Removal of alkali metal ions from aqueous solution by foam separation method

Foam separation method can effectively remove alkali metal ions from anionic surfactant solution by a combination of an air bubble generator and the flotation system. The alkali metal ions in the solution electrostatically adsorb onto the many bubbles by bubbling ionic surfactant solution and they transfer to the air/water interface. These bubbles change to the foam state that is removed using a long glass tube. The typical experimental system was composed of sodium dodecyl sulfate (SDS) as an anionic surfactant and the target alkali ions (Li, K, Rb, and Cs). Therefore, selective competition for air bubbles exists between the sodium (counterion of the surfactant) and target alkali ions. The removal rate of the alkali metal was noticeably dependent on surfactant concentration. The surfactant was effective in removing the alkali metal below the critical micelle concentration (CMC). The removal rate increased with increasing atomic number or crystal ion radius: Li < K < Rb < Cs. The best removal rate of the alkali metals involved 80% Cs elimination by using a microbubble generator (5 h) at the initial SDS concentration of 4 mmol/L and Cs 2.5 mmol/L. On the other hand, the removal rates of dodecylsulfate and counterion (Na) respectively decreased to 25% and 42% for the system. These results indicated that Cs was more selectively adsorbed onto the bubble than Na. A smaller hydration radius can easily bind the surfactant sulfate ions. This study showed that the foam separation method could selectively remove hazardous metals from contaminated water. (C) 2018 Elsevier B.V. All rights reserved.