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The anti-fouling action of polymers preadsorbed on ultrafiltration and microfiltration membranes
Journal ofMembrane Science, 76 (1993) 281-291 Elsevier Science Publishers B.V., Amsterdam
281
The anti-fouling action of polymers preadsorbed ultrafiltration and microfiltration membranes
on
L.E.S. Brink, S.J.G. Elbers, T. Robbertsen and P. Both Netherlands
Institute for Dairy Research, P.O. Box 20,671O BA Ede (The Netherlands)
(Received March 18,1992; accepted in revised form October 9,1992)
Abstract Preadsorption of hydrophilic polymers on hydrophobic ultrafiltration membranes can reduce the susceptibility of the modified membranes to protein fouling. The mechanisms of this anti-fouling action were investigated. Polysulfone and nuclear track-etched membranes with different average pore diameters were hydrophilized by preadsorption of two water-soluble polymers. The fouling of the unmodified and modified membranes due to filtration of a whey protein solution or to adsorption of the whey protein at the membrane surface was characterized by flux measurements and by electron microscopy. Adsorption of protein at the pore walls of ultrafiltration membranes, resulting in the narrowing of pores, is prevented by partly sealing off the pore entrances by polymer molecules presorbed at the external membrane surface. The observed blockage of pores of microfiltration membranes cannot be averted by the preadsorption technique. Keywords: fouling; protein adsorption; polysulfone membranes; nuclear track-etched membranes; membrane modification; pore size distribution
Introduction Several techniques have been proposed to reduce the protein fouling of ultrafiltration (UF) membranes. One class of these techniques is based on the importance of interactions between the protein molecules and the membrane surface for the membrane fouling characteristics. Modification of the membrane surface to minimize the effects of protein/surface interaction (adsorption) seems then to be a logical step to prevent fouling [l-8]. It has already Correspondence to: L.E.S. Brink, Netherlands Institute for Dairy Research, P.O. Box 20, 6710 BA Ede, The Netherlands.
0376-7388/93/$06.00
been demonstrated that preadsorption of more or less hydrophilic compounds (usually polymers) on UF membranes in order to reduce protein adsorption at and in these membranes decreases the susceptibility of the membranes to fouling [ 9-13 1. Two main mechanisms of irreversible protein fouling are often considered: pore narrowing as a result of protein adsorption and pore plugging. However, the mechanisms involved in the prevention of fouling by the preadsorption technique are still not clear. The aim of this study was to investigate the different mechanisms of the anti-fouling action of preadsorbed polymers. For this purpose membrane fouling experiments were conducted in
0 1993 Elsevier Science Publishers B.V. All rights reserved.
L.E.S. Brink et al.lJ. Membrane Sci. 76 (1993) 281-291
282
which the (average) pore diameter of the UF and microfiltration (MF) membranes was the main experimental parameter. The surface of these membranes was modified by preadsorption of two hydrophilic, water-soluble polymers. The fouling behaviour of the unmodified and modified membranes during UF/MF of a whey protein solution was determined. Also, the fouling due to adsorption of the whey protein and of the hydrophilic polymers on the membranes was established separately. Furthermore, the effects of adsorption of protein and polymers on the pore size distributions of the nuclear track-etched membranes were examined by electron microscopy. Experimental Chemicals
*
/?-Lactoglobulin (/?-LG, molecular weight dimer - 36,000) was isolated from desalted, clarified casein whey by ion-exchange chromatography (Pharmacia Stack KS 370/15, DEAE Sepharose fast flow anion exchanger). Bovine serum albumin (BSA, molecular weight -66,000) was supplied by Sigma, St. Louis, MO, USA (cat. no. A 0281). The McIlvaine buffer (0.01 M citric acid/O.02 M phosphate) was used to dissolve the proteins (pH 4.7). Poly(viny1 methyl ether) and methyl cellulose (Fig. 1) were obtained from Aldrich, Milwaukee, WI, USA (cat. no. l&272-9, average mo$H20CH3
b I”-“\ -I ""\;"
,CH-'$
CH
CH
lecular weight -66,000 [14] ) and Sigma, St. Louis, MO, USA (cat. no. M 7140, average molecular weight - 14,000), respectively. All aqueous solutions were prepared using demineralized water, which was prefiltered before use by reverse osmosis (Osmonics PA 99 ROmodule). Before use the polysulfone membranes were cleaned at 50’ C for 30 min with a 0.5 vol.% alkaline cleaning and disinfectant solution commonly used in the dairy industry (Reca VL, obtained from Centrale Aankoop FNZ, Arnhem, The Netherlands; contains no surfactants). Membranes The nuclear track-etched membranes were supplied by Nuclepore, Pleasanton, CA, USA. The polysulfone (type Udel) membranes were obtained from DDS, Nakskov, Denmark. The molecular weight cut-offs and/or the estimated average pore diameters of the used membrane types are given in Table 1. Electron microscopy Pore size distributions of the clean and fouled track-etched membranes were obtained from scanning electron microscopy (SEM) and transmission electron microscopy (TEM) micrographs (Jeol JEM-1200 EX) using a Nikon profile projector coupled to a Mitutoyo digital counter. The SEM samples were prepared by sputtering 10 nm gold layers on freeze-dried membrane strips. The TEM replica samples were prepared by applying platinum/carbon layers (2 nm Pt/C followed by 10 nm C) onto freeze-dried membrane pieces and subsequently dissolving the polycarbonate substratum in chloroform. Flux decline measurements
OCH3
Fig. 1. Structural formulae of poly(viny1 methyl ether) (a) and methyl cellulose (b).
The degree of membrane flux decline due to (bio)polymer adsorption or filtration of a pro-
L.E.S. Brink et al.fJ. Membrane Sci. 76 (1993) 281-291
283
TABLE 1 Average pore diameter and/or molecular weight cut-off (MWCO) of the membranes used Manufacturer
Material
Type
UF or MF
Average pore diameter (nm)
Nuclepore
polycarbonate
N10 N15 N30 N50 N80 N200 N400
UF UF UF MF MF MF MF
81 11’ 25l 36l 62l 148l 3071
DDS
polysulfone
GR81 GR61 GR51 GR40
UF UF UF MF
approx. 52
MWCO @DA)
6 20 50 100
‘From SEM/TEM micrographs. ‘Determined by permporometry [ 191.
tein solution was established by measuring the relevant water fluxes of the clean, presorbed or fouled membranes in a stirred dead-end filtration cell (Amicon model 202, dp = 1.5 x lo5 Pa, measurement time 30 min). All experiments were carried out at room temperature and performed several times in view of the considerable differences between the measured water fluxes and degrees of fouling of different membrane disks (effective surface area 25.5 X lo-* m” ). The polysulfone membranes were cleaned in Reca VL, while the track-etched membranes were wetted with ethanol. Membrane fouling of clean membranes due to adsorption of P-LG, methyl cellulose (MC) or poly(viny1 methylether) (PVME ) was accomplished by passive adsorption from a (bio)polymer solution in water or buffer for 30 min (proteins: 1 g/l, synthetic polymers: 10 g/l). Membrane fouling of clean or presorbed membranes due to UF/MF of a protein solution was effected by filtration of a buffered solution of P-LG or BSA (180 ml, 1 g/l, pH 4.7) in the stirred filtration cell (dp= 1.5 x lo5 Pa) for 2 hr. If necessary, the
filtration cell was refilled with protein solution in the case of MF membranes. Results and discussion Membrane fouling due to (bio)polymer adsorption Adsorption of a biopolymer, like @-LG, as well as of synthetic polymers, like MC and PVME, on UF membranes by contacting the membranes with the polymer solution for 30 min can already result in considerable fouling. Examination of the top and bottom surfaces of clean and fouled nuclear track-etched membranes by SEM and TEM revealed that pore narrowing due to internal adsorption is the most likely cause of this flux decrease. This type of membrane fouling has been observed previously [ 13,151. Figure 2 shows an example of the effect of protein adsorption on the sizes of the pore entrances of an N80 membrane. The pore (opening) size distributions of the clean and fouled track-etched membranes were deter-
284
L. E.S. Br&k et al./J. Membrane Sci. 76 (1993) 281-291
Fig. 2. SEM micrographs (top view) of a clean (a) and a protein-fouled (b) NSO membrane.
mined by estimating for each sample the dimensions (diameters) of several hundreds of pore entrances visible on the SEM and TEM micrographs. To verify the accuracy of the dimensions thus obtained, the surface diameter distributions of some membrane samples were computed by image analysis of the micrographs. As the results of these two methods were found to be in agreement (with respect to average and width of the distribution), only the first-mentioned method applied was subsequently. The effect of (bio)polymer adsorption on the pore size distribution of the top side of a tracketched N50 membrane is given in Fig. 3. The top side is the side of the membrane which is exposed to the (bio)polymer solution. Adsorption both of fi-LG and of PVME on the N50 membrane resulted in a significant shift of the pore size distribution to smaller diameters. A similar decrease of the pore size at the top side of the membrane was found in case of fi-LG adsorption on the N30 and N80 membranes (Table 2 ) . The magnitude of this shift can be used to obtain a crude estimate of the thickness of the layer of adsorbed (bio)polymer, 6, in the membrane pores. The calculated values for the adsorbed layer thickness are given between
brackets in Table 2. From the pore size measurements at the top side of the membranes, it can be concluded that the adsorbed layer thickness of the protein P-LG (molecular weight dimer -36,000) is of the same order of magnitude as that of the synthetic polymer PVME (average molecular weight - 66,000). Furthermore, the values determined for the adsorbed layer thickness seem to indicate monolayer adsorption [ 161. This agrees with earlier work on protein adsorption on UF membranes [ 13,151. The pore opening size distributions of the bottom side of clean and fouled track-etched membranes were also determined by electron microscopy (Table 2). The bottom side is the side of the membrane which is not directly exposed to the (bio)polymer solution. In the case of unmodified N30, N50 and N80 membranes, only minor differences are found between the average pore sizes of the top side and those of the bottom side. This indicates that these tracketched membranes have a well-defined, symmetrical pore structure. Furthermore, there is remarkably good agreement between the average pore diameters of the top and bottom side of the N80 membrane determined with TEM and the corresponding diameters determined with SEM. These observations demonstrate the
L.E.S. Brink et al.fJ. Membrane Sci. 76 (1993) 281-291 frequency (X) 20
a
16 12 f
8 4
0
+ lu, 204060800204060800204060800
-H-H
20406080 dphn)
Fig. 3. The effects of adsorption of/SLG and PVME on the pore size distribution of a N50 membrane (de: membrane pore diameter); (a) clean, top side, (b) /3-LG, top side, (c) PVME, top side, (d) PVME, bottom side.
applicability of electron microscopy for obtaining pore size distributions of the track-etched membranes used. Adsorption of 8-I-G on the N30, N50 and N80 membranes was also found to narrow the pore openings at the bottom side of these membranes (Table 2). This suggests that, despite the pore narrowing, the adsorption period used (30 min) was sufficient for the protein mole-
285
cules to be transported (mainly by diffusion) to the bottom side of the membranes (thickness 6 pm). The estimated adsorbed layer thickness in case of the N50 and N80 membranes (8 nm) is in agreement with the observed layer thickness of adsorbed /?-LG at the top side of the membranes. In case of the N30 membrane, the layer thickness is somewhat smaller (5 nm). This might indicate that the adsorption time used has some limiting effect on the penetration depth of protein in the narrowed pores ( - 11 nm, top side) of the N30 membrane. Interestingly, no pore narrowing could be observed on the micrographs of the bottom side of the N50 membrane fouled by PVME adsorption (Fig. 3d). This can be explained by strong limitation of the rate of diffusion of PVME in the membrane pores. In addition to electron microscopy, the fouling of the track-etched membranes by adsorption of a (bio)polymer was also characterized by membrane flux measurements (Fig. 4). The increase in the total, relative resistance (RJR,, with R, the resistance of the clean membrane) of the membranes fouled by the synthetic polymers MC and PVME is greater than that observed for protein adsorption. In the case of MC
TABLE 2 The average pore diameter, dp, and, between brackets, adsorbed layer thickness, 6, of unmodified and fouled Nuclepore membranes Membrane type
Unmodified dr (nm)
PVME adsorption dp (&ME) (nm)
p-LG adsorption dr (&-I.& (nm)
toP
bottom
toP
bottom
N30
26
24
ii (a)
14 (5)
N50
38
34
16 (11)
ia (a)
N60
62 601
64 601
45 (6)
47 (3)
‘From SEM micrographs; other values from TEM micrographs.
tJJP
bottom
25 (7)
36 (-0) _
286
adsorption (pore diameter > 20 nm) and protein adsorption an increase in the (average) pore diameter will reduce the degree of fouling (i.e. the relative membrane resistance RJR,). This suggests internal adsorption, as a larger pore will narrow comparatively less through adsorption at the pore wall. The relatively slight fouling show in in this context by the N15 membrane as a result of MC and PVME adsorption can be explained by the adsorption time adopted (30 min), which, especially for the small pore diameters, may have been too short to achieve full coverage of the internal surface. The latter is in agreement with the above-mentioned observation that the pore size distribution of the bottom side of the N50 membrane is not affected by adsorption of PVME (Table 2). The mechanism of adsorption of MC and PVME thus revealed is depicted in Fig. 5. Incomplete internal adsorption to the N50 membrane could also explain why, in the case of PVME adsorption, the N80 membrane shows a higher fouling level than the N50 membrane. The relatively low degree of pore narrowing of the comparatively large pores of the N80 membranes (about 62 nm) after adsorption of the first polymer molecules makes it plausible that the internal surface of this membrane type can be covered completely by a (mono)layer of polymer. If this last is assumed, an estimate of the thickness of the adsorbed layer, S, can be obtained in a simple way by using the Poiseuille equation:
*+(1-@-‘l’) in which d, is the average diameter of the clean membrane pores (about 62 nm). For/SLG, MC and PVME this gives layer thicknesses of, respectively, 3.3,6.8 and 11 (nm). The order of magnitude of these values is comparable to that of the thicknesses obtained from the shift of the pore size distributions
L.E.S. Brink et al&J. Membrane
Sci. 76 (1993) 281-291
%n I
Fig. 4. Total, relative membrane resistance (RJR,) of the track-etched membranes as a function of the average pore diameter (d,) of the clean membrane; adsorption of /I-LG (O),MC (+) orPVME (0).
N15
N30
N50
N80
Fig. 5. Mechanism of adsorption of MC and PVME to the polycarbonate model membranes.
(Table 2). The protein layer thickness found is smaller than the previously determined value ( N 8 nm) . However, both estimates are in reasonable agreement with known values of the dimension of a P-LG dimer (two spheres of N 3.5 nm diameter each [ 171). The value obtained for the thickness of adsorbed MC (a polysaccharide, molecular weight about 14,000) compares favourably with the corresponding value of polysaccharides adsorbed at a water-silica
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Sci. 76 (1993) 281-291
interface (4 to 5 nm at molecular weight of - 15,000 [ 181). The PVME layer thickness is greater than the TEM value (7 nm, Table 2)) which might, however, be explained by shrinking of the adsorbed layer during TEM sample preparation. Finally, the magnitude of the MC and PVME layer thicknesses lends support to the mechanism of adsorption to the 11 nm membrane depicted in Fig. 5. Soon after the first contact with the membrane, the MC and PVME molecules will partly block the pore entrance, thereby protecting the rest of the pore against internal fouling.
15 13 11
c 2
E
9 7 5 3 1
GR81
19 I-
Membrane fouling due to filtration of a protein solution
PVNE
GR61
GR51
GR'KI
b
17 15 13
Figure 6 compares the resistance increases (flux decreases) found for different types of polysulfone membranes as a result of filtration of a P-LG solution. A distinction is made between protein fouling by UF/MF of an unmodified polysulfone membrane (first bar), the ‘fouling’ resulting from preadsorption with MC (Fig. 6a) or PVME (Fig. 6b) (second bar), protein fouling by UF/MF of the MC or PVMEmodified membrane (third bar), and lastly the total fouling level produced by preadsorption and filtration (fourth bar). The first phenomenon that catches the eye is that fouling of an unmodified membrane (first bar) turns out to be strongly dependent on the (average) pore diameter of the membrane type. An increase of the pore diameter (i.e. an increase of the molecular weight cut-off - see Table 1) results in a considerably lower fouling level of the UF membranes GR81/61/51. This is readily accounted for by pore narrowing through internal protein adsorption, which has a particularly marked effect when the pores are relatively small (see also the discussion at Fig. 4). The strong increase in the degree of fouling that results from use of the GR40 membrane,
qE 11 0.Y 9 7 5 3 1
n 0
q q
GR81 GR61 GR51 GRW fouling of unmodifiedmembrane by UF/MF 'fouling'by presorptionwith IIC(a) or PVME (b) fouling of modified membrane by lJF/MF fouling by wesorption and UF/W
Fig. 6. Effect of pore diameter on the relative membrane resistance (RJR,) of polysulfone membranes.
which has considerably larger pores and which in fact is not an UF but a MF membrane, indicates that larger pores ( > approx. 30 nm) might be fouled by a different mechanism. The latter was investigated by performing comparable filtration experiments with track-etched membranes (Fig. 7). The degree of protein fouling of an unmodified track-etched membrane (first bar) increases when membranes with larger pore diameters are used. Assuming that the average pore diameter of the N15 membrane is comparable to that of the GR51 membrane (Table 1) , this observed trend is in
288
L.E.S. Brink et al./J. Membrane Sci. 76 (1993) 281-291 71 MC
a
Nl5
N30
N50
N80
125 PVnE
b
32
1 N15 I 0
N30
fouling 'fouling'
N80
1150
of umdified
menWane
by wesorptipn
q
fouling
of modified
m
fouling
by Presorption
with
membrane and
N200 by UF/E IIC (a) or PVE
(b)
by UF/W lJF/W
Fig. 7. Effect of pore diameter on the relative membrane resistance (RJR,) of track-etched membranes.
agreement with the similar tendency of the GR51/40 membranes (greater fouling at larger pores). SEM micrographs of the fouled N200 membrane reveal partly blocked pores. This phenomenon is even more pronounced in case of the N400 membrane (Fig. 8a,b). A similar kind of fouling of the N400 membrane is observed in the case of filtration of a solution of an other whey protein, BSA (Fig. 8c). Pore
narrowing as a result of (ultra) filtration of a protein solution was also measured. The decrease of the average pore diameters of the N30/ 50/80/200/400 membranes (top and bottom side) is comparable to those listed in Table 2. However, the results of Figs. 6 and 7 (first bars) as well as the electron micrographs of Fig. 8 suggest that in all likelihood pore blocking is the main fouling mechanism in the case of membranes with larger pores ( > about 30nm). In view of the molecular dimensions of the whey proteins /?-LG and BSA in comparison to the pore dimensions of the MF membranes used, it seems likely that (large) protein aggregates and/or multilayer adsorption play an important role in the pore-blocking mechanism. Next we consider the ‘fouling’ caused by preadsorption of MC and PVME (second bar ) . Especially the adsorption of PVME to the polysulfone membranes used (Fig. 6b) causes a substantial resistance increase, which is in agreement with the results shown in Fig. 4. The ‘fouling’ of polysulfone membranes due to both MC and PVME becomes less with an increase in the pore diameter (GR81/61/51), but more serious again in the membrane having the largest pores (GR40). Taking into account the above-estimated layer thicknesses of adsorbed MC and PVME, and the fact that these polymers can only penetrate to a limited extent into a pore of approx. 11 nm (Fig. 5)) it is possible to visualize a mechanism of MC and PVME adsorption to the polysulfone membranes as depicted in Fig. 9. Of course, the membrane pores shown in Fig. 9 only present a highly simplified picture of the complex pore morphology of the polysulfone membranes. The scale is roughly comparable to that of Fig. 5 (average pore size of the GR61 membrane about half the size of the N15 membrane). Internal adsorption of MC and PVME is regarded as unlikely in the case of the GR81 and GR61 membranes, given the average pore diameter of the GR61 membrane (approx. 5 nm, determined by permporometry
L.E.S. Brink et al./J. Membrane Sci. 76 (1993) 281-291
GR81
GR61
GR51
GR40
GR61
GR51
GR40
b: PVRE
GR81
Fig. 9. Mechanism of adsorption of MC (a)and PVME (b) to polysulfone membranes.
Fig. 8. SEM micrographs (top view) of protein-fouled tracketched membranes; (a) N200, b-LG, (b) N400, p-LG, (c) N400, BSA.
[ 191) . The pore diameter dependence of ‘fouling’ bypreadsorption of the GR61/61/51 membranes can be explained by partial clearing of the pore entrance where larger pores are involved. Internal adsorption does play a significant role in the GR40 membrane. The latter phenomenon can explain the relatively high degree of ‘fouling’ due to preadsorption in case of the GR40 membrane (Fig. 6a and b). Finally, the third and fourth bar in Figs. 6 and 7 represent, respectively, the fouling by protein and the total fouling level (MC or PVME + protein) of membranes presorbed with MC or PVME. For the three UF polysulfone membranes (GR61/61/51), preadsorption with MC (Fig. 6a) gives an overall resistance increase that is smaller than the increase found for an unmodified membrane (first bar). In practical terms this implies the possibility of achieving a given degree of concentration of the protein solution in a shorter time, or a higher degree of concentration in the same period of time. The same applies to ultrafiltration by means of the PVME-presorbed GR51 membrane (Fig. 6b ). For the GR61 and GR61 membranes PVME preadsorption seems less useful on account of the strong ‘fouling’ effect produced by modification with PVME. A further notable point is that the GR61/61/51 mem-
290
branes pretreated with PVME hardly become fouled during ultrafiltration (third bar). This lends additional support to the theory underlying the adsorption mechanism outlined in Fig. 9 (b). The pore entrances are effectively shielded by PVME, so that the protein molecules are incapable of penetrating into the pores. In this case a variation in the pore diameter no longer has any influence on protein fouling. A less efficient pore shielding by MC (Fig. 9a) might explain why the fouling of MC-modified membranes is still dependent on the pore size. As already discussed above, (unmodified) membranes with pores larger than about 30 nm (GR40, N50, N80, N200) are strongly fouled during filtration of a protein solution. Pore blockage seems to be a major cause of this membrane fouling. From comparing the degree of fouling of unmodified membranes with the fouling by preadsorption and UF/MF (first and fourth bar in Figs. 6 and 7)) it can be concluded that preadsorption cannot prevent this kind of fouling. Protein molecules (aggregates) can enter the pores of the mentioned (MF) membranes as the adsorbed polymers cannot cover the pore openings (Figs. 5 and 9). Furthermore, adsorption of MC or PVME at the pore wall might modify the internal surface characteristics and reduce internal protein adsorption, but, apparently, cannot prevent blockage of the membrane pores.
Conclusions Regarding the UF membranes the conclusion can be drawn that it is possible to reduce membrane fouling through protein adsorption by presorbing the membrane surface with a hydrophilic polymer which protects it against internal adsorption of protein. The polymer derives its protective action from the fact that it occupies adsorption positions and is hard to displace by the protein molecules, but also - and perhaps especially - from the fact that the
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polymer partly seals off the pore entrance and prevents internal protein adsorption by steric repulsion. In the case of protein fouling of MF membranes the preadsorption technique does not seem to prevent the blockage of the membrane pores. Acknowledgements These investigations were supported (in part) by the Programme Commission Membrane Technology (PCM) of the Ministry of Economics. References I. Jitsuhara and S. Kimura, Structure and properties of charged ultrafiltration membranes made of sulfonated polysulfone, J. Chem. Eng. Japan, 16 (1983) 389393. 2 F.F. Stengaard, Characteristics and performance of new types of ultrafiltration membranes with chemically modified surfaces, Desalination, 70 (1988) 207224. 3 F.F. Stengaard, Preparation of asymmetric microfiltration membranes and modification of their properties by chemical treatment, J. Membrane Sci., 36 (1988) 257-275. 4 A. Higuchi, N. Iwata, M. Tsubaki and T. Nakagawa, Surface-modified polysulfone hollow fibers, J. Appl. Polym. Sci., 36 (1988) 1753-1767. 5 A. Higuchi, N. Iwata and T. Nakagawa, Surface-modified polysulfone hollow fibers; Fibers having CH,CH,CH,S03 segments and immersed in HCl solution, J. Appl. Polym. Sci., 40 (1990) 709-717. 6 A. Higuchi and T. Nakagawa, Surface-modified polysulfone hollow fibers; Fibers having a hydroxide group, J. Appl. Polym. Sci., 41 (1990) 1973-1979. 7 L. Breitbach, E. Hinke and E. Staude, Heterogeneous functionalizing of polysulfone membranes, Angew. Makromol. Chem., 184 (1991) 183-196. 8 M. Nystriim and P. Jiirvinen, Modification of polysulfone ultrafiltration membranes with UV irradiation and hydrophilicity increasing agents, J. Membrane Sci., 60 (1991) 275-296. 9 0. Tozawa and D. Nomura, UF-treatment of soy milk using membrane coated with soybean lecithin, Membrane, 8 (1983) 361-363. 10 M.S. Le and J.A. Howell, The fouling of ultrafiltration membranes and its treatment, in: C. Cantarelli and C. Peri (Eds.), Progress in Food Engineering, ForsterVerlag, Kiisnacht, 1983, pp. 321-326. 1
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A.G. Fane, C.J.D. Fell and K.J. Kim, The effect of surfactant pretreatment on the ultrafiltration of proteins, Desalination, 53 (1985) 37-55. 12 K.J. Kim, A.G. Fane and C.J.D. Fell, The performance of ultrafiltration membranes pretreated by polymers, Desalination, 70 (1988) 229-249. 13 L.E.S. Brink and D.J. Romijn, Reducing the protein fouling of polysulfone surfaces and polysulfone ultrafiltration membranes; Optimization of the type of presorbed layer, Desalination, 78 (1990) 209-233. 14 H. Park, E.M. Pearce and T.K. Kwei, Thermal oxidation of blends of polystyrene andpoly (vinyl methyl ether), Macromolecules, 23 (1990) 434-441. 15 J.H. Hanemaaijer, T. Robbertsen, Th. van den Boomgaard and J.W. Gunnink, Fouling of ultrafiltration membranes. The role of protein adsorption and salt
16
17
18
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precipitation, J. Membrane Sci., 40 (1989) 199-217. T. Sato and R. Ruth, Stabilization of Colloidal Dispersions by Polymer Adsorption, Surfactant Sci. Ser., Vol. 9, Marcel Dekker, New York, NY, 1980, p. 24. H.A. McKenzie (Ed.), Milk proteins. Chemistry and Molecular Biology, Vol. 2, Academic Press, New York, NY, 1971, p. 307. I. Baudin, A. Ricard and R. Audebert, Adsorption of dextrans and pullulans at the silica-water interface. Hydrodynamic layer thickness measurements. Role in the fouling of ultrafiltration membranes, J. Colloid Interface Sci., 138 (1990) 324-331. F.P. Cuperus, Characterization of ultrafiltration membranes. Pore structure and top layer thickness, Ph.D. Thesis, University of Twente, Enschede, The Netherlands, 1990, p. 76.
281
The anti-fouling action of polymers preadsorbed ultrafiltration and microfiltration membranes
on
L.E.S. Brink, S.J.G. Elbers, T. Robbertsen and P. Both Netherlands
Institute for Dairy Research, P.O. Box 20,671O BA Ede (The Netherlands)
(Received March 18,1992; accepted in revised form October 9,1992)
Abstract Preadsorption of hydrophilic polymers on hydrophobic ultrafiltration membranes can reduce the susceptibility of the modified membranes to protein fouling. The mechanisms of this anti-fouling action were investigated. Polysulfone and nuclear track-etched membranes with different average pore diameters were hydrophilized by preadsorption of two water-soluble polymers. The fouling of the unmodified and modified membranes due to filtration of a whey protein solution or to adsorption of the whey protein at the membrane surface was characterized by flux measurements and by electron microscopy. Adsorption of protein at the pore walls of ultrafiltration membranes, resulting in the narrowing of pores, is prevented by partly sealing off the pore entrances by polymer molecules presorbed at the external membrane surface. The observed blockage of pores of microfiltration membranes cannot be averted by the preadsorption technique. Keywords: fouling; protein adsorption; polysulfone membranes; nuclear track-etched membranes; membrane modification; pore size distribution
Introduction Several techniques have been proposed to reduce the protein fouling of ultrafiltration (UF) membranes. One class of these techniques is based on the importance of interactions between the protein molecules and the membrane surface for the membrane fouling characteristics. Modification of the membrane surface to minimize the effects of protein/surface interaction (adsorption) seems then to be a logical step to prevent fouling [l-8]. It has already Correspondence to: L.E.S. Brink, Netherlands Institute for Dairy Research, P.O. Box 20, 6710 BA Ede, The Netherlands.
0376-7388/93/$06.00
been demonstrated that preadsorption of more or less hydrophilic compounds (usually polymers) on UF membranes in order to reduce protein adsorption at and in these membranes decreases the susceptibility of the membranes to fouling [ 9-13 1. Two main mechanisms of irreversible protein fouling are often considered: pore narrowing as a result of protein adsorption and pore plugging. However, the mechanisms involved in the prevention of fouling by the preadsorption technique are still not clear. The aim of this study was to investigate the different mechanisms of the anti-fouling action of preadsorbed polymers. For this purpose membrane fouling experiments were conducted in
0 1993 Elsevier Science Publishers B.V. All rights reserved.
L.E.S. Brink et al.lJ. Membrane Sci. 76 (1993) 281-291
282
which the (average) pore diameter of the UF and microfiltration (MF) membranes was the main experimental parameter. The surface of these membranes was modified by preadsorption of two hydrophilic, water-soluble polymers. The fouling behaviour of the unmodified and modified membranes during UF/MF of a whey protein solution was determined. Also, the fouling due to adsorption of the whey protein and of the hydrophilic polymers on the membranes was established separately. Furthermore, the effects of adsorption of protein and polymers on the pore size distributions of the nuclear track-etched membranes were examined by electron microscopy. Experimental Chemicals
*
/?-Lactoglobulin (/?-LG, molecular weight dimer - 36,000) was isolated from desalted, clarified casein whey by ion-exchange chromatography (Pharmacia Stack KS 370/15, DEAE Sepharose fast flow anion exchanger). Bovine serum albumin (BSA, molecular weight -66,000) was supplied by Sigma, St. Louis, MO, USA (cat. no. A 0281). The McIlvaine buffer (0.01 M citric acid/O.02 M phosphate) was used to dissolve the proteins (pH 4.7). Poly(viny1 methyl ether) and methyl cellulose (Fig. 1) were obtained from Aldrich, Milwaukee, WI, USA (cat. no. l&272-9, average mo$H20CH3
b I”-“\ -I ""\;"
,CH-'$
CH
CH
lecular weight -66,000 [14] ) and Sigma, St. Louis, MO, USA (cat. no. M 7140, average molecular weight - 14,000), respectively. All aqueous solutions were prepared using demineralized water, which was prefiltered before use by reverse osmosis (Osmonics PA 99 ROmodule). Before use the polysulfone membranes were cleaned at 50’ C for 30 min with a 0.5 vol.% alkaline cleaning and disinfectant solution commonly used in the dairy industry (Reca VL, obtained from Centrale Aankoop FNZ, Arnhem, The Netherlands; contains no surfactants). Membranes The nuclear track-etched membranes were supplied by Nuclepore, Pleasanton, CA, USA. The polysulfone (type Udel) membranes were obtained from DDS, Nakskov, Denmark. The molecular weight cut-offs and/or the estimated average pore diameters of the used membrane types are given in Table 1. Electron microscopy Pore size distributions of the clean and fouled track-etched membranes were obtained from scanning electron microscopy (SEM) and transmission electron microscopy (TEM) micrographs (Jeol JEM-1200 EX) using a Nikon profile projector coupled to a Mitutoyo digital counter. The SEM samples were prepared by sputtering 10 nm gold layers on freeze-dried membrane strips. The TEM replica samples were prepared by applying platinum/carbon layers (2 nm Pt/C followed by 10 nm C) onto freeze-dried membrane pieces and subsequently dissolving the polycarbonate substratum in chloroform. Flux decline measurements
OCH3
Fig. 1. Structural formulae of poly(viny1 methyl ether) (a) and methyl cellulose (b).
The degree of membrane flux decline due to (bio)polymer adsorption or filtration of a pro-
L.E.S. Brink et al.fJ. Membrane Sci. 76 (1993) 281-291
283
TABLE 1 Average pore diameter and/or molecular weight cut-off (MWCO) of the membranes used Manufacturer
Material
Type
UF or MF
Average pore diameter (nm)
Nuclepore
polycarbonate
N10 N15 N30 N50 N80 N200 N400
UF UF UF MF MF MF MF
81 11’ 25l 36l 62l 148l 3071
DDS
polysulfone
GR81 GR61 GR51 GR40
UF UF UF MF
approx. 52
MWCO @DA)
6 20 50 100
‘From SEM/TEM micrographs. ‘Determined by permporometry [ 191.
tein solution was established by measuring the relevant water fluxes of the clean, presorbed or fouled membranes in a stirred dead-end filtration cell (Amicon model 202, dp = 1.5 x lo5 Pa, measurement time 30 min). All experiments were carried out at room temperature and performed several times in view of the considerable differences between the measured water fluxes and degrees of fouling of different membrane disks (effective surface area 25.5 X lo-* m” ). The polysulfone membranes were cleaned in Reca VL, while the track-etched membranes were wetted with ethanol. Membrane fouling of clean membranes due to adsorption of P-LG, methyl cellulose (MC) or poly(viny1 methylether) (PVME ) was accomplished by passive adsorption from a (bio)polymer solution in water or buffer for 30 min (proteins: 1 g/l, synthetic polymers: 10 g/l). Membrane fouling of clean or presorbed membranes due to UF/MF of a protein solution was effected by filtration of a buffered solution of P-LG or BSA (180 ml, 1 g/l, pH 4.7) in the stirred filtration cell (dp= 1.5 x lo5 Pa) for 2 hr. If necessary, the
filtration cell was refilled with protein solution in the case of MF membranes. Results and discussion Membrane fouling due to (bio)polymer adsorption Adsorption of a biopolymer, like @-LG, as well as of synthetic polymers, like MC and PVME, on UF membranes by contacting the membranes with the polymer solution for 30 min can already result in considerable fouling. Examination of the top and bottom surfaces of clean and fouled nuclear track-etched membranes by SEM and TEM revealed that pore narrowing due to internal adsorption is the most likely cause of this flux decrease. This type of membrane fouling has been observed previously [ 13,151. Figure 2 shows an example of the effect of protein adsorption on the sizes of the pore entrances of an N80 membrane. The pore (opening) size distributions of the clean and fouled track-etched membranes were deter-
284
L. E.S. Br&k et al./J. Membrane Sci. 76 (1993) 281-291
Fig. 2. SEM micrographs (top view) of a clean (a) and a protein-fouled (b) NSO membrane.
mined by estimating for each sample the dimensions (diameters) of several hundreds of pore entrances visible on the SEM and TEM micrographs. To verify the accuracy of the dimensions thus obtained, the surface diameter distributions of some membrane samples were computed by image analysis of the micrographs. As the results of these two methods were found to be in agreement (with respect to average and width of the distribution), only the first-mentioned method applied was subsequently. The effect of (bio)polymer adsorption on the pore size distribution of the top side of a tracketched N50 membrane is given in Fig. 3. The top side is the side of the membrane which is exposed to the (bio)polymer solution. Adsorption both of fi-LG and of PVME on the N50 membrane resulted in a significant shift of the pore size distribution to smaller diameters. A similar decrease of the pore size at the top side of the membrane was found in case of fi-LG adsorption on the N30 and N80 membranes (Table 2 ) . The magnitude of this shift can be used to obtain a crude estimate of the thickness of the layer of adsorbed (bio)polymer, 6, in the membrane pores. The calculated values for the adsorbed layer thickness are given between
brackets in Table 2. From the pore size measurements at the top side of the membranes, it can be concluded that the adsorbed layer thickness of the protein P-LG (molecular weight dimer -36,000) is of the same order of magnitude as that of the synthetic polymer PVME (average molecular weight - 66,000). Furthermore, the values determined for the adsorbed layer thickness seem to indicate monolayer adsorption [ 161. This agrees with earlier work on protein adsorption on UF membranes [ 13,151. The pore opening size distributions of the bottom side of clean and fouled track-etched membranes were also determined by electron microscopy (Table 2). The bottom side is the side of the membrane which is not directly exposed to the (bio)polymer solution. In the case of unmodified N30, N50 and N80 membranes, only minor differences are found between the average pore sizes of the top side and those of the bottom side. This indicates that these tracketched membranes have a well-defined, symmetrical pore structure. Furthermore, there is remarkably good agreement between the average pore diameters of the top and bottom side of the N80 membrane determined with TEM and the corresponding diameters determined with SEM. These observations demonstrate the
L.E.S. Brink et al.fJ. Membrane Sci. 76 (1993) 281-291 frequency (X) 20
a
16 12 f
8 4
0
+ lu, 204060800204060800204060800
-H-H
20406080 dphn)
Fig. 3. The effects of adsorption of/SLG and PVME on the pore size distribution of a N50 membrane (de: membrane pore diameter); (a) clean, top side, (b) /3-LG, top side, (c) PVME, top side, (d) PVME, bottom side.
applicability of electron microscopy for obtaining pore size distributions of the track-etched membranes used. Adsorption of 8-I-G on the N30, N50 and N80 membranes was also found to narrow the pore openings at the bottom side of these membranes (Table 2). This suggests that, despite the pore narrowing, the adsorption period used (30 min) was sufficient for the protein mole-
285
cules to be transported (mainly by diffusion) to the bottom side of the membranes (thickness 6 pm). The estimated adsorbed layer thickness in case of the N50 and N80 membranes (8 nm) is in agreement with the observed layer thickness of adsorbed /?-LG at the top side of the membranes. In case of the N30 membrane, the layer thickness is somewhat smaller (5 nm). This might indicate that the adsorption time used has some limiting effect on the penetration depth of protein in the narrowed pores ( - 11 nm, top side) of the N30 membrane. Interestingly, no pore narrowing could be observed on the micrographs of the bottom side of the N50 membrane fouled by PVME adsorption (Fig. 3d). This can be explained by strong limitation of the rate of diffusion of PVME in the membrane pores. In addition to electron microscopy, the fouling of the track-etched membranes by adsorption of a (bio)polymer was also characterized by membrane flux measurements (Fig. 4). The increase in the total, relative resistance (RJR,, with R, the resistance of the clean membrane) of the membranes fouled by the synthetic polymers MC and PVME is greater than that observed for protein adsorption. In the case of MC
TABLE 2 The average pore diameter, dp, and, between brackets, adsorbed layer thickness, 6, of unmodified and fouled Nuclepore membranes Membrane type
Unmodified dr (nm)
PVME adsorption dp (&ME) (nm)
p-LG adsorption dr (&-I.& (nm)
toP
bottom
toP
bottom
N30
26
24
ii (a)
14 (5)
N50
38
34
16 (11)
ia (a)
N60
62 601
64 601
45 (6)
47 (3)
‘From SEM micrographs; other values from TEM micrographs.
tJJP
bottom
25 (7)
36 (-0) _
286
adsorption (pore diameter > 20 nm) and protein adsorption an increase in the (average) pore diameter will reduce the degree of fouling (i.e. the relative membrane resistance RJR,). This suggests internal adsorption, as a larger pore will narrow comparatively less through adsorption at the pore wall. The relatively slight fouling show in in this context by the N15 membrane as a result of MC and PVME adsorption can be explained by the adsorption time adopted (30 min), which, especially for the small pore diameters, may have been too short to achieve full coverage of the internal surface. The latter is in agreement with the above-mentioned observation that the pore size distribution of the bottom side of the N50 membrane is not affected by adsorption of PVME (Table 2). The mechanism of adsorption of MC and PVME thus revealed is depicted in Fig. 5. Incomplete internal adsorption to the N50 membrane could also explain why, in the case of PVME adsorption, the N80 membrane shows a higher fouling level than the N50 membrane. The relatively low degree of pore narrowing of the comparatively large pores of the N80 membranes (about 62 nm) after adsorption of the first polymer molecules makes it plausible that the internal surface of this membrane type can be covered completely by a (mono)layer of polymer. If this last is assumed, an estimate of the thickness of the adsorbed layer, S, can be obtained in a simple way by using the Poiseuille equation:
*+(1-@-‘l’) in which d, is the average diameter of the clean membrane pores (about 62 nm). For/SLG, MC and PVME this gives layer thicknesses of, respectively, 3.3,6.8 and 11 (nm). The order of magnitude of these values is comparable to that of the thicknesses obtained from the shift of the pore size distributions
L.E.S. Brink et al&J. Membrane
Sci. 76 (1993) 281-291
%n I
Fig. 4. Total, relative membrane resistance (RJR,) of the track-etched membranes as a function of the average pore diameter (d,) of the clean membrane; adsorption of /I-LG (O),MC (+) orPVME (0).
N15
N30
N50
N80
Fig. 5. Mechanism of adsorption of MC and PVME to the polycarbonate model membranes.
(Table 2). The protein layer thickness found is smaller than the previously determined value ( N 8 nm) . However, both estimates are in reasonable agreement with known values of the dimension of a P-LG dimer (two spheres of N 3.5 nm diameter each [ 171). The value obtained for the thickness of adsorbed MC (a polysaccharide, molecular weight about 14,000) compares favourably with the corresponding value of polysaccharides adsorbed at a water-silica
L.E.S. Brink et al/J. Membrane
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Sci. 76 (1993) 281-291
interface (4 to 5 nm at molecular weight of - 15,000 [ 181). The PVME layer thickness is greater than the TEM value (7 nm, Table 2)) which might, however, be explained by shrinking of the adsorbed layer during TEM sample preparation. Finally, the magnitude of the MC and PVME layer thicknesses lends support to the mechanism of adsorption to the 11 nm membrane depicted in Fig. 5. Soon after the first contact with the membrane, the MC and PVME molecules will partly block the pore entrance, thereby protecting the rest of the pore against internal fouling.
15 13 11
c 2
E
9 7 5 3 1
GR81
19 I-
Membrane fouling due to filtration of a protein solution
PVNE
GR61
GR51
GR'KI
b
17 15 13
Figure 6 compares the resistance increases (flux decreases) found for different types of polysulfone membranes as a result of filtration of a P-LG solution. A distinction is made between protein fouling by UF/MF of an unmodified polysulfone membrane (first bar), the ‘fouling’ resulting from preadsorption with MC (Fig. 6a) or PVME (Fig. 6b) (second bar), protein fouling by UF/MF of the MC or PVMEmodified membrane (third bar), and lastly the total fouling level produced by preadsorption and filtration (fourth bar). The first phenomenon that catches the eye is that fouling of an unmodified membrane (first bar) turns out to be strongly dependent on the (average) pore diameter of the membrane type. An increase of the pore diameter (i.e. an increase of the molecular weight cut-off - see Table 1) results in a considerably lower fouling level of the UF membranes GR81/61/51. This is readily accounted for by pore narrowing through internal protein adsorption, which has a particularly marked effect when the pores are relatively small (see also the discussion at Fig. 4). The strong increase in the degree of fouling that results from use of the GR40 membrane,
qE 11 0.Y 9 7 5 3 1
n 0
q q
GR81 GR61 GR51 GRW fouling of unmodifiedmembrane by UF/MF 'fouling'by presorptionwith IIC(a) or PVME (b) fouling of modified membrane by lJF/MF fouling by wesorption and UF/W
Fig. 6. Effect of pore diameter on the relative membrane resistance (RJR,) of polysulfone membranes.
which has considerably larger pores and which in fact is not an UF but a MF membrane, indicates that larger pores ( > approx. 30 nm) might be fouled by a different mechanism. The latter was investigated by performing comparable filtration experiments with track-etched membranes (Fig. 7). The degree of protein fouling of an unmodified track-etched membrane (first bar) increases when membranes with larger pore diameters are used. Assuming that the average pore diameter of the N15 membrane is comparable to that of the GR51 membrane (Table 1) , this observed trend is in
288
L.E.S. Brink et al./J. Membrane Sci. 76 (1993) 281-291 71 MC
a
Nl5
N30
N50
N80
125 PVnE
b
32
1 N15 I 0
N30
fouling 'fouling'
N80
1150
of umdified
menWane
by wesorptipn
q
fouling
of modified
m
fouling
by Presorption
with
membrane and
N200 by UF/E IIC (a) or PVE
(b)
by UF/W lJF/W
Fig. 7. Effect of pore diameter on the relative membrane resistance (RJR,) of track-etched membranes.
agreement with the similar tendency of the GR51/40 membranes (greater fouling at larger pores). SEM micrographs of the fouled N200 membrane reveal partly blocked pores. This phenomenon is even more pronounced in case of the N400 membrane (Fig. 8a,b). A similar kind of fouling of the N400 membrane is observed in the case of filtration of a solution of an other whey protein, BSA (Fig. 8c). Pore
narrowing as a result of (ultra) filtration of a protein solution was also measured. The decrease of the average pore diameters of the N30/ 50/80/200/400 membranes (top and bottom side) is comparable to those listed in Table 2. However, the results of Figs. 6 and 7 (first bars) as well as the electron micrographs of Fig. 8 suggest that in all likelihood pore blocking is the main fouling mechanism in the case of membranes with larger pores ( > about 30nm). In view of the molecular dimensions of the whey proteins /?-LG and BSA in comparison to the pore dimensions of the MF membranes used, it seems likely that (large) protein aggregates and/or multilayer adsorption play an important role in the pore-blocking mechanism. Next we consider the ‘fouling’ caused by preadsorption of MC and PVME (second bar ) . Especially the adsorption of PVME to the polysulfone membranes used (Fig. 6b) causes a substantial resistance increase, which is in agreement with the results shown in Fig. 4. The ‘fouling’ of polysulfone membranes due to both MC and PVME becomes less with an increase in the pore diameter (GR81/61/51), but more serious again in the membrane having the largest pores (GR40). Taking into account the above-estimated layer thicknesses of adsorbed MC and PVME, and the fact that these polymers can only penetrate to a limited extent into a pore of approx. 11 nm (Fig. 5)) it is possible to visualize a mechanism of MC and PVME adsorption to the polysulfone membranes as depicted in Fig. 9. Of course, the membrane pores shown in Fig. 9 only present a highly simplified picture of the complex pore morphology of the polysulfone membranes. The scale is roughly comparable to that of Fig. 5 (average pore size of the GR61 membrane about half the size of the N15 membrane). Internal adsorption of MC and PVME is regarded as unlikely in the case of the GR81 and GR61 membranes, given the average pore diameter of the GR61 membrane (approx. 5 nm, determined by permporometry
L.E.S. Brink et al./J. Membrane Sci. 76 (1993) 281-291
GR81
GR61
GR51
GR40
GR61
GR51
GR40
b: PVRE
GR81
Fig. 9. Mechanism of adsorption of MC (a)and PVME (b) to polysulfone membranes.
Fig. 8. SEM micrographs (top view) of protein-fouled tracketched membranes; (a) N200, b-LG, (b) N400, p-LG, (c) N400, BSA.
[ 191) . The pore diameter dependence of ‘fouling’ bypreadsorption of the GR61/61/51 membranes can be explained by partial clearing of the pore entrance where larger pores are involved. Internal adsorption does play a significant role in the GR40 membrane. The latter phenomenon can explain the relatively high degree of ‘fouling’ due to preadsorption in case of the GR40 membrane (Fig. 6a and b). Finally, the third and fourth bar in Figs. 6 and 7 represent, respectively, the fouling by protein and the total fouling level (MC or PVME + protein) of membranes presorbed with MC or PVME. For the three UF polysulfone membranes (GR61/61/51), preadsorption with MC (Fig. 6a) gives an overall resistance increase that is smaller than the increase found for an unmodified membrane (first bar). In practical terms this implies the possibility of achieving a given degree of concentration of the protein solution in a shorter time, or a higher degree of concentration in the same period of time. The same applies to ultrafiltration by means of the PVME-presorbed GR51 membrane (Fig. 6b ). For the GR61 and GR61 membranes PVME preadsorption seems less useful on account of the strong ‘fouling’ effect produced by modification with PVME. A further notable point is that the GR61/61/51 mem-
290
branes pretreated with PVME hardly become fouled during ultrafiltration (third bar). This lends additional support to the theory underlying the adsorption mechanism outlined in Fig. 9 (b). The pore entrances are effectively shielded by PVME, so that the protein molecules are incapable of penetrating into the pores. In this case a variation in the pore diameter no longer has any influence on protein fouling. A less efficient pore shielding by MC (Fig. 9a) might explain why the fouling of MC-modified membranes is still dependent on the pore size. As already discussed above, (unmodified) membranes with pores larger than about 30 nm (GR40, N50, N80, N200) are strongly fouled during filtration of a protein solution. Pore blockage seems to be a major cause of this membrane fouling. From comparing the degree of fouling of unmodified membranes with the fouling by preadsorption and UF/MF (first and fourth bar in Figs. 6 and 7)) it can be concluded that preadsorption cannot prevent this kind of fouling. Protein molecules (aggregates) can enter the pores of the mentioned (MF) membranes as the adsorbed polymers cannot cover the pore openings (Figs. 5 and 9). Furthermore, adsorption of MC or PVME at the pore wall might modify the internal surface characteristics and reduce internal protein adsorption, but, apparently, cannot prevent blockage of the membrane pores.
Conclusions Regarding the UF membranes the conclusion can be drawn that it is possible to reduce membrane fouling through protein adsorption by presorbing the membrane surface with a hydrophilic polymer which protects it against internal adsorption of protein. The polymer derives its protective action from the fact that it occupies adsorption positions and is hard to displace by the protein molecules, but also - and perhaps especially - from the fact that the
L.E.S. Brink et al./J. Membrane
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polymer partly seals off the pore entrance and prevents internal protein adsorption by steric repulsion. In the case of protein fouling of MF membranes the preadsorption technique does not seem to prevent the blockage of the membrane pores. Acknowledgements These investigations were supported (in part) by the Programme Commission Membrane Technology (PCM) of the Ministry of Economics. References I. Jitsuhara and S. Kimura, Structure and properties of charged ultrafiltration membranes made of sulfonated polysulfone, J. Chem. Eng. Japan, 16 (1983) 389393. 2 F.F. Stengaard, Characteristics and performance of new types of ultrafiltration membranes with chemically modified surfaces, Desalination, 70 (1988) 207224. 3 F.F. Stengaard, Preparation of asymmetric microfiltration membranes and modification of their properties by chemical treatment, J. Membrane Sci., 36 (1988) 257-275. 4 A. Higuchi, N. Iwata, M. Tsubaki and T. Nakagawa, Surface-modified polysulfone hollow fibers, J. Appl. Polym. Sci., 36 (1988) 1753-1767. 5 A. Higuchi, N. Iwata and T. Nakagawa, Surface-modified polysulfone hollow fibers; Fibers having CH,CH,CH,S03 segments and immersed in HCl solution, J. Appl. Polym. Sci., 40 (1990) 709-717. 6 A. Higuchi and T. Nakagawa, Surface-modified polysulfone hollow fibers; Fibers having a hydroxide group, J. Appl. Polym. Sci., 41 (1990) 1973-1979. 7 L. Breitbach, E. Hinke and E. Staude, Heterogeneous functionalizing of polysulfone membranes, Angew. Makromol. Chem., 184 (1991) 183-196. 8 M. Nystriim and P. Jiirvinen, Modification of polysulfone ultrafiltration membranes with UV irradiation and hydrophilicity increasing agents, J. Membrane Sci., 60 (1991) 275-296. 9 0. Tozawa and D. Nomura, UF-treatment of soy milk using membrane coated with soybean lecithin, Membrane, 8 (1983) 361-363. 10 M.S. Le and J.A. Howell, The fouling of ultrafiltration membranes and its treatment, in: C. Cantarelli and C. Peri (Eds.), Progress in Food Engineering, ForsterVerlag, Kiisnacht, 1983, pp. 321-326. 1
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A.G. Fane, C.J.D. Fell and K.J. Kim, The effect of surfactant pretreatment on the ultrafiltration of proteins, Desalination, 53 (1985) 37-55. 12 K.J. Kim, A.G. Fane and C.J.D. Fell, The performance of ultrafiltration membranes pretreated by polymers, Desalination, 70 (1988) 229-249. 13 L.E.S. Brink and D.J. Romijn, Reducing the protein fouling of polysulfone surfaces and polysulfone ultrafiltration membranes; Optimization of the type of presorbed layer, Desalination, 78 (1990) 209-233. 14 H. Park, E.M. Pearce and T.K. Kwei, Thermal oxidation of blends of polystyrene andpoly (vinyl methyl ether), Macromolecules, 23 (1990) 434-441. 15 J.H. Hanemaaijer, T. Robbertsen, Th. van den Boomgaard and J.W. Gunnink, Fouling of ultrafiltration membranes. The role of protein adsorption and salt
16
17
18
I9
precipitation, J. Membrane Sci., 40 (1989) 199-217. T. Sato and R. Ruth, Stabilization of Colloidal Dispersions by Polymer Adsorption, Surfactant Sci. Ser., Vol. 9, Marcel Dekker, New York, NY, 1980, p. 24. H.A. McKenzie (Ed.), Milk proteins. Chemistry and Molecular Biology, Vol. 2, Academic Press, New York, NY, 1971, p. 307. I. Baudin, A. Ricard and R. Audebert, Adsorption of dextrans and pullulans at the silica-water interface. Hydrodynamic layer thickness measurements. Role in the fouling of ultrafiltration membranes, J. Colloid Interface Sci., 138 (1990) 324-331. F.P. Cuperus, Characterization of ultrafiltration membranes. Pore structure and top layer thickness, Ph.D. Thesis, University of Twente, Enschede, The Netherlands, 1990, p. 76.