Wastewater generated from textile, leather, paper and plastic industries contains toxic dyes, which can make serious damages on water resources and can thus results in severe economic and environmental problems [1-3]. Synthetic dyes are widely used in food products, cosmetics, pharmaceuticals, paper, printing etc. Due to their good solubility, synthetic dyes are also common water pollutants and they may be frequently found in large quantities in industrial wastewater. Meanwhile, many of these dyes are also toxic and even carcinogenic . Then, it is necessary to eliminate dyes from wastewater before it is discharged. However, wastewater containing dyes is very difficult to treat because the existing synthetic dyes are recalcitrant organic molecules, resistant to aerobic digestion, and stable to light, heat and oxidizing agents . During the past decades, several physical, chemical and biological removal methods have been studied for the dyes wastewater purification strategies, like adsorption, filtration, coagulation, biodegradation [6-10]. However, physical methods are limited because further treatment was demanded, where the removed dyes only being transferred from liquid phase to solid phase. Among various techniques of dyes removal, Advanced Oxidation Processes (AOPs) is the procedure of choice and gives the best results as its strong oxidizing property and environmentally benign characters [11, 12].
Photocatalysis is an advanced oxidation technique to use the catalytic activity of the semiconducting metal oxides. This technique produces OH•, which has stronger oxidation power than ordinary oxidants normally used in the oxidation process, and decomposes the organic compounds into harmless compounds, such as CO2, H2O, or HCl . Many types of semiconductors have been used as photocatalyst including TiO2, ZnO, CdS, WO3, etc. Most of these semiconductor photocatalysts have band gap in the ultraviolet (UV) region, equivalent to or
larger than 3.2eV (λ = 387 nm). When photocatalyst absorbs ultraviolet (UV) radiation from sunlight or illuminated light source, it will produce pairs of electrons and holes. The excess energy of this excited electron promotes the electron to the conduction band therefore creating the negative-electron (e-) and positive-hole (h+) pair. This stage is referred as the semiconductor’s ‘photo-excitation’ state. The energy difference between the valence band and the conduction band is known as the ‘Band Gap’. The positive-hole of photocatalyst breaks apart the water molecule to form hydrogen gas and hydroxyl radical. The negative-electron reacts with oxygen molecule to form super oxide anion. The activation of ZnO by UV light can be represented by the following steps [14, 15]
ZnO+ hv → e- + h+ (1)
e-+ O2→ O2. - (2)
h++ Organic compound→CO2 (3)
h++ H2O→ •OH + H+ (4)
OH+ Organic compound→CO2 (5)
Present study involves the use of synthesized nano ZnO catalyst for degradation of the industrial effluent and the effect of various parameters like pH, catalyst dose and H2O2 addition through ultrasonic, photocatalytic and sonophotocatalytic degradation techniques.
MATERIALS AND METHODS
The set of experiments was conducted using textile industry effluents from dye house of textile industries. The some Physical parameters of textile industrial effluents shown in Table 1. The degradation efficiency of ZnO was determined after treatment with textile effluents from Shyama Trandze Pvt. Ltd, Pandesara, Surat (India). Rate of degradation of textile effluent was determined by using total organic carbon (TOC) analysis. The photocatalytic degradation was carried out in photocatalytic reactor, having 400W mercury lamp. Temperature inside the reactor was maintained by using cooling water jacket. Different nano catalyst doses were added in 100 mL solution and placed in a photocatalytic reactor. Concentration of the effluent was estimated using absorbance recorded on UV-Vis spectrophotometer (Systronics model-2203) at the λmax 400 nm. Sonication was performed with ultrasonic Probe Sonicator-20 KHz, 150 W (Dakshin Ultrasound, Mumbai, India) equipped with a titanium probe (10 mm of diameter), a water-circulating unit and a temperature controller. The pH of dye solution is maintained by 0.1M NaOH and 0.1M HCl using pH meter (EQ-615). After certain time intervals, adequate amount of sample is withdrawn and centrifuged to remove the nano catalyst and analyzed for TOC.
% TOC removal can be determined by using the following equation,
% TOC removal =
(TOC)0 = Initial TOC of industrial effluent
(TOC)t = TOC of industrial effluent at specific reaction time during treatment with ZnO nano catalyst
Synthesis of ZnO nano catalyst
ZnO nano catalyst was synthesized by using sol-gel method. Zinc nitrate solution was obtained by dissolving 0.05 mole in 500 mL water. NaOH solution was prepared in water by dissolving 0.75 mole in water with the ratio 1:15 (NaOH; water). The prepared NaOH solution was added drop wise to zinc nitrate solution with vigorous agitation at 70 oC. After complete addition of NaOH solution the reaction mixture (precipitate) was agitated for two h. Then, solution was allowed to settle for overnight. Finally supernunt liquid was discarded slowly after centrifugation . The precipitate was thoroughly washed using water and ethanol. Finally filtered precipitate was dried at 75 oC. 
RESULTS AND DISCUSSION
SEM and EDX analysis
Morphological characteristics of ZnO nano catalyst was studied by using SEM. SEM micrographs of the ZnO nano catalyst is shown in Fig. 1 (a). SEM micrographs showed that ZnO nano catalyst have oval type shape. The EDX analysis shown in Fig. 2 (b) indicates that ZnO nano catalyst contains Zn K (79.95 %), O K (20.05%).
X-ray diffraction analysis
The XRD pattern of ZnO is shown in Fig. 3. The sample was scanned from 20oto 80o (2θ) at a scanning rate. It shows main peak at 2θ of 30.87o, 33.52o, 35.35o, 46.65o, 55.71o, 61.98o, 65.53o, 67.08o, 68.20oand 76.8o in Fig. 2 corresponding to (1 0 0), (0 0 2),(1 0 1), (1 0 2), (1 1 0), (1 0 3), (2 2 0), (2 0 0), (1 1 2) and (2 0 2) diffraction planes of oval type shape on nano ZnO. The high intensity of peaks indicates the highly crystalline nature of the ZnO nano catalyst .
Fig. 1. (a) The SEM image of nano ZnO (b) The EDX spectrum of nano ZnO catalyst
Fig. 2. XRD diagram of ZnO nano catalyst
Table 1. Various parameters of industrial effluent samples
Effect of pH
The pH value of aqueous solution is a key parameter for ultrasonic, photocatalytic and sonophotocatalytic degradation of wastewater because it affects the degradation of pollutants. The wastewater of textile, dairy and pharmaceutical industries generally have a wide range of pH values. In addition, the generation of hydroxyl radicals which is necessary for the ultrasonic, photocatalytic and sonophotocatalytic reaction also depends on pH of the solution. Therefore, pH plays a significant role for both cases, in the chemical nature of wastes and generation of hydroxyl radicals [18, 19]. Hence, many attempts have been taken to investigate the effect of pH in the degradation of wastewater in the visible light irradiations. Effect of pH on degradation efficiency of ZnO nano catalyst on textile industry effluent is shown in Fig. 1. The plot of % TOC removal vs pH (Fig. 3) shows that as pH increases from 3 to 7, % TOC removal increases but further increases in pH up to 11 decreases the % TOC removal . This suggests that pH 7 is suitable pH for % TOC removal for the industrial effluent studies. The % TOC removal for ultrasound (US) is very low (below 21% TOC) for all the pH studied. The photocatalytic (UV+ZnO) study shows an increase in % TOC up to 73 %. The combination of (US+UV+ZnO) that is sonophotocatalyst shows an increase in % TOC removal up to 94%. This suggests that additive of both systems.
Effect of contact time
Effect of contact time on textile industrial effluent in terms of % TOC removal is shown in Fig. 4. The equilibrium time for % TOC removal in photocatalytic and sonophotocatalytic was found to be 160 min. at pH 3. On the basis of % TOC values, as initially as contact time increases % TOC removal also increases rapidly, though it increases slowly after 160 min.
Fig. 3. Effect of pH on textile industry effluent in presence of ZnO nano catalyst (catalyst dose- 1g/L, reaction time 160 min.)
Fig. 4. Effect of contact time on % TOC removal efficiency of textile industry effluent (catalyst dose 1 g/L and pH 7)
Fig. 5. Effect of catalyst concentration on % TOC removal of textile industry effluent at pH-7 and contact time 160 min
Effect of catalyst on % TOC removal
The effect of catalyst dosage on photocatalytic and sonophotocatalytic % TOC removal is observed by taking different catalyst dosages, while effluent concentration was constant. It is evident that as the amount of ZnO catalyst is increased, the photocatalytic and sonophotocatalytic % TOC removal also increased. On the basis of the experimental results, the % TOC removal increase with increase in amount of catalyst. On increase in photocatalyst and sonophotocatalyst dose from 0.25 mg/L to 1 mg/L, % TOC removal also increases from 22.7 to 71.08% for photocatalyst and 33.88 to 92.55 % for sonophotocatalyst after 160 min reaction time and pH 7.
Comparing the ultrasonic, photocatalytic and sonophotocatalytic degradation of textile industrial effluent
The further experiments were conducted under ultrasonic (US), photocatalytic (UV+ZnO) and sonophotocatalytic (US+UV+ ZnO) processes to study their effect on percentage TOC removal at catalyst dose 1 g/L, pH-7 and contact time 160 min. It is found that after 160 min, 92.55%, 71.08% and 34.83 % textile industrial effluent, % TOC removal takes place under sonophotocatalytic, photocatalytic and ultrasonic processes, respectively. Fig. 6 shows that the sonophotocatalytic % TOC removal efficiency is better than ultrasonic and photocatalytic % TOC removal.
Fig. 6. Comparison of ultrasonic, photocatalytic and sonophotocatalytic processes for degradation of textile industrial effluent at pH 7, ZnO dose 1 g/L, contact time 160 min. and US power 150 W
Fig. 7. Reusability of ZnO nanocatalyst
Recycle performance of ZnO nanocatalyst
To find out the stability and efficiency of ZnO nanocatalyst as well as cost effectiveness of the process, the reusability of ZnO nanocatalyst was investigated for the % TOC removal of textile industrial effluents under UV radiation. To study its reusability, the powdered nanocatalyst was centrifuged after completion of each sonophotocatalytic experiment. The recovered sample was reused for 3 times under the same experimental conditions. Fig. 7 shows that % TOC removal of the textile industrial effluents by ZnO nanocatalyst achieves up to 92.55% (160 min) after 1st run. After 4th run it decreases down to 89.62%. The catalytic activity was found to decrease slightly after 4th run. This decrease may be attributed to loss of the reused catalyst during sampling each time and irreversible changes of the sonophotocatalyst surface by pollutants. Fig. 7 shows that ZnO has excellent stability and does not suffer from corrosion during sonophotocatalytic % TOC removal.
In present study, the effectiveness of using ZnO nano catalyst by photocatalysis and sonophotocatalysis processes in removing toxic organic pollutants from the selected wastewater influents from textile industries. The collected samples were treated with ZnO nano catalyst and different parameters were analyzed and found that ZnO photocatalyst and sonophotocatalyst have good ability in removing the trace pollutants present in industrial wastewater. The increase in catalyst concentration increases the % TOC removal at neutral pH. Neutral pH is effective for removal of organic pollutants in textile industrial effluent. The comparison for ultrasonic, photocatalytic and sonophotocatalytic treatment of textile industrial effluents for treated wastewater samples using ZnO nano catalyst showed almost constant pH (pH-7). A maximum % TOC removal was observed in 92.55% for sonophotocatalytic, 71.08% for photocatalytic and 34.83 % for ultrasonic in 160 min contact time. All the above results clearly indicate that the sonophotocatalyst is effective for % TOC removal in textile industrial effluents than ultrasonic and photocatalytic process. Furthermore, the ZnO nano catalyst with high mineralization efficiency, high stability and recyclability could be the potential material for the wastewater treatment on an industrial scale
The authors have gratefully acknowledged to Dr. Shirish Sonawane (NIT Warangal) for providing TOC analysis and for his valuable suggestions. Authors are also thankful to Principal, Kisan Arts, Commerce and Science College, Parola, Dist. Jalgaon (M.S.) for providing necessary laboratory facilities.