Dual-Frequency Ultrasonic Inactivation of Escherichia coli and Enterococcus faecalis Using Persulfate: A Synergistic Effect

Dual-frequency ultrasound (DFUS) has received considerable attention for enhanced inactivation of microbial pathogens for medical treatment, but remains little investigated for water disinfection. This study is focused on inactivation of E. coli and E. faecalis in aqueous solution under dual-frequency ultrasonication at 120 + 1700 kHz using persulfate. Single-frequency ultrasonic inactivation showed the higher efficiency of 1700 kHz, compared to 120 kHz. Under the experimental conditions used, no measurable synergy between two frequencies was observed in the absence of persulfate. A high time-based synergistic effect in terms of total inactivation (5-log) of both bacterial species was achieved by DFUS-activated persulfate with synergistic indices of 1.8-5.0. We assume that this is attributed to increased generation of reactive oxygen species (primarily, sulfate anion (SO4 center dot(-)) and hydroxyl (center dot OH) radicals) as a result of enhanced acoustic cavitation. Radical probing and scavenging tests confirmed the generation of radicals and showed a nearly equal contribution of center dot OH and SO4 center dot(-). This method could be an attractive alternative to ultraviolet technology for fast and effective water disinfection.