ZwitterionâAg Complexes That Simultaneously Enhance Biofouling

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Zwitterion-Ag complexes that simultaneously enhance biofouling resistance and silver binding capability of thin film composite membranes Ming Yi, Cher Hon Lau, Shu Xiong, Wen-Jie Wei, Rong-Zhen Liao, Liang Shen, Ang Lu, and Yan Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 15 Apr 2019 Downloaded from http://pubs.acs.org on April 15, 2019

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ACS Applied Materials & Interfaces

Zwitterion-Ag Complexes that Simultaneously Enhance Biofouling Resistance and Silver Binding Capability of Thin Film Composite Membranes

Ming Yia,b, Cher Hon Lauc, Shu Xionga,b, Wenjie Weia,b, Rongzhen Liaoa,b, Liang Shena,b, Ang Lud, and Yan Wanga,b*

a

Key Laboratory of Material Chemistry for Energy Conversion and Storage

(Huazhong University of Science and Technology), Ministry of Education, Wuhan, 430074, P.R. China b

Hubei Key Laboratory of Material Chemistry and Service Failure, School of Chemistry and Chemical Engineering, Huazhong University of Science & Technology, Wuhan, 430074, P.R. China

c School

of Engineering, University of Edinburgh, Robert Stevenson Road, Edinburgh EH9 3FB, UK

d School

of Life Science and Technology, Huazhong University of Science & Technology, Wuhan, 430074, P.R. China

* Corresponding author. Tel.: 86 027-87543032; fax: 86 027-87543632. E-mail address: [email protected] (Yan Wang)

KEYWORDS: Nanofiltration; antifouling; zwitterion modification; metallization; mineralization

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ABSTRACT Biofouling can be overcome with zwitterion grafts and anti-microbial, metallic nanoparticles. However, the mechanism underpinning this effective approach remains unclear. To elucidate the role of each component in this system whilst maximizing membrane anti-fouling and anti-microbial properties, here we performed a comparative study to investigate the impact of zwitterion type and their interactions with Ag of various states. Two different zwitterions (SO3--based and COO--based) were employed to modify polyamide (PA) thin film composite (TFC) membranes, and the metallized and mineralized membranes were developed via in-situ formation of silver (Ag) nanoparticles and deposition of silver chloride (AgCl) particles on the zwitterion-modified TFC membranes. The presence of zwitterions was key to enhancing Ag content, resulting in significantly improved anti-microbial and anti-fouling

properties

without

compromising

the

nanofiltration

separation

performance. COO--based zwitterions were found more favourable towards Ag metallization and mineralization compared to SO3--based zwitterions. The underlying mechanisms underpinning this discovery were further revealed using the Density Functional Theory (DFT) to reveal Gibbs free energy of the binding between zwitterions and Ag+ ions. This fundamental knowledge is crucial for designing next-generation anti-biofouling strategies.


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Introduction Fouling is an omni-present problem plaguing polymer membrane separation during gas purification, solvent recovery, and water treatment. Fouling often leads to reduced separation efficiency as membrane pores are blocked by cake formation,1 organic adsorption,2 inorganic precipitation or biofouling.3-4 Amongst various fouling processes, biological or microbial fouling that involves complex physicochemical and biological mechanisms remains the most difficult to resolve. The adhesion of biological substances and proliferation of microorganisms often form a stubborn biofilm that underpins flux reduction and consequently lower product quality, shorter membrane lifespan whilst increasing energy consumption.1, 5-6 Clearly, there is a need to eradicate the problem of membrane fouling.

Efforts to minimize membrane fouling can be categorized into two main approaches: (1) optimization of the feed composition and operation conditions,7 and (2) tailoring physico-chemical properties of membrane surfaces to imbue anti-fouling properties.8-12 The latter approach is preferred as this will require minimal changes to existing protocols and set-ups.6 For example, rough and hydrophobic surfaces of polyamide-based thin film composite (TFC) membranes that are extremely prone to fouling are modified with functional compounds to minimize the build-up of biomatter, hence inhibiting fouling.10,

13

This

approach can be further classified as active or passive strategies.6 Active strategies focus on eliminating proliferative fouling with anti-microbial agents to inactivate cells,14-17 while passive approaches involve introducing of hydrophilic or low surface energy materials on to membrane surfaces to reduce fouling and facilitate foulant release. 18-19

A common active strategy to inhibit biofouling is to incorporate silver-based (Ag) nanoparticles that are known to possess strong anti-bacterial activity against a broad spectrum



of micro-organisms via direct blending,20-21 chemical immobilization,22-23 or in situ formation of Ag nanoparticles during membrane formation.24 Though effective for preventing the build-up of biofilms on membrane surfaces, active strategies are inefficient to prevent the accumulation of dead bacteria on membrane surfaces. Moreover, the hydrophobicity of Ag-based nanoparticles also diminishes anti-fouling properties. Hence, a combination of both the active and passive strategies is preferred. This is best demonstrated by incorporating Ag nanoparticles modified with hydrophilic compounds on to TFC membranes, and in situ formation of Ag nanoparticles on TFC membranes pre-grafted with hydrophilic anchors such as graphene oxide nanosheets,25-26 polydopamine coatings and zwitterions. 27-29

Compared to other hydrophilic anchors, zwitterions can also form a dense hydration layer to restrict foulant adhesion,30-31 whilst immobilizing Ag+ ions via the terminal group for subsequent formation of Ag nanomaterials. Hence, zwitterions offer a route towards combining both active and passive strategies to overcome biofouling polymer membranes. This combinatory strategy is best demonstrated by simultaneous grafting of zwitterions and Ag nanoparticles on to membrane surfaces. For example, zwitterionic poly(sulfobetaine methacrylate) were grafted on to membrane surfaces to facilitate the formation of Ag nanoparticles.29 Anti-fouling properties of such membranes surpass those of conventional membranes modified by a single strategy. However, the underlying mechanism of zwitterions and Ag+ interaction and the effect of zwitterion structure on Ag nanoparticles formation remain unclear.

To elucidate these mechanisms, here we modified conventional polyamide TFC membranes with two different zwitterions containing SO3- and COO- functional groups. Using complimentary characterization techniques, we were able to differentiate the impact of two


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modification routes – Ag metallization and Ag mineralization during complexation between the zwitterion and Ag+ ions. In metallization, Ag+ was reduced into Ag (0) nanoparticles with NaBH4, while the approach of mineralization was based on the reaction between Ag+ and NaCl to form AgCl nanoparticles. For the first time, this was validated with theoretical calculations of the Gibbs free energy of the binding between zwitterions and Ag+. This was employed to maximize anti-fouling and anti-microbial properties in polyamide thin film composite membranes studied here, and their impact on nanofiltration separation performances.

EXPERIMENTAL SECTION

Materials Polysulfone (PSf, Mw=800,000 Da) was obtained from Beijing HWRK Chem Co., Ltd. (China), and dried in a vacuum oven at 80 °C for overnight before use. Piperazine (PIP, 99.5%), 1, 3, 5-benzene tricarbonyl trichloride (TMC, 98%), 1,3-propane sultone (PS, 99%) and 3-bromopropionic acid (3-BPA, purity 98%) were purchased from Aladdin Industrial Corporation and stored in the refrigerator before use. N,N-diethylethylenediamine (DEDA, 99%) was supplied by Xiya regent research center. Polyethylene glycol 400 (PEG 400, CP), N-methyl pyrrolidone (NMP, anhydrous, 99.5%), n-hexane (97%, anhydrous), concentrated hydrochloric acid (HCl, 37%), disodium hydrogen phosphate dodecahydrate (Na2HPO4 ∙ 12H2O, 99%), sodium hydroxide (NaOH, 99%), potassium dihydrogen phosphate (KH2PO4, 99%), potassium chloride (KCl, 99%), glutaraldehyde (99%), silver nitrate (AgNO3, 99%) and nitric acid (HNO3, 65%) were all purchased from Sinopharm Chemical Reagent Co., Ltd. Sodium borohydride (NaBH4, 98%) was obtained from Fisher Scientific (Fair Lawn, NJ). Bovine Serum Albumin (BSA) was provided by Shanghai Yuanye Bio-Technology Co., Ltd.



Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus) were obtained from Beijing ComWin Biotech Co., Ltd. Luria-Bertani (LB) power was supplied by Thermo Fisher Scientific. All reagents were of analytical grade and used as received.

Preparation and modification of NF membranes PSf substrate was prepared by casting a dope solution (18/16/66 PSf/PEG400/NMP wt%) on a glass plate with a casting knife (3520-8, Elcometer, UK) of 150 m at room temperature, which was then transferred immediately into a water coagulation bath at room temperature for phase inversion. The as-fabricated PSf susbtrate was stored in DI water before use.

The control TFC membrane was fabricated via a conventional interfacial polymerization (IP) method. The PSf substrate was immersed into 0.2 wt% PIP aqueous solution for 5 min. After removing excess PIP solution from the membrane surface with a rubber roller, TMC hexane solution (0.15 wt%) was poured on top of the PIP-saturated PSf substrate. Within 1 min of IP reaction, an ultrathin PA film was formed.

Zwitterion-grafted TFC membranes were prepared as reported in previous works (Fig. S1).32-33 Briefly, the pristine TFC membrane was first placed into a DEDA aqueous solution (2.0 wt%, pH=4.0) at 40 °C for 20 min, resulting in DEDA-grafted TFC membrane by an amidation reaction between DEDA amide groups and acyl chloride groups on the PA membrane surface. Subsequently, this membrane was immersed in excess PS aqueous solution (pH=7.0) or 3-BPA aqueous solution (pH=4.0) for 6 h at 30 °C to graft SO3--based or COO--based zwitterion onto the surface of TFC membranes. The as-prepared zwitterionic TFC membranes were washed thoroughly with DI water (pH=6.5-6.7) prior use.


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Membrane modifications of metallization and mineralization Following a reported protocol,29 we metallized membranes decorated with zwitterions by in-situ synthesis of Ag nanoparticles (Fig. 1). Briefly, the surface of TFC membranes were exposed to an AgNO3 solution (15 mL, 5 mM) for 10 min to immobilize Ag+ ions, and washed with DI water to remove excess solution. Afterwards, a NaBH4 solution (15 mL, 5 mM) was employed to reduce nucleated Ag+ ions for subsequent in situ formation of Ag nanoparticles on the membrane surface. After 5 min, the NaBH4 solution was discarded and the Ag-loaded membranes were cleaned with DI water.

Mineralization of TFC membranes decorated with zwitterions was performed with an alternate soaking process according to literature.34 The membrane sample was sequentially immersed in a 5mM AgNO3 aqueous solution, DI water, a 5 mM NaCl aqueous solution and a final soak in DI water. Each step lasted for 60 s at 25.0 °C. The above procedures were repeated twice to complete the mineralization. Finally, the samples were rinsed thoroughly with DI water and stored away from light.



Fig. 1 (a) Thin-film composite membranes comprising selective layers of polyamide (yellow) and porous support (grey), (b) membrane functionalized with zwitterions (green), followed by (c) Ag ions (silver) deposition via immersion in AgNO3 solution. These membranes were immersed in different solutions to obtain (d) metallized membranes that contain Ag nanoparticles (purple) and (e) mineralized membranes that contain AgCl nanoparticles (black).

Membrane characterizations All membranes were dried in vacuum oven before characterizations. The chemical property of the membrane surface was investigated by an attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR, Brucker VERTEX-70, Germany) with a range of 650– 4000 cm-1 and a resolution of 2 cm-1. The formation silver-based nanoparticles was evaluated


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by X-ray diffraction (XRD, SmartLab-SE). The elementary composition of the membrane surface was analyzed by an X-ray photoelectron spectroscopy (XPS, VG Multilab 2000, Thermo VG Scientific, UK) using a monochromatic A1 Ka as the radiation source. The membrane surface morphology was observed by a Scanning electron microscopy (SEM, Tescan VEGA 3 SBH). Hydrophilicity of the membrane surface was evaluated by measuring water contact angles (WCA) with a Contact angle goniometer (DSA 25, KRÜSS, Germany) using the sessile drop method at the ambient temperature, and at least 8 points were taken for each membrane to calculate the average data. The charge property of the membrane surface was measured by an Electrokinetic analyzer (Anton Paar GmbH, Austria). 1 mmol KCl aqueous solutions were used to determine the zeta potential of the membrane with pH ranging from 3.0 to 10.0 at 25.0 °C.

Computational details Density functional calculations were performed to calculate the Gibbs free energy of the reaction between Ag+ ions and different zwitterions. Gaussian09 program with the hybrid B3LYP-D3 functional which included Grimme’s empirical D3 dispersion correction (with the default D3 damping function) was used here for the calculations.35-38 Geometry optimizations were carried out with the def2-TZVP basis set for all elements.39 The final and solvation energies in water (ε=80) were obtained by performing single-point calculations using SMD continuum solvation model with a larger def2-TZVPP basis set.40-41 Analytic frequency calculations were performed at the same level of the theory as the geometry optimizations to obtain Gibbs free energy corrections. The energies reported herein (B3LYP-D3) include Gibbs free energy corrections, solvation corrections, and D3 dispersion corrections.



Membrane NF performance NF performances of TFC membranes were characterized using a lab-scale cross-flow filtration setup at 25.0 °C with an effective area of 7.07 cm2. All membrane samples were pre-filtrated using DI water as the feed solution at 0.3 MPa for 1 h to reach a steady state before testing. Then the membrane performance was examined with 1000 ppm NaCl, Na2SO4, and MgSO4 aqueous feed solutions under 0.2 MPa, with a cross-flowrate of 80 L/h. The water flux (Jw) and salt rejection(R) were calculated by Equations (1) and (2)，

𝑉

𝐽𝑤 = 𝐴∆𝑡 (1)

(

)

𝐶𝑝

𝑅 = 1 ― 𝐶𝑓 × 100% (2)

where V is the volume of the permeated water (L), A is the effective membrane area (m2), and Δt is the permeation time (h); The parameters Cp and Cf are the concentrations of the permeate and feed solution, respectively. The salt concentration was obtained by measuring the conductivity of the aqueous solution with a conductivity meter (DDS-11A, Hangzhou Dong Xing Instrument Plant, China). At least three samples were examined for each membrane to obtain the average result.

Membrane anti-fouling property Fouling resistant property of membranes was evaluated by a quantitative static protein adsorption test. At room temperature, a membrane sample with an effective area of 8 cm2 was immersed in the phosphate buffer solution (PBS, 0.24 g KH2PO4, 3.63 g Na2HPO4·12H2O, 8 g NaCl, 0.2 g KCl and 1 L DI water) for 2 h and then in 10mL BSA solution (1000ppm, in PBS solution) for 5 h. The BSA concentrations before and after the membrane adsorption


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were detected by a UV–vis spectrometer (AOE INSTRUMENTS, UV–1800PC, China) at wavelength of 278nm. The absorption capacity was calculated by Equation (3),

𝐴𝑑𝑠𝐵𝑆𝐴 =

(𝐶1 ― 𝐶2) ∙ 𝑉𝐵𝑆𝐴 𝐴

(3)

where Ads is the protein absorption capacity (g/m2), C1 is the initial concentration of BSA solution (g/L), C2 is the concentration of BSA solution after the membrane adsorption (g/L), and VBSA is the volume of BSA solution (L).

The long-term NF test was also carried out with 1000 ppm BSA feed solution (pH=7.0, PBS solution) to investigate the long-term anti-fouling behavior of membranes. After stabilization, the membrane was filtrated with DI water for 200 min to test the initial water flux, and then the feed solution was replaced with BSA foulant solution to conduct the fouling test for 700 min. Subsequently, a water cleaning was conducted for three times before the water flux was tested again with DI water feed solution for 200 min.

Membrane anti-microbial property The anti-microbial property of the TFC membranes was evaluated by the disk diffusion method using E. coli and S. aureus as typical Gram-negative and Gram-positive bacteria. In short, the bacteria were cultured in 5 mL (25 g/L) fresh liquid LB in an incubator at 37 °C with continuous shaking (250 rpm) for overnight to reach an optical cell density at 600 nm (OD600) of 0.5, and then were diluted to approximately 107 colony forming unit (CFU) mL−1. After the above solution (100 μL) reinoculated onto the fresh solidified LB agar by a uniform spread, the disinfected membrane sample (diameter = 12 mm) was placed (face down) on the surface of LB agar and incubated at 37 °C for an overnight. The inhibition zone near the



membrane sample was then observed and recorded by an automatic digital gel imaging analysis system (Tanon 2500, China). The CFU counting method was employed for quantitative assessment of the anti-microbial abilities of metallized membranes following the reported protocol.23 Briefly, the disinfected membrane sample was immersed in the suspension of 4.9 mL liquid LB with 100 μL well-cultured (OD600=0.5) bacterial solution, and subsequently incubated at 37 °C in an incubator shaker (250 rpm) for an overnight. Upon removing the membrane sample, the remaining suspension was diluted and spread on LB agar plates. CFU content was then determined after overnight incubation at 37 °C. The dynamic biofouling tests with bacterial feed solution were carried out with simulated synthetic wastewater containing S. aureus. The composition of synthetic wastewater (pH 7.5) was as follows: 1.16 mM C6H5Na3O7·2H2O, 0.94 mM NH4Cl, 0.45 mM KH2PO4, 0.5 mM CaCl2·2H2O, 0.5 mM NaHCO3, 2.0 mM NaCl and 0.6 mM MgSO4·7H2O.42 S. aureus suspension was then added into the feed solution with bacterial concentration of 107 CFU L−1. The membrane was pre-filtrated with DI water at 0.3 MPa and then filtrated with bacterial feed solution at 0.2 MPa to conduct the biofouling test for 720 min. The temperature of solutions was kept at 25 ± 0.3 °C.

Silver stability Silver release test was conducted to investigate the stability of loaded Ag nanoparticles on the metallized membrane surface. The membrane sample with an effective area of 8 cm2 was soaked in a centrifuge tube with 10 mL ultrapure water on a shaking platform at room temperature. Then the membrane sample was taken out and soaked in 10 mL fresh ultrapure water every two days. The ultrapure water after the membrane immersion was collected to determine the released silver amount subsequently. On the other side, a membrane sample


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with the same effective area was soaked in a 10 ml 5 wt% nitric acid aqueous solution to dissolve all Ag nanoparticles on membrane surface, to obtain the total amount of loaded Ag nanoparticles. The silver concentration was analyzed by an Atomic absorption spectroscopy (AA300, Agilent Technologies) to calculate the released and total silver amounts.

RESULTS AND DISCUSSION

Physicochemical properties of thin film composite membranes ATR-FTIR spectroscopy was employed here to track the changes in chemical structures of polyamide-based thin film composite membranes (Fig. 2a). The absorption peak centred at 1624 cm-1 attributed to the C=O stretching vibration of the amide group was present in all membranes studied here. New peaks centred at 1035 and 1730 cm-1 that were correlated to O=S=O stretching vibrations of sulfonic acid groups and C=O stretching vibrations of carboxyl groups, could be observed in membranes modified by SO3- and COO--based zwitterions, respectively.33,

43-44

This was indicative of successful grafting of various

zwitterions on to the surfaces of the polyamide thin film composite membranes. Crystallographic studies of Ag and AgCl that decorated on the membrane surface were carried out by XRD analysis. The strong reflection at 38.11°, 44.32°, 64.56°, and 77.59° corresponding to (1,1,1), (2,0,0), (2,2,0), and (3,1,1) planes of face-centred metallic silver (Fig. 2b), respectively.45 Another prominent peaks of the corresponding silver chloride presented in Fig. S2 were observed at 27.84°, 32.27°, 46.27°, 54.80° and 57.55°, could be indexed to (1,1,1), (2,0,0), (2,2,0), (3,1,1) and (2,2,2) crystal planes of AgCl.46 All these results manifested the states of Ag-based nanomaterials on the membrane surface.



Fig. 2 (a) ATR-FTIR spectra of the control and zwitterions modified membranes; (b) XRD patterns; (c) peak deconvolution of narrow-scan spectra; (d) SEM images of metallized membranes.

XPS analyses revealed two peaks of Ag 3d spectral region at 367.95 and 373.58 eV, corresponding to Ag3d5/2 and Ag3d3/2 respectively (Fig. 2c).47 The deconvolution of these peaks indicated two strong peaks at 368.53 and 373.17 eV attributed to the Ag (0) state, while the two weaker peaks centred at 367.34 and 374.02 eV corresponded to the Ag (I) state.46 The appearance of Ag (0) state indicated the successful reduction of Ag ions on metallized membranes. Peak deconvolution results of AgCl mineralized membrane (Fig. S4) showed


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two strong peaks corresponding to Ag (I) state and two weak peaks corresponding to Ag (0) state, indicating the presence of Ag-Cl species on mineralized membranes.

SEM micrographs in Fig. S5a-c showed that the typical granular structure of polyamide membranes produced by interfacial polymerization between piperazine and trimesoyl chloride did not change much after grafting with both zwitterions. Compared with metallized pristine polyamide membranes (Fig. 2d), metallization only altered the surface morphologies of zwitterion-grafted polyamide membranes as the grainy surface structures were smoothened. The difference in surface morphology is mainly ascribed to the promoted loading of Ag nanoparticles on zwitterion-modified membranes. The impact of zwitterion grafting on surface morphologies was more prominent in mineralized samples （ Fig. S5d-f ） . The changes in surface morphologies were due to the nature of zwitterion modification and their complexation with Ag ions. This was validated from XPS (Table S1) analyses where Ag concentration increased in the presence of zwitterions where there was a stronger affinity between Ag+ ions and COO--based zwitterions. More importantly, the presence of Ag-based nanomaterials in polyamide membranes studied here also impacted on surface charges – a key parameter towards inhibiting fouling. The surface charges of polyamide membranes studied were determined by zeta-potential tests (Fig. S6). The presence of electrically-neutral COO- and SO3- zwitterion grafts enhanced the zeta potential of polyamide membranes that were originally decorated with negatively-charge carboxylic groups by 68% and 71%, respectively. The similar surface zeta potentials of two zwitterion-grafted membranes indicate the approximate grafting degrees of SO3- and COO--based zwitterions on the membrane surface. In addition, the incorporation of negatively-charged Ag and AgCl nanoparticles via metallization and mineralization, respectively, reduced the surface zeta potentials of polyamide membranes studied here.34, 48



The surface zeta potential of membranes was reduced as Ag content increased (Fig. 3a), which is particularly pronounced in COO- zwitterion modified one. For example, the zeta potential of COO- zwitterion modified membrane studied here reduced from -11.4 mV to -45.3 mV as Ag content increased to 3.76% after mineralization, whilst that of control membrane only reduced from -36.4 mV to -37.2 mV with Ag increased 0.42%. The concentration of Ag-based nanomaterials on the membrane surface follows the order of COOzwitterion modified membrane > SO3- zwitterion modified membrane > control membrane. These results indicated that zwitterion grafts were beneficial to Ag introduction onto the membrane surface. Ag concentration in the polyamide membranes and zwitterionic modification also impacted on the surface hydrophilicity (Fig. 3b). Zwitterion grafts resulted in smaller water contact angles i.e. enhanced hydrophilicity. Ag metallization slightly increased the water contact angles by about 8%. However, AgCl mineralization of zwitterion-modified polyamide membranes reduced water contact angles by 31-48%. This distinctive difference in water affinity between Ag metallization and mineralization was attributed to type of Ag-based nanomaterial immobilized on the membrane surface. Ag nanoparticles produced by the metallization route are hydrophobic, whilst AgCl nanoparticles formed by mineralization are hydrophilic.34, 49


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Fig. 3 (a) Correlation between zeta potential (pH=7) and Ag content (atomic percentage) and (b) WCA of different TFC membranes.

Thermodynamics between various zwitterions and Ag+ Theoretical density functional calculations were performed here to elucidate the mechanism underpinning the interactions between Ag+ ions and zwitterions. We first assumed that three complex structures were formed from each type of zwitterion (Fig. 4a) where one Ag+ cation can interact with one SO3- or COO- group of the zwitterion (complexes 1 and 4), or ligate one zwitterion and one nitrate ion (complexes 2 and 5), or ligate two zwitterions (complexes 3 and 6). We also assumed that the functional group next to the quaternary amine in these hypothetical structures was not involved in any relevant interaction with SO3- or COO- group or Ag+. For simulation convenience, we replaced these inactive groups with a hydrogen atom as it was fairly safe to assume that this replacement had little effect on the calculated results and obtained optimized structures of these zwitterion-Ag+ complexes (Fig. 4b).50 Compared to SO3- zwitterion-Ag+ complexes, the Gibbs free energy generated by complexation of COO--based zwitterion with Ag+ ions was lower (Fig. 4c), indicating that the formation of COO- zwitterion-Ag+ complexes was thermodynamically favourable. In other words, the stronger ionic interaction between Ag+ and COO- groups led to the more nucleation of Ag nanoparticles on the surface of polyamide membranes grafted with COO--based zwitterions. Amongst the six hypothetical structures, the most stable structures contained Ag ions ligated with two zwitterions i.e. Complexes 3 and 6. This could explain why the control membrane studied here also contained carboxylic groups but demonstrated limited Ag+ binding capability. Carboxyl groups with short chain on the pristine polyamide surface might hinder the formation of stable COO- zwitterion-Ag+ complexes. Ultimately, the presence of



COO--based zwitterions were crucial and preferred for forming complexes with Ag and AgCl nanoparticles that were keys for demonstrating resistance towards biofouling.

Fig. 4 (a) Possible interaction ways of zwitterion-Ag+ complexes; (b) optimized structures (Atoms marked with red circles were fixed during the geometry optimizations) of zwitterion-Ag+ complexes; (c) calculated G for the formation of zwitterion-Ag+ complexes.

Anti-microbial and anti-fouling properties silver release of membranes modified with zwitterion-Ag complexes Inhibition zone tests via the agar diffusion method were employed here to evaluate the anti-microbial ability of polyamide membranes modified with zwitterion-Ag nanoparticles. A large number of bacterial colonies grew around the control membrane after a 24h incubation


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period (Fig. 5a), indicating a lack of anti-bacterial properties to E. coli and S. aureus. Meanwhile, a distinct zone free of bacterial colonies appeared on metallized polyamide membranes that contained zwitterion-Ag nanoparticles. Ag nanoparticles inactivate bacteria by releasing free Ag+ ions that disrupt the growth and metabolism of bacteria, which is the main principle for the anti-bacterial properties of almost all Ag-modified membranes in the previous literatures.6 As COO--based zwitterions could form more Ag nanoparticles compared to SO3--based one, the higher amount Ag+ ions released from Ag-loaded membranes and killed bacteria around, forming significantly larger inhibition zone. CFU tests were carried out to quantify anti-microbial properties of membranes studied here. The number of E. coli and S. aureus bacteria colonies decreased significantly for metallized membranes that contained zwitterion grafts, while the control membrane exhibited only minimal anti-microbial properties (Fig. 5b). The exposure to the metallized membrane with COO- zwitterion modified led to a reduction of 93.1% for E. coli and 95.7% for S. aureus, respectively, demonstrating its great disinfection effect towards various types of bacteria. These results further revealed that, zwitterions, especially COO--based zwitterion, promoted the metallization degree of the membrane and further maximized its anti-microbial property. To evaluate the long-term anti-biofouling performances of membranes modified with zwitterion-Ag nanoparticles under dynamic biofouling conditions, we performed 12-h dynamic biofouling experiments with simulated synthetic wastewater containing S. aureus (Fig. 5c). The normalized water flux for the control membrane declined by 72.9% due to severe biofouling, and declined by 47.8% after Ag loading. Under the same condition, the normalized flux of membranes with SO3- zwitterion-Ag and COO- zwitterion-Ag nanoparticles only declined by 21.6% and 10.5%, respectively, indicating the superior antibacterial property and stability of zwitterion-Ag complexes modified membranes under dynamic conditions.



Fig. 5 (a) Inhibition zone (membrane diameter = 12 mm); (b) colony forming units (CFU) tests of metallized membranes for E. coli and S. aureus; (c) dynamic biofouling performance of membranes modified with zwitterion-Ag nanoparticles with simulated synthetic wastewater containing S. aureus.

The functionality of membranes modified by zwitterion-Ag complexes was dependent on both total loaded Ag and the release rate of Ag+ ions. Hence the Ag+ release is of undisputed importance in the membranes studied here. Ag+ release was determined by a shaking test in ultrapure water (Fig. 6). The initial amount of Ag+ ions released from control membrane was 0.11 g/cm2 day, while 0.31 and 0.5 g/cm2 day of Ag+ ions were released from membranes modified with SO3- and COO--based zwitterions, respectively, depending on the total Ag


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loading amount on the membrane surface. The releasing amount of Ag+ was relatively rapid during the first two days and then gradually level off in the next several days, which is consistent with the result reported in previous literatures.26 However, the Ag+ release rates of zwitterion-modified membranes stabilized after 5 – 7 days of testing, while the Ag+ release rate of control polyamide membranes remained unstable. The total Ag loading amount was 1.39, 4.88, and 10.05 g/cm2 for control, SO3- and COO- zwitterions modified membranes, respectively, which is consistent with XPS results. These results clearly showed that zwitterion grafts were essential for anchoring immobilized Ag-based nanoparticles on to polyamide surfaces. Compared to other Ag-incorporated membranes reported in the literature, the membranes with zwitterion-Ag complexes in this work presented satisfactory silver release rate to achieve the effective killing efficiency against bacteria in practical applications. Detailed benchmarking Table (Table S3) and description can be referred in the Supporting Information.

Fig. 6 (a) Ag+ release and (b) total Ag amount from the surface of metallized membranes.

Protein adsorption is a key indicator for membrane fouling. Here we investigated the static protein adsorption behavior of control and membranes modified with zwitterion-Ag complexes with an aqueous solution containing 1000 ppm BSA (pH 7.4). BSA adsorption on



zwitterion modified membranes was significantly lower when compared to pristine polyamide membranes (Fig. 7a). This was due to the formation of a dense hydration layer by zwitterions on the membrane surface.51 The hydrophilicity of this layer inhibited the attachment of protein molecules. For metallized membranes with zwitterion modified, the relative smoother surface (Fig. 2d) with more Ag nanoparticles mitigates the foulant deposition on the membrane surface. Meanwhile, the hydration layer formed by zwitterion on the membrane surface weakened the interaction between BSA and the membrane surface. Clearly, these dense hydration layers were able to offset the hydrophobicity of Ag nanoparticles in metallized membranes as well. Mineralized membranes that contained hydrophilic AgCl nanomaterials demonstrated a stronger resistance to BSA adsorption, which could be attributed to their hydrophilicity and negative charge leading to electrostatic repulsion of negatively-charged BSA molecules (isoelectric point is about 4.9). Besides, owing to the lower surface charge, membranes with COO- zwitterion grafts presented less BSA adsorption despite the relatively lower hydrophilicity compared to that with SO3zwitterion. Overall, in both instances, the highest loading degree of Ag-based nanomaterials on COO- zwitterion modified membranes minimized the BSA adsorption.


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Fig. 7 (a) BSA adsorption amount of different TFC membranes; and anti-fouling properties of (b) control, zwitterion-grafted TFC membranes, as well as the corresponding (c) metallized and (d) mineralized membranes. (Pressure: 2 bar; cross flow rate: 80 L/h).

To illustrate the excellent anti-fouling properties of membranes modified with zwitterion-Ag complexes in practical applications, we performed long-term dynamic fouling tests (Fig. 7b-d). The normalized water fluxes of pristine polyamide membranes were reduced by 58 % after 900 min. Meanwhile, the water fluxes of membranes modified with zwitterion-Ag complexes were only reduced by 28% and could be recovered in a faster manner upon cleaning with DI water. Crucially, regeneration of slightly fouled modified membranes could recover up to 98 % of the water flux. Moreover, after 1000-min filtration, membranes modified with zwitterion-Ag complexes were more resistant to fouling than control membranes loaded with Ag or AgCl nanoparticles where flux was reduced to 34-42%. This was due to less Ag loading amount, as well as the weak interaction between Ag+ and COO-



groups on the control membrane surface that led to poorly immobilized Ag-based nanoparticles, and hence unstable anti-fouling properties. These results indicated that zwitterion modification, COO--based zwitterion especially, endowed metallized and mineralized membrane with promoted anti-fouling performance.

Membrane NF performance

NF performances of different membranes towards different solutes (1000 ppm Na2SO4 and MgSO4 feed solutions) were also investigated (Fig. 8). The water fluxes of zwitterion-grafted membranes were both higher than that of the control membrane. This was ascribed to the superior water bonding ability of zwitterions. In addition, SO3- zwitterion modified membranes possessed better permeability compared to membranes grafted with COOzwitterion. This was because membranes grafted with SO3--based zwitterion were more hydrophilic than membranes containing COO--based zwitterions.52 The rejection rates of corresponding membranes followed the order of control membrane > COO- zwitterion modified membrane > SO3- zwitterion modified membrane for Na2SO4 (Fig. 8a) but a reverse trend for MgSO4 (Fig. 8b), which could be explained by the decreased surface negative charge after zwitterion modification. According to the Donnan salt exclusion effect, a strong electrostatic repulsion exists between multivalent anions (SO42-) and negative-charged membrane surface, which impedes the transport of anions while facilitating the passage of cations.42-43 The water fluxes of all metallized membranes decreased, resulted from the higher Ag nanoparticles amount of corresponding metallized membrane and therefore the higher water permeation resistance. Accordingly, higher rejection rates towards both Na2SO4 and MgSO4 were obtained, because of the resulted denser selective layer. The water flux of mineralized membranes showed a significant increase compared to both control and


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zwitterion modified membranes, owing to the high surface hydrophilicity with the deposition of plentiful hydrophilic AgCl particles on the membrane surface. The rejection increased slightly towards Na2SO4 while decreased towards MgSO4, owing to the more negatively charged surface after mineralization.

Fig. 8 NF performance of different TFC membranes for (a) Na2SO4 and (b) MgSO4 feed solutions (Pressure: 2 bar; cross flow rate: 80 L/h).

CONCLUSION

In this study, Ag metallization and mineralization are performed on control and zwitterion-grafted TFC membranes, and the effect of zwitterion type (SO3--based and COO--based) are studied and compared systematically. Various techniques are applied to characterize the surface chemical composition and morphology of membranes with and



without zwitterion modification. The results confirm that zwitterion grafting endows the membrane surface with promoted metallization and mineralization degree. Additionally, it is found that this promotion effect was much more prominent for COO--based zwitterion, and the mechanism is revealed by the theoretical calculation, which confirms the stronger interaction between Ag+ and COO- zwitterion and the stable bonding of silver-based particles on the TFC membrane surface. The inhibition zone tests and silver release tests confirm that metallized membrane with COO--based zwitterion possesses maximized anti-microbial property and silver stability. In addition, the improved anti-fouling property is also obtained for both metallized and mineralized zwitterion-grafted membranes with no compromise in NF performance. This study is therefore believed to provide a useful guideline to maximize the anti-fouling and anti-microbial properties for the passive and active modification strategies of separation membranes with zwitterion and Ag+ involved.

ASSOCIATED CONTENT

Supporting Information Schematic illustration of the preparation of zwitterions (COO--based and SO3--based) grafted TFC membranes, XRD patterns of mineralized membranes, XPS wide scans spectra of metallized and mineralized membranes, peak deconvolution of narrow-scan spectra of mineralized membranes, chemical compositions of metallized and mineralized membranes, SEM images of control, SO3- and COO--based zwitterions modified membranes before and after mineralization, surface zeta potential (pH=7) of different TFC membranes, rationale behind the structure of COO- zwitterion in theoretical section, silver release rate of recently Ag incorporated membranes.


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ACKNOWLEDGEMENT

The authors thank the financial supports from National Key Technology Support Program (no. 2014BAD12B06) and Opening Project of Key Laboratory of Biomedical Polymers of Ministry of Education at Wuhan University (no. 20140401). We would also like to thank the Analysis and Testing Center, the Analysis and Testing Center of Chemistry and Chemical Engineering School, the State Key Laboratory of Materials Processing and Die & Mould Technology, in Huazhong University of Science and Technology for their help with material characterizations. We would like to give a special thanks to Prof. Zhong Fangrui (Huazhong University of Science and Technology) for his kindly guidance.

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