Extraction of metal ions with non-fluorous bipyridine derivatives as chelating ligands in supercritical carbon dioxide

Three new non-fluorous bipyridine derivatives, bis(2-(2-butoxyethoxy)ethyl)-2,2′-bipyridine-4,4′-dicarboxylate (ligand 1), bis(2-(2-ethoxyethoxy)ethyl)-2,2′-bipyridine-4,4′-dicarboxylate (ligand 2), and bis(2-butoxyethyl)-2,2′-bipyridine-4,4′-dicarboxylate (ligand 3), were synthesized as chelating ligands to remove metal ions from solid matrix into supercritical CO₂ (scCO₂). These produced compounds 1-3 showed considerable solubilities in scCO₂ (8.0 g/l, 4.8 g/l, 7.8 g/l for ligands 1-3 at 313 K, respectively) and the tested solubility data were then calculated and correlated with semiempirical model at different pressures and temperatures, which showed satisfactory agreement with each other and the average absolute relative deviation were in the range of 0.1-28.3%. The effects of pressure, temperature, time, and ligand to metal ratio (5:1 to 75:1) on the extraction efficiency of metal ions were also systematically investigated. The extraction efficiency was 100% for Ni²⁺ and 95.9% for Cu²⁺ in scCO₂ with the system of ligand 1, ultrapure water, and perfluoro-1-octanesulfonic acid tetraethylammonium salt (PFOAT) under the optimized conditions (25 MPa, 313 K, 90 min, and ligand to metal ratio of 10). Although all ligands exhibited good efficiency for Ni²⁺ (>85%) and Cu²⁺ (>70%) extraction, the extraction of mixed metal ions indicated that the bipyridine derivatives had low selectivity. Finally, the detailed calculation results exhibited that the extraction constants (K_ex) of the metal ions increased with the increase of the extraction efficiency in the same extraction system for each same metal ion.