“Treating Waste with Waste”: Utilizing Phosphogypsum to Synthesize Porous Calcium Silicate Hydrate for Recovering of Fe2+ from Pickling Wastewater
Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsThis is an interesting study that falls within the scope of journal. Please find comments below to further improve the quality of this work:
-I think you could shorten the title. Maybe erase “Sustainable Utilization of Phosphogypsum” – right now you have phosphogypsum in the title two times which feels a bit repetitive.
-how much of this pickling wastewater is produced per year? Or at one facility? I think some production numbers would really help here to understand the scale of this problem.
-there are actually nearly 300 million metric tons of phosphogypsum produced annually worldwide (https://doi.org/10.1016/j.jclepro.2023.137561)
-“treating waste with waste” is a great slogan. Maybe use this in your title
-can you explain the washing process of the PG in more details. I think this is crucially important. PG is usually highly acidic as well (https://doi.org/10.1007/s10967-023-09079-w). Through the washing process that you describe I think you are essentially removing a lot of the acidity of the PG. So this is a really important step that can be explained in more detail. I think it would also be relevant to include this in your analysis/judgement of potential industrial application.
-Table 1: is this before or after the washing. Please specify.
-please provide an analysis of the simulated pickling wastewater and ideally also describe how pickling wastewater from an industrial plant is different from the one that you used now.
-the experiments are ok and we would usually describe the results in a similar way. In the abstract you say that this approach is very promising for industrial application. Why is that? From the presented information it is not clear to me. Please elaborate more.
Author Response
Comments 1: I think you could shorten the title. Maybe erase “Sustainable Utilization of Phosphogypsum” – right now you have phosphogypsum in the title two times which feels a bit repetitive.
Response 1: Thank you for pointing this out. I agree with this comment. Therefore, I have removed “Sustainable Utilization of Phosphogypsum” from the article title. The modifications are made on page 1, line 2.
Comments 2: how much of this pickling wastewater is produced per year? Or at one facility? I think some production numbers would really help here to understand the scale of this problem.
Response 2: Thank you for pointing this out. In recent years, China has been the world’s largest steel producer, generating millions of tons of pickling wastewater annually. The modifications are made on page 1, line 30.
Comments 3: there are actually nearly 300 million metric tons of phosphogypsum produced annually worldwide (https://doi.org/10.1016/j.jclepro.2023.137561)
Response 3: Thank you for pointing this out. I agree with this comment. Therefore, I will add information about the annual production of phosphogypsum. The modifications are made on page 2, line 51.
Comments 4: “treating waste with waste” is a great slogan. Maybe use this in your title.
Response 4: Thank you for pointing this out. I agree with this comment. Therefore，I will also add “Waste Treatment with Waste” to the title. The modifications are made on page 1, line 2.
Comments 5: Can you explain the washing process of the PG in more details. I think this is crucially important. PG is usually highly acidic as well (https://doi.org/10.1007/s10967-023-09079-w). Through the washing process that you describe I think you are essentially removing a lot of the acidity of the PG. So this is a really important step that can be explained in more detail. I think it would also be relevant to include this in your analysis/judgement of potential industrial application.
Response 5: Thank you for pointing this out. Prior to use, the washing process for PG involves placing it into a beaker containing deionized water, subjecting it to ultrasonic treatment for 10 minutes, and then filtering it. This operation is repeated twice, dried at 105 °C for 48 hours, and then crushed, ground, and sieved through 200-mesh screen. The modifications are made on page 2, line 78.
Comments 6: Table 1: is this before or after the washing. Please specify.
Response 6: Thank you for pointing this out. Table 1 will show the main chemical components of phosphogypsum after washing and drying. The modifications are made on page 2, line 84.
Comments 7: please provide an analysis of the simulated pickling wastewater and ideally also describe how pickling wastewater from an industrial plant is different from the one that you used now.
Response 7: Thank you for pointing this out，The simulated pickling wastewater studied in the experiment was primarily prepared using iron powder and hydrochloric acid to create a 1,000 mg/L Fe²⁺ standard solution. Unlike the actual pickling wastewater, the real pickling wastewater may contain trace amounts of other heavy metals, such as Mn²⁺, Cu²⁺, Ni²⁺, etc. The modifications are made on page 4, line 125.
Comments 8: the experiments are ok and we would usually describe the results in a similar way. In the abstract you say that this approach is very promising for industrial application. Why is that? From the presented information it is not clear to me. Please elaborate more.
Response 8: Thank you for pointing this out，I agree with this comment. This study aims to solve the industrial utilization problem of phosphogypsum by synthesizing a high-efficiency, cost-effective adsorbent, porous hydrated calcium silicate, while also using the inexpensive adsorbent to recover iron from iron-containing pickling wastewater. The modifications are made on page 2, line 70.
Reviewer 2 Report
Comments and Suggestions for AuthorsAn interesting work
1)Is the phosphogypsum in the form of dihydrate or hemihydrate?
2)Describe the process of phosphogypsum generation.
3)For Table 1, how did you determine the SiO4 content?
4)The sum of the percentages is around 99.35%. What does the remaining percentage represent?
5)Is this percentage close to the crystallization water content of hemihydrate phosphogypsum?
6)Are you working in the field of reagent supersaturation?
7)Why do you use NaOH not NH4OH….?
8)How does supersaturation affect the reaction?
9)What is the specific surface area of the phosphogypsum?
10)For Table 3, if the R² value is below 95%, how do you address this hypothesis?
Author Response
Comments 1: Is the phosphogypsum in the form of dihydrate or hemihydrate?
Response 1: Thank you for pointing this out. The raw material used in this study is phosphogypsum that has been washed and dried, primarily consisting of hemihydrate.
Comments 2: Describe the process of phosphogypsum generation.
Response 2: Thank you for pointing this out. Phosphogypsum is produced in the process of wet-process phosphoric acid production (mainly through the reaction of phosphate rock with sulfuric acid to generate phosphoric acid), and it is primarily extracted by precipitation from the acid solution.
Comments 3: For Table 1, how did you determine the SiO4 content?
Response 3: Thank you for pointing this out. In Table 1, the SiO₂ content was determined using an X-ray fluorescence (XRF) spectrometer.
Comments 4: The sum of the percentages is around 99.35%. What does the remaining percentage represent?
Response 4: Thank you for pointing this out. Analysis of the instrument’s detection results shows that the oxides in phosphogypsum contain trace amounts of heavy metal impurities, which constitute a certain percentage of the total content; this suggests that there may be minimal impurities that were not detected.
Comments 5: Is this percentage close to the crystallization water content of hemihydrate phosphogypsum?
Response 5: Thank you for pointing this out. In fact, the detection results approximate the crystal water content of hemihydrate phosphogypsum.
Comments 6: Are you working in the field of reagent supersaturation?
Response 6: Thank you for pointing this out. I conducted experiments under conditions of reagent supersaturation.
Comments 7:Why do you use NaOH not NH4OH….?
Response 7: Thank you for pointing this out. Since NaOH has a higher alkalinity, it can effectively regulate the alkalinity range of the hydrothermal synthesis product. Compared to NH4OH, NaOH is more conducive to promoting the hydrothermal reaction. The involvement of NH4OH would produce NH3, thereby increasing the pressure inside the sealed reaction autoclave and potentially affecting the reaction.
Comments 8: How does supersaturation affect the reaction?
Response 8: Thank you for pointing this out. During the experiment, supersaturation is primarily influenced by the pH of the reaction system; OH⁻ may cause partial metal hydrolysis, thereby affecting the reaction process.
Comments 9: What is the specific surface area of the phosphogypsum?
Response 9: Thank you for pointing this out. The specific surface area of the phosphogypsum used in this study has not been measured; however, based on previous measurements of different batches of phosphogypsum by our research group, its specific surface area ranges from 10 to 20 m²/g.
Comments 10: For Table 3, if the R² value is below 95%, how do you address this hypothesis?
Response 10: Thank you for pointing this out. If the R² value is below 95%, it is necessary to analyze the correlation between the fitting curve and the experimental value trend, as well as the fitting parameters. Generally, the closer the R² value of the model is to 1 and the smaller the fluctuation range of the R² value under different concentration gradients, the more the kinetic model can represent the adsorption process.
Reviewer 3 Report
Comments and Suggestions for AuthorsThe authors have carried out a large volume of research, the results of which are of certain scientific interest. The structure of the article and the content correspond to the requirements of the publication. However, there are a number of the following comments that require correction and clarification:
1. It is not clear from the Introduction whether similar studies have been conducted before. If such studies have been conducted, then how did those presented in the current manuscript differ from similar ones;
2. It is necessary to clearly indicate the purpose of the study;
3. The manuscript does not have a description and explanation for Figure 1;
4. Information on phosphogypsum (Table 1, Figure 1) should be moved to Section 2.1. Materials;
5. Figure 7, please clarify what is highlighted with a yellow dashed line;
Technical comments:
L98 «….in Table S1…» remove «S»;
L145 2Ca(OH)2(s); L158 CaSiO3 - use subscripts;
L273 "Fig.10(a) show The relationship..." replace with "t";
L282 "where Qe (mg/g) is……" remove paragraph.
Author Response
Comments 1: It is not clear from the Introduction whether similar studies have been conducted before. If such studies have been conducted, then how did those presented in the current manuscript differ from similar ones;
Response 1: Thank you for pointing this out. I agree with this comment. Therefore, I will provide an explanation in the introduction, with modifications made at line 58 on page 2.
Comments 2: It is necessary to clearly indicate the purpose of the study;
Response 2: Thank you for pointing this out，I agree with this comment. The modifications are made on page 2, line 70.
Comments 3: The manuscript does not have a description and explanation for Figure 1;
Response 3: Thank you for pointing this out，I agree with this comment. The modifications are made on page 2, line 84.
Comments 4: Information on phosphogypsum (Table 1, Figure 1) should be moved to Section 2.1. Materials;
Response 4: Thank you for pointing this out，I agree with this comment. The modifications are made on page 3, line 98.
Comments 5: Figure 7, please clarify what is highlighted with a yellow dashed line;
Technical comments:
L98 «….in Table S1…» remove «S»;
L145 2Ca(OH)2(s); L158 CaSiO3 - use subscripts;
L273 "Fig.10(a) show The relationship..." replace with "t";
L282 "where Qe (mg/g) is……" remove paragraph.
Response 5: Thank you for pointing this out，I agree with this comment. The yellow dashed lines in Figure 7 indicate unreacted materials of the product PCSH under SEM observation with different parameters. I will further revise the above issues.
Round 2
Reviewer 1 Report
Comments and Suggestions for AuthorsComments have been addressed - this is fine with me now.
Reviewer 3 Report
Comments and Suggestions for AuthorsThe authors have made all the necessary corrections. The manuscript can be accepted for publication in its current form.
