Abstract
Membranes hold great potential to be used for the successful treatment of oily waste water, but membrane fouling leads to substantial decreases in performance. Here we study the impact of ionic strength on membrane fouling from an emulsion stabilized by the anionic surfactant sodium dodecyl sulfonate (SDS). For this we use a unique combinatorial approach where droplet adhesion to a cellulose surface in a flow cell is compared to membrane fouling (flux decline) on a cellulose membrane. In the initial membrane fouling stages droplet adhesion dominates. While the flow cell demonstrates a high number of droplets adhering especially at high ionic strengths (100 mM NaCl), the strongest flux decline is observed at intermediate (10 mM NaCl) ionic strength. This suggests that the fouling mechanism must be different, with pore blocking expecting to dominate at intermediate ionic strength. At the later fouling stages the porosity of the cake layer plays a key role in the flux reduction. At low ionic strength, oil droplets repel each other strongly and an open, more permeable, cake layer is formed. However at higher ionic strength, a screening of charge interactions leads to a lower porosity and thereby a lower flux. This leads to a clear trend: with a higher ionic strength a higher flux decline is observed. Flux recovery is high at all ionic strengths, in line with the observation in the flow cell that oil droplets can easily be sheared of a cellulose surface at all ionic strengths. This work thus highlights the critical effect of the ionic strength on membrane fouling by anionically stabilized emulsions. Moreover it shows how the use of an optical flow cell can provide key insights to help explain observations in more standard membrane fouling experiments.
Highlights
Membrane filtration of oil-in-water emulsions is a cost-effective and selective way of separating water from oil
We can distinguish two regimes, initially the rate of flux decline is coupled to droplets adhering to the membrane surface, while for longer times the build up of a cake layer will dominate the flux decline
At the start of the experiment, the flux decline is higher at 100 mM than at 1 mM, which can be coupled to the higher droplet adhesion expected at high salt concentration on the basis of the flow-cell results
Summary
Membrane filtration of oil-in-water emulsions is a cost-effective and selective way of separating water from oil. This is especially the case for stable emulsions, with droplets < 10 μm, which cannot be separated efficiently by, for instance, flotation or other gravity-driven processes [1,2,3,4,5]. The more microscopic interactions between particles or, in this case, oil droplets, and the membrane surface are harder to study in-situ. Understanding the interactions between oil droplets and the membrane surface is a first step towards better tailoring membrane filtration processes to their feed stream in the future
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