Study On The Stability And Complexation Mechanism Of Me₂-CA-BTP Ligand In The Separation Of Minor Actinides From HLLW

High level liquid waste?HLLW?is generated from the post-PUREX process after a large amount of U and Pu recovered.A trace amount of minor actinides in HLLW is one of the main contributors of long-term radiotoxicity and has a potential long-term radioactive hazards to environment.Partitioning and transmuting MA?III?from HLLW into short-lived or stable nuclides is the key step for reducing the potential long-term radiological risk.However,large amounts of lanthanides?Ln?with large neutron absorption cross-sections existing in HLLW,which will significantly reduce the transmutation efficiency of MA?III?,must be separated with MA?III?before the transmutation process.Whereas on account of similar chemical properties of MA?III?and Ln?III?,the mutual separation of them is one of the most challenging technical problems which has not been solved for decades.In this work,with the objective of effectively separating MA from HLLW,the stability of Me₂-CA-BTP/SiO₂-P in nitric acid solution has been studied.Besides,the extraction behavior of Me₂-CA-BTP and the selective complexation mechanism with MA?III?and Ln?III?were also studied.The main results are listed as follows.Novel macroporous silica-based soft ligand material,i.e.,Me₂-CA-BTP/SiO₂-P,was synthesized through vacuum impregnation of Me₂-CA-BTP ligand into the pores of the macroporous SiO₂-P support.The microstructure,chemical composition and composite mechanism of the material are characterized by SEM-EDS,BET,FT-IR and TG-DSC.After a 60-days Me₂-CA-BTP/SiO₂-P soaking in 0.1 M HNO₃ solution or 3.2M HNO₃ solution,it shows good resistance to HNO₃ solution according to TOC analysis of solution and UV analysis of solid sample.The radiolytic stability of adsorbent decreases with the HNO₃ acidity increasing.The UV spectra and FT-IR spectra of the samples after irradiated in 0.01 M HNO₃-0.99 M NaNO₃ and0.1 M HNO₃ solution almost remained unchanged,indicating that the structure of the adsorbent is unbroken.However,the UV spectra of the samples after irradiated in 1 M HNO₃ and 3.2 M HNO₃ shows an absorption peak at 319 nm,which is attributed to the protonation of the ligand.The TOC leakage rate is only26.5%even in the conditions of 3.2 M HNO₃ solution and 480 kGy of absorption dose.The above results that Me₂-CA-BTP/SiO₂-P has good resistance for nitric acid and irradiation.The effects of HNO₃ concentration,equilibrium time and extraction thermodynamics on the extraction behavior of Me₂-CA-BTP towards typical fission products have been studied.It is found that the extractant only extracts Pd?II?and hardly extracts other fission products in 4 M HNO₃ solution.The extraction equilibrium of Eu?III?in n-octanol can be obtained in 15 h.As temperature increased,the extraction of Me₂-CA-BTP towards Ru?III?,Zr?IV?,Mo?VI?increases,and the extraction of Y?III?,Sm?III?,Eu?III?,Gd?III?decreases.In addition,the extraction process of Eu?III?by Me₂-CA-BTP in n-octanol is a spontaneous exothermic reaction.The complexation mechanism research of Me₂-CA-BTP towards heavy REEs has been studied by time resolved laser fluorescence spectroscopy?TRLFS?and X-ray crystal structure.It indicates that 1:3 complexes are formed from the solvated metal ion upon addition of ligand,and no water is found in the inner coordination sphere.The coordination postions of Eu?III?are N atoms of the pyridine fragment and the double N atoms closer to pyridine in triazine N-N fragment.The conditional stability constants?log??are 14.3±0.04 in 0.01 M HClO₄ and 13.2±0.03 in 0.01 M HNO₃.By comparing the bond length of the complexes[Me₂-CA-BTP]M?M=Sm?III?,Eu?III?,Dy?III??,the binding strength between ligand and metal was following the order of Dy?III?>Eu?III?>Sm?III?,which is consistent with the results in extraction and adsorption experiment,and elucidate the reason of better selectivity of Me₂-CA-BTP towards MA?III?than Ln?III?to some extent.These results above have important value for the application of Me₂-CA-BTP extractant in MAREC process.