Highly stable macro-porous N-donor CyMe4-BTPhen/SiO2-P adsorbent for efficient selective extraction of actinides from HLLW: An experimental, mechanism, theoretical study

2,9-bis(5,5,8,8-tetramethyl-5,6,7,8-tetrahydro-1,2,4-benzotriazin-3-yl)-1,10-phenanthroline (CyMe4-BTPhen) is one of the most prospective ligands for selective separation of trivalent actinides from high level liquid waste (HLLW). To address its low solubility and irradiation resistance in liquid-liquid extraction systems, herein it was loaded into a porous silica-polymer carrier SiO2-P by vacuum impregnation technique to obtain a high loading rate macro-porous adsorbent CyMe4-BTPhen/SiO2-P with CyMe4-BTPhen as high as 30.48 wt%. BET, SEM, XRD, FT-IR and TOC analysis proved the stable macro-porous CyMe4-BTPhen/SiO2-P was successfully prepared. It exhibited high adsorption efficiency (>98 %) towards trace amount of radioactive 241Am(III) in a broad acidity range of 0.01–4 M nitric acid solutions in less than 5 min, which was even up to 99.5 % in 1–2 M HNO3 solution. Highly efficient separation between Am(III) and Eu(III) was achieved with the separation factor SFAm/Eu up to 123 in 4 M HNO3 solution. The kinetic and adsorption isotherm of Eu(III) are in good agreement with pseudo-second-order kinetic and Langmuir models, respectively. FT-IR and XPS analysis reveal that the complexation mechanism involves the immediate coordination of Eu(III) to N in phenanthroline group and triazine ring. Coordination behavior was further studied by UV titration that the ligand and Nd formed a 2:1 complex in acetonitrile solution. MBO analysis indicated that the Am-N bond in the complex has a shorter bond length and more covalent properties than the Eu-N bond. Meanwhile, thermodynamic results indicate that the ligand binding to Am(III) has a lower Gibbs free energy than that of Eu(III), which all reveal the reasons for the preferential bonding of the ligand to Am(III). In a word, CyMe4-BTPhen/SiO2-P proves to be a very promising adsorbent for separation minor actinides from HLLW.