Photocatalytic decoloration of water-soluble azo dyes by reduction based on bisulfite-mediated borohydride

Investigation of the photocatalytic decoloration of nine water-soluble anionic azo dyes widely used for the coloration of textiles was conducted in a photoreactor by a selected reduction based on bisulfite-mediated borohydride. The influence of the irradiation power on their decoloration was discussed in this work. The decoloration processes of two typical azo dyes, Reactive Red MS and C.I. Direct Blue 15 were examined with UV–vis measurement and their decoloration kinetics was also investigated. Moreover, identification of benzidine compounds released from the photoreduction of four direct dyestuffs was undertaken. The results indicated that the reduction of the dyes was significantly enhanced under the increasing irradiation. It was found that the overall degradation followed pseudo-first-order kinetics and two distinct stages were observed in the decoloration processes of Reactive Red MS and C.I. Direct Blue 15. The decay rates at first stage were slightly changed, but the decay rates at second stage were significantly affected by the light intensity. In addition, UV–vis spectra showed that the azo linkages in molecule structure of the dyes were destroyed during the decoloration. Finally, dimethoxyl benzidine or benzidine produced from the photoreduction of four direct dyes was identified as the decoloration products through a rapid spectrophotometric method.