Obtaining Iron Chelates and Iron Oxide Nanoparticles via Multispark Discharge Treatment of EDTA Solutions in Argon Atmosphere

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Comment:

1. The authors mentioned that many metallic residues were found in the liquid solutions. These residues were obviously from the erosion of the electrode materials (stainless steel) in the multi-electrode ring. It is known that after the prolonged usage of the discharge between electrodes (typical gap = 2mm) may not be the same as measured in previous experiments. Has the peak voltage in the discharge been monitored for stability?
2. While question 1 is concerned with the problems in the erosion of the metal materials of the electrodes, there is a new question regarding the liquid solution. The authors foresee the role of their results in future applications. However, if you are doing clean solution chemistry, it is not recommended to let metal particles mix with the solution. Due to the tiny size of metal particles, it is very easy for these metal particles to catalyze some reactions in the solution, which makes your experimental data qcom[licated and unreliable. It is good to know that perhaps EDTA can remove metal ions under a certain pH range. However, there is a limit. Not all reactions react in one way only; there is always an equilibrium between reactants and products. Thus, there is always a certain amount of reactants and products present.
3. The above issue in solutions generates a 3 rd question. Since the electrode surface composition may have become nonuniform due to metal corrosion, the newly produced metal particles may have a different composition from those produced in the beginning of the experiment. Thus the amount of EDTA and pH values usually used may not be able to remove all sort of metal particles completely as before. The optimal amount of EDTA used every time has to be recalibrated.
4. Usually, if we say the use of plasma treatment of water, it often emphasize the  better efficiency of removing contaminants and improving water quality than conventional methods. But now, before the plasma treatment, the water could have been very pure, but after the ring electrode discharge, metal particles appear in the water. Then, in order to adjust the pH value and chelating the metal particles with EDTA and other chemical agents are added to the water to form a messy solution resulting from plasma treatment.

 

In conclusion, this investigation has shown that the authors have tried to use EDTA to clean up the mess created by their experimental tool (ring electrode). This doesn’t give you any positive impression of this work. To let this research work appear to be outstanding and significant, it is suggested that the authors should come up with a concept/proposal to prevent the occurrence of metal corrosion. It will be noticed by a very wide range of people doing research of science and technology. Prevention is better than a cure.

Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

The presented work addresses the relevant topic of controlling nanomaterial synthesis in plasma-liquid systems. The authors demonstrate an original approach by using a multispark discharge in argon to treat solutions of EDTA and its disodium salt, aiming for the directed synthesis of either stable iron chelates or iron oxide nanoparticles. The experimental part is performed at a high level, utilizing a comprehensive set of diagnostic methods (OES, UV spectroscopy, TEM, SEM, conductivity measurement). The obtained results convincingly prove that the introduction of a complexing agent (EDTA) radically alters the chemical pathways in the system, suppressing hydrogen peroxide formation and promoting the chelation of iron ions. The work is of significant interest to the scientific community working at the intersection of plasma chemistry, nanotechnology, and materials science.

Despite the high quality of the research, the reviewer has several questions and comments that, in their opinion, could deepen the interpretation of the results and outline prospects for future research.

Comments:
1.  Was any attempt made to perform XRD on the obtained nanoparticles? TEM and EDS data provide insight into morphology and elemental composition but do not give a definitive answer about their phase composition (magnetite, maghemite, hematite, etc.). This is critical for understanding their properties and potential applications. 

2. Authors note that the size distribution results differ from a previous study without ultrasound, pointing to a potential stabilization issue. Did there are any plan for studies on the long-term stability of the obtained suspensions? Were measurements of the ζ-potential conducted to assess the effectiveness of particle stabilization in the presence of EDTA?

3. The UV spectrum shows a band at 260 nm, characteristic of the Fe-EDTA complex. However, could the authors comment in more detail on the predominant oxidation state of iron in this complex (Fe(II) or Fe(III)) under plasma treatment conditions? Does the plasma or its reaction products reduce Fe(III) to Fe(II)?

4. The TEM/EDS data for the sample with 30 mM EDTA-Na₂ indicate the formation of particles enriched with chromium, while iron appears to be effectively chelated. Are stable Cr-EDTA and Ni-EDTA complexes also formed, remaining in the solution? Was the filtrate analyzed for the content of these metals? This is a key question both for controlling the synthesis and for assessing the potential toxicity of the products.

5. The study uses one, albeit optimized, set of discharge parameters (frequency, pulse duration, current). In the authors' opinion, how could a variation of these parameters (e.g., energy density per pulse) affect the balance between the processes of electrode erosion/evaporation and chelation? Could this shift the ratio between the output of nanoparticles and chelates?

Author Response

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Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

High current pulsed multispark discharge is employed for nanoparticle generation, with a focus on the physicochemical processes during plasma-liquid interaction. It is found that the EDTA and its disodium salt can change the reaction pathways remarkably, and proper explanations are given. This work demonstrates the critical role of complex-forming agents in tuning the plasma-liquid interaction performance. My comments are as follows.

1. First paragraph, more recent studies and reviews should be given to elucidate the physical and chemical approaches for NP generation.
2. In the introduction, list out detailed methods for nanomaterial production, along with the nanomaterial synthesis using plasma, and compare the pros and cons of plasma approaches.
3. In the introduction, the discussion on plasma approaches should be more specific, which type of plasma, e.g. DBD, glow, arc, RF/microwave are used, what are the advantages of each types.
4. Authors should clarify why EDTA is of particular interest in this work, clarifying main motivations.
5. Describe the high-voltage pulsed-periodic multi-electrode ring discharge system with schematic to illustrate the setup.
6. “The experimental spectra were acquired by averaging over at least 50 discharge events in frequency-resolved mode” can authors comment on the reproducibility and error bar?
7. Figure 3 and 4, how many repeats are performed, and how about the reproducibility of the trends over the repeats.
8. Figure 7, it seems the nanoparticle size is not very uniform, have authors investigated the NP shape distribution after the processing?
9. It is suggested that authors discuss on tuning or optimizing the discharge setup to achieve optimized NP production. Now the results include interesting observations and solid explanations, but it is better to show how to improve the technic in general.
10. Regarding the NP generation performance, have authors performed comparison with some state-of-the-art methods for producing the same product, is yours better in some aspects?

Author Response

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Author Response File: Author Response.pdf

Reviewer 4 Report

Comments and Suggestions for Authors

Comments, Questions, and Suggestions for the Authors:

1. Comments on the structure of the article. I recommend adding a “Conclusions” section to present the main results obtained. The material from lines 114-134 is best moved to Section 2, “Materials and Methods”. Section 4 “Discussion” contains virtually no analysis of the results obtained and is devoted to a review of the work of other authors. This text is best moved to Section 1, “Introduction”.
2. P.5, Figure 3, line 159. Increasing the concentration of disodium salt EDTA should lead to greater binding of iron ions into chelate complexes and a decrease in the Fe3+ concentration, according to chemical analysis. However, according to Figure 3, the Fe3+ concentration for a 30 mM solution is even slightly higher than for a 3 mM solution. This fact requires explanation.
3. P.6, Table 1, line 190. Neither the text of the article nor the table caption explains the designations pH1, pH2, S1, and S2. Of course, one can guess that these are the pH and conductivity values before and after plasma treatment, but it's better to write this down.
4. P. 7, Figure 5. In row 2 of the table in Figure 5c, the sum of the contents of all the elements presented is only 54.31 at.%. Thus, almost half of the composition is "spurious impurity" (line 195). This fact requires explanation. How many measurements were performed on each sample? Why does the composition vary so greatly? Data on the average sample composition and the characteristic scale of compositional heterogeneity are needed.
5. P. 8, Fig. 6. The particle size distribution for EDTA 3mM has an unusual appearance – one narrow peak with a maximum value of less than 50% and practically zero values for other particle sizes. Since the area under the curves for different samples should be the same, a significant portion of the particles for EDTA 3mM are likely outside the presented size range. Furthermore, the SEM image of this sample, shown in Figure 7b, does not accurately represent this distribution.
6. Abstract, Line 18; p. 2, line 86. No "metallic nanoparticles" were formed in this study. Under the synthesis conditions, metal hydroxide or oxide particles are formed.
7. P. 8, lines 217-218, Figure 7. Under the influence of EDTA, the particles undergo a change in morphology, becoming spherical. The text of the article lacks an explanation for this interesting result.

Comments for author File: Comments.pdf

Author Response

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Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

Well done! publish as it is.

Reviewer 3 Report

Comments and Suggestions for Authors

I recommend acceptance.

Reviewer 4 Report

Comments and Suggestions for Authors

Good Luck!