Study On The Mechanism And Potential Environmental Risks Of Microcystis Aeruginosa Removal In Water By Discharge Plasma

Microcystis aeruginosa（M.aeruginosa）,as the most common and widely distributed algae in cyanobacterial blooms.While the overgrowth of algal cells and the release of large amounts of intracellular organic matter can pose serious environmental risks to the health of the water body and the safety of the water supply.Therefore,it is important to find a technology that can quickly and efficiently remove algal cells,effectively degrade algal organic matter,and avoid new environmental risks during the treatment process.Discharge plasma technology,as a new type of green advanced oxidation technology,has a certain research foundation because it can remove algae quickly and efficiently.However,the treatment process can rupture algal cells,resulting in the release of large amounts of intracellular dissolved organic matter（DOM）,which may pose new potential environmental risks.Therefore,in order further investigate the mechanism of algae removal by discharge plasma technology and promote its practical application in dealing with the risk of cyanobacterial blooms,this study established a dielectric barrier discharge system to remove M.aeruginosa from water to investigate the effect and mechanism of algae removal,the release and degradation pattern of intracellular DOM,and to assess the potential environmental risks during the treatment process.The main results are as follows:（1）By controlling the discharge conditions,the type and quantity of active substances in the discharge plasma system can be affected.Compared with O₃,·OH plays a more critical role in the removal process.Increasing the discharge voltage and using O₂as the discharge gas can produce more O₃and·OH.Increasing the p H of the solution can promote the conversion of O₃to·OH,thus improving the removal effect.Under the optimal discharge conditions（discharge voltage is 11.2KV,p H=11,and discharge gas is O₂）,the removal rate of M.aeruginosa（400 m L）with cell density of 10⁷cells·m L^-1can reach 96.2%within 40 min of discharge plasma treatment.The discharge plasma can completely destroy the algal cell membrane within 2 min,degrade more than 95%of chlorophyll a and phycobiliprotein within 5 min,destroy cell structure,lyse the nucleus,and the leakage of intracellular substances.At the same time,the algal cell solution after a short time of treatment（5 min）loses the ability of photosynthesis,reproduction,and nutrient absorption,and cannot resume growth within 8 d of culture,so as to achieve the goal of rapid,effective and irreversible removal of M.aeruginosa.（2）The discharge plasma treatment made the M.aeruginosa cells break and released a large amount of intracellular organic matter.The content of dissolved organic carbon and organic nitrogen in solution increased 5.5 and 5.5 times,respectively,at 40 min compared to0 min.In the treatment process,inorganic nitrogen and inorganic phosphorus increased with the treatment time,indicating that the DOM released from the cell was also being degraded and mineralised,but due to a large amount of intracellular material released in the early stages of the treatment,the concentration of DOM generally showed a pattern of first increase and then decrease.The results of ultraviolet spectroscopy showed that the relative molecular weight and humification degree of DOM decreased with treatment time.Infrared spectroscopy also found that the peak strength of functional groups of organic compounds such as alcohols and aliphatic compounds decreased after plasma treatment.After 40 min of discharge plasma treatment,the total protein and polysaccharide concentrations decreased by 63.9%and 84.7%,respectively.The fluorescence intensity of aromatic amino acids,humic acids,and other components increased first and then decreased rapidly,further indicating that the released DOM was effectively degraded.（3）The release of DOM resulted in a significant increase in the concentration of microcystin-LR,reaching 61.0μg·L^-1at 3 min,but was completely removed at 20 min.The released DOM can also be used as a precursor for the subsequent disinfection process,which in turn promotes the production of disinfection by-products（DBPs）.However,by extending the treatment time,the DBPs were further oxidatively degraded,and after 40 min of discharge treatment,the production potential of total DBPs and their calculated toxicity decreased by 19.1%and 19.3%,respectively.Of these,halogen acetaldehydes,haloacetic acids,and trihalomethanes from carbon DBPs dominated the formation of DBPs,followed by haloactonitriles from nitrogen DBPs which made a greater toxic contribution.The CHO cytotoxicity results for DOM showed that the toxicity of DOM did not change significantly throughout the treatment and that its toxicity was not lethal to zebrafish cultured in DOM solution.Inhibition of the relative activity of CHO cells by chlorinated DOM reached48.4%at 20 min of discharge treatment,further demonstrating the potential risk of disinfection by-product toxicity associated with DOM release,but decreased to 3.9%at 40min after prolonged treatment.These results indicate that discharge plasma can effectively reduce potential environmental risks during the treatment process.This study provides a method that can rapidly and effectively remove M.aeruginosa from water by constructing a dielectric barrier discharge plasma system,elucidates the effect and mechanism of discharge plasma for the removal of M.aeruginosa from water,and analyses and evaluates the potential environmental risks during the treatment of M.aeruginosa by discharge plasma.The study has some theoretical and practical significance for the treatment of cyanobacterial blooms by discharge plasma.