Design of Polymer-Embedded Heterogeneous Fenton Catalysts for the Conversion of Organic Trace Compounds

Round 1

Reviewer 1 Report

In this work the authors investigated the embedding of heterogeneous Fenton catalysts into thermoplastic polimer to hinder the iron leaching.

As the result of the embedding, the iron leaching decreased together with the catalytic activity. However, also the specific surface area of the catalysts were changed and checked the consequences.

1) My main problem with this manuscript appeared on the page 14 in the lines 473-486:  “Surprisingly, no degradation of phenol with F2 could be detected, while 85 RB5 was converted by F2 after 2 h… The RB5 assay mostly reflects the scission of the azo group, while degradation of phenol mostly reflects the conversion of phenol in an oligomeric compound [1,17,34]. This resulted in different activities of the Fenton catalysts.”
This situation is totally strange, and the explanation is unconvincing for me. A typical Fenton reaction results in the “generation of non-selective, highly reactive hydroxyl radicals which are among the most powerful oxidizing agents” (cited from the manuscript, page 2 lines 51-52), which should react with the phenol, too. Probably, the F2 is not able to catalyse the formation of hydroxyl radical.
RB5 is degraded by the F2 through another mechanism. Therefore the authors should be directly prove the formation of hydroxyl radical during the use of F2 catalyst.

2) My second problem is the whole Figure 8. “Residual concentration of model substances after 24 h degradation by F2 under Fenton conditions in citrate buffer at pH 4.5 (blue) and water at pH 7 (red), (pCP was not measured in water”
Why was parachloro-phenol (pCP) not measured in water? When its solubility in water is not too low: 27.1 g/L. This experiment should be made in addition!
Furthermore, the results, presented on Figure 8, are totally chaotic; and their explanation in the main text is confused, unpretentious, unscientific (on the page 15 in the lines 512-517:)  “In citrate buffer, complete degradation of EED (100%), followed by BPA (77%), DFC (64%) and pCP (21%) was obtained after 24 h, while in water almost no degradation was observed for BPA (3%). In contrast, DFC (98%) was almost completely converted in water. The experiments showed the suitability of the catalyst studied to degrade these trace substances, and the influence of the pH value during the Fenton process. The higher conversions at lower pH were ascribed to the different chemical structure of the substrates.”
Into a scientific publication should be used much more concrete facts, e.g. the protonation constants of substrats, etc.

3) For the Figure 6, the data series 2, 3, 5 should be measured again {Residual concentration of phenol after 120 min degradation under Fenton conditions catalyzed by (1) PP-g-MA-g-PEO600/APTES/F2 (65/35) (2) F2, (3) PP-g-MA-g-PEO1000/APTES/F2 (65/35), (4) PP-g-MA/F2 (65/35), (5) PP-g-MA/F1 (80/20), and (6) F1.}!
Then the percentage of the ratios should be represented on the scale of the y-axis (instead of decimal fraction) according to the title of the axis.

4) Tetrametoxy-silane was presented in Scheme 1 (Schematic presentation of the preparation of low-cost Fe2O3/SiOx Fenton catalysts 100 by embedding the thermally treated sol-gel materials into a thermoplastic matrix resulting in 101 composites). However, tetraethoxysilane was mentioned in the main text (on the page in the line 140). If the ethoxy was used for the experiments, then it should be presented in the Scheme 1. Nevertheless, what is the significance of ethoxy compared to methoxy?

5) On the page 6 in the line 235 “50 mL of a reaction solution containing phenol (100 μg/mL) and H2O2 (200 mg/L, 235 6 mmol/L)”.
Unit mg/L should be used for both reactants.

6) (On the page 8 in the line 332) Delete the negative sign before the “r” (rate) in Equation 1 because the reaction rate is a non-negative data on the basis of its determination.

7) Under Table 5: (Kinetic parameters of the RB5 conversion catalyzed by Fenton polymer composites and iron leaching after 120 min.)
“(c) determined by ICP-OES., n.d.: not determined”
The situation is not clear for me:   n.d. should be mean rather “not detectable”, e.g. it is under the LoD (limit of detection). If these data would be detectable, then measure them!

In conclusion, I do not recommend to be accepted the manuscript, only after major revisions, based on my comments.

Author Response

We thank the reviewers for the time they took to evaluate the manuscript and took their comments carefully into account. The manuscript was revised in track mode. The rebuttal to the reviewer comments is attached to this letter.

 

Reviewer 1

Open Review

( ) I would not like to sign my review report
(x) I would like to sign my review report

English language and style

( ) Extensive editing of English language and style required
(x) Moderate English changes required

The manuscript was checked carefully by a native speaker from UK and revised accordingly. These revisions are seen by the track mode and are not summarized here.
( ) English language and style are fine/minor spell check required .
( ) I don't feel qualified to judge about the English language and style

 

	Yes	Can be improved	Must be improved	Not applicable
Does the introduction provide sufficient background and include all relevant references?	( )	(x)	( )	( )
Is the research design appropriate?	( )	( )	(x)	( )
Are the methods adequately described?	( )	(x)	( )	( )
Are the results clearly presented?	( )	( )	(x)	( )
Are the conclusions supported by the results?	( )	(x)	( )	( )

Comments and Suggestions for Authors

In this work the authors investigated the embedding of heterogeneous Fenton catalysts into thermoplastic polymer to hinder the iron leaching.

As the result of the embedding, the iron leaching decreased together with the catalytic activity. However, also the specific surface area of the catalysts were changed and checked the consequences.

• My main problem with this manuscript appeared on the page 14 in the lines 473-486:  “Surprisingly, no degradation of phenol with F2 could be detected, while 85 RB5 was converted by F2 after 2 h… The RB5 assay mostly reflects the scission of the azo group, while degradation of phenol mostly reflects the conversion of phenol in an oligomeric compound [1,17,34]. This resulted in different activities of the Fenton catalysts.”
  This situation is totally strange, and the explanation is unconvincing for me. A typical Fenton reaction results in the “generation of non-selective, highly reactive hydroxyl radicals which are among the most powerful oxidizing agents” (cited from the manuscript, page 2 lines 51-52), which should react with the phenol, too. Probably, the F2 is not able to catalyse the formation of hydroxyl radical.
  RB5 is degraded by the F2 through another mechanism. Therefore the authors should be directly prove the formation of hydroxyl radical during the use of F2 catalyst.

Some additional experiments were carried out in the last week to indirectly prove the occurrence of radicals. We added on line 236:

The influence of radical scavengers on the degradation of RB5 was investigated by adding O₂^●-(chloroform) and OH^• (t-butanol) radical scavengers. The scavengers were added to the RB5 assay prior to H₂O₂ in the molar ratios of scavenger/RB5 of 50:1 and 100:1. The RB5 assay was carried out as described above.

The experiments were incorporated into the discussion on line 369:

It is known that H₂O₂ can generate O₂^●- radicals in addition to hydroxyl radicals during Fenton reaction. To determine the influence of O₂^●- and OH^• on RB5 degradation, degradation studies of RB5 catalyzed by F2 with and without chloroform or t-butanol, respectively, in the molar ratios scavenger/RB5 of 50:1 and 100:1 were carried out (see SM, Figure S4). Chloroform has been widely used as an O₂^●- scavenger, while t-butanol has been used as OH^• scavenger [33–35]. No significant influence of O₂^●- on the B5 degradation could be observed for both molar ratios of chloroform to RB5 (50:1, 100:1) (see SM, Figure S4a). The degradation efficiency after 60 min decreased by 23% for a molar ratio t-butanol to RB5 of 50:1 and by 48% for the molar ratio 100:1 when t-butanol was used as OH^• scavenger. These results indicated indirectly that RB5 was oxidatively degraded by mainly OH^• radicals.

• My second problem is the whole Figure 8. “Residual concentration of model substances after 24 h degradation by F2 under Fenton conditions in citrate buffer at pH 4.5 (blue) and water at pH 7 (red), (pCP was not measured in water”
  Why was parachloro-phenol (pCP) not measured in water? When its solubility in water is not too low: 27.1 g/L. This experiment should be made in addition!

Thank you for suggestion! pCP was measured in water and the results were incorporated into Figure 8. We revised the introductory text in section 3.3 to explain the choice of trace compounds and degradation conditions more deeply on line 519:

The Fenton catalyst F2 was employed to carry out orienting preliminary degradation experiments of organic substances that are considered to be trace pollutants in wastewater. The development of F2 was aimed at applications in a modular two-step system for wastewater treatment consisting of a Fenton AOP module and a subsequent enzymatic treatment, as reported recently [39]. Therefore, degradation conditions were kept similar to the ones employed before [39] and the same four organic trace compounds were selected for this study.

Furthermore, the results, presented on Figure 8, are totally chaotic; and their explanation in the main text is confused, unpretentious, unscientific (on the page 15 in the lines 512-517:)  “In citrate buffer, complete degradation of EED (100%), followed by BPA (77%), DFC (64%) and pCP (21%) was obtained after 24 h, while in water almost no degradation was observed for BPA (3%). In contrast, DFC (98%) was almost completely converted in water. The experiments showed the suitability of the catalyst studied to degrade these trace substances, and the influence of the pH value during the Fenton process. The higher conversions at lower pH were ascribed to the different chemical structure of the substrates.”
Into a scientific publication should be used much more concrete facts, e.g. the protonation constants of substrats, etc.

We added the differences between the chemical structures of the selected trace compounds and their consequences on line 534:

While DFC is a carboxylic acid with pKa = 4.15 [47], BPA represents a typical phenol with a pKa = 10.3 [47,48], pCP a chloro-substituted, electron-deficient phenol with pKa = 9.47 [49]. This structure represents a compound that is difficult to oxidize. EED is a phenol substituted by bulky aliphatic cycles terminated by a triplebond with pKa = 10.7 [48]. In this case, the oxidation of the triple bond was expected to occur first and fastly.

The four substances DFC, BPA, EED and pCP were tested at a concentration of 10 µM for conversion with the Fenton catalyst F2 in both citrate buffer (pH 4.5) and water (pH 7), respectively. The tests in water represent the conditions in water treatment, while pH 4.5 is close to the conditions for standard (homogeneous) Fenton processes.

 

And for the discussion on line 549:

 

In citrate buffer, complete degradation of EED (100%), followed by BPA (77%), DFC (64%) and pCP (21%) was obtained after 24 h, while in water almost no degradation was observed for pCP (0%) and BPA (3%). In contrast, DFC (98%) was almost completely converted in water. The dissociation equilibrium of DFC at pH 7 is shifted to the side of the carboxylate ion which is more easily oxidizable than the neutral form. BPA, pCP, and EED possess higher pKa values, indicating that the neutral form was predominant at pH 7. The enhanced degradation of BPA, pCP and EED at lower pH could be attributed to higher iron leaching and thus homogeneous Fenton reaction contributions. In addition, the oxidation potential of the hydroxyl radical increases with lower pH which also favored the conversion of the trace pollutants. Same trends are reported in literature [50,51]. The experiments showed the potential of the catalyst studied to degrade these trace substances, and the influence of the pH value during the Fenton process.

• For the Figure 6, the data series 2, 3, 5 should be measured again {Residual concentration of phenol after 120 min degradation under Fenton conditions catalyzed by (1) PP-g-MA-g-PEO600/APTES/F2 (65/35) (2) F2, (3) PP-g-MA-g-PEO1000/APTES/F2 (65/35), (4) PP-g-MA/F2 (65/35), (5) PP-g-MA/F1 (80/20), and (6) F1.}!
  Then the percentage of the ratios should be represented on the scale of the y-axis (instead of decimal fraction) according to the title of the axis. (this was changed)

Thank you for the suggestion! The measurements were repeated several times in the last week. And the curves were corrected accordingly. It could be shown in particular that the phenol degradation catalyzed by F2 took place. Therefore, the sentence “Surprisingly, no degradation of phenol with F2 could be detected, while 85% RB5 was converted by F2 after 2 h.” was deleted.

4) Tetrametoxy-silane was presented in Scheme 1 (Schematic presentation of the preparation of low-cost Fe2O3/SiOx Fenton catalysts 100 by embedding the thermally treated sol-gel materials into a thermoplastic matrix resulting in 101 composites). However, tetraethoxysilane was mentioned in the main text (on the page in the line 140). If the ethoxy was used for the experiments, then it should be presented in the Scheme 1. Nevertheless, what is the significance of ethoxy compared to methoxy?

Thank you for the suggestion. We proved and changed Scheme 1. Although indeed TEOS was shown, we slightly altered the structure to make it more clear.

Significance of ethoxysilane versus methoxysilane: ethoxysilanes are more often used because their reactivity in both, hydrolysis and polycondensation are lower and, therefore, more homogeneous products are obtained (compare: C.J. Brinker, G.W. Scherer, Sol-Gel Science, The Physics and Chemistry of Sol-Gel Processing, Academic Press, 1990). We did not outline this in detail because the focus of the paper was not on the sol-gel process. Moreover, it is cheaper. This might be the reason that Martinez et al. (reference [25]) used it, and we tried to employ the Martinez procedure, also to get a comparison.

5) On the page 6 in the line 235 “50 mL of a reaction solution containing phenol (100 μg/mL) and H2O2 (200 mg/L, 235 6 mmol/L)”.
Unit mg/L should be used for both reactants.

Thank you for the suggestion. We changed the phenol concentration to 0.1 mg/mL.

6) (On the page 8 in the line 332) Delete the negative sign before the “r” (rate) in Equation 1 because the reaction rate is a non-negative data on the basis of its determination.

We removed the minus. Thank you for the suggestion!

7) Under Table 5: (Kinetic parameters of the RB5 conversion catalyzed by Fenton polymer composites and iron leaching after 120 min.)
“(c) determined by ICP-OES., n.d.: not determined”
The situation is not clear for me:   n.d. should be mean rather “not detectable”, e.g. it is under the LoD (limit of detection). If these data would be detectable, then measure them!

Thank you for the suggestion. All samples were measured for iron leaching, but some results were below the lower limit of quantitation. This was clarified.

In conclusion, I do not recommend to be accepted the manuscript, only after major revisions, based on my comments.

Author Response File: Author Response.docx

Reviewer 2 Report

The manuscript should be accepted with minor typographical errors. There are some typographical mistakes in the manuscript. Please correct it.

Author Response

We thank the reviewers for the time they took to evaluate the manuscript and took their comments carefully into account. The manuscript was revised in track mode. The rebuttal to the reviewer comments is attached to this letter.

 

Reviewer 2

Open Review

(x) I would not like to sign my review report
( ) I would like to sign my review report

English language and style

( ) Extensive editing of English language and style required
( ) Moderate English changes required
(x) English language and style are fine/minor spell check required

Spell check done.
( ) I don't feel qualified to judge about the English language and style

 

	Yes	Can be improved	Must be improved	Not applicable
Does the introduction provide sufficient background and include all relevant references?	(x)	( )	( )	( )
Is the research design appropriate?	(x)	( )	( )	( )
Are the methods adequately described?	(x)	( )	( )	( )
Are the results clearly presented?	(x)	( )	( )	( )
Are the conclusions supported by the results?	( )	( )	( )	( )

Comments and Suggestions for Authors

The manuscript should be accepted with minor typographical errors. There are some typographical mistakes in the manuscript. Please correct it.

We carefully checked the revised manuscript for typos and corrected. Thank you for the hint!

 

 

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

I thank the authors to have taken my advices and my questions, comments were almost answered. In my opinion, the professional quality of the manuscript has significantly increased; hence, I can suggest the editor to accept the manuscript.