Chelate extraction of transition metal ions by ethylene diamine diacetic acid with n-substituted octadecyl groups

Abstract

An organic-soluble chelating agent, N,N′-di-n-octadecyl ethylenediamine-N,N′-diacetic acid (DOED) has been synthesized in two steps by (i) condensation of dibromoethane with 1-amino octadecane (octadecylamine) and (ii) subsequent reaction with sodium chloro acetate in alcohol–water mixture. The chelating agent in 2-ethylhexanol-cyclohexanol (1:1) solutions shows extremely high chelating ability for Ni(II), Co(II), Cu(II), Fe(III), Cr(III), and Cd(II) ions in neutral aqueous solutions. Extractions take place by the formation of (1:1) ligand-to-metal complexes, and extraction coefficients are in the range 0.74–0.99. The ligand can be regenerated almost quantitatively by washing its complexes with 1 M HCl and 0.2 M NaOH solutions successively. In each extraction step, the organic solution exhibits a clear-cut phase separation and does not need salting out. The chelating agent can be regenerated and recycled more than 24 times, without losing its extracting ability, due to nonhydrolyzability of linkages in its structure. Experiments indicate that in high metal concentrations (more than 1 M), the solvent mixture itself is able to extract appreciable amounts of metal ions (33–53%). The solubility in organic solvents, induced by long aliphatic chains, seems to be general and the method presented offers possibility of large scale chelate extraction of metal ions of relatively low concentrations.