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Environmental Science & Technology
Nanometric Graphene Oxide Framework Membranes with Enhanced Heavy Metal Removal via Nanofiltration
Yu Zhang†, Sui Zhang‡, Tai-Shung Chung†, ‡, *
†
NUS Graduate School for Integrative Science and Engineering,
National University of Singapore, 28 Medical Drive, Singapore 117456
‡
Department of Chemical and Biomolecular Engineering,
National University of Singapore, 4 Engineering Drive 4, Singapore 117585
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Abstract
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A novel dual-modification strategy, including (1) the crosslinking and construction of a GO
3
framework by ethylenediamine (EDA) and (2) the amine-enrichment modification by
4
hyperbranched polyethyleneimine (HPEI), has been proposed to design stable and highly
5
charged GO framework membranes with the GO selective layer thickness of 70 nm for
6
effective heave metal removal via nanofiltration (NF). Results from sonication experiments
7
and positron annihilation spectroscopy confirmed that EDA crosslinking not only enhanced
8
structural stability, but also enlarged the nanochannels among the laminated GO nanosheets
9
for higher water permeability. HPEI 60K was found to be the most effective post-treatment
10
agent that resulted in GO framework membranes with a higher surface charge and lower
11
transport resistance. The newly developed membrane exhibited a high pure water
12
permeability of 5.01 L m-2 h-1 bar-1 and comparably high rejections towards Mg2+, Pb2+, Ni2+,
13
Cd2+ and Zn2+. These results have demonstrated the great potential of GO framework
14
materials in wastewater treatment and may provide insights for the design and fabrication of
15
the next generation 2D-based NF membranes.
16 17
Keywords:
18
Graphene oxide framework, hyperbranched polyethyleneimine, nanofiltration, heavy metal
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1. INTRODUCTION
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The scarcity of fresh water is a global challenge due to the rapid growth in population and
21
industrialization 1. This leads to a great demand on exploring cost-effective and highly
22
efficient water reuse and desalination technologies 2. Compared with traditional wastewater
23
treatments such as chemical, precipitation and sorption methods, nanofiltration (NF)
24
technology has several advantages, including smaller footprint, lower operation cost and
25
energy consumption, and elimination of chemical residuals 3-7. One important application of
26
NF is for heavy metal removal, which is achieved by combining the size exclusion and the
27
Donnan exclusion separation mechanisms 8-11.
28 29
The ideal NF membrane should have a narrow pore size distribution to achieve a high
30
selectivity and a thin and highly porous structure to ensure a good permeability. Generally,
31
polymeric NF membranes have acceptable separation performance and are widely used
32
owing to easy fabrication with relatively low costs. However, polymeric membranes have
33
some drawbacks including poor chemical and thermal resistance, fouling and physical aging
34
12, 13
35
weaknesses of polymeric membranes and achieve high separation performance
36
Nevertheless, inorganic NF membranes are expensive and only used for special applications.
. Inorganic porous membranes, such as Al2O3, can address the aforementioned 14
.
37 38
Recently, carbon-based materials, e.g. graphene and its derivative graphene oxide (GO), have
39
shown promising to be membrane materials for their easy accessibility, high chemical and
40
mechanical stabilities 15-18. GO is only one atom thick with a lateral length of several hundred 3
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nm (Figure S-1)
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42
self-assemblable
20-22
43
achievements have been made for various applications, including gas separation
44
pervaporation dehydration 25-28, and ultra/nanofiltration 29-32.
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. This high aspect ratio makes GO nanosheets highly stackable and . By controlling the d-spacing of GO membranes, remarkable 23, 24
,
45 46
Nair et al. found that a sub-micrometer-thick GO membrane can completely block the
47
passage of liquids and gases under a dry state while facilitating the permeation of water vapor
48
33
49
by the empty inter-space between the nonoxidized regions of GO sheets. However, hydration
50
would increase the d-spacing of the GO membrane when it was immersed in water and allow
51
a faster permeation of molecules with sizes smaller than 0.45 nm while blocking ions or
52
molecules larger than that 34. Inspired by Nair’s findings, Huang et al. reported a modified
53
nanostrand-channeled GO membrane using copper hydroxide nanostrands as a sacrificial
54
template to enhance the membrane permeability
55
membranes of tens-of-nanometer thick for dye retention with similar rejection performance
56
but much higher water flux than commercial NF membranes 30. These results suggested that
57
laminar GO membranes are a promising candidate to develop NF membranes with an
58
ultrahigh permeability.
. They attributed this fast transport of water vapor to the low-friction nanocapillaries formed
32
. Han et al. also prepared ultrathin GO
59 60
However, since the integrity of pure GO films is solely maintained by hydrogen bond
61
interactions between the oxygen-containing groups of GO nanosheets, pure GO films suffer 4
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from low structural and physicochemical stability. When the membrane is immersed in an
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aqueous solution, the hydration effect will destroy the hydrogen bond and enlarge the
64
d-spacing among GO nanosheets, leading to a significant expansion of the nanochannels in
65
the membrane. Therefore, there is an urgent need to improve the GO membrane design and
66
self-assembly technique to enhance its stability under various conditions 18, 35. So far studies
67
on the separation of small ionic species using GO membranes are limited. It was reported that
68
GO based membranes had lower rejections towards ions than common NF membranes 29, 30.
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Several approaches have been proposed to address these issues, including layer-by-layer
70
(LbL) dip-coating 36 and construction of GO frameworks 27. However, the LbL membranes
71
assembled via electrostatic interaction tend to swell up in ionic solutions, while fully
72
crosslinked GO framework membranes suffer from a low liquid water permeability.
73 74
In this study, a novel dual-modification strategy, including (1) the crosslinking and
75
construction of a GO framework by ethylenediamine (EDA) and (2) the amine-enrichment
76
modification by hyperbranched polyethyleneimine (HPEI), is designed to fabricate stable and
77
highly charged NF membranes. The first step includes the mixing of EDA with the GO
78
solution and then deposits it on a polycarbonate substrate via a pressure-assisted assembly
79
technique, while the second step modifies and stabilizes the as-prepared membrane by HPEI.
80
Different chemical, physicochemical and morphological characterizations are conducted to
81
investigate the stability and surface properties of the pristine and modified membranes. The
82
newly developed GO composite membrane shows good stability, high water permeability and 5
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rejections towards a variety of cationic heavy metal ions, including Pb2+, Ni2+, Cd2+ and Zn2+.
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This study may provide useful insights on the design and fabrication of new generation NF
85
membranes.
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86 87
2. EXPERIMENTAL
88
2.1. GO framework membrane preparation
89
Prior to the GO assembly process, the substrates were modified with polydopamine to
90
enhance the adhesion between the substrate and the GO layer 37. Polydopamine coating has
91
been widely employed in membrane fabrication processes to increase substrate hydrophilicity
92
as well as to provide feasible modification sites 38, 39. Firstly, the coating solution (2 mg/mL)
93
was prepared by dissolving 0.16 g dopamine hydrochloride into an 80 mL 0.01 M tris buffer
94
at pH 8.5. The substrates were then immersed in the solution under ambient temperature for 3
95
hours. During this period, dopamine will undergo self-polymerization and form an adhesive
96
layer on the substrate to provide GO nanosheets anchor sites (Figure S3). After that, the
97
substrates were rinsed in DI water to remove the residues.
98 99
In order to prepare the GO framework, a certain amount of GO solution was fully dissolved in
100
a 20 mL 1 wt% EDA water solution. The blend was then filtrated through the modified
101
polycarbonate membrane in a dead-end filtration cell under 1 bar. After the filtration, the
102
membrane was rinsed in DI water to remove the residual EDA. GO framework membranes
103
fabricated under this condition were referred to as GO&EDA. For comparison, a control
104
membrane was prepared following the same procedure but without EDA in water. This 6
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Environmental Science & Technology
membrane was referred to as pristine GO.
106 107
The surface charge properties of the as-fabricated GO&EDA membrane were further
108
modified by immersing it into an aqueous solution containing either 1 wt% amine HPEI 60K,
109
PEI 2K or dendrimer G(2,0) for a certain period of time and rinsed by DI water. The resultant
110
membrane is denoted as GO&EDA_HPEI 60K if it is modified by HPEI 60K.
111 112
2.2. Pure water permeability and salt rejection of the GO membranes
113
Pure water permeability (PWP, L m-2 h-1 bar-1, abbreviated as LMH bar-1) and salt rejection
114
(R, %) of these composite GO membranes were tested at 1 bar by a dead-end permeation
115
cell at room temperature (Figure S4). The effective membrane area was 3.14 cm2. PWP of
116
each membrane was measured with DI water and the salt rejection was determined using a
117
1000 ppm ion solution (i.e., NaCl, MgCl2). The membrane was firstly conditioned under the
118
NF mode for 2 hours before collecting permeate samples. During the test, the feed solution
119
was stirred at 500 rpm. PWP and R are calculated as follows:
120
PWP =
121
= 1 − × 100%
(1) (2)
122
where Q is the water flow rate (L/h) at the permeate side of DI water. A is the effective
123
membrane area (m2) and ∆P is the trans-membrane pressure (bar). Cp and Cf are the
124
concentrations of the ion solution in the permeate and feed, respectively, which are
125
determined by a conductivity meter (Metrohm AG). 7
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2.3. Membrane characterizations
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X-ray photoelectron spectroscopy was employed to determine the surface chemistry of the
129
membranes using a monochromatized Al Kα X-ray source (1486.6 eV photons) at a
130
constant dwell time of 100 ms and a pass energy of 40 eV. The chemical structure of the GO
131
framework layer was analyzed by Fourier transform infrared spectroscopy (FTIR) (Bio-Rad
132
FTS-3500) over the range of 400-4000 cm-1. The polydopamine coating was not applied on
133
the polycarbonate membrane surface for the specific sample where the GO layer was peeled
134
off from the substrate, A pellet was prepared by grinding a 0.5 mg GO layer sample with
135
99.5 mg KBr powder and compressed under 8 bar, which was then characterized by the
136
direct transmittance mode. The total number of scans for each sample was 16.
137 138
The morphologies of the GO framework membranes were observed by field emission
139
scanning electron microscopy (FESEM JEOL JSM-6700LV). Prior to FESEM observation,
140
the membrane was freeze-dried before fractured in liquid nitrogen. The membrane samples
141
were then fixed on stubs and coated with a thin layer of platinum under a vacuum condition
142
by a Jeol JFC-1100e Ion Sputtering device. The GO nanosheets and membrane thicknesses
143
were examined by an Atomic force microscope (AFM) (Agilent Technologies, USA) under
144
the tapping mode. A membrane stability test was carried out via sonicating the membrane
145
sample under the degas mode by Elmasonic (S 30 H) for 10 minutes.
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The surface charge properties of the pristine GO, GO&EDA and GO&EDA_HPEI 60K
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framework membranes were analyzed by a SurPASS electrokinetic analyzer (Anton Paar
149
GmbH, Austria). A 450 mL 0.01 M NaCl solution was used to measure the ζ-potential of the
150
membranes under neutral pH. In order to determine the ζ-potential as a function of pH from
151
2.5 to 10, the 0.01 M NaCl solution was firstly auto-titrated with 0.1 M HCl to pH 2.5
152
followed by auto-titration with 0.1 M NaOH to pH 10.
153 154
The microstructural evolution of GO framework membranes was investigated by Doppler
155
broadening energy spectroscopy (DBES). This measurement was conducted using positron
156
annihilation spectroscopy (PAS) in our laboratory. 22Na was used as the source of positrons
157
and experiments were performed at a counting rate of 3000-4000 counts s-1. For each
158
spectrum, a total of one millions counts was taken. The detailed testing procedures have
159
been elaborated elsewhere
160
used to determine the microstructural changes along the membrane depth profile. The mean
161
depth of the materials associated with the incident positron energy can be calculated by the
162
following equation:
163
40
. S- parameter derived from the annihilation spectra, can be
Z ( ) = × .
(3)
164
Where Z is the mean depth (nm), is the material density (g cm-3) and E+ is the incident
165
positron energy. The average density employed in the fitting was calculated via the weight
166
and volume of the membrane as 0.95 g cm-3.
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3. RESULTS AND DISCUSSION
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3.1. Chemical
169
membranes
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Figure 1 shows the FTIR spectra of the pristine GO, GO&EDA and GO&EDA_HPEI 60K
171
membranes under the transmission mode. The pristine GO (Figure 1a) has characteristic
172
peaks of –OH stretching at 3400 cm-1, -C=O stretching at 1732 cm-1 and -OH stretching at
173
1414 cm-1 in -COOH, C-O-C stretching at 1224 cm-1, which well corresponds to its
174
structure as illustrated in Figure S-2a in the Supporting Information 17. After crosslinked by
175
EDA, new peaks appear at about 1357 cm-1 and 3250 cm-1 (Figure 1b), which are
176
representatives of the –CN in secondary amine and the -NH bond of primary amines,
177
respectively 41. In addition, compared with the spectrum of pristine GO, the epoxy content
178
almost disappears and the intensity of carboxyl group decreases significantly. This indicates
179
that crosslinking reactions and electrostatic interaction have taken place between the amine
180
group of EDA and the epoxy or carboxylic acid group of GO. For GO&EDA_HPEI 60K, a
181
new peak can be observed at 1570 cm-1, which is attributed to the abundant secondary
182
amine on the HPEI chains. In addition, part of the free amine groups of HPEI molecules
183
may also react with the residual epoxy group in GO to contribute to the peak intensity.
and
morphological
characterizations
of
go
framework
184 185
The reactions are further confirmed by XPS as shown in Table S-1 in the Supporting
186
Information. The pristine GO membrane contains an O/C mass ratio of 0.4 and no nitrogen
187
content could be detected. An increment in N1s content and a decline in O1s content for 10
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GO&EDA and GO&EDA_HPEI 60K are observed Correspondingly, O-C=O, C=O, C-O
189
and C-C signals are detected on the surface of the pristine GO membrane in Figure S-5a.
190
For the GO&EDA framework membrane, the peak intensities of O-C=O, C=O and C-O
191
decline while a new peak of C-N appears due to the introduction of EDA into the GO layers
192
(Figure S-5b). Figure S-6a shows the peaks of primary, secondary and quaternary amine
193
groups on the surface of GO&EDA, indicating the presence of free amine, formation of
194
covalent bonding and electrostatic interaction between GO and EDA. The peak intensity of
195
C-N in GO&EDA_HPEI 60K increases substantially owing to the C-N bond of HPEI 60K
196
on the membrane surface (Figure S-5c). In the meantime, the peak intensities of C-O and
197
C=O further decrease while the O-C=O peak disappears, which may be due to the formation
198
of the covalent bond and ionic interaction between oxygen-containing groups of GO with
199
amine groups.
200 201
Figure 2b and 2c elucidates the chemical structures on the surfaces of GO&EDA and
202
GO&EDA_HPEI 60K membranes. It is worth noting that EDA may either perpendicularly
203
crosslink two parallel GO nanosheets or two adjacent nanosheets. Moreover, there might be
204
only one amine group of EDA reacting with GO nanosheets, which explains the presence of
205
amine group in the FTIR spectrum of the GO&EDA composite membrane. Therefore, the
206
dual modification strategy; namely, the first step by EDA to crosslink the GO sheets and the
207
second step by HPEI to impart the surface with abundant positive charge, works well on the
208
GO membranes. 11
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The structural integrity of the GO layer over the polycarbonate support is greatly enhanced
211
by the dual modifications as evidenced in Figure S-7 in the Supporting Information, which
212
shows the digital images of pristine GO and GO&EDA_HPEI 60K after a vigorous
213
sonication process. Originally, both samples have the same brown appearance indicative of a
214
thin GO layer. After the sonication process. ¾ of the pristine GO membrane becomes
215
translucent, implying most GO nanosheets have been removed by the sonication, whilst
216
GO&EDA_HPEI 60K maintains its appearance. This phenomenon suggests the structural
217
stability of GO&EDA_HPEI 60K has been improved as compared with pristine GO.
218 219
In order to optimize the preparation conditions, Figure S-8 shows the PWP and MgCl2
220
rejection of GO framework membranes as functions of GO loading and immersion duration
221
in the HPEI 60K solution. The GO framework membrane comprising 0.09375 mg GO
222
(specific deposition: 0.083 g/m2) and being modified by 1 wt% HPEI 60K aqueous solution
223
for 20 min has the best balanced separation performance; namely, a MgCl2 rejection of 96.3%
224
± 1.3% and a PWP of 5.01 ± 0.24 LMH bar -1. It is therefore chosen as a representative of
225
the GO&EDA_HPEI 60K for the subsequent studies.
226 227
Figure 3a-d displays the surface and cross section morphologies of the substrate and the
228
composite GO&EDA_HPEI 60K membrane. The polycarbonate substrate has a smooth
229
surface with uniform pores of 0.2 µm in diameter. After the deposition of GO nanosheets, a 12
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well packed laminated film is formed on the substrate surface (Figure 3b). The wrinkles on
231
the top surface of the GO membrane are formed from the edges and folding of GO
232
nanosheets. The thickness of the GO layer was measured by FESEM and AFM. Figure 3d
233
shows its FESEM cross section image on the substrate with a thickness of less than 100 nm,
234
while Figure 3e displays its height profile along the line across the membrane surface drawn
235
by AFM with an average thickness of 69 nm. Since the specific GO amount for this NF
236
membrane is only 0.083 g/m2, the newly developed NF membrane could be cost effective
237
and prospective for practical wastewater treatment.
238 239
3.2. Effects of EDA crosslinking and HPEI modification on rejection
240
performance in NF processes
241
Figure 4a compares the PWP values of the pristine GO, GO&EDA and GO&EDA_HPEI
242
60K membranes. GO&EDA has the highest PWP of 9.79 ± 2.20 LMH bar-1, which is
243
significantly higher than that of the pristine GO membrane. One possible explanation is that
244
the orientation of GO nanosheets in the pristine GO membrane is highly ordered and
245
densely packed
246
For perpendicularly crosslinked GO nanosheets, the expansion of d-spacing under the wet
247
condition is retarded, which would reduce water permeability. In contrast, if EDA
248
crosslinks two adjacent nanosheets, it would enlarge water transport channels and increase
249
PWP. Similarly, the incorporation of free amine and un-ordered EDA molecules among GO
250
nanosheets may introduce defects and lower their packing density, thus increasing PWP.
42
. For GO&EDA, EDA may crosslink GO nanosheets in different ways.
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These competing mechanisms result in an enhanced pure water permeability for the
252
GO&EDA membrane. The S- parameter values measured by DBES spectroscopy confirm
253
our hypotheses. Figure 5 shows the S- parameter as a function of the incident positron
254
energy for these three membranes. A smaller S- parameter indicates either a smaller free
255
volume size or a lower free volume content of the membrane
256
(i.e., corresponding to the thin GO layer), the S- parameter of the pristine GO membrane is
257
much lower than the other two. It is hence reasonable to conclude that the pristine GO has
258
the densest and most ordered packing, while GO&EDA and GO&EDA_HPEI 60K have a
259
comparable packing density. Although the packing densities of GO nanosheets in
260
GO&EDA_HPEI 60K and GO&EDA are similar, HPEI 60K crosslinks the top layer of
261
GO&EDA_HPEI 60K and thus its pure water permeability is reduced to 5.01 ± 0.24 LMH
262
bar -1.
25
. At low incident energy
263 264
The ζ-potential of the pristine GO, GO&EDA and GO&EDA_HPEI 60K as a function of
265
pH is shown in Figure 6. In a wide pH range from 3 to 11, the pristine GO membrane is
266
negatively charged, which is mainly due to the deprotonation of the carboxyl group at the
267
edges of GO nanosheets. The GO&EDA is slightly positive charge below pH 4.5 but
268
becomes neutral at pH 4.5 and then negative charge at higher pH values owing to the
269
complicated functional groups on the GO&EDA membrane surface such as hydroxyl,
270
unreacted carboxyl acid, unreacted amine and amide groups. For GO&EDA_HPEI 60K,
271
since additional amine groups are introduced on the GO framework surface, the isoelectric 14
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point shifts to pH 10.5 due to the protonation of the free amine groups. Moreover, its
273
ζ-potential is around 100 mV at neutral pH, which is higher than most reported data for
274
positively charged membranes 43, 44.
275 276
Figure 4 also displays the rejections of these membranes against MgCl2, MgSO4, NaCl and
277
Na2SO4 solutions. The pristine GO has the highest rejection towards Na2SO4, which is
278
consistent with the literature data 30, then the rejection follows an order of R (Na2SO4) > R
279
(MgSO4) > R (NaCl) > R (MgCl2). However, the rejection order is reversed for
280
GO&EDA_HPEI 60K. This phenomenon can be explained by the Donnan exclusion effect.
281
GO is negatively charged in a wide pH range. Therefore, it tends to extrude co-ions, such as
282
SO42- and Cl-1. In order to maintain the electroneutrality of the solutions at each side of the
283
GO composite membrane, the counter ions Na+ and Mg2+ have to be rejected as well.
284
According to Donnan exclusion theory, the rejection rate is related to the valences of the ion
285
species, following the order of Zco-ions/Zcount-ions (Z refers to the valence). On the contrary, the
286
GO&EDA_HPEI 60K membrane is positively charged after being crosslinked by
287
amine-rich HPEI molecules. In this case, the co-ions are Mg2+ and Na+. The membrane
288
thereby shows a higher rejection against divalent cations (Mg2+). Since the GO&EDA has a
289
larger free volume and close-to-neutral charge at neutral pH, the rejections towards different
290
salts are lower compared with the other two membranes.
291
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3.3. Effects of different amine-enrichment modifications
293
Besides HPEI 60K, PEI 2K and dendrimer G(2,0) were employed to investigate the effects
294
of various amine modifications. PEI 2K has a smaller molecular weight and dendrimer G(2,
295
0) is a micelle shaped molecule with abundant amine groups. The post-treatment conditions
296
for PEI 2K and dendrimer G(2,0) have been optimized and Table 1 tabulates the best NF
297
performance for each membrane.
298 299
The PEI 2K post-treatment is unfavorable for both permeability and rejections because the
300
resultant membrane has a lower rejection of 89% towards MgCl2 and a significantly
301
reduced PWP of 0.39 LMH bar -1. This is due to the fact that the molecular size of PEI 2K is
302
smaller. It can easily block surface pores of the GO layer and react with GO nanosheets 45,
303
resulting in a denser surface with a reduced water permeability. For the membrane
304
post-treated by dendrimer G(2,0), the rejection remains high while the PWP drops 4-fold.
305
This arises from the fact that dendrimer G (2.0) may form a denser crosslinking layer than
306
that from HPEI 60K because HPEI 60K has a longer molecular chain and higher chain
307
flexibility. As a consequence, the membrane post-treated by the former has a comparable
308
rejection but a lower PWP. Clearly, HPEI 60K is the most efficient agent to modify the GO
309
framework membrane with higher surface charge but lower transport resistance.
310 311
3.4. Heavy metal rejection of the HPEI 60K modified GO framework membrane
312
The newly developed GO&EDA_HPEI 60K membrane was further tested by 1000 ppm 16
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Pb(NO3)2, NiCl2, CdCl2 and ZnCl2 solutions, respectively. High rejections to these divalent
314
cationic heavy metal ions have been achieved as summarized in Table 2, especially for
315
Pb(NO3)2 and NiCl2. Since the newly developed GO framework membrane is highly
316
positively charged at neutral pH, it can effectively repel the cationic heavy metal ions. Table
317
2 shows a benchmarking of the current GO framework membrane with some other NF
318
membranes
319
this work.
8-10, 46, 47
. A much higher PWP with comparable rejections has been achieved in
320 321
ASSOCIATED CONTENT
322
Supporting Information
323
Materials, AFM images of GO nanosheets, chemical structures of the different compounds
324
used in this work, FESEM images of substrate membrane before and after polydopamine
325
modification, experimental apparatus, XPS C 1s and N 1s narrow scan spectra of different
326
membranes, digital photo images of GO framework membranes, effect of GO loading and
327
post-treatment duration on NF performance and XPS characterizations results. This material
328
is available free of charge via the Internet at http:// pubs. acs. org.
329 330
AUTHOR INFORMATION
331
Corresponding author
332
* Tel: +65-65166645. Fax: +65-67791936. Email:
[email protected].
333
Notes 17
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The authors declare no completing financial interest.
335 336
ACKNOWLEDGEMENT
337
The authors would like to acknowledge the financial support from the Singapore National
338
Research Foundation (NRF) Competitive Research Program for the project entitled
339
“Advanced FO membranes and Membrane systems for Wastewater Treatment, Water Reuse
340
and Seawater Desalination” (Grants R-279-000-336-281 and R-278-000-339-281). Special
341
thanks are due to Dr. Y. Tang, Dr. K.S. Liao, Ms. J. Gao and Mr. S. Japip for their suggestions
342
on the experimental work and paper writing.
343 344
ABBREVIATIONS
345
AFM, atomic force microscopy; DBES , doppler broadening energy spectroscopy; EDA,
346
ethylenediamine; FESEM, field emission scanning electron microscopy; FTIR, Fourier
347
transform
348
polyethyleneimine; LbL, layer-by-layer, NF, nanofiltration; PAS, positron annihilation
349
spectroscopy; PWP, pure water permeability; XPS, X-ray photoelectron spectroscopy.
infrared
spectroscopy;
GO,
graphene
oxide;
HPEI,
hyperbranched
350 351
REFERENCES
352
1.
353
P.; Glidden, S.; Bunn, S. E.; Sullivan, C. A.; Liermann, C. R.; Davies, P. M., Global threats
354
to human water security and river biodiversity. Nature 2010, 467, 555-61.
Vorosmarty, C. J.; McIntyre, P. B.; Gessner, M. O.; Dudgeon, D.; Prusevich, A.; Green,
18
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355
2.
Escobar, I. C.; Van der Bruggen, B., Modern applications in membrane science and
356
technology. Oxford University Press: Oxford, 2011.
357
3.
358
solutes during NF/RO treatment--a literature review. Water Res. 2004, 38, 2795-809.
359
4.
360
membranes for water environmental remediation. J. Membr. Sci. 2015, 476, 95-104.
361
5.
362
Osmosis and Nanofiltration Membranes. Environ. Sci. Technol. 2003, 37, 4435-4441.
363
6.
364
fiber membranes through a single-step direct spinning technique. Environ. Sci. Technol.
365
2014, 48, 13933-40.
366
7.
367
der Bruggen, B., Fractionation of direct dyes and salts in aqueous solution using loose
368
nanofiltration membranes. J. Membr. Sci. 2015, 477, 183-193.
369
8.
370
Approach To Fabricate a Chitosan Rejecting Layer over Poly(ether sulfone) Support for
371
Heavy Metal Removal. Ind. Eng. Chem. Res. 2015, 54, 472-479.
372
9.
373
nanofiltration (NF) hollow fiber membranes for high efficient heavy metal removal. Water
374
Res. 2014, 63, 252-61.
375
10. Thong, Z.; Han, G.; Cui, Y.; Gao, J.; Chung, T. S.; Chan, S. Y.; Wei, S., Novel
Bellona, C.; Drewes, J. E.; Xu, P.; Amy, G., Factors affecting the rejection of organic
Cheng, X. Q.; Shao, L.; Lau, C. h., High flux polyethylene glycol based nanofiltration
Escobar, I. C.; Peng, W., Rejection Efficiency of Water Quality Parameters by Reverse
Ong, Y. K.; Chung, T. S., Mitigating the hydraulic compression of nanofiltration hollow
Lin, J.; Ye, W.; Zeng, H.; Yang, H.; Shen, J.; Darvishmanesh, S.; Luis, P.; Sotto, A.; Van
Zhang, S.; Peh, M. H.; Thong, Z.; Chung, T.-S., Thin Film Interfacial Cross-Linking
Gao, J.; Sun, S. P.; Zhu, W. P.; Chung, T. S., Chelating polymer modified P84
19
ACS Paragon Plus Environment
Environmental Science & Technology
Page 20 of 33
376
nanofiltration membranes consisting of a sulfonated pentablock copolymer rejection layer
377
for heavy metal removal. Environ. Sci. Technol. 2014, 48, 13880-7.
378
11. Fu, F.; Wang, Q., Removal of heavy metal ions from wastewaters: a review. J. Environ.
379
Manage. 2011, 92, 407-18.
380
12. Van der Bruggen, B.; Mänttäri, M.; Nyström, M., Drawbacks of applying nanofiltration
381
and how to avoid them: A review. Sep. Purif. Technol. 2008, 63, 251-263.
382
13. Xu, P.; Drewes, J. E.; Kim, T.-U.; Bellona, C.; Amy, G., Effect of membrane fouling on
383
transport of organic contaminants in NF/RO membrane applications. J. Membr. Sci. 2006,
384
279, 165-175.
385
14. Van Gestel, T.; Van decasteele, C.; Buekenhoudt, A.; Dotremont, C.; Luyten, J.; Leysen,
386
R.; Van der Bruggen, B.; Maes, G., Salt retention in nanofiltration with multilayer ceramic
387
TiO2 membranes. J. Membr. Sci. 2002, 209, 379-389.
388
15. Geim, A. K.; Novoselov, K. S., The rise of graphene. Nature materials 2007, 6,
389
183-191.
390
16. Geim, A. K., Graphene: status and prospects. Science 2009, 324, 1530-4.
391
17. Dreyer, D. R.; Park, S.; Bielawski, C. W.; Ruoff, R. S., The chemistry of graphene
392
oxide. Chem. Soc. Rev. 2010, 39, 228-40.
393
18. Huang, H.; Ying, Y.; Peng, X., Graphene oxide nanosheet: an emerging star material for
394
novel separation membranes. J. Mater. Chem. A 2014, 2, 13772.
395
19. Meyer, J. C.; Geim, A. K.; Katsnelson, M. I.; Novoselov, K. S.; Booth, T. J.; Roth, S.,
396
The structure of suspended graphene sheets. Nature 2007, 446, 60-3. 20
ACS Paragon Plus Environment
Page 21 of 33
Environmental Science & Technology
397
20. Chen, C.; Yang, Q.-H.; Yang, Y.; Lv, W.; Wen, Y.; Hou, P.-X.; Wang, M.; Cheng, H.-M.,
398
Self-Assembled Free-Standing Graphite Oxide Membrane. Adv. Mater. 2009, 21,
399
3007-3011.
400
21. Dikin, D. A.; Stankovich, S.; Zimney, E. J.; Piner, R. D.; Dommett, G. H.; Evmenenko,
401
G.; Nguyen, S. T.; Ruoff, R. S., Preparation and characterization of graphene oxide paper.
402
Nature 2007, 448, 457-60.
403
22. Smith, Z. P.; Freeman, B. D., Graphene oxide: a new platform for high-performance
404
gas- and liquid-separation membranes. Angew. Chem. Int. Ed. Engl. 2014, 53, 10286-8.
405
23. Kim, H. W.; Yoon, H. W.; Yoon, S. M.; Yoo, B. M.; Ahn, B. K.; Cho, Y. H.; Shin, H. J.;
406
Yang, H.; Paik, U.; Kwon, S.; Choi, J. Y.; Park, H. B., Selective gas transport through
407
few-layered graphene and graphene oxide membranes. Science 2013, 342, 91-5.
408
24. Li, H.; Song, Z.; Zhang, X.; Huang, Y.; Li, S.; Mao, Y.; Ploehn, H. J.; Bao, Y.; Yu, M.,
409
Ultrathin, molecular-sieving graphene oxide membranes for selective hydrogen separation.
410
Science 2013, 342, 95-8.
411
25. Tang, Y. P.; Paul, D. R.; Chung, T. S., Free-standing graphene oxide thin films
412
assembled by a pressurized ultrafiltration method for dehydration of ethanol. J. Membr. Sci.
413
2014, 458, 199-208.
414
26. Hung, W.-S.; An, Q.-F.; De Guzman, M.; Lin, H.-Y.; Huang, S.-H.; Liu, W.-R.; Hu,
415
C.-C.; Lee, K.-R.; Lai, J.-Y., Pressure-assisted self-assembly technique for fabricating
416
composite membranes consisting of highly ordered selective laminate layers of amphiphilic
417
graphene oxide. Carbon 2014, 68, 670-677. 21
ACS Paragon Plus Environment
Environmental Science & Technology
Page 22 of 33
418
27. Hung, W.-S.; Tsou, C.-H.; De Guzman, M.; An, Q.-F.; Liu, Y.-L.; Zhang, Y.-M.; Hu,
419
C.-C.; Lee, K.-R.; Lai, J.-Y., Cross-Linking with Diamine Monomers To Prepare Composite
420
Graphene Oxide-Framework Membranes with Varyingd-Spacing. Chem. Mater. 2014, 26,
421
2983-2990.
422
28. Huang, K.; Liu, G.; Lou, Y.; Dong, Z.; Shen, J.; Jin, W., A graphene oxide membrane
423
with highly selective molecular separation of aqueous organic solution. Angew. Chem. Int.
424
Ed. Engl. 2014, 53, 6929-32.
425
29. Hu, M.; Mi, B., Enabling graphene oxide nanosheets as water separation membranes.
426
Environ. Sci. Technol. 2013, 47, 3715-23.
427
30. Han, Y.; Xu, Z.; Gao, C., Ultrathin Graphene Nanofiltration Membrane for Water
428
Purification. Adv. Funct. Mater. 2013, 23, 3693-3700.
429
31. Xu, C.; Cui, A.; Xu, Y.; Fu, X., Graphene oxide–TiO2 composite filtration membranes
430
and their potential application for water purification. Carbon 2013, 62, 465-471.
431
32. Huang, H.; Song, Z.; Wei, N.; Shi, L.; Mao, Y.; Ying, Y.; Sun, L.; Xu, Z.; Peng, X.,
432
Ultrafast viscous water flow through nanostrand-channelled graphene oxide membranes.
433
Nat. Commun. 2013, 4, 2979.
434
33. Nair, R. R.; Wu, H. A.; Jayaram, P. N.; Grigorieva, I. V.; Geim, A. K., Unimpeded
435
permeation of water through helium-leak-tight graphene-based membranes. Science 2012,
436
335, 442-4.
437
34. Joshi, R. K.; Carbone, P.; Wang, F. C.; Kravets, V. G.; Su, Y.; Grigorieva, I. V.; Wu, H.
438
A.; Geim, A. K.; Nair, R. R., Precise and ultrafast molecular sieving through graphene oxide 22
ACS Paragon Plus Environment
Page 23 of 33
Environmental Science & Technology
439
membranes. Science 2014, 343, 752-4.
440
35. Mi, B., Materials science. Graphene oxide membranes for ionic and molecular sieving.
441
Science 2014, 343, 740-2.
442
36. Hu, M.; Mi, B., Layer-by-layer assembly of graphene oxide membranes via
443
electrostatic interaction. J. Membr. Sci. 2014, 469, 80-87.
444
37. Dreyer, D. R.; Miller, D. J.; Freeman, B. D.; Paul, D. R.; Bielawski, C. W., Perspectives
445
on poly(dopamine). Chem. Sci. 2013, 4, 3796.
446
38. Wang, Z.; Jiang, X.; Cheng, X.; Lau, C. H.; Shao, L., Mussel-Inspired Hybrid Coatings
447
that Transform Membrane Hydrophobicity into High Hydrophilicity and Underwater
448
Superoleophobicity for Oil-in-Water Emulsion Separation. ACS Appl. Mater. Interfaces
449
2015, 7, 9534-45.
450
39. Li,
451
polyglycerol-grafted poly(ether sulfone) hollow fiber membranes for osmotic power
452
generation. Environ. Sci. Technol. 2014, 48, 9898-907.
453
40. Zhang, S.; Wang, K. Y.; Chung, T.-S.; Chen, H.; Jean, Y. C.; Amy, G., Well-constructed
454
cellulose acetate membranes for forward osmosis: Minimized internal concentration
455
polarization with an ultra-thin selective layer. J. Membr. Sci. 2010, 360, 522-535.
456
41. Socrates, G., Infrared and Raman Characteristic Group Frequencies: Tables and
457
Charts. Wiley: 2004.
458
42. Tsou, C.-H.; An, Q.-F.; Lo, S.-C.; De Guzman, M.; Hung, W.-S.; Hu, C.-C.; Lee, K.-R.;
459
Lai, J.-Y., Effect of microstructure of graphene oxide fabricated through different
X.;
Cai,
T.;
Chung,
T.
S.,
Anti-fouling
behavior
of
hyperbranched
23
ACS Paragon Plus Environment
Environmental Science & Technology
Page 24 of 33
460
self-assembly techniques on 1-butanol dehydration. J. Membr. Sci. 2015, 477, 93-100.
461
43. Gao, J.; Sun, S.-P.; Zhu, W.-P.; Chung, T.-S., Polyethyleneimine (PEI) cross-linked P84
462
nanofiltration (NF) hollow fiber membranes for Pb2+ removal. J. Membr. Sci. 2014, 452,
463
300-310.
464
44. Wang, H.; Zhang, Q.; Zhang, S., Positively charged nanofiltration membrane formed by
465
interfacial polymerization of 3,3′,5,5′-biphenyl tetraacyl chloride and piperazine on a
466
poly(acrylonitrile) (PAN) support. J. Membr. Sci. 2011, 378, 243-249.
467
45. Sun, S. P.; Hatton, T. A.; Chung, T. S., Hyperbranched polyethyleneimine induced
468
cross-linking of polyamide-imide nanofiltration hollow fiber membranes for effective
469
removal of ciprofloxacin. Environ. Sci. Technol. 2011, 45, 4003-9.
470
46. Zhu,
471
polybenzimidazole/polyethersulfone (PBI/PES) nanofiltration (NF) hollow fiber membranes
472
for heavy metals removal from wastewater. J. Membr. Sci. 2014, 456, 117-127.
473
47. Al-Rashdi, B. A. M.; Johnson, D. J.; Hilal, N., Removal of heavy metal ions by
474
nanofiltration. Desalination 2013, 315, 2-17.
W.-P.;
Sun,
S.-P.;
Gao,
J.;
Fu,
F.-J.;
Chung,
T.-S.,
Dual-layer
475 476 477
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TOC Art
479 480
481
25
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C-O-C
-OH hydroxyl
(a)
C=O C=C
(b)
C-OH -CN- secondary amine
-NH secondary amine
(c)
pristine GO GO&EDA GO&EDA_HPEI 60K
-NH Primary amine 4500
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4000
3500
3000
2500
Wavenumber
2000
1500
1000
500
0
(cm-1)
Figure 1. FTIR spectra of the GO layers: (a) pristine GO, (b) GO&EDA and (c) GO&EDA_HPEI 60K framework membranes, measured in the transmission mode.
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O
OH HO2C
OH
O
OH
O
O
O
O
CO2H
OH
OH
OH
O
O
OH
HO2C
CO2H
CO2H
OH
OH
OH
CO
OH
O
(a)
OH
HO2C
OH
OH
OH
CO
OH OH
O
OH CO H 2 OH
HO2C
OH HO2C
OH
O
(b)
HO2C
OH
OH
O
(c)
Figure 2. Schematic diagrams of the purposed structures: (a) pristine GO, (b) GO&EDA and (c) GO&EDA_HPEI 60K frameworks. ACS Paragon Plus Environment
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1 μm
(b)
1 μm
(c)
100 nm
(d)
100nm nm 100
(e)
20 nm
(a)
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0
-20 -40
Membrane average thickness: 69.41 ± 3.85 nm
-60 -80 -100 0
2
4
6
8
10
12
Figure 3. The top surface morphology of (a) the Whatman® Cyclopore® polycarbonate membrane and (b) the GO&EDA_HPEI 60K composite membrane. The cross section morphology of (c) the substrate layer and (d) the GO layer. (e) The AFM image of the GO layers on a mica film. ACS Paragon Plus Environment
14 μm
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PWP (LMH bar -1)
(a) 15
10
5
0 pristine GO
Salt Rejection (%)
(b)100
MgCl2
GO&EDA
NaCl
MgSO4
GO&EDA_HPEI 60K Na2SO4
80 60 40 20
0 pristine GO
GO&EDA
GO&EDA_HPEI 60K
Figure 4. (a) The PWP of the three GO framework membranes at a transmembrane pressure of 1 bar, and (b) the rejection performance ofACS theParagon membranes for four different salt solutions (each 1000 Plus Environment ppm).
Environmental Science & Technology
0
0.55
Mean depth (µm) 1.68 3.21 5.08
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7.26
9.72
S- parameter
0.52 0.5 0.48 pristine GO
0.46
GO&EDA
0.44
GO&EDA_HPEI 60K
0.42 0
0
5
4.6
10 15 20 25 Incident positron energy (keV)
30
Mean depth (nm) 13.9 26.6 40.0 60.2 80.6 103.1 127.6
S- parameter
0.47 0.46 GO framework layer
0.45 0.44 0.43 0.42 0
0.25
0.5 0.75 1 1.25 1.5 1.75 Incident positron energy (keV)
2
Figure 5 . S- parameters of the three GO framework membranes against the incident positron energy. ACS Paragon Plus Environment
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150
ζ-potential (mV)
100 50 0 0
2
4
6
8
12
pH
-50
-100
10
pristine GO GO&EDA
-150
GO&EDA_HPEI 60K
Figure 6. ζ-potential as a function of pH of the three GO framework membranes. ACS Paragon Plus Environment
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Table 1. Effects of post-treatment agents on NF performance of 1000 ppm MgCl 2 .
Reaction duration*
PWP (LMH bar -1)
Rejection (%)
ζ-potential (mV)
30 s
0.39
89.0
72.6
HPEI 60K
20 min
5.01
96.3
92.6
Dendrimer G(2,0)
20 min
1.20
97.2
101.0
PEI 2K
*
The reaction duration was optimized to the highest rejection performance
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Table 2. A benchmarking of NF membranes for heavy metal removal.
Membrane Chitosan PES composite membrane
PWP (LMH bar -1)
Ion
Testing condition
Rejection (%)
Refs.
3.45
NiCl2
1000 ppm, 10 bar
96.3
6
Pb(NO3)2
93.0
Chelating polymer modified P84
˃1
Pb(NO3)2
1000 ppm, 10 bar
˃ 99
7
Kraton matrimid composite membrane
2.4
Pb(NO3)2
1000 ppm, 10 bar
99.8
8
˃ 98
CdCl2 PBI/PES dual-layer hollow fiber
0.826
Pb(NO3)2
200 ppm, 1 bar
93
42
Dow membrane NF270
13.2
Pb(NO3)2
1000 ppm, 4 bar
≈ 60
43
≈ 68
CdCl2 HPEI modified GO&EDA framework membrane
5.01
Pb(NO3)2
1000 ppm, 1 bar
95.7 ± 0.7
NiCl2
96.0 ± 3.8
ZnCl2
97.4 ± 2.0
CdCl2
90.5 ± 0.1
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