Titan Yellow and Congo Red Removal with Superparamagnetic Iron-Oxide-Based Nanoparticles Doped with Zinc

Round 1

Reviewer 1 Report

This is an interesting paper looking into novel magnetic nanomaterials for water remediation.

I do, however, have a number of issues with the manuscript.

Lines 71-73 have accidental accreditation information, please remove.

The characterization section only mentions SEM but TEM is mentioned in the results. Please include information about how this analysis was conducted.  

Figures 1 and 2: Please include zoomed images of individual nanoparticles in each image. Please also conduct a size analysis of the nanoparticles and generate a histogram showing size distribution.

I note that the adsorption experiments were conducted for 30 minutes while it was also the case that the kinetic studies were close to an hour. Can you guarantee that equilibrium absorption was reached in that short time.

Was there an effort to remove the nanoadsorbent from the TY so that it didn't effect the UV-VIS adsorption experiments?

Was there a dark adsorption period used in the photocatalysis experiments in order to separate the reduction in dye from physical adsorption and that from photocatalytic degradation?

Table 3: Do you have the results for the other adsorbents (Fe3O4@1%Zn, Fe3O4@2%Zn, Fe3O4@5%Zn)? Please at least mention them if you have them.

I would suggest that additional experiments should be included in the conclusions as possible future work. I would suggest including 

1) Investigating the cyclical reuse of these adsorbents. 

2) Testing adsorption for other dye molecules

3) Testing their adsorption behaviour under "real" conditions, with other materials such as metal salts and changes in pH in solution.      

Author Response

We would like to thank you for your valuable comments and suggestions. Thanks to them we could improve our work. All changes are included in the main text and marked in yellow.

Lines 71-73 have accidental accreditation information, please remove.

Thank you for this comment. We have removed it from the manuscript.

The characterization section only mentions SEM but TEM is mentioned in the results. Please include information about how this analysis was conducted.  

Indeed, we missed the technical information about TEM. The draft has been modified.

Figures 1 and 2: Please include zoomed images of individual nanoparticles in each image. Please also conduct a size analysis of the nanoparticles and generate a histogram showing size distribution.

We have zoomed in on particular particles and included that in the TEM images. According to SEM images, they were just surveyed images to check if there are no artefacts and show uniform morphology.

I note that the adsorption experiments were conducted for 30 minutes while it was also the case that the kinetic studies were close to an hour. Can you guarantee that equilibrium absorption was reached in that short time.

Presented spectra were recorded for 30 minutes (an equilibrium for Fe3O4@10%Zn) contact time as an overview to present the absorbance drop with the increase of the adsorbent mass for a particular type of sample. Since there was no effect of photodegradation after 30 min, the photodegradation of TY experiments lasted longer. The equilibrium adsorption for titan yellow for the Fe3O4@10%Zn sample ensues after 90, while in the case of photodegradation, a higher effect was observed after 120 min than 90 min.

Was there an effort to remove the nanoadsorbent from the TY so that it didn't effect the UV-VIS adsorption experiments?

Nanosorbent has magnetic properties, so it’s easy to remove nanoparticles from the solution within the magnetic separation. In our work, we exposed solutions at the separate beakers, then nanoparticles were separated magnetically. The solution without nanoparticles was then placed in the quartz cuvette.

Was there a dark adsorption period used in the photocatalysis experiments in order to separate the reduction in dye from physical adsorption and that from photocatalytic degradation?

Yes. We have included that in the text.

Table 3: Do you have the results for the other adsorbents (Fe3O4@1%Zn, Fe3O4@2%Zn, Fe3O4@5%Zn)? Please at least mention them if you have them.

We have shown undoped Fe3O4 and Fe3O4 doped with 1%, 2%, 5, and 10% zinc dopant. Only these samples were proposed for the UV-vis studies. For these sopants the magnetic properties remain similar. Higher dopants content e.g. 15% and 20% decreased the magnetic properties, so magnetic separation was poor. That is why we have decided to focus only on the mentioned samples In this work, we presented an alternative to commonly used ZnO, where the main advantage is magnetic separation.

I would suggest that additional experiments should be included in the conclusions as possible future work. I would suggest including 

1) Investigating the cyclical reuse of these adsorbents. 

Thank you for this suggestion. It is very important to find a solution to reuse samples. In this work we focused on the adsorption of particular dyes. Due to the physicochemical properties of the nanoparticles they work well with e.g. sulphonic dyes. The reuse of the nanoparticles will be presented within the following paper, where we are going to present the second life of magnetic particles post-dye treatment. Currently, we have submitted a patent application that deals with the reuse of magnetic particles. The application is proceeding, so hopefully soon we can share our results in a paper.

2) Testing adsorption for other dye molecules

In fact, we have also tested different dyes. Depending on their structure, the results are different. For example, crystal violet is not removed effectively within the proposed particles, while when we tested biochar - it worked well. Therefore, we have tested other sulphonic dyes like Congo red, being in the same class of dyes as titan yellow, and the effect was similar. However, we have recorded higher photocatalytic activity but poorer adsorption. We have decided to add these data in this paper. Thank you for this comment.

3) Testing their adsorption behaviour under "real" conditions, with other materials such as metal salts and changes in pH in solution.      

We have performed measurements in the different pH and ionic strengths for titan yellow and Congo red dyes. All results are included in the text. Thank you for this suggestion.

 

All changes performed in the manuscript are marked in yellow.

Reviewer 2 Report

The paper describes the synthesis of Zn-doped magnetite nanoparticles and their evaluation as potential adsorbent for the capture titan yellow from wastewater. The study shows interest though the level of novelty is limited in both synthesis part and applicability. Authors should consider the following comments in order to improve the impact of the manuscript.

·         It is unclear if the resulting phase of Zn doping is a zinc ferrite or anything else. No evidence is given on this.

·         There is no clear correlation (if any) between properties such as the specific surface area, the magnetization or the crystal size with the Zn content. It can be said that values are fluctuating randomly around a more or less mean value.

·         For the magnetization values it is surprizing that the Zn-rich nanoparticles show the highest value. It is known that Zn ferrites generally provide lower magnetization than the pure Fe3O4.

·         In the first part of Section 3.6, I cannot understand why authors prefer to give the qualitative variation of the UV/Vis spectra instead of providing quantitative concentration data (after carrying out the proper calibration by such spectra)?

·         The legend in Figure 7 should be “Adsorption efficiency” instead of “UV-vis spectra adsorption titan yellow dye”.

·         I wonder how comes the conclusion that the material can be reused because of the degradation observation during TG analysis. This assumption should be checked experimentally.

·         The discussion/conclusion section should be improved instead of being a repetition of major measurements.

Author Response

We would like to thank you for your valuable comments and suggestions. Thanks to them we could improve our work. All changes are included in the main text and marked in yellow.

•     It is unclear if the resulting phase of Zn doping is a zinc ferrite or anything else. No evidence is given on this. - EDS

We have added EDS mapping results for all samples, while in our opinion, the resolution of the EDS technique is not proper for samples having very low dopant content like 1-2%. The Zn on the map is poorly seen. However, we have tested 3 samples, and the results were almost the same - uniform distribution of dopant.

•     There is no clear correlation (if any) between properties such as the specific surface area, the magnetization or the crystal size with the Zn content. It can be said that values are fluctuating randomly around a more or less mean value.

Indeed, only minor changes are seen in the results, especially for optical properties; however, even a small change significantly affects sorption properties. According to the XRD studies, there is a shift in agnle for doped samples. Each measurement from magnetization, morphology and optical properties studies to UV-vis analysis was repeated min. Three times to be sure that the results are not artifacts. Each time we recorded similar results, so the presented data are average for min. 3 experiments. 

•     For the magnetization values it is surprizing that the Zn-rich nanoparticles show the highest value. It is known that Zn ferrites generally provide lower magnetization than the pure Fe3O4.

We repeated measurements min. 3hree times to be sure that the results are not artifacts. The results were each time the same. However, higher Zn content of about 15% and 20% by wt.% worsened the magnetic properties. The magnetization saturation for 15 and 20% was dropping, so we have decided to focus only on the samples that will be the most suitable for the magnetic separation of nanoparticles from the tested dye solution.

•     In the first part of Section 3.6, I cannot understand why authors prefer to give the qualitative variation of the UV/Vis spectra instead of providing quantitative concentration data (after carrying out the proper calibration by such spectra)?

The measurements were performed in the following order: %H was evaluated to optimize the time and dose of a particular adsorbent to present the highest efficiency with the lowest costs of practical application. Therefore, measurements were performed in various pH and ions concentrations to reveal the effect of experimental conditions on the adsorption efficiency. Then, the kinetics and isotherms were measured for the sample and conditions that give the highest efficiency. It is a standard procedure to optimize the sorbent effectiveness on the particular model pollution. Since we proposed different adsorbents (different Zn content), the measurements were started in the order presented in our work.

•     The legend in Figure 7 should be “Adsorption efficiency” instead of “UV-vis spectra adsorption titan yellow dye”.

We have corrected the labels.

•     I wonder how comes the conclusion that the material can be reused because of the degradation observation during TG analysis. This assumption should be checked experimentally.

The TGA was performed complementary to the UV-vis measurements showing that the organics are adsorbed onto the SPIONs’ surface. Indeed, based on the TG analysis, it is also seen that the organics are removed, and SPIONs can be reused. The nanoparticle reuse will be presented in the following paper, presenting the second life of magnetic particles post-dye treatment. We have submitted a patent application dealing with the reuse of magnetic particles. The application is proceeding. Hopefully, we can present the results of this work soon.

    The discussion/conclusion section should be improved instead of being a repetition of major measurements.

We have modified the discussion.

Additionally, we have added the data for Congo red removal showing a similar effect. We have chosen the dye that is from the same class of dyes - the sulfonic one.
We hope that all changes meet the suggestions.

Round 2

Reviewer 2 Report

After the proper additions the manuscript was improved satisfactory