Extraction of soft metals from acidic media with nitrogen-donor ligand TPEN and its analogs

Soft metals were extracted from acidic solution (pH 1 to 4) using two hydrophobic analogs of N,N,N',N'-tetrakis[2-pyridyl-methyl]-1,2-ethylenediamine (TPEN). N,N,N',N'-tetrakis[4-(2-butyloxy)-2-pyridyl-methyl]-1,2-ethylenediamine (TBPEN) and N,N,N',N'-tetrakis(2-quinolinylmethyl)-1,2-ethylenediamine (TQEN) have shown enhanced extraction performance in more acidic media than TPEN. TBPEN showed enhanced extraction of Cd(II) from highly acidic solutions (pH < 2). This is because the distribution of this ligand to the aqueous phase is suppressed by the formation of a metal complex encapsulating Cd(II) with TBPEN. The percent extraction of Cd(II) for TBPEN was 100% at pH 1, whereas TPEN extracts less than 10% of the Cd(II) under these conditions. TBPEN also exhibited selective extraction of a variety of soft metals, such as Au(III), Pt(II), Pd(II), and Cd(II), from the acidic solution at pH 1. These results suggest that a hydrophobic TPEN analog combining alkyl chains to four pyridyl groups is applicable to the practical extraction process of soft metals.