Comparison of catalyzed and noncatalyzed oxidation of azo dye and effect on biodegradability

Catalytic and noncatalytic oxidations of diluted (0.5 g/L) aqueous azo dye (Orange II) solutions were performed at temperatures from 180 to 240 degrees C and oxygen partial pressure of 10 bar. The catalyst consisted of alumina and copper and zinc oxides. Dye-decay curves indicate that nearly complete removal of Orange II was achieved after 10 min at 240 degrees C in experiments without the catalyst while only about 3 min was needed in the presence of the catalyst at 180 degrees C. The TOC removal rates were found lower than those of the dye, thus implying the formation of intermediate products that are more resistant to further oxidation to carbon dioxide. On the basis of concentration-time behavior and the intermediate products identified, it is concluded that both catalytic and noncatalytic oxidations of Orange II obey a parallel consecutive reaction pathway involving free radicals. Aerobic biodegradability of Orange II solutions treated by catalytic and noncatalytic oxidation processes increases with time. The ratio of BOD5/COD vs temperature exhibited a slight maximum at about 200 degrees C in the case of noncatalytic oxidation. The results of this investigation indicate that intermediate product distribution plays a dominant role in biodegradability: the solutions treated by catalytic oxidation were found to be more amenable to aerobic biodegradation, simply because smaller quantities of intermediate products were formed in this process.