Extraction separation of Cd(II) and Zn(II) with Cyanex301 and aqueous nitrogen-donor ligand TPEN

The extraction separation of Cd(II) and Zn(II) with an organodithio-phosphorus extractant, bis(2,4,4-trimethylpentyl)dithiophosphinic acid (Cyanex301), and a hexadentate ligand containing six nitrogen donors, N,N,N',N'-tetrakis(2-pyridylmethyl)ethylenediamine (TPEN), was studied. When TPEN in amounts equimolar with divalent metal was added in the aqueous phase, most of TPEN molecules (>99%) were complexed with these divalent metals in the pH range above 2. Then, the extraction of Cd with Cyanex301 was little affected by the addition of TPEN, however, that of Zn was depressed remarkably and the extraction constant of Zn was reduced to less than 10(-4) times that without TPEN. TPEN acts as a strong masking agent only for Zinc. The binary extraction tests of Cd and Zn were carried out under the conditions that the initial concentration of Cyanex301 was 0.0025 mol/L and TPEN in amounts equimolar with divalent metals was added in the aqueous phase. The separation factor of Cd was evaluated as 1000 at pH 4. TPEN is an excellent ligand that promotes the separation of Cd by adding slightly to the aqueous phase.