Selective extraction and transport of copper(II) with new alkylated pyridinecarboxylic acid derivatives

Two kinds of N-(6-alkylamido)-2-pyridine carboxylic acid with a pyridine moiety and a carboxylic acid as chelating ligands were newly synthesized for the selective extraction and the transport of copper(II) from aqueous solution. Liquid–liquid extraction was carried out to examine the extraction ability of extractants with metal ions. The selectivity for the metal ions with N-6-(2-ethylhexylamido)-2-pyridine carboxylic acid (EHPA) was in the following order: Cu(II)≫Zn(II) ≈ Pb(II) ≈ Ni(II) ≈ Co(II) > Cd(II) > Mn(II). All metals tested in this study were selectively extracted at a lower pH by 2–3 units compared with commercial available alkyl carboxylic acids such as naphthenic acid and Versatic 10. The extraction equilibria of copper(II) were measured by a batchwise method at 303 K with EHPA and N-6-(t-dodecylamido)-2-pyridinecarboxylic acid (t-DAPA). Copper(II) was extracted as a 1:2 complex according to the following reaction: C u 2 + + 2 ( HR ¯ ) 2 = Cu R 2 2 HR ¯ + 2 H + . The extraction equilibrium constant, Kex was evaluated and the values were found to be 1.13 and 0.31 for EHPA and t-DAPA, respectively. It was demonstrated that t-DAPA was very stable to be incorporated into a polymer inclusion membrane (PIM) consisting of cellulose triacetate (CTA) as a base polymer and 2-nitrophenyl octylether (NPOE) as a plasticizer. This novel PIM containing t-DAPA as a carrier exhibited an excellent copper(II) transport characteristic and a high selectivity for copper(II) over cadmium(II) from aqueous solution.