Use of Ligand-Modified Micellar-Enhanced Ultrafiltration in the Selective Removal of Toxic Metal Ions from Polluted water

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This proposal is concerned with removing divalent metal ions, present in dilute amounts, from aqueous solutions by ligand modified micelle enhanced ultrafiltration. In this method, surfactant molecules are added to the aqueous solution in concentrations exceeding the CMC so that the molecules form micelles. Micelles are aggregates of surfactant molecules in which the nonpolar chains of different molecules extend and stack together to form a ball-like hydrophobic core with the polar parts surrounding the core and remaining in contact with the aqueous phase. Also added to the aqueous solution are metal complexing ligands consisting of a hydrophic tail and a moiety which complexes with the metal ion. The ligand complexes the metal ion, and the ligand-metal assembly attaches itself to the micelle by solubilization of the hydrophobic tail into the membrane interior. The aqueous solution is then forced through a membrane which does not allow passage of the micelles. Thus the metal ion is retained, attached to the micelle, in the retentate on one side of the membrane, and the permeate contains a reduced level of the ion. The feature of this separation which is most attractive is its selectivity. Ligands can be engineering which exclusively bind to a particular metal ion. Therefore an aqueous solution that has only one ion at an undesirable high concentration can be cleansed of that ion without wasting materials on the removal of other ions. The PI and CoPIs received an NSF grant to develop the process and study its selectivity in 1988. As reported in the section of the proposal "Results from Prior NSF Support", research supported by the previous grant demonstrated the feasibility of the separation. In particular, in a stirred-cell ultrafiltration of a solution containing a mixture of Cu2+/Ca2+, using the ligand NIDA which binds the copper ion prefentially, rejection of copper was upwards of 90 percent. The present proposal extends the technique developed in the previous grant to the specific selective removal of divalent Pb2+, UO22+, Hg2+, Cd2+, Cu2+, Zn2+ and Ca2+ and synthesizing new ligands. Binding constants of the ligands with ions, and their protination constants will be measured when the ligand is free in solution and solubilized in a micelle. These measurements will be undertaken by a combination of titration and dialysis experiments. Ultrafiltration experiments will be undertaken using a spiral wound membrane module. The proposal remarks that this configuration allows for modelling of the laminar boundary layer, and concentration polarization phenomena.