Performance enhancement of TFC FO membranes with polyethyleneimine modification and post-treatment

Author’s Accepted Manuscript Performance enhancement of TFC FO membranes with polyethyleneimine modification and posttreatment Liang Shen, Xuan Zhang, Jian Zuo, Yan Wang www.elsevier.com/locate/memsci

PII: DOI: Reference:

S0376-7388(17)30261-2 http://dx.doi.org/10.1016/j.memsci.2017.04.008 MEMSCI15172

To appear in: Journal of Membrane Science Received date: 26 January 2017 Revised date: 1 April 2017 Accepted date: 5 April 2017 Cite this article as: Liang Shen, Xuan Zhang, Jian Zuo and Yan Wang, Performance enhancement of TFC FO membranes with polyethyleneimine modification and post-treatment, Journal of Membrane Science, http://dx.doi.org/10.1016/j.memsci.2017.04.008 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. 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.

Performance enhancement of TFC FO membranes with polyethyleneimine modification and post-treatment

Liang Shena,b,c, Xuan Zhanga,b,c, Jian Zuod and Yan Wanga,b,c*

a

Key Laboratory of Material Chemistry for Energy Conversion and Storage (Huazhong

University of Science and Technology), Ministry of Education, Wuhan, 430074, P.R. China b

Hubei Key Laboratory of Material Chemistry and Service Failure, School of Chemistry and

Chemical Engineering, Huazhong University of Science & Technology, Wuhan, 430074, P.R. China c

Shenzhen Institute of Huazhong University of Science & Technology, Shenzhen 518000, PR China d

Singapore Institute of technology, 10 Dover Drive, Singapore 138683, Singapore

*

Corresponding author. Tel.: 86 13871464406; fax: 86 027-87543632. E-mail address: [email protected] (Yan Wang)

Abstract

In this work, simple and effective second interfacial polymerization (SIP) of thin-film composite (TFC) membranes is performed with polyethyleneimine (PEI) of various molecular weights, in order to improve the separation performance for forward osmosis (FO) applications. Various characterization techniques are employed to examine the modification mechanism. Compared to the control TFC membrane, PEI-modified membranes exhibit higher water permeabilities, acceptable salt rejection and lower fouling propensity. The 1

effects of PEI molecular weight on the membrane morphology and separation performance are

investigated

systematically

with

various

characterizations.

Post-treatment

of

PEI-modified membranes in alkaline and acidic aqueous solutions is further investigated, and its effects on the FO performance, intrinsic separation properties and antifouling properties of these post-treated membranes are studied. In comparison with pristine PEI-modified TFC membranes, the post-treated TFC membranes show further enhanced water flux and improved anti-fouling properties.

Graphical Abstract

2

Abbreviations AFM, atomic force microscopy; ATR-FTIR, attenuated total reflectance Fourier transform infrared; CaCl2, calcium chloride; DBES, doppler broadening energy spectroscopy ; DI, deionized; DS, draw solution; FO, forward osmosis; FRR, flux recovery ratio; FS, feed solution; HCl, hydrochloric acid; IP,

interfacial polymerization; KH2PO4, potassium

dihydrogen phosphate ; MgSO4, magnesium sulfate ; MPD, m-phenylenediamine; NaCl, sodium chloride ; NaHCO3, sodium bicarbonate ; NaOH, sodium hydroxide; NH4Cl, ammonium chloride ; NMP, N-methyl pyrrolidone; PA, polyamide; PAS, position annihilation spectroscopy; PEI, polyethyleneimine; PEG, polyethylene glycol; PRO, pressure retarded osmosis; PSf, polysulfone; RO, reverse osmosis; SA, sodium alginate ; SDS, sodium dodecyl sulfate; SEM, scan electron microscopy; SIP, second interfacial polymerization; TFC, thin-film composite; TMC, 1,3,5-trimesoyl chloride; WCA, water contact angle ; XPS, X-ray photoelectron spectroscopy

Keywords: second interfacial polymerization, thin-film composite membrane, forward osmosis, polyethyleneimine, post-treatment

3

Nomenclatures

A

: water permeability

AL-DS

: active layer facing draw solution

AL-FS

: active layer facing feed solution

Am

: effective membrane area

B

: salt permeability

Cf

: feed concentration

Cp

: permeate

Ct

: salt concentration

D

: diffusion coefficient

FRR%

: flux recovery ratio

J

: pure water flux

Js

: reverse salt flux

Jv

: water flux

Rs

: salt rejection

ΔP

: hydraulic pressure

∆t

: test time

∆V

: volume change

Δπ

: osmotic pressure

concentration

4

1. Introduction

Forward osmosis (FO), an emerging membrane separation process utilizing the difference of osmotic pressure between the feed and draw solution as the driving force, has drawn growing attention ascribed to its unique advantages of relative low energy consumption, high water recovery, and low fouling propensity. Therefore, it has been widely applied into various fields, such as wastewater treatment[1-4], brackish water or seawater desalination[5], power production[6, 7] and so on[8]. The development of high performance FO membrane is of paramount importance for the implementation of FO technology. Massive endeavors have been devoted to a variety of high performance FO membranes with desirable characteristics including enough mechanical strength, good chemical stability, high water permeability, low solute permeability and low internal concentration polarization. The thin-film composite (TFC) membrane, a kind of promising candidate formed by interfacial polymerization of the amine monomer and acyl chloride monomer on a porous substrate, has gained much attention in the field of water treatment ascribed to its excellent separation performance in terms of relative high water flux, high salt rejection and stability under a wide range of operation temperature and pH.

Despite the outstanding separation performance, TFC membranes still faces problems of low water flux and high fouling propensity, especially for traditional TFC membranes formed by TMC and MPD, which are of insufficient hydrophilicity due to the highly crosslinked aromatic PA networks[9]. A large number of research works has therefore been reported in this direction. Firstly, additives and/or co-solvent can be incorporated in the aqueous or organic phase during the interfacial polymerization process to tune the PA layer structure and 5

the corresponding properties, such as cetyl trimethyl ammonium bromide[10, 11], triethyl benzyl ammonium bromide[12], trimethylamine[13], sodium dodecyl sulfate (SDS)[6]. Secondly, embedment of nano-materials into the PA layer can significantly enhance the overall properties of the TFC membranes, including carbon nanotubes[14, 15], graphene oxide[16-18], zeolite[19-22], silica[23, 24], titanium oxide[25-27] and so on[28-30]. Thirdly, surface modification of PA layer can be performed since the negatively charged moieties on the membrane surface can act as reactive sites to bind functional monomers, polymers or nanoparticles. Fourthly, post-treatment of nascent TFC membranes by immersing in certain solutions (including SDS/glycerol[31], alcohols[7, 32], N, N-dimethyl formamide[6] ) can improve the water flux owing to the increased fractional free volume.

Among them, surface modification is one of the most common approaches to enhance the separation performance and antifouling properties, owing to its simplicity and flexibility. Because of the incomplete reaction between the amine and acyl chloride monomers during the interfacial polymerization process, negatively charged surface moieties of TFC membrane cane be exploited as reactive sites for surface modification by various functional materials[33]. A typical surface modification is to graft PEG-based polymers[34-37], which always exhibit large extended molecular volume in water and create a hydration boundary layer with coordinated water molecules to render surface resistance to foulant adsorption. Polydopamine[38, 39], as a supramolecular aggregate, can also effectively improve the membrane surface high hydrophilicity and thus render reduced fouling propensity. In addition, zwitterionic grafting is another promising method for membrane surface modification contributed by its more strong water binding ability via electrostatically induced hydration[40, 41].

6

As well known, PEI, is a cationic polyelectrolyte, with high hydrophilicity, flexible long chain, large expanded molecular volume, abundant reactive groups (primary amine groups) and high charge density (23.3meq/g in aqueous solution if fully protonated)[42]. PEI is also of high reactivity with acyl chloride, imide, epoxy, acid and isocyanate groups, and thus holds great potential for the preparation or surface modification of membranes in various separation applications, such as ultrafiltration (UF), nanofiltration (NF) or FO membranes[42-46]. In the preparation of TFC membranes, PEI is acted as an amine monomer for interfacial polymerization bringing out a higher water flux because of the high surface wettability and large fractional free volume. PEI surface modification has also been reported for NF or UF, and the results show that modified membranes generally exhibit improved separation performance and antifouling properties because of the enhanced hydrophilicity, formed hydration layer and higher steric hindrance. Above studies show that, PEI can play a multiple role of reactive monomer, hydrophilicity promoter and cross-linker. However, no work has been reported yet to systematically investigate the comprehensive effect by PEI modification. In addition, limited studies have been reported on the surface modification of TFC membranes by PEI, especially for FO applications.

In this study, second interfacial polymerization (SIP), a convenient and effective route for modifying TFC membranes[47], is performed by the reaction between the pristine PA layer and PEI with three different molecular weights. Membrane hydrophilicity is expected to be significantly improved after surface modification by PEI. In addition, the bulky spatial structure of PEI molecules may prevent the compact chain packing and endow the crosslinked polymer with enhanced fractional free volume, which is opposite to the general phenomenon that cross-linking modification leads to the tightened polymer chains and reduced fractional free volume. State-of-the-art characterization techniques were applied to 7

confirm the modification mechanism, including attenuated total reflectance Fourier transform infrared (ATR-FTIR), X-ray photoelectron spectroscopy (XPS) and positron annihilation spectroscopy (PAS). Changes in chemical properties and surface wettability by PEI modification are systematically investigated. Besides, further post-treatment of PEI-modified TFC membranes in basic and acidic aqueous solution is explored and its effect on the surface hydrophilicity and microstructure of PA selective layer is investigated. Moreover, the corresponding separation performance and antifouling properties of PEI-modified and post-treated TFC membranes are studied systematically.

2. Materials and Methods

2.1 Materials

Polysulfone (PSf) (Mw= 800,000 Da) and polyethyleneimine (PEI) (Mw= 600, 1200, 1800 Da) were purchased from Beijing HWRK Chem co. Ltd. (China) and dried in the vacuum oven at 80 °C for overnight before use. The m-phenylenediamine (MPD, 99.5%) and 1, 3, 5-benzenetricarbonyl trichloride (TMC, 98%) were obtained from Aladdin and kept in refrigerator before use. Polyethylene glycol 400 (PEG 400, CP), N-methyl pyrrolidone (NMP, anhydrous, ≥99.5%), n-hexane (≥97%, anhydrous), sodium hydroxide (NaOH, ≥96%), concentrated hydrochloric acid (HCl, ≥37%), sodium chloride (NaCl, ≥99.5%), sodium alginate (SA, Mw: 98.11), potassium dihydrogen phosphate (KH2PO4, 99.5%), magnesium sulfate (MgSO4, 99%), sodium bicarbonate (NaHCO3, 99.5%), calcium chloride (CaCl2, 96%) and ammonium chloride (NH4Cl, 99.5%) were all purchased from China National Medicine Corporation.

8

2.2 Preparation of PSf substrate

PSf support layer was prepared by non-solvent induced phase separation method. A prepared dope solution of 18/16/66 wt% PSf/PEG-400/NMP was cast onto a pre-cleaned glass plate with a casting knife of 100 μm thickness, and followed by being immersed into a water coagulation bath at room temperature for phase inversion immediately. To remove the residual solvent in membrane, water was changed every 12 hours for 2 days. The as-fabricated membranes were then stored in deionized (DI) water before use.

2.3 Preparation and modification of TFC membranes

PSf membranes were washed with ultra-pure water before interfacial polymerization, and then immersed in 2 wt% MPD aqueous solution for 2 min. A rubber roller was used to remove the excess MPD solution before pouring a 0.1 wt% TMC/hexane solution onto the membrane top surface. After 1-min contact, TMC/hexane solution was drained off from the membrane surface. Further SIP modification was carried out by immersing the nascent TFC membrane into 3 wt% PEI aqueous solution for 20 min immediately after the excess hexane solution was evaporated completely. After that, some control and modified TFC membranes were exposed to a further post-treatment in an aqueous solution with fixed pH (1, 5, 9 and 13) for 1 h. The PEI-modified TFC membranes before and after post-treatment were washed with DI water and stored in DI water before use. These modified TFC membranes were donated as sPA-X-Y, where X refers to the molecular weight of PEI employed, Y indicates the aqueous solution pH for post-treatment.

9

2.4 Characterizations of TFC membranes

Changes in surface chemical properties of the control and modified membranes were characterized by the attenuated total reflectance Fourier transform infrared (ATR-FTIR, Brucker VERTEX-70, Germany) with a resolution of 2 cm−1 and a range of 700-4000 cm−1. X-ray photoelectron spectroscopy (XPS, VG Multilab 2000, Thermo VG Scientific, UK) was also employed to analyze the chemical composition changes of TFC membranes using a monochromatic A1 Ka X-ray source. The scanning electron microscope (SEM, VEGA3, TESCAN, Czech) was employed to observe the surface morphology of the PA active layer. Membrane samples for SEM characterizations were all sprayed with an ultrathin and uniform gold film by a sputter coater (Q150RS, Quorum, England) before observation. The atomic force microscope (AFM, SPM9700, Shimadzu, Japan) was applied to investigate the surface topologies of the selective layer under dynamic mode in the air and obtain the mean roughness (Ra). Membrane hydrophilicity evaluated by the water contact angle (WCA) was measured using a contact angle goniometer (DSA 25, KRÜSS, Germany) by the sessile drop method at room temperature. At least 10 points of each sample were taken to calculate the average data. A doppler broadening energy spectroscopy (DBES) using a positron annihilation spectroscopy (PAS, National University of Singpore) coupled with a variable mono-energy slow positron beam was employed to inspect the micro-structure of PA layer as a function of depth from surface. For each membrane sample, the DBES spectra were obtained at different positronium energies ranging from 0.1 to 27 keV by a HP Ge detector (EG&G Ortec).

10

2.5 Evaluation of separation performance

A lab-scale reverse osmosis (RO) set-up was applied to measure the intrinsic separation properties of TFC membranes, i.e., water permeability (A), salt permeability (B) and salt rejection (Rs). After system stabilization with DI water at 3 bar for 30 min, the test was conducted at room temperature with an applied pressure of 2 bar, and all samples were repeated for at least three times. The water permeability were measured using DI water as the feed solution, while the salt permeability and salt rejection were measured using a 1000 ppm NaCl aqueous solution as the feed solution. Detailed definitions are listed in Eqs (1) to (4). ∆𝑉

J=𝐴

(1)

𝑚,𝑅𝑂 ×∆𝑡

𝐽

A = ∆𝑃 𝑅𝑆 = (1 − 1−𝑅𝑠 𝑅𝑠

(2) 𝐶𝑝 𝐶𝑓

) × 100

𝐵

= 𝐴(∆𝑃−∆𝜋)

(3) (4)

where J is the pure water flux, Am, RO is the effective membrane area (17.35 cm2 in this study), ∆V is the permeate volume change over the test time ∆t, ΔP and Δπ are the applied hydraulic pressure across the membrane and the osmotic pressure of the feed solution, respectively. Cp and Cf are the salt concentrations in the feed and the permeate solutions, respectively, determined by a conductivity meter (FE30, Mettler Toledo, Switzerland).

FO tests were carried out by a lab-scaled FO set-up at temperature about 22±0.5 C. DI water and aqueous NaCl solution (2.0 M) were used as feed and draw solution circulated by using two variable speed pumps (BT300-2J, Longer, China) with fixed flow rates of 0.3 L min-1 11

(150 rpm).The conductivity meter (FE30, Mettler Toledo, Switzerland) and digital weight balance (FX3000-GD, AND, Japan) were conducted to monitor the concentration change of the feed solution and the weight change of draw solution which was outputted to a computer. Each test was pre-conditioned for 30 minutes to reach the steady state and repeated for at least three times to yield an average value. All TFC membranes were tested in two different operating modes (FO and PRO mode).

Water flux (Jv) and reverse salt flux (Js) were employed to evaluate the FO performance, as calculated by Eqs. (5) and (6). 𝐽𝑣 = 𝐴

∆𝑉

𝑚,𝐹𝑂 ∆𝑡

𝐽𝑠 =

∆(𝐶𝑡 𝑉𝑡 ) 𝐴𝑚 ∆𝑡

(5) (6)

where Am, FO is the effective membrane surface area in the FO test (3.87 cm2), Ct and Vt are the salt concentration and volume of the feed solution, respectively.

2.6 Antifouling test

The dynamic antifouling test was conducted on a lab-scale FO set-up under FO mode at room temperature. The detailed procedure is described in our previous work[16]. Firstly, a clean membrane sample was stabilized for 1 h with DI water as both feed and draw solutions. Secondly, synthetic wastewater without SA and NaCl aqueous solution were employed as the feed and draw solutions to obtain the initial water flux for 1 h before the accelerated fouling stage. Subsequently, an 18-h fouling stage was conducted using the synthetic wastewater containing 250 ppm SA as the feed solution. The cross-flow rates of both feed and draw solutions were fixed at 0.3 L/min (150 rpm) for all above three stages. In order to estimate the fouling reversibility, the physical cleaning stage was immediately conducted after the fouling 12

stage, with 0.15 mM NaCl solution circulated through the feed and draw sides for 30 min at a cross-flow velocity of 0.6 L/min (300 rpm). After that, the water flux of the fouled membrane was measured again. In order to eliminate the effect of initial water flux on the membrane antifouling property, the concentration of NaCl draw solution was adjusted (0.5-2.0 M) to fix the initial water flux at 25±1 LMH. In addition, a large volume of draw solution about 1.5 L was applied to mitigate the draw solution dilution effect as much as possible.

Flux recovery ratio (FRR%), an index defined as the final water flux after cleaning normalized by the initial water flux, is employed to assess the reversibility of membrane fouling and determined by Eq. (7). 𝐽

𝐹𝑅𝑅% = 𝐽𝑐 × 100% 0

(7)

where J0 is the initial flux, Jc is the water flux after the physical cleaning.

3. Results and discussion

3.1 Effects of PEI modification on chemical properties of TFC membranes

As well known, the nascent formed PA layer of TFC membranes after the IP process has some residual acyl chloride groups on the surface which can provide reactive sites for further chemical modification. In this work, PEI with different molecular weights is employed to modify pristine TFC membranes by SIP method, and the reaction mechanism is shown in Fig. 1.

Fig. 2 displays the FTIR spectra of the control and modified membranes. Characteristic peaks of amide groups at 1660 and 1544 cm-1 can be observed in both spectra, attributed to the 13

carbonyl stretching vibration of amide I and the coupling of the in-plane N-H bending and C-N stretching vibrations of amide II groups, respectively. As well known, the peak intensity at 1660 cm-1 can be considered constant since no more carbonyl groups are introduced with the surface modification by PEI. Therefore, taking the peak (1660 cm-1) as a reference, the peak intensity at 1544 cm-1 exhibits a sharp increase as compared to that of the control membrane, indicating the increment in newly formed amide linkages between amine groups of PEI and residual acyl chloride groups of TMC. Besides, the peak intensity at 1610 cm -1 (attributed to the deformation stretching of N-H bond in amine groups) in the spectrum of the modified membrane also increases obviously as compared to that of the control membrane, ascribed to the introduction of amine groups in PEI. Moreover, the decline in the peak intensity of hydroxyl groups at 3410 cm-1 is also found, which could be due to the less carboxyl groups converted from acyl chloride groups by hydrolysis. It also exhibits a slight red shift from 3410 cm-1 to 3330 cm-1 probably because of the overlap with the adsorption band of unreacted amine groups in PEI, and the hydrogen bonding between them. In summary, all above peak changes prove the successful surface modification by PEI.

Chemical changes of modified TFC membranes are further confirmed by XPS. Fig. S-1 displays the XPS spectra of the control TFC membrane and those modified with PEI of different molecular weights, showing predominant peaks of oxygen, nitrogen and carbon. The elemental composition of the PA layer in the control and modified membranes are listed in Table 1. We can see that, in comparison with the control membrane, O content in the modified TFC membranes is lower and shows an ascending tendency with the increase of PEI molecular weight, suggesting that less transformed carboxyl groups by hydrolysis is resulted with modification by PEI of a lower molecular weight. This behavior could be because of the smaller steric hindrance of PEI molecules with lower molecular weight and therefore the 14

higher reactivity with the residual acyl chloride groups of TMC. Moreover, N content of modified membranes is all larger than that of the control TFC membrane, and increases with the increase of PEI molecular weight, because of higher N content in PEI with higher molecular weight. In addition, O/N ratio in the surface of the control and modified membranes are also investigated and the results are listed in Table 1. According to previous studies[47], the theoretical O/N ratios corresponding to the fully crosslinked and fully linear PA network formed by TMC and MPD are 1.0 and 2.0, respectively. As listed in Table 1, the O/N ratio for the control membrane is an intermediate value of 1.70, indicating the PA layer of the control membrane is partially crosslinked which is beneficial for further PEI modification. After modification by PEI, O/N ratios of all modified TFC membrane are much lower than that of the control membrane, which might be ascribed to the enhanced cross-linking degree and successful introduction of PEI into the PA network.

The deconvolution of C 1S and O 1S spectra are further performed for the control and modified TFC membranes and results are shown in Fig. 3 and Table 2. In C 1S spectra of the control and modified TFC membranes (Fig. 3 (a)), three components at 285, 286.2 and 288.1 eV are observed owing to the CI (*C-C/*C=C), CII (*C-O/*C-N/*C-Cl) and CIII (O=C*-N/*C=NH/O=C*) bands, respectively[48]. For CIII peak, the counted amount of carbon atoms for both control and modified membranes is constant, since the carbon atoms are all from acyl chloride groups of TMC. Therefore, the peak intensity ratio of ICII/ICIII can indicate the amount change of amine groups in the PA layer, and the higher ICII/ICIII ratio suggests more amine groups. As listed in Table 2, in comparison with the control membrane, spectra of all modified TFC membranes exhibit higher ratios of ICII/ICIII peak intensity, and those ratios show a rising trend with the increasing PEI molecular weight, demonstrating the success of bonding PEI onto the PA network. Furthermore, in O 1S spectra, two components 15

peaks at 531.2 and 532.6 eV are found, attributed to the OI (N-C=O* and O-C=O* bonds) and OII (*O-C=O bond) band, respectively[48]. For OI peak, the amount of oxygen atoms in N-C=O* and O-C=O* bonds is considered constant, since they are entirely from the carbonyl groups in acyl chloride of TMC. While for OII peak, oxygen atoms of the bond *O-C=O are from the carboxyl groups which are translated from the residual acyl chloride groups by hydrolysis. Therefore, the higher intensity ratio IOI/IOII of modified membranes than that of the control membrane as shown in Table 2 suggests less carboxyl groups on the membrane surface, indicating that some residual acyl chloride groups of nascent PA layer have successfully reacted with the amine groups of PEI by SIP. Additionally, the IOI/IOII intensity ratios increase with the increase of PEI molecular weight, since more acyl chloride groups of nascent PA layer have been involved in SIP. Therefore, both ATR-FTIR and XPS results confirm successfully incorporation of PEI into the pristine PA layer.

3.2 Effects of PEI modification on membrane microstructure and surface properties

The effect of PEI modification on the micro-structure of the PA layer is studied by PALS and SEM techniques. By PALS characterization, S parameter is obtained and employed as an index to reflect the changes in free volume properties (free volume size and amount) of the TFC membrane from the dense PA active layer to the porous support layer[49]. It can be found in Fig. 4, profiles of S parameters for both control and modified TFC membranes exhibit the similar trend. All curves present a sharp increase with the increase in the positron incident energy firstly, resulted from the back diffusion and scattering of positroniums near the membrane surface. It then gradually decreases after reaching the maximum, indicating the gradual transition from the dense PA layer to the transition layer. With the increase of positron incident energy, S parameter increases again, because of the morphology transition 16

from the dense PA layer to the porous substrate. This typical phenomenon of S parameter is so-called “three-layer model” of TFC membranes, i.e. the PA dense layer, transition layer from dense skin to porous support layer and the porous layer[7]. Fig. 4 also shows that, S parameters of modified TFC membranes are all higher than that of the control TFC membrane and increase with the increase of PEI molecular weight, suggesting the larger fractional free volume of the resultant PA layer after PEI modification, especially with a higher molecular weight. Previous study[50] also showed that, the cross-linking reaction between the cross-linker and pendant functional groups (acyl chloride groups in this work) in polymer chains, would contribute to a higher d-space of the resultant polymer chains, if the resultant cross-linking bridges are larger and bulkier. In addition, the space-filling effect of PEI with bulk molecular structure endows the crosslinked polymer larger d-spacing[51].

Membrane morphology and surface topology of the control and modified TFC membranes are characterized by SEM and AFM and displayed in Fig. 5. It can be found that, both the control and modified membranes exhibit typical ridge-and-valley structures and no obvious changes are found after surface modification by PEI.

It is expected that the surface modification by PEI with abundant polar amine groups may result in an enhanced surface hydrophilicity. It is confirmed by the significantly lower WCAs of modified membranes than that of the control membrane as shown in Fig. 6. In addition, WCAs of modified TFC membranes decrease with the ascending PEI molecular weight, owing to the higher amount of amine groups introduced on the membrane surface.

As illustrated in Table 3, the zeta potential for all TFC membranes is negative, because of the dissociation of carboxyl groups from residual acyl chloride groups in nascent PA layer by 17

hydrolysis. Compared to the control TFC membrane, zeta potential of modified TFC membranes increases ascribed to the combined effects of grafted PEI molecules and less carboxyl groups. On one side, grafted PEI molecules on the membrane surface are of positive charge due to the protonation of amine groups. On the other side, less carboxyl groups are resulted by reaction with PEI and responsible for the reduction in the negative charge. Moreover, zeta potentials of modified TFC membranes reduce slightly with the increase of PEI molecular weight owing to that the poorer mobility of larger PEI molecules and resulted poorer reactivity for further modification.

3.3 Effects of PEI modification on separation performance of TFC membranes

The FO performance of the control and modified TFC membranes are displayed in Fig. 7. It shows that, in comparison with the control membrane, modified TFC membranes exhibit much higher water fluxes, especially for the membrane sPA-1800. This behavior should be mainly ascribed to the enhanced hydrophilicity and larger fractional free volume as discussed in the previous section. In addition, both water fluxes and reverse salt fluxes of those modified TFC membranes increase with the increase of PEI molecular weight, following a trade-off relationship between the water permeability and salt rejection. The higher reverse salt flux is probably caused by the increased fractional free volume and reduced electrostatic repulsive of declined surface negative charge as confirmed by the zeta potential test[52].

Intrinsic transport properties of the control and PEI-modified TFC membranes are also evaluated and corresponding results are listed in Table 4, which shows a good accordance with the FO results. In comparison with the water permeability of the control TFC membrane (1.99±0.10 LMH/Bar), water permeabilities (ranging from 2.29±0.13 to 2.85±0.14 LMH/Bar) 18

of modified TFC membranes are higher and increase with the increase of PEI molecular weight. Moreover, salt permeabilities of the control and modified TFC membranes present the similar increasing trend. Therefore, salt rejections (Rs) of those TFC membranes decrease, while B/A ratios increase.

3.4 Effect of post-treatment

Since PEI is a cationic polyelectrolyte, some amine groups of PEI are protonated. The chemical forms of amine groups will change with the pH variation of PEI solution, i.e. unprotonated amine groups will be ionized in a solution of a lower pH beneath the isoelectric point, and protonated amine groups will be deionized at a higher pH over the isoelectric point. The introduction of ionized amine groups in PEI will contribute to the improved hydrophilicity of the membrane; on contrary, the deionized amine groups will lead to the reduction in hydrophilicity. Additionally, the degradation of PA chains with partial hydrolyzation of amide groups may occur in the strong basic or acidic environment, causing the reduced cross-linking degree and higher surface hydrophilicity[48], and resulting in a larger water flux and reverse salt flux[33] . The possible reaction mechanism is displayed in Fig. 8.

Chemical properties changes of the control, PEI-1800 modified membranes with and without post-treatment are confirmed by their FTIR and XPS spectra and shown in Figs. 9, 10 and S-2. FTIR result in Fig. 9 shows that, compared to the constant peak intensity at 1660 cm-1, peak intensities at 1544 cm-1 decrease in the spectra of sPA-1800-1 and sPA-1800-13 membranes as compared to that of the sPA-1800 membrane, indicating the degradation of the PA chains. Besides, corresponding XPS results in Table 1 also show that, in comparison with sPA-1800 19

membrane, membranes with post treatment (sPA-1800-1 and sPA-1800-13) possess higher O/N ratios, ascribed to the hydrolysis of PA chains and resulted lower cross-linking degree. As listed in Table 5, intensity ratios IOI/IOII of sPA-1800-1 and sPA-1800-13 membranes are lower than those of the control and sPA-1800 membranes.

The ionization or deionization of amine groups in PEI as well as the degradation of PA chains by post-treatment in turn affects the surface charge of resultant membranes significantly. As displayed in Table 3, in comparison with sPA-1800 membrane, sPA-1800-1 and sPA-1800-13 membranes exhibit lower zeta potential values. As for sPA-1800-1 membrane, two opposite factors exists, i.e., the protonation of amine groups in PEI (enhancement in positive charges) and the degradation of PA chains (increment in negative charges), which result in the slight higher zeta potential than that of sPA-1800 membrane. While for sPA-1800-13 membrane, both the deprotonation of amine groups in PEI and the degradation of PA chains contribute to the higher negative surface charge. Fig. 11 displays WCAs of the control, PEI-modified membranes with and without post-treatment. It can be found that, in comparison with sPA-1800 membrane, sPA-1800-1 and sPA-1800-13 membranes exhibit higher hydrophilicity ascribed to ionized amine groups in PEI and the degradation of PA chains. Fig. S-3 also shows the surface morphology and topology of the PEI-modified membranes before and after the post-treatment. It can be seen that, there is no obvious change in the surface morphology and roughness of sPA-1800-1 and sPA-1800-13 membranes after post-treatment, although the chemical properties of PA chains may be changed[53].

Aforementioned changes with the post-treatment significantly impact on the separation performance of resultant TFC membranes. Fig. 12 shows that, in comparison with the pristine sPA-1800 membrane, all sPA-1800 membranes after post-treatment exhibit increased water 20

flux and reverse salt flux, due to the improved hydrophilicity and decreased crosslinking degree by the partial hydrolysis. Among them, sPA-1800-5 and sPA-1800-9 membranes post-treated in relatively neutral pH conditions exhibit smaller increment in the water flux and reverse salt flux as compared to those membranes post-treated in stronger pH conditions, probably because of the stability of amide groups in the pH scope (2-11) of the PA-based membrane applications[54]. It can be also found that, the reverse salt flux of sPA-1800-1 membrane is lower than that of sPA-1800-13 membrane, which may be attributed to the formation of two layers with opposite charges (the positive-charged layer with ionized amine groups in PEI, and the negatively charged layer owing to more hydroxyl groups after partial degradation of PA chains) [43].

The basic transport properties of the control, PEI-modified membranes with and without post-treatment are summarized in Table 6. Water permeabilities of sPA-1800 membranes with post-treatments (3.96±0.14 to 2.87±0.06 LMH/Bar) are all higher than that of sPA-1800 membrane without post-treatment (2.85±0.14 LMH/Bar), which are in accordance with FO water fluxes. But the corresponding salt rejections also decline from 88.35±1.35% to 83.48±1.27%. Accordingly, salt permeabilities and B/A ratios of those membranes increase with post acidic or basic treatment.

Post-treatments at pH=1 and pH =13 were also conducted on the control TFC membrane for a comparison and the corresponding separation performance are shown in Fig. S-4 and Table S1. The results show that post-treatment on control TFC membranes leads to severer increment in the reverse salt flux as compared to PEI-modified membranes due to the lower crosslinking degree. Detailed results and discussion are elaborated in the Supporting Information. 21

3.5 Dynamic anti-biofouling performance of TFC membranes

In this study, effects of PEI modification and post-treatment on antifouling properties of resultant TFC membranes are also investigated. Sodium alginate, a surrogate for polysaccharide, is employed as the foulant in the synthetic wastewater containing Ca2+ ions. Generally, polysaccharide is the main constituents of extracellular polymeric substances, leading to the membrane fouling. Based on previous studies[55], the existence of Ca2+ ions in the SA-containing wastewater can exacerbate the fouling phenomenon by acting as a bridge between the SA and PA layer. In addition, Ca2+ ions bind preferentially to carboxyl groups of SA molecules and become the intermolecular bridge between neighboring SA molecules, resulting in a crosslinked gel network on the membrane surface. Therefore, the membrane water flux sharply declines due to the amplified hydraulic resistance for water transport. What’s more, the gel layer also retains the draw solute permeated to the feed solution and hinders its back diffusion, resulting in the salt accumulation within the gel layer. That phenomenon, known as cake-enhanced osmotic pressure, causes a decline in the osmotic pressure difference, and consequently leads to a significant water flux reduction[55].

The dependence of normalized water flux on the fouling testing time over the dynamic fouling process is displayed in Fig. 13. It can be seen that the water flux drop of the control membrane is approximately to 45% during the 18-h test, while that of PEI-modified membranes is significantly less, especially for those modified with PEI of higher molecular weights, indicating their improved fouling resistance. The following factors could explain the improved anti-fouling property. Firstly, the introduction of PEI on the nascent PA layer enhances the surface hydrophilicity. Nitrogen atoms can attract water molecules as hydrogen 22

acceptors to form a hydration layer to hinder the SA adsorption. Secondly, the reaction between the residual acyl chloride groups and amine groups in PEI results in the reduced active site of carboxylic groups (which favors the strong adhesion to Ca2+ ions by complexation), and therefore contributes to a higher bio-fouling resistance. Thirdly, the declined fouling propensity can also owe to the existence of stereo-hindrance of PEI molecules. On the other side, despite the PEI-modified TFC membranes exhibit negative-charged surface in the alkalescence feed solution (pH=7.4), some protonated amine groups can still attract SA molecules by electrostatic interaction, resulting in deteriorative anti-biofouling properties.

Fig. 14 also displays the FRR values of the control and modified membranes after fouling and cleaning processes, respectively. As show in Fig. 14, the water flux decline of the control membrane is about 45.0±1.4% with a low FRR of 70.3±2.0% after physical cleaning. In a comparison, modified TFC membranes not only exhibit much less water flux reduction, but also possess much higher FRRs, especially for PEI-1800 modified TFC membrane, with a high FRR value of 94.9±1.5%.

Effect of post-treatment in strong acidic and basic aqueous solutions on the fouling propensity of resultant membranes is also investigated. From Fig. S-5 we can see, in comparison with sPA-1800 membrane, the water flux drop of sPA-1800-13 membrane is much less because of the enhanced hydrophilicity, reduced complexation sites (carboxyl groups) and steric hindrance. However, sPA-1800-1 membrane exhibits a higher fouling propensity due to the strong electrostatic attraction between the membrane surface with positive charge and SA molecules with negative charge. FRR values of the control and PEI-modified membranes before and after post-treatment also give the similar results as 23

shown in Fig. S-6.

3.6 Benchmarking

Table 7 summarizes the FO performance of recently developed TFC membranes[5, 56-60]. Here, the decline ratio of the specific reverse salt flux was employed to evaluate the membrane selectivity, that is, the higher decline ratio indicates the better membrane J

selectivity, and vice versa. The specific reverse salt flux decline ratio is defined as ∆(J S,C − w,C

JS,m Jw,m

) ∗ 100%, here

JS,C Jw,C

and

JS,m Jw,m

present the specific reverse salt flux of the control and

modified TFC membranes, respectively.

It can be seen that, most modified TFC membranes exhibit improved water flux but sacrificed membrane selectivity as compared to the corresponding control membranes. Alternatively, the modified TFC membrane in the present study exhibits not only an enhanced water flux, but also a lower reverse salt flux than the control one. This performance enhancement could be ascribed to the higher surface hydrophilicity and larger free volume as confirmed by the aforementioned WCA and PAS characterizations.

4. Conclusion In this study, PEI is employed to conduct the surface modification on the nascent PA layer by SIP method. Various characterization techniques are applied to confirm the modification mechanism including FTIR, XPS and PALS. Remarkable enhancement in the surface hydrophilicity and fractional free volume of PA layer are resulted after modification, and the enhancement magnifies with the increase of the PEI molecular weight. Both the surface 24

hydrophilicity and fractional free volume of those modified TFC membranes increase with, contributing to the significantly enhanced water flux. In addition, with a post-treatment on PEI-modified membrane by the acidic or basic solution, chemical forms of amine groups in PEI bonded on PA chains translate from the ionic (non-ionic) state to the non-ionic (ionic) state, and PA chains are degraded by hydrolysis, resulting in higher water fluxes and larger reverse salt fluxes modestly. Moreover, almost all modified TFC membranes exhibit enhanced anti-fouling behavior in the SA-containing wastewater as compared to that of the control TFC membrane, because of their improved hydrophilicity, less active complexion sites of carboxyl groups and enhanced steric-hindrance with PEI modification. But sPA-1800-1 membrane exhibits a severer membrane fouling due to the electrostatic interaction with SA molecules.

Acknowledgement

We thank the financial support from National Natural Science Foundation of China (no. 21306058), the Free Exploring Fundamental Research Project from Shenzhen Research Council, China (no. JCYJ20160408173516757), and Open Research Fund Program of Collaborative Innovation Center of Membrane Separation and Water Treatment of Zhejiang Province (no. 2016YB03). Special thanks are also given to the Analysis and Testing Center, the Analysis and Testing Center of Chemistry and Chemical Engineering School, and the State Key Laboratory of Materials Processing and Die & Mould Technology, in Huazhong University of Science and Technology for their help with material characterizations.

25

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33

Fig. 1 Reaction mechanism of second interfacial polymerization between nascent PA and PEI

34

C=O

N-H

1660

1610

-OH -OH/-NH2

1650

1600

N-H/C-N

sPA-1800

PA-Control

1700

1544

3330 3410

3500

1550

1500

Wavenumbers (cm-1) Fig. 2 ATR-FTIR spectra of the control and PEI-modified membranes

35

(a) C 1S

*C-O/*C-N/*C-Cl *C-C/*C=C O=C*-N/*C=NH/O=C*

292

290

288

286

284

282

280

*C-O/*C-N/*C-Cl *C-C/*C=C O=C*-N/*C=NH/O=C*

292

290

Binding energy (eV)

C 1S

O=C*-N/*C=NH/O=C*

286

284

284

282

280

282

C 1S

*C-O/*C-N/*C-Cl

Intensity (a.u.)

Intensity (a.u.)

*C-C/*C=C

288

286

sPA-1800

*C-O/*C-N/*C-Cl

290

288

Binding energy (eV)

sPA-1200

292

C 1S

sPA-600

Intensity (a.u.)

Intensity (a.u.)

PA-Control

280

*C-C/*C=C O=C*-N/*C=NH/O=C*

292

290

Binding energy (eV)

288

286

284

282

280

Binding energy (eV)

(b)

*O=C-NH/*O=C-O

O=C-O*

536

534

sPA-600

O 1S

532

530

528

Intensity (a.u.)

Intensity (a.u.)

PA-Control

O 1S *O=C-NH/*O=C-O

O=C-O*

534

532

530

532

530

528

528

526

sPA-1800

Intensity (a.u.)

Relative intensity (a.u.)

sPA-1200

Binding energy (eV)

534

526

Binding energy (eV)

Binding energy (eV)

536

*O=C-NH/*O=C-O

O=C-O*

536

526

O 1S

O 1S *O=C-NH/*O=C-O

O=C-O*

536

534

532

530

528

526

Binding energy (eV)

Fig. 3 (a) C 1S and (b) O 1S XPS spectra and peak deconvolution of the control and modified TFC membranes.

36

0.46

S

0.45

0.44

PA-Control sPA-600 sPA-1200 sPA-1800

S

0.46

0.43

0.45

0.44 1.0

1.5

2.0

2.5

3.0

Positron Incident Energy (keV)

0

5

10

15

20

25

Positron Incident Energy (keV) Fig. 4 Depth profiles of S parameters of control and PEI-modified TFC membranes.

37

(A)

(B)

(C)

Fig. 5 SEM and AFM images of the control and PEI-modified membranes: (A) surface morphology, (B) cross-sectional morphology and (C) surface topology.

38

80

WCA (°)

60

40

20

0 0

600

1200

1800

Mw-PEI (g/mol) Fig. 6 Water contact angles of the control and PEI-modified membranes.

39

Jv (LMH)

80 60 40 20 0 0

600

1200

1800

Mw-PEI (g/mol) 25

Js-FO Js-PRO

Js (gMH)

20 15 10 5 0 0

600

1200

1800

Mw-PEI (g/mol) Fig. 7 FO performance of the control and PEI-modified membranes. (2 M NaCl aqueous solution and DI water are used as the draw solution and feed solution, respectively.)

40

(a)

(b)

Fig. 8 Reaction mechanism of post-treatment: (a) the degradation of PA chain by hydrolysis and (b) the ionization or deionization of amine groups in PEI under alkaline or acidic conditions

41

1660

1544

C=O

N-H/C-N

sPA-1800-13

sPA-1800-1

sPA-1800

PA-Control

1750 1700 1650 1600 1550 1500 1450 1400

Wavenumbers (cm-1)

Fig.

9 ATR-FTIR spectra of the control TFC membrane and PEI-modified ones before and after post-treatment

42

*O=C-NH/*O=C-O

O=C-O*

536

534

sPA-1800-1

O 1S

532

530

528

Intensity (a.u.)

Intensity (a.u.)

PA-Control

*O=C-NH/*O=C-O O=C-O*

536

526

Binding energy (eV)

*O=C-NH/*O=C-O

534

532

530

Binding energy (eV)

532

530

sPA-1800-13

O 1S

O=C-O*

536

534

528

526

Binding energy (eV)

528

526

Relative intensity (a.u.)

Relative intensity (a.u.)

sPA-1800-1

O 1S

O 1S *O=C-NH/*O=C-O

O=C-O*

536

534

532

530

528

526

Binding energy (eV)

Fig. 10 O 1S XPS spectra of the control TFC membrane and PEI-modified ones before and after post-treatment

43

80

WCA (°)

60

40

20

0

PA-Control

sPA-1800

sPA-1800-1 sPA-1800-13

Fig. 11 Water contact angles of the control TFC membrane and PEI-modified ones before and after post-treatment

44

120

Jv(LMH)

100

FO PRO

80 60 40 20 0

PA -

sP sP sP sP sP AAAAACo 18 18 18 18 1 8 ntr 0 0 00 00 00 0 0 -5 -9 -13 -1 ol

sPA -18 009

30 25

FO PRO

Js(gMH)

20 15 10 5 0

PA -

sP sP sP sP sP AAAAACo 1 1 18 1 1 8 8 8 8 ntr 00 0 00 00 00 0 -9 -5 -13 -1 ol

sPA -18 009 before and after Fig. 12 FO performance of the control TFC membrane and PEI-modified ones

post-treatment (2 M NaCl aqueous solution and DI water are applied as the draw solution and feed solution, respectively.)

45

Normalized water flux Jw/Jw,0

1.0 0.9 0.8 0.7 PA-Control sPA-600 sPA-1200

0.6

sPA-1800 Baseline

0

2

4

6

8

10

12

14

16

18

Testing time (h) Fig. 13 Dynamic forward osmosis fouling tests of the control and PEI-modified membranes. (2 M NaCl solution and synthetic wastewater are used as draw solution and feed solution, respectively; the Jw/Jw,0 ratio is taken with a 1-hour interval during the fouling test.)

46

Normalized water flux Jw/Jw,0

1.0

0.8

1.0 After fouling

0.8

After cleaning

0.6

0.6

0.4

0.4

0.2

0.2 0

600

1200

1800

Mw-PEI (g/mol) Fig. 14 Dynamic forward osmosis fouling tests of the control and PEI-modified membranes.

47

Table 1 Surface elemental composition of the control and PEI-modified membranes by XPS analysis

Code

C

O

N

O/N

PA-Control

72.21

17.51

10.28

1.70

SIP-PEI-600

72.55

13.80

13.64

1.01

sPA-1200

72.08

13.89

14.03

0.99

sPA-1800

70.76

14.22

15.01

0.95

sPA-1800-1

69.00

19.79

11.21

1.77

sPA-1800-13

69.48

20.02

10.50

1.91

48

Table 2 Surface chemical compositions of the control and PEI-modified membranes by XPS analysis

Code

PA-Control

sPA-600

sPA-1200

sPA-1800

CI

88173.87

88927.7

57666.35

94842.57

CII

35858.38

41180.14

25858.59

38605.96

CIII

29036.99

25865.34

14955.84

21168.62

CII/

1.23

1.59

1.73

1.82

OI

65020.37

55994.29

57713.18

88216.71

OII

38861.06

31192.81

29113.58

41180.42

OII/

1.67

1.80

1.98

2.14

CIII

OIII * CI: *C-C/*C=C, BE=285 eV CII: *C-O/*C-N/*C-Cl, BE=286.2 eV CIII: O=C*-N/*C=NH/O=C*, BE=288.1 eV O I: N-C=O*/O-C=O*, BE=531.2 eV O II: O=C-O*, BE=532.6 eV

49

Table 3 Zeta potential of the control and modified membranes Code

Zeta potential (mV)

Code

Zeta potential (mV)

PA-Con sPA-1800 -42.63 -25.18 trol sPA-60 sPA-1800-1 -28.49 -24.86 0 sPA-12 sPA-1800-13 -25.95 -33.07 00 * Zeta potentials of all membranes are measured in the solution of pH 7.

50

Table 4 Intrinsic transport properties of the control and PEI-modified membranes Membrane ID

Aa, LMH/Bar

PA-Contro

1.99±0.10

Bb, LMH

Rejection Rs, %

B/A, Bar

94.72±1.05 0.09

0.18±0.05 l sPA-600

2.29±0.13

0.35±0.01

91.16±0.70

0.15

sPA-1200

2.59±0.15

0.45±0.03

90.02±1.02

0.18

sPA-1800

2.85±0.14

0.59±0.05

88.35±1.35

0.21

a

DI water is used as the feed solution in the RO test with an applied pressure of 3 bar (2.5 rpm); b 1000 ppm NaCl solution is used as the feed solution in the RO test with an applied pressure of 3 bar (2.5 rpm).

51

Table 5 Surface chemical compositions of the control TFC membrane and PEI-modified ones with and without post-treatment by XPS analysis

Code

PA-Control

sPA-1800

sPA-1800-1

sPA-1800-13

OI

57542.79

73904.96

54162.85

75165.02

O II

41994.9

41486.83

42139.94

66950.13

O I/

1.51

1.78

1.29

1.12

O II * O I: N-C=O*/O-C=O*, BE=531.2 eV O II: O=C-O*, BE=532.6 eV

52

Table 6 Intrinsic transport properties of the control TFC membrane and PEI-modified ones before and after post-treatment Membrane ID

Aa, LMH/Bar

Bb, LMH

Rejection Rs, %

B/A, Bar

PA-Control

1.99±0.10

0.18±0.05

94.72±1.05

0.09

sPA-1800

2.85±0.14

0.59±0.05

88.35±1.35

0.21

sPA-1800-1

3.96±0.14

0.92±0.08

87.09±1.37

0.23

sPA-1800-5

2.90±0.05

0.61±0.03

88.19±0.64

0.21

sPA-1800-9

2.87±0.06

0.67±0.03

87.17±0.34

0.22

sPA-1800-13

3.53±0.11

1.10±0.07

83.48±1.27

0.31

a

DI water is used as the feed solution in the RO test with an applied pressure of 3 bar (2.5 rpm); b 1000 ppm NaCl solution is used as the feed solution in the RO test with an applied pressure of 3 bar (2.5 rpm).

53

Table 7 FO performance of recently developed TFC membranes Water flux

Reverse salt flux

Membrane code (LMH)

(gMH)

MPD+TMC on PSF

24.9/45.4

9.8/18.5

PEI-MPD/TMC on PSF

46.5/87.8

12.8/22.0

MPD+TMC on PSF

20.2/35

3.41/4.60

MPD+TMC on PSF/ BPSH100-BPS0

40.9/74.4

9.32/11.88

MPD+TMC on PES water

32.1/52.7

6.15/6.93

MPD+TMC on PES water/NMP/PEG

34.5/65.1

9.87/12.34

MPD+TMC on PSF

13.9/32.3

5.3/14.2

MPD+TMC on PSF/TiO2

33.0/59.4

15.7/31.0

MPD+TMC on PSF

22.8/36.7

5.7/8.9

MPD+TMC/UiO-66 on PSF

27/51.3

6.1/12.3

MPD+TMC on PSF

13.3/26.01

6.02/14.20

MPD+TMC on PSF/HNT

36.62/53.95

20.30/38.91

MPD+TMC on PSF/SPEK

30/40

7/8

MPD+TMC on PSF/SPEK/DEG

35/50

7/9

J

J

w,C

w,m

∆(J S,C − J S,m )

References

%

11.8/15.7

This work

-5.9/-2.8

[1]

-9.44/-5.8

[2]

-9.4/-8.2

[3]

2.4/0.3

[4]

-10.2/-17.5

[5]

3.3/2

[6]

*Feed solution: DI water; draw solution: 2M NaCl solution

54

Highlights 

SIP modification of TFC membranes by PEI with various molecular weights.



Post-treatment of PEI-modified membranes in basic or acidic solution is conducted.



Modified membranes exhibit higher hydrophilicity and fractional free volume.



Modified membranes possess better separation performance and antifouling property.

55