Photooxidative N-de-ethylation of anionic triarylmethane dye (sulfan blue) in titanium dioxide dispersions under UV irradiation

The TiO₂-mediated photocatalysis process has been successfully applied to degradation of dye pollutants. Our results indicate that the TiO₂ surface is negatively charged, and the sulfan blue (SB) adsorbs onto the TiO₂ surface through the positive di-ethylamino groups while the TiO₂ surface is positively charged and the SB adsorbs onto the TiO₂ surface through the negative sulfonyl groups. In order to obtain a better understanding of the mechanistic details of this TiO₂-assisted photodegradation of the SB dye with UV irradiation, five intermediates of the processes were separated, identified, and characterized by the HPLC-ESI-MS technique, which included a positive- and negative-ion mode. The results indicated that the N-de-ethylation process continues until the N-de-ethylated SB dye is completely formed. The probable photodegradation pathways were proposed and discussed. The reaction mechanisms of UV/TiO₂ proposed in this study should be useful for future applications of the technology to the decolorization of dyes.