Liquid–liquid extraction of transition metal ions with an alkylhistidine extractant

Liquid–liquid extraction of transition metal ions using an alkylhistidine (histidine-2-ethylhexyl amide, abbreviated as His-EH) as a new extractant was studied. Intermediate metal cations in the Hard and Soft Acids and Bases (HSAB) sense, such as Cu(II), Ni(II), Zn(II) and Co(II), were selectively extracted with His-EH in the presence of other metal cations. The order of extractability for divalent transition metal ions was Cu(II) ≫ Ni(II) > Zn(II) > Co(II) ≈ Cd(II) > Mn(II), which is approximately the order of the stability constants for complexes of histidine and the metal ions. From the results of slope analysis for Co(II) extraction, the complex stoichiometry was found to be 2:1 for His-EH and Co(II). In addition, the ion-pair extraction reaction of Co(II) with His-EH was in accordance with the equation: Co(aq)2++2His-EH(org)+2ClO4(aq)-=Co2+(His-EH)2(ClO4-)2(org). The extraction equilibrium constant (Kex) of Co(II) using His-EH was evaluated as 3.03 × 1010 [(mol dm−3)−4].

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► An alkylhistidine extractant histidine-2-ethylhexyl amide was prepared as a new extractant for metal ions.
► Selective extraction of intermediate transition metal ions with the extractant was confirmed.
► The extraction reaction for Co(II) using the extractant was determined.
► Conditions for back extraction of the extracted metal were clarified.