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, the separation characteristics of a novel, anionically charged membrane with UF and NF capabilities were elucidated. This study included three distinct techniques to characterize the separation potential; solute challenges to demonstrate the apparent Molecular Weight Cut Off (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 Cut Off (MWCO)
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 (Fig 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 crossflow 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 buildup of solute near the membrane surface.