Adsorption effects in rejection of macromolecules by ultrafiltration membranes

Rejection of adsorbing solutes by ultrafiltration membranes is not adequately described by the steric rejection theory [3]. Solute adsorption (fouling) changes the shape of the rejection curve. Typically, the measured curves are steeper than the theoretical curve. The shape of the curve can be predicted qualitatively from simple theoretical considerations. For adsorbing solutes, single-solute and multiple-solute ultrafiltration experiments give different results. Relative thickness of adsorbed solute layer in a membrane pore was found to depend on (1) solute size, (2) solute hydrophobicity, (3) pH and ionic strength for a protein solute, (4) solute concentration, and (5) time of adsorption. Large differences observed between water fluxes and fluxes of very dilute polymer solutions through the same membrane are also interpreted in terms of solute adsorption.