Micellar-enhanced ultrafiltration (MEUF), a surfactant-based separation process, can remove heavy metal from aqueous stream with little energy, relatively low pressure and low cost compared to reverse osmosis and nanofiltration. At the critical micelle concentration, surfactant monomers form micelles in aqueous stream. Anionic surfactants such as sodium dodecyl sulfate (SDS) have negatively charged head group on the shell of micelles and metal ions adsorb or bind on the surface of micelles. The micelles and associated metal ions can be separated from the waste stream through an ultrafiltration membrane with pore size small enough to reject the aggregates.
The characteristics of flux, removal of metal and rejection of SDS were investigated for four metal ions ($Cd^{2+}$, $Cu^{2+}$, $Co^{2+}$, $Zn^{2+}$) using sodium dodecyl sulfate (SDS). The flux decline in membrane with MWCO of 10,000 was mainly due to the concentration polarization, whereas that with MWCO of 3,000 might be due to membrane pores plugged by surfactant monomers. The removal efficiency of metal ions was at least 95 % at the > 10 of the surfactant/metal ratio. In the mixture of metal ions, there was a small difference of removal efficiency on the order of Cd > Cu > Co \thickapprox Zn. The difference diminished as the concentration of SDS increased. To explain the difference of removal efficiency, complexation of metal cations and counterions in solution was considered, but it was not successful. The higher removal of Cd than other metal ions may be due to the larger ion size. Further research is necessary for the clear explanation.
