A Study on the Influence and Mechanism of Temperature and Dosage on PCDD/Fs Adsorption via Coal-Based Activated Carbon

1. Summary

Thank you very much for taking the time to review this manuscript. Please find the detailed responses below and the corresponding revisions in track changes in the re-submitted files.

2. Questions for General Evaluation

Reviewer’s Evaluation

Response and Revisions

Does the introduction provide sufficient background and include all relevant references?

Can be improved

Corresponding responses have been given in the point-by-point response letter.

Are all the cited references relevant to the research?

Yes

Is the research design appropriate?

Yes

Are the methods adequately described?

Yes

Are the results clearly presented?

Yes

Are the conclusions supported by the results?

Can be improved

3. Point-by-point response to Comments and Suggestions for Authors

Comments 1: Abstract: abbreviations need to be explained the first time they are used - PCDD/Fs; Cl-PCDD/Fs; I-TEQ; TCDD/Fs and PeCDD/Fs; HpCDD/Fs, HxCDD/Fs and OCDD/Fs; MSWI.

The same note for the title of the article. It is not clear what the abbreviation PCDD/Fs means.

Response 1: Thank you for pointing this out. We agree with the comment of the abstract, and we have added the full names of the above-mentioned abbreviations in the abstract, page 1. Besides, “Cl-PCDD/Fs”, extending from “PCDD/Fs”, was a new defined concept, which had been well explained in line 178-184, instead of the abbreviation of an independent noun.

As for the title of this article, it’s a common writing method that “PCDD/Fs” is directly used in the title of articles in this field, as shown below:

Mokhtar Mutahharah M., Taib Rozainee M., Hassim Mimi H., 2014. Measurement of PCDD/Fs emissions from a coal-fired power plant in Malaysia and establishment of emission. ATMOSPHERIC POLLUTION RESEARCH 5, 388-397.

Shibata E, Yamamoto S, Kasai, E, Nakamura T, 2003. Formation behavior of PCDD/Fs in PVC pyrolysis with copper oxide. CHEMOSPHERE 50, 1235-1242.

Guerriero Ettore, Guarnieri Alessandra, Mosca Silvia, Rossetti Gianluca, Rotatori Mauro, 2009. PCDD/Fs removal efficiency by electrostatic precipitator and wetfine scrubber in an iron ore sintering plant. JOURNAL OF HAZARDOUS MATERIALS 172, 1498-1504.

Comments 2: Introduction: References should be numbered in order of appearance and indicated by a numeral or numerals in square brackets—e.g., [1] or [2,3], or [4–6]. What is the main purpose of the study?

Response 2: Agree. We have adjusted the references citation style, and the revisions could be found in track changes in the re-submitted files.

The main purpose of this study is to figure out the most effective operating mode of the activated carbon in the PCDD/Fs removal filed, which could be achieved by studying on the relationship between working temperature/ activated carbon dosage and the PCDD/Fs removal efficiencies. This main purpose has been clarified in the introduction section (line 112-114), and the revised part is as follows:

“Targeted at applying AC in a way with better cost performance, this article discussed the effect and mechanism of temperature and dosage on PCDD/Fs adsorption by coal-based AC.”

Comments 3: Results and discussion. Figure 3 - very poor quality. It is necessary to improve the quality of the figures a), b) and c)

Response 3: Thanks very much for this comment. We have replaced the original figure with a version of right quality. 

Comments 4: The article does not indicate the novelty of this research.

Response 4: Thank you for pointing this out. We have further clarified the novelty of this research in the introduction section (line 114-124), and the revised part is as follows:

“As previous studies of temperature and AC dosage influence usually carried out on site or adopted model compounds of PCDD/Fs, a trace PCDD/Fs stabilizing generator was innovatively employed in this study to explore the influence law in the lab-scale, which is expected to be more accurate and interference factors free. The adsorption performance, including PCDD/Fs concentration and I-TEQ value before and after adsorption under different temperatures, PCDD/Fs distribution, were investigated. The correlations between temperature/AC dosage and AC adsorption capacity were further explored. Furthermore, with the adoption of XPS analysis, the influence mechanism of temperature on PCDD/Fs chemical adsorption was firstly discussed.”

Comments 5: Author Xiaodong Li cites his articles 8 times? Did I understand correctly from the references list?

Response 5: Thanks very much for this comment. You are right in this question. Xiaodong Li, the director of our research team, lead our team conducted systemic work on PCDD/Fs removal field, especially pioneered PCDD/Fs adsorption study in the mainland of China. Thus, previous articles of our team played an irreplaceable guiding role in this study, and we consider there existed necessity to cite those articles.

1. Summary

Thank you very much for taking the time to review this manuscript. Please find the detailed responses below and the corresponding revisions in track changes in the re-submitted files.

2. Questions for General Evaluation

Reviewer’s Evaluation

Response and Revisions

Does the introduction provide sufficient background and include all relevant references?

Yes

Are all the cited references relevant to the research?

Yes

Is the research design appropriate?

Must be improved

Corresponding responses have been given in the point-by-point response letter.

Are the methods adequately described?

Must be improved

Are the results clearly presented?

Must be improved

Are the conclusions supported by the results?

Must be improved

3. Point-by-point response to Comments and Suggestions for Authors

Comments 1: The authors carefully prepared the Introduction chapter, diligently described the experiment, presented and justified the results by discussing them, and presented the conclusions. Unfortunately, they did not present the purpose of the research.

Response 1: Thank you for pointing this out. The main purpose of this study is to figure out the most effective operating mode of the activated carbon in the PCDD/Fs removal filed, which could be achieved by studying on the relationship between working temperature/ activated carbon dosage and the PCDD/Fs removal efficiencies. This main purpose has been clarified in the introduction section (line 112-114), and the revised part is as follows:

“Targeted at applying AC in a way with better cost performance, this article discussed the effect and mechanism of temperature and dosage on PCDD/Fs adsorption by coal-based AC.”

Comments 2: Analyzing the content of the chapters on descriptions of the conducted experiment and discussion of the results, I do not see the purpose/sense of the research carried out in the presented manner. Although the problem of removing toxic PCDD/F from exhaust gases is very important, the research carried out by the authors does not bring anything new - neither in the field of science nor economically. This statement of mine is fully reflected in the cited bibliographic items (in the Introduction and Results and Discussion chapters).

Response 2: Thanks very much for this comment. The novelty of this research is mainly embodied in two aspects: 1) Lab-scale experiments based on the trace PCDD/Fs stabilizing generator. Owing to the fact that PCDD/F is a kind of trace organic pollutants which is hard to generate with a steady flow, previous studies of temperature and AC dosage influence (stated in the introduction section, line 71-98) usually carried out on site or adopted model compounds of PCDD/Fs. However, field experiments often have many interference factors, and model compounds are usually unsatisfactory in adsorption research due to the high molecular weight of PCDD/Fs. Therefore, this lab-scale research with real PCDD/Fs adsorption could afford a more accurate and reliable result of the temperature and AC dosage influence study. 2) Effect of temperature change on the AC chemical adsorption. This research firstly studied the variation of the AC surface functional groups after the temperature change, which could help explain the mechanism of the temperature influence from the perspective of chemical adsorption.

Based on the above statement, we have further clarified the novelty of this research in the introduction section (line 114-124), and the revised part is as follows:

“As previous studies of temperature and AC dosage influence usually carried out on site or adopted model compounds of PCDD/Fs, a trace PCDD/Fs stabilizing generator was innovatively employed in this study to explore the influence law in the lab-scale, which is expected to be more accurate and interference factors free. The adsorption performance, including PCDD/Fs concentration and I-TEQ value before and after adsorption under different temperatures, PCDD/Fs distribution, were investigated. The correlations between temperature/AC dosage and AC adsorption capacity were further explored. Furthermore, with the adoption of XPS analysis, the influence mechanism of temperature on PCDD/Fs chemical adsorption was firstly discussed.”

Comments 3: The authors used commercially produced and widely used activated carbon for PCDD/F adsorption (it was not specified what raw material it is made from) - "One kind of commercial coal-based AC". This means that the manufacturers of exhaust gas purification installations (including PCDD/F adsorption) for MSW incinerators have developed the operating conditions of these devices - e.g., temperature, exhaust gas flow rate, AC dose, etc. Therefore, it is not a scientific discovery, e.g., that an increase in temperature reduces the effectiveness PCDD/F sorption. This is known not only for the adsorption of PCDD/Fs. Adsorption is an exothermic process.

Response 3: Thanks very much for this comment. The coal-based AC used in this study is one kind of commercial AC which have already been adopted in the solid waste incineration plants. We considered that it is representative to apply this AC to the operating conditions research. Actually, in the practical application, there have no very normative instructions of the AC operating conditions. As mentioned in line 71-74, the temperature at the fabric filter is generally fluctuated between 140 and 160 ℃, and would increase to 220-240 ℃ when catalytic fabric was used. While the AC dosage are usually determined based on experience and are likely to use excessive AC to ensure the standard emission of PCDD/Fs. Therefore, we think this research is meaningful and the pick of the often-used AC is also reasonable.

Besides, instead of the simple conclusion that an increase in temperature reduced the effectiveness PCDD/F sorption, the study of the temperature influence in this article is more focused on the exponential correlation between temperature and effectiveness PCDD/F sorption, which have tangible guiding implications in PCDD/Fs adsorption.

Comments 4: These studies would make sense if the authors comparatively performed an analogous series of tests for e.g., modified ACs (it would then be possible to obtain materials with much better adsorption and catalytic properties or with a strictly defined hydrophobic-hydrophilic character). It was possible to find a substance that modifies the surface of the adsorbent in order to significantly improve the adsorption properties towards PCDD/F. Or the effectiveness of AC made from different raw materials could be comparatively tested. The variety of raw materials from which activated carbons are produced is very large, e.g. hard coal, brown coal, peat, wood, natural raw materials of organic origin, fruit seeds, polymer precursors, polyvinylidene fluoride, polyacrylonitrile, polyfurfuryl alcohol, polyaniline, polymides, polystyrene , polydivinylbenzene, polythiophene, phenol-formaldehyde resins, polyethylene terephthalate, formaldehyde and resorcinol, sulfonated styrene-divinylbenzene resin, polymer waste, car tires, polyethylene terephthalate-1,4-phenyldiamide fibers known under the trade name Kevlar®.

Response 4: Thanks very much for this comment. This study aimed at researching on the appropriate operating conditions of AC on PCDD/Fs removal, and it’s expected to have good generality. Therefore, we consider it unsuitable to develop modified AC or other materials in this study, as the widely used material in PCDD/Fs removal field is normal porous AC. We still appreciate your comment, which is valuable and instructive for our follow-up studies.

Comments 5: Please provide the conditions for GC analysis, i.e., dispenser temperature, column temperature. Was the column temperature programmed? If so, please provide.

Was the gas chromatograph equipped with its own detector (in addition to being coupled to the mass spectrometer)? If so, please provide the type of detector.

Response 5: Thank you for pointing this out. The column temperature program was added in section 2.3 (line 182-186), and the revised part is as follows:

“Following was the column temperature program of GC: splitless injection of 1 μl at the in-itial temperature (150 ℃), which was kept constant for 1 min and then increased to 190 ℃ at a 25 ℃/min rate. Finally, the temperature was increased to 280 ℃ as a rate of 3 ℃/min, and that temperature was held for 20 min.”

The gas chromatograph was coupled to the mass spectrometer and doesn’t equipped with other detector.