Study on the Adsorption of Tetracycline Hydrochloride in Water by Modified Highland Barley Straw Biochar

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The manuscript presents a systematic experimental investigation into the adsorption of tetracycline hydrochloride (TCH) onto chemically modified biochars derived from Tibetan highland barley straw. The manuscript is scientifically promising and methodologically sound in experimental scope but requires major revision before publication.

1. The literature section should introduce cation bridging and surface complexation theories earlier to guide the reader toward the later mechanistic discussion. The discussion could also integrate adsorption capacities or mechanistic distinctions from some advanced modified biochar (KOH or Fe/Mg modified) for contextual benchmarking.
2. The pyrolysis procedure is described adequately but lacks explicit mass ratio between straw and activating agent. The molar concentration range (2–5 mol L⁻¹) alone is insufficient for reproducibility.
3. Justify why the pyrolysis temperature of 750 °C was chosen without showing optimization data. Citing previous thermogravimetric or yield studies would strengthen the rationale.
4.  The use of HCl/NaOH titration is appropriate, but buffering effects or ionic strength adjustment (e.g., background electrolyte) are not discussed; this could influence adsorption equilibrium.
5. The equilibration time of 2880 min (48 h) contact time seems excessive for batch adsorption. Please justify. 
6. No zeta potential or elemental composition (CHN or XPS) data are reported. These are critical for linking functional group density to electrostatic interactions.
7. The lack of control samples in comparison for “unmodified straw” and “unmodified biochar under identical pyrolysis conditions.” Inclusion would better quantify modification effects, especially in XRD and FTIR analysis.
8. In Figure 3 and Figure 4, although removal efficiencies are compared across modifiers, no standard deviation or error bars are shown. Statistical analysis (ANOVA or t-test) is needed to validate “significant difference.”
9. In Figure 5, magnification factors should be included.
10. In Table 1, the reported surface area of 862 m² g⁻¹ for Zn-BC is unusually high for 750 °C biochar. The authors should confirm degassing temperature and cross-validate with pore volume data.
11. In XRD Analysis, the claim of increased “graphitization degree” is speculative without crystallite size or d002 spacing data from Scherrer analysis.
12. The study states “optimum at 5 mg,” but the test volume is 50 mL. This equates to 0.1 g L⁻¹; unit conversions and mass-normalized capacities should be clarified.
13. The experimental condition for adsorption performance (pH, dosages, concentration, ionic strength) shall be included in the Figure caption for better readability. 
14. For kinetic modeling in Figure 14 and Table 2, although R² values are high, the small difference between pseudo-first- and second-order models (ΔR² ≈ 0.002) is statistically insignificant. Discussion should highlight the mechanistic rationale (surface diffusion vs. chemisorption) rather than rely solely on R².
15. Recheck isotherm parameter in Table 3. The Freundlich n values are < 1 (−0.56, −0.62), implying inverse adsorption behavior; this is physically implausible and likely a sign-convention or regression error.
16. The authors qualitatively discuss temperature effects but did not calculate ΔH°, ΔS°, or ΔG°. These parameters are essential to substantiate the “endothermic/physical” claims.
17. The mechanistic Figure 17 is visually clear but oversimplified. The inclusion of charge distribution, π–π interactions, or specific complexation pathways (e.g., Zn²⁺–O–C coordination) would improve scientific value.
18. The study lacks benchmarking of qₘₐₓ values versus existing ZnCl₂- or H₃PO₄-modified biochars (e.g., 239–325 mg g⁻¹). This prevents readers from gauging the improvement magnitude.
19. The text alternates between “pseudo-first-order indicates physisorption” and “coexistence of chemisorption.” The evidence base (FTIR before/after adsorption or XPS binding shift) is missing to support chemisorption claims.
20. The assertion that Zn²⁺ forms complexes with TCH is not experimentally verified and quantifying released Zn²⁺ would strengthen credibility of the manuscript.
21. The recovery of adsorption at pH 9 is attributed to cation bridging, yet Zn(OH)₂ precipitation or surface charge reversal could also explain this. Alternative interpretations should be discussed.
22. The claim of “complementary suitability” is conceptually appealing but should be backed by comparative capacity and stability data under identical conditions (e.g., reusability, desorption cycles) for Zn-BC vs. P-BC samples.

Author Response

Dear Reviewer,

Thanks very much for taking your time to review our manuscript. We highly appreciate all your reasonable, scientific and insightful comments and suggestions. Please find our itemized responses below, and the relevant revisions have been highlighted in the resubmitted files.

Point-by-point response to Comments and Suggestions for Authors

 

Comments 1: The literature section should introduce cation bridging and surface complexation theories earlier to guide the reader toward the later mechanistic discussion. The discussion could also integrate adsorption capacities or mechanistic distinctions from some advanced modified biochar (KOH or Fe/Mg modified) for contextual benchmarking.

Response 1:  Thank you very much for your insightful comment, which has effectively improved the logical coherence and comprehensiveness of our manuscript. We fully accept your suggestion and have made corresponding revisions to the manuscript. Specifically, the introductions to the cation bridging theory and surface complexation theory have been added to the literature review section (lines 76 - 96) to guide readers in understanding the later mechanistic discussion. In the discussion section (lines 552 - 559), we have also integrated the adsorption capacity data and mechanistic distinction analysis of KOH-modified and Fe/Mg-modified advanced biochar, so as to achieve contextual benchmarking with related studies. We sincerely appreciate your advice which contributes significantly to the improvement of our manuscript.

 

Comments 2: The pyrolysis procedure is described adequately but lacks explicit mass ratio between straw and activating agent. The molar concentration range (2–5 mol L⁻¹) alone is insufficient for reproducibility.​

Response 2: Thank you for pointing out the issues that the mass ratio of straw to activating agent in the pyrolysis process is not specified, and the molar concentration range alone cannot ensure experimental reproducibility. The experimental design of this study is as follows: 6 g of highland barley straw powder was weighed uniformly, and 100 mL of modifier with concentrations of 2, 3, 4, and 5 mol·L^-1 was added respectively. Only the modifier concentration was changed in all groups, while other conditions (such as solid-liquid contact method, reaction temperature, etc.) were completely consistent to eliminate interference from factors other than concentration. At present, the above experimental details have been supplemented in Lines 182-185 of the manuscript.

 

Comments 3: Justify why the pyrolysis temperature of 750 °C was chosen without showing optimization data. Citing previous thermogravimetric or yield studies would strengthen the rationale.

Response 3: Thank you for your suggestion. The selection of 750 °C as the pyrolysis temperature for highland barley straw biochar is based on two key reasons: first, highland barley straw has a similar composition ratio of cellulose, hemicellulose, and lignin to wheat straw, allowing reference to relevant research logic; second, both in-house preliminary experiments and literature research indicate that the highland barley straw biochar prepared at this temperature exhibits the optimal adsorption performance. This explanation has been added to the revised manuscript (Lines 152-164).

 

Comments 4: The use of HCl/NaOH titration is appropriate, but buffering effects or ionic strength adjustment (e.g., background electrolyte) are not discussed; this could influence adsorption equilibrium.

Response 4: Thank you for pointing out this detail. We have confirmed through CaCl₂ ionic strength tests that ionic strength has almost no effect on the experimental adsorption equilibrium at low concentrations; additionally, the ionic strength introduced by HCl/NaOH titration is relatively low, so no discussion was provided on this aspect. This explanation has been added to the revised manuscript (Lines 424-427).

 

Comments 5: The equilibration time of 2880 min (48 h) contact time seems excessive for batch adsorption. Please justify.

Response 5: Thank you for your question. The reason for selecting 2880 min (48 h) as the equilibration time is as follows: both modified biochars have significantly increased specific surface areas and formed abundant micro-mesoporous structures, which require tetracycline (TCH) molecules to undergo a multi-step process of "surface adsorption → pore diffusion → micropore filling"; the diffusion resistance inside micropores prolongs the time to reach adsorption equilibrium. Additionally, the phosphate nanoparticles attached to the surface of P-BC may partially block the pores, further slowing down the internal diffusion rate of TCH molecules and requiring more time to complete adsorption. Moreover, the fitting of the isothermal adsorption model shows that the adsorption process is multi-molecular layer adsorption, where TCH molecules can continue to accumulate on the surface of already adsorbed molecules rather than being limited to monolayer saturation. Therefore, the adsorption capacity increases continuously with time, requiring a longer time to reach equilibrium. This explanation has been added to the revised manuscript (Lines 396-400，and line 503-505).

 

Comments 6: No zeta potential or elemental composition (CHN or XPS) data are reported. These are critical for linking functional group density to electrostatic interactions.

Response 6: Thank you for pointing out the deficiency in the characterization data of this study. We fully agree that zeta potential and elemental composition (CHN or XPS) data are crucial for linking functional group density to electrostatic interactions. Due to the limitations of current experimental conditions and resources, this study cannot supplement the above determinations temporarily, but the relevant correlation has been indirectly verified through existing data: FTIR shows that Zn-BC and P-BC have more oxygen-containing functional groups (-OH, -COOH, etc.) on their surfaces than BC; combined with pH experiments, it is known that the adsorption effect of the two is better when TCH is in cationic state under acidic conditions, and the removal rate of Zn-BC rebounds at pH>8 due to the cation bridging effect of Zn²⁺ in Zn(OH)Cl (confirmed by XRD), which confirms that the increased surface negative charge caused by more functional groups enhances electrostatic attraction. BET shows that the specific surface area, pore volume and adsorption capacity of the two are much higher than those of BC, providing more sites for functional groups, which is consistent with the logic that functional groups enhance adsorption. Although the current study cannot be accurately quantified due to the lack of direct characterization and has limitations, a "structure-performance-mechanism" evidence chain has been formed by combining experiments such as SEM, BET, FTIR, XRD, adsorption kinetics and isotherms, which initially confirms that adsorption is a synergistic effect of pore filling, hydrogen bonding, electrostatic attraction and cation bridging, and the conclusion is reliable.

 

Comments 7: The lack of control samples in comparison for "unmodified straw"and "unmodified biochar under identical pyrolysis conditions." Inclusion would better quantify modification effects, especially in XRD and FTIR analysis.

Response 7: Thank you for pointing out the deficiencies in control samples and characterization analysis of this study. Your suggestion to include "unmodified straw" and "unmodified biochar" for quantifying modification effects is of great value. In this study, unmodified biochar (BC) has been prepared and its FTIR characterization has been completed (the spectrum is shown in Figure 7 of the manuscript). By comparing with Zn-BC and P-BC, it is confirmed that oxygen-containing functional groups (-OH, -COOH, etc.) are significantly enhanced after modification. In the XRD test, although multiple attempts were made to characterize BC, due to its low graphitization degree after pyrolysis, many impurity peaks of amorphous carbon, weak signal of target characteristic peaks (e.g., graphite (002) crystal plane peak at 2θ=22°) and severe baseline interference, effective comparison data could not be extracted, so it was not included.​

 

Comments 8: In Figure 3 and Figure 4, although removal efficiencies are compared across modifiers, no standard deviation or error bars are shown. Statistical analysis (ANOVA or t-test) is needed to validate “significant difference.”

Response 8: Thank you for pointing out the deficiencies in the presentation of graph data and statistical analysis in this study. This is crucial for improving the scientificity and rigor of the conclusions, which we fully agree with and attach great importance to. Error bars have now been added to Figure 3 and Figure 4.

 

Comments 9: In Figure 5, magnification factors should be included.

Response 9: Thank you for pointing out the deficiency in the magnification factor labeling of Figure 5. We have added the magnification factor (MAG) at the bottom of Figure 5 to ensure readers accurately understand the magnification ratio of the SEM images and improve the completeness and readability of the figure information.

 

Comments 10: In Table 1, the reported surface area of 862 m² g⁻¹ for Zn-BC is unusually high for 750 °C biochar. The authors should confirm degassing temperature and cross-validate with pore volume data.

Response 10: Thank you for your suggestion. The degassing condition for Zn-BC is 120 °C for 1 hour, and the linear fitting degree of the BET test is > 0.999 with small data deviation; meanwhile, the specific surface area of biochar is positively correlated with the total pore volume— the more developed the pore structure, the more synchronous the improvement of both, which indicates that the BET characterization results are true and reliable. This explanation has been added to the revised manuscript(Lines 309-315).

 

Comments 11: In XRD Analysis, the claim of increased “graphitization degree” is speculative without crystallite size or d002 spacing data from Scherrer analysis.

Response 11: Thank you for pointing out the deficiency in the article's statement. Regarding the speculative issue of the "graphitization degree" statement, we have supplemented Scherrer analysis data: the graphite (002) crystallite size and d₀₀₂ spacing of P-BC are 4.86 nm and 0.407 nm, while those of Zn-BC are 3.85 nm and 0.399 nm. Meanwhile, the relevant statement has been revised in the manuscript (Lines 362-367).

 

Comments 12: The study states “optimum at 5 mg,” but the test volume is 50 mL. This equates to 0.1 g L^-1; unit conversions and mass-normalized capacities should be clarified.

Response 12: Thank you for pointing out the issues regarding unit conversion and mass normalization. As suggested, we have supplemented the key information in the manuscript, and the relevant supplements have been marked at the corresponding positions in the manuscript (Lines 375–380).

 

Comments 13: The experimental condition for adsorption performance (pH, dosages, concentration, ionic strength) shall be included in the Figure caption for better readability. 

Response 13: Thank you for pointing out this issue. We have fully supplemented key experimental conditions, including pH value, dosage, initial concentration, and ionic strength, in the captions of all figures related to adsorption performance. The relevant revisions have been added to the manuscript.

 

Comments 14: For kinetic modeling in Figure 14 and Table 2, although R² values are high, the small difference between pseudo-first- and second-order models (ΔR² ≈ 0.002) is statistically insignificant. Discussion should highlight the mechanistic rationale (surface diffusion vs. chemisorption) rather than rely solely on R².

Response 14: Thank you for pointing out the issues regarding the statistical insignificance of the R² difference in the kinetic models (Figure 14 and Table 2) and the insufficient discussion on the mechanism. We have revised the relevant content (see Lines 463–483 in the manuscript): instead of relying solely on R² values, we now clarify that physical adsorption and chemisorption coexist in the adsorption process, with the overall mechanism centered on physical adsorption dominated by surface diffusion and supplemented by chemical effects, based on experimental phenomena and characterization results.

 

Comments 15: Recheck isotherm parameter in Table 3. The Freundlich n values are < 1 (−0.56, −0.62), implying inverse adsorption behavior; this is physically implausible and likely a sign-convention or regression error.

Response 15: Thank you for pointing out the contradiction in the physical meaning of the Freundlich n values (-0.56, -0.62) in Table 3. This phenomenon arises from the difference in parameter definitions between different expression forms of the Freundlich model; after standardized conversion, the actual n values are all greater than 1, which fully conforms to the physical meaning of "easy adsorption".

 

Comments 16: The authors qualitatively discuss temperature effects but did not calculate ΔH°, ΔS°, or ΔG°. These parameters are essential to substantiate the “endothermic/physical” claims.

Response 16: Thank you for pointing out the issue regarding the thermodynamic parameters ΔH°, ΔS°, and ΔG°. When calculating these parameters for Zn-BC and P-BC, we found that the R² of linear fitting had low reliability. The key reason is that significant desorption occurred in the adsorption system at 35°C, leading to a sharp drop in the equilibrium constant K at this temperature. This changed the relationship between "lnK and 1/T" from the expected "linear increase" to "first increase then decrease", which destroyed the basis of linear fitting. Therefore, the values of ΔH°, ΔS°, and ΔG° were not presented in the manuscript.

 

Comments 17: The mechanistic Figure 17 is visually clear but oversimplified. The inclusion of charge distribution, π-π interactions, or specific complexation pathways (e.g., Zn²⁺ -O-C coordination) would improve scientific value.

Response 17: Thank you for your valuable suggestions on improving the scientific value of Mechanism Figure 17. We agree that supplementing details such as charge distribution, π-π interactions, and Zn²⁺ -O-C coordination pathways can further enrich the molecular-level mechanism explanation.​

Based on the core positioning of this study—focusing on the analysis of the correlation between adsorption performance and key interaction types—the current Figure 17 can already intuitively present the core interaction mechanisms of the adsorption process (such as electrostatic attraction, hydrogen bonding, etc.), which is sufficient to support the conclusion demonstration needs of this study. Therefore, no additional supplements to the details in the figure have been made temporarily. In subsequent extended studies focusing on in-depth mechanism analysis, we will combine methods such as XPS high-resolution spectra and DFT simulations to improve the above molecular-level details, so as to reveal the adsorption mechanism more comprehensively.

 

Comments 18: The study lacks benchmarking of q_mₐₓ values versus existing ZnCl₂- or H₃PO₄-modified biochars (e.g., 239–325 mg g⁻¹). This prevents readers from gauging the improvement magnitude.

Response 18: Thank you for pointing out the lack of literature benchmarking analysis for the q_mₐₓ values. As suggested, we have supplemented relevant content in Lines 572–574 of the manuscript。

 

Comments 19: The text alternates between “pseudo-first-order indicates physisorption” and “coexistence of chemisorption”. The evidence base (FTIR before/after adsorption or XPS binding shift) is missing to support chemisorption claims.

Response 19: Thank you for pointing out the contradictions in the description of the adsorption mechanism and the lack of characterization evidence for chemisorption in the manuscript. We agree with this comment, and due to the absence of remaining samples after the experiment, we are temporarily unable to supplement the FTIR or XPS characterization tests (before and after adsorption) to verify the chemisorption process.

 

Comments 20: The assertion that Zn²⁺ forms complexes with TCH is not experimentally verified and quantifying released Zn²⁺ would strengthen credibility of the manuscript.

Response 20: We agree with this suggestion, but due to experimental constraints, we are currently unable to directly quantify the concentration of Zn²⁺ released in the system. Therefore, we have provided indirect evidence through existing experimental data: the nonlinear characteristics of adsorption isotherms, multi-stage changes in adsorption kinetic curves, and the positive correlation between initial TCH concentration and Zn-BC adsorption capacity. These phenomena are consistent with the characteristics of Zn²⁺-mediated complexation, which can support the inference of their interaction.

 

Comments 21: The recovery of adsorption at pH 9 is attributed to cation bridging, yet Zn(OH)₂ precipitation or surface charge reversal could also explain this. Alternative interpretations should be discussed.

Response 21: As suggested, we have supplemented relevant discussions in Lines 435–437 and 546–548 of the manuscript, further improving the integrity of the pH impact mechanism.

 

Comments 22: The claim of “complementary suitability” is conceptually appealing but should be backed by comparative capacity and stability data under identical conditions (e.g., reusability, desorption cycles) for Zn-BC vs. P-BC samples.

Response 22: Thank you for pointing out the key issue that the claim of "complementary suitability" requires supplementary comparative data between Zn-BC and P-BC samples. Due to the limitations of the current research design and experimental resources, stability data analysis has not been conducted in this study. Relevant content will be incorporated as a core direction in subsequent research to more comprehensively verify the compatibility of the two materials. Currently, the expression "complementary applicability" in the paper is proposed based on the performance differences between the two under different adsorption conditions (e.g., pH range, pollutant concentration interval). Although lacking direct comparative data support, combined with the existing adsorption kinetics, isotherm, and characterization results, it can still initially reflect their potential complementary value in application scenarios. Follow-up research will supplement and improve relevant verification.

 

 

We would also like to thank you for your valuable comments and suggestions, which have greatly helped improve the quality of our manuscript. We are pleased to resubmit the revised version as requested.

 

We hope that the revisions address all your concerns and that the revised manuscript is now suitable for publication.

 

Sincerely,

Jian Xiong

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

This is a strong and practical study that turns local farm waste into a useful tool for cleaning up antibiotic pollution. The real strength is that it doesn't just make one "magic" biochar, but instead creates two with different specialties; one for high-power removal and another for tough, salty water, giving a smart, tailored solution for different types of contaminated waters. There are several issues which needs authors attention.

1. The literature cited in introduction isn't enough to validate your proposed claim. You must add recent/relevant researches with proper citation. Also, the title should be more targeted and concise.
2. I observed several subscript and superscript issues in chemical formulas. Also, there are other typos which must be addressed.
3. Most of the captions of the added Figures are misleading and the given information is incomplete. For example Figure 5. SEM images of biochar. There are four SEM images and this caption is totally lacking. In addition, check the captions of all Figures. 
4. Figure 7. FTIR spectra of biochar, Figure 8. XRD pattern of biochar. In both these Figures, the captions as well the peak assignment is incomplete. The added discussion is misleading and need corrective measures. Please take guidelines from similar studies.
5. The ithenticate report shows 28% verbatim overlap which is quite high. Rephrase to avoid this issue.
6. In these two adsorption models: Freundlich model Langmuir model; which one best fitting and how?
7. What is mechanism of adsorption of the target pollutant by the prepared adsorbate?

Author Response

Dear Reviewer,

Thanks very much for taking your time to review our manuscript. We highly appreciate all your reasonable, scientific and insightful comments and suggestions. Please find our itemized responses below, and the relevant revisions have been highlighted in the resubmitted files.

Point-by-point response to Comments and Suggestions for Authors

 

Comments 1: The literature cited in introduction isn't enough to validate your proposed claim. You must add recent/relevant researches with proper citation. Also, the title should be more targeted and concise.

Response 1: Focusing on the core argument of the introduction, we have selected and supplemented three relevant studies published in recent years in the manuscript (see Lines 76 - 96 of the manuscript). All these selected literatures are from authoritative journals in the field, which can fully ensure the timeliness and relevance of supporting the argument.

 

Comments 2: I observed several subscript and superscript issues in chemical formulas. Also, there are other typos which must be addressed.

Response 2: Regarding the issues you pointed out, we have reviewed all chemical formulas in the manuscript one by one, focusing on correcting the subscript and superscript labeling errors to ensure the format of all formulas complies with academic standards. Additionally, we have thoroughly checked and corrected all typos to avoid affecting the accurate communication of the research content.

 

 

Comments 3: Most of the captions of the added Figures are misleading and the given information is incomplete. For example Figure 5. SEM images of biochar. There are four SEM images and this caption is totally lacking. In addition, check the captions of all Figures.

Response 3: Thank you very much for pointing out the issues of misleading and incomplete information in the figure captions; your suggestions are crucial for improving the readability and rigor of the figures in the manuscript. We have comprehensively revised all figure captions in accordance with the principles of "clearly distinguishing subfigure contents, supplementing core experimental information, and unifying caption formats" to ensure each caption accurately reflects the core content of the corresponding figure.

 

 

Comments 4: Figure 7. FTIR spectra of biochar, Figure 8. XRD pattern of biochar. In both these Figures, the captions as well the peak assignment is incomplete. The added discussion is misleading and need corrective measures. Please take guidelines from similar studies.

Response 4: Thank you for your suggestion. Referring to the standards of similar studies, we have revised the captions of Figures 7 and 8. Peaks that do not affect the core experimental conclusions have not been labeled. Meanwhile, we have fully corrected the analytical discussions on FTIR and XRD to eliminate misleading expressions, with the relevant revisions in Lines 335 - 339, 341 - 345, 354 - 357 and 362- 367 of the manuscript.

Comments 5: The ithenticate report shows 28% verbatim overlap which is quite high. Rephrase to avoid this issue.

Response 5: Thank you for your reminder. As we do not have access to an iThenticate account, we have conducted a thorough review of the entire manuscript and revised content that may involve verbatim overlap, ensuring original expression while preserving the accuracy of research information.

 

Comments 6: In these two adsorption models: Freundlich model Langmuir model; which one best fitting and how?

Response 6: A comprehensive analysis of the two adsorption models (Freundlich and Langmuir) was conducted from three aspects: model assumptions, fitting parameters, and adsorption mechanisms. The Langmuir model assumes monolayer adsorption, uniform adsorption sites, no interactions between adsorbates, and reversible adsorption-desorption. In contrast, the Freundlich model assumes multilayer adsorption, heterogeneous adsorption sites, and interactions between adsorbates. Both models exhibited a good fitting degree with R² > 0.9 in this study, while the Freundlich model showed a slightly higher R² value. Considering the modified biochars possess a structure with abundant micro-mesopores and diverse functional groups, their adsorption sites are heterogeneous, which is consistent with the Freundlich model’s assumptions of "multilayer adsorption and heterogeneous surface". Additionally, the adsorption of tetracycline (TCH) on the biochar surface is a synergistic process of "pore filling (physical adsorption) + functional group interaction (chemical adsorption)". The superposition of multiple mechanisms leads to interactions between adsorbates, which is consistent with the relevant assumptions of the Freundlich model. Therefore, the Freundlich model provides a better fitting effect.

 

Comments 7: What is mechanism of adsorption of the target pollutant by the prepared adsorbate?

Response 7: The adsorbent prepared in this study possesses a large specific surface area and well-developed porous structure, enabling the adsorption of tetracycline (TCH) through physical adsorption mechanisms including pore-filling and surface diffusion dominated by the porous structure. Meanwhile, electrostatic attraction occurs between the surface charge of the adsorbent and TCH molecules, further enhancing the adsorption effect. In addition, the surfaces of Zn-BC and P-BC are rich in -OH and -PO₃H₂ groups, respectively. The hydroxyl groups of these functional groups can form hydrogen bonds with the amino and hydroxyl groups of TCH molecules, improving adsorption affinity. Under alkaline conditions, Zn-BC can further enhance TCH adsorption through cation bridging or the formation of Zn(OH)₂ precipitates from Zn²⁺. In summary, the adsorption of TCH by the adsorbent is a synergistic result driven by physical adsorption, electrostatic attraction, hydrogen bonding, and (Zn-BC-specific) cation bridging/precipitation.

 

 

 

We would also like to thank you for your valuable comments and suggestions, which have greatly helped improve the quality of our manuscript. We are pleased to resubmit the revised version as requested.

 

We hope that the revisions address all your concerns and that the revised manuscript is now suitable for publication.

 

Sincerely,

Jian Xiong

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

This research addresses the important issue of aquatic environmental contamination with pharmaceutical compounds. Currently, the search for effective methods to reduce/eliminate these compounds from water is in full swing. This work aligns with this trend and also addresses the circular economy approach. However, the work requires improvement before publication in a journal. Detailed comments are provided below:

• Lines 38-40 „Measured concentrations of antibiotics in water bodies of Xainza Town, Xi Zang, are between 0.01 and 11.27 ng·L-1” - Please include the tetracycline concentration. Concentrations in various aqueous matrices can also be included. Please also use superscript when notating units.
• Lines 48-50 „The current arsenal for aqueous TC remediation comprises conventional biological processes, bioelectrochemical (electrogenic) methods, advanced oxidation technologies, membrane separation, and adsorption [7].” - Please include any limitations or drawbacks of the methods used. This will make the advantages of adsorption more visible.
• Abstract - Using barley straw to obtain biochar is certainly beneficial and consistent with the circular economy. However, obtaining biochar from various plant materials or waste is a common phenomenon, as is the sorption of pharmaceuticals onto carbon. Please indicate achievements in this field. Describe the gap the research fills in existing knowledge. Emphasize the novelty of the research. The authors can also specify the research goal and research hypotheses.
• Please check the entire work and correct the superscripts and subscripts.
• Please provide full details of the companies from which the materials and reagents were obtained for testing. Please indicate the region/state, city, and country.
• 4. Batch Adsorption Experiment - The authors used quite high concentrations of tetracycline. How does this compare to tetracycline concentrations in the environment? Secondly, the authors used a spectrophotometric method to study the change in tetracycline concentration during sorption. What are the detection and quantification limits of the method? Did the authors consider that tetracycline is easily oxidized in an aqueous environment and that transformation products can be formed, which may have similar absorption spectra? The absorption spectrum also changes under the influence of different pH. Chromatographic studies are recommended to verify the results.
• Figures 3-4 - Was the experiment conducted once or repeated several times?
• Please mark the error bars on the all graphs in the manuscript.
• The authors discussed the adsorption mechanism. The reviewer believes that the entire results section should be commented on in conjunction with the international literature. The reviewer suggests a joint section for results and discussion. In each subsection, the authors are requested to comment on their results or observations with those of other scientists who have studied similar biochars and tetracycline or other pharmaceutical compounds. This discussion is essential and will enhance the scientific quality of the work
• Conclusions - How can the research results be used in practice? What are the limitations of the research? The authors should critically address them. What are the future research prospects?

Comments for author File: Comments.pdf

Author Response

Dear Reviewer,

Thanks very much for taking your time to review our manuscript. We highly appreciate all your reasonable, scientific and insightful comments and suggestions. Please find our itemized responses below, and the relevant revisions have been highlighted in the resubmitted files.

Point-by-point response to Comments and Suggestions for Authors

 

Comments 1: Lines 38-40 “Measured concentrations of antibiotics in water bodies of Xainza Town, Xi Zang, are between 0.01 and 11.27 ng·L^-1” - Please include the tetracycline concentration. Concentrations in various aqueous matrices can also be included. Please also use superscript when notating units.

Response 1: Thank you for your valuable suggestions. After systematic retrieval and verification of relevant research literature, only the total antibiotic concentration range (0.01~11.27 ng·L^-1) in the water bodies of Xainza Town, XiZang, is available in the current public reports. Specific measured data of tetracycline (TC) and targeted concentration records in different aqueous matrices in this region have not been retrieved yet. To ensure the authenticity and rigor of the research, we have not speculated or fabricated unvalidated information and have supplemented the description of this current situation in the revised manuscript. Meanwhile, we have standardized the unit notation throughout the text by converting the exponential parts of relevant units to superscript format.

 

Comments 2: Lines 48-50 “The current arsenal for aqueous TC remediation comprises conventional biological processes, bioelectrochemical (electrogenic) methods, advanced oxidation technologies, membrane separation, and adsorption [7].” - Please include any limitations or drawbacks of the methods used. This will make the advantages of adsorption more visible.

Response 2: Thank you for your suggestion. As requested, we have supplemented the limitations and drawbacks of conventional biological processes, bioelectrochemical (electrogenic) methods, advanced oxidation technologies, and membrane separation in Lines 54-66 of the manuscript. The advantages of the adsorption method are further highlighted through comparison.

 

Comments 3: Abstract - Using barley straw to obtain biochar is certainly beneficial and consistent with the circular economy. However, obtaining biochar from various plant materials or waste is a common phenomenon, as is the sorption of pharmaceuticals onto carbon. Please indicate achievements in this field. Describe the gap the research fills in existing knowledge.

Emphasize the novelty of the research. The authors can also specify the research goal and research hypotheses.

Response 3: Thank you for your valuable suggestions. We have refined the content of the abstract, and elaborated on the research achievements in this field, existing knowledge gaps, and the innovation of the research.

Comments 4: Please check the entire work and correct the superscripts and subscripts.

Response 4: Thank you for the suggestion. We have checked and corrected all superscripts and subscripts throughout the manuscript.

 

 

Comments 5: Please provide full details of the companies from which the materials and reagents were obtained for testing. Please indicate the region/state, city, and country.

Response 5: Thank you for the suggestion. Detailed information of the suppliers of the materials and reagents used for testing has been indicated in Lines 140 - 148 of the manuscript.

 

Comments 6: 4. Batch Adsorption Experiment - The authors used quite high concentrations of tetracycline. How does this compare to tetracycline concentrations in the environment? Secondly, the authors used a spectrophotometric method to study the change in tetracycline concentration during sorption. What are the detection and quantification limits of the method? Did the authors consider that tetracycline is easily oxidized in an aqueous environment and that transformation products can be formed, which may have similar absorption spectra? The absorption spectrum also changes under the influence of different pH. Chromatographic studies are recommended to verify the results.

Response 6: Thank you for your valuable comments. Although the tetracycline concentration used in this study is higher than that in the general environment, it is scientifically reasonable and practically meaningful. The experimental concentration refers to the tetracycline concentration level (0.1~50 mg·L^-1) in severely polluted areas such as livestock breeding wastewater and pharmaceutical wastewater discharge outlets. Moreover, experiments under high-concentration conditions can accurately quantify the maximum adsorption capacity, adsorption kinetic and thermodynamic parameters of the adsorbent, providing data and theoretical support for the adsorption and remediation of tetracycline in low-concentration actual water bodies. The UV-Vis spectrophotometric method (λ=357 nm) was used to determine the tetracycline concentration in this study, with a limit of detection (LOD) of 0.05 mg·L^-1 and a limit of quantification (LOQ) of 0.17 mg·L^-1, which is suitable for the detection of the high-concentration system in this experiment. Regarding the issue that tetracycline is prone to oxidation to form transformation products with similar absorption spectra, light-shielding operations were adopted throughout the experiment to reduce the oxidation risk. Your suggestion of verifying the results by chromatographic studies is very pertinent, but it cannot be implemented temporarily due to current conditional constraints. We will use chromatographic studies to verify the experimental results when conditions are available in the future.

 

Comments 7: Figures 3-4 - Was the experiment conducted once or repeated several times?

Response 7: Thank you for your suggestion. The experiments corresponding to Figures 3 - 4 were each repeated three times, and error bars have been added to the figures.

 

Comments 8: Please mark the error bars on the all graphs in the manuscript.

Response 8: Thank you for your valuable suggestion. We have added error bars to the three bar graphs supported by replicate experimental data. For graphs including line graphs, their results are derived from deterministic single-observation experiments and thus do not require error bars to illustrate data fluctuations, so no extra annotations have been added.

 

Comments 9: The authors discussed the adsorption mechanism. The reviewer believes that the entire results section should be commented on in conjunction with the international literature. The reviewer suggests a joint section for results and discussion. In each subsection, the authors are requested to comment on their results or observations with those of other scientists who have studied similar biochars and tetracycline or other pharmaceutical compounds. This discussion is essential and will enhance the scientific quality of the work

Response 9: Thank you for your valuable suggestions. To improve the scientific quality of the study, we have optimized the expression logic of the discussion section to strengthen its close connection with the research results on the one hand. On the other hand, in Lines 553 - 560, we have introduced research results from other scholars on similar biochars, tetracycline and related pharmaceutical compounds for comparative analysis, making the research conclusions more convincing.

 

Comments 10: Conclusions - How can the research results be used in practice? What are the limitations of the research? The authors should critically address them. What are the future research prospects?

Response 10: Thank you for the suggestion. Relevant content added in Lines 595 - 612 of the conclusion: Zn - BC (531.41 mg・g^-1) fits end - of - pipe treatment of medium - high concentration TC. P - BC, low - cost with wide pH adaptability, suits in - situ remediation of trace pollution. Both provide a solid waste recycling scheme for high - altitude areas. The study has limitations like insufficient consideration of coexisting substrates, without dynamic adsorption and pilot tests. Future research will focus on water adaptability, regeneration processes and pilot studies to promote technology application.

 

 

 

We would also like to thank you for your valuable comments and suggestions, which have greatly helped improve the quality of our manuscript. We are pleased to resubmit the revised version as requested.

 

We hope that the revisions address all your concerns and that the revised manuscript is now suitable for publication.

 

Sincerely,

Jian Xiong

 

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The authors have made meaningful improvements to the manuscript, including clearer methodological descriptions, improved figures, error bars, and more consistent kinetic/isotherm interpretations. These changes have strengthened the clarity and reproducibility of the study. However, several important scientific issues remain unresolved, and the manuscript still contains mechanistic and application-related claims that are not adequately supported by the available evidence. I therefore recommend acceptance after minor revision. The manuscript is potentially publishable, but the following issues must be addressed before it can be accepted:

1. Throughout the Discussion and Conclusion, the authors describe metal–ligand coordination, Zn-bridging, and chemisorption-dominated mechanisms as confirmed outcomes. The explanations are currently based on indirect inferences from isotherm/kinetic fitting, XRD, and surface functional groups. These observations can suggest possible interactions, but they do not confirm them. As no XPS/FTIR before–after adsorption data or Zn-leaching measurements are provided, these mechanisms should be presented as **plausible hypotheses rather than facts.  A clear acknowledgement of this limitation is needed in both the discussion and conclusion.

2. The authors mentioned data at 35 °C cause non-linear lnK vs 1/T because of desorption and so they did not present ΔH°, ΔS°, ΔG°. That is an acceptable explanation only if the authors quantify and show the lnK vs 1/T plot with discussion; simply omitting the parameters without showing the problematic data is insufficient. Since no thermodynamic parameters are reported, conclusions about endothermicity or spontaneity are not supportable.

3. Avoid over-interpretation of isotherm and kinetic fits. The manuscript still implies that pseudo-second-order fitting “proves” chemisorption or that Freundlich behaviour demonstrates multilayer adsorption. Model fits alone cannot conclusively establish mechanisms, especially when R² differences are small. Reword these sections to show that model fits suggest possible behaviours but do not definitively determine mechanistic pathways.

4. Table 3 originally had negative Freundlich n values; the authors say they corrected the sign convention. The final manuscript must present the corrected n (and explain the conversion) so readers can trust the reported qm and model fits. A footnote explaining the parameter form used for fitting and conversion is required.

 

 

Author Response

Dear Reviewer,

Thanks very much for taking your time to review our manuscript. We highly appreciate all your reasonable, scientific and insightful comments and suggestions. Please find our itemized responses below, and the relevant revisions have been green highlighted in the resubmitted files.

Comments 1: Throughout the Discussion and Conclusion, the authors describe metal–ligand coordination, Zn-bridging, and chemisorption-dominated mechanisms as confirmed outcomes. The explanations are currently based on indirect inferences from isotherm/kinetic fitting, XRD, and surface functional groups. These observations can suggest possible interactions, but they do not confirm them. As no XPS/FTIR before–after adsorption data or Zn-leaching measurements are provided, these mechanisms should be presented as **plausible hypotheses rather than facts.  A clear acknowledgement of this limitation is needed in both the discussion and conclusion.

Response 1: We would like to sincerely thank the reviewers for their valuable suggestions. The limitation has been clearly acknowledged in the Discussion section of the manuscript.The relevant revisions have been made in Lines 565–570 of the manuscript.

Comments 2:The authors mentioned data at 35 °C cause non-linear lnK vs 1/T because of desorption and so they did not present ΔH°, ΔS°, ΔG°. That is an acceptable explanation only if the authors quantify and show the lnK vs 1/T plot with discussion; simply omitting the parameters without showing the problematic data is insufficient. Since no thermodynamic parameters are reported, conclusions about endothermicity or spontaneity are not supportable. 

Response 2:We appreciate the reviewer's suggestions. Additional thermodynamic parameters have been incorporated into the manuscript to support the conclusions regarding adsorption performance and spontaneity, as shown in Table 2.The relevant revisions have been completed in Lines 455–459 of the manuscript.

Comments 3: Avoid over-interpretation of isotherm and kinetic fits. The manuscript still implies that pseudo-second-order fitting “proves” chemisorption or that Freundlich behaviour demonstrates multilayer adsorption. Model fits alone cannot conclusively establish mechanisms, especially when R² differences are small. Reword these sections to show that model fits suggest possible behaviours but do not definitively determine mechanistic pathways.

Response 3:We would like to thank the reviewer for their suggestions. The relevant explanations in the manuscript have been revised accordingly.The relevant revisions have been completed in Lines 507–512 of the manuscript.

Comments 4:Table 3 originally had negative Freundlich n values; the authors say they corrected the sign convention. The final manuscript must present the corrected n (and explain the conversion) so readers can trust the reported qm and model fits. A footnote explaining the parameter form used for fitting and conversion is required.

Response 4: We would like to thank the reviewer for their suggestions. The Freundlich equation in the manuscript has been corrected to ensure the accurate presentation of the n value. The revised content is shown in Table 4.

 

We would also like to thank you for your valuable comments and suggestions, which have greatly helped improve the quality of our manuscript. We are pleased to resubmit the revised version as requested.

 

We hope that the revisions address all your concerns and that the revised manuscript is now suitable for publication.

 

Sincerely,

Jian Xiong

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

I agree with the authors responses and suggest acceptance 

Author Response

Dear Reviewer,

Your careful review and recognition of the work’s significance have been highly encouraging to our team. We greatly appreciate the efforts you dedicated to assessing our manuscript—your insights not only validated the core contributions of our research but also helped us further refine the clarity and rigor of the content.

Please accept my wishes for your good health and smooth work.

Sincerely,

Jian Xiong

Reviewer 3 Report

Comments and Suggestions for Authors

The authors responded to all of the reviewer's comments and questions and made the necessary corrections. In its current form, the work is suitable for publication.

Author Response

Dear Reviewer,

Your careful review and recognition of the work’s significance have been highly encouraging to our team. We greatly appreciate the efforts you dedicated to assessing our manuscript—your insights not only validated the core contributions of our research but also helped us further refine the clarity and rigor of the content.

Please accept my wishes for your good health and smooth work.

Sincerely,

Jian Xiong