Combined effect of membrane and foulant hydrophobicity and surface charge on adsorptive fouling during microfiltration

Abstract

This study provides insight into the combined effect of membrane and foulant hydrophobicity and surface charge on adsorptive fouling during microfiltration. A series of polyvinylidene fluoride (PVDF) membranes were used with dextran (DEX), bovine serum albumin (BSA) and Aldrich humic acid (HA) employed as model foulants representative of polysaccharides, proteins and humic substances, respectively. Hydrophobicity and surface charge of the fouling participants (i.e. membranes and foulants) were indicated by water contact angle ( θ) and zeta potential ( ζ), respectively. The water contact angles of the foulants were measured at a buffered pH, with the contact angle disturbance induced by the buffer salt (sodium bicarbonate) eliminated via an extrapolation method. The steric properties of fouling participants were carefully examined ensuring that the inter-membrane or inter-foulant steric disparity as well as the membrane–foulant size exclusion effect was reduced to an acceptable level for insight into the hydrophobic and electrostatic effects. The extent of adsorptive fouling between each membrane–foulant pair was parameterized as adsorption equilibrium constant ( K (0)), which was determined using Thomas’ dynamic adsorption model. To interpret the combined effect of hydrophobicity and surface charge of membrane (m) and foulant (f) on adsorptive fouling, a semi-empirical model, derived from the extended Derjaguin–Landau–Verwey–Overbeek (XDLVO) theory, of the form: ln K (0)/ S c = −0.143 × (cos θ m + cos θ f) − 4.20 × 10 −5 × ζ m ζ f + 0.357 (where S c in nm 2 is the contactable area between membrane and a foulant molecule, and zeta potentials are given in mV) was found to adequately describe the results obtained. Statistical analysis of the results indicates that the contact angle term contributes significantly to this relationship while zeta potential term exhibits marginal impact. This suggests that hydrophobic interaction, rather than electrostatic interaction, may be the predominant mechanism affecting adsorptive fouling.