Reaction kinetics of azo dye by ozonation

Textile industries have shown a significant increase in the use of synthetic complex organic dyes as the coloring material. The annual world production of textiles is about 30 million tones requiring 0.7 million tones of different dyes per year. Among these dyes, over 50 % of them are azo dyes. The utilized dyes with about 10similar to20 % of them lost in the effluents produces large quantities of highly colored effluents. It was reported that conventional treatment processes do not readily remove dyes from textile wastewater, because of their stability to light and biological degradation. Although some treatment processes, such as chemical coagulation and carbon adsorption, may remove certain categories of dye to about 90%, the main drawback of these processes is the generation of a large amount of sludge or solid waste, resulting in high operational costs for sludge treatment and disposal. Ozonation, as an affordable and easy-operated control technology, is very effective in treating wastewaters containing recalcitrant compounds. The high oxidation potential of ozone (2.07volts) allows it to degrade most organic compounds. During the ozonation process, dyes lose their color by the oxidative cleavage of the chromophores. The cleavage of carbon-carbon double bonds and other functional groups with high electron densities will shift the absorption spectra of the molecule out of the visible region. In this paper, Diacryl Red X-GRL, an azo dye that is broadly used in the textile, was selected for investigation because it was found non-biodegradable by the conventional activated sludge process. The ozonation of Diacryl Red X-GRL in a semi-batch reactor was studied by modifying the gas flow rate, initial Diacryl Red X-GRL concentration, temperature, and pH. By the evaluation of liquid mass transfer coefficient, and the interfacial area between ozone and Diacryl Red X-GRL previously, the rate constant and the kinetic regime of the reaction between ozone and Diacryl Red X-GRL were investigated applying the experimental data to a model based on the film mass transfer theory. The results obtained support a second-order overall reaction, first-order with respect to both ozone and dye, and the rate constants were correlated as a modified Arrhenius Equation of temperature and pH with activation energy of 18.06kJ mol(-1). Hatta number of the reaction between 0.026 and 0.041 proves that the reaction occurs in the liquid bulk, corresponding to the slow kinetic regime.