Lanthanide extraction using a thiodiglycolamic acid extractant: effect of S-donor on lanthanide separation

Liquid-liquid extraction of lanthanide (Ln) ions was investigated using N,N-dioctylthiodiglycolamic acid (DOTDGAA), which is a sulfur donor ligand with an amide group and a carboxyl group connected by a thioether chain. The extraction performance and selectivity of DOTDGAA for Ln ions were compared with those of N,N-dioctyldiglycolamic acid (DODGAA), which is also an oxygen donor ligand with a similar chemical structure, to assess the effect of the soft/hard donor atom on Ln separation. DOTDGAA quantitatively extracted all Ln ions while being selective toward the light and middle Ln ions, in contrast to the selectivity of DODGAA for heavier Ln ions. Slope analysis demonstrated that the Ln³⁺ transfer using DOTDGAA proceeded through a proton-exchange reaction, forming a 1:3 complex, Ln(DOTDGAA)₃. The back-extraction of Ln ions from the extracting phase was successfully achieved under acidic conditions.