Studying Ni(II) Adsorption of Magnetite/Graphene Oxide/Chitosan Nanocomposite
In this paper, Fe3O4/graphene oxide/chitosan (FGC) nanocomposite was synthesized using coprecipitation method for application to removal of nickel ion (Ni(II)) from aqueous solution by adsorption process. To determine residue Ni(II) ions concentration in aqueous solution after adsorption process, we have used UV-Vis spectrophotometric method, which is an effective and exact method for Ni(II) monitoring at low level by using dimethylglyoxime (DMG) as a complex reagent with Ni(II), which has a specific adsorption peak at the wavelength of 550 nm on UV-Vis spectra. A number of factors that influence Ni(II) ions adsorption capacity of FGC nanocomposite such as contact time, adsorption temperature, and adsorbent dosage were investigated. Results showed that the adsorption equilibrium is established after 70 minutes with the adsorbent dosage of 0.01 g.mL−1 at 30°C (the room temperature). The thermodynamic and kinetic parameters of this adsorption including free enthalpy change (∆G0), enthalpy change (∆H0), entropy change (∆S0), and reaction order with respect to Ni(II) ions were also determined. The Ni(II) ions adsorption equilibrium data are fitted well to the Langmuir isotherm and the maximum monolayer capacity () is 12.24 mg.g−1. Moreover, the FGC adsorbent can be recovered by an external magnet; in addition, it can be regenerated. The reusability of FGC was tested and results showed that 83.08% of removal efficiency was obtained after 3 cycles. The synthesized FGC nanocomposite with many advantages is a promising material for removal of heavy metal ions from aqueous solution to clean up the environment.
Nickel ion (Ni(II)) is mainly generated from petroleum procedure, plating industry wastewater which is released into the natural environment and is toxic to nature. Drinking of nickel(II) polluted water for a long time will cause cancer, lungs and nervous system problem, or dry cough . Therefore, removal of Ni(II) from the aquatic environment is a serious environmental problem in view of public health. Graphene with high specific surface area, chemical stability, and excellent electrical, thermal properties  recently has received increasing attention of researchers all over the world in the area of adsorption. However, the disadvantages of graphene sheets are that, in the water environment, they are poorly soluble and tend to aggregate, which significantly reduces the surface area and adsorption capacity. Thus, graphene oxide (GO), the intermediate product of oxidation of graphite, with many oxygen-rich functional groups (epoxy, hydroxyl, and carbonyl groups), is an attractive object for many research areas such as detection of DNA , matrix composite membranes, or film [4–6], especially in removal of heavy metal ions and organic pollutants from aqueous solutions. In recent years, a number of reports have been published on the adsorption of heavy metal ions by using GO and GO-based materials as an adsorbent. Table 1 listed the maximum adsorption capacity of some heavy metal ions on GO and some GO-based materials. From that, GO proves the strong adsorption affinity and is a good adsorption material.
In this study, we have synthesized Fe3O4/graphene oxide/chitosan (FGC) nanocomposite and used it as recoverable adsorbent for the adsorption of Ni(II) ions in aqueous solution with the ambition of using functional groups (epoxy, carboxyl) of graphene and amino, hydroxyl groups of chitosan to enhance the adsorption interactions with heavy metal ions to reduce the high cost of graphene materials and increase the efficiency of the process. By using a magnetic material (Fe3O4), the separation of small particle size of adsorbent will be rapid and the secondary discharge to environment will be avoided. The different equilibrium conditions and kinetics of adsorption Ni(II) are also investigated in detail.
2.1. Chemical and Reagents
Graphite was extracted from pencils which were purchased from a local bookstore. Dimethylglyoxime (CH3C(NOH)C(NOH)CH3) (DMG), sulfuric acid (H2SO4) 98 wt.%, sodium nitrate (NaNO3), potassium permanganate (KMnO4), NiCl2.6H2O, iron(III) chloride hexahydrate (FeCl3.6H2O), iron(II) sulfate heptahydrate (FeSO4.7H2O), chitosan (CS), acetic acid (CH3COOH) solution 30 wt.%, and sodium hydroxide (NaOH) were purchased from Sigma Aldrich.
2.2. Preparation of Fe3O4/Graphene Oxide/Chitosan (FGC) Nanocomposite
Fe3O4/graphene oxide/chitosan (FGC) materials were synthesized using coprecipitation method as previously reported  with several modifications. Three precursors, FeSO4.4H2O, FeCl3.6H2O solution with the molar ratio of 2:1; GO dispersion, and chitosan (2.5 wt.%) solution, are used directly without any purification. The composition of sample in this study is Fe3O4 : GO : CS = 42.5 : 7.5 : 50 (in mass), whereby a mixture of three precursors with various volumes was stirred continuously (using IKA magnetic stirrer with stirring rate of 200 rpm) in a flask for 30 minutes to obtain a homogeneous mixture. NaOH solution is dropped slowly to this flask to obtain the Fe3O4/GO/CS suspension. This suspension was kept at room temperature for 18 hours without stirring and then washed by distilled water for many times to remove all base (pH reaches 7). It was dried in vacuum at 78°C for 18 hours; we obtained the FGC adsorbent. XRD pattern and transmission electron microscopy (TEM) of GO, Fe3O4, and FGC adsorbent are shown in Figure SI.1 and Figure SI.2 in supporting information (SI), respectively.
2.3. Batch Experiments
A stock of Ni(II) solution is prepared by dissolving NiCl2.6H2O in distilled water (5.8 g.L−1). Experimental solutions with different concentrations are obtained by diluting a stock solution. A typical adsorption experiment was carried out through the following procedure: 10 mg of FGC powder was added to 1 mL of Ni(II) containing solution. This mixture was stirred for adsorption process in a water batch for 70 minutes. Then, FGC nanocomposite was removed by an external magnet and residue Ni(II) in solution will be determined.
Working solutions were prepared daily, consisting of 1.725 mL distilled water, 0.1 mL DMG solution, 0.3 mL ammonia solution, 0.25 mL saturated Br2 solution, and 0.125 mL experimental solution. The intensity of color was measured using UV-Vis spectrophotometer. The effects of contact time, temperature, and kinetics of Ni(II) adsorption on FGC nanocomposite were studied. After that, the adsorbent was regenerated to see the reusability of this material.
The reaction order with respect to Ni(II) ions was studied using Lambert-Beer equation as described in where A is absorbance, ε(λ) is a molar absorbed factor (this factor changes by changing λ and it characterizes each substance), l is a length of cuvette, and C is concentration of Ni(II) solution vs. time. Thus, if absorbance of a substance is measured at a known wavelength (λ) and l = 1 cm constant, so absorbance (A) only depends on concentration (C). Therefore, to estimate the reaction order with respect to Ni(II) ions, the relation between absorbance A and time t is investigated, instead of investigating the relation between C and time t, using the following Eq.:where A0 and are the initial and equilibrium absorbance, respectively; A is the absorbance at time t; C0 and are the initial concentration and the concentration at time t, respectively. Thus, the reaction order with respect to Ni(II) ions is extracted by drawing the plot of ln(A-) vs. t.
The amount of Ni(II) ions uptake by FGC (, mg.g−1) was calculated by the following equation:where C0 and (mg.L−1) are the initial and equilibrium concentrations of Ni(II) ions in solution, respectively; is the concentration of FGC (g.L−1).
The thermodynamic parameters of the Ni(II) ions adsorption such as enthalpy change (∆H0), entropy change (∆S0), and free enthalpy change (∆G0) are also calculated using the following equations :where and are the initial and equilibrium concentrations of Ni(II) ions (mol.L−1), respectively; R is gas constant (R = 8,314 J.mol−1.K−1); T is absolute temperature (K).
The Langmuir equation is given as in the following Eq.: where (mg.g−1) is the amount of Ni ions adsorbed at equilibrium, (mg.g−1) is the maximum Ni ions adsorption amount, and is the equilibrium adsorption constant.
The Freundlich isotherm is given as:where and n are Freundlich constant and obtained from the intercept and the slope of the linear plot of log vs. .
3. Results and Discussion
3.1. Calibration Curve
DMG was used to recognize the small amount of Ni(II) in the solution by spectroscopy. In the presence of Ni(II) ion, the solution containing DMG color will change from colorless to red color. The red color is darker if the Ni ion content is high. 1.2% alcoholic DMG solutions were obtained by dissolving the amounts of the solid in absolute ethanol and used within no more than 2 weeks. To generate a calibration curve for determination Ni(II) residue in solution, various solutions with different Ni(II) concentrations in DMG were prepared. Ni(II) ions in aqueous solution with low concentration values were determined effectively and exactly by UV-Vis spectrophotometric method using DMG as a complex reagent at wavelength of 550 nm. Figure 1(a) shows UV-Vis spectra of Ni(II) solutions with different Ni(II) concentrations. Figure 1(b) shows the calibration curve for determining of Ni(II) concentration in solution that has been generated by drawing the optical density at wavelength of 550 nm () vs. Ni(II) concentration. The remaining concentrations of Ni(II) in solutions after adsorption process have been determined by measuring UV-Vis spectra and using the calibration curve (Figure 1(b)).
3.2. Effect of Contact Time
The UV-Vis spectra of 135 mg.L−1 Ni (II) solution after adsorption process are shown in Figure 2(a). From these experimental results, the concentration of remaining Ni(II) ions in the solution has been determined by using the calibration curve, Figure 1(b). The effect of contact time on the Ni(II) ions adsorption capacity is shown in Figure 2(b). The result indicates that, from 5 to 70 minutes, the concentration of the remaining Ni(II) ions in the solution decreases, so the adsorption capacity increases with the increasing contact time. In the first 70 minutes, the Ni(II) ions are adsorbed rapidly. The adsorption equilibrium is established after 70 minutes.
Using (1) and (2), the reaction order with respect to Ni(II) ions is extracted by drawing the plot of ln(A-) vs. t, Figure 3. As can be seen in Figure 3, the obtained plot is nearly linear with the correlation coefficient (R2 = 0.9705), which means the reaction order with respect to Ni(II) is 1 (the first order).
3.3. Effect of Temperature
The UV-Vis spectra of 135 mg.L−1 Ni (II) solutions after 70 minutes of adsorption process at different temperatures are shown in Figure 4(a). From these experimental results, the concentration of the remaining Ni(II) ions in the solution has been calculated by using the calibration curve, Figure 1(b). Then the amount of Ni(II) ions uptake by FGC (, mg.g−1) was calculated by using (3).
The effect of temperature on the Ni(II) ions adsorption capacity is shown in Figure 4(b). It can be seen in Figure 4(b) that the adsorption capacity increases with the increasing temperature from 30°C to 35°C. When the temperature continues to increase from 35°C to 50°C, the adsorption capacity decreases. This result can be attributed to the fact that at the high temperature and the pore sites of chitosan on the adsorbent surface are miniature and inactivated.
The values of ∆G0 calculated by using (4), (5), and (6) at 303, 308, 313, 318, and 323 K are -2.95, -3.69, -2.39, -1.42, and -1.21 (kJ.mol−1), respectively. The negative values of ∆G0 indicate that the adsorption Ni(II) ions process is spontaneous. From the linear fit of lnK0 vs. 1/T (see (7)), ∆H0 and ∆S0 values calculated are -30.97 kJ.mol−1 and -92.2 J.mol−1.K−1, respectively. The negative value of ∆H0 indicates that the Ni(II) ions adsorption process is exothermic. The negative value of ∆S0 shows the decreasing of randomness during the adsorption process of Ni(II) ions onto FGC surface.
3.4. Effect of Adsorbent Dosage
The effect of adsorbent dosage on the Ni(II) ions adsorption capacity of FGC nanocomposite is shown in Figure 5. The amount of FGC nanocomposite has been changed from 0.01 to 0.03 g.mL−1 in order to optimize the adsorbent dosage condition. As can be seen in Figure 5, the optimized amount of the adsorbent (FGC nanocomposite) is 0.01 g.mL−1. With adsorbent dosage of 0.01 g.mL−1, the Ni(II) ions adsorption capacity on FGC nanocomposite reaches the highest value ( = 10.30 mg.g−1). This result indicates that the adsorbent has a high adsorption capacity, resulting in a small amount of adsorbent being able to adsorb maximum amount of Ni(II) ions in the solution.
3.5. Adsorption Isotherm
Langmuir and Freundlich adsorption isotherms (see (8) and (9)), respectively, were used to analyze the adsorption data. The Langmuir isotherm is assumed for monolayer and homogeneous site of adsorbent surface without transmigration in the plane and uniform adsorption . The Freundlich isotherm is valid for heterogeneous surface. The results are shown on Figure 6. Langmuir model with a higher correlation coefficient (R2 = 0.9513) compares to R2 of Freundlich (R2 = 0.7857), indicating that the adsorption was fitted well to the Langmuir isotherm.
The maximum capacity is 12.24 mg.g−1; and n are 1.79 and 2.55, respectively. With this value of n, the adsorption is favored  and the adsorption process is physical (n > 1) . Compared to other adsorbents, the adsorption capacity of FGC is higher than some adsorbents as clay  and CS1501  as shown in Table 2.
3.6. Regeneration Studies
FESEM images of FGC nanocomposite before and after Ni(II) ions adsorption process are shown in Figures 7(a), 7(b), 7(c), and 7(d), respectively. It can be seen that the surface morphologies of FGC before and after Ni(II) ions adsorption are not changed. However, on FGC surface after Ni(II) adsorption (Figures 7(c) and 7(d)), the surface is coated by a layer of small particles with different contrast (Figures 7(a) and 7(b)), which can be attributed to uptake of Ni(II) onto FGC surface.
After recovery by an external magnet, FGC nanocomposite was regenerated by 0.1 M NaOH solution in 3 hours for deadsorption of Ni(II) ions. Then the adsorbent was cleaned many times with distilled water and dried for reuse.
The Ni(II) ions efficient removal of regenerated materials was compared to the original materials (as synthesized) and the results are shown in Figure 8. Figure 8(a) shows UV-Vis spectra of Ni(II) solutions before and after 1 to 3 adsorption-regeneration cycles. As can be seen in Figure 8(b), the Ni(II) ions efficient removal of FGC nanocomposite was still 83.08% after 3 adsorption-regeneration cycles. The adsorption capacity for Ni(II) ions slightly decreased; that is probably a result of losing binding sites after each desorption step . In our future works, optimizing the pH of the solution used for deadsorption is really necessary in order to increase the Ni(II) ions adsorption capacity after a large number of adsorption-regeneration cycles. However, the above result has indicated that the synthesized FGC nanocomposite has a long-term stability and is fit for treating the heavy metal ions. In fact, in our previous work on adsorption Cr (VI) from aqueous solution, with the same material (FGC), can reach 200 mg.g−1 at 298 K, pH 3 . Therefore, this synthesized material can be used as a promising adsorbent for removal of heavy metal ions from aqueous solution.
This study indicated that Ni(II) ions can be removed from aqueous solution using synthesized FGC nanocomposite by adsorption method. Ni(II) ions in aqueous solution with lower concentration than 58.69 mg.L−1 were determined effectively and exactly by UV-Vis spectrophotometric method using dimethylglyoxime (DMG) as a complex reagent. The results show that the optimized adsorbent dosage for adsorption is 10 mg.mL−1 FGC with the Ni(II) ions initial concentration of 135 mg.L−1. The thermodynamic parameters indicate that the adsorption process is spontaneous and exothermic and decreases the randomness. The adsorption fitted the Langmuir model well and the highest adsorption capacity is 12.24 mg.g−1. After 3 cycles, the Ni(II) ions adsorption capacity of FGC was about 83.08% comparing to the original sample, proving that it is a good material for removal of heavy metal ions in aqueous solution.
The data used to support the findings of this study are included within the article and included within the supplementary information file(s).
Conflicts of Interest
The authors declare that they have no conflicts of interest.
XRD pattern and transmission electron microscopy (TEM) of GO, Fe3O4, and FGC adsorbent are shown in Figure SI.1 and Figure SI.2 in supporting information (SI), respectively. (Supplementary Materials)
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