Synthesis Of Novel BTP/SiO₂-P Adsorbents And The Study On Separation Minor Actinides From HLLW

The minor actinides?MA=Am,Cm,etc.?in high level liquid waste?HLLW?,which are?-emitters with long half-lives,are one of the main contributors of long-term radiotoxicity of HLLW.Partitioning MA from HLLW and then transmuting them into short-lived or stable nuclides in an advanced nuclear reactor or transmutation system is the key step for reducing both the HLLW amount and the potential long-term radiological risk.However,the content of lanthanides?Ln?is an order of magnitude higher than that of MA?III?,while some Ln have large neutron absorption cross sections and are neutron poisons so Ln?III?should be separated with MA?III?before the transmutation step.But considering the same oxidation valence and the similar ionic radius of MA?III?and Ln?III?,the mutual separation is very difficult,which is one of the key challenging problems in HLLW separation.According to the hard and soft acid-base theory,MA?III?and Ln?III?are both belong to hard acid,but MA?III?are a little softer than Ln?III?and have stonger coordination complexation with ligands containing soft coordination atoms N or S.Using ligands containing soft coordination atoms N or S is expected to achieve the mutual separation of MA?III?and Ln?III?from HLLW.In this work,we have improved the structure of R-BTP?2,6-bis?5,6-dialkyl-1,2,4-triazin-3-yl?pyridine,R:alkyl group?,which is one of the most promising N-containing soft ligand to achieve the separation of MA from HLLW,and designed and successfully synthesized a new type of ligands,i.e.,Me₂-CA-BTP?2,6-bis?5,6,7,8-tetrahydro-5,8,9,9-tetramethyl-5,8-methano-1,2,4-benzotriazin-3-yl?pyridine?.Then Me₂-CA-BTP/SiO₂-P adsorbent was prepared by impregnating Me₂-CA-BTP into a stable macroporous silica/polymer composite support particles?SiO₂-P?.Meanwhile,CA-BTP/SiO₂-P adsorbent was prepared for comparison.In this paper,we evaluated the adsorbent basic properties of separating MA?III?from HLLW on the aspects of selectivity,adsorption kinetics,adsorption isotherm,thermodynamics,chemical stability,?-irradiation stability by batch adsorption experiments.Finally,we studied the possibility of using the novel Me₂-CA-BTP/SiO₂-P adsorbent to separate MA?III?from HLLW by MAREC process?Minor Actinides Recovery by Extraction Chromatography Process?.The research indicated that in 0.5-1 M HNO₃ solution,CA-BTP/SiO₂-P showed some adsorption ability towards ²⁴¹Am?III?with the uptake rate of 50%,while it exhibited almost no adsorption towards ¹⁵²Eu?III?.Overall,the adsorption capacity is weak in pure nitric acid solution system.While in 0.01 M HNO₃-NaNO₃ solution system,when NO₃^-concentration was 2 M,CA-BTP/SiO₂-P showed good adsorption towards ²⁴¹Am?III?with the uptake rate of 97%,and the separation factor SF_Am/Eu was70 indicating good separation ability.In 0.01 M HNO₃-1.99 M NaNO₃ solution,the adsorption of CA-BTP/SiO₂-P towards ²⁴¹Am?III?reached equilibrium within half an hour with the uptake rate of 95%showing rapid adsorption kinetics.And in 0.01 M HNO₃-2.99 M NaNO₃ solution,for the adsorption of CA-BTP/SiO₂-P towards Dy?III?and Eu?III?,the adsorption kinetics both matched the quasi-second order dynamic equation,and adsorption isotherms were both in line with Langmuir isotherm model with the adsorption capacity of about 0.3 mmol/g,and the adsorption were spontaneous endothermic reaction.The ²⁴¹Am?III?adsorbed on CA-BTP/SiO₂-P can be eluted by H₂O,0.001 M HNO₃,0.01 M DTPA.Furthermore,CA-BTP/SiO₂-P adsorbent has a certain acid stability,showed good resistance to?irradiation in dry state.When CA-BTP/SiO₂-P was acceptted?irradiation in 0.5 M nitric acid solution,nitric acid exhibited a bad impact on CA-BTP/SiO₂-P stability.Overall,the stability of CA-BTP/SiO₂-P needs be further improved.Accordingly,in 0.01 M HNO₃-NaNO₃ solution system when NO₃^-concentration was in the range of 0.1-4 M,and in pure nitric acid system when HNO₃ concentration was in the range of 0.1-4 M,Me₂-CA-BTP/SiO₂-P showed both good adsorption towards ²⁴¹Am?III?with SF_Am/Eu more than two digits,indicating Me₂-CA-BTP/SiO₂-P had good adsorption and separation effect towards ²⁴¹Am?III?in above two solution systems.In both 0.01 M HNO₃-0.99 M NaNO₃ solution and 1 M HNO₃ solution,the adsorption of Me₂-CA-BTP/SiO₂-P towards ²⁴¹Am?III?reached equilibrium within an hour with the uptake rate of 96%showing rapid adsorption kinetics,and for the adsorption towards Dy?III?and Eu?III?,the adsorption kinetics both matched the q quasi-second order dynamic equation,and adsorption isotherms were both in line with Langmuir isotherm model with the adsorption capacity of about 0.2 mmol/g,and the adsorption were spontaneous endothermic reaction.The ²⁴¹Am?III?adsorbed on Me₂-CA-BTP/SiO₂-P can be eluted by 0.01 M DTPA.In 0.1 M HNO₃,0.01 M HNO₃-0.49M NaNO₃,0.01 M HNO₃-0.99 M NaNO₃,1 M HNO₃,3 M HNO₃ solution,with the increase of?irradiation absorbed dose,the adsorption performance of Me₂-CA-BTP/SiO₂-P towards ²⁴¹Am?III?did not change significantly.When?irradiation absorbed dose reached to 207 kGy,Me₂-CA-BTP/SiO₂-P maintained good adsorption selectivity towards ²⁴¹Am?III?,which indicating Me₂-CA-BTP/SiO₂-P has good resistance to?irradiation.Further more,the continuous column separation experimental results showed that Me₂-CA-BTP/SiO₂-P can effectively separate ²⁴¹Am?III?from 0.01 M HNO₃-0.34 M NaNO₃ and 0.1 M HNO₃ simulation HLLW with the ²⁴¹Am?III?recovery rate of 99.5%and 98.6%respectively in ²⁴¹Am?III?elution solution.Meanwhlie,the content of fission products Ln?III?was less than 1%,which basically meet the requirement of salt-free for the MAREC two-cycle process.Moreover,Me₂-CA-BTP/SiO₂-P can directly separate²⁴¹Am?III?from 3 M HNO₃ simulated HLLW with ²⁴¹Am?III?recovery rate of 95.5%in ²⁴¹Am?III?elution step,meanwhlie,the content of typical FP was less than 3%.Me₂-CA-BTP/SiO₂-P has great application potential in the MAREC single cycle process,and is expected to establish an advanced simplified MA?III?separation process.In a word,from the aspects of the adsorption ability towards ²⁴¹Am?III?,the applicable concentration range of nitric acid,the amount of NaNO₃ needing to be added,the resistance to acid and?iraradiation stability,Me₂-CA-BTP/SiO₂-P adsorbent is more suitable for the separation of MA?III?from HLLW compared with CA-BTP/SiO₂-P and is a very promising adsorbent in the application of the separation of MA from HLLW.