Selective separation of cadmium(II) from zinc(II) by a novel hydrophobic ionic liquid including an N,N,N′,N′-tetrakis(2-methylpyridyl)-1,2-phenylenediamine-4-amido structure: a hard–soft donor combined method

A novel hydrophobic ionic liquid including an N,N,N′,N′-tetrakis(2-methylpyridyl)-1,2-phenylenediamine-4-amido structure ((IL-1,2-tpbd)⁺NTf₂⁻) was successfully synthesized. (IL-1,2-tpbd)⁺NTf₂⁻ combined one amido (O-hard donor) and four pyridine (N-soft donor) groups. Its Cd²⁺ and Zn²⁺ separation behavior in nitric acid solution was investigated as a function of the extraction time, effect of pH etc. by dissolving (IL-1,2-tpbd)⁺NTf₂⁻ in a room temperature ionic liquid, 1-hexyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ((C₆mim)⁺NTf₂⁻). The extraction kinetics were fairly fast and could reach equilibrium within 4 h. When pH_eq ≥ 1.8, the extraction percentage of Cd²⁺ and Zn²⁺ remained constant and the maximum separation factor was calculated as 12.78 at pH_eq = 3.1; when pH_eq < 1.8, the extraction percentage of Cd²⁺ and Zn²⁺ decreased drastically due to the protonation of the pyridine groups. Complete stripping of the extracted Cd²⁺ and Zn²⁺ from the ionic liquid phase into an aqueous phase was successfully achieved under highly acidic conditions ([HNO₃] = 2 M) without adding any other metal complex forming agents. The extraction mechanism was summarized as a cation exchange due to the independence of nitrate ions in the extraction process. Additionally, the results of the slope analysis and UV-vis titration revealed the formation of a 1 : 2 complex. Furthermore, (IL-1,2-tpbd)⁺NTf₂⁻ showed a higher preference for Cd²⁺ even under the interference of various co-existing metal ions.