Nanofiltration of solutions containing surfactants - prediction of flux decline and modelling of mass transfer

Nanofiltration (NF) membranes are capable of separating surfactants from aqueous solutions with very high retentions. For the anionic surfactant sodium dodecylether sulfate (SDES) retentions exceeding 99.9% referring to the feed concentration have been found. The decrease of the permeate flux compared to the pure water flux in membrane filtration of solutions with surfactants is difficult to predict. In order to simplify membrane selection for a separation problem, an attempt is made to correlate flux decline due to adsorption to the contact angle measured on UF/NF membranes. The vague dependence shown cannot replace membrane screening. In the second part of the paper, a mass transfer model for NF of surfactant solutions based on the extended Nernst-Planck equation is presented. Convective mass transfer of the solute is included in the model, as well as the influence of adsorption of surfactant and concentration polarisation.