Degradation of organic water pollutants through sonophotocatalysis in the presence of TiO2

The degradation of 2-chlorophenol and of the two azo dyes acid orange 8 and acid red 1 in aqueous solution was investigated kinetically under sonolysis at 20 kHz and under photocatalysis in the presence of titanium dioxide particles, as well as under simultaneous sonolysis and photocatalysis, i.e. sonophotocatalysis. The influence on the degradation and mineralisation rates of the initial substrate concentration and of the photocatalyst amount was systematically investigated to ascertain the origin of the synergistic effect observed between the two degradation techniques. The evolution of hydrogen peroxide during kinetic runs was also monitored. Small amounts of Fe(III) were found to affect both the adsorption equilibria on the semiconductor and the degradation paths. Ultrasound may modify the rate of photocatalytic degradation by promoting the deaggreagation of the photocatalyst, by inducing the desorption of organic substrates and degradation intermediates from the photocatalyst surface and, mainly, by favouring the scission of the photocatalytically and sonolytically produced H2O2, with a consequent increase of oxidising species in the aqueous phase.