Foam Fractionation and Ion Flotation of Simple and Complex Anions with Cationic Surfactants

Experimental data are presented and interpreted on the foam fractionation of an extensive series of simple and complex anions from dilute (of the order 10⁻⁶−10⁻⁴ molar) aqueous solutions, utilizing a quaternary ammonium surfactant with which the anions form soluble ion pairs in competition with the surfactant's counterion. Selectivity coefficients, based on a bubble-interface, ion exchange model, are established in a single-equilibrium-stage, continuous-flow, foam fractionation unit for each of a series of 13 anions and oxyanions versus the surfactant's bromide counterion. Three additional series of batch experiments with multi-metal solutions establish the separation and concentration in the foam of the oxyanions of Re(VII), Mo(VI), Cr(VI), W(VI), and V(V); of the cyanide complex anions of Zn(II), Cd(II), Hg(II), and Au(III); and of the chloride complex anions of Zn(II), Cd(II), Hg(II), and Au(III). The metal oxyanions and metal cyanide and chloride complex anions can be separated from each other and from competing chloride, cyanide, or nitrate. Ion flotation of anionic species with a cationic surfactant involves an entirely different mechanism, in which a precipitation reaction occurs and particle flotation follows. Hexavalent chromium (primarily HCrO⁻₄) is ion-floated with each of a series of variable chain length quaternary ammonium surfactants, elucidating the effects of temperature, the surfactant/Cr(VI) feed ratio, mixing time, and surfactant chain length in terms of the roles of the surfactant as precipitant, dispersant, collector, and frother.