Effect of solution chemistry on flux decline during high concentration protein ultrafiltration through a hydrophilic membrane
Introduction
Ultrafiltration (UF) is a pressure driven separation process, with membranes having pore sizes ranging from 1 to 100 nm. It is currently applied for the concentration of a wide range of protein products, including recombinant therapeutics, nutraceuticals, industrial enzymes, diagnostic products and a variety of food and beverages product [1], [2]. However, membrane fouling by irreversible adsorption and/or deposition of solutes on and/or within the membrane, is the major drawback in membrane separation processes. In view of the fact that flux decline can severely affect the throughput and commercial feasibility of a manufacturing process, it is crucial to develop techniques to improve flux by understanding the cause of flux decline and elucidating the mechanism of fouling.
In general, the rate and extent of membrane fouling in protein ultrafiltration is affected by four factors, namely, membrane material, solution conditions, operating conditions and protein properties [3], [4], [5], [6], [7], [8]. Particularly, electrokinetic effects such as membrane and solute charge, pH, ionic strength, have been shown to greatly influence the membrane fouling, water flux and solute retention [9], [10], [11]. Conversely, special structures and properties of different types of proteins can make separation process a complicated task. Proteins tend to interact with other components in the feed solution as well as adsorb onto polymeric membrane by the interaction mechanisms such as ionic, entropic, Van der Waals interactions and hydrogen bonding [12]. In most of protein ultrafiltration studies, fouling and adsorption have been found to be strongly dependent on protein–protein and protein–membrane interactions [13]. Protein–protein interactions affect the porosity of the cake layer on the membrane, while protein–membrane interactions affect irreversible adsorption onto the membrane [14]. Both interactions are usually affected by the pH and ionic strength of the feed solution. In fact, protein adsorption is maximum at pH values near the protein isoelectric point (IEP) under static and dynamic filtration mode [13], [14]. At higher pH, where the protein and membrane (for negatively charged membranes) are of same sign, electrostatic repulsion created by the charged molecules and charged surface result in less fouling. Similar observations were reported by several researchers [15], [16], [17] that the protein rejection was highest under conditions where the membrane and protein had like charge due to strong electrostatic repulsion. At pH away from protein's IEP, as higher ionic strength solutions screen the charges in the system. This shielding effect reduces the hydrodynamic diameter of a protein, and also naturally decreases the charge of the membrane, which in turn increases the adsorption at membrane surface [18]. However, when proteins are at their IEP, normally the opposite effect is seen. These effects have been shown by Fane et al. [13], [19].
For food proteins such as gelatin, a number of studies [20], [21], [22], [23] have shown that ultrafiltration (UF) is feasible to concentrate mammalian and fish gelatin solution which contains 4–10% solids in water. However, limited study has been conducted on fouling characteristics and mechanism of gelatin ultrafiltration through manipulation of solution chemistry. Therefore, the objective of this paper is thus to obtain better understanding of the extent of adsorptive and dynamic fouling behavior in ultrafiltration of gelatin solution through the analysis of protein–protein and protein–membrane interactions. For this study, a hydrophilic type membrane, regenerated cellulose acetate, with 30 kDa MWCO was employed with a dead-end UF filtration mode. Typical fouling model centered on pore blocking is used to examine the effects of solution chemistry on fouling mechanisms in gelatin ultrafiltration.
Section snippets
Theory
Usually, flux decline behavior under constant pressure filtration can be analyzed in terms of four classic blocking laws, namely: standard blocking model, intermediate blocking model, cake filtration model and complete blocking model [24]. In a standard model, membranes have straight cylindrical pores that decline in effective radii as solid matter adsorb into the pore walls. When a portion of the membrane pores are unavailable for flow due to blockage of the pore, or solute attach to
Reagents
Hydrochloric acid from System ChemAR® and sodium hydroxide from R&M Chemicals, Essex, United Kingdom were used. Appropriate amount of sodium chloride (NaCl) were dissolved into the feed solution to control the ionic strength of the solution. The pH of solutions was measured with InoLab pH/Cond Level 1 meter (Weilheim, Germany) and pH of the medium was adjusted by adding either HCl or NaOH solutions. All reagents used in the experiments were of analytical grade unless otherwise stated. Water
Gelatin characterization
In designing ultrafiltration process, it is important that the emphasis is on studying all the parameters affecting the membrane performance and achieving the desired level of separation for the application of interest. In protein ultrafiltration, one of the crucial parameters is the protein properties which affect the permeability and the retention of the membrane process. Table 3 shows the main physicochemical properties of gelatin solution used in the experiment. Since the molecular weight
Conclusion
It has been shown that solution pH and ionic strength have significant effect on the extent of protein fouling in membrane processes. With regard to the permeate flux, the maximum flux decline occurred at pH 5.3, i.e., around the isoelectric point of gelatin. This could be due to the development of membrane fouling caused by the formation of dense and compact foulant layer at membrane surface. On the other hand, increasing fluxes were observed at pH away from IEP (pH 4 and pH 6.8) since
Acknowledgements
The authors would like to acknowledge the financial grant funded by Universiti Kebangsaan Malaysia via grant UKM-GUP-KPB-08-32-129 and TF0206A084. The authors would also like to acknowledge Halagel (M) Sdn. Bhd. for donation of the gelatin.
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On study leave from Faculty of Chemical Engineering, Universiti Teknologi MARA, 40450 Shah Alam, Selangor, Malaysia.