Effective removal of tetracycline antibiotics from wastewater using practically applicable iron(III)-loaded cellulose nanofibres

The non-toxic and completely biodegradable cellulose within bamboo is one of the most abundant agricultural polysaccharide wastes worldwide, and can be processed into cellulose nanofibres (CNFs). Iron(III)-loaded CNFs (Fe(III)@CNFs) derived from bamboo were prepared to improve the adsorption of tetracycline (TC), chlortetracycline (CTC) and oxytetracycline (OTC) from an aqueous solution. The preparation conditions of Fe(III)@CNFs suitable for the simultaneous adsorption of three tetracycline antibiotics (TCs) were investigated. Various analyses proved the abundance of oxygen-containing functional groups and the existence of Fe(III) active metal sites in Fe(III)@CNFs. In batch experiments, Fe(III)@CNFs were applied under a wide pH range and the maximum adsorption capacities were 294.12, 232.56 and 500.00 mg g−1 (for TC, CTC and OTC, respectively). In addition, different concentrations and types of coexisting anions have a weak effect on TCs adsorption. The original TCs adsorption capacities of Fe(III)@CNFs remained stable (greater than 92%) after five cycles when UV + H2O2 was used as the regeneration method. Four adsorption mechanisms (surface complexation, hydrogen bonding, electrostatic interaction and van der Waals force) were obtained for the endothermic adsorption of TCs, among which surface complexation between Fe(III) and TCs always dominates. The practically applicable Fe(III)@CNFs adsorbents are promising for TCs enrichment and remediation in engineering applications.

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Once again, thank you for submitting your manuscript to Royal Society Open Science and I look forward to receiving your revision. If you have any questions at all, please do not hesitate to get in touch. ********************************************** RSC Associate Editor: Comments to the Author: The research presented in this draft is original and of interest to RSOS audience, however more details on the experimental conditions as well as a deeper material characterization and explanation on sorption mechanisms and trends are needed. Please read carefully each of the comments from the reviewers and address each of them.
In this manuscript, the authors prepared Fe(III)@CNFs and characterized the composites in detail. The prepared composites were applied as adsorbents for the removal of OTC, TC and CTC from aqueous solutions. The contents are important for the removal of TCs from aqueous solutions. The manuscript is well organized. After reading the manuscript, I think it can be accepted for publication after revision. Special comments: 1. It is necessary to give the experimental conditions for the sorption capacities as the sorption capacity is dependent on solution conditions. 2. In the Introduction section, several critical reviews should be added in the revised form such as The Innovation, 2021, 2 (1) [1653][1654][1655][1656][1657][1658][1659][1660] in the revised manuscript. 5. The para. from line 113 to 123 can be described in section 2.6 and is independent of the novelty and scientific problems of this work. 6. The detail conditions and process of TCs determination by spectrophotometric method should be given. The concentrations of TCs were determined usually by HPLC. 7. Characterizations of the materials are too simple to present its physicochemical properties. More information from TG, TEM and XRD analysis, so on could be offered if possible. 8. From the images of SEM, no nanofibers were found. Why? The SEM images of iron(III)-loaded cellulose nanofibers should be exhibited in the text. 9. The linear equations of the kinetic, thermodynamics and isothermal models and should be described in text. 10

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Accept as is

Comments to the Author(s)
The authors revised the manuscript carefully and I recommend for publication.

Review form: Reviewer 2
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Are the interpretations and conclusions justified by the results? Yes
Is the language acceptable? Yes

Recommendation? Accept as is
Comments to the Author(s) I consider that the presented work can be published .

Decision letter (RSOS-210336.R1)
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Dear Miss Lu:
Title: Effective removal of tetracycline antibiotics from wastewater using practically applicable iron(III)-loaded cellulose nanofibers Manuscript ID: RSOS-210336.R1 It is a pleasure to accept your manuscript in its current form for publication in Royal Society Open Science. The chemistry content of Royal Society Open Science is published in collaboration with the Royal Society of Chemistry.
The comments of the reviewer(s) who reviewed your manuscript are included at the end of this email.
Please see the Royal Society Publishing guidance on how you may share your accepted author manuscript at https://royalsociety.org/journals/ethics-policies/media-embargo/. After publication, some additional ways to effectively promote your article can also be found here https://royalsociety.org/blog/2020/07/promoting-your-latest-paper-and-tracking-yourresults/. Response: Thanks a lot for your comment. We have carefully read these critical reviews and added them in the Introduction section (Lines 66 and 79 in the revised manuscript).
The modified BiOCl is a promising photocatalyst in photocatalysis and tetracycline Appendix A antibiotics (TCs) can be photocatalyzed by the modified BiOCl [1]. For example, Al-doped BiOCl [2], Y 3+ -doped BiOCl [3], and BiOCl/TiO2 [4] can reach photocatalytic efficiency of over 84% in a short time. The application of carbon materials derived from metal organic frameworks (MOFs) have become a rapidly expanding research field [5]. For example, Co3O4/CNTs [6] and MIL/CNT-Fe [7] performed well as catalysts in the applications of removing oxytetracycline (OTC) and tetracycline (TC) respectively. Covalent organic frameworks-based (COF) materials as superior adsorbents for the efficient removal of organic pollutants [8]. A novel COF-based material named as NCCT reached the high removal capacities of TC (388.52 mg/g) [9]. 4 12.00 respectively and reacted for 24h. The iron content in the leachate was tested and analyzed by atomic absorption spectrophotometer (GGX-600, HAIGUANG INSTRUMENT). The specific parameters were designed as follows: air flow rate is 7.6 L/min, acetylene flow rate is 1.3 L/min, 370 V, 10 mA, and measurement was performed at a wavelength of 248.3nm. The experimental results showed that the iron content of Fe(III)@CNFs at pH 7.05 and 12.00 was less than 0.03 mg/L (detection limit), while the iron content at pH 3.07 was 0.241 mg/L. Although Fe(III)@CNFs has iron leaching at low pH, the leaching amount is very low. In addition, the adsorbent is mostly used in sewage with neutral pH, so the stability of the composites can be guaranteed.  Response: We greatly appreciate your comments. We have carefully read articles you recommended and the mechanism was discussed more detailly (Lines 529-533 and 535-538 in the manuscript).
TCs' electron-rich ketone, carboxyl, amino, and hydroxyl groups contribute to their strong tendency to complex with metals. In Chen et al.'s study [10], TC, chlorotetracycline (CTC) and OTC adsorb strongly to aluminum oxide (Al2O3). Adsorption sites were also simulated force fields, which protonated carbonyl and amino groups of TCs that can bind cations effectively and form complexes [11]. The Fe(III) ions with TC, CTC and OTC form the stable complexes with the phenolic β-diketone structure of TCs [12]. In our study, the presence and absence of Fe(III) has a significant effect on the adsorption of TCs, thus the surface complexation between Fe(III) and TCs can be verified. Fe(III)@CNFs contain carboxyl groups and hydroxyl functional groups that can lead to the formation of a hydrogen bond with phenolic hydroxyl, amine, hydroxyl, and ketene moieties of TCs [13]. In addition, the FTIR spectra of Fe(III)@CNFs before and after TCs adsorption also proved the formation of hydrogen bond. In addition, the adsorption tendency of the Fe(III)@CNFs to Response: We greatly appreciate your constructive comments. The reason for the sorption amounts of OTC>TC>CTC has been discussed more detailly (Lines 491-500 in the revised manuscript).
The difference of chemical structure among OTC, TC and CTC is the substituent. The adding amount of hydroxyl in OTC resulted in the increase of combination sites with Fe(III)@CNFs. Fe(III)@CNFs contain carboxyl groups and hydroxyl functional groups that can lead to the formation of a hydrogen bond with hydroxyl of OTC [13]. Thus, the adsorption capacities of Fe(III)@CNFs to OTC is higher than adsorption capacity to TC. In addition, the additional chlorine atom at C7 position in CTC have the potential effect on electric density of the whole molecules and enhanced polarity of CTC's functional groups [10], therefore the adsorption capacity of Fe(III)@CNFs to CTC is just 232.56 mg/g.
Comparing to CTC, Fe(III)@CNFs can form the more complex with OTC and TC.

Comments to the Author(s)
The removal of TCs is of environmental importance due to their toxicity. The work in this manuscript shows enough novelty and scientific soundness to meet the requirement for publishing in Royal Society Open Science after major revision. There are some problems in 8 this work as follow: Response: Thanks a lot for your constructive comments and we have prepared a detailed reply to your comments.

The highlights do not contain sufficient novel features of the work and should be rewritten.
Response: Thanks a lot for your comments. The novel features of the work have been carefully summarized and the highlights have been rewritten as follows (Revised-highlights): (1) The renewable and non-sintered Fe(III)@CNFs derived from bamboo were optimized.
(3) Fe(III)@CNFs exhibited stable adsorption and excellent tolerance in a wide pH range.
(5) Adsorption of TCs fitted well with Langmuir and pseudo-second-order models.

What is the novelty of the work? Mention the novelty of the work clearly.
Response: Thanks a lot for your comments. The novelty of the work have been mentioned (Lines 84-86, 103-104, 573, 580-582, and 586-588 in the revised manuscript).
Bamboo grows fast and the bamboo wastes are abundant in Sichuan province of China.
The development and utilization of bamboo resources is conducive to broadening the application of bamboo resource products. The non-toxic and completely biodegradable cellulose within bamboo is one of the most abundant agricultural polysaccharide wastes in the world, and can be processed into CNFs by mechanical shearing method in this study.
Fe(III) has the advantages of being environmentally friendly, almost harmless to organisms and being able to complex with TCs. Therefore, the renewable and non-sintered Fe(III)@CNFs were developed and optimized to adsorb TC, CTC, and OTC from different metal and Fe(III) salts. The manufactured Fe(III)@CNFs exhibited stable adsorption and excellent tolerance in a wide pH range that tend to reduce the cost of pH adjustment. In addition, The strong reproducibility, firm Fe(III) fixation and the characteristics of not being easily affected by coexisting ions greatly broaden the application possibilities of Fe(III)@CNFs in actual sewage treatment.

What scientific problems can be solved by the design? The statements about novelty of the manuscript should be stressed if any.
Response: We greatly appreciate your comments. TCs are ubiquitously detected in wastewater and exist in the aquatic environment persistently because they are incompletely metabolized in organisms, are difficult to degrade, and have high hydrophilicity and low volatility. TCs' electron-rich ketone, carboxyl, amino, and hydroxyl groups contribute to their strong tendency to complex with metals. CNFs can be used as a framework to loading Fe(III) to remove TC, CTC, and OTC through the synergistic interactions of the surface complexation, the hydrogen bond, the electrostatic interactions and the van der Waals forces.
Through the design, a renewable and non-sintered adsorbent with stable adsorption performance is desired to solve the scientific problem of simultaneous adsorption of TC, CTC and OTC. The statements about novelty of the manuscript have been stressed (Lines 122-124 in the revised manuscript). Response: We greatly appreciate your comments. We have read these papers and sorbents for TCs have been updated (Lines 79-82 in the revised manuscript).

Sorbents for TCs should be updated. Recent sorbents reported should be described. Authors should consult and include the following papers (such as Applied Surface
The nanocomposites of reduced graphene oxide with the ZrO2 (ZrO2@rGO) [14], the hybrid nanocomposites of zero-valent iron loaded the activated carbon (ZVI@ACCS) [12], and the hybrid sorbent of α-iron oxide/reduced graphene oxide (α-Fe2O3@RGO) [ Response: We greatly appreciate your comments. We have reorganized the structure of Section 2.6 and introduced the regeneration method in Section 3.4 (Lines 394-396 in the revised manuscript).

The detail conditions and process of TCs determination by spectrophotometric method
should be given. The concentrations of TCs were determined usually by HPLC.
Response: Thanks a lot for your comments. In this study, the concentration of TCs is at the mg/L level and spectrophotometric method can be used. The ultraviolet spectrum of TC, CTC and OTC was scanned in the wavelength range of 190-400 nm using ultravioletvisible spectrophotometer (N4, INESA, China) and the wavelength corresponding to the strongest absorbance value was obtained ( Figure R2). In addition, the concentration of TCs has an excellent linear relationship with absorbance at the corresponding wavelength ( Figure   R2). Therefore, the concentration of TCs was tested by spectrophotometric method. The detailed conditions and process of TCs determination by spectrophotometric method have been given (Lines 186-190). Response: We greatly appreciate your constructive comments. We have added thermogravimetric analysis-differential scanning calorimetry (TGA-DSC) and X-ray diffraction (XRD) analysis to present the physicochemical properties of materials further.
Since sample preparation of TEM requires the sample to be dispersed in a solvent, and in theory, the thinner the sample is, the easier it is to observe, it is necessary to grind the sample sufficiently and disperse it in the solvent. We have tried to test our materials for TEM.
However, it is difficult to meet the TEM sample preparation requirements of the thickness of 100-200 nm and the time for returning the manuscript is limited, thus TEM has not been completed. We will pay attention to this test in future research.  The crystalline phases in CNFs and Fe(III)@CNFs samples were tested and analyzed by XRD (EMPYREAN, PANalytical) ( Figure R4). The specific parameters are designed as of the material at the nanometer level, and the fibrous material is not noticeable due to the insufficient magnification of the SEM image. In Figure S1 of Zhang's article [19], it can be seen that the material is obviously fibrous at magnification of 80,000×. The SEM images of iron(III)-loaded cellulose nanofibers have been exhibited in the text ( Figure 2 in the revised manuscript).