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 th e assessment of the separations efficiency led to the selection of a strong hydrophilic nanofiltration membrane, negatively charged, with an active la yer made of a proprietary polymer. Further experiments were made with this membrane, at various feed surfactant concentrations (up to 20 x cmc), tempe ratures, 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 i nteractions between the membrane and the ionic species in the aqueous solut ion. To explain the changes of the permeate flux with the feed surfactant c oncentration, namely the flux increase in the micellar critic region, a mec hanism based on physical-chemical interactions between the membrane and the surfactant is proposed. Because of the high values obtained for the permea te flux and rejection (maximum of 204 L/(m(2) . h) and 99.5%, respectively) , environmental applications of the process appear to be interesting, speci ally in the pretreatment of industrial effluents with a significant amount of anionic surfactants.