Study on the photoreductive decolorization of azo dyes by sulfite aqua

Abstract

In this paper, the decolorization of a series of azo dyes by sulfite aqua was studied under UV light irradiation and in the dark, respectively. Soluble azo dyes were easily decolorized with high decolorization efficiency under UV light irradiation, while their decolorization efficiency was much lower in the dark. The ultraviolet light is of the utmost importance for the rapid decolorization of azo dyes. As for the 10 mg/L Methyl Orange and Congo Red exposed to UV light, the decolorization efficiency reaches up to 95.9% in 10 min and 95.8% in 20 min, using 1.0 g/L and 8.0 g/L sodium sulfite, respectively. Whereas, for the same concentration of azo dyes and sodium sulfite, the decolorization efficiency of Methyl Orange and Congo Red is only 13.8% and 25.6%, respectively, even if they are laid in the dark with sodium sulfite for 48 h. The decolorization process of Methyl Orange and Congo Red follows the first order rate kinetics. The location of absorption peak in the visible region is blue-shifted in the course of the reaction. It is proposed that the decolorization of azo dyes is a reduction reaction. The chromophore of azo dye reacts with hydrogen and results in their decolorization. The possible mechanisms of the reductive decolorization of azo dyes were also discussed.