Degradation pathways of ethyl violet by photocatalytic reaction with ZnO dispersions

The photodegradation of ethyl violet (EV; N,N,N',N',N',N'-hexaethyl-pararosaniline; C.I. 42600) dye was examined in aqueous ZnO dispersions under UV light irradiation. The techniques of HPLC-ESI-MS have been used to identify the various intermediates 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 oxidative degradation. Twenty-six intermediates have been detected by HPLC-ESI-MS. The N-de-ethylation took place in a stepwise manner to yield mono-, di, tri-, tetra-, penta-, and hexa-ethylated EV species while the oxidative degradation yielded 4-diethylaminophenol (DAP), 4-diethylamino-4'-diethylaminobenzophenone (DDBP) and their N-de-ethylated products, which were found for the first time in the ZnO-mediated photocatalysis process. Moreover, the hydroxyethylated intermediates formed in the N-de-ethylated EV, DDBP, and DAP processes were also separated and identified for the first time. For decolorization of dyes, the possible photodegradation pathways were proposed and discussed. (c) 2006 Elsevier B.V. All rights reserved.