MSc lab 2

Introduction Textile industries produce large volume of colored dye effluents, which are toxic and non-biodegradable. These dyes create severe environmental pollution problems by releasing toxic and potential carcinogenic substances into the aqueous phase. Over the last decades, the increasing demand for dyes by the textile industry has shown a high pollutant potential. It is estimated that around 10-15% of dyes are lost in the effluent during the dyeing processes. Various chemical and physical processes such as precipitation, adsorption, air stripping, flocculation, reverse osmosis and ultra filtration can be used for color removal from textile effluents. However these techniques are non-destructive, since they only transfer the non-biodegradable matter into sludge, giving rise to new type of pollution, which needs further treatment.Recently there has been considerable interest in the utilization of advanced oxidation processes (AOPs) for the complete destruction of dyes. AOPs are based on generation of reactive species such as hydroxyl radicals that oxidizes a broad range of organic pollutants quickly and non-selectively. AOPs include photocatalysis systems such as combination of semiconductors and light, and semiconductor and oxidants. Heterogeneous photocatalysis has emerged as an important destructive technology leading to the total mineralization of most of the organic pollutants including organic reactive dyes. Titanium dioxide (TiO2) is generally considered to be the best photo catalyst and has the ability to detoxification water from a number of organic pollutants. However widespread use of TiO2 is uneconomical for large-scale water treatment, thereby interest has been drawn towards the search for suitable alternatives to TiO2. Many attempts have been made to study photo catalytic activity of different semiconductors such as SnO2, ZrO2, CdS and ZnO. Dye removal techniques Oxidative process: Oxidation is the most commonly used chemical decolorization processes due to its simple handling. As described in the early years of research, modern dyes are resistant to mild oxidation conditions such as those, must be accomplished by more powerful oxidizing agents such as chlorines, ozone, Fenton’s reagents, UV/peroxide, UV/ozone or other oxidizing techniques or combinations. Fenton’s reagent: Fenton’s reagent (hydrogen peroxide, activated with Fe(II) salts) is very suitable for the oxidation of toxicants present in wastewaters, which inhibit biological treatment. Chemical separation uses the action of sorption or bonding to remove dissolved dyes from wastewaters and shown to be effective in decolorizing both soluble and insoluble dyes. Advantages of this process include COD, color and toxicity reduction. Ozonation: The use of ozone first pioneered in the early 1970’s and is a very good oxidizing agent due to its high instability (Eo = 2.07 V) compared to chorine (Eo =1.36 V) and H2O2 (Eo =1.78 V). It can selectivity oxidize unsaturated bonds (e.g.-C=C- or –N=N-) and aromatic structures. Oxidation by ozone will lead to the degradation of chlorinated hydrocarbons, phenols, pesticides and aromatic hydrocarbons. Electrochemical oxidation: Electrochemical treatment of colored wastewater is considered as one of the advanced processes, and a potentially powerful method of pollution control, offering high removal efficiencies. Electrochemical processes generally have lower temperature requirement when compared to other non- electrochemical treatments and hence do not require any additional chemicals. Coagulation and precipitation: Hydrolyzing metal salts of iron and aluminium are widely used as primary coagulants to promote the formation of aggregates in wastewater and reduce the concentration of colorants and other dissolved organic compounds. Short detention time and low capital cost makes chemical coagulation a widely used technique. Biological methods: The ability of biological treatment process for decolorization of industrial effluents is ambiguous, different and divergent. Observations indicate that dyes themselves are not biologically degradable since microorganisms do not utilize the color constituents as a source of food. Photocatalytic degradation: The photo catalytic degradation of harmful organic compounds is of great interest and importance for environmental protection consideration. Photocatalytic processes are rapidly developing for the degradation of resistant pollutants. A variety of semiconductor powders (oxides, sulphides, etc) acting as photo catalysts have been used. Most attention has been given to MOx because of their high photo catalytic activity, resistance to photocorrosion, biological immunity and low cost. Many organic compounds are decomposed in aqueous solution in the presence of Metal oxide illuminated with UV and visible light. Mechanism of the undoped MOx Photocatalysed degradation There is evidence supporting the idea that hydroxyl radical (.OH) is the main oxidizing species responsible for photo-oxidation of the majority of the studied organic compounds. Generally MOx performed photocatalysed degradation under the uv light and its mechanism is given below Fig . The photo catalytic mechanism under UV radiation Kinetics : The rate of photo catalytic degradation depends on several factors including illumination intensity, catalyst type, oxygen concentration, pH, presence of inorganic ions and the concentration of organic reactant. The destruction rates of organics in photo catalytic oxidation have been modeled by different kinetic models. Langmuir-Hinshelwood (L-H) kinetics seems to describe many of the reactions fairly well. The rate of destruction is given by Eq. : - dC/ dt = k1 k2 C/(1 + k2 C) In the ideal case, for which the L-H model is derived, C is the bulk solute concentration, k1 the reaction rate constant, k2 the equilibrium adsorption constant and t represents time. The L-H reaction rate constants are useful for comparing the reaction rate under different experimental conditions. Once the reaction constants k1 and k2 have been evaluated, the disappearance of the reactant can be estimated if all other factors are held constant. For low solute concentration C, the L-H expression reduces to a pseudo first order expression: - dC/ dt = k1 k2 C = k C This equation has been shown to apply to many photocatalysed reactions. The industrial pollutants levels are typically of the order of ppm, which are low enough for the reaction rate to follow pseudo first order kinetics. Besides this ‘k’ reaction rate constant is not a traditional rate-constant used in reaction engineering due to the nature of the photo catalytic reaction, it is also a function of external system parameters such as UV intensity, pH, catalyst loading, geometry of photo reactor and initial concentration. Literature review There are many works have been done on degradation of methylene blue by ZnO photo catalyst. Thou-Jen Whang, Mu-Tao Hsieh, and Huang-Han Chen worked on “Visible-light photocatalytic degradation of methylene blue with laser-induced Ag/ZnO nanoparticles”. HougangFan, XiaotingZhao, JinghaiYang, XiaonanShan , LiliYangYongjunZhang, XiuyanLi, and Ming Gao worked on “ZnO–graphene composite for photocatalytic degradation of methylene blue dye”. TianLv, Likun Pan, XinjuanLiu and ZhuoSun worked on “Enhanced photocatalytic degradation of methylene blue by ZnO–reduced graphene oxide–carbon nanotube composites synthesized via microwave-assisted reaction”. VenkateshamVuppala, MadhuGattumaneMotappa, SatyanarayanaSuggalaVenkata, and PreethamHalugondanahalliSadashivaiah worked on “Photocatalytic degradation of methylene blue using a zinc oxide-cerium oxide catalyst”. WenzhongShen, Zhijie Li, Hui Wang, Yihong Liu, QingjieGuo, and Yuanli Zhang worked on “Photocatalytic degradation for methylene blue using zinc oxide prepared by codeposition and sol-gel methods”. WenzhongShen, Zhijie Li, Hui Wang, Yihong Liu, QingjieGuo, and Yuanli Zhang worked on “Photocatalytic degradation for methylene blue using zinc oxide prepared bycodeposition and sol–gel methods”. Objectives of the experiment : Preparation of 20 ppm and 30 ppm methylene blue solution. Photo degradation studies of methylene blue and this intermediates using semiconductor catalysts ZnO under UV light. Comparism of their photo catalytic activity on the basis of their Degradation activity/ Decolorization efficiency (%). Efficiency (%) =(C0 – Ct)/C0 × 100 ≈(Ao-At)/Ao × 100 where C= concentration, A= absorbance,0= initial time, t-at t time. Kinetic studies of the degradation process. Experimental Section Materials Dye Methylene blue C16H18N3SCl Molar mass: 319.85 g/mol Fig: structural formula of Methylene blue Photocatalysts and chemicals: Zinc oxide was obtained from BD and Methylene blue purchased from Merck were used without further purification. Double distilled water was used for preparation of various solutions. pH of the solutions was adjusted with 1M HCl or 1M NaOH. Equipment and Instruments pH Meter 2. UV-Vis Spectrophotometer 3.Centrifuge machine 4. UV chamber photo reactor 5. Volumetric flask 6. Beaker 7. Magnetic stirrer etc. Experimental Procedures: At first 10mg or 0.01g and 15mg or 0.015g methylene blue was weighed to prepare 20ppm & 30ppm 500ml methylene blue solution in a 500ml volumetric flask. pH of the solutions was measured. 1gm ZnO was weighed and mixed in both 100ml methylene blue solutions. After remaining 30 min in dark for adsorption, the UV of the centrifuged solutions were taken. Then the solutions were kept in a UV chamber with continuous magnetic stirring. The UV of the solutions were taken after 30 min for 6 times by centrifuging the solution. The data of UV was collected for calculations. Results and Discussion Characterization of methylene blue Fig.1: Typical UV-Visible spectrum of methylene blue (20 ppm). Fig.2: Typical UV-Visible spectrum of methylene blue (30 ppm). In the UV-Visible spectrum of methylene blue (Fig. 1) and (Fig. 2) three absorption bands are observed at 392 nm (low intensity), 612 nm (medium intensity) and 663 nm (highest intensity).But lower intensity for 20 ppm than 30 ppm. The major distinct band at 663 nm is due to substitution of various functional groups to heteroaromatic ring and is responsible for the color of the dye. The λmax was found to be 663 nm, which proved the relatively higher conjugation in MB molecule. UV–visible spectra of dyes For 20 ppm MB solution The UV-visible spectra of methylene blue were taken 6 times within 30 min interval. The initial absorbance at 663 nm and 612nm wavelengths was decreased with increasing time. The rate of decolorization was recorded with respect to the change in intensity of absorption peaks at 663 and 612 nm. The absorption peaks, corresponding to dyes diminished and finally disappeared during reaction, which indicated that the dyes had been degraded. Fig. 3. Absorbance spectra of Methylene blue(20 ppm) during the course of reaction under UV-visible light For 30 ppm MB solution Fig. 4. Absorbance spectra of Methylene blue(30 ppm) during the course of reaction under UV-visible light Effect of concentration When the concentration of methylene blue increases the efficiency of photocatalyst decrease. Hence the efficiency of ZnO for 20 ppm methylene blue is greater than 30 ppm methylene blue solution. Efficiency calculation The efficiency (%) = (A0-At)/A0*100 Where, A0 = Absorbance at initial time At = Absorbance at time t For 20 ppm methylene blue solution Time(min) A0 At A0-At (A0-At)/A0 Efficiency(%)= (A0-At)/A0 *100 0 3.058 3.058 0 0 0 30 3.053 0.005 0.001635 0.163506 60 1.854 1.204 0.393721 39.37214 90 0.771 2.287 0.747874 74.78744 120 0.066 2.992 0.978417 97.84173 150 -0.048 3.106 1.015697 101.5697 Fig.5: % Efficiency vs. time plot for the degradation efficiency of ZnO catalyst on MB under UV light irradiation (dye concentration 20 ppm). For 30 ppm methylene blue solution Time(min) A0 At A0 -At A0 -At/A0 Efficiency(%)= (A0-At)/A0 *100 0 3.413 3.413 0 0 0 30 3.359 0.054 0.015822 1.582186 60 2.936 0.477 0.13976 13.97597 90 2.399 1.014 0.297099 29.70993 120 1.431 1.982 0.580721 58.07208 150 1.192 2.221 0.650747 65.07471 Fig.6: % Efficiency vs. time plot for the degradation efficiency of ZnO catalyst on MB under UV light irradiation (dye concentration 30 ppm). It has been observed from plot efficiency increases with increasing time. Akpan stated that in alkaline condition, OH° radicals are easier to be generated by oxidizing OH available on ZnO surface, therefore the efficiency is increased (Akpan, 2009). On the other hand in alkaline condition there is a coulombic repulsion between the negative charged surface of photocatalyst and the hydroxide anions. This status could prevent the OH° formation and therefore decline the photooxidation process (Fox, 1993). This is an indicative of the significant role of the photocatalyst ZnO. Kinetic study For 20 ppm methylene blue solution Time(min) A0 At A0/At lnA0/At 0 3.058 3.058 1 0 30 3.053 1.001638 0.001636 60 1.854 1.649407 0.500416 90 0.771 3.966278 1.377828 120 0.066 46.33333 3.835862 150 -0.048 -63.7083 4.154315 Fig. 7. Kinetic analysis of Methylene blue(20 ppm) under UV-vis light. For 30 ppm methylene blue solution Time(min) A0 At A0 /At lnA0 /At 0 3.413 3.413 1 0 30 3.359 1.016076 0.015948 60 2.936 1.162466 0.150544 90 2.399 1.422676 0.35254 120 1.431 2.385045 0.869218 150 1.192 2.863255 1.051959 Fig. 8. Kinetic analysis of Methylene blue(30 ppm) under UV-vis light. The semi logarithmic plots of the absorbance data give a straight line. By this straight line, we can consider that the photo catalytic decolorization followed pseudo-first order kinetics. Precaution & Conclusion Precaution is very important in this experiment. During the adsorption and desorption the methylene blue solution with ZnO must be kept in dark so that any light can interfere. During the keeping the solution in UV light, the switch must be off so that UV light can enter in our body because it is harmful for our skin. After centrifuging, the precipitate solution must be returned in the existing solution. After the experiment, the solution must be disposed off in a proper way. In this experiment, the efficiency of ZnO has been shown. It has been found that ZnO is a very effective photo catalyst for methylene blue degradation which is responsible for polluting water. Then the kinetics of this reactions have been studied. It has been found that the photo catalytic decolorization followed pseudo-first order kinetics.