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Original article
12 (
4
); 588-596
doi:
10.1016/j.arabjc.2014.09.006

A novel green synthesis of Fe3O4 magnetic nanorods using Punica Granatum rind extract and its application for removal of Pb(II) from aqueous environment

, , , ,
a Analytical & Inorganic Division of Chemistry, S.V. University, Tirupati 517502, Andhra Pradesh, India
b DST-PURSE Centre, S.V. University, Tirupati 517502, Andhra Pradesh, India

⁎Corresponding author: Tel.: +91 9912366219. nvvjyothi01@gmail.com (Nimmagadda Venkata Vijaya Jyothi)

Received: , Accepted: ,
© The Author(s) 2014
Disclaimer:
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.

Peer review under responsibility of King Saud University.

Abstract

We described a novel and eco-friendly approach to remove toxic heavy metal of Pb(II) by using dimercaptosuccinic acid (DMSA) anchored Fe3O4 magnetic nanorods (MNRs) which were synthesized via facile method utilizing Punica Granatum rind extract which was a non toxic waste material. The DMSA@Fe3O4 MNRs were characterized by X-ray diffraction (XRD), Fourier transformed infrared analysis (FT-IR), thermogravimetric analysis (TGA), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDX), nitrogen adsorption and desorption techniques, and vibrating sample magnetometer (VSM). These DMSA@Fe3O4 MNRs have been used for the removal of Pb(II) from aqueous solution. The adsorption isotherm data fitted well with Langmuir isotherm and Freundlich model, the monolayer adsorption capacity was found to be 46.18 mg/g at 301 K. The experimental kinetic data fitted very well with the pseudo-second-order model. The results indicate that the biogenic synthesized DMSA@Fe3O4 MNRs act as significant adsorbent material for removal of Pb (II) from aqueous environment.

Keywords

Punica Granatum rind extract
DMSA@Fe3O4 MNRs
VSM
TGA
Pb(II) removal
1

1 Introduction

Due to the rapid industrialization the environment and all the living systems on the earth were facing a very serious threat from the heavy metal pollution. Heavy metals were not biodegradable and tend to accumulate in living organisms causing serious health problems (Jang et al., 2008). Among these heavy metals, Pb(II) was one of the most toxic heavy metal which constitute major environmental health disorders (Ni et al., 2011; Wu et al., 2009). The Pb(II) and their compounds were widely used in modern society, such as printing, fuels and storage battery industries (Selatnia et al., 2004). The Pb(II) exposure may cause damage to organs including kidneys, liver, heart and also cause edema to the immune system (Shah et al., 2011). Thus effective removal of Pb(II) from aqueous system is very important and has become a challenging task for scientists. There were many processes that have been developed to curtail heavy metal pollution like membrane filtration, chemical precipitation, solvent extraction, ion-exchange, electrode deposition, and bio adsorption (Uluozlu et al., 2008; Suc et al., 2013; Hua et al., 2012; Madadrang et al., 2012; Hajdu et al., 2012; Barakat and Schmidt, 2010; Stafej and Pyrzynska, 2007). Among these methods, adsorption is one of the most promising and widely used methods because it is simple and relatively low-cost. Many researchers have tried to find low-cost and easily available adsorbents to remove hazardous metal ions, such as clays, zeolites and plant waste materials (Zong et al., 2011; Du et al., 2011; Aldaco et al., 2011). However, each process has some limitations in its applications.

Recently, nano materials have become attractive adsorbent materials because of their high surface, abundant functional groups and enhanced activity sites on the surface than bulk particles. Moreover, some nano materials can be functionalized with various chemical groups to increase their affinity for target compounds (Musico et al., 2013; Marzo et al., 2013). Among these materials, magnetic nanoparticles have shown to be promising for removal of heavy metal ions from aqueous solutions because of no centrifugation, filtration is required and also less secondary wastes are produced. Especially, Fe3O4 has generated great interest in the magnetic material field and has established its wide applications, such as magnetic resonance imaging (MRI) (Xuan et al., 2011), spintronics (Zhang et al., 2004; Zhan et al., 2011), lithium ion battery field (Xu and Zhu, 2012), catalysis (Lu et al., 2004), targeted drug delivery (Zhu et al., 2010; Chen et al., 2010) and environmental remediation (Linley et al., 2013; Liu et al., 2012; Cheng et al., 2012). However, one-dimensional (1D) nanostructures such as nanorods and nanowires have stimulated enormous interest due to their significant promising potential applications in nanoscale device systems with novel electrical, magnetic, optical and adsorption properties (Hornbaker et al., 2002; Shen et al., 2009; Han et al., 2011). Despite their technological importance, various traditional synthetic methods have been employed to synthesize Fe3O4 1D magnetite nanorods based on hard template directing technique (Zierold et al., 2011), solgel (Woo et al., 2003), microwave-assisted synthesis (Yin et al., 2010) and thermal methods (Wan et al., 2005; Wei et al., 2012). Using organic solvents like benzene, sodium borohydride, hydrazine and carbon monoxide is highly reactive and potentially hazardous to the environment. Therefore, the existing synthesis methods were environmentally unfriendly, expensive, complex and time consuming. So, it is still a great challenge to develop eco-friendly synthesis methods. To the best of our knowledge, no reports on the synthesis of magnetite nanorods using agricultural by-product such as Punica Granatum rind (fruit waste) are available.

Herein, we report the first successful green synthesis of Fe3O4 1D magnetic nanorods with P. Granatum rind extract as the reducing material, the use of naturally available agricultural by-product material has not been investigated so far, for such applications. P. Granatum rinds are one of the classical examples. The P. Granatum is native from Iran to the Himalayas. The total area under cultivation of P. Granatum in India is 116.4 thousand ha and production is around 849.1 thousand tons. The P. Granatum was a rich source of polyphenols and carbohydrates (Qu et al., 2012). In the literature there were a few applications of these rinds (Ahmad and Sharma, 2012; Ahmad et al., 2012; Ashtoukhy et al., 2008). The main advantage of this method was that it is relatively easy to handle and environmentally benign as well as less time consuming. The obtained nanorods have been characterized by using X-ray diffraction (XRD), Fourier transformed infrared analysis (FT-IR), thermogravimetric analysis (TGA), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDS), nitrogen adsorption and desorption techniques and vibrating sample magnetometer (VSM). The Fe3O4 MNRs are anchored by DMSA. According to the Hard-Soft Acid-Base (HSAB) theory (Person, 1988), Pb(II) is borderline acid, which tends to form very strong bonds with –NH2 and –SH groups. The optimum adsorption conditions for the removal of Pb(II), adsorption isotherms and kinetic studies were extensively studied.

2

2 Experimental section

2.1

2.1 Materials

Ferric chloride hexa hydrate (FeCl3·6H2O), sodium acetate (NaAc), dimercaptosuccinic acid (DMSA), HCl, NaOH and (CH3COO)2 Pb 3H2O were all purchased from Sigma Aldrich. P. Granatum rinds were collected from local market in Tirupati, Chittoor, A.P., South India.

2.2

2.2 Extraction of P. Granatum rind

The collected rinds were thoroughly rinsed with double distilled water to remove dust particle. These P. Granatum rinds were shade dried at room temperature for about 14 days under dust free condition. Dried rinds were cut into small pieces. 10 g of dried pieces was blended with 100 ml double distilled water in a 250 ml round bottom flask, and refluxed for 1 h at 70 °C until the color of aqueous solution changes from watery to brown color. Then the resultant extract was cooled to room temperature and filtered with a cheese cloth and the filtrate was stored at −4 °C in order to use for further experiments.

2.3

2.3 Green synthesis and DMSA anchoring of Fe3O4 MNRs

Fe3O4 MNRs were prepared through a simple and eco-friendly method. In a unique reaction procedure, 2.16 g of FeCl3·6H20 and 6.56 g of sodium acetate were dissolved in 40 ml of freshly prepared P. Granatum rind extract solution and then the mixture was stirred vigorously for 2 h at 70 °C in a 100 ml round bottom flask, after 2 h the resulting solution turned into homogenous black color and the temperature of the mixture is increased to 120 °C for 10 h. The obtained black product was isolated by applying an external magnetic field and washed three times with ethanol and dried in a vacuum oven at 90 °C overnight and stored in a stoppered bottle for further use. Typically, 40 ml of double distilled water was taken in a 100 ml round bottom flask to this 0.926 g of dried Fe3O4 MNRs and 0.7288 g dimercaptosuccinic acid were mixed together by ultrasonication for 10 h at room temperature and pH adjustment was done by adding 0.01 M NaOH solution drop wise until pH 8 is reached. After 10 h reaction the obtained DMSA anchored Fe3O4 MNRs were separated by using external magnetic field, washed with double distilled water and ethanol for triplicates, and finally dried at 90 °C under vacuum. The detailed mechanism is shown in scheme 1 ESI†.

2.4

2.4 Characterization

The crystal structure information was obtained using Seifert 3003 TT X-ray diffractometer with Cu Kα radiation with a wave length of 1.54 Å. Morphology and size distribution of Fe3O4 magnetic nanorods were determined with Jeol JEM-2100 transmission electron microscope (TEM), Quantitative elemental analyses of the nanoparticles were carried out with Oxford instruments Inca Penta FET x 3 electron diffraction spectrum (EDS). The magnetization loops for magnetite nanoparticles washed with ethanol were measured at room temperature using a vibrating sample magnetometer (VSM, LKSM-7410). Specific surface area and pore diameter were determined using a Micromeritics ASAP 2020 V3.00 H (BET). FTIR measurements of P. Granatum rind extract and prepared sample were made with Thermo Nicolet FTIR-200 thermo electron corporation. TGA experiment was carried out using NETZSCH STA 449 F3 Jupiter.

2.5

2.5 Batch adsorption studies

The adsorption of Pb(II) ion on to the DMSA@Fe3O4 MNR nano composites was investigated in aqueous solution by batch adsorption experiments with pH range varied from 2 to 7 at 301 K. Pb(II) stock solution was prepared with different concentrations, and then 2.5 mg of magnetic nano adsorbent was added to 25 mL of each Pb(II) ion solution. The initial pH of Pb(II) ion was adjusted by using 0.1 M HCl/NaOH solution. The solution mixture was ultrasonicated at room temperature for 5 min and transferred to 100 mL Erlenmeyer flask was shaken in a thermostatic incubator (200 rpm) at 301 K. After that the magnetic nano adsorbent was removed magnetically from the solution. The concentration of Pb(II) ions was determine using FAAS (Shimadzu AA – 6300). To understand the pH effect (Elico LI 120) the DMSA@Fe3O4 MNR dosage was maintained at 0.1 g/L. All the adsorption experiment was repeated triplicate. The amount of Pb(II) adsorbed by the magnetic nano adsorbent at equilibrium was obtained using the following equation.

(1)
q e = ( C i - C e ) V M where qe (mg/g) is the equilibrium adsorption capacity of Pb(II). Ci and Ce were initial and equilibrium concentrations (mg/L) of Pb(II) respectively, M is the adsorbent dosage (mg), V is the volume of the solution (L), and also the adsorption percentage was defined as follows
(2)
Adsorption ( % ) = ( C i - C e ) C i × 100

3

3 Results and discussion

3.1

3.1 Characterization

The crystal structure of the synthesized Fe3O4 MNRs and DMSA@Fe3O4 MNRs was determined by using X-ray diffraction (XRD) analysis. Fig. 1a and b shows the XRD pattern of MNRs and DMSA@Fe3O4 MNRs obtained using the P. Granatum rind extract. The XRD patterns of the Fe3O4 MNRs display six relatively strong reflection peaks in the 2θ region of 10–70°. It was found that all reflection peaks at (1 1 1), (2 0 0), (2 2 0), (3 1 1), (4 0 0), and (4 4 0) can be well indexed with the cubic inverse spinel structure of both Fe3O4 MNRs and DMSA@Fe3O4 MNRs (JCPDS cardno.79-0416) according to the reflection peak positions and relative intensities, which confirms that the nanoparticles synthesized in this study are the Fe3O4 MNRs. The diffraction peak width of Fe3O4 MNRs slightly differs from that of DMSA@Fe3O4 MNRs, which conform to the slightly grain-size variation the result coincide with TEM. This indicates that the DMSA anchored does not result in phase change of Fe3O4 MNRs.

XRD patterns of (a) Fe3O4 MNRs and (b) DMSA@ Fe3O4 MNRs.
Figure 1
XRD patterns of (a) Fe3O4 MNRs and (b) DMSA@ Fe3O4 MNRs.

FTIR analysis was used for characterizing the synthesized Fe3O4 MNRs and binding of DMSA on the surface of Fe3O4 MNRs and also for understanding the existence of surface functional groups in metal interactions. The P. Granatum rind was constituted by rich polyphenols, carbohydrates, acid derivatives, proteins, lipids, and fibers (Cama and Hısıl, 2010). Fig. 2 represents the FT-IR spectra of P. Granatum rind extract curve (a), Fe3O4 curve (b) and DMSA anchored Fe3O4 MNRs curve (c). The three curves show that there was a variation in the intensity of bands at different regions. A major peak was identified at 3580 cm−1 corresponds to the O–H stretching vibrations (polyphenolic group). This shifting of peak from 3580 to 3460 cm−1 indicates the possible involvement of polyphenols in the synthesis of nanorods and the peak shift from 2925 to 2815 cm−1 was assigned to the C–H stretching vibration of methyl and methoxy groups and their roles. The peak at 1735 cm−1 in curve (a) is shifted to 1690 cm−1 in curve (b) which revealed the involvement of C⚌O stretching vibration of aldehyde derivatives and the band at 1310 cm−1 was attributed to C–O stretching of acid groups. Curve (b) indicates the characteristic band of Fe–O at 585 cm−1 which was an indication of Fe3O4 MNRs. From curve (c) the bands at 2945 and 2522 cm−1 are C–H stretching and free S–H stretching vibrations and the band at 1680 cm−1 derives the C⚌O stretching in addition the bands at 1480 and 1280 cm−1 reveal the C–OO symmetric and C–O stretching vibrations. Above FT-IR results indicate the presence of polyphenols and other bio molecules in the P. Granatum rind extract and these bio molecules may participate in the formation of Fe3O4 MNRs from curve (c) in the absence of free C⚌O peak at 1720 cm−1 and the appearance of new peaks at 1680, 1480 and 1280 cm−1 shows the binding nature of COO− on the surface of Fe3O4.

FTIR spectra of (a) extract, (b) Fe3O4 MNRs and (c) DMSA@ Fe3O4 MNRs.
Figure 2
FTIR spectra of (a) extract, (b) Fe3O4 MNRs and (c) DMSA@ Fe3O4 MNRs.

Thermogravimetric analysis was done to understand the amount of DMSA anchored on the surface of MNRs. Fig. 3a and b presents the TGA curves of Fe3O4 and DMSA@Fe3O4 MNRs. Curve (a) shows that the mass loss of Fe3O4 MNRs over the temperature range from 42 °C to 600 °C is about 4.7%, which was assigned to the mass loss of water and the other bio functional molecules in the compound. In the meanwhile cure (b) reveals two mass loss steps. One at 160 °C that is less weight loss due to the removal of water and the second one is mass loss at 250 °C which is due to the burning of DMSA ligands over Fe3O4 MNRs (Menelaou et al., 2014). When the temperature was increased from 300 °C to 600 °C there was no mass loss showing that only iron oxide is present at this temperature. The average mass content of DMSA in Fe3O4 MNRs is about 11.5%.

TGA graphs of (a) Fe3O4 MNRs and (b) DMSA@ Fe3O4 MNRs.
Figure 3
TGA graphs of (a) Fe3O4 MNRs and (b) DMSA@ Fe3O4 MNRs.

Transmission electron microscope (TEM) studies of Fe3O4 MNRs and DMSA@Fe3O4 MNRs were carried out to understand the shape and size of the prepared particles. Fig. 4a and c shows representative transmission electron microscopy (TEM) images of Fe3O4 and DMSA@Fe3O4 MNRs green synthesized, with an average diameter of 40 nm and length above 200 nm. It can be seen that the nanorods slightly change in Fig. 4c because of DMSA ligands which became interlinked on the surface of Fe3O4 MNRs through COO- groups this favors the stability of colloidal dispersion. In addition, the selected area diffraction (SEAD) pattern demonstrates the poly crystalline nature of nanorods. Meanwhile, the corresponding selected area diffraction (SEAD) also displays poly crystalline diffraction rings of the Fe3O4 MNRs. The spectrum was used to determine the element present in the composition which was revealed by the EDS analysis. Fig. 4b shows the presence of only iron, oxygen and carbon in addition Fig. 4d shows the presence of sulfur in the sample with other elements after surface modification with DMSA. These results conform the anchoring of DMSA on the surface of Fe3O4 MNRs.

(a) and (b) show the TEM image and EDX pattern of Fe3O4 MNRs and corresponding SAED pattern in inset of (a). While (c) and (d) show the TEM image and EDX pattern of DMSA@ Fe3O4 MNRs.
Figure 4
(a) and (b) show the TEM image and EDX pattern of Fe3O4 MNRs and corresponding SAED pattern in inset of (a). While (c) and (d) show the TEM image and EDX pattern of DMSA@ Fe3O4 MNRs.

Fig. 5 shows the adsorption and desorption isotherms of nitrogen used to determine the surface area and pore size of DMSA@Fe3O4 MNRs. The MNRs display a characteristic hysteresis in the desorption isotherm at relative pressure (P/P0) in the range 0.01–0.9. This behavior is indicative of mesopore structure of the tested sample (Castro et al., 2011). The pore size distribution was determined by the Barret–Joyner–Halender (BJH) method using desorption isotherm. Fig. 5 inset shows the pore size distribution of the sample indicating that most of the mesopores is around 12.6 nm. According to the BET method, the specific surface area of the DMSA@Fe3O4 MNRs is determined to be 10.88 m2/g calculated from the linear part of the Brunauer–Emmett–Teller (BET) plot. The single-point adsorption total volume at P/P0 = 0.9896 is 0.07063 cm3 g−1. The BET surface area and pore volume support the fact that the DMSA@Fe3O4 MNRs have a mesopore structure. These DMSA@Fe3O4 mesopore MNRs might have the potential to remove Pb(II) from aqueous medium.

Adsorption and desorption isotherms of nitrogen and pore distribution (inserted in figure) of DMSA@Fe3O4 MNRs.
Figure 5
Adsorption and desorption isotherms of nitrogen and pore distribution (inserted in figure) of DMSA@Fe3O4 MNRs.

Vibrating sample magnetometer (VSM) analysis was performed at room temperature of DMSA@Fe3O4 MNRs. Fig. 6 shows the hysteresis loop demonstration of ferromagnetic behavior with the saturation magnetization (Ms) value about 22.7 emu/g, coercivity (Hc) of 334.22G and remanence (Mr) 4.2 emu/g, lower right inserted shows the enlarged hysteresis loop which conforms the ferromagnetic behaviour. Fig. 6 upper left inserted bottles shows the behavior of DMSA@Fe3O4 MNRs before and after external magnetic field. The MNRs are easily dispersed in double distilled water and also could be drawn from the solution to the side wall of the vial by an external magnet and the black suspended aqueous solution turns transparent within seconds when it is placed nearby, suggesting that the obtained DMSA@Fe3O4 MNRs had an excellent magnetic responsivity and also good recyclable property.

Magnetization-hysteresis (M-H) loops of DMSA@Fe3O4 MNRs measured at room temperature. Upper inset shows DMSA@Fe3O4 MNRs dispersed in water and its magnetic separation and lower inset shows the enlargement of the hysteresis loop at low magnetic field.
Figure 6
Magnetization-hysteresis (M-H) loops of DMSA@Fe3O4 MNRs measured at room temperature. Upper inset shows DMSA@Fe3O4 MNRs dispersed in water and its magnetic separation and lower inset shows the enlargement of the hysteresis loop at low magnetic field.

3.2

3.2 Influence of pH value

The pH of solution is one of the most important variables, influence of the pH value on the removal of Pb(II) ion by Fe3O4 and DMSA@Fe3O4 MNRs was investigated at pH 2–7, 301 K and an different Pb(II) initial concentrations of 20, 40 and 60 mg/L. As shown in Fig. 7, the percentage removal of Pb(II) increased with an increase in pH from 2 to 5 and decreased with an increased pH 6–7 and the nanocomposite displayed a maximum removal of 96.68% at pH 5.0, initial concentration of 20 mg/L. Decrease in Pb(II) removal at higher pH (pH > 5) is because of the formation of Pb(II) as Pb(OH)+, Pb(OH)2o, and Pb(OH)3− at different pH values (Weng, 2004). While less affect was observed when the initial concentration of Pb(II) was as 40 or 60 mg/L and the adsorption capacity of Fe3O4 MNRs was below 50% (Figure not shown). The pH value of the solution can affect the surface charge of DMSA@Fe3O4 MNRs, this impacts the adsorption of metal ions on the surface of the magnetic adsorbent. At lower pH Pb(II) removal was inhibited because of the H+ competed with Pb(II) for adsorption sites (Yang et al., 2011), which significantly affected Pb(II) adsorption at low pH medium. Moreover, at higher pH electrostatic attraction increases between the ligand and metal so it increases adsorption capacity. The change in the adsorption with pH solution was clearly explained by the following mechanism:

(3)
MNRs-SH + H + → MNRs- SH 2 +
(4)
MNRs-SH + Pb 2 + → MNRs- Pb + + H +
(5)
MNRs-SH + PbOH + → MNRs-PbOH + H +
(6)
MNRs-SH + Pb ( OH ) 2 → MNRs-PbOH + H 2 O Eq. (3) shows the protonation of the mercapto group at lower pH, Eq. (4) indicated the formation of chelation of Pb(II) with mercapto groups, and Eqs. (5) and (6) describe the hydroxylated lead forms (Pb(OH)+, Pb(OH)2o) which resulted in the formation of neutral complexes form (MNRs-S-PbOH). All these reactions prove the adsorption characteristics at different pH solutions.
Effect of pH value on the adsorption of Pb(II) by DMSA@Fe3O4 MNRs at different initial concentrations of Pb(II) (initial concentrations of Pb(II): 20, 40, 60 mg/L, material dosage: 0.1 g/L, solution volume: 20 m/L, time: 120 min, temperature: 301 K).
Figure 7
Effect of pH value on the adsorption of Pb(II) by DMSA@Fe3O4 MNRs at different initial concentrations of Pb(II) (initial concentrations of Pb(II): 20, 40, 60 mg/L, material dosage: 0.1 g/L, solution volume: 20 m/L, time: 120 min, temperature: 301 K).

3.3

3.3 Adsorption kinetic studies

Fig. 8 shows the adsorption behavior of Pb(II) on the DMSA@Fe3O4 MNRs at an initial concentration of 20 mg/L, pH at 5.0 and 301 K as a function of contacting time. The adsorption rate of the Pb(II) on DMSA@Fe3O4 MNRs was almost finished within 30 min, and then gradually reached equilibrium in 60 min. The pseudo-first-order (Lagergren, 1898; Yao et al., 2010) and pseudo-second-order (Ho and Mckay, 1999) kinetic models were used to investigate the kinetics of removal on the magnetic nanocomposites.

Effect of contact time on the extraction of Pb(II) by DMSA@Fe3O4 MNRs (initial concentration of Pb(II) 20 mg/L, material dosage: 0.1 g/L, solution volume: 20 mL, pH: 5.0, temperature: 301 K).
Figure 8
Effect of contact time on the extraction of Pb(II) by DMSA@Fe3O4 MNRs (initial concentration of Pb(II) 20 mg/L, material dosage: 0.1 g/L, solution volume: 20 mL, pH: 5.0, temperature: 301 K).

The linear form of pseudo-first-order kinetic model is described by the equation:

(7)
log ( q e - q t ) = log q e - k 1 2.303 t where k1 (min−1) is the pseudo-first-order rate constant of adsorption, qe (mg/g) and qt (mg/g) are the amount of the Pb(II) adsorbed at equilibrium and at time t. The pseudo-first-order kinetic constant was determined from the slope of the plot of log (qe − qt) vs t. The R2 value is very less suggesting that the adsorption of Pb(II) ions does not follow pseudo-first-order kinetic model. Data were not shown.

The kinetic data were further analyzed using pseudo-second-order kinetic model. The linearized form of the equation is represented as

(8)
t q t = 1 k 2 q e 2 + 1 q e t where k2 (g/mg min−1) is the pseudo-second-order rate constant, qe (mg/g) and qt (mg/g) are the amount of the Pb(II) adsorbed at equilibrium and at time t. The values of k2 and qe can be calculated from the slope and intercept of a plot of t/qt vs t. From the removal kinetics shown in Fig. 9, the slope shows good linearity with the correlation coefficient value (R2) 0.9986, indicating that the removal kinetic follow the pseudo-second-order model. The corresponding parameters of the pseudo-second-order model are listed in Table S1, ESI†. The adsorption system obeyed the pseudo-second-order kinetic model for the entire adsorption period and thus supported the assumption that the adsorption was the chemisorption process (Crini et al., 2007).
Pseudo second-order adsorption kinetics of Pb(II) on DMSA@Fe3O4 MNRs.
Figure 9
Pseudo second-order adsorption kinetics of Pb(II) on DMSA@Fe3O4 MNRs.

3.4

3.4 Adsorption isotherm

To evaluate the maximum adsorption capacity of adsorbent the equilibrium adsorption of Pb(II) on DMSA@Fe3O4 MNRs was analyzed using the Langmuir and Freundlich isotherm models. The Langmuir equation can be expressed in the linearized form:

(9)
C e q e = C e q m + 1 q m b where qe is the equilibrium adsorption capacity of metal on concentration on the adsorbent (mg/g), Ce is the equilibrium metal ion concentration in the solution (mg/L), qm is the maximum capacity of adsorbent (mg/g), and b (L/mg) is the equilibrium constant relating to the sorption energy. Fig. 10 shows that the experimental data fits the Langmuir adsorption isotherm well, maximum adsorption capacity was found to be 46.18 mg/g as prepared DMSA@Fe3O4 MNRs at pH = 5.0 and correlation coefficients, equilibrium constants are summarized in Table S2 ESI†. In addition, another parameter in the Langmuir adsorption isotherm a dimension less factor (RL) is described by the following equation.
(10)
R L = 1 1 + bC i where Co (mg/g) is initial metal concentration, b (L/mg) is the Langmuir constant. For favorable sorption, 0 < RL < 1; for unfavorable sorption, RL > 1; for irreversible sorption RL < 0; for linear sorption, RL < 1. In this study, the RL value is 0.0769 which lies between 0 and 1. This indicates that the adsorption of Pb(II) on DMSA@Fe3O4 MNRs is favorable.
Linear plot of Freundlich isotherm of Pb(II) on DMSA@Fe3O4 MNRs.
Figure 10
Linear plot of Freundlich isotherm of Pb(II) on DMSA@Fe3O4 MNRs.

The Freundlich isotherm can be applicable for modeling the adsorption of metal ions on heterogeneous surfaces and the linearized form of isotherm is expressed as:

(11)
log q e = log k f + 1 n log C e where kf (mg/g) and n are the Freundlich isotherm constants that represent the adsorption and the intensity of adsorbents, Fig. 11 shows the linear plot of Freundlich isotherm of Pb(II) adsorption on DMSA@Fe3O4 MNRs at 301 K. The fitted constants for the Freundlich isotherm model values of kf, n and correlation coefficient (R2) are calculated from the intercept and slope of the plot and are presented in Table S2, ESI†. The values of n > 1 represent favorable adsorption condition (Hameed et al., 2008; Gong et al., 2011) and the n value 4.5187 suggests that the DMSA@Fe3O4 MNRs are favorable for the adsorption of Pb(II) ions. Both the Langmuir and Freundlich isotherm models fit well with the adsorption data and have similar correlation coefficients. This may be attributed to a good anchoring of DMSA on Fe3O4 MNRs, and the uniform distribution of active sites for metal ion adsorption with monolayer coverage. The adsorption capacity of as prepared DMSA@Fe3O4 MNRs for Pb(II) from Langmuir isotherm model compared with that of various adsorbents is given in Table S3, ESI†.
Linear plot of Langmuir isotherm of Pb(II) on DMSA@Fe3O4 MNRs.
Figure 11
Linear plot of Langmuir isotherm of Pb(II) on DMSA@Fe3O4 MNRs.

3.5

3.5 Desorption and reusability

Owing to the economic efficiency and environmental sustainability, the regeneration and reuse of an adsorbent are essential. From the pH study, the adsorption percentage of Pb(II) is lower at lower pH value, acidic medium is expected to be a feasible approach for the regeneration of Pb(II) loaded DMSA@Fe3O4 MNRs. Thus 0.01 M HCl solutions of different pHs were used to test the desorption study. It was found that desorption percentages were 91%, 90% and 78% in the HCl solutions of pH 1.5, 2.0 and 3.0, respectively. At lower pH it shows higher desorption because of the sufficiently high hydrogen ion concentration, which led to the strong competitive adsorption. It is worth noting that the regenerated adsorbents can still be used for Pb(II) removal. Under same adsorption conditions the percentage of Pb(II) removed can be 75.1%, indicating that the DMSA@Fe3O4 MNRs has a good reusability.

4

4 Conclusions

In brief, by using agriculture by-product we investigated a novel and environmentally benign route for the synthesis of Fe3O4 MNRs and anchored with DMSA and can be used as a reusable adsorbent. TEM image shows the rod shape structure, XRD, FT-IR and TGA results reveal the surface characterization of DMSA@Fe3O4 MNRs. VSM measurement displays ferromagnetic properties. A strong electrostatic attraction is present between Pb(II) and DMSA@Fe3O4 MNRs at various pHs and plays a vital role in the adsorption mechanism. The maximum adsorption capacity was found to be 46.18 mg/g at pH 5, dose 0.1 g/L and temperature 301 K. The pseudo-second-order kinetic model fits rather than the pseudo-first-order kinetic model, These MNRs could achieve a rapid removal of Pb(II) from water with external magnet. These green synthesized DMSA@Fe3O4 MNRs have promising application in future environmental remediation process and nano biotechnology, respectively.

Acknowledgements

The author Sada Venkateswarlu expresses his sincere gratitude to UGC-BSR, New Delhi, India for providing financial support in the form of an award of Meritorious Research Fellowship. The authors thank IIT-Madras, NEHU, Shillong for providing instrument facilities.

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Appendix A

Supplementary data

Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.arabjc.2014.09.006.

Appendix A

Supplementary data

Supplementary data 1

Supplementary data 1


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