Combination of sunlight with hydrogen peroxide generated at a modified reticulated vitreous carbon for drinking water disinfection

Journal of Cleaner Production 252 (2020) 119794

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Combination of sunlight with hydrogen peroxide generated at a modified reticulated vitreous carbon for drinking water disinfection Yanchao Jin a, b, Yijun Shi a, Ziyu Chen a, Riyao Chen a, b, *, Xiao Chen a, b, Xi Zheng a, b, Yaoxing Liu a, b a b

College of Environmental Science and Engineering, Fujian Normal University, Fuzhou, 350007, China Fujian Key Laboratory of Pollution Control & Resource Reuse, Fuzhou, 350007, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 12 September 2019 Received in revised form 13 December 2019 Accepted 18 December 2019 Available online 19 December 2019

Long treatment times limit the application of solar disinfection (SODIS). The concentrations of electrolytes in drinking water are low, so it is difficult to generate a sufficient quantity of H2O2 for drinking water disinfection via electrochemical reduction. In this study, SODIS and electrochemical reduction were combined together. A reticulated vitreous carbon (RVC) cathode was anodised to improve its performance and characterized using scanning electron microscopy and X-ray photoelectron spectroscopy. The contributions of the electrochemical process and SODIS to disinfection were studied. The influences of current, temperature and humic acid (HA) were also investigated. The results showed that the modification of RVC added oxygen-bearing functional groups and doubled the quantity of H2O2 generated at the cathode. When the hybrid process was employed to remove E. coli from water, The E. coli count was reduced from approximately 106 colony-forming units (CFUs) per mL to below the detection threshold (<4 CFU/mL) after 120 min. The disinfection time of the hybrid method was lower than those of SODIS and electrochemical disinfection alone by 60% and 20%, respectively. Increasing the current reduced the treatment time from 150 to 90 min, although the treatment consumed more electricity. HA in a low concentration (1 mg/L) could enhance the disinfection process. However, a relatively high HA concentration (4 mg/L) suppressed the inactivation of E. coli. Increasing the temperature from 20 to 40
C reduced the treatment time from 120 to 90 min, and the electricity consumption per log of E. coli decreased from 102.2 to 64 Wh/m3. This study demonstrates that combining SODIS and H2O2 electrogeneration is an effective and energy-efficient disinfection strategy. © 2019 Elsevier Ltd. All rights reserved.

Handling editor: Jin-Kuk Kim Keywords: Electrochemical reduction Hydrogen peroxide Solar disinfection Anodic polarization Escherichia coli

1. Introduction Microorganisms contaminate the water of approximately 800 million people worldwide (Carratala et al., 2016). Disinfection is thus the most important aspect of drinking water treatment (Ortega-Gomez et al., 2012). Conventional methods for water disinfection, such as chlorination and ozonation require infrastructure and often involve a considerable infusion of capital (Shannon et al., 2008). It is thus difficult for many regions in the world, with limited economies, to implement these methods. Water distribution introduces additional contamination risk. Pointof-use drinking water disinfection is a feasible and effective to

* Corresponding author. College of Environmental Science and Engineering, Fujian Normal University, Fuzhou, 350007, China. E-mail address: [email protected] (R. Chen). https://doi.org/10.1016/j.jclepro.2019.119794 0959-6526/© 2019 Elsevier Ltd. All rights reserved.


rez overcome economic limitations and reduce risk (Castro-Alfe et al., 2017a). The disinfecting effect of sunlight has been recognized for many years, and sunlight has been used for water disinfection since 1980 (Acra et al., 1980). The primary disinfecting agent in SODIS (solar disinfection) is the ultraviolet (UV) light in sunlight. Microorganisms can absorb UVb portion of the spectrum, which damages deoxyribonucleic acid (DNA) or ribonucleic acid (RNA) directly (Mbonimpa et al., 2012). The UVa portion of the solar spectrum has an indirect role in sterilization. UVa light is absorbed by endogenous chromophores, which act as sensitizers, and induces the
rez et al., production of reactive oxygen species (ROS) (Castro-Alfe 2017b). Proteins and lipids are damaged via oxidation by ROS, and the membrane permeability is altered. In addition, DNA ruptures appear (Berney, 2006). Solar energy is clean, abundant and readily accessible. SODIS is commonly used for household drinking

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2. Materials and methods Nomenclature 2.1. Modification and characterization of the RVC cathode CE CFUs DT DNA EC E.coli HA NOM RNA ROS RVC SODIS SEM XPS

current efficiency colony-forming units detection threshold deoxyribonucleic acid electricity consumption, Wh/m3 Escherichia coli humic acid nature organic matters ribonucleic acid reactive oxygen species reticulated vitreous carbon solar disinfection scanning electron microscope X-ray photoelectron spectrometer

water disinfection in developing countries (McGuigan et al., 2012). However, complete disinfection by SODIS takes at least 6 h (Castro
rez et al., 2018). Alfe Additives have been employed in SODIS to shorten the treatment time and improve water treatment efficiency. Amin and Han (2011) confirmed that lemon and vinegar could place additional stress on cells, which increases the SODIS disinfection rate by approximately 40%. However, it is difficult to avoid the emergence of problematic odours and tastes when these additives are used. H2O2 can increase the production of intracellular ROS and enhance microorganism inactivation. The removal of total Vibrio from marine aquaculture effluent via SODIS was significantly improved through H2O2 addition (Villar-Navarro et al., 2019). However, difficulties in transporting and storing H2O2 must be overcome. H2O2 can be generated in situ by electrochemical reduction. Dissolve oxygen is reduced to H2O2 at the cathode through a twoelectron process, while its reduction to H2O is a four-electron process (Siahrostami et al., 2013; Zhang et al., 2019). The properties of the cathode are critical to the oxygen reduction process. Carbon materials prefer the two-electron process and are always used in electro-Fenton systems (Lan et al., 2016). Basova et al. (1999) showed that oxygen functional groups on the cathode surface could improve H2O2 generation by decreasing diffusion resistance and providing more active surface sites. Various efforts have been made to increase the number of oxygen containing functional groups, such as eC]O, eCHO and eCOOH, on carbon cathodes to improve their performance. These include anodizing in an acidic medium or boiling in concentrated sulphuric and nitric acids (Miao et al., 2014; Zhong et al., 2013). Although disinfection could be achieved by the in situ generated H2O2 with modified cathode, it needed long treatment time and more electricity consumption (Jin et al., 2019). It can be combined with SODIS, which would reduce the required treatment time and consumption of electricity. The aim of this study was to construct an effective, energyefficient drinking water disinfection device that combined SODIS and H2O2 generation through electrochemical reduction. A reticulated vitreous carbon cathode (RVC) electrode modified via anodic polarization was used as the cathode, owing to its large void volume and surface area, and E. coli was chosen as the indicator bacteria (Jin et al., 2017). The roles of SODIS and the electrochemical process during the disinfection were investigated. The effects of current, water temperature, and nature organic matters (NOM) concentration on disinfection were also determined.

The RVC cathode (40.0  50.0  10.0 mm) was supplied by Jiangsu Tengerhui Technology Co. Ltd. China. It was first thoroughly cleaned in ethanol using an ultrasonicator and washed in deionized water. The RVC cathode served as the working electrode for subsequent modification via anodic polarization (Jin et al., 2019). An Ag/AgCl electrode and a platinum sheet were employed as the reference electrode and counter-electrode, respectively. The RVC cathode was oxidized from 0.0 to 2.0 V in 20 wt % H2SO4 at a rate of 10 mV/s over six cycles. The electrode was then washed with sulphuric acid and deionized water, and dried under vacuum. The surface morphology of the modified RVC cathode was examined using a Regulus 8100 microscope (SEM, Hitachi, Japan) at an accelerating voltage of 5.0 kV. The oxygen-containing functional groups of the RVC cathode surface were measured with X-ray photoelectron spectrometer (XPS, Shimadzu, Japan). Resolution of the XPS spectra was performed with the XPS Peak 4.1 software package using an asymmetrical GaussianeLorentzian peak shape and a Shirley background function to account for contributions from inelastic scattering.

2.2. Solar-electrochemical reactor Experiments were performed at a constant temperature under stable irradiation. A solar simulator (Jinzhou Sunshine Technology Co. Ltd, China) equipped with a xenon lamp integrated with an AM1.5 filter was used to simulate the standard terrestrial AM1.5 spectrum. A portion of 3% of the emitted photons in 280e400 nm range. After 400 nm, the emission spectrum follows the solar spectrum. The global irradiance was set at to 835 W/m2 and tested using a model SPP pyranometer (Eppley Laboratory, Inc., USA) with a sensitivity of 8.43 (mV/W)∙m2 from 300 to 3000 nm. The UV irradiance was set to 25 W/m2 and tested from 280 to 400 nm with a model TUVR UV radiometer (Eppley Laboratory, Inc., USA) at a sensitivity of 150 (mV/W)∙m2. The electrochemical reactor is shown in Fig. 1. A 1 L Pyrex beaker was placed in a DF-101S water bath (Yuhua Co. Ltd., China) to control the water temperature. A Ti/RuO2 (35.0  50.0  1.0 mm) electrode supplied by Guangrui Co. Ltd., China was used as the

Fig. 1. Structure of the reactor used during this study.

Y. Jin et al. / Journal of Cleaner Production 252 (2020) 119794

anode, and either an RVC or a modified RVC electrode was used as the cathode. The electrodes were aligned vertically in the reactor at a distance of 10 mm. A constant current was provided by a power source, and the voltage was followed at 30 min intervals. To match the standard of World Health Organization for drinking-water, sodium sulphate (250 mg/L) was selected as an electrolyte. The water in the reactor was agitated at 150 rpm during the disinfection process. O2 was bubbled into the reactor at a constant rate of 400 mL/min to increase the amount of dissolved oxygen in the water. 2.3. Microbiological analysis E. coli K-12 (ATCC 25922) was selected as the indicator bacteria during this study (Jin et al., 2016). A single colony was used to inoculate 5 mL lysogeny broth (LB, Oxoid Ltd, England) and the inoculated LB was incubated at 37
C for approximately 15 h. The bacteria were collected via centrifugation at 4500 rpm for 5 min. To thoroughly eliminate the culture medium, suspension and centrifugation steps were performed 3 times. The isolated bacteria were then added to 0.5 L water to a concentration of ~106 colony-forming units (CFUs) per mL. The bacterial concentration (CFU/mL) in the samples was determined by the standard plate count method. To avoid the influence of residual H2O2 to the bacteria, excess sodium thiosulfate (10 mM) was added to the sample. Serial 10-fold dilutions were prepared in water, and 0.3 mL appropriate diluted samples were spread onto the LB Agar plates (Oxoid Ltd., England) and incubated at 37
C for 24 h. The CFUs on each plate was were counted and the detection threshold (DT) was 4 CFU/mL. 2.4. Analytical methods The titanium potassium oxalate method was used to measure H2O2 concentration. Absorbance was measured at 400 nm (lmax) (Gallard and De Laat, 2000) on a Cary 5000 spectrophotometer (Cary5000, Agilent, US) at a resolution of 0.005 nm. The current efficiency (CE) of electrochemical H2O2 generation was calculated with Eq. (1) (Zhou et al., 2012):

CE ¼

zFCH2 O2 V  100% MH2 O2 It

(1)

where z represents the number of electrons transferred for H2O2 production, F is the Faraday constant (96485 C/mol), CH2 O2 is the H2O2 concentration (g/L), V is the water volume (L), MH2 O2 is the molar mass of the H2O2, I means the constant current (A), and t represents the performed time (s). The electricity consumption (EC, Wh/m3) per log of E. coli reduction was calculated based on the time required to reach the DT using:

EC ¼

IUt Vðlog10 N0  0:6021Þ

(2)

where I means the constant current (A), U represents the cell voltage (V), t (h) means the time required for disinfection to achieve the DT, V means the water volume (m3), N0 represents the number of initial E. coli, and 0.6021 means the log10 (DT). 2.5. Experimental procedures The RVC cathode was modified and characterized, and the disinfection performance of the cathode was then tested. Appropriate E. coli was added to the water, and the first sample was taken,

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and kept in dark condition as controls. O2 was then bubbled near the cathode for 30 min. The water temperature was regulated and kept at 20
C, except during experiment studying the effect of temperature. The solar simulator was turned on to start the experiment, except during experiment studying the disinfection enhancement due to electrode modification. Samples were taken at 30 min intervals to determine the concentration of surviving E. coli and that of accumulated H2O2. The electrodes were washed via ultrasonication in deionized water, and then dried under vacuum for the next experiment. Each experiment was performed in triplicate to establish repeatability. The replicates average and error bars calculated as standard deviation are shown in the figures. 3. Results and discussion 3.1. Characterization of the modified RVC electrode SEM images of the unmodified and modified RVC electrodes showed that the surface morphology of the RVC electrode was much rougher following modification (Fig. 2). Numerous small holes were visible, which were caused by electrochemical corrosion. The O 1s XPS spectra of the RVC cathodes are shown in Fig. 3. The peak 531.9 eV was attributed to carbonyl oxygen (Lin et al., 2018). The peaks at 532.8 and 533.5 eV were attributed to oxygen in hydroxyl and carboxyl groups, respectively (Miao et al., 2014). The results demonstrated that many oxygen-containing were generated on the RVC during anodic polarization. Oxygen functional groups can increase the hydrophilic character of the cathode, which decreases the dissolved oxygen diffusion resistance and facilitates H2O2 formation (Miao et al., 2014). 3.2. Electrochemical disinfection enhancement due to electrode modification tested in dark conditions The electrochemical disinfection performance of each RVC cathode was determined. The disinfection rate was very low in the first 150 min of operation with the unmodified RVC cathode. Only an ~3.5 log reduction was achieved after 240 min (Fig. 4a). When the modified RVC cathode was used, the number of E. coli in the reactor was remarkably reduced within 90 min (1.5-log removal). After 150 min of treatment, the number of E. coli was below the DT (<4 CFU/mL). The concentration of H2O2 in the neutral medium containing 0.25 g/L Na2SO4 was also measured. The H2O2 concentration increased to ~26 mg/L over 150 min when the unmodified RVC was used (Fig. 4b). When the modified RVC cathode was used, the concentration of H2O2 approximately doubled over the same treatment time. Previous studies have shown that disinfection via electrochemical oxidation with Ti/RuO2 anodes is quite poor and negligible (Jeong et al., 2009; Jin et al., 2019). Therefore, the main cause of disinfection during this experiment was H2O2 generated at the cathode via electrochemical reduction. H2O2 attacked the bacterial membranes and increased their permeability (Villar-Navarro et al., 2019). H2O2 could also penetrate the cells and induce OH production via the Fenton reaction with intracellular iron (Porras et al., 2018). In addition, H2O2 damaged the ironesulphur clusters in the cells, resulting in the release of Fe3þ (Imlay, 2003). Due to the selfprotection mechanisms and the relatively low doses of H2O2, E. coli concentrations were stable at the beginning of the experiment. However, the bacteria were quickly inactivated as damage accumulated. Characterization of the RVC electrodes demonstrated that the properties of the modified RVC cathode were more conducive to H2O2 generation. The concentration of accumulated H2O2 was

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Fig. 2. SEM images of (a) the unmodified RVC and (b) modified RVC electrodes.

Fig. 3. XPS O1s spectra of (a) the modified RVC and (b) unmodified RVC electrodes.

Fig. 4. Concentrations of E. coli (a) and H2O2 (b) with the unmodified and modified RVC cathodes (I: 25 mA, T: 20
C).

significantly higher with the modified RVC cathode, and the disinfection rate was clearly higher.

3.3. Disinfection by the SODIS hybrid method with H2O2 electrogeneration Disinfection was performed using SODIS, H2O2 electrogeneration, and the hybrid method. E. coli was inactivated, only when the damage has accumulated to the lethal dose levels. A“shoulder” was observed onset of disinfection (Fig. 5a). Approximately 3-log removal was achieved by SODIS after 180 min, and complete disinfection to below the DT required 300 min of

treatment (no present). Although the extent of disinfection performance via H2O2 electrogeneration was similar to that of SODIS in the first 90 min of the experiment, the bacterial count had decreased to below the DT after 150 min with H2O2 generation. The time needed for disinfection was shortened to 120 min when the H2O2 electrogeneration was combined with SODIS (Fig. 5a). The concentration of H2O2 had no difference when the electrochemical process was performed under sunlight (Fig. 5b). The UV portion of sunlight can be absorbed by various cell components. UV light causes direct damage in several ways, including the breakage of double-stranded DNA and the formation of cyclobutane pyrimidine dimers (Pfeifer et al., 2005). Some

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Fig. 5. Concentration of E. coli (a) and H2O2 (b) during different disinfection processes (I: 25 mA, T: 20
C).

enzymes are also affected by UV absorption. It has been suggested that UVa irradiation suppresses peroxidase activity (Eisenstark, 1998). Iron- and sulphur-containing molecules in the cell, such as dihydroxy acid dehydratase can also be damaged by UV irradiation (Giannakis et al., 2016). ROS generation is also induced by sunlight, ROS oxidize proteins and lipids. Due to the self-defence mechanisms against sunlight induced damage, a “shoulder” could not be avoided when the water was treated solely with solar irradiation. It was consistent with that obtained by others during E. coli disinfection under sunlight (Vivar et al., 2015). When the water was treated using the hybrid method, both UV in the sunlight and H2O2 damaged the E. coli. In addition, ROS were generated as H2O2 absorbed UV photons. Therefore, the disinfection rate of the hybrid method was faster, compared with that of SODIS or H2O2 electrogeneration. The water did not contain a catalyst, so the effect of the sunlight on H2O2 generation was not obvious. With the exception of hydrogen evolution at the cathode, the anode and the cathode were in the same tank, and a portion of the H2O2 decomposed at the anode (Brillas et al., 2000). The side reactions increased as H2O2 accumulated. The CE of the hybrid process was 72.49%. While, that of the electrochemical process alone was 68.35% due to the longer treatment time. The EC per log of E. coli reduction was calculated based on the time required to reach the DT, which represented 5.6log removal. EC was reduced from 132.4 to 102.2 Wh/m3 when sunlight was used to enhance the electrochemical process (Table 1). Even compared with that of previous electrochemical disinfection processes in a high concentration of electrolyte, the EC of this hybrid method is low (Table S1).

3.4. Effect of the current on the hybrid disinfection process Sterilization at an E. coli concentration of ~106 CFU/mL was studied at various currents. No change in the bacterial concentration was observed in the first hour at a current of 15 mA (Fig. 6a), and the concentration decreased by only 2.7 orders of magnitude after 90 min of treatment. Complete disinfection to below the DT required 150 min of treatment. The disinfection rate increased

Table 1 Electricity consumption for per log E. coli reduction and the current efficiency of H2O2 generation based on the time to reach the DT.

Electro Electroþsolar

Voltage (V)

Time (h)

EC (Wh/m3)

CE (%)

5.89 ± 0.06 5.86 ± 0.09

2.5 2.0

132.4 ± 1.2 102.2 ± 1.6

68.35 ± 2.71 72.49 ± 4.35

when the current was increased to 35 mA, and complete disinfection was achieved within 90 min. The concentration of H2O2 was also monitored during the experiment (Fig. 6b), and it increased from 26 to 46 mg/L over 90 min as the current increased from 15 to 35 mA. Except the increase of cell membrane damage, more H2O2 would diffuse into the cell and enhance the intracellular Fenton along with the increase of the H2O2 in the water. It is consistent
ndez et al., 2012). with previous studies (García-Ferna The H2O2 production rate increased with the current. Thus, the treatment time needed to reach a certain lethal dose was shortened. On the other hand, the operating voltage increased from 3.73 to 6.52 V when the current increased from 15 to 35 mA. Therefore, power consumption increased markedly. The EC per log of E. coli reduction was 51.5 Wh/m3 at 15 mA, which increased to 128.3 Wh/ m3 at 35 mA (Table S2). Side reactions, such as hydrogen evolution at the cathode and H2O2 decomposition, were not significant at a relatively low current. The CE was about 81% at 15 mA and decreased to about 69% when the current was increased to 35 mA. Increasing the current reduced the treatment time, but more electricity was consumed. By comprehensive consideration of the electricity consumption and treatment time, the current of 25 mA was suitable for this study. 3.5. Effect of HA on the hybrid disinfection process NOM is commonly found in drinking water. HA has generally been used to model NOM, so it was selected to study the influence of NOM on E. coli inactivation during hybrid disinfection. The disinfection rate was reduced by HA addition at different concentrations in the first 60 min of disinfection (Fig. 7a). However, the E. coli concentration fell below the DT as disinfection continued for another 30 min at an HA concentration of 1 mg/L. Compared to water that did not contain HA, the time needed for disinfection was reduced from 120 to 90 min. Suppression by HA never altered at HA concentrations of 2 and 4 mg/L. The DT was not reached at an HA concentration of 4 mg/L, even when the water was treated for 180 min. The addition of HA did not affect the production of H2O2 (Fig. 7b), but increased the conductivity of the water. The voltage was thus reduced by the addition of HA (Table S3). Previous studies have demonstrated that HA can filter sunlight, which has a detrimental effect on bacterial inactivation (Porras et al., 2018). HA also reacts with ROS like singlet oxygen, and plays the role of a scavenger in aquatic photochemistry (Kohantorabi et al., 2019). On the other hand, Porras et al. (2014) have found that HA can enhance the formation of singlet oxygen by facilitating charge transfer, which can damage the cell membranes and DNA. HA excited by sunlight

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Fig. 6. Concentration of E. coli (a) and H2O2 (b) at different currents (T: 20
C).

Fig. 7. Concentration of E. coli (a) and H2O2 (b) in drinking water containing HA at different concentrations (I: 25 mA, T: 20
C).

can also transfer electrons to the bacterial membrane, which causes sterilization. These mechanisms explain why HA at a concentration of 1 mg/L significantly increased the disinfection rate. The time needed to reach DT was shortened, and the EC decreased from 102.2 to 68 Wh/m3 (Table S3). If more HA was added to the water, suppression predominated. Consequently, the disinfection rate decreased when 4 mg/L of HA was added.

3.6. Effect of temperature on the hybrid disinfection process The temperature of the water increased when it was disinfected under sunlight. To investigate the role of temperature during the disinfection process, experiments were performed at 20, 30 and 40
C under simulated sunlight. The E. coli concentrations did not differ from those in the control samples at corresponding temperature under dark conditions, which indicated that increasing temperature alone in the range from 20 to 40
C could not inactive E. coli. The hybrid disinfection process was enhanced at higher temperatures in the range from 20 to 40
C (Fig. 8a). A 3.5-log decrease was observed over 90 min of treatment at 20
C, While, the E. coli concentration fell below the DT over the same treatment period at to 40
C. H2O2 was stable at temperatures from 20 to 40
C. In addition, oxygen was bubbled around the cathode, and the amount of dissolved oxygen for electroreduction was sufficient. Therefore, Changes in the H2O2 concentration were negligible when the temperature was increased from 20 to 40
C (Fig. 8b). Thus, the influence of temperature on disinfection was not due to changes in H2O2 concentration. The optimum E. coli growth temperature is

37
C. At temperatures near 37
C, cellular metabolism becomes active, and genetic material is unpacked and completely functional. For these reasons, the microorganisms are more vulnerable (Ortega-Gomez et al., 2012). The time needed to reach the DT decreased from 120 to 90 min when the temperature was adjusted from 20 to 40
C. To meet the drinking water quality standards issued by the World Health Organization, the Na2SO4 concentration in the water could be no higher than 0.25 g/L. Therefore, the conductivity of water was very low. Increasing the temperature increased the ionic activity, so the resistance and voltage were decreased. As shown in Table S4, the voltage decreased from 5.87 to 4.86 V. Increasing the temperature not only shortened the treatment time but also reduced the consumption of electric power. Thus, by raising water temperature, the EC for per log E. coli reduction decreased from 102.2 to 64 Wh/m3 when the temperature was increased (Table S4).

4. Conclusions The oxygen functional groups on the RVC cathode was increased by anodic polarization, which increased the H2O2 yield approximately two-fold. Combining H2O2 electrogeneration with sunlight shortened the disinfection time to 120 min and the EC for per log E. coli reduction to 102.2 Wh/m3. Increasing the current increased the disinfection rate. However, it also increased electricity consumption. The treatment time was reduced from 150 to 90 min when the current was increased from 15 to 35 mA, while, and the EC for per log E. coli reduction increased from 51.5 to 128.3 Wh/m3. HA in a low concentration (1 mg/L)

Y. Jin et al. / Journal of Cleaner Production 252 (2020) 119794

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Fig. 8. Concentration of E. coli (a) and H2O2 (b) at different temperatures (I: 25 mA).

enhanced disinfection, but that in a relatively high concentration (4 mg/L) inhibited disinfection. Higher temperatures also promoted the disinfection process. When the temperature was increased from 20 to 40
C, the EC for per log E. coli reduction fell from 102.2 to 64 Wh/m3. By comprehensive consideration of the electricity consumption and treatment time, a current of 25 mA, a low concentration of HA (1 mg/L) and a high temperature (40
C) were suitable for this system. The working voltage of the reactor is ~6 V, and its power consumption was quite low. Thus, photovoltaics can be convenient power sources for the hybrid process. Author contribution statement Riyao Chen and Yanchao Jin designed the study. Yanchao Jin, Yijun Shi, Ziyu Chen, Xiao Chen, Xi Zheng and Yaoxing Liu performed the experiments. Yanchao Jin and Riyao Chen analyzed the data and wrote the manuscript. All authors read and approved the final manuscript. Declaration of competing interest The authors declare that they have no competing interests. Acknowledgments We gratefully acknowledge the financial support of this research by the Natural Science Foundation of Fujian Province, China (Grant No. 2019J05069; 2018J01672); the Scientific Research Project of the Education Department of Fujian Province, China (Grant No. JT180083) and the Key Project of Fujian Provincial Department of Science and Technology (Grant No. 2019Y0010; 2017Y0026). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jclepro.2019.119794. References Acra, A., Karahagopian, Y., Raffoul, Z., Dajani, R., 1980. Disinfection of oral rehydration solutions by sunlight. Lancet 2, 1257e1258. Amin, M.T., Han, M.Y., 2011. Improvement of solar based rainwater disinfection by using lemon and vinegar as catalysts. Desalination 276, 416e424. Basova, Y.V., Hatori, H., Yamada, Y., Miyashita, K., 1999. Effect of oxidation-reduction surface treatment on the electrochemical behavior of PAN-based carbon fibers. Electrochem. Commun. 1, 540e544. Berney, M., 2006. Flow-cytometric study of vital cellular functions in Escherichia coli during solar disinfection (SODIS). Microbiology 152, 1719e1729. Brillas, E., Calpe, J.C., Casado, J., 2000. Mineralization of 2,4-D by advanced

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