Visible-Light-Driven Peroxymonosulfate Activation for Accelerating Tetracycline Removal Using Co-TiO₂ Nanospheres

Round 1

Reviewer 1 Report

Comments for author File: Comments.pdf

Author Response

Review 1：

This work “Visible-light-driven peroxymonosulfate activation for accelerating tetracycline removal by Co-TiO2 nanospheres” reports the detailed study Co-doped TiO2 nanospheres for peroxymonosulfate activation and photocatalytic TC degradation. The catalyst is studied thoroughly for effect of different parameters such as anion conc., pH, catalyst dose along with degradation studies of TC in different water environment as well as for different pollutants.

I recommend this work for publication after minor comments.

General Comments:

1. What is the percentage doping of Co? It seems to be 1:1 for Ti:Co, although EDX results reflects 5:1. Clarify and calculate, and correct in title as well as in the manuscript.

Response:Thanks for the reviewer’s suggestion. The percentage of Co is calculated and displayed in Table S1 based on the results of XPS and SEM Mapping. The atomic ratio of Ti/Co in Co-TiO₂ sample was approximately 5:1 according to these two techniques.

Table S1. Atomic percent of elements measured by different characterization techniques.

Technique	Ti (Atom%)	Co (Atom%)	O (Atom%)
XPS	24.89	5.42	69.69
SEM Mapping	28.74	4.60	66.66

 

2. What is the conc. of other pollutants (RhB, MB, etc.)?

Response:Thanks for the reviewer’s suggestion. The concentration of other pollutants is also 20 mg/L. We have added the relate contents in the manuscript.

 

3. Why there is significant decrease in activity after cyclic studies as there observed almost 30% fall in degradation efficiency after 3rd cycle. How you comment about the reusability of these catalysts, may be after 6-7 cycles, catalyst become unusable.

Response:Thanks for the reviewer’s suggestion. The main reason for the decrease in catalytic activity after cycle tests may be due to the increase by-products that occupy some active site of the catalyst. The catalyst may become unavailable after 6-7 cycles, suggesting that further improvements are needed to immobilize the metal ions. Thank you for pointing out the issue, we will be aware of this in our future study.

 

4. The stability data is missing? What happened to catalyst after 3 cycles of degradation? Catalyst not characterized after reaction as a standard practice by xrd, xps, SEM/TEM. Add data and discuss.

Response:Thanks for the reviewer’s suggestion. The used Co-TiO₂ sample is also examined by XRD and XPS (Fig. S4). The XRD and XPS peaks of the used Co-TiO₂ show almost no change, indicating that the chemical structure of Co-TiO₂ retains stable after used. The decline of the performance in the 3th cycle may be attributed to the increase by-products that occupy some active site of the catalyst.

 

Fig. S4 The XRD (a) and XPS (b) of the fresh and used Co-TiO₂

 

5. In XPS analysis, calculate and mention the ratio of Co2+ : Co3+, why the peak area for Co3+(2p1/2) is lesser than Co2+(2p1/2). A shoulder peak is visible in Ti 2p spectra around 471 eV. Explain.

Response:Thanks for the reviewer’s suggestion. According to XPS analysis, the ratio of Co²⁺/Co³⁺ is 1.12:1, suggesting that the amount of Co²⁺/ is higher than that of Co³⁺ in Co-TiO₂ sample, and this phenomenon is suitable for PMS activation. Moreover, the shoulder peak appearing in the Ti 2p spectrum near 471 eV is the satellite peak of Ti 2p. We have added the related content in the manuscript.

 

6. Check line 40, 47 and the whole manuscript for typos and grammatical errors.

Response: Thanks for the reviewer’s suggestion. We have checked the whole manuscript and corrected them in the manuscript.

 

Thank you again for your precious comments and advice. Those comments are all valuable and very helpful for revising and improving our paper.

 

 

Reviewer 2 Report

This work reports the preparation and photocatalytic activity of cobalt-doped TiO2 for activating peroxymonosulfate (PMS) to remove tetracycline. The results are of interest due to the degradation of antibiotics by using (PMS) as an alternative to hydrogen peroxide has received significant attention. The authors present an interesting theoretical study for TC degradation through intermediates identification. Maybe the main drawback of this study is the poor analysis of the characterization results.

Then, this paper could be accepted with the revisions noted below:

What is the percentage by weight of the Co added to the TiO2? Is it totally implanted into the TiO2 lattice?

What is the observed shoulder at 2q at about 27.5 and 30 in the XRD analysis related to? Is it possible that the added cobalt is only partially incorporated into the TiO2 lattice, and the rest is forming any other compound?

Please explain the high background in the pattern.

How is explained the presence of oxygen vacancy in this material?

Also, in the experimental part, the role of Co in the acceleration of PMS activation is not well explained.

 

How are the results of the TC degradation compared with other results in the literature?

Author Response

Review2：

 

This work reports the preparation and photocatalytic activity of cobalt-doped TiO2 for activating peroxymonosulfate (PMS) to remove tetracycline. The results are of interest due to the degradation of antibiotics by using (PMS) as an alternative to hydrogen peroxide has received significant attention. The authors present an interesting theoretical study for TC degradation through intermediates identification. Maybe the main drawback of this study is the poor analysis of the characterization results.

Then, this paper could be accepted with the revisions noted below:

• What is the percentage by weight of the Co added to the TiO2? Is it totally implantedinto the TiO2 lattice?

Response: Thanks for the reviewer’s suggestion. The weights of the elements were measured with different characterization techniques and the results are shown in Table S1. According to the XRD result, we speculated most of the Co may be implanted into the titanium dioxide lattice.

Table S1. Atomic percent of elements measured by different characterization techniques.

Technique	Ti (Atom%)	Co (Atom%)	O (Atom%)
XPS	24.89	5.42	69.69
SEM Mapping	28.74	4.60	66.66

 

• What is the observed shoulder at 2qat about 27.5 and 30 in the XRD analysis related to? Is it possible that the added cobalt is only partially incorporated into the TiO2 lattice, and the rest is forming any other compound? Please explain the high background in the pattern.

Response: Thanks for the reviewer’s suggestion. From the XRD result, no obvious peak of Co species or other compounds are observed, we speculated that most of the Co species may be incorporated into the TiO₂ lattice. The high background in the pattern may be related to the bottom noise of the XRD technique.

 

• How is explained the presence of oxygen vacancy in this material?

Response: Thanks for the reviewer’s suggestion. The O 1s spectrum of Co-TiO₂ exhibited three peaks at 528.9 eV, 530.1 eV, and 531.8 eV (Fig. 1d) corresponding to the lattice oxygen (such as Ti-O and Co-O), oxygen defects, and surface-adsorbed oxygen, respectively.From the O1s spectra of the fresh and used samples, the oxygen vacancy does not change much. Thank you for pointing out the issue, we will use other techniques to detect the oxygen defects in our future study.

 

Fig. I The O 1s spectra of the fresh and used Co-TiO₂.

 

• Also, in the experimental part, the role of Co in the acceleration of PMS activation is not well explained.

Response:Thanks for the reviewer’s suggestion. We have added the relate content in the article. 

“Briefly, the photogenerated electrons can reduce the Co³⁺ to Co²⁺, and the formation of Co²⁺ play positive effect on PMS activation.”

 

• How are the results of the TC degradation compared with other results in the literature?

Response: Thanks for the reviewer’s suggestion.The performance of the Vis/Co-TiO₂/PMS system is among the excellent ones of some other catalyst/PMS system listed as follow (Table I).

Table I Comparison of the performance of different catalysts coupled with PMS to remove TC.

Samples	PMS	Pollutant (mg/L)	Catalyst （g/L）	Degradation rate	Ref.
BC300-MoS2-1	1 mM	20 mg/L (TC)	0.05	~80%(120 min)	https://doi.org/10.1016/j.seppur.2021.120118
Fe-N-CS-800	1 mM	20 mg/L (TC)	0.2	~93.74%(120 min)	https://doi.org/10.1016/j.jes.2022.03.018
G/0.1BC500	1 mM	30 mg/L (TC)	0.1	~73.0%(60 min)	https://doi.org/10.1016/j.scitotenv.2021.147102
PFSC-900	0.3 g/L	20 mg/L (TC)	0.4	~90.1%(120 min)	https://doi.org/10.1016/j.jhazmat.2021.126495
Co-TiO2	0.5 mM	20 mg/L (TC)	0.1	~100%(30 min)	This study

 

Thank you again for your precious comments and advice. Those comments are all valuable and very helpful for revising and improving our paper.

 

Reviewer 3 Report

The manuscript presented for review, entitled Visible-light-driven peroxymonosulfate activation for accelerating tetracycline removal by Co-TiO₂ nanospheres, presents interesting results on the feasibility of using the Co-TiO2 system to remove tetracycline from water. The research was well planned and described. My only concern is the fit of the model of the kinetics of the contaminant decomposition. All lines should start at 0.0. In addition, it can be seen that in some cases the pseudo-first-order kinetics used does not fit the data. This is the case with Fig.3b red straight line, Fig.4e pink and green straight lines, Fig.4f red and orange straight lines. It would be necessary to consider the possibility of using a different kinetic model or two ranges of straight fit.

After this minor correction, the work can be recommended for publication.

Author Response

Review3：

 

The manuscript presented for review, entitled Visible-light-driven peroxymonosulfate activation for accelerating tetracycline removal by Co-TiO2 nanospheres, presents interesting results on the feasibility of using the Co-TiO2 system to remove tetracycline from water. The research was well planned and described.

 

My only concern is the fit of the model of the kinetics of the contaminant decomposition. All lines should start at 0.0. In addition, it can be seen that in some cases the pseudo-first-order kinetics used does not fit the data. This is the case with Fig.3b red straight line, Fig.4e pink and green straight lines, Fig.4f red and orange straight lines. It would be necessary to consider the possibility of using a different kinetic model or two ranges of straight fit.
Response:Thanks for the reviewer’s suggestion. The pseudo-first-order kinetics were carried out according to the reference [Water Research 168 (2020) 115093. Water Research 207 (2021) 117849], and the fit of the model of the kinetics did not need to start at 0.0. Thank you again for your precious comments and advice.

 

Thank you again for your precious comments and advice. Those comments are all valuable and very helpful for revising and improving our paper.