Review Reports - Comparative Effect of the Type of a Pulsed Discharge on the Ionic Speciation of Plasma-Activated Water

Round 1

Reviewer 1 Report

The content of the article is of interest to the readers of the journal and the article may be published. The main novelty lies in the use of two original designs of reactors, as well as in the fact that for the first time the effect of the discharge was studied not on some model solutions, but on a real natural solution.

The reviewer does not have any significant comments on the results obtained. But their interpretation from the point of view of chemistry looks not very reasonable and contradictory.

1. The authors write that “It can be assumed that the pH growth is due mainly to the increase in the concentration of calcium and magnesium ions during the first 2 minutes of treatment”. The pH of a solution is the decimal logarithm of the concentration of H⁺ ions with the opposite sign. The causal relationship is broken. The correct conclusion from these data is that the formation of calcium and magnesium ions is accompanied by the consumption of H⁺ ions.

2. The authors claim that "The low concentration of Са⁺⁺ и Mg⁺⁺ in the reference sample (Table 2) and the slightly alkaline environment of ground water imply the presence of these ions in the solution in the form of insoluble salts, for example, calcium and magnesium carbonates. ". If so, then the solution must contain a precipitate. Nothing has been written about him. At the concentrations indicated in the table, these should not be carbonates, but bicarbonates. But bicarbonates are highly soluble in water. And the bicarbonate ion (HCO₃^-) does not contain calcium and magnesium ions. That is, its reactions cannot lead to the formation of metal ions and to an increase in their concentration. NO₃^- and NO₂^- ions by themselves cannot react with carbonates.

In our opinion, a more reasonable explanation may be as follows. Calcium and magnesium hydroxides are weak bases. Therefore, Mg(OH)⁺ and Ca(OH)⁺ ions must be present in the solution. Under the action of the discharge, nitric acid, that is, H⁺ and NO₃^- ions, should be formed in the solution. This is evidenced by the drop in the pH of the solution during the treatment of distilled water. And then the reaction Me(OH)⁺ + H⁺→ Me²⁺+ H₂O follows, as a result of which Ca and Mg ions appear. The consumption of H⁺ ions explains the observed increase in pH.

Author Response

Reply to Referee 1

Comments and Suggestions for Authors

The reviewer does not have any significant comments on the results obtained. But their interpretation from the point of view of chemistry looks not very reasonable and contradictory.

The authors write that “It can be assumed that the pH growth is due mainly to the increase in the concentration of calcium and magnesium ions during the first 2 minutes of treatment”. The pH of a solution is the decimal logarithm of the concentration of H+ ions with the opposite sign. The causal relationship is broken. The correct conclusion from these data is that the formation of calcium and magnesium ions is accompanied by the consumption of H+ ions.

We are grateful to the referee for the clarification. An extended explanation is given in the text of the manuscript (red text).

The authors claim that "The low concentration of Са++ и Mg++ in the reference sample (Table 2) and the slightly alkaline environment of ground water imply the presence of these ions in the solution in the form of insoluble salts, for example, calcium and magnesium carbonates. ". If so, then the solution must contain a precipitate. Nothing has been written about him. At the concentrations indicated in the table, these should not be carbonates, but bicarbonates. But bicarbonates are highly soluble in water. And the bicarbonate ion (HCO3-) does not contain calcium and magnesium ions. That is, its reactions cannot lead to the formation of metal ions and to an increase in their concentration. NO3- and NO2- ions by themselves cannot react with carbonates.

We are grateful to the reviewer for the comment.

We believe that initially these elements are present in solution in the form of carbonates CaCO₃ and MgCO₃. This may be due to the fact that hydroxide ions Mg(OH)⁺ и Ca(OH)⁺ are easily converted into carbonates in groundwater in air. Insoluble salts in the form of carbonates in solution do not precipitate due to the small size of (fine) particles in suspension, and determine the constant hardness of water. Further, under the discharge action, both hydroxide ions and nitric acid (i.e. H⁺ and NO₃¯ ions) can be formed, as a result of which the formation of ions is possible, including as a result of the reaction Me(OH)⁺ + H⁺ → Me⁺⁺ + H₂O. The ion formation process must be accompanied by the consumption of hydrogen ions H⁺, since an increase in the hydrogen index from 8.1 to 8.5 for the barrier discharge and to 8.6 for the bubble discharge is observed (Fig. 3). It should be noted that the question of the formation of calcium and magnesium ions under these conditions requires a separate, more detailed study. Nevertheless, it can be assumed that the pH growth is due mainly to the increase in the concentration of calcium and magnesium ions during the first 2 minutes of treatment.

However, this issue is the subject of our further more detailed studies.

An extended explanation is given in the text of the manuscript (red text).

Regards, team of authors!

Author Response File: Author Response.pdf

Reviewer 2 Report

See the attached Word file.

Comments for author File: Comments.pdf

Author Response

Reply to Referee 2

The manuscript compares discharge in gas flow versus the discharge in gas bubbles with respect to incorporation of ions into distilled and ground water. The presented results are new, especially the measurement of cations is a new topic in the plasmachemical community. However, major revision should be done according to the specified comments.

1/ The authors should provide images of the discharge and/or any other data which will show the difference between two types of discharge. Predominantly, in the first type, it should be clear whether the discharge is ignited inside the bubbles or at the interface of bubble-water.

We are grateful to the reviewer for the comment.

A description of possible options for the design of the reactor (Fig. 1) is presented in the manuscript is given, as well as explanations are given where and how the discharge plasma is formed. Both the barrier discharge and the bubble discharge have been repeatedly described in the literature, so we believe that this will be redundant.

2/ Since a new type of reactor is provided here, the measurement of electric characteristics is needed, if not available in a previous work which can be cited.

We are grateful to the reviewer for the comment.

As an example, an oscillogram of the voltage at the high-voltage electrode for the case of the discharge in air bubbles was added to the manuscript (see Fig. 2). Since the breakdown in bubbles can occur in different phases of voltage change (during the rise time, at the peak or during the fall time of the pulse), which at this stage leads to a scatter and ambiguity in the electrical characteristics, we used the thermodynamic approach to estimate the input energy. This approach, in our opinion, makes it possible to quite correctly estimate the energy input for both types of excitation. In the future, it will be possible to devote a separate study to the development of a method and a system for recording the electrical characteristics of the reactor.

An extended explanation is given in the text of the manuscript (red text).

3/ It is not discussed why the difference in NO3- concentration is so high when the difference in thermalized power between two types of discharge is only twice, at most.

This fact has indeed been discovered. We have made an attempt to give an explanation, but the exact reason, of course, at this stage of research has not been established. This is a topic for a separate study, since when we started our experiments, this fact had not yet been established.

4/ The fact that more power is dissipated in the case of ground water than distilled water is not discussed.

We are grateful to the reviewer for the comment. From the point of view of electrolytic physics, it can be assumed that ground water differs from distilled one by a significant difference in the concentrations of ions in initial solution (mineralization of water samples). For ground water, a higher concentration of ions in the solution causes its greater conductivity, and, consequently, a higher dissipation of the discharge energy in water. One more reference has been added .

An extended explanation is given in the text of the manuscript (red text).

5/ The presented pH value of 1.5 seems to be too low in comparison with the discharge energizing. The maximum presented concentration of NO3- for distilled water is 8 mg/l = 0.00013 mol/l. This concentration can be roughly equal to HNO3 and so H+ value, which results in an estimate of pH ~ 3.9. The authors should re-check the pH measurement with an independent pH-meter.

We are grateful to the reviewer for the comment and agree with it. The typo has been corrected (see red text).

6/ The great increase of Mg2+ and Ca2+ cations is the most important (and surprising) result of this work. Therefore, it should be more discussed. It is indicated that this could be an interaction with NO3-. However, the provided references does not introduce any information about this phenomenon. CaCO3 can by hydrolyzed with HNO3, but in the case of ground water, there is a pH > 8. Isn’t this a physical effect? Does the discharge disintegrate CaCO3 crystals?

We are grateful to the reviewer for the comment.

We believe that initially these elements are present in solution in the form of carbonates CaCO3 and MgCO3. This may be due to the fact that hydroxide ions Mg(OH)+ и Ca(OH)+ are easily converted into carbonates in groundwater in air. Insoluble salts in the form of carbonates in solution do not precipitate due to the small size of (fine) particles in suspension, and determine the constant hardness of water. Further, under the discharge action, both hydroxide ions and nitric acid (i.e. H+ and NO3¯ ions) can be formed, as a result of which the formation of ions is possible, including as a result of the reaction Me(OH)+ + H+ → Me++ + H2O. The ion formation process must be accompanied by the consumption of hydrogen ions H+, since an increase in the hydrogen index from 8.1 to 8.5 for the barrier discharge and to 8.6 for the bubble discharge is observed (Fig. 3). It should be noted that the question of the formation of calcium and magnesium ions under these conditions requires a separate, more detailed study. Nevertheless, it can be assumed that the pH growth is due mainly to the increase in the concentration of calcium and magnesium ions during the first 2 minutes of treatment.

However, this issue is the subject of our further more detailed studies.

An extended explanation is given in the text of the manuscript (red text).

7/ The term “physicochemical properties” in the abstract and other parts of the manuscript is used. Such a term would rather refer to thermodynamic properties, diffusion coefficients, electrochemical potential on so on. I recommend to use a term like e.g., “concentration of ions”, which will reflect also the pH and conductivity

We are grateful to the reviewer for the comment. The Abstract of the manuscript has been corrected (red text).

Regards, team of authors!

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

The authors answered all rised questions. I recommend to publish manuscript in the revised version.

Author Response

Hello dear Reviewer!

We are grateful for the comments. Minor revisions have been made to the manuscript. (see file with the tracking of revisions)