Sonolytic, photocatalytic and sonophotocatalytic degradation of malachite green in aqueous solutions

The degradation of malachite green (MG) in water by means of ultrasound irradiation and its combination with heterogeneous (TiO2) and homogeneous photocatalysis (photo-Fenton) was investigated. Emphasis was given on the effect of key operating conditions on MG conversion and mineralization rates and the elucidation of major reaction by-products. Eighty-kilohertz of ultrasound irradiation was provided by a horn-type sonicator, while a 9 W lamp was used for UV-A irradiation. The extent of sonolytic degradation increased with increasing ultrasound power (in the range 75-135 W) and decreasing initial concentration (in the range 2.5-12.5 mg L-1), while the presence of TiO2 in the dark generally had little effect on degradation. Sonolysis under argon was substantially faster than under air, oxygen or helium leading to complete MG degradation after 120 min at 10 mg L-1 initial concentration and 135 W ultrasound power. On the other hand, TiO2 photocatalysis or photo-Fenton led to complete MG degradation in 15-60 min with the rate increasing with increasing catalyst loading (in the range 0.1-0.5 g L-1 for TiO2 and 7-20 mg L-1 for Fe3+) and also depending on the gas used. TiO2 Sonophotocatalysis was always faster than the respective individual processes due to the enhanced formation of reactive radicals as well as the possible ultrasound-induced increase of the active surface area of the catalyst. For instance, the pseudo-first order rate constant for the sonophotocatalytic degradation at 0.5 mg L-1 TiO2 under air was 136.7 x 10(-3) min(-1) with the respective values for photocatalysis and sonolysis being 112.6 x 10(-3) and 11.6 x 10(-3) min(-1). Irrespective of the process employed, mineralization was slower than MG decomposition implying the formation of stable by-products accompanied by the release of nitrates in the solution. GC/MS analysis verified the identity of primary intermediates and a reaction pathway based on them was proposed. Depending on the conditions employed, ecotoxicity of MG to marine bacteria was partly or fully eliminated. (c) 2007 Published by Elsevier B.V.