Combined Effect of Propyl Gallate and Tert-Butyl Hydroquinone on Biodiesel and Biolubricant Based on Waste Cooking Oil

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The present study concerns a significant environmental problem: the finding of biodegradable alternatives to petroleum products such as biodiesel and biolubricants, based on affordable formulations such as waste cooking oil (WCO). The authors explore the effect of antioxidants (propyl gallate /PG/, tert-butylhydroquinone /TBHQ/ and a mixture of both) on biodiesel and biolubricants in order to enhance their oxidative stability and improve their performance under extreme oxidation conditions.  The effect of PG is found to be more pronounced and optimal concentration of 300ppm for biolubricants is proposed. The authors also successfully proposed a voltametric method for PG and TBHQ quantitative determination in WCO biolubricant as an alternative to traditional, more time consuming analytical techniques. The research and the results obtained are useful for better understanding of biolubricants performance and might lead to their efficient applications in various industrial process.

 

 

Technical remarks:

In my opinion a unification of the antioxidant concentration units in fig. 9, 10, 11, 12 (mg/L) and in the text (ppm) would be appropriate and would favor the result interpretation.

 

 

 

Comments on the Quality of English Language

The English could definitely be improved. More clear expressions am less words repetition would significantly improve the  understanding of the information. Especially in the results Interpretation and discussion.

 

Author Response

Dear reviewer,

Firstly, thank you for your point of view and recommendations, which have contributed to the improvement of this work. Our replies are the following:

The present study concerns a significant environmental problem: the finding of biodegradable alternatives to petroleum products such as biodiesel and biolubricants, based on affordable formulations such as waste cooking oil (WCO). The authors explore the effect of antioxidants (propyl gallate /PG/, tert-butylhydroquinone /TBHQ/ and a mixture of both) on biodiesel and biolubricants in order to enhance their oxidative stability and improve their performance under extreme oxidation conditions.  The effect of PG is found to be more pronounced and optimal concentration of 300ppm for biolubricants is proposed. The authors also successfully proposed a voltametric method for PG and TBHQ quantitative determination in WCO biolubricant as an alternative to traditional, more time consuming analytical techniques. The research and the results obtained are useful for better understanding of biolubricants performance and might lead to their efficient applications in various industrial process.

 Thank you for your encouraging feedback.

 

Technical remarks:

In my opinion a unification of the antioxidant concentration units in fig. 9, 10, 11, 12 (mg/L) and in the text (ppm) would be appropriate and would favor the result interpretation.

There was a mistake, as all the units were the same (ppm). We have corrected this mistake in the final text and the figures.

 

Again, thank you for your help and sincerely,

Dr. Sergio Nogales Delgado.

Reviewer 2 Report

Comments and Suggestions for Authors

The article "Combined effect of propyl gallate and tert-butyl hydroquinone on biodiesel and biolubricant based on waste cooking oil" has undertaken a very interesting and important task from the point of view of circular economy solutions.

 Production of biodiesel from waste raw materials of cooking vegetable oils is an interesting alternative, which is characterized by obtaining waste products during transesterification. For instance, methanol, being a waste product, can be reused. The only limitation of the use of waste cooking (vegetable) oils is their tendency to self-oxidize, resulting in a loss of physicochemical properties. Thus, some  alternative is to add antioxidants into vegetable oils to inhibit this process.

An interesting research hypothesis has been put forward related to the possibility of converting used waste vegetable oils, into diesel fuel additives. The aforementioned waste can be converted into biodiesel and biosolids. Of course, this is possible if the physicochemical requirements of waste cooking oils are met. Authors have studied the effect of antioxidants (PG and TBHQ) during the production of biodiesel and biolubricant on their physicochemical properties.

The described research plan is very detailed. The research methods and descriptions of the obtained results are presented in a clear manner.

The list of literature, providing the basis for the research conducted, is correct.

The results described in the article have important impact on the development of the alternative fuel sector.

Comments of a polemical nature that would expand the state of the art:

1. What was the reason for using lettuce in phytotoxicity analyses? What criterion determined such a choice?

2. What is the effect of the antioxidants used on the possible results of engine tests on the dynamometer? Whether the biodiesel used have a beneficial effect on the operating conditions of Diesel engines? This element of the study is missing in the analyzes. Such a test would significantly improve the potential applicability of the obtained results.

(3) Does the tested biodiesel have proper viscosity at low ambient temperatures?

Author Response

Dear reviewer,

First of all, thank you so much for your review, which has contributed to the improvement of the quality of this work. Our replies to your comments are the following:

The article "Combined effect of propyl gallate and tert-butyl hydroquinone on biodiesel and biolubricant based on waste cooking oil" has undertaken a very interesting and important task from the point of view of circular economy solutions.

 Production of biodiesel from waste raw materials of cooking vegetable oils is an interesting alternative, which is characterized by obtaining waste products during transesterification. For instance, methanol, being a waste product, can be reused. The only limitation of the use of waste cooking (vegetable) oils is their tendency to self-oxidize, resulting in a loss of physicochemical properties. Thus, some  alternative is to add antioxidants into vegetable oils to inhibit this process.

An interesting research hypothesis has been put forward related to the possibility of converting used waste vegetable oils, into diesel fuel additives. The aforementioned waste can be converted into biodiesel and biosolids. Of course, this is possible if the physicochemical requirements of waste cooking oils are met. Authors have studied the effect of antioxidants (PG and TBHQ) during the production of biodiesel and biolubricant on their physicochemical properties.

The described research plan is very detailed. The research methods and descriptions of the obtained results are presented in a clear manner.

The list of literature, providing the basis for the research conducted, is correct.

The results described in the article have important impact on the development of the alternative fuel sector.

Thank you for your encouraging comments.

 

Comments of a polemical nature that would expand the state of the art:

 

1. What was the reason for using lettuce in phytotoxicity analyses? What criterion determined such a choice?

Lettuce is a species that is highly sensitive to phytotoxic products, as observed in the literature. Even though the method is not the same, we have added references to support the selection of lettuce. There are plenty of phytotoxicity tests that could have been equally used in this context.

2. What is the effect of the antioxidants used on the possible results of engine tests on the dynamometer? Whether the biodiesel used have a beneficial effect on the operating conditions of Diesel engines? This element of the study is missing in the analyzes. Such a test would significantly improve the potential applicability of the obtained results.

This is a very interesting point, as the effect of antioxidants on combustion performance has been widely studied in the literature. However, it is not possible to carry out such a experiment in our facilities. In any case, the use of antioxidants normally contributes to a better performance of biodiesel in the corresponding engine, with lower emissions and better flow properties over time. We have added this point in the conclusion section, as further studies.

 

3. Does the tested biodiesel have proper viscosity at low ambient temperatures?

We have added a new determination (cold filter plugging point) to point out this fact. Indeed, for hot climates, it complied with the standard (1 °C, within the limits established of -20 to 5 °C), assuring the good performance in Diesel engines under these circumstances.

 

Again, thank you for your attention and sincerely,

Dr. Sergio Nogales Delgado.

Reviewer 3 Report

Comments and Suggestions for Authors

 

The paper shown the research on combined effect of propyl gallate and tert-butyl hydroquinone on biodiesel and biolubricant based on waste cooking oil. As noted in this paper, biodiesel and biolubricants offer multiple advantages, but they also face challenges, such as low oxidation stability. This work focuses on antioxidants, which can improve the overall performance of these products during storage (or use). Therefore, in my opinion, the work is based on valid assumptions. The aim of this work was to assess, through voltammetry techniques, the combined and separate use of two different antioxidants on biodiesel and biolubricant based on waste cooking oil. It was noted that voltammetry was a suitable and fast method to quantify both propyl gallate and tert-butylhydroquinone. However, the combined use of antioxidants did not result in a synergistic effect. The paper rightly highlights the issue of oil oxidation, focusing research on the analysis of antioxidants. Studying antioxidants can extend product shelf life, thereby reducing environmental impact (fewer expired oils for disposal). Therefore, the topic addressed in this work is both relevant and necessary. Additionally, the study offers a new and interesting approach to the subject.   The work is well-described and presented clearly.  The methodology employed in this research is well-suited to the topic and thoroughly explained.  The experimental results are presented in a clear and understandable manner, and the references are appropriately selected to support the research. The references are well-chosen and relevant to the topic.   Some questions and conclusions follow: - Were other research methods considered when developing the research methodology? If so, it would be helpful to describe in a few sentences why the particular method used in this work was chosen. - What challenges or difficulties were encountered during the measurements? Even if no significant issues arose, it would be valuable to mention this or discuss any potential limitations. - It would be beneficial to redraw Figure 3 to include larger images, which will improve readability. - In Figure 4, the axis label is incorrect – it should be: time (days). Why is "PG" in square brackets in Figure 13? - In Figure 14, the axis label should be corrected to: Viscosity (cSt), in accordance with the formatting of other unit descriptions in brackets. - Figures 9-12 are readable, but it would be beneficial to increase their resolution. The same applies to Figures 13-15. - In my opinion, the Conclusions chapter would benefit from a more concise presentation, focusing on the most important findings of this work. Including too many points (discoveries) may give the impression that this study addressed all the questions in the analyzed area. Some points should instead be highlighted in the Results and Discussion chapter. Therefore, the Conclusions chapter should be rewritten.

Author Response

Dear reviewer,

Thank you for your report. Your recommendations and comments have been really useful to enhance the quality of this work. Our comments and changes are the following:

The paper shown the research on combined effect of propyl gallate and tert-butyl hydroquinone on biodiesel and biolubricant based on waste cooking oil. As noted in this paper, biodiesel and biolubricants offer multiple advantages, but they also face challenges, such as low oxidation stability. This work focuses on antioxidants, which can improve the overall performance of these products during storage (or use). Therefore, in my opinion, the work is based on valid assumptions. The aim of this work was to assess, through voltammetry techniques, the combined and separate use of two different antioxidants on biodiesel and biolubricant based on waste cooking oil. It was noted that voltammetry was a suitable and fast method to quantify both propyl gallate and tert-butylhydroquinone. However, the combined use of antioxidants did not result in a synergistic effect. The paper rightly highlights the issue of oil oxidation, focusing research on the analysis of antioxidants. Studying antioxidants can extend product shelf life, thereby reducing environmental impact (fewer expired oils for disposal). Therefore, the topic addressed in this work is both relevant and necessary. Additionally, the study offers a new and interesting approach to the subject.   The work is well-described and presented clearly.  The methodology employed in this research is well-suited to the topic and thoroughly explained.  The experimental results are presented in a clear and understandable manner, and the references are appropriately selected to support the research. The references are well-chosen and relevant to the topic.  

Thank you for your encouraging words.

 

Some questions and conclusions follow:

- Were other research methods considered when developing the research methodology? If so, it would be helpful to describe in a few sentences why the particular method used in this work was chosen.

There are different methods to produce biolubricants. For instance, the use of epoxidation. In this case, considering the biorefinery context of this double transesterification, we consider that this process is interesting for us. On the other hand, voltammetry could be an alternative for HPLC, which is typically used in antioxidant determination, but it is normally more expensive and takes longer. We have pointed out these facts in subsection 1.7.

-What challenges or difficulties were encountered during the measurements? Even if no significant issues arose, it would be valuable to mention this or discuss any potential limitations.

The main challenges found during the measurements, especially concerning PG and TBHQ content through voltammetry, was the differences in physico-chemical properties between WCO biodiesel and biolubricant. This fact provokes that both antioxidants present different behaviour in this determination, requiring the adjustment of the technique to the specific requirements. We have pointed out this fact in the final text (Section 3.3).

-It would be beneficial to redraw Figure 3 to include larger images, which will improve readability.

We have included larger images. Thank you for your suggestion.

- In Figure 4, the axis label is incorrect – it should be: time (days). Why is "PG" in square brackets in Figure 13?

We have changed the axis label in Figure 4. Thank you for your remark. The brackets indicated PG concentration. In any case, we have changed the label in this figure to clarify this variable.

- In Figure 14, the axis label should be corrected to: Viscosity (cSt), in accordance with the formatting of other unit descriptions in brackets.

Thank you for the suggestion, we have made the change.

- Figures 9-12 are readable, but it would be beneficial to increase their resolution. The same applies to Figures 13-15.

The resolution of these figures has been improved when possible, with a legend redistribution and the increase in font size, among other changes.

- In my opinion, the Conclusions chapter would benefit from a more concise presentation, focusing on the most important findings of this work. Including too many points (discoveries) may give the impression that this study addressed all the questions in the analyzed area. Some points should instead be highlighted in the Results and Discussion chapter. Therefore, the Conclusions chapter should be rewritten.

The conclusions have been rewritten. Thank you for your suggestion.

 

Again, thank you for your recommendations and sincerely,

Dr. Sergio Nogales Delgado.

Reviewer 4 Report

Comments and Suggestions for Authors

In this work, Nogales-Delgado et al. have investigated the combined effect of two antioxidants, namely propyl gallate and tert-butylhydroquinone, on biodiesel and biolubricant derived from waste cooking oil.  Their results suggested that the combined usage of two antioxidants was less efficient than the separate usage. The authors have also demonstrated the advantage of voltametric method in simultaneously analyzing both antioxidants. This work is of interest to the audience ofApplied Sciences. There are several minor issues, though, that need to be addressed before acceptance.

1.     Figure 8: How many samples of PG and TBHQ mixture are tested? How are the ratios between PG and TBHQ determined? Is the number of samples sufficient to cover the whole concentration regions?

2.     Line 50-52: “As a result, the consumption of the most 50 valuable vegetable oil in Mediterranean diet has decreased, depending on the kind of extraction process, from 22.6 to 52.5%.” These two numbers are likely in reverse order.

3.     Line 99: “Both natural and synthetic antioxidants, they are mainly phenolic compounds.” Grammar error.

Author Response

Dear reviewer,

Thank you for your review, our answers to your questions and remarks are the following:

In this work, Nogales-Delgado et al. have investigated the combined effect of two antioxidants, namely propyl gallate and tert-butylhydroquinone, on biodiesel and biolubricant derived from waste cooking oil.  Their results suggested that the combined usage of two antioxidants was less efficient than the separate usage. The authors have also demonstrated the advantage of voltametric method in simultaneously analyzing both antioxidants. This work is of interest to the audience of Applied Sciences. There are several minor issues, though, that need to be addressed before acceptance.

Thank you for your encouraging words.

 

1. Figure 8: How many samples of PG and TBHQ mixture are tested? How are the ratios between PG and TBHQ determined? Is the number of samples sufficient to cover the whole concentration regions?

For each kind of product (WCO biodiesel and biolubricant), 10 samples were determined, carrying out the corresponding repetitions. The ratios were distributed as follows: 3 samples with variable PG concentration, from the lowest to the highest range included in this work; 3 samples with variable TBHQ concentration; and 4 samples with mixtures of both antioxidants, at high, medium and low concentrations according to our experience. In this sense, the samples were distributed throughout the range of minimum and maximum PG and TBHQ concentrations, considering it enough to cover these regions, offering good adjustments to polynomic functions. We have pointed out this fact in the final text.

 

2. Line 50-52: “As a result, the consumption of the most 50 valuable vegetable oil in Mediterranean diet has decreased, depending on the kind of extraction process, from 22.6 to 52.5%.” These two numbers are likely in reverse order.

It is not in reverse order. It is a range of decreases depending on the kind of oil (according to their extraction process). We have clarified this term in the final text (changes in red).

 

3. Line 99: “Both natural and synthetic antioxidants, they are mainly phenolic compounds.” Grammar error.

We have simplified the sentence, removing the article “they”. Thank you for your suggestion.

 

Again, thank you for your comments, which have contributed to the improvement of the quality of this work. Sincerely,

Dr. Sergio Nogales Delgado.

Reviewer 5 Report

Comments and Suggestions for Authors

The manuscript entitled “Combined effect of propyl gallate and tert-butyl hydroquinone on biodiesel and biolubricant based on waste cooking oil” deals with the field of biodiesel’s properties improvement (in this case the oxidative ability) produced from non-edible biomass feedstocks. However, the following deficiencies ought to be addressed prior to any potential publication:

- Abstract: Fairly clarify the innovative part of your research

- Keywords: The last word is too generic. Please fix.

- Line 38: Authors seem to distinguish frying oil from WCO. However, on section 1.2 the authors refer to both of them as WCO. Please clarify in the entire text.

- Introduction: A brief report on waste-biomass feedstocks should also be mentioned, as these feedstocks also belong to non-edible biomass and comprise promising feedstocks for the production of advanced biofuels, including biodiesel falling under the perspective “Waste-to-Energy”. Suggested papers: doi.org/10.1016/j.renene.2023.02.113 , doi.org/10.1016/j.molstruc.2024.137870 , doi.org/10.1007/978-981-97-0515-3_6

- Section 2: There is no mention on the scientific equipment along with the method used for the chemical analysis (qualitative and quantitative) of the BD samples.

- Section 3.1: A typical spectrum of the FAMEs found in the BD would be helpful.

- Table 6: How did authors test linearity?

- There are many abbreviations in the manuscript. Please insert a list of abbreviations for readers’ convenience.

Author Response

Dear reviewer,

Thank you for your comments! Our replies are the following:

The manuscript entitled “Combined effect of propyl gallate and tert-butyl hydroquinone on biodiesel and biolubricant based on waste cooking oil” deals with the field of biodiesel’s properties improvement (in this case the oxidative ability) produced from non-edible biomass feedstocks. However, the following deficiencies ought to be addressed prior to any potential publication:

- Abstract: Fairly clarify the innovative part of your research

The innovative part was added to the abstract (changes in red).

 

- Keywords: The last word is too generic. Please fix.

We have specified the keyword.

 

- Line 38: Authors seem to distinguish frying oil from WCO. However, on section 1.2 the authors refer to both of them as WCO. Please clarify in the entire text.

Both terms are exchangeable in this context. We have removed frying oil to avoid misunderstandings in the final text.

 

- Introduction: A brief report on waste-biomass feedstocks should also be mentioned, as these feedstocks also belong to non-edible biomass and comprise promising feedstocks for the production of advanced biofuels, including biodiesel falling under the perspective “Waste-to-Energy”. Suggested papers: doi.org/10.1016/j.renene.2023.02.113 , doi.org/10.1016/j.molstruc.2024.137870 , doi.org/10.1007/978-981-97-0515-3_6

We have considered these works in the introduction section. Thank you for your suggestion.

 

- Section 2: There is no mention on the scientific equipment along with the method used for the chemical analysis (qualitative and quantitative) of the BD samples.

You are right about this point, but there is one reason to do so, as we have worked with these equipments in previous research works. That is the reason why the references point out these details, which could be included in this work, but similarity index of this work would be increased in this case. Nevertheless, the most representative equipments (as in the case of voltammetry) were specified.

 

- Section 3.1: A typical spectrum of the FAMEs found in the BD would be helpful.

It is an interesting question. For more information about a typical chromatogram of FAMEs, indicating the most representative elution times, is included in a previous work, which was added as a reference at this point.

 

- Table 6: How did authors test linearity?

Linearity was tested according to a set of different standards at different concentrations (7 in this case for each kind of antioxidant), doing the experiments in triplicate. We have specified this point in the final text.

 

- There are many abbreviations in the manuscript. Please insert a list of abbreviations for readers’ convenience.

We have added a list of abbreviations at the end of the work. Thank you for your suggestion.

 

Again, thank you for your time and sincerely,

Dr. Sergio Nogales Delgado.

Round 2

Reviewer 3 Report

Comments and Suggestions for Authors

The authors have addressed all the suggestions and questions. Additionally, the manuscript has been revised and supplemented appropriately.

In my opinion, the current (improved) version of the manuscript is ready for submission for publication.

Reviewer 5 Report

Comments and Suggestions for Authors

Authors successfully revised and justified their manuscript according to reviewer's comments-suggestions.