Influence of Ammonium on the Adsorption and Desorption of Heavy Metals in Natural Zeolites

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This manuscript explores the competitive adsorption and desorption behavior of ammonium and six heavy metals (Cd, Cr, Cu, Ni, Pb, Zn) on two natural zeolites using batch and column experiments with synthetic wastewater. The experiments were well designed, the description for the experiments was clear and the mechanisms on adsorption were analyzed. The paper could be accepted after minor revision. The suggestions are following.

1. In the Experimental Section, specify the types of natural zeolites used in the study.

2. Ensure that Table 1 is cited before Figure 1, so that readers are informed early on about the origin of the zeolites and the meaning of the nomenclature used throughout the manuscript.

3. Table 2 should be explicitly referenced in Section 3.1.1, as the discussion presented there is based on the HM properties summarized in the table.

4. Clearly define the acronym PSO at its first occurrence in the manuscript. While commonly used to refer to the pseudo-second-order kinetic model, it should be explicitly stated to avoid ambiguity for readers unfamiliar with the term.

5. Provide an explanation for the differences observed in adsorption performance between the two zeolites, both in the kinetic tests and column experiments. Indicate which physicochemical property or structural feature may account for the higher or lower adsorption capacity observed. This discussion will strengthen the interpretation of results and the scientific contribution of the work.

Author Response

We wish to thank the reviewer for the valuable comments and allocating their time to review this manuscript. For the ease of reference, we numbered the comments and addressed all the comments in the revised manuscript. The main revisions relate to the structure of the article, the abstract, introducing isotherm data, enhancing citations in the results and discussion part and adding further information to the conclusions section. Please find the detailed response below and the corresponding revisions are highlighted as markups in the attached manuscript.

Reviewer 1:

This manuscript explores the competitive adsorption and desorption behavior of ammonium and six heavy metals (Cd, Cr, Cu, Ni, Pb, Zn) on two natural zeolites using batch and column experiments with synthetic wastewater. The experiments were well designed, the description for the experiments was clear and the mechanisms on adsorption were analyzed. The paper could be accepted after minor revision. The suggestions are following.

1. In the Experimental Section, specify the types of natural zeolites used in the study.

Thank you for this remark. We have analytically specified the BET-surface area and composition of the 2 zeolites in Table 1. In line 75 we also mentioned the specific type of natural zeolite we used in our experiments.

2. Ensure that Table 1 is cited before Figure 1, so that readers are informed early on about the origin of the zeolites and the meaning of the nomenclature used throughout the manuscript.

You are right. We switched the complete sections section 2.1 and 2.2 as it improves the logical structure of the manuscript.

3. Table 2 should be explicitly referenced in Section 3.1.1, as the discussion presented there is based on the HM properties summarized in the table.

Thank you for pointing this out. We have explicitly a mentioned table 2 in line 213.

4. Clearly define the acronym PSO at its first occurrence in the manuscript. While commonly used to refer to the pseudo-second-order kinetic model, it should be explicitly stated to avoid ambiguity for readers unfamiliar with the term.

Indeed, we have already done that in line 148.

5. Provide an explanation for the differences observed in adsorption performance between the two zeolites, both in the kinetic tests and column experiments. Indicate which physicochemical property or structural feature may account for the higher or lower adsorption capacity observed. This discussion will strengthen the interpretation of results and the scientific contribution of the work.

In line with your comment, we have added a detailed explanation of the adsorption behaviour depending on the properties of the two natural zeolites in lines 278-289.

Reviewer 2 Report

Comments and Suggestions for Authors

Comments to authors

• The title should be revised
• The abstract should be revised and contain more valuable results.
• Explain the use of these heavy metals
• Remove all old articles in the introduction and add more recent articles in the introduction section.
• In figure 1, it's particle size, not particle thickness
• The competitive adsorption protocol must be added in detail
• In competitive adsorption, you can't use equation 3, there are system equations
• Add the plot of Qt=f(time) for different pollutants
• All figures must be revised
• For the PSO model, R2 equal to 0.86 does not validate the model. The PSO model is not suitable
• For adsorbent dose, all % removals increase with adsorbent dose, why use 5000 mg/L as maximum value.
• More details in the desorption study
• The results and discussion are poorly referenced.
• Add the adsorption technique to indicate the type of intercation
• Add a comparison table with other articles
• Adsorbent regeneration is very important, add it to the manuscript
• The conclusion section should be revised
• Figure S1 and S2 are not useful, remove them, the specific surface area in the table is sufficient.

Author Response

We wish to thank the reviewer for the valuable comments and allocating their time to review this manuscript. For the ease of reference, we numbered the comments and addressed all the comments in the revised manuscript. The main revisions relate to the structure of the article, the abstract, introducing isotherm data, enhancing citations in the results and discussion part and adding further information to the conclusions section. Please find the detailed response below and the corresponding revisions are highlighted as markups in the attached manuscript.

The title should be revised

We have revised the title according to your suggestion to better explain the actual research content of the conducted experiments.

The abstract should be revised and contain more valuable results.

We agree with you and we added fundamental isotherm data to the abstract along with maximum loading rates.

Explain the use of these heavy metals

We added detailed explanation of the usage of the heavy metals in lines 38-44.

Remove all old articles in the introduction and add more recent articles in the introduction section.

Thank you for pointing this out. We deleted references older than 10 years in case they were not irreplaceable due to their comprehensive data base and exchanged them with more recent literature.

In figure 1, it's particle size, not particle thickness

Thank you for this comment. The mistake was that we wrote particle sice. We have corrected it into particle size.

The competitive adsorption protocol must be added in detail

We have added our data during the submission process according to the MDPI guidelines. Adding all measurement data in the manuscript would go beyond limits. We also think that figures 2-5 show and explain the data in a sufficient way.

In competitive adsorption, you can't use equation 3, there are system equations

In accordance with your remark, we added Hill isotherms (Figure 3 and Table 4) as they fitted the best.

Add the plot of Qt=f(time) for different pollutants

Thank you for the remark. We plotted the suggested figures and added them as Figure S1.

All figures must be revised

We have revised figures 4 and 5 were changed the x-axis scale for a better understanding of the actual breakthrough. We also added further information in the subtitles of figures 2-5.

For the PSO model, R2 equal to 0.86 does not validate the model. The PSO model is not suitable

Thank you for this comment. We agree that R² isn’t as high as desired. However, for a multicomponent solution and natural zeolites like in our case, the PSO model shows the best fit compared to other models. In lines 197-198 we also cited literature, that this model is the way to go with multicomponent solutions.

For adsorbent dose, all % removals increase with adsorbent dose, why use 5000 mg/L as maximum value.

Thank you for this question. We have used the range of adsorbent dosage based on earlier experiments we did in the lab. It is also important to cover a braod range of dosages in order to make proper removal and/or loading plots.

More details in the desorption study

In our opinion, we described our data in detail and also cited recent literature to strengthen the discussion.

The results and discussion are poorly referenced.

After going through the manuscript, we agree with your observation, so we added references to better discuss our data. However, there are not so many studies that dealt with competitive NH₄/heavy metal adsorption and desorption with a similar approach so we could not cite a lot of recent studies.

Add the adsorption technique to indicate the type of intercation

Indeed, that would enhance the quality of the manuscript. In lines 506-513 we added the interaction type based on the isotherm data.

Add a comparison table with other articles

We agree with your comment. In our opinion we do not want to insert too many tables and figures, so we added selectivity comparisons with other references in lines 255 onwards as flow-text.

Adsorbent regeneration is very important, add it to the manuscript

Indeed, regeneration of used natural zeolites is a very important topic which needs further consideration. However, we think that our work already covers two comprehensive topics with competitive adsorption and desorption (which also part of regeneration in a very broad view). Therefore, also considering regeneration as a 3^rd topic would be too much and should be part of another standalone publication.

The conclusion section should be revised

Thank you for pointing this out. We have revised parts of the conclusion section for more clarity and to better point out the main points of this study. In lines 506-513 we added the results of the isotherms to deepen the understanding of the underlying adsorption process.

Figure S1 and S2 are not useful, remove them, the specific surface area in the table is sufficient.

Indeed, we removed these figures from the Supplementary Information.

Reviewer 3 Report

Comments and Suggestions for Authors

This manuscript discusses the adsorption characteristics of zeolite for heavy metal ions and its application in column experiments. It is recommended to revise the discussion by addressing the following four points:
1
The quality of Supporting Information Figures S1 and S2 is very poor, making them difficult to read. Moreover, they appear to be reproduced from a doctoral thesis or a similar source. These figures should be clearly redrawn and presented in a high-quality format.
2
Figure 2 should be presented as a conventional adsorption isotherm. Instead of using the initial concentrations, it would be more appropriate to plot the data against the equilibrium concentrations and fit them using the Langmuir or Freundlich model (or similar).
3
In Figure 3, the areas showing irregularities or bumps should be explained in detail. Additionally, the exchange capacity of the zeolite obtained from the adsorption curve should be compared with the theoretical exchange capacity to better interpret the results presented.
4
In the section on competitive adsorption experiments, it would be valuable to discuss the affinity toward each ion and the selectivity of the zeolite in more detail. This discussion should be supported by comparisons with existing literature and related to the specific characteristics of the zeolite used in this study.

Author Response

We wish to thank the reviewer for the valuable comments and allocating their time to review this manuscript. For the ease of reference, we numbered the comments and addressed all the comments in the revised manuscript. The main revisions relate to the structure of the article, the abstract, introducing isotherm data, enhancing citations in the results and discussion part and adding further information to the conclusions section. Please find the detailed response below and the corresponding revisions are highlighted as markups in the attached manuscript.

Reviewer 3:

This manuscript discusses the adsorption characteristics of zeolite for heavy metal ions and its application in column experiments. It is recommended to revise the discussion by addressing the following four points:

1
The quality of Supporting Information Figures S1 and S2 is very poor, making them difficult to read. Moreover, they appear to be reproduced from a doctoral thesis or a similar source. These figures should be clearly redrawn and presented in a high-quality format.

Indeed, these are the datasheets we got from the laboratory. As the results are also mentioned in Table 1, we removed it from the Supplementary Information.

2
Figure 2 should be presented as a conventional adsorption isotherm. Instead of using the initial concentrations, it would be more appropriate to plot the data against the equilibrium concentrations and fit them using the Langmuir or Freundlich model (or similar).

Very good remark. In our experience, depcting the removal over the adsorbent dosage delivers an easy to understand, intuitive approach on the effects of the adsorbent. However, we agree with you and added the plot of Hill isotherms for all HMs combined for further understanding as this model showed a very good fit along with a new section (3.1.3) (Figure 3 and Table 4).

3
In Figure 3, the areas showing irregularities or bumps should be explained in detail. Additionally, the exchange capacity of the zeolite obtained from the adsorption curve should be compared with the theoretical exchange capacity to better interpret the results presented.

Thanks for pointing this out. This was caused by unfortunate x-axis scaling. We changed that for figures 4 and 5. In lines 292-298 we added a comparison between qmax from the isotherm tests and the loading in the collumn test, as well as an explanation why batch test loading is generally lower.

4
In the section on competitive adsorption experiments, it would be valuable to discuss the affinity toward each ion and the selectivity of the zeolite in more detail. This discussion should be supported by comparisons with existing literature and related to the specific characteristics of the zeolite used in this study.

We partly agree with your remark. We fully agree that further discussion with regard to the zeolite properties is needed. Therefore, we added a detailed explanation of the adsorption behaviour depending on the properties of the two natural zeolites in lines 278-289. We also added selectivity comparisons with other references in line 255 onwards. However, we have excessively discussed the selectivity of the zeolites based on the respective ion properties (Table 2) in section 3.

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

No comment

Reviewer 3 Report

Comments and Suggestions for Authors

The authors have adequately addressed my comments, and I recommend acceptance of the manuscript in its present form.