Plasma-Assisted Rotating Disk Reactor toward Disinfection of Aquatic

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Kinetics, Catalysis, and Reaction Engineering

A Plasma-Assisted Rotating Disk Reactor towards Disinfection of Aquatic Microorganisms Yong Cai, Xiang-Sen Wu, Yong Luo, Meng-Jun Su, Guang-Wen Chu, Bao-Chang Sun, and Jian-Feng Chen Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b02562 • Publication Date (Web): 15 Jul 2019 Downloaded from pubs.acs.org on July 17, 2019

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Industrial & Engineering Chemistry Research

A Plasma-Assisted Rotating Disk Reactor towards Disinfection of Aquatic Microorganisms

Yong Cai a, b, c, Xiang-Sen Wu b, c, Yong Luo c, Meng-Jun Su a, b, c, Guang-Wen Chu a, b, c, *, Bao-Chang Sun c, *, Jian-Feng Chen a, b, c

a Beijing

Advanced Innovation Center for Soft Matter Science and Engineering, b State

Key Laboratory of Organic-Inorganic Composites and Technology, and c Research Center of the Ministry of Education for High Gravity Engineering Technology, Beijing University of Chemical Technology, Beijing 100029, PR China

* Corresponding author. Tel: +86 10 64446466; Fax: +86 10 64434784. E-mail address: [email protected]; [email protected]

1 ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Abstract: Plasma discharge in contact with water is a promising technology for disinfection. Developing efficient gas-liquid non-thermal plasma reactors for disinfection is very urgent. In this work, a plasma-assisted rotating disk reactor (plasma-RDR) was developed towards disinfection of Escherichia coli (E. coli). Experimental results show that the number of killed E. coli increased by 3.07  105 CFU·mL-1 within 60 min when the rotational speed increased from 0 to 500 rpm. Effects of discharge characteristics and other operating conditions on the disinfection efficiency were evaluated. Synergistic effects of plasma and acid or alkali on the disinfection efficiency were obvious. Based on Weibull distribution model, the critical pH values of resistance were approximate 10.4 and 5.3. The main disinfection process of cell electroporation was inferred based on observations of the cell morphology. Compared with other reported plasma reactors and disinfection technologies, plasma-RDR has better disinfection efficiency at a lower energy consumption. Keywords: Plasma-assisted rotating disk reactor; Pulsed discharge; Escherichia coli; Disinfection process; Energy density


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1. Introduction Disinfection of hazardous microbes in water is a significant issue in terms of environmental safety and human health.1 By far, special attentions are given the development of various nanomaterials for disinfection, such as nano-silver2 and carbon nanotubes.3 These strong antimicrobial substances are limited by the possible side-effects of the nanomaterial release that affected human health and the environment.4 Also, the practiced chemical and physico-chemical methods, including chlorination,5,6 ozonation,7 and ultraviolet irradiation8 etc., or their combinations, have been commonly used in inactivation of microbes. Some of these methods have complex processes with the production of toxic by-products.9-11 These open an avenue for the development of friendly physical technologies, for example, cavitation.12,13 Extreme environments (T ~ 1000-10000 K, P ~ 100-5000 bar) were generated, which led in homolytic water cleaving to generate the reactive oxygen species such as hydroxyl radicals due to the collapse.14 The produced reactive oxygen species favor disinfection. Simultaneously, the fluid turbulence was highly demanded to generate cavity collapse in the hydrodynamic cavitation reactors.15 Among the various emerging physical technologies, plasma, which attracted much attention, was able to generate electric discharge, UV radiation, shock wave, etc. to inactivate a wide range of harmful microorganisms.16 The lethal effect of plasma discharge on microorganisms was significant, inducing damages on DNA, protein, and biomacromolecule.17 It is meaningful to pay considerable attention on the related plasma reactor innovation for efficient utilization of discharge for disinfection.



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Several plasma reactors, including dielectric barrier discharge reactor,18 wire-plate corona discharge reactor,19 and multi-plate corona discharge reactor,20 have displayed prospects in the application of disinfection.21 However, the plasma filaments generated in these reactors merely impact on the surface of the liquid layer,22 leading to discrepant disinfection performance with different liquid layer thicknesses. Also, liquid flow causes the instability of water surface, triggering the electrical breakdown.23 A thin liquid film and uniform discharge may enhance the cell electroporation, which was deemed to be one of the most important disinfection processes.24,25 Developing an efficient disinfection technology involving a thin liquid film with steady flow was meaningful. Different from the plasma reactors mentioned above, a plasma-assisted rotating disk reactor (plasma-RDR) has been developed,26 including the coupled fields of plasma and high gravity. The rotating disk was used as a ground electrode to activate more discharge volume and improve the discharge uniformity. Cieplak et al.27 adopted rotating electrode for enhancing the ozone generation and the ozone generation efficiency increased about 15%. Bae et al.28 introduced a plasma rotating electrode to improve particle size distribution and yield of single-walled carbon nanotubes. To the best of our knowledge, however, the improvement of discharge uniformity in a gas-liquid non-thermal plasma reactor by rotating electrodes for disinfection has rarely been reported. A micron-scale liquid film could be achieved by the rotating disk, corresponding to characteristic sizes of microorganisms; for instance, Escherichia coli (E. coli) with about 3.9 µm length, 1.5 µm width and Listeria with


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about 2.4 µm length, 0.9 µm width.17,29,30 Therefore, more E. coli microorganisms could be exposed to plasma environment. It could be exceedingly expected to achieve high disinfection efficiency in the plasma-RDR. E. coli, a common indicative microorganism for bacterial contaminant of water that could cause disease and death was selected as the model hazardous microbe.6,31 In this work, the disinfection performance of the plasma-RDR was firstly investigated. Then the synergistic effects of plasma and acid or alkali on the disinfection efficiency were analyzed according to Weibull distribution model. The main disinfection process of E. coli was inferred based on observations of the cell morphology. Comparisons of the energy consumption with other reported plasma reactors and disinfection technologies have been conducted.

2. Experimental 2.1. Plasma generator The input voltage of the plasma generator (Beijing Ruiant Technology Co., Ltd., China) was an AC pulse signal. The voltage and current in the plasma-RDR were measured by a Rigol PVP2150 high voltage probe and a Cybertek CP0030A current probe connected to a Tektronix TDS 3032C oscilloscope. The typical voltage and current waveforms for discharge of plasma-RDR were tested. Voltage pulses with amplitudes of 36 kV were obtained and current pulses had amplitudes of 19.6 A (Figure S1, Supporting Information). The pulse repetition was set at 200 Hz. 2.2. Experimental procedure



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Figure 1(a) shows the experimental setup for disinfection of E. coli cells. The deionized water containing E. coli cells and oxygen was fed into the plasma-RDR by the tube-in-tube structure as shown in Figure 1(b). The oxygen flow rate had less influence on the disinfection efficiency and was set at 0.5 m3·h-1 (Figure S2, Supporting Information). The detailed geometrical parameters of the tube-in-tube structure were listed in the Supporting Information. Oxygen was fed into the plasma-RDR for 15 min before the start of the disinfection experiments to generate more reactive oxygen species and then evaluate the plasma activity.32 Then the deionized water containing E. coli cells, together with oxygen, were introduced into the plasma-RDR, flowed radially outwards through the rotating ground electrode, and were collected in a fluid reservoir below. The solution was sampled in triplicate every 10 min at the liquid outlet. 2.3. Analysis of E. coli Cultivation of E. coli cells was prepared in a traditional Luria-Bertani (LB) liquid medium, consisting of 10 g·L-1 of tryptone, 5.0 g·L-1 of yeast, and 5.0 g·L-1 of sodium chloride.33 Then, E. coli cells were incubated on a shaker for 8-10 h under the rotational speed of 150 rpm and temperature of 37 °C. Finally, E. coli cells were harvested by centrifugation and suspended in deionized water. According to the relationship of OD600 nm and the concentration of E. coli,34 the sample containing E. coli was diluted to the initial concentration required and then stored in the refrigerator under the temperature of 4 °C within 24 hours. The number density of E. coli cells used in all experiments was calculated by means of the heterotrophic plate count


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method,35 which was separately tested on two plates for calculating the mean value (Figure S3, Supporting Information).

3. Results and discussion 3.1. Investigation of the disinfection performance of plasma-RDR It is of great significance to study the effects of the high gravity and plasma field on the disinfection efficiency, respectively. Effects of rotational speed on the disinfection efficiency were firstly investigated. Then effects of discharge characteristics on the disinfection efficiency were evaluated. Effects of initial concentration of E. coli, liquid flow rate, and water conductivity were also studied systematically. 3.1.1. Effects of rotational speed on the disinfection efficiency In order to reveal the effects of rotational speed of plasma-RDR on disinfection, the disinfection efficiency was analyzed. As shown in Figure 2, compared with the bacterial removal, the form of log reduction could better clarify the trend of disinfection with time when the initial colony number was large. In the following experiments, the form of log reduction was adopted to calculate the disinfection efficiency. Comparisons of disinfection efficiency under different rotational speeds were plotted. It is evident that the disinfection efficiency increased with the increment of rotational speed from 0 to 800 rpm. The value of log reduction under 0 rpm was obviously lower than that of the rotational speed from 500 to 800 rpm. The different disinfection efficiencies may be caused by different liquid film thicknesses that varied



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from millimeter to micron-scale as previous study reported.26 A steady flow of micron-scale liquid film spreads over the rotating ground electrode when the rotational speed exceeded 500 rpm, which could significantly enhance the plasma-liquid interface renewal process and improve the discharge uniformity. 3.1.2. Effects of discharge characteristics on the disinfection efficiency Discharge parameters were also deemed essential for the disinfection in the coupled fields. Previous studies have illustrated that various discharge parameters have different disinfection effects in the plasma reactors.36 In this work, peak voltage and electrode gap were selected as the significant discharge parameters to evaluate the disinfection efficiency, as shown in Figure 3. The narrow electrode gap of 6 mm favors the stable plasma discharge with the increase of the peak voltage from 33 to 39 kV. Figure 3(a) exhibits that the similar trends were observed for the peak voltage of 36 kV and 39 kV tested. Approximate 2.2-log reduction was achieved in both peak voltages of 36 kV and 39 kV. However, the disinfection efficiency of the lower peak voltage of 33 kV was merely 1.6-log reduction. At a lower peak voltage of 33 kV, there were a few weak streamers. Increasing peak voltage to 39 kV, the discharge pattern converted from streamer to spark discharge that could act strongly on microorganisms. The results demonstrated that the effect of cell electroporation was much obvious in a higher peak voltage. Increasing peak voltage powerfully enhances the lethal effects of E. coli. The rules of disinfection under different electrode gaps are consistent with that of the peak voltages. As demonstrated in Figure 3(b), with the increment of the electrode


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gap from 4 to 10 mm, the disinfection efficiency sharply decreased. It has little difference of log reduction between the electrode gap of 4.0 mm and 6.0 mm, where 2.25-log reduction could be achieved. Nevertheless, the log reduction of 8.0 mm and 10.0 mm is merely 1.3 and 0.3, respectively. Under a larger electrode gap, very few corona discharges appeared above the liquid surface, indicating that the cell electroporation was difficult to occur. Considering the disinfection efficiency of E. coli, priority should be given to a narrower electrode gap. Robinson et al.37 has reported that the fluctuation of the water surface like Taylor cones appeared during the dielectric barrier discharge process when the peak voltage was high enough. Compared with the electrode gap of 6 mm, the electrode gap of 4 mm may trigger the electrical breakdown due to the instability of water surface at a narrower electrode gap under the condition of high peak voltage.38 Therefore, an optimal electrode gap of 6 mm is more appropriate for the disinfection of E. coli. 3.1.3. Effect of initial concentration of E. coli on the disinfection efficiency Investigating the effect of different initial concentrations on the disinfection efficiency, performed in deionized water, peak voltage of 36 kV, and electrode gap of 6 mm, was regarded as an important parameter. Figure 4 presents the typical log reduction in terms of the initial concentration and treatment time. The disinfection efficiency curve is approximately parallel with the increment of initial concentration from 5.20-log10 to 6.30-log10. Further increasing initial concentration to 6.81-log10, only approximate 1.2-log reduction could be achieved after 60 min treatment. For an initial concentration of 6.5 × 106 CFU·mL-1, the disinfection process became much



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less efficient. Previous research also demonstrated that the dead cells always adhered to the active cells for the initial density exceeding 106 CFU·mL-1, which affected the disinfection efficiency.32 According to the above experimental results, a higher initial concentration of E. coli was detrimental to the disinfection efficiency. Hence, it is assured that all the optimizations of the disinfection efficiency in the next steps were exclusively a result of initial concentration of 5.78-log10. 3.1.4. Effect of liquid flow rate on the disinfection efficiency In the traditional plasma-based water treatment reactor, the increase of liquid flow rate often improves the treatment efficiency of wastewater.39 Meanwhile, the rotating disk reactor favors the plasma-liquid interface renewal with the increment of liquid flow rate.40,41 Liquid flow rates were deemed to be important to evaluate the disinfection efficiency in the plasma-RDR. The disinfection efficiency of E. coli increased by 0.42-log reduction with the increment of the liquid flow rate from 60 to 80 L·h-1 and then slightly changed from 80 to 120 L·h-1, as shown in Figure 5. The final disinfection efficiency of 2.5-log reduction has been achieved. At a lower liquid flow rate, the plasma-liquid interface renewal rate decreased. Higher liquid flow rate means that extra fresh bacterial water was exposed to the plasma environment. Based on the above experimental results, the liquid flow rate of 100 L·h-1 was selected for disinfection. 3.1.5. Effect of water conductivity on the disinfection efficiency It was important to reveal the effect of water conductivity on the formation of plasma and therefore on the disinfection efficiency. Aiming to investigate the effect of


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water conductivity on the disinfection efficiency, Figure 6 exhibits the relationship between the disinfection efficiency and water conductivity varied from 4 to 4000 S·cm-1, which was adjusted by sodium sulfite. Obviously, increasing water conductivity was detrimental to the disinfection efficiency. The worse disinfection efficiency, approximate 1.0-log reduction, was obtained under the condition of 2000 S·cm-1 and 4000 S·cm-1. Further decreasing water conductivity to 1000 S·cm-1 and 4 S·cm-1, the disinfection efficiency increased by 0.65-log reduction and 1.2-log reduction, respectively. At higher water conductivities beyond 1000 S·cm-1, it was not conducive to form the plasma channel. Under these conditions, the discharge intensity is weak. For lower water conductivities, the spark discharge could be initiated from the high voltage plate, which favors the disinfection processes. Similarly, Wang et al.42 also found that increasing the ionic conductivity of the treated water was detrimental to the formation of plasma channels. These results mean that the lower conductivity is more conducive to disinfection. 3.2. Synergistic effects of plasma and acid or alkali on the disinfection efficiency It was well accepted that acid or alkali considerably affected the plasma disinfection.43 In order to clarify the synergistic effects of plasma and acid or alkali on the disinfection efficiency, effects of pH on the disinfection efficiency were studied. The shape parameter of disinfection curve was further calculated to analyze the synergistic effects of plasma and acid or alkali on the disinfection efficiency according to Weibull distribution model. 3.2.1. Effect of pH on the disinfection efficiency



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As shown in Figure 7(a) and (b), the profiles of disinfection efficiency have great changes with the increase and decrease of pH values, with an approximate 5.78-log reduction for the pH value of 4.3 and 11.5, respectively. In contrast, the closer it was to the neutral or weakly alkaline conditions, the lower disinfection efficiency was. It was, indeed, a similar result in the plasma-RDR as previous report,43,44 where disinfection efficiency varied obviously with the change of pH values. To further explore the enhanced effect of cell electroporation under different pH values, synergistic effects of plasma and acid or alkali on the disinfection efficiency were investigated, as shown in Figure 8. The disinfection efficiency decreased sharply without discharge, with the log reduction of 1.3 and 1.0 for pH value of 4.8 and 10.8, respectively. Similarly, the disinfection efficiency was merely 2.3-log reduction with the discharge alone. The disinfection efficiency increased by 3.5-log reduction and 2.9-log reduction in view of the synergistic effects of plasma and acid and alkali, respectively. 3.2.2. Disinfection model For further analyzing synergistic effects of plasma and acid or alkali on the disinfection efficiency under different pH values, it is meaningful to develop a disinfection model to describe microbial death. The classical method for calculating the number of microorganisms assumed as a first-order kinetic model has been accepted and practiced in some cases.45 The survival curve always does not follow a linear relationship, resulting from a cumulative form of a temporary distribution of lethality events.46 A model originated


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from Weibull distribution was proposed to match the nonlinear curve.47,48 This model was used for describing microbial death, enzymatic degradation, and chemical degradation kinetics.49,50 The death of microorganism and the dispersion of individual resistance was regulated by Eq. (1).

N  N 0  e  kt

n

(1)

where N0 represents the initial cell density and N represents cell density for a given treatment time. k is the special value of key influencing factor parameter and n is the value of shape parameter. Peleg and Cole have developed the model into the following form:49

log

N0  b  tn N

(2)

where parameter b of Eq. (2) has no physical meaning but is the time dimension. The concrete meaning of the shape parameter is illustrated by Eq. (3).

 n  1 (downward concave curves)   n  1 (linear)  n  1 (upward concave curves) 

(3)

The specific meaning of the shape parameter is depicted as follows. The value of n  1 represents that the model curve shows a trend of increasing growth with treatment time and it has a cumulative killing effect on microorganism. But the value of n  1 represents that the model curve shows a trend of decreasing growth and the resistance of microorganism becomes stronger with treatment time. Comparisons of experimental and calculated data by using Eq. (2) were presented in Figure 9. The pH values of 4.3, 5.5, 7.0, 9.3, and 10.8 were selected. As



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shown in Figure 9, the deviation between the calculated values and the experimental data is negligible. Thus, the kinetics of disinfection could be interpreted by Weibull distribution model. With the increase or decrease of pH value, the lethal curve of E. coli has been well described. Effects of all pH values tested on b, n, and R2 of Weibull distribution model have been evaluated, which were similar to the above results (Table S2, Supporting Information). According to the above data, analysis of synergistic effects of plasma and acid or alkali on the disinfection efficiency has been carried out, as shown in Figure 10. Based on the change of pH values, Figure 10 was divided into three regions, named as I, II, and III, respectively. In regions I and III, the leather effect of E. coli cells was distinct, which was similar to the research by Aronsson et al.44 In other words, further increasing pH value to approximate 10.4 or decreasing pH value to approximate 5.3, the cumulative killing effect became obvious, which also meant the significant synergistic effects of plasma and acid or alkali on the disinfection efficiency. 3.3. Disinfection process of E. coli The study of disinfection process is significant for more efficient sterilization in the plasma-RDR. For the sake of revealing the disinfection process, characterizations of cell morphology of E. coli during plasma treatment were evaluated. Then effects of long-life active products on the disinfection process were carried out. 3.3.1. Characterization of cell morphology of E. coli during plasma treatment SEM was conducted to confirm the impact of the disinfection process on E. coli and to infer the potential disinfection process during plasma treatment. Figure 11


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presents the SEM photos of E. coli before and after plasma treatment. To better clarify the disinfection process, the SEM photo of a single E. coli has been evaluated, as shown in Figure 11(a) and Figure 11(b). The fresh E. coli cell under the initial state was full and smooth (Figure 11(a) and (c)). However, from the result of Figure 11(b), the disruption of E. coli membrane integrity was serious, and the phenomenon of cell electroporation was obvious, as outlined by the white line in Figure 11(b). After the formation of micropores, leakage of intracellular materials occurred (Figure 11(d)) and the cytoplasm started oozing out, leading to cell clumping (Figure 11(e)). 3.3.2. Effect of active products on the disinfection process For further revealing effects of long-life active products on the disinfection process, variations of hydrogen peroxide, ozone, and nitrate ions were firstly evaluated. Aqueous hydrogen peroxide and ozone generated, indicating the plasma activity, have been detected by using the iodide and indigo methods in this work.51,52 Figure 12(a) presents the variation of hydrogen peroxide and ozone in the plasma-RDR under 4 S·cm-1. After 60 min plasma treatment, the final concentrations of hydrogen peroxide and ozone reached 32.78 mol·L-1 and 3.83 mol·L-1, respectively. Plasma discharge also leads to the electric breakdown of air to produce reactive nitrogen. Figure 12(b) showed that the concentration of nitrate ions increased up to 0.634 mmol·L-1 within 60 min. Variations of water conductivity and pH value during plasma treatment under 4 S·cm-1 have been investigated, as shown in Figure 12(c). The water conductivity increased from 4 to 12 S·cm-1, while the pH value decreased



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from 6.30 to 4.86. The increment of water conductivity and decrement of pH value were mainly caused by the formation of organic acids and inorganic ions, particularly, the generated nitrate ions. Then effects of long-life active products on the disinfection efficiency without discharge were investigated. Plasma discharge lasted until the concentration of hydrogen peroxide, ozone, and nitrate ions could be arrived at the same level in the previous experiments. As shown in Figure 13, the number of E. coli cells decreased from 6.0  105 to 5.6  105 CFU·mL-1, which was much lower than the treatment of plasma discharge. Based on the SEM photos of E. coli cells, there were few changes in the cell morphology of E. coli cells under the treatment of long-life active products (Figure S4, Supporting Information). In comparison with the result from Figure 13, it could be concluded that long-life active products affected the disinfection process insignificantly. The results also showed that the limited pH changes have little effect on disinfection process, which was similar to the previous research.36 According to the above results, cell electroporation was the main reason for disinfection during plasma treatment, although the previous study showed that the reactive oxygen species (ROS), such as ·OH, HO2˙, and O2-, may cause atom etching on cell membrane of bacteria,53 Then a schematic diagram of possible disinfection process was proposed based on the above experimental results, as shown in Figure 14. 3.4. Compared with other plasma reactors and disinfection technologies The calculation of average power (P) of the plasma-RDR was proposed as P

  U  Idt   f T

0

(4)


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where U was the voltage, I was the current, T was the pulse duration time, and f was the pulse repetition frequency. Each experiment was mainly performed at an applied voltage of 36 kV. Based on the above discharge parameter, the calculated average discharge power of plasma-RDR was 14.65 W. The input energy density (ED) of the discharge was calculated by Eq. (5):

ED  J  mL1  

P(W)  3600 L  L  h 1   1000

(5)

where L is the liquid flow rate with an initial concentration of 5.78-log10. The liquid flow rate of 100 L·h-1 was selected for disinfection and thus the input energy density was approximately 0.53 J·mL-1, which could achieve 2.30-log reduction of E. coli and 5.78-log reduction of E. coli with an initial pH of 4.3. Comparisons of the input energy density for disinfection of E. coli with other reported plasma reactors were firstly evaluated. The input energy density of various types of plasma reactors for disinfection were given in Table 1. Compared to the traditional plasma reactors,16,54-56 plasma-RDR could achieve higher disinfection efficiency with lower input energy density. Furthermore, comparisons of energy consumption with other disinfection technologies were also investigated. As shown in Table 2, Plasma-RDR could achieve 2.30-log reduction of E. coli with the energy consumption of 52.74 kJ, which is better than that of the sonication,57 a little worse than that of the UV-C irradiation,58 and similar to that of the microwave.59 The results suggested that plasma-RDR could effectively increase the energy utilization for disinfection.



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4. Conclusions Disinfection of E. coli in a plasma-RDR was studied. Compared to the non-rotated experiments, the disinfection efficiency increased by 2.2-log reduction in the plasma-RDR under the rotational speed of 500 rpm. Higher peak voltage and liquid flow rate favor disinfection efficiency, while higher initial cell density, electrode gap, and water conductivity are unfavorable to disinfection efficiency. The shape parameter of n  1 determined by Weibull distribution model revealed the significant synergistic effects of plasma and acid or alkali on the disinfection efficiency. Electroporation was inferred as the main role in the disinfection process through SEM analysis for the cell morphology of E. coli cells. The average power of plasma-RDR was 14.65 W and the disinfection efficiency of 2.3-log reduction with an input energy density of 0.53 J·mL-1 could be achieved. Compared with other reported plasma reactors and disinfection technologies, plasma-RDR has higher disinfection efficiency with lower energy consumption.

Acknowledgments This work was supported by the National Natural Science Foundation of China (Nos. 21725601, 21676009, and 21436001).

Supporting Information The Supporting Information is available free of charge on the ACS Publications website.


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Discharge characteristics, effect of oxygen flow rate on the disinfection efficiency, photographs of cultured E. coli cells, and SEM photos of E. coli treated by long-life active products.



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Nomenclatures d = electrode gap (mm) f = pulse repetition frequency (Hz) I = current (A) k = value of key influencing factor L = liquid flow rate (L·h-1) N0 = initial cell density (CFU·mL-1) N = cell density for a given treatment time (CFU·mL-1) n = shape parameter P = pH value R = rotational speed t = treatment time (min) T = pulse duration time (s) U = voltage (V) Vpp= peak voltage (kV) log10 (N0/N) = disinfection efficiency

Greek Letters δ = film thickness

Abbreviation EC = energy consumption rpm = r·min-1 LVP = low voltage plate HVP = high voltage plate 20 ACS Paragon Plus Environment

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AIChE J. 2012, 58, 247. (42) Wang, B. W.; Dong, B.; Xu, M.; Chi, C. M.; Wang, C. Degradation of methylene blue using double-chamber dielectric barrier discharge reactor under different carrier gases. Chem. Eng. Sci. 2017, 168, 90. (43) Singh, R. K.; Babu, V.; Philip, L.; Ramanujam, S. Disinfection of water using pulsed power technique: Effect of system parameters and kinetic study. Chem. Eng. J. 2016, 284, 1184. (44) Aronsson, K.; Rönner, Ulf. Influence of pH, water activity and temperature on the inactivation of Escherichia coli and Saccharomyces cerevisiae by pulsed electric fields. Innov. Food Sci. Emerg. 2001, 2, 105. (45) Lacasa, E.; Tsolaki, E.; Sbokou, Z.; Rodrigo, M. A.; Mantzavinos, D.; Diamadopoulos, E. Electrochemical disinfection of simulated ballast water on conductive diamond electrodes. Chem. Eng. J. 2013, 223, 516. (46) Peleg, M.; Cole, M. B. Reinterpretation of microbial survival curves. Critic. Rev. Food Sci. 1998, 38, 353. (47) Fernández, A.; Salmerón, C.; Fernández, P. S.; Martı́nez, A. Application of a frequency distribution model to describe the thermal inactivation of two strains of bacillus cereus. Trends Food Sci. Tech. 1999, 10, 158. (48) Mafart, P.; Couvert, O.; Gaillard, S.; Leguerinel, I. On calculating sterility in thermal preservation methods: Application of the Weibull frequency distribution model. Int. J. Food Microbiol. 2002, 72, 107. (49) Cunha, L. M.; Oliveira, F. A. R.; Oliveira, J. C. Optimal experimental design for


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Page 29 of 46 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(a)

(b) Figure 1. Schematic diagrams of (a) experimental setup and (b) tube-in-tube structure.



3500

2.8 2.4

80

2.0

70

1.6

Vpp = 48 kV

2500

d = 8 mm

2000



log(N0/N)

1.2

60

3000

N0 = 5.78-log10

1500

Bacterial Removal

-6

90

 10 m

100

Bacterial Removal (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 46

1000

0.8

50

500 0.4

40

0

400

500

600

700

800

0

R (rpm)

Figure 2. Effect of rotational speed on the disinfection efficiency.


Page 31 of 46

2.4

-1

N0 = 5.78-log10, d = 6 mm, L = 80 Lh

log (N0/N)

2.0

33.0 kV 36.0 kV 39.0 kV

1.6 1.2 0.8 0.4 0.0 0

10

20

30

40

50

60

Time (min)

(a)

2.5

-1

N0 = 5.78-log10, Vpp = 36 kV, L = 80 Lh 4.0 mm 6.0 mm 8.0 mm 10.0 mm

2.0

log (N0/N)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.5 1.0 0.5 0.0 0

10

20

30

40

50

60

Time (min)

(b) Figure 3. Effects of discharge characteristics of (a) peak voltage and (b) electrode gap on the disinfection efficiency.



-1

Vpp = 36 kV, d = 6 mm, L = 80 Lh

2.4

6.81-log10

2.0

log (N0/N)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 46

6.30-log10 5.78-log10

1.6

5.20-log10

1.2 0.8 0.4 0.0 0

10

20

30

40

50

60

Time (min)

Figure 4. Effect of initial cell density on the disinfection efficiency.


Page 33 of 46

3.0

N0 = 5.78-log10, Vpp = 36 kV, d = 6 mm

2.5

-1

60 Lh -1 80 Lh -1 100 Lh -1 120 Lh

2.0

log (N0/N)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.5 1.0 0.5 0.0

0

10

20

30

40

50

60

Time (min)

Figure 5. Effect of liquid flow rate on the disinfection efficiency.



-1

N0 = 5.78-log10, Vpp = 36 kV, d = 6 mm, L = 100 Lh

2.5

4 S/cm 1000 S/cm 2000 S/cm 4000 S/cm

2.0

log (N0/N)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 46

1.5 1.0 0.5 0.0 0

10

20

30

40

50

60

Time (min)

Figure 6. Effect of water conductivity on the disinfection efficiency.


Page 35 of 46

8

-1


7

11.5 9.3

6

10

5

10

Cell density (cfu/mL)

log (N0/N)

6 5 4

10.8 8.2

10.2 7.0

4

10

3

10

2

10

11.5 10.8 10.2 9.3 8.2 7.0

1

10

0

10

-1

10

0

3

10

20

30

40

50

60

Time (min)

2 1 0

0

10

20

30

40

50

60

Time (min)

(a) 8

-1


7

6

7.0 4.8

10

5

10


6 5

log (N0/N)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


4

6.3 4.3

5.5

4

10

3

10

2

10

7.0 6.3 5.5 4.8 4.3

1

10

0

10

-1

10

0

10

20

30

40

50

60

Time (min)

3 2 1 0 0

10

20

30

40

50

60

Time (min)

(b) Figure 7. Effect of pH on the disinfection efficiency.



6

+

H : pH = 4.8

OH : pH = 10.8

5

log (N0/N)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 46

4 3 2 1 0

OH Plasma+OH Plasma

Plasma+H+

H+

Figure 8. Synergistic effects of plasma and acid or alkali on the disinfection efficiency.


Page 37 of 46

pH=4.30 Exp. pH=4.30 Cal. W-B pH=5.50 Exp. pH=5.50 Cal. W-B pH=7.00 Exp. pH=7.00 Cal. W-B pH=9.30 Exp. pH=9.30 Cal. W-B pH=10.8 Exp. pH=10.8 Cal. W-B

6 5

log (N0/N)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


4 3 2

b=0.0561 n=1.154

b=0.0284 n=1.248

b=0.0670 n=0.942

1 0

b=0.0682 n=0.829

0

10

20

30

40

50

b=0.0651 n=0.716

60

Time (min)

Figure 9. Comparisons of experimental and calculated data by using Weibull distribution model.



Page 38 of 46

Figure 10. Analysis of acid and alkali resistance.


Page 39 of 46 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 11. SEM photos: (a) an untreated cell, (b) a treated cell with plasma, (c) untreated cells, (d) treated cells with plasma, and (e) cell debris after plasma treatment.



35 H2O2

4.0

O3

3.5

25

3.0

20

2.5 2.0

15

1.5

10

1.0

5 0

O3 (mol/L)

H2O2 (mol/L)

30

0.5 0

10

20

30

40

50

60

50

60

0.0

Time (min)

(a) 0.7 CNO-

0.6

3

CNO- (mmol/L)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 40 of 46

3

0.5 0.4 0.3 0.2 0.1 0.0

0

10

20

30

40

Time (min)

(b)


Page 41 of 46

12

Water conductivity

pH

6.3

10

6.0

8

5.7

6

5.4

4

5.1

2

4.8 0

10

20

30

40

50

pH

Water conductivity (Scm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


60

Time (min)

(c) Figure 12. Variation of (a) aqueous hydrogen peroxide and ozone concentrations, (b) nitric acid concentration, and (c) water conductivity and pH during plasma treatment under 4 S/cm.


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



6



5



4



3

Page 42 of 46



2



0

5

10

15

20

Time (min)

Figure 13. Effect of long-life active products on the disinfection efficiency.


Page 43 of 46 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 14. A schematic diagram of possible disinfection process.



Page 44 of 46

Table 1. Comparisons of the input energy density for disinfection of E. coli with other reported plasma reactors. Reactor types Rod-rod reactor Wire-plate reactor Needle-plate reactor Multi-electrode reactor Plasma-RDR

Items Conditions N0: 4.0  107 CFU·mL-1, pH: 7.4, V0: 3 L, Vpp: 10.2 kV, d: 4 mm N0: ~1.0  104 CFU·mL-1, pH: 7.6, V0: 0.01 L, Vpp: 130 kV, d: 23 mm N0: 105 ~ 106 CFU·mL-1, pH: 7.2, V0: 1 L, Vpp: 13 kV, d: 20 mm N0: 1.0  106 CFU·mL-1, potable water, f: 10 Hz N0: 6.0  105 CFU·mL-1, pH: 6.5, V0: 2 L, Vpp: 36 kV, d: 6 mm

ED (J·mL-1)

log reduction

Refs.

100

2.1

(16)

10

3.0

(54)

23

1.0

(55)

0.3

1.0

(56)

0.53

2.3

This work


Page 45 of 46 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 2. Comparisons of the energy consumption for disinfection of E. coli with other reported physical technologies. Disinfection technologies

Sonication UV-C irradiation Microwave

Plasma-RDR

Conditions N0: 3~6  106 CFU·mL-1, t: 1.0 h, V0: 0.3 L, P: 80 W N0: 107~108 CFU·mL-1, t: 0.0583 h, 25 °C, P: 64 W N0: 107~108 CFU·mL-1, t: 0.0306 h, f: 915 MHz, P: 1200 W N0: 6.0  105 CFU·mL-1, t: 1.0 h, V0: 2.0 L, P: 14.65 W

Items EC (kJ)

log reduction

Refs.

288

3.0

(57)

13.43

1.53

(58)

132

5.86

(59)

52.74

2.3

This work



Page 46 of 46

Table of Contents:


Plasma-Assisted Rotating Disk Reactor toward Disinfection of Aquatic

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