Extraction of Cd 2+ and Pb 2+ from dilute aqueous solution using metal-sorbing vesicles in a hollow-fiber cartridge

Abstract

Continuous extraction of heavy metal ions from dilute aqueous solution using engineered surfactant vesicles as metal ion sorbents has been investigated. In this experimental process, the metal-sorbing vesile solution circulates through the tube-side of a hollow-fiber cartridge, while metal bearing solution circulates through the shell-side. The vesicle sorbents are stable and rapidly take up and concentrate by orders of magnitude both Cd 2+ and Pb 2+. A lipophilic metal carrier in the phosphatidylcholine vesicle wall shuttles heavy metal ions to the aqueous vesicle core where an encapsulated water-soluble chelating agent, nitrilotriacetate, complexes the ion, thereby providing the driving force for metal ion sorption. Two metal carriers have been studied: natural antibiotic, A23187, and a novel synthetic carrier. Vesicles containing the synthetic carrier show selective uptake of Pb 2+ over Cd 2+ at pH 5.5 from a mixed solution. A pseudo-steady state model simulates the overall extraction process well. The rate of extraction has been found to be limited by the rate of diffusion of ions through the hollow fiber membrane.