Composite Nanofiltration Polyamide Membrane: A Study on the

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Ind. Eng. Chem. Res. 2004, 43, 8074-8082

Composite Nanofiltration Polyamide Membrane: A Study on the Diamine Ratio and Its Performance Evaluation A. L. Ahmad,*,† B. S. Ooi,† A. W. Mohammad,‡ and J. P. Choudhury† School of Chemical Engineering, Engineering Campus, Universiti Sains Malaysia, Seri Ampangan, 14300 Nibong Tebal, S.P.S, Penang, Malaysia, and Department of Chemical and Process Engineering, Universiti Kebangsaan Malaysia, 43600, Bangi, Selangor, Malaysia

Membranes were prepared by reacting diamine mixtures with trimesoyl chloride to produce composite nanofiltration polyamide membranes. The flux behavior as well as rejection profiles of monovalent and divalent ions were elucidated pertaining to the 3,5-diaminobenzoic acid (BA) content in the copolymer membranes. The polyamide characteristic group was confirmed by using the ATR-FTIR method. The pore size (rp), effective thickness/porosity (∆x/Ak), and surface charge density (X) of the membrane were investigated using the Donnan steric pore flow model (DSPM). The membrane with the addition of 0.05% BA shows some improvement in terms of flux. However, at higher loading of BA, flux is reduced. The rejection of divalent ions (Mg2+, SO42-) is higher than that of monovalent ions (Na+, Cl-). It was found that the diamine content and reaction time play key roles in determining the permeate flux, membrane parameters, and membrane separation capabilities. Introduction A major breakthrough in the field of membrane separation was the development of composite membranes, which are characterized by an ultrathin separating “barrier” layer supported on a chemically different asymmetric porous substrate, wherein the benefits of two separate polymeric layers could be combined to obtain the desired performance properties for a number of applications, most notably the reverse osmosis (RO) desalination of brackish water. Composite membranes have advantages over single-material asymmetric membranes in which the top-separating layer is formed in situ and, hence, the chemistry and performance of the top barrier layer and the bottom porous substrate can be independently studied and modified to maximize the overall membrane performance.1 The common materials used for the polymerization process include aliphatic or aromatic diamines,2-5 polysulfonamide,6 aromatic diols, and a combination of diols and amides,7 and the common cross-linkers used are trimesoyl chloride, isophthaloyl chloride, and terephthaloyl chloride. Piperazine and m-phenylenediamine, for example, are the most popular primary and secondary diamines used in the preparation of highrejection ultrathin reverse osmosis and nanofiltration membranes. The interfacial polymerization process is utilized to fabricate RO membrane, but less attention has been paid to nanofiltration membranes. Advanced thin-film composite membranes now have improved water-flux and solute-rejection characteristics compared to cellulose acetate membranes, which increases their potential for many new applications. Extensive literature and patents exist on modifying the membrane surface for higher hydrophilicity. Various techniques that are used for this purpose include acid * To whom correspondence should be addressed. E-mail: [email protected]. Tel.: +6-04-5937788. Fax: +6-04-5941013. † Universiti Sains Malaysia. ‡ Universiti Kebangsaan Malaysia.

treatment,8-10 radical grafting,11 dip-coating,12 plasma modification,13 and addition of ionic polymers.14 It has been concluded that polymeric materials with hydrophilic groups could be used to develop RO membranes of improved quality. In view of these observation, efforts have been made to develop membrane-grade polyamides having a pendant carboxylic group at their backbones.15 An analysis of reverse osmosis data revealed that the membranes prepared from the 3,5diaminobenzoic acid and 4,4′-diphenyldicarboxylic acid chloride are superior.15 These membranes were prepared by interfacially reacting trimesoyl chloride with 0.5 wt % diaminobenzoic acid, the resulting composite having a salt rejection rate of 94% and a water permeability of 62 gfd (105.4 L‚m-2‚h-1) at 50 atm pressure using a 5000 ppm synthetic brine.16 3,5-Diaminobenzoyl piperazine was prepared as a dimer that was then interfacially reacted with trimesoyl chloride and/or terephthaloyl chloride to form a composite reverse osmosis membrane. Compared to the wholly aromatic m-phenylene diamine membranes, the novel benzoyl piperazine membrane produces higher transport of both water and salt. These membranes have performances ranging from reverse osmosis to nanofiltration and thus have been utilized in low-pressure desalination and/or organic solute separation17 processes. Copolymers with highly reduced viscosity were obtained by the lowtemperature solution polymerization of iso- or terephthaloyl with mixed 3,3′-diaminodiphenyl sulfone and 3,5-diaminobenzoic acid. The membrane showed not only good RO performance, but also high chlorine resistance. However, it was reported that this copolymer with highly reduced viscosity values could not be obtained by interfacial polymerization.18 Given that the incorporation of 3,5-diaminobenzoic acid could improve the membrane properties, it has become the objective of the current research to develop and characterize composite nanofiltration polyamide membranes based on 3,5-diaminobenzoic acid. The effect of the diamine content on the membrane performance was explored and analyzed. The membranes produced

10.1021/ie0497994 CCC: $27.50 © 2004 American Chemical Society Published on Web 11/11/2004

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under different ratios of diamine and reaction times were characterized in terms of water permeability (flux) and salt rejection. Membrane properties such as pore size and effective thickness/porosity could be predicted using an established model. In this current work, the membrane properties pore size (rp), effective thickness/ porosity (∆x/Ak), and surface charge density (X) are analyzed using the Donnan steric pore flow model. The effective thickness/porosity (∆x/Ak) is a cumulative term for effective thickness and porosity. The higher the value of ∆x/Ak, the thicker the active layer or the more reduced the porosity. Salt rejection and flux values of the membranes are reported for various salts such as NaCl, Na2SO4, MgCl2, and MgSO4, and finally, the polyamide layer is confirmed using the ATR-FTIR method.

relating the finite size of the solute and pore size

Φ ) (1 - λ)2

where λ is the ratio of the solute radius (rs) to the pore size (rp). In terms of real rejection, eq 1 becomes

Rreal ) 1 -

Ci,p Ki,cΦ )1(7) Ci,m 1 - exp(- Pem)[1 - ΦKi,c]

where Ci,m and Ci,p refer to the concentrations (mol‚m-3) on the feed side of the membrane and in the permeate, respectively. The Peclet number, Pem, is defined as

Pem )

Theory Model Assumptions. To determine the pore size, effective thickness/porosity, and membrane volume charge (X), the Donnan steric pore flow model (DSPM) was used.19,20 Below are the assumptions applied to the derivation of DSPM model: (i) The effective membrane volume charge (X) is constant throughout the membrane and is strongly dependent on the feed concentration. (ii) The membrane consists of a bundle of identical straight cylindrical pores of radius rp and length ∆x. (iii) A very dilute system was used, which enables the effects of coupling between the components in the solution to be neglected. (iv) For porous membranes, the fluxes, concentrations, potentials, and velocities were all defined in terms of radially averaged quantities. Fundamental Equations of the DSPM. The extended Nernst-Planck equation proposed by Schlogl and Dresner21 forms the basis of the description of ion transport through the membranes. The equation can be expressed as

dci F dψ - ziciDi,p + Ki,cciv ji ) -Di,p dx RT dx

Ki,c Jv∆x Ki,d Di,∞Ak

(8)

where Di,∞ is the bulk diffusivity (m2‚s-1), Jv is the volume flux (based on membrane area) (m‚s-1), ∆x is the effective thickness (m), and Ak refers to the porosity of the membrane. The Hagen-Poiseuille equation gives the relationship between the pure-water flux and the applied pressure across the membrane22

Jw )

rp2∆P

(9)

8µ(∆x/Ak)

where Jw is the water flux (m3‚m-2‚s-1), ∆P is the applied transmembrane pressure (kPa), and µ is the viscosity of the solution (kPa‚s). To find the film layer concentration, the concentration polarization equation was employed. For a stirred cell configuration, the observed rejection was related to the real rejection by volume flux, Jv, and the mass transfer coefficient, k, as follows22

ln

(1)

where

(6)

(

) (

)

Jv 1 - Robs 1 - Rreal ) ln + Robs Rreal k k ) k′ω0.567

(10) (11)

where

Di,p ) Ki,dDi,∞

(2)

ji is the flux of ion i and the terms on the right-hand side of eq 1 represent the transport due to diffusion, electromigration, and convection, respectively. Because the pore radius is very small, a homogeneous velocity for the transport of solute across the membrane was assumed. Ki,d and Ki,c are related to the hydrodynamic coefficients as -1

2

Ki,d ) K (λ,0) ) 1.0 - 2.30λ + 1.154λ + 0.224λ

3

(3)

Ki,c ) G(λ,0) ) 1.0 + 0.054λ - 0.988λ2 + 0.441λ3 (4) For uncharged solute (glucose), the electrical potential gradient in the axial direction can be eliminated; thus eq 1 can be reduced to

ji ) -Dip

dci + Ki,cciv dx

(5)

For purely steric interactions between the solute and the pore wall, the notation, Φ, is the steric terms

k′ ) 0.23

() ( ) r2 v

0.567

v D∞

0.33D∞

r

(12)

For charged solutes, the basic equation that governs the transport of ions inside the membrane is given in eq 1. The conditions of electroneutrality in the bulk solution and inside the membrane are expressed as n

ziCi ) 0 ∑ i)1

(13a)

n

zici ) -X ∑ i)1

(13b)

respectively, where Ci is the bulk concentration of ion i, ci is the concentration of ion i inside the membrane, and X is the effective volumetric membrane charge density. X is assumed to be constant at all points in the active part of the membrane. Because the electric potential gradient is common for every ion inside the membrane, the electric potential

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Ind. Eng. Chem. Res., Vol. 43, No. 25, 2004

and concentration gradients can be derived from eq 1 as

zici dψ dci Jv (K c - Ci,p) ) F dx Di,p i,c i RT dx

Table 1. Contents of Membranes with Different Diamine Ratios aqueous-phase diamine mixtures

(14)

3,5-diaminobenzoic acid, wt %

trimesoyl chloride, w/v%

2.00 1.95 1.90 1.85 1.80

0.00 0.05 0.10 0.15 0.20

0.1 0.1 0.1 0.1 0.1

where the flux of ion i, ji, is expressed as

ji ) JvCi,p

(15)

By assuming constant X throughout the membrane, for a binary salt such as NaCl, eq 14 can be written as

z1

dc1 dc2 + z2 )0 dx dx

(16)

ion concentration inside the membrane is remarkably higher than the co-ion concentration.24 In the case of negatively charged membranes, with NaCl as the separating solute, the following approximation is obtained

By solving eqs 13-15, the following expression for the potential gradient can be obtained

ziJ v (Ki,cci - Ci,p) i)1 Di,p

dx

∑ )

n

F

(17)

(zi2ci) ∑ RT i)1

(

2

)

(18)

Determination of rp and ∆x/Ak Using an Uncharged Solute (Glucose). In this approach, an average pore size (rp) was fitted by solving eqs 7-9 for the pure-water flux, Jw, versus ∆P giving the slope of

slope )

rp2 8µ(∆x/Ak)

(19)

and

∆x/Ak )

rp2 8µ × slope

{[

Rreal ) 1 D2,p

Equation 14 is integrated across the membrane thickness with the solute concentration in the membrane at the upper (x ) 0) and lower (x ) ∆x) boundaries expressed in terms of the external concentrations (Ci,m and Ci,p) using the Donnan steric equilibrium partition as follows23

ziF ci ) Φ exp ∆ψ Ci RT D

z1cc . |z2|c2 w z1c1 ≈ X

(21)

The analytical relationship for salt rejection could be derived as

n

dψ

organic-phase content

piperazine, wt %

(20)

Equation 20 was substituted into eq 8 and the rp value was obtained by obtaining the best fit to the Jv-Rreal curve of glucose solution based on eq 7. The ∆x/Ak value could be obtained directly from the permeability data (eq 9). Determination of Charge Density (X) Using a Charged Solute (NaCl). The charge density, X, was found by the best fit of the Jv-Rreal curve of NaCl solution with eq 22. The rp and ∆x/Ak values were obtained from the previous fitting of the glucose solution rejection. An analytical solution was derived to express the value of Rreal on the basis of the membrane parameters rp, ∆x/Ak, X, and Jv for NaCl. By assuming that the membrane is negatively charged, a simplified approach for parameter (X) calculation could be performed through the curve fitting of Rreal and Jv using SigmaPlot 2000. It could be reasonably assumed that the counter-

Jv

( )x( ) ∆x Ak

X Cm

2

x[ ( )( )] [ ( )]

+ 4Φ1Φ2 Jv

∆x Ak

Jv

2

∆x Ak

X Cm

- Φ1Φ2D2,p2

2

]}

+ (2Φ1Φ2D2,p)2

(22) Experimental Section Materials. The polysulfone Udel P-1700 (Mn ) 17 000) was purchased from Solvay Advanced Polymers, LLC. Piperazine, 3,5-diaminobenzoic acid, n-hexane, sodium chloride, sodium sulfate, magnesium chloride, magnesium sulfate, and glucose were supplied by Merck Company. N-Methylpyrrolidone and trimesoyl chloride were purchased from Fluka and poly(vinylpyrrolidone) from Sigma-Aldrich Co. The tightly woven polyester style 0715 Dacron fabric was supplied by Texlon Corporation (Torrance, CA). Preparation of Polysulfone Support Layer. The microporous polysulfone membrane was used as a support layer for the composite membrane. The polysulfone support was prepared by dissolving 15% polysulfone (Udel P-1700) in N-methylpyrrolidone with 18% poly(vinylpyrrolidone) as a pore-former. The solution was cast onto a tightly woven polyester fabric with a nominal thickness of 150 µm. Then the membrane was immersed in a water bath and was kept in the water bath for 24 h until most of the solvent and water-soluble polymer were removed.25

Figure 1. Schematic diagram of the membrane permeation test.


Figure 2. ATR-FTIR spectra of polyamide with different concentrations of BA.

Figure 4. Effect of pressure on pure-water, NaCl, and glucose solution fluxes.

Figure 3. ATR-FTIR spectra of membrane skin layer for 0.15% and 0.20% BA.

Fabrication of Thin-Film Composite Membranes. An active skin layer was prepared by interfacial polymerization method. The support layer, which was taped to a glass plate, was immediately dipped into an aqueous diamine solution containing a mixture of piperazine (PIP) and 3,5-diaminobenzoic acid (BA), as listed in Table 1. The polysulfone support was kept in the aqueous solution for 5 min at ambient temperature. The excess solution from the impregnated membrane surface was then removed using a rubber roller. The membrane was immediately dipped into an n-hexane solution, which consisted of 0.1 w/v% trimesoyl chloride (TMC) for interfacial reaction. The reaction was carried out for a predetermined time of 10 s or 30 s. Membrane Permeation Characterization. Membrane permeation tests were carried out using an Amicon 8200 stirred cell at five different pressures: 150, 250, 350, 400, and 450 kPa. The experimental apparatus is shown schematically in Figure 1. For each operating pressure, a fresh solution was used as the feed. Nitrogen gas was used to pressurize the water flux through the membrane. The membrane was cut into a disk of 5.5cm diameter (effective area of 28.27 cm2) and then mounted at the bottom of the stirred cell. The solution

Figure 5. Pure-water permeability for membrane prepared under conditions of 10-s reaction time with 0.05% BA.

was stirred at a speed of 350 rpm to reduce concentration polarization. The feed solutions are pure water, 0.01 M sodium chloride, 0.01 M sodium sulfate, 0.01 M magnesium chloride, 0.01 M magnesium sulfate solution, and 300 ppm glucose solution. The permeate and feed concentrations of salts were measured using a conductivity meter (Hanna Instruments, Padova, Italy, model HI8633), and feed and permeate concentrations of glucose solution were analyzed using a spectrophotometer (Thermo Spectronic, Rochester, NY, model GENESYS 20) at 485 nm. The bulk feed concentration was calculated as an average of the initial and final feed concentrations.

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was checked by using attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy instrument (Perkim-Elmer Series II) with an ATR accessory fixed at an angle of 45°, which causes a 5-µm depth penetration of the IR beam into the surface of the membrane. Results and Discussion

Figure 6. Effect of BA concentration on membrane flux at 450 kPa.

Figure 7. Effect of BA concentration on membrane pore size and effective thickness/porosity (10-s reaction time).

Each membrane was subjected to pressure pretreatment at 500 kPa for 1 h before the permeation experiments. The flux was equilibrated for the passage of the first 20 mL of permeate, and the following 10 mL of permeate was collected for concentration analysis. All results presented are averaged data obtained using three membrane samples with a variation of (10%. Analytical Technique for Bonding Confirmation. Bonding characterization of the membrane surface

Polyamide Confirmation Using ATR-FTIR Spectroscopy. Figure 2 shows an analysis of the ATR-FTIR spectra. From the spectra, it is found that the carboxyl group is introduced into the active skin layer of the polyamide composite membrane by interfacial polymerization. The most characteristic feature in the spectrum of a carboxylic acid (3,5-diaminobenzoic acid) is the extremely broad O-H absorption occurring in the region from 3600 to 2800 cm-1. The presence of carboxylic acid is further confirmed by the stretching of CdO in the region from 1725 to 1600 cm-1. The peaks at 1586 and 1638 cm-1 correspond to the carboxylic acid salt and amide (in the solid state), respectively. When the BA content is increased from 0.00% to 0.15% BA in the diamine solution, the peaks intensity for the O-H and CdO regions (1638 cm-1) are increased. However, Figure 3 shows that the intensity ratio of amide (in the solid state) to carboxylic acid salt is decreased from 0.15% to 0.20% BA. These results strongly suggest that increasing the BA content results in an increase of the carboxylic group content in the skin layer. However, at higher BA content (>0.20%) the free -OH further reacted with the piperazine to form an ionic bond and three-dimensional polymer networks, which reduced the free carboxylic group content. Concentration Polarization and Permeate Flux. All experiments were carried out at a stirring speed of 350 rpm. The same stirring speed was adopted by Bowen et al. in characterizing nanofiltration using 0.01 M NaCl and 300 ppm glucose.19 The effect of concentration polarization can be neglected at a 350 rpm stirring

Figure 8. Space charge model of (a) PIP-TMC polyamide, (b) BA-TMC polyamide and tube model of (c) PIP-TMC polyamide, (d) BA-TMC model.


Figure 9. Effect of BA concentration on membrane pore size and effective thickness/porosity (30-s reaction time).

speed as illustrated by the hydrodynamic study in which the permeate fluxes of the solution vs the operating pressure remain linear and close to the pure-water flux (see Figure 4). A similar study relating concentration polarization to hydrodynamic effects was also reported by Dorange et al.26 It can be noted that the flux difference between the pure water, glucose, and salt solutions is less than 5%, which means that at a stirring speed of 350 rpm, the flux difference caused by concentration polarization is negligible (as concentration polarization would result in higher osmotic pressure that would reduce the flux). However, the effect of concentration polarization cannot be completely eliminated; therefore, the rejection throughout this paper is reported as real rejection (with respect to the wall feed concentration) rather than observed rejection (with respect to the bulk feed concentration). The concentration polarization effect under the same stirring speed is taken into account by relating the real rejection (Rreal) to the observed rejection (Robs) by comparing the values obtained from a correlation for stirred cells19 as shown in eqs 10-12. To obtain the rp and ∆x/Ak values, the slope of Jv∆p graph is first determined. Figure 5 shows a plot of flux against applied pressure for the membrane prepared under conditions of 10-s reaction time with 0.05% BA. The slope of the line (pure-water permeability or permeance) is given by rp2/8µ(∆x/Ak) ) 3.3615 × 10-8 m‚s-1 kPa-1. This value is comparable to those measured for commercially available membranes such as DDS HC-50 (0.577 × 10-8 m‚s-1 kPa-1), Hydranautics HN-7450 (0.609 × 10-8 m‚s-1 kPa-1), and FilmTec NF45 (1.267 × 10-8 m‚s-1 kPa-1).27 Effect of BA Concentration on Volume Flux and Membrane Properties. Figure 6 shows the effects of the 3,5-diaminobenzoic acid (BA) concentration on the membrane flux. For both 10- and 30-s polymerization times, the flux generally increase upon addition of 0.05% 3,5-diaminobenzoic acid. The increase in the volume flux could be due to the introduction of a pendant carboxylic group.1,25,28 Higher contents of BA lead to a higher ratios of hydrophilic groups in the membrane.29 Consequently, excessive water uptake produces pores with weaker wall strength. As a result, the pores tend to coalesce together to produce a larger pore size. This phenomenon can be further demonstrated by the fitted pore size shown in Figure 7, in which the pore size is found to be increasing as the BA content increases from 0.00% to 0.05% BA, by as much as 10% in a 10-s reaction time. Figure 6 shows that the further addition of BA does not contribute to the flux because the planar benzoic

Figure 10. Effect of pressure on salt rejection at different BA contents for (a) NaCl, (b) Na2SO4, (c) MgCl2, and (d) MgSO4.

ring produces a more compact structure than the zigzag piperazine. On the basis of molecular models built using the program of SpartanBuild (McGraw-Hill Companies), the molecular geometry of piperazine-based polyamide and 3,5-diaminobenzoic acid-based polyamide could be distinctively differentiated. The BA-based polyamide was found to be denser than the PIP-based polyamide, which is nonplanar and produces channeling effects as shown in Figure 8. As can be seen from the error bars in Figure 7, the errors for the pore sizes cover a very small range of less than (0.1 × 10-10m. Because a variation of 1 × 10-10 m in pore size will result in a 10% difference in glucose rejection, the pore size should be considered constant from 0.05% to 0.20% BA content for a 10-s reaction time. If it is assumed that the thickness is constant or decreasing for higher BA/PIP contents (this is possible

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Figure 11. Effect of BA concentration on salt rejection at 450 kPa.

because the reactivity of the monomer for higher BA/ PIP contents is lower), then an increase in ∆x/Ak is an indication of decreasing porosity (Ak). Because porosity can be expressed in terms of pore size, pore size distribution, and effective number of pores,30 the decrease of porosity under the conditions of increasing pore size is mainly caused by the decrease of effective number of pores. As indicated by the space-filling model shown in Figure 8, when the BA content is increased, the channeling effect will be reduced to produce a compact/ dense structure that results in a decrease in the number of pores. On the other hand, Figure 9 shows that, for a 30-s reaction time, the variation of pore size is within 0.3 × 10-10 m, which is very close to the error bars limit of 0.2 × 10-10 m. Therefore, at a 30-s reaction time, the effect of the BA content (within the range of 0.050.15%) on pore size is not significant. Nonetheless, small-magnitude increases of pore size and flux are observed at 0.20% BA content compared to 0.05% BA content. It can be concluded that the effect of the BA content on the membrane production rate (flux) is due to the degree of hydrophilicity, pore size, and number of effective pore numbers. The amount of BA in the active skin layer depends not only on the diamine loading ratio but also on the reaction time. A low BA content at a 10-s reaction time could improve the flux due to the hydrophilicity and larger pore size. Nevertheless, at higher BA contents, a skin layer with a compact structure is produced, which reduces the water permeability. At higher reaction times (30 s) and higher BA contents, the flux is increased because of the coalescence of adjacent pores to produce an interconnected pore structure that becomes defective if not properly controlled. Effect of BA Concentration and Measurement of Its Retention. Figure 10 shows the real rejections of NaCl, Na2SO4, MgCl2, and MgSO4 at different operating pressures and BA contents. It was found that the retention increases with increasing transmembrane pressure. This phenomenon is due to the increasing water flux at higher transmembrane pressure while the

Table 2. Bulk Diffusion Coefficients and Solute Sizes for Ions19,31-33 ion Na+ Mg2+ ClSO42-

D∞ (10-9 m2/s)

rs (nm)

1.33 0.70 2.01 1.06

0.18 0.35 0.12 0.23

salt flux is sterically and electrically hindered by the pores and charge.31 The expected rejection sequence should be Na2SO4 > MgSO4 > NaCl > MgCl2 because of the negative charge of the carboxylic group. However, it is found from Figure 11 that the real rejection profile for all of the membranes follows the rejection sequence of MgSO4 > Na2SO4 > MgCl2 > NaCl. The same trend was observed for nanofiltration membranes of CA30 (Hoechst), NF40 (Dow), and UTC20 (Toray Industries) as reported by Schaep et al.30 The phenomenon of better retention of sulfate salts compared to chloride salts can be understood because sulfate salts are more easily repelled by negatively charged surfaces. An overview of the diffusion coeffiecients of all ions used in this paper is provided in Table 2. The effective diffusion coefficients, Di,p, are calculated by multiplying the bulk diffusion coefficients of the respective ion, Di,∞ with the diffusive steric hindrance factor, which is given by Bowen as in eq 2. Such ionic diffusion data have also been reported in the literature by other researchers.31-33 Moreover, the salt bulk diffusion coefficient (Di,∞) and the solute size (rs)19,23 show that the ion diffusivity (Di,∞) follows the sequence Cl- > SO42- whereas the solute size follows the order SO42- > Cl-. These trends can explain why the rejection trend shown in Figure 11 was obtained as the higher diffusivity and smaller solute size of the Na+ ion allow it to be more easily transported through the membrane than the SO42- ion. Another phenomenon observed in Figure 11 is that MgCl2 is better retained than NaCl. On the basis of the Donnan exclusion, the opposite trend is expected in the case of a negatively charged membrane. This can be explained only if the charge density is not constant but depends strongly on the type of salt being transported.


charge density is reduced from -464.3 to -245.7 mol‚m-3 when the concentration of BA increased from 0.05% to 0.20%. Conclusion

Figure 12. Effect of BA content on membrane charge density and real rejection.

It is suggested that each individual ion could make its individual contribution to the membrane charge by means of adsorption.19,34,35 The membrane might become positively charged in the presence of magnesium salts. This would explain why MgCl2 is better retained compared to the NaCl. According to the Donnan exclusion theory, a highervalence counterion leads to a lower rejection of the salt.31 However, neither Mg2+ nor SO42- exhibits poor rejection, so it can be concluded that electromigration effects did not play an important role compared to the steric hindrance mechanism and the membranes most probably are weakly charged, especially when the feed solution is very dilute (∼0.01 M). The membrane charge could change its sign depending on the type of solute passing through it. This phenomenon is best described by a dielectric repulsion mechanism that result in a higher rejection of multivalent ions (Mg2+ and SO42-) compared to monovalent ions (Na+ and Cl-) regardless of their sign (positive or negative). Rejections of sulfate-containing salts such as MgSO4 and Na2SO4 are higher than those of chloride-containing salts such as MgCl2 and NaCl, especially at higher BA contents. This phenomenon can be explained if the electromigration effect is taken into consideration. The sulfate ions with the lower diffusion coefficient (1.06 × 10-9 m2‚s-1) and larger solute size (2.3 × 10-10 m) compared to the Na+ ions will most likely face steric hindrance resistance of the pores as well as the repulsion of the negatively charged and dense structure due to the carboxyl groups (-COOH) of the incorporated 3,5diaminobenzoic acid. This can be clearly seen in Figure 11, where the Na2SO4 and MgSO4 salts are highly rejected compared to MgCl2 and NaCl especially at high BA content (0.20%). Consequently, the rejections for sulfate-containing salts are higher. Figure 12 shows the separation performance of composite polyamide membrane reacted for 10 s. It is observed that the NaCl rejection ability is reduced steadily when the BA content is increased. The rejection profile can be related to the pore structure as well as the charge density. From 0.00% to 0.10%, the pore size is growing, which results in a poor steric hindrance effect. Although the pore size is slightly less for the membrane with 0.20% compared to 0.15% BA, the rejection is not improved. The reason for this phenomenon can be found from Figure 12, where the

The increase of the carboxylic acid group content is confirmed by the increasing intensity of the stretching band of CdO in the region from 1725 to 1600 cm-1, and also the broad O-H absorption band in the region from 3600 to 2800 cm-1. The final content of BA in the polyamide layer depends on the diamine loading as well as the reaction time. The increase in volume flux at low BA content is due to the introduction of the pendant carboxylic group. However, a further increase in the content of BA causes a more compact structure because of its planar structure and eventually reduces the porosity of the membrane. Further increasing BA content will give a poorer rejection because of the defective active layer. The membrane shows excellent rejection of multivalence ions compared to the monovalent ions because of electrostatic, dielectric, and steric effects. Overall, the membranes produced are negatively charged and showed better rejection of co-ions such as sulfate ions compared to counterions such as magnesium. In addition, the membranes could remove more than 95% of sulfate-containing salts and gave a mild rejection within 90-95% of MgCl2 with the poorest rejection of NaCl, which was below 70%. The rejection behavior is a typical characteristic of nanofiltration membranes. Acknowledgment The authors are grateful for the financial support provided by the Ministry of Science and Technology Malaysia through its Fundamental Research and IRPA grants. Nomenclature Ak ) porosity of the membrane ci ) concentration of ion i in the membrane (mol‚m-3) Ci,m ) concentration of ion i on the feed side of the membrane (mol‚m-3) Ci,p ) concentration of ion i in the permeate (mol‚m-3) Di,p ) hindered diffusivity of ion i (m2‚s-1) Di,∞ ) bulk diffusity of ion i (m2‚s-1) F ) Faraday constant (C‚mol-1) ji ) ion flux (based on membrane area) (mol‚m-2‚s-1) Jv ) volume flux (based on membrane area) (m3‚m-2‚s-1) Jw ) water flux (based on membrane area) (m3‚m-2‚s-1) k ) mass-transfer constant (m‚s-1) k′ ) mass-transfer constant defined by eq 9 K-1 ) hydrodynamic enhanced drag coefficient Ki,c ) hindrance factor for convection Ki,d ) hindrance factor for diffusion Pem ) Peclet number r ) radius of stirrer rp ) effective pore radius (m) rs ) Stokes radius of solutes or ions (m) Robs ) observed rejection Rreal ) real rejection T ) absolute temperature (K) V ) solute velocity x ) distance normal to the membrane (m) ∆x ) effective membrane thickness (m) X ) effective membrane volume charge (mol‚m-3) zi ) valence of component i Φ ) steric partition term

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λ ) ratio of solute radius to pore radius ω ) stirring speed ψ ) electric potential in the axial direction (V) ∆ψD ) Donnan potential (V)

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Received for review March 15, 2004 Revised manuscript received September 7, 2004 Accepted September 29, 2004 IE0497994

Composite Nanofiltration Polyamide Membrane: A Study on the

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