Adsorption of Naphthalene in Liquid Paraffin by Using Boron-Containing Nanoclay Derived from the Boron Enrichment Process Waste

Comments 1: Line 12: What is bu?

Response 1: Thank you for pointing this out. The typographical error “bu” has been corrected to “by” in Line 12 of the revised manuscript. The manuscript has been revised accordingly.

Comments 2: Please use the full term of abbreviations (XRD, SEM, XPS) in the abstract, as abstract is standalone.

Response 2: Thank you so much for useful comment. The full names of the abbreviations, namely X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), and X-ray Photoelectron Spectroscopy (XPS), have been provided at their first occurrence in the abstract. The abstract has been revised accordingly to improve clarity and readability.

Comments 3: Abstract seems generic. Please use quantitative values to support findings.

Response 3: Thank you so much for useful comment. We agree that the original abstract was overly descriptive and lacked quantitative information. Therefore, the abstract has been revised to include the most significant numerical findings of the study. Specifically, quantitative results such as the increase in BET surface area from 35.42 to 112.15 m²/g after nanoclay synthesis, the maximum Langmuir adsorption capacity of 365.20 mg/g for BNC, the adsorption equilibrium time of 90 min, and the excellent fitting of the adsorption data to the Langmuir isotherm model (R²=0.995) have been incorporated. These additions provide a clearer summary of the adsorption performance and scientific significance of the developed adsorbent.

Comments 4: Line 44: Please avoid clustered references [11][12][13][14][15].

Response 4: Thank you for pointing this out. The clustered references have been revised throughout the manuscript and consolidated into an appropriate citation format in accordance with the journal style. The manuscript has been carefully checked to ensure citation consistency.

Comments 5: Could the authors justify the conditions used for the preparation of nanoclay from boron waste in section 2.1 (calcination at 800 °C for 4 hours, hydrothermal synthesis method at 500 °C)? Why were these temperatures selected?

Response 5: Thank you for this important comment. Upon re-examination of our experimental records, we identified an error in the manuscript. The hydrothermal synthesis temperature was incorrectly reported as 500 °C. The correct hydrothermal treatment temperature was 270 °C for 2–4 h. We sincerely apologize for this oversight, and the manuscript has been revised accordingly. The calcination temperature of 800 °C for 4 h was selected to promote the dehydroxylation and structural transformation of the boron-containing minerals, facilitating the development of a porous framework and increasing the accessibility of active adsorption sites. Subsequently, hydrothermal treatment at 270 °C was employed to promote the reorganization of the borate–silicate framework and the formation of boron-containing nanoclay structures with enhanced surface area and adsorption properties. To address the reviewer’s concern, additional explanations regarding the rationale for the selected preparation conditions have been incorporated into Section 2.2 of the revised manuscript.

Comments 6: Why is the Figure 1 (Scheme 1) given and cited in the Materials and Methods section? It describes the potential mechanism of naphthalene adsorption. It is advised to move this figure in the last section of the mechanistic description. Authors can place the schematic illustration of processes used to synthesize nanoclay in this section.

Response 6: Thank you so much for useful comment. We agree with the reviewer that the original Scheme 1 illustrates the proposed adsorption mechanism rather than the experimental preparation procedure. Therefore, the scheme has been moved from the Materials and Methods section to the end of the mechanistic discussion in the Results and Discussion section, where it is more appropriately associated with the interpretation of the adsorption results. Furthermore, the caption of Scheme 1 has been revised to explicitly indicate that it presents the proposed adsorption mechanism of naphthalene onto boron-containing nanoclay (BNC) based on the experimental findings. The manuscript has been revised accordingly to improve the logical flow and presentation of the study.

Comments 7: In section 2.4 (Adsorption Experiments). The authors used the repetitive information about isotherms and kinetics and so on at multiple places, making the section too lengthy. It is advised to review and rearrange the whole section. Reduce the repeated information.

Response 7: Thank you so much for useful comment. We agree that some descriptions of the adsorption kinetics, diffusion, and isotherm models were unnecessarily detailed and partially repetitive. Therefore, Section 2.4 has been carefully revised and reorganized to improve clarity and readability. Repeated explanations have been removed or condensed, while essential methodological information has been retained. The revised section now provides a more concise description of the experimental procedures and analytical models, avoiding redundancy with the Results and Discussion section.

Comments 8: Can authors explain, clarify, and justify how pH was measured and adjusted for liquid paraffin? Liquid paraffin is nonpolar and nonconductive. How the reported pH values physically meaningful in this non-aqueous system?

Response 8: Thank you so much for useful comment. We agree that pH cannot be directly measured or adjusted in liquid paraffin because it is a nonpolar and nonconductive medium. We would like to clarify that the reported pH values do not refer to the liquid paraffin phase. In our experiments, the pH was adjusted using dilute HCl and NaOH solutions in the naphthalene solution prior to the adsorption studies. Therefore, the pH values reported in the manuscript correspond to the naphthalene solution and not to the paraffin medium itself. To avoid any misunderstanding, the relevant descriptions in the Materials and Methods and Results sections have been revised and clarified accordingly. The following statement has been added to the revised manuscript and highlighted in red: “For the pH-dependent adsorption studies, the pH of the naphthalene solution was adjusted to the desired values (pH 4-9) using dilute HCl or NaOH solutions before the adsorption experiments. The reported pH values correspond to the naphthalene solution and not to the liquid paraffin phase, since pH is not physically defined for nonpolar, nonconductive media such as liquid paraffin.”

Comments 9: Line 188-189: Authors mentioned that “The chemical composition of BEW is presented in Table 1. BEW primarily consists of SiO2 and B2O3, with minor contributions from CaO, MgO, and Na2O, while other oxides are detected only in trace amounts”. The statement contradicts the values in Table 1. How 17.52% of CaO, 10.32% of MgO, 14.20% of SO3, 7.52% of Na2O are minors’ contributors?

Response 9: Thank you so much for useful comment. We agree with the reviewer that the term “minor contributions” was not appropriate considering the oxide contents reported in Table 1. The statement has been revised to more accurately reflect the chemical composition of BEW. The revised text now indicates that, in addition to SiO₂ and B₂O₃, significant amounts of CaO, MgO, SO₃, and Na₂O are present, while the remaining oxides occur at relatively low concentrations or trace levels. The manuscript has been corrected accordingly. The revised statement, highlighted in red in the revised manuscript, is as follows: “The chemical composition of BEW is presented in Table 1. BEW is mainly composed of SiO₂ and B₂O₃, together with significant amounts of CaO, MgO, SO₃, and Na₂O. The remaining oxides are present in relatively low concentrations.”

Comments 10: Line 217-218: Authors mentioned that “As shown in Fig. 3B, the SEM image of BNC reveals that calcination of the BEW in the development of a porous structure with a smoother surface and an increased specific surface area”. However, the SEM images do not reflect the statement; it seems BEW is more porous than BCN.

Response 10: Thank you so much for useful comment. We agree that the original statement may have overstated the conclusions that can be drawn directly from the SEM images. SEM analysis primarily provides qualitative information regarding surface morphology and particle distribution, whereas specific surface area is more accurately determined by BET analysis. Therefore, the relevant discussion has been revised. The revised text emphasizes that the morphology of BNC differs markedly from that of BEW, exhibiting a more homogeneous distribution of finely aggregated particles and nanostructured features. These morphological changes indicate that the calcination and hydrothermal treatment processes significantly altered the microstructure of the precursor material. While SEM analysis provides qualitative evidence of these structural modifications, the enhancement in specific surface area was quantitatively verified by BET measurements rather than direct SEM observations. In addition, higher-magnification SEM images have been included in the revised manuscript to better illustrate the morphological differences between BEW and BNC. The revised discussion has been highlighted in red in the revised manuscript.

Comments 11: The whole discussion about the pH effect seems meaningless in normal conditions. What is meant by "positively charged naphthalene species"? The proposed mechanism does not make chemical sense. Could authors justify that?

Response 11: Thank you so much for useful comment. We agree with the reviewer that the original discussion of the pH effect was not sufficiently accurate. Naphthalene is a neutral, non-ionizable aromatic hydrocarbon under the experimental conditions investigated in this study, and therefore the expression “positively charged naphthalene species” was inappropriate. We sincerely appreciate the reviewer for identifying this issue. Accordingly, the relevant discussion has been revised. References to the ionization of naphthalene and electrostatic interactions involving positively charged naphthalene species have been removed from the manuscript. The revised interpretation emphasizes that the observed pH-dependent behavior is more reasonably associated with changes in the surface properties of the adsorbent, including the protonation/deprotonation of surface functional groups and variations in the accessibility of adsorption sites. The adsorption mechanism is primarily attributed to π–π interactions, van der Waals forces, hydrophobic interactions, and pore-filling effects rather than electrostatic attraction involving ionized naphthalene species. The corresponding text has been revised and highlighted in red in the revised manuscript.

Comments 12: Why the adsorption capacity of BEW is higher than BCN in Figure 4?

Response 12: Thank you so much for useful comment. We agree with the reviewer that the original Figure 4 was inconsistent with the adsorption performance and characterization results discussed throughout the manuscript. Upon careful re-examination of the original data, we identified an error in the figure labeling. The legends corresponding to BEW and BNC were inadvertently interchanged during figure preparation. The figure has been corrected in the revised manuscript. After correction, the adsorption capacity of BNC is consistently higher than that of BEW over the investigated pH range, in agreement with the BET surface area measurements and the overall adsorption performance of the synthesized nanoclay. We sincerely apologize for this oversight. The corrected Figure 4 has been included in the revised manuscript and highlighted in red.

Comments 13: The manuscript contains many formatting errors particularly references citations.

Response 13: Thank you for this important comment. We have carefully reviewed the entire manuscript and corrected the formatting inconsistencies, with particular attention to the reference citations. The citation style has been revised throughout the manuscript to ensure consistency and compliance with the journal guidelines. The revised manuscript has been thoroughly checked for formatting accuracy.

1. Point-by-point response to Comments

Comments 1: “hydrothermal synthesis at 500 °C for 2–4 h” – Hydrothermal synthesis is usually conducted in a closed vessel under autogenous pressure at temperatures ≤ 300 °C. Is 500 °C truly feasible? Additionally, the pH is adjusted to 5–9 during synthesis—how is this adjustment performed, and what is its purpose?

Response 1: Thank you for pointing this out. Upon re-examination of our experimental records, we identified an error in the manuscript. The hydrothermal synthesis temperature was incorrectly reported as 500 °C. The correct hydrothermal treatment temperature was 270 °C for 2–4 h. We sincerely apologize for this oversight, and the manuscript has been revised accordingly. The calcination temperature of 800 °C for 4 h was selected to promote the dehydroxylation and structural transformation of the boron-containing minerals, facilitating the development of a porous framework and increasing the accessibility of active adsorption sites. Subsequently, hydrothermal treatment at 270 °C was employed to promote the reorganization of the borate–silicate framework and the formation of boron-containing nanoclay structures with enhanced surface area and adsorption properties. Regarding the pH adjustment, the synthesis medium was adjusted to the desired pH range (5–9) using dilute HCl and NaOH solutions. The purpose of pH adjustment was to control the hydrothermal reaction environment, thereby influencing the dissolution–reprecipitation processes, structural reorganization, and formation of the boron-containing nanoclay. Additional clarification regarding the hydrothermal synthesis conditions and the rationale for pH adjustment has been incorporated into Section 2.2 of the revised manuscript and highlighted in red.

Comments 2: In the XPS spectrum (Fig. 2), the O²⁻ peak is labelled as 513.1 eV, but the standard binding energy is around 530–533 eV—please verify. The TEM image (Fig. 3C) has insufficient resolution to clearly show the nanolayered structure. The BET surface area increased from 35.42 (BEW) to 112.15 m²/g (BNC), but the isotherm type, pore‑size distribution, and raw data are not provided; these should be added.

Response 2: Thank you so much for useful comment. We carefully re-examined the characterization data and revised the manuscript accordingly. First, we identified a typographical error in the XPS analysis. The O 1s binding energy was incorrectly reported as 513.1 eV. The correct value is 531.1 eV, which is consistent with the characteristic binding energy range of oxygen-containing species. This error has been corrected in both the figure and the manuscript. Second, to better demonstrate the nanostructured morphology of the synthesized material, the original TEM image has been replaced with a higher-resolution image. The revised TEM micrograph more clearly illustrates the nanostructural features of the boron-containing nanoclay. All corresponding revisions have been highlighted in red in the revised manuscript. Regarding the BET characterization, we agree that N₂ adsorption–desorption isotherms, pore-size distribution analysis, and the corresponding raw data would provide additional insight into the textural properties of the materials. However, these data were not included in the original scope of the study and could not be incorporated within the revision period. Nevertheless, the BET surface area measurements (35.42 m²/g for BEW and 112.15 m²/g for BNC) clearly demonstrate the substantial increase in specific surface area following the synthesis process. We acknowledge this limitation and have clarified the discussion accordingly in the revised manuscript.

Comments 3: The experiments were performed in “liquid paraffin”, yet pH values (4.0–9.0) were adjusted—pH adjustment is typically carried out in aqueous systems. Liquid paraffin is a non‑polar, hydrophobic medium. How was the pH adjusted and measured in it? The manuscript provides no methodological description (e.g., whether a buffer or an emulsion system was used). Please explain the operational principle and feasibility in detail.

Response 3: Thank you so much for useful comment. We agree that pH cannot be directly measured or adjusted in liquid paraffin because it is a non-polar and nonconductive medium. We would like to clarify that the reported pH values do not refer to the liquid paraffin phase itself. In our experiments, the pH was adjusted using dilute HCl and NaOH solutions in the naphthalene solution prior to the adsorption studies. Therefore, the reported pH values correspond to the naphthalene solution and not to the liquid paraffin medium. No buffer or emulsion system was employed in this study. To avoid any misunderstanding, the Materials and Methods section has been revised to explicitly clarify the experimental procedure. The following statement has been added to the revised manuscript and highlighted in red: “For the pH-dependent adsorption studies, the pH of the naphthalene solution was adjusted to the desired values (pH 4-9) using dilute HCl or NaOH solutions before the adsorption experiments. The reported pH values correspond to the naphthalene solution and not to the liquid paraffin phase, since pH is not physically defined for nonpolar, nonconductive media such as liquid paraffin.” We appreciate the reviewer’s observation, which helped us clarify this point and improve the description of the experimental methodology.

Comments 4: Naphthalene concentration in paraffin solution was measured by UV spectrophotometry at 275 nm. However, paraffin itself has significant absorption in the UV region, which may interfere with the determination. A blank spectrum of paraffin and spike‑recovery experiments are recommended.

Response 4: Thank you so much for useful comment. We agree that potential matrix effects should be considered when UV-Vis spectroscopy is applied to determine naphthalene concentrations in liquid paraffin. In the present study, the calibration curve was established using naphthalene standards prepared directly in the liquid paraffin medium rather than in an aqueous solvent. Therefore, the absorbance contribution of the paraffin matrix was inherently accounted for during the calibration procedure. The concentration of naphthalene was subsequently determined using this matrix-matched calibration curve, minimizing potential interference from the liquid paraffin phase. We acknowledge that blank spectrum measurements and spike-recovery experiments could provide additional validation of the analytical method. However, such experiments were not included within the scope of the present study. Nevertheless, the excellent linearity of the calibration curve (R² = 0.998) and the reproducibility of the adsorption results indicate that the UV-Vis method provided reliable concentration measurements under the experimental conditions employed. To improve clarity, additional information regarding the calibration procedure has been included in the revised manuscript.

Comments 5: page 8: The authors state that “increasing pH increases the negative surface charge, enhancing electrostatic attraction with positively charged naphthalene species.” But naphthalene is a neutral polycyclic aromatic hydrocarbon and does not become positively charged over the pH range 4-9. Please provide additional explanation and solid supporting data.

Response 5: Thank you so much for useful comment. We fully agree with the reviewer that naphthalene is a neutral polycyclic aromatic hydrocarbon under the experimental conditions investigated in this study and does not become positively charged within the pH range of 4-9. Therefore, the original statement referring to “positively charged naphthalene species” was inaccurate, and we sincerely appreciate the reviewer for identifying this issue. Accordingly, the relevant discussion has been revised. References to the ionization of naphthalene and electrostatic attraction involving positively charged naphthalene species have been removed from the manuscript. The revised interpretation emphasizes that the observed pH-dependent adsorption behavior is more likely related to changes in the surface properties of the adsorbents, including the protonation and deprotonation of surface functional groups and the resulting variations in the accessibility of adsorption sites. Since naphthalene remains neutral throughout the investigated pH range, the adsorption process is more reasonably attributed to π-π interactions, van der Waals forces, hydrophobic interactions, and pore-filling effects rather than electrostatic attraction involving ionized naphthalene species. The corresponding section has been revised and highlighted in red in the revised manuscript.

Comments 6: The Langmuir constants *K_L* for BEW and BNC are –0.0068 and –0.0179 L/mg, respectively. *K_L* should be positive (reflecting affinity). What does a negative value indicate? Can it be explained?

Response 6: Thank you so much for useful comment. We agree with the reviewer that the Langmuir constant (KL), which reflects the adsorption affinity between the adsorbent and adsorbate, should be positive. Upon re-examination of the original calculations and fitting results, we identified a typographical error in the reported values. During manuscript preparation, the positive KL values were inadvertently presented with negative signs. The Langmuir fitting results themselves were correctly calculated, and only the signs reported in the table were erroneous. The KL values have been corrected in the revised manuscript and are now reported as positive values, consistent with the physical interpretation of the Langmuir adsorption model. We sincerely apologize for this oversight. The corrected table has been revised accordingly and highlighted in red.

Comments 7: In Fig. 6, the adsorption capacity of BEW at 120 min is ~250 mg/g, whereas the Langmuir *q_m* in Table 4 is only 102.98 mg/g. Similarly, in Fig. 7 the capacity of BNC is ~400 mg/g, but Table 4 gives a *q_m* of 214.60 mg/g. Please explain the large discrepancies.

Response 7: Thank you so much for useful comment. We agree with the reviewer that there was a clear inconsistency between the adsorption capacities shown in Figures 6 and 7 and the Langmuir maximum adsorption capacities reported in Table 4. Upon careful re-examination of the original data and calculations, we identified an error in the calculation of the Langmuir qm values. The Langmuir maximum adsorption capacities have been recalculated and corrected as 247.59 mg/g for BEW and 365.20 mg/g for BNC. These corrected values are now consistent with the adsorption capacities observed in the kinetic experiments. Table 4 and the related discussion have been revised accordingly in the revised manuscript, and the corrected values have been highlighted in red. We sincerely apologize for this oversight.

Comments 8: The text is rather disordered, with many writing errors. The authors are urged to carefully correct and revise the manuscript.

Response 8: Thank you so much for useful comment. We carefully revised the entire manuscript to improve its clarity, organization, and overall presentation. The manuscript was thoroughly proofread, and numerous grammatical, typographical, formatting, and stylistic errors were corrected throughout the text. In addition, several sections were reorganized and clarified to improve the logical flow of the manuscript and the interpretation of the experimental results. Reference formatting, figure captions, table presentations, and in-text citations were also carefully checked and revised. We believe that these revisions have significantly improved the readability and scientific quality of the manuscript.

Comments 9: The reference list is inconsistent, with duplicate numbering. Self‑citations are excessive.

Response 9: Thank you so much for useful comment. We carefully reviewed the reference list and corrected the inconsistencies in the citation numbering and formatting throughout the manuscript. Duplicate numbering and formatting issues have been resolved in the revised version. Regarding self-citations, we re-evaluated all cited references and retained only those that were directly relevant to the objectives, methodology, and discussion of the present study. Several unnecessary self-citations were removed, and additional references from the broader literature were considered where appropriate. We believe that the revised reference list now provides a more balanced and accurate representation of the current state of research in this field.

Comments 1: About the pH adjustment, authors mentioned that the pH was adjusted for the naphthalene solution, not liquid paraffin. So the question arises: how was the naphthalene solution prepared prior to adding to the liquid paraffin? Which solvent was used? Naphthalene has very low solubility in water. How exactly were the pH-dependent experiments conducted? Did they prepare a naphthalene solution first with solvent, then added it to liquid paraffin, and then added adsorbents? Or they performed the experiments in naphthalene solution without liquid paraffin. Please mention these things very clearly in the paper.

Response 1: Thank you for pointing this out. We appreciate the reviewer's concern regarding the experimental procedure used for the pH-dependent adsorption studies. To clarify, the pH-dependent experiments were not carried out by adjusting the pH of the liquid paraffin phase. Instead, a dilute aqueous naphthalene solution was first prepared at the concentrations employed in the adsorption experiments. The pH of this solution was adjusted to the desired values (pH 4-9) using dilute HCl or NaOH solutions. Subsequently, the pH-adjusted naphthalene solution was brought into contact with the liquid paraffin phase, and the adsorption experiments were carried out in the presence of the adsorbent. No buffer or emulsion system was used. To avoid any ambiguity, the Materials and Methods section has been revised to provide a clear step-by-step description of the experimental procedure, including the preparation of the naphthalene solution, the pH adjustment process, and the subsequent adsorption experiments in the liquid paraffin medium. The corresponding revisions have been highlighted in red in the revised manuscript.