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Antibiotic amoxicillin removal from aqueous solution using magnetically modified graphene nanoplatelets
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JIEC 2820 1–8 Journal of Industrial and Engineering Chemistry xxx (2016) xxx–xxx
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Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec 1 2 3 4 5 6
Antibiotic amoxicillin removal from aqueous solution using magnetically modified graphene nanoplatelets ¨ zge Q1 O a b
Kerkez-Kuyumcu a, S¸ahika Sena Bayazit a, Mohamed Abdel Salam b,*
Beykent University, Faculty of Engineering & Architecture, Chemical Engineering Department, P.O. Box 34396, Istanbul, Turkey Chemistry Department, Faculty of Science, King Abdulaziz University, P.O. Box 80200, Jeddah 21589, Saudi Arabia
A R T I C L E I N F O
A B S T R A C T
Article history: Received 7 December 2015 Received in revised form 26 January 2016 Accepted 30 January 2016 Available online xxx
The removal of antibiotic amoxicillin (AA) from aqueous solution was investigated using magnetically modified graphene nanoplatelets (M-GNPs). M-GNPs were prepared by mixing GNPs with freshly prepared magnetite nanopartilcles, and characterized using TEM, and XRD. The characterization results revealed the homogenous distribution of the magnetite nanopartilcles over the surface of transparent platelet-like graphene platlets. The M-GNPs proved to possess superior adsorption capacity compared with the pristine GNPs and the magnetite nanopartilces. The effects of different operational parameters which affect the removal process were explored; adsorbent amounts, contact time, initial pH, temperature, and the initial concentration of AA. The results showed the great affinity of the M-GNPs toward the AA and the maximum adsorption capacity was found to be 14.10 mg g1. The adsorption mechanism of AA by the M-GNPs involved p–p stacking and electrostatic interaction. The adsorption was studied kinetically and thermodynamically, and was found to mainly follow pseudo-second-order kinetic model, and was spontaneous and exothermic in nature. ß 2016 Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering Chemistry.
Keywords: Adsorption Antibiotic amoxicillin Graphene Kinetics Magnetic separation Q2 Thermodyanmics
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1. Introduction
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Water pollution is a serious threat for human health, ecosystem, and in terms of depletion of clean water resources. One of the sources that causes water pollution, even in small concentrations, are the pharmaceuticals [1], as about 30–90% of the given dose can remain un-degradable in the human or animal body, and is largely excreted as an active compound [2,3]. Pharmaceutical residues have adverse effects towards non-target organisms [4]. Especially release of antibiotics to the environment causes enhanced resistant bacteria, which makes related treatment difficult [5]; therefore the removal of the antibiotics from waste water is very important to protect the ecosystem in long term. Amoxicillin is a broad spectrum b-lactam antibiotic (aminopenicilin antibiotic), used as veterinary medicine for treatment of infections encountered in gastro-intestinal and systemic infections, and also, commonly used as human prescription medicine (against infections caused by bacteria) [5,6]. Amoxicillin consumption is higher than the other types in the penicillin family because of the oral
* Corresponding author. Tel.: +966 541886660; fax: +966 2 6952292. E-mail address: [email protected] (M.A. Salam).
absorption ability [7], and unfortunately, it is non-biodegradable and also an inhibitor of the photosynthesis mechanism of algae Synechocystis sp. [8,9]. Several treatment methods used for the removal of antibiotics, such as adsorption, advanced oxidation processes (heterogeneous Fenton reaction, UV, UV/H2O2, UV/H2O2/ TiO2 processes, etc.), biodegradation, membrane filtration, and coagulation/flocculation/sedimentation [1,10–14]. Adsorption is a promising procedure due to its effectiveness, mild operational costs, generation of secondary pollutants are non-existant, regeneration and reuse possibilities, short retention time, and high efficiency, in addition to the high levels of harmful organic substances that could be accumulatig on the solid surface [15,16]. There have been many reports for adsorption of antibiotics from waste water, and the most used adsorbents are activated carbon/carbon based materials and clays/organoclays [15– 21]. Putra et al. have reported a comperative study of activated carbon and bentonite for amoxicillin adsorption [6]. Recently, Carrales-Alvarado et al. used different carbon based materials (activated carbon, activated carbon cloth, mesoporous activated carbon, and carbon nanotubes) for removal of the antibiotic metronidazole [22]. Nanographene is a newly developed member of carbon based materials and has been used in the environmental remediation for the removal of different pollutants from water
http://dx.doi.org/10.1016/j.jiec.2016.01.040 1226-086X/ß 2016 Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering Chemistry.
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[23,24]. Also, graphene-based materials are used for the adsorption of different pharmaceuticals adsorption such as tetracycline [25], sulfamethoxazole [26,27], ciprofloxacin [27], aspirin, acetaminophen, and caffeine [28]. One of the limitations of the adsorption process is the difficulty of separating the adsorbent from the aqueous medium. Lately, magnetic separation using magnetic adsorbents has been found to be an interesting solution to overcome such limitations in terms of ease and fast separation of the adsorbent from bulk solution using the external magnets [29,30]. There are several reports for the application of magnetic graphene for the adsorption of different pollutants from aqueous solution [31–34]. In the presented study, magnetically modified graphene nanoplatelets (were prepared by co-precipitation method and used for the adsorption of AA from aqueous solution. The selected graphene nanoplatelets used in this study characterized with the high surface area, special structure, and its high adsorption ability is proved previously [23,28], as well as its great hydrophobicity compared with previous work performed using graphene oxide [25–27]. This hydrophobicity enables a better separation of the graphene nanoplatelets from the bulk solution. The physical characterization for the M-GNPs was studied to explore their morphological properties first, then the effect of different environmental conditions; solution pH, temperature, and adsorption time, which affect the removal process, were investigated. Additionally, the adsorption process was studied kinetically and thermodynamically to predict the adsorption rate in order to understand the adsorption behavior, the mechanism of adsorption, and its spontaneity by calculating different thermodynamic parameters. The experimental data have been modeled by using the Langmuir, Freundlich, and Temkin isotherms. Finally, the desorption process was performed to evaluate the recovery of the magnetic adsorbent.
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2. Experimental
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2.1. Materials
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Graphene nanoplatelets (GNPs) were obtained from XG Science (xGnP1-C-750), USA. xGnP1 Graphene Nanoplatelets are unique nanoparticles consisting of short stacks of graphene sheets having a platelet shape with thickness ranging from 1 to 20 nm and width ranging from 1 to 50 mm. FeCl36H2O (Merck), Fe(SO4)7H2O (Merck), HCl (Merck, 36.5%), and ammonia solution (Merck, 25%) were used in the synthesis of magnetite (Fe3O4). Amoxicillin was obtained from Sigma-Aldrich. All the reagents employed in the experiments were used without further purification. A stock solution of 1000 mg/l containing amoxicillin was prepared, and dilution was performed for the calibration solutions and adsoption study solutions.
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2.2. Synthesis of magnetically modified graphene nanoplatelets (M-GNPs)
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Two hundred milliliter of aqueous solutions of graphene nanoplatelets (GNPs) were prepared under ultrasound. The aqueous solution in the molar ratio of Fe2+:Fe3+ = 1:2 was prepared and a few drops of concentrated HCl was added for complete dissolution of iron salts. It was added to the graphene solution under ultrasound and the mixture was kept under ultrasound for 10 min. The mass ratio of graphene:Fe3O4 was adjusted to be 75:25. 8 M NH4OH solution was added dropwise till the pH reached 11–12. Finally, the mixture was kept at 50 8C under mechanical stirring. The precipitation was magnetically separated from the aqueous phase and the composites were washed with deionized water and ethanol, dried at 60 8C in vacuum oven. To
adjust the mass ratio of GNPs: Fe3O4, a set of previous experiments were performed first for the preparation of the Fe3O4 alone. The yield of Fe3O4 was measured and accordingly, the amount of GNPs were determined and added to the experiment. The mass of the MGNPs was measured by the end of the experiment substracted from the orignal mass of the GNPs used, and the mass of ratio of GNPs: Fe3O4 was mostly 75:25.
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2.3. Characterization
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X-ray diffraction (XRD) patterns were recorded for phase analysis and the measurement of crystallite size on a Philips X pert pro diffractometer, which was operated at 40 mA and 40 kV by using Cu-Ka radiation and a nickel filter in the 2u range from 2 to 908 in steps of 0.028, with a sampling time of one second per step. TEM/STEM (JEOL 2100F) operating at 200 KV with a Field Emission Gun, obtaining a point resolution of 0.19 nm was used to study the morphological structure of the prepared Fe3O4 nanoparticles, GNPs, and magnetically modified graphene nanoplatelets. The specific surface areas of the GNPs, Fe3O4 nanoparticles, and the prepared M-GNPs were determined from nitrogen adsorption/ desorption isotherms measured at 77 K using a model NOVA 3200e automated gas sorption system (Quantachrome, USA).
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2.4. Adsorption studies
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The adsorption experiments were carried out in batch mode by mixing a specific amount of adsorbent and 10 mL of amoxicillin solution in the stoppered conical flask under constant shaking (120 rpm) in a thermostat shaker. The effects of contact time, amount of adsorbent, initial amoxicillin concentration, pH, and temperature of the amoxicillin solutions were investigated. For determination of the equilibrium time of adsorption, the experiments were carried out specific time intervals. The adsorption conditions were chosen as 5 mg of adsorbents and 10 mg/l amoxicillin solution for determination of the effect of contact time. In the effect of amount of adsorbent experiments, 1–10 mg of adsorbents was used. For the effect of initial amoxicillin concentration experiments, the concentration range was chosen as 3–30 mg/ l. To investigate the pH effect, 10 mg/l of amoxicillin solutions were prepared at pH 3, 7, 9, and 11. The 0.1 M NaOH and 0.1 M H2SO4 were used for pH adjusting. The temperature effect was investigated for three different temperatures, 20 8C, 30 8C, and 40 8C. After the completion of the adsorption experiment, a magnet was brought to the side of the flask, which attract the entire magnetically modified graphene nanoplatelets and a clear solution was left behind as shown in Fig. 1. The clear solution was collected
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Fig. 1. Magnetic separation of the adsorbents from aqueous media.
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by a glass pasture pipette and the concentration of amoxicillin was measured. The amoxicillin concentration was followed by UV–vis spectrophotometer (PG Instruments) at 230 nm. The uptakes of adsorbents (qe, mg/g) were calculated by the Eq. (1). C0 and Ce (mg/ l) are initial concentration and equilibrium concentration of amoxicillin, respectively; V (l) is the volume of the solution; m (g) is the amount of adsorbent. qe ¼
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ðC 0 C e ÞV m
The removal efficiency was calculated by Eq. (2). Removal% ¼
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(1)
ðC 0 C e Þ 100 C0
(2)
It is noteworthy to mention that the amoxicillin solution was freshly prepared daily before the adsorption studies, and controlled by UV spectrohotometer analysis; when amoxicillin hydrolyzed, it did not give a peak at 230 nm wavelength. This is due to the high sensitivity of the amoxicillin for hydrolysis. Also, the experimental values mainly were mean values based on three different experimental measurements, and the standard deviation was calculated and mostly less than 5% of the reported value.
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3. Results and discussion
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3.1. Characterization of the adsorbent
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The structure and composition of the prepared Fe3O4 nanoparticles, GNPs, and magnetically modified graphene nanoplatelets were investigated using XRD and TEM. The XRD patterns of the prepared Fe3O4 nanoparticles, GNPs, and magnetically modified graphene nanoplatelets are shown in Fig. 2. The diffraction peaks of the prepared Fe3O4 nanoparticles revealed the presence of magnetite Fe3O4 (JCPDS No. 01-075-0449), with their corresponding peaks. Also, the XRD pattern of GNPs revealed the presence of pristine graphene in the form of nanoplatelets, same as that of native graphite (JCPDS No. 00-008-0415). Accordingly, the XRD pattern of magnetically modified graphene nanoplatelets shows the characteristic diffraction peak of the magnetite Fe3O4 nanoparticles and graphene nanoplatelets. Fig. 3 showed the morphological structures of the prepared Fe3O4 nanoparticles, and magnetically modified graphene nanoplatelets using TEM. The TEM revealed that the prepared Fe3O4 nanoparticles appeared to be non-uniform in size and shape with average diameter of 20 nm. Also, the TEM showed that the prepared Fe3O4 nanoparticles tend to agglomerate together, and this is mainly due to their permanent magnetic moment [35]. Also, it is clear from the GNPs/ Fe3O4 nanoparticles TEM image the distribution of the Fe3O4
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Fig. 2. XRD pattern of the Fe3O4 nanoparticles, GNPs, and M-GNPs.
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Fig. 3. TEM images of the Fe3O4 nanoparticles and M-GNPs.
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nanoparticles over the transparent GNPs. The specific surface areas of the GNPs, Fe3O4 nanoparticles, and the prepared M-GNPs were measured using the nitrogen adsorption/desorption isotherms at 77 K. The measurements showed that the BET-specific surface areas were 677.5 m2/g, 11.6 m2/g, and 543.2 m2/g for the GNPs, Fe3O4 nanoparticles, and the prepared M-GNPs, respectively. It is obvious that the GNPs had a considerably high specific surface area, which was reduced upon the mixing with the Fe3O4 nanoparticles.
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3.2. Effect of operational parameters
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The effect of the adsorbent amount was investigated in two terms: the uptake of the adsorbent (qe, mg/g) and removal % of amoxicillin (Fig. 4). A comparison between prepared Fe3O4 nanoparticles, pristine GNPs, and M-GNPs for amoxicillin adsorption was performed at the begining of the experiments. The results showed that a remarkable enhancement in uptake was obtained by M-GNPs compared with the prepared Fe3O4 nanoparticles, and pristine GNPs. The effect of M-GNPs mass on the adsorption profile of amoxicillin was studied using 1.6, 3.6, 5.4, 7.2, and 9.6 mg, and the % adsorption acheived were 39.4%, 66.5%, 78.3%, 79.6%, and 84.0%, respectively. The removal percentage increased almost linearly with the increase of M-GNPs mass until an adsorbent mass of 5.4 mg was used. Further increases in M-GNPs mass beyond 5.4 mg slightly affect adsorption, as most of the amoxicillin had already been adsorbed. Accordingly, further experiments were conducted using 5 mg of M-GNPs to be able to monitor the change in % adsorption upon the change in the other factors. Fig. 5a shows the effect of the initial pH of the solution. In order to find out the pH through which the maximum % removal was
Fig. 4. Comperative amoxicillin adsorption results in terms of uptake (a) and removal % (b) (10 mg/l initial concentration, 1–10 mg adsorbent amount, 293 K, pH 5, 90 min contact time).
achieved, pH values from 3.0 to 11.0 was examined. The results showed that there was no significant change in % removal of amoxicillin in the pH range of 3.0–5.0, but beyond pH 5.0 the % removal decreased as the pH increase. It is well known that amoxicillin contains three main function groups with different pKa values, –COOH (pK a1 2.69), –NH2 (pK a2 7.49), –OH (pK a3 9.63). These functional groups ionized at different pH values and create negtively charged amoxicillin. Increasing the solution pH associated with an increase in the electrostatic repuslion between the negatively charged amoxicillin and the M-GNPs surface. Also, a competition arises between both –OH ions present in the solution; as a result of increasing the solution pH, and the negatively charged amoxicillin to the active sites presents at the M-GNPs surface. The pH around 5.0 was selected as the optimum pH value for adsorption of amoxicillin onto M-GNP as indicated from Fig. 5a. This result is in good agreement with the previous studies used carbon-based adsorbents for the amoxicillin adsorption. Putra et al. reported that the Langmuir isotherm paramater qm reached the highest value when pH of the initial solution was pH 4.98 in the amoxicillin adsorption by activated carbon [6]. Fazelirad et al. showed the optimum pH was 4.62 for amoxicillin adsorption by multi-walled carbon nanotubes [36]. Q3 The influence of ionic strength on the adsorption of pharmaceutical compounds is significant, as it creates different adsorption situations in which the electrostatic interactions
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Fig. 5. pH effect on amoxicillin adsorption by M-GNPs (a); ionic strength effect on amoxicillin adsorption by M-GNPs (b) (10 mg/l initial concentration, 5 mg adsorbent amount, 293 K, 90 min contact time).
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between the M-GNPs’ surface and the pharmaceutical compounds are either attractive or repulsive [28]. Different amounts of KNO3 were added to the solution containing the amoxicillin to investigate the effect of ionic strength on the adsorption behavior, and the result is presented in Fig. 5b. It is clear that the percentage of adsorption was not significantly affected by an increase in KNO3 from 0.0001 M to 0.001 M; but further increase in KNO3 concentration to 0.01 M caused a serious decrease in the percentage of adsorption from 68.3% to 33.9%. This is attributed to hindering effect of high concentration of K+ ions. Finally, the ionic strength investigation showed that the adsorption behavior of amoxicillin on M-GNP involved electrostatic interaction due to p–p stacking.
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3.3. Equilibrium study
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The adsorption isotherms mainly depend upon the equilibrium equations, which provide valuable information about the surface properties of the adsorbent by calculating the maximum adsorption capacity [37,38]. Adsorption isotherms describe the relationship between the amount of adsorbate adsorbed by the adsorbent (qe) and the adsorbate concentration remaining in the solution after the system has attained the equilibrium state (Ce) at constant temperature. The adsorption of amoxicillin on M-GNP was analyzed using three different well-known equilibrium equations: Langmuir, Freundlich, and Temkin isotherm models, at three different temperatures: 293 K, 303 K, and 313 K, and the results are presented in Fig. 6.
Fig. 6. Langmuir isotherm model (a), Freundlich isotherm model (b), Temkin isotherm model (c) plots for amoxicillin adsorption by M-GNPs (2–32 mg/l initial concentration, 5 mg adsorbent amount, pH 5, 90 min contact time).
Langmuir isotherm model, which defines a monolayer adsorption, is given in Eq. (3) where, Ce is the equilibrium concentration of adsorbates in the solution (mg L1), qe is the uptake of the adsorbates (mg g1), qm is the monolayer adsorption capacity of adsorbents for adsorption of adsorbates (mg g1), and KL is the Langmuir adsorption equilibrium constant (L mg1) [39]. 1 1 1 ¼ þ qe qm K L C e qm
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(3)
Freundlich isotherm model is expressed by Eq. (4) where, KF (L/mg) and n are the Freundlich constants [40]. 1/n is defined as
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heterogeneity factor in this isotherm. Freundlich isotherm defines a heterogeneous adsorption with different surface energy sites and assumes the change of uptake with exponential distribution of adsorption sites and energies [41–43]. log qe ¼ log K F þ
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1 log C e n
(4)
Temkin isotherm model is given in Eq. (5) where, KT = RT/bt, bt is the Temkin constant related to heat of sorption (J mol1), f (L g1) is the equilibrium binding constant corresponding to the maximum binding energy, R is the universal gas constant (8.314 J mol1 K1), and T is the solution temperature (K). Temkin model is a proper model for the chemical adsorption based on strong electrostatic interaction between positive and negative charges [25]. qe ¼ K T ln C e þ K T ln f
(5)
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The parameters for each adsorption isotherm models were calculated and are given in Table 1. The average of the R2 values of the Langmuir and Temkin isotherm models are approximately the same. Accordingly, the adsorption mechanism composed of both a monolayer adsorption and electrostatic interaction between the surface and the adsorbate through the p–p stacking, which agreed well with ionic strength investigation.
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3.4. Kinetics study
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The variation of the amount of amoxicillin adsorbed by M-GNPs at 293 K as a function of the adsorption time was studied and the experimental results are presented in Fig. 7. The other experimental conditions were 9.5 mg L1 amoxicillin solution concentration, 5 mg adsorbent mass, and pH = 5. Adsorption was reached equilibrium in 90 min, no further improvement in adsorption % was observed, and the adsorption capacity reached 14.10 mg g1. Also, it is clear from the figure that amoxicillin adsorption occurred in two stages. The first stage occurred during the first 30 min of adsorption, and was characterized by the high number of active binding sites on the M-GNPs’ surface during this phase. Adsorption occurred rapidly in this step, which indicates that the adsorption was controlled by the diffusion process of the amoxicillin molecules from the bulk phase to the M-GNPs’ surface. In the second step, adsorption was most likely an attachmentcontrolled process due to the decrease in the number of active sites available for the amoxicillin molecules on the GNPs’ surface. The experimental results based on the effect of contact time on the adsorption of amoxicillin by M-GNPs were used to investigate the adsorption kinetically using pseudo-second-order model,
Table 1 Langmuir, Freundlich, and Temkin adsorption isotherms fitting parameters of amoxicillin adsorption on M-GNPs. Isotherm parameters
Langmuir KL (L mg1) qm (mg g1) R2 Freundlich 1/n KF (L mg1) R2 Temkin KT f R2
M-GNPs 293 K
303 K
313 K
0.029 106.38 0.99
0.066 60.61 0.99
0.12 33.00 0.83
0.78 3.89 0.97
0.92 2.67 0.84
0.65 4.17 0.92
13.94 0.57 0.99
12.39 0.67 0.96
10.62 0.63 0.89
Fig. 7. The effect of contact time on amoxicillin adsorption by M-GNPs (10 mg/l initial concentration, 5 mg adsorbent amount, 293 K, pH 5, 90 min contact time).
Weber–Morris intraparticle diffusion, and Bangham models. Eq. (6) describes the pseudo-second-order kinetic model where, k2 is the rate constant of pseudo-second-order model (g mg1 min1). t 1 1 ¼ þ t qt k2 qe 2 qe
(6)
Intraparticle diffusion model is expressed by Eq. (7) where, ki (mg g1 min1/2) is the intraparticle diffusion rate constant, c (mg g1) is a constant proportional to the thickness of the boundary layer. If the c = 0, the adsorption carries on intraparticle diffusion model, but if the c 6¼ 0 then the adsorption carries on both film diffusion and intraparticle diffusion. c was calculated as 8.63 for amoxicillin adsorption by M-GNP, so the adsorption mechanism performs both by film diffusion and intraparticle diffusion. This indicates that intra-particle diffusion is a part of the adsorption mechanism, but it is not the rate determining step for the adsorption. qt ¼ ki ðt Þ1=2 þ c
C0 k0 m ¼ log þ alog t 2:303V C 0 qt m
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(7)
Bangham model is given by Eq. (8) where, C0 (mg L1) is the initial concentration of amoxicillin, V (L) is the volume of the solution, m (g L1) is the weight of the adsorbent used per liter of the solution, qt (mg g1) is the uptake for amoxicillin per gram of the adsorbent at time t, a, and k0 are constants. log log
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(8)
Fitting curves of kinetic models to experimental data points is presented in Fig. 8. Kinetic parameters of the models are given in Table 2. It was concluded that the application of M-GNPs for the removal of amoxicillin from an aqueous solution followed mainly pseudo-second-order kinetic model, as observed by the excellent correlation coefficient (>0.99), which confirms that the adsorption process of amoxicillin on M-GNPs is mainly due to the p–p electron donor–acceptor interaction which is the rate determining step.
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3.5. Thermodynamics study
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Thermodynamic parameters: enthalpy change (DH), free energy change (DG), and entropy change (DS), were calculated
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The more negative the DG value, the more spontaneous the adsorption was; accordingly, a more negative DG was associated with a higher percentage of adsorption or adsorption capacity. This was supported by the experimental results that 53.42%, 47.03%, and 43.65% removal (29.45 mg g1, 28.38 mg g1, and 27.91 mg g1) were achieved by 5 mg M-GNPs, with 30 mg L1 initial amoxicillin concentration in 90 min at 293 K, 303 K, and 313 K, respectively. Therefore, at 293 K the adsorption capacity was higher as observed more negative DG. The DH value of amoxicilin adsorption on M-GNPs was calculated as 9.34 kJ mol1, this indicated the adsorption process was exothermic in nature. Also, the DS value of amoxicilin adsorption on M-GNPs is negative, which showed that while amoxicillin molecules adsorbed on the adsorbents, randomness was decreased on the surface.
Fig. 8. Adsorption kinetic models (pseudo-second-order model, intraparticle diffusion model, Bangham model) for amoxicillin adsorption by M-GNPs.
Table 2 Adsorption kinetic models’ parameters. Kinetic parameters
M-GNPs
Pseudo second order k2 (g mg1 min1) qe (mg g1) R2
0.0235 13.44 0.9945
Intraparticle diffusion model ki (mg g1 min1/2) c (mg g1) R2
0.4018 8.63 0.9809
Bangham model
a
0.2142 0.0079 0.9075
k0 R2
365
according to the following equations. D¼
qe Ce
367 366 ln D ¼ 369 368 372 371 370 373 374 375 376 377 378
(9)
DS R
DH 1
(13)
R T
DG ¼ DHT DS
(11)
D is the equilibrium constant, qe is the equilibrium uptake (mg g1), and Ce is the equilibrium concentration (mol L1). R is the gas constant (8.314 J mol1 K1), T is the temperature (K). The values of DH and DS are determined from the slope and the intercept of the plots of ln Kc versus 1/T. The calculated thermodynamic parameters are given in Table 3. Negative DG values indicate the process was spontaneous at all temperatures.
Table 3 Thermodynamic parameters of amoxicillin adsorption on M-GNPs. Thermodynamic parameters
1
DG (kJ mol ) DH (kJ mol1) DS (J mol1)
M-GNPs 293 K
303 K
313 K
1.66 9.34 26.31
1.30
1.14 Fig. 9. Desorption results with using different eluents (a); desorption equilibrium time (b); adsorption results with recovered adsorbent (c).
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3.6. Recovery of the adsorbent
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M-GNP was found to be an effective adsorbent for amoxicillin removal with a superior property that it can be separated from the aqueous media easily using a NdFeB magnet, and specific amount of M-GNPs can be used a number of times in amoxicillin adsorption. The adsorption/desorption experiments to investigate the number of times M-GNPs could be used efficiently for the removal of amoxicillin from aquoeus solution. Firstly, three different eluent were tried to desorb amoxicillin; deionized water, 0.15 M aqueous NaOH solution, and isotonic NaCl solution. Fig. 9a presents the percentage of desorption with these eluents; the maximum desorption capacity was achieved by 0.15 M NaOH solution. Accordingly, 0.15 M NaOH was chosen as desorption eluent. Fig. 9b shows the desorption equilibrium study, the maximum desorption % was reached in 8 h. So, the desorption studies were performed for 8 h in shaker. Recovered adsorbent was used repeatedly in adsorption process, and the adsorption– desorption cycles were repeated for 4 times using the maximum desorbed adsorbents (Fig. 9c). The adsorption capacity was still 35% for the fourth time. The magnetic property of the adsorbent showed resistance during the adsorption–desorption cycles. So it could be concluded that M-GNPs can be used many times for the amoxicillin adsorption process.
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4. Conclusions
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Magnetic graphene nanoplatelets were prepared by mixing graphene nanoplatelets with freshly prepared magnetite (Fe3O4) nanoparticles and used for the removal of antibiotic amoxicillin (AA). The prepared magnetic graphene nanoplatelets were chacterized using XRD, and TEM, which shows the homogenous distribution of the magnetite nanopartilces over the surface of the graphene nanoplatelets. The adsorption behavior of AA by high surface area M-GNPs from aqueous solutions was investigated, and the results showed that most of the AA was removed within 90 min using 9.6 mg adsorbent, at pH 5.0 and 293 K. Adsorption equilibrium data were fitted using both Langmuir and Temkin isotherms, and the Langmuir maximum adsorption capacity was found to be 106.38 mg g1. Adsorption for AA was studied kinetically and thermodynamically, and found to be well fitted by the pseudo-second-order kinetic model, spontaneous and exothermic in nature. The adsorption behavior of AA on M-GNPs involved p–p stacking and electrostatic interaction, and the rate determining step was the p–p electron donor–acceptor interaction. The recovery and reusability of the adsorbent was investigated, and the adsorbents showed great ability for the removal of AA for many times without losing its stability. Generally, M-GNPs
could be considered a potential and promising adsorbent for the removal of organic pollutants from polluted water.
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References
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JIEC 2820 1–8 Journal of Industrial and Engineering Chemistry xxx (2016) xxx–xxx
Contents lists available at ScienceDirect
Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec 1 2 3 4 5 6
Antibiotic amoxicillin removal from aqueous solution using magnetically modified graphene nanoplatelets ¨ zge Q1 O a b
Kerkez-Kuyumcu a, S¸ahika Sena Bayazit a, Mohamed Abdel Salam b,*
Beykent University, Faculty of Engineering & Architecture, Chemical Engineering Department, P.O. Box 34396, Istanbul, Turkey Chemistry Department, Faculty of Science, King Abdulaziz University, P.O. Box 80200, Jeddah 21589, Saudi Arabia
A R T I C L E I N F O
A B S T R A C T
Article history: Received 7 December 2015 Received in revised form 26 January 2016 Accepted 30 January 2016 Available online xxx
The removal of antibiotic amoxicillin (AA) from aqueous solution was investigated using magnetically modified graphene nanoplatelets (M-GNPs). M-GNPs were prepared by mixing GNPs with freshly prepared magnetite nanopartilcles, and characterized using TEM, and XRD. The characterization results revealed the homogenous distribution of the magnetite nanopartilcles over the surface of transparent platelet-like graphene platlets. The M-GNPs proved to possess superior adsorption capacity compared with the pristine GNPs and the magnetite nanopartilces. The effects of different operational parameters which affect the removal process were explored; adsorbent amounts, contact time, initial pH, temperature, and the initial concentration of AA. The results showed the great affinity of the M-GNPs toward the AA and the maximum adsorption capacity was found to be 14.10 mg g1. The adsorption mechanism of AA by the M-GNPs involved p–p stacking and electrostatic interaction. The adsorption was studied kinetically and thermodynamically, and was found to mainly follow pseudo-second-order kinetic model, and was spontaneous and exothermic in nature. ß 2016 Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering Chemistry.
Keywords: Adsorption Antibiotic amoxicillin Graphene Kinetics Magnetic separation Q2 Thermodyanmics
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1. Introduction
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Water pollution is a serious threat for human health, ecosystem, and in terms of depletion of clean water resources. One of the sources that causes water pollution, even in small concentrations, are the pharmaceuticals [1], as about 30–90% of the given dose can remain un-degradable in the human or animal body, and is largely excreted as an active compound [2,3]. Pharmaceutical residues have adverse effects towards non-target organisms [4]. Especially release of antibiotics to the environment causes enhanced resistant bacteria, which makes related treatment difficult [5]; therefore the removal of the antibiotics from waste water is very important to protect the ecosystem in long term. Amoxicillin is a broad spectrum b-lactam antibiotic (aminopenicilin antibiotic), used as veterinary medicine for treatment of infections encountered in gastro-intestinal and systemic infections, and also, commonly used as human prescription medicine (against infections caused by bacteria) [5,6]. Amoxicillin consumption is higher than the other types in the penicillin family because of the oral
* Corresponding author. Tel.: +966 541886660; fax: +966 2 6952292. E-mail address: [email protected] (M.A. Salam).
absorption ability [7], and unfortunately, it is non-biodegradable and also an inhibitor of the photosynthesis mechanism of algae Synechocystis sp. [8,9]. Several treatment methods used for the removal of antibiotics, such as adsorption, advanced oxidation processes (heterogeneous Fenton reaction, UV, UV/H2O2, UV/H2O2/ TiO2 processes, etc.), biodegradation, membrane filtration, and coagulation/flocculation/sedimentation [1,10–14]. Adsorption is a promising procedure due to its effectiveness, mild operational costs, generation of secondary pollutants are non-existant, regeneration and reuse possibilities, short retention time, and high efficiency, in addition to the high levels of harmful organic substances that could be accumulatig on the solid surface [15,16]. There have been many reports for adsorption of antibiotics from waste water, and the most used adsorbents are activated carbon/carbon based materials and clays/organoclays [15– 21]. Putra et al. have reported a comperative study of activated carbon and bentonite for amoxicillin adsorption [6]. Recently, Carrales-Alvarado et al. used different carbon based materials (activated carbon, activated carbon cloth, mesoporous activated carbon, and carbon nanotubes) for removal of the antibiotic metronidazole [22]. Nanographene is a newly developed member of carbon based materials and has been used in the environmental remediation for the removal of different pollutants from water
http://dx.doi.org/10.1016/j.jiec.2016.01.040 1226-086X/ß 2016 Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering Chemistry.
¨ . Kerkez-Kuyumcu, et al., J. Ind. Eng. Chem. (2016), http://dx.doi.org/10.1016/j.jiec.2016.01.040 Please cite this article in press as: O
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[23,24]. Also, graphene-based materials are used for the adsorption of different pharmaceuticals adsorption such as tetracycline [25], sulfamethoxazole [26,27], ciprofloxacin [27], aspirin, acetaminophen, and caffeine [28]. One of the limitations of the adsorption process is the difficulty of separating the adsorbent from the aqueous medium. Lately, magnetic separation using magnetic adsorbents has been found to be an interesting solution to overcome such limitations in terms of ease and fast separation of the adsorbent from bulk solution using the external magnets [29,30]. There are several reports for the application of magnetic graphene for the adsorption of different pollutants from aqueous solution [31–34]. In the presented study, magnetically modified graphene nanoplatelets (were prepared by co-precipitation method and used for the adsorption of AA from aqueous solution. The selected graphene nanoplatelets used in this study characterized with the high surface area, special structure, and its high adsorption ability is proved previously [23,28], as well as its great hydrophobicity compared with previous work performed using graphene oxide [25–27]. This hydrophobicity enables a better separation of the graphene nanoplatelets from the bulk solution. The physical characterization for the M-GNPs was studied to explore their morphological properties first, then the effect of different environmental conditions; solution pH, temperature, and adsorption time, which affect the removal process, were investigated. Additionally, the adsorption process was studied kinetically and thermodynamically to predict the adsorption rate in order to understand the adsorption behavior, the mechanism of adsorption, and its spontaneity by calculating different thermodynamic parameters. The experimental data have been modeled by using the Langmuir, Freundlich, and Temkin isotherms. Finally, the desorption process was performed to evaluate the recovery of the magnetic adsorbent.
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2. Experimental
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2.1. Materials
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Graphene nanoplatelets (GNPs) were obtained from XG Science (xGnP1-C-750), USA. xGnP1 Graphene Nanoplatelets are unique nanoparticles consisting of short stacks of graphene sheets having a platelet shape with thickness ranging from 1 to 20 nm and width ranging from 1 to 50 mm. FeCl36H2O (Merck), Fe(SO4)7H2O (Merck), HCl (Merck, 36.5%), and ammonia solution (Merck, 25%) were used in the synthesis of magnetite (Fe3O4). Amoxicillin was obtained from Sigma-Aldrich. All the reagents employed in the experiments were used without further purification. A stock solution of 1000 mg/l containing amoxicillin was prepared, and dilution was performed for the calibration solutions and adsoption study solutions.
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2.2. Synthesis of magnetically modified graphene nanoplatelets (M-GNPs)
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Two hundred milliliter of aqueous solutions of graphene nanoplatelets (GNPs) were prepared under ultrasound. The aqueous solution in the molar ratio of Fe2+:Fe3+ = 1:2 was prepared and a few drops of concentrated HCl was added for complete dissolution of iron salts. It was added to the graphene solution under ultrasound and the mixture was kept under ultrasound for 10 min. The mass ratio of graphene:Fe3O4 was adjusted to be 75:25. 8 M NH4OH solution was added dropwise till the pH reached 11–12. Finally, the mixture was kept at 50 8C under mechanical stirring. The precipitation was magnetically separated from the aqueous phase and the composites were washed with deionized water and ethanol, dried at 60 8C in vacuum oven. To
adjust the mass ratio of GNPs: Fe3O4, a set of previous experiments were performed first for the preparation of the Fe3O4 alone. The yield of Fe3O4 was measured and accordingly, the amount of GNPs were determined and added to the experiment. The mass of the MGNPs was measured by the end of the experiment substracted from the orignal mass of the GNPs used, and the mass of ratio of GNPs: Fe3O4 was mostly 75:25.
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2.3. Characterization
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X-ray diffraction (XRD) patterns were recorded for phase analysis and the measurement of crystallite size on a Philips X pert pro diffractometer, which was operated at 40 mA and 40 kV by using Cu-Ka radiation and a nickel filter in the 2u range from 2 to 908 in steps of 0.028, with a sampling time of one second per step. TEM/STEM (JEOL 2100F) operating at 200 KV with a Field Emission Gun, obtaining a point resolution of 0.19 nm was used to study the morphological structure of the prepared Fe3O4 nanoparticles, GNPs, and magnetically modified graphene nanoplatelets. The specific surface areas of the GNPs, Fe3O4 nanoparticles, and the prepared M-GNPs were determined from nitrogen adsorption/ desorption isotherms measured at 77 K using a model NOVA 3200e automated gas sorption system (Quantachrome, USA).
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2.4. Adsorption studies
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The adsorption experiments were carried out in batch mode by mixing a specific amount of adsorbent and 10 mL of amoxicillin solution in the stoppered conical flask under constant shaking (120 rpm) in a thermostat shaker. The effects of contact time, amount of adsorbent, initial amoxicillin concentration, pH, and temperature of the amoxicillin solutions were investigated. For determination of the equilibrium time of adsorption, the experiments were carried out specific time intervals. The adsorption conditions were chosen as 5 mg of adsorbents and 10 mg/l amoxicillin solution for determination of the effect of contact time. In the effect of amount of adsorbent experiments, 1–10 mg of adsorbents was used. For the effect of initial amoxicillin concentration experiments, the concentration range was chosen as 3–30 mg/ l. To investigate the pH effect, 10 mg/l of amoxicillin solutions were prepared at pH 3, 7, 9, and 11. The 0.1 M NaOH and 0.1 M H2SO4 were used for pH adjusting. The temperature effect was investigated for three different temperatures, 20 8C, 30 8C, and 40 8C. After the completion of the adsorption experiment, a magnet was brought to the side of the flask, which attract the entire magnetically modified graphene nanoplatelets and a clear solution was left behind as shown in Fig. 1. The clear solution was collected
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Fig. 1. Magnetic separation of the adsorbents from aqueous media.
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by a glass pasture pipette and the concentration of amoxicillin was measured. The amoxicillin concentration was followed by UV–vis spectrophotometer (PG Instruments) at 230 nm. The uptakes of adsorbents (qe, mg/g) were calculated by the Eq. (1). C0 and Ce (mg/ l) are initial concentration and equilibrium concentration of amoxicillin, respectively; V (l) is the volume of the solution; m (g) is the amount of adsorbent. qe ¼
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ðC 0 C e ÞV m
The removal efficiency was calculated by Eq. (2). Removal% ¼
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(1)
ðC 0 C e Þ 100 C0
(2)
It is noteworthy to mention that the amoxicillin solution was freshly prepared daily before the adsorption studies, and controlled by UV spectrohotometer analysis; when amoxicillin hydrolyzed, it did not give a peak at 230 nm wavelength. This is due to the high sensitivity of the amoxicillin for hydrolysis. Also, the experimental values mainly were mean values based on three different experimental measurements, and the standard deviation was calculated and mostly less than 5% of the reported value.
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3. Results and discussion
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3.1. Characterization of the adsorbent
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The structure and composition of the prepared Fe3O4 nanoparticles, GNPs, and magnetically modified graphene nanoplatelets were investigated using XRD and TEM. The XRD patterns of the prepared Fe3O4 nanoparticles, GNPs, and magnetically modified graphene nanoplatelets are shown in Fig. 2. The diffraction peaks of the prepared Fe3O4 nanoparticles revealed the presence of magnetite Fe3O4 (JCPDS No. 01-075-0449), with their corresponding peaks. Also, the XRD pattern of GNPs revealed the presence of pristine graphene in the form of nanoplatelets, same as that of native graphite (JCPDS No. 00-008-0415). Accordingly, the XRD pattern of magnetically modified graphene nanoplatelets shows the characteristic diffraction peak of the magnetite Fe3O4 nanoparticles and graphene nanoplatelets. Fig. 3 showed the morphological structures of the prepared Fe3O4 nanoparticles, and magnetically modified graphene nanoplatelets using TEM. The TEM revealed that the prepared Fe3O4 nanoparticles appeared to be non-uniform in size and shape with average diameter of 20 nm. Also, the TEM showed that the prepared Fe3O4 nanoparticles tend to agglomerate together, and this is mainly due to their permanent magnetic moment [35]. Also, it is clear from the GNPs/ Fe3O4 nanoparticles TEM image the distribution of the Fe3O4
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Fig. 2. XRD pattern of the Fe3O4 nanoparticles, GNPs, and M-GNPs.
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Fig. 3. TEM images of the Fe3O4 nanoparticles and M-GNPs.
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nanoparticles over the transparent GNPs. The specific surface areas of the GNPs, Fe3O4 nanoparticles, and the prepared M-GNPs were measured using the nitrogen adsorption/desorption isotherms at 77 K. The measurements showed that the BET-specific surface areas were 677.5 m2/g, 11.6 m2/g, and 543.2 m2/g for the GNPs, Fe3O4 nanoparticles, and the prepared M-GNPs, respectively. It is obvious that the GNPs had a considerably high specific surface area, which was reduced upon the mixing with the Fe3O4 nanoparticles.
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3.2. Effect of operational parameters
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The effect of the adsorbent amount was investigated in two terms: the uptake of the adsorbent (qe, mg/g) and removal % of amoxicillin (Fig. 4). A comparison between prepared Fe3O4 nanoparticles, pristine GNPs, and M-GNPs for amoxicillin adsorption was performed at the begining of the experiments. The results showed that a remarkable enhancement in uptake was obtained by M-GNPs compared with the prepared Fe3O4 nanoparticles, and pristine GNPs. The effect of M-GNPs mass on the adsorption profile of amoxicillin was studied using 1.6, 3.6, 5.4, 7.2, and 9.6 mg, and the % adsorption acheived were 39.4%, 66.5%, 78.3%, 79.6%, and 84.0%, respectively. The removal percentage increased almost linearly with the increase of M-GNPs mass until an adsorbent mass of 5.4 mg was used. Further increases in M-GNPs mass beyond 5.4 mg slightly affect adsorption, as most of the amoxicillin had already been adsorbed. Accordingly, further experiments were conducted using 5 mg of M-GNPs to be able to monitor the change in % adsorption upon the change in the other factors. Fig. 5a shows the effect of the initial pH of the solution. In order to find out the pH through which the maximum % removal was
Fig. 4. Comperative amoxicillin adsorption results in terms of uptake (a) and removal % (b) (10 mg/l initial concentration, 1–10 mg adsorbent amount, 293 K, pH 5, 90 min contact time).
achieved, pH values from 3.0 to 11.0 was examined. The results showed that there was no significant change in % removal of amoxicillin in the pH range of 3.0–5.0, but beyond pH 5.0 the % removal decreased as the pH increase. It is well known that amoxicillin contains three main function groups with different pKa values, –COOH (pK a1 2.69), –NH2 (pK a2 7.49), –OH (pK a3 9.63). These functional groups ionized at different pH values and create negtively charged amoxicillin. Increasing the solution pH associated with an increase in the electrostatic repuslion between the negatively charged amoxicillin and the M-GNPs surface. Also, a competition arises between both –OH ions present in the solution; as a result of increasing the solution pH, and the negatively charged amoxicillin to the active sites presents at the M-GNPs surface. The pH around 5.0 was selected as the optimum pH value for adsorption of amoxicillin onto M-GNP as indicated from Fig. 5a. This result is in good agreement with the previous studies used carbon-based adsorbents for the amoxicillin adsorption. Putra et al. reported that the Langmuir isotherm paramater qm reached the highest value when pH of the initial solution was pH 4.98 in the amoxicillin adsorption by activated carbon [6]. Fazelirad et al. showed the optimum pH was 4.62 for amoxicillin adsorption by multi-walled carbon nanotubes [36]. Q3 The influence of ionic strength on the adsorption of pharmaceutical compounds is significant, as it creates different adsorption situations in which the electrostatic interactions
¨ . Kerkez-Kuyumcu, et al., J. Ind. Eng. Chem. (2016), http://dx.doi.org/10.1016/j.jiec.2016.01.040 Please cite this article in press as: O
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Fig. 5. pH effect on amoxicillin adsorption by M-GNPs (a); ionic strength effect on amoxicillin adsorption by M-GNPs (b) (10 mg/l initial concentration, 5 mg adsorbent amount, 293 K, 90 min contact time).
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between the M-GNPs’ surface and the pharmaceutical compounds are either attractive or repulsive [28]. Different amounts of KNO3 were added to the solution containing the amoxicillin to investigate the effect of ionic strength on the adsorption behavior, and the result is presented in Fig. 5b. It is clear that the percentage of adsorption was not significantly affected by an increase in KNO3 from 0.0001 M to 0.001 M; but further increase in KNO3 concentration to 0.01 M caused a serious decrease in the percentage of adsorption from 68.3% to 33.9%. This is attributed to hindering effect of high concentration of K+ ions. Finally, the ionic strength investigation showed that the adsorption behavior of amoxicillin on M-GNP involved electrostatic interaction due to p–p stacking.
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3.3. Equilibrium study
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The adsorption isotherms mainly depend upon the equilibrium equations, which provide valuable information about the surface properties of the adsorbent by calculating the maximum adsorption capacity [37,38]. Adsorption isotherms describe the relationship between the amount of adsorbate adsorbed by the adsorbent (qe) and the adsorbate concentration remaining in the solution after the system has attained the equilibrium state (Ce) at constant temperature. The adsorption of amoxicillin on M-GNP was analyzed using three different well-known equilibrium equations: Langmuir, Freundlich, and Temkin isotherm models, at three different temperatures: 293 K, 303 K, and 313 K, and the results are presented in Fig. 6.
Fig. 6. Langmuir isotherm model (a), Freundlich isotherm model (b), Temkin isotherm model (c) plots for amoxicillin adsorption by M-GNPs (2–32 mg/l initial concentration, 5 mg adsorbent amount, pH 5, 90 min contact time).
Langmuir isotherm model, which defines a monolayer adsorption, is given in Eq. (3) where, Ce is the equilibrium concentration of adsorbates in the solution (mg L1), qe is the uptake of the adsorbates (mg g1), qm is the monolayer adsorption capacity of adsorbents for adsorption of adsorbates (mg g1), and KL is the Langmuir adsorption equilibrium constant (L mg1) [39]. 1 1 1 ¼ þ qe qm K L C e qm
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(3)
Freundlich isotherm model is expressed by Eq. (4) where, KF (L/mg) and n are the Freundlich constants [40]. 1/n is defined as
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heterogeneity factor in this isotherm. Freundlich isotherm defines a heterogeneous adsorption with different surface energy sites and assumes the change of uptake with exponential distribution of adsorption sites and energies [41–43]. log qe ¼ log K F þ
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1 log C e n
(4)
Temkin isotherm model is given in Eq. (5) where, KT = RT/bt, bt is the Temkin constant related to heat of sorption (J mol1), f (L g1) is the equilibrium binding constant corresponding to the maximum binding energy, R is the universal gas constant (8.314 J mol1 K1), and T is the solution temperature (K). Temkin model is a proper model for the chemical adsorption based on strong electrostatic interaction between positive and negative charges [25]. qe ¼ K T ln C e þ K T ln f
(5)
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The parameters for each adsorption isotherm models were calculated and are given in Table 1. The average of the R2 values of the Langmuir and Temkin isotherm models are approximately the same. Accordingly, the adsorption mechanism composed of both a monolayer adsorption and electrostatic interaction between the surface and the adsorbate through the p–p stacking, which agreed well with ionic strength investigation.
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3.4. Kinetics study
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The variation of the amount of amoxicillin adsorbed by M-GNPs at 293 K as a function of the adsorption time was studied and the experimental results are presented in Fig. 7. The other experimental conditions were 9.5 mg L1 amoxicillin solution concentration, 5 mg adsorbent mass, and pH = 5. Adsorption was reached equilibrium in 90 min, no further improvement in adsorption % was observed, and the adsorption capacity reached 14.10 mg g1. Also, it is clear from the figure that amoxicillin adsorption occurred in two stages. The first stage occurred during the first 30 min of adsorption, and was characterized by the high number of active binding sites on the M-GNPs’ surface during this phase. Adsorption occurred rapidly in this step, which indicates that the adsorption was controlled by the diffusion process of the amoxicillin molecules from the bulk phase to the M-GNPs’ surface. In the second step, adsorption was most likely an attachmentcontrolled process due to the decrease in the number of active sites available for the amoxicillin molecules on the GNPs’ surface. The experimental results based on the effect of contact time on the adsorption of amoxicillin by M-GNPs were used to investigate the adsorption kinetically using pseudo-second-order model,
Table 1 Langmuir, Freundlich, and Temkin adsorption isotherms fitting parameters of amoxicillin adsorption on M-GNPs. Isotherm parameters
Langmuir KL (L mg1) qm (mg g1) R2 Freundlich 1/n KF (L mg1) R2 Temkin KT f R2
M-GNPs 293 K
303 K
313 K
0.029 106.38 0.99
0.066 60.61 0.99
0.12 33.00 0.83
0.78 3.89 0.97
0.92 2.67 0.84
0.65 4.17 0.92
13.94 0.57 0.99
12.39 0.67 0.96
10.62 0.63 0.89
Fig. 7. The effect of contact time on amoxicillin adsorption by M-GNPs (10 mg/l initial concentration, 5 mg adsorbent amount, 293 K, pH 5, 90 min contact time).
Weber–Morris intraparticle diffusion, and Bangham models. Eq. (6) describes the pseudo-second-order kinetic model where, k2 is the rate constant of pseudo-second-order model (g mg1 min1). t 1 1 ¼ þ t qt k2 qe 2 qe
(6)
Intraparticle diffusion model is expressed by Eq. (7) where, ki (mg g1 min1/2) is the intraparticle diffusion rate constant, c (mg g1) is a constant proportional to the thickness of the boundary layer. If the c = 0, the adsorption carries on intraparticle diffusion model, but if the c 6¼ 0 then the adsorption carries on both film diffusion and intraparticle diffusion. c was calculated as 8.63 for amoxicillin adsorption by M-GNP, so the adsorption mechanism performs both by film diffusion and intraparticle diffusion. This indicates that intra-particle diffusion is a part of the adsorption mechanism, but it is not the rate determining step for the adsorption. qt ¼ ki ðt Þ1=2 þ c
C0 k0 m ¼ log þ alog t 2:303V C 0 qt m
331 333 332 334 335 336 337 338 339 340 341 342 343
(7)
Bangham model is given by Eq. (8) where, C0 (mg L1) is the initial concentration of amoxicillin, V (L) is the volume of the solution, m (g L1) is the weight of the adsorbent used per liter of the solution, qt (mg g1) is the uptake for amoxicillin per gram of the adsorbent at time t, a, and k0 are constants. log log
328 329 330
344 346 345 347 348 349 350
(8)
Fitting curves of kinetic models to experimental data points is presented in Fig. 8. Kinetic parameters of the models are given in Table 2. It was concluded that the application of M-GNPs for the removal of amoxicillin from an aqueous solution followed mainly pseudo-second-order kinetic model, as observed by the excellent correlation coefficient (>0.99), which confirms that the adsorption process of amoxicillin on M-GNPs is mainly due to the p–p electron donor–acceptor interaction which is the rate determining step.
353 352 351 354 355 356 357 358 359 360 361
3.5. Thermodynamics study
362
Thermodynamic parameters: enthalpy change (DH), free energy change (DG), and entropy change (DS), were calculated
363 364
¨ . Kerkez-Kuyumcu, et al., J. Ind. Eng. Chem. (2016), http://dx.doi.org/10.1016/j.jiec.2016.01.040 Please cite this article in press as: O
G Model
JIEC 2820 1–8 O¨. Kerkez-Kuyumcu et al. / Journal of Industrial and Engineering Chemistry xxx (2016) xxx–xxx
7
The more negative the DG value, the more spontaneous the adsorption was; accordingly, a more negative DG was associated with a higher percentage of adsorption or adsorption capacity. This was supported by the experimental results that 53.42%, 47.03%, and 43.65% removal (29.45 mg g1, 28.38 mg g1, and 27.91 mg g1) were achieved by 5 mg M-GNPs, with 30 mg L1 initial amoxicillin concentration in 90 min at 293 K, 303 K, and 313 K, respectively. Therefore, at 293 K the adsorption capacity was higher as observed more negative DG. The DH value of amoxicilin adsorption on M-GNPs was calculated as 9.34 kJ mol1, this indicated the adsorption process was exothermic in nature. Also, the DS value of amoxicilin adsorption on M-GNPs is negative, which showed that while amoxicillin molecules adsorbed on the adsorbents, randomness was decreased on the surface.
Fig. 8. Adsorption kinetic models (pseudo-second-order model, intraparticle diffusion model, Bangham model) for amoxicillin adsorption by M-GNPs.
Table 2 Adsorption kinetic models’ parameters. Kinetic parameters
M-GNPs
Pseudo second order k2 (g mg1 min1) qe (mg g1) R2
0.0235 13.44 0.9945
Intraparticle diffusion model ki (mg g1 min1/2) c (mg g1) R2
0.4018 8.63 0.9809
Bangham model
a
0.2142 0.0079 0.9075
k0 R2
365
according to the following equations. D¼
qe Ce
367 366 ln D ¼ 369 368 372 371 370 373 374 375 376 377 378
(9)
DS R
DH 1
(13)
R T
DG ¼ DHT DS
(11)
D is the equilibrium constant, qe is the equilibrium uptake (mg g1), and Ce is the equilibrium concentration (mol L1). R is the gas constant (8.314 J mol1 K1), T is the temperature (K). The values of DH and DS are determined from the slope and the intercept of the plots of ln Kc versus 1/T. The calculated thermodynamic parameters are given in Table 3. Negative DG values indicate the process was spontaneous at all temperatures.
Table 3 Thermodynamic parameters of amoxicillin adsorption on M-GNPs. Thermodynamic parameters
1
DG (kJ mol ) DH (kJ mol1) DS (J mol1)
M-GNPs 293 K
303 K
313 K
1.66 9.34 26.31
1.30
1.14 Fig. 9. Desorption results with using different eluents (a); desorption equilibrium time (b); adsorption results with recovered adsorbent (c).
¨ . Kerkez-Kuyumcu, et al., J. Ind. Eng. Chem. (2016), http://dx.doi.org/10.1016/j.jiec.2016.01.040 Please cite this article in press as: O
379 380 381 382 383 384 385 386 387 388 389 390 391 392
G Model
JIEC 2820 1–8 O¨. Kerkez-Kuyumcu et al. / Journal of Industrial and Engineering Chemistry xxx (2016) xxx–xxx
8
393
3.6. Recovery of the adsorbent
394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415
M-GNP was found to be an effective adsorbent for amoxicillin removal with a superior property that it can be separated from the aqueous media easily using a NdFeB magnet, and specific amount of M-GNPs can be used a number of times in amoxicillin adsorption. The adsorption/desorption experiments to investigate the number of times M-GNPs could be used efficiently for the removal of amoxicillin from aquoeus solution. Firstly, three different eluent were tried to desorb amoxicillin; deionized water, 0.15 M aqueous NaOH solution, and isotonic NaCl solution. Fig. 9a presents the percentage of desorption with these eluents; the maximum desorption capacity was achieved by 0.15 M NaOH solution. Accordingly, 0.15 M NaOH was chosen as desorption eluent. Fig. 9b shows the desorption equilibrium study, the maximum desorption % was reached in 8 h. So, the desorption studies were performed for 8 h in shaker. Recovered adsorbent was used repeatedly in adsorption process, and the adsorption– desorption cycles were repeated for 4 times using the maximum desorbed adsorbents (Fig. 9c). The adsorption capacity was still 35% for the fourth time. The magnetic property of the adsorbent showed resistance during the adsorption–desorption cycles. So it could be concluded that M-GNPs can be used many times for the amoxicillin adsorption process.
416
4. Conclusions
417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437
Magnetic graphene nanoplatelets were prepared by mixing graphene nanoplatelets with freshly prepared magnetite (Fe3O4) nanoparticles and used for the removal of antibiotic amoxicillin (AA). The prepared magnetic graphene nanoplatelets were chacterized using XRD, and TEM, which shows the homogenous distribution of the magnetite nanopartilces over the surface of the graphene nanoplatelets. The adsorption behavior of AA by high surface area M-GNPs from aqueous solutions was investigated, and the results showed that most of the AA was removed within 90 min using 9.6 mg adsorbent, at pH 5.0 and 293 K. Adsorption equilibrium data were fitted using both Langmuir and Temkin isotherms, and the Langmuir maximum adsorption capacity was found to be 106.38 mg g1. Adsorption for AA was studied kinetically and thermodynamically, and found to be well fitted by the pseudo-second-order kinetic model, spontaneous and exothermic in nature. The adsorption behavior of AA on M-GNPs involved p–p stacking and electrostatic interaction, and the rate determining step was the p–p electron donor–acceptor interaction. The recovery and reusability of the adsorbent was investigated, and the adsorbents showed great ability for the removal of AA for many times without losing its stability. Generally, M-GNPs
could be considered a potential and promising adsorbent for the removal of organic pollutants from polluted water.
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¨ . Kerkez-Kuyumcu, et al., J. Ind. Eng. Chem. (2016), http://dx.doi.org/10.1016/j.jiec.2016.01.040 Please cite this article in press as: O