Removal of Bound PAH Residues in Contaminated Soils by Fenton Oxidation
, Pengfei Cheng, Bing Yang, Jian Wang and Wanting Ling *Round 1
Reviewer 1 Report
Catalysts
Title: Removal of bound PAH residues in contaminated soils by Fenton oxidation
The authors investigated the removal of PAH residues in contaminated soils by Fenton oxidation. The topic is interesting but it has been deeply investigated in the literature. Accordingly, the relevance and novelty of this work should be more clearly stated. On the other hand, the introduction requires an overhaul showing the results obtained in previous works dealing with the application of chelating agents for the treatment of PAHs in contaminated soils by Fenton oxidation, as this is the main goal of the current study. Furthermore, the results section is too descriptive and the results obtained are not very promising. Only two of the four PAHs investigated were completely eliminated and long reaction times were required (36 h). Apart from these general comments, please consider the following ones:
Major:
· Both the introduction and objectives of this work were focused on the use of citric acid as chelating agent. However, in the results section, oxalic acid led to the best results. The introduction should be revised taking this into account and showing previous results in the literature dealing with the application of this acid as chelating agent.
· The manuscript should be checked for grammar. There are several mistakes throughout the text. For instance, P3L136 “the optimum conditions… is similar to”.
· The selected H2O2:Fe2+ ratio was extremely high (30:1 w/w). Why? The contribution of the soil as catalyst should be carefully analyzed in blank experiments.
· It must be noted that, according to the results depicted in Figure 3a, the H2O2:Fe2+ ratios between 15:1 and 33:1 led to the same results with fluorene and anthracene, which clearly shows that H2O2 was in excess at ratios above 15:1. On the other hand, the degradation of acenaphthylene was not remarkably increased by increasing the H2O2:Fe2+ ratio. The highest removal yield was below 30%. The discussion about these results should be improved. Furthermore, more experiments at intensified operating conditions should be carried out with the most persistent PAH acenaphthylene.
· In general, the results do not seem very promising as long reaction times (up to 36 h) were required to achieve the complete removal of the target pollutants (only fluorine and anthracene were completely eliminated). Do the results obtained in this work improve those previously reported in the literature? The results should be discussed in the context of the literature. A table summarizing the results obtained in this work and those of previous studies should be provided.
· The evolution of H2O2 along the reactions should have been considered.
· To better evaluate the efficiency of the proposed catalytic system, the amount of PAHs removed per gram of H2O2 decomposed should be calculated.
· Did the authors evaluate the formation of reaction intermediates? The generation of different hydroxylated species is expected and their toxicity could even be higher than that of the parent compounds. In the same line, the mineralization yield should be considered.
Minor:
· The recent review on the field of PAH treatment, “Treatment technologies for PAH-contaminated sites: a critical review” by Gitipour et al. (2018), should be considered to be cited in the introduction.
· The kinetic constant k must be written in lower case throughout the text.
· There are informal expressions such as “one notes that” which should be removed from the manuscript.
· “m/m” should be replaced by “w/w”.
· The same Y axis scale should be used in Fig. 1 for the different PAH residues for the sake of comparison.
· Figure 2a should be removed as the same information can be seen in Figure 2b.
· The legend in all figures are hardly to be seen.
· The concentration of H2O2 should be given in mg/L instead of %.
Author Response
Thank you for your kind suggestions. We carefully revised the manuscript according to your comments and journal guidelines, with the revisions are identified in the point-by-point response as follows.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
V
ID: catalysts-490772Removal of bound PAH residues in contaminated soils by Fenton oxidation
The paper discusses the Fenton oxidation of four PAHs, naphthalene (NAP), acenaphthene (ACP), fluorene (FLU) and anthracene (ANT). The first question that should be raised what is the definition of the bound PAH. It is not given in the text. The authors collected soil samples from the neighbourhood of an oil refinery plant. They dried the samples and then applied a dichloromethane: acetone extraction to remove what can be removed in this way. Then they applied NaOH treatment on the treated soil samples followed by acidification. A second extraction was applied with dichloromethane, and what was extracted was considered as bound PAH. The Fenton treatment was applied after the first extraction. I think the treatment processes highly modify the structure of soil.
The next question how the PAH’s are bound to the soil. Certainly by not covalent bonds. One can think of some electric type interactions. This is important question, since the authors investigated also the pH effect. They attributed the pH dependence to the pH dependence of the Fenton process. However, I think the soil-PAH interaction should be also pH dependent and this pH dependence also influences the degradation rate.
The manuscript has a rather unusual structure. 1. Introduction is followed by 2. Results, in this part under heading 2.6 in one block there are 11 figures and 2 tables, 3. Discussion, 4. Materials and Methods, 5. Conclusions after that Supplementary material and References.
The English of the manuscript is rather poor. Seemingly the authors have no practice in paper writing and there are lot of editorial type problems with the MS. For instance in Table 1 references are missing to the partition coefficients or to the ionization potentials.
I mention as examples problematic places in the first part of the manuscript:
Line 36: Therefore, it is imperative to minimize Needs rewriting
Line 41: Bound residue formation may toxify the soil by permanently binding the contaminant to the soil. Wrong sentence
Line 44. Over time, the bound residues tightly attached to soil organic matter become indistinguishable What does indistinguishable mean?
Line 45: Therefore, an environmental solution is considered when reducing Funny!
Line 55: Then, hydroxyl radical oxidize organic compounds (RH or R) by hydrogen abstraction or hydroxyl addition: Hydrogen abstraction of the hydrogen atoms of an aromatic ring is impossible due to the very high bond energy. Only addition can take place.
Line 80: was up to 77.2% after 7d What is 7d?
Line 95: residues were to study the degradation efficiency Wrong English.
Line 97: After 36 h, the degradation of bound PAH residues up to 83%, 27%, 100%, and 100%, respectively. Wrong
Line 98: The concentration of bound PAH residues in soil with different reaction time, as shown in Table S4. Wrong English.
Line 99: The concentration of bound PAH residues in soil with different reaction time, as shown in Table S4. The degradation of
Line 100: test bound PAH What is it?
Line 108: C0 should be c
Line 99: The degradation of test bound PAH residues in soil by Fenton oxidation treatment followed pseudo-first order kinetics through the whole reaction duration of 36 h. In the case of pseudo-first order reaction the time dependence is exponential-like. Here only the anthracene curve seems to be exponential, i.e. pseudo-first order, the other curves shown more-or-less linear dependence.
Line 122. This suggests that the degradation rate of individual bound PAH residues is related to the degree of selectivity. Needs explanation.
Line 126. Hydroxyl radicals react with aromatic compounds in a hydrogen abstraction manner equivalent to electrophilic substitution. No reaction.
Line 127: aromatic compounds in a hydrogen abstraction manner equivalent to electrophilic substitution. Therefore, the reactivity of PAHs was attributed to their IP. Here H-abstraction does not take place.
Line 131: Although ANT has one more aromatic ring than NAP, the IP of ANT is higher than NAP contribute to its higher chemical reactivity. ???
Line 144: and the removal tends to balance I don’t understand.
Line 151: In addition to NAP and ANT have been degraded, FLU and ACP removal remain balance. ??
I do not suggest publication of the paper in the present form. At the same time I want to encourage the authors to continue the work by asking someone who has practice I paper writing, evaluation the results and in scientific English.
Author Response
Thank you for your kind suggestions. We carefully revised the manuscript according to your comments and journal guidelines, with the revisions are identified in the point-by-point response as follows.
Author Response File: Author Response.pdf
Reviewer 3 Report
The paper deals with the oxidation of PAHs ins soil via Fenton reaction. The scope of the works belongs to Catalysts
The authors used different chelating agents (oxalic, citric acid) for the Fenton reaction while they demonstrated the efficiency of the process for different PAHs. The main novelty of the work is that they use Fenton to remove bound PAH residues
Some points:
Figure 1. I suggest the authors use the scale 0-1 for Y axe. The scale that the authors used (different for each graph) maybe lead to a first wrong interpretation of the data for the reader that is no so familiar
(also negative at y axe (-0.05) has no physical meaning
Have the authors measure the concentration of peroxide? Can they show us at least the variation of the peroxide in the optimum conditions?
My overall impression is that the work is well written and easy to follow. However, the novelty is low to moderate. Something is missing in order this manuscript to have a novelty and higher scientific interest to the reader in comparison with the literature in order to published in a high impact factor journal like catalyst
For example, I can suggest performing just one more experiment (in the optimum conditions) with stepwise addition of peroxide in order to check of the step wise addition of peroxide can improve the efficiency
Author Response
Thank you for your kind suggestions. We carefully revised the manuscript according to your comments and journal guidelines, with the revisions are identified in the point-by-point response as follows.
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
Although the authors have modified their previous version of the manuscript, there are still important concerns that have not been addressed:
- The introduction has been slightly modified. The state-of-the-art on the field is not clearly stated.
- The experiments under intensified operating conditions with the aim of reducing the amount of H2O2 used (up to 102 g/L) have not been carried out. In the same line, the evolution of H2O2 is not provided and thus, the efficiency of the system cannot be calculated.
- A table summarizing the results obtained in this work compared to those previously achieved in the literature is still missing. Even in the cases that the PAHs were extracted prior the application of Fenton oxidation, the removal yields could be compared and explained.
- The reaction intermediates generated along the process have not been evaluated, which is important considering that even more toxic compounds could be formed. At least, an explanation of the intermediates expected to be obtained considering previous work could have been included.
- There are still grammar mistakes that should be corrected.
Author Response
Thank you for your valuable feedback.We carefully revised the manuscript according to your comments and journal guidelines, with the revisions are identified in the point-by-point response as follows.
Author Response File: Author Response.pdf
Reviewer 2 Report
catalysts-490772
The authors made some changes during rewriting, but the paper mainly remained as it was before the revision.
Most of my suggestions were not considered.
1. The authors express the degradation results in percentages relative to the untreated system. They call it as efficiency. Percentage is not efficiency.
2. The authors wrote a 2 pages long discussion chapter. 80% of this part is just mentioning literature results. The comparison with literature results is important. However, the place of the literature results is the introduction.
Specific comments:
Line 78: can blind iron and maintain ??
Line 112: The citric-Fenton reaction experiments in contaminated soils containing only bound PAH residues were studied the degradation efficiency and kinetics of bound PAH residues with Fenton oxidation Wrong sentence!
Line 118: The degradation of test bound PAH residues in the soil involved the Fenton oxidation treatment, followed by pseudo-first-order kinetics in the 36 h duration. Wrong!
Line 122: whereby the reaction time is proportional to the degradation rate of tested bound PAH residues. How is it?
Line 123: After 36 h, the incomplete oxidation of ACP may be due to its low reactivity (to be mentioned below) (Figure.1(b)). Hence, the citric-Fenton oxidation can be used for the removal of bound PAH residues in contaminated soils. Not clear, why?
Lines 127-132: the authors should use only one of the equations. The correct way of writing (3) is -dCt/dt=kCt.
Line 138: From Figure 2, the preferred oxidation reaction (ANT>FLU>NAP>ACP) was observed as the citric-Fenton reaction wrong sentence!
Line 194: Hydroxyl radicals react with aromatic compounds by a single electron transfer followed by a nucleophilic addition reaction equivalent to a redox process Hydroxyl radicals react with aromatic compounds by addition to the ring!
Line 200: The kinetic reaction rate constants are proportional to the ionization potential, are inversely proportional to the
Line 203: The PAHs removal efficiency appears to be effect by
Line 297: When the solid: liquid (g/mL) was low, the soil did not completely make contact with water. Why the solid simply precipitated?
Line 349: in artificially aged soils by Fenton oxidation at different aged days aging days
Line 354. Fenton treatment has become an increasingly established method, so citric-Fenton oxidation was applied to remove bound PAH residues in contaminated soils and reduce their associated human health and ecosystem risks. Fenton reaction is known for more than 100 years!
Line 380: Forsey et al. [38] studied the correlation between the reaction rate of pyrene, fluoranthene, phenanthrene, and naphthalene and their IP, with their decreasing IP increasing the rate of reaction Wrong sentence!
Line 393: These conclusions corroborated to the findings results in this study Revision is needed!
Line 403: our study did not observe the decrease in bound PAH residues degradation rate ???
Line 414: When pH is above the optimal pH, iron may precipitate to iron oxyhydroxides lead to deactivation of the catalyst, these complexes would further form [Fe(OH)4]- [44], Wrong sentence!
Line 422: the consumption of •OH by hydrogen ion (H+) and lower pH What is the reaction?
As a conclusion a real rewriting is needed!
Author Response
Thank you for your helpful comment.We carefully revised the manuscript according to your comments and journal guidelines, with the revisions are identified in the point-by-point response as follows.
Author Response File: Author Response.pdf
Reviewer 3 Report
The authors revised their manuscript, however they did not perform any additional experiment since they lost the original soil. However, the fate of peroxide is crucial for the manuscript and in order to publish in a high impact factor journal like Catalyst is necessary
Therefore i advise the authors to perform additional experiments in order to investigate the decomposition of peroxide together with the degradation of PAHS in different soil sample if it is necessary
I cannot recommend publication at least at this stage
Author Response
Thank you for your valuable feedback.We carefully revised the manuscript according to your comments and journal guidelines, with the revisions are identified in the point-by-point response as follows.
Author Response File: Author Response.pdf
Round 3
Reviewer 2 Report
I think we have to remain with the manuscript as it is now and publish it.
Reviewer 3 Report
The authors revised their manuscript according to the reviewer suggestions.
