Effects of Surface Force Interactions on an NF/UF Membrane


Appreciation of a membrane's surface chemistry and steric exclusion character is needed to truly understand and predict membrane performance for specific industrial separations. The interpreter of membrane characterization data must consider both factors in the assessment of separation potential. The test conditions employed will strongly influence the separation data outcome, as will inherent surface force interactions between the membrane and solution components. Sepa® CF cell solute challenges and affinity chromatography methods are useful tools to characterize the pore size and surface character of ultrafiltration and nanofiltration membranes.

The characterization data presented demonstrate the separation potential of B-type membranes. For example, the combination of the B-type membrane surface charge and pore size affords economical separations of salts from organics. The anionic surface charge of B-type membranes also makes them competitive for high fouling applications. Dye concentration, paper pulp waste treatment and similar applications appear promising for B-type membranes where traditional membranes are not well suited.


Membrane separations in the ultrafiltration (UF) and especially the nanofiltration (NF) regions of the filtration spectrum are governed by a complex combination of both steric exclusion and surface force interactions. UF and NF membranes bearing formal surface charges display unusual selectivity behavior not predicted on the basis of physical pore size alone. Therefore, practical characterization should employ several techniques to gain insight on membrane function.

In this work we elucidated the separation characteristics of a novel, anionically charged membrane with UF and NF capabilities. This study included three distinct techniques to characterize the separation potential solute challenges to demonstrate the apparent molecular weight cutoff (MWCO) of the membrane; a special affinity chromatography technique intended to qualitatively demonstrate surface force interactions between the solutes and the membrane resin; and pilot scale operation of actual industrial applications.


Molecular weight cutoff

The membrane characterized in this work is a novel thin layer composite. (TLC® ). The permselective barrier layer is produced from a unique polymer material. The commercial membrane manufactured from this proprietary resin is referred to as B-type membrane. B-type resin features a formal negative surface charge which is responsible for unique separation characteristics of B-type membranes.

Solute challenges were completed using a bench-top laboratory test device known as a Sepa® CF cell (Figure 1). This device is unique among test cells: it is designed to model the fluid dynamics of commercial spiral-wound membrane elements ('sepralators'). Like the larger sepralators, the Sepa CF cell functions in a true cross-flow or tangential flow mode. Challenge solutions are pumped under pressure through feed channel spacers. and permeate is routed through a woven fabric spacer called the permeate carrier. Both of these spacers are the same materials used in a spiral-wound element (sepralator). The presence of the mesh spacer in the feed channel in Figure 1 is designed to promote turbulence in the flow channel to reduce the build-up of solute near the membrane surface.

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