polyethyleneimine for nanofiltration

Author’s Accepted Manuscript Nanocomposite Membranes of Polydopamine/Electropositive Nanoparticles/ Polyethyleneimine for Nanofiltration Yan Lv, Yong Du, Zhi-Xiong Chen, Wen-Ze Qiu, Zhi-Kang Xu www.elsevier.com/locate/memsci

PII: DOI: Reference:

S0376-7388(17)31692-7 http://dx.doi.org/10.1016/j.memsci.2017.09.066 MEMSCI15606

To appear in: Journal of Membrane Science Received date: 14 June 2017 Revised date: 25 July 2017 Accepted date: 22 September 2017 Cite this article as: Yan Lv, Yong Du, Zhi-Xiong Chen, Wen-Ze Qiu and ZhiKang Xu, Nanocomposite Membranes of Polydopamine/Electropositive Nanoparticles/ Polyethyleneimine for Nanofiltration, Journal of Membrane Science, http://dx.doi.org/10.1016/j.memsci.2017.09.066 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Nanocomposite Membranes of Polydopamine/Electropositive Nanoparticles/ Polyethyleneimine for Nanofiltration Yan Lv, a,b Yong Du, a,b Zhi-Xiong Chen, a,b Wen-Ze Qiu, a,b Zhi-Kang Xu*, a,b

a

MOE Key Laboratory of Macromolecular Synthesis and Functionalization, b Key Laboratory of

Adsorption and Separation Materials & Technologies of Zhejiang Province, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China

Abstract Nanocomposite membranes (NCMs) provide inspiration to combine the superiorities of inorganic nanomaterials and polymeric matrices for outstanding nanofiltration performance. Herein, novel NCMs have been fabricated via co-deposition of polydopamine (PDA), polyetheylenimine (PEI) and electropositive gold nanoparticles (GNPs) followed by crosslinking. The GNPs distribute in the formed selective layer uniformly without obvious aggregation due to their good dispersion and compatibility with the positively charged

*

Corresponding author. E-mail: [email protected]; fax: + 86 571 8795 1773. 1

PDA/PEI matrix. Thus the selective layer remains defect-free and the surface potential is enhanced by the positively charged GNPs. These endow the NCMs with high retention ratio (>90%) for bivalent cations, such as Mg2+, Ca2+, and various heavy metal ions. Meanwhile, the permeate flux of the NCMs doubles compared with the PDA/PEI co-deposited nanofiltration membranes (NFMs) attributed to the hydrophilicity of the embedded GNPs and the loosened selective layer structures. The compact resistance and the structural stability of the NCMs are also improved effectively in contrast with the PDA/PEI ones, which are of significant importance for the practical use of NFMs. Moreover, the quaternary amine moieties on GNPs improve the antibacterial activity of NCMs against S. aureus and E. coli. Keywords:

Nanocomposite

membrane,

Nanofiltration,

Electropositive

nanoparticles,

Polydopamine, Antibacterial activity

2

1. Introduction Nanofiltration is drawing great attention since the last decades due to its unique advantages of superior retention rate for multivalent ions and organic molecules (200-1000 Da) accompanied with high permeate flux under low operation pressure.[1-3] It has been widely applied in the fields of drinking water purification, seawater desalination and wastewater recycling to meet the increasing demands for clean water.[4-6] Nanofiltration membranes (NFMs) are the most critical parts of the nanofiltration technologies for water treatment. Generally, most high-performance NFMs have been prepared via interfacial polymerization to form thin-film composite (TFC) structures with a selective layer on an ultrafiltration support.[1] In these cases, the nanofiltration performance is mainly determined by the selective layer.[7] However, polymeric membranes are usually suffering from the disadvantages of unsatisfying mechanical, structural, thermal and physicochemical stabilities.[4,8] To address these issues, various inorganic nanomaterials have been incorporated into the polymeric selective layers, aiming to combine the merits of inorganic materials and polymeric matrices as well as to obtain synergistic effects between them.[8-14] For example, Deng et al. introduced porous silica nanoparticles into the interfacial polymerization system to improve the membrane permeability.[15] Shao et al. incorporated titanium dioxide nanoparticles into polypyrrole selective layer through the in-situ hydrolysis of organic precursor to obtain high permeation flux, high rejection and good stability in organic solvent nanofiltration.[46] Inorganic additives can also bestow the nanofiltration membranes with different functions such as anti-bacterial ability and photocatalytic activity.[16,47] Furthermore, plenty of works have been conducted to facilitate the 3

compatibility between the inorganic fillers and the polymeric matrices, which mainly include the surface modification of nanomaterials with reactive moieties, the development of fillers containing organic motifs such as metal organic framework and covalent organic framework, and the utilization of organic precursors for in-suit synthesis of inorganic nanoparticles.[18-23] The aim of all these efforts are to develop nanocomposite membranes (NCMs) with high nanofiltration performance. It is normally that most of the thin-film NCMs are negatively charged because they have been prepared by the interfacial polymerization of diamine and trimesoyl chloride.[9] However, positively charged NCMs is of significant importance for nanofiltration due to their particular applications in water softening and metal ion removing, which is calling for new designs of materials and preparation strategies.[24-27] For example, Tang et al. prepared a positively charged high-performance NFM by solvent casting chitosan with metal organic framework on the top surface of polysulfone ultrafiltration support.[22] Recently, we have developed a highly efficient way to fabricate positively charged NCMs with satisfying nanofiltration performance based on the co-deposition of polydopamine (PDA) and polyethyleneimine (PEI) with silica nanoparticles.[28] In our cases, the PDA/PEI co-deposited layer serves as a dense and adhesive matrix due to the universal adhesive and film-forming ability of PDA, accompanied with the promoted homogeneity and uniform structure endowed by PEI. These characteristics ensure us to prepare NCMs with high nanofiltration performance and structural stability.[29-34] Additionally, the “bio-glue” effect of PDA enhances the interactions between the inorganic nanoparticles and the PDA/PEI matrix.[35-38,48] However, it should be noticed that aggregation may be caused by the 4

electrostatic attraction between the negatively charged fillers and the positively charged PEI molecules. Another issue is that the positive potentials of the NCM surfaces are weakened by the silica nanoparticles. To solve these problems, we report here a facile strategy to fabricate positively charged NCMs via the co-deposition of PDA/PEI with electropositive gold nanoparticles (GNPs) followed by crosslinking. The incorporation of GNPs loosens the selective layer, enhances the hydrophilicity and thus improves the permeate flux of the NCMs. The positive charges of GNPs not only promote the compatibility between GNPs and PDA/PEI matrix via cation- interactions [42] and then the uniform dispersion of GNPs in PDA/PEI, but also strengthen the positive surface potentials of the prepared NCMs. The as-prepared NCMs also exhibit more stable nanofiltration performance compared to those without GNPs during a long-term filtration process, which results from the reinforced structure by rigid GNPs. Furthermore, owing to the adequate quaternary amine moieties on the surface of GNPs, our NCMs also show improved antibacterial activity for S. aureus and E. coli.[44,45]

2. Experimental 2.1 Materials Polyacrylonitrile (PAN) ultrafiltration membranes (UF, molecular weight cut-off is 50 kDa) were obtained from AMFOR Inc. (China). Dopamine hydrochloride, gold (III) chloride trihydrate, 4-mercaptophenol, sodium borohydride, glycidyltrimethyl- ammonium chloride and N,N,N’,N’-tetramethylethylenediamine were purchased from Sigma-Aldrich (USA). Polyethyleneimine (PEI, Mw is 600 Da) was purchased from Aladdin (China). Trypticase soy 5

broth (TSB) and tryptic soy agar (TSA) were acquired from Hangzhou Baisi Biotechnology Co., Ltd. E. coli (ATCC 8739) and S. aureus（ATCC 6538）were obtained from China General Microbiological Culture Collection Center and

China General Microbiological Culture

Collection Center, respectively. Other chemicals, including methanol, isopropanol, acetic acid, sodium hydroxide, hydrochloric acid solution (18 mol/L), ethanol, tris(hydroxymethyl) aminomethane, glutaraldehyde (GA) solution (50 wt% in H2O), and inorganic salts, were procured from Sinopharm Chemical Reagent Co., Ltd (China). All of the chemical agents were used without further purification. Ultrapure water (18.2 MΩ) was produced from an ELGA Lab Water System (France). 2.2 Synthesis and characterization of GNPs GNPs with a diameter of about 5 nm were prepared according to a method reported by Brust et al. (Fig. 1a).[38,39] Firstly, 20 mg gold(III) chloride trihydrate and 15.3 mg 4-mercaptophenol were dissolved in 5 mL methanol, respectively. Then the two solutions were mixed and 30.1 mg sodium borohydride dissolved in 2 mL pure water was added into it with 0.2 mL acetic acid under vigorous stirring at 25 C for 1 hour. After reaction, 250 mL pure water was added into the brown solution to precipitate the gold nanoparticles, which was further washed by water for several times. To modified the GNPs with positively charged groups, the obtained GNPs were re-dissolved in 10 mL isopropanol and reacted with 1.5 mL glycidyltrimethylammonium chloride with 0.75 mL N,N,N’,N’-tetramethylethylene-diamine as catalyst at 60 C for 24 hours.[37] Finally, the GNP dispersion was dissolved in water, neutralized with acetic acid and dialyzed for 3 days to remove residual reactant and catalyst. The obtained modified GNP solution is deep brown and clear. Chemical structures decorated 6

on the GNPs were detected by a Fourier transform infrared spectrometer (Bruker Alpha-T FT-IR instrument, Germany). IR spectra were recorded from KBr pellets in the region of 4000-400 cm-1 and collected by accumulating 32 scans at a resolution of 4 cm-1. The hydrodynamic diameter and surface potential of the GNPs were detected by dynamic light scattering measurement (DLS, Zeta PALS, Brookhaven Instruments Corp., USA). Further transmission electron microscope (TEM, Hitachi 7700, Japan) images were observed at 100 kV by dip-coating GNP solution on copper grids coated with a pure carbon support film. 2.3 Fabrication of NCMs Fig. 1b presents the typically fabrication process of NCMs. Firstly, the UF substrates were hydrolyzed in sodium hydroxide solution (1.5 mol/L) for 60 min at 50 C, and then immersed into hydrochloric acid solution (1 mol/L) at 25 C for protonization overnight. The treated substrates were rinsed by ultrapure water for several times and then immersed in pure water for further use. Dopamine hydrochloride and PEI were dissolved in Tris-HCl buffer solution (pH = 8.5, 50 mmol/L) at same concentration of 2 mg/mL, then GNPs with designed concentration was added to prepare deposition solutions. The UF substrates were immersed into the fresh solution and shaken at 25 C for certain time. The as-prepared membranes (referred as PDA/GNPs/PEI modified membranes) were washed by ultrapure water overnight and crosslinked by glutaraldehyde (2% in ethanol) for 20 min at 50 C as described in literatures.27 Finally, the obtained NCMs were rinsed several times and stored in ultrapure water for further characterization and evaluation. Fig. 1 2.4 Membrane characterization 7

The chemical structures and components of membrane surfaces were characterized by Fourier transform infrared spectrometer-attenuated total reflection spectrometer (FT-IR/ATR, Nicolet 6700, USA) with spectra collected from 400 to 4000 cm-1 by cumulating 32 scans at a resolution of 4 cm−1 and X-ray photoelectron spectrometer (XPS, PerkinElmer, USA) using Al Kα excitation radiation (1486.6 eV), respectively. The surface and cross-section morphologies of the membranes were observed by field emission scanning electron microscopy (FESEM, Hitachi S4800, Japan). And the distribution of GNPs in the selective layer was detected using transmission electron microscopy (Hitachi 7700, Japan). The membranes were dehydrated by graded ethanol solutions and dried in vacuum oven adequately. Then they were fractured in liquid nitrogen for FESEM cross-sectional samples. For TEM detection, the membranes were frozen and cut using cryoultramicrotome (Leica UC7/FC7, Germany) into ultrathin films as cross-sectional samples. Dynamic water contact angles measured by a DropMeter A-200 contact angle system (MAIST VisionInspection & Measurement Co. Ltd., China) in ambient environment were adopted to evaluate the wettability of membrane surfaces. A streaming potential method was utilized to measure the surface potentials of the membrane at different pH values using an electrokinetic analyzer (SurPASS Anton Paar GmbH, Austria) with KCl (1 mmol/L) solution as electrolyte solution. The pH value was adjusted by NaOH (0.05 mol/L) and HCl (0.05 mol/L) solutions. 2.5 Evaluation of nanofiltration performance Nanofiltration performance of the prepared NCMs was tested by a laboratory scale cross-flow flat membrane module with effective membrane area of 7.07 cm2 under 0.4 MPa at 301 C. Generally, the membranes should be pre-compacted under 0.5 MPa for 2 hours 8

before tests. PEG molecules with different molecular weights (200, 400, 600, 1000, 1500 and 2000 Da) and various salts, including MgCl2, CaCl2, MgSO4, Na2SO4 and NaCl, at a concentration of 1000 mg/L were used as solute in aqueous solutions to measure the molecular weight cut-off (MWCO) and nanofiltration performance of the NCMs. The cross-flow rate was fixed at 30 L/h, and the permeate flux (Fw, L/m2h) and rejection ratio (R, %) were calculated by the following equations: 𝑄

𝐹𝑤 = 𝐴∙𝑡

(1)

where Q (L), A (m2) and t (h) are the volume of permeated water, the effective membrane area and the permeation time, respectively. 𝐶𝑝

𝑅 = (1 − 𝐶 ) × 100% 𝑓

(2)

where Cp (mg/L) and Cf (mg/L) are the solute concentrations in permeate and feed, respectively. The concentration of inorganic salt solution was detected by an electrical conductivity meter (METTLER TOLEDO, FE30, China), while the PEG concentrations were determined by spectrophotometry after iodine complexation. In detail, 4 mL PEG solution (< 1000 ppm) was mixed with 1 ml 5% (w/v) BaCl2 in HCl (1 mol/L) solution. Then a solution with 1.27 g I2 in 100ml 2% KI (w/v) was prepared and diluted 10 times, 1 ml of which was added into the above mixed PEG solution. After color was developed for 15 min at room temperature, absorption at 535 nm was read using an ultraviolet spectrophotometer (Shimadzu, UV 2450) against a reagent blank. The exact amount of PEG can be obtained according to the standard curve of absorption-concentration.[28,49] Additionally, the feasibility of using these nanocomposite membranes for heavy metal rejection was assessed with ZnCl2, BaCl2, NiCl2, and CdCl2 as representative metal salts. The solution was adjusted to pH 5.8 using HCl 9

solution (0.1 mol/L) to avoid the hydrolysis of metal ions. The concentration of metal ions in feed and filtrate was detected by inductively coupled plasma mass spectrometry (ICP-MS, XSENIES, USA). The structural stability of the as-prepared NCMs was evaluated according to the performance variation during a long-term operation process. More specifically, the NCMs were continuously tested on the apparatus for 72 h without pre-compacting, meanwhile the permeate flux and salt rejection were real-time monitored. Besides, the concentration of gold with operation time in feed was also monitored by inductively coupled plasma mass spectrometry (ICP-MS, XSENIES, USA) to detect the release of GNPs into water. All experimental results presented were repeated adequately. 2.6 Evaluation of antibacterial activity Antibacterial assay was conducted using S. aureus and E. coli as typical model bacteria. Firstly, S. aureus and E. coli incubated in TSB fluid medium for 12 h to obtain late-log phase bacteria, respectively, which were then centrifugalized and resuspended in PBS. Then circular membrane samples with size of 1 square centimeter were immersed into the diluted bacteria solutions and incubated at 37 C for 48 h. The obtained bacteria solutions were further diluted to 106 folds and transferred on TSA solid plates for another 12 h’s incubation at 37 C. Finally, the amount of viable bacteria was detected and used for calculation of bacteria viability rate. Besides, inhibition zone test was conducted to verify the bactericidal mechanism. The membrane samples were put in the TSA solid plates containing 100 L bacteria suspended solution. After incubation of 12 h at 37 C, the inhibition zone was observed and the sized was measured.

10

3. Results and discussion It can be seen from Fig. 1a, GPNs were synthesized via Brust-Schiffrin method and then modified with glycidyltrimethylammonium chloride. The originally synthesized GNPs are near sphere with a relatively uniform size. After cationization with quaternary ammonium groups, the nanoparticle still maintains in spherical shape but the size increases slightly (Fig. S1 in supporting information, SI). DLS analyses indicate that the original GNPs have a diameter range from 4 nm to 8 nm with a peak value at ~5 nm, while the cationized ones show a broader range of 3 nm ~ 20 nm and the peak value increases to ~8 nm (Fig. 2a). These results are consistent with those of TEM. (Fig. S1 in SI) Furthermore, FT-IR spectrum of the original GNPs shows a wide peak around 3400 cm-1 ascribed to the stretching vibration of O-H, and two peaks arising at 1582 cm-1 and 1490 cm-1 due to the stretching vibration of aromatic ring, which decrease slightly after cationization (Fig. S2 in SI). At the same time, a series of peaks enhance obviously, which include the C-H stretching vibration peaks at 2923 cm-1 and 2852 cm-1, C-H deforming vibration peak at 1438 cm-1 and C-N stretching vibration peak at 1383 cm-1. It is expected that the surface potential is 19.03  2.01mV for the cationized GNPs. Therefore, the positively charged GNPs can be well dispersed in aqueous solutions, making the dopamine/PEI/GNPs solutions maintain clarity rather than turbidity after 6 hours without visible precipitates when the GNP concentration is ranged from 0.05 to 2 mg/mL (Fig. 2b). These results are attributed to the electrostatic repulsion between the cationized GNPs and the positively charged PDA/PEI aggregates. It will be beneficial for the uniform distribution of GNPs in PDA/PEI coatings compared with using electronegative nanofillers, which are co-deposited as the selective layers for nanofiltration [28,29] 11

Fig. 2 PDA/GNPs/PEI coatings were then co-deposited onto the polyacrylonitrile ultrafiltration substrate to fabricate NCMs with different amount of GNPs. FT-IR/ATR spectra were used to analyze the chemical structures of the membrane surfaces. It can be seen form Fig. 3a that the PDA/PEI coating shows absorption peaks at 1665 cm-1, 1580 cm-1, 1560 cm-1 and 1487 cm-1 due to the C=N, N-H and C=C stretching vibrations, respectively.

27

For the PDA/GNPs/PEI

coating, the absorption peaks at 1560 cm-1 and 1487 cm-1 enhance slightly ascribed to the abundant phenyl on the surfaces of GNPs. XPS spectra (Fig. 3b) were further used to prove the chemical compositions of the co-deposited coatings. It can be seen that the binding energy peaks of Au 4d5 and Au 4f7 appear in the spectrum of PDA/GNPs/PEI coating. The atomic percentage of Au changes from 0.47% to 3.49% with GNP concentration meanwhile the O/C ratios increase compared with the PDA/PEI coatings due to the high carbon content on the surfaces of GNPs (Table 1). Furthermore, TEM and SEM were used to analyze distribution of GNPs in the co-deposition layer and morphology of the membrane surface. Fig. 4a shows that GNPs mainly locate near the top surface of the co-deposited layer without large aggregation while no nanoparticles can be observed in PDA/PEI coatings (Fig. S3). SEM images also exhibit no obvious aggregation of nanoparticles on NCM surface even the GNP concentration increases to 0.4 mg/mL, indicating a much uniform distribution of GNPs in the selective layer compared with our previous work (Fig. S4 in SI).[28] The good dispersion of GNPs in PDA/PEI matrix will promote high nanofiltration performance of NCMs. Fig. 3, Table 1, Fig. 4 12

It is well known that the nanofiltration performance of TFC NFMs is mainly dominated by the selective layers. We evaluated the surface wettability of PDA/GNPs/PEI layers by measuring the time-dependently dynamic and static water contact angles (WCA). Fig. 5a shows the ultrafiltration substrate exhibits the best wettability with a WCA value decreasing to 0 in 20 seconds due to those abundant hydrophilic groups and the porous surface structures (Fig. S5). This value is higher than 70o for the prepared PDA/PEI NFMs assigned to the reduced hydrophilic groups and the compacted surface structure. However, the WCA of NCMs decreases to lower than 60o with the concentration of GNPs, revealing enhanced wettability due to the inherent hydrophilicity of GNPs and the loosened surface structures (Fig. S6 in SI). On the other hand, the surface charges also play an important role in the separation performance of NFMs for charged solutes, which will exclude co-ion near the membrane surface due to the Donnan effect.[3] Fig. 5b compares the surface -potentials of the prepared NFMs and substrates measured under different pH values. The isoelectric point of the substrate and PDA/PEI NFMs is pH 4.0 and pH 7.3, respectively. It increases to pH 7.8 for the prepared NCMs, which is higher than similar NCMs containing silica nanoparticles even with PEI grafting.[28] This is attributed to the adequate quaternary ammonium cations on GNPs. Moreover, the surface potential of NCMs at pH 6.0 increases proportionally with the concentration of GNPs for the co-deposited PDA/GNPs/PEI selective layers (Fig. S7 in SI). Therefore, these NCMs can be expected to show high rejection performance for positively charged solutes in nanofiltration process (conducted at pH 6.0). Fig. 5 13

Nanofiltration performance of the as-prepared NCMs was evaluated using a lab-scale cross-flow filtration module in detail. Fig. 6a shows that the rejection for MgCl2 decreases with the concentration of GNPs though the membrane surface potential is enhanced, which should be ascribed to the loosened co-deposition layer structures by GNPs. MWCOs of the NCMs were measured to verify the compactness change of the selective layers. It can be seen from Fig. 6b, the MWCO value increases from 580 Da to over 2000 Da as the concentration of GNPs changes from 0 to 0.4 mg/mL, indicating loose structures at high concentration of GNPs. This is reasonable because of the possible nonselective interfacial voids among the rigid nanoparticle aggregations or in the polymer-filler interfaces.[43] Meanwhile, the permeate flux increases from 31 L/m2 h to 240 L/m2 h with the concentration of GNPs, which is attributed to both the improved surface wettability and the loosened surface structures (Fig. 6b, and Fig. S6 in SI). Therefore, the optimized concentration was chosen at 0.05 mg/mL for GNPs to prepare NCMs for further evaluation. Besides, the variation of nanofiltration performance was also investigated with deposition time. Fig. 6c shows an increased rejection for MgCl2 of the NCMs with deposition time and simultaneously a declined permeate flux. This is because thickness and compactness of the selective layer are both improved with the deposition time, and nearly reaches a steady state after 6 h deposition (Fig. S8, Table S1). Deposition time of 6 h is considered appropriate for the fabrication of NCMs given both the rejection and the permeation performances. The NCMs prepared under optimized conditions were applied to reject different inorganic ions in aqueous solutions. Fig. 6d illustrates that the rejection ratio for MgCl2 reaches above 90% while that for Na2SO4 is lower than 30%. The rejection for different salts follows a 14

sequence of MgCl2  CaCl2 > MgSO4 > NaCl > Na2SO4, which is divinable for the positively charged NCMs according to Donnan effects. Moreover, the permeate flux of NCMs for different solutions reaches nearly 70 L/m2 h, which is twice of the PDA/PEI NFM without GNPs. Besides, it can be seen in Table 2 that the NCMs show high rejections of various heavy metal ions, which should be attributed to the small mean effective pore diameter accompanied by positively charges of the selective layer under the evaluation conditions. Therefore, the NCMs show great potential for metal cations removal in deep treatment of water. Furthermore, we evaluated the structural stability and the compaction resistance of the NCMs, which is critical for the service performance of NFMs in practical operation. Fig. 7a illustrates that the NCMs exhibits stable permeate flux and high rejection capability for MgCl2 during an operation of 72 h, revealing an excellent structural stability. In contrast, the permeate flux declines by about 25% for the PDA/PEI NFMs without GNPs during the whole filtration process, accompanied by slightly increased rejection ratio (Fig. 7b). It could be inferred that the improved stability of the NCMs is attributed to the enhanced rigidity by GNPs.[28] Additionally, there is scarcely any GNPs releasing into the solution during the long-term filtration, indicating robust incorporation of GNPs in the selective layer and thus avoid secondary pollution to the treated water (Table S2). Fig. 6, Fig. 7, Table 2 In addition, quaternary amine cations are also known for their bactericidal ability, so the antibacterial function of the NCMs was evaluated against typical S. aureus and E. coli.[44,45] Fig. 8a shows the pristine PAN substrates exhibit no biocidal capability. The PDA/PEI NFMs provides about 15% and 25% reduction of S. aureus and E. coli, respectively. In contrast, the 15

NCMs show over 50% reduction of both bacteria, revealing much improved antibacterial activity. Inhibition zone test was also conducted but no obvious inhibition zone can be observed for all membranes. These results indicate that the antibacterial property is derived from contact inhibition of quaternary amine moieties on the membrane surface rather than the release of GNPs into culture (Table S2). Though the biocidal capability of NCMs is limited due to the small amount of GNPs in selective layer and their cladding by PDA/PEI matrix, this improved surface antibacterial function is conducive for antimicrobial fouling and thus lifespan in the practical nanofiltration process (Table S3). Fig. 8

4. Conclusion Novel positively charged NCMs are fabricated via co-deposition of PDA/GNPs/PEI onto the ultrafiltration substrates followed by crosslinking. The electropositive GNPs disperse uniformly in the selective layers and enhance the surface potential of the NCMs. Thus their rejection ratio for bivalent cations (e.g. MgCl2) is higher than 90% while the value is lower than 30% for bivalent anions (e.g. NaCl). The NCMs also shows great capability for heavy metal ions removal from water with the combination of both Donnan and size exclusion effects. Permeate flux of the NCMs is doubles compared with those without GNPs. The structural stability of the NCMs is also improved effectively during the long-term filtration owing to the inherent rigidity of the GNPs and the good compatibility between GNPs and PDA/PEI matrix. Additionally, the quaternary amine moieties on GNPs endow the NCMs with improved antibacterial activity. In summary, we propose here an effective approach to 16

fabricate positively charged NCMs for enhanced nanofiltration performance.

Acknowledgements Financial support is acknowledged to National Natural Science Foundation of China (Grant no. 21534009). The authors thank Prof. Jian Ji and PhD. Deng-feng Hu in Zhejiang University for the antibacterial experiments of the prepared membranes.

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Table 1. Surface chemical compositions of different membranes calculated from XPS spectra (in atomic percent). Concentration unit of GNPs is mg/mL. Sample

C 1s (%)

O 1s (%)

N 1s (%)

Au (%)

O/C

PDA/PEI NFMs

72.89

19.08

5.89

0

3.82

NCMs-0.05

72.47

18.75

7.56

0.47

3.87

NCMs-0.1

73.58

16.52

6.92

1.87

4.45

NCMs-0.2

72.52

15.65

7.09

2.81

4.63

NCMs-0.4

72.62

14.45

8.34

3.49

5.03

Table 2 Rejection performance for metal ions. Metal salt (ion)

Rejection (%)

Hydrated diameter of cation (nm) [50]

ZnCl2

88.3

0.860

BaCl2

90.6

0.808

NiCl2

90.4

0.808

CdCl2

86.8

0.852

20

Fig. 1. Schematic diagram of (a) synthesis approach of positively charged GNPs and (b) fabrication and nanofiltration process of the prepared NCMs. Fig. 2. (a) Size distribution of the synthesized GNPs. (b) Photographs of PDA/PEI solutions with the positively charged GNPs at different concentrations. The unit of the insert is mg/mL and the solution was diluted to quadruple after standing for 6 hours. Fig. 3. (a) FT-IR/ATR and (b) XPS spectra of different membrane surfaces. Concentration of GNPs for NCMs fabrication is 0.4 mg/mL. Fig. 4. Typical TEM images from the cross-section of the selective layer for the prepared NCMs. Concentration of GNPs for NCMs fabrication is 0.4 mg/mL. Fig. 5. (a) Dynamic water contact angles and (b) -potentials at different pH values of the substrate and the nanofiltration membrane surfaces. Concentration of GNPs for NCMs fabrication is 0.05 mg/mL. Fig. 6. (a) Effect of GNPs concentration on nanofiltration performances of the NCMs (MgCl2 as solute). (b) MWCOs of the NCMs with GNPs at different concentrations. (c) Effects of deposition time on nanofiltration performances of the NCMs (MgCl2 as solute). (d) Nanofiltration performances of the NCMs for different salts. Fig. 7. Long-term operation test of the (a) NCMs and (b) PDA/PEI NFMs. MgCl2 is used as solute. Fig. 8. (a) Antibacterial performance of the PAN substrates, PDA/PEI NFMs and NCMs against S. aureus and E. coli. Photograph of inhibition zone experiments of the PAN substrates, PDA/PEI NFMs and NCMs against (b) S. aureus and (c) E. coli. Concentration of GNPs for NCMs fabrication is 0.4 mg/mL.

21

Figure 1

(a)

(b)

22

Figure 2

23

Figure 3

24

Figure 4

Figure 5

25

Figure 6

26

Figure 7

27

Figure 8

(b)

(c)

28

Table of Contents Graphic

Highlights  Nanocomposite membranes are fabricated by the co-deposition of PDA/GNPs/PEI.  GNPs distribute uniformly in the co-deposited selective layer for nanofiltration.  The enhanced surface potentials guarantee high retention ratio to bivalent cations.  Structural stability and antibacterial activity are effectively improved for the membranes.

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