Removal of Microcystis aeruginosa by ultrasound: Inactivation mechanism and release of algal organic matter

The efficacy of ultrasonic irradiation for removal of Microcystis aeruginosa and release of algal organic matter (AOM) was investigated under different ultrasound conditions, including ultrasonic frequency, power density, and time. Laboratory results suggested that the ultrasonic efficiency and the release of AOM were influenced by frequency, power density, and time. The mechanism of AOM algae removal by ultrasound was systematically explored. The inactivation of algae resulted from mechanical and chemical effects caused by ultrasound. Mechanical destruction and free-radical oxidation considerably affected the structure and physiological function of algal cells. The SEM and TEM images indicated that ultrasound could damage the cell membrane, wall, and organelle. Flow cell cytometry results showed decreases in the size, internal granularity, integrity, and activity of algal cells, revealing that ultrasound exerted severe damage to the structure and function of algal cells. The activity of the antioxidant system of algal cells was then studied by investigating changes in MDA, SOD, and CAT concentration after ultrasound to confirm the inactivation of the cells. The release of AOM was explored by determining changes in water quality indices (UV254, DOC, and SUVA) at 10 min and 48 h after ultrasound. This study provides information about the safety of ultrasound usage on algae removal and references for ultrasonic parameters to be selected to ensure effective and safe algae removal.