In the recent decades, contaminated waters are one of the main problems in developing countries. Wastewater that contaminated with die chemical products is one of the major industrial water pollution sources . Various industries such as textile, food, plastics and cosmetics use different types of dyestuffs which also appear in the wastewaters discharged from these industries. Usually, synthetic dyes are toxic and therefore they must be removed immediately from aquatic sources, and otherwise they will lead to bad effects on the individual health. For example of these chemical dies, malachite green (MG) is one of the most commonly dyes used for cotton, paper, silk and paints or printing inks. The chemical structure of MG is shown in Fig. 1.
Removing of MG from aqueous solutions is difficult, because it belongs to the same group of tri-phenyl-methane dyes and they have properties that make them difficult to remove from aqueous solution. Some important properties of MG were reported in literature such as teratogenic , carcinogenic  and reproductive abnormalities  which spanning its effect from various fish to mammals .
Treating of dye contaminated wastewater studied by different methods of adsorption [6-11], membrane processes [12-15], coagulation-precipitation [16-18], biological processes [19,20] and etc. These methods are usually non-destructive and they may generate secondary pollution, because of these methods dyes are not destroyed and they transferred to another phase. Therefore, methods in which dye chemicals were destroyed are efficiently needed because they lead to degradation of toxic and refractory organics [21-29].
For removing chemical wastes from discharging wastewater, Nanomaterials proved to be useful and environmentally friendly candidates because they are easy to regenerate and are effective in dye removal from contaminated wastewater [30, 31]. In literature, different nanoparticles were successfully used to degrade dye compounds from aqueous solutions . Nanoparticles like Titanium dioxide, Cupper oxide, Zinc oxide, Silver oxide and etc, were used widely by different researchers [33-36].
Zinc Oxide based composites exhibits different band gaps and they are being used by different research groups for degradation of organic dyes . All of them use ultraviolet or visible light as a source of energy to create an electron hole pair for dye degradation.
In our present research pure nano ZnO and ZnO-TiO2 nanoparticles were fabricated and the photocatalytic ability of these nanoparticles was studied. Moreover, the effects of some important effective parameters on the degradation efficiency were reported.
MATERIAL AND METHODS
Analytical grade of chemicals was used in all of experiments. Titanium Isopropoxide (C12H28O4Ti with 97% purity and Mw=284.22 g/mol), Hydrated Zinc Nitrate (Zn(NO3)2.6H2O with 99% purity and Mw=297.49), Sodium Hydroxide (99% Purity), Hydrogen Peroxide (H2O2, 30% purity) and Malachite Green (C23H15ClN2 with 99% purity and Mw=364/9 g/mol) were purchased from Merck (Germany). Ethanol (C2H5OH with purity of 99/5%), Hydrochloric Acid (HCl with 37% purity) and other used solvents were purchased from Sigma-Aldrich (Germany). Double distilled water (DW) is used in all experiments.
Production of Nano ZnO
ZnO nanoparticles were produced by co-precipitation method in which, 1 mole of Hydrated Zinc Nitrate is dissolved in 100 ml deionized water and Sodium Hydroxide solution was added to the first solution draperies with continued mixing until pH=12. The precipitated white powder was collected and washed three times with deionized water and then it dried in 90°C oven for 26 hours. Finally, the produced powder calcined in 500°C for 1 hour.
Production of Nano composite of ZnO-TiO2
To prepare ZnO-TiO2 nano composite, at first, 5 ml of TTIP was added to ethanol and mixed for about 1 hour with a magnetic sitter until a clear solution was produced. Then deionized water and HCl solution were added to first solution and mixed vigorously until clear and yellow solution of TiO2 were formed. The solution of Zinc Nitrate – Ethanol was added to TiO2 solution very slowly and the mixture was mixed continuously in the room temperature for 2 hours. The produced solution dried in 60°C for 20 hours and 110°C for 2 hours. Finally, to prepare a crystalline structure, the produced powder was calcined in 400°C for 3 hours.
To study the degree of crystallinity, structure and characteristics of the synthesized ZnO and ZnO-TiO2 nanoparticles XRD, FE SEM and UV-Visible spectroscopy analyses have been performed.
X-ray diffraction (XRD) pattern of the produced particles was performed in a Bruker X-ray powder diffractometer (XRD, Rigaku smart lab, Cu K α1, Rigaku, Japan). All samples were swept from 2Ɵ =10° to 80° with a speed of 1°/min.
Spectroscopic measurements of the synthesized nanomaterials were studied using a HITACHI (S-4160) field electron scanning microscope with maximum voltage of 30 KV and different magnitude of 20X to 30000X. In order to prevent charging, these samples were coated with carbon.
The UV-visible spectroscopy experiments were conducted on a Perkin Elmer (lambada 25) with a Peltier temperature programmer and a water bath (Lab Teach LCB).
Photocatalytic degradation method
To study on the degradation of MG, different values of synthesized nanocatalyst (0.01, 0.02, 0.03 and 0.04 g) is added to 25 ml of water–MG solution with concentration of 3 ppm. The mixture is continuously mixed by a magnetic heater stirrer for about 30 min in the light irradiation. A mercury vapor pressure lamp (250W) is used for its light source. The used lamp is a cheap and commercial light source which irradiates in UV & Vis spectrum range. This lamp emits about 90% of its power in visible regime and 10% of its power in ultra-visible regime (λcentrifuging (4000 RPM & 20 min) and the remained concentration of MG in the solution is measured by UV-Vis spectroscopy. For the measurements, the calibration curves were plotted and degradation percent is calculated by the following formula:
In this equation C0 and Ct are the initial and final concentration of MG in the solution (ppm).
In this work various effective parameters (catalyst concentration, reaction time and H2O2 concentration) on the MG degradation were studied for two synthesized nanocatalysts of ZnO and ZnO-TiO2. For studding on the effects of each parameter, the values of other parameters were remained constant in its optimum value.
RESUTLT AND DESCUSSION
Characterization of nanocatalysts
XRD analysis results
The crystalline structures of synthesized nanocatalysts recognized with XRD patterns (Fig. 2). The X-ray diffraction pattern of synthesized TiO2 nanoparticles shows that TiO2 was successfully constructed. The 2θ at peak 25.4° confirms the TiO2 anatase structure. Strong diffraction peaks at 25° and 48° indicating TiO2 in the anatase phase. For ZnO nanoparticles, Sharpe peaks located at typical 2θ angles about 32, 35, 37,47, 56, 64 and 68° show that ZnO was successfully synthesized and crystallized. The XRD patterns of ZnO are closely same of the pure ZnO powders reported in literature without any impurity peak; therefore produced ZnO have a wurtzite hexagonal phase .
For ZnO-TiO2, the values of 2Ө ranges obtained in the spectrum corresponded to the reference pattern of ZnO-TiO2 nanoparticles. According to this pattern, zinc oxide and titanium oxide are both synthesized. The crystallite sizes of the catalysts were calculated using the Scherrer’s formula (Eq.2) :
Where D is taken as average crystallite size, λ is 1.5406 A˚, K is a constant equals to 0.9, β is the full width at half maximum (FWHM) in radians on the 2Ө scale and Ө is Bragg angle for the diffraction peaks. According to the above equation, the average crystallite size of the synthesized TiO2, ZnO and ZnO-TiO2 nanoparticles was estimated to be 21.5, 29.26 and 26.08 nm, respectively. The presence of TiO2 has a characteristic effect on the particles size of the sample. This reduction is due to increased surface energy of ZnO-TiO2 nanoparticles.
FE-SEM analysis results
FE-SEM image of ZnO and ZnO-TiO2 nanoparticles are shown in Figs. 3 and 4, respectively.
The results of these images clearly show that by forming of ZnO-TiO2 nano composite, the morphology and size of ZnO nanoparticles were changed completely.
Particle size distribution his to grams of ZnO and ZnO-TiO2 nanoparticles are shown in Fig.5 and 6,respectively.As noted above, synthesized composite nanoparticles are smaller in size than Zinc Oxide nanoparticles, which are well illustrated in the diagrams below. The average area and
diameter of obtaining nanoparticles are reported in Table 1.
Study of optical properties of synthesized nanoparticles
The absorption spectra of ZnO and ZnO-TiO2, is shown in Fig. 7. The energy band gap for samples was calculated by Eq (3) :
Were h is Planck constant (h= 4.14×10-15 ev.s), c is the speed of light (c= 2.99×10+8 m/s) and λ is wavelength of nanoparticles.
The band gaps of the nanoparticles were estimated 3.66 eV and 3.39 eV for ZnO and ZnO-TiO2, respectively. It’s predicted that by decreasing of the band gap, the catalytic activity is increased.
Effect of Photocatalysts dose on degradation of malachite green
Fig. 8, shows the effect of ZnO photocatalyst dose on degradation of malachite green. The degradation reaction was carried out with different doses of photocatalysts. Various values of photocatalyst weight in the reaction medium (0.01, 0.02, 0.03 and 0.04 g) were examined. According to the results reported in this figure the optimal photocatalyst weight was 0.03 g with 44% destruction and this optimum value was used in all other experiments.
The effect of time of light irradiation on degradation of malachite green
In this section, the effect of visible light irradiation for removal of MG was examined. For this purpose, different radiation times were evaluated. A mercury vapor pressure lamp (250 W) is used for light source and the samples located in about 30 cm from the light source with continuous stirring. The results are shown in Fig.9. As shown in this figure, with increasing of the irradiation time, the degradation of MG increased and ZnO-TiO2 composite nanoparticles exhibit a better effect on the MG photocatalytic degradation.MG destroyed about 66% in 60 min Vis light irradiation.
The effect of H2O2 dose on degradation of malachite green
These experiments were carried out in the presence of 0.03 g photocatalysts and 60 minutes of visible light irradiation. This effect is shown in Fig.10. Like the reported results in other papers , with increasing the dose of H2O2, removal efficiency increased [2, 41]. The accepted mechanism for the photolysis of H2O2is the cleavage of the molecule into hydroxyl radicals, which are very strong oxidizing types. These radicals can oxidize organic compounds (RH) producing organic radicals (Rº), which are highly reactive and can undergo further oxidation. The possible reactions, which occur during process, are as follows:
If the operation is carried out under suitable conditions, the final products are H2O, CO2 and
low molecular weight aliphatic acids .
More than 10,000 different types of synthetic dyes are produced and used in various industries in all over the world. Their waste products contain significant amounts of dye organic compounds that create irreparable damage in the environment. Therefore, the treatment of dye wastewater is necessary before it is discharged into the environment. Today, removing these contaminants using nanoparticles is a very simple, low-cost, and efficient way. The Malachite Green is one of the pollutants used both in dyeing industry and as a medicine for fish breeding. The results showed that ZnO and ZnO-TiO2 nanoparticles could be prepared successfully. The use of both photocatalytic synthesized in nanoscale has shown very good results in removing the MG. The results of photocatalytic experiments showed that in the absence of an oxidant, the optimal photocatalyst value is 0.03 g and the time of visible light irradiation is 60 minutes. In this condition, about 66% of malachite green was removed from aqueous solution using ZnO-TiO2 photocatalysts but in the presence amount of oxidizing agents, the decolorization process was completed.
CONFLICT OF INTEREST
The authors declare that there is no conflict of interests regarding the publication of this manuscript.