Effects of proteins on electrokinetic properties of inorganic membranes during ultra- and micro-filtration

Effect of solution physico-chemistry on the charge property of nanofiltration membranes

Electrokinetic phenomenon is an important factor in pressure-driven membrane water filtration process. Zeta potential is a measurable and reliable parameter to characterize the charge property of membranes. This study investigated the effects of solution physico-chemistry on the zeta potential of nanofiltration membranes. A comprehensive semi-empirical mathematical model based on the Poisson-Boltzmann equation, Membrane Surface Charge model, was established. The correlations between the zeta potential, surface charge density and solution chemistry were well delineated by this new model. The empirical coefficients in the model are dependent on neither solution pH nor the ionic strength. They only reflect the characteristics of the electrokinetic property of the membrane--solution interface. The point of zero charge (PZC) can be calculated by combination of those empirical coefficients. The Membrane Surface Charge model also provides more information on understanding the charge interaction involved in membrane filtration system. The effects of potential determining ions, indifferent ions and specific adsorption of substances onto the membrane could be revealed by analyzing the coefficients of the model.

Role of electrophoretic mobility of protein on its retention by an ultrafiltration membrane. Comparison to chromatography mechanisms

Lysozyme and lactoferrin, two globular proteins, were first studied separately in order to elaborate a strategy for the improvement of their separation by ultrafiltration (UF) with zirconia-based membranes of different charge sign and pore radius. The electrophoretic mobility (mu) at fixed pH and variable ionic strength was used for the characterisation of both proteins and zirconia particles, similar to the active layer of the membrane during the UF run. Specific adsorption of phosphate ions was shown for both proteins resulting in new isoelectric points. The occurrence of electrostatic exclusion mechanism in addition to the molecular sieving in UF of charged solutes was shown for: * Low molecular weight solute: multivalent citrate at pH 6 was specifically adsorbed on zirconia and its transmission through the membrane (defined as the ratio of the concentration in the permeate to that of the feed solution) was reduced in the range 0.001-0.01 mol l(-1) of citrate concentration * Proteins: their transmissions increase when the ionic strength increases (ion-exchange is not the relevant mechanism because transmission is irrespective of the initial charge of the membrane compared with the protein charge). A model based on convection, diffusion and electrophoretic migration mechanisms (CDE model) was proposed to take into account this behaviour. The CDE model predicts the possible existence of a depleted sub-layer of the charged protein in the concentration polarisation layer, located in the close vicinity of the membrane surface. A strategy for the separation of two proteins in mixed solution was proposed by varying both the physico-chemical environment in the feed solution (pH, ionic strength, chemical nature of the electrolyte) and the membrane pore radius. Maximum selectivity was obtained when the target protein (to be transmitted in the permeate side) is close to being uncharged due to specific adsorption of electrolyte ions. Ultrafiltration selectivity is enhanced with membrane of large pore radius, which provides high transmission of the target protein and efficient electrostatic exclusion of the solute to be retained in the retentate side. This UF approach corresponds roughly to the separation of one uncharged and one charged protein from a mixed solution by size exclusion chromatography of the uncharged protein combined with electrostatic exclusion of the charged protein due to packing of similar charge.

Ionic interactions in nanofiltration of beta casein peptides

Nanofiltration (NF) membrane technology shows interesting potentials for separating organic components on the basis of solute charge and size in the range of 300-1000 g mol-1. Separation properties of two inorganic NF membranes were studied with a set of 10 small peptides (molecular mass range: 300-900 g mol-1; 3 < pI < 10) contained in a well-characterized tryptic beta casein hydrolysate. Peptides transmission strongly depended on ionic interactions in the system. Physicochemical conditions such as ionic strength and especially pH were crucial to the separation, because the membrane and peptides showed amphoteric properties. Thus, the three categories of peptides (acid, basic, neutral) were separated according to their pI because of presumed concentration gradients of charged peptides at the membrane: positive for basic peptides and negative for acid peptides. At optimum pH 8 this led to high transmissions of basic peptides (even over 100%), intermediate transmissions for neutral peptides, and low transmissions for acid peptides. The addition of multicharged cationic and anionic species in the hydrolysate induced a markedly enhanced selectivity when the polyelectrolyte was a membrane coion and a complete reversion of selectivity when it was a membrane counterion. Copyright 1998 John Wiley & Sons, Inc.