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

doi:10.21203/rs.3.rs-3549665/v1

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

https://doi.org/10.21203/rs.3.rs-3549665/v1

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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 identification 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 flow rates, gas pressures, water volume, and plasma discharge powers on the concentration of H₂O₂ in PATW, providing valuable insights into optimizing the production process.

Plasma-activated tap water

alkaline water

reactive oxygen-nitrogen species

emission spectra

different plasma-forming gases

Plasma-Activated Water (PAW) is an emerging plasma technology that demonstrates substantial potential across a diverse range of applications, from sterilization to agriculture. Previous researchers have explored various applications of PAW, such as sterilization, food preservation, selective elimination of cancer cells, medicinal uses (e.g., wound healing), inactivation of pathogens (bacteria, fungi, viruses, pests), enhancement of seed germination and plant growth, and as a nitrogen source in agriculture and aquaculture (1–19). Raud et al. (1) studied the impact of Plasma-Activated Water (PAW) on two distinct cancer cell lines: 4T1 (breast cancer) and PPC-1 (prostate cancer). Their findings revealed a decrease in cell viability in both cancer cell lines following PAW treatment. Wang et al. (2) reported the wound healing ability of PAW, noting superior performance in comparison to medical alcohol and a control group in a mouse full-thickness skin wound model.

Ten Bosch et al. (7) previously explored the insecticidal efficacy 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, 8–10). 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 H₂O₂, dissolved O₃, ONOOˉ, etc., play a crucial role in microbial inactivation, food preservation, wound healing, and other applications. Furthermore, the existence of reactive nitrogen species, such as NO₃ˉ ions and NO₂ˉ, establishes PAW as an environmentally friendly nitrogen source for agriculture and aquaculture (20–24).

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, NO₃ˉ ions, and NO₂ˉ ions (25–34).

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 (35–37). 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 significantly 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. (42–44) Nevertheless, the acidic solution waste leads to soil and water acidification, 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 (38–40).

This study addresses these concerns and introduces a novel approach for producing alkaline plasma-activated 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.

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, significantly 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 (O₂), were introduced into the vacuum chamber, with the flow rate of the feed gas being controlled using a flow controller. The voltage-current waveform was monitored using an oscilloscope, and plasma species/radicals were identified 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 flow rate varied from 0 to 8 l min-1 (air, O₂, 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 semi-quantitatively employing MACHEREY-NAGEL QUANTOFIX semi-quantitative test strips.

3.1 Plasma Characterization

3.1.1 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 plasma-water interaction, as depicted in Fig. 1 within the vacuum chamber. This observation was supported by the presence of multiple peaks representing current filaments in Fig. 2. These current filaments 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).

3.1.2 Identification of Plasma Species/Radicals

The identification of plasma radicals/species under sub-atmospheric conditions using various plasma-forming gases is depicted in Fig. 3. This figure 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 (N₂) of the second (2nd ) positive system (N₂ (C ³П_u → B ³П_g)) (equations (1–2)), accompanied by the N₂⁺ first negative system (N₂⁺ (B ²Σ_u⁺ → X ²Σ_g⁺)) (equations (3–4)), hydroxyl radical rotational band, and hydrogen lines. The observed OH (A ²Σ → X ²П) and H_α lines indicate the dissociation of H₂O molecules (air moisture) to form OH and H (equations (10–11) (50–54). The excited N₂ and N₂⁺, along with OH and H, interact with the various reactive oxygen-nitrogen species in PATW (equations (5–11, 14–25)) (8, 29, 48, 55–72). The observed emission spectra in Fig. 3(a) represent the radiative decay transition of N₂, N₂⁺, OH band peaks, and the H_α line (equations (2, 4, 10)). The strong electric field (high voltage) applied accelerates the electrons, exciting, ionizing, and dissociating the N₂, O₂, H₂O, etc., atoms/molecules present in the air (equations (1–11)). The emission intensity of observed emission band peaks N₂, N₂⁺, OH, and H_α line dominates over other species/radicals formed during air discharge (equations (2, 4, 10)). As a result, their domination is observed only in Fig. 3(a).

Figure 3(b) displays the emission spectra of argon (Ar) plasma, characterized by strong emission lines of atomic Ar (4p → 4s) in the range of 680 to 850 nm (equations (12–13)) (65, 73), along with relatively weak vibrational band peaks from N2 (C ³П_u → B ³П_g) and N₂⁺ (B ²Σ_u⁺ → X ²Σ_g⁺) systems, and the H_α line (equations (2, 4, 10)). The observed nitrogen vibrational band peaks and the H_α line are attributed to nitrogen and moisture impurities present in the vacuum chamber under sub-atmospheric pressure.

In Fig. 3(c), the emission spectra of oxygen (O₂) plasma predominantly feature a strong atomic line of atomic oxygen (O (3p → 3s)) (74), accompanied by vibrational band peaks from the N₂ (C ³П_u → B ³П_g) and N₂⁺ (B ²Σ_u⁺ → X ²Σ_g⁺) systems, and O₂⁺ first negative system (b ⁴Σ_g → a ⁴П_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(d) illustrates a significant reduction in the emission intensity of atomic argon lines (Ar (4p → 4s)), indicating the dominance of N₂ and N₂⁺ emission vibrational band peaks (N₂ (C ³П_u → B ³П_g) and N₂⁺ (B ²Σ_u⁺ → X ²Σ_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 (Fig. 2(d)).

3.2 Physicochemical Properties and Concentration of Reactive Species in PATW

The 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 O₂ 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⁺, NO₂ˉ, NO₃ˉ, H₂O₂, etc., whereas reducing PATW has a high concentration of reducing species like OHˉ or a lower concentration of oxidizing species like NO₂ˉ, NO₃ˉ, H₂O₂, etc. (48, 63, 65, 70–72)

The proposed reactions occur in the reactive plasma phase (50–54, 65, 73):

$${N}_{2}\left(X\right)+{e}^{-}\to {N}_{2}\left(C\right)+{e}^{-}$$

$${N}_{2}\left(C\right)\to {N}_{2}\left(B\right)+hv$$

$${N}_{2}\left(X\right)+{e}^{-}\to {N}_{2}^{+}\left(B\right)+2{e}^{-}$$

$${N}_{2}^{+}\left(B\right)\to {N}_{2}^{+}\left(X\right)+hv$$

$${N}_{2}+{e}^{-}=2N+{e}^{-}$$

$${O}_{2}+{e}^{-}\to {O}_{2}^{+}+2{e}^{-}$$

$${O}_{2}^{+} \left(b\right)\to {O}_{2}^{+} \left(a\right)+hv$$

$${O}_{2}+ {e}^{-}\to 2O+{e}^{-}$$

$$xO+yN\to {N}_{y}{O}_{x} (NO, {NO}_{2}, {NO}_{3}, {N}_{2}{O}_{5}, etc.)$$

$${H}_{2}O\to H+OH$$

$$OH \left(A\right)\to OH \left(X\right)+hv$$

Only occurs in Ar plasma:

$$Ar+{e}^{-}\to {Ar}^{*}+{e}^{-}$$

$$Ar \left(p\right)\to Ar \left(s\right)+hv$$

The proposed reactions occur in the reactive liquid phase (8, 29, 48, 55–72):

$${H}^{.}\to {\varvec{H}}^{+}+{e}^{-}$$

$$OH+OH\to {\varvec{H}}_{2}{\varvec{O}}_{2}$$

$${NO}_{2}+{e}^{-}\to {\varvec{N}\varvec{O}}_{2}^{-}$$

$${NO}_{3}+{e}^{-}\to {\varvec{N}\varvec{O}}_{3}^{-}$$

$${H}^{+}+{NO}_{2}^{-}\leftrightharpoons {HNO}_{2}$$

$${H}^{+}+{NO}_{3}^{-}\leftrightharpoons {HNO}_{3}$$

$${\varvec{N}\varvec{O}}_{2}^{-}+{\varvec{H}}_{2}{\varvec{O}}_{2}\to {\varvec{N}\varvec{O}}_{3}^{-}+{H}_{2}O$$

$$OH+{e}^{-}\to {\varvec{O}\varvec{H}}^{-}$$

$$2{OH}^{-}+NO\to {\varvec{N}\varvec{O}}_{2}^{-}+{H}_{2}O$$

$${2OH}^{-}+NO\to {\varvec{N}\varvec{O}}_{3}^{-}+{H}_{2}$$

$${HNO}_{3}+{OH}^{-}\to {\varvec{N}\varvec{O}}_{3}^{-}+{H}_{2}O$$

$${HNO}_{2}+{OH}^{-}\to {\varvec{N}\varvec{O}}_{2}^{-}+{H}_{2}O$$

The pH of PATW produced using air and Ar as plasma-forming gases exhibited the minimum (acidic) and maximum (basic) values compared to other plasma-forming gases, with corresponding values of 3.7 and 8.3, respectively. Additionally, the plasma-water interaction facilitates the transfer of heat from plasma to water, consequently raising the temperature. To maintain the temperature of Plasma-Activated Tap Water (PATW) below 50°C, continuous cooling of the vacuum chamber was employed, as illustrated in Fig. 4 (a). The decrease in pH of water after treatment is in line with previous reported work by Tian et al. (60), Ma et al. (64), and Lu et al. (63), etc.

Figure 4 (b) presents the concentration of various reactive species formed in PATW using different plasma-forming gases. The proposed reactions involved in the formation of these RONS in PATW are shown by equations (14–25) (8, 29, 48, 55–72). PATW prepared using Ar + Air and Air as plasma-forming gases exhibited a significantly higher concentration of NO₃ˉ ions compared to other plasma-forming gases. This was attributed to the higher concentration of N₂ carried by the inserted air compared to the presence of air impurities at sub-atmospheric pressure while preparing PATW using O₂ and Ar as plasma-forming gases. The elevated concentration of high-energy N₂ species/radicals reacts with oxygen molecules in the high-energy plasma reactive environment, forming NO_x, which dissolves in water to produce NO₃ˉ and NO₂ˉ ions in PATW (equations (16–17)) (8, 14, 23, 32, 56, 63). Moreover, these NO₃ˉ and NO₂ˉ ions combine with H⁺ that was generated due to the dissociation of H₂O molecule into H radicals (also observed in the emission spectra as H_α line) to form nitric (HNO₃) and nitrous acid (HNO₂) in PATW (equations (18–19)) (8, 14, 23, 32, 56, 63). Hence, the presence of these acids significantly decreases the pH of PATW when prepared using Air and Air + Ar mixture as a plasma-forming gas. Along with that, the observed NO₃ˉ ions in PAW (O2) and PAW (Ar) were due to air impurities present at sub-atmospheric 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 NO2ˉ and NO3ˉ ions as shown in equations (22, 23) (60, 70–72).

Moreover, the concentration of NO₂ˉ ions in PATW prepared using Air and Air + Ar was slightly higher compared to O₂ and Ar. This can be attributed to the highly reactive environment of PATW (Air) and PATW (Air + Ar), promoting the reaction between NO₂ˉ ions with H₂O₂, resulting in the formation of stable NO₃ˉ ions as shown in Eq. (20) (32, 55). Consequently, even with an excess of NO₂ˉ ions compared to PATW (Air) and PATW (Air + Ar), their concentration decreased due to their reactivity with H₂O₂.

The reaction between NO₂ˉ ions and H₂O₂ also led to a reduction in the concentration of H₂O₂ in PATW (Air) and PATW (Air + Ar) (Eq. (20)) (32, 55). The presence of moisture in the air increased the concentration of H₂O₂ 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, H₂O₂ formed in PATW (O₂) 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 (O₂) or PATW (Ar), the possibility of a reaction between NO₂ˉ and H₂O₂ was significantly low (Eq. (20)). Consequently, there was little to no degradation of H₂O₂ in PATW (O₂) or PATW (Ar).

The results demonstrate a notable increase in the pH of water when Ar or O₂ is used for water activation (Fig. 5a), aligning with the discussed findings 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. (23), Xiang et al. (62), Shen et al. (58), etc. The heightened pH in PATW (Ar) or PATW (O₂) signifies a substantial increase in the hydroxyl ion concentration with prolonged plasma treatment time (Eq. (21)). The maximum percentage increase in pH for PATW (Ar) and PATW (O₂) 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 (NO₃ˉ) 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 NO₃ˉ ion concentration followed by a decrease with prolonged plasma treatment time. This pattern highlights the lower stability of NO₃ˉ ions in PATW under vacuum conditions. The high-energy electrons from the plasma dissociate the dissolved NO₂ˉ and NO₃ˉ ions in PATW to corresponding N₂ and O₂, resulting in a decrease in the concentration of NO₂ˉ and NO₃ˉ ions with increasing plasma treatment time. A similar trend is observed in NO₂ˉ ion concentration, where an initial rise is followed by a continuous decrease in concentration. The highest concentrations of NO₃ˉ and NO₂ˉ 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 (O₂)), 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 NO₂ˉ and NO₃ˉ ions is observed with increasing plasma treatment time. This is due to the dissociation of NO₂ˉ and NO₃ˉ 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 NO₂ˉ and NO₃ˉ ions is observed in PATW with increasing plasma treatment time (Fig. 5 (b, c)).

The variation in H₂O₂ concentration in PATW prepared using different plasma-forming gases is shown in Fig. 5(d). In this figure, a monotonous increase in H₂O₂ 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 H₂O₂ concentration was not observed in PATW (air) with increasing plasma treatment time. This was due to the reducing concentration of NO₂ˉ and NO₃ˉ ions in PAW, limiting the reaction between H₂O₂ and NO₂ˉ. As a result, the decrease in the concentration of H₂O₂ in PATW (air) was not observed, and PAW (air) showed the highest concentration of H₂O₂ compared to other plasma-forming gases.

The graph presented in Fig. 6 illustrates the fluctuation in H₂O₂ concentration within PATW (O₂) and PATW (Ar) as the flow rate varies. An evident trend shows that an increase in plasma-water treatment leads to a consistent rise in H₂O₂ concentration across all specified flow rates for both PATW (O₂) and PATW (Ar) (32, 48, 49). However, when focusing on PATW (O₂), a noticeable rise and fall in H₂O₂ concentration is observed at different flow rates, as depicted in Fig. 6 (a). The optimal concentration of H₂O₂ (79 mg L^− 1) in PATW (O₂) is achieved at a flow rate of 4 L min^− 1. The reduction in H₂O₂ concentration at higher flow rates can be attributed to the reduced discharge of O₂ molecules. This is due to the high molecular density of O₂, causing rapid energy dissipation and reduced ionization, resulting in lower H₂O₂ formation in PATW (O₂) under the given input power.

In contrast, this fluctuating behavior in H₂O₂ concentration is not observed in PATW (Ar). A higher flow rate of Ar, indicative of a higher Ar atom density, results in an elevated concentration of H₂O₂ in PATW (Ar). The supplied energy is sufficient to ionize Ar and other atoms/molecules even at high flow rates, leading to a significant increase in H₂O₂ concentration (63 mg L^− 1) observed at a flow rate of 6 L min^− 1.

In Fig. 7(a), we observe the variation in H₂O₂ 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 significantly higher concentration of H₂O₂ 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 H₂O₂ is observed in PATW (Ar) at 76 torr gas pressure.

Additionally, Fig. 7(b) showcases the impact of increasing plasma discharge power on the H₂O₂ concentration in PATW (Ar). Maintaining a fixed treatment time of 20 minutes, elevating the plasma discharge power from 24 W to 136 W results in a 100% increase in the H₂O₂ 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 amplified concentration of H₂O₂ 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 H₂O₂ during storage and its impact on H₂O₂ concentration in PATW (Ar). In Fig. 8 (a), the concentration of H₂O₂ 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 significantly higher concentration of H₂O₂ 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 H₂O₂ density in the lower volume of PATW compared to the higher volume, while all other process parameters and power remain constant (48).

In Fig. 8 (b), the stability of H₂O₂ in PATW (Ar) over time is presented. After approximately 113 hours (around 5 days) from production, a 40% decrease in H₂O₂ concentration is observed. However, between approximately 5 to 7 days (113 to 162 hours), there is a 10% increase in H₂O₂ concentration. This fluctuation indicates that after the initial increase and subsequent decrease in H₂O₂ concentration in PATW (Ar), it stabilizes over time (48, 58).

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 O₂ as the plasma-forming gas at sub-atmospheric pressure. The concentrations of NO₃ˉ and NO₂ˉ 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 H₂O₂ kept increasing with treatment time using different plasma-forming gases.

Moreover, the gas flow rate, plasma discharge power, volume of water, and gas pressure significantly influence the concentration of dissolved H₂O₂ in PAW. Stability studies showed an initial decrease in H₂O₂ concentration with storage, which then stabilized over time.

In conclusion, these findings contribute to the understanding of plasma-water interactions and offer a promising avenue for tailoring PATW to specific 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 field of plasma-activated water technology.

Acknowledgements

We would like to thank Agricultural Research Development Agency (ARDA), International Research Network (IRN), Thailand Institute of Nuclear Technology (TINT) and Walailak university, for their funding supports.

Authors’ Contribution

KR, SK, DS, AT and MN designed and conducted the experiment. MN & VR analysis the data and wrote the first draft of manuscript. DB performed the review and editing of manuscript.

Funding

Agricultural Research Development Agency (ARDA), International Research Network (IRN), Thailand Institute of Nuclear Technology (TINT) and Walailak university.

Data Availability

All data are will be made available by the corresponding author upon reasonable request.

Conflict of Interest: Authors have no competing interests

Ethical Approval: Not applicable

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No competing interests reported.

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You are reading this latest preprint version

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

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Abstract

Figures

1. Introduction

2. Material and Methods

3. Results and Discussion

3.1 Plasma Characterization

3.1.1 Voltage-Current Waveform

3.1.2 Identification of Plasma Species/Radicals

3.2 Physicochemical Properties and Concentration of Reactive Species in PATW

3. Conclusion

Declarations

References

Additional Declarations

Status:

Version 1