DOI:
10.1039/C6RA22710D
(Paper)
RSC Adv., 2016,
6, 113492-113502
Facile synthesis of methyl propylaminopropanoate functionalized magnetic nanoparticles for removal of acid red 114 from aqueous solution
Received
11th September 2016
, Accepted 28th November 2016
First published on 29th November 2016
Abstract
Methyl propylaminopropanoate-coated Fe3O4@SiO2 nanoparticles (Fe3O4@SiO2–MPAP NPs) was prepared as a novel adsorbent and used for the removal of acid red 114 (AR-114) from aqueous solution through hydrogen bonding interactions between the sulfonate groups of the dye and the ester carbonyl groups of the adsorbent. Characterization of the obtained new adsorbent was achieved by FT-IR, XRD and TEM. According to the experimental results, about 95.8% of AR-114 was removed from aqueous solution at the adsorbent amount of 0.24 g L−1 at pH = 2 in 60 min. The adsorption kinetics and isotherm results demonstrate that the kinetics and equilibrium adsorptions can be well-described by the pseudo-second-order kinetics and Langmuir models, respectively. Thermodynamics studies depicted that the adsorption of hazardous dye AR-114 was spontaneous and endothermic with randomness of the process. Furthermore, the Fe3O4@SiO2–MPAP NPs could be simply recovered by an external magnet and it exhibited striking recyclability and reusability for five cycles. Such functional nanoparticles are appropriate adsorbents for removal of dyes containing sulfonate groups from aqueous solutions.
1. Introduction
Synthetic dyes, with complex aromatic structures, are present in wastewater streams from many industries including textile, leather, food, cosmetics, electronics, paper, rubber, plastic, printing and pharmaceutical.1–3 These dyes and their degradation species are toxic in nature and have potentially carcinogenic and mutagenic effects on human beings, micro-organisms and aquatic life.4–10 A wide range of physical and chemical procedures such as electrochemical degradation, ozonation, oxidation, photocatalysis, ion-exchange, membrane separation, adsorption and biological process have been developed for the treatment of industrial wastewater containing dyes before they are delivered to the environment.11–30 Among these possible techniques for water treatment, adsorption has been considered as a more feasible process for most pollutant removal due to its low cost, simplicity of design, ease of operation without harmful residues and the possibility of regeneration of the adsorbent.31–33 Currently, a variety of potential adsorbents have been employed for the removal a large amount of molecule dyes from water.34–36
Magnetic nanoparticles (M-NPs) have received considerable attention as a new adsorbent with large surface area and small diffusion resistance for the adsorb contaminants such as metals, dyes and gases.37–40 Furthermore, M-NPs can be separated easier and faster with an external magnet without centrifugation or filtration.41 In the current study, for the first time the use of Fe3O4@SiO2 coated with new ester-terminal ligands as a sorbent for removal of acid red 114 (AR-114) was studied. Recently, ester linkages have been demonstrated for effective removal of phenolic compounds from wastewater mainly through the hydrogen bonding interaction between the hydroxyl group of phenolic compounds and the carbonyl group of ester moiety.42,43 Hydrogen bonds are principle noncovalent interactions to construct supramolecular nanoporous architectures since they are highly selective and directional.44 In addition, the hydrogen bonded materials have appealing properties and have promise in fields such as filtration, separation, adsorption, ion conductivity, enantioselective separation and molecular recognition.45
Acid red 114 (AR-114), as a bis-azo biphenyl dye, with carcinogenic effects has been widely used in large quantities in dyeing of woven fabrics and wool and cotton textiles and cause many environmental troubles.46,47 The azo dyes with an aromatic structure, are resistant to aerobic digestion, and are stable to oxidizing agents. The aim of this study is to modify the surface of Fe3O4@SiO2 NPs with methyl propylaminopropanoate (MPAP) as a new magnetic adsorbent for significant removal of AR-114 from aqueous solutions through the hydrogen bonding interaction between the sulfonate groups of AR-114 and the ester carbonyl groups of adsorbent. The effect of principle factors including pH, contact time, the sorbent amount and dye concentration were studied. The kinetic and thermodynamic parameters of adsorption were also analyzed. The results obtained could be useful in further application of modified NPs, and effective way for treatment of dye containing wastewaters.
2. Results and discussion
In continuation to our prior work,9,10,48 herein, the modified magnetic NPs Fe3O4@SiO2–MPAP was synthesized by the steps shown in Scheme 1. Firstly, Fe3O4 NPs were prepared by co-precipitation of Fe2+ and Fe3+ ions in basic solution49 and then in order to avoid possible aggregation or oxidation of the Fe3O4 NPs surfaces, a layer of SiO2 using sol–gel process, through the hydrolysis of TEOS was coated on Fe3O4 NPs surfaces and successfully, Fe3O4@SiO2 core–shell microspheres were prepared.50 Subsequently, Fe3O4@SiO2 core–shell was reacted with 3-aminopropyltriethoxysilane (APTES) as a spacer to obtain amino-functionalized Fe3O4@SiO2–APTES.50 Finally, methyl propylaminopropanoate-modified Fe3O4@SiO2 (Fe3O4@SiO2–MPAP) was prepared by the reaction of methyl acrylate (MA) with aminopropyl grafted Fe3O4@SiO2 during the double Michael addition in EtOH solution and the success of this immobilization was monitored with FT-IR, XRD and TEM.
 |
| Scheme 1 (a) Preparation of Fe3O4@SiO2–MPAP NPs and (b) main interactions between Fe3O4@SiO2–MPAP NPs and AR-114. | |
2.1. Fourier transform infrared spectroscopy
The FT-IR spectra of Fe3O4 (black line), Fe3O4@SiO2 (violet line), Fe3O4@SiO2–APTES (red line) and Fe3O4@SiO2–MPAP (blue line) are shown in Fig. 1. The typical absorption peak for Fe3O4 at 570 cm−1 is related to the stretching vibration of Fe–O bond and two bonds at 3423 and 1633 cm−1 are attributed to the symmetric and asymmetric stretch and bending vibrations of O–H, respectively which is attached to the surface iron atoms. For Fe3O4@SiO2, the strong broad band at 1089 cm−1 with a shoulder at 1209 cm−1 are assigned to the asymmetric stretching bonds of Si–O–Si in SiO2. The weaker bands at 796, 466 and 952 cm−1 correspond to the Si–O–Si symmetric stretch, Si–O–Si or O–Si–O bending modes and Si–O symmetric stretch, respectively. The band at 567 cm−1 is an indication of the presence of Si–O–Fe. In the spectrum of Fe3O4@SiO2–APTES, the characteristic absorptions band at 1000, 3417 and 2869–2906 cm−1 are corresponded to C–N, N–H and C–H stretching modes of the alkyl chain, respectively and the N–H bending mode appeared at 1627 cm−1 that confirm the successful coating of APTES on Fe3O4@SiO2. In the FT-IR spectrum of the Fe3O4@SiO2–MPAP, the absorption band at 1722 and 1012 cm−1 was attributed to the C
O and C–O stretching of the ester groups. Also, the absence of the N–H stretching and bending modes in the spectrum, clearly show that the Michael additions proceed to completion and two ester groups have been successfully coated on the surface of Fe3O4@SiO2 NPs.
 |
| Fig. 1 The FT-IR spectra of Fe3O4 (black line), Fe3O4@SiO2 (violet line), Fe3O4@SiO2–APTES (red line) and Fe3O4@SiO2–MPAP (blue line). | |
2.2. X-ray diffraction
The X-ray diffraction (XRD) patterns of Fe3O4 (black line), Fe3O4@SiO2 (red line), Fe3O4@SiO2–APTES (blue line) and Fe3O4@SiO2–MPAP (green line) are shown in Fig. 2. The sharp diffraction peaks with 2θ at 30.3°, 35.6°, 43.3°, 57.2°, and 62.5° are observed, which indicate that the Fe3O4 particles have highly crystalline cubic spinel structure of the magnetite and the same sets of characteristic peaks were also observed for Fe3O4@SiO2, Fe3O4@SiO2–APTES and Fe3O4@SiO2–MPAP indicating the stability of the crystalline phase of Fe3O4 NPs during silica coating and surface amino- and ester-functionalization.51
 |
| Fig. 2 The XRD patterns of Fe3O4 (black line), Fe3O4@SiO2 (red line), Fe3O4@SiO2–APTES (blue line) and Fe3O4@SiO2–MPAP (green line). | |
The average crystallite size D was determined by the Scherrer formula, D = Kλ/(β
cos
θ), where λ = 1.54 Å is the wavelength of Cu-Kα radiation used, β is the full width at half-maximum (FWHM) intensity of the diffraction line, θ is the Bragg angle for the measured hkl peak and K is a constant equal to 0.94. The particle sizes of the magnetite calculated using the Scherrer equation were 12.1, 13.8, 15.7 and 18.0 nm for Fe3O4, Fe3O4@SiO2, Fe3O4@SiO2–APTES and Fe3O4@SiO2–MPAP, respectively.52 These values are in good agreement with the TEM images.
2.3. Transmission electron microscopy image
Fig. 3a–d show the transmission electron microscopy (TEM) images of the Fe3O4@SiO2–MPAP and Fe3O4 microspheres. Fig. 3a–c show that the MPAP (with light color) is immobilized on the surface of Fe3O4@SiO2 NPs (with dark color), obviously. The pure Fe3O4 NPs (Fig. 3d) appear to be almost spherical in shape with a diameter range about <15 nm. The TEM analysis demonstrates that the presence of SiO2–MPAP shell increases the size of magnetite NPs.30 The synthesized Fe3O4@SiO2–MPAP NPs showed a spherical shape with an average diameter of about <30 nm, however, the NPs tended to aggregate to large particle.
 |
| Fig. 3 The TEM micrographs of synthesized (a and b) Fe3O4@SiO2–MPAP NPs, (c) Fe3O4@SiO2–MPAP core–shell and (d) Fe3O4 NPs. | |
2.4. Effect of pH on adsorption
The pH of the aqueous solution is the very crucial parameter for its effect on the adsorption property of adsorbent and adsorbate. The effect of initial pH on the AR-114 adsorption by Fe3O4@SiO2–MPAP surfaces at different values, ranging 2–11 and with a constant time 120 min is shown in Fig. 4. The initial concentration of dye and adsorbent dosage were kept fixed at 20 mg L−1 and 0.2 g L−1, respectively. As was expected, the results demonstrated that the uptake amount strongly depends on solution pH. Maximum removal efficiency of AR-114 onto Fe3O4@SiO2–MPAP was observed at the initial pH = 2 (86.25%), and decreased dramatically when solution pH increased and turned to alkaline, which could be due to the ionization of sulfonic groups of AR-114 at pH above the pKa. The pKa value of benzenesulfonic acid is 2.1 at 298 K and change to sulfonate anion at pH > pKa with hydrophilic nature. Therefore, sulfonate groups of AR-114 existed as molecular form (sulfonic form) in the range of pH < pKa and adsorbed effectively onto the Fe3O4@SiO2–MPAP surface through van der Waals interactions. In addition, the intermolecular hydrogen bonds occurred between hydroxyl groups of adsorbate and the carbonyl and methoxy groups of adsorbent as an adsorption driving force to catch the AR-114 molecules from water (Scheme 1b).42 Therefore, pH = 2 was chosen as the optimum pH for all further adsorption experiments. In order to obtain information about the surface charge of the adsorbent, the point of zero charge (pHZPC) was determined. The plot of pH difference versus initial pH of the solutions is illustrated in Fig. 5, which shown the point of zero charge is around 6. This means that at pH values below 6, the Fe3O4@SiO2–MPAP surface has a net positive charge, while at pH greater than 6, the surface has a net negative charge and the electrostatic repulsive force exists between adsorbent and adsorbate. Hence, the acidic pH facilitate the adsorption of AR-114 onto Fe3O4@SiO2–MPAP surface and maximum removal percentage and adsorption capacity take place at pH = 2.
 |
| Fig. 4 The effect of pH on the adsorption of AR-114 on Fe3O4@SiO2–MPAP in different time interval ([AR-114]0 = 20 mg L−1 and adsorbent dosage = 0.2 g L−1). | |
 |
| Fig. 5 Determination of the pH of point of zero charge (pHZPC). | |
2.5. Effect of adsorbent dose on adsorption
The adsorption of dye on Fe3O4@SiO2–MPAP was investigated by changing the quantity of adsorbent range of 0.1–0.3 g L−1, with the dye concentration of 20 mg L−1, room temperature (25 ± 1 °C) and pH of 2.0 for different time intervals (0–120 min). The results in Fig. 6, show that with the increase in adsorbent dosage from 0.1–0.3 g L−1, the percentage adsorption increases from 53.71 to 97.34% over the entire contact time due to increased active sites. Furthermore, the rate of removal of AR-114 at all dosages is primarily rapid in the first stage of contact time and then it is gradually slowed until reactions reach equilibrium at about 60 min. The rapid adsorption at the initial contact time is attributed to the large quantity of free active sites on the Fe3O4@SiO2–MPAP surface and convenient accessibility of them for AR-114 molecules.53 Since the removal efficiency of AR-114 does not significant difference between dosage 0.24 and 0.3 g L−1, further bath adsorption experiments were carried out at 0.24 g L−1. In fact, the removal efficiency enhanced from 55.24 to 95.75% by increasing the contact time from 0.5 to 60 min at pH = 2 and adsorbent dosage equal to 0.24 g L−1.
 |
| Fig. 6 The effect of adsorbent dose on the adsorption of AR-114 on Fe3O4@SiO2–MPAP in different time interval ([AR-114]0 = 20 mg L−1 and pH = 2). | |
2.6. Effect of contact time on adsorption
To know the equilibration time for maximum adsorption and the kinetics of the adsorption process, the adsorption of AR-114 on the Fe3O4@SiO2–MPAP surface was studied as a function contact time as shows in Fig. 7. The sorption of AR-114 increases fast in the first and almost 90% adsorption was completed within 10 min on the Fe3O4@SiO2–MPAP surface, and then slowed down until the sorption process reaches equilibrium after 60 min. In general, the removal process is quite fast and 30 min is enough to achieve equilibrium. In this section, the concentration of AR-114 was 20 mg L−1, pH = 2, temperature 298 K and adsorbent dosage 0.24 g L−1. The dye adsorption processes are shown in the inset of Fig. 7.
 |
| Fig. 7 The effect of contact time on the adsorption of AR-114 on Fe3O4@SiO2–MPAP in different contact time interval. The inset figures show photographs of aqueous solutions of AR-114 before (0 min vortex) and after adsorption (2–15 min vortex). ([AR-114]0 = 20 mg L−1, adsorbent dosage = 0.24 g L−1 and pH = 2). | |
2.7. Effect of initial dye concentration on adsorption
The adsorbate initial concentration acts as an important driving force to overcome the mass transfer resistance of dye between the aqueous and the solid phases.54 The effects of initial dye concentrations on the rate of adsorption by Fe3O4@SiO2–MPAP were studied in the range from 10 to 100 mg L−1 (Fig. 8). By increasing the initial dye concentration the percentage of dye removal decreased, these results suggest that the vacant sites on the adsorbent are saturated by dye molecules.
 |
| Fig. 8 The effect of initial dye concentrations on the adsorption of AR-114 on Fe3O4@SiO2–MPAP in different contact time interval (adsorbent dosage = 0.24 g L−1 and pH = 2). | |
2.8. Adsorption kinetics
Adsorption kinetic experiments were analyzed at different AR-114 concentration = 10, 20, 40, 60, 80 and 100 mg L−1, constant adsorbent dosage = 0.24 g L−1 and pH = 2. The pseudo-first-order, pseudo-second-order and intra-particle-diffusion models were applied in order to find an efficient model for the description of adsorption mechanism. The pseudo-first-order kinetic model is expressed as follow:55 |
ln(qe − qt) = ln(qe) − k1t
| (1) |
where qe and qt are the amount of dye adsorbed at equilibrium (mg g−1) and amount of dye adsorbed at time t (mg g−1), respectively; k1 (min−1) is the equilibrium rate constants of the pseudo-first-order adsorption. Using this equation, the values of k1 and qe were calculated from the slope and intercept of the plot of ln(qe − qt) versus t, respectively.56
The pseudo-second-order kinetic model can be expressed as follow:
|
 | (2) |
where
k2 is the equilibrium rate constant of pseudo-second-order adsorption (g mg
−1 min
−1). The slope and intercept of the plot of
versus t were used to calculate the second-order rate constant
k2.
The possibility of intra-particle diffusion resistance affecting adsorption was investigated using the intra-particle diffusion model as:57
where
kp (mg g
−1 min
−1/2) is the intra-particle diffusion rate constant which can be evaluated from the slope of the linear plot of
qt versus t1/2, and
C (mg g
−1) is intercept. Values of
C give an idea about the thickness of the boundary layer: the larger the intercept, the greater the boundary layer effect (
Fig. 9).
 |
| Fig. 9 The linear plots of pseudo-second-order model on the AR-114 removal by Fe3O4@SiO2–MPAP in different time and concentration (pH = 2 and adsorbent dosage = 0.24 g L−1). | |
The best-fit model was selected based on the linear regression correlation coefficient R2 values. The kinetic parameters for the removal AR-114 at different initial concentrations by three models are summarized in Table 1. The results show that first-order kinetic and intra-particle diffusion models are not suitable for the present adsorption system due to low correlation coefficients R2. For the first-order kinetic model very difference exists between qe,exp and qe,cal that demonstrate a poor pseudo-first-order fit to the experimental data. Therefore, it is necessary to fit the experimental data to another model. The kinetic data for AR-114 adsorption showed the best fitting (R2 = 0.9999) with the pseudo-second-order model and the calculated qe values also agree very well with the experimental data. Moreover, when the initial AR-114 concentration increased from 10 to 100 mg L−1, the value of k2 (g mg−1 min−1) and R2 for the pseudo-second-order model were decreased from 0.062 to 0.0071 g mg−1 min−1 and 0.9998 to 0.9996, respectively. Also, qe,cal (mg g−1) increased from 46.08 to 108.7 mg g−1. This result indicated that adsorption data were in agreement with this model. Furthermore, the curve-fitting plots of pseudo-second-order kinetic model gave a straight line, passed through the origin and confirmed the best fit of this kinetic model in the present system (Fig. 9). Based on the kinetic model obtained, a chemisorptions bond takes place during adsorption of AR-114 on the Fe3O4@SiO2–MPAP NPs surface. For the intra-particle diffusion model the value of C was calculated as 67.162 mg g−1 (C ≠ 0), indicating that intra-particle diffusion is not the only rate-limiting step for AR-114 adsorption and the sorption process is rather complex and involves more than one diffusive resistance.
Table 1 Kinetic parameters for AR-114 adsorption onto Fe3O4@SiO2-MPA
Kinetic models |
[AR-114]0 (mg L−1) |
10 |
20 |
40 |
60 |
80 |
100 |
qe,exp (mg g−1) |
46.5 |
83.71 |
104.08 |
102.69 |
98.76 |
108.71 |
![[thin space (1/6-em)]](https://www.rsc.org/images/entities/char_2009.gif) |
Pseudo-first-order |
k1 (min−1) |
0.0144 |
0.0303 |
0.0382 |
0.0656 |
0.0397 |
0.0309 |
qe,cal (mg g−1) |
14.17 |
12.33 |
6.03 |
5.79 |
4.73 |
6.45 |
R2 |
0.4092 |
0.369 |
0.5013 |
0.6382 |
0.5988 |
0.3083 |
![[thin space (1/6-em)]](https://www.rsc.org/images/entities/char_2009.gif) |
Pseudo-second-order |
k2 (g mg−1 min−1) |
0.062 |
0.0253 |
0.0099 |
0.0097 |
0.0081 |
0.0071 |
qe,cal (mg g−1) |
46.08 |
83.33 |
103.09 |
104.17 |
99.01 |
108.7 |
R2 |
0.9998 |
0.9999 |
0.9981 |
0.9999 |
0.9995 |
0.9996 |
![[thin space (1/6-em)]](https://www.rsc.org/images/entities/char_2009.gif) |
Intra-particle diffusion |
kp (mg g−1 min−1/2) |
0.8195 |
1.9076 |
3.1404 |
4.2669 |
3.4093 |
4.1794 |
C0 (mg g−1) |
39.006 |
67.162 |
75.635 |
66.697 |
68.336 |
71.158 |
R2 |
0.4595 |
0.5183 |
0.4704 |
0.6133 |
0.6016 |
0.7457 |
2.9. Adsorption isotherm studies
The evaluation of adsorption isotherms are important for developing a model for interaction of adsorbent–adsorbate and provide inclusive information about the nature of interactions.58,59 The adsorption isotherms of AR-114 on the Fe3O4@SiO2–MPAP with 20 mg L−1 AR-114 as an initial concentration using various adsorbent dosages (0.04–0.24 g L−1) at pH = 2 for 72 h at different temperatures are given in Fig. 10, and the equilibrium adsorption data were evaluated according to the renowned models Langmuir and Freundlich isotherms. The Langmuir isotherm supposes that monolayer adsorption take place at binding sites with homogenous energy levels, without interactions between adsorbed molecules and transmigration of adsorbed molecules onto adsorption surface. The Langmuir equations can be expressed as:60 |
 | (4) |
or |
 | (5) |
where Ce is the equilibrium concentration of the AR-114 solution (mg L−1), qe is the adsorption capacity at equilibrium (mg g−1), kl is the constant related to free energy of adsorption (L mg−1), and qm is the maximum adsorption capacity at monolayer coverage (mg g−1).
 |
| Fig. 10 The (a) Langmuir isotherm and (b) Freundlich isotherm on the AR-114 removal by Fe3O4@SiO2–MPAP at different temperatures (pH = 2, [AR-114]0 = 20 mg L−1, adsorbent dosage = 0.04–0.24 g L−1 and time = 72 h). | |
The Freundlich isotherm is a practical equation that supposes heterogeneous adsorbent surface with its adsorption sites at changeable energy levels.61 The corresponding equations are commonly represented by:
or
|
 | (7) |
kf (mg
1+n g
−1 L
−n) and
n are the Freundlich constants characteristics of the system, indicating the adsorption capacity and the adsorption intensity, respectively. If the value of 1/
n is lower than 1, it indicates a normal Freundlich isotherm; if not, it is indicative of cooperative adsorption.
62
The calculated isotherm parameters from both models are shown in Table 2. The adsorption of AR-114 was fit to the Langmuir isotherm model better than Freundlich with the higher R2 (0.9913). It indicates that the adsorption occurred at specific homogeneous sites within the adsorbent forming monolayer coating of AR-114 at the surface of the absorbent. The Freundlich constant 1/n was smaller than 1, indicating a high adsorption intensity (Table 2). Table 1, also shows that the maximum monolayer adsorption capacity (qm) of AR-114 by Fe3O4@SiO2–MPAP was in 293 K and decreased above it. In this temperature, interaction between the AR-114 and Fe3O4@SiO2–MPAP is probably larger due to the presence of a higher number of potential adsorption sites. However, at high temperatures, the interaction between AR-114 and Fe3O4@SiO2–MPAP is weakened due to decreased of hydrogen bonds and van der Waals interactions.63
Table 2 Adsorption isotherm parameters for AR-114 adsorption on the Fe3O4@SiO2–MPAP
T (K) |
Langmuir isotherm |
Freundlich isotherm |
qm (mg g−1) |
k1 (L mg−1) |
R2 |
RL |
kf (mg1+n g−1 L−n) |
n |
R2 |
293 |
105.2632 |
1.2025 |
0.9913 |
0.0399 |
71.1377 |
7.5075 |
0.7108 |
308 |
70.4225 |
1.5778 |
0.9839 |
0.0307 |
88.5523 |
25.1889 |
0.1927 |
318 |
75.7576 |
2.129 |
0.9741 |
0.023 |
63.6502 |
18.4162 |
0.1815 |
The adsorption isotherm process favorability was also evaluated using the dimensionless separation factor (RL) that were calculated using the following equation:64
|
 | (8) |
The adsorption process can be defined as favorable (0 < RL < 1), unfavorable (1 < RL), linear (RL = 1) and irreversible in nature (RL = 0).65 In this study, the value of RL calculated for the adsorption of AR-114 by Fe3O4@SiO2–MPAP fall between 0 and 1 (RL = 0.0399), therefore, the adsorption of AR-114 onto the adsorbent appears to be a favorable process.
2.10. Thermodynamic studies
Thermodynamics parameters such as standard enthalpy change (ΔH°), standard Gibbs free energy change (ΔG°) and standard entropy change (ΔS) were estimated to check the temperature effects on the adsorption of AR-114 on the Fe3O4@SiO2–MPAP NPs. The thermodynamic parameters can be evaluated with the Van't Hoff equations:66 |
ΔG° = −RT ln K
| (9) |
|
 | (10) |
where K is adsorption equilibrium constant (L mol−1), R the universal gas constant (8.314 J mol−1 K−1) and T the absolute temperature (K).
Thermodynamic experiments were performed at different temperature from 293 to 318 K at constant adsorbent dosage (0.24 g L−1) and at pH = 2. The values of ΔH° and ΔS° was determined by plotting ln
K versus 1/T (Fig. 11), and the ΔG° values were obtained from eqn (11). Table 3 presents the thermodynamic results at various temperatures. The positive value of ΔH° indicates that the adsorption is endothermic in nature. The positive value of ΔS° shows the higher degree of disorder at the solid–solution interface during the adsorption process. Furthermore, the negative value ΔG° confirms the spontaneous nature of sorption and affinity of Fe3O4@SiO2–MPAP for the AR-114 dye and the increase of these values with the temperature increasing indicate a higher adsorption with increase in the temperatures. Since the reaction process is endothermic, this can be attributed to increased surface coverage at higher temperature, expansion and creation of reactive and active sites.
 |
| Fig. 11 The Van't Hoff plot for ln K vs. 1/T ([AR-114]0 = 20 mg L−1, adsorbent dosage = 0.24 g L−1 and pH = 2). | |
Table 3 Thermodynamics parameters of AR-114 adsorption on the Fe3O4@SiO2–MPAP
ΔH° (kJ mol−1) |
ΔS° (J mol−1 K−1) |
ΔG° (kJ mol−1) |
293 K |
308 K |
318 K |
250.31 |
60.42 |
−0.45 |
−1.17 |
−2 |
2.11. Desorption and recycling studies
Several (adsorption–desorption) regeneration cycles with Fe3O4@SiO2–MPAP NPs were performed by 20 mL of NaOH 0.01 M solution, as shown in Fig. 12. In fact, the AR-114 molecules on the surface of Fe3O4@SiO2–MPAP NPs could be replaced with hydroxyl ions of the basic solution in adsorbent washing step. The result showed the recovery values were not significantly decreased and reused for at least five successive removal processes with removal efficiency higher than 80% that display the stability of adsorbent. Fig. 12d shows the TEM image of the Fe3O4@SiO2–MPAP after five adsorption–desorption cycles that indicate the Fe3O4@SiO2–MPAP NPs was not significantly altered during the regeneration process. Also, after five cycles of the desorption–adsorption process, the high magnetic sensitivity of Fe3O4@SiO2–MPAP NPs still retained and was collected from the solution using a magnet of 1.4 T (right inset in Fig. 12a–c). Therefore, the Fe3O4@SiO2–MPAP NPs can be potentially used as a magnetic adsorbent for further dye adsorption from water.
 |
| Fig. 12 The removal efficiency during five cycles of Fe3O4@SiO2–MPAP. The inset figures show aqueous solutions of AR-114 (a) before adsorption, (b) after adsorption, (c) after five cycles of desorption–adsorption and (d) the TEM image of the Fe3O4@SiO2–MPAP after five adsorption–desorption cycles. | |
A detailed understanding of dye mineralization by bio- or chemical degradation is required for the conversion of recovered AR-114 to inorganic compounds such as NO3, CO2 and H2O.67–69 Experiments are planned to further examine the pathways in the complete detoxification of this recovered dye.
2.12. A comparison with other adsorbents
A comparison between results of the performance of Fe3O4@SiO2–MPAP with other adsorbents on removal of dyes in the previous studies is presented in Table 4.24,70–81 As the results show, the absorption capacity of Fe3O4@SiO2–MPAP NPs due to hydrogen bond interaction could be comparable or even better than the other adsorbent. In Fe3O4@SiO2–MPAP NPs compared to Fe3O4 NPs (entry 18), a thin and dense SiO2 layer along with a desired thickness of MPAP shell can protect the iron oxide core from leaching out under acidic conditions.82 Hence, the stability of Fe3O4@SiO2–MPAP NPs increases. It is noticeable that the higher adsorption capacity of SBA-3/PEHA (entry 9) is due to electrostatic interaction and hydrogen bond formation between the surface of the adsorbent and AR-114, simultaneously.24 Additionally, the adsorption capacity of Fe3O4@SiO2–NH2 as a result of electrostatic interaction is lower than Fe3O4@SiO2–MPAP.72 Also, the adsorption capacity of Fe3O4@SiO2 on removal of Congo red (CR) as a AR-114 derivative is lower than Fe3O4@SiO2–MPAP NPs (entry 19).80 These results demonstrate that the hydrogen bond formation plays more important function in AR-114 adsorption than electrostatic interaction. Therefore, Fe3O4@SiO2–MPAP NPs could be considered as an alternative and efficient adsorbent for removing organic compounds with hydrogen bonding ability such as amino-, hydroxyl- and sulfonate-substituted dyes, phenols and amines because of their high adsorption capacities. Furthermore, the magnetic properties of Fe3O4@SiO2–MPAP make it more efficient adsorbent for the removal of contaminant from aqueous solution.
Table 4 Comparison of various adsorbents for the removal of dye
Adsorbents |
Dye |
qm (mg g−1) |
Adsorbent dosage (g L−1) |
Concentration (mg L−1) |
Best fit isotherm |
Ref. |
Acid-activated Eichornia crassipes (an activated plant biomass). Kattamanakku tree leaf powder carbon. Ceiba pentradenta wood waste-activated carbon@H3PO4. Ipomea carnea stem waste@phosphoric acid. Activated pongam seed shells. Activated cotton seed shells. Activated sesame seed shells. Pentaethylene hexamine functionalized SBA-3. Magnetite/silica/pectin nanoparticles. Methyl orange. Eriochrome black T. Magnetite/pectin nanoparticles. Chitosan coated magnetic mesoporous silica nanoparticles. Methylene blue. Cetyltrimethylammonium bromide coated magnetite nanoparticles. Disperse red 167. Disperse blue 183. |
Activated carbon |
AR-114 |
103.73 |
0.25 |
250 |
Toth |
70 |
AAECa |
AR-114 |
112.34 |
1.25 |
100 |
Langmuir |
71 |
KTCb |
AR-114 |
450.02 |
2 |
40 |
Freundlich |
72 |
CPACc |
AR-114 |
69.45 |
0.10 |
20 |
Langmuir |
73 |
ICACd |
AR-114 |
80.86 |
0.10 |
20 |
Langmuir |
74 |
Activated carbon-Ppe |
AR-114 |
204.08 |
3 |
100 |
Langmuir & Freundlich |
75 |
Activated carbon-Cpf |
AR-114 |
153.85 |
3 |
100 |
Langmuir & Freundlich |
75 |
Activated carbon-Spg |
AR-114 |
102.04 |
3 |
100 |
Langmuir & Freundlich |
75 |
SBA-3/PEHAh |
AR-114 |
1000 |
0.20 |
100 |
Freundlich |
24 |
MSP NPsi |
MOj |
26.75 |
2 |
100 |
Freundlich & Sips |
30 |
MSP NPs |
EBTk |
65.35 |
2 |
100 |
Sips |
30 |
MP NPsl |
MO |
27.22 |
2 |
100 |
Freundlich |
30 |
MP NPs |
EBT |
72.35 |
2 |
100 |
Freundlich |
30 |
CMMSNsm |
MBn |
43.03 |
0.2 |
20 |
Freundlich |
76 |
CTAB–Fe3O4 NPso |
DR-167p |
2000 |
— |
80 |
— |
77 |
CTAB–Fe3O4 NPs |
DB-183q |
2000 |
— |
80 |
— |
77 |
Fe3O4@SiO2 |
MB |
— |
1 |
20 |
— |
78 |
Fe3O4 |
AR-114 |
111 |
1 |
100 |
Freundlich |
79 |
Fe3O4@SiO2 |
CR |
50.54 |
3 |
30 |
Langmuir |
80 |
Fe3O4@SiO2–NH2 |
AR-114 |
84.75 |
0.40 |
20 |
Langmuir |
81 |
Fe3O4@SiO2–MPAP |
AR-114 |
105.26 |
0.24 |
20 |
Langmuir |
This work |
3. Experimental
3.1. Materials and apparatus
The following reagents were purchased from Merck and used without further modification: ferric chloride hexahydrate (FeCl3·6H2O), ferrous chloride tetrahydrate (FeCl2·4H2O), ammonium hydroxide (NH3, 25 wt%), tetraethylorthosilicate (TEOS), 3-aminopropyl-trimethoxysilane (APTES), and methyl acrylate. Acid red 114 (AR-114) was purchased from Sigma-Aldrich. A pH meter (Metrohm, model 713, Swiss) was used for pH measurements. The UV-Vis absorption spectra in the range of 200–700 nm were measured with a Shimadzu UV-2100 spectrometer. Spectral data were obtained using quartz cuvettes with 1 cm optical path length. The FT-IR spectra for the samples were obtained using Shimadzu FT-IR-8900 spectrophotometer by using KBr pellets. The X-ray diffraction (XRD) patterns were recorded in air at ambient temperature by Phillips (pw-1840) X-ray diffractometer with Cu-Kα radiation source (λ = 1.54056 Å) at 40 kV voltage and 25 mA current in a wide angle range (2θ = 20–70). The TEM samples were prepared by suspending the nanoparticles in EtOH followed by sonication for several minutes. One drop of the dilute nanoparticles/EtOH suspension was placed on a carbon-coated holey TEM copper grid and was dried in air. The dried grid was then loaded into a double tilt sample holder. The sample was then examined with a Philips CM-20 STEM equipped with a Gatan UltraScan 1000 CCD camera and an energy dispersive X-ray spectrometer: INCA Energy TEM 200. TEM images were taken at 200 kV.
3.2. Preparation of magnetic nanoparticles of Fe3O4@SiO2–MPAP
Fe3O4@SiO2–MPAP NPs were synthesized in the following steps (Scheme 1): first, Fe3O4 nanoparticles were synthesized by co-precipitation process of a mixture of the FeCl3·6H2O and FeCl2·4H2O (molar ratio 1
:
2) according to the previous work,49 and then were coated with silica layers via a sol–gel method.50 Subsequently, aminopropyl modified silica coated Fe3O4 magnetic nanoparticles (Fe3O4@SiO2–APTES) was synthesized according to the procedure reported in the previous literature.50 The obtained Fe3O4@SiO2–APTES (6.9 mmol, 6 g) was suspended in dry EtOH (300 mL) and stirred heavily for 30 min under N2 gas. Then, a solution of methyl acrylate (140 mmol, 11.82 g) in dry EtOH (100 mL) was added drop-wise via a dropping funnel under mechanical stirring. The reaction mixture was vigorously stirred at 50 °C for 5 days at N2 atmosphere. The resulted mixture was cooled and the solid was separated magnetically and then washed with absolute EtOH several times (3 × 50 mL). The precipitate was then dried in oven at 50 °C for 8 h and the black nanopowder Fe3O4@SiO2–MPAP was obtained.
3.3. Adsorption studies
The adsorptive removal of AR-114 was carried by batch experiments. Tests were conducted in 500 mL conical flasks containing 20 mg L−1 AR-114 solution in a water bath to elucidate the values of the test parameters including solution pH (2–11), dye concentration (10–100 mg L−1), temperature (298 K), contact time (0–120 min) and absorbent dosage (0.1–0.3 g L−1). The pH of the solution was adjusted by dropwise addition of 0.1 M HCl or 0.1 M NaOH. After each removal condition experiments, the samples were separated from the dye solution with a permanent magnet and the residual dye molecules concentrations in the solution were determined by UV-Vis spectrophotometer at 515 nm for AR-114.
3.4. Desorption and reusability experiments
Desorption was investigated separately for 20 mg L−1 AR-114 in 500 mL. The sorbent was rinsed with 20 mL of 0.01 M NaOH in H2O that was sufficient for complete desorption of the dye. In order to increase the recoveries; 10 min vortex was used after each washing step. The UV-Vis test of the upper layer solution after vortex was applied to make sure all the AR-114 was recovered. The concentration of each eluents was measured using the obtained standard curve from spectrophotometry method. For reusability test, the sorbent was washed 5 times with 20 mL of 0.01 NaOH aqueous solutions (each step with 4 mL solvent and 10 min vortex) after dye extraction. Then, the sorbent was rinsed with H2O to remove the NaOH excess and dried at 40 °C in an oven.
4. Conclusions
Here, for the first time diester-modified Fe3O4@SiO2–MPAP NPs have been successfully prepared by surface modification of Fe3O4@SiO2 with methyl acrylate and used as a new adsorbent for the removal of AR-114 from aqueous solution. The adsorption kinetics, isotherms and thermodynamics were examined in detail. The adsorption followed pseudo-second-order kinetics. The equilibrium data fitted well the Langmuir isotherm and thermodynamics studies depicted that the adsorption of hazardous dye AR-114 was spontaneous and endothermic with randomness of the process. The experimental results showed that the Fe3O4@SiO2–MPAP NPs could be utilized as a promising and efficient adsorbent for the environmental cleanup.
Acknowledgements
This study was supported in part by the Research Committee of University of Guilan & Iran Nanotechnology Initiative Council (INIC).
Notes and references
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