Solvent extraction separation of Cd(II) and Zn(II) with the organophosphorus extractant D2EHPA and the aqueous nitrogen-donor ligand TPEN

Solvent extraction separation of divalent metal ions, Cd(II) and Zn(II), with an organophosphorus extractant, D2EHPA (di(2-ethylhexyl)phosphoric acid), and an aqueous hexadentate ligand with nitrogen donors, TPEN (N,N,N′,N′-tetrakis(2-pyridylmethyl)ethylenediamine), was studied. The extraction process consists of two steps, namely, the complex formation between TPEN and the divalent metals in the aqueous phase, M(TPEN)2+ (M=Cd or Zn), and the formation of a hydrophobic complex between M(TPEN)2+ and D2EHPA in the organic phase. The stability constants for the formation of Cd(TPEN)2+ and Zn(TPEN)2+, which were measured by potentiometric titration technique, were evaluated as higher than 1013. By the addition of TPEN in amounts equimolar with the divalent metal in the aqueous solution, more than 99% of divalent metal was coordinated with TPEN at pH above 2.5, where the protonation of TPEN is negligible. Under these conditions, the extraction of Cd(II) with D2EHPA was promoted; on the contrary, that of Zn(II) was depressed remarkably. TPEN acts as a synergist for the extraction of Cd(II) and a masking agent for that of Zn(II). Then, the separation factor of Cd(II) was increased by more than 500 times than that without TPEN. TPEN is an excellent synergist that functions by small additions in the aqueous solution and promotes extraction selectively for soft metals such as Cd(II).