Effects of Surfactants on Decomposition of Water-Soluble Dyes by Sodium Hypochlorite.

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Abstract
The oxidative decomposition of seven water-soluble dyes with sodium hypochlorite at 20°C under various pHs was investigated in the presence and absence of surfactants, such as sodium dodecylbenzenesulfonate (DBS), sodium dodecylsulfate (SDS), hexadecyltrimethylammonium bromide (CTAB), and polyoxyethylene (23) dodecyl ether (B-35). On the basis of pH-profiles of the apparent decomposition-rate constants of the dyes obtained from their half-life periods, the dyes were classified into two categories in the absence of surfactants. The origin was inferred mainly from oxidation mechanism of functional groups (amino or hydroxyl group) in the dyes. The effects of surfactants on the decomposition-rate were also investigated for typical two dyes, which showed contractive pH-profiles. As results, the cationic CTAB over the cmc enhanced oxidative decomposition remarkably, and on the contrary, the anionic DBS and SDS micelles and the non-ionic B-35 micelle rather slightly retarded the decomposition. These phenomena were interpreted in terms both of binding of the dyes with the micelle due to ionic interactions, and of solubilization of oxidation species (ClO_??_, HOCl, and Cl2) into the micelles. The measurements of partition coefficient of sodium hypochlorite to organic and water layers at 20°C supported the enhanced decomposition mechanism for the cationic micelle.

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