Selective separation of heavy metal ions from dilute aqueous solutions by foams and micelles of surfactants

Abstract

Traditional metal ion separation by surfactant foams is dependent on the interaction difference of various metal ions with surfactant monomers rather than surfactant aggregates, because the binding of metal ions with surfactant aggregates retains the metal ions in bulk solution. This kind of separation method is only effective for the metal ions with obvious differences in valence, size or coordination ability. The present study proposes a novel separation method based on the binding affinity difference of metal ions with micelles and monomers of two surfactants to selectively separate multivalent ions Cr³⁺, Ni²⁺ and Cu²⁺ from their dilute mixed aqueous solution. The two surfactants are single-chain surfactant sodium dodecyl sulfate (SDS) and gemini surfactant 1,3-bis(N-dodecyl-N-propanesulfonate sodium)-propane (C₁₂C₃C₁₂(SO₃)₂), which show negligible synergism because they are both negatively charged and hold a significantly different self-assembling ability, thus allowing the coexistence of SDS/C₁₂C₃C₁₂(SO₃)₂ micelles with SDS monomers. At first, Cr³⁺ ions were separated from Cu²⁺ and Ni²⁺ ions by the foam generated by the SDS monomers due to more intensive electrostatic interaction of Cr³⁺ ions with the SDS monomers. Afterwards Ni²⁺ ions were separated from Cu²⁺ ions by utilizing the high binding affinity of Cu²⁺ with the SDS/C₁₂C₃C₁₂(SO₃)₂ micelles in the bulk solution and Ni²⁺ with the SDS monomers in the foam. This work has proved that micelles can assist the selective separation of “twin-like” metal ions Ni²⁺ and Cu²⁺ when the concentrations of monomers and micelles are properly adjusted.