The potential for photocatalytic oxidation of dyes in textile wastewater | |
| Author | Mallika Iangphasuk |
| Call Number | AIT Diss. no. EV-97-01 |
| Subject(s) | Sewage--Purification--Oxidation Photocatalysis |
| Note | A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Technical Science |
| Publisher | Asian Institute of Technology |
| Abstract | This research examined the photocatalytic decolorization of synthetic and real dyeing wastewater. Synthetic wastewater was a solution containing 15-30 mg/L of reactive black 5, a reactive azo dye. Ti02 as well as CdS photocatalysis were carried out in the synthetic wastewater studies whereas only Ti02 photocatalysis was applied to decolorize real wastewater collected from cotton dyeing, printing and finishing process. An aerated semiconductor slurry was illuminated in a batch-lab-scale reactor. Effects of the influencing factors:-initial pH, catalyst dosage, light intensity, V/IA ratio, solution temperature and initial dye concentration-were explored. Even under anaerobic conditions, the decolorization could be observed in both photocatalysis. External aeration played less important role in CdS than in Ti02 photocatalysis. The minimum first order rate constant (k1st) of RB5-Ti02 photocatalysis was recorded at pH 7 and k1st increased in acidic as well as in alkaline range while for CdS photocatalysis, the maximum rate constant was found at pH 4. This different pattern reflected the difference in reaction mechanisms of these two reactions. Solution pH affects the radical formation, the position band shift and the electrostatic adsorption of dye onto catalyst surface, depending on molecular structures of dye, investigated by FTIR and amphoteric behavior of catalyst surface. The main mechanism of Ti02 photocatalysis was the oxidation by radicals, especially hydroxyl radical. The enhancement in alkaline region was owing to the increase of hydroxyl ions affecting the hydroxyl radical formation whereas the enhancement in acid region was due to increase of adsorption capacity. Direct oxidation by valence band holes was expected to be the main mechanism of CdS photocatalysis. Thus, the increase in hydroxide ions did not enhance the reaction rate (pH 4 to 11). The decrease of reaction rate with increase in pH may be attributed by the decrease of RBS adsorbed onto CdS surface and by the increase of Cd deposition and Cd(OH)2 precipitation on CdS surface. The first order rate constants of both Ti02 and CdS photocatalysis increased with increase in the amount of catalyst (0-1.0 g/L for Ti02 and 0-1.5 g/L for CdS) and remained almost constant above a certain level. The k1st values of both reactions increased in a nonlinear pattern with increased light intensity. Ti02 as well as CdS photocatalysis followed the Arrhenius equation in the range of 303-333 °K. The activation energy of both reactions was of the same order of magnitude. The reaction rate of both reactions were significantly magnified as V/IA ratio decreased continuously from 7.62 to 1.27 mL/cm2. A linear relationship between k1st and the initial concentration (15-75 mg/L) was observed for Ti02 photocatalysis, and in contrast, the non-linear relationship was found for CdS photocatalysis. Although lower percentage of color removal was associated with higher initial dye concentration, the total amount of dye removal increased. Energy per volume of Ti02 and CdS photocatalysis were 38.S and 47.7 kWh/m3, respectively and EE/O of both systems were 46.7 and S7.9 kWh/order/m3, respectively. CdS photocatalysis followed both Langmuir Hinshelwood and Eley-Rideal kinetics, (at pH 7) while Ti02 photocatalysis of RBS could not be explained by two mentioned kinetics. The RBS photocatalytic degradation pathway identified by UV-visible absorption and FTIR studies was proposed that the azo linkages were the sites of attachment and then, the RBS molecule was cleaved at the azo linkages and phenyl-N bonds. The complete mineralization of RBS did not occur during the 180 min treatment period. For real wastewater photocatalysis, the obvious enhancement of photocatalytic decolorization in alkaline region was found owing to the increase of hydroxyl ions and the repulsion between catalyst surface charges and inhibitor ions at pH > 6.3 (IP of Ti02). Photocatalysis was also reinforced by the increase of temperature. The original pH and temperature of wastewater at the collected station were 9.8-11.2 and 48-S2 °C respectively; thus, pH and temperature adjustments before photocatalysis were not necessary. The optimum V/IA was 1.27 mL/cm2 (solution depth = 1.2 cm). The effect of Ti02 dosage and UV light intensity on real wastewater photocatalysis was similar to RBS wastewater. The optimum Ti02 dosage was 2 g/L and the optimum UV light intensity was 14.2 mW/cm2. Energy per volume was 157.5 kWh/m3 and EE/O was 191.2 kWh/order/m3. The potential of decolorization by Ti02-solar light was demonstrated. With constant light intensity, treatment efficiency increased when V/IA ratio was decreased. During the 3-hr photocatalysis, the color and the COD in the wastewater continuously decreased whereas the BOD increased. Indicated by the BOD:COD ratio, the biodegradability of treated wastewater was higher than the raw one. Under the optimum operating conditions, time to achieve 300 ADMI for synthetic and real wastewater were 18 and 63 min, respectively. The numbers of Ti02 reuse times in synthetic and real wastewater photocatalysis were 9 and 7, respectively. The catalyst activity of reused CdS was significantly lower than the new one due to Cd deposition and Cd(OH)2 precipitation on CdS surface. Results of Microtox® test showed that synthetic as well as real wastewater were detoxified by Ti02 photocatalysis. In contrast, toxicity of RBS wastewater treated by CdS photocatalysis significantly increased due to the CdS photocorrosion. In conclusion, this research showed the potential of Ti02 photocatalytic decolorization of dyeing wastewater without any increase of toxicity in wastewater. |
| Year | 1997 |
| Type | Dissertation |
| School | School of Environment, Resources, and Development (SERD) |
| Department | Department of Energy and Climate Change (Former title: Department of Energy, Environment, and Climate Change (DEECC)) |
| Academic Program/FoS | Environmental Engineering and Management (EV) |
| Chairperson(s) | Samorn Muttamara; |
| Examination Committee(s) | Chongrak Polprasert;Visvanathan, C.;Nagarur, Nagendra N.;Reutergardh, Lars Baetz;Ohgaki, Shinichiro |
| Scholarship Donor(s) | Royal Thai Government; |
| Degree | Thesis (Ph.D.) - Asian Institute of Technology, 1997 |