Plasma-Activated Tap Water Production and Its Application in Atomization Disinfection

: Plasma-activated tap water (PATW) is a new technology for obtaining a high concentration of active aqueous plasma substances by discharging underwater. Commonly plasma-activated water (PAW) is realized by activating deionized water or distilled water, which has problems such as high cost, a small discharge area, and insufﬁcient dissolution of active substances. This paper reports the development of a dielectric barrier discharge array to generate a high concentration of active aqueous plasma substances. The device can realize a uniform, stable, and large-area discharge in a large volume of tap water, and it has the advantages of low cost, high integration, and reusability. Using the device to treat 1000 mL of tap water for 1 h can reduce the pH of the tap water from 8.10 to 2.54, and the logarithmic value of killing E. coli is greater than 5.0. We sprayed PATW onto clothes to sterilize the bacteria when people were through the disinfection door and veriﬁed that the PATW had a good sterilization effect. The short-lived substances, singlet oxygen, and superoxide anion radicals played a key role in the sterilization process by PATW.


Introduction
In recent years, the population on the earth has grown rapidly.It is estimated that the world's population will reach 10 billion by the middle of this century.The resulting problems of food safety, hygiene, and health have become increasingly prominent [1].The SARS, Ebola, COVID-19, and other diseases that broke out in this century remind us always to be alert to possible safety hazards, take timely cleaning and disinfection measures, and inhibit the spread of germs.
Traditional disinfectants commonly used in sterilization, such as chlorine-containing disinfectants, alcohol disinfectants, and aldehyde disinfectants, etc., can effectively disinfect and sterilize the surface of objects, but they are irritating when used, and their chemical residues cause particular harm to the environment and the human body [2,3].Some bacteria even develop strong resistance to these disinfectants.Human society urgently needs a new disinfection method that can effectively kill pathogens.Plasma-activated water (PAW) is a new technology that uses low-temperature plasma to treat water.Reactive oxygen and nitrogen species (RONS) are transferred from the plasma, where they are produced, into water so that the water contains nitrates, nitrites, hydrogen peroxide, singlet oxygen, superoxide, etc. [4].These aqueous RONS react with the components of pathogenic microorganisms and effectively inactivate these microorganisms [5].The bactericidal effect of PAW is significantly enhanced in an acidic environment [6].
PAW has the advantages of convenient preparation and portability.It can sterilize efficiently and be combined with existing disinfection systems.In recent years, it has been used in medical disinfection [7][8][9][10], food preservation [11][12][13], and wound treatment [14][15][16], and it does not produce apparent side effects on the human body and the environment.It is a green, clean, safe, and compelling new sterilization and freshness preserving technology.Matthew J. Traylor et al. treated distilled water with a dielectric barrier discharge (DBD) to generate PAW and used it to treat E. coli for 15 min.The active bacterial population was reduced by five orders of magnitude [17].Li Y. et al. used a DC-driven needle array to activate a chemical solution containing nitrite and hydrogen peroxide, reducing the active E. coli population by 6 orders of magnitude [18].
There are mainly two methods to generate PAW: discharges on water and discharges in water.The existing PAW preparation systems primarily use water surface discharges and include plasma jets, needle water discharges, surface dielectric barrier discharges, sliding arc discharges, etc. [19][20][21][22][23].This method wastes some gaseous RONS and is often used to activate small amounts of deionized or distilled water.When dealing with cheap and readily available tap water, the acid produced by plasma is consumed by the high level of carbonate and bicarbonate in the tap water, which act as buffers, so it is difficult to lower the pH value of tap water [19,24], resulting in a poor bactericidal effect.In contrast to water surface discharges, discharges in water increase the contact area between the plasma and water significantly and promote the dissolution of the gaseous RONS [25].
Aiming at the shortcomings of traditional devices, we developed a plasma-activated tap water (PATW) preparation device based on an air dielectric barrier discharge (DBD) in tap water.The plasma generated during the discharge was in direct contact with the tap water through the discharge channel, and the gaseous RONS dissolved easily through the discharge channel and the subsequent bubbling.The high concentration of aqueous RONS had a good bactericidal effect.The atomized PATW was also sprayed onto clothes to sterilize bacteria when people were through the disinfection door.By studying the bactericidal effects of various long-lived and short-lived aqueous RONS, we verified that short-lived singlet oxygen and superoxide anion radical RONS played a key role.

DBD-Based PAW Device
An overall structure diagram of the DBD-based PAW is shown in Figure 1, showing in particular 4 DBD devices in a water tank.The voltage of discharge was measured by a voltage probe (Tektronix P6015A, USA), and the current was measured by a current probe (Tektronix TCP312A, USA).The homemade kHz AC power supply was used to drive the discharge.The optical emission spectrum of the plasma was measured by a spectrometer (Princeton Instruments Acton SpectraHub 2500i, USA).
The DBD device consisted of a metal rod electrode, an inner glass wall, and an outer glass wall from the inside to the outside (Figure 2).The metal rod electrode was closely attached to the inner glass wall and was directly connected to the high-voltage pole of the power supply.There are air inlets and outlet holes on the outer glass wall.When the air flow rate was 10 L/min (provided by an air pump), a discharge environment of pure air was formed under the water surface, thereby generating a DBD discharge between the metal rod electrode, and the water, which was used as the ground electrode.The glass tube's length and diameter were 150 mm and 11 mm, respectively.The diameter of the metal rod electrode was 4 mm.The thickness of the glass wall was 1 mm.The diameter of the air inlet on the outer glass wall was 6 mm, the diameter of the air outlet hole was 2 mm, and the distance between the inner and outer glass wall was 2 mm.The DBD device consisted of a metal rod electrode, an inner glass wall, and an outer glass wall from the inside to the outside (Figure 2).The metal rod electrode was closely attached to the inner glass wall and was directly connected to the high-voltage pole of the power supply.There are air inlets and outlet holes on the outer glass wall.When the air flow rate was 10 L/min (provided by an air pump), a discharge environment of pure air was formed under the water surface, thereby generating a DBD discharge between the metal rod electrode, and the water, which was used as the ground electrode.The glass tube's length and diameter were 150 mm and 11 mm, respectively.The diameter of the metal rod electrode was 4 mm.The thickness of the glass wall was 1 mm.The diameter of the air inlet on the outer glass wall was 6 mm, the diameter of the air outlet hole was 2 mm, and the distance between the inner and outer glass wall was 2 mm.The DBD device consisted of a metal rod electrode, an inner glass wall, and an outer glass wall from the inside to the outside (Figure 2).The metal rod electrode was closely attached to the inner glass wall and was directly connected to the high-voltage pole of the power supply.There are air inlets and outlet holes on the outer glass wall.When the air flow rate was 10 L/min (provided by an air pump), a discharge environment of pure air was formed under the water surface, thereby generating a DBD discharge between the metal rod electrode, and the water, which was used as the ground electrode.The glass tube's length and diameter were 150 mm and 11 mm, respectively.The diameter of the metal rod electrode was 4 mm.The thickness of the glass wall was 1 mm.The diameter of the air inlet on the outer glass wall was 6 mm, the diameter of the air outlet hole was 2 mm, and the distance between the inner and outer glass wall was 2 mm.

Measurement of Aqueous RONS
We treated 1 L of tap water (the living water provided by Wuhan Water Affairs Group Co., Ltd.Wuhan, China) and distilled water (high -purity industrial distilled water, Honghuangzhili Co., Ltd., Guangzhou, China) for 1 h using the DBD source shown in Figure 1, and then measured the concentrations of aqueous NO − 3 , NO − 2 , and H 2 O 2 , pH, and conductivity in the two activated waters.We used absorption spectrometry to measure the concentration of NO − 3 , NO − 2 ion, and H 2 O 2 .The analytical instrument was a microplate reader (SpectraMaxM4, SRI 8610C GC, USA).As for nitrate and nitrite, in the presence of sulfuric acid and phosphoric acid, nitrate reacts with 2,6-dimethylphenol to produce 4-nitro-2,6-dimethylphenol, and its absorbance is measured at 324 nm to obtain the corresponding nitrate concentration.The interference of nitrite can be eliminated by aminosulfonic acid.When the pH is around 1.9 and phosphoric acid is present, nitrite reacts with 4-aminobenzenesulfonamide to form diazonium salt and then reacts with (1-naphthyl)-1,2-ethylenediamine dihy-drochloride to form a pink dye.The absorbance is measured at 540 nm to obtain the concentration of nitrite.As for hydrogen peroxide, it can react with titanium sulfate in an acid medium to form a yellow colored precipitate.We can measure its absorbance at around 410 nm to define the concentration of hydrogen peroxide.The pH value and conductivity of the activated water were measured by a pH probe (Testo 206-pH1, Hangzhou China) and a conductivity tester (Hengxin AZ86031, Hangzhou China), respectively.We used E. coli (A TCC25922) provided by the School of Life Science and Technology of Huazhong University of Science and Technology to determine the bactericidal effect of PAW.To do this, 1 L of tap water and distilled water were treated by plasma for 1 h, respectively, and the obtained PATW and PADW were immediately used to cultivate E. coli with a concentration of 10 8 CFU/mL.5 mL of normal saline to the centrifuged colony as the control group and 5 mL of PATW and PAW as the experimental group, mixed well and reacted with E. coli for 15 min and 30 min.After oscillating evenly, 100 µL of the corresponding mixed solution was used to coat the petri dish (90 mm CITOCIST) and was incubated in a constant temperature incubator at 37 • C for 18 h.We used a CFU counting method to measure the number of surviving colonies after 18 h of cultivation.Each experiment was repeated 3 times.By comparing the number of surviving colonies in the control group and the experimental group, the bactericidal ability of PATW and PADW were determined.

Disinfection of Clothing Surface with Atomized PATW
In order to determine the bactericidal effect of PATW in a particular application, we atomized PATW in a disinfection door (an engineering prototype, the structure diagram is shown in Figure 3) and disinfected the surface of the bacteria-contaminated clothing on a dummy.We first put the stainless steel sampling plate (G01 Bikeman) on the surface of the experimental cloth, used a sterile cotton swab to take an E. coli suspension with an initial concentration of 10 8 CFU/mL, and spread it evenly on the blank area of the stainless steel sampling plate and dried it naturally.Afterward, we sprayed the atomized PATW onto the surface of the contaminated clothes in the disinfection door for 1 min and 2 min, waited for 10 min for disinfection, and then sampled and cultured the collected bacteria.

Comparison of Sterilization Effects of PATW and Chemical Solutions
In order to verify the specific effects of NO − 3 , NO − 2 , H 2 O 2 (long-lived RONS) in PATW sterilization, we used sodium NO − 3 , sodium nitrite, and hydrogen peroxide solution to prepare a slightly higher concentration NO − 3 (5600 µmol/L), NO − 2 (50 µmol/L), and H 2 O 2 (850 µmol/L) of the chemical solution and adjusted its pH to about 2.5 with dilute hydrochloric acid.This chemical solution was added to the centrifuged E. coli solution and incubated for 15 min.
Active particle removers, such as 50 mM L-histamine (singlet oxygen scavenger), 100 U/mL SOD (superoxide anion radical scavenger), and 50 mM mannitol (hydroxyl radical scavenger), were added to the PATW.Afterward, this solution was mixed with the centrifuged E. coli suspension to test whether these short-lived substances, 1 O 2 2 , O − 2 , and OH, played a key role in the sterilization process of PATW [26][27][28].
2.5 with dilute hydrochloric acid.This chemical solution was added to the centrifuged E. coli solution and incubated for 15 min.
Active particle removers, such as 50 mM L-histamine (singlet oxygen scavenger), 100 U/mL SOD (superoxide anion radical scavenger), and 50 mM mannitol (hydroxyl radical scavenger), were added to the PATW.Afterward, this solution was mixed with the centrifuged E. coli suspension to test whether these short-lived substances, O

Discharges of Plasma Sources in Tap Water
Figure 4 shows DBD source discharges in tap water.The discharge started at about 16 kV (peak-to-peak), and a blue-purple uniform filamentary discharge appeared in the glass tube.As the bubbles extended outward from the small holes in the outer glass tube (10 L/min air provided by an air pump, Figure 1), part of the plasma extended to the tap water outside the tube through the channel formed by many bubbles, generating many bubbles with plasma.The extended volume of the plasma outside the tube increased significantly as the voltage increased.

Discharges of Plasma Sources in Tap Water
Figure 4 shows DBD source discharges in tap water.The discharge started at about 16 kV (peak-to-peak), and a blue-purple uniform filamentary discharge appeared in the glass tube.As the bubbles extended outward from the small holes in the outer glass tube (10 L/min air provided by an air pump, Figure 1), part of the plasma extended to the tap water outside the tube through the channel formed by many bubbles, generating many bubbles with plasma.The extended volume of the plasma outside the tube increased significantly as the voltage increased.
2.5 with dilute hydrochloric acid.This chemical solution was added to the centrifuged E. coli solution and incubated for 15 min.
Active particle removers, such as 50 mM L-histamine (singlet oxygen scavenger), 100 U/mL SOD (superoxide anion radical scavenger), and 50 mM mannitol (hydroxyl radical scavenger), were added to the PATW.Afterward, this solution was mixed with the centrifuged E. coli suspension to test whether these short-lived substances, O

Discharges of Plasma Sources in Tap Water
Figure 4 shows DBD source discharges in tap water.The discharge started at about 16 kV (peak-to-peak), and a blue-purple uniform filamentary discharge appeared in the glass tube.As the bubbles extended outward from the small holes in the outer glass tube (10 L/min air provided by an air pump, Figure 1), part of the plasma extended to the tap water outside the tube through the channel formed by many bubbles, generating many bubbles with plasma.The extended volume of the plasma outside the tube increased significantly as the voltage increased.

The voltage and current characteristics of the discharge array are shown in Figu
The peak-to-peak voltage was 20 kV, the frequency was 10 kHz, and the peak-tocurrent was about 100 mA.The plasma power calculated by Lissajous graphic was W. The optical emission spectrum (OES) is shown in Figure 7.The emission spect N2 and N2 + in excited states dominated the OES.Because the DBD discharge formed i air injection zone underwater (Figure 5), the OES was dominated by N2 species as mon air plasma [29,30].This measurement result is consistent with the mass spectrom measurement result of the discharge when the air humidity was high [31].Mass spect etry shows that N2 in vibrational states easily reacted with O, O3, and OH to generate and NO2 and further generated particles such as NO 2 − , NO 3 − , and NO 3 − (H 2 O) n .The voltage and current characteristics of the discharge array are shown in Figure 6.The peak-to-peak voltage was 20 kV, the frequency was 10 kHz, and the peak-to-peak current was about 100 mA.The plasma power calculated by Lissajous graphic was 17.5 W.

The voltage and current characteristics of the discharge array are shown in Figu
The peak-to-peak voltage was 20 kV, the frequency was 10 kHz, and the peak-tocurrent was about 100 mA.The plasma power calculated by Lissajous graphic was W. The optical emission spectrum (OES) is shown in Figure 7.The emission spect N2 and N2 + in excited states dominated the OES.Because the DBD discharge formed in air injection zone underwater (Figure 5), the OES was dominated by N2 species as c mon air plasma [29,30].This measurement result is consistent with the mass spectrom measurement result of the discharge when the air humidity was high [31].Mass spect etry shows that N2 in vibrational states easily reacted with O, O3, and OH to generate and NO2 and further generated particles such as NO 2 − , NO 3 − , and NO 3 − (H 2 O) n .The optical emission spectrum (OES) is shown in Figure 7.The emission spectra of N 2 and N 2 + in excited states dominated the OES.Because the DBD discharge formed in the air injection zone underwater (Figure 5), the OES was dominated by N 2 species as common air plasma [29,30].This measurement result is consistent with the mass spectrometry measurement result of the discharge when the air humidity was high [31].Mass spectrometry shows that N 2 in vibrational states easily reacted with O, O 3 , and OH to generate NO and NO 2 and further generated particles such as NO − 2 , NO − 3 , and NO − 3 (H 2 O) n .

Aqueous RONS Measurement
We measured the concentration of NO 3 − , NO 2 − , and H 2 O 2 in PATW and PADW as a function of plasma activation time (Figure 8).During the plasma activation process, we also measured the pH and conductivity of PATW and PADW (Figure 9).The RONS concentrations in PATW and PADW were within 90% of each other.However, the pH and conductivity of PATW and PADW considerably differed.The concentration of NO 3 − and H 2 O 2 increased, the pH decreased, and the conductivity increased with the treatment time, while the concentration of NO2 -increased first and then

Aqueous RONS Measurement
We measured the concentration of NO − 3 , NO − 2 , and H 2 O 2 in PATW and PADW as a function of plasma activation time (Figure 8).During the plasma activation process, we also measured the pH and conductivity of PATW and PADW (Figure 9).

Aqueous RONS Measurement
We measured the concentration of NO 3 − , NO 2 − , and H 2 O 2 in PATW and PADW as a function of plasma activation time (Figure 8).During the plasma activation process, we also measured the pH and conductivity of PATW and PADW (Figure 9).The RONS concentrations in PATW and PADW were within 90% of each other.However, the pH and conductivity of PATW and PADW considerably differed.The concentration of NO 3 − and H 2 O 2 increased, the pH decreased, and the conductivity increased with the treatment time, while the concentration of NO2 -increased first and then the pH value to drop from 8 to 7 during the first 30 min, and afterward, due to the generation and accumulation of hydrogen peroxide, nitric acid, and peroxynitrite, the pH value dropped from 7 to 2.5 during the last 30 min.The initial conductivity of PATW and PADW was 360 uS/cm and 0 uS/cm, respectively.The conductivity of the two treated liquids increased to ~1800 uS/cm at 60 min (Figure 9b).NO 3 − and NO 2 − produced by underwater discharges increased the conductivity (Figure 8).

Sterilization by PATW and PADW
Figure 10 shows that the bactericidal effect of PATW and PADW (tap water and deionized water treated by plasma for 1 h) was almost the same.The concentration of E. coli colonies on the culture medium in the control group was 10 7 CFU/mL, while in the experimental group, when PATW and PADW were mixed with E. coli for 15 min, the concentration of colonies dropped from the initial 10 7 CFU/mL to 10 2 CFU/mL.When the mixed reaction increased to 30 min, the concentration of colonies dropped from the initial 10 7 CFU/mL to 10 1 CFU/mL, and it was difficult to find surviving E. coli on the medium.Therefore, the PATW and PADW both had a good bactericidal effect.The RONS concentrations in PATW and PADW were within 90% of each other.However, the pH and conductivity of PATW and PADW considerably differed.The concentration of NO − 3 and H 2 O 2 increased, the pH decreased, and the conductivity increased with the treatment time, while the concentration of NO 2 − increased first and then decreased.NO − 2 accumulated in the PATW and PADW in the initial stage of plasma treatment.After 30 min of plasma treatment, due to the accumulation of more oxidizing substances, NO − 2 reacted with hydrogen peroxide and ozone to generate NO − 3 and peroxynitrite [32], and its consumption rate was greater than the production rate, resulting in a decrease in concentration.
The differences of concentration of the three aqueous RONS between PATW and PADW were less than 5%, but unlike distilled water, due to the presence of various impurities in tap water, most of the acids produced during plasma treatment in the first 30 min were consumed by carbonates and bicarbonates (707-935 mg/L CaCO 3 ) [33], causing the pH value to drop from 8 to 7 during the first 30 min, and afterward, due to the generation and accumulation of hydrogen peroxide, nitric acid, and peroxynitrite, the pH value dropped from 7 to 2.5 during the last 30 min.The initial conductivity of PATW and PADW was 360 uS/cm and 0 uS/cm, respectively.The conductivity of the two treated liquids increased to ~1800 uS/cm at 60 min (Figure 9b).NO − 3 and NO − 2 produced by underwater discharges increased the conductivity (Figure 8).

Sterilization by PATW and PADW
Figure 10 shows that the bactericidal effect of PATW and PADW (tap water and deionized water treated by plasma for 1 h) was almost the same.The concentration of E. coli colonies on the culture medium in the control group was 10 7 CFU/mL, while in the experimental group, when PATW and PADW were mixed with E. coli for 15 min, the concentration of colonies dropped from the initial 10 7 CFU/mL to 10 2 CFU/mL.When the mixed reaction increased to 30 min, the concentration of colonies dropped from the initial 10 7 CFU/mL to 10 1 CFU/mL, and it was difficult to find surviving E. coli on the medium.Therefore, the PATW and PADW both had a good bactericidal effect.
Many short-lived vital bactericidal substances in PAW, such as superoxides, singlet oxygen, hydroxyl radicals, etc., are essential in sterilization [34,35].Storage often leads to the inactivation of these short-lived substances and affects the bactericidal effect of PATW [35].Therefore, we studied the sterilization effect of PATW stored at low temperature (2 • C) and room temperature (30 • C) for 6 h and 24 h (Figure 11).Compared with the unpreserved PATW, the sterilizing ability of the PATW stored at 2 • C for 6 h did not change, while the sterilizing ability of the PATW stored at 30 • C decreased.When mixed with bacteria for 15 min, the bactericidal effect of PATW stored 2 • C was 6 orders of magnitude, while the bactericidal effect of PATW stored at 30 • C decreased to 4. When the reaction time increased to 30 min, the PATW stored at 2 • C still killed all E. coli in the culture medium, while less than 0.001% of E. coli treated with PATW stored at 30 • C still survived.Many short-lived vital bactericidal substances in PAW, such as superoxides, sing oxygen, hydroxyl radicals, etc., are essential in sterilization [34,35].Storage often leads the inactivation of these short-lived substances and affects the bactericidal effect of PAT [35].Therefore, we studied the sterilization effect of PATW stored at low temperature °C) and room temperature (30 °C) for 6 h and 24 h (Figure 11).Compared with the unp served PATW, the sterilizing ability of the PATW stored at 2 °C for 6 h did not chan while the sterilizing ability of the PATW stored at 30 °C decreased.When mixed w bacteria for 15 min, the bactericidal effect of PATW stored 2 °C was 6 orders of magnitu while the bactericidal effect of PATW stored at 30 °C decreased to 4. When the reacti time increased to 30 min, the PATW stored at 2 °C still killed all E. coli in the cultu medium, while less than 0.001% of E. coli treated with PATW stored at 30 °C still surviv The bactericidal components in the PATW gradually decomposed, and the bacte cidal effect of the PATW decreased by about 1 order of magnitude after 24 h of 2 °C a 30 °C storage (compare Figure 11a,b).Therefore, the PATW was suitable for storage at l temperatures for 24 h or for immediate use.We also used the disinfection door (Figure 12) to spray the atomized PATW onto surface of clothes to study its disinfection effect.We evenly spread the E. coli solution  Many short-lived vital bactericidal substances in PAW, such as superoxides, singlet oxygen, hydroxyl radicals, etc., are essential in sterilization [34,35].Storage often leads to the inactivation of these short-lived substances and affects the bactericidal effect of PATW [35].Therefore, we studied the sterilization effect of PATW stored at low temperature (2 °C) and room temperature (30 °C) for 6 h and 24 h (Figure 11).Compared with the unpreserved PATW, the sterilizing ability of the PATW stored at 2 °C for 6 h did not change, while the sterilizing ability of the PATW stored at 30 °C decreased.When mixed with bacteria for 15 min, the bactericidal effect of PATW stored 2 °C was 6 orders of magnitude, while the bactericidal effect of PATW stored at 30 °C decreased to 4. When the reaction time increased to 30 min, the PATW stored at 2 °C still killed all E. coli in the culture medium, while less than 0.001% of E. coli treated with PATW stored at 30 °C still survived.
The bactericidal components in the PATW gradually decomposed, and the bactericidal effect of the PATW decreased by about 1 order of magnitude after 24 h of 2 °C and 30 °C storage (compare Figure 11a,b).Therefore, the PATW was suitable for storage at low temperatures for 24 h or for immediate use.We also used the disinfection door (Figure 12) to spray the atomized PATW onto the surface of clothes to study its disinfection effect.We evenly spread the E. coli solution on the surface of the clothes, used the disinfection door to atomize PATW for disinfection for 1 min and 2 min, respectively, and sampled and cultured the collected bacteria.
Figure 12 shows the sterilization efficiency (the number of bacteria reduced after PATW treatment divided by the initial amount of bacteria) of treating the clothes with The bactericidal components in the PATW gradually decomposed, and the bactericidal effect of the PATW decreased by about 1 order of magnitude after 24 h of 2 • C and 30 • C storage (compare Figure 11a,b).Therefore, the PATW was suitable for storage at low temperatures for 24 h or for immediate use.
We also used the disinfection door (Figure 12) to spray the atomized PATW onto the surface of clothes to study its disinfection effect.We evenly spread the E. coli solution on the surface of the clothes, used the disinfection door to atomize PATW for disinfection for 1 min and 2 min, respectively, and sampled and cultured the collected bacteria.
Figure 12 shows the sterilization efficiency (the number of bacteria reduced after PATW treatment divided by the initial amount of bacteria) of treating the clothes with atomized PATW in the disinfection door for 1 min was 91.8%, and the sterilization efficiency increased to 96.7% for 2 min of treatment.By prolonging the preparation time of PATW and increasing the spray volume of PATW, the sterilization efficiency of atomized PATW might be further improved.
atomized PATW in the disinfection door for 1 min was 91.8%, and the sterilization efficiency increased to 96.7% for 2 min of treatment.By prolonging the preparation time of PATW and increasing the spray volume of PATW, the sterilization efficiency of atomized PATW might be further improved.
PATW and increasing the spray volume of PATW, the sterilization efficiency of atomiz PATW might be further improved.

Discussion
The underwater DBD first produced UV, charged particles, and gaseous RON These substances interacted with water to produce aqueous RONS, such as peroxynitr NO 3 − , NO 2 − , OH, hydrogen peroxide, ozone, etc. (the reactions pathways are summariz in Figure 13) [36][37][38][39], while reducing the PAW's pH.active ingredients (short lifetime RONS) decomposed in ms-s time range [20].However, other scholars thought that hydrogen peroxide alone could not play a decisive role.The bactericidal effect of PAW is jointly determined by various long-lived and short-lived active substances [40][41][42].Whether PAW has a strong bactericidal effect not only depends on the individual effects of various substances, but also depends on the complex reactions between various species.In addition to several long-lived substances, the substances with bactericidal effects in PAW also include short-lived substances such as peroxynitrite, singlet oxygen, hydroxyl radicals, and superoxide anion radicals.Among them, peroxynitrite is produced by the reaction of NO − 2 , H 2 O 2 , and H + .It is a strong oxidant, which can oxidize and thus denature the protein in the bacteria, thereby changing the bacterial morphology and thus inactivating the bacteria [43].At low pH, peroxynitrite decomposes into hydroxyl radicals and nitrogen dioxide.Hydroxyl free radicals have strong oxidative properties and can attack bacterial cell membranes, increasing their permeability, inhibiting the regeneration ability of biofilms, and severely damaging bacteria [44].Singlet oxygen is the excited state of oxygen molecules and has potent cytotoxicity.It can interact with biological macromolecules such as proteins and DNA in cells and cause damage to the cell membrane system by combining with molecules, causing oxidative stress in bacteria [45].
Figure 14 compares the sterilization effect of PATW and the chemical mixed solution.It can be seen that the chemical mixed solution had a killing effect of less than one logarithm on E. coli, which was far less than the five logarithm inactivation effect of PATW on E. coli.Therefore, long-lived RONS are not the most important factor for the bactericidal effect in PATW, and we should focus on the impact of other short-lived RONS.
bactericidal effect of PAW is jointly determined by various long-lived and shor tive substances [40][41][42].Whether PAW has a strong bactericidal effect not onl on the individual effects of various substances, but also depends on the comple between various species.In addition to several long-lived substances, the subst bactericidal effects in PAW also include short-lived substances such as peroxyn glet oxygen, hydroxyl radicals, and superoxide anion radicals.Among them, pe is produced by the reaction of NO 2 − , H 2 O 2 , and H + .It is a strong oxidant, whic dize and thus denature the protein in the bacteria, thereby changing the bact phology and thus inactivating the bacteria [43].At low pH, peroxynitrite decom hydroxyl radicals and nitrogen dioxide.Hydroxyl free radicals have strong properties and can attack bacterial cell membranes, increasing their permeabili ing the regeneration ability of biofilms, and severely damaging bacteria [44].S gen is the excited state of oxygen molecules and has potent cytotoxicity.It ca with biological macromolecules such as proteins and DNA in cells and cause the cell membrane system by combining with molecules, causing oxidative str teria [45].
Figure 14 compares the sterilization effect of PATW and the chemical mixe It can be seen that the chemical mixed solution had a killing effect of less than rithm on E. coli, which was far less than the five logarithm inactivation effect of E. coli.Therefore, long-lived RONS are not the most important factor for the b effect in PATW, and we should focus on the impact of other short-lived RONS Figure 15 shows that these scavengers alone had no appreciable steriliza on E. coli. Figure 16 reflects the bactericidal effect of PATW after adding sever gers.L-histidine almost completely eliminated the sterilizing effect of PATW adding L-histidine to remove singlet oxygen, E. coli survived almost completel SOD also greatly weakened the effect of PATW on E. coli, and the bacteria we activated by about 1 order of magnitude.In contrast, the addition of mannit significantly affect the sterilization effect of PATW.Therefore, singlet oxygen a oxide anion radicals play a key role in the sterilization by PATW. Figure 15 shows that these scavengers alone had no appreciable sterilization effect on E. coli. Figure 16 reflects the bactericidal effect of PATW after adding several scavengers.L-histidine almost completely eliminated the sterilizing effect of PATW, and after adding L-histidine to remove singlet oxygen, E. coli survived almost completely.Adding SOD also greatly weakened the effect of PATW on E. coli, and the bacteria were only inactivated by about 1 order of magnitude.In contrast, the addition of mannitol did not significantly affect the sterilization effect of PATW.Therefore, singlet oxygen and superoxide anion radicals play a key role in the sterilization by PATW.According to the Reactions ( 1)-( 6), we speculate that the peroxynitrite produced by the plasma treatment further slowly reacts with hydrogen peroxide to form peroxynitrate and the decomposition of peroxynitrate produces a singlet oxygen and superoxide anion radicals.These reactions allow the continuous production of important short-lived (msrange lifetime) reactive species after the discharge, and the strongly oxidative environ ment of the PATW slows down the decomposition of short-lived ROS [46] so that the ster ilizing effect is maintained.According to the Reactions (1)-( 6), we speculate that the peroxynitrite produced by the plasma treatment further slowly reacts with hydrogen peroxide to form peroxynitrate, and the decomposition of peroxynitrate produces a singlet oxygen and superoxide anion radicals.These reactions allow the continuous production of important short-lived (ms-s range lifetime) reactive species after the discharge, and the strongly oxidative environment of the PATW slows down the decomposition of short-lived ROS [46] so that the sterilizing effect is maintained.According to the Reactions (1)-( 6), we speculate that the peroxynitrite produced by the plasma treatment further slowly reacts with hydrogen peroxide to form peroxynitrate, and the decomposition of peroxynitrate produces a singlet oxygen and superoxide anion radicals.These reactions allow the continuous production of important short-lived (ms-s range lifetime) reactive species after the discharge, and the strongly oxidative environment of the PATW slows down the decomposition of short-lived ROS [46] so that the sterilizing effect is maintained.

Conclusions
In this paper, the DBD discharge effectively decreased the pH value of tap water and generated high-concentration aqueous RONS.DBD treatment of tap water for 1 h and then treatment of E. coli for 15 min and 30 min, produced a bactericidal effectiveness of 5 and 6 orders of magnitude, respectively.The bactericidal effects of PATW and PADW were almost the same.Atomizing PATW onto the surface of clothes contaminated with bacteria through a disinfection door produced good sterilization and has application potential.It was determined that short-lived singlet oxygen and superoxide anion radicals played a dominant sterilization role.The suspected chemical pathway includes hydrogen peroxide reacting with NO 2 − continuously to generate peroxynitrite during the DSD discharge.After the discharge, peroxynitrite further reacts with hydrogen peroxide to generate peroxynitric acid.The singlet oxygen and superoxide anion free radicals produced by the decomposition of peroxynitric acid play a vital sterilization role.

Figure 1 .
Figure 1.The overall structure diagram of the DBD-based PAW system.

Figure 1 .
Figure 1.The overall structure diagram of the DBD-based PAW system.

Figure 4 .
Figure 4. Discharges in the tap water at different applied voltages (Pictures taken by Nikon D750, exposure time 50 ms).

Figure 5
Figure 5 captured the simultaneous discharge of four DBD devices driven by 20 kV pulses.The 4-DBD discharge array generated a stable discharge below the water surface, producing aqueous RONS, which can generate 500 mL PATW at one time.

Figure 4 .
Figure 4. Discharges in the tap water at different applied voltages (Pictures taken by Nikon exposure time 50 ms).

Figure 5
Figure 5 captured the simultaneous discharge of four DBD devices driven by 2 pulses.The 4-DBD discharge array generated a stable discharge below the water sur producing aqueous RONS, which can generate 500 mL PATW at one time.

Figure 5 .
Figure 5. Four DBD devices discharge simultaneously under 20 kV high voltage drive (Pic taken by Nikon D750, exposure time 10 ms).

Figure 6 .
Figure 6.The voltage and current waveform of the 4−DBD tube array.

Figure 5 .
Figure 5. Four DBD devices discharge simultaneously under 20 kV high voltage drive (Pictures taken by Nikon D750, exposure time 10 ms).

Figure 5
Figure 5 captured the simultaneous discharge of four DBD devices driven by 2 pulses.The 4-DBD discharge array generated a stable discharge below the water sur producing aqueous RONS, which can generate 500 mL PATW at one time.

Figure 5 .
Figure 5. Four DBD devices discharge simultaneously under 20 kV high voltage drive (Pic taken by Nikon D750, exposure time 10 ms).

Figure 6 .
Figure 6.The voltage and current waveform of the 4−DBD tube array.

Figure 6 .
Figure 6.The voltage and current waveform of the 4−DBD tube array.

Figure 7 .
Figure 7. Optical emission Spectroscopy of the plasma.

Figure 7 .
Figure 7. Optical emission Spectroscopy of the plasma.

15 Figure 7 .
Figure 7. Optical emission Spectroscopy of the plasma.

Figure 9 .
Figure 9.The (a) pH value and (b) conductivity of PATW and PADW as a function of activation time.

Figure 9 .
Figure 9.The (a) pH value and (b) conductivity of PATW and PADW as a function of activation time.

Figure 10 .
Figure 10.Comparison of the number of surviving E. coli after PATW and PADW treatment.

Figure 10 .
Figure 10.Comparison of the number of surviving E. coli after PATW and PADW treatment.

Figure 10 .
Figure 10.Comparison of the number of surviving E. coli after PATW and PADW treatment.

Figure 14 .
Figure 14.Comparison of the sterilization effect of PATW and a chemical mixed solution higher concentration.

Figure 14 .
Figure 14.Comparison of the sterilization effect of PATW and a chemical mixed solution of a slightly higher concentration.

1 Figure 15 .
Figure 15.The short-lived ROS scavenger solutions alone had no appreciable sterilization effect on E. coli.

Figure 16 .
Figure 16.The bactericidal effect of PATW after adding different scavengers.

Figure 15 . 15 Figure 15 .
Figure 15.The short-lived ROS scavenger solutions alone had no appreciable sterilization effect on E. coli.

Figure 16 .
Figure 16.The bactericidal effect of PATW after adding different scavengers.

Figure 16 .
Figure 16.The bactericidal effect of PATW after adding different scavengers.