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.