Removal of Fluoride from Phosphogypsum Leaching Solution with Phosphate Tailing Based Layered Double Hydroxides: Kinetics and Equilibrium Isotherms
, Zhenyue Zhang 4, Lulu Xu 1, Qingrong Cheng 1, Hong Zhou 1, Junxia Yu 1, Zhiquan Pan 1 and Dongsheng Wang 5
Anjaneyulu Putta
George William Kajjumba
Round 1
Reviewer 1 Report
Yanming Liu et al reported a research article " Removal of fluoride from phosphogypsum leaching solution with phosphate tailing based double layered hydroxides : Kinetics and equilibrium isotherms". Overall the article is well written with meaningful conclusions, should be considered to publish in the journal "Minerals".
Author Response
We wish to thank the reviewers for the valuable comments on our paper. Our manuscript entitled “Removal of fluoride from Phosphogypsum leaching solution with phosphate tailing based layered double hydroxides: Kinetics and Equilibrium Isotherms”, was carefully revised according to your comments. The comments and suggestions are incorporated in the revision of the manuscript. The changes in the manuscript have also been marked in red color for your evaluation. The itemized response to each reviewer’s comments is attached. Many thanks for your suggestions.
Yanming Liu et al reported a research article “Removal of fluoride from phosphogypsum leaching solution with phosphate tailing based double layered hydroxides : Kinetics and equilibrium isotherms”. Overall the article is well written with meaningful conclusions, should be considered to publish in the journal "Minerals".
Response: We are really grateful to you for the valuable advices and comments on our paper.
Reviewer 2 Report
I thank the authors for their contribution to the understanding of fluoride removal. Below are some of the comments that need to be addressed before the paper is published
Abstract
1. Define XRD, SEM, FTIR, and BET in full—first-time appearance; otherwise, remove them.
2. Rewrite “PTB-LDO3 exhibited better adsorption performance when the pH value was ranged from 5 to 6, and dosages decided at 0.04 mg L-1”. Maybe: PTB-LDO3 exhibited better adsorption performance at pH 5-6 and a dosage of 0.04 mg L-1.
3. Confirm the unit style; should there be space between mg and L-1?
Introduction
1. “Studies have shown that the fluorine concentration in the groundwater near the phosphogypsum dump site has exceeded the standard by several times as the research says [3]. Write the sentence and mention the reported fluoride concentration in the groundwater.
2. “Severe fluorosis can lead to bone abnormalities, damage to the nerve center and brain, and even be harmful to the reproductive system…” The term fluorosis applies to only teeth. Damage to the central nervous system, brain, and reproductive system.
3. References 4-6 never investigated the effect of fluoride on humans; their research is about fluoride removal. Find references that have investigated the effect of fluoride on the nervous system, and brain, …
4. “Therefore, it is urgent to difluoride the phosphogypsum leaching solution.” Reduce the tone of the sentence “urgent??” Maybe say: there is a need to difluoride….
5. How is reference 7 related to the electrocoagulation of fluoride. Please cite the right references
6. References 8 and 9 are about sorption of fluoride not reverse osmosis and ion exchange
7. “Among these methods, adsorption has attracted extensively attention because of its mature development, low cost, simple operation, environmental-friendly and high efficiency [11].” This is wrong, adsorption is not a cheap technology, it depends on material—besides you need to process the adsorbents. Adsorption is not easy to operate compared to other technologies like coagulation in water treatment. High efficiency—based on what? Environmentally friendly—no, all the adsorbents like acidic alumina and iron oxides must be mined. The amount of energy expended in their making makes adsorption expensive; co-precipitation is an expensive process
8. “…iron oxides and ores [13],” ores are not adsorbents
9. Check the grammar of the paragraph that starts with “Layered double hydroxides (LDHs) is a brucite-like…”
10. …thermodynamics were “investivated” to understand the adsorption mechanisms. It is investigated
Experimental design
11. Change the heading of the chapter2 to experimental plan. Experimental is an adjective that must relate to something
Synthesis
12. Why do you adjust pH with ammonia and later with NaOH? What could happen if you used NaOH alone? What form of ammonia was used; NH4OH?
13. …” the filtrate was neutral, dried in vacuum at 65 oC, and the PTB-LDH3 and PTB-LDH4 was obtained.” Consider changing it to “…the filtrate was neutral, then vacuum dried at 60 oC to obtain PTB-LDH3 and PTB-LDH4. The resulting materials were placed in a tubular furnace and calcinated at 400 oC for 4 hours to obtain PTB-LDO3 and PTB-LDO4.
Characterization
14. “For the adsorbents adsorbed on the surface...” Adsorption can take place on the surface or in pores. Consider: For the adsorbents adsorbed, specific surface area and …
15. “…was used to understand the internal functional groups and effective substances of the samples.” The word “effective substances” sound wrong in this sentence
Adsorption
16. Why did you choose 298,308, and 318 K temperatures? Operators may want to know the performance of your product in the winter and summer seasons; however, your temperatures don’t represent these conditions
SEM analysis
17. Rewrite the entire paragraph, use present tense or simple past tense. “SEM was used to understand the micro-and nanoparticle imaging characterization of hydrotalcite products, Figure 1. PTB-LDH3 (Figure 1a) and PTB-LDH4 (Figure 1b) elicit multi-layer scale shapes, which are in accordance with their lamellar structure. After calcination, LDHs lost their interlamellar water molecules and anions, causing the lamellar structure to collapse forming LDOs (Figures 1 c and d). Comparing LDHs and LDOs, the scale shapes in LDHs disappeared.” Please present or simple past tense in your description.
18. Figure 1a-b and Figure 1c-d are on different magnification scales; how can you compare them?
19. How do you know that water molecules were lost without EDS analysis?
20. Which anions were lost?
BET analysis
21. Table 1: maintain the same decimal points or significant figures in a given column. Do this to all tables
22. Remove the text between Table 1 and Figure 2
FTIR analysis
23. In the SEM analysis it was speculated that H2O molecules were lost but in FTIR analysis the size of the OH peak did not significantly change (Figure 3). Check your explanation
24. In the SEM analysis, it was cited that anions were lost—but in FTIR analysis, nitrate and carbonate anions were incorporated into the interlayer. Which correct?
XRD analysis
25. Make sure the description matches the previous explanations
pH effect
26. Grammar is bad
Kinetics
27. Cite the original authors of pseudo-first and second order, not 35 and 36
28. Equations 3 and 4 are linear forms but Figure 7 shows nonlinear forms. Change equations to nonlinear forms.
29. Cite the original author of the intra-particle diffusion model
30. “…PTB-LDOs was more consistent with the pseudo-first-order kinetic equation.” Did you run the statistical significance between the pseudo-first/second-order and experimental data? The chances are high that they are not significantly different at ρ = 0.05. So you can’t use R as a concluding factor to say that the first order is better. Keep in mind that there are a lot of mistakes in the literature
Isotherms
31. Cite the original authors of D-R, Temkin, Langmuir and Freundlich isotherms
32. Check the spelling of Freundlich isotherm
33. Define D-R
34. It is well-known that logarithmic expressions do not have any unit. Eq. (8) is used for gases. In this study, for aqueous solutions, how could the authors calculate the epsilon values?? In literature, many article results published about the Dubinin-Radushkevich isotherm model are incorrect. See https://doi.org/10.1016/j.jenvman.2019.06.088 for some clarification
35. In adsorption isotherm experiments, Ce is in a very narrow window. For industrial and practical applications, it should be increased to higher concentrations in order to see the maximum adsorption capacity value or the fitting of experimental data with various isotherm models reliably.
36. These statements given as "Gibbs free energy, entropy, and enthalpy were analysed", "Assessment of Gibbs free energy" and "entropy (<DELTA>S)" are incorrect!!!
Response report
When responding to comments please state the comment, reply, and correction in the main article. Also include line numbering to the article for easy tracking
For example
Comment 31: Cite the original authors of D-R, Temkin, Langmuir and Freundlich isotherms
Response: The original authors have been cited
Modification: Line 321-324: The equations for the two of them are as follows [62, 78].
Author Response
Author’s reply to Editor and Reviews’ comments
We wish to thank the editor and reviewers for the valuable comments on our paper. Our manuscript entitled “Removal of fluoride from Phosphogypsum leaching solution with phosphate tailing based layered double hydroxides: Kinetics and Equilibrium Isotherms”, was carefully revised according to your comments. The comments and suggestions are incorporated in the revision of the manuscript. The changes in the manuscript have also been marked in red color for your evaluation. The itemized response to each reviewer’s comments is attached. Many thanks for your suggestions.
Reviewer 2：
I thank the authors for their contribution to the understanding of fluoride removal. Below are some of the comments that need to be addressed before the paper is published.
Abstract
- Define XRD, SEM, FTIR, and BET in full-first-time appearance; otherwise, remove them.
Response: Many thanks for pointing this out. Corrections have been made.
- Rewrite “PTB-LDO3 exhibited better adsorption performance when the pH value was ranged from 5 to 6, and dosages decided at 0.04 mg L-1”. Maybe: PTB-LDO3 exhibited better adsorption performance at pH 5-6 and a dosage of 0.04 mg L-1.
Response: Sorry for our carelessness. Corrections have been made.
- Confirm the unit style; should there be space between mg and L-1?
Response: Sorry for our carelessness. Corrections have been made.
Introduction
- “Studies have shown that the fluorine concentration in the groundwater near the phosphogypsum dump site has exceeded the standard by several times as the research says [3]. Write the sentence and mention the reported fluoride concentration in the groundwater.
Response: Thanks for your suggestion. Corrections have been made. “……The fluorine concentration in the groundwater near the phosphogypsum dump site were 4.13-32.38 mg L-1, which has exceeded the Quality Standard for Ground Water…….”.
- “Severe fluorosis can lead to bone abnormalities, damage to the nerve center and brain, and even be harmful to the reproductive system…” The term fluorosis applies to only teeth. Damage to the central nervous system, brain, and reproductive system.
Response: Many thanks for pointing this out. Corrections have been made. “….. Severe fluorosis can lead to abnormal teeth, even endanger bone health and renal function in infants and young children [4-6]….”
- References 4-6 never investigated the effect of fluoride on humans; their research is about fluoride removal. Find references that have investigated the effect of fluoride on the nervous system, and brain, …
Response: Many thanks for pointing this out. Corrections have been made. “….. Severe fluorosis can lead to abnormal teeth, even endanger bone health and renal function in infants and young children [4-6]….”
[4] Fallahzadeh, R. A.; Miri, M.; Taghavi, M.; Gholizadeh, M. Anbarani, A. Spatial variation and probabilistic risk assessment of exposure to fluoride in drinking water. Food Chem. Toxicol. 2018, 113. 314-321.
[5] Augustsson, A.; Berger, T. Assessing the risk of an excess fluoride intake among Swedish children in households with private wells-Expanding static single-source methods to a probabilistic multi-exposure-pathway approach. Environ. Int. 2014, 68, 192-199.
[6] Wambu, E. W.; Onindo, C. O.; Ambusso, W.; Muthakia, G. K. Removal of Fluoride from Aqueous Solutions by Adsorption Using a Siliceous Mineral of a Kenyan Origin. Acta Hydroch. Hydrob. 2013, 41(4), 340-348.
- “Therefore, it is urgent to difluoride the phosphogypsum leaching solution.” Reduce the tone of the sentence “urgent??” Maybe say: there is a need to difluoride….
Response: This is a good suggestion. Corrections have been made.
- How is reference 7 related to the electrocoagulation of fluoride. Please cite the right references.
Response: Sorry for our carelessness. Corrections have been made.
[7] Emamjomeh, M. M.; Sivakumar, M. An empirical model for defluoridation by batch monopolar electrocoagulation/flotation (ECF) process. J. Hazard. Mater. 2006, 131 (1-3), 118-125.
- References 8 and 9 are about sorption of fluoride not reverse osmosis and ion exchange.
Response: Sorry for our carelessness. Corrections have been made.
[8] Lhassani, A.; Rumeau, M.; Benjelloun, D.; Pontie, M. Selective demineralization of water by nanofiltration application tO the defluofination ofbrackish water. Water Res. 2001, 35 (13), 3260-3264.
[9] Qureshi, S. Z.; Khan, M. A.; Rahman, N. Removal of fluoride ion by zirconium(IV) arseniate vanadate using ion-selective electrode. Water Treat. 1995, 10, 307-312.
- “Among these methods, adsorption has attracted extensively attention because of its mature development, low cost, simple operation, environmental-friendly and high efficiency [11].” This is wrong, adsorption is not a cheap technology, it depends on material-besides you need to process the adsorbents. Adsorption is not easy to operate compared to other technologies like coagulation in water treatment. High efficiency-based on what? Environmentally friendly-no, all the adsorbents like acidic alumina and iron oxides must be mined. The amount of energy expended in their making makes adsorption expensive; co-precipitation is an expensive process.
Response: Many thanks for pointing this out. Corrections have been made.
- “…iron oxides and ores [13],” ores are not adsorbents.
Response: Sorry for our carelessness. Corrections have been made. We have removed this reference.
- Check the grammar of the paragraph that starts with “Layered double hydroxides (LDHs) is a brucite-like…”
Response: Many thanks for pointing this out. Corrections have been made.
- …thermodynamics were “investivated” to understand the adsorption mechanisms. It is investigated
Response: Sorry for our carelessness. Corrections have been made.
Experimental design
- Change the heading of the chapter2 to experimental plan. Experimental is an adjective that must relate to something
Response: Many thanks for pointing this out. Corrections have been made.
Synthesis
- Why do you adjust pH with ammonia and later with NaOH? What could happen if you used NaOH alone? What form of ammonia was used; NH4OH?
Response: This is a good question. In this work, we used PTs as raw materials to prepare LDHs. When PTs are decomposed by hydrochloric acid, phosphorus in PTs mainly coexists with Ca2+, Mg2+ and Al3+ in solution in the form of PO43-. PO43- easily enters the interlayer of synthetic LDHs and affects the adsorption effect. So it is necessary to purify the acid leaching solution of PTs before obtaining mixed metal salt solution. In the course of our study, it was found that when the pH of acid leaching solution of PTs was adjusted to 6 by ammonia water (NH3·H2O), almost all phosphates in the solution were precipitated in the form of ammonium magnesium phosphate complex salt, while the pH adjustment by sodium hydroxide could not completely remove phosphates in the solution.
- …” the filtrate was neutral, dried in vacuum at 65 oC, and the PTB-LDH3 and PTB-LDH4 was obtained.” Consider changing it to “…the filtrate was neutral, then vacuum dried at 60 oC to obtain PTB-LDH3 and PTB-LDH4. The resulting materials were placed in a tubular furnace and calcinated at 400 oC for 4 hours to obtain PTB-LDO3 and PTB-LDO4.
Response: Many thanks for pointing this out. Corrections have been made.
Characterization
- “For the adsorbents adsorbed on the surface...” Adsorption can take place on the surface or in pores. Consider: For the adsorbents adsorbed, specific surface area and …
Response: Many thanks for pointing this out. Corrections have been made.
- “…was used to understand the internal functional groups and effective substances of the samples.” The word “effective substances” sound wrong in this sentence
Response: Sorry for our carelessness. Corrections have been made. “….. Fourier transform infrared spectroscopy (FT-IR, Vertex 70, Bruker, Optic, Inc. USA) was used to understand the surface functional groups and interlayer anions of the samples”.
Adsorption
- Why did you choose 298,308, and 318 K temperatures? Operators may want to know the performance of your product in the winter and summer seasons; however, your temperatures don’t represent these conditions
Response: This is a good question. In this work, We mainly choose these three temperatures (298,308, and 318 K) to study the isothermal adsorption characteristics and adsorption thermodynamic parameters of the PTB-LDO3 and PTB-LDO4. Indeed, the adsorption properties of materials in winter and summer seasons are very necessary to study their application possibility in actual real environment. In our future work, we will further study the possibility of practical application of materials.
SEM analysis
- Rewrite the entire paragraph, use present tense or simple past tense. “SEM was used to understand the micro-and nanoparticle imaging characterization of hydrotalcite products, Figure 1. PTB-LDH3 (Figure 1a) and PTB-LDH4 (Figure 1b) elicit multi-layer scale shapes, which are in accordance with their lamellar structure. After calcination, LDHs lost their interlamellar water molecules and anions, causing the lamellar structure to collapse forming LDOs (Figures 1 c and d). Comparing LDHs and LDOs, the scale shapes in LDHs disappeared.” Please present or simple past tense in your description.
Response: Many thanks for pointing this out. Corrections have been made.
- Figure 1a-b and Figure 1c-d are on different magnification scales; how can you compare them?
Response: Sorry for our carelessness. Corrections have been made.
- How do you know that water molecules were lost without EDS analysis?
Response: In this work, PTB-LDOs was products of PTB-LDHs calcined at 400 oC. When hydrotalcite is calcined at 400 °C, interlayer and structural water will be lost and converted to LDO [1].
- Which anions were lost?
Response: Considering that this absorption peak was caused by the vibration of CO32- and NO3- anions intercalated between layers [29]. After calcination, the interlayer anions of hydrotalcite disappear.
[29] Zaghloul, A.; Benhiti, R.; Ait Ichou, A.; Carja, G.; Soudani, A.; Zerbet, M.; Sinan, F.; Chiban, M. Characterization and application of MgAl layered double hydroxide for methyl orange removal from aqueous solution. Mater. Today: Proc. 2021, 37, 3793-3797.
BET analysis
- Table 1: maintain the same decimal points or significant figures in a given column. Do this to all tables
Response: Many thanks for pointing this out. Corrections have been made.
- Remove the text between Table 1 and Figure 2
Response: This is a good suggestion. Corrections have been made.
FTIR analysis
- In the SEM analysis it was speculated that H2O molecules were lost but in FTIR analysis the size of the OH peak did not significantly change (Figure 3). Check your explanation.
Response: Many thanks for pointing this out. Corrections have been made. “….. After calcination, layered double hydroxides were transformed to mixed metal oxides, causing the lamellar structure collapsed……”.
- In the SEM analysis, it was cited that anions were lost-but in FTIR analysis, nitrate and carbonate anions were incorporated into the interlayer. Which correct?
Response: PTB-LDOs was products of PTB-LDHs calcined at 400 oC. Nitrate and carbonate anions mainly exist between hydrotalcite layers to balance positive charges. After calcination, layered double hydroxides were transformed to mixed metal oxides, causing the lamellar structure collapsed. In the meantime, there was no anions in PTB-LDOs structure.
XRD analysis
- Make sure the description matches the previous explanations
Response: Many thanks for pointing this out. Corrections have been made.
pH effect
- Grammar is bad
Response: Sorry for our carelessness. Corrections have been made.
Kinetics
- Cite the original authors of pseudo-first and second order, not 35 and 36
Response: This is a good suggestion. Corrections have been made.
- Equations 3 and 4 are linear forms but Figure 7 shows nonlinear forms. Change equations to nonlinear forms.
Response: Many thanks for pointing this out. Corrections have been made.
- Cite the original author of the intra-particle diffusion model
Response: This is a good suggestion. Corrections have been made.
- “…PTB-LDOs was more consistent with the pseudo-first-order kinetic equation.” Did you run the statistical significance between the pseudo-first/second-order and experimental data? The chances are high that they are not significantly different at ρ = 0.05. So you can’t use R as a concluding factor to say that the first order is better. Keep in mind that there are a lot of mistakes in the literature
Response: Many thanks for pointing this out. Corrections have been made. “….. The model fittings were shown in Figures 7a. and 7b, and the corresponding kinetic parameters for the adsorption of fluoride ions by PTB-LDO3 and PTB-LDO4 were listed in Table S1 in Support Information (SI). It was found that the pseudo-first-order kinetic model exhibited a higher correlation coefficient (R2) for both PTB-LDO3 and PTB-LDO4. Moreover, qe calculated by pseudo-first-order for both PTB-LDO3 and PTB-LDO4 were all very close to the experimental data, which indicated that fluoride ions adsorption reaction with PTB-LDO3 and PTB-LDO4 could be properly described by pseudo-first-order kinetic model……”.
Isotherms
- Cite the original authors of D-R, Temkin, Langmuir and Freundlich isotherms
Response: This is a good suggestion. The original authors have been cited. The equations for the two of them are as follows [38-41].
- Check the spelling of Freundlich isotherm
Response: Sorry for our carelessness. Corrections have been made.
- Define D-R
Response: Many thanks for pointing this out. Corrections have been made. D-R model was based on Dubinin and Radushkevich’s model for studying micropore porosity of activated carbon.
- It is well-known that logarithmic expressions do not have any unit. Eq. (8) is used for gases. In this study, for aqueous solutions, how could the authors calculate the epsilon values?? In literature, many article results published about the Dubinin-Radushkevich isotherm model are incorrect. See https://doi.org/10.1016/j.jenvman.2019.06.088 for some clarification
Response: Many thanks for pointing this out. Corrections have been made. “……The value of E based on D-R model can provide correlative information about the adsorption mechanism, especially the type of adsorption of fluoride ions on PTB-LDO3 and PTB-LDO4 can be determined [43]: (1) E < 8 kJ mol1, physical adsorption prevails; (2) 8 kJ mol1 < E < 16 kJ mol1, ion exchange adsorption dominates; (3) E > 16 kJ mol1, the interaction may be chemical adsorption. It can be seen from Table S2 that the adsorption energy under all conditions was below 8 kJ/mol, which indicated that removal of fluoride ions by PTB-LDO3 and PTB-LDO4 might depend on physical interaction……..”.
- In adsorption isotherm experiments, Ce is in a very narrow window. For industrial and practical applications, it should be increased to higher concentrations in order to see the maximum adsorption capacity value or the fitting of experimental data with various isotherm models reliably.
Response: This is a good question. Indeed, initial and equilibrium concentrations generally occur in a wide range of concentrations in practical applications. In this work, we aimed to explore the possibility of hydrotalcite based on phosphate tailings for the adsorption of fluorine in phosphogypsum leaching solution. In the future work, we will study the application characteristics of phosphate tailings-based hydrotalcite in wastewater with wider concentration range.
- These statements given as “Gibbs free energy, entropy, and enthalpy were analysed”, “Assessment of Gibbs free energy” and “entropy (<DELTA>S)” are incorrect!!!
Response: Many thanks for pointing this out. Here we have a writing error. Based on thermodynamic theory and the data obtained from our thermodynamic calculation, ∆H of F- adsorption process calculated by PTB-LDO3, PTB-LDO4 were negative, which means that the adsorption of fluoride by PTB-LDO3, PTB-LDO4 was exothermic. Also redescribed here, “……The negative value of ∆G (calculated at 298K, 308K and 318K for PTB-LDO3 and PTB-LDO4 to be -12.45, -12.81, -13.17 and -12.77, -12.99, -13.21 KJ·mol-1 respectively) indicated spontaneous nature of the adsorption of F− by PTB-LDO3 and PTB-LDO4……”
Author Response File: 
Author Response.docx
Reviewer 3 Report
#Overall Merit
1. The authors presented a manuscript with plenty of formatting issues. The lines are not numbered, parts with different font colors and size and acronym errors are the major points that hinder the analysis.
2. In terms of quality of manuscript, the document has serious flaws in the XRD and thermodynamics.
#Abstract:
1. The abstract is not standardized in terms of font size.
#Introduction
1. The manuscript hypothesis is clearly presented. However, the novelty is obscure. A clear statement must be provided to assess the manuscript merit accordingly.
2. Besides using an interesting number of references, the review of the state-of-the-art is fairly short in this topic.
#Methods:
1. The grade of the involved chemicals must be presented for all process reagents.
2. In the characterization part, once again there is a not standardized font size.
FT-IR is incorrect. Authors must use FTIR as the acronym for Fourier transform infrared spectroscopy.
#Results and Discussion:
1. The title of the topic is in capitalized letters while the previous topics are not.
2. Figure 1 does not present scale bar or magnification.
3. In Figure 2, the BJH acronym has not been previously defined.
4. In Figure 4, the XRD results are irrelevant. The benchmark in this area is to develop a quantitative analysis by means of Rietveld method. The qualitative assessment to be valid must at least has the peak marks associated with each phase. Here, only the signal in presented.
5. Based on the thermodynamics, the authors affirm textually that the adsorption process was endothermic. This in incorrect as adsorption is by nature an exothermic event.
Author Response
Author’s reply to Editor and Reviews’ comments
We wish to thank the editor and reviewers for the valuable comments on our paper. Our manuscript entitled “Removal of fluoride from Phosphogypsum leaching solution with phosphate tailing based layered double hydroxides: Kinetics and Equilibrium Isotherms”, was carefully revised according to your comments. The comments and suggestions are incorporated in the revision of the manuscript. The changes in the manuscript have also been marked in red color for your evaluation. The itemized response to each reviewer’s comments is attached. Many thanks for your suggestions.
Reviewer 3：
- The authors presented a manuscript with plenty of formatting issues. The lines are not numbered, parts with different font colors and size and acronym errors are the major points that hinder the analysis.
Response: Sorry for our carelessness. We have carefully checked and modified the manuscript.
- In terms of quality of manuscript, the document has serious flaws in the XRD and thermodynamics.
Response: Many thanks for pointing this out. Corrections have been made. “…..The X-ray diffraction patterns of PTB-LDH3,PTB-LDH4, PTB-LDO3 and PTB-LDO4 were shown in Figure 4. It can been seen from Figure 4a and 4c that PTB-LDHs formed a series of characteristic peaks of hydrotalcite, and their specific 2θ angles were 11.10, 22.25, 34.36, 38.02 and 50.06, respectively [31]. Especially from the specific 2θ angles of 11.10 and 22.25……..”
#Abstract:
- The abstract is not standardized in terms of font size.
Response: Many thanks for pointing this out. Corrections have been made.
#Introduction
- The manuscript hypothesis is clearly presented. However, the novelty is obscure. A clear statement must be provided to assess the manuscript merit accordingly.
Response: This is a good suggestion. At the end of the preface, We have added a summary of the novelty of the manuscript at the end of the Introduction. “……This green and novel adsorption material using solid waste as raw material is expected to realize the reduction and resource utilization of phosphorus tailings, and effectively realize the removal of fluorine in phosphogypsum leaching solution..…”.
- Besides using an interesting number of references, the review of the state-of-the-art is fairly short in this topic.
Response: This is a good suggestion. Corrections have been made.
#Methods:
- The grade of the involved chemicals must be presented for all process reagents.
Response: Many thanks for pointing this out. Corrections have been made.
- In the characterization part, once again there is a not standardized font size. FT-IR is incorrect. Authors must use FTIR as the acronym for Fourier transform infrared spectroscopy.
Response: Sorry for our carelessness. Corrections have been made.
#Results and Discussion:
- The title of the topic is in capitalized letters while the previous topics are not.
Response: Many thanks for pointing this out. Corrections have been made.
- Figure 1 does not present scale bar or magnification.
Response: Many thanks for pointing this out. Corrections have been made.
- In Figure 2, the BJH acronym has not been previously defined.
Response: Many thanks for pointing this out. BJH have been defined in section 2.3. “…..Simultaneously, Barret-Joyner-Halenda (BJH) method was used to calculate the pore size distribution……”.
- In Figure 4, the XRD results are irrelevant. The benchmark in this area is to develop a quantitative analysis by means of Rietveld method. The qualitative assessment to be valid must at least has the peak marks associated with each phase. Here, only the signal in presented.
Response: Many thanks for pointing this out. Corrections have been made. “……The X-ray diffraction patterns of PTB-LDH3,PTB-LDH4, PTB-LDO3 and PTB-LDO4 were shown in Figure 4. It can been seen from Figure 4a and 4c that PTB-LDHs formed a series of characteristic peaks of hydrotalcite, and their specific 2θ angles were 11.10, 22.25, 34.36, 38.02 and 50.06, respectively [31]. Especially from the specific 2θ angles of 11.10 and 22.25…..”.
- Based on the thermodynamics, the authors affirm textually that the adsorption process was endothermic. This in incorrect as adsorption is by nature an exothermic event.
Response: Many thanks for pointing this out. Here we have a writing error. Based on thermodynamic theory and the data obtained from our thermodynamic calculation, ∆H of F- adsorption process calculated by PTB-LDO3, PTB-LDO4 were negative, which means that the adsorption of fluoride by PTB-LDO3, PTB-LDO4 was exothermic.
Author Response File: Author Response.docx
Round 2
Reviewer 2 Report
Most of the comments under the results section were not sufficiently answered. Please consider visiting the comments to improve your work better.
Author Response
Author’s reply to Reviews’ comments
We wish to thank the reviewers for the valuable comments on our paper. Our manuscript entitled “Removal of fluoride from Phosphogypsum leaching solution with phosphate tailing based layered double hydroxides: Kinetics and Equilibrium Isotherms”, was carefully revised according to your comments. The comments and suggestions are incorporated in the revision of the manuscript. The changes in the manuscript have also been marked in red color for your evaluation. The itemized response to each reviewer’s comments is attached. Many thanks for your suggestions.
Reviewer 2：
Most of the comments under the results section were not sufficiently answered. Please consider visiting the comments to improve your work better.
Response: Many thanks for pointing this out. We examined the manuscript carefully and revised it further on the basis of previous comments.
Reviewer 3 Report
The authors were not efficient on their reply.
The XRD issue was not answered at all. The manuscript still relies on the qualitative assessment of the spectrum without any quantitative measurement associated with this analysis. The figure 4 do not have a single peak marking…
The authors defined the BJH acronym but did not present the equipment detail (manufacturer, model and operation conditions). This type of approach hinders any reproducible use of this material by its peers in the future and must not be accepted nowadays.
Regarding the correction of the “writing error” on my 11th comment, the authors did not correct the corresponding issue on the Conclusions (line 412). Now, their adsorption is both “exothermic and endothermic”, depending on the section of their manuscript. This kind of flaw is the example of how much little effort was deployed to improve the document.
In my 1st review, I recommend its rejection and I will sustain with my position. This manuscript does not have the requirements for publishing in this journal.
Author Response
Author’s reply to Reviews’ comments
We wish to thank the reviewer for the valuable comments on our paper. Our manuscript entitled “Removal of fluoride from Phosphogypsum leaching solution with phosphate tailing based layered double hydroxides: Kinetics and Equilibrium Isotherms”, was carefully revised according to your comments. The comments and suggestions are incorporated in the revision of the manuscript. The changes in the manuscript have also been marked in red color for your evaluation. The itemized response to each reviewer’s comments is attached. Many thanks for your suggestions.
Reviewer 3：
- The XRD issue was not answered at all. The manuscript still relies on the qualitative assessment of the spectrum without any quantitative measurement associated with this analysis. The figure 4 do not have a single peak marking…
Response: Many thanks for pointing this out. Corrections have been made. We have revised Figure 4, added the single peak marking and made a corrections of the analysis. “……The X-ray diffraction patterns of PTB-LDH3,PTB-LDH4, PTB-LDO3 and PTB-LDO4 were shown in Figure 4. It can been seen from Figure 4a and 4c that the clearly exposing planes of crystals were (003), (006) and (009) except for the obvious diffracction peaks of LDHs phase, implying the successful formation of PTB-LDHs with typical layered double hydroxide structure [30]…….”.
Figure 4. XRD patterns of (a) PTB-LDH3, (b) PTB-LDH4, (c) PTB-LDO3 and (d) PTB-LDO4.
- The authors defined the BJH acronym but did not present the equipment detail (manufacturer, model and operation conditions). This type of approach hinders any reproducible use of this material by its peers in the future and must not be accepted nowadays.
Response: Many thanks for pointing this out. Corrections have been made.
- Regarding the correction of the “writing error” on my 11th comment, the authors did not correct the corresponding issue on the Conclusions (line 412). Now, their adsorption is both “exothermic and endothermic”, depending on the section of their manuscript. This kind of flaw is the example of how much little effort was deployed to improve the document
Response: Sorry for our carelessness. We now revise the corresponding conclusions and keep the exothermic reaction consistent.
Author Response File: Author Response.docx
Round 3
Reviewer 3 Report
The authors improved the manuscript accordingly.
