PONE-D-24-25807electro-assisted loading of magnesium/aluminium onto sludge-based biochar for the simultaneous adsorption of ammonia nitrogen and phosphatePLOS ONE

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Chu-Ya Wang was supported by the natural science foundation of Jiangsu Province (BK20211047); Guangcan Zhu was supported by the natural science foundation of Jiangsu Province (BK20220038).

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This work was supported by the Natural Science Foundation of Jiangsu Province (BK20211047 and BK20220038).

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Chu-Ya Wang was supported by the natural science foundation of Jiangsu Province (BK20211047); Guangcan Zhu was supported by the natural science foundation of Jiangsu Province (BK20220038).

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Additional Editor Comments:

Please 1. replace the keywords by biochar; ammonia adsorption; phosphate adsorption; adsorption mechanism; electro modification or others that are common are can be readily searched by others; 2. provide complete captions for Figure and Tables that can be understood with no need to read the text.

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Reviewer #1: Partly

Reviewer #2: Yes

Reviewer #3: No

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2. Has the statistical analysis been performed appropriately and rigorously?

Reviewer #1: Yes

Reviewer #2: I Don't Know

Reviewer #3: No

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Reviewer #1: Yes

Reviewer #2: Yes

Reviewer #3: No

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Reviewer #1: Yes

Reviewer #2: Yes

Reviewer #3: No

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5. Review Comments to the Author

Please use the space provided to explain your answers to the questions above. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. (Please upload your review as an attachment if it exceeds 20,000 characters)

Reviewer #1: This study describes the production of a Mg/Al modified sludge-based biochar tested in the removal of phosphate and ammonia nitrogen from water. Despite some interesting results, several aspects should be improved and cleaned before a final decision. Specific comments are given below:

1. On page 13, line 3 of the ‘Batch adsorption experiments’ section, author mention that 'Mix a solution containing 50 mg·L-1 of PO4 3- -P and 100 mg·L-1 of NH4 + -N.' How did the authors determine the concentrations of the two pollutants and is there a correlation between the ratios of nitrogen and phosphorus? Please give the reasoning.

2. Why did author use H2SO4 for acidity adjustment in the 'Preparation of materials' section, but why use HCl for acidity adjustment in the 'Batch adsorption experiments' section?

3. In the section 'Batch adsorption experiments', 'Kinetic, isothermal and thermodynamic experiments', author do not give the experimental conditions such as pH value.

4. In the section 'Effect of different factors on NH4+-N and PO43--P capture', page 30, line 16, there is an error in 'Fe4(PO4)4'.

5. Page 36, 'Nitrogen and phosphorus recovery' section, lines 11-12, author suggest that 'Group A might promote the growth and development of mung bean sprouts because it absorbed rich N and P.' However, the ability of MgSBC-0.5(@Al)-NP to release N and P has not been studied in the manuscript, there is a lack of evidence to support this. It is suggested to supplement the related research on the performance of MgSBC-0.5(@Al)-NP in releasing N and P for a combined supplementary explanation.

6. Do the Mg/Al ions in MgSBC-0.5(@Al) leach? How about the release? This can be discussed in conjunction with the corresponding emission standards.

Reviewer #2: This study prepared biochar for simultaneous removal of ammonium nitrogen and phosphate. To help the authors improve this manuscript, specific suggestions and comments are put forward as follows:

1. Please re arrange the introduction. For example; you should mentioned kind of materials used to make biochar, then explain what is biochar.

2. Is that enough to remove moisture content at 105 C for 2 hour?

3. Do you measure soil characteristics? Please add in the manuscript

4. Germination rate calculation? please add in the manuscript

5. Soil experiments under room temperature or constant temperature? Under greenhouse or field experiment? Please make a detail, because next researcher need to understand the methodology and developed it.

6. I think the title should be change by adding its application on plant

Reviewer #3: The manuscript entitled “Electro-assisted loading of magnesium/aluminium onto sludge-based biochar for the simultaneous adsorption of ammonia nitrogen and phosphate” by Qi Wang et al. presents the modification of sewage sludge with Mg and Al through electrochemical methods. The Mg/Al modified sewage sludge is then pyrolyzed and the modified biochar is used for simultaneous adsorption of ammonium and phosphate. The premise of the study is quite unique because electrochemical modification is not commonly used for biochar modification. The manuscript has several major drawbacks:

(i) The mechanism of the electrically assisted impregnation is not clearly explained. To what extent does the impregnation method affect the biochar properties, and what is the mechanism?

(ii) The authors have prepared only two biochars (one with Mg/Al and one without). There is no variation of the impregnation conditions (e.g., impregnation time, current density) and pyrolysis conditions (e.g., pyrolysis temperature). The authors did vary the Mg acetate concentration, but those biochars were only tested in the adsorption experiment and not characterized. Thus, the material preparation conditions and the adsorption performance cannot be clearly related.

(iii) The role of aluminum is not clear. What was the role of Al in the adsorption of ammonium and phosphate? Or did the Al have a different role?

(iv) The main adsorption mechanism of ammonium and phosphate is explained to be due to struvite crystallization and formation of MgHPO4 and CaHPO4, but insufficient evidence is given. Such claims should be supported by a more in-depth characterization of the post-adsorption material (e.g., XRD measurements) supported by solubility calculations.

(v) A plant study was conducted to test the performance of the post-adsorption biochar as a soil amendment. This experiment has several problems namely (1) the number of replicates is not stated, (2) no positive control group was included, (3) no statistical analysis was carried out, and (4) the N and P contents in the sprouts were not measured.

(vi) Presentation and discussion of the results is confusing, both due to the language used and conclusions drawn based on the presented (insufficient) data.

More specific comments to improve the manuscript are given below:

1. Introduction: “Magnesium (Mg)/aluminum (@Al) bilayer metal … adsorption capability of BC”. Please provide a reference to this study.

2. Introduction: “When PO43--P, Mg2+, and NH4+-N are combined in solution, NH4+-N and PO43--P can be removed.”. Why is this? Please clarify.

3. Introduction: “Therefore, sludge has unique advantages over other BC materials in practical application [11].”. It is not clear how the authors come to this conclusion based on the information given in the discussed paragraph. Please clarify.

4. Materials and methods, preparation of materials: please explain where and how the sludge was collected.

5. Materials and methods, preparation of materials: “Subsequently, the initial pH was modified to 3.0”. Why was this done? Please clarify.

6. Materials and methods, preparation of materials: the experimental setup of the electrochemical modification is not clear. What was the anode material, and what was the cathode? Was a reference electrode used? Please clarify.

7. Materials and methods, preparation of materials: how were the NH4+-N and PO43—P solutions prepared?

8. Materials and methods, preparation of materials: please provide the purity and supplier of all reagents used.

9. Materials and methods, batch adsorption experiments: the authors have carried out the adsorption experiments using a solution containing 50 mg·L-1 of PO43--P and 100 mg·L-1 of NH4+-N. How/why were these concentrations selected? Please clarify.

10. Please refer to all equations in the text.

11. Please include the units of all symbols used in the equations.

12. Materials and methods, batch adsorption experiments: “At the temperature of 35 °C, for the pH experiment”. Why was a temperature of 35 °C used for the pH experiment, while 25 °C was used for the previous experimentss?

13. Equation 11: what is ‘D’? Please clarify.

14. Materials and methods, pot experiments: here the authors mention both MgSBC-0.5(@Al) and MgSBC-0.5(@Al)-NP. What is the difference between these materials?

15. Materials and methods, pot experiments: what is the “leaching process” that the authors are referring to here?

16. Materials and methods, pot experiments: the number of replicates in this experiment was not clear. Was only one pot used for each condition? Please clarify.

17. Materials and methods, pot experiments: in each pot experiment both a negative control and positive control should be used. How did the performance of the MgSBC-0.5(@Al)-NP compare to a commercial fertilizer? And what was the performance of the raw sludge (SP) in the mung bean growth experiment?

18. Materials and methods, characterization and analytical method: how were the samples prepared for ICP-MS measurements? Please clarify.

19. Results and Discussion, characterization and morphology: “Fig 1d shows the energy spectrum of MgSBC-0.5(@Al),demonstrating that the nano flake mainly consisted of Mg, Al, Si, chlorine (Cl), and oxygen (O) [12]”.

- What “nano flake” are the authors referring to here?

- Which sample does Fig. 1d refer to?

- What was the carbon content in the EDX analysis? Given that the authors have prepared biochars, the material is expected to contain a substantial amount of carbon.

20. Results and Discussion, characterization and morphology: “indicating that the electrical assisted modification could increase the SBET of BC, thereby increasing the adsorption capacities of pollutants.” A higher SBET may not necessarily lead to a higher adsorption capacity, because the SBET is not the only factor that affects the adsorption process. In various situations, materials with a low SBET have reached outstanding adsorption capacities because of the unique functional groups. The second part of the statement (“thereby…”) should be removed.

21. The number of significant figures should be checked and reduced in all tables. For example in Table 1: the measured pore volumes are not known with such precision.

22. Results and Discussion, characterization and morphology: “The peak observed at a wavelength of 465 cm-1 in SBC attributed to the Si-O-Si vibration. The peak at 465cm-1 corresponded to the SiO-Si or Al-O-Al vibration.” These two sentences can be combined into a single sentence.

23. Results and Discussion, characterization and morphology: “The results of FTIR further confirmed the existence of Mg and Al in MgSBC-0.5(@Al)”. This is not correct. The FTIR analysis cannot provide any information about the elemental composition. Please remove this statement.

24. Results and Discussion, adsorption kinetics and isotherm: at the beginning of this paragraph, the authors explain that they have prepared sludge-based biochars with varying Mg acetate solution concentrations. However, no explanation of these materials is given in the Materials and methods, and these materials have not been characterized. Please explain their preparation in the Materials and methods section, and provide a characterization of these biochars with the same techniques as MgSBC-0.5(@Al). This may provide insight into how the Mg acetate concentration affects the NH4+-N and PO43—P adsorption capacity.

25. Results and Discussion, adsorption kinetics and isotherm: “However, a high concentration of Mg2+ could cause the transformation of struvite crystals into Mg3(PO4)2.” Please provide experimental evidence for this statement or otherwise remove it.

26. Table 5: please mention the adsorbent in the caption.

27. Results and Discussion, Effect of different factors on NH4+-N and PO43--P capture: “The pHpzc values of MgO, AlOOH and Al2O3 were 12.0, 8.2 and 9.1, respectively [39, 40].” It is not clear why this is relevant here. Instead, what was the pHPZC of the adsorbents used in this study?

28. Results and Discussion, Effect of different factors on NH4+-N and PO43--P capture: “Simultaneously, the loss of NH4+ in the form of NH3 gas occurred at elevated pH levels. This led to a little decrease in the adsorption of NH4+-N”.

- What does “elevated pH levels” mean? Please specify the values.

- How does loss of N via NH3 gas decrease the adsorption capacity? If NH3 gas would escape, wouldn’t this lead to an overestimation of the adsorption capacity?

29. Results and Discussion, Effect of different factors on NH4+-N and PO43--P capture: “The existence of K+ and Na+ in the solution would compete with NH4+-N to form struvite potassium (KMgPO4·7H2O) and sodium magnesium PO43--P heptahydrate [NaMgPO4·7H2O].” In theory this is possible, but the solubility of K-struvite and Na-struvite may not be the same as that of regular struvite (depending on the pH). Please check the solubility of K-struvite, Na-struvite, and regular struvite at the pH of the experimental conditions used.

30. Fig. 5a: What was the pH of the solutions after the adsorption experiments?

31. Fig. 6: did the XPS signal of Al change after the adsorption experiment?

32. Results and Discussion, Nitrogen and phosphorus recovery:

- Were the treatments significantly different? Please include a statistical analysis.

- In Fig. 7 please include error bars for the wet weight

- Please include dry weight data.

33. Results and Discussion, Nitrogen and phosphorus recovery: “Therefore, MgSBC-0.5(@Al)-NP had the potential to replace N and P fertilizers.” This is not true, because no comparison is made with traditional N and P fertilizers. This statement should be removed.

34. Results and Discussion, Nitrogen and phosphorus recovery: what were the N and P contents in the seedlings?

35. The authors mention several times that adsorption of ammonium and phosphate was due to struvite formation. However, no evidence is given on the formation of struvite. X-ray diffraction analysis of the biochars after adsorption should be carried out to confirm whether any struvite was formed.

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Reviewer #1: No

Reviewer #2: No

Reviewer #3: No

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Simultaneous adsorption of ammonia nitrogen and phosphate phosphorus on electro-assisted magnesium/aluminum-loaded sludge-based biochar and its utilization as a plant fertilizer

PONE-D-24-25807R1

Dear Dr. Wang,

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G Yang

Academic Editor

PLOS ONE

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