Occurrence State And Oxidative Degragation Mechanism Of Cyanobacterial Neurotoxin BMAA In Water

?-methylamino-L-alanine(BMAA)is a neurotoxic cyanobacterial toxin that caused the incidence of amyotrophic lateral sclerosis(ALS/PDC)100 times higher than the world average in Guam in the 1950s.There is still a lot of controversy in the academic community on whether BMAA is presence widely in the world.There are still few reports on how to solve the problem of BMAA in drinking water.In recent years,the eutrophication of freshwater resources in our country has been seriously polluted,and the outbreak of cyanobacterial blooms has seriously affected our drinking water safety.Therefore,it is urgent to investigate the current status of water polluted by BMAA in China and oxidative degradation methods of BMAA.In this study,it is starting from the establishment of an analytical method for BMAA in water.Subsequently,the effect of different nutrients on the production of BMAA in cyanobacteria was evaluated;the occurrence status of BMAA in actual water in China was analyzed,and the degradation efficiency of different oxidation systems for BMAA was explored.This study first established the analysis method of BMAA in water,gradually optimized the operating parameters in each step,compared the difference between the direct detection method and the derivative detection method in instrument detection,and finally verified the analysis method of intracellular BMAA.The results showed that both the direct detection method and the derivatization detection method can accurately quantify the content of BMAA and its isomers in actual water.Solid phase extraction(SPE)was a key step in the sample pretreatment stage.The nature of the SPE cartridge and the SPE operating parameters greatly affect the effectiveness of SPE.The competition effect of natural organic matter in the actual water body was the main reason that caused the recovery rate of BMAA in the SPE process to decrease from 90%to 38.11%.Using C18 type SPE cartridge to pre-purify the sample and reduce the volume of the sample solution from 100mL to 50mL can effectively improve the recovery rate of the actual sample.The final optimized BMAA analysis method can achieve the recovery rate of BMAA>88%in both algae solutions and actual samples.The relative standard deviation(RSD)of the intracellular BMAA analysis method was less than 7%,and the sample recovery rate was more than 70%.The analysis method of BMAA used in this study can accurately and effectively analyze various forms of BMAA in the sample.The samples from 16 different lakes or reservoirs were collected and 9 samples obtained positive results,indicating that there was indeed a risk of BMAA pollution in China.No dissolved BMAA was detected in the collected samples.In order to analyze the relationship between different forms of BMAA and environmental factors,six strains of cyanobacteria were cultivated and the toxicity characteristics of different cyanobacteria were analyzed.Four of six strains of cyanobacteria can produce BMAA.By changing the initial concentration of nitrate(NaNO3)and phosphate(K2HPO4)in the medium,the effect of nutrient materials in the environment on the toxicity characteristics of Anabaena FACHB-418 was studied.The results showed that the yield of bound BMAA in cyanobacteria was not affected by environmental factors,and its concentration was between 50 and 100 ng·g-1.When the nitrogen concentration in the environment was lower than 1.7mg·L-1(121.43?mol),free BMAA was detected in the sample,and the statistical analysis results showed that there was a strong positive correlation between the free BMAA and the nitrogen in the environment.Normally growing cyanobacteria cannot expel BMAA from the body,and the death and cytolysis of cyanobacteria can dissolve the free BMAA into the water.In addition,exogenous BMAA can inhibit the growth of cyanobacteria that cannot produce BMAA,and as the concentration of exogenous BMAA increased,its effect increased.This phenomenon implied that the production of BMAA may be a stress response of certain cyanobacteria to low nitrogen conditions to kill other cyanobacteria,namely their competitors.This subject studied the efficiency of NaClO oxidative degradation of BMAA and the effect of pH on the degradation mechanism of BMAA.The results showed that the reaction between NaClO and BMAA was followed the second-order kinetic model,and the degradation reaction rate of BMAA accelerated with the increase of NaClO concentration.The pH value of the solution significantly affected the oxidation reaction rate.When the pH increased from 4.5 to 9,the pseudo-first-order reaction rate constant(kobs)decreased from 1.69 min-1 to 0.015 min-1.The presence forms of active chlorine and BMAA at different pH values were the main reasons for this phenomenon.The oxidation reaction of NaClO and BMAA consisted of four elementary reactions,and hydrogen ions were beneficial to the reaction.The activation energy of this reaction was 16.6 ± 1.99 kJ·mol-1,and BMAA was converted into 6 different products through chlorination,dechlorination and decarboxylation.In order to obtain a more efficient BMAA degradation method,this study further explored the effectiveness of advanced oxidation technology to remove BMAA.The UV/PDS oxidation system can efficiently remove BMAA.Therefore,the influence of reaction parameters and water matrix on the degradation efficiency of BMAA in the UV/PDS oxidation system was analyzed in the study.Through steady-state kinetic model analysis,,the concentration of SO4-·?is two orders of magnitude higher than that of ·OH in the UV/PDS system.Capture experiments and model calculations confirmed that SO4-4· was the main oxide species that caused BMAA degradation.The pH value can significantly affect the degradation rate of BMAA.At pH 8,the degradation rate of BMAA was the fastest,and the kobs was 1.039 min-1.NOM and Cl-inhibit the oxidative degradation of BMAA,while HCO3-promoted the degradation of BMAA.In addition,the activation energy of this reaction was calculated to be 15.85±1.29kJ·mol-1.Free radicals convert BMAA into 5 products by attacking the C-N bond.The results of this study help to resolve the current academic disputes about BMAA analysis methods,provide basic data for assessing the risk of BMAA risks facing China's current water environment,and have important significance for solving the BMAA pollution problems in drinking water.