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Impregnation of GO with Cu2+ for enhancement of aniline adsorption and antibacterial activity
Journal of Water Process Engineering 20 (2017) 160–167
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Impregnation of GO with Cu2+ for enhancement of aniline adsorption and antibacterial activity
MARK
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Siamak Zavareha, , Elham Norouzib a b
Department of Applied Chemistry, Faculty of Science, University of Maragheh, P.O. Box: 533, Maragheh, 55181-83111, Iran Department of Chemistry, Islamic Azad University, Tabriz branch, Tabriz, Iran
A R T I C L E I N F O
A B S T R A C T
Keywords: Modified GO Adsorption Bactericidal activity Aniline Aquatic environment
In this work, Cu2+-binded graphene oxide (Cu2+-GO) as a modified form of GO was prepared and characterized using FTIR, XRD, SEM, EDX and TEM techniques. The modified GO showed an increased antibacterial activity based on its minimum inhibitory concentrations (MIC) against several bacterial strains. It exhibited a high capacity for adsorption of Aniline as compared to pristine GO with almost no adsorption. Maximum adsorption of Aniline by Cu2+-GO was observed at neutral pH range. The results of kinetic studies revealed that the process of Aniline adsorption is rather fast and follows a pseudo-second-order rate model. The adsorption isotherm data were fitted well to non-linear form of the Langmuir model with maximum adsorption capacity (qm) of 79.3 mg/g. A mechanism of chemisorption was proposed for removal of Aniline by Cu2+-GO based on the experimental results. An acceptable performance of selectivity was observed for Aniline adsorption by Cu2+-GO in the presence of higher concentrations of natural waters common anions. It also showed a desired performance for adsorptive removal of Aniline from a simulated real sample of polluted water. The results of this study exhibited that Cu2+-GO can be used for practical purposes such as water treatment plants and drinking water disinfection filters.
1. Introduction
electrochemical oxidation [16–19], photocatalytic degradation [19–22], biological treatment [23], membrane process [24] and adsorption [25] for removal of aromatic amines from water. Adsorption is the widely-used method for removal of many pollutants from water due to its simplicity, low cost and effectiveness [26–30]. Many adsorbents such as activated carbon [31], carbon nanotubes [32,33], Fe3O4/activated carbon magnetic nanosorbent [34], layered clay minerals [35] and modified silica [36] have been used successfully for removal of Aniline from water. Graphene, a two dimensional carbon nanostructure with one carbon atom thick, has attracted intensive interest due to its excellent electrical, thermal and mechanical properties [37–39]. The oxidized from of graphene named as graphene oxide (GO) has a wide range of oxygencontaining functioned groups on its surface such as hydroxyl, carboxyl and epoxy groups. The presence of these hydrophilic functional groups allows GO to be stably dispersed in polar solvents such as water [40]. These properties along with relatively simple and low cost procedure for preparation of GO from graphite makes it as a promising adsorbent for removal of many pollutants from aqueous solutions [41]. Antibacterial properties of GO has recently attracted much interest to develop contact-based antibacterial surfaces [42]. It has been
The occurrence of synthetic organic compounds in natural environment, especially natural waters, is a great environmental problem due to its adverse impact on human health and the environment [1]. Aromatic amines, a group of organic compounds, are used widely for production of azo-dyes. It is also used in pharmaceuticals, pesticides and plastics industries [2,3]. The discharge of wastewaters of these industries is a major source of aromatic amines in the aquatic environment [4–6]. Furthermore, these compounds are formed during decomposition of azo-dyes in the environment and biological treatment of the dye-containing wastewaters [7–12]. The exposure to aromatic amines has serious health problems for the human bodies. It has been proved that a large number of aromatic amines are carcinogen [13]. Aniline, a typical and important member of aromatic amines, has very industrial applications. Aniline is very toxic for human and the exposure to it causes cyanosis, decreased appetite, anemia, weight loss, nervous system damage, kidney and liver failure [14]. Therefore removal of aromatic amines such as Aniline from natural waters is of great importance. There are many methods such as coagulation [15], chemical and
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Corresponding author. E-mail address: [email protected] (S. Zavareh).
http://dx.doi.org/10.1016/j.jwpe.2017.10.012 Received 20 May 2017; Received in revised form 24 September 2017; Accepted 26 October 2017 2214-7144/ © 2017 Elsevier Ltd. All rights reserved.
Journal of Water Process Engineering 20 (2017) 160–167
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constant solution volume (100 mL) and adsorbent amount (0.2 g) with varying Aniline concentrations (50–120 mg/L) at room temperature. The solutions were allowed to be in contact with the adsorbent by agitating at 150 rpm in a water bath for 4 h to reach equilibrium state. Then, the adsorbent was separated by filtration and the concentration of Aniline was determined in the resulting. The adsorption capacity, qe (mg/g), defined as the mass of Aniline adsorbed per mass unit of the adsorbent was calculated by the following equation:
reported that GO has higher antibacterial activity compared to reduced GO and graphite [43]. Few attempts have been made to improve antibacterial performance of GO. For instance, it was reported that ultraviolet irradiation to GO surface increases its antibacterial activity considerably [44]. The objective of the present work is to remove Aniline from water using a modified GO. The modification was performed with the aim of promoting its selectivity and capacity for adsorption of Aniline. It is also aimed to increase antibacterial activity of GO. Thus, a new adsorbent was prepared simply by saturation of GO with Cu2+ ions. The modified GO, Cu2+-GO, was characterized using Fourier Transform Infrared Spectroscopy (FTIR), X-ray Diffraction (XRD), scanning electron microscopy (SEM), Energy-dispersive X-ray spectroscopy (EDX) and Transmission Electron Microscopy (TEM). The adsorption behaviors and antimicrobial properties of Cu2+-GO and pristine GO were investigated and compared.
qe =
(Ci − Ce ) V m
(1)
where Ci (mg/L) is initial concentration of Aniline in solution, Ce (mg/ L) is concentration of Aniline at equilibrium state, m (g) is the mass of dry adsorbent and V (L) is volume of solution. For kinetic studies, the concentration of Aniline was measured periodically. The experiment was performed using the solution with the initial Aniline concentration of 70 mg/L and the adsorbent amount of 2 g/L.
2. Experimental 2.1. Materials Graphite powder was obtained from Daejung Chemicals. The chemicals employed, including H2SO4 (concentrated), H3PO4 (concentrated), KMnO4 and H2O2 (30%), were supplied by Sigma-Aldrich Company.
2.5. Desorption and regeneration The reusability of Cu2+-GO was determined by performing adsorption-desorption experiments in 5 repeated cycles with the same adsorbent. After each adsorption experiment using the adsorbent dosage of 2 g/L and initial Aniline concentration of 70 mg/L, the loaded adsorbent was separated and then washed with distilled water. Afterwards, it was stirred with a 100 mL CuSO4 solution (10 g/L) at a rate of 150 rpm for 5 h followed by filtration, washing with distilled water and drying in an oven at 70 °C.
2.2. Preparation of adsorbents Graphene oxide was prepared according to the improved Hummers’s method [45]. Briefly, 400 mL of a 9:1 v/v mixture of concentrated H2SO4/H3PO4 was added to 21 g of a 1:6 wt/wt mixture of graphite/KMnO4. Temperature of the stirring mixture was kept at 50 °C for 12 h. Then, the mixture was cooled to room temperature and added to a mixture of 400 mL ice and 3 mL 30% H2O2. Finally, the mixture was purified using filtration, multiple washings, centrifugations and decanting and drying. The modified adsorbent, Cu2+-GO was prepared simply by mixing of GO with 100 mL aqueous solution of CuSO4 (10 g/L) for 2 h followed by separation, washing and drying in an oven at 50 °C.
2.6. Selectivity experiments The interfering effect of natural waters common anions (nitrate, chloride, sulfate and phosphate) on the adsorption of Aniline by Cu2+GO was determined by performing adsorption experiments using a series of solutions containing 70 mg/L Aniline and 400 mg/L of interfering anions at neutral pH values. The comparison of the capacity for Aniline adsorption in the presence and absence of interfering anions can provide an evaluation of the adsorbent selectivity.
2.3. Characterization The technique of scanning electron microscopy (SEM) was used to study morphological properties of the samples. The micrographs of samples were obtained on a Vega Tescan SEM (Czech Republic). The spectra of Fourier transform infrared (FTIR) spectra for samples were obtained on a Bruker Tensor 27 spectrometer (Germany). X-ray diffraction (XRD) diffraction patterns were recorded on a Siemens D-500 X-ray diffractometer (Germany) equipped with a Cu-Kα X-ray source. The pH at the point of zero charge (pHpzc) was measured using batch equilibrium method [46]. A 0.2 g of each sample was added to a set of solutions (100 mL) with different pH values and the same ionic strength. In all experiments, the ionic strength of solutions was kept constant by addition of 0.1 mol/L KNO3 as inert background electrolyte. The pH values of solutions were adjusted from 1 to 10 (pHi) by addition of 0.1 M KOH or HNO3 solutions. Suspensions were agitated using a shaker for 48 h at room temperature to reach equilibrium state. Then, the final pH (pHf) values of the solutions were measured. The plot of ΔpH (pHi–pHf) versus pHi gives pHpzc, that is the pH at which ΔpH = 0.
2.7. Analytical method For analysis of Aniline in aqueous solutions, UV spectrophotometry method [9,47] at the wavelength of maximum absorbance (290 nm) was employed. Experiments were conducted by a Shimadzo UV-1800 spectrophotometer.
2.8. Antibacterial activity The antibacterial activity of GO and Cu2+-GO was studied by determination of their minimum inhibitory concentrations (MIC) against certain bacterial strains. The measurements were performed based on National Committee on Clinical Laboratory Standard (NCCLS) protocol [48]. The standard microbial strains used in this study were Escherichia coli, Bacillus subtilis and Staphylococcus aureus. A series of test mixtures was prepared by mixing of the microbial suspension with final concentration of approximately 105 organisms per mL (0.5 McFarland Standard) and the sample with varying concentrations. Then, test mixtures were added into diffusion wells and placed in a shaker incubator at 37 °C for 24 h. The minimum concentration of the antibacterial material to reach irreversible inhibition of bacteria growth is considered as MIC value.
2.4. Batch adsorption experiments All adsorption studies were performed in batch mode using aqueous solutions of Aniline. The isotherm measurements were carried out using 161
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Fig. 2. XRD patterns for (A) pristine GO and (B) Cu2+-GO.
3.2. XRD studies Fig. 2 presents XRD patterns for GO and Cu2+-GO. The GO shows a characteristic peak at 2θ = 10.6° corresponding to interlayer spacing of 8.33 Ǻ. For Cu2+-GO, three crystalline phases, in addition to GO, were identified from its diffractogram. The peaks appeared at 2θ = 32.3°, 24° and 16.9° correspond to Cu(OH)2 phase on the adsorbent surface. The reflections at 2θ = 24° and 22.4° relate to CuSO4·5H2O phase on the surface. The peaks at 26.9° and 31.3° may be attributed to the presence of Na2Cu(SO4)2·2H2O on the surface. The formation of Cu (OH)2 phase may be due to interaction between Cu2+ and carboxylic groups on the surface as confirmed by FTIR studies. Again, XRD studies indicate the presence of Cu(II) on the modified adsorbent with strong tendency to aromatic amines.
Fig. 1. FTIR spectra for (A) pristine GO, (B) and (C) Cu2+-GO before and after Aniline adsorption.
3. Results and discussion 3.1. FTIR studies FTIR spectra for pristine GO and Cu2+-GO before and after adsorption are presented in Fig. 1. In the spectrum of pristine GO, the broad band at 3421 cm−1 is assigned to OeH stretching vibration. The peak at 1734 cm−1 is attributed to stretching vibration of C]O bond in carbonyl functional group. The band observed at 1627 cm−1 corresponds to stretching vibration of aromatic C]C bond. The peak at 1400 cm−1 and 1036 cm−1 are related to CeO stretching of CeO bond in carboxyl and alkoxyl groups, respectively. The band appeared at 1229 cm−1 can be assigned to CeO bond in epoxides. Treatment of GO with Cu2+ solution to prepare Cu2+-GO caused some changes in its spectrum. The band at 1734 cm−1 associated to carbonyl group for GO was eliminated due to interaction of Cu2+ ions and carboxylic functional groups. Such behaviors with detailed explanation have been reported in the literature [49,50]. Furthermore, the position of bands related to CeO bond in carboxyl, alkoxyl and epoxy groups were changed because of relatively strong interaction with Cu2+ ions. Therefore, the bands for vibration of CeO bond in carboxyl, alkoxyl and epoxy groups were appeared at 1386, 1109 and 1271 cm−1, respectively. The new peak appeared at 1449 cm−1 may relate to asymmetric vibration of COO groups after modification as result of interaction with Cu2+ ions [49,50]. The modified adsorbent was employed for removal of Aniline from aqueous solution. In the FTIR spectrum of Cu2+-GO after adsorption, the characteristic band appeared around 3188 cm−1 corresponds to stretching vibration of aromatic CeH. An increase was observed in the relative intensity of C]C stretching band at around 1614 cm−1 as a result of adsorption of Aniline on the surface. The peak of carboxyl group was also observed in the spectrum. It may be due to the interaction of amine group of Aniline and Cu(II) bonded to carboxylic groups on the adsorbent surface so that the C]O bond can be formed somewhat. This indicates that the chemisorption of Aniline on Cu2+-GO is the most probable mechanism.
3.3. Morphological properties Morphological properties of Cu2+-GO were explored using SEM and TEM techniques. As obvious in the TEM image (Fig. 3A), it is fully exfoliated so that individual and highly electron transparent nanosheet can be seen after dispersion in aqueous media. The SEM image of Cu2+GO is shown in Fig. 3B. The material Cu2+-GO is composed of randomly-stacked GO flakes. The sample in solid state was subjected to SEM imaging and so such morphology was expected in the SEM micrograph of Cu2+-GO. The SEM micrograph of neat GO is also presented in Fig. 3C. The SEM images revealed that the layers have lower agglomeration in neat GO compared to Cu2+-GO. The decrease in negative charges of the layers as a result of impregnation with Cu2+ cations may be the reason of the observed change for Cu2+-GO compared to neat GO. The sample surface was also studied for elemental analysis using energy-dispersive X-ray spectrometry (EDX). The EDX spectrum for Cu2+-GO (Fig. 4A) showed the peaks of O, C, S and Cu elements. The peak of Cu observed in the spectrum indicates the presence of Cu on the sample surface that may be binded to the GO surface as confirmed formerly by FTIR studies. The distribution of Cu on the modified adsorbent before and after adsorption is shown in Fig. 4A and 4B. A uniform distribution of element copper is seen on the modified adsorbent before adsorption. After adsorption, no considerable changes in the copper distribution are observed. It implies that Cu2+ cations are tightly binded to the surface so that Aniline in the solution can be complexed by the Cu(II) on the surface without any desorption of copper ions to the solution. 162
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Fig. 3. (A) TEM image of Cu2+-GO, (B) and (C) SEM micrographs of Cu2+-GO and neat GO.
Aniline by neat GO was observed, while Cu2+-GO adsorbed Aniline from aqueous solution considerably. It demonstrates that the modification has important effect on GO performance for removal of Aniline. The presence of Cu(II)-binded to the GO surface provides the
3.4. Adsorption studies Adsorption studies for removal of Aniline were examined by two adsorbents, pristine GO and modified GO. No measurable adsorption of
Fig. 4. (A) EDX spectrum of Cu2+-GO; (B) and (C) related EDX mapping of copper element before and after adsorption.
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ability to remove Aniline. The tendency of Aniline to Cu(II) to form a strong interaction may be the reason for such behavior. To study adsorption isotherms for removal of Aniline by Cu2+-GO, aqueous solutions of Aniline with initial concentrations varying from 50 to 125 mg/L were employed. The adsorption data were fitted with two well-known isotherm models, namely, Langmuir and Freundlich models. Langmuir isotherm is expressed by the following equation:
qe = qm kL Ce /(1 + KL Ce )
Table 1 Langmuir and Freundlich isotherm constants and related fitting parameters for adsorption of Aniline on Cu(II)-binded GO.
(2)
where qe (mg/g) is the amount of Aniline adsorbed per unit mass of adsorbent, Ce (mg/L) is the equilibrium concentration of Aniline, and qm (mg/g) and KL (L/mg) are the maximum adsorption capacity related to monolayer coverage and the Langmuir content corresponding to the free energy of adsorption, respectively. The Freundlich isotherm model is giving by the following equation:
qe = KF Ce1/ n
Isotherm model
Isotherm constants
Fitting parameters
Langmuir
qm(mg/g) = 79.34 KL(L/mg) = 0.0251
Freundlich
KF[(mg/g)/(mg/L)1/n] = 4.89 n = 1.783
SSE = 1.556 R2 = 0.995 Adj-R2 = 0.993 RMSE = 0.702 SSE = 9.6808 R2 = 0.9689 Adj-R2 = 0.9586 RMSE = 1.7964
SSE: Sum square error; RMSE: Root mean square error. Table 2 Comparison of adsorption capacity of some adsorbents.
(3) 1/n
where KF [(mg/g)/(mg/L) ] is the Freundlich constant corresponding to the relative adsorption capacity and n indicates the adsorption intensity. The adsorption data for removal of Aniline were fitted to non-linear from of Langmuir and Freundlich isotherms, as shown in Fig. 5. Isotherms constants and fitting parameters for Langmuir and Freundlich models are given in Table 1. The regression coefficients (R2) for both models are close to 1, indicating that two models describe satisfactorily the adsorption data. Considering R2 values, Langmuir isotherm is a better model than Freundlich. The greater than 1 value for the Freundlich constant n can be attributed to relatively intense adsorption of Aniline on Cu2+-GO. The maximum adsorption capacity for Cu2+-GO was calculated to be 70.34 mg/g based on the Langmuir model. To evaluate the adsorption performance of Cu2+-GO for removal of Aniline, the maximum adsorption capacity of Cu2+-GO was compared with results of other adsorbents reported in the literature [36,51–54]. As shown in Table 2, the adsorbent Cu2+-GO has higher capacity for removal of Aniline from aqueous solution.
Adsorbent
pH
Max. capacity (mg/g)
Reference
Cr-bentonite Polyacrylamide-grafted SiO2 Chitosan-coated activated carbon KMnO4-modified MWNT MCM-41 Cu2+-saturated GO
4–5 8 6–7
21.60 52.00 40.65
[40] [27] [41]
Not reported 7 7
46.70 16.64 79.34
[42] [43] Present study
kinetic models as expressed by the following equations:
qt = qe [1 − exp (−k1 t )]
(4)
qt = qe2 k2 t /(1 + qe k2 t )
(5)
where qe is the amount of Aniline adsorbed per mass unit of the adsorbent at equilibrium, qt is the adsorption capacity at time t, k1 and k2 are pseudo-first-order and pseudo-second-order rate constant, respectively. The kinetic data and the fitted curves of both models are shown in Fig. 6. The adsorption process is rapid and high percentage of the contaminant is adsorbed within the first 30 min of the contact. The correlation parameters and the value of rate constants for both models are given in Table 3. The fitting parameters, especially R2-values, show that pseudo-second-order model presents better model to describe kinetic data. Furthermore, the value of qe(model), the adsorption capacity
3.5. Kinetics of adsorption The study on the rate of adsorption for removal of a contaminant is of great importance for practical purposes such as process design and scale-up. The kinetic data of Aniline adsorption by Cu2+-GO were fitted to non-linear forms of pseudo-first-order and pseudo-second-order
Fig. 5. Fitting of adsorption data to Langmuir and Freundlich isotherm models for adsorption of Aniline on Cu2+-GO.
Fig. 6. Kinetic data for adsorption of Aniline by Cu2+-GO.
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Table 3 Parameters of pseudo-first-order and pseudo-second-order kinetic models fitting of Aniline adsorption data on Cu(II)-binded GO. Kinetic model
Kinetic constants
Fitting parameters
Pseudo-first-order
qe(model) = 23.61 qe(Experimental) = 25.54 kl = 0.04936
Pseudo-second-order
qe(model) = 27.17 qe(Experimental) = 25.54 k2 = 0.0023
SSE = 5.64 R2 = 0.9233 Adj- R2 = 0.9169 RMSE = 1.142 SSE = 3.068 R2 = 0.985 adj- R2 = 0.9837 RMSE = 0.5056
SSE: Sum square error; RMSE: Root mean square error.
at equilibrium predicted by the pseudo-second-order model, has better compliance with experimental equilibrium uptake. Hence, the nonlinear form of pseudo-second-order model describes well the kinetics of adsorption. It is well-known that pseudo-second-order kinetic model gives better description of chemisorption as rate controlling mechanism [55].
Fig. 8. Determination of pHpzc for Cu2+-GO.
3.7. Interfering effect The effect of natural waters common cations and anions on the adsorption of Aniline by Cu2+-GO was examined. The cations Ca2+ and Mg2+ and anions chloride, nitrate, sulfate and phosphate were selected as interfering agents. Considering that Cu2+-GO showed desired performance for adsorption of Aniline at neutral pH values, the selectivity experiments were also performed in neutral pH conditions. It was found that the presence of Ca2+ and Mg2+ in concentrations higher than Aniline has no considerable effect on the adsorption of Aniline by the adsorbent. Such behavior was expected due to the presence of binded Cu(II) on the adsorbent surface with no possible interaction with the cations. As shown in Fig. 9, the coexistence of the interfering anions with Aniline decreases the capacity of Cu2+-GO for Aniline adsorption. The anions chloride, nitrate and sulfate slightly interfere with Aniline adsorption whereas the anion phosphate compete with Aniline adsorption and decreases Aniline adsorption somewhat. The interfering anions may have different interactions with the adsorbent. The anions chloride, nitrate and sulfate with electrostatic interaction have slight influence on the chemisorption of Aniline with stronger interaction. The anion phosphate can form a complex with Cu(II) binded to the adsorbent surface, resulting in reduction of Aniline adsorption.
3.6. Effect of initial pH The effect of initial pH on the adsorption of Aniline by Cu2+-GO is shown in Fig. 7. With the increase of pH form 3–7, the adsorption capacity increases gradually to reach its maximum value at pH = 7. At pH values above 7, a gradual decrease is observed in the adsorption capacity. In order to find out the reason of the adsorption change with pH, the pH at the point of zero charge (pHpzc) for Cu2+-GO was determined as presented in Fig. 8. The pHpzc was obtained to be ∼2.5. The surface of an adsorbent is positively charged at pH values lower than its pHpzc, whereas at pH values higher than its pHpzc, the surface is negatively changed. Considering the pKa value for Aniline, it is in protonated form at pH < 4.6 and in deprotonated (non-ionic) form at pH > 4.6. According to the values of pKa for Aniline and pHpzc for the adsorbent, electrostatic interactions do not control the adsorption process. The observed behavior may be followed by complex formation between Aniline and Cu(II) on the GO surface. At pH values higher than 7, the Aniline adsorption decreases considerably. It can be as a result of formation of copper hydrate on the adsorbent surface at pH values higher than 7 and the increase of hydroxide ion concentration at alkaline condition.
Fig. 9. Adsorption capacity of Cu2+-GO for Aniline adsorption in the presence of interfering anions (400 mg/L).
Fig. 7. Effect of initial pH on Aniline adsorption by Cu2+-GO.
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Cu2+ enhances its antibacterial properties considerably (2–4 folds). The presence of Cu(II) with antibacterial properties on the modified adsorbent surface may increase its antibacterial properties. 4. Conclusions
• The new adsorbent, Cu -GO was prepared and employed for removal of Aniline from water. • FTIR, EDX and XRD studies indicated the presence of bonded Cu(II) on the adsorbent surface. • SEM and TEM images showed a dispersion of the nanosheets as morphological properties of the adsorbent. • Aniline had no measurable adsorption on pristine GO while Cu GO adsorbed Aniline considerably. • The adsorption data were better fitted by non-linear form of Langmuir isotherm model. • The kinetic experimental data were fitted well by non-linear form of pseudo-second-order model. • The modified GO showed a good performance for removal of Aniline from a simulated real sample and favorable regeneration ability. • Antibacterial activity of Cu -GO was 2–4 folds higher than that of neat GO. • The mechanism of chemisorption is proposed for adsorption of 2+
2+
Fig. 10. Adsorption efficiency of Cu2+-GO versus regeneration cycle.
2+
Table 4 MIC values of neat and modified GO against various bacterial strains. Bacterial strain
Escheriachia coli ATCC 25922 Staphylococcus aureus ATCC 25923 Bacillus subtilis ATCC 6633
Aniline by Cu2+-GO according to the results of characterization and adsorption experiments.
MIC (μg/mL) GO
Cu(II)-binded GO
125 250 250
31.25 125 62.5
Acknowledgment The authors are most grateful for the continuing financial support of this research project by the University of Maragheh (IRAN). References
3.8. Reusability of adsorbent
[1] R.P. Schwarzenbach, T. Egli, T.B. Hofstetter, U. Von Gunten, B. Wehrli, Global water pollution and human health, Annu. Rev. Environ. Resour. 35 (2010) 109–136. [2] H. Egan, L. Fishbein, I.A.f.R.o. Cancer, W.H. Organization, U.N.E. Programme, Some Aromatic Amines and Azo Dyes in the General and Industrial Environment, International Agency for Research on Cancer, 1981. [3] A.o.L.S.C.o. Amines, D.B. Clayson, Aromatic Amines: An Assessment of the Biological and Environmental Effects, National Academy Press, 1981. [4] R. Wegman, G. De Korte, Aromatic amines in surface waters of the Netherlands, Water Res. 15 (1981) 391–394. [5] B. Jurado-Sanchez, E. Ballesteros, M. Gallego, Occurrence of aromatic amines and N-nitrosamines in the different steps of a drinking water treatment plant, Water Res. 46 (2012) 4543–4555. [6] G.E. Parris, Environmental and Metabolic Transformations of Primary Aromatic Amines and Related Compounds, Residue Reviews, Springer, 1980 pp. 1–30. [7] H.S. Rai, M.S. Bhattacharyya, J. Singh, T. Bansal, P. Vats, U. Banerjee, Removal of dyes from the effluent of textile and dyestuff manufacturing industry: a review of emerging techniques with reference to biological treatment, Crit. Rev. Environ. Sci. Technol. 35 (2005) 219–238. [8] F.P. Van der Zee, S. Villaverde, Combined anaerobic–aerobic treatment of azo dyes–a short review of bioreactor studies, Water Res. 39 (2005) 1425–1440. [9] H. Pinheiro, E. Touraud, O. Thomas, Aromatic amines from azo dye reduction: status review with emphasis on direct UV spectrophotometric detection in textile industry wastewaters, Dyes Pigm. 61 (2004) 121–139. [10] S. Karthikeyan, R. Boopathy, G. Sekaran, In situ generation of hydroxyl radical by cobalt oxide supported porous carbon enhance removal of refractory organics in tannery dyeing wastewater, J. Colloid Interface Sci. 448 (2015) 163–174. [11] T. Saeed, R. Afrin, A. Al Muyeed, G. Sun, Treatment of tannery wastewater in a pilot-scale hybrid constructed wetland system in Bangladesh, Chemosphere 88 (2012) 1065–1073. [12] T.P. Sauer, L. Casaril, A.L.B. Oberziner, H.J. José, R.D.F.P.M. Moreira, Advanced oxidation processes applied to tannery wastewater containing Direct Black 38–Elimination and degradation kinetics, J. Hazard. Mater. 135 (2006) 274–279. [13] K.T. Chung, Mutagenicity and carcinogenicity of aromatic amines metabolically produced from azo dyes, J. Environ. Sci. Health C 18 (2000) 51–74. [14] G.-Q. Wu, X. Zhang, H. Hui, J. Yan, Q.-S. Zhang, J.-L. Wan, Y. Dai, Adsorptive removal of Aniline from aqueous solution by oxygen plasma irradiated bamboo based activated carbon, Chem. Eng. J. 185 (2012) 201–210. [15] Y.-L. Yuan, Y.-Z. Wen, X.-Y. Li, S.-Z. Luo, Treatment of wastewater from dye manufacturing industry by coagulation, J. Zhejiang Univ. Sci. A 7 (2006) 340–344. [16] P. Faria, J. Órfão, M. Pereira, Ozonation of Aniline promoted by activated carbon, Chemosphere 67 (2007) 809–815.
The reusability of an adsorbent is so important for industrial and commercial applications. The reusability of Cu2+-GO was examined by a series of repeated adsorption-desorption experiments. The results of reusability of the adsorbent are shown in Fig. 10. After 8 cycles, only 17% reduction in its efficiency was observed. It implies that Cu2+-GO has high durability so that it can be regenerated and reused several times without significant loss in its performance. After each adsorption process and filtration of adsorbent, the solution was analyzed for Cu2+ ions using atomic absorption spectroscopy. The results showed no leakage of Cu2+ ions to the solution during 8 cycles. This observation confirms the results of EDX mapping of copper on the adsorbent after and before adsorption. 3.9. Removal of aniline from simulated contaminated water The concentration of Aniline in surface waters has been reported to be less than 200 μg/L [4]. Therefore, the simulated contaminated water was prepared by spiking 200 μg/L Aniline into a real surface water collected from Sufichay river (Maragheh, Iran). The adsorption experiment was performed for the water sample using 2 g/L of Cu2+-GO. After adsorption process, the filtered water was subjected to HPLC analysis for Aniline measurement. No Aniline was detected in the sample within 1 ppb. It reveals that Cu2+-GO can be employed as effective adsorbent for removal of Aniline from contaminated natural waters. 3.10. Antibacterial activity Antibacterial properties of GO and Cu2+-GO were evaluated against several bacteria strains as summarized in Table 4. The modified GO showed higher antibacterial properties than pristine GO against both Gram-positive Gram-negative bacteria. The impregnation of GO with 166
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[35] O. Homenauth, M. McBride, Adsorption of Aniline on layer silicate clays and an organic soil, Soil Sci. Soc. Am. J. 58 (1994) 347–354. [36] F. An, X. Feng, B. Gao, Adsorption of Aniline from aqueous solution using novel adsorbent PAM/SiO 2, Chem. Eng. J. 151 (2009) 183–187. [37] W. Choi, I. Lahiri, R. Seelaboyina, Y.S. Kang, Synthesis of graphene and its applications: a review, Crit. Rev. Solid State Mater. Sci. 35 (2010) 52–71. [38] A. Fakhri, M. Naji, Degradation photocatalysis of tetrodotoxin as a poison by gold doped PdO nanoparticles supported on reduced graphene oxide nanocomposites and evaluation of its antibacterial activity, J. Photochem. Photobiol. B 167 (2017) 58–63. [39] A. Fakhri, D.S. Kahi, Synthesis and characterization of MnS 2/reduced graphene oxide nanohybrids for with photocatalytic and antibacterial activity, J. Photochem. Photobiol. B 166 (2017) 259–263. [40] Y. Zhu, S. Murali, W. Cai, X. Li, J.W. Suk, J.R. Potts, R.S. Ruoff, Graphene and graphene oxide: synthesis, properties, and applications, Adv. Mater. 22 (2010) 3906–3924. [41] V. Chabot, D. Higgins, A. Yu, X. Xiao, Z. Chen, J. Zhang, A review of graphene and graphene oxide sponge: material synthesis and applications to energy and the environment, Energy Environ. Sci. 7 (2014) 1564–1596. [42] F. Perreault, A.F. De Faria, S. Nejati, M. Elimelech, Antimicrobial properties of graphene oxide nanosheets: why size matters, ACS Nano 9 (2015) 7226–7236. [43] S. Liu, T.H. Zeng, M. Hofmann, E. Burcombe, J. Wei, R. Jiang, J. Kong, Y. Chen, Antibacterial activity of graphite, graphite oxide, graphene oxide, and reduced graphene oxide: membrane and oxidative stress, ACS Nano 5 (2011) 6971–6980. [44] M. Veerapandian, L. Zhang, K. Krishnamoorthy, K. Yun, Surface activation of graphene oxide nanosheets by ultraviolet irradiation for highly efficient anti-bacterials, Nanotechnology 24 (2013) 395706. [45] D.C. Marcano, D.V. Kosynkin, J.M. Berlin, A. Sinitskii, Z. Sun, A. Slesarev, L.B. Alemany, W. Lu, J.M. Tour, Improved synthesis of graphene oxide, ACS Nano 4 (2010) 4806–4814. [46] E. Cristiano, Y.-J. Hu, M. Siegfried, D. Kaplan, H. Nitsche, A comparison of point of zero charge measurement methodology, Clays Clay Miner. 59 (2011) 107–115. [47] A. Łabudzińska, K. Gorczyńska, Ultraviolet spectrophotometric method for the determination of aromatic amines in chemical industry waste waters, Analyst 119 (1994) 1195–1198. [48] S.J. Cavalieri, Microbiology, Manual of Antimicrobial Susceptibility Testing, American Society for Microbiology, 2009 (A.S.f.). [49] N. Kovtjukhova, G. Karpenko, The interaction of Cu2+ ammine ions with graphite oxide, Mater. Sci. Forum Trans. Technol. Publ. (1992) 219–223. [50] S.K. Papageorgiou, E.P. Kouvelos, E.P. Favvas, A.A. Sapalidis, G.E. Romanos, F.K. Katsaros, Metal–carboxylate interactions in metal–alginate complexes studied with FTIR spectroscopy, Carbohydr. Res. 345 (2010) 469–473. [51] H. Zheng, D. Liu, Y. Zheng, S. Liang, Z. Liu, Sorption isotherm and kinetic modeling of Aniline on Cr-bentonite, J. Hazard. Mater. 167 (2009) 141–147. [52] Q. Liu, L. Zhang, P. Hu, R. Huang, Removal of Aniline from aqueous solutions by activated carbon coated by chitosan, J. Water Reuse Desalin. 5 (2015) 610–618. [53] X. Xie, L. Gao, J. Sun, Thermodynamic study on Aniline adsorption on chemical modified multi-walled carbon nanotubes, Coll. Surf. A 308 (2007) 54–59. [54] X. Yang, Q. Guan, W. Li, Effect of template in MCM-41 on the adsorption of Aniline from aqueous solution, J. Environ. Manage. 92 (2011) 2939–2943. [55] W. Plazinski, W. Rudzinski, A. Plazinska, Theoretical models of sorption kinetics including a surface reaction mechanism: a review, Adv. Colloid Interface Sci. 152 (2009) 2–13.
[17] X. Xie, Y. Zhang, W. Huang, S. Huang, Degradation kinetics and mechanism of Aniline by heat-assisted persulfate oxidation, J. Environ. Sci. 24 (2012) 821–826. [18] M. Matsushita, H. Kuramitz, S. Tanaka, Electrochemical oxidation for low concentration of Aniline in neutral pH medium: application to the removal of Aniline based on the electrochemical polymerization on a carbon fiber, Environ. Sci. Technol. 39 (2005) 3805–3810. [19] E. Brillas, E. Mur, R. Sauleda, L. Sanchez, J. Peral, X. Domènech, J. Casado, Aniline mineralization by AOP's: anodic oxidation photocatalysis, electro-Fenton and photoelectro-Fenton processes, Appl. Catal. B: Environ. 16 (1998) 31–42. [20] J. Anotai, C.-C. Su, Y.-C. Tsai, M.-C. Lu, Effect of hydrogen peroxide on Aniline oxidation by electro-Fenton and fluidized-bed Fenton processes, J. Hazard. Mater. 183 (2010) 888–893. [21] A. Kumar, N. Mathur, Photocatalytic oxidation of aniline using Ag+-loaded TiO 2 suspensions, Appl. Catal. A: Gen. 275 (2004) 189–197. [22] H. Tang, J. Li, Y. Bie, L. Zhu, J. Zou, Photochemical removal of aniline in aqueous solutions: switching from photocatalytic degradation to photo-enhanced polymerization recovery, J. Hazard. Mater. 175 (2010) 977–984. [23] S. Zhang, A. Li, D. Cui, J. Yang, F. Ma, Performance of enhanced biological SBR process for Aniline treatment by mycelial pellet as biomass carrier, Bioresour. Technol. 102 (2011) 4360–4365. [24] A.M. Hidalgo, G. León, M. Gomez, M.D. Murcia, E. Gomez, J.L. Gómez, Modeling of Aniline removal by reverse osmosis using different membranes, Chem. Eng. Technol. 34 (2011) 1753–1759. [25] H. Al-Johani, M.A. Salam, Kinetics and thermodynamic study of Aniline adsorption by multi-walled carbon nanotubes from aqueous solution, J. Colloid Interface Sci. 360 (2011) 760–767. [26] A. Fakhri, S. Adami, Adsorption and thermodynamic study of Cephalosporins antibiotics from aqueous solution onto MgO nanoparticles, J. Taiwan Inst. Chem. Eng. 45 (2014) 1001–1006. [27] A. Fakhri, Assessment of ethidium bromide and ethidium monoazide bromide removal from aqueous matrices by adsorption on cupric oxide nanoparticles, Ecotoxicol. Environ. Saf. 104 (2014) 386–392. [28] A. Fakhri, S. Behrouz, Comparison studies of adsorption properties of MgO nanoparticles and ZnO?MgO nanocomposites for linezolid antibiotic removal from aqueous solution using response surface methodology, Process Saf. Environ. Prot. 94 (2015) 37–43. [29] A. Fakhri, Investigation of mercury (II) adsorption from aqueous solution onto copper oxide nanoparticles: optimization using response surface methodology, Process Saf. Environ. Prot. 93 (2015) 1–8. [30] A. Fakhri, Adsorption characteristics of graphene oxide as a solid adsorbent for aniline removal from aqueous solutions: kinetics, thermodynamics and mechanism studies, J. Saudi Chem. Soc. (2013). [31] K. Laszlo, Adsorption from aqueous phenol and Aniline solutions on activated carbons with different surface chemistry, Coll. Surf. A 265 (2005) 32–39. [32] K. Yang, W. Wu, Q. Jing, L. Zhu, Aqueous adsorption of Aniline, phenol, and their substitutes by multi-walled carbon nanotubes, Environ. Sci. Technol. 42 (2008) 7931–7936. [33] J.-G. Yu, X.-H. Zhao, H. Yang, X.-H. Chen, Q. Yang, L.-Y. Yu, J.-H. Jiang, X.-Q. Chen, Aqueous adsorption and removal of organic contaminants by carbon nanotubes, Sci. Total Environ. 482 (2014) 241–251. [34] B. Kakavandi, A. Jonidi, R. Rezaei, S. Nasseri, A. Ameri, A. Esrafily, Synthesis and properties of Fe 3 O 4-activated carbon magnetic nanoparticles for removal of Aniline from aqueous solution: equilibrium, kinetic and thermodynamic studies, Iran. J. Environ. Health Sci. Eng. 10 (2013) 1.
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Journal of Water Process Engineering journal homepage: www.elsevier.com/locate/jwpe
Impregnation of GO with Cu2+ for enhancement of aniline adsorption and antibacterial activity
MARK
⁎
Siamak Zavareha, , Elham Norouzib a b
Department of Applied Chemistry, Faculty of Science, University of Maragheh, P.O. Box: 533, Maragheh, 55181-83111, Iran Department of Chemistry, Islamic Azad University, Tabriz branch, Tabriz, Iran
A R T I C L E I N F O
A B S T R A C T
Keywords: Modified GO Adsorption Bactericidal activity Aniline Aquatic environment
In this work, Cu2+-binded graphene oxide (Cu2+-GO) as a modified form of GO was prepared and characterized using FTIR, XRD, SEM, EDX and TEM techniques. The modified GO showed an increased antibacterial activity based on its minimum inhibitory concentrations (MIC) against several bacterial strains. It exhibited a high capacity for adsorption of Aniline as compared to pristine GO with almost no adsorption. Maximum adsorption of Aniline by Cu2+-GO was observed at neutral pH range. The results of kinetic studies revealed that the process of Aniline adsorption is rather fast and follows a pseudo-second-order rate model. The adsorption isotherm data were fitted well to non-linear form of the Langmuir model with maximum adsorption capacity (qm) of 79.3 mg/g. A mechanism of chemisorption was proposed for removal of Aniline by Cu2+-GO based on the experimental results. An acceptable performance of selectivity was observed for Aniline adsorption by Cu2+-GO in the presence of higher concentrations of natural waters common anions. It also showed a desired performance for adsorptive removal of Aniline from a simulated real sample of polluted water. The results of this study exhibited that Cu2+-GO can be used for practical purposes such as water treatment plants and drinking water disinfection filters.
1. Introduction
electrochemical oxidation [16–19], photocatalytic degradation [19–22], biological treatment [23], membrane process [24] and adsorption [25] for removal of aromatic amines from water. Adsorption is the widely-used method for removal of many pollutants from water due to its simplicity, low cost and effectiveness [26–30]. Many adsorbents such as activated carbon [31], carbon nanotubes [32,33], Fe3O4/activated carbon magnetic nanosorbent [34], layered clay minerals [35] and modified silica [36] have been used successfully for removal of Aniline from water. Graphene, a two dimensional carbon nanostructure with one carbon atom thick, has attracted intensive interest due to its excellent electrical, thermal and mechanical properties [37–39]. The oxidized from of graphene named as graphene oxide (GO) has a wide range of oxygencontaining functioned groups on its surface such as hydroxyl, carboxyl and epoxy groups. The presence of these hydrophilic functional groups allows GO to be stably dispersed in polar solvents such as water [40]. These properties along with relatively simple and low cost procedure for preparation of GO from graphite makes it as a promising adsorbent for removal of many pollutants from aqueous solutions [41]. Antibacterial properties of GO has recently attracted much interest to develop contact-based antibacterial surfaces [42]. It has been
The occurrence of synthetic organic compounds in natural environment, especially natural waters, is a great environmental problem due to its adverse impact on human health and the environment [1]. Aromatic amines, a group of organic compounds, are used widely for production of azo-dyes. It is also used in pharmaceuticals, pesticides and plastics industries [2,3]. The discharge of wastewaters of these industries is a major source of aromatic amines in the aquatic environment [4–6]. Furthermore, these compounds are formed during decomposition of azo-dyes in the environment and biological treatment of the dye-containing wastewaters [7–12]. The exposure to aromatic amines has serious health problems for the human bodies. It has been proved that a large number of aromatic amines are carcinogen [13]. Aniline, a typical and important member of aromatic amines, has very industrial applications. Aniline is very toxic for human and the exposure to it causes cyanosis, decreased appetite, anemia, weight loss, nervous system damage, kidney and liver failure [14]. Therefore removal of aromatic amines such as Aniline from natural waters is of great importance. There are many methods such as coagulation [15], chemical and
⁎
Corresponding author. E-mail address: [email protected] (S. Zavareh).
http://dx.doi.org/10.1016/j.jwpe.2017.10.012 Received 20 May 2017; Received in revised form 24 September 2017; Accepted 26 October 2017 2214-7144/ © 2017 Elsevier Ltd. All rights reserved.
Journal of Water Process Engineering 20 (2017) 160–167
S. Zavareh, E. Norouzi
constant solution volume (100 mL) and adsorbent amount (0.2 g) with varying Aniline concentrations (50–120 mg/L) at room temperature. The solutions were allowed to be in contact with the adsorbent by agitating at 150 rpm in a water bath for 4 h to reach equilibrium state. Then, the adsorbent was separated by filtration and the concentration of Aniline was determined in the resulting. The adsorption capacity, qe (mg/g), defined as the mass of Aniline adsorbed per mass unit of the adsorbent was calculated by the following equation:
reported that GO has higher antibacterial activity compared to reduced GO and graphite [43]. Few attempts have been made to improve antibacterial performance of GO. For instance, it was reported that ultraviolet irradiation to GO surface increases its antibacterial activity considerably [44]. The objective of the present work is to remove Aniline from water using a modified GO. The modification was performed with the aim of promoting its selectivity and capacity for adsorption of Aniline. It is also aimed to increase antibacterial activity of GO. Thus, a new adsorbent was prepared simply by saturation of GO with Cu2+ ions. The modified GO, Cu2+-GO, was characterized using Fourier Transform Infrared Spectroscopy (FTIR), X-ray Diffraction (XRD), scanning electron microscopy (SEM), Energy-dispersive X-ray spectroscopy (EDX) and Transmission Electron Microscopy (TEM). The adsorption behaviors and antimicrobial properties of Cu2+-GO and pristine GO were investigated and compared.
qe =
(Ci − Ce ) V m
(1)
where Ci (mg/L) is initial concentration of Aniline in solution, Ce (mg/ L) is concentration of Aniline at equilibrium state, m (g) is the mass of dry adsorbent and V (L) is volume of solution. For kinetic studies, the concentration of Aniline was measured periodically. The experiment was performed using the solution with the initial Aniline concentration of 70 mg/L and the adsorbent amount of 2 g/L.
2. Experimental 2.1. Materials Graphite powder was obtained from Daejung Chemicals. The chemicals employed, including H2SO4 (concentrated), H3PO4 (concentrated), KMnO4 and H2O2 (30%), were supplied by Sigma-Aldrich Company.
2.5. Desorption and regeneration The reusability of Cu2+-GO was determined by performing adsorption-desorption experiments in 5 repeated cycles with the same adsorbent. After each adsorption experiment using the adsorbent dosage of 2 g/L and initial Aniline concentration of 70 mg/L, the loaded adsorbent was separated and then washed with distilled water. Afterwards, it was stirred with a 100 mL CuSO4 solution (10 g/L) at a rate of 150 rpm for 5 h followed by filtration, washing with distilled water and drying in an oven at 70 °C.
2.2. Preparation of adsorbents Graphene oxide was prepared according to the improved Hummers’s method [45]. Briefly, 400 mL of a 9:1 v/v mixture of concentrated H2SO4/H3PO4 was added to 21 g of a 1:6 wt/wt mixture of graphite/KMnO4. Temperature of the stirring mixture was kept at 50 °C for 12 h. Then, the mixture was cooled to room temperature and added to a mixture of 400 mL ice and 3 mL 30% H2O2. Finally, the mixture was purified using filtration, multiple washings, centrifugations and decanting and drying. The modified adsorbent, Cu2+-GO was prepared simply by mixing of GO with 100 mL aqueous solution of CuSO4 (10 g/L) for 2 h followed by separation, washing and drying in an oven at 50 °C.
2.6. Selectivity experiments The interfering effect of natural waters common anions (nitrate, chloride, sulfate and phosphate) on the adsorption of Aniline by Cu2+GO was determined by performing adsorption experiments using a series of solutions containing 70 mg/L Aniline and 400 mg/L of interfering anions at neutral pH values. The comparison of the capacity for Aniline adsorption in the presence and absence of interfering anions can provide an evaluation of the adsorbent selectivity.
2.3. Characterization The technique of scanning electron microscopy (SEM) was used to study morphological properties of the samples. The micrographs of samples were obtained on a Vega Tescan SEM (Czech Republic). The spectra of Fourier transform infrared (FTIR) spectra for samples were obtained on a Bruker Tensor 27 spectrometer (Germany). X-ray diffraction (XRD) diffraction patterns were recorded on a Siemens D-500 X-ray diffractometer (Germany) equipped with a Cu-Kα X-ray source. The pH at the point of zero charge (pHpzc) was measured using batch equilibrium method [46]. A 0.2 g of each sample was added to a set of solutions (100 mL) with different pH values and the same ionic strength. In all experiments, the ionic strength of solutions was kept constant by addition of 0.1 mol/L KNO3 as inert background electrolyte. The pH values of solutions were adjusted from 1 to 10 (pHi) by addition of 0.1 M KOH or HNO3 solutions. Suspensions were agitated using a shaker for 48 h at room temperature to reach equilibrium state. Then, the final pH (pHf) values of the solutions were measured. The plot of ΔpH (pHi–pHf) versus pHi gives pHpzc, that is the pH at which ΔpH = 0.
2.7. Analytical method For analysis of Aniline in aqueous solutions, UV spectrophotometry method [9,47] at the wavelength of maximum absorbance (290 nm) was employed. Experiments were conducted by a Shimadzo UV-1800 spectrophotometer.
2.8. Antibacterial activity The antibacterial activity of GO and Cu2+-GO was studied by determination of their minimum inhibitory concentrations (MIC) against certain bacterial strains. The measurements were performed based on National Committee on Clinical Laboratory Standard (NCCLS) protocol [48]. The standard microbial strains used in this study were Escherichia coli, Bacillus subtilis and Staphylococcus aureus. A series of test mixtures was prepared by mixing of the microbial suspension with final concentration of approximately 105 organisms per mL (0.5 McFarland Standard) and the sample with varying concentrations. Then, test mixtures were added into diffusion wells and placed in a shaker incubator at 37 °C for 24 h. The minimum concentration of the antibacterial material to reach irreversible inhibition of bacteria growth is considered as MIC value.
2.4. Batch adsorption experiments All adsorption studies were performed in batch mode using aqueous solutions of Aniline. The isotherm measurements were carried out using 161
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Fig. 2. XRD patterns for (A) pristine GO and (B) Cu2+-GO.
3.2. XRD studies Fig. 2 presents XRD patterns for GO and Cu2+-GO. The GO shows a characteristic peak at 2θ = 10.6° corresponding to interlayer spacing of 8.33 Ǻ. For Cu2+-GO, three crystalline phases, in addition to GO, were identified from its diffractogram. The peaks appeared at 2θ = 32.3°, 24° and 16.9° correspond to Cu(OH)2 phase on the adsorbent surface. The reflections at 2θ = 24° and 22.4° relate to CuSO4·5H2O phase on the surface. The peaks at 26.9° and 31.3° may be attributed to the presence of Na2Cu(SO4)2·2H2O on the surface. The formation of Cu (OH)2 phase may be due to interaction between Cu2+ and carboxylic groups on the surface as confirmed by FTIR studies. Again, XRD studies indicate the presence of Cu(II) on the modified adsorbent with strong tendency to aromatic amines.
Fig. 1. FTIR spectra for (A) pristine GO, (B) and (C) Cu2+-GO before and after Aniline adsorption.
3. Results and discussion 3.1. FTIR studies FTIR spectra for pristine GO and Cu2+-GO before and after adsorption are presented in Fig. 1. In the spectrum of pristine GO, the broad band at 3421 cm−1 is assigned to OeH stretching vibration. The peak at 1734 cm−1 is attributed to stretching vibration of C]O bond in carbonyl functional group. The band observed at 1627 cm−1 corresponds to stretching vibration of aromatic C]C bond. The peak at 1400 cm−1 and 1036 cm−1 are related to CeO stretching of CeO bond in carboxyl and alkoxyl groups, respectively. The band appeared at 1229 cm−1 can be assigned to CeO bond in epoxides. Treatment of GO with Cu2+ solution to prepare Cu2+-GO caused some changes in its spectrum. The band at 1734 cm−1 associated to carbonyl group for GO was eliminated due to interaction of Cu2+ ions and carboxylic functional groups. Such behaviors with detailed explanation have been reported in the literature [49,50]. Furthermore, the position of bands related to CeO bond in carboxyl, alkoxyl and epoxy groups were changed because of relatively strong interaction with Cu2+ ions. Therefore, the bands for vibration of CeO bond in carboxyl, alkoxyl and epoxy groups were appeared at 1386, 1109 and 1271 cm−1, respectively. The new peak appeared at 1449 cm−1 may relate to asymmetric vibration of COO groups after modification as result of interaction with Cu2+ ions [49,50]. The modified adsorbent was employed for removal of Aniline from aqueous solution. In the FTIR spectrum of Cu2+-GO after adsorption, the characteristic band appeared around 3188 cm−1 corresponds to stretching vibration of aromatic CeH. An increase was observed in the relative intensity of C]C stretching band at around 1614 cm−1 as a result of adsorption of Aniline on the surface. The peak of carboxyl group was also observed in the spectrum. It may be due to the interaction of amine group of Aniline and Cu(II) bonded to carboxylic groups on the adsorbent surface so that the C]O bond can be formed somewhat. This indicates that the chemisorption of Aniline on Cu2+-GO is the most probable mechanism.
3.3. Morphological properties Morphological properties of Cu2+-GO were explored using SEM and TEM techniques. As obvious in the TEM image (Fig. 3A), it is fully exfoliated so that individual and highly electron transparent nanosheet can be seen after dispersion in aqueous media. The SEM image of Cu2+GO is shown in Fig. 3B. The material Cu2+-GO is composed of randomly-stacked GO flakes. The sample in solid state was subjected to SEM imaging and so such morphology was expected in the SEM micrograph of Cu2+-GO. The SEM micrograph of neat GO is also presented in Fig. 3C. The SEM images revealed that the layers have lower agglomeration in neat GO compared to Cu2+-GO. The decrease in negative charges of the layers as a result of impregnation with Cu2+ cations may be the reason of the observed change for Cu2+-GO compared to neat GO. The sample surface was also studied for elemental analysis using energy-dispersive X-ray spectrometry (EDX). The EDX spectrum for Cu2+-GO (Fig. 4A) showed the peaks of O, C, S and Cu elements. The peak of Cu observed in the spectrum indicates the presence of Cu on the sample surface that may be binded to the GO surface as confirmed formerly by FTIR studies. The distribution of Cu on the modified adsorbent before and after adsorption is shown in Fig. 4A and 4B. A uniform distribution of element copper is seen on the modified adsorbent before adsorption. After adsorption, no considerable changes in the copper distribution are observed. It implies that Cu2+ cations are tightly binded to the surface so that Aniline in the solution can be complexed by the Cu(II) on the surface without any desorption of copper ions to the solution. 162
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Fig. 3. (A) TEM image of Cu2+-GO, (B) and (C) SEM micrographs of Cu2+-GO and neat GO.
Aniline by neat GO was observed, while Cu2+-GO adsorbed Aniline from aqueous solution considerably. It demonstrates that the modification has important effect on GO performance for removal of Aniline. The presence of Cu(II)-binded to the GO surface provides the
3.4. Adsorption studies Adsorption studies for removal of Aniline were examined by two adsorbents, pristine GO and modified GO. No measurable adsorption of
Fig. 4. (A) EDX spectrum of Cu2+-GO; (B) and (C) related EDX mapping of copper element before and after adsorption.
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ability to remove Aniline. The tendency of Aniline to Cu(II) to form a strong interaction may be the reason for such behavior. To study adsorption isotherms for removal of Aniline by Cu2+-GO, aqueous solutions of Aniline with initial concentrations varying from 50 to 125 mg/L were employed. The adsorption data were fitted with two well-known isotherm models, namely, Langmuir and Freundlich models. Langmuir isotherm is expressed by the following equation:
qe = qm kL Ce /(1 + KL Ce )
Table 1 Langmuir and Freundlich isotherm constants and related fitting parameters for adsorption of Aniline on Cu(II)-binded GO.
(2)
where qe (mg/g) is the amount of Aniline adsorbed per unit mass of adsorbent, Ce (mg/L) is the equilibrium concentration of Aniline, and qm (mg/g) and KL (L/mg) are the maximum adsorption capacity related to monolayer coverage and the Langmuir content corresponding to the free energy of adsorption, respectively. The Freundlich isotherm model is giving by the following equation:
qe = KF Ce1/ n
Isotherm model
Isotherm constants
Fitting parameters
Langmuir
qm(mg/g) = 79.34 KL(L/mg) = 0.0251
Freundlich
KF[(mg/g)/(mg/L)1/n] = 4.89 n = 1.783
SSE = 1.556 R2 = 0.995 Adj-R2 = 0.993 RMSE = 0.702 SSE = 9.6808 R2 = 0.9689 Adj-R2 = 0.9586 RMSE = 1.7964
SSE: Sum square error; RMSE: Root mean square error. Table 2 Comparison of adsorption capacity of some adsorbents.
(3) 1/n
where KF [(mg/g)/(mg/L) ] is the Freundlich constant corresponding to the relative adsorption capacity and n indicates the adsorption intensity. The adsorption data for removal of Aniline were fitted to non-linear from of Langmuir and Freundlich isotherms, as shown in Fig. 5. Isotherms constants and fitting parameters for Langmuir and Freundlich models are given in Table 1. The regression coefficients (R2) for both models are close to 1, indicating that two models describe satisfactorily the adsorption data. Considering R2 values, Langmuir isotherm is a better model than Freundlich. The greater than 1 value for the Freundlich constant n can be attributed to relatively intense adsorption of Aniline on Cu2+-GO. The maximum adsorption capacity for Cu2+-GO was calculated to be 70.34 mg/g based on the Langmuir model. To evaluate the adsorption performance of Cu2+-GO for removal of Aniline, the maximum adsorption capacity of Cu2+-GO was compared with results of other adsorbents reported in the literature [36,51–54]. As shown in Table 2, the adsorbent Cu2+-GO has higher capacity for removal of Aniline from aqueous solution.
Adsorbent
pH
Max. capacity (mg/g)
Reference
Cr-bentonite Polyacrylamide-grafted SiO2 Chitosan-coated activated carbon KMnO4-modified MWNT MCM-41 Cu2+-saturated GO
4–5 8 6–7
21.60 52.00 40.65
[40] [27] [41]
Not reported 7 7
46.70 16.64 79.34
[42] [43] Present study
kinetic models as expressed by the following equations:
qt = qe [1 − exp (−k1 t )]
(4)
qt = qe2 k2 t /(1 + qe k2 t )
(5)
where qe is the amount of Aniline adsorbed per mass unit of the adsorbent at equilibrium, qt is the adsorption capacity at time t, k1 and k2 are pseudo-first-order and pseudo-second-order rate constant, respectively. The kinetic data and the fitted curves of both models are shown in Fig. 6. The adsorption process is rapid and high percentage of the contaminant is adsorbed within the first 30 min of the contact. The correlation parameters and the value of rate constants for both models are given in Table 3. The fitting parameters, especially R2-values, show that pseudo-second-order model presents better model to describe kinetic data. Furthermore, the value of qe(model), the adsorption capacity
3.5. Kinetics of adsorption The study on the rate of adsorption for removal of a contaminant is of great importance for practical purposes such as process design and scale-up. The kinetic data of Aniline adsorption by Cu2+-GO were fitted to non-linear forms of pseudo-first-order and pseudo-second-order
Fig. 5. Fitting of adsorption data to Langmuir and Freundlich isotherm models for adsorption of Aniline on Cu2+-GO.
Fig. 6. Kinetic data for adsorption of Aniline by Cu2+-GO.
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Table 3 Parameters of pseudo-first-order and pseudo-second-order kinetic models fitting of Aniline adsorption data on Cu(II)-binded GO. Kinetic model
Kinetic constants
Fitting parameters
Pseudo-first-order
qe(model) = 23.61 qe(Experimental) = 25.54 kl = 0.04936
Pseudo-second-order
qe(model) = 27.17 qe(Experimental) = 25.54 k2 = 0.0023
SSE = 5.64 R2 = 0.9233 Adj- R2 = 0.9169 RMSE = 1.142 SSE = 3.068 R2 = 0.985 adj- R2 = 0.9837 RMSE = 0.5056
SSE: Sum square error; RMSE: Root mean square error.
at equilibrium predicted by the pseudo-second-order model, has better compliance with experimental equilibrium uptake. Hence, the nonlinear form of pseudo-second-order model describes well the kinetics of adsorption. It is well-known that pseudo-second-order kinetic model gives better description of chemisorption as rate controlling mechanism [55].
Fig. 8. Determination of pHpzc for Cu2+-GO.
3.7. Interfering effect The effect of natural waters common cations and anions on the adsorption of Aniline by Cu2+-GO was examined. The cations Ca2+ and Mg2+ and anions chloride, nitrate, sulfate and phosphate were selected as interfering agents. Considering that Cu2+-GO showed desired performance for adsorption of Aniline at neutral pH values, the selectivity experiments were also performed in neutral pH conditions. It was found that the presence of Ca2+ and Mg2+ in concentrations higher than Aniline has no considerable effect on the adsorption of Aniline by the adsorbent. Such behavior was expected due to the presence of binded Cu(II) on the adsorbent surface with no possible interaction with the cations. As shown in Fig. 9, the coexistence of the interfering anions with Aniline decreases the capacity of Cu2+-GO for Aniline adsorption. The anions chloride, nitrate and sulfate slightly interfere with Aniline adsorption whereas the anion phosphate compete with Aniline adsorption and decreases Aniline adsorption somewhat. The interfering anions may have different interactions with the adsorbent. The anions chloride, nitrate and sulfate with electrostatic interaction have slight influence on the chemisorption of Aniline with stronger interaction. The anion phosphate can form a complex with Cu(II) binded to the adsorbent surface, resulting in reduction of Aniline adsorption.
3.6. Effect of initial pH The effect of initial pH on the adsorption of Aniline by Cu2+-GO is shown in Fig. 7. With the increase of pH form 3–7, the adsorption capacity increases gradually to reach its maximum value at pH = 7. At pH values above 7, a gradual decrease is observed in the adsorption capacity. In order to find out the reason of the adsorption change with pH, the pH at the point of zero charge (pHpzc) for Cu2+-GO was determined as presented in Fig. 8. The pHpzc was obtained to be ∼2.5. The surface of an adsorbent is positively charged at pH values lower than its pHpzc, whereas at pH values higher than its pHpzc, the surface is negatively changed. Considering the pKa value for Aniline, it is in protonated form at pH < 4.6 and in deprotonated (non-ionic) form at pH > 4.6. According to the values of pKa for Aniline and pHpzc for the adsorbent, electrostatic interactions do not control the adsorption process. The observed behavior may be followed by complex formation between Aniline and Cu(II) on the GO surface. At pH values higher than 7, the Aniline adsorption decreases considerably. It can be as a result of formation of copper hydrate on the adsorbent surface at pH values higher than 7 and the increase of hydroxide ion concentration at alkaline condition.
Fig. 9. Adsorption capacity of Cu2+-GO for Aniline adsorption in the presence of interfering anions (400 mg/L).
Fig. 7. Effect of initial pH on Aniline adsorption by Cu2+-GO.
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Cu2+ enhances its antibacterial properties considerably (2–4 folds). The presence of Cu(II) with antibacterial properties on the modified adsorbent surface may increase its antibacterial properties. 4. Conclusions
• The new adsorbent, Cu -GO was prepared and employed for removal of Aniline from water. • FTIR, EDX and XRD studies indicated the presence of bonded Cu(II) on the adsorbent surface. • SEM and TEM images showed a dispersion of the nanosheets as morphological properties of the adsorbent. • Aniline had no measurable adsorption on pristine GO while Cu GO adsorbed Aniline considerably. • The adsorption data were better fitted by non-linear form of Langmuir isotherm model. • The kinetic experimental data were fitted well by non-linear form of pseudo-second-order model. • The modified GO showed a good performance for removal of Aniline from a simulated real sample and favorable regeneration ability. • Antibacterial activity of Cu -GO was 2–4 folds higher than that of neat GO. • The mechanism of chemisorption is proposed for adsorption of 2+
2+
Fig. 10. Adsorption efficiency of Cu2+-GO versus regeneration cycle.
2+
Table 4 MIC values of neat and modified GO against various bacterial strains. Bacterial strain
Escheriachia coli ATCC 25922 Staphylococcus aureus ATCC 25923 Bacillus subtilis ATCC 6633
Aniline by Cu2+-GO according to the results of characterization and adsorption experiments.
MIC (μg/mL) GO
Cu(II)-binded GO
125 250 250
31.25 125 62.5
Acknowledgment The authors are most grateful for the continuing financial support of this research project by the University of Maragheh (IRAN). References
3.8. Reusability of adsorbent
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The reusability of an adsorbent is so important for industrial and commercial applications. The reusability of Cu2+-GO was examined by a series of repeated adsorption-desorption experiments. The results of reusability of the adsorbent are shown in Fig. 10. After 8 cycles, only 17% reduction in its efficiency was observed. It implies that Cu2+-GO has high durability so that it can be regenerated and reused several times without significant loss in its performance. After each adsorption process and filtration of adsorbent, the solution was analyzed for Cu2+ ions using atomic absorption spectroscopy. The results showed no leakage of Cu2+ ions to the solution during 8 cycles. This observation confirms the results of EDX mapping of copper on the adsorbent after and before adsorption. 3.9. Removal of aniline from simulated contaminated water The concentration of Aniline in surface waters has been reported to be less than 200 μg/L [4]. Therefore, the simulated contaminated water was prepared by spiking 200 μg/L Aniline into a real surface water collected from Sufichay river (Maragheh, Iran). The adsorption experiment was performed for the water sample using 2 g/L of Cu2+-GO. After adsorption process, the filtered water was subjected to HPLC analysis for Aniline measurement. No Aniline was detected in the sample within 1 ppb. It reveals that Cu2+-GO can be employed as effective adsorbent for removal of Aniline from contaminated natural waters. 3.10. Antibacterial activity Antibacterial properties of GO and Cu2+-GO were evaluated against several bacteria strains as summarized in Table 4. The modified GO showed higher antibacterial properties than pristine GO against both Gram-positive Gram-negative bacteria. The impregnation of GO with 166
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