Experimental Investigation of Water-Retaining and Unsaturated Infiltration Characteristics of Loess Soils Imbued with Microplastics
Round 1
Reviewer 1 Report
The authors have explored the effect of microplastics on unsaturated infiltration characteristics of loess soils by simulating the rainfall irrigation process, by using a one-dimensional vertical soil column rainfall infiltration test device. Although such a research topic is innovative and of some significance to agriculture, this manuscript shows the lack of rigor in the results analysis and other parts. Therefore, the manuscript must undergo major revisions if it is to be accepted.
Specific comments:
1. Line 21: The phrase “clean up” used here seems inappropriate. It is suggested that the author use a more authentic expression.
2. Line 24: What is the purpose of putting this picture here? Why is there no figure number, name, or description?
3. Line 105-106: Dose the I value of 2:1 mean 60 minutes for rain and 60 minutes for rest? Obviously, here is a mistake. It will mislead the reader and the results following will be incomprehensible.
4. Line 108-111: I am not sure whether the washing of the soil will affect the simulation results of this experiment to represent the actual soil level. Are there any similar studies that show the feasibility of doing this? Will this damage the soil's original properties?
5. Fig. 2: All the pictures in this manuscript are not clear enough, which makes it difficult for readers to read (This problem will not be repeated later in these comments).
6. Line 184-186: There is no rational explanation or inference for particular phenomena.
7. Line 191-192: Here is a serious mistake. The analysis statement here is not consistent with the data in Fig. 3. Apparently, the authors have confused the numbering of the sensors.
8. Fig. 6: Why wasn't the average conductivity C similarly analyzed here as in Fig. 2?
9. Line 205-210: The discussion about the effect of microplastic content should be put in Section 3.1. Besides, the data results of supplementing rainwater infiltration tests could be included in the supplementary attachment.
10. Line 394-395: This conclusion is not consistent with the results of this experiment and is even wrong. Unique experimental phenomena were not taken into account, such as those for microplastics with a size of 5μm at a content of 0.05% or 8μm at a content of 0.25%. Why not try to be more rigorous?
11. There is no separate discussion section in this manuscript regarding the experimental data results. According to the only analysis content in the manuscript, the author's analysis and discussion of the data results are not very sufficient. It must be mentioned here that, except for the introduction, no references are cited in other sections of this manuscript, which makes this manuscript very imprecise. At least, in my opinion, when analyzing and discussing the data results, the author should make reasonable explanations or inferences about the causes of the phenomenon, and cite relevant literature as the basis.
Author Response
Dear Editors and Reviewers:
Thank you for your letter and for the reviewers’ comments concerning our manuscript entitled “Experimental investigation of water-retaining and unsaturated infiltration characteristics of loess soils imbued with microplastics” (ID: sustainability-2016132). Those comments are all valuable and very helpful for revising and improving our paper, as well as the important guiding significance to our researches. We have studied comments carefully and have made correction which we hope meet with approval. Revised portion are marked in red in the paper. The main corrections in the paper and the responds to the reviewer’s comments are as following.
Response to Reviewer 1 Comments
Point 1: Line 21: The phrase “clean up” used here seems inappropriate. It is suggested that the author use a more authentic expression.
Response 1: Thanks to the reviewer's suggestions. We have changed expression. After careful consideration, we use “dispose” to substitute for the phrase “clean up”. If it is still inappropriate, please let us know. We appreciate it very much.
Point 2: Line 24: What is the purpose of putting this picture here? Why is there no figure number, name, or description?
Response 2: This picture is a Graphical Abstract which is an image that appears alongside the text abstract in the Table of Contents. In addition to summarizing the content, it should represent the topic of the article in an attention-grabbing way. When we read the guideline for authors, we notice that a Graphical Abstract is needed. We are sorry that there is no description and have made the supplement. We have redrawn the picture.
Point 3: Line 105-106: Dose the I value of 2:1 mean 60 minutes for rain and 60 minutes for rest? Obviously, here is a mistake. It will mislead the reader and the results following will be incomprehensible.
Response 3: Thanks to the reviewer’s suggestions. We are sorry for making the mistake. The rain interval 2:1 indicates 60 min for rain and 30 minutes for rest. The mistake has been corrected in Line 108-109.
Point 4: Line 108-111: I am not sure whether the washing of the soil will affect the simulation results of this experiment to represent the actual soil level. Are there any similar studies that show the feasibility of doing this? Will this damage the soil's original properties?
Response 4: Thanks to the reviewer’s suggestions. According to the same experiments conducted before (Liu, Q., Xi, P., Miao, J., Li, X., Wang, K. 2020. Applicability of wetting front advancing method in the sand to silty clay soils. Soil and Foundation, 60(5), 1215-1225.), it is feasibility to wash soil to reduce disturbance before conducting experiments. In our study, there exist invasive substances such as litter, plant rhizomes, microplastics and so on in original soils. We explore water-retaining and the unsaturated infiltration characteristics of soil imbued with microplastics under rainfall conditions. The existing microplastics must be removed to avoid confuse with microplastics added in experiments. Other substances like litter and plant rhizomes must be removed because they would also affect rainwater infiltration, influencing experiment results.
Point 5: Fig. 2: All the pictures in this manuscript are not clear enough, which makes it difficult for readers to read (This problem will not be repeated later in these comments).
Response 5: Thanks to the reviewer’s suggestions. We are sorry for the difficulty of reading due to the fuzzy pictures. We have tried our best to make improvements.
Point 6: Line 184-186: There is no rational explanation or inference for particular phenomena.
Response 7: Thanks to the reviewer’s suggestions. The existence of polystyrene microplastics in soils could fill the soil particle skeletons to bond and block water, and polystyrene microplastics themselves have strong hydrophobicity. Current research (Yang et al., 2015, Yang and Wu, 2020, You et al., 2022, Keatts et al., 2018) shows that soil particles are often regarded as completely hydrophilic solids. The contact angle of soil increases and soil particles represent hydrophobic properties owing to the addition of hydrophobic materials. Meanwhile, microplastics would fill pores of soil particles and decrease the soil permeability coefficient, representing blocking effects. The two effects are contradictory and would play a dominant role in different situations with the change of soil compactness. When p is constant and q is relatively small, the strong hydrophobicity of microplastics plays a dominant role. Microplastics promote water infiltration. With q increasing, microplastics compact soil particles, representing blocking effects overall. The blocking effects enhance with q increase.
This explanation has been added in Line 195-205.
Reference:
Keatts, M.I., Daniels, J.L., Langley, W.G., Pando, M.A., Ogunro, V.O., (2018). Apparent contact angle and water entry head measurements for organo-silane modified sand and coal fly ash. Journal of Geotechnical and Geoenvironmental Engineering, 144(6), 1090-0241. http://10.1061/(ASCE)GT.1943-5606.0001887.
Yang, S., Gong, A.M., Wu, J.H., Lu, T.H., (2015). Effect of contact angle on matric suction of unsaturated soil. Rock and Soil Mechanics. 36(3), 674-678.
http://10.16285/j.rsm.2015.03.010. http://doi.org/10.16285/j.rsm.2019.2.2187.
Yang, S., Wu, J.H., (2020). Evaporation characteristics of liquid drops on silt soil surface and the influences on soil-water characteristic curve. Rock and Soil Mechanics. S(2), 1-9.
You, Y., Xiao, H.L., Tan, Y., (2022). Experimental research on permeability of hydrophobic material modified clay. Journal of China Three Gorges University. 44(1):14-19. http://10.13393/j.cnki.issn.1672-948X.2022.01.002.
Point 7: Line 191-192: Here is a serious mistake. The analysis statement here is not consistent with the data in Fig. 3. Apparently, the authors have confused the numbering of the sensors.
Response 7: Thanks to the reviewer’s suggestions. We are sorry for making this mistake. We have confused the numbering of the sensors and have corrected it in Line 208-210.
Point 8: Fig. 6: Why wasn't the average conductivity C similarly analyzed here as in Fig. 2?
Response 8: Thanks to the reviewer’s suggestions. We are sorry for lack of roger and the time-history curves of average conductivity C under different microplastic particle size (p) and soil bulk density (γ) have been added.
Point 9: Line 205-210: The discussion about the effect of microplastic content should be put in Section 3.1. Besides, the data results of supplementing rainwater infiltration tests could be included in the supplementary attachment.
Response 9: Thanks to the reviewer’s suggestions. In Part.3, we explore the influence of the microplastic content (q), microplastic particle size (p), soil bulk density (γ) and intermittent rainfall ratio (i) on loess soil rainfall infiltration. In each section of Part.3, we discuss the effects of one parameter. In section 3.2, according to supplement experiments, it could be obtained that microplastics with p of 3μm q of 0.05%, 0.10%, 0.25% promote water infiltration while microplastics with q of 0.50% block water infiltration. Controlling the mass of microplastics as constant, if p decrease, the amount of microplastics that compact soil particles and have blocking effects should increase. That explains the microplastic content range that promotes infiltration increases with p decreasing. We would like to emphasize effects of the parameter microplastic particle size (p) on water infiltration. Therefore, we put this part in Section 3.2.
We are sorry for have not included the data results of supplementing rainwater infiltration tests in the supplementary attachment. We have included the experiment data in the supplementary attachment file.
Point 10: Line 394-395: This conclusion is not consistent with the results of this experiment and is even wrong. Unique experimental phenomena were not taken into account, such as those for microplastics with a size of 5μm at a content of 0.05% or 8μm at a content of 0.25%. Why not try to be more rigorous?
Response 10: Thanks to the reviewer’s suggestions. We are sorry for our lack of rigor. We have checked the experiment data and reviewed relevant literature (Su, Y., Zhu, Jian., Wang, Ping., Wang, D.B., Lin, Y. Research progress in soil water holding capacity. 2013. Chinese Agriculture Science Bulletin. 29(14):140-145.). We noticed that the conclusions are not roger. It could only be concluded that the existence of microplastics reduces soil water-retaining capacity. The peak soil moisture contents (θmax) are all less than that of the blank experiment control group. However, the changes of θmax with the changes of q, p and g are not obvious. It could not get the conclusion like the smaller the microplastic particles are, the weaker the soil water-retaining capacity. The experiment analysis part and conclusion part are all corrected.
As to the specific changing regulations of soil water-retaining capacity under the influence of single or coupled parameters still needed further explore. We are going to do further research in this part.
Point 11: There is no separate discussion section in this manuscript regarding the experimental data results. According to the only analysis content in the manuscript, the author's analysis and discussion of the data results are not very sufficient. It must be mentioned here that, except for the introduction, no references are cited in other sections of this manuscript, which makes this manuscript very imprecise. At least, in my opinion, when analyzing and discussing the data results, the author should make reasonable explanations or inferences about the causes of the phenomenon, and cite relevant literature as the basis.
Response 11: Thanks to the reviewer’s suggestions. The suggestions are very helpful and help us to make depth analysis of the reasons behind the experimental phenomenon. We appreciate it very much and have made improvements in Part.3.
We tried our best to improve the manuscript and made some changes in the manuscript. These changes will not influence the content and framework of the paper. And here we did not list the changes but marked in red in revised paper.
We appreciate for Editors/Reviewers’ warm work earnestly, and hope that the correction will meet with approval.
Once again, thank you very much for your comments and suggestions.
Author Response File: 
Author Response.docx
Reviewer 2 Report
In this manuscript, a one-dimensional vertical soil column rainfall infiltration test device was used to study the unsaturated infiltration characteristics of loess soil imbued with microplastics under rainfall conditions. This research has certain innovation and practical value, and the manuscript is well written. I think this manuscript can be accepted with minor revisions:
(1) Incorrect formatting of symbols in the introduction or even throughout the manuscript. It should be written in "the physical meaning of the symbol (symbol)" format. There is no need for parentheses when using symbols alone to express physical quantities. However, when the symbol is used with the physical quantity it represents, parentheses should be placed on the symbol, otherwise it will cause duplication.
(2) The theme of this manuscript is the effects of microplastics on soil water-retaining ability and water permeability. However, there are few literature reviews on soil water retaining ability and water permanence in the introduction. It is suggested that the author should add the latest literature in this field. The following literature is for reference:
Study of permeability of calcareous silty layer of foundation at an artificial reclamation island.
Experimental study on permeability characteristics of calcareous soil.
(3)In line 76-83: It is suggested to draw the particle size distribution curve of loess.
(4)In line 149-151: ‘The minimum infiltration rate λmin with a q value of 0.05% is larger than that of the blank experimental control group, while λmin with a q of 0.10%, 0.25%, and 0.50% is smaller than that of the blank experimental control group.’ The author should briefly describe the causes of this experimental phenomenon.
(5)In line 229-230: ‘the smaller the microplastic particles are, the weaker the soil water-retaining capacity.’ This conclusion seems to be inconsistent with our common sense. It is generally believed that the smaller the particle size is, the stronger the soil water-retaining capacity is.
(6)In line 363: The physical meaning of the parameter in Equation 19 should be added.
(7) The conclusion of the article is a little long. Can you simplify it appropriately.
Author Response
Dear Editors and Reviewers:
Thank you for your letter and for the reviewers’ comments concerning our manuscript entitled “Experimental investigation of water-retaining and unsaturated infiltration characteristics of loess soils imbued with microplastics” (ID: sustainability-2016132). Those comments are all valuable and very helpful for revising and improving our paper, as well as the important guiding significance to our researches. We have studied comments carefully and have made correction which we hope meet with approval. Revised portion are marked in red in the paper. The main corrections in the paper and the responds to the reviewer’s comments are as following.
Response to Reviewer 2 Comments
Point 1: Incorrect formatting of symbols in the introduction or even throughout the manuscript. It should be written in "the physical meaning of the symbol (symbol)" format. There is no need for parentheses when using symbols alone to express physical quantities. However, when the symbol is used with the physical quantity it represents, parentheses should be placed on the symbol, otherwise it will cause duplication.
Response 1: Thanks to the reviewer's suggestions and all incorrect formatting of symbols have been checked in paper.
Point 2: The theme of this manuscript is the effects of microplastics on soil water-retaining ability and water permeability. However, there are few literature reviews on soil water retaining ability and water permanence in the introduction. It is suggested that the author should add the latest literature in this field. The following literature is for reference:
Response 2: Thanks to the reviewer's suggestions and we have added the relevant references about soil water retaining ability in paper.
Point 3: In line 76-83: It is suggested to draw the particle size distribution curve of loess.
Response 3: Thanks to the reviewer's suggestions. We have drawn the particle size distribution curve of loess in Fig.1 of the supplementing attachment file.
Point 4: In line 149-151: ‘The minimum infiltration rate λmin with a q value of 0.05% is larger than that of the blank experimental control group, while λmin with a q of 0.10%, 0.25%, and 0.50% is smaller than that of the blank experimental control group.’ The author should briefly describe the causes of this experimental phenomenon.
Response 4: Thanks to the reviewer’s suggestions. The existence of polystyrene microplastics in soils could fill the soil particle skeletons to bond and block water, and polystyrene microplastics themselves have strong hydrophobicity. Current research (Yang et al., 2015, Yang and Wu, 2020, You et al., 2022, Keatts et al., 2018) shows that soil particles are often regarded as completely hydrophilic solids. The contact angle of soil increases and soil particles represent hydrophobic properties owing to the addition of hydrophobic materials. Meanwhile, microplastics would fill pores of soil particles and decrease the soil permeability coefficient, representing blocking effects. The two effects are contradictory and would play a dominant role in different situations with the change of soil compactness. When p is constant and q is relatively small, the strong hydrophobicity of microplastics plays a dominant role. Microplastics promote water infiltration. With q increasing, microplastics compact soil particles, representing blocking effects overall. The blocking effects enhance with q increase.
This explanation has been added in Line 195-205.
Reference:
Keatts, M.I., Daniels, J.L., Langley, W.G., Pando, M.A., Ogunro, V.O., (2018). Apparent contact angle and water entry head measurements for organo-silane modified sand and coal fly ash. Journal of Geotechnical and Geoenvironmental Engineering, 144(6), 1090-0241. http://10.1061/(ASCE)GT.1943-5606.0001887.
Yang, S., Gong, A.M., Wu, J.H., Lu, T.H., (2015). Effect of contact angle on matric suction of unsaturated soil. Rock and Soil Mechanics. 36(3), 674-678.
http://10.16285/j.rsm.2015.03.010. http://doi.org/10.16285/j.rsm.2019.2.2187.
Yang, S., Wu, J.H., (2020). Evaporation characteristics of liquid drops on silt soil surface and the influences on soil-water characteristic curve. Rock and Soil Mechanics. S(2), 1-9.
You, Y., Xiao, H.L., Tan, Y., (2022). Experimental research on permeability of hydrophobic material modified clay. Journal of China Three Gorges University. 44(1):14-19. http://10.13393/j.cnki.issn.1672-948X.2022.01.002.
Point 5: In line 229-230: ‘the smaller the microplastic particles are, the weaker the soil water-retaining capacity.’ This conclusion seems to be inconsistent with our common sense. It is generally believed that the smaller the particle size is, the stronger the soil water-retaining capacity is.
Response 5: Thanks to the reviewer's suggestions. We are sorry for our lack of rigor. We have checked the experiment data and reviewed relevant literature (Su, Y., Zhu, Jian., Wang, Ping., Wang, D.B., Lin, Y. Research progress in soil water holding capacity. 2013. Chinese Agriculture Science Bulletin. 29(14):140-145.). We noticed that the conclusions are not roger. It could only be concluded that the existence of microplastics reduces soil water-retaining capacity. The peak soil moisture contents (θmax) are all less than that of the blank experiment control group. However, the changes of θmax with the changes of q, p and g are not obvious. It could not get the conclusion like the smaller the microplastic particles are, the weaker the soil water-retaining capacity. The experiment analysis part and conclusion part are all corrected.
As to the specific changing regulations of soil water-retaining capacity under the influence of single or coupled parameters still needed further explore. We are going to do further research in this part.
Point 6: In line 363: The physical meaning of the parameter in Equation 19 should be added.
Response 6: Thanks to the reviewer's suggestions. The physical meaning of the parameter in Equation 19 is the same with Equation 4. We have added the explanation before Equation 19.
Point 7: The conclusion of the article is a little long. Can you simplify it appropriately.
Response 7: Thanks to the reviewer's suggestions and we have simplified the conclusion.
We tried our best to improve the manuscript and made some changes in the manuscript. These changes will not influence the content and framework of the paper. And here we did not list the changes but marked in red in revised paper.
We appreciate for Editors/Reviewers’ warm work earnestly, and hope that the correction will meet with approval.
Once again, thank you very much for your comments and suggestions.
Reviewer 3 Report
Review of the manuscript coded as sustainability-2016132
General comment: the manuscript is written properly and considers a very important problem as the behavior of microplastics in soils. In the case of the Authors’ studies and elimination of organic remnants the loess had properties more similar to the mather rock than the soil.
Below I have listed some minor comments on the text, especially on the Materials and test methods chapter. I suggest re-editing it partly.
- Add particle size of microplastics and explain why they differed in the study;
- Comment table 1 in the text because the test scheme is hard to understand without reading the Results chapter,
- In this section point out that the structure and porosity of loess were not disturbed or underline that it was disturbed what may influence the results in the field conditions,
- Check the rain interval ratios 60 min for rain and 60 minutes for rest should be indicated as 2:2, correct also the Results chapter;
- Paragraph 2.2 rather considers the preparation of the device and the column filled with loess, than the test scheme. Therefore I suggest providing a proper description of the experimental design which in fact considered 4 steps. This information is included in the Results chapter, but I suggest providing a short description in the Material and Methods chapter as well.
- Add the heading in the first column of table 2 – Results chapter,
- There is no discussion in the manuscript. Although the last paragraph is entitled Conclusions and Discussion in fact no discussion is included. According to the Instructions for Authors, the Discussion is a separate chapter. I suggest eliminating some equations and shortening the text to fulfill the Sustainability journal’s requirements.
Based on the novelty of the research and its importance but also considering the suggested changes, I recommend publishing the manuscript coded as sustainability-2016132 after implementing corrections.
Author Response
Dear Editors and Reviewers:
Thank you for your letter and for the reviewers’ comments concerning our manuscript entitled “Experimental investigation of water-retaining and unsaturated infiltration characteristics of loess soils imbued with microplastics” (ID: sustainability-2016132). Those comments are all valuable and very helpful for revising and improving our paper, as well as the important guiding significance to our researches. We have studied comments carefully and have made correction which we hope meet with approval. Revised portion are marked in red in the paper. The main corrections in the paper and the responds to the reviewer’s comments are as following.
Response to Reviewer 3 Comments
Point 1:Add particle size of microplastics and explain why they differed in the study;
Response 1: Thanks to the reviewer's suggestions and the particle size of microplastics has been shown in Table 1. When we explore the water-retaining and the unsaturated infiltration characteristics of soil imbued with microplastics under rainfall conditions, we assume that there exist three influence aspects soil, microplastics and rainfall conditions. As to one factor microplastics, we study microplastic content q, microplastic particle size p respectively. Microplastics vary in size in nature.
Point 2:Comment table 1 in the text because the test scheme is hard to understand without reading the Results chapter,
Response 2: Thanks to the reviewer's suggestions and some supplemental information has been added for reading clearly in Line 104-105.
Point 3:In this section point out that the structure and porosity of loess were not disturbed or underline that it was disturbed what may influence the results in the field conditions,
Response 3: Thanks to the reviewer's suggestions. We do agree that the disturbance of loess soil structure and porosity have great influence on experiment results. Because the experiments we conducted are in the laboratory. We suppose that the structure and porosity of loess were not disturbed during experiments. We point out this in Line 122-123.
Point 4:Check the rain interval ratios 60 min for rain and 60 minutes for rest should be indicated as 2:2, correct also the Results chapter;
Response 4: Thanks to the reviewer’s suggestions. We are sorry for making the mistake. The rain interval 2:1 indicates 60 min for rain and 30 minutes for rest. The mistake has been corrected in Line 108-109.
Point 5:Paragraph 2.2 rather considers the preparation of the device and the column filled with loess, than the test scheme. Therefore I suggest providing a proper description of the experimental design which in fact considered 4 steps. This information is included in the Results chapter, but I suggest providing a short description in the Material and Methods chapter as well.
Response 5: Thanks to the reviewer’s suggestions. We have made adjustment about Paragraph 2.2 Test scheme and experiment procedure and added a short description about experiment procedure. We also add another paragraph 2.3 about the preparation of the device and the column filled with loess which were included in Paragraph 2.2 Test scheme before.
Point 6:Add the heading in the first column of table 2 – Results chapter,
Response 6: Thanks to the reviewer’s suggestions. We have added the heading “Microplastic contents q/%” in the first column of table 2.
Point 7:There is no discussion in the manuscript. Although the last paragraph is entitled Conclusions and Discussion in fact no discussion is included. According to the Instructions for Authors, the Discussion is a separate chapter. I suggest eliminating some equations and shortening the text to fulfill the Sustainability journal’s requirements.
Reponses 7: Thanks to the reviewer's suggestions. The last paragraph entitled Conclusions and Discussion has been changed to Conclusions.
We tried our best to improve the manuscript and made some changes in the manuscript. These changes will not influence the content and framework of the paper. And here we did not list the changes but marked in red in revised paper.
We appreciate for Editors/Reviewers’ warm work earnestly, and hope that the correction will meet with approval.
Once again, thank you very much for your comments and suggestions.
Author Response File: Author Response.docx
Round 2
Reviewer 1 Report
The whole paper was carefully revised and can be published in present form.
