Sustainable Phosphate Recovery Using Novel Ca–Mg Bimetallic Modified Biogas Residue-Based Biochar

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

 

 The current work deals with Adsorption of phosphate on a novel Ca-Mg bimetallic-modified biogas residue-based biochar. The study explicitly addresses gaps identified in previous research, providing a more complete understanding of the topic, however, there are some points that need to be supported by previous research. Below are some comments before this work can be accepted.

• Remove the repeated statement in page 4 lines (141-143), as was stated on page 3.
• The words “While” in page 4, “Capacity” in page10 should in lower case.
• It is better to add all (hkl) indices for all XRD figures.
• More discussion about why 0.5 g/L is considered as the best dosage of Ca-Mg/BC, although the adsorbent dosage around 1.33 g/L has the same removal efficiency.
• The section 3.4 “Analysis of factors affecting phosphate adsorption by biochar” needs more supported references.
• In pot-planting experiment, the environmental conditions better to be added as (temperature, humidity, pH, etc).

Author Response

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Reviewer 2 Report

Comments and Suggestions for Authors

This study investigates the synthesis and application of Ca-Mg bimetallic-modified biogas residue-based biochar (Ca-Mg/BC) for phosphate adsorption and its subsequent use as a soil amendment.  The research proposes a sustainable solution by repurposing biogas residues.  The experimental design is comprehensive, combining material synthesis, adsorption studies, and practical application in agriculture.  However, several aspects require clarification to strengthen the manuscript.

1. The preparation of Ca-Mg/BC (Section 2.1) lacks critical details: The rationale for selecting 0.3 mol/L Mg/Ca(CH₃COO)₂ and 700°C pyrolysis temperature should be supported by preliminary data or references；The drying step (65°C for 12 h) may alter precursor properties; justify this choice.

2. Clarify whether experiments were conducted in triplicate and if error bars in figures represent standard deviations. Statistical significance of differences in adsorption capacities (e.g., between Ca-Mg/BC and BC) should be explicitly stated.

3. FTIR and XPS results (Figures 2b, 8a–d) lack quantitative analysis. For instance, peak intensities for Ca-O/Mg-O bonds before and after adsorption should be quantified to support the proposed mechanisms (e.g., precipitation vs. surface complexation).

4. The pH adaptability claim (Section 3.4.2) requires deeper mechanistic insight. Explain how Ca-Mg/BC maintains high adsorption at pH > 9 despite electrostatic repulsion between PO₄³⁻ and negatively charged surfaces.

5. The role of Fe in phosphate adsorption (Section 3.1) is underdiscussed. XPS data (Table 2) show Fe content in Ca-Mg/BC (1.24%), yet its contribution to phosphate removal is not explored.

6. Improve resolution and label axes/legends clearly. For example, Figure 3a–c lacks units for the x-axis (salt concentration, temperature)；Table 3: Correct typographical errors (e.g., "Ca-Mg/BC)" → "Ca-Mg/BC").

Author Response

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Reviewer 3 Report

Comments and Suggestions for Authors

The topic of the paper is interesting and current. You have presented a lot of results, however, I would like to make some remarks and suggestions.
Modify the title of the paper.
Be concise in the introductory part of the paper.
What does ,, ground biogas residue powder (BR) ,, refer to?
What does ,, cracked at a constant temperature for 2 h,, mean? and why was it done?
The calculation from paragraph 2.2 could have gone into additional material.
Figure 1. is not clear, neither the diameter, magnification, nor the morphology can be seen in the picture. Also, the EDS spectrum is not visible, present it in a table.
Figure 2. must be separated and discussed separately for each method.
With the BET method, you did not show the pore distribution?
I also did not understand the essence of what you did when your specific surface area is miserable?
Why did you not do a proximate and ultimate analysis of your initial sample, the precursor?
Technically fix the work, images and text.
Why didn't you do the equilibrium model according to Temkin?
Enlarge image 8, resolution.

Author Response

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Reviewer 4 Report

Comments and Suggestions for Authors

The research presented in the article on “Adsorption of phosphate on a novel Ca-Mg bimetallic-modified biogas residue-based biochar” is interesting from a scientific as well as practical point of view. The authors proposed a new sorbent tailored to remove phosphate from wastewater. The sorbent used has also been tested as a formulation to affect the growth of mung beans. The article presents a high scientific level, and the comments made are mainly explanatory and supplementary:

1. verses 124-125 The doses of the adsorbent in g/L should also be given.
2. verse 138. was the study of the effect of pH on adsorption conducted from single-component solutions containing only phosphate? This is not clearly described.
3. verse 157-161. no information on the dose of adsorbent and the adsorption time chosen.
4. formula 9 Provide a literature source for the formula for Kc. How was Mw determined?
5. formula numbering errors
6. error in chapter numbering (there are two chapters 3.4.3)
7. no information whether adsorption isotherms and kinetics equations were determined from nonlinear models. If so, it should be specified in the methodology what program was used.
8. table 4. why no R2 value is given for Intraparticle diffusion model steps.

Author Response

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Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

Accept in present form.