Fabrication of eco-friendly nanofibrous membranes functionalized with carboxymethyl-β-cyclodextrin for efficient removal of methylene blue with good recyclability

Article abstract of DOI:10.1039/c8ra07523a

Considering the excellent thermo-mechanical properties, chemical stability and low cost of biodegradable aliphatic-aromatic copolyesters, they are an ideal matrix when functionalized for capturing pollutants in wastewater. In this work, biodegradable poly((butylene succinate-co-terephthalate)-co-serinol terephthalate) (PBSST) copolyesters with amino side group (-NH₂) were first synthesized through copolymerization, followed by grafting carboxymethyl-β-cyclodextrin (CM-β-CD) into PBSST molecular chains via amidation reaction to prepare PBSST-g-β-CD. The corresponding nanofibrous membranes were then fabricated by electrospinning as adsorbents for efficiently removing cationic dye methyl blue (MB) from aqueous solutions. The adsorption performance of the nanofibrous membranes was fitted well with pseudo-second-order model and Langmuir isotherm model. The maximum adsorption capacity was 543.48 mg g^?1 for MB along with a removal efficiency of 98% after five regeneration cycles, indicating the high adsorption capacity and good recyclability of nanofibrous membranes. The adsorbents possess features of high adsorption capacity, eco-friendliness and easy operation, and exhibit great potential for disposing of printing-dying wastewater.

Full text of DOI:10.1039/c8ra07523a

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PAPER
Modification of poly(amide-urethane-imide) (PAUI)
thin film composite reverse osmosis membrane
with nano-silver particles
Cite this: RSC Adv., 2018, 8, 37817
ac
d
ac
Hao Wu,†ab Rui-Han Li,†ac Chun-yang Yu,
Xue-Ting Zhao
*
*
Li-Fen Liu,
and Cong-Jie Gaoac
A novel reverse osmosis (RO) composite membrane, poly(amide-urethane-imide@Ag) (PAUI@Ag), was
prepared on a polysulfone supporting film through two-step interfacial polymerization. First, in the 1st
interfacial polymerization procedure, a new tri-functional crosslinking agent with –OCOCl and –COCl
groups, 5-choroformyloxyisophaloyl chloride (CFIC), was reacted with 4-methyl-phenylenediamine
(MMPD) without curing treatment to obtain the poly(amide-urethane) base membrane with a CFIC–
MMPD precursor separation layer. And then N,N0-dimethyl-m-phenylenediamine (DMMPD) with nano-
Ag particle dispersion was introduced onto the base membrane to further construct a CFIC–DMMPD
modified ultrathin separation layer via the 2nd interfacial polymerization. Thus, the PAUI@Ag RO
membrane with poly(amide-urethane-imide) bi-layer skin was obtained. The membrane was
characterized for the chemical composition of separation layer, the membrane cross-section structure
and the membrane surface morphology. Permeation experiment was employed to evaluate the
PAUI@Ag membrane performance including salt rejection rate and water flux. The results revealed that
the PAUI@Ag membrane composed the highly cross-linked separation layer with entire ridges and
valleys, small surface roughness, and well dispersed nano-Ag particles. Upon exposure of the
membranes to high concentration of free chlorine solutions, the PAUI@Ag RO membrane showed
a slightly less chlorine-resistant property compared with the nascent PAUI RO membrane, but was still
superior to the conventional polyamide MPD-TMC RO membrane, meanwhile it processed higher anti-
biofouling property.
Received 8th June 2018
Accepted 31st October 2018
DOI: 10.1039/c8ra04906h
rsc.li/rsc-advances
two major obstacles for the aromatic polyamide RO
membranes, which limit further application of RO membrane
technology.15–20 To address these two problems, various
1. Introduction
Reverse osmosis (RO) technology is increasingly used in
seawater desalination, ultrapure water production and waste-
water treatment today.1–4 The reverse osmosis membrane is the
core of RO technology and various polymers have been tried for
the production of thin-lm composite (TFC) RO membranes.5–13
Due to the excellent separation performances including water
ux and salt rejection, aromatic polyamide (PA) membranes
have been widely accepted as the optimal among such RO
membranes.6,14 However, oxidation and fouling are currently
approaches have been performed including pretreatment of the
RO feed solution, development of new membrane materials,
surface modication of commercial RO membranes,21–36 opti-
mization of process conditions, and periodic cleaning.37–51 Even
aer long periods of such developments, however, biofouling is
still the major reason for performance deterioration of
conventional PA membranes including water ux decline and
salt rejection reduction.52–56
Membrane biofouling is attributed to the biolm formation
that is caused by the adhesion and growth of microbes on the
surface of RO membrane during use.57–60 At present, the
common strategy in preventing membrane biofouling is to add
bactericide such as free chlorine into the feed in RO process.
However, chlorination cannot completely kill all of microor-
ganisms,61 especially causes a rapid damage of most conven-
tional PA membrane.62 It is thus desirable to nd a practicable
solution to collaboratively enhance the resistance to bacteria
and chlorine of RO membrane during use.
aCenter for Membrane and Water Science and Technology, Ocean College, Zhejiang
University of Technology, Hangzhou 310014, China. E-mail: lifenliu@zjut.edu.cn;
Zhaoxt@zjut.edu.cn
bCollege of Chemical Engineering, Zhejiang University of Technology, Hangzhou
310014, China
cCollaborative Innovation Center of Membrane Separation and Water Treatment of
Zhejiang Province, Hangzhou 310014, China
dSchool of Chemistry & Chemical Engineering, State Key Laboratory of Metal Matrix
Composites, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai, China
200240
† These authors contributed equally.
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It is well known that silver has an excellent antibacterial hydrophilic amino group (–NH2) instead of carboxylic acid
capability and even very low concentrations of silver ions group (–COOH) on membrane surface aer the second inter-
(<0.001 mg lꢀ1) is effective in killing bacteria,63–65 so it has been facial polymerization, that the chlorine-resistant and anti-
used in many application elds such as burn wound and ulcer fouling of the resultant Ag-loaded poly(amide-urethane-imide)
healing,66 catheter biolm inhibiting, and microorganism membrane (PAUI@Ag) can be enhanced simultaneously. The
deactivating etc. Furthermore, there are also various chemical composition and morphology of the fabricated
membranes reported by loading silver (silver ion or nano- membrane surface were measured via attenuated total reec-
particle) onto the membrane surface through different methods tance mode Fourier transform infrared spectroscopy (ATR-
to improve the anti-biofouling performance.67–76 For example, FTIR), X-ray photoelectron spectroscopy (XPS), scanning elec-
Dong et al. in situ immobilized silver nanoparticles (AgNPs) tronic microscopy (SEM) and atomic force microscopy (AFM).
onto the commercial polyamide RO membrane (virgin Exposure of the membranes to high concentration of free
membrane) surface via a two-step surface modication process chlorine solutions was carried out to evaluate the chlorine-
to improve the membrane's anti-biofouling property.68 Chou resistant property of the fabricated RO membrane. A prelimi-
et al. prepared the silver-loaded cellulose acetate hollow ber nary investigation of antifouling performance of the silver-
membrane by dissolving AgNO3 in the polymeric casting solu- loaded TFN RO membrane was also performed.
tion, and the hybrid membrane was evaluated antibacterial
activity against both Escherichia coli and Staphylococcus aureus.70
Liu et al. also demonstrated that the hybrid PA/Ag nano-
composite membrane possessed dramatic anti-biofouling effect
by using layer-by-layer (LbL) assembly method.73,74 Wu and Yang
2. Materials and methods
2.1. Materials and chemicals
5-Choroformyloxyisophaloyl chloride (CFIC, Fig. 1) was
synthesized via triphosgene (BTC) method in the presence of
composite catalyst imidazole-pyridine,91 and N,N0-dimethyl-m-
phenylenediamine (DMMPD, Fig. 1) was prepared by the
reduction method with composite NaBH4–I2.92 m-
et al. loaded Ag nanoparticles by using polydopamine (PDA) as
nano-template on the PA layer and the resultant nanocomposite
membranes showed the potential towards long lasting and
regenerable anti-biofouling ability in membrane systems.75,76
Obviously, loading silver onto the polymeric membrane
makes signicant contribution to membrane anti-biofouling
performance. However, the chlorine resistance property of
these Ag-loaded PA composite membranes has scarcely been
reported. In reality, chlorine is frequently added to feed water
for disinfection and to inhibit biolm formation and the
conventional TFC aromatic polyamide RO membrane is poor
resistance to continual exposure to free chlorine, so the modi-
ed TFC RO membrane with silver nanoparticles is also ex-
pected to be simultaneously less prone to chlorination. It is
generally considered that the chlorination of polyamide attri-
butes to the N-chlorination by substituting the hydrogen on
amide nitrogen, followed by ring-chlorination via Orton rear-
rangement.77–79 Based on this understanding, some approaches
for preventing chlorination have been conducted to improve the
chlorine resistances of aromatic polyamide RO membrane,
such as introducing electron-withdrawing groups in the
aromatic ring, increasing steric hindrance of aromatic amide
bond with ortho-position methyl group, or directly substituting
hydrogen atom for methyl on aromatic amide bond, and surface
modication by coating with high chlorine-resistance
polymers.80–86
Phenylenediamine-4-methyl (MMPD, purity
> 99.5%) was
purchased from J&K Scientic Ltd. Nanosilver colloid solution
(nano-Ag, 10–30 nm) was provided by the Shanghai Weilang
Nanomaterial Co. Ltd. All other chemicals are analytic purity
grade unless otherwise specied and used as received without
further purication. Deionized (DI) water with conductivity less
than 5 mS cmꢀ1 was produced by a two-stage reverse osmosis
system (Scheme 1).
Fig. 1 Schematic illustration of fabrication process of PAUI@Ag RO
membrane via two-step interfacial polymerization.
In this study, therefore, the new crosslinking agent, 5-cho-
roformyloxyisophaloyl chloride (CFIC) with trifunctional groups
(including –OCOCl and –COCl), was proposed to react succes-
sively with 4-methyl-phenylenediamine (MMPD) and modied
functional diamine N,N0-dimethyl-m-phenylene diamine
(DMMPD) via two-step interfacial polymerization to obtain the
chlorine resistant poly(amide-urethane-imide) (PAUI) TFC RO
membrane.87–90 At the same time, the nanosilver was introduced
synchronously in the membrane separation skin layer through
the addition in the second aqueous phase of DMMPD. Due to
the introduction of nano-Ag and the existence of residual Scheme 1 Chemical structure of the used functional monomers.
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2.2. Fabrication of RO membranes
2.2.1 Conventional aromatic polyamide membrane. The
polysulfone (PSF) substrates with Mw of 200 000 Da were supplied
by the Center of Water Treatment Technology, Hangzhou, China.
The conventional aromatic polyamide RO membrane (MPD-TMC)
was fabricated by the conventional interfacial polymerization
method. First, the PSF substrate was clamped between two Teon
frames with the inner length and width of 20 cm and 15 cm,
respectively. The top surface of PSF substrate was immersed into
the aqueous solution for 2 min which was composed of 2.0 wt%
MPD, triethyl amine (TEA, 3.0 wt%), sodium dodecyl sulfate (SDS,
0.15 wt%) and (+)-10-camphor sulfonic acid (CSA, 4.0 wt%). The
excess solution was then drained, and the membrane was air-dried
at ambient temperature until no visible liquid drops were on the
surface. Subsequently, the organic solution containing 0.15 wt%
TMC was reacted with residual MPD on the surface of PSF
substrate for 60 s. The resultant membrane was nally heated at
60 ꢁC in the air dryer to further polymerization. The obtained
MPD-TMC membrane was washed by DI water and stored in
1.0 wt% NaHSO3 solution for further modication.
2.2.2 Poly(amide-urethane-imide) membranes including
PAUI and PAUI@Ag. Both of the poly(amide-urethane-imide)
TFC RO membranes including PAUI and PAUI@Ag were
prepared through two-step interfacial polymerization tech-
nology in a clean room (Fig. 1),93 and the PAUI@Ag membrane
was based on the modication of PAUI nascent membrane with
nano-Ag particles by introduction in the second aqueous phase
solution.
The rst aqueous phase solution was prepared with 2.0 wt%
MMPD, 3.0 wt% TEA, 0.15 wt% SDS and 4.0 wt% CSA, and the
second aqueous solution was composed of 1.0 wt% DMMPD,
3.0 wt% TEA, 0.15 wt% SDS, 4.0 wt% CSA and 100 mg lꢀ1
nanosilver particle (particle diameter: 10–30 nm). It should be
noticed that the second aqueous phase for fabricating PAUI@Ag
membrane contained nanosilver. Firstly, the rst aqueous
phase solution was poured into the top surface of the PS
support to soak for 2 min. Aer removing the excess solution
from the dip-coated surface and air-drying at ambient temper-
ature, the organic phase solution of CFIC (0.15 wt%) in hexane
was then poured rapidly into the MMPD-saturated surface for
the rst polymerization reaction. Aer polymerization of 40 s,
the excess organic solution was drained from the surface of the
obtained nascent poly(amide-urethane) base membrane and
air-dried adequately at ambient temperature until no remaining
liquids. Subsequently, the second aqueous phase solution was
fast poured into the top surface of the poly(amide-urethane)
base membrane for the second interfacial polymerization (60
s) of DMMPD and the residual unreacted CFIC or functional
groups (–COCl and/or –OCOCl). Aer removing the excess
second aqueous solution, the membrane with frame was heated
to 100 ꢁC (8 min) in the air dryer for further polymerization,
leading to the formation of skin bi-layer including poly(amide-
urethane) CFIC–MMPD precursor separation layer and ultra-
thin polyimide CFIC–DMMPD modied separation layer.
Finally, the resultant PAUI and PAUI@Ag membranes were
washed in DI water and stored in NaHSO3 solution (1.0 wt%).
Fig. 2 Device for the permeation tests.
2.3. Membrane separation performance test
Water ux and salt-rejection performance tests were performed
in duplicates for at least 3 times using a homemade stainless
steel cross-ow membrane ltration unit (Fig. 2). The ltration
unit contained
5 parallel test units, and the averaged
measurements were reported. The membrane samples were
picked carefully under a uorescent lamp to eliminate samples
with decits before testing. Membrane coupons with a diameter
of 40 mm (effective area: 12 cm2) were soaked in DI water (room
temperature, >30 min) prior to loading to the test unit. The
membrane was equilibrated with DI water (1.55 MPa, 25 ꢁC, 2 h)
before the permeation tests to ensure stable membrane.
When ux was maintained steady for at least 30 minutes, an
appropriate volume of NaCl solution was added to provide
desired salt concentration. The running unit was also allowed to
equilibrate until a satisfactory steady-state ux was again
reached. Permeate, retentate, and feed samples were drawn,
and salt rejection rate (R, %) and water ux (Jv, L mꢀ2 hꢀ1) were
calculated respectively.
Cp
R ¼ 1 ꢀ
(1)
Cf
where R is the percent solute rejection, and Cp and Cf (g lꢀ1) are
the concentrations of solute in the permeate and feed, respec-
tively. The salt concentration in the feed and permeate was
calculated according to the electrical conductivity of the corre-
sponding salt solution using an electrical conductivity (DDS-
11A, Hangzhou Dongxing Instrument Co., China).
V
J ¼
(2)
A ꢂ Dt
where A (m2) is the effective area of the membrane for perme-
ation, and V (l) is the volume of permeated water over a time
interval Dt (h).
2.4. Membrane active chlorine exposure experiment
It has been reported that the short-time exposure of membrane
to high concentration of active chlorine is equivalent to that of
long-time exposure to low concentration of active chlorine.94,95
In this study, therefore, membrane chlorination experiments
were carried out by soaking in high concentration of active
chlorine solutions at constant pH 8.0 for a short time to evaluate
the membrane chlorine-tolerant property. The membranes were
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exposed respectively to 8000 mg lꢀ1 chlorine solutions for 1 h.
The soaking tests were performed in Pyrex glass bottles covered
with PTFE (polytetrauoroethylene) caps, and the contents were
mixed on a shaker. Exact concentrations were determined by
titration with a sodium thiosulfate standard. Aer that, the
separation performances of the membranes were tested aer
being thoroughly rinsed with deionized water. Data in this
paper are normalized to make the comparison more
straightforward.
3. Results and discussion
The poly(amide-urethane) membrane was fabricated based on
MMPD and CFIC monomers via conventional interfacial poly-
merization method without curing treatment. Nano-Ag particles
were successfully applied to modify the poly(amide-urethane)
membrane along with DMMPD monomers through two-step
interfacial polymerization method for anti-biofouling purpose.
Thus, the PAUI@Ag RO membrane was obtained without scar-
ifying the chlorine-tolerant property. For comparison, the PAUI
RO membranes were also fabricated without adding nano-Ag
particles. Several results were obtained as below.
2.5. Membrane fouling test
In this study, the membrane fouling test was carried out via
dynamic soaking in the original Westlake water with bacteria
(Table 1) for a long time (not less than 7 days) with vigorous
stirring to evaluate the membrane resistance to biofouling.
Aer soaking, the membrane samples were observed via SEM
whether the bacteria adhere to the membrane surface aer
being thoroughly rinsed with deionized water and dried under
vacuum. Meanwhile, the separation performances of the soaked
membranes were also measured.
3.1. Chemical composition of the separation layer
The ATR-FTIR was employed to analyze the chemical composi-
tion of the separation layer of TFC reverse osmosis membrane
and the ATR-FTIR spectra of PAUI and PAUI@Ag RO
membranes were presented in Fig. 3. The nC]O peak at
1724 cmꢀ1 is an evidence of formation of urethane bond
(–OCONH–) and/or the methylurethane bond (–OCON(CH3)–) in
the separation layer. The amide I (nC]O, 1639 cmꢀ1), II (dNH,
1531 cmꢀ1) and III (nC–N, 1242) peaks of the separation layer
respectively are clearly identied as evidences of existing of
functional acylamide bond (–CONH–) and/or the methyl-
acrylamide bond (–CON(CH3)–). The peaks are located at 1589
and 1488 cmꢀ1 as expected for the benzene ring vibration in the
body frame of aromatic polymer. The double peaks at 1150 and
1100 cmꢀ1 as expected for the C–O stretching vibration from
urethane and/or methylurethane bond. At the same time, it is
clearly seen that the PAUI@Ag RO membrane exhibited sharper
and larger characteristic peaks at 1531 and 1242 cmꢀ1 for amide
bond and at 1150 and 1100 cmꢀ1 for urethane bond, indicating
the higher contents of amide and urethane bond in the sepa-
ration layer. The higher contents of amide and urethane bonds
were believed to be attributed to the higher cross-linking degree
of the membrane separation layer.
2.6. Characterization of membrane
ATR-FTIR (Nicolet560 FTIR, US Nicolet Corp., USA) spectra was
employed to analysis the functional groups and bonds of the
near-surface region of the membrane skin layer. Surface
chemical characterization was performed using XPS (PHI 5000C
ESCA System, USA), and the obtained data further were
analyzed through PHI ACCESS ESCA-V6.0 F soware package.
SEM of the fractured surface and cross section for the TFC RO
membrane with polysulfone supporting lm was conducted on
JSM-5610LV. The surface morphology was determined by AFM
(Park Instrument Auto Probe CT) combined with SEM (S-4700,
Hitachi Ltd., Japan). The existence and morphology of nano-
silver particles in the membrane separation layer was observed
by TEM (Tecnai G2 F30 S-Twin, 300 kV, Philips FEI Co., Holand).
All membrane samples were removed from storage in NaHSO3
solution and soaked in DI water (room temperature, >30 min),
then rinsed in DI water for 2–3 times to remove any remaining
NaHSO3, and lastly dried under vacuum before measurements.
The XPS was further employed to analyze the chemical
composition of the top-most separation layer of TFC reverse
osmosis membrane with the sampling depth about 10–90 A.
˚
Table 1 Composition of water from the Westlake
Species
Quantity
K+ and Naꢀ+,1mg lꢀ1
Ca2+, mg l
11.96
30.78
2.330
0.03
79.64
20.00
18.43
<5
Mg2+, mg lꢀ1
Fe(Total), mg lꢀ1
HCO3ꢀ, mg lꢀ1
Clꢀ, mg lꢀ1
SO42ꢀ, mg lꢀ1
HA, mg lꢀ1
NO2ꢀ, mg lꢀ1
10.78
8.30
SiO2, mg lꢀ1
Turbidity, NTU
COD(Mn), mg lꢀ1
Bacteria, CFU mlꢀ1
20.5
7.12
757
Fig. 3 ATR-FTIR spectrum of PAUI and PAUI@Ag RO membrane,
respectively.
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PAUI and PAUI@Ag membranes. The surface and cross-section
morphologies of PAUI and PAUI@Ag RO membranes are shown
in Fig. 5 and 6. The surface of PAUI RO membranes exhibited
the typical ridge-and-valley structure, thus leading to a relatively
high surface roughness. Upon Nano-Ag loading, the surface of
PAUI@Ag RO membranes showed much smaller ridge-and-
valley structure, and thus the surface roughness of PAUI@Ag
RO membranes is obviously decreased. This phenomenon
could be ascribed to the fact that the nanoscale size of nano-Ag
particles and their affinity to the multiple functional groups of
PAUI network could conne the further growing of large ridge-
and-valley structure, leading to a signicantly reduced surface
roughness. This tendency of the ridge-and-valley morphology
affected by mixing with nanoscale particles was also reported by
other work.97 The cross-section of both PAUI and PAUI@Ag RO
membranes exhibited the typical thin-lm composite structure
with thickness of the separation layer about 200–300 nm. With
the introduction of nano-Ag particles, the thickness of the
selective layer increases from 231 ꢃ 31 nm for nascent PAUI RO
Fig. 4 XPS spectrum of PAUI and PAUI@Ag RO membranes.
Table
2 Elemental composition of the PAUI and PAUI@Ag RO
membranes
Item
C%
O%
N%
C/N
C/O
O/N
PAUI
PAUI@Ag
73.14
74.88
14.94
15.04
11.92
13.08
6.14
5.72
4.9
4.98
1.25
1.15
The quantitative elemental compositions of the top-most layer
of the PAUI and PAUI@Ag RO membranes were calculated from
the spectrum (Fig. 4) and summarized in Table 1. The XPS wide
scans spectra of both nascent PAUI and PAUI@Ag RO
membranes revealed three characteristic signals of C, N, and O
elements at 282 397, and 528 eV, respectively, which was
consistent with the chemical compositions of the separation
layer of membranes. Different with spectra of the nascent PAUI
RO membrane, the spectra of PAUI@Ag RO membrane showed
the appearance of new binding energy peak of Ag at 368 eV, and
the high-resolution XPS spectra exhibited two contributions
from Ag 3d5/2 and Ag 3d3/2 at around 373 eV and 366 eV,
respectively. Both of the peaks were attributed to the AgO
species, indicating the presence of Ag NPs on the PAUI@Ag RO
membrane surface.96
Fig. 5 SEM images of PAUI and PAUI@Ag RO membranes.
The separation layers of the PAUI and PAUI@Ag membranes
derived from interfacial polymerization is typically highly
crosslinked to ensure the high salt rejection. In reality, the
reaction scheme could vary between full cross-linked routes, full
linear routes and their combination. The lower C/N and O/N
ratio of separation layers usually predict higher cross-linking
degree. The relative atomic concentration of C, O, and N, as
well as the corresponding C/N, C/O, and O/N ratios were
summarized in Table 2. The N% content of the PAUI@Ag RO
membrane was increased upon nano-Ag particle loading
compared with that of the nascent PAUI RO membrane.
Accordingly, the corresponding C/N and O/N ratios were
decreased, indicating the enhanced cross-linking degree of
membrane separation layer. The XPS analysis was accordant
with the result of ATR-FTIR.
3.2. Physical morphology of the separation layer
The combination of FESEM, AFM and TEM was employed to
observe the physical morphology of separation layers of the Fig. 6 AFM images of the PAUI and PAUI@Ag RO membranes.
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Fig.
7 TEM images of poly(amide-urethane-imide) membrane
without and with silver-loaded.
Fig. 8 Separation performance of PAUI and PAUI@Ag RO membranes.
Testing conditions: 1.55 MPa, 2000 ppm NaCl, 25.0 ꢃ 2.0 ꢁC.
membrane to 260 ꢃ 42 nm for PAUI@Ag RO membranes. The
cross-section TEM images in Fig. 7 are also used to further
conrm the encapsulation of the nano-Ag particles into the
separation layer of PAUI@Ag RO membranes. The presence of
nano-Ag particles within the separation layer is evident and the
embedded nano-Ag particles were mostly enriched at the near-
surface region, indicating the successful incorporation of nano-
Ag particles into the CFIC–DMMPD modied separation layer of
PAUI@Ag RO membranes.
PAUI@Ag RO membranes before and aer chlorination as
compared to that of the conventional aromatic polyamide RO
membrane MPD-TMC. It can be seen that the salt rejection rate
of MPD-TMC was greatly decreased aer exposing in 8000 mg
lꢀ1 chlorine solutions and correspondingly its water ux was
dramatically increased. For PAUI RO membrane, there pre-
sented only a slight performance decline with lower salt rejec-
tion and higher water ux. The chlorine resistance of the PAUI
RO membrane can be attributed to the unique chemical struc-
ture of CFIC–DMMPD modied separation layer. For conven-
tional aromatic polyamide, the Orton-rearrangement and
subsequent N-chlorination and ring-chlorination reactions
could disrupt the formation of intermolecular hydrogen bonds
and destroy the symmetry of polyamide network, leading to the
severe conformational deformations of polyamide chains and
the remarkable performance decline.77–79 The CFIC–DMMPD
modied separation layer contains a relative large amount of
polyimide bonds with N-position substituted methyl group.
These characters improved the chlorine resistance of PAUI RO
membrane though increasing the steric hindrance of active
chlorine attacking and decreasing the of aromatic amide
bond.80,81,93 For PAUI@Ag RO membrane, the salt rejection
3.3. Reverse osmosis performance of the membranes
The separation performance of the PAUI and PAUI@Ag RO
membranes was evaluated under the test condition of 2000 ppm
ꢁ
NaCl solution, 1.55 MPa and 25 C. The desalination rate and
water ux were the average values respectively tested from three
different samples at the same time presented in Fig. 8. As
compared with the nascent PAUI RO membrane, the water ux
of the PAUI@Ag RO membrane with nano-Ag particle loading
was decreased to some extent. The decrease of water ux could
due to the fact that the higher cross-linking degree and thick-
ness of the separation layer of PAUI@Ag RO membrane
increased hydraulic resistance of separation layer and thus
affected the pressure drop of membrane. Meanwhile the salt
rejections of the PAUI@Ag RO membrane was increased upon
nano-Ag particle loading compared with that of the nascent
PAUI RO membrane. It is no doubt that the high cross-linking
degree of the separation layer of PAUI@Ag RO membrane
ensured the high rejection ability. Obviously, this result indi-
cated that the introduction of nano-Ag particle did not cause
defects in the separation layer of RO membrane. It was
purposed that the affinity of nano-Ag particles to the multiple
functional group of separation layer inhibited the formation of
membrane defects and resulted in a denser PAUI network in the
separation layer of RO membrane.
3.4. Chlorine-tolerant property of the membranes
The chlorine-tolerant property of the PAUI and PAUI@Ag RO
membranes was evaluated by comparing the water ux and salt
rejection of membranes before and aer exposing in
8000 mg lꢀ1 chlorine solutions, under the test condition of
1.55 MPa, 2000 ppm NaCl, 25.0 ꢃ 2.0 ꢁC. Fig. 9 represented the
Fig. 9 Chlorine-tolerant property of the PAUI and PAUI@Ag RO
membranes compared to conventional polyamide membrane MPD-
change in the water uxes and salt rejections of PAUI and TMC. Testing conditions: 1.55 MPa, 2000 ppm NaCl, 25.0 ꢃ 2.0 ꢁC.
37822 | RSC Adv., 2018, 8, 37817–37827
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Fig. 10 TEM images of silver-loaded poly(amide-urethane-imide)
membrane before and after chlorination. (a) Before chlorination; (b)
after chlorination.
slightly dropped from 95% to 90% and the water ux increased
from 22 to 40 L mꢀ2 hꢀ1 aer chlorination, so the modied
PAUI@Ag membrane had relatively slightly weaker chlorine-
resistant property than that of the vacant PAUI membrane but
still superior to MPD-TMC membrane. Considering the robust
chlorine-tolerant property of CFIC–DMMPD modied separa-
tion layer, the decline performance could be attributed to the
oxidation of nano-Ag particles to AgCl species under high
concentration of active chlorine. It was suggested that the
oxidation of nano-Ag particles would change the micro-
structure of the membrane separation layer and generated
some micro defects within the PAUI network (Fig. 10(b)), but did
not signicantly alter the chemical structure and the degree of
cross-linking as demonstrated by.93 Obviously, the two
poly(amide-urethane-imide) membranes including PAUI and
PAUI@Ag have favourable chlorine-resistant property than
conventional polyamide MPD-TMC membrane.
Fig. 11 SEM images of membranes before and after being fouled by
dipping in the Westlake water for 7 days.
PAUI@Ag RO membrane. Biolm formed on the fouled
membranes was further investigated using SEM. Fig. 11 showed
the change in the surface morphologies of fouled RO
membranes. For MPD-TMC and PAUI RO membranes, there
existed visible bacterial metabolites on membrane surface,
indicating severe biofouling. Meanwhile, the surface of the
PAUI@Ag RO membrane seems clean aer biofouling with no
obvious change in surface morphology. On one hand, the
smoother surface of PAUI@Ag RO membrane could limit the
adsorption tendency of bacterial metabolites or extracellular
polymeric substances on the surface and resist the formation of
biolm.98 On the other hand, the antibacterial activity of nano-
Ag particles could inhibit the bacterial reproduction and
proliferation on the membrane surface and avoid the gradual
formation of mature biolm.99
3.5. Antifouling property of the membranes
Fouling study was evaluated for the membrane aer fouled by
the real lake water. The permeate performances of the fouled
PAUI and PAUI@Ag RO membranes were tested as compared to
that of the conventional MPD-TMC membrane. Table 3 repre-
sented the change in the ux decline rate of RO membranes
aer dynamic soaking in the Westlake water for a long time.
The decline in water ux was observed due to the formation of
fouling on the membrane surfaces (Fig. 10). The presence of
microorganism in the Westlake water resulted in a signicant
decline in ux of the MPD-TMC and PAUI membranes. In
contrast, the PAUI@Ag RO membrane showed only 3.3% of ux
decline, implying the excellent anti-biofouling property of
4. Conclusions
In conclusion, a novel PAUI@Ag reverse osmosis composite
membrane containing bi-layer of the poly(amide-urethane)
CFIC–MMPD precursor separation layer and a antifouling and
chlorine-resistant functional layer of ultrathin polyimide CFIC–
DMMPD lm modied with nano-Ag particle dispersion was
prepared through two-step interfacial polymerization. Charac-
terizations were used to exhibit the chemical composition and
physical morphology of the PAUI@Ag RO membrane. The RO
performance of the PAUI@Ag RO membrane evaluated that the
introduction of nano-Ag particles endowed membrane with
higher rejection abilities and relatively lower permeability. The
change of RO performance of the PAUI@Ag RO membrane aer
chlorine exposure evaluated that the PAUI@Ag RO membrane
could still maintain high level of rejection abilities without
Table 3 Flux stability of membranes after be immersed in the Westlake
water
Flux before be
Flux aer be
Flux decline
ratea (%)
Membrane
fouled (L mꢀ2 hꢀ1
)
fouled (L mꢀ2 hꢀ1
)
MPD-TMC
PAUI
PAUI@Ag
34
30.2
21.5
26.5
27.3
20.8
22.2
9.7
3.3
a
Test conditions: (1) the fouling experiment was carried out for 6
hours; (2) the ux decline rate was calculated from the equation of (1
ꢀ JW/JW ) ꢂ 100%.
0
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obvious sacrice on the membrane performance. Furthermore, 12 S. D. Arthur, Reverse-osmosis membranes of polyamide-
the PAUI@Ag RO membrane also exhibited better antifouling
ability in natural water system. Therefore, the PAUI@Ag RO
urethanes, in: Application: US, E.I. du Pont de Nemours
and Co., USA, 1992, p. 6.
membrane would provide chlorine-resistant and anti- 13 M. Liu, S. Yu, J. Tao and C. Gao, Preparation, structure
biofouling candidate for practical applications.
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Conflicts of interest
14 R. Baker, Membrane Technology and Applications, John Wiley
& Sons Ltd, England, 2nd edn, 2004.
15 G.-d. Kang and Y.-m. Cao, Development of antifouling
reverse osmosis membranes for water treatment: a review,
Water Res., 2012, 46, 584–600.
16 D. Rana and T. Matsuura, Surface modications for
antifouling membranes, Chem. Rev., 2010, 110, 2448–2471.
17 L. Zhao, P. C. Y. Chang, C. Yen and W. S. W. Ho, High-ux
and fouling-resistant membranes for brackish water
desalination, J. Membr. Sci., 2013, 425–426, 1–10.
18 Y.-N. Kwon and J. O. Leckie, Hypochlorite degradation of
crosslinked polyamide membranes: II. Changes in
hydrogen bonding behavior and performance, J. Membr.
Sci., 2006, 282, 456–464.
There are no conicts to declare.
Acknowledgements
The current study was supported by the National Natural
Science Foundation of China (No. 21776253 & 21774077), the
National key research and development program of China (No.
2016YFC0401508), the National Basic Research Program of
China (No. 2015CB655303) and the Open Research Fund
Program of Collaborative Innovation Center of Membrane
Separation and Water Treatment of Zhejiang Province.
19 G.-D. Kang, C.-J. Gao, W.-D. Chen, X.-M. Jie, Y.-M. Cao and
Q. Yuan, Study on hypochlorite degradation of aromatic
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