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Protein fouling of surface-modified polymeric microfiltration membranes
journal of MEMBRANE SCIENCE
ELSEVIER
Journal of Membrane Science 116 (1996) 47-60
Protein fouling of surface-modified polymeric microfiltration membranes Jeffrey Mueller, Robert H. Davis * Department of Chemical Engineering, University of Colorado, Boulder, CO 80309-0424, USA
Received 5 July 1995; revised 2 November 1995; accepted 18 December 1995
Abstract The effects of varying morphology and surface chemistry on protein fouling of microfiltration membranes were investigated. In part I of the study, on the effects of varying morphology, results show that 0.2 ~ m track-etched polycarbonate (PC) membranes internally foul, with external fouling becoming the dominant means of fouling only at later times. A 0.2 /zm cellulose acetate (CA) membrane showed only internal fouling, while 0.2 /xm polysulfone (PS) and polyvinylidene fluoride (PVDF) membranes showed only external fouling. It is hypothesized that the low surface porosities of the PS and PVDF membranes lead to almost immediate external fouling, while the higher surface porosities of the PC and CA membranes allow for a significant period of time for internal fouling to occur. For each membrane, protein transmission remained constant or only slightly decreased during internal fouling, while a significant loss of protein transmission was observed during external fouling. In part II of the study, on the effects of various surface modifications, results show that surface-modified polyethylene and polypropylene membranes have lower initial fluxes than the unmodified membranes. However, the hydrophilic modified membranes demonstrated comparable final fluxes and lower percent flux declines than the unmodified membranes. The azlactone modified membranes showed very low long-term fluxes and large decreases in permeate protein concentration due to efficient protein binding. Again, protein transmission remained constant or only slightly decreased during internal fouling, while a significant loss of protein transmission was observed during external fouling. Keywords: Surface modified membranes; Fouling; Microfiltration; Biotechnology
1. I n t r o d u c t i o n Microfiltration is capable of separating fluids and particles below the 0.02 to l 0 /~m size range from larger particles. This separation is useful in many
* Corresponding author. Tel: (303) 492-7314; FAX: (303) 4 9 2 434l; E-mail: [email protected],
industries, particularly the pharmaceutical, biotechnology, and food and beverage industries. When microfiltration is performed with protein solutions, the flux (volume permeate collected per time per membrane area) decreases dramatically over time due to fouling, even though proteins are generally more than an order of magnitude smaller than the membrane pores. Internal fouling takes place when particles enter the membrane pores and deposit or
0376-7388/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved PH S0376-73 88(96)00017-8
48
J. Mueller, R.H. Davis/Journal of Membrane Science 116 (1996) 47-60
adsorb to the pore walls or entrance. External fouling occurs when particles deposit or adsorb as a layer on the surface of the membrane. The particular case of protein fouling in a microfiltration membrane process has been examined by other researchers. Flux decline due to fouling is attributed in a small part to protein adsorption, but the major contribution is from protein deposition during the dynamic and convective flow conditions, as reviewed recently by Belfort et al. [1]. Questions remain about the location of protein deposition and the mechanism of deposition. For example, Bowen and Gan [2] concluded that deposition of protein inside the membrane pores is an important mechanism for flux loss, whereas Kim et al. [3] concluded that protein fouling of various ultrafiltration and microfiltration membranes is a surface phenomenon, Other researchers provide evidence which attributes fouling behavior to the deposition of protein aggregates on the membrane surface [3-6]. Tracey and Davis [7] demonstrated that both phenomena, internal and external fouling, occur with track-etched polycarbonate membranes. Internal fouling was described using the standard and pore blocking models, while external fouling was described using a cake filtration model. A two-stage fouling mechanism was also proposed by Kelly et al. [8]. They attributed the initial flux decline to the deposition of large protein aggregates, and subsequent flux decline to the attachment of bulk protein to these aggregates, Recently, interest has focused on the development of surface modifications which can enhance flux or the transmission/rejection characteristics of specific proteins. For example, a hydrophilic surface coating is expected to reduce protein binding, increasing flux. Coatings can also be applied to chemically bind specific proteins and effectively separate these proteins from a solution. Dudley et al. [9] treated PVDF membranes with a surface grafting of phosphorylcholine. They showed that membranes with this non-protein-binding coating experience less severe flux declines and a 95% reduction in protein adsorption. Kim et al. [10] reduced nonselective adsorption of proteins by using a radiation-induced grafting of hydrophilic alcoholic hydroxyl (diol) groups onto polyethylene membranes. They found that protein saturation capacities could be reduced to 1 m g / m 3, the binding interaction was reversible rather than
irreversible, and the saturation capacity correlated well with the diol group density. Rasmussen et al. [11] studied the effectiveness of crosslinked, hydrophilic, azlactone-functional beads at binding vailous proteins. They found that the beads bound proteins at very high densities, e.g. Protein A at 397 mg/g. Recently, Gagnon and Coleman [12] made polyethylene microfiltration membranes, grafted with azlactone and derivatized with Protein A, and studied their reversible protein binding ability. They found static binding kinetics are limited by diffusion, and not reaction rate. The research which our group has undertaken examines both the mechanisms of protein fouling and the effects of various membrane morphologies and surface modifications on these mechanisms. Previous work focused on where protein fouling occurs and how operating conditions and membrane pore size affect fouling mechanisms in track-etched polycarbonate membranes [7]. In part I of the present study, we investigated the effects of varying membrane morphology on fouling mechanisms. In part II of this study, the effects of membrane surface modifications on membrane flux performance and fouling mechanisms were examined. Fouling mechanisms were identified in the present work using a method described by Tracey and Davis [7]. This method involves examining the total resistance (which is inversely proportional to flux) versus time curve to see whether it is concave up or concave down. A concave up curve indicates internal fouling, which can be described by a standard blocking (pore radii decreasing) or pore blocking (number of pores decreasing) model. A concave down curve indicates external fouling, which can be described by a cake filtration (gradual surface layer build-up) model. In addition, Tracey and Davis [7] showed that the protein transmission is nearly 100% during internal fouling, but decreases during external fouling.
2. Materials and methods
Protein fouling experiments were conducted using a 10 cm 3 Nuclepore stir cell (model $25-10). The complete experiment apparatus is shown in Fig. 1. A 4-1, 304-stainless-steel pressurized vessel (Alloy Products Corp., no. 72-01) served as the feed reser-
J. Mueller, R.H. Davis~Journal of Membrane Science 116 (1996) 47-60
49
airfilter ~~~ pressure ,permeate[ line~~pressuri )r~egul~1at°~uage r~-z~edPVaTve t I ~~sti~l i ~)~vessel ~ feed r stirrer magneticmicrobalancecomputer reservoi Fig. 1. Schematic of the experimental apparatus.
voir for the water or protein solution. It was pressurized with air run through a Matheson dual stage regulator (8-H) and a 0.02 /xm Matheson filter (model 6190). A 0 - 6 0 psig pressure gauge on the feed reservoir measured the apparatus pressure with a maximum error of 1-2%. The Nuclepore stir cell was stirred using a magnetic stirrer (Fisher Scientific model 115004) with a speed setting of 8. This setting corresponds to a stir speed of 575 rpm and was sufficient to eliminate concentration polarization [7]. The permeate exiting the stir cell was collected in a vessel placed upon a mass balance. The balance (Mettler PE3600) was interfaced with a computer (Data Store 386-20D) to collect mass and time data using a computer program written in Quickbasic 4.0. The mass versus time data were differentiated numerically to obtain flux values. The total resistance was then determined by RTo T = AP/tzJ, where A p is the transmembrane pressure, J is the permeate flux, and/~ is the permeate viscosity. Samples of the permeate were collected from the permeate line at specific time intervals for later protein analysis,
The protein used in all the experiments was bovine serum albumin (BSA). The powdered BSA, supplied by Sigma (catalog no. A-3803, < 0.005% fatty acid content, initial fractionation made by heat shock), was dissolved in deionized and prefiltered water just prior to each experiment run. The pH of the 0.1 g/1 BSA feed solution was 6.6, and the pH of the 1.0 g/1 BSA feed solution was 7.1. The protein concentrations of the permeate samples were measured following each experiment using a Sigma protein assay kit (Sigma catalog number P-5656) and a Hewlett Packard diode array spectrophotometer (model 8452A), employing a modified Lowry method. There were four membranes tested in part I of the experiments, listed in Table 1. Each had a reported nominal pore size of 0.2 /zm. The track-etched polycarbonate (PC) membranes supplied by Nuclepore have a poly(vinylpyrrolidone) coating to make them hydrophilic. They have a regular pore structure of cylindrical pores with a uniform diameter and a narrow size distribution. Their track-etched morphol-
Table 1 Characteristics of the polymeric membranes used in part I Membrane type
Membrane thickness (/xm)
Nominal pore diameter (/xm)
Bulk porosity (%)
Surface porosity (%)
Surface character
Track-etched polycarbonate Cellulose acetate Polysulfone Polyvinylidene fluoride
10 125 305 279
0.2 0.2 0.2 0.2
9 66 65-80 65-80
9 26 12 1
Slightly hydrophilic Highly hydrophilic Slightly hydrophilic Slightly hydrophilic
50
J. Mueller, R.H. Davis~Journal of Membrane Science 116 (1996)47-60
Table 2 Characteristics of the polymericmembranesused in part II Membrane type
Membrane thickness (/zm)
Maximumpore diameter (t~m)
Coating thickness (/zm)
Bulk porosity (%)
Surface porosity (%)
Surface character
Base polyethylene Hydrophilic (PVA) polyethylene Azlactonecoated polyethylene Base polypropylene Hydrophilic (PVA) polypropylene Azlactone coated polypropylene
60 60 60 60 60 60
0.5 0.5 0.5 0.5 0.5 0.5
N/A 0.01 0.01 N/A 0.01 0.01
75-80 75-80 75-80 75-80 75-80 75-80
34 7.5 N/A N/A N/A N/A
Hydrophobic Hydrophilic Chemically active Hydrophobic Hydrophilic Chemically active
ogy gives equal surface and bulk porosities (9%). The low-protein-binding and highly hydrophilic cellulose acetate (CA) membranes supplied by Micro Filtration Systems have an irregular pore structure and a high bulk porosity (66%). They also have a high surface porosity (26%). The GRMO.2PP polysulfone ( P S ) a n d FSMO.2PP polyvinylidene fluoride (PVDF) membranes supplied by Dow Chemical have lower surface porosities (12% and 1%, respectively) and high bulk porosities (65-80%). These membranes are both slightly hydrophilic due to a bydrophilic additive present, In part II of the research, six membranes supplied by 3M were tested, listed in Table 2. The unmodified
polypropylene (PP) and polyethylene (PE) merebranes were coated with polyvinylalcohol (PVA) to make them hydrophilic, or grafted with azlactone and derivatized with Protein A to present a high-protein-binding surface [12-14]. The membranes all had a reported m a x i m u m pore size of 0.5 p~m, as determined by bubble point, and high bulk porosities, 7 5 - 8 0 % . The surface coatings grafted onto the membrane were reported by the manufacturer to be approximately 10 n m thick. The coating was primarily on the external surface, but it did extend slightly into the membrane structure and the membrane pores. The bubble-point pore size, which is a measure of the largest pores, was not affected by the coatings.
Table 3 Summary of results for experimentsin part I; the three sets of data are for 10 psig and 0.1 g/l, 20 psig and 0.1 g/1 and 10 psig and 1.0 g/1 (top, middle and bottom) Membranetype
Waterflux Initial (1/m e h)
Polycarbonate Cellulose acetate Polysulfone Polyvinylidene fluoride Polycarbonate Cellulose acetate Polysulfone Polyvinylidene fluoride Polycarbonate Cellulose acetate Polysulfone Polyvinylidene fluoride
BSA solutionflux Final (1/m 2 h)
Average Initial decline (1/m 2 h) (%)
Final (1/m2 h)
3100 ± 200 1800± 500 41 6200± 500 6000± 500 3 210 ± 50 150 ± 40 29 240 ± 150 140 ± 60 42
3000 ± 400 6200 ± 500 220 ± 20 64 ± 23
110 _+60 380 77 ± 28 35 ± 18
7000 ± 300 2800± 500 60 11000 9400 10 460 + 20 300 ± 20 33 800 ± 300 220 ± 30 73
5500 ± 700 N/A N/A 120 ± 20
3100 ± 200 1800± 500 41 6200_+500 6000± 500 3 210 ± 50 150 ± 40 29 240 ± 150 140 _+60 42
2000 ± 300 N/A 190 ± 20 100 ± 20
BSA concentration Average Feed(g/l) decline (%) 96 94 66 45
Permeate(g/l)
Average rejection (%)
0.13 ± 0.03 0.10 ± 0.01 0.10 _+0.01 0.10 ± 0.01
0.03± 0.02 0.01 ± 0.01 0.03± 0.01 0.013
77 10 71 86
110 ± 40 98 N/A N/A N/A N/A 65 ± 21 47
N/A N/A N/A N/A
N/A N/A N/A N/A
N/A N/A N/A N/A
49 ± 13 98 N/A N/A 49 ± 2 74 20 ± 8 80
N/A N/A N/A N/A
N/A N/A N/A N/A
N/A N/A N/A N/A
J. Mueller, R.H. Davis / Journal of Membrane Science 116 (1996) 47-60
For the PE membrane, the surface porosity changed from 34% for the unmodified PE membrane to 7.5% for the hydrophilic coated PE membrane. Each membrane was immersed in isopropanol, and then rinsed with water immediately prior to placement in the stir cell. This insured that the new membranes were fully wetted before the experiment. In addition, the azlactone coated membranes were kept desiccated until immediately before use to prevent hydrolysis of the azlactone coating. Fresh bovine serum albumin solution and a new membrane were loaded into the apparatus for each experiment run. A typical experiment was run for 3 h at constant pressure and room temperature (2223°C). Each experiment was repeated three times, Most experiments were run at a transmembrane pressure of 10 psig and a protein solution feed concentration of 0.1 g BSA/1. Permeate samples were collected and analyzed for protein content for each of the experiments at these conditions. In part I, some experiments were run at a different pressure (20 psig) or a different feed concentration (1.0 g BSA/1) to study the effects of these operating variables on the fouling mechanism. The apparatus was thoroughly cleaned after each experiment run with a rinse of 0.5 wt% NaOH solution followed by a rinse of prefiltered, deionized water. Tests showed this
51
cleaning procedure was sufficient to remove any remaining protein from the apparatus.
3. Results and discussion
3.1. Part 1. Effects of membrane morphology 3.1.1. Water flux A set of water flux tests at 10 and 20 psig transmembrane pressures was completed for each membrane. Table 3 reports the membrane water flux as the arithmetic mean of the three experiment runs, each with a new membrane. The plus/minus numbers shown represent one standard deviation about the mean. Table 3 shows that the initial water flux of the CA membrane is about 2 times larger than for the PC membrane and 25-30 times larger than for the PS and PVDF membranes. Fig. 2 depicts the water fluxes for the four membranes. The lines in Fig. 2 represent best-fit polynomials through the arithmetic means of the data for three repeated experiments. The large differences in fluxes are due to the differences in morphology, thickness, and surface characteristics of the four membranes (see Table 1). The water flux of each membrane decreased over the 1500 s period of each run (Fig. 2). Using a 10
7000
Cellulose Acetate
6ooo
5000
NE 4 0 0 0
E~'3°°°= 2000
Polysulfone
~
1000
0
Polyvinyldiene Fluoride
. . . .
,
~
I
I-
200
400
' | = -
-
I .
.
.
600
.
I-
-
-
800 Time
1 1000
-
-
"
-II 1200
"
-
-
"i 1400
-
eILt 1600
(seconds)
Fig. 2. Water flux at 10 psig transmembrane pressure for the four polymeric membranes used in part I.
J. Mueller, R.H. Davis~Journal of Membrane Science 116 (1996) 47-60
52
psig transmembrane pressure, the water flux declines were about 30-40% of the initial flux (Table 3). The flux decline is attributed in part to the compressibility of the polymeric membranes. There may also be minute particulates in the feed or from the tubing connecting the feed reservoir and the stir cell, as evidenced by membrane discoloration after the water flux experiments. The feed water was not degassed; this may have led to the formation of microscopic bubbles on the permeate side of the membrane, which would heave contributed to the water flux decline. The water flux did not in general increase linearly with pressure (see Table 3). As Bowen and Gan [2] found, membranes are reversibly compressed at higher pressures, leading to greater resistance and lower flux.
amount of protein solution that is carried to and fouls the membrane. In the high feed concentration experiment, the increased flux loss may also be caused by increased protein aggregation. For a higher concentration of protein, the aggregates are expected to be larger and more numerous than the aggregates at a lower concentration. Fig. 3 is a plot of the flux versus time curves for the four membranes tested under identical conditions of 10 psig transmembrane pressure and 0.1 g BSA/1 feed concentration. The cellulose acetate membrane exhibited the highest flux and least flux decline, followed by the polycarbonate, polysulfone, and polyvinylidene fluoride membranes. The lower fluxes for the PS and PVDF membranes are due, in part, to their greater thicknesses. The CA membrane had a higher flux because it is thinner and exhibits very low binding of proteins. The PC membrane had an intermediate flux, despite its small thickness, because of its low porosity.
3.1.2. Protein solution flux Experiments were carried out at 20 psig transmembrane pressure and 0.1 g B S A / I feed concentration, and at 10 psig transmembrane pressure and 1.0 g BSA feed concentration, in addition to the typical experiment, which was run at 10 psig transmembrane pressure and 0.1 g BSA/1 feed concentration. Results in Table 3 show that a higher pressure or a higher feed concentration led to a higher overall resistance and a greater percentage of flux loss for each of the membranes. This is due to the increased
3.1.3. Total resistance curues The total resistance versus time curves for the polycarbonate membrane under different conditions are shown in Fig. 4. The PC membrane exhibited internal fouling (concave up part of the curve), followed by a transition to external fouling (concave down part of the curve) after about 1 h of operation
7000 6000 ~
e
5000 4000 ", . . . . x 3000 ~ P'o,ycarDonate =
~
~
/
~ , , Polysulfone
2000 1000 0 I.'.'.'.'.'.'.'.'.'E-_. . . . . . . . . . . . 0 2000
4000
~"~" ~ 6000
_
_ 8000
~ 10000
12000
Time (seconds) Fig. 3. Flux decline at 10 psig transmembrane pressure and 0.1 g B S A / I feed solution for the four polymeric membranes used in part I.
J. Mueller, R.H. Davis~Journal of Membrane Science 116 (1996) 47-60
8.OE+~2 10 ps,O, l o
o BSA/,
/
~03 -I.IJ
' ""
I
./
,"
.."
/
3.0E+12 --
2.0E+12
observed previously by Lee et al. [15] for particle filtration with microporous membranes. Increased transmembrane pressure and increased feed concentration also resulted in more severe fouling of the PS and PVDF membranes (Table 3), although external fouling behavior was observed from the start of protein filtration for these membranes. The four membranes tested are made of different materials with different morphologies, indicating that they may foul in different ways. Fig. 5 shows the total resistance curves of the four membranes tested.
.-:2.'.
4.OE+12
/
: ~
"
•
20 psio, 0.1 g BSA/I
•
/
..
10ps~g.o.1 g BSA/~
1.oE.~2
~08E+0 2000
4000
6000
I 10000 12000
8000
TIME (seconds)
53
The PC and CA membranes have higher fluxes Fig. 4. The effect of various operating conditions on the total resistance versus time curves for the polycarbonate membrane.
(lower resistances)
for the experiments with 10 psig transmembrane pressure and 0.1 g B S A / 1 feed concentration. This behavior was observed for both transmembrane pressures, but considerably more fouling (higher total resistance) occurred at the higher pressure. When the feed concentration was increased, the resistance increased quickly and sharply, and external fouling became the dominant mechanism within 30 min. Tracey and Davis [7] also saw this rapid switch from internal to external fouling at higher protein concentrations for 0.2 /~m PC membranes. Internal pore fouling followed by external cake filtration has been
whereas this transition had not occurred, or was just beginning to occur, by the end of the experiment for the low-fouling CA membrane. In contrast, the PS and PVDF membranes have lower fluxes (higher resistances), and the fouling behavior was external from the start.
and
less
fouling. Their fouling
behavior was initially internal. Then, a transition to external fouling occurred for the PC membrane,
3.1.4. Protein transmission In order to assess the effects of internal and external fouling on protein transmission, and to provide more evidence that two distinct fouling mechanisms do occur, the permeate protein concentration
6.0E+12
Polyvinyldiene Fluoride 5.0E+12
~ ~v 4.0E+12 c
'~ 3.0E+12 '~ (9
Polysulfone
2.0E+12
1.0E+12
~
..."
""
""
~'
/f
Cellulose
O.OE+O0 2000
Acetate
,
I
I
I
4000
6000
8000
10000
Time
12000
(seconds)
Fig. 5. Total resistance versus time curves at 10 psig transmembrane pressure and 0.l g B S A / I feed solution for the four polymeric membranes used in part I.
J. Mueller, R.H. Davis~Journal of Membrane Science 116 (1996) 47-60
54
0.12
from each membrane at 10 psig transmembrane pressure and 0.1 g B S A / 1 feed was measured as the experiment progressed. Fig. 6 shows the results ob-
7"
2 01o ~~
"
" "
~
'
~
0.08
meate protein concentration data for the PC membrane. The transition phase between internal fouling (concave up resistance curve), and external fouling (concave down resistance curve) was matched by a sharp decrease in protein transmission. Similar resuits were obtained by Tracey and Davis [7]. The observed increase in protein concentration in the permeate near the end of the experiment may be from experimental variation (Table 3), or for an increase in protein concentration in the stir cell due to the protein rejection [7]. Fig. 7 shows the permeate protein concentration data for the CA, PS, and P V D F membranes versus time for filtration at 10 psig transmembrane pressure and 0.1 g B S A / 1 feed concentration. The permeate protein concentration remained near 0.1 g B S A / 1 (100% transmission) for the CA membrane, until about 9000 s of filtration, after which the transition from internal to external fouling began and the protein concentration in the permeate declined. In contrast, a large decrease in protein concentration occurred almost immediately for the P V D F membrane, which exhibited much higher resistance and external fouling from the start. The PS membrane, which
z m 008 Ocn o E
zE ,v,~ 0.04 ~ 002 o_ o
I
I
aooo
4000
I
I
6000
8000
I
10000 12000
TIME (seconds)
Fig. 7. Permeate protein concentration versus time data at l0 psig transmembranepressure and 0.1 g BSA/I feed solution for the CA ( • ) , PS ( • ) and PVDF ( • ) membranes.
exhibited external fouling with an intermediate resistance, suffered a more gradual decrease in protein transmission.
3.1.5. Discussion of results of part I The permeate flux and protein concentration data for the four 0.2 /zm microfiltration membranes examined in part I of this study indicate that internal fouling of the membranes is followed by external fouling. Internal membrane fouling is characterized by a concave up resistance curve and nearly complete transmission of protein. During internal foul-
2.5E+12
0.1
2E+12
0.09
A
0.08
~
0.07
I~
i.
E t~
1.5E+12
0.06
o.o,
I1~
1E+12
0.04
I-
0.03
C ¢.)
.=_ 5E+11
0.02 0.01
0
i
I
I
2000
4000
6000
o 8000
I 10000
a.
0 12000
Time (seconds)
Fig. 6. Total resistance and permeate protein concentration versus time at 10 psig transmembrane pressure and 0.1 g BSA/1 feed solution for the polycarbonate membrane.
J. Mueller, R.H. Davis~Journal of Membrane Science 116 (1996) 47-60
ing, it is hypothesized that protein molecules or aggregates attach to the pore walls or mouths so that the flux is reduced due to the resulting pore blockage and constriction. However, some of the pores remain at least partially open, with the openings being considerably larger than the characteristic size of individual protein molecules, and so protein transmission occurs without significant rejection as the permeate solution flows through these openings. Eventually, the pore blockage and constriction become sufficiently severe that a significant fraction of the protein is rejected and an external fouling deposit begins to form on the membrane surface. External fouling is
55
characterized by decreased protein transmission and a concave down resistance versus time curve due to the growing fouling layer. For the PC membrane, the period of internal fouling lasted for less than one-half of the 3-h experiment, whereas for the high-flux, low-fouling CA membrane, internal fouling was extended for almost the entire experiment. In contrast, external fouling occurred almost immediately for the low-flux PS and PVDF membranes. In order to characterize the surface morphologies of the membranes and relate these to the fouling behavior, scanning electron micrographs (SEMs) of the clean membranes were taken
Fig. 8. Scanning electron micrographs of the membrane surfaces of (a) polycarbonate, (b) cellulose acetate, (c) polysulfone and (d) polyvinylidene fluoride.
J. Mueller, R.H. Davis~Journal of Membrane Science 116 (1996) 47-60
56
(see Fig. 8). The PC membrane has uniform pores with 0.2 /xm openings and a measured surface porosity of 9%. In contrast, the CA membrane has much larger openings (0.3-1.0 /xm) and a larger surface porosity (26o/0); these characteristics, along with its highly hydrophilic nature, account for the lower surface fouling of this membrane. The PS membrane has intermediate-sized surface openings (0.2-0.5 /xm) and surface porosity (12%); its lower flux and almost immediate external fouling may be due to its greater thickness and lower hydrophilicity. Finally, the PVDF membrane has a wide range of surface openings (0.1-1.0 /zm) but very low surface porosity (1%), with the latter likely contributing to its low flux and rapid external fouling. Tracey and Davis [7] also presented scanning electron micrographs, which showed the deposition of protein aggregates in the surface of 0.2 /zm polycarbonate membranes, followed by the formation of a continuous fouling layer. Tracey and Davis [7] showed that external fouling occurs more rapidly when the membrane pore size is decreased, and they hypothesized that this was because the protein aggregates were larger than the pore openings for the smaller pore sized membrane. Both our data and that of Tracey and Davis [7] show that increased protein concentrations lead to rapid onset of external fouling, as the protein aggregates which deposit on the rnembrane are more numerous and perhaps larger. More recently, Kelly and Zydney [8] also implicated protein aggregates in the initiation of protein fouling of microfiltration membranes, and they presented a two-step model of the fouling process in which aggregates deposit on the membrane surface and within the pore mouths, and then these aggregates
i ~1
~
~ ,
I
i;
i L~~ ~
!mier,_ 1 .... , ~ u
_l.Lql
~
i
] Fig. 9. Scanning electron micrographs of the membrane surfaces of (a) unmodified polyethylene and (b) hydrophilic coated
polyethylene.
serve as nucleation sites for the formation of a deposit layer which eventually covers the membrane surface.
Table 4 Summary of results for experiments in part II Membrane type
Base polyethylene Hydrophilic PE AzlactonePE Base polyethylene Hydrophilic PE Azlactone PP
Water flux
BSA solution flux
Initial (1/m 2 h)
Final ( l / m 2 h)
Average decline
Initial ( l / m 2 h)
Final ( l / m 2 h)
4 2 0 0 + 200 510 _+ 100 980_+450 5400_+ 700 1700 _+ 500 980_+ 450
2300_+ 500 250 _+ 60 400_+ 150 2200_+ 100 1200 _+ 500 210 + 100
46 50 59 59 30 74
2400 + 500 500 _+ 90 360_+ 310 4100+400 1700 _+ 300 150_+ 50
77_+ 5 64 _+ 15 0 97_+ 42 62 _+ 16 34 + 11
(%)
"
i
BSA concentration Average decline
Feed
97 87 100 98 96 77
0.14_+0.08 0.09 _+ 0.01 0.11 _+0.02 0.09_+ 0.02 0.09 _+ 0.01 0.09
(%)
Permeate (g/l)
Average rejection
0.06_+ 0.01 0.05 _+ 0.01 0 0.07_+ 0.03 0.05 _+ 0.03 0
60 48 100 20 51 100
(%)
J. Mueller, R.H. Davis~Journal of Membrane Science 116 (1996) 47-60
57
4000
~ Base PP
3500
\ 3000
k
~" 2500
/ B a s e PE
/~
2000
•
Hydrophilic PP "
~
/Azlactone PP
E 1500
oOOoo
.
0 2000
4000
6000
8000
10000
12000
Time (seconds) Fig. 10. Flux decline at l0 psig transmembrane pressure and 0.1 g BSA/1 feed solution for the six polymeric membranes used in part II.
3.2. Part II. Effects of membrane surface modifications
branes were consistently lower than those of the unmodified membranes. Water flux loss after surface modification of PE membranes was also observed by Kim et al. [10]. This seems surprising, since the surface coating is reported to be only about 10 nm thick. It is thought that the surface coating, entangled with the unmodified membrane surface, contracts when it dries, effectively shrinking the surface pore
3.2.1. Waterflux Water fluxes were examined at 10 psig transmembrahe pressure for all six membranes used in part II. As shown in Table 4, the water fluxes for the hydrophilic and azlactone surface modified mem6E+12
/" i
HydrophilicPP 5E+12
/
I-,llI, J~..
i
m AzlactonePP
Hydroph|lic PE
4E+12
Base PE
~ 2E+12
.- /.
1E÷12
.
.
8000
10000
0 0
2000
4000
6000
12000
Time (seconds) Fig. l l . Total resistance versus time curves at 10 psig transmembrane pressure and 0.1 g B S A / I feed solution for the six polymeric membranes used in part II.
J. Mueller, R.H. Davis/Journal of Membrane Science 116 (1996) 47-60
58
sizes. Fig. 9, which presents scanning electron micrographs of the unmodified PE membrane and the hydrophilic PE membrane, shows an example of this hydrophilic The unmodified PE membrane phenomenon. a has an open structure and a high surface porosity, while the coated PE membrane has more closed or coated structure with a low surface porosity. Large water flux declines of 3 0 - 7 0 % were observed during testing of these membranes (Table 4). These declines were observed for the modified as well as unmodified membranes. As for the membranes in part I, the water flux decline is attributed to membrane compaction and possibly to the presence of microscopic particulates and bubbles,
3.2.2. Protein solution flux and resistance All experiments for the modified and unmodified membranes used in part II were run at 10 psig transmembrane pressure and 0.1 g B S A / I feed concentration. Even with this low feed concentration and a relatively large, 0.5 /xm pore size, severe flux declines were observed for both the PE and PP membranes with and without surface modifications (see Fig. 10). The resistance versus time curves (Fig. 11) show that a period of internal fouling occurred for both unmodified membranes, but external fouling took over after about 5000 s for the PE membrane and after about 8000 s for the PP membrane. The resistance behavior for the hydrophilic modified PE and PP membranes is similar to that for their unmodified, hydrophobic counterparts (internal followed by external fouling). However, the azlactone-coated 012 ~_ O.lO ~ ¥ z~ z ~ 0.o6
.
008~ ~ "
°° z ~ ,~ m~
0.04
~o ~_°: 0.020
I
=
L
I
I
2000
4000
6000
BOO0
10000
O,tla| ~ ~
0.10
•
z!~~~ 0.o8 z ~ 0.06 0.04
• ~> ~ 0020
"
*
~
*
~
f~1. I
0
2000
•
4000
I
6000
±
I±
6000
T{M(E seconds)
±1
10000 12000
Fig. 13. Permeate protein concentration versus time at 10 psig transmembrane pressure and 0.1 g BSA/1 feed solution for the unmodified PP ( • ) , hydrophilic PP ( • ) and azlactone PP ( • ) membranes.
membranes fouled very rapidly, with very low longterm fluxes and large increases in the total resistance.
3.2.3. Protein transmission Figs. 12 and 13 show the protein concentration in the permeate data versus time for the polyethylene and polypropylene membranes, respectively. For the unmodified (hydrophobic) and PVA-coated (hydrophilic) PE membranes, the protein transmission was near 100% initially, and then it decreased after about 5000-6000 s, which corresponds to the transition from internal to external fouling. In contrast, the azlactone-coated PE membrane exhibited substantial protein rejection almost immediately. For the unmodified (hydrophobic) PP membrane, the protein concentration in the permeate declined only slightly from that in the feed, which is consistent with the observation in Fig. 11 that internal fouling continued for the majority of the experiment. In contrast, a distinct decrease in protein transmission through the PVA-coated (hydrophilic) PP membrane occurred after about 5000 s, representing the transition from internal to external fouling for this case. For the azlactone-coated PP membrane, which exhibited severe external fouling, the protein concentration in the permeate stream dropped to essentially zero (100% rejection) after 2000 s.
12000
TIME (seconds)
Fig. 12. Permeate protein concentration versus time at 10 psig transmembrane pressure and 0.1 g BSA/1 feed solution for the unmodified PE (•), hydrophilic PE (0) and azlactone PE ( • ) membranes,
3.2.4. Discussion of results of part H The permeate flux and protein concentration data for the six 0.5 /xm microfiltration membranes examined in part II of this study indicate that internal
J. Mueller, R.H. Davis/Journal of Membrane Science 116 (1996) 47-60
fouling was followed by external fouling for the unmodified membranes. The internal fouling is likely due to the relatively high surface porosity and membrane tortuosity (see Fig. 9a), so that a depth filtration mechanism was important. A hydrophilic coating did not significantly affect fouling characteristics, while an azlactone coating led to immediate and severe external fouling. The morphology of the hydrophilic coated microfiltration membranes apparently was more important than the low-protein-binding characteristics provided by the coating. This is consistent with a fouling mechanism which is initiated by the physical rejection of aggregates. The azlactone modified membranes showed immediate fouling and an immediate decrease in permeate protein concentration. This indicated the protein was efficiently bound by the azlactone grafted, derivatized Protein A surface coating. Thus, a different fouling mechanism was dominant for the azlactone coatings.
59
membrane surface, is characterized by a concave down total resistance versus time curve and significant protein rejection. Part II of this study examined the effects of various surface modifications on microfiltration membrane performance and protein fouling mechanisms. Results showed that the flux of the surface modified membranes was less than the unmodified m e m b r a n e s . The fouling behavior of the hydrophilic-coated membranes was similar to that for the unmodified, hydrophobic membranes. In contrast, the azlactone modified membranes showed immediate fouling, an immediate decrease in permeate protein concentration, and very low long-term fluxes. The protein was efficiently bound by the azlactone grafted, derivatized Protein A coating. In all of the membranes tested, the transition from internal to external fouling was matched by a corresponding decrease in protein transmission.
Acknowledgements 4. Concluding r e m a r k s Part I of this study examined the relationship between membrane morphology and the protein fouling mechanism through tests on four polymeric microfiltration membranes. The polycarbonate membrane exhibited internal fouling followed by a transition to external fouling. The cellulose acetate membrane primarily exhibited internal fouling, with a shift to external fouling near the end of the 3-h experiment. The polysulfone and polyvinylidene fluoride membranes showed immediate external fouling. The low surface porosity and large thickness of the PS and PVDF membranes reduced permeate flux and allow protein aggregates to quickly plug the few pores, leading to immediate external fouling. The higher surface porosity and small thickness of the CA and PC membranes yield higher fluxes and extensive internal fouling before external fouling
This work was supported by the National Science Foundation (grant CTS-9107703) and by the Center for Separations Using Thin Films at the University of Colorado, Boulder. The authors thank undergraduates Keith Hohn, supported by the National Science Foundation's Research Experiences for Undergraduares program, and Christina Nacos, supported by the Hughes Foundation, for their assistance with the experiments. The experiments on the cellulose acetate membrane were performed by Eve Tracey. Appreciation is also extended to David Gagnon of 3M and Richard Fibiger of Dow Chemical for supplying membranes.
References [1] G. Belfort, R.H. Davis and A.L. Zydney, The behavior of suspensions and macromolecular solutions in crossflow mi-
takes over. Increasing the protein concentration gives a more rapid transition to external membrane fouling. Internal fouling, which refers to the blockage or constriction of membrane pores, is characterized by a
crofiltration, J. MembraneSci., 96 (1994) 1-58. [2] W.R. Bowen and Q. Gan, Properties of microfiltrationmembranes: adsorption of bovine serum albumin at polyvinyldienefluoride membranes, J. Colloid Interface Sci., 144
c o n c a v e up total resistance versus time curve and
[3] K.J. Kim, A.G. Fane and C.J.D. Fell, Fouling mechanisms of
high protein transmission. External fouling, which refers to the build-up of a deposit layer on the
membranes during protein ultrafiltration, J. Membrane Sci.,
(1991) 254-262.
68 (1992)79.
60
J. Mueller, R.H. Davis/Journal of Membrane Science 116 (1996) 47-60
[4] S.T. Kelly, W.S. Opong and A.L. Zydney, The influence of protein aggregates on the fouling of microfiltration membranes during stirred cell filtration, J. Membrane Sci., 40 (1993) 175-187. [5] A.S. Chandavarkar, Dynamics of Fouling of Microporous Membranes by Proteins, Ph.D. Thesis Digest, MIT, Cambridge, MA, 1990. [6] M. Hlavacek and F. Bouchet, Constant flowrate blocking laws and an example of their application to dead-end microfiltration of protein solutions, J. Membrane Sci., 82 (1993) 285. [7] E.M. Tracey and R.H. Davis, Protein fouling of track-etched polycarbonate microfiltration membranes, J. Colloid Interface Sci., 167 (1994) 104-116. [8] S.T. Kelly and A.L. Zydney, Mechanisms for BSA fouling during microfiltration, J. Membrane Sci., 107 (1995) 115127. [9] L.Y. Dudley, P. Stratford, S. Aktar, C. Hawes, B. Reuben, O. Perl and I.M. Reed, Coatings for the prevention of fouling of microfiltration membranes, Trans. ICbemE, 71 (1993) 327328.
[10] M. Kim, J. Kojima, K. Saito, S. Furusaki and T. Sugo, Reduction of nonselective adsorption of proteins by hydrophilization of microfiltration membranes by radiation-induced grafting, Biotecbnol. Prog., 10 (1994) 114-119. [11] J.K. Rasmussen, S.M. Heilmann, L.R. Krepski, K.M. Jensen, J. Mickelson, K. Johnson, P.L. Coleman, D.S. Milbrath and M.M. Walker, Crosslinked, hydrophilic, azlactone-fnnctional polymeric beads: A two-step approach, React. Polym., 16 (1991 / 1992) 199-212. [12] D.R. Gagnon and P.L. Coleman, Azlactone-grafted microporous membranes, ACS-PMSE Prepr., 70 (1994) 262. [13] D.R. Gagnon, Article Having a Polymeric Shell and a Method of Preparing Same, US Pat., 5443727, 1995, [14] D.R. Gagnon, P.L. Coleman, G.J. Drtina, C.S. Lyons, D.S. Milbrath, J.K. Rasmussen and J.B. Stahl, Porous Supports Having Azlactone-functional Surfaces, US Pat., 5344701, 1994. [15] J.-K. Lee, B.Y.H. Liu and K.L. Rubow, Latex sphere retention by microporous membranes in liquid filtration, J. Inst. Environ. Sci., 36(January/February) (1993) 26-36.
ELSEVIER
Journal of Membrane Science 116 (1996) 47-60
Protein fouling of surface-modified polymeric microfiltration membranes Jeffrey Mueller, Robert H. Davis * Department of Chemical Engineering, University of Colorado, Boulder, CO 80309-0424, USA
Received 5 July 1995; revised 2 November 1995; accepted 18 December 1995
Abstract The effects of varying morphology and surface chemistry on protein fouling of microfiltration membranes were investigated. In part I of the study, on the effects of varying morphology, results show that 0.2 ~ m track-etched polycarbonate (PC) membranes internally foul, with external fouling becoming the dominant means of fouling only at later times. A 0.2 /zm cellulose acetate (CA) membrane showed only internal fouling, while 0.2 /xm polysulfone (PS) and polyvinylidene fluoride (PVDF) membranes showed only external fouling. It is hypothesized that the low surface porosities of the PS and PVDF membranes lead to almost immediate external fouling, while the higher surface porosities of the PC and CA membranes allow for a significant period of time for internal fouling to occur. For each membrane, protein transmission remained constant or only slightly decreased during internal fouling, while a significant loss of protein transmission was observed during external fouling. In part II of the study, on the effects of various surface modifications, results show that surface-modified polyethylene and polypropylene membranes have lower initial fluxes than the unmodified membranes. However, the hydrophilic modified membranes demonstrated comparable final fluxes and lower percent flux declines than the unmodified membranes. The azlactone modified membranes showed very low long-term fluxes and large decreases in permeate protein concentration due to efficient protein binding. Again, protein transmission remained constant or only slightly decreased during internal fouling, while a significant loss of protein transmission was observed during external fouling. Keywords: Surface modified membranes; Fouling; Microfiltration; Biotechnology
1. I n t r o d u c t i o n Microfiltration is capable of separating fluids and particles below the 0.02 to l 0 /~m size range from larger particles. This separation is useful in many
* Corresponding author. Tel: (303) 492-7314; FAX: (303) 4 9 2 434l; E-mail: [email protected],
industries, particularly the pharmaceutical, biotechnology, and food and beverage industries. When microfiltration is performed with protein solutions, the flux (volume permeate collected per time per membrane area) decreases dramatically over time due to fouling, even though proteins are generally more than an order of magnitude smaller than the membrane pores. Internal fouling takes place when particles enter the membrane pores and deposit or
0376-7388/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved PH S0376-73 88(96)00017-8
48
J. Mueller, R.H. Davis/Journal of Membrane Science 116 (1996) 47-60
adsorb to the pore walls or entrance. External fouling occurs when particles deposit or adsorb as a layer on the surface of the membrane. The particular case of protein fouling in a microfiltration membrane process has been examined by other researchers. Flux decline due to fouling is attributed in a small part to protein adsorption, but the major contribution is from protein deposition during the dynamic and convective flow conditions, as reviewed recently by Belfort et al. [1]. Questions remain about the location of protein deposition and the mechanism of deposition. For example, Bowen and Gan [2] concluded that deposition of protein inside the membrane pores is an important mechanism for flux loss, whereas Kim et al. [3] concluded that protein fouling of various ultrafiltration and microfiltration membranes is a surface phenomenon, Other researchers provide evidence which attributes fouling behavior to the deposition of protein aggregates on the membrane surface [3-6]. Tracey and Davis [7] demonstrated that both phenomena, internal and external fouling, occur with track-etched polycarbonate membranes. Internal fouling was described using the standard and pore blocking models, while external fouling was described using a cake filtration model. A two-stage fouling mechanism was also proposed by Kelly et al. [8]. They attributed the initial flux decline to the deposition of large protein aggregates, and subsequent flux decline to the attachment of bulk protein to these aggregates, Recently, interest has focused on the development of surface modifications which can enhance flux or the transmission/rejection characteristics of specific proteins. For example, a hydrophilic surface coating is expected to reduce protein binding, increasing flux. Coatings can also be applied to chemically bind specific proteins and effectively separate these proteins from a solution. Dudley et al. [9] treated PVDF membranes with a surface grafting of phosphorylcholine. They showed that membranes with this non-protein-binding coating experience less severe flux declines and a 95% reduction in protein adsorption. Kim et al. [10] reduced nonselective adsorption of proteins by using a radiation-induced grafting of hydrophilic alcoholic hydroxyl (diol) groups onto polyethylene membranes. They found that protein saturation capacities could be reduced to 1 m g / m 3, the binding interaction was reversible rather than
irreversible, and the saturation capacity correlated well with the diol group density. Rasmussen et al. [11] studied the effectiveness of crosslinked, hydrophilic, azlactone-functional beads at binding vailous proteins. They found that the beads bound proteins at very high densities, e.g. Protein A at 397 mg/g. Recently, Gagnon and Coleman [12] made polyethylene microfiltration membranes, grafted with azlactone and derivatized with Protein A, and studied their reversible protein binding ability. They found static binding kinetics are limited by diffusion, and not reaction rate. The research which our group has undertaken examines both the mechanisms of protein fouling and the effects of various membrane morphologies and surface modifications on these mechanisms. Previous work focused on where protein fouling occurs and how operating conditions and membrane pore size affect fouling mechanisms in track-etched polycarbonate membranes [7]. In part I of the present study, we investigated the effects of varying membrane morphology on fouling mechanisms. In part II of this study, the effects of membrane surface modifications on membrane flux performance and fouling mechanisms were examined. Fouling mechanisms were identified in the present work using a method described by Tracey and Davis [7]. This method involves examining the total resistance (which is inversely proportional to flux) versus time curve to see whether it is concave up or concave down. A concave up curve indicates internal fouling, which can be described by a standard blocking (pore radii decreasing) or pore blocking (number of pores decreasing) model. A concave down curve indicates external fouling, which can be described by a cake filtration (gradual surface layer build-up) model. In addition, Tracey and Davis [7] showed that the protein transmission is nearly 100% during internal fouling, but decreases during external fouling.
2. Materials and methods
Protein fouling experiments were conducted using a 10 cm 3 Nuclepore stir cell (model $25-10). The complete experiment apparatus is shown in Fig. 1. A 4-1, 304-stainless-steel pressurized vessel (Alloy Products Corp., no. 72-01) served as the feed reser-
J. Mueller, R.H. Davis~Journal of Membrane Science 116 (1996) 47-60
49
airfilter ~~~ pressure ,permeate[ line~~pressuri )r~egul~1at°~uage r~-z~edPVaTve t I ~~sti~l i ~)~vessel ~ feed r stirrer magneticmicrobalancecomputer reservoi Fig. 1. Schematic of the experimental apparatus.
voir for the water or protein solution. It was pressurized with air run through a Matheson dual stage regulator (8-H) and a 0.02 /xm Matheson filter (model 6190). A 0 - 6 0 psig pressure gauge on the feed reservoir measured the apparatus pressure with a maximum error of 1-2%. The Nuclepore stir cell was stirred using a magnetic stirrer (Fisher Scientific model 115004) with a speed setting of 8. This setting corresponds to a stir speed of 575 rpm and was sufficient to eliminate concentration polarization [7]. The permeate exiting the stir cell was collected in a vessel placed upon a mass balance. The balance (Mettler PE3600) was interfaced with a computer (Data Store 386-20D) to collect mass and time data using a computer program written in Quickbasic 4.0. The mass versus time data were differentiated numerically to obtain flux values. The total resistance was then determined by RTo T = AP/tzJ, where A p is the transmembrane pressure, J is the permeate flux, and/~ is the permeate viscosity. Samples of the permeate were collected from the permeate line at specific time intervals for later protein analysis,
The protein used in all the experiments was bovine serum albumin (BSA). The powdered BSA, supplied by Sigma (catalog no. A-3803, < 0.005% fatty acid content, initial fractionation made by heat shock), was dissolved in deionized and prefiltered water just prior to each experiment run. The pH of the 0.1 g/1 BSA feed solution was 6.6, and the pH of the 1.0 g/1 BSA feed solution was 7.1. The protein concentrations of the permeate samples were measured following each experiment using a Sigma protein assay kit (Sigma catalog number P-5656) and a Hewlett Packard diode array spectrophotometer (model 8452A), employing a modified Lowry method. There were four membranes tested in part I of the experiments, listed in Table 1. Each had a reported nominal pore size of 0.2 /zm. The track-etched polycarbonate (PC) membranes supplied by Nuclepore have a poly(vinylpyrrolidone) coating to make them hydrophilic. They have a regular pore structure of cylindrical pores with a uniform diameter and a narrow size distribution. Their track-etched morphol-
Table 1 Characteristics of the polymeric membranes used in part I Membrane type
Membrane thickness (/xm)
Nominal pore diameter (/xm)
Bulk porosity (%)
Surface porosity (%)
Surface character
Track-etched polycarbonate Cellulose acetate Polysulfone Polyvinylidene fluoride
10 125 305 279
0.2 0.2 0.2 0.2
9 66 65-80 65-80
9 26 12 1
Slightly hydrophilic Highly hydrophilic Slightly hydrophilic Slightly hydrophilic
50
J. Mueller, R.H. Davis~Journal of Membrane Science 116 (1996)47-60
Table 2 Characteristics of the polymericmembranesused in part II Membrane type
Membrane thickness (/zm)
Maximumpore diameter (t~m)
Coating thickness (/zm)
Bulk porosity (%)
Surface porosity (%)
Surface character
Base polyethylene Hydrophilic (PVA) polyethylene Azlactonecoated polyethylene Base polypropylene Hydrophilic (PVA) polypropylene Azlactone coated polypropylene
60 60 60 60 60 60
0.5 0.5 0.5 0.5 0.5 0.5
N/A 0.01 0.01 N/A 0.01 0.01
75-80 75-80 75-80 75-80 75-80 75-80
34 7.5 N/A N/A N/A N/A
Hydrophobic Hydrophilic Chemically active Hydrophobic Hydrophilic Chemically active
ogy gives equal surface and bulk porosities (9%). The low-protein-binding and highly hydrophilic cellulose acetate (CA) membranes supplied by Micro Filtration Systems have an irregular pore structure and a high bulk porosity (66%). They also have a high surface porosity (26%). The GRMO.2PP polysulfone ( P S ) a n d FSMO.2PP polyvinylidene fluoride (PVDF) membranes supplied by Dow Chemical have lower surface porosities (12% and 1%, respectively) and high bulk porosities (65-80%). These membranes are both slightly hydrophilic due to a bydrophilic additive present, In part II of the research, six membranes supplied by 3M were tested, listed in Table 2. The unmodified
polypropylene (PP) and polyethylene (PE) merebranes were coated with polyvinylalcohol (PVA) to make them hydrophilic, or grafted with azlactone and derivatized with Protein A to present a high-protein-binding surface [12-14]. The membranes all had a reported m a x i m u m pore size of 0.5 p~m, as determined by bubble point, and high bulk porosities, 7 5 - 8 0 % . The surface coatings grafted onto the membrane were reported by the manufacturer to be approximately 10 n m thick. The coating was primarily on the external surface, but it did extend slightly into the membrane structure and the membrane pores. The bubble-point pore size, which is a measure of the largest pores, was not affected by the coatings.
Table 3 Summary of results for experimentsin part I; the three sets of data are for 10 psig and 0.1 g/l, 20 psig and 0.1 g/1 and 10 psig and 1.0 g/1 (top, middle and bottom) Membranetype
Waterflux Initial (1/m e h)
Polycarbonate Cellulose acetate Polysulfone Polyvinylidene fluoride Polycarbonate Cellulose acetate Polysulfone Polyvinylidene fluoride Polycarbonate Cellulose acetate Polysulfone Polyvinylidene fluoride
BSA solutionflux Final (1/m 2 h)
Average Initial decline (1/m 2 h) (%)
Final (1/m2 h)
3100 ± 200 1800± 500 41 6200± 500 6000± 500 3 210 ± 50 150 ± 40 29 240 ± 150 140 ± 60 42
3000 ± 400 6200 ± 500 220 ± 20 64 ± 23
110 _+60 380 77 ± 28 35 ± 18
7000 ± 300 2800± 500 60 11000 9400 10 460 + 20 300 ± 20 33 800 ± 300 220 ± 30 73
5500 ± 700 N/A N/A 120 ± 20
3100 ± 200 1800± 500 41 6200_+500 6000± 500 3 210 ± 50 150 ± 40 29 240 ± 150 140 _+60 42
2000 ± 300 N/A 190 ± 20 100 ± 20
BSA concentration Average Feed(g/l) decline (%) 96 94 66 45
Permeate(g/l)
Average rejection (%)
0.13 ± 0.03 0.10 ± 0.01 0.10 _+0.01 0.10 ± 0.01
0.03± 0.02 0.01 ± 0.01 0.03± 0.01 0.013
77 10 71 86
110 ± 40 98 N/A N/A N/A N/A 65 ± 21 47
N/A N/A N/A N/A
N/A N/A N/A N/A
N/A N/A N/A N/A
49 ± 13 98 N/A N/A 49 ± 2 74 20 ± 8 80
N/A N/A N/A N/A
N/A N/A N/A N/A
N/A N/A N/A N/A
J. Mueller, R.H. Davis / Journal of Membrane Science 116 (1996) 47-60
For the PE membrane, the surface porosity changed from 34% for the unmodified PE membrane to 7.5% for the hydrophilic coated PE membrane. Each membrane was immersed in isopropanol, and then rinsed with water immediately prior to placement in the stir cell. This insured that the new membranes were fully wetted before the experiment. In addition, the azlactone coated membranes were kept desiccated until immediately before use to prevent hydrolysis of the azlactone coating. Fresh bovine serum albumin solution and a new membrane were loaded into the apparatus for each experiment run. A typical experiment was run for 3 h at constant pressure and room temperature (2223°C). Each experiment was repeated three times, Most experiments were run at a transmembrane pressure of 10 psig and a protein solution feed concentration of 0.1 g BSA/1. Permeate samples were collected and analyzed for protein content for each of the experiments at these conditions. In part I, some experiments were run at a different pressure (20 psig) or a different feed concentration (1.0 g BSA/1) to study the effects of these operating variables on the fouling mechanism. The apparatus was thoroughly cleaned after each experiment run with a rinse of 0.5 wt% NaOH solution followed by a rinse of prefiltered, deionized water. Tests showed this
51
cleaning procedure was sufficient to remove any remaining protein from the apparatus.
3. Results and discussion
3.1. Part 1. Effects of membrane morphology 3.1.1. Water flux A set of water flux tests at 10 and 20 psig transmembrane pressures was completed for each membrane. Table 3 reports the membrane water flux as the arithmetic mean of the three experiment runs, each with a new membrane. The plus/minus numbers shown represent one standard deviation about the mean. Table 3 shows that the initial water flux of the CA membrane is about 2 times larger than for the PC membrane and 25-30 times larger than for the PS and PVDF membranes. Fig. 2 depicts the water fluxes for the four membranes. The lines in Fig. 2 represent best-fit polynomials through the arithmetic means of the data for three repeated experiments. The large differences in fluxes are due to the differences in morphology, thickness, and surface characteristics of the four membranes (see Table 1). The water flux of each membrane decreased over the 1500 s period of each run (Fig. 2). Using a 10
7000
Cellulose Acetate
6ooo
5000
NE 4 0 0 0
E~'3°°°= 2000
Polysulfone
~
1000
0
Polyvinyldiene Fluoride
. . . .
,
~
I
I-
200
400
' | = -
-
I .
.
.
600
.
I-
-
-
800 Time
1 1000
-
-
"
-II 1200
"
-
-
"i 1400
-
eILt 1600
(seconds)
Fig. 2. Water flux at 10 psig transmembrane pressure for the four polymeric membranes used in part I.
J. Mueller, R.H. Davis~Journal of Membrane Science 116 (1996) 47-60
52
psig transmembrane pressure, the water flux declines were about 30-40% of the initial flux (Table 3). The flux decline is attributed in part to the compressibility of the polymeric membranes. There may also be minute particulates in the feed or from the tubing connecting the feed reservoir and the stir cell, as evidenced by membrane discoloration after the water flux experiments. The feed water was not degassed; this may have led to the formation of microscopic bubbles on the permeate side of the membrane, which would heave contributed to the water flux decline. The water flux did not in general increase linearly with pressure (see Table 3). As Bowen and Gan [2] found, membranes are reversibly compressed at higher pressures, leading to greater resistance and lower flux.
amount of protein solution that is carried to and fouls the membrane. In the high feed concentration experiment, the increased flux loss may also be caused by increased protein aggregation. For a higher concentration of protein, the aggregates are expected to be larger and more numerous than the aggregates at a lower concentration. Fig. 3 is a plot of the flux versus time curves for the four membranes tested under identical conditions of 10 psig transmembrane pressure and 0.1 g BSA/1 feed concentration. The cellulose acetate membrane exhibited the highest flux and least flux decline, followed by the polycarbonate, polysulfone, and polyvinylidene fluoride membranes. The lower fluxes for the PS and PVDF membranes are due, in part, to their greater thicknesses. The CA membrane had a higher flux because it is thinner and exhibits very low binding of proteins. The PC membrane had an intermediate flux, despite its small thickness, because of its low porosity.
3.1.2. Protein solution flux Experiments were carried out at 20 psig transmembrane pressure and 0.1 g B S A / I feed concentration, and at 10 psig transmembrane pressure and 1.0 g BSA feed concentration, in addition to the typical experiment, which was run at 10 psig transmembrane pressure and 0.1 g BSA/1 feed concentration. Results in Table 3 show that a higher pressure or a higher feed concentration led to a higher overall resistance and a greater percentage of flux loss for each of the membranes. This is due to the increased
3.1.3. Total resistance curues The total resistance versus time curves for the polycarbonate membrane under different conditions are shown in Fig. 4. The PC membrane exhibited internal fouling (concave up part of the curve), followed by a transition to external fouling (concave down part of the curve) after about 1 h of operation
7000 6000 ~
e
5000 4000 ", . . . . x 3000 ~ P'o,ycarDonate =
~
~
/
~ , , Polysulfone
2000 1000 0 I.'.'.'.'.'.'.'.'.'E-_. . . . . . . . . . . . 0 2000
4000
~"~" ~ 6000
_
_ 8000
~ 10000
12000
Time (seconds) Fig. 3. Flux decline at 10 psig transmembrane pressure and 0.1 g B S A / I feed solution for the four polymeric membranes used in part I.
J. Mueller, R.H. Davis~Journal of Membrane Science 116 (1996) 47-60
8.OE+~2 10 ps,O, l o
o BSA/,
/
~03 -I.IJ
' ""
I
./
,"
.."
/
3.0E+12 --
2.0E+12
observed previously by Lee et al. [15] for particle filtration with microporous membranes. Increased transmembrane pressure and increased feed concentration also resulted in more severe fouling of the PS and PVDF membranes (Table 3), although external fouling behavior was observed from the start of protein filtration for these membranes. The four membranes tested are made of different materials with different morphologies, indicating that they may foul in different ways. Fig. 5 shows the total resistance curves of the four membranes tested.
.-:2.'.
4.OE+12
/
: ~
"
•
20 psio, 0.1 g BSA/I
•
/
..
10ps~g.o.1 g BSA/~
1.oE.~2
~08E+0 2000
4000
6000
I 10000 12000
8000
TIME (seconds)
53
The PC and CA membranes have higher fluxes Fig. 4. The effect of various operating conditions on the total resistance versus time curves for the polycarbonate membrane.
(lower resistances)
for the experiments with 10 psig transmembrane pressure and 0.1 g B S A / 1 feed concentration. This behavior was observed for both transmembrane pressures, but considerably more fouling (higher total resistance) occurred at the higher pressure. When the feed concentration was increased, the resistance increased quickly and sharply, and external fouling became the dominant mechanism within 30 min. Tracey and Davis [7] also saw this rapid switch from internal to external fouling at higher protein concentrations for 0.2 /~m PC membranes. Internal pore fouling followed by external cake filtration has been
whereas this transition had not occurred, or was just beginning to occur, by the end of the experiment for the low-fouling CA membrane. In contrast, the PS and PVDF membranes have lower fluxes (higher resistances), and the fouling behavior was external from the start.
and
less
fouling. Their fouling
behavior was initially internal. Then, a transition to external fouling occurred for the PC membrane,
3.1.4. Protein transmission In order to assess the effects of internal and external fouling on protein transmission, and to provide more evidence that two distinct fouling mechanisms do occur, the permeate protein concentration
6.0E+12
Polyvinyldiene Fluoride 5.0E+12
~ ~v 4.0E+12 c
'~ 3.0E+12 '~ (9
Polysulfone
2.0E+12
1.0E+12
~
..."
""
""
~'
/f
Cellulose
O.OE+O0 2000
Acetate
,
I
I
I
4000
6000
8000
10000
Time
12000
(seconds)
Fig. 5. Total resistance versus time curves at 10 psig transmembrane pressure and 0.l g B S A / I feed solution for the four polymeric membranes used in part I.
J. Mueller, R.H. Davis~Journal of Membrane Science 116 (1996) 47-60
54
0.12
from each membrane at 10 psig transmembrane pressure and 0.1 g B S A / 1 feed was measured as the experiment progressed. Fig. 6 shows the results ob-
7"
2 01o ~~
"
" "
~
'
~
0.08
meate protein concentration data for the PC membrane. The transition phase between internal fouling (concave up resistance curve), and external fouling (concave down resistance curve) was matched by a sharp decrease in protein transmission. Similar resuits were obtained by Tracey and Davis [7]. The observed increase in protein concentration in the permeate near the end of the experiment may be from experimental variation (Table 3), or for an increase in protein concentration in the stir cell due to the protein rejection [7]. Fig. 7 shows the permeate protein concentration data for the CA, PS, and P V D F membranes versus time for filtration at 10 psig transmembrane pressure and 0.1 g B S A / 1 feed concentration. The permeate protein concentration remained near 0.1 g B S A / 1 (100% transmission) for the CA membrane, until about 9000 s of filtration, after which the transition from internal to external fouling began and the protein concentration in the permeate declined. In contrast, a large decrease in protein concentration occurred almost immediately for the P V D F membrane, which exhibited much higher resistance and external fouling from the start. The PS membrane, which
z m 008 Ocn o E
zE ,v,~ 0.04 ~ 002 o_ o
I
I
aooo
4000
I
I
6000
8000
I
10000 12000
TIME (seconds)
Fig. 7. Permeate protein concentration versus time data at l0 psig transmembranepressure and 0.1 g BSA/I feed solution for the CA ( • ) , PS ( • ) and PVDF ( • ) membranes.
exhibited external fouling with an intermediate resistance, suffered a more gradual decrease in protein transmission.
3.1.5. Discussion of results of part I The permeate flux and protein concentration data for the four 0.2 /zm microfiltration membranes examined in part I of this study indicate that internal fouling of the membranes is followed by external fouling. Internal membrane fouling is characterized by a concave up resistance curve and nearly complete transmission of protein. During internal foul-
2.5E+12
0.1
2E+12
0.09
A
0.08
~
0.07
I~
i.
E t~
1.5E+12
0.06
o.o,
I1~
1E+12
0.04
I-
0.03
C ¢.)
.=_ 5E+11
0.02 0.01
0
i
I
I
2000
4000
6000
o 8000
I 10000
a.
0 12000
Time (seconds)
Fig. 6. Total resistance and permeate protein concentration versus time at 10 psig transmembrane pressure and 0.1 g BSA/1 feed solution for the polycarbonate membrane.
J. Mueller, R.H. Davis~Journal of Membrane Science 116 (1996) 47-60
ing, it is hypothesized that protein molecules or aggregates attach to the pore walls or mouths so that the flux is reduced due to the resulting pore blockage and constriction. However, some of the pores remain at least partially open, with the openings being considerably larger than the characteristic size of individual protein molecules, and so protein transmission occurs without significant rejection as the permeate solution flows through these openings. Eventually, the pore blockage and constriction become sufficiently severe that a significant fraction of the protein is rejected and an external fouling deposit begins to form on the membrane surface. External fouling is
55
characterized by decreased protein transmission and a concave down resistance versus time curve due to the growing fouling layer. For the PC membrane, the period of internal fouling lasted for less than one-half of the 3-h experiment, whereas for the high-flux, low-fouling CA membrane, internal fouling was extended for almost the entire experiment. In contrast, external fouling occurred almost immediately for the low-flux PS and PVDF membranes. In order to characterize the surface morphologies of the membranes and relate these to the fouling behavior, scanning electron micrographs (SEMs) of the clean membranes were taken
Fig. 8. Scanning electron micrographs of the membrane surfaces of (a) polycarbonate, (b) cellulose acetate, (c) polysulfone and (d) polyvinylidene fluoride.
J. Mueller, R.H. Davis~Journal of Membrane Science 116 (1996) 47-60
56
(see Fig. 8). The PC membrane has uniform pores with 0.2 /xm openings and a measured surface porosity of 9%. In contrast, the CA membrane has much larger openings (0.3-1.0 /xm) and a larger surface porosity (26o/0); these characteristics, along with its highly hydrophilic nature, account for the lower surface fouling of this membrane. The PS membrane has intermediate-sized surface openings (0.2-0.5 /xm) and surface porosity (12%); its lower flux and almost immediate external fouling may be due to its greater thickness and lower hydrophilicity. Finally, the PVDF membrane has a wide range of surface openings (0.1-1.0 /zm) but very low surface porosity (1%), with the latter likely contributing to its low flux and rapid external fouling. Tracey and Davis [7] also presented scanning electron micrographs, which showed the deposition of protein aggregates in the surface of 0.2 /zm polycarbonate membranes, followed by the formation of a continuous fouling layer. Tracey and Davis [7] showed that external fouling occurs more rapidly when the membrane pore size is decreased, and they hypothesized that this was because the protein aggregates were larger than the pore openings for the smaller pore sized membrane. Both our data and that of Tracey and Davis [7] show that increased protein concentrations lead to rapid onset of external fouling, as the protein aggregates which deposit on the rnembrane are more numerous and perhaps larger. More recently, Kelly and Zydney [8] also implicated protein aggregates in the initiation of protein fouling of microfiltration membranes, and they presented a two-step model of the fouling process in which aggregates deposit on the membrane surface and within the pore mouths, and then these aggregates
i ~1
~
~ ,
I
i;
i L~~ ~
!mier,_ 1 .... , ~ u
_l.Lql
~
i
] Fig. 9. Scanning electron micrographs of the membrane surfaces of (a) unmodified polyethylene and (b) hydrophilic coated
polyethylene.
serve as nucleation sites for the formation of a deposit layer which eventually covers the membrane surface.
Table 4 Summary of results for experiments in part II Membrane type
Base polyethylene Hydrophilic PE AzlactonePE Base polyethylene Hydrophilic PE Azlactone PP
Water flux
BSA solution flux
Initial (1/m 2 h)
Final ( l / m 2 h)
Average decline
Initial ( l / m 2 h)
Final ( l / m 2 h)
4 2 0 0 + 200 510 _+ 100 980_+450 5400_+ 700 1700 _+ 500 980_+ 450
2300_+ 500 250 _+ 60 400_+ 150 2200_+ 100 1200 _+ 500 210 + 100
46 50 59 59 30 74
2400 + 500 500 _+ 90 360_+ 310 4100+400 1700 _+ 300 150_+ 50
77_+ 5 64 _+ 15 0 97_+ 42 62 _+ 16 34 + 11
(%)
"
i
BSA concentration Average decline
Feed
97 87 100 98 96 77
0.14_+0.08 0.09 _+ 0.01 0.11 _+0.02 0.09_+ 0.02 0.09 _+ 0.01 0.09
(%)
Permeate (g/l)
Average rejection
0.06_+ 0.01 0.05 _+ 0.01 0 0.07_+ 0.03 0.05 _+ 0.03 0
60 48 100 20 51 100
(%)
J. Mueller, R.H. Davis~Journal of Membrane Science 116 (1996) 47-60
57
4000
~ Base PP
3500
\ 3000
k
~" 2500
/ B a s e PE
/~
2000
•
Hydrophilic PP "
~
/Azlactone PP
E 1500
oOOoo
.
0 2000
4000
6000
8000
10000
12000
Time (seconds) Fig. 10. Flux decline at l0 psig transmembrane pressure and 0.1 g BSA/1 feed solution for the six polymeric membranes used in part II.
3.2. Part II. Effects of membrane surface modifications
branes were consistently lower than those of the unmodified membranes. Water flux loss after surface modification of PE membranes was also observed by Kim et al. [10]. This seems surprising, since the surface coating is reported to be only about 10 nm thick. It is thought that the surface coating, entangled with the unmodified membrane surface, contracts when it dries, effectively shrinking the surface pore
3.2.1. Waterflux Water fluxes were examined at 10 psig transmembrahe pressure for all six membranes used in part II. As shown in Table 4, the water fluxes for the hydrophilic and azlactone surface modified mem6E+12
/" i
HydrophilicPP 5E+12
/
I-,llI, J~..
i
m AzlactonePP
Hydroph|lic PE
4E+12
Base PE
~ 2E+12
.- /.
1E÷12
.
.
8000
10000
0 0
2000
4000
6000
12000
Time (seconds) Fig. l l . Total resistance versus time curves at 10 psig transmembrane pressure and 0.1 g B S A / I feed solution for the six polymeric membranes used in part II.
J. Mueller, R.H. Davis/Journal of Membrane Science 116 (1996) 47-60
58
sizes. Fig. 9, which presents scanning electron micrographs of the unmodified PE membrane and the hydrophilic PE membrane, shows an example of this hydrophilic The unmodified PE membrane phenomenon. a has an open structure and a high surface porosity, while the coated PE membrane has more closed or coated structure with a low surface porosity. Large water flux declines of 3 0 - 7 0 % were observed during testing of these membranes (Table 4). These declines were observed for the modified as well as unmodified membranes. As for the membranes in part I, the water flux decline is attributed to membrane compaction and possibly to the presence of microscopic particulates and bubbles,
3.2.2. Protein solution flux and resistance All experiments for the modified and unmodified membranes used in part II were run at 10 psig transmembrane pressure and 0.1 g B S A / I feed concentration. Even with this low feed concentration and a relatively large, 0.5 /xm pore size, severe flux declines were observed for both the PE and PP membranes with and without surface modifications (see Fig. 10). The resistance versus time curves (Fig. 11) show that a period of internal fouling occurred for both unmodified membranes, but external fouling took over after about 5000 s for the PE membrane and after about 8000 s for the PP membrane. The resistance behavior for the hydrophilic modified PE and PP membranes is similar to that for their unmodified, hydrophobic counterparts (internal followed by external fouling). However, the azlactone-coated 012 ~_ O.lO ~ ¥ z~ z ~ 0.o6
.
008~ ~ "
°° z ~ ,~ m~
0.04
~o ~_°: 0.020
I
=
L
I
I
2000
4000
6000
BOO0
10000
O,tla| ~ ~
0.10
•
z!~~~ 0.o8 z ~ 0.06 0.04
• ~> ~ 0020
"
*
~
*
~
f~1. I
0
2000
•
4000
I
6000
±
I±
6000
T{M(E seconds)
±1
10000 12000
Fig. 13. Permeate protein concentration versus time at 10 psig transmembrane pressure and 0.1 g BSA/1 feed solution for the unmodified PP ( • ) , hydrophilic PP ( • ) and azlactone PP ( • ) membranes.
membranes fouled very rapidly, with very low longterm fluxes and large increases in the total resistance.
3.2.3. Protein transmission Figs. 12 and 13 show the protein concentration in the permeate data versus time for the polyethylene and polypropylene membranes, respectively. For the unmodified (hydrophobic) and PVA-coated (hydrophilic) PE membranes, the protein transmission was near 100% initially, and then it decreased after about 5000-6000 s, which corresponds to the transition from internal to external fouling. In contrast, the azlactone-coated PE membrane exhibited substantial protein rejection almost immediately. For the unmodified (hydrophobic) PP membrane, the protein concentration in the permeate declined only slightly from that in the feed, which is consistent with the observation in Fig. 11 that internal fouling continued for the majority of the experiment. In contrast, a distinct decrease in protein transmission through the PVA-coated (hydrophilic) PP membrane occurred after about 5000 s, representing the transition from internal to external fouling for this case. For the azlactone-coated PP membrane, which exhibited severe external fouling, the protein concentration in the permeate stream dropped to essentially zero (100% rejection) after 2000 s.
12000
TIME (seconds)
Fig. 12. Permeate protein concentration versus time at 10 psig transmembrane pressure and 0.1 g BSA/1 feed solution for the unmodified PE (•), hydrophilic PE (0) and azlactone PE ( • ) membranes,
3.2.4. Discussion of results of part H The permeate flux and protein concentration data for the six 0.5 /xm microfiltration membranes examined in part II of this study indicate that internal
J. Mueller, R.H. Davis/Journal of Membrane Science 116 (1996) 47-60
fouling was followed by external fouling for the unmodified membranes. The internal fouling is likely due to the relatively high surface porosity and membrane tortuosity (see Fig. 9a), so that a depth filtration mechanism was important. A hydrophilic coating did not significantly affect fouling characteristics, while an azlactone coating led to immediate and severe external fouling. The morphology of the hydrophilic coated microfiltration membranes apparently was more important than the low-protein-binding characteristics provided by the coating. This is consistent with a fouling mechanism which is initiated by the physical rejection of aggregates. The azlactone modified membranes showed immediate fouling and an immediate decrease in permeate protein concentration. This indicated the protein was efficiently bound by the azlactone grafted, derivatized Protein A surface coating. Thus, a different fouling mechanism was dominant for the azlactone coatings.
59
membrane surface, is characterized by a concave down total resistance versus time curve and significant protein rejection. Part II of this study examined the effects of various surface modifications on microfiltration membrane performance and protein fouling mechanisms. Results showed that the flux of the surface modified membranes was less than the unmodified m e m b r a n e s . The fouling behavior of the hydrophilic-coated membranes was similar to that for the unmodified, hydrophobic membranes. In contrast, the azlactone modified membranes showed immediate fouling, an immediate decrease in permeate protein concentration, and very low long-term fluxes. The protein was efficiently bound by the azlactone grafted, derivatized Protein A coating. In all of the membranes tested, the transition from internal to external fouling was matched by a corresponding decrease in protein transmission.
Acknowledgements 4. Concluding r e m a r k s Part I of this study examined the relationship between membrane morphology and the protein fouling mechanism through tests on four polymeric microfiltration membranes. The polycarbonate membrane exhibited internal fouling followed by a transition to external fouling. The cellulose acetate membrane primarily exhibited internal fouling, with a shift to external fouling near the end of the 3-h experiment. The polysulfone and polyvinylidene fluoride membranes showed immediate external fouling. The low surface porosity and large thickness of the PS and PVDF membranes reduced permeate flux and allow protein aggregates to quickly plug the few pores, leading to immediate external fouling. The higher surface porosity and small thickness of the CA and PC membranes yield higher fluxes and extensive internal fouling before external fouling
This work was supported by the National Science Foundation (grant CTS-9107703) and by the Center for Separations Using Thin Films at the University of Colorado, Boulder. The authors thank undergraduates Keith Hohn, supported by the National Science Foundation's Research Experiences for Undergraduares program, and Christina Nacos, supported by the Hughes Foundation, for their assistance with the experiments. The experiments on the cellulose acetate membrane were performed by Eve Tracey. Appreciation is also extended to David Gagnon of 3M and Richard Fibiger of Dow Chemical for supplying membranes.
References [1] G. Belfort, R.H. Davis and A.L. Zydney, The behavior of suspensions and macromolecular solutions in crossflow mi-
takes over. Increasing the protein concentration gives a more rapid transition to external membrane fouling. Internal fouling, which refers to the blockage or constriction of membrane pores, is characterized by a
crofiltration, J. MembraneSci., 96 (1994) 1-58. [2] W.R. Bowen and Q. Gan, Properties of microfiltrationmembranes: adsorption of bovine serum albumin at polyvinyldienefluoride membranes, J. Colloid Interface Sci., 144
c o n c a v e up total resistance versus time curve and
[3] K.J. Kim, A.G. Fane and C.J.D. Fell, Fouling mechanisms of
high protein transmission. External fouling, which refers to the build-up of a deposit layer on the
membranes during protein ultrafiltration, J. Membrane Sci.,
(1991) 254-262.
68 (1992)79.
60
J. Mueller, R.H. Davis/Journal of Membrane Science 116 (1996) 47-60
[4] S.T. Kelly, W.S. Opong and A.L. Zydney, The influence of protein aggregates on the fouling of microfiltration membranes during stirred cell filtration, J. Membrane Sci., 40 (1993) 175-187. [5] A.S. Chandavarkar, Dynamics of Fouling of Microporous Membranes by Proteins, Ph.D. Thesis Digest, MIT, Cambridge, MA, 1990. [6] M. Hlavacek and F. Bouchet, Constant flowrate blocking laws and an example of their application to dead-end microfiltration of protein solutions, J. Membrane Sci., 82 (1993) 285. [7] E.M. Tracey and R.H. Davis, Protein fouling of track-etched polycarbonate microfiltration membranes, J. Colloid Interface Sci., 167 (1994) 104-116. [8] S.T. Kelly and A.L. Zydney, Mechanisms for BSA fouling during microfiltration, J. Membrane Sci., 107 (1995) 115127. [9] L.Y. Dudley, P. Stratford, S. Aktar, C. Hawes, B. Reuben, O. Perl and I.M. Reed, Coatings for the prevention of fouling of microfiltration membranes, Trans. ICbemE, 71 (1993) 327328.
[10] M. Kim, J. Kojima, K. Saito, S. Furusaki and T. Sugo, Reduction of nonselective adsorption of proteins by hydrophilization of microfiltration membranes by radiation-induced grafting, Biotecbnol. Prog., 10 (1994) 114-119. [11] J.K. Rasmussen, S.M. Heilmann, L.R. Krepski, K.M. Jensen, J. Mickelson, K. Johnson, P.L. Coleman, D.S. Milbrath and M.M. Walker, Crosslinked, hydrophilic, azlactone-fnnctional polymeric beads: A two-step approach, React. Polym., 16 (1991 / 1992) 199-212. [12] D.R. Gagnon and P.L. Coleman, Azlactone-grafted microporous membranes, ACS-PMSE Prepr., 70 (1994) 262. [13] D.R. Gagnon, Article Having a Polymeric Shell and a Method of Preparing Same, US Pat., 5443727, 1995, [14] D.R. Gagnon, P.L. Coleman, G.J. Drtina, C.S. Lyons, D.S. Milbrath, J.K. Rasmussen and J.B. Stahl, Porous Supports Having Azlactone-functional Surfaces, US Pat., 5344701, 1994. [15] J.-K. Lee, B.Y.H. Liu and K.L. Rubow, Latex sphere retention by microporous membranes in liquid filtration, J. Inst. Environ. Sci., 36(January/February) (1993) 26-36.