Study On Multivariate Coupling Mechanism Of Microorganism,Minerals And Radionuclides In Groundwater

The development of nuclear weapons and the leakage of nuclear power plants have caused a large amount of radionuclides to enter the environment.Due to the great harm of radionuclides to the environment and human body,the migration of radionuclides in groundwater had attracted great attention.The migration characteristics of radionuclides in underground water were mainly related to the interactions between the oxidation state of radionuclides,minerals,and some organic ligands,such as microorganisms and their extracellular polymeric substances(EPS).Therefore,the study of species states of radionuclides in groundwater and the interaction between radionuclides and environmental media(minerals,microorganisms,etc.)have become a hot topic in environmental radiochemistry.This paper is focused on the oxidation state of radionuclides in the"radionuclides-microorganisms"system and"radionuclides-microorganisms-mineral"system,and their interaction mechanisms.In this study,U(?)and Pu(?)were selected as the target nuclides,Shewanella putrefaciens(S.putrefaciens)was selected as a typical microorganisms,and the minerals were kaolinite and hematite(?-Fe₂O₃).Based on the batch experiment and infrared and three-dimensional fluorescence spectroscopy,the goal of this study is to analyze the oxidation state of U and Pu,the distribution of U(?)and Pu(?)in microorganisms,EPS and minerals,and to explore the interation mechanisms among U(?)and Pu(?),microorganism,EPS and minerals.The research results can further understand the migration of U(?)and Pu(?)in groundwater.The main results and conclusions are showing below:(1)For the mixtures of S.putrefaciens and U(?)system,non-reductive biomineralization was the main mechanism of microbial immobilization of U(?),and its maximum contribution to U(?)removal by S.putrefaciens was 57.4±8.4%.As well,the contributions of EPS,bioreductive and intracellular accumulation could not be ignored.Biosorption and biomineralization occured in the process of U(?)immobilization by EPS.For the mixtures of S.putrefaciens and Pu(?)system,the existence of EPS significantly affected the immobilization of Pu(?)by S.putrefaciens.Under the experimental conditions,it was found that the EPS of S.putrefaciens accounted for 24.7%of the total immobilization Pu(?).The presence or absence of EPS had a great impact on microbial adsorption and reduction of Pu(?).(2)For the mixtures of S.putrefaciens,minerals and U(?)system,the immobilization of U(?)by S.putrefaciens bacteria was much higher than that of?-Fe₂O₃ and kaolinite.For minerals,the functional groups in?-Fe₂O₃ and kaolinite have no obvious chemical adsorption or complexation with U(?).S.putrefaciens bacteria system had an extremely adsorption and reduction ability to U(?).After adding?-Fe₂O₃ and kaolinite into the S.putrefaciens bacteria system,S.putrefaciens immobilization ability to U was significantly reduced.The reason was that the combination of mineral particles and bacteria lead to the masking of bacteria,thereby reducing S.putrefaciens adsorption and reduction of U(?).The desorption behavior of U(?)from in U(?)-bearing minerals by microorganisms and their EPS also further proved this conclusion.EPS could mobilize U from minerals through competitive adsorption with U(?)on minerals surfaces as well as forming stable soluble U(?)-EPS complexes.As high as 64%of U(?)on kaolinite could be desorbed by EPS,while 90%desorption could be reached by EPS on?-Fe₂O₃.Therefore,the interaction of microorganisms,EPS and minerals could significantly affect the migration behavior of U(?)in the environment.(3)For the mixtures of S.putrefaciens,?-Fe₂O₃ and Pu(?)system,the composite system of S.putrefaciens-?-Fe₂O₃ had higher adsorption and reduction effects on Pu(?)than that of S.putrefaciens.The experimental results showed that microorganisms and?-Fe₂O₃ had strong adsorption and reduction ability to Pu(?),which could affect the migration behavior of Pu(?).It was found that the reaction behavior of different kinds of nuclide ions on microorganism-mineral was completely different,through compared with the the mixtures of S.putrefaciens,mineral and U(?)system.The adsorption of Pu(?)in microorganism-mineral system was greater than that in microorganism system.The adsorption of U(?)in microorganism-mineral system was less than that in microorganism system.(4)In the reaction of EPS with U(?)and Pu(?),the results of three-dimensional fluorescence spectra showed that U(?)mainly binded to humic acid,while Pu(?)mainly binded to protein.All fluorescence intensity quenching belonged to static quenching.The results of UV-Vis absorption spectrum showed that there were differences in the binding of EPS with U(?)and Pu(?),indicating that there was complexation.When EPS combined with Pu(?),new peaks appeared at 300 nm,indicating the formation of new complexes.The fourier transform infrared spectroscopy showed that U(?)may bind to carboxyl functional groups in EPS,and Pu(?)may bind to-CONH-and carboxyl functional groups in EPS.In addition,both soluble and particulate U was detected in EPS analyzed by single particle ICP-MS,confirmed that biosorption and biomineralization occur in the process of U(?)immobilization by EPS.EPS could react with U(?)and Pu(?)through different mechanisms and components,and EPS was the key component of dissolved organic matter in groundwater(up to 50%).Therefore,EPS in underground environment was expected to be an important factor affecting the migration of U(?)and Pu(?).