High-performance thin-film composite membranes with surface functionalization by organic phosphonic acids

Journal of Membrane Science 563 (2018) 284–297

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High-performance thin-film composite membranes with surface functionalization by organic phosphonic acids Liang Shena,b,c, Fangqian Wanga, Lian Tiana,b, Xuan Zhanga,b,c, Chun Dinga,b,c, Yan Wanga,b,c,

T ⁎

a Key Laboratory of Material Chemistry for Energy Conversion and Storage (Huazhong University of Science and Technology), Ministry of Education, Wuhan 430074, PR China b Hubei Key Laboratory of Material Chemistry and Service Failure, School of Chemistry and Chemical Engineering, Huazhong University of Science & Technology, Wuhan 430074, PR China c Shenzhen Institute of Huazhong University of Science and Technology, Shenzhen 518000, PR China

A R T I C LE I N FO

A B S T R A C T

Keywords: Forward osmosis Thin-film composite polyamide membrane Organic phosphonic acid Fouling resistance Chemical stability

Forward osmosis (FO), a novel promising technology for water treatment, has not progressed significantly beyond the conceptualization. A bottleneck is the lack of suitable membranes with high separation performance, good antifouling properties and chemical stability. In this work, a 3-step surface modification approach is employed to develop a thin-film composite polyamide (TFC PA) membrane, i.e., PEI grafting, acid functionalization (organic phosphonic acid or carboxylic acid), and the calcium mineralization. Effects of the organic acid type on membrane properties are investigated with various characterizations, in terms of chemical changes, surface properties, and morphology. The separation performance, antifouling properties and chemical stability of modified membranes are studied systematically. In comparison with the control membrane and those modified by organic carboxylic acids, membranes modified by organic phosphonic acids exhibit the higher FO water flux, lower reverse salt flux, lower fouling propensity, a better pH tolerance, as well as a superior chlorine resistance. The surface functionalized TFC membrane developed in this work therefore holds a great potential for wide membrane-based separation applications in various harsh environments.

1. Introduction Sustainable water supply has drawn growing worldwide concerns for sustaining the public health and economic prosperity [1]. Among various technologies to alleviate the water shortage stresses, the advance in membrane technology is one of the most direct, effective and feasible approaches to solve these sophisticated issues [2]. However, traditional pressure-driven membrane processes requiring high hydraulic pressure are energy-intensive which leads to the high operational cost. Alternatively, forward osmosis (FO), an emerging membrane technology for water treatment and desalination, is considered energy-efficient, since it utilizes the transmembrane osmotic pressure between the feed solution and draw solution as the driving force for the water permeation through a semipermeable membrane spontaneously [3,4]. In addition, FO process also exhibits the high water recovery ratio and low membrane fouling propensity, therefore holding a great potential in various fields, such as wastewater treatment [5,6], desalination of sea water or brackish water [7,8], power regeneration [9,10], food processing [11], pharmaceutical [12] and so on [13].

The desirable FO membrane is the key prerequisite for the development of FO technology. Among various types of developed FO membranes, the thin film composite (TFC) membrane formed by the interfacial polymerization on a porous substrate is the dominant membrane type, due to its easy fabrication and excellent separation performance. However, traditional TFC membranes prepared by aromatic amines and acyl chlorides may suffer a high fouling tendency and the relative low water flux due to its relatively hydrophobic PA layer, which further lead to the deteriorated water product quality and short membrane lifespan [14]. Additionally, the trade-off relationship usually exists between the superior water permeability and the high salt rejection [15,16]. Moreover, the applicable pH range for PA-based TFC membranes is about 2–11 [17], and the sensitive amide groups in PA chains can be easily attacked by active chlorine species [18], therefore the harsh environment of strong acidic, alkaline or chloric condition may be detrimental for the use of TFC membranes. Massive endeavors have been devoted to designing or modifying FO membranes with improved separation performance, fouling resistance and chemical stability. The enhancement of separation performance generally can be

⁎ Corresponding author at: Key Laboratory of Material Chemistry for Energy Conversion and Storage (Huazhong University of Science and Technology), Ministry of Education, Wuhan 430074, PR China. E-mail address: [email protected] (Y. Wang).

https://doi.org/10.1016/j.memsci.2018.05.071 Received 20 February 2018; Received in revised form 28 May 2018; Accepted 29 May 2018

Available online 31 May 2018 0376-7388/ © 2018 Elsevier B.V. All rights reserved.

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realized via the optimization of the membrane morphology, surface properties and micro-structure of the formed PA layer [19]. The improvement of membrane fouling resistance could be achieved through alleviating the attractive interactions or enhancing the repulsive interactions between foulants and the membrane surface by modulating the membrane surface hydrophilicity, roughness and charge [20,21]. And common strategies to enhance the chemical stability of TFC membranes are through modifications of the PA structure, elimination or protection of sensitive sites (amide group). According previous studies, the incorporation of novel functional monomers [22–24] or nanomaterials [25,26] into the PA layer for insitu modification can exert significant impacts on the microstructure and morphology of resultant membranes, contributing to the improved separation performance, fouling or chlorine resistance. Additionally, surfactants [27,28] or additives [29–31] in the aqueous or organic phase, can finely tune the microstructure and surface properties of PA layer, by affecting the interfacial polymerization reaction, and in turns influence the separation performance, antifouling or anti-chlorine properties of as-fabricated TFC membranes. Among various modification strategies, surface modification is an effective and simple approach to address above issues, since the residual acyl chloride groups on the surface of the PA network can be further exploited as reactive sites for further modification [16]. Common functional materials for the surface modification including polyethyleneglycol (PEG)-based polymers [32,33], zwitterions [34,35], polydopamine [36] and inorganic minerals [37,38], can render resultant membranes improved water permeability and fouling resistance. But few studies [17,39,40] have been reported on the chemical stability enhancement of TFC membranes, especially the acid-base resistance properties. Moreover, to the best of our best knowledge, previous studies are generally aimed at promoting one or two aspects of the membrane separation performance, fouling resistance or chemical stability, but no study has been reported yet to improve the overall membrane properties in all aspects simultaneously. In this study, a surface modification of the nascent TFC membrane with various organic phosphonic acids is proposed for the first time, via a three-step modification – PEI grafting, organic acid grafting and mineralization sequentially. Surface modification by PEI grafting has been reported in our previous work to enhance the surface hydrophilicity and provide abundant amine groups for the subsequent organic acid grafting [41]. Organic phosphonic acid has been widely applied as water treatment agent attributed to its high stability, low toxicity and corrosion inhibition activity in neutral aqueous media [42,43]. Additionally, it is also an excellent scale inhibitor and can be used in cooling water for industrial circulating water because of its strong chelating ability with various metal ions, especially for controlling the calcium scale [44,45]. Moreover, compared with most other reported functional groups (-OH, -NH2, -COOH), the phosphonic acid group is expected to render modified membranes a higher surface hydrophilicity for its stronger polarity. Meanwhile, the introduction of alkaline PEI and organic acid groups (with abundant lone pair electrons of O and N atoms) could work as the sacrificial structure to inhibit the attack on sensitive amide linkages because of their higher reactivity to active species (H+, OH-, Cl+) in harsh environments. Therefore, the 3-step surface modification on the PA layer of TFC membrane is expected to improve the separation performance, the fouling resistance (inorganic, organic and microbial fouling, especially for gypsum scale) and the chemical stability simultaneously. State-of-the-art characterization techniques are employed to testify the proposed modification mechanism. Changes in the chemical properties, surface properties, membrane morphology, corresponding separation performance, antifouling properties and chemical stability of resultant membranes are systematically investigated. Effects of the organic acid type and organic phosphonic acid molecular structure on properties and separation performance of as-fabricated membranes are also studied.

Fig. 1. Molecular structures of the carboxylic acid and various phosphonic acids employed in this study.

2. Materials and methods 2.1. Materials Polysulfone (PSf) (Mw = 800,000 Da) and polyethyleneimine (PEI) (Mw = 600 Da) were purchased from Beijing HWRK Chem co. Ltd. (China) and dried in the vacuum oven at 80 °C for overnight before use. Four organic acids, diethylenetriaminepentakis (methylphosphonic acid) (50 wt% in water, DTPMP, denoted as 5P), ethylene bis(nitrilodimethylene) tetraphosphonic acid (98%, EDTMP, denoted as 4P), tris(phosphonomethyl) amine (50 wt% in water, TPMA, denoted as 3P), diethylenetriaminepentaacetic acid (99%, DTPA, denoted as 5C), employed in this study for the surface modification of TFC membranes, were obtained from Aladdin and kept in the refrigerator before use. Their molecular structures are shown in Fig. 1. M-phenylenediamine (MPD, 99.5%), 1, 3, 5 – trimesoyl chloride (TMC, 98%) and (1S) – (+) – 10 – camphorsulfonic acid (CSA, ≥ 99) were also purchased from Aladdin. Polyethylene glycol 400 (PEG 400, CP), N-methyl pyrrolidone (NMP, anhydrous, ≥ 99.5%), sodium chloride (NaCl, ≥ 99.5%), sodium hydroxide (NaOH, 98%), sodium alginate (SA, Mw: 98.11), potassium dihydrogen phosphate (KH2PO4, 99.5%), magnesium sulfate (MgSO4, 99%), sodium sulfate (Na2SO4, 99%), bovine serum albumin (BSA, 98%), sodium bicarbonate (NaHCO3, 99.5%), calcium chloride (CaCl2, 96%), hexane (anhydrous, AR) and ammonium chloride (NH4Cl, 99.5%) were all obtained from China National Medicine Corporation. 2.2. Preparation of PSf substrate PSf substrate membrane was prepared via the non-solvent induced phase separation method. The degassed dope solution (with a composition of 18/16/66 wt% PSf/PEG/NMP) was poured onto a pre-cleaned glass plate and cast by a casting knife (3520-8, Elcometer, UK) with a height of 90 µm. The obtained membrane was transferred into a water bath immediately to launch the initial phase separation at ambient temperature. The as-fabricated PSf substrate membrane after complete phase separation was then stored in the tap water with water changed every 12 h before use. 2.3. Preparation of TFC membranes The PA selective layer of TFC membrane was fabricated by the interfacial polymerization method. Firstly, the as-fabricated PSf substrate was immersed in 3.4 wt% MPD aqueous solution for 2 min before being washed in the ultrapure water. Subsequently, the excess MPD aqueous 285

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pre-conditioned for 30 min before a steady water flux being recorded, and FO test was repeated for at least three times to get an average data. FO performance of TFC membranes was evaluate with the water flux (Jv, LMH) and reverse salt flux (Js, gMH) as calculated by Eqs. (1) and (2).

solution was removed from the PSf substrate by a rubber roller. Next, TMC/hexane solution (0.15 w/v%) was poured onto MPD-wetted membrane surface for 1-min contact to initiate the interfacial polymerization, followed by the air dry for 1 min after the excess TMC solution was poured off. The obtained TFC membranes were stored in DI water and denoted as PA-Control membrane. Surface modifications were further conducted on the as-fabricated nascent TFC membranes. After hexane evaporation, membranes were firstly transferred into a 2 wt% PEI aqueous solution at 40 °C for 20 min to allow PEI grafting on the surface, and then washed by DI water. The obtained membrane is referred as PA-PEI membrane. The as-obtained PA-PEI membrane was later immersed into an aqueous solution containing 1 wt% organic acid solution (DTPMP, EDTMP, TPMA, or DTPA solutions) (pH adjusted to 3 by NaOH solution), and then washed by DI water. These as-modified membranes were coded as PA-PEI-3P (/4P/ 5P) or PA-PEI-5C, accordingly. Here the number (3, 4, 5) presents the amount of acidic group in the molecular structure of organic acids, while P and C refers to the organic phosphonic acid or carboxylic acid, respectively. Finally, a mineralization treatment was conducted by immersing the as-modified membranes into a 0.001 M aqueous NaOH solution (pH of 11) for 10 min followed by a 5 wt% CaCl2 aqueous solution for 6 h at room temperature. The obtained membranes were then washed by DI water, and denominated as PA-PEI-3P (/4P/5P)-Ca or PAPEI-5C-Ca, respectively. TFC membranes with various modifications were listed in Table 1 with schematic diagrams of their chemical structures.

Jv =

∆V Am, FO ∆t

(1)

Js =

∆ (Ct Vt ) Am, FO ∆t

(2)

where ΔV is the volume change of the draw solution over the predetermined time Δt (30 min), Am,FO is the effective membrane area (3.87 cm2), Ct and Vt are the salt concentration and the volume of the feed solution at the pre-determined time, respectively. 2.6. Dynamic FO fouling tests The dynamic fouling test was also performed using the lab-scale FO set-up under FO mode to evaluate the fouling propensity of the control and modified membranes. 3 different feed solutions are used in this study, i.e., the synthetic waste water (containing 0.45 mM KH2PO4, 9.20 mM NaCl, 0.61 mM MgSO4, 0.5 mM NaHCO3, 0.5 mM CaCl2, 0.93 mM NH4Cl and 250 ppm SA in DI water with adjusted pH of 7.4), the gypsum solution (containing 35 mM CaCl2, 20 mM Na2SO4, and 19 Mm NaCl in DI water), and the local lake water from the East Lake (Wuhan) which was allowed to stand for 24 h and filtrated by the suction filtration using the nonwoven (CU 434 – UF, Crane, USA) for 5 times. The detailed experimental parameters can be referred to our previous works [41,46,47], except that a larger draw solution volume (2.0 L) was utilized in this work to mitigate the draw solution dilution and the water flux decline arising therefrom. In brief, DI water was utilized as both draw and feed solutions for the first 1 h to stabilize membrane samples. The feed solution was then replaced with the synthetic wastewater, the gypsum solution, or the local lake water; while 2 M NaCl aqueous solution was used as the draw solution, to perform an 18-h accelerated fouling test. The flow velocity of both solutions was fixed at 0.3 L/min (150 rpm) during the whole test. Whereafter, the physical cleaning was conducted immediately at a crossflow rate of 0.6 L/min (300 rpm) for 30 min using DI water circulated through the feed and draw solution sides. After that, the water flux of the cleaned membrane sample was measured again. Additionally, a long-term anti-scaling test was also conducted with the gypsum solution for 72 h. Flux recovery ratio (FRR%) was calculated by Eq. (3) to estimate the antifouling property of TFC membranes.

2.4. Characterizations of TFC membranes Changes in chemical structures of the membrane surface were characterized by a X-ray photoelectron spectroscopy (XPS, VG Multilab 2000, Thermo VG Scientific, UK) using a monochromatic A1 Ka X-ray source, as well as an 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. The surface hydrophilicity of TFC membranes was evaluated using a contact angle goniometer (DSA 25, KRÜSS, Germany) at room temperature. Each kind of membrane was measured with at least 4 samples of 10 points each to yield an average data of the water contact angle (WCA). The surface streaming potentials of TFC membranes were detected by a zeta potential analyzer (SurPASS™ 3, Anton Paar, Austria) using 0.001 M KCl aqueous solution at 25 °C with an adjusted pH of 2–10. The surface morphology of TFC membranes were observed by Scanning Electron Microscopy (SEM, VEGA3, TESCAN, Czech). Membrane samples were attached onto the sample stud by the conducting tape and coated with gold by a sputter coater (Q150RS, Quorum, England). The atomic force microscope (AFM, SPM9700, Shimadzu, Japan) was also employed to observe the surface topology of the PA layer under the dynamic mode, to obtain the mean roughness (Ra).

FRR% =

Jw, c × 100% Jw,0

(3)

where Jw,0 and Jw,c are the initial water flux and the water flux after the physical cleaning, respectively. Static protein adsorption tests were conducted to further investigate the fouling resistance of TFC membranes. The fresh membrane sample with an area of 2 * 4 cm2 was immersed in the phosphate buffer solution (PBS, a mixture solution of 60.1 mg NaH2PO4·2H2O, 218.5 mg Na2HPO4·10H2O, and 10 mL ultrapure water, pH = 7.0) for 2 h and then soaked in a BSA solution (0.5 g/L, in PBS solution) for 5 h. Lastly, the BSA concentration changes in BSA/PBS solution before and after adsorption tests of membrane samples were examined by a UV–vis spectrometer (AOE INSTRUMENTS, UV–1800PC, China) at 278 nm. The relative BSA adsorption capacity of TFC membranes was evaluated by the BSA adsorption capacity of TFC membranes (Ads BSA, g/m2) and the capacity ratio of the modified membrane versus the control membrane (RAds), as calculated by Eqs. (4) and (5) respectively.

2.5. FO test The separation performance of the control and modified TFC membranes was evaluated by a lab-scale FO set-up at 22 ± 0.5 °C. DI water and 2 M NaCl aqueous solution were employed as the feed and draw solutions, respectively, which were co-currently circulated into cell channels by two variable speed pumps (BT300-2J, Longer, China) with the fixed flow speed of 0.3 L/min (150 rpm). The concentration change in the feed solution was measured by a conductivity meter (FE30, Mettler Toledo, Switzerland). The weight change in the draw solution was detected by a digital weight balance (FX3000-GD, AND, Japan), which was connected to a computer to output the recorded data. FO performance of each membrane sample was evaluated under two different operation modes, i.e. FO mode (the active layer facing the feed solution, i.e. AL-FS orientation) and PRO mode (the active layer facing the draw solution, i.e. AL-DS orientation). Each membrane was

AdsBSA = (C1 − C2)⋅V / A 286

(4)

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Table 1 Chemical structures of the control and modified membranes. Membrane code

Grafting structure

PA-Control

Membrane code

Grafting structure

/

PA-PEI-5C

PEI/DTPA

PA-PEI

PEI

PA-PEI-3P-Ca

PEI/TPMA/Ca

PA-PEI-3P

PEI/TPMA

PA-PEI-4P-Ca

PEI/EDTMP/Ca

PA-PEI-4P

PEI/EDTMP

PA-PEI-5P-Ca

PEI/DTPMP/Ca

PA-PEI-5P

PEI/DTPMP

PA-PEI-5C-Ca

PEI/DTPA/Ca

RAds = Adscontrol/ Adsmodified

Chemical structure diagram

Chemical structure diagram

3. Results and discussions

(5)

where C1 and C2 are BSA concentrations of the initial solution and the solution after adsorption equilibrium (g/L), V is the volume of BSA solution (L), A is the effective membrane adsorption area (m2), Ads control and Ads modified present the BSA adsorption capacity of the control and modified membranes.

3.1. Surface modification mechanism In this work, a 3-step surface modification is carried out on the nascent TFC membrane, i.e., PEI grafting, organic acid functionalization, and mineralization with calcium ions. The schematic illustration of the sequential surface modifications is proposed in Fig. 2. In the first step, polyamide chains on the membrane surface are crosslinked with PEI via the chemical interaction between the residual acyl chloride groups of TMC and amine groups of PEI. The abundant amine groups on PEI chains allow the further grafting of organic acid groups in the second step. In the last step, calcium ions can chelate with O and N atoms of the organic acid groups (organic phosphonic or carboxylic acid), endowing the membrane a mineralized surface. The detailed reaction mechanism involved can be found in Fig. S-1 of the Supporting information.

2.7. Chemical stability tests The chemical stability of TFC membranes in acidic, alkaline or chloric environment is evaluated by immersing the membrane in HCl aqueous solution (pH of 1), NaOH aqueous solution (pH of 13), or 2000 ppm NaClO aqueous solution (pH of 7) for 24 h and washing them by DI water for several times before the FO test.

287

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Fig. 2. Schematic illustration of surface modification of the nascent TFC membrane by grafting PEI, organic phosphonic acid (OPa) and calcium ions mineralization sequentially.

As well known, the TFC membrane formed by MPD and TMC is generally negative charged, due to the existence of carboxylic groups translated from the residual acyl chloride groups by hydrolysis [16]. The expected impacts of more carboxylic acid groups on the PA layer on the separation performance and anti-scale fouling properties are investigated, and a comprehensive comparison between organic carboxylic acid and phosphonic acid modified membranes is conducted to explore the effects of acid type. Chemical changes in the surface modification of TFC membranes are investigated by FTIR and XPS as shown in Figs. 3 and 4. Fig. 3 shows that, all spectra of TFC membranes exhibit characteristic peaks of amide groups at 1660 and 1544 cm−1, attributed to the stretching vibrations of amide I (C˭O) and the coupling vibrations of amide II (C-N stretching vibration and N-H in-plane blending vibration) groups, respectively [48,49]. In comparison with the spectra of the control membrane, the intensity of absorption band at 3380 cm−1 of all modified membranes (attributed to the stretching vibration of hydroxyl groups in carboxylic acid groups translated from the residual acyl chloride groups by hydrolysis) significantly increases, ascribed to the grafted amine and acid groups. It can also be seen in the spectrum of PA-PEI membrane, the above wide adsorption band splits into two peaks ascribed to the symmetric and asymmetric stretching vibrations of amine groups, indicating the successful bonding PEI onto PA chains. In addition, two new peaks at 1365 and 975 cm−1 can be observed in the spectrum of PA-PEI-5P membrane, due to the stretching vibrations of P=O and P–O bonds respectively [50,51], demonstrating the successful grafting of phosphonic acid on the PA layer. Similarly, a new characteristic peak at 1780 cm−1 appears in the spectrum of PA-PEI-5C membrane, attributed to the stretching vibration of the carbonyl bond in carboxyl groups, indicating the successful attachment of organic carboxylic acid chains.

Fig. 4. XPS spectra of PA-PEI-5P-Ca and PA-PEI-5C-Ca membranes.

It also can be found that, the peak intensity at 1610 cm−1 (deformation vibration of N–H bond in amine groups of PEI) decreases in the spectra of PA-PEI-5P and PA-PEI-5C membranes as compared to that of PA-PEI membrane, probably due to the reaction between amine groups and acid groups. All above changes prove the successful surface modification of nascent PA layer as proposed in Fig. S-1. Chemical changes are further confirmed by XPS characterization. It can be seen from Fig. 4 that, besides to predominant peaks of C, N and O elements, peaks of P and/or Ca can also be observed in the curves of PA-PEI-5P-Ca and PA-PEI-5C-Ca membranes. The detailed surface elemental composition of the control and modified membranes are listed in Table 2. It can be seen that, O/N ratio of PEI-modified membrane is lower than that of the control membrane, ascribed to the higher crosslinking degree and introduced amine groups by PEI modification [41]. While in comparison with PA-PEI membrane, PA-PEI-5C and PAPEI-5P membranes exhibit higher O/N ratios, demonstrating the successful grafting of DTPA or DTPMP onto PEI-modified membranes.

Table 2 Surface elemental composition of the control and modified membranes by XPS analysis.

Fig. 3. FTIR spectra of the control membrane and modified ones with PEI and different organic acids. 288

Code

C

O

N

P

Ca

O/N

Ca/N

Ca/P

PA-Control PA-PEI PA-PEI-3P PA-PEI-4P PA-PEI-5P PA-PEI-5C PA-PEI-3P-Ca PA-PEI-4P-Ca PA-PEI-5P-Ca PA-PEI-5C-Ca

72.21 72.55 69.43 69.03 70.11 70.26 71.06 69.13 69.9 69.88

17.51 13.89 15.76 16.11 16.17 16 15.45 17.4 14.84 15.37

10.28 13.55 14.64 14.57 13.28 13.75 13.08 12.23 14.41 14.45

0 0 0.17 0.29 0.44 0 0.13 0.35 0.37 0

0 0 0 0 0 0 0.28 0.89 0.48 0.35

1.70 1.02 1.08 1.11 1.22 1.16 / / / /

/ / / / / / 0.021 0.073 0.033 0.024

/ / / / / / 2.15 2.54 1.30 0

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Fig. 5. EDX mapping of PA-PEI-5P, PA-PEI-5C-Ca and PA-PEI-5P-Ca membranes.

functionalization and mineralization exerts no significant impacts on the surface morphology of resultant membranes as confirmed in Figs. S2 and S-3. As shown Fig. 7(a), WCAs of all modified membranes are lower than that of the control membrane, and follow an order of PAControl > PA-PEI > PA-PEI-5C > PA-PEI-5C-Ca > PA-PEI-5P > PAPEI-5P-Ca, which is consistent with the water flux results. The improved surface hydrophilicity of modified TFC membranes should be brought by the introduction of hydrophilic amine groups and acid groups for PA-PEI, PA-PEI-5C and PA-PEI-5P membranes. In addition, the mineralization is responsible for the further improved hydrophilicity of PA-PEI-5P-Ca and PA-PEI-5C-Ca membranes as compared to corresponding organic acid functionalized membranes. Moreover, WCAs of PA-PEI-5P and PA-PEI-5P-Ca membranes are both lower than those of PA-PEI-5C and PA-PEI-5C-Ca membranes, which may be ascribed to the stronger polarity and more chelated calcium ions of phosphonic acid groups than those of carboxylic acid groups. And the changes in the reverse salt fluxes of modified membranes are believed to be resulted from the variation in the surface negative charges [52]. The surface zeta potentials of the control and modified TFC membranes are studied and shown in Fig. 7(b). In comparison with the curve of the control TFC membrane, zeta-potential curves of PA-PEI membrane and mineralized membranes shift towards the upper right side, demonstrating the higher isoelectric point values. The reduced negative surface charge is ascribed to the grafted PEI structure with positive charge by the protonation of amine groups, less carboxyl groups with negative charge by the surface modification reaction with PEI and the introduction of Ca2+ ions. On the contrary, zeta-potential curves of PA-PEI-5P and PA-PEI-5C membranes even shift lower-left because of the introduction of negative-charged acid groups. It also can be found that, the isoelectric point value of PA-PEI-5P membrane is lower than that of PA-PEI-5C membrane, which could be ascribed to the stronger dissociation of phosphonic acid groups. Moreover, the isoelectric point value of PA-PEI-5P-Ca membrane is higher than that of PA-PEI-5C-Ca membrane, probably because of more chelated calcium ions.

Moreover, P element is detected in the surfaces of PA-PEI-5P and PAPEI-5P-Ca membranes, and Ca is also observed in the surface of PA-PEI5P-Ca and PA-PEI-5C-Ca membranes, confirming the proposed chemical structures of modified membranes. Ca/P ratio of PA-PEI-5P-Ca membrane is found to be around 1.30, indicating that one DTPMP molecule can chelate with one more calcium ions to form a stereostructure. Furthermore, PA-PEI-5P membrane also exhibits a higher Ca/ N ratio than PA-PEI-5C membrane, indicating more chelated Ca2+ ions with DTPMP than with DTPA, probably because of the stronger chelating ability of organic phosphate groups. EDX mapping tests are conducted to further confirm the existence and distribution of phosphor and calcium elements on the surfaces of PA-PEI-5P, PA-PEI-5C-Ca and PA-PEI-5P-Ca membranes and corresponding results are shown in Fig. 5 and Table 3. It can be found that, P and Ca elements are distributed uniformly on the surfaces of above membranes. Additionally, as shown in Table 3, Ca element amount on PA-PEI-5P-Ca membrane is much higher than that on PA-PEI-5C-Ca membrane, which is consistent with the XPS results. 3.2. Separation performance Fig. 6 displays the FO performance of the control and modified TFC membranes. It can be seen that, water fluxes of modified membranes are all higher than that of the control membrane, especially for PA-PEI5P-Ca membrane, while reverse salt fluxes of modified membranes are also slightly higher than that of the control membrane, except for PAPEI-5P and PA-PEI-5C membranes. It is also found that, water fluxes of PA-PEI-5P membrane and corresponding mineralized membrane are higher than those of PA-PEI-5C membrane and its mineralized membrane. Moreover, the reverse salt flux of PA-PEI-5P membrane is lower than that of PA-PEI-5C membrane, while that of the corresponding mineralized membrane is slight higher. The water flux enhancement of modified membranes is believed to be ascribed to their improved hydrophilicity, since the acid Table 3 Surface elemental composition of in PA-PEI-5P, PA-PEI-5C-Ca and PA-PEI-5PCa membranes by EDX analysis. Element

C

O

N

P

Ca

PA-PEI-5P PA-PEI-5P-Ca PA-PEI-5C-Ca

55.47% 58.05% 55.58%

35.03% 34.79% 34.74%

8.26% 5.01% 9.14%

1.24% 0.86% /

/ 1.29% 0.54%

3.3. Effect of the molecular structure of organic phosphonic acid The effect of the molecular structure of organic phosphonic acid on the separation performance of resultant membranes is further investigated. Table 2 shows that, the counted amount of P atoms in functionalized membranes obeys the order of PA-PEI-5P > PA-PEI4P > PA-PEI-3P, indicating more introduced phosphonic acid groups in 289

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Fig. 6. FO performance of the control membrane and modified ones by PEI grafting, organic acids functionalization, and corresponding mineralization sequentially: (a) water flux; (b) reverse salt flux (2 M NaCl aqueous solution and DI water are used as the draw solution and feed solution, respectively.).

Fig. 7. (a) WCAs and (b) Zeta potentials of the control membrane and modified ones by PEI grafting, organic acids functionalization and corresponding mineralization sequentially.

Fig. 8(a) exhibits WCAs of the control and various functionalized TFC membranes. Compared with the control membrane, WCAs of modified membranes are all lower, and decrease with more phosphonic acid groups in the molecular structure. Fig. 8(b) also shows that, WCA results of corresponding mineralized membranes show the opposite trend with Ca/N ratios, since more calcium ions bonded contributes to a higher surface hydrophilicity. Zeta potentials of the control and modified membranes as a function of pH are studied and shown in Fig. 9. It shows that, isoelectric point values of modified membranes decrease with the increase of phosphonic acid group number in their molecular

the membrane modified with an organic phosphonate acid with a larger molecular weight. The result is also consistent with their O/N ratios, where PA-PEI-5P membrane exhibits the highest O/N ratio among these membranes functionalized with different organic phosphonic acids. In addition, Ca/N ratios in corresponding mineralized membranes present an order of PA-PEI-4P-Ca > PA-PEI-5P-Ca > PA-PEI-3P-Ca, demonstrating more Ca2+ ions introduced in PA-PEI-4P-Ca membrane. Moreover, Ca/P ratios of mineralized membranes, which are all larger than 1, show the similar trend, indicating that EDTMP owns the strongest chelation ability with calcium ions.

Fig. 8. WCAs of (a) the control and modified membranes with various phosphonic acids; (b) the control membrane and mineralized membranes after acid functionalization. 290

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Fig. 9. Zeta potentials of (a) the control and modified membranes with various phosphonic acids; (b) the control membrane and mineralized membranes after acid functionalization.

much higher, and increase with the rising phosphonic acid group number in the molecular structure, because of the improved surface hydrophilicity. In addition, compared to the control membrane, reverse salt fluxes of modified membranes are even lower, following an order of PA-PEI-3P > PA-PEI-4P > PA-PEI-5P, ascribed to the more surface negative charge with more phosphonic acid groups grafted. In terms of the mineralized membranes, Fig. 10(b) also shows that, the water flux and

structure (Fig. 9(a)), while isoelectric point values of corresponding membranes after mineralization obey the sequence of PA-PEI-4PCa > PA-PEI-5P-Ca > PA-PEI-3P-Ca (Fig. 9(b)), since more introduced calcium ions can offset more negative charges. The separation performance of modified membranes is studied. As shown in Fig. 10(a), in comparison with the control membrane, water fluxes of modified membranes with grafted organic phosphonic acid are

Fig. 10. FO performance of (a) the control and modified membranes with various phosphonic acids; (b) the control membrane and mineralized membranes after acid functionalization (DI water and 2 M NaCl aqueous solution are used as the feed and draw solution, respectively). 291

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Fig. 11. Organic fouling test results of PA-Control, PA-PEI, PA-PEI-5P and PA-PEI-5P-Ca membranes: (a) water flux decline; (b) water flux decline ratio and water flux recovery ratio (the synthetic wastewater containing SA and Ca2+ ions and 2 M NaCl aqueous solution are used as the feed solution and draw solution, respectively).

hydrophilicity is responsible for the low fouling propensity, because the abundant N and O atoms in grafted PEI and organic phosphonic acid can act as hydrogen bond acceptors contributing to the formation of a hydration layer which inhibits SA adsorption. Secondly, since carboxylic groups generally work as active complexion sites with foulants, PEI grafting results in less residual acyl chloride groups which can be hydrolyzed into carboxylic acid groups, thus benefiting the enhanced fouling resistance. Lastly, the introduction of PEI and organic phosphonic acid of expanded molecular structures also can prevent the adsorption of SA foulants because of their steric-hindrance effects. In addition, the even lower surface negative charge of PA-PEI-5P membrane is also responsible for its lower fouling propensity. As to the server fouling tendency of the PA-PEI-5P-Ca membrane than that of the control one, the introduction of Ca2+ ions on the membrane surface may serve as “bridges” to link foulants to the membrane via carboxylic acid groups on the surface [53]. Protein adsorption capability on TFC membrane surfaces is further evaluated by the quantitative static protein adsorption tests and corresponding results are displayed in Fig. 12. It can be seen that, modified membranes exhibit much lower protein adsorption rate as compared to that the control membrane, indicating the better antifouling properties of modified membranes. The higher protein adsorption rate of the control membrane is ascribed to the hydrophobic–hydrophobic interactions between BSA molecules and the PA layer. While the higher hydrophilicity, bigger steric hindrance, and the stronger electrostatic repulsion with more negative charges on the membrane surface (BSA molecules in the PBS solution with pH of 7.4 are negatively charged since the isoelectric point of BSA is 4.7) all contribute to the lower protein adsorption rate of modified membrane, especially for PA-PEI-5P membrane with the higher hydrophilicity and more negative charges. The anti-scaling properties of the control and acid functionalized membranes towards inorganic foulants are also investigated in this study with gypsum as the model foulant and the results are displayed in Fig. 13. Compared to the organic fouling on TFC membranes, much fewer studies on the fouling mechanism [56–59] and especially alleviation solutions [60] of the inorganic fouling has been reported. According to previous studies, the gypsum scaling of membranes is controlled by both the surface heterogeneous crystallization (crystallization on the membrane surface) and the bulk crystal deposition (bulk crystallization followed by its deposition on the membrane surface) [61]. Crystallization starts with the formation of prenucleation clusters, which aggregate to form amorphous nanoparticles then polycrystals, and the formation of crystalline domains after reaching the critical size [61]. As well known, the PA layer of TFC membrane is negatively charged due to the existence of carboxylic acid groups, which can chelate with calcium ions to form complexes. The specific interaction between calcium ions and the PA layer results in the higher

Fig. 12. Relative BSA adsorption of the control membrane and modified membranes by different organic acids.

reverse salt flux are all higher than that of the control membrane, and obey the increasing order of PA-PEI-4P-Ca > PA-PEI-5P-Ca > PA-PEI3P-Ca, ascribed to the increased surface hydrophilicity and declined surface negative charges [51]. 3.4. Antifouling properties of membranes The anti-fouling behavior of the control and modified membranes are studied here using the synthetic wastewater containing Ca2+ (CaCl2) and 250 ppm SA as the feed solution. According to previous studies [53,54], the presence of Ca2+ in wastewater effluents results in the membrane fouling, which can act as “bridges” between SA molecules, thus leading to the formation of a crosslinked SA gel layer. On one side, this gel layer amplifies the hydraulic resistance to water transport, causing the deterioration in water flux; on the other side, this gel layer also causes the cake-enhanced osmotic pressure (CEOP) which leads to the reduction in osmotic pressure difference and therefore a significant water flux decline [53,54]. Dynamic fouling tests results in Fig. 11 shows that, the water flux of the control membrane drops sharply during the 18-h test, and its water flux recovery ratio after the physical cleaning is only about 70%. Comparatively, water flux declines of PA-PEI and PA-PEI-5P membranes slow down, and their water flux recovery ratios are much higher than that of the control membrane. According to previous studies, membrane fouling can be alleviated by weakening these specific interactions between the foulants and membrane surface, such as the hydrophobic interaction, van der Waals attraction, electrostatic interaction, and hydrogen bond [16,55]. In this study, the improved anti-fouling properties of modified membranes could be attributed to the following factors. Firstly, the improved 292

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Fig. 13. Inorganic fouling test results of the control membrane and modified membranes by different organic acids: (a) water flux decline; (b) water flux decline ratio and water flux recovery ratio (the gypsum solution with saturation index of 1.3 and 2 M NaCl aqueous solution are used as the feed solution and draw solution, respectively).

Fig. 14. SEM images of gypsum scale on surfaces of the control membrane and modified membranes by different organic acids.

as compared to 5C-Ca crystal (as shown in Fig. 14), resulting in the better dispersity in water. Actually, one DTPMP molecule can chelate two or two more metal ions to form a two-cycle or multi-cycle complexes with stereostructure, however, one DTPA molecule can only chelate one metal ion to form the complex with a plane structure. The improved anti-scaling property of PA-PEI-5P membrane could be ascribed to the following reasons. Firstly, Ca2+ in the gypsum solution tends to interact with DTPMP rather than with sulfate ions (SO42-) of the unsaturated CaSO4 complex in the feed solution, since the chelation between DTPMP and Ca2+ by O and N atoms is more stable. Secondly, it is more difficult for the formation of prenucleation cluster on the surface of PA-PEI-5P membrane, due to its higher critical saturation concentration. Thirdly, the existence of phosphonic acid groups can restrict the crystallization of gypsum. According to previous studies, the organic phosphonic acid can work as a scale inhibitor in the cooling water system by chelating with calcium ions and inhibiting the growth of gypsum crystals [44,45,62]. Thereby, even if 5P-Ca crystals form on the membrane surface, they are too loose to deposit on the membrane surface tightly, leading to the severe performance deterioration. Additionally, a long-term gypsum fouling test of PA-PEI-5P membrane was also conducted. As displayed in Fig. S-4(a), after 72-h fouling test, the water flux decline of PA-PEI-5P membrane is only about 15%, which may partially due to the dilution effect of draw solution, and then becomes steady after 20 h. Fig. S-4(b) also shows that the slope of weight versus time approximately keep constant, demonstrating the negligible gypsum scale fouling on the PA-PEI-5P membrane. The antifouling behavior of the control and modified membranes is further studied using the local lake water as the feed solution. Compared to the simple organic or gypsum fouling, the membrane fouling in the local lake water is more complicated, involving microbial, organic and inorganic fouling. Fig. 16 shows that, compared to the control membrane, the water flux decline of modified membranes is

calcium ion concentration on the membrane surface, thus initiating the formation of gypsum crystals [61]. Fig. 13 shows that, water fluxes of both the control and PA-PEI-5C membranes decline sharply. The especially severer fouling behavior of PA-PEI-5C membrane is due to the existence of plenty of carboxylic acid groups on the PA layer surface [58,61]. It can be further confirmed by SEM images of the contaminated membranes in Fig. 14, which shows that the gypsum scale formed on PA-PEI-5C membrane is denser than that on the control membrane. It is also worth noting that, the water flux decline of PA-PEI5P membrane caused by the gypsum scale fouling is nearly ignorable, which is consistent with the SEM image of PA-PEI-5P membrane surface where no gypsum scale deposition can be observed. Physical cleaning is also conducted to investigate the scale fouling reversibility of the control and acid functionalized membranes. Fig. 13 also shows that, flux recovery ratios (FFRs) of the control membrane and PA-PEI-5C membrane are only approximately 70%; while FRR of PA-PEI-5P membrane can reaches 100%, indicating the gypsum scale fouling on PA-PEI-5P membrane is complete reversible. To find the reasons behind the superior anti-scaling property of PAPEI-5P membrane, the critical saturation concentrations of DTPMP and DTPA solutions are studied by CaCl2 titration tests. As listed in Table S1, only 1.14 g CaCl2 is needed to reach the turbidity point of 100 mL 0.05 M DTPA solution (pH of 11), while for DTPMP solution, the turbidity point is observed with a much higher CaCl2 amount (3.64 g) added, indicating the higher critical saturation concentration of DTPMP. Additionally, Fig. 15 shows that, after two-week standing, DTPMP solution with CaCl2 added is still a stable emulsion, while precipitates in the bottom of DTPA solution are observed. This phenomenon suggests that, in comparison with the complex formed by calcium ions and carboxylic groups (abbreviated as 5C-Ca here) on the PA layer, the complex formed by calcium ions and phosphonic acid groups (abbreviated as 5P-Ca here) is of a much looser packing, probably because that 5P-Ca crystal has more regular shape and smaller size 293

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Fig. 15. Top: photos of 5P-Ca and 5C-Ca solutions after two-week standing; bottom: SEM images of 5P-Ca and 5C-Ca crystals.

still much less, and the water flux recovery ratios are also much higher (> 93%) after the physical cleaning, especially for PA-PEI-5P membrane, demonstrating the improved fouling resistance against the local lake water. Here PA-PEI-5P-Ca membrane still exhibits a slightly higher fouling propensity than that of PA-PEI-5P membrane in spite of its higher surface hydrophilicity, which is believed to due to the existence of organic fouling again.

3.5. Chemical stability of TFC membranes The chemical stability of the modified TFC membranes is studied by immersing the membrane in the acidic (pH of 1), alkaline (pH of 13) or chloric solution for 24 h before the FO test. Fig. 17 shows that, the control TFC membrane after acidic or alkaline immersion exhibits the increased water flux and reverse salt flux obviously as compared to the pristine control membrane, probably due to the higher surface hydrophilicity and/or larger free volume with the hydrolysis of PA chains

Fig. 16. Organic fouling test results of PA-Control, PA-PEI, PA-PEI-5P and PA-PEI-5P-Ca membranes: (a) water flux decline; (b) water flux decline ratio and water flux recovery ratio (the local lake water and 2 M NaCl aqueous solution are used as the feed solution and draw solution, respectively). 294

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Fig. 17. FO performance of the PA-control, PA-PEI-5P and PA-PEI-5C membranes with and without immersion treatment in (a) acidic solution (HCl aqueous solution, pH = 1) or (b) alkaline solution (NaOH aqueous solution, pH = 13) (DI water and 2 M NaCl aqueous solution are used as the feed solution and draw solution, respectively).

Fig. 18. FO performance of the PA-control, PA-PEI-5P and PA-PEI-5C membranes with and without immersion treatment in chlorine solution (2000 ppm NaClO aqueous solution) (DI water and 2 M NaCl aqueous solution are used as the feed solution and draw solution, respectively).

[41]. While, the chlorination of the PA layer occurs when active chlorine species (Cl+, hypochlorous acid) attack the lone electron pair of N atoms on amide linkages [18]. While for PA-PEI-5P and PA-PEI-5C TFC membranes, H+ ions prefer to interact with more reactive –OH or O- moieties in acidic groups and/or basic amine groups in PEI instead of carbonyl groups in amide linkages, and OH- also favors to react with H+ ions released from acidic groups. Similarly, electrophilic Cl+ ions tend to bind with N and O atoms in PEI and organic acid groups with higher electron cloud density instead of N atoms on amide linkages, since the electron cloud of N atom in amide linkages of PA network tends to delocalize to benzene rings and thus lower its electron cloud density. The proposed reaction mechanisms are displayed in Figs. S-5 and S-6. Moreover, the grafted hyper-branched PEI and branched

(formed by TMC and MPD), which can only tolerate a relatively narrow pH application range (2–11). While for PA-PEI-5P and PA-PEI-5C TFC membranes, the variations in the water flux and reverse salt flux are negligible after post-treatment. Similar results also can be found in Fig. 18 which displays FO performance of TFC membranes with and without being immersed in chloric solution. It can be seen that, in comparison with the control membrane, both PA-PEI-5P and PA-PEI-5C membranes exhibits much less performance decline after the chlorine immersion. Above results demonstrate the excellent chemical stability of modified membranes, and their great potential in applications of harsh operational conditions. Generally, the degradation of PA chains in acidic or basic condition is caused by the H+ or OH- attack on carbonyl groups of amide linkages

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References

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4. Conclusion In the present study, TFC membranes with surface functionalization are developed and exhibit improved FO performance, superior antifouling properties and chemical stability in harsh environments. The surface functionalization is performed in three steps, i.e., PEI grafting, organic acid grafting, and mineralization. The reaction mechanism involved is proposed and reasonably testified by FTIR and XPS characterizations. In comparison with the control membrane, modified membranes exhibit much higher hydrophilicity and changed surface charge type and density. Accordingly, in comparison with the control membrane, all modified membranes show obviously increased water flux with the acceptable sacrifice of salt rejections, or even lower reverse salt fluxes of PA-PEI-5P membrane. In addition, compared with the DTPA (5C) functionalized membrane, the DTPMP (5 P) functionalized membrane presents more surface negative charge, higher surface hydrophilicity and thus higher water flux and lower reverse salt flux. Correspondingly, PA-PEI-5P-Ca membrane exhibits less surface negative charge, higher surface hydrophilicity and thus higher water flux and slight higher reverse salt flux than that of PA-PEI-5C-Ca membrane due to more calcium ions chelated with DTPMP. Moreover, the surface negative charge, surface hydrophilicity and water flux of modified membranes increase with the increasing phosphonic acid group amount in the molecular structure of different organic phosphonic acids, and the results of reverse salt flux presents the opposite trend, which is attributed to the more introduced phosphonic acid groups. Meanwhile, a series of surface modification approaches enhance the antifouling properties of resultant membranes compared to that of the control membrane against SA, BSA and local lake water (excluding PA-PEI-5PCa membrane), particularly for PA-PEI-5P membrane. Moreover, PAPEI-5P membrane also exhibits superior anti-scaling property. Lastly, grafted PEI and acids renders resultant TFC membranes excellent chemical stability, in terms of acid-base resistance and chlorine resistance. This work therefore puts forward novel approaches for modifying TFC membranes with outstanding separation performance, fouling resistance and chemical stability, ensuring the potential applications in harsh environment.

Acknowledgements We thank the financial support from National Key Technology Support Program (no. 2014BAD12B06), National Natural Science Foundation of China (no. 21306058), Natural Science Foundation of Hubei Scientific Committee (no. 2016CFA001), and the Free Exploring Fundamental Research Project from Shenzhen Research Council, China (no. JCYJ20160408173516757). 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.

Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.memsci.2018.05.071. 296

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