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May 10, 2017 - PANI/AgNWs-CC Composite Nanofiber Membrane. Junjie Wen,. ‡,§ ... nanofibers offers a new way to achieve the separation and reuse...
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Filtration and electrochemical disinfection performance of PAN/PANI/AgNWs-CC composite nanofiber membrane Junjie Wen, Xiaojun Tan, Yongyou Hu, Qian Guo, and Xuesen Hong Environ. Sci. Technol., Just Accepted Manuscript • Publication Date (Web): 10 May 2017 Downloaded from http://pubs.acs.org on May 16, 2017

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Environmental Science & Technology

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Filtration and electrochemical disinfection performance of PAN/PANI/AgNWs-CC composite

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nanofiber membrane

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Junjie Wenb, c, Xiaojun Tanb, c, Yongyou Hua,b, * , Qian Guoa , Xuesen Hongb, c

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a

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Guangzhou 510006, PR China

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b

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University of Technology, Guangzhou Higher Education Mega Centre, Guangzhou, 510006, PR China

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c

School of Environment and Energy, South China University of Technology, Guangzhou Higher Education Mega Centre,

The Key Lab of Pollution Control and Ecosystem Restoration in Industry Clusters, Ministry of Education, South China

School of Civil Engineering and Transportation, South China University of Technology, Guangzhou, 510640, PR China

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*Corresponding author: Yongyou Hu

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Tel: +86 13602746125; Fax: +86-20-39380508; Email: [email protected]

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Address: School of Environment and Energy, South China University of Technology, Guangzhou, 510006, China

14 15

Other authors

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First author:Junjie Wen

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Second author:Xiaojun Tan

(Tel: +86 15914336545; Email: [email protected]) (Tel: +86 18613170205; Email: [email protected])

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Abstract The removal and inactivation of waterborne pathogens from drinking water are important for

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human

health.

Here,

a

polyacrylonitrile/polyaniline/silver

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(PAN/PANI/AgNWs-CC) composite nanofiber membrane was fabricated using a simple and rapid

22

co-electrospinning process, and an electrical device was applied with the PAN/PANI/AgNWs-CC

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filter for water electrochemical disinfection. The effects of voltage, flow rate and microbial

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concentration on the filtration and electrochemical disinfection performance of the nanocomposite

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membrane were investigated. The characterization results show that PAN/PANI/AgNWs with

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uniform diameters and without beads were successfully fabricated on CC. AgNWs were uniformly

27

distributed in the PAN/PANI/AgNWs. The PAN/PANI/AgNWs-CC filter was an effective sieve for

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completely removing both Escherichia coli and Staphylococcus aureus in the absence of applied

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voltage, and the sieved bacteria were completely inactivated by the released silver within 8 h. Over

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99.999% inactivation of the sieved bacteria was achieved within a few seconds by concurrent

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filtration and electrochemical disinfection under a voltage of 3 V. This high performance is enabled

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by means of an electrical mechanism, and an extremely high electric field induces sharp AgNWs tips

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to generate electroporated pores in the bacteria. The electrochemical PAN/PANI/AgNWs-CC

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membrane is an excellent material with potential application value in point-of-use drinking water

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treatment.

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nanowires-carbon

fiber

cloth

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TOC/Abstract art

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Introduction

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The diseases and death caused by waterborne pathogens in developing countries are receiving

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increasing attention as primary public health concerns 1. According to the 2014 World Health

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Organization report, more than 700 million people, particularly in developing and rural areas, lack

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adequate safe drinking water, which results in 2 million deaths from diarrhea annually

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insufficient funds, a centralized water supply is limited in dispersed rural areas. Additionally, owing

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to the generation of toxic disinfection by-products (DBPs), ineffectiveness against resistant

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microorganisms and difficulty in managing equipment, conventional disinfection methods are not

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suitable for developing and rural areas that lack water utilities. Developing innovative point-of-use

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(POU) methods for the removal and inactivation of waterborne pathogens is urgently needed.

2, 3

. Due to

50

Although membrane filtration is a preferred disinfection method for POU 4, pathogens can only be

51

removed but not inactivated by traditional membrane filtration, and the consequent membrane

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fouling problem and potential environmental risks cannot be ignored. Developing a new type of

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membrane that simultaneously removes and inactivates water pathogens is thus important in

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membrane application.

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Electrospinning has emerged as a powerful technique for fabricating polymer nanofiber membrane

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because of its adaptability and utility 5. Electrospun polymer fibers possess characteristics such as a

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high specific surface area, lightweight and flexibility that make them applicable to such fields as

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substrates or scaffolds 5, 6. Because the electrospun nanofibrous membrane porosity is adjustable 7, it

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can overcome the low-flux and high-fouling performance of porous polymeric membranes that are

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manufactured by conventional methods. Moreover, by incorporating superior electrical conducting

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materials into the conducting polymer, electrospun conducting fibers can possess better electrical

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properties and can be used extensively in electronic devices 8. Silver nanomaterial is known for its high electrical conductivity, highest antibacterial activity, 9, 10

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biocompatibility and infrequent release of harmful DBPs into water

. Although it has

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disadvantages, such as difficulty in separation, recovery and reuse, immobilizing nanosilver on

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electrospun nanofibers offers a new way to achieve the separation and reuse of nanosilver both

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simply and effectively.

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Through the inactivation mechanisms of electroporation and reactive oxygen species (ROS),

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electrochemical disinfection is effective for both viral and bacterial inactivation in water in a short

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time at relatively low voltage and without DBPs 11-15. Electrochemical devices can be designed to be

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portable and compact 13, 16, 17. Therefore, it is expected that electrochemical disinfection devices with

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polymer/nanosilver electrospun nanofiber membranes would be effective and promising for POU

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water disinfection.

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In this study, membrane filtration and an electrochemical disinfection process were combined.

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PAN nanofibers were fabricated by electrospinning using N,N-Dimethylformamide (DMF) as a

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solvent. The obtained PAN nanofibers, with their large specific surface areas and good solvent

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resistance properties, can act as an effective substrate. But the poor electrical conductivity of PAN

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nanofiber is a setback in our electrochemical system, which can be enhanced by doping conducting

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materials. Given PANI’s special conduction mechanism, unique electrical property, relative

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environmental stability and controllable electrochemical property and processability

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PANI into PAN nanofibers to fabricate PAN/PANI composite nanofibers with a higher electrical

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conductivity. Silver nanowires (AgNWs) were added to the electrospun solution to compose

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PAN/PANI/AgNWs, thus ensuring the antibacterial and conductive performance of the composite

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, we doped

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nanofibers. Allowing for the not high stiffness and mechanical strength of the nanofiber membrane,

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carbon fiber cloth (CC) was chosen as the collected electrospinning nanofiber substrate, which would

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improve its strength and thus expand the application of nanofiber membrane. Importantly, the porous

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and conductive carbon fiber cloth can be well run in our filtration/electrochemical disinfection

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system. Then, the antimicrobial and conductive PAN/PANI/AgNWs-CC nanocomposite membrane

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was fabricated through a simple electrospinning process. Field emission scanning electron

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microscopy (FESEM), x-ray diffraction (XRD), transmission electron microscope (TEM), energy

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dispersive x-ray (EDS) and graphite furnace atomic absorption spectrometry (GFAAS) were used to

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analyze the physicochemical properties of the synthesized nanocomposites. Then, an electrochemical

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filter device with a PAN/PANI/AgNWs-CC nanocomposite membrane was structured for the

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simultaneous removal and inactivation of both Gram-negative Escherichia coli (E. coli) and

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Gram-positive Staphylococcus aureus (S. aureus). The effects of microbial concentration, voltage

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and flow rate were investigated to evaluate the membrane filtration and electrochemical disinfection

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performance. The filtration and electrochemical mechanism was investigated by measuring the silver

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concentration of the effluent and observing SEM images of bacteria after electrolysis at different

99

voltages. This work offers insights for new silver nanocomposites synthesis strategies for

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electrochemical POU drinking water disinfection.

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2. Materials and Methods

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2.1 Materials and chemicals

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PAN with an average Mw of 150,000 was obtained from Shanghai Macklin Biochemical Co. Ltd

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(Shanghai, China). PANI (Mw ~ 20,000) was obtained from Sigma–Aldrich (USA). AgNWs with a

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diameter of approximately 50 nm and length of approximately 20 µm were acquired from Changsha

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Wei Xi New Materials & Technology Company (Changsha, China). CC with a thickness of 0.5 mm

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was purchased from Tianxiang Textile Technology Company (Shanghai, China). All chemicals used

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in the study were of analytical grade.

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2.2 Preparation of electrospun solution

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First, 1.8 g PAN was dissolved in 18.2 g DMF at 80 °C and magnetically stirred for 50 min to

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obtain the PAN electrospun solution (9 wt %). Then, 0.4 g PANI was added to the PAN solution (9

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wt %) and magnetically stirred at room temperature for 12 h to acquire the PAN/PANI electrospun

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solution (PAN 9 wt %, PANI 2 wt %). To prepare the PAN/PANI/AgNWs electrospun solution, first,

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the mixture of 1.8 g PAN powder and 12.85 g DMF was magnetically stirred for 50 min at 80 °C.

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After PAN was dissolved completely in DMF, 5 ml AgNWs solution (dispersed in DMF, 20 mg/ml)

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was added to the solution and magnetically stirred at room temperature (~ 28 °C) for 4 h. Ultimately,

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0.4 g PANI was added to the solution and magnetically stirred at room temperature (~ 28 °C) for 12

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h followed by probe sonication (JY92-IIDN, Scientz biotechnology Co. Ltd, Ningbo, China) for 1 h.

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2.3 Electrospinning nanofibers

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Electrospinning machines, DT-200 from Dalian Dingtong Science and Technology Development

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Co. Ltd. (China), were utilized to prepare the nanofibers. The obtained PAN electrospun solution was

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fed into a 10 mL 19 gage stainless steel needle syringe. The stainless steel needle was linked to the

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positive electrode, which was applied at 21 kV. The solution flow rate in the syringe was maintained

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at 0.3 mL/h by a syringe pump. The stainless needle and collection cylinder were separated by 16 cm.

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Aluminum foil or a CC (prior to use, CC was immersed in acetone overnight to remove the residual

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organics, rinsed with deionized (DI) water and dried) was taped on the collected cylinder. The PAN

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nanofibers were collected on the collective cylinder to produce an electrospun PAN nanofiber film

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on aluminum foil or an electrospun PAN-CC composite film. The preparation of PAN/PANI-CC and

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PAN/PANI/AgNWs-CC resembled that of PAN-CC.

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2.4 Characterization

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The surface and cross section morphologies of the as-obtained composites were investigated using

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a field emission scanning electron microscope (FESEM, Zeiss Merlin) and an accelerating voltage of

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5 kV. Transmission electron microscope (TEM) observations were made using a Hitachi TEM

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system at 80 kV. The structure phase was analyzed by XRD (Bruker D8 Advance). Energy dispersive

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X-ray spectra (EDS) were obtained on a JEOL-2010 microscope operating at an accelerating voltage

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of 200 kV.

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2.5 Disinfection experiments

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The disinfection performance of PAN/PANI/AgNWs-CC nanofiber membrane was evaluated using both Gram-negative E. coli (ATCC 25922) and Gram-positive S. aureus (ATCC 6538). For the filtration removal and inactivation tests, the same nanocomposite membrane filtration 19

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device described in our previous study

was applied. First, 10 mL of bacteria suspensions at

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different concentrations (105, 107 CFU/mL) was passed through PAN/PANI/AgNWs-CC (PAN-CC,

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PAN/PANI-CC) membranes. Effluents were collected in autoclaved containers. Aliquots (100 µL) of

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effluents were spread over an agar plate and incubated at 37 °C for 24 h. To assess the viability of the

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bacteria adsorbed on nanocomposite membrane, the membrane was removed from the device and

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transferred into an autoclaved glass vial containing 10 mL of PBS after filtration. The vial was bath

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sonicated for 8 min to remove bacteria from the membrane. Then, 100 µL of suspension was added

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to a standard agar culture medium and incubated at 37 °C for 24 h. After cultivation, the numbers of

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colony cells were counted to identify the cell concentrations. Bacterial inactivation efficiency was

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calculated using the following equation: Bacterial inactivation efficiency = Log10 (C0/C)

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(1)

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where C0 and C were the number of activated colony cells before and after each experiment,

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respectively.

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For static bactericidal tests, 1 x 2.5 cm nanofiber composite membranes were positioned in petri

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dishes and sterilized on both sides by exposure to UV for 30 min. The solutions containing bacteria

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(50 µL, 107 and 105 CFU/mL) were introduced onto the membranes. Samples with the bacterial

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solutions were incubated at 37 °C for 8 h. To resolve the potential problem of water evaporation from

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the bacterial solution, petri dishes were sealed with sealing rubber. At the same time, a beaker filled

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with DI water was put at the bottom of the incubator. To dissociate bacteria that had adhered to

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membrane surfaces, we positioned membranes in 10 mL centrifuge tubes and added 3 mL phosphate

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buffer solution (PBS), followed by ultrasonication for 8 min. Finally, 100 µL of suspension was

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added to a standard agar culture medium and incubated at 37 °C for 24 h.

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For the electrochemical disinfection test, an electrochemical composite membrane filtration device 19

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was used as described in our previous study

. NaCl, the background electrolytic throughout the

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experiment, is a common electrolytic water treatment and is ubiquitous in aquatic systems. All

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electrolysis experiments were completed at 10 mM NaCl. The electrochemistry was driven by a

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PS-302DM DC power supply (Longwei Instruments Co. Ltd, HK). In all cases, electrolysis was

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completed at constant voltages of 1.5, 3, 4.5 and 6 V. Here, 10 mL of bacteria suspension (107

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CFU/mL) was pumped through composite film filters under different voltages (0, 1.5, 3, 4.5, 6 V) at

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different flow rates (1, 2, 4 mL/min) using a peristaltic pump. In addition, to investigate the

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reusability and stability of the composite nanofiber membrane, a 5-recycles and a 10 h continuous

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operating electrochemical disinfection tests were carried out. In the 5-recycles test, 10 mL of bacteria

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suspension (107 CFU/mL) was pumped through composite filters under different voltages (3, 6 V) at

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the flow rate of 1 mL/min. And membrane was washed by DI water before each recycle. For the 10 h

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continuous operating test, at a flow rate of 1 mL/min, bacteria suspension (107 CFU/mL) was

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continuously pumped through composite filters under different voltages (3, 6 V) for 10 h. The

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viability of bacteria adsorbed on nanocomposite membrane was assessed in the same manner as the

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filtration test.

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FESEM was performed to examine the effects of electrochemistry on E. coli cell morphology on

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PAN/PANI/AgNWs-CC membrane. After the completion of electrolysis, membranes were fixed with

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2.5% glutaraldehyde for 2 h and cleaned three times with 0.2 M phosphate buffer. Then, samples

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were dehydrated in a series of ethanol solutions (30, 50, 70, 85, 95 v/v%) for 10 min sequentially,

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with the final dehydration completed by immersion twice in pure ethanol for 20 min. Samples were

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freeze dried and sputtered with gold for SEM imaging.

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Three parallel experiments were run throughout the study, and every culture was prepared in

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triplicate.

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2.6 Released silver concentrations from nanocomposite membranes

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To probe the inactivation mechanism and determine potential for composite nanofibers health risks,

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the silver concentrations in the effluents were scaled. The total silver (ion and metal) concentrations

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in effluents were analyzed by graphite furnace atomic absorption spectrometry (GFAAS,

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PerkinElmer, PinAAcle 900T). The test procedures were described previously in detail 19.

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3. Results and Discussion

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3.1 Characterization of composite nanofibers

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The morphology of composite nanofibers is shown in Fig. 1. As shown in Fig. 1 (a) and (d), PAN

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nanofibers without beads were randomly deposited to form a nonwoven mat. Smooth surfaces and

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round cross sections of PAN nanofibers were clearly observed. PAN nanofibers diameters were

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uniform and ranged from 140 to 200 nm. Compared with PAN nanofibers, PAN/PANI nanofibers had

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similar morphology, smooth surfaces and uniform diameters without beads (Fig. 1 (b) and (e)).

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Furthermore, as can be seen from the FTIR spectra of PAN nanofiber, PANI powder and PAN/PANI

200

composite nanofiber (Fig.S2), PANI is present in the composite and there was no chemical reaction

201

between PANI and PAN. These results illustrate that high quantities of PAN nanofibers and

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PAN/PANI nanofibers can be successfully manufactured by electrospinning.

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Fig. 1 (c) and (f) show the PAN/PANI/AgNWs composite nanofiber morphology. The diameters of

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PAN/PANI/AgNWs composite nanofibers were not as uniform as those of PAN nanofibers and

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PAN/PANI nanofibers, which might have been caused by AgNWs traversing the composite

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nanofibers (Fig. 2 (a)). However, there were no obvious beads on the PAN/PANI/AgNWs composite

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nanofibers. These results indicate that a relatively high quantity of PAN/PANI/AgNWs composite

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nanofibers can be fabricated by co-electrospinning.

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Fig. 1. FESEM images of (a, d) PAN, (b, e) PAN/PANI, (c, f) PAN/PANI/AgNWs nanofibers at different magnifications.

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TEM was used to identify the existence and distribution of AgNWs in the composite nanofibers.

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Fig. 2 (a) and (b) show representative TEM images of PAN/PANI/AgNWs composite nanofibers,

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which reveal the successful embedment of AgNWs with 50-nm-diameters in composite nanofibers.

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Fig. 2 (b) shows that some of the incorporated AgNWs were dispersed inside the nanofibers and

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completely aligned along the nanofibers’ axes without any aggregation, causing nanofiber electrical

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conductivity improvement. From Fig. 2 (a), it can be confirmed that some of the incorporated

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AgNWs crossed the fiber and were partly exposed to the external of the nanofibers instead of being

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parallel to the fiber. The distribution of AgNWs concurrently enhanced the electrical conductivity

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and intrinsic antibacterial performance of the composite nanofibers. Moreover, nanofibers could limit

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the excess of released silver and provide composite nanofibers with persistent antimicrobial activity.

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AgNWs embedded in nanofibers were further verified by EDS analysis, and the loading amount

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was estimated as 5.22 wt.%, as shown in Fig. 2 (c). According to EDS mapping images (Fig. S4) of

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the PAN/PANI/AgNWs, AgNWs were uniformly distributed in the composite nanofibers.

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Fig. 2. (a) (b) TEM images of PAN/PANI/AgNWs at different magnifications. (c) EDS analysis of

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PAN/PANI/AgNWs.

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3.2 Disinfection efficacy of nanocomposite membranes

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3.2.1 Filtration and static bactericidal performance of nanocomposite membrane

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The bacterial filtration removal efficiencies of nanocomposite membranes are shown in Fig. 3. No

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E. coli and S. aureus colonies formed on the re-incubated agar culture medium of effluents with

232

initial microbial concentrations of both 105 and 107 CFU/mL, which indicated that all bacteria were

233

removed by a sieving mechanism. This result is consistent with the results of 13.8 nm pore size

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nanofibrous membrane filtration. In our previous study 19, AgNW-CC only removed 2.2 log E. coli at

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most by filtration. The results illustrate that the electrospun nanofibrous membrane is of great

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significance because it can act as an effective filter for the complete removal of E. coli and S. aureus.

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Fig. 3. Removal efficiency of E. coli and S. aureus at different concentrations in filtration tests: (a) E. coli, (b)

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S. aureus.

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E. coli and S. aureus were completely removed from the influent by the sieving mechanism.

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However, sieved bacteria can be released and cause membrane biofouling during continuous

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filtration. Therefore, it is necessary to evaluate the extent of bacterial inactivation sieved on the

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nanocomposite membrane. The inactivation efficiencies of sieved bacteria on different

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nanocomposite membranes are presented in Fig. 4; those of E. coli and S. aureus on PAN-CC

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membranes were measured as 1.9 log and 1.4 log, respectively. For the PAN/PANI-CC membrane,

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inactivation efficiencies of E. coli and S. aureus were measured as 1.8 log and 1.6 log, respectively.

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These phenomena were caused by nonculturable bacteria. The inactivation efficiencies of E. coli and

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S. aureus for the PAN/PANI/AgNWs-CC membrane were slightly higher than those of the PAN-CC

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and PAN/PANI-CC membranes, which suggests that the released silver from AgNWs can contribute

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to inactivation efficiency.

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Fig. 4 Inactivation efficiencies of sieved bacteria on different nanocomposite membranes in filtration tests (107

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CFU/mL of bacterial concentration of influent)

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The static bactericidal effects of the three types of nanocomposite membranes are shown in Fig. 5.

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Fig. 5 provides typical photographs of E. coli and S. aureus colonies on three types of nanocomposite

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membranes starting with different bacterial concentrations. For PAN-CC and PAN/PANI-CC,

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continuous bacterial colonies were prevalent on the agar culture medium at both bacterial

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concentrations, implying that E. coli and S. aureus could survive on the surface of PAN-CC and

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PAN/PANI-CC. As shown in Fig. 5 left, no bacteria colonies were seen on agar medium at either

260

bacterial concentration, indicating that PAN/PANI/AgNWs-CC has significant antimicrobial activity

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against E. coli and S. aureus. The results above indicate that PAN-CC and PAN/PANI-CC exhibited

262

no antimicrobial effect but that AgNWs played a key role in the antibacterial performance of the

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composites. It can be concluded that the silver released from AgNWs is the primary cause of this

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result. The silver released included Ag+ and AgNWs, which was generated from the oxidation of

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AgNWs

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oxidative damage, and DNA damage in pathogens

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excellent intrinsic antibacterial activity. The results shown in Fig. 5 also indicate that sieved bacteria

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can be completely inactivated within 8 h from the silver released.

20

and detachment from composite nanofibers. They can cause cell membrane damage, 21, 22

; thus, PAN/PANI/AgNWs-CC possesses

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Fig. 5 Typical photographs of re-cultivated (a) E. coli colonies and (b) S. aureus colonies on agar culture

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plates, with seeded bacterial concentrations on composite nanofiber membrane of 107 CFU/mL and 105

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CFU/mL.

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3.2.2 Electrochemical disinfection performance of nanocomposite membrane

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Electrochemical bacterial inactivation efficiencies using different membranes under different

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voltages and flow rates are presented in Fig. 6. The effects of voltage are shown in Fig. 6 (a) and (b).

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E. coli and S. aureus inactivation efficiencies were both greatly enhanced with increasing voltage,

278

which indicated that a stronger electric field results in more efficient disinfection. In PAN-CC

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experiments, the maximum inactivation efficiencies of E. coli and S. aureus were only 3.6 log and

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3.3 log, respectively. For PAN/PANI-CC, which had higher electrical conductivity, the maximum

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inactivation efficiencies of E. coli and S. aureus were improved to 4.3 log and 4.5 log, respectively.

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According to the Nyquist plots of different nanofiber membranes (Fig. S5), compared with PAN-CC

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membrane, PAN/PANI-CC membrane might exhibit a faster electron transfer rate and better

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electrochemical performance under the relatively high applied voltages, resulting in higher

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inactivation efficiencies of sieved bacteria of PAN/PANI-CC membrane. However, when the applied

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voltage was below 3 V, the inactivation efficiencies of sieved bacteria of PAN/PANI-CC membrane

287

had no significant increase to PAN-CC membrane, for the improvement of conductivity of doping

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PANI

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PAN/PANI/AgNWs-CC

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PAN/PANI/AgNWs-CC, at 6 V applied voltage, E. coli and S. aureus were both completely

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inactivated (7 log), which illustrated that AgNWs on nanofibers had an important germicidal effect.

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The nanocomposites without AgNWs exhibited lower disinfection performance.

on

the

composite

material

exhibited

might much

not better

tremendous

enough.

disinfection

In

comparison,

performance.

For

293

The disinfection performance of nanocomposite membranes was also investigated at various flow

294

rates, and the results are shown in Fig. 6 (c) and (d). With increasing flow rates, the E. coli and S.

295

aureus inactivation efficiencies both decreased, which indicates that longer contact time can enhance

296

disinfection efficiency.

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297 298

Fig. 6. Inactivation efficiencies of sieved E. coli and S. aureus in electrochemical disinfection tests: (a), (c)

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effect of voltage (1 mL/min flow rate ); (b), (d) effect of flow rate (6 V).

300

According to Liu’s report, silver nanowire-carbon nanotube coated polyurethane sponge only

301

inactivated 0.2 log of E. coli without an electric field but achieved over 6 log in an electric field 14.

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AgNW/CNT cotton inactivated 77% of E. coli at + 20 V, whereas at 0 V, it inactivated approximately

303

20% 16. Our experimental results are consistent with the above studies. Fig. 6 (a) and (b) show that

304

increasing voltage played an effective role in enhancing the disinfection efficiency. Increasing

305

voltage can likewise improve energy consumption. Flow rate was also an important factor:

306

decreasing the flow rate provided better disinfection. However, the filtration efficiency and energy

307

consumption were also influenced by changing flow rate. Thus, comprehensive considerations must

308

be taken into account for both energy consumption and disinfection performance in practical

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applications.

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Increasing voltage and decreasing flow rate are conducive to enhancing the disinfection

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performance of nanocomposites. However, considering efficiency and economy, a 3 V applied

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voltage and 1 mL/min flow rate, which inactivated more than 99.999% bacteria under this condition,

313

is recommended.

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Additionally, as shown in the left of Fig. 7, in the 5-recycles test, the inactivation efficiencies of

315

E.coli of each recycle were 5, 5.2, 4.7, 4.9 and 4.6 log under the applied voltage of 3 V, respectively.

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As shown in the right of Fig. 7, when E. coli was electrolyzed at 3 V, the inactivation efficiencies of

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E.coli was decreased to 4.1 log after 10 h continuous operation. Similarly, the electrochemical

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inactivation efficiencies of E. coli were both slightly decreased after 5-recycles and 10 h continuous

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operation at 6 V. These results revealed that PAN/PANI/AgNWs-CC has a relatively good

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disinfection reusability and stability under a relatively low flow rate.

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According to the changes of the enfluent flow rate in 10 h continuous operating test with different

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E.coli influent concentration (Fig. S6), it could be inferred that PAN/PANI/AgNWs-CC has a

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relatively good flux under a relatively low bacterial concentration influent, but its flux would be

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obviously decreased when the bacterial concentration and flow rate of influent were high enough.

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Fig. 7. Inactivation efficiencies of sieved E. coli in 5-recycles and 10 h continuous operating electrochemical

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disinfection tests at 3, 6 V (1 mL/min flow rate ).

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3.2.3 Electrochemical disinfection mechanism of nanocomposite membrane

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With applied voltages of 0 to 6 V, the effluent silver concentrations were all below 15 ppb, which

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is far below the national drinking water standard of 100 ppb. This result shows that silver release can

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be controlled by embedding AgNWs on nanofibers and that the electrochemical filtration system is

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safe for drinking water disinfection. Although the contact time between silver and bacteria was very

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short, these results also indicate that the intrinsic antimicrobial activity of silver (metal 23, 24 and Ag+

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ions 20, 25) could not account for the dramatic disinfection efficiency enhancement in an electric field.

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The excellent disinfection capability against E. coli and S. aureus in this study revealed that the

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electric field plays an important role in electrochemical disinfection. FESEM was performed to

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evaluate the voltage effects on cell morphology of model bacteria E. coli on PAN/PANI/AgNWs-CC

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membrane. Typical FESEM images of E. coli in contact with PAN/PANI/AgNWs-CC membranes

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after electrolysis at 0, 3, and 6 V are presented in Fig. 8. FESEM images (Fig. 8 (a) − (d)) clearly

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show morphological changes after electrolysis at different applied voltages. As shown in Fig. 8 (a),

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when no voltage was applied, cells were still intact and plump. When E. coli was electrolyzed on

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PAN/PANI/AgNWs-CC at 3 V, the cells were shriveled and dehydrated but maintained the

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macroscopic cell membrane structures (Fig. 8 (b)). When the applied voltage was increased to 6 V,

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small pores were observed on bacteria surfaces, and the cell membrane integrity was destroyed (Fig.

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8 (c) and (d)), which resulted in the release of cellular contents and the ultimate death of the

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microorganism.

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In electrochemical disinfection, a high-intensity electric field of brief duration can generate 14, 16, 26

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electroporated pores on the bacteria surfaces

. This was a pre-existing microbicidal

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electroporation mechanism. Electroporation causes damage to bacteria membranes, including the

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loss of semipermeability and occurrence of pores, which can lead to ion leakage and escape of

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metabolites. Severe electroporation-generated pores are irreparable and give rise to cell death. Dye

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staining experiments further confirmed that electroporation involves cellular membrane damage and

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eventual cell death 14, 26.

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In this study, as a result of the nanocomposites’ special one-dimensional nanostructure, an

355

extremely high electric field can be induced on the sharp nano-tips of AgNWs. Once the bacterial

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cells were attached to AgNWs, which exposed the external nanofibers (as shown in Fig. 2 (a)), the

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transmembrane potential induced by the high-intensity electric field would exceed the critical value.

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This process would cause tiny pores on the cell membranes. Electroporation could occur repeatedly

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and cause more pores on the membranes, leading to eventual cell death.

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Fig. 8. FESEM images of E. coli on PAN/PANI/AgNWs-CC filter after electrolysis in 10 mM NaCl at applied

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voltages of (a) 0 V, (b) 3 V, and (c), (d) 6 V.

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3.3 Potential environmental applications of PAN/PANI/AgNWs-CC nanofiber membrane

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filtration/electrochemical system

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The novel electrochemical PAN/PANI/AgNWs-CC membrane presented here can be applied as a

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drinking water purification technology for pathogen removal and inactivation. In this study, we

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combined physical membrane filtration with electrochemical disinfection, which provides

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advantages over using one process alone. Our electrochemical filtration system can be competitive

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with other POU technologies based on the following advantages:

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(1) Low cost of energetic requirements. For instance, at 3 V, the average current was 10 mA, and the

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average power was 30 mW; thus, the electricity usage only requires 0.125 KW·h to treat 1 m3 of

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water (4167 h at 4 mL/min).

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(2) Efficient bacterial removal and inactivation. Without applied voltage, PAN/PANI/AgNWs-CC

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membrane reduced the number of culturable bacteria in the effluent to 0, and the sieved bacteria

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could be completely inactivated in 8 h upon silver release. At the applied voltage of 6 V, the system

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could completely inactivate the sieved bacteria within a few seconds. According to Liu’s report,

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silver nanowire-carbon nanotube coated polyurethane sponge inactivated over 6 log of E. coli with

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applied voltage above 10 V 14. AgNW/CNT cotton inactivated 77% of E. coli at + 20 V and at a flow

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rate of 1 L/h with several seconds 16. rGO-Ag-CF inactivated 100% E. coli at 1.5 V with 5 min 17.

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Compared with these existed reports, the electrochemical disinfection performance (inactivated 7-log

381

at 6 V with several seconds) of our composite material can be very competitive.

382

(3) Short disinfection contact time. Our experiment results showed that a 7 log E. coli and S.aureus

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inactivation were achieved within only few seconds.

384

(4) The silver released was less than 15 ppb, which is far below the national drinking water standard

385

of 100 ppb.

386

There have also reported some nanocomposite materials applied in the bacteria filtration and 16

387

inactivation, such as AgNW/CNT cotton

, silver nanowire-carbon nanotube coated polyurethane

388

sponge 14, multiwalled carbon nanotube microfilter 13, etc. However, as far as we know, there is no

389

report about the PAN/PANI/AgNWs-CC composite nanofiber filtration membrane so far, which has

390

physical interception, conductive and antimicrobial performance and can be prepared by a facile and

391

rapid co-electrospinning technology. Nowadays, with its rapid development, electrospinning

392

technology makes it possible to prepare larger membrane which has a greater scope of application

393

and a better prospect. Importantly,

compared with other silver nanocomposite materials, the

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PAN/PANI/AgNWs-CC membrane in this work can restrict the excess of the released silver, thus

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endowing the composite nanofibers with persistent antimicrobial activity. Hence our study gives a

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new viewpoint of POU drinking water disinfection.

397

However, the expensive silver nanowire has limited the application of the membrane

398

filtration/electrochemical system. The synthesis of other nanomaterial (such as cheaper CuO wire)

399

may address this challenge. Moreover, the actual performance of PAN/PANI/AgNWs-CC in field

400

applications is unknown. It is likely that raw water quality could affect the performance and lifespan

401

of PAN/PANI/AgNWs-CC, such as ion species and dissolved oxygen. Additionally, the disinfection

402

of viruses and other microbial pathogens by this system should be investigated before putting into

403

practical application. In brief, although there are many human health problems that still need to be

404

solved urgently, the superb disinfection performance and the simple and rapid synthesis method

405

offered by PAN/PANI/AgNWs-CC make it a great potential option to provide safe drinking water in

406

less developed or rural areas.

407

Supporting Information

408

Detail information were presented in this part, including further characterization of the PAN/PANI

409

nanofiber (FESEM, FTIR spectra) and AgNWs (FESEM, TEM, XRD), EDS mapping images of

410

PAN/PANI/AgNWs, Nyquist plots of different nanofiber membranes and the tabulation of previous

411

reports about silver nanocomposite material for water disinfection.

412

Acknowledgments

413

The authors are grateful for the financial support provided by the National Natural Science Fund

414

of China (No. U1401235). The authors thank Professor Weijia Zhou, Jian Sun, Yaping Zhang,

415

Junfeng Chen, Zhuyu Niu, Cao Yang, Yuancai Lv for valuable comments on this manuscript.

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