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Fouling of unmodified and modified polysulfone ultrafiltration membranes by ovalbumin
Journal of Membrane
Science,
Elsevier Science Publishers
44 (1989) 183-196 B.V., Amsterdam - Printed
183 in The Netherlands
FOULING OF UNMODIFIED AND MODIFIED POLYSULFONE ULTRAFILTRATION MEMBRANES BY OVALBUMIN
MARIANNE
NYSTROM
Laboratory of Physical Chemistry, Department of Chemical Technology, Lappeenranta University of Technology, P.O. Box 20, SF-53851, Lappeenranta (Finland)
(Received April 26,1988; accepted in revised form September
13,1988)
Summary Dilute ovalbumin solutions were ultrafiltered with unmodified and modified polysulfone membranes. With unmodified membranes, flux reduction was greatest at the isoelectric point (IP) of the protein and decreased on both the acidic and the basic side of the IP. Modification of the membrane with the polyelectrolyte polyethylenimine decreased flux reduction when coulombic repulsion between the protein molecules and the membrane surface was achieved. High ionic strength enhanced flux, as it increased repulsion by increasing the electric charge of the protein molecule or by lowering the electroviscous effects in the pores. However, the hydrophilicity of the membrane seemed to be a more important factor in flux reduction than its actual charge.
Introduction Ultrafiltration (UF) of protein-containing solutions is severely limited by the fouling of the membrane surface and its pores, resulting in a decrease of permeate flux. This phenomenon is especially prominent with hydrophobic polysulfone membranes, and it has been shown to be directly related to the amount of protein adsorbed [ 1,2]. Consequently, the membrane performance should depend strongly on properties of the feed solution, such as the nature and the concentration of the protein, pH, ionic strength and the character of the small amounts of impurities present, and especially also on the properties of the membrane. The last-named properties can in principle be altered by modifying the surface of the membrane. An important objective of this study is to investigate the effect on protein fouling of enhanced electrostatic repulsion or attraction between the membrane surface and the macromolecules in solution. Protein adsorption from solutions on solid surfaces is a complex phenomenon comprising coulombic and non-coulombic interactions and possible conformational changes [ 31. Quantitatively, adsorption can be analyzed with several techniques, including radioactive labelling [ 11, ellipsometry [ 4,5 1, micro-
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B.V.
184
weighing [6] and certain spectroscopic methods [5,7]. In addition, the effects of the adsorbed species on the electrostatic properties of the surface can be characterized by electrokinetic measurements [ 8,9]. In the present study the UF data obtained with dilute ovalbumin (OA) solutions by using unmodified and modified polysulfone (PSu) membranes are correlated with the results of streaming potential (SP) experiments. The UF and SP experiments were carried out with commercial membranes. Experimental Material Membranes
UF membranes (type GR 61 PP and GS 61 PP, nominal molecular weight cut-off 20,000) received from De Danske Sukkerfabrikker (DDS) were washed extensively before use with purified water in order to remove preservatives. The polysulfone membrane (GR) carries a small negative charge, the nominal value of which increases almost linearly with pH at values above 4 [9]. According to streaming potential measurements carried out in our laboratory, the charge is zero or slightly positive at lower pH values. The sulfonated polysulfone membrane (GS) carries a stronger, almost constant negative charge in the pH range 3-7, which results from the anionic character of the membrane material. Ovalbumin and polyethylenimine
Crystallized and lyophilized ovalbumin supplied by Sigma Chemicals (Cat. No. A-5503 ) and polyethylenimine (PEI) 50% solution (MW = 50,000-60,000) supplied by Aldrich Chemie (Cat. No. 18,197-B) were used. Before use, the PEI was dialyzed in order to remove low-molecular-weight fractions and the OA solutions were filtered in order to remove precipitates. All solutions used in the experiments were prepared in ion-exchanged, distilled and prefiltered (UF membrane, cut-off 1500) water, and all the electrolytes used were of p.a. grade. Ovalbumin is a globular protein with a molar mass of 45,000 g/mol. It consists of about 300 amino acid residues and is ampholytic in nature. It is supposed to contain 36 titratable basic and 33 titratable acidic groups per molecule [lo]. Most of the basic amine groups result from lysine and arginine and the carboxylic acid groups from glutamic and aspartic acids. The isoelectric point, IP, of OA is 4.58 and the isoionic point is 4.9. The Stokes radius of the molecule is 2.8 nm [ 111. The diffusion coefficient of OA in water at 25 “C, D, was calculated from molar frictional ratio data [ 121 to be 7.5 x 10-l’ m2/sec. Kinematic viscosities of 1% wt. OA solutions measured at three pH values in three concentrations of KC1 are given in Table la.
185 TABLE I (a) Kinematic viscosities, v, of 1% wt. ovalbumin solutions at different pH and KC1 concentrations (t=25”C) PH
C (KCl) (mol-dm-3)
10.2 7.6 5.3
0.15 0.15
10.3 7.7 5.3
10.1 7.4 5.5
(b) Viscosities, r],of 2.5% wt. PEI solutions at different pH and KC1 concentrations (t=25”C) PH
C (KCl) (mol-dm-3)
1 (CP)
0.9110 0.9100 0.9000
10.9
0.15 0.15 0.15
1.692 2.053 2.100
0.015 0.015 0.015
0.9243 0.9228
10.5
0.9195
5.2 4.0
0.015 0.015 0.015
1.733 2.673 2.735
0.0015 0.0015 0.0015
0.9394
10.7
0.9299 0.9205
5.2 4.0
0.0015 0.0015 0.0015
2.855 2.939
0.15
YcSt)
5.1 4.1
1.791
Polyethylenimine used for modification of the membranes in this study is a cationic polyelectrolyte with its point of zero charge, pzc, =10.8 [13]. The alkaline demand in titration of PEI from pH 3 to pH 11 is measured to be 18.3 mmol KOH/g PEI. At high ionic strengths PEI is slightly more dissociated than in dilute electrolyte solutions. The PEI molecule has a slight tendency to expand in acidic solutions, especially at low ionic strengths. Viscosities of 2.5% wt. PEI solutions at some pH values and in different concentrations of KC1 were measured and the results are given in Table lb. The tabulated viscosities for PEI and OA show that viscosity increases with increasing charge of the molecule, i.e., when moving away from its IP or pzc. The viscosity decreases with increasing electrolyte concentration. The charge effect is greater for PEI than for OA, and it indicates a higher charge density and a less structured form of the PEI molecule. Methods Potentiometric titration of OA In order to study how the ampholytic nature of OA varied with ionic strength, potentiometric titrations of OA with both HCl and KOH were carried out. Thus, 0.5 g of OA was dissolved in 300 ml of KCl+HCl solution (pH 3) of a specific ionic strength. The solution was titrated to pH 3 with 0.1 M HCl and subsequently to pH 11 with 0.1 M KOH. The buffering capacity of OA, at different pH values and ionic strengths, was determined as the difference in alkali consumption between the sample solution and a blank solution without OA.
186
Modification
with PEI
In some experiments the surfaces of the membranes were modified by exposing them for 30 min to dilute solutions of PEI at pH 10. No pressure was applied, in order to minimize intrusion of PEI into the pores. After each PEI treatment the membranes were thoroughly washed with water in order to remove non-adsorbed polyelectrolyte. Flux declines, with respect to pure water flux, caused by pH and electrolyte are tabulated in Table 2 for the “pure” membrane, for the membrane modified with PEI at pH 10 and for the calculated pure PEI layer. Streaming potential experiments
The streaming potentials of the membranes at three different pH values (4, 5 and 6) and at increasing levels of OA adsorption were measured at 25°C in a cell provided with Ag/AgCl electrodes and described in detail elsewhere [ 91. The streaming potential, i.e., the potential difference between the electrodes on both sides of the membrane, AE, was measured, along with the pressure difference across the membrane, AP. The relationship AE vs. AP is linear in the pressure range under study (AP=O-70 mbar) and the parameter AE/AP indicates the amount of electric charge on the pore walls. The streaming potential measurements were carried out with 0.5 mM KC1 solution. For each piece of membrane, a reference value (AE/AP), was first measured in pure electrolyte at one of the specified pH values (4, 5 or 6) and at pH 6.7. Then the cell was filled with a 0.15 M KC1 solution adjusted to the same specified pH and the membrane was fouled at the lowest concentration of OA under study. The fouling was carried out by slow circulation of the protein solution in the cell for 2 hours. The solution was then replaced with the 0.5 mM KC1 solution, and AE/AP for the fouled membrane was measured both at the pH of interest and at pH 6.7. This procedure was repeated by increasing stepwise the OA concentration over the fouling period. At the end of each series of experiments the membrane was removed, and similar experiments were carried out TABLE 2 Flux decline (%) with respect to pure water flux caused by pH and electrolyte concentration: (a) for the pure polysulfone GR 61 membrane; (b) for the membrane after treatment with 1 ppm PEI solution at pH 10; (c) for the PEI layer calculated as the difference between (b) and (a) C (KCl) (mol-dm-3)
0.15 0.015
(c ) Calculated PEI layer at pH
(a) Pure GR 61 membrane at pH
(b) Membrane modified with PEI at pH
3.5
4.0
3.5
4.0
16.8 11.0
11.7 6.6
20.7 13.6
32.1 36.1
.
3.5
4.0
3.9 2.6
20.4 29.5
187
with a fresh piece of membrane, which was fouled at another specified pH value. The measured streaming potentials were used to calculate the respective zeta potentials from eqn. (1) , which is valid for cylindrical capillaries [ 81: +g 0 r
(1)
In this equation q is the viscosity of the solution, K the conductivity of the bulk solution, e. the permittivity of vacuum and E, the dielectric constant of the solvent. In order to compare the results from OA application experiments at different pH values a relative value [/co (co calculated from (dE/dP), using eqn. 1) was calculated from the measurements at pH 6.7. As at this pH the zeta potential of the pure membrane is the same in all experiments, the change in [must arise from the adsorbed ovalbumin layer. UF experiments The ultrafiltration experiments were carried out at 25°C in a plate module with an effective membrane area of 19.3 cm2. The module is described in detail elsewhere [ 141. The feed solution was circulated through the slit-shaped channel with a constant velocity, which corresponds to a Reynolds number, Re, of 5300. The pressure across the membrane, AP, was 1.5 bar in all experiments. The permeate was recirculated after weighing. Before each series of experiments with the OA solutions, the membranes were stabilized for 2.5 hours by circulating pure water at AP= 1.5 or 3 bar and then a KC1 solution of a specified ionic strength at AP= 1.5 bar. During each experiment the pH was kept at a constant specific value. When a stable permeation rate with the saline solution was attained, the first OA concentration in each experiment was set by injecting an aliquot of a stock solution into the circulation system. The protein was allowed to adsorb on to the membrane and the permeate mass flux, J,, was measured. The OA concentration was adjusted to a new value and the corresponding flux was measured. This procedure was repeated stepwise until the desired OA concentration range was covered. Finally, the system was rinsed and the membrane was removed. A new membrane was installed and the whole series of experiments was repeated at another fixed PH. The flux decline due to protein adsorption was expressed as relative flux reduction, S, defined by eqn. (2): S=l-J,/Jo,
(2)
where J”, is the mass flux measured with pure KC1 solution at the same pH after modification, if any. In UF, the adsorption equilibrium is determined by the solute concentration
188
prevailing at the wall of the membrane, C,, rather than by the bulk concentration, C,,. Consequently, the concentration polarization in the diffuse layer must be taken into account. According to Jonsson, the wall concentration of OA for the same type of UF module can be calculated from eqns. (3 ) and (4) [151. (3)
WG=exp(JJk) h~0.023
D
ReO.’ SCO.~~-
dh
(4)
In these equations J, is the volume flux, SC is the Schmidt number and dh ( = 1.9 mm) the effective diameter of the flow channel. The expression for the mass transfer coefficient k is valid in the turbulent flow region [ 151 and its value under the prevailing experimental conditions was calculated to be 1.9x low5 m/set. k can be considered constant because the viscosity and the density of the dilute OA solutions can be replaced by the corresponding values for water. Therefore the value of C&/C,, varied only with J, and it was usually smaller than 2.0. However, eqn. (3) can be applied only if the protein is completely rejected by the membrane. In this study this condition was fulfilled, as verified with absorbance measurements of the permeate at 3,= 280 nm. Results and discussion
Potentiometric
titration of OA
The H+/OHconsumptions vs. pH for OA at four different ionic strengths were calculated as described above. The results are depicted in Fig. 1.As expected, at high ionic strengths the OA is charged to a relatively higher degree. As titrations were made at several ionic strengths, a large jump could be detected between the titration curves recorded at 0.015 M and 0.15M KCl. This can be interpreted as meaning that, for the charge to be fully developed, the ionic strength must be > 0.015 M KCl. Streaming potential
results
The streaming potential experiments were carried out at three different pH values (4,5 and 6) and the results are shown in Fig. 2 (a). It can be seen that the value of the zeta potential increases at pH 4 and 5 with increasing OA concentration. This is due to adsorption of OA. The OA layer on the membrane seems to form a secondary porous layer, thus the streaming potential is formed partly across the “semipores” of the OA layer and partly across the pores of the membrane. The thicker the adsorbed layer, the more is the streaming potential value determined by the charge of OA and not by the charge of the membrane. As the OA is positively charged at pH 4, the zeta potential will become positive at some OA concentrations. At pH 5 the OA is so slightly negatively charged that the negative charge of the membrane seems to be “di-
189
Us1 :
2 0.4..
KCI cont./M
3
Fig. 1. Buffer capacity IP = isoelectric point.
4
5
of ovalbumin
6
at different
1
0-o
0.3
u-u
0.1s
8
9
KC1 concentrations
10
11
as a function
P”
of pH.
minished” by the OA adsorption. At pH 6 adsorption is weak, as the negative charge of OA is strongly developed as indicated above, and the charge densities are about the same for the membrane and OA. Consequently, no difference can be seen between low and high OA concentrations. In Fig. 2 (b) the reference measurements at pH 6.7 are given. The results more or less verify the results at the respective pH values, but seem to indicate that OA is slightly less charged than the membrane at this pH and that some adsorption also takes place. Ultrafiltration results
In the UF experiments with OA, the aim was to study how much the electrostatic repulsion on one hand and the hydrophilicity on the other hand affect flux reduction. The results of the UF experiments with OA carried out with unmodified membranes are given in Fig. 3. In this figure, flux reduction at different pH values is plotted as a function of OA wall concentration. As expected, flux reduction is greatest at pH 4.5, i.e., near the isoelectric point of OA, where the net charge of the.protein molecule is almost zero. Correspondingly, flux reduction is rather low at pH 8, where both the protein molecules and the membrane surface carry a negative charge and the electrostatic repulsion opposes fouling. The repulsive force becomes smaller as pH approaches the IP of the protein and the pzc of the membrane. This has also been verified
pH4 .*W*-
1
2
3
4 5 COA [kg/m’]
b 1
2
3
4
5 Co,
[kg/m’]
Fig. 2. Zeta potentials calculated from streaming potential measurements at different pH for the polysulfone GR 61 membrane. (a) Zeta potential as a function of applied OA concentration. Measurements made at the pH of OA application. (b) Reduced zeta potential, c/c,,, as a function of applied OA concentration. Measurements made at the reference pH, 6.7; OA application at the pH indicated in the figure.
in the ultrafiltration of amino acids with charged membranes by Kimura and Tamano [ 161.At pH values lower than the IP of OA, flux reduction diminishes again, accompanied by an increasing positive net charge of the protein. This is normally the case for protein adsorption at surfaces [ 1,3,17]. If the membrane is not positively charged in this pH region, which seems to be the case with the GR 61 membrane, the result cannot be explained by electrostatic repulsion alone. When the protein is charged the solubility, i.e., the relative stability of the protein in water, increases [ 201 and the affinity for the membrane material decreases. At pH 3-4.5 the polysulfone membrane was detected [9] to have a very slight positive charge or no charge at all (calculated zeta potential z 0.5 mV at pH 4). In order to increase the positive charge density of the membrane and to induce stronger repulsion, the membrane was modified with PEI, which has a
191
PH8 ’ o-
__
” .1
0.2
_
0.3
0.4
0.5
0.6
0.7
0.8
“’ Cw [kg/m’]
Fig. 3. Flux reduction in ultrafiltration of ovalbumin with unmodified GR 61 membranes as a function of wall concentration, C!,, of OA at different pH in 0.15 M KCl. dP= 1.5 bar, t=25”C. (OA stock solutions were made in water without pH control. Membrane stabilization pressure= 1.5 bar.)
high positive net charge in this pH range (calculated zeta potential at pH 4 z + 6 mV [ 211). According to the potentiometric titration experiments of PEI carried out in our laboratory, the titration curve is almost completed at pH 4, which means that the zeta potential at pH 4 almost corresponds to a fully developed charge of the membrane modified with PEI. A series of modification experiments with increasing PEI concentrations was carried out and the results are depicted in Fig. 4. At high PEI concentration the pores were probably blocked with excess PEI molecules, which resulted in an even higher flux reduction than without PEI. The best result was obtained at a PEI concentration of 1 ppm. It can also be seen from Fig. 4 that the positive effect of PEI is greatest at low OA concentrations. A strong flux reduction effect of PEI was also noticed when flux of pure water through the unmodified membrane and flux of saline solution through the modified membrane were compared (see Table 2 ) . This effect was even stronger at higher modifying concentrations of PEI, and resulted in a minimal
0.6
PEI
0.1
0.2
0.3
0.4
cont.
Q-0 *-+
1
pm ”
*-P
5
”
t-+
10
”
0.5
0.6
o.7
p
rL
Fig. 4. Ultrafiltration of ovalbumin with polysulfone GR 61 membranes at different modification concentrations of PEI. Flux reduction as a function of OA wall concentration, C,, at pH 4 in 0.015 M KCl. (OA stock solution prepared in water without pH control. Membrane stabilization pressure = 3 bar.)
net permeate flux, even lower than the permeate flux obtained without modification. Using a concentration of 1 ppm in PEI modifications, experiments were run with different concentrations of KCl. It was found that flux reduction was greatest at the lowest ionic strength (Fig. 5). This result is in accordance with the results obtained by Matthiasson [ 1 ] in UF of bovine serum albumin with GR 61 membranes without PEI, and also with the adsorption studies by Turker and Hubble [ 181 with polysulfone membranes at pH 7.5. This effect might be due to the lower degree of dissociation of OA at low ionic strength, as verified by the titration results above. Also OA, like most proteins, seems to be less stable at low ionic strengths, so that adsorption followed by flux reduction can result. The theoretical influence of electrolyte on adsorption is contradictory. On one hand, a high electrolyte concentration increases and stabilizes the charges of OA and PEI, and thus repulsion diminishes adsorption, but on the other
lY3
KCI cont. / M e-0 *-r(r l
0.1
0.2
0.3
0.4
-a
0.5
0.15 0.015 0.0015
0.6
'.'
C,[kg/m”]
Fig. 5. Ultrafiltration of ovalbumin with polysulfone GR 61 membranes at different ionic strengths. PEI modification concentration = 1 ppm. Flux reduction as a function of wall concentration, C,, at pH=4. (OA stock solution prepared in water without pH control. Membrane stabilization pressure = 3 bar. )
hand the electrolyte can have a shielding influence. The results seem to indicate that the effect of the stabilized charge is important, although the repulsive force of OA is fairly small at pH 4, as its positive charge is only partially developed, as indicated by the titration experiments depicted in Fig. 1. The electroviscous effect of the fouling OA layer leads one to predict stronger flux reduction at low ionic strengths, which is in accordance with the results of Fig. 5 and Table 1. When OA was dissolved in pure water, the pH of the resulting stock solution, appeared to be higher than the IP of OA. It was assumed that, during pH adjustment of the stock solution, OA could possibly adhere irreversibly to the membrane as its IP was passed. In order to avoid this effect, OA was in some experiments dissolved at pH 3 during preparation of the stock solution. Subsequent UF experiments were run at pH 3.5 at two KC1 concentrations, with and without PEI. The effect of PEI is remarkable also at this pH, as indicated in Fig. 6; the effect can result from the positive effect of electrostatic repulsion on UF results, and is greater at low ionic strengths. The strong effect of elec-
_ "'
CW [ kg/m’]
Fig. 6. Flux reduction as a function of wall concentration, C,, in ultrafiltration of ovalbumin with polysulfonated GR 61 membranes with and without modification with PEI (1 ppm at pH 10) at two different ionic strengths at pH = 3.5. (OA stock solution prepared in water at pH 3. Membrane stabilization pressure = 3 bar. )
trostatic repulsion has also been observed with other process solutions [ 16,191. At pH 3.5, the charge of OA in 0.015 M KC1 solution is approximately the same as at pH 4.0 in 0.15 M KC1 solution. Comprehensive studies of the flux reduction results with PEI show that the curves are rather similar. At pH 3.5 in 0.15 M KC1 solution, the positive net charge on OA molecules should be about twice that at pH 4.0. This would predict a strong repulsion. Figure 6 shows that this was not the case. According to Fane et al. [ 171, this is in accordance with experiments with BSA on a polysulfone membrane, where ions lowered the flux at higher charges of the protein. This effect can possibly be explained by the assumption that the positive PEI-ovalbumin layer on the membrane acts in a somewhat similar way as a charged layer, which excludes positive ions and therefore increases flux reduction, especially at high ionic strengths. Under the same UF conditions as above, a sulfonated polysulfone membrane (GS 61) , which is negatively charged at pH 3.5 [ 91, was also studied. As the charges of the membrane and the protein have opposite signs, severe fouling might be expected to result if only repulsive electrostatic forces influence the result. The results shown in Fig. 6 turned out to be almost as good as those with PEI. It may therefore be concluded that the charge on the protein and the
195
hydrophilicity, rather than the charge of the membrane, determine the degree of adsorption. This can be illustrated by a model in which the protein molecules adhere to the membrane and form a modifying layer of their own, and the membrane then repels continued adsorption of similar molecules. This suggested phenomenon also explains why the electrostatic repulsion of the strongly charged PEI is more effective at small concentrations of OA when OA adsorption has not yet taken place. Under such conditions, the energy of electrostatic repulsion most probably is not strong enough to prevent irreversible bond formation between the membrane surface and the adhering OA molecules. The hydrophilicity of the membrane is probably more important than its electrostatic repulsive strength, as the hydrophilic environment prevents irreversible adsorption of the protein. Conclusions
From the results given above, it can be seen that the flux in ultrafiltration is very much determined by the behavior of the membrane used and the properties of the material to be retained from solution at different pH values. If the component to be ultrafiltered is hydrophilic, it is more stable in a solution of high ionic strength. At the same time, fouling of the membrane is also counteracted when a hydrophilic membrane is used. The optimum pH with respect to flux reduction is one where the component in the solution is highly charged, and preferably with the same sign of charge as the membrane. If the chargecarrying component adheres to the membrane, it can counteract the lack of charge or the opposite charge of the membrane by forming a kind of modification layer of its own. This leads to the adsorbed layer repelling further adsorption. If adsorption does not take place, and the charge of the membrane is not such that it repels the component to be ultrafiltered, the charge of the membrane can be changed by proper modification of the membrane. Increase in ionic strength also seems to improve the ultrafiltration flux by increasing and stabilizing the charge of the protein and so enhancing electrostatic repulsion. When the modification or adsorption layer is strongly charged, the electroviscous and ion exclusion effects have to be taken into account. Finally, one can state that in ultrafiltration of OA the ionic character of the protein molecule is more important for the result than the charge of the membrane. Acknowledgements
The author is indebted to Nordisk Industrifond, The Finnish Academy of Sciences and Maj and Tor Nessling Foundation for financial support. The author also wishes to thank Lit. Techn. Markku Laatikainen for his theoretical help and inspired discussions and Miss Tuija Silonsaari for her assistance.
196
References 1
2 3 4
5
6 7 8 9 10 11 12 13
14 15 16
17 18 19 20 21
E. Matthiasson, Macromolecular adsorption and fouling in ultrafiltration and their relationships to concentration polarization, Ph.D. Thesis, Lund University, 1984. P. Aimar, Mecanismes de transfert de matiere en ultrafiltration, Ph.D. Thesis, Universite Paul Sabatier, 1987. W. Norde, Adsorption of proteins from solution at the solid-liquid interface, Adv. Colloid Interface Sci., 25 (1986) 267. I. Lundstrom, Protein conformation at surfaces, in: P.-O. Glantz, S.A. Leach and T. Ericson (Eds.), Oral Interfacial Reactions of Bone, Soft Tissue and Saliva, IRL Press, Oxford, 1985, p. 9. C.-G. Golander, H. Arwin, J.C. Eriksson, I. Lundstrom and R. Larsson, Heparin surface film formation through adsorption of colloidal particles studied by ellipsometry and scanning electron microscopy, Colloids Surf., 5 (1982) 1. M. Laatikainen and M. Lindstrijm, Determination of adsorption isotherms with quartz crystal microbalance in liquid phase, J. Colloid Interface Sci., 125 (1988) 610. R.W. Paynter, B.D. Ratner, T.A. Horbett and H.R. Thomas, XPS studies on the organization of adsorbed protein films on fluoropolymers, J. Colloid Interface Sci., 101 (1984) 233. R.J. Hunter, Zeta Potential in Colloid Science, Principles and Applications, Academic Press, London, 1981. M. Lindstriim and M. Nystrom, Streaming potential of UF membranes at different pH, Finn. Chem. Lett., 14 (1987) 123. A. White, P. Handler and E.L. Smith, Principles of Biochemistry, McGraw-Hill, New York, NY, third edn., 1964, p. 123. J.A. Howell, 0. Velicangil, MS. Le and A.L. Herrera Zeppelin, Ultrafiltration of protein solutions, Ann. N.Y. Acad. Sci., 369 (1981) 355. J.S. Fruton and S. Simmonds, General Biochemistry, Wiley, New York, NY, 1958, p. 150. G.M. Lindquist and R.A. Stratton, The role of polyelectrolyte charge density and molecular weight on the adsorption and flocculation of colloidal silica with polyethylenimine, J. Colloid Interface Sci., 55 (1976) 45. M. Lindstriim, M. Nystrijm and M. Laatikainen, Interactions between chlorolignin and polysulfone ultrafiltration membranes, Sep. Sci. Technol., 23 (1988) 703. G. Jonsson, Boundary layer phenomena during ultrafiltration of dextran and whey protein solutions, Desalination, 51 (1984) 61. S. Kimura and A. Tamano, Separation of amino acids by charged ultrafiltration membranes, in E. Drioli and M. Nakagaki (Eds.), Membranes and Membrane Processes, Proc. Eur.-Jpn. Congr. Membr. Membr. Processes, Plenum, New York, NY, 1986, p. 191. A.G. Fane, C.J.D. Fell and A. Suki, The effect of pH and ionic environment on the ultrafiltration of protein solutions with retentive membranes, J. Membrane Sci., 16 (1983) 195. M. Turker and J. Hubble, Membrane fouling in a constant-flux ultrafiltration cell, J. Membrane Sci., 34 (1987) 267. A. Bindoff, C.J. Davies, C.A. Kerr and CA. Buckley, The nanofiltration and reuse of effluent from the caustic extraction stage of wood pulping, Desalination, 67 (1987) 455. L. Zeman, Adsorption effects in rejection of macromolecules by ultrafiltration membranes, J. Membrane Sci., 15 (1983) 213. M. Nystrom, M. Lindstriim and E. Matthiasson, Streaming potential as a tool in the characterization of ultrafiltration membranes, Colloids Surf., accepted.
Science,
Elsevier Science Publishers
44 (1989) 183-196 B.V., Amsterdam - Printed
183 in The Netherlands
FOULING OF UNMODIFIED AND MODIFIED POLYSULFONE ULTRAFILTRATION MEMBRANES BY OVALBUMIN
MARIANNE
NYSTROM
Laboratory of Physical Chemistry, Department of Chemical Technology, Lappeenranta University of Technology, P.O. Box 20, SF-53851, Lappeenranta (Finland)
(Received April 26,1988; accepted in revised form September
13,1988)
Summary Dilute ovalbumin solutions were ultrafiltered with unmodified and modified polysulfone membranes. With unmodified membranes, flux reduction was greatest at the isoelectric point (IP) of the protein and decreased on both the acidic and the basic side of the IP. Modification of the membrane with the polyelectrolyte polyethylenimine decreased flux reduction when coulombic repulsion between the protein molecules and the membrane surface was achieved. High ionic strength enhanced flux, as it increased repulsion by increasing the electric charge of the protein molecule or by lowering the electroviscous effects in the pores. However, the hydrophilicity of the membrane seemed to be a more important factor in flux reduction than its actual charge.
Introduction Ultrafiltration (UF) of protein-containing solutions is severely limited by the fouling of the membrane surface and its pores, resulting in a decrease of permeate flux. This phenomenon is especially prominent with hydrophobic polysulfone membranes, and it has been shown to be directly related to the amount of protein adsorbed [ 1,2]. Consequently, the membrane performance should depend strongly on properties of the feed solution, such as the nature and the concentration of the protein, pH, ionic strength and the character of the small amounts of impurities present, and especially also on the properties of the membrane. The last-named properties can in principle be altered by modifying the surface of the membrane. An important objective of this study is to investigate the effect on protein fouling of enhanced electrostatic repulsion or attraction between the membrane surface and the macromolecules in solution. Protein adsorption from solutions on solid surfaces is a complex phenomenon comprising coulombic and non-coulombic interactions and possible conformational changes [ 31. Quantitatively, adsorption can be analyzed with several techniques, including radioactive labelling [ 11, ellipsometry [ 4,5 1, micro-
0376-7388/89/$03.50
0 1989 Elsevier Science Publishers
B.V.
184
weighing [6] and certain spectroscopic methods [5,7]. In addition, the effects of the adsorbed species on the electrostatic properties of the surface can be characterized by electrokinetic measurements [ 8,9]. In the present study the UF data obtained with dilute ovalbumin (OA) solutions by using unmodified and modified polysulfone (PSu) membranes are correlated with the results of streaming potential (SP) experiments. The UF and SP experiments were carried out with commercial membranes. Experimental Material Membranes
UF membranes (type GR 61 PP and GS 61 PP, nominal molecular weight cut-off 20,000) received from De Danske Sukkerfabrikker (DDS) were washed extensively before use with purified water in order to remove preservatives. The polysulfone membrane (GR) carries a small negative charge, the nominal value of which increases almost linearly with pH at values above 4 [9]. According to streaming potential measurements carried out in our laboratory, the charge is zero or slightly positive at lower pH values. The sulfonated polysulfone membrane (GS) carries a stronger, almost constant negative charge in the pH range 3-7, which results from the anionic character of the membrane material. Ovalbumin and polyethylenimine
Crystallized and lyophilized ovalbumin supplied by Sigma Chemicals (Cat. No. A-5503 ) and polyethylenimine (PEI) 50% solution (MW = 50,000-60,000) supplied by Aldrich Chemie (Cat. No. 18,197-B) were used. Before use, the PEI was dialyzed in order to remove low-molecular-weight fractions and the OA solutions were filtered in order to remove precipitates. All solutions used in the experiments were prepared in ion-exchanged, distilled and prefiltered (UF membrane, cut-off 1500) water, and all the electrolytes used were of p.a. grade. Ovalbumin is a globular protein with a molar mass of 45,000 g/mol. It consists of about 300 amino acid residues and is ampholytic in nature. It is supposed to contain 36 titratable basic and 33 titratable acidic groups per molecule [lo]. Most of the basic amine groups result from lysine and arginine and the carboxylic acid groups from glutamic and aspartic acids. The isoelectric point, IP, of OA is 4.58 and the isoionic point is 4.9. The Stokes radius of the molecule is 2.8 nm [ 111. The diffusion coefficient of OA in water at 25 “C, D, was calculated from molar frictional ratio data [ 121 to be 7.5 x 10-l’ m2/sec. Kinematic viscosities of 1% wt. OA solutions measured at three pH values in three concentrations of KC1 are given in Table la.
185 TABLE I (a) Kinematic viscosities, v, of 1% wt. ovalbumin solutions at different pH and KC1 concentrations (t=25”C) PH
C (KCl) (mol-dm-3)
10.2 7.6 5.3
0.15 0.15
10.3 7.7 5.3
10.1 7.4 5.5
(b) Viscosities, r],of 2.5% wt. PEI solutions at different pH and KC1 concentrations (t=25”C) PH
C (KCl) (mol-dm-3)
1 (CP)
0.9110 0.9100 0.9000
10.9
0.15 0.15 0.15
1.692 2.053 2.100
0.015 0.015 0.015
0.9243 0.9228
10.5
0.9195
5.2 4.0
0.015 0.015 0.015
1.733 2.673 2.735
0.0015 0.0015 0.0015
0.9394
10.7
0.9299 0.9205
5.2 4.0
0.0015 0.0015 0.0015
2.855 2.939
0.15
YcSt)
5.1 4.1
1.791
Polyethylenimine used for modification of the membranes in this study is a cationic polyelectrolyte with its point of zero charge, pzc, =10.8 [13]. The alkaline demand in titration of PEI from pH 3 to pH 11 is measured to be 18.3 mmol KOH/g PEI. At high ionic strengths PEI is slightly more dissociated than in dilute electrolyte solutions. The PEI molecule has a slight tendency to expand in acidic solutions, especially at low ionic strengths. Viscosities of 2.5% wt. PEI solutions at some pH values and in different concentrations of KC1 were measured and the results are given in Table lb. The tabulated viscosities for PEI and OA show that viscosity increases with increasing charge of the molecule, i.e., when moving away from its IP or pzc. The viscosity decreases with increasing electrolyte concentration. The charge effect is greater for PEI than for OA, and it indicates a higher charge density and a less structured form of the PEI molecule. Methods Potentiometric titration of OA In order to study how the ampholytic nature of OA varied with ionic strength, potentiometric titrations of OA with both HCl and KOH were carried out. Thus, 0.5 g of OA was dissolved in 300 ml of KCl+HCl solution (pH 3) of a specific ionic strength. The solution was titrated to pH 3 with 0.1 M HCl and subsequently to pH 11 with 0.1 M KOH. The buffering capacity of OA, at different pH values and ionic strengths, was determined as the difference in alkali consumption between the sample solution and a blank solution without OA.
186
Modification
with PEI
In some experiments the surfaces of the membranes were modified by exposing them for 30 min to dilute solutions of PEI at pH 10. No pressure was applied, in order to minimize intrusion of PEI into the pores. After each PEI treatment the membranes were thoroughly washed with water in order to remove non-adsorbed polyelectrolyte. Flux declines, with respect to pure water flux, caused by pH and electrolyte are tabulated in Table 2 for the “pure” membrane, for the membrane modified with PEI at pH 10 and for the calculated pure PEI layer. Streaming potential experiments
The streaming potentials of the membranes at three different pH values (4, 5 and 6) and at increasing levels of OA adsorption were measured at 25°C in a cell provided with Ag/AgCl electrodes and described in detail elsewhere [ 91. The streaming potential, i.e., the potential difference between the electrodes on both sides of the membrane, AE, was measured, along with the pressure difference across the membrane, AP. The relationship AE vs. AP is linear in the pressure range under study (AP=O-70 mbar) and the parameter AE/AP indicates the amount of electric charge on the pore walls. The streaming potential measurements were carried out with 0.5 mM KC1 solution. For each piece of membrane, a reference value (AE/AP), was first measured in pure electrolyte at one of the specified pH values (4, 5 or 6) and at pH 6.7. Then the cell was filled with a 0.15 M KC1 solution adjusted to the same specified pH and the membrane was fouled at the lowest concentration of OA under study. The fouling was carried out by slow circulation of the protein solution in the cell for 2 hours. The solution was then replaced with the 0.5 mM KC1 solution, and AE/AP for the fouled membrane was measured both at the pH of interest and at pH 6.7. This procedure was repeated by increasing stepwise the OA concentration over the fouling period. At the end of each series of experiments the membrane was removed, and similar experiments were carried out TABLE 2 Flux decline (%) with respect to pure water flux caused by pH and electrolyte concentration: (a) for the pure polysulfone GR 61 membrane; (b) for the membrane after treatment with 1 ppm PEI solution at pH 10; (c) for the PEI layer calculated as the difference between (b) and (a) C (KCl) (mol-dm-3)
0.15 0.015
(c ) Calculated PEI layer at pH
(a) Pure GR 61 membrane at pH
(b) Membrane modified with PEI at pH
3.5
4.0
3.5
4.0
16.8 11.0
11.7 6.6
20.7 13.6
32.1 36.1
.
3.5
4.0
3.9 2.6
20.4 29.5
187
with a fresh piece of membrane, which was fouled at another specified pH value. The measured streaming potentials were used to calculate the respective zeta potentials from eqn. (1) , which is valid for cylindrical capillaries [ 81: +g 0 r
(1)
In this equation q is the viscosity of the solution, K the conductivity of the bulk solution, e. the permittivity of vacuum and E, the dielectric constant of the solvent. In order to compare the results from OA application experiments at different pH values a relative value [/co (co calculated from (dE/dP), using eqn. 1) was calculated from the measurements at pH 6.7. As at this pH the zeta potential of the pure membrane is the same in all experiments, the change in [must arise from the adsorbed ovalbumin layer. UF experiments The ultrafiltration experiments were carried out at 25°C in a plate module with an effective membrane area of 19.3 cm2. The module is described in detail elsewhere [ 141. The feed solution was circulated through the slit-shaped channel with a constant velocity, which corresponds to a Reynolds number, Re, of 5300. The pressure across the membrane, AP, was 1.5 bar in all experiments. The permeate was recirculated after weighing. Before each series of experiments with the OA solutions, the membranes were stabilized for 2.5 hours by circulating pure water at AP= 1.5 or 3 bar and then a KC1 solution of a specified ionic strength at AP= 1.5 bar. During each experiment the pH was kept at a constant specific value. When a stable permeation rate with the saline solution was attained, the first OA concentration in each experiment was set by injecting an aliquot of a stock solution into the circulation system. The protein was allowed to adsorb on to the membrane and the permeate mass flux, J,, was measured. The OA concentration was adjusted to a new value and the corresponding flux was measured. This procedure was repeated stepwise until the desired OA concentration range was covered. Finally, the system was rinsed and the membrane was removed. A new membrane was installed and the whole series of experiments was repeated at another fixed PH. The flux decline due to protein adsorption was expressed as relative flux reduction, S, defined by eqn. (2): S=l-J,/Jo,
(2)
where J”, is the mass flux measured with pure KC1 solution at the same pH after modification, if any. In UF, the adsorption equilibrium is determined by the solute concentration
188
prevailing at the wall of the membrane, C,, rather than by the bulk concentration, C,,. Consequently, the concentration polarization in the diffuse layer must be taken into account. According to Jonsson, the wall concentration of OA for the same type of UF module can be calculated from eqns. (3 ) and (4) [151. (3)
WG=exp(JJk) h~0.023
D
ReO.’ SCO.~~-
dh
(4)
In these equations J, is the volume flux, SC is the Schmidt number and dh ( = 1.9 mm) the effective diameter of the flow channel. The expression for the mass transfer coefficient k is valid in the turbulent flow region [ 151 and its value under the prevailing experimental conditions was calculated to be 1.9x low5 m/set. k can be considered constant because the viscosity and the density of the dilute OA solutions can be replaced by the corresponding values for water. Therefore the value of C&/C,, varied only with J, and it was usually smaller than 2.0. However, eqn. (3) can be applied only if the protein is completely rejected by the membrane. In this study this condition was fulfilled, as verified with absorbance measurements of the permeate at 3,= 280 nm. Results and discussion
Potentiometric
titration of OA
The H+/OHconsumptions vs. pH for OA at four different ionic strengths were calculated as described above. The results are depicted in Fig. 1.As expected, at high ionic strengths the OA is charged to a relatively higher degree. As titrations were made at several ionic strengths, a large jump could be detected between the titration curves recorded at 0.015 M and 0.15M KCl. This can be interpreted as meaning that, for the charge to be fully developed, the ionic strength must be > 0.015 M KCl. Streaming potential
results
The streaming potential experiments were carried out at three different pH values (4,5 and 6) and the results are shown in Fig. 2 (a). It can be seen that the value of the zeta potential increases at pH 4 and 5 with increasing OA concentration. This is due to adsorption of OA. The OA layer on the membrane seems to form a secondary porous layer, thus the streaming potential is formed partly across the “semipores” of the OA layer and partly across the pores of the membrane. The thicker the adsorbed layer, the more is the streaming potential value determined by the charge of OA and not by the charge of the membrane. As the OA is positively charged at pH 4, the zeta potential will become positive at some OA concentrations. At pH 5 the OA is so slightly negatively charged that the negative charge of the membrane seems to be “di-
189
Us1 :
2 0.4..
KCI cont./M
3
Fig. 1. Buffer capacity IP = isoelectric point.
4
5
of ovalbumin
6
at different
1
0-o
0.3
u-u
0.1s
8
9
KC1 concentrations
10
11
as a function
P”
of pH.
minished” by the OA adsorption. At pH 6 adsorption is weak, as the negative charge of OA is strongly developed as indicated above, and the charge densities are about the same for the membrane and OA. Consequently, no difference can be seen between low and high OA concentrations. In Fig. 2 (b) the reference measurements at pH 6.7 are given. The results more or less verify the results at the respective pH values, but seem to indicate that OA is slightly less charged than the membrane at this pH and that some adsorption also takes place. Ultrafiltration results
In the UF experiments with OA, the aim was to study how much the electrostatic repulsion on one hand and the hydrophilicity on the other hand affect flux reduction. The results of the UF experiments with OA carried out with unmodified membranes are given in Fig. 3. In this figure, flux reduction at different pH values is plotted as a function of OA wall concentration. As expected, flux reduction is greatest at pH 4.5, i.e., near the isoelectric point of OA, where the net charge of the.protein molecule is almost zero. Correspondingly, flux reduction is rather low at pH 8, where both the protein molecules and the membrane surface carry a negative charge and the electrostatic repulsion opposes fouling. The repulsive force becomes smaller as pH approaches the IP of the protein and the pzc of the membrane. This has also been verified
pH4 .*W*-
1
2
3
4 5 COA [kg/m’]
b 1
2
3
4
5 Co,
[kg/m’]
Fig. 2. Zeta potentials calculated from streaming potential measurements at different pH for the polysulfone GR 61 membrane. (a) Zeta potential as a function of applied OA concentration. Measurements made at the pH of OA application. (b) Reduced zeta potential, c/c,,, as a function of applied OA concentration. Measurements made at the reference pH, 6.7; OA application at the pH indicated in the figure.
in the ultrafiltration of amino acids with charged membranes by Kimura and Tamano [ 161.At pH values lower than the IP of OA, flux reduction diminishes again, accompanied by an increasing positive net charge of the protein. This is normally the case for protein adsorption at surfaces [ 1,3,17]. If the membrane is not positively charged in this pH region, which seems to be the case with the GR 61 membrane, the result cannot be explained by electrostatic repulsion alone. When the protein is charged the solubility, i.e., the relative stability of the protein in water, increases [ 201 and the affinity for the membrane material decreases. At pH 3-4.5 the polysulfone membrane was detected [9] to have a very slight positive charge or no charge at all (calculated zeta potential z 0.5 mV at pH 4). In order to increase the positive charge density of the membrane and to induce stronger repulsion, the membrane was modified with PEI, which has a
191
PH8 ’ o-
__
” .1
0.2
_
0.3
0.4
0.5
0.6
0.7
0.8
“’ Cw [kg/m’]
Fig. 3. Flux reduction in ultrafiltration of ovalbumin with unmodified GR 61 membranes as a function of wall concentration, C!,, of OA at different pH in 0.15 M KCl. dP= 1.5 bar, t=25”C. (OA stock solutions were made in water without pH control. Membrane stabilization pressure= 1.5 bar.)
high positive net charge in this pH range (calculated zeta potential at pH 4 z + 6 mV [ 211). According to the potentiometric titration experiments of PEI carried out in our laboratory, the titration curve is almost completed at pH 4, which means that the zeta potential at pH 4 almost corresponds to a fully developed charge of the membrane modified with PEI. A series of modification experiments with increasing PEI concentrations was carried out and the results are depicted in Fig. 4. At high PEI concentration the pores were probably blocked with excess PEI molecules, which resulted in an even higher flux reduction than without PEI. The best result was obtained at a PEI concentration of 1 ppm. It can also be seen from Fig. 4 that the positive effect of PEI is greatest at low OA concentrations. A strong flux reduction effect of PEI was also noticed when flux of pure water through the unmodified membrane and flux of saline solution through the modified membrane were compared (see Table 2 ) . This effect was even stronger at higher modifying concentrations of PEI, and resulted in a minimal
0.6
PEI
0.1
0.2
0.3
0.4
cont.
Q-0 *-+
1
pm ”
*-P
5
”
t-+
10
”
0.5
0.6
o.7
p
rL
Fig. 4. Ultrafiltration of ovalbumin with polysulfone GR 61 membranes at different modification concentrations of PEI. Flux reduction as a function of OA wall concentration, C,, at pH 4 in 0.015 M KCl. (OA stock solution prepared in water without pH control. Membrane stabilization pressure = 3 bar.)
net permeate flux, even lower than the permeate flux obtained without modification. Using a concentration of 1 ppm in PEI modifications, experiments were run with different concentrations of KCl. It was found that flux reduction was greatest at the lowest ionic strength (Fig. 5). This result is in accordance with the results obtained by Matthiasson [ 1 ] in UF of bovine serum albumin with GR 61 membranes without PEI, and also with the adsorption studies by Turker and Hubble [ 181 with polysulfone membranes at pH 7.5. This effect might be due to the lower degree of dissociation of OA at low ionic strength, as verified by the titration results above. Also OA, like most proteins, seems to be less stable at low ionic strengths, so that adsorption followed by flux reduction can result. The theoretical influence of electrolyte on adsorption is contradictory. On one hand, a high electrolyte concentration increases and stabilizes the charges of OA and PEI, and thus repulsion diminishes adsorption, but on the other
lY3
KCI cont. / M e-0 *-r(r l
0.1
0.2
0.3
0.4
-a
0.5
0.15 0.015 0.0015
0.6
'.'
C,[kg/m”]
Fig. 5. Ultrafiltration of ovalbumin with polysulfone GR 61 membranes at different ionic strengths. PEI modification concentration = 1 ppm. Flux reduction as a function of wall concentration, C,, at pH=4. (OA stock solution prepared in water without pH control. Membrane stabilization pressure = 3 bar. )
hand the electrolyte can have a shielding influence. The results seem to indicate that the effect of the stabilized charge is important, although the repulsive force of OA is fairly small at pH 4, as its positive charge is only partially developed, as indicated by the titration experiments depicted in Fig. 1. The electroviscous effect of the fouling OA layer leads one to predict stronger flux reduction at low ionic strengths, which is in accordance with the results of Fig. 5 and Table 1. When OA was dissolved in pure water, the pH of the resulting stock solution, appeared to be higher than the IP of OA. It was assumed that, during pH adjustment of the stock solution, OA could possibly adhere irreversibly to the membrane as its IP was passed. In order to avoid this effect, OA was in some experiments dissolved at pH 3 during preparation of the stock solution. Subsequent UF experiments were run at pH 3.5 at two KC1 concentrations, with and without PEI. The effect of PEI is remarkable also at this pH, as indicated in Fig. 6; the effect can result from the positive effect of electrostatic repulsion on UF results, and is greater at low ionic strengths. The strong effect of elec-
_ "'
CW [ kg/m’]
Fig. 6. Flux reduction as a function of wall concentration, C,, in ultrafiltration of ovalbumin with polysulfonated GR 61 membranes with and without modification with PEI (1 ppm at pH 10) at two different ionic strengths at pH = 3.5. (OA stock solution prepared in water at pH 3. Membrane stabilization pressure = 3 bar. )
trostatic repulsion has also been observed with other process solutions [ 16,191. At pH 3.5, the charge of OA in 0.015 M KC1 solution is approximately the same as at pH 4.0 in 0.15 M KC1 solution. Comprehensive studies of the flux reduction results with PEI show that the curves are rather similar. At pH 3.5 in 0.15 M KC1 solution, the positive net charge on OA molecules should be about twice that at pH 4.0. This would predict a strong repulsion. Figure 6 shows that this was not the case. According to Fane et al. [ 171, this is in accordance with experiments with BSA on a polysulfone membrane, where ions lowered the flux at higher charges of the protein. This effect can possibly be explained by the assumption that the positive PEI-ovalbumin layer on the membrane acts in a somewhat similar way as a charged layer, which excludes positive ions and therefore increases flux reduction, especially at high ionic strengths. Under the same UF conditions as above, a sulfonated polysulfone membrane (GS 61) , which is negatively charged at pH 3.5 [ 91, was also studied. As the charges of the membrane and the protein have opposite signs, severe fouling might be expected to result if only repulsive electrostatic forces influence the result. The results shown in Fig. 6 turned out to be almost as good as those with PEI. It may therefore be concluded that the charge on the protein and the
195
hydrophilicity, rather than the charge of the membrane, determine the degree of adsorption. This can be illustrated by a model in which the protein molecules adhere to the membrane and form a modifying layer of their own, and the membrane then repels continued adsorption of similar molecules. This suggested phenomenon also explains why the electrostatic repulsion of the strongly charged PEI is more effective at small concentrations of OA when OA adsorption has not yet taken place. Under such conditions, the energy of electrostatic repulsion most probably is not strong enough to prevent irreversible bond formation between the membrane surface and the adhering OA molecules. The hydrophilicity of the membrane is probably more important than its electrostatic repulsive strength, as the hydrophilic environment prevents irreversible adsorption of the protein. Conclusions
From the results given above, it can be seen that the flux in ultrafiltration is very much determined by the behavior of the membrane used and the properties of the material to be retained from solution at different pH values. If the component to be ultrafiltered is hydrophilic, it is more stable in a solution of high ionic strength. At the same time, fouling of the membrane is also counteracted when a hydrophilic membrane is used. The optimum pH with respect to flux reduction is one where the component in the solution is highly charged, and preferably with the same sign of charge as the membrane. If the chargecarrying component adheres to the membrane, it can counteract the lack of charge or the opposite charge of the membrane by forming a kind of modification layer of its own. This leads to the adsorbed layer repelling further adsorption. If adsorption does not take place, and the charge of the membrane is not such that it repels the component to be ultrafiltered, the charge of the membrane can be changed by proper modification of the membrane. Increase in ionic strength also seems to improve the ultrafiltration flux by increasing and stabilizing the charge of the protein and so enhancing electrostatic repulsion. When the modification or adsorption layer is strongly charged, the electroviscous and ion exclusion effects have to be taken into account. Finally, one can state that in ultrafiltration of OA the ionic character of the protein molecule is more important for the result than the charge of the membrane. Acknowledgements
The author is indebted to Nordisk Industrifond, The Finnish Academy of Sciences and Maj and Tor Nessling Foundation for financial support. The author also wishes to thank Lit. Techn. Markku Laatikainen for his theoretical help and inspired discussions and Miss Tuija Silonsaari for her assistance.
196
References 1
2 3 4
5
6 7 8 9 10 11 12 13
14 15 16
17 18 19 20 21
E. Matthiasson, Macromolecular adsorption and fouling in ultrafiltration and their relationships to concentration polarization, Ph.D. Thesis, Lund University, 1984. P. Aimar, Mecanismes de transfert de matiere en ultrafiltration, Ph.D. Thesis, Universite Paul Sabatier, 1987. W. Norde, Adsorption of proteins from solution at the solid-liquid interface, Adv. Colloid Interface Sci., 25 (1986) 267. I. Lundstrom, Protein conformation at surfaces, in: P.-O. Glantz, S.A. Leach and T. Ericson (Eds.), Oral Interfacial Reactions of Bone, Soft Tissue and Saliva, IRL Press, Oxford, 1985, p. 9. C.-G. Golander, H. Arwin, J.C. Eriksson, I. Lundstrom and R. Larsson, Heparin surface film formation through adsorption of colloidal particles studied by ellipsometry and scanning electron microscopy, Colloids Surf., 5 (1982) 1. M. Laatikainen and M. Lindstrijm, Determination of adsorption isotherms with quartz crystal microbalance in liquid phase, J. Colloid Interface Sci., 125 (1988) 610. R.W. Paynter, B.D. Ratner, T.A. Horbett and H.R. Thomas, XPS studies on the organization of adsorbed protein films on fluoropolymers, J. Colloid Interface Sci., 101 (1984) 233. R.J. Hunter, Zeta Potential in Colloid Science, Principles and Applications, Academic Press, London, 1981. M. Lindstriim and M. Nystrom, Streaming potential of UF membranes at different pH, Finn. Chem. Lett., 14 (1987) 123. A. White, P. Handler and E.L. Smith, Principles of Biochemistry, McGraw-Hill, New York, NY, third edn., 1964, p. 123. J.A. Howell, 0. Velicangil, MS. Le and A.L. Herrera Zeppelin, Ultrafiltration of protein solutions, Ann. N.Y. Acad. Sci., 369 (1981) 355. J.S. Fruton and S. Simmonds, General Biochemistry, Wiley, New York, NY, 1958, p. 150. G.M. Lindquist and R.A. Stratton, The role of polyelectrolyte charge density and molecular weight on the adsorption and flocculation of colloidal silica with polyethylenimine, J. Colloid Interface Sci., 55 (1976) 45. M. Lindstriim, M. Nystrijm and M. Laatikainen, Interactions between chlorolignin and polysulfone ultrafiltration membranes, Sep. Sci. Technol., 23 (1988) 703. G. Jonsson, Boundary layer phenomena during ultrafiltration of dextran and whey protein solutions, Desalination, 51 (1984) 61. S. Kimura and A. Tamano, Separation of amino acids by charged ultrafiltration membranes, in E. Drioli and M. Nakagaki (Eds.), Membranes and Membrane Processes, Proc. Eur.-Jpn. Congr. Membr. Membr. Processes, Plenum, New York, NY, 1986, p. 191. A.G. Fane, C.J.D. Fell and A. Suki, The effect of pH and ionic environment on the ultrafiltration of protein solutions with retentive membranes, J. Membrane Sci., 16 (1983) 195. M. Turker and J. Hubble, Membrane fouling in a constant-flux ultrafiltration cell, J. Membrane Sci., 34 (1987) 267. A. Bindoff, C.J. Davies, C.A. Kerr and CA. Buckley, The nanofiltration and reuse of effluent from the caustic extraction stage of wood pulping, Desalination, 67 (1987) 455. L. Zeman, Adsorption effects in rejection of macromolecules by ultrafiltration membranes, J. Membrane Sci., 15 (1983) 213. M. Nystrom, M. Lindstriim and E. Matthiasson, Streaming potential as a tool in the characterization of ultrafiltration membranes, Colloids Surf., accepted.