Preventative Inhibition And Emergency Removal Of Microcystis Aeruginosa Upon Ultrasound: Mechanism Exploration And Security Optimization

The eutrophication of surface waters can lead to many undesired issues,among which cyanobacterial blooms are the most alarming ones and have attracted considerable attention due to their increasing frequency and intensity in recent decades.Blooms of cyanobacteria can not only destroy the stability and service function of the aquatic ecosystem but also severely threaten the security of drinking water.Microcystis aeruginosa(M.aeruginosa)is one of the most common bloom-forming cyanobacterium types in freshwater bodies.In this study,ultrasound as an environmentally-friendly technology was applied to explore its capacity for M.aeruginosa control via preventative growth inhibition and emergency removal by enhanced coagulation.By focusing on the characteristics of the acoustic field,the physiological property variations of algal cells,and the behaviors of algal cells and algal organic matter(AOM)in short(immediate)and/or long terms,the objectives of this study were to excavate the discrepancies in the effects,mechanisms and security of ultrasound at a low and two high frequencies of 29.4,470 and 780 k Hz of typical frequency ranges on the growth inhibition and enhanced coagulation removal of algae.Comparatively,the low-frequency ultrasound at 29.4 k Hz propagated in significantly high dispersity and slower attenuation in algae-containing water,whereas the high-frequency ultrasound at 470 and 780 k Hz had a more centralized and attenuated propagation.With regard to ultrasonic effects,the 29.4 k Hz ultrasound displayed 3-4.5 times stronger mechanical shock than the 470 and 780 k Hz ultrasound,while the latter two exhibited 3.8-6 times higher yields of hydroxyl radicals(·OH)than the former one.Under the restriction of acoustic intensity to control the threat of AOM leakage to water security,ultrasound at either the low or the high frequencies was not capable of efficiently removing algal cells instantly.However,the growth of algae was inhibited to different degrees with different frequencies,which is of great significance for ecosystem protection.Different frequency ranges led to distinct action modes of ultrasound to M.aeruginosa cells.The 29.4,470 and 780 k Hz ultrasound exerted primarily mechanical shock,chemical oxidation and cell resonance effects,respectively,under which sonicated algal cells responded differently over the subsequent growth period.The extent of physical damage to algal cells by ultrasound dominated the recovery/resuspension progress.Specifically,in the 29.4 k Hz scenario,massive algal cells suffering from severe structure rupture and photosynthesis destruction sedimented significantly,under which the limited energy transformation obstructed algal cells to recover/resuspend.In contrast,the sedimentary algal cells at the 470 and 780 k Hz scenarios showed no legible physical rupture in the exterior and retained higher photosynthetic activity,therefore,allowing a conspicuous resuspension.Enhanced coagulation removal of M.aeruginosa cells by polymeric aluminum chloride(PAC)via ultrasonic pretreatment was ascribed to the immediate reduction in cell activity and negatively-charged AOM(i.e.,deteriorated cell stability).For activity impairment,the mechanical shock was more efficient due to its more violent contact with algal cells.For stability deterioration,there existed two routes.One was a physical disturbance that included mechanical shock and cell resonance leading to the detachment of AOM from the cell surface.The other was chemical oxidation causing the degradation of AOM.In addition,mechanical shock facilitated the aggregation of algal cells by enabling a better mixing and collision situation,favorable for PAC trapping and sedimentation.Of note,a too-high intensity of mechanical shock(e.g.,at29.4 k Hz and >6.84 J/m L)would result in the excessive exposures of negatively-charged AOM,intensifying electrostatic repulsion among algal cells and consequently deteriorating PAC coagulation.In terms of removal effects,the high-frequency ultrasound at 470 and 780 k Hz was preferable for cell removal.However,the application of ultrasonic pretreatment was not able to stimulate the AOM removal by PAC and even reduced the removal in the case of algal cells being seriously ruptured(e.g.,at 29.4 k Hz and >3.42 J/m L).Ultrasound application at restricted intensity for water security had poor AOM mineralization performance.Notably,the physical disturbance could aggravate AOM release over the inhibited growth period.Having regard to growth inhibition efficiency,water security and economy,the ultrasound characterized by significant chemical oxidation was more suitable for preventatively inhibiting algal growth(e.g.,at 470 k Hz and 1.77～6.84 J/m L).The application of ultrasonic pretreatment can conspicuously abate the disinfection by-product(DBP)yields and the corresponding cytotoxicity upon chlorination after ultrasound-coagulation,especially at high frequencies of 470 and780 k Hz.Besides,the 470 and 780 k Hz ultrasound-coagulation can slightly mitigate the acute toxicity in the coagulated water,thus exhibiting greater security.In terms of cell removal performance,water security and economy,the intensities of the 29.4,470 and 780 k Hz sonications were optimized at ～1.77,1.77～3.42 and 1.77～3.42 J/m L,respectively.Not only that,the laboratory-scale optimized ultrasound-coagulation had satisfactory performance in the field water(dominated by Pseudanabaena limnetica),even though the algae-containing water treated had different algal conditions(type,shape and amount)and water quality conditions,thus,highlighting the practicability of the optimized ultrasound-coagulation.