Utilization of Phosphogypsum as Sustainable Adsorbent for Removal of Crystal Violet Dye from Wastewater: Kinetics, Thermodynamics, and Applications in Textile Effluent Treatment

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The comments are in attached file

Comments for author File: Comments.pdf

Comments on the Quality of English Language

The English of the manuscript must be checked.

Author Response

• The English of the manuscript must be checked.

Response:

Thank you for your suggestion. The manuscript has been thoroughly checked for English language accuracy, and necessary revisions have been made to improve clarity, grammar, and readability. We have conducted a proofreading via MDPI Author Services (attached English editing Certification).

 

• The introduction lacks structure, and it is recommended that the author provide an overall situation and some key results to demonstrate the depth of the analysis. And provide the contribution of this article. The authors can emphasize the research work with a detailed explanation of the whole process, considering past, present, and future scope. It needs to be strengthened in terms of recent research in this area, with possible research gaps. Explain why this research is important.

Response:

Thank you for your suggestion. The introduction section was modified and re-written.

 

• Should authors discuss the feasibility/industrial application of this process in more detail? Justify the usability of this process in industry, where the processes are continuous at the expense of large quantity.

Response:

We thank the reviewer for their feedback. In this study, we utilized phosphogypsum waste, a low-cost and readily available byproduct, making it a strong economic competitor to other adsorbents. In future work, we will conduct a comprehensive cost analysis to further validate the economic feasibility of this approach.

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• How many parallel experiments did the authors make? Are the results the only ones or means or what? It is recommended to insert the error bar into the figures where the error bar can be inserted.

Response:

We thank the reviewer for raising this important point. To ensure statistical reliability:

1. All experiments were conducted in triplicate, and the results presented in the figures represent the mean values of these replicates.
2. Error bars (standard deviation) have now been added to all relevant figures (e.g., adsorption capacity vs. time, removal efficiency vs. pH) to reflect data variability

 

• To model the adsorption isotherm, the authors use Langmuir and Freundlich models, two simple models that are limited in the interpretation of the results. There are many advanced models that can better fit the adsorption isotherms (10.1039/C7RA08387D https://doi.org/10.1016/j.molliq.2016.07.011). Moreover, the linearization of the equations in the fit gives relatively systematic errors which can be avoided by nonlinear fit.

Response:

We appreciate the reviewer’s suggestion. In our study, we applied the Langmuir, Freundlich, and Temkin models to describe the adsorption behavior. The Langmuir model (R² = 0.996) provided the best fit, indicating monolayer adsorption. The Temkin model (R² = 0.93) offered additional insights into adsorption energy distribution, while the Freundlich model (R² = 0.842) suggested some surface heterogeneity.

While advanced models exist, our focus is on fundamental adsorption behavior rather than complex fittings. Regarding linearization, while nonlinear methods can reduce systematic errors, linearized forms remain widely accepted for their simplicity and ease of comparison with literature. Given the strong fit of the Langmuir model, we believe our approach is appropriate.

We thank the reviewer for their valuable input.

 

• The study of adsorption in real situation from wastewater can give competitive adsorption between different constituents (adsorbates), do the authors take account of this?

Response:

Thank you for you valuable comment. In our study, adsorption experiments on real textile effluent showed no significant selectivity, indicating that phosphogypsum effectively adsorbed all pollutants based on their concentrations. This suggests that the adsorption mechanism is not highly specific to crystal violet and can accommodate multiple contaminants simultaneously.

Reviewer 2 Report

Comments and Suggestions for Authors

Well-written article. The content of the article presents the potential of phosphogypsum, a by-product of the phosphoric acid production process, as a cheap and sustainable adsorbent for removing the crystal violet (CV) dye from aqueous solutions.

Comments:
1. The article addresses a very interesting topic within the area of ​​circulatr economy, while realizing the potential of phosphogypsum as a by-product and the possibility of using it as a cheap adsorbent. The area of ​​sustainable development is realized here through economic and ecological aspects
2. Abstract - very well developed - very well described most important research results
3. Literature analysis quite modest, but sufficient and indicating a "research gap", a small number of scientific literature items justify a very specialized thematic scope, which may be important in the interest of scientists in this subject
4. very well developed research methodology
5. please improve the formatting, e.g. formula 2?
6. Please be sure to include a reference to the formulas in the article
7. Please discuss Figure 1 in more detail - it is not enough to write the results are in the figure ... - these are very important results of the analysis
8. Cost-effectiveness, low-cost ??? is it worth using values? Although costs may constitute a separate topic of the article
9. In the summary, please be sure to emphasize whether the aim of the article has been achieved

Comments on the Quality of English Language

English is correct

Author Response

Well-written article. The potential of phosphogypsum, a by-product as a cheap adsorbent for removing the dye.

• The article addresses a very interesting topic within the area of ​​circulatr economy, while realizing the potential of phosphogypsum as a by-product and the possibility of using it as a cheap adsorbent. The area of ​​sustainable development is realized here through economic and ecological aspects.
• Abstract - very well developed - very well described most important research results.
• Literature analysis quite modest, but sufficient and indicating a "research gap", a small number of scientific literature items justify a very specialized thematic scope, which may be important in the interest of scientists in this subject.
• Very well developed research methodology.
• Please improve the formatting, e.g. formula?
• Please be sure to include a reference to the formulas in the article.
• Please discuss Figure 1 in more detail - these are very important results of the analysis.

Response:

Thank you for your valuable comment and constructive feedback. We appreciate your recognition of the study's relevance to the circular economy and sustainable development, as well as your positive remarks on the abstract and research methodology. Regarding your suggestions:

• The formatting has been carefully reviewed, and necessary improvements have been made, including refining the presentation of formulas.
• References to the formulas have been incorporated throughout the manuscript for clarity and coherence.

• More details about XRD pattern (became Figure 2) were added
• We have conducted a proofreading via MDPI Author Services (attached English editing Certification).

 

 

 

• Cost-effectiveness, low-cost? Is it worth using values? Although costs may constitute a separate topic of the article.

Response:

Thank you for your comment. The cost-effectiveness of phosphogypsum as an adsorbent is an advantage due to its abundant availability as an industrial by-product. Unlike commercial adsorbents such as activated carbon, phosphogypsum has negligible raw material expenses, making it a promising low-cost alternative for wastewater treatment. While a detailed economic analysis is beyond the scope of this study, the results highlight its potential for large-scale applications, especially in regions where phosphogypsum waste management is a concern. Future studies can further assess operational costs, regeneration efficiency, and cost per unit of pollutant removal

 

• In the summary, please be sure to emphasize whether the aim of the article has been achieved.

Response:

Thank you for your valuable comment. This conclusion section has been modified to explicitly emphasize that the study's aim has been achieved, aligning with the reviewer’s suggestions.

 

 

Reviewer 3 Report

Comments and Suggestions for Authors

Thank you for submitting this work. I think it is an excellent fit for “Sustainability”. Please find more specific comments to further improve this work below:

-“large quantities worldwide, with limited utilization, leading to significant disposal challenges” – there are approximately 300 million tons of phosphogypsum produced every year – this really is a large amount (https://doi.org/10.1016/j.jclepro.2023.137561) strengthening your proposed approach of using this material.

-I find the idea to use phosphogypsum for wastewater treatment fascinating. The review above mentions that phosphogypsum can be washed and then better used as a construction material because some toxic materials are essentially washed out. This leads to my first question if the wastewater that you want to treat might receive other substances from the phosphogypsum if these volatile components (I am thinking for instance of P) are washed out?

-a lot of phosphogypsum in Morocco is currently discharged in water bodies, and this study (https://doi.org/10.1007/s10967-023-09079-w) claims for instance that this works fairly well since the phosphogypsum is soluble in seawater...could this become an issue of phosphogypsum is to be used for wastewater treatment?

-4.65 P2O5 seems very high. Maybe also do analysis using ICP-MS if this is possible.

-Fig. 4 looks interesting. Could you describe these results in more detail and provide an explanation? I think this would be really helpful.

Author Response

1. “large quantities worldwide, with limited utilization, leading to significant disposal challenges” – there are approximately 300 million tons of phosphogypsum produced every year – this really is a large amount (https://doi.org/10.1016/j.jclepro.2023.137561) strengthening your proposed approach of using this material.
2. Find the idea to use phosphogypsum for wastewater treatment fascinating. The review above mentions that phosphogypsum can be washed and then better used as a construction material because some toxic materials are essentially washed out. This leads to my first question if the wastewater that you want to treat might receive other substances from the phosphogypsum if these volatile components (I am thinking for instance of P) are washed out?
3. A lot of phosphogypsum in Morocco is currently discharged in water bodies, and this study (https://doi.org/10.1007/s10967-023-09079-w) claims for instance that this works fairly well since the phosphogypsum is soluble in seawater...could this become an issue of phosphogypsum is to be used for wastewater treatment?

Response for 1 -3:

Thank you for your valuable comments and references.

We acknowledge the 300 million tons/year global phosphogypsum production, highlighting the urgency of sustainable applications like wastewater treatment. This point will be emphasized in the revised manuscript.

Regarding potential leaching, our XRF analysis confirms that phosphogypsum mainly consists of CaSO₄·2H₂O (91%), with minor impurities. Our experiments with real textile wastewater showed no unexpected contamination, but we agree that further leaching tests could ensure environmental safety.

While phosphogypsum is soluble in seawater, its use as an adsorbent in wastewater treatment relies on surface interactions, not dissolution. The Langmuir isotherm fit and pseudo-second-order kinetics confirm adsorption rather than dissolution. However, we acknowledge that long-term stability requires further study.

These clarifications were added into the manuscript.

We have conducted a proofreading via MDPI Author Services (attached English editing Certification).

 

 

 

4. 65 P2O5 seems very high. Maybe also do analysis using ICP-MS if this is possible.

Response:

Thank you for your comment. The P₂O₅ content of phosphogypsum was confirmed through atomic absorption analysis. Its relatively high value may indicate insufficient washing during the phosphoric acid production process

 

5. 4 looks interesting. Could you describe these results in more detail and provide an explanation?

Response:

Thank you for your valuable comment. More details were added to explain the Zeta potential results (Fig.4).

Reviewer 4 Report

Comments and Suggestions for Authors

In the work the phosphogypsum (91% Calcium Sulfate Di-Hydrate with 3.34 % silica

and 4.65 % P2O5) was tested as adsorbent for cation dye. The sorption capacity increases as the solution pH increased up to pH 5. Increasing the initial dye concentration increased the uptake capacity but decreased the removal efficiency. The sorption results fit by Langmuir model, which meant the sorption process, is chemically adsorption. The equilibrium adsorption was achieved after 60 min. The pseudo second- order model was fit well the results which demonstrated higher sorption on the exterior surface rather than in the pores. The adsorption capacity is inversely proportional to temperature which indicates that the adsorption is an exothermic process. The thermodynamics parameters showed that the reaction is exothermic, and it is higher than 70kJ/mol; indicates the chemically controlled process. The entropy is negative indicates the decreasing of randomness at the solid/liquid interface during the adsorption. The free energy is negative at 20°C and 30°C which indicate that this adsorption process is spontaneous in nature. The increase in the free energy value with increasing temperature indicates that the adsorption process becomes less favorable at high temperatures.

The regeneration of the adsorbent showed that the adsorbent can be reuse. The removal efficiency was reduced to 71% after the 5th reuse cycle. The phospho-gypsum was employed to treat textile effluent. It could reduce BOD by 71% while zinc was reduced by 91% and the coloring materials (dyes) were reduced by 87%.

The paper content valuable experimental data but it could be published after minor revision as below.

 

General Remarks

 

Abstract

Please do not use abbreviation in abstract and keyword or provide the full name.

 

Phosphogypsum is the hazardous waste containing many impurities (heavy metals, radioactivity. Its use as adsorbent is original, but more information justifying this choice should be added.

 

Detailed remarks

 

1. Tables 3,4,5 should be self-explaining. Please add proper information/changes.
2. Table 1 could be removed, and its contentment incorporated into the text.
3. Figures 2,3,6,8,10 should be self-explaining. Please add proper information/changes.

Author Response

General Remarks

1. Please do not use abbreviation in abstract and keyword or provide the full name.

Response:

Thank you for your valuable comment. We have revised the abstract and keywords to either remove abbreviations or provide their full names to ensure clarity and readability. We have conducted a proofreading via MDPI Author Services (attached English editing Certification).

 

 

2. Phosphogypsum is the hazardous waste containing many impurities (heavy metals, radioactivity. Its use as adsorbent is original, but more information justifying this choice should be added.

Response:

Thank you for your valuable comment. Although phosphogypsum contains impurities such as heavy metals and traces of radioactivity, its use as an adsorbent is justified because these impurities exhibit low solubility under the experimental conditions. Additionally, as mentioned in the introduction, phosphogypsum has been reported as an effective adsorbent for heavy metals such as cadmium, copper, and zinc from aqueous solutions (Balkaya & Cesur, 2008; Es-Said et al., 2020). It has also been successfully used for the removal of various organic dyes, including acid red and malachite green, from wastewater (Ghouaybi et al., 2023; Panda et al., 2024).

 

3. Tables 3,4,5 should be self-explaining. Please add proper information/changes.
4. Figures 2,3,6,8,10 should be self-explaining. Please add proper information/changes.

Response for 3 & 4:

Thank you for your insightful comments. We have revised Tables 3, 4, and 5 to include additional details. We have updated Figures 2, 3, 6, 8, and 10 with appropriate labels and captions. Additionally, we have included clarifications of the experimental conditions in the figure captions to enhance their comprehensibility.

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

I am satisfied with the authors corrections

Reviewer 3 Report

Comments and Suggestions for Authors

Comments have been addressed. This is largely fine with me with small things I would change that are really at the authors discression.

Comments on the Quality of English Language

Sometimes the language can be improved - this is not a major issue though - everything can be understood as it is.