| Wastewaters from metal finishing facilities often contain a variety of toxic substances in dilute solution. Surfactant-based ultrafiltration (UF), which combines the high flux of UF with the high selectivity of surfactants, is applied to separate dissolved heavy metals from simulated and real wastewaters using natural surfactants such as derivatives of cholesterol and lecithins. The underlying principle is to increase the size of target metal ions by fixing them to larger surfactant macromolecules so they can be retained by a compatible membrane. Selective and total removal of metal cations from waste streams are achieved by varying the ratio of total surfactant-to-metal (S/M) concentrations.; Five heavy metals (cadmium, copper, nickel, lead, and zinc) of different concentrations in a simulated wastewaters, alone or together, are successfully removed by this technique. The optimal S/M ratio for substantial metal removal (rejection ratio {dollar}>{dollar}99.9+%) is 2.5 for deoxycholic acid (DCA, {dollar}rm Csb{lcub}24{rcub}Hsb{lcub}39{rcub}Osb4{dollar}Na) and 5.0 for sodium dodecyl sulfate (SDS, {dollar}rm Csb{lcub}12{rcub}Hsb{lcub}25{rcub}Osb4{dollar}SNa). This optimal S/M ratio for substantial metal removal (rejection ratio {dollar}>{dollar}99.9+%) obtained from bench-scale experiments is confirmed in the pilot-scale trials using DCA to process simulated and real wastewaters of metal finishing industries. A process model based on material balances and sorption isotherms was used to predict the permeate concentration. A method called continuous diafiltration was performed to study the binding behavior of five heavy metals (Cd, Ni, Pb, Cu, and Zn), individually and collectively, to various surfactants (SDS, DCA, and taurocholic acid). Further, the results of the binding study was interpreted by using the theory of diafiltration and differential mass balances. Overall, surfactant-based UF has outstanding potential to remove heavy metals, to produce high quality treated water with reasonable flux, and to reduce the cost of industrial wastewater treatment by reducing energy consumption and waste volumes. |
