Application of foam separation method for removal of alkaline earth metal ions from solution

Abstract

Foam separation was used to separate dissolved alkaline earth metal ions from aqueous solution. The mechanism is based on anionic surfactant adsorption onto the bubbles generated in the aqueous alkaline earth metal solution. The negatively charged bubbles rise while adsorbing the positive metal ions and change to foam at the surface. The foam in the glass tube drains excess water as it rises and is finally turned into dry foam. The foam separation method with sodium decyl sulfate (SDeS) (8 mmol L−1, below the critical micelle concentration (CMC)) was used to remove alkaline earth metal (Mg, Ca, and Sr) chlorides in the respective systems. The initial concentrations of the surfactant and alkaline earth metal chloride were 8 mmol L−1 (below the CMC) and 2.5 mmol L−1 in 250 mL of solution, respectively. The results showed that the removal rate in the one-component alkaline earth metal system increased with increasing atomic number: Mg (46%) < Ca (58%) < Sr (77%) in 5 h of foam separation. A similar trend was observed in the foam separation in a mixed system of alkaline earth metals. However, the concentration of the surfactant counterion (Na+) remained almost constant after the operation, indicating that the divalent ions, rather than the monovalent surfactant counterions, were preferentially adsorbed onto the sulfate group of the anionic surfactant. The changes in the concentration of the alkaline earth metal ions were analyzed using kinetics. The plot of the first-order rate constant against the ionic radius of each metal suggested a linear relationship in each series of alkaline earth metals or alkali metals. The present study indicates that foam separation using SDeS can be used to effectively remove large alkaline earth metal ions from contaminated water.