Use of micellar-enhanced ultrafiltration at low surfactant concentrations and with anionic-nonionic surfactant mixtures

Micellar-enhanced ultrafiltration is a separation technique which can be us ed to remove metal ions or dissolved organics from water. Metal ions bind t o the surface of negatively charged micelles of an anionic surfactant while organic solutes tend to dissolve or solubilized within the micelles. The m ixture is then forced through an ultrafiltration membrane with pore sizes s mall enough to block passage of the micelles and associated metal ions and/ or dissolved organics. Monomeric or unassociated surfactant passes through the membrane and does not contribute to the separation. This paper consider s advantages of addition of small concentrations of nonionic surfactant to an anionic surfactant; the resulting anionic-nonionic mixed micelles exhibi t negative deviation from ideality of mixing which leads to a smaller fract ion of the surfactant being present as monomer and a subsequently larger fr action present in the micellar form. The addition of nonionic surfactant im proved the separation of divalent zinc substantially at total concentration s above the critical micelle concentration (cmc) of the anionic surfactant. Both zinc and tert-butylphenol (a nonionic organic solute) show unexpected rejection at surfactant concentrations moderately below the cmc, where mic elles are absent. This is considered as due to a higher surfactant concentr ation in the gel layer adjacent to the membrane where micelles are present. Reduction of this rejection at lower transmembrane pressure drops supports this mechanism. Some rejection of zinc was observed in the absence of surf actant but not of tert-butylphenol, indicating an additional effect of memb rane charge for ionic solutes. (C) 1999 Academic Press.