Production of Alkaline Plasma Activated Tap Water using Different Plasma Forming Gas at Sub-Atmospheric Pressure

The present study demonstrates the successful production of alkaline plasma-activated tap water (PATW), addressing the challenge of acidity in traditional PATW for various applications. By carefully controlling the plasma-forming gases (oxygen, air, argon) and process parameters, such as PATW production at sub-atmospheric pressure, it is possible to shift the pH of acidic PATW towards the alkaline range, making it suitable for applications like agriculture, aquaculture, sterilization, wound healing, disinfection, and food preservation. The investigation involved the characterization of plasma and the identi�cation of various plasma species/radicals. The impact of different plasma-forming gases on the pH of PATW and the concentration of reactive species in PATW was thoroughly analyzed. Plasma created using oxygen and argon led to the production of reducing or alkaline PATW, while air and air-argon mixtures resulted in an acidic or oxidizing nature. The study also discussed the stability of nitrate ions, nitrite ions, and hydrogen peroxide in PATW, shedding light on their behavior over varying plasma treatment times and plasma-forming gas. Finally, the investigation explored the effects of gas �ow rates, gas pressures, water volume, and plasma discharge powers on the concentration of H 2 O 2 in PATW, providing valuable insights into optimizing the production process.

Ten Bosch et al. (7) previously explored the insecticidal e cacy of PAW, reporting a high mortality rate of approximately 90% among insects after 24 hours of PAW treatment.Guo et al. (6) delved into the virus inactivation mechanism by PAW, highlighting that oxidizing species such as single oxygen damage the structure of various viruses, including double-stranded DNA, single-stranded DNA, and RNA bacteriophages.Consequently, virus inactivation occurs post-PAW treatment.
Our previously published reports discuss the diverse applications of PAW in antimicrobial, antifungal, food preservation, agriculture, and aquaculture (3,4,(8)(9)(10).In experiments, a few seconds of PAW exposure led to over a 6 + CFU ml − 1 log reduction in pathogenic bacteria such as Staphylococcus aureus and Pseudomonas aeruginosa (8).Additionally, a brief PAW exposure completely inhibited pathogenic fungi, including Candida albicans and the food spoilage agent Citrus limon fungi (4).Washing Citrus limon with PAW extended its shelf life by over two months, preserving sensory and nutritional properties (3).
Our past research also indicated that PAW treatment enhances seed germination and plant growth in peas (Pisum sativum L.).Compared to the control group, seeds treated with PAW exhibited a 37% increase in germination and a 95% increase in plant growth (9).Moreover, PAW can serve as a nitrogen source for freshwater algae growth.In a PAW medium as a nitrogen source, algae growth increased by up to 626% compared to conventional Bold's Basal Medium (10).
The effectiveness of PAW in these applications can be attributed to the presence of diverse reactive oxygen-nitrogen species.Components like H 2 O 2 , dissolved O 3 , ONOO , etc., play a crucial role in microbial inactivation, food preservation, wound healing, and other applications.Furthermore, the existence of reactive nitrogen species, such as NO 3 ions and NO 2 , establishes PAW as an environmentally friendly nitrogen source for agriculture and aquaculture (20)(21)(22)(23)(24).
Reports on the physicochemical properties of PAW by numerous researchers indicate its acidic and oxidizing nature, which is integral to its activities in disinfection, sterilization, food preservation, etc.The acidic nature of PAW results from the formation of nitrous and nitric acid during plasma-water interactions, as evidenced by the increased concentration of H + ions, NO 3 ions, and NO 2 ions (25)(26)(27)(28)(29)(30)(31)(32)(33)(34).
However, the acidic pH of PAW renders it unsuitable for various medicinal and agricultural applications.Low pH solutions are not recommended for medicinal use due to their potential to oxidize skin cells upon contact, leading to damage and skin irritation, as well as eye irritation (35)(36)(37).In food preservation, a low pH solution may oxidize the outer surface of food, leading to a loss in nutritional value, texture, taste, and overall food quality.Likewise, in agriculture, the low pH of PAW is a concern as it can signi cantly impact crop health and soil health, considering the desired pH range for agricultural and aquaculture applications is neutral to slightly basic (6.5 to 8) (38-40).Sivachandiran et al. (41) also demonstrated that high exposure of plasma to water makes the water acidic, negatively affecting seed germination and plant growth.Moreover, during surface sterilization or disinfection, the low pH of PAW can cause problems like rusting and corrosion, etc. (42)(43)(44) Nevertheless, the acidic solution waste leads to soil and water acidi cation, which can harm aquatic life, plants, and the ecosystem.Hence, the use of a low pH solution always comes with environmental regulatory compliance for proper use and exploitation (38)(39)(40).
This study addresses these concerns and introduces a novel approach for producing alkaline plasmaactivated tap water (PATW).A sub-atmospheric pressure vacuum system has been designed and developed to enable large-volume production of plasma-activated alkaline tap water.The generation of PATW utilizes a radio frequency (RF) power source.Characterization of PATW involves studying changes in pH and the concentrations of reactive oxygen and nitrogen species, including nitrate ions, nitrite ions, and hydrogen peroxide.Additionally, the study highlights the effects of different plasma-forming gases and their combinations on the properties of PATW.

Material and Methods
The schematic illustrating the production process of plasma-activated tap water (PATW) is presented in Fig. 1.To produce alkaline water, preventing interference from nitrogen molecules in the surrounding air is imperative.The discharge of nitrogen plasma species results in the formation of nitrous and nitric acid in the water, signi cantly lowering its pH and undermining the objective of this study.Consequently, all experiments were conducted under sub-atmospheric pressure, facilitated by a vacuum pump.Figure 1 depicts the vacuum chamber, boasting a volume of 5 L. The vacuum in the system was created using a Joto 2BV2060 vacuum pump (45).
The plasma electrode, crucial for plasma generation, is crafted from stainless steel and placed within the vacuum chamber.It is energized by a high-voltage radio frequency (RF) power supply, reaching a maximum power of 500 W. The voltage and frequency range for this power supply were 1 to 10 kV and 50 to 500 kHz, respectively.Plasma-forming gases, including air, argon (Ar), and oxygen (O 2 ), were introduced into the vacuum chamber, with the ow rate of the feed gas being controlled using a ow controller.The voltage-current waveform was monitored using an oscilloscope, and plasma species/radicals were identi ed using a spectrometer in the range of 290 nm to 875 nm, respectively.
In the preparation of PATW, 100 to 500 ml of tap water was placed in a cup within the vacuum chamber for activation.The gas ow rate varied from 0 to 8 l min-1 (air, O 2 , Ar, air + Ar).The electrode was energized with a frequency of 20 kHz and 0 to 500 W RF power to activate the water.Cooling for the chamber was achieved by utilizing metallic pipes containing cooling water wrapped around the vacuum chamber, connected to a water chiller, as illustrated in Fig. 1.
Monitoring the pH of PATW was carried out using a pH meter (Mettler Toledo Seven Compact pH/Ion meter).The determination of reactive oxygen-nitrogen species (RONS) in PATW was conducted semiquantitatively employing MACHEREY-NAGEL QUANTOFIX semi-quantitative test strips.

Voltage-Current Waveform
Figure 2 illustrates the voltage-current waveform of air plasma generated during plasma-water interaction under sub-atmospheric pressure within the vacuum chamber.The recorded peak-to-peak voltage was 13.4 kV, displaying a sinusoidal waveform.Filamentary discharges were noticeable during the plasmawater interaction, as depicted in Fig. 1 within the vacuum chamber.This observation was supported by the presence of multiple peaks representing current laments in Fig. 2.These current laments indicate the formation of various reactive species and radicals during air discharge in each rising and falling current half-cycle (46).The resulting radicals and species dissolved in water during the plasma-water interaction, giving rise to plasma-activated water (6, 15, 18, 19, 22, 26, 47-49).

Identi cation of Plasma Species/Radicals
The identi cation of plasma radicals/species under sub-atmospheric conditions using various plasmaforming gases is depicted in Fig. 3.This gure illustrates the emission spectra of air plasma, argon plasma, oxygen plasma, and air + argon plasma, offering insights into their impact on various physicochemical properties and the concentration of reactive species in PATW.
In Fig. 3(a), the emission spectra of air plasma primarily exhibit strong vibrational band peaks from nitrogen gas (N 2 ) of the second (2nd  and N 2 + (B 2 Σ u + → X 2 Σ g + ) systems, and O 2 + rst negative system (b 4 Σ g → a 4 П u ) (equations (2, 4, 6-8)) (75).The emission lines and molecular band peaks of oxygen dominate in oxygen plasma along with air impurities in the form of nitrogen and hydroxyl molecular band peaks (Fig. 3(c)).
When air is introduced into argon plasma, Fig. 2 The proposed reactions occur in the reactive plasma phase (50-54, 65, 73):  gases.This was attributed to the higher concentration of N 2 carried by the inserted air compared to the presence of air impurities at sub-atmospheric pressure while preparing PATW using O 2 and Ar as plasmaforming gases.).Hence, the presence of these acids signi cantly decreases the pH of PATW when prepared using Air and Air + Ar mixture as a plasma-forming gas.Along with that, the observed NO 3 ions in PAW (O2) and PAW (Ar) were due to air impurities present at subatmospheric pressure in the vacuum chamber.However, the formed nitrous and nitric acid are neutralized by the higher concentration of hydroxide ions as shown in equations (24, 25) (60, 70-72).Moreover, the presence of hydroxide ions supports the formation of NO2 and NO3 ions as shown in equations (22,23) (60, 70-72).
Moreover, the concentration of NO 2 ions in PATW prepared using Air and Air + Ar was slightly higher compared to O 2 and Ar.This can be attributed to the highly reactive environment of PATW (Air) and PATW (Air + Ar), promoting the reaction between NO 2 ions with H 2 O 2 , resulting in the formation of stable NO 3 ions as shown in Eq. ( 20) (32,55).Consequently, even with an excess of NO 2 ions compared to PATW (Air) and PATW (Air + Ar), their concentration decreased due to their reactivity with H 2 O 2 .
The reaction between NO 2 ions and H 2 O 2 also led to a reduction in the concentration of H 2 O 2 in PATW (Air) and PATW (Air + Ar) (Eq.( 20)) (32,55).The presence of moisture in the air increased the concentration of H 2 O 2 in PATW (Air) + PATW (Air + Ar), in addition to the moisture from the evaporation of water kept in the vessel inside the vacuum chamber (equations (10,15)).However, H 2 O 2 formed in PATW (O 2 ) or PATW (Ar) was solely due to the dissociation of residual air moisture and the evaporation of water molecules.Given the low reactivity (neutral or basic pH) and low oxidizing potential of PATW (O 2 ) or PATW (Ar), the possibility of a reaction between NO 2 and H 2 O 2 was signi cantly low (Eq.( 20)).
Consequently, there was little to no degradation of H 2 O 2 in PATW (O 2 ) or PATW (Ar).
The results demonstrate a notable increase in the pH of water when Ar or O 2 is used for water activation (Fig. 5a), aligning with the discussed ndings above.Conversely, a decrease in the pH of water is observed when air is employed as the plasma-forming gas.This decrease in pH with prolonged plasma treatment time is consistent with the previously published literature of Sajib et al.  21)).The maximum percentage increase in pH for PATW (Ar) and PATW (O 2 ) compared to the pH of control (tap water) is reported as 41.5% and 44.5%, respectively.Furthermore, Fig. 5(b) illustrates the variation in nitrate (NO 3 ) ion concentration in PATW prepared with different plasma-forming gases, correlating with increasing plasma treatment time.The PATW prepared with various gases exhibit a consistent pattern: an initial rise in NO 3 ion concentration followed by a decrease with prolonged plasma treatment time.This pattern highlights the lower stability of NO 3 ions in PATW under vacuum conditions.The high-energy electrons from the plasma dissociate the dissolved NO 2 and NO 3 ions in PATW to corresponding N 2 and O 2 , resulting in a decrease in the concentration of NO 2 and NO 3 ions with increasing plasma treatment time.A similar trend is observed in NO 2 ion concentration, where an initial rise is followed by a continuous decrease in concentration.The highest concentrations of NO 3 and NO 2 ions are observed at 10 minutes of treatment time, recorded as 99.8 mg L − 1 and 6.95 mg L − 1 (PATW (Ar)), 40.1 mg L − 1 and 4.9 mg L − 1 (PATW (O 2 )), and 162.6 mg L − 1 and 11.0 mg L − 1 (PATW (Air)).After 10 minutes of plasma treatment time, a substantial decrease in the concentration of NO 2 and NO 3 ions is observed with increasing plasma treatment time.This is due to the dissociation of NO 2 and NO 3 ions by high-energy electrons from the plasma, breaking down the dissolved nitrogen oxide ions into corresponding nitrogen and oxygen gas, which is then removed by the vacuum pump.As a result, the declining nature of NO 2 and NO 3 ions is observed in PATW with increasing plasma treatment time (Fig. 5 (b, c)).
The variation in H 2 O 2 concentration in PATW prepared using different plasma-forming gases is shown in Fig. 5 In contrast, this uctuating behavior in H 2 O 2 concentration is not observed in PATW (Ar).A higher ow rate of Ar, indicative of a higher Ar atom density, results in an elevated concentration of H 2 O 2 in PATW (Ar).The supplied energy is su cient to ionize Ar and other atoms/molecules even at high ow rates, leading to a signi cant increase in H 2 O 2 concentration (63 mg L − 1 ) observed at a ow rate of 6 L min − 1 .
In Fig. 7(a), we observe the variation in H 2 O 2 concentration within PATW (Ar) under different gas pressures, maintaining a constant plasma power of 105 W. Notably, lower Ar gas pressure (36 torr) demonstrates a signi cantly higher concentration of H 2 O 2 compared to the 76 torr Ar gas pressure.This variance can be attributed to the dissipation of plasma energy by the high density of Ar atoms at elevated gas pressure, consequently reducing the formation of OH radicals.As a result, a lower concentration of H 2 O 2 is observed in PATW (Ar) at 76 torr gas pressure.
Additionally, Fig. 7(b) showcases the impact of increasing plasma discharge power on the H 2 O 2 concentration in PATW (Ar).Maintaining a xed treatment time of 20 minutes, elevating the plasma discharge power from 24 W to 136 W results in a 100% increase in the H 2 O 2 concentration in PATW (Ar).
This effect is attributed to the heightened plasma power at a consistent gas pressure, promoting increased gas ionization and, consequently, a greater formation of hydroxyl radicals (32,48,49).This increase in hydroxyl radicals is mirrored by the ampli ed concentration of H 2 O 2 in PATW.
Moreover, an extension in plasma treatment time leads to a rise in the water temperature, causing water loss during plasma treatment, as depicted in Fig. 7(a).Initially, the loss of water is considerably higher for low Ar gas pressure (36 torr) compared to high Ar gas pressure (76 torr).However, towards the end of 120 minutes, the loss of water stabilizes and becomes comparable for both low and high gas pressures.
Figure 8 illustrates the stability of H 2 O 2 during storage and its impact on H 2 O 2 concentration in PATW (Ar).In Fig. 8 (a), the concentration of H 2 O 2 in PATW (Ar) remains comparable for both 200 ml and 500 ml volumes initially, with slight variations observed during a 10-minute treatment.Notably, a signi cantly higher concentration of H 2 O 2 is observed in the 200 ml PATW compared to the 500 ml PATW at extended plasma-water treatment times.This difference is attributed to the higher H 2 O 2 density in the lower volume of PATW compared to the higher volume, while all other process parameters and power remain constant (48).

Conclusion
This study has introduced an innovative method for producing alkaline plasma-activated tap water, overcoming the limitations posed by the acidic nature of conventional PATW.Alkaline PATW was prepared using Ar or O 2 as the plasma-forming gas at sub-atmospheric pressure.The concentrations of NO 3 and NO 2 were substantially lower compared to air as the plasma-forming gas.However, at high plasma treatment times, a reduction in RNS concentration was observed at sub-atmospheric pressure due to the dissociation of dissolved reactive nitrogen species in PATW.Simultaneously, the concentration of H 2 O 2 kept increasing with treatment time using different plasma-forming gases.
Moreover, the gas ow rate, plasma discharge power, volume of water, and gas pressure signi cantly in uence the concentration of dissolved H 2 O 2 in PAW.Stability studies showed an initial decrease in H 2 O 2 concentration with storage, which then stabilized over time.
In conclusion, these ndings contribute to the understanding of plasma-water interactions and offer a promising avenue for tailoring PATW to speci c applications, particularly in the realms of agriculture, aquaculture, food preservation, wound healing, disinfection, and sterilization, where a neutral to slightly basic pH is desired.The insights gained pave the way for further advancements and applications in this burgeoning eld of plasma-activated water technology.

Figure 4 Impact
Figure 4

Figure 5 Impact
Figure 5

Figure 8 (
Figure 8 (d) illustrates a signi cant reduction in the emission intensity of atomic argon lines (Ar (4p → 4s)), indicating the dominance of N 2 and N 2 + emission vibrational band peaks (N 2 (C 3 П u → B 3 П g ) and N 2 + (B 2 Σ u + → X 2 Σ g + )) over argon lines.Furthermore, the moisture content present in the inserted air enhances the intensity of the H α line in comparison to Ar plasma species and radicals discussed above, generated using various plasma-forming gases, undergo changes in the physicochemical properties of water when they come into contact with water inside the vacuum chamber (18, 49, 58, 60, 64, 65).These alterations are depicted in Fig. 4. PATW production utilizing O 2 and Ar as plasma-forming gases indicates the generation of alkaline or reducing PATW.Conversely, when air and an air + Ar mixture are used for PATW production, an acidic or oxidizing nature is observed.This variation in pH is due to the formation of different reactive species in different concentrations.The acidic PATW is dominated by oxidizing species like a high concentration of H + , NO 2 , NO 3 , H 2 O 2 , etc., whereas reducing PATW has a high concentration of reducing species like OH or a lower concentration of oxidizing species like NO 2 , NO 3 , H 2 O 2 , etc. (48, 63, 65, 70-72) The 14,23,32,ted concentration of high-energy N 2 species/radicals reacts with oxygen molecules in the high-energy plasma reactive environment, forming NO x , which dissolves in water to produce NO 3 and NO 2 ions in PATW (equations (16-17)) (8,14,23,32, 56, 63).Moreover, these NO 3 (32, was due to the reducing concentration of NO 2 and NO 3 ions in PAW, limiting the reaction between H 2 O 2 and NO 2 .As a result, the decrease in the concentration of H 2 O 2 in PATW (air) was not observed, and PAW (air) showed the highest concentration of H 2 O 2 compared to other plasma-forming gases.The graph presented in Fig.6illustrates the uctuation in H 2 O 2 concentration within PATW (O 2 ) and PATW (Ar) as the ow rate varies.An evident trend shows that an increase in plasma-water treatment leads to a consistent rise in H 2 O 2 concentration across all speci ed ow rates for both PATW (O 2 ) and PATW (Ar)(32, 48, 49).However, when focusing on PATW (O 2 ), a noticeable rise and fall in H 2 O 2 concentration is observed at different ow rates, as depicted in Fig.6 (a).The optimal concentration of H 2 O 2 (79 mg L − 1 ) in PATW (O 2 ) is achieved at a ow rate of 4 L min − 1 .The reduction in H 2 O 2 concentration at higher ow rates can be attributed to the reduced discharge of O 2 molecules.This is due to the high molecular density of O 2 , causing rapid energy dissipation and reduced ionization, resulting in lower H 2 O 2 formation in PATW (O 2 ) under the given input power.
(d).In this gure, a monotonous increase in H 2 O 2 concentration is observed for all plasma-forming gases.This trend is also observed in previously reported works by Tian et al. (60), Lu et al. (63), Arda et al. (14), etc.Additionally, a decrease in H 2 O 2 concentration was not observed in PATW (air) with increasing plasma treatment time.
(b), the stability of H 2 O 2 in PATW (Ar) over time is presented.After approximately 113 hours