SYNTHESIS AND CHARACTERIZATION OF Na-Y ZEOLITE FROM COAL FLY ASH AND ITS APPLICATION IN REMOVAL OF AZO DYE (CONGO RED) BY CATALYTIC WET PEROXIDE OXIDATION

Abstract

According to the latest report of Central Pollution Control Board, there are 81 coal based thermal power plants comprising total installed capacity of 60263.5 MW. Thermal power plants are mainly responsible for air and water pollution besides problems related to solid waste (fly ash) management. Sixteen percent of total fly ash generation. (80 million tonnes) in the country is utilized mainly for manufacturing cement, bricks and construction of roads and embankments. Now-a-days several . new approaches have been made for proper utilization of fly ash, either to reduce the cost of disposal or to minimize the environmental impact. One of the approaches is the conversion of fly ash to zeolites, which have wide applications in ion exchange, as molecular sieves, catalysts, and adsorbents (Breck 1974). Zeolite synthesized from coal fly ash has been used extensively for adsorbents but using zeolite synthesized from coal fly ash as catalyst is a novel approach. Dyes are the effluents for various industries like dyeing industry . and textile industry. Reactive azo dyes, particularly, pose problem with their degradation. Various methods are described for dye removal like adsorption/absorption, coagulation, electrocoagulation, using Fenton's reagent method and combination of these processes. Wet oxidation (WAO & WPO) is a method where there is no sludge disposal or minimum sludge disposal. All Other methods produce sludge in large amounts. In wet oxidation the sludge is disposed off to a 'great extent by oxidizing the organic pollutant. Catalytic wet oxidation method (CWAO & CWPO) is gaining more popularity because of more efficient ways of processing than wet oxidation method. CWPO process in particular has advantages like better oxidation ability than using oxygen as the former is carried out at lower pressure (atmospheric pressure). Present study involves CWPO of azo dye (Congo red) using iron exchanged Na-Y zeolite. The objectives of the present study include (a) Synthesis of Na-Y zeolite from coal fly ash and to study the effect of various factors like temperature ramp, fusion temperature, mixing time, crystallization time and temperature and acid treatment on % crystallinity of the zeolite product. Zeolite samples were characterized for BET surface area, Si/Al ratio, % purity and iron present. Studies of other miscellaneous effects like formation of Na-P zeolite, iron exchanging, presence of mullite and quartz phases were also done. (b) To study the dye concentration, color and COD removal as a function of temperature, initial pH°, hydrogen peroxide concentration and. catalyst loading using CWPO of azo dye (Congo red) in presence of iron exchanged Na-Y commercial zeolite as catalysts and to compare the result with iron exchanged Na-Y zeolite synthesized from coal fly ash. Synthesis of Na-Y zeolite catalyst was carried out in five stages. Initially the fly ash was screened through 355 micron mesh to eliminate larger particles and calcined at 800+ 10°C to eliminate the unburnt carbon and volatiles. Fly ash and sodium hydroxide (in form of powder) was taken in the ratio of 4:5 (by weight), grinded and mixed to form a homogeneous mixture. The mixture was fused at 550°C with increasing temperature ramp rate of 30°C/ 10 min. The fused mixture was cooled, grinded and mixed in about 500 ml distilled water for about 10 hrs (aging). The resulting (sodiumaluminate) slurry/gel which was formed is subjected to hydrothermal crystallization in oven at 90- 100°C for 17 hrs. After crystallization period, it was cooled and filtered by Whatman 1 filter paper and washed with distilled water to remove excess aluminum and dried in oven at 50- 60°C. The catalyst that is formed is powdered and stored.The zeolite formed is characterized by XRD, SEM/EDAX, surface area and FTIR. From the XRD, the d spacing peak values and their intensities are identified with the help for PDF (JCPDS) from which the % crystallinity is calculated (sum of the peaks/ total number of peaks). Composition (Si and Al and hence Si/Al ratio) and SEM images are obtained from SEMIEDAX. The surface area (m2/g) is analyzed with Chemisorb 2720 and. the formation of zeolites is confirmed with FTIR peaks from zeolite database. Among various heating rates, step of 30°C for 10 min resulted in more number of peaks than other temperature ramps. Fusion temperature (three temperatures 350°C, 550°C and 750°C) is optimum at 550°C and % crystallinity (65.79%) and BET surface area (74.39 m2/g) are the maximum obtained at that temperature. Mixing time (three mixing times l0hrs, 24 hrs and 48 hrs) is optimum at 48°C. Crystallinity (74.96%) and BET surface area (78.7 m2/g) is maximum for mixing time of 48 hrs. With crystallization time and temperature (three combined 100°C for 10 hrs, 100°C for 17 hrs and 90°C for 17 hrs), 100°C for 17 hrs forms a perfect crystalline Na-Y zeolite. Acid (1H2SO4, HC1, HNO3) treated fly ash shows improved performance in all factors like crystallinity, BET surface area and Si/Al (as it dealuminates the sample). HCl treated fly ash gives best results for % crystallinity (53.15%), BET surface area (160.43 m2/g) and Si/Al ratio (1.61). % purity increases (maximum of 84.38% by H2SO4) with acid treatment as it dissolves, unnecessary oxides which are present in fly- ash and :especially removes iron (which hinders the catalyst performance) to a large extent (60.67%). Formation of zeolite Na-P which is a competitive phase in the formation of Na-Y also varies with factors like temperature ramps, fusion temperature, mixing time, crystallization time and temperature and acid treatment showing the least formation for 90°C crystallization temperature for 17 hrs (only 5 peaks identified). Mullite and Quartz phases which are present in fly ash diminished in zeolite Y. Iron exchanging for synthesized Na-Y from coal fly ash is more compared to commercial zeolite as confirmed by XRD and EDAX results. The experimental studies for CWPO process were carried out in a 0.5 1 three= necked. glass reactor equipped with a magnetic stirrer with heater and a total reflux. Water containing Congo red dye is transferred to the three necked glass reactor. Thereafter, the catalyst was added to the solution. The temperature of the reaction mixture was raised using heater to the, desired value and maintained by a P.I.D. temperature controller, which was fitted in one of the necks through the thermocouple. The raising of the temperature of the reaction mixture to 90°C from ambient took about 30 min. The total reflux. prevents any loss of vapor and magnetic stirrer to agitate the mixture. Hydrogen peroxide is added with the runs were conducted at 90°C and the samples were taken at periodic intervals. The samples after collection were raised to pH-11 by adding 0.1N NaOH (so that no further reaction, takes place) and the residual hydrogen peroxide is removed by adding Mn02 which catalyses the decomposition of peroxide to water and oxygen. The samples are allowed to settle for overnight or one day (or centrifuged) and filtered. The supernatant was tested for color and COD. After the completion of the run, the mixture is allowed to cool and settle overnight. The samples are analyzed with visible spectrophotometer (amount of dye present in ppm), calorimeter (color in PCU) and COD meter (COD mg/1). The treatment by CWPO led to dye concentration removal of 99.1% (in 3 h), color removal of 100 % (in 30 min) and COD removal of 64% (in 3 h) at 100°C. Since the solution starts vaporizing at 100°C, 90°C was chosen as optimum operating temperature for CPWO showing the dye removal of 99.1% (in 4 h) , color removal of 100% (in 45 min) and COD removal of 75 % (in 5 h). The above results were obtained at pH 7, H2O2 cone. 0.6 ml in 350 ml solution, catalyst loading of 1 g/l. The performance of the catalyst is strongly dependent on initial pH of the solution. pH° 2 seemed to be the best with dye removal of 99% (in 4 hrs), color removal of 100 % (in 10 min) and COD removal of 69% (in 4 hrs). As the leaching of Fe ions is predominant in for acidic medium pH 7 was taken as operating pH° with dye removal of 99.1% (in 5 h) , color removal 100% in 45 min and COD removal 75 % in 5 h. The effect H202 concentration on CPWO was studied and a maximum dye removal of 99.02% (in 4 h), color removal of 100 % (in 1 h) and COD removal of 63% (in 4 h) was observed using 3 ml of H202. The results are comparable with Iml and to reduce peroxide concentration, 1 ml is taken as operating H202. concentration with maximum dye removal of 97.3% (in 3 h) , color removal of 100% (in 3h).and COD removal of 54 % (in 4 h). The studies on the effect of catalyst loading gave at 1.5 g/l loading, 98.6% dye (in 4 h), 100 % color (in 1 %z . h) and COD removal of only 51.8% . The results are comparable with 1 g/1 i.e. dye removal 97.25% (in 3 h) and 100% color removal (in 3 h). Hence 1 g/l has been effectively used. On the basis of above results, the following can be concluded. (1) HC1 treated fly ash with operational parameters; heating rate of 30°C in 10 min, fusion temperature of 550°C, mixing time of 48 h, crystallization at 100°C for 17 h is the best Na-Y zeolite synthesized. It formed perfect crystals with 54% crystallinity and 160 m2/g Surface area. (2) CWPO of azo dye Congo red at 90°C operating temperature, initial pH° 6.97, H202 concentration of 1 ml and catalyst concentration of 1 g/l is best suited conditions for dye removal (along with color and COD removal) showing the results as given above.