Colloids and Surfaces A: Physicochem. Eng. Aspects 470 (2015) 52–59 Contents lists available at ScienceDirect Colloids and Surfaces A: Physicochemical and Engineering Aspects j ourna l h om epa ge: www.elsevier.com/locate/colsurfa Evaluation of ion separation coefficients by foam flotation using a carboxylate surfactant Cyril Micheau, Andreas Schneider, Luc Girard, Pierre Bauduin ∗ Institut de Chimie Séparative de Marcoule, ICSM-UMR5257 (CEA/CNRS/UM2/ENSCM), 30206 Bagnols sur Céze, France h i g h l i g h t s • Selectivity coefficients between ions have been evaluated from ion flota- tion 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. g r a p h i c a l a b s t r a c t a r t i c l e i n f o Article history: Received 13 November 2014 Received in revised form 12 January 2015 Accepted 13 January 2015 Available online 29 January 2015 Keywords: Ion flotation Selectivity Lanthanide Zeta potential Micelle Separation a b s t r a c t Separation of metal cations of different charges, Na + , Li + , Ca 2+ , Sr 2+ , Cu 2+ , Nd 3+ and Eu 3+ was investi- gated 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 bal- ances. 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 + < Sr 2+ < Ca 2+ < Cu 2+ < Nd 3+ < Eu 3+ . 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 Nd 3+ that leads Li + to be repelled from the foam. © 2015 Elsevier B.V. All rights reserved. 1. Introduction Ion flotation was first described by Sebba in 1959 [1] and was used for the treatment of trace elements from aqueous phases. In ∗ Corresponding author. Tel.: +33 466339288. E-mail address: Pierre.bauduin@cea.fr (P. Bauduin). this process, an ionic surface-active agent (collector or surfactant) is used to concentrate and extract a non-surface active ion of the opposite charge (colligend) at the liquid/gas interface. Sparging gas through the aqueous phase enables the generation of a gas dis- persion at the top of the solution. Small bubbles and low water content increase the interfacial area as well as the efficiency of the ion extraction process. When the collector/colligend couple has a solubility lower than 10 -6 –10 -7 M and for concentrations http://dx.doi.org/10.1016/j.colsurfa.2015.01.049 0927-7757/© 2015 Elsevier B.V. All rights reserved.