Evaluation of ultrasound as a preventative algae-controlling strategy: Degradation behaviors and character variations of algal organic matter components during sonication at different frequency ranges

Abstract To date, the removal of harmful algae and specific algae metabolites (e.g., microcystins, odorous compounds and phycocyanin) by ultrasound has been confirmed. However, the effects of ultrasound on algal organic matter (AOM) variation have not been systematically investigated, which is of great necessity for the management of water safety and the practical application of ultrasound for algae control. This study investigated the effects of ultrasound at three typical frequencies on the degradation behaviors and characters of AOM, including their fluorescence properties, molecular weight and especially sample toxicity. The results showed effective growth inhibition of algae over 7 days after ultrasonic treatment at frequencies of 29.4, 470 and 780 kHz; however, extracellular AOM increased by 1.4%–147.2%, −2.1%–82.5% and 0.9%–90.8%, respectively. High-frequency ultrasound (470 and 780 kHz) had significantly better AOM degradation. The fluorescence pseudo first-order rate constant refers to the decrease in fluorescence and implicates AOM degradation. The rate constants of AOM components were 0.00178–0.00295, 0.00880–0.01763 and 0.00670–0.01319 for the 29.4, 470 and 780 kHz treatments (0.059 W/mL), respectively. Low-frequency sonication was dominated by limited pyrolysis and supercritical water oxidation (SCWO) and mainly caused non-fracture damage. In addition to pyrolysis and SCWO, high-frequency sonication included conspicuous hydroxyl radical oxidation that accounted for more than 60% of the degradation. During high-frequency sonication, some AOM (~25.1 kDa) was degraded into smaller products (0.8–2.0 kDa), and the degradation of the humic-like components was greater than that of the protein-like components. Moreover, high-frequency sonication caused an evident decrease in the toxicity of AOM. This study indicates that high-frequency ultrasound was a promising algae-controlling strategy with respect to effectiveness and safety.