Enhancement and Mechanism of Rhodamine B Decomposition in Cavitation-Assisted Plasma Treatment Combined with Fenton Reactions

Round 1

Reviewer 1 Report

General comments

In line with their previous paper, the authors provide an additional study on their ACAP system. This study is interesting however some major modifications and revision should be done to add a little more novelty and consistency in this study.

Experimental part:

The authors should clearly indicate the volume of treated solution and the ultrasonic condition used fort the ACAP treatment so that the experiments can be reproduced by another team. Amplitude is given however since this value is not always given by the US manufacturer, at least acoustic power (not the electric power) delivered into the medium should be indicated as already stated in the well know and dedicated sonochemistry literature.

Results

It would have been interesting to know what is the part of RhB degradation due to the cavitation phenomenon and the part from the plasma discharge. This is actually not clear in this paper whether plasma discharge is really required. Moreover, without a further understanding about the effective part of both phenomena it would be difficult to enhance the global ACAP efficiency.

I do appreciate a paper with other team/study results comparison however, the authors should also integrate results at higher ultrasonic frequency because, to be honest, in well-chosen conditions under ultrasonic irradiation alone much higher RhB degradation rate can be achieved as seen elsewhere (https://doi.org/10.3390/catal11080928). Please comment the choice of sticking to low US frequency which is actually low on the H2O2/HO°/HO2°(almost zero here) production rate compare to high frequency conditions.

In the same way, the authors identify that the radical formation rate as the limiting step in their experimental conditions, why did the authors stick to Ar gas and not Ar/O2 since this atmosphere is well known to lead to more oxidative specie production rate under US?

Since RhB concentration is initially low, what is the amount on RhB adsorbed on the vessel/reactor?

The authors use both stainless steel and titanium alloy sonotrode, is the sonochemical activity the same with both sonotrode?

Morevoer, TOC abatment measurement should be considered.

Discussion

Authors speak about “water pyrolysis” at the origin of HO radical formation under US. This is actually not accurate since radicals and excited species can be formed by hot particles collision / formation of a plasma within the cavitation bubble (https://doi.org/10.1016/j.ultsonch.2016.02.003).

The authors proposed a quite detailed RhB decomposition pathway, however within the main text, only degradation of RhB based on UV/Vis spectra are given and the audience cannot really figure out whether this pathway is based on real analyses or just hypotheses. HPLC-TOFSIMS measurements are claimed to be done however, no values, mass balance or carbon speciation for instance are given. Please give more pieces of information about this part and the % of the various identified compounds (are the same compounds identified whatever the experimental conditions?). In the same, for several steps of Fig 8 pathway, CO2 formation and release was identified by the authors, meaning that the total amount of carbon changed during the RhB degradation, thus what is the final TOC abatement under their various conditions?  

For the kinetics comparison in Fig 10, it would be fair to add high frequency US study and not only US bath and low frequency treatment since this is well known that such conditions are not the best at all for organic compound oxidation/degradation (see other comments on this subject)

Minor comments :

Please be careful with false accuracy, 3 significant digits are usually enough and more cannot be experimentally justified (ex : [Fe] = 1.734 mg/L ; same with table 1 %).

How was determined the iron concentrations in the various media ?

Error bars should be given in figures in order to compare whether the difference are relevant or not

Please check equation/reaction (5)

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

(1)      Line 16: “that use hydrogen peroxide and ferrous ions as reagents” should be deleted.

(2)      Some relevant references about emerging AOPs, such as 10.1016/j.seppur.2022.120716, 10.1016/j.cej.2020.128176, 10.1016/j.cclet.2021.10.087 should be cited in introduction.

(3)      Line 92: “Ferrous chloride, FeCl2”, pls keep one.

(4)      Sections 2.1 and 2.2 should be combined.

(5)      The method for degradation of byproducts should be provided.

(6)      Figures 3-6: Pls add error bars.

(7)      The writing style of radicals should be corrected according to the following reference: “Koppenol, W. (2000). "Names for inorganic radicals (IUPAC Recommendations 2000)." Pure and Applied Chemistry 72(3): 437-446.”

(8)      Pls improve the quality of Figure 10.

(9)      More recently references should be cited.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Thank you for having taking into account my various comments. In my opinion, this paper is now acceptable for publication.