Impacts of Mixing Mode on Photocatalytic Reduction of Hexavalent Chromium over Titanium Dioxide Nanomaterial under Various Environmental Conditions

Round 1

Reviewer 1 Report

The manuscript describes the photocatalytic reduction of Cr(VI) by titanium dioxide photocatalyst in various conditions. This is a very hot current topic and new data are welcomed. However some weakness in the manuscript must be addressed before publication (see major and minor issues below).

Major issues

- In the manuscript it is stated that most of Cr(III) is absorbed on the catalyst surface. This can lead to the reach of an equilibrium condition with possible partial catalyst deactivation or to a complete catalyst deactivation. Some specifically targeted experiments should be performed to clarify this point. Moreover, this should be discussed in the manuscript also in the prospect of possible applications (e.g. if it is foreseeable the need for a catalyst recovery/cleaning process).

- Rows 245-248: at 185 nm the UV absorption by water can be quite important (the final contribution depending also by the reactor geometry). It is thus possible that the measured reduced efficiency is at least partially due to irradiance depletion at the catalyst surface. This should be ruled-out by specific experiments or at very least considered in the comments.

- A schematics of the experimental setup for catalytic activity measurement (including UV source specifications and geometry) should be included in the manuscript (see also previous point).

- Obtained results must be better related to the existing literature.

Minor issues

– Row 36 (and throughout the manuscript): Composed measure units must be separated in each components. “mgl^–1” must be corrected in “mg L^–1” or “mg/L” (liter symbol “L” is better than “l” for clarity).

– Row 187: “Background experiments” are typically indicated as “control experiments”.

Author Response

Manuscript ID WATER-1304358

Dear editor and reviewers,

The authors would like to thank the editor and the reviewers for having this paper carefully reviewed and offering many valuable comments. The point-to-point responses to all reviewers’ comments are listed below. We hope the revised paper and responses meet with your favorable consideration. We will be very happy to answer any other questions regarding the revised paper.

Reviewer(s)' Comments to Author:
Reviewer: 1
The manuscript describes the photocatalytic reduction of Cr(VI) by titanium dioxide photocatalyst in various conditions. This is a very hot current topic and new data are welcomed. However some weakness in the manuscript must be addressed before publication (see major and minor issues below).

Major issues

- In the manuscript it is stated that most of Cr(III) is absorbed on the catalyst surface. This can lead to the reach of an equilibrium condition with possible partial catalyst deactivation or to a complete catalyst deactivation. Some specifically targeted experiments should be performed to clarify this point. Moreover, this should be discussed in the manuscript also in the prospect of possible applications (e.g. if it is foreseeable the need for a catalyst recovery/cleaning process).

<Reply> Thanks a lot for reviewer’s comment. The manuscript has been revised. We have revised the manuscript “comma “in 3.4 Effect of wavelength on distribution of chromium species, line 280-305 , Page 8.

<Reply> “The prospect of possible applications ” We have added some new discussion in line 488-490. Using to one-pot, multi-step reaction sequences with catalyst recovery in wastewater by combining magnetic catalysts with traditional gravimetrically recoverable that the Cr was absorbed on the catalyst .

- Rows 245-248: at 185 nm the UV absorption by water can be quite important (the final contribution depending also by the reactor geometry). It is thus possible that the measured reduced efficiency is at least partially due to irradiance depletion at the catalyst surface. This should be ruled-out by specific experiments or at very least considered in the comments.

<Reply> Thanks a lot for reviewer’s comment. We have added some new description in section 2.4 Slurry reactor, in line 173-179.

- A schematics of the experimental setup for catalytic activity measurement (including UV source specifications and geometry) should be included in the manuscript (see also previous point).

<Reply> Thanks a lot for reviewer’s comment. We have added some new description in 2.4 Slurry reactor, in line 173-179.

- Obtained results must be better related to the existing literature.

<Reply> Thanks a lot for reviewer’s comment. The authors have added the sentences suggested by the reviewer and more references to Introduction in the revised manuscript.

Minor issues

– Row 36 (and throughout the manuscript): Composed measure units must be separated in each components. “mgl^–1” must be corrected in “mg L^–1” or “mg/L” (liter symbol “L” is better than “l” for clarity).

<Reply> Thanks a lot for reviewer’s comment. The manuscript has been revised. “mgl^–1 “ have been revised to “ mg L^–1”.

– Row 187: “Background experiments” are typically indicated as “control experiments”.

<Reply> Thanks a lot for reviewer’s comment. The manuscript has been revised. “Background experiments “ have been changed to “Control experiments” , in line 195.

Author Response File: Author Response.pdf

Reviewer 2 Report

I am very sorry for that. And I thank you for your understanding. But I had to read the manuscript several times. It is written i very detail. Please find below my review.

Review

„The effects of wavelength, hole-scavanger and oxygen on photocatalytic reduction of hexavalent chromium over titanium dioxide photocatalyst "submitted to water“.

The paper is of interest and in general worth for publication. The influence of the above mentioned paramter on the photocatalytic reduction of toxic chromate to less toxic chromiun(III) species. The resuts are disscribed in very detail. Blank tests are included in order to ensure that the observed effects are really catalytic ones. Also the adsorption, and, hence, surface blockage by chromate species was studied. However, the paper contains a couple of editorial and other errors errors. In part the language should be improved. In part the paper can be a

little bit shortened by focusing on the main things, because all is considered in detail.Therefore, the paper needs some improvements, before it can be published.

The recommendation is: accepted with revision.

1. The abstract should be shortened and should not contain the precise results, values.

The autors should use the trems longer and shorter wavelengths instead of high wavelenghts, and higer or lower energetic irradiation.

2. The headline should be improved:on the photocatalytic...“

3. Further editorials

line 35 industry

line 35 production ? or use ? of dyes

line 37: currently used

line 47 because it is non-toxic ...

line 56: the titania is Not active under visible light

line 68 Illumination of titania with UV light exits the transition of valence band electrons to the conduction band. As a result electron- hole pairs are formed which migrate to the surface of the photocatalyst and initiate photocatalytic reactions.

line 76. unclear expressed sentence: organics act as hole scavenger

and more

3.Materials and methods

The experimental part is not complete.

The light sources used for the irradiation with different wavelengths as well as the reactor and the illumination arrangement, lamp parameter are not given.

The determination of solid Cr and used chracterization methods should be given here anHow was determined the formation of the Cr_solid?

4. Characterization

FTIR: In case that the sample were not dehydrated prior to themeasureement, the observed OH vibration bands are mainly due to the adsorbed water.

What about the nature of the Cr_solid? - should be presented and discussed here.

5. Results

As mentioned, the authors describe and discuss the findings in very detail which makes it sometimes difficult for the readaer to follow.

To 3.3 Interactions between wavelenghts and hole scavenger

The discussion of the influence of the wavelength of irradiation against the hole scavenger is not really meaningful.

I see only a combined action. Without irradiation the photocatalyst does not run. And the photocatalytic treatment itself forms organic radicals R˙which can reduce Cr(VI) to Cr(II). i.e., salicylic acid acts not only as a simple hole scavenger.

The loss in activity observed with 185 nm compared to 254 nm irradiation can be also due to a catalyst surface blockage by formed products with higher energetic irradiation.

To 3.4 Effect of wavelength and distribution chromium species.

Was the formation of chromium(III)oxide precipitates proven?

Besides, also chromium carboxylates can be formed. The caboxylates can be formed as intermediates in the photocatalytic degradation. Carbxylates attach the catalyst surface and may poison the catalyst, especially at higer chromium concentration.

The set of photocatalytic reactions given in the introduction should be improved by adding the formation of organic radicals and carboxlic species as intermediates before mineralization to carbon dioxide and water is achieved.

To 3.6 Effect of oxygen on the distribution of chromium species

The term aeration can be used only for the injection of air or oxygen into the reaction solution. The injection of nitrogen canot be called „aeration“. It is called purging and used to removed solved oxygen from the reaction mixture.

The used terms „reduction rate“ or „mineralization rate „are misleading, because it is not a velocity. Better is to use the „degree“or precentatge or achived mineralization of %.

The lower degree of reduction achieved with nitrogen purging could be also due to the direct reduction of Cr(IV) by organic radicals formed directly beside surface chromates, with corresponding blocking of the catalystsurface.

7. The captions of the figures should be improved. They should be self-explaining.

Fig.2: What means „effects on the residuals of Cr“. It is shown the relative decrease of the chromium(VI) concentration during the course of photocatalytic treatment. Fig. 2B, inlet: the 185nm 10ppm SA is missing.

t is Fig. 3a. What is Cr alone. Do the authors mean the blank experiment without the catalyst?

Fig. 5. The wavelength of irradiation should be given.

8. The conclusion should focus on the main points.

I hope It will be helpful, and it is friendly ment.

Sincerely yours and thank you gain for your understanding,

Author Response

Manuscript ID WATER-1304358

Dear editor and reviewers,

The authors would like to thank the editor and the reviewers for having this paper carefully reviewed and offering many valuable comments. The point-to-point responses to all reviewers’ comments are listed below. We hope the revised paper and responses meet with your favorable consideration. We will be very happy to answer any other questions regarding the revised paper.

Review-2

The effects of wavelength, hole-scavanger and oxygen on photocatalytic reduction of hexavalent chromium over titanium dioxide photocatalyst "submitted to water“.

The paper is of interest and in general worth for publication. The influence of the above mentioned paramter on the photocatalytic reduction of toxic chromate to less toxic chromiun(III) species. The resuts are disscribed in very detail. Blank tests are included in order to ensure that the observed effects are really catalytic ones. Also the adsorption, and, hence, surface blockage by chromate species was studied. However, the paper contains a couple of editorial and other errors errors. In part the language should be improved. In part the paper can be a

little bit shortened by focusing on the main things, because all is considered in detail.Therefore, the paper needs some improvements, before it can be published.

The recommendation is: accepted with revision.

1. The abstract should be shortened and should not contain the precise results, values.

The autors should use the trems longer and shorter wavelengths instead of high wavelenghts, and higer or lower energetic irradiation.

<Reply> Thanks a lot for reviewer’s comment. The abstract has been revised. We have not contain precise values, line 14-15, Page 1.

2. The headline should be improved: on the photocatalytic...“

<Reply> Thanks a lot for reviewer’s comment. The title has been changed to “Impacts of mixing mode on photocatalytic reduction of hexavalent chromium over titanium dioxide nanomaterial under various environmental conditions” , Page 1.

3. Further editorials

line 35 industry

line 35 production ? or use ? of dyes

line 37: currently used

line 47 because it is non-toxic ...

line 56: the titania is Not active under visible light

line 68 Illumination of titania with UV light exits the transition of valence band electrons to the conduction band. As a result electron- hole pairs are formed which migrate to the surface of the photocatalyst and initiate photocatalytic reactions.

line 76. unclear expressed sentence: organics act as hole scavenger and more

<Reply> Thanks a lot for reviewer’s comment. Line 35 the steel industrial was production that contain a minimum of approximately 11% chromium.

line 37: “currently adopted” have revised “currently used...”, in line 36.

line 47: “due to its “ have revised “because it is non-toxic...” , in line 46.

line 56: “Ultraviolet and visible lights can offer .. “have revised “Ultraviolet lights can offer..” , in line 55.

line 68 :have revised “Illumination of titania with UV light exits the transition of valence band electrons to the conduction band(reaction (1). As a result electron- hole pairs are formed which migrate to the surface of the photocatalyst and initiate photocatalytic reactions, in line 67-69.

line 76: have revised “That is to say, in the presence of salicylic acid (COOH-), the photo-generated hole…”, in line 76 and 122.

We have added some new description in line 79-81. In this study, we attempted to formation of the oxidation products of COOH- in the photocatalytic conversion of Cr(III) using chrome an aqueous solution of COOH-, which was added as a hole scavenger.

3.Materials and methods

The experimental part is not complete.

The light sources used for the irradiation with different wavelengths as well as the reactor and the illumination arrangement, lamp parameter are not given.

The determination of solid Cr and used chracterization methods should be given here an How was determined the formation of the Cr solid?

<Reply> Thanks a lot for reviewer’s comment. We have added some new description in 2.4 Slurry reactor. The X-Ray Photoelectron Spectroscopy (XPS) analysis proved that the as Cr(III) and Cr(VI) forms, that the description in line 276-309. In this study, we also concluded that the reduction of Cr(VI) to Cr(III) occurred not only on the surfaces of TiO2, but also on the surface of their composite. However, it is difficult to distinguish the amounts of Cr(III) and Cr(VI) from the total chromium on the surface. We tried to balance the chromium mass in solution and on the surface of the catalyst. However, only the masses of Cr(VI) in solution, Cr(III) in solution, and Cr(VI),Cr(III) on the surface could be obtained. Identifying the amount of Cr(VI), which was further reduced to Cr(III) on the nanomaterial surfaces, is important to the understanding of the mechanisms developed on the surface of nanomaterials, so further studies are needed.

4. Characterization

FTIR: In case that the sample were not dehydrated prior to the measurement, the observed OH vibration bands are mainly due to the adsorbed water. What about the nature of the Cr solid? - should be presented and discussed here.

<Reply> Thanks a lot for the valuable comments. In case that sample was dehydrated before have measured. In this study, we attempted to formation of the oxidation products of Ti-O stretching and O-Ti-O bending at ~500-660cm^-1, that Cr solid was adsorbed on catalytic surface.

5. Results

As mentioned, the authors describe and discuss the findings in very detail which makes it sometimes difficult for the readaer to follow.

To 3.3 Interactions between wavelenghts and hole scavenger

The discussion of the influence of the wavelength of irradiation against the hole scavenger is not really meaningful.

I see only a combined action. Without irradiation the photocatalyst does not run. And the photocatalytic treatment itself forms organic radicals R˙ which can reduce Cr(VI) to Cr(II). i.e., salicylic acid acts not only as a simple hole scavenger.

The loss in activity observed with 185 nm compared to 254 nm irradiation can be also due to a catalyst surface blockage by formed products with higher energetic irradiation.

<Reply> Thanks a lot for the valuable comments. In this study, when low concentration Cr(VI) with salicylic acid obtain better photo-reduction than high concentration Cr(VI). But high energy light excites more electrons and electron-hole pairs, thereby increasing the possibility of collision and recombination for electrons and hole. This phenomenon disappeared when SA (hole-scavenger) was present in the high concentration solution.

To 3.4 Effect of wavelength and distribution chromium species.

Was the formation of chromium(III)oxide precipitates proven?

Besides, also chromium carboxylates can be formed. The caboxylates can be formed as intermediates in the photocatalytic degradation. Carbxylates attach the catalyst surface and may poison the catalyst, especially at higer chromium concentration.

The set of photocatalytic reactions given in the introduction should be improved by adding the formation of organic radicals and carboxlic species as intermediates before mineralization to carbon dioxide and water is achieved.

<Reply> Thanks a lot for the valuable comments. In study demonstration of chromium species shows that the lower the energy of UV illumination was, the more the amount of trivalent chromium ions remained in the solution and the less the amount of chromium retained on TiO₂ surface.

To 3.6 Effect of oxygen on the distribution of chromium species

The term aeration can be used only for the injection of air or oxygen into the reaction solution. The injection of nitrogen canot be called „aeration“. It is called purging and used to removed solved oxygen from the reaction mixture.

The used terms „reduction rate“ or „mineralization rate „are misleading, because it is not a velocity. Better is to use the „degree“or precentatge or achived mineralization of %.

The lower degree of reduction achieved with nitrogen purging could be also due to the direct reduction of Cr(IV) by organic radicals formed directly beside surface chromates, with corresponding blocking of the catalyst surface.

<Reply> Thanks a lot for the valuable comments. The authors have revised the manuscript as directed.

7. The captions of the figures should be improved. They should be self-explaining.

Fig.2: What means „effects on the residuals of Cr“. It is shown the relative decrease of the chromium(VI) concentration during the course of photocatalytic treatment. Fig. 2B, inlet: the 185nm 10ppm SA is missing.

t is Fig. 3a. What is Cr alone. Do the authors mean the blank experiment without the catalyst?

Fig. 5. The wavelength of irradiation should be given.

<Reply> Thanks a lot for the valuable comments. The authors have revised the manuscript in Fig.2 Effects of the wavelength factor on residuals of Cr(VI) after photo-reduction by TiO2 (a) Cr(VI) 3mgL^-1 (b) Cr(VI) 20mgL^-1, in line 266.

In Fig 3a and Fig 3c were Cr alone without SA.

Fig. 5. The wavelength of description in line 421-422.

8. The conclusion should focus on the main points. I hope It will be helpful, and it is friendly ment.

<Reply> The authors would like to thank the editor and the reviewers for having this paper carefully reviewed and offering many valuable comments.

Author Response File: Author Response.pdf