Systematic Review of Degradation Processes for Microplastics: Progress and Prospects

Round 1

Reviewer 1 Report

Comments for author File: Comments.pdf

Author Response

Dear Reviewers:

Thank you for your letter and for the comments concerning our manuscript entitled “Systematic review of degradation methods for microplastics: Progress and Prospects” (sustainability-2494820). Those comments are all valuable and very helpful for revising and improving our paper. We have studied comments carefully and have made corrections which we hope meet with approval. The main corrections in the paper and the responds to the reviewer’s comments are as flowing:

1: In the direct photo Degradation section on Page 4 of the manuscript, the author extensively focuses on the UV irradiation-based degradation of microplastics. But the latest and current technologies such as Mie Resonance, Plasmonic based catalysis have proven their mark in the synthesis and degradation of polymer materials, where visible light and solar radiation can be exploited for performing chemical reactions using metal, semiconductor-based nanomaterials of singular characteristics. I think the author has to include these current and upcoming technologies as described in detail in Point 5.

A: Thank you for your suggestion. We have added descriptions of the latest techniques such as Mie Resonance, Plasmonic based catalysis, etc. to the article. As follow:

And in recent years, some studies have shown that the photocatalytic reaction based on Mie resonance can lead to the coupling reaction of carbon-carbon (C-C) in aromatic polymers to achieve the degradation effect(Addanki Tirumala, 2022). In addition, Kwon et al.(Kwon et al., 2018) synthesized different Cu2-xS nanoparticles (CuS and Cu1.8S NPs) with localized surface plasmon resonance (LSPR) absorbance in the near-infrared (NIR) region with photodegradation properties leading to polydimethylsiloxane (PDMS) polymer.

References cited:

Addanki Tirumala, R.T., 2022. Exploiting the Optical Properties of Earth Abundant Cuprous Oxide Nanocatalysts for Energy and Health Applications.

Kwon, Y.T., Lim, G.D., Kim, S., Ryu, S.H., Hwang, T.Y., Park, K.R., Choa, Y.H., 2018. Near-infrared absorbance properties of Cu2-xS/SiO2 nanoparticles and their PDMS-based composites. Journal of Materials Chemistry C 6(4), 754-760. https://doi.org/10.1039/c7tc04784c.

2. In Section 3.2 Photocatalytic Oxidation, Page 4 The author focusses on TiO2 and hybrid structures such as ZnO-Pt nanorods, MXene/ZnxCd1-xS photocatalysts show potential to degrade microplastics, the author concluded that “large-scale application of photo degradation technology in the future is limited”. There are other novel materials such as Cu₂O, alpha-Fe₂O₃, metals such as Au, Ag, Cu, and other hybrid structures with which photocatalytic technology can be made feasible economically on a large scale and is not discussed in this work.

A: Thank you for your suggestion. We have added a discussion on the application of Cu₂O, α-Fe₂O₃, metals such as Au, Ag, Cu and other hybrid structures in photocatalytic oxidation technology.  As follow:

Zhou et al.(Zhou et al., 2017) concluded that Cu₂O is an excellent visible light photocatalyst. In 2021, Zhu et al.(Zhu et al., 2021) successfully prepared a material (rGO@Fe₃O₄/Cu₂O@ZnO) with strong hydrophobicity, high photocatalytic performance, and recyclability, and its photocatalytic degradation efficiency of acrylamide (AM) reached 97.3%. In addition to this, a large number of literatures have reported the study of photocatalysts such as Cu₂O(Pary et al., 2021; Tirumala et al., 2022), α-Fe₂O₃(Zhang et al., 2020; Zhang et al., 2022), Au(Yin et al., 2015), Ag(Xu et al., 2013), and Cu(Xu et al., 2023) for polymer degradation.

References cited:

Pary, F.F., Addanki Tirumala, R.T., Andiappan, M., Nelson, T.L., 2021. Copper(i) oxide nanoparticle-mediated C-C couplings for synthesis of polyphenylenediethynylenes: evidence for a homogeneous catalytic pathway. Catalysis Science & Technology 11(7), 2414-2421. https://doi.org/10.1039/d1cy00039j.

Tirumala, R.T.A., Gyawali, S., Wheeler, A., Ramakrishnan, S.B., Sooriyagoda, R., Mohammadparast, F., Khatri, N., Tan, S., Kalkan, A.K., Bristow, A.D., Andiappan, M., 2022. Structure–Property–Performance Relationships of Cuprous Oxide Nanostructures for Dielectric Mie Resonance-Enhanced Photocatalysis. ACS Catalysis 12(13), 7975-7985. https://doi.org/10.1021/acscatal.2c00977.

Xu, C.Y., Chen, W.Q., Wang, J.J., Wu, Q.Y., Wu, P.Y., Tang, L., 2023. Two Cu(I\II) Coordination Polymers for Photocatalytic Degradation of Organic Dyes and Efficient Detection of Fe3+ Ions. Journal of Inorganic and Organometallic Polymers and Materials 33(4), 885-894. https://doi.org/10.1007/s10904-022-02489-5.

Xu, X.X., Cui, Z.P., Qi, J., Liu, X.X., 2013. Fabrication of Ag/CPs composite material, an effective strategy to improve the photocatalytic performance of coordination polymers under visible irradiation. Dalton Transactions 42(37), 13546-13553. https://doi.org/10.1039/c3dt51178b.

Yin, X.N., Wang, J., Zhou, J.J., Li, L., 2015. Mussel-inspired modification of Microporous polypropylene membranes for functional catalytic degradation. Chinese Journal of Polymer Science 33(12), 1721-1729. https://doi.org/10.1007/s10118-015-1726-8.

Zhang, T.Y., Liu, J.D., Zhou, F., Zhou, S.Y., Wu, J.C., Chen, D.Y., Xu, Q.F., Lu, J.M., 2020. Polymer-Coated Fe2O3 Nanoparticles for Photocatalytic Degradation of Organic Materials and Antibiotics in Water. Acs Applied Nano Materials 3(9), 9200-9208. https://doi.org/10.1021/acsanm.0c01829.

Zhang, X.Q., Xia, L., Liu, C., Cheng, X.B., Yang, Z., 2022. A direct Z-scheme heterojunction g-C3N4/alpha-Fe2O3 nanocomposite for enhanced polymer-containing oilfield sewage degradation under visible light. Environmental Science-Water Research & Technology 8(9), 1965-1975. https://doi.org/10.1039/d2ew00342b.

Zhou, X.S., Jin, B., Luo, J., Gu, X.X., Zhang, S.Q., 2017. Photoreduction preparation of Cu2O@polydopamine nanospheres with enhanced photocatalytic activity under visible light irradiation. Journal of Solid State Chemistry 254, 55-61. https://doi.org/10.1016/j.jssc.2017.07.007.

Zhu, B.J., Jiang, G.F., Lv, Y., Liu, F., Sun, J., 2021. Photocatalytic degradation of polyacrylamide by rGO@Fe3O4/Cu2O@ZnO magnetic recyclable composites. Materials Science in Semiconductor Processing 131. https://doi.org/10.1016/j.mssp.2021.105841.

3. Through careful reading the manuscript, the authors claim for slight increase in band gap due to Mn doping however, it is still unclear that what would be the potential applications and its significant impacts and also there are several other studies carried out on doping crystal to tailor the optical properties. The manuscript can be made impactful by adding more details.

A: Thank you for your suggestion. We have discussed more of the literature.  As follow:

The soil containing clay, iron oxides, and MnO2 enhanced the degradation rate, while the soil containing organic carbon inhibited the degradation rate, electrostatic interactions may be the dominant factor influencing the rate of photodegradation of MPs in soils with different properties. Wang et al.(Wang et al., 2020) found that natural organic acids in the aqueous environment can promote the aging of PVC microplastics, which may be related to hydroxyl radicals produced by photolysis of organic acids.

References cited:

Wang, C., Xian, Z., Jin, X., Liang, S., Chen, Z., Pan, B., Wu, B., Ok, Y.S., Gu, C., 2020. Photo-aging of polyvinyl chloride microplastic in the presence of natural organic acids. Water Research 183, 116082. https://doi.org/https://doi.org/10.1016/j.watres.2020.116082.

4. The introduction is not well written and it needs to be further elaborated, which will connect the audience of researchers carrying out work on developing novel degradation technologies. Microplastics though have resistance to degrading due to its high stability they do undergo physical, chemical, biological wear, degradation. I think the authors have to discuss about the effect of pH and conditions in the rate of degradation and what byproducts are formed in these scenarios which will help us in developing technologies favoring the decomposition of microplastics. The lines on Page 4 and 5 of the manuscript the author has mentioned TiO2 and ZnO/Pd that they have been widely studied and applied for microplastics degradation but, the most significant advantage of light-harnessing materials which can convert solar-chemical energy lies in its characteristics for high photocatalytic activity, stability, and low toxicity, making it a promising photocatalytic material. The author did not provide an extensive overview of other photocatalytic materials. To enhance the impact of the study, the author can discuss several strategies that can be employed to tune the optical properties of materials. These strategies include laserassisted surface modifications, doping techniques, and chemical synthesis routes that can tailor particle morphologies for specific applications such as for micro plastics degradation. In order to provide a comprehensive overview, I recommend the authors to include an introduction to metal-oxides, semiconductor photocatalysis, and their role in decomposition of microplastics and challenges faced by these materials. The following articles are suggested for citation as they directly focus on describing these methods and their applications:

o Visualizing undyed microplastic particles and fibers with plasmon-enhanced fluorescence

https://www.sciencedirect.com/science/article/pii/S1385894722016151

o Copper( i ) Oxide Nanoparticle-Mediated C–C Couplings for Synthesis of

Polyphenylenediethynylenes: Evidence for a Homogeneous Catalytic Pathway.

https://doi.org/10.1039/D1CY00039J.

o Plasmonic Carbonaceous Nanotemplates for Microplastics Raman Detection.

DOI: 10.1149/MA2021-01611638mtgabs

o Structure–Property–Performance Relationships of Cuprous Oxide Nanostructures for

Dielectric Mie Resonance-Enhanced Photocatalysis.

https://doi.org/10.1021/acscatal.2c00977.

o Capturing colloidal nano- and microplastics with plant-based nanocellulose networks.

https://doi.org/10.1038/s41467-022-29446-7

o Microplastics in the environment: Occurrence, perils, and eradication.

doi: 10.1016/j.cej.2020.127317

Including these articles in the study will provide a solid foundation for understanding the methods used to tune the optical properties of materials and their specific applications, as well as shedding light on metal oxides, semiconductor photocatalysis, and other relevant mechanisms for methylene blue degradation.

A: Thank you for your advice. The introductory section has been modified according to your suggestions to include a discussion of the effects of degradation conditions and pH on degradation efficiency and by-products. The prospects for the application of photocatalytic materials are discussed, as well as an introduction to metal oxides, semiconductor photocatalysis and their roles in the decomposition of microplastics as well as the challenges faced by these materials, with references to relevant literature. As follow:

And the selection of biodegradation conditions is a key factor in improving the efficiency of degradation. pH is a critical factor for the survival of microorganisms, as it has a key influence on their life activities and substance metabolism(Xu et al., 2011). Increase or decrease in pH during biodegradation may be due to the production and accumulation of alkaline aromatic compounds or other metabolites during degradation.

References cited:

Xu, L., Crawford, K., Gorman, C.B., 2011. Effects of Temperature and pH on the Degradation of Poly(lactic acid) Brushes. Macromolecules 44(12), 4777-4782. https://doi.org/10.1021/ma2000948

Anand, U., Dey, S., Bontempi, E., Ducoli, S., Vethaak, A.D., Dey, A., Federici, S., 2023. Biotechnological methods to remove microplastics: a review. Environmental Chemistry Letters 21(3), 1787-1810. https://doi.org/10.1007/s10311-022-01552-4.

Leppänen, I., Lappalainen, T., Lohtander, T., Jonkergouw, C., Arola, S., Tammelin, T., 2022. Capturing colloidal nano- and microplastics with plant-based nanocellulose networks. Nature Communications 13(1), 1814. https://doi.org/10.1038/s41467-022-29446-7.

Pary, F.F., Addanki Tirumala, R.T., Andiappan, M., Nelson, T.L., 2021. Copper(i) oxide nanoparticle-mediated C-C couplings for synthesis of polyphenylenediethynylenes: evidence for a homogeneous catalytic pathway. Catalysis Science & Technology 11(7), 2414-2421. https://doi.org/10.1039/d1cy00039j.

Tirumala, R.T.A., Gyawali, S., Wheeler, A., Ramakrishnan, S.B., Sooriyagoda, R., Mohammadparast, F., Khatri, N., Tan, S., Kalkan, A.K., Bristow, A.D., Andiappan, M., 2022. Structure–Property–Performance Relationships of Cuprous Oxide Nanostructures for Dielectric Mie Resonance-Enhanced Photocatalysis. ACS Catalysis 12(13), 7975-7985. https://doi.org/10.1021/acscatal.2c00977.

5. Figure 3 describing “Pathways for animal decomposition of microplastics” can be made more legible with the text bold.

A: Thank you for your suggestion. We have bolded the font of Figure 3 to make the image clearer.

6. A combined figure of various ways of degradation of microplastics can add value to this work.

A: Thank you for your suggestion. We show a combination of the various degradation methods in the graphic summary.

 

Thank you again for your suggestion.

 

 

 

 

 

Reviewer 2 Report

I have now completed reviewing the manuscript entitled" Systematic Review of Degradation Methods for Microplastics: Progress and Prospects ". My comments below are for your attention:

 

1. What has been the efficiency of various degradation methods for these microplastics concerning Physical and chemical methods? This is not mentioned in the manuscript and Section 4 should be reworked. It will make sense to make each method (either physical or chemical) a section on its own. Otherwise, the manuscript heading should be revised to Systematic Review of Biodegradation Methods for Microplastics: Progress and Prospects.
2. Sections 2 and 3 should be rewritten as part of Section 4.
3. There is no more profound analysis of the author's input other than what is already available in the scientific literature. Authors should try to interrogate these literature review findings while adding their perspectives.
4. Other relevant recent references should be included, such as 1007/s10311-022-01552-4.
5. In its current form, the paper requires major rework before publication to make it a one-stop resource regarding degradation methods for microplastics.

Author Response

Dear Reviewers:

Thank you for your letter and for the comments concerning our manuscript entitled “Systematic review of degradation methods for microplastics: Progress and Prospects” (sustainability-2494820). Those comments are all valuable and very helpful for revising and improving our paper. We have studied comments carefully and have made corrections which we hope meet with approval. The main corrections in the paper and the responds to the reviewer’s comments are as flowing:

1.What has been the efficiency of various degradation methods for these microplastics concerning Physical and chemical methods? This is not mentioned in the manuscript and Section 4 should be reworked. It will make sense to make each method (either physical or chemical) a section on its own. Otherwise, the manuscript heading should be revised to Systematic Review of Biodegradation Methods for Microplastics: Progress and Prospects.

A: Thank you for your suggestion. We have added a description of the degradation efficiencies of the various degradation methods to the article and have rewritten the contents of Part IV.

 

2.Sections 2 and 3 should be rewritten as part of Section 4.

A: Thanks to your suggestion, we have restructured the article by rewriting the second and third sections as part of the fourth section.

 

3.There is no more profound analysis of the author's input other than what is already available in the scientific literature. Authors should try to interrogate these literature review findings while adding their perspectives.

A: Thank you for your suggestion. We have added an analytical discussion of the literature and summarized our views after each degradation method.

 

4.Other relevant recent references should be included, such as 1007/s10311-022-01552-4.

A: Thank you for your suggestion. We cited recent relevant literature in our article.

References cited:

Anand, U., Dey, S., Bontempi, E., Ducoli, S., Vethaak, A.D., Dey, A., Federici, S., 2023. Biotechnological methods to remove microplastics: a review. Environmental Chemistry Letters 21(3), 1787-1810. https://doi.org/10.1007/s10311-022-01552-4.

 

5.In its current form, the paper requires major rework before publication to make it a one-stop resource regarding degradation methods for microplastics.

A: Thank you very much for reviewing my article, I have carefully read your review comments and made changes according to your suggestions, and I hope to meet your requirements.

 

Reviewer 3 Report

The authors need to proofread the manuscript thoroughly. I suggest the following changes and improvements:

1.     The first two sentences of the abstract should be improved considering the research problem or background of the study.

2.     A conclusive critical view should be included in the abstract.

3.     The authors need to improve the Abstract section considering the important findings and underscore the scientific value added to your manuscript in your abstract.

4.     The current structure of the introduction is not well organized. The authors need to be improved. Additionally, the last part needs to be revised considering the main theme/objectives and findings of the research.

5.     The novelty and research significance of this work should be stated clearly in the introduction section.

6.     Add a section regarding the literature review and search results. The literature review methodology is not well explained, and the procedures followed for the study need to be described in more detail. Please clarify that from the ### articles you found up to this point, you made a first assessment by screening titles and abstracts. When was this research conducted? This is very important because databases are updated also for past periods of time.

7.     This not only requires a lot of lands but also, because of the………. Please rewrite and eliminate the grammatical issue.

8.     The timeline for the development of various photocatalytic materials for microplastic degradation should be presented.

9.     Please ensure the proper credit when reproducing the material/Figures.

 

10.  The authors need to provide detailed research direction advising young researchers, especially Ph.D. students who would be reading with interest to learn more about this research topic.

Mentioned in the comment section.

Author Response

Dear Reviewers:

Thank you again for your letter and for the comments concerning our manuscript entitled “Systematic review of degradation methods for microplastics: Progress and Prospects” (sustainability-2494820). Those comments are all valuable and very helpful for revising and improving our paper. We have studied comments carefully and have made corrections which we hope meet with approval. The main corrections in the paper and the responds to the reviewer’s comments are as flowing:

1: The first two sentences of the abstract should be improved considering the research problem or background of the study. 

A: Thank you for your suggestions. We have revised the content of the abstract. As follows:

MPs have been shown to be more hazardous than large plastics. In recent years, many studies have confirmed the hazards of MPs to organisms and summarized various MPs degradation techniques, but there is a lack of discussion on the prospects of the application of degradation techniques and the degradation efficiency.

 

2: A conclusive critical view should be included in the abstract.

A: Thank you for your suggestions. We have revised the summary content based on your comments.

 

3: The authors need to improve the Abstract section considering the important findings and underscore the scientific value added to your manuscript in your abstract.

A: Thank you for your suggestions. We have revised the content of the abstract and emphasized the scientific value of the study. As follows:

Microplastics (MPs) have been shown to be more hazardous than large plastics. In recent years, many studies have confirmed the hazards of MPs to organisms and summarized various MPs degradation techniques, but there is a lack of discussion on the prospects of the application of these degradation techniques and their degradation efficiency. Therefore, this paper reviewed the  degradation techniques of MPs, such as adsorption, direct photodegradation, photocatalytic oxi-dation, electrochemical oxidation and biological methods and their application prospects. By fo-cusing on the biodegradation mechanism and degradation efficiency, the potential for efficient and sustainable development of biodegradation methods and the prospect of large-scale application are highlighted, enabling readers to better understand the current status of research on MPs biodeg-radation. This review provides direction for research on MPs degradation, suggestions for gov-ernmental environmental governance and policy development, and references for the sustainability and large-scale application of MPs biodegradation.

 

4: The current structure of the introduction is not well organized. The authors need to be improved. Additionally, the last part needs to be revised considering the main theme/objectives and findings of the research.

A: Thank you for your suggestions. We have modified the content of the introduction. As follows:

In general, a problem that human beings must face is that MPs will only be pro-duced by human beings, after a series of migrations, they will eventually return to hu-man beings. With the passage of time and the accumulation of MPs, it is time to think and solve these problems. As of May 2022, a total of 113 articles on microplastic deg-radation were found through a literature analysis is performed using the Stork softwareand screening of article titles and abstracts. By ana-lyzing the above articles, this study reviews various degradation techniques for MPs as well as discusses for the first time the efficiency, sustainability, and prospects for large-scale application of biodegradation methods. This review provides direction for research on the degradation of MPs, suggestions for governmental environmental gov-ernance and policy development, and references for the sustainability and large-scale application of MPs biodegradation.

 

5: The novelty and research significance of this work should be stated clearly in the introduction section.

A: Thank you for your suggestions. We have revised the preface section to clearly state the novelty and research significance of this work. As follows:

By analyzing the above articles, this study reviews various degradation techniques for MPs as well as discusses for the first time the efficiency, sustainability, and prospects for large-scale application of biodegradation methods. This review provides direction for research on the degradation of MPs, suggestions for governmental environmental governance and policy development, and references for the sustainability and large-scale application of MPs biodegradation.

 

6: Add a section regarding the literature review and search results. The literature review methodology is not well explained, and the procedures followed for the study need to be described in more detail. Please clarify that from the ### articles you found up to this point, you made a first assessment by screening titles and abstracts. When was this research conducted? This is very important because databases are updated also for past periods of time.

A: Thank you for your suggestions. We have added a presentation of the literature review and search results in the introduction. As follows:

As of May 2022, a total of 113 articles on microplastic degradation were found through a literature analysis is performed using the Stork software and screening of article titles and abstracts.

 

7: This not only requires a lot of lands but also, because of the………. Please rewrite and eliminate the grammatical issue.

A: Thank you for your suggestions. We have made the change. As follows:

At present, most of the waste plastics in the world are disposed of by landfills (Zhang et al., 2021a) , which requires a large amount of land. Due to the strong stability of plastic, it is not easy to decompose in the soil, which seri-ously affects the sustainable use of the soil. Moreover,  a large number of microorgan-isms breed in the soil, producing harmful gases that adversely affect the surrounding air and environment.

 

8: The timeline for the development of various photocatalytic materials for microplastic degradation should be presented.

A: Thank you for your suggestions. We have added the development time of various photocatalytic materials to the text. As follows:

Nabi et al. (Nabi et al., 2020) showed in their 2020 paper that the TiO2 nanoparticle film had a high degradation efficiency of 98.40% for 400 nm PS within 12 h and the highest degradation efficiency for PE after 36 h. In addition, it was mentioned in a paper in 2021 that N-TiO2 materials synthesized by two routes showed a good degradation effect on MPs (Luo et al., 2021a). Not only on nano-TiO2, Tofaet al. (Tofa et al., 2019b) found in 2019 that zinc oxide (ZnO) nanorods (ZnO-Pt) also have a good effect on the degradation of MPs in water. The use of solar energy as an energy source is the greatest advantage of photocatalytic oxidation. Cao et al. (Cao et al., 2022) successfully prepared a series of MXene/ZnxCd1-xS photocatalysts in 2022, which can degrade MPs and utilize light energy to catalyze hydrogen evolution.

 

9: Please ensure the proper credit when reproducing the material/Figures.

A: Thank you for your suggestions. Images and materials in the paper are cited and licensed.

 

10: The authors need to provide detailed research direction advising young researchers, especially Ph.D. students who would be reading with interest to learn more about this research topic

A: Thank you for your suggestions. In our summary, we suggest several directions for future research. As follows:

First, we can screen plants that can effectively absorb MPs in the soil to improve the soil environment. Second, screen for microorganisms that can efficiently degrade MPs, especially fungi and mixed flora. Third, remediation of MPs-contaminated soil by in-oculation with degrading bacteria.

Thank you again for your suggestions!

 

Round 2

Reviewer 3 Report

The authors have answered the comments satisfactorily.

Author Response

Thank you again for your suggestions. These comments are valuable and helpful in revising and improving our paper.