Influence of Lanthanum Doping on the Photocatalytic and Antibacterial Capacities of Mg_0.33Ni_0.33Co_0.33Fe₂O₄ Nanoparticles

Round 1

Reviewer 1 Report

In this manuscript, the authors synthesized a solar-driven photocatalyst by doping La into spinel ferrite nanoparticles. The synthesized materials were characterized using XRD, TEM, UV-vis spectroscopy, and photoluminescence. The degradation rate constants (first order) on methylene blue by the solar photocatalytic oxidation were compared at various catalyst dosages, pH, temperature, and addition of graphene. Further, the antibacterial properties of the catalyst were evaluated. The manuscript is solid with plenty of experimental data. However, a few questions needed to be addressed before publishing.

1.      Why did the authors choose a spinel ferrite with three metals (Mg, Ni, Co) in equal amount? How different was the catalyst compared with other spinel ferrites with 1-2 metals?  Was the combination promoting the performance of the ferrite NP?

 

2.      How did the author confirm that the product was Mg0.33Ni0.33Co0.33LaxFe2-xO4 NPs, not a mixture with hematite?

3.      Why does the addition of La increase the oxygen vacancy?

4.      Table 2, how the catalyst with La at 0.02 had the lowest rate constant among other catalysts?

5.      The discussion on the effect of pH was wrong, Spinel ferrite should be positively charged at pH below zero point charge, and positively charged at high pH. The author should analyze the zeta potential and find out the zero point charge of the catalyst. High pH may fade MB.

6.      Figure 8 has a missing symbol on the X aisle.

7.      What about using Gr only? Gr can absorb MB as well. It is hard to tell whether MB was removed from adsorption or photodegradation.

8.      Figure 12, why the absorbance at 60 min was increased?

9.      Were the antibacterial tests conducted under solar irradiation?

10.   “microbial water pollution” should be “microbiological water pollution”,

 

11.   Some grammar mistakes were found.

Author Response

Manuscript ID: catalysts-2300436 - Revised Version

We would like to submit the revised version of the research article titled “Influence of Lanthanum Doping on the Photocatalytic and Antibacterial Capacities of Mg_0.33Ni_0.33Co_0.33Fe₂O₄ Nanoparticles” for publication in catalysts.

The authors would like to thank the editor and the reviewers for their precious time, efforts, and invaluable comments. The comments and suggestions were very helpful and allowed us to explain and improve more aspects of the manuscript. We have carefully addressed all the comments and we are pleased to learn that the manuscript has been substantially improved.

Below is our point-by-point response to the Reviewer 1 comments as the following:

Reviewer 1 Comments, Suggestions, and Authors’ Responses:

In this manuscript, the authors synthesized a solar-driven photocatalyst by doping La into spinel ferrite nanoparticles. The synthesized materials were characterized using XRD, TEM, UV-vis spectroscopy, and photoluminescence. The degradation rate constants (first order) on methylene blue by the solar photocatalytic oxidation were compared at various catalyst dosages, pH, temperature, and addition of graphene. Further, the antibacterial properties of the catalyst were evaluated. The manuscript is solid with plenty of experimental data. However, a few questions needed to be addressed before publishing.

Point 1: Why did the authors choose a spinel ferrite with three metals (Mg, Ni, Co) in equal amount? How different was the catalyst compared with other spinel ferrites with 1-2 metals? Was the combination promoting the performance of the ferrite NP?

Response 1: The choice of a spinel ferrite with three metals (Mg, Ni, Co) in equal amounts is owed to the properties of MgFe₂O₄, NiFe₂O₄ and CoFe₂O₄. The following paragraph is added in the last paragraph of the Introduction. “It is known that MgFe₂O₄ is a soft magnetic semiconducting material with a normal spinel structure. NiFe₂O₄, having an inverse spinel structure, is a soft magnetic semiconducting material. Whereas, CoFe₂O₄, characterized by its inverse spinel structure, is classified as a semi-hard material” Based on the listed characteristics, it is interesting to study the properties of Mg_0.33Ni_0.33Co_0.33Fe₂O₄ NPs, prepared by combining the metals in equal amounts. As listed in Table 5, the prepared nanoparticles exhibited superior photocatalytic properties compared to nano-ferrites reported in previous studies such as Mg_0.5Zn_0.5Fe₂O₄. The structural, optical properties and photocatalytic performance of the prepared samples were further improved upon doping with La.

Point 2: How did the author confirm that the product was Mg_0.33Ni_0.33Co_0.33La_xFe_2-xO₄ NPs, not a mixture with hematite?

Response 2: The formed phases were confirmed by fitting the XRD patterns on the Material Analysis Using Diffraction (MAUD) software, applying Rietveld refinement. The presence of hematite in pure NPs was confirmed by the presence of an additional peak, located at 2θ = 32.9°. Though, upon doping the NPs with La the intensity of the peak, referring to the hematite phase, disappears. This confirms that the product was Mg_0.33Ni_0.33Co_0.33La_xFe_2-xO₄ NPs, not a mixture with hematite.

Point 3: Why does the addition of La increase the oxygen vacancy?

Response 3: Upon doping Mg_0.33Ni_0.33Co_0.33Fe₂O₄ NPs with La, the oxygen vacancies are reduced. The intensity of the PL peak, indicating the presence of oxygen vacancies and located around 3 eV, ranges between 20 and 30 in pure NPs (Figure 5a). However, it ranges between 8 and 15 in La-doped samples (Figure 5b). Thus, pure NPs have more oxygen vacancies than doped samples. This might be attributed to the presence of hematite as a secondary phase in pure NPs; whereas no secondary phases were detected in La-doped samples. From here, the addition of La reduces the presence of oxygen vacancies.

Among the doped samples, Mg_0.33Ni_0.33Co_0.33La_0.01Fe_1.99O₄ NPs exhibited the lowest intensity of the PL peak in the UV region and the highest PL peak intensity in the visible region. Thus, the improved photocatalytic activity might be attributed to the slowest recombination rate of photogenerated electron-hole pair accompanied by the presence of oxygen vacancies.

Point 4: Table 2, how the catalyst with La at 0.02 had the lowest rate constant among other catalysts?

Response 4: Upon rechecking the results and linear plot of ln (C₀/C_t), we noticed a typing mistake for the rate constant of the degradation reaction performed in the presence of Mg_0.33Ni_0.33Co_0.33La_xFe_2-xO₄ NPs with x = 0.02. The rate constant is 5 × 10^-4 min^-1 instead of 0.5 × 10^-4 min^-1. As the La content increases from 0.01 to 0.02, the rate constant decreases from 37 × 10^-4 to 5 × 10^-4 min^-1. Among the La-doped samples, the slowest rate was revealed by Mg_0.33Ni_0.33Co_0.33La_xFe_2-xO₄ NPs with x = 0.02. This might be attributed to the fast recombination rate of the photogenerated electron-hole pair since Mg_0.33Ni_0.33Co_0.33La_xFe_2-xO₄ NPs with x = 0.02 revealed the highest PL intensity of the peak located in the UV region.

Point 5: The discussion on the effect of pH was wrong, Spinel ferrite should be positively charged at pH below zero point charge, and positively charged at high pH. The author should analyze the zeta potential and find out the zero point charge of the catalyst. High pH may fade MB.

Response 5: Although analysing the zeta potential and finding out the zero point charge of the catalyst is interesting and significant, we don’t have the needed instrument in our laboratory to perform this experiment. High pH value above 12 may fade MB. To avoid this, the highest pH value, in our study, was 10.73.

The discussion on the effect of pH was rewritten and became as follows (2.2.2. Effect of pH):

“One of the key variables in the photocatalytic of substances is the solution's pH. This is explained by the fact that the pH affects the adsorption behavior of the pollutants as well as the chemical characteristics of the photocatalyst. So, at pH values ranging from 2.47 to 10.73 (acidic, neutral and basic pH values), the effect of pH on the photocatalytic degradation of MB in the presence of Mg_0.33Ni_0.33Co_0.33La_0.01Fe_1.99O₄ NPs was investigated. As shown in Table 3, the rate constant increases from 7 × 10^-4 to 517 × 10^-4 min^-1 when pH rises from 2.47 to 10.73. Thus, the rate of the photodegradation reaction of MB is 73.8 and 14.7 times higher in basic medium (pH = 10.73) compared to that studied in acidic (pH = 2.47) and neutral (pH = 6.27) medium, respectively. So, the solution's ideal pH is basic. As reported in previous studies, the point of zero charge (PZC) of MgFe₂O₄, NiFe₂O₄ and CoFe₂O₄ NPs were 8.4, 6.4 and 7.2, respectively [38-40]. If the pH is less than PZC, the surface of NPs will be positively charged. Whereas when the pH is greater than the PZC, the negative form of the NPs is more probable to exist [37]. Being a cationic dye, MB is positively charged in the solution. Consequently, the MB dye is attracted to the negatively charged catalyst surface due to opposite charges. Thus, increasing the pH of the solution, improves the photocatalytic degradation of MB. Identical results were reported in a previous study where the pH = 12 was the optimum medium for the photodegradation of MB by biosynthesized ZnFe₂O₄ NPs [41].”

Point 6: Figure 8 has a missing symbol on the X aisle.

Response 6: Figure 8 was corrected based on the comment.

Point 7: What about using Gr only? Gr can absorb MB as well. It is hard to tell whether MB was removed from adsorption or photodegradation.

Response 7: The effect of Gr alone was studied and the results were displayed in Figure 11. The mixture of Gr with MB was stirred in dark for 30 min. Then, 3 mL of solution was extracted from the mixture and UV-vis analysis was performed. It was noticed that around 7% was adsorbed by Gr. Since the amount adsorbed was small, the adsorption phenomena were not considered in our study.

Point 8: Figure 12, why the absorbance at 60 min was increased?

Response 8: After rechecking the obtained results, we noticed that there was a mistake in Figure 12. So, it is replaced with the corrected Figure. When exposure time increases from 0 to 60 min, the intensity of the MB peak, which is positioned around 675 nm, decreases.

Point 9: Were the antibacterial tests conducted under solar irradiation?

Response 9: The antibacterial tests were performed under normal conditions without any solar irradiation. Solar irradiation can by itself lead to the inactivation of bacteria. This technique has been applied in many previous studies to treat bacteria present in waste water [1–3]. Exposing waste water to radian energy like β-rays and  g-rays may lead to bacterial cell lysis due to their high penetration power. For that, the investigation of the antibacterial effect of the nanoparticles was not conducted under solar irradiation.

Point 10: “microbial water pollution” should be “microbiological water pollution”,

Response 10: The comment has been addressed in the manuscript.

Point 11: Some grammar mistakes were found.

Response 11: The whole manuscript was rechecked and all mistakes were corrected.

References

1. Al-Gheethi A, Lalung J, Omar Ab Kadir M, S AL-Gheethi AA, Ismail N, Talib A. Reduction of faecal indicators and elimination of pathogens from sewage treated effluents by heat treatment. Casp J Appl Sci Res [Internet]. 2013;2(2):39–55. Available from: http://www.cjasr.com
2. Al-Gheethi AA, Efaq AN, Bala JD, Norli I, Abdel-Monem MO, Ab. Kadir MO. Removal of pathogenic bacteria from sewage-treated effluent and biosolids for agricultural purposes. Appl Water Sci [Internet]. 2018;8(2):1–25. Available from: https://doi.org/10.1007/s13201-018-0698-6
3. Romdhana MH, Lecomte D, Ladevie B, Romdhana MH, Lecomte D, Ladevie B, et al. Monitoring of pathogenic microorganisms contamination during heat drying process of sewage sludge To cite this version : HAL Id : hal-01651373 Monitoring of pathogenic microorganisms contamination during heat drying process of sewage sludge. 2019

Author Response File: Author Response.pdf

Reviewer 2 Report

This paper described the synthesis of La doped Mg0.33Ni0.33Co0.33Fe2O4 and their performance in photodegradation of dyes and antibacterial applications. The structure of catalysts and the influence of experimental conditions on the photocatalytic activity are investigated. Nevertheless, the novelty of current manuscript is not good enough for publication. Therefore, a major revision is suggested.

1.     The composition of materials is so complicated, and the essential roles of each element should be further clarified.

2.     The abstract should be rewritten to show the novelty of this research.

3.     The enhancement contributed by structure modulation is slight. What about using some other photoactive elements that commonly used in photocatalytic applications?

4.     Graphene is very light, so the utilization of 20% graphene in the composite seems too high. The contribution of adsorption to the pollutant removal should be considered.

 

5.     More discussion should be provided to clarify the possible mechanism.

Author Response

Manuscript ID: catalysts-2300436 - Revised Version

We would like to submit the revised version of the research article titled “Influence of Lanthanum Doping on the Photocatalytic and Antibacterial Capacities of Mg_0.33Ni_0.33Co_0.33Fe₂O₄ Nanoparticles” for publication in catalysts.

The authors would like to thank the editor and the reviewers for their precious time, efforts, and invaluable comments. The comments and suggestions were very helpful and allowed us to explain and improve more aspects of the manuscript. We have carefully addressed all the comments and we are pleased to learn that the manuscript has been substantially improved.

Below is our point-by-point response to the Reviewer 2 comments as the following:

Reviewer 2 Comments, Suggestions, and Authors’ Responses:

This paper described the synthesis of La doped Mg_0.33Ni_0.33Co_0.33Fe₂O₄ and their performance in photodegradation of dyes and antibacterial applications. The structure of catalysts and the influence of experimental conditions on the photocatalytic activity are investigated. Nevertheless, the novelty of current manuscript is not good enough for publication. Therefore, a major revision is suggested.

Point 1: The composition of materials is so complicated, and the essential roles of each element should be further clarified.

Response 1: The choice of a spinel ferrite with three metals (Mg, Ni, Co) in equal amounts is owed to the properties of MgFe₂O₄, NiFe₂O₄ and CoFe₂O₄. This paragraph is added to the last paragraph in the introduction. “It is known that MgFe₂O₄ is a soft magnetic semiconducting material with a normal spinel structure. NiFe₂O₄, having an inverse spinel structure, is a soft magnetic semiconducting material. Whereas, CoFe₂O₄, characterized by its inverse spinel structure, is classified as a semi-hard material.” Based on the listed characteristics, it is interesting to study the properties of Mg_0.33Ni_0.33Co_0.33Fe₂O₄ NPs, prepared by combining the metals in equal amounts.

Point 2: The abstract should be rewritten to show the novelty of this research.

Response 2: The abstract has been modified and represented as follows:

“The increase in environmental pollution, especially water pollution, intensified the requirement for new strategies for the treatment of water sources. Furthermore, the improved properties of nano-ferrites permit their usage in wastewater treatment. In this regard, novel Mg_0.33Ni_0.33Co_0.33La_xFe_2-xO₄ nanoparticles (NPs), where 0.00≤ x≤0.08, were synthesized to test their photocatalytic, antibacterial and antibiofilm activities. The structural and optical properties of the prepared NPs were investigated by X-ray powder diffraction (XRD), transmission electron microscopy (TEM), UV-vis spectroscopy and photoluminescence (PL) analysis. As La content increases, the bandgap energy increases, whereas the particle size decreases. The photocatalytic activity of the prepared NPs is evaluated in the degradation of methylene blue (MB) dye under sunlight irradiation. Superior activity is exhibited by Mg_0.33Ni_0.33Co_0.33La_0.01Fe_1.99O₄ NPs. The influence of catalyst dosage, pH, temperature and addition of graphene (Gr) on the photodegradation reaction was studied. Increasing the pH and temperature improved the rate of the photodegradation reaction. The antibacterial and antibiofilm activities of the NPs were assessed against Escherichia coli, Leclercia adecarboxylata, Staphylococcus aureus and Enterococcus faecium. Mg_0.33Ni_0.33Co_0.33Fe₂O₄ NPs inhibited bacterial growth. It had a bacteriostatic activity on all isolates, with a better effect on Gram-positive bacteria. All tested nano-ferrites had significant antibiofilm activities against some biofilms.”

Point 3: The enhancement contributed by structure modulation is slight. What about using some other photoactive elements that commonly used in photocatalytic applications?

Response 3: Since the enhancement of the photocatalytic activity of the prepared samples contributed by structure modulation was slight, the effect of catalyst amount, pH, temperature, and graphene addition were studied. It is worth mentioning that graphene is considered as a photoactive component. Graphene offers several advantages for photocatalytic degradation of dyes. Its unique electronic structure and high surface area enable efficient light absorption and charge separation, which is critical for driving the photocatalytic reaction. In addition, graphene-based photocatalysts have been shown to exhibit high catalytic activity and selectivity for the degradation of various dyes, including those commonly used in textile industries.

The papers, listed below, describe the use of graphene and graphene-based materials as photoactive elements in photocatalytic degradation of various organic pollutants, including dyes.

• Ramalingam, G., Nagapandiselvi, P., Priya, A. K., & Rajendran, S. (2022). A review of graphene-based semiconductors for photocatalytic degradation of pollutants in wastewater. Chemosphere, 134391.
• Ahmed, M. A., & Mohamed, A. A. (2023). Recent progress in semiconductor/graphene photocatalysts: synthesis, photocatalytic applications, and challenges. RSC advances, 13(1), 421-439.
• Wang, H. X., Wang, Q., Zhou, K. G., & Zhang, H. L. (2013). Graphene in light: design, synthesis and applications of photo‐active graphene and graphene‐like materials. Small, 9(8), 1266-1283.

Point 4: Graphene is very light, so the utilization of 20% graphene in the composite seems too high. The contribution of adsorption to the pollutant removal should be considered.

Response 4: The choice of graphene content is based on literature where similar wt.% of graphene was used:  

• Mishra, S., Acharya, R., & Parida, K. (2023). Spinel-Ferrite-Decorated Graphene-Based Nanocomposites for Enhanced Photocatalytic Detoxification of Organic Dyes in Aqueous Medium: A Review. Water, 15(1), 81.
• Fu, Y., Chen, H., Sun, X., & Wang, X. (2012). Combination of cobalt ferrite and graphene: high-performance and recyclable visible-light photocatalysis. Applied Catalysis B: Environmental, 111, 280-287.
• Fu, Y., Chen, H., Sun, X., & Wang, X. (2012). Graphene‐supported nickel ferrite: A magnetically separable photocatalyst with high activity under visible light. AIChE Journal, 58(11), 3298-3305.

The effect of Gr alone was studied and the results were displayed in Figure 11. The mixture of Gr with MB was stirred in dark for 30 min. Then, 3 mL of solution was extracted from the mixture and UV-vis analysis was performed. It was noticed that around 7% was adsorbed by Gr after 30 min. Since the amount adsorbed was small, the adsorption phenomenon was not considered in the current study.

Point 5: More discussion should be provided to clarify the possible mechanism.

Response 5: The possible mechanism for the photodegradation reaction of MB is stated as follows (2.2. Photocatalytic Activity of Mg_0.33Ni_0.33Co_0.33La_xFe_2-xO₄ NPs):

“Whenever the mixture of MB and NPs is placed under sunlight, light absorption by NPs will occur. Consequently, electron-hole pairs are generated. Since Mg_0.33Ni_0.33Co_0.33La_0.01Fe_1.99O₄ NPs revealed the slowest recombination rate of the photogenerated electron-hole pair, the electron-hole pair will migrate to the NPs’ surface. It is worth mentioning that the uncombined electron-hole pair interacts with the O₂ and H₂O and produces reactive species such as hydroxyl radicals (OH) and superoxide radicals (O₂^-). Finally, the produced reactive species are responsible for the degradation of MB dye through a direct oxidation process”.

Author Response File: Author Response.pdf

Reviewer 3 Report

The article by M. Rabaa et al. titled “Influence of Lanthanum Doping on the Photocatalytic and Antibacterial Capacities of Mg_0.33Ni_0.33Co_0.33Fe₂O₄ Nanoparticles” describes the synthesis and antibacterial evaluation of some substituted ferrite Mg_0.33Ni_0.33Co_0.33La_xFe_2-xO₄ nanoparticles, with x=0.00, 0.01, 0.02, 0.04 and 0.08.  The ferrite showing antibacterial activity was the unsubstituted, mixed-cation one (x=0.00), with a greater effect on Gram-positive bacteria.

The draft is well-written, with some minor observations:

-          check the formula on page 3 : Ni0.3Zn0.5Co0.2LaxFe1.98-xO4

-          Figure 1 (XRD) : Why is the hematite peak disappearing upon La substitution increase?

-          Table 1 has no/wrong title (page 3)

-          Figure 4 (page 5) – wrong title, and all formulae must comply to usage of subscripts (general observation)

-          the natural logarithm : “ln”, not “Ln” which might allude to other meanings

-          Page 6: repeating word “thus” in the same phrase

-          Table 2: The drop in k value when switching from 0 to 0.01 La substitution seems very high. Have any intermediate gradients been used to substantiate this? (page 7)

-          Same page (7): “sites increase with increasing in the amount”

-          How was the UV test performed to obtain the Eg ? This should be explained in detail.

-          What proof is there that La3+ substitutes Fe3+ in the ferrite structure? There is no elemental analysis shown.

-          Also, the lack of bacterial activity of La-substituted ferrites (in contrast to prev. studies like those in ref [44]) might suggest that La is not a ferrite constituent

-          What is the result for E. faecium after 24h (Table 9)?

-          Under conclusion, it’s rather deceiving to include “La_0.00” in the formula of NPs with bacterial activity, since only those without La exhibited some.

Author Response

Manuscript ID: catalysts-2300436 - Revised Version

We would like to submit the revised version of the research article titled “Influence of Lanthanum Doping on the Photocatalytic and Antibacterial Capacities of Mg_0.33Ni_0.33Co_0.33Fe₂O₄ Nanoparticles” for publication in catalysts.

The authors would like to thank the editor and the reviewers for their precious time, efforts, and invaluable comments. The comments and suggestions were very helpful and allowed us to explain and improve more aspects of the manuscript. We have carefully addressed all the comments and we are pleased to learn that the manuscript has been substantially improved.

Below is our point-by-point response to the Reviewer 3 comments as the following:

Reviewer 3 Comments, Suggestions, and Authors’ Responses:

The article by M. Rabaa et al. titled “Influence of Lanthanum Doping on the Photocatalytic and Antibacterial Capacities of Mg_0.33Ni_0.33Co_0.33Fe₂O₄ Nanoparticles” describes the synthesis and antibacterial evaluation of some substituted ferrite Mg_0.33Ni_0.33Co_0.33La_xFe_2-xO₄ nanoparticles, with x=0.00, 0.01, 0.02, 0.04 and 0.08.  The ferrite showing antibacterial activity was the unsubstituted, mixed-cation one (x=0.00), with a greater effect on Gram-positive bacteria. The draft is well-written, with some minor observations:

Point 1: Check the formula on page 3: Ni_0.3Zn_0.5Co_0.2La_xFe_1.98-xO₄

Response 1: After double-checking, the formula written on page 3 is correct. Referring to reference 21, the formula given in the reference is as follows Ni_0.3Zn_0.5Co_0.2RE_xFe_1.98-xO₄ ferrites (where RE is either Gd or La (x = 0.0, 0.02, 0.06)). in our manuscript we cited the formula Ni_0.3Zn_0.5Co_0.2La_xFe_1.98-xO₄ ferrite which is in accordance with the formula in reference 21

Point 2: Figure 1 (XRD): Why is the hematite peak disappearing upon La substitution increase?

Response 2: The decrease in intensity of the hematite peak might be attributed to the occupation of La³⁺ ions in the host lattice. In other words, the absence of extra peaks of dopants or secondary phases revealed the successful replacement of iron ions by the dopant ions (La³⁺). Similar results are reported in previous studies:

• Kamran, M., & Anis-ur-Rehman, M. (2023). Influence of La3+ substitutions on structural, dielectric and electrical properties of spinel cobalt ferrite. Ceramics International, 49(4), 7017-7029.
• Naik, C. C., & Salker, A. V. (2019). Structural, magnetic and dielectric properties of Dy3+ and Sm3+ substituted Co–Cu ferrite. Materials Research Express, 6(6), 066112.

Point 3: Table 1 has no/wrong title (page 3)

Response 3: The title of table 1 was corrected and it is represented as follows:

“Table 1. The values of lattice parameter (a), particle size (D_TEM) and bandgap energy (E_g) of Mg_0.33Ni_0.33Co_0.33La_xFe_2-xO₄ NPs where 0.00 x 0.08.”

Point 4: Figure 4 (page 5) – wrong title, and all formulae must comply to usage of subscripts (general observation)

Response 4: The title of Figure 4 was corrected and it is represented as follows:

“Figure 4. Tauc’s plot of Mg_0.33Ni_0.33Co_0.33La_xFe_2-xO₄ NPs where 0.00 x 0.08.”

Point 5: the natural logarithm: “ln”, not “Ln” which might allude to other meanings

Response 5: The logarithm abbreviation was corrected and denoted as “ln” instead of “Ln” in the whole manuscript.

Point 6: Page 6: repeating word “thus” in the same phrase

Response 6: Based on this comment the phrase was corrected as follows:

“Thus, the photocatalytic performance of NPs is in good accordance with the PL results.”

Point 7:  Table 2: The drop in k value when switching from 0 to 0.01 La substitution seems very high. Have any intermediate gradients been used to substantiate this? (page 7)

Response 7: Upon rechecking the results and linear plot of ln (C₀/C_t), we noticed a typing mistake for the rate constant of the degradation reaction performed in the presence of Mg_0.33Ni_0.33Co_0.33La_xFe_2-xO₄ NPs with x = 0.02. The rate constant is 5 × 10^-4 min^-1 instead of 0.5 × 10^-4 min^-1. As La content increases from 0.01 to 0.02, the rate constant decreases from 37 × 10^-4 to 5 × 10^-4 min^-1. Among the La-doped samples, the slowest rate was revealed by Mg_0.33Ni_0.33Co_0.33La_xFe_2-xO₄ NPs with x = 0.02. This might be attributed to the fast recombination rate of the photogenerated electron-hole pair since Mg_0.33Ni_0.33Co_0.33La_xFe_2-xO₄ NPs with x = 0.02 revealed the highest PL intensity of the peak located in the UV region.

Point 8:  Same page (7): “sites increase with increasing in the amount”

Response 8: The sentence is corrected and is written as follows:

“This is because the active sites of the catalyst increase with increasing the amount of photocatalyst.”

Point 9: How was the UV test performed to obtain the Eg? This should be explained in detail.

Response 9: The comment has been addressed and the following details have been included in the manuscript (4.2. Characterization techniques):

“To perform the UV test, 0.01 g of each of the prepared NPs was dissolved in 50 ml of 1 M HCl solution. Afterward, the mixtures were sonicated for 5 min. Subsequently, the optical properties of the samples were estimated by ultraviolet-visible (UV–Vis) spectroscopic examinations that were performed at room temperature in the range of 300-700 nm using Jasco spectrophotometer V-670.”

Point 10: What proof is there that La3+ substitutes Fe3+ in the ferrite structure? There is no elemental analysis shown.

Response 10: Although no elemental analysis is shown, the absence of extra peaks of dopants or secondary phase revealed the replacement of iron ions by the dopant ions (La³⁺). Furthermore, the lattice parameter of La-doped NPs is greater than that of pure (undoped) NPs. This is also owed to the substitution of Fe³⁺ with La³⁺ knowing that the ionic radius of La³⁺ (1.06 Å) is greater than that of Fe³⁺ (0.65 Å). Similar behavior was reported in previous studies upon doping ferrite NPs with rare earth ions:

• Xueyun, Z., Dongsheng, Y., & Liling, Z. (2021). Improved cut-off frequency in Gd/La doped NiZnCo ferrites. Materials Science and Engineering: B, 272, 11533
• Singh, R. K., Shah, J., & Kotnala, R. K. (2016). Magnetic and dielectric properties of rare earth substituted Ni0.5Zn0.5Fe1.95R0. 05O4 (R= Pr, Sm and La) ferrite nanoparticles. Materials Science and Engineering: B, 210, 64-69.

Point 11: Also, the lack of bacterial activity of La-substituted ferrites (in contrast to prev. studies like those in ref [44]) might suggest that La is not a ferrite constituent

Response 11: In the mentioned study [44], La-doped sodium tantalate nanoparticles were tested and not ferrites. Their composition is different. In addition, the antibacterial activity of the tantalate nanoparticles was attributed to a reduction in the bandgap with doping. However, in our study the La-doped nano-ferrites showed an increase in the bandgap energy with doping. This leads to a decrease in the antibacterial activity [1]. In addition, the decrease in oxygen vacancies decreases the ability of the nanoparticles to produce reactive oxygen species (ROS), which in turn decreases their antibacterial activity [1–3]. Therefore, the mentioned nanoparticles are ferrite constituents, but La doping had an adverse effect on their antibacterial activity. This clarification has been added to the manuscript (page 13)

Point 12: What is the result for E. faecium after 24h (Table 9)?

Response 12: As mentioned in table 9, the biofilm of Enterococcus faecium was eradicated by the Mg_0.33Ni_0.33Co_0.33La_xFe_2-xO₄ NPs where 0.00 ≤ x ≤ 0.04 at concentrations ranging between 0.375 to 1.5 mg/mL after 24 hours of incubation. The best activity was recorded for Mg_0.33Ni_0.33Co_0.33La_0.02Fe_2-xO₄. This result is consistent with previous studies stating that the anti-biofilm effects are not concentration-dependent [4]. This variation might be due to the absorbance and cell enumeration.

Point 13: Under conclusion, it’s rather deceiving to include “La_0.00” in the formula of NPs with bacterial activity, since only those without La exhibited some.

Response 13: The term “La_0.00” was used in the whole manuscript to express the undoped nanoparticles. In addition, despite their inactive effect on bacteria, the La doped nanoparticles exhibited an antibiofilm activity. A sentence is added in the conclusion to clarify this point, as follows: “An antibiofilm action was observed for some of the doped NPs, especially those where x = 0.01, 0.02 and 0.04, mainly against biofilms of Gram-positive bacteria”.

References

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Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

The quality of paper has been improved, thus can be accepted by this journal.