Evaluation of Indirect-Heated Microwave Thermal Desorption Treatment on Engineering Properties of Lubricant-Contaminated Soil

Round 1

Reviewer 1 Report

Lee et al.,

This manuscript presents experimental results demonstrating the implications of indirect-heated microwave thermal desorption on the geotechnical properties of lubricant oil-contaminated soil.  The experiments appear to have been well-performed, and the findings demonstrate that this thermal desorption technique can be an effective remediation method that also restores the geotechnical integrity of the soil to pre-contaminated conditions.  My comments below are principally focused on the experimental variability and improving the clarity and conciseness of the manuscript.  I hope the authors find this feedback valuable.

General Comments

1. It would be ideal to include more information on the repeatability of the experimental results. This variability would help improve the interpretation of the findings and help other researchers conducting similar experiments.

2. While the authors do indeed show that the geotechnical properties largely returned to initial values upon thermal desorption, it would be valuable to comment briefly on the differences between the uncontaminated and treated soil. One potential explanation may be the heterogeneity in the soil or repeatability of the analyses. 

Specific Comments

1. Lines 131-133: From Figure 4, it looks like 400, 500, and 600 °C was achieved after ~80, 110, and 180 minutes, respectively, from applying 32 kW only. As this paragraph is currently written, I think the authors claim that that: (i) 400 °C was achieved after 80 minutes using 16 kW, (ii) 500 °C was achieved after 110 minutes using 24 kW, and (iii) 600 °C was achieved after 180 minutes using 32 kW.  Perhaps this is a grammar error?

2. Section 2.3.1.: Can the authors please provide more details on their experimental methodology. Currently, there limited information is provided for other researchers to replicate the methods.  g., (i) What was the make/model of gas chromatography instrument used? (ii) What was the purity of their reagents – like sodium sulfuric anhydride and dichloromethane? (iii) What was the make/model of the ultrasonic extractor used? (iv) What type of filters were used?

3. Can the authors please define the acronyms NP and SM used in Table 4.

4. Line 191: the authors note the permeability units in cm/sec. Are the authors referring to the hydraulic conductivity here – or perhaps there is a typo in the permeability units, which should be length^2.

5. Section 3.3: I think this short section is interesting; however, I am not sure where the authors gathered their values from. Can the authors please expand here on the assumptions they made for their comparison and cite where they got their information?

6. Figures 9 can be improved. Typically, it is best practice to make figures as clear and accessible as possible, e.g., without unnecessary 3D effects and in a greyscale for colourblind readers.  In addition, I suggest the authors use different line types to differentiate the data in Figure 8 to also improve this figure’s accessibility.

7. There are grammar errors throughout, I suggest the article be reviewed by a native English speaker. Below are a few issues I noticed *starred*:
   • (Line 267) Treated soil is *hope* to be reusable… - grammar error
   • (Lines 221-222) 2.45GHz microwave stimulates microwave absorber to *highly generate heat* the surface of the inner container… -grammar error

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Reviewer 2 Report

The subject of the paper is of interest, however, the paper is poorly-written, the manuscript is not clear and the presented data are not properly supported by experimental evidence.
Due to the aforementioned problems, publication in the current for is not recommended, however, it should be considered after a major revision.

Remarks and observations:

• The wording of the paper should be revised, as the microwave (MW) induced thermal desorption is an existing technique (DOI: 10.1016/j.jaap.2007.07.004, DOI: 10.1016/j.chemosphere.2019.01.079, etc), and the concept was not developed by the Authors as mentioned in for instance: abstract, lines 14-15. Ex-situ thermal desorption methods might include direct or indirect heating by steam, oil, electrical resistance heating, radio-frequency heating etc. Please present the current state of soil-remediation methods, with the existing MW heating techniques and compare your results with them.
• The irreversible structural alteration of soils heated above cc. 350°C should also be considered when evaluating the effect of thermal treatment. At elevated temperatures, the thermal dehydroxylation of various oxy-hydroxydes (T>250°C) and clay minerals (T>350-400°C) and their subsequent transformation are also taking place, which would alter soil composition.
• line 95: please describe the term “Atterberg limits”, as it might not be evident for all Readers.
• lines 97-104: Was the investigated soil originally contaminated? What is the composition, properties of the applied artificial lubricant oil contaminant?
• Table 1, Table 4: please describe the applied abbreviations, such as SM, NP
• Figure 1: Why was the soil particle-distribution not investigated in the fine fraction region? Most of the pollutants are expected to be adsorbed by and therefore are expected to be found in these soil constituents.
• line 116-124: the role of the external MW adsorbent is not clear. Soil constituents and the present pollutants having a dipole is susceptible to MW adsorption and subsequent heating. Please clarify the role and properties of the adsorbents.
• Figure 4: graphs could be improved, there is an empty space above 250°C. Where was the temperature measured? Please indicate it on Figure 2. Was the heat-distribution effect, the uneven heat-zone considered when the temperature was measured?
• line 142-143: Please elaborate, what do you mean by “depending on the degree of pollution”?
• Chapter 2.3. The description and measurement parameters of GC is not given. Please amend.
• Chapter 2.3.2. Please describe the measurement protocol, parameters and applied equipment.
• Please apply an uniform terminology in the text (lubricant oil vs lubricating oil, line 97 vs Figure 6)
• Figure 6: Please provide a typical chromatogram as an experimental evidence supporting the results. Figure 6/b, y-axis label should be revised, it is incorrectly displayed.
• Figure 8/a: evaluation of the results are not given in the text. Please amend.
  Figure 8/b: How was the water content modified? Please describe and evaluate.
• lines 199-121: text should be revised, data might be given in a Table for a better comprehension.
• Chapter 3.3.: the presented data is not supported by experimental evidence or calculations. How was the CO2 and the operation cost values determined? What equipment/method was utilized? Given the sources or the calculation parameters for the cost estimation. Please clarify as the results are no comprehensible.
• Conclusion should be revised and improved. Soil and contaminant properties should also be included in the evaluation, specially when the comparing the results with other methods.
• Extensive English revision is required, there are many grammatical errors. A few examples:
  • line 19: “to reach as target” should be “to reach target”
  • line 23: The term “Treated soil is hope to be reusable” is incorrect (“is hoped to be”) and should be avoided and objective statements should be applied.
  • line 55: “using the microwave” should be “using microwave”
  • line 109-113: sentence should be revised
  • line 141: “gas chromatography” should be “by gas chromatography”
  • line 143: please revise “sodium sulfuric anhydride”. Is it supposed to be sodium-sulfate anhydride?
  • line 177: terms such as “we can decide” should be avoided.
  • line 190: terms such as “the liquid limit came back to NP” should be avoided.
  • line 215: “cost was shown” should be “cost is shown”

Author Response

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Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

Lee et al.,
Thank you for providing robust responses to my questions. I think the manuscript has significantly improved; however, I still have outstanding comments below. I hope the authors find this additional feedback valuable.

Please see the attachment.

Comments for author File: Comments.pdf

Author Response

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Author Response File: Author Response.docx

Reviewer 2 Report

The answers for the questions are well prepared and appreciated. The Authors have improved the article and properly addressed the questions, however, issues are still present, and many valuable information are not incorporated to the manuscript. These few concerns should be addressed before publication of this otherwise informative article.

Remarks and observations:

• A major problem is that the changes implemented via the revision are not traced! Part of the manuscript is highlighted in yellow, but changes are not easily assessed and not all of the changes are highlighted (e.g. lines 215-226 (v1) vs lines 230-241 (v2, revised) is changed without indication). It would be more beneficial if a corrected version is provided, where the changes are tracked.
• Answer to Q1 is convincing and appreciable, however no related changes are implemented in the manuscript. It would be beneficial to articulate the novelty and placement/comparison of this new MW methods – e.g. Comparison of Microwave heating [MWH] technology presented in the answer seems perfect for the case.
• Answer to Q2 is acceptable, however there is no information about exclusive treatment of sandy-soils, therefore the general disadvantage of this method should be articulated, as not only desorption is taking place (lines 62-63, 250-251), but the soil-constituents can also be transformed via the structural dehydroxylation, which could result in the limitation of soil function. This could be beneficial information for the Readers, when assessing the applicability of the method.
• The chemical composition of the applied pollutant should also be given in the manuscript, as the Readers will not see your answers to Q4. The inclusion of Table Composition of lubricant oil is recommended.
• The answer to Q6 is appreciated, however it should be included in the manuscript. The addition of relevant data to Table 1 is proposed.
• No (typical TPH) chromatogram was provided as an experimental evidence supporting the results displayed in Figure 6. It could be supplemented in an annex or supplementary file.
• Relevant information about the applied instrumentation and methods should be included in the manuscript, not only as an answer to Q10. For instance, the GC instrument model/manufacturer, column type, flow rate of gas, oven temperature should be given for a proper evaluation of the method.
• Figure 6/b, y-axis label should still be revised: currently it reads “Residual TPH efficiency in soil” which would imply that the residual TPH concentration is increasing, and the removal efficiency is decreasing by the treatment time. The label should be “removal efficiency” and expressed in (%). Please revise.
• Figure 8 revision and its evaluation is appreciated, however, it is still not clear how the water content of the samples was varied between cc. 2-16% (Figure 8/b). Also, the applied measurement protocols should be given in chapter 2.3. – please clearly indicate what protocol was applied for the determination of the indicated water content.
• The answer to Q16 regarding the energy costs and CO2 emission is well appreciated, however, it should also be presented in the article or as supplementary material to support the claims. 

Author Response

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Author Response File: Author Response.docx