Preparation and Performance Evaluation of Nanokaolinite-Particle

Article pubs.acs.org/IECR

Preparation and Performance Evaluation of NanokaoliniteParticle-Based Polyacrylonitrile Mixed-Matrix Membranes R. Saranya,† G. Arthanareeswaran,*,† S. Sakthivelu,‡ and P. Manohar‡ †

Membrane Research Laboratory, Department of Chemical Engineering, National Institute of Technology, Tiruchirappalli 620015, India ‡ Department of Ceramic Technology, Alagappa College of Technology Campus, Anna University, Chennai 600025, India ABSTRACT: The synergistic effects of polyacrylonitrile (PAN) and nanokaolinite particles were studied using PAN/ nanokaolinite mixed-matrix membranes (MMMs). Nanokaolinite was obtained from naturally available kaolin clay by an intercalation/exfoliation method. The kaolinite nanoparticles were added in varying compositions from 2.5 to 10 wt % at an increment of 2.5 wt % to PAN in the presence of the solvent N,N′-dimethylformamide (DMF). The uniform dispersion of nanokaolinite in the PAN matrix was achieved with the help of ultrasonication. The PAN/nanokaolinite material was characterized by attenuated-total-reflectance-infrared (ATR-IR) spectroscopy. Contact-angle measurements showed increased hydrophilicity due to the nanokaolinite addition that, in turn, helped reduce membrane fouling. Thermal stability and miscibility were characterized using thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC). The pure-water permeability flux increased from 122.8 to 264.93 L m−2 h−1 with increasing nanokaolinite concentration from 0 to 7.5 wt %. Rejection studies using protein showed an improved rejection efficiency of 92.7%, which is higher than that of the neat PAN membrane. PAN/nanokaolinite MMMs were also investigated in the separation of synthetic rhodamine B dye with and without the macroligand poly(diallyldimethylammonium chloride) (PDDA) for which the dye removal efficiency and flux were compared. Nanokaolinite-based MMMs are an inexpensive material and provide enhanced properties such as porosity, hydrophilicity, thermal stability, rejection, and productivity.

1. INTRODUCTION Many membranes have the ability to selectively pass one component in a mixture while rejecting others and have a number of advantages over conventional separation processes as they are more compact, energy-efficient, economical, and environmentally friendly.1 Membranes are becoming more popular in the wastewater treatment field because they can be designed to meet stringent standards while requiring significantly less space and time.2 The emerging trend in separation and purification technology necessitates membranes having robust performance that operate with a high rate of permeability, have a high degree of selectivity, and have a high resistance to fouling. The widely used polymeric membranes have an inherent drawback, namely, a tradeoff between permeability and selectivity, which means that, in general, more permeable membranes have lower selectivities and vice versa.3,4 As there is growing anticipation in overcoming the tradeoff between selectivity and permeability in membrane separation process, numerous studies have focused on novel and facile membrane fabrication appraoches using both new technologies and new materials.5−8 Inorganic membrane formed from metals, ceramics, or pyrolyzed carbon have been found to have higher selectivities than traditional organic polymeric membranes.9 However, high costs and processing difficulties prevent them from being used in large-scale applications.10 Mixed-matrix membranes (MMMs) have the potential to achieve higher selectivities and permeabilities, resulting from the mixing of the inorganic particles with their inherent superior separation characteristics,11 whereas continuous polymer matrix offers flexibility for membrane processing.12 © 2012 American Chemical Society

Novel MMMs have been proposed that use nanosized particles as the inorganic solid added to the polymer. The presence of such nanoparticles allows much higher selectivities to be obtained without compromising the flux. As the selectivity is linked to parameters such as the polymer-free volume, the particle size and surface area, and the presence of covalent bonding,13 it is expected that, by mixing the nanosize particles with the polymer, the accessible free volume in the polymer matrix might be further increased. A higher magnitude of masstransport behavior is linked with the incorporation of nanoparticles. Also, a higher selectivity is attained from the extremely narrow pore size distribution and selective nature of the solid nanoparticles The transport behavior of polymer/inorganic MMMs is strongly influenced by a variety of factors, including (1) the properties of the polymer and inorganic materials used, (2) their compatibility and absence of interfacial defects, (3) their morphology, and (4) the membrane formation process.5 First, the potential of MMMs depends on the selection of a suitable polymer and inorganic nanoparticles based on their properties and processing capabilities. Second, a uniform dispersion of nanoparticles in the polymer matrix is desirable to exploit the advantageous effects of both organic polymer and inorganic nanoparticles. Preventing nanoparticle agglomeration is important because such agglomeration would increase the diffusion Received: Revised: Accepted: Published: 4942

January 4, 2012 March 6, 2012 March 9, 2012 March 10, 2012 dx.doi.org/10.1021/ie300026s | Ind. Eng. Chem. Res. 2012, 51, 4942−4951

Industrial & Engineering Chemistry Research

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As the aim of this work is also to assess the applicability of PAN/nanokaolinite MMMs toward industrial wastewater treatment, synthetic rhodamine B dye solution was considered for a membrane separation study. With the anticipation of reducing the dye concentration and color, an analysis of poly(diallyldimethylammonium chloride)- (PDDA-) assisted UF treatment in conjugation with PAN/nanokaolinite MMMs was also performed. The flux and rejection results were then compared for rhodamine B dye solution in the presence and absence of PDDA.

distance and generate gaps between individual solid particles. Third, the dispersed phase of solid particles should have enough compatibility with the polymer phase to avoid any adverse change in membrane morphology. Finally, the method of membrane formation also has an impact on the separation characteristics of MMMs. The higher agglomeration of nanoparticles and viscosity of solvent-dissolved polymer/inorganic solutions would hinder the exchange rate between solvent and nonsolvent during membrane formation. In the present work, to fabricate MMMs, the polymer polyacrylonitrile (PAN) was chosen for its versatility, good solvent stability, ease of modifying its nitrile (−CN) group to offer membranes with increased hydrophilicity, and low cost compared to other materials.14,15 PAN is a well-known material for ultrafiltration (UF) and has been investigated for wastewater treatment.16 MMMs that are hydrophilic in nature will have better antifouling effects and flux performance that can be used with ease in water treatment processes. Inorganic nanosize kaolinite particles were used as the solid dispersed phase along with the PAN. Kaolinite, with the chemical composition Al2Si2O5(OH)4, is a clay mineral formed by the weathering of feldspar and is one of the most common minerals on Earth. It has high crystallinity and unique structure: One side of the interlayer space is covered with hydroxyl groups of Al2(OH)4 octahedral sheets, and the other side is covered with oxygen molecules of SiO4 tetrahedra.17 It is soft, has a low viscosity at high solids contents in many systems, is readily wetted and dispersed in water and some organic systems, and can be produced with a controlled particle size distribution. It is used largely in the ceramics and paper industries and also has been used for the treatment of polluted water.18−20 So far, numerous research works21,22 have been based on investigations of polymer/inorganic composite membranes in applications such as gas separation, pervaporation separation of organic mixtures, and so on. New hybrid materials that result from the interaction between clay minerals and organic polymers have received much attention that increased rapidly throughout the past decade.23 Earlier, works on PAN/smectite and PAN/montmorillonite were performed, and the first method for the synthesis of PAN/nanokaolinite composite was described in 1988.24 In the current work, an attempt was made to prepare and characterize PAN/nanokaolinite MMMs based on permeability and rejection to assess their applicability for industrial wastewater treatment. Different weight percentages of nanokaolinite were mixed with PAN by means of solution mixing. The phase-inversion method25 was employed to form PAN/nanokaolinite MMMs that were then investigated in membrane flux and rejection studies. Novel PAN/nanokaolinite MMMs seemed to have improved the performance in terms of both membrane flux and separation efficiency. PAN/nanokaolinite MMMs were later subjected to characterization studies wherein the hydrophilicity of the membranes was examined by contact-angle measurements and the thermal stability of the membranes was examined using thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC) measurements. X-ray diffraction (XRD) studies were performed to assess the presence of crystallites, and attenuated-total-reflectance-infrared (ATR-IR) spectroscopy was used to evaluate the chemical structure of the membrane. These characterization results were compared with the membrane transport properties measured on a batch-type ultrafiltration test kit with ultrapure water and aqueous protein solution (1000 ppm) at a transmembrane pressure of 60 psi.

2. EXPERIMENTAL SECTION 2.1. Materials. PAN was obtained from Aldrich Chemical Company (Milwaukee, WI). The Indian clay form kerala (kannur) was used for nanokaolinite preparation. The solvents N,N′-dimethylformamide (DMF) and sodium lauryl sulfate (SLS) were purchased from Qualigens Fine Chemicals, Glaxo India Ltd., Mumbai, India. Analytical-grade acetone and eggalbumin powder (Mw = 45000) were obtained from SRL Chemicals Pvt Ltd., Mumbai, India. Anhydrous disodium hydrogen orthophosphate and monosodium dihydrogen orthophosphate dihydrate were obtained from CDH Chemicals Ltd., Delhi, India, for the preparation of sodium phosphate buffer. Rhodamine B was acquired from Merck Chemicals. Poly(diallyldimethylammonium chloride (PDDA) with a molecular weight of 200−400 kDa was supplied by Aldrich Chemical Company (Milwaukee, WI). The chemical structures of PDDA and rhodamine B are shown in Figure 1. Ultrapure water was produced in the laboratory using a Millipore pilot plant.

Figure 1. Chemical structures of PDDA and rhodamine B.

2.2. Preparation of Kaolinite Nanoparticles. 2.2.1. Purification of Raw Kaolin. Naturally available kaolin clay was chosen to extract purified kaolinite following the procedure shown in Figure 2. The first sedimentation process helped in removing nonplastic clay components. Purification was further intensified by removing free iron oxide and quartz based on the citrate−bicarbonate−dithionite (CBD) procedure26 and applying H2O2 treatment to remove unwanted organic matter. The purified kaolinite was then characterized by wet sieve and chemical analysis to understand the extent of purification. A 45-μm sieve (AIMIL) was used for wet sieve analysis, whereas chemical analysis was done using energy-dispersive X-ray fluorescence (ED XRF) analyzer (Minipal 4). Specific surface area was measured indirectly by the cation-exchange-capacity (CEC) method using methylene blue trihydrate (Acros, Morris Plains, NJ). 2.2.2. Intercalation and Exfoliation of Purified Kaolinite. The purified kaolinite was subjected to intercalation with the organic molecule potassium acetate (KAc), thus resulting in the formation of nanolayered organo complex.27 The intercalated 4943

dx.doi.org/10.1021/ie300026s | Ind. Eng. Chem. Res. 2012, 51, 4942−4951


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A schematic representation of nanokaolinite dispersion in the PAN matrix is shown in Figure 3. The homogeneous solution

Figure 3. Schematic presentation of nanokaolinite dispersion in the PAN matrix.

was then allowed to stand at room temperature to eliminate air bubbles. Before being cast, the solution mix was again sonicated for 30 min, The solution was then cast into a thin film of 400 μm thickness over a clean, leveled glass plate using a stainless steel film applicator at room temperature. Then, the film was kept for partial evaporation of solvent for 30 s and was later quenched in a water bath maintained at 10 °C. The PAN/ nanokaolinite MMMs thus formed were left in the water for 24 h. The resultant membranes were stored in 1% formalin solution to preserve them from any microbial growth. 2.4. Characterization of PAN/Nanokaolinite MMMs. 2.4.1. Membrane Hydrophilicity. Contact-angle measurements enable the identification of the extent of hydrophilicity with increasing nanokaolinite addition in PAN/ nanokaolinite MMMs. The water contact angle of the prepared membranes was measured using a goniometer (model 250-F1, Ramé-Hart Instruments (Succasunna, NJ). The static sessile drop method was employed by which the water droplet was introduced by means of a syringe on the surface of the membranes to determine the average equilibrium water contact angle, as mentioned by Marmur.28 The mean of the left and right contact angles resulted in the equilibrium water contact angle, and measurements were taken at five different locations to get the average equilibrium water contact angles of the respective membranes. 2.4.2. X-ray Diffraction Studies. X-ray diffraction (XRD) pattern has been observed for the neat PAN and the PAN/ nanokaolinite MMMs using powder X-ray diffractometer (D8 advance, Bruker, Germany) using the monochromatic source of Cu Kα radiation. The samples were studied in the 2θ range with an angle of 10−80 °C and with a constant operating voltage of 40 kV and a tube current of 30 mA. The presence of crystalline and nonmolecular particles in the MMMs can be revealed by the peaks of XRD profile. From the XRD pattern shown in Figure 7 (below), the MMMs were analyzed based on a comparison of peak intensities and crystallite size. The size, D, of the crystal particles can be determined with the help of the Scherrer equation,29 given by

Figure 2. Schematic diagram of the preparation of purified kaolinite.

kaolinite formed is designated as KPI. The kaolinite intercalated compounds were then subjected to exfoliation using a wet milling process to delaminate the layered kaolinite complex. The exfoliation of kaolinite led to the formation of xerogel-type materials that were then dried to prepare nanokaolinite. A comparative characterization between KPI and nonintercalated kaolinite, KPN, was performed by X-ray diffraction (XRD) using a D8 Advance X-ray diffractometer (Bruker, Germany). Scanning electron microscopy (SEM) images of KPN were also recorded using a JSM-6360 (JEOL) instrument. 2.3. Synthesis of PAN/Nanokaolinite MMMs. Neat PAN membrane and PAN/nanokaolinite MMMs were prepared by the phase-inversion method. PAN and various contents of kaolinite nanoparticles, namely, 2.5, 5, 7.5, and 10 wt %, were mixed by dissolving them in DMF solvent. The compositions of the casting solution for the synthesized MMMs are given in Table 1. Table 1. Compositions of PAN and PAN/Nanokaolinite MMMs casting solution composition membrane type

PAN (g)

nanokaolinite (g)

DMF solvent (mL)

PAN PK-1

4.34 4.26

− 0.10

21.7 21.7

PK-2

4.15

0.21

21.7

PK-3

4.04

0.32

21.7

PK-4

3.93

0.43

21.7

membrane description neat PAN PAN/2.5 wt % nanokaolinite PAN/5 wt % nanokaolinite PAN/7.5 wt % nanokaolinite PAN/10 wt % nanokaolinite

β=

0.9λ D cos θ

(1)

where β represents the full width at half-maximum (fwhm) of the intensity peak corresponding to 2θ and λ is the X-ray wavelength. 2.4.3. Thermal Stability Studies. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) measurements were conducted on the prepared membranes using a thermal analyzer from TA Instruments (model SDT 2000, New Castle, DE) interfaced to a computer. The thermal properties of the PAN and PAN/nanokaolinite MMMs were

The PAN/nanokaolinite mixture was stirred mechanically for nearly 3 h with subsequent heating. The solution mixture was also sonicated in an ultrasonication (Sonics) bath for 1 h to ensure uniform dispersion of nanokaolinite in the PAN matrix. 4944



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calculated using eq 2 and denoted as Jv. The percentage rejection of egg-albumin solution was determined using the equation

characterized based on TGA and DSC analyses. Initially, the test membranes were pretreated by being dried overnight in a vacuum oven at 180 °C to remove water content. Later, 10− 20 mg of the sample was loaded into an alumina crucible, and analysis was performed at temperatures between 10 and 600 °C at the heating rate of 10 °C/min under a supply of nitrogen gas. This study was carried out to compare the thermal decomposition temperature and percentage weight losses in the respective temperature range between PAN and PAN/nanokaolinite MMMs. By incorporating heat-flow signals, DSC measurements were obtained during TGA at the same heating rate and under isothermal conditions that were maintained for 10 min. The glass transition temperatures (Tg) of the formed PAN and PAN/nanokaolinite MMMs were compared to assess the improvement in thermal stability against inorganic nanokaolinite addition into the organic PAN polymer. 2.4.4. ATR-IR Spectroscopy. A Fourier transform infrared (FTIR) spectrophotometer (model Spectrum 1000, PerkinElmer, Wellesley, MA) interfaced with a continuously variable attenuated-total-reflectance (ATR) accessory consisting of a ZnSe crystal with an incident angle of 45° was used for ATR-IR spectral measurements. The ATR mode offers the possibility of concentrating on a micrometer-thin surface layer, which makes it a highly suitable tool for studying the active layer of composite membranes.30 Hence, IR spectra of all synthesized membranes (PAN, PK-1, PK-2, PK-3, and PK-4) were recorded in ATR mode, and percentage transmittance values were plotted for wavenumbers ranging from 4000 to 500 cm−1. A total of 40 scans was employed and the maximum resolution of wavenumbers of about 4 cm−1. Before ATR-IR analysis, all membrane samples were washed with deionized water and completely dried in a vacuum oven. 2.5. Performance of PAN/Nanokaolinite MMMs. 2.5.1. Permeability Studies. The pure-water permeabilities of the neat PAN and PAN/nanokaolinite MMMs were measured at 60 psi transmembrane pressure (TMP) in a dead-end standard ultrafiltration stirred cell (model Cell-XFUF076, Millipore, Billerica, MA). The membranes were initially compacted until a steady permeate volume was collected. Once the steady state had been attained, permeate volume was collected for each membrane having the effective area of 38.5 cm2 for the period of 10 min. Later, the flux analysis has been done for neat PAN membrane and also for PAN/nanokaolinite MMMs using the equation Jw1 =

V At

⎛ Cp ⎞ rejection(%) = ⎜1 − ⎟ × 100 Cf ⎠ ⎝

(3)

where Cp is the permeate concentration and Cf is the feed concentration of the egg-albumin solution. For macromolecular solutes such as proteins, the solute concentration at the membrane surface (Cm) exceeds the bulk feed concentration (Cf). This causes some additional resistance to the permeate flux (Jv), and hence, it can be expressed in terms of the total driving force of the concentration as ⎛ Cm − C p ⎞ ⎟ Jv = k ln⎜⎜ ⎟ ⎝ Cf − Cp ⎠

(4)

This expression is based on the concentration polarization model.31 The correlation for the mass-transfer coefficient (k) of eq 4 can be obtained based on the diffusive transport of the protein as D⎛ ν ⎞ k = 0.0443 ⎜ ⎟ r ⎝D⎠

0.33⎛

ωr 2 ⎞ ⎜⎜ ⎟⎟ ⎝ ν ⎠

0.8 (5)

where r is the radius of the UF cell, ω is the angular velocity of the stirrer (350 rpm), ν is the kinetic viscosity of the protein found using the relation η/ρ (where the density of egg albumin, ρ, and the viscosity of egg albumin at 20 °C, η, were taken as 1269 kg/m3 and 0.16 Pa s, respectively), and D is the bulk diffusivity of egg albumin. The diffusivity of egg albumin was found to be 3.557 × 10−8 from the equation D = 2.74 × 10−9M −1/3

(6)

where M is the molecular weight of egg albumin. The concentration polarization also affects the observed rejection efficiency (Robs) of protein as calculated by eq 3. Hence, the real rejection efficiency (Rreal) of the membranes in the pressure-driven transport process can be derived from the membrane concentration (Cm) as ⎛ Cp ⎞ R real (%) = ⎜1 − ⎟ × 100 Cm ⎠ ⎝

(7)

2.5.3. Flux Recovery Ratio. The flux recovery ratio (FRR) was estimated to compare the fouling resistance ability32 of neat PAN and PAN/nanokaolinite MMMs. The FRR is the ratio between initial pure-water flux (Jw1) and the final pure-water flux (Jw2) found after protein separation. Once the protein separation had been completed using UF, the membranes were allowed to run using deionized water for 20 min so that the membranes were completely washed. Then, the cleaned membranes were run using ultrapure water in the UF cell at 60 psi TMP to measure the final permeate flux (Jw2). The FRRs of PAN and PAN/nanokaolinite MMMs were calculated using the equation

(2)

where V is the permeate volume in liters, A is the membrane effective area in square meters, and t is the operation period in hours. 2.5.2. Protein Separation Studies Based on Transport Properties. The UF performance of the MMMs was evaluated based on protein separation studies. For these experiments, 1000 ppm egg-albumin solution was prepared by dissolving egg-albumin powder in sodium phosphate buffer (0.5 M, 6.8 pH). The protein permeates of neat PAN and PAN/nanokaolinite MMMs were collected at 60 psi TMP, for which the absorbance was recorded using a UV spectrophotometer (model Jasco UV/vis/NIR 570 V, Tokyo, Japan) at 485 nm. From the permeate volume, the protein flux was also

J FRR(%) = w2 × 100 Jw1

(8)

2.5.4. Treatment of Dye Solution. A synthetic rhodamine B dye solution of 100 ppm was used as the feed solution. The feed dye solution was run using neat PAN and PAN/ nanokaolinite MMMs in the UF stirred cell at 60 psi. After 4945



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reaching steady state, the dye permeate of each membrane was collected, and the absorbance was recorded at 485 nm using a UV spectrophotometer. Then, the concentration of each permeate dye solution was calculated to determine the Rdye using the equation ⎛ Cp ⎞ R dye (%) = ⎜1 − ⎟ × 100 Cf ⎠ ⎝

(9)

To improve the effect of dye removal, polyelectrolyte-assisted UF was carried out in conjugation with MMMs. The polyelectrolyte PDDA is a water-soluble chelating polymer that acts as a complexing agent by interacting with the dye compounds. The interaction of ion-rich PDDA ligand and rhodamine B dye leads to an increase in the molecular weight, and an improvement in the rejection efficiency can be observed. Membrane-based UF easily allows this separation by means of the liquid-phase polymer-based retention (LPR) technique.33,34 In this study, 100 ppm dye solution was dissolved completely in 1 wt % PDDA dissolved in ultrapure water, and the run was performed in the same UF setup. The Rdye value of PDDAcomplexed rhodamine B solution was determined using eq 6. Then, the Rdye values of rhodamine B and PDDA-mixed rhodamine B were compared between neat PAN and PAN/ nanokaolinite MMMs. The permeate fluxes of rhodamine B in the absence and presence of PDDA were also measured and compared simultaneously.

Figure 4. XRD patterns of KPN and KPI samples.

3. RESULTS AND DISCUSSION 3.1. Effect of Purification and Intercalation on Raw Clay. The purification process reduced the iron content and free quartz, as is evident from the results of the sieve and chemical analyses on purified kaolinite as shown in Table 2. Table 2. Characterization of Purified Kaolinite characteristics

raw kaolin

Sieve Analysis >45 μm (wt %) 1.8 Chemical Analysis (wt %) SiO2 48.40 Al2O3 35.62 Fe2O3 1.42 loss on ignition 13.95 Specific Surface Area CEC (mequiv/100 g) 10.9

purified kaolinite 0.2 46.90 38.11 0.16 14.23

Figure 5. SEM images of KPN and KPI samples.

added in PK-1 MMM. The increasing trend continued to 7.5 wt % kaolinite, as shown in Figure 6, with a contact angle of 58.7°. However, in PK-4 MMM having 10 wt % kaolinite, the equilibrium contact angle increased slightly to 61.6°. This could be due to the steric hindrance and electrostatic interactions among kaolinite nanoparticles and between kaolinite and PAN. Subsequently, this has an effect on the flux and antifouling properties of the membrane.35 Zhang et al. reported that enhanced hydrophilicity of membrane surfaces can effectively reduce membrane fouling.36 3.3. Effect of PAN/Nanokaolinite MMMs on Crystallinity. The XRD diffractograms of neat PAN and PAN/ nanokaolinite MMMs were recorded as shown in Figure 7. The presence of kaolinite was confirmed by the intense peak in the range of 12.25−12.35° 2θ in all PAN/nanokaolinite MMMs. As the kaolinite content increased to 10 wt % in PK-4 MMM, two characteristic peaks at ∼25° and ∼26.5° 2θ corresponding to kaolinite were observed. This indicates an increase in the degree of crystallinity in PK-4 MMM. The effect of crystallinity provides mechanical and thermal stability to PK-4 MMM that

11.7

The increase in CEC values also confirms that the iron impurities were removed from the raw kaolin. The kaolinite intercalation with KAc in sample KPI was confirmed by XRD in Figure 4, as the basal spacing was increased considerably from 0.704 to 1.404 nm. The intercalation step helped make the kaolinite reactive and offered surface properties essential for nanoscale kaolinite. Figure 5 displays SEM images from which the reduction in particle size of KPI compared to KPN can be observed. 3.2. Effect of PAN/Nanokaolinite MMMs on Hydrophilicity. The equilibrium contact angle of the PAN/ nanokaolinite MMMs was observed to be less than that of the neat PAN membrane. It is evident that the presence of hydroxyl groups in kaolinite imparts a higher hydrophilicity to PAN/nanokaolinite MMMs. The contact angle of neat PAN decreased from 70.2° to 66.9° when 2.5 wt % of kaolinite was 4946



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MMMs are shown in the TGA profiles in Figure 8. The neat PAN membrane showed decomposition in the temperature

Figure 6. Effect of PAN/nanokaolinite MMMs on hydrophilicity. Figure 8. TGA profiles of PAN and PAN/nanokaolinite MMMs.

range of 296−310 °C. The decomposition stage of almost all PAN/nanokaolinite MMM sample also occurred at the similar but slightly higher temperature range of 297−315 °C, nearly reaching 330 °C for PK-4 MMM. Thus, a significant increase in thermal decomposition temperature could be possible with higher loadings of nanokaolinite. However, the weight loss due to decomposition of the samples decreased considerably with increasing kaolinite content. From 17.5 wt % loss for the neat PAN membrane, a continuous decrease in weight loss to 2.48 wt % was observed for the PAN/ nanokaolinite MMMs, as reported in Table 4. This suggests a Table 4. Thermal Behavior of PAN/Nanokaolinite MMMs decomposition stage

Figure 7. XRD patterns of PAN and PAN/nanokaolinite MMMs.

can be seen by its thermal characteristics. However, complete intermolecular bonding was not effective this membrane, and hence, the contact angle was less than that for PK-3. The broadening of the peak corresponding to neat PAN at ∼16.5° 2θ in PK-3 MMM indicates that the kaolinite nanoparticles were completely miscible with PAN. Table 3 lists the crystallite

peak intensity (deg, 2θ)

fwhm (deg)

crystallite size (nm)

PK-1 PK-2 PK-3 PK-4

12.36 12.50 12.15 12.35

0.24 0.59 1.02 0.74

29.42 11.97 6.92 9.54

temperature range (°C)

weight loss (%)

PAN PK-1 PK-2 PK-3 PK-4

296−310 298−311 300−315 297−312 301−330

17.5 8.05 7.96 5.47 2.48

slight improvement in thermal stability owing to the addition of inorganic kaolinite to the organic PAN. The glass transition temperatures (Tg) of neat PAN and PAN/nanokaolinite MMMs are described by DSC thermograms (Figure 9). The increase in Tg to 136.9 °C for PK-1 and to a higher value of about 160 °C for PK-4 compared 132.7 °C for neat PAN polymer is shown in Figure 10. The higher Tg values can be engineered by altering the degree of branching or cross-linking in PAN through the addition of inorganic kaolinite nanoparticles, and this provides necessary evidence for the improvement in thermal characteristics being rendered by these nanoparticles.38 3.5. ATR-IR Spectroscopy of PAN/Nanokaolinite MMMs. Figure 11 shows the ATR-IR spectra of neat PAN and PAN/nanokaolinite MMMs. The presence of kaolinite in samples PK-1−PK-4 was revealed by the IR band at 912 cm−1 indicating the deformation of the inner surface −OH group in Al2OH such that hydrophilicity of MMMs was significantly increased. The IR spectra of PAN/nanokaolinite hybrid nanofiber

Table 3. Crystallite Size of PAN/Nanokaolinite MMMs membrane type

membrane type

size of each PAN/nanokaolinite MMM, along with the fwhm and peak intensity. Also, it can be observed that all of the XRD profiles of the MMMs in Figure 7 are complex because of the presence of several overlapping peaks, and the reason for this would be the positioning of the crystal at relatively short distances.37 3.4. Thermal Behavior of PAN/Nanokaolinite MMMs. The thermal characteristics of neat PAN and PAN/nanokaolinite 4947



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the presence of PAN in all of the PAN/nanokaolinite MMMs. The severity in the drift of this band indicates an increase in PAN content. Another IR band characteristic of the strong interaction between the nitrile group of PAN and the hydroxyl group of kaolinite nanoparticles can be attributed by the presence of a strong band between 1100 and 1115 cm−1. Also, the band intensity seems to increase with increasing kaolinite content. However, PK-4, containing 10 wt % of kaolinite, shows a slight decrease in intensity compared to PK-3, resulting in lower hydrophilicity. The characteristic band at 1114 cm−1 reflects the presence of silica ascribable to kaolinite nanoparticles and also suggests the longitudinal mode stretching of Si−O bond, which might have influenced a better interaction between PAN and nanokaolinite. This interaction is favorable for the even distribution of kaolinite and the stability of the PAN/nanokaolinite blend solution before membrane formation. 3.6. Performance of PAN/Nanokaolinite MMMs Based on Permeation. The pure-water flux of PAN/nanokaolinite MMMs is comparatively higher than that of neat PAN membrane. The permeability flux increases from 122.8 to 264.93 L m−2 h−1 as the nanokaolinite addition increases to 7.5 wt %. The two factors influencing the improvement in flux are (1) hydrophilicity and (2) the membrane formation process. First, the hydrophilicity of MMMs is enhanced by means of the hydroxyl group (−OH) in the kaolinite chemical structure. This increase in hydrophilicity subsequently results in an improvement in flux and also reduces membrane fouling. Second, the method of phase inversion employed during the membrane formation process has its effect in enhancing the flux. In brief, during the phase-inversion method, an exchange between solvent and nonsolvent occurs. The adsorption and hydrophilicity of the incorporated kaolinite nanoparticles lead to an increase in the exchange rate, which, in turn, increases the size of the pores in the membrane.40 Therefore, the addition of inorganic nanokaolinite to PAN/nanokaolinite MMMs contributes to the flux enhancement. However, from Figure 12, it is

Figure 9. DSC patterns of PAN and PAN/nanokaolinite MMMs.

Figure 10. Effect of PAN/nanokaolinite MMMs on glass transistion temperature (Tg).

Figure 12. Effect of PAN/nanokaolinite MMMs on pure-water flux and egg-albumin flux and rejection. Figure 11. ATR-IR spectra of PAN and PAN/nanokaolinite MMMs.

observed that, for PK-4, there is a slight decrease in flux from 264.93 to 239.68 L m−2 h−1 due to agglomeration of the inorganic nanoparticles in the membrane casting solution. This causes a more viscous solution that severely hinders the exchange between solvent and nonsolvent during the phase

mats were studied earlier,39 in which the existence of kaolinite and PAN in the nanofiber mats was confirmed by the appearance of absorption bands at 1099 and 2244 cm−1. The similar characteristic band at 2250 cm−1 in Figure 11 confirms 4948



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inversion and slows the precipitation of the membrane.41 Hence, a membrane with fewer pores is formed, leading to a decrease in flux. 3.7. Antifouling Properties of PAN/Nanokaolinite MMMs. Recyclability and reproducibility are essential parameters for the large-scale continuous application of PAN/ nanokaolinite MMMs toward wastewater treatment. The antifouling properties of the membranes were evaluated based on FRR values. The FRRs of neat PAN and PAN/nanokaolinite MMMs are listed in Table 5. It is observed that the FRRs of

membrane concentration was observed. Table 6 presents the membrane concentrations and the difference between the Rreal and Robs values for the protein. Table 6. Protein Rejection through PAN/Nanokaolinite MMMs rejection efficiency (%)

Table 5. Membrane Performance As a Function of Nanokaolinite Loading in PAN Membrane membrane type

pure-water flux (L m‑2 h‑1)

contact angle (deg)

FRR (%)


122.8 195.43 219.74 264.93 236.68

70.2 66.9 62.5 58.7 61.6

52 59.03 69.42 71.18 55.93

membrane type

Cm (ppm)

Robs

Rreal


2228.5 2294.2 2276 2245.3 2235.5

88 92.7 91 89.2 88.5

94.6 96.7 95.14 95.15 96.13

3.9. Performance of PAN/Nanokaolinite MMMs Based on Dye Removal. When rhodamine B dye solution was employed as the feed, the Rdye value of the neat PAN membrane was observed as 13.3%, whereas PK-1 MMM showed an Rdye value as high as 30%. However, when PDDA-assisted rhodamine B dye solution was treated using the same membranes, the Rdye value of the neat PAN membrane increased to 40%, whereas an appreciable Rdye value of 83.8% was noted for PK-1 MMM. Figure 13 shows the Rdye values of

PAN/nanokaolinite MMMs increased continuously due to nanokaolinite addition to neat PAN membrane. The FRR reached a maximum value of 71.18 for PK-3 MMM, and this increase is well correlated with the higher hydrophilicity of the respective MMM. Thus, the membrane hydrophilicity and FRR confirmed the higher antifouling properties of PK-3 MMM. Because the FRR values of all of the PAN/ nanokaolinite MMMs were higher than that of neat PAN, it can be concluded that the MMMs have better antifouling abilities. 3.8. Performance of PAN/Nanokaolinite MMMs Based on Protein Rejection. From protein rejection studies, the rejection efficiency of egg-albumin solution was determined for MMMs and neat PAN. Owing to the properties of the MMMs, the selectivity was not compromised with that of the flux, which means that both the rejection efficiency and the permeability flux of PAN/nanokaolinite MMMs remained higher than those of the neat PAN membrane. Typically, these superior properties are attributed to the better-structured kaolinite nanoparticles in the PAN/nanokaolinite MMMs. The influence of inexpensive nanokaolinite on improved rejection of proteins should be suitable for biomolecule separations in the food and biotechnogical industries. The protein rejection efficiency of PAN membrane was 88%, from which it reached 92.7% upon 2.5 wt % addition of kaolinite in PK-1 MMM (Figure 12). On the other hand, when the kaolinite content reached 5 wt %, the protein rejection efficiency of the PK-2 MMM decreased to 91%, and the decreasing trend continued at the high nanokaolinite loading of 10 wt %. The flux of egg albumin (Jv) was also found simultaneously, and the results corresponded to the pure-water flux as observed before (Figure 12). This would suggest that the inorganic kaolinite loading significantly modifies the polymer network structure and, consequently, the transport properties.40 The real rejection efficiencies (Rreal) of the protein solute were determined for neat PAN and PAN/nanokaolinite MMMs. It was inferred that the R real values of both neat PAN and PAN/ nanokaolinite MMMs remained higher than the Robs values. This is due to concentration polarization, and the effect remains higher for dead-end ultrafiltration. However, no effect in terms of a decrease in permeate flux with respect to

Figure 13. Effect of PAN/nanokaolinite MMMs on the percentage rejection of rhodamine B and PDDA−rhodamine B solution.

both PAN and MMMs upon treatment of rhodamine B dye solution with and without PDDA. It is also evident that PDDA complexation results in a decrease of the permeate flux due to the increase in dye rejection efficiency (Figure 14). An appreciable extent of dye color removal was also evident in the treatment of the PDDA−rhodamine B solution. The reason for the increase in dye removal efficiency is the high ion-exchange capacity of PDDA chloride and the degree of cross-linking between rhodamine B and PDDA chloride.34 The improvement in rejection efficiency is because of the increase in the molecular size of the feed solution. This property was utilized for the removal of chromium metal ions due to the higher binding ability of macroligand PDDA.42 It can also be concluded that the extent of removal of dye compounds depends on the formation of macromolecules using the PDDA complexing agent, as well as the pore size and porosity of the membranes. 4949



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(3) Robeson, L. M.; Burgoyne, W. F.; Langsam, M.; Savoca, A. C.; Tien, C. F. High performance polymers for membrane separation. Polymer 1994, 35, 4970. (4) Freeman, B. D. Basis of permeability/selectivity trade-off relations in polymeric gas separation membranes. Macromolecules 1999, 32, 375. (5) Ahn, J.; Chung, W. J.; Pinnau, I.; Michael, D. Polysulfone/silica nanoparticle mixed-matrix membranes for gas separation. J. Membr. Sci. 2008, 314, 123. (6) Peng, F. B.; Lu, L. Y.; Sun, H. L.; Wang, Y. Q.; Liu, J. Q.; Jiang, Z. Y. Hybrid organic inorganic membrane: Solving the tradeoff between permeability and selectivity. Chem. Mater. 2005, 17, 6790. (7) Uragami, T.; Okazaki, K.; Matsugi, H.; Miyata, T. Structure and permeation characteristics of an aqueous ethanol solution of organic− inorganic hybrid membranes composed of poly(vinyl alcohol) and tetraethoxysilane. Macromolecules 2002, 35, 9156. (8) Yang, Y.; Zhang, H.; Wang, P.; Zheng, Q; Li, J. The influence of nano-sized TiO2 fillers on the morphologies and properties of PSF UF membrane. J. Membr. Sci. 2007, 288, 231. (9) Morooka, S.; Kusakabe, K. Microporous inorganic membranes for gas separation. MRS Bull. 1999, 24, 25. (10) Monash, P.; Majhi, A.; Pugazhenthi, G. Separation of bovine serum albumin (BSA) using γ-Al2O3−clay composite ultrafiltration membrane. J. Chem. Technol. Biotechnol. 2010, 85, 545. (11) Arthanareeswaran, G.; Thanikaivelan, P. Fabrication of cellulose acetate−zirconia hybrid membranes for ultrafiltration applications: Performance, structure and fouling analysis. Sep. Purif. Technol. 2010, 74, 230. (12) Iqbal, A.; Yusof, Z. A. M.; Beg, M. D. H. Fabrication of Polymer Based Mix Matrix MembraneA Short Review. Int. J. Basic Appl. Sci. 2010, 10, 17. (13) Pabby, A. K.; Rizvi, S. S. H.; Sastre, A. M. Handbook of Membrane Separations; CRC Press: London, 2008. (14) Saufi, S. M.; Ismail, A. F. Development and characterization of polyacrylonitrile (PAN) based carbon hollow fiber membrane. Songklanakarin J. Sci. Technol. 2002, 24, 843. (15) Scharnagl, N.; Buschatz, H. Polyacrylonitrile (PAN) membranes for ultra- and microfiltration. Desalination 2000, 139, 191. (16) Nouzak, K.; Nagata, M.; Arai, J.; Idemoto, Y.; Koura, N.; Yanagishita, H.; Negishi, H.; Kitamoto, D.; Ikegami, T.; Haraya, K. Preparation of polyacrylonitrile ultrafiltration membranes for wastewater treatment. Desalination 2002, 144, 53. (17) Benco, L.; Tunega, D.; Hafner, J.; Lischka, H. Upper limit of the O−H−O hydrogen bond: Ab initio study of the kaolinite structure. J. Phys. Chem. B 2001, 105, 10812. (18) Klein, C.; Hurlbut, C. S., Jr. Manual of Mineralogy (After J. D. Dana), 20th ed.; Wiley: New York, 1985; p 28. (19) Breck, D. W. Zeolite Molecular Sieves; Robert E. Krieger Publishing Company: Malabar, FL, 1984, 314. (20) Bellotto, M.; Gualtieri, A.; Artioli, G.; Clark, S. M. Kinetic study of the kaolinite−mullite reaction sequence. Part I: Kaolinite dehydroxylation. Phys. Chem. Miner. 1995, 22, 207. (21) Lue, S. J.; Chien, C. F.; Mahesh, K. P. O. Pervaporative concentration of ethanol−water mixtures using heterogeneous polydimethylsiloxane (PDMS) mixed matrix membranes. J. Membr. Sci. 2011, 384, 17−26. (22) Hillock, A. M. W.; Miller, S. J.; Koros, W. J. Crosslinked mixed matrix membranes for the purification of natural gas: Effects of sieve surface modification. J. Membr. Sci. 2008, 314, 193. (23) Choi, Y. S.; Wang, K. H.; Xu, M.; Chung, I. J. Synthesis of exfoliated polyacrylonitrile/Na−MMT nanocomposites via emulsion polymerization. Chem. Mater. 2002, 14, 2936. (24) Sugahara, Y.; Satokawa, S.; Kuroda, K.; Kato, C. Preparation of a kaolinite−polyacrylamide intercalation compound. Clay Miner. 1990, 38, 137. (25) Arthanareeswaran, G.; Sriyamuna Devi, T. K.; Raajenthiren, M. Effect of silica particles on cellulose acetate blend ultrafiltration membranes: Part I. Sep. Purif. Technol. 2008, 64, 38. (26) Aguilera, N. H.; Jackson, M. L. Iron oxide removal from soils and clays. Soil Sci. Soc. Am. Proc. 1953, 17, 359.

Figure 14. Effect of PAN/nanokaolinite MMMs on the permeate flux of rhodamine B and PDDA−rhodamine B solution.

4. CONCLUSIONS Kaolinite nanoparticles were successfully synthesized by the intercalation/exfoliation method and incorporated into polyacrylonitrile (PAN) to prepare novel PAN/nanokaolinite mixed-matrix membranes (MMMs). The characterization results showed the appropriate nanokaolinite loading and dispersion in PAN. Further, the hydrophilic properties of PAN/ kaolinite MMMs increased because of the hydrophilic-grouprich nanokaolinite addition, and hence, reduced membrane fouling occurred. The PAN/nanokaolinite MMM containing about 10 wt % nanokaolinite was more thermally stable than the other PAN/nanokaolinite MMMs and the neat PAN membrane. The characterization results correlated well with the UF experiments, as was evident from the improvement in permeability flux as a consequence of hydrophilicity. Excellent performance in terms of protein and dye rejection was also identified for PAN/nanokaolinite MMMs. Moreover, these MMMs showed dye removal as high as 83.8% when separating PDDA-complexed dye solution. The enhancement of the permeability (productivity) and rejection efficieny (selectivity) due to nanokaolinite addition would certainly suggest the applicability of PAN/nanokaolinite MMMs to rapid and effective industrial wastewater treatment. Cost-effective PAN/ nanokaolinite MMMs could also be applied for other energyintensive operations such as pervaporation and gas separation.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +91431-2503118. Fax: +91431-2500133. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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Preparation and Performance Evaluation of Nanokaolinite-Particle

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