Tuning the composition of plasma-activated water by a surface-wave microwave discharge and a kHz plasma jet

The atmospheric pressure plasma jet consists of a quartz tube with the outer and inner diameters of 1.5 and 1 mm, respectively, and a copper wire of 100 microns diameter, which is inserted in the capillary and serves as the powered electrode [24]. The powered electrode is connected to a high voltage power supply, which provides a sinusoidal waveform of 28 kHz with 12 kV maximum voltage. The discharge is ignited in N₂ gas (99.996% purity), which is supplied into the capillary typically with a flow rate of 500 sccm. The purified water—commercial purified water of Pharmaceutical degree (Pharmacopoeia Europaea, Ph. Eur. 9) with pH 6.5 from the KEMIG d.o.o. Croatian company—in Berzelius beaker is brought in contact with the plasma jet by placing the water surface from the capillary orifice at distances in the 1–10 mm range. The treated volumes have been 25, 125  and 200 ml.

The surface-wave microwave discharge is generated with the help of a surfatron launcher (Sairem, Surfatron 80) in a quartz tube of outer diameter 6 mm and inner diameter 4 mm, using as a main gas Ar at gas flow rates of 1500–2000 sccm. During experiments Ar-N₂/O₂ binary and ternary mixtures are also used with the O₂ and N₂ gas flow rates ranging between 10 and 100 sccm. The input power is varied between 25 and 30 W, while the reflected power can be up to a maximum of 1 W. The quartz tube is fixed inside the surfatron in such a way, that downstream from the closing plate of the surfatron the edge of the quartz tube is at 14.5 mm distance. This insures that at the powers and gas flow rates used the plasma plume outside the tube is long enough to allow different contact points with the water surface. A Berzelius beaker of 35 ml filled with 32 ml of purified water is positioned below the plasma plume with the water surface being at 5.5, 8.5  or 10.5 mm distances from the edge of the quartz tube. The purified water is produced with the ELGA Purelab Option-R 7 purifier and is characterized by Total Organic Carbon (TOC) <20 ppb, Bacteria <1 CFU/ml, Inorganic-Typical >15 MΩcm. In order to avoid the overheating of the quartz tube during treatments, compressed air is applied along the quartz tube with a gas flow rate of 8 slm, which on the other hand has also shielding effect. The surface-wave microwave discharge being an electrodeless discharge can have a significant advantage by avoiding the contamination of liquid with nanoparticles originating from the sputtering of the electrodes, as observed in some cases [25]. Due to their advantages surface wave-microwave discharges have also been studied for biological applications [26].

The surface-wave microwave discharges have also the advantage of the high flexibility in tuning the plasma composition [27]. The surface-wave microwave discharge consists of a plasma column, characterized with a decreasing electron density profile along the column, with the critical electron density occurring at the end of it [28]. At atmospheric pressure the plasma column can extend over the discharge tube (with a slight change in the electron density profile at the exit of the quartz tube due to the change of the guiding dielectric [26]), as shown in figure 1. Due to this property, by changing the distance between the discharge tube and the water surface, different electron density can be achieved at the interaction surface. In our study, as a reference condition it is chosen the 2000 sccm Ar and 25 W discharge, with the discharge tube-water surface distance of 5.5 mm. In order to vary in the plasma plume the density of NO and NO₂ molecules (which contribute to the formation of nitrite and nitrate ions in the PAW), N₂ and O₂ gases can be added to Ar. The sustainment of the discharge and the length of the plasma column depend on the power absorption of electrons from the electromagnetic field, which is determined by the electron collision frequencies [29]. When molecular gasses are added to argon, due to the change of the power absorption, larger input power is necessary to sustain a stable discharge, while the length of the plasma column also decreases, as illustrated in figure 1. Therefore, using different Ar/N₂/O₂ mixtures and treatment distances, both the electron and the NO and NO₂ densities can be tuned at the plasma-water interaction surface.

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Table 2 shows the different treatment conditions applied and the characteristics of the generated plasma activated water. First of all, different Ar discharge conditions have been tested by changing the gas flow rate and the treatment distance. As the plasma plume is characterized by a decreasing electron density profile, by changing the treatment distance the electron density is also changed at the plume-water surface interaction point. The electrons interacting with water molecules at the gas–water interface can create OH radicals, which afterwards in the OH + OH $\longrightarrow$ H₂O₂ recombination reaction form H₂O₂. This is well illustrated by the obtained results, namely, at the lower treatment distance, where higher electron density occurs at the water surface, higher H₂O₂ concentration is obtained (condition Ar_1 versus Ar_2). By decreasing the gas flow rate (the Ar_3 condition), the plasma plume becomes slightly shorter, while more air is able to diffuse into the plume, which favoures the formation of nitrate and nitrite radicals.

Table 2.  The treatment conditions: gas flow rates, input power and quartz tube-water surface distance, and the surface-wave microwave discharge produced PAW's characteristics at different ageing moments. The PAWs could be reproduced with a maximum error of 5%.

I.D.	Conditions	t	[H2O2]	[NO${}_{2}^{-}]$	[NO${}_{3}^{-}]$	[pH]
		[min]	[mg l−1]	[mg l−1]	[mg l−1]
Ar_1.	2000 sccm Ar	0	84	6.8	92	5.7
	25 W, 5.5 mm	95	63	0.5	49	4.6
		83 567	50	0.5	40	6.6
Ar_2.	2000 sccm Ar	0	60	16	178	5.8
	25 W, 10.5 mm	90	28	0.5	55	4.4
Ar_3.	1500 sccm Ar	0	46	12	123	5.4
	25 W, 5.5 mm	93	25	1.6	61	5.1
		83 554	21	0.5	55	6.2
ArN2_1.	2000 sccm Ar-40 sccm N2	0	40	12	136	5.3
	27 W, 5.5 mm	82	36	1.3	63	4.3
		83 535	23	0.5	56	6.3
ArN2_2.	1500 sccm Ar-40 sccm N2	0	9	9.6	104	5.6
	27 W, 5.5 mm	86	9	5.6	80	5.5
		83 540	2.4	0.5	40	6.6
ArN2_3.	2000 sccm Ar-40 sccm N2	0	2	6.6	73	5.8
	27 W, 10.5 mm	81	2	5.9	79	5.4
		83 529	0.5	1.6	36	6.5
ArN2_4.	2000 sccm Ar-40 sccm N2	0	4	14	141	5
	27 W, 10.5 mm, 10 min	92	3	6.3	101	5.3
		83 488	0.5	0.5	61	5.8
ArN2_5.	2000 sccm Ar-60 sccm N2	0	17	8.7	128	5.5
	27 W, 5.5 mm	68	25	2.8	68	4.8
		83 508	15	0.5	51	6.3
ArN2_6.	2000 sccm Ar-60 sccm N2	0	3	4.9	62	5.8
	27 W, 8.5 mm	79	4	4	55	5.8
		83 520	0.5	0.5	24	6.6
ArN2_7.	1500 sccm Ar-60 sccm N2	0	3	4.5	59	6
	27 W, 5.5 mm	66	3	3.4	52	6
		83 503	0.5	0.5	22	6.7
ArN2_8.	2000 sccm Ar-100 sccm N2	0	11	6.5	73	5.8
	30 W, 5.5 mm	84	11	3.8	52	5.5
		83 484	5	0.5	29	6.4
ArN2O2_1.	2000 sccm Ar-40 sccm N2-10 sccm O2	0	20	8.8	104	5.4
	25 W, 5.5 mm	76	19	1.8	59	5.5
		83 486	16.6	0.5	53	5.8
ArN2O2_2.	2000 sccm Ar-40 sccm N2-20 sccm O2	0	37	12	128	5.3
	25 W, 5.5 mm	74	14	2.7	64	4.9
		83 484	8.5	0.5	43	5.9
ArO2_1.	2000 sccm Ar-20 sccm O2	0	80	9.3	113	5.5
	25 W, 5.5 mm	73	37	0.5	50	4.9
		83 451	43	0.5	45	5.8
ArO2_2.	2000 sccm Ar-40 sccm O2	0	75	13	122	5
	27 W, 5.5 mm	84	41	1	66	4.4
		83 388	35	0.5	57	5.9

Figure 2 shows the spectra of the plume close to the water surface in the case of two different quartz tube-water surface distances. The intensity of the OH band (the OH(A–X) transition (0, 0) bandhead at 308.5 nm) is considerably lower comparing to the N₂ bands and Ar lines intensity, suggesting, that the OH radicals are created at the gas–liquid interphase, instead of the plasma plume. The spectra show, that the emission of the Ar states excited by electrons (in the 602–760 nm spectral range) are lower at the higher treatment distance, indicating a lower electron density. Additionally, we can also observe the emission of the NOγ and NOβ bands (200–300 nm range)—which is a good indication for the presence of NO molecules—with higher intensity at the higher treatment distance. Meanwhile at the lower treatment distance, there is a marked signal of the NO₂ green–yellow continuum, showing the presence of NO₂ molecules. The dissolved NO and NO₂ molecules contribute to the creation of nitrate and nitrite radicals in PAW through the following reactions:

$$\begin{eqnarray}&&{\mathrm{NO}}_{2(\mathrm{aq})}+{\mathrm{NO}}_{2(\mathrm{aq})}+{{\rm{H}}}_{2}{{\rm{O}}}_{{\rm{l}}}\longrightarrow {\mathrm{NO}}_{2}^{-}+{\mathrm{NO}}_{3}^{-}+2{{\rm{H}}}^{+},\end{eqnarray} \tag{ 1 }$$

$$\begin{eqnarray}&&{\mathrm{NO}}_{(\mathrm{aq})}+{\mathrm{NO}}_{2(\mathrm{aq})}+{{\rm{H}}}_{2}{{\rm{O}}}_{{\rm{l}}}\longrightarrow 2{\mathrm{NO}}_{2}^{-}+2{{\rm{H}}}^{+},\end{eqnarray} \tag{ 2 }$$

$$\begin{eqnarray}&&{\mathrm{NO}}_{(\mathrm{aq})}+{\mathrm{OH}}_{(\mathrm{aq})}\longrightarrow {\mathrm{NO}}_{2}^{-}+{{\rm{H}}}^{+},\end{eqnarray} \tag{ 3 }$$

$$\begin{eqnarray}&&{\mathrm{NO}}_{2(\mathrm{aq})}+{\mathrm{OH}}_{(\mathrm{aq})}\longrightarrow {\mathrm{NO}}_{3}^{-}+{{\rm{H}}}^{+}.\end{eqnarray} \tag{ 4 }$$

Additionally, the formation of NO₃⁻ is also promoted through the NO₂⁻ + H₂O₂ + H⁺ $\longrightarrow$ NO₃⁻ + H₂O + H⁺ reaction, which results in relatively lower NO₂⁻ concentration comparing to NO₃⁻.

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By adding N₂ to Ar (conditions Ar_N₂ in table 2) the length of the plasma plume decreases, while the plasma becomes stable only at a higher 27 W power. This effect is well reflected by the lower H₂O₂ concentrations obtained with the Ar-N₂ discharges comparing to Ar discharges (see Ar_1 versus ArN₂_1 condition). Figure 3 shows the spectra close to the water surface when adding 40 sccm, 60 sccm and 100 sccm N₂, respectively, to Ar in the case of 5.5 mm treatment distance. Comparing to the spectra of the Ar discharge (figure 2) the intensity of Ar lines gradually decreases with the N₂ addition, which also indicates the lowering of the electron density. With the addition of 100 sccm of N₂ the emission intensity decreases significantly, which indicates, that the plasma plume becomes so short, that the water surface interacts with the plasma afterglow, as also illustrated by the plasma plume images in figure 1. Along the afterglow the electron density drops fast, while the density of NO and NO₂ radicals also decrease. This results, as expected, in a lower H₂O₂ and nitrite/nitrate concentrations.

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The NOγ emission is also well observed in the afterglow region due to the fact that here the excited NO(A) molecules are created mostly through the three body recombination of N and O atoms. The afterglow conditions are also approached when increasing the treatment distance in the case of 40 sccm and 60 sccm N₂ mixture discharges, resulting in lower species concentrations in the PAW: ArN₂_1 versus ArN₂_3 conditions, and ArN₂_5 versus ArN₂_6, respectively. ArN₂_6 and ArN₂_7 conditions show, that similar effect can be obtained by decreasing the gas flow rate instead of changing the treatment distance. It is also found, that the doubling of the treatment time (ArN₂_3. versus ArN₂_4. condition) practically doubles the active species concentrations in PAW.

When adding O₂ to the Ar-N₂ mixture, the discharge can be sustained at a lower power of 25 W, similar to the case of pure Ar. Due to the lower input power and the different power absorption along the plasma column, the electron density along the plasma plume is lower than in the case of the 27 W input power Ar-N₂ mixture, which is well reflected by the lower H₂O₂ concentration obtained in the PAW: ArN₂O₂_1 versus ArN₂_1 in table 2. On the other hand, when O₂ is added to the pure Ar, such in the case of 20 sccm O₂ to 2000 sccm Ar (ArO₂_1), the discharge's structure is very similar to that of the pure Ar discharge (Ar_1), which is also shown by the obtained similar H₂O₂ concentrations (and the measured spectra, not shown here, where the emission intensity of Ar I lines were found to be similar as in the case of Ar discharge), while in the case of the Ar-O₂ mixture higher nitrate/nitrite concentrations are obtained, due to the higher creation probability of the NO₂ radicals in the gas phase. With the increase of the O₂ gas flow rate in the Ar-O₂ mixture (ArO₂_2) the density of NO₂ radicals can be further increased, resulting in higher nitrate and nitrite production in PAW.

The ageing of the PAW at room temperature, already shown by the data presented in table 2, is further illustrated in figure 4 at conditions with very different initial hydrogen peroxide levels in the PAW. The figures indicate that the ageing dynamics depends on the H₂O₂ concentration, and in each case the concentrations stabilize after one week.

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The most sensitive radical is found to be the NO₂⁻, also due to its low concentration. In the case of initially high H₂O₂ concentrations the NO₂⁻ radicals recombine within the first hour, while at considerably lower H₂O₂ concentrations, figure 4(d) and (f), the recombination of NO₂⁻ slows down. At very low initial H₂O₂ concentrations the NO₂⁻ can survive more than one month. This indicates, that the main recombination pathway for NO₂⁻ could be the NO₂⁻ + H₂O₂ + H⁺ $\to$ NO₃⁻ + H₂O + H⁺ process. This reaction is found to be effective in acidic conditions (pH < 6) [30], a constant rate being reported for conditions with pH between 4 and 6 at 25 °C [31]. In the case of H₂O₂ and NO₃⁻ radicals we can observe a sharp decrease of concentrations within the first hour, when both initial concentrations are higher than about 60 mg l⁻¹, figures 4(a) and (b). At lower initial H₂O₂ concentrations (10–40 mg l⁻¹), figures 4(c) and (d), the recombination rates for both radicals become smaller, more pronouncedly in the case of H₂O₂. Furthermore, when the initial H₂O₂ concentration is lower than 5 mg l⁻¹ the H₂O₂ radicals disappear totally within one week.

Another important characteristics of PAW is the pH, which is also shown in table 2, as well as its post-discharge evolution. Comparing the different PAWs at t = 0 min, as expected based on the reactions (1)–(4), the pH correlates with the nitrite/nitrate concentrations, namely, at higher nitrate concentration lower pH is obtained. However, during the first hour of ageing, when a sharp decrease of concentrations occurs, the pH also decreases, except for the conditions where the H₂O₂ concentration is initially very low e.g. ArN₂_4 and ArN₂_6 conditions. This suggests the influence of the first phase H₂O₂ recombination on the pH, i.e. on the H⁺ ion concentration. In order to understand the complex dynamics of the system, a detailed investigation of the processes is needed, which will be the focus of a next work.

Figure 5 (a) and (b) summarizes the different PAW compositions obtained with the treatments listed in table 2 right after the treatment and after one hour of storage, respectively. As already shown in figure 4, during the first hour both the nitrate and hydrogen peroxide concentrations decrease, resulting in a considerable change of the PAW's composition. Figure 5 illustrates the possible PAWs to be obtained with a surface-wave microwave discharge. We note, that with doubling the treatment time, the doubling of the concentrations has been achieved (ArN₂_3 versus ArN₂_4). Comparing to the condition with the largest H₂O₂ in figure 5(a) (84 mg l⁻¹ H₂O₂ and 92 mg l⁻¹ NO₃⁻), higher H₂O₂ concentration and corresponding lower NO₃⁻ concentration can be achieved by further decreasing the treatment distance or increasing the input power in the case of the Ar discharge. For the given condition, the corresponding NO₃⁻ concentration can be further decreased by increasing the Ar gas flow rate, which results in lower air inflow into the plasma plume from the surroundings. On the other extreme, at very low H₂O₂ concentrations, the treatments occur close to the afterglow conditions. By further increasing the treatment distance (or decreasing the input power), only neutral species would interact with the water surface, which results in H₂O₂ free PAWs. The nitrite-hydrogen peroxide concentration map, figure 5(b), shows the limit for the H₂O₂ content of PAW where NO₂⁻ radicals can survive longer than one hour, namely in the case of H₂O₂ concentrations below 30 mg l⁻¹.

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Finally, figure 6 shows the ratio of the nitrate to hydrogen peroxide concentration as a function of ageing for the different treatment conditions as listed in table 2. By changing the treatment distance and the initial gas mixture composition of the discharge, the [NO₃⁻]/[H₂O₂] concentration ratio can be varied over three orders of magnitude from about 0.5 to 150 (we note that the lowest detection limit of H₂O₂ is 0.5 mg l^–1, thus in the ratio calculations the concentrations that are lower than that value are taken as 0.5). In the case of the treatment conditions where initially (t = 10 s) low H₂O₂ concentrations are obtained, the ageing has a more pronounced effect on the concentrations ratio, due to the strong recombination of H₂O₂, while the NO₃⁻ recombination is much slower (see figures 4(e)–(f)).

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Comparing to the surface-wave microwave discharge, the kHz plasma jet can be more easily ignited in molecular gases, such as nitrogen and oxygen, and consequently in air. For large scale applications, from the economical point of view, this system could have the advantage of using air or air like mixtures instead of more costly rare gases. Figure 7 shows the optical emission spectra of a nitrogen and an argon kHz plasma jet. In both cases the nitrogen band exhibit similar intensities, while in argon discharge a strong OH band (309 nm) emission also appears. This could be related to the difference in the electron density, which has also been reflected by the H₂O₂ concentrations obtained in PAW, namely higher concentration in the Ar discharge treated water. The emission intensity of the NOγ bands are also stronger in the argon discharge, however, due to the strong quenching of the excited NO molecules by N₂, this emission can indicate well the presence of NO molecules. In the present work the possibilities given by the nitrogen jet are investigated in detail.

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First of all, the evolution of the PAW composition with the treatment time in different treated volumes is investigated. Figure 8 shows the concentrations of the nitrate, nitrite, hydrogen peroxide, as well as the pH as a function of treatment time in the case of 25, 125  and 200 ml treated water. The concentrations have been measured at every six minutes during the treatment. It is found that during the treatment the H₂O₂ concentration increases linearly independently of the treated volume. On the other hand, the and NO₃⁻ and NO₂⁻ concentrations show a saturation at the lower 25 ml treated volume, which can be related to the recombination processes involving H₂O₂, that is produced here in higher concentration. This effect does not appear at the higher treated volumes, thanks to the lower H₂O₂ concentrations obtained during the same treatment time. The pH is found to decrease linearly with the treatment time, which is related to the increase of the nitrite and nitrate concentrations. The error bars shown in figure are obtained by averaging four independent sample treatments. The treatment of the smallest volume is the most sensitive case, the errors are due to the adjustment of the capillary orifice—water surface distance, which can slightly change also during treatment due to the water evaporation.

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Figure 9 shows the evolution of the concentrations and pH during the storage at room temperature of PAWs produced in different volumes. Similarly to the PAW produced by the surface-wave microwave discharge, the post-discharge evolution of radicals in PAW strongly depends on the absolute concentrations. Accordingly, the sharpest first phase decrease can be observed in the smallest 25 ml volume PAW, where the highest concentrations have been obtained. Especially, the NO₂⁻ concentration drops fast, due to the recombination of NO₂⁻ with H₂O₂. In the larger, less concentrated volume, with the lowest initial H₂O₂ concentration, the nitrite and nitrate recombination is more moderate, with the NO₂⁻ being preserved for months.

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In the case of the nitrogen kHz plasma jet one controlling parameter can be the treatment distance. Figure 10 shows the concentrations obtained by treating the 25 ml volume at different capillary orifice to the water surface distances. As expected, with the decrease of the distance, due to the increase of the electron density the formation of H₂O₂ radicals is strongly enhanced, while the nitrate and nitrite concentrations change just slightly. Accordingly, with this method the [NO₃⁻]/[H₂O₂] concentration ratio could be varied in the 2–12 range. However the ageing tends to equalize this ratio.

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As we have seen, the most affected radical by storage is the NO₂⁻, and its evolution strongly depends on the H₂O₂ concentration. In order to preserve the NO₂⁻, the concentration of H₂O₂ and its recombination should be controlled. One possibility is to rely on a Fenton type reaction, which controls the H₂O₂ recombination. This reaction is believed to have more pathways: (i) the Fenton M²⁺ + H₂O₂ $\to$ M³⁺ + OH + OH⁻ or (ii) the catalase M²⁺ + H₂O₂ $\to$ MO²⁺ + H₂O, MO²⁺ + H₂O₂ $\to$ M²⁺ + H₂O + O₂. In our study we use copper as a metal.

In order to test this effect, we compared the evolution of two different samples produced under same treatment conditions: (i) plasma-activated water (PAW) and (ii) plasma-activated water with a copper plate inserted during the treatment (PAW+Cu). The applied copper plate is 1 mm thick of 70 × 10 mm² size. Figure 11 shows the ageing of the samples activated with the surface-wave microwave discharge. The results illustrate, that copper contributes to the increase of the H₂O₂ recombination, while it slows down that of the NO₂⁻. In this way NO₂⁻ can be preserved longer (We note that the effect depends on the concentration of H₂O₂.). This indicates that the copper mediated H₂O₂ recombination changes the system's chemical kinetics. In order to be able to control the ageing of PAW, a deeper understanding of the chemical pathways is required, which can be the focus of a future work.

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