American Fisheries Society Symposium 38:387–401, 2003© 2003 by the American Fisheries Society

Sepiolite Membrane for Ultrafiltration

Q. K. WANG1, TAKESHI MATSUURA2, C. Y. FENGIndustrial Membrane Research Institute, Department of Chemical Engineering

University of Ottawa, Ottawa, Ontario K1N 6N5, Canada

M. R. WEIR, CHRISTIAN DETELLIEROttawa-Carleton Chemistry Institute, Department of Chemistry

University of Ottawa, Ottawa, Ontario K1N 6N5, Canada

E. RUTIDINKA, R. LE VAN MAODepartment of Chemistry and Biochemistry, Laboratories for Inorganic Materials

Concordia University, Montreal, Quebec H3G 1M8, Canada

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tion of sepiolite membranes by pore size and poresize distribution is another objective.

Many works have been published on the poresize and pore size distribution of synthetic mem-branes (Michaels 1980; Zeman and Wales 1981;Singh et al. 1998). One of the methods of determin-ing pore size and pore size distribution is based onthe separation of solutes of known sizes. Michaelsconcluded that the lognormal probability functionwas generally accounted as means for describingsieving curves for ultrafiltration membranes, and acomplete sieving curve could be constructed for agiven membrane using only two experimental siev-ing coefficient values for two different solutes ofknown Einstein–Stokes radius. On the basis of thisobservation, Michaels proposed a method to deter-mine pore size and pore size distribution. Singh et al.studied various sulfonated poly(2,6-dimethyl-1,4-phenylene oxide) membranes and found that thesieving coefficients and Einstein–Stokes radii werecorrelated well with the lognormal distributionfunction. Aimar et al. (1994) described the prob-lems that may be encountered when developing amethod for membrane characterization based onsolute sieving (e.g., concerning macromoleculartransport through capillaries). They also showed theapplicability of the lognormal curve to fit the soluteseparation–solute size correlation.

Scanning electron microscopy (SEM) is apowerful tool for investigating membrane struc-ture. However, because of the low conductivity ofthe membrane surface, the sample must be coatedwith a heavy metal, and the coating process maycause some damage to the membrane. Therefore,SEM is not a reliable method for measuring poresize (Hsieh et al. 1979; Aimar et al. 1994). Hence,in this work, membrane pore size and pore size dis-tribution are determined by a method proposed byMichaels (1980).

In recent years, a great deal of research has beendevoted to the development of a new kind of inor-ganic membrane that exhibits improved resistanceto heat, chemicals, and corrosion. Rapid develop-ment and innovation have already been realized inthis area (Cot 1998). Clay minerals are a well-known class of naturally occurring inorganic mate-rials with well-known structural adsorption, rheo-logical, and thermal properties (Nagata et al. 1974;Serna et al. 1975; Jones and Galán 1988; Pérez-Rodríguez and Galán 1994). Research on clay as amembrane material has concentrated mainly onpillared clays (Cool et al. 1997; Mishra and Parida1997). Studies of membranes prepared entirelyfrom clay have begun (Ishiguro et al. 1995; Le VanMao et al. 1999).

Sepiolite, one of the most important gel-form-ing clays, can give rise to stable suspensions of highviscosity at relatively low concentrations. It is char-acteristically fibrous as observed under an electronmicroscope (Jones and Galán 1988). Its structuraland morphological changes that occur on heatingcan be divided into three phases: low-temperature(600°C) regions. In the high-temperatureregion, dehydroxylation of the structure takes placeat about 800°C, together with a change in entropydue to structural collapse (Jones and Galán 1988).This thermal behavior suggests that there is a limi-tation in temperature for sintering the membrane.

The objective of the present study was toestablish the preparation procedure for pure sepio-lite membranes and to investigate their propertiesand potential for ultrafiltration. The characteriza-

1Present address: Department of Food Science and Engineering,Dalian Fisheries University, Heishijiao, Dalian 116023, PRChina. E-mail: jane98@mail.dlptt.ln.cn or qiukuan@hotmail.com2Corresponding author. E-mail: matsuura@eng.uottawa.ca

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TheoreticalMean pore size and pore size distribution

According to the equation for solute separation (f; in %),

ƒ = (1–Cp/Cf)×100 (1)

where Cp and Cf are the solute concentration in thepermeate and in the feed solution, respectively. InEquation (1), the effects of dispersive or electro-static interactions and concentration polarizationare not considered. Solute separation of an ultrafil-tration membrane, plotted versus the solute diame-ter on a lognormal probability paper, can yield alinear relationship (Michaels 1980). If solute diam-eter correlates with the solute separation accordingto the lognormal probability function, then therelationship is given as

ƒ = erf (z) = —— ∫z e 2 du (2)

where

lnds –ln µsz = ————— (3)lnσg

and ds is the solute diameter, µs is the geometricmean diameter of a solute molecule, and σg is thegeometric standard deviation about the meandiameter. On lognormal probability coordinates,Equations (2) and (3) linearize in the form of

F(ƒ) = A0 + A1(1nds) (4)

where A0 and A1 are the intercept and the slope,respectively (Michaels 1980); µs can be determinedfrom ds corresponding to f = 50%, and σg can bedetermined from the ratio of ds at f = 84.13% andat 50%. By neglecting the effects of steric andhydrodynamic interaction between solute andpore, the mean pore size (µp) and the geometricstandard deviation (σp) of the membrane can beconsidered to be the same as µs and σg, respective-ly (Michaels 1980; Ishiguro et al. 1995). Depend-ing on µp and σp, the pore size distribution of anultrafiltration membrane can be expressed by thefollowing probability density function (Youm andKim 1991):

df(dp) 1 (lndp –lnµp)2—— = —————— exp [– —————] (5)ddp dp (lnσp) ���� 2(lnσp)2

where dp is the pore diameter.

Pore density and surface porosity

Pore density can be calculated from the permeabil-ity data of the membrane through the Hagen–Poiseuille equation. According to this equation,the solvent flux (Ji) through the pores of diameterdi can be expressed as

Niπdi4∆pJi = —————128η δ

where Ni is the number of pores per unit area hav-ing the diameter di, δ is the length of the pores, ∆pis the pressure drop from one end to the other endof the pore, and η is the solvent viscosity. The totalflux through membrane is the sum of all the fluxesthrough the pores of different sizes; therefore,

J = Σ Jiπ∆pJ = ——— (N1d14 + N2d24 + N3d34 + Nndn4)

128η δ

π∆pJ = ———— (ƒ1Nd14 + ƒ2Nd24 + ƒ3Nd34 + ƒnNdn4)128η δ

π∆pN d maxJ = ————— Σ fidi4 (6)128η δ d min

where N is the number of pores per unit area andcalled pore density and fi is the fraction of the num-ber of pores with diameter di. From Equation (6),the total number of pores can be expressed as

128ηδJN = ————— (7)d max

π∆p Σ fidi4d min

The pore length δ is assumed to be equal to thethickness of the membrane based on the consider-ation that the solvent can pass through only thepores with the shortest length; otherwise, the poreswill be dead ended.

The surface porosity (Ps), defined as the ratiobetween the area of pores to the total membranesurface area, can be calculated as

Nπd max

Ps = ( —— Σ fidi2) × 100 (8)4d min

Einstein–Stokes radius

Einstein–Stokes radius is used as a parameter tocharacterize the size of a macrosolute in the ultra-

WANG ET AL.

1����2π

�u2__– �

2π

389SEPIOLITE MEMBRANE FOR ULTRAFILTRATION

filtration study. It is defined as the “apparent equiv-alent spherical radius” of the macromolecule byusing Einstein–Stokes equation

DAB = kT/6πηα (9)

where DAB is diffusivity, k is Boltzmann’s constant,and α is the Einstein–Stokes radius. Therefore, ds asmacrosolute diameter can be calculated from α.

Singh et al. (1998) have given the followingequations to obtain Einstein–Stokes radius of poly-ethylene glycol (PEG) and polyethylene oxide (PEO)solutes from their molecular weights. For PEG,

� = 16.73 × 10–12 M0.557 (10)

And for PEO,

� = 10.44 × 10–12 M0.587 (11)

Experimental MethodsPurification of sepiolite

Brute sepiolite was purchased from the Source ClayMinerals Repository, University of Missouri–Columbia. Crude sepiolite (30 g) was ground witha small amount of water to form a thick paste, thenadded to 400 mL of distilled water and stirredovernight. The suspension was centrifuged. Thesupernatant was discarded, and the clay particleson the top layer of the sediment were scraped offwith a spatula.

The clay particles were suspended in distilledwater and acidified with 1 M HCl to pH 3.5 (todestroy the carbonates), then centrifuged. The sed-iment was washed with dilute HCl several times,centrifuged, and finally dispersed in distilled water.The pH of the suspension was increased to 8 byadding 0.1 M NaOH and leaving it overnight forsedimentation. The upper layer of the suspensionwas removed. Water (200 mL) was added to theremaining sediment and stirred to homogenize.The saturated NaCl solution was added, andexchange occurred overnight. The suspension wastransferred to dialysis bags to expel the chlorideions. The contents in the dialysis bags were finallycentrifuged and dried at 60°C.

Measurement and observation of sepiolite particle size

Two drops of the suspension sample were placed onthe glass slides. The sepiolite particles wereobserved by using an Olympus-BX40 optical micro-

scope connected to a Polaroid model DMCI digitalmicroscope camera.

Membrane preparation

The aqueous suspension of clay fibers was preparedby dispersing 0.25 or 0.5 g of sepiolite in 30 or 60mL of distilled water. The mixture was stirred witha magnetic stirrer for about 24 h or subjected toultrasonic agitation for about 30 s. Then, the sus-pension was poured into a 9-cm-diameter petri dishand left at room temperature to evaporate to dry-ness. The air-dried membrane was calcined at120°C for 2 h, then the temperature was increasedto 550°C for 5 h.

The membranes were labeled as 0.5A, 0.25A,0.5B, and 0.25B, where the numbers indicate theamount of sepiolite used and the letters representthe method of dispersion (A, magnetic stirring; B,ultrasonic wave). Although the mechanicalstrength of sepiolite membrane was not measured,the membranes were self-sustaining during theultrafiltration experiments.

Ultrafiltration

Ultrafiltration was carried out in a dead-end stirredcell configuration. The cell was pressurized withnitrogen, and the transmembrane pressure was readon a calibrated gauge. The feed concentration wasassumed to remain constant during ultrafiltration,because small amounts of permeate were collected.Five solutes were used: PEG with 35,000 molecularweight (MW) and PEO with 100,000, 200,000,300,000, and 400,000 MW. Feed concentrationwas 200 ppm in each experiment. The permeatewas collected for a predetermined period to meas-ure the permeation rate. The total organic carbonwas measured with a Folio DC-190 Total OrganicCarbon Analyser, and the solute separation wascalculated as in Equation (1).

Scanning electron microscopy

Scanning electron microscopic images wereobtained by using a JEOL 6400 scanning electronmicroscope, and the images were captured digitallyon a personal computer.

Results and DiscussionPurity of sepiolite and structure of sepiolitesuspension

The purity and structure of sepiolite were examinedby X-ray diffraction (Philips pw3710 diffractome-ter). The X-ray powder patterns showed that themajor impurities in the crude material are calcite,

dolomite, and quartz, which were removed by acidtreatment and sedimentation during purification.The main fraction on the powder X-ray diffractionpatterns of purified material is at 12.1 Å, which is acharacteristic reflection associated with sepiolitestructure (Serna et al. 1975), whereas the calcite,dolomite, and quartz are no longer detectable.

The sepiolite fibers observed under the elec-tron microscope are shown in Figure 1. The lengthof the fiber was 1.0–7.0 µm, and the diameter was0.2–0.4 µm. The ultrasonified sample was morehighly dispersed than the magnetically stirred sam-ple. In the latter, aggregated sepiolite fiber bundleswere apparent.

The membrane thickness before and after cal-cination was measured by using a micrometer;results are listed in Table 1. The membranes made

by applying magnetic stirring were thinner thanthose made by applying ultrasonic wave. This dis-crepancy probably results because less fiber aggre-gation occurred when the suspension was sonified.As a result, the porosity of membranes preparedfrom ultrasonified dispension was greater.

Ultrafiltration

The permeate flux changed almost linearly withapplied pressure (30–120 pounds per square inchgauge [psig]; Figure 2). The flux depends mainly onthe thickness of the membrane. For the membranesmade from magnetically stirred suspensions, thethinner the membrane, the higher the permeateflux. Similarly, for the membranes made from soni-fied suspensions, the thickness of membrane is the

390 WANG ET AL.

Figure 1. Sepiolite fibers: magnetic stirred suspension (a) and sonified suspension (b).

Table 1. Thickness of sepiolite membranes before and after calcination.

Membrane Thickness (µm)

Magnetically stirred

0.5A 97 before85 after

0.25A 52 before43 after

Sonified

0.5B 123 before114 after

0.25B 64 before57 after

main factor that controls flux rate. The membranesmade by sonified suspension tend to get higher fluxthan the membranes made by magnetic stirring,probably because of the higher porosity in themembranes made by sonified suspension.

Even though the flux was lowered significant-ly in the presence of PEG and PEO solutes, this

391SEPIOLITE MEMBRANE FOR ULTRAFILTRATION

effect was not permanent, because the flux of waterobtained after filtration experiments with PEG andPEO became closer to the initial water flux.

Mean pore size and pore size distributions

Figure 3 illustrates the separations of PEG and PEOof different molecular weights. Although the flux is

Figure 2. Flux rates of four different sepiolite membranes (0.5A, 0.25A, 0.5B, and 0.25B) calcined at 550ºC.

paper. High correlation coefficients were obtained(r2 = 0.92-0.96). The slope of the line indicates thepore size distribution, and all slopes are quite similar.

Table 2 lists the geometric mean pore size andthe geometric standard deviation for the sepiolitemembranes. Again, the values of the geometricmean pore size (23–26 nm) and the geometric

392 WANG ET AL.

quite different, the separation of all these mem-branes is quite similar.

The pore size and the pore size distribution ofmembranes were calculated from the data in Figure3. As illustrated in Figure 4, straight lines weredrawn between the separation and Einstein–Stokesdiameters of solutes using a lognormal probability

Figure 2. continued

standard deviation (1.91–2.04) are quite similar.The different dispersion means for membranepreparation did not obviously affect the pore sizes.This finding demonstrates that sepiolite mem-branes can be quite easily prepared, and the poresizes can be kept stable even when the preparationmethod is altered.

Figure 5 illustrates the cumulative pore sizedistributions for different membranes. More than80% of the pores have diameters less than 50 nm.The largest pore size is more than 130 nm.

Probability density function curves are illus-trated in Figure 6 by using the values of mean poresize and geometrical standard deviation. All the

393SEPIOLITE MEMBRANE FOR ULTRAFILTRATION

Figure 3. Separations of four different sepiolite membranes (0.5A, 0.25A, 0.5B, and 0.25B) calicined at 550ºC.

membranes have quite similar trends, and pore sizescover a broad range. That is why the separation isquite low, even for PEO, with 400,000 MW. Asshown in Figure 7, the pores of sepiolite mem-branes are neither spherical nor cylindrical asviewed with SEM; the sepiolite membrane also was

formed by layers of fibers. This may be the reasonfor the broad distribution of pore sizes.

Pore density and surface porosity

Pore density and surface porosity were calculatedaccording to Equations (7) and (8); results are listed

394 WANG ET AL.

Figure 3. continued

395SEPIOLITE MEMBRANE FOR ULTRAFILTRATION

Figure 4. Solute separation curves (solute diameter vs. their separation) plotted on a lognormal probabilitypaper for 0.5A, 0.25A, 0.5B, and 0.25B membranes.

Figure 5. Cumulative pore size distribution of four different sepiolite membranes (0.5A, 0.25A, 0.5B, and 0.25B).

Table 2. Geometric mean pore size (µp) and geometric standard deviation (σp) of sepiolite membranes.

Membrane µp (nm) σp

0.5A 25.7 1.950.25A 25.1 1.910.5B 23.4 2.040.25B 23.0 1.99

90

70

50

30

10

Solu

te s

epar

atio

n (

%)

10 100Solute diameter (nm)

0.5A (r2=0.92)0.25A (r2=0.93)0.5B (r2=0.96)0.25B (r2=0.96)

in Table 3. Pore density and surface porosity did notdepend very much on the amount of sepiolite used.Instead, they depended highly on the method of claydispersion. The aggregation of the fiber bundles wasprevented by sonifying the clay suspension. As a

396 WANG ET AL.

Figure 5. continued

consequence, the pore density and surface porosityof the membrane increased, resulting in higher flux,despite the fact that the membrane thicknessincreased as a result of sonification. The thickness ofthe membrane (43–114 µm) is 107.5–570 times as

397SEPIOLITE MEMBRANE FOR ULTRAFILTRATION

Figure 5. continued

Figure 6. Probability density function curves for four different sepiolite membranes (0.5A, 0.25A, 0.5B, and 0.25B).

large as the diameter of the sepiolite fiber (0.2–0.4µm); this difference indicates that the membraneswere formed by assembling layers of completely dis-ordered fibers. Therefore, a large portion of the pores

formed are dead ended or blocked between layers offibers. The pore density and surface porosity calcu-lated above are only for the pores that could passthrough the entire cross section of membrane.

Characterization by SEM images

Figure 7 shows SEM images of membranes con-structed of layers of disordered sepiolite. It is easyto prepare pinhole- and crack-free membranesfrom sepiolite because of its fibrous nature. The

SEM image also shows that fibers are better dis-persed in the membrane prepared from ultrasoni-fied suspension, unlike the membrane made byapplying magnetic stirring, where a network offiber bundles is observed.

398 WANG ET AL.

Figure 6. continued

Arribas, J. I., F. Martinez, A. Hernández, P. Prádanos, andG. Caruana. 1991. Morphological study of surfaceinorganic membranes by scanning electron micro-scopy and image analysis. Key Engineering Materials61/62:371–374.

Cool, R., A. Clearfield, V. Mariagnanam, L. J. McElli-strem, R. M. Crooks, and E. F. Vansant. 1997. Self-assembly of aluminum-pillared clay on a gold sup-port. Journal of Materials Chemistry 7(3):443–448.

Cot, L. 1998. Inorganic membranes: academic exercise orindustrial reality. In Proceedings of the Fifth Inter-nal Conference on Inorganic Membranes, June22–26, Nagoya, Japan.

Hsieh, F.-U., T. Matsuura, and S. Sourirajan. 1979. Reverseosmosis separation of polyethylene glycols in diluteaqueous solutions using porous cellulose acetate mem-brane. Journal of Applied Polymer Science 23:561–573.

Ishiguro, M., T. Matsuura, and C. Detellier. 1995. Reverseosmosis separation for a montmorillonite mem-brane. Journal of Membrane Science 107:87–92.

Jones, B. F., and E. Galán. 1988. Sepiolite and palygorskite.Pages 631–674 in S. W. Bailey, editor. Reviews inmineralogy. Volume 19: Hydrous phyllosilicates.Mineralogical Society of America, Washington, DC.

Le Van Mao, R., E. Rutinduka, C. Detellier, P. Gougay, V.Hascoet, S. Tavakoliyan, S. V. Hoa, and T. Matsuu-ra. 1999. Mechanical and pore characteristics of zeo-lite composite membrane. Journal of MaterialsChemistry 9:783–788.

Michaels, A. S. 1980. Analysis and prediction of sieving curvesfor ultrafiltration membranes: a universal correlation?Separation Science Technology 15 (6):1305–1322.

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Figure 6. continued

Conclusions• Ultrafiltration membranes can be prepared

from sepiolite clay material.• The method of preparing sepiolite mem-

branes requires only one step (spreadingthe clay suspension on a smooth surface)before calcination, whereas the sol–gelmethod requires three steps (precipitation,peptization, and gelling). Thus, sepiolitemembrane preparation is much simpler.

• The sepiolite membrane has a broad poresize distribution because of its multilayerstructure of sepiolite fibers.

• Sepiolite fiber is better dispersed by sonifi-cation than by magnetic stirring.

AcknowledgmentsThe authors thank the China Scholarship Councilfor providing a scholarship. They also thank theNatural Sciences and Engineering Research Coun-cil of Canada (NSERC) for a research grant (toCD) and a postdoctoral fellowship.

ReferencesAimar, P., M. Meireles, and V. Sanchez. 1994. A contri-

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Figure 7. Scanning micrographs of sepiolite membranes: cross sections of 0.5A (a), 0.5B (b), 0.25A (e), and0.25B (f); surfaces of 0.5A (c), 0.5B (d), 0.25A (g), and 0.25B (h).

Mishra, T., and K. Parida. 1997. Transition-metal oxidepillared clays. 2. A comparative study of textural, andacidic properties of manganese(III) pillared montmo-rillonite, and pillared acid-activated montmoril-lonite. Journal of Materials Chemistry 7(1):147–152.

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Table 3. Pore density and surface porosity of sepiolite membranes calculated from solute transport data.

Membrane Pore density (pores/µm2) Surface porosity (%)

0.5A 27 3.150.25A 28 3.040.5B 67 7.150.25B 70 7.03