Introduction

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, including 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 [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19]. For instance, Raud et al. [1] demonstrated decreased cell viability in breast and prostate cancer cell lines following PAW treatment. Wang et al. [2] reported superior wound healing ability of PAW 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 h of treatment. Guo et al. [6] investigated the virus inactivation mechanism by PAW. They highlighted 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 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, 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 while preserving sensory and nutritional properties [3].

Our previous research also indicated that PAW treatment enhances seed germination and plant growth in peas (Pisum sativum L.), with seeds treated showing a 37% increase in germination and a 95% increase in plant growth compared to the control group [9]. Additionally, PAW can serve as a nitrogen source for freshwater algae growth, showing up to 626% increase compared to conventional Bold’s Basal Medium [10].

The effectiveness of PAW in various 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 [20,21,22,23,24].

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, NO3ˉ ions, and NO2ˉ ions [25,26,27,28,29,30,31,32,33,34].

However, the acidic pH of PAW poses challenges for medicinal and agricultural applications, impacting crop health, soil health, and overall environmental balance. 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 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) [38,39,40]. 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 such as rusting and corrosion, etc. [42,43,44]. 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 [38,39,40].

This study addresses these concerns and introduces a novel approach for producing alkaline plasma-activated tap water (PATW). Previously, Pang et al. [45] highlighted the significance of alkaline plasma-activated water in cancer treatment. However, their method involved using an alkaline sodium hydroxide solution for the production of alkaline PAW. In contrast, the present study endeavors to produce alkaline PAW utilizing commonplace household tap water. 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.

Materials and Methods

The production process of plasma-activated tap water (PATW) is illustrated in Fig. 1. To achieve alkaline water production, preventing interference from nitrogen molecules in the surrounding air is crucial. 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 study’s objective. Consequently, all experiments were conducted under sub-atmospheric pressure (also known as negative pressure or vacuum), facilitated by a vacuum pump. Figure 1 depicts the vacuum chamber, boasting a volume of 5 L. The vacuum in the system (∼ 10− 2 torr) was created using a Joto 2BV2060 vacuum pump [46]. The base gas (air, Ar, O2) pressure varies from 36 to 49 torr.

Fig. 1
figure 1

Schematic of the high-volume plasma-activated tap water production process

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The plasma electrode, a critical component for plasma generation, was crafted from stainless steel and was positioned within the vacuum chamber. It was 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 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) such as NO2ˉ ions, NO3ˉ ions and H2O2 in PATW was conducted semi-quantitatively employing MACHEREY-NAGEL QUANTOFIX semi-quantitative test strips. The test strips used were MACHEREY-NAGEL QUANTOFIX NO3ˉ/NO2ˉ ions for NO3ˉ and NO2ˉ ions determination, and MACHEREY-NAGEL QUANTOFIX Peroxide for H2O2 determination. The temperature of the water was measured using a mercury thermometer.

Results and Discussion

Plasma Characterization

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 (see Fig. 2). The non-uniform dielectric barrier 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 [47]. The resulting radicals and species dissolved in water during the plasma-water interaction give rise to plasma-activated water [6, 15, 18, 19, 22, 26, 48,49,50].

Fig. 2
figure 2

Voltage-current waveform of air plasma generated under sub-atmospheric pressure

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Identification of Plasma Species/Radicals

Figure 3 illustrates the emission spectra of air plasma, argon plasma, oxygen plasma, and air + argon plasma under sub-atmospheric conditions, providing insights into their impact on various physicochemical properties and the concentration of reactive species in PATW.

Fig. 3
figure 3

Emission spectra of (a) Air, (b) Argon (Ar), (c) Oxygen (O2) gas, and (d) Air + Ar gas mixture produced under sub-atmospheric pressure

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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 3Пu → B 3Пg)) (Eqs. (1–2)). This is accompanied by the N2+ first negative system (N2+ (B 2Σu+ → X 2Σg+)) (Eqs. (3–4)), hydroxyl radical rotational band, and hydrogen lines. The observed OH (A 2Σ → X 2П) and Hα lines indicate the dissociation of H2O molecules (air moisture) to form OH and H (Eqs. (10–11) [51,52,53,54,55]. The excited N2 and N2+, along with OH and H, interact with the various reactive oxygen-nitrogen species in PATW (Eqs. (5–11, 14–25)) [8, 29, 49, 56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72,73]. The observed emission spectra in Fig. 3(a) represent the radiative decay transition of N2, N2+, OH band peaks, and the Hα line (Eqs. (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 (Eqs. (1–11)). The emission intensity of observed emission band peaks N2, N2+, OH, and Hα line dominates over other species/radicals formed during air discharge (Eqs. (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 (Eqs. (12–13)) [66, 74], along with relatively weak vibrational band peaks from N2 (C 3Пu → B 3Пg) and N2+ (B 2Σu+ → X 2Σg+) systems, and the Hα line (Eqs. (2, 4, 10)). The observed nitrogen vibrational band peaks and the Hα line were 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)) [75], accompanied by vibrational band peaks from the N2 (C 3Пu → B 3Пg) and N2+ (B 2Σu+ → X 2Σg+) systems, and O2+ first negative system (b 4Σg → a 4Пu) (Eqs. (2, 4, 6–8)) [76]. 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 was 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 3Пu → B 3Пg) and N2+ (B 2Σu+ → X 2Σg+)) over argon lines. Furthermore, the moisture content present in the inserted air enhances the intensity of the Hα line in comparison to Ar plasma (Fig. 2 (d)).

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 contact water inside the vacuum chamber [18, 50, 59, 61, 65, 66]. 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 were used for PATW production, an acidic or oxidizing nature was observed.

Fig. 4
figure 4

Impact of various plasma-forming gases on physicochemical properties (a) pH and concentration of reactive species (b) nitrate (NO3ˉ) ions, nitrite (NO2ˉ) ions, and hydrogen peroxide (H2O2) in PATW

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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 HOˉ or a lower concentration of oxidizing species like NO2ˉ, NO3ˉ, H2O2, etc. [49, 64, 66, 71,72,73].

The proposed reactions occur in the reactive plasma phase [51,52,53,54,55, 66, 74]:

$${N}_{2}\left(X\right)+{e}^{-}\to {N}_{2}\left(C\right)+{e}^{-}$$
(1)
$${N}_{2}\left(C\right)\to {N}_{2}\left(B\right)+hv$$
(2)
$${N}_{2}\left(X\right)+{e}^{-}\to {N}_{2}^{+}\left(B\right)+2{e}^{-}$$
(3)
$${N}_{2}^{+}\left(B\right)\to {N}_{2}^{+}\left(X\right)+hv$$
(4)
$${N_2} + {e^ - } \to 2N + {e^ - }$$
(5)
$${O}_{2}+{e}^{-}\to {O}_{2}^{+}+2{e}^{-}$$
(6)
$${O}_{2}^{+} \left(b\right)\to {O}_{2}^{+} \left(a\right)+hv$$
(7)
$${O}_{2}+ {e}^{-}\to 2O+{e}^{-}$$
(8)
$$xO+yN\underrightarrow{M}{N}_{y}{O}_{x} (NO, {NO}_{2}, {NO}_{3}, {N}_{2}{O}_{5}, etc.)$$
(9)
$${H}_{2}O\underrightarrow{M}H+OH$$
(10)
$$OH \left(A\right)\to OH \left(X\right)+hv$$
(11)

Only occurs in Ar plasma:

$$Ar+{e}^{-}\to {Ar}^{*}+{e}^{-}$$
(12)
$$Ar \left(p\right)\to Ar \left(s\right)+hv$$
(13)

The proposed reactions occur in the reactive liquid phase [8, 29, 49, 56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72,73]:

$${H}^{.}\underrightarrow{M}{\varvec{H}}^{+}+{e}^{-}$$
(14)
$$OH+OH\to {\varvec{H}}_{2}{\varvec{O}}_{2}$$
(15)
$${NO}_{2}+{e}^{-}\to {\varvec{N}\varvec{O}}_{2}^{-}$$
(16)
$${NO}_{3}+{e}^{-}\to {\varvec{N}\varvec{O}}_{3}^{-}$$
(17)
$${H}^{+}+{NO}_{2}^{-}\leftrightharpoons {HNO}_{2}$$
(18)
$${H}^{+}+{NO}_{3}^{-}\leftrightharpoons {HNO}_{3}$$
(19)
$${\varvec{N}\varvec{O}}_{2}^{-}+{\varvec{H}}_{2}{\varvec{O}}_{2}+{H}^{+}\to ONOOH+{H}_{2}O\to {\varvec{N}\varvec{O}}_{3}^{-}+{H}^{+}+{H}_{2}O$$
(20)
$$OH+{e}^{-}\to {\varvec{O}\varvec{H}}^{-}$$
(21)
$$2{OH}^{-}+NO\to {\varvec{N}\varvec{O}}_{2}^{-}+{H}_{2}O$$
(22)
$${2OH}^{-}+NO\to {\varvec{N}\varvec{O}}_{3}^{-}+{H}_{2}$$
(23)
$${HNO}_{3}+{OH}^{-}\to {\varvec{N}\varvec{O}}_{3}^{-}+{H}_{2}O$$
(24)
$${HNO}_{2}+{OH}^{-}\to {\varvec{N}\varvec{O}}_{2}^{-}+{H}_{2}O$$
(25)

Where ‘M’ represents high-energy third bodies, energetic electrons, radiation, etc. providing an energetic environment for reactions to occurs in both the plasma phase and liquid phase.

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. The corresponding values were 3.7 and 8.3, respectively. Additionally, plasma-water interaction facilitates heat transfer from plasma to water, 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 the pH of water after treatment is in line with previous reported work by Tian et al. [61], Ma et al. [65], and Lu et al. [64], 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 Eqs. (14–25) [8, 29, 49, 56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72,73]. 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 NOx, which dissolves in water to produce NO3ˉ and NO2ˉ ions in PATW (Eqs. (16–17)) [8, 14, 23, 32, 57, 64]. Moreover, these NO3ˉ and NO2ˉ ions combine with H+ to form nitric (HNO3) and nitrous acid (HNO2) in PATW (Eqs. (18–19)) [8, 14, 23, 32, 57, 64]. The H+ ions were generated through the dissociation of H2O molecules into H radicals, also evident in the emission spectra as the Hα line (Fig. 3).

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 the observed NO3ˉ ions in PAW (O2) and PAW (Ar) were due to air impurities present at sub-atmospheric pressure. However, the formed nitrous and nitric acid were neutralized by the higher concentration of hydroxide ions, as shown in Eqs. (24, 25) [61, 71,72,73]. Moreover, the presence of hydroxide ions supports the formation of NO2ˉ and NO3ˉ ions, as shown in Eqs. (22, 23) [61, 71,72,73].

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), attributed to the highly reactive environment promoting the reaction between NO2ˉ ions with H2O2, resulting in the formation of stable NO3ˉ ions as shown in Eq. (20) (32, 56). 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, 56]. 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 (Eqs. (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)), resulting in little to no degradation of H2O2 in PATW (O2) or PATW (Ar).

Figure 5 illustrates the change in physicochemical properties (pH) and the concentration of reactive species (NO3ˉ, NO2ˉ, H2O2) over varying plasma treatment time with water.

Fig. 5
figure 5

Impact of plasma treatment duration on physicochemical properties: (a) pH, and reactive species concentration: (b) nitrate (NO3ˉ) ions, (c) nitrite (NO2ˉ) ions, and (d) hydrogen peroxide (H2O2) in PATW.

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The results demonstrate a notable increase in the pH of water when Ar or O2 was used for water activation (Fig. 5 (a)), aligning with the discussed findings above. Conversely, a decrease in the pH of water was observed when air was employed as the plasma-forming gas. This decrease in pH with prolonged plasma treatment time was consistent with the previously published literature of Sajib et al. [23], Xiang et al. [63], Shen et al. [59], 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) was 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 was observed in NO2ˉ ion concentration, where an initial rise was followed by a continuous decrease in concentration. The highest concentrations of NO3ˉ and NO2ˉ ions were observed at 10 min 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 min of plasma treatment time, a substantial decrease in the concentration of NO2ˉ and NO3ˉ ions was observed with increasing plasma treatment time. This was 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 was then removed by the vacuum pump. As a result, the declining nature of NO2ˉ and NO3ˉ ions was 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 was observed for all plasma-forming gases. This trend was also observed in previously reported works by Tian et al. [61], Lu et al. [64], 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, 49, 50]. However, when focusing on PATW (O2), a noticeable rise and fall in H2O2 concentration was observed at different flow rates, as depicted in Fig. 6 (a). The optimal concentration of H2O2 (79 mg L− 1) in PATW (O2) was 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.

Fig. 6
figure 6

Variation in H2O2 concentration in (a) PATW with oxygen (O2) gas and (b) PATW with argon (Ar) gas at different flow rates

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In contrast, this fluctuating behavior in H2O2 concentration was 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 was 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 was observed in PATW (Ar) at 76 torr gas pressure.

Fig. 7
figure 7

Variation in H2O2 concentration in PATW with respect to gas pressure and power: (a) variations in H2O2 concentration, temperature, and volume loss in PATW concerning plasma treatment time. (b) variation in H2O2 concentration with respect to plasma discharge power

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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 min, elevating the plasma discharge power from 24 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, 49, 50]. 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 was considerably higher for low Ar gas pressure (36 torr) compared to high Ar gas pressure (76 torr). However, towards the end of 120 min, 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 was 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 [49].

Fig. 8
figure 8

(a) Variations in H2O2 concentration in PATW (Ar) with varying volume and (b) time-dependent stability of H2O2 concentration in PATW (Ar)

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In Fig. 8 (b), the stability of H2O2 in PATW (Ar) over time is presented. After approximately 113 h (around 5 days) from production, a 40% decrease in H2O2 concentration was observed. However, between approximately 5 to 7 days (113 to 162 h), there was 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 [49, 59].

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 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.