Extraction and complexation of trivalent rare earth elements with tetraalkyl diglycolamides

The extraction and complexation behaviors of four tetraalkyl diglycolamide (TRDGA, where R represents n-butyl, n-hexyl, n-octyl and 2-ethylhexyl) ligands with linear chain or branched chain structure were described with respect to La3+, Pr3+, Nd3+; Eu3+, Gd3+; Y3+, Er3+ and Yb3+ as the representatives of light, middle and heavy rare-earth group ions in HNO3 solution. Effects of acidity, ligand concentration and temperature on the distribution ratio of these ions were examined. High acidity was good for the extraction. For same metal ion, the extractability of DGA ligands followed the order of T2EHDGA < THDGA < TODGA < TBDGA at the same acidity, which was also supported by nuclear magnetic resonance (NMR) analyses. Meanwhile, for different metal ions, their extracted efficiencies by the same DGA ligand increased in the sequence of lanthanide contraction. The average separation factors of adjacent lanthanides within light, middle and heavy rare-earth groups could attain about 2.4, 1.7 and 2.1, respectively. Slope analysis indicated that the formation of predominant 1:3 and 1:2 metal/ligand extracted species for linear chain and branched DGA, respectively. In methanol solution, 1:1, 1:2 and 1:3 Eu3+/DGA species were also observed using fluorescence spectroscopy titration. Fourier transform infrared spectroscopy (FT-IR) analyses suggested that the carbonyl oxygens compared with ether oxygen play a dominant role in coordination. Moreover, the stability constants (log beta) and the thermodynamic parameters (Delta H, Delta S and Delta G) for the complexation of Eu3+ with DGAs were also presented.