Investigation of electrical conductivity and temperature
Active species and ions created during plasma exerting were dissolved in water and changed the EC of the water. Electrical conductivity and temperature changes due to plasma exerting four gases were measured for NH and H waters. Figures 2 and 3 show electrical conductivity and temperature during time change for NH water. Figures 4 and 5 are for H water data. The 0 min is the time before plasma exerting in all plots. As shown in Fig. 2, electrical conductivity in all plasmas and times was more than before plasma exerting, and the process of its change was ascending and descending at different periods. The increase in electrical conductivity was caused by entering nitrogen oxide and hydrogen oxide byproducts into the water due to ionizing ambient air, water vapor, and feed gas [11].
It appears that the increasing concentration of reactive ions in water has reduced the electrical conductivity of water in a short amount of time. In other words, making the water more electrically conductive, the diffusion layer near the electrodes with a concentration differing from its value in the volume of water has reached saturation and blocked the entrance of more ions into the water. The escape of ions as reaction products from the diffusion layer in water under the effect of the electric field and chemical potential gradient owing to the difference in concentration, or the consumption of ions, has disrupted the saturation state [12]. This process has been repeated several times for EC. According to Fig. 3, the temperature changes were ascending and descending at various periods and also higher than before plasma exerting.
For all plasmas, percentages of temperature and electrical conductivity variations in NH water in each period have been compared to before plasma in Table 1.
Table 1
Percentages of temperature and electrical conductivity variations in NH water in each period compared to before argon, nitrogen, air, and oxygen plasma.
\(\varDelta \left(\mathbf{t}\right)\left(\mathbf{m}\mathbf{i}\mathbf{n}\right)\)
|
\({\varDelta \left(\mathbf{T}\right)}_{\mathbf{A}\mathbf{r}}\left(\mathbf{\%}\right)\)
|
\({\varDelta \left(\mathbf{E}\mathbf{C}\right)}_{\mathbf{A}\mathbf{r}}\left(\mathbf{\%}\right)\)
|
\({\varDelta \left(\mathbf{T}\right)}_{{\mathbf{N}}_{2}}\left(\mathbf{\%}\right)\)
|
\(\varDelta {\left(\mathbf{E}\mathbf{C}\right)}_{{\mathbf{N}}_{2}}\left(\mathbf{\%}\right)\)
|
\(\varDelta {\left(\mathbf{T}\right)}_{{\mathbf{O}}_{2}}\left(\mathbf{\%}\right)\)
|
\(\varDelta {\left(\mathbf{E}\mathbf{C}\right)}_{{\mathbf{O}}_{2}}\left(\mathbf{\%}\right)\)
|
\(\varDelta {\left(\mathbf{T}\right)}_{\mathbf{A}\mathbf{i}\mathbf{r}}\left(\mathbf{\%}\right)\)
|
\(\varDelta {\left(\mathbf{E}\mathbf{C}\right)}_{\mathbf{A}\mathbf{i}\mathbf{r}}\left(\mathbf{\%}\right)\)
|
0.5
|
5.42
|
8.92
|
13.72
|
24
|
2.89
|
11.08
|
14.08
|
11.38
|
1
|
24.55
|
15.38
|
31.77
|
24
|
20.22
|
20.62
|
26.71
|
16.62
|
1.5
|
40.79
|
23.69
|
44.40
|
36
|
41.16
|
18.15
|
51.99
|
25.84
|
2
|
48.01
|
25.85
|
58.84
|
42.77
|
41.88
|
20.62
|
59.21
|
31.08
|
2.5
|
62.45
|
42.15
|
67.87
|
44
|
76.17
|
13.85
|
88.09
|
45.23
|
3
|
21.3
|
37.85
|
82.31
|
32.62
|
85.92
|
19.69
|
107.94
|
40.31
|
3.5
|
93.14
|
50.46
|
93.14
|
50.15
|
97.11
|
11.08
|
106.14
|
16.62
|
4
|
103.97
|
39.08
|
103.97
|
45.54
|
105.05
|
7.08
|
106.14
|
54.15
|
As shown in Figs. 4 and 5, EC and T changes in H water were increasing at all periods and plasmas in comparison to the prior periods and before plasma exerting.
Stirring and lack of saturation in the water diffusion layer were the causes. Electrical conductivity has altered with temperature variations in all plasmas. Increasing the temperature of the water has also raised the ions' mobility and the number of ions due to the separation of molecules in water.
For all plasmas, percentages of temperature and electrical conductivity variations in H water in each period have been compared to before plasma in Table 2. Plasma-activated water (PAW) generated by cold atmospheric plasma (CAP)-water interaction employing controllable parameters has been reported to have higher conductivity [13]. According to the results of the significant increase in conductivity for H water, and its high rise in PAW reported in several study cases, H water was selected for further investigation.
Table 2
Percentages of temperature and electrical conductivity variations in H water in each period compared to before argon, nitrogen, air, and oxygen plasma.
\(\varDelta \left(\mathbf{t}\right)\left(\mathbf{m}\mathbf{i}\mathbf{n}\right)\)
|
\({\varDelta \left(\mathbf{T}\right)}_{\mathbf{A}\mathbf{r}}\left(\mathbf{\%}\right)\)
|
\({\varDelta \left(\mathbf{E}\mathbf{C}\right)}_{\mathbf{A}\mathbf{r}}\left(\mathbf{\%}\right)\)
|
\({\varDelta \left(\mathbf{T}\right)}_{{\mathbf{N}}_{2}}\left(\mathbf{\%}\right)\)
|
\(\varDelta {\left(\mathbf{E}\mathbf{C}\right)}_{{\mathbf{N}}_{2}}\left(\mathbf{\%}\right)\)
|
\(\varDelta {\left(\mathbf{T}\right)}_{{\mathbf{O}}_{2}}\left(\mathbf{\%}\right)\)
|
\(\varDelta {\left(\mathbf{E}\mathbf{C}\right)}_{{\mathbf{O}}_{2}}\left(\mathbf{\%}\right)\)
|
\(\varDelta {\left(\mathbf{T}\right)}_{\mathbf{A}\mathbf{i}\mathbf{r}}\left(\mathbf{\%}\right)\)
|
\(\varDelta {\left(\mathbf{E}\mathbf{C}\right)}_{\mathbf{A}\mathbf{i}\mathbf{r}}\left(\mathbf{\%}\right)\)
|
0.5
|
8.30
|
7.38
|
17.33
|
17.85
|
8.30
|
7.38
|
11.91
|
8.92
|
1
|
26.35
|
11.38
|
33.57
|
25.85
|
34.30
|
17.54
|
24.55
|
20
|
1.5
|
38.99
|
16.92
|
49.82
|
34.15
|
46.57
|
23.69
|
48.01
|
30.15
|
2
|
53.43
|
23.38
|
67.87
|
38.46
|
62.45
|
30.77
|
64.26
|
37.85
|
2.5
|
66.06
|
33.84
|
76.90
|
44.92
|
84.12
|
40.62
|
84.12
|
45.23
|
3
|
84.12
|
42.15
|
87.73
|
52.92
|
90.25
|
51.08
|
100.36
|
52.31
|
3.5
|
98.56
|
48.62
|
103.97
|
57.23
|
109.39
|
61.23
|
109.39
|
59.08
|
4
|
109.39
|
56.92
|
113.00
|
61.85
|
141.52
|
80.62
|
127.44
|
71.08
|
Investigation of pH and concentration of hydrogen and hydroxide ions
Although measuring and analyzing pH levels is one of the major metrics to certify the standards of the water industry, it can play a fundamental role across a wide range of industries including the food industry and agriculture. The pH standing for the power of hydrogen describes the concentration of hydrogen ions in a solution. Figures 6 to 9 display the pH, hydrogen cation, and hydroxide anion concentration variations of H water for each plasma as a function of time. As can be seen in Fig. 6(a) for argon plasma, there was a low rise in the sample's pH after 0.5 min. It decreased in 0.5–1.5 min, increased at 2 min, and then decreased at 2.5 min. The pH went up at 3 min and reduced again with a low slope at 4 min. However, the water was acidic during the whole experiment. Figure 6(b) shows that the concentration of hydrogen cation was always more than hydroxide anion in argon plasma.
According to Fig. 7, the water had higher concentrations of hydrogen cations than hydroxide anion and was acidic due to the exerting of nitrogen gas plasma. After plasma exerting for 0.5 min, a rise in pH was seen. It went down at 1min and went up at 2 min. It decreased at 2.5 min. After increasing at 3 min, there was a decline at 3.5 min and ultimately rise at 4 min.
In Fig. 8, air plasma has increased the concentration of hydrogen cation relative to hydroxide anion and acidified the water. The 0.5 min of electric discharge resulted in a pH drop. The changes trend was upward in 0.5-2 min and downward in 2–3 min. It rose at 3.5 min and then fell at 4 min.
The results of oxygen plasma on the pH and concentrations of hydrogen cations and hydroxide anions are shown in Fig. 9. It has made the water acidic. The 0-1.5 min of electric discharge resulted in a pH decrease. The pH increased at 2 min. The changes trend was downward in 2.5-3 min and upward at 3.5 min. It reduced at 4 min.
The prior stated results of this study were in agreement with the acidic values pH of PAW generated by CAP in previous reports [14–18]. So, an attempt was made to create a possibility to increase the water pH with the same setup instead of decreasing it. The tap water was filtered with inert argon gas before exerting the plasma to remove other gases inside the water. Oxygen gas, which caused the most acidic property in water, was selected for plasma production.
The results of oxygen plasma after filtering are shown in Fig. 10. As can be seen in Fig. 10(a), pH increased after 0.5 min of electric discharge and subsequently decreased at 1 min. The pH enhanced at 1.5 min, reduced at 2-2.5 min, then climbed again in 3-3.5 min and finally went down at 4 min. According to Fig. 10(b), water had more hydroxide anion than hydrogen cation in the whole experiment, indicating that oxygen plasma behaved differently from before. The composition and reaction of the active species in oxygen plasma led to basic water after clearing. As shown in Figs. 10(c) and (d), EC and T changes at all periods obtained more than the prior periods and before plasma exerting. For oxygen plasmas, percentages of temperature and electrical conductivity variations after filtering in each period have been compared to before plasma in Table 3.
Table 3
Percentages of temperature and electrical conductivity variations in H water in each period after filtering compared to before oxygen plasma.
\(\varDelta \left(\mathbf{t}\right)\left(\mathbf{m}\mathbf{i}\mathbf{n}\right)\)
|
\({\varDelta \left(\mathbf{T}\right)}_{{\mathbf{O}}_{2}}\left(\mathbf{\%}\right)\)
|
\({\varDelta \left(\mathbf{E}\mathbf{C}\right)}_{{\mathbf{O}}_{2}}\left(\mathbf{\%}\right)\)
|
0.5
|
5.415
|
10.15
|
1
|
15.52
|
26.769
|
1.5
|
24.548
|
44.92
|
2
|
33.57
|
49.538
|
2.5
|
49.819
|
52.92
|
3
|
69.675
|
60.30
|
3.5
|
89.53
|
63.69
|
4
|
112.996
|
74.46
|
As illustrated in Fig. 11, the plasma exerting resulted in the generation of reactive oxygen species (ROS), reactive nitrogen species (RNS), and reactive hydrogen species (RHS) in the water due to direct interactions and several indirect cascade phenomena including liquid evaporation, molecules collision, mass transfer, sputtering, and ultra-violet radiation at the plasma phase, plasma-water interface, and liquid phase [19].
Table 4 displays the common expected reactions that resulted in acidic and basic characteristics of tap water. Ar, N2, Air, and O2 symbols, respectively, denoted the major reactions of argon, nitrogen, air, and oxygen plasmas. The NOx species were generated by argon, nitrogen, air, and oxygen (before filtering) plasmas interacting with water and water vapor. The hydrogen cations making acidic qualities in water were created by the interaction of NOx with hydrogen and oxygen species in water. The feed gas has a significant impact on the quantity of OH• radical generated in water, with oxygen plasma producing most of it [20]. As a result, this radical produced more hydroxide anion than hydrogen cation in the oxygen glow discharge (after filtering) in reaction with the electron, which led to the basic property of water. In all plasmas, hydrogen gas around the cathode and oxygen gas in the vicinity of the anode was produced [8, 21–22].
This research showed that in using the capacity of CAP, a small variation in reactor design can make a different property in water under plasma exerting depending on the application type compared to the state before. Here, water filtering with argon gas before exerting oxygen plasma made water basic instead of acidic which is different from previous reports [38]. According to these characteristics, cold plasma can be introduced as an emerging technology compatible with the environment, which can have a unique position in modern applications required by societies, including improving agricultural methods and food industries (increasing shelf life and quality characteristics of fresh products, seed germination, and plant growth), health and medical usages (anti-infection of medical equipment, treatment of skin, digestive, and cancer diseases), and water industry (urban and industrial water and wastewater treatment, electrolysis, and hydrogen fuel) by forming RHS, RNS, and ROS, and changing electrical conductivity and the chemical composition of water [19, 39–58].