Separation and Purification Technology 41 (2005) 83–89

Characterization of chemically modified zeolite–clay compositemembranes using separation of trivalent cations

Anupam Shukla, Anil Kumar∗

Department of Chemical Engineering, Indian Institute of Technology, Kanpur 208016, India

Received in revised form 17 April 2004; accepted 5 May 2004

Abstract

Three zeolite–clay composite membranes have been used for separation of chloride salts of trivalent cations (FeCl3 and AlCl3) in anunstirred batch cell. The membranes used are the unmodified analcime zeolite (Z1), the modified zeolite containing oxynitride groups (Z2)and the modified zeolite containing imine/amine groups (Z3). The experimental data has been analyzed by the irreversible thermodynamicsapproach and the concentration polarization is also taken into account in the model used. The calculations show that modification causesan increase in the intrinsic rejection (from ∼70% for the unmodified to ∼94% for the Z2 and ∼96% for the Z3 membrane in case of FeCl3

solution and from ∼84% for Z1 to 90% for both Z2 and Z3 membranes in the case of AlCl3 solution) of the membrane and a decrease in thesolute permeability. The reflection coefficients of the modified membranes are found to be more than that of the unmodified membrane andthat of Z3 membrane is found to increased to a value of about 1 (σ = 0.978 for FeCl3 and σ = 0.99 for AlCl3).© 2004 Published by Elsevier B.V.

Keywords: Zeolite membrane; Trivalent salts; Unstirred batch cell

1. Introduction

Inorganic micropollutants are non-biodegradable andtoxic. The traditional techniques (e.g. reduction process orlime precipitation) [1] for their removal are incapable ofreducing their concentrations to the required level. Expen-sive techniques like ion-exchanging and/or reverse osmosisare invariably used for reducing the pollution to acceptablelimits. Some of the advantages, of using membranes forseparation of inorganic pollutants, are easy inclusion in agiven process, room temperature operation and easy ad-justment of modular membrane area. Majority of inorganicmicropollutants consists of metal ion carrying charge andthis fact can be used to separate them using membranes withcomparatively bigger pore size and having charge on theirsurface. In fact, development of nanofiltration membranes(membranes having pore size of the order of 1nm and havingsurface charge) have improved the economics of the processbecause they perform comparable separation at much lowerpressure as compared to the reverse osmosis membranes. In

∗ Corresponding author. Tel.: +91 512 2597195;fax: +91 512 2590104.

E-mail address: anilk@iitk.ac.in (A. Kumar).

this work, we report the use of zeolite membranes havingmuch bigger pore size (∼15 to 20 nm) and high surfacecharge density for separation of ions at still lower pressureand higher flux as compared to nanofiltration membranes.

Inorganic zeolite membranes offer many advantages overorganic polymer membranes and some of these are theirability to withstand high temperature, high structural sta-bility, stability against organic solvents and stability underextreme pH. In addition, pore size of the membrane canbe adjusted by choosing appropriate type of zeolite. Its hy-drophilic/hydrophobic nature can be modified by introduc-ing different dopants in the zeolite framework at the sol–gelsynthesis stage [2]. Zeolites contain well-defined cavitiesand channels of molecular dimensions and these are moreor less uniform in size giving a sharp pore size distributionwithin the inorganic separating layer.

This work reports the use of zeolite–clay compositemembranes for separation of the chloride salts of thetrivalent metal ion (FeCl3 and AlCl3) from their aque-ous solutions. The three types of membranes used are theanalcime-o-zeolite clay composite membrane (hereafter,referred to as Z1 membrane), the Z2 membrane obtainedby reaction of Z1 membrane with NOx and the Z3 mem-brane obtained by reaction of Z2 membrane with hydrazine

1383-5866/$ – see front matter © 2004 Published by Elsevier B.V.doi:10.1016/j.seppur.2004.05.001

84 A. Shukla, A. Kumar / Separation and Purification Technology 41 (2005) 83–89

hydrate [3]. The experiments were carried out in the pres-sure range of 140–400 kPa and both the observed and theintrinsic rejection coefficients were computed. It was foundthat for FeCl3 the intrinsic rejection increased on modifi-cation from a value of from ∼70% for the unmodified to∼94% for the Z2 and ∼96% for the Z3 membrane. ForAlCl3 the modification leads to an increase in the intrinsicrejection coefficient from ∼84% for Z1 to 90% for both Z2and Z3 membranes. The reflection coefficient of the mem-brane increases to a value very close to 1 (σ = 0.978 forFeCl3 and σ = 0.99 for AlCl3 in the case of Z3 zeolite) onmodification.

2. Theory

For an unstirred batch cell, used in this study, rejectionof membrane can be reported in two ways. The first oneis called the observed rejection (Robs) which is based onthe bulk concentration of retentate (Cb). The second one isdetermination of the intrinsic rejection (Rint) and is basedon the concentration on the surface of the membrane (Cm)and these are defined as

Robs = 1 − Cp

Cb(1)

Rint = 1 − Cp

Cm(2)

where Cp is the concentration of solute in the permeate. Itmay be observed that Rint is an inherent property of the mem-brane while Robs depends strongly on the operating condi-tions. It is therefore, desirable to report separation perfor-mance of a membrane in terms of Rint even though the de-termination of Cm is difficult. The latter can be determinedby either of the following two techniques.

1. Direct measurement Cm through interoferometric [4,5]and optical shadow measurements [6].

2. Cm can also be determined by solving transport equationsin the polarization layer. Accuracy of the estimated Cmdepends upon the validity of the hydrodynamic modelused.

In the second technique, the following relations are usedfor determination of Cm. The membrane surface concentra-tion is calculated using the osmotic pressure model [7]

Jv = Lp(P − σπ) (3)

where Jv is the permeate flux, Lp the permeability of themembrane, P the applied pressure difference, σ the mem-brane reflection coefficient and π is the osmotic pressuredifference. The osmotic pressure difference between two so-lutions is calculated using the van’t Hoff equation for elec-trolytes.

π = νRTC (4)

where ν is the number of moles of ions given by each moleof the electrolyte (is equal to four for trivalent salt consid-ered in this work) in the solution, R is universal gas con-stant, T the temperature (Kelvin units) and C (=Cm −Cp)is the difference in the concentration of the solute at themembrane surface (upstream side) and in the permeate. Thereflection coefficient is related to the intrinsic rejection ofthe membrane through the equation given by Spiegler andKedem [8].

Rint = σ(1 − F)

1 − σF(5)

where F is given by

F = exp

{−(1 − σ)Jv

Pm

}(6)

where Pm is the solute permeability of the membrane. TheCm, σ, and Pm are calculated using Eqs. (3)–(6) followingthe iterative technique given by Ghose et al. [7] and theflow chart of computation is shown in Fig. 1. The techniqueinvolves assuming a σ value and using Eqs. (3) and (4)to calculate the values of Cm. The Pm values for variousexperimental data are then calculated using Eqs. (5) and (6)and the standard deviation in the Pm values is calculated andminimized by adjusting the σ value. The σ value obtainedis used to calculate the surface concentration (Cm) and theconvergence criterion is taken as a change of less than 0.5%in the value of the surface concentration of the membrane(Cm).

3. Experimental

3.1. Preparation and characterization of the membranes

The support for the membrane is synthesized using claymixture and the analcime zeolite film is grown over it byin situ hydrothermal crystallization. Our earlier work gavethe procedure for the synthesis of the analcime–clay com-posite membrane [3] and it also describes the two chemicalmodifications of the analcime zeolite membrane (Z1 mem-brane). The first one is the reaction of Z1 membrane withNOx gas at 225 ◦C and atmospheric pressure that leads tointroduction of oxynitride groups on the zeolite surface (Z2membrane). The second modification is the reaction of Z2membrane with hydrazine hydrate at 60 ◦C that reduces theoxynitride groups to imine/amine groups (Z3 membrane).

The ESCA analysis of the modified silica gel and theFTIRs of the Z1, Z2 and Z3 zeolites have been used toconfirm the occurrence of the modification reactions. Thestructural characterization of membranes have been done bytaking their X-ray diffraction pattern (XRD) and the sur-face morphology has been studied using scanning electronmicroscope (SEM). The zeolite membranes have also beencharacterized with respect to their exchange capacities, pore

A. Shukla, A. Kumar / Separation and Purification Technology 41 (2005) 83–89 85

Start

Input Jv, Cb, CP, ∆P, LP

Assume σ

For each JV

Calc. ∆Π Using eqn. 3

Calc. Cm Using eqn 4

Calc. Rint & Pm using eqn 5 & 6

Calc. avg value of Pm & its std. deviation

Calc. Cm value for each JV using avg value of Pm

Do the Cm values calc. in previous step & step 4 differ by <2%

Is std. deviation

minimum??

END

YES

NO

NO

Fig. 1. Flowchart showing the algorithm used to find Cm and the intrinsic rejection.

size distribution (using the biliquid displacement technique)and elemental analysis.

3.2. Water flux and separation of trivalent ions

The membranes were compacted using double distilledwater at a pressure 136 kPa higher than the maximum op-erating pressure till a constant water flux is obtained. Theunstirred batch cell used for the water flux determination is

the same as described in the previous work [3]. For waterflux determination, at each pressure 50 ml of permeate isallowed to pass and the time required for collection of thenext 10 ml permeate is used for the water flux determina-tion. The electrolyte solutions were prepared using doubledistilled water and the concentration was determined usingconductivity measurements.

The feed concentration of the salts is kept at 500 mg/l andthe applied pressure is varied in the range of 130–450 kPa.

86 A. Shukla, A. Kumar / Separation and Purification Technology 41 (2005) 83–89

The batch cell is filled with 400 ml of the salt solution andthe first 50 ml of permeate is discarded. The next 50 ml ofpermeate is collected and used for determination of the rejec-tion coefficient. The membrane is cleaned after each readingby first passing 1% nitric acid followed by 1% NaOH solu-tion and then passing distilled water through it till all acid-ity/alkalinity is removed as measured by the pH of permeate.This process ensures proper cleaning of the membrane asdetermined by the fact that the water flux is regained within5% of the original value.

4. Result and discussion

4.1. Characterization of membranes [3]

The zeolite in the top layer of the Z1 membrane hasbeen identified as analcime zeolite (NaAlSi2O6·H2O, Or-thorhombic crystal structure with lattice parameters a =13.72 Å, b = 13.714 Å, c = 13.714 Å) by matching itsXRD pattern with those given in JCPDS files. The XRDpattern of the Z2 and Z3 zeolite are identical but are dif-ferent from that of Z1 zeolite and do not match with anyof the patterns of the sodium zeolites given in the JCPDSfiles. The pattern is indexed and it shows that the crystalstructure of modified zeolite is different from that of the Z1zeolite.

Occurrence of the gas phase modification reaction hasbeen shown by subjecting silica gel to identical reaction andtaking its ESCA spectra. The spectra show a shift in the Si2p binding energy and the appearance of N 1s binding en-ergy peak in the spectrum of the modified silica gel, thisway confirming formation of Si–N bonds. The modificationreactions are further confirmed by taking FTIR spectra of

0 100 200 300 400 5000.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

Wat

er F

lux

(m3 /m

2 -s)

X10

5

P (kPa)

Z1 membrane

Z2 membrane

Z3 membrane

Fig. 2. Water flux data of the Z1, Z2, and Z3 membranes.

Table 1Effect of modification on the pore diameter range of zeolite membranes

Type of sample Pore diameter range (Å)

Clay support 1091–272Z1 zeolite membrane 273–156Z2 zeolite membrane 218–121Z3 zeolite membrane 181–109

Z1–Z3 zeolites. The spectrum of Z2 zeolite shows an ad-ditional peak at 1383 cm−1 wavenumber showing presenceof NO2 groups while the spectrum of Z3 zeolite does nothave peak at 1383 cm−1 and shows peaks corresponding toNH/NH2 groups.

The cation exchange capacity of Z1–Z3 zeolites is deter-mined to be 4.5 meq/dry g of zeolite. The Z3 zeolite hasanion exchange capacity due to imine/amine groups and isdetermined to be 0.9 meq/dry g of zeolite. The pore sizerange of the membranes has been determined using biliquiddisplacement technique and the results are given in Table 1.The data in Table 1 show that the pore size of the membranedecrease on modification.

4.2. Trivalent ion separation

4.2.1. Water flux and Lp

Fig. 2 shows the water permeability data for the threekinds of membranes. The data clearly indicate that for allthe three types of membranes the water flux varies linearlywith pressure, i.e. it follows Darcy’s law.

Jv = LpP (7)

where Lp is the permeability of the membrane. The waterflux data was regressed subject to the condition of zero flux

A. Shukla, A. Kumar / Separation and Purification Technology 41 (2005) 83–89 87

Table 2Membrane reflection coefficients and solute permeability for FeCl3 and AlCl3

Membrane type Reflection coefficient (σ) Solute permeability (Pm) (mol/N-s) × 107

FeCl3 AlCl3 FeCl3 AlCl3

Z1 0.95 0.9945 7.184 3.896Z2 0.95 0.9985 5.065 2.800Z3 0.9885 0.9985 1.761 1.249

at no applied pressure and the solid lines in the figure givethe regression results. The values of Lp thus determined are7.946×10−11, 5.542×10−11, and 3.891×10−11 m3/m2-s-Pafor unmodified, Z2 and Z3 membranes, respectively. A de-crease in Lp value with successive modification is consistent

0 50 100 150 200 250 300 350 400 450 5000

2

4

6

8

10

12 Z1 membrane

Z2 membrane

Z3 membrane

Per

mea

te fl

ux (

m/s

) x1

06

∆P (kPa)

0 50 100 150 200 250 300 350 400 450 5000.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

Z1 membrane

Z2 membrane

Z3 membrane

Per

mea

te fl

ux (

m/s

) x1

06

∆P (kPa)

(a)

(b)

Fig. 3. Flux of aqueous solutions of (a) FeCl3 and (b) AlCl3 through the membranes.

with the fact the pore size of the membrane decreases onmodification. However, the decrease in flux is much lowerthan expected from Hagen–Poiseuille equation and this in-dicates that the hydrophilicity of the membrane increases onmodification.

88 A. Shukla, A. Kumar / Separation and Purification Technology 41 (2005) 83–89

100 150 200 250 300 350 400 450

30

40

50

60

70

80

90R-Z

3

R-Z2

R-Z1

Robs

-Z3

Robs

-Z2

Robs

-Z1

Rej

ectio

n (%

)

P (kPa)

Fig. 4. Variation of the observed and intrinsic rejection coefficients of the membranes with pressure for FeCl3 solution.

4.2.2. Variation of ion rejection with applied pressureFor a feed concentration of 500 mg/l of FeCl3 (pH ∼ 2.1)

and AlCl3 (pH ∼ 5.0), the separation experiments were per-formed and Fig. 3 shows the permeate flux for the differ-ent membranes as a function of applied pressure. It showsthat for all the membranes the flux varies linearly with thepressure. Permeate flux and salt concentrations in the per-meate were used to determine the intrinsic rejection coeffi-cients of the membranes following the procedure describedin Section 2. It was assumed that the membrane reflectioncoefficients remained constant in the range within which the

100 150 200 250 300 350 400 450

10

20

30

40

50

60

70

80

90R-Z

3

R-Z2

R-Z1

Robs

-Z3

Robs

-Z2

Robs

-Z1

Rej

ectio

n (%

)

P (kPa)

Fig. 5. Variation of the observed and intrinsic rejection coefficients of the membranes with pressure for AlCl3 solution.

Cm values varied. Table 2 gives the membrane reflectioncoefficients and solute permeabilities of the membranes forboth the salts used in this work. Figs. 3 and 4 show the in-trinsic rejection and the observed rejection data against theapplied pressure for FeCl3 and AlCl3 solutions. It can beseen that the intrinsic rejection coefficient increases withthe increase in the applied pressure (and the permeate flux),though the rate of increase decreases with increase in pres-sure. This trend is typical of the separation of ionic so-lutes through charged membranes. The observed rejectionof AlCl3 is found to be lower although the intrinsic rejec-

A. Shukla, A. Kumar / Separation and Purification Technology 41 (2005) 83–89 89

tion is calculated to be slightly more than that of FeCl3.This may be due to the fact that there is more accumulationof AlCl3 in the boundary layer on the upstream side of themembrane. As the surface concentration increases the per-meate concentration is also increased. This leads to a lowervalue of the observed rejection, which is based on the bulkconcentration of feed (Eq. (1)).

The chemical modification of the membrane causes a de-crease in its pore size and consequently, a decrease in the per-meate flux. In a membrane of pores having surface charge,the ion (called the co-ion) in the solution whose charge isof same sign as that of the pore wall is repelled. The totalcurrent flow in a pressure driven flow is zero and thus thesalt flux is determined by the flux of the co-ion. The ca-pacity to exclude co-ion depends both on the magnitude ofthe surface charge density of its pore walls and the radiusof the pore. As the modification causes a decrease in thepore radius the effect of wall charge on exclusion of co-ionsis increased because the double layer occupies more vol-ume fraction of the pore. This should lead to an increase inboth the rejection and the reflection coefficients. The mod-ification also changes the nature of the surface to some ex-tent and would, therefore, change the surface charge den-sity. The salt rejection behavior of the membrane is influ-enced by both the decrease in the pore size and the changein the nature of the surface and it is the combined effectof these two phenomena that will determine the rejectioncharacteristics of the modified membranes. It is not possi-ble to determine the individual effect of these two factorson the separation performance of the membrane by the ex-periments reported in this work. The results, however, showthat the combined effect of these two factors, for both theelectrolytes, leads to an increase in the rejection and the re-flection coefficients of the modified membranes. The saltsare almost completely rejected by the modified membraneswith reflection coefficients very close to 1 for both thesalts.

The observed rejection of all the three types of membraneas shown in Figs. 4 and 5 are significantly lower than the in-trinsic rejection coefficients. This indicates that the concen-tration polarization effects are significant and lead to con-siderable loss of efficiency. This clearly indicates that Robsis strongly dependent on the operating conditions while Rintrepresents the intrinsic property of the membrane accurately.

5. Conclusion

Zeolite clay composite membranes are characterized us-ing the separation of chloride salts of the trivalent cations(FeCl3 and AlCl3) in an unstirred batch cell. The experi-mental results are interpreted in term of the Spiegler andKedem’s approach based on the irreversible thermodynam-ics. The concentration polarization is also taken into accountin the calculations. The calculations show that modificationresults in an increase in the intrinsic rejection coefficientfrom ∼70% for the unmodified to ∼94% for the Z2 and∼96% for the Z3 membrane in case of FeCl3 solution andfrom ∼84% for Z1 to 90% for both Z2 and Z3 membranesin the case of AlCl3 solution. It also leads to a decrease inthe permeate flux. It has also been shown that separationin an unstirred batch cell leads to significant concentrationpolarization and thus, considerable loss in efficiency.

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