Carbon Fiber-Based Flow-Through Electrode System (FES) for Water

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Remediation and Control Technologies

Carbon Fiber-Based Flow-Through Electrode System (FES) for Water Disinfection via Direct Oxidation Mechanism with a Sequential Reduction-Oxidation Process Hai Liu, Xin-Ye Ni, Zheng-Yang Huo, Lu Peng, Guo-Qiang Li, Chun Wang, Yin-Hu Wu, and Hong-Ying Hu Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b07297 • Publication Date (Web): 15 Feb 2019 Downloaded from http://pubs.acs.org on February 16, 2019

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Environmental Science & Technology

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Carbon Fiber-Based Flow-Through Electrode System (FES) for Water

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Disinfection via Direct Oxidation Mechanism with a Sequential Reduction-

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Oxidation Process

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Hai Liu†, Xin-Ye Ni†, Zheng-Yang Huo†, Lu Peng†, ‡, Guo-Qiang Li†, Chun Wang†,

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Yin-Hu Wu*,†, and Hong-Ying Hu*,†, ‡

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†Environmental

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Environmental Protection Key Laboratory of Microorganism Application and Risk

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Control (SMARC), School of Environment, Tsinghua University, Beijing 100084, PR

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China.

Simulation and Pollution Control State Key Joint Laboratory, State

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‡Shenzhen

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Laboratory, Tsinghua-Berkeley Shenzhen Institute, Shenzhen 518055, PR China.

Environmental Science and New Energy Technology Engineering

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* Corresponding author:

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Yin-Hu Wu: E-mail: [email protected]; Tel: +86-10-62797265

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Hong-Ying Hu: E-mail: [email protected]; Tel: +86-10-6279-4005

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† Supporting information (SI) available.

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1

ACS Paragon Plus Environment


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ABSTRACT ART

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ABSTRACT

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Flow-through configuration for electrochemical disinfection is considered as a

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promising approach to minimize the formation of toxic byproducts and energy

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consumption via the enhanced convective mass transport as compared with

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conventional flow-by one. Under this hydrodynamic condition, it is essential to

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ascertain the effect of sequential electro-redox processes with the cathode/anode then

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anode/cathode arrangements on disinfection performance. Here, carbon fiber felt (CFF)

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was utilized to construct two flow-through electrode systems (FESs) with sequential

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reduction-oxidation (cathode-anode) or oxidation-reduction (anode-cathode) processes

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to systematically compare their disinfection performance towards a model Escherichia

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coli (E. coli) pathogen. In-situ sampling and live/dead backlight staining experiments

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revealed that E. coli inactivation mainly occurred on anode via an adsorption-

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inactivation-desorption process. In reduction-oxidation system, after the cathode-

34

pretreatment, bulk solution pH increased significantly, leading to the negative charge

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of E. coli cells. Hence, E. coli cells were adsorbed and inactivated easily on the

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subsequent anode, finally resulting in its much better disinfection performance and

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energy efficiency than the oxidation-reduction system. Application of 3.0 V resulted in

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~6.5 log E. coli removal at 1500 L m-2 h-1 (50 mL min-1), suggesting that portable

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devices can be designed from CFF-based FES with potential application for point-of-

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use water disinfection.

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INTRODUCTION

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Disinfection for drinking water has been an indispensable process to the reduction

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in global mortality and morbidity by eliminating life-threatening diseases such as

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dysentery, cholera and typhoid. Despites decades of effort and progress, up to 660

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million people still do not have access to improved drinking-water source and sanitation

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estimated by the WHO/UNICEF Joint Monitoring Programme.1 Electrochemical

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disinfection has emerged as one of the most feasible technique with extended prospects

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for non-centralized water disinfection in rural areas due to its high energy efficiency,

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small occupied area, convenient automatic operation, process adaptability, and

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environment-friendly nature.2

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During electrochemical process, generally, pathogen can be inactivated near the

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electrode-electrolyte interface via direct and indirect oxidation pathways. Direct

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oxidation involves electron transfer between anode and cell membrane,3 which is

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affected mainly by mass diffusion, electrode surface area, and anode potential. Indirect

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oxidation relies on the production of reactive species from electrolyte redox reaction,

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such as hydroxyl radical (•OH) and hydrogen peroxide (H2O2) that subsequently

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mediate the pathogen inactivation in solution.4 The species and concentration of these

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generated oxidants depend primarily on the composition of electrolyte and electrode

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potential. In an electrochemical system, the contributions and functions of direct and

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indirect oxidation to pathogen inactivation are strongly dependent on electrode

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configurations, besides the basic properties of electrode material, experimental

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conditions, and electrolyte composition.

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For industrial electrochemical processes, parallel-plate electrode reactors operated

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in flow-by or sequencing batch modes have been applied successfully owing to their

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cost effectiveness, practicability and maneuverability. On account of the strong mass

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transfer limitation and low ratio of electrode area to reactor volume, direct oxidation is

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not significant in these reactors, and higher applied voltages or current densities are

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required to promote the generation of reactive species (Cl2, H2O2, and •OH), which leads 4


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to a drastic enhancement of energy consumption and undesired side reactions, such as

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oxygen evolution, and reactive chlorine and toxic byproducts generation.5-7 In addition,

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previous works also confirmed that direct oxidation of both bacteria and virus required

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lower anodic potentials than water electrolysis for oxygen or oxidants generation.8, 9 To

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address these issues, flow-through electrochemical reactors using porous three-

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dimensional electrodes with high surface area have been developed to enhance the

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convective transport of pathogen cells to the electrode surface. In turn, the enhanced

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mass transfer enables the pathogen inactivation via direct oxidation at low applied

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voltages,10 resulting in reduction of energy requirements, and avoiding generation of

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undesirable disinfection byproducts.

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Numerous

bench-scale

researches

have

demonstrated

the

ability

of

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electrochemical disinfection process to inactivate a range of bacteria and viruses, which

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also systematically investigated the effects of operational variables, water quality

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parameters, and electrode materials on disinfection performance. It has been established

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that the electrochemical process is accompanied by production of H+ ions on anode and

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OH- ions on cathode as resultants, or by consumption of OH- ions on anode and H+ ions

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on cathode as reactants.11, 12 Unlike flow-by reactor, in the flow-through reactors, the

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pathogen suspension will be reduced on the first cathode or oxidized on the first anode,

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followed by oxidation on the second anode or reduction on the second cathode. As a

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result, the disinfection ability of second electrode is bound to be affected by first

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electrode when the pathogens can permeate to the second electrode. However, the flow-

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through electrode system (FES) with sequential oxidation-reduction4,

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reduction-oxidation11, 12, 16, 17 processes were both utilized in electrochemical process.

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Additionally, limited relevant studies can be found in literature about using sequential

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oxidation-reduction process to recognize the synergism during flow through E-Fenton

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oxidation of oxalate,18 and to demonstrate the ability of the sequential reduction-

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oxidation process for nitrobenzene mineralization.19 Thus, it is of paramount

13-15

and

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importance to investigate the impacts of electrode arrangements on disinfection

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performance, and reveal the underlying mechanisms in a flow-through system.

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Various types and textures of materials have been used successfully as flow-

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through electrodes, such as granular activated carbon or graphite fixed beds,20 carbon

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fiber cloths/felts,21-23 packed metal mesh,13 nanowires-decorated foam,24, 25 and carbon

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nanotube (CNT) membrane.9, 11, 17, 26 The elemental carbon-based porous materials are

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most highly recommended with features of flexible texture, high surface area and no

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toxic metal content, which achieved significant inactivation of both viruses and bacteria

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under applied voltage below 3.0 V. The most studied CNT nano-membranes with a

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fiber diameter of ~15 nm and aerial pore diameter of ~100 nm can achieve

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simultaneously filtration and inactivation of bacteria and virus. However, the large-

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scale application of the nano-membranes is limited by the toxic effects if ingested or

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released into the environment, and easy fouling by micron-sized bacteria. Carbon fiber

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felt (CFF) is a relatively inexpensive material with excellent electrical conductivity,

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microfibrous structure, and high surface area. Unlike the fiber bundles or tow structures

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of carbon fiber cloth, the randomly netlike structure of CFF (a kind of non-woven)

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ensures its high contact area, completely accessible surface of each micro-fiber for

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microorganisms, and large void space between fibers (up to 90%) for high fluid

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permeability, which make CFF promising for flow-through-design electrode

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application. However, previous works focused mainly on H2O2-mediated disinfection

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mechanism by CFF cathode,22, 27, 28 and limited attention is paid on the electrochemical

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inactivation of pathogen via direct oxidation with flow-through CFF anode.

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Herein, we utilize CFF as flow-through electrodes, and systematically compare

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the disinfection performance of two flow-through electrode systems (FESs) with

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sequential reduction-oxidation and oxidation-reduction processes towards a model

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pathogen (Escherichia coli) commonly detected in the aqueous environment. First, the

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operation performance of the two FESs as functions of applied voltage and liquid flow

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rate is examined, in terms of E. coli inactivation, active chlorine production, effluent 6


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pH, and current. Then, a combination of in-situ sampling experiments and

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electrochemical analyses are used to distinguish the disinfection functions of individual

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cathode and anode in the two FESs, and monitor the main factors causing the

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differences in operation performance of the two systems. Finally, batch experiments are

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conducted to further elucidate the underlying mechanisms of bacteria inactivation of

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individual cathode and anode in the two systems.

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MATERIALS AND METHODS

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Chemicals and Materials.

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All aqueous solutions were prepared using distilled deionized (DI) water with a

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resistivity of ≥18 MΩ cm-1 (Millpore, MilliQ Water System, USA). Carbon fiber felt

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(CFF) sheet was purchased from Liaoning Jingu Carbon Material (Co., LTD. China).

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The CFF electrodes with diameter of 5 cm and thickness around 4.7 ± 0.2 mm (Figure

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1) were washed and wetted by 1:1 DI-H2O:EtOH with sonication for 5 min to remove

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any impurities, washed with excess DI-H2O to remove the residual EtOH, and stored in

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the DI-H2O until use. The CFF electrode has a fiber diameter of 9.6 ±0.7 μm, pore size

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between the fibers of around 50-200 μm (see Figure 1), void content of ~90%, BET

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surface area of 4.5 m2 g-1, elemental carbon composition of ~98.0 wt.%, and

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conductivity of 3.8 S cm-1.

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CFF-Based Flow-Through Electrode Cells and Apparatus.

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The CFF-based flow-through electrode system (FES) used in the experiments (see

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Figure 1) was made of Plexiglas and constituted by a conic water distributor, two

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cylindrical electrode chambers, and a conic water collector. The electrode chamber had

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an inner diameter of 5 cm and a height of 4.7 mm, and the distributor/collector with a

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cone angle of 45o was settled with a PC perforated plate with pore diameter of 3 mm

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and thickness of ~2 mm. A piece of quantitative filter paper with pore sizes of 30-50

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μm and thickness of ~0.15 mm was used as insulator for the CFF cathode and anode.

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After assembling, the CFF electrodes were flatly fixed in electrode chambers by the

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two perforated plates, and electrically connected through titanium wires with a DC

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power supply source (DG1718E-5) and a CHI electrochemical analyzer (CHI660E, CH

154

Instruments).

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After influent was pumped into the FES and distributed vertically along the flow

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direction by the conic distributor, it can be reduced by cathode or oxidized by anode

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first, and further oxidized by anode or reduced by cathode, finally collected by the conic

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collector and discharged from FES (Figure 1). Therefore, according to the

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electrochemical reaction sequences of influent, FES can be involved in two systems,

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namely the reduction-oxidation (red-ox) system with cathode then anode sequence and

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the oxidation-reduction (ox-red) system with anode then cathode sequence. The two

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systems can be switched simply by changing the connection of the electrodes with the

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positive and negative poles of DC power supply.

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FES General Operation.

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Pure cultured Escherichia coli (ATCC 15597), provided by Institute of

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Microbiology, Chinese Academy of Sciences, was used as a model pathogen, and its

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cultivation and enumeration procedures were described in Supporting Information. E.

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coli containing approximately 106-107 CFU mL-1 was used for all disinfection

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experiments. The base electrolyte was constituted with aqueous solutions of 10 mM

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NaCl without pH adjustment (~6.15) to normalize the ionic strength and conductivity,

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which is commonly used for disinfection of synthetic drinking water.9, 29 A lake water

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and a reclaimed water (effluent from a water reclamation plant) from Beijing China

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were also used to investigate the disinfection ability of CFF-based FES (Table S1).

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Without application of cell voltage, influent was firstly pumped through the FES

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at 50 mL min-1 for 5 min using a peristaltic pump to rinse CFF electrodes and to remove

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any air in the distributor and collector that could affect water distribution. Our

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preliminary experiment also confirmed that the E. coli concentration in effluent was

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almost equal to that in influent after rinsing, meaning insignificant interception and

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adsorption abilities of CFF electrodes and filter paper towards E. coli cells. The results 8


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indicated that with application of cell voltage, the removal/inactivation of E. coli was

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attributed to the electro-adsorption and electro-inactivation. Then electrochemical

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disinfection was conducted at an applied voltage over the range of 2.0 to 3.5 V and a

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flow rate from 5 to 125 mL min-1, which corresponded to the flux of 150-4000 L m-2

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h-1 with permeability above 1.5*105 L m-2 h-1 bar-1.

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Effluent samples were collected in autoclaved centrifugal tubes, and the spread

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plate count was conducted immediately to avoid bacterial death during storage. Based

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on the change in the bacterial count of each sample after a certain disinfection

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experiment, the inactivation rate was calculated as the logarithmic reduction of bacteria

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(log N0/N), where N0 and N represent the bacterial concentrations in influent and

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effluent. All the disinfection experiments were performed in triplicate at least, and

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average results were given with error bars displaying the standard deviation.

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Analytical Methods

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The open-circuit potential (OCP) and electrochemical impedance spectroscopy

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(EIS) were completed with a CHI electrochemical analyzer with a three-electrode

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system: a CFF working electrode, a CFF counter electrode and an Ag/AgCl (3.5 M

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KCl) reference electrode. All electrode potential values were reported in V vs Ag/AgCl.

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The concentration of electrochemical oxidants (active chlorine) was determined by

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using N,N-diethyl-p-phenylenediamine (DPD) colorimetric method, and hydrogen

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peroxide concentrations were measured spectrophotometrically by the iodide method

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with detection limit of ∼1 μM.30 The CFF electrode were characterized with a field

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emission scanning electron microscopy (FE-SEM, FEI STRATA DB235) at a voltage

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of 10 kV. All characterization details can be found in Supporting Information.

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RESULTS AND DISCUSSION

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Impact of Electrode Arrangements on Electrochemical Disinfection.

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A series of experiments were conducted to study the dependency of E. coli

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inactivation on the applied voltage (2.5-3.5 V) under flow rates from 5 to 125 mL min-1.

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Figure 2A depicts a significant increase of log removal with increase in applied voltage

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or with decrease in flow rate for the two systems. Similar results were also reported that

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electrochemical disinfection was enhanced at smaller flow rate and higher applied

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voltage.7, 23, 31 Obviously, the disinfection performance of red-ox system was much

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better than that of ox-red system. In ox-red system, less than 3 log removal was obtained

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at 2.5 V/5-20 mL min-1 and 3.0 or 3.5 V/25-125 mL min-1. While in red-ox system, at

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applied voltage of 2.5, 3.0 or 3.5 V, above 6.5 log removal of E. coli was achieved

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under flow rates below 10, 50 or 100 mL min-1, respectively. There was no error on the

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log removal since in all experiments no culturable E. coli were measured in the effluents.

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Although the higher applied voltage resulted in a significant improvement of E.

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coli inactivation in the two systems via the enhancement of direct oxidation and

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oxidant-induced indirect oxidation, it is likely to consume more energy for other

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undesired side reactions, such as oxygen and hydrogen evolution, and to form harmful

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chlorine by-products which have carcinogenic potentials at 10 mM Cl- concentration.

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The currents of the two systems at applied voltage of 2.5, 3.0 and 3.5 V were further

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compared under flow rates from 5 to 125 mL min-1 (See Figure 2B). Overall, the

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currents of ox-red system were slightly higher than red-ox system, meaning more

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energy in ox-red system was consumed for other undesired side reactions. The energy

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consumption of red-ox system at applied voltage from 2.0 to 3.5 V and the

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corresponding highest flow rate under which no live E. coli cells were detectable in the

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effluent solutions were calculated (Figure S1) to be 13.3±0.8 W h m-3 at 2.0 V/4 mL

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min-1 (data not shown), 29.4±1.8 W h m-3 at 2.5 V/10 mL min-1, 46.7 ± 1.6 W h m-3 at

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3.0 V /50 mL min-1, 52.1 ±2.1 W h m-3 at 3.5 V/100 mL min-1. The lower applied

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voltage achieved less energy consumption since water electrolysis was promoted at

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higher applied voltage. The energy consumption of the CFF-based red-ox system

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compared favorable to earlier results using CNT/AgNWs/CuONWs electrodes (see

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Table S2).

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An in-situ sampling method was set up (Figure 2C) to assess active chlorine

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formation by anodic oxidation of Cl- ions in the two FES systems. The active chlorine

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concentration for influent after anode-treatment in the two systems, namely sample #1

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in ox-red system and sample #2 in red-ox system were investigated with DPD

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colorimetric method. Oxidant was not detectable with the DPD detection limit of 10 µg

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L-1 as Cl2 in sample #1 under applied voltage below 3.5 V in ox-red system, and in

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sample #2 under applied voltage below 3.0 V in red-ox system. Only at 3.5 V, the

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concentration of oxidants raised up to 0.025-0.35 mg L-1 in red-ox system (see Figure

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2D). As to the DPD colorimetric method for active chlorine measurement, it should be

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noted that, besides active chlorine (Cl2 and HClO), other oxidants such as H2O2 can

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also react with DPD and account for the magenta color.32 In red-ox system, with the

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cathode-pretreatment, H2O2 species can be produced from O2 reduction, and

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subsequently consumed by anodic oxidation. The residual H2O2 species in effluent may

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also react with DPD reagent. Therefore, for the red-ox system, the influent was also

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treated with Ar sparging to exclude the effect of H2O2 formed by cathodic reduction of

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dissolved O2. The generation of oxidant species under 3.5 V was also observed in

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sample #2, supporting that the oxidants in effluent from red-ox system were not H2O2

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species produced from cathodic reduction of O2 molecules (see Figure S2A).

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Since F- ion (2.66 V vs Ag/AgCl) has a much higher standard evolution potential

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than the Cl- ion (1.28 V vs Ag/AgCl), 10 mM NaF electrolyte (106-107 CFU mL-1 E.

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coli and pH 6.15) was utilized to confirm whether these oxidant species were active

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chlorine generated from anodic oxidation of Cl- ions. Under applied voltage of 3.5 V,

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no oxidant was detectable with both DPD and iodide colorimetric methods in sample

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#2 for the red-ox system, meaning that for the disinfection experiments using 10 mM

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NaCl electrolyte, these oxidants produced at 3.5 V in sample #2 were derived from Cl-

259

oxidation. The Cl-free electrolyte (10 mM NaF) was further utilized to distinguish

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between direct and indirect oxidation for E. coli inactivation under applied voltage of

261

3.0 or 3.5 V. There was a negligible difference in the E. coli log removals by using 10 11



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mM NaCl or NaF electrolyte at applied voltage of 3.0 V (data not shown), and at 3.5 V

263

a slight decrease in E. coli inactivation was observed using NaF electrolyte as compared

264

with using NaCl electrolyte (see Figure S2B), which can be attributed to the absence of

265

active chlorine generation. The above 6.5 log removal at 3.5 V and 75 mL min-1 using

266

NaF electrolyte also gave additional support for predominant direct oxidation

267

mechanism at 3.0 and 3.5 V in red-ox system.

268

On the basis of above results, from the aspects of energy requirements, operation

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of CFF-based FES with sequential reduction and oxidation processes at lower applied

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voltages (2.0 and 2.5 V in this study) were recommended, but its application may be

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restricted by its low amount of water production in a certain time or long operation time

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for treating a certain water amount. For the purpose of inhibiting the re-contamination

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of water between the time of clean water production and consumption, applied voltages

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above 3.5 V with active chlorine residual in effluent can meet these requirements. While,

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for the production of instant drinking water, applied voltage of 3.0 V was recommended,

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since relatively large amount of water treatment, low energy consumption, and low

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potential of byproducts formation were achieved here. In the following parts of this

278

study, the applied voltage of 3.0 V was used to further recognize the underlying

279

mechanisms causing the differences of the two systems in disinfection performance,

280

and to recognize the disinfection functions of the individual cathode and anode in the

281

two systems.

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The disinfection process was investigated by the live/dead fluorescent assays of

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the E. coli cells in effluent solution at operation parameters of 3.0 V and 50 mL min-1

284

with a laser scanning fluorescence microscopy. The water samples were stained with

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mixtures of SYTO9 and PI, which are a cell-permeable green-fluorescent stain labeling

286

both live and dead cells and a cell-impermeable red fluorescent stain labeling only dead

287

cells, respectively. As shown in Figure 3A, after operation for 1 min, the amount of

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dead cells in effluent (A2) (sample #2) was much less than the total amount of live and

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dead cells in influent (A1), meaning that a part of E. coli cells were adsorbed on the 12


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electrodes. However, after operation for 5 (A3) and 30 min (A4), insignificant

291

difference in cell amounts (510-550 cells per image) were observed for the dead cells

292

in two effluent solutions and the total cells in influent (A1), indicating an adsorption-

293

inactivation-desorption process under the existence of both electrostatic attraction and

294

flow scouring effect.

295

To confirm the flow scouring effect on the desorption of E. coli cells adsorbed on

296

the electrodes, the fluorescence microscopic images of E. coli cells attached on the

297

cathode and anode in red-ox system were investigated. After operation using E. coli

298

suspension (106-107 CFU mL-1, [NaCl] = 10 mM) at 3.0 V and 50 mL min-1 for 10 min

299

to reach adsorption and desorption equilibrium of E. coli cells, the system was

300

immediately flushed with 10 mM NaCl (without E. coli cells) at 50 mL min-1. During

301

the flushing process, to recognize the scouring effect, the continuous electrostatic

302

interactions between the E. coli cells and the electrodes were maintained by using

303

applied voltage of 3.0 V. After flushing for 2 (B) and 5 min (C), the cathode and anode

304

were taken out and sonicated in 30 mL DI water. The E. coli cells in the water samples

305

were harvested and used for staining experiments (see more details in SI). For the

306

electrodes after flushing with 10 mM NaCl for 2 min, more cells were retained by the

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anode (B3) as compared with the cathode (B1), reflecting the attraction interaction

308

between the E. coli cells and the anode. In addition, a small part of E. coli was

309

inactivated on the cathode (B2), whereas a significant decrease in cell viability was

310

observed for the cells on the anode (B4). These results also revealed the primary

311

contribution of anode to E. coli inactivation, which was consistent with the direct

312

oxidation mechanism. After flushing with 10 mM NaCl for 5 min, the few cells retained

313

by the two electrodes reflected that the adsorbed cells were released into bulk solution

314

by the flow scouring effect (Figure 3C). As shown in Figure 3D, main disinfection

315

occurred on anode in red-ox system. For the electrochemical inactivation process,

316

bacteria approached the electrodes via hydraulic action or electrostatic attraction (see

317

more details in the next part), then they were removed from aqueous solution by 13



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adsorption on the anode via electrostatic attraction, followed by inactivation at the

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anodic surface and release from the anode by flow souring effect.

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Generation of H+, OH-, and H2O2 species from Individual Anode and Cathode.

321

As discussed above, direct oxidation was the primary mechanism for E. coli

322

inactivation at applied voltage of 3.0 V. Electroporation was unlikely to be responsible

323

for the observed E. coli inactivation due to the flat carbon fiber tips (see Figure 1) and

324

low electric field strength used in our experiments (~200 V cm-1), which was much

325

weaker than the reported value of 105 V cm-1 for induction of breaking down cell

326

membrane.33 Other solution chemical conditions may be also responsible for E. coli

327

death, such as formation of H+-, OH--, and H2O2-concentrated interfaces of the

328

electrode surface and electrolyte.34, 35 During the electrochemical process, the H+, OH-

329

and H2O2 species can be generated via anodic oxidation of H2/H2O2/H2O (equations

330

(1)-(3)) and cathodic reduction of O2/H2O (equations (4)- (6).

331

𝐻2←→2𝐻 + + 2𝑒 ―

(1)

332

𝐻2𝑂2←→𝑂2 + 2𝐻 + + 2𝑒 ―

(2)

333

2𝐻2𝑂←→𝑂2 + 4𝐻 + + 4𝑒 ―

(3)

334

𝑂2 + 2𝐻2𝑂 + 2𝑒 ― ←→𝐻2𝑂2 + 2𝑂𝐻 ―

(4)

335

𝑂2 + 2𝐻2𝑂 + 4𝑒 ― ←→4𝑂𝐻 ―

(5)

336

2𝐻2𝑂 + 2𝑒 ― ←→𝐻2 + 2𝑂𝐻 ―

(6)

337

According to the in-situ sampling method (Figure 2C), the bulk solution pH and

338

H2O2 concentration in sample #1 (influent treated by cathode in red-ox system or by

339

anode in ox-red system) and sample #2 (influent treated by both electrodes) were

340

measured under operational parameters of 3.0 V and 25-125 mL min-1. As shown in

341

Figure 4A, after treatment with the cathode in red-ox system or the anode in ox-red

342

system, the pH of the collected solution (sample #1) exhibited strong alkaline (pH of

343

9.8-10.3) or acidic values (pH of 3.4-4.1), in agreement with recent results about

344

generation of H+ ions on anode and OH- ions on cathode.14 While the bulk solution pH 14


Page 15 of 41


345

of effluents (sample #2) in the two systems were close to the pH of influent (~6.15),

346

which indicated the produced OH- or H+ ions from the first cathode or anode were

347

neutralized or consumed by the subsequent anode or cathode in the two systems.

348

Thus, the Nernst equation was introduced to explain the effect of solution pH on

349

the equilibrium potential of electrochemical reactions, which can be expressed as

350

following equation under nonstandard conditions:36

351

𝐸 = 𝐸0 +

352

where E0 is the standard electrode potential, n is the number of electrons transferred,

353

and [Ox] and [Red] are the activities of oxidized and reduced species. Using water

354

electrolysis for oxygen production (2H2O --- O2 + 4H+ + 4e-) as example, the E can be

355

calculated by E = E0 + 0.01475log([H+]4Po2/[H2O]2), and apparently, the E value will

356

decrease with increase in solution pH, or increase with decrease in solution pH. The

357

dependency of E value on solution pH calculated by Nernst equation are also shown in

358

Figure S3. Base on this calculation, the anodic oxidation of H2O/H2O2 to O2 can be

359

improved under alkaline condition, and the cathodic reduction of O2 to H2O2 can be

360

enhanced under acidic condition. From this aspect, the anodic direct oxidation via the

361

electron transfer between the anode and E. coli cell membrane in red-ox system was

362

also promoted under the alkaline condition (sample #1, solution pH of 9.8-10.3), hence

363

resulting in its better disinfection performance than the ox-red system (see Figure 2A).

364

As shown in Figure 4B, the generation of H2O2 from the cathode (sample#2, 100-

365

135 μM) in ox-red system was slightly more than that from the cathode (sample#1, 75-

366

115 μM) in red-ox system, also supporting that the reduction of O2 in ox-red system

367

was enhanced under acidic condition. Limited H2O2 concentration (below the detection

368

limit of 1 μM) was detected in the sample #2 for red-ox system and in sample #1

369

(influent only treated by anode) for ox-red system, which was coincident with the

370

results of undetectable concentration of active chlorine in the anode-treated influent

371

solution at 3.0 V for the two systems. It also indicated that in red-ox system, the

0.059 [𝑂𝑥] 𝑛 𝑙𝑜𝑔10[𝑅𝑒𝑑]

(7)

15



Page 16 of 41

372

produced H2O2 species on the first cathode were oxidized subsequently on the anode

373

into O2 molecules (see the potentials for H2O2 oxidation in Figure S3). The

374

electrochemical O2 reduction for H2O2 species production was confirmed by

375

disinfection experiments using influent with [O2] below 0.25 mg L-1 (Ar sparging) (see

376

more details in Figure S4) that H2O2 were only detected in the sample #2 (~20-30 μM)

377

in ox-red system due to the production of O2 from anodic oxidation of H2O that were

378

subsequently reduced into H2O2 on the cathode.

379

Based on the results above, the solution pH and O2 transformation pathways under

380

applied voltage of 3.0 V in the two systems are illustrated in Figure 4C and 4D. In red-

381

ox system, with the pretreatment of cathode, the solution turned into alkaline (pH of

382

9.8-10.3) and some dissolved O2 molecules were reduced into H2O2 species. By further

383

oxidation on anode, the produced OH- and H2O2 species were exhausted and oxidized

384

to O2 molecules, causing a decrease in solution pH. In ox-red system, after pretreating

385

by the anode, H2O molecules were oxidized into O2 and H+ species. Then, under acidic

386

conditions (pH of 3.4-4.1), the reduction of H2O and O2 molecules to H2O2 and OH-

387

species was enhanced, in turn, resulting in the increase in solution pH. Thus, a large

388

part of energy was consumed via the H2O and O2 electro-redox in the two systems, and

389

the solution pH and O2 transformation pathways could account for current differences

390

in the two systems.

391

The energy consumption for anodic oxidation of H2O2 and H2O for O2 production

392

were evaluated at operational parameters of 3.0 V/50 mL min-1 and 3.5 V/100 mL min-1

393

in red-ox system. The production of total [O2] on the anode were obtained by the [O2]

394

differences in sample#1 and sample #2 under 3.0 V/50 mL min-1 (119 μM) and 3.5

395

V/100 mL min-1(137 μM), and the corresponding [O2] produced from [H2O2] oxidation

396

were also investigated to be 116 μM and 127 μM calculated by the [H2O2] differences

397

in sample #1 and #2. Hence, the [O2] produced from oxidation of H2O/OH- were

398

calculated by the differences of total [O2] and [O2] produced from [H2O2] oxidation.

399

The 116 μM [H2O2] oxidation at 50 mL min-1 and 127 μM [H2O2] oxidation at 100 mL 16


Page 17 of 41


400

min-1 can result in 1.2*1017 and 2.7*1017 e s-1 corresponding to 19.3 and 42.3 mA,

401

respectively. The 3 μM [O2] at 50 mL min-1 and 10 μM [O2] at 100 mL min-1 produced

402

by oxidation of H2O/OH- can lead to around 1.1 and 6.2 mA current. Therefore, the

403

current consumption for [O2] production from anodic oxidation of H2O2 and H2O were

404

43.6% to its total current (46.7 mA) at 3.0 V/50 mL min-1 and 54.4% to the total current

405

(89.3 mA) at 3.5 V/100 mL min-1, thus resulting in the higher energy efficiency at 3.0

406

V/50 mL min-1 compared with that at 3.5 V/100 mL min-1.

407

The electrochemical properties of the two systems were measured according to the

408

schematic diagram in Figure S5. The open circuit potential versus time for the cathode

409

and anode in the two systems are displayed in Figure 5. Apparently, at applied voltage

410

above 1.5 V, the red-ox system exhibited much higher cathodic potential and less

411

anodic potential than the ox-red system. This result gave additional evidence that under

412

acid/oxidant or alkali/reductant conditions (See Figure 4C and 4D), the reduction on

413

cathode in ox-red system or the oxidation on anode in red-ox system were promoted,

414

hence causing their less potential distribution. These results were also confirmed by

415

EIS analyses about the interfacial electrochemical behaviors of the anodes in the two

416

systems (SI, Figure S6). The two Nyquist plots were similar in form, and included an

417

incomplete semicircle and a straight line, which were associated with charge transfer

418

resistance and diffusion resistance at the anode and electrolyte interface.10, 12 The anode

419

in red-ox system showed a smaller radius (~40 Ω) and a more vertical shape (slope of

420

2.36) than that in ox-red system (~54 Ω and slope of 0.812), which implied the lower

421

charge transfer resistance and ion diffusion resistance for anodic oxidation.

422

In our previous work,37 the isoelectric point of E. coli cell was estimated to be

423

around 4.6. The E. coli surface was positively charged at pH below its isoelectric point

424

and negatively charged at pH above its isoelectric point. The electrostatic interactions

425

between the charged cells and the electrodes in the two systems were further analyzed

426

approximately based on the bulk solution pH in influent, samples #1 and #2. In ox-red

427

system, the bulk solution pH of the samples #1 and #2 are around 3.4-4.1 and 8.8-6.2. 17



Page 18 of 41

428

When the influent with initial solution pH of 6.15 was treated by the anode, the

429

negatively charged cells can be electro-adsorbed by the anode. Along the flow direction,

430

the produced H+ ions by anodic oxidation were accumulated in bulk solution, and its

431

pH value turned to lower than the isoelectric point of E. coli cells, leading to the

432

electrostatic repulsion between the positively charged cells and the anode. When the

433

anode-treated influent with bulk solution pH of 3.4-4.1 was pumped into the cathode,

434

electrostatic attraction existed between the positively charged cells and the cathode.

435

Also, along the flow direction, the H+ consumption or OH- generation by cathodic

436

reduction led to the increase in solution pH, and the surface charge of E. coli cells turned

437

to negative, also resulting in the electrostatic repulsion between the negatively charged

438

cells and the cathode.

439

In red-ox system, the bulk solution pH of the samples #1 and #2 are around 9.8-

440

10.3 and 5.0-6.1. For the whole cathode-reduction process, electrostatic repulsion was

441

present between the negatively charged cells (pH of influent (6.15) and sample #1 (9.8-

442

10.3)) and cathode. Also, for the whole anode-oxidation process, obvious electrostatic

443

attraction existed between the negatively charged E. coli cells (pH of sample #1 (9.8-

444

10.3) and sample #2 (5.0-6.1)) and the anode.

445

The interactions between the charged E. coli cells and the four electrodes in the

446

two systems at applied voltage of 3.0 V were also semi-quantified via Derjaguin-

447

Landau-Verwey-Overbeek (DLVO) theory.38,

448

qualitative agreement with above statement. In ox-red system, the attractive interaction

449

between the charged E. coli cells on the anode and cathode existed in the beginning

450

when the influent or anode-treated influent passed through the anode or cathode, but

451

along the flow direction repulsive interaction increased between the E. coli cells and

452

the two electrodes. While in red-ox system, the repulsive interaction existed for the

453

whole reduction process, and increased significantly along the flow direction in cathode,

454

and attractive interaction existed for the whole oxidation process and decreased along

455

the flow direction in anode (See more details in SI Figure S7). Therefore, the better

39

The DLVO analyses were in

18


Page 19 of 41


456

disinfection ability of red-ox system can also be attributed to the stronger electrostatic

457

attraction between the negatively charged E. coli cells and the anode.

458

E. coli Inactivation at H+-, OH--, and H2O2-Concentrated Interfaces.

459

Given the incomplete E. coli removal, the operational parameters of applied

460

voltage of 3.0 V and flow rate of 75 mL min-1 were chosen to get insight into the

461

differences in disinfection contribution of the four electrodes in the two systems. The

462

E. coli inactivation (%) of the individual electrode in the two systems were investigated

463

based on the in-situ sampling method as shown in Figure 2C. The E. coli inactivation

464

(%) by the first cathode/anode and the subsequent anode/cathode in the two systems

465

were calculated by (N0-N1)/N0*100% and (N1-N2)/N0*100%, where the N0, N1 and N2

466

are the culturable E. coli concentrations in influent, sample #1 and sample #2.

467

As shown in Table 1, the E. coli inactivation of the individual electrode in the two

468

systems followed an order of: anode (91.68%) in red-ox system > anode (76.61%) in

469

ox-red system > cathode (21.09%) in ox-red system > cathode (8.30%) in red-ox system.

470

As expected, the E. coli inactivation mainly occurred on the anode in the two systems,

471

and the anode in red-ox system made larger contribution to E. coli inactivation than that

472

in ox-red system, since E. coli cells were much easier to access the anode and were

473

inactivated subsequently by direct oxidation. However, it was still difficult to

474

distinguish the contribution of H+-concentrated interface and the direct oxidation on the

475

anode to E. coli inactivation (%), and to distinguish the contribution of the OH- and

476

H2O2-concentrated interface on the cathode to E. coli inactivation (%).

477

Overall, the [H+], [OH-], or [H2O2] at the electrode/electrolyte interface must be

478

much higher than the bulk solution, and these species were accumulated as the influent

479

passed through the electrode, hence it was difficult to confirm the exposed time and

480

concentration for the E. coli cells at the H+-, OH--, and H2O2-concentrated interfaces.

481

According to the [H+] (0.202 mM), [OH-] (0.086 mM) and [H2O2] (0.12 mM)

482

determined by in-situ sampling experiments in the two systems (see Table 1), batch

483

experiments were designed to explore approximately the contributions of H+-, OH--, 19



Page 20 of 41

484

and H2O2-concentrated interfaces to E. coli inactivation. For these batch experiments,

485

100 mL of 10 mM NaCl solution was adjusted with HCl, NaOH or H2O2 to the [H+],

486

[OH-], or [H2O2] (Ci) of 10-1000 times (Ci/C0) higher than their values (C0) determined

487

from in-situ sampling experiments, and the residence time was chosen as the HTR

488

(around 7 s) for each piece of CFF electrode at flow rate of 75 mL min-1. Then 2 mL

489

fresh E. coli culture was added into the 100-mL solution with an initial E. coli

490

concentration of 106-107 CFU mL-1. After stirring at 500 rpm for 7 s for each sample,

491

the residual H+/OH-/ H2O2 in 100 μL sample were quenched with 10 mL of 10 mM

492

phosphate-buffered saline or thiosulfate solution, followed by bacteria quantification

493

using the plate count method.

494

As shown in Figure 6A, E. coli cells were very impressionable to the enlargement

495

of OH- concentration. For example, its viability dropped to 86% at Ci/C0 of 100, and

496

reached to almost 0 at Ci/C0 above 250. While under exposure in acidic solution, E. coli

497

cells began to be inactivated until Ci/C0 above 250, and the viability showed a slow

498

drop with increasing Ci/C0 values. However, almost no E. coli inactivation occurred

499

even after treatment with 1000 times higher H2O2 concentration, reflecting that E. coli

500

cells in the fresh made solution were invulnerable to the H2O2-concentrated interface.

501

Regarding to the E. coli inactivation (8.30%) of cathode in red-ox system, the

502

equivalent E. coli inactivation for the OH- or OH-+H2O2 batch experiments achieved at

503

Ci/C0 of 10-100, while almost no E. coli cells were inactivated at Ci/C0 of 10-100 for

504

H+ batch experiments. These results indicated that negligible E. coli inactivation was

505

attributed to H+-concentrated interface on the anode in the two systems, and also gave

506

another support for the primary direct oxidation mechanism.

507

The cathode in red-ox system possessed OH-- and H2O2-concentrated interface,

508

and the synergistic effects of OH- and H2O2 just achieved a poor E. coli inactivation

509

(8.30%). While the cathode in ox-red system had much weaker alkaline strength, and a

510

bit higher H2O2 concentration (0.12 mM H2O2) than the cathode (0.098 mM) in red-ox

511

system, it achieved much higher E. coli inactivation (21.09%), which seems to be 20


Page 21 of 41


512

contradicted with the results from H2O2 batch experiments. To further confirm the

513

disinfection function of H2O2, additional disinfection experiments were performed

514

under applied voltage of 3.0 V and flow rate of 25-125 mL min-1 for the two systems

515

by using anaerobic E. coli suspension ([O2] below 0.25 mg L-1). In red-ox system, the

516

excluding oxygen yielded an increase in E. coli log removal in red-ox system (Figure

517

6B), since the H2O2 formed from oxygen reduction on cathode competed the reactive

518

sites on anode with E. coli cells as shown in Figure 4C. While, the obvious decrease in

519

E. coli log removal in ox-red system gave an experiment evidence that the formed H2O2

520

on cathode played an important role on E. coli inactivation.

521

As analyzed by the DLVO theory (see Figure S7), much weaker electrostatic

522

repulsion exhibited between the charged E. coli cells and the cathode in ox-red system

523

as compared with the cathode in red-ox system, meaning that the E. coli cells were

524

earlier to access the H2O2-concentrated interface and inactivated on the cathode in ox-

525

red system. The E. coli sources for two disinfection processes could be another reason

526

causing the differences in disinfection ability of the cathode (fresh made influent) in

527

red-ox system and the cathode (anode-pretreated influent) in ox-red system. Additional

528

experiments were performed to explore the storage time on the E. coli cells viability in

529

the sample #1 taken from the two systems, namely the influent after treatment with

530

cathode in red-ox system or with the anode in ox-red system. After in-situ sampling,

531

the residual H+, OH- or H2O2 were quenched with 10 mL of 10 mM phosphate-buffered

532

saline or thiosulfate solution, followed by dilution in 10 mM NaCl solution and bacteria

533

quantification using the plate count method at different time intervals. As shown in

534

Figure 6C, since the cathode in red-ox system had limited cell inactivation ability, its

535

E. coli inactivation during storage was insignificant as compared with the untreated E.

536

coli sample. However, about 40% cell inactivation occurred after 120 min storage for

537

the anode-pretreated sample, reflecting that E. coli cells still can be culturable on

538

nutrient agar after limited damage. Therefore, after the anode-pretreatment, the

539

impaired cells were easily inactivated on the cathode by H2O2- or/and OH-21



Page 22 of 41

540

concentrated interface, resulting in its higher E. coli inactivation than the cathode in

541

red-ox system.

542

The disinfection mechanisms in red-ox and ox-red systems are concluded and

543

shown in Figure S8A and 8B at electrochemical parameters of 3.0 V and 75 mL min-1.

544

In red-ox system, the fresh made influent was treated by cathode first, and the dissolved

545

O2 and H2O molecules were reduced to OH- and H2O2 species. Under bulk solution pH

546

from 6.15 (influent) to 9.98 (after cathode treatment), strong electrostatic repulsion

547

existed between the cathode and negatively charged E. coli cells, resulting in the limited

548

E. coli inactivation (8.30%) at the OH-- and H2O2-concentrated interface. Then, the

549

cathode-pretreated influent with high solution pH of 9.98 was pumped into the anode,

550

the negatively charged E. coli cells were easily to be adsorbed and inactivated on the

551

anode via direct oxidation with a major E. coli inactivation of 91.68%.

552

In ox-red system (Figure S8B), the fresh made influent was treated by anode first,

553

and the H2O molecules were oxidized to H+ and O2 species. Along the flow direction

554

in the anode, the H+ ions were accumulated in the bulk solution and caused the

555

electrostatic repulsion existed between the positively charged E. coli cells and the anode,

556

finally resulting in less E. coli inactivation (76.61%) via direct oxidation as compared

557

to the anode in red-ox system. With the pretreatment of anode, the E. coli suspension

558

became acidic with bulk solution pH of 3.70, and some E. coli cells were impaired, but

559

were still culturable on nutrient agar. At this condition, the H2O2 generation via

560

reduction of dissolved O2 molecules on the cathode was promoted, and the H2O2- or

561

OH--concentrated interface near the cathode was more accessible to the (impaired) E.

562

coli cells, significantly improving the E. coli inactivation of cathode to 21.09%.

563

Implications and Applications of Water Disinfection with FES.

564

CFF-based FES with sequential reduction-oxidation process showed much better

565

disinfection performance than that with sequential oxidation-reduction process, and

566

achieved above 6.5 log E. coli inactivation via direct oxidation-mediated mechanism at

567

high flow rate and low applied voltage, such as 3.0 V/50 mL min-1 and 3.5 V/100 mL 22


Page 23 of 41


568

min-1. The operation at low applied voltage also enabled the fabrication of CFF-based-

569

portable devices with button battery for point-of-use water disinfection. Also, portable

570

electrochemical devices with larger amount of water treatment can be designed by

571

connecting CFF electrodes in parallel or in series for point-of-use water disinfection.

572

Recent studies have revealed the application potentials of electrochemical cells as

573

point-of-use drinking water purification devices for pathogen inactivation in

574

developing countries where waterborne pathogens cause millions of deaths annually.

575

As shown in Table S2, the disinfection performance, energy consumption, and

576

electrode cost of CFF-based flow-through electrode used in this study compared

577

favorably with the flow-through electrodes in previously published work. As a kind of

578

commercially available and microfiber-based products, CFF electrode not only can be

579

used as received without further synthesis process, but also offers several advantages

580

over other CNT/AgNWs/CuONWs-based ones9, 29, 40-44, such as favorable price (see

581

more details in Table S2) and no toxic heavy metal ions/nano-particles release during

582

electrolysis.

583

The disinfection ability of CFF-based FES was also investigated towards a lake

584

water and an effluent from a water reclamation plant in Beijing, China, based on the

585

heterotrophic plate count (HPC) using R2A agar. Obviously, as shown in Figure 7, the

586

red-ox system still exhibited better disinfection abilities than the ox-red system for the

587

two water sources. However, the disinfection abilities for the two natural water sources

588

were lower than synthetic drinking water. It is likely that water quality could affect

589

significantly the disinfection performance, such as microbial species and concentration,

590

buffering capacity and natural organic matter. Also, the long-term disinfection

591

performance of the FES towards viruses and other germ positive/negative bacteria

592

should be tested before its practical application.

593

ASSOCIATED CONTENT

594

Supporting Information. Details of characterization of CFF electrodes, the cultivation

595

and enumeration of E. coli, CFF electrochemistry measurement, determination of 23



Page 24 of 41

596

hydrogen peroxide concentrations, and live/dead baclight staining experiment.

597

Characteristics of the natural water sources used in the experiments (Table S1);

598

Disinfection performance of flow-through reactors in previously published work and

599

the cost assessment of the electrode used (Table S2); Energy consumption of red-ox

600

system at applied voltage of 2.0-3.5 V (Figure S1); Active chlorine concentration in the

601

effluent in red-ox system using 10 mM NaCl as electrolyte at 0.5-3.5 V under anoxic

602

condition and Log E. coli removal using 10 mM NaF and NaCl as electrolyte at 3.5 V

603

in red-ox system (Figure S2); Electrode potentials for electro-oxidation of H2O/H2O2

604

and electro-reduction of O2 as a function of solution pH calculated by Nernst equation

605

(Figure S3); Bulk H2O2 concentration for the influent with [O2] below 0.25 mg L-1 after

606

treatment by the first cathode in red-ox system, by the first anode in ox-red system, and

607

by the both electrode (effluent) in the two systems (Figure S4.); Schematic diagram of

608

red-ox and ox-red systems for electrochemical characterization (Figure S5);

609

Electrochemical impedance spectra for red-ox and ox-red systems (Figure S6);

610

Interaction energies between charged E. coli cells and the four electrodes in red-ox and

611

ox-red systems under applied voltage of 3.0 V (Figure S7); and Depiction of

612

electrochemical inactivation of E. coli cells in red-ox and ox-red systems (Figure S8).

613

AUTHOR INFORMATION

614

Corresponding Author

615

* Phone: +86-10-62797265; e-mail: [email protected];

616

*Phone: +86-10-6279-4005; e-mail: [email protected].

617

Present Addresses

618

Environmental Simulation and Pollution Control State Key Joint Laboratory, State

619

Environmental Protection Key Laboratory of Microorganism Application and Risk

620

Control (SMARC), School of Environment, Tsinghua University, Beijing 100084, PR

621

China.

24


Page 25 of 41


622

ACKNOWLEDGMENTS

623

This study was supported by National Key R&D Program of China (No.

624

2016YFE0118800), Key Program of the National Natural Science Foundation of China

625

(No. 51738005), China Postdoctoral Science Foundation (No. 2018M630168), and the

626

Collaborative Innovation Center for Regional Environmental Quality, China.

627

REFRENCES

628

(1) World Health Organization UN-water global analysis and assessment of sanitation

629

and drinking-water (GLAAS) 2017 report: financing universal water, sanitation and

630

hygiene under the sustainable development goals. 2017

631

(2) Radjenovic, J.; Sedlak, D. L., Challenges and opportunities for electrochemical

632

processes as next-generation technologies for the treatment of contaminated water.

633

Environ. Sci. Technol. 2015, 49, (19), 11292-11302.

634

(3) Okochi, M.; Nakamura, N.; Matsunaga, T., Electrochemical killing of

635

microorganisms using the oxidized form of ferrocenemonocarboxylic acid.

636

Electrochim. Acta 1999, 44, (21), 3795-3799.

637

(4) Monasterio, S.; Mascia, M.; Di Lorenzo, M., Electrochemical removal of

638

microalgae with an integrated electrolysis-microbial fuel cell closed-loop system. Sep.

639

Purif. Technol. 2017, 183, 373-381.

640

(5) Rajab, M.; Heim, C.; Letzel, T.; Drewes, J. E.; Helmreich, B., Electrochemical

641

disinfection using boron-doped diamond electrode – The synergetic effects of in situ

642

ozone and free chlorine generation. Chemosphere 2015, 121, 47-53.

643

(6) Bruguera-Casamada, C.; Sirés, I.; Brillas, E.; Araujo, R. M., Effect of

644

electrogenerated hydroxyl radicals, active chlorine and organic matter on the

645

electrochemical inactivation of Pseudomonas aeruginosa using BDD and dimensionally

646

stable anodes. Sep. Purif. Technol. 2017, 178, 224-231.

647

(7) Ghasemian, S.; Asadishad, B.; Omanovic, S.; Tufenkji, N., Electrochemical

648

disinfection of bacteria-laden water using antimony-doped tin-tungsten-oxide

649

electrodes. Water Res. 2017, 126, 299-307. 25



Page 26 of 41

650

(8) Matsunaga, T.; Nakasono, S.; Takamuku, T.; Burgess, J. G.; Nakamura, N.; Sode,

651

K., Disinfection of drinking water by using a novel electrochemical reactor employing

652

carbon-cloth electrodes. Appl. Environ. Microbiol. 1992, 58, (2), 686-689.

653

(9) Vecitis, C. D.; Schnoor, M. H.; Rahaman, M. S.; Schiffman, J. D.; Elimelech, M.,

654

Electrochemical multiwalled carbon nanotube filter for viral and bacterial removal and

655

inactivation. Environ. Sci. Technol. 2011, 45, (8), 3672-3679.

656

(10) Schnoor, M. H.; Vecitis, C. D., Quantitative examination of aqueous ferrocyanide

657

oxidation in a carbon nanotube electrochemical filter: effects of flow Rate, ionic

658

strength, and cathode material. J. Phys. Chem. C 2013, 117, (6), 2855-2867.

659

(11) Gao, G.; Vecitis, C. D., Electrochemical carbon nanotube filter oxidative

660

performance as a function of surface chemistry. Environ. Sci. Technol. 2011, 45, (22),

661

9726-9734.

662

(12) Guo, L.; Ding, K.; Rockne, K.; Duran, M.; Chaplin, B. P., Bacteria inactivation at

663

a sub-stoichiometric titanium dioxide reactive electrochemical membrane. J. Hazard.

664

Mater. 2016, 319, 137-146.

665

(13) Mascia, M.; Monasterio, S.; Vacca, A.; Palmas, S., Electrochemical treatment of

666

water containing Microcystis aeruginosa in a fixed bed reactor with three-dimensional

667

conductive diamond anodes. J. Hazard. Mater. 2016, 319, 111-120.

668

(14) Kerwick, M. I.; Reddy, S. M.; Chamberlain, A. H. L.; Holt, D. M., Electrochemical

669

disinfection, an environmentally acceptable method of drinking water disinfection?

670

Electrochim. Acta 2005, 50, (25), 5270-5277.

671

(15) Mollah, M. Y. A.; Pathak, S. R.; Patil, P. K.; Vayuvegula, M.; Agrawal, T. S.;

672

Gomes, J. A. G.; Kesmez, M.; Cocke, D. L., Treatment of orange II azo-dye by

673

electrocoagulation (EC) technique in a continuous flow cell using sacrificial iron

674

electrodes. J. Hazard. Mater. 2004, 109, (1), 165-171.

675

(16) Zhu, R.; Yang, C.; Zhou, M.; Wang, J., Industrial park wastewater deeply treated

676

and reused by a novel electrochemical oxidation reactor. Chem. Eng. J. 2015, 260, 427-

677

433.

26


Page 27 of 41


678

(17) Liu, Y.; Liu, H.; Zhou, Z.; Wang, T.; Ong, C. N.; Vecitis, C. D., Degradation of

679

the common aqueous antibiotic tetracycline using a carbon nanotube electrochemical

680

filter. Environ. Sci. Technol. 2015, 49, (13), 7974-7980.

681

(18) Gao, G.; Zhang, Q.; Vecitis, C. D., CNT–PVDF composite flow-through electrode

682

for single-pass sequential reduction–oxidation. J. Phys. Chem. A 2014, 2, (17), 6185-

683

6190.

684

(19) Gao, G.; Zhang, Q.; Hao, Z.; Vecitis, C. D., Carbon nanotube membrane stack for

685

flow-through sequential regenerative electro-fenton. Environ. Sci. Technol. 2015, 49,

686

(4), 2375-2383.

687

(20) Matsunaga, T.; Nakasono, S.; Masuda, S., Electrochemical sterilization of bacteria

688

adsorbed on granular activated carbon. FEMS Microbiol. Lett. 1992, 93, (3), 255-259.

689

(21) Laxman, K.; Myint, M. T. Z.; Al Abri, M.; Sathe, P.; Dobretsov, S.; Dutta, J.,

690

Desalination and disinfection of inland brackish ground water in a capacitive

691

deionization cell using nanoporous activated carbon cloth electrodes. Desalination

692

2015, 362, 126-132.

693

(22) Miao, J.; Zhu, H.; Tang, Y.; Chen, Y.; Wan, P., Graphite felt electrochemically

694

modified in H2SO4 solution used as a cathode to produce H2O2 for pre-oxidation of

695

drinking water. Chem. Eng. J. 2014, 250, 312-318.

696

(23) Chen, S.; Hu, W.; Hong, J.; Sandoe, S., Electrochemical disinfection of simulated

697

ballast water on PbO2/graphite felt electrode. Mar. Pollut. Bull. 2016, 105, (1), 319-

698

323.

699

(24) Marracino, J. M.; Coeuret, F.; Langlois, S., A first investigation of flow-through

700

porous electrodes made of metallic felts or foams. Electrochim. Acta 1987, 32, (9),

701

1303-1309.

702

(25) Liu, C.; Xie, X.; Zhao, W.; Liu, N.; Maraccini, P. A.; Sassoubre, L. M.; Boehm,

703

A. B.; Cui, Y., Conducting nanosponge electroporation for affordable and high-

704

efficiency disinfection of bacteria and viruses in water. Nano Lett. 2013, 13, (9), 4288-

705

4293.

27



Page 28 of 41

706

(26) Rahaman, M. S.; Vecitis, C. D.; Elimelech, M., Electrochemical carbon-nanotube

707

filter performance toward virus removal and inactivation in the presence of natural

708

organic matter. Environ. Sci. Technol. 2012, 46, (3), 1556-1564.

709

(27) Zhou, S.; Huang, S.; Li, X.; Angelidaki, I.; Zhang, Y., Microbial electrolytic

710

disinfection process for highly efficient Escherichia coli inactivation. Chem. Eng. J.

711

2018, 342, 220-227.

712

(28) Pandiyan, R.; Delegan, N.; Dirany, A.; Drogui, P.; El Khakani, M. A., Correlation

713

of sp2 carbon bonds content in magnetron-sputtered amorphous carbon films to their

714

electrochemical H2O2 production for water decontamination applications. Carbon 2015,

715

94, 988-995.

716

(29) Wen, J.; Tan, X.; Hu, Y.; Guo, Q.; Hong, X., Filtration and electrochemical

717

disinfection performance of PAN/PANI/AgNWs-CC composite nanofiber membrane.

718


719

(30) Ge, J.; Qu, J., Ultrasonic irradiation enhanced degradation of azo dye on MnO2.

720

Appl. Catal. B Environ. 2004, 47, (2), 133-140.

721

(31) Zhang, Y.; Zuo, S.; Zhang, Y.; Li, M.; Cai, J.; Zhou, M., Disinfection of simulated

722

ballast water by a flow-through electro-peroxone process. Chem. Eng. J. 2018, 348,

723

485-493.

724

(32) Schmalz, V.; Dittmar, T.; Haaken, D.; Worch, E., Electrochemical disinfection of

725

biologically treated wastewater from small treatment systems by using boron-doped

726

diamond (BDD) electrodes – Contribution for direct reuse of domestic wastewater.

727

Water Res. 2009, 43, (20), 5260-5266.

728

(33) Tsong, T. Y., Electroporation of cell membranes. Biophys. J. 1991, 60, (2), 297-

729

306.

730

(34) Mendonca, A. F.; Amoroso, T. L.; Knabel, S. J., Destruction of gram-negative

731

food-borne pathogens by high pH involves disruption of the cytoplasmic membrane.

732

Appl. Environ. Microbiol. 1994, 60, (11), 4009-4014.

28


Page 29 of 41


733

(35) Issa-Zacharia, A.; Kamitani, Y.; Tiisekwa, A.; Morita, K.; Iwasaki, K., In vitro

734

inactivation of Escherichia coli, Staphylococcus aureus and Salmonella spp. using

735

slightly acidic electrolyzed water. J. Biosci. Bioeng. 2010, 110, (3), 308-313.

736

(36) Ge, X.; Sumboja, A.; Wuu, D.; An, T.; Li, B.; Goh, F. W. T.; Hor, T. S. A.; Zong,

737

Y.; Liu, Z., Oxygen reduction in alkaline media: from mechanisms to recent advances

738

of catalysts. ACS Catal. 2015, 5, (8), 4643-4667.

739

(37) Huo, Z.-Y.; Li, G.-Q.; Yu, T.; Feng, C.; Lu, Y.; Wu, Y.-H.; Yu, C.; Xie, X.; Hu,

740

H.-Y., Cell transport prompts the performance of low-voltage electroporation for cell

741

inactivation. Sci. Rep. 2018, 8, (1), 15832.

742

(38) Zhang, Q.; Vecitis, C. D., Conductive CNT-PVDF membrane for capacitive

743

organic fouling reduction. J. Membr. Sci. 2014, 459, 143-156.

744

(39) Zhang, Q.; Nghiem, J.; Silverberg, G. J.; Vecitis, C. D., Semi-quantitative

745

performance and mechanism evaluation of carbon nanomaterials as cathode coatings

746

for microbial fouling reduction. Appl. Environ. Microbiol. 2015.

747

(40) Huo, Z.-Y.; Li, G.-Q.; Yu, T.; Lu, Y.; Sun, H.; Wu, Y.-H.; Yu, C.; Xie, X.; Hu, H.-

748

Y., Impact of water quality parameters on bacteria inactivation by low-voltage

749

electroporation: mechanism and control. Environ. Sci.-Water. Res. Tech. 2018, 4, (6),

750

872-881.

751

(41) Schoen, D. T.; Schoen, A. P.; Hu, L.; Kim, H. S.; Heilshorn, S. C.; Cui, Y., High

752

speed water sterilization using one-dimensional nanostructures. Nano Lett. 2010, 10,

753

(9), 3628-3632.

754

(42) Hong, X.; Wen, J.; Xiong, X.; Hu, Y., Silver nanowire-carbon fiber cloth

755

nanocomposites synthesized by UV curing adhesive for electrochemical point-of-use

756

water disinfection. Chemosphere 2016, 154, 537-545.

757

(43) Huo, Z.-Y.; Xie, X.; Yu, T.; Lu, Y.; Feng, C.; Hu, H.-Y., Nanowire-modified three-

758

dimensional electrode enabling low-voltage electroporation for water disinfection.

759


29



Page 30 of 41

760

(44) Huo, Z.-Y.; Luo, Y.; Xie, X.; Feng, C.; Jiang, K.; Wang, J.; Hu, H.-Y., Carbon-

761

nanotube sponges enabling highly efficient and reliable cell inactivation by low-voltage

762

electroporation. Environ. Sci. Nano 2017, 4, (10), 2010-2017.

763

30


Page 31 of 41


764

Table 1. Disinfection functions of individual anode and cathode in the two systems at

765

applied voltage of 3.0 V and flow rate of 75 mL min-1. Systems

766

Red-Ox system

Ox-Red system

Cathode

Anode

Anode

Cathode

E. coli Inactivation (%)

8.30±3.4

91.68±0.17

76.61±4.8

21.09±1.8

OPC (V)

1.07

-1.64

1.28

-1.31

Bulk solution pH

9.98±0.05

5.85±0.10

3.70±0.26

6.53±0.06

H2O2 (μM)

98±3.9

UD

120±8.4

UD

OCP represents the open-circuit potential at 3.0 V. UD = under detection limit.

767

31



Page 32 of 41

768 769

Figure 1. (A) Vertically expanded depiction of the carbon fiber-based flow-through

770

electrode system (FES), consisting of a conic water distributor, two carbon fiber felt

771

(CFF) electrodes, an insulating filter paper between the two electrodes, and a conic

772

water collector. (B) FESs for E. coli disinfection (up one) and electrochemical

773

characterization (down one). (C) Images of the CFF electrode.

774

32


Page 33 of 41


A

Red-Ox: Ox-Red:

Log Removal

8

2.5 V; 2.5 V;

3.0 V; 3.0 V;

3.5 V 3.5 V

6

4

2

0

0

5

10

15

20 25

50 75 100 125 150 -1

Flow Rate (mL min )

775 B 120 Red-Ox:

Current (mA)

2.5 V; Ox-Red: 90 2.5 V;

3.0 V;

3.5 V

3.0 V;

3.5 V

60

30

0

0

5

10

15

20 25 50 75 100 125 150 -1

Flow Rate (mL min )

776

777

33



Page 34 of 41

D Activate Chlorine (as Cl2 mg/L)

0.4 0.3 0.2

Red-Ox: -1 25 mL min -1 50 mL min -1 75 mL min -1 100 mL min -1 125 mL min

0.1 0.0 0.0

0.5

1.0

1.5 2.0 2.5 Applied Voltage (V)

3.0

3.5

778 779

Figure 2. Log E. coli removal (A) and current (B) as functions of applied voltage and

780

flow rate in red-ox and ox-red systems. E. coli suspension (106-107 CFU mL-1, [NaCl]

781

= 10 mM, pH 6.15) was electrolyzed at 2.5, 3.0 or 3.5 V and a flow rate of 5-125 mL

782

min-1. (C) Schematic diagram of in-situ determination of disinfection functions for

783

individual anode and cathode. A needle tip with external diameter of ~0.65 mm was

784

inserted horizontally into the reactor, and fixed between the filter paper and the

785

measured electrode with the tip bevel facing to the measured electrode. The measured

786

electrode can be the cathode in red-ox system or the anode in ox-red system. During

787

electrochemical disinfection, samples were withdrawn by 1 mL syringe with a pulling

788

speed of 500 μL min-1 to minimize the effect to the disinfection performance of the

789

whole system.

790

(D) Active chlorine production using 10 mM NaCl as electrolyte at

0.5-3.5 V in red-ox system (106-107 CFU mL-1, [NaCl] = 10 mM, pH 6.15).

791

34


Page 35 of 41


792 793

Figure 3. (A) Fluorescence microscopic images of E. coli cells in influent (A1) and E.

794

coli cells in effluent after operation for 1 (A2), 5 (A3) and 30 (A4) min at 3.0 V and

795

50 mL min-1 in red-ox system. Fluorescence microscopic images of E. coli cells

796

attached on the cathode and anode in red-ox system. The system was continuously

797

operated using E. coli suspension (106-107 CFU mL-1, [NaCl] = 10 mM) at 3.0 V and

798

50 mL min-1 for 10 min, followed immediately by flushing with 10 mM NaCl

799

(without E. coli cells) at 3.0 V and 50 mL min-1 for 2 (B) and 5 min (C). Scale bar in

800

the images presents 50 μm. (D) Depiction of electrochemical process of E. coli

801

inactivation on the anode in red-ox system at 3.0 V and 50 mL min-1.

802

35



A 14

Red-Ox:

12

Ox-Red:

Page 36 of 41

treated by cathode treated by both electrodes treated by anode treated by both electrodes

Solution pH

10 8 6 4 2

25

50

75

100

125

-1

Flow Rate (mL min )

803

-6

H2O2 concentration (*10 M)

B 210

treated by cathode treated by both electrodes treated by anode treated by both electrodes

Red-Ox:

180

Ox-Red:

150 120 90 60 30 0 25

50

75

100

125

-1

Flow Rate (mL min )

804

805 36


Page 37 of 41


806 807

Figure 4. Bulk solution pH (A) and H2O2 concentration (B) for the influent after

808

treatment by the first cathode in red-ox system, by the first anode in ox-red system,

809

and by the both electrode (effluent) in the two systems. E. coli suspension (106-107

810

CFU mL-1, [NaCl] = 10 mM, pH 6.15) was electrolyzed at 3.0 V and 25-125 mL min-

811

1.

Dash line in Figure 4A represents the solution pH of influence. Depiction of the

812

primary electrode surface reaction for the change of solution pH and H2O2

813

concentration in red-ox (C) and ox-red (D) systems.

37



Red-Ox: Ox-Red:

Potential (V vs. Ag/AgCl)

1.8

Anode; Anode;

Page 38 of 41

Cathode Cathode

1.2 0.6

1.0 V

1.5 V

2.0 V

2.5 V

3.0 V

3.5 V

0.0 -0.6 -1.2 -1.8 0

815

210

420

630 840 Time (s)

1050

1260

816

Figure 5. Anode and cathode potential in V vs. Ag/AgCl as a function of total cell

817

potential applied in V. E. coli suspension (106-107 CFU mL-1, [NaCl] = 10 mM, pH

818

6.15) was electrolyzed at 0-3.5 V and 75 mL min-1.

819

38


Page 39 of 41


A

100

E. coli Viability (%)

80 60 +

H OH OH +H2O2

40 20

H2O2

0 10

100

1000 Ci/C0

820

B

Red-Ox:

10

10

-1

[O2] ＜ 0.25 mg L

Ox-Red:

8 Log Removal

-1

[O2] ~ 8.0 mg L

-1

[O2] ~ 8.0 mg L

8

-1

[O2] ＜ 0.25 mg L

No Live E. coli

6

6

4

4

2

2

0

25

50

75

100

125

0

-1

Flow Rate (mL min )

821

39



Page 40 of 41

E. coli Viability (%)

C 100

75

50

Fresh made E. coli suspension Treated by cathode in red-ox system Treated by anode in ox-red system

25

0

0

20

40 60 80 Storage Time (min)

100

120

822 823

Figure 6. (A) Batch experiments for evaluating the E. coli inactivation abilities of H+-,

824

OH--, and H2O2-concentrated interfaces. Ci and C0 represented the adjusted [H+],

825

[OH-] or [H2O2] and the ones determined by in-situ sampling experiments (see Table

826

1). (B) Log E. coli removal under oxygen-saturated (8.0 mg L-1) and anoxic

827

conditions ([O2] below 0.25 mg L-1) in red-ox and ox-red systems. (C) The effect of

828

storage time in 10 mM NaCl on the viability of E. coli cells in the sample #1 for the

829

two systems, namely influent after treatment by the cathode in red-ox system or by

830

the anode in the ox-red system. E. coli suspension was electrolyzed at 3.0 V and 75

831

mL min-1.

832

40


Page 41 of 41


6

Lake water: Reclaimed water:

Log Removal

5

6

Red-Ox; Red-Ox;

Ox-Red Ox-Red No Live HPC

4

No Live HPC

4

3 2

2

1 0

0

20

40

60

80

100

0

-1

Flow Rate (mL min )

833 834

Figure 7. The disinfection ability of red-ox and ox-red systems for a lake water and a

835

effluent from a water reclamation plant under applied voltage of 3.0 and a flow rate of

836

10-100 mL min-1.

41


Carbon Fiber-Based Flow-Through Electrode System (FES) for Water

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