New aspect of the effect of liquid temperature on sonochemical degradation of nonvolatile organic pollutants in aqueous media

The literature reports on the effect of liquid temperature on sonochemical degradation of hazardous substances are very controversial. Here, we show for the first time experimental evidence that the effect of liquid temperature at high frequency ultrasound is strongly dependent to the initial substrate concentration (C0). Toluidine blue (TB) was taken as substrate model and experiments were conducted at 1700 kHz for a wide range of liquid temperature (25-70 °C) at various initial TB concentrations in the range of 0.5-20 mg L−1. The obtained results showed that ultrasound at 1700 kHz efficiently degraded TB. For low initial TB concentrations, C0 = 0.5 and 2 mg L−1, the rise of the liquid temperature up to 60 °C had practically no significant impact on the degradation rate of the dye. For higher initial dye concentrations (>2 mg L−1), the degradation rate increased significantly with increasing liquid temperature up to 50 °C and decreased afterward. In this case, the higher the initial concentration of TB, the higher was the rate of the sono-oxidation. Computational analysis based on single bubble sonochemistry model as well as direct dosing of H2O2 reveals that 50 °C gives the maximum sonochemical activity, OH yield, and thus, the obtained liquid temperature effect dependence of initial substrate concentration would be closely related to the difference in the reaction zone at low and high initial pollutants concentrations.