SYNTHESIS AND CHARACTERIZATION OF NANOCOMPOSITE NiFe2O4@SalenSi AND ITS APPLICATION IN EFFICIENT REMOVAL OF Ni(II) FROM AQUEOUS SOLUTION

In this work, nano ferrite spinel NiFe2O4 was synthesized by sol-gel method and characterized by SEM, XRD, FT-IR, and VSM. In second step Schiff base made from salicylaldehyde and amino propyl triethoxy silane was used for modification of the synthesized nano ferrit. In the third step removal of Ni(II) was done using modified adsorbent and 95% efficiency was achieved. The removal rate was determined by atomic absorption spectroscopy. These studies showed that the Freundlich isotherm model was fitted well with adsorption data. Moreover, the pseudo-second order kinetic model was fitted very well with experimental data. The results demonstrated that NiFe2O4@SalenSi nanoadsorbent can be used for the removal and recovery of metal ions from wastewater over a number of cycles, indicating its suitability for the design of a continuous process.


INTRODUCTION
The scarcity of water in terms of both quantity and quality has become a significant threat to the well-being of humanity. In particular, the quality of drinking water has become a serious concern, with the rapid escalation of industrialization towards a developed society. The waste products generated from the textiles, chemicals, mining and metallurgical industries are mainly responsible for contaminating the water [1]. This contaminated water contains nonbiodegradable effluents, such as heavy metal ions (arsenic, zinc, copper, nickel, mercury, cadmium, lead and chromium, etc.) and organic materials that are carcinogenic to human beings and harmful to the environment [2]. According to the WHO, the limit of the toxicity value for nickel is 130 μg/L, assuming a 60 kg adult, drinking two liters of water per day. However, the presence of nickel at higher levels in the human body can cause serious lung and kidney problems as well as gastrointestinal distress, pulmonary fibrosis and skin dermatitis [3].
Several conventional methodologies such as precipitation [4], ion exchange [5], filtration membrane technology [6], electrochemical processes [7] and adsorption process [8] are available for heavy metal removal and other pollutants from wastewaters. Generally, they are expensive or ineffective sometimes, especially when the metal concentration is higher than 100 mg/L [9]. Among all the treatments proposed, adsorption using sorbents is one of the most popular methods. It is now recognized as an effective, efficient and economic method for water decontamination applications and for separation for pilot purpose.
Some nanoadsorbents can be easily recovered or manipulated with an external magnetic field and show a good capacity for the rapid and efficient adsorption of multivalent metal cations from aqueous solutions. Composite microspheres can be applied to remove heavy metal ions from industrial wastewater because the surface of the microspheres is covered with SiO 2 , and the SiO 2 is inactive and can adsorb heavy metal ions (such as Hg 2+ and Pb 2+ ) [10], or using supplementary material modification process magnetic nano particles, for example: capped by thiol [11], capped by amine [12] or capped by Schiff base [13]. The use of monodisperse amine-terminated Fe 3 O 4 @SiO 2 -NH 2 for the removal of metal ions was recently reported. These amineterminated core-shell magnetic nanoparticles show effective removal of Pb 2+ [14] and removal heavy metals hybrid composite such as: Fe 3 O 4 -SiO 2 -poly(1,2-diaminobenzene) [15], amine-Fe 3 O 4 @SiO 2 @meso-SiO 2 [16] and EDTA-γ-Fe 3 O 4 @SiO 2 /Thiol-γ-Fe 3 O 4 @SiO 2 [17]. The effective removal of heavy metal ions from an aqueous solution using modified magnetic nanoparticles by polymer [18] for example polypyrrole [19] has been developed by the covalent binding of poly(acrylic acid) on the surface of Fe 3 O 4 nanoparticles followed by sulfonation using sulfanilic acid via carbodiimide activation [20] capped by EDA polymers [21]. Aminefunctionalized magnetite chitosan nanocomposites as a recyclable tool was used for the removal of heavy metals (amine-Fe 3 O 4 @SiO 2 ) [22], magnetic nanoparticles surface modification using acids such as humic acid (humic acid-Fe 3 O 4 ) [11] and ascorbic acid (ascorbic acid-Fe 3 O 4 ) [23]. Also to remove heavy metals by magnetic nanoparticles surface modification using some chemicals such as DMSA (DMSA-Fe 3 O 4 ) [24]. There is recent research from composite to remove heavy metal ions from aqueous solutions used. The optimum conditions for this research pH 6, 24 h as well as the reaction efficiency100% (Cu 2+ , Pb 2+ , Hg 2+ , As 3+ ), 40% (Co 2+ ), 22% (Cd 2+ ) and 25% (Ni 2+ ) [25,26].
In this work, nano ferrite spinel NiFe 2 O 4 was synthesized. In second step Schiff base made from salicylaldehyde and amino propyl triethoxysilane was used for modification of synthesized nano ferrit, also the characterization of such new phases and their capabilities toward Ni(II) removal were done, respectively. Therefore, in the batch system the effects of pH, adsorbent dosage, initial concentration and contact time on the adsorption capacity were investigated.

Preparation of NiFe 2 O 4
The nickel ferrite was synthesized by sol-gel method. First the aqueous solution of metal nitrate with nominal atomic ratios Fe:Ni = 2:1 was mixed together in deionized water. Citric acid (CA) was proportionally added to the metal ion solution to gain the same amounts of equivalents. The solution was concentrated by evaporation at approximately 50 o C with simultaneous stirring for 1 h to convert it to stable (Fe, Ni)/CA complex. The solution was heated (while being stirred) at approximately 75 o C to remove excess water. Then the dry gel was obtained by letting the sol into an oven heating it slowly up to 110 o C and keeping it for 6 h in a baking oven. The gel pieces were grounded in an agate mortar to form fine powder. Finally NiFe 2 O 4 nanoferrite was obtained.

Preparation of the Schiff base
The Schiff base was synthesized according to the literature [27]. A solution of salicylaldehyde (4 mmol) in CH 3 CN or CH 3 OH (20 mL) was treated with amino propyl triethoxysilane in a molar ratio of 1:1.5. The reaction mixture was refluxed for four hours. The result was a pale yellow Schiff base that is shown in (Scheme 1). 79 Scheme 1. Preparation process of Schiff base bonding to nickel ferrite.

Preparation of nano composites NiFe 2 O 4 @SalenSi
At this stage nickel ferrite (1 mmol) was added to the solution. Then the reaction mixture was refluxed for twenty-four hours. The solid residue was filtered and washed with CH 3 OH (8 drops) and chloroform (3 drops). After that it was laid in the desiccators for a few hours in order to be dried, shown in ( Figure 1).

Adsorbent characterization
Several techniques were employed to analyze and validate the synthesized powder. For structural investigation of calcinated powder at 700 o C, powder X-ray diffraction (XRD) measurements were carried out in the region of (2θ = 20 to 70 o ) using CuK α radiation on a Rigaku D/MAX RB XRD diffract meter equipped with a curved graphite monochromator. To study the surface morphology scanning electron microscopy (SEM) images were carried out. The complex polymeric gel and derived powders have been also analyzed by Fourier transform infrared spectroscopy (FTIR) on Perkin Elmer BX II FTIR spectrometer. The magnetic properties (M-H curve) were evaluated on a BHV-55 vibrating sample magnetometer (VSM). This analysis was measured at room temperature using the magnetic field 1000 Oe to -1000 Oe. Finally with the help of atomic absorption spectroscopy AAS final concentration of nickel ions was achieved.

Ni(II) removal experiments
A prepared solution of Ni(II) was distributed into different flasks (1 L capacity) and pH was adjusted with the help of the pH meter (HORIBA F 11 E, Japan made). The initial pH value of the soluble metal ion was adjusted to the desired levels, using either HCl (0.5 M) or NaOH (0.5 M). A known mass of NiFe 2 O 4 @SalenSi powder (adsorbent dosage) was then added to 20 mL of soluble [Ni(II)], and the obtained suspension was immediately stirred for a predefined time. All experiments were done at room temperature. The investigated ranges of the experimental variables were as follows: concentrations of pollutants (10-50 mg/L), pH of solution (2-11), adsorbent dosage (0.02, 0.03 and 0.04 g) and mixing time (30-120 min).
Adsorption capacity and removal efficiency were calculated according to the following equations [28]: where qe (mg/g) is adsorption capacity; E (%) is removal efficiency; C 0 (mg/L), and C e (mg/L) are initial and equilibrated adsorbate concentrations, respectively;

Reusability of nanoadsorbents
To test the reusability of the nanoadsorbents, the NiFe 2 O 4 @SalenSi nanoadsorbents containing Ni(II) were washed with CH 3 OH. After that NiFe 2 O 4 @SalenSi nanoadsorbents were thoroughly washed with deionized water till. Then, the washed nanoadsorbents were at 150 o C for 15 min and reused for the subsequent adsorption cycle. Five cycles of consecutive adsorptiondesorption-regeneration were carried out to validate the reusability of NiFe 2 O 4 @SalenSi nanoadsorbents for the removal and recovery of Ni(II).

X-Ray diffraction
XRD patterns of the synthesized powder which was calcinated at temperature 700 o C for 9 h (heating rate: 3 o C/min) is shown in (Figure 2). XRD results reveal the existence of nano ferrite for sol-gel method. When the precursor was calcinated at 700 o C for 9 hours, several sharp peaks were observed attributed to the nano ferrite where D hkl is the particle size perpendicular to the normal line of (hkl) plane, β hkl is the full width at half maximum, θ hkl is the Bragg angle of (hkl) peak, and λ is the wavelength of X-ray. The particle size of nanoparticles calcinated at 700 o C is about 52.75 nm.

SEM analysis
Scanning electron microscopy (SEM) of nanoferrite prepared by the sol-gel method and modified by Schiff base is shown in Figure 3 (a, b). The micrograph represents formation of agglomerated particles with Schiff base addition. However, appearance of bigger particles on the surface seems to be dominant.

FT-IR spectroscopy
The FT-IR spectra of the NiFe 2 O 4 fresh xerogel and calcinated xerogel and NiFe 2 O 4 @SalenSi in the range of 500-4000 cm -1 are shown in Figure 4 (a, b, c). The dried gel of the sample shows the characteristic peaks at about 1778.17, 1643.81 and 1427.58 cm -1 , which correspond to the symmetric and anti-symmetric stretching mode of carboxylate group [29,30]. The peak around 1027.55 cm -1 is attributed to C-O bond [31]. As seen in (Figure 4), the bands correspond to O-H group, NO 3 , and carboxylate ligand disappears as the gel which was calcinated at 700 o C. This indicates the possibility of metal carboxylate dissociation and conversion to metal oxides during the calcination process. Also, the observed peak at about 607.20 cm -1 is due to the stretching internal vibrations of octahedral oxygen in nickel ferrite oxide structure.
In the FT-IR spectrum Schiff base nickel ferrite nanoparticles deposited on the surface, Figure 4 (c), the broad peak in 1200-1000 cm -1 belongs to Si-O-Si stretching vibration. In addition, a peak at 1652 cm -1 observed is attributed to the stretching vibration of C=N bond.

Vibrating sample magnetometer (VSM) analyses
Ferromagnetism properties of the NiFe 2 O 4 at room temperature are shown in Figure 5 (a, b).

Adsorption study Effect of pH
The solution pH is an important parameter that affects the adsorption of Ni(II). The effect of the initial solution pH on the metals removal efficiency of Ni(II) by NiFe 2 O 4 @SalenSi nanoparticles was evaluated at different pH values, ranging from 2 to 11, with a stirring time of 45 min. The initial concentrations of Ni(II) and adsorbent dosage were set at 10 mg/L and 15.0 mg/L, respectively. As shown in Figure 6, pH 9 seems to lead to the best result; so this pH was selected to run further experiments.

Effect of Ni(II) concentration
The Ni(II) concentration is another important parameter that can affect the adsorption process. The effect of initial Ni(II) concentration on metal removal efficiency by NiFe 2 O 4 @SalenSi particles was studied by varying the initial Ni(II) concentration from 10 to 50 mg/L at pH 9, a nanoadsorbent dosage of 15.0 mg/L and contact time of 45 min, as shown in Figure 8. Results verify that metal removal of Ni(II) decreases with increasing the initial concentration. Obviously, the percentage removal of Ni(II)was around 94.6% at a concentration of 10 mg/L . Figure 9 shows the removal efficiency, R (%), of Ni(II), during five cycles of adsorptiondesorption-regeneration, from a 20 mL solution of initial Ni(II) concentration 10 mg/Lat pH 9.

Reusability of nano adsorbent NiFe 2 O 4 @SalenSi
As can be seen, no significant decrease in the adsorption capacity of NiFe 2 O 4 @SalenSi nanoadsorbent during the five cycles was observed. The results demonstrated that NiFe 2 O 4 @SalenSi nanoadsorbent can be used for the removal and recovery of metal ions from wastewater over a number of cycles, indicating its suitability for the design of a continuous process. It is noteworthy that commercially available adsorbents such as activated carbon are costly, time-consuming and their regeneration is essential. Actually, the regeneration and recovery of spent activated carbons is the most expensive part of the adsorption technology and it accounts for about 75% of the total operating and maintenance costs [32]. However, it seems that this is not the case for the regeneration of spent NiFe 2 O 4 @SalenSi nanoadsorbents. Therefore, NiFe 2 O 4 @SalenSi nanoadsorbent is a cost-effective alternative with fast adsorption rate and its regeneration is simple and may be not necessary, especially for in situ applications [33].

Adsorption kinetic studies
The kinetic studies describe that the solute uptake rate for choosing optimum operating conditions for designing purposes. In order to investigate the mechanism of adsorption and potential rate controlling steps such as chemical reaction, diffusion control and mass transport process, kinetic models have been used to test experimental data from the adsorption of Ni(II). These kinetic models were analyzed using pseudo-first-order and pseudo-second-order which were respectively presented as follows in Eqs. where t is the contact time of adsorption experiment (min); q respectively the adsorption capacity constant, k 1 and k 2 are -0.053 and 0. 16 The adsorption process of Ni(II) The linear regression coefficient value Kinetics was closer to unity than the R the pseudo-second-order kinetic model provided good correlation for the ions onto NiFe 2 O 4 @SalenSi.

Adsorption isotherm studies
Adsorption isotherm studies analysis of the equilibrium data is important to develop which accurately represents the results which obtained for adsorption of Ni(II) the Langmuir, Freundlich and Temkin isotherm models. ion concentration was varied whereas solution pH and amount of adsorbent were max max where C e is the Ni(II) concentration in the solution (mg/L), q solid adsorbent (mg/g), q m is the maximum adsorption capacity (mg/g), K the energy of adsorption (L/g), K F represents the capacity of the adsorbent for the adsorbate and n is related to the adsorption distribution (mg 1-1/n L 1/n /g), interactions and B (mg .L -1 ) is another constant related with adsorption heat.  32(1) where t is the contact time of adsorption experiment (min); q e (mg/g) and q t (mg/g) are respectively the adsorption capacity at equilibrium and at any time t. The values of the rate 0.053 and 0.161 (M −1 min −1 ), respectively. The adsorption process of Ni(II) can be well fitted by using the pseudo-second order rate. The linear regression coefficient value R 2 (0.999) obtained for pseudo-second-order constant.
to unity than the R 2 value (0.907) for first-order kinetics. This indicates that kinetic model provided good correlation for the adsorption of Ni(II) analysis of the equilibrium data is important to develop an equation which accurately represents the results which could be used for designing purposes. The results Ni(II) ions were analyzed by the use of well-known models given by the Langmuir, Freundlich and Temkin isotherm models. For the sorption isotherms, initial metal was varied whereas solution pH and amount of adsorbent were held constant.
concentration in the solution (mg/L), q e is the Ni(II) concentration in the is the maximum adsorption capacity (mg/g), K l is a constant related to energy of adsorption (L/g), K F and n are constants of the Freundlich equation. The constant represents the capacity of the adsorbent for the adsorbate and n is related to the adsorption A (L/g) is Temkin constant representing adsorbent-adsorbate is another constant related with adsorption heat.
concentration in the is a constant related to and n are constants of the Freundlich equation. The constant represents the capacity of the adsorbent for the adsorbate and n is related to the adsorption adsorbate values for the Freundlich model (R 2 = 0.999) are closer to unity than those for the other isotherm models (Langmuir: R 2 = 0.958 and Temkin: R 2 = 0.955) for adsorption process onto NiFe 2 O 4 @SalenSi. This indicates that the adsorption of Ni(II) on NiFe 2 O 4 @SalenSi particles is better described by the Freundlich model than the Langmuir and Temkin models and generally it has been found better suited for characterizing monolayer adsorption process onto the homogenous adsorbent surface (Figure 10 (a, b, c)) [34].

CONCLUSION
This study showed that NiFe 2 O 4 @SalenSi nanoadsorbent could be used as an alternate to conventional adsorbents for the removal of metal ions from wastewater in a very short time with high removal efficiency, whereas, single NiFe 2 O 4 was not be able to removal of metal ions. The removal of Ni(II), as a typical metal ion commonly present in wastewater, by adsorption onto NiFe 2 O 4 @SalenSi nanoadsorbents was successfully accomplished. Adsorption was very rapid and equilibrium was achieved in less than 45 min. Also the adsorption was highly dependent on the initial concentration of Ni(II) and pH. Maximum removal was observed at pH 9. Adsorption increased as the initial concentration of Ni(II). The adsorption isotherms were also determined and were appropriately described by both Langmuir and Freundlich models, with a better fitting to the Freundlich model than the Langmuir model. Therefore, NiFe 2 O 4 @SalenSi nanoadsorbents are recommended as fast, effective and inexpensive adsorbents for rapid removal and recovery of metal ions from wastewater effluents. The pseudo-second-order kinetic model provided good correlation for the adsorption of Ni(II) ions onto NiFe 2 O 4 @SalenSi. The desorption and regeneration studies indicated that nanoadsorbents can be used repeatedly, without impacting the adsorption capacity. Therefore, NiFe 2 O 4 @SalenSi nanoadsorbents are recommended as fast, effective and inexpensive adsorbents for rapid removal and recovery of metal ions from wastewater effluents.