Adsorption of oxytetracycline on kaolinite.

As antibiotic contamination increases in wastewater and aqueous environments, the reduction of antibiotics has become a pertinent topic of research regarding water treatment. Clay minerals, such as smectite or kaolinite, are important adsorbents used in water treatment, and sufficient removal of antibiotics by clay minerals is expected. In this study, the adsorption of oxytetracycline (OTC) on kaolinite was investigated. The experimental data of OTC adsorption on kaolinite fit the pseudo-second-order kinetics model well (R2>0.98). After 24 h, adsorption equilibrium of OTC on kaolinite was reached. The Langmuir model was better fitting with the adsorption isotherms generated from experimental data and OTC adsorption occurred on the external surface of kaolinite. The analysis of several thermodynamic parameters indicated that the adsorption of OTC on kaolinite was spontaneous and thermodynamically favorable. With the increase of the pH of a solution, the adsorption capacity increased and then decreased. The adsorption coefficient (Kd) of 102-103 were obtained for adsorption process of OTC on kaolinite.

Introduction

Currently, antibiotic contamination in the environment has received considerable attention [1-3]. Many antibiotics have been detected in wastewater and surface water [4-6], which results in the deterioration of the aquatic environment and the production of antibiotic resistant bacteria [7, 8]. Oxytetracycline (OTC) is a member of tetracyclines (TCs) antibiotics which are used widely in the world. The presence of OTC in wastewater treatment plant effluents, aqueous environments and even in drinking water has been reported in some studies [9-11]. Therefore, it is important to develop an efficient method to remove OTC from the aqueous phase. Several studies to remove OTC during water treatment have been reported [12-14].

Adsorption is widely used for pollutant removal during water treatment processes. Some studies have reported the adsorption of TCs by adsorbents such as activated carbon, biochar and clay minerals [15-17]. Previous studies on interactions between TCs and clay minerals have mostly focused on smectite because of a high cation exchange capacity and big surface area of smectite [18, 19]. However, the adsorption capability of TCs on kaolinite has less literature reports owing to its low cation exchange capacity and surface area. Figueroa et al reported that the adsorption capacity of kaolinite was lower than that of montmorillonite [20]. Some studies of the interaction of antibiotics and kaolinite mainly involved factors such as pH, organic matter or ionic strength. Zhao et al investigated the effects of some factors such as pH, background electrolyte cations and humic acid on kaolinite for TC adsorption, and the results indicated that TC adsorption by kaolinite was influenced by changes in the above mentioned solution conditions [21].The study on the interactions between TC and kaolinite found that the TC adsorption on kaolinite mainly focused on cation exchange of the external surfaces rather than due to complexation [22]. Some studies on kaolinite adsorbing TC have been carried out; however, few studies on the adsorption of OTC on kaolinite have been conducted. In this study, the adsorption capacities of OTC on kaolinite were investigated, and the adsorption mechanism was discussed for OTC on kaolinite.

Materials and methods

Materials

In this study, oxytetracycline hydrochloride was purchased from Dr. Ehrenstorfer (Germany). Acetonitrile (HPLC grade) and methyl alcohol (HPLC grade) were purchased from Merck (America). The kaolinite sample was obtained from Macklin (Shanghai). The kaolinite samples were filtered through a 300 mesh sieve and no further purification. The specific surface of kaolinite sample was 4.3 m2/g measured with the N2/BET method by ASAP 2020 Plus of Micromeritics (America). The measure of the particle size was conducted with an automatic laser particle size analyzer (LAP-W2000H, Xiamen), and most of the kaolinite was approximately 2.1 μm in size. The infrared spectroscopic analysis was conducted by fourier transform infrared spectrometer (VERTEX 70, Bruker, Germany) in the 400–4000 cm-1 wavenumber range. KBr pressed-disc method was adopted in this study. Kaolinite treated with OTC was pretreated with freeze drying. A certain mass of dried sample and KBr were mixed and grinded together in an agate bowl. Then, the mixed powder was pressed into discs and detected with spectrometer.

Adsorption experiments

The kinetic adsorption of OTC on kaolinite was conducted in a batch experiment. Accordingly, 40 mg kaolinite and 20mL 0.01 M CaCl2 electrolyte solution with different OTC concentration (at 5, 10 and 25 mg/L) were combined in 40 mL brown glass vials, and the mixed samples were shaken on a reciprocal shaker at 150 rpm and 298 K for 0.16, 0.33, 0.3, 1, 2, 4, 6, 8, 12, 16, 20, 24, 36 and 48 h at pH 5.5. The above samples were centrifuged at 5000 rpm for 10 min, and the supernatant solution was filtered through a 0.22μm membrane. Subsequently, the OTC concentration of the filtered solution was determined by HPLC with a UV detector (e2695, Waters, USA).

The initial OTC concentrations of 1 to 35 mg/L were used to generate adsorption isotherms under three temperature conditions (288 K, 298 K and 308 K). Then, 20 mL of OTC solutions at different concentrations and 40 mg of kaolinite were mixed at pH 5.5 in 40 mL brown glass vials. The mixed samples were shaken with the shaker at 150 rpm until reaching adsorption equilibrium. The OTC concentration in the equilibrium samples was obtained using the same method as described above (refer to kinetic adsorption).

The influence of solution pH on the OTC adsorption by kaolinite was evaluated in a series of batch experiments. The variation of solution pH was from 3.5 to 11.5, and the concentration of OTC was 5 mg/L or 10 mg/L. pH of OTC and kaolinite mixed samples were adjusted according to the above series of pH values and then adjusted samples were shaken at 150 rpm to adsorption equilibrium. Finally, the OTC concentration was measured.

Control samples including no kaolinite or no OTC conditions were conducted simultaneously in each experiment and all experiments were run in triplicate.

Data analysis

Adsorption kinetics and isotherms models are showed in Table 1.

Table 1

Adsorption kinetics and isotherms models.

Adsorption model		Model equation	Parameters
kinetic models	pseudo-first-order	log(q_e−q_t)=logq_e−k_1t2.303	qe: the adsorption capacity at the equilibrium time (mg/g)qt: the adsorption capacities at the time t (mg/g)k1: the rate constant of the pseudo-first-order model (h-1)
	pseudo-second-order	tq_t=1k_2qe2+tq_e	k2: the rate constant of the pseudo-second-order (g/(mg·h))
isotherm models	Langmuir	q_e=k_lq_maxC_e1+k_lC_e	Ce: the OTC equilibrium concentration (mg/L)kl: the adsorption coefficientqmax: the maximum adsorption capacity(mg/g)
	Freundlich	q_e=k_fCen	kf: the sorption coefficients (mg1-n·Ln/g)n: the linear coefficients
	Tempkin	q_e=RTB_Tln(k_tC_e)	kt: the Tempkin constant that corresponds to the maximum binding energy (L/mg)R: universal gas constant (8.314 J/(mol·K))T: the absolute temperature (K)BT: obtained after solving the Tempkin equation

Thermodynamic analysis was conducted using Eqs (1)–(3) to obtain thermodynamic parameters, such as the standard Gibbs free energy (ΔG), enthalpy (ΔH) and entropy (ΔS), as follows: Where T (K) is the absolute temperature and R (8.314 J/(mol·K)) is the universal gas constant.

Results and discussion

OTC adsorption kinetics

The adsorption kinetics of OTC by kaolinite is shown in Fig 1. For OTC, the adsorption capacity of kaolinite had a varying trend with increasing time. During the initial adsorption time, apparent adsorption was observed, and the adsorption capacity reached approximately 50% at the sorption time of 4 h. From the figure, a steep curve was presented before 8 h. A previous study found that a substantial mass transfer driving force occurred between the adsorbent and solution because of the high concentration of antibiotic, which caused the antibiotic to rapidly occupy the adsorption sites of the adsorbent [18, 22]. In the range of 8-24h, adsorption capacity increased slowly than that during the initial time with the increment of time. Subsequently, the adsorption capacity changed slightly, and a steady curve could be found during 24–48 h, which indicated that adsorption equilibrium was obtained after 24 h. The adsorption kinetics of the three concentrations of OTC presented the same adsorption trend, and low concentrations of OTC had faster equilibrium times than those of high concentrations. Compared to smectite, the adsorption capacity of OTC on kaolinite is much lower [18]. In this study, OTC adsorption capacity of 7mg/g on kaolinite was obtained during initial OTC concentration of 25mg/L, which is similar to the results of Zhao’s study [21].

Fig 1

Adsorption kinetics of OTC on kaolinite: (a) pseudo-second-order model; (b) the linear plot of the pseudo-second-order model.

Table 2 shows the adsorption kinetics parameters of the pseudo-first-order and pseudo-second-order models. The R2 of the pseudo-second-order model of the experimental data reached above 0.98 but was only 0.747–0.873 for the pseudo-first-order model. The experimental data obviously fit a pseudo-second-order model, and the adsorption of OTC on kaolinite mainly involved chemical adsorption processes. This is consistent with previous studies [23]. From the table, the value of qe of the pseudo-second-order model increased with increasing initial OTC concentration. Moreover, the fitted rate constants of 0.044–0.149 g/(mg·h) during initial OTC concentration of 5-25mg/L were higher than that of TC on smectite. However, compared with TC adsorption on smectite [22], the initial rates of 0.65–2.60mg/(mg·h) in this experiment were much lower.

Table 2

Pseudo-first-order and pseudo-second-order parameters of OTC adsorption on kaolinite.

OTC concentrations	Pseudo-first-order model			Pseudo-second-order model
	qe (mg/g)	k1 (h-1)	R2	qe (mg/g)	k2 (g/(mg·h))	R2
5 mg/L	1.331	0.047	0.782	2.088	0.149	0.988
10 mg/L	0.739	0.051	0.747	3.846	0.086	0.993
25 mg/L	0.386	0.059	0.873	7.692	0.044	0.986

Adsorption isotherms

In this study, three models including Langmuir model, Freundlich model and Tempkin model were adopted to evaluate the adsorption equilibrium isotherms of OTC on kaolinite. Fig 2 shows the adsorption isotherms for the adsorption of OTC on kaolinite. With increasing temperature, the capacity of OTC adsorption on kaolinite presented a significant difference: the higher the temperature, the larger the adsorption capacity. The largest adsorption capacity reached 8.92 mg/g under 308 K condition, and a capacity of only 4.53 mg/g was reached at 288 K.

Fig 2

Adsorption isotherms for the OTC adsorption on kaolinite.

The experimental data was better fitted to the Langmuir model than Freundlich model, and the correlation coefficient (R2) was above 0.95 (0.953–0.980) for Langmuir model (Table 3). However, the correlation coefficient of the Tempkin model was worse than that of the above two models. This indicated the sorption of OTC on kaolinite was monolayer adsorption and may occur on the external surface of kaolinite [22]. The trend of qmax in the Langmuir model is increasing with the increase of temperature. The value of qmax changed from 6.342 mg/g to 15.236 mg/g over the range of 288 K-308 K. The adsorption capacity of OTC on kaolinite was similar to that of TC adsorption on kaolinite [22] and quinolone antibiotic nalidixic acid onto kaolinite [24]. The Freundlich model constant n represents the sorption intensity, and an n value <1 implied good adsorption and a concentration-dependent process. In this study, n values <1 were obtained in the Freundlich model.

Table 3

Langmuir, Freundlich and Tempkin models parameters for OTC adsorption on kaolinite.

Temperature(K)	Langmuir model			Freundlich model			Tempkin model
	qmax (mg/g)	KL (L/mg)	R2	Kf (mg1-n·Ln/g)	n	R2	KT (L/mg)	BT (×103)	R2
288	6.342	0.133	0.953	0.875	0.511	0.921	1.737	1.2329	0.911
298	8.749	0.137	0.965	1.227	0.549	0.944	1.9513	1.6258	0.915
308	15.236	0.171	0.980	2.220	0.645	0.971	2.634	2.6526	0.921

To further analyze the mechanisms of adsorption, FTIR spectroscopic analysis was conducted. Fig 3 shows the changes of FTIR spectrum plot for raw kaolinite and kaolinite treated with 10mg/L OTC. The FTIR spectrum of OTC adsorbed by kaolinite changed for the two bands. The peak intensity at 1633, corresponding to the–C = O groups, was observed after adsorption which implied that OTC was adsorbed on kaolinite [25]. Jia suggested that the intense stretching band between 3300 and 3500 cm−1 is due to the O-H of OTC [26]. In this study, the peak intensity at 3466 increased after adsorbing OTC which indicated the interaction between OTC and kaolinite.

Fig 3

Fourier transform infrared spectroscopic analysis of kaolinite before and after adsorption.

Analysis of thermodynamics

During the thermodynamics analysis, three thermodynamic parameters including ΔG, ΔH and ΔS were investigated. The effect of temperature on the adsorption coefficient (Kd) for OTC sorption on kaolinite is shown in Fig 4. Here, Kd was calculated by qe/Ce (L/kg) and the ΔG values were derived from lnKd. Table 4 shows the parameters of thermodynamics of OTC adsorption on kaolinite. All ΔG values were negative during different adsorption temperatures, which indicated that the adsorption of OTC on kaolinite was spontaneous and thermodynamically favorable. Moreover, the absolute ΔG values increased with the adsorption temperature, which implied that high temperature is favorable for adsorption of OTC on kaolinite. The positive ΔH values of OTC adsorption on kaolinite implied an endothermic adsorption process which agrees with the results of the favored high temperatures indicated by ΔG. The positive ΔS suggested that the adsorption process favored sorption stability. In this study, the ΔS value was 245.64 J/(mol·K), which implied that disorder increased at the interface between OTC and kaolinite during the adsorption process.

Fig 4

Effect of temperature on the sorption coefficient (Kd) for OTC sorption on kaolinite.

Table 4

Thermodynamic parameters for OTC adsorption on kaolinite.

Temperature (K)	ΔH (kJ/mol)	ΔS (J/(mol·K))	ΔG (kJ/mol)	R2
288	55.92	245.64	-14.99	0.972
298			-16.34
308			-18.64

The effect of pH on OTC adsorption

To further analyze the effects of different OTC fractions on the adsorption process, an empirical model (eqn.4) of K was used in this study. Where K is the adsorption coefficient (L/kg); K+00, K+-0, K+--, and K0-- are the adsorption coefficients of the four OTC fractions; f+00, f+-0, f+--, and f0—are the cationic fraction, zwitterionic fraction, amination anionic fraction and bivalent anionic fraction, respectively.

The effect of pH on the adsorption capacity for OTC sorption on kaolinite is shown in Fig 5. There were substantial influences of pH on the adsorption capacities of kaolinite for OTC. With the increase of pH, the adsorption capacity increased and then decreased. A maximum adsorption capacity existed at a pH of approximately 5.5. The surface charges of kaolinite and OTC changed with different pH values, which influenced the adsorption properties. For amphoteric OTC, there are three pKa values (3.57, 7.49 and 9.88), and OTC was divided into four fractions as follows under different pH conditions: the cationic OTC (OTC+00) fraction with pH<3.57, the zwitterionic OTC fraction (OTC+-0) during pH of 3.57–7.49, the amination anionic OTC (OTC+--) or bivalent anionic OTC (OTC0--) with pH >7.49 (Fig 6). It is believed that surface of kaolinite had a constant structural charge and edge charge depending on the solution pH [27]. The surface charge of kaolinite is normally considered to be a negative surface charge and some positive charge exists on kaolinite under acidic conditions, while negative charges presents under alkaline conditions [21]. In this study, when the pH was greater than 7.49, the same charges existed on both OTC and kaolinite and resulted in electrostatic repulsion. Thus, the adsorption capacity of kaolinite for OTC in the pH range >7.49 was worse than that during in pH range <7.49. At pH<3.57, the positive charge of OTC can be adsorbed by the negative charge of kaolinite; with the increase of pH value, the charge of OTC becomes neutral. The adsorption capacities presented an increasing trend, which is similar to the results of previous studies [28, 29]. Some studies have suggested that the mechanism is complexation [20, 30]. However, cation exchange mechanism for antibiotics adsorption on clays was supported by more researchers. Li et al found that there existed simultaneous H+ uptake during the adsorption process of tetracycline onto smectites and cation exchange occurred even under neutral pH conditions [18]. Zhao et al suggested that the adsorption mechanism between tetracycline and the kaolinite surface was similar to an outer-sphere cation exchange reaction [21]. Many studies agreed low pH was beneficial to the adsorption of TC and the adsorption mechanism was cation exchange [20, 22, 29].

Fig 5

Effect of pH on OTC adsorption capacity on kaolinite.

Fig 6

Distribution of OTC species during different pH values.

The adsorption coefficients (Kd) for the OTC species at different pH values are shown in Table 5. A relatively good fit of the Kd data was obtained, and R2 reached 0.826 for 5 mg/L and 0.982 for 10 mg/L. From the table, it can be seen that the four OTC species exhibited different adsorption coefficients. The value of Kd+-0 was greater than the other three values, Kd+00 and Kd0-- were in the middle, and Kd+--was the lowest. This illustrated that the highest adsorption affinity was obtained by the zwitterionic species during the four species. The contribution results of different species to OTC adsorption indicated that more than 55% contribution rates of the zwitterionic species were obtained for two OTC concentrations (5 mg/Land 10 mg/L). It seems that interaction between the zwitterionic species and the negative surface charge of kaolinite is easy. In addition, positive OTC species had more contribution to OTC adsorption than negative species did, which is also confirmed by the above data (refer to Fig 5).

Table 5

Calculated adsorption coefficients for the OTC species.

	Kd+00 (L/kg)	Kd+-0 (L/kg)	Kd+-- (L/kg)	Kd0-- (L/kg)	Radj2
5 mg/L OTC	976.69	2636.21	373.59	603.73	0.826
10 mg/L OTC	821.74	2023.27	94.45	402.04	0.982
Contribution rate (%) (5 mg/L)	21.29	57.45	8.10	13.16
Contribution rate (%) (10 mg/L)	24.59	60.56	2.82	12.03

Adsorption affinity

Fig 7 shows the adsorption coefficient (Kd) values for the OTC adsorption on kaolinite. With an increasing OTC equilibrium concentration, the values of Kd decreased. Obviously, the adsorption affinity between OTC and kaolinite was highly correlated with OTC concentration, and the lower the concentration, the larger the adsorption affinity. In this study, the adsorption capacity was evaluated by Kd, and the large value of Kd is favorable for adsorption. The study indicated that the Kd values of some natural adsorbents were 102−103 L/kg [20]. Similar results were obtained in this study, which were Kd values ranging from 350 to 1600 L/kg.

Fig 7

Values of the adsorption coefficient (Kd) for the sorption of OTC on kaolinite.

Conclusions

The adsorption of OTC on kaolinite indicated that the adsorption equilibrium was obtained after 24 h, and the adsorption experimental data fit the pseudo-second-order model well. The adsorption isotherms for OTC by kaolinite fit very well with the Langmuir model. The thermodynamic analysis revealed that a spontaneous and endothermic process occurred between OTC and kaolinite. The solution pH had a great effect on the adsorption processes, and a relatively higher adsorption capacity could be obtained for the zwitterionic OTC species. The values of the adsorption coefficient (Kd) presented the order of 102–103.

Adsorption kinetics of OTC on kaolinite.

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9 Sep 2019

PONE-D-19-21989

Adsorption of oxytetracycline on kaolinite

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Reviewer #1: This manuscript presents a study about the adsorption of oxytetracycline on kaolinite. It is shown that the adsorption of OTC on kaolinite indicated that the adsorption equilibrium was obtained at 24h and the adsorption experimental data well fitted second -pseudo-order model. The adsorption isotherms for OTC by kaolinite had high fitting with Langmuir model and Freundlich model. The thermodynamic analysis revealed that the adsorption process of OTC on kaolinite was spontaneous and endothermic. The solution pH had a great effect on the adsorption processes and relatively higher adsorption capacity could be obtained for the zwitterionic OTC species. In my opinion it will be of contemporary importance to the readers of PLOS ONE. Hence, upon properly addressing the comments listed below the manuscript should be suitable for publication.

1. On Section 2.3 Data analysis:

The author mentioned that: " The variation of solution pH was from 3.5 to 11.5, and the concentration of OTC was 5mg/L or 10mg/L. ". Since fine kaolinite will change the pH of the solution, it is recommended to specify whether it is the initial or equilibrium pH of the solution.

2. On Section 3.4 The effect of pH on adsorption:

Kaolinite is a 1:1 layer mineral, in which one layer consists of an alumina octahedral sheet and another consists of silica tetrahedral sheet. The charge characteristics of these two layers and edges vary greatly in solutions with different pH values. Therefore, it is suggested to describe in detail the charged characteristics of kaolinite particles at different pH values. It is helpful for the authors to analyze the effect of pH on adsorption capacity for OTC sorption on kaolinite surface.

Reviewer #2: This paper discusses an interesting and relevant topic. Ultimately I would like to see this data published, but I think considerable improvements must be made to the description of methods and analysis before this work is publishable.

The English and language and gramma are poor throughout. This could be improved, this is not a huge job, but it does need to be corrected to improve the clarity and intent of some of the arguments and reduce ambiguity.

Page 8, why SA to 4 decimal places? The SA is very low compared to many kaolinite samples…why? Large average particle size? Very low porosity? How do we know all of the sand and other contaminating minerals were removed? Is there XRD information?

Page 8, what was the pH of the kinetic and isotherm experiments…it would be useful if this was stated in the methods section.

Page 8, is there any replication of the experiments? What is the uncertainty, what controls of data quality were used?

Page 8, what was the ionic strength? The introduction suggests that ionic strength has been tested by other researchers, but there is no mention of the ionic strength for the suspensions in this study.

Page 8, How was the pH controlled for the sorption experiments…adjustments were made between pH 3.5 and 11, but was this measured at the beginning or at the end of the equilibration period? Was there any change in the pH over the equilibration time (it would be unusual if the pH did not change a little)?

Page 8, Why would an intra-particle diffusion model be appropriate here, the SA is very low indicating little porosity, and no estimate of pore volume in given form the BTE SA measurements?

Page 10, I find the results and discussion of a diffusion based mechanism a little puzzling. Given that the porosity is low, and that kaolinite is a classic non-swelling clay, I find it extremely unlikely that there is a diffusion process at play. I do not think there is sufficient evidence to conclude that there is a movement of OTC form external to internal surfaces. Kinetic modelling of this type is really just a curve fitting process, and the models with more adjustable parameters will naturally provide a better fit. Conclusions about actual kinetic mechanisms really need more substantiation that just the fit of a particular semi-empirical model.

Page 11, the commentary on the application of the isotherm modelling is similarly superficial as the kinetic modelling mentioned above, and hence I think there is an over-interpretation of the data sets based on the model parameters, though the overall all conclusion is rather weak in that the authors suggest that there are ‘multiple’ mechanisms responsible for the uptake.

Page 12, the IR spectra and the interpretation is very unconvincing. How were these spectra collected? Is this diffuse reflectance or ATR of dried samples or pastes? There is no description in the methods section, and it is simply not possible to make conclusions from the spectra unless more detail is known about what these spectra represent (ie wet/dry samples, ATR etc). What was the surface concentration of the OTC when absorbed, can you actually see it in the IR, was the OTC just crystalline sample or a solution etc?

Overall I think the commentary through the discussion and analysis of results is too speculative, and this needs significant improvement. I do however, think that much of this could be tidied up quite quickly by the authors thinking more carefully about what the data is telling them. Certainly more description of the methods would help the paper a lot. The data set look very interesting, and is a valuable contribution, though I would insist on some mention of quality control and uncertainty.

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Reviewer #1: No

Reviewer #2: No

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24 Oct 2019

Responds to the editor’s and reviewer’s comments:

Responds to the editor

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Response: We have modified the manuscript according to PLOS ONE’s style requirements.

2. We suggest you thoroughly copyedit your manuscript for language usage, spelling, and grammar. If you do not know anyone who can help you do this, you may wish to consider employing a professional scientific editing service.

Response: Considering the editor’s comment, the manuscript has been edited by AJE.

3. We noticed you have some minor occurrence of overlapping text with the following previous publication(s), which needs to be addressed: * Sorption of tetracycline on biochar derived from rice straw and swine manure, doi: 10.1039/C8RA01454J

Response: Some sentences have been modified to avoid overlapping text in the manuscript.

4. PLOS authors have the option to publish the peer review history of their article (what does this mean?). If published, this will include your full peer review and any attached files.

Response: No

Responds to the reviewer #1

1. On Section 2.3 Data analysis: The author mentioned that: "The variation of solution pH was from 3.5 to 11.5, and the concentration of OTC was 5mg/L or 10mg/L. ". Since fine kaolinite will change the pH of the solution, it is recommended to specify whether it is the initial or equilibrium pH of the solution.

Response: The solution pH is the initial pH and we have added the description of pH in the manuscript.

2. On Section 3.4 The effect of pH on adsorption: Kaolinite is a 1:1 layer mineral, in which one layer consists of an alumina octahedral sheet and another consists of silica tetrahedral sheet. The charge characteristics of these two layers and edges vary greatly in solutions with different pH values. Therefore, it is suggested to describe in detail the charged characteristics of kaolinite particles at different pH values. It is helpful for the authors to analyze the effect of pH on adsorption capacity for OTC sorption on kaolinite surface.

Response: As the reviewer’s comment, we added the charged characteristics of kaolinite at different pH values inferred from previous studies in the part of discussion. At same time, we discussed how charged characteristics of kaolinite influence the OTC sorption at different pH values.

Responds to the reviewer #2

1. The English and language and grammar are poor throughout. This could be improved, this is not a huge job, but it does need to be corrected to improve the clarity and intent of some of the arguments and reduce ambiguity.

Response: The English language and grammar has been improved. Discussion part has also been tried to clear.

2. Page 8, why SA to 4 decimal places? The SA is very low compared to many kaolinite samples…why? Large average particle size? Very low porosity? How do we know all of the sand and other contaminating minerals were removed? Is there XRD information?

Response: The equipment of measuring SA gave a 4 decimal value. Considering to small value of SA, the SA is kept 2 decimal places. The kaolinite used in this experiment was bought from Macklin (Shanghai) and pretreatment of kaolinite was relatively simple (just filtered through a 300 mesh sieve), so low SA is obtained. We have found that kaolinite SA of 6.45m2/g was reported by Yandan Li. Most particle size focused on 2.1um and only porosity of kaolinite of 0.0047cm3/g. There is no XRD information for kaolinite sample because of limit of experimental condition.

3. Page 8, what was the pH of the kinetic and isotherm experiments…it would be useful if this was stated in the methods section.

Response: The pH of the kinetic and isotherm experiments is 5.5 and manuscript has been revised for this question.

4. Page 8, is there any replication of the experiments? What is the uncertainty, what controls of data quality were used?

Response: We have added the contents in section 2.2 in the manuscript.

5. Page 8, what was the ionic strength? The introduction suggests that ionic strength has been tested by other researchers, but there is no mention of the ionic strength for the suspensions in this study.

Response: In this study, 0.01 M CaCl2 was used as background electrolyte of solution and in the manuscript we have added the description of solution background electrolyte.

6. Page 8, How was the pH controlled for the sorption experiments…adjustments were made between pH 3.5 and 11, but was this measured at the beginning or at the end of the equilibration period? Was there any change in the pH over the equilibration time (it would be unusual if the pH did not change a little)?

Response: pH was measured at the beginning of the equilibration experiment. The pH changed by 1 to 2 units of pH after equilibration.

7. Page 8, Why would an intra-particle diffusion model be appropriate here, the SA is very low indicating little porosity, and no estimate of pore volume in given form the BTE SA measurements?

Response: An intra-particle diffusion model is not appropriate for OTC adsorption of kaolinite, so we have removed the contents about intra-particle diffusion model from the manuscript.

8. Page 10, I find the results and discussion of a diffusion based mechanism a little puzzling. Given that the porosity is low, and that kaolinite is a classic non-swelling clay, I find it extremely unlikely that there is a diffusion process at play. I do not think there is sufficient evidence to conclude that there is a movement of OTC form external to internal surfaces. Kinetic modeling of this type is really just a curve fitting process, and the models with more adjustable parameters will naturally provide a better fit. Conclusions about actual kinetic mechanisms really need more substantiation that just the fit of a particular semi-empirical model.

Response: According to some studies about kinetics of kaolinite adsorption, intra-particle diffusion process is very little. So it is not appropriate to use an intra-particle diffusion model here and the contents have been deleted from the manuscript.

9. Page 11, the commentary on the application of the isotherm modeling is similarly superficial as the kinetic modeling mentioned above, and hence I think there is an over-interpretation of the data sets based on the model parameters, though the overall all conclusion is rather weak in that the authors suggest that there are ‘multiple’ mechanisms responsible for the uptake.

Response: We have revised some detail description of isotherm model and discussion has been added.

10. Page 12, the IR spectra and the interpretation is very unconvincing. How were these spectra collected? Is this diffuse reflectance or ATR of dried samples or pastes? There is no description in the methods section, and it is simply not possible to make conclusions from the spectra unless more detail is known about what these spectra represent (ie wet/dry samples, ATR etc). What was the surface concentration of the OTC when absorbed, can you actually see it in the IR, was the OTC just crystalline sample or a solution etc?

Response: We have added the method of IR spectra collection in section 2.1. In this study, KBr pressed-disc method was adopted. OTC of 10mg/L adsorbed by kaolinite was used and all samples for IR measuring were solid.

References

1. Yandan Li. Sorption behavior of typical flueroquinolone antibiotics on kaolinite：batch experiments. Bioresour Technol. Chia university of geosciences. 2017.

Submitted filename: Response to Reviewers.doc

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4 Nov 2019

Adsorption of oxytetracycline on kaolinite

PONE-D-19-21989R1

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Reviewers' comments:

8 Nov 2019

PONE-D-19-21989R1

Adsorption of oxytetracycline on kaolinite

Dear Dr. Song:

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