Round 1

Reviewer 1 Report

Please find attached my remarks

Comments for author File: Comments.pdf

Author Response

Response to Reviewer 1 Comments

 

Thanks for the advice on revisions to the paper

This paper has been revised according to the recommendation of revision Reviewer 1, and the changed contents are colored in red.

1. Recommendation: a critical aspect that needs to be mentioned as most make plastic what it is.

=> Thanks for the comment. I added “Plastics produced in 2019 are PE 27.2%, PP 19.3%, PVC 10%, PA 7.9%, PET 7.7%, PS 6.4%, and others 19% [1].”

2. Recommendation: plastic additives leach from plastic and give its toxic nature as a pollutant, from a chemical point of view.

=> Thanks for the comment. I added “Plastics as well as various additives to improve the functionality in the manufacture of plastics should be checked. Plastic additives including plasticizers, flame retardants, antioxidants, acid scavengers, light and heat stabilizers, lubricants, pigments, antistatic agents, can play a role in improving the functional properties of plastics. Representative plastic additives include bisphenols [161], phthalates [162], and adipates, and it is necessary to confirm the toxicity and removal method of the additives.” in the introduction part and “  Pop et al. (2021) concluded that bisphenol A (BPA) had a direct or indirect effect on chlorophyll in water microorganisms, and it was confirmed that it was a problem for cell membrane division, integrity, and survival [161]. Bhatnagar and Anastopoulos (2016) reported that BPA was used as an endocrine disruptor as a major raw material in cosmetics and was found in surface water, groundwater, wastewater, and landfill runoff [162]. Wang and Qian (2021) confirmed that phthalates negatively affect the endocrine system and various organ functions, especially the reproductive organs such as pregnancy, child growth and development [163].” in the toxicity part

3. Recommendation: Table one, correct "Microplastics in comsmetics“

=> I have changed it to “Microplastics in additives”

4. Recommendation: Line 387-406 should be made clearer, methods should be briefly described.

=> I have added a detailed method figure at Figure 7.

Author Response File: Author Response.docx

Reviewer 2 Report

My main criticism is the same as for the first review of this manuscript: the authors present a vast amount of information, but do not provide any synthesis or understanding of the various topics related to microplastics.  Instead, they extracted one or two key facts from each referenced paper and connected these facts together into a paragraph.  As an example, nearly all the sentences in the toxicology section follow the same format: X did this (reference). Y found this (reference).  And so on.  The reader is required to sort through this information and then form some sort of summary.  It is the author's job to do this for the reader.

This problem exists in the other sections as well.  The text in the analytical discussion provides very little insight into advances in the field.  The tables, on the other hand, contain so much detail that it is difficult to grasp where the field is progress, what works and what does not work.

The section on waste water treatment, likewise needs improvement because it suffers from the same problems.

While it's appreciated that the authors desire to summarize the current research regarding microplastics, merely reporting a few key findings or facts from published papers is not enough to constitute a review paper.  There needs to be a critical synthesis of the information into knowledge that the reader can use.  The reader should not have to perform the critical synthesis using only a few facts per referenced paper.

As before, I recommend the authors to break this giant review manuscript into several smaller manuscripts, each focusing on a section topic.  In these smaller review manuscripts, provide a critical synthesis of the current literature and show where and how the field is making advancements.

In closing, I do not see much difference, if any, from the previous manuscript.  Nor do I see much attempt to address the issues highlighted in the first review.

 

Author Response

Response to Reviewer 2 Comments

 

Thanks for the advice on revisions to the paper

This paper has been revised according to the recommendation of revision Reviewer 2, and the changed contents are colored in blue. Other changes were made by other reviewers recommendation.

1. Recommendation: The reader is required to sort through this information and then form some sort of summary.

=> Thanks for the comment. I added “Microplastics are an environmental problem that only occurs through humans, and it has spread widely until it becomes a threat to our lives. Compared to the microplastic pollution situation, the distribution, toxicity, analysis method, removal method, and policy of microplastic are not clearly established. Accordingly, in this paper, research in various fields related to microplastics was reviewed on distribution, toxicity, analysis” in the introduction part

=> “Research on microplastic pollution in the ocean has mainly focused on the number of mi-croplastic particles in the seawater layer, especially surface water”, “These study results confirmed that there is a limit to the scope of microplastic collection depending on the size of the mesh used for sampling in the microplastic pollution research”, “As the current research focuses on only one type of vegetation zone, there is a limit to ascertaining the degree of contamination of microplastics according to the vegetation characteristics.”, “As research on microplastics in the marine sector expands and research cases are accumulated, additional research on the occurrence, distribution, and characteristics of microplastics considering climate and spatial differences in the marine environment is needed.” in 2.1 Microplastic Distribution in Marine.

=> “Freshwater serves as a major route and source of microplastics transport between land and ocean”, “Accordingly, freshwater is more accessible to humans than the ocean, so if it is adjacent to an industrialized or urbanized area, a higher degree of microplastic pollution than the ocean might appear.”, “Microplastics, which are confirmed at a high rate, are found more in sludge than in treat-ed wastewater, so reuse of sludge can serve as a source of microplastics in soil.”, “To summarize the above contents, microplastic research is being actively conducted in the soil field especially on agricultural land. Since sludge deposited with microplastics in wastewater treatment facilities can diffuse into the soil, it is necessary to check the use of sludge recycling fertilizers used in agricultural land.” in 2.2. Microplastic Distribution in Land

=> “ As freshwater is considered to be a major part of the migration of microplastics to the ocean, studies on the degree of microplastic pollution in the air field are insufficient com-pared to the water and sediment fields. However, light microplastics can easily be trans-ported through the air to other environments on land and in the ocean.”, “To summarize, it is only possible to confirm that air is one of the pathways contributing to the movement and diffusion of microplastics. Accordingly, more research should be con-ducted in order to identify the clear source of origin, migration route, spatial distribution characteristics, and toxicity to the human body” in 2.3.  Microplastic Distribution in Air

=> “In addition, studies related to the toxic effects of microplastics using organisms and cell lines are being actively conducted, and many research papers on toxicity have been published recently”, “Recently, several studies have been conducted to evaluate the risk of microplastics in ecosystem”, “Although risk assessments for microplastics are continuously being conducted on humans and ecology, it is considered that more research is needed for a standard of con-centration, and accuracy in microplastic risk.” in 3. Microplastic toxicity

=> “ To summarize the above, net sampling is mainly used to collect the amount of microplastics in a large area, and a net of 100µm-350µm is used depending on the size of the microplastics. When collecting microplastics in a limited space or in a narrow area, grab sampling or bulk sampling is recommended, and a large number of samples is recommended.” in 4.1.1 Water sampling

=> A new Figure 7 was added.

=> “Oxidation of NOM removal of microplastics may take 24h – 30days depending on the type of sample” in 4.2. Pretreatment

=> “According to the analysis of microplastic studies, FTIR-microscope and pyr or TD-GC/MS are most used analysis methods. LC/MS/UV and Nile Red Method are often used. The unit used in the result by FTIR-microscope and Nile Red method is expressed as the number of MP/L or m3, and the unit used in the result by GC/MS, LC/MS is expressed as weight (ng, g)/L or m3.”

=> “The size and concentration of microplastics in the wastewater entering the wastewater treatment plant require more research depending on the location of the wastewater treatment plant, the type of wastewater, and the purpose of treatment. The concentration of microplastics in the effluent from the wastewater treatment plant was 1 MPs/L or less in many studies when the primary, secondary, and tertiary treatments were completed. In addition, it is difficult to ascertain a clear average removal rate of the re-viewed studies because the microplastic treatment rate varies at each treatment stage, but the concentration of the inlet is different. Most of the high concentration of microplastics is removed from the primary treatment and contained in the sludge, so the use of recycled fertilizers by the sludge can cause secondary pollution of microplastics.” in 5.1 Wastewater treatment plants (WWTP)

 

 

Author Response File: Author Response.docx

Reviewer 3 Report

Review: Review of microplastic distribution, analysis methods, and removal technologies (revised)

The manuscript has been improved in comparison to the first version, but there is still a lot that needs to be done about it. This time I judge it reparable and therefore advise a major revision. Below I detail my critique, which concerns grammatical and idiomatic mistakes and several more conceptual point.

Abstract:

Line 7: “exacerbating” is the wrong word here.

Introduction:

Line 27: “less than 5mm and less than 1um plastics particles …” – this sentence should be rewritten.

Line 38: “microplastics are becoming more serious around the world” – I guess pollution by microplastics is meant

Line 40: “Among 192 countries” -> “Among the 192 countries” – “the 44” -> “44”

(Note: I will not mark down all sentences that need to be corrected. There are many: the text requires a thorough investigation)

Line 48: “produced and used for secondary microplastics” – that is a contradiction. Secondary microplastics are not expressly produced. They are deteriorated plastics.

Lines 56-58: Curiously formulated. Please revise the sentence.

Line 63: “analysed with PET and PP” – I guess what is meant is the plastics particles were analysed and found to consist of PET and PP.

Microplastic distribution:

Line 78: “250,000 tons” – that is a fraction of the yearly (!) amount mentioned on page 5, in relation to the findings of Lebreton et al. Please sort this out.

Lines 83-87: These sentences are not entirely clear and rather badly structured. Please revise.

Lines 106-108: This sentence is still about the Scottish waters? In that case I wonder about the different geographic ranges …

Line 109: only 104 MPs/km2? How on Earth would you find that few pieces in such a large area?

Line 119: “same irradiation site” – what has irradiation got to do with measuring microplastics?

Line 129: “lower” – lower than what?

Lines 142-143: Odd grammatical construction. This happens more often in a slightly different form: the authors are mentioned and then the research in a passive sentence. For instance at line 258.

Line 149: “they found … varied spatially and temporally” – that is hardly surprising. The opposite would have been!

Line 160: “showed some sign of microplastic contamination” – what does that mean? How should I interpret that?

Line 167: “higher influence of human activity” – again a rather odd expression.

Note: this section is really long. Perhaps split it up in subsections?

Lines 189-191: Incomplete sentence, just a list at the moment.

Line197: “morphological type and size … in terms of chemical composition” – the size and the form of the particles is independent of their composition. Please correct.

Lines 207 and 209: “air field” versus “road dust” – those are two different media.

Microplastic toxicity:

Line 275: “2.0 ng/l” – what is the substance for which this is a toxicological limit?

Lines 281-287: All very well, these formulae, but they are not used in any way in the text, so they are superfluous and should be removed.

Microplastic analysis:

Figure 4. “madta” -> “manta”

Line 302: “microplastic sampling” – the examples are all media, not sampling methods.

Lines 306-310: These sentences need attention.

Line 322: “at a low speed” – then there will no or less clogging?

Lines 329-330: “sampling range is narrow” – what does that mean?

Line 333: “the amount of microplastics could vary” – it is the measured amount that varies.

Lines 353 and 354: “300g-500g” versus “bottles of 200g” – that won’t fit!

Lines 357-359: The recommendations to sieve the soil with 1, 2 and 5 mm are all very well, but you will not capture the very small particles then. Any suggestions?

Lines 366-368: This sentence is meaningless as it does not detail when to do the separation/oxidation before or after.

Line 373: No formula for sodium polytungsate?

Line 419: “number of papers in the sample” – what is meant here?

Line 426: “(FTIR)” – the abbreviation was used before without this explanation. Please move that to the first line where the abbreviation is used.

Lines 439-445: This piece of text has little or nothing to do with analysis methods. It describes some results, but that is all. Please remove.

Line 451: “Previously, FTIR …” – what has changed? Why is the method now successful?

Lines 454-456: Rather garbled sentence. Please revise.

Line 464: “microplastic particles are excited” – it is the molecules that are excited, not the particles as a whole.

Microplastic Removal Technology:

Line 497: “Most abundant … are through wastewater” – this sentence is grammatically and idiomatically incorrect. Please revise.

Lines 505-513: This piece of text says nothing about the removal efficiency, it simply does not belong here. Please remove.

Table 8: The reference [139] apparently found much lower concentrations than the rest – roughly a factor 100. What is the explanation?

Line 543: “less than > 10 um” – less than larger than 10 um? What does this mean?

Table 9: The result mentioned in the first row is unrelated to the results in the other rows. What is the removal rate?

Line 559: “PAM” – the more common abbreviation of polyacrylamide is PAA.

Conclusions:

Line 624: “On the other hand” – where is the contradiction?

Comments for author File: Comments.docx

Author Response

Response to Reviewer 3 Comments

 

Thanks for the advice on revisions to the paper

This paper has been revised according to the recommendation of revision Reviewer 3, and the changed contents are colored in purple. Other changes were made by other reviewers recommendation.

1. Recommendation: Line 7: “exacerbating” is the wrong word here.

• “became a problem”

2. Recommendation: Line 27: “less than 5mm and less than 1um plastics particles …” – this sentence should be rewritten.

• “Microplastics are small plastic particles smaller than 5 mm and plastic particles smaller than 1 μm are defined as nanoplastics.”

3. Recommendation: Line 38: “microplastics are becoming more serious around the world” – I guess pollution by microplastics is meant

• “pollution from microplastics are becoming a serious problem around the world.”

4. Recommendation: Line 40: “Among 192 countries” -> “Among the 192 countries” – “the 44” -> “44”

• “Among the 192 countries” – “the 44” -> “44””

5. Recommendation: Line 48: “produced and used for secondary microplastics” – that is a contradiction. Secondary microplastics are not expressly produced. They are deteriorated plastics.

• “Polyamide (PA) is often found on the beach as secondary microplastics”

6. Recommendation: - Lines 56-58: Curiously formulated. Please revise the sentence.

• The sewage treatment plant removes many microplastics contained in sewage, but nano-sized microplastics still remains in the treated sewage water.

7. Recommendation: Line 63: “analysed with PET and PP” – I guess what is meant is the plastics particles were analysed and found to consist of PET and PP.

• “most of them were identified by PET and PP”

8. Recommendation: Line 78: “250,000 tons” – that is a fraction of the yearly (!) amount mentioned on page 5, in relation to the findings of Lebreton et al. Please sort this out.

• “ A global estimate of plastic emissions from rivers into the world’s oceans is between 1.15 and 2.41 million tonnes per year, and most of the river plastic input is coming from Asian countries with rapid economic development and poor waste management.”

9. Recommendation: Lines 83-87: These sentences are not entirely clear and rather badly structured. => “<Figure 1> shows the source of microplastic contamination. Microplastics in water can be caused by all areas of human life, including agriculture, industry, landfill, household, and roads. Because microplastics are introduced through various sources and routes, and it is difficult to establish a clear removal plan. <Figure 2> shows the path of contamination and distribution of microplastics in the sea. Microplastics in the ocean through rivers, land, and air, are firstly absorbed by marine animals and plants. Human digests various marine organisms (eg: fish, salt, clam, seaweed etc.) containing microplastics.”
10. Recommendation: Line 40: Lines 106-108: This sentence is still about the Scottish waters? In that case I wonder about the different geographic ranges

• Yes, Ressell et al. reported that they could not identify different geographic and temporal ranges of the data and trends.

11. Recommendation: only 104 MPs/km2? How on Earth would you find that few pieces in such a large area?

• I am sorry. It was miswritten. It is 1 x 104 MPs/km²

12. Recommendation: Line 119: “same irradiation site” – what has irradiation got to do with measuring microplastics?

=> I changed it to “the same sampling site”.

13. Recommendation: Line 129: “lower” – lower than what?

• I changed “lower” to “low”. Piarulli et al. (2020) did not provide any comparison.

14. Recommendation: Lines 142-143: Odd grammatical construction. This happens more often in a slightly different form: the authors are mentioned and then the research in a passive sentence. For instance at line 258.

• “Studies of riversides and beaches around the world, Tibbetts et al. (2018), a microplastic survey from a heavily urbanized catchment, the River Tame and four of its tributaries, which flows through the city of Birmingham, UK was reported”

15. Recommendation: “they found … varied spatially and temporally” – that is hardly surprising. The opposite would have been!

• Park et al. (the author) provide these words. but I think that ‘spatially and temporarily’ means the concentration of MPs changes by seasonal effects (weather, sampling place etc). Korea has a special rainy season in summer. I don’t feel that I can change the word that the original author mentioned.

16. Recommendation: Line 160: “showed some sign of microplastic contamination” – what does that mean? How should I interpret that?

• “Mason et al. (2018) investigated eleven globally sourced brands of bottled water, purchased in 19 locations in nine different countries and found that 93 % of them contained microplastic synthetic polymer particles among the 259 total bottles”

17. Recommendation: Line 167: “higher influence of human activity” – again a rather odd expression.”

• “Microplastic pollution by fresh water is highly dependent on human activity, and factors such as population density and industrialization in the river area can be a major factor affecting marine microplastic pollution”

18. Recommendation: Note: this section is really long. Perhaps split it up in subsections?

• I have divided into the marine, the land, the air section.

19. Recommendation: Lines 189-191: Incomplete sentence, just a list at the moment.

• “Studies on land contamination of microplastics include a study on open land in an industrial area [52], a study on the concentration of microplastics in soil samples in an open land of flood area in Switzerland [53], a study on mixed agricultural land [54].”

20. Recommendation: - Line197: “morphological type and size … in terms of chemical composition” – the size and the form of the particles is independent of their composition. Please correct.

• “Dioses-Salinas et al. (2020) explained similarities in the morphological type and size of the microplastics in terms of the product purpose.”

21. Recommendation: Lines 207 and 209: “air field” versus “road dust” – those are two different media.

• “I agreed with your opinion. However, microplastic in air field is not much valuable research paper that I included road dust in air field. Please understand it.

22. Recommendation: Line 275: “2.0 ng/l” – what is the substance for which this is a toxicological limit?

• “It includes many ecological substances in aquatic environment. I will attached the original work done by Besseling. It was too many I can’t write down in the paper.”

23. Recommendation: Lines 281-287: All very well, these formulae, but they are not used in any way in the text, so they are superfluous and should be removed.

• They are all removed. Thanks for the comment

24. Recommendation: Figure 4. “madta” -> “manta”

• Thanks for the comment. It has changed.

25. Recommendation: Line 302: “microplastic sampling” – the examples are all media, not sampling methods.

• Thanks for the comment. It has changed.
• “There are various types of microplastic sampling methods, and they are applied differently depending on water (freshwater, seawater), sediment, soil, cosmetics, and living organisms. “

26. Recommendation: Lines 306-310: These sentences need attention.

• Please let me know what attention I need? Sorry, I did get it

27. Recommendation: Line 322: “at a low speed” – then there will no or less clogging?

• The author provide a low speed will offer less clogging.
• I changed it to “it is important to collect it at a low speed to avoid from fast clogging”

28. Recommendation: Lines 329-330: “sampling range is narrow” – what does that mean?

• The bulk sampling or grab sampling is collected the sample using the bottle or similar water sampler, which can not sample a large amount of water at once.
• I have changed it to “ A bulk sampling or grab sampling is collected as much as a fixed volume, and the sampling range is narrower than the net sampling.”

29. Recommendation: Line 333: “the amount of microplastics could vary” – it is the measured amount that varies.

• Thanks for the comment
• I have changed it to “the measured amount of microplastics”

30. Recommendation: Lines 353 and 354: “300g-500g” versus “bottles of 200g” – that won’t fit!

• Thanks for the comment
• I have changed it to “bottles of 300g or more”

31. Recommendation: Lines 357-359: The recommendations to sieve the soil with 1, 2 and 5 mm are all very well, but you will not capture the very small particles then. Any suggestions?

• Thanks for the comment
• I added the sentence “Standard sieve sizes vary from 37.5 mm to 0.075 mm, so 0.5 mm or 0.15 mm sieves are recommended to collect smaller microplastics.”

32. Recommendation: Lines 366-368: This sentence is meaningless as it does not detail when to do the separation/oxidation before or after.

• I added the detailed pretreatment step in Figure 7. I wish it would help this explanation.

33. Recommendation: Line 373: No formula for sodium polytungsate?

• Thanks for the comment
• I have changed it to “Na₆[H₂W₁₂O₄₀]”

34. Recommendation: Line 419: “number of papers in the sample” – what is meant here?

• It was miswritten.
• “Another easy method is to find the number of moles of solute in a sample by using Beer's law based on the UV-Vis measurement result.”

35. Recommendation: “(FTIR)” – the abbreviation was used before without this explanation. Please move that to the first line where the abbreviation is used.

• I have changed it to the first line where the abbreviation was used.

36. Recommendation: Lines 439-445: This piece of text has little or nothing to do with analysis methods. It describes some results, but that is all. Please remove.

• Thanks for the comment. It has been removed.

37. Recommendation “Previously, FTIR …” – what has changed? Why is the method now successful?

• Thanks for the comment. a new micro(μ)-FTIR microscopy is a recent technology now.
• Previously, it was difficult to analyze microplastics under 500 μm and contaminated plastics with FTIR spectroscopy, but recently, microplastics as small as 10 μm to 5 μm can be analyzed using a new micro(μ)-FTIR microscopy[101].

38. Recommendation: Lines 454-456: Rather garbled sentence. Please revise.

• Thanks for the comment. I have changed it.
• “ FTIR spectroscopy can quickly measure all the frequencies of the infrared source at the same time through pretreated filter paper.[113]”

39. Recommendation: Line 464: “microplastic particles are excited” – it is the molecules that are excited, not the particles as a whole.

• Thanks for the comment. I have changed it to “when the molecules of microplastic particles are excited”

40. Recommendation: Line 497: “Most abundant … are through wastewater” – this sentence is grammatically and idiomatically incorrect. Please revise.

• Thanks for the comment. I have changed it to “Industrial wastewater, domestic wastewater, agricultural wastewater, and livestock wastewater contain many microplastics, and wastewater treatment plants cannot remove all microplastics.”

41. Recommendation: Lines 505-513: This piece of text says nothing about the removal efficiency, it simply does not belong here. Please remove.

• Thanks for the comment. I have removed it.

42. Recommendation: Table 8: The reference [139] apparently found much lower concentrations than the rest – roughly a factor 100. What is the explanation?

• I thought it was an improper reference. I have searched another reference and changed it

43. Recommendation: Line 543: “less than > 10 um” – less than larger than 10 um? What does this mean?

• Thanks for the comment. I have miswritten it. It means “larger than 10 um”

44. Recommendation: Table 9: The result mentioned in the first row is unrelated to the results in the other rows. What is the removal rate?

• Thanks for the comment. I couldn’t find the values. I have searched another reference and changed it.

45. Recommendation: Line 559: “PAM” – the more common abbreviation of polyacrylamide is PAA.

• I have changed to “PAA”

46. Recommendation: Line 624: “On the other hand” – where is the contradiction?

• I have changed it to “Accordingly”

 

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

The authors have revised accordingly and I find the article of good quality and suitable for publication

Author Response

Please see attachment. 

Author Response File: Author Response.docx

Reviewer 2 Report

My previous comments still stand.  The additions the authors added do not provide an in-depth or detailed scholarly assessment of the papers they cite and briefly describe. 

Author Response

Please see attachment. 

Author Response File: Author Response.docx

This manuscript is a resubmission of an earlier submission. The following is a list of the peer review reports and author responses from that submission.

Round 1

Reviewer 1 Report

The work of Hanbai et al summarizes information on the 
distribution, toxicity, analytical methods, and removal techniques for microplastics.

The title is relevant and the paper is of importance and indeed insufficient data regarding microplastic  analytical methods, and removal technique are currently present.

The abstract is well written, concise and simple.

Introduction is clear and well cited

Microplastic toxicity chapter lack an important aspect, additives used in microplastic which leak from the plastic, such as Bisphenol, they have a detrimental effect not only on humans but on all eukariotic and prokariotic life.

The MS analysis is mass on electrical charge, the authors need to understand mass spectrometry better before writing about gas chromatography/mass spectrometry, still the chapter is not incorrect, just not fully understood and explained.

Microplastic Removal Technology

I urge the authors to speak about the PETase  and other enzymes used in plastic degradation as well as microorganisms such as Ideonella sakaiensis.

 

I am seriously considering to propose a major revision but the work has been done with care, therefore I suggest a minor revision but only if the above stated are fixed.

 

Congratulations!

Reviewer 2 Report

Review: Review of microplastic distribution, analysis methods, and removal technologies

The manuscript presents results on a variety of aspects of microplastic pollution but fails to come to the point. The information is mostly rather superficial, I am sorry to say, and the text needs quite some editing for it to become proper English. It is, however, the lack of depth that leads me to the advice to reject this. I do not do so lightly but it is the outcome of my review. I give some detailed criticism below.

Introduction:

Line 25: “Once large plastic item is thrown” -> “Once a large plastic item …”

Lines 26-28: the sentence is not proper English and I may be mistaken, but “nanoplastics” are usually classified as much smaller than 1 mm, more in the order of 1 um (please use the Greek letter mu for this!)

Line 28: “are absorbed” -> “consumed”

Line 32: “Primary microplastics defines plastic made …” -> “is defined as plastic …”

Line 33: “include” -> “is often included” – I will not comment on all textual mistakes from here on.

Line 40: “only 22.9%” – 22.9% of 192 countries are 44 countries – please use the number instead of a percentage for such small number of items.

Line 41: “impacts … have mostly targeted” – I guess that the studies (!) targeted the impacts on fish.

Lines 42-43: “studies … not well documented” – I guess that is meant that there are few studies regarding other organisms than fish. Whether the studies themselves are not well documented is something completely different.

Line 48: “are mostly used for secondary microplastics” – I guess that it meant that secondary microplastics mostly consist of PA. The sentence now suggests that secondary microplastics are also expressly produced.

Lines 63-65: this sentence is quite hard to understand.

Microplastics Distribution:

Line 75: “more than 5 trillion” – what is the unit of this number? Is it the number of individual plastic particles? How bad is that in view of the size of the ocean?

Lines 77-79: “are generated … sewage treatment plants” – sewage treatment plants are not so much sources of microplastics, but transporters.

Lines 80-84: The ingestion of microplastics by humans is postulated here without any reference. Is there a relevant publication? (Also the sentence needs revising)

Lines 91-114: This is just a conglomerate of individual findings from all over the world. A presentation in the form of a table – if you want it – would be more attractive. Something else: the references are garbled: the name “Joseph T.” is actually the first name of the author and the first letter of his surname. The same for the co-authors of this publication (ref. 37). This happened with several publications.

Microplastic toxicity:

Line 121: “BPA” – this term is not explained in the manuscript. As there is only one occurrence, I suggest use of the full name (bisphenol-A?)

Microplastics Analysis:

Line 149: The described method may be useable in water systems, but it is not for sediment or soil. What are the methods for these media?

Lines 158-161: This fragment of text is rather vague. Are there any more precise guidelines for choosing a sampling method?

Tables 1 and 2: I doubt that the extensive tabulation of results in this way is useful, specifically the concentration ranges. These will depend greatly on where the samples were taken. It would be more interesting to see if and how the observed concentrations are related to the reported size ranges.

Lines 225-228: The sentence is garbled, even though you can make out what is meant. For instance: the FTIR method is said to be “small in size” – that is not a property for a measurement method.

Lines 243: “[88]” – Primke et al. are mentioned under no. 77 in the references section.

Line 2623: “Advantages … analyse …” – again a garbled sentence.

Line 263: “<10 ug” – that is an amount, not a concentration.

Microplastic Removal Technology:

Line 282: Waste water may be an important source, but actual source is the use of plastics and microplastics.

Line 286: “microplastics and nanoplastics are not designed to be treated …” – I think it is the other way around: treatment plants are generally not designed to remove plastic particles.

Lines 291-297: The reference to Talvitie et al. is puzzling. It is not ref. 125 or 117. (Talvitie et al. appears in ref. 20 and 95). References 111 and 120 do not correspond with the names given in the text either.

Table 6: the removal rate of 99.3% seems unlikely. None of the other entries come close. Also the given reference [94] is unlikely to be the source of this information – the title refers to very different research.

Line 320: “less than > 10 um” – this is a contradiction

Line 322: “[126]” – perhaps [127]?

Line 342: “[104]” – more likely [125] is meant.

Lines 346-348: If wastewater treatment plants can remove up to 99% of the plastic particles, why are they considered an important source?

Lines 356-358: Why are daphnia and microalgae connected in these sentences?

Lines 350-362: the references [131] and [132] should be respectively [132] and [133], judging from the authors’ names.

Conclusion:

Line 363: “plastic contamination has been deteriorating” – it is not the contamination that has been deteriorating, but rather the contamination has caused global deterioration of the environment.

Lines 396-399: a rather obligatory statement.

Comments for author File: Comments.docx

Reviewer 3 Report

The manuscript “Review of microplastic distribution, analysis methods, and removal technologies” presents very poorly presents an “overview” (most is about wastewater effluents) of microplastics. The first mistake was setting such a broad objective, which could never be covered in a few paged. The manuscript is filled with misconceptions, such as that wastewater treatment plants are the main responsible for microplastic release (which is highly dependent on each area) and other that I enumerated below. A striking misconception, possibly originating from language, is that toxicity testing in which microplastics are internalized by organisms and therefore removed from the medium (e.g., in Daphnia) can be used as a removal technology. On the bright side, the authors present beautiful figures which could indeed improve any manuscript. However, the content is poor and misleading, and completely lacks critical discussion needed in a review. Therefore, it must be rejected.

 

The objective of the manuscript is too broad. If the objective was removal technologies, there was no need to address so many different issues. This only dilutes information.

L44 – 48 makes no sense. Of course microplastics are made of the same materials of plastics, they are plastics.

L49 – Flash water? Also, effluent discharges may not be the major source everywhere. It really depends on each location.

Section 2 on microplastics distribution adds nothing to what is already known in the literature. A review must be critical and critically compare studies. This must consider, for instance, different sampling methods. Figure 1 and 2 are good representations, but alone cannot justify the need for this section. Figure 3 is being reused from a published article, for which the authors must ask for permission from the publisher. Most of the section is focused on wastewater effluents, despite this not being the objective of the work.

Section 3 on toxicity is also not clear that all toxicity is from monomers and oligomers. Microplastics have other ways of interacting with biologic systems. Everything is presented as definite proof, which it is not. Citing a dozen of papers is not enough to clearly cover the literature. Some studies even found that microplastics remove other contaminants from the water or are used as substrates by species helping them reproduce. What about those cases? There is a severe lack of critical thinking in these sections.

Section 3 on sampling methods starts with a paragraph completely devoid of content. What is the interest to the reader that researchers are developing methods (are they not always doing that?). What methods are being developed? Which failed? What are the pros and cons of each method? Presentation of sampling devices only considers water and specifically trawl nets, which is very limited. On pre-treatment, separation by density is mixed with biogenic organic matter removal – these are different treatments. The question of removing organic matter without damaging plastics is poorly addressed. The most frequently used method of analysis is visual inspection (unfortunately, because it produces errors), which has not been addressed. Staining dyes are also frequently use, possibly more than MS methods, and also not addressed.

The use of images to improve the work is commendable and clearly a strong suit of the authors. Unfortunately, the lack of structure and deep misunderstanding the topics are not in the same level as the illustration.

Section 4 on removal strategies, there should be a discussion of pros and cons of each technique. Some final conclusion must be taken from the review. For instance, what type of removal methods is more likely to succeed? Biological treatments are not damaging to microplastics, but organisms capable of degrading plastics could have been introduced here. There is clearly a confusion with the removal in toxicity testing, meaning plastics have been ingested / internalized by organisms, with removal technologies.

Conclusion is only made of generic statements without any structure. This is not surprising given the lack of a precise objective and misunderstanding of topics.

Reviewer 4 Report

The authors need to be commended for attempting to develop a comprehensive review of the rapidly growing and changing field of microplastics.  However, in their attempt to capture of the breadth of microplastics, ranging from environmental distribution, methods of extraction and characterization, impacts on living organisms, and fate and transport in wastewater treatment systems, the review ends up being very shallow and not providing much new information or understanding of any of the topics.  Because of this, the review is unable to identify knowledge gaps in the quickly developing field of microplastics.

 

For example, the analytical method sections briefly discuss methods that have been established for some time, and in fact, are being modified in multiple ways.  New directions being pursued for extraction and characterization are, at best, briefly mentioned in this review.  For example, new methods for the extraction of microplastics from high organics matrices (e.g. biosolids, organisms) or clay (e.g. clayey solids) are not addressed.  Additionally, the many analytical challenges faced as smaller and smaller microplastic particles are desired to be quantified are not addressed.  Likewise, there is no discussion of the analytical methods being developed to explore the interaction of microplastics with metals and organic compounds.

 

Similar problems exist with the section focusing on the health impacts of microplastics on organisms.  Little or no mention, much less an insightful discussion, is provided of how microplastics impact organisms as a function of decreasing microplastic size.  There are many studies published that are examining how particles in the submicron size range can enter the bloodstream and effect various organs. 

 

There are similar issues with the section on wastewater treatment and microplastics.  For example, the observation that microplastics have a negative impact on methanogenesis processes in anaerobic digestors is not explored.

 

In closing, this review doesn’t explore the current issues facing examination of microplastics with respect to extraction/characterization, health effects, and wastewater treatment, nor does it provide enough depth to identify or propose knowledge gaps that are critical to address.  It is recommended that the authors select one of the sections and prepare an in-depth review of that specific area.  Otherwise, this review doesn’t add much new to the understanding or overview of the microplastic field.