Improved Anti-Biofouling Performance of Thin -Film Composite

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Environmental Processes

Improved Antibiofouling Performances of Thin Film Composite Forward Osmosis Membrane Containing Passive and Active Moieties Longbin Qi, Yunxia Hu, Zhongyun Liu, Xiaochan An, and Edo Bar-Zeev Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b06382 • Publication Date (Web): 03 Aug 2018 Downloaded from http://pubs.acs.org on August 5, 2018

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Environmental Science & Technology

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Improved Antibiofouling Performances of Thin Film

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Composite Forward Osmosis Membrane Containing Passive

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and Active Moieties

4 5 Longbin Qi1,2, 3, Yunxia Hu1*, Zhongyun Liu2, Xiaochan An2, 3, Edo Bar-Zeev4

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1. State Key Laboratory of Separation Membranes and Membrane Processes, School of Materials

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Science and Engineering, Tianjin Polytechnic University, Tianjin 300387, P. R. China

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2. CAS Key Laboratory of Coastal Environmental Processes and Ecological Remediation; Research

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Center for Coastal Environmental Engineering and Technology of Shandong Province; Yantai Institute

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of Coastal Zone Research, Chinese Academy of Sciences, Yantai, Shandong Province 264003, P. R.

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China

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3. University of Chinese Academy of Sciences, Beijing 100049, PR China

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4. Department of Environmental Hydrology & Microbiology, Zuckerberg Institute for Water Research

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(ZIWR), Ben-Gurion University of the Negev, 8499000, Israel

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*Corresponding author, Tel: +86-22-83955129;

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E-mail: [email protected]

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ACS Paragon Plus Environment


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ABSTRACT

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Forward osmosis (FO) has been gaining increasing attentions in desalination, wastewater treatment,

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and power generation. However, biofouling remains a major obstacle for the sustainable

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development of FO process. Both passive and active strategies have been developed to mitigate

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membrane biofouling. A comprehensive understanding of different strategies and mechanisms has

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fundamental significance for the antifouling membrane development. In this study, thin-film

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composite (TFC) FO membranes were modified with polydopamine (PDA) coating as a passive

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antibacterial moiety and silver nanoparticles (Ag NPs) as an active antibacterial moiety. Their

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antibiofouling performances were investigated both in static and dynamic conditions. In static

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exposure, the PDA coated membranes exhibited great passive anti-adhesive property, and the Ag

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NPs generated membranes presented both of excellent passive anti-adhesive property and active

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antibacterial performance. While, in dynamic cross-flow running condition, Ag NPs effectively

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mitigated the membrane water flux decline due to their inhibition of biofilm growth, whereas the

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PDA coating failed because of its inability to inactivate the attached bacteria growth. Moreover, Ag

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NPs were stable and active on membrane surfaces after 24 h cross-flow operation. These findings

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provide new insights into the performances and mechanisms of passive and active moieties in the

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FO process.

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TOC Art

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INTRODUCTION

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Increasing demand on freshwater and clean energy is one of the greatest global challenges due to

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the speeding population growth, growing urbanization and changing global climate.1, 2 Forward

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osmosis (FO) membrane process works as a state-of-the-art technology to desalinate seawater,3,4

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reuse wastewater,5-7 and produce clean energy using salinity gradients,8, 9 alleviating fresh water

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shortage and clean energy crisis locally and globally. The development of high-performance

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thin-film composite (TFC) FO membranes has been gaining increasing attention striving to achieve

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high water flux without compromising salt rejection during a long-term operation under

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environmental conditions.10-12 However, membrane fouling and particularly biofouling, remains a

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bottleneck for the development and application of large-scale FO systems, which significantly

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reduces membrane performance and increases operation costs as well as energy inputs.13-15

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Passive and active strategies have been developed to mitigate biofilm by minimizing the deposition

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and growth of microorganisms on membrane surface.16-18 The passive strategy generally involves

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membrane surface modification by grafting,19-21 coating or blending hydrophilic materials22, 23 to

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weaken the foulant-membrane interactions and prevent foulants adsorption on membrane surface.16,

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24

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and prevent biofilm formation.25-27 Foulant adsorption on membrane surface is an initial stage of

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membrane fouling. The passive strategy presents effective performances against fouling under low

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or mild fouling conditions. However, under severe fouling conditions, the passive strategy may lose

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its efficacy because the membrane surface is quickly masked by the foulant layer and the

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foulant-foulant interaction is dominant to cause membrane fouling.28, 29 Moreover, biofouling is

The active strategy normally incorporates biocides on membrane surface to deactivate bacteria

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more severe and complicate than inorganic and organic fouling because microorganisms are living

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foulants to secrete sticky proteins and polymers and induce strong interaction with membrane

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surfaces.28 The membrane performances may not be impaired if the attached microorganisms can

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be deactivated by biocides and then washed off easily. Therefore, an ideal antibiofouling FO

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membrane should have both passive and active functions to minimize bacterial absorption and

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deactivate bacterial growth on the membrane surface.30 A holistic understanding of different

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strategies and mechanisms has fundamental significance for the antifouling membrane design.

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However, few strategies have been developed to incorporate both passive and active moieties on

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membrane surfaces for the fabrication of ideal antibiofouling FO membranes. Our previous work

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and other reported research have demonstrated that mussel-inspired dopamine chemistry provides a

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simple, yet, universal approach to incorporate both passive and active moieties on most types of

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inorganic and organic substrates.31-33 Dopamine can self-polymerize into polydopamine (PDA) on

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various substrates with controllable film thickness and durable stability.34 PDA has been confirmed

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to exhibit antifouling properties of alleviating the absorption of protein and microorganism.35, 36

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Silver nanoparticles (Ag NPs) can form in situ on the PDA coating upon incubation with silver salt

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solution, and act as efficient biocide.33, 37-39 The O- and N-sites of PDA can serve as anchors for the

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Ag NPs to enhance their stability on membrane. Furthermore, Ag NPs are rechargeable on the PDA

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coated surfaces and have exhibited sustainable bactericidal efficacy.33 Thus, dopamine chemistry

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provides a facile process to incorporate both passive PDA coating and active Ag NPs on FO

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membrane surfaces for antibiofouling properties. However, no work has been done to investigate

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and compare the passive and active antibiofouling performances of PDA and Ag NPs functionalized 5



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TFC FO membranes in the continuous cross-flow forward osmosis test system.

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Herein, the thin-film composite (TFC) FO membranes were fabricated and then modified with PDA

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coating as a passive antibacterial moiety and Ag NPs as an active antibacterial moiety. Their

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antibiofouling performances were investigated in both static and dynamic conditions and also

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compared on both operation modes of FO (active layer facing feed solution) and pressure retarded

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osmosis (PRO, active layer facing draw solution) to gain comprehensive understanding on the

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antibiofouling performances induced by PDA and Ag NPs. Pseudomonas aeruginosa was selected

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as a model bacterial stain and synthetic wastewater was used as a low osmotic feed solution.

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Dynamic biofouling experiments were conducted in a continuous cross-flow FO system. The

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structure and composition of biofilms were analyzed to clarify the mechanisms of biofouling

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mitigation by PDA and Ag NPs. Our findings aim to provide new insights on the performances and

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mechanisms of passive and active moieties on the antibiofouling efficiency of TFC FO membranes.

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MATERIALS AND METHODS

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Materials and Chemicals.

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A commercial polyester nonwoven fabric (PET, grade 3249, 40 µm thickness) was purchased from

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Ahlstrom (Helsinki, Finland). Polysulfone (PSf, Mn: 22,000 Da), m-phenylenediamine

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(MPD, >99%), 1,3,5-benzenetricarbonyl trichloride (TMC, 98%), N-Methyl pyrrolidone (NMP)

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and dopamine were supplied by Sigma Aldrich (St. Louis, MO, USA). SYTO®9 green fluorescent

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nucleic acid stain and Concanavalin A (Con A, Alexa Flour 633) were obtained from Invitrogen

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(Eugene, Oregon, USA) and propidium iodide (PI) was received from Sigma-Aldrich (St. Louis, 6


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MO, USA). 1 M Tris-HCl buffer (pH 8.5) was purchased from Beijing Solarbio Science &

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Technology Co., Ltd., China. Pseudomonas aeruginosa (P. aeruginosa, ATCC 27853) was obtained

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from American Type Culture Collection. Yeast extract, tryptone and agar were purchased from

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Oxoid Limited, UK. Ag NO3, NaCl, MgSO4·7H2O, NaHCO3, CaCl2·H2O, KH2PO4, NH4Cl, EDTA

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and sodium citrate were purchased from Sinopharm Chemical Reagent Beijing Co., Ltd, China. All

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chemicals were used as received.

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TFC Membrane Fabrication.

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TFC FO membranes were fabricated following our previous protocol through two steps including

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the casting of PSf ultrafiltration support layer by a typical non-solvent induced phase separation

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(NIPS) method, and the formation of polyamide active layer by interfacial polymerization on top of

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PSf support.40, 41 12 wt.% PSf casting solution in NMP was casted with a steel casting knife (150

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µm gate height) on a clean glass plate covered by PET nonwoven fabric, and precipitated in a water

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coagulation bath at room temperature to produce a PET integrated PSf support membrane. During

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the casting process, the humidity was controlled at 40 ± 5 RH% in a clean room. Polyamide layer

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was formed by interfacial polymerization of MPD in aqueous phase and TMC in organic phase on

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top of the PSf support layer. A wet PSf membrane was taped to a clean glass plate and then

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immersed in a 30 mL aqueous solution of 3.4 wt.% MPD for 2 min. A rubber roller was used to

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remove excess MPD solution from the PSf membrane surface. The MPD-containing PSf membrane

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was then immersed in a 0.15 wt.% TMC solution in hexane for 1 min to form polyamide films.

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Finally, the membranes were cured in DI water at 95 °C for 2 min. After thorough rinse with DI

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water, the TFC FO membranes were stored in DI at 4 °C before characterization. 7



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TFC Membrane Surface Functionalization.

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In situ growth of Ag NPs on both surfaces of TFC FO membranes was performed according to our

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previous work.33 Briefly, the TFC FO membrane coupons were immersed in 2 mg/mL dopamine

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solution at 10 mM Tris-HCl buffer (pH 8.5) for 1 h at room temperature under 60 rpm shaking to

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fabricate PDA coated membranes. To generate Ag NPs on membrane surfaces, the PDA coated

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membrane coupons were immersed in 50 mM silver nitrate aqueous solution at room temperature,

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and the incubation time was varied from 1 h, 2 h, 4 h, to 8 h to optimize the generation conditions

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of Ag NPs. Finally, the membrane coupons were rinsed three times with DI water, air-dried at room

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temperature, and stored in a light-proof dark chamber for further characterization.

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TFC Membrane Characterization.

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The surface morphologies of pristine and modified membranes were observed using a scanning

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electron microscope (SEM, S-4800, Hitachi). Ag NPs on the membrane surfaces were detected

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using EX-350 Energy Dispersive X-ray Microanalyzer (EDX, Horiba, Tokyo, Japan) with 15.0 kV

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accelerating voltage. All samples were coated with 10 nm thick platinum (Pt) for 100 s using an

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EMITECH SC7620 sputter coater before SEM and EDX observation. The particle size and number

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of Ag NPs were measured using Nano Measurer software and Image J software, respectively. The

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static water contact angles of membranes were measured using the sessile drop method on an

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optical instrument (OCA 20, Data Physics, Germany). Each sample (1 × 5 cm2) was dried in a

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vacuum oven overnight and then mounted on a clean glass slide. A 2 µL deionized (DI) water

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droplet was dropped on membrane surface using a microsyringe, and two seconds later the droplet 8


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shape was recorded and analyzed using the automated Drop Shape Analysis software (SCA20

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version 2). The surface zeta potential of membranes was measured using an Anton Paar SurPASS

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electrokinetic analyzer (Anton Paar, Austria) using 1.0 mM KCl solution as the background at pH

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of 7.5. Zeta potential was determined using the associated software from the streaming potential

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slope versus pressure plots based on the Helmholtz-Smoluchowski approach. All measurements

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were performed on at least three individual samples and the data were reported as the average value

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and the standard deviation.

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Static Anti-adhesion Tests.

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P. aeruginosa was used as a model bacterial strain to evaluate the bacteria adhesion and viability on

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both surfaces of TFC FO membrane, following our previous protocol with slight changes.42 P.

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aeruginosa was initially cultured in Luria-Bertani (LB) broth to reach a mid-exponential growth

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phase. Bacteria were collected after 1 min of centrifugation at 5,000 rpm and then re-suspended in

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sterile synthetic wastewater solution to reach an optical cell density at 600 nm (OD600) of 0.15,

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where the initial concentration of P. aeruginosa was approximately 108 CFU mL−1. Synthetic

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wastewater composed of 8.0 mM NaCl, 0.15 mM MgSO4·7H2O, 0.5 mM NaHCO3, 0.2 mM

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CaCl2·H2O, 0.2 mM KH2PO4, 0.4 mM NH4Cl, and 0.6 mM sodium citrate (pH 7.5) was prepared

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based on the reported protocol.43

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Bacterial attachment was evaluated by placing circular membrane coupons with the diameter of 1.6

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cm were placed in sterile plastic tubes with 5 mL of P. aeruginosa suspension at 37 °C for 1 h.

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Biofouled membrane coupons were then collected and gently rinsed using sterile synthetic 9



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wastewater to remove unattached cells. Live and dead cells were stained with 3.34 µM SYTO®9

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and 20 µM PI solution respectively for 30 min in dark. The membranes were then rinsed three times

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with sterile synthetic wastewater to remove unbound stains. The fluorescence images of the stained

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membrane coupons were collected using a confocal laser scanning microscopy (CLSM, Fluo View

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FV1000, Olympus, Japan). SYTO®9 was excited with an argon laser at 488 nm and PI was excited

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with a diode-pumped solid state laser at 559 nm. Live cells and dead cells were recognized as green

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and red spots, respectively. The CLSM images were analyzed using Image J Pro software (National

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Institutes of Health, MD). For quantitative image analysis, three individual samples were measured

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parallelly, and the results were reported as the average values with the standard deviations.

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Antimicrobial Activity Measurements.

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Antimicrobial activity of Ag NPs-containing TFC FO membranes was assessed using the reported

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colony forming unit (CFU) method.38 Circular membrane coupons with 1.6 cm in diameter were

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cut from the freshly prepared membranes, and then incubated with P. aeruginosa suspensions (108

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CFU mL−1) in the mid-exponential growth phase for 5 h at 37 °C. After gentle rinse with sterile

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physiological saline solution (0.15 M NaCl, 20 mM NaHCO3, pH 7.5), the bacteria attached on the

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membrane coupons were removed through 7 min bath-sonication in 10 mL sterile physiological

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saline solution. The bacterial suspensions were serially diluted 100 times, and 100 µL of the

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dilution was taken to spread onto an LB agar plate. Then bacteria colonies were counted after 24 h

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incubation at 37 °C. The antibacterial efficiency Eb was calculated by the following equation:

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=

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Where Np and Nm are the numbers of colonies formed on pristine membranes and modified

( ‒ )

× 100%

( 1 )

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membranes, respectively.

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Dynamic Biofouling Experiments and Biofilm Characterization.

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The dynamic biofouling measurements were carried out in a lab-scale FO cross-flow set-up with

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the effective surface area of 8 cm2 reported in our previous work.41 The FO set-up was stabilized

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using DI water as both feed solution and draw solution to obtain the water flux of 0 for 20 min. The

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synthetic wastewater stock and NaCl stock was added into the feed solution side and draw solution

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side separately to achieve a permeate water flux of 20 L m-2h-1. P. aeruginosa suspension was then

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added into the feed solution side to reach a bacterial concentration of 6.0 × 107 CFU L−1. The

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volumes of feed and draw solutions were 2 L for all of the experiments. The pH of feed solution

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was adjusted to pH 7.5 during the dynamic biofouling experiment. The cross-flow rates of both

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feed and draw solutions were maintained at 0.5 L min-1 and the temperatures of solutions were kept

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at 25±0.5 °C. The water flux was continuously monitored by measuring the weight increment of

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the draw solution using an electronic balance (ME3002, Mettler, Switzerland). The FO filtration

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tests were operated under both FO and PRO modes, respectively.

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Biofilms were analyzed upon staining using CLSM. Briefly, 1 cm2 of biofouled membrane coupon

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was cut from the tested membranes and then stained with SYTO®9, PI and 50 µM Con A for 30

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min in dark to label live cells, dead cells and extracellular polymeric substance (EPS), respectively.

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A Z-stack scanning with a slice thickness of 2 µm was performed to collect the images of biofilm.

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Three-dimensional structure of biofilm was reconstructed using the FV1000 Viewer software

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(Olympus, Japan). Con A was excited with a diode-pumped solid state laser at 633 nm.

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Stability of Ag NPs on TFC Membrane.

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To evaluate the stability of Ag NPs on TFC membrane, the membranes coupons were tested for 24

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h in the FO cross-flow system, and then taken out from the FO cell to cut as 1 cm × 1 cm

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membrane coupons. Their antimicrobial activity was determined using the CFU method following

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the abovementioned protocol. The residual amount of Ag NPs on the tested membranes was

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quantified using an inductively coupled plasma mass spectrometer (ICP-MS). Membrane coupons

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were immersed in 10 mL 3.5% HNO3 solution for 48 h to completely dissolve Ag NPs in the

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membrane. The concentrations of silver ions in the solutions were measured using ICP-MS (ELAN

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DRC II, PerkinElmer).

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Total Organic Carbon (TOC) and Total Protein Analysis.

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TOC measurement was performed according to the reported protocol,19 membrane coupons were

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cut (1×1 cm2) from the center of biofouled membranes and placed into sterile glass bottles with 20

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mL hydrogen chloride aqueous solution (20 mM). The biofilm was removed from the membrane

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coupons through 60 s sonication using a probe sonicator (JY92-IIDN, Scientz biotechnology Ning

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Bo Co., Ltd, China), and TOC in the solution was quantified using a TOC analyzer (TOC-VCPH,

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Shimadzu, Japan) according to the operation manual of manufacture. The TOC of biofilm was

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reported as the average value from three parallel samples and the standard deviation as the error bar

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after normalized by the surface area of membrane coupons.

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Total protein biomass was quantified using the reported BCA method.44, 45 1×1 cm2 biofouled 12


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membrane coupons were placed into 2 mL Eppendorf tubes with 1 mL PBS buffer and then

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sonicated for 7 min to break bacteria cells. After centrifugation at 13000 rpm for 10 min, the protein

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in the supernatant solution was quantified using a BCA protein assay kit (Institute of biological

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engineering Co., Ltd, China) according to the operation manual of manufacture. The total protein

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biomass of biofilm was reported as the average value from three parallel samples and the standard

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deviation as the error bar after normalized by the surface area of membrane coupons.

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RESULTS AND DISCUSSION

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PDA Coating and Ag NPs Generation Increase Surface Hydrophilicity without Affecting

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Transport Properties of TFC FO Membranes.

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Our previous work had demonstrated that Ag NPs were in situ generated on both surfaces of TFC

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FO membranes including top polyamide surface and bottom PET-strengthened polysulfone surface

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via mussel-inspired dopamine chemistry.33 Here, we further investigated the optimized

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experimental conditions of in situ Ag NPs generation on both surfaces of TFC FO membranes.

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SEM analysis indicates that the PDA coating did not present morphology change between the PDA

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coated membranes (Figure 1b and 1e) and pristine membranes (Figure 1a and 1d) on the polyamide

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surface and the polysulfone surface. Ag NPs were densely populated and uniformly distributed on

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characteristic ridge-and-valley polyamide surface (Figure 1c and Figure S1) as well as smooth

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polysulfone surface (Figure 1f). Interestingly, the Ag NPs generated on the polysulfone surface

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was 28.1 ± 4.7 nm in diameter, which is bigger than 10.6 ± 2.5 nm in diameter of Ag NPs formed

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on the polyamide surface. This change in Ag NPs size is likely due to the different surface

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morphologies and properties of polysulfone surface and polyamide surface which affects the 13



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growth of PDA coating and thus tailors the nucleation and growth of Ag NPs.33, 46 In addition, the

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Ag NPs generated on the polysulfone surface presented stronger silver signals than that on the

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polyamide surface, indicating the more silver mass deposited on the polysulfone surface.

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Figure 1. Representative SEM micrographs of top polyamide surface and bottom polysulfone surface from pristine (a and d), PDA coated (b and e) and Ag NPs generated (c and f) TFC membranes, and corresponding EDX spectra of Ag NPs generated polyamide surface (g) and polysulfone surface (h). Scale bars in SEM are 200 nm.

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PDA imparts its hydrophilicity to membrane surfaces once its coating forms, since it decreased the

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water contact angles of both the polyamide layer and PSf support from 70.8 ± 4.6 °, and 125.4 ±

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3.6 °, to 43.8 ± 5.7 ° and 79.1 ± 8.7 ° respectively, as shown in Table S1. The Ag NPs generation

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further decreased the water contact angles of polyamide layer and PSf support to 37.9 ± 3.8 ° and

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75.1 ± 8.6 ° respectively, and thus improved the membrane surface hydrophilicity because of the

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hydrophilic nature of Ag NPs and the improved surface roughness of Ag NPs modified membranes. 14


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Noticeably, the pristine PSf support surface showed a quite high water contact angle, due to the

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hydrophobic PET nonwoven fabric and polysulfone on the on the back surface of the TFC FO

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membrane (Figure S2). Furthermore, zeta potential measurements shown in Table S1 found that

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the PDA coating and Ag NPs generation did not affect the zeta potentials of TFC FO membranes,

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and therefore did not change their surface charges.

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Impacts of PDA coating and Ag NPs on transport properties of TFC FO membrane were

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investigated to optimize surface modification conditions. PDA coating had no major effect on

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membrane separation performance within 1 h of modification due to the formation of thin PDA

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layer.33, 39 Under the condition of 1 h PDA coating, Ag NPs generation had negligible effect on

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membrane water fluxes within 2 h of modification, and then resulted in 10% flux reduction with

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increasing the incubation time to 8 h (Figure S3a). In addition, the reverse salt flux of the TFC FO

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membrane was not affected by Ag NPs generation. Under the optimized conditions of 1 h PDA

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coating and 2 h AgNO3 immersion time, the PDA coated and Ag NPs generated TFC FO

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membranes were prepared and their intrinsic transport parameters were measured following the

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reported protocol.49 Results shown in Figure S3b illustrate that the PDA coating and Ag NPs

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generation had no significant impact on membrane transport properties.

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PDA Coating and Ag NPs Impart Membrane Antiadhesive and Antimicrobial Properties.

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Bacteria attachment on membrane surface is a critical step of biofilm formation and development

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during biofouling.17 Thus, the investigation of bacterial adhesion on membrane surface is very

301

conducive to understand the mechanism of biofouling control. As shown in Figure 2a, the total 15



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bacterial cells attached on the membrane surface decreased by 70% from 7.6×105 cells per cm2 to

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2.3×105 cells per cm2 for the pristine and PDA coated polysulfone support, respectively (Figure 2b).

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Concurrently, the cell viability on the PDA coated membrane surface was 47%, lower than 86% of

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cell viability on the pristine membrane surface (Figure 2c). Moreover, Ag NPs generation further

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decreased the total attached cells by 65% to 0.8×105 cells cm2, and reduced the cell viability by

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96%, compared with the PDA coated membrane (Figure 2b and 2c). On the polyamide surface of

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TFC FO membranes, PDA coating has reduced the number of attached cells by 85% and

309

deactivated 30% cells compared with the pristine membrane (Figure S4). Ag NPs generation

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further decreased the cell viability by 94%, compared with the PDA coated membrane (Figure S4).

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It can conclude that the PDA coating exhibits both anti-adhesive property and antibacterial

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performance. The anti-adhesive effect of PDA coating has been extensively reported due to the

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formation of a hydration layer from the enhanced surface hydrophilicity mitigating the adsorption

314

of contaminants.35 The bacterial biocide effect of PDA coating is related to the protonated amine

315

groups, which can cause bacteria lysis and deactivate bacteria by contacting the cell wall of

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bacteria.50 Even though the surface charge of membrane surface is negative at pH 7.5, the

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protonation degree of amine groups in PDA was reported as 94%.35, 51 The less viability of bacteria

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on PDA coated membranes is not only due to the antibacterial properties of PDA but also due to

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the less number of bacteria attached on PDA coted surfaces. Above all, these results illustrate that

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the PDA coating could effectively prevent the bacterial adhesion on membrane surface and the Ag

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NPs generation could successfully deactivate the attached bacteria under static conditions.

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Figure 2. Static anti-adhesion properties of TFC FO membranes. (a) Representative CLSM images

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of P. aeruginosa cells attached on the pristine, PDA coated and Ag NPs generated polysulfone

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surface of TFC FO membranes; and total number (b) and viability (c) of P. aeruginosa cell

327

attached on membrane surfaces. Results are presented as an average of at least three different

328

membrane samples and error bars are given as standard deviations. Bacterial cells were stained

329

with SYTO®9 (green), and PI (red) for live and dead cells, respectively. Scale bars in CLSM are 10

330

µm. Asterisks indicate statistical significance determined by a Student t-test (P-value

Improved Anti-Biofouling Performance of Thin -Film Composite

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