Degradation of Malachite green using heterogeneous nanophotocatalysts (NiO/TiO2, CuO/TiO2) under solar and microwave irradiation

Heterogeneous photocatalysis is an advanced oxidation process (AOP). This technique is used to degrade a wide range of pollutants in water. In this study, photocatalytic oxidation and mineralization of malachite green in an aqueous suspension containing nickel-based catalysts and copper supported on TiO2 prepared by wet diffusional impregnation was studied using two sources of irradiation: solar and microwave. Photodegradation kinetics were studied according to several parameters, such as catalyst type, dye concentration, photocatalyst mass and microwave power. The results showed that the photodegradation of malachite green is faster in the presence of CuO/TiO2 catalyst than NiO/TiO2 catalyst than TiO2. Dye degradation by microwave irradiation is faster than that by solar irradiation.


Introduction
Several industries such as the textile, plastic paper, leather, pharmaceutical, cosmetic and nutrition use organic dyes (soluble or pigmented), to color their products and discharge their wastewater, into the environment without any previous treatment [8,20].These colored effluents pose a serious risk to human health and aquatic environments [9,10].
The most used methods for the treatment of the industrial wastewaters including biological and physico-chemical treatments, which in most cases cannot be sufficient to completely eliminate the organic load contained in these effluents [1]. To resolve these problems, new techniques were then developed to oxidize non-biodegradable organic compounds. Among the innovative techniques are advanced oxidation processes based on the production and use of a highly reactive and non-selective oxidant (the radical hydroxyl HO . ) to mineralize partially or completely organic pollutants [3,7,11], Zhang and Deng 2011, [4] The use of catalysts improves the performance and reduces the cost of these technologies, leading to more efficient removal of pollutants and more selective use of the oxidizing agent [19,24] The degradation process consists of a succession of radical oxidation initiated by strong oxidants such as OH . . OH . are directly generated by the photolysis of water molecules adsorbed on the active sites of the catalysts. The organic pollutants adsorbed on the catalyst are then degraded by successive radical reactions into non-toxic mineral species [22] TiO 2 is the most commonly used catalyst in advanced oxidation processes [17,26] The interest in TiO 2 is mainly due to its low-cost, nontoxicity chemical stability. However, TiO 2 with his large band gap absorbs photons in UV region which represent 5% of the solar spectrum. Thus, the rate of photocatalytic activity of TiO 2 is limited to the recombination rate of photoproduced electron-hole. To enhance the utilization of solar light for photocatalytic and to reduce electron-hole recombination, TiO 2 is modified by various metallic species and coupling with other semi-conductors [12,13] Many photocatalys is studies have been published using TiO 2 and doped TiO 2 as heterogeneous catalysts for the degradation of various organic pollutants [14,15,23].
The multiplex compound of TiO 2 with oxide sis one of the most effective methods for enhancing the photocatalyticactivity of modified TiO 2 (Pengwei et al. 2010). Several studies have indicated that oxide as a modifier can effectively increase the photocatalystactivity.
Zhang et al. prepared the SeO/TiO 2 nanocomposite, in which they proved that the recombination of electron-hole pairs has been effectively suppressed due to the formation of heterostructure and absorption has been extended to visible light due to the narrow band gap of 2.0 eV of Se 0 by the reduction of Se 4+ (Zhang et al. 2007 The aim of the present work is to synthesize two composite semiconductors based on nickel and copper supported on titanium dioxide (TiO 2 ) in order to increase its photocatalytic activity and investing their efficiency in the degradation of Malachite Green using sunlight and microwave irradiation.

Preparation of catalysis
We impregnation method was used to prepare supported catalysts. The suspension of pure TiO 2 powder in a distilated water was mixed with the required amount of the nitrate copper nitrate trihydrate and nickel nitrate trihydrate, stirred for ½ hour. After that, we added an amount of NaOH to ajustate the pHat 10. Then, the mixture wasstirred for 3 h. The sample was placed in a sand bath at 100 °C to remove the water. The obtained material was washed and filtred to eliminate undesirable impurities. Drying in Oven over night and calcination at 400 °C. Monometallic supported TiO 2 are prepared according to the 10% in weight. The experiments were carried out during the months of May-June-July 2019 between 12 p.m. and 4 p.m. Skikda city North East of Algeria.

Effect of microwave power
In an Erlenmeyer flask, we put 200 ml of a solution of 20 ml / g in malachite green.
The tests were carried out without catalyst and in the presence of 0.1 g of catalyst. Then 1 ml of hydrogen peroxide is added. The tests were carried out in the microwave at (350 W) and (500 W). Synthesized catalysts are characterized using infrared spectroscopy to determine functional groups (Table 1).

Infrared spectroscopic analysis (FTIR)
FTIR spectra were performed using a "Perkin-Elmer Fourier Transform 1720-x" spectrometer, over a range of 400 to 4000 cm −1 with a resolution of 2 cm −1 . The results obtained in Figs. 1 and 2 show that all IR spectrum reveal the existence of the following essential bands: The band in the range 3775-3600 cm −1 , with an intense peak at 3750 cm −1 , corresponds to the elongation vibrations of the O-H group binding.
A band appearing between 1629 and 1320 cm −1 that could be attributed to the valence vibrations (elongation) of the O-H bond of the constituent water and to the deformation vibrations of the bonds of the water molecules adsorbed in the pores. There is also no band between 3500 and 3120 cm −1 corresponding to the vibrations of surface hydroxyl groups, this band being characterized by uncalcined solids.

X-ray diffraction analysis (XRD)
X-ray diffraction technique was performed to identify phase formation and crystallographic information of the samples. The XDR patterns of CuO/TiO 2 , and NiO/ TiO 2 (Figs. 3, 4 respectively), were carried out by the XPERT-PRO diffractometer type with Cu K α radiation (λ = 0.15406 nm) and scanning over the 2θ range from 10 to 85°. The pattern exhibits a narrow diffraction peaks which is due to the well crystallization. According to JCPDS data, the peaks at 25.

Effect of the initial concentration of malachite green
The photocatalytic activity of as prepared catalyst CuO/ TiO 2 and NiO/TiO 2 was evaluated for the degradation of malachite green dye concentration. Figure 5 shows the photocatalytic degradation of malachite green at different initial concentration in the range from 20 to 80 mg/l. The curves show that the rate of degradation increased with time at diverse initial concentrations. The results obtained show that the degradation of GM solutions is faster in the presence than in the absence of catalyst. It was also noted that CuO/TiO 2 catalyst gives better results than NiO/TiO 2 and TiO 2 catalyst for all concentrations. The decolorization efficiency decreased with the increasing concentration which is possibly due to the difficulty of penetration of UV irradiations into a large concentration gradient. Then, it can be due to the adsorption of molecular dyes on the catalyst surface occupying the active sites (Mishra et al. 2015).

Effect of catalyst dosage
To optimize the dose of the photocatalyst, the photocatalytic degradation kinetics of malachite green is performed for different catalyst amount (0.1, 0.2, 0.3 g/l).
The results obtained and plotted in Fig. 6 show that the mass of the catalyst has a very significant effect on the dye degradation. It has been noted that the C t /C 0 ratio decreases more rapidly over time in the presence of different catalyst masses and that the best result is obtained with 0.3 g of CuO /TiO 2 catalyst.
The particles of catalysts that are semiconductors are at the origin of this activation. They will absorb solar radiation to give rise to highly reactive species that will lead to the degradation of the dye in solution. The degradation efficiency increase as the catalyst dose increase, due to the increase of the active sites on the catalyst surface. The CuO/TiO 2 , NiO/TiO 2 photocatalyst show a higher activity than the TiO 2 this can be explained by the fact that TiO 2 GAP has decreased by doping copper and nickel. According to literature, we can give the explanation below (Fig. 7).
Heterogeneous photo catalysis process can be divided into five independent steps (Eskandarloo et

Reactions:
The electrons in the metal's conduction band can reduce dissolved oxygen with the formation of radical superoxide ion O 2 − Molecular oxygenacts as an electron acceptor in the transfer reaction [25] The radical superoxide ion O 2 ·− reactwithH 2 O to give HO · , OH − et O 2 (1)  The photocatalysis of hydrogenperoxide regenerates the free hydroxyl radical OH · While the h + (positive holes) react with the adsorbed H 2 O or OH‾, thus giving an OH · radical, according to the reactions(Eqs. 5 and 7), we can also witness direct oxidation by electron transfer from the adsorbed substrate (pollutant) on the surface depending on the reaction(Eq. 6) (Mungondori et al. 2013) Direct Oxydation, As an example of the last process, the hydroxyl radicals oxidize the C-H bond to give a carboxylic group which is decarboxylate according to Eq. 10.
The hydroxyl radicals formed, are also involved in the degradation of pollutants (RX) The next step is of great importance, especially because of the high concentration of OH − , due to the dissociation of water [25].
Much of the electron-hole pairs recombine in the particle volume or on the surface which reduces the quantum yield (Mungondori et al. 2013

Effect of microwave power
The influence of the microwave power in the degradation of green malachite was examinated for CuO/TIO 2 , NiO/TiO 2 and TiO 2 catalysts under power of 350 W and 500 W.
The results obtained and plotted in Figs. 8, 9 show that the power of the microwave oven has a very significant effect on the degradation. It was noted that the C t /C 0 ratio decreases rapidly over time. At 500 power, the two supported catalysts have almost the same efficiency. The CuO /TiO 2 catalyst has the best efficiency for GM degradation at 350 W power. However, we notice that TiO 2 has not a good reactivity via microwave. Indeed, when the power has increased from 350 to 500 W, the reaction in the absence of catalyst is more effective than that in the presence of TiO 2 . This can be explained by the increase in combination reactions that inhibits the degradation reaction (Figs. 10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25).

Kinetic study of the degradation reaction of malachite green
Kinetics study of removal of green malachite by photocatalytic degradation was investigated at different (15) Cu + + O 2 , H 2 O 2 , other oxidants → Cu 2+ + e −       In the present work, pseudo first order kinetic model is used to find the rate of photodegradation of GM.

Kinetics of catalysts with different microwave powers
The results obtained in the

Conclusion
In this work, supported nickel-copper catalysts were synthesized on titanium dioxide by the liquid or "wet" diffusional impregnation process. These catalysts are tested to remove malachite green (GM), which is a synthetic dye present in textile and tanning industry effluents by the advanced oxidation process (AOP) in the presence of hydrogen peroxide and catalysts under solar and microwave irradiation.
The results show that Malachite green is easily removed with the advanced oxidation process. Mass and concentration are important factors in the study of dye photo-degradation. The photo-degradation reaction via microwave is faster than that via solar irradiation.
Infrared (IR) spectrometric analysis revealed the presence of M-O or M-O-M oxide characteristic bands (M = Ni or Cu), and the absence of strange species or surface hydroxyl groups (bound OH). The DRX spectra of our solids have made it possible to identify the different crystalline phases present, such as CuO, NiO and TiO 2 supports.