The Study On The Synthesis And Ion Exchange Properties Anionic Rare Earth Metal Organic Frameworks

With the increasing depletion of fossil energy sources,clean and efficient nuclear energy has received widespread attention.The development of nuclear energy,however,inevitably produces radioactive waste.¹³⁷Cs and ⁹⁰Sr are radioisotopes produced by the fission of ²³⁵U or ²³⁹Pu,both of which have long half-life(t_1/2=30.2 for ¹³⁷Cs,and 28.8years for ⁹⁰Sr,respectively).¹³⁷Cs produces strongγ-ray radiation whereas ⁹⁰Sr produces strongβ-ray radiation.In the water environment they often exist in ionic form of ¹³⁷Cs⁺and⁹⁰Sr²⁺,respectively,having strong water solubility and environmental mobility.They both have strong biotoxicity and may cause cancer.Thus,they are extremely harmful to human health.On the other hand,¹³⁷Cs and ⁹⁰Sr have a wide range of applications in medical,industrial,and agricultural fields.Therefore,the development of efficient ¹³⁷Cs and ⁹⁰Sr adsorbent materials is very important for the treatment of radioactive contamination and the recovery of radionuclides.However,the selective removal of ¹³⁷Cs⁺and ⁹⁰Sr²⁺from complex aqueous environments remains a great challenge because the adsorption of ¹³⁷Cs⁺and ⁹⁰Sr²⁺is highly susceptible to the influence of interfering ions.The research goal of this thesis is the effective removal of Cs⁺and Sr²⁺ions.We aimed to design and synthesize water-stable rare earth metal-organic frameworks（MOF）as ion-exchange materials and systematically investigate their ion-exchange performance for Cs⁺and Sr²⁺ions.Two series of anionic rare earth metal-organic framework materials have been synthesized and screened,which exhibit good removal performance for Cs⁺and Sr²⁺ions,respectively.1.Two new three-dimensional microporous rare earth MOF compounds（Me NH₂）_0.5（H₃O）_0.25Na_0.25Ln（OH）（stp）·0.25H₂O（FJSM-Ln MOF;Ln=Eu,Tb,H₃stp=2-sulfophenyl terephthalate）have been prepared by the solvothermal methods,which possess good water stability and show fast kinetics and high adsorption capacity for Cs⁺ions(q_m^Cs of 229.25 mg/g and 211.28 mg/g,respectively).As the representative,the Cs⁺adsorption performance of FJSM-Eu MOF was studied systematically.FJSM-Eu MOF shows a good selectivity for Cs⁺ions(K_d^Cs values up to 2.18×10~3 m L/g)and shows selective adsorption properties for Cs⁺ions even in the presence of excess Na⁺,K⁺,Mg²⁺,and Ca²⁺ions.The single crystal structure of the Cs⁺adsorption products was investigated.The Cs⁺adsorption mechanism of FJSM-Eu MOF is confirmed to be ion exchange through single crystal structure analysis combined with various characterizations such as XPS,IR,EDS,and EA.The results show that the efficient adsorption of Cs⁺ions by FJSM-Eu MOF mainly originates from the strong interactions between the COO^-and R-SO₃^-functional groups on organic ligands in the lanthanide metal-organic anionic framework and Cs⁺ions,and the presence of easily exchangeable[Me₂NH₂]⁺and[H₃O]⁺cations in the channels.2.We screened and synthesized a series of rare earth oxalates with three-dimensional（3D）microporous anionic frameworks,namely[Me₂NH₂][Ln（C₂O₄）₂（H₂O）]·3H₂O（Ln-ox;Ln=Y,Nd,Sm,Eu,Gd,Er,Tm,Yb）.Ln-ox can be easily synthesized on a large scale and has high water stability and radiation resistance.As a representative,Eu-ox achieves fast and highly selective uptake of Sr²⁺ions with fast kinetics（3 min）and K_d^Sr values up to2.61×10~5 m L/g.At high concentrations of interfering metal ions(M=Na⁺,Cs⁺,Mg²⁺and Ca²⁺),Eu-ox exhibits high selectivity for Sr²⁺with a high separation coefficient SF_Sr/M.The Sr²⁺removal mechanism of Eu-ox is revealed,which is attributed to the ion exchange between Sr²⁺and[Me₂NH₂]⁺cations in the channels.Furthermore,the comparison of a series of rare-earth oxalate frameworks reveals that the occurrence of ion exchange in three-dimensional microporous rare-earth oxalate materials is related to the channel size and the interaction of exchangeable cations in the anionic frameworks.The strong complexation ability of oxalate groups for Sr²⁺,the large channel size of Eu-ox,and exchangeable[Me₂NH₂]⁺cations are the key factors for Sr²⁺ion exchange.The innovation of this thesis is the successful synthesis of microporous rare-earth metal-organic frameworks with efficient ion-exchange properties for Cs⁺and Sr²⁺.The materials can achieve rapid,high adsorption capacity and selective capture of Cs⁺and Sr²⁺ions,while the adsorption mechanism is revealed systematically.Moreover,the two series of three-dimensional microporous rare-earth metal-organic frameworks have excellent water stability and can selectively remove Cs⁺or Sr²⁺in complex aqueous environments.This is the first time that rare earth oxalate frameworks have been used as ion exchange materials for Sr²⁺removal,confirming the potential of rare earth oxalates for radionuclide remediation.This thesis provides a reference for the use of new MOFs materials for the removal of radioactive ions and provides a new idea for the construction of water-stable anionic MOFs.