Characterization of the adsorption-fouling layer using globular proteins on ultrafiltration membranes

Journal of Membrane Science, 76 (1993) 101-120 Elsevier Science Publishers B.V., Amsterdam

101

Characterization of the adsorption-fouling layer using globular proteins on ultrafiltration membranes Myong K. Ko, John J. Pellegrino, Ryan Nassimbene and Paul Marko National Institute

of Standards

and Technology,

Thermophysics

Division, 838.01,X%

Broadway, Boulder,

CO 80303

f USA)

(Received March 25,199l; accepted in revised form August 28, 1992)

Abstract The mechanisms of membrane fouling and the effects of properties and structure of the adsorbedprotein layer on membrane fouling were studied using bovine serum albumin (BSA) andp-lactoglobulin (P-LG) as test proteins, and poly(vinylpyrrolidone)-coated polycarbonate (PC) and regenerated cellulose (RC ) as membranes. Ultrafiltration (UF) experiments were modeled with a 3-parameter resistance model to separately determine resistances due to osmotic pressure and adsorption. An analytical method was also developed to determine protein loadings under UF and static conditions. The dynamic behavior of osmotic pressure or interfacial co centration is discussed on the basis of the material balance around the interface. Dejaguin-Landau an1 Verwey-Overbeek (DLVO) theory and the concept of a rate-limiting step are applied to phenomena hat occur at the interface during protein adsorption. A steric hinIt drance repulsion due to PVP tails on;the PC membrane has a pronounced effect on protein loading. The hydrophilic matrix of RC may act a$ a water reservoir that maintains continuous hydration of the adsorbed layer, which can be dehydrated during protein adsorption. We believe that hydration results in a lower fouling resistance. Additional1 the size of the pores relative to the protein molecule has a pronounced effect on the properties and,8> structure of the adsorbed layer and its fouling resistance. ultrafiltration; protein adsorption; fouling; regenerated cellulose; polycarbonate; bovine serum albumin; /3-lactoglobulin

Keywords:

Introduction Ultrafiltration (UF ) membrane processes experience reversible (i.e. concentration polarization) and irreversible (i.e. solute adsorption) declines in flux. Irreversible protein adsorption subsequently causes a shorter membrane service life and higher operating Correspondence to: Myong K. Ko or John J. Pellegrino, National Institute of Standards and Technology, Thermophysics Division, 838.01, 325 Broadway, Boulder, CO 80303 (USA).

0376-7388/93/$06.00

cost. Irreversible adsorption refers to the adsorbed proteins that are not readily desorbed from the membrane surface when washed with starting buffer or water [ 11. Improvements in the economics and efficiency of UF processes can be obtained by understanding the fundamental mechanisms of irreversible adsorption and the properties and structure of the adsorbed-protein layer. Membrane fouling and the properties of the adsorbed-layer depend on hydrodynamics and the physico-chemical properties of proteins in

0 1993 Elsevier Science Publishers B.V. All rights reserved.

102

solution and the chemistry and structure of the membrane. These properties influence the protein-membrane interactions at the interface and within the pores. However, fundamental aspects of protein adsorption and membrane fouling at the interface (initially at the membrane surface) have not been fully understood due to the complexity of the UF process and the lack of reliable and convenient methods for determining protein loading under filtration conditions. Thus far only a few UF studies have attempted to include solution environment effects in a formal manner. Fane and his colleagues [ 2-51 have studied the effect of protein adsorption on flux decline in various solutions and hydrodynamic conditions. Also the effect of protein aggregation near the interface on adsorption (in the context of DLVO theory) [ 61 and electrokinetic aspects of the adsorbed layer [ 31 have had only limited previous discussion. Many investigators [2-51 use a sodium dodecyl sulfate (SDS) stripping technique to determine dynamic (UF) protein loading on membranes. There is uncertainty with regards to the stripping efficiency of this approach. Protein-membrane interactions on statically fouled membranes have been studied as a function of contact time and solution environment [7,8], using an isotope-labeling technique for protein loading measurements. This technique is generally used for static protein adsorption and poses some problems for dynamic (UF) protein adsorption, because of the use of large quantities of an expensive isotope-labelled protein and the difficulty in handling during UF. Additionally, Robertson and Zydney [9] reported the preferential adsorption of isotopelabeled proteins compared to the unlabeled proteins. This study includes ultrafiltration, static and dynamic adsorption experiments, and scanning electron microscope (SEM) views of fouled membranes. The resistances due to osmotic pressure and fouling are determined from

Myong K. Ko et al./J. Membrane Sci. 76 (1993) 101-120

filtration experiments and a simple 3-parameter resistance model. The static and dynamic protein loadings are determined in a consistent manner using a new method developed in our laboratory. The resistance data are combined with the static and dynamic protein-loading data to discuss phenomena within the concentration-polarized layer, both at the interface and within the adsorbed layer. Membranes and proteins with different properties are used to vary both chemical and geometric parameters. We present our current measurements of protein adsorption and adsorptive-layer resistances in the context of membrane-fouling mechanisms and implications about the structure of the adsorbed-layer. We use a DLVO [6,10] viewpoint for the static adsorption mechanisms of protein on a solid surface, and the concept of a rate-determining step [ 111 as a theoretical basis for interpreting our current results regarding fouling in UF. Flux model The osmotic pressure-adsorption model [12,13] is used in this study. The build-up of protein accelerates membrane fouling due to surface adsorption and pore plugging. Based on the irreversible nature of protein adsorption, the adsorbed protein is assumed to form a permanently bound immobile layer that may increase both in thickness and bulk density during filtration. The immobile adsorbed layer is an additional resistance layer formed during UF. The interfacial concentration (Cmi) and concentration at the membrane surface (C,) are the mobile protein concentration in the vicinity of the interface and membrane surface, respectively. C, may differ from C,i (e.g. C,i > C, >/ C,) due to rejection through the adsorbed layer (C, is the permeate concentration). For a highly-retentive membrane, C, should be close to zero because the channels developed within the adsorbed layer are probably

Myong K. Ko et al./J. Membrane Sci. 76 (1993) 101-120

smaller than the protein molecules. For a partially-retentive membrane, C,‘s value would vary depending on the geometry of the channels developed. In the osmotic pressure-adsorption model, the filtration rate decreases due to a decrease in the hydrodynamic driving force (from osmotic pressure build-up) and an increase in the fouling resistance (R,) (from surface adsorption and pore plugging for a partially-retentive membrane). A schematic diagram of UF is illustrated in Fig. 1. In the osmotic pressure-adsorption model the solution flux (J,) can be phenomenologically represented as

J,=(dP,-ada)/[~(R,+R,)l

(1)

where AP, is the transmembrane pressure, o is the reflection coefficient, a is the osmotic pressure, R, is the hydraulic membrane resistance, and p is the viscosity of the solution. We have further defined relative specific resistance (SR,) as: SR, = (RI%)

(2)

l’ya

SR, is a qualitative measure of the adsorbed layer’s structure or porosity averaged over the

\

Concentration

Membrane

\ \

Adsorbed Protein Layer

Fig. 1. Schematic of ultrafiltration process.

103

entire layer’s thickness (including surface adsorption and/or pore wall adsorption), ya is the protein loading. Hence the adsorbed layer’s resistance is normalized by both the clean membrane’s resistance and the amount of protein that constitutes it. C,i depends on the total mass of mobile proteins (M,) accumulated within the polarized layer. This is strongly influenced by the geometric properties of the membrane and the hydrodynamic conditions. Equation (3) is a summation of transport rates that leads to the buildup of mobile proteins within the polarized layer:

Ma=

(Ji -Jo -Jp -yi)dt,

(3)

where, at time t

Ji =JvCb J, =J& where Ji is the transport rate of the mobile proteins from the bulk phase to the polarized layer, Jo is the back-diffusion rate, and Jp is the permeate flux; 7: is the rate of protein incorporation into the adsorption layer. Ma becomes constant when Ji equals the sum of Jo, Jp, and 76. As J, approaches its steady value, Ji and Jo also approach their steady values. Jo depends on hydrodynamics and protein concentration in the boundary layer. At steady state, Jo equals (Ji- J,) unless the change in )J~is significant. C,i should therefore be relatively constant unless y: is significantly greater than zero throughout the UF. The quantities, aAn, R,, yaya, and SR,, are functions of the solution environment, the hydrodynamic conditions, and the physical, chemical and geometric properties of the protein and the membrane. In general, the solution environment refers to C,,, temperature (77, pH, and salt concentration (2). The hydrodynamic conditions include AP, and the crossflow velocity (U,). The quantities in the flux model de-

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Myong K. Ko et al./J. Membrane Sci. 76 (1993) 101-120

pend primarily on the interfacial concentration (Cmi) and are functions of time. Static adsorption mechanisms Proteins in water interact with and adhere to a solid surface. Binary interactions at the interface include: water-membrane (a function of hydrophilicity of membrane), water-protein (solubility ), protein-protein (association), protein-membrane surface (monolayer adsorption), and protein/adsorbed protein and interactions of laterally and adjacently adsorbed proteins (multilayer adsorption). Norde and coworkers [11,14-161 have discussed the relationship between structural properties of proteins and their behavior in the adsorption process (including the role of small ions). They stated that protein adsorption is an irreversible process and is usually entropically driven by the structural rearrangement of the adsorbed protein molecules. Several binding steps occur, including: charge redistribution in overlapping electrical double layers of protein and solid surface, dehydration of the solid surface and protein, and structural rearrangement of the adsorbed protein molecules. The DLVO theory [10,17,18] describes the long-range interactions between proteins and a solid surface in terms of dispersion, electrostatic, steric hindrance, and polymer bridging forces by treating the dissolved protein molecules as a colloid. Schematic energy profiles as a function of distance between the protein and solid surface are illustrated in Fig. 2 for two cases: (a) both the protein and the solid are negatively charged, and (b) the solid surface has negatively charged polymer ( [ - ]polymer) tails attached with an extended loop and brush conformation. When the [ - ] polymer is attached to the surface, the protein molecules experience an electrostatic repulsion with this polymer layer rather than the surface. If the [ - Ipolymer has a flat conformation or if the

Fig. 2. Qualitative potential energy versus distance due to long-range interactions between proteins and the membrane surface: (a) a bare surface, and (b) a surface with polymer loop and brush arrangement. Gtis the total energy. G, is the contribution from electrostatic forces. G. is the contribution from dispersion forces. G,,, is a contribution due to polymer bridging. G. is due to steric hindrance.

extended loops and brushes are much shorter than the Debye length of the electrical double layer, then case (a) is more applicable. Otherwise, the electrostatic repulsion occurs near the tip of the loops and brushes (outside the region of the electrical double layer), which can be viewed as a steric hindrance repulsion. As the protein molecules penetrate the electrical double layer, polymer bridging attraction is anticipated. In this study, the steric hindrance effect is considered with the long-range interactions. During adsorption proteins rearrange their structure to reach the most energetically-favorable state. Therefore short-range interactions between protein and the interface are important, especially in aqueous media [ 111. Short-range interactions include hydrogen bonding, ion-pair, hydrophobic, dipole-dipole, and dipole-induced dipole interactions. As a result of intermolecular hydrogen bonding and dipole interactions, water molecules form dynamic aggregates thereby introducing a phase boundary between hydrophobic and hydrophilic regions. As adsorption occurs the Hz0

Myong K. Ko et al./J. Membrane Sci. 76 (1993) 101-120

aggregates are removed from the solid surface, dehydrating it. Internal structural rearrangement of the protein occurs simultaneously resulting in an orders of magnitude increase in the number of exposed binding sites per molecule for hydrophobic and other interactions. Near the isoelectric point (IEP ), the electrostatic contribution on adsorption is small and the simultaneous dehydration and structural rearrangement of the adsorbed proteins are particularly important.

105

However, under the UF conditions, the protein at the interface is at its interfacial concentration Cmi, and thus the adsorption rate and equilibrium level is decided by Cmi.C,i can be estimated from dn data obtained from the filtration experiment and independently measured n vs. concentration data [ 19,201. Thus a static adsorption experiment with C, the same as the Cmiobtained from a UF experiment will help us to identify the rate-controlling step of adsorption by comparing the protein loading rate under static and dynamic conditions.

Adsorption kinetics The final loading and kinetics of protein adsorption depend on the magnitudes of the binary interactions and mechanical forces involved at the interface. Important mechanical forces include dP,, U,, permeate flow velocity ( U,), and the shear stress at the boundary layer. The binary interactions and mechanical forces can have both competitive and additive effects on the kinetics of protein adsorption. The kinetics [ 111 involve: (1) transport towards the interface, (2) surface binding reaction accompanied by structural rearrangements in the adsorbed state, (3) detachment from the interface, and (4 ) transport away from the interface. The slowest of these becomes the controlling step for the rate of protein adsorption. However, steps (3) and (4) will not be rate limiting because proteins are not readily desorbed. Therefore, in the absence of significant mechanical forces, diffusion of the protein to the interface and/or the surface binding reaction will control the rate of adsorption. If the diffusion process controls (i.e. binding rate is fast), the concentration of the protein at the interface drops from C, to some lower value, Cbl, and the adsorption rate depends on Cbl, until equilibrium is reached. On the other hand, if the binding rate is slow, the protein concentration in the vicinity of the interface remains at C,,.

Experimental Materials The water used in all the experiments was distilled, deionized, and filtered through a reverse osmosis membrane to a resistance of 18.3 MR-cm. At each time of use, the purified water was refiltered through a 0.2 pm membrane to remove any particulate contamination during handling. The proteins used were bovine serum albumin (BSA, fraction V, A-4503 from Sigma’ ) and /I-Lactoglobulin (@G, A and B, L-2506 from Sigma). BSA and j&G have a molecular mass of 67,000 and 18,300 and an IEP of pH 4.8 and 5.2, respectively. The pH of the protein solutions (for concentrations up to 0.4 kg/L) were nearly constant at 5.2 for BSA and 6.15 for/&G without any buffer salt. At these pH’s, fiG is somewhat more negatively charged than BSA. The dimensions for the ovaloidal BSA is 3.3 x 10.7 nm and for the spherical KG r= 1.8 nm. At a concentration above 0.02 kg/L more than 80% of @G exists in dimer form [ 201. Dimerit bG has a rod like shape with dimensions of 1.8x7.2 nm [20]. ‘NIST does not endorse any particular brand of product or company. Commercial names are used only for precise description of experimental materials and procedures.

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Myong K. Ko et al./J. Membrane Sci. 76 (1993) 101-120

The membranes used in this study were a polycarbonate (Nuclepore PC 0.015 ,um designated as PC015) and regenerated cellulose (Amicon YM 5K and YM 1OOKdesignated as RC5 and RClOO, respectively). The PC015 membrane is a homoporous membrane with its polycarbonate matrix having highly hydrophobic properties (no water wetting, sorption, or swelling). Its surface is treated with poly (vinylpyrrolidone ) (PVP ) which is expected to form slightly negatively charged hydrophilic PVP “tails” on the surface. Free PVP polymer is readily soluble in water, but it has an amphiphilic character containing highly polar amide groups and apolar methylene methine groups that confer hydrophobic properties and a loose, random-coil conformation [ 211. The PVP coated membrane has - 3.2 mV zeta potential at 10m4A4NaCl and no pH buffer ions [ 221. The manufacturer’s data’ (MW, surface coverage, and processing condition) [23] suggests that, to some extent, the PVP layer has a nearly full monolayer coverage and favors the extended loop and brush conformation, but the length of the loops and brushes are unknown. Conformational aspects of the adsorbed polymer have been discussed elsewhere [ 17,24,25]. The RC5 and RClOO membranes have an asymmetric membrane structure with a bare surface. The regenerated cellulose is a polysaccharide made up of p-0 ( + )-glucose residues with three hydroxyl groups resulting in highly hydrophilic properties (very high water wetting, sorption and swelling). Therefore, the RC membranes are more negatively charged and hydrophilic than the PC015 membrane. The physico-chemical and geometrical characteristics of the membranes used in this study are summarized in Table 1.

‘Few details were disclosed due to proprietary

nature.

Filtration A three-stage experimental strategy based on the osmotic pressure-adsorption resistance model [ 131, eqn. (1)) was used to carry out filtration experiments to determine R,, dir, and R,, separately. The experimental set-up and procedure are described in detail in Ref. [ 131. A brief summary of the procedure is: ( 1) filtration of clean water through a fresh membrane (determines R,) followed by (2) filtration of a protein solution (determines CT-R with R, and R, known ) and finally ( 3 ) filtration of clean water through the fouled membrane until a steady water flux is reached (determines R,) . The time reported in each table and figure is the solution filtration time in stage 2. For each filtration experiment, a fresh solution was used. In order not to disturb the adsorbed layer during the removal of mobile proteins in the boundary layer (stage 3 ), the flow rate used for all 3 stages was 1.26 mL/sec (or U, of 26 mm/set). After each filtration experiment, the fouled membrane was analyzed for the total protein loading on the membrane. Some SEM pictures of fresh and fouled membranes were taken to help visualize the adsorbed protein layers.

Adsorption Adsorption isotherms and static and dynamic adsorption kinetics of BSA and /ILG on the PC015, RC5 and RClOO membranes were determined using the external surface area of the membranes as the total available surface area for protein adsorption. These experiments were carried out in two parts: first, preparation of protein-adsorbed membrane samples; second, analysis of total protein adsorbed. The statically adsorbed membrane samples were prepared by contacting fresh membranes (0.025 m2 external surface area) with protein solution, allowing the protein to diffuse and ad-

107

Myong K. Ko et al./J. Membrane Sci. 76 (1993) 101-120 TABLE 1 Material and geometric characteristics of the three membranes used in this study PC015 Membrane material

Polycarbonate coated with PVP

Nature of surface Surface hydrophilicity Matrix hydrophilicity

PVP tails

Type Pore size MWCO Pore size distribution Number pore density Surface pore density” Retention

Homoporous 0.015 pm* 15%

(BSA&BLG)

RC5

RClOO Regenerated cellulose

Bare surface RC5, RClOO> PC015 RC5, RClOO >> PC015

Narrow 6 X 10” m-*

5,000 Wide

Asymmetric PC015 > RClOO > RC5 100,000 Wide

RClOO 2 RC5 > PC015 Partial

Total

Partial

“Ref. [13].

sorb on the membrane surface and within the pores. Only the active side of the membrane surface, normally exposed to solution during UF, was exposed to the solution during the adsorption experiments. The membrane samples were then rinsed with purified water four times (0.25 hr each time) to remove any unbound protein molecules. For the measurement of static adsorption rates, the contact time of protein solution with the membrane was varied. For the adsorption isotherms, protein concentration was varied with a contact time of 48 hr to ensure a maximum value of protein loading without bacterial spoilage for each measurement. Prior to the static adsorption experiments, control experiments (adsorption and gel-electrophoresis) were carried out to define the experimental protocol. When purified water and a sterilized adsorption apparatus were used, there was no evidence of bacterial growth on the membrane or in solution. Fouled membranes from UF experiments were used as the samples in the analysis of the dynamic adsorption kinetics.

Measurement technique for total protein adsorption To analyze the total protein loading (whether statically or dynamically adsorbed) in a consistent manner, an analytical strategy involving protein hydrolysis was developed. A brief summary of the analytical technique is given here. Each membrane sample was placed in a flamesealed glass tube with 5 mL of 6N HCl solution for acid hydrolysis of the adsorbed protein. The sample tubes were immersed in a hot oil bath for 6 hr at 423 K in order to completely hydrolyze the adsorbed protein into its constituent amino acids. The pH of 1 mL of the resulting hydrolysate solution was adjusted to near 6 by adding 0.85 mL of 6N NaOH and 1 mL of 6 N sodium acetate buffer. The pH-adjusted solution was mixed with a ninhydrin reagent (6 : 5 ratio) under an inert N2 environment. This mixture was then placed in a boiling water bath for 0.4 hr and was periodically agitated to enhance the reaction. After the reaction was completed, the solution was cooled (under ambient

108

Myong K. Ko et d/J.

conditions) to 298 K for a stable absorbency reading at 570 nm. For all cases, a cooling time of 1 hr was sufficient and used to maintain consistency in each measurement. The amino acid concentration was calibrated relative to control samples of known protein amounts with and without membrane materials. Interference of the membrane materials was negligible when the pH of the hydrolysate solution was adjusted to near 6. Under identical conditions, deviation.was only a few percent for calibrations, but up to 10% for statically adsorbed samples. The deviation in protein loading may result from the rinsing step. Results and discussion Ultrafiltration resistances The osmotic pressure and membrane fouling resistances and the relative flux reductions were determined from the three-stage flux measurements. For each filtration experiment the largest deviation of the fluxes, averaged over 15 set intervals, was + 4%. Table 2 illustrates the flux data for six typical experiments. Jw is the water flux through a fresh membrane and J,, is the water flux through the fouled membrane. The

Membrane Sci. 76 (1993) 101-120

time reported is the solution filtration time during stage 2. The time dependences of the calculated dn for all the experimental conditions are shown in Fig. 3. The rejection coefficient, a, is greater than 0.97 for all cases studied [ 131. The rapid increase in dn at the early stage of UF is due to the rapid build-up of protein molecules near the interface. Initially, Ji is greater than the sum of J,,, Jp, and 7: because of the high J, and apparently low Jo (Table 3 ) . The polarized layer is rapidly developed at the early stage of UF. The RClOO/BSA system has a high J,, (Table 2) and a relatively low BSA transmission (Fig. 4a). The dn reaches its maximum value very early with little change thereafter. The observed BSA transmission through the RClOO membrane is nearly 0 after 2 hr, and therefore Ji may nearly equal Jo. The calculated ML is nearly zero after the initial increase. Therefore, interfacial protein concentration (C,i and Ma) probably remains constant during UF for the RClOO/BSA case. There seems to be more variation in dir for RC5, PC015 and RClOO/fiG cases. This implies that C,i varies. For RC5 (BSA and /_?LG ) and RClOO/fiG cases, d7c decreases after a maximum value and then gradually increases

TABLE 2 Fluxes of BSA and&G Membrane

solutions through the subject membranes during the three filtration stages

Protein solution

Jw

ml/ (m2-set)

J”

ml/ (m2-see)

Filtration time (hr)

J IT;;/(ms-set)

PC015

BSA PLG

9.04 1.27

1.68 0.88

3.0 3.5

1.98 0.97

RC5

BSA PLG

8.90 7.18

6.08 6.82

3.5 3.5

8.51 7.15

RClOO

BSA aLG

100.90 82.50

7.20 13.91

3.5 3.5

90.15 68.87

The filtration conditions are: BSA (C,=O.5 wt.% and pH 5.2), PLG (C&=0.1 wt.% AP,= 138 kPa, and U,=2.6 cm/set.

andpH 6.151,no addedsalt,T=303K,

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Myong K. Ko et al./J. Membrane Sci. 76 (1993) 101-120

- 1.0 0.0

0.6

c

0

IO00

\

3004

2000

0

Time, s

0.0 10

5

Timex

Fig. 3. Time dependent behavior of the calculated osmotic pressure for BSA and /ILG proteins on PC015, RC5 and RClOO membranes. Filtration conditions are: no salt added, T=303 K, AP,=138 kPa and UC=26 mm/set; for BSA (C,=O.5 t.%, pH=5.2) and for jX,G (Cs=O.l wt.%, pH=6.15).

10J,

15

20

s 1.0

0.0

f ii ;

0.6

B

0.4

f C. 8 3

TABLE 3 Calculated individual transport rates of BSA during UF at the initial time and after 1 hr

F .? 0.2

Rate (mg/m’-set)

Time (set)

PC015

RC5

RClOO

+----------G 0 5

0

Ji

3600

JP

3600

JO

3600

Y:

M:,

0

0

45.2 9.6

44.5 28.3

43 6.9

0 0

-0 2.7 0.0135 0

0

3600

2.2

0 3600

-0

504.5 39.1 15.1 2.2 -0

-0 29.3

0.0382 0.0084 44.5 0.0084

36.9

0.0 10

Time Y 10m3,

s

Fig. 4. Time dependent behavior of the observed transmission, Tobe= C,,/C,, (open symbols) and normalized flux reduction, Jr= J,/J, (filled symbols) for PC015, RC5 and RClOO membranes with (a) BSA (C,=O.5 wt.%, pH=5.2) and (b) /ILG (C,,=O.l wt.%,pH=6.15). Filtration conditions are: no salt added, T=303 K, Al’,=138 kPa and U,= 26 mm/set. (0 ) Tob/RC5, (0 ) T,,/PCO15, (A ) T,,/RClOO; (0 ) J,/RC5, (m) J,/PCO15, (A) J,/RClOO.

0.0993 0

353.5 -0

The filtration conditions are: BSA (C&=0.5 wt.% and pH 5.2), mG (C,,=O.l wt.% and pH 6.15), no added salt, T=303K, AP,=138 kPa, and U,=2.6cm/sec.

again. This variation in dx is possibly due to changes in Ji, Jo, and ~6. As dn approaches its maximum value, Jo increases. The sum of Jo and yi may gradually exceed the steady value of Ji until accumulation of protein within the

boundary layer is sufficiently lowered. For the PC015 membrane, An increases and then slowly decreases. This may result from a change in & (see yafor PC015 dynamic adsorption in Figs. 9 and 10). As shown in Figs. 3 and 4, the Aa and Tabs vary depending on the proteins and the membranes. The higher value of Tabsfor #ILGversus BSA with the RClOO membrane is consistent with protein transmission based on a size difference. When the initial flux is high, AK is only

110

slightly affected by the difference between the Tabsfor /3LG and BSA. With the highly-retentive RC5 membrane, the An value for BSA is about 5 times higher than fiG. This is consistent with the difference in their C,, values (0.5 wt.% for BSA and 0.1 wt.% for/&G). On the other hand,/ILG and BSA show a transmission behavior through the PC015 membrane, which does not follow their relative sizes. The lower value of Tabsfor bG than BSA indicates that the movement of the /3LG molecules is more hindered near the interface and within the pores of the PC015 membrane. Somewhat similar values of Ale for /3LG and BSA (in PC015) indicates that the &G molecules are probably retained near the interface. This can be due to a stronger interaction between the fiG dimers and the PVP polymer tails. The time dependencies of the fouling resistance, R,, are shown in Fig. 5. R, increases as the filtration time increases. The shapes of the curves depend on the differences in size between the protein and the membrane pores. When the retention value is close to 1 (i.e. for RC5 and BSA on RClOO), R, approaches a plateau value. A similar trend was observed for highly-retentive membranes by other researchers [2,7,8]. However, when the protein size is much smaller than the pore size, R, continues to increase even after 7 hr (PCO15) and may only reach a plateau value later. The normalized solution flux (J, = J,/J_ ) vs. time for all three membranes is also shown in Fig. 4. Several quantities related to Jr are shown in Fig. 6 for BSA UF. These are discussed more fully in Ref. [ 131, but, briefly, the extent of flux reduction, Jti= 1 -Jr, is broken into two components, (1) fouling Jr,= 1 -J,,/ J_, and (2) osmotic pressure J,, = Jti - J+ The value of these parameters, as calculated at the end of the UF, is presented in Fig. 6 (b ) . Based on these values we can imply that the solution flux through the PC015 (surface-

Myong K. Ko et al./J. Membrane Sci. 76 (1993) 101-120

31

Timex

10’3, s

Fig. 5. Time dependent behavior of the measured fouling resistance for PC015, RC5 and RClOO membranes with BSA (C,=O.5 wt.%, pH=5.2) and j?LG (C,,=O.l wt.%, pH = 6.15 ) . Filtration conditions are: no salt added, T= 303 K, dP,= 138 kPa and U,= 26 mm/set.

treated) membrane is mainly reduced due to irreversible membrane fouling. Fouling causes a much smaller reduction of flux on the RC5 and RClOO membranes (entirely hydrophilic). For these latter membranes the osmotic pressure resistance causes the major flux reduction. These data suggest that irreversible fouling of an entirely hydrophilic membrane will be less than that of a membrane which is only surfacetreated to be hydrophilic. Adsorption isotherms Adsorption isotherms of BSA andfiG on the PCO15, RC5 and RClOO membranes are shown in Fig. 7. The isotherm values (based on the

111

Myong K. Ko et al/J. Membrane Sci. 76 (1993) 101-120 (W

i

J rt=

I

Jr, +J rf

PC015 r-l

” Time x IO”,

PC015

RClOO

RC.5

s

Fig. 6. Extent of flux reduction due to osmotic pressure and fouling resistances for PC015, RC5 and RClOO membranes with BSA (C,,=O.5 wt.%, pH=5.2). Filtration conditions are: no salt added, T=303 K, dP,= 138 kPa and UC=26 mm/set. Filtration time was 3.5 hr for RC membranes and 3 hr for PC015. J, values are as listed in Table 2.

(0)

‘7-----7 50

.Ol BSA

.l

Concentration,kg/l

1

.OOl

.Ol RLG

.1

1

Concentration,kg/l

Fig. 7. Adsorption isotherms for (a) BSA and (b) j3LG on UF membranes. pH 5.2 for BSA and pH 6.15 for /3LG, no added salt. T=303 K and 48 hr contact time.

external surface area) indicate formation of a thick multilayer (a BSA monolayer requires approximately 3 mg/m2) as reported by others [ 2,7,8]. Differences in protein adsorption (loading) would result from the use of different membranes, experimental conditions (temperature, concentration, pH, ionic strength, etc. ), and analytical technique. Only BSA monolayer

formation was concluded by Robertson and Zydney [ 91, using the total (external and internal) surface area. The total surface area is probably more than the actual available surface area for adsorption. The SEM pictures (Fig. 8) of dynamically fouled membranes (whose protein-loading data is similar to the isotherm values), as well as observations by

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Myong K. Ko et al./J. Membrane Sci. 76 (1993) 101-120

(a) PC015

(b) PC015

(c) PCO15, front side

(d) PCO15, back side

(e) RC5

(f) RC5

Myong K. Ko et al.jJ. Membrane Sci. 76 (1993) 101-120

others [ 26,271, seem to support the idea of formation of a multilayer during filtration. The isotherm values for both BSA and /?LG follow the order RC5 > RClOO > PC015 A comparison between the RC5 and RClOO membranes is a comparison between the same material but with different pore size distributions. The highly-retentive RC5 membrane has a greater external surface area for adsorption than the RClOO membrane, and adsorbed more of both BSA and /?LG. Tovey and Baldo [ 281 have reported similar behavior for protein adsorption on membrane surfaces. The PC015 has the largest pore size among the three membranes (0.015 pm, 10” pores/m’). Even though it has much larger pores than the RClOO membrane (Table 2 ) it has a 10 times lower J,. This suggests that its available surface area for adsorption is significantly higher. Thus a higher protein adsorption on the PC015 membrane is expected, however, the observed protein adsorption is the lowest among the three membranes, implying that the composition and structure of the adsorbed PVP layer plays a significant role in equilibrium (static) protein adsorption. As seen in Fig. 4 (a), Tabsfor BSA is higher through the PC015 than through the RClOO membrane under filtration conditions. This result and the SEM picture of a fresh PC015 membrane indicates that the pathways to the pore mouth (dark spots shown in Fig. 8a) and the major portion of the pore mouth is unblocked ( > BSA dimension). Interestingly enough though is that under passive diffusion conditions BSA did not diffuse through the PC015 but did diffuse through the RClOO. One interpretation of these results is that the BSA can penetrate into the PVP tails during diffusion but during convection it doesn’t move off

113

the streamlines to a significant degree. Conversely, the PC015 membrane transmits less &G than the RClOO membrane under UF (Fig. 4b). This implies that it is more likely to penetrate into and interact with the PVP polymer segments before reaching the pore mouth. The difference between BSA and /lLG transmission seems to also correlate with the difference in their static adsorption. Both increased static adsorption and lower transmission would result from greater penetration into the PVP layer. This is consistent with the assumption that the PVP layer has an extended loop and brush conformation with a Debye length of 5 to 7 nm (under our experimental conditions). Loosely structured adsorbed polymer layers often extend 5 t,o 20 nm [ 111. The lower adsorptive-behavior of the PC015 membrane can be interpreted in the context of applied DLVO theory [lo] as follows. The PC015 membrane has PVP (loops and brushes) on a polycarbonate film, whereas the RC5 and RClOO membranes are entirely regenerated cellulose. As proteins approach the PVP tails, there is a steric hindrance repulsion between the PVP and the protein near the tip of the tails, thereafter, electrostatic, dispersion, and polymer bridging interactions become important. The steric hindrance effect would be greatest at the onset before the polymer tails are covered with the adsorbed proteins. The steric hindrance repulsion is absent in the cases of the RC5 and RClOO membranes. The adsorption energy barrier at the distance of the tip of the PVP polymer is much higher for the PC015 than for the RC5 and RClOO, resulting in a lower collision frequency of protein molecules onto the surface. The low protein loading on the PC015 membrane compared to the RC

Fig. 8. Scanning electron micrograph pictures for fresh and BSA-fouled (during UF) PC015 and RC5 membranes. Filtration conditions are: C,, = 0.5 wt.%, pH = 5.2, no salt added, T= 303 K, AP, = 138 kPa and U, = 26 mm/set. Times indicated on the photographs are for stage 2 of the filtration experiment.

114

Myong K. Ko et al/J. Membrane Sci. 76 (1993) 101-120

membranes would, therefore, be due to this steric hindrance repulsion. Kinetics of protein adsorption The adsorption kinetics of BSA and j?LG on all three membranes were studied under static and dynamic (UF) conditions. The results are shown in Figs. 9 and 10. The C, values used to study the kinetics of static adsorption were estimated from the independently determined R vs. concentration data for BSA [ 191 and /3LG [ 201 and the calculated, average osmotic pressures from our UF experiments. The values of average osmotic pressure, the corresponding values of Cmiand the maximum static protein loading (Yap) determined at C,i (from Fig. 7) are given in Table 4. A visual illustration of the rate of the adsorbed-protein buildup is presented in the SEM pictures in Fig. 8. The pictures show BSAfouled PC015 (Fig. 8a-8d) and RC5 (Figs. 8e,

8f) from UF and a 0.5 mass% BSA solution. The BSA adsorbed PC015 membranes show a definite boundary of the multilayer. The adsorbed layer at 5 hr is approximately 0.3 pm in thickness (in a dry state) and shows channel openings on the surface of the adsorbed layer. TABLE 4 Calculated interfacial protein concentration (evaluated at average osmotic pressure) and protein loading estimated from their adsorption isotherms System BSA-PC015 BSA-RC5 BSA-RClOO bG-PC015 /3LG-RC5 /3LG-RClOO

lr

Gni

Y (GJC

&Pa)

(kg protein/L)

(mg/m’ )

0.180” 0.245” 0.400” 0.100s 0.063b 0.288b

17.5 102.0 93.0 18.7 110.0 80.0

17.2 34.5 127.5 13.8 6.9 127.5

“Ref. [ 191. bRef. [20]. “Est. from Fig. 7. 120

120 RClOO

100

___.-._._..--- --___. __-.* ,-.-

80

*

CT

5 M

..-’

RC5 p__-”

1

___Q__. RClOO 0

;:’

RC5

___________..-------

.’

RCloO

.:’ 60

fg**.

E s

.,: 0

__..-_d

__.-

RC5

RC5 c_.*--

PC015

: :

40

4 0

10

20

30

Timex10m3,s

Fig. 9. Static (---, open symbols) and dynamic (-, closed symbols) adsorption kinetics of BSA on UF membranes. Dynamic case: C,=O.5 mass%, pH= 5.2, no salt added, T=303 K, dP,= 138 kPa and U,= 26 mm/set. Static case: C,,=Cmi (=0.18 kg/L for PC015; =0.25 kg/L for RC5; =0.4 kg/Lfor RClOO),pH=5.2, no saltaddedand T=303 K. (A) PCO15-s, (A ) PC015-d, (0 ) RC5-s, (m) RC5-d, (0) RClOO-s, (0) RClOO-d.

0

10

20 Time

x 10s3,

30

s

Fig. 10. Static (---, open symbols) and dynamic (-, close symbols) adsorption kinetics of /3LG on UF membranes. Dynamic case: C,=O.l mass%, pH=6.15, no salt added, T= 303 K, dP, = 138 kPa and U, = 26 mm/set. Static case: Cb=Cmi (~0.1 kg/L for PC015; ~0.063 kg/L for RC5; co.288 kg/L for RClOO), pH~6.15, no salt added and T=303 K. (A ) PCO15-s, (A ) PC015-d, (Cl ) RC5-s, (m) RC5-d, (0) RClOO-s, (0) RClOO-d.

Myong K. Ko et al./J. Membrane Sci. 76 (1993) 101-120

At 7 hr, a protein deposit can be seen within the membrane pores, and in addition, a thick layer of protein is also observed on the back side. The scraped section of the BSA adsorbed layer on the RC5 membrane indicates that the adsorbed layer is loosely packed and more hydrated than that on the PC015 membrane during the filtration. Moreover, no channel opening can be seen on the surface of the adsorbed layer on the RC5 membrane. In all cases, static protein loading sharply increases at the very early stage of adsorption and then approaches its maximum plateau (pseudoequilibrium) values, yae (Figs. 9 and 10). The order of the static protein loadings is RClOO> RC5 > PC015 at the early stage of adsorption for both BSA andfiG. This order corresponds with the implied interfacial concentration. However, the order changes to RC5 > RClOO> PC015 at later times. This implies that & and ya can be strongly influenced by the local protein concentration at the early stages of adsorption, but the observed limit, yaYae, is more dependent on available surface sites at later times. The shape of the dynamic protein loading curves depends largely on the retentive characteristics of the membranes. The dynamic adsorption kinetics of BSA and/3LG on the highly retentive RC5 membrane shows a similar trend as in static adsorption (Figs. 9 and 10). But for the less retentive RClOO and PC015 membranes, the dynamic protein loadings of BSA an fiG far exceed their yaevalues at some time during filtration. This behavior and the upward swing are presumably due to increasing pore wall adsorption (PC015 and RClOO) and/ or protein deposition on the back-side of the membranes (PC015, Fig. 8d). The time required for ya to exceed yaedepends both on the applied protein concentration and on the relative size of membrane pores versus the proteins. The order of these times is BSA on PC015 (<0.15hr)<@LGonPC015(<0.15hr)
115

on RClOO( < 0.25 hr) 3 hr ) . Moreover, the large-pore, homoporous PC015 membrane seems to have a higher level of pore adsorption and/or back-side protein deposition than the asymmetric RClOO membrane. The static and dynamic BSA adsorption kinetics (Fig. 9) on the RC membranes are compared to identify whether the mass transfer to the interface or the binding reaction is ratecontrolling. Since the rate of dynamic BSA adsorption on the RC5 membrane is only somewhat slower than the static adsorption and the protein loadings on the RClOO membrane under static and dynamic conditions are practically the same for a considerable length of time, we can assume that the adsorption is reactioncontrolled. In such cases, the dynamic protein loading can be predicted from the static protein adsorption kinetics. Hydrodynamics plays a major role in bringing the proteins to the interface, while interactions between proteins and the interface are at the molecular level. Thus, when the structural properties of dynamically and statically adsorbed protein layers are similar, fouling resistance may also be predicted from static data. This notion needs to be confirmed in a study of structural properties of the adsorbed layer. Comparison between the static and dynamic /3LG adsorptions (Fig. 10) on the RC membranes indicates that mass transfer is the controlling step. The dynamic protein loading of fiG on the RC membranes rises rapidly to the corresponding ya’ae values at early stages of filtration. The large deviations between the dynamic and static protein loadings at the early stage of adsorption gradually become smaller with increasing time. The binding reaction of @LG on the RC membranes is, therefore, faster than diffusion of /3LG, which causes a steep concentration gradient near the interface for static adsorption. The interfacial concentra-

116

Myong K. Ko et al./J. Membrane Sci. 76 (1993) 101-120

tion is significantly concentration.

lower than the bulk

Properties of the adsorbed layer The dynamic response of the relative specific resistance (SR,) for both BSA and fiG on all three membranes is shown in Fig. 11. SR, is a qualitative measure of the properties and structure of the adsorbed layer averaged over its entire thickness. The protein loading on the RC membranes under both static and dynamic adsorption conditions is higher than that on the PC015 membrane (Figs. 7,9, and lo), but the RC suffers much less irreversible fouling than the PC015 (Fig. 6). Therefore, the SR, of RC membranes is significantly lower than those of the PC015. Apparently, the protein-adsorbed layers on the entirely hydrophilic RC membranes are loosely packed and/or more hydrated (hydrophilic) than on the PC015 membrane. The following hypothesis, a mechanism of dehydration-hydration of the membrane surface and its adsorbed layer, could be consistent

*““T-----l

3

0 r(

.l

[

0

10 Time

20 x 10s3, s

I

30

Fig. 11. Relative specific resistance (SR,) of the adsorbed protein layer on UF membranes. Filtration conditions are: no salt added, T= 303 K, dP, = 138 kPa and U, = 26 mm/ sec;forBSA (C,=Odwt.%,pH=5.2) andfor/LG (&=O.l wt.%, pH=6.15).

with the above observations. Upon static protein adsorption, the adsorbed layer is continually dehydrated at the interface (initially at the membrane surface) until it reaches the thermodynamically most favorable state [ 111. The native protein molecules in the boundary layer are more hydrophilic than the denatured adsorbed molecules. Therefore, the highly concentrated protein molecules near the interface are striving for water molecules, resulting in a net removal of water from the surface. The adsorbed protein molecules are denatured to a different extent according to the dehydration level within the adsorbed layer. Once the adsorbed proteins are dehydrated and denatured, they do not readily regain their original conformation. During UF, the solid phase of the membrane matrix may gain water from the filtrate (in the pores) depending on its hydrophilicity. The water gained can be transported from the solid phase of the matrix to the surface and from the surface to the adsorbed layer leading to a less dehydrated-state of that layer. The dehydration level within the adsorbed layer is therefore influenced by hydration from the matrix. Two extreme cases of this dehydration-hydration mechanism are depicted in Fig. 12. In the context of this hypothesis some of our current results can be interpreted as follows. The hydrophobic nature of the polycarbonate matrix (PC015 ) inhibits the supply of water from the filtrate to the membrane surface and the adsorbed layer independent of whether or not the surface layer is hydrophilic. Once water is removed from the surface, the PVP tails, which have amphiphilic characteristics [ 211, may also form pairs with denatured proteins to maintain electroneutrality and minimize the energy level within the adsorbed layer. The denatured proteins can become strongly adsorbed in a very compact manner due to this combination of interactions (mainly by dispersion, polymer bridging and hydrophobic). As a result, the adsorbed layer is dense and strongly

117

Myong K. Ko et al. jJ. Membrane Sci. 76 (1993) 101-120 cb

the membrane surface, but also the membrane matrix must be highly hydrophilic in order to reduce the irreversible fouling,

Bulk Phase

Structure of the adsorbed layer Membrane

Matrix

(a) Dehydration

cb

Bulk Phase

(b) Rehydration

Fig. 12. A schematic illustration of (a) dehydration and (b) rehydration of the adsorbed layer. The arrows indicate directions of water transport.

hydrophobic, causing a high fouling resistance. The adsorbed layer on the RC membranes can be resupplied with water from the hydrophilic membrane matrix as its surface is being dehydrated. In such a case, the membrane matrix may act as a water reservoir, and dehydration is lowered. The structure of the adsorbed proteins may then be only mildly compacted (if at all) by a low degree of hydrophobic interactions. Additionally, the polymer bridging interaction is absent. The low fouling behavior of the RC membranes is, therefore, due to the hydrated, loosely packed, and less hydrophobic adsorbed layer. Bauser et al. [ 291 have also observed a considerable enhancement in solution flux during UF when they deposited a hydrophilic activated-carbon layer on hydrophobic membrane surfaces. We conclude that not only

As shown in Fig. 4 (a), the observed BSA transmission for the PC015 membrane was slowly reduced to a steady value, but that for the RClOO membrane gradually becomes zero at around 2 hr. There must be clear channels developed within the adsorbed layer on the PC015 membrane allowing protein molecules through, and the pores are not completely filled by adsorbed proteins. The dark spots shown in Fig. 8(b) may correspond to channel openings in the adsorbed-protein layer on the PC015 membrane. For the RClOO membrane, channels larger than BSA may initially develop within the adsorbed layer, and then become gradually blocked, thereby forming a proteinfilled cake layer with tortuous paths as in the case of the totally retentive RC5 membrane. This cake layer is shown in Fig. 8 (f). The magnitude of SR, depends largely on the physico-chemical properties of the membranes, but the shape of the SR, curves depends on the retention characteristics of the membranes (Fig. 11). For the high retention cases (BSA/RClOO and all RC5), SR, sharply decreases at the early stage of filtration, and then approaches its steady value. In such cases, properties (porosity and hydrophilicity) and structure (thickness, channel size, and tortuosity) of the surface-adsorbed-protein layer depend primarily on physico-chemical interactions among proteins and interface. The SR, value is expected to be at its maximum for the first layer adsorption because the total chemical potential difference between protein solution and membrane surface is at its maximum at the beginning of UF. As protein adheres more onto the surface forming a multilayer, the total chemical potential difference at the interface

118

would decline. On the other hand, for the less retentive cases (PC015 and /lLG/RClOO) SR, increases with increasing filtration time. The adsorbed layer is developed both on the membrane surface and within pores with channels smaller than the original pores, but larger than protein molecules. The increasing trend in SR, is mainly due to reduction of the effective hydraulic radius of pores, causing a significant increase in (RJR,) with a marginal increase in y*. In such cases, the total hydraulic resistance (R,+ R,) is proportional to thickness of the layer and the l/4 power of channel size [9]. The SR, value for the BSA/PC015 case is higher during the pore plugging period, and then SR, may gradually decrease to a new steady value. Thus SR, may continually increase until the major portion of the channels becomes smaller than the protein molecules (see also Fig. 5).

Conclusions An analytical method was developed to measure total adsorbed protein both on membrane surface and within pores; hence it was possible to determine protein loadings under static and UF conditions in a consistent manner. Comparison between static and dynamic (UF) adsorption kinetics demonstrated the role of hydrodynamics in adsorption during UF, bringing protein molecules to the interface where interactions at the interface are at the’ molecular level. Protein loading under UF can be predicted from static protein loading when protein adsorption is controlled by the binding reaction. This seemed to be the case for RC membranes and BSA. For diffusion-controlled adsorption, results of static tests will not accurately predict protein loading under UF. This seems to be the case for KG and RC membranes.

Myong K. Ko et al./J. Membrane Sci. 76 (1993) 101-120

Variation in osmotic pressure (da) or interfacial concentration (Cmi) during UF has been interpreted using the material balance around the interface. When the rate of protein loading is appreciable, C,i varies with filtration time even at a steady-state condition. The fouling resistance (R,) and relative specific fouling resistance (SR,) differ widely between membranes with hydrophobic (PC015) and hydrophilic (RC5 and RClOO) compositions. We feel that entirely hydrophilic membranes are likely to form a porous and hydrated adsorbed layer with low resistance due to its effective hydration. Non-hydrophilic matrices are more likely to form a dehydrated and compact adsorbed layer. During UF, the hydrophilic membrane matrix may act as a water reservoir for continuous rehydration of the adsorbed layer. A steric hindrance repulsion between protein and polymer tails (i.e. PVP coated PC015) reduces protein loading. The resistance of the adsorbed layer (both R, and SR,) are also strongly influenced by the difference in size between protein molecules and pores. For the case of a highly retentive membrane, a protein-filled cake layer is formed on the surface with channel sizes smaller than those of protein molecules. R, and SR, of the surface-adsorbed layer are determined primarily by physico-chemical interactions among protein molecules and the interface. For a permeable membrane, the adsorbed layer is formed both on the surface and within pores, with channel sizes larger than those of protein molecules, but smaller than the original pores. The size of channels will depend on the relative sizes of pores to protein molecules, hence R, (or SR,) is determined by the effective hydraulic radius of channels and total thickness of the layer. Overall, the interfacial concentration, protein loading, and properties and structure of the adsorbed layer are interrelated, and determine R, and da.

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Myong K. Ko et al./J. Membrane Sci. 76 (1993) 101-120

Acknowledgement We are grateful to A.G. Fane and Paul Todd for their helpful comments. We express sincere gratitude to S. Sikdar for continuous support and encouragement during this investigation, and also gratefully thank Doug Wray at University of Colorado, Department of Chemical Engineering for assistance with the SEM analysis. List of symbols C

Cb Cbl Gm

Gni

CP G&3

protein concentration (kg/L) bulk protein concentration (kg/L) bulk protein concentration at the interface (kg/L) mobile protein concentration at the membrane surface (kg/L) mobile protein concentration at the interface (kg/L) permeate protein concentration (kg/L) dispersion interaction energy [J/ (kg-

m”)l

Gbr

Gt?

polymer bridging interaction energy [J/ (kg-m2) 1 electrostatic interaction energy [J/ (kg-

m”)l

G

G

JIf

relative flux reduction due to fouling resistance relative flux reduction due to osmotic pressure resistance total relative flux reduction due to both fouling and osmotic pressure resistances J” solution flux [ mL/ ( m2-sec ) ] clean water flux through a fouled memJ “a brane [mL/(m’-set)] JVW clean water flux through a fresh membrane [ml,/ ( m2-set ) ] Ml total protein accumulation in the boundary layer ( mg/m2 ) rate of protein accumulation in the M:, boundary layer [ mg/ ( m2-set) ] Apt transmembrane pressure (kPa) R, resistance due to fouling (m-l ) &I membrane resistance (m-l ) R obs observed rejection SRa relative specific resistance averaged over the total adsorbed-protein layer (m"/mg ) time (set :) t T temperature (K) T obs observed transmission UC cross-flow velocity (cm/set) 4 permeate flow velocity (cm/set ) 2 salt concentration (M) Greek symbols

steric hindrance interaction energy [J/ (kg-m2) 1 total long-range interaction energy between protein and solid surface [J/ (kg-

reflection coefficient viscosity, ( kPa-set ) osmotic pressure (kPa) normalized osmotic pressure (An/Apt) protein loading ( mg/m2) rate of protein loading [ mg/ ( m2-set) ]

m”)l

H

Ji JO

JP Jr

distance between protein and solid surface [nm] protein transport rate from bulk phase to the interface [ mg/ ( m2-set) ] transport rate of protein from the boundary layer to the bulk phase by back-diffusion [ mg/ ( m2-sec ) ] protein flux in permeate [ mg/ ( m2-sec ) ] solution flux normalized by clean water flux through a clean membrane (J,/J_)

maximum plateau protein loading (mg/ m2) References 1

W. Norde, F. MacRitchie, G. Nowicka and J. Lyklema, Protein adsorption at solid-liquid interfaces: reversibility and conformational aspects, J. Colloid Interface Sci., 112 (1986) 447.

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3

4

5

6

7

8

9

10

11

12

13

14 15

Myong K. Ko et al./J. Membrane Sci. 76 (1993) 101-120 A. Suki, A.G. Fane and C.J.D. Fell, Flux decline in protein ultrafiltration, J. Membrane Sci., 21 (1984) 269. A.G. Fane, C.J.D. Fell and A. Suki, The effect of pH and ionic environment on the ultrafiltration of protein solutions with retentive membranes, J. Membrane Sci., 16 (1983) 195. A.G. Fane, C.J.D. Fell and A.G. Waters, Ultrafiltration of protein solutions through partially permeable membranes. The effect of adsorption and solution environment, J. Membrane Sci., 16 (1983) 211. M.W. Chudacek and A.G. Fane, The dynamics of polarization in unstirred and stirred ultrafiltration, J. Membrane Sci., 21 (1984) 145. A. Suki, A.G. Fane and C.J.D. Fell, Modeling fouling mechanisms in protein ultrafiltration, J. Membrane Sci., 27 (1986) 181. E. Matthiasson, The role of macromolecular adsorption in fouling of ultrafiltration membranes, J. Membrane Sci., 16 (1993) 23. P. Aimar, S. Baklouti and V. Sanchez, Membranesolute interactions: influence on pure solvent transfer during ultrafiltration, J. Membrane Sci., 29 (1986) 207. B.C. Robertson and A.L. Zydney, Protein adsorption in asymmetric ultrafiltration membranes with highly constricted pores, J. Colloid Interface Sci., 134 (1990) 563. W. Norde, Physicochemical aspects of the behavior of biological components at solid/liquid interfaces, in: S.S. Davis, L. Illum, J.G. McVie and E. Tomlinson (Eds.), Microsphere and Drug Therapy, Pharmaceutical, Immunological and Medical Aspects, Elsevier Science Publishers B.V., 1984, p. 39. W. Norde, Adsorption of proteins from solution at the solid-liquid interface, Advances in Colloid and Interface Sci., 25 (1986) 257. H. Nabetani, M. Nakajima, A. Watanabe, S. Nakao and S. Kimura, Effects of osmotic pressure and adsorption on ultrafiltration of ovalbumin, AIChE J., 36 (1990) 907. M.K. Ko and J.J. Pellegrino, Determination of osmotic pressure and fouling resistances and their effects on performance of ultrafiltration membranes, J. Membrane Sci., 74 (1992) 141. W. Norde, Ion participation in protein adsorption at solid surfaces, Colloids and Surfaces, 10 (1984) 21. W. Norde and J. Lyklema, The adsorption of human plasma albumin and bovine pancreas ribonuclease at negatively charged polystyrene surfaces. I. Adsorp-

tion isotherms. Effects of charge, ionic strength, and temperature, J. Colloid Interface Sci., 66 (2) (1978) 259. 16 W. Norde, The role of charged groups in the adsorption of proteins at solid surface, Chemica Acta, CCACAA, 56 (4) (1983) 705. 17 B. Vincent, The effect of adsorbed polymers on dispersion stability, Adv. Colloid Interface Sci., 4 (1974) 193. 18 D.H. Napper, Steric stabilization, J. Colloid Interface Sci., 58 (2) (1977) 390. 19 V.L. Vilker, C.K. Colton, K.A. Smith and D.L. Green, The osmotic pressure of concentrated protein and lipoprotein solutions and its significance to ultrafiltration, J. Membrane Sci., 20 (1984) 63. 20 G.B. Van den Berg, J.H. Hanemaaijer and C.A. Smolders, Ultrafiltration of protein solutions; the role of protein association in rejection and osmotic pressure, J. Membrane Sci., 31 (1987) 307. 21 P. Molyneux, Water-Soluble Synthetic Polymers: Properties and Behavior, Vol. I, CRC Press, Boca Raton, FL, 1983,146. 22 J.A. Ibaiiez, J. Forte, A. Hemandez and F. Tejerina, Streaming potential and phenomenological coefficients in Nuclepore membranes, J. Membrane Sci., 36 (1988) 45. 23 Personal communication with D. Peterson affiliated with Costar Corp., March (1992). 24 F.R. Eirich, The conformational states of macromolecules adsorbed at solid-liquid interfaces, J. Colloid Interface Sci., 58(2) (1977) 423. 25 P.G. de Gennes, Polymers at an interface; a simplified view, Adv. Colloid Interface Sci., 27 (1987) 189. 26 A. Lenhoff, Proteins in ordered arrays on graphite surfaces, C&EN, Sept. 10,1990,p. 24. 27 I.M. Reed and J.M. Sheldon, Investigation of protein fouling characteristics of ultrafiltration membranes, in: M.K. Turner (ed.), Effective Industrial Membrane Processes: Benefits and Opportunities, Elsevier Applied Science, London, 1991, p. 91. 28 E.R. Tovey and B.A. Baldo, Protein binding and nitrocellulose, nylon and PVDF membranes in immunoassays and electroblotting, J. Biochem. Biophys. Meth., 19(2-3) (1989) 169. 29 H. Bauser, H. Chmiel, N. Stroh and E. Walitza, Control of concentration polarization and fouling of membranes in medical, food and biotechnical applications, J. Membrane Sci., 27 (1986) 195.