Fe-Doped g-C₃N₄ for Enhanced Photocatalytic Degradation of Brilliant Blue Dye

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The manuscript aligns with the journal’s aims and scope and will appeal to readers in Water. It presents a systematic investigation into the synergistic photocatalytic-Fenton degradation of brilliant blue using Fe/g-C₃N₄ composites, demonstrating promising practical implications. Overall, the study has potential scientific merit; however, several issues should be addressed before the manuscript can be considered for publication:

1. The introduction section fails to clearly articulate the novelty and distinctive contributions of this work relative to existing literature. The authors are encouraged to highlight the specific innovation and scientific gap addressed by their study. In addition, several sentences would benefit from refinement for improved clarity and flow.
2. The abstract lacks essential quantitative details (e.g., degradation efficiency, reaction rate constants, or kinetic performance). Please include representative numerical data to strengthen the abstract.
3. The coordinates in Figure 3c are not clear enough.
4. For the photocatalysis experiment section, please specify the intensity of the light source.
5. The conclusion section should be revised to provide a concise summary that integrates the major findings, highlights the mechanistic insights, and briefly discusses the broader implications of the work.
6. The current conclusion section contains numerous colloquial, repetitive, and imprecise expressions. For instance, the consecutive use of “The results are as follows” and “The results showed that” appears redundant. It is recommended that the entire section be restructured and rewritten using more concise and professional academic language.

Author Response

To the first referee’s comments, we make the following responses and changes in the manuscript:

The paper presents the synthesis and characterization of a 15wt%Fe/g-C₃N₄ composite photocatalyst aimed at degrading p-nitrophenol (PNP) in water. The authors utilized thermal polycondensation and co-precipitation methods to prepare the catalyst, followed by extensive characterization techniques including XRD, SEM, and UV-Vis DRS. The study explores optimal preparation conditions and evaluates catalytic performance under various experimental parameters. The findings indicate that 15wt%Fe/g-C₃N₄ demonstrates high degradation efficiency for brilliant blue under visible light, with both •OH and •O₂^- playing significant roles in the photocatalytic process. However, minor revisions are required before acceptance.

 

1. The introduction section fails to clearly articulate the novelty and distinctive contributions of this work relative to existing literature. The authors are encouraged to highlight the specific innovation and scientific gap addressed by their study. In addition, several sentences would benefit from refinement for improved clarity and flow.

Answer: Thanks for the reviewer’s suggestion, the innovation and advantages of this experiment compared with the existing literature have been added in the preface.

Manuscript: Compared with carbon nitride, iron-doped carbon nitride has the advantages of accurately adjusting catalytic performance and optimizing electronic structure, optimizing the shortcomings of pure carbon nitride such as narrow visible light response range and easy recombination of current-carrying electrons. (Page 4, Lines 61 to 64)

 

2. The abstract lacks essential quantitative details (e.g., degradation efficiency, reaction rate constants, or kinetic performance). Please include representative numerical data to strengthen the abstract.

Answer: Thanks for the reviewer’s suggestion，details have been added to the summary section and a certain amount of numerical data has been added to enhance the summary.

Manuscript: In this paper, g-C₃N₄ and Fe/g-C₃N₄ photocatalysts were prepared by thermal polycondensation method, Fe/g-C₃N₄ of 15wt% can reach 98.59% under the best degradation environment, and the degradation rate of g-C₃N₄ is only 7.6% under the same conditions，and the photocatalytic activity of the catalysts was further studied. (Page 2, Lines 13 to 16)

In addition, under the optimal experimental conditions obtained by response surface experiments, the fitting degree of first-order reaction kinetics is 0.96642, and the fitting degree of second-order reaction kinetics is 0.57884. Therefore, this reaction is more in line with first-order reaction kinetics，The adsorption rate is only proportional to the concentration of Fe/g-C₃N₄. (Page 2, Lines 22 to 25)

 

3. The coordinates in Figure 3c are not clear enough.

Answer: Fig 3 (c) has been revised in the revised draft.

Manuscript:

Fig. 3. SEM characterization of the catalyst (a) g-C₃N₄; (b)15% Fe/g-C₃N₄，（c）EDS energy spectrum of 15 wt% Fe/g-C₃N₄.（Page 8, Lines 140 to 141）

 

4. For the photocatalysis experiment section, please specify the intensity of the light source.

Answer: In the revised draft, add the name of the light source in Section 2.1, and supplement the specific parameters of the light source intensity in Section 2.4.

Manuscript: Sunlight was simulated using a 500W xenon lamp, with a brightness of approximately 37,000 cd/cm² and a radiation intensity of 6230 mW/sr in the wavelength range of 350-450 nm. (Page 6, Lines 99 to100)

 

5. The conclusion section should be revised to provide a concise summary that integrates the major findings, highlights the mechanistic insights, and briefly discusses the broader implications of the work.

Answer: We sincerely thank the reviewers for their valuable comments! In the revised draft, we have streamlined and optimized the conclusion section, eliminating redundant content to make it more concise and rigorous, and more in line with academic expression norms.

Manuscript: In this study, g-C₃N₄ and Fe-doped g-C₃N₄ (Fe/g-C₃N₄) photocatalysts were prepared by a simple and low-cost thermal condensation polymerization method. The optimal preparation process of Fe/g-C₃N₄ was determined through characterization and the visible light-H₂O₂ synergistic degradation experiment of bright blue: calcination temperature 450℃, calcination time 4 hours, and Fe doping amount 15wt%. Characterization confirmed that Fe was successfully embedded in the g-C₃N₄ layered structure in the form of Fe-N bonds, significantly regulating its microscopic morphology (increasing specific surface area, pore size and pore volume), expanding the visible light response range, and enhancing visible light absorption and photocatalytic activity by inhibiting the growth of g-C₃N₄ grains and reducing the interplanar spacing. The response surface method optimization obtained the optimal reaction conditions: H₂O₂ 1.4mol/L, catalyst 1g/L, bright blue concentration 46mg/L, pH=4.3. At this time, the degradation rate of bright blue reached 98.59%, which was highly consistent with the predicted value of 98.86%, and the reaction followed the first-order kinetic model. The influence law of inorganic ions: Mg²⁺ has no significant effect; Cation inhibition effect: Cu²⁺ > Mn²⁺, anion inhibition effect: HCO₃⁻ > CO₃²⁻ > H₂PO₄⁻ > Cl⁻. The degradation mechanism indicates that H₂O₂ reacts with Fe²⁺ to form •OH, which efficiently oxidizes bright blue and mineralizes it into CO₂ and H₂O. The Fe-N bond can reduce the dissolution of Fe and accelerate the Fe³⁺/Fe²⁺ cycle. The O₂ produced by the decomposition of H₂O₂ reacts with photogenerated e⁻ to form •O₂⁻, which not only reduces Fe³⁺ to regenerate Fe²⁺ but also directly participates in degradation, synergically enhancing catalytic performance. (Page 28, Lines 456 to 472)

 

6. The current conclusion section contains numerous colloquial, repetitive, and imprecise expressions. For instance, the consecutive use of “The results are as follows” and “The results showed that” appears redundant. It is recommended that the entire section be restructured and rewritten using more concise and professional academic language.

Answer: We sincerely thank the reviewers for their valuable comments! The revised draft has removed the colloquial expressions in the text and optimized and improved the content presentation to make it more accurate and rigorous, in line with the norms of academic writing.

Manuscript: In this study, g-C₃N₄ and Fe-doped g-C₃N₄ (Fe/g-C₃N₄) photocatalysts were prepared by a simple and low-cost thermal condensation polymerization method. The optimal preparation process of Fe/g-C₃N₄ was determined through characterization and the visible light-H₂O₂ synergistic degradation experiment of bright blue: calcination temperature 450℃, calcination time 4 hours, and Fe doping amount 15wt%. Characterization confirmed that Fe was successfully embedded in the g-C₃N₄ layered structure in the form of Fe-N bonds, significantly regulating its microscopic morphology (increasing specific surface area, pore size and pore volume), expanding the visible light response range, and enhancing visible light absorption and photocatalytic activity by inhibiting the growth of g-C₃N₄ grains and reducing the interplanar spacing. The response surface method optimization obtained the optimal reaction conditions: H₂O₂ 1.4mol/L, catalyst 1g/L, bright blue concentration 46mg/L, pH=4.3. At this time, the degradation rate of bright blue reached 98.59%, which was highly consistent with the predicted value of 98.86%, and the reaction followed the first-order kinetic model. The influence law of inorganic ions: Mg²⁺ has no significant effect; Cation inhibition effect: Cu²⁺ > Mn²⁺, anion inhibition effect: HCO₃⁻ > CO₃²⁻ > H₂PO₄⁻ > Cl⁻. The degradation mechanism indicates that H₂O₂ reacts with Fe²⁺ to form •OH, which efficiently oxidizes bright blue and mineralizes it into CO₂ and H₂O. The Fe-N bond can reduce the dissolution of Fe and accelerate the Fe³⁺/Fe²⁺ cycle. The O₂ produced by the decomposition of H₂O₂ reacts with photogenerated e⁻ to form •O₂⁻, which not only reduces Fe³⁺ to regenerate Fe²⁺ but also directly participates in degradation, synergically enhancing catalytic performance. (Page 28, Lines 456 to 472)

Reviewer 2 Report

Comments and Suggestions for Authors

The work presented by Su et al. provides a substantial amount of information regarding the development of a photocatalytic system based on Fe-doped g-C3N4 photocatalyst. However, before publication, there are several points that need to be clarified, which I outline below

1. Line 94 2.3. Characterization Please use the same order as for data presentation in section 3. Results and discussion: first XRD, then SEM, XPS , FTIR and specific surface area.
2. Line 110 please revised the sentences specify that 250 mlis the volume of solution and 40 mg/l is the intial concetration of dye.
3. Provide information about the ligth soureces employed during the photocatalytic experiment( Type of lamp
4. model and manufacturer; Spectral range / dominant wavelength (nm); Light intensity or irradiance (mW·cm⁻²) at the catalyst surface and the temeparture of systems if the cooling system are not present,
5. Line 128-130 “Fe doping changes the C/N ratio during the thermal polycondensation process, which leads to different degrees of polycondensation reflected, and further leads to smaller crystal plane spacing of g-C3N4, thus increasing the….”To support the claim, would it be useful to include the crystallite size and an evaluation of the lattice parameters?"Moreover, how change the XRD patterns changing the Fe-doped mass ratio (x=0, 5%, 7.5%, 10%, 12.5%, 15%, 17.5%, 20%)? And how change the morphology?
6. Line 147 Please revised the sentences “It indicates that Fe is successfully doped into graphitic carbon nitride” If doping occurs only on the surface or within the lattice (substitutional or interstitial sites), EDS cannot distinguish it.
7. Line 215 and line 224 Please revised the values of the degradation efficiency of brilliant blue.
8. To better demonstrate the effectiveness of the system, a TOC analysis should be added to highlight that the system is not only capable of decolorizing Brilliant Blue but also of mineralizing it to CO₂ and H₂O
9. Lines 503 to 521 should be moved to the supplementary material.
10. The analysis of reaction intermediates using LC/MS or ESI/MS should be added to provide stronger support for the proposed reaction mechanism proposed in Figure 14.

Author Response

To the second referee’s comments, we make the following responses and changes in the manuscript:

The paper presents the synthesis and characterization of a 15wt%Fe/g-C₃N₄ composite photocatalyst aimed at degrading p-nitrophenol (PNP) in water. The authors utilized thermal polycondensation and co-precipitation methods to prepare the catalyst, followed by extensive characterization techniques including XRD, SEM, and UV-Vis DRS. The study explores optimal preparation conditions and evaluates catalytic performance under various experimental parameters. The findings indicate that 15wt%Fe/g-C₃N₄ demonstrates high degradation efficiency for brilliant blue under visible light, with both •OH and •O₂^- playing significant roles in the photocatalytic process. However, minor revisions are required before acceptance.

 

1. Line 94 2.3. Characterization Please use the same order as for data presentation in section 3. Results and discussion: first XRD, then SEM, XPS , FTIR and specific surface area.

Answer: Thanks for the reviewer’s suggestion, the corresponding sequence in 2.3 characterization and Chapter 3 has been readjust. Additionally, the solid ultraviolet analysis section has been moved to 3.1.5 for one-to-one correspondence.

Manuscript: X-ray diffractometer (XRD, D/max-rB, Rigaku, Japan); scanning electron microscope (SEM, Sigma500, Zeiss, Germany), the kinds and contents of elements in the micro-region of the catalyst were analyzed by EDS, and X-ray photoelectron spectroscopy (XPS, ESCALAB250, ThermoVG Company, USA). The chemical composition, valence distribution and element content of the sample surface were determined by Fourier transform infrared spectrometer (Spectrum100, PerkinElmer, USA). The surface area and porosity analyzer (BET, ASAP2460, McMurray (Shanghai) Instrument Co., Ltd.) was used to analyze the pore distribution and specific surface area of the catalyst. The solid UV-Vis diffuse reflectance spectra of the catalysts were measured using a UV-Vis DRS spectrophotometer (UV-Vis DRS, Lambda950, PerkinElmer Company, USA) to determine their spectral response range. (Page 5, Lines 89 to 97)

 

2. Line 110 please revised the sentences specify that 250 mlis the volume of solution and 40 mg/l is the intial concetration of dye.

Answer: Thanks for the reviewer’s suggestion. In response to the issues you pointed out, we have made corresponding modifications and improvements to the relevant sentences in line 110 of Section 2.4 to ensure that the expression is more precise and standardized.

Manuscript: The volume of brilliant blue solution is specified as 250ml, and the initial dye concentration is 40mg/L in quartz reactor, and 0.25g g-C₃N₄ or Fe/g-C₃N₄ catalyst was added. (Page 6, Lines 104 to 106)

 

3. Provide information about the ligth soureces employed during the photocatalytic experiment( Type of lamp).

Answer: Thank you very much to the reviewers for pointing out the missing information in the article. In the returned manuscript, the lighting type was added in 2.1.

Manuscript: The 500w long-arc xenon lamp was purchased from Nubit Technology (Beijing, China). (Page 5, Line 71)

 

4. model and manufacturer; Spectral range / dominant wavelength (nm); Light intensity or irradiance (mW·cm⁻²) at the catalyst surface and the temeparture of systems if the cooling system are not present.

Answer: The model and manufacturer of the experimental light source have been supplemented in Section 2.1 of the returned manuscript. In Section 2.4, supplement the brightness of the light source and the specific radiation intensity within the wavelength range used. In the experiment, the light source was suspended in a condenser tube for cooling, regardless of its temperature.

Manuscript: The 500w long-arc xenon lamp was purchased from Nubit Technology (Beijing, China). (Page 5, Line 71)

Sunlight was simulated using a 500W xenon lamp, with a brightness of approximately 37,000cd/cm² and a radiation intensity of 6230mW/sr in the wavelength range of 350-450nm. (Page 6, Lines 99 to 100)

 

5. Line 128-130 “Fe doping changes the C/N ratio during the thermal polycondensation process, which leads to different degrees of polycondensation reflected, and further leads to smaller crystal plane spacing of g-C₃N₄, thus increasing the….”To support the claim, would it be useful to include the crystallite size and an evaluation of the lattice parameters?"Moreover, how change the XRD patterns changing the Fe-doped mass ratio (x=0, 5%, 7.5%, 10%, 12.5%, 15%, 17.5%, 20%)? And how change the morphology?

Answer: We sincerely apologize for not being able to supplement the relevant experiments as you suggested! At present, no samples from the same batch of experiments have been retained. Due to the current time arrangement and the use and allocation of experimental instruments, it is temporarily impossible to complete the relevant supplementary tests when preparing new samples. We will further conduct in-depth research in related studies to improve this part.

In addition, due to the limitations of the research budget and the rental conditions of related detection equipment, this study only conducted XRD detection on the Fe/g-C₃N₄ samples with the optimal doping ratio, and failed to cover the lattice characterization of all doping ratios. In response to the conclusion that the doping amount of Fe affects the g-C₃N₄ lattice, we have added two authoritative literatures in the relevant field in this paper as support to further verify the rationality and reliability of this conclusion.

Manuscript: Iron doping in g-C₃N₄ can significantly change the lattice spacing, and the lattice spacing is optimal at 15 wt%[20-21]. (Page 7, Lines 123 to 124)

 

6. Line 147 Please revised the sentences “It indicates that Fe is successfully doped into graphitic carbon nitride” If doping occurs only on the surface or within the lattice (substitutional or interstitial sites), EDS cannot distinguish it.

Answer: Thank you for your correction! The original sentence on line 147 has been revised.

Manuscript: It is shown that the surface or lattice of Fe-doped graphitic carbon nitride is observed, which accords with XRD pattern. (Page 8, Lines 137 to 138)

 

7. 7. Line 215 and line 224 Please revised the values of the degradation efficiency of brilliant blue.

Answer: We are very sorry for this oversight. In the returned manuscript, the brilliant blue degradation rate of lines 215 and 224 originally in 3.2.1 has been corrected.

Manuscript: The results show that the degradation efficiency of 15wt% Fe/g-C₃N₄ composite photocatalyst is the highest, and the degradation efficiency is 87.1% under visible light for 60 min, which may be due to the optimal Fe content ratio, specific surface area and pore volume of 15wt% Fe/g-C₃N₄, which is similar to other literatures[24-25]. (Page 13, Lines 219 to 222)

 

8. 8. To better demonstrate the effectiveness of the system, a TOC analysis should be added to highlight that the system is not only capable of decolorizing Brilliant Blue but also of mineralizing it to CO₂ and H₂O.

Answer: We sincerely apologize for not being able to carry out the supplementary experiment as you suggested! Due to the current research budget and the actual difficulties in leasing and allocating related experimental equipment, it is currently impossible to complete this supplementary experimental work.

The conclusion regarding "the decolorization and mineralization of brilliant blue into CO₂ and H₂O" is mainly supported by the authoritative literature research results published in the relevant field. We have made standardized references to the relevant literature in the article.

We fully recognize the significant importance of the supplementary experimental suggestions you put forward in improving the research conclusion. If we receive corresponding resource support in the future, we will actively carry out this part of the experiment to further verify and deepen the research. Once again, we sincerely apologize to you. We are deeply grateful for your understanding, tolerance and valuable guidance!

 

9. 9. Lines 503 to 521 should be moved to the supplementary material.

Answer: In the return manuscript, transfer the original parts from lines 503 to 521 to SI as Text S2. And Fig. S5.

SI:

Fig. S5. Reaction kinetics (a) first-order reaction kinetics (b) second-order reaction kinetics.

 

10. 10. The analysis of reaction intermediates using LC/MS or ESI/MS should be added to provide stronger support for the proposed reaction mechanism proposed in Figure 14.

Answer: We sincerely apologize for not being able to carry out the supplementary experiment as you suggested! Due to the current research budget and the rental and allocation restrictions of related experimental equipment, the supplementary experimental work cannot be completed for the time being. We fully recognize the academic value of the supplementary experimental suggestions you put forward. They are of great significance for improving the research conclusion. If there are relevant resource supports in the future, we will actively carry out this part of the experiment to further deepen the research. We sincerely apologize to you once again. Thank you for your understanding and valuable guidance!

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

The authors have addressed all the comments and observations raised previously in a thorough and satisfactory manner. I therefore consider that the manuscript can be accepted in present form for publication in Water.