Coupling Ion-Pair Extraction with Adsorption for the Separation of Acidic Solutes from Water

Abstract

In addition to energy considerations, the cost and potential liability for waste disposal are requiring the chemical process industries to invest in separations for waste minimization (e.g., for the recovery and recycling of reusable materials). To achieve selectivity for the adsorptive separations of acidic solutes, the authors examined the coupling of ion pair extraction with adsorption. Specifically, they used the quaternary ammonium extractant tricaprylmethylammonium chloride (Aliquat 336, Henkel), the inert solvent hexane, and a polar acrylic ester sorbent (Amberlite XAD-7, Rohm and Haas), and studied the selective extraction/adsorption of benzenesulfonate relative to benzoate. At high pH, when both acids are deprotonated, benzenesulfonate was selectively removed from water in this coupled system and a separation factor of 4.8 [+-] 0.5 was measured. This separation factor resulted because the benzenesulfonate ion pair is preferentially partitioned (2.8-fold) and preferentially adsorbed (1.7-fold), compared to the benzoate ion pair. At lower pH's when benzoate, but not benzenesulfonate, is protonated, the separation factor for benzenesulfonate was considerably larger. Thus by exploiting differences in ion pair partitioning and adsorption, and differences in pK[sub a]'s, the acids could be separated using the coupled extraction and adsorption system.