Synergy of combining megahertz ultrasound frequency and heat-activated persulfate for wastewater decontamination: micromodeling of acoustic cavitation and its role in the sono-hybrid process

Abstract Both sonolysis and heat-activated persulfate (HAPS) are effective advanced oxidation processes (AOPs) for the destruction of organic pollutants in wastewater. Although ultrasound can activate persulfate at ambient temperature, the synergy between ultrasound and HAPS was poorly investigated, especially when megahertz frequency ultrasound is used. The current chapter focuses on studying the synergism between too high-frequency sonolysis (1.7 MHz, 15 W) and HAPS toward the degradation of organic pollutants taking toluidine blue, a persistent textile dye, as a substrate model. An approach of coupling experimental results to simulation of cavitation bubble dynamics has been made to identify synergistic links between individual mechanisms of AOPs in the sono-hybrid system. It is revealed that in sono-hybrid process, chemical effects of ultrasound, and cavitation (generation of oxidants) are important in both substrate degradation and persulfate activation. A higher flux of bubbles has been predicted to be formed in the sonicating solution, generating a higher concentration of hydroxyl radical, particularly at higher liquid temperature. The synergy between individual processes was not always possible. Another notable observation of our study is that the synergy is strongly dependent on operational conditions. Synergisms were only observed at elevated temperatures, neutral pH, moderate substrate concentration, and low persulfate dosage. Overall, the crucial factor of mechanistic interaction between AOPs, resulting in either positive or negative synergy, needs to be accounted for in design and optimization of wastewater treatment processes using the studied sono-hybrid technique.