Heat treatment effects on the characteristics and sonocatalytic performance of TiO2 in the degradation of organic dyes in aqueous solution

The ambient sonocatalytic degradation of congo red, methyl orange, and methylene blue by titanium dioxide (TiO2) catalyst at initial concentrations between 10 and 50 mg/L, catalyst loadings between 1.0 and 3.0 mg/L and hydrogen peroxide (H2O2) concentrations up to 600 mg/L is reported. A 20 kHz ultrasonic processor at 50 W was used to accelerate the reaction. The catalysts were exposed to heat treatments between 400 and 1000 °C for up to 4 h to induce phase change. Sonocatalysts with small amount of rutile phase showed better sonocatalytic activity but excessive rutile phase should be avoided. TiO2 heated to 800 °C for 2 h showed the highest sonocatalytic activity and the degradation of dyes was influenced by their chemical structures, chemical phases and characteristics of the catalysts. Congo red exhibited the highest degradation rate, attributed to multiple labile azo bonds to cause highest reactivity with the free radicals generated. An initial concentration of 10 mg/L, 1.5 g/L of catalyst loading and 450 ppm of H2O2 gave the best congo red removal efficiency of above 80% in 180 min. Rate coefficients for the sonocatalytic process was successfully established and the reused catalyst showed an activity drop by merely 10%.