Faradaic Processes at the Interaction of Low-Temperature Plasma with Water and Aqueous Solutions

The paper considers the mechanism of reactions that lead to a change in the physicochemical properties of water in the case of its electrochemical activation with non-equilibrium plasma. The uniqueness of the method of the low-temperature plasma electrolysis (LTPE) consists in the fact that one electrode is placed in the liquid phase while the other is placed at some distance from the surface of the liquid, which offers new possibilities for the electrochemical method. It is shown that in the LTPE treatment of water, some destructive changes occur, which can result in the formation of peroxide and superoxide compounds with high oxidation properties. It is supposed that the mechanism of these processes is based on the appearance and functioning of a bipolar electrode at the liquid–gas interface.

Very few works have considered the mechanism of such processes for the passage of faradaic current, generated under the action of low-temperature plasma.
In this work, we study changes in the properties of water and aqueous solutions under these conditions. The processes under consideration are conventionally divided into electrochemical (transformations on the electrodes) and plasma-chemical (contact action on liquid-phase systems by ultraviolet radiation, ionization of a gaseous medium with the formation of charged particles, and ultrasonic irradiation) interactions.
The main point of low-temperature plasma electrolysis is that one electrode in the electrolyzer is in the liquid phase, and the other is placed at some distance from the surface of the solution; thus, a rather high faradaic current passes in the circuit, leading to certain oxidation-reduction reactions at the liquid-gas interface that should be according to classical electrolysis or for the action on the electrolyte of a contaminantfree plasma with arc, corona, Townsend, or barrier discharges.

EXPERIMENTAL
Distilled water with an electric conductivity of 6 × 10 -6 -1 × 10 -5 Ohm -1 m -1 , natural water with an average salt content of 280 mg L -1 (рН 7.2) corresponding to GOST 2874-82 type drinking water, as well as the sodium chloride solutions with a concentration of 0.005 mole L -1 and copper sulphate with a concentration of 0.02 mole L -1 were chosen as the study objects.
The investigations were carried out using an installation diagramed in Fig. 1a. The electric circuit of the supply source is shown in Fig. 1b. The power supply of the system was brought from an alternating current network with a current frequency of 50 ± 1 Hz and a voltage of 220 V through the alternating current control, with a limiting current value of 9 A. The transformer that stepped up the voltage had a coefficient of 7.9. The KTs201E diodes were used in the diode bridge. Ballast resistance of not less than 27 kOhm was used in the work, and it provided all the required conditions for the realization of the glow discharge. The smoothing condenser had a capacity value of 0.5 ± 5% μF.
On ignition, the plasma assumed the shape of a cone, point up, between the electrode and the electrolyte surface. About 600 V were required to maintain the discharge, and the current was maintained at a level of 0.002-0.2 A, depending on the conditions. In the main set of experiments the volume of the solution under treatment in the cell was 30 cm 3 .
Main and auxiliary cells were used in the experiment, and the vacuum space was divided to carry out tests in the volumes of 500 and 10000 cm 3 . A hydrogen-oxygen mixture with water vapor or with natural air was applied as a gas phase.
For the hydrogen-oxygen mixture, the experiment was as follows. The solution under study was poured into cell 2 and into auxiliary cell 3. Then, the system was vacuumed until the solutions in cells 2 and 3 boiled, and this was held for 5 min until the chemical decontamination of the solutions was complete. Then, a minor circle was separated from valve 9 from the main vacuum system, and in cell 3, a glow discharge was put into place as the pressure in the minor circle grew to 20.27-20.33 kPa. When that pressure was attained, the cell was switched off, the excess pressure was relieved through valve 9 into receiver 6, and the minor circle was again disconnected from the general system. After these operations, cell 2 was used to carry out the experiment. For natural air, the system was vacuumed to obtain the desired conditions, and the experiment was carried out. The classical electrolysis with insoluble platinum electrodes was carried out for comparison, using the same equipment.

RESULTS AND DISCUSSION
The functions of the change in pH of the solution and the formation of peroxide compounds in distilled and drinking water and in salt solutions are shown in Fig. 2.
As can be seen, in some cases the solution pH in the course of the low-temperature plasma electrolysis grows, and in other cases it decreases. Let us consider the observed phenomenon from the point of view of electrochemistry.
In the cathode space the following reactions occur: (1) (4) Correspondingly, in the anode space d the following reactions occur: The analysis of the mentioned reactions shows that in the case of reaction (2) in the solution under investigation the pH value is decreased due to the formation of acid (curve 4 in Fig. 2b). If reaction (2) is absent, then the solution pH remains constant due to the balance of the cathode-anode reactions after the cathode-anode spaces are mixed. In actuality, as Fig. 2b shows, for classical electrolysis in the above-mentioned solutions, the pH remains almost unchanged only at the electrolysis of distilled water. In the case of drinking water and water containing a certain quantity of NaCl, a growth trend is observed. It should be noted that in low-temperature plasma electrolysis, the trend of increase in pH is more pronounced. In no cases of conventional electrolysis was the generation of any noticeable amounts of hydrogen peroxide found.
At the low-temperature plasma electrolysis, hydrogen peroxide was generated to a greater or lesser degree in all cases, with no exceptions. Under the influence of UV radiation on the cathode there the following reactions appear: (10) Thus, hydrogen peroxide can be generated both on the cathode and in the area of the solution-gas interface as a result of the consecutive reactions: The analysis of the presented data shows that at the action of the low-temperature plasma on the water cleared from inorganic impurities using distillation method its properties differ vastly from those at the conventional electrolysis.
Thus, for the action of the low-temperature plasma electrolysis, the pH quantity of this water is abruptly decreased to a certain constant value and hereafter keeps these properties ( Table 1). The value of the specific electric conductivity grows, and it maintains these properties as well. The corrosion properties of the activated water are greatly improved.
According to the data shown in Table 1, the pH and electric conductivity of chemically pure water activated in the discharge are comparable to the similar characteristics for H 2 SO 4 0.01 M solution; however, the corrosiveness of water is higher than that of the sulfuric acid 0.01 M solution. The analysis of the data, found due to the corrosion measurements using voltammetry and presented in Table 2, shows that for the activated water this value is 1.5-2 times higher than for the H 2 SO 4 solution. The acquired properties of the activated water lead to the expectation of some changes in its structure.
However, as Fig. 3 shows, the chemical shift in the hydrogen protons before and after the treatment of distilled water remained almost unchanged. This is related to the fact that the amount of the agent generated in water is lower than a threshold quantity detectable with this method, which can change the structure of the water under investigation. Nevertheless, some changes in the spectra are seen that can be explained from the appearance of hydrogen peroxide. As the analysis of data in Fig. 2 shows, the quantity of hydrogen peroxide produced under certain conditions is not proportional to the process time, and after some time the rate of its generation falls sharply, which makes measurement difficult.
Thus, the quantity of the products in the weight ratio to the total volume is not large in the course of the low-temperature plasma synthesis, and one can deduce that the change in the structure of the total solution weight is hardly possible, especially, that the nature of the appearing bonds does not greatly differ from those exist. Taking into account that the acquired properties of the activated water are rather steady one can determine its main electrochemical characteristics. Table 3 presents the values of transfer numbers, mobility, and dissociation constant of the formed products under the action of the low-temperature plasma electrolysis.
Comparing the mobilities of the positively charged particles with the table data, it is possible to deduce that a hydrogen proton can act as a proton of the compound generated at the electrolysis. The anion-active particle greatly differs in its characteristics (mobility and transfer number) from the ОНanion and approaches , thus, the formation of anions, typical for the generation of the over-peroxide compounds of alkaline and alkaline earth metals, can be suggested. Such is also shown by the comparison of Thus, for the low-temperature plasma electrolysis of distilled water, the generation of the Н 2 О 3 and Н 2 О 4 type of over-peroxide compounds can be suggested, along with hydrogen peroxide. One possible mechanism of these reactions is expressed by the following scheme: It should be underlined that the reaction of generation of Н 2 О 4 and its subsequent dissociation plays a leading role in the acidification process: (18) Thus, when chemically pure water is treated with the help of the low-temperature plasma electrolysis destructive changes in it proceed, which result in the generation of peroxide and over-peroxide compounds with high oxidizing properties. It is likely that the mechanism of these processes is based on the appearance and operation of a bipolar electrode.
The same results are confirmed by the measurements of the formal oxidation-reduction potentials (ORP) of the electrolyte treated with the help of the LTPE. The measurements were carried out on the platinum electrode in reference to the silver chloride electrode in the cell with separated cathode and anode spaces.
It was found that initially the ORP quickly grows, attains a value of 1.2-1.3 V, and then it becomes slower; the variation depends both on the quantity of the polarizing current and the electrolyte nature. The ORP grows the most for distilled water. In particular tests the ORP values for distilled water were high enough (about 1.5 V).
Let us consider the possible appearance and operation of a bipolar electrode at the water-gas interface (Fig. 4). When an electric field is imposed between the cathode and the anode the water molecules at the interface are oriented in accordance to the data in Fig. 4a. Then the surface molecules dissociate according to the following equation: (19) with the formation of a double layer insert (Fig. 4b).
Finally, after a bipolar charged surface is formed, the ОН − ions emit electrons into the gaseous phase (Fig. 4c). Here, the reaction (20) is possible, as are reactions (14)  in the water pH at the low-temperature plasma electrolysis.
The formation of a bipolar surface allows for the conclusion that a new type of the second kind of electrochemical fluid electrodes exists, which appears due to a specific process but only in cases where a particular field intensity is achieved.

CONCLUSIONS
Thus, for the LTPE activation of water there proceed destructive changes in it that cause the formation of oxide and over-peroxide compounds with high oxidizing properties. The mechanism of these processes is based on the formation and operation of a bipolar fluid electrode, which can be effectively used at the realization of other processes in discharges.