Study On Degradation Characteristics Of Niclosamide By Microbial Electrochemical System Under The Action Of Rhamnolipid

As we all know,schistosomiasis is one of the most serious parasites that affects human health.Oncomelania snails are its main intermediate host.The unique climatic conditions and geographical environment of Poyang Lake provide good growth conditions for oncomelania snails.Oncomelania snails grow arbitrarily,which makes Poyang.The lake basin has become one of the most severe areas of schistosomiasis in China.In recent years,Niclosamide(NCL)as a molluscicide has been widely used in the snail control of lakes and grasslands.Part of the excess niclosamide is naturally degraded into intermediate products in the environment.A part remains in the bottom mud or soil.Niclosamide remaining in the environment has a toxic effect on some organisms,has a certain environmental risk,and may enter the human body through the way of bio-enrichment in the food chain,endangering human health.Therefore,it is extremely important to control the content of niclosamide and its intermediate products in the water or sediment.Based on the research on microbial electrochemical technology and biosurfactant rhamnolipid at home and abroad,this subject used a microbial electrochemical system(single-chamber MEC,MEC-UASB coupling system)to study the degradation of the molluscicide niclosamide.Through single factor experiments,The factors such as the applied voltage,initial concentration,glucose dosage,and rhamnolipid dosage were investigated,which influented the degradation of niclosamide by the microbial electrochemical system,and the orthogonal test method was used to optimize the operating parameters of the MEC-UASB coupling system.Niclosamide and its degradation products were analyzed by HPLC and LC/MS.The main results are as follows:(1)The microbial electrochemical system(R1)and the pure microbial system(R3)had a certain degradation effect on NCL.The former was better than the latter.The pure electrochemical system(R2)had almost no degradation effect on NCL.In addition,when testing the sludge in R1 and R3 after 48 h,it was found that a small amount of NCL remained in the sludge.(2)The applied voltage,the initial concentration of NCL,the dosage of glucose and the dosage of rhamnolipid had varying degrees of influence on the degradation of NCL by the microbial electrochemical system.The experimental results showed that under the control of other variables,the applied voltage is controlled between1.0 and 1.5 V to have a better degradation effect.The microbial electrochemical system had high removal efficiency for treating low-concentration of NCL.Under the conditions of the applied voltage was 1.0 V,the dosage of glucose was 1 g·L^-1,and the initial concentration was 1 mg·L^-1?5 mg·L^-1 and 10 mg·L^-1,respectively,the removal rates of NCL by the system after 48 h were 91.7%,82.14%,and 87.57%,respectively.Compared with the system without added glucose(co-substrate),adding glucose can promote the degradation of NCL by the microbial electrochemical system.Within a certain range of glucose addition,the degradation efficiency of NCL was increased with the increase of glucose content.In addition,the sludge in the system was tested after 48 hours,and it was found that the sludge under different conditions contained NCL.The higher the initial concentration of NCL,the more residual NCL in the sludge.The presence of rhamnolipid also affects the degradation efficiency of the microbial electrochemical system for NCL.The greater the amount of rhamnolipid added,the less the residual amount of NCL in the mud.(3)The orthogonal experiment was used to further study the influence of applied voltage,initial NCL concentration,glucose dosage,and rhamnolipid dosage on the degradation of NCL by the MEC-UASB coupling system.Orthogonal experiment results showed that the primary and secondary order of the experimental factors were the amount of rhamnolipid added,the applied voltage,the initial concentration of NCL,and the amount of glucose added.The best combination in this experiment was the applied voltage of 1.25 V,and the initial concentration of NCL was 10 mg·L^-1,the dosage of glucose was 5 g·L^-1,and the dosage of rhamnolipid was 2 g·L^-1.Considering that the dosage of glucose was not the dominant factor and the problem of reducing operating costs,it was advisable to reduce the dosage of glucose to 3?4 g·L^-1.(4)According to the HPLC/MS analysis results,it can be inferred that the degradation products of niclosamide were mainly 5-chlorosalicylic acid and2-chloro-4-nitroaniline.Under the action of the biological cathode,5-chlorosalicylic acid was produced Dechlorination reaction produces salicylic acid,which was then converted into 2,5-dihydroxybenzoic acid.Under the action of anode microorganisms,it was oxidized by breaking the bonds and opening the rings,and further metabolized into Krebs intermediates(such as oxalic acid,etc.).It was mineralized into CO₂ and H₂O;the 2-chloro-4-nitroaniline is converted into2-chloro-4nitrophenol,and then 2-chloro-4nitrophenol had a denitration reaction to generate 2-chlorophenol.Under the action of the anode microorganisms,2-chlorophenol was generated chlorinated fatty acids by breaking the bonds and opening the rings.The chlorinated fatty acids had a dechlorination reaction on the surface of the biological cathode to degrade into valeric acid,oxalic acid and other substances,and finally mineralized into CO₂ and H₂O.