Pulse injection technique for membrane characterization: Ultrafiltration of dextran solutions

The current standard technique for evaluating the macromolecule sieving behavior for the ultrafiltration (UF) of dextran solutes relies on steady-state operation, at constant trans-membrane pressure and it provides no information on membrane fouling or concentration polarization. This paper describes the use of a pulsed sample injection technique (PSIT), a technique that obviates these limitations, to characterize UF of dextran solutes. Using neutral dextran solutions and a well characterized thin-film composite (TFC) membrane, PSIT yields apparent sieving coefficients (Sapp) that accurately follow trends from previous studies where the standard technique was used. To demonstrate the versatility of PSIT as a characterization method, a series of experiments were done with charged dextran solutes. As compared to neutral dextran, the Sapp value for polycationic diethylaminoethyl (DEAE)-dextran and polyanionic dextran sulfate sodium salt (DSSS) were lower than neutral dextran due to the combined steric and electrostatic repulsion effects. The degree of solute–membrane interaction was assessed using solute-transport times and indicated significant interactions between DEAE-dextran and the membrane but minimal interactions between DSSS and the membrane. Permeate refractive index profiles were used to estimate the amount of solute bound to the membrane and good correlation was found between the amount of DEAE-dextran bound and the change in membrane permeability. The charged dextran sieving behavior was explained according to solute–solute and solute–membrane interactions using the exclusive information on solute transport kinetics and fouling behavior that is available from the PSIT. Overall, the PSIT is shown to be an excellent characterization technique for dextran UF with TFC membranes.