Enhanced Visible-Light-Driven Photocatalytic Bacterial Inactivation by

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Enhanced Visible Light–Driven Photocatalytic Bacterial Inactivation by Ultrathin Carbon-Coated Magnetic Cobalt Ferrite Nanoparticles Tianqi Wang, Zhifeng Jiang, Taicheng An, Guiying Li, Huijun Zhao, and Po Keung Wong Environ. Sci. Technol., Just Accepted Manuscript • Publication Date (Web): 26 Mar 2018 Downloaded from http://pubs.acs.org on March 26, 2018

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Environmental Science & Technology

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Enhanced Visible-Light-Driven Photocatalytic Bacterial Inactivation by

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Ultrathin Carbon-Coated Magnetic Cobalt Ferrite Nanoparticles

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§

§

Tianqi Wang,† Zhifeng Jiang,†, ‡ Taicheng An,*, Guiying Li, Huijun Zhao,ǁ and Po Keung Wong*,†

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†

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China.

8

‡

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Zhenjiang, Jiangsu 212013, China.

School of Life Sciences, The Chinese University of Hong Kong, Shatin, NT, Hong Kong SAR,

Institute for Energy Research, School of Chemistry and Chemical Engineering, Jiangsu University,

§

Guangzhou Key Laboratory of Environmental Catalysis and Pollution Control, Institute of Environmental Health and Pollution Control, School of Environmental Science and Engineering, Guangdong University of Technology, Guangzhou 510006, Guangdong, China

ǁ

Centre for Clean Environment and Energy, Griffith Scholl of Environment, Griffith University, Queensland 4222, Australia

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Corresponding authors:

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__________________________________

Tel: +86 20 2388 3536, Fax: +86 20 8529 1501, E-mail: [email protected] (T.C. An); Tel: +852 3943 6383, Fax: +852 2603 5767, E-mail: [email protected] (P.K. Wong). 14

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ACS Paragon Plus Environment


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ABSTRACT: Ultrathin hydrothermal carbonation carbon (HTCC)–coated cobalt ferrite (CoFe2O4)

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composites with HTCC coating thicknesses between 0.62 and 4.38 nm were fabricated as novel,

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efficient, and magnetically recyclable photocatalysts via a facile, green approach. The

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CoFe2O4/HTCC composites showed high magnetization and low coercivity, which favored

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magnetic separation for reuse. The results show that the close coating of HTCC on CoFe2O4

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nanoparticles enhanced electron transfer and charge separation, leading to a significant

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improvement in photocatalytic efficiency. The composites exhibited superior photocatalytic

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inactivation toward Escherichia coli K-12 under visible-light irradiation, with the complete

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inactivation of 7 log10 cfu·mL−1 of bacterial cells within 60 min. The destruction of bacterial cell

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membranes was monitored by field-effect scanning electron microscopy analysis and fluorescence

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microscopic images. The bacterial inactivation mechanism was investigated in a scavenger study,

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and •O2, H2O2, and h+ were identified as the major reactive species for bacterial inactivation.

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Multiple cycle runs revealed that these composites had excellent stability and reusability. In

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addition, the composites showed good photocatalytic bacterial inactivation performance in authentic

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water matrices such as surface water samples and secondarily treated sewage effluents. The results

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of this work indicate that CoFe2O4/HTCC composites have great potential in large-scale

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photocatalytic disinfection operations.

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INTRODUCTION

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threat to human health because they can cause infectious diseases. However, conventional water

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disinfection strategies such as chlorination, ozonation, and ultraviolet irradiation are all facing some

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technical challenges, including secondary pollution, recolonization, and high energy consumption.1-

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3

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because it only requires reusable photocatalyst(s) irradiated by an appropriate light source such as

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sunlight,4-5 and especially because no harmful disinfection byproduct is formed. Various kinds of

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reactive species (RSs) are produced during the photocatalytic process, and these RSs are highly

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capable of inactivating bacteria.6 Recently, numerous photocatalysts such as CdIn2S4, Ni2P/g-C3N4,

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and B-doped BiOBr have been developed for the photocatalytic disinfection of bacteria.7-9

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However, some challenges have yet to be fully addressed for large-scale photocatalytic bacterial

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disinfection. The high cost of material synthesis and the difficulty in the recovery of photocatalysts

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from a heterogeneous system lie in the major problems that significantly hinder the large-scale

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application of photocatalytic disinfection.10 Thus, low-cost, highly recyclable, and visible-light-

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driven (VLD) photocatalysts are urgently needed for an efficient, green, and cost-effective water

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disinfection process.

The emergence of biohazards such as bacteria and viruses in water sources poses a serious

Photocatalysis is considered a promising clean and cost-effective method of bacterial disinfection

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Nanosized cobalt ferrite (CoFe2O4) nanoparticles (NPs) are a good alternative photocatalyst for

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VLD photocatalytic bacterial disinfection because of their high magnetic anisotropy, narrow band

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gap, ample availability, and high stability.11-13 These properties make CoFe2O4 NPs a highly

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recyclable VLD photocatalyst. For instance, amorphous and crystalline CoFe2O4 NPs have been

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fabricated and used as recyclable photocatalysts for water splitting and degradation of 2-

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phenylbenzimidazole-5-sulfonic acid.14-15 Nevertheless, to obtain better magnetic properties, better

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photocatalytic efficiency, and smaller particle sizes of CoFe2O4, many organic solvents or templates

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are commonly used, including ethanol, benzyl ether, oleic acid, 1,2-hexadecanediol, and polyvinyl

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alcohol.15-18 To develop a greener process, the synthesis of CoFe2O4 NPs should avoid or minimize 3



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the use of the above-mentioned hazardous organic chemicals.

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More importantly, bare CoFe2O4 NPs exhibit only moderate photocatalytic activity.15-16 Thus,

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to address this issue, CoFe2O4 NPs are commonly combined with many other carbon materials such

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as graphene and C3N4 to enhance the separation of photogenerated electrons and holes. For

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instance, CoFe2O4–graphene and CoFe2O4/g–C3N4 composites have already been fabricated as

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efficient photocatalysts for dye degradation.16,

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been hindered by high cost and scarcity in supply of the above-mentioned carbon materials. This

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boosts great interests in seeking for low-cost substitute of graphene and C3N4.

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However, their practical applications have still

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It was recently reported that hydrothermal carbonation carbon (HTCC) can be easily obtained

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via a simple hydrothermal treatment of carbohydrates such as glucose, starch, or even grass with a

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relatively low cost.20,21 The synthesis of HTCC is considered as a much less energy-consuming

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process because no energy is required to remove moisture.20 The conversion of carbohydrates into

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HTCC rather than burning them contributes to mitigate greenhouse gas release. Moreover, HTCC

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can closely coat metal oxide catalysts, leading to a large contact area, less leakage of metal irons,

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and high photocatalytic efficiency. For example, Hu et al. reported a MoO2/HTCC heterojunction

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with enhanced photocatalytic activity for O2 evolution and dye degradation.21 HTCC-coated CdS

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NPs were also reported to exhibit high photocatalytic activity in dye degradation.22 However, to the

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best of our knowledge, CoFe2O4 NPs coated with HTCC have not been studied, and thus the

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feasibility of photocatalytic water disinfection over a magnetic CoFe2O4/HTCC composite is

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unknown.

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Herein, for the first time, magnetic CoFe2O4 NPs with an ultrathin HTCC coating were

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designed and fabricated via a facile hydrothermal method. By a novel two-step hydrothermal

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treatment, the CoFe2O4 NPs were first synthesized without using any harmful organic solvent or

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template. The resulting HTCC with a controllable thickness was then coated onto CoFe2O4 NPs by a

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hydrothermal method in the presence of glucose. The CoFe2O4/HTCC composite showed greatly

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enhanced VLD photocatalytic inactivation activity toward Escherichia coli K-12 in both saline 4


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solution and authentic water matrices. This work provides a facile approach to synthesize magnetic

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HTCC-coated photocatalysts as a cost-effective method for biohazards inactivation.

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2. EXPERIMENTAL

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2.1. Synthesis of CoFe2O4 NPs. The CoFe2O4 NPs were synthesized via a novel two-step

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hydrothermal approach. Typically, 0.82 g of CoCl2·6H2O and 0.25 g of FeCl3·6H2O were dissolved

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in 32.5 mL of deionized water to form solution A; 0.6 g of urea was dissolved into 32.5 mL of

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deionized water to form solution B. Solution B was then added to solution A dropwise under

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vigorous stirring, and the mixed solution was transferred to a 100-mL Teflon-lined stainless-steel

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autoclave and maintained at 130 °C for 12 h. The autoclave was then cooled to room temperature

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(~25 °C). The resulting dark gray product was filtered with a membrane filter (0.45 µm; Millipore),

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washed several times with deionized water and ethanol, and finally dried at 60 °C for 12 h. Then,

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0.15 g of the solid product was evenly dispersed into 65 mL of deionized water by ultrasonication.

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The suspension was transferred to a 100-mL Teflon-lined stainless-steel autoclave and

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hydrothermally heated in a temperature-programmed oven. The temperature was increased from 25

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to 180 °C at a rate of 8 °C min-1 and maintained at 180 °C for 10 h. After cooling to room

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temperature, the black product was washed thoroughly with deionized water and ethanol and dried

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at 60 °C for 12 h.

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2.2. Synthesis of CoFe2O4/HTCC Composite. The CoFe2O4/HTCC composite was

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synthesized via a facile hydrothermal method. For a typical process, 0.1 g of the as-prepared

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CoFe2O4 NPs and 0.05 g of glucose were mixed in 65 mL of deionized water under ultrasonication.

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The mixed solution was stirred for 30 min and then transferred to a 100-mL Teflon-lined stainless-

108

steel autoclave. The autoclave was heated at 180 °C for 10 h and then allowed to cool to room

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temperature (~25 °C). The resultant product was washed several times with deionized water and

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ethanol and dried at 60 °C for 12 h. CoFe2O4/HTCC composites with different HTCC contents were

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prepared by varying the dosage of glucose (i.e., 0.025, 0.05, 0.1, and 0.2 g). The products were 5



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accordingly

denoted

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CoFe2O4/HTCC-4.

as

CoFe2O4/HTCC-1,

CoFe2O4/HTCC-2,

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CoFe2O4/HTCC-3,

and

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2.3. Characterization. The field-effect scanning electron microscopic (SEM) images were

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obtained with a Quanta 400F field-effect scanning electron microscope (FEI Company, USA).

116

Transmission electron microscopic (TEM) and high-resolution TEM (HRTEM) images were

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obtained with a Tecnai F20 high-resolution transmission electron microscope (FEI). The X-ray

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diffraction (XRD) patterns were recorded with a SmartLab X-ray diffractometer (Rigaku

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Corporation, Japan) operating at 40 mA and 40 kV with Cu Kα radiation. X-ray photoelectron

120

spectroscopy (XPS) analyses were performed with a ESCALAB 250XI X-ray photoelectron

121

spectrometer (Thermo Scientific, USA). Fourier transform infrared (FT-IR) spectra were recorded

122

with a NICOLET 5700 FT-IR spectrometer (Thermo Scientific, USA). The magnetic properties of

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the as-prepared samples were determined with a SQUID XL-7 magnetic property measurement

124

system (Quantum Design Inc., USA) from −15 to +15 kOe. Raman spectra were recorded with a

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Horiba Jobin-Yvon LabRAM HR800 Raman spectrometer (HORIBA Scientific, France).

126

Thermogravimetric (TG) analysis was performed with a SDT-Q500 thermal analyzer (TA

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Instruments, USA). UV–vis diffuse reflectance spectra (DRS) were recorded with a UV–vis

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spectrometer (Hitachi UH4150, Japan). Room-temperature photoluminescence spectra were

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obtained using a FP-6500 fluorescence spectrometer (Jasco, Japan) with the excited wavelength

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(λex) of 365 nm.

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2.4.

Photoelectrochemical

Measurements.

The

transient

photocurrent

responses,

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electrochemical impedance spectroscopy, and Mott–Schottky plots were recorded on an

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electrochemical workstation (CHI 660D, Shanghai Chen Hua Instrument Company, China) in a

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three-electrode quartz cell with a Na2SO4 electrolyte solution (0.1 M). Ag/AgCl and Pt were used as

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the reference and counter electrodes, respectively. Typically, 5 mg of the sample powder and 10 µL

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of a Nafion@ 117 solution (5 wt%) were well dispersed in 0.5 mL of a water/isopropanol mixed

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solvent (3:1 v/v) by sonication to form a homogeneous colloid. A 100-µL sample of the colloid was 6


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then deposited on fluorinated-tin-oxide glass with an area of 1 cm2 and dried in air at room

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temperature. The electrodes were held at the potential of +0.6 V. A 300-W Xenon lamp was used as

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the light source in the photocurrent measurement.

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2.5. Photocatalytic Bacterial Inactivation. E. coli K-12 was chosen as the model bacterium to

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investigate the photocataytic inactivation performance of the as-prepared samples. The bacterial

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cells were inoculated into 50 mL of Nutrient Broth (Lab M, Lancashire, UK), incubated at 37 °C for

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16 h in a shaking incubator and then harvested via centrifugation at 1300 rpm for 1 min. The

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bacterial pellets were washed twice with a sterilized saline solution (0.9% NaCl) in a centrifuge

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tube and resuspended in a sterilized saline solution. A 25-mg sample of CoFe2O4 NPs was then

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dispersed in 25 mL of sterilized saline solution with the aid of ultrasonication, followed by the

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addition of a suitable amount of bacterial suspension. The final cell density of the bacterial

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suspension was about 1×107 colony forming units per milliliter (cfu·mL-1). The mixed solution was

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irradiated using a 300-W Xenon lamp (PLS-SXE300C, Beijing Perfect Light Technology Co., Ltd

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China) with a 400-nm ultraviolet cut-off filter. A water cooling system was used to maintain the

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temperature at 25 °C throughout the experiment. At given time intervals, 1 mL of the mixed

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solution was collected and serially diluted with sterilized saline solution. After that, 0.1 mL of the

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diluted solution was uniformly spread onto Nutrient Agar (Lab M, Lancashire, UK) plates. Finally,

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the plates were incubated at 37 °C for 18 h to determine the viable cell count. All photocatalytic

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bacterial inactivation experiments were conducted in triplicate.

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Bacterial inactivation was also conducted in authentic water matrices, including surface water

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samples collected from the Tai Po Kau Stream and the Lam Tsuen River, and in secondarily treated

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sewage effluent samples collected from Tai Po Sewage Treatment Works and Sha Tin Sewage

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Treatment Works. The key physiochemical parameters of the water samples were determined and

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are listed in Table S1. Before use, the water samples were filtered by using 0.45-µm glass fiber

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membrane papers. Then, 1 × 107 cfu·mL-1 of E. coli K-12 cells was added into the water or effluent

163

samples. Photocatalytic bacterial inactivation by the composite in these authentic water and effluent 7



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samples was conducted by the procedure mentioned above. 2.6. Analytical Method. The concentration of potassium ions (K+) was measured with a

165 166

polarized Zeeman atomic

absorption spectrophotometer (Hitachi Z-2300, Japan).

The

167

concentrations of cobalt (Co2+) and iron (Fe3+) ions were measured with Agilent 700 ICP optical

168

emission spectrometers (Agilent Technologies Company, USA). The total carbon, total organic

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carbon (TOC), and total nitrogen of the authentic water samples were determined by a Shimadzu

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TOC-L analyzer (Shimadzu Corporation, Japan). The pH values were measured using a Thermo

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Orion 420 pH meter (Thermo Scientific, USA). The amount of hydroxyl radicals (•OH) was

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analyzed by a Hitachi F-4500 fluorescence spectrophotometer based on the reaction of •OH with

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terephthalic acid to produce a highly fluorescent compound, 2-hydroxyterephthalic acid, with an

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excitation wavelength of 315 nm.

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2.7. Scavenger Study. To investigate the bacterial inactivation mechanism of the

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CoFe2O4/HTCC composite, various kinds of scavengers were used to quench the corresponding

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RSs during the bacterial inactivation process. The scavengers with predetermined optimized

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concentration used in this work were Cr(VI) (0.1 mM, for e-), Fe(II)-EDTA (0.2 mM, for H2O2),

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isopropanol (5 mM, for •OH), sodium oxalate (1 mM, for h+), and TEMPOL (2 mM, for •O2). The

180

roles of the different RSs were examined by comparing the inactivation efficiencies with and

181

without the addition of corresponding scavengers. Bacterial inactivation in the absence of O2 was also conducted to further investigate the role of

182 183

•

184

•

185

photogenerated e- alone toward bacterial cells, sodium oxalate was added to remove photogenerated

186

h+ in the absence of O2.

O2. Nitrogen gas was purged into the solution to remove the O2 and thus eliminate the formation of O2 coming from the reaction between photogenerated e- and O2. To determine the impact of

187 188 189

3. RESULTS AND DISCUSSION 3.1. Characterization of Materials. As shown in the XRD patterns (Figure 1), for the as8


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synthesized CoFe2O4 NPs via a two-step hydrothermal method, all peaks are well indexed to

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CoFe2O4 (PDF card No.: 22-1086), indicating that CoFe2O4 was successfully prepared with a pure

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phase. The XRD patterns of the products after being coated by HTCC remained the same as those

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of the CoFe2O4 NPs owing to the amorphous structure of HTCC, which was confirmed by the

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absence of noticeable diffraction in its XRD pattern. As shown in the SEM and TEM images

195

(Figure 2), the pure CoFe2O4 NPs displayed a nearly cubic shape, with an average particle size of

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45 nm. The selected-area electron diffraction (SAED) pattern obtained from the position labeled in

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Figure S1a revealed an angle of 45° between the (220) and (400) planes of CoFe2O4, which was

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identical to the theoretical angle between (220) and (400) planes. The predominantly exposed facet

199

of CoFe2O4 crystals was calculated to be {0, 0,1} facet according to the crystallographic structure

200

of CoFe2O4. The lattice spacings (d) of 0.21 and 0.29 nm matched well with the interplanar

201

spacings of (400) and (220) planes of CoFe2O4, respectively. After loading of HTCC, thin and

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semitransparent layers were clearly observed on the surfaces of the CoFe2O4 NPs (Figure 2e). The

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HTCC layers closely coated the CoFe2O4 NPs, which favored the contact and charge transfer

204

between CoFe2O4 and HTCC. No lattice fringe was found in the HRTEM image of HTCC (Figure

205

2f), further suggesting its amorphous phase. As displayed in Figure S2, by adjusting the dosage of

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glucose, the thickness of the HTCC coating could be controlled from 0.62 to 4.38 nm. The mass

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ratio of the HTCC to the CoFe2O4 was estimated by thermogravimetric (TG) analysis in air (Figure

208

S3). For these four CoFe2O4/HTCC composites, the slight mass loss before 200 °C was due to the

209

evaporation of adsorbed water, and the significant mass loss in the temperature range of 200-400 °C

210

was ascribed to the decomposition of HTCC.21 The mass losses of 3.90, 7.84, 10.02 and 13.21% at

211

200-400 °C were observed for CoFe2O4/HTCC-1, -2, -3, and -4 samples, respectively. No further

212

mass loss was observed after 400 °C, suggesting the thermal stability of CoFe2O4. Therefore, for

213

CoFe2O4/HTCC-1, -2, -3, and -4 samples, the HTCC/CoFe2O4 mass ratios were estimated to be

214

3.90% : 93.24%, 7.84% : 90.01%, 10.02% : 87.45%, and 13.21% : 85.12%, respectively.

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The magnetic properties of the products were measured by their respective hysteresis loops 9



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(Figure 3). The saturation magnetization values of CoFe2O4 and the CoFe2O4/HTCC-1, -2, -3, and -

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4 composites were obtained as 42.0, 40.7, 39.9, 38.4, and 37.1 emu/g, respectively. The slight

218

decrease in the saturation magnetization of the CoFe2O4/HTCC composites was due to the

219

introduction of nonmagnetic HTCC. Even so, the saturation magnetization values were strong

220

enough to separate the composites from the solution. As demonstrated in the inset of Figure 3, the

221

CoFe2O4/HTCC composite was rapidly separated by an external magnetic field within 15 s.

222

Moreover, the coercivity (Hc) values of CoFe2O4 and the CoFe2O4/HTCC composites were

223

determined to range from 47.4 to 174.5 Oe, values much lower than those of previously reported

224

CoFe2O4 NPs.17, 23-24 The low Hc values mean that CoFe2O4 and the CoFe2O4/HTCC composites can

225

be magnetically recycled without significant aggregation, which is desirable for large-scale

226

applications.

227

FT-IR analyses were conducted to investigate the chemical structure of the as-synthesized

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samples. As shown in Figure S4, a series of polyfuran bands were observed in the FT-IR spectrum

229

of HTCC. The wide bands located at 1612 and 1384 cm-1 and two shoulder bands at 1440 and 960

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cm-1 are ascribed to the vibrational modes of the furan monomer.25 The band centered at 1514 cm-1

231

is assigned to the C=C stretching of the furan ring in the polymer. The bands at 1171, 870, and 622

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cm-1 are ascribed to the C–H bending, which comes from the C–H groups in the furan rings.26 The

233

band at 796 cm-l is due to the α,α′-coupling of the carbon backbone in the linear structure of

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polyfuran. Also, the bands at 2926 and 1702 cm-1 are ascribed to the aliphatic C–H stretching mode

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and C=O stretching, respectively, revealing that some of the furan rings were open, which is a

236

common observation for polyfuran.21 For the bare CoFe2O4, the most obvious band at 612 cm-1 is

237

ascribed to metal–oxygen stretching, whereas the band at 1632 cm-1 is ascribed to the adsorbed

238

water.27 The major characteristic bands of polyfuran at 1600, 1563, 1356, 964, and 796 cm-1 were

239

observed after coupling HTCC with CoFe2O4, suggesting that the HTCC maintained the polyfuran

240

components in the CoFe2O4/HTCC composite.

241

The Raman spectra of the samples were recorded to examine the defects and disorder nature of 10


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the carbon-based products. As displayed in Figure S5, the HTCC and CoFe2O4/HTCC samples

243

exhibited two peaks at around 1365 and 1585 cm-1, which were ascribed to the disorder (D) band

244

and graphitic (G) band, respectively. The G band is a result of the E2g vibrational mode of the sp2

245

bonded carbon atoms, and the D peak is due to the out-of-plane vibrations attributed to the presence

246

of structural defects.28 The intensity ratio of the D and G bands (ID/IG ratio) helps to estimate the

247

defects of carbon materials. The higher ID/IG ratio means more defects in carbon materials.29 The

248

calculated ID/IG ratios of HTCC and the CoFe2O4/HTCC composite were 0.73 and 1.06,

249

respectively. The higher ID/IG ratio of the CoFe2O4/HTCC composite clearly depicts the defective

250

nature of HTCC coating. By contrast, no D/G bands were observed in the Raman spectrum of

251

CoFe2O4, suggesting the absence of carbon structures. The peaks located at around 679 and 600 cm-

252

1

253

symmetric stretching of O atoms with respect to metal ions in tetrahedral sites.30

in the Raman spectra of CoFe2O4 and the CoFe2O4/HTCC composite were assigned to the A1g

254

The elemental compositions and valence states of the as-prepared samples were investigated by

255

XPS. As displayed in Figure S6a and 6b, CoFe2O4 and the CoFe2O4/HTCC-2 composite showed the

256

same peak locations of Fe 2p and Co 2p, respectively, indicating that the chemical states of Co and

257

Fe remained the same after coupling with HTCC. Two major peaks of Fe(III) together with two

258

shake-up satellites were found at 711.3 (Fe 2p3/2), 725.2 (Fe 2p1/2), 719.4 (Fe 2p3/2 sat.), and 733.2

259

(Fe 2p1/2 sat.) eV, respectively, indicating that the chemical state of Fe in the products was 3+.31-32

260

The Co chemical state of 2+ was evidenced by the peaks at 796.5 (Co 2p1/2), 803.5 (Co 2p1/2 sat.),

261

780.5 (Co 2p3/2), and 786.4 (Co 2p3/2 sat.) eV.33-35 The C 1s spectrum of HTCC (Figure S6c) could

262

be deconvoluted into three peaks located at 284.7, 285.7, and 288.1 eV, which are attributed to C–C,

263

C–O–C, and C=O bonding, respectively.21, 36 Similarly, the C 1s spectrum of the CoFe2O4/HTCC-2

264

composite (Figure S6c) could also be decomposed into these three components. The O 1s spectrum

265

of HTCC (Figure S6d) could be divided into two peaks centered at 532.7 and 531.6 eV, which are

266

ascribed to the C–O–C and C=O bonds, respectively.37-38 In the O 1s spectrum of CoFe2O4/HTCC-2

267

(Figure S6d), besides the similar peaks centered at 532.8 and 531.8 eV, an additional peak with a 11



268

binding energy (BE) of 530.1 eV was observed, which is attributed to the Co/Fe–O bond.27, 39

269

To understand the band structure and semiconductive nature of the as-prepared samples, the

270

UV-Vis DRS spectra were examined. As shown in Figure S7a, the HTCC sample displayed an

271

intrinsic semiconductor-like absorption in a wide range. The CoFe2O4 and the CoFe2O4/HTCC-2

272

composite showed similar light absorption profiles, whereas the adsorption intensity of the

273

CoFe2O4/HTCC composite was higher than that of bare CoFe2O4, suggesting a stronger light

274

capturing ability. The presence of HTCC coating also led to a slight red shift of absorption,

275

indicating a narrower band gap. The positions of valence-band maximum (VBM) and conduction

276

band minimum (CBM) of the as-prepared samples were estimated by valence-band XPS spectra and

277

Mott–Schottky plots, respectively. As demonstrated in Figure S7b and 7c, the VBM/CBM positions

278

of CoFe2O4, HTCC, and the CoFe2O4/HTCC composite were 0.84/-0.86, 1.14/-0.31, and 0.92/-0.65

279

eV, respectively. Compared with bare CoFe2O4, an obvious negative shift of VBM and a slight

280

positive shift of CBM of the CoFe2O4/HTCC composite were observed, which eventually narrowed

281

the band gap. Based on the results, a diagram of the band structures of CoFe2O4, HTCC, and the

282

CoFe2O4/HTCC composite is shown in Figure S7d. The well-aligned straddling band structures can

283

facilitate photo-induced carrier separation because of the flow of generated electrons from the CB

284

of CoFe2O4 to that of HTCC and the transfer of holes from the VB of HTCC to that of CoFe2O4.

285

Such a staggered bandgap alignment has been widely reported as an efficient way to enhance the

286

charge separation efficiency in a heterojunction system.40-41

287

3.2. Efficiency and Mechanisms of Photocatalytic Bacterial Inactivation. The photocatalytic

288

efficiency of the products was evaluated by the inactivation of a common waterborne bacterium E.

289

coli K-12 under VL irradiation (Figure 4). In the dark and light control experiments (Figure 4a), the

290

bacterial cell density remained unchanged within 150 min, suggesting no toxic effects of the

291

photocatalysts and no photolysis of bacterial cells under VL irradiation. As shown in Figure 4b,

292

HTCC alone had no apparent photocatalytic bacterial inactivation within 150 min. Bare CoFe2O4

293

showed moderate photocatalytic inactivation efficiency toward E. coli K-12, with a 0.9 log10 12


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294

cfu·mL-1 reduction of cell density within 150 min. Impressively, a rapid decrease of the bacterial

295

concentration

296

CoFe2O4/HTCC-2 composite with an HTCC thickness of ~1 nm exhibited the highest bacterial

297

inactivation efficiency, with the complete inactivation of 7 log10 cfu·mL-1 cell densities within 60

298

min of VL irradiation. The other three CoFe2O4/HTCC composites also showed remarkably

299

enhanced photocatalytic activity for bacterial inactivation in comparison to that of bare CoFe2O4.

300

Compared to CoFe2O4/HTCC-2, the lower photocatalytic bacterial inactivation efficiency of the

301

CoFe2O4/HTCC-1 composite is due to the insufficient HTCC content. The HRTEM images of the

302

CoFe2O4/HTCC-1 composite (Figure S2a) show that the HTCC layer was rarely found on the

303

surface of CoFe2O4, which led to a low yield of electrons produced by HTCC and thus inhibited the

304

photocatalytic activity. Furthermore, the less HTCC content means a smaller contact area between

305

HTCC and CoFe2O4, which is unfavorable for charge separation at the contacted interface. In

306

addition, the decrease in the photocatalytic activity of the CoFe2O4/HTCC-3 and CoFe2O4/HTCC-4

307

composites is probably due to the blocking of light by the thickened HTCC coating (2.21–4.38 nm

308

thick). Therefore, the optimized HTCC thickness in this work is ~1 nm, as suggested by the highest

309

photocatalytic activity of the CoFe2O4/HTCC-2 composite. A similar phenomenon was reported by

310

Zhang et al.,42 where the TiO2@carbon core–shell photocatalyst exhibited the highest activity when

311

the carbon thickness was 1 to 2 nm, whereas the 4- to 8-nm carbon thickness blocked the passage of

312

light and prevented the reactant from reaching the TiO2 surface.

was

observed

once

the

CoFe2O4/HTCC

composites

were

added.

The

313

To understand the destruction of the bacterial cell membrane, field-effect SEM images of intact

314

and photocatalytically treated E. coli K-12 cells were compared. As shown in Figure 5a, before

315

photocatalytic treatment, the bacterial cells exhibited a well-preserved rod shape and a smooth

316

surface. After 20 min of photocatalytic inactivation by the CoFe2O4/HTCC-2 composite, some pits

317

were observed on the cell surfaces, revealing initial damage to the membrane (Figure 5b). By

318

prolonging the inactivation time to 40 and 60 min, respectively (Figures 5c and 5d), the cell shape

319

became abnormal and was distorted with obvious holes, suggesting more severe damage and 13



320

increased cell membrane permeability according to the earlier study.4 Finally, the cells displayed

321

completely disorganized morphology after 120 min of photocatalytic inactivation (Figure 5e),

322

implying that the cells were essentially destroyed. These results suggest that sustainably generated

323

RSs can cause a myriad of adverse effects on bacterial cell membranes.

324

Potassium ion (K+) is an important intracellular cation in bacterial cells, and it will immediately

325

leak out from damaged cells due to changes in the cell membrane permeability.43 As shown in

326

Figure 5f, the concentration of leaked K+ increased linearly from 0 to nearly 800 ppb in the first 40

327

min and then gradually reached 935 ppb at the end of the reaction. Comparatively, no obvious K+

328

leakage was observed in the control experiments. This result reveals that the cell membrane

329

permeability of E. coli K-12 changed significantly during photocatalytic inactivation by the

330

CoFe2O4/HTCC composite.

331

The bacterial membrane integrity during the photocatalytic inactivation process was also

332

monitored by BacLight Bacterial Viability Kit for fluorescence microscopy. The live bacterial cells

333

only accumulate SYTO 9 stain and emit green fluorescence, while the dead ones with damaged

334

membranes can accumulate both SYTO 9 and PI stains and emit red fluorescence. As shown in

335

Figure 5g, the untreated cells with intact cell membranes exhibited intense green fluorescence. The

336

number of red fluorescent staining cells increased with prolonged treatment, suggesting that cell

337

membrane was gradually damaged during the photocatalytic inactivation process. All bacterial cells

338

were stained red at 60 min (Figure 5j), revealing that the membrane integrity of all the cells was

339

lost.

340

To analyze the presence and contributions of the specific RSs, scavenger study was carried out

341

by adding various kinds of scavengers to the photocatalytic inactivation system. Before conducting

342

the experiment, the applied concentration of each scavenger was optimized to ensure their

343

maximum scavenging effect and no toxicity toward the bacterial cells. As shown in Figure 6, the

344

photocatalytic inactivation efficiencies were virtually inhibited after the addition of sodium oxalate

345

and Cr(VI) to capture holes (h+) and electrons (e-), respectively, revealing the critical roles of h+ and 14


Page 14 of 31

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346

e- in the bacterial inactivation process. In addition, the important role of •O2 was confirmed by the

347

remarkable decrease in the inactivation efficiency after adding TEMPOL into the system. The redox

348

potential for O2/•O2 is -0.33 V vs NHE,44 so the CBM of CoFe2O4 (-0.86 V vs NHE) was negative

349

enough to reduce O2 to •O2 [Equation (1)]. It is known that •O2 not only directly inactivates bacterial

350

cells, it also produces H2O2 in the conduction band.7 Therefore, the quenching of •O2 will also

351

suppress the formation of H2O2, leading to a significant reduction of the bacterial inactivation.

352

Furthermore, H2O2 has been found to play a moderate role in the inactivation process, as affirmed

353

by the residual of 2.5 log10 cfu·mL-1 of bacterial cells when using Fe-EDTA to quench H2O2.

354

However, with the addition of isopropanol, the inactivation efficiency remained almost the same as

355

that without scavenger addition, indicating that •OH did not play a major role in this bacterial

356

inactivation system. Accordingly, low fluorescence intensities of •OH were observed by using the

357

fluorescence method, with terephthalic acid as the •OH-trapping agent (Figure S8), revealing a low

358

concentration of produced •OH. This further confirms that •OH was not heavily involved in this

359

photocatalytic inactivation process. Thus, the amounts of H2O2 generated by the coupling of two

360

•

361

convert H2O to H2O2 (1.78 V vs NHE), H2O2 was mainly formed through the following equations:

OH were also small. Because the VBM of HTCC (1.14 eV) was not positive enough to directly

362

O2 + e- → •O2

(1)

363

2H+ + •O2 → H2O2

(2)

364

To further investigate the roles of •O2 and e-, bacterial inactivation experiments in the absence

365

of O2 with/without the addition of sodium oxalate were also carried out. The solution was

366

continuously purged with nitrogen gas to remove O2 to eliminate the formation of •O2 from the

367

conduction band of the CoFe2O4/HTCC composites. Under anaerobic conditions, the

368

photocatalytic inactivation efficiency decreased remarkably, showing the role of O2 as the •O2

369

precursor in the inactivation system. When sodium oxalate was added into the anaerobic system,

370

both the e--generated RSs and h+ were removed, leaving only e- in the inactivation system. In this

371

case, the photocatalytic inactivation was almost completely inhibited and only about 0.5 log10 15



372

cfu·mL-1 of cell reduction was achieved, indicating that photogenerated e- is not an effective RS to

373

directly inactivate bacterial cells. Based on the above discussion, a photocatalytic bacterial

374

inactivation mechanism by the CoFe2O4/HTCC composites as depicted in Scheme 1 is proposed.

375

Electrochemical experiments were conducted for the in-depth investigation of the enhanced

376

photocatalytic inactivation over the CoFe2O4/HTCC composites. As demonstrated in Figure S9a,

377

the photocurrent responses of bare CoFe2O4 and HTCC are insignificant. By contrast, the

378

CoFe2O4/HTCC composites exhibited significantly enhanced photocurrent responses, except the

379

CoFe2O4/HTCC-4 sample. The CoFe2O4/HTCC-2 electrode showed the highest photocurrent

380

intensity, followed by the CoFe2O4/HTCC-3 and CoFe2O4/HTCC-1 electrodes. The higher

381

photocurrent densities imply a higher photogenerated e- transfer efficiency. However, for the

382

CoFe2O4/HTCC-4 composite, the enhancement of its photocurrent intensity was not high, which

383

was probably due to the blocking of light by the excessively thickened HTCC coating, as mentioned

384

above. As demonstrated in the electrochemical impedance spectroscopy Nyquist plots (Figure S9b),

385

in comparison with bare CoFe2O4 and HTCC, the semicircular arcs of the CoFe2O4/HTCC

386

composites displayed much smaller diameters, indicating faster interfacial charge transfers to the

387

electron acceptors. In addition, room-temperature PL spectra (Figure S10) were used to examine the

388

recombination of photogenerated electron–hole pairs in the as-prepared materials. The

389

CoFe2O4/HTCC composites exhibited much lower PL intensities as compared with those of pure

390

CoFe2O4 and HTCC. This result suggests that the presence of the HTCC coating can facilitate the

391

electron transfer and suppress the recombination of charge carriers, and thus eventually favor the

392

photocatalytic performance of bacterial inactivation.

393

3.3. Stability and Reusability of CoFe2O4/HTCC Composite. The stability and reusability of

394

CoFe2O4/HTCC composites were tested by repeating the photocatalytic bacterial inactivation

395

experiments with recycled CoFe2O4/HTCC composites. After being magnetically collected by an

396

external magnet, the CoFe2O4/HTCC composites were washed thoroughly with ethanol and distilled

397

water before another run. As shown in Figure S11, a slight decrease in the photocatalytic efficiency 16


Page 16 of 31

Page 17 of 31


398

was initially observed at the third run, and no significant loss of photocatalytic activity was found

399

even after the fifth run. The photocatalytic inactivation efficiency in the fifth run was calculated to

400

be (1 – 101.5/107) × 100% = 99.99965%. Therefore, the CoFe2O4/HTCC composite can serve as a

401

highly stable, recyclable, and cost-effective composite for VLD photocatalytic bacterial

402

inactivation.

403

The leakage of Co2+ and Fe3+ cations from the photocatalysts during the inactivation process

404

was monitored. As shown in Figure S12 and Table S1, the leakage of Co2+ from the

405

CoFe2O4/HTCC-2 composite was less than one-tenth of that from bare CoFe2O4, suggesting that the

406

HTCC coating could significantly reduce the leakage of metal ions from the catalyst. The

407

concentration of released Fe3+ from these two samples was less than 1 × 10-4 mg·L-1, indicating

408

almost no Fe3+ leakage. The determined concentrations of released Co2+and Fe3+ from the

409

CoFe2O4/HTCC-2 composite were far below the discharge limits (0.05 mg·L-1 for Co2+and 1mg·L-1

410

for Fe3+, GB 18918-2002). Thus, the CoFe2O4/HTCC composite can be used as a stable and

411

environmentally friendly photocatalyst for water disinfection.

412

3.4. Environmental Implications for Authentic Water Treatment. To evaluate the

413

environmental application of this newly developed photocatalyst, photocatalytic inactivation by the

414

CoFe2O4/HTCC-2 composite was also conducted in authentic water matrices, including surface

415

water and secondarily treated sewage effluent samples. As shown in Figure 7, the photocatalytic

416

inactivation performance in the surface water samples collected from Tai Po Kau Stream and Lam

417

Tsuen River was almost the same as that in saline solution, with a total inactivation of 7 log10

418

cfu·mL−1 of bacterial cells within 60 and 80 min, respectively. For the bacterial inactivation in

419

secondarily treated sewage effluent samples, it is worth noting that the cell density increased

420

slightly in the first 20 min, probably due to the rich nutrients in the effluents, which are favorable

421

for cell proliferation. After that, the bacterial inactivation kinetics showed a typical “shoulder + log-

422

linear” model.5 The shoulder length can be considered the cumulative damage period induced by the

423

photocatalytic reactions before cell proliferation.43 Therefore, the wide shoulders mean that it took a 17



424

relatively long time to generate enough RSs to inhibit cell proliferation in the secondary wastewater

425

effluents, which is owing to the competition for the RSs by the organic components in the effluents.

426

It is noted that the values of total carbon, TOC, and total nitrogen were much higher in the

427

secondary wastewater effluents than in the surface water (Table S2). After 160 min of VL

428

irradiation, a complete 7-log10 reduction of bacterial cells was achieved in the secondarily treated

429

sewage effluents. The photocatalytic inactivation performance of the CoFe2O4/HTCC composite in

430

the secondarily treated sewage effluents was satisfactory because it was even better than that

431

performed in saline solution or ultrapure water, as reported previously.45-46 Thus, the

432

CoFe2O4/HTCC composite is able to serve as a promising photocatalyst for authentic water

433

disinfection.

434 435

ASSOCIATED CONTENT

436

Supporting Information

437

HRTEM images of CoFe2O4 and the CoFe2O4/HTCC composites; SAED pattern of CoFe2O4;

438

Thermogravimetric (TG) curves of the CoFe2O4/HTCC composites; FT-IR spectra, Raman spectra,

439

UV-Vis DRS patterns, valence band XPS spectra, Mott-Schottky, and band structures of HTCC,

440

CoFe2O4 and the CoFe2O4/HTCC-2 composite;

441

CoFe2O4/HTCC-2 composite and TiO2 (P25); Transient photocurrent responses, EIS Nyquist plots

442

and room-temperature photoluminescence spectra of the as-prepared samples; Recycling

443

experiment of the photocatalytic inactivation; Leakage of Co2+ and Fe3+ cations from CoFe2O4 and

444

the CoFe2O4/HTCC-2 composite. Parameters of the surface water and the secondarily treated

445

sewage effluent samples.

Fluorescence emission intensities of the

446 447

AUTHOR INFORMATION

448

Corresponding Authors Tel: +86 20 2388 3536, Fax: +86 20 8529 1501, E-mail: [email protected] (T.C. An); Tel: +852 3943 6383, Fax: +852 2603 5767, E-mail: [email protected] (P.K. Wong). 18


Page 18 of 31

Page 19 of 31


449 450

Notes

451

The authors declare no competing financial interest.

452 453

ACKNOWLEDGEMENTS

454

This research was financially supported by the research grant (GRF14100115) from Research Grant

455

Council, Hong Kong SAR Government, National Natural Science Foundation of China (41573086,

456

21706102), and Natural Science Foundation of Jiangsu Province (BK20170527). P.K. Wong was

457

also supported by the CAS/SAFEA International Partnership Program for Creative Research Teams

458

(2015HSC-UE004) of Chinese Academy of Sciences, China. The authors also would like to

459

acknowledge the technical assistance from Ms. Hongli Sun and Dr. Bo Wang of School of Life

460

Sciences, The Chinese University of Hong Kong, and Ms. Mingzhe Sun of School of Energy and

461

Environment, City University of Hong Kong.

462 463

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(44) Wang, W.; Yu, J. C.; Xia, D.; Wong, P. K.; Li, Y., Graphene and g-C3N4 nanosheets cowrapped elemental

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α-sulfur as a novel metal-free heterojunction photocatalyst for bacterial inactivation under visible-light.

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Figures

Figure 1. XRD patterns of HTCC, CoFe2O4 and CoFe2O4/HTCC composites.

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Figure 2. (a) SEM, (b) TEM and (c) HRTEM images of CoFe2O4; (d) SEM, (e) TEM and (f) HRTEM images of the CoFe2O4/HTCC composite.

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Figure 3. The hysteresis loops of pure CoFe2O4 and CoFe2O4/HTCC composites (the upper left inset shows the CoFe2O4/HTCC-2 composite dispersed in water and separated by a magnet, the lower right inset shows the low field region of the hysteresis loops).

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(a)

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Figure 4. (a) Experimental controls and (b) Photocatalytic inactivation by the as-prepared CoFe2O4/HTCC composites under visible light irradiation.

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(a)

(b)

(c)

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(e)

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(g)

(h)

(i)

(j)

Figure 5. FE-SEM images of E. coli K12 cells being photocatalytically inactivated by the CoFe2O4/HTCC-2 composite for (a) 0 min, (b) 20 min, (c) 40 min, (d) 60 min and (e) 120 min (scale bar: 2 µm); (f) Potassium ion (K+) leakage from E. coli K-12 under different treatment conditions; Fluorescence microscopic images of E. coli K12 cells being photocatalytically inactivated by the CoFe2O4/HTCC-2 composite for (g) 0 min, (h) 20 min, (i) 40 min, and (j) 60 min (scale bar: 20 µm)

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Figure 6. Photocatalytic inactivation efficiencies toward E. coli K-12 by the CoFe2O4/HTCC-2 composite with different scavengers.

Figure 7. Photocatalytic inactivation efficiencies toward E. coli K-12 by the CoFe2O4/HTCC-2 composite in different water matrices (surface water and secondary wastewater effluent).

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Scheme 1. Proposed VLD photocatalytic bacterial inactivation mechanism by the CoFe2O4/HTCC composite.

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Graphical Abstract

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Enhanced Visible-Light-Driven Photocatalytic Bacterial Inactivation by

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