Study On The Rapid Degradation Of Microcystins By Hydroxyl Radical In Algal Bloom Water

Microcystin-LR is a kind of hepatotoxin consisting of a seven amino acid ring,which is released by blooming cyanobacteria.When algal bloom breaks out,MC-LR is difficult to be removed by conventional drinking water treatment,which seriously threatens the health and safety of drinking water.Aiming at the national strategic demand of developing safety drinking water technology,in this paper,·OH was efficiently produced by strong ioniation discharge conbined with water jet cavitation at atmospheric pressure,and the ·OH degradation of MC-LR in algal bloom water treatment was studied.The main results are as follows:(1)Based on the strong ionization discharge at atmospheric pressure,·OH was prepared and realized the rapid mineralization of MC-LR.High concentrated oxygen active species(OAS)were prepared,which was combined with water jet cavitation to generate·OH.The total reactive oxygen(TRO)reached an equilibrium concentration in 1 s in the main pipe.And the·OH concentration was 41.91 ?M with a generating rate of 2514.6 ?M/min at a TRO of 7.99 mg/L,which was far superior to other advanced oxidation technologies.The studies of dose-effect and time-effect indicated that ·OH degraded 100.91 ?g/L and 1094.27 ?g/L MC-LR at a TRO of 0.75 mg/L and 2.44 mg/L in 3 s,respectively.The Data of TOC,TN and NO3-indicated that MC-LR could be fully mineralized at a mass ratio TRO/MC-LR of 10.1(2)Based on LC/MS/MS and GC/MS,the chemical reaction mechanism of ·OH mineralization of MC-LR was clarified.Based on the solid phase extraction and LC/MS/MS detection,polypeptides(m/z 1029.4,795.4,815.3,835.4 and 829.3)and benzene ring product m/z 232.1 were found,and we firstly discovered the characteristic benzene ring-opening product of m/z 158.0.And we firstly found 4 amino acid intermediates including Ala,Leu,Asp and Glu,together with a series of aldehydes and ketones with small moleculars by GC/MS detection.Based on the analysis of molecular structures and generation trends of intermediates,we ensured the main reaction sites of MC-LR including the benzene ring and carbon-carbon conjugated diene bonds in Adda side chain,C=C bonds in Mdha amino acid and multiple amide bonds in peptide ring.·OH attacked MC-LR molecular at the conjugated diene bonds and benzene ring in Adda side chain,the double bonds in Mdha amino acid and amido bonds in the peptide ring,resulted in the destruction of peptide ring and benzene ring.Through above reactions,MC-LR was degraded into amino acids,benzene ring-opening products of binary carboxylic acids and aldehydes and ketones,which were transformed into CO2,H2O and inorganic ions.(3)The ·OH degradation effect,intermediates and reaction pathways of MC-LR were compared with conventional chemical oxidants including O3,ClO2,NaClO and KMnO4,and it revealed that the benzene ring opening was the key to mineralize MC-LR.It was only 3 s for ·OH totally degradation of MC-LR,which was far below those conventional oxidants.For ·OH,O3 and KMnO4 degradation,MC-LR was transformed into hydroxylation products,carboxylic acid compounds,aldehydes and ketones.But the chlorination of MC-LR generated large amounts of harmful chlorobenzene products.·OH was the only oxidant to open the benzene ring and peptide ring simultaneously,furtherly mineralize MC-LR.While the other oxidants were difficult to open the benzene ring to form the characteristic ring-opening product m/z 158.0,or even mineralize MC-LR.(4)The release of intracellular MC-LR during ·OH/ClO2 inactivation of typical cyanobacteria Microcystis aeruginosa was compared,which provided the safe TRO dose for ·OH treatment of algal bloom water.The lethal thresholds for ·OH inactivation of M.aeruginosa with a density of 97.9 × 104 cells/mL and 188.2 × 104 cells/mL were 1.35 mg/L and 1.81 mg/L,respectively,which were identified by photosynthetic activity parameter Fv/Fm.Within the safe thresholds(higher thanlethal threshold,residual TRO?0.4 mg/L),while all algal cells were inactivated,the intracellular MC-LR was not released and all extracellular MC-LR was degraded to lower than detection limit fastly.However,low dose of C4O2 would lead to large amounts of MC-LR release,and about 44%intracellular MC-LR was released at the leathal threshold.With the increase of ClO2 concentration,the extracellular MC-LR firstly increased and then decreased,which has negative effects on water safety.These results provided positive evidences for ·OH engineering application of algal bloom water treatment.(5)An ·OH drinking water treatment system with a capacity of 480 m3/day was built in the Xinglin water plant in Xiamen,which realized the rapid inactivation of algae and degradation of MC-LR.The pilot study results showed that,with ·OH treatment(TRO=0.88 mg/L)of algal bloom water in the main pipe within 9.8 s,the algae density decreased from 3.5 × 104 cells/mL to 220 cells/mL,resulting in no algae go through the sand filter or even enter into the network.Thus the algae removal rate was improved by 30.1%compared with conventional pre-treatment,which effectively avoided the second pollution for drinking water distribution system.At the same time,·OH rapidly degraded MC-LR to not detected in algal bloom water,and no disinfection by-products were formed obviously.All water quality indexes satisfed with the Chinese Sanitary Standards for Drinking Water(GB/T 5749-2006).Based on the ·OH generation by strong ionization discharge at atmosphere pressure,the rapid mineralization of MC-LR was realized,the chemical reaction mechanism of·OH mineralization of MC-LR was revealed,and the safety threshold of treatment of M.aeruginosa was confirmed.All above work provided technical support for developing noval algal bloom water treatment process,which has important application prospect.