Use of Mordenite Surface Acidity Properties for the Selective

ARTICLE pubs.acs.org/IECR

Use of Mordenite Surface Acidity Properties for the Selective Separation of Halide Salts: Modification of Dielectric Effects Elodie Chevereau,† Lionel Limousy,*,‡ and Patrick Dutournie‡ †

Laboratoire d’Ingenierie des MATeriaux de Bretagne (LIMATB-EA 4250), Universite de Bretagne Sud, rue de Saint-Maude, BP 92116, 56 321 Lorient, France ‡ Laboratoire de Gestion des Risques et Environnement, Universite de Haute Alsace, 25 rue de Chemnitz, 68200 Mulhouse, France ABSTRACT: The Mordenite membrane was synthesized onto commercial ceramic tubular support (R-alumina), and surface characterizations were carried out on Mordenite powder. The surface charge density remains stable when Mordenite was in contact with monovalent or divalent salts. Pore diameter was estimated in the range of 8-10 nm. Filtrations tests were performed using the Mordenite membrane with single monovalent halide salt solutions. No rejection was observed. Then, the Mordenite membrane was treated with a divalent anionic salt (Na2CO3). Weak rejections were observed according to the order of hydration energy (NaF > NaCl > NaBr > NaI). After an acid cleaning, the membrane recovered its initial properties. A new estimation of average pore radius proved that steric effects were not responsible for rejection themselves. A new filtration test of mixed monovalent salt solution was carried out. The fluoride ion was retained by a majority. A preferential transfer was observed for chloride, bromide, and iodide ions.

1. INTRODUCTION Membrane processes, low cutoff ultrafiltration and nanofiltration, are effective separation techniques, at low-consumption, and that avoids or limits the resort to chemical treatments. This separation technique is governed by different phenomena: steric (pore size and shape), electrostatic (surface charge, polarity), and dielectric.1,2 In the current environmental context, these processes have taken an important place in our industry. There are two kinds of membranes: ceramic and organic ones. Among these membranes, ceramic membranes are able to operate in a significant range of temperature and pH, contrary to polymeric ones. Most of the time, commercial ceramic membranes consist of a support (made of R-alumina) and an active layer developed on this support (often titanium dioxide or zirconium dioxide). Some works report the use of zeolite as a membrane filtration layer. Zeolites have uniform and molecular-sized pores that cause significant differences in transport rates for some molecules, and allow molecular sieving in some cases.3 For these reasons, zeolite membranes have been used for gas phase separations, for pervaporation applications,4 or membrane bioreactors5-7 but, rarely for ultra or nanofiltration processes. Mordenite is a high-silica zeolite with an ideal composition of Na8[Al8Si40O96] 3 nH2O, and it is known that the Si/Al ratio of the synthetic Mordenite varies from 5 to 10 depending on the chemical composition of the reactant mixture without using an organic structure directing agent (OSDA) under hydrothermal condition.8 This zeolite, was first synthesized by Barrer et al.9 in 1948. In 1990, a first Mordenite membrane was obtained by Suzuki et al.10 using a hydrothermal synthesis method. The purpose of this research in this area is to develop a zeolite membrane that could be used as low cutoff ultrafiltration or nanofiltration membrane (ionic filtration) for water desalination or more precisely for water rectification, or specific treatment of industrial wastewater (metallurgy, semiconductors industries). Mordenite was selected as one of the potential membrane r 2011 American Chemical Society

materials owing to its strong hydrophilicity, ion exchange properties, its controllable inter- and intraparticle pore size, and for its Br€onsted and Lewis acid site density that depends on the Al/Si ratio.11,12 Especially, Br€onsted and Lewis acid sites may interact with hydrated anions (dipole-dipole interactions) and can modify the membrane selectivity. The crystalline structure of Mordenite (intraparticle porosity) is constituted by two different channels: small pores with 8 T-atoms channels (0.57 0.26 nm); large pores with 12 T-atoms channels whose diameter sizes vary to 0.65-0.7 nm. Indeed, the intraparticle pore size is controlled by chemical reagents during synthesis while interparticle pore size will depend on thermal or hydrothermal treatment. Under these conditions, the transport of ionic species through the Mordenite membrane layer may follow two pathways: interparticle and/or intraparticle. Concerning ion exchange properties, some works report that metal ions (generally sodium ions) may be exchanged with other alkaline ions, under certain conditions, by contact with salt or alkaline ion hydroxide solution.13-15 The aim of this work is to synthesize a Mordenite membrane and to understand the rejection mechanism. The charge density of the Mordenite powder is investigated by zeta potential and streaming induced potential measurements. Transport characteristics (as average pore radius and membrane permeability) are estimated by studying pure water flux and the rejection of neutral molecule and nitrogen adsorption. Finally, experimental studies of the membrane selectivity are presented for pure single monovalent salt-water solutions and mixed salt-water solution, before and after the filtration of divalent salt-water solution.

Received: September 30, 2010 Accepted: February 17, 2011 Revised: February 11, 2011 Published: March 02, 2011 4003

dx.doi.org/10.1021/ie1019968 | Ind. Eng. Chem. Res. 2011, 50, 4003–4010

Industrial & Engineering Chemistry Research

2. EXPERIMENTAL SECTION 2.1. Materials and Methods. A gel of Mordenite is prepared (in collaboration with the Institute of Research on Advanced Materials: IRMA, Ploemeur, France) according to the operating procedure detailed by Hincapie et al.16 First, a gel is obtained, from which a Mordenite powder is produced as well as a Mordenite membrane by seeding hydrothermal synthesis onto commercial alumina tubular support (length = 25 10-2m; internal radius = 3.5 10-3 m) provided by Pall Exekia (Bazet, France). The powder and the membrane are calcinated at 400 °C for 4 h. The mean pore radius is estimated by filtration of vitamin B12 (purity of 98%, Alfa Aesar) solutions and pore size distribution is obtained by nitrogen adsorption experiments at 77 K (Brunauer Joyner Halenda methodology, BJH) carried out on a Micromeritics ASAP 2000 apparatus. Filtration experiments are performed with different salts: NaCl (99%), NaF (99%), NaBr (99%), NaI (99.5%), Na2CO3 (99%) diluted in pure water (conductivity lower than 0.1 μS/cm). Hydrochloric acid (HCl) and sodium hydroxide (NaOH) 100 mM solutions are used for pH adjustment (zeta potential experiments). 2.2. Zeolite Morphology. X-ray diffraction is used at room temperature on a Philips θ/θ Bragg-Brentano X’pert MPD PRO diffractometer (Cu KRþ1 radiations) equipped with the X’celerator detector to confirm the synthesis of pure Mordenite. Specific surface area measurements are performed by nitrogen adsorption (BET methodology) at 77 K (Micromeritics ASAP 2000) after sample degassing at 110 °C for 24 h. Membrane and powder surface observations are carried out at various enlargements using a JEOL 6340 LV microscope (SEM). Fourier Transform Infrared Reflectance (FTIR) spectroscopy is used to analyze surface species adsorbed on the surface of Mordenite samples. FTIR spectra are recorded with a PerkinElmer spectrum BX spectrometer (Courtaboeuf, France) with a resolution of 2 cm-1 in the range of 4000 to 400 cm-1, using the KBr wafer technique. Wafers are prepared from the mixture of 1 mg of the sample and 100 mg of KBr. This mixture is compacted in a manual hydraulic press at 10 tons during 1 min. 2.3. Electrical Properties: Mordenite Powder Charge. Analyses are performed with Mordenite powder (not with Mordenite membrane powder), but the DRX spectrum obtained on both the powder and the membrane are identical. Previous experiments also showed that surface Mordenite properties were identical when they were performed on Mordenite powder or on Mordenite extracted from the support. This was not a surprise because the gel of Mordenite was made before the impregnation. Mordenite powder is produced as well as a Mordenite membrane by seeding hydrothermal synthesis at the same time in the same autoclave with the same gel. 2.3.1. Zeta Potential Measurements. Zeta potential measurements are performed with a Malvern Nanosizer instrument (Malvern Instrument) to obtain the nature and the sign of the Mordenite surface. Suspensions are prepared using 25 mg of Mordenite dispersed with the help of an ultrasonic device (samples have been submitted to ultrasonic waves for 1 min to destroy aggregates and to have a good dispersion) in 50 mL of solutions containing either pure water or NaF, NaCl, NaBr, or NaI at 3.4 mM, or Na2CO3 at 6.6 mM. pH is adjusted (in the range of 2-11) using 50 mM NaOH or 100 mM HCl solutions. Then, solutions are shaken during 24 h. Each experiment is repeated three times.

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2.3.2. Streaming Induced Potential Measurements. Surface charge density (μeq m-2) of Mordenite particle is obtained with a particle charge detector (μMutek PCD-03- Streaming potential measurements, Noviprofibre, Eybens, France). The detailed experimental protocol used is available in the literature.17 2.4. Experimental Filtration Setup. The experimental unit has been described in previous work.23 The unit is operated in the cross-flow mode, with a flow rate from 0 to 700 L/h, pressure from 0 to 5 bar using a volumetric pump. Flow is controlled by an electromagnetic flow meter, and pressure is measured by two sensors upstream and downstream of the membrane and is regulated by a manual valve. Temperature is maintained at 25 °C through circulation in a counter current heat exchanger cooled by a refrigeration unit. Concentrations are kept constant by recycling permeate into the feeding tank (5 L) except during measurements for permeate flow rate calculation and sampling for concentration analyses. 2.5. Analyses. Ion concentrations are analyzed by an ionic chromatography apparatus (ICS 1000, Dionex, Voisins le Bretonneux, France) equipped with a conductivity detector. Single salt solutions are analyzed with a double electrode probe conductimeter (CRISON, GLP 31, France). Vitamin B12 concentration is determined at 362 nm with a UV-visible spectrophotometer (Varian, Cary 50 Bio, Les Ulis, France). 2.6. Experimental Protocol. Before each experiment, the pilot unit is cleaned and filled with demineralized water. Water flux is then measured for various transmembrane pressures to estimate membrane performances. Membrane hydraulic permeability is also deduced from water flux experiments. The feed tank is emptied and filled with salt solution, which is homogenized by high flow velocity circulation for 30 min. Then, the pilot unit is run during 12 h to reach ionic adsorption equilibrium.

3. RESULTS AND DISCUSSION 3.1. Morphology and Structure of Mordenite. Observations of membrane cross sections and Mordenite crystals were carried out by Scanning Electron Microscopy (SEM) and presented in Figure 1. Figure 1a is a microscopy photograph of the membrane cross section at large scale. It shows the impregnated active layer and the heterogeneity of this one. Figure 1b (SEM photograph) shows the three main layers: support, intermediate layer, and the impregnated layer. This last one is around 80-100 μm thick and seems homogeneous enough. The size of the alumina crystals which constitute the support is approximately 25 μm. The average pore size of the two intermediate layers is 0.8 and 0.2 μm (manufacturer data). The last photograph (Figure 1c) shows a zoom of the active layer. The Mordenite crystals which constitute this layer are around 2-3 μm length and 1.5 μm thick. To verify the structure of the active layer, the synthesized powder is characterized by X-ray diffraction (XRD). The XRD-pattern shows different reflection intensities (22.3°, 25.7°; 26.3°; 27.9°; 30.8°) which are characteristics of the Mordenite structure. Nitrogen adsorption/desorption experiments (BET and BJH measurements) were carried out with Mordenite powder. Results show that the specific surface area is 19.8 m2 g-1 ((0.26 m2 g-1), among which microporous surface represents 16.0 m2 g-1. This value is extremely lower than the one of Mordenite before thermal treatment (around 300 m2 g-1). The hysteresis of adsorption isotherm is type H3, that is, characteristic of 4004

dx.doi.org/10.1021/ie1019968 |Ind. Eng. Chem. Res. 2011, 50, 4003–4010


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Figure 1. SEM photographs of membrane cross section and Mordenite crystals at different enlargements (A, 1 ; B, 370 ; C, 27000).

Figure 2. Variation of ζ according to the pH for different saline solutions (NaF, NaCl, NaBr, and NaI at 5 mM and Na2CO3 at 10 mM) at 25 °C.

particle aggregates or agglomerates forming slit shaped pores (plates or edged particle like cube), with nonuniform size and shape. This kind of hysteresis is typical of zeolite materials.18 Total pore volume is close to 0.03 cm3 g-1. A formation of both mesopores (diameter: 0.8 nm) and macropores (diameter: 8.4 nm) is observed corresponding respectively to Mordenite intracrystallinity and to Mordenite intercrystallinity. 3.2. Electrical Properties. Zeta potential investigations were performed with the different electrolytes in a pH range 2 to 11, except for sodium carbonate experiments which were carried out at native pH. Figure 2 shows zeta potential measurements versus pH for the different water-electrolyte solutions. Zeta potential curves are characteristic of amphoteric surfaces,19,20 or surfaces with both acidic and basic functional groups. Results show that the membrane surface is negatively charged in the studied range of pH. The isoelectric point (i.e.p.) of Mordenite powder is close to pH = 2 while it shifts to lower pH values when suspensions contain F-, Cl-, Br-, or I- electrolytes. This phenomenon can be attributed to the adsorption of F-, Cl-, Br-, and I- ions at low pH values.21

Knowing the nature of the material charge, induced streaming potential experiments were performed. Table 1 summarizes the quantitative estimations of the surface charge density for different solutions at native pH (close to pH = 7). It seems that the Mordenite surface charge density slightly decreases when F-, Cl-, Br-, and I- ions are present. Zeta potentials of the different studied solutions at pH = 7 are not representative of this behavior (Figure 1). This result is explained by the fact that, at a given pH, zetametry potentials depend on the ionic strength and on the nature of the electrolyte. The Zeta potential is relative to the surface charge sign but not to the surface charge density. This result indicates that streaming induced potential measurements are necessary to understand the real influence of electrolytes on oxide surface charges. 3.3. Filtration Performances. These experiments are carried out with a new membrane. First, the membrane is stabilized by measuring the hydraulic permeability, and until it reaches a stable value (5.5.10-14 m3 m-2memb after 100 h). 3.3.1. Rejection of Pure Monovalent Salts. To understand the selectivity of saline solutions with a low cutoff ultrafiltration membrane, rejection rates of single salt-water solutions are studied. The first experiment is conducted in the presence of 3.4 mM NaCl-water solution, at different transmembrane pressures (1-5 bar), after stabilization at room temperature (25 °C). No rejection is observed. This result was expected to take into account the ratio between the membrane mean pore radius (4.2 nm) and the Stokes radii of the two studied ions (Table 2). After rinsing the pilot unit with pure water, the hydraulic permeability increases slightly (5.5.10 -14 f 6.6.10 -14 m 3 m -2 memb ). Similar filtration tests were carried out with single halide salt-water solutions: NaF, NaI, and NaBr. The results are the same in terms of rejection rates and increase of hydraulic permeability. All the phenomena which govern this filtration process1 (i.e., steric, electric, and dielectric ones) seem to have no influence on the filtration of pure monovalent salt-water filtration. 3.3.2. Modification of Mordenite Surface Properties 3.3.2.1. Rejection of Pure Monovalent Salts after Treatment with Carbonate-Water Solution. As no rejection is observed for 4005



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Table 1. Surface Charge of Mordenite Powder Suspensions in Contact with Different Salts solution

H2O

ionic strength (mM)

NaF 3.4

NaCl

NaBr

3.4

3.4

NaI 3.4

mixture of 4 monovalent salts 3.4

Na2CO3 6.6

pH

6.2

6.9

6.8

6.8

6.8

6.8

10.2

surface charge (μeq.g-1)

-11.4

-12.7

-13.3

-12.3

-13.9

-12.0

-13.9

standard deviation (μeq g-1)

1.95

1.10

0

1.34

1.10

1.10

1.10

Table 2. Physicochemical Properties of Halide Ions Stokes radius (nm)

Ehyd (kJ mol-1)

F-

0.166

-483

Cl-

0.121

-340

Br-

0.118

-309

I-

0.120

-296

CO32-

0.266

HCO3-

0.207

Vitamin B12

0.740

ions or molecule

the monovalent halide salt-water solutions, a new filtration experiment is investigated with a divalent salt Na2CO3 (6.6 mM) at 5 bar, 25 °C, during 1 h. Indeed, divalent anion size and its charge are larger than those of halogenated anions (Table 2). The pH of the carbonate solution is controlled by the equilibrium of HCO3-/CO32- (pH = 10.3; [HCO3-] = 3.6 mM (54,5%); [CO32-] = 3 mM (45,5%)) and the pH remains stable from the beginning to the end of the filtration test (10.3f10.2). Following the pure water rinsing cycle, hydraulic permeability dropped from 6.55 to 2.22 10-14 m3 m-2memb. Taking into account the drop of permeability, it is necessary to ensure the non modification of the membrane morphology (because of adsorption or chemical modification). For this, filtration of a neutral molecule-water solution (Vitamin B12),22,23 before and after Na2CO3-water filtration tests, was carried out. Actually, the study of steric effect requires the elimination of all other effects. The average pore radius is estimated by fitting experimental rejection rates by a model of neutral molecule, described by Bowen et al.24 This molecule was chosen because it is often used to study the steric effect of UF membrane at low cutoff. Its size is given in Table 2 as the studied ions. No rejection of vitamin B12 is observed in the two filtration tests. These results show that the mean pore radius has not been modified significantly. Moreover, infrared investigations were carried out to search a possible presence of carbonate species in the material (adsorbed species). For all that, Mordenite powder has been in contact with sodium carbonate-water (6.6 mM) solution during 24 h and then was analyzed. The infrared spectrum revealed that neither carbonate nor hydrogeno-carbonate ions have been adsorbed onto the Mordenite. These different investigations show that the filtration performances of the membrane can not be attributed to the significant modification of steric effects by pore size modification or selective adsorption of ions in the pore. To make sure that the Mordenite surface is not modified, a further test is carried out with a 3.4 mM NaCl-water solution. Figure 3 shows the observed rejection rates of sodium chloride at different transmembrane pressures. Contrary to what was

previously observed (section 3.3.1), the sodium chloride is partially retained and the maximal rejection reaches 10% at 3 bar (Table 3). Filtration properties of the membrane have been modified after carbonate filtration. The hydraulic permeability of the membrane increases slightly after the filtration test (Table 3). Taking into account that the modification of steric effects cannot explain itself this phenomenon and the estimation of surface charge density (constant whatever the tested solution is), the only explanation for this filtration property modification is a significant evolution of dielectric effect in the pore. These effects are often studied in nanofiltration, because it is not possible to simulate NF performances without taking account of these effects. They are the consequence of different causes: - Confinement:25,26 the electrical field is modified in the pore by the presence of the walls (difference of dielectric constant between the material and the liquid phase). - Electrical charge:27 in addition to the electrical forces with ions in solution, the surface charges induce forces on water molecule (charge/dipole). - Chemical nature of the surface: the presence of dipolar group (as hydroxide ones for example) induces forces as dipole/dipole (van der Waals) with water molecules, dipole/ion with ion in solution, or dipole/dipole with the hydrated ions. - Nature of the solvent: the water molecule is highly dipolar and its dielectric constant is very sensitive to the forces previously described. Theses different points have the major effect to modify the solvation properties of water in regard with ions in solution. In a modeling point of view, different authors have tried to take into account the dielectric effects in their models. For example, some authors25,28,29 have introduced these effects by a global decrease of water dielectric constant in the pore. Others authors have introduced a new model called “Image Forces” which describes the modification of the electrical field in the pore because of the presence of the material with a different dielectric constant.30-32 Others have connected the two models.33,34 At the sight of initial chemical composition of Mordenite, the surface reveals an important silicon content. The adsorption and other properties of the rich-silica surface are known to depend frequently on the surface hydroxyl groups (silanol). By changing the concentration of Br€onsted and Lewis acid sites on the zeolite surface, it is possible to modify its properties.35 According to Moritz et al.,36 if the Mordenite membrane surface is exposed to water, it becomes hydrated. The membrane is electrically charged because of the amphoteric behavior of hydroxyl groups coming from the metal oxide hydrated surfaces (Br€onsted and Lewis acid sites). The surface charge of oxide materials can be achieved by two mechanisms which often act simultaneously: the dissociation of surface groups and the adsorption of ions from the solution. The dissociation of the 4006



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Figure 3. Rejection rate of sodium chloride (3.4 mM) as a function of transmembrane pressure at 25 °C.

Table 3. Evolution of the Hydraulic Permeability and Rejection Rate at 25°C for Single Monovalent Salt Solutions after Carbonate-Water (6.6 mM) Filtration Experiments solution

NaF

NaCl

concentration (mM)

3.4

3.4

3.4

3.4

hydraulic permeability before and after filtration test (10-14 m3 m-2membrane)

3.86f4.27

2.17f2.78

4.44f5.21

6.14f7.15

maximal rejection rate (%)

14%

10%

8%

5%

hydroxyl surface groups strongly depends on the pH of the solution and can be expressed by eq 1 and eq 2: MOH þ Hþ f MOH2 þ T Mþ þ H2 O

ð1Þ

MOH þ OH- f MðOHÞ2 - T MO- þ H2 O

ð2Þ

In this case, the pH of sodium carbonate-water solution is close to 10.3 and thus hydroxyl species are present in the liquid phase and may react with the Mordenite surface. The rejection rates of NaCl-water solution after a carbonate filtration can be explained by a hydroxylation of the Mordenite membrane surface (modification of dielectric effect). Moreover, the presence of these hydroxyl groups at the surface decreases the degree of water molecule freedom (polar solvent). 3.3.2.2. Reversibility of Mordenite Surface Properties. The rejection of monovalent salts seems to depend on surface Br€onsted and Lewis acid site density. To verify the influence of hydroxyl groups on the separation process a surface treatment of the membrane surface is carried out with hydrochloric acid (10 mM) for 1 h, to dehydroxylate the Mordenite surface. After this filtration, the hydraulic permeability rises but does not return to its initial value (Lp = 5.5.10-14 m3 m-2memb). Assuming that the filtration property modifications are due to the presence of hydroxyl groups upon the surface, it is possible to recover initial properties by carrying out an acid cleaning. Indeed, after an acid cleaning, it is supposed that hydroxyl groups are not in the majority, compared to hydronium groups upon the Mordenite membrane surface. To validate this hypothesis, a further filtration test of sodium chloride-water solution is carried out after the acid cleaning. Once more, the sodium chloride is not at all retained. This test confirms that hydroxyl groups, present upon membrane surface, involve a rejection of monovalent salts. The membrane seems to recover its initial filtration properties. This

NaBr

NaI

result confirms that rejection rate depends on surface chemistry, specifically on surface hydroxylation rate. The reversibility of the membrane property modifications was validated by a second test series following the established protocol presented in Figure 4. Filtration tests are carried out with single halogenated saltwater solutions (3.4 mM): NaF, NaI, and NaBr (STEP 1). Whatever the monovalent salt used, after an acid cleaning, no rejection rate is observed. Then, a filtration of sodium carbonatewater solution (6.6 mM) is performed (STEP 3). Once the surface of the membrane is modified by carbonate treatment, filtration tests of monovalent halide salt-water solutions (STEP 5) are carried out. As it was observed in the first NaCl-water solution filtration test (Figure 4), each salt is now retained. Table 3 sums up the hydraulic permeabilities and rejection rates of each halogenated salt. The maximal rejection rates of fluoride, chloride, bromide, and iodide ions are respectively 14%, 10%, 8%, and 5%. The rejection rates of fluoride ion are the highest among all halide ions, especially at low transmembrane pressure where diffusive transport is significant, as reported by Lhassani et al.37 in the case of nanofiltration experiments. The more retained salts are respectively NaF > NaCl > NaBr > NaI; this follows the order of Stokes radius and also hydration energy of co-ion (Table 2). After an acid cleaning (STEP 7), the membrane recovers its initial properties, that is, there is no rejection of monovalent salts (STEP1). These different experimental tests show that filtration of sodium carbonate-water solution allows to reversibly change filtration performances of the membrane in regard to pure monovalent salt-water solutions. 3.3.3. Rejection of Mixed Monovalent Salts Solution. A filtration test (STEP 5) is made with a mixture of four monovalent 4007



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Figure 4. Experimental protocol of filtration tests.

salts (NaF, NaCl, NaBr, NaI) at native pH and after conditioning the membrane according to the protocol (step 1 to 4). In this test, ionic strength (3.4 mM) is maintained constant in regard with all previous pure monovalent salt experiments. For this, the concentration of each salt is 0.85 mM. Figure 5a and Figure 5b show rejection rates of halogenated ions as a function of the transmembrane pressure. As shown in Figure 5a, the fluoride ion is retained by a majority (around 24%, at 1 bar). The three other ions (Cl-, Br-, and I-) are only weakly retained (inferior to 2%). It seems that the surface chemistry modification induces a preferential transport of three ions in regard with fluoride transport. Figure 5b shows the reaction rates of the nonretained ions (scale enlargement). At 2 bar, the maximal rejection is reached for each halogenated salt. The maximal rejection rate of chloride ions (∼2%) is higher than that of bromide ions (∼0.5%) and iodide ions (0.15%). Moreover, when the transmembrane pressure increases, the rejection rates of these three ions become negative. In all experimental tests, chloride ions are respectively more retained than bromide and iodide ions. In comparison to the filtration tests made with single salt solutions (see section 3.3.2.2), fluoride ion rejection increases while chloride, bromide, and iodide rejections decrease with the mixed solution. Specific rejection of fluoride ion can not be explained by electrostatic effects because the surface charge density of the membrane is the same for the four anions, as mentioned in section 3.2. But, this result should be explained by both steric and dielectrical phenomena that notably depend on the polarity (dipolar moment) and the hydration energy of ions. Indeed, as reported by Diawara et al.,38 the fluoride ion, which has hydration energy (see Table 2) higher than chloride, bromide, and iodide ions, is preferentially retained.

Figure 5. Rejection rates of mixed halogenated salts solution (NaF, NaCl, NaBr, and NaI, at 0.85 mM each) (a), and zoom in on rejection rates of chloride (0.85 mM), iodide (0.85 mM), and bromide (0.85 mM) (b) versus transmembrane pressure at 25 °C.

As reported by Pontalier et al.,39 hydration energy can be considered as a force necessary to extract the solute from the solvent to push it into the pores. In this case, it would require more energy to extract F- ion and to push it into the pores in comparison with the Cl-, Br-, or I- anions (see Table 2). According to Volkov et al.,40 these phenomena dependent on the size of solvated ions (i.e., diffusion coefficient) will also depend on the extent of solvation (the number of solvent molecules surrounding an ion and bound to it) and the steric hindrance of the solvent. To understand the interactions between ions and the surface, it is necessary to have more information on the ions' environment in water as well as on the solvent properties and on their mutual interaction. In this case, there are different electrical interactions present in solution: Ion-dipole interaction which depends on the oriented attraction of ions by the solvent partial charge of opposite sign and that corresponds to hydration of ions in solution. Ion-dipole forces are more important for solution of ionic compound in polar solvent where there are solvated species. In the case of NaF solution in H2O, two species are found Na(H2O)xþ and F(H2O)y-.41 The ion-dipole forces are directional insofar as these forces involve preferential orientations of molecules. Dipole-dipole interactions (van der Waals forces) depend on orientation (Keesom forces), dispersion (London forces), and induction (Debye forces) effects.42 Keesom forces are associated molecules that have a permanent dipole 4008


Industrial & Engineering Chemistry Research moment via chemical composition; Debye and Falkenhagen induction forces are linked molecules that polarize by induction under the effect of dipolar molecules; London forces are joined molecules that, under the effect of fluctuations of electron and nucleus positions, present, at any instant, a dipolar moment (though they appear neutral on average). In this work, it is assumed that rejection of monovalent ions is related to Mordenite surface interaction with the solvated ions and also to the size of the hydrated ions. This required to focus on the dipole-dipole interactions and especially on the Keesom forces which are the most sensitive ones. A plausible explanation for the Keesom forces effect is that if we consider the Mordenite surface after carbonate solution filtration, it presents a high Br€onsted and Lewis acid site density. Under these conditions, hydroxyl groups may be orientated leading to the formation of an ultrapolar layer all over the active layer (pore and surface). This phenomenon induces a modification of the boundary layer, very close to the surface, and modifies the mass transfer in the polarization layer especially for ionic species. Then, the more important the hydration energy of ionic species is, the more difficult mass transfer through the polarization layer is. Considering these explanations, experimental results suggest that sodium fluoride is preferentially retained because of its important Keesom forces. Other halide salts (NaCl, NaBr, NaI), with quasi-similar hydration energies (but lower than the one of sodium fluoride), have a preferential transport through membrane structure.

4. CONCLUSION Mordenite is synthesized by the sol-gel method. It is impregnated onto a commercial support and at the same time prepared in the form of powder. Morphological, electrical, and surface property characterizations were made. The surface charge is negative in the range of pH 2-11. Zetametry and streaming induced potential experiments have shown that the membrane charge is almost stable when it is in contact with different saline solutions. The experimental tests carried out on a laboratory pilot unit show, as expected, that the membrane does not retain monovalent salts. Nevertheless, after filtration of sodium carbonate-water solution, monovalent salts are partially retained. The filtration properties of the membrane are significantly modified by a treatment with sodium carbonate-water solution. Filtration with this divalent salt must modify surface chemistry, especially Br€onsted and Lewis surface acid site density, because experimental characterizations exclude a modification of electric effects. After an acid cleaning, the membrane recovers its initial properties, thus allowing the implementation of a reversible experimental protocol. According to this protocol, a permeation test with a mixture of four halide monovalent salt-water solution shows a preferential transport of chloride, bromide, and iodide ions in regard with fluoride ones. This result may be both explained by Keesom forces which strongly depend on the hydration energy of ions and by the modification of the boundary layer resulting from the appearance of an ultrapolar layer all over the Mordenite surface. To conclude, after surface hydroxylation, the Mordenite membrane seems to behave as an ultrafiltration membrane toward neutral molecules and as a nanofiltration membrane toward monovalent salt-water solutions.

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’ AUTHOR INFORMATION Corresponding Author

*Phone: (33) 3-89-32-76-62. Fax: (33) 3-89-32-76-61. E-mail: [email protected].

’ ACKNOWLEDGMENT The authors would like to thank the Pall Exekia company (Bazet, France) and IRMA Society (Ploemeur, France) for technically supporting this research. ’ REFERENCES (1) Deon, S.; Dutournie, P.; Bourseau, P. Transfer of Monovalent Salts through Nanofiltration Membranes: A Model Combining Transport through Pores and the Polarization Layer. Ind. Eng. Chem. Res. 2007, 46 (21), 6752. (2) Tavolaro, A.; Drioli, E. Zeolite membranes. Adv. Mater. 1999, 11, 975. (3) Asghari, M.; Mohammadi, T.; Aziznia, A.; Danayi, M. R.; Moosavi, S. H.; Alamdari, R. F.; Agand, F. Preparation and characterization of a thin continuous faujasite membrane on tubular porous mullite support. Desalination 2008, 220, 65. (4) Caro, J.; Noack, M.; Zeolite Membranes - Status and Prospective. In Advances in Nanoporous Materials; Ernst, S., Ed.; Elsevier: New York, 2010; Vol. 1. (5) Buetehorn, S.; Koh, C. N.; Wintegens, T.; Volmering, D.; Vossenkaul, K.; Melin, T. Investigating the impact of production conditions on membrane properties for MBR applications. Desalination 2008, 231, 191. (6) Bienati, B.; Bottino, A.; Capannelli, G.; Comite, A. Characterization and performance of different types of hollow fibre membranes in a laboratory-scale MBR for the treatment of industrial wastewater. Desalination 2008, 231, 133. (7) Maximous, N.; Nakhla, G.; Wan, W.; Wong, K. Preparation, characterization and performance of Al2O3/PES membrane for wastewater filtration. J. Membr. Sci. 2009, 341, 67. (8) Itabashi, K.; Matsumoto, A.; Ikeda, T.; Kato, M.; Tsutsumi, K. Synthesis and characteristic properties of Rb-Mordenite. Microporous Mesoporous Mater. 2007, 101, 57. (9) Barrer, R. M. Synthesis and reactions of Mordenite. J. Chem. Soc. 1948, 127, 2158. (10) Suzuki, K.; Kizoyumi, Y.; Sekine, T.; Obata, K.; Shindo, Y.; Shin, S. Preparation and characterization of a zeolite layer. Chem. Express 1990, 5, 793. (11) Olah, G. A.; Prakash, G. K. S.; Sommer, J.; Superacids; Wiley: New York, 1985. (12) Alberti, A. Location of Brønsted sites in mordenite. Zeolites 1997, 19, 411. (13) Barthomeuf, D.; Quenach de Quivillic, V. New catalysts based on zeolites modified by alkaline elements in metallic form, their preparation and their application to the alkylation of alkyl aromatic derivatives. European Patent EP0389530 B1, 1993. (14) Barrer, R. M.; Klinowski, J. Ion exchange in mordenite. J. Chem. Soc. 1974, 70, 2362. (15) Colella, C. Ion exchange equilibria in zeolite minerals. Miner. Deposita 1996, 31, 554. (16) Hincapie, B. O.; Garces, L. J.; Zang, Q.; Sacco, A.; Suib, S. L. Synthesis of mordenite nanocrystals. Microporous Mesoporous Mater. 2004, 67, 19. (17) Limousy, L.; Mahzoul, H.; Hamon, L.; Stiffert, B. Determination by zetametry and ‘streaming induced potential’ measurements of the amounts of catalytic precursors necessary to saturate a support. Colloids Surf. A 2001, 181, 91. (18) Kaneko, K. Determination of pore size and pore size distribution: 1.Adsorbents and catalysts. J. Membr. Sci. 1994, 96, 59. 4009


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Use of Mordenite Surface Acidity Properties for the Selective

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