Surfactant specific ionic strength effects on membrane fouling during produced water treatment

Membrane filtration is a technique that can be successfully applied to remove oil from stable oil-in-water emulsions. This is especially interesting for the re-use of produced water (PW), a water stream stemming from the petrochemical industry, which contains dispersed oil, surface-active components and often has a high ionic strength. Due to the complexity of this emulsion, membrane fouling by produced water is more severe and less understood than membrane fouling by more simple oil-in-water emulsions. In this work, we study the relation between surfactant type and the effect of the ionic strength on membrane filtration of an artificial produced water emulsion. As surfactants, we use anionic sodium dodecyl sulphate (SDS), cationic hexadecyltrimethylammonium bromide (CTAB), nonionic Triton ^TMX-100 (TX) and zwitterionic N-dodecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate (DDAPS), at various ionic strengths (1, 10, 100 mM NaCl). Filtration experiments on a regenerated cellulose ultrafiltration (UF) membrane showed a pronounced effect of the ionic strength for the charged surfactants SDS and CTAB, although the nature of the effect was quite different. For anionic SDS, an increasing ionic strength leads to less droplet-droplet repulsion, allowing a denser cake layer to form, resulting in a much more pronounced flux decline. CTAB, on the other hand leads to a lower interfacial tension than observed for SDS, and thus more deformable oil droplets. At high ionic strength, increased surfactant adsorption leads to such a low oil-water surface tension that the oil droplets can permeate through the much smaller membrane pores. For the nonionic surfactant TX, no clear effect of the ionic strength was observed, but the flux decline is very high compared to the other surfactants. For the zwitterionic surfactant DDAPS, the flux decline was found to be very low and even decreased with increasing ionic strength, suggesting that membrane fouling decreases with increasing ionic strength. Especially promising is that at lower surfactant concentration (0.1 CMC) and high ionic strength no flux decline was observed, while a high oil retention (85%) was obtained. From our results, it becomes clear that the type of the surfactant used is crucial for a successful application of membrane filtration for PW treatment, especially at high ionic strengths. In addition, they point out that the application of zwitterionic surfactants can be highly beneficial for PW treatment with membranes.