Dual-functional acyl chloride monomer for interfacial polymerization: Toward enhanced water softening and antifouling performance

Journal Pre-proofs Dual-functional acyl chloride monomer for interfacial polymerization: toward enhanced water softening and antifouling performance Ping Hu, Zewen Xu, Chi Jiang, Bizhuo Tian, Xin Guo, Ming Wang, Q. Jason Niu PII: DOI: Reference:

S1383-5866(19)34940-8 https://doi.org/10.1016/j.seppur.2019.116362 SEPPUR 116362

To appear in:

Separation and Purification Technology

Received Date: Revised Date: Accepted Date:

28 October 2019 27 November 2019 27 November 2019

Please cite this article as: P. Hu, Z. Xu, C. Jiang, B. Tian, X. Guo, M. Wang, Q. Jason Niu, Dual-functional acyl chloride monomer for interfacial polymerization: toward enhanced water softening and antifouling performance, Separation and Purification Technology (2019), doi: https://doi.org/10.1016/j.seppur.2019.116362

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Dual-functional acyl chloride monomer for interfacial polymerization: toward enhanced water softening and antifouling performance Ping Hua, Zewen Xua, Chi Jianga, Bizhuo Tiana, Xin Guoa, Ming Wanga, Q. Jason Niua, b, * a State

Key Laboratory of Heavy Oil Processing, China University of Petroleum (East China), Qingdao, 266580, P.R. China

b Institute

for Advanced Study, Shenzhen University, Shenzhen 518060, P.R. China.

* Corresponding

author, Tel: +86 532 86981850; fax: +86 546 8395190.

E-mail address: [email protected] Author: [email protected] (P. Hu)

Abstract Here, a strategy to control surface negative charge, pore size and active layer thickness by manipulating the polyamide (PA) layer structure is proposed to fabricate an advanced water softening nanofiltration (NF) membrane. Based on this strategy, a dual-functional acyl dichloride monomer containing a protected amino group, 5-sulfinyl amino isophthaloyl dichloride (NSO), is designed as organic phase alternative. Because the protected amine group of NSO has higher hydrolytic activity and is prone to de-protect and form free amino group, it has important impact on the interfacial polymerization (IP) process and formed polyamide (PA) structure. To elucidate the role of the protected amino group in regulating PA structure, the desalination performances and structural characteristics of the three PA membranes prepared by NSO monomer and its structurally similar monomers (isophthalyl chloride (IPC) and trimesoyl chloride (TMC)) are systematically investigated. Compared with TMC-based NF membrane, the NSO-based membrane shows a smoother and lower carboxyl group density surface, decreased pore size and a thinner PA rejection layer (~ 30 nm). As a result, the novel NF membrane exhibits outstanding water softening performance in terms of ultrahigh divalent salts (Na2SO4, MgSO4, MgCl2 and CaCl2) rejections (above 98%) and preferable high water flux (138.25+6.32 LMH), together with superior antifouling property. More importantly, its water softening capability remain stable even under the mixed salt solution including abundant SO42-, which far outperforms the typical poly(piperazine-amide)-based and state-of-the-art commercial NF membranes. This work further establishes the structure-property relationship and enables the rational design of high performance of NF membrane at the molecular level.

Key words: Nanofiltration membrane; dual-functional acyl chloride monomer; interfacial polymerization; outstanding water softening performance; superior antifouling property

1. INTRODUCTION Relying on its dual benefits of energy-saving and low-cost, membrane-based separation technology has become an indispensable platform for water purification, seawater or brackish water desalination as well as wastewater reuse.1 The design of thin-film composite (TFC) membrane is a milestone breakthrough in the membrane separation field, which incorporates an ultrathin selective barrier layer and a microporous support, wherein the advantages of two independent polymeric layers can be maximized to obtain the ideal properties for nanofiltration (NF).1-3 Since sieving of salt ions and small organic molecules occur within the polyamide (PA) rejection layer, the surface property and structural characteristic of PA layer thus dominate the water permeability, solute retention, selectivity and antifouling property of the TFC membrane.4-5 The separation layer of the state-of-the-art commercial NF membrane is assembled by piperazine (PIP) and trimesoyl chloride (TMC) with interfacial polymerization (IP) process. To enable wider deployment of NF technology, highly permeable NF membrane is desired for the significant reductions of energy-cost and capital cost, thus rendering it more affordable.3 To achieve this goal, two main strategies are reported. One is to regulate the IP variables (centrally are the distribution and diffusion rate of IP monomer and IP reaction rate) by doping various additives2,

6

(i.e., catalyst, co-solvent and macromolecule), constructing available

interlayers7 and tailoring reaction temperature8 so as to enhance the separation performance of traditional poly(piperazine-amide) NF membrane. However, significant improvement in the permeability of the typical PA layer is hard to achieve because its intrinsic structure is essentially unchanged in this way. Another is to manipulate the intrinsic structure of the rejection layer via designing alternative monomer. As the IP reaction is diffusion controlled at oil phase interface, extensive efforts have attempted to design alternative organic phase monomers towards preparing the novel NF membranes with desired performance, which contains biphenyl acyl chloride,9 alicyclic acyl chloride,10 multi-acyl chloride11-13 and rigidly twisted aromatic acyl chloride.14 However, to our best knowledge, the dual-functional organic phase monomer (multi-chlorinated monomer comprising the protected amino group, can play acyl chloride role as well as amine role during the IP process) has rarely been reported.15 The dual-functional acyl chloride monomer is commonly utilized to synthesize dendrimer.16-17 Dendrimer PA not only remains the excellent chemical and mechanical properties of conventional aromatic PA, but also can interact with IP-prepared PA layer owing to the similarity in their chemical structures, making it preferred alternative to modify PA membrane.17 Thus, the dual-functional acyl chloride monomers have great potential in tailoring the IP-prepared PA chemical structures. Herein, we exploit dual-functional acyl chloride monomer as organic monomer alternative to prepare the novel NF membrane. The dual-functional acyl dichloride monomer comprising a protected amino group (sulfinamide group (-NSO)), 5-sulfinyl amino isophthaloyl dichloride (NSO), is designed and synthesized. Its molecular structure is similar to that of TMC, with the only difference is to replace an acyl chloride group (-

COCl) with a -NSO group, while the minor difference has significant impact on the IP process. Since the NSO group is readily converted into -NH2 group upon contacting with H + (refer as de-protection process), it would be able to serve the H+ acceptor role that consumes the produced acid in the interface of aqueousoil phase to promote the IP reaction. Moreover, -NSO group possesses a higher hydrolytic activity than COCl and is more inclined to diffuse to the interface, therefore, it can restrain the hydrolysis of -COCl groups in interface and facilitate more -COCl groups to involve in the IP reaction. Both the inhibition effect of -NSO group and the less number of -COCl groups of NSO monomer results in the formation of PA active layer with decreased surface carboxyl group (-COOH) density, which is closely related to biofouling,18 scaling19 and organic foulant accumulation initiated by membrane-metal-organic bridges.20-21 Besides, the -NH 2 groups formed by de-protection of -NSO not only contribute to elevate surface positive charges, but also participate in the IP reaction to increase the network cross-linking of PA structure or in-situ generate dendrimer by self-polymerization22 (Figure S1). Incorporating organic nanoparticles into the PA active layer can significantly alter the structure and performance of NF membrane.23 Thus, the dual-functional NSO monomer is used in the IP process for the fabrication of advanced water softening NF membrane with ultra-high retentions against multivalent cations, preferable water permeability and superior fouling resistance. To investigate the tailoring mechanism of NSO monomer structure on the PA structure, two structurally similar monomers, i.e., TMC and isophthaloyl dichloride (IPC), are purposely chosen to systematically compare the extrinsic (thickness, roughness and surface functionality) and intrinsic (pore size, permeability and fractional free volume) properties of the as-prepared three PA separation layers. Compared with the typical TMC-based NF membrane, its thinner PA layer, less surface negative charges and smaller effective pore size are cardinally responsible for the outstanding water softening performance of the newly developed NSO-based NF membrane. Moreover, its water softening capability is further evaluated using a high salinity feed solution with abundant SO42-. 2. EXPERIMENTAL SECTION 2.1 Materials Poly (ether sulfone) (PES) UF membrane (its physicochemical properties and permeability as illustrated in Table S1 and Figure S2) is supported by Vontron Co., Ltd. PIP, IPC, TMC and 5-amino isophthalic acid are purchased from TCI. The other chemicals are supplied from Sino-pharm. NF 270 and NF 90 commercial membranes are from Dow Film-Tec. Secondary effluents from a reverse-osmosis purification unit are used as DI water. 2.2 Synthesis of NSO monomer 5-sulfinyl amino isophthaloyl dichloride (NSO) is synthesized via the nucleophilic substitution reaction (Figure S3). Briefly, 35 g of 5-amino isophthalic acid is dissolved in SOCl2 and reflux for 24 h. After excess

SOCl2 is distilled off, 18 g of glassy yellow products are distilled under 135℃/0.6 mm Hg. The FTIR and 1H NMR (8.77(2H), 8.80(1H)) spectra (Figure S4) of product are determined. 2.3 Fabrication of the novel NF membrane The three PA (PIP-TMC, PIP-NSO and PIP-IPC) NF membranes are fabricated via IP, and the prepared PA chemical structures and IP process are schematically illustrated in Figure 1. First, the PES substrate is immersed in 1.0% w/v PIP aqueous solution for 120 s, then the surplus aqueous is poured off and dried by an air knife. Subsequently, 0.15% w/v NSO n-hexane is dispensed on the PIP-saturated support surface for 60 s. Finally, the fresh TFC membrane is dried in oven at 60 °C for 120 s, and then is stored in DI water before testing. To guarantee the same organic phase monomer concentration, either 0.15% (w/v) TMC or 0.115% (w/v) IPC substitutes NSO in the organic phase. The optimal IP parameters (i.e., the concentrations of PIP and NSO ae well as the reaction time) of PIP-NSO membrane is obtained from Figure S5-S7.

Figure 1 Preparation of the PIP-NSO NF membrane via IP. (a) Synthesis of polyamide membranes. (b) The de-protection process of -NSO group. (c) Visualization of IP process. (d) Water softening process of the novel PIP-NSO membrane.

2.4 Characterization The purity of the synthesized NSO monomer is detected by a nuclear magnetic resonance spectroscopy (Bruker). Membrane surface inherent functional groups are analyzed by X-ray photoelectron spectroscopy (XPS, PHI 5300) and an ATR-FTIR (Nicolet 6700). Surface and cross-section morphologies are captured by a scanning electron microscopy (SEM, Hitachi S-4800) and an atomic force microscopy (AFM, SPM-9700). The thickness of active layer is monitored by a spectroscopic ellipsometry (HORIBA) with a 70° angle of incidence.24-25 The fitting results with 2<0.2 are recorded. The separation layer thickness is also measured by AFM.26 Prior to measurement, the PA active layer is peeled off by dissolving PES substrate with DMF and

then is elaborately transferred onto a silicon wafer. The PA nanofilm covered on the silicon wafer is scratched by a tip to expose the height from silicon wafer to PA layer. Surface wettability is evaluated using a surface tension analyzer (DSA30, Krüss). Surface charged property is detected by a SurPASS 3.0 electro-kinetic analyzer. The toluidine blue method 20-21 is used to measure surface -COOH density of NF membrane, and the process as described in ESI S6. 2.5 Separation performance Separation tests are conducted on a lab-scale cross-flow filtration unit comprising 6 filter cells with an effective permeate area of 18.55 cm2 (Figure S8). The NF performances (water/salt permeate flux and solute rejection (R)) of the prepared membranes are executed by separating various 2,000 ppm single salt solutions at 10 bar and 25 °C. The MWCO, mean pore size and pore size distribution of the three NF membranes are determined via the solute transport method27 using 200 ppm PEGs (Mw = 200-800) solutions, and the test process as shown in ESI S8. Water softening capability of the novel NF membrane is evaluated by a 13,500 ppm mixed salt feed solution10 containing 3000 ppm Ca2+, 500 ppm Mg2+, 1000 ppm Na+, 7000 ppm Cl- and 2000 ppm SO42- under operation pressure varying from 10 bar to 30 bar. The water/salt flux (L m-2 h-1, LMH) is obtained from the volume of permeate (Q) at the fixed permeate time (ΔT) and filtrated area (A), as given in equation 1， Flux

 Q

A Δ T  . R is calculated from equation 2,

R  1  Cp Cf   100%, where

Cf and Cp are the solute concentrations in feed and permeate solutions, respectively, which are determined using an electrical conductivity for individual salt, a total organic carbon measurement (Shimadzu) for PEG, and an ion chromatography (883 Basic IC Plus) for anions and cations. 2.6 Antifouling testing Fouling behaviors of the three fabricated NF membranes are performed using sodium alginate (SA) as pollutant. Before fouling, the tested membranes are allowed to reach a steady stage by prepressing with background solution (1 mM CaCl2, 16 mM NaCl, and 1 mM NaHCO3) under 10 bar for 2 h. To ensure that fouling behavior can be compared between different membranes, all the initial fluxes are adjusted to 100 LMH via varying the applied pressure.28 To begin fouling, SA is added into the background solution (50 ppm SA) and the fouling process lasts for 14 h, during which permeate flux is determined at fixed intervals. Antifouling property is evaluated by flux attenuation and cleaning efficiency attained by physical cleaning29, as described in ESI S9. Besides, fouling behaviors of the three membranes are also evaluated by 500 ppm bovine serum albumin (BSA) feed solution. After pre-compacting under 10 bar for 2 h, all the water fluxes (Jw0) of the test samples are identically adjusted to the same flux (100 LMH). Subsequently, the permeate flux (Jp) of fouling stage is measured at specific intervals and continues for 13 h. Finally, water flux (Jw1) is re-determined after repeatedly

rising with DI water for 2 h. The fouling resistance indexes (total flux decline ratio (FDR) and flux recovery ratio (FRR)) are calculated from equations 3-4:

FDR  (1  J p J wo )  100% (3) and FRR  J w1 J w0  100% (4). 3. RESULTS AND DISCUSSION 3.1 Surface morphology and structure characterization

Figure 2. The surface morphologies of the prepared NF membrane: (a) SEM images, (b) AFM topography images, and (c) the three-dimensional AFM images.

Surface morphologies of the prepared PA membranes are characterized by SEM and AFM. In Figure 2a-b, it is clear that the three NF membranes exhibit typical nodular structures, where nodules are associated with the distribution and diffusion rate of PIP monomers during the IP process.30-31 However, the globules distributed on the PIP-NSO membrane surface become smaller and closer compared to PIP-TMC membrane, which suggests that it has relatively smoother surface. Moreover, the three-dimensional AFM images (Figure 2c) coupled with the measured surface roughness (Table 1) also confirm that the NSO-based NF membrane surface is smoother. It is well known that, during IP process, PIP monomer diffuses toward organic phase to react with TMC, and the rapid migration of PIP may push around and twist the generated nascent ultrathin PA nanofilm, ultimately resulting in the formation of the grainy morphology.30-32 Therefore, it can be reasonably concluded that the formation of the smoother PIP-NSO polyamide layer is closely related to the accelerated amidation, which results from the -NSO group de-protection process that consumes the formed acid in the

interface. This promotes more PIP monomers to participate in the IP reaction, and forms a denser initial PA nanofilm, which blocks the further diffusion of PIP towards organic phase to react with TMC, leading to the reduction in the formation of grainy structures.30-32 Table 1 The characteristic properties of the prepared composite membranes. a

Membrane s PIP-TMC PIP-NSO PIP-IPC

Roughness Ra

Rq

12.54 6 4.096 9.309

15.38 7 6.508 12.38 6

b

c The

Contact angle (℃)

Zeta potential (mV) (pH=7.0)

Isoelectrical Point (IEP)

The PA thickness by ellipsometry (nm)

The PA thickness by SEM (nm)

PA thickness by AFM (nm)

48.2+3.1

-35.63

3.95

66.92+2.3

76.5

70.1

39.1+2.7 46.8+3.3

-10.02 -23.02

5.02 4.01

31.11+1.2 37.03+1.6

37.4 40.2

32.5 --

The roughness values are obtained from 4×4 m three-dimensional AFM images (Figure 2c). thickness of PA rejection layer are measured by ellipsometry (Figure S9). c The thickness of PA rejection layer are measured by ellipsometry. a

b The

Figure 3. The cross-section images of the (a) PIP-TMC, (b) PIP-NSO and (c) PIP-IPC membranes; (d) and (e) represent the thickness of the PIP-TMC and PIP-NSO nanofilms on the silicon wafer, respectively.

From the cross-section micrographs (Figure 3a-c), the thicknesses of the PA selective layer of PIP-NSO and PIP-IPC membranes are thinner than that of PIP-TMC membrane (76.5 nm), whereas it is as thin as 37.4 nm for the PIP-NSO membrane. Furthermore, the ellipsometry and AFM methods are also employed to monitor the PA active layer thickness. The examined results of ellipsometry (Table 1) and AFM method (Figure 3d-e) further demonstrate that the thickness of NSO-based PA layer is the thinnest among them.

Compared with PIP-TMC membrane, the decreased thickness of PIP-IPC layer can be ascribed to the more planar and linear initial PA nanofilm. For NSO, its acyl groups involve in linear polymerization reaction with PIP, whereas its -NSO group after de-protecting may take part in cross-linking. Simultaneously, the deprotection process of -NSO groups promotes the IP reaction, which facilitates to form a tighter initial PA nanofilm, thereby leading to the formation of a thinner PA active layer. More importantly, the thickness of PA layer is negatively correlated to the water permeability of TFC membrane.24 The decrease in the PA layer thickness is equated to declining hydraulic resistance, which has been deem as an advisable strategy to achieve highly permeable NF membrane. 24 In the recent report, 7 constructing hydrophilic interlayer is deemed as a conventional way to reduce the PA layer thickness. In this work, a thinner PA layer can be readily obtained by dual-functional NSO monomer. 3.2 Surface chemical property As depicted in Figure 4a, a new peak appears at 1623 cm-1 for the three prepared TFC membranes and another additional peak at 1667 cm-1 is present only for the PIP-NSO membrane compared to PES substrate, while the characteristic absorption peak intensity for PIP-TMC membrane is higher than that of the PIP-IPC membrane. Although both peaks at 1623 and 1667 cm-1 correspond to amide Ι band, they can be identified to the C=O stretching vibration of tertiary and secondary amide groups, respectively.33-34 It indicates that the PA rejection layers of PIP-TMC and PIP-IPC membranes are solely composed of semi-aromatic polyamide, whereas the active layer of PIP-NSO membrane consists of semi-aromatic and aromatic polyamides. Besides, the broad adsorption peak at 3200-3600 cm-1 suggests the presence of unreacted amine and carboxyl groups. Nevertheless, for the PIP-NSO membrane, the aromatic PA typical peaks at 1541 cm-1 associated with amide II band and 1610 cm-1 assigned to deformation vibration of N-H may be too weak to be obviously observed, which reveals that the IP reaction between PIP and NSO is governed by PIP.

Figure 4. The ATR-FTIR spectra (a) and XPS survey spectra (b) of the fabricated three NF membranes.

Table 2 Elemental composition of the fabricated three NF membranes. a

Membranes C(%)

N(%)

O(% )

PIP-TMC

69.99

13.11

PIP-NSO

71.31

PIP-IPC

71.27

Atomic composition form XPS (%)

b-

b -N-C=O/-NH-

c >N+H /2

O/N

O/C

NH3+

COOH

C=O-

16.90

30.61

69.39

2.41

1.29

0.241

13.86

14.83

20.18

79.82

8.30

1.07

0.208

11.54

15.69

26.85

73.15

4.76

1.36

0.225

a

The penetration depth of the XPS probe is around 10 nm. The surface -COOH groups and –N-C=O contents are calculated from the core-level O 1s XPS spectra (Table S2) c The surface>N+H / -N+H content is estimated from the core-level N 1s XPS spectra (Figure 5(a-c) and Table S2) 2 3 b

To confirm that the protected amine groups further involve in the IP reaction, the chemical composition of PIP-NSO membrane is analyzed by XPS. From Figure 4b, it is found that the PIP-NSO membrane possesses the highest N content and the lowest O content. The N elements of both PIP-TMC and PIP-IPC membranes are entirely derived from the reacted PIP monomer, whereas for PIP-NSO membrane, apart from this, the protected amino groups of NSO monomer also contribute to the N elements of PA structure, which is mainly responsible for the increased N content. The lower O content of the PIP-NSO layer may result from the extra consumption of -COCl groups by the de-protected -NSO group and inhibition effect of -NSO in IP process. Based on the examined O/N ratio (Table 2), it is concluded that the network cross-linked PA structure are formed via the further reaction of -NSO groups in IP, which is apparently different from the fully linear PIPIPC structure. Moreover, both N 1s (Figure 5a-c) and O 1s (Figure 5d-f) core-level spectra indicate that the NSO-base PA layer exhibits higher amide bond content. For PIP-NSO membrane, its surface protonated amine (>N+H2 /-NH3+) group proportion obtained from the core-level N1s spectra is approximately 8.3%, which is higher than those of the PIP-TMC (2.41%) and PIP-IPC (4.76%) membranes, revealing that its surface has more positive charges. Besides, its surface -COOH content (20.18%) calculated from the O 1s core-level spectrum is lower compared with the PIP-TMC (30.61%) and PIP-IPC (26.85%) membranes, and this tendency is in agreement with the obtained -COOH group content from the C 1s high-resolution spectra (Figure S10 and Table S2).

Figure 5. The deconvolution of N1s (a-c) and O1s (d-f) core level XPS spectra of the prepared composite membranes.

Figure 6. (a) The measured surface carboxyl group density with the TBO method; (b) surface potential of the membranes in the feed solution (without Ca2+, 9 mM NaCl and 1 mM NaHCO3; with Ca2+, 1 mM CaCl2, 6 mM NaCl, and 1 mM NaHCO3; Total ionic strength 10 mM and pH 8.0 ± 0.1).

Since surface carboxyl group of NF membrane is critical to surface charge and antifouling performance, it is further determined by the TBO method. As presented in Figure 6a, carboxyl density of PIP-TMC and PIP-IPC membranes are 11.4±0.8 nm-2 and 6.7±0.9 nm-2, respectively, which are within the range of the reported results,20-21 and are higher compared with the PIP-NSO membrane (4.2±1.1 nm-2), strongly evincing that the NSO-based PA TFC membrane surface has a lower carboxyl density. Moreover, for the PIP-NSO membrane, the reduction in the absolute value of its surface ζ potential caused by Ca2+ is smaller compared with the other membranes (Figure 6b). The decreased absolute value of surface ζ potential can be rationalized to the specific binding interaction between Ca2+ with surface -COOH groups.29 Thus, the smaller degree of decline in the ζ potential further verifies that the NSO-based membrane surface displays lower -COOH group density, which is consistent with the measured result by the TBO means. Since surface -COOH groups are originated from the hydrolysis of unreacted -COCl groups,20-21 hence the lower -COOH density means less

unreacted -COCl groups residual in the PIP-NSO layer, which further demonstrates the catalytical role of the de-protection process of -NSO group and the further reaction of the de-protected -NSO group in IP reaction. This indicates that the dual-functional NSO monomer is beneficial to preparing the NF membrane with less surface carboxyl groups, undoubtedly, which imparts less negative charges and fouling propensity to the NSObased TFC membrane. 3.3 Surface hydrophilicity and surface charge Surface roughness and chemical property (the dipole moment and hydration numbers of polar groups) jointly command surface hydrophilicity that is critical to permeability and antifouling performance.35 Amide, carboxyl and amino groups are primarily presented polar groups on the three PA membrane surfaces, among which the hydrophilic of amide is considerably more important due to its larger dipole moment and more hydration numbers36 (Table S3). As shown in Figure 7a, the water contact angles of the three NF membranes are in the range of 40° to 50°, whereas the surface hydrophilicity of PIP-NSO membrane is slightly higher. Because its surface is smoother, the elevated surface wettability may result from its more amide bonds as proved by the XPS analysis (Table 2). Figure 7b illustrates that surface zeta potential of the prepared NF membranes at pH ranging from 3.0 to 10.0, and the iso-electric point (IEP) and zeta potential value at neutral pH are given in Table 1. The IEPs of PIP-TMC and PIP-IPC membranes are almost identical (about 4.0) and in line with the reported value in elsewhere,6 while the PIP-NSO membrane displays an apparent IEP shift towards a larger pH (4.93) simultaneously with higher zeta potential at the same pH condition, suggesting that it has lower surface electronegativity thanks to its surface lower -COOH density.

Figure 7. (a) Water contact angle and (b) surface zeta potential of the prepared NF membranes.

3.4 Pore size distribution Solute transport method27, 37 and potential analysis34 are two the preferred ways for measuring the pore size of NF membrane. Here, the solute transport method is selected because both steric partitioning coefficient and convective hindrance factor contribute to average pore size in this case. In this method, the rejections of PEGs with different molecular weights for the three NF membrane are determined (Table S4),

and the rejection curve is plotted by nonlinear fitting (Figure 8a). Based on the rejection curve and the fitted probability density function between rejection and Stokes diameter (Figure S11), the log-normal probability plot of pore radius distribution and mean pore size (p) are obtained as presented in Figure 8b. MWCO are 302, 243 and 285 Da for the PIP-TMC, PIP-NSO and PIP-IPC membranes, respectively, and the corresponding p are 0.421, 0.351 and 0.397 nm (inset image in Figure 8b), suggesting that the NSO monomer is conducive to form a progressively tight PA active layer with the smaller mean pore size. Moreover, as revealed in Figure 8b, the PIP-NSO membrane exhibits the narrowest pore size distribution. In addition, the interconnected cavity size distributions of PIP-TMC and PIP-NSO nanofilms are further calculated by molecular simulation,10, 38 and the estimated process is illustrated in ESI 16. As seen in Figure 9, the PIP-NSO polymer depicts the decreased cavity size distributions in the range of 1.5-3.5 Å compared with PIP-TMC polymer.

Figure 8. MWCO (a), pore size distribution (b) and mean pore size (c) of PIP-TMC, PIP-NSO and PIP-IPC membranes.

Figure 9. Simulated cavity size distributions in the PIP-TMC and PIP-NSO polymers.

3.5 Desalination performance NF performance of the novel membrane is evaluated and compared with the prepared PIP-TMC and PIP-IPC membranes as well as commercial NF membranes (NF 270 and NF 90). The salt-sieving characteristic of the PIP-NSO membrane is completely different from that of NF 270 membrane,

contrarily behaves like NF 90 membrane (Figure 10a), which implies that its salts retentions mainly rely on size exclusion. In comparison with the typical TMC-based NF membranes (PIP-TMC and NF 270), all the salts rejections of the PIP-NSO membrane are significantly enhanced, especially, its NaCl rejection increases to 78+3.7%. Meantime, its divalent salt rejections (Na 2 SO 4 , MgSO 4 , MgCl 2 and CaCl 2 ) are up to above 99%, revealing its substantially higher Ca 2+ and Mg 2+ removal capability. For PIP-NSO membrane, its decreased effective pore size (0.351 nm, smaller than the hydrated radius of Ca 2+ /Mg 2+ (Table S5)) and reduced cavity radius distribution in the range of 1.0-3.5 Å as proved by molecular simulation (Figure 9) impose an increased steric hindrance on the passages of Ca 2+ /Mg 2+ . Therefore, in addition to the low surface electronegativity, the decreased pore size also contributes to the enhancement of the divalent salts rejections. Furthermore, the water flux of PIP-NSO membrane also is greatly improved, as high as 138.25+6.32 LMH, which is approximately twice of magnitude higher than that of the PIP-TMC membrane (76.1+3.92 LMH) and more than three orders of that of the PIP-IPC membrane (36.25+3.77 LMH). Both elevated surface wettability and reduced PA layer thickness are mainly responsible for the improvement in water flux.

Figure 10. (a) Comparison of the NF performance and (b) correlation between the water permeability and permselectivity of the prepared NF membranes. (The detail calculation process of water permeability, salt permeability and perm-selectivity is described in ESI S18, as illustrated in elsewhere.4, 39 Where Jw stands water flux, Rs is solute retention, R is gas constant, Δπ is the osmotic pressure of 2000 ppm MgSO4 solution, h is the PA layer thickness. The date of NF 270 and NF 90 are consulted the results of Figure 10a and Table S6).

To further understand the structure-property relationship of the fabricated membranes with similarly structural monomers, their intrinsic properties containing water permeability (Pw), salt permeability (Ps) and perm-selectivity (α),4 are measured. As shown in Figure 10b, the Pw of PIP-NSO membrane is slightly smaller than that of PIP-TMC membrane, which demonstrates that the elevated permeate flux is mainly attributed to the decrease of the PA layer thickness. However, compared with PIP-NSO membrane, the Pw of PIP-IPC membrane is lower, although their PA layer thicknesses are similar. This is probably explained by the increased fractional free volume from the further reaction of -NSO groups in IP process than that of full linear

PIP-IPC structure as proved by the molecular simulation results (Figure S12). Moreover, the Ps of the PIPNSO membrane is magnitude lower than that of the PIP-TMC membrane, which results in a higher permselectivity. Therefore, it can be reasonably concluded that manipulating polyamide structure with the welldesigned dual-functional acyl chloride monomer is an advisable strategy to overcome the longstanding permeability-permselectivity trade-off of the traditional TFC membrane. 3. 6 Water softening performance To highlight the water softening capability of the novel NSO-based NF membrane, its permeate flux and CaCl 2 /MgCl 2 rejection are bench-marked with several laboratory-prepared membranes (i.e., positively charged NF membranes, nanomaterial incorporated nanocomposite membranes and layer-by-layer assembly polyelectrolyte membranes) and commercially advisable NF membranes (Table S7). In Figure 11, it is clearly seen that the divalent cations rejections of PIP-NSO membrane are ultra-high and superior to all the listed membranes. Simultaneously, its permeance is competitive and outperforms most the compared NF membranes, indicating its outstanding water softening performance. However, other presented co-ions/ counter-ions and total ionic concentration of feed solution have important impacts on the removal efficiency of hardness ions,40-41 thus assessing the water softening performance of TFC membrane only by using sole salt feed is unconvincing. A 13,500 ppm simulated hard water (described in 2.5) is used to further evaluate its water softening performance.

Figure 11. Water softening performance comparison between our NF membrane and other reported NF membranes.

From Figure 12a and Figure S13, it is seen that, except for the PIP-NSO membrane, the ions rejections of the other membranes are linearly increased as the operating pressure elevates from 10 to 20 bar, but they fiercely drop with a further increases in operation pressure. Also a similar trend is observed for the permeate flux (Figure 12b). Moreover, the order of ions retentions is closely related to their hydration radius and diffusion coefficient (Table S5). Both the PIP-TMC and NF 270 membranes are ineffective in removing the hard ions at high ionic strength and high osmotic pressure, which may be imputed to the severe membrane fouling induced by the formed fouling layer between the surface abundant -COOH groups and metal cations.

For NF 90 membrane, though it exhibits high hard ion rejection, its salt permeate flux is lower compared with PIP-NSO membrane. Besides, its water softening capability is readily destroyed upon elevating the operation pressure owing to its rougher surface. However, for the PIP-NSO membrane, even though the presence of plentiful SO42- ions can cause the shielding effect,40 its rejections towards both Mg2+ and Ca2+ are still up to 99.56% and 99.33%, respectively, which further reveals that its hardness ions removal mainly dependents on size exclusion. Furthermore, its water softening performance significantly enhances with elevating the operating pressure. Even being tested at 30 bar, its flux achieves approximately 162.2 LMH and divalent cations rejections still remain above 98%. This comparison indicates that PIP-NSO membrane exhibits outstanding water softening performance under the high ionic strength and high osmotic pressure.

Figure 12. The water softening performance of the prepared PIP-NSO membrane in the mixed salt solutions (the feed solution contains 500 ppm Mg2+, 3000 ppm Ca2+, 1000 ppm Na+, 2000 ppm SO42- and 7000 ppm Cl-)

3.7 Antifouling performance Fouling behaviors of the three NF membranes in the feed solution containing 50 ppm SA and 1 mM Ca2+ are compared in Figure 13a. Once SA is added, the fluxes of the three membranes decline sharply, which results from the generation of the compact SA fouling layer induced by the accumulation of calcium bridges between the -COOH groups of membrane and those of SA molecules.20-21 Even so, the ultimate flux of PIPNSO membrane is the highest among them, which is ascribed to its lower -COOH density. Moreover, its cleaning efficiency (71.5+3.5%) is higher than those of PIP-TMC (45+10.7%) and PIP-IPC (60.5+4.9%) membranes (Figure S14), suggesting that the formed contaminated layer on membrane surface is readily washed away by physical cleaning.

Figure 13. Fouling behaviors of the prepared NF membranes (a) in SA feed solution (containing 50 ppm SA, 1 mM CaCl2, 16 mM NaCl and 1 mM NaHCO3) and (b) in the 500 ppm BSA feed solution.

Moreover, their fouling behaviors are further evaluated by using BSA feed solution. As shown in Figure 13b, all the permeate fluxes are fiercely down in the initial stage until reaching plateau due to the unavoidable deposition and adsorption of BSA onto the membrane. However, the ultimate flux and FRR (Figure S14) of PIP-NSO membrane are higher compared with the other membranes, which can be ascribed to its smoother and higher hydrophilic surface. Besides, the long-term running stability of the PIP-NSO membrane is excellent (Figure S15). Therefore, advantages of outstanding water softening capability, superior antifouling performance and excellent long-term stability, make the novel PIP-NSO membrane a promising candidate in water softening filed. 4. CONCLUSIONS A facile strategy to design a novel dual functional acyl chloride monomer NSO comprising a protected amino group is originated in this work for the fabrication of an advanced water softening NF membrane with ultra-high rejections of hardness ions, high permeate flux and superior antifouling performance. The surface characteristic (surface morphology, roughness, chemical functionality, hydrophilicity and charge), structural property (thickness and pore size) and separation performance of the PIP-NSO membrane are thoroughly investigated, and compared with the prepared PA membranes with two structurally similar monomers (TMC and IPC). The results demonstrate that the significant enhancements of the permeate flux and salt rejections are ascribed to the -NSO group. The -NSO group readily de-protects and forms free amino group once contacting with H+, while the de-protection process restrains the hydrolysis of acyl chloride groups at interface and simultaneously facilitates the IP reaction, ultimately resulting in the reductions in the surface -COOH density, surface negative charges, surface roughness and PA layer thickness. Moreover, the formed amino groups can further involve in IP reaction, which is responsible for the increased fractional free volumes of the PIP-NSO polymer compared with the PIP-IPC polymer, as proved by molecular simulation. Besides, the generated compacter PIP-NSO structure initiated by the accelerated IP reaction results in the

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Abstract Graphic

Highlights  A novel dual-functional acyl chloride monomer, 5-sulfinyl amino isophthaloyl dichloride (NSO), was synthesized;  The prepared polyamide layer with decreased surface -COOH group density, pore size and thickness;  The novel NSO-based membrane exhibits outstanding water softening performance and superior antifouling property.

No conflict of interest exits in the submission of this manuscript, and the manuscript is approved by all co-authors for publication. I would like to declare on behalf of my co-authors that the work described is original research that has not been published previously, and not under consideration for publication elsewhere, in whole or in part. All the authors listed have approved the manuscript that is enclosed. The authors declared that they have no conflicts of interest to this work. We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted