Eco-Friendly Utilization of Phosphogypsum via Mechanical Activation for Sustainable Heavy Metal Removal from Wastewater

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This article presents an eco-friendly method to use phosphogypsum waste material from phosphate production for efficient heavy metal extraction from wastewater. It applies mechanical activation through grinding to improve phosphogypsum's properties by converting gypsum into anhydrite while increasing surface area and reducing pore size and modifying zeta potential for better adsorption performance. Also, characterization methods including X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, and Brunauer–Emmett–Teller BET analysis was adopted. However, certain points must treat:

• The manuscript is well-written but contains minor grammatical errors and awkward phrasing in some sections.
• The adsorption capacity values should be compared with literature data to contextualize the improvements made through mechanical activation.
• The study presents R² values for adsorption models but does not include statistical validation like error bars, ANOVA, or confidence intervals to confirm the reliability of experimental data.
• Some figures, such as XRD and XPS spectra, need better labeling for improved readability.

Comments on the Quality of English Language

• The manuscript is well-written but contains minor grammatical errors and awkward phrasing in some sections.

Author Response

1. The manuscript is well-written but contains minor grammatical errors and awkward phrasing in some sections.

Response:

We thank the reviewer for their positive feedback and for pointing out the minor grammatical errors and awkward phrasing in the manuscript. We have conducted a proofreading via MDPI Author Services (attached English editing Certification).

 

2. The adsorption capacity values should be compared with literature data to contextualize the improvements made through mechanical activation.

Response:

Thank you for your comment, the adsorption capacity values were compared with literature data and presented in Table 4.

 

3. The study presents R² values for adsorption models but does not include statistical validation like error bars, ANOVA, or confidence intervals to confirm the reliability of experimental data.

Response:

Thank you for your comment. All figures were modified to show the error bars to confirm the reliability of experimental data.

 

 

4. Some figures, such as XRD and XPS spectra, need better labeling for improved readability.

Response:

We thank the reviewer for their feedback on improving the readability of the figures. We have revised the XRD and XPS spectra and other figures by enhancing the labeling, including clearer axis titles, legends, and annotations.

 

Reviewer 2 Report

Comments and Suggestions for Authors

The present manuscript details a utilization process of phosphogypsum induced by mechanical activation achieving effective Cd and Pb contaminants removal from wastewater. It seems that these technologies could obtain a remarkable performance for the removal of the heavy metal via adsorption. The topic is meaningful and the research data is credible. However, certain issues should be addressed to improve the quality of paper. The comments are listed in the attachment.

Comments for author File: Comments.pdf

Comments on the Quality of English Language

Ensure consistent formatting for chemical notations (such as CaSO4.2H2O, 1.0×10-3, 1.0^-9, SO42-...). The entire manuscript needs to be checked for English writing tenses.

Author Response

1. Introduction: With regard to the literature on heavy metal removal and comprehensive phosphogypsum utilization, a rudimentary list of references has been merely provided. However, a more thorough analysis is required to elucidate the full implications, deficiencies or difficulties mentioned therein. The current writing is inadequate for adequately conveying the significance and innovation of the research in this manuscript. Furthermore, the cited literature is outdated and does not reflect recent research developments.

Response:

We thank the reviewer for their insightful feedback regarding the introduction section. We acknowledge that the current discussion on heavy metal removal and phosphogypsum utilization does not fully highlight the significance and innovation of our research. Additionally, we agree that the cited literature should be updated to reflect recent advancements in the field.

In the revised manuscript, we have:

1. Expanded the literature review to provide a more thorough analysis of the challenges, deficiencies, and recent developments in heavy metal removal and phosphogypsum utilization.
2. Updated the references to include the most recent and relevant studies, ensuring that the discussion reflects current research trends.
3. Clearly expressed the innovation of our work, emphasizing the use of mechanically activated phosphogypsum as a sustainable and cost-effective solution for heavy metal removal, and its potential to address existing limitations in the field.

 

In addition, we have conducted a proofreading via MDPI Author Services (attached English editing Certification).

 

2. Materials and methods: In the subsequent study, the SEM technique was utilized; however, its depiction is excluded from this section. Moreover, it is not essential to elaborate on the precise parameters of a standard characterization technique (such as XRD, XPS, …), but rather, emphasis should be placed on salient aspects. The information (such as model number, manufacturer, …) for some of the instruments and materials does not appear to be listed in full.

Response:

Thank you for your comments, I’ve revised the Materials and Methods section to:

1. Remove unnecessary details about standard characterization techniques (e.g., XRD, XPS).
2. Add missing information about instruments and materials (e.g., model numbers, manufacturers).
3. Ensure the section is concise and focuses on salient aspects.

 

 

 

3. Result and discussion: Figure and Table: Firstly, the presence of an excessive number of tables and figures is observed, with the recommendation being made that non-essential charts and tables be relegated to the annexes of supporting documentation. Secondly, the format of the tables is found to be non-uniform, with garbled codes and unclear expressions noted. Errors have been identified in some figures, such as the designation "A" in Fig. 2 and the omission of words in Fig.7. It is imperative that these errors are rectified.

Response:

We thank the reviewer for their valuable feedback regarding the presentation of figures and tables in the Results and Discussion section. We have carefully reviewed the section and made all the reviewer recommendations.

 

4. Finally, it is recommended that the experimental conditions be listed in detail under the title of the figure, and that they not be included in the figure itself.

Response:

We thank the reviewer for their valuable feedback. The Figures were modified and we listed the experimental conditions under the title of the figure.

 

5. Experimental condition: The detailed experimental conditions or procedure (Line 205-207, Line 219-221, ...) should not be included in this section; rather, they can be incorporated into "2. Materials and methods" or listed in the figure’s caption.

Response:

6. We thank the reviewer for their valuable suggestion. As recommended, we have moved the detailed experimental conditions and procedures (Lines 205-207, 219-221, etc.) from the results section to Section 2 (Materials and Methods)

 

 

7. XRD analysis: At high grinding speeds, how to solve the problem of temperature control during the mechanical activation process? As a reminder, the localized high temperature of on the surface of the particles may be the real cause of the conversion of CaSO4·2H2O to CaSO4 (dehydration in the lattice).

Response:

Thank you for this valuable comment. We agree with the reviewer that the conversion of CaSO₄·2H₂O to CaSO₄ during high-speed grinding is likely due to localized high temperatures generated during the process, causing dehydration. While no specific temperature control measures were implemented in our study, the observed phase transformation highlights the significant impact of mechanical energy on the material’s structure. Future work could explore temperature regulation (e.g., cooling systems) to better control this transformation.

 

8. Line 134-135： This statement seems to be inconsistent with the results presented in Figure 2.

Response:

Thank you for your comment. We have revised the statement to better align with the data.

 

 

9. Adsorption study: Although the authors obtained extensive data and plotted graphs accordingly, they did not fully utilize these date and graphs during the discussion session, instead presenting their conclusions immediately.

Response:

We thank the reviewer for their feedback. In the revised manuscript, we have expanded the discussion to thoroughly analyze all data and graphs, linking them to the adsorption mechanisms and performance. This ensures a more comprehensive interpretation and better supports the conclusions.

 

10. Innovation: This paper has employed the phosphogypsum induced by mechanical activation to treat Cd and Pb pollutants. The research paper should reflect the innovation. The authors are advised to condense the innovative features according to their own research.

Response:

We thank the reviewer for their valuable feedback. The innovation of this study, focusing on the use of mechanical activation to enhance phosphogypsum’s adsorption capacity, has been explicitly highlighted in both the Introduction and Conclusion sections, as suggested.

 

 

11. Conclusion: Further condense this section to focus solely on key findings. The last sentence (Line 396-398) can be removed.

Response:

We thank the reviewer for their suggestion. The conclusion section has been condensed to focus solely on the key findings, as requested.

 

12. 11 Formatting Issue: Ensure consistent formatting for chemical notations (such as CaSO4.2H2O, 1.0×10-3, 1.0^-9, SO42-...).

Response:

Thank for your comment. The formatting for chemical notations has been corrected.

 

Reviewer 3 Report

Comments and Suggestions for Authors

Review Comments

Manuscript: “Eco-Friendly Utilization of Phosphogypsum via Mechanical Activation for Sustainable Heavy Metal Removal from Wastewater.”

General Comments

5. The manuscript reports a study on heavy metals removal from wastewater via mechanically modified phosphogypsum. The mechanically activation was achieved via intensive grinding using a planetary ball mill. The authors characterized the modified phosphogypsum by analyzing the surface area, and zeta potential using X-ray diffraction, Brunauer–Emmett–Teller analysis, zeta potential measurements, X-ray photoelectron spectroscopy, and scanning electron microscopy. The authors have shown that there was an increased in surface area from the unmodified adsorbent from 5.8 m²/g to 17.8 m²/g, and decreased pore radius from 76.6 nm to 9.3 nm. The authors demonstrated that heavy metals such as Pb and Cd with removal efficiency of ~90% for Cd and ~80% for Pb.

Specific comments that illustrate the concerns with the manuscript, and provide suggestions for improving the study, are provided below with the intent of assisting the authors to improve their study:

1. The manuscript is well written. Nonetheless, column study will be helpful in understanding the real applicability of such process.
2. The authors also claimed that the proposed approach is economically efficient for the removal of heavy metals from wastewater. However, there is no technoeconomic evaluation of the approach.
3. One of the concern from employing such approach is the issue chemical waste and high concentrated waste generated when the adsorbents have been exhausted. There is also the environmental and regulatory concern regarding the exhausted adsorbents.
4. In section 3.2.6, the authors also showed that the maximum adsorbent capacity was observed at a temperature of 70 ^oC. What is the practicality of employing such adsorbent since most wastewater is at room temperature. What is the impact of increasing temperature to get the maximum efficiency relative to overall system cost?
5. The authors have shown a good removal efficiency with respect to heavy metals. However, the concentrations of the treated wastewater still contain a concentration higher than allowable concentrations.

Author Response

1. The manuscript is well written. Nonetheless, column study will be helpful in understanding the real applicability of such process.

Response:

We thank the reviewer for their positive feedback and valuable suggestion. We agree that column studies would enhance the practical applicability of our findings. While the current study focuses on batch experiments to establish fundamental mechanisms, we plan to conduct column studies in future work to evaluate the process under dynamic, real-world conditions.

In addition, we have conducted a proofreading via MDPI Author Services (attached English editing Certification).

 

 

2. The authors also claimed that the proposed approach is economically efficient for the removal of heavy metals from wastewater. However, there is no techno economic evaluation of the approach.

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.

 

3. One of the concerns from employing such approach is the issue chemical waste and high concentrated waste generated when the adsorbents have been exhausted. There is also the environmental and regulatory concern regarding the exhausted adsorbents.

Response:

We thank the reviewer for their comment. We agree with the concerns regarding environmental and regulatory issues related to exhausted adsorbents. To address this, we propose regeneration and reuse of the adsorbent to minimize waste. Additionally, metal recovery (Cd and Pb) can be considered to further enhance sustainability.

 

4. In section 3.2.6, the authors also showed that the maximum adsorbent capacity was observed at a temperature of 70° What is the practicality of employing such adsorbent since most wastewater is at room temperature. What is the impact of increasing temperature to get the maximum efficiency relative to overall system cost?

Response:

Thank you for this valuable feedback. This study showed that adsorption efficiency increases with temperature; however, the process remains effective at room temperature. The results indicate that at room temperature, adsorption efficiency decreases by approximately 12% for cadmium and 27% for lead compared to the efficiency observed at 70°C. While higher temperatures enhance adsorption, the material can still be used effectively under normal operating conditions, making it a practical and cost-effective option for wastewater treatment.

 

5. The authors have shown good removal efficiency with respect to heavy metals. However, the concentrations of the treated wastewater still contain a concentration higher than allowable concentrations.

Response:

Thank you for your comment. Although the treated water may still contain heavy metal concentrations above permissible limits, these levels can be further reduced by implementing an additional treatment cycles. This approach can help achieve compliance with regulatory standards while maintaining the efficiency of the process.

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors The questions have been addressed. Should be ready for publications.