Removal And Mechanism Of Microcystis Aeruginosa And Algal Toxins By Photo/Electrochemical Oxidation

With the acceleration of industrialization,a large amount of nutrients such as nitrogen and phosphorus that comes from urban domestic and industrial wastewater were discharged into natural waters,and resulting in the frequent outbreak of eutrophication.During the metabolism of bloomed algae,the surface water was contaminated with a large amount of algal toxins released by the algae.Algal toxins can enter into human body through the digestive tract,causing diarrhea,nerve paralysis,liver damage,and in severe cases can cause poisoning or even death.However,the conventional water treatment processes(i.e.,coagulation,precipitation,filtration,and disinfection)can hardly remove them.Therefore,Microcystis aeruginosa and its derivatives were chosen to discuss the degradation efficiency and mechanism of microcystis inactivation and algal toxins by using UV/chlorine,solar/chlorine and boron-doped diamond(BDD)anode/sulfate electrochemical combined processes.For the treatment of Microcystis aeruginosa by UV/chlorine process.First,flow cytometry as well as confocal laser scanning microscopy(CLSM)were used to characterize the inactivation of Microcystis aeruginosa in UV/chlorine process,and the model of chlorine dacay was established.Secondly,the microcystin MC-LR and endotoxin released by the algal during the reaction were detected by liquid chromatography-mass spectrometry(LC-MS)and limulus reagent.Finally,the effect of coagulation sedimentation as a post-treatment process on algae removal efficiency was investigated.The results showed that the UV/chlorine process can significantly change the surface characteristics of Microcystis aeruginosa and enhance the coagulation efficiency.Moreover,due to the destruction of algal cells,microcystin-LR would be released and then degraded during pre-oxidation,and the released endotoxins could be removed in the subsequent coagulation-sedimentation process.For the typical algal toxin MC-LR released by Microcystis aeruginosa.First,the improvement of degradation efficiency of the solar/chlorine combined process compared with sunlight alone and chlorination alone was studied.Secondly,the effect of chlorine dosage,p H,bicarbonate and natural organic matter on the removal efficiency of solar/chlorine process was investigated,and the contribution of each free radical in the process was calculated by using some specific compounds as the probes.Finally,the degradation products were detected by LC-MS to speculate the possible degradation pathways,and the protein phosphatase 2A(PP2A)was used to study the time-dependant changes of hepatotoxicity of the degradation products.Compared with solar alone and chlorination alone,the combination of solar and chlorine can significantly improve the removal efficiency of MC-LR.The higher chlorine dosage and p H can effectively improve the removal effect of MC-LR,however,bicarbonate and natural organic mater showed a great inhibit effect.Quenching experiments confrmed that reactive chlorine species(RCS),ozone and hydroxyl radical(HO~·)were the predominant reactive species in solar/chlorine process at neutral condition,and the respective contribution on MC-LR removal could be ranked as:RCS>HO~·>ozone.The proposed degradation pathway of MC-LR generated during the solar/chlorine process contained 15 intermediate and final products,of which the Adda group on MC-LR is the most vulnerable position,the hepatotoxicity of MC-LR and its transformation products continuesly decreased with increasing treatment time.The degradation of ?-methylamino-L-alanine(BMAA)by UV/chlorine process was studied,and the impacts of chlorine dosage,p H,NOM dosage and alkalinity were investigated.Results showed that compared with UV alone and chlorine alone,the UV/chlorine process has huge advantages in handling BMAA.The degradation effect of BMAA under alkaline conditions was better,and the degradation rate was not significantly inhibited within a certain range of increasing bicarbonate concentration,but the presence of natural organic matter significantly inhibited the degradation of BMAA.Subsequently,LC-MS was used to analyze the degradation products of BMAA.Based on the 11 intermediate products and their relative abundances,the degradation pathway of BMAA and the evolution profile of intermediate products in the UV/chlorine process were speculated.Besises,the degradation pathway was further verified using Gaussian16 software based on Fukui function.The electrochemical oxidation inactivation of Microcystis aeruginosa,was studied using the boron-doped diamond(BDD)electrode.First,the photosynthetic capacity and cell integrity of Microcystis aeruginosa were characterized by chlorophyll fluorometer,flow cytometry and CLSM.Secondly,the effect of different anodes(BDD,MMO and Pt)and different electrolytes(nitrate,phosphate and sulfate)coupling on the inactivation of Microcystis aeruginosa was discussed.Finally,a kinetic model based on two consecutive irreversible first-order reactions was established to simulate the release and degradation of MC-LR.In the coupled system of BDD anode and sulfate,the in-situ generated and activated persulfate plays an important role in the inactivation of Microcystis aeruginosa.The inactivation efficiency increased along with the increase of applied currents.In addition,the established kinetic model was able to successfully predict the concentration of extracellular,intracellular and total MC-LR under different applied currents and extended exposure time.