Efficient Removal of Azo Dye from Wastewater Using the Non-Toxic Potassium Ferrate Oxidation–Coagulation Process

Round 1

Reviewer 1 Report

The manuscript by Jargalsaikhan et al. describes an oxidation-coagulation process of dye removal from wastewater. The paper shows that the potassium ferrate can outperform a single-stage process (either oxidation or flocculation), using alternative chemicals. Overall, I enjoyed reading the manuscript. It is well-written, with some room to improvement. Therefore, I recommend minor revision. Below the authors can find my comments to the paper:

Major comments:
1) Lines 70-77: Authors suggest that theirs is the first work suggesting the oxidating-coagulating nature of potassium ferrate, yet quick search in the literature found some other papers mentioning this, e.g. https://doi.org/10.3390/ma12223784. Please modify your statement, check the literature thoroughly and add the necessary references.
2) General comment to results and discussion: I do not recall any mentions about coagulation/precipitation/sedimentation during your experiments when reading through the manuscript. If the high removal efficiency is actually due to two-stage process, this has to be proved in the paper, either by microscopic analysis, or filtration and gravimetric analysis.
3) Figure 2: While I agree with the authors that at pH 3 the kinetics of removal were the quickest, the removal efficiency after 10-15 minutes was 
essentially the same in all pH systems for potassium ferrate. Please comment.
4) Comment to all figures: Even though it was mentioned that the measurements were done in triplicates, there is no error bars in the figures. There are some shaded areas on some of the figures (e.g. Fig. 3, 4 etc.), but personally, I am not familiar with this kind of data visualization and it is not really explained. I would assume that these show the range of obtained results (maximum maybe?), yet some of the scatter data points are on the top edge of these figures, some below, and also in some cases it is impossible to read the data properly (Figure 5). Please consider using more readable format.
5) I believe that molar dosage comparison of different additives could add some value to the discussion. While mass is more important from an operational point of view, molar ratios would be more interesting from an academic side.
6) Line 256: I doubt that the authors can claim that potassium ferrate process can reduce CAP/OP-EX already based on simple jar test. Please modify that statement.

Minor comments:
Line 35-36: This statement requires a reference.
Line 52-55: The topic of the dye being a hazard has been mentioned already in the previous paragraph. Please join those statements together in one place.
Line 109-110: Was any dye lost on the filter due to adsorption?
Line 163-165: This statement (decomposition at lower pH) requires a reference.
Figure 2: Graph for OD pH 9 - please change the y axis to the same as all the other graphs.
Figures 4 and 5: Put those two graphs horizontally, like in Figure 3.

Author Response

Responses to reviewers’ comments

Reviewer 1

Development of methods for removal of azo dyes is one of the key issues of research and development in the field of environmental engineering and wastewater treatment, respectively. Considering the practical applicability the simpler methods can be preferred. The analysis of the process efficiency and the effects of process parameters can provide useful information not just for the science but also for practice. Manuscript applsci-1287148 focuses on the applicability and efficiency of potassium ferrate compare it to NaOCl dosage and FeCl₃ coagulation processes. The manuscript is well structured. Introduction summarizes briefly but clearly the problems of azo dyes in industry effluents, and the generally used processes for azo dye removal. The advantages of using of potassium ferrate is given in more details. Research motivations are well defined. Applied methods are adequate to the aims of the research/testing the hypothesis. Materials and experimental procedures are described clearly. Manuscript contains interesting and valuable results, but the presentation and discussion of experimental results is superficial.

Comments, suggestions:

Q1-1. Please define clearly and highlight the novelty of the study.

A1-1) The authors appreciate of the valuable comments on this manuscript. The potassium ferrate oxidation-coagulation process exhibited complete removal (nearly 100%) of Azo dye compared to the individual FeCl₃·6H₂O coagulation and NaOCl oxidation processes. Basically, Azo dye is difficult to remove completely by conventional treatment. The novelty of this study has been highlighted in the revised manuscript:

(Page 1 Line 19-20, Line 22-24)

mg/L for K₂FeO₄, and it removed nearly 100% of orange II azo dye (OD) and lissamine green B dye (LGB). However, their removal efficiencies decreased when the pH increased to 12. In all process, dye removal was completed in 5 min of the reaction. Overall, OD and LGB was effectively removed by K₂FeO₄, compared to the NaOCl and FeCl₃·6H₂O. It indicates that combination of oxidation and coagulation- K₂FeO₄, outperformed the individual treatment process with non-toxic by-product production.

Q1-2. How was selected the concentration range of dye solutions? Did it come from the general characteristics of real industry effluents?

A1-2) The authors would explain that the concentration of the dye solution in this study was determined by the general characteristics of real industry effluents and literatures from similar studies.

Q1-3. Figures do not contain the standard deviation. Please present these data as well.

A1-3) The authors have double-checked the raw data and would kindly mention that the experiment was proceeded triplicate for accuracy and reproducibility. Further, the standard deviation of the results was less than 3 %, thus, error bars were not shown in the plot. The authors apologize that it was not explained in the first submission. It has been clearly mentioned in the manuscript:

(Page 3 Line 130-132)

All experiments were conducted triplicate for reproducibility and reliability. In each plot, the data set were presented with standard deviation (error bar). The standard deviation of the results was less than 3%, thus, error bars were not clearly shown in the plot.

Q1-4. In general, the Results and discussion is mainly descriptive, the results are not discussed with references. I suggest the authors to consider discussing more relevant references in this section.

A1-4) More detailed discussion has been added in the revised manuscript.

Q1-5. It should be more informative to analyze the effect of process parameters by ANOVA.

A1-5) The authors would agree with the importance of statistical analysis for informativeness. The ANOVA test was conducted to experimental results, for determining statistical significance. The statistical results showed that there was a statistical significance on azo dye removal as a function of different pH levels (3 to 12).

All information was inserted to the manuscript.

(Page 3 Line 134-137)

2.4. Statistics

An analysis of variance was performed on the dye removal experimental group. Three groups were tested for statistical significance between different pH conditions selected as one of process parameters.

(Page 4 Line 159-164)

The effect of pH on azo dye removal efficiency was verified by ANOVA. The difference pH varying from 3 to 12 had a statistical significance on the OD removal by K₂FeO₄. For OD removal by K₂FeO₄, F value (46. 526) was higher than F-critical (3.239) and p value was 3.98E-08 (<0.05). For LGB removal by K₂FeO₄, F value (55.94) was higher than F-critical (3.239) and p value was 1.06E-08 (<0.05). It indicates that controlling pH for dye removal experiment affected the statistical significance.

Q1-6. Authors not provide data/not discuss the cost of the different processes (but in line 256 the costs are mentioned).

A1-6) The authors agree with that the data of cost for different processes (Ferrate, single FeCl₃·6H₂O coagulation and NaOCl oxidation process) should be discussed. The authors would explain that this study was not focused on in-depth of study of economic evaluations on those three types of processes. Thus, those explanation was deleted for clarity of this manuscript.

(Page 10 Line 275-277)

K₂FeO₄ has combined effect of oxidation and coagulation, which is useful and important for improving water quality. On comparison with the conventional two-step unit processes (oxidation and coagulation),

Q1-7. Please check the typos in the manuscript (‘K₂FeO₄’ in line 144, for instance).

A1-7) The authors have carefully checked whole manuscript, and corrected all the typos.

Author Response File: Author Response.docx

Reviewer 2 Report

Development of methods for removal of azo dyes is one of the key issues of research and development in the field of environmental engineering and wastewater treatment, respectively. Considering the practical applicability the simpler methods can be preferred. The analysis of the process efficiency and the effects of process parameters can provide useful information not just for the science but also for practice. Manuscript applsci-1287148 focuses on the applicability and efficiency of potassium ferrate compare it to NaOCl dosage and FeCl3 coagulation processes. The manuscript is well structured. Introduction summarizes briefly but clearly the problems of azo dyes in industry effluents, and the generally used processes for azo dye removal. The advantages of using of potassium ferrate is given in more details. Research motivations are well defined. Applied methods are adequate to the aims of the research/testing the hypothesis. Materials and experimental procedures are described clearly. Manuscript contains interesting and valuable results, but the presentation and discussion of experimental results is superficial.

Comments, suggestions:

Please define clearly and highlight the novelty of the study.

How was selected the concentration range of dye solutions? Did it come from the general characteristics of real industry effluents?

Figures do not contain the standard deviation. Please present these data as well.

In general, the Results and discussion is mainly descriptive, the results are not discussed with references. I suggest the authors to consider discussing more relevant references in this section.

It should be more informative to analyse the effect of process parameters by ANOVA.

Authors not provide data/not discuss the cost of the different processes (but in line 256 the costs are mentioned).

Please check the typos in the manuscript ( ‘K2FeO4’ in line 144, for instance).

Author Response

Responses to reviewers’ comments

Reviewer 2

This work has a good structure, and the quality is high. I only suggest these minor revisions before publication on this Journal.

Q2-1. Line 38-54. Also AOPs can be used to degrade dyes. Please, also discuss this aspect. I suggest you a recent work on this topic [https://doi.org/10.1016/j.jhazmat.2019.121668].

A2-1) The authors appreciate of the valuable comments on this manuscript and agree with AOPs for dye degradation. The suggested reference was inserted in the manuscript.

(Page 2 Line 56-57)

environmental problems. A recent study reported that H₂O₂ assisted photo electrocatalysis for decolorization and biodegradation of a pharmaceutical wastewater [1].

Q2-2. In figures 3, 4 and 5 I see colored areas. What do they represent? If it is a mistake, the figures should be amended. If it is not a mistake, the meaning of colored areas should be clarified in the captions.

A2-2) The authors have checked it and the plot with colored areas that presenting each data points were not mistakenly inserted. The authors would intend to highlight the outperformance of the ferrate compared to other coagulants/oxidants. However, it may look like a mistake, and/or not clearly present the authors’ intention. Thus, the highlight was strengthened by changing thickness and color of the line. Thank you very much for the reviewer’s valuable comments.

Figure 3. Effect of potassium ferrate dose on the removal efficiency on (a) Orange II azo dye (b) and Lissamine green B dye.

Figure 4. Comparison of potassium ferrate with NaOCl and FeCl₃·6H₂O with respect to the removal efficiency of (a) 25 mg/L and (b) 50 mg/L of Orange II Azo Dye at pH 3.

Figure 5. Comparison of potassium ferrate with NaOCl and FeCl₃·6H₂O with respect to the removal efficiency of (a) 25 mg/L and (b) 50 mg/L of Lissamine green B dye at pH 3.

Q2-3. What about possible application on real scale of your results? Please, briefly discuss this point in the conclusions.

A2-3) The authors agree with explaining the possible application of the K₂FeO₄ coagulation-oxidation process. It has been inserted to the conclusion of the revised manuscript.

(Page 10 Line 280-281)

scale. The K₂FeO₄ process is a possible application to wastewater treatment plant to treat the dye containing effluent on real scale.

Q2-4. A list of abbreviations is strongly suggested at the end of the manuscript.

A2-4) The authors appreciate of the comments, and a list of abbreviations has been inserted in the revised manuscript:

(Page 11, Line 288-292)

A list of abbreviations

OD: orange II azo dye 

LGB: lissamine green B dye 

K2FeO4: Potassium ferrate 

Ferrate: [FeO₄]^2-

Q1-5. Please, literature should be cited according to the instruction of the Journal.

A1-5) The authors appreciate of the comments, and a format of the literature has been revised according to the journal instruction.

(Page 11-12)

Author Response File: Author Response.docx

Reviewer 3 Report

This work has a good structure, and the quality is high. I only suggest these minor revisions before publication on this Journal.

• Line 38-54. Also AOPs can be used to degrade dyes. Please, also discuss this aspect. I suggest you a recent work on this topic [https://doi.org/10.1016/j.jhazmat.2019.121668].
• In figures 3, 4 and 5 I see coloured areas. What do they represent? If it is a mistake, the figures should be amended. If it is not a mistake, the meaning of coloured areas should be clarified in the captions.
• What about possible application on real scale of your results? Please, briefly discuss this point in the conclusions.
• A list of abbreviations is strongly suggested at the end of the manuscript.
• Please, literature should be cited according to the instruction of the Journal.

Author Response

Responses to reviewers’ comments

Reviewer 3

This work has a good structure, and the quality is high. I only suggest these minor revisions before publication on this Journal.

Q2-1. Line 38-54. Also AOPs can be used to degrade dyes. Please, also discuss this aspect. I suggest you a recent work on this topic [https://doi.org/10.1016/j.jhazmat.2019.121668].

A2-1) The authors appreciate of the valuable comments on this manuscript and agree with AOPs for dye degradation. The suggested reference was inserted in the manuscript.

(Page 2 Line 56-57)

environmental problems. A recent study reported that H₂O₂ assisted photo electrocatalysis for decolorization and biodegradation of a pharmaceutical wastewater [1].

Q2-2. In figures 3, 4 and 5 I see colored areas. What do they represent? If it is a mistake, the figures should be amended. If it is not a mistake, the meaning of colored areas should be clarified in the captions.

A2-2) The authors have checked it and the plot with colored areas that presenting each data points were not mistakenly inserted. The authors would intend to highlight the outperformance of the ferrate compared to other coagulants/oxidants. However, it may look like a mistake, and/or not clearly present the authors’ intention. Thus, the highlight was strengthened by changing thickness and color of the line. Thank you very much for the reviewer’s valuable comments.

Figure 3. Effect of potassium ferrate dose on the removal efficiency on (a) Orange II azo dye (b) and Lissamine green B dye.

Figure 4. Comparison of potassium ferrate with NaOCl and FeCl₃·6H₂O with respect to the removal efficiency of (a) 25 mg/L and (b) 50 mg/L of Orange II Azo Dye at pH 3.

Figure 5. Comparison of potassium ferrate with NaOCl and FeCl₃·6H₂O with respect to the removal efficiency of (a) 25 mg/L and (b) 50 mg/L of Lissamine green B dye at pH 3.

Q2-3. What about possible application on real scale of your results? Please, briefly discuss this point in the conclusions.

A2-3) The authors agree with explaining the possible application of the K₂FeO₄ coagulation-oxidation process. It has been inserted to the conclusion of the revised manuscript.

(Page 10 Line 280-281)

scale. The K₂FeO₄ process is a possible application to wastewater treatment plant to treat the dye containing effluent on real scale.

Q2-4. A list of abbreviations is strongly suggested at the end of the manuscript.

A2-4) The authors appreciate of the comments, and a list of abbreviations has been inserted in the revised manuscript:

(Page 11, Line 288-292)

A list of abbreviations

OD: orange II azo dye 

LGB: lissamine green B dye 

K2FeO4: Potassium ferrate 

Ferrate: [FeO₄]^2-

Q1-5. Please, literature should be cited according to the instruction of the Journal.

A1-5) The authors appreciate of the comments, and a format of the literature has been revised according to the journal instruction.

(Page 11-12)

Round 2

Reviewer 2 Report

The manuscript has an interesting topic that can provide useful information not just for the science but also for practice. Authors have revised the manuscript thoroughly according to reviewers’ comments and suggestions. The overall scientific quality of manuscript has improved significantly due to revision. Rephrasing, amendments, further references discussed, more detailed discussion of experimental results made the manuscript more complete and clear. I accept all modifications and answers by the authors an recommend the manuscript for publishing.