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

CS with the deacetylation degree of 90% was prepared from our lab. Acrylic acid (AA) was obtained from Aldrich-Sigma Chem. Co. Potassium persulfate (K₂S₂O₈) was recrystallized from distilled water, glutaraldehyde (GLA), iron (III) chloride hexahydrate (FeCl₃.6H₂O), cobalt (II) chloride hexahydrate (CoCl₂.6H₂O), 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(NO₃)₂.6H₂O 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.

The synthesis of CoFe₂O₄ magnetic nanoparticles was prepared by the coprecipitation method and based on the mixture of FeCl₃ · 6H₂O and CoCl₂ · 6H₂O salts with the molar ratio of 2:1. Briefly, 1.84 g FeCl₃ · 6H₂O and 0.8 g CoCl₂ · 6H₂O 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, CoFe₂O₄ magnetic nanoparticle was obtained.

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 K₂S₂O₈ was added into the above-mentioned solution and stirring was continued at 60 °C for 1 h. Then, CoFe₂O₄ magnetic nanoparticle was added into the CS-graft-PAA solution and CoFe₂O₄/CS-graft-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.

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.

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(NO₃)₂ · 6H₂O. The stock solution was then diluted to give a standard solution of appropriate concentration.

2.5.1. Effect of initial concentration and contact time

2.5.2. Effect of pH

2.5.3. Effect of adsorbent dosage

2.5.4. Effect of temperature

The preparation of CoFe₂O₄/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 –NH₂ of CS and –COOH of AA, and then, the polymerization of AA was initiated by K₂S₂O₈ [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 CS solution. As the polymerization time extended, the amount of PAA in the solution increased, the CoFe₂O₄/CS-graft-PAA nanocomposite was formed when CoFe₂O₄ magnetic fluid was added into CS-graft-PAA solution. FTIR spectra of CoFe₂O₄ nanoparticles, CS, CS-graft-PAA and CoFe₂O₄/CS-graft-PAA nanocomposites are shown in figure 1. Absorption peak at 617 cm⁻¹ is assigned to Fe–O group in CoFe₂O₄. For the spectrum of CS, the characteristic absorption bands appeared at around 3449 cm⁻¹ corresponding to the stretching vibration of O–H and N–H bonds. The peaks at 2923 cm⁻¹, 1641 cm⁻¹, 1388 cm⁻¹, 1029 cm⁻¹ are ascribed to C–H of alkyl group, C=O of amide I, -NHCO of amide III and C–OH bond, respectively.

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The surface morphology of nanocomposite is described in figure 2. The result illustrates the surface texture and porosity of CoFe₂O₄/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].

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XRD is also used to examine the crystallite structure of CoFe₂O₄ nanoparticles, CS and CoFe₂O₄/CS-graft-PAA nanocomposites. The pattern of pure CoFe₂O₄ 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. Additionally, XRD parttern of CoFe₂O₄/CS-graft-PAA nanocomposite shows the mixed peaks of CS and the pure CoFe₂O₄ nanoparticles. However, the diffraction intensity of CoFe₂O₄/CS-graft-PAA nanoparticles is lower than that of CoFe₂O₄ due to lower CoFe₂O₄ content. This result suggests that the CoFe₂O₄ nanoparticles are incorporated into CS-graft-PAA and the structure of CoFe₂O₄ nanoparticles is not changed during the formation of CoFe₂O₄/CS-graft-PAA composite particles.

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The magnetization curves of the naked CoFe₂O₄ nanoparticles and CoFe₂O₄/CS-PAA nanocomposite are illustrated in figure 4, which shows that the magnetization of the samples would approach the saturation values when the applied magnetic field increases to 10 000 Oe. The saturation magnetization of naked CoFe₂O₄ nanoparticles is 16.579 emu g⁻¹. For the CoFe₂O₄/CS-graft-PAA nanocomposite, the saturation magnetization is about 2.632 emu g⁻¹. The loss of magnetization was attributed to the non-magnetization organic 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.

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According to TGA analysis, the content of CoFe₂O₄ nanoparticles in the CoFe₂O₄/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 CoFe₂O₄ upon thermal treatment [14].

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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 CoFe₂O₄/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 CoFe₂O₄/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.

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3.2.2. Effect of pH solution

The pH of the aqueous solution is an important variable, which controls the adsorption of the metals on the solid–water interfaces. The pH affects the availability of metal ions in solution and the metal binding sites of the adsorbent. The obtained CoFe₂O₄/CS-graft-PAA particles were incubated in buffer solution with different pH values. Results of these experiments showed that these nanocomposites were stable in distilled water and acidic media in a range of pH values from 4.1 to 7.6. However, the concentration of OH⁻ is high when pH is higher than 7.6, and OH⁻ reacts with Ni²⁺ to form the corresponding 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 CoFe₂O₄/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, CoFe₂O₄/CS-graft-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.

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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.

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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 the dose of adsorbents in the solution, the greater the availability of exchangeable sites for the ions.

3.2.4. Effect of temperature on absorption of CoFe₂O₄/CS-graft-PAA

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.

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