CHAPTER 3 MATERIALS AND METHODS

Transcription

1 86 CHAPTER 3 MATERIALS AND METHODS 3.1 GENERAL Experimental investigations were carried out in order to develop an integrated MW-UV reactor indigenously by incorporating a conventional UV lamp in microwave reactor by overcoming the difficulties experienced in previous studies. The effect of MW-UV on degradation of model phenolic pollutants using phenol, o, m, and p- cresols in aqueous solution was studied. The study reactor, materials and experimental methodology employed during this research study are explained in detail in this chapter. 3.2 EXPERIMENTAL METHODOLOGY The methodology adapted to carry out the research is detailed hereunder and depicted in Fig. 3.1.

2 Studies on Development of Indigenous Microwave-Ultraviolet Reactor for Degradation of Model Phenolic Pollutants Dark adsorption Model Pollutant mm Catalyst dose g and Contact time 24 h Development of MW-UV reactor using modified microwave oven (800W, 2450MHz) and UV lamp (6W, 352 nm) Degradability Studies on model phenolic pollutants using MW-UV reactor Photolytic Studies (without catalysyst) Photocatalytic Studies (With Catalyst) UV MW MW-UV MW-TiO 2 UV-TiO 2 MW-UV-TiO 2 Effect of ph- 4, 5, 6, 7, 8, 9 & actual ph, Initial Concentration: 0.1 mm, contact time 180 min Effect of ph- 4, 5, 6, 7, 8, 9 & actual ph, Initial Concentration: 0.1 mm, Catalyst dose -0.5 g, contact time 180 min Effect of Initial Concentration -0.1mM, 0.2mM, 0.3mM, 0.4mM, 0.5mM, ph actual ph, contact time 180 min Effect of Initial Concentration -0.1mM, 0.2mM, 0.3mM, 0.4mM, 0.5mM ph actual ph, Catalyst dose 0.5g, contact time 180 min Effect of Catalyst dose 0.2g, 0.4g, 0.6g, 0.8g and 1.0g, Initial concentration -0.1mM, ph actual ph, contact time 180 min Mineralization of model phenolic Pollutants Kinetics of removal of model phenolic pollutants Figures of Merit for Treatment Alternatives Figure 3.1 Methodology flow chart 87

3 Development of MW-UV reactor Preliminary studies on development of MW-UV reactor was carried out with microwaves generated using a magnetron and transported through a rectangular wave-guide in to the cavity. A quartz cylinder (volume-200 ml) filled with sample solution was inserted vertically in to a cavity and a UV lamp ( max = 254 nm, power intensity 15W) was placed in to the quartz reactor in such a way that both the radiation sources were orthogonal to each other. However, the above reactor failed due to breakage of UV lamp in view of overheating within 15 minutes in spite of repeated attempts. Subsequently the MW-UV reactor was developed using domestic microwave oven (Kenstar-Ken chef, Made in India) with output power 800W and frequency of 2450 MHz was used in this study. A quartz cylinder (Air Blow Equipments Pvt. Ltd., India) of 50 mm diameter and 350 ml of volume was placed into the microwave oven and a Mercury lamp (Sankyodenki 7B made in Japan; supplied by Heber Scientific-India) of wavelength ( ) 352 nm and output power of 6W of 24 cm length was inserted into quartz tube and positioned in such a way that the metallic part of the electrode of the lamp is not directly exposed to microwaves. The bottom and top of the quartz reactor was connected using a silicon tube through a peristaltic pump (Model Watson Marlow 313S; made in England) and a glass condenser (supplied by Air Blow Equipments Pvt. Ltd., India). The condenser was cooled by a cooler (Model LAUDA WK 1400; made in Germany) and the reactor was checked for any possible leakages of microwave using an electrical field probe (C.A.43 Field Meter Model-CHAUVIN ARNOUX, made in FRANCE) - an antenna combined with high frequency detector with power and sensitivity ranging from 0.1 V- m to 200 V-m and 100 KHz to 2.5 GHz respectively.

4 89 The schematic diagram and pictorial view of the experimental setup is shown in Figures UV power supply 2. UV lamp 3. Quartz reactor 4. MW oven 5. MW power supply 6. Three way valve 7. Peristaltic pump 8. Cooler Figure 3.2 Schematic Diagram of Experimental Set up Estimation of Microwave Power Microwave power was estimated using the following equation (3.1) proposed by Cha et al (1999) P=cm T/t (3.1) where P is the absorbed power of microwave (W), m is mass of water (g), c is the heat capacity of water (4.184 J/g C), T is the temperature rise (C), and t is irradiation time (s). The temperature of water or reacting fluid during experiments was measured by a temperature indicator with a sensor (range C, type PT 100) immediately after stopping microwave irradiation.

5 Degradation studies on model Phenolic pollutants Dark Adsorption All the experiments were conducted in triplicate. The degradation studies were conducted in three stages as given below. In a typical dark adsorption experiment, a 100 ml sample containing 0.5 mm of phenolic compound was mixed with 1.0 g TiO 2 and covered with aluminum foil sheet and kept for 24 hours in a place where no stray light. The initial ph of the phenolic solutions was maintained as the actual ph without modifications. The dependence of adsorption on the concentration of phenolic compounds on TiO 2 was assessed. Preliminary experiments had shown that, after 24-h contact, less than 10% of phenols (initial concentration 50 mg/l) were adsorbed on the surface of TiO 2 particles (1 g/l) at natural ph of aqueous solutions, likely due to the effect of electrostatic repulsion as reported by Chiou et al (2008a) Studies on MW-UV reactor for the degradation of model Phenolic Pollutants The degradation studies of phenol, o, m, and p- cresols were carried out in six phases as follow: (i) direct Microwave irradiation in the absence of TiO 2 (MW), (ii) Microwave irradiation in the presence of TiO 2 (MW-TiO 2 ), (iii) Coupled UV light and microwave radiations in the absence of TiO 2 (UV- MW), (iv) TiO 2 photoassisted degradation with coupled UV light and microwave radiations (UV-MW-TiO 2 ), (v) photodegradation with UV light only (UV) and (vi) TiO 2 photoassisted degradation with UV light only (UV- TiO 2 ). A 450 ml of phenolic solution with known initial concentration was filled in the reactor from the top of the quartz cylinder. The peristaltic

6 91 pump was switched on and allowed to circulate the sample throughout the length of the reactor. The cooler was switched on before the reactor and allowed to stabilize the temperature of the solution in the reactor as 20±1 0 C. The output power regulator of microwave oven was kept at the warmer stage, which is the lowest level of heating. The initial sample (5 ml) was collected immediately after the microwave oven was switched on. The intermediate samples were collected once in every 30 minutes and the total duration of the experiments was 180 minutes. The collected samples were immediately analyzed for the residual concentration of the model compound. The combined effects of UV and microwave irradiation on model compounds were studied by keeping both radiation sources on simultaneously, whereas, in case of UV degradative method only the UV lamp was switched on keeping the oven off. In this experiment, the variables of photodegradation such as effect of ph, effect of Initial concentration of pollutant and effect of Catalyst dosage on degradation of model compounds were studied. 3.3 EFFECT OF OPERATING VARIABLES ON PHENOL DEGRADATION IN MW-UV REACTOR Effect of ph for Degradation of Phenolic Compounds in MW- UV Reactor An important parameter in any heterogeneous photo oxidation is the initial ph of the suspensions and the ph value has a dominant effect on the photocatalytic reaction because many properties, such as the semiconductor s surface state, the flat-band potential, the dissociation of organic contaminant are all strongly ph dependent. The change in ph affects the ionization state of the phenols and also the surface charge of the photo catalyst.

7 92 Experiments were carried out by varying the initial ph of the sample solution in order to study the effect of hydrogen ion concentration on degradation of model compounds under MW-UV irradiation, combined effect of MW-UV-TiO 2 and UV-TiO 2 photocatalysis. 0.1 mm of phenolic solution was prepared and its initial ph of the solution was varied from 4.0 to 9.0 using concentrated sulphuric acid (H 2 SO 4 ) and sodium hydroxide (NaOH). The TiO 2 dose was 0.5 g/l of phenolic solution. Experiment was also conducted with the actual ph of the solution. The initial ph of the solution in which maximum removal occurred at 180 minutes on effects of MW, UV and UV-MW was considered for further studies Effect of Initial Concentration for Degradation of Phenolic Compounds in MW-UV reactor The influence of the initial concentration of pollutant on the oxidation by MW-UV irradiation, combined effect of MW-UV-TiO 2 and UV- TiO 2 methods has been analyzed, since the pollutant concentration is a very important parameter in wastewater treatment. The initial phenols concentrations were varied as 0.1 mm, 0.2 mm, 0.3 mm, 0.4 mm and 0.5 mm. The initial ph of the solution was maintained as the one in which maximum degradation was obtained and the total experimental duration was 180 minutes. The TiO 2 dose was 0.5 g/l of phenolic solution. The initial pollutant concentration in which maximum degradation occured for each treatment method was considered further studies Effect of Dosage of Titanium di Oxide for Degradation of Phenolic Compounds in MW-UV reactor Many studies have demonstrated that the rates of photodegradation for organic pollutants are strongly affected by the number of active sites and the photo-absorption ability of the catalyst used. Adequate dosage of the

8 93 catalyst increases the generation rate of electron-hole pairs; thus, the formation of OH radicals for enhancing photodegradation. However, an excess dosage of the catalyst decreases the light penetration via shielding effect of the suspended particles and hence reduces photodegradation rate. In the TiO 2 photocatalytic process, the limiting reagent is TiO 2. In this sense, a series of experiments was carried out in order to study the effect of catalyst dosage on the photodegradation of phenolic solution. The dosage of TiO 2 was varied as 0.2 g, 0.4 g, 0.6 g, 0.8 g and 1.0 g for 0.1 mm solution and 0.5 g, 1.0 g, 1.5 g and 2.0 g for 0.5 mm phenolic solutions. The initial ph of the phenolic solution was maintained as the ph value at which maximum phenolics were removed in previous experiments. 3.4 CHEMICALS Stock Phenolic Solution Commercial reagents in their highest available grades in analar (AR) were used for preparing the solutions without further purification. Phenol, o-cresol, m-cresol, and p-cresol were purchased from Central Drug House Ltd., New Delhi, India. A stock solution of 1000 mg/l was prepared by dissolving exactly 1.0 g of the corresponding phenolic compound in double distilled water and made up to 1 L Titanium Dioxide Titanium dioxide, (Degussa P25) procured from Germany was used as photocatalyst. The properties of the catalyst as furnished by the manufacturer are presented in Table 3.1.

9 94 Table 3.1 Characteristics of Titanium dioxide Sl.No Parameter Description 1 Mean particle size Crystalline form Anatase-rutile (3.6-1) 3 Colour White powder (solid) 4 Odour Odourless 5 Surface area 56 m 2 /g 6 Band gap energy 3.2eV Glasswares All glasswares like beakers, pipettes, burettes, etc., used for experiments and analyses were cleaned with grade I detergent and then soaked with chromic acid solution (a solution of potassium dichromate dissolved in concentrated sulphuric acid). They were rinsed several times with fresh water and finally in distilled water. 3.5 ANALYTICAL METHODS ph The ph of the solutions was measured using a ph meter (model: Elico 120, Make: India) equipped with combined glass-calomel electrode. The instrument was calibrated daily using buffer solutions of ph 4.0 and Determination of Total Organic Carbon Total organic carbon (TOC) values are the total concentration of organics in solution and the changes of TOC mirror the degree of mineralization as a function of irradiation time. Mineralization occurs when an organic-carbon containing compound is oxidized to carbon dioxide (CO 2 )

10 95 and water. Hence, mineralization of various phenolic compounds during oxidation was determined by the removal of TOC in the samples. Profiles of TOC concentration were monitored using TOC analyzer micro C model 1997 (Analytica Jena, Germany) with auto-sampler liquid injector ALS-C-104. Samples were pretreated by adjusting the ph<2.0 with 15% H 3 PO 4 to remove any inorganic carbon formed during oxidation reaction and analyzed immediately without storage. The analytical procedures involve purging the samples with oxygen for 1 min, followed by injecting 100 µl of the sample into the combustion tube maintained at C. Inside the combustion tube samples were oxidized by oxygen, which flows at a rate of 12L/h, in the presence of platinum coated alumina, which act as a catalyst. Calibration graphs were prepared by using standard potassium hydrogen phthalate solutions. The minimum detection limit was 0.2 mg/l Analysis of Phenol and Cresols Phenol and methyl phenols present in initial, intermediate and final sample solutions were directly measured spectrophotometrically. An UV- Visible spectrophotometer SPEKOL 1200 (Analytic Jena, Germany) with wavelength range 215 nm to 625 nm and wavelength accuracy of ±0.5nm was used for the measurement. The concentrations of phenol o, m and p- cresols were measured at their maximum wavelength ( max) of 270 nm, 273 nm, 274 and 279 nm respectively. 3.6 CALCULATIONS Degradation / Mineralization Efficiency The degradation / mineralization efficiencies for the oxidation of various phenolic solutions by different treatment methods were calculated from the following equation (3.2)

11 96 DE / ME = [M 0 -M t ] x 100 / M 0 (3.2) where DE / ME is degradation / mineralization efficiency in % M 0 is concentration of pollutant / TOC in mg/l before the oxidation M t is concentration of Pollutant / TOC in mg/l during oxidation after timet Figures of Merit and Operating Cost for Treatment Alternatives The standard figures-of merit are valuable in that they give a direct link to the electrical efficiency of an advanced oxidation process, independent of the nature of the system and therefore allow for comparison of widely disparate AOP technologies. Such figures-of-merit are necessary not only to compare AOP technologies, but also to provide the requisite data for scale-up and economic analyses for comparison with conventional treatment technologies. The evaluation of the treatment costs is today one of the most important aspects. The overall costs are represented by the sum of the capital costs, the operating costs and maintenance. For a full-scale system these costs strongly depend on the nature and the concentration of the pollutants, the flow rate of the effluent and the configuration of the reactor. An estimation of costs has been made in this thesis regarding the operating costs for the various treatment processes studied. Degradation of phenolic compounds by direct MW, combined MW-UV and direct UV studies are energy intensive processes and it is important to use figure of merit proposed by Bolten et al (1996), which relates directly the process efficiency and treatment cost. The appropriate figure of merit for the above processes is electrical energy per order (EE/O). This figure-of-merit is best used for situations where particular organic content (C)

12 97 is low (i.e., cases that are overall first-order in (C) because the amount of electrical energy required to bring about a reduction by one order of magnitude in [C] is independent of [C]. Thus it would take the same amount of electrical energy to decrease from 0.1 mm to 0.01 mm in a given volume as it would to decrease from 10 ppb to 1 ppb. Electrical Energy per Order (EE/O) is the electrical energy in kilowatt hours (kwh) required to bring about the degradation of a contaminant C by one order of magnitude in 1 m 3 (1000 L) of contaminated water or air. EE/O values (in kwh per order per m 3 ) can be calculated using the following formula given in equation (3.3): EE/O=P x t x 1000/V x 60 x log(c i /Cf) (3.3) where, P is the rated power (kw) of the AOP system, V is the volume (in liters, L) of effluent treated in the time t (min), C i and C f are the initial and final concentrations (mol/l) of C and the factor of 1000 converts g to kg.