عنوان مقاله [English]
In this work, a new type of solid adsorbent was prepared by functionalization of carbon nanotubes and the structure and morphologies of the amine-functionalized Multi-walled carbon nanotubes were characterized by Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA) and scanning electron microscope (SEM). Its adsorption behavior towards Pd(II) and Mo(VI) ions was investigated. For this purpose, a solid phase extraction process using the prepared adsorbent was carried out in the pH range of 1 to 9. The effect of various parameters such as initial concentration, time and interfering ions on the adsorption amount was investigated, and the optimal conditions for the maximum extraction of the two ions were obtained. The adsorption of these ions by the synthesized MWNTs in real samples was examined and good results were achieved. The negative value of the change in standard Gibbs free energy (ΔG°<0) for Mo(VI) sorption by the adsorbent specify a spontaneous and realizable sorption process. The changes in free energy (ΔG °), enthalpy (ΔH °) and entropy (ΔS °) for Pd(II) ions associated with Mo(VI) ions are reversed. finally, The compatibility of the experimental data with the adsorption isotherms of Langmuir, Freundlich and Redlich-Peterson were investigated for the adsorption of Mo(VI) and Pd (II) the modified carbon nanotubes.
Keywords: Adsorption; amine-functionalized MWNT; Kinetic Study.
Palladium is a rare silver and white metal from the platinum group. Palladium (Pd) is chemically similar to platinum, is extracted from copper and nickel ores and is used to manufacture industrial catalysts and jewelry. This metal, with its low density, has the lowest melting point among the metals of the platinum group and combines strongly with sulfuric and nitric acids and slowly dissolves in hydrochloric acid. Pd does not combine with oxygen at normal temperatures. It is found along with platinum, copper, and mercury minerals. Natural palladium isotopes consist of six isotopes. The most stable radioactive isotopes of the metal is Pd-107, with a half-life of 6.5 million years. The Pd-103 has a half-life of 17 days and the Pd-100 has a half-life of 3.63 days. Another 18 radioactive isotopes are classified according to their atomic weight. With the exception of Pd-101 with a half-life of 8.47 hours, Pd-109 with a half-life of 13.7 hours, and Pd-112 with a half-life of 21 hours, the other unstable isotopes of this metal have a half-life of less than half an hour .
The abundance of molybdenum in the earth's crust is 0.007%. Molybdenum (Mo) is a white or off-white metal and its mineral is molybdenite (MoS2), which is an essential element for plant nutrition.
Molybdenum is insoluble in hydrochloric acid, sodium hydroxide, ammonia and sulfuric acid and reacts with hot or concentrated sulfuric acid or nitric acid and boiling hydrochloric acid.
Studies of the environmental hazards of molybdenum have shown that it is generally one of the most dangerous substances that can cause acute poisoning when consumed. Molybdenum can cause severe gastrointestinal irritation with diarrhea, coma, and death from heart failure. When it is absorbed into the skin, it is very harmful and irritating to the skin. It also affects the nervous system, lungs, and respiratory tract, and can cause skin allergies and severe damage to the skin, eyes, and airways. Molybdenum is used in the aerospace, automotive and electronics industries. Beans need molybdenum to remove bacterial nitrogen . Approximately 25 to 75% of the molybdenum in food is absorbed by the gastrointestinal tract and quickly excreted in the urine. Animal data show that the metabolism of molybdenum compounds is closely related to the metabolism of copper and sulfur compounds .
In recent years, various methods have been proposed to eliminate metal ions, such as chemical precipitation, reverse osmosis, electrolytic recycling, ion exchange or adsorption by various adsorbents. The adsorption process is widely used because of its simplicity and cost-effectiveness. Various sorbents such as seaweed, crab skin, dry activated sludge and activated carbon made from almond kernels have been used to remove metal ions from water. The effect of particle size of the adsorbent was discussed and a direct relationship between particle size and adsorption rate was obtained and researchers became aware of the ability of nanoparticles as adsorbents for the pollutant materials. For this purpose, many studies on the application of carbon nanotubes to remove metal ions have been carried out over the past ten years .
. Synthesis of amine-functionalized MWNTs
In order to prepare the MWNT-COOH, raw MWNTs (0.54 g) were added to a mixture of sulfuric acid and nitric acid in a volumetric ratio of 3:1 and stirred in an ultrasonic bath (40 kHz) for about 30 minutes while it was refluxed. Then the solution was washed with distilled water until the pH of the filtrate reached approximately seven. The obtained material was dried under vacuum at 60 ° C for 12 hours to give COOH-functionalized MWNT. Then 0.5 g of MWNT-COOH in 30 ml of thionyl chloride was exposed and stirred at 65 ° C for 24 hours and then filtered, washed with anhydrous tetrahydrofuran and dried under vacuum at 25 ° C for 24 hours. The yield of MWNT-COCl was obtained. Then 0.48 g of acyl chloride-functionalized MWNT in 20.0 ml of C2H8N2 were exposed to an ultrasonic bath (40 kHz) at 60 ° C for 5 hours and the solution was then stirred at 60 ° C for a further 24 hours. The yield obtained was collected by vacuum filtration followed by washing with anhydrous methanol. After repeated washing and filtering, the resulting crop was dried in a vacuum for 24 hours to give amine-functionalized MWNT (Figure 1).
The use of infrared spectrum is one of the most common methods for investigating the functionalization of carbon nanotubes. In this study, the infrared spectrum of pristine nanotubes and functionalized nanotubes has been obtained by an FTIR device model Vector22 made in Bruker Company of Germany. The sampling method was potassium bromide tablets.
Figure. 2 shows the pronounced peak developments and the peak in the range of 1670 cm-1 belongs to carbonyl group. The peak in 1051 cm-1 can be related to C-O, whose presence indicates the formation of the carboxyl functional group.
From the comparison of the two spectra, the creation of new functional groups that have formed some new absorption peaks in the spectrum is clear. In the spectrum of the adsorbent, around a wavelength of 3100 cm-1, a small wide peak is observed, which can be related to the tensile adsorption of the N – H bond. An absorption peak of about 3478 cm-1 is also observed in functionalized nanotubes, which can be due to the integration of the absorption band of the O-H group and the tensile adsorption of NH2. The peak of 1660 cm-1 in the primary nanotube can be related to the C = C bond in its structure, which after being actuated by a relatively wide peak in the range of 1670 cm-1 of the carbonyl amide group is covered
The TGA thermograms of the primary nanotube are shown in Figure 3A and the thermograms of the functionalized nanotubes are shown in Figure 3B. These thermograms were performed under the inert gas of argon from ambient temperature to 800 °C at a rate of 10 °C per minute. As can be seen in these figures, the functionalized nanotube has lost weight in three stages before the temperature of 500 degrees, but in the initial nanotube, the main weight loss occurs at about 550 °C and with a steep slope. This observation is consistent with the presence of significant functional groups on the surface of the synthesized nanotubes.
The SEM image of the primary nanotube is shown in Figure 4A and the SEM images of the functionalized nanotubes are shown in Figures 4B and C. As can be seen from these photos, it can be said that the functional groups have almost uniformly covered the surface of the nanotubes.
According to the results obtained in this project, significant points can be shown in connection with the operational nanotubes and their adsorption of Pd (II) and Mo (VI) ions:
1- The results of examining the relationship between the adsorption percentage and initial concentrations show interesting differences. For example, it has been shown that the percent of Mo(VI) and Pd(II) adsorption through nanotubes depends on their initial concentrations in the solution. The adsorption percentage concentrations of about 2 mg/L for palladium ions and 1 to 2 mg/L for molybdenum ions. The different optimal pH and the initial concentrations for the two ions can be successfully used for separation of these ions from each other. In addition, the difference in behavior of different detergents increases the chances of achieving this goal.2- The table of the ion adsorption time shows that the behavior of ions in the adsorption rate is different, which can be taken into account and used.3- Examination of the percentage of saturation of synthesized nanotubes at different times .