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Deposition of MnO2 nanoparticles on the magnetic halloysite nanotubes by hydrothermal method for lead(II) removal from aqueous solutions
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Deposition of MnO2 nanoparticles on the magnetic halloysite nanotubes by hydrothermal method for lead(II) removal from aqueous solutions Daryoush Afzali∗, Maryam Fayazi∗ Department of Environment, Institute of Science and High Technology and Environmental Sciences, Graduate University of Advanced Technology, Kerman, Iran
a r t i c l e
i n f o
Article history: Received 24 October 2015 Revised 31 January 2016 Accepted 17 February 2016 Available online xxx Keywords: Lead Adsorption Manganese oxide Halloysite nanotubes Composite Removal
a b s t r a c t The magnetic halloysite nanotubes@manganese oxide (MHNTs@MnO2 ) nanocomposite was synthesized and applied as a novel sorbent for removal of lead(II) ions. The prepared sorbent was characterized by using Fourier transform infrared spectroscopy (FT-IR), field emission scanning electron microscopy (FESEM), energy-dispersive X-ray spectroscopy (EDX) and vibrating sample magnetometer (VSM) analysis. Batch mode adsorption studies were performed to evaluate the adsorption kinetics, adsorption isotherms, and selective recognition of MHNTs@MnO2 . The equilibrium data was evaluated using Langmuir and Freundlich isotherm. The kinetic data were analyzed using Lagergren pseudo-first-order and pseudo-secondorder equations. The pseudo-second-order exhibited the best fit for the kinetic studies (R2 = 0.9999). Equilibrium data fitted by Langmuir adsorption isotherm and the maximum adsorption capacity was calculated as 59.9 mg/g. The adsorption was analyzed thermodynamically, and the results revealed that the removal process was spontaneous and endothermic. In addition, no obvious decrease was observed after up to five adsorption cycles, indicating that the MHNTs@MnO2 adsorbent has a good stability and reusability. It was concluded that MHNTs@MnO2 nanocomposite is an effective material for the removal of Pb(II) from aqueous solutions. © 2016 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
1. Introduction Presently, pollution of the environment by heavy metals especially some toxic metals has received considerable attention. Unlike organic contaminants, heavy metals are essentially nonbiodegradable in the environment and can accumulate in living tissues particularly in human bodies causing various diseases and disorders [1]. Lead is one of the most important trace elements due to its serious toxicity even at very low concentrations [2,3]. It is accumulated in body tissues by long-term exposure to low concentration level because of the rather slow rate of excretion. During the past few years, magnetic nanoparticles (MNPs) as an efficient adsorbent with the large specific surface area and small diffusion resistance have been recognized [4,5]. The magnetic separation provides a desirable path for online separation. A distinctive superiority of this technique is that the MNPs with affinity to target species can be readily isolated from sample solutions using
∗
Corresponding authors. Tel.: +98 34 26226611; fax: +98 34 26226617. E-mail addresses: [email protected] (D. Afzali), maryam.fayazi@ yahoo.com (M. Fayazi).
an external magnetic field without additional filtration or centrifugation steps. Manganese oxide (MnO2 ) is a widely used material featuring high energy density, environmental pollution free and nature abundance [6]. Compared to Fe or Al oxides, MnO2 has a higher affinity for many heavy metal ions [7,8]. However, pure MnO2 as the adsorbent is not favorable for both economic reasons and unfavorable physical and chemical characteristics, but the manganese oxide coating changes substrate surface area, particle size, porosity, and surface electrochemical properties, which greatly influences sorption behavior of the substrates [9,10]. To take all advantage of the specific adsorption behavior of MnO2 and the readily isolation ability of magnetic iron oxide (Fe3 O4 ), carbon-based nanomaterials such as graphene and carbon nanotube (CNT) have been used to improve separation efficiency [11]. But the high cost of these materials detracts from its practical applications. Halloysite nanotube (HNT) is a kind of aluminosilicate clay with a predominantly hollow tubular structure in the submicron range [12]. Compared with other nanomaterials such as graphene and CNT, HNT is an economically available raw material that can be mined from the corresponding deposit as a raw mineral. Due to some excellent physicochemical properties, HNTs have been widely
http://dx.doi.org/10.1016/j.jtice.2016.02.025 1876-1070/© 2016 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
Please cite this article as: D. Afzali, M. Fayazi, Deposition of MnO2 nanoparticles on the magnetic halloysite nanotubes by hydrothermal method for lead(II) removal from aqueous solutions, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.02.025
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used in various fields such as catalysis, biomedicine, electronics and absorbent materials [13-15]. Considering HNTs have got reactive hydroxyl groups on the surface of the tubes [16] they can be modified with some organic and inorganic compounds to increase the sorption selectivity of metal ions. Herein, we modified for the first time the magnetic HNTs with wire-like MnO2 nanoparticles through a facile hydrothermal method. The fabricated magnetic halloysite nanotubes@manganese oxide (MHNTs@MnO2 ) nanocomposite can be magnetically separated from aqueous solution due to superparamagnetic property. The behavior of Pb(II) adsorption on the novel prepared sorbent was described in detail.
2.3. Adsorption studies
2. Experimental
Removal efficiency =
2.1. Reagents and instruments All reagents and chemicals used in this study were of analytical grade. The HNT powder was obtained from New Zealand China Clays Ltd., (Auckland, New Zealand) as a gift. Ferric chloride hexahydrate (FeCl3 . 6H2 O), ferrous sulfate heptahydrate (FeSO4 . 7H2 O), ammonium hydroxide (NH4 OH, 25%, w w-1 ), potassium permanganate (KMnO4 ), ammonium persulfate ((NH4 )2 S2 O8 ) were supplied by Merck (Darmstadt, Germany). The standard stock solution of Pb(II) (10 0 0 mg/L) was obtained from Merck and diluted as required. A Varian Spectra AA 220 atomic absorption spectrometer (Varian, Melbourne, Australia) was used for determination of lead ions. The pH values were measured with a Metrohm digital pH-meter model 827 (Herisau, Switzerland) supplied with a glass-combined electrode. The Fourier transform infrared (FT-IR) spectra (40 0 0–40 0 cm−1 ) were on a Spectrum One FTIR spectrometer (PerkinElmer, USA) using the KBr disk method with a ratio sample/KBr of 1:100 by mass. The morphology and composition of the synthesized composites were analyzed with field emission scanning electron microscope (FE-SEM, CARL ZEISS- AURIGA 60 microscope, Jena, Germany), which was equipped with an energy-dispersive X-ray analyzer (EDX). The magnetic property was analyzed by using a vibrating sample magnetometer (VSM) model MDKFD (Danesh Pajohan Kavir Co., Kashan, Iran). The magnetic separation was done by a super magnet with 7,0 0 0 Gs magnetic field (Nd–Fe–B, 10 × 5 × 4 cm). 2.2. Preparation of MHNTs@MnO2 nanocomposite The MHNTs was prepared according to a previously reported [12]. In brief, 0.5 g HNTs was dispersed into a 200 mL deionized water and then added to 1.165 g of FeCl3 ·6H2 O and 0.6 g of FeSO4 ·7H2 O at 60 °C under N2 . Afterward, ∼20 mL NH4 OH solution (8 mol/L) was added dropwise with vigorously stirring to precipitate the surface-immobilized Fe2+ and Fe3+ ions. The pH of the final mixtures was maintained at ∼11 to ensure the complete transformation of iron ions to iron oxides. The black mixture product was aged at 70 °C for 4 h, separated by a magnet and then washed with deionized water repeatedly. The obtained MHNTs composite was dried at 70 ºC for 12 h. The MHNTs@MnO2 nanocomposite was synthesized by hydrothermal technique. Briefly, 4.2 g of MHNTs was dispersed in 60 mL of deionized water, and 4.424 g of ammonium persulfate and 3.7 g of potassium permanganate were then added under stirring. The mixed solution was poured into a 250 mL of Teflon-lined stainless-steel autoclave, which was heated at 105 °C for 12 h. After cooling to room temperature, the precipitate was collected by magnetic separation, followed by repeated washing with deionized water, and then drying at 60 °C for 24 h.
To study the effect of important parameters on the adsorptive removal of lead(II) ions, batch experiments were carried out. For each experimental run, 20 mL of Pb(II) solution at specified concentration and pH was mixed completely with adsorbent for 60 min using an incubator shaker at 150 rpm. After adsorption, the MHNTs@MnO2 nanocomposite was separated by a magnet and the eluent was transferred into a test tube for the determination of residual lead concentration by flame atomic absorption spectrometry (FAAS). The removal efficiency was calculated using the following equation:
Co − Ce × 100 Co
(1)
where Co and Ce are the initial and final concentrations (mg/L) of Pb(II) ions in solution, respectively. All experiments were performed in triplicate, and the mean of the three measures are taken for all calculations. The relative errors of the data were about 3%.
3. Results and discussion 3.1. Characterization The FT-IR spectra of HNTs, MHNTs and MHNTs@MnO2 are shown in Fig. 1. Absorption peaks appearing at 3697 cm−1 and 3624 cm−1 in the FT-IR spectrum (Fig. 1(a)) are attributed to the stretching vibrations of hydroxyl groups at the surface of bare HNTs [12]. The peak at 1662 cm−1 is assigned to O–H stretching vibration of adsorbed water [17]. The band at 1034 cm−1 is related to the stretching mode of apical Si-O-Si. The peak at 911 cm−1 arises from O–H deformation vibration of inner Al–O–H groups. The peak at 538 cm−1 can be assigned to Al–O–Si deformation vibration and the band at 466 cm−1 is related to O–H bending vibration. The absorption peak at 797 cm−1 can also be attributed to Si–O symmetric stretching vibration. The peaks mentioned above all appeared in FT-IR of MHNTs (Fig. 1(b)), in which the slightly broadened band at 3442 cm−1 was due to stretching vibrations of hydroxyl groups from iron oxide [18]. The FT-IR spectrum of MHNTs@MnO2 (Fig. 1 (C)) show two new absorption peaks at 601 and 1441 cm−1 , the former is assignable to the Mn–O bending vibration [19], whereas the latter was attributable to the bending vibration of –OH group attached to the Mn atoms [20]. The SEM photographs of the surface of the MHNTs and MHNTs@MnO2 are shown in Fig. 2. It is evident that the Fe3 O4 nanoparticles slightly modified the HNTs structure and made the nanotubes surface rougher (Fig. 2(a)). The attachment could be related to the structure of HNTs, such as the large pore volume, large surface area and hydroxyl-containing surface, which enabled metal ions to access and adsorb on the surface easily [21]. As can be seen in Fig. 2(b), there are a large number of wire-like MnO2 nanoparticles randomly deposited on the surface of the MHNTs. The EDX spectrum of the MHNTs@MnO2 nanocomposite (Fig. 2(c)) also confirms the presence of the elements Fe, Al, Si, Mn and oxygen in the prepared nanocomposite. The magnetic hysteresis loops of MHNTs and MHNTs@MnO2 at 298 K are illustrated in Fig. 3. The saturation magnetization was found to be 25.1 emu/g for MHNTs@MnO2 , less than the MHNTs nanocomposite (27.8 emu/g1 ).The saturation magnetization of the particle can be affected by their structure, such as size and crystallinity [22]. The high saturated magnetization of MHNTs@MnO2 directly demonstrated the strong magnetic sensitivity of the magnetic nanocomposite.
Please cite this article as: D. Afzali, M. Fayazi, Deposition of MnO2 nanoparticles on the magnetic halloysite nanotubes by hydrothermal method for lead(II) removal from aqueous solutions, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.02.025
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Fig. 1. FT-IR spectra of (a) HNTs, (b) MHNTs and (c) MHNTs@MnO2 .
3.2. Effect of pH To determine the optimal pH, the effect of pH on the removal of Pb(II) ions was studied over the range of 2.0–8.0 and the result are shown in Fig. 4a. The removal efficiency for Pb(II) increased with pH from 2.0 to 5.5, while sorbents showed a high efficiency in the range of pH 5.5–8.0 due to protonation weakened and the both surface of MHNTs and MHNTs@MnO2 became less positively
charged. At pH values lower than 8.0, lead ions exist in the soluble forms Pb2+ and Pb(OH)+ [23], and the removal of lead ions from a solution is mainly due to the adsorption on the surface of MHNTs and MHNTs@MnO2 nanocomposites. In this study, pH 6.0 was selected as the optimum pH in subsequent work. In order to evaluate the stability of MHNTs@MnO2 regarding iron and manganese leaching, leaching test was carried out in the pH range between 2.0 and 8.0. Fig. 4b shows iron and manganese
Please cite this article as: D. Afzali, M. Fayazi, Deposition of MnO2 nanoparticles on the magnetic halloysite nanotubes by hydrothermal method for lead(II) removal from aqueous solutions, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.02.025
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Fig. 2. FE-SEM images of (a) MHNTs and (b) MHNTs@MnO2 , (c) EDX spectrum of MHNTs@MnO2 .
Fig. 4. (a) Effect of pH on the removal efficiency of MHNTs and MHNTs@MnO2 for Pb(II) ions: concentration of Pb(II) = 50 mg/L, sorbents = 0.035 g, time = 60 min, temperature = 25 °C. (b) Leached iron and manganese concentration at different pH values. The error bars were calculated from the standard deviation of the three replicate samples per experiment.
3.3. Effect of adsorbent dosage The adsorption of Pb(II) on MHNTs and MHNTs@MnO2 nanocomposites was evaluated by changing the dosage of adsorbent range of 0.02 to 0.05 g. As can be seen in Fig. 5, the removal efficiency did not increase linearly with the increase in the sorbents amount. As reported [24], with the increase in sorbent dosage the removal efficiency also increases up to 98.0% at 0.035 g for 50 mg/L Pb(II) solution. After that, even though the adsorbent dosage increases in the system, because of the unavailability of the lead ions, the removal efficiency remains constant. Fig. 3. VSM magnetization curves of MHNTs and MHNTs@MnO2 .
leaching as a function of pH for Pb(II) removal by MHNTs@MnO2 adsorbent. Leaching of Fe and Mn was insignificant at pH > 4.0 and increased considerably at pH < 4.0. According to these results, the prepared sorbent is unstable at lower pH due to higher leaching of iron and manganese ions. Hence, a pH of 6.0 was chosen for further experiments.
3.4. Effect of shaking time The effect of shaking time on the amount of Pb(II) ions adsorbed on MHNTs and MHNTs@MnO2 was investigated in the range of 5–200 min using the optimum dosage of sorbents (0.035 g) in 20 mL of 50 mg/L lead(II). As can be seen in Fig. 6, the removal efficiency proceeds through a two-stage process, involving a rapid initial adsorption of lead(II) on the sorbent surface followed by a second stage with a much slower adsorption. It is evident that most of the lead(II) ions are removed within 60 min for both
Please cite this article as: D. Afzali, M. Fayazi, Deposition of MnO2 nanoparticles on the magnetic halloysite nanotubes by hydrothermal method for lead(II) removal from aqueous solutions, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.02.025
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Fig. 5. Effect of adsorbent dosage on the Pb(II) removal efficiency: concentration of Pb(II) = 50 mg/L, pH = 6.0, time = 60 min, temperature = 25 °C.
Fig. 7. Adsorption kinetics: (a) Pseudo- first order and (b) Pseudo- second order models for adsorption of Pb(II) on the MHNTs and MHNTs@MnO2 .
Fig. 6. Effect of shaking time on the removal efficiency of MHNTs and MHNTs@MnO2 : concentration of Pb(II) = 50 mg/L, pH = 6.0, sorbents = 0.035 g, temperature = 25 °C.
adsorbents and then the curve levels off after this period, corresponding to the saturation of the adsorbents. The fast initial removal rate is probably due to the fast diffusion of Pb(II) from the solution onto the external surface of composites. As the sites are gradually occupied, the adsorbed Pb(II) ions tend to be transported from the bulk phase to the actual sorption sites (i.e., inner-sphere pores of magnetic composites). Such a slow diffusion process will decrease the sorption rate of Pb(II) at later stages. Accordingly, the reaction time of 60 min was enough for further experiments.
3.5. Adsorption kinetic The kinetic model of solute adsorption at solid/solution interfaces is usually complex. Two of the most widely used kinetic models, i.e., pseudo-first-order [25] equation, and pseudo-secondorder [26] equation were used to research the adsorption kinetic behavior of Pb(II) onto magnetic adsorbents. The consistency between the experimental and the model-predicted data was investigated by calculating correlation coefficients (R2 values closer to 1 means more applicability of the model). The formula of Lagergren
pseudo-first-order model is given as follows:
ln (qe − qt ) = lnqe − k1 t
(2)
where qe and qt (mg/g) are the adsorption capacities at equilibrium and at time t, respectively, and k1 is the rate constant of the pseudo-first-order adsorption (min−1 ). Using this well-known equation, the values of k1 and qe were calculated from the slope and intercept of the plot of log(qe − qt ) versus t, respectively [27]. The corresponding kinetic parameters from both models are listed in Table 1. The correlation coefficients (R2 ) for the pseudo-firstorder model are relatively low (Fig. 7(a)), and the calculated qe values (qe,cal ) from the pseudo-first-order model do not agree with the experimental data (qe,exp ), suggesting that the lead(II) adsorption on the composites cannot be explained a pseudo-first-order model. Therefore, it is necessary to fit the experimental data to another model. The adsorption may be described by pseudo-secondorder kinetic model as follows:
t 1 1 = + t qt qe k2 q2e
(3)
where k2 is the equilibrium rate constant of pseudo-second-order adsorption (g/mg min). The slope and intercept of the plot of t/qt versus t were used to calculate the second-order rate constant, k2 (Fig. 7(b)). The pseudo-second order rate constants, k2 , was obtained as 0.0094 g/mg min and the equilibrium adsorption capacity was obtained as 40.0 mg/g. The linear correlation coefficient
Please cite this article as: D. Afzali, M. Fayazi, Deposition of MnO2 nanoparticles on the magnetic halloysite nanotubes by hydrothermal method for lead(II) removal from aqueous solutions, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.02.025
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D. Afzali, M. Fayazi / Journal of the Taiwan Institute of Chemical Engineers 000 (2016) 1–9 Table 1 Adsorption kinetics parameters for Pb(II). Sorbent
MHNTs@MnO2 MHNTs
Pseudo-first-order
Pseudo-second-order
qe,cal (mg/g)
qe,exp (mg/g)
k1 (min−1 )
R2
qe,cal (mg/g)
k2 (g/mg min)
R2
11.66 1.86
39.1 10.99
0.0456 0.0587
0.9741 0.9874
40.0 11.12
0.0094 0.0675
0.9999 1
Fig. 8. Adsorption isotherm of Pb(II) on the MHNTs and MHNTs@MnO2 .
value (R2 ) for the pseudo-second-order adsorption model has high value (>99%) for adsorbents. The kinetic model of system showed that pseudo-second order kinetic model had best fit, suggesting the rate-limiting step of lead(II) onto MHNTs and MHNTs@MnO2 sorbents could be chemical sorption or chemisorptions [28]. 3.6. Adsorption isotherm Isotherms studies can describe how the adsorbate interact with an adsorbent, affording the most important parameter for designing a desired adsorption system [29]. The adsorption isotherm of the prepared sorbents at different initial concentrations was investigated, and the results are given in Fig. 8. It is obvious that the adsorbtion was dependent on the initial Pb(II) concentration since the increase in the initial concentration increased the amount of the lead adsorbed on the both nanocomposite. The adsorption equilibrium data were analyzed by the well-known Langmuir and Freundlich isotherm models. The Langmuir sorption isotherm is often used to describe adsorption of a solute from a liquid solution [30]. The Langmuir equation can be expressed as:
Ce 1 1 = + Ce qe kl qm qm
(4)
where Ce is the equilibrium concentration of the lead(II) solution (mg/L), qe is the adsorption capacity at equilibrium (mg/g), kl is the constant related to free energy of adsorption (L/mg), and qm is the maximum adsorption capacity at monolayer coverage (mg/g). The Freundlich isotherm is an empirical equation that assumes heterogeneous adsorbent surface with its adsorption sites at varying energy levels [31]. The corresponding equation is commonly represented by:
l n qe = ln k f +
1 l n Ce n
(5)
Kf (mg/g (L/mg)1/n ) and n are the Freundlich constants characteristics of the system, indicating the adsorption capacity and the adsorption intensity, respectively.
Fig. 9. (a) Langmuir and (b) Freundlich plots for the adsorption of Pb(II) ions.
Langmuir and Freundlich isotherm models for lead(II) removal on proposed adsorbent using linear regression are shown in Fig. 9. The Langmuir and Freundlich constants and the calculated coefficients are listed in Table 2. According to the correlation coefficients (R2 ) of isotherms, the Langmuir isotherm model was more favorable rather than Freundlich isotherm model. Moreover, the theoretical maximum adsorption capacity (59.9 mg/g) agreed very well with their experimental values (59.2 mg/g) in the case of Langmuir model, indicating a good Langmuir isotherms fit to the experimental data. The essential features and the feasibility of the Langmuir isotherm can be expressed in terms of a dimensionless constant separation factor (RL ) given by the following relationship [32]:
RL =
1 1 + kl C0
(6)
Please cite this article as: D. Afzali, M. Fayazi, Deposition of MnO2 nanoparticles on the magnetic halloysite nanotubes by hydrothermal method for lead(II) removal from aqueous solutions, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.02.025
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Table 2 Isotherm parameters for lead(II) adsorption. Sorbent
MHNTs@MnO2 MHNTs
Langmuir isotherm
Freundlich isotherm
Kl (L/mg)
qm (mg/g)
RL
R2
Kf (mg/g (L/mg)1/n )
n
R2
0.1327 0.0148
59.9 23.1
0.029 0.57
0.9972 0.9939
34.6 1.93
11.24 2.46
0.9266 0.9704
Table 3 Thermodynamic parameters at different temperatures.
H (kJ/mol)
98.21
S (kJ/mol K)
0.349
G (kJ/mol) 298K
313K
328K
–5.72
–9.62
–17.0
Table 4 Influences of EDTA concentration on the desorption capacity and desorption efficiency of Pb(II) ions.
Fig. 10. Van’t Hoff plot for the adsorption of Pb(II) ions on the MHNTs@MnO2 nanocomposite.
where kl (L/mg) is the Langmuir constant and C0 (mg/L) is the initial concentration in the liquid phase. The value of RL indicates the shape of the isotherm to be either unfavorable (RL >1), linear (RL =1), favorable (0
kd = qe /Ce
(7)
G0 = −RT lnkd ln kd =
S R
0
−
H RT
(8) 0
(9)
where kd is the distribution coefficient, T (k) is the temperature, and R (8.314 J/mol K) is the gas constant. The values of (H 0 ) and (S0 ) were calculated from the slope and intercept of the Van’t Hoff linear plot of ln (kd ) against 1/T (Fig. 10). Based on Table 3, the negative values of G0 at all of the experimental temperatures indicated the spontaneous nature of Pb(II)
Concentration of EDTA (M)
Desorption capacity (mg/g)
Desorption efficiency (%)
0.01 0.025 0.05 0.75 0.1
12.6 28.1 36.2 44.5 56.6
21.3 47.5 61.1 75.2 95.6
adsorption by MHNTs@MnO2 nanocomposite. On the other hand, the positive value of H 0 (Table 3) suggested that the adsorption process is endothermic. The positive value of S0 implied the increase in randomness at the solid-solution interface and a good affinity of Pb(II) ions with MHNTs@MnO2 . 3.8. Desorption and regeneration From a practical point of view, the desorption and recyclability of a sorbent are two key evaluation factors [35]. The desorption properties of the MHNTs@MnO2 nanocomposite saturated with adsorbed Pb(II) ions were investigated. To study the desorption, different concentrations of EDTA were used as desorbing media. The desorption capacity and desorption efficiency were calculated according to the following equations [36]:
Desorption capacity =
CaVa m
Desorption efficiency =
Ca Va /m × 100 qe
(10)
(11)
where Ca , Va , m and qe represented the Pb(II) concentration (mg/L) in aqueous solution, the volume of EDTA solution (L), the mass of the adsorbent (g) and the amount of Pb(II) adsorbed (mg/g) at equilibrium, respectively. The effect of concentration of EDTA on the desorption capacity and desorption efficiency of Pb(II) from the representative Pb(II)loaded MHNTs@MnO2 nanocomposite is shown in Table 4. As can be seen, the maximum desorption efficiency was 95.6% using 0.1 N EDTA solution. For evaluating the reuse value of the MHNTs@MnO2 , 0.035 g of the used sorbent was agitated with 20 mL of 0.1 N EDTA solution for 60 min. The sample was separated from the solution by a magnet, washed with distilled water three times, and then dried in an oven at 70 °C for reuse study. As shown in Fig. 11, after five repeated adsorption–desorption cycles, the adsorption capacity of MHNTs@MnO2 sorbent was decreased about 10%. The results showed that the MHNTs@MnO2 nanocomposite could repeatedly be used without considerable loss of adsorption capacity.
Please cite this article as: D. Afzali, M. Fayazi, Deposition of MnO2 nanoparticles on the magnetic halloysite nanotubes by hydrothermal method for lead(II) removal from aqueous solutions, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.02.025
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Fig. 11. Recycling of MHNTs@MnO2 adsorbent for the removal of Pb(II) ions. Table 5 Comparison of various adsorbents for removal of Pb(II) ions. Adsorbents
Adsorption capacity (mg/g)
Best fit isotherm
References
Mangrove-alginate composite bead Multi-walled carbon nanotubes/manganese oxide Magnetic ion-imprinted polymer Magnetic alginate beads Magnesium–aluminum layered double hydroxides/manganese oxide Activated sludge biomass Exfoliated graphene nanosheets β - Manganese oxide Aluminum oxide nanoparticles Multi-walled carbon nanotubes/polyacrylamide Magnetic halloysite nanotubes/manganese oxide
10.84 19.97 32.58 50 49.87 44.75 35.46 13.57 34.10 29.71 59.9
Freundlich – Langmuir Langmuir – Langmuir Langmuir Langmuir Langmuir Langmuir Langmuir
[2] [37] [38] [39] [40] [41] [42] [43] [44] [45] This work
3.9. Comparison of MHNTs@MnO2 with various adsorbents
Acknowledgments
A comparison between the performance of proposed sorbent with those reported previously is shown in Table 5. As can be seen from the Table, the MHNTs@MnO2 in this work has a higher adsorption capacity than most other adsorbents reported in the literature [2,37-45], suggesting that it may be a potential application for the elimination of Pb(II) ion from aqueous solution.
We gratefully acknowledge the financial support provided for this project (No. 4618) by Institute of Science and High Technology and Environmental Sciences, Graduate University of Advanced Technology, Kerman, Iran.
References 4. Conclusion A novel MHNTs@MnO2 nanocomposite was synthesized by chemical precipitation and hydrothermal methods. The synthesized composite material was then characterized by FE-SEM, VSM, FTIR and EDX analysis. The resulting composite combined the both features of Fe3 O4 and MnO2 , and thus exhibited extraordinary adsorption capacity and fast removal rate for lead(II) ions. The adsorption isotherms and kinetics were investigated and indicated that the equilibrium and kinetics adsorption were well-modeled by the Langmuir isotherm model and the pseudo-second-order kinetic model, respectively. It is concluded that the proposed could be utilized as a novel magnetically separable and efficient adsorbent for the removal of Pb(II) from aqueous solution.
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Deposition of MnO2 nanoparticles on the magnetic halloysite nanotubes by hydrothermal method for lead(II) removal from aqueous solutions Daryoush Afzali∗, Maryam Fayazi∗ Department of Environment, Institute of Science and High Technology and Environmental Sciences, Graduate University of Advanced Technology, Kerman, Iran
a r t i c l e
i n f o
Article history: Received 24 October 2015 Revised 31 January 2016 Accepted 17 February 2016 Available online xxx Keywords: Lead Adsorption Manganese oxide Halloysite nanotubes Composite Removal
a b s t r a c t The magnetic halloysite nanotubes@manganese oxide (MHNTs@MnO2 ) nanocomposite was synthesized and applied as a novel sorbent for removal of lead(II) ions. The prepared sorbent was characterized by using Fourier transform infrared spectroscopy (FT-IR), field emission scanning electron microscopy (FESEM), energy-dispersive X-ray spectroscopy (EDX) and vibrating sample magnetometer (VSM) analysis. Batch mode adsorption studies were performed to evaluate the adsorption kinetics, adsorption isotherms, and selective recognition of MHNTs@MnO2 . The equilibrium data was evaluated using Langmuir and Freundlich isotherm. The kinetic data were analyzed using Lagergren pseudo-first-order and pseudo-secondorder equations. The pseudo-second-order exhibited the best fit for the kinetic studies (R2 = 0.9999). Equilibrium data fitted by Langmuir adsorption isotherm and the maximum adsorption capacity was calculated as 59.9 mg/g. The adsorption was analyzed thermodynamically, and the results revealed that the removal process was spontaneous and endothermic. In addition, no obvious decrease was observed after up to five adsorption cycles, indicating that the MHNTs@MnO2 adsorbent has a good stability and reusability. It was concluded that MHNTs@MnO2 nanocomposite is an effective material for the removal of Pb(II) from aqueous solutions. © 2016 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
1. Introduction Presently, pollution of the environment by heavy metals especially some toxic metals has received considerable attention. Unlike organic contaminants, heavy metals are essentially nonbiodegradable in the environment and can accumulate in living tissues particularly in human bodies causing various diseases and disorders [1]. Lead is one of the most important trace elements due to its serious toxicity even at very low concentrations [2,3]. It is accumulated in body tissues by long-term exposure to low concentration level because of the rather slow rate of excretion. During the past few years, magnetic nanoparticles (MNPs) as an efficient adsorbent with the large specific surface area and small diffusion resistance have been recognized [4,5]. The magnetic separation provides a desirable path for online separation. A distinctive superiority of this technique is that the MNPs with affinity to target species can be readily isolated from sample solutions using
∗
Corresponding authors. Tel.: +98 34 26226611; fax: +98 34 26226617. E-mail addresses: [email protected] (D. Afzali), maryam.fayazi@ yahoo.com (M. Fayazi).
an external magnetic field without additional filtration or centrifugation steps. Manganese oxide (MnO2 ) is a widely used material featuring high energy density, environmental pollution free and nature abundance [6]. Compared to Fe or Al oxides, MnO2 has a higher affinity for many heavy metal ions [7,8]. However, pure MnO2 as the adsorbent is not favorable for both economic reasons and unfavorable physical and chemical characteristics, but the manganese oxide coating changes substrate surface area, particle size, porosity, and surface electrochemical properties, which greatly influences sorption behavior of the substrates [9,10]. To take all advantage of the specific adsorption behavior of MnO2 and the readily isolation ability of magnetic iron oxide (Fe3 O4 ), carbon-based nanomaterials such as graphene and carbon nanotube (CNT) have been used to improve separation efficiency [11]. But the high cost of these materials detracts from its practical applications. Halloysite nanotube (HNT) is a kind of aluminosilicate clay with a predominantly hollow tubular structure in the submicron range [12]. Compared with other nanomaterials such as graphene and CNT, HNT is an economically available raw material that can be mined from the corresponding deposit as a raw mineral. Due to some excellent physicochemical properties, HNTs have been widely
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used in various fields such as catalysis, biomedicine, electronics and absorbent materials [13-15]. Considering HNTs have got reactive hydroxyl groups on the surface of the tubes [16] they can be modified with some organic and inorganic compounds to increase the sorption selectivity of metal ions. Herein, we modified for the first time the magnetic HNTs with wire-like MnO2 nanoparticles through a facile hydrothermal method. The fabricated magnetic halloysite nanotubes@manganese oxide (MHNTs@MnO2 ) nanocomposite can be magnetically separated from aqueous solution due to superparamagnetic property. The behavior of Pb(II) adsorption on the novel prepared sorbent was described in detail.
2.3. Adsorption studies
2. Experimental
Removal efficiency =
2.1. Reagents and instruments All reagents and chemicals used in this study were of analytical grade. The HNT powder was obtained from New Zealand China Clays Ltd., (Auckland, New Zealand) as a gift. Ferric chloride hexahydrate (FeCl3 . 6H2 O), ferrous sulfate heptahydrate (FeSO4 . 7H2 O), ammonium hydroxide (NH4 OH, 25%, w w-1 ), potassium permanganate (KMnO4 ), ammonium persulfate ((NH4 )2 S2 O8 ) were supplied by Merck (Darmstadt, Germany). The standard stock solution of Pb(II) (10 0 0 mg/L) was obtained from Merck and diluted as required. A Varian Spectra AA 220 atomic absorption spectrometer (Varian, Melbourne, Australia) was used for determination of lead ions. The pH values were measured with a Metrohm digital pH-meter model 827 (Herisau, Switzerland) supplied with a glass-combined electrode. The Fourier transform infrared (FT-IR) spectra (40 0 0–40 0 cm−1 ) were on a Spectrum One FTIR spectrometer (PerkinElmer, USA) using the KBr disk method with a ratio sample/KBr of 1:100 by mass. The morphology and composition of the synthesized composites were analyzed with field emission scanning electron microscope (FE-SEM, CARL ZEISS- AURIGA 60 microscope, Jena, Germany), which was equipped with an energy-dispersive X-ray analyzer (EDX). The magnetic property was analyzed by using a vibrating sample magnetometer (VSM) model MDKFD (Danesh Pajohan Kavir Co., Kashan, Iran). The magnetic separation was done by a super magnet with 7,0 0 0 Gs magnetic field (Nd–Fe–B, 10 × 5 × 4 cm). 2.2. Preparation of MHNTs@MnO2 nanocomposite The MHNTs was prepared according to a previously reported [12]. In brief, 0.5 g HNTs was dispersed into a 200 mL deionized water and then added to 1.165 g of FeCl3 ·6H2 O and 0.6 g of FeSO4 ·7H2 O at 60 °C under N2 . Afterward, ∼20 mL NH4 OH solution (8 mol/L) was added dropwise with vigorously stirring to precipitate the surface-immobilized Fe2+ and Fe3+ ions. The pH of the final mixtures was maintained at ∼11 to ensure the complete transformation of iron ions to iron oxides. The black mixture product was aged at 70 °C for 4 h, separated by a magnet and then washed with deionized water repeatedly. The obtained MHNTs composite was dried at 70 ºC for 12 h. The MHNTs@MnO2 nanocomposite was synthesized by hydrothermal technique. Briefly, 4.2 g of MHNTs was dispersed in 60 mL of deionized water, and 4.424 g of ammonium persulfate and 3.7 g of potassium permanganate were then added under stirring. The mixed solution was poured into a 250 mL of Teflon-lined stainless-steel autoclave, which was heated at 105 °C for 12 h. After cooling to room temperature, the precipitate was collected by magnetic separation, followed by repeated washing with deionized water, and then drying at 60 °C for 24 h.
To study the effect of important parameters on the adsorptive removal of lead(II) ions, batch experiments were carried out. For each experimental run, 20 mL of Pb(II) solution at specified concentration and pH was mixed completely with adsorbent for 60 min using an incubator shaker at 150 rpm. After adsorption, the MHNTs@MnO2 nanocomposite was separated by a magnet and the eluent was transferred into a test tube for the determination of residual lead concentration by flame atomic absorption spectrometry (FAAS). The removal efficiency was calculated using the following equation:
Co − Ce × 100 Co
(1)
where Co and Ce are the initial and final concentrations (mg/L) of Pb(II) ions in solution, respectively. All experiments were performed in triplicate, and the mean of the three measures are taken for all calculations. The relative errors of the data were about 3%.
3. Results and discussion 3.1. Characterization The FT-IR spectra of HNTs, MHNTs and MHNTs@MnO2 are shown in Fig. 1. Absorption peaks appearing at 3697 cm−1 and 3624 cm−1 in the FT-IR spectrum (Fig. 1(a)) are attributed to the stretching vibrations of hydroxyl groups at the surface of bare HNTs [12]. The peak at 1662 cm−1 is assigned to O–H stretching vibration of adsorbed water [17]. The band at 1034 cm−1 is related to the stretching mode of apical Si-O-Si. The peak at 911 cm−1 arises from O–H deformation vibration of inner Al–O–H groups. The peak at 538 cm−1 can be assigned to Al–O–Si deformation vibration and the band at 466 cm−1 is related to O–H bending vibration. The absorption peak at 797 cm−1 can also be attributed to Si–O symmetric stretching vibration. The peaks mentioned above all appeared in FT-IR of MHNTs (Fig. 1(b)), in which the slightly broadened band at 3442 cm−1 was due to stretching vibrations of hydroxyl groups from iron oxide [18]. The FT-IR spectrum of MHNTs@MnO2 (Fig. 1 (C)) show two new absorption peaks at 601 and 1441 cm−1 , the former is assignable to the Mn–O bending vibration [19], whereas the latter was attributable to the bending vibration of –OH group attached to the Mn atoms [20]. The SEM photographs of the surface of the MHNTs and MHNTs@MnO2 are shown in Fig. 2. It is evident that the Fe3 O4 nanoparticles slightly modified the HNTs structure and made the nanotubes surface rougher (Fig. 2(a)). The attachment could be related to the structure of HNTs, such as the large pore volume, large surface area and hydroxyl-containing surface, which enabled metal ions to access and adsorb on the surface easily [21]. As can be seen in Fig. 2(b), there are a large number of wire-like MnO2 nanoparticles randomly deposited on the surface of the MHNTs. The EDX spectrum of the MHNTs@MnO2 nanocomposite (Fig. 2(c)) also confirms the presence of the elements Fe, Al, Si, Mn and oxygen in the prepared nanocomposite. The magnetic hysteresis loops of MHNTs and MHNTs@MnO2 at 298 K are illustrated in Fig. 3. The saturation magnetization was found to be 25.1 emu/g for MHNTs@MnO2 , less than the MHNTs nanocomposite (27.8 emu/g1 ).The saturation magnetization of the particle can be affected by their structure, such as size and crystallinity [22]. The high saturated magnetization of MHNTs@MnO2 directly demonstrated the strong magnetic sensitivity of the magnetic nanocomposite.
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Fig. 1. FT-IR spectra of (a) HNTs, (b) MHNTs and (c) MHNTs@MnO2 .
3.2. Effect of pH To determine the optimal pH, the effect of pH on the removal of Pb(II) ions was studied over the range of 2.0–8.0 and the result are shown in Fig. 4a. The removal efficiency for Pb(II) increased with pH from 2.0 to 5.5, while sorbents showed a high efficiency in the range of pH 5.5–8.0 due to protonation weakened and the both surface of MHNTs and MHNTs@MnO2 became less positively
charged. At pH values lower than 8.0, lead ions exist in the soluble forms Pb2+ and Pb(OH)+ [23], and the removal of lead ions from a solution is mainly due to the adsorption on the surface of MHNTs and MHNTs@MnO2 nanocomposites. In this study, pH 6.0 was selected as the optimum pH in subsequent work. In order to evaluate the stability of MHNTs@MnO2 regarding iron and manganese leaching, leaching test was carried out in the pH range between 2.0 and 8.0. Fig. 4b shows iron and manganese
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Fig. 2. FE-SEM images of (a) MHNTs and (b) MHNTs@MnO2 , (c) EDX spectrum of MHNTs@MnO2 .
Fig. 4. (a) Effect of pH on the removal efficiency of MHNTs and MHNTs@MnO2 for Pb(II) ions: concentration of Pb(II) = 50 mg/L, sorbents = 0.035 g, time = 60 min, temperature = 25 °C. (b) Leached iron and manganese concentration at different pH values. The error bars were calculated from the standard deviation of the three replicate samples per experiment.
3.3. Effect of adsorbent dosage The adsorption of Pb(II) on MHNTs and MHNTs@MnO2 nanocomposites was evaluated by changing the dosage of adsorbent range of 0.02 to 0.05 g. As can be seen in Fig. 5, the removal efficiency did not increase linearly with the increase in the sorbents amount. As reported [24], with the increase in sorbent dosage the removal efficiency also increases up to 98.0% at 0.035 g for 50 mg/L Pb(II) solution. After that, even though the adsorbent dosage increases in the system, because of the unavailability of the lead ions, the removal efficiency remains constant. Fig. 3. VSM magnetization curves of MHNTs and MHNTs@MnO2 .
leaching as a function of pH for Pb(II) removal by MHNTs@MnO2 adsorbent. Leaching of Fe and Mn was insignificant at pH > 4.0 and increased considerably at pH < 4.0. According to these results, the prepared sorbent is unstable at lower pH due to higher leaching of iron and manganese ions. Hence, a pH of 6.0 was chosen for further experiments.
3.4. Effect of shaking time The effect of shaking time on the amount of Pb(II) ions adsorbed on MHNTs and MHNTs@MnO2 was investigated in the range of 5–200 min using the optimum dosage of sorbents (0.035 g) in 20 mL of 50 mg/L lead(II). As can be seen in Fig. 6, the removal efficiency proceeds through a two-stage process, involving a rapid initial adsorption of lead(II) on the sorbent surface followed by a second stage with a much slower adsorption. It is evident that most of the lead(II) ions are removed within 60 min for both
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Fig. 5. Effect of adsorbent dosage on the Pb(II) removal efficiency: concentration of Pb(II) = 50 mg/L, pH = 6.0, time = 60 min, temperature = 25 °C.
Fig. 7. Adsorption kinetics: (a) Pseudo- first order and (b) Pseudo- second order models for adsorption of Pb(II) on the MHNTs and MHNTs@MnO2 .
Fig. 6. Effect of shaking time on the removal efficiency of MHNTs and MHNTs@MnO2 : concentration of Pb(II) = 50 mg/L, pH = 6.0, sorbents = 0.035 g, temperature = 25 °C.
adsorbents and then the curve levels off after this period, corresponding to the saturation of the adsorbents. The fast initial removal rate is probably due to the fast diffusion of Pb(II) from the solution onto the external surface of composites. As the sites are gradually occupied, the adsorbed Pb(II) ions tend to be transported from the bulk phase to the actual sorption sites (i.e., inner-sphere pores of magnetic composites). Such a slow diffusion process will decrease the sorption rate of Pb(II) at later stages. Accordingly, the reaction time of 60 min was enough for further experiments.
3.5. Adsorption kinetic The kinetic model of solute adsorption at solid/solution interfaces is usually complex. Two of the most widely used kinetic models, i.e., pseudo-first-order [25] equation, and pseudo-secondorder [26] equation were used to research the adsorption kinetic behavior of Pb(II) onto magnetic adsorbents. The consistency between the experimental and the model-predicted data was investigated by calculating correlation coefficients (R2 values closer to 1 means more applicability of the model). The formula of Lagergren
pseudo-first-order model is given as follows:
ln (qe − qt ) = lnqe − k1 t
(2)
where qe and qt (mg/g) are the adsorption capacities at equilibrium and at time t, respectively, and k1 is the rate constant of the pseudo-first-order adsorption (min−1 ). Using this well-known equation, the values of k1 and qe were calculated from the slope and intercept of the plot of log(qe − qt ) versus t, respectively [27]. The corresponding kinetic parameters from both models are listed in Table 1. The correlation coefficients (R2 ) for the pseudo-firstorder model are relatively low (Fig. 7(a)), and the calculated qe values (qe,cal ) from the pseudo-first-order model do not agree with the experimental data (qe,exp ), suggesting that the lead(II) adsorption on the composites cannot be explained a pseudo-first-order model. Therefore, it is necessary to fit the experimental data to another model. The adsorption may be described by pseudo-secondorder kinetic model as follows:
t 1 1 = + t qt qe k2 q2e
(3)
where k2 is the equilibrium rate constant of pseudo-second-order adsorption (g/mg min). The slope and intercept of the plot of t/qt versus t were used to calculate the second-order rate constant, k2 (Fig. 7(b)). The pseudo-second order rate constants, k2 , was obtained as 0.0094 g/mg min and the equilibrium adsorption capacity was obtained as 40.0 mg/g. The linear correlation coefficient
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D. Afzali, M. Fayazi / Journal of the Taiwan Institute of Chemical Engineers 000 (2016) 1–9 Table 1 Adsorption kinetics parameters for Pb(II). Sorbent
MHNTs@MnO2 MHNTs
Pseudo-first-order
Pseudo-second-order
qe,cal (mg/g)
qe,exp (mg/g)
k1 (min−1 )
R2
qe,cal (mg/g)
k2 (g/mg min)
R2
11.66 1.86
39.1 10.99
0.0456 0.0587
0.9741 0.9874
40.0 11.12
0.0094 0.0675
0.9999 1
Fig. 8. Adsorption isotherm of Pb(II) on the MHNTs and MHNTs@MnO2 .
value (R2 ) for the pseudo-second-order adsorption model has high value (>99%) for adsorbents. The kinetic model of system showed that pseudo-second order kinetic model had best fit, suggesting the rate-limiting step of lead(II) onto MHNTs and MHNTs@MnO2 sorbents could be chemical sorption or chemisorptions [28]. 3.6. Adsorption isotherm Isotherms studies can describe how the adsorbate interact with an adsorbent, affording the most important parameter for designing a desired adsorption system [29]. The adsorption isotherm of the prepared sorbents at different initial concentrations was investigated, and the results are given in Fig. 8. It is obvious that the adsorbtion was dependent on the initial Pb(II) concentration since the increase in the initial concentration increased the amount of the lead adsorbed on the both nanocomposite. The adsorption equilibrium data were analyzed by the well-known Langmuir and Freundlich isotherm models. The Langmuir sorption isotherm is often used to describe adsorption of a solute from a liquid solution [30]. The Langmuir equation can be expressed as:
Ce 1 1 = + Ce qe kl qm qm
(4)
where Ce is the equilibrium concentration of the lead(II) solution (mg/L), qe is the adsorption capacity at equilibrium (mg/g), kl is the constant related to free energy of adsorption (L/mg), and qm is the maximum adsorption capacity at monolayer coverage (mg/g). The Freundlich isotherm is an empirical equation that assumes heterogeneous adsorbent surface with its adsorption sites at varying energy levels [31]. The corresponding equation is commonly represented by:
l n qe = ln k f +
1 l n Ce n
(5)
Kf (mg/g (L/mg)1/n ) and n are the Freundlich constants characteristics of the system, indicating the adsorption capacity and the adsorption intensity, respectively.
Fig. 9. (a) Langmuir and (b) Freundlich plots for the adsorption of Pb(II) ions.
Langmuir and Freundlich isotherm models for lead(II) removal on proposed adsorbent using linear regression are shown in Fig. 9. The Langmuir and Freundlich constants and the calculated coefficients are listed in Table 2. According to the correlation coefficients (R2 ) of isotherms, the Langmuir isotherm model was more favorable rather than Freundlich isotherm model. Moreover, the theoretical maximum adsorption capacity (59.9 mg/g) agreed very well with their experimental values (59.2 mg/g) in the case of Langmuir model, indicating a good Langmuir isotherms fit to the experimental data. The essential features and the feasibility of the Langmuir isotherm can be expressed in terms of a dimensionless constant separation factor (RL ) given by the following relationship [32]:
RL =
1 1 + kl C0
(6)
Please cite this article as: D. Afzali, M. Fayazi, Deposition of MnO2 nanoparticles on the magnetic halloysite nanotubes by hydrothermal method for lead(II) removal from aqueous solutions, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.02.025
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7
Table 2 Isotherm parameters for lead(II) adsorption. Sorbent
MHNTs@MnO2 MHNTs
Langmuir isotherm
Freundlich isotherm
Kl (L/mg)
qm (mg/g)
RL
R2
Kf (mg/g (L/mg)1/n )
n
R2
0.1327 0.0148
59.9 23.1
0.029 0.57
0.9972 0.9939
34.6 1.93
11.24 2.46
0.9266 0.9704
Table 3 Thermodynamic parameters at different temperatures.
H (kJ/mol)
98.21
S (kJ/mol K)
0.349
G (kJ/mol) 298K
313K
328K
–5.72
–9.62
–17.0
Table 4 Influences of EDTA concentration on the desorption capacity and desorption efficiency of Pb(II) ions.
Fig. 10. Van’t Hoff plot for the adsorption of Pb(II) ions on the MHNTs@MnO2 nanocomposite.
where kl (L/mg) is the Langmuir constant and C0 (mg/L) is the initial concentration in the liquid phase. The value of RL indicates the shape of the isotherm to be either unfavorable (RL >1), linear (RL =1), favorable (0
kd = qe /Ce
(7)
G0 = −RT lnkd ln kd =
S R
0
−
H RT
(8) 0
(9)
where kd is the distribution coefficient, T (k) is the temperature, and R (8.314 J/mol K) is the gas constant. The values of (H 0 ) and (S0 ) were calculated from the slope and intercept of the Van’t Hoff linear plot of ln (kd ) against 1/T (Fig. 10). Based on Table 3, the negative values of G0 at all of the experimental temperatures indicated the spontaneous nature of Pb(II)
Concentration of EDTA (M)
Desorption capacity (mg/g)
Desorption efficiency (%)
0.01 0.025 0.05 0.75 0.1
12.6 28.1 36.2 44.5 56.6
21.3 47.5 61.1 75.2 95.6
adsorption by MHNTs@MnO2 nanocomposite. On the other hand, the positive value of H 0 (Table 3) suggested that the adsorption process is endothermic. The positive value of S0 implied the increase in randomness at the solid-solution interface and a good affinity of Pb(II) ions with MHNTs@MnO2 . 3.8. Desorption and regeneration From a practical point of view, the desorption and recyclability of a sorbent are two key evaluation factors [35]. The desorption properties of the MHNTs@MnO2 nanocomposite saturated with adsorbed Pb(II) ions were investigated. To study the desorption, different concentrations of EDTA were used as desorbing media. The desorption capacity and desorption efficiency were calculated according to the following equations [36]:
Desorption capacity =
CaVa m
Desorption efficiency =
Ca Va /m × 100 qe
(10)
(11)
where Ca , Va , m and qe represented the Pb(II) concentration (mg/L) in aqueous solution, the volume of EDTA solution (L), the mass of the adsorbent (g) and the amount of Pb(II) adsorbed (mg/g) at equilibrium, respectively. The effect of concentration of EDTA on the desorption capacity and desorption efficiency of Pb(II) from the representative Pb(II)loaded MHNTs@MnO2 nanocomposite is shown in Table 4. As can be seen, the maximum desorption efficiency was 95.6% using 0.1 N EDTA solution. For evaluating the reuse value of the MHNTs@MnO2 , 0.035 g of the used sorbent was agitated with 20 mL of 0.1 N EDTA solution for 60 min. The sample was separated from the solution by a magnet, washed with distilled water three times, and then dried in an oven at 70 °C for reuse study. As shown in Fig. 11, after five repeated adsorption–desorption cycles, the adsorption capacity of MHNTs@MnO2 sorbent was decreased about 10%. The results showed that the MHNTs@MnO2 nanocomposite could repeatedly be used without considerable loss of adsorption capacity.
Please cite this article as: D. Afzali, M. Fayazi, Deposition of MnO2 nanoparticles on the magnetic halloysite nanotubes by hydrothermal method for lead(II) removal from aqueous solutions, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.02.025
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Fig. 11. Recycling of MHNTs@MnO2 adsorbent for the removal of Pb(II) ions. Table 5 Comparison of various adsorbents for removal of Pb(II) ions. Adsorbents
Adsorption capacity (mg/g)
Best fit isotherm
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Acknowledgments
A comparison between the performance of proposed sorbent with those reported previously is shown in Table 5. As can be seen from the Table, the MHNTs@MnO2 in this work has a higher adsorption capacity than most other adsorbents reported in the literature [2,37-45], suggesting that it may be a potential application for the elimination of Pb(II) ion from aqueous solution.
We gratefully acknowledge the financial support provided for this project (No. 4618) by Institute of Science and High Technology and Environmental Sciences, Graduate University of Advanced Technology, Kerman, Iran.
References 4. Conclusion A novel MHNTs@MnO2 nanocomposite was synthesized by chemical precipitation and hydrothermal methods. The synthesized composite material was then characterized by FE-SEM, VSM, FTIR and EDX analysis. The resulting composite combined the both features of Fe3 O4 and MnO2 , and thus exhibited extraordinary adsorption capacity and fast removal rate for lead(II) ions. The adsorption isotherms and kinetics were investigated and indicated that the equilibrium and kinetics adsorption were well-modeled by the Langmuir isotherm model and the pseudo-second-order kinetic model, respectively. It is concluded that the proposed could be utilized as a novel magnetically separable and efficient adsorbent for the removal of Pb(II) from aqueous solution.
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Please cite this article as: D. Afzali, M. Fayazi, Deposition of MnO2 nanoparticles on the magnetic halloysite nanotubes by hydrothermal method for lead(II) removal from aqueous solutions, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.02.025