Enhanced Anion Exchange for Selective Sulfate Extraction: Overcoming the Hofmeister Bias

Abstract

Synergism in liquid-liquid extraction, typified by the combination of a neutral extractant with a cation-exchanger to enhance selectively cation extraction strength, has been used and understood for over five decades.1 Surprisingly, analogous synergism in anion extraction has not yet been developed. In this Communication we present a simple way to achieve non-Hofmeister selectivity in liquid-liquid anion exchange by combining a synthetic hydrogen-bond-donating (HBD) anion receptor with a standard quaternary ammonium type extractant. Specifically, we show that the fluorinated calixpyrroles 1 and 22 and the tetraamide macrocycles 3�5,3 may be used to enhance the solvent extraction of sulfate from nitrate by Aliquat 336-nitrate (A336-nitrate).