Study On Adsorption And Separation Behavior Of Lanthanides And Minor Actinides With R-BTP/SiO₂-P Adsorbents

In advanced nuclear-fuel reprocessing systems,the separation of minor actinides?MA=Am,Cm,etc.?is an important task.Minor actinides,which are?-emitters with long half-life,dominate the long-term radiotoxicity of high-level liquid waste?HLLW?.If they can be partitioned from HLLW and transmuted to short-lived or stable nuclides in an advanced nuclear reactor or transmutation system,the potential radiological risk induced by the final waste disposal will be significantly reduced.However,the separation of minor actinides from lanthanides has always been one of the most challenge works owing to their chemical similarity.In the present work,two kinds of novel silica-based adsorbents were prepared by impregnating organic chelating extractants isoBu-BTP?2,6-bis?5,6-dibutyl-1,2,4-triazin-3-yl?pyridine?and isoHex-BTP?2,6-bis?5,6-bis?4-methylpentyl?-1,2,4-triazin-3-yl?pyridine?into an inert silica-based macro porous support.IsoBu-BTP and isoHex-BTP are chosen as the extractant not only for their promising separation ability to Am?III?and RE?III?,but also for the composition of containing C,H,O and N atoms only.The adsorption of RE?III?onto the adsorbents were studied by batch adsorption experiments.The effect of HNO₃ concentration,NO₃^-concentration,contact time and temperature were investigated in detail,respectively.The results showed that the adsorption of RE?III?onto R-BTP/SiO₂-P was significantly increased with the increase of HNO₃ concentration.The adsorption ability of trivalent RE onto isoBu-BTP and isoHex-BTP adsorbent in nitric solution was:heavy RE>middle RE>light RE.The adsorption and separation phenomena of RE?III?were elucidated by EXAFS?Extended X-ray Absorption Fine Structure?spectroscopy.The results suggested that with the increase of atomic number of RE?III?the distance of R-r decreases gradually,which indicates the bond length between atoms decreased and the interatomic binding force increased.The pseudo-second-order model and Langmuir equation were more suitable to describe the adsorption process of Dy?III?onto isoBu-BTP and isoHex-BTP adsorbent than the pseudo-first-order model and Freundlich equation in this experiment,respectively.The adsorptions of ²⁴¹Am?III??¹⁵²Eu?III??²³⁹Pu?IV??⁹⁹Tc?VII??U?VI?onto R-BTP/SiO₂-P were studied by batch adsorption experiments.The effect of HNO₃concentration and NO₃^-concentration were investigated,respectively.The results showed that the adsorption of ²⁴¹Am?III??¹⁵²Eu?III??²³⁹Pu?IV?onto R-BTP/SiO₂-P increased with the increase of HNO₃ concentration,while the adsorption of ⁹⁹Tc?VII?decreased.U?VI?was weakly adsorbed with the change of HNO₃ concentration in the experiment,which indicates that the separation of minor actinides would not be affected by the residual U?VI?in HLLW.The adsorption of ²⁴¹Am?III??¹⁵²Eu?III??²³⁹Pu?IV?onto R-BTP/SiO₂-P increased with the increase of nitrate concentration under low acid condition and different nuclides can be separated individually according to the different distribution coefficients.¹⁵²Eu?III??²³⁹Pu?IV?were barely adsorbed on isoHex-BTP/SiO₂-P and the distribution coefficients of ²⁴¹Am?III?is less than ⁹⁹Tc?VII?and U?VI?.The chemical and irradiation stability of the adsorbents were evaluated by ⁶⁰Co?-irradiation source.The effect of contact time,HNO₃ concentration and absorbed dose were investigated,respectively.It was found that in the absence of irradiation isoBu-BTP and isoHex-BTP adsorbents showed good chemical stability in 0.1M HNO₃.The increase of HNO₃ concentration enhanced hydrolysis of the extractants which caused a negative effect on the adsorption capacity and ion selectivity of the adsorbent.In irradiation experiments isoBu-BTP/SiO₂-P degraded rapidly in 1M HNO₃ with the increase of absorbed dose.After the absorbed dose reached 69kGy the adsorbents finally lost its adsorption capacity.However,the separation factor of²⁴¹Am/¹⁵²Eu was still kept at 99.9 in 0.01M HNO₃ with 0.99M NaNO₃ at the absorbed dose reached 207kGy,which reveals the isoBu-BTP/SiO₂-P adsorbent could keep the original selective adsorption ability after a long time when the concentration of nitric acid was low.IsoHex-BTP/SiO₂-P adsorbent also showed good resistance to irradiation in 0.01M HNO₃.But the adsorbent completely lost its adsorption capacity of Dy?III?at the absorbed dose of 46kGy in 2M and above HNO₃,which indicates that isoHex-BTP/SiO₂-P adsorbent could not be used for a long time in high concentration nitric acid solution with gamma irradiation.The separation of ²⁴¹Am?III?from simulated HLLW was studied by extraction chromatography.The column separation experiment was carried out at room temperature with the solution flow velocity of 0.2mL/min.²⁴¹Am?III?was successfully separated from simulated HLLW by isoBu-BTP/SiO₂-P adsorbent and the recovery rate reached 100%which offers helpful reference for the advanced partitioning process of HLLW by two column separation.