Influence of bulk concentration on the organisation of molecules at a membrane surface and flux decline during reverse osmosis of an anionic surfactant

• The fouling of a reverse osmosis membrane by amphiphilic molecules is studied.
• Permeate flux, contact angles, mass balance are combined to characterize the fouling.
• A local organisation at the interface is deduced from the surfactant phase diagram.

Surfactants are extensively used in household and industrial products. Several processes exist to treat industrial wastewaters, including membrane filtration such as ultrafiltration, nanofiltration and reverse osmosis (RO). We studied fouling of RO membranes during filtration of aqueous anionic surfactant solutions under different conditions. The aim was to describe the local organisation of the surfactant at the membrane interface. To this end, the typical surfactant sodium dodecyl sulfate (SDS) and a polyamide membrane (SG, GE Water & Process Technologies) were selected. A marked surfactant mass loss was experimentally quantified and attributed to the accumulation of surfactants on the membrane surface and adsorption on the non-membrane materials in the filtration system. The concentration of surfactant in the polarisation layer compared with the SDS phase diagram, combined with contact angle measurements and flux decline analysis, enabled us to deduce a structure for the fouling. The fouling layer presented different structures according to the surfactant concentration: from a dense hydrophobic layer at very low concentration to a lamellar hexagonal phase in the gel layer at concentrations above 35 wt% in water.

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