Hydrochar Loaded with Nitrogen-Containing Functional Groups for Versatile Removal of Cationic and Anionic Dyes and Aqueous Heavy Metals
, Yongshan Wan 2,*, Yulin Zheng 1, Yicheng Yang 1, Jinsheng Huang 1, Hao Chen 3, Jianjun Chen 4, Ahmed Mosa 5 and Bin Gao 6
Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsComments on the manuscript “Hydrochar loaded with nitrogen-containing functional groups for versatile removal of cationic and anionic dyes and aqueous heavy metals” (water-3309060)
This manuscript explores the synthesis of hydrochar from wheat straw and its modification with nitrogen-containing functional groups. The characterization and adsorption performance of both adsorbents are compared and analyzed, with discussions on adsorption kinetic and isotherm.
1. Additional information and clarification are needed in the introduction:
- The rationale for choosing hydrochar over biochar should be clearly stated, including its advantages and disadvantages, despite hydrochar's relatively low specific surface area and pore volume compared to biochar.
- Explain the choice of Pb and Cu as target pollutants in this study, as only two dyes are currently mentioned.
- Add references that demonstrate the use of APTES as a modification agent to enhance adsorption performance.
2. If yield calculations for APTES-HyC and HyC were performed, this information should be included in the manuscript.
3. Clarify how the trendlines for first-order, second-order, and Elovich kinetic in Fig. 6, as well as the Langmuir and Freundlich isotherms in Fig. 7, were determined. Indicate whether the R² values in Table 1 were derived from these figures.
4. Provide linear graphs and equations in the supplementary document to identify the best-fitting models for the first-order, second-order, and Elovich kinetic, as well as the Langmuir and Freundlich isotherm. Include explanations on how to calculate k, qe, and other constant values, along with the equations used.
5. From the experimental results, the addition of APTES does not significantly improve the surface properties and adsorption capacities of hydrochar compared to other green adsorbents. In the final paragraph of section 3, provide suggestions for enhancing the adsorption capacity of APTES-HyC (increasing the APTES solution concentration?).
6. In Section 2.4, explain why the initial concentrations of each pollutant varied (150 mg/L for Pb²⁺, 30 mg/L for Cu²⁺, 100 mg/L for MB, and 50 mg/L for Red). What evidence supports these experimental conditions? If Fig. 6 supports this setup, please include this information in the figure caption.
Author Response
Please see the attachment.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
Comments and Suggestions for AuthorsThe topic investigated in this manuscript is timely and of great interest to Water readers. However, it requires major revisions before it can be reconsidered for potential publication. Kindly find my comments below:
1) Line 17: “… that modified hydrochar …” – modified by what? Please specify the modifying agent.
2) The distinction between the two N-containing functionalities is unclear. Do the authors mean various types of amines were observed in the samples?
3) When reporting adsorption capacities, the authors should include operational conditions like pH, temperature, etc. This applies to the entire text.
4) The authors should mention textural properties, such as specific surface area and pore volume, in the abstract.
5) The abstract requires significant revision; the current version is insufficient.
6) In the introduction, the authors should cite these relevant, up-to-date references to strengthen the wastewater treatment discussion: (1) doi.org/10.1016/j.dwt.2024.100446 (for tools, techniques, and materials used in heavy metal removal) and (2) doi.org/10.1016/j.envres.2024.119029 (to support micropollutant adsorption in water/wastewater).
7) Line 85: “Absorption” should be replaced with “adsorption.” These are distinct processes and should not be used interchangeably; this applies throughout the text.
8) Chemical and reagent specifications, such as purity, are missing. Please include these details.
9) Section 2.4: A primary concern is the use of different filter pore sizes (0.45 μm for metal ions and 0.22 μm for dyes), which may introduce systematic bias. Smaller pores for dyes may retain additional particulates or colloids, potentially skewing the sorption measurements and complicating sorbent performance comparison between metals and dyes. The rationale for using different filters needs clarification.
10) Could the authors explain how N-containing functional groups affect the electrical conductivity of hydrochar?
11) Please use superscript for “2” in R² throughout the text.
12) How do the authors differentiate between chemical and physical adsorption mechanisms, and what additional experiments or analyses could confirm the dominant mechanism?
13) The conclusion is too brief; please expand it to be more comprehensive, covering data and results obtained in this study.
Author Response
Please see the attachment.
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
Comments and Suggestions for AuthorsThe authors' responses are clear and concise, and this revised version is suitable for publication.
Reviewer 2 Report
Comments and Suggestions for AuthorsThe revised version is acceptable.
