Removal Of Nitrate Contamination In Groundwater By Magnetic Chitosan Gel Beads Immobilized Microorganisms

Among different denitrification methods,biological denitrification has the advantages of high recovery,thorough denitrification,low operating costs,and sustainability.In particular,hydrogen autotrophic denitrifying bacteria show unique advantages due to clean denitrification.As a new material synthesized from natural biomass,magnetic chitosan gel beads(MCHBs)are currently widely used in the water treatment field,however,there are few reports on the application of immobilized microorganism technology.In this study,MCHBs were synthesized by mature methods,and then the surface structure of MCHBs was modified to produce modified magnetic chitosan gel beads(MMCHBs)and performance parameters of gel spheres before and after modification were compared.Then,screened out hydrogen autotrophic denitrifying bacteria from the activated sludge and fixing microbes on MCHBs and MMCHBs to prepare magnetic chitosan gel ball immobilized bacterias(MCHBs-HDB)and modified magnetic chitosan Gum ball immobilized bacterias(MMCHBs-HDB)investigating their denitrification properties of stability,slow release performance,etc.In order to utilize magnetic chitosan gel ball immobilized bacteria to fill the permeable reaction wall(PRB)and provide data for in-situ repair of nitrate contamination.The main conclusions of this study are:(1)Characterization showes that hydrogen autotrophic denitrifying bacterias can be successfully fixed on MCHBs.Compared with free bacterias,MCHBs-HDB can remove nitrate nitrogen better with concentration of 100 mg/L.It shows that magnetic chitosan gel spheres increase the tolerance of free hydrogen autotrophic denitrifying bacteria to nitrate nitrogen concentration to some extent.When the initial p H is 7～9,it is beneficial for immobilized hydrogen autotrophic denitrifying bacteria to remove nitrate nitrogen and it still works when the p H is 6.That indactes the magnetic chitosan gel sphere broadens the p H tolerance range of the hydrogen autotrophic denitrifying bacteria.The addition of MCHBs did not significantly optimize the adaptation range of hydrogen autotrophic denitrifying bacterias in C / N and temperature,and was still within the adaptation range of the microorganism itself.(2)After the fifth cycle,the removal rate of nitrate nitrogen by MCHBs-HDB was still greater than 90%,the accumulation of nitrite nitrogen was less than 5 mg/L,and there was no accumulation of ammonia nitrogen.All of them indicate MCHBs-HDB is still stable after 5 repetitive experiments and maintain good denitrification ability.(3)Due to chitosan can be dissolved in acidic solution,the surface structure of MCHBs is modified to obtain MMCHBs.It is concluded MMCHBs are equivalent to MCHBs in the parameters of diameters,expansion coefficients and mechanical strength after comparison.Besides,the specific surface area of MMCHBs is 45.8 times than that of MCHBs,and the mass transfer performance is increased by 73.92%.(4)Comparing with MMCHBs,MCHBs-HDB,and MMCHBs-HDB,it can be concluded that denitrification by hydrogen autotrophic denitrifying bacterias is the main factor in the nitrate removal process of MMCHBs-HDB and MMCHBs-HDB exhibits higher removal ability.Through the study of nitrate removal under different single factors indicates that MMCHBs have better environmental regulation ability than MCHBs,mainly in environmental p H adjustment.In addition,MMCHBs-HDB further increased the tolerance of autotrophic hydrogen denitrifying bacteria to nitrate concentration,which is reached 125 mg/L.Furthermore,MMCHBs-HDB has good stability and slow-release properties.(5)The kinetic simulation of nitrate reduction by MMCHBs-HDB is accords with zero order reaction kinetics.In addition,the kinetic constant showed an increasing trend with the increase of the initial concentration,indicating that MMCHBs-HDB can efficiently reduce higher concentrations of nitrate under suitable conditions.MMCHBs adjusts the environmental p H so that hydrogen autotrophic denitrifying bacteria can work in a suitable climate.(6)The ability MMCHBs-HDB and MCHBs-HDB reduce nitrate in the condition of actual groundwater components indicates that MMCHBs-HDB and MCHBs-HDB can effectively remove nitrate.The removal rates are 83.51% and 63.56% and nitrate nitrogen was 5.52 mg/L and 9.47 mg/L respectively after 72 h.However,the composition of the actual groundwater is complicated.Some elements can compete with nitrate such as sulfate resulting in the kinetics of MMCHBs-HDB and MCHBsHDB in the early episode of the reaction can not meet the zero-order reaction but when the reaction entered the second it meet the zero-order,kinetic constants of them are 0.714 and 0.522 respectivly.