Removal Of Radionuclides By Nanoadsorbents And The Mechanism Investigation

As the exploitation of nonrenewable resources,such as coal,oil and gas,energy crisis has became an urgent problem for human beings,so it is more and more important to exploit new energy.Nuclear power source has been used by countries all over the world,because of its efficient energy conversion and lower envirinmental pollutant emissions.With the quick development of nuclear science and energy,more and more nuclear power plants have been built and large amounts of nuclear wastes have been generated.In the nuclear fuel cycle options（i.e.,from the extraction of uranium for the fabrication of nuclear fuel,the application of nuclear fuel in nuclear power plants,and at last in the spent fuel process to the geological disposal,etc.）,it is inevitable to produce large volumes of wastewater containing different kinds of long-lived radionuclides,especially some important fission products,lanthanides and actinides,which should be eliminated from the radioactive wastewater before they are discharged into the natural environment.Sorption technique has widely been applied in large scale and in real environmental pollution management because of its simple operation,low cost and applications in large scale.In the last decades,nanoadsorbents have been investigated widely and exhibited great potential in the field of environmental remediation due to their excellent chemical and physical properties,such as large surface area,chemical stability,and effective adsorption.In this paper,the adsorption studies of radionuclides on carbon nanotubes（CNTs）,graphene oxides（GOs）and aluminium oxide（Al₂O₃）were investigated by batch experiments,spectrum analysis and theoretical calculations.The main research findings are listed as follows:（1）The adsorption of Eu（Ⅲ）and 243Am（Ⅲ）on CNTs were studied,and the results showed that Eu（Ⅲ）and 243Am（Ⅲ）could form strong inner-sphere surface complexes on CNTs surfaces.However,the sorption of Eu（Ⅲ）on CNTs was stronger than that of 243Am（Ⅲ）on CNTs,suggesting the difference in the interaction mechanisms or properties of Eu（Ⅲ）and 243Am（Ⅲ）with CNTs,which was quite different from the results of Eu（Ⅲ）and 243 Am（Ⅲ）interaction on natural clay minerals and oxides.On the basis of the results of density functional theory calculations,the binding energies of Eu（Ⅲ）on CNTs were much higher than those of 243Am（Ⅲ）on CNTs,indicating that Eu（Ⅲ）could form stronger complexes with the oxygen-containing functional groups of CNTs than 243Am（Ⅲ）,which was in good agreement with the experimental results of higher sorption capacity of CNTs for Eu（Ⅲ）.The oxygen-containing functional grops contribute significantly to the uptake of Eu（Ⅲ）and 243Am（Ⅲ）,and the binding affinity increased in the order of（?）S-OH<（?）S-COOH<（?）S-COO.This paper highlights the interaction mechanism of Eu（III）and 243Am（Ⅲ）with different oxygen-containing functional groups of CNTs,which plays an important role for the potential application of CNTs in the preconcentration,removal,and separation of trivalent lanthanides and actinides in environmental pollution cleanup.（2）The GOs were applied as adsorbents to remove U（Ⅵ）from aqueous solutions at different experimental conditions,and the results showed that the adsorption of U（VI）on GOs was strongly dependent on pH and was not affected by ionic strength obviously.The sorption capacity of U（Ⅵ）on GOs was calculated to be 1330 mg/g at pH 4.5 and T = 20 ℃,which was much higher than any other today’s materials.The interaction mechanism of U（Ⅵ）on GOs was simulated by the computational DFT calculations,and the results evidenced that the variation in pH values led to significant changes in the circumstance of functional groups on GOs.As the pH value increased,the weak physical sorption/outer-sphere surface complexation between the GOs and U（Ⅵ）at low pH turned into strong chemical sorption/inner-sphere surface complexation with the negative charged-O or-COO groups induced by the deprotonation of the functional groups at high pH.（3）To contribute to the comprehension of Eu（Ⅲ）adsorption properties in oxide-organic material systems,the influence of humic acid（HA）on Eu（Ⅲ）adsorption on bare and HA-bounded γ-A12O3 was studied using batch experiments and time resolved laser fluorescence spectroscopy（TRLFS）.The effect of pH and addition sequences of Eu（Ⅲ）/HA/A1203 on Eu（Ⅲ）adsorption was carried out at different experimental conditions,and the results showed that the adsorption of Eu（Ⅲ）was strongly dependent on pH and ionic strength.The adsorption of Eu（Ⅲ）was mainly dominated by ion exchange or outer-sphere surface complexation at low pH,and by inner-sphere surface complexation at high pH values.The presence of HA enhanced Eu（Ⅲ）adsorption at low pH,but reduced Eu（Ⅲ）adsorption at high pH values.The addition sequences of Eu（Ⅲ）/HA to Al₂O₃ suspensions did not affect Eu（Ⅲ）adsorption,which was also evidenced by the TRLFS measurements.The surface complexes were characterized by their emission spectra[the ratio of emission intensities of 5D0 → 7F1（λ = 594 nm）and 5D0 → 7F2（λ = 619 nm）transitions]and their fluorescence lifetime.The TRLFS results showed clearly that no effect of addition sequences on the adsorption and speciation of Eu（Ⅲ）in HA-Al₂O₃ hybrids after equilibration These findings on the effect of humic substances in the complexation properties of Eu（Ⅲ）indicated that the clarification of the environmental behavior of humic substances was necessary to understand the physicochemical properties of Eu（Ⅲ）,or its analog trivalent lanthanide and actinide ions in the natural environment.（4）The association of 243Am（Ⅲ）with humic acid-bound γ-Al₂O₃（HA-Al₂O₃）was investigated using batch technique and the kinetic dissociation properties were studied using chelating resin.The kinetics of desorption of 243Am（Ⅲ）forom HA-Al₂O₃ hybrids were studied at three different pH values（pH 4.0,5.0 and 6.0）after aging time for 2 h,4 days,and 1 month by adding chelating resin to remove the desorbed free 243Am（Ⅲ）ions from the HA-Al₂O₃ hybrids.The fraction of 243Am（Ⅲ）on the irreversible binding sites of the HA-Al₂O₃ hybrids increased as the pH increased.Two different 243Am（Ⅲ）-HA-Al₂O₃ species（i.e.,"fast" and "slow" dissociation kinetics,which represented the "weak" and "strong" species on reversible sites）were required to fit the kinetic dissociation results.Although 243Am（Ⅲ）was initially a "fast"dissociating species,it became a "slow" dissociating species as the aging time increased and at low HA concentrations.When the HA concentration was high,the"fast" and "slow" dissociating species of 243Am（Ⅲ）bound to HA-Al₂O₃ particles were not influenced by the aging time.These results describe the physicochemical behavior of trivalent lanthanides and actinides in the natural environment,which is important to understand the potential pollution of long-lived radionuclides.In conclusion,nanoadsorbents have high efficiency to remove radionuclides.The ambient environment conditions（e.g.pH,contact time,temperature and HA）have great effect on the sorption of radionuclides.The thesis investigated the microscopic interaction mechanism of radionuclides on nanoadsorbents under different hydrochemical conditions from a molecular perspective.These findings might likely lead to a better understanding of the migration and transformation of radionuclides in the natural environment.