Facile Techniques to Synthesize Reduced Graphene Oxide for Removing Tetracycline From Water: Kinetics and Thermodynamics Studies

16 In this study, reduced graphene oxide (rGO) was successfully produced from graphite 17 precursor by chemical oxidation and exfoliation processes which were followed by a reduction 18 process in mild conditions. rGO was then applied in the adsorption of tetracycline (TC) in water. 19 SEM/EDX, XRD, FT-IR, BET, pH pzc were conducted to characterize the synthesized materials. 20 The adsorption efficiency of TC from water was evaluated by changes in several factors such as 21 contact time, temperature, pH of the solution, adsorbent load, and tetracycline concentration. 22 Furthermore, adsorption kinetics, thermodynamics, and isotherms were also investigated. As the 23 result, the adsorption process of TC onto rGO was spontaneous, endothermic, and governed by both physisorption and chemisorption. The maximum uptake calculated from Langmuir isotherm 25 model was 58.03 mg/g. rGO material could be regenerated by using methanol and diluted NaOH 26 solutions. The findings in this work provides a complete data on the TC adsorption process onto 27 rGO and the process of recovery and reuse of rGO.


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Since their discovery in 1928, antibiotics have played a very important role in human health 34 protection and the livestock industry. They have been extensively and effectively used in human 35 and veterinary medicines and their benefits have also been recognized in agriculture, aquaculture, 36 bee-keeping, and livestock as growth promoters [1]. It is estimated that millions of people have 37 been saved from diseases (smallpox, cholera, typhoid fever, syphilis, etc.) thanks to antibiotics. . 56 Several techniques have been developed to efficiently and effectively remove TC residues 57 from water such as membrane filtration [11] adsorption [12], and advanced oxidation processes 58 [13][14][15][16]. Among these methods, adsorption was a simple treatment method; it was affordable, easy 59 to handle with simple equipment, and low cost. In addition, the adsorbent could be recycled and 60 able to reuse several times. There were various mechanisms that affect the accumulation of 61 adsorbates on the surface of adsorbent like π -π interaction, electrostatic interaction, and pore-62 filling mechanism [17, 18].

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Graphene is a single layer of carbon with thickness as a carbon molecule, dense with sp 2 64 carbon molecules in honeycomb lattice [19]. Graphene oxide (GO) is oxidized graphene and has 65 the presence of oxygen-containing functional groups while reduced graphene oxide (rGO) is 66 obtained from the reduction of GO by removing oxygen-containing functional groups. There were 67 differences in functional groups or C:O ratio between GO and rGO [20,21]. Although the presence 68 of oxygen-containing groups make GO able to be hydrophilic which is suitable for water treatment, 69 they usually weaken the π-electron activity linked to a high fraction of sp 3 C atoms, which is 70 4 important interaction in the adsorption process [20]. On the other hand, rGO had a large specific 71 surface area and significantly fewer functional groups than GO [22,23]. Therefore, rGO is a 72 promising adsorbent for the treatment of different pollutants. Huízar-Félix et al. [24] reported 73 about the removal of TC using magnetic rGO material, which was expected to increase 74 electrostatic interaction between rGO with the TC and recoverability. rGO also showed a rather 75 high adsorption capacity for TC/sulfamethazine mixture (277.76 mg/g) than each substance 76 (219.10mg/g for TC and 174.42mg/g for sulfamethazine) [25]. However, there is a lack of research 77 conducting a complete study on the effect of different parameters on the adsorption of TC by rGO, 78 which provide fully information of the adsorption of TC by rGO.

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This paper aims to synthesize rGO from graphite precursor by chemical reduction of GO 80 using L-ascorbic acid and employ rGO material to study the TC adsorption process. The effect of 81 cintact time, temperature, initial pH, adsorbent dosage, initial TC concentration, and stability and 82 reusability of the material will be investigated. Furthermore, the adsorption isotherms, adsorption 83 kinetics, adsorption thermodynamics will be reported.     95 GO was synthesized by using modified Hummer's methods [15,16]. The oxidation of 96 graphite was conducted with the mixture of KMnO4, H2SO4, NaNO3. The obtained product was 97 washed several times with doubly distilled water and then dried at 50°C overnight.   Finally, the impact of temperature was examined with three temperature 298 K, 308K, and 318K.

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It is noted that all the experiments were conducted in triplicate.  where C0 (mg/L) and Ct (mg/L) are the TC initial concentrations and at time t (min), respectively.

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V is the volume of TC solutions (L); m is rGO mass (g).

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The Langmuir (Eq. 3) and Freundlich (Eq. 4) are the two most commonly used isotherm models  In this work, pseudo-first-order, pseudo-second-order, and intra-particle diffusion models were 155 deployed to examine the adsorption kinetics. They can be written as the following where the 156 pseudo-first-order, pseudo-second-order, and intra-particle diffusion models are presented in 157 equations 5, 6, and 7, respectively. (1/min) is the adsorption rate constant, 2 (g/mg.min) is the rate constant of the second-order 163 model, 3 (mg/g.min 1/2 ) is the rate constant of the intra-particle diffusion model, and C is the   of an intense peak at 1591 cm -1 confirmed the restoration of the sp 2 carbon networks [32]. The 212 obtained results suggested the reduction of oxygen-containing groups by L-Ascorbic acid. This 213 observation was also confirmed by the EDX analysis presented in the subsequent section.

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The images of graphite and rGO are described in Fig. 3a and 3b, respectively. It was shown 215 that the graphite's structure was characterized by several layers stacked while rGO took a typical 216 wrinkled morphology. The elemental composition of the material was examined by the EDX method. The C/O 221 ratio in GO and rGO materials were respectively 1.07 and 7.10 (Table 1), implying that the removal 222 of oxygen-containing functional groups led to the reduction of oxygen element in the rGO sheets.

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The obtained results in this work were comparable with previously reported results, which applied 224 different reduction methods (Table 1). It is noted that the reduction of GO by L-ascorbic acid is 225 milder and more acceptable than the use of NaBH4 or hydrazine, which can result in the formation  The BET analyses are displayed in Fig. 4   force. In the range of pH4 -pH7, the TC was in the form of zwitterion + − 0, the interaction 259 between rGO and TC molecules was impacted by both electrostatic repulsion and attraction forces.

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As a result, the adsorption of TC molecules on rGO was controlled by the stronger one. In pH > 261 7.7 solutions, TC was in the form of monovalent anion, + − −, or a divalent anion, 0 − − [15,41].

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This explained a decline in adsorption capacity of rGO in the pH  8 solutions (Fig. 5b) due to the 263 electrostatic repulsion between rGO surface negatively charged and TC anions.

Effect of adsorbent dosage 272
The adsorbent dosage might affect the adsorption process because it supplies more 273 available sites for TC molecules to be adsorbed. The results in Fig. 6a show that the adsorption     324 The results were better fitted with the pseudo-first-order model than the pseudo-second-325 order (regression coefficient (R 2 ) = 0,991 for the pseudo-second-order model compared with R 2 = 326 0,998 for the pseudo-first-order model). In addition, the percentage deviations of qe for the pseudo-327 first-order model was significantly lower than that obtained from the pseudo-second order model 328 (4.91% vs 34.2%), implying physisorption was predominant in the TC adsorption process (Table   329 2). The constant rate K estimated from the first-order model was 0.6356 h -1 .  339 To assess energy exchange phenomenon of the TC sorption process, Gibbs free energy 340 (ΔG), enthalpy (ΔH), and entropy (ΔS) were estimated. ΔG values were negative at all temperature 341 (-7.79, -9.31, and -9.54 kJ/mol for 298, 308, and 318K) which confirmed the spontaneous nature 342 of the adsorption of TC onto rGO (Table 3).

Adsorption thermodynamics and regeneration of adsorbent
343 Table 3. Thermodynamic parameters of TC adsorption process onto rGO. It is noted that the successive desorption-adsorption trial was repeated three times in which 354 adsorption test was performed at optimal conditions (10 mg rGO, 5 mg/L MB solution (pH=7), 355 and a contact time of 6 hours). The adsorption efficiency remained very high (> 89%) after three 356 20 cycles for TC adsorption (Fig 6b). This suggests that methanol and 0.1 M NaOH solutions could 357 be deployed to regenerate rGO for further adsortion process.

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The authors declare that they have no known competing financial interests or personal 378 relationships that could have appeared to influence the work reported in this paper.