Study On Adsorption And Separation Properties Of Lanthanides And Actinides With HDEHP/SiO₂-P Adsorbent And Its Adsorption Mechanism

To develop the nuclear energy in a sustainable and safe way,it is necessary to use maximally of uranium resource and minimize nuclear waste emission.In the advanced nuclear fuel cycle system,the PUREX process is utilized to recover and recycle the uranium and plutonium from the spent fuel.The minor actinides?MA?in the high level liquid waste?HLLW?are the?-activity and long half-life nuclides.It is a basic solution to reduce the emission of HLLW and decrease the long-term potential hazards when transmutation the MA to short life or stable nuclides by the fast neutrons.The HLLW has many lanthanides?Ln?which contents are twenty times of MA.However,lanthanides are the neutron poisons with higher neutron capture cross sections than MA,which lead to reduce the neutron absorption efficiency for actinides during transmutation process.Therefore,it is necessary to separate minor actinides from lanthanides before the nuclides transmutation process.The lanthanides and minor actinides have approximate ionic radius and the same valence due to lanthanides contraction,it makes Ln and MA have the similar chemical properties and their separation procedure is still a significantly technological challenge.Concerning the separation of Ln and MA,other researchers had carried out many studies both on theory and experiment and acquired many achievements.Wei et al.reported a separation technique called MAREC?Minor Actinides Recovery from HLLW by Extraction Chromatography?process by extraction chromatography techniques,which had a satisfying group partition behavior of nuclides and fission products in HLLW.But it was found that it is still a difficult challenge to separate MA and middle-heavy rare earths?mRE?elements only by the MAREC process.This thesis mainly utilized the common and low-cost extractant?HDEHP?and immobilized it into macroporous silica resin?SiO₂-P?to form the solid phase extractant,HDEHP/SiO₂-P.We discussed the adsorption and separation behaviors of Ln and MA by HDEHP/SiO₂-P and put emphasis on MA and mRE?Sm?III?,Eu?III?,Gd?III?,and Tb?III??.The final aim of this study was to optimize and improve the MAREC process to achieve a reasonable process for separation of MA and middle-heavy rare earth elements.This research found that HDEHP/SiO₂-P had many advantages of solid phase extractants,such as fast kinetic,easily phase separating,and simple operation.HDEHP/SiO₂-P adsorbent had well adsorption properties for both lanthanides and minor actinides,especially,Ce?III?had a similar adsorption behavior to ²⁴¹Am?III?by HDEHP/SiO₂-P adsorbent at the acidity range from 0.1 M to 0.3 M.HDEHP/SiO₂-P adsorbent showed reasonably selective properties for mRE and ²⁴¹Am?III?which separation factors were all approximately equals to 100.The static adsorption research demonstrated that Gd?III?and Ce?III?adsorption kinetics onto HDEHP/SiO₂-P adsorbent were very fast that reached the equilibrium within 2 h and the pseudo-second-order model well explained the experimental results.Results indicated that Freundlich model matched well with the experimental data of both Ce?III?and Gd?III?adsorption towards HDEHP/SiO₂-P and the adsorption process of Gd?III?by HDEHP/SiO₂-P was a spontaneously exothermic reaction.Ce?III?,as a simulated element of ²⁴¹Am?III?,can be desorbed from loaded HDEHP/SiO₂-P under moderate and simple conditions.The results of column experiment demonstrated that Ce?III?could be separated with mRE with well performance which improved the MAREC process.This research evaluated systematically the chemical stability of HDEHP/SiO₂-P adsorbent and the obtained results indicated that the adsorbent had satisfactory resistance properties against nitric acid and high temperature.The results illustrated that HDEHP/SiO₂-P adsorbent also had an excellent stability against?-irradiation.The irradiated adsorbent still maintained the excellent adsorption properties and the TOC leakage from HDEHP/SiO₂-P could be negligible after irradiated in air condition,0.1 M,and 3 M HNO₃ solutions in the absorbed dose of500 kGy by?-ray.Therefore,HDEHP/SiO₂-P was thought to be a potential application in the MAREC process to separate mRE and MA.In the TALSPEAK process,the synergic extraction was utilized to combine HDEHP with other extractants to improve separation factor and extraction efficiency.Inspired by TALSPEAK process,this research also immobilized HDEHP and CMPO into macroporous silica-based resin to form different proportions of synergic solid phase extractants,CMPO-HDEHP/SiO₂-P and discussed their adsorption behavior of lanthanides by static experiment.It was found that the synergic extractants had a good adsorption capacity to lanthanides under both low and high concentrations of HNO₃.HDEHP played the leading role of adsorption in low concentrations of HNO₃,while CMPO played the dominant role of adsorption in high concentrations,and these made the synergic extractants extend their scope of application.However,CMPO in the synergic extractants showed no evident effect on selective properties of Ln and MA and it made HDEHP proportions decrease in the synergic extractants.Therefore,the separation ability towards Ln and minor actinides used by synergic CMPO-HDEHP/SiO₂-P was inferior to use HDEHP/SiO₂-P adsorbent solely.This thesis put emphasis on adsorption mechanism of HDEHP and lanthanides and analyzed the coordination complex of HDEHP and Ce?III?,Sm?III?,Eu?III?,Gd?III?,and Tb?III?by UPLC-Q-TOF-MS technology.The results indicated that HDEHP could form different coordination structures with different lanthanides ions.Ce?III?and HDEHP formed mainly 1:8 mononuclear complex,Sm?III?and HDEHP can form 1:8 mononuclear complex as well as 2:8 dinuclear complex,Eu?III?and HDEHP could form both 1:6 mononuclear complex and 2:8 dinuclear complex.The reason why HDEHP has the selective and separation ability is that it could be formed different complex with different lanthanides.