Degradation of Ketamine and Methamphetamine by the UV/H2O2 System: Kinetics, Mechanisms and Comparison
Md. Rabiul Awual
Round 1
Reviewer 1 Report
water-933518
The authors studied the degradation of ketamine (KET) and methamphetamine (METH) in the presence of UV light and H2O2 in water. They observed that neither UV nor H2O2 alone react with KET or METH.
The degradation were carried out in a photo-reactor. A qualitative scheme of the set-up is shown in Fig.1. Any quantitative information concerning the dimensions of the installation is omitted. No temperature control seems to be installed.
To describe the degradation kinetics, a 2nd order irreversible reaction is supposed. But, as most of the experiments are carried out with a very high molar excess of H2O2, a quasi 1st order model is used to estimate an efficient rate constant, supposing a constant concentration of ·OH-radicals. The evaluation of experimental results obtained at low H2O2 concentrations with a quasi 1st order model is not acceptable. These results must be described with a 2nd order model.
From Figure 5 it becomes evident that the concentration of ·OH-radicals is not proportional to the initial H2O2 concentration. Therefore, for correct kinetic studies, the knowledge of ·OH-concentration as function of the experimental conditions is necessary.
A further strange observation concerns the different influence of KET and METH on the ·OH-concentration. Whereas for KET the efficient rate constant seems to reach asymptotically a final value, the efficient rate constant pass through a maximum for METH degradation. The authors explain the observation with a self-scavenging effect at high H2O2 concentration leading to a decreasing ·OH-concentration. But, they didn’t explain, why this effect doesn’t occur for KET degradation.
In further chapters, the authors report on the influence of pH, carbonate ions, chloride ions and humic acid on the degradation behavior. Exact experimental conditions should be given.
In a final chapter the degradation products are presented. It seems that the presented process cannot be used for a complete degradation of KET and METH.
In summary, I propose to reject the manuscript in its actual form.
Author Response
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Author Response.docx
Reviewer 2 Report
The authors were investigated the ketamine and methamphetamine removal using degradation process by H2O2 effect. The authors were designed the work systematic way with performing some valuable experimental works accordingly. It is also necessary to critically evaluate new data and do not make hasty conclusions which may lead to misinterpretations. However, several points are important to be addressed before going to possible publication in this high-quality journal. Also, the authors need to address all points in the revision stage for broad range readers understanding.
Some comments are:
-Title should be revised to be precise and reflecting the contents;
-No need to mention the abbreviation in the abstract unless it is reused. There are many abbreviation words used without given full name. The authors need to seriously consider this issue.
-In the abstract; "The results indicated that UV or H2O2 alone had no effect on their removal. ….." however, the first sentence was completely different this statement…
-Abstract should be rewritten to summarize the work; the abstract should state briefly the purpose of the research, the principal results and major conclusions. An abstract is often presented separately from the article, so it must be able to stand alone.
-More elaboration on the chemical interaction between the components is required.
-The introduction should be clarified in term of uniqueness and advantage what is the novelty of this work over the previous related work. There are many long sentences should be refined.
-The Introduction should be focused and goes over the topic
-Why the authors think about using this method? What is new? What is the aim of this work; what is the gab to cover.
-Most of the cited references are not proper. Probably, the references were selected by the student instead of researchers. Then it is like a learning report by a student.
-There are many studies reported in the literature regarding the diverse substance and components removal as defined by Awual group articles. Based on this, do the authors think that the present process is an improvement when compared to other reported method? If so, please provide some discussions on the advantage and disadvantage as the novelty in the Introduction section. And the authors should highlight the scientific value of the present work with citing high impact journals of the above indicated group articles……….
-Result and discussion section must explain with more experimental findings, such as experimental results, significance of work, finding of results and choice of materials. Result and discussion part must be supported in the main manuscript by the following references: Journal of Environmental Chemical Engineering, 6 (2018) 218–227; Chemical Engineering Journal, 266 (2015) 368–375; Journal of Molecular Liquids, 294 (2019) 111679 and so on…
-Under influencing of H2O2; please discuss with scientific way how and why; the efficiency is better.
-How did you confirm no hazardous components existed in your method; Please prove it….
I WOULD LIKE TO SEE THE REVISED MANUSCRIPT
Author Response
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Author Response File: Author Response.docx
Round 2
Reviewer 1 Report
water-933518-V2
The authors answered to the questions and remarks pronounced in the 1st review.
Point2:
This point concerns the use of the quasi 1st order model for low low H2O2 concentrations. In the corrected version the authors specified that experiments were carried in the range from 0.1 to 2 mM H2O2, which is high compared to the initial concentration of the studied reactants: 0.42 µM for KET and 0.67 µM for METH and justifies the quasi 1st order model. Nevertheless, the domain of 0 to 2 mM H2O2 is still mentioned in the text.
The authors tried to prove the constant ·OH-concentration with the terephthalic acid/ H2O2 reaction. It turns out that the reaction rate seems to be higher for 0.1 mM H2O2 compared to 1 mM H2O2 concentration. Whatever the reason for these results may be, it is not a proof for constant ·OH-concentrations.
Point 3:
The answer to this point is confusing. The authors claim that ·OH-radicals are formed quicker at high H2O2 concentrations; these means that the radical concentration is a function of time and in consequence not constant?!
Point 4:
Whereas for KET the efficient rate constant seems to reach asymptotically a final value, the efficient rate constant pass through a maximum for METH degradation. The authors explain the observation with the speculation that the concentration of 2 mM H2O2 is not enough for ·OH degradation in the presence of KET but, sufficiently high in the presence of METH in spite of the ca 30% higher concentration compared to KET. In addition, the supposed scavenging results in an irreversible decomposition of H2O2 concentration with time. Again leading to diminishing concentrations of H2O2, in contradiction to the claimed constant concentration.
Even more confusing is that the type of reactants used influences the ·OH decomposition. If this is true, the estimation of the 2nd order constant based on the benzoic acid/ H2O2 reaction as proposed in Equ. (2) and (3) seems to be not correct.
A more general remark concerns the presentation: It would be helpful to give concentrations as mol/volume for all reactants (H2O2, KET, METH…) and to present the graphs with the same scaling.
In summary, the manuscript in its actual form is not jet acceptable for publication
Author Response
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Reviewer 2 Report
The authors were revised the manuscript based on the remarks to improve the manuscript for readers understanding. This can be accepted.
Author Response
The expert has agreed to the first revision.
Round 3
Reviewer 1 Report
Whereas most of the questions were answered satisfactorily, my remark concerning the presentations are not considered.
I strongly propose to use molar concentrations of all mentioned reactants: H2O2, KET, METH and BA in the Figures and tables.
It would be also of interest to explain, why the initial concentration of METH is fixed to 0.67 µmol/L and a much lower concentration of 0.42 µmol/L is used for KET.
The same remark concerns the comparison of rate constants with the BA/ H2O2 reaction. The used initial BA concentration and in consequence the initial molar BA/ H2O2 ratio is roughly twice compared to KET.
In summary, the manuscript can be accepted after considering the mentioned corrections and explanations.
Author Response
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Author Response File: Author Response.docx
This manuscript is a resubmission of an earlier submission. The following is a list of the peer review reports and author responses from that submission.
