Selective extraction and transport of UO22+ with calixarene-based uranophiles

Abstract

Hexacarboxylate derivatives of p-t-butylcalix[6]arene (2b) and p-hexylcalix[6]arene (3b) extract uranyl ion (UO₂²⁺) efficiently and selectively from water into organic media. This high selectivity can be attributed to the nature of calix[6]arene; this is suitably structured for the binding of UO₂²⁺, which requires a pseudo-planar hexaco-ordination. When trioctylmethylammonium cation (TMA⁺) is added to the organic phase, the stoicheiometry of the extraction species [UO₂²⁺/(2b)] changes from 1:3 to 1:1–2. This implies that an additional 1–2 mol of UO₂²⁺ are adsorbed to (2b) to form ‘neutral’ extraction species and can be displaced by added TMA⁺. UO₂²⁺ is transported by (2b) across a liquid membrane from a neutral IN (source) aqueous phase (pH 8.5) to an acidic OUT (receiving) aqueous phase (0.1 mol dm^–3 HCl) in an active transport manner. The rate-determining step is the release of UO₂²⁺ from the membrane to the OUT aqueous phase. On the basis of these basic data, we have prepared polymer/liquid-crystal (N-4′-ethoxybenzylidene-4-butylaniline, EBBA/TMA⁺Cl^–) composite membranes immobilising (2b) and have found that the rate of UO₂²⁺ transport is efficiently controlled by the changes in pH and temperature. These are novel examples for the selective transport of UO₂²⁺.