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Reusable nanocomposite of CoFe2O4/chitosan-graft-poly(acrylic acid) for removal of Ni(II) from aqueous solution

In this paper, CoFe2O4/chitosan-graft-poly(acrylic acid) (CoFe2O4/CS-graft-PAA) nanocomposites were prepared successfully by coprecipitation of the compounds in alkaline solution and were used for removal of nickel (II) ions from aqueous solution. The sorption rate was affected significantly by the initial concentration of the solution, sorbent amount, and pH value of the solution. Batch experiments were conducted to investigate the adsorption capacity under different initial concentration (ranging from 25 to 150 mg L−1), solution pH (4.1, 5.3, 6.4 and 7.6), and contact time. These nanocomposites can be recycled conveniently from water with the assistance of an external magnet because of their exceptional properties. The prepared nanocomposites were characterized by Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), x-ray powder diffraction (XRD), and thermogravimetric analysis (TGA).


Introduction
The toxic heavy metals present in wastewater, effluents and soils are becoming an increasingly important issue because of growing economic and environmental concerns. One of the metals released to the environment is nickel from a number of sources such as electroplating processes, nickel batteries, alloys, and steels [1]. Several methods for removal of the heavy metals from wastewater have been reported including precipitation, filtration, ion exchange, biological process, chemical reactions, and adsorption [2,3]. Among these, adsorption is considered as an effective and economical method for the removal of pollutants from wastewater due to ease of handling, and effective and economical treatment process. There are many materials used for adsorption of heavy metal such as active carbon, chitosan (CS) bead, betonies and so on. However, these materials are difficult or not able to be recycled after adsorption, if the adsorbent was prepared with a very small size for improvement of the adsorption efficiency by increase of the surface area. Therefore, it is interesting to improve the separability of the adsorbents having small size. Recently, attention has been paid to the magnetic separation technology. With the aid of magnetic force, magnetic matters can be separated from the water efficiently regardless of its size.
CS has also increasingly been studied as an adsorbent for the removal of heavy metal ions from aqueous solutions because the amino and hydroxyl groups on the CS chain act as chelation or reaction sites for the substances. These groups function as the coordination sites for heavy metal ions [4,5]. Several studies of metal ion adsorption by CS have been carried out in recent years, such as the removal of copper [6], chromium [7], cadmium, nickel and lead ions from aqueous solution [3]. The aim of this study was to synthesize magnetic chitosan-graft-poly(acrylic acid) composite (CoFe 2 O 4 /CSgraft-PAA) for removal of the heavy metal from aqueous solution. The sorption of CoFe 2 O 4 /CS-graft-PAA nanoparticles for nickel ion was also evaluated. The effect of various experimental conditions such as the initial concentration, sorbent amount, temperature and pH value of the solution on the adsorption capacity of the magnetic composite for nickel ions were investigated.

Materials
CS with the deacetylation degree of 90% was prepared from our lab. Acrylic acid (AA) was obtained from Aldrich-Sigma Chem. Co. Potassium persulfate (K 2 S 2 O 8 ) was recrystallized from distilled water, glutaraldehyde (GLA), iron (III) chloride hexahydrate (FeCl 3 .6H 2 O), cobalt (II) chloride hexahydrate (CoCl 2 .6H 2 O), sodium hydroxide (NaOH) were purchased from Aldrich-Sigma Chem. Co. All the other materials were reagent grade and used without further purification. Metal stock solutions were prepared (1000 ppm) by dissolving the Ni(NO 3 ) 2 .6H 2 O metal salts in deionized distilled water. The pH of 4-6 and 8 of medium solutions was adjusted by adding acetate buffer solutions and ammonium chloride solution, respectively. Deionized distilled water was used in all experiments.

Preparation of CoFe 2 O 4 magnetic nanoparticle
The synthesis of CoFe 2 O 4 magnetic nanoparticles was prepared by the coprecipitation method and based on the mixture of FeCl 3 · 6H 2 O and CoCl 2 · 6H 2 O salts with the molar ratio of 2:1. Briefly, 1.84 g FeCl 3 · 6H 2 O and 0.8 g CoCl 2 · 6H 2 O were dissolved in 85 mL of deionized water. 2 M solution of NaOH was prepared and slowly added to the salt solution dropwise. The pH of the solution was constantly monitored by adding the NaOH solution under a magnetic stirrer until a pH level of 11-12 was reached. The precipitation immediately occurred to change the reaction solution to dark-black. Stirring of the resulting solution was continued for one hour and the product was cooled to room temperature. After that, the precipitation was magnetically separated by an external magnet, and the pH level was reduced to approximately 7-8 by washing with deionized water and then was washed with ethanol to remove the excess agents from the solution. Finally, CoFe 2 O 4 magnetic nanoparticle was obtained.

Preparation of CoFe 2 O 4 /CS-graft-PAA nanocomposite
The CS-graft-PAA copolymer was obtained by polymerization of AA in CS solution. 4 g of CS flakes was dissolved in 400 ml of 1% (w/w) AA solution under mechanical stirring. Until the solution became clear, 1 mmol of K 2 S 2 O 8 was added into the above-mentioned solution and stirring was continued at 60°C for 1 h. Then, CoFe 2 O 4 magnetic nanoparticle was added into the CS-graft-PAA solution and CoFe 2 O 4 /CSgraft-PAA nanocomposite was obtained by adding 2 M NaOH solution; final pH of the solution was 9-10 and the reaction was allowed to proceed at 60°C for 2 h. At the end of polymerization, 4 mL of 25% GLA was added to the reaction system at 40°C and cross-linking reaction was conducted for 1 h. The final product was washed with distilled water several times until pH reached neutral, then collected by magnetic separation and dried for further application.

Characterization
Scanning electron microscopy (SEM) images were taken with 7410F-JMS-JEOL electronic scanning microscope (Japan) to examine the morphology and surface structure of the adsorbent. The transmission Fourier transform infrared spectroscopy (FTIR) spectra were recorded with a Vector 22-Bruker (Germany) and the range of the scanning wave numbers was 500-4000 cm −1 . The structural properties of synthesized nanoparticles were analyzed by x-ray powder diffraction (XRD) with a D8 Advance-Bruker System (Germany). Thermogravimetric analysis (TGA), thermal stability as well as the content of cobalt ferrite in the nanocomposites were analyzed by a thermogravimetric analyzer (Q500, Japan) in air atmosphere. The temperature ranged from 30 to 800°C at a scanning rate of 10°C min −1 . Magnetic characterization of the nanoparticles was conducted using vibrating sample magnetometer (EV11-VSM, USA) at room temperature with a magnetic field in the range of −10 kOe to 10 kOe, where parameters such as saturation magnetization and coercive field were evaluated.

Batch adsorption studies
The adsorption of Ni(II) was analyzed in a batch system at room temperature with varied concentrations ranging from 25-50 mg L −1 . The solutions were prepared in deionized distilled water using the Ni(NO 3 ) 2 · 6H 2 O. The stock solution was then diluted to give a standard solution of appropriate concentration.
2.5.1. Effect of initial concentration and contact time. In brief, 50 mL of metal ion solution was placed in a 100 mL beaker, and then 0.5 g CoFe 2 O 4 /CS-graft-PAA nanocomposite was added. The mixtures were then kept at room temperature. The aqueous samples were taken at various contact times: 0.5, 1, 2, 3, 4 and 5 h. The concentrations of Ni ions were measured using standard methods recommended for examination of water and wastewater [8]. The amount of adsorption at time t, , where C 0 and C t (mg L −1 ) are the liquid-phase concentrations of Ni ions at initial and any time t, respectively; V is the volume of the solution (L); W is the mass of used dry adsorbent (g). The percentage adsorption was determined by , where ′ C 0 and C e are the initial and equilibrium concentration of metal ion solution (mg L −1 ).

Characterization of CoFe 2 O 4 /CS-PAA nanocomposites
The preparation of CoFe 2 O 4 /CS-graft-PAA nanocomposites was conducted through coprecipitation of the mixture containing CS-graft-PAA copolymer and ferric compounds in alkaline solution were described in the experimental part. First, when CS is added into AA aqueous solution, CS-AA salts will be formed through the strong electrostatic interaction between -NH 2 of CS and -COOH of AA, and then, the polymerization of AA was initiated by K 2 S 2 O 8 [9]. When the polymerization of AA reached a certain level, the inter-and intra-molecular linkages occurred between carboxyl groups from PAA and positively charged amino groups of CS. These linkages could make the macromolecular chains of CS roll up, which was responsible for the formation of the gelation of the The surface morphology of nanocomposite is described in figure 2. The result illustrates the surface texture and porosity of CoFe 2 O 4 /CS-graft-PAA nanocomposite with holes and small openings on the surface, thereby increasing the contact area, which facilitates the pore diffusion during adsorption [10].
XRD is also used to examine the crystallite structure of CoFe 2 O 4 nanoparticles, CS and CoFe 2 O 4 /CS-graft-PAA nanocomposites. The pattern of pure CoFe 2 O 4 is shown in figure 3, the main peaks are at 2θ values of 30°, 35.8°, 43.2°, 57°and 62.9°. These peaks match well with face-centered cubic cobalt ferrite. These results are consistent with the previous results [11,12]. The XRD patterns of the CS exhibit two characteristic peaks at 2θ values of 11.3°and 20.2°. The peak at 20.2°shows the allomorphic tendon form of CS, which resulted in a strong decrease in the sorption capacities. components, which reduced the total magnetization [13]. The composite particles show strong magnetic properties and can be separated easily from the solution with the help of an external magnetic force. A narrow hysteresis loop can be seen in figure 4. There is a small remnant magnetization. It could be explained that some particles are magnetically blocked. According to TGA analysis, the content of CoFe 2 O 4 nanoparticles in the CoFe 2 O 4 /CS-PAA nanocomposites is about 17.7% ( figure 5). There are three stage weight losses in the temperature ranges from 40 to 800°C. First weight loss is about 11.1% ranged 40-100°C, which corresponds to the adsorbed and bound water in the sample. The second weight loss is about 32.3% in the range from 100-380°C, which was ascribed to the decomposition of CS and the carboxyl groups of PAA. Moreover, the weight loss of 38.9% in the range from 380 to 800°C is due to the breakage of PAA chain and the dehydroxylation of CoFe 2 O 4 upon thermal treatment [14].

Batch adsorption experiment
3.2.1. Effect of initial concentration and contact time. The adsorption capacity depends on the initial concentration and the contact time. As shown in figure 6, it is remarkable that an increase in the initial concentration (25-150 ppm) for Ni(II) ions led to an increase in the adsorption capacity of CoFe 2 O 4 / CS-graft-PAA nanocomposite at various contact times. Other parameters such as dose of the adsorbent, pH of the solution and agitation speed were kept constant. This can be attributed to the mass transfer effects and the driving force of the concentration gradient being directly proportional to the initial concentrations. The plateau values indicated that the adsorption equilibrium was gradually attained. The equilibrium adsorption capacity of Ni(II) ions was 2.46, 4.96, 9.86 and 13.84 mg g −1 CoFe 2 O 4 /CS-graft-PAA for 25, 50, 100 and 150 ppm, respectively. The sorption percentage was higher than 98% with concentration of 100 ppm and decreased to 92.3% at 150 ppm. It appears that as adsorbate concentration was raised, binding capacity of the adsorbent reached instantaneous saturation resulting in diminishing the overall % adsorption of Ni ions. precipitations, which causes the decline of absorption capacity. The result is graphically represented in figure 7.
The maximum adsorption capacity of Ni(II) ions occurred at pH 5.3. In addition, the maximal Ni(II) uptakes of CoFe 2 O 4 / CS-graft-PAA was 4.92 (mg g −1 ) and the amount of sorption percentage was 99.2%. After combination of PAA and CS, the resultant composite could have not only improved Ni(II) uptake but also good biodegradability. Overall, CoFe 2 O 4 /CSgraft-PAA could be considered as a promising adsorbent for removal of metal ions for high adsorption capacity, good biodegradability and rapid separation ability from aqueous solutions.

3.2.3.
Effect of adsorbent dosage. The dependence of metal ion sorption on dose was studied by varying the amount of adsorbents from 0.1 g to 0.5 g, while keeping the volume (50 mL) and concentration (50 ppm) of the solution constant at room temperature and corresponding to optimal pH of solution. The results are graphically represented in figure 8. It can be seen that the rate of removal of Ni ions increases with the increase in the dose of adsorbent. The percentages of removal increased from 93 to 95% with dose from 0.1 g to 0.5 g, respectively. There was a substantial increase in adsorption when the dose of adsorbents was increased from 0.1 g to 0.5 g. This was expected due to the fact of the higher   It was observed that temperature had a pivotal role in the complex of metal ion and CS derivative. As shown in figure 9, the absorption of metal ions appeared to increase as the temperature increased to 50°C. The absorption capacity increases from 4.62 to 4.88 mg g −1 . It could be explained that the absorbent molecular and absorbed ions move much faster at the high temperature, the interactions among molecules decrease, and the metal ions can more easily move into absorbent. As a result, the contact area between absorbent and metal ions increases, which leads to the increase of the absorption capacity of absorbent for metal ions. On the other hand, the absorption here is an endothermic process, therefore, the rise of temperature promotes the absorption of absorbent for metal ions; the temperature has to be optimum, preferably at 50°C.

Conclusions
Magnetic CS-graft-PAA nanocomposite with a high CoFe 2 O 4 loading content have been readily prepared by polymerization of AA with CS. The magnetic composite was characterized by FTIR, XRD, SEM, TGA and VSM. This study has evidenced the improved adsorption behavior of CoFe 2 O 4 /CSgraft-PAA to remove nickel ions from water. In addition, the magnetic composite adsorbents also showed superparamagnetic property and significant improvement of the separability from aqueous solutions.