Separation of an anionic surfactant by nanofiltration

The separation by nanofiltration of an anionic surfactant belonging to the alkyl-polyether-sulfate family (critical micellar concentration, cmc = 300 mg/L) was studied. Different asymmetric membranes were tested first, but the assessment of the separations efficiency led to the selection of a strong hydrophilic nanofiltration membrane, negatively charged, with an active layer made of a proprietary polymer. Further experiments were made with this membrane, at various feed surfactant concentrations (up to 20 x cmc), temperatures, and cross-flow velocities. The results revealed a complex behavior, mainly for the permeate flux, showing that the separation depends much on the physical-chemical properties of the surfactant and the electrostatic interactions between the membrane and the ionic species in the aqueous solution. To explain the changes of the permeate flux with the feed surfactant concentration, namely the flux increase in the micellar critic region, a mechanism based on physical-chemical interactions between the membrane and the surfactant is proposed. Because of the high values obtained for the permeate flux and rejection (maximum of 204 L/(m(2) . h) and 99.5%, respectively), environmental applications of the process appear to be interesting, specially in the pretreatment of industrial effluents with a significant amount of anionic surfactants.