Organic pollutant such as dye is one of these pollutants that require much oxygen to decompose and very stable to many water treatment methods1,2. Dye is used widely in several industries such as producing fabric, food, furniture and paint. The waste water released from factories contains enormous amount of dyes and organic chemicals produced by industrial processes which is discharged into the environment3,4.
Many synthetic dyes and their metabolites are toxic, mutagenic and carcinogenic5. Also, methylene blue can cause permanent injury to humans and animals on inhalation and ingestion and the risk of the presence of this dye in water may be arisen from the burning effect of eye, nausea, vomiting and diarrhoea6,7. Generally, the presence of these pollutants in water causes severe problems to human beings and aquatic life8.
Acute heavy metal intoxications may damage central nervous function, the cardiovascular and gastrointestinal systems, lungs, kidneys, liver, endocrine glands and bones. Increased heavy metal exposure has been implicated in several degenerative diseases of these same systems and may increase the risk of some cancers9.
Photocatalysis has attracted attention since the discovery of the Fujishima and Honda in the early 1970s10, (they discovered that photolysis of water could occur using semiconductor photocatalysts). This discovery suggested a large number of potential applications, such as photovoltaic cells, degradation of pollutants and photolysis of water. Since then, photocatalysis has been a subject of serious research.
One of the major advantages of photocatalytic processes over the existing technologies is that there is no further need for secondary treatments. The process can be summarized as follows:
The ZnO is one of the most important and widely used photocatalysts as it has the advantages of being cheap, efficient, safe and eco-friendly11. In this respect, Advanced Oxidation Process (AOPs) have emerged as promising water treatment technologies for the complete destruction of organic contaminants in water12,13.
This study was aimed to synthesize nontoxic, environmental friendly and affordable heterogeneous photocatalyst able to mineralize and decompose dyes in wastewater and adsorb heavy metals from water. Therefore, the photodegradation efficiency of ZnO and SnO2/ZnO was studied against methylene blue dye as a water pollutant. Also, the adsorption of various heavy metals on ZnO and SnO2/ZnO photocatalysts in neutral conditions were studied.
MATERIALS AND METHODS
Reagents: All inorganic and organic chemicals (zinc acetate dihydrate, tin chloride dihydrate, sodium hydroxide, 2-methoxy ethanol and methylene blue) were of analytical grade.
Preparation of pure tin oxide and zinc oxide catalysts: Zinc acetate dihydrate was used as a precursor to prepare ZnO by sol-gel method. Zinc acetate dihydrate was dissolved in 0.1 M 2-methoxy ethanol by stirring at 70°C for 30 min. Then 28% sodium hydroxide solution was added to the prepared solution drop wise till the solution reached pH 9 to form gel14. The obtained gel was left overnight then dried for 20 h at 130 °C to produce xerogel. The xerogel was calcined for 2 h at different temperatures (400, 500, 600 and 700°C) to obtain crystalline nanoparticles of zinc oxide (ZnO). Pure tin oxide (SnO2) was prepared in the same manner using tin chloride dihydrate as starting material14.
Preparation of tin oxide doped zinc oxide catalysts: Starting with zinc acetate dihydrate, 8.12 g of zinc acetate dihydrate were dissolved in 370 mL of 0.1 M 2-methoxy ethanol by stirring at 70°C for 30 min. Preparation of 0.1 M 2-methoxy ethanol solution of tin chloride dihydrate was by the same way and added drop wise under vigorous stirring to the previously prepared solution of zinc acetate dihydrate. The obtained solution was vigorously stirred for 2 h at 70°C. The amount of tin chloride dihydrate solution was with molar ratio equal to 0.01/1, 0.02/1, 0.04/1 and 0.05/1 SnO2/ZnO. The NaOH solution was added drop wise until pH reached the value of 9 where gel is actually formed. The obtained gels were dried for 20 h at 130°C to produce xerogel. Finally, the photocatalysts were obtained by calcining the xerogels for 2 h at different temperatures (400, 500, 600 and 700°C)14.
Catalyst characterization: X-ray diffraction (XRD) is a non-destructive method for identification and quantitative analysis of various crystalline forms of metal oxides15. Patterns were obtained with hand pressed samples mounted on a Philips PW 1830 goniometer using the Cu Kα line (λ = 0.15458 nm) radiation under 40 kV and 100 mA and scanning with the 2θ ranging from 10-80°.
The ZnO and SnO2 doped ZnO topography was determined by a Jeol JSM-840 Scanning Electron Microscope (SEM) under high vacuum and acceleration voltage of 200 KeV. The samples were deposited onto carbon tape and coated with gold in a Blazers plasma sputterer (30 sec at 30 mA).
Transmission Electron Microscopy (TEM) was carried out using a JEOL 2010 instrument with resolving power 0.2 nm, accelerating tension being 200 keV. Samples diluted in ethanol solution were deposited onto Formvar-coated, carbon-reinforced, 3 mm diameter copper electron microscopes grids and left to air-dry prior to analysis16.
The UV-Vis transmission spectra of the as-synthesized ZnO and SnO2 doped ZnO were recorded by the HP Hewlett Packard 8452 A Diode Array Spectrophotometer over the wavelength range of 200-1000 nm. The DRS measurements were used to obtain band gap values17.
The photocatalytic activities of photocatalysts were evaluated by measuring the degradation ratio of methylene blue as a model pollutant in water under a Shimadzu MPC-2200 UV/Vis spectrophotometer. The methylene blue degradation was carried out at 20°C with external lamp (400 W UV/Vis lamps, Mercury lamp).
The prepared ZnO and SnO2 photocatalysts were used to remove heavy metals (Cu, Cd, Fe and Pb) from water via adsorption process.
The total metal concentration in solution was determined by Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES).
RESULTS AND DISCUSSION
XRD analysis for ZnO and SnO2 doped ZnO: The XRD patterns of ZnO, SnO2 and SnO2/ZnO samples of different molar ratios calcined at 600°C are shown in Fig. 1. The indexed peaks showed that ZnO nanoparticles exhibited hexagonal wurtzite structure (JCPDS card number: 36-1451) and SnO2 nanoparticles had tetragonal structure (JCPDS card number: 41-1445) and no other phases were detected. ZnO nanoparticles were well crystalline at 600°C whereas SnO2 particles were almost amorphous at the same calcination temperature.
The average grain sizes of the SnO2/ZnO samples were determined by using the diffraction peak (101) for ZnO. The crystallinity and the average grain sizes of ZnO decreased with increasing the amount of SnO2 from 35-18 nm18-21. The mean grain sizes decreased with increasing SnO2 molar ratio because Sn4+ ion (0.71°A) is smaller than Zn2+ ion (0.74 °A). Therefore, the more Zn2+ ions are substituted with Sn4+ ions, the more reduction in crystals size occur.
SEM analysis for SnO2 doped ZnO: The SEM images of selected samples of pure ZnO and SnO2 doped ZnO photocatalysts with different concentrations of SnO2 (0.01, 0.04 and 0.05) are illustrated in Fig. 2. The SEM images show that all the samples were agglomerated to some extent. The average sizes appeared quite close to those calculated from the XRD patterns.
||XRD pattern for ZnO and SnO2 doped ZnO calcined at 600°C
SEM images for (a) ZnO, (b) 0.01 M SnO2/ZnO, (c) 0.04 M SnO2/ZnO and (d) 0.05 M SnO2/ZnO
The particles of pure ZnO photocatalyst was successfully synthesized22 as shown in Fig. 2a. The micrographs of the mixed oxides in Fig. 2b-d revealed the presence of both the hexagonal phase for ZnO and tetragonal phase for SnO2. The SEM images showed also that tetragonal phase increased with increasing SnO2 molar ratio. The small crystallites of the different phases were interwoven with each other forming tightly bound nanoclusters23.
TEM analysis SnO2 doped ZnO: The representative TEM images of selected samples, pure ZnO and SnO2 doped ZnO (0.01, 0.04 and 0.05) are illustrated in Fig. 3. The TEM observations showed relative coincidence with XRD analysis in size of crystals and the phase of both ZnO and SnO2. According to the TEM image in Fig. 3a, ZnO sample was composed of nanoparticles with sizes in the range of about 30 nm and each nanoparticle was attached to several other nanoparticles. The TEM observations also showed that ZnO nanoparticles had a spherical shape and were agglomerated to some extent. The images in Fig. 3(b-d) showed a mixture of hexagonal and tetragonal phases indicating that SnO2 tetragonal phase is present and indicated that tetragonal phase increased with increasing SnO2 molar ratio. The images showed that crystals size decreases by increasing amount of SnO2 in accordance with XRD analysis22.
UV-Vis absorption spectra: The absorbance values were substituted in Kubelka-Munk equation to get (αhν)2 values. The (αhν)2 values were plotted against the light energy and the band gap values were determined by extrapolation of the linear part of (αhν)2 to hν axis17-24.
The Eg values of ZnO and SnO2 were 3.27 and 3.55 e.V, respectively. The band gap values of the samples containing ZnO and SnO2 ranged between 3.10 and 3.16 e.V and were lower than those of both pure oxides. In general, the band gap energies of mixed oxides were very close to that of pure ZnO because the amount of SnO2 was relatively low.
Effect of SnO2 doping concentration on photoactivity of ZnO: The effect of SnO2 as a dopant on the photocatalytic activity of ZnO in decomposing methylene blue is illustrated in Fig. 4a.
TEM images for (a) ZnO, (b) 0.01M SnO2 doped ZnO, (c) 0.04 M SnO2/ZnO and (d) 0.05 M SnO2/ZnO
The SnO2 had very law photocatalytic activity due to its law crystallinity. The ZnO showed appreciable photocatalytic activity due to its crystallinity. All the SnO2/ZnO samples were more active than the two pure oxides. The sample 0.04 M SnO2/ZnO revealed the highest photocatalytic activity then the photocatalytic activity decreased at 0.05 M SnO2/ZnO.
Effect of calcination on photoactivity of SnO2 doped ZnO: The effect of the calcination temperature on the photocatalytic activity of the 0.04 M SnO2/ZnO sample are illustrated in Fig. 4b. The best photocatalytic activity was obtained when the sample was calcined at 600°C, whereas the photocatalysts calcined at 500 and 700°C were less active. The increase in calcination temperature from 400-600°C enhanced the photocatalytic activity due to increase of the crystallinity. At 700°C the photocatalytic activity decreased due to growth of the size of the particles leading to a low surface area.
Effect of initial pH: The effect of initial pH due to the amphoteric property of ZnO was studied, it is very important to investigate the effect of pH in the dye solution on the reactions that take place on ZnO surface, as pH is a main factor that influences the surface charge profile of the photocatalysts25.
In this study, experiments were carried out at the same conditions except that pH of the methylene blue aqueous solution was adjusted within the range of 3-12 using diluted NaOH or HNO3 solution.
The photocorrosion of ZnO is most rapid in a strong acidic environment (pH lower than 4). Photocorrosion of ZnO is less severe with increasing pH and no photocorrosion takes place at pH higher than 1026.
Photodegradation curves of methylene blue on (a) ZnO, SnO2 and SnO2 doped ZnO and (b) 0.04 M SnO2/ZnO, treated under different temperatures
(a) Effect of initial concentration of methylene blue on the photodegradation efficiency of 0.04 SnO2/ZnO,
(b) Degradation for methylene blue after 60 min at pH 10 over 0.04 M SnO2/ZnO
More importantly, in alkaline solution, large quantities of OH- ions are present on the catalyst surface and in the reaction medium, which promotes the formation of hydroxyl radicals (OH▪)27-30.
It is very well-known that for pH values higher than the point of zero charge (PZC), which is about pH = 9.48 for 0.04 M SnO2/ZnO, the surface becomes negatively charged and it is the opposite for pH<9.48.
Being a cationic dye methylene blue molecule is easily adsorbed to ZnO particles with negative surface charge. These methylene blue cations can be oxidized directly by photo-generated OH-. That is why the high degradation ratios were achieved in alkaline and neutral pH regions31.
Effect of initial concentration of methylene blue: Dye concentration was investigated using initial concentrations of methylene blue (20, 50, 75 and 100 ppm), 0.05 mg of photocatalyst and 50 mL of dye solution at pH = 10. The Photodegradation of 0.04 M SnO2/ZnO at different dye concentrations is illustrated in Fig. 5a. It can be observed that the photodegradation efficiency decreased by increasing the dye concentration.
Rate constants and correlation coefficients for photocatalytic degradation of methylene blue for 0.04 M SnO2/ZnO
Adsorption of heavy metals over ZnO and SnO2 doped ZnO
As the concentration of dye increases, amount of dye adsorbed on the photocatalyst surface increases, consequently generation of hydroxyl radicals will be reduced. Furthermore, as the concentration of methylene blue increases, the fewer photons reach the photocatalyst surface. As a result, the production of hydroxyl radicals that can attack the pollutants will decrease and photocatalytic efficiency decreases32,33.
The percentage of degradation dropped from 98.3-84.2% when the initial dye concentration increased from 20-50 ppm after 60 min of irradiation was observed at Fig. 5b and to 47.8% for 75 ppm dye solution. Beyond 75 ppm, the reduction in photodegradation efficiency is not very significant.
Kinetic studies of methylene blue degradation using ZnO and SnO2/ZnO under UV/Vis light: The results show that the photocatalytic decomposition of methylene blue by 0.04 M SnO2/ZnO can be described by the first-order kinetic model. The plots of the concentration data gave a straight line. The correlation coefficient (R) for the fitted line and the rate constants (kapp) are calculated and their values are grouped in Table 1. R2 is a measure of the strength and direction of the linear relationship between ln (Co/C) and time (t). The more R2 values approach 1, the more the relation will be linear indicating that the reaction is first order reaction27.
Adsorption of heavy metals over ZnO: Many factors can affect the efficiency of heavy metal removal such as nanoparticles size, morphology and pH of the medium34. The ZnO and SnO2 doped ZnO were investigated to remove heavy metals from water by adsorption process.
Adsorption of heavy metals on ZnO and SnO2 doped ZnO is illustrated in Table 2. The amount of adsorbed metals on SnO2 doped ZnO surface increased by increasing SnO2 molar ratio then decreased at 0.05 M SnO2 doped ZnO. All samples showed high efficiency in removal of all metals.
Pure ZnO, SnO2 and SnO2 doped ZnO photocatalysts were synthesized using sol-gel method. It was found that all the doped samples had stronger light absorption in UV/Vis light range when compared with the pure ZnO. The doped ZnO photocatalysts showed enhanced photocatalytic activity in methylene blue photodegradation greater than pure ZnO under UV/Vis light irradiation. The 0.04 M SnO2 doped ZnO photocatalyst demonstrated the highest photodegradation efficiency which was about 100% in 90 min and higher adsorption efficiency to all studied heavy metals which was nearly 100% to all metals in 1 h. The 0.04 M SnO2/ZnO sample was used to study the effect of calcination, pH and initial dye concentration of dye. It showed best photodegradation efficiency at 600°C and the photocatalytic activity increased by increasing pH of the medium. Also, it was observed that photodegradation rates decreased by increasing initial concentration of methylene blue.
This study indicated that the best photodegradation efficiency showed at 600°C and the photocatalytic activity increased by increasing pH of the medium. As well SnO2 doped ZnO remove heavy metals from water by adsorption process.
The technically contents and ideas presented in this study are solely the authors interpretations. The authors thank the reviewers for their critical and constructive reviews which helped in improving this study.