Journal Pre-proof Effect of ultrasonication parameters on forward osmosis performance of thin film composite polyamide membranes prepared with ultrasound-assisted interfacial polymerization Liang Shen, Wei-song Hung, Jian Zuo, Lian Tian, Ming Yi, Chun Ding, Yan Wang PII:
S0376-7388(19)32868-6
DOI:
https://doi.org/10.1016/j.memsci.2020.117834
Reference:
MEMSCI 117834
To appear in:
Journal of Membrane Science
Received Date: 14 September 2019 Revised Date:
9 January 2020
Accepted Date: 10 January 2020
Please cite this article as: L. Shen, W.-s. Hung, J. Zuo, L. Tian, M. Yi, C. Ding, Y. Wang, Effect of ultrasonication parameters on forward osmosis performance of thin film composite polyamide membranes prepared with ultrasound-assisted interfacial polymerization, Journal of Membrane Science (2020), doi: https://doi.org/10.1016/j.memsci.2020.117834. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.
Author Statement
Liang Shen: Conceptualization, Methodology, Validation, Formal analysis, Investigation, Data Curation, Writing Original Draft, Writing-Review&Editing, Visualization Wei-song Hung: Investigation Jian Zuo: Investigation Lian Tian: Investigation Ming Yi: Investigation Chun Ding: Investigation Yan Wang: Conceptualization, Methodology, Validation, Writing-Review&Editing, Supervision, Project administration, Funding acquisition
Graphic Abstract for Effect of Ultrasonication Parameters on Forward Osmosis Performance of Thin Film Composite Polyamide Membranes Prepared with Ultrasound-Assisted Interfacial Polymerization” Liang Shen, Wei-song Hung, Jian Zuo, Lian Tian, Ming Yi, Chun Ding, and Yan Wang a,b*
Ultrasound power: 360 W
Ultrasound frequency: 60 KHz
480-600 W
40 KHz
Ultrasound power: 360 W Ultrasound frequency: 40 KHz
Ultrasound time: Short
Modification efficiency Balance
Ultrasound frequency: 40 KHz 60 KHz
Ultrasound time: Long
Weak Strong
Morphology change Thinner PA layer with smoother surface & smaller free volume
Thicker PA layer with rougher surface & larger free volume
1
Effect of Ultrasonication Parameters on Forward Osmosis Performance of Thin
2
Film Composite Polyamide Membranes Prepared with Ultrasound-Assisted
3
Interfacial Polymerization
4 5
Liang Shen a,b, Wei-song Hung c,d, Jian Zuo e, Lian Tian a,b, Ming Yi a,b, Chun Ding a,b
6
and Yan Wang a,b*
7 a
8
Key Laboratory of Material Chemistry for Energy Conversion and Storage
9
(Huazhong University of Science and Technology), Ministry of Education, Wuhan,
10
430074, China b
11
Hubei Key Laboratory of Material Chemistry and Service Failure, School of
12
Chemistry and Chemical Engineering, Huazhong University of Science and
13
Technology, Wuhan, 430074, P.R. China c
14
Graduate Institute of Applied Science and Technology, National Taiwan University of Science and Technology, Taipei, 10607, Taiwan
15 16
d
R&D Centre for Membrane Technology, Chung Yuan Christian University, Taoyuan, 32023, Taiwan
17 18
e
Singapore Institute of technology, 10 Dover Drive, Singapore 138683, Singapore
19 20 21
* Corresponding author. Tel.: 86 027-87543032; fax: 86 027-87543632.
22
E-mail address:
[email protected] (Yan Wang)
1
23
ABSTRACT
24 25
High-performance thin-film composite (TFC) membranes with a high water
26
permeability and high salt rejection are requisites for the successful development of
27
the forward osmosis technology. Based on our previous work on high-performance
28
TFC membranes obtained by a novel ultrasound-assisted interfacial polymerization
29
with different ultrasonication powers, this work conducts a comprehensive
30
investigation of the effects of various ultrasonication parameters including
31
ultrasonication power, frequency, and time on the membrane performance. The effects
32
of the ultrasonication parameters on the chemical and surface properties, morphology,
33
free volume, and the corresponding separation performance characteristics were
34
investigated systematically. The modified TFC membrane obtained at the optimized
35
ultrasonication conditions shows a water flux of 120.1±2.1 LMH (2.6 times higher
36
than that of the control membrane) and a reverse salt flux of 12.1±0.7 gMH (34.2%
37
reduction relative to that of the control membrane) in the pressure retarded osmosis
38
mode with the draw solution of 2 M NaCl and the feed solution of deionized water.
39 40
Keywords:
Ultrasound-assisted
interfacial
41
Thin-film composite membrane; Polyamide; Ultrasonication parameters
42
2
polymerization;
Forward
osmosis;
43
1. Introduction
44 45
Due to their ultrathin selective layer, thin-film composite (TFC) membranes with
46
nanometer or sub-nanometer scale pores have emerged as mainstream candidates for
47
use in a wide variety of separation fields, including nanofiltration [1-4], reverse
48
osmosis [5-7] and forward osmosis (FO) [8-10]. Commonly, the polyamide (PA)
49
selective layer is formed by aromatic amine and acyl halide via interfacial
50
polymerization (IP), and is highly-crosslinked and inherently hydrophobic, leading to
51
the low water flux. Additionally, the permeability-rejection trade-off relationship also
52
limits the improvement in the membrane separation properties.
53
It is well-known that the bulk properties of the PA layer are important factors for
54
determining the separation performance of TFC membranes, and are controlled by the
55
interplay between the diffusion and reaction of both monomers [11, 12]. Since the IP
56
reaction generally occurs in the organic phase due to the solubility differences
57
between the two monomers [11, 12], the morphology and microstructure of the
58
resultant PA layer and the separation performance of the TFC membranes are mainly
59
affected by the absolute IP reaction rate, that is governed by the diffusion of the amine
60
monomers into the organic phase [13].
61
Massive efforts have been made to optimize and alter the IP reaction by various
62
modifications to improve the overall separation performance of TFC membranes. One
63
widely-used strategy is to utilize additives (such as the phase transfer catalyst) in one
64
of the two phases to facilitate the penetration of amine monomers into the reaction
65
zone [14], or reduce (accelerate) the IP reaction rate with an inhibitor [15] (catalyst
66
[16]), resulting in the optimized separation properties. Another strategy is to use a
67
co-solvent (such as dimethyl sulfoxide, acetone, or alcohols) in the aqueous phase to 3
68
form a transition layer between the two phases to enhance the phase miscibility,
69
increasing the amount of diffused amine monomer, and resulting in the formation of a
70
rougher PA layer [17]. Furthermore, the addition of nanomaterials into the monomer
71
solution may also affect the IP process, giving rise to the formation of a thinner and
72
smoother PA layer, and hence the improved separation performance [18, 19].
73
Moreover, the molecular layer-by-layer method that utilizes a single toluene solvent to
74
dissolve both monomers has been reported recently for the fabrication of the PA layer
75
of TFC membranes [20]. This method can control the thickness and roughness of the
76
resulting PA layer by overcoming the kinetic and mass transfer limitations of the
77
traditional IP, contributing to the optimized separation properties of the resultant TFC
78
membranes [20].
79
Recently, we developed a novel green method called ultrasound-assisted
80
interfacial polymerization (UAIP) for fabricating TFC membranes with excellent
81
separation properties [21]. The effect of the ultrasonication power on the bulk
82
properties and separation performance of the obtained TFC membranes was also
83
studied. The sonochemical effect of the ultrasonication not only enlarges the mixing
84
interface to increase the polymerization area, but also promotes the diffusion of amine
85
monomers, resulting in the better mixing efficiency of the two monomers, and thus a
86
more complete IP reaction. Additionally, the introduced ultrasound waves also
87
disrupted the PA chain packing, generated more nanobubbles, and increased the
88
amount of the amine penetrating into the organic phase, giving rise to the formation of
89
a looser PA layer with a larger free volume [22, 23]. With the increased
90
ultrasonication power, the resultant membrane exhibits a rougher and thicker PA layer
91
with a larger free volume, achieving greatly improved separation performance.
92
This study is a continuation of our previous work and investigates the effects of 4
93
different ultrasonication parameters (including power, frequency, and time) on the
94
bulk and separation properties of the resultant TFC membranes. Theoretically, when a
95
higher ultrasonication frequency is applied, more ultrasound waves with relatively
96
lower energy can be generated [24], exerting more frequent but less intense effects on
97
the formation of the PA layer during the IP process. Meanwhile, a longer
98
ultrasonication time may facilitate the diffusion of more amine monomers into the
99
organic phase, increasing the amount of the amine monomer involved in the IP
100
reaction and therefore obtaining a rougher, thicker and looser PA layer. As a
101
continuation of our previous work [21], here we comprehensively explore the
102
interplay between the ultrasonication power and frequency, as well as its impacts on
103
the morphology and microstructure, and the separation performance of the obtained
104
TFC membranes. Moreover, the effect of the ultrasonication time on these properties
105
is also studied systematically. Therefore, the aim of this work is to comprehensively
106
explore and optimize the ultrasonication parameters in order to develop a TFC
107
membrane with the desirable morphology, microstructure, and improved separation
108
performance.
109 110
2. Materials and methods
111 112
2.1 Materials and chemicals
113 114
Polysulfone (PSF, Mw: 800 kDa), N-methyl pyrrolidone (NMP, ≥99.5%) and
115
polyethylene glycol 400 (PEG-400, 99%) were applied for fabricating the substrate
116
membrane. M-phenylenediamine (MPD, 99.5%), trimesoyl chloride (TMC, 98%), and
117
n-hexane (≥99%) were used for preparing the PA layer. Sodium chloride (NaCl, 5
118
≥99.5%) was employed in FO and reverse osmosis (RO) tests.
119 120
2.2 Preparation of TFC membranes
121 122
PSF substrates and PA active layers were prepared by the phase inversion and IP
123
methods, respectively. The detailed procedures can be referred to our previous works
124
[25-29]. A brief description about the fabrication of the PA layer under ultrasonication
125
is shown as below [21]. First, the substrate washed by ultrapure water was immersed
126
in 2.0 wt% MPD aqueous solution 2 min. After the removal of excessive amine
127
solution, 0.1 wt% TMC/hexane solution was brought to contact with the substrate for
128
1 min under an ultrasonication circumstance. After the organic solution discarded, the
129
obtained TFC membrane was stored in deionized (DI) water. The ultrasonication
130
powers and frequencies applied in this work were 360, 480, 600 W, and 40, 60 kHz,
131
respectively. These obtained membranes were denoted as PA-p-f, which p and f
132
represent the ultrasonication power and frequency, respectively. Meanwhile, these
133
modified membranes formed with different ultrasonication time (0 - 60 seconds) (here
134
the ultrasonication power and frequency were fixed at 360 W and 40 kHz) were
135
denoted as PA-T, which T stands for the ultrasonication time. For example, PA-15s
136
refers to the membrane formed under UAIP for 15 seconds.
137 138
2.3 Membrane characterizations
139 140
Surface and bulk properties of obtained TFC membranes were examined by
141
various characterization techniques, and a detailed description can be found in the
142
Supporting Information. Specifically, the crosslinking degree (CD) value of the PA 6
143
network (Fig. 1) can be calculated from the X-ray Photoelectron Spectroscopy (XPS)
144
results based on Eqs. (1) and (2). Here x and y refer to the repeat unit numbers of
145
fully-crosslinked and linearly-crosslinked parts in the PA network, respectively.
146
147
=
(1)
CD =
× 100% =
×
(2)
× 100%
148
149 150 151
Fully-crosslinked PA network
Linearly-crosslinked PA network
Fig. 1. Molecular structures of fully and linearly crosslinked PA networks
152 153
2.4 Evaluation of separation and antifouling properties of TFC membranes
154 155
The intrinsic transport properties of the TFC membranes were determined by RO
156
tests using a RO system. The FO performance and antifouling capacity were
157
evaluated by FO tests using a lab-scale FO set-up. The detailed descriptions also can
158
be found in the Supporting Information.
159 160
3. Results and discussion
161 162
3.1 Effect of ultrasonication power and frequency
163 7
164
In this work, two ultrasonication frequencies (40 and 60 kHz) are used for the
165
preparation of TFC membranes by the UAIP process. On the one hand, ultrasound
166
waves generated at a low frequency are generally energetic, and may destroy the
167
nascent PA layer easily at a high power density [21]. On the other hand, at a
168
high-frequency condition, the sound is readily attenuated, so that the cavitation
169
bubbles are smaller and collapse less energetically with lower temperature and
170
pressures [30], resulting in the weaker cavitation effect. This is due to the rarefaction
171
cycles are too short to allow the growth of developed cavitation bubbles to the
172
equilibrium size [30, 31]. When the applied power density is low, the generated sound
173
waves have low energy, particularly for the high-frequency condition with more
174
sound waves where a lower amount of effective sound waves leads to a weaker
175
sonochemical effect in the IP process. On the other hand, when the power density
176
increases, the sonochemical effect is enhanced because both the number of the
177
cavitation bubbles and the size of the cavitation zone increase with the higher pressure
178
amplitude of the sound waves [30, 32]. Moreover, hydrodynamic turbulence also
179
increases with the higher power density, as the result of the combined effects of the
180
higher bubble implosion density, larger number of bubbles, and greater absorption of
181
the acoustic energy by the medium [30, 32]. Therefore, for a sufficiently high power
182
density, more sound waves will be energetic and effective. Accordingly, the
183
sonochemical effect in the high-frequency condition is more pronounced than that in
184
the low-frequency condition due to more effective cavitation bubbles.
185
Additionally, ultrasonication power in the range of 360-600 W is applied in this
186
work due to the relatively better sonochemical effect of ultrasonication in this power
187
range as found in our previous work [21]. According to the above analysis, for the
188
ultrasonication power of 360 W, the modification effect of the ultrasonication with a 8
189
higher frequency is expected to be weaker. By contrast, when the applied power
190
density increases to 480-600 W, the modification effect of the ultrasonication with a
191
higher frequency is expected to be stronger than that of the lower-frequency condition.
192
The different modification effects are verified by the examination of the variations in
193
the chemical properties, micro-morphology, and the separation properties of the
194
resultant TFC membranes as described below.
195
The chemical changes in the PA layers due to the use of ultrasonication are
196
examined by XPS, and the corresponding results are displayed in Figs. 2 and S1 and
197
Table S1. Fig. 2 shows that the O/N ratios of the modified membranes increase
198
compared to that of the control membrane, suggesting the higher crosslinking degrees
199
(CDs) of the modified PA layers, particularly under a higher ultrasonication power.
200
This behavior is resulted from the more complete IP reaction achieved by
201
ultrasonication assistance, as discussed in our previous work [21]. It is also observed
202
that with the exception of the PA-360-60 membrane, the O/N ratios of the modified
203
membranes formed with a frequency of 60 kHz are lower than those of the modified
204
membranes formed with a frequency of 40 kHz. This is ascribed to the more efficient
205
monomer mixing under the higher ultrasonication frequency condition with a
206
relatively high ultrasonication power, as discussed above.
207
The micro-structural changes introduced to the resultant PA layers by
208
ultrasonication are further examined by Positron Annihilation Lifetime Spectra (PALS)
209
characterization. Figs. 3a and S2-a show that the S values of the modified membranes
210
increase compared to that of the control one, suggesting the formation of a looser PA
211
layer. This is believed to be due to the looser PA chain packing [21], greater
212
generation of nanovoids [22], and greater MPD penetration [23] obtained with the
213
assistance of ultrasonication in the IP process. With the exception of the membranes 9
214 215 216
217 218
Fig. 2. O/N ratios and crosslinking degrees (CD) of the control membrane and
219
modified ones formed with different ultrasonication powers and frequencies
220 221
formed at the ultrasonication power of 360 W, the S values of all modified membranes
222
formed under ultrasonication at 60 kHz are higher than those of the membranes
223
formed under ultrasonication at 40 kHz with the same ultrasonication power, as
224
shown in Fig. 3a. The larger S values indicate the looser structure of the PA layers
225
formed under a higher frequency that is most likely due to the greater amount of free
226
volume cavities generated with a larger pore size as derived from the orth-positron
227
(o-Ps) lifetime results. The smaller S value of the PA-360-60 membrane compared to
228
that of the PA-360-40 membrane is believed to be due to the less energetic cavitation
229
effect.
10
230
The lifetime results presented in Figs. 3b and 3c further reveal the free volume
231
properties of the obtained PA layers. Fig. 3b shows that the free volume pore radii (R)
232
of the modified PA layers (2.932-3.063 Å) are all larger than that of the control one
233
(2.878 Å). Additionally, the order of the pore radius (R) values is consistent with the
234
order of the S values, that is, PA-0 < PA-600-40 < PA-360-60 < PA-480-40 <
235
PA-360-40 < PA-600-60 < PA-480-60. Moreover, as observed from Fig. 3c, despite
236
the different sequence of the free volume intensity (I3) values, i.e., PA-0 < PA-360-60
237
< PA-600-40 < PA-480-40 < PA-360-40 < PA-600-60 < PA-480-60, the fractional free
238
volume (FFV) result is still consistent with the S value result that is positively related
239
to the free volume pore radius and amount (density). With an intermediate
240
ultrasonication power density (360 W), the modification efficiency for a lower
241
ultrasonication frequency (40 kHz) is higher than that with a higher frequency (60
242
kHz). This may be ascribed to the greater impact of the acoustical cavitation on the
243
above-described factors. By contrast, a high ultrasonication power density (480-600
244
W) coupled with a high frequency favors stronger sonochemical effects that are most
245
likely due to enhanced acoustical cavitation.
246 247
(a)
(b)
248 249
(c) 11
250 251
Fig. 3. (a) S values at 2 keV, (b) o-Ps lifetime distribution, and (c) free volume pore
252
parameters of the control membrane and modified ones formed with various
253
ultrasonication powers and frequencies
254 255
The more efficient monomer mixing under ultrasonication is expected to benefit
256
the formation of a thicker and rougher PA layer. As observed from Fig. 4a that
257
compared to the control membrane, the ridge-and-valley structural feature becomes
258
more pronounced on the surface of the modified membranes, which also can be
259
confirmed by AFM images (Fig. S3). Additionally, rougher and thicker PA layers are
260
formed at a higher ultrasonication frequency compared to those formed at a lower
261
frequency (except for the PA-360-60 membrane). This is ascribed to the more
262
efficient monomer mixing and a larger reaction interface for the interfacial
263
polymerization. Specifically, for the PA-600-40 membrane, unlike for the PA-600-60
264
membrane, the roughness and thickness of the PA layer is even lower than that of the
12
265
PA-480-40 membrane. This phenomenon could be due to the fact that the
266
ultrasonication power (600 W) is very strong and provides the energy for the
267
destruction of the nascent formed PA layer, leading to the defective PA layer with
268
non-uniformly distributed ridge-and-valley structures [21]. Due to the higher surface
269
roughness, the WCA values of the membranes formed by UAIP are lower than that of
270
the membrane formed by the traditional IP as displayed in Fig. 4b, indicating the
271
improved surface hydrophilicity. Except for the PA-360-60 membrane, better surface
272
hydrophilicity of the formed TFC membrane can be obtained with a higher
273
ultrasonication frequency of 60 kHz employed in UAIP.
274 275
(a)
276 277
(b)
13
278 279
Fig. 4. (a) SEM images, (b) average roughness (Ra) and WCAs of the control
280
membrane and modified membranes formed with various ultrasonication powers and
281
frequencies
282 283
The RO results of these membranes prepared under different ultrasonication
284
powers and frequencies are shown in Fig. 5a. It is observed that the water permeances
285
(A) of the modified membranes increase compared to that of the control one due to the
286
better surface hydrophilicity, larger FFV, and rougher surface. Additionally, it is
287
interesting to note that the water permeances of the modified membranes formed
288
under ultrasonication at 60 kHz show non-monotonic changes with increasing power,
289
while those of the modified membranes formed under ultrasonication at 40 kHz
290
decrease monotonically. With the further increase in the ultrasonication power, a
291
reduction in the water fluxes is observed for both the PA-480-40 and PA-600-60
292
membranes, caused by the thicker PA layer and the smoother surface, respectively.
293
Furthermore, with the exception of the PA-360-60 membrane, the water permeances 14
294
of the modified membranes under ultrasonication at 60 kHz are all larger than those of
295
the modified membranes obtained under ultrasonication at 40 kHz, due to the further
296
improved membrane hydrophilicity, surface roughness, and FFV.
297
On the other hand, modified membranes with a thicker PA layer also show higher
298
salt rejections (Rs), except for the PA-600-40 membrane due to the formation of a
299
defect-containing PA layer [21]. In addition, with the exception of the PA-360-60
300
membrane, modified membranes formed under ultrasonication at 60 kHz exhibit
301
higher salt rejections than those of the modified membranes formed under
302
ultrasonication at 40 kHz with the same ultrasonication power, which is consistent
303
with the variation of the PA layer thickness. Moreover, the salt rejections of the
304
modified membranes obtained under both ultrasonication frequencies increase with
305
increasing power (with the exception of the PA-600-40 membrane) because of the
306
formed thicker PA layer. Additionally, except for the PA-600-60 membrane, the salt
307
permeability (B) of the control membrane is lower than or comparable to those of
308
modified ones. Accordingly, the B/A ratio of the control membrane is lower than those
309
of the modified ones (with the exception of the PA-600-40 membrane), indicating the
310
improved permselectivity.
311
The FO performance characteristics of these membranes are also evaluated (Fig.
312
5b). In accordance with their intrinsic transport properties, the water flux of the
313
control membrane is lower than those of the modified membranes, as the result of the
314
lower surface hydrophilicity, smoother surface, and smaller FFV. Accordingly, except
315
for the PA-360-60 membrane, the modified membranes formed under ultrasonication
316
at 60 kHz exhibit increased water fluxes compared to those of the modified ones
317
formed under ultrasonication at 40 kHz with the same power condition. In addition,
318
with the exception of the PA-600-40 membrane, the reverse salt fluxes of the 15
319 320
(a)
321 322
(b)
323 324
Fig. 5. (a) Intrinsic transport properties and (b) FO separation performance of the 16
325
control membrane and modified ones formed with different ultrasonication powers
326
and frequencies
327 328
modified membranes with thicker PA layers decrease compared to that of the control
329
one, and decrease with increasing ultrasonication power. Moreover, with the
330
exception of the PA-360-60 membrane, the reverse salt fluxes of the modified
331
membranes obtained under ultrasonication at 60 kHz are lower than those of the
332
modified ones obtained under ultrasonication at 40 kHz at the same power condition.
333 334
3.2 Effect of ultrasonication time
335 336
In theory, with a longer ultrasonication time, more amine monomers can diffuse
337
into the organic phase due to the longer duration of the sonochemical effect on the IP
338
process, resulting in a higher IP reaction degree, and therefore the formation of a
339
thicker, rougher, and looser PA layer.
340
XPS results in Fig. S4 reveals that the O/N ratios of the obtained membranes
341
decrease with increasing ultrasonication time, indicating a higher crosslinking degree
342
(20.59-48.96%) due to the more complete IP reaction. Additionally, Fig. 6a shows
343
that the roughness and thickness of these membranes increase monotonically with
344
increasing ultrasonication time. Correspondingly, the WCA values of the obtained
345
membranes decrease with increasing ultrasonication time, as displayed in Fig. 6b,
346
indicating the improved surface hydrophilicity due to the rougher surface.
347
PALS characterizations of the PA layers formed under ultrasonication for different
348
time are performed to study the microstructure changes. As displayed in Fig. 7, the S
349
values, free volume pore radii, and FFVs of the modified membranes increase with 17
350
increasing ultrasonication time, indicating the formation of an increasingly loose PA
351
layer with the longer ultrasonication time.
352 353
(a)
354 355
(b)
356 357
Fig. 6. (a) SEM images, (b) average roughness (Ra) and WCAs of as-fabricated
358
membranes with different ultrasonication time
18
359
(a)
(b)
360
(c)
361
362 363
Fig. 7. (a) S values and (b) free volume pore parameters of as-fabricated membranes
364
formed with different ultrasonication time
365 366
Accordingly, the FO performance characteristics of the obtained TFC membranes
367
are studied as shown in Fig. 8. It can be found that the water fluxes of the membranes
368
increase monotonically with increasing ultrasonication time, while the reverse salt 19
369
fluxes present non-monotonic changes with increasing ultrasonication time. Due to
370
the increased free volume pore size, the reverse salt fluxes of the membranes formed
371
by UAIP with a shorter ultrasonication time (15-30 s) increase compared to that of the
372
control one. By contrast, with a further increase in the ultrasonication time (45-60 s),
373
the thicker PA layer contributes to the lower reverse salt flux of the modified
374
membranes. The RO performance results (Table S2) are consistent with the FO
375
performance results. The water permeances of the resultant membranes increase with
376
longer ultrasonication time. Meanwhile, the salt rejection (Rs) values of the modified
377
membranes first tend to decrease and then increase with the longer ultrasonication
378
time. Moreover, the salt permeabilities (B) of the resultant membranes decrease
379
monotonically with the increase in the ultrasonication time. Accordingly, the B/A
380
results of as-fabricated membranes are opposite to their Rs results.
381
382 383
Fig. 8. FO performance of TFC membranes formed with various ultrasonication time
384 385
To further explore the relationship between the pore size and the separation
386
performance, Fig. 9 presents the results for the water flux and salt rejection of the 20
387
as-fabricated membranes as a function of the mean free volume pore radius. It is
388
observed that the water flux presents a rough rising trend with the increase of free
389
volume pore radius. Generally, a membrane with a larger free volume should have a
390
high water flux because the flux is proportional to the size and density of the free
391
volume in the PA layer [23, 33]. However, since both the free volume pore density
392
and surface roughness are also important factors that determine the membrane
393
permeability, the water flux of a membrane with a large free volume pore size but a
394
low pore density and/or smooth surface may not follow the trend strictly. By contrast,
395
salt rejection is not directly affected by the changes in the free volume pore radius,
396
because it is mainly governed by the amount of the free volume pores larger than the
397
hydrated salt [23] and the PA layer thickness rather than by the mean free volume pore
398
radius.
399
400 401 402
Fig. 9. Water flux and salt rejection of obtained TFC membranes as a function of mean free volume pore radius
403 404
3.3 Antifouling properties of TFC membranes
405 21
406
The effects of the ultrasonication introduced in the IP on the antifouling properties
407
are also studied using the PA-0 membrane (smoothest surface, lowest crosslinking
408
degree and hydrophilicty) and the PA-600-60 membrane (roughest surface, highest
409
crosslinking degree and hydrophilicty) against three foulant systems, namely
410
inorganic gypsum foulant, organic sodium alginate (SA) foulant, and mixed foulant
411
containing both SA and Ca2+ ions.
412
The inorganic fouling test results are presented in Fig. 10a and show that in
413
comparison with the PA-0 membrane, the PA-600-60 membrane exhibits a lower flux
414
drop and a higher flux recovery. This implies that the modified membrane has
415
improved anti-scaling property that is mainly attributed to its highly-crosslinked PA
416
layer with fewer carboxylate groups. The gypsum scale fouling on PA-based
417
membranes is predominantly governed by the surface chemistry of the PA layer, i.e.,
418
the carboxylate groups [34-36]. It was reported in previous studies that the gypsum
419
fouling on the PA layer is mainly caused by surface heterogeneous crystallization that
420
proceeds through the stages of prenucleation cluster, amorphous nanoparticle, and
421
polycrystal [34]. The complexion of negatively-charged carboxylate groups with Ca2+
422
ions increases the Ca2+ ion concentration on the PA layer surface, inducing the
423
occurrence of the gypsum prenucleation [34, 36]. Therefore, a greater amount of
424
carboxylate groups in the PA-600-60 membrane leads to the more severe gypsum
425
fouling [34, 36].
426
The organic fouling results presented in Fig. 10b reveal that the PA-600-60
427
membrane exhibits a greater water flux decrease and less water flux recovery than
428
those of the PA-0 membrane. This is due to the rougher surface of the PA layer with
429
less negative charges. The SA fouling layer growth on the membrane surface
430
undergoes two stages, namely the stage driven by the membrane - foulant interaction 22
431
(a)
432 433
(b)
434 435
(c)
436 437
Fig. 10 Dynamic fouling testing results of the PA-0 and PA-600-60 membranes: (a)
438
inorganic fouling using gypsum as foulant, (b) organic fouling using SA as foulant, (c)
439
organic fouling using SA and Ca2+ ions as foulant
440 441
and the stage driven by the foulant - foulant interaction [37]. The formation of an 23
442
initial SA gel layer in the first stage is the necessary step for the SA membrane fouling
443
[37]. Generally, a rough PA surface is prone to the accumulation of SA molecules due
444
to its large surface area [37, 38] and uneven flux distribution [37, 39]. Additionally, a
445
PA layer with a greater amount of negatively-charged carboxylate groups can
446
electrostatically repel the negatively-charged SA molecules. Accordingly, the initial
447
formation of the SA layer in the modified PA layer with larger roughness and fewer
448
negative charges is easier, enhancing the further growth of the SA gel layer caused by
449
the foulant – foulant interaction.
450
The combined fouling results are presented in Fig. 10c, and show that both
451
membranes exhibit a more severe water flux reduction and less water flux recovery
452
compared to the SA-only fouling, because the presence of the Ca2+ ions in the SA
453
foulant solution aggravates the fouling behavior through its “bridge” effect for the
454
crosslinking of SA molecules that leads to the formation of a denser SA gel layer [37,
455
40]. However, unlike the organic fouling caused by SA, the PA-600-60 membrane
456
exhibits a slightly better antifouling capacity than that of the PA-0 membrane. This
457
may be resulted from the formation of a relatively looser SA gel layer in the
458
membrane-foulant interaction stage.
459 460
3.4 Benchmarking
461 462
Table 1 summarizes the separation performance benchmarking of the TFC
463
membrane developed in this work and some other TFC membranes recently reported
464
[41-50]. An examination of the listed data reveals that compared to the other TFC
465
membranes obtained using various modification approaches, the PA-480-60
466
membrane shows a much higher water flux and a comparable reverse salt flux. 24
467
Accordingly, the specific reverse salt flux (Js/Jv) of the PA-480-60 membrane is much
468
lower than those of most other reported membranes, indicating its much better
469
membrane permselectivity. The superior separation performance of the PA-480-60
470
membrane is attributed to the optimized microstructure and morphology of the PA
471
layer prepared via the UAIP process with optimized parameters.
472 473
Table 1 FO performance benchmarking of TFC membranes Membrane code
Jv Js Js/Jv Feed (LMH) (gMH) (g/L) solution 120.10
12.10
0.10
73.87
7.90
0.11
PA-PSF/LDH
34.60
12.70
0.37
PA-PES/SPES
35.10
9.90
0.28
PA-PSf/HNT
26.01
14.20
0.55
PA-AMPES/3
56.30
9.50
0.17
PA-PSf/zeolite
~85.00
~55.00
0.65
PA-PVDF nanofiber
30.40
6.40
0.21
PA-PSF/BP
74.40
11.88
0.16
PA-PVDF/SiO2 @MWCNT
22.10
4.10
0.19
PA-PSf/TiO2
31.20
6.66
0.21
PA-PSf/LDH /GO
13.5
5.5
0.41
PA-PSf/GO
19.77
3.36
0.17
PA-480-60
Draw solution
Operation mode
DI water
2M NaCl
PRO
DI water DI water DI water DI water DI water DI water DI water DI water DI water DI water DI water
1M NaCl 2M NaCl 2M NaCl 2M NaCl 2M NaCl 1M NaCl 2M NaCl 1M NaCl 1M NaCl 1M NaCl 0.5 M NaCl
Ref.
FO
This work
PRO
[41]
PRO
[42]
PRO
[43]
PRO
[44]
PRO
[45]
PRO
[46]
PRO
[47]
FO
[51]
FO
[48]
FO
[49]
FO
[50]
474 475
4. Conclusion
476 477
The present study is a continuation of our previous study for the development of 25
478
high-performance TFC membranes by the UAIP method with different ultrasonication
479
parameters (power, frequency, and time). The introduction of ultrasonication in the IP
480
process promotes efficient monomer mixing, resulting in the more complete IP
481
reaction. With a relatively high ultrasonication power (480-600 W), a high
482
ultrasonication frequency (60 kHz) with greater amount of effective cavitation
483
bubbles favors the efficient modification of the TFC membrane, resulting in the
484
higher crosslinking, larger free volume pore size, and rougher and thicker PA layer,
485
and therefore the superior separation performance of this membrane compared to
486
those of the TFC membrane formed under low-frequency (40 kHz) ultrasonication.
487
However, with a low power intensity at 360 W, the modification at the lower
488
ultrasonication frequency (40 kHz) is more effective than that at the higher frequency
489
(60 kHz), due to the stronger sonochemical effect of acoustical cavitation. As a result,
490
compared to the PA-360-40 membrane, the PA-360-60 membrane shows a lower
491
crosslinking degree, smaller free volume pore size, smoother and thinner PA layer,
492
and inferior separation performance. On the other hand, the crosslinking degree, S
493
value, FFV, roughness, thickness, and hydrophilicity of the PA layer in the resultant
494
TFC membrane increase with increasing ultrasonication time. Consequently, the
495
water fluxes of the resultant membranes increase monotonically with longer
496
ultrasonication time, while reverse salt fluxes present non-monotonic changes.
497
Moreover, despite the formation of a rougher PA layer under ultrasonication, the
498
modified membrane with fewer carboxylate groups exhibits improved antifouling
499
capacities against gypsum scaling and mixed SA/Ca2+ foulants, but poor resistance to
500
organic foulants of SA.
501 502 26
503
Acknowledgment
504 505
We thank the financial support from National Natural Science Foundation of
506
China (no. 21306058), and the Free Exploring Fundamental Research Project from
507
Shenzhen Research Council, China (no.JCYJ20160408173516757). We are indebted
508
to Prof. Tai-Shung Chung’s group in the National University of Singapore for his help
509
with PALS characterization. Special thanks are also given to the Analysis and Testing
510
Center, the Analysis and Testing Center of Chemistry and Chemical Engineering
511
School, and the State Key Laboratory of Materials Processing and Die & Mould
512
Technology, in Huazhong University of Science and Technology for their help with
513
material characterizations.
514 515
27
516
List of abbreviations and nomenclatures
517 518
Abbreviations
519 520
AFM
: atomic force microscopy
521
CD
: crosslinking degree
522
DI
: deionized
523
FFV
: fractional free volume
524
FO
: forward osmosis
525
IP
: interfacial polymerization
526
mLBL
: molecular layer-by-layer
527
MPD
: m-phenylenediamine
528
NaCl
: sodium chloride
529
NMP
: N-Methyl pyrrolidone
530
o-Ps
: orth-positron
531
PA
: polyamide
532
PALS
: position annihilation lifetime spectroscopy
533
PDF
: probability density function
534
PEG
: polyethylene glycol
535
PRO
: pressure retarded osmosis
536
PSf
: polysulfone
537
RO
: reverse osmosis
538
SEM
: scan electron microscopy
539
TFC
: thin-film composite
540
TMC
: 1,3,5-trimesoyl chloride 28
541
UAIP
: ultrasound-assisted interfacial polymerization
542
WCA
: water contact angle
543
XPS
: X-ray photoelectron spectroscopy
544 545
Nomenclatures
546 547
A
: water permeance
548
Am,FO
: effective membrane area in FO process
549
Am,RO
: effective membrane area in RO process
550
B
: salt permeability
551
Cf
: feed concentration
552
Cp
: permeate
553
Ct
: salt concentration
554
I3
: free volume intensity
555
J
: pure water flux
556
Js
: reverse salt flux
557
Js/Jv
: specific reverse salt flux
558
Jv
: water flux
559
LMH
: L·m-2·h-1
560
gMH
: g·m-2·h-1
561
R
: free volume pore size radius
562
Ra
: average roughness
563
Rs
: salt rejection
564
S
: ratio of total annihilation counts at 511 keV
565
∆P
: hydraulic pressure
concentration
29
566
∆t
: test time
567
∆V
: volume change
568
∆π
: osmotic pressure
569
τ3
: orth-positron lifetime
570
30
571
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37
Research Highlights for the manuscript “Effect of Ultrasonication Parameters on Forward Osmosis Performance of Thin Film Composite Polyamide Membranes Prepared with Ultrasound-Assisted Interfacial Polymerization” by Liang Shen, Wei-song Hung, Jian Zuo, Lian Tian, Ming Yi, Chun Ding, and Yan Wang a,b*
Effects of ultrasonication parameters on the PA layer formation were studied Modification efficiency varies with the ultrasonication power and frequency conditions Higher modification efficiency benefits the rougher, thicker and looser PA layer Longer ultrasonication time favors the high separation performance of TFC membranes Optimized ultrasonication condition benefits the high water permeance and salt rejection
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☒The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
No financial interest/personal relationship is considered as potential competing interests.