Foam Separation of Anions from Aqueous Solution: Selectivity of Cationic Surfactants

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Abstract
Anions are selectively separated and concentrated from dilute aqueous solution by foam fractionation. Selectivity coefficients are established from steady-state equilibrium data (solution concentrations 10−4 to 10−3 M) for SCN-, I-, ClO3-, NO3-, BrO3 −, and NO2 −, each vs Br−, with the quaternary ammonium surfactant modeled as a soluble anion exchanger. Studies are reviewed on the foam separation of Re(VII), Mo(VI), and V(V) oxyanions; Au(I), Ag(I), Ni(II), and Co(III) cyanide complexes; and Pt(IV), Pd(II), and Au(III) chloro complexes. In a five-component system, the oxyanions of Re(VII), Mo(VI), Cr(VI), W(VI), and V(V) are foam fractionated from 10−6 M solutions with the cationic surfactant, hexadecyldimethylbenzylammonium chloride. In the batch, time-dependent experiments, the metals are monitored by radiotracers and gamma radiation spectrometry. At pH 6.0 and a chloride (NaCl) concentration of 10−2 M, and at pH 2.0, adjusted with HCl, Re(VII) and Mo(VI) oxyanions can be separated completely from Cr(VI), W(VI), and V(V) oxyanions. The selectivity sequences are discussed in terms of acid-base equilibria and in terms of the absolute partial molal entropy of each anion in aqueous solution, as a measure of surfactant cation-anion interaction.

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