Study On Regulation Of Electron Flow In Denitrification Path Of Alcaligenes Sp.TB By Pd-Fe/MWCNTs

Nitrate（NO₃^--N）is one of the typical inorganic pollutants in water.Excessive intake of NO₃^--N will lead to the occurrence of methemoglobinemia（blue baby syndrome）.Biological denitrification can reduce NO₃^--N to N₂,which has the advantages of low operation cost and no secondary pollution.However,there are still some problems in the early stage of biological denitrification,such as low microbial activity,slow degradation rate and long hydraulic retention time.Therefore,improving the efficiency of biological nitrate removal is one of the hot issues.In this thesis,two kinds of solutions are proposed to improve the efficiency of biological NO₃^--N removal.The first type is combined NO₃^--N removal process,which combines multiple physical,chemical,and biological methods to achieve rapid and efficient removal of NO₃^--N.The second type is to solve the efficiency problem by improving the degradation rate of microorganisms themselves,that is,to find some medium that can enhance the process of biological NO₃^--N removal,accelerate the utilization of carbon sources,optimize the metabolic pathway,and promote electron transfer,so as to enhance the metabolism of microorganisms.Therefore,a kind of nanocomposite with the characteristics of both chemical reduction and redox mediator was prepared in this study.The basic theory of accelerated denitrification biological regulation technology of nanocomposite was discussed from two levels of biochemistry and electrochemistry.The main results are as follows:（1）By examining the effect of Pd-Fe structure type and carrier properties on the efficiency of chemical reduction of NO₃^--N,the best Pd-Fe structure type is determined and a suitable reductant carrier is selected and its influence mechanism is explored:different junction structured Pd-Fe nanocomposites have higher reduction activity and N₂selectivity.At the same time,when MWCNTs（c）modified with“10 mol·L^-1 HNO₃,353K,12 h”acidification,the metal nanoparticles have the smallest particle size and the highest reduction activity;the electron density change of the Pd surface in the alloy structure Pd-Fe may change the adsorption energy of H*,NO*and other species,thereby affecting their reduction activity and selectivity.（2）Comprehensively investigate the reduction activity of the Pd-Fe loading ratio on the nanocomposite and its selectivity to the reaction product,and select the Pd-Fe/MWCNTs with the best ratio of Pd to Fe:when Pd:Fe=2:1,the reduction activity of the corresponding nanocomposite Pd₂-Fe₁/MWCNTs was 1.413 mg/(g_red·min),the N₂selectivity was 85.27%,and the reduction performance was optimal.The apparent activation energy of Pd₂-Fe₁/MWCNTs chemical reduction nitrate is 31.62 k J/mol,and the chemical reduction reaction conforms to the Langmuir-Hinshelwood kinetic mechanism.The kinetic model is ln（c₀/c_t）/（c₀-c_t）+0.0249=0.0513×w_red×t/（c₀-c_t）.With the increase of the use frequency of Pd-Fe nanocomposites,although its reducing activity and its selectivity to N₂ have decreased to a certain extent,it can still maintain a high reducing activity and N₂selectivity of more than 40%.（3）By investigating the biological metabolic pathway combining the nanocomposite Pd-Fe/MWCNTs and the denitrifying bacteria Alcaligenes sp.TB,the results confirmed that Pd-Fe/MWCNTs can quickly remove nitrate in the process of coupled denitrifying bacteria denitrification,enriching the microbial denitrification path and accelerating the electron transfer rate in the microbial denitrification path.Through the investigation of oxidation-reduction potential,dissolved oxygen and p H,the influence of Pd-Fe/MWCNTs on the electron transfer path in the denitrification process of Alcaligenes sp.TB was discussed.The main conclusions are as follows:10-50 mg/L Pd-Fe/MWCNTs all significantly promoted their metabolic activity,and showed a dose response.30 mg/L and 50 mg/L Pd-Fe/MWCNTs showed optimal values for microbial metabolic activity and denitrification performance,respectively.Enzyme activity experiments showed that30 mg/L of Pd-Fe/MWCNTs significantly increased the NIR and N₂OR activities of microorganisms.The intermediate products NO₂^--N and by-product NH₄^--N produced by it could be used in the process of microbial denitrification,enriching the microbial reaction and accelerating the denitrification process.In addition,through the discussion of the denitrification electron flow mechanism,using the principle of electronic competition and redox potential,the redox potentials of related substances in the denitrification process are sorted,and the position of Pd-Fe/MWCNTs in this path is preliminarily predicted.（4）The electron depressant speculates that the site of Pd-Fe/MWCNTs accelerated denitrification is located near cytochrome c,that is,the path of receiving electrons from cytochrome c to its downstream nitrite reductase or molecular oxygen.Through electrochemical testing and analysis,the redox potential of Pd-Fe/MWCNTs was 266 m V,and the redox potentials of related substances in the process of denitrification were sorted,so that the transfer sites in the electron transfer chain of Pd-Fe/MWCNTs in the denitrification process could be accurately predicted.By analyzing the relationship between the exogenous and endogenous electron mediators and the relevant indicators of the denitrification process,it was determined that the endogenous electron mediators produced by Pd-Fe/MWCNTs-induced strains are closely related to the NAP activity of the denitrification process relationship,and the redox mediator characteristic of Pd-Fe/MWCNTs itself has a significant positive correlation with NIR activity.In summary,this study focuses on the improvement of biological denitrification efficiency by nanocomposites,and has carried out two aspects of chemical-biological combined denitrification and redox mediator accelerated denitrification.Clarified the relationship between metal heterostructure and its reduction activity in the field of environmental governance,explained the law of nanocomposite coupling and enhanced microbial denitrification performance,and explored the regulation mechanism of electron flow branch transfer path during denitrification.