Evaluation of ion separation coefficients by foam flotation using a carboxylate surfactant

• Selectivity coefficients between ions have been evaluated from ion flotation results using a polyethoxy-carboxylate surfactant.
• Ion selectivity in the foam correlates with the ion selectivity at the micelle surface.
• A depletion effect of lithium ions from the foam was observed in the presence of neodymium (III) ions.

Separation of metal cations of different charges, Na+, Li+, Ca2+, Sr2+, Cu2+, Nd3+ and Eu3+ was investigated through ion foam flotation using a pH sensitive surfactant, nonaoxyethylene oleyl ether carboxylic acid. We propose here a method to evaluate ion selectivity coefficients using mass and volume balances. This method yields selectivity coefficients in agreement with those obtained with the classical slope method. The ion selectivity obtained by the flotation experiments was found to correlate well with the apparent charge of the surfactant micelles (zeta potential values) in the presence of different salts and is therefore not influenced by the surface curvature. Consequently the ion specificity order has been established according to the surfactant–ion affinity at the air–water and micelle–water interfaces as Na+ < Li+ < Sr2+ < Ca2+ < Cu2+ < Nd3+ < Eu3+. It has been noticed that the selectivity coefficients between the different metal ions, obtained by ion flotation, differ from the ones predicted by using metal ion complexation constants of acetate, which is considered here as the non-surface active complexing part of the surfactant. This discrepancy was attributed to the surface complexation effects at the air–water interface in flotation experiments and at the micelle–water interface. For the separation of lithium and neodymium, a depletion phenomenon of lithium ions from the interface, hence from the foam, has been observed, i.e. once the flotation experiment was finished, the lithium concentration in the remaining foaming solution was indeed higher compared to the initial one. This phenomenon was explained by the strong adsorption of Nd3+ that leads Li+ to be repelled from the foam.

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