Interaction between surfactants of different classes and nanofiltration membranes

The presence of surfactants in aquatic environments may alter the water quality parameters. Nanofiltration is a promising technique that can separate surfactants from water with high rejections. Nevertheless, negative charges on nanofiltration membranes could facilitate the occurrence of electrostatic interactions between the charged surfactants and the surface of these membranes, increasing the membrane-fouling tendency. In this work, the interaction of surfactants commonly found in residential laundry wastewater with membranes used in nanofiltration was investigated. For this purpose, the surface, and transport properties of four commercial membranes were analyzed. The zeta potential of the NF90 membrane in the presence of individual and mixture of surfactants was performed to evaluate the effect of adsorption of these surfactants on the performance of the membrane. Additionally, a central composite rotatable design (CCDR) and response surface model were applied to investigate the simultaneous effect of the initial concentration of CTAB and SDS ([CTAB + SDS]0) and pH on the NF90 permeate fluxes and rejections. It was shown that surfactant adsorption varies with the type of surfactant and membrane. Moreover, adsorption played an important role in NF90 membrane fouling, mainly for the cationic surfactant in individual solutions and in the mixture of surfactants. Besides adsorption, pore blocking may have affected the performance of nanofiltration for higher concentrations of surfactants. Finally, by the application of the central composite rotatable design (CCDR), it was possible to find the best conditions of pH and initial concentration of surfactants for achieving higher fluxes and rejections by the NF90 membrane.