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.

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