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Elevated Performance of Thin Film Nanocomposite Membranes Enabled by Modified Hydrophilic MOFs for Nanofiltration Junyong Zhu, Lijuan Qin, Adam Andrew Uliana, Jingwei Hou, Jing Wang, Yatao Zhang, Xin Li, Shushan Yuan, Jian Li, Miaomiao Tian, Jiuyang Lin, and Bart Van der Bruggen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b14412 • Publication Date (Web): 27 Dec 2016 Downloaded from http://pubs.acs.org on December 28, 2016

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ACS Applied Materials & Interfaces

Elevated Performance of Thin Film Nanocomposite Membranes Enabled by Modified Hydrophilic MOFs for Nanofiltration Junyong Zhu, a Lijuan Qin, b Adam Uliana, c Jingwei Hou, d Jing Wang, a, b Yatao Zhang, b* Xin Li, a Shushan Yuan, a Jian Li, a Miaomiao Tian, a Jiuyang Lin, e Bart Van der Bruggen a* a

Department of Chemical Engineering, KU Leuven, Celestijnenlaan 200F, B-3001 Leuven, Belgium b School of Chemical Engineering and Energy, Zhengzhou University, Zhengzhou 450001, China c Department of Chemical Engineering, The Pennsylvania State University, University Park, PA 16802, USA d UNESCO Centre for Membrane Science and Technology, School of Chemical Engineering, University of New South Wales, Sydney, NSW 2052, Australia e School of Environment and Resources, Qi Shan Campus, Fuzhou University, No. 2 Xueyuan Road, University Town, 350116 Fuzhou, Fujian, China.

*Corresponding authors: Yatao Zhang, [email protected] Bart Van der Bruggen, [email protected]

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Abstract: Metal-organic frameworks (MOFs) are studied for the design of advanced nanocomposite membranes, primarily due to their ultra-high surface area, regular and highly tunable pore structures, and favorable polymer affinity. However, the development of engineered MOF-based membranes for water treatment lags behind. Here,

thin-film

nanocomposite

(TFN)

membranes

containing

poly(sodium

4-styrenesulfonate) (PSS) modified ZIF-8 (mZIF) in a polyamide (PA) layer were constructed via a facile interfacial polymerization (IP) method. The modified hydrophilic mZIF nanoparticles were evenly dispersed into an aqueous solution comprising piperazine (PIP) monomers, followed by polymerizing with trimesoyl chloride (TMC) to form a composite PA film. FT-IR spectroscopy and XPS analyses confirm the presence of mZIF nanoparticles on the top layer of the membranes. SEM and AFM images evince a retiform morphology of the TFN-mZIF membrane surface, which is intimately linked to the hydrophilicity and adsorption capacity of mZIF nanoparticles. Furthermore, the effect of different ZIF-8 loadings on the overall membrane performance was studied. Introducing the hydrophilizing mZIF nanoparticles not only furnishes the PA layer with a better surface hydrophilicity and more negative charge, but it also more than doubles the original water permeability, while maintaining a high retention of Na2SO4. The ultra-high retentions of reactive dyes (e.g., reactive black 5 and reactive blue 2, > 99.0%) for mZIF functionalized PA membranes

ensure

their

superior

nanofiltration

performance.

This

facile,

cost-effective strategy will provide a useful guideline to integrate with other modified hydrophilic MOFs to design nanofiltration for water treatment. 2


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Keywords: hydrophilizing; ZIF-8; interfacial polymerization; nanofiltration; water treatment; MOF modification

1. Introduction

The global challenge of lacking potable water increasingly urges for progress in the pursuit of membrane based nanotechnologies for water desalination and purification. The established superiority of membrane systems over conventional separation technologies lies in the high separation efficiency, low energy consumption, small footprint, and eco-friendliness.1 Nanofiltration, a pressure-driven separation process, is deemed a promising nanotechnology to efficiently fractionate molecules ranging from 200 to 1000 Da or multivalent ions.2-4 Recently, through extensive research studies focused on NF membranes, their surface physicochemical properties such as pore size, charge characteristic, roughness, and hydrophilicity have been properly tailored for a series of specific water treatment requirements, e.g., dye purification,5 water desalination,6 heavy metals removal,7 etc. In view of the membrane performance, solute selectivity and water permeability are ubiquitously recognized as the most paramount characteristics, yet these features are typically subjected to the trade-off effect. In recent years, advanced multifunctional nanomaterials have been introduced for integration with the polymer, continuously break through the upper bound of the flux-selectivity trade-off via physical blending or surface functionalization.8-10

Mixed matrix membranes blended with multifunctional nanomaterials were 3



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successfully fabricated and found to have an elevated water permeability and antifouling properties.11,12 Nevertheless, due to the poor compatibility between hard inorganic nanoparticles and the soft polymer matrix, problems such as aggregation of nanoparticles, poor dispersion, and non-selective interface voids arise frequently and lead to non-improved membranes that have the risk of declined solute selectivity and performance stability. Despite the use of organically modified nanocomposites to optimize the interface compatibility for designing NF membranes with high water flux, the relatively low retention of multivalent salts makes these types of NF membranes unsuitable for water desalination.13-15 As an enhancement of phase inversion formed membranes, the development of thin-film nanocomposite (TFN) nanofiltration membranes through interfacial polymerization rapidly emerges as an efficacious alternative to advancing both permeance and selectivity.16-18 TFN membranes developed to date are integrally composed of an ultrathin polyamide (PA) layer containing functional nanomaterials (e.g., zeolite, GO, CNTs, COFs, MOFs, etc.), and a porous polymer support.3,19-22 The ultimate aim of introducing such nanomaterials is to create additional pathways for increased molecular transport within the PA layer through their intrinsic nanopores and interface voids, which should not distinctly alter the overall selectivity of the membranes. However, the existing problem of the affinity between the polymer and nanomaterials still remains a challenge to develop high-performance TFN membranes.23 In this regard, porous organic frameworks may be outstanding nanofillers to remedy the flaws induced by hard inorganic nanoparticles. 4


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Metal-organic frameworks (MOFs) are promising nanomaterials for various applications in catalysis, molecular sensing, gas storage/separation, etc.24,25 MOFs, a class of hybrid organic–inorganic porous crystalline materials, are formed by integrating inorganic metal-containing ions with organic units by strong bonds. An increasing number of MOF types is developed by varying their morphologies, sizes, and chemical compositions to meet various specific demands.26 In contrast to stiff inorganic materials, a more ingenious design consists in the accurate control of the pore architecture. In this context, MOFs emerged as structurally flexible materials to construct advanced composite membranes for gas and liquid separations, by making use of the precisely controlled pore size and shape of MOFs.27-29 In the light of the architecture of composite membranes, the main advantage of crystalline MOFs over inorganic porous zeolites lies in their better compatibility/affinity with the soft polymer matrix. This allows for the formation of relatively ideal interface voids that do not distinctly sacrifice the solute rejection.30 Consequently, polymer-MOFs membranes have been intensively studied and reported.31 Nevertheless, such merits of MOFs are difficult to be merged into polymers to fabricate NF membranes for water treatment. This is mainly attributed to the hydrophobic nature of most MOFs, which adversely affects the overall performance of such composite membranes, including their antifouling property and water permeability. The hydrophobicity of MOFs also inspires researchers to disperse them into the organic phase; the subsequent polymerization with diamine makes MOFs randomly distributed into the PA layer. More importantly, well-dispersed MOFs in an organic solvent are mostly absent from 5



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the oil/water interface and fail to be entrapped into the PA layer, thus posing a large loss of MOFs. This not only results in an impossibility of resource maximization, but also restricts their latent functionalities to improve membrane surface properties. In this regard, ensuring the existence of MOFs at the water/oil interface (or the surface of support) prior to interfacial polymer is of paramount importance to further optimize the membrane performance.

Among other types of MOF nanomaterials, ZIF-8 is extensively studied because of its better hydrothermal, thermal, and chemical stabilities.32 In this study, the first example of modified ZIF-8 (mZIF) nanoparticles used in the design of TFN nanofiltration membranes for water-related nanofiltration is reported. Poly(sodium 4-styrenesulfonate) solution (PSS), a class of water-soluble polymers,33 is used as an efficient dispersant for fabricating mZIF nanoparticles to avoid aggregation in aqueous solution. The homogeneous dispersion of mZIF in an aqueous solution containing piperazine monomers was applied on top of a hydrolyzed polyacrylonitrile (HPAN) support, followed by a sufficient contact to ensure a maximum mZIF adsorption and deposition. After the removal of the aqueous solution, a gradual evaporation under atmospheric condition was allowed to maximize the exposure of mZIF. The PA nanofilm formed via interfacial polymerization with TMC enables the entrapment of positioned mZIFs.

2. Experimental section

2.1 Materials and Chemicals. Commercially available polyacrylonitrile UF flat sheet 6


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membranes (PAN, MWCO = 100 kDa) were kindly supplied by Ultura (USA). Zinc nitrate hexahydrate (Zn(NO3)2·6H2O, 98%), 2-methylimidazole (Hmim, 99%), sodium hydroxide (NaOH, 98%) and poly(sodium 4-styrenesulfonate) solution (PSS, average Mw = ca. 70,000 Da, 30 wt. % in H2O) were purchased from Sigma-Aldrich. Triethylamine (TEA) was obtained from Acros Organics (USA). Reactive blue 2 (RB2, pure, Sigma-Aldrich) and reactive black 5 (RB5, dye content: >50%, Sigma-Aldrich) were selected as model dyes; their chemical structures are reported elsewhere.15,34 Sodium chloride (NaCl, 99%), and sodium sulfate (Na2SO4, 99%) were purchased from Sigma-Aldrich and were tested for salt permeation. Unless otherwise specified, all solutions were prepared in deionized water.

2.2 Synthesis of ZIF-8 nanocrystals. The ZIF-8 nanocrystals were synthesized via a rapid room temperature synthesis method in aqueous solution.35 In a typical synthesis, 1.466 g of Zn(NO3)2·6H2O (4.93 mmol) was dissolved in 100 mL DI water, and 6.488 g of Hmim (79.03 mmol) and 8 g (79.06 mmol) TEA was dissolved in 100 mL deionized water. The molar ratio of Zn(NO3)2·6H2O, Hmim, and TEA equals to 1:16:16. Afterwards, the two solutions were sufficiently mixed and reacted under vigorous stirring for 10 min. Then another 8 g of TEA was added to the above mixture, which was kept stirring for 1 h. The precipitates were collected after three times of washing with water. After centrifugation, the products were dried in a vacuum oven at 40 oC.

2.3 Hydrophilization of ZIF-8 nanocrystals with PSS solution. The synthesis of 7



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modified ZIF-8 (mZIF) was achieved by mixing ZIF-8 and PSS in aqueous solution. Herein, 2 wt.% PSS aqueous solution was prepared for ZIF-8 modification via diluting a highly-concentrated PSS solution. In detail, 0.2 g ZIF-8 powder was well dispersed in 50 mL PSS aqueous solution under 1 h sonication, followed by a vigorous stirring for 24 h. After keeping the suspension standing for 2 h, the supernatant was collected and centrifuged at 4000 rpm for 10 min. The acquired precipitates were 3 times rinsed with water prior to vacuum drying at 40 oC.

2.4 Support modification. Prior to interfacial polymerization (IP), PAN membranes were hydrolyzed by immersing them into NaOH aqueous solution (2 M) at room temperature for 20 h, subsequently soaking in DI water for 24 h to create hydrolyzed PAN (HPAN) membranes. All the membrane supports used in this study were stored in deionized water before use.

2.5 Preparation of TFN-mZIF membranes. TFN-mZIF nanofilms were constructed on supporting HPAN membranes through interfacial polymerization (Fig. 1). The HPAN support was fixed in a membrane holder, which was reported in detail elsewhere.34 Prior to polymerization, mZIF nanoparticles with different amounts (0.05% w/v, 0.10% w/v, and 0.20% w/v) were evenly dispersed in PIP solution (0.35% w/v) through sonication for 1 h. First, the aqueous suspension (40 mL) was slowly poured into the membrane holder where only the membrane surface can contact with the solution for 20 mins for a sufficient particle deposition, subsequently removing the excess solution by using bibulous paper and superficially drying under a ventilated 8


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environment for ca. 15 min. Secondly, the supports saturated with solution inside were submerged in an organic solution containing 0.20% w/v TMC in n-hexane. After 1 min reaction, the organic solution was removed, followed by a further polymerization at 70 oC for 10 min. Finally, the prepared membranes were rinsed and stored in DI water before use. For comparison, TFC membranes were also fabricated without adding nanoparticles.

2.6 Characterization methods. The chemical structure of ZIF-8 and mZIF was characterized by an FT-IR Thermo Nicolet IR 200 spectrometer (Thermo Nicolet Corporation, USA). TGA measurements were carried out using a TG-DTA, DT-40 system (Shimadzu, Japan). Powder specimens (5 mg) were heated from 30 oC to 650 o

C at 10 oC min-1 in a nitrogen atmosphere under a flow of nitrogen. Powder X-Ray

diffraction (XRD) patterns were recorded at room-temperature with a PANalytical X' Pert Pro diffractometer (PANalytical, Netherlands). The samples for TEM were well dispersed on a copper-supported carbon film. The morphology of ZIF-based nanoparticles was visualized using an FEI Tecnai G2 transmission electron microscope (JEOL JEM 2100F) under 200 kV acceleration voltage.

An ATR-FTIR spectrometer (Perkin-Elmer Spectrum 100, Germany) and PHI QUANTERA X-ray photoelectron spectrometer (XPS, PHI, USA) were used to analyze the surface chemical functionality of as-prepared membranes. The surface morphologies of membranes were inspected by SEM imaging through a Philips Scanning Electron Microscope XL30 FEG (Netherlands). The surface wettability of 9



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membranes was characterized by a contact angle goniometer (OCA20, Dataphysics Instruments, Germany) at room temperature with 45% relative humidity. The zeta potential of the membrane surface was obtained via streaming potential measurements on an electrokinetic analyzer (SurPASS Anton Paar, Austria). A KCl solution (1.0 mM) was chosen as the background solution. HCl (0.1 M) and KOH (0.1 M) were used to adjust the pH over a range of 3–10.

The separation and permeation performance (e.g., water flux, salt retention, and dye retention) of TFC and TFN membranes synthesized in this work were evaluated via a lab-made cross-flow filtration apparatus.34 Typically, to make the NF system stable, membrane coupons (22.9 cm2) were pre-pressurized at 8 bar for at least 30 min. Afterwards, pure water fluxes were recorded under different pressures (2, 4, 6, and 8 bar). The water flux (J) was calculated as J = V/(A·∆t), where V is the permeated volume (L), A is the effective membrane area (m2), ∆t is the filtration time (h). The water permeability (WP, LMH/bar) was calculated as follows: WP = J/∆p, where ∆p is the applied pressure (bar). Salt (NaCl, Na2SO4, 1 g L-1) retention was separately studied at different pressures and room temperature. RB2 and RB5 retention (0.5 g L-1) were tested separately under similar conditions than those used to measure the salt retention at 4 bar. The retention (R) was calculated using the following equation: R = 1 – Cp/Cf, where Cp and Cf are the solute concentrations in the permeate and feed (g L-1), respectively. The salt selectivity, SNaCl/Na2SO4 = (100-RNaCl)/(100-RNa2SO4), was calculated from NaCl and Na2SO4 retention. Herein, the salt concentration was

10


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measured with a conductivity meter (Thermo Scientific Orion Star A212). The concentration of reactive dye was measured with a UV-Vis spectrophotometer (Shimadzu, Japan).

3. Results and discussion

3.1 Characterization of ZIF-8 based nanocrystals.

The intrinsic hydrophobicity of ZIF-8 renders it difficult to obtain a good dispersion in water due to the presence of organic segments. After surface modification with polar PSS, the nanocrystals can be evenly dispersed in water for at least 24 h (see Fig. 2a and insert an image in Fig. 2b). This distinct comparison of particle dispersion implies a favorable integration of PSS with ZIF-8 nanoparticles. PSS, a class of water-soluble polymers, comprises both hydrophilic sulfonate groups and oleophilic organic chains; this can be used as dispersing agent to enable an even dispersion of hydrophobic ZIF-8 in deionized water. Besides, an anionic polymer (PSS) can impart enhanced negative charges to ZIF-8,33 combined with an increased steric hindrance between nanoparticles, thus making them highly stable in aqueous solution.36 Fig. 2b presents the thermogravimetric (TG) curves of ZIF-8 and PSS modified ZIF-8 performed under nitrogen flow. The weight loss of ZIF-8 to ca. 700 oC mainly corresponds to the degradation of organic units including the encapsulated ligand. The difference of mass loss demonstrates that the loading density of PSS onto ZIF-8 is 48.9 mg/g. To further confirm a well-synthesized ZIF-8 and the existence of PSS, FT-IR curves (Fig. 2c) were used to analyze their chemical composition. The 11



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spectrum of the material is generally in agreement with the pattern of ZIF-8 reported in the literature,37 confirming a successful preparation in this study. In view of mZIF, no obvious difference of characteristic peaks compared with ZIF-8 was observed, which indicates that PSS modification does not substantially impact its chemical structure. A new peak at 1037 cm-1 attributed to sulfonate groups confirms that PSS is successfully attached to ZIF-8.15 XRD patterns of the nanocrystals are shown in Fig. 2d. Both ZIF-8 and mZIF match well with the theoretical pattern and data reported in the literature.16,32 This illustrates not only the excellent nanocrystal structure, but also implies that PSS functionalization has no adverse effect on the inherent nanostructure of ZIF-8. In summary, the results demonstrate that ZIF-8 was well synthesized and their modification with PSS was favorably conducted.

Fig. 2

To visually inspect the morphology of the nanocrystals, TEM characterization of ZIF-8 and mZIF was carried out; images with different magnifications are shown in Fig. 3 a-d. The homogeneously distributed nanoparticles were shown to have sharp hexagonal facets with a relatively uniform size, visible with the naked eye. Such nanosized crystal texture was reported in the literature,32 and can be confirmed as ZIF-8. For mZIF, despite the verification that PSS is attached to ZIF-8, there is no apparent distinction in the morphology and size. Thus, the attached PSS is capable of maintaining the intrinsic superior features of ZIF-8. As shown in Fig.3e, the marked samples selected from Fig. 3c with the size from 33 to 83 nm occupy 94%, 12


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demonstrating a narrow size distribution conducive to a rational design of PA film with mZIF nanoparticles. Apart from their aqueous dispersibility, the abundance of the organic component in mZIF enables a good compatibility with PA films prepared by IP and thereby facilitates the formation of a stable TFN layer constructed on a membrane support.

Fig. 3 3.2 Characterization of TFC and TFN-mZIF membranes.

The hydrophilization of ZIF-8 aims to develop hydrophilic TFN membranes for water treatment, with the purpose of an improved water permeability. Thus, mZIFs used as hydrophilic and nanoporous fillers were uniformly dispersed into an aqueous solution containing PIP, followed by firmly binding mZIF into the PA layer through an IP method. FTIR spectra of TFC and TFN-mZIF membranes are presented in Fig. 4a. Compared with the spectrum of the HPAN support, two new peaks at 3399 and 1368 cm-1 that correspond to the N-H band were observed for TFC and TFN-mZIF membranes.38 Additionally, the characteristic peak at 1623 cm-1 is attributed to the stretch vibration of C=O in amide groups,39 strongly evincing a PA film formed on the HPAN support via interfacial polymerization. To further confirm whether mZIF nanoparticles exist in the PA layer, a comparison of FTIR spectra of mZIF, TFC, TFN-mZIF membranes is displayed in Fig. 4b. As shown in this figure, four emerging peaks ranging from 700 cm-1 to 1400 cm-1 associated with the absorption of C-N bonds were found in the spectra of both mZIF and FN-mZIF membranes,25 but not in 13



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that of the TFC membrane, thus demonstrating the successful loading of mZIF nanoparticles on top of the HPAN membranes.

Fig. 4 XPS survey spectra of TFC, TFN-mZIF1, and TFN-mZIF3 membranes are given in Fig. 5a. After being fully hydrolyzed, the nitrile groups of PAN were transformed to hydroxyl groups in HPAN. Thus, the emission peak at 400.8 eV (N 1s) for TFN membranes mainly originates from the established polyamide layer and anchored mZIF-8 nanoparticles. In comparison with TFC membranes, two new characteristic peaks of TFN membranes emerged (Fig. 5b), i.e., Zn 2p3 (1020.6 eV), Zn 2p1 (1043.9 eV) which further confirms the existence of modified ZIF-8 nanoparticles. In addition, the chemical composition and the atomic percentage of the membrane surface calculated from XPS are listed in Table 1. It was observed that the Zn content increases from 0.2% to 0.8% as the added amount of mZIF increases from 0.05% to 0.20% w/v. The resultant Zn content is higher than that of TFN membranes containing ZIF-8,40,41 indicating an effective method of anchored PSS modified ZIF-8. The presence of elemental sulfur that stems from sulfonate groups of PSS indicates that the mZIF nanoparticles were smoothly loaded on the membrane surface.

Fig. 5 Table 1 Surface morphology of membranes. In order to further study the influence of mZIF fillers on the surface morphology of the PA layer, SEM images of the top layer were 14


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made for the TFC and TFC-mZIF membranes (Fig. 6). For the TFC-PIP membrane without adding nanoparticles, a typical nodular structure packed by spherical globules was dispersed evenly and closely on the top layer of HPAN (Fig. 6a, b). The appearance of such morphology is closely related to the initial position, the diffusion rate of PIP monomers, and physicochemical properties of underlying polymer support.42 More concretely, the hydrolyzed PAN support has hydrophilic surface pores so that the meniscus for an aqueous PIP solution remains concave after superficially drying of the membrane surface.43 When contacting with TMC, the ensuing formed polyamide tufts are greatly confined by surface pores of the membrane support, combined with a slow upward diffusion of PIP limited by the hydrophilic support and organic solvent, eventually giving rise to relatively smooth nodular structures.42,44 The surface morphology of the membranes prepared by adding PSS modified ZIF-8 nanoparticles was visibly altered. The emerging reticulate structure makes the surface morphology much rougher compared to the pure TFC membranes, which potentially increases the membrane surface area and thereby enhances the water flux. The better wetting of modified ZIF-8 with anionic PSS enables the adsorption of a certain amount of PIP monomers on its surface; this results in an increased PIP concentration around ZIF nanoparticles on the membrane surface, not just below their surface pores. The PIP monomers exposed outside primarily polymerize with TMC to form small nuclei of polyamide above the pore openings, which further evolves into polyamide tufts in the vicinity of mZIF. The PIP diffusing continuously from membrane inner pores grows laterally into a film that can connect the scattered polyamide tufts.43 The 15



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different growth orientations of tufts and lateral diffusion of PIP may lead to a rough net-like structure. This yields a volcano-like structure (Fig. 6) when PIP erupts from membrane surface pores and penetrates through the incipiently formed film. Additionally, as the mZIF amount increases, the grid within the net-like structure becomes smaller and denser (Fig. 6d, f, and h). This was probably because the enhanced active sites of mZIF can absorb more PIP monomers on the membrane surface. The ensuing formation of more initial polyamide nuclei may eventually make the grid arrangement more close and compact.

Fig. 6 Fig. 7 shows AFM images of the HPAN, TFC, and TFN-mZIF membranes. The TFC membranes have an Rrms (root mean square roughness) of 47.0 nm and Ra (average roughness) of 35.6 nm, which matches well with the data of TFC-PIP membranes reported in the literature.39 After the introduction of mZIF nanoparticles, the surface morphology was completely changed and became much rougher with an overall high Rrms ranging from 56.2 to 71.3 nm. The net-like structure of TFN membranes is also well-reflected in AFM three-dimensional images, which confirms the results acquired from SEM images (Fig. 6c-h). Though the roughly reticular morphology may potentially increase the fouling propensity, the enhanced surface area induced by such morphology is beneficial for a higher water flux.8

Fig. 7 Hydrophilicity and charge characteristics of membranes. Previous studies focus 16


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on the use of ZIF-8 for organic solvent nanofiltration partially because of the hydrophobic nature of ZIF-8. Therefore, the study of the influence of mZIF nanoparticles on the surface hydrophilicity of TFN membranes is of paramount importance for water treatment. Fig. 8a illustrates the water contact angles of HPAN, TFC, and TFN membranes. Hydrolyzed PAN membranes were prepared in this study as a hydrophilic support (31.7o). The goal is to obtain a membrane surface with low roughness by controlling the majority of PIP monomers within the pores and limiting the size of initial PA tufts. The water contact angle of TFC membranes increased to 50.6o due to the formation of a polyamide layer covering the surface of the HPAN support. The improved hydrophilicity of TNF-mZIF membranes was confirmed by the decline of the contact angle to 39.3o and 44.1o when the mZIF amount was below 0.10% w/v. This is mainly attributed to the presence of modified ZIF-8 with polar sulfonate groups on its surface, which facilitates a better water wettability compared to pure TFC membranes.45 Nevertheless, with further increase of mZIF content to 0.20% w/v, water contact angle of TFN-mZIF3 membrane elevates to 61.0o. The decreased surface hydrophilicity possibly originates from the rougher net-like surface, increased surface roughness as confirmed by Table 2, and even the non-uniform distribution of mZIF induced by partial aggregation as the added mZIFs content increase. To study the surface charge properties of the membrane, zeta potential measurements were performed; the results are shown in Fig. 8b. The TFC membranes were found to have a far-ranging negative charge, which is in accordance with that of TFC-PIP membranes reported in other studies.39 The negative charge is strongly linked to 17



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carboxyl groups generated from hydrolysis of acyl chloride groups. More negative zeta potential values were measured after mZIF was introduced, demonstrating that TFN-mZIF membranes are more negatively charged. Multiple anionic sulfonate groups of PSS are present on the surface of the mZIF, causing the nanoparticles to carry a more negative charge density. In conclusion, mZIF nanoparticles make the PA layer not only more hydrophilic, but also more negatively charged.

Fig. 8 Permeation through membranes. Fig. 9 presents the water flux at different pressures of TFC membranes and TFN membranes containing different mZIFs. It was found that all water fluxes increased linearly with the applied pressure, confirming a stable NF system. The water permeability of TFC membranes, based on a linear fit of the experimental data, is 6.94 LMH bar-1. Compared with pure TFC membranes, TFN membranes containing mZIF nanoparticles have an obviously increased water flux. In detail, 0.10% w/v mZIF suspension endows TFN membranes with the highest water permeability at 14.9 LMH bar-1. In addition to the rough surface caused by the net-like structure, this great enhancement in permeability is largely attributed to the combination of the high surface hydrophilicity and the relatively loose PA layer induced by introduction of nanoporous mZIF. Generally, the better compatibility of mZIF with the PA matrix compared to inorganic nanofillers effectively avoids the formation of interface defects (e.g., non-selective voids).30 The generated interface voids between entrapped mZIF nanoparticles and PA macromolecules potentially

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constitute continuous flow channels for transporting water molecules. The high hydrophilicity of mZIF with PSS can absorb water molecules and facilitate their transport through the mentioned interfacial channels.17 Due to the presence of open pores throughout the mZIF, it is suggested that some water molecules partly pass through their pores at high pressure; but this is not the leading factor due to the presence of non-activated mZIF as confirmed by TGA. This also inspires us to explore the influence of fully activated MOFs on the overall performance of TFN membranes in the future work. When the added concentration reaches 0.2% w/v, the water permeability of membranes reduced to 12.0 LMH bar-1. An excess of mZIF used in an IP process will inevitably cause the uneven distribution of nanoparticles on the top layer, thus resulting in a partially compact surface structure. Combined with the decreased hydrophilicity, the water flux of TFN-mZIF3 membranes decreased distinctly.

Fig. 9

Separation properties of membranes. Two types of salts (NaCl, Na2SO4) and two types of reactive dyes were used to explore the NF separation performance of the membranes. Fig. 10 summarizes their retention and permeation flux of TFC and mZIF functionalized TFN membranes. As shown in Fig. 10a, pure TFC membranes generally show high Na2SO4 retention and low NaCl retention, which is due to the combination of strongly negative charge of membrane surface and densely formed polyamide layer. After incorporating mZIF nanocrystals into PA layer, the retention of 19



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multivalent salts slightly decreases to ca. 93% at 4 bar when the mZIF is below 0.10%. This indicates that the introduction of mZIF could maintain a high Na2SO4 retention. Meanwhile, the NaCl retention of TFN membranes obviously decreases due to the assembled interfacial paths, which can also allow the penetration of small chloride ions, thus enabling a higher NaCl permeation. Therefore, the selectivity between Na2SO4 and NaCl for TFN membranes is elevated due to the increased retention differences. However, when the added content is 0.2% w/v, the salt retention decreases distinctly to 87.4%. This is possibly due to the formation of partial interface defects caused by the particles aggregation, which on the other hand this implies that the optimum dosage of added mZIF is 0.10% w/v.

Fig. S1a, b shows that as the applied pressure rises from 2 bar to 8 bar, TFC membranes evince a slight increase of the NaCl retention from 24.1% to 25.3%, with the permeation flux ranging from 10.7 to 46.5 LMH. After incorporation of mZIF, TFN membranes have an overall reduced NaCl retention; the retention reduced from 19.5% to 11.5% at 4 bar as the mZIF concentration increases from 0.05% min to 0.10% w/v. On the other hand, the permeation flux of TFN membranes is boosted. As presented in Fig. S1c, d, the enhanced salt retention with pressure can be explained by the combination of steric (compact pure layer) and electrostatic (negative electric charge) repulsive effects. Nevertheless, each type of TFN membranes shows different levels of retention reduction as a function of applied pressure. Combined with the improved negative charge density of TFN membrane, such minor reduction in Na2SO4

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retention is mainly due to the formation of a relatively loose PA-based matrix structure on top of HPAN. For a traditional TFC NF membrane, the Donnan effect plays an important role in the multivalent salt rejection based on a dense and compact PA nanofilm. In this work, since mZIF nanoparticles are introduced into a pure PA layer, surface properties of TFN membranes involving the morphology, compactness, hydrophilicity, and charge density are apparently altered. TFN membranes have a relatively high retention at low pressure due to a dominant role of the Donnan repulsion effect caused by their strongly negative charge. At higher pressures, pressure-driven forces can compromise electrostatic repulsion to some extent. Herein, the distinct decrease of salt retention in TFN-mZIF3 membranes was probably because most sulfate ions pass through the formed non-selective interface voids that are closely associated with uneven distribution of mZIFs. Among all as-prepared membranes, the TFN-mZIF2 membrane has the highest permeation flux, 57.3 LMH at 4 bar. These results confirm the advantages of TFN membranes: high water permeability and retention of bivalent ions.

Fig. S1

To further study the NF performance of TFN membranes, the retention of two water-soluble dyes (RB5 and RB 2) was measured at room temperature and 4 bar. Fig. 10b shows the effect of different mZIF concentrations on the retention and permeation flux of dye solutions. As described above, the polyamide-based top layer becomes rougher and more loose after the introduction of PSS modified ZIF-8. This net-like 21



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and internal loose layer substantially contributes to promoting a high permeation flux. All synthesized TFC and TFN-mZIF membranes display a high retention for both dyes, which meets the prime separation requirement of NF membranes. This excellent trait is mainly due to the integration of size exclusion of compact PA-based nanocomposite layer and electrostatic repulsion between negatively charged dye molecules and membrane surface. For TFC membranes, the low permeation fluxes of reactive dyes inevitably impede the practical application for dye wastewater treatment. Although a few lower dye retentions were measured than for TFC (RB2: 99.42%, RB5: 99.92%, compared to TFN-mZIF2 RB2: 99.12, RB5: 99.03), the water flux of TFN-mZIF2 enhanced by 199.3% at 4 bar. Furthermore, the solutes retention (RB2, Na2SO4) and corresponding permeation flux of TFN-mZIF2 membranes as a function of filtration time were tested for evaluation of membrane stability (see Fig. S2). During the filtration process, except that small increases in dye and salt retention occurred at the very early stage, the membranes show a relatively stable condition both in retention and permeation flux. This confirms a favorable membrane stability after the introduction of mZIF to polyamide layer via interfacial polymerization. Overall, the obtained high dye retention and permeation flux render TNF-mZIF membranes promising for the treatment of dye wastewater.

Fig. 10 Fig. S2 Comparison to other reported membranes.

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To compare the NF performance of TFN membranes in water permeability, salt and dye retention, Table 3 summarizes a comparison of established commercial or synthesized membranes to TFN membranes prepared in this work. For facile physically blended membranes, the intrinsically loose nanostructure stemming from the decreased interface compatibility and wet-phase inversion gives a high water permeability, but this is at the cost of a significantly reduced rejection of multivalent ions, which makes them not suitable for multivalent/monovalent salt separation. The commonly studied method is interfacial polymerization of PIP and TMC monomers, and the formed PA thin layers enable a superior retention of dye and NaCl/Na2SO4 selectivity (> 10.0). However, the compact nanofilm correspondingly limits the water permeability (6-10 LMH bar-1). In spite of the change of aqueous monomers, pre-treatment of membrane surface improved the NF performance, it still remains a challenge to achieve a satisfactory water permeability. Other surface modifications like cross-linking or coating substantially boost the salt selectivity and dyes retention, yet the water permeability decreased substantially, which would also impede the application of membranes in dye wastewater treatment. Furthermore, the solutes retention (RB2, Na2SO4) and corresponding permeation flux of TFN-mZIF2 membranes as a function of filtration time were tested for evaluation of membrane stability (see Fig. S2). During the filtration process, except that small increases in dye and salt retention occurred at the very early stage, the membranes show a relatively stable condition both in retention and permeation flux. This confirms a favorable membrane stability after the introduction of mZIF to polyamide layer via interfacial 23



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polymerization. Overall, an efficient and facile introduction of porous and hydrophilic mZIF-8 largely improves the water permeability of TFN membranes to 14.9 LMH bar-1, while maintaining a good salt selectivity (13.1) and retention of reactive dyes (> 99.0%). These advantages of TFN membranes over relevant NF membranes indicate a great potential in coping with dye/salt mixtures. Table 3 4. Conclusions

Thin film nanocomposite containing PSS modified ZIF-8 (mZIF) membranes were facilely prepared via interfacial polymerization. As confirmed by a series of characterizations, PSS used as an anionic dispersant not only can allow mZIF-8 to be evenly dispersed in aqueous solution, but also it preserves its original features, thus enabling the design of high-performing TFN-mZIF membranes for nanofiltration. ATR-FTIR, XPS measurement of membrane surface confirmed the presence of mZIF nanoparticles in the PA layer. SEM images show a net-like morphology of the TFN-mZIF membrane surface, which was closely correlated with the hydrophilicity and adsorption capacity of mZIF nanoparticles. The introduction of mZIF to the PA layer greatly improved the surface hydrophilicity and water flux, without sacrificing much of the salt retention. The overall results highlight the promising potential of modified hydrophilic MOFs in the development of TFN nanofiltration membranes for water treatment.

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Fig. 1 Preparation process of thin-film-nanocomposite (TFN) containing PSS modified ZIF-8 nanoparticles via interfacial polymerization

Fig. 2 (a) Photo images of aqueous suspension containing ZIF-8 and PSS modified ZIF-8 (mZIF) nanoparticles after 24 h standing. (b) Thermogravimetric analysis (TGA) curves of ZIF-8 and mZIF, and insert photo images of DI water, and mZIF suspension (0.05 w/v%, 0.1 w/v%, and 0.2 w/v%) after 24 h standing. (c) FTIR spectra of ZIF-8 and mZIF. (d) XRD patterns of ZIF-8 and mZIF powders.

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Fig. 3 TEM images of ZIF-8 (a, b) and mZIF-8 (c, d) in different magnifications. (e) Size distribution of the marked samples (mZIF) selected from figure 2c.

Fig. 4 (a) FT-IR spectra of hydrolyzed PAN, TFC, TFN-mZIF1, TFN-mZIF2 and TFN-mZIF3 membranes. (b) FT-IR spectra of mZIF powders, TFC, and TFN-mZIF2 membranes 26


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Fig. 5 (a) XPS survey spectra of pristine TFC, TFN-mZIF1 and TFN-mZIF3 membranes. (b) Zn 2p core-level spectra of TFC, TFN-mZIF1 and TFN-mZIF3 membranes.

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Fig. 6 SEM images of as-prepared membrane surfaces: TFC (a, b), TFN-mZIF1 (c, d), TFN-mZIF2 (e, f), TFN-mZIF3 (g, h)

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Fig. 7 AFM 3D images of mZIF functionalized membranes: (a) TFC, (b) TFN-mZIF1, (c) TFN-mZIF2 and (d) TFN-mZIF3.

Fig. 8 (a) Water contact angles of hydrolyzed PAN, TFC, and TFN-mZIF membranes. (b) Zeta potential of TFC, TFN-mZIF1, TFN-mZIF2, and TFN-mZIF3 membranes versus pH value

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Fig. 9 Water flux of TFC, TFN-mZIF1, TFN-mZIF2, and TFN-mZIF3 membranes. WP: water permeability

Fig. 10 (a) Salts retention and permeation flux of mZIF modified TFN membranes, (b) Reactive dyes retention and permeation flux of mZIF functionalized TFN membranes (4 bar, 25±2 oC)

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Table 1 Atomic concentrations of different elements for TFC, TFN-mZIF1, and TFN-mZIF3 membranes Membrane ID TFC TFN-mZIF 1 TFN-mZIF 3

Atomic concentrations (%) of different elements C O N S Zn 76.6 12.3 11.1 70.6 16.2 12.7 0. 0.4 2 71.5 15.7 11.2 0. 0.8 2

Table 2 AFM surface roughness parameters of TFC and TFN membranes: Ra (average roughness), and Rrms (root mean square roughness) Membrane ID TFC TFN-mZIF1 TFN-mZIF2 TFN-mZIF3

Ra (nm) 35.6 45.8 43.0 53.1

Rrms (nm) 47.0 56.2 56.7 71.3

Table 3 Performance comparisons between membranes prepared in this work and previously reported NF membranes in water permeability, dye retention, and salt selectivity (NaCl/Na2SO4) . Permeability Dye Membrane Types of Dye α1 Ref. -1 (LMH bar ) retention RB2 99.9 This TFC 6.94 9.6 RB5 99.4 work RB2 99.2 This TFN-mZIF2 14.90 13.1 RB5 99.0 work 46 SPECMs 6.71 Methyl blue 99.9 13.1 4 QPEI-PES 12.56 RB5 97.1 1.2 47 Sepro NF 2A 10.50 Congo Red 99.9 48 TFC-SR2 11.75 RB5 99.0 49 SiO2-PIL-PES 11.63 RB5 95.0 1.1 50 TFC-sericin 11.90 Congo Red 99.8 13.2 51 PSS/PVA–PSF 8.34 Congo red 99.7 26.1 52 TFC-PEI-g-SBMA 13.2 Orange GII 90.1 1.9 53 PA/PD-PES 11.40 10.8 31



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Supporting Information Figures showing salt retention and permeation flux of NaCl and Na2SO4 for the mZIF functionalized TFN membranes, short-term stability of mZIF-8 functionalized TFN membranes

Author Contribution Junyong Zhu, Yatao Zhang and Bart Van der Bruggen conceived the research. The primary experimental work was conducted by Junyong Zhu, Lijuan Qin and Adam Uliana. All authors contributed to manuscript writing and editing.

Notes The authors declare no competing financial interests.

Acknowledgements J. Y. Zhu would like to acknowledge the support provided by China Scholarship Council (CSC) of the Ministry of Education, P. R. China. Ultura, USA, is greatly acknowledged for providing the ultrafiltration membrane samples.

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