Study On The Synergistic Removal Effect And Mechanism Of Microcystis Aeruginosa And Microplastics In Coagulation Proces

In lakes,reservoirs and other water sources,Microcystis aeruginosa,which is the dominant algae species,has been the key water pollution problem in China.However,under the condition that it has not been effectively contained,another new pollutant-microplastics（MPs）has entered the water source in large quantities,posing a serious threat to the water safety of the water source.Coagulation process is the key to remove Microcystis aeruginosa and microplastics.However,when Microcystis aeruginosa and microplastics coexist,the interfacial action during coagulation and stirring may affect the synergistic removal of the two from many angles.However,at present,the interaction mechanism between algae cells and microplastics and the coagulation removal efficiency of algae-microplastics composite system are still unclear.Therefore,aiming at the combined pollution of Microcystis aeruginosa and microplastics in water,this paper studies the synergistic removal effect and mechanism of typical Microcystis aeruginosa-microplastics composite system in different coagulation processes,taking the coagulation process as the breakthrough point.The specific results are as follows:（1）Coagulation process is the key to remove Microcystis aeruginosa and microplastics.However,when Microcystis aeruginosa and microplastics coexist,the interfacial action during coagulation and stirring may affect the synergistic removal of the two from many angles.However,at present,the interaction mechanism between algae cells and microplastics and the coagulation removal efficiency of algae-microplastics composite system are still unclear.Therefore,aiming at the combined pollution of Microcystis aeruginosa and microplastics in drinking water,this paper studies the synergistic removal effect and mechanism of typical Microcystis aeruginosa-microplastics composite system in different coagulation processes,taking the coagulation process as the breakthrough point.The specific results are as follows:（2）The optimal dosage of HTTC and Al₂（SO₄）₃ for treating a single Microcystis aeruginosa system（10⁶ cells/m L）is 1 mg/L and 40 mg/L respectively.Under the optimum dosage,the removal rate of algae cells by both of them reached more than 76%.HTTC and Al₂（SO₄）₃ have good removal effects on protein substances,but the removal effect on UV₂₅₄ is not obvious.It is worth noting that the removal efficiency of TOC by HTTC is much higher than that of Al₂（SO₄）₃.The removal efficiency of HTTC and Al₂（SO₄）₃ coagulants for single microplastics system is not ideal,and the removal rate is low（<15%）,which may be due to the following reasons:First,the density of PS is close to that of water（1.05 g/m³）,which makes it difficult to precipitate during coagulation.Second,the flocs formed in the coagulation process are small,which can not effectively adsorb microplastics particles.（3）In the process of removing microplastics-Microcystis aeruginosa composite pollutants by HTTC and Al₂（SO₄）₃ coagulation,the removal effect of microplastics in composite pollutants was significantly improved,and the smaller the particle size of microplastics,the more obvious the removal effect of microplastics was.This may be that the composite system is easier to form uniform and larger flocs,and more microplastics particles can be wrapped in cyanobacteria flocs,and the smaller the particle size of microplastics,the easier it is to be wrapped in cyanobacteria flocs,thus enhancing the coagulation effect of the composite system.However,with the addition of HTTC coagulant,the removal rate of algae cells in the composite pollution system was obviously reduced by the addition of microplastics.It may be that the addition of negatively charged microplastics requires a higher dose of HTTC to improve the ability of electric neutralization and adsorption bridging.With the addition of Al₂（SO₄）₃ coagulant,the removal effect of algae cells was improved by adding microplastics.It may be that the addition of microplastics increased the compactness of its flocs,thus enhancing its coagulation and sedimentation effect on algae cells.The removal efficiency of TOC and microcystins in the composite system is also obviously improved.First,microplastics itself has adsorption,and second,the floc formed by microplastics and algae may have stronger ability to capture organic matter.（4）In the process of sediment stacking in microplastics-Microcystis aeruginosa mixed pollution system,with the addition of HTTC coagulant,UV₂₅₄ began to accumulate on the 8th day of stacking.With the addition of coagulant Al₂（SO₄）₃,UV₂₅₄ began to accumulate on the 4th day.The change of EOM is similar to that of UV₂₅₄.It shows that the flocs formed in the process of HTTC coagulation have stronger protective ability to cells or stronger binding ability to extracellular substances.The concentration of UV₂₅₄ and microcystin and the content of EOM in the composite system are higher,which may be due to the mechanical damage caused by the collision between microplastics and algal cells during coagulation and stirring and the oxidative damage caused during contact,which makes the rupture of algal cells more thorough.In the process of stacking,the microcystins in the mixed pollution system with HTTC coagulant are lower than those with Al₂（SO₄）₃ coagulant.It shows that when the coagulant type is HTTC in the process of sediment stacking,the algae cells in the combined pollution system are relatively stable and will not burst and release algal toxins in large quantities.During the 8-day stacking process,the microbial community abundance of the sediment was improved to varying degrees.The community abundance of the cyanobacteria sediment containing microplastics was significantly higher than that of the cyanobacteria sediment without microplastics,and the community abundance was most obviously improved in the composite pollution system with the addition of Al₂（SO₄）₃ coagulant,especially higher than that of the algae-containing sediment with the addition of HTCC coagulant.This may be because firstly,microplastics can be used as a colonization site for microorganisms,thus promoting their reproduction;secondly,HTCC has good antibacterial activity,thus reducing the proliferation of associated bacteria of Microcystis aeruginosa.