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Enhanced Adsorption of Aqueous Ciprofloxacin Hydrochloride by a Manganese-Modified Magnetic Dual-Sludge Biochar

Water 2025, 17(8), 1229; https://doi.org/10.3390/w17081229

by Jingxi Tie^1,*, Mengjia Yan¹, Sihao Shao¹ and Xiaohan Duan²

Reviewer 1:

Hassan Anwer

Reviewer 2:

Bhaskar Bhaduri

Reviewer 3: Anonymous

Reviewer 4: Anonymous

Reviewer 5: Anonymous

Water 2025, 17(8), 1229; https://doi.org/10.3390/w17081229

Submission received: 28 January 2025 / Revised: 26 March 2025 / Accepted: 27 March 2025 / Published: 20 April 2025

(This article belongs to the Section Wastewater Treatment and Reuse)

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The topic is novel and relevant considering that the potential of manganese-modified magnetic dual-sludge 10 biochar (Mn@MDSBC) is still unexplored. Furthermore, the health impacts of Ciprofloxacin hydrochloride (CIP) make it a significant environmental pollutant. The introduction does not explicitly mention the environmental and health impacts of CIP including its amount present in the environment, how it affects living beings and ecosystems and what amount of CIP in aquatic bodies is deemed safe.
The specific amount of adsorbent used for adsorption studies has not been mentioned anywhere. 3. Hyperlinking of references is absent in most parts of the paper.
Methodology is not quite clear. Specific mass/volume of various chemicals used during the preparation is missing.
In introduction, important background information is missing on how the individual adsorbents from the composites have previously been used in adsorption studies. There is only one with specific removal rate and parameters. Gap has been identified in the last paragraph of introduction, but the significance of filling this gap is missing.
In Methodology, Chemicals and Materials including Sludge procurement part is missing.
Line 79 please mention the amount of mass used in grams or milligrams to prepare varying ratios of MnCl₂and IBWS+PMS is missing.
Line 85 please mention the specific amount of CIP dissolved in deionized water.
Line 86 please mention the value of pH adjusted through HCl and NaOH solution.
Line 90 please mention the specific concentration of CIP used in 20 mL of CIP-bearing solution.
Fig S1 description: please mention the specific concentration of Mn@DSBC used.
Fig S1 please describe Figure C as well.
In Discussion section, Effect of mass ratio (3.1.1) and pyrolysis temperature (3.1.2) has been reported but the reasons for the results have not been explored and reported.
Line 130 and 131, there is no literature reference to confirm the BET values.
SEM and XRD analysis results are also not backed up with the literature about similar materials.
Overall characterization results have been reported but not discussed thoroughly how these specific characteristics of Mn@DSBC make it an efficient adsorbent for CIP.
The results for Zeta potential results were not reported and discussed, although it has been mentioned that the characterization techniques were performed using specific instruments in line 100 to 102.
Line 152 to 155 discussed the existence of CIP in cationic form but didn’t provide enough literature to justify it.
Line 227, Figure 8 is not present in the document or Appendix.
In the description of figure 5 mention the concentration of Adsorbent used for reusability experiments.
Line 310 and 311 please mention reference.
Line 323 please mention the specific mass ratio or varying mass ratios of mMnCl2:m(IBWS+PMS). Also mention the amount of adsorbent used for the adsorption of CIP.

Comments on the Quality of English Language

NIL

Author Response

Dear Reviewer:

Thank you very much for your comments concerning our manuscript. Those comments are all valuable and very helpful for revising and improving our paper. The manuscript has already been revised by taking all these suggestions into account and substantial corrections have been added to the revised manuscript which we hope to meet with approval. The revised portions are marked in red in the paper. The main corrections in the paper and the responds to the your comments are as following.

Answer 1: Thank you for the comments on our manuscript. The following content was incorporated into the first paragraph of the manuscript to provide an introduction to CIP, highlighting its concentration levels and negative impacts in paragraph 1:

The concentration of CIP detected in different types of wastewater ranges from less than 1 μg to tens of mg, while in the aquatic environment, the concentration of CIP is generally below 1 microgram [4]. CIP exhibits resistance to microbiological degradation, primarily attributed to its highly stable chemical structure. The presence of CIP in water can trigger the development of antibiotic resistance in microbial communities, thereby posing a significant threat to the aquatic ecosystems. Moreover, CIP exhibits chronic toxicity to aquatic organisms, which can result in bioaccumulation within their tissues. This bioaccumulation can potentially propagate through the food chain, ultimately posing risks to human health via dietary intake [5-7].

Comment 2: Methodology is not quite clear. Specific mass/volume of various chemicals used during the preparation is missing.

Answer2: Given the diverse objectives of material synthesis, particularly during the initial exploratory phase of optimizing preparation conditions, it is impractical to specify precise reagent quantities for each synthesis process. Nevertheless, we have provided the ratios of raw materials, which can serve as a foundation for conducting experiments. we give the parameters and procedures as follows:

To prepare the BC, PMS was first rinsed with purified water until the its reached neutrality. Both PMS and IBWS were then rinsed and dehydrated at 105°C for 2 h, followed by pulverization to pass through an 80-mesh sieve. The two raw materials were blended at a mass ratio of IBWS to PMS of 1:2.5, a proportion previously determined in our study to be optimal for obtaining a 105 g mixture. A 3g mixture was calcined in a tube furnace at 500°C for 3 h, under a constant N₂ flow of 300 mL/min. The cooled black powder was smashed and sieved through an 80-mesh screen, and labeled as MDSBC.

MnCl₂ was combined with the mixture of IBWS and PMS (IBWS+PMS) at various mass ratios of MnCl₂to (IBWS+PMS)= 2:1, 1:1, 1:2, 1:3, 1:4, 1:5, 1:6, and 1:7 to prepare Mn@MDSBC. These mixtures were added to 100 mL of 0.1 M NaOH solution, sonicated for 30 mins, mixed for 6 h, and then filtered and dried to pass through an 80-mesh sieve again. The resulting combinations were treated under the same conditions as described previously. The final composites were identified as Mn@MDSBC.

Comment 5 : In introduction, important background information is missing on how the individual adsorbents from the composites have previously been used in adsorption studies. There is only one with specific removal rate and parameters. Gap has been identified in the last paragraph of introduction, but the significance of filling this gap is missing.

Answer 5: Two examples were cited in the introduction section, and the experimental conditions were detailed as follows:

See paragraph 3: For example, Yu et al. employed a magnetic S/N co-doped BC to remove tetracycline from aqueous solutions. The results demonstrated that the composite exhibited a magnetization of 17.80 emu/g, enabling it to be easily separated from water. After four cycles of reuse, the composite retained 88.3% of its initial adsorption capacity, with only an 11.7% loss compared to the fresh composite under conditions of 4 g/L adsorbent dosage, 298 K reaction temperature, a solution pH value of 7 and tetracycline concentration of 100 mg/L [25].

See paragraph 4: For instance, Shao et al. developed a rice straw biochar modified with KMnO4 and NaOH, and evaluated its performance in removing CIP from water. The study revealed that manganese oxide particles (MnOₓ) roughened the surface of the biochar, thereby significantly increasing its specific surface area, which enhanced the biochar’s adsorption efficiency for CIP. The biochar achieved a maximum CIP adsorption capacity of 32.25 mg/g under conditions of 0.3 g/L adsorbent dosage, 298 K reaction temperature, and a solution pH value of 3 [27].

Comment 6: In Methodology, Chemicals and Materials including Sludge procurement part is missing.

Answer 6: We added a new section as follows:

2.1 Chemicals and materials

MnCl₂ (>99%, Shanghai McLean Reagent Co., Ltd), NaOH (AR,Tianjin Kermel Chemical Reagent Co.,Ltd), HCl (Guangzhou Chemical Reagent Factory), CIP (>99%, Baikesaisi Biotechnology Co., Ltd). PMS and IBWS used in this study were obtained from a paper mill in Xinmi in Henan Province and from a drinking water treatment plant in Huludao in Liaoning Province, respectively.

Comment 7: Line 79 please mention the amount of mass used in grams or milligrams to prepare varying ratios of MnCl₂ and IBWS+PMS is missing.

Answer 7: Thank you for your kind reminder. See the change in paragraoh 1 in section 2.2 as follows:

The two raw materials were blended at a mass ratio of IBWS to PMS of 1:2.5, a proportion previously determined in our study to be optimal for obtaining a 105 g mixture.

Comment 8: Line 85 please mention the specific amount of CIP dissolved in deionized water.

Answer 8: See section 2.2 : A stock solution was prepared by dissolving 1 g of CIP (Baikesaisi Biotechnology Co., Ltd) in 1 L of water, which was subsequently diluted to achieve the desired concentrations for each group of static adsorption tests.1M HCl (Tianjin Kermel Chemical Reagent Co.,Ltd) and NaOH solutions were employed to adjust the solution pH from 3 to 9.

Comment 9 : Line 86 please mention the value of pH adjusted through HCl and NaOH solution.

Answer 9: Revised as follows: 1 M HCl and NaOH solutions were employed to adjust the solution pH from 3 to 9.

Comment 10: Line 90 please mention the specific concentration of CIP used in 20 mL of CIP-bearing solution.

Answer 10: see section 2.3: Static CIP adsorption tests were conducted in triplicate using Erlenmeyer flasks, with each holding 20 mL (80-360mg/L) of artificial CIP-bearing wastewater and 0.05 g of adsorbent.

Comment 11: Fig S1 description: please mention the specific concentration of Mn@DSBC used.

Comment 12: Fig S1 please describe Figure C as well.

Answer 12: We did give the description of Fig S1C in section 3.1.2 as follows:

3.1.2 Influence of pyrolysis temperature on CIP adsorption

CIP adsorption by Mn@MDSBC increased from 75.46 mg/g to 79.28 mg/g as the pyrolysis temperature rose from 400 ℃ to 500 ℃ ( Fig.S1c), but decreased to 36.14 mg/g as the temperature increased to 700 ℃. As a result, 500 ℃ was determined as the optimal temperature for Mn@MDSBC preparation.

Comment 13: In Discussion section, Effect of mass ratio (3.1.1) and pyrolysis temperature (3.1.2) has been reported but the reasons for the results have not been explored and reported.

Answer 13:The reasons underlying the results presented in Sections 3.1.1 and 3.1.2 are highly complex and multifaceted. Therefore, they are beyond the scope of this manuscript. We will address these reasons in a subsequent publication.

Comment 14: Line 130 and 131, there is no literature reference to confirm the BET values.

Answer 14: Two reference has cited as follows：
These values confirm that Mn@MDSBC was a typical composite with both micropores and mesopores, which contributed to its high adsorption capacity [38, 39].

We cited two references to confirm the BET values.

Huang, Y., Li, S.X., Chen, J.H., Zhang, X.L., Chen, Y.P.: Adsorption of Pb(II) on mesoporous activated carbons fabricated from water hyacinth using H3PO4 activation: Adsorption capacity, kinetic and isotherm studies. Appl. Surf. Sci. 293, 160-168 (2014). http://dx.doi.org/10.1016/j.apsusc.2013.12.123
Samson, T., Aminah, U., Rika, T.Y., Ridla, B., Budi, R.P., Wulan, T.W., Arifutzzaman, A., Mohamed, K.A., Munawar, K.: Mesoporous metal oxide via nanocasting: Recent advances on types of templates, properties, and catalytic activities. Mater. Today Commun. 40, 110152 (2024). http://dx.doi.org/10.1016/j.mtcomm.2024.110152

The two cited references explicitly define mesoporous materials as those with pore sizes ranging from 2 to 50 nm. Our material exhibits an average pore size of 14.53 nm (Table S1), yet it also features pores smaller than 2 nm. Consequently, it can be characterized as a material that encompasses both micropores and mesopores.

Comment 15: SEM and XRD analysis results are also not backed up with the literature about similar materials.

Answer 15: The morphological characteristics observed via Scanning Electron Microscopy (SEM) are significantly influenced by the preparation conditions of the material, including the types and ratios of raw materials used, as well as variations in the preparation process. Given these factors, SEM results primarily serve to describe the unique surface and structural features of the material itself and are generally not supported by specific references.

In contrast, we have identified relevant references to support our X-ray Diffraction (XRD) characterization results, which are detailed as follows:

Fig.S3 presents the XRD spectrum of Mn@MDSBC, which closely matched the standard diffraction patterns of carbon (C) (PDF#04-007-2136), Fe₃O₄ (PDF#04-001-7822), and MnO₂(PDF#04-007-3892). The peaks at 2θ = 29.50°, 42.21°, 52.33°, 61.22°, and 69.40° corresponded to the (110), (200), (211), (220), and (013) carbon crystallographic planes, respectively. The peaks at 2θ = 35.45°, 43.08°, 47.17°, 56.97°, and 62.56° corresponded to the (110), (200), (211), (220), and (311) crystal planes of Fe₃O₄, respectively [42]. The peaks at 2θ = 28.85°, 43.03°, 56.35°, 57.76°, 59.76°, 66.82°, and 68.57° corresponded to the (110), (111), (211), (121), (220), (310), and (130) crystal planes of MnO₂, respectively, confirming that manganese was effectively incorporated onto the surface of MDSBC [43]. The lower peak intensities of Fe₃O₄ and MnO₂ were ascribed to the dominant intensity of the carbon peaks, which overshadowed the weaker signals of these compounds.

Comment 16: Overall characterization results have been reported but not discussed thoroughly how these specific characteristics of Mn@DSBC make it an efficient adsorbent for CIP.

Answer 16: The biochar developed in this study exhibits two key characteristics: first, its magnetism, which arises from the high-temperature calcination of iron-based waterworks sludge under anaerobic conditions; and second, its manganese modification. The two characteristics are effectively manifested through both its physicochemical characterization and adsorption performance.

We depicted the improvement of the composite by modification as follows:

In section 3.1.1: MDSBC had a CIP adsorption capacity of 51.132 mg/g, which was significantly lower than those of all eight types of Mn@MDSBCs, confirming that manganese modification enhanced CIP adsorption. As the mass ratio of MnCl2 to (IBWS+PMS) increased from 2:1 to 1:2, the CIP adsorption capacity increased from 76.58 mg/g to 80.39 mg/g. However, as the mass ratio continued to decrease from 1:2 to 1:3, 1:4, 1:5, 1:6, and 1:7, the adsorption capacity gradually decreased to 79.72 mg/g, 79.39 mg/g, 79.28 mg/g, 76.14 mg/g, and 74.67 mg/g, respectively (Fig. S1b). Statistical analysis revealed that as the mass ratio increased from 1:2 to 1:5, no significant differences (p>0.05) in the adjacent adsorption capacity were detected among the groups. In contrast, when the mass ratio was further increased to 1:6, significant differences (p<0.05) in adsorption capacity emerged. Meanwhile, the saturation magnetic induction values of the seven Mn@MDSBCs synthesized at different ratios were 6.43 emu/g, 7.54 emu/g, 7.75 emu/g, 7.82 emu/g, 7.97 emu/g, 8.11 emu/g and 8.57 emu/g, respectively. The results indicate that the magnetism of the synthesized material increased as the mass ratio of the two components decreased from 2:1 to 1:7. This enhancement was attributed to the increased iron content in the raw materials as the mass ratio was reduced. As a result, more magnetic substances were formed during the high-temperature calcination process under anaerobic conditions.

Comment 17: The results for Zeta potential results were not reported and discussed, although it has been mentioned that the characterization techniques were performed using specific instruments in line 100 to 102.

Answer 17: Figure 1 shows the effect of initial solution pH on CIP uptake by Mn@MDSBC and its zeta potential under different pH values

Figure 1. Effect of initial solution pH on CIP uptake by Mn@MDSBC and its zeta potential under different pH values (Adsorbent dosage: 0.05 g/ 20 mL, Temperatue: 25 ℃, Time: 12 h).

We discussed the relationship between CIP adsorption and zeta potential in section 3.1 as follows:

The pH value is a critical determinant in the adsorption procedure as it influences both the form of adsorbate and surface properties of the adsorbent. CIP existed in its positively charged cationic state (CIP⁺) when the solution pH was less than 5.9 due to the fact that its amine group was protonated [44]. It adopts a zwitterionic form within the pH range of 5.9 to 8.9. When the pH exceeds 8.9, the carboxyl group ionizes, resulting in an anionic form of CIP^- [44].

As depicted in Fig.2, Mn@MDSBC has an isoelectric point at pH 3.28, indicating that it carries a positive charge when the pH was below 3.28 and a negative charge when the pH is above 3.28. At pH 3, the positively charged Mn@MDSBC repeled CIP⁺, leading to a low CIP uptake of 56.48 mg/g. When the pH increased to 5, the CIP adsorption capacity rose to 75.86 mg/g. This increase was attributed to the charge-related interaction between Mn@MDSBC surface carrying a negative charges and the cationic form of CIP (CIP⁺), which was prevalent at pH values below 5.9. When the pH reached 9, the anionic form of CIP (CIP^-) reduced CIP adsorption through electrostatic repulsion, causing the adsorption capacity to drop to 56.46 mg/g. Similar observations have been made when magnetic activated carbon derived from eucalyptus sawdust and oil sludge was used for CIP adsorption [45].Raheem et al. observed that the adsorption of CIP by MILDH increased progressively as the pH of the solution rose from 3 to 7, reaching a peak at pH 7. Conversely, when the pH was further elevated to 10, the adsorption capacity of the material for CIP declined [45].

Comment 18: Line 152 to 155 discussed the existence of CIP in cationic form but didn’t provide enough literature to justify it.

Answer 18:see section 3.3.1 :The pH value is a critical determinant in the adsorption procedure as it influences both the form of adsorbate and surface properties of the adsorbent. CIP existed in its positively charged cationic state (CIP⁺) when the solution pH was less than 5.9 due to the fact that its amine group was protonated [44]. It adopts a zwitterionic form within the pH range of 5.9 to 8.9. When the pH exceeds 8.9, the carboxyl group ionizes, resulting in an anionic form of CIP^- [44].

Comment 19: Line 227, Figure 8 is not present in the document or Appendix.

Answer 19: Thank you for your kind reminder. It has been changed into Fig.3.

Comment 20: In the description of figure 5 mention the concentration of Adsorbent used for reusability experiments.

Answer 20: See Answer 3.

Comment 21: Line 310 and 311 please mention reference.

Answer 21: As shown in Fig.S5k, the peaks at 641.8 eV and 653.6 eV corresponded to MnO₂ [86], indicating that Mn has been successfully loaded onto Mn@MDSBC. The peak at 644.8 eV was the satellite peak of Mn²⁺ [87]. After CIP adsorption, the peaks at 641.3 eV and 653.1 eV belonged to Mn³⁺ [88, 89], suggesting that Mn³⁺ had strong oxidizing properties and adsorbs CIP through oxidation reactions (Fig.S5l).

Comment 22: Line 323 please mention the specific mass ratio or varying mass ratios of mMnCl2:m(IBWS+PMS). Also mention the amount of adsorbent used for the adsorption of CIP.

Answer 22: See the first paragraph of section 5: The optimal conditions for composite preparation were determined to be a mass ratio of MnCl₂to (IBWS+PMS) of 1:5 .

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

Journal: Water
Manuscript ID: water-3476319
Type of manuscript: Article
Title: Enhanced adsorption of aqueous ciprofloxacin hydrochloride by a
manganese modified magnetic dual-sludge biochar
Authors: Jingxi Tie *, Mengjia Yan, Sihao Shao, Xiaohan Duan

Comments:

Please add some literature review that deals with the adsorption of ciprofloxacin hydrochloride on various other adsorbents in the second last paragraph of the Introduction section. Try to add at least seven to eight literature data.
There should be one materials section before methods. Add all the chemicals (also mention its purity) used in the study. Also, mention the solubility of ciprofloxacin hydrochloriode in water.
Figure 1- add the zeta potential and adsorbed amount data up to pH 12.
Bring the SEM images of the materials in the main manuscript.
Use non-linearized form of Langmuir and Freundlich isotherm models. Also, change Figure 3 and Table 2 accordingly.
Improve the BET adsorption isotherm (surface area) part. Take help from the following manuscript:

https://doi.org/10.1016/j.ces.2023.118655; Improve the discussion on the adsorption kinetic and isotherm studies.

For comparison purposes, add the SEM images and EDS spectrum of the spent adsorbent. Also, it would be interesting to see the morphology of the material after the completion of five regeneration cycles. Therefore, the authors are encouraged to add the SEM and EDS data of the regenerated sample after 5^th

Overall, this is a good study. However, the above mentioned points are required to be addressed before publication in the journal.

Author Response

Response to Reviewer

Dear Reviewer:

Comments 1: Please add some literature review that deals with the adsorption of ciprofloxacin hydrochloride on various other adsorbents in the second last paragraph of the Introduction section. Try to add at least seven to eight literature data.

Answer 1:Thank you for your suggestion. We added the following content at the end of the second paragraph:
To date, a wide variety of materials, including metallic materials, non-metallic materials, and metal–non-metal hybrid composites, have been utilized for the adsorptive removal of CIP from water [14-17]. Among these materials, the application of biochar for CIP removal has emerged as a prominent research focus in recent years [13, 18-20].

Comments 2:There should be one materials section before methods. Add all the chemicals (also mention its purity) used in the study. Also, mention the solubility of ciprofloxacin hydrochloriode in water.

Answer 2: Thank you for your suggestion. We added a new section as follows :

2.1 Chemicals and materials

Comments 3: Figure 1- add the zeta potential and adsorbed amount data up to pH12.

Answer 3: Our preliminary experiments have shown that excessively high or low pH levels can cause degradation of the material. Drawing on the existing literature, we have established the pH range for this study as 3 to 9.

Fig. 3 The efect of environmental factors on adsorption of CIP on virgin PS and aged PS. a pH

Li, T., Lan, J., Wang, Y. et al. Enhanced biotoxicity by co-exposure of aged polystyrene and ciprofloxacin: the adsorption and its influence factors. Environ Geochem Health 46, 185 (2024). https://doi.org/10.1007/s10653-024-01961-0

Fig. Adsorption characteristics and mechanisms of ciprofloxacin on polyanionmodified laterite material

Comments 4: Bring the SEM images of the materials in the main manuscript.

Answer 4: SEM images have been moved into the main manuscript as Fig.1.

Comments 5: Use non-linearized form of Langmuir and Freundlich isotherm models. Also, change Figure 3 and Table 2 accordingly.

Answer 5:We use the non-linearized fitting to analyzed the kinetics and isotherm. The content is as follows:

3.3.2 Adsorption kinetics

The adsorption of CIP by Mn@MDSBC exhibited a rapid increase within the initial 30 minutes, regardless of the two distinct initial CIP concentrations. This sharp rise was ascribed to the abundant reactive sites on Mn@MDSBC surface. Subsequently, as these active sites became progressively occupied, the rate of CIP adsorption decelerated, and both reactions reached equilibrium at 960 min (Fig.3a).To further elucidate the adsorption process, three kinetic models were employed. The first two models are the pseudo-first-order (PFO) model (Eqs.1) and the pseudo-second-order (PSO) model (Eqs.2). These models are designed to describe physical adsorption and chemical adsorption, respectively [46]. The third model is the intra-particle diffusion model (Eqs.3), which is typically used to depict the the inner or pore interaction processes [47].

&nbsp（1）

&nbsp（2）

I（3）

where q_e(mg/g) and q_t(mg/g) represent the CIP adsorption capacities at equilibrium and at time t, respectively. k₁ (min⁻¹) was the PFO rate constant, k₂ (g/mg·min) was the PSO rate constant. k_b (mg/(g·h^0.5)) and I (mg/g) were the rate constant and the intecept constant, respectively.

The nonlinear fitting results are depicted in Fig.3a, and the fitting parameters are summarized in Table 1. The coefficients of determination (R²) for the PSO model exceeded those for the PFO and Elovich models. Moreover, the experimental q_e values approached the values obtained from the PSO model, indicating that the PSO model was appropriate for describing the kinetic process and that chemisorption was the predominant mechanism [48].

Figure 3. Nonlinear fitting of kinetic processes using PFO kinetic equation and PSO (a), and the intra-particle diffusion model (b) (Adsorbent dosage: 0.05 g/ 20 mL, Temperatue: 25 ℃, Time: 12 h, pH: 5).

The intra-particle diffusion model was employed to further explore the rate-limiting steps during the process of CIP adsorption by Mn@MDSBC. As shown in Fig.2d, each fitting curve consisted of three linear segments, corresponding surface diffusion, intra-particle diffusion, and equilibrium adsorption, respectively [49]. Multiple adsorption mechanisms, rather than a single rate-limiting step was involved into the adsorption process due to the fact that none of the linear segments passed through the coordinate origin [50]. Furthermore, for each of the two initial CIP concentration, the intra-particle diffusion rate constants followed the order kip₁ > kip₂ > kip₃, suggesting that surface diffusion played a more dominant role in the initial stages of the adsorption process compared to intra-particle diffusion [49].

Table 1. Parameters obtained from the three kinetic models.

Kinetics	Parameters	Adsorbents
Kinetics	Parameters	100 mg/L	200 mg/L
Pseudo-first order	q_e,exp(mg/g)	36.572	76.233
	q_e,cal(mg/g)	35.112	72.779
	k₁(min^-1)	0.02337	0.02669
	R²	0.989	0.983
Pseudo-second order	k₂[g/(mg·min)]	38.368	79.026
	q_e,cal(mg/g)	0.000912	0.000517
	R²	0.999	0.999
Intra-particle diffusion	k_i1[mg/(g·min^0.5)]	2.126	3.753
	C₁	8.339	24.726
	R₁²	0.955	0.875
	k_i2[mg/(g·min^0.5)]	0.821	1.716
	C₂	22.318	45.807
	R₂²	0.980	0.999
	k_i3[mg/(g·min^0.5)]	0.149	0.334
	C₃	32.550	67.657
	R₃²	0.990	0.867

3.3.3 Adsorption isotherm and thermodynamics analysis

Fig.4a illustrates the relationship between q_e and C_e. It is evident that the CIP adsorption capability of Mn@MDSBC declined as the reaction temperature increased. To analyze the adsorption data, the Langmuir and Freundlich models (Eqs. 4 and 5) were employed:

&nbsp（4）

&nbsp（5）

where C_e (mg/L) was the equilibrium CIP concentration, q_m (mg/g) was the theoretical CIP uptake, b (L/mg) was the Langmuir constant, k_f (Lⁿ·mg^1–n/g) and n are the Freundlich constants.

Fig.4a depicts the nonlinear fitting results for the Langmuir and Freundlich models, respectively, and the fitting parameters are summarized in Table 2. Compared to the R²values for the Freundlich model, the R²values for the Langmuir model were closer to 1 in all three tests, indicating that the Langmuir model provided a better description of CIP adsorption by Mn@MDSBC. This suggests that the adsorption process likely involved the formation of a single layer of CIP on the homogeneous surface of Mn@MDSBC [48].

Figure 4. Nonlinear fitting of adsorption isotherms using Langmuir equation and Freundlich equation (a), and the relation between 1/T and lnK(b (Adsorbent dosage: 0.05 g/ 20 mL, Temperatue: 25 ℃, Time: 12 h).

Table 2. Parameters obtained from the two isotherm equations and thermodynamic exploration.

T/(K)	Langmuir parameters			Freundlich parameters			Thermodynamic parameters
T/(K)	q_m	k_L	R²	k_f	n	R²	∆G⁰	∆H⁰	∆S⁰
288	145.175	0.0967	0.999	23.859	-0.433	0.977	-1.986	-12.695	-0.039
298	141.985	0.0690	0.999	17.337	-0.480	0.959	-1.601
308	138.479	0.0434	0.999	11.497	-0.531	0.952	-1.134

Comments 6: Improve the BET adsorption isotherm (surface area) part. Take help from the following manuscript:https://doi.org/10.1016/j.ces.2023.118655; Improve the discussion on the adsorption kinetic and isotherm studies.

Answer 6:Thank you for your very meaningful suggestions. We have made the following revisions to the BET section.

Fig. S2 presents the N₂adsorption-desorption isotherms of Mn@MDSBC within the relative pressure range from 0 to 1 , which exhibited the typical type IV isotherms accompanied by a H3 hysteresis loop, suggesting the existence of a mesoporous structure [37]. The specific surface area (S_BET), total pore volume (V_tot), and average pore diameter of Mn@MDSBC were 48.42 m²/g, 0.18 cm³/g, and 14.54 nm, respectively (Table S1). These values confirm that Mn@MDSBC was a typical composite with both micropores and mesopores, which contributed to its high adsorption capacity [38, 39]. Fig.S2 also reveals that the pore size distribution of Mn@MDSBC was characterized by a distinct bimodal pattern. The two peaks in the distribution corresponded to pore sizes ranging from 3.6 nm to 4.1 nm and from 11.51 nm to 12.93 nm, respectively [40]. The smaller pores (3.6 nm to 4.1 nm) originated from the pyrolysis of PMS in the raw materials, whereas the larger pores (11.51 nm to 12.93 nm) were formed through the pyrolysis of IBWS in the raw materials [41].

Comments 7: For comparison purposes, add the SEM images and EDS spectrum of the spent adsorbent. Also, it would be interesting to see the morphology of the material after the completion of five regeneration cycles. Therefore, the authors are encouraged to add the SEM and EDS data of the regenerated sample after 5^th

Answer 7: We highly appreciated your suggestions and have conducted EDS analysis on the samples. We have also revised the relevant sections as follows:

3.2.2 SEM images and SEM-EDS results

As shown in Fig. 1a and 1b, the pristine Mn@MDSBC consisted of flake-like structures and small particles, with a relatively rough surface appearance. After being used and regenerated five times, the number of small particles on the surface decreased, and the flake-like structures became more pronounced (Fig. 1c and 1d). After the fifth repeated use, due to the adsorption of CIP on the surface, the number of small particles on the surface increased again, making the surface rough once more (Fig. 1e and 1f).

Fig.1g and 1h illustrates the surface chemical composition of pristine Mn@MDSBC as determined by EDS analysis. Initially, the contents of C, O, Fe, and Mn were 48.9%, 39.2%, 6.7%, and 5.3%, respectively, with no detectable N. After undergoing five regeneration cycles (Fig.1i and 1j ), the elemental proportions shifted to 55.6% for C, 28.0% for O, 11.7% for Fe, and 3.7% for Mn, while N emerged with a content of 1.0%. This change colud be attributed to the compoiste's final preparation step involving magnesium modification, which positioned magnesium on the outermost layer. Following five regenerations, the surface magnesium was depleted, exposing the underlying C, O, and Fe and thereby increasing their measured contents. The newly detected N was likely sourced from adsorbed ciprofloxacin.

Fig.1 SEM images of virgin Mn@MDSBC at magnifications of 16K (a) and 50K (b), Mn@MDSBC regenerated for 5 cycles at magnifications of 16K (c) and 50K (d), Mn@MDSBC reused for 5 cycles at magnifications of 16K (e) and 50K (f), SEM-EDS imaging of virgin Mn@MDSBC (g) and mapping of corresponding elements (h), Mn@MDSBC regenerated for 5 cycles (i) and mapping of corresponding elements (j)

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

Enhanced adsorption of aqueous ciprofloxacin hydrochloride 2

by a manganese-modified magnetic dual-sludge biochar

Water-3476319

Comments’ paper

Although numerous studies focus on ciprofloxacin (CIP) removal, this work integrates three key innovations: industrial byproducts, manganese modification, and magnetic properties. The composite material, Mn@MDSBC, synthesises paper mill sludge (PMS) and iron-based waterworks sludge (IBWS), promoting waste recycling and sustainability. Manganese enhancement significantly boosts adsorption capacity, while magnetisation ensures ease of recovery and reusability.

Regarding the keywords, their presentation in alphabetical order would be more appropriate.

Line 106

Section 3.1.1; it is noted that Mn@MDSBC demonstrates an enhanced capacity for CIP adsorption as the mass ratio of MnCl₂ to (IBWS+PMS) transitions from 2:1 to 1:2. However, this capacity subsequently diminishes as the ratio progresses further to 1:7. The text does not provide a comprehensive explanation for this decline beyond the 1:2 ratio, leaving room for greater exploration into the intricate interactions among the components and their influence on adsorption dynamics. A deeper analysis of these mechanisms would enhance understanding and lend greater insight into the observed trends.

Line 119

Section 3.1.2 highlights that the optimal preparation temperature for Mn@MDSBC is 500°C, as it yields the highest CIP adsorption capacity. However, when the temperature rises to 700°C, there is a significant decrease in adsorption efficiency. The study does not comprehensively explain why higher temperatures lead to such a substantial reduction in performance. A more detailed analysis of the structural or chemical changes occurring at elevated temperatures could help clarify this phenomenon.

Line 168

Kinetic analysis

While the kinetic models employed in this study—such as the pseudo-first-order, pseudo-second-order, and Elovich models—are widely recognised and have been used for decades, they may be considered overly simplistic in addressing the complexities of adsorption processes. Though valuable for basic interpretations, these models fail to capture the nuanced interactions and mechanisms underlying the adsorption of ciprofloxacin hydrochloride (CIP) by Mn@MDSBC. To enhance this work's scientific rigour and relevance, exploring more advanced modelling approaches that can provide deeper insights into the system’s dynamics is recommended.

Advanced kinetic modelling techniques, such as non-linear regression analysis or multi-component reaction models, could offer a more comprehensive understanding of adsorption. Such methods would allow for evaluating factors like surface heterogeneity, pore diffusion, and intra-particle transport, which are critical in determining the efficiency and mechanism of CIP removal.

Line 236

3.3.4. Effect of co-existing anions

It is observed that Cl⁻ and SO₄²⁻ ions enhance the adsorption of ciprofloxacin (CIP), while PO₄³⁻ decreases it. While the explanations provided in the study are plausible, it would be beneficial to include a more detailed comparison with findings from previous research on the impact of these coexisting ions on the adsorption of similar compounds. Such an approach would help validate the conclusions presented and strengthen the robustness of the proposed interpretations.

Line 105

Line 236

When referencing other studies—particularly those concerning the chemical modification of biochar or the impact of coexisting ions—it would be advantageous to directly compare the results achieved in this study and those documented in previous research. By emphasising the similarities and differences, such an analysis could enhance the contextual framework and underscore the significance of the presented findings. This method will contribute to validating the current work and situating it more firmly within the broader scientific discourse.

Line 249

3.3.5. Reusability of the adsorbent

The reusability of Mn@MDSBC was demonstrated through its ability to maintain 82.83% of its initial ciprofloxacin (CIP) adsorption capacity after five cycles, highlighting its potential for reducing operational costs in practical applications. However, a more detailed explanation of the material's degradation mechanisms and a comparison with similar studies would strengthen the findings and provide deeper insights into its performance relative to existing solutions.

Detailed Analysis of Degradation Mechanisms:

Investigating the structural and functional changes in the material after multiple usage cycles using advanced techniques such as XPS spectroscopy or SEM microscopy could clarify the specific causes of the gradual efficiency reduction.

Comparison with Competitive Materials: Including a comparative table of other adsorbents reported in the literature, highlighting metrics like initial adsorption capacity, decay rate after reuse, and production costs, would help position Mn@MDSBC relative to existing solutions.

Author Response

Response to Reviewer

Dear Reviewer:

Comments: Although numerous studies focus on ciprofloxacin (CIP) removal, this work integrates three key innovations: industrial byproducts, manganese modification, and magnetic properties. The composite material, Mn@MDSBC, synthesises paper mill sludge (PMS) and iron-based waterworks sludge (IBWS), promoting waste recycling and sustainability. Manganese enhancement significantly boosts adsorption capacity, while magnetisation ensures ease of recovery and reusability.

Q1: Regarding the keywords, their presentation in alphabetical order would be more appropriate.

A1: Thank you very much for your suggestions. The keywords has been presented in alphabetical order: Adsorption; Ciprofloxacin hydrochloride; Iron-based waterworks sludge; Manganese modified magnetic dual-sludge biochar, Paper mill sludge;

Q2: Line 106, Section 3.1.1; it is noted that Mn@MDSBC demonstrates an enhanced capacity for CIP adsorption as the mass ratio of MnCl₂ to (IBWS+PMS) transitions from 2:1 to 1:2. However, this capacity subsequently diminishes as the ratio progresses further to 1:7. The text does not provide a comprehensive explanation for this decline beyond the 1:2 ratio, leaving room for greater exploration into the intricate interactions among the components and their influence on adsorption dynamics. A deeper analysis of these mechanisms would enhance understanding and lend greater insight into the observed trends.

A2:Thank you very much for your suggestions. The objective of Section 3.1.1 is to identify the optimal preparation conditions for Mn@MDSBC, guided by the following principles: (1) Ensuring the material exhibits superior CIP adsorption performance, which is the primary goal of its synthesis; (2) Endowing the material with strong magnetic properties to facilitate rapid solid-liquid separation and reusability under an external magnetic field, thereby enhancing its practical applicability; and (3) Minimizing the use of MnCl₂ while maximizing the utilization of two solid waste materials, thereby reducing the overall preparation cost and promoting waste valorization.

To achieve these goals, the CIP adsorption capacity and magnetic properties of the material were evaluated as key indicators, while also considering the usage of raw materials. After comprehensive assessment, the optimal ratio of MnCl₂ to (IBWS+PMS) was determined to be 1:5.

The Description is as follows:

3.1.1 Effect of MnCl₂ to (IBWS+PMS) mass ratio on CIP adsorption

Figure S1a illustrates the CIP adsorption capacities of MDSBC and eight distinct Mn@MDSBCs synthesized at varying mass ratios of MnCl₂to (IBWS+PMS). MDSBC had a CIP adsorption capacity of 51.132 mg/g, which was significantly lower than those of all eight types of Mn@MDSBCs, confirming that manganese modification enhanced CIP adsorption. As the mass ratio of MnCl₂ to (IBWS+PMS) increased from 2:1 to 1:2, the CIP adsorption capacity increased from 76.58 mg/g to 80.39 mg/g. However, as the mass ratio continued to decrease from 1:2 to 1:3, 1:4, 1:5, 1:6, and 1:7, the adsorption capacity gradually decreased to 79.72 mg/g, 79.39 mg/g, 79.28 mg/g, 76.14 mg/g, and 74.67 mg/g, respectively (Fig. S1b). Statistical analysis revealed that as the mass ratio increased from 1:2 to 1:5, no significant differences (p>0.05) in the adjacent adsorption capacity were detected among the groups. In contrast, when the mass ratio was further increased to 1:6, significant differences (p<0.05) in adsorption capacity emerged. Meanwhile, the saturation magnetic induction values of the seven Mn@MDSBCs synthesized at different ratios were 6.43 emu/g, 7.54 emu/g, 7.75 emu/g, 7.82 emu/g, 7.97 emu/g, 8.11 emu/g and 8.57 emu/g, respectively. The results indicate that the magnetism of the synthesized material increased as the mass ratio of the two components decreased from 2:1 to 1:7. This enhancement was attributed to the increased iron content in the raw materials as the mass ratio was reduced. As a result, more magnetic substances were formed during the high-temperature calcination process under anaerobic conditions. A lower mass ratio implies the use of less Mn and more PMS and IBWS in the composite synthesis, which was advantageous as it minimized chemical usage and promotes waste recycling. Considering both CIP adsorption and magnetic properties, a mass ratio of MnCl₂to (IBWS+PMS) = 1:5 was selected for composite synthesis.

Q3: Line 119 Section 3.1.2 highlights that the optimal preparation temperature for Mn@MDSBC is 500°C, as it yields the highest CIP adsorption capacity. However, when the temperature rises to 700°C, there is a significant decrease in adsorption efficiency. The study does not comprehensively explain why higher temperatures lead to such a substantial reduction in performance. A more detailed analysis of the structural or chemical changes occurring at elevated temperatures could help clarify this phenomenon.

A3:Thank you very much for your suggestions. The main focus of this paper was to study the adsorption performance of the material for CIP. Therefore, it is necessary to understand how the material was prepared, although this part is not the primary emphasis. When the preparation temperature was increased from 500°C to 700°C, a significant decrease in the adsorption capacity of the material was observed. The reasons for this phenomenon are too complex and require a comprehensive analysis in conjunction with the material's characterization. This will be discussed in detail in another paper.

Q4: Line 168 Kinetic analysis. While the kinetic models employed in this study—such as the pseudo-first-order, pseudo-second-order, and Elovich models—are widely recognised and have been used for decades, they may be considered overly simplistic in addressing the complexities of adsorption processes. Though valuable for basic interpretations, these models fail to capture the nuanced interactions and mechanisms underlying the adsorption of ciprofloxacin hydrochloride (CIP) by Mn@MDSBC. To enhance this work's scientific rigour and relevance, exploring more advanced modelling approaches that can provide deeper insights into the system’s dynamics is recommended.

A4:Thank you very much for your suggestions. We have revised the relevant content accordingly. In the updated version, we have incorporated three kinetic equations. The PFO and PSO models retained as they represent reaction-based kinetic models. The original third equation was replaced with an intraparticle diffusion model, which is a diffusion-based kinetic model. This modification enables a more comprehensive understanding of the adsorption process. The revised content is presented as follows:

3.3.2 Adsorption kinetics

To further elucidate the adsorption process, three kinetic models were employed. The first two models are the pseudo-first-order (PFO) model (Eqs.1) and the pseudo-second-order (PSO) model (Eqs.2). These models are designed to describe physical adsorption and chemical adsorption, respectively [46]. The third model is the intra-particle diffusion model (Eqs.3), which is typically used to depict the the inner or pore interaction processes [47].

&nbsp（1）

&nbsp（2）

I（3）

Table 1. Parameters obtained from the three kinetic models.

Kinetics	Parameters	Adsorbents
Kinetics	Parameters	100 mg/L	200 mg/L
Pseudo-first order	q_e,exp(mg/g)	36.572	76.233
	q_e,cal(mg/g)	35.112	72.779
	k₁(min^-1)	0.02337	0.02669
	R²	0.989	0.983
Pseudo-second order	k₂[g/(mg·min)]	38.368	79.026
	q_e,cal(mg/g)	0.000912	0.000517
	R²	0.999	0.999
Intra-particle diffusion	k_i1[mg/(g·min^0.5)]	2.126	3.753
	C₁	8.339	24.726
	R₁²	0.955	0.875
	k_i2[mg/(g·min^0.5)]	0.821	1.716
	C₂	22.318	45.807
	R₂²	0.980	0.999
	k_i3[mg/(g·min^0.5)]	0.149	0.334
	C₃	32.550	67.657
	R₃²	0.990	0.867

Q5: Line 236 3.3.4. Effect of co-existing anions It is observed that Cl⁻and SO₄²⁻ ions enhance the adsorption of ciprofloxacin (CIP), while PO₄³^- decreases it. While the explanations provided in the study are plausible, it would be beneficial to include a more detailed comparison with findings from previous research on the impact of these coexisting ions on the adsorption of similar compounds. Such an approach would help validate the conclusions presented and strengthen the robustness of the proposed interpretations.

A5:Thank you very much for your suggestions. We have revised the relevant content accordingly. In the updated version, more references were cited. The new expression is as follows：

3.3.4 Effect of coexisting anions

The impact of three common anions including Cl^-, SO₄^2- and PO₄^3- on the adsorption of CIP by Mn@MDSBC was investigated. As depicted in Fig.5, both Cl^- and SO₄^2- enhanced CIP adsorption in a concentration-dependent manner, with SO₄^2- having a more pronounced effect than Cl^-. The salting-out effect of Cl^- and SO₄^2- reduced the solubility of CIP, thereby promoting its diffusion towards the surface of Mn@MDSBC and improving adsorption. Lu et al. discovered that the addition of 3% sodium chloride to the solution significantly enhanced the adsorption capacities of ciprofloxacin (CIP) by resins XAD-4 and MN-202, with respective increases of 93.48% and 53.94% [58]. Another studies have demonstrated that the presence of salts in the solution coluld diminish the adsorption of CIP. For instance, Seibert et al. observed that the addition of KCl and Na₂SO₄ to the solution notably reduced the adsorption capacity of cork for CIP. This reduction is primarily due to the competitive interaction between the added ions and the ionic form of CIP for the limited active adsorption sites on the adsorbent's surface [59]. Conversely, CIP adsorption significantly decreased with the increase in PO₄^3- concentration rose from 100 to 300 mg/L. This reduction is attributed to the hydrolysis of PO₄^3-, which generates OH^-, raising the solution's pH and creating competition between OH^-and CIP^-.The same phenomena was observed by Afzal et al. when they used chitosan/biochar hydrogel beads as adsorbents to remove CIP from water [60].

Q6: Line 105 Line 236 When referencing other studies-particularly those concerning the chemical modification of biochar or the impact of coexisting ions-it would be advantageous to directly compare the results achieved in this study and those documented in previous research. By emphasising the similarities and differences, such an analysis could enhance the contextual framework and underscore the significance of the presented findings. This method will contribute to validating the current work and situating it more firmly within the broader scientific discourse.

A6: We appreciate the reviewer’s insightful suggestion. In response, we have conducted a comprehensive comparison of the CIP adsorption performance among 10 different materials, including the adsorbent developed in this study. This comparison provides a more transparent and detailed understanding of the performance of our material relative to others. The content of this section is presented below.

Table 3 demonstrates that the adsorption capacity of Mn@MDSBC for CIP ranges between 137.931 and 145.985 mg/g. Among the ten materials compared in the table, Mn@MDSBC ranks third. This highlights that the material developed in this study exhibits a robust CIP adsorption ability from aqueous solutions.

Table 3 The CIP adsorption capacities of different adsorbents determined by Langmuir model

No.	Adsorbents	q_m (mg/g)	Reaction conditions	Reference
1	MMT	416.66	-, -	[51]
2	FMB	357	298 K, pH 5	[46]
3	Mn@MDSBC	145.985 141.643 137.931	288 K, pH 5 298 K, pH 5 308 K, pH 5	This work
4	SBC Zn-SBC Fe/Zn-SBC	13.5 77.3 74.2	-, - -, - -, -	[52]
5	Dopa-CoF NPs Gluta-CoFN PsMela-CoF NPs	16.5 14.0 7.18	298 K, pH 7 298 K, pH 7 298 K, pH 7	[53]
6	CoFe-LDH-modified sludge biochar	19 16 12	298 K, - 303 K, - 308 K, -	[54]

“-” in the table means the parameter did not show in the study

Q7: Line 249

3.3.5. Reusability of the adsorbent

Detailed Analysis of Degradation Mechanisms:

Comparison with Competitive Materials: Including a comparative table of other adsorbents reported in the literature, highlighting metrics like initial adsorption capacity, decay rate after reuse, and production costs, would help position Mn@MDSBC relative to existing solutions.

A7: Thank you for your insightful suggestion. We have compiled and analyzed the reuse data for five materials. The detailed findings of this analysis are presented below. Given that the majority of existing studies remain at the laboratory scale, focusing predominantly on evaluating material performance through static adsorption experiments, the preparation costs are often not accounted for. As a result, the compiled statistical data on these materials do not encompass their preparation costs. The material's degradation mechanisms will be thoroughly investigated in a subsequent study.

The evaluation of the material's reusability is as follows:

Table 4 presents the reusability five different CIP adsorbents. Given that the number of uses varied among these materials, the analysis focuses on the loss rate per individual use to ensure a fair comparison. The average loss rate across the five materials is 3.3%. Among them, Fe₃O₄@SiO₂/l-CRG/GPTMS exhibits the highest loss rate at 4.8%, while ACAF/Fe₃O₄/ZnO has the lowest at 1.3%. The material developed in this study demonstrates a loss rate of 2.9%, which is below the average. This suggests that the material possessed excellent reusability and maintained a relatively stable adsorption capacity over multiple cycles.

Table 4 Reusability of CIP Adsorbents

No.	Adsorbents	Cycles of use	Adsorption capacity loss	Reference
1	Fe₃O₄@SiO₂/ι-CRG/GPTMS	4	19%	[61]
2	ACAF/Fe₃O₄/ZnO	6	7.9%	[62]
3	MBC	5	17.01%	[63]
4	MNPC-700-0.4	4	17%	[64]
5	Mn@MDSBC	6	17.17%	This study

Author Response File: Author Response.pdf

Reviewer 4 Report

Comments and Suggestions for Authors

Dear Author,

I read the manuscript 3476319-peer-review-v1: ”Enhanced adsorption of aqueous ciprofloxacin hydrochloride 2 by a manganese modified magnetic dual-sludge biochar ”, and I present below a few of my observations:

4 line 170: What is HCIP ?
The interpretation of the results in the kinetic study (Figure 2 and Table 1) is quite poor for each method used and regarding the conclusions that emerge from this study.
In eqs 6-8: ” k was the distribu-226 tion coefficient (L/g) which could be obtained from Fig. 8.” It is about K or k and where is Figure 8 ? There is some confusion here regarding the meaning of "K". It is worth reviewing the literature to establish the connection with the Langmuir constant k(L)
About subchapter 3.3.5.: How and with what was CIP desorption done to reuse the absorbent?

The work needs major corrections to go to the publication stage

Sincerely yours,

Author Response

Response to Reviewer

Dear Reviewer:

Q1: 4 line 170: What is HCIP ?

A1: Sorry,it is CIP instead of HCIP. We corrected it.

Q2: The interpretation of the results in the kinetic study (Figure 2 and Table 1) is quite poor for each method used and regarding the conclusions that emerge from this study.

A2: We appreciate your valuable suggestions, which have enabled us to significantly enrich the content of this section as follows：

3.3.2 Adsorption kinetics

&nbsp（1）

&nbsp（2）

I（3）

Table 1. Parameters obtained from the three kinetic models.

Kinetics	Parameters	Adsorbents
Kinetics	Parameters	100 mg/L	200 mg/L
Pseudo-first order	q_e,exp(mg/g)	36.572	76.233
	q_e,cal(mg/g)	35.112	72.779
	k₁(min^-1)	0.02337	0.02669
	R²	0.989	0.983
Pseudo-second order	k₂[g/(mg·min)]	38.368	79.026
	q_e,cal(mg/g)	0.000912	0.000517
	R²	0.999	0.999
Intra-particle diffusion	k_i1[mg/(g·min^0.5)]	2.126	3.753
	C₁	8.339	24.726
	R₁²	0.955	0.875
	k_i2[mg/(g·min^0.5)]	0.821	1.716
	C₂	22.318	45.807
	R₂²	0.980	0.999
	k_i3[mg/(g·min^0.5)]	0.149	0.334
	C₃	32.550	67.657
	R₃²	0.990	0.867

Q3: In eqs 6-8: ” k was the distribution coefficient (L/g) which could be obtained from Fig. 8.” It is about K or k and where is Figure 8 ? There is some confusion here regarding the meaning of "K". It is worth reviewing the literature to establish the connection with the Langmuir constant k(L)

A3: Thank you very much for your suggestions. In the literature we reviewed, two approaches for calculating K were identified. The first approach directly utilizes the coefficient K from the Langmuir model, whereas the second is derived from the relationship between ln(qₑ/cₑ) and cₑ, as depicted in Fig.3a(which was erroneously written as Figure 8 in the manuscript) . In this study, we employed the second method for the calculation of K.

We sincerely apologize for the incorrect expression that resulted from our oversight. We have changed it into “The values of ΔS⁰ and ΔH⁰ were derived from Fig. 4d in accordance with Equation 8. K⁰ was the distribution coefficient (L/g) which was calculated based on the relationship between ln(q_e/c_e) and c_e using the data shown in Fig.4a. ” You can find it below Table 3.

Q4: About subchapter 3.3.5.: How and with what was CIP desorption done to reuse the absorbent?

A4: We add a new section to explain it as follows:

2.4 Adsorbent regeneration

After the adsorption, the adsorbent was separated and thoroughly washed five times with distilled water. Subsequently, it was dried in a vacuum oven at 80 °C for 12 h and calcined at 500 °C for 3 h. All other conditions were maintained as described in Section 2.2. The regenerated adsorbent was sieved through an 80-mesh screen. The adsorption experiments were then conducted following the procedures outlined in Section 2.3.

Author Response File: Author Response.pdf

Reviewer 5 Report

Comments and Suggestions for Authors

In this research, the authors synthesized a Mn@MDSBC composite from IBWS, PMS, and manganese to adsorb CIP from water. The composite retained over 80% effectiveness after five uses, with electrostatic and chemical interactions, hydrogen bonding, and π-π stacking as key mechanisms.

The article is logically written, the material is well-presented, and the explanations provided are essential for studying adsorption materials and their adsorption processes. Congratulations to the authors! Just a few minor comments: 1. Section 2.3 - the static adsorption test was conducted with shaking; either it was dynamic or it was conducted without shaking, i.e., you are writing about a contradiction. 2. According to Fig. S2, the pore size is not only 14 nm, but there are also pores of smaller size, as indicated by the intense peak around 5-6 nm, so it is necessary to mention the bimodal pore distribution of the samples.

Author Response

Response to Reviewer

Dear Reviewer:

Comment: In this research, the authors synthesized a Mn@MDSBC composite from IBWS, PMS, and manganese to adsorb CIP from water. The composite retained over 80% effectiveness after five uses, with electrostatic and chemical interactions, hydrogen bonding, and π-π stacking as key mechanisms. The article is logically written, the material is well-presented, and the explanations provided are essential for studying adsorption materials and their adsorption processes. Congratulations to the authors!

Answer: Thank you for the positive feedback on our manuscript.

Q1: Section 2.3 - the static adsorption test was conducted with shaking; either it was dynamic or it was conducted without shaking, i.e., you are writing about a contradiction.

A1: We conducted the static adsorption tests using a shaker operating at 120 rpm. The purpose of shaking was to ensure thorough contact and interaction between the adsorbate and the adsorbent. Alternatively, static adsorption tests can also be performed in a completely static state. Dynamic adsorption typically involves the continuous flow of the solution over the adsorbent material, facilitating an adsorption reaction. This is different from our experiment.

Q2: According to Fig. S2, the pore size is not only 14 nm, but there are also pores of smaller size, as indicated by the intense peak around 5-6 nm, so it is necessary to mention the bimodal pore distribution of the samples.

A2: Thank you for your suggestion. We added additional content in section 3.2.1 as follows：

Fig.S2 also reveals that the pore size distribution of Mn@MDSBC was characterized by a distinct bimodal pattern. The two peaks in the distribution corresponded to pore sizes ranging from 3.6 nm to 4.1 nm and from 11.51 nm to 12.93 nm, respectively [40]. The smaller pores (3.6 nm to 4.1 nm) originated from the pyrolysis of PMS in the raw materials, whereas the larger pores (11.51 nm to 12.93 nm) were formed through the pyrolysis of IBWS in the raw materials [41].

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The manuscript may be considered for publication in its present form.

Author Response

Dear Respected Reviewer: We sincerely appreciate your insightful and valuable suggestions, which have greatly enhanced the quality of our manuscript. Additionally, we are deeply grateful for your agreement to accept our paper.

Reviewer 2 Report

Comments and Suggestions for Authors

Author Response

Reviewer 4 Report

Comments and Suggestions for Authors

Dear Author,

I read the manuscript 3476319-peer-review-v2: ”Enhanced adsorption of aqueous ciprofloxacin hydrochloride 2 by a manganese modified magnetic dual-sludge biochar ”, and I present below a few of my observations:

Line 282:” As shown in Fig.2d, each” - Where is this figure?
In Table 1: It would be good to mark the two values of 100 and 200mg/L as the initial concentration (Co).
In eqs 6-8: Who is "k"? ” It is worth reviewing the literature to establish the connection with the Langmuir constant k(L). These two (k and K^o) should actually be the kL parameter - Langmuir parameter. In these 3 equations there is only one K and not two.
About subchapter 3.3.5.: How and with what was CIP desorption done to reuse the absorbent?

Although the authors have made consistent improvements to the work, there are still some unclear elements that are essential. It is essential to present with which solvent the retained species (CIP) was eluted from the adsorbent and how this regeneration was achieved. In the thermodynamic equations, it is necessary to establish what the exact value of the constant K is and the fact that it is a single parameter and not two.

Unfortunately, I can only recommend major corrections so that the work can be subsequently published.

Sincerely yours,

Author Response

Response to reviewer Dear Reviewer: Thank you very much for your comments concerning our manuscript. Those comments are all valuable and very helpful for revising and improving our paper. The manuscript has already been revised by taking all these suggestions into account and substantial corrections have been added to the revised manuscript which we hope to meet with approval. The revised portions are marked in red in the paper. The main corrections in the paper and the responds to the your comments are as following. Comment 1: Line 282:” As shown in Fig.2d, each” - Where is this figure? Answer 1: We truly appreciate your meticulous attention to detail. It is Fig.2b instead of Fig.2d, we have corrected it. Comment 2: In Table 1: It would be good to mark the two values of 100 and 200mg/L as the initial concentration (Co). Answer 2: Thank you for your valuable suggestions. We have refined the table headers to improve clarity and usability, with the following updates: Kinetics Parameters Initial concentration ( C0 ) (mg/L) 100 200 Comment 3: In eqs 6-8: Who is "k"? ” It is worth reviewing the literature to establish the connection with the Langmuir constant k(L). These two (k and Ko) should actually be the kL parameter - Langmuir parameter. In these 3 equations there is only one K and not two. Answer 3: We have thoroughly examined the relevant literature and have verified that the coefficient k in Equation 6 corresponds to the Langmuir coefficient kL. We are truly grateful for your valuable suggestion. We have accepted your suggestion and have made the following revisions accordingly: （4）（5） where Ce (mg/L) was the equilibrium CIP concentration, qm (mg/g) was the theoretical CIP uptake, KL (L/mg) was the Langmuir constant, kf (Ln·mg1–n/g) and n are the Freundlich constants. where ΔG0 (kJ/mol) was the Gibbs free energy change , ΔS0 (kJ/mol·K) was the standard entropy change, and ΔH0 (kJ/mol) was the standard enthalpy change. The values of ΔS0 and ΔH0 were derived from Fig. 4d in accordance with Equation 8. KL (L/g) was the Langmuir constant. R (8.314 J/mol·K) was the ideal gas constant, and T was the reaction temperature (K). As indicated in Table 2, ΔG0 values, ranging from 5.558 to 7.947 kJ/mol, are all positive, confirming that CIP adsorption by Mn@MDSBC was a non-spontaneous process [55]. Moreover, the increase in ΔG0 values with rising reaction temperature suggests that lower temperatures were more conducive to CIP adsorption [56]. The negative ΔH0 value indicates that the adsorption process was exothermic. Furthermore, the negative ΔS0 value suggests a decrease in randomness at the interface between Mn@MDSBC and the liquid phase throughout the CIP adsorption [56, 57]. Table 2. Parameters obtained from the two isotherm equations and thermodynamic exploration. T/(K) Langmuir parameters Freundlich parameters Thermodynamic parameters Qm (mg/g) kL (L/mg) R2 Kf (Ln·mg1–n/g) n R2 ∆G0 (kJ/mol) ∆H0 (kJ/mol) ∆S0 (kJ/mol·K) 288 145.985 0.0955 0.999 26.941 2.513 0.972 5.558 -28.842 -0.119 298 141.643 0.0693 0.999 21.946 2.404 0.958 6.753 308 137.931 0.0436 0.999 16.253 2.258 0.959 7.947 Comment 4 : About subchapter 3.3.5.: How and with what was CIP desorption done to reuse the absorbent? Although the authors have made consistent improvements to the work, there are still some unclear elements that are essential. It is essential to present with which solvent the retained species (CIP) was eluted from the adsorbent and how this regeneration was achieved. Answer 4: The previously provided descriptions of the regeneration and reuse methods lacked clarity and completeness. Following further verification, the comprehensive and detailed methods are now presented as follows: 窗体底端 2.4 Regeneration and reuse of adsorbent The adsorption experiment was conducted using the pristine Mn@MDSBC under the following conditions: adsorbent dosage of 0.05 g per 20 mL of solution, temperature at 25℃, duration of 12 h, pH value of 5, and initial concentration (C0) of 80 mg/L. Upon completion of the experiment, the used adsorbent was collected and immersed in 50 mL of distilled water, followed by stirring at 150 rpm for 5 min. Subsequently, it was dried in vacuum at 80 ℃ for 12 h and calcined at 500 ℃ for 3 h. All other conditions were maintained as described in Section 2.2. The regenerated adsorbent was sieved through an 80-mesh screen. The adsorption experiment was conducted using the regenerated Mn@MDSBC as the adsorbent, under the identical conditions specified previously. The regeneration process of the composite and the subsequent CIP adsorption experiments using the regenerated material were each conducted 5 times.

Author Response File: Author Response.pdf

Round 3

Reviewer 4 Report

Comments and Suggestions for Authors

Dear Author,

I read the manuscript 3476319-peer-review-v3: ”Enhanced adsorption of aqueous ciprofloxacin hydrochloride 2 by a manganese modified magnetic dual-sludge biochar ”, and I present below a few of my observations:

Line 288: ” As shown in Fig.2b” – I think that it is about Fig.3b !!!!
In eq 8: I think that it is about „K_L” instesad of ”k”

The authors have made consistent improvements to the work, but there are 2 small inaccuracies that can be corrected.

In this context, I recommend publishing after minor corrections

Sincerely yours,

Author Response

Dear Reviewer:

We are extremely grateful for your patience and meticulous attention to detail in identifying the errors in our article. Your valuable input has significantly enhanced the quality of our manuscript. We have diligently addressed and corrected the issues you highlighted and have conducted a thorough review of the entire text to ensure its accuracy and coherence. Your approval of the revised version of our paper for publication is highly appreciated.

Commengt 1: Line 288: ” As shown in Fig.2b” - I think that it is about Fig.3b !!!!

Answer 1: We are truly grateful for your meticulous attention to detail. We changed it into Fig.3b.

Commengt 2: In eq 8: I think that it is about „KL” instesad of ”k”

Answer 2: Thank you.We changed it into K_L.

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Enhanced Adsorption of Aqueous Ciprofloxacin Hydrochloride by a Manganese-Modified Magnetic Dual-Sludge Biochar

Fig. 3 The efect of environmental factors on adsorption of CIP on virgin PS and aged PS. a pH

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