UV light induced photodegradation of malachite green on TiO2 nanoparticles

The photodegradation of malachite green (MG), a cationic triphenylmethane dye, is examined both under different pH values and amounts of TiO2. After 15 W UV-365 nm irradiation for 4 h, ca. 99.9% of MG was degraded with addition of 0.5 g L-1 TiO2 to solutions containing 50 mg L-1 of the MG dye. The HPLC-PDA-ESI-MS technique was used to obtain a better understanding on the mechanistic details of this TiO2-assisted photodegradation of the MG dye with UV irradiation. Five intermediates of the process were separated, identified, and characterized for the first time. The results indicated that the N-de-methylation degradation of MG dye took place in a stepwise manner to yield mono-, di-, tri-, and tetra-N-de-methylated MG species generated during the processes. Under acidic conditions, the results indicated that the photodegradation mechanism is favorable to cleavage of the whole conjugated chromophore structure of the MG dye. Under basic conditions, the results showed that the photodegradation mechanism is favorable to a formation of a series of N-de-methylated intermediates of the MG dye. (c) 2006 Elsevier B.V. All rights reserved.