In recent years, extensive attention has been paid to environmental problems. Photocatalysis technology is a useful strategy for solving the energy and environment issues due to the generation of photo-induced electrons and holes through light irradiating on the photocatalysts that would simplify the removal of pollutants in water, air, and organic synthesis [1-2]. Some of the dyes such as azo dyes are used for their high performance in metallurgy, carpet, paper, textile, and leather industries. Azo dyes are toxic, stable in nature; they have high thermal stability, carcinogenicity, and many detrimental effects on human health, aquatic systems, and animals . Congo red is some type of the anionic synthetic dyes which has two azo (–N=N–) chromophores. This compound is more stable than other azo dyes, because of its complex aromatic structure . Recently, many scientists have been attempting to eliminate these pollutants. Available methods for removal of azo dyes include chemical oxidation, coagulation , adsorption, and electrochemical processes . These methods are not effective for removal of the mentioned organic materials from industrial wastes  since in these methods; photocatalytic processes are very slow at high concentrations or rather ineffective at very low amounts of contaminants. Also, the contaminant changes from one phase to another phase. Among all these methods, photocatalytic technology has attracted much attention through employing the semiconductors for the water treatment because of their functions in green chemistry, their nontoxicity, their energy-conserving applications, and low-cost functionality, as it employs only UV and Visible light to decompose pollutants [8-17]. Spinel oxides with the general chemical formula of AB2O4, A, and B are two different cations with different oxidation numbers. A is a divalent ion and B is a trivalent ion. They have been used as ceramics, optical materials, catalysts, and photocatalysts because of their unique properties [18, 19]. Several spinels are semiconductor materials that have been used in photocatalytic removal of pollutants or photocatalytic composition in water. The great benefit of this material is that their band gaps and other properties can be handled easily.
Magnesium aluminate spinel (MgAl2O4) is one of the most important oxidizing spinels and ceramic materials that has good mechanical properties and encompasses a wide range of applications that can tolerate very high temperatures [20-21]. Since it has a very high melting temperature (2135 0C), it is resistant to mechanical stress, and thermal shock, at room temperature and higher temperatures. Also, its unique optical properties and low dielectric constant provide special applications for the spinel. Recently, MgAl2O4 has been received attention as a photocatalyst and many research groups have studied and used it as a photocatalyst on different materials [22, 23]. Magnesium aluminate spinel has a relatively high band gap as a semiconductor. It can be used to reduce the band gap and improve photocatalytic properties by doping these nanoparticles with metals such as Ni2+, Cu2+, and Zn2+.
Different methods such as sol–gel [24, 25], co-precipitation , solid state method, have been reported for the synthesis of (MgAl2O4). Among these methods, sol–gel, and co-precipitation are very common. Due to it can produce smaller nanoparticles and requires a lower synthesis temperature compared to other methods. Using a hybrid sol–gel combustion method at relatively low temperature is a new method to produce magnesium aluminate nanoparticles suitable for different applications, especially the photocatalytic.
In this study, Magnesium aluminate spinel (MgAl2O4) nanoparticles were synthesized and doped with different mass percentages of Ni2+ by using the sol-gel auto combustion method. These nanoparticles were used as a photocatalyst for the removal of Congo red dye. Photocatalytic activity of these materials was studied under UV light radiation for Congo red, with magnesium aluminate spinel doped with different percentages of Ni2+ for 90 minutes.
Materials and methods
All chemicals were purchased from Merck or Sigma-Aldrich with the analytical grade. Fourier transform infrared (FT-IR) spectra were obtained using the FT BOMEM MB102 IR spectrophotometer. X-ray diffraction (XRD) patterns of the synthesized samples were obtained with a Philips X-ray diffractometer (model PW1730). The SEM and EDX images were obtained by Hitachi Scanning Electron Microscope. The photocatalytic reactions were performed using a photoreactor in which four UV lamps with λmax= 254 nm, manufactured by Holland (UV-C, 6 W, PHILIPS) were operated.
Preparation of nanoparticles
The analytical grades of 6.8 mmol magnesium nitrate Mg(NO3)2.6H2O, 6 mmol aluminum nitrate Al(NO3)2.9H2O, and different percentages of nickel nitrate Ni(NO3)2 were dissolved in 5 ml of distilled water and stirred at 60 0C for 10 minutes. The quantities of nickel nitrate Ni (NO3)2.6H2O are shown in Table 1. After that, 44.25 mmol of urea was dissolved in 25 ml of distilled water and added to this mixture and refluxed at 80 0C for 1 hour and then refluxed at 120 0C for 5 hours. It was then cooled slowly at room temperature and was placed for 2 hours at 200 0C in an oven and then calcined at 1000 0C for 4 hours . The photocatalytic activity of the synthesized spinel catalyst was assessed by monitoring the removal of the Congo red solution under the UV light irradiation. A UV lamp was used as the UV light source and was positioned parallel to the quartz tube. In each experiment, 0.1 g of the catalyst was dispersed in 50 mL of Congo red solutions with a concentration of 10 mg/L, at room temperature. To earn the adsorption/desorption equilibrium, the suspensions were stirred in a dark room and ultrasonic bath for 30 min before the irradiation. During the irradiation process, stirring was continued to hold the mixture in the suspension form. At given time intervals, around 2 mL of the suspension was taken every 10 minutes and centrifuged to remove the photocatalyst. After that, the solution was analyzed by UV-Vis spectrophotometry at a wavelength of 505 nm. The decolorization rate D was calculated by equation (1), where C0 is the absorbance of the first solution, and C is the absorbance of the solution at different times, and different irradiation.
D= A0-A/A0100 (1)
RESULTS AND DISCUSSION
MgAl2O4 was prepared by sol-gel auto combustion method. The FT-IR spectra of MgAl2O4 and Mg1-XNiXAl2O4 (x= 0.1, 0.05, 0.03) are presented in Fig.1. (see Fig.1 a, b, c, d). In the IR spectra, the MgAl2O4 samples displayed two stretching bands at 555 cm-1 and 700 cm-1 consecrated to the [AlO6] groups, the vibration of Mg–O stretching, and displaying the formation of MgAl2O4 spinel. Peak intensity increased at 700 cm-1 and 555 cm-1 [28, 29] with increasing nickel, which indicates that the crystalline properties of the samples increase. The absorption spectra in the region of 1630 cm-1 and 3437 cm-1 are the bending vibrations associated with water .
The XRD patterns of MgAl2O4 and Mg1-XNiXAl2O4 are shown in Fig.2 a, b, c, d. The pattern of magnesium aluminate spinel with the standard card (JCPDS card no. 01-075-1798), indicates the retention of the spinel structure of MgAl2O4 during the doping with Ni2+. The diffraction peaks related to Bragg’s reflections from (111), (220), (311), (400), (422), (511), (440), and (533) planes correspond to the standard spinel structure of MgAl2O4. No extra peaks of the phase have been observed, in following the standard X-ray powder diffraction pattern of magnesium aluminate spinel. The crystallite size of MgAl2O4 nanoparticles was about 40 nm, as determined by using the Debye–Scherrer formula. As indicated in Fig 2. the intensity of peaks related to (220), (311), (400), and (511) planes increases with the increase in the amount of Ni2+ substitutions in comparison with the MgAl2O4 and Mg1-XNiXAl2O4 indicating the formation of the Mg1-XNiXAl2O4 solid solution and the increase in the crystallinity of the samples [31, 32].
Scanning Electron Microscopy (SEM)
SEM images of magnesium aluminate spinel nanoparticles, Mg0.97Ni0.03Al2O4, Mg0.95Ni0.05Al2O4, Mg0.9Ni0.1Al2O4, are shown in Fig. 3 a, b, c, d. According to the SEM images, magnesium aluminate spinel nanoparticles have the spherical morphology and are uniform. SEM images of magnesium aluminate spinel nanoparticles doped with nickel are shown in Fig.3 [33, 34]. Magnesium aluminate spinel nanoparticles retain their spherical morphology and have a good quality despite being doped with Ni2+( Fig. 3 b, c, d). The EDX pattern of magnesium aluminate spinel doped with 4% nickel is shown in Fig.4. As indicated in Fig 4, the atoms of Mg, Al, O, and Ni are shown in various percentages, and the data obtained for nickel match the expected values. The amount of Ni2+ in Mg0.97Ni0.03Al2O4, Mg0.95Ni0.05Al2O4, Mg0.9Ni0.1Al2O4 was determined by the ICP-AES analysis to be 2.98, 4.99, 9.98 mol%.
Photocatalytic activity of Mg1-xNixAl2O4 spinel powders
As indicated in Fig. 5 a, b, c, d. Congo red dye cannot be decolorized under visible light irradiation in the absence of photocatalysts. It can be seen that Congo red is very stable and no serious decolorization is observed after the visible light irradiation exposure. The concentration of Congo red dye dramatically decreases with the increasing irradiation time, as soon as the photocatalysts were added. The absorption of the samples shows a decrease in the concentration of Congo red solution in the presence of the catalyst. The best result for decolorization of Congo red was calculated by (1) after 90 min of exposure to the UV light irradiation by using, 50 mL of Congo red solution 10 mg/L, and Mg0.9Ni0.1Al2O4 with decolorization of 99.3%. To remove the Congo red, 0.1 g from Mg0.9Ni0.1Al2O4 was used, which demonstrated a yield of 99.3%, while showed a higher efficiency compared to magnesium aluminate spinel and others with similar conditions (Fig. 6). The Results of this project are compared with similar researches (Table 2).
The DRS spectra of MgAl2O4, Mg0.97Ni0.03Al2O4, Mg0.95Ni0.05Al2O4, and Mg0.9Ni0.1Al2O4 are presented in Fig. 7 a, b, c, d. The band that is displayed next to the visible light region for each spectrum perhaps can be attributed to the O2 Al3+ charge transition because of the excitation of electrons from the valence band of O (2p) to the transference band of Al (3d). The band gap energy of the synthesized photocatalysts can be calculated according to the formula Eg=1240/. Eg is the band gap energy and λ is the absorption edge. The optical band gap energy values are 4.42, 4.27, 4.13, and 4.07 eV for MgAl2O4, Mg0.97Ni0.03Al2O4, Mg0.95Ni0.05Al2O4, and synthesized Mg0.9Ni0.1Al2O. When Ni2+ was added to MgAl2O4 the orbitals between Ni3d8 and O2p were hybridized, due to the Ni3d8 orbitals were filled with eight electrons. The replacement of Ni2+ with Mg2+ has improved the photocatalytic activity of magnesium aluminate spinel [39, 40].
Recovering of the catalyst
The Mg0.9Ni0.1Al2O was isolated from the final product by simple filtration. After that, the catalyst was washed several times with water. It was reused in a new reaction after drying. The recovered catalyst could be reused five times with no changes in the photocatalytic activity. The FT-IR spectrum of the recovered catalyst shows that it has been used for five successive times (Fig. 8). The results are listed in Fig 9.
In this study, MgAl2O4 nanoparticles were produced by sol-gel auto combustion method and were doped with different mass percentages of Ni2+ (x= 0, 0.1, 0.05, and 0.03). The prepared nanoparticles were characterized by X-ray diffraction (XRD), SEM, FT-IR, EDX, and ICP-AES. These nanoparticles were used as a catalyst for decolorization of Congo red 10 mg/L solution in the presence of UV light. The nanoparticles used in this reaction have the potential to be used in the degradation of Congo red. The results showed that Mg0.9Ni0.1Al2O4 had better decolorization percentages of Congo red compared to MgAl2O4, Mg0.95Ni0.05Al2O4, and, Mg0.97Ni0.03Al2O4 with UV light irradiation using a 0.1g catalyst and, 50 mL of Congo red solution 10 mg/L after 90 min. The efficiency of the photocatalyst (Mg0.9Ni0.1Al2O4) with similar papers for decolorization of Cong red was compared. The results show the high efficiency of the photocatalyst.
Financial support of this paper by Tehran North Branch, Islamic Azad University, and Tarbiat Modares University, is thankfully acknowledged. Reza Hajavazzade Ph.D. student who did all the experience. Maryam Kargar Razi; Supervisor of the project. Alireza Mahjoub; Advisor of the project.
CONFLICT OF INTEREST
The authors declare that there is no conflict of interest.