Selectivity of bis[n-butyl(hydroxyl)phosphoryl]dibenzo crown ether endoreceptors in metal extraction from nitric acid solutions

Abstract

Diphosphorylated dibenzo crown ethers with 4,4′(5′)-bis[n-butyl(hydroxy)]phosphoryl substituents at the benzene rings of dibenzo-18-crown-6 (I), dibenzo-21-crown-7 (II), and dibenzo-24-crown-8 (III) were prepared and studied with the aim of developing new highly effective and selective extractants for recovery and separation of metals. Extraction of a solution containing Al, B, Ba, Bi, Ca, Cd, Co, Cr, Cu, Fe, Ga, Ge, In, K, La, Li, Mg, Mn, Mo, Na, Ni, Pb, Re, Sc, Sr, Tl, V, Y, and Zn (10 ppm each) and an Am(III) solution in 0.01–5 M HNO₃ with 0.05 M solutions of the endoreceptors in chloroform was studied. For endoreceptor III, the maximal distribution ratios of metals D ^max_m are ranked in the following order: Sc (>10³) > Am (162) > Mo (129) > V (75.9) > In (29.3) > Bi (16.5) > Fe (4.7) > Rb (1.0) > Pb (0.34) > Ga (0.18) > Ge (0.11) > Al, B, Ba, Ca, Cd, Co, Cr, Cu, K, La, Li, Mg, Mn, Na, Ni, Re, Sr, Tl, V, Zn (<0.1). The distribution ratio D _m in all the examined equilibrium systems decreases with increasing HNO₃ concentration in the range 0.01–3 M. The fact that this decrease is significantly less pronounced than that for the known organophosphorus acids suggests complex extraction mechanism, presumably via formation of scorpion-like host-guest complexes. Thus, diphosphorylated dibenzo crown ethers with phosphoric acid substituents are very strong and highly selective endoreceptors in extraction of Sc, Mo, In, and Am from nitric acid solutions and can be used in practice for preconcentration of these elements and their separation from concomitant impurities.