Evaluation of ·OH Production Potential of Particulate Matter (PM2.5) Collected on TiO2-Supporting Quartz Filters
Round 1
Reviewer 1 Report
This paper presents valuable experimental data that can be the starting point for an efficient air depollution technology . It is appropriate for publication in present form but some aspects should be detailed:
- It is possible to evaluate the mineralization of OC ( components of PM) under UV exposure,in the presence of TiO2 layer deposited on the surface of quartz fibers?
-The toxicity of the resulted UV degradation products can be also evaluated?
It could be of interest to provide the fluorescence spectra used for 2-OHTA identification at least for a sample ( In addition to Figure 7).
Author Response
Response to reviewers’ comment
We greatly appreciate the constructive and useful recommendations. We have studied your comments carefully and made a serious effort to make appropriate changes. Our answers to listed comments are described below.
1.It is possible to evaluate the mineralization of OC (components of PM) under UV exposure, in the presence of TiO2 layer deposited on the surface of quartz fibers?
(Answer) Thank you very much for your positive concern on the performance of TiO2 filter. In our previous report [28], the mineralization of OC (components of PM) had been already reported considering the mass balance. So, respecting your comments, we have added the following descriptions to improve the manuscript.
P2 L54-58: The carbonaceous components in PM2.5 were successfully converted CO2 by irradiating ultraviolet (UV) rays on the surface of PM2.5 collected on the TiO2 filter [28]. The TiO2 filter also degraded the polycyclic aromatic hydrocarbons in PM2.5, which are potential precursors of PAH quinones [29] and reduced the O2-･production potential of PM2.5 [30].
2. The toxicity of the resulted UV degradation products can be also evaluated?
(Answer) As well as the referee’s concern, we have also mentioned that the redox-active substances can be produced by the UV-indued photocatalytic reaction in our previous report [30]. Therefore, the resulting ･OH production potential after UV irradiation, as shown in Figure 7, can be caused by such degradation products. However, it is very difficult to prove because the individual redox-active substances in PM2.5 samples were too trace to determine at present.
3. It could be of interest to provide the fluorescence used for 2-OHTA identification at least for a sample (In addition to Figure 7).
(Answer) Since the fluorescence spectra of 2-OHTA can be found in literatures, we have added the source of the detection wavelength used in this study, as follows.
P8 L241-242: The detection wavelength of 2-OHTA was referred to Page et al. [36].
Reviewer 2 Report
Sohara et. al studied the evaluation of hydroxyl radical production over PM2.5 over TiO2/quartz filters. They concluded that the PM2.5 samples exhibited ･OH production potential mostly attributed to organic aerosols. The effect was verified using a TiO2-supporting quartz filter for the collection and subsequent degradation of PM2.5. The ･OH production potential was significantly reduced from 0.58 ± 0.40 pmol/ (min m3) to 0.22 ± 0.13 pmol/ (min 19 m3) through ultraviolet irradiation for 24 h. This suggests that the photocatalytic reaction of the TiO2 filter is effective in reducing the ･OH production potential of PM2.5. The manuscript is written in a simple way and the results were clearly shown. However, there are a few comments that need to be clarified before the acceptance of this manuscript.
1- Introduction: the authors should highlight why they choose a TiO2-based filter in their study.
2- The authors are recommended to cite some related references working on the TiO2-based materials in several fields like photocatalysis. Some of the recommended papers are
- https://0-doi-org.brum.beds.ac.uk/10.1088/1361-6528/ac6816
- https://0-doi-org.brum.beds.ac.uk/10.1016/j.colsurfa.2022.128564
- https://0-doi-org.brum.beds.ac.uk/10.1016/j.jphotochem.2022.113906
3- Some characterization like SEM, SBET, and XRD of the TiO2 could be added even in a supporting material for reference of the obtained phases of TiO2 and their structure and surface properties.
4- How about the recyclability of the TiO2-filter, can it be reused ??
So, if the authors can respond to these comments, the manuscript can be further for acceptance.
Author Response
Response to reviewers’ comment
We greatly appreciate the constructive and useful recommendations. We have studied your comments carefully and made a serious effort to make appropriate changes. Our answers to listed comments are described below.
1. Introduction: the authors should highlight why they choose a TiO2-based filter in their study.
(Answer) Respecting the comment, we have added the description as follows.
P2 L47-52: Photocatalysis has been receiving considerable attention worldwide for its diverse potential applications to solve the global energy crisis and environmental problems in a sustainable and ecologically friendly manner [20-22]. Titanium dioxide (TiO2) is by far the most studied photocatalytic material for the decomposition of organic pollutants [23,24], removal of heavy metal ions [25,26] and production of hydrogen [27].
2. The authors are recommended to cite some related references working on the TiO2-based materials in several fields like photocatalysis.
(Answer) We greatly appreciate the useful comment on this point. We have added the following description with citation as recommended by the reviewer.
P2: 50-52: Titanium dioxide (TiO2) is by far the most studied photocatalytic material for the decomposition of organic pollutants [23,24], removal of heavy metal ions [25,26] and production of hydrogen [27].
3. Some characterization like SEM, SBET, and XRD of the TiO2 could be added even in a supporting material for reference of the obtained phases of TiO2 and their structure and surface properties.
(Answer) Thank you very much for your constructive comment. We have added descriptions with SEM images (Figure 4) and XRD pattern (Figure 5) of the TiO2 filter. Since the formed TiO2 did not have a certain morphology, characterization of SBET was not performed in this study.
P5 L150-154: Figure 4 shows the images of the quartz filter before and after depositing TiO2, observed by a field-emission scanning electron microscope (FE-SEM). The TiO2 layer was formed on the surface of the quartz fibre without constant morphology and its crystalline structure was identified to be anatase using X-ray diffraction (XRD) analysis as shown in Figure 5.
4. How about the recyclability of the TiO2-filter, can it be refused??
(Answer) This is an important aspect on the sustainable use of the developed TiO2 filter. As the reviewer knows, PM2.5 samples usually contain significant amounts of sulfate and nitrate which are inert to the photocatalytic degradation. So, post treatment of such residual components on the TiO2 filter will be required for recycling.
Reviewer 3 Report
1.The contribution of this manuscript in this field is not highlighted in the introduction.
2.Lines 80 to 85 describe changes in the concentration of 2-OHTA. However, there are discrepancies between the description and the figure. For example, the units of them are not uniform.
3. Lines 86 to 91 compare OH production potential in different regions. How does this description relate to this chapter?
4.Figure 2 shows the variation of 2-OHTA concentration, however, some of the data are not clearly presented.
5. The manuscript uses a lot of abbreviations, so it is recommended to add a list of abbreviations.
6. Chapter 2.2 uses Pearson's correlation coefficient to compare the correlations of the parameters, why r=0.42 represents "no apparent" (line 99) and r=0.68 represents "significant correlations" (line 110).
7.The description does not match the picture, lines 159 to 165, the OH production potential in Figure 5a has changed from 0.66 to 0.85, an increase of 28.8%, why is it described as "No significant changes"?
8. The section of Materials and Methods is placed after the Results and Discussion, so please explain the reason?
9. From line 265 to line 272, "To quantify the color change of PM2.5 ....", where is the corresponding data support? Why does it jump directly to the conclusion?
Author Response
Response to reviewers’ comment
We greatly appreciate the constructive and useful recommendations. We have studied your comments carefully and made a serious effort to make appropriate changes. Our answers to listed comments are described below.
1. The contribution of this manuscript in this field is not highlighted in the introduction.
(Answer) Respecting the comment, we have added the following description to improve the manuscript.
P2 L71-75: In this study, we aimed to investigate the effect of a photocatalytic reaction on the ･OH production potential of PM2.5 collected on a TiO2 filter, using the TPT assay coupled flow injection analysis (FIA). The result showed the photocatalytic reaction of the TiO2 filter is effective in reducing the ･OH production potential of PM2.5 and will be useful for controlling environmental pollution caused by PM2.5.
2. Lines 80 to 85 describe changes in the concentration of 2-OHTA. However, there are discrepancies between the description and the figure. For example, the units of them are not uniform.
(Answer) Thank you very much for your careful attentions on this point. We have corrected the Y-axis in Figure 2 from “Concentration of 2-OHTA (nM)” to “Produced 2-OHTA (pmol)” and corresponding description in the text body.
3. Lines 86 to 91 compared OH production potential in different regions. How does this description relate to this chapter?
(Answer) As the reviewer pointed out, the comparison with other regions is not essential in this study. However, such comparison has been often asked by reviewers and/or readers in our previous reports. So, I have added the following description in order to clarify the reason.
P3 L102-103 : As a reference, the ･OH production potential was compared with previous reported values.
4. Figure 2 shows the variation of 2-OHTA concentration, however, some of the data are not clearly presented.
(Answer) Thank you very much for your careful attention. As pointed out, some chart legends did not appear in Figure 2. So, we have corrected.
5. The manuscript uses a lot of abbreviations, so it is recommended to add a list of abbreviations.
(Answer) Thank you very much for your recommendation. We have added the list of abbreviations as new Table 1.
6. Chapter 2.2 uses Pearson’s correlation coefficient to compare the correlations of the parameters, why r=0.42 represents “no apparent” (line 99) and r=0.68 represents “significant correlations” (line110).
(Answer) This confusion may be caused by our omission of p-value in the original manuscript. We added the p=0.26 for r=0.42, as below.
P4 L113-114: The diagram suggests no apparent relationship between the two variables (Pearson’s correlation coefficient, r=0.42, p=0.26);
7. The description does not match the picture, lines 159 to 165, the OH production potential in Figure 5a has changed from 0.66 to 0.85, an increase of 28.8%, why is it described as “No significant changes”?
(Answer) This is simply because the pared t-test showed p=0.051 for the Quartz filter, which means “no significant change” in this study.
8. The section of Materials and Methods is placed after the Results and Discussion, so please explain the reason?
(Answer) We can understand the reviewer’s question, but we exactly followed the format of the journal template.
9. From line 265 to line 272, “To quantify the color changed of PM2.5…”, where is the corresponding data support? Why does it jump directly to the conclusion?
(Answer) Thank you very much for your careful comment. Our typographical errors in the original text body probably invited this confusion. The typographical errors have been corrected as follows.
P6 L165-168: As shown in Figure 6, the color of the collection filters was originally white, with a relative luminance (L) of 0.48 for the quartz filter and 0.47 for the TiO2 filter. After trapping PM2.5, the color turned black with a relative luminance of 0.20, partly owing to the carbonaceous components in PM2.5.
Round 2
Reviewer 2 Report
The authors have responded to the reviewers and the submitted manuscript is suitable for publication in Catalysts
