Effect of frequency and power on the piezocatalytic and sonochemical degradation of dyes in water

For the very first time, the effect of frequency on the piezocatalytic degradation of dyes has been systematically evaluated. To achieve this, a combination of systems and experimental setups operating at different ultrasonic frequencies ranging from 20 kHz up to 1 MHz were used. In addition, the effect of ultrasonic power was investigated at a low ultrasonic frequency of 20 kHz and higher ultrasonic frequency of 576 kHz to shed more light into the controversial discussion surrounding the ‘true’ mechanisms behind piezocatalysis. The results revealed that mechanical effects derived from acoustic cavitation, predominant at lower ultrasonic frequencies (<100 kHz), indeed enhanced the piezocatalytic degradation of the dye, Rhodamine B, to some extent (from 53% to 64% RhB degradation after 2 h). However, it was again demonstrated that sonochemical production of radicals remains a significant contributor for the overall degradation of the dye. Moreover, at higher ultrasonic frequencies (>100 kHz), the chemical effects derived from acoustic cavitation were so remarkable, that it raised the question of whether a piezocatalyst is really necessary when the optimisation of frequency and power may be enough for sonochemistry to fully degrade organic pollutants at a fast rate (pseudo first-order degradation reaction rate constant up to 0.037 min ⁻¹).