar, N.; Bakr, A. R.; Rahaman, M. S. Highly conductive ultrafiltration membrane via vacuum filtration assisted layer-by-layer deposition of functionalized carbon nanotubes. Ind. Eng. Chem. Res. 2017, 56, (30), 8474-8484. (10) Pan, S.; Li, J.; Noonan, O.; Fang, X.; Wan, G.; Yu, C.; Wang, L. Dual-functional ultrafiltration membrane for simultaneous removal of multiple pollutants with high performance. Environ. Sci. Technol. 2017, 51, (9), 5098-5107. (11) Musico, Y. L. F.; Santos, C. M.; Dalida, M. L. P.; Rodrigues, D. F. Surface modification of membrane filters using graphene and graphene oxide-based nanomaterials for bacterial inactivation and removal. ACS Sustainable Chem. Eng. 2014, 2, (7), 1559-1565. (12) Gao, G.; Zhang, Q.; Hao, Z.; Vecitis, C. D. Carbon nanotube membrane stack for flowthrough sequential regenerative electro-Fenton. Environ. Sci. Technol. 2015, 49, (4), 2375-2383. (13) Perreault, F.; Fonseca de Faria, A.; Elimelech, M. Environmental applications of graphene-based nanomaterials. Chem. Soc. Rev. 2015, 44, (16), 5861-5896. (14) Mauter, M. S.; Zucker, I.; Perreault, F.; Werber, J. R.; Kim, J. H.; Elimelech, M. The role of nanotechnology in tackling global water challenges. Nat. Sustain. 2018, 1, (4), 166-175. (15) Kim, S. H.; Kwak, S.-Y.; Sohn, B.-H.; Park, T. H. Design of TiO2 nanoparticle selfassembled aromatic polyamide thin-film-composite (TFC) membrane as an approach to solve biofouling problem. J. Membrane Sci. 2003, 211, (1), 157-165. (16) Geng, Z.; Yang, X.; Boo, C.; Zhu, S. Y.; Lu, Y.; Fan, W.; Huo, M. X.; Elimelech, M.; Yang, X. Self-cleaning anti-fouling hybrid ultrafiltration membranes via side chain grafting of poly(aryl ether sulfone) and titanium dioxide. J. Membrane Sci. 2017, 529, 1-10. 19 ACS Paragon Plus Environment
Environmental Science & Technology
526 527 528 529 530 531 532 533 534 535 536 537 538 539 540 541 542 543 544 545 546 547 548 549 550 551 552 553 554 555 556 557 558 559 560 561 562 563 564 565 566 567 568 569 570
(17) Damodar, R. A.; You, S.-J.; Chou, H.-H. Study the self cleaning, antibacterial and photocatalytic properties of TiO2 entrapped PVDF membranes. J. Hazard. Mater. 2009, 172, (2), 1321-1328. (18) Xu, Z. W.; Wu, T. F.; Shi, J.; Teng, K. Y.; Wang, W.; Ma, M. J.; Li, J.; Qian, X. M.; Li, C. Y.; Fan, J. Y. Photocatalytic antifouling PVDF ultrafiltration membranes based on synergy of graphene oxide and TiO2 for water treatment. J. Membr. Sci. 2016, 520, 281-293. (19) Lu, D.; Zhang, T.; Gutierrez, L.; Ma, J.; Croué, J.-P. Influence of surface properties of filtration-layer metal oxide on ceramic membrane fouling during ultrafiltration of oil/water emulsion. Environ. Sci. Technol. 2016, 50, (9), 4668-4674. (20) Shimotori, T.; Nuxoll, E. E.; Cussler, E. L.; Arnold, W. A. A polymer membrane containing Fe0 as a contaminant barrier. Environ. Sci. Technol. 2004, 38, (7), 2264-2270. (21) Huang, R.; Zhu, H.; Su, R.; Qi, W.; He, Z. Catalytic membrane reactor immobilized with alloy nanoparticle-loaded protein fibrils for continuous reduction of 4-nitrophenol. Environ. Sci. Technol. 2016, 50, (20), 11263-11273. (22) Zirehpour, A.; Rahimpour, A.; Arabi Shamsabadi, A.; Sharifian Gh, M.; Soroush, M. Mitigation of thin-film composite membrane biofouling via immobilizing nano-sized biocidal reservoirs in the membrane active layer. Environ. Sci. Technol. 2017, 51, (10), 5511-5522. (23) Xu, Z.; Ye, S.; Zhang, G.; Li, W.; Gao, C.; Shen, C.; Meng, Q. Antimicrobial polysulfone blended ultrafiltration membranes prepared with Ag/Cu2O hybrid nanowires. J. Membrane Sci. 2016, 509, 83-93. (24) Zhao, L.; Lu, X.; Wu, C.; Zhang, Q. Flux enhancement in membrane distillation by incorporating AC particles into PVDF polymer matrix. J. Membrane Sci. 2016, 500, 46-54. (25) Liang, Y. H.; Zhou, B. M.; Li, N.; Liu, L. S.; Xu, Z. W.; Li, F. Y.; Li, J.; Mai, W.; Qian, X. M.; Wu, N.; Enhanced dye photocatalysis and recycling abilities of semi-wrapped TiO2@carbon nanofibers formed via foaming agent driving. Ceram. Int. 2018, 42, (2), 1711-1718. (26) Yang, X.-j.; Xu, X.-m.; Xu, J.; Han, Y.-f. Iron Oxychloride (FeOCl): An efficient Fentonlike catalyst for producing hydroxyl radicals in degradation of organic contaminants. J. Am. Chem. Soc. 2013, 135, (43), 16058-16061. (27) Sun, M.; Chu, C.; Geng, F.; Lu, X.; Qu, J.; Crittenden, J.; Elimelech, M.; Kim, J.-H. Reinventing Fenton Chemistry: Iron Oxychloride Nanosheet for pH-Insensitive H2O2 Activation. Environ. Sci. Technol. Lett. 2018, 5, (3), 186-191. (28) Walling, C.; Goosen, A. Mechanism of the ferric ion catalyzed decomposition of hydrogen peroxide. Effect of organic substrates. J. Am. Chem. Soc. 1973, 95, (9), 2987-2991. (29) Ensing, B.; Buda, F.; Baerends, E. J. Fenton-like chemistry in water: oxidation catalysis by Fe(III) and H2O2. J. Phys. Chem. A 2003, 107, (30), 5722-5731. (30) Wang, Y. Y.; Zhang, H. W.; Zhu, Y. D.; Dai, Z. F.; Bao, H. M.; Wei, Y.; Cai, W. P. AuNP-decorated crystalline FeOCl nanosheet: facile synthesis by laser ablation in liquid and its exclusive gas sensing response to HCl at room temperature. Adv. Mater. Interfaces 2016, 3, (9), 1500801. (31) Sun, M.; Zhang, G.; Qin, Y.; Cao, M.; Liu, Y.; Li, J.; Qu, J.; Liu, H. Redox conversion of chromium(VI) and arsenic(III) with the intermediates of chromium(V) and arsenic(IV) via AuPd/CNTs electrocatalysis in acid aqueous solution. Environ. Sci. Technol. 2015, 49, (15), 9289-9297. (32) Lousada, C. M.; Jonsson, M. Kinetics, mechanism, and activation energy of H2O2 decomposition on the surface of ZrO2. J. Phys. Chem. C 2010, 114, (25), 11202-11208.
20 ACS Paragon Plus Environment
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Environmental Science & Technology
(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.
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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.
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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.
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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
Normalized Water Flux
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
Normalized Water Flux
Water Flux Recovery Rate (%)
A
0.0 150
130
Normalized Water Flux
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
Normalized Water Flux
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
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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.
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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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Environmental Science & Technology
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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