Hydrophilic Hollow Nanocube-Functionalized Thin Film

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Functional Nanostructured Materials (including low-D carbon)

Hydrophilic Hollow Nanocubes Functionalized Thin Film Nanocomposite Membrane with Enhanced Nanofiltration Performance Zhipeng Liao, Xiaofeng Fang, Jia Xie, Qin Li, Dapeng Wang, Xiuyun Sun, Lianjun Wang, and Jiansheng Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b19121 • Publication Date (Web): 14 Jan 2019 Downloaded from http://pubs.acs.org on January 15, 2019

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Hydrophilic Hollow Nanocubes Functionalized Thin Film Nanocomposite Membrane with Enhanced Nanofiltration Performance

Zhipeng Liao, Xiaofeng Fang, Jia Xie, Qin Li, Dapeng Wang, Xiuyun Sun, Lianjun Wang, Jiansheng Li*

Key Laboratory of New Membrane Materials, Ministry of Industry and Information Technology, School of Environment and Biological Engineering, Nanjing University of Science and Technology, Nanjing 210094, China *Corresponding author. Tel/Fax: +86 25 84315351. E-mail addresses: [email protected] (J. Li).

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ABSTRACT: The performance of thin film nanocomposite (TFN) membrane is significantly determined by the inherent structure and composition of the incorporated nanofillers. In this work, hydrophilic hollow nanocubes (HHNs) derived from zeolitic imidazolate framework 8 (ZIF-8) were incorporated into the polyamide layer via interfacial polymerization approach. The HHNs with abundant hydroxyl groups on surface were obtained by etching solid ZIF-8 using tannic acid (TA). Benefiting from the hydrophilicity, hollow structure and negative charge of HHNs, the outstanding nanofiltration performance of the composite membrane was achieved. With the assistance of HHNs, the permeance and Na2SO4 rejection of the TFN membrane increased up to 19.4 ± 0.6 L/(m2·h·bar) and 95.2 ± 1.4 %, corresponding to the improvement of 190 % of the permeance and 2.0 % of the rejection compared to the pristine thin film composite (TFC) membrane, respectively. Comparatively, the performance of TFN membranes prepared with solid ZIF-8 only shows 116 % improvements of the permeance with slightly increased salt rejection. More importantly, the antifouling property of the TFN-4H membrane was also elevated. The flux recovery ratios of TFN-4H membrane are 93.2 % and 84.7 % corresponding to humic acid and bovine serum albumin solutions, respectively. The results indicate the nanofiltration performance of the composite membrane was significantly enhanced with the incorporation of HHNs. KEYWORDS: Nanocubes; thin film nanocomposite; hollow structure; hydrophilicity; antifouling properties.

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1. Introduction Inspired by the excellent permeability, selectivity and antifouling performance, the organic-inorganic hybrid thin film nanocomposite (TFN) membrane, combining the intriguing processability of polymers with the excellent performance of nanomaterials, has prevailed in the realm of gas separation, 1 wastewater treatment, 2 solvent reuse 3 and desalination.

4

Fundamentally, the performance of TFN membrane significantly

relies on the property of incorporated nanofillers. To date, a variety of nanofillers, including titanium dioxide, 5 graphene oxide, 6 layered double hydroxide, 7 polypyrrole, 8

polyrhodanine nanoparticles 9 and zwitterionic polymeric nanoparticles, 10 have been

extensively introduced to the polyamide (PA) layer of TFN membrane. With the assistance of the incorporated nanofillers, the performance of the composite membrane was substantively enhanced. Compared with the sole inorganic or organic nanofillers, hybrid nanomaterials are more advantageous for the membrane separation due to their unique components and synergistic effect of compositions.

11-12

For example, the

organic and inorganic segments of polyhedral oligomeric silsesquioxane promote the compatibility between the nanofillers and the PA polymers and hydrophilic of the TFN membranes. 13-14 Owing to the synergistic effect of the tannic acid (TA) and graphene oxides (GO), the chlorine resistances and antimicrobial properties of TFN membranes prepared with TA coated GO were better than that of the sole TA or GO modified membranes. 15 Therefore, it is of great importance to develop newly hybrid nanofillers for elevating the performance of TFN membranes. Metal-organic frameworks (MOFs), a class of porous hybrid materials, consist of inorganic and organic segments, have emerged in catalysis, separation

18

16

energy storage

17

and

at the right moment. The superiority of MOF-based membranes, which

utilizes MOFs as the selective layer or the fillers in gas or liquid separation, has been confirmed. 16 The uniform and porous structure of MOFs serves as the molecular sieve or additional pathway for selective permeation.

19-21

For example, ZIF-8 has been

successfully incorporated into the selective layers of TFN membranes with different strategies and pleasing results were obtained.

22-24

Except for the composition, the

structure of nanofillers is another key role for the composite membrane. Derived from 3


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the internal cavity, the hollow nanofillers in the membrane could greatly decrease the mass diffusion resistance. 25 The water molecules could transfer more freely from the feed side to the permeate side through the hollow space of the fillers compared with the diffusion through the solid ones.

26

Recently, ZIF-8 derived hydrophilic hollow

nanocubes (HHNs), etched by polyphenols such as TA and ellagic acid (EA), 27 were considered to be an enthusiasm, and to have potential applications in multidisciplinary areas.

28

The synthesized HHNs were featured as structural stability, surface

hydrophilicity and internal hollow characteristics.

29

The surface hydrophilicity and

hollow structure of the nanocubes are favorable for the mass transport in separation fields. Correlated with the judicious nanomaterials expected by TFN membranes, it can be envisioned that these hydrophilic, hollow hybrid nanocubes are prone to endow the TFN membrane with enhanced performance. However, as far as our concern, the hybrid HHNs modified TFN membrane has not been reported yet. In this work, the organic-inorganic hybrid HHNs derived from ZIF-8 by etching with TA were firstly incorporated into the polyamide (PA) layer of nanofiltration membrane via interfacial polymerization (IP) approach. Surface properties of the obtained

membranes

were

substantially

characterized.

Membrane

filtration

performances were investigated with the separation of monovalent and divalent salt solutions. To highlight the compositional and structural superiority of the HHNs, the TFN membrane was also prepared with the incorporation of solid ZIF-8 as control. Besides, the antifouling property of the TFN membrane was investigated using bovine serum albumin and humic acid as model pollutants. The results indicate the obtained TFN membrane with the introduction of HHNs possesses enhanced permeability, improved selectivity and ascendant antifouling performance, simultaneously. 2. Experimental 2.1 Materials and reagents Trimesoyl

chloride

(TMC),

(+)-10-camphorsulfonic

acid

(CSA),

2-

methylimidazole (2-MeIM), tannic acid (TA) and Zinc nitrate hexahydrate (Zn(NO3)2·6H2O) were supplied by Aladdin Co., Ltd (China). Cetyltrimethyl ammonium bromide (CTAB) was purchase from Shanghai Titan Scientific Co., Ltd 4



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(China). Triethylamine (TEA), sodium sulfate (Na2SO4), sodium chloride (NaCl), bovine serum albumin (BSA), humic acid (HA) and n-hexane were received from Sinopharm Chemical Reagent Co., Ltd. (China). Polyethyleneglycol (PEG) with different molecular weight (200, 400, 600, 1000, 1500 and 2000) and Piperazine (PIP) were received from J & K Chemical Reagent Co., Ltd. (China). Commercial polysulfone ultrafiltration membrane (PSF, MWCO=20 kDa) was obtained from RisingSun Membrane Technology Co., Ltd. (China). All chemicals above mentioned are analytical grade, and used without further purification. Deionized (DI) water was provided by a Millipore water purification system. 2.2 Preparation of ZIF-8 nanoparticles ZIF-8 nanoparticles were synthesized in room temperature based on the previous work. 30 Briefly, 9 mL of 0.01 M CTAB was dissolved in 200 mL solution containing 21.6 g 2-MeIM under stirring for 15 min. Then another 200 mL solution containing 1.42 g Zn (NO3)2·6H2O was added. The mixed solution was vigorously stirred for 5 min. The resulting emulsion was kept for 3 h without disturbance. After that, the ZIF8 nanoparticles were centrifuged at 11000 rpm/min for 10 min and purified with ethanol for 3 times. The obtained ZIF-8 white powder was dried under 60 oC in oven. 2.3 Preparation of ZIF-8 derived HHNs The preparation of HHNs was followed the procedure proposed by Hu et al.

31

Specially, 600 mg of ZIF-8 nanoparticles was uniformly dispersed in 150 mL DI water under ultrasonic for 30 min, then added into a 150 mL of 10 g/L TA solution. The mixture was stirred for 5 min and centrifuged at 11000 rpm/min for 5 min. The gray products were washed with methanol and water in turn for 3 times. The formation of HHNs is based on synergistic etching and surface functionalization mechanism. When mixing TA with ZIF-8 in solution, a thin coating will be formed on ZIF-8 due to the surface binding affinity of TA. The relatively large molecular size allows TA to block the pores of ZIF-8. The released free protons from TA further etch internal segment of ZIF-8 to form hollow space, leading to the formation of hydrophilic hollow structure. 2.4 Preparation of TFC and TFN membranes The TFC and TFN membranes were fabricated via a typical IP process on 5


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commercial PSF ultrafiltration support. The PSF membrane was fully rinsed with DI water and stabilized overnight. First, the water on the membrane surface was rolled out and air dried for 2 min, then the membrane was taped into a plastic frame. Aqueous solution consists of 0.35 wt% PIP, 2 wt% TEA and 1 wt% CSA was poured onto the membrane surface and saturated for 2 min. Excessive aqueous solution on the membrane surface was rubbed away, and the membrane was kept air dried for another 2 min. After that, the membrane surface was immersed in an organic solution containing 0.1 wt% TMC for 1min. Finally, the membrane was cured at 60oC for 15 min for further polymerization. The fabricated membrane was stored in DI water and maintained overnight. The TFN membranes were synthesized via adding various amounts of HHNs (0.02, 0.04 and 0.06 wt%) into the organic solution during IP process. The resultant membranes were denoted as TFN-2H, TFN-4H and TFN-6H, respectively. To highlight the positive effect of the composition and structure of nanofillers for the TFN membranes, solid ZIF-8 with 0.04 wt% dosage was utilized to prepare the composite membrane (denoted as TFN-4S). 2.5 Characterization methods The morphologies of the prepared membranes were characterized by FESEM (FEI Quanta 250F, USA). The characteristic structures of the nanofillers were analyzed by TEM (FEI TECNAI G20, USA), XRD (Bruker AXS D 8, Germany), and FTIR (Thermo Scientific Nicolet iS5, USA). The zeta potential of the nanofillers was measured by Zeta potential Analyzer (Brookhaven ZetaPALS, USA) at a pH ranged from 3 to 10. The results of potential were averaged from three replications. The particle sizes and dispersion of nanofillers were investigated by DLS analysis (Brookhaven ZetaPALS, USA). The thermogravimetric analysis of the nanofillers and membranes was conducted by thermogravimetric analyzer (TGA, SDT Q600, USA) with a heating rate of 10 oC/min from 30 oC to 950 oC under the nitrogen atmosphere. The N2 adsorption and desorption isotherms and pore sizes of nanofillers were collected by micromeritics (ASAP-2020, USA). The samples were degassed at 250 oC for 240 min. A contact angle measuring system (WCA, Krüss DSA30, Germany) was employed to measure the water contact angle of nanofillers and various membranes. The root mean 6



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square (RMS) roughness of the membranes was investigated using atomic force microscopy (AFM, Bruker MultiMode8, Germany) within a 2 μm×2 μm area by smart mode, and the data were obtained from three regions. Surface charges of the membranes were inspected by zeta potential analyzer (Anton Paar SurPASS 3, Austria) with pH ranging from 3 to 10. Each data is averaged from three results in the same pH value. The chemical compositions of the membrane surface were estimated by X-ray photoelectron spectroscopy (XPS, PHI Quantera II, Japan). The loading of nanofillers in the selective layer of the composite membrane was calculated by measuring the concentration of zinc ions from the digestion experiment of unit area with 1 mol/L hydrochloric acid and sodium hypochlorite. The concentration of zinc ions was analyzed by inductively coupled plasma spectrometry (ICP-OES, PerkinElmer Optima 7000 DV). The values of the concentration are obtained from the average data of three random areas. 2.6 Membrane performance The permeability of TFC and TFN membranes was evaluated by a lab-scale crossflow apparatus at ambient temperature. The feed solution was controlled by a diaphragm pump with an flow of 4.8 L/min (SF-52848-800P, China). The temperature was maintained at 25 oC with the cooling circulation system. The effective filtration area of the cell is 12.56 cm2. The permeance (P) and salt rejection (R) of the membranes were investigated at 6 bar with 1 L of 1 g/L Na2SO4 and NaCl, respectively. Each membrane was pre-pressured at 8 bar with DI water for 0.5 h to stabilize the membrane performance, and then the DI water was replaced by the salt solution. The equations for the calculation of J and R were listed as follows:

P

V S  t  p

R（%） (1 

Cp Cf

) 100%

(1)

(2)

Where V is the obtained water volume (L), S is the filtration membrane area (m2), Δt is the penetration time (h) and Δp is the pressure difference between the feed and permeated solutions. Cp and Cf (g/L) are the solute concentrations of the permeation 7


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and the feed solution, respectively, which were measured by a conductance meter (INESA DDSJ 308A, China). The antifouling properties of the membranes were investigated using 1 g/L Na2SO4 + 1 g/L HA and 1 g/L Na2SO4 + 1 g/L BSA as the feed solutions, respectively. The membranes were compacted under 8 bar for 0.5 h to achieve a steady state, and Na2SO4 solution was represented as the feed solution, the initial permeance (P0) was recorded for 0.5 h at 6 bar. Then the filtration of Na2SO4 solution was lasted for 5 h. Subsequently, a solution containing 1 g/L Na2SO4 and 1 g/L BSA solution or HA solution was served as the feed solution for the antifouling experiment for 5 h at 6 bar. The totally fouled membranes were washed with DI water for 1 h at 2 bar. After that, Na2SO4 solution was replaced as the feed solution again. 2.5 cycle experiments were conducted on each membrane. Normalized permeance ratio Pc and permeance decline ratio Pd were calculated as following equations.

Pc 

Pt 100% P0

Pd （1 -

Pt ）100% P0

(3)

(4)

Where Pt is the permeance at t time. Permeance recovery ratio Pr is equal to Pc mathematically. 2.7 Membrane stability The stability of the TFN-4H membrane was investigated by continuous filtration experiment as well as acid and alkali resistance tests. For the short-time filtration experiment, TFN-4H membrane was pre-pressured with DI water at 8 bar for 0.5 h, then DI water was replaced with 1 g/L Na2SO4 solution. Performance of the membrane was measured versus time at 6 bar for 60 h. To further explore the stability of the composite membrane, performance of the TFN-4H membrane was investigated with filtering Na2SO4 solution at different pH conditions. The permeance and salt rejections were tested during the process. The data was obtained from three independent samples. 3. Results and discussion 3.1 HHNs and ZIF-8 characterization 8



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Figure S1 presents the characterization results of the prepared nanofillers. The HHNs (SEM image in Figure S1a) possess cubic morphology with a little shrinkage on the surface as compared with the solid ZIF-8 nanoparticles (SEM image in Figure S1b). Well dispersed particles with sizes of 100-200 nm have barely changed after etching, which is consistent with the DLS results (Figure S2). The shell thickness of the HHNs is around 18 nm (as marked in Figure S1a). The XRD result reveals that the diffraction peaks of HHNs is well matched with that of ZIF-8, indicating the crystal structure is well maintained after TA etching (Figure S1c). 31 The difference of the N2 adsorption and desorption isotherms (Figure S3) indicates the specific surface area was changed after the etching process. Because of the pore blockage with TA, the pore size analysis data manifests only slight difference between HHNs and the pristine ZIF-8 (Figure S4). These results indicate the internal hollow structure is appeared, whereas the exterior structure of ZIF-8 is maintained after etching process. To further investigate the chemical properties of the HHNs, the FTIR analysis was inspected, as shown in Figure S1d. The characteristic peaks at 1540 cm-1, 1620 cm-1 and 1720 cm-1 of HHNs are corresponding to aromatic C=C stretching, O-H bending and C=O stretching vibration 32-34 from TA, respectively, as marked by the violet arrow, indicating the presence of TA on HHNs. The water contact angle (WCA) of nanoparticles is also measured (inset in Figure S1d). Compared with the solid ZIF-8 (WCA: 122.8 ± 5.4 o), the water drop can not reside on the flake of HHNs, confirming the super hydrophilicity, which is due to the existence of abundant oxygen-containing hydrophilic groups from TA. Furthermore, as presented in the TGA curve (Figure S5), the weight of HHNs and TA decreased sharply during 200 to 300 oC compared with that of solid ZIF-8, which is mainly attributed to the decomposition of oxygen functional groups from the TA. 15 These results apparently indicate the existence of TA on HHNs. Owing to the dual roles of etching and hydrophilic modification from TA, the obtained new filler with hydrophilic hollow structure is expected to endow TFN membranes with enhanced NF performance. 3.2 Membrane characterization 9


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-

Figure 1. SEM and corresponding AFM images of various membranes: TFC (a and d), TFN-4H (b and e) and TFN-4S (c and f) membrane. The surface morphologies of the membranes were investigated with SEM and AFM (Figure 1). As shown in Figure 1a-c, all membrane surfaces present as the nodular structure, which belongs to the typical appearance of the PA layer of nanofiltration membranes.

35

Compared with the TFC membrane, the introduction of nanofillers

generates more small nodular structures on the membrane surface, leading to rougher surfaces than that of the TFC membrane, which is consistent with the AFM results (Figure 1d-f) and cross-section images (Figure S6). The rougher surface may be induced by two aspects: on one hand, the addition of nanofillers in the organic phase could influence the diffusion of PIP during the IP process, leading to a rougher PA layer. 36

On the other hand, the incorporated nanofillers are embedded in or stacked on the

membrane surface, which also contributes to the increment of roughness. During the interfacial polymerization process, once the contact of aqueous and organic solution, the HHNs in the organic phase would promote the diffusion of PIP molecules from the membrane surface to react with TMC, owing to the force such as hydrogen bond and surface free energy. Besides, it can be obtained from Figure S6, the thickness of various membranes is all around 100 nm, indicating the incorporation of nanofillers has negligible effect on the thickness of PA layer.

10



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Figure 2. AFM results of nanofillers on the TFN-4H membrane surface: AFM image (a) and corresponding horizontal and vertical profile of nanofillers (b). The presence of nanofillers on the TFN membrane surface can be demonstrated by the surface and cross-section morphologies (Figure 1 and Figure S6). Owing to the difference between the particle size of HHNs (~150 nm) and the thickness of PA layer (~100 nm), the cubic nanofillers could only be half embedded in or tightly immobilized on the PA polymer instead of fully wrapped in the selective layer. Evidently, the smooth surface of the cubic nanofillers (SEM images of Figure S1a and b) was changed to be rougher, this is because the nanofillers are covered by the typical nodular structures of PA polymer. The abundant functional groups on the HHNs surface could take part in the IP reaction. 8 Therefore, the typical PA polymer was presented on the surface of HHNs. This phenomenon directly proves the polymerization reaction was taken place on the surface of decorated MOFs. The half-imbedding of nanofillers in the PA layer could also be demonstrated by the AFM results of TFN-4H membrane (Figure 2). The nanofillers show a cubic particle on the upper-right corner of Figure 2a. The corresponding particle size (horizontal distance between the red broken line of Figure 2b) is around 150 nm. The vertical distance between the top of the nanofillers and the PA substrate is calculated as ~52 nm (much less than the side length of nanofillers), which confirms the intercalation of nanofillers on the membrane surface. Besides, Figure S7 demonstrates the width of nanofillers were uniformly around 150 nm and the vertical distances are all distributed from 40 to 60 nm, which is consistent with that of Figure 2. 11


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Figure 3. XPS spectra of various nanofillers and membranes. Table 1. Elemental compositions of nanofillers and the surface of various membranes. Sample ID

C (%)

O (%)

N (%)

Zn (%)

C/N

O/N

HHNs

62.5

30.1

4.7

2.7

13.3

6.4

ZIF-8

65.9

9.5

19.7

4.9

3.3

0.5

TFC

70.7

18.8

10.5

0.0

6.7

1.8

TFN-4H

63.9

31.2

4.4

0.5

14.8

7.3

TFN-4S

61.4

31.7

6.2

0.7

15.3

8.9

The elemental compositions of nanofillers and the PA layer of membranes were characterized by XPS as listed in Figure 3 and Table 1. The XPS results were used to investigate the crosslinking degree of the membranes. It can be observed that there is no zinc appearing in the PA layer of the pristine TFC membrane. After the incorporation of HHNs, the content of zinc increases to 0.5 %. Besides, the C/N and O/N ratio of TFN-4H and TFN-4S membranes (excluding element concentration from HHNs and ZIF-8) are higher than those of the TFC membrane. In regard to the PA layer of the nanofiltration membrane, the higher C/N and O/N ratio indicates a lower crosslinking degree. 37-38 Therefore, a relative loose PA structure was obtained with the incorporation of HHNs or ZIF-8 during IP process. 12



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Figure 4. Zeta potential of various nanofillers and membranes. The charge properties of nanofillers and membranes were characterized by zeta potential curves, as shown in Figure 4. The HHNs are negatively charged within the tested pH range, which is quite different from ZIF-8 nanoparticles. This is mainly owing to the presence of abundant phenolic hydroxyl groups from the surrounded TA molecules. 30 Besides, the surface of the TFN-4H membrane possesses more negative charges than that of the pristine TFC membrane, which is ascribed to the exposed or half-embedded HHNs on the membrane surfaces. Meanwhile, the addition of nanofillers influences the IP process as proved by XPS results abovementioned, leading to the hydrolysis of more acyl chloride groups on the PA layer. 8 Thus, more carboxyl groups, providing additional negative charge for the surface of TFN-4H membranes, are generated. The incorporation of positively charged ZIF-8 could justifiably increase the zeta potential of the TFN-4S membrane. However, the surface charge property of the TFN-4S membrane is comparable to that of the TFC membrane. This is because negatively charged carboxyl groups are also generated with the introduction of ZIF-8 during IP process.

20

Therefore, the charge property of the TFN-4S membrane is

determined by the integration of the two factors (incorporated positively charged ZIF8 and increased negatively charged carboxyl groups) and is approximate to the charge property of TFC membrane.

13


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Figure 5. Dynamic water contact angles (a) and MWCO (b) of various membranes. The water contact angle is an important index to estimate the membrane performance. The wettability of the TFC and TFN membranes was examined with the dynamic water contact angle. It can be observed from Figure 5a that, the water contact angle of the TFN-4H membrane is much smaller than that of the TFC and TFN-4S membranes. The minor water contact angle of the TFN-4H membrane is owing to the addition of hydrophilic nanofillers, which promote the wettability of membrane surfaces.

39

Compared with the TFC membrane, the slightly decreased water contact

angle of the TFN-4S membrane may be owing to the rougher surface introduced by the incorporation of ZIF-8 during IP process, 40 which is positive to the wettability of the membrane. The molecular weight cutoffs (MWCO) of the TFC and TFN membranes were investigated by PEG with different molecular weight. As shown in Figure 5b, the rejection rates of PEG are decreased after incorporation of the nanofillers compared with that of the TFC membrane. The MWCO of the TFC, TFN-4H and TFN-4S membranes are 332 Da, 397 Da and 426 Da, respectively, which indicates the selective layer of membranes becomes looser with the introduction of nanofillers. The relative loose structure of the PA selective layer of the TFN membranes is owing to the reduced crosslinking degree, which could be confirmed by the XPS results mentioned above. 8 To accurately determine the actual loading amount of incorporated nanofillers in the composite membrane, the concentration of zinc ions was analyzed using ICP. The measured concentration of zinc ions was only from the selective layer of the TFN 14



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membrane because the nanofillers only exists in or on the PA polymer. The actual amounts of HHNs and ZIF-8 from the TFN-4H and TFN-4S membranes are 0.061 mg/cm2 and 0.049 mg/cm2, which is acceptable for the enhanced permeability of the TFN membrane. 3.3 Separation performance of the TFC and TFN membranes

Figure 6. Performances of various membranes: permeance (a) and salt rejection (b) tested with 1 g/L Na2SO4 and NaCl solution (pH=7) at ambient temperature and 6 bar. Separation performance of the various membranes was investigated with the filtration of 1 g/L Na2SO4 and NaCl solution at 6 bar, and the results were shown in Figure 6a and b. It can be clearly seen that, the permeance of TFN membranes increases significantly as compared with the TFC membrane. The permeance of Na2SO4 and NaCl solutions of the TFN-4H membrane escalates to 19.4 ± 0.6 and 14.5 ± 0.7 (L/(m2·h·bar)) LMH/bar, respectively. The permeance of the TFN-4H membrane for Na2SO4 solution is almost 2 folder higher than that of the TFC membrane. Meanwhile, the permeance of the TFN-4S membrane is much smaller than the permeance of TFN4H membrane. The greatly stimulated water permeance was assisted by the incorporated HHNs through following aspects: (1) the intrinsic hydrophilicity of nanofillers improve the wettability of the membrane surface. 41-42 The water molecules tend to be absorbed into the internal pores of the membrane; (2) the internal hollow space of HHNs supplies preferential flow paths and reduces the mass transfer resistance by shorting the diffusion distance; 43 (3) the generated border area between HHNs and PA polymer provides more channels for water transport from the feed side to permeate 15


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side. 44 Straightforwardly, the results confirm the contribution of the hydrophilicity and internal hollow space of the nanofillers for the membrane performance enhancement. The rejection of Na2SO4 is much higher than the rejection of NaCl on various membranes. This should be ascribed to the negatively charged surface of nanofiltration membrane, which presents stronger repulsive force to high valent anions than to low valent anions.

6

Besides, the larger ion size of SO42- also contributes to the higher

rejection. It can be concluded from Figure 6b, the rejection of both TFN membranes is better than the control TFC membrane. Specifically, TFN-4H membrane exhibits 95.2 ± 1.4 % and 47.4 ± 3.5 % rejection for Na2SO4 and NaCl, respectively. The rejection of the TFN-4S membrane is relatively smaller than that of the TFN-4H membrane. After the incorporation of the negatively charged HHNs, the charge density of the TFN4H membrane surface is upgraded, which enhances the electrostatic repulsion between the PA layer and anions, thereby leading to the considerable rejection of salts. In addition, the micropores of the nanofillers are smaller than the diameter of SO42-, which also contributes to the rejection of salts. Especially, the incorporation of HHNs into the PA layer of composite membrane overcomes the trade-off phenomenon of the conventional membranes. The results of the TFN-4S membrane can be explained by the low charge density of the TFN-4S membrane weakens the repulsive force to the ions. Under this circumstance, based on the charge effect of nanofillers, HHNs are superior to ZIF-8 for the rejection elevation of the TFN membranes. The dose of HHNs was optimized by comparing the performance of the TFN-2H, TFN-4H and TFN-6H membranes as shown in Figure S8. When the dose of HHNs raises from 0.02 % to 0.04 %, the corresponding permeance of Na2SO4 escalates from 14.6 ± 0.5 to 19.4 ± 0.6 LMH/bar, respectively. The increment of the permeance is owing to the elevated dose of HHNs, which enhances the hydrophilicity (as shown in Figure S9) of the membrane and provides more internal water pathways for the transport of water molecules. However, the permeance suffered a little decrease as the dose of HHNs further increased to 0.06 %. The slightly dwindled permeance of TFN6H membrane is mainly caused by the agglomeration of HHNs (SEM image in Figure S10).

39

The increase of the salt rejection may be ascribed to the reinforced negative 16



Page 18 of 27

charge of the TFN membrane (seeing zeta potential results in Figure S11), which enhances the electrostatic repulsion between the PA layer and anions, leading to the enhanced rejection of salts. Besides, the diminutions of the selectivity of TFN membranes are obviated, even at high dose, which indicates no non-selective defects are formed in the PA layer. This exciting result is fostered by the organic attribute and the abundant chemical groups (hydroxyl groups) from the TA molecules of HHNs,

1

which favors the compatibility and generates strong hydrogen bonds between the PA polymer and HHNs. 3.4 Antifouling properties of the TFC and TFN membranes

Figure 7. Time-dependent permeance of various membranes tested with 1 g/L Na2SO4 and 1 g/L Na2SO4 + 1 g/L HA solution (a) and 1 g/L Na2SO4 and 1 g/L BSA solution (b) at ambient temperature and 6 bar. Table 2. Permeance ratios of various membranes during filtration Membrane type TFC TFN-4H TFN-4S

Pc

Pr

Pd

HA

BSA

HA

BSA

HA

BSA

74.1 % 84.9 % 66.6 %

72.1 % 78.7 % 59.7 %

89.4 % 93.2 % 84.7 %

79.0 % 84.7 % 75.0 %

10.6 % 6.8 % 15.3 %

21.0 % 15.3 % 25.0 %

The antifouling properties of the membranes were investigated and the results were presented in Figure 7 and Table 2. The permeance decline is associated with the adsorption and accumulation of pollutants on the membrane surface via hydrophobic and electrostatic interaction and van der Waals force.

45

The antifouling properties of

the TFN membranes are usually vulnerable to be deteriorated owing to the rougher 17


Page 19 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


surface. 10 However, for both feed solutions, the permeance decline ratios of the TFN4H membrane are much slower than those of the TFC membrane and TFN-4S membrane. The enhanced antifouling properties of the TFN-4H membrane may be induced by the improved hydrophilicity and stronger negative charge. The hydrophilicity maximizes the hydration on the membrane surface, which could weaken the interaction between pollutant molecules and membrane.

9

In addition, the

strengthened negative charge of the membrane reinforced the repulsive force to the HA or BSA molecules, which ulteriorly minimized the possibility of adhesion effect. 46 For the TFN-4S membrane, the weak antifouling properties, even poorer than the TFC membrane, are ascribed to the adsorption of HA and BSA to the incorporated ZIF-8. Therefore, owing to the various characteristics of nanofillers, the antifouling property of the TFN-4H membrane was enhanced, while that of the TFN-4S membrane was deteriorated. 3.5 Stability of the TFN-4H membranes

Figure 8. Stability of the TFN-4H membrane: time dependent stability (a) pH stability (b). The stability is a key point for the composite membrane during application process. It can be concluded from Figure 8a that, under continuous filtration for 60 h, the TFN4H membrane only presents insignificant permeance decline with almost unchanged salt rejection. The results indicate the excellent stability of the prepared TFN-4H membrane. In addition, the permeability of the TFN-4H membrane under various pH values was investigated, as shown in Figure 8b. Apparently, the performance of the TFN membrane is stable at the pH ranged from 5 to 9. Under the alkaline condition, 18



Page 20 of 27

the rejection of the membrane slightly decreased. However, the rejection of the membrane suffered drastic decrease at acid condition. This may be owing to the instability of HHNs in strong acid condition, 31 which leads to the formation of defects in the PA layer. Base on the results, the TFN-4H membrane could be applied in neutral or weak acid or alkaline condition. 3.6 Comparisons with other TFN membranes

Figure 9. Comparisons of the separation performance of the TFN-4H membrane with the state-of-the-art TFN nanofiltration membranes. To distinguish the superiority of our TFN membrane, comparisons of the permeability between TFN-4H membrane and other reported TFN membranes were made in Figure 9. Apparently, the permeability of the TFN-4H membrane is pleasing compared with the reported works. The striking performance of our TFN membrane is intensified by the distinctive HHNs. To sum up, the wettability of the membrane is enhanced with the addition of hydrophilic HHNs. Besides, the hollow structure of HHNs provides extra pathway for water diffusion. More importantly, the pressure resistance for conveying water molecules is tremendously slashed by the internal hollow structure of HHNs. 4. Conclusion In this work, a novel TFN membrane was fabricated by incorporating HHNs into the PA layer for desalination. Owing to the enhanced hydrophilicity as well as the internal hollow structure and negative charge of HHNs, the prepared TFN membranes present an excellent permeability of 19.4 ± 0.6 (LMH/bar) with elevated Na2SO4 19


Page 21 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


rejection rates (95.2 ± 1.4 %), which is corresponding to the improvement of 190 % of the permeance and 2.0 % of the rejection compared to the pristine thin film composite (TFC) membrane, respectively. Meanwhile, the performance of TFN membranes prepared with solid and hydrophobic ZIF-8 only shows 116 % and slight improvements of the permeance and rejection, respectively. Compared with the solid and hydrophobic ZIF-8, HHNs are more suitable for the modification of the membranes owing to their hollow and hydrophilic characteristics. Better antifouling property of the TFN membrane was also endowed with the assistance of the incorporated HHNs. Based on the present work, we believe this strategy could also be extended to the development of adjacent reverse osmosis (RO), forward osmosis (FO), organic solvent nanofiltration (OSN) and even gas separation membranes. ASSOCIATED CONTENT Supporting Information Details on the materials used for the experiments, characterization of nanofillers: SEM, TEM, XRD, FTIR of nanofillers (Figure S1). DLS analysis of HHNs (Figure S2). N2 adsorption and desorption isotherms of nanofillers (Figure S3). Pore size analysis of nanofillers (Figure S4). TGA curves of nanofillers and TA (Figure S5). Cross-section images of various membranes (Figure S6). AFM results of HHNs on the TFN-4H membrane surface (Figure S7). Comparisons of the performance of TFN membranes (Figure S8). Water contact angles of TFN membranes (Figure S9). SEM image of TFN6H membrane (Figure S10). Zeta potential of TFN membranes (Figure S11). ACKNOWLEDGEMENTS This work was financially supported by the National Natural Science Foundation of China (Grant No. 51678307). REFERENCES 1.

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Hydrophilic Hollow Nanocube-Functionalized Thin Film

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