Impact of Natural Weathering on Stabilization of Heavy Metals (Cu, Zn, and Pb) in MSWI Bottom Ash
Round 1
Reviewer 1 Report
In the paper, the influence of natural weathering on the stabilization of heavy metals in MSWI bottom ash is investigated. The results are interesting in terms of bottom ash treatment and disposal according to the circular economy.
Some revisions are required:
1. The language style needs correction. There are numerous minor stylistic issues throughout the article, for example:
in line 16 stabilization is used two times in the sentence,
in line 130 "(1-8%)" should rather be at the end of the sentence,
in lines 183-184 the word order seems strange,
in line 310 a word seems to be missing.
These are just a few examples. Such issues throughout the article make it harder to read in places.
2. Please update the introduction section. Similar research from the literature should be briefly elaborated and their results should be presented, so that they can be compared to your results.
3. The methodology needs a better explanation. Please provide more details and parameters about the weathering procedure. Section 2.2 is not covering this issue enough.
4. Table 1: please add unit (wt%) in the table caption.
5. Table 1 and Table 2: Why is the composition of BA presented as elemental composition in table 1 and as oxide composition in table 2? Usually, oxide analysis is used for ash characterization.
6. Table 3: Why don't the authors present specific values?
5. Figure 3: What is the purpose to compare the elemental composition of BA to the Earth's crust? Please explain.
6. Figure 6 and Figure 7: Please add chart legends
7. Figures 8-10 Please evaluate the concentrations against the European Union limits rather than Lithuanian limits. In my opinion, the EU limits will be more relevant to the international audience.
8. I suggest the conversion of graphs 8-10 into the color mode to increase their readability.
Author Response
Thanks for your review. We provide our response and a revised manuscript.
Author Response File: 
Author Response.docx
Reviewer 2 Report
This work includes a characterisation of bottom ash before and after an ageing process. The topic of the work is actual and interesting. However, the paper is unclear in several parts and needs to be developed further. In the Reviewer´s opinion, the following questions should be answered in the paper.
- The term “natural weathering” is used in many areas to designate the degradation suffered by materials or products exposed outdoors to atmospheric conditions. In this work, the use of natural weathering is ambiguous: e.g., Line 9: “natural weathering (ageing)”; Line 21: “natural weathering (laboratory aging); Line 26 “(weathering or aging)”; Line 47 “natural weathering (aging or carbonation). This confusion of terms should be avoided. The authors should also analyse whether the term natural weathering is in fact the correct term to designate the aging process to which BA was subjected.
- It is not clear what the innovation of this work is. What are the difference from previous works?
- Reference should be made to existing works on the topic of the current research. A more in-depth introduction is needed.
- Authors should explain why they studied copper, zinc and lead and not other metals.
- Abbreviations must be defined the first time they appear in the text.
- The methods of the EN 933 do not recommend the use of a greater number of sieves? Why was the characterisation not performed below 5.6 mm?
- The description of the natural weathering experiments (section 2.2) is not clear. It is difficult to understand the characteristics of the tests. This sections should be highly improved, including a complete description of the ageing conditions and procedures.
- The purpose of analysing BA in different fractions is not clear. Why not analyse it as a whole? What do the different fractions represent?
- What was the criterion for choosing and testing these fractions? One of them included more than 50% in weight. Information is also needed about how the different fractions were obtained.
- The fraction “< 40.0 mm” is correct? Does this represent the whole sample without the particles with dimensions higher than 40 mm? Why to test it like this?
- Figures 1a and 1b show pretty much the same thing, only one is needed.
- Errors are missing in Figure 2.
- In Section 3, it is not always clear which technique was used for the analyses (e.g. in the analyses of the metals).
- How were the error ranges determined? Are they confidence limits?
- Why does Figure 3 compare only 7 elements and not the 10 shown in Table 1?
- Line 142: Why these 5 metals and not others? What was the reason for choosing these?
- Lines 144-148: This discussion is relatively poor.
- Figure 4: The meaning of <40.0 N and <40.0L was not explained.
- The presentation of results in Section 3.4 is sometimes confusing and needs improvement.
- Tables 2 and 3 should be improved for better reading.
- How was Mn and Sr determined?
- Equation 2: it is silicon (Si), not sulphur (S) and iodine (I).
- Equation 3 is not correct (e.g. sulphur is in the products but not in the reagents).
- Line 247: what is the reason to compare with Lithuania’s limits? Why not to compare it with more broad requirements (e.g. European requirements)?
- What use do the authors foresee for BA? This is not clear in the paper. Mentioning only civil engineering is too vague. Depending on the application, different requirements may be required.
- The conclusions sections is practically a repetition of the results obtained. The authors should make a more general analysis of the results and what their implications are.
- Authors should be careful when using text/information in parentheses. In many cases, they make sentences confusing.
- In general, the English quality is acceptable/good. However, some grammatical errors and misspellings can be found in the paper. The purpose of this review is not to correct the English, but is suggested that authors pay attention to this aspect.
Author Response
Thanks for your review. We provide our response and a revised manuscript.
Author Response File: Author Response.docx
Reviewer 3 Report
The manuscript presents results on the impact of natural weathering of heavy metals of MSWI bottom ash. The authors did not explain this impact. Moreover, there are some problems in the writing and experimental analysis, so we suggest that it can be accepted after careful revision.
- In the experimental section, the authors should provide more details about weathering experiments in the laboratory (ex. temperature, laboratory stand, etc.).
- The authors mentioned that "Mineral composition analysis was performed using an X-ray diffractometer SmartLab (Rigaku) with a 9kW rotating Cu anode X-ray tube. X-rays are recorded in the 20-10-75 degree range using the Bragg-Brentan method" (line 84-86). In the manuscript, these XRD results are unavailable. The authors should provide the XRD pattern for all measurements with appropriate PDF cards for all detected phases (ex. in Supporting Information).
- The line and points visible in Figure 7 should be signed.
- In the Conclusion section, the authors should discuss why natural and laboratory weathering gives similar results and explain what affects the stabilization of heavy metals of MSWI bottom ash.
Author Response
Thanks for your review. We provide our response and a revised manuscript.
Author Response File: Author Response.docx
Round 2
Reviewer 3 Report
Accept in present form
Author Response
The text has been checked for correct use of grammar and common
technical terms, and edited to a level suitable for reporting research in a scholarly journal by MDPI uses experienced, native English speaking editors.
Author Response File: 
Author Response.pdf
