Review on low- and high-frequency sonolytic, sonophotolytic and sonophotochemical processes for inactivating...

Ultraviolet and ultrasound-based advanced oxidation processes (AOPs) are gaining considerable research attention for water treatment and disinfection. Compared to low-frequency ultrasound (LFUS, <100 kHz), high-frequency ultrasound (HFUS, >100 kHz and MHz range) for water disinfection remains much less investigated. The present review aims at surveying and discussing literature data on microbial inactivation in non-food aqueous media using HFUS alone and with AOPs. More specifically, the review covers sonophotolytic (US/UV) processes under sequential and simultaneous modes as well as sonophotochemical processes, where both low and high frequencies were applied. Addressing a state-of-the-art biomedical research, we have attempted to provide more insight into mechanical and sonochemical mechanisms of inactivation under ultrasonic exposure. Sonoporation, intracellular generation of reactive oxygen species (ROS), energy stimulation of aquaporins to deliver ROS, and injection of extracellular ROS into sonoporated cells have all been identified as primary ways of inactivation. Application of ultrasound in the 0.2-2 MHz range and mercury-free light sources to support the Minamata Convention on Mercury is an ongoing challenge for effective elimination of microbial pathogens from water and wastewater through sonophotolytic and sonophotochemical AOPs.