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Polypyrrole-coated magnetic nanoparticles as an efficient adsorbent for RB19 synthetic textile dye: Removal and kinetic study
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 149 (2015) 481–486
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Polypyrrole-coated magnetic nanoparticles as an efficient adsorbent for RB19 synthetic textile dye: Removal and kinetic study Maryam Shanehsaz a,⇑, Shahram Seidi b,⇑, Yousefali Ghorbani a, Seyed Mohammad Reza Shoja a, Shohre Rouhani c,d a
Analytical Chemistry Research Group, Research Institute of Petroleum Industry (RIPI), P.O. Box 14665-137, Tehran, Iran Department of Analytical Chemistry, Faculty of Chemistry, K.N. Toosi University of Technology, Tehran, Iran Department of Organic Colorants, Institute for Color Science and Technology, 1668814811 Tehran, Iran d Center of Excellence for Color Science and Technology, Institute for Color Science and Technology, 16656118481 Tehran, Iran b c
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
The simple, rapid and efficient
method for removal of RB19 has been introduced. PPy@Fe3O4 MNPs were synthesized by chemical co-precipitation route. PPy@Fe3O4 MNPs Langmuir adsorption capacity was 112.36 mg g1. Magnetic separation permits process flexibility to recover and reused the adsorbent.
a r t i c l e
i n f o
Article history: Received 4 December 2014 Received in revised form 18 March 2015 Accepted 30 April 2015 Available online 7 May 2015 Keywords: Magnetic nanoparticle Pyrrole Adsorption Reactive blue 19 Textile dye
a b s t r a c t The present work deals with the first attempt to study the removal of synthetic textile dye, reactive blue 19 (RB19), using the magnetic Fe3O4 nanoparticles modified by pyrrole (PPy@Fe3O4 MNPs) as an efficient adsorbent. The nanoadsorbent was synthesized using chemical co-precipitation. Scanning electron microscopy and FT-IR were used to characterize nanoparticles. Factors affecting the dye adsorption including the pH of the dye solution, amount of adsorbent and contact time were also further investigated. Sorption of the RB19 on PPy@Fe3O4 MNPs reached to equilibrium at contact time less than 10 min and fitted well to the Langmuir adsorption model with a maximum adsorption capacity of 112.36 mg g1. Experiments for adsorption kinetic were carried out and the data fitted well according to a pseudo-second-order kinetic model. Moreover, the MNPs were recovered with over than 90% efficiency using methanol as elution agent. Ó 2015 Published by Elsevier B.V.
Introduction Dyes are one of the most easily recognizable environmental pollutants. Various industries such as textile, leather, cosmetic, paper, printing, plastic, rubber, pharmaceutical and food are the
⇑ Corresponding authors. Tel.: +98 2148251; fax: +98 2144739712 (M. Shanehsaz). Tel.: +98 (21)23064228; fax: +98 (21)22853650 (S. Seidi). E-mail addresses: [email protected], [email protected] (M. Shanehsaz), [email protected] (S. Seidi). http://dx.doi.org/10.1016/j.saa.2015.04.114 1386-1425/Ó 2015 Published by Elsevier B.V.
sources of pollutant dyes since they all use it in their products [1]. Decomposition of these compounds is controversial due to their aromatic structure and molecular size that lead to their good stability and solubility [2,3]. Existence of dyes in water bodies decreases the depth of light penetration, inhibit the growth of biota and impart toxicity to aquatic life [4]. Hence, the water contaminated by dye at even a concentration of 1.0 mg L1 would cause a change in color and make it improper for human usage [5]. Consequently, removing of dyes from wastewaters before discharging them into rivers and natural streams is one of the serious environmental concerns.
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Some physical and chemical methods have been introduced to remove the dyes from aquatic bodies. These include photocatalytic oxidation [6], membrane separation flocculation [7], irradiation and ozonation [8] etc. But they are expensive routes with initial and operational high costs and cannot remove dyes from wastewater with very low concentration of contaminant [9]. New attempts to introduce efficient methods are under study. The adsorption processes have been preferred to remove colors, odors, oils and organic pollutants from waste effluents because of their low initial costs, simplicity of design and ease of operation [10–12]. There is an increasing demand for the use of nano-sized materials specially magnetic nanoparticle as effective absorbents for removal of dyes with regard to adsorption processes [13–16]. The attractiveness of magnetic nanoparticles lies primarily in their higher surface area-to-volume ratio and magnetic characteristics that led to higher removal efficiency in a short period of time and extraction capacity by an external magnetic field without internal diffusion resistance. Moreover, after the magnetic separation, the recovered MNPs can be reused by removing the contamination using suitable desorption agents [17–20]. In comparison with centrifugation or filtration, an easier and much faster removal or extraction procedure is provided by MNPs [21]. Interest in the iron oxide magnetic nanoparticle (Fe3O4 MNPs) is spawned by its vast utilization in adsorption process to remove the dyes [22–24]. This fact is referred to easy synthesis of iron oxide MNPs by the addition of a basic solution such as ammonia into a heated and degassed aqueous solution containing Fe2+/Fe3+ salts [25]. However, low stability in strong acidic aqueous media, low dispersibility in various sample matrices and low selectivity toward target analytes are the main drawbacks that can be accounted for Fe3O4 MNPs. To overcome these barriers, surface modification of MNPs has been introduced as an efficient strategy. Increasing interest to establish various coating materials on the surface of MNPs is a testimony implicating this verity [26–29]. Application of conductive polymers is an alternative coating agent that can eliminate the mentioned drawbacks and improve selectivity [21]. Polyaniline [30], polypyrrole [31] and polythiophene [32] have extremely become popular for coating on the surface of nanomaterial. In this work, polypyrrole-coated Fe3O4 MNPs were chemically synthesized and applied for removal of RB19. Kinetic models and adsorption isotherm along with the effects of the pH of the sample solution, contact time and amount of MNPs on the adsorption efficiency of RB19 were investigated. It is introduced as a simple, efficient and short time – consuming method to remove RB19 from aqueous solutions.
Experimental Reagents and materials All reagents were of analytical reagent grade and used without further purification. Reactive blue 19 (RB19), an anionic textile dye, was purchased from Aldrich (Milwaukee, WI, USA) and its molecular structure is shown in Fig. 1. Ferric chloride (FeCl3H2O), ferrous chloride (FeCl24H2O), ammonium solution (25%), acetone, methanol and propanol were purchased from Merck (Darmstadt, Germany). Ultra-pure water was used throughout the process which was produced by an Aqua Max-Ultra Youngling ultrapure water purification system (Dongan-du, South Korea). A stock standard solution of RB19 was prepared at the concentration of 1000 mg L1. The standard working solutions were prepared daily by appropriate dilution of the stock standard solution with ultrapure water to the required concentrations.
Fig. 1. The molecular structure of reactive blue 19.
Apparatus All of spectrophotometric measurements were done by a Cecil CE1010 UV–Vis spectrophotometer (London, England). A supermagnet with 1.4 Tesla magnetic field (10 5 4 cm) was used for magnetic separation. A Heidolph motor stirrer (Schwabach, Germany) was applied to stir dye solutions equipped with a glassware stirrer. The pH measurements and adjustments were carried out using a Metrohm 713 pH-meter equipped with a combined glass electrode. The size and morphology of the PPy@Fe3O4 MNPs were obtained using a EM3200 scanning electron microscope (SEM, KYKY Zhongguancun, China). To study surface modification of Fe3O4 NPs a Thermo Scientific Nicolet IR100 (Madison, WI, USA) Fourier-transform infrared (FT-IR) spectroscopy was used in the frequency range of 4000–400 cm1 by pelletizing a homogenized powder of the synthesized NPs and KBr. The obtained Py@Fe3O4 core/shell MNPs were characterized on a 1840 Philips X-ray powder diffraction (XRD) with Co Ka radiation (k = 1.7890 Å). A Zeta sizer Nano ZS (Malvern, Worcestershire, UK) was employed to measure the zeta potential. Preparation of polypyrrole-coated magnetic nanoparticles A two-step strategy was used for synthesis of PPy@Fe3O4 MNPs previously reported in literature [33]. In the first step, Fe3O4 MNPs were synthesized by co-precipitation method. Typically, 8.4 g of FeCl36H2O and 2.25 g of FeCl24H2O were dissolved in 400 mL deionized water via vigorous mechanical stirring under nitrogen atmosphere at 80 °C. By the addition of 20 mL ammonia solution (25%wt), the color of solution changed from orange to black and the solution was allowed to be stirred for 5 min. After the reaction, the obtained MNPs precipitate was separated from the reaction medium by an external magnetic field, washed with deionized water several times and dried in a vacuum oven at 70 °C for 5 h. In the second step, 1.0 g of the dried Fe3O4 MNPs was added to 400 mL of deionized water at pH of 9.0 under stirring for 5 min and then, 8.0 mL of pyrrole monomers was added and stirred for 10 min. Subsequently, 1.5 g of FeCl36H2O as an oxidant was dissolved in 50 mL of deionized water and added drop wise to the mixture under stirring. After the addition of oxidant, the mixture was stirred for 12 h at room temperature. The obtained PPy@Fe3O4 MNPs were collected by a magnet field and washed with deionized water and ethanol several times to remove the unreacted materials and dried at 50 °C under vacuum for 3 h. Removal procedure Briefly, 20 mg of dried PPy@Fe3O4 MNPs was added onto 25 mL of sample solution to a known initial concentration (C0) of RB19. The pH of sample solution was adjusted to the desired value by the addition of 1.0 mol L1 of HCl or NaOH. Then, the mixture was stirred at 500 rpm for 8 min and allowed to reach equilibrium.
M. Shanehsaz et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 149 (2015) 481–486
After dye adsorption, MNPs were separated from the sample solution using an external magnetic field and residual dye concentration in the supernatant was determined using spectrophotometer at 569 nm. Removal efficiency of RB19 was calculated using the following equation:
Dye removal efficiency ð%Þ ¼ ðC 0 C r =C 0 Þ 100
ð1Þ
where C0 and Cr (mg L1) are the initial concentration and residual concentration of RB19, respectively. Results and discussion Characterization of PPy@Fe3O4 MNPs SEM was used to determinate the size and morphology of synthesized PPy@Fe3O4 MNPs. According to Fig. 2, the PPy@Fe3O4 MNPs have a nearly spherical shape with a smooth and uniform surface morphology with average particle size less than 80 nm. The FT-IR analysis was also used for more confirmation of PPy coating. Both bare and PPy@Fe3O4 MNPs were analyzed by FT-IR between 4000 and 400 cm1 and the obtained spectra are shown
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in Fig. 3a and b, respectively. Two characteristic absorption peaks at 3400 cm1 and 580 cm1 in Fig. 3(a and b) are attributed to the stretching vibrations of hydrogen-bonded surface water molecules as well as hydroxyl groups and the Fe–O transverse vibration, respectively. The appearance of characteristic PPy bands in spectrum (b) confirms the coating of PPy on the surface of Fe3O4 NPs. The weak bands at 2800 and 2900 cm1 are corresponding to the stretching vibrations of C–H bonds. The absorption peak at 1050 cm1 is assigned to the bending vibration of C–H bond in the pyrrole ring. The absorption band at 1314 cm1 indicates the C–N stretching vibration. The absorption bands at 1549 and 1460 cm1 could be attributed to C–C asymmetric and symmetric stretching vibrations of the pyrrole ring, respectively. Finally, successful coating of PPy on the surface of Fe3O4 NPs can be concluded from the obtained results. A typical X-ray diffraction (XRD) was used to characterize the crystal structure of Py@Fe3O4 core/shell MNPs. The presence of a broad high-angle asymmetric scattering peak stretching from 2h = 14.5 to 30° is associated with the amorphous PPy and confirms the existence of amorphous PPy on the surface of Fe3O4 [34]. Fig. 4, XRD pattern of the synthesized Py@Fe3O4 core/shell MNPs with Cu Ka radiation (k = 1.54178 Å).
Fig. 2. SEM micrographs of PPy@Fe3O4 MNPs at (a) 20,000 and (b) 40,000 magnifications.
Fig. 3. FT-IR spectra of (a) bare Fe3O4 NPs and (b) PPy@Fe3O4 MNPs.
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Removal efficiency (%)
100
80
60
40
20 0
Fig. 4. X-ray diffraction pattern of PPy@Fe3O4 MNPs.
2
4
6 pH
8
10
12
Fig. 6. Effect of pH on the adsorption of dye on MNPs. (RB19: 25 mL, 50 mg L1, agitation time = 10 min, 20 mg PPy@Fe3O4 MNPs).
Zeta potential measurements
100 Removal efficiency (%)
Very useful information can be provided about charge density on the surface of an adsorbent at different pH by zeta potential measurements. Isoelectric point (IEP) is known as an important characteristic of an adsorbent assisting to control the charge density of its surface so that the adsorbent surface is negatively charged when the pH value is above the IEP and is positively charged when the pH value is below the IEP. Therefore, the charge density of the surface of an adsorbent can easily be adjusted by knowledge about IEP and variation of the sample solution pH. The zeta potential values of synthesized Py@Fe3O4 core/shell MNPs were investigated at different pH in the range of 2–12. The results are shown in Fig. 5 indicating a positive surface charge at pH values below 3.15 for Py@Fe3O4 core/shell MNPs.
80
60
40
20 5
15
25
35
45
Weight (mg) Effect of pH
Fig. 7. Effect of different amount of MNPs on removal efficiency (pH = 3.0, RB19: 25 mL, 50 mg L1, agitation time = 10 min).
In order to gain more concrete information about removing procedure, study of pH variation is inevitable. Firstly, an investigation was taken on UV spectra of the dye solution in the various pH range of 3–12. The results reveal that, there is no changes in color absorbance and wavelengths within pH variation. This phenomenon is strongly confirmed that every change can be attributed to interaction of dye with PPy@Fe3O4 MNPs. In the acidic medium, (pH = 3) the nanoparticles have positive surface charge. Thus, under this circumstance, the nanoparticles can interact with anion dye molecules, then it was chosen as the optimum pH (Fig. 6).
individually carried out by adding different amounts of MNPs from 5 to 40 mg into 25 mL of dye solution (50 mg L1, pH = 3.0, room temperature) with contact time of 10 min. After a dye removal, the MNPs were separated with an external magnetic field from sample solution and the residue concentration of supernatant was determined. It was found that an increase in amount of absorbent up to 20 mg increased the removal efficiency. At higher amounts, no considerable changes in removal efficiency were observed (Fig. 7).
Effect of adsorbent amount
Effect of contact time
The amount of adsorbent is an important factor to determine adsorption capacity. For this purpose, some experiments were
The effect of contact time on removal efficiency was studied at the range of 0–60 min. The experiments were carried out in 25 mL
Fig. 5. Zeta-potential variation of synthesized Py@Fe3O4 core/shell MNPs under different pH conditions.
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1.2
100
1 t/qt (min g/mg )
Removal efficiency (%)
y = 0.0181x + 0.0051 R² = 0.9999
80
60
0.8 0.6 0.4 0.2
40
0
0
10
20
30
40
50
60
0
10
20
Contact time (min)
60
70
0.2 y = 0.0783x + 0.0089 R² = 0.9978
0.16
1/qe (g/mg)
Adsorption kinetics
50
Fig. 9. Fitting of kinetic data to pseudo-second order kinetic model.
Fig. 8. Effect of contact time on adsorption efficiency (pH = 3.0, RB19: 25 mL, 50 mg L1, 20 mg PPy@Fe3O4 MNPs).
of dye solution with pH of 3.0 (50 mg L1, 20 mg PPy@Fe3O4 MNPs, room temperature). According to Fig. 8, the adsorption capacity of the dye was increased by increasing contact time up to 8 min and then remained constant indicating that a maximum removal was attained. Therefore, 8 min was selected as optimum contact time for subsequent experiments.
30 40 time (min)
0.12 0.08
The kinetic study provides important information to choose the optimum operating conditions for the full-scale bath process, the type of the adsorption mechanism and adsorption characteristics. Three adsorption kinetics models include pseudo-first-order, pseudo-second-order and intraparticle diffusion model were used to analyze the adsorption kinetic data [35–37]. The correlation coefficients (R2) were used to confirm the agreement between experimental data and the model-predicted values. Kinetic studies were performed in 25 mL of dye solution (50 mg L1, 20 mg PPy@Fe3O4 MNPs, pH = 3.0, room temperature). The residual RB19 concentration in the solution was analyzed using spectrophotometer during the intervals ranging from 0 to 60 min. Within our experiments, the best R2 value (0.999) was obtained by pseudo-second-order model, whereas fitting of kinetic data to a pseudo-first-order and intraparticle diffusion models resulted in R2 values of 0.961 and 0.956, respectively. The kinetic rate equation for pseudo-second order reaction can be written as follows:
maximum adsorption capacity. Langmuir’s model states that the maximum adsorption capacity consists of a monolayer adsorption, and the adsorption energy is distributed homogeneously over the entire coverage surface. The Freundlich isotherm model is an empirical equation which describes the surface heterogeneity of the sorbent. It considers multilayer adsorption with a heterogeneous energetic distribution of active sites, accompanied by interactions between adsorbed molecules. Both the linearized forms of Langmuir and Freundlich equations are given by the following equations, respectively:
1=qt 1=qe ¼ 1=k2 q2e t
C e =qe ¼ 1=K L qmax þ 1=qmax C e
ð3Þ
Log qe ¼ Log K F þ ð1=nÞ Log C e
ð4Þ
ð2Þ
where qt and qe, are the value of adsorbed dye at each time and at equilibrium time and k2 is the pseudo-second order rate constant. The fitting of kinetic data to pseudo-second-order model depicted in Fig. 9 indicates that the present system shows the adsorption of dye followed by chemisorption mechanism via electrostatic attraction. Adsorption isotherm The experimental sorption data were described by equilibrium isotherm equations. Information including affinity of the sorbent, the surface properties and also important information on the sorption mechanism can be obtained from different adsorption models [38,39]. Langmuir [40] and Freundlich [41] are the most important isotherms to evaluate the adsorption properties and determine the
0.04 0 0
0.5
1 1.5 1/Ce (l/mg)
2
2.5
Fig. 10. Langmuir adsorption isotherm of dye for MNPs.
where Ce is the equilibrium dye concentration into the sample solution (mg L1), qe is the amount of dye absorbed per gram of adsorbent (mg g1) at Ce, qmax is the maximum sorption capacity (mg g1) which depends on the number of adsorption site. KL and KF are the Langmuir and Freundlich constants (L mg1), respectively and n (dimensionless) is the heterogeneity factor. The adsorption equilibrium curves were evaluated by adding weighted amounts of MNPs to 25 mL of sample solution with different concentrations of RB19 at pH of 3.0. Then, the mixture was stirred for 8 min. The amount of dye adsorption by PPy@Fe3O4 MNPs at equilibrium time, qe (mg g1), was calculated by:
qe ¼ ðC 0 C e Þ V=ms
ð5Þ
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where C0 and Ce (mg L1) are the initial and the equilibrium concentrations of the dye in the sample solution, respectively. V is the volume of the sample solution (L) and ms is the mass of dry nanoparticles (g). The obtained correlation coefficients (R2Langmuir = 0.997, 2 RFreundlich = 0.938) showed that the adsorption equilibrium could be better fitted with the Langmuir isotherm (1/qe = 0.008 + 0.078/Ce) rather than Freundlich isotherm (Log qe = 1.533 Log Ce 1.614, n = 0.65) which indicates the homogeneous distribution of active sites on the surface of MNPs. The plotting 1/qe (g mg1) against 1/Ce (L mg1) gives a straight line, with the slope and intercept equal to 1/Kl qmax and 1/qmax, respectively (Fig. 10). The maximum monolayer capacity qmax and KL that calculated by Langmuir model were as 112.36 mg g1 and 0.113 L mg1, respectively. Desorption and reused study Dye adsorption on the surface of MNPs is a reversible process. Then, the MNPs can be reused after desorpting of dye from the absorbent. Three eluents including methanol, acetone and propanol were selected to elute of RB19 from the surface of MNPs. The results showed that the higher desorption efficiency was obtained using 3.0 mL of methanol in comparison with other eluents. To evaluate the reused ability of adsorbent, several adsorption/desorption cycles were carried out. After each extraction process, the adsorbent was eluted using 3.0 mL of methanol at 5 times and then used in the subsequent extractions. It was found that more than 90% of RB19 can be removed from the sample solution by the regenerated sorbent obtained after 5 cycles of adsorption/desorption. Conclusion In the present work, PPy@Fe3O4 MNPs were synthesized and successfully utilized as efficient absorbents for remove of RB19 textile dye. Sorption of the RB19 on MNPs reached equilibrium in less than 10 min. The kinetic data and adsorption isotherm fitted well with the pseudo-second-order and Langmuir models, respectively. According to the Langmuir isotherm model, the maximum adsorption capacity for RB19 was 112.36 mg g1. From economical point of view, MNPs can be washed and recycled for additional dye removal. Moreover, high removal efficiency, fast adsorption and simplicity of the magnetic separation from sample body are the advantages that make the PPy@Fe3O4 MNPs as unique adsorbents for removal of RB19.
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Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Polypyrrole-coated magnetic nanoparticles as an efficient adsorbent for RB19 synthetic textile dye: Removal and kinetic study Maryam Shanehsaz a,⇑, Shahram Seidi b,⇑, Yousefali Ghorbani a, Seyed Mohammad Reza Shoja a, Shohre Rouhani c,d a
Analytical Chemistry Research Group, Research Institute of Petroleum Industry (RIPI), P.O. Box 14665-137, Tehran, Iran Department of Analytical Chemistry, Faculty of Chemistry, K.N. Toosi University of Technology, Tehran, Iran Department of Organic Colorants, Institute for Color Science and Technology, 1668814811 Tehran, Iran d Center of Excellence for Color Science and Technology, Institute for Color Science and Technology, 16656118481 Tehran, Iran b c
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
The simple, rapid and efficient
method for removal of RB19 has been introduced. PPy@Fe3O4 MNPs were synthesized by chemical co-precipitation route. PPy@Fe3O4 MNPs Langmuir adsorption capacity was 112.36 mg g1. Magnetic separation permits process flexibility to recover and reused the adsorbent.
a r t i c l e
i n f o
Article history: Received 4 December 2014 Received in revised form 18 March 2015 Accepted 30 April 2015 Available online 7 May 2015 Keywords: Magnetic nanoparticle Pyrrole Adsorption Reactive blue 19 Textile dye
a b s t r a c t The present work deals with the first attempt to study the removal of synthetic textile dye, reactive blue 19 (RB19), using the magnetic Fe3O4 nanoparticles modified by pyrrole (PPy@Fe3O4 MNPs) as an efficient adsorbent. The nanoadsorbent was synthesized using chemical co-precipitation. Scanning electron microscopy and FT-IR were used to characterize nanoparticles. Factors affecting the dye adsorption including the pH of the dye solution, amount of adsorbent and contact time were also further investigated. Sorption of the RB19 on PPy@Fe3O4 MNPs reached to equilibrium at contact time less than 10 min and fitted well to the Langmuir adsorption model with a maximum adsorption capacity of 112.36 mg g1. Experiments for adsorption kinetic were carried out and the data fitted well according to a pseudo-second-order kinetic model. Moreover, the MNPs were recovered with over than 90% efficiency using methanol as elution agent. Ó 2015 Published by Elsevier B.V.
Introduction Dyes are one of the most easily recognizable environmental pollutants. Various industries such as textile, leather, cosmetic, paper, printing, plastic, rubber, pharmaceutical and food are the
⇑ Corresponding authors. Tel.: +98 2148251; fax: +98 2144739712 (M. Shanehsaz). Tel.: +98 (21)23064228; fax: +98 (21)22853650 (S. Seidi). E-mail addresses: [email protected], [email protected] (M. Shanehsaz), [email protected] (S. Seidi). http://dx.doi.org/10.1016/j.saa.2015.04.114 1386-1425/Ó 2015 Published by Elsevier B.V.
sources of pollutant dyes since they all use it in their products [1]. Decomposition of these compounds is controversial due to their aromatic structure and molecular size that lead to their good stability and solubility [2,3]. Existence of dyes in water bodies decreases the depth of light penetration, inhibit the growth of biota and impart toxicity to aquatic life [4]. Hence, the water contaminated by dye at even a concentration of 1.0 mg L1 would cause a change in color and make it improper for human usage [5]. Consequently, removing of dyes from wastewaters before discharging them into rivers and natural streams is one of the serious environmental concerns.
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Some physical and chemical methods have been introduced to remove the dyes from aquatic bodies. These include photocatalytic oxidation [6], membrane separation flocculation [7], irradiation and ozonation [8] etc. But they are expensive routes with initial and operational high costs and cannot remove dyes from wastewater with very low concentration of contaminant [9]. New attempts to introduce efficient methods are under study. The adsorption processes have been preferred to remove colors, odors, oils and organic pollutants from waste effluents because of their low initial costs, simplicity of design and ease of operation [10–12]. There is an increasing demand for the use of nano-sized materials specially magnetic nanoparticle as effective absorbents for removal of dyes with regard to adsorption processes [13–16]. The attractiveness of magnetic nanoparticles lies primarily in their higher surface area-to-volume ratio and magnetic characteristics that led to higher removal efficiency in a short period of time and extraction capacity by an external magnetic field without internal diffusion resistance. Moreover, after the magnetic separation, the recovered MNPs can be reused by removing the contamination using suitable desorption agents [17–20]. In comparison with centrifugation or filtration, an easier and much faster removal or extraction procedure is provided by MNPs [21]. Interest in the iron oxide magnetic nanoparticle (Fe3O4 MNPs) is spawned by its vast utilization in adsorption process to remove the dyes [22–24]. This fact is referred to easy synthesis of iron oxide MNPs by the addition of a basic solution such as ammonia into a heated and degassed aqueous solution containing Fe2+/Fe3+ salts [25]. However, low stability in strong acidic aqueous media, low dispersibility in various sample matrices and low selectivity toward target analytes are the main drawbacks that can be accounted for Fe3O4 MNPs. To overcome these barriers, surface modification of MNPs has been introduced as an efficient strategy. Increasing interest to establish various coating materials on the surface of MNPs is a testimony implicating this verity [26–29]. Application of conductive polymers is an alternative coating agent that can eliminate the mentioned drawbacks and improve selectivity [21]. Polyaniline [30], polypyrrole [31] and polythiophene [32] have extremely become popular for coating on the surface of nanomaterial. In this work, polypyrrole-coated Fe3O4 MNPs were chemically synthesized and applied for removal of RB19. Kinetic models and adsorption isotherm along with the effects of the pH of the sample solution, contact time and amount of MNPs on the adsorption efficiency of RB19 were investigated. It is introduced as a simple, efficient and short time – consuming method to remove RB19 from aqueous solutions.
Experimental Reagents and materials All reagents were of analytical reagent grade and used without further purification. Reactive blue 19 (RB19), an anionic textile dye, was purchased from Aldrich (Milwaukee, WI, USA) and its molecular structure is shown in Fig. 1. Ferric chloride (FeCl3H2O), ferrous chloride (FeCl24H2O), ammonium solution (25%), acetone, methanol and propanol were purchased from Merck (Darmstadt, Germany). Ultra-pure water was used throughout the process which was produced by an Aqua Max-Ultra Youngling ultrapure water purification system (Dongan-du, South Korea). A stock standard solution of RB19 was prepared at the concentration of 1000 mg L1. The standard working solutions were prepared daily by appropriate dilution of the stock standard solution with ultrapure water to the required concentrations.
Fig. 1. The molecular structure of reactive blue 19.
Apparatus All of spectrophotometric measurements were done by a Cecil CE1010 UV–Vis spectrophotometer (London, England). A supermagnet with 1.4 Tesla magnetic field (10 5 4 cm) was used for magnetic separation. A Heidolph motor stirrer (Schwabach, Germany) was applied to stir dye solutions equipped with a glassware stirrer. The pH measurements and adjustments were carried out using a Metrohm 713 pH-meter equipped with a combined glass electrode. The size and morphology of the PPy@Fe3O4 MNPs were obtained using a EM3200 scanning electron microscope (SEM, KYKY Zhongguancun, China). To study surface modification of Fe3O4 NPs a Thermo Scientific Nicolet IR100 (Madison, WI, USA) Fourier-transform infrared (FT-IR) spectroscopy was used in the frequency range of 4000–400 cm1 by pelletizing a homogenized powder of the synthesized NPs and KBr. The obtained Py@Fe3O4 core/shell MNPs were characterized on a 1840 Philips X-ray powder diffraction (XRD) with Co Ka radiation (k = 1.7890 Å). A Zeta sizer Nano ZS (Malvern, Worcestershire, UK) was employed to measure the zeta potential. Preparation of polypyrrole-coated magnetic nanoparticles A two-step strategy was used for synthesis of PPy@Fe3O4 MNPs previously reported in literature [33]. In the first step, Fe3O4 MNPs were synthesized by co-precipitation method. Typically, 8.4 g of FeCl36H2O and 2.25 g of FeCl24H2O were dissolved in 400 mL deionized water via vigorous mechanical stirring under nitrogen atmosphere at 80 °C. By the addition of 20 mL ammonia solution (25%wt), the color of solution changed from orange to black and the solution was allowed to be stirred for 5 min. After the reaction, the obtained MNPs precipitate was separated from the reaction medium by an external magnetic field, washed with deionized water several times and dried in a vacuum oven at 70 °C for 5 h. In the second step, 1.0 g of the dried Fe3O4 MNPs was added to 400 mL of deionized water at pH of 9.0 under stirring for 5 min and then, 8.0 mL of pyrrole monomers was added and stirred for 10 min. Subsequently, 1.5 g of FeCl36H2O as an oxidant was dissolved in 50 mL of deionized water and added drop wise to the mixture under stirring. After the addition of oxidant, the mixture was stirred for 12 h at room temperature. The obtained PPy@Fe3O4 MNPs were collected by a magnet field and washed with deionized water and ethanol several times to remove the unreacted materials and dried at 50 °C under vacuum for 3 h. Removal procedure Briefly, 20 mg of dried PPy@Fe3O4 MNPs was added onto 25 mL of sample solution to a known initial concentration (C0) of RB19. The pH of sample solution was adjusted to the desired value by the addition of 1.0 mol L1 of HCl or NaOH. Then, the mixture was stirred at 500 rpm for 8 min and allowed to reach equilibrium.
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After dye adsorption, MNPs were separated from the sample solution using an external magnetic field and residual dye concentration in the supernatant was determined using spectrophotometer at 569 nm. Removal efficiency of RB19 was calculated using the following equation:
Dye removal efficiency ð%Þ ¼ ðC 0 C r =C 0 Þ 100
ð1Þ
where C0 and Cr (mg L1) are the initial concentration and residual concentration of RB19, respectively. Results and discussion Characterization of PPy@Fe3O4 MNPs SEM was used to determinate the size and morphology of synthesized PPy@Fe3O4 MNPs. According to Fig. 2, the PPy@Fe3O4 MNPs have a nearly spherical shape with a smooth and uniform surface morphology with average particle size less than 80 nm. The FT-IR analysis was also used for more confirmation of PPy coating. Both bare and PPy@Fe3O4 MNPs were analyzed by FT-IR between 4000 and 400 cm1 and the obtained spectra are shown
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in Fig. 3a and b, respectively. Two characteristic absorption peaks at 3400 cm1 and 580 cm1 in Fig. 3(a and b) are attributed to the stretching vibrations of hydrogen-bonded surface water molecules as well as hydroxyl groups and the Fe–O transverse vibration, respectively. The appearance of characteristic PPy bands in spectrum (b) confirms the coating of PPy on the surface of Fe3O4 NPs. The weak bands at 2800 and 2900 cm1 are corresponding to the stretching vibrations of C–H bonds. The absorption peak at 1050 cm1 is assigned to the bending vibration of C–H bond in the pyrrole ring. The absorption band at 1314 cm1 indicates the C–N stretching vibration. The absorption bands at 1549 and 1460 cm1 could be attributed to C–C asymmetric and symmetric stretching vibrations of the pyrrole ring, respectively. Finally, successful coating of PPy on the surface of Fe3O4 NPs can be concluded from the obtained results. A typical X-ray diffraction (XRD) was used to characterize the crystal structure of Py@Fe3O4 core/shell MNPs. The presence of a broad high-angle asymmetric scattering peak stretching from 2h = 14.5 to 30° is associated with the amorphous PPy and confirms the existence of amorphous PPy on the surface of Fe3O4 [34]. Fig. 4, XRD pattern of the synthesized Py@Fe3O4 core/shell MNPs with Cu Ka radiation (k = 1.54178 Å).
Fig. 2. SEM micrographs of PPy@Fe3O4 MNPs at (a) 20,000 and (b) 40,000 magnifications.
Fig. 3. FT-IR spectra of (a) bare Fe3O4 NPs and (b) PPy@Fe3O4 MNPs.
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Removal efficiency (%)
100
80
60
40
20 0
Fig. 4. X-ray diffraction pattern of PPy@Fe3O4 MNPs.
2
4
6 pH
8
10
12
Fig. 6. Effect of pH on the adsorption of dye on MNPs. (RB19: 25 mL, 50 mg L1, agitation time = 10 min, 20 mg PPy@Fe3O4 MNPs).
Zeta potential measurements
100 Removal efficiency (%)
Very useful information can be provided about charge density on the surface of an adsorbent at different pH by zeta potential measurements. Isoelectric point (IEP) is known as an important characteristic of an adsorbent assisting to control the charge density of its surface so that the adsorbent surface is negatively charged when the pH value is above the IEP and is positively charged when the pH value is below the IEP. Therefore, the charge density of the surface of an adsorbent can easily be adjusted by knowledge about IEP and variation of the sample solution pH. The zeta potential values of synthesized Py@Fe3O4 core/shell MNPs were investigated at different pH in the range of 2–12. The results are shown in Fig. 5 indicating a positive surface charge at pH values below 3.15 for Py@Fe3O4 core/shell MNPs.
80
60
40
20 5
15
25
35
45
Weight (mg) Effect of pH
Fig. 7. Effect of different amount of MNPs on removal efficiency (pH = 3.0, RB19: 25 mL, 50 mg L1, agitation time = 10 min).
In order to gain more concrete information about removing procedure, study of pH variation is inevitable. Firstly, an investigation was taken on UV spectra of the dye solution in the various pH range of 3–12. The results reveal that, there is no changes in color absorbance and wavelengths within pH variation. This phenomenon is strongly confirmed that every change can be attributed to interaction of dye with PPy@Fe3O4 MNPs. In the acidic medium, (pH = 3) the nanoparticles have positive surface charge. Thus, under this circumstance, the nanoparticles can interact with anion dye molecules, then it was chosen as the optimum pH (Fig. 6).
individually carried out by adding different amounts of MNPs from 5 to 40 mg into 25 mL of dye solution (50 mg L1, pH = 3.0, room temperature) with contact time of 10 min. After a dye removal, the MNPs were separated with an external magnetic field from sample solution and the residue concentration of supernatant was determined. It was found that an increase in amount of absorbent up to 20 mg increased the removal efficiency. At higher amounts, no considerable changes in removal efficiency were observed (Fig. 7).
Effect of adsorbent amount
Effect of contact time
The amount of adsorbent is an important factor to determine adsorption capacity. For this purpose, some experiments were
The effect of contact time on removal efficiency was studied at the range of 0–60 min. The experiments were carried out in 25 mL
Fig. 5. Zeta-potential variation of synthesized Py@Fe3O4 core/shell MNPs under different pH conditions.
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1.2
100
1 t/qt (min g/mg )
Removal efficiency (%)
y = 0.0181x + 0.0051 R² = 0.9999
80
60
0.8 0.6 0.4 0.2
40
0
0
10
20
30
40
50
60
0
10
20
Contact time (min)
60
70
0.2 y = 0.0783x + 0.0089 R² = 0.9978
0.16
1/qe (g/mg)
Adsorption kinetics
50
Fig. 9. Fitting of kinetic data to pseudo-second order kinetic model.
Fig. 8. Effect of contact time on adsorption efficiency (pH = 3.0, RB19: 25 mL, 50 mg L1, 20 mg PPy@Fe3O4 MNPs).
of dye solution with pH of 3.0 (50 mg L1, 20 mg PPy@Fe3O4 MNPs, room temperature). According to Fig. 8, the adsorption capacity of the dye was increased by increasing contact time up to 8 min and then remained constant indicating that a maximum removal was attained. Therefore, 8 min was selected as optimum contact time for subsequent experiments.
30 40 time (min)
0.12 0.08
The kinetic study provides important information to choose the optimum operating conditions for the full-scale bath process, the type of the adsorption mechanism and adsorption characteristics. Three adsorption kinetics models include pseudo-first-order, pseudo-second-order and intraparticle diffusion model were used to analyze the adsorption kinetic data [35–37]. The correlation coefficients (R2) were used to confirm the agreement between experimental data and the model-predicted values. Kinetic studies were performed in 25 mL of dye solution (50 mg L1, 20 mg PPy@Fe3O4 MNPs, pH = 3.0, room temperature). The residual RB19 concentration in the solution was analyzed using spectrophotometer during the intervals ranging from 0 to 60 min. Within our experiments, the best R2 value (0.999) was obtained by pseudo-second-order model, whereas fitting of kinetic data to a pseudo-first-order and intraparticle diffusion models resulted in R2 values of 0.961 and 0.956, respectively. The kinetic rate equation for pseudo-second order reaction can be written as follows:
maximum adsorption capacity. Langmuir’s model states that the maximum adsorption capacity consists of a monolayer adsorption, and the adsorption energy is distributed homogeneously over the entire coverage surface. The Freundlich isotherm model is an empirical equation which describes the surface heterogeneity of the sorbent. It considers multilayer adsorption with a heterogeneous energetic distribution of active sites, accompanied by interactions between adsorbed molecules. Both the linearized forms of Langmuir and Freundlich equations are given by the following equations, respectively:
1=qt 1=qe ¼ 1=k2 q2e t
C e =qe ¼ 1=K L qmax þ 1=qmax C e
ð3Þ
Log qe ¼ Log K F þ ð1=nÞ Log C e
ð4Þ
ð2Þ
where qt and qe, are the value of adsorbed dye at each time and at equilibrium time and k2 is the pseudo-second order rate constant. The fitting of kinetic data to pseudo-second-order model depicted in Fig. 9 indicates that the present system shows the adsorption of dye followed by chemisorption mechanism via electrostatic attraction. Adsorption isotherm The experimental sorption data were described by equilibrium isotherm equations. Information including affinity of the sorbent, the surface properties and also important information on the sorption mechanism can be obtained from different adsorption models [38,39]. Langmuir [40] and Freundlich [41] are the most important isotherms to evaluate the adsorption properties and determine the
0.04 0 0
0.5
1 1.5 1/Ce (l/mg)
2
2.5
Fig. 10. Langmuir adsorption isotherm of dye for MNPs.
where Ce is the equilibrium dye concentration into the sample solution (mg L1), qe is the amount of dye absorbed per gram of adsorbent (mg g1) at Ce, qmax is the maximum sorption capacity (mg g1) which depends on the number of adsorption site. KL and KF are the Langmuir and Freundlich constants (L mg1), respectively and n (dimensionless) is the heterogeneity factor. The adsorption equilibrium curves were evaluated by adding weighted amounts of MNPs to 25 mL of sample solution with different concentrations of RB19 at pH of 3.0. Then, the mixture was stirred for 8 min. The amount of dye adsorption by PPy@Fe3O4 MNPs at equilibrium time, qe (mg g1), was calculated by:
qe ¼ ðC 0 C e Þ V=ms
ð5Þ
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where C0 and Ce (mg L1) are the initial and the equilibrium concentrations of the dye in the sample solution, respectively. V is the volume of the sample solution (L) and ms is the mass of dry nanoparticles (g). The obtained correlation coefficients (R2Langmuir = 0.997, 2 RFreundlich = 0.938) showed that the adsorption equilibrium could be better fitted with the Langmuir isotherm (1/qe = 0.008 + 0.078/Ce) rather than Freundlich isotherm (Log qe = 1.533 Log Ce 1.614, n = 0.65) which indicates the homogeneous distribution of active sites on the surface of MNPs. The plotting 1/qe (g mg1) against 1/Ce (L mg1) gives a straight line, with the slope and intercept equal to 1/Kl qmax and 1/qmax, respectively (Fig. 10). The maximum monolayer capacity qmax and KL that calculated by Langmuir model were as 112.36 mg g1 and 0.113 L mg1, respectively. Desorption and reused study Dye adsorption on the surface of MNPs is a reversible process. Then, the MNPs can be reused after desorpting of dye from the absorbent. Three eluents including methanol, acetone and propanol were selected to elute of RB19 from the surface of MNPs. The results showed that the higher desorption efficiency was obtained using 3.0 mL of methanol in comparison with other eluents. To evaluate the reused ability of adsorbent, several adsorption/desorption cycles were carried out. After each extraction process, the adsorbent was eluted using 3.0 mL of methanol at 5 times and then used in the subsequent extractions. It was found that more than 90% of RB19 can be removed from the sample solution by the regenerated sorbent obtained after 5 cycles of adsorption/desorption. Conclusion In the present work, PPy@Fe3O4 MNPs were synthesized and successfully utilized as efficient absorbents for remove of RB19 textile dye. Sorption of the RB19 on MNPs reached equilibrium in less than 10 min. The kinetic data and adsorption isotherm fitted well with the pseudo-second-order and Langmuir models, respectively. According to the Langmuir isotherm model, the maximum adsorption capacity for RB19 was 112.36 mg g1. From economical point of view, MNPs can be washed and recycled for additional dye removal. Moreover, high removal efficiency, fast adsorption and simplicity of the magnetic separation from sample body are the advantages that make the PPy@Fe3O4 MNPs as unique adsorbents for removal of RB19.
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