Tris(2-aminoethyl)amine in-situ modified thin-film composite membranes for forward osmosis applications

Author’s Accepted Manuscript Tris(2-aminoethyl)amine in-situ modified thin-film composite membranes for forward osmosis applications Liang Shen, Jian Zuo, Yan Wang www.elsevier.com/locate/memsci

PII: DOI: Reference:

S0376-7388(16)32433-4 http://dx.doi.org/10.1016/j.memsci.2017.05.035 MEMSCI15269

To appear in: Journal of Membrane Science Received date: 3 December 2016 Revised date: 22 March 2017 Accepted date: 7 May 2017 Cite this article as: Liang Shen, Jian Zuo and Yan Wang, Tris(2aminoethyl)amine in-situ modified thin-film composite membranes for forward osmosis applications, Journal of Membrane Science, http://dx.doi.org/10.1016/j.memsci.2017.05.035 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.

Tris(2-aminoethyl)amine in-situ modified thin-film composite membranes for forward osmosis applications Liang Shena,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

Huazhong University of Science and Technology, Research Institute in ShenZhen, Shenzhen 518000, P. R. China d

Department of Chemical & Biomolecular Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260, Singapore *Corresponding author. Tel.: 86 13871464406; fax: 86 027-87543632. [email protected] (Yan Wang)

Abstract Forward osmosis (FO) has drawn growing attention in recent years, while the lack of desirable FO membranes has been restricting its further development in industrial applications. In this work, a novel tripodal amine — tris(2-aminoethyl)amine (TAEA), with a dual role of catalyst and reactive amine monomer, is incorporated in the PA selective layer for the first time, to in-situ modify the thin-film composite (TFC) membrane. A series of characterization techniques are employed to investigate the modification mechanism involved, as well as changes in chemical properties and the microstructure of the PA layer in terms of the TAEA content and the amine solution

pH. The separation performance and anti-fouling behavior of the TAEA-modified TFC membranes are studied correspondingly. In comparison with the control TFC membrane, modified TFC membranes possess higher water permeability, higher salt rejection, and much lower fouling propensity, and may hold a great potential for FO applications. Abbreviations

AFM,

atomic force microscopy; ATR-FTIR, transform infrared; CaCl2,

attenuated total reflectance Fourier

calcium chloride; CSA,

-10-camphorsulfonic acid; DBES, spectroscopy; DI, osmosis; FS,

deionized; DS,

(1S) - (+)

doppler broadening energy draw solution; FO,

feed solution; IP,

forward

interfacial polymerization;

KH2PO4, potassium dihydrogen phosphate; MgSO4, magnesium sulfate; MPD,

m-phenylenediamine; NaCl,

NaHCO3, sodium bicarbonate; NH4Cl, N-methyl pyrrolidone; PA, spectroscopy; PALS, PRO,

tris(2-aminoethyl)amine; TEA,

sodium alginate; SDS,

scan electron microscopy; TAEA, triethylamine; TFC,

1,3,5-trimesoyl chloride; WCA,

thin-film

water contact

wide-angle X-ray diffractometer; XPS,

photoelectron spectroscopy

Keywords

position annihilation

position annihilation lifetime spectroscopy;

sodium dodecyl sulfate; SEM,

angle; WXRD,

ammonium chloride; NMP,

polyamide; PAS,

pressure retarded osmosis; SA,

composite; TMC,

sodium chloride;

X-ray

forward osmosis, tris(2-aminoethyl)amine, thin-film composite membrane, polyamide, in-situ modification 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

FDR%

: flux decline ratio

FRR%

: flux recovery ratio

FV

: free volume

I3

: Positron intensity

ICP

: internal concentration polarization

J

: pure water flux

Js

: reverse salt flux

Jv

: water flux

R

: free volume radius

Rs

: salt rejection

t3

: Positron lifetime

ΔP

: hydraulic pressure

concentration

∆t

: test time

∆V

: volume change

Δπ

: osmotic pressure

1. Introduction The development of sustainable technologies for wastewater reclamation and seawater desalination to produce clean water has drawn growing global attention owing to the severer water scarcity, especially for these arid and environmentally contaminated regions. Among various technologies, membrane-based separation processes, not limited by the heat consumption, hold greater potential than most other conventional thermal-based separation processes[1-8]. However, most pressure-driven membrane processes, such as microfiltration, ultrafiltration, nanofiltration and reverse osmosis, require external energy to impel water recovery through the semipermeable membrane, causing a high-energy consumption and therefore the huge operation cost. Alternatively, FO, as a potential solution for water treatment, has received considerable attention in recent years, since it utilizes the osmotic pressure difference between the draw solution (DS) and feed solution (FS) as the driving force to facilitate spontaneous water transport across the semipermeable membrane. The operation with no or low hydraulic pressure endows FO process with many advantages as compared to those conventional pressure-driven membrane processes, i.e., (1) relative low energy consumption[9], (2) high water recovery[10], and (3) low propensity towards membrane fouling[11, 12]. Potential applications of FO process in various fields are therefore explored, including water treatment[13-15], power regeneration[16], liquid food processing and juice concentration, pharmaceutical[17], essential products and protein enrichment[18].

For the development of FO technology, lack of desirable membranes is still the major obstacle at present stage. An ideal FO membrane should meet the following standards[19-21]: (1) sufficient mechanical strength, (2) good chemical stability, (3) high water permeability, (4) high solute rejection, and (5) high fouling resistance. State-of-the-art technology for FO membrane fabrication is the thin-film composite (TFC) membrane, by the formation of an ultrathin active polyamide (PA) layer on the porous substrate via interfacial polymerization (IP), because of its easy fabrication and excellent separation performance over a wide range of operational temperature and pH.

However, traditional TFC membranes formed by m-phenylenediamine (MPD) and trimesoyl chloride (TMC) generally suffer relative low water flux and high fouling tendency due to the relative hydrophobic PA layer and its rough surface[22]. Extensive researches have been launched to solve above problems via appropriate modifications on the active layer of TFC membrane. Among them, incorporation of novel functional monomers[23, 24] or nanoparticles[25, 26] into the PA layer could bring about significant changes in the chemical and morphological properties of the membrane and its resultant separation performance, chlorine resistance and fouling tendency. Surfactants or other additives can also be added into the aqueous or organic phase to affect the reaction between the amine monomers and acyl chloride monomers, resulting in an enhancement in the overall separation performance. For examples, triethyl benzyl ammonium bromide[27], sodium dodecyl sulfate (SDS)[16], trimethylamine (TEA)[28], cetyl trimethyl ammonium bromide[29, 30] etc are all typical additives in the fabrication of TFC membranes to alter the resultant separation

performance. Besides that, surface modification as an effective and simple way to improve the membrane surface properties, is widely reported to enhance the water permeability and anti-fouling behavior of the membrane, including polyethylene glycol based hydrophilic modification[31, 32], polydopamine coating[33], zwitterion grafting[34, 35] and so on[36, 37]. Additionally, post-treatment of nascent TFC membranes in certain solutions/solvents can also be conducted to swell up PA chains, dissolve PA fragments or remove unreacted monomers, resulting in the increased fractional free volume of PA layer and therefore the water flux, such as alcohols[38, 39], N, N-dimethyl formamide[16], SDS/glycerol[40].

In this study, a novel tripodal amine, TAEA with three primary amine groups and one tertiary amine group, is incorporated into the MPD aqueous solution to perform an in-situ modification of the TFC membrane. Here TAEA not only plays an active role as an amine monomer, but also acts as a catalyst to accelerate the reaction rate of interfacial polymerization. Based on previous studies, the semi-aromatic PA layer formed with aliphatic amine/acyl chloride involved generally exhibits looser structure, and smoother surface as compared to the fully aromatic PA layer[41-43]. Additionally, tertiary amine may act as the catalyst to accelerate the reaction rate between MPD and TMC by absorbing the by-product hydrogen chloride during the amide formation, resulting a thinner PA layer of a higher crosslinking [28]. However, to the best of our knowledge, no study has been reported yet on the modification of TFC membranes with an amine of the dual role. In this work, TAEA-modification of the TFC membrane is investigated using such a novel amine with a dual role for the first time to optimize the morphology, physicochemical properties, and separation performance of resultant TFC membranes. A series of characterizations (FTIR, XPS, WXRD and

PALS) are employed to verify the proposed modification mechanism. Significant changes in TFC membranes in terms of the chemical properties, membrane hydrophilicity and microstructure of the modified PA layer are also investigated. The corresponding separation performance and anti-biofouling property of the TAEA-modified TFC membranes are further studied systematically in terms of various TAEA contents and amine solution pHs. This work shows that, the in-situ modification by TAEA can significantly improve the overall performance of TFC membranes.

2. Experiment 2.1 Materials Polysulfone (PSf) (Mw: 800,000 Da) were provided by Beijing HWRK Chem co. Ltd. (China). M-phenylenediamine (MPD, 99.5%), 1, 3, 5- trimesoyl chloride (TMC, 98%) and (1S) - (+) -10-camphorsulfonic acid (CSA, ≥99) were purchased from Aladdin. Tris (2-aminoethyl)amine (TAEA, 98%) was supplied by Tokyo Chemical Industry Co. Ltd., and stored in the refrigerator before use. N-methyl pyrrolidone (NMP, anhydrous, ≥99.5%), n-hexane (≥97%, anhydrous), polyethylene glycol 400 (PEG 400, CP), triethylamine (TEA, 99%) 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 obtained from China National Medicine Corporation. 2.2 Preparation of TFC membranes The PSf substrate was fabricated by non-solvent induced phase separation method by casting the dope solution with a composition of PSf/PEG400/NMP (18/16/66 wt%) on

a glass plate with a casting knife of 100 μm thickness. The nascent membrane was immersed into a water coagulation bath immediately at room temperature to initiate the phase separation and transferred into the water coagulation bath for 2 days with water changed every 12 h to remove the residual solvent.

Both the control and TAEA-modified TFC membranes were prepared via interfacial polymerization on PSf substrates. PSf substrates were firstly immersed into a MPD/TAEA aqueous amine solution, where MPD concentration is fixed at 3.4 wt% while TAEA concentration is varied in the range of 1-3 wt%. The solution pH was adjusted to 9 by a CSA solution unless otherwise stated. The excess amine solution was then carefully removed from the membrane surface by a rubber roller after 2-min immersion. A 0.15 w/v% TMC/hexane solution was then gently poured onto the surface of the soaked PSf substrate which was fixed in a plastic frame for 1-min contact to initiate the interfacial polymerization. Afterwards, the excess TMC solution was poured out and the as-fabricated membrane was transferred into an 80 C hot water bath to conduct the post treatment for 5 min after hexane evaporation in the air. The resultant TFC membranes were then stored in DI water with water changed daily before use. The TAEA-modified TFC membranes were donated as PA-X-TAEA-Y, where X refers to the TAEA content in the aqueous amine solution, while Y indicates the solution pH.

2.3 Characterizations of TFC membranes Changes in the chemical structures of TFC membranes by TAEA modification were examined by the attenuated total reflectance Fourier transform infrared (ATR-FTIR, Brucker, VERTEX-70) with a range of 500-3500 cm−1 and a resolution of 2 cm−1, and

X-ray photoelectron spectroscopy (XPS, VG Multilab 2000, Thermo VG Scientific, UK) using a monochromatic A1 Ka X-ray source.

The doppler broadening energy spectroscopy (DBES), was employed using a Position Annihilation Spectroscopy (PAS) coupled with a variable monoenergy slow positron beam (0-30 keV), to explore the microstructure of TFC membranes in terms of S parameters. The DBES spectra were measured using an HP Ge detector (EG&G Ortec) at a counting rate of approximately 3000 counts per second (cps). The free volume, pore size and pore size distribution in the PA layer of the TFC membrane were also characterized by Position Annihilation Lifetime Spectroscopy (PALS), and analyzed by PATFIT and MELT softwares.

The inter-chain spacing distance (d-spacing) of TFC membranes were also estimated by a wide-angle X-ray diffraction (WXRD) (PAnalytical X'pert Pro diffractometer), using CuK α radiation, in the range of 10–60°. Based on the Bragg’s Law, the d-spacing (d) values can be determined by Eq. (1). 𝑛𝜆 = 2𝑑𝑠𝑖𝑛𝜃

(1)

where n is an integer (n=1 in this study), λ represents the X-ray radiation wave length (1.5406 Å), and θ refers to the X-ray diffraction angle of the peak.

The membrane morphology was observed by Scanning Electron Microscopy (SEM, VEGA3, TESCAN, Czech) and Atomic Force Microscopy (AFM, SPM9700, Shimadzu, Japan). SEM samples were prepared by being sprayed with a uniform gold layer by a sputter coater (Q150RS, Quorum, England) for a fixed time. Samples for cross-sectional morphology observation were fractured in liquid nitrogen. For AFM

sample, an area of 3×3 μm was scanned at a rate of 1 Hz to observe the surface topology and determine the mean roughness (Ra) under the dynamic mode in the air.

Membrane hydrophilicity of TFC membranes was evaluated by a Contact Angle Goniometer (DSA 25, KRÜSS, Germany) using sessile drop method to get the water contact angle (WCA) at room temperature, and at least ten points were taken to yield an average data of each membrane sample.

Mechanical properties of the control and modified membranes were measured by the tensile test using an Instron 5542 Material Testing Instrument at room temperature. Each membrane sample was clamped at both ends and stretched vertically at a speed of 1 mm/min until breakage. Five samples were tested for each membrane to obtain an average value. The results were shown in Fig. S-1 in the Supporting Information.

2.4 Evaluation of intrinsic properties

Intrinsic transport properties of TFC membranes were characterized by a lab-scale RO filtration apparatus (Suzhou Faith Hope Membrane Technology). The tested membrane was fixed in a stainless-steel cell with the active layer facing the feed solution and pre-compacted under a pressure of 3 bar by DI water for 0.5 h prior to the test. Each test was repeated for at least three times to yield an average data for all membrane samples. The water permeability A (L m-2 h-1/bar, abbreviated as LMH/bar) was calculated by the pure water permeation (J, LMH) over a trans-membrane pressure (P) of 3 bar with DI water as the feed solution according to Eqs. (2) and (3),

J=𝐴

∆𝑉

(2)

𝑚 ×∆𝑡

𝐽

A = ∆𝑃

(3)

where ∆V is the permeate volume change over the predetermined testing time (∆t) of 0.5 h, Am is the effective membrane area (17.35 cm2), ΔP is the applied hydraulic pressure across the membrane. The salt rejection Rs (%) and salt permeability B (g m-2 h-1, abbreviated as gMH) were measured using a 1000 ppm NaCl aqueous solution as the feed solution over a constant applied pressure of 3 bar and determined according to Eqs. (4) and (5), 𝐶𝑝

𝑅𝑆 = (1 − 𝐶 ) × 100 𝑓

1−𝑅𝑠 𝑅𝑠

𝐵

= 𝐴(∆𝑃−∆𝜋)

(4) (5)

where Cp and Cf are the solute concentrations in the permeate and feed solutions, detected by a conductivity meter (FE30, Mettler Toledo, Switzerland), while Δπ is osmotic pressure of the feed solution.

2.5 Evaluation of FO performance

A lab-scale cross-flow FO set-up was employed to evaluate the FO performance of the control and TAEA-modified TFC membranes in terms of water flux and reverse salt flux. DI water and 2 M NaCl aqueous solution of 0.5 L were employed as the feed solution and draw solution, respectively, and recirculated by two peristaltic pumps (BT300-2J, Longer, China) with a fixed crossflow velocity of 0.3 L min-1 (150 rpm) at 22.5±0.5 C. The FO test was performed under two operational modes, i.e. FO mode (AL–FS mode, active layer facing the feed solution) and pressure retarded osmosis (PRO) mode (AL–DS mode, active layer facing the draw solution). It was

pre-conditioned for 30 minutes to reach a steady water flux before the data collection. The volumetric flux (Jv, LMH) was determined by the weight change of the draw solution monitored by a digital balance (FX3000-GD, AND, Japan) which could output the collected data to a computer. The reverse salt flux (Js, gMH) was calculated by the concentration changes of the feed solution by a conductivity meter (FE30, Mettler Toledo, Switzerland). The water flux Jv a reverse salt flux Js could be calculated by Eqs. (6) and (7). ∆𝑉

𝐽𝑣 = 𝐴

𝑚 ∆𝑡

𝐽𝑠 =

∆(𝐶𝑡 𝑉𝑡 ) 𝐴𝑚 ∆𝑡

(6) (7)

where ΔV is the volume change of draw solution over a predetermined test time (Δt) of 0.5 h, Am is the effective membrane area (3.87 cm2), Ct and Vt are the salt concentration and the feed solution volume in the FO test, respectively. At least three tests were repeated for each tested membrane sample.

2.6 Dynamic FO fouling tests

The dynamic anti-fouling test was also conducted to evaluate the fouling propensity of TFC membranes by the laboratory-scale FO system using sodium alginate (SA) as the foulant. The whole fouling test contained four stages and the detailed experiment procedure was elaborated in previous work[44, 45]. Firstly, the system was run with DI water as both draw and feed solutions to stabilize the membrane samples for 1 h. In the second stage, synthetic wastewater and 2 M NaCl aqueous solution were used as the feed and draw solutions respectively for 1 h to obtain the initial water flux. After that, the accelerated fouling test started using 2 M NaCl aqueous solution as the draw solution and the synthetic wastewater with 250 ppm SA as the feed solution for

an 18-h duration. Next, the physical cleaning was conducted immediately at a crossflow rate of 0.6 L/min (300 rpm) for 30 minutes using 15 mM NaCl solution circulated through the feed and draw sides. Finally, the water flux with the cleaned membrane was measured again. Here a large volume of the draw solution (1.8 L) was used during fouling test to minimize its dilution effect.

The antifouling properties of TFC membranes are evaluated in terms of the flux decline ratio (FDR%) and flux recovery ratio (FRR%) by Eqs. (8) and (9) 𝐹𝐷𝑅% =

𝐽0 −𝐽𝑡 𝐽0

× 100%

𝐽

𝐹𝑅𝑅% = 𝐽𝑐 × 100% 0

(8) (9)

where J0 is the initial flux, Jt is the flux after the accelerated fouling test, while Jc is the water flux after the physical cleaning.

The actual brackish water from a local lake (East Lake) was also used to conduct the dynamic fouling test for 24 h. The experimental details and corresponding results (Fig. S-2) are elaborated in the Supporting Information.

2.7 Chemical stability tests

The chlorine-resistance and acid-resistance tests of the control and TAEA-modified membranes were conducted to investigate the effect of TAEA modification on the chemical stability of the membrane. The experimental details and corresponding results (Figs S-3 and S-4) are elaborated in the Supporting Information.

3. Results and discussion

3.1 Chemical properties of modified TFC membranes

TAEA, a tertiary amine containing three primary amine groups, is employed as an additive in the amine solution to perform the in-situ modification of TFC membranes, acting as not only a catalyst but also an active amine monomer. The proposed reaction mechanism of modification by TAEA is shown in Fig. 1. On one side, the tertiary amine groups of TAEA can perform as a catalyst to speed up the rate of positive reaction between TMC and MPD by neutralizing the product of the hydrogen chloride generated

during

the

interfacial

polymerization

process,

resulting

in

a

highly-crosslinked PA network[28]. On the other side, primary amine groups of TAEA can also participate in the interfacial polymerization, playing an active role of amine monomer to react with TMC, contributing to a semi-aromatic PA network. Additionally, it also acts as a crosslinker to interlink adjacent PA chains, further improving the crosslinking degree. The proposed modification mechanism is confirmed by various characterization techniques including FTIR, XPS, WXRD and PALS.

Fig. 2 displays the FTIR spectra of the control and modified TFC membranes. Characteristic peaks of amide groups at 1655 and 1530 cm-1 can be observed for all TFC membranes, attributed to stretching vibrations of amide I and amide II groups, respectively. In comparison with the spectrum of the control membrane, new peaks at 1360 and 3065 cm-1 appear in those of TAEA-modified TFC membranes, which could be

ascribed to stretching vibrations of C-NR2 (R= C) and -CH2- groups in TAEA. Additionally, the band intensity at 1610 cm-1, attributed to the deformation vibration of N-H bond of amine groups, shows to be much stronger in the spectra of modified TFC membranes than that in the control one. Meanwhile, the intensities of the N-H adsorption band increase with the increase in the TAEA content of TFC membranes. What’s more, the peak intensities at 3406 cm-1 (ascribed to the stretching vibrations of hydroxyl groups) in the spectra of TAEA-modified TFC membranes sharply decline as compared to that of the control membrane, which could be assigned to less converted carboxyl groups from acyl chloride groups by hydrolysis because of the higher cross-linking degree of the PA layer with TAEA involved. It can also be observed that, the intensities of that adsorption band in the spectra of modified TFC membranes decrease with the increase of the TAEA content. Moreover, those peaks shift right gradually, since adsorption bands of more unreacted primary amine groups of TAEA overlap with those of hydroxyl groups. All above peak changes demonstrate the successful bonding of TAEA onto the PA chains and are consistent with the proposed reaction mechanism.

Chemical composition changes of the TFC membranes are also analyzed by XPS using wide-scan (low-resolution) and the results are shown in Fig. S-5 and Table 1. It is observed in Table 1 that, O contents in the surfaces of TFC membranes decrease with the increase of TAEA content, and N contents exhibit the opposite trend. Besides, O/N ratios decrease from 1.78 to 1.08 continuously with the TAEA content increase from 0 to 3 wt% in the amine solution, which should be contributed to the combined

effects of higher crosslinking degree and the presence of high nitrogen-containing TAEA into PA network.

Table 1 Surface elemental composition of TFC membranes with different TAEA contents by XPS analysis Code

PA-Control

PA-1-TAEA-9 PA-2-TAEA-9 PA-3-TAEA-9

C

76.76

75.25

74.36

74.21

O

14.87

14.29

13.89

13.37

N

8.37

10.46

11.74

12.42

O/N

1.78

1.37

1.17

1.08

Table 2 Surface chemical compositions of TFC membranes with different TAEA contents by XPS O 1S spectra analysis

Code

OI

O II

O I/ O II

PA-Control

59254.08

28682.33

2.07

PA-1-TAEA-9

63993.91

17729.09

3.61

PA-2-TAEA-9

67264.20

16950.49

3.97

PA-3-TAEA-9

66863.97

14786.88

4.52

*O I: N-C=O*/O-C=O*, BE=531.5 eV O II: O=C-O*, BE=533.3 eV

The high-resolution narrow-scanned spectra analyzed by XPS are employed to further quantitatively investigate changes in surface functional groups of TFC membranes and results are shown in Fig. 3. Besides, corresponding peak areas of O 1S are summarized in Table 2. The deconvolution of O 1S spectra of TFC membranes (Fig. 3(a)) involves two kinds of peaks, i.e., (1) N-C=O* and O-C=O* (OI, BE= 531.2 eV), and (2) *O-C=O (OII, BE=532.6 eV)[46]. As well known, for the second deconvoluted peak of O 1S, the oxygen atoms in *O-C=O bond are from those carboxyl groups converted from residual acyl chloride groups. In addition, the counted amount of oxygen atoms from N-C=O* and O-C=O* bonds also keep in constant, since oxygen atoms are entirely from the carbonyl groups in acyl chloride of TMC. Consequently, the IOI/IOII can be employed to estimate the reduction degree of carboxyl groups. It can be found that, the intensity ratios of IOI/IOII of modified TFC membranes increases with the increase of the TAEA content, demonstrating that less residual acyl chloride groups on nascent PA layer have been translated into carboxyl groups. Moreover, Fig. 3 (b) displays the deconvolution of N 1S spectra of TFC membranes containing three peaks: (1) -C=N*- and R-*NH2 (NI, BE=399.2 eV), (2) O=C-N* and C=N*H (NII, BE=400.4 eV), (3) R-*N+H3 and R-*N+H2R’ (NIII, BE=401.5 eV)[47]. All modified TFC membranes exhibit NIII peaks as compared to that of the control one, owing to the presence of protonated primary amine groups. The XPS results are in perfect accordance with FTIR results and testify the suggested reaction mechanism again.

To further explore the modification mechanism by TAEA, WXRD is applied to examine the microstructure changes of resultant PA layers. Generally, a higher crosslinking degree of an amorphous glassy polymer indicates the smaller d-spacing

between adjacent polymeric chains. Fig. 4 displays WXRD patterns of the control and modified TFC membranes. Based on Bragg’s Equation, a higher 2θ value demonstrates smaller d-spacing. Therefore, it can be found that, in comparison with the control membrane (d-spacing of 5.12 Å), d-spacing values of all modified TFC membranes are smaller and decrease with the increase of the TAEA content, indicating a more compacted PA chain after TAEA modification because of the higher crosslinking degree.

Fig. 5 also illustrates S parameters of the control and modified TFC membranes to evaluate changes of the free-volume in the PA layer. In this study, a three-layer model is applied to analyze the obtained results, i.e. (I) dense PA layer, (II) transition layer from dense layer to porous PSf support layer, and (III) porous PSf support layer. As displayed in Fig. 5, depth profiles of S parameters for all TFC membranes exhibit similar trends. All curves increase sharply with the increase of the positron incident energy firstly, due to the back diffusion and scattering of positroniums near the membrane surface. Subsequently, S parameter decreases gradually after reaching the maximum, demonstrating the gradual transition from the dense PA layer to the support layer (transition layer). Lastly, S parameter increases slowly, corresponding to the transformation from the transition layer to the porous support layer. Basically, S parameter increases with an increase in free volume cavity size and/or the free volume number. In comparison with the control membrane, modified TFC membranes exhibit lower S values, which are consistent with WXRD results, suggesting the formation of the denser PA network.

Table 3 summarizes the positron lifetime results of the control and modified membranes by PATFIT analysis. We can see that, the pore size and free volume of modified membranes are all lower than that of the control membrane, which is ascribed to the increased crosslinking degree by TAEA modification. Fig. S-6 displays the o-Ps lifetime distributions of the control and modified membranes by MELT analysis, which exhibit similar trends. It can be seen that, the lifetime values (the positron lifetime t3 and positron intensity I3 ) of modified membranes are both lower than that of the control membrane, corresponding to the smaller free volume radius (R), as listed in Table 2, which is ascribed to the increased crosslinking degree by TAEA modification. In addition, the distribution widths of modified membranes are also observed to be larger than that of the control membrane, which can be resulted from the formed more complicated PA network by TMC, TAEA and MPD, and thus a wide free volume distribution.

Table 3 Positron lifetime results of the control and modified membranes by PATFIT analysis Membrane code

t3 (ns)

I3 (%)

R (Å)

Free Volume (Å3)

PA-Control

2.001±0.020 5.979±0.192 2.857±0.015

97.623±1.529

PA-2-TAEA-9

1.767±0.016 7.425±0.088 2.631±0.014

76.309±1.232

PA-1-TAEA-11 1.892±0.015 8.815±0.085 2.754±0.012

87.514±1.130

3.2 Morphology and surface hydrophilicity of modified TFC membranes

The surface and cross-sectional morphology of TFC membranes with different TAEA content is also investigated and shown in Fig. 6. The typical ridge-and-valley structures of PA selective layers are observed in all TFC membranes. However, the characteristic ridge-and-valley structures become less obvious in TAEA-modified TFC membranes, where the leaf-like structure gradually disappears and a relative denser and smoother surface with more nodular-like structure is observed. In addition, the PA layer thicknesses of those modified TFC membranes also decrease with the increase of the TAEA content. PA layer thicknesses of the control and modified membranes are also obtained from the PAS characterization and shown in Table S-1 in the Supporting information, which are in accordance with the SEM results. Correspondingly, AFM result (Fig. 7) also shows that the surface roughness of modified TFC membranes decease with the ascending TAEA content, consistent with their SEM surface morphology. Similar phenomenon has also been reported in the previous work with TEA as catalyst[28].

Changes in surface morphologies of modified TFC membranes may be mainly ascribed to the accelerated reaction rate between TMC and MPD with the presence of TAEA. During the interfacial polymerization process, MPD molecules in the water phase transfer towards the organic phase to react with TMC molecules at the interface, resulting in the formation of the nascent cross-linked ultrathin layer with an

inconspicuous ridge-and-valley structure[48]. Additionally, the Marangoni effect of the surface tension between two phases can reinforce the rapid migration of MPD towards the organic phase, pushing and twisting the initially formed PA layer to form a more obvious ridge-and-valley structure[48]. As for those modified TFC membranes, since both TMC and MPD are active monomers to form PA layer immediately, the PA layer would form even more quickly with the accelerated effect by TAEA, which restricts the further diffusion of MPD molecules toward the organic phase and prevent the formation of the more obvious ridge-and-valley structure. In addition, the formation of the semi-aromatic PA network with the aliphatic amine (TAEA) incorporated into the PA chain may also result in the formation of a smoother membrane surface[41, 42, 49]. Other possible reasons for the denser surface morphology can be ascribed to the more complete reaction degree between MPD and TMC with higher crosslinking degree, as well as the crosslinking effect of PA chains by TAEA.

Changes in the chemical properties and microstructure of the formed PA layer will in turn affect the membrane hydrophilicity, which is evaluated by the water contact angle result as shown in the Fig. 8. In comparison with the control membrane, modified TFC membranes exhibit slightly lower WCA values, except the membrane PA-3-TAEA-9. The enhanced membrane hydrophilicities of both PA-1-TAEA-9 and PA-2-TAEA-9 are ascribed to the incorporated TAEA with hydrophilic amine groups.

Additionally, WCAs of those modified TFC membranes increase with the increase of the TAEA content, due to the formation of the denser and smoother PA layer.

3.3 Separation performance of TFC membranes

Intrinsic transport properties of the control and modified TFC membranes are evaluated by RO tests and listed in Table 4. It can be found that, the water permeabilities of PA-1-TAEA-9 and PA-2-TAEA-9 membranes are both higher than that of the control membrane, while that of the PA-3-TAEA-9 membrane is somewhat lower, which is consistence with the membrane surface hydrophilicity as shown in Fig. 8. Meanwhile, they show a descending tendency with the increase of the TAEA content due to the lower fractional free volume of PA layer. With a low TAEA content ( 2 wt%), the incorporation of TAEA plays a positive role in the improvement of the separation performance because of the higher hydrophilicity and thinner PA layer thickness of modified TFC membranes. However, with a further increase in the TAEA content, extra TAEA can only act as the catalyst to accelerate the reaction between TMC and MPD resulting in a denser PA network, since the amount of residual acyl chloride groups of TMC is fixed. In addition, the salt permeability and B/A ratio of the TFC membranes exhibit the similar trends to that of the water permeability, while the salt rejection of TFC membranes exhibits an opposite trend because of the higher cross-linking degree.

Table 4 Intrinsic transport properties of TFC membranes with different TAEA

contents Aa,

Rejection

B/A,

Rs, %

Bar

b

B , LMH

Membrane ID LMH/Bar

PA-Control

0.84±0.05

0.19±0.03

92.09±0.55

0.22

PA-1-TAEA-9

1.04±0.06

0.25±0.02

91.58±0.57

0.24

PA-2-TAEA-9

0.91±0.02

0.18±0.01

92.93±0.24

0.19

PA-3-TAEA-9

0.80±0.01

0.12±0.01

94.53±0.56

0.15

a

DI water is used as the feed solution in 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);

Correspondingly, Fig. 9 summarizes the FO performance of the control and modified TFC membranes with different TAEA contents, which coincides with the RO results. In comparison with the control TFC membrane, both the water flux and reverse salt flux of the modified TFC PA membranes increase firstly then decrease with the increase of TAEA content.

3.4 PH optimization of the amine aqueous solution According to previous studies, the pH of amine solution plays a significant role on the formation of PA chains[50]. In addition, chemical forms of amine groups in TAEA also change with the variation of amine solution pH resulting in the reactivity change. In this study, CSA was employed to adjust the solution pH from 11 to 8 (the pristine pH of amine solution containing 1 wt% TAEA is 11) in order to investigate the comprehensive influence on the performance of resultant TFC membranes.

The reaction mechanism between TAEA and CSA is shown in the Fig. S-7. With the change of the solution pH, TAEA in the amine solution exhibits four kinds of different amine groups, i.e. primary amine, protonated primary amine, tertiary amine and protonated tertiary amine. When the pH of TAEA-containing amine solution is lowered from 11 to 8 or 9 by CSA, tertiary amine and primary amine groups in TAEA will be partially protonated and lose their functions of the catalyst and the reactive amine monomer, leading to a lower reaction degree between TMC and MPD, as well as the lower molecular weight and looser morphology of the formed PA network [50]. In addition, the lower pH of the amine solution means less amine groups available for IP, contributing to a high surface hydrophilicity with more unreacted protonated amine groups. However, if the pH of the amine solution is too high, the hydrolysis of TMC may occur and also lead to a looser PA network with a lower crosslinking degree.

The influence of the amine solution pH on the chemical properties of the membrane surface of the resultant TFC membrane can be examined through FTIR and XPS characterizations. Fig. 10 displays FTIR spectra of TFC membranes modified with TAEA-containing amine solutions of different pHs. It can be observed that, characteristic peaks of amide groups at 1655 and 1530 cm-1 appear in all spectra of the control and modified TFC membranes. Besides, new peaks at 1360 and 3065 cm-1 also emerge in the spectra of all modified TFC membranes, ascribed to the stretching vibrations of C-NR2 and –CH2- bonds in TAEA. Moreover, the band intensities at 1610 cm-1 (the deformation stretching of N-H bonds in amine groups) of modified TFC membranes are much stronger than that in the control membrane, and exhibit an

ascending trend with the increment of the amine solution pH. Meanwhile, the adsorption bands of hydroxyl groups become weaker and slightly shift from 3406 to 3375 cm-1 in the spectra of membranes PA-1-TAEA-8 and PA-1-TAEA-9, due to the overlap with the adsorption band of unreacted primary amine groups. However, this phenomenon is not observed in that of the membrane PA-1-TAEA-11, since there is no protonated amine group and all primary amine groups in TAEA are active enough to react with acyl chloride groups in TMC. All above analysis confirms the successful attachment of TAEA on PA network. Besides, by pH adjustment of the amine solution from 11 to 8 or 9, reactions between TAEA and TMC or PA chains are different under different pH condition, which is consistent with our hypothesis above.

XPS results in the Figs. 11, S-8, Tables 5 and 6 also give the similar results. In comparison with the control membrane, the N contents of modified TFC membranes are higher and increase with the increment in amine solution pH. Meanwhile, results of O contents and O/N ratios show opposite trends, demonstrating the successful grafting of TAEA onto PA networks. Moreover, the intensity ratios of IOI/IOII of modified TFC membranes increase, and show an ascending tendency with the increase of pH, demonstrating less carboxyl groups translated from acyl chloride groups. Meanwhile, only spectra of membranes PA-1-TAEA-8 and PA-1-TAEA-9 exhibit the N III peaks, indicating the successful binding of TAEA with protonated amine groups on PA chains. Therefore, the suggested reaction mechanism between TAEA and CSA is reliable.

Table 5 Surface elemental composition of the control and modified TFC membranes by XPS analysis Code

PA-Control

PA-1-TAEA-8

PA-1-TAEA-9 PA-1-TAEA-11

C

76.76

75.34

75.25

75.37

O

14.87

14.78

14.29

13.32

N

8.37

9.89

10.46

11.31

O/N

1.78

1.49

1.37

1.18

Table 6 Surface chemical compositions the control and modified TFC membranes of by XPS O 1S spectra analysis

Code

OI

O II

O I/ O II

PA-Control

59254.08

28682.33

2.07

PA-1-TAEA-8

64478.59

21684.74

2.97

PA-1-TAEA-9

63993.91

17729.09

3.61

PA-1-TAEA-11

68290.56

16547.05

4.12

*O I: N-C=O*/O-C=O*, BE=531.5 eV O II: O=C-O*, BE=533.3 eV

The microstructure change of the PA layer is also studied by WXRD. Fig. 12 shows that, d-spacing values of modified TFC membranes exhibit a descending tendency with the increase of the amine solution pH, and are all smaller than that of the control membrane except for the PA-1-TAEA-8 membrane. This result is ascribed to the combined effect of the following three factors. Firstly, TAEA can act as an aliphatic

amine to react with TMC resulting in a semi-aromatic PA network, which is of looser structure than that of the fully-aromatic PA network formed by MPD and TMC. Secondly, TAEA also plays a role of the cross-linker to bind adjacent PA chains contributing to a denser PA network. Thirdly, TAEA with tertiary amine groups can also work as a catalyst to accelerate the reaction between MPD and TMC favoring the formation of a denser PA network. When the pH of the amine solution is 11, TAEA with three un-protonated primary amine groups can react with TMC, resulting in a fully crosslinked and denser PA network. Generally, the PA network formed by a tri-amine and a tri-acyl chloride is denser than that formed by a di-amine and tri-acyl chloride. Additionally, PA network crosslinked by three primary amine groups of TAEA is denser than that crosslinked by two primary amine groups. However, since more amine groups of TAEA are protonated under a lower pH condition, i.e. less protonated primary amine groups can react with the TMC or bind the adjacent PA chains, leading to a lower crosslinking degree and looser structure.

Furthermore, PALS results in Fig. 13 also show that, all modified TFC membranes exhibit lower S values as compared to the control membrane, except for the membrane PA-1-TAEA-8. Additionally, S values of the modified TFC membranes decreases with the increase of the pH in the amine solution, ascribed to the increased crosslinking degree.

The surface and cross-sectional morphology of the control and modified TFC

membranes are also displayed in Fig. 14. The typical ridge-and-valley surface morphology can be observed in all TFC membranes. However, the leaf-like structure gradually disappears and is substituted by the nodular-like structure. In addition, the surface smoothness of modified TFC membranes increase with the increase of amine solution pH, due to the formation of semi-aromatic PA network with the increased crosslinking degree. From the cross-sectional morphology, it can be found that the PA layer thickness of those modified TFC membranes reduces with the increase of the amine solution pH and are all lower than that of the control TFC membrane, because of the formed denser PA layer and smoother surface. AFM images (Fig. 15) of TFC membranes also show the similar results that the TAEA-modified TFC membranes exhibit a denser and smoother PA layer with a lower surface roughness, and the roughness decreases with the increase of the amine solution pH.

Changes in the chemical properties and microstructure of the formed PA layer will in turn affect the membrane hydrophilicity, which is evaluated by the water contact angle result as shown in Fig. 16. Different with PA-1-TAEA-11membrane, other two TAEA-modified TFC membranes exhibit lower water contact angles than that of the control membrane, demonstrating the higher surface hydrophilicity, which could be ascribed to the introduced un-protonated primary groups in PA chains by TAEA. The higher water contact angle of the PA-1-TAEA-11 membrane is probably caused by the formed denser PA layer.

Fig. 17 summarizes the FO performance of the control and modified TFC membranes

under both FO and PRO modes. It can be found that, the water fluxes of PA-1-TAEA-8 and PA-1-TAEA-9 membranes are higher than those of the control membrane, which should be contributed by the enhanced membrane hydrophilicity, larger fractional free volume and the thinner PA layer, as discussed above. However, the water fluxes of modified TFC membranes show a descending trend with the increase of the amine solution pH, due to the reduced membrane hydrophilicity and higher crosslinking degree. The water flux of the PA-1-TAEA-11 membrane is even lower than that of the control membrane. On the other side, the reverse salt fluxes also exhibit the similar trend to that of the water fluxes, due to changes in the crosslinking degree and thickness of the PA layer.

Table 7 Intrinsic transport properties of the control and modified TFC PA membranes Aa,

Bb ,

Rejection Rs,

B/A,

LMH/Bar

LMH

%

Bar

PA-Control

0.84±0.05

0.19±0.03

92.09±0.55

0.22

PA-1-TAEA-8

1.25±0.09

0.35±0.05

90.39±0.82

0.27

PA-1-TAEA-9

1.04±0.06

0.25±0.02

91.58±0.57

0.24

PA-1-TAEA-11

0.81±0.02

0.14±0.01

93.64±0.31

0.18

Membrane ID

a

DI water is used as the feed solution in 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);

The basic transport properties of the control and modified TFC membranes are also

evaluated, and the results summarized in Table 7 show good consistency with FO results. The water permeabilities of membranes PA-1-TAEA-8 and PA-1-TAEA-9 (1.25±0.09, 1.04±0.06 LMH/bar) are both higher than that of the control membrane (0.84±0.05 LMH/bar), while that of the PA-1-TAEA-11 membrane is slightly lower (0.81±0.02 LMH/bar). Correspondingly, the salt permeabilities of those membranes show the similar trend to the water permeabilities. The resultant salt rejections exhibit the contrary trend, with the highest value of 93.64±0.31% for the PA-1-TAEA-11 membrane, which are consistent with the B/A ratios.

3.5 Dynamic antifouling performance of TAEA-modified TFC membranes The membrane fouling propensity is further investigated in this study with the synthetic wastewater containing SA as the model foulant. Sodium alginate, a surrogate for polysaccharides, is a kind of extracellular polymeric substances existing in wastewater effluents and may lead to the severe membrane fouling[51, 52]. An extremely exigent testing condition with a high SA concentration of 250 ppm (much higher than its common concentration about 33 ppm in the wastewater effluent), is applied to conduct the fouling experiments within a time scale of 18 h. A cross-linked SA gel layer may form with Ca2+ ions in the aqueous solution as bridges to interlink SA molecules, which would enlarge the hydraulic resistance to the water transportation resulting in a water permeability decline[45]. Additionally, the cake-enhanced osmotic pressure in the feed solution by the salt accumulation within the SA gel layer would also reduce the osmotic pressure difference, and therefore a significant flux decline[45].

Fig. 18 displays the normalized water flux as a function of the testing time for the control and modified TFC membranes prepared with amine solutions of various pHs. It can be found that, compared to the sharp water flux decline (58.9±7.3%) in the control TFC membrane, the water flux declines in modified TFC membranes are much less over the 18-h test, especially for the membrane PA-1-TAEA-11 (15.2±1.2%), indicating the lower fouling propensity. The enhanced antifouling property of modified TFC membranes could be ascribed to smoother surface and less reactive site of carboxyl groups. Firstly, the reduced surface roughness can also contribute to the improved anti-biofouling property. Generally, a membrane with a smoother surface can inhibit the accumulation of SA molecules effectively due to less adhesion sites[45, 53]. Additionally, water permeates through a membrane with a low surface roughness will form a spatially uniform flux distribution, to avoid the higher fouling tendency of those regions with locally high water flux[45, 54]. Secondly, a more compete reaction between TMC and MPD and the additional reaction between TMC and TAEA both lead to the less amount of carboxyl groups on the membrane surface, which serve as the complexation sites for alginate in the presence of Ca2+, therefore contributing to the better anti-biofouling property. In addition, improved membrane hydrophilicity of PA-1-TAEA-8 and PA-1-TAEA-9 is also responsible for the enhanced fouling resistance. The presence of the nitrogen atoms of TAEA in PA network may act as hydrogen bond acceptors to attract water molecules to form a thin hydration layer as a barrier to the adsorption of foulants. Despite the protonated primary amines introduced into the PA network may promote the membrane hydrophilicity, they may also form the surface electrostatic interactions with carboxyl groups in SA and thus aggravate the membrane fouling. That is why the membrane

PA-1-TAEA-9 exhibit a bit better anti-biofouling resistance than that of the membrane PA-1-TAEA-8. Among those modified TFC membranes, PA-1-TAEA-11 membrane with the smoothest surface and least reactive sites therefore exhibits the best anti-biofouling property.

The membrane fouling reversibility is also evaluated by the flux recovery ratio (FRR) as displayed in Fig. 19. It can be found that, the control TFC membrane exhibits a dramatic flux drop after fouling and a low FRR about 59.4±2.5% after cleaning, indicating the fouling is nearly irreversible. Alternatively, the water flux drops of those TAEA-modified TFC membranes are much less (all FFRs > 94%) with the highest FRR up to 98.9±0.7% for the membrane PA-1-TAEA-11, suggesting the reversible fouling phenomena in the modified TFC membranes.

The fouling behaviors of TFC membranes with different TAEA contents are also studied and the results are shown in Figs. 20 and 21. It can see that, with the increase in the TAEA content, the TFC membrane shows an enhanced antifouling property with less water flux decline, attributed to the higher hydrophilicity, smoother surface and less adsorption sites. In addition, FRR values of all modified TFC membranes are higher than 95%. Particularly for the membrane PA-3-TAEA-9, the FRR value can reach up to 100%, demonstrating the SA-caused fouling is almost fully reversible.

3.6 Benchmarking

Table 8 summarizes the FO performance of recently developed TFC membranes. Here, the decline ratio of the specific reverse salt flux was employed to evaluate the membrane selectivity, that is, a higher decline ratio indicates a better membrane selectivity, vice versa. The specific reverse salt flux decline ratio is defined as J

J

JS,C

w,C

w,m

Jw,C

∆(J S,C − J S,m ) ∗ 100%, here

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 TAEA-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 is believed to be contributed by the higher crosslinking degree of PA network and lower PA layer thickness.

Table 8 FO performance of recently developed TFC membranes

Membrane code

Water flux (LMH)

Reverse salt flux (gMH)

MPD+TMC on PSF

22.7/43.4

12.1/19.6

MPD/TAEA+TMC on PSF

26.9/51.7

9.3/15.5

MPD+TMC on PES

17/21.5

7.5/8.1

MPD+TMC on PES/SPES

35.1/42.1

9.9/11.1

MPD+TMC on PSF MPD+TMC on PSF/ BPSH100-BPS0

20.2/35

3.41/4.60

40.9/74.4

9.32/11.88

MPD+TMC on PSF

12.7/27.7

5.5/9.2

MPD+TMC on PSF/LDH

18.1/34.6

8.1/12.7

J

J

w,C

w,m

∆(J S,C − J S,m ) (%)

18.7/15.2

Draw solution 2 M NaCl

Refs

This work

2 M NaCl 15.9/11.3

2 M NaCl

[55]

2 M NaCl -5.9/-2.8

2 M NaCl

[56]

2 M NaCl -1.4/-3.5

1M NaCl 1 M NaCl

[57]

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

13.3/26.01

6.02/14.20

MPD+TMC on PSF/HNT

13.3/26.01

6.02/14.20

MPD+TMC on PPSU

10/10

2.1/2.3

-9.4/-8.2

2M NaCl

[58]

2M NaCl -10.2/-17.5

2M NaCl

[59]

2M NaCl 4.7/7.1

MPD+TMC on sPPSU 54/48 8.8/7.6 * The feed solution is DI water in all studies in this Table.

2M NaCl 2M NaCl

[60]

4. Conclusion

In this work, a novel tertiary amine TAEA is incorporated in the MPD solution to in-situ modify TFC membranes for FO applications. It plays a dual-role as the catalyst and the reactive amine monomer in interfacial polymerization, not only to accelerate the reaction rate between TMC and MPD, but also to react with TMC to form a semi-aromatic PA network. Chemical properties, microstructure of the PA layer and corresponding membrane performance of the modified TFC membranes are investigated via various characterization techniques. Following conclusions can be drawn from this study.

With the increase of TAEA content, the PA network of TFC membranes becomes denser because of the higher crosslinking degree and smaller d-spacing between polymer chains. The surface hydrophilicity exhibits an up-and down trend as a result of the incorporated amine groups and smoother surface. Nodular-like structure gradually appears to replace the leaf-like structure, and the PA later become thinner, due to the accelerated reaction rate and higher crosslinking degree. Correspondingly, most modified TFC membranes exhibit higher water fluxes and lower reverse salt fluxes compared to that of the control membrane.

With the pH adjustment of the amine monomer solution from 11 to a lower value (8 or 9), the PA layer shows a looser microstructure, thicker PA layer, and higher surface hydrophilicity due to less available unprotonated amine groups in TAEA. Correspondingly, the water flux and reverse salt flux of modified TFC membranes are almost all higher than that of the control membrane, and exhibit a descending trend with the increase of the amine solution pH. Compared to the control TFC membrane, modified TFC membranes also exhibit much higher 35

anti-fouling property with higher FRR values. In addition, the anti-fouling property can be further enhanced with the increase in the pH of the amine solution or TAEA content in the PA layer.

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), Open Research Fund Program of Collaborative Innovation Center of Membrane Separation and Water Treatment of Zhejiang Province, and Opening project of Key Laboratory of Biomedical Polymers of Ministry of Education at Wuhan University (no. 20140401). 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.

References [1] X. Zhang, W.-Z. Lang, H.-P. Xu, X. Yan, Y.-J. Guo, L.-F. Chu, Improved performances of PVDF/PFSA/O-MWNTs hollow fiber membranes and the synergism effects of two additives, Journal of Membrane Science, 469 (2014) 458-470. [2] X. Zhang, W.-Z. Lang, X. Yan, Z.-Y. Lou, X.-F. Chen, Influences of the structure parameters of multi-walled carbon nanotubes(MWNTs) on PVDF/PFSA/O-MWNTs hollow fiber ultrafiltration membranes, Journal of Membrane Science, 499 (2016) 179-190. [3] Y.C. Xu, X.Q. Cheng, J. Long, L. Shao, A novel monoamine modification strategy toward 36

high-performance organic solvent nanofiltration (OSN) membrane for sustainable molecular separations, Journal of Membrane Science, 497 (2016) 77-89. [4] X.Q. Cheng, Y. Liu, Z. Guo, L. Shao, Nanofiltration membrane achieving dual resistance to fouling and chlorine for “green” separation of antibiotics, Journal of Membrane Science, 493 (2015) 156-166. [5] P. Sukitpaneenit, T.-S. Chung, Molecular elucidation of morphology and mechanical properties of PVDF hollow fiber membranes from aspects of phase inversion, crystallization and rheology, Journal of Membrane Science, 340 (2009) 192-205. [6] S.P. Sun, T.A. Hatton, S.Y. Chan, T.-S. Chung, Novel thin-film composite nanofiltration hollow fiber membranes with double repulsion for effective removal of emerging organic matters from water, Journal of Membrane Science, 401-402 (2012) 152-162. [7]

S.P.

Sun,

K.Y.

Wang,

N.

Peng,

T.A.

Hatton,

T.-S.

Chung,

Novel

polyamide-imide/cellulose acetate dual-layer hollow fiber membranes for nanofiltration, Journal of Membrane Science, 363 (2010) 232-242. [8] S.P. Sun, T.A. Hatton, T.S. Chung, Hyperbranched polyethyleneimine induced cross-linking of polyamide-imide nanofiltration hollow fiber membranes for effective removal of ciprofloxacin, Environmental science & technology, 45 (2011) 4003-4009. [9] R.L. McGinnis, M. Elimelech, Energy requirements of ammonia–carbon dioxide forward osmosis desalination, Desalination, 207 (2007) 370-382. [10] C.R. Martinetti, A.E. Childress, T.Y. Cath, High recovery of concentrated RO brines using forward osmosis and membrane distillation, Journal of Membrane Science, 331 (2009) 31-39. [11] A. Achilli, T.Y. Cath, E.A. Marchand, A.E. Childress, The forward osmosis membrane bioreactor: A low fouling alternative to MBR processes, Desalination, 239 (2009) 10-21. [12] B. Mi, M. Elimelech, Organic fouling of forward osmosis membranes: Fouling 37

reversibility and cleaning without chemical reagents, Journal of Membrane Science, 348 (2010) 337-345. [13] P. Sukitpaneenit, T.S. Chung, High performance thin-film composite forward osmosis hollow fiber membranes with macrovoid-free and highly porous structure for sustainable water production, Environmental science & technology, 46 (2012) 7358-7365. [14] Y. Wang, T. Xu, Anchoring hydrophilic polymer in substrate: An easy approach for improving the performance of TFC FO membrane, Journal of Membrane Science, 476 (2015) 330-339. [15] Y. Wang, R. Ou, H. Wang, T. Xu, Graphene oxide modified graphitic carbon nitride as a modifier for thin film composite forward osmosis membrane, Journal of Membrane Science, 475 (2015) 281-289. [16] Y. Cui, X.-Y. Liu, T.-S. Chung, Enhanced osmotic energy generation from salinity gradients by modifying thin film composite membranes, Chemical Engineering Journal, 242 (2014) 195-203. [17] C.A. Nayak, N.K. Rastogi, Forward osmosis for the concentration of anthocyanin from Garcinia indica Choisy, Separation and Purification Technology, 71 (2010) 144-151. [18] K.Y. Wang, M.M. Teoh, A. Nugroho, T.-S. Chung, Integrated forward osmosis– membrane distillation (FO–MD) hybrid system for the concentration of protein solutions, Chemical Engineering Science, 66 (2011) 2421-2430. [19] T.Y. Cath, A.E. Childress, M. Elimelech, Forward osmosis: Principles, applications, and recent developments, Journal of Membrane Science, 281 (2006) 70-87. [20] N.-N. Bui, J.R. McCutcheon, Hydrophilic Nanofibers as New Supports for Thin Film Composite Membranes for Engineered Osmosis, Environmental science & technology, 47 (2013) 1761-1769. [21] L. Huang, J.T. Arena, S.S. Manickam, X. Jiang, B.G. Willis, J.R. McCutcheon, 38

Improved mechanical properties and hydrophilicity of electrospun nanofiber membranes for filtration applications by dopamine modification, Journal of Membrane Science, 460 (2014) 241-249. [22] D. Rana, T. Matsuura, Surface Modifications for Antifouling Membranes, Chemical Reviews, 110 (2010) 2448-2471. [23] H. Wang, L. Li, X. Zhang, S. Zhang, Polyamide thin-film composite membranes prepared from a novel triamine 3,5-diamino-N-(4-aminophenyl)-benzamide monomer and m-phenylenediamine, Journal of Membrane Science, 353 (2010) 78-84. [24] M. Liu, D. Wu, S. Yu, C. Gao, Influence of the polyacyl chloride structure on the reverse osmosis performance, surface properties and chlorine stability of the thin-film composite polyamide membranes, Journal of Membrane Science, 326 (2009) 205-214. [25] S. Sorribas, P. Gorgojo, C. Tellez, J. Coronas, A.G. Livingston, High flux thin film nanocomposite membranes based on metal-organic frameworks for organic solvent nanofiltration, Journal of the American Chemical Society, 135 (2013) 15201-15208. [26] N. Ma, J. Wei, R. Liao, C.Y. Tang, Zeolite-polyamide thin film nanocomposite membranes: Towards enhanced performance for forward osmosis, Journal of Membrane Science, 405-406 (2012) 149-157. [27] J. Jegal, S.G. Min, K.-H. Lee, Factors affecting the interfacial polymerization of polyamide active layers for the formation of polyamide composite membranes, Journal of Applied Polymer Science, 86 (2002) 2781-2787. [28] A.K. Ghosh, B.-H. Jeong, X. Huang, E.M.V. Hoek, Impacts of reaction and curing conditions on polyamide composite reverse osmosis membrane properties, Journal of Membrane Science, 311 (2008) 34-45. [29] Y. Mansourpanah, K. Alizadeh, S.S. Madaeni, A. Rahimpour, H. Soltani Afarani, Using different surfactants for changing the properties of poly(piperazineamide) TFC nanofiltration 39

membranes, Desalination, 271 (2011) 169-177. [30] Y. Mansourpanah, S.S. Madaeni, A. Rahimpour, Fabrication and development of interfacial polymerized thin-film composite nanofiltration membrane using different surfactants in organic phase; study of morphology and performance, Journal of Membrane Science, 343 (2009) 219-228. [31] S. Romero-Vargas Castrillón, X. Lu, D.L. Shaffer, M. Elimelech, Amine enrichment and poly(ethylene glycol) (PEG) surface modification of thin-film composite forward osmosis membranes for organic fouling control, Journal of Membrane Science, 450 (2014) 331-339. [32] B.D. McCloskey, H.B. Park, H. Ju, B.W. Rowe, D.J. Miller, B.D. Freeman, A bioinspired fouling-resistant surface modification for water purification membranes, Journal of Membrane Science, 413-414 (2012) 82-90. [33] B.D. McCloskey, H.B. Park, H. Ju, B.W. Rowe, D.J. Miller, B.J. Chun, K. Kin, B.D. Freeman, Influence of polydopamine deposition conditions on pure water flux and foulant adhesion resistance of reverse osmosis, ultrafiltration, and microfiltration membranes, Polymer, 51 (2010) 3472-3485. [34] S. Azari, L. Zou, Using zwitterionic amino acid l-DOPA to modify the surface of thin film composite polyamide reverse osmosis membranes to increase their fouling resistance, Journal of Membrane Science, 401-402 (2012) 68-75. [35] R. Yang, J. Xu, G. Ozaydin-Ince, S.Y. Wong, K.K. Gleason, Surface-Tethered Zwitterionic Ultrathin Antifouling Coatings on Reverse Osmosis Membranes by Initiated Chemical Vapor Deposition, Chemistry of Materials, 23 (2011) 1263-1272. [36] D. Wu, X. Liu, S. Yu, M. Liu, C. Gao, Modification of aromatic polyamide thin-film composite reverse osmosis membranes by surface coating of thermo-responsive copolymers P(NIPAM-co-Am). I: Preparation and characterization, Journal of Membrane Science, 352 (2010) 76-85. 40

[37] S. Yu, Z. Lü, Z. Chen, X. Liu, M. Liu, C. Gao, Surface modification of thin-film composite

polyamide

reverse

osmosis

membranes

by

coating

N-isopropylacrylamide-co-acrylic acid copolymers for improved membrane properties, Journal of Membrane Science, 371 (2011) 293-306. [38] S. Zhang, F. Fu, T.-S. Chung, Substrate modifications and alcohol treatment on thin film composite membranes for osmotic power, Chemical Engineering Science, 87 (2013) 40-50. [39] G. Han, S. Zhang, X. Li, T.-S. Chung, High performance thin film composite pressure retarded osmosis (PRO) membranes for renewable salinity-gradient energy generation, Journal of Membrane Science, 440 (2013) 108-121. [40] R.C. Ong, T.-S. Chung, J.S. de Wit, B.J. Helmer, Novel cellulose ester substrates for high performance flat-sheet thin-film composite (TFC) forward osmosis (FO) membranes, Journal of Membrane Science, 473 (2015) 63-71. [41] C.Y. Tang, Y.-N. Kwon, J.O. Leckie, Effect of membrane chemistry and coating layer on physiochemical properties of thin film composite polyamide RO and NF membranes, Desalination, 242 (2009) 168-182. [42] P.R. Buch, D. Jagan Mohan, A.V.R. Reddy, Preparation, characterization and chlorine stability of aromatic–cycloaliphatic polyamide thin film composite membranes, Journal of Membrane Science, 309 (2008) 36-44. [43] N.K. Saha, S.V. Joshi, Performance evaluation of thin film composite polyamide nanofiltration membrane with variation in monomer type, Journal of Membrane Science, 342 (2009) 60-69. [44] L. Shen, S. Xiong, Y. Wang, Graphene oxide incorporated thin-film composite membranes for forward osmosis applications, Chemical Engineering Science, 143 (2016) 194-205. [45] X. Lu, S. Romero-Vargas Castrillon, D.L. Shaffer, J. Ma, M. Elimelech, In situ surface 41

chemical modification of thin-film composite forward osmosis membranes for enhanced organic fouling resistance, Environmental science & technology, 47 (2013) 12219-12228. [46] V.T. Do, C.Y. Tang, M. Reinhard, J.O. Leckie, Degradation of polyamide nanofiltration and reverse osmosis membranes by hypochlorite, Environmental science & technology, 46 (2012) 852-859. [47] M.J. Ariza, J. Benavente, E. Rodriguez-Castellon, L. Palacio, Effect of hydration of polyamide membranes on the surface electrokinetic parameters: surface characterization by x-ray photoelectronic spectroscopy and atomic force microscopy, Journal of colloid and interface science, 247 (2002) 149-158. [48] C. Klaysom, S. Hermans, A. Gahlaut, S. Van Craenenbroeck, I.F.J. Vankelecom, Polyamide/Polyacrylonitrile (PA/PAN) thin film composite osmosis membranes: Film optimization, characterization and performance evaluation, Journal of Membrane Science, 445 (2013) 25-33. [49] N.K. Saha, S.V. Joshi, Performance evaluation of thin film composite polyamide nanofiltration membrane with variation in monomer type, Journal of Membrane Science, 342 (2009) 60-69. [50] M. Liu, S. Yu, J. Tao, C. Gao, Preparation, structure characteristics and separation properties of thin-film composite polyamide-urethane seawater reverse osmosis membrane, Journal of Membrane Science, 325 (2008) 947-956. [51] A.C. Fonseca, R.S. Summers, A.R. Greenberg, M.T. Hernandez, Extra-Cellular Polysaccharides, Soluble Microbial Products, and Natural Organic Matter Impact on Nanofiltration Membranes Flux Decline, Environmental science & technology, 41 (2007) 2491-2497. [52] M. Herzberg, S. Kang, M. Elimelech, Role of Extracellular Polymeric Substances (EPS) in Biofouling of Reverse Osmosis Membranes, Environmental science & technology, 43 42

(2009) 4393-4398. [53] E.M. Vrijenhoek, S. Hong, M. Elimelech, Influence of membrane surface properties on initial rate of colloidal fouling of reverse osmosis and nanofiltration membranes, Journal of Membrane Science, 188 (2001) 115-128. [54] G.Z. Ramon, E.M.V. Hoek, Transport through composite membranes, part 2: Impacts of roughness on permeability and fouling, Journal of Membrane Science, 425–426 (2013) 141-148. [55] S. Sahebi, S. Phuntsho, Y.C. Woo, M.J. Park, L.D. Tijing, S. Hong, H.K. Shon, Effect of sulphonated polyethersulfone substrate for thin film composite forward osmosis membrane, Desalination, 389 (2016) 129-136. [56] X. Zhang, J. Tian, Z. Ren, W. Shi, Z. Zhang, Y. Xu, S. Gao, F. Cui, High performance thin-film composite (TFC) forward osmosis (FO) membrane fabricated on novel hydrophilic disulfonated poly(arylene ether sulfone) multiblock copolymer/polysulfone substrate, Journal of Membrane Science, 520 (2016) 529-539. [57] P. Lu, S. Liang, L. Qiu, Y. Gao, Q. Wang, Thin film nanocomposite forward osmosis membranes based on layered double hydroxide nanoparticles blended substrates, Journal of Membrane Science, 504 (2016) 196-205. [58] D. Emadzadeh, W.J. Lau, T. Matsuura, A.F. Ismail, M. Rahbari-Sisakht, Synthesis and characterization of thin film nanocomposite forward osmosis membrane with hydrophilic nanocomposite support to reduce internal concentration polarization, Journal of Membrane Science, 449 (2014) 74-85. [59] M. Ghanbari, D. Emadzadeh, W.J. Lau, H. Riazi, D. Almasi, A.F. Ismail, Minimizing structural parameter of thin film composite forward osmosis membranes using polysulfone/halloysite nanotubes as membrane substrates, Desalination, 377 (2016) 152-162. [60] N. Widjojo, T.-S. Chung, M. Weber, C. Maletzko, V. Warzelhan, A sulfonated 43

polyphenylenesulfone (sPPSU) as the supporting substrate in thin film composite (TFC) membranes with enhanced performance for forward osmosis (FO), Chemical Engineering Journal, 220 (2013) 15-23.

Fig. 1 In-situ modification of TFC membrane by TAEA with different reaction mechanisms. a) TAEA acts as the amine monomer; b) TAEA acts as the crosslinker to react with the formed PA chains; c) TAEA acts as the catalyst. Fig. 2

ATR-FTIR spectra of TFC membranes with various TAEA contents

Fig. 3

High-resolution XPS spectra of TFC membranes with various TAEA contents, (a) O

1S, (b) N 1S. Fig. 4

WXRD patterns of TFC membranes with various TAEA contents.

Fig. 5

Depth profiles of S parameter of TFC membranes with various TAEA contents.

Fig. 6

SEM images of TFC membranes with various TAEA contents. (Top: surface

morphology, bottom: cross-sectional morphology.) Fig. 7

AFM images of TFC membranes with various TAEA contents.

Fig. 8

Water contact angles of TFC membranes with various TAEA contents

Fig. 9

FO performance of TFC membranes with various TAEA contents. (2 M NaCl

aqueous solution and DI water are employed as the draw solution and feed solution, respectively.) Fig. 10 ATR-FTIR spectra of the control and TAEA -modified TFC membranes. Fig. 11 High-resolution XPS spectra and their deconvolution of control and TAEA -modified TFC membranes, (a) O 1S, (b) N 1S. 44

Fig. 12 WXRD patterns of the control and TAEA-modified TFC membranes. Fig. 13 Depth profiles of S parameter of control and TAEA -modified TFC membranes by PALS characterization. Fig. 14

SEM images of control and TAEA-modified TFC PA membranes. (Top: surface

morphology, bottom: cross-sectional morphology). Fig. 15 AFM images of control and TAEA-modified TFC PA membranes. Fig. 16 Water contact angle of the control and TAEA-modified TFC membranes prepared with amine monomer solutions of different pH (containing 1 wt% TAEA). Fig. 17 FO performance of the control and TAEA-modified TFC membranes. (2 M NaCl aqueous solution and DI water are used as the draw solution and feed solution, respectively.) Fig. 18 Dynamic forward osmosis fouling tests of control and TAEA-modified TFC 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.) Fig. 19 Dynamic forward osmosis fouling tests of the control and TAEA-modified TFC membranes. Fig. 20 Dynamic forward osmosis fouling tests of TFC membranes with various TAEA contents. (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.) Fig. 21 Dynamic forward osmosis fouling tests of TFC membranes with various TAEA contents.

Highlights 

Novel tripodal amine TAEA is employed to in-situ modify TFC membranes. 45



TAEA exhibits dual role of catalyst and reactive amine monomer.



Significant changes in overall properties are introduced after modification.



Modified membranes possess better separation performance and antifouling property.

46