Superchaotropic ion flotation: A new concept for the extraction and separation of nanometer-sized ions by non-ionic surfactant-based foams

Ion flotation is a separation technology for recovering ions from dilute aqueous solutions. All ion flotation processes known so far make use of ionic foaming agents (surfactants) to selectively extract ions of opposite charge by electrostatic interactions. Recently, it was shown that nanometric-sized ions (nano-ions) with low charge density, such as certain polyoxometalates (POMs), strongly adsorb to neutral hydrated surfaces. In this study, we provide proof-of-concept for a method that we call "superchaotropic ion flotation," which uses nonionic surfactant foams to selectively recover and separate superchaotropic POMs from other ions, including non-superchaotropic ions. Specifically, we investigated the extraction of isopolyoxomolybdates, formed by pH and concentration variation of molybdate aqueous solutions, using foams produced with a commercial polyethoxylated surfactant (BrijO10). The speciation of polyoxomolybdates was investigated by Raman spectroscopy and small angle X-ray scattering (SAXS). SAXS has also confirmed the superchaotropic behaviour of the polyoxomolybdates species via the characterization of their strong adsorption on surfactant micelles. The flotation recovering is high and maximal at pH 1, a pH for which Mo36O1128  , an anion with a low charge density, is the predominant molybdate species. It was found that the surfactant has a strong influence on the molybdate speciation. It was also demonstrated that molybdate may be separated from tungstate at pH 2 by superchaotropic ion flotation. In conclusion, we propose a novel ion separation method via non-ionic surfactant aqueous foams and we show that the superchaotropic effect makes it possible to foresee various applications in separation science.