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Review
Peer-Review Record

Microplastics in the Aquatic Environment—The Occurrence, Sources, Ecological Impacts, Fate, and Remediation Challenges

Reviewer 1: Anonymous
Reviewer 2: Anonymous
Reviewer 3: Anonymous
Received: 19 April 2021 / Revised: 12 June 2021 / Accepted: 16 June 2021 / Published: 18 June 2021
(This article belongs to the Special Issue Marine Pollutants)

Round 1

Reviewer 1 Report

Q1) Microplastic pollution is a hot topic widely discussed in numerous reviews or articles. Which are the differences between this new review and the others reported in the literature? Other reviews are more complete in terms of concentration, distribution and analysis of microplastics. This review deals with the toxicological aspects, but in different parts of the paper, it is written that there aren't enough data available in the literature, so it seems more focused on the gaps and the future rather than the results achieved. The authors should emphasize the difference with the other reviews and add more information about the risks.

Q2) The authors should do a deeper research of the literature. There are several reviews and articles about the presence and the effects of microplastics in soils. They can be cited and properly discussed. Here it follows a list of some articles about microplastics in soil. 

- Sorption Behavior and Mechanisms of Organic Contaminants to Nano and Microplastics (https://0-doi-org.brum.beds.ac.uk/10.3390/molecules25081827)

- Plastics in soil: Analytical methods and possible sources (https://0-doi-org.brum.beds.ac.uk/10.1016/j.scitotenv.2017.08.086)

- Effects of polystyrene microplastics on the fitness of earthworms in an agricultural soil (doi :10.1088/1755-1315/61/1/012148)

- Current research trends on plastic pollution and ecological impacts on
the soil ecosystem: A review (https://0-doi-org.brum.beds.ac.uk/10.1016/j.envpol.2018.05.008)

- Impacts of Microplastics on the Soil Biophysical Environment (https://0-doi-org.brum.beds.ac.uk/10.1021/acs.est.8b02212)

There are also several reviews and articles about the presence and the distribution of microplastics in freshwater. Two recently published reviews that show maps and/or tables about the distribution in freshwater and seawater are:

-Microplastics in Freshwater: What Is the News from
the World? (doi:10.3390/d12070276).

- Microplastics in the Aquatic Environment: occurrence, persistence, analysis, and human exposure (https://0-doi-org.brum.beds.ac.uk/10.3390/w13070973).

In the first review there are also some data about the presence of microplastics in African waterbodies.

Q3) The paragraph devoted to the microplastics in Africa is interesting. Few data are available while most of the microplastic pollution is due to the improper waste management in several countries of Africa and Asia. Do you have other information of microplastic distribution in other countries of Africa? Could you do a geographical map or a table with the distribution of microplastics in the African continent? This aspect is missing in other reviews and give more originality to your work.

Q4) Minor corrections recommended are:

Line 77. The passage is a little abrupt. Could you connect better this part with the previous sentence?

Line 92. Polyethylterephtalate should be replaced with polyethylenterephtalate

Line 135 and Line 360. Could you write the long name of PBTs. Even if it can be obvious it is better to indicate what this acronym stands for.

Line 224. Probably the word is researchers instead of researches.

Line 246. Add the final s to microplastic.

Line 304. A concentration of ... was reported instead of were reported.

Line 608. Revenues instead of revenue

Author Response

Responses to Reviewer’s Comments

Reviewer #1

Comment

Q1) Microplastic pollution is a hot topic widely discussed in numerous reviews or articles. Which are the differences between this new review and the others reported in the literature? Other reviews are more complete in terms of concentration, distribution and analysis of microplastics. This review deals with the toxicological aspects, but in different parts of the paper, it is written that there aren't enough data available in the literature, so it seems more focused on the gaps and the future rather than the results achieved. The authors should emphasize the difference with the other reviews and add more information about the risks.

Response

Thank you for the comment. The toxicity aspect is covered briefly under section 6.1 in this revised version. However, the microplastic toxicity on human being and animals are scarce in the literature, and we have covered the toxicities to human and animals available in the literature. Unlike several review articles on microplastics that are available in literature which address specific issue of microplastics, this review addressed briefly all necessary aspects that needs attention for the safety of human and environment such as their occurrence, source, toxicity to aquatic species, ecological impacts, their concentration in different environmental compartments, their fate and remediation challenges. 

Comment

Q2) The authors should do a deeper research of the literature. There are several reviews and articles about the presence and the effects of microplastics in soils. They can be cited and properly discussed. Here it follows a list of some articles about microplastics in soil.

- Sorption Behavior and Mechanisms of Organic Contaminants to Nano and Microplastics (https://0-doi-org.brum.beds.ac.uk/10.3390/molecules25081827) Wang et al. (2020)

- Plastics in soil: Analytical methods and possible sources (https://0-doi-org.brum.beds.ac.uk/10.1016/j.scitotenv.2017.08.086) Blasing and Amelung. (2018)

- Effects of polystyrene microplastics on the fitness of earthworms in an agricultural soil (doi :10.1088/1755-1315/61/1/012148) Cao et al. (2017)

- Current research trends on plastic pollution and ecological impacts on

the soil ecosystem: A review (https://0-doi-org.brum.beds.ac.uk/10.1016/j.envpol.2018.05.008) Chae and An, (2018)

- Impacts of Microplastics on the Soil Biophysical Environment (https://0-doi-org.brum.beds.ac.uk/10.1021/acs.est.8b02212) Machado et al. (2018)

There are also several reviews and articles about the presence and the distribution of microplastics in freshwater. Two recently published reviews that show maps and/or tables about the distribution in freshwater and seawater are:

-Microplastics in Freshwater: What Is the News from

the World? (doi:10.3390/d12070276). Cera et al. (2020)

- Microplastics in the Aquatic Environment: occurrence, persistence, analysis, and human exposure (https://0-doi-org.brum.beds.ac.uk/10.3390/w13070973). Ricciardi et al. (2021)

In the first review there are also some data about the presence of microplastics in African waterbodies.

Response

We have used the references in the comment to improve the manuscript. However, to avoid an overly broad review that included soils we decided to focus on aquatic systems with more emphasis of Africa. In any case, data on microplastics in African terrestrial ecosystems is lacking as indicated in a review by Alimi et al. (2021).

Comment

Q3) The paragraph devoted to the microplastics in Africa is interesting. Few data are available while most of the microplastic pollution is due to the improper waste management in several countries of Africa and Asia. Do you have other information of microplastic distribution in other countries of Africa? Could you do a geographical map or a table with the distribution of microplastics in the African continent? This aspect is missing in other reviews and give more originality to your work.

Response

We thank the reviewer. We have gathered data from literature and tabulated the distribution of microplastics in African countries (Table 2).

Q4) Minor corrections recommended are:

Comment

Line 77. The passage is a little abrupt. Could you connect better this part with the previous sentence?

Response

We have revised as follows:

Microplastics have been detected in a number of environmental compartments around the world. For instance, scientists reported over 400 000 particles of microplastics per square km in the Great Lakes, USA [32], while in a separate study, researchers reported 60 microplastic particles per 100 mL of sediment at Sydney Harbour [33].

Comment

Line 92. Polyethylterephtalate should be replaced with polyethylenterephtalate

Response

Revised, thank you.

Comment

Line 135 and Line 360. Could you write the long name of PBTs. Even if it can be obvious it is better to indicate what this acronym stands for.

Response

Thank you. Written in full at point of first use as polybutylterephthalates.

Comment

Line 224. Probably the word is researchers instead of researches.

Response

Corrected, thank you.

Comment

Line 246. Add the final s to microplastic.

Response

Corrected, thank you.

Comment

Line 304. A concentration of ... was reported instead of were reported.

Response

Corrected, thank you.

Comment

Line 608. Revenues instead of revenue

Response

Corrected, thank you.

Author Response File: Author Response.docx

Reviewer 2 Report

This article is a review of literature of the past 30+ years, to promote further research in this area.
Since this is a standard part of literature search, it may be beneficial to list concrete and tangible results obtained as a part of this investigation.
Authors may wish to highlight results that show the life cycle analysis of the MPs.

Since the review concerns the marine environment a specific table that summarizes the MP abundance in water ecosystems is needed.

Today biosorption techniques are considered important for the removal of pollutants. For this reason, this section should be expanded considering these techniques. Likewise with biotransformation studies.

The section focused on human health is unspecific. Expand specific information on this topic.

Author Response

Responses to Reviewer #2

Comment

This article is a review of literature of the past 30+ years, to promote further research in this area.

Response

Indeed, the review covered literature in the period 1960-2021. Thank you.

Comment

Since this is a standard part of literature search, it may be beneficial to list concrete and tangible results obtained as a part of this investigation.

Response

Thank you. We summarized these as:

Importantly, while there is significant microplastics pollution in the aquatic environment, pollution in terrestrial systems are not widely studied. Besides, there is a dearth of information on microplastics in African aquatic systems.

Comment

Authors may wish to highlight results that show the life cycle analysis of the MPs.

Response

Thank you for the comment. We included a schematic diagram summarising the life cycle of plastics from production industrial uses, disposal and behaviour and fate in aquatic systems (Fig 2).

Comment

Since the review concerns the marine environment a specific table that summarizes the MP abundance in water ecosystems is needed.

Response

Indeed, we have summarized data on microplastics in African aquatic systems in Table 2.

Comment

Today biosorption techniques are considered important for the removal of pollutants. For this reason, this section should be expanded considering these techniques. Likewise with biotransformation studies.

Response

We considered the comment. However, we are no sure if biosorption can effectively remove plastics, and have commented on that aspect. The aspect on bio-uptake, bioaccumulation and biotransformation is covered under behaviour. We have included the statement:

A number of methods, including the conventional wastewater treatment process, can be used to reduce microplastics pollution, especially the aquatic environment. A more de-tailed description of these and other methods is provided in a recent review [154].

Comment

The section focused on human health is unspecific. Expand specific information on this topic.

Response

Thank you for the comment. The manuscript already discusses potential human exposure routes documented in literature. We however, found it difficult to provide a detailed treatment of human health risks without making the manuscript cumbersomely long. We have therefore included:

A detailed treatment of specific human risks posed by microplastics in the environment is outside the scope of this review. Nevertheless, humans are exposed to microplastics through a number of routes such as drinking water and food.

Author Response File: Author Response.docx

Reviewer 3 Report

I have reviewed the manuscript entitled " Microplastics in the aquatic environment - the occurrence, sources, ecological impacts, fate, and remediation challenges" by Chaukura et al for publication in Pollutants. This review discusses sources and pathways through which microplastics enter the environment and evaluates the fate and behaviour of microplastics with the aim to identify research gaps and recommend remediation strategies. However, the present manuscript offers little that is either new or contributes meaningfully to the current discussion in the literature. The manuscript is overly ambitious in its stated objectives, poorly organized. Although the subject matter is very interesting, there are several issues with the manuscript as currently written that need to be considered:

1)Since this is a standard part of literature search, it may be beneficial to list a concrete and tangible results obtained as a part of this investigation. Authors must explain how this review is unique and different from the others.

2)As a review paper, it requires figures or tables to help support the discussion. In line 368 table 1 was mentioned, but it is not present in the paper. As currently written, comparison of literature data results very difficult to understand.

3)As stated in line 139, authors considered literature published until June 2020, however in the last year several works and especially reviews on regarding microplastic in the aquatic environment are reported that should be taken into account. (10.3390/w13070973; 10.1016/j.scitotenv.2020.141948; 10.1016/j.envpol.2021.116552; 10.1016/j.jwpe.2021.101966; 10.1016/j.scitotenv.2020.144655; 10.1016/j.scitotenv.2021.14559; 10.1016/j.scitotenv.2020.142428;

10.1016/j.scitotenv.2020.144479; 10.1016/j.chemosphere.2020.128502)

4)Only few examples regarding microplastic occurrence and abundance in aquatic environment are reported and discussed by the authors. Add more references or remove occurrence from the title.

5)Lines 58-60: provide a more detailed definition of microplastics.

6)Lines 77: replace particles of microplastics per square km with particles/km2.

7)Lines 85-87: the sentence is incorrect; several reviews are reported concerning occurrence of microplastic in freshwater and lakes. (10.1016/j.watres.2015.02.012; 10.1016/j.scitotenv.2020.137512; 10.1016/j.watres.2019.02.054; 10.3390/w13070973; 1016/j.scitotenv.2020.141948; 10.1016/j.envpol.2021.116552, 10.1016/j.jwpe.2021.101966, 10.1016/j.scitotenv.2020.144655)

8)Lines 88-90 and 257: The exponential number must be superscripted.

9)Lines 119-130: these sentences are not appropriate for the introduction section, seems to be a conclusion.

10)Improve quality and resolution of figure 1.

11)The text from section 2 to the end of manuscript must be justified.

12)Section 3 must be revised and rewritten. The subsections are unnecessary because some notions can be incorporated and the comparison of the mechanisms of plastic degradation should be more easily presented. I suggest making a figure to summarize these processes. Results on degradation of plastics must be illustrated in detail, otherwise the section 3 is unnecessary since such general sentences should be moved to introduction.

13)Lines 270-271: sentence incorrect, again several reviews are reported in literature!!! It seems that the authors have not studied in detail literature about microplastic pollution.

14)Table 1? Some consideration about it?

15)Some styles errors are present in references that must be corrected. Moreover, in my opinion, the references are few for a review article.

Author Response

Responses to Reviewer #3

Comment

I have reviewed the manuscript entitled " Microplastics in the aquatic environment - the occurrence, sources, ecological impacts, fate, and remediation challenges" by Chaukura et al for publication in Pollutants. This review discusses sources and pathways through which microplastics enter the environment and evaluates the fate and behaviour of microplastics with the aim to identify research gaps and recommend remediation strategies. However, the present manuscript offers little that is either new or contributes meaningfully to the current discussion in the literature. The manuscript is overly ambitious in its stated objectives, poorly organized. Although the subject matter is very interesting, there are several issues with the manuscript as currently written that need to be considered:

Response

We thank the reviewer for finding the subject matter of the manuscript interesting. We have taken all reviewers’ comments into consideration and improved the manuscript as a result. Thank you.

Comment

1) Since this is a standard part of literature search, it may be beneficial to list a concrete and tangible results obtained as a part of this investigation. Authors must explain how this review is unique and different from the others.

Response

Thank you. We included:

Importantly, while there is significant microplastics pollution in the aquatic environment, pollution in terrestrial systems are not widely studied. Besides, there is a dearth of infor-mation on microplastics in African aquatic systems.

Comment

2)As a review paper, it requires figures or tables to help support the discussion. In line 368 table 1 was mentioned, but it is not present in the paper. As currently written, comparison of literature data results very difficult to understand.

Response

We agree. We have included Table 1 and Table 2 to summarize the data. These complement Fig 1 and Fig 2 in the manuscript.

Comment

3)As stated in line 139, authors considered literature published until June 2020, however in the last year several works and especially reviews on regarding microplastic in the aquatic environment are reported that should be taken into account. (10.3390/w13070973 (Ricardi et al., 2021); 10.1016/j.scitotenv.2020.141948 (Yang et al., 2021); 10.1016/j.envpol.2021.116552; 10.1016/j.jwpe.2021.101966 (Koutnik et al., 2021); 10.1016/j.scitotenv.2020.144655 (Kabir et al., 2021); 10.1016/j.scitotenv.2021.14559; 10.1016/j.scitotenv.2020.142428 (Zhang et al., 2021); 10.1016/j.scitotenv.2020.144479 (de Carvalho et al., 2021);  10.1016/j.chemosphere.2020.128502 (Manikanda et al., 2021)

Response

We thank the reviewer for the insightful literature. We have used the literature as far as we could to improve the manuscript.

Comment

4)Only few examples regarding microplastic occurrence and abundance in aquatic environment are reported and discussed by the authors. Add more references or remove occurrence from the title.

Response

We thank the reviewer. We have added more examples from literature using the recommended literature and more.

Comment

5)Lines 58-60: provide a more detailed definition of microplastics.

Response

Thank you. We have revised to:

Microplastics, or microbeads, are micron-sized plastics or polymers particles (ap-proximately 5000 μm) whose size has been miniaturized or fragmented enough that there might be a need for the use of microscopy techniques to study them [19, 20].

Comment

6)Lines 77: replace particles of microplastics per square km with particles/km2.

Response

Corrected, thank you.

Comment

7)Lines 85-87: the sentence is incorrect; several reviews are reported concerning occurrence of microplastic in freshwater and lakes. (10.1016/j.watres.2015.02.012 (Eerkes-Medrano et al., 2015); 10.1016/j.scitotenv.2020.137512 (Wong et al., 2020); 10.1016/j.watres.2019.02.054 (Koelmans et al., 2019); 10.3390/w13070973 (Riccardi et al., 2021); 1016/j.scitotenv.2020.141948 (Yang et al., 2021); 10.1016/j.envpol.2021.116552 (Koutnik et al., 2021), 10.1016/j.jwpe.2021.101966 (Kumar et al., 2021); 10.1016/j.scitotenv.2020.144655 (Kabir et al., 2021)

Response

After checking the literature, we have revised to:

Water reservoirs such as lakes and dams are particularly susceptible to microplastic pollution, hence there is considerable data on microplastic pollution characteristics in these ecosystems.

Comment

8)Lines 88-90 and 257: The exponential number must be superscripted.

Response

Corrected, thank you.

Comment

9)Lines 119-130: these sentences are not appropriate for the introduction section, seems to be a conclusion.

Response

We agree. We have deleted the paragraph to avoid confusing our readers.

Comment

10)Improve quality and resolution of figure 1.

Response

Revised, thank you.

Comment

11)The text from section 2 to the end of manuscript must be justified.

Response

Text justified, thank you.

Comment

12)Section 3 must be revised and rewritten. The subsections are unnecessary because some notions can be incorporated and the comparison of the mechanisms of plastic degradation should be more easily presented. I suggest making a figure to summarize these processes. Results on degradation of plastics must be illustrated in detail, otherwise the section 3 is unnecessary since such general sentences should be moved to introduction.

Response

We have revised by summarizing section 3 in a Fig 2.

Comment

13)Lines 270-271: sentence incorrect, again several reviews are reported in literature!!! It seems that the authors have not studied in detail literature about microplastic pollution.

Response

We thank the reviewer. We have deleted the misleading statement.

Comment

14)Table 1? Some consideration about it?

Response

Thank you. We have included Table 1 in the manuscript.

Comment

15)Some styles errors are present in references that must be corrected. Moreover, in my opinion, the references are few for a review article.

Response

Thank you. We have corrected by using a referencing software, and used the literature recommended by the reviewers to improve the manuscript.

Round 2

Reviewer 1 Report

I found the last version definitely improved, The authors didn't add all the citations suggested, but they made important changes to the paper. Anyway, before the final pubblication, I recommend to adjust the two tables reported. The sizes of columns should be modified according to the content. The column that includes just the references or the size of the particles could be decreased in order to enlarge the other columns and avoid the split of the words. In Table 2 it is not clear what place is "Euaries of"

Author Response

Comment

I found the last version definitely improved, The authors didn't add all the citations suggested, but they made important changes to the paper. Anyway, before the final pubblication, I recommend to adjust the two tables reported. The sizes of columns should be modified according to the content. The column that includes just the references or the size of the particles could be decreased in order to enlarge the other columns and avoid the split of the words. In Table 2 it is not clear what place is "Euaries of"

Response

We would like to thank the reviewer for the comment. To address this, we have reformatted the tables and corrected ‘Euaries of’ to ‘Estuaries of KwaZulu-Natal River system, South Africa’.

Reviewer 2 Report

ok

Author Response

Reviewer #2

Comment

ok

Response

We thank the reviewer for finding our manuscript acceptable for publication.

Reviewer 3 Report

After reviewers’ comments, authors have improved the manuscript organization and content. However, in my opinion, as currently written the review should have a title that specifically focus on the African aquatic systems making it more fascinating. Moreover, other corrections must be made:

-The definition of microplastics is still unspecific, see https://0-doi-org.brum.beds.ac.uk/10.1016/j.marpolbul.2018.11.022; https://0-doi-org.brum.beds.ac.uk/10.3390/w13070973.

-Since it is a review article, the number of references should be increased adding all reviews of the last year that assess microplastic occurrence in the aquatic compartment. Some of them (https://0-doi-org.brum.beds.ac.uk/10.1016/j.scitotenv.2019.135578; https://0-doi-org.brum.beds.ac.uk/10.3390/w13070973)  show microplastic distribution all over the worlds, and highlight as the occurrence in Africa, South America and North Asia remain poorly investigated.

Author Response

Reviewer #3

Comment

After reviewers’ comments, authors have improved the manuscript organization and content. However, in my opinion, as currently written the review should have a title that specifically focus on the African aquatic systems making it more fascinating.

Response

We are grateful to the reviewer for the generous comments. We agree that a title that specifically focuses on the African aquatic systems will be more fascinating. However, because of the smaller content on African aquatic systems, we have addressed this comment by maintaining the title while adding an objective that focuses on Africa. Thank you.

Comment

Moreover, other corrections must be made:

-The definition of microplastics is still unspecific, see https://0-doi-org.brum.beds.ac.uk/10.1016/j.marpolbul.2018.11.022 (Frias and Nash, 2019); https://0-doi-org.brum.beds.ac.uk/10.3390/w13070973 (Ricciardi et al., 2021).

Response

Thank you for the insightful comment. We have used the literature to revise the definition of microplastics to:

Microplastics, or microbeads, are plastic particles <5 mm diameter , and they include nano-sized particles (1 nm) [19, 20].

Comment

-Since it is a review article, the number of references should be increased adding all reviews of the last year that assess microplastic occurrence in the aquatic compartment. Some of them (https://0-doi-org.brum.beds.ac.uk/10.1016/j.scitotenv.2019.135578 (Li et al., 2019); https://0-doi-org.brum.beds.ac.uk/10.3390/w13070973 (Ricciardi et al., 2021)  show microplastic distribution all over the worlds, and highlight as the occurrence in Africa, South America and North Asia remain poorly investigated.

Response

Thank you for this comment. We have used the suggested and other references to enrich the manuscript.

 

 

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