Reviews Pune 411 008, India Science, Part C: Polymer ...folk.ntnu.no/andersty/3. Klasse/Felleslab (SepTek... · the intrinsic rejection of the membrane R', = define 1 - CpICdm a)

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Effect of Membrane-SoluteInteractions on UltrafiltrationPerformanceD. A. Musale & S. S. Kulkarnia Polymer Science and Engineering Group ChemicalEngineering Division National Chemical Laboratory,Pune 411 008, Indiab Polymer Science and Engineering Group ChemicalEngineering Division National Chemical Laboratory,Pune 411 008, India

Version of record first published: 10 Mar 2008.

To cite this article: D. A. Musale & S. S. Kulkarni (1998): Effect of Membrane-SoluteInteractions on Ultrafiltration Performance, Journal of Macromolecular Science, PartC: Polymer Reviews, 38:4, 615-636

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J.M.S.—REV. MACROMOL. CHEM. PHYS., C38(4), 615-636 (1998)

Effect of Membrane-SoluteInteractions on UltrafiltrationPerformance

D. A. MUSALE1'2 and S. S. KULKARNIPolymer Science and Engineering GroupChemical Engineering DivisionNational Chemical LaboratoryPune411 008, India

1. INTRODUCTION 6161.1. Membrane-Solute Interactions 6171.2. Mechanism of Flux Decline: Role of Membrane

Characteristics 618

2. MEMBRANE SURFACE CHARACTERISTICS 6192.1. Hydrophilicity/Hydrophobicity 6192.2. Electrostatic Effects 6212.3. Surface Roughness 6232.4. Pore Size/Pore Size Distribution 624

3. STRATEGIES TO MODIFY MEMBRANE SURFACECHARACTERISTICS 6243.1. Polymer Modification 6243.2. Polymer Blends 6263.3. Surface Modification 626

'To whom correspondence should be addressed.2Present address: Institute for Chemical Process and Environmental Technology, Na-

tional Research Council of Canada, M-12, Room 246, Montreal Road, Ottawa, ON, KIAOR6, Canada. E-mail: [email protected]

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616 MUSALE AND KULKARNI

4. FEED VARIABLES THAT MODIFY MEMBRANE-SOLUTE

INTERACTIONS 631

LIST OF ACRONYMS 632

REFERENCES 633

1. INTRODUCTION

The major limitations for application of ultrafiltration (UF) processes arethe membrane fouling and concentration polarization phenomena arising fromrejection of solute molecules at the membrane surface. Concentration polariza-tion can be controlled by engineering parameters such as module design andsystem'hydrodynamics, whereas fouling is influenced by various membrane-solute interactions, membrane morphology, and solute-solute interactions. Stud-ies of the membrane surface chemistry and solution environment, which areresponsible for membrane-solute interactions, are important for understandingmembrane performance and fouling during UF.

Several membrane characteristics and solution parameters are known to af-fect membrane performance and fouling behavior. Hydrophilic membranes tendto adsorb less protein and exhibit higher fluxes and slower flux declines thanhydrophobic membranes with similar other characteristics. Also, it is knownthat electrostatic charges on both protein and membrane affect the permeateflux, protein transmission, and fouling mechanism. The pH, ionic strength, andconcentration of protein-containing solutions have been shown to affect fluxand protein transmission through membranes. Filtration characteristics are alsoaffected by electrostatic and hydrophobic interactions between the proteinsthemselves and between protein and membrane. This review focuses on the roleof membrane surface characteristics in determining UF performance.

Transport through microporous membranes involves flow through pores, withthe flow modified by factors such as pore size distribution, pore tortuosity, andinteractions of the feed components between themselves and with the membrane[1]. Initially, UF was considered to be purely a molecular sieving process inwhich the permeate flux is described by the Hagen-Poiseuille equation, whilethe intrinsic rejection of the membrane R', defined as (R' = 1 - CpICm) can berelated to the ratio X of radii of the solute r, and the pore rp by either Ferry'sequation [2] or its extension by Zeman and Wales [3], where Cp and Cm are thesolute concentration in the permeate and on the membrane surface, respectively.However, more-comprehensive models were developed, such as the surfaceforce-pore flow [4] or the hindered transport model [5], that take into accountnot only the steric parameters r, and rp, but also the effect of frictional forces and

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MEMBRANE-SOLUTE INTERACTION EFFECTS ON UF 617

membrane-solute interactions. The membrane-solute interactions are describedthrough a potential function E(r) for the force exerted by the pore wall on thesolute.

1.1. Membrane-Solute Interactions

Membrane-solute interactions are described variously as hydrophilic/hydro-phobic interactions, electrostatic interactions, or polar/nonpolar interactions. Hy-drophobic or nonpolar (dispersive) interactions are controlled by van der Waalsforces. This interaction energy {Ev(r)) as a function of r, the distance betweenthe particle and the surface, is given as [6]

A\2rs(r+rs) Jr + 2r,\6[rir+2rs) "\ r J (1)

where A, the effective Hamaker coefficient of the system, represents the net vander Waals interactions among the solute particle, membrane surface, and thesolvent.

Even in the absence of specific ionizable groups, both membrane and solutesgenerally carry an electrostatic charge. According to the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory, the electrostatic effects are governed byelectrical double-layer interactions outside the charged surface. Considering themembrane as an infinitely wide, flat surface and the solute (protein) as a sphere,the electrostatic interaction {EE (r)} is approximately given as [6]

(2)1-expH/r)

where er and Eo are the dielectric constant of the medium and the permittivityof vacuum, respectively, and §, and <j>2 are the potentials at the boundary be-tween the diffuse layer of ions in the solvent and membrane surface or soluteparticle, respectively. The reciprocal Debye length d depends on the ionicstrength of the medium:

where e is the elementary charge, k is the Boltzmann constant, and C, and Z, arethe concentration and valency of ion /, respectively. Electrostatic interactionsare more significant in solutions with low ionic strength.

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The total interaction energy E(r) is the sum of £v and EE. The total energy isnegative for net attractive interactions and positive for net repulsive interactions.

1.2. Mechanism of Flux Decline:Role of Membrane Characteristics

The mechanisms of flux decline in UF and the corresponding significantmembrane parameters are summarized in Table 1. In this review, we focus onthe role of membrane-solute interactions that affect the last three flux declinemechanisms in the table. The flux reduction resulting from pore size reductiondue to solute adsorption at the pore wall can be empirically modeled [7] as

(4)

where Jxi and Jy{ are the initial and final fluxes, respectively, ld is the adsorbedlayer thickness, and rpi is the initial pore radius. Similarly, flux decline by solutedeposition at the membrane surface is modeled [8] as an additional resistanceRc in series with the membrane resistance Rm:

(5)

where Rc is the product of the specific cake resistance and the cake-layer thick-ness.

TABLE 1

Role of Membrane Characteristics in Flux Decline

No. Mechanism of flux decline Membrane parameter

1 Concentration polarization at membranesurface

2 Membrane compaction

3 Pore narrowing through solute adsorp-tion

4 Pore blocking through solute deposition

5 Solute adsorption/deposition at mem-brane surface

Module hydrodynamics

Membrane morphology, polymerstrength

Membrane-solute interactionsSurface roughnessMembrane-solute interactionsPore size and tortuosityMembrane-solute interactionsModule hydrodynamicsSurface roughness

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A more rigorous time-dependent model of the interaction between proteinadsorption and concentration polarization was proposed by Gekas et al. [9]. Theflux at any time / is given by

where All is the osmotic pressure difference across the membrane and Rad is theresistance due to solute adsorption. The general order kinetics equation proposedby Aimar, Baklouti, and Sanchez [10] was used to relate Rad to its equilibriumvalue Rae:

dRJdt =pC\Rae - RJ) (7)

where p is the adsorption rate constant. Rae is assumed to depend on concentra-tion in accordance with either the well-known Langmuir or Freundlich models.This model has been recently significantly improved by Ruiz-Bevia et al. [11]using a single kinetic expression for the amount of protein adsorbed and usingthis value to calculate R^ directly.

2. MEMBRANE SURFACE CHARACTERISTICS

Various membrane surface characteristics (such as hydrophilicity, electro-static effects, surface roughness, and pore size/pore size distribution) affect themembrane-solute interactions. These are discussed below.

2.1. Hydrophilicity/Hydrophobicity

Though membrane hydrophilicity is not the only factor affecting fouling [12],it is definitely important and deserves to be discussed first.

2.1.1. Characterization

Hydrophilicity of the membrane material is generally measured by (1) watercontact angle measurements on the surface of either the membrane or the corre-sponding polymer film or (2) by solvent/water swelling measurements of themembrane material, though other methods such as bubble release [13] or chro-matography [14] have also been used.

Zhang, Wahlgren, and Sivik [15] and Uyama et al. [16] have compared thevarious methods of contact angle measurements on UF membranes and polymerfilms, respectively. Zhang et al. [15] measured both advancing and recedingwater contact angles by the sessile drop method and captive bubble method.

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620 MUSALE AND KULKARN1

Both methods gave consistent results for polyolefin (PO), polysulfone (PSF),cellulose acetate (CA), and poly(acrylonitrile) (PAN) UF membranes. The CAand PAN membranes showed less hysteresis than the PSF and PO membranes.Uyama et al. [16] measured the water contact angles on films of regeneratedcellulose (RC), poly(vinyl alcohol) (PVA), polystyrene (PS), nylon 6, and so onby sessile drop (with a telescope or a laser beam) and Wilhelmy plate methods.They concluded that all three methods lead to similar contact angles; however,use of the laser beam was simpler and clearer.

Contact angle measurements of various liquids on the membrane surface canbe used to construct a Zisman plot [17]. The surface energy can then be de-scribed in terms of its polar and dispersive force components:

1 + cos0 = 2(Y?)O5[(Y?)°5/Y,,] + 2(Y?)°5[(Yf)° 5/Y,v] (8)

where Y, and Y, are the surface energy of the membrane polymer and contactingliquid, respectively, and the superscripts d and p indicate the dispersion andpolar components of surface energy, respectively.

Keurentjes et al. [13] developed a sticking bubble technique for measurementof membrane hydrophilicity. The contact angle measured by this method isthought to be less affected by the presence of pores. The membrane is sub-merged in liquids of different surface tensions. Air bubbles are attached to themembrane, and the reference surface tension yd, for the membrane is the one atwhich the air bubble detaches from the membrane with 50% probability. Thereference surface tension value is lower for more-hydrophobic membranes. Keu-rentjes et al. [13] found ydl and yc (reported critical surface tension for the poly-mer) to be in good agreement for polypropylene (PP), poly(tetrafluoroethylene)(PTFE), and polydimethylsiloxane (PDMS); however, polyvinylidene fluoride(PVDF), PSF, and polyethersulfone (PES) membranes were found to be morehydrophilic than expected from the reported yc values.

Silva et al. [18] measured the hydrophilicity of copolymers of acrylonitrilewith 2-hydroxyethyl acrylate (HEA) or 4-hydroxybutyl acrylate (HBA) by mea-suring the swelling of polymer films in water. The water content in the mem-brane was determined by immersing the films in water, removing excess waterby gentle blotting, weighing the water-swollen films, and then drying forat least 12 h in vacuum before reweighing. The equilibrium water uptake byAN/HEA and AN/HBA was found to be 83% and 81%, respectively.

2.1.2. Adsorption of Proteins on the Membrane and Specific Interactions

It is well documented that hydrophilic membranes are less prone to proteinadsorption than hydrophobic ones [19-28].

The hydrophobicity of the membrane surface also influences the nature ofthe deposited protein. For example, Sheldon, Reed, and Howes [27] character-• •• • , u — i . / D C A ^ ^ ^ t i ^ n n in.kn MWCO (10 x 103 dalton

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molecular weight cutoff, defined as the solute molecular weight for which themembrane shows 90% rejection) PSF and RC membranes using various trans-mission electron microscopy (TEM) techniques. The TEM studies showed thatBSA was globular both in solution and on the surface of the RC membrane;however, BSA deposited on the PSF membrane was long and filamentous. Sincethe globular shape was observed both in solution and on the RC membranesurface, disruption due either to freeze damage or to the filtration process wasruled out. It was concluded that the disruption of the BSA tertiary structurewas due to its interactions with hydrophobic polysulfone, which unfolds thehydrophobic sites of BSA and hence denatures it.

2.1.3. Effect of Hydrophilicity on Membrane Permeability

Higher permeate fluxes have been achieved when the membrane surface ismore hydrophilic [25, 29-35]. This would be expected from the adsorption trenddiscussed above.

Membrane hydrophilicity also affects protein transmission. For example,Rolchigo, Raymond, and Hildebrandt [36] considered the behavior of a proteinsolution (myoglobin, BSA, and ferritin) with two 100-kD Ultrafilic™ membranes,one that had a water contact angle of 4° and an unmodified precursor with acontact angle of 46°. They found that BSA and myoglobin transmissions werehigher with the hydrophilic membrane in both crossflow and rotary systems.

Higuchi, Ishida, and Nakagawa [37] compared the separation of y-globulinand BSA through surface-modified (incorporating hydroxyl groups) and unmod-ified PSF membranes. The transmission of y-globulin through the surface-modi-fied membranes was higher than BSA at pH 7.2 and 9.0, even though the molec-ular weight of y-globulin is higher than that of BSA. They attributed this to thebalance of hydrophilic and hydrophobic segments on the surface of the modifiedmembranes, rather than to charge repulsion or sieving mechanisms.

Musale and Kulkarni [34, 35] studied the UF of BSA and hemoglobin (Hb)at pH 4.0-7.5 through hydrophobic PAN and hydrophilic poly(acrylonitrile-co-acrylamide) (PAN-AAm) membranes with a similar pore size. They observedthat protein transmission of both BSA (hydrophilic) and Hb (hydrophobic) inindividual protein UF (0.1 g/dL) through the hydrophilic membrane was higherthan through the hydrophobic one. Similar results were obtained in the UF ofmixed protein UF (0.1 g/dL each of BSA and Hb).

2.2. Electrostatic Effects


Charges on the membrane surface can be ionic or electrostatic in nature.Ionic charges are generally measured by potentiometric titration or as ion ex-change capacity [38]. The electrostatic charge on the membrane surface is mea-

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622 MUSALE AND KULKARN1

sured in terms of the zeta potential by electroosmosis [39-41] or streamingpotential [42-44] methods, both of which are based on electrokinetic phe-nomena.

The electrostatic charge on a membrane is dependent upon the membranematerial, the pH, and the ionic strength of the feed solution [40, 45-48]. Poly-meric membranes are generally negatively charged at neutral pH. For example,PSF PM10 and Dynel XM-300 membranes carry negative charges at pH 2-10,while cellulosic YC-05 membranes have an isoelectric point at pH 9 [49].

2.2.2. Adsorption on Membrane Surface

Godjevargova and Dimov [50] found less BSA adsorption and higher perme-ability with membranes based on hydrolyzed PAN (containing carboxyl groups)than on PAN itself at pH 7.4. This effect may also have been due to the in-creased hydrophilicity. Hosch and Staude [51] found less protein adsorptionwith a negatively charged poly amide membrane at pH values above the BSAisoelectric point (IEP) (pH 4.8) due to protein-membrane repulsion. Correspond-ingly, more adsorption was seen at these pH values with a positively chargedmembrane due to protein-membrane attraction.

2.2.3. Effect of Surface Charge on Membrane Permeability

Generally, higher permeate fluxes are obtained if the membrane has a chargesimilar to that of the protein being ultrafiltered [42, 52-54].

Kobayashi et al. [55] studied the UF of synthetic wastewater containing pep-tone with positively charged (benzyltrimethylammonium chloride) and negativelycharged poly(acrylonitrile-co-sodium styrene sulfonate) [poly(AN-co-NaSS)]membranes. The data were compared with an uncharged PAN membrane withsimilar sieving characteristics. They found a lower rate of flux decline for thepositively charged membrane compared to the uncharged membrane. This wasattributed to a thinner peptone layer formation on the positively charged mem-brane surface compared to that on PAN or poly(AN-co-NaSS). The infrared(IR) spectral data showed that the peptone parts containing the —COOH groupsare mainly concentrated on the surface of the positively charged membrane,which may suppress the gel layer formation.

The electrical charge on the membrane has also been used to enhance theseparation of charged proteins/solutes. For example, Nakao et al. [38] measuredthe transmission of myoglobin and cytochrome C (both proteins are of similarsize; myoglobin has an IEP of 7, and cytochrome C has an IEP of 10.7) throughpositively charged (quaternary ammonium chloride) and negatively charged (so-dium sulfonate) PSF membranes. They found greater rejection for whicheverprotein was of the same charge as the membrane (either positive or negative).

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The rejection of both proteins at their IEP was low, despite a reduction in poresize due to increased protein deposition at this pH.

Similarly, Kobayashi et al. [56] reported that the separation of dextran overdextran sulfate by negatively charged UF membranes of poly(AN-co-NaSS)with stearyltrimethyl ammonium as a counterion increases as the styrene sulfo-nate group content in the membrane increases.

For UF of BSA-y-globulin at pH 4.3 (both proteins with positive charge),Miyama, Yoshida, and Nosaka [57] obtained greater permeability of BSA withnegatively charged poly(acrylonitrile)-gra//-poly(sodium-p-styrene sulfonate)(PAN-£-NaSS) membranes compared to that with brominated poly(acrylonitrile)(PANBr) membrane.

Kontturi, Kontturi, and Vuoristo [58] investigated separation of human serumalbumin (HSA) and Hb (which are again of similar size) by convective electro-phoresis using a hydrophilic PVDF membrane (Durapore DVPP, 0.65 [im) incombination with standard cation and anion exchange membranes. They foundthat, with decreasing pH in the range of 7.2-5.2 (i.e., as HSA becomes lessnegative and Hb becomes more positive), the HSA/Hb separation factor in-creases.

2.3. Surface Roughness


Surface roughness can be measured directly by various electron microscopytechniques, such as TEM [59-61] or atomic force microscopy (AFM) [61]. It isalso possible to characterize surface roughness by adsorptive measurements/fractal dimension measurements [62, 63].

2.3.2. Effect of Surface Roughness

Static adsorption studies of BSA at pH 7 on PM30 and PTTK membranesby Chen, Kim, and Fane [60] showed that PTTK adsorbed more BSA thanPM30 even though both the membranes had approximately similar skin layerthickness, pore size, and substructure thickness. They attributed the greater ad-sorption of BSA on PTTK membrane to its increased surface roughness.

Surface roughness may affect concentration polarization/fouling through ei-ther a (1) change in surface free energy or (2) liquid stagnation in the surfacedepressions. Larsson [17] speculated that an increase in surface roughness in-creases surface free energy, which in turn would increase adhesion of surface-active molecules. Fell et al. [64] suggested that surface roughness affects thetransverse feed flow, causing feed solution to stagnate in the surface depressionsin which pores usually exist. The resulting high local concentration polarizationat the pore entrance causes reduced flux and increased fouling.

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Fane and Kim [65] found greater flux loss with an increase in surface rough-ness of the membrane. Kim, Fane, and Fell [59] found that smoothing of thesurface by application of Langmuir-Blodgett layers decreased flux loss andfouling.

2.4. Pore Size/Pore Size Distribution

It is well documented that increasing the membrane pore size, and hencereducing the intrinsic membrane resistance, results in increased membrane foul-ing, which in turn may actually result in lower long-term process fluxes. Severalinvestigators [66-71] have observed higher membrane fouling for microfiltra-tion (MF) of various solutions compared to UF. Tarleton and Wakeman [72]found that steady-state filtrate fluxes during suspension filtration decreased withincreasing pore size, indicating fouling in pores. The increased fouling in largerpores is mainly related to concentration polarization phenomena or pore blocking.

Mueller and Davis [73] studied the effect of surface porosities on the foulingof PSF, PVDF, and CA membranes. The low surface porosities of PSF andPVDF membranes led almost immediately to external (surface) fouling; withthe higher surface porosity CA membranes, internal (pore) fouling was moreimportant. Protein transmission remained constant or decreased only slightlydue to internal fouling, while significantly decreased protein transmission wasobserved with external fouling.

The membrane pore size distribution also affects protein transmission. Forexample, Urase, Yamamoto, and Ohgaki [74] studied the transmission of coli-phage QB and T4 viruses through membranes with various pore sizes. SomeMF membranes showed higher virus rejection than UF and nanofiltration (NF)membranes. The greater leakage of viruses through these membranes was attrib-uted to a few pores being considerably larger than the average pore size.

3. STRATEGIES TO MODIFYMEMBRANE SURFACE CHARACTERISTICS

Modification of the membrane material/surface to increase hydrophilicity orto modify the surface charge has been investigated for reducing adsorption/fouling of UF membranes by biomolecules. The techniques investigated for thispurpose include (1) using hydrophilic polymers/copolymers, (2) blending of hy-drophilic or charged polymers with hydrophobic polymers, or (3) surface modi-fication of the membrane itself. These techniques are described below.

3.1. Polymer Modification

Various commercial membranes are based on cellulosic materials that havegood fouling resistance due to their low adsorption of hydrophobic solutes.

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However, the disadvantage with these materials is their restricted chemical orthermal stability. Therefore, membranes based on more-stable materials havebeen investigated. These polymers can be modified to increase hydrophilicity orto change surface charge characteristics. Usually, modifications of the polymerstructure by introducing polar or ionic groups, which are intended to change thesurface charge characteristics, also increase the polymer hydrophilicity.

Polysulfone is a widely used material for UF membranes because of its rela-tively good thermal/chemical stability and processibility. Sulfonation of hydro-phobic PSF by sulfuric acid, chlorosulfonic acid [75-77], or SO3-triethyl phos-phate complex [38, 76] gives a negatively charged hydrophilic material. Sincesulfuric acid may cause polymer degradation, the other two reagents are pre-ferred. Similarly, polyetheretherketone (PEEK), which is extremely chemicallystable but rather hydrophobic, can be modified by partial sulfonation to get amore hydrophilic polymer [78].

Nakao et al. [38] synthesized positively charged PSF by introducing a quater-nary ammonium group on the PSF backbone. This was accomplished by firstreacting PSF with chloromethyl methyl ether at 50°C in tetrachloroethane usingZnCl2 as catalyst. The resulting chloromethyl PSF was then reacted with trieth-ylamine in N,JV-dimethyl formamide (DMF) at 50°C.

A considerable amount of chemical modification work has also been donewith PAN. Miyama et al. [57] grafted sodium p-styrene sulfonate (NaSS) onPANBr. PANBr and NaSS were dissolved in dimethylsulfoxide, and the solutionwas photoirradiated at 30°C for 5 h in argon flow with a 100-W high-pressuremercury lamp. The reacted mixture was concentrated at reduced pressure at50°C and precipitated in aqueous methanol. Membranes made from PAN-g-NaSS showed higher separation of BSA and y-globulin at pH 4.3 than thosemade from PANBr.

Kobayashi et al. [56] prepared negatively charged UF membranes of poly-(AN-co-NaSS) followed by ion exchange of sodium with amphiphilic counter-ions such as stearyltrimethyl ammonium ion. The UF separation of dextran/dextran sulfate was enhanced by increasing the content of the styrene sulfonategroup in the membrane. The uptake of a fluorescence probe (8-anilino-l-naph-thalene-sulfonic acid sodium salt) by the membrane was measured to show thatthe hydrophobicity of the charged membrane increases due to the ion exchangeof sodium with stearyltrimethyl ammonium. Contrary to most other reports onthe effect of hydrophilicity, Kobayashi et al. [56] found that the flux for dextranUF increased as a result of this ion exchange, that is, as the membrane becamemore hydrophobic.

Copolymers of acrylonitrile (AN) with HEA or HBA have been prepared bySilva et al. [18] as suitable hydrophilic materials for dialysis and UF mem-branes. The monomers were copolymerized (AN:HEA/HBA = 95:5) with am-monium persulfate as catalyst in aqueous suspension.

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Musale and Kulkarni [34] prepared copolymers of AN with 10-30 mol% ofacrylamide in DMF using concentrated nitric acid as catalyst. The acrylamideinsertion resulted in increased hydrophilicity and an increase in the polar surfaceenergy compared to the dispersive surface energy, coupled with a decrease inthe negative electrostatic charge. The BSA UF fluxes and flux recoveries werehigher for acrylamide-containing membranes compared to PAN. This improve-ment in membrane performance was attributed to the increased hydrophilicityin spite of the reduced negative charge.

3.2. Polymer Blends

Blending of a miscible hydrophilic polymer with a chemically, or thermallystable hydrophobic polymer has been investigated for achieving stable poly-meric materials with improved hydrophilicity. However, the choice of polymersis limited since only a few polymers form homogeneous blends [79].

Polyvinylpyrrolidone (PVP) (a hydrophilic polymer) is known to form blendswith PSF [80], PES, polyimide, and polyetherimide [81]. Membranes from ho-mogeneous blends of these polymers can be prepared by phase inversion. PVPis water soluble; hence, it is expected to be removed in the water used to precipi-tate the membrane. However, there are indications that sufficient PVP remainsentangled with the matrix polymer to impart a more-hydrophilic character to themembrane [82]. As described in patent literature, it is also possible to insolubi-lize the PVP by reaction with peroxydisulfate.

Chen, Chiao, and Tseng [83] have prepared mechanically stable, partiallycharged hydrophilic membranes from blends of aminated PSF or sulfonated PSFwith PSF.

Blends of CA and PSF have been prepared by Sivakumar et al. [84]. Theycharacterized the blends by differential scanning calorimetry (DSC). The mem-branes prepared from these blends were tested for pure water permeability andseparation of metal ion in aqueous medium.

3.3. Surface Modification

Another widely investigated method is to modify the surface of an existinghydrophobic membrane so that the surface has the desired characteristics forUF, while the matrix itself retains its nonswelling characteristics. This has someadvantages over bulk polymer reaction, for which the extent of chemical modifi-cation may be limited by solubility or strength properties. However, there maybe long-term loss of the desired surface characteristics via loss of the functional-ized moieties through abrasion, diffusion into the bulk, or chemical reaction.Various surface modification methods are reviewed by Mulder [78], Nystrom

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and Howell [85], and Garg et al. [86]. These methods include chemical reaction,plasma treatment, and adsorption coating.

3.3.1. Chemical Reaction

Hydrophobic surfaces can be chemically modified by the introduction ofcharged and/or hydrophilic groups such as carboxylic, amino, hydroxyl, sul-fonic, or quaternary ammonium groups. Generally, hydrophobic membranessuch as PVDF, PO, PAN, or PSF are modified by this technique. Examples ofthis technique are discussed below.

3.3.1.1. Modification of PVDF Membrane. Stengaard [32] reported thatether or amine groups can be introduced onto a PVDF surface by reaction withan alcohol (R—OH) or a primary amine (/?—NH2) in the presence of a strongbase (NaOH). The probable mechanism for this reaction is the elimination ofhydrogen and fluorine from PVDF and subsequent double-bond formation. Thisis followed by nucleophilic addition, introducing amine or ether groups on thePVDF backbone. Increases up to 20-50% for whey UF flux were seen with themodified membranes. Also, the water flux recovery after UF of solutions con-taining BSA, dextran, and Triton X-100 was almost complete in modified mem-branes.

Similarly, Sternberg [87] reacted PVDF membrane samples, prewetted bymethanol, with glycine in the presence of aqueous sodium hydroxide at 132°Cfor 15 min, followed by washing with hot running water. These treated mem-brane samples were found to be instantly water wettable.

Shoichet and McCarthy [88] reported the surface modification of PVDF tointroduce carboxyl groups. They reacted PVDF films with aqueous sodium hy-droxide in the presence of tetrabutyl ammonium bromide at 40°C for 3 min.This was followed by washing with water, methanol, and dichloromethane, fol-lowed by drying under vacuum at 50°C for 48 h. These films were then reactedwith potassium chlorate in sulfuric acid for 2 h at room temperature, followedby a similar washing and drying procedure. The increase in hydrophilicity ofthe treated films was confirmed by contact angle measurements.

Crowe and Badyal [89] studied surface modification of PVDF by LiOH.PVDF pellets were dipped overnight in an aqueous solution of LiOH, followedby washing with water or isopropyl alcohol, and then vacuum drying. X-rayphotoelectron spectroscopy (XPS) studies showed that the reduction in fluorinecontent was increased after isopropanol washing compared to water washing.The appearance of oxygen on the PVDF surface was attributed to the formationof —COOH or —OH groups.

3.3.1.2. Modification of PSF Membrane. Polysulfone membranes havebeen modified by the introduction of hydroxyl groups [37, 90, 91] or sulfonate

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groups [77, 92, 93] or by surface fluorination [94, 95]. For example, Higuchi,Mishima, and Nakagawa [91] found increased separation between BSA andy-globulin after surface modification of PSF membranes by reacting with pro-pylene oxide in A1C13 and hexane at 5°C for 5 min to obtain hydroxyl groups onthe PSF surface. This modification proceeds via the well-known Friedel-Craftsreaction mechanism.

Chang, Kulkami, and Funk [77] exposed PSF membranes to vapors of 9 Nsulfuric acid for about 10 min at ambient temperature. This exposed membranewas then quenched in 1 N HC1 to stop the sulfonation reaction and then rinsedwith deionized water.

Sedath, Taylor, and Li [95] carried out the fluorination of 100K PSF mem-brane by 0.025 vol% elemental fluorine in an inert gas (N2) with varying reac-tion times. They found higher initial permeate fluxes and lower flux declinewith the fluorinated membrane compared to an untreated membrane for UF ofpotato waste streams.

3.3.1.3. Modification of PAN Membrane. Godjevargova and Dimov [50]studied the permeability of vitamin B-12 through 10K surface-modified, chargedmembranes of AN copolymer. They found that surface modification by eitherNaOH (hydrolysis with 5-20% aqueous NaOH at 50°C for 60 min) or hydroxyl-amine increases the hydrophilicity of the membrane and thereby the permeabil-ity of vitamin B-12. In the case of surface modification by hydroxylamine, themembrane was swollen in 5% aqueous DMF for 30 min at room temperatureand then immersed in 2.5-15 wt% aqueous hydroxylamine solution at 40°C for120 min. However, modification with diethylaminoethyl methacrylate and fur-ther quaternization with dimethyl sulfate resulted in increased vitamin B-12 per-meability only up to a 1.6% degree of grafting. The decreased permeabilityabove this level was attributed to decreased pore volume in the skin layer of themodified membrane.

Ulbricht et al. [96] studied grafting of hydrophilic monomers such as acrylicacid, 2-hydroxyethyl methacrylate (HEMA), and various poly(ethylene glycol)(PEG) methacrylates on PAN UF membranes by either simultaneous or sequen-tial ultraviolet (UV) irradiation methods. UV irradiation leads to formation ofperoxides on the membrane surface; the decomposition of these peroxides at ahigher temperature initiates monomer grafting. They found that, for membraneswith a high degree of grafting (>400 ng/cm2), very little protein (y-globulin,BSA, and cytochrome C) adsorption occurred, and almost no fouling due toBSA adsorption was observed.

3.3.1.4. Modification of Polyolefin Membranes. Garg et al. [86] studiedsurface modification of PP membranes (Celgard® 2400 and 2500) by hydroxyla-tion with potassium peroxydisulfate solution, followed by grafting with acryl-

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amide using cerric ammonium nitrate as an initiator. Subsequently, the acryl-amide groups at the membrane surface were partially hydrolyzed to carboxylgroups.

Kim et al. [97] showed that nonselective adsorption of BSA and y-globulinon polyethylene MF membranes can be reduced by hydrophilizing the mem-branes through radiation-induced grafting of hydrophilic monomers such asHEMA, vinyl acetate, and glycidyl methacrylate. Radiation-induced grafting ofhydrophilic monomers has also been reported on PP hollow-fiber membranes[98].

3.3.1.5. Modification of Poly amide Membranes. Hosch and Staude [51]studied the fouling of heterogeneously modified polyamide UF membranes byHSA solutions at different pH values. The base materials chosen for modifica-tion were asymmetric membranes made from fully aliphatic, fully aromatic, oraliphatic-aromatic polyamides. The membranes made from aromatic-aliphaticpolyamide were heterogeneously reacted with glycidyl methacrylate to obtainepoxy groups on the membrane surface. These epoxy groups were further re-acted with trimethyl amine and iminodiacetic acid to get positively and nega-tively charged groups, respectively.

3.3.2. Plasma Treatment

Dudley et al. [99] modified 0.2-nm PVDF membranes by first introducinghydroxyl groups by oxygen plasma etching, followed by grafting of 2-methacry-loyloxyethyl phosphoryl choline using cerric ammonium nitrate as an initiator.They found that the treated membrane had increased flux and a lower rate offlux decline for BSA UF. Similar results were obtained by Akhtar et al. [100]with phospholipid-grafted CA membranes for filtration of BSA solution.

Plasma treatment followed by grafting of hydrophilic monomers such asacrylic acid and methacrylic acid on PAN and PSF membranes has also beenshown to improve permeate fluxes for BSA UF [101]. Surface modification byplasma treatment has also been studied by Wolff, Steinhauser, and Ellinghorst[102], Lai and Chao [103], Vigo, Nicchia, and Uliana [104], and Karakelle andZdrahala [105].

The surface chemistry resulting from plasma treatment is complex. If thekinetic energy of the electrons created in the plasma is higher than the excitationenergy level of the gas molecules, the electron-gas molecule collisions are in-elastic. The resulting excited gas molecules may decay and emit photons orundergo homolytic splitting to create free radicals. Treatment with O2 plasmagenerally leads to polymer degradation, cross-linking, or carbonylation/hydrox-ylation of the surface through reaction with atmospheric moisture [102].

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3.3.3. Adsorbed Coatings

In the adsorbed coating method, membranes are made hydrophilic by pre-treatment with hydrophilic polymers, surfactants, and the like. Though the ad-sorbed coating constitutes an additional mass transfer resistance, the decreasedfouling tendency can result in better process flux.

3.3.3.1. Pretreatment with Hydrophilic Polymers. Preadsorption or coat-ing of UF or MF membranes with hydrophilic polymers sometimes results inimproved permeate fluxes and reduced protein adsorption.

Adsorption coating of two hydrophilic polymers, poly(vinyl methyl ether)(PVME) and methyl cellulose (MC), on polycarbonate (PC) (Nuclepore) andPSF membranes led to reduced protein adsorption within the membrane pores[106]. This was attributed to the polymers preoccupying the protein adsorptionsites. Also, the coated polymer partly seals off the pore entrance and preventsinternal protein adsorption due to increased steric hindrance. They found that,compared to the unmodified membrane, PSF membranes pretreated with MCexhibited a smaller increase in overall transport resistance during UF of (i-lacto-globulin at pH 4.7.

Nystrom [42] found that adsorption coating of PSF membranes with polyeth-yleneimine (PEI) decreased flux reduction during ovalbumin UF. This was at-tributed to the increase in hydrophilicity rather than to charge modification.Similarly, surface coating of PES membranes by polyurea/polyurethane showedimproved permeate fluxes and flux recoveries for BSA, dextran, and PEG UFcompared to standard PSF membranes [107].

Kim, Fane, and Fall [108] studied the effect of pretreating PSF-based PM30membranes with hydrophilic polymers such as MC, polyvinylalcohol (PVA),and PVP by passive or convective adsorption methods. Average flux improve-ments of 20-40% relative to untreated membranes were obtained for BSA UF.In multiple usage cycles, the MC-treated membrane showed 100% higher fluxafter 5 cycles.

Brink and Romijn [54] studied the effect of preadsorbing PSF membraneswith anionic, nonionic, and cationic polymers and surfactants on the adsorptionof whey proteins and flux stability. The application of the nonionic hydrophilicpolymers PEG, poly(acrylamide), PVP, and PVME was found effective in re-ducing protein adsorption, as well as membrane resistance, during UF. The ap-plications of other surfactants and ionic polymers such as PEI, poly(acrylicacid), carboxymethyl cellulose, and poly(sodium 4-styrene sulfonate) (PNaSS)were generally less successful.

Bauser et al. [109] found that permeate fluxes for UF of bovine blood serumwere increased by two and three times after coating a 0.4-|im Nuclepore PCmembrane with PAN and isotropic carbon, respectively.

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3.3.3.2. Pretreatment with Surfactants. Pretreatment of membranes withionic or nonionic surfactants has also been shown to improve permeate fluxesand reduce flux decline.

Coating of alcohols and nonionic surfactants on PSF UF membranes causedreduced fouling during UF of model fermentation broths containing either anti-foam agents or yeast extract and glucose [110]. Nonionic surfactants were gen-erally more effective than alcohols in reducing antifoam agent fouling.

Chen, Fane, and Fell [111] studied the effect of various surfactant coatingson the performance of PM30 membranes used in protein UF. They observedthat, in contrast to long-chain nonionic surfactants, the small anionic surfactant(AOT) reduces protein adsorption by altering protein-membrane electrostaticinteractions. AOT was less effective at reducing flux decline at pH values belowthe BSA IEP, that is when the protein was positively charged. When AOT wasused in conjunction with other nonionic surfactants or with polyethylene oxide,significant flux improvement for BSA UF was observed compared to that wheneither AOT or a nonionic surfactant was used alone.

Kim et al. [59] showed that membranes pretreated with nonionic surfactantmonolayers by the Langmuir-Blodgett (LB) technique exhibited 30% higherflux increase than untreated membranes for albumin UF. They attributed thiseffect to the increase in membrane smoothness and homogeneity rather than theincrease in hydrophilicity because similar flux improvement was also observedwith a coating of strongly hydrophobic stearic acid.

3.3.3.3. Other Pretreatment Methods. Howell and Velicangil [112] sug-gested increasing flux by enzyme immobilization on the membrane surface tohydrolyze adsorbed solute. They immobilized various proteases and papain onPSF (PM-10) membranes and showed flux improvements for BSA, Hb, andcheese whey UF. The net protein loss due to cleavage of filtered albumin byactive enzyme was found to be between 5% and 1% of the total protein pro-cessed. Szaniawski and Spencer [113] also found a flux increase during pectinconcentration when pectinase was immobilized on the membrane surface com-pared to addition of enzyme in the feed solution.

4. FEED VARIABLES THAT MODIFYMEMBRANE-SOLUTE INTERACTIONS

The common feed variables that affect the membrane-solute interactions arepH, ionic strength /, solute concentration, and specific interactions with othersolutes.

The solution pH determines the net charge on the protein, thereby affectingboth protein-protein and membrane-protein electrostatic interactions. These, inturn, affect the amount of protein adsorption on the membrane surface, the per-

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meability of the deposited protein layer, the protein rejection/fractionation prop-erties, and permeate flux. For example, the flux minima commonly observed atthe protein IEP [33, 42, 114, 115] can be attributed to the increased adsorption/deposition of protein due to reduction in membrane-solute or increase in cakedensity due to reduced solute-solute repulsive electrostatic interactions.

Changing / affects the protein charge through charge shielding at pH valuesabove and below IEP and anion binding at IEP. The charge shielding effect ismore apparent at low ionic strength values, typically below 0.1 M. Above thislevel of /, dispersive (van der Waals) interactions are expected to dominate. Forexample, for BSA UF through an XM100A membrane, Kim and Fane [33]found decreased flux with increasing I at pH values above and below IEP; thiseffect was attributed to charge shielding. The flux increase at IEP with increas-ing / was attributed to anion binding. The effect of / on membrane performancehas also been studied by Palecek, Mochizuki, and Zydney [114] and Iritani,Mukai, and Murase [116].

The change in feed concentration affects the extent of protein adsorption andsubsequent membrane performance. For example, in a static adsorption study ofBSA at pH 7 and 25°C on CA and PSF UF membrane surfaces, Matthiasson[20] found that the protein adsorption increased up to 0.2% BSA concentrationfor CA and up to 0.8% for PSF; above these BSA levels, adsorption remainedconstant up to 1.2%.

LIST OF ACRONYMS

AFM atomic force microscopyAN acrylonitrileBSA bovine serum albuminCA cellulose acetateCFMF cross-flow microfiltrationDLVO Derjaguin-Landau-Verwey-OverbeekDMF MN-dimethyl formamideDSC differential scanning calorimetryHb hemoglobinHBA 4-hydroxybutyl acrylateHEA 2-hydroxyethyl acrylateHEMA 2-hydroxyethyl methacrylateHSA human serum albuminIEP isoelectric pointL-B Langmuir-BlodgettMC methyl celluloseMF microfiltration

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NaSS sodium-styrene sulfonatePAN poly(acrylonitrile)PC polycarbonatePDMS poly(dimethylsiloxane)PEEK poly(etheretherketone)PEG poly(ethylene glycol)PEI poly(ethyleneimine)PES polyethersulfonePO poly(olefin)PP poly(propylene)PS polystyrenePSF polysulfonePTFE poly(tetrafluoroethylene)PVA polyvinylalcoholPVDF poly(vinylidene fluoride)PVME poly(vinyl methyl ether)PVP poly(vinyl pyrrolidone)RC regenerated celluloseTEM transmission electron microscopyUF ultrafiltratonXPS x-ray photoelectron spectroscopy

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Reviews Pune 411 008, India Science, Part C: Polymer ...folk.ntnu.no/andersty/3. Klasse/Felleslab (SepTek... · the intrinsic rejection of the membrane R', = define 1 - CpICdm a)