Water is known to be the most abundant chemical compound in the Universe. Due to its properties, water, at least on the Earth surface, does not exist as a pure hydrogen-oxygen (H₂O) compound. In scientific terms, water is an aqueous solution with a complex molecular structure and a system containing various inorganic, organic or gas substances. Recently, the structure and properties of water at the molecular level have become again a subject of discussions thanks to the availability of new measurement techniques and advanced computational methods [1] [2] [3] [4] [5].

Knowledge of the properties of water (water solution) at the molecular level controlled and forced by external factors allows not only better understanding of the impact of water on living organisms at the cellular level, but also permits deeper insight into the processes in living organisms. These two factors: the cluster structure of water forced by the formation of hydrogen bonds between H₂O molecules and the solubility of chemical compounds and gases in water, modify the water properties in response to physical and chemical external factors [6] [7] [8]. Recently, thanks to the progress in plasma technology, changes in the properties of water (aqueous solutions) induced by external factors are mainly studied by the use of plasma [9] [10] [11]. Non-thermal plasma (NTP) is used to induce complex oxidation processes in water solutions and has been increasingly often used to change the properties of water and for wastewater treatment [12] [13] [14] [15] [16]. The impact of plasma on the properties of water is realized through the induced electrical discharges in water, in air above the water level or simultaneously in water and in gas [17] [18] [19]. Lacke et al. [20] have shown the effect of interaction of non-thermal plasma with water as direct in situ generation of many types of highly reactive chemical species (radicals), quenching in the gas phase and liquid phase radical reaction products, generation of UV light and shock waves. Chwastowski et al. [21] have presented methods of water treatment with low-temperature, low-pressure glow discharge plasma. The same authors have described the glow discharge plasma treated water under anaerobic nitrogen. This operation produced nitrogen in the water in different excited states depending on the treatment time. The excited nitrogen molecules built up water clathrates around them. The number and structures of these clathrates depended on the duration of treatment with the glow discharge plasma. Ciesielska et al. [22] have reported the treatment of CO₂-saturated water using low-temperature, low-pressure, low-frequency glow plasma. The macrostructure of the water treated with this method consisted mainly of tetrahedral and deformed stabilized tetrahedral structural units with long-lived free radicals. Hiratsuka et al. [23] have presented application of sound waves for the improvement of water treatment process. In that study, various sound waves such as: ultrasonic wave, music box, and wind-bell were used to water and wastewater for removing contaminants. As a result, a possibility of improvement of water and wastewater treatment process using sound waves with various frequencies was proposed. Implementation of a similar approach by Acoustic Wave Separation, led to industrial water cleaning process using a pattern of ultrasonic waves to exert acoustic forces on hydrocarbons and other water contaminants [24] [25].

In a previous paper, we presented the apparatus for inducing changes in water properties in closed vessels as a result of treatment with plasma generated by pulsed direct current glow discharges, hypothesizing that the action of plasma on the structure of water consists in resonant excitation of water aggregates [26]. The method presented in the above mentioned paper is characterized by the transmission of electromagnetic radiation generated by plasma maintained by pulsed direct current glow discharge formed in special conditions. The impact of plasma is exerted through a dielectric and not necessary transparent shield and results in changes in the composition of aqueous solutions in closed vessels induced in a contactless manner. Previous methods of modifying the properties of water solutions with low-temperature plasma have been focused on the treatment of a certain amount of water in vessels of volumes not exceeding a few liters.

In this work, we present for the first time an efficient flow-through method that allows changes in water properties in flow as a result of treatment with plasma generated by pulsed direct current glow discharges which can efficiently produce a wideband electromagnetic noise. In order to assess the influence of the process on the physicochemical parameters of the solutions, different methods of analysis were applied. The method described above can be used to improve water solution and wastewater physicochemical parameters on an industrial scale, and in particular, to enhance absorption of different gases for example carbon dioxide, oxygen and acetylene.

A highly efficient stochastic-resonance system to change the parameters of water or water solutions was built. The systems permitted the treatment of 4000 l of water per hour. The construction of the flow system, plasma noise-generating lamps and the control system has been presented in the details in the experimental part. The flow stochastic-resonant system for changing the parameters of water or aqueous solutions is based on the use of broadband electric noise generated in the plasma and transmitted in the contactless manner through the dielectric wall to the flowing water. Stochastic resonance is the phenomenon that occurs in dynamical systems whose response to an external signal reaches the optimal value in the presence of noise of certain intensity [30].

Stochastic resonance refers to the enhancement of a weak periodic signal by addition of an external white noise representing a wide range of frequencies. When the sum of the subthreshold periodic signal and the noise amplitude crosses the threshold, the excitable system responds with a burst signal [31] [32]. The stochastic system uses a plasma discharge noise-generating and noise transmitting ability in a wide frequency range. In the stochastic system, the energies corresponding to the frequencies of the generated noise are in resonance with those of vibrations of different water structures, different chemical species and molecules contained in water.

Plasma noise can be easily modified by adding components present in the treated water as seeds to the plasma gas and thus obtaining a coherent resonant energy coupling. Such a system in terms of energy is optimal from the point of view of resonant energy transfer to water. An important benefit is that very significant changes to the structure of the water can be made as a result of exposition to noisy plasma, while the energy consumption by the entire treatment process may be really low. The questions that arise at this point are what kind of water/aqueous solution is the most susceptible to the stochastic resonance interaction and which analytical methods should be used to permit experimental observation of the changes that occur in the solution under the influence of the plasma discharge.

Three types of water were tested in the constructed system: super-clean water, tap water and spring water from the intake in Besko. The water before and after treatment in the flow-through apparatus was analyzed in accordance with the applicable standards for the contents of basic components (Table 1).

Table 2 shows the values of FWHM of the signal of water were determined by ¹⁷O NMR measurements. The basic physicochemical tests of water before and after plasma treatment revealed that in the case of super-clean water, apart from a slight but measurable increase in the pH of the solution, no significant changes in the other water parameters were observed.

For the samples of tap water and the water from the Besko intake, containing of calcium and magnesium compounds, an increase in the value of pH was observed. The value of the permanganate index and the content of dissolved anions-nitrates, used as a parameter in assessment of water quality, also increased for both types of examined water. Moreover, a significant decrease, by about 10%, in the value of electric conductivity and an ascending trend in the value of oxidation-reduction potential (ORP) were detected. A slight increase in the ORP value of water after the water treatment process proves an increase in the oxidizing properties of the water solution. The increase in ORP value, depending on the type of water, varied in the range 5% - 17% confirmed the results of our previous studies, proving the lack of free radicals generated in the process of stochastic resonance water treatment. The FWHM of the ¹⁷O NMR signal in the spectra of the tested water samples increased from 76 - 86 Hz to 109 - 114 Hz after the plasma treatment. On the other hand, after saturation of water treated with plasma with CO₂ the FWHM of the ¹⁷O NMR signal decreased to the value of 48 - 52 Hz. The water molecules in aqueous solution formed aggregates/clusters thanks to the connecting ability of hydrogen bonds. The sizes of the H₂O molecular clusters are continually changing and strongly depend on the presence of ions (H⁺ and Mⁿ⁺) or neutral particles in the solution [33] [34]. Li et al. [35] have reported a decrease in the FWHM values of ¹⁷O-NMR spectral line of pure water from 123.94 Hz to 65.13 Hz, and correlated these values with the strength of water molecular hydrogen bond. Lei et al. [36] have studied the influence of nanoparticles on the FWHM of the ¹⁷O NMR signal, showing that the added nanoparticles can effectively weaken the hydrogen bonding between water molecules, changing the network structure.

The nanoparticles ability to weaken the association of molecules (atoms) via hydrogen bonds formation is manifested by a reduction in the FWHM of the ¹⁷O NMR signal. The FWHM values of the H₂O peak in the ¹⁷O-NMR spectra reflect the velocity of molecular movement, which is directly related to the degree of association of the molecules.

As mentioned by Hiraoka et al [37], it has been generally accepted that the sizes of H₂O molecules clusters cannot be evaluated on the basis of the ¹⁷O NMR spectra as the correlation between the H₂O cluster sizes and the spectral data also depends on other factors. The width of the bands depends on the pH, the degree of association of molecules, ions, neutral molecules and nanoparticles added to water solution. The results obtained in our study refer to a situation in which, as a result of the interaction of water with plasma radiation, an increase in the pH value of the solution and changes in some basic physicochemical parameters of water are observed, Table 1.

Raman spectroscopy (RS) is widely applied for qualitative and quantitative analysis of solid, liquid and gas samples. This method is based on the phenomenon of electromagnetic radiation scattering associated with the change in energy of scattered photons. The mentioned change in energy of the scattered photons provides information about the oscillating energy levels of the test substance. Raman spectra contain vibrational bands corresponding to the normal vibrations of molecules. In this study, Raman spectroscopy was used to analyze changes in the water from the Besko intake appearing as a result of flow through the plasma reactor. Figure 3 shows the Raman spectra of the water sample from the Besko intake before and after treatment in the reactor. The spectra of this

sample of water recorded before and after the treatment differed only in the region of 3150 - 3350 cm⁻¹.

In the Raman spectrum, the peaks at 3250 cm⁻¹ unambiguously indicate the presence of the stretching vibrations of the O-H bands in the fully hydrogen-bonded water molecule that forms tetrahedron-like structure by connecting other four water molecules through hydrogen bonds.

The Raman peaks between 3300 cm⁻¹ and 3630 cm⁻¹ are ascribed to Raman scattering of partly hydrogen bonded water [38] [39] [40]. In the water after treatment in the flow reactor, the number of water molecules with tetrahedron-like structure formed by connecting other four water molecules through hydrogen bonds decreases.

The Electrospray Ionization Mass Spectrometry (ESI MS) method was used to study the water samples before and after exposure to radiation produced by the plasma discharge. ESI mass spectra at various cone voltages recorded in positive mode (m/z 2 - 300) of super-clean water, tap water and the water from Besko intake before (a) and after (b) plasma treatment are shown in Figures 4-6.

The ESI mass spectra at various cone voltages recorded in positive mode (m/z 2 - 300) of super-clean water, tap water and the water from Besko intake before (a) and after (b) plasma treatment with formic acid (1:1) are presented in Figures 7-9.

The ESI MS spectra of water samples before and after the plasma treatment, depending on the degree of water purity, show signals from the protonated forms of water aggregates [H₂O + H]⁺, calcium and magnesium compounds and their hydrated forms. In the case of pure water, an increase in the intensity of signals [H₂O+H]⁺ m/z 19 and 37 at cv = 5 - 15 V is observed, while for the remaining water samples with a high concentration of calcium and magnesium salts, an increase in the intensity of the signals of non-solvated compounds is observed (m/z 41 and 85).

In the previous paper [26], on the basis of ESI MS study of water samples before and after plasma treatment, we showed that this process significantly affects the solvation of ions in a solution. In this paper we examined the following aqueous solutions: super clean water: formic acid (1:1), tap water intake: formic acid (1:1) and water Besko intake: formic acid (1:1) before and after plasma treatment in a batch reactor, using the ESI MS method.

For super-pure water after the treatment with plasma in the technological process studied, with the addition of formic acid, the ESI MS spectrum showed only a slight increase in the intensity of the m/z [HCOOH + H]⁺ 47 and [2xHCOOH + H]⁺ 93 signals with respect to those of untreated water.

However, in the spectra of the tap water and the water from the Besko intake, a significant increase in intensity of the signal assigned to low-molecular solvated metal ions is observed, with a simultaneous decrease in intensity of the signals corresponding to better solvated species.

Water subjected to the treatment with plasma is characterized by a different

way of ion solvation with a smaller number of water molecules in the aqua complexes than in those made with the untreated water. The process of treatment with plasma changes the ability of water to solvation of ions present in the treated water samples, which is manifested by the m/z signals in the recorded ESI MS spectra. The water samples after the treatment in the flow-through reactor show a specific ability to dissolve gases, which is different from that of the water before the treatment. The fractions of dissolved carbon dioxide, carbonic acid and carbonate in the water samples as a function pH are shown in Figure 10 [42] [43].

The pH of the solutions after CO₂ saturation was 5.6 - 5.8, at this pH, CO₂ is present in water mainly in the form of gas. The solubility of CO₂ gas in water was measured as a function of temperature at atmospheric pressure (Table 3 and Figure 11).

The solubility of CO₂ in the water treated with the plasma compared to the starting water differs at higher temperatures, the effect increases with temperature, e.g., the solubility of CO₂ is lower by 30% - 40% at 25˚C.

Gas oxygen dissolves in water. This process is of fundamental importance for biological life in the aquatic environment. Oxygen dissolves in water because the polar charges in the water molecule induce a dipole in the oxygen molecule. When water and oxygen molecules meet, the negative dipole of water repels electrons around the oxygen molecule, creating a temporary dipole in the oxygen molecule. Solubility of different gases in water was studied when gas was fed into water at 4˚C, at a pressure of 7 bar for CO₂ and O₂ and 2.2 bar for acetylene, respectively, followed by decompression at a pressure of 993 hPa (Table 4).

The solubility of CO₂ is 0.2871 g/100g in water [43] [44]. Under conditions of elevated pressure, the solubility of CO₂ in the water treated with the plasma increases sharply compared to the starting water (e.g., 3.6 times at 5˚C). Under these conditions, water can be used to absorb CO₂.

Gas oxygen molecule O₂ is symmetrical and non-polar, but it can be the recipient of several intermolecular attractions: hydrogen bonding, Debye forces and London dispersion forces. The solubility of oxygen thus depends on the interaction of gas oxygen with water dipoles. An important role in the increased solubility of gas oxygen under increased pressure in the water samples after the

treatment with plasma play the voids in the water structure that can be filled with oxygen.

Molecular oxygen in liquid water becomes a dipole upon interaction with water molecules. The presented technological process significantly increases the interaction of gas oxygen with water, making it possible to obtain highly oxygenated water without the presence of reactive oxygen species. The solubility of oxygen at 4˚C is 0.006232 g/100g in water [44]. The solubility of oxygen in water after the action of white plasma noise is unexpectedly very high (100 times highs) and surprising.

The water solubility of acetylene (gas) was also tested. Acetylene is only slightly soluble in water, 0.120 g/100g at 25˚C [44] [45]. Tzeli et al. [46] have presented a theoretical study of the interaction between acetylene and water molecules, which is very weak.

Acetylene in water dissolves under high pressure by being trapped in the liquid structure.

Using this precondition of introducing gas under increased pressure, higher amounts of gas have been dissolved in water. We hypothesize that at high gas pressures the introduction of gas to water after plasma treatment is readily enhanced as gas molecules can be deposited in the spaces of the altered water structure.

The ORP values of water before and after gas absorption were measured. These values change after CO₂ absorption from 135 to 140, after oxygen saturation from 135 to 170, while for acetylene from 135 to 115. This parameter may be a marker of the process of water saturation with gases.

We present a flow plasma reactor permitting modification of the physicochemical properties of water/aqueous solutions by stochastic resonance amplification of vibrations of selected chemical species in water with electromagnetic noise generated during plasma discharge. The impact of the treatment on the properties of water was characterized by a number of methods. The basic parameters of water quality for the samples of super-pure water, tap water and the water from the intake in Besko before and after the process in the plasma reactor were measured and compared. In addition, ¹⁷O NMR (FWHM), Raman spectroscopy (RS) and electrospray ionization mass spectrometry (ESI MS) methods were used to determine differences between the raw water samples and plasma-treated ones. The water samples after the treatment in the apparatus proposed were characterized by much different gas absorption properties (examples provided for CO_2, oxygen and acetylene) as a function of temperature and pressure, which allows the use of the water after treatment in industrial processes. Worth stressing are both, the great capacity of the new reactor (4000 L/h) and the appreciably low energy consumption (20 MJ) spent for the treatment of that amount of solution. Further research on gas absorption will be carried out with other gases important from the point of view of industrial and laboratory applications.

The authors declare no conflicts of interest regarding the publication of this paper.

Yelkin, I., Reszke, E., Binkiewicz, G. and Schroeder, G. (2021) The Industrial Production of Water Dedicated to Absorption of Gases. Journal of Water Resource and Protection, 13, 632-653. https://doi.org/10.4236/jwarp.2021.138034