Journal of Colloid and Interface Science 299 (2006) 806–814 www.elsevier.com/locate/jcis Effect of ionic environments on the adsorption and diffusion characteristics of serine alkaline protease enzyme in polyethersulfone ultrafiltration membranes Sema Salgın ∗,1 , Serpil Takaç, Tunçer H. Özdamar Department of Chemical Engineering, Faculty of Engineering, Ankara University, 06100 Tando˘ gan, Ankara, Turkey Received 29 November 2005; accepted 10 March 2006 Available online 20 March 2006 Abstract Static adsorption of serine alkaline protease (SAP) enzyme on hydrophobic polyether sulfone (PES) ultrafiltration membranes in different ionic environments was investigated. The amount of SAP adsorbed on membranes was the lowest at its isoelectric point (IEP) where the maximum adsorption was obtained below the IEP of the enzyme. The extent of SAP adsorption in the phosphate buffer solutions including different salts followed the order: (NH 4 ) 2 HPO 4 > KH 2 PO 4 > Na 2 HPO 4 −NaH 2 PO 4 (buffer) > CaCl 2 > ((NH 4 ) 2 HPO 4 + H 2 PO 4 + CaCl 2 ), which was consistent with the Hofmeister series. The zeta potentials of membranes contacted with the ionic species were calculated by streaming potential measurements and found that the increase in ionic strength decreased the electrical double layer thickness leading to a decrease in adsorption. A model based on mass balance was developed to calculate the diffusion coefficient of SAP in PES membranes. Employing experimental data evaluated in a diffusion cell along with the data of adsorption isotherms, diffusion coefficients of SAP in PES membranes in the presence of different ionic species were calculated. To detect the structural changes occurred, membrane surfaces were analysed by Fourier transform infrared- attenuated total reflectance (FTIR-ATR) measurements.  2006 Elsevier Inc. All rights reserved. Keywords: Adsorption; Diffusion coefficient; Protease; Hofmeister series 1. Introduction Membrane separations are used in biotechnological pro- cesses preferably because of their effectiveness in purifying and concentrating biomolecules such as proteins. The crucial prob- lem limiting their practical applications is membrane fouling due to adsorption. The main factors that influence the foul- ing can be classified into two categories: (i) hydrodynamics of the separation process and (ii) characteristics of the mem- brane and solution. To explain the fouling mechanism through protein–membrane interactions, a major group of literatures has been devoted to the interaction of serum proteins with poly- meric membranes. Many parameters including pH, type of salts * Corresponding author. Fax. +90 346 219 11 65. E-mail address: ssalgin@cumhuriyet.edu.tr (S. Salgın). 1 Present address: Department of Chemical Engineering, Faculty of Engi- neering, Cumhuriyet University, 58140 Sivas, Turkey. and buffers, and protein concentration are taken into account in these studies [1–5]. Almost all of these investigations explain membrane fouling by means of electrokinetic measurements, colloidal theory, and the changes occurred in the membrane and protein structures in various ionic environments. Although the literature has a consensus that the ionic environment strongly affects the extent of adsorption, the results reported on the ef- fects of pH and ionic strength on protein–membrane interac- tions are not fully converged. Ions, mainly originated in the buffer and salts of the protein production media, create an ionic environment for biomole- cules and membranes; and have strong impacts on protein– membrane interactions. In addition to the solution pH, i.e., the H + concentration, which alters the electrostatic charge of the proteins and membranes, the type and concentration of other ions of the solution affect the adsorption through complex in- terfacial interactions. Although several researchers report the effects of pH and ionic strengths on the electrokinetic prop- 0021-9797/$ – see front matter  2006 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2006.03.020