Evaluation of Heavy Metal Content in Plastic Bags Used as Improvised Food Cooking Covers: A Case Study from the Mozambican Community

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The study investigated the physical and chemical characteristics of various plastic bags collected from markets and stores or shops in Mozambique and the presence of different harmful heavy metals in them. It can be an impactful research from the perspective of human health and environmental safety to limit exposure to harmful heavy metals. However, some critical concerns remain regarding the experimental design, data interpretation, and data representation of this study. Therefore, it is suggested that the authors address the following specific comments to improve the quality of the manuscript.

 

Comment 1. The overall manuscript has quite a few typos, grammatical mistakes, punctuation mistakes, and use of scientifically extreme/inaccurate words. Please correct them throughout the manuscript.

 

Comment 2. The “Title” is confusing. How are plastic bags part of food cooking containers? What kind of food cooking containers, e.g., metal pots, plastic pots, skillets? Please explain the perspective and consider rewriting the “Title” for clarity.

 

Comment 3. The “Abstract” talks about the similar characteristic peaks for LDPE and HDPE from the FTIR analysis, making it unclear how they were distinguished. Please discuss the characteristics peak information that enabled you to distinguish between LDPE and HDPE.

 

Also, it includes the relative abundances of heavy metals in LDPE compared to HDPE bags. Consider reporting heavy metal quantities as well so that the audiences can understand the extent of heavy metal additives easily.

 

Comment 4. The “Introduction” section discussed the classification of plastics, including “thermoplastics, elastomers, thermosets, microplastics, macroplastics, nanoplastics, single-use plastics and bioplastics” as categories. Thermoplastics, elastomers, and thermosets are categories depending on the polymer characteristics, while microplastics, macroplastics, and nanoplastics are categories based on size. Single-use plastics and bioplastics are categorized based on reusability and degradability, respectively. Categorizing all of them together seems inappropriate.

 

The role of heavy metals as additives in plastic bags should be more comprehensively discussed with support from more appropriate/relevant references.

 

Moreover, the “Introduction” section needs to be more organized in terms of explaining general plastic use trends in Mozambique and then shifting to the extent of plastic use in food cooking practices.

 

Finally, the novelty of the study should be specifically highlighted, and the objectives of this study should be stated clearly in the last paragraph of the “Introduction” as testable hypotheses.

 

Comment 5. The plastic bags were sourced from 17 different origins. Was only one plastic bag collected from each origin? Or were replicates also collected? How many replicates of each plastic bag sample from each origin were collected? If replicates were not collected, how did you ensure that the plastic bags had consistent characteristics throughout the manufacturing process or between manufacturing processes taking place on different days?

 

Comment 6. For FTIR analysis, samples were cleaned with ethanol. Even though ethanol evaporates, did you do any “control study” to track whether ethanol wash had any effect on plastic surface characteristics? If not, please design and conduct a “control study” to provide us with information on whether the interaction between ethanol and plastic bags affected the characteristics identified by the FTIR.

 

Comment 7. There is no information provided on AAS calibration standards and calibration curve parameters, such as R² values, limits of detection, and limits of quantification for different metals. Please provide these specific values.

 

Comment 8. How were the plastic bag samples cleaned before aqua regia digestion? There is no information on that. Also, was any field control study carried out during sample collection? If not, how did you ensure that the quantity of heavy metals in plastics was not the result of environmental exposure from the markets/shops/stores? The quantity of heavy metals reported in your study can be an overestimation if those bags were kept in the store exposed to open space and there was environmental deposition of heavy metals onto the bags.

 

Comment 9. Were the samples replicated for digestion and AAS analysis?

 

Comment 10. Consider removing Figures 1 and 2 and presenting the experimental procedures in detail in writing.

 

Comment 11. The density of the plastic bags calculated in the study is way lower than the standard LDPE and HDPE density. You reported that it may be because of the additives and production process. However, the discussion is inadequate and used references are scant. A more comprehensive and critical discussion regarding the contribution of these factors on density variation needs to be included in the manuscript based on appropriate and adequate number of references.

 

Comment 12. Similar characteristic peaks for LDPE and HDPE were discussed. However, the characteristic peak or peaks that differentiated them were not well discussed. Moreover, the LDPE and HDPE spectra represented in Figures 4 and 5, respectively, look more or less similar. Please explain how you differentiated them based on those figures.

 

Comment 13. Standard deviation was not reported for the heavy metal values in Tables 4 and 5. Therefore, it is not possible to understand the uncertainty level. Please perform a replicated heavy metal analysis and report the standard deviation.

 

Comment 14. Consider removing Tables 4 and 5. Rather, consider representing the data from Tables 4 and 5 as graphs, e.g., bar graphs, for easier visualization and comparison for the readers. Also, please include error bars to represent the standard deviation/uncertainty level of results.

 

Comment 15. Some important discussions in section 3.3 did not include references to support the reasoning/implications/conclusion. For example, the following discussion does not refer to the specific literature where the indicated regulatory limit can be found: “The sample with the highest metal content was Cod.06 for LDPE, while Cod.03 had the highest levels for HDPE. Both values significantly exceed the limits established in the relevant regulation.”

 

Similar practices can be seen throughout the manuscript. Please fix them throughout the manuscript and include appropriate references to support your reasoning/implication/conclusion.

 

Comment 16. Discuss the limitations of this study in the “Conclusion” section, then briefly discuss the findings of this study, and finally suggest safe plastic bag use practices to limit human heavy metal exposure from plastic bags.

Comments on the Quality of English Language

English used in the manuscript does not restrict the understanding of the research. However, the overall manuscript has quite a few typos, grammatical mistakes, punctuation mistakes, and use of scientifically extreme/inaccurate words. Therefore, there is room for improving the quality of English.

Author Response

Journal: Sustainability MDPI Manuscript ID: Sustainability-3350158

Title: Evaluation of Heavy Metals content in Plastic Bags Used as Improvised Food Cooking Covers: A Case Study from the Mozambican Community

 

Manença Cristiano Nhanga, Dulce Geraldo, Sérgio Leonardo Nhapulo, Afonso Filipe João and Manuel F. M. Costa.

We are thankful to receive the Reviewers’ comments on our manuscript "Evaluation of Heavy Metals content in Plastic Bags used as Improvised Food Cooking Covers: A Case Study from Mozambican Community", and we truly appreciate the time have taken to read, analyse and comment on our manuscript. To improve the quality of our manuscript, several improvements were made, aiming to comply with their observations. We restructured our references list, to include more relevant literature. This came along with the reformulation of some sections of the manuscript, to improve the contents of our text. The answers to each of the points raised by the Reviewers are listed below.

All queries and comments from the Reviewers are addressed below. Our direct changes have been highlighted in yellow for easy identification in the manuscript file. We truly hope that you now consider our work suitable for publication in the journal.

Staying at your disposal for any further requests, comments, and questions.

On behalf of the authors,

Manença Nhanga

 

Reviewer 1:

The study investigated the physical and chemical characteristics of various plastic bags collected from markets and stores or shops in Mozambique and the presence of different harmful heavy metals in them. It can be an impactful research from the perspective of human health and environmental safety to limit exposure to harmful heavy metals. However, some critical concerns remain regarding the experimental design, data interpretation, and data representation of this study. Therefore, it is suggested that the authors address the following specific comments to improve the quality of the manuscript:

 

The authors sincerely appreciate the reviewer's careful evaluation of the work detailed in the manuscript and the insightful comments provided.

Comment 1: The overall manuscript has quite a few typos, grammatical mistakes, punctuation mistakes, and use of scientifically extreme/inaccurate words. Please correct them throughout the manuscript?

Reply: The authors sincerely appreciate the insightful comment regarding the typos, grammatical mistakes, punctuation errors, and the use of scientifically extreme or inaccurate words throughout the manuscript.

Action: We thoroughly reviewed the text and made the necessary corrections to ensure clarity, accuracy, and scientific rigor in the revised version.

 

Comment 2: The “Title” is confusing. How are plastic bags part of food cooking containers? What kind of food cooking containers, e.g., metal pots, plastic pots, skillets? Please explain the perspective and consider rewriting the “Title” for clarity.

Reply: The Mozambican community often uses plastic bags as pot lids, creating a greenhouse effect that facilitates steaming, similar to pressure cookers. This method is especially used for cooking tubers, rice and other foods.

 

Action: We have revised the title of the manuscript (Evaluation of Heavy Metals content in Plastic Bags Used as Improvised Food Cooking Covers: A Case Study from the Mozambican Community)

 

Comment 3: The “Abstract” talks about the similar characteristic peaks for LDPE and HDPE from the FTIR analysis, making it unclear how they were distinguished. Please discuss the characteristics of peak information that enabled you to distinguish between LDPE and HDPE.

Also, it includes the relative abundances of heavy metals in LDPE compared to HDPE bags. Consider reporting heavy metal quantities as well so that the audiences can understand the extent of heavy metal additives easily.

Reply: The authors sincerely appreciate the comment regarding the clarity of the FTIR analysis and the discussion of the distinguishing characteristics of LDPE and HDPE.

Action: Changes were made in the Section “Results” of the manuscript. (pag.9)

Comment 4: The “Introduction” section discussed the classification of plastics, including “thermoplastics, elastomers, thermosets, microplastics, macroplastics, nanoplastics, single-use plastics and bioplastics” as categories. Thermoplastics, elastomers, and thermosets are categories depending on the polymer characteristics, while microplastics, macroplastics, and nanoplastics are categories based on size. Single-use plastics and bioplastics are categorized based on reusability and degradability, respectively. Categorizing all of them together seems inappropriate. 

The role of heavy metals as additives in plastic bags should be more comprehensively discussed with support from more appropriate/relevant references.

 

Moreover, the “Introduction” section needs to be more organized in terms of explaining general plastic use trends in Mozambique and then shifting to the extent of plastic use in food cooking practices.

 

Finally, the novelty of the study should be specifically highlighted, and the objectives of this study should be stated clearly in the last paragraph of the “Introduction” as testable hypotheses.

 

Reply: The authors sincerely appreciate the insightful comment regarding the classification of the plastic.

Action: Changes were made in the section “Introduction” (pag. 1 to 3) of the manuscript.

Comment 5: The plastic bags were sourced from 17 different origins. Was only one plastic bag collected from each origin? Or were replicates also collected? How many replicates of each plastic bag sample from each origin were collected? If replicates were not collected, how did you ensure that the plastic bags had consistent characteristics throughout the manufacturing process or between manufacturing processes taking place on different days?

Reply: The authors sincerely appreciate the comment and the opportunity to clarify the sampling process. A total of 17 plastic bags were collected from different origins, not 17 separate origins. Each plastic bag was sourced from a different location, and only one bag was collected from each origin. Replicates were not collected for each plastic bag sample. However, to ensure consistency, all plastic bags were freshly collected, and each bag was carefully selected to represent the typical properties of bags from that particular origin. While replicates were not included in the study, we believe that the characteristics of each bag were representative of the manufacturing process, as they were all new and sourced directly from different origins, minimizing variability due to age or prior use.

Comment 6:  For FTIR analysis, samples were cleaned with ethanol. Even though ethanol evaporates, did you do any “control study” to track whether ethanol wash had any effect on plastic surface characteristics? If not, please design and conduct a “control study” to provide us with information on whether the interaction between ethanol and plastic bags affected the characteristics identified by the FTIR.

Reply: The authors sincerely appreciate the comment and the valuable suggestion regarding the use of ethanol in the FTIR analysis. To clarify, ethanol was not used to clean the surfaces of the plastic bags themselves. Instead, ethanol was used exclusively to clean the surfaces of the equipment, specifically the Perkin Elmer IR, as part of the mandatory procedure to prevent contamination between samples and ensure accurate measurements.

As such, there was no direct interaction between ethanol and the plastic bags that could potentially affect the surface characteristics identified by the FTIR. Since ethanol was not in contact with the plastic surfaces, a control study to track its effect was not necessary.

Comment 7: There is no information provided on AAS calibration standards and calibration curve parameters, such as R² values, limits of detection, and limits of quantification for different metals. Please provide these specific values.

Reply: The measurements were performed at the LCR - Waste Characterization Laboratory of CVR, Campus de Azúrem, University of Minho, Guimaraes, Portugal (https://www.cvresiduos.pt/en/) an internationally accredited laboratory according to standard NP EN ISO / IEC 17025. The report provided information on the metal concentrations and the quantification limits of the method.

Action: The data about quantification limits was incorporated into the text to address the request (pag.11)

Comment 8: How were the plastic bag samples cleaned before aqua regia digestion? There is no information on that. Also, was any field control study carried out during sample collection? If not, how did you ensure that the quantity of heavy metals in plastics was not the result of environmental exposure from the markets/shops/stores? The quantity of heavy metals reported in your study can be an overestimation if those bags were kept in the store exposed to open space and there was environmental deposition of heavy metals onto the bags.

Reply: The authors sincerely appreciate the thoughtful questions and would like to clarify the sample preparation and collection process. Regarding the cleaning of the plastic bag samples before aqua regia digestion, no additional cleaning process was carried out prior to digestion. However, the plastic bags were carefully handled to minimize any potential contamination during sample collection. As highlighted in Comment 5, all plastic bags were new and collected directly from different origins, ensuring that they were not exposed to environmental conditions that could affect the results.

Regarding the field control study, no formal field control study was conducted during sample collection. However, to address concerns about environmental exposure, special care was taken to avoid any unnecessary exposure of the plastic bags to environmental elements that could lead to the deposition of heavy metals. We believe that the careful handling and the use of only new plastic bags provided a reasonable assurance that the reported heavy metal concentrations are primarily due to the materials used in manufacturing the bags, rather than environmental exposure.

Comment 9: Were the samples replicated for digestion and AAS analysis?

Reply: The authors sincerely appreciate the insightful comment and the opportunity to clarify this aspect of our methodology.

We confirm that our study included replicates for the digestion and AAS analysis to ensure the reliability and consistency of the results. Additionally, we varied the mass of plastic within the same volume to further validate the findings and explore potential variations. This approach was critical to ensuring the robustness of the data presented in our study.

Comment 10: Consider removing Figures 1 and 2 and presenting the experimental procedures in detail in writing.

Reply: The authors sincerely appreciate the suggestion to remove Figures 1 and 2 and present the experimental procedures in more detail in writing. However, we believe that the inclusion of these figures provides clear and concise visual representation of the experimental setup, which enhances the reader's understanding of the methodology. Therefore, we would prefer to retain the figures in the manuscript for better clarity and presentation.

Comment 11: The density of the plastic bags calculated in the study is way lower than the standard LDPE and HDPE density. You reported that it may be because of the additives and production process. However, the discussion is inadequate and used references are scant. A more comprehensive and critical discussion regarding the contribution of these factors on density variation needs to be included in the manuscript based on appropriate and adequate number of references.

Reply: We thank you for the valuable comment regarding the density of the plastic bags.

Action: The manuscript has been modified highlighting a comprehensive and critical discussion of the factors on density variation (pag. 9).

Comment 12: Similar characteristic peaks for LDPE and HDPE were discussed. However, the characteristic peak or peaks that differentiated them were not well discussed. Moreover, the LDPE and HDPE spectra represented in Figures 4 and 5, respectively, look more or less similar. Please explain how you differentiated them based on those figures.

Reply: The authors sincerely appreciate the comment regarding the differentiation of LDPE and HDPE based on the FTIR spectra presented in Figures 4 and 5. While the spectra for LDPE and HDPE indeed share similar characteristic peaks, as both are types of polyethylene, their differentiation was based on differences of the intensity and position of specific peaks.

Action: We have revised the manuscript to explicitly highlight the specific peaks and characteristics that enabled the differentiation of LDPE and HDPE, providing greater clarity. (Pag. 9).

 

Comment 13:  Standard deviation was not reported for the heavy metal values in Tables 4 and 5. Therefore, it is not possible to understand the uncertainty level. Please perform a replicated heavy metal analysis and report the standard deviation.

Reply: We appreciate the reviewer's comment, which provided us with the opportunity to clarify this point. To evaluate the global precision of the method, replicate sampling and digestion of the samples were carried out. The maximum relative standard deviation observed was 20%.

Action: A sentence was added to the manuscript to include this information.

Comment 14: Consider removing Tables 4 and 5. Rather, consider representing the data from Tables 4 and 5 as graphs, e.g., bar graphs, for easier visualization and comparison for the readers. Also, please include error bars to represent the standard deviation/uncertainty level of results.

Reply: Thank you for your comments. We thought that the representation in table form was very clear, as in different studies such as [1], [2] and [3] they represent it in table form. However, we found the reviewer's suggestion relevant, which is why we modified the tables into graphs.

[1] https://doi.org/10.1016/j.jenvman.2018.02.034

[2] https://doi.org/10.1016/j.heliyon.2024.e25384

[3] https://doi.org/10.1007/s40201-019-00337-2

Action: We have modified the tables 4 and 5 into figure 6 and 7, (pag.12 to 13)

Comment 15: Some important discussions in section 3.3 did not include references to support the reasoning/implications/conclusion. For example, the following discussion does not refer to the specific literature where the indicated regulatory limit can be found: “The sample with the highest metal content was Cod.06 for LDPE, while Cod.03 had the highest levels for HDPE. Both values significantly exceed the limits established in the relevant regulation.”

Similar practices can be seen throughout the manuscript. Please fix them throughout the manuscript and include appropriate references to support your reasoning/implication/conclusion.

 

Reply: We thank the reviewer for the suggestion. Below goes the revised passage with the specific reference.

 

Action: “The sample with the highest metal content was Cod.06 for LDPE, while Cod.03 had the highest levels for HDPE. Both values significantly exceed the limits established in the relevant regulation. [36]”.

 

Comment 16: Discuss the limitations of this study in the “Conclusion” section, then briefly discuss the findings of this study, and finally suggest safe plastic bag use practices to limit human heavy metal exposure from plastic bags.

Reply: The authors sincerely appreciate the comment of the reviewer.

Action: Relevant discussions have been addressed in the manuscript, (pag.14).

Comments on the Quality of English Language

English used in the manuscript does not restrict the understanding of the research. However, the overall manuscript has quite a few typos, grammatical mistakes, punctuation mistakes, and use of scientifically extreme/inaccurate words. Therefore, there is room for improving the quality of English.

Actions:  We reviewed some linguistic aspects of the manuscript and believe that they are sufficient for acceptance and subsequent publication.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

I congratulate you on your excellent work.

I believe a review would be interesting. I see no need to repeat terms already used in the title in key words.

I believe that the methodology is a bit extensive and unnecessary. It would be interesting to cite only the references that guided the analyses. What reference was the concept presented in Figure 1 based on?

Comments for author File: Comments.pdf

Author Response

Journal: Sustainability MDPI Manuscript ID: Sustainability-3350158

Title: "Evaluation of Heavy Metals content in Plastic Bags used as Improvised Food Cooking Covers: A Case Study from Mozambican Community"

Manença Cristiano Nhanga, Dulce Geraldo, Sérgio Leonardo Nhapulo, Afonso Filipe João and Manuel F. M. Costa.

We are thankful to receive the Reviewer’s comments on our manuscript "Evaluation of Heavy Metals content in Plastic Bags used as Improvised Food Cooking Covers: A Case Study from Mozambican Community", and we truly appreciate the time have taken to read, analyse and comment on our manuscript. To improve the quality of our manuscript, several improvements were made, aiming to comply with their observations. We restructured our references list, to include more relevant literature. This came along with the reformulation of some sections of the manuscript, to improve the contents of our text. The answers to each of the points raised by the Reviewers are listed below.

All queries and comments from the Reviewers are addressed below. Our direct changes have been highlighted in yellow for easy identification in the manuscript file. We truly hope that you now consider our work suitable for publication in the journal.

Staying at your disposal for any further requests, comments, and questions.

On behalf of the authors,

Manença Nhanga

Reviewer 2:

I congratulate you on your excellent work.

I believe a review would be interesting. I see no need to repeat terms already used in the title in key words.

Reply: Keywords: Public health; FTIR; AAS; LDPE; HDPE; Plastic Bags Density.

I believe that the methodology is a bit extensive and unnecessary. It would be interesting to cite only the references that guided the analyses.

The authors sincerely appreciate the reviewer's careful evaluation of the work detailed in the manuscript and the insightful comments provided. Further revisions were added throughout the manuscript in order to make it scientifically concise and clear.

What reference was the concept presented in Figure 1 based on?

Reply: Figure 1 is based on Reference 3 of the manuscript. To provide additional details, the relevant reference was added alongside the figure.

Figure 1. FTIR analysis procedure [3].

[3] Jiang, Y.; Son, G.; Zhang, H. Material identification and heavy metals characterization of plastic packaging bags used in Chinese express delivery. Frontiers in Environmental Science. 2023, 11:1253108. doi:10.3389/fenvs.2023.1253108.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

Thank you for inviting me to review this manuscript, which focuses on the evaluation of heavy metals in plastic bags used as food cooking containers in Mozambique, analyzed using Atomic Absorption Spectroscopy (AAS). The topic is interesting based on the introduction and title; however, the manuscript noticeably lacks innovation. Numerous studies have already reported the presence of both organic and inorganic substances in microplastics and plastic items, including plastic bags. The manuscript does not sufficiently emphasize its novelty or explain how it differentiates itself from existing research.

 

Moreover, the results section requires significant revision. The authors should provide more in-depth explanations regarding the presence of these metals in plastic bags, rather than simply listing their findings. For these reasons, I recommend major revisions before the manuscript can be considered for publication.

 

Additional Comments:

 

 

1)    Please address the following points in the manuscript:

a)     Why was Atomic Absorption Spectroscopy (AAS) chosen as the method to detect heavy metals?

b)    Why were these specific heavy metals selected for the study?

c)     Why is it important to determine the density of the plastics?

d)    In Figure 1, explain why the plastic samples were washed with EtOH.

e)     Clarify the differences between samples Cod.07 and Cod.07R, and explain why the plastic fingerprint area varies, given that both samples are from the same plastic.

 

2)    Minor Corrections:

a)     Rephrase the following sentence for clarity and fluency:“For example, studies have revealed accumulations of heavy metals in children's plastic toys and in blood samples from plastics workers with heavy metal content [14,15]. Therefore, it is almost unknown to talk about the use of plastic bags to prepare food, however, the migration of heavy metals from plastic bags to food when subjected to thermal conditioning is generally known [16,17].”

b)    Move section headings to a new page when they appear as the last line on the current page (e.g., sections 2.2 and 3.2).

c)     In Figure 2, remove the red error lines.

d)    In Figure 3, remove the numbers from the Y-axis.

Author Response

Journal: Sustainability MDPI Manuscript ID: Sustainability-3350158

Title: "Evaluation of Heavy Metals content in Plastic Bags used as Improvised Food Cooking Covers: A Case Study from Mozambican Community"

Manença Cristiano Nhanga, Dulce Geraldo, Sérgio Leonardo Nhapulo, Afonso Filipe João and Manuel F. M. Costa.

We are thankful to receive the Reviewer’s comments on our manuscript "Evaluation of Heavy Metals content in Plastic Bags used as Improvised Food Cooking Covers: A Case Study from Mozambican Community", and we truly appreciate the time have taken to read, analyse and comment on our manuscript. To improve the quality of our manuscript, several improvements were made, aiming to comply with their observations. We restructured our references list, to include more relevant literature. This came along with the reformulation of some sections of the manuscript, to improve the contents of our text. The answers to each of the points raised by the Reviewers are listed below.

All queries and comments from the Reviewers are addressed below. Our direct changes have been highlighted in yellow for easy identification in the manuscript file. We truly hope that you now consider our work suitable for publication in the journal.

Staying at your disposal for any further requests, comments, and questions.

On behalf of the authors,

Manença Nhanga

 

Reviewer 3:

Thank you for inviting me to review this manuscript, which focuses on the evaluation of heavy metals in plastic bags used as food cooking containers in Mozambique, analyzed using Atomic Absorption Spectroscopy (AAS). The topic is interesting based on the introduction and title; however, the manuscript noticeably lacks innovation. Numerous studies have already reported the presence of both organic and inorganic substances in microplastics and plastic items, including plastic bags. The manuscript does not sufficiently emphasize its novelty or explain how it differentiates itself from existing research.

 

Moreover, the results section requires significant revision. The authors should provide more in-depth explanations regarding the presence of these metals in plastic bags, rather than simply listing their findings. For these reasons, I recommend major revisions before the manuscript can be considered for publication.

 

 The authors sincerely appreciate the reviewer's careful evaluation of the work detailed in the manuscript and the insightful comments provided.

1. Please address the following points in the manuscript:
2. a)Why was Atomic Absorption Spectroscopy (AAS) chosen as the method to detect heavy metals?

Reply: We thank you for the valuable question.

We have already addressed the importance of using Atomic Absorption Spectroscopy in the manuscript section “Methods” pag.6.

1. b)Why were these specific heavy metals selected for the study?

Reply: The authors sincerely thank you for the question.

Selected heavy metals were quantified due to their potential toxicology in the public health and the environment.

For example, Pb has been extensively studied and is proven to cause neurotoxicity, cognitive impairments, behavioral abnormalities, infertility, miscarriage, and reduced reproductive capacity. Additionally, studies have demonstrated that cancerous breast tissue biopsies often exhibit elevated levels of Cd. Prolonged exposure to Zn can result in Cu deficiency, which manifests as hypocupremia, anemia, leukopenia, and neutropenia. Excessive Cu exposure, on the other hand, is associated with anemia, diarrhea, and infertility. [3,36,37].

Action: We have included the explanation in the manuscript.

 

1. c)Why is it important to determine the density of the plastics?

Reply: We thank you for the relevant question regarding the importance of determining the density of the plastic bags.

Determining the density of the plastic bags was essential for comparing their physical properties, particularly in identifying the similarities and mechanical differences between LDPE and HDPE. Additionally, this measurement played a crucial role in investigating the hypothesised correlation between density and heavy metals concentration.

Action: We have revised the manuscript (pag.8).

1. d)In Figure 1, explain why the plastic samples were washed with EtOH.

Reply: The authors sincerely appreciate the comment and the valuable suggestion regarding the use of ethanol in the FTIR analysis. To clarify, ethanol was not used to clean the surfaces of the plastic bags themselves. Instead, ethanol was used exclusively to clean the surfaces of the equipment, specifically the Perkin Elmer IR, as part of the mandatory procedure to prevent contamination between samples and ensure accurate measurements.

As such, there was no direct interaction between ethanol and the plastic bags that could potentially affect the surface characteristics identified by the FTIR. Since ethanol was not in contact with the plastic surfaces, a control study to track its effect was not necessary.

1. e)Clarify the differences between samples Cod.07 and Cod.07R, and explain why the plastic fingerprint area varies, given that both samples are from the same plastic.

Reply: The authors sincerely appreciate the insightful comment and the opportunity to clarify. We have already included the explanation in the manuscript. (pag.9).

“The difference in transmittance shows that the sample is, in part, composed of other molecular structures, which results in a lower transmittance. Through the equipment's software, it was possible to identify the presence of nylon in the Cod.07R sample, which may have been added to the plastics to increase the tensile and impact resistance of the base material, making them more durable and capable of withstanding greater loads without deformation and even improving thermal resistance [40]. These factors contribute to the observed variations in the plastic fingerprint area despite both samples originating from the same plastic.”

1. Minor Corrections:
2. a)Rephrase the following sentence for clarity and fluency: “For example, studies have revealed accumulations of heavy metals in children's plastic toys and in blood samples from plastics workers with heavy metal content [14,15]. Therefore, it is almost unknown to talk about the use of plastic bags to prepare food, however, the migration of heavy metals from plastic bags to food when subjected to thermal conditioning is generally known [16,17].”

Reply: The authors sincerely appreciate the comment and the opportunity to enhance the clarity of the sentence.

Action: We clarified the passage as follows and it has been changed in manuscript: " For instance, studies have documented the accumulation of heavy metals in children's plastic toys and in the blood samples of plastics industry workers due to heavy metal exposure [25,26]. While the use of plastic bags for food preparation is rarely discussed, it is well-documented that heavy metals can migrate from plastic bags into food when exposed to thermal conditions [27,28].

2. b)Move section headings to a new page when they appear as the last line on the current page (e.g., sections 2.2 and 3.2).

Reply: The authors sincerely appreciate the valuable suggestion and it has been corrected in the manuscript.

1. c)In Figure 2, remove the red error lines.

Reply: We thank you for the valuable suggestion.

Action: The red error lines have been removed in the figure 2.

1. d)In Figure 3, remove the numbers from the Y-axis.

Reply: We thank you for the valuable suggestion regarding the figure 3.

Action: We have transformed the figure 3 of the manuscript.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The study investigated the physical and chemical characteristics of various plastic bags collected from markets and stores or shops in Mozambique and the presence of different harmful heavy metals in them. It can be an impactful research from the perspective of human health and environmental safety to limit exposure to harmful heavy metals.

 

The authors addressed most of the comments properly from the previously suggested revisions. However, they still failed to appropriately address some of the critical concerns regarding the experimental design and data representation. Therefore, it is suggested that the authors address the following specific comments to further improve the quality of the manuscript.

 

Comment 1. The first two paragraphs of the "Introduction" section are too generalized in terms of the focus of this study. Therefore, it is suggested that the authors shorten the discussion in the first two paragraphs and present them concisely to preserve the focus of the manuscript.

 

Comment 2. Instrument surface cleaning is not a part of the sample pre-processing. Therefore, it is suggested that "cleaning the surface" part be removed from Figure 1. You can discuss the instrument cleaning in the writing portion.

 

Comment 3. R² value information for the AAS calibration is still missing in the manuscript, as was requested previously. It is very important to know the calibration parameters appropriately to determine whether there was proper quality control and quality assurance during the study.

 

Additionally, the discussion on the calibration information, quality control, and detection or quantification limit should be included in section 2.4 rather than the discussion section.

 

Comment 4. Table data was converted to graphs as requested. However, error bars to demonstrate the uncertainty level were not included in the graphs according to the suggestion. Please include error bars depending on the sample replication data. Please note that one sample analyzed twice by the AAS is not considered replication. AAS showing a 20% deviation between two analyses for the same sample reflects the precision of AAS. However, multiple digested replicates of the same sample are needed to be analyzed by the AAS to understand the variation in heavy metal content in a single plastic bag. Please represent their standard deviation as error bars in Figures 6 and 7.

 

Comment 5. Please design an appropriate control study and share the control data to present the degree of contamination from the stores/marketplaces where the samples were collected.

 

It is important because no sample cleaning/washing procedure was performed before sample digestion for AAS analysis and FTIR analysis. Therefore, it is possible that atmospheric deposits from the stores/marketplaces can lead to an overestimation of heavy metal contents. Additionally, if there were any organic deposits, FTIR could have recorded spectra from those organic deposits due to not cleaning the plastic bag surfaces, leading to a wrong spectral representation.

 

Comment 6. Please move the "Limitations and Suggestions" section as section 4 before the "Conclusion" section and include the “Conclusion” section as section 5.

Author Response

Journal: Sustainability MDPI Manuscript ID: Sustainability-3350158

Title: "Evaluation of Heavy Metals content in Plastic Bags used as Improvised Food Cooking Covers: A case Study from Mozambican Community"

Manença Cristiano Nhanga, Dulce Geraldo, Sérgio Leonardo Nhapulo, Afonso Filipe João, Joaquim Carneiro and Manuel F. M. Costa.

We are thankful to receive the Reviewer comments on our manuscript "Evaluation of Heavy Metals content in Plastic Bags used as Improvised Food Cooking Covers: A case Study from Mozambican Community", and we truly appreciate the time they have taken to read, analyse and comment on our manuscript. To improve the quality of our manuscript, several improvements were made, aiming to comply with your observations. We restructured our references list, to include more relevant literature. This came along with the reformulation of some sections of the manuscript, to improve the contents of our text. The answers to each of the points raised by the Reviewer are listed below.

All queries and comments from the Reviewers are addressed below. Our direct changes have been highlighted in yellow for easy identification in the manuscript file. We truly hope that you now consider our work suitable for publication in the journal.

Staying at your disposal for any further requests, comments, and questions.

On behalf of the authors,

Manença Nhanga

 

Reviewer 1; Round 2:

The study investigated the physical and chemical characteristics of various plastic bags collected from markets and stores or shops in Mozambique and the presence of different harmful heavy metals in them. It can be an impactful research from the perspective of human health and environmental safety to limit exposure to harmful heavy metals.

The authors addressed most of the comments properly from the previously suggested revisions. However, they still failed to appropriately address some of the critical concerns regarding the experimental design and data representation. Therefore, it is suggested that the authors address the following specific comments to further improve the quality of the manuscript.

The authors sincerely appreciate the reviewer's careful evaluation of the work detailed in the manuscript and the insightful comments provided.

Comment 1. The first two paragraphs of the "Introduction" section are too generalized in terms of the focus of this study. Therefore, it is suggested that the authors shorten the discussion in the first two paragraphs and present them concisely to preserve the focus of the manuscript.

Reply: We thank you for the insightful suggestion of the referee regarding the shortened discussion in the first two paragraphs and the opportunity to improve and make it concise throughout the manuscript. 

We have modified the paragraphs in the manuscript (pag.1).

Comment 2. Instrument surface cleaning is not a part of the sample pre-processing. Therefore, it is suggested that the "cleaning the surface" part be removed from Figure 1. You can discuss the instrument cleaning in the writing portion.

Reply: We sincerely appreciate the valuable suggestion and the opportunity to adjust the figure 1.

We have modified the schematic structure of the figure.

Comment 3. R2 value information for the AAS calibration is still missing in the manuscript, as was requested previously. It is very important to know the calibration parameters appropriately to determine whether there was proper quality control and quality assurance during the study.

Additionally, the discussion on the calibration information, quality control, and detection or quantification limit should be included in section 2.4 rather than the discussion section.

Reply: We thank the referee for emphasizing the importance of including calibration parameters and R^2 values information in the manuscript. We would like to clarify that the calibration parameters, including the R^2 values, have been included in the manuscript as requested.

In line with the suggestion, we have ensured that the discussion on calibration information, and detection or quantification limits is now presented in the Section 2.4 of the manuscript, as this seems to be the most appropriate location for such details.

Comment 4. Table data was converted to graphs as requested. However, error bars to demonstrate the uncertainty level were not included in the graphs according to the suggestion. Please include error bars depending on the sample replication data. Please note that one sample analyzed twice by the AAS is not considered replication. AAS showing a 20% deviation between two analyses for the same sample reflects the precision of AAS. However, multiple digested replicates of the same sample are needed to be analyzed by the AAS to understand the variation in heavy metal content in a single plastic bag. Please represent their standard deviation as error bars in Figures 6 and 7.

Reply: We appreciate the referee's suggestion regarding the inclusion of error bars to demonstrate the uncertainty level in the graphs. However, we would like to clarify that the 20% deviation reported between two AAS analyses of the same sample reflects the overall uncertainty of the method, which takes into account the variability introduced by the entire process. This includes the sampling process, the digestion procedure, and the AAS analysis itself.

The AAS analysis, while important, is the factor contributing the least to the overall uncertainty. The primary sources of variability stem from the sampling and digestion steps, which can introduce greater variability than the AAS analysis. Therefore, the reported 20% deviation is representative of the combined uncertainty from all stages of the method, and not solely from the AAS analysis. Error bars were added to the graphs of figures 6 and 7.

Comment 5. Please design an appropriate control study and share the control data to present the degree of contamination from the stores/marketplaces where the samples were collected.

It is important because no sample cleaning/washing procedure was performed before sample digestion for AAS analysis and FTIR analysis. Therefore, it is possible that atmospheric deposits from the stores/marketplaces can lead to an overestimation of heavy metal contents. Additionally, if there were any organic deposits, FTIR could have recorded spectra from those organic deposits due to not cleaning the plastic bag surfaces, leading to a wrong spectral representation.

Reply: We appreciate the referee's insight regarding the potential impact of contamination from the stores/marketplaces on the samples' results. However, it is important to clarify that the primary objective of this study is to assess the metal content in the plastic bags as they are collected, reflecting their real-world condition and usage.

Since these plastic bags are intended for practical applications without any pre-cleaning or washing procedures, our study deliberately avoids altering their state. This approach ensures that our findings are representative of their actual use and aligns with the intended application scenarios.

Moreover, while atmospheric or organic deposits could contribute to the observed results, these are inherently part of the environment in which the bags are used. Therefore, the degree of contamination from the stores/marketplaces is not only relevant but also integral to understanding the overall impact of these plastic bags in their native state.

We acknowledge the importance of contamination studies in other contexts and agree that such investigations could provide valuable insights. However, in this study, our focus remains on evaluating the bags as they are, without any modifications.

Comment 6. Please move the "Limitations and Suggestions" section as section 4 before the "Conclusion" section and include the “Conclusion” section as section 5.

Reply: We thank you for the relevant comment on changing sections.

We have made changes in the manuscript (pag. 13 to 14).

Author Response File: Author Response.docx

Round 3

Reviewer 1 Report

Comments and Suggestions for Authors

Please address the following comments for the manuscript to be accepted:

Comment 1. There are quite a few mistakes in the sentence structure where one sentence was started and then another sentence was infused without finishing the prior sentence (specifically paragraphs 3 and 4 of the Introduction section). Please correct those sentences.

Comment 2. The following statement about heavy metals in the Introduction section is factually incorrect: "Their atomic weights vary between 63.5 and 200.6 and they occur on the surface of the Earth [13,14]."

Pb is a heavy metal and its atomic weight is 207.2, which is greater than 200.6. Please correct the facts in the manuscript.

Comment 3. Discussion regarding the LOD/LOQ still exists in section 3.3, which should be moved to section 2.4 and added to the discussion on calibration.

Comment 4. The following sentence was included in the manuscript in section 3.1: "This hypothesis is supported by the total heavy metal content values obtained in this study, which provide a basis for further verification."

However, no supporting discussion to correlate density and heavy metal content was provided. Please include precise relevant discussion.

Comment 5. " an extremely low density" used in the manuscript (section 3.1) is a scientifically unacceptable term. Remove/correct extremity in any sentences throughout the manuscript.

Comment 6. Make the following corrections to this paragraph in the manuscript in section 3.2: "Peaks around 1.462 cm⁻¹, specifically 1462.64 cm⁻¹ in LDPE and 1462.07 cm⁻¹ in HDPE, correspond to C–H bending vibrations, indicating a shared structural feature. Additionally, the peaks at 875.88 cm⁻¹ in LDPE and 875.89 cm⁻¹ in HDPE are attributed to C=CH bending vibrations of alkenes, demonstrating functional composition similarities. The peak at 718.86 cm⁻¹, common to both LDPE and HDPE, further supports these shared characteristics."

1.462 cm^-1 is wrong; it should be 1462. Additionally, references to claims made in this paragraph should be added.

Comment 7. Please add error bars to total heavy metal concentrations as well, like the other ones.

Comments on the Quality of English Language

Please improve the paragraph structures throughout the manuscript to improve the flow of reading.

Author Response

Journal: Sustainability MDPI Manuscript ID: Sustainability-3350158

Title: "Evaluation of Heavy Metals content in Plastic Bags used as Improvised Food Cooking Covers: A case Study from Mozambican Community"

Manença Cristiano Nhanga, Dulce Geraldo, Sérgio Leonardo Nhapulo, Afonso Filipe João, Joaquim Carneiro and Manuel F. M. Costa.

We are thankful to receive the Reviewer´s comments on our manuscript "Evaluation of Heavy Metals content in Plastic Bags used as Improvised Food Cooking Covers: A case Study from Mozambican Community", and we truly appreciate the time they have taken to read, analyse and comment on our manuscript. To improve the quality of our manuscript, several improvements were made, aiming to comply with your observations. We restructured our references list, to include more relevant literature. This came along with the reformulation of some sections of the manuscript, to improve the contents of our text. The answers to each of the points raised by the Reviewer are listed below.

All queries and comments from the Reviewers are addressed below. Our direct changes have been highlighted in yellow for easy identification in the manuscript file. We truly hope that you now consider our work suitable for publication in the journal.

Staying at your disposal for any further requests, comments, and questions.

On behalf of the authors,

Manença Nhanga

 

Reviewer 1; Round 3:

The authors sincerely appreciate the reviewer's careful evaluation of the work detailed in the manuscript and the insightful comments provided.

Comment 1. There are quite a few mistakes in the sentence structure where one sentence was started and then another sentence was infused without finishing the prior sentence (specifically paragraphs 3 and 4 of the Introduction section). Please correct those sentences.

Reply: The authors sincerely appreciate the relevant comment regarding the sentence structure on paragraphs 3 and 4.

We have modified the paragraphs throughout the manuscript (pag 2).

Comment 2. The following statement about heavy metals in the Introduction section is factually incorrect: Their atomic weights vary between 63.5 and 200.6 and they occur on the surface of the Earth [13,14].

Pb is a heavy metal and its atomic weight is 207.2, which is greater than 200.6. Please correct the facts in the manuscript.

Reply: The authors sincerely appreciate the insightful comment regarding the statements about heavy metals in the manuscript.

We thoroughly reviewed the text and made the necessary corrections to ensure clarity, accuracy, and scientific rigor in the revised version.

Comment 3. Discussion regarding the LOD/LOQ still exists in section 3.3, which should be moved to section 2.4 and added to the discussion on calibration.

Reply: We thank you for the valuable comment regarding the LOD/LOQ. We have made changes to the section in the manuscript.

Comment 4. The following sentence was included in the manuscript in section 3.1: "This hypothesis is supported by the total heavy metal content values obtained in this study, which provide a basis for further verification."

However, no supporting discussion to correlate density and heavy metal content was provided. Please include precise relevant discussion.

Reply: We thank the referee for the valuable and insightful comment regarding the correlation of the density and the results of the heavy metals contents. We have addressed relevant explanations about the correlation between the density and heavy metals content (pag.13).

Comment 5. " an extremely low density" used in the manuscript (section 3.1) is a scientifically unacceptable term. Remove/correct extremity in any sentences throughout the manuscript.

Reply: We thank you for the relevant observation and the opportunity to correct. We have corrected the passage in the manuscript (pag.8).

Comment 6. Make the following corrections to this paragraph in the manuscript in section 3.2: "Peaks around 1462 cm⁻¹, specifically 1462.64 cm⁻¹ in LDPE and 1462.07 cm⁻¹ in HDPE, correspond to C–H bending vibrations, indicating a shared structural feature. Additionally, the peaks at 875.88 cm⁻¹ in LDPE and 875.89 cm⁻¹ in HDPE are attributed to C=CH bending vibrations of alkenes, demonstrating functional composition similarities. The peak at 718.86 cm⁻¹, common to both LDPE and HDPE, further supports these shared characteristics."

1462 cm-1 is wrong; it should be 1462. Additionally, references to claims made in this paragraph should be added.

Reply: The Authors sincerely appreciate the insightful comment and the opportunity to correct and make it clear. We have corrected the mistake (pag. 9).

Comment 7. Please add error bars to total heavy metal concentrations as well, like the other ones.

Reply: We thank you for the valuable comment regarding the error bars. We have added the error bars to total heavy metals concentrations.

 

Author Response File: Author Response.docx