Photocatalytic degradation of basic violet 4: Degradation efficiency, product distribution, and mechanisms

The photocatalytic degradation of Basic Violet 4 (BV-4) was examined in aqueous TiO2 suspensions under UV light irradiation. After 30 W UV 365 nm irradiation for 20 h, approximately 99.9% of BV-4 was degraded. The techniques of high performance liquid chromatography-photodiode array-electrospray ionization-mass spectrometry (HPLC-PDA-ESI-MS), gas chromatography-mass spectrometry (GC-MS), and total organic carbon (TOC) have been used to identify the various photodecomposed products and to investigate the photocatalytic mechanism of the dye after low watt irradiation. The degradation progresses through competitive reactions such as N-de-ethylation and destruction of the conjugated structure. The N-de-ethylation took place in a stepwise manner to yield mono-, di-, tri-, tetra-, penta-, and hexa-N-de-ethylated BV-4 species while the destruction of the conjugated structure yielded 4-diethylaminophenol (DAP), 4-diethylamino-4'-diethylaminobenzophenone (DDBP), and their N-de-ethylated products in the TiO2-mediated photocatalysis process. Moreover, the hydroxyethylated intermediates formed by the N-de-ethylated BV-4, DDBP, and DAP processes were separated and identified. The GC-MS analysis showed the formation of N,N-diethylaminobenzene, N-ethylaminobenzene, aminobenzene, acetamide, 2-propenoic acid, and acetic acid. Nonetheless, we clearly observed the changes in the distribution of each intermediate during the photodegradation of BV-4. For decolorization of dyes, the possible photodegradation pathways were proposed and discussed on the basis of the evidence of intermediates formation.