Effects of dye additives on the ozonation process and oxidation by-products: A comparative study using hydrolyzed C.I. Reactive Red 120.

Decolorization of azo reactive dyes by ozone has been described by several authors. However, the efficiency of ozonation and the by-products produced by dye additives/impurities such as synthetic precursors, by-products, salts and dispersing agents in commercial azo dyes during the oxidative treatment have not been reported. To investigate the effects of such impurities on the ozonation process and on the formation of oxidation by-products, a commercially available azo reactive dye C.I. Reactive Red 120 was chosen as a model compound. Experiments were conducted in aqueous solutions of hydrolyzed C.I. Reactive Red 120 (200 mg/l) either with or without purification. Treatment efficiencies of purified and unpurified dye were evaluated in terms of COD, BOD28, absorbance and initial decolorization rates. Additionally, organic and inorganic anions were analyzed by HPIC. After 28 days of incubation the BOD28/COD ratio increased from 0.22 to 0.48 and from 0.05 to 0.26 for purified and unpurified C.I. Reactive Red 120, respectively, indicating an enhancement of biodegradable compounds in the ozonated solutions. The results also illustrate that the dye additives/impurities in commercial dyes affect the microbial activity as well as the biodegradability. It is therefore recommended that dyes should be purified before ozonation, if detailed information on the degradation processes and the resulting oxidation by-products are required