Impact of surfactants on membrane fouling during separation of an oil-water emulsion

While membrane separation has proven to be an outstanding method for separating oil in water (O/W) emulsions containing droplets smaller than 20 µm, it is severely limited due to fouling. Much research has been aimed at understanding the mechanism behind membrane fouling, particularly for ultrafiltration (UF) membranes. Interestingly, studies pointed out that the emulsifier, namely surfactant, is the main source of fouling in nanofiltration and reverse osmosis, while in the case of UF, oil is generally regarded as the main source of fouling. Herein, we study the fouling of UF membranes during separation of O/W emulsions stabilized by surfactants, with the explicit goal of determining the relative impact of the oil and surfactant present on fouling severity and dynamics. Results obtained from flux decline measurements, complimented by visualization using confocal microscopy, show that oil causes irreversible fouling to a certain extent, however, surfactant fouling dominates the observed membrane performance. The degree of fouling and flux recovery appears to be closely related to the properties the surfactant, namely charge and molecular weight, as has been observed in the past but attributed to the oil-membrane interactions, mediated by the surfactant. Further, to visualize the fouling mechanisms, direct observation via a confocal microscope set-up is used to capture real-time images of the membrane surface, which reveal that surface coverage of oil is not directly related to flux decline during the separation process. Our results suggest that membrane flux decline and fouling is dominated by membrane-surfactant interactions, the exact nature of which is a topic for future extensions of this work.