organically modified montmorillonite nanocomposite membranes in removal of pesticides

Journal of Membrane Science 382 (2011) 135–147

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Preparation, characterization and performance of polyethersulfone/organically modified montmorillonite nanocomposite membranes in removal of pesticides Negin Ghaemi a,b , Sayed S. Madaeni a,∗ , Abdolhamid Alizadeh c , Hamid Rajabi d , Parisa Daraei a a

Membrane Research Centre, Department of Chemical Engineering, Razi University, Tagh Bostan, 67149 Kermanshah, Iran Department of Chemical Engineering, Kermanshah University of Technology, Kermanshah, Iran Nanoscience and Nanotechnology Research Centre (NNRC), Department of Chemistry, Razi University, Tagh Bostan, 67149 Kermanshah, Iran d Department of Soil Mechanics and Geotechnical Engineering, Tarbiat Modares University, Tehran, Iran b c

a r t i c l e

i n f o

Article history: Received 16 February 2011 Received in revised form 31 July 2011 Accepted 2 August 2011 Available online 9 August 2011 Keywords: Nanofiltration Polymer–clay nanocomposite membranes Organically modified Montmorillonite Pesticide Polyethersulfone

a b s t r a c t Nanocomposite membranes containing polyethersulfone (PES) and organically modified montmorillonite (OMMT) were prepared by a combination of solution dispersion and wet-phase inversion methods and accordingly, the effect of OMMT addition to the properties and performance of fabricated nanofiltration membranes was investigated. The membranes were characterized by contact angle measurement, scanning electron microscopy (SEM), atomic force microscopy (AFM), transmission electron microscopy (TEM), X-ray diffraction (XRD), mechanical strength evaluation, thermogravimetric analysis (TGA), and zeta potential. The performance of the membranes was elucidated by the removal of pesticides (nitrophenols (NPs)) in neutral and acidic pHs. The hydrophilicity of the membranes was strongly enhanced by increasing the OMMT concentration. The SEM and AFM images showed that addition of OMMT to the casting solution resulted in nano-structure membranes with a thinner skin layer and a smaller surface pore size. XRD patterns revealed the formation of intercalated and exfoliated layers of mineral clays in the PES matrix which was also confirmed by TEM images. The addition of OMMT improved the mechanical properties and thermal stability of the membranes. Moreover, the pure water flux, permeation and, rejection of NPs were significantly improved. The performance of fabricated NF membranes in removal of NPs varied depending on the solute and membrane properties as well as solution condition. Finally, a comparison between fabricated membranes and a commercial NF membrane (NF45, Dow Filmtec) proved that the OMMT addition is a convenient procedure for producing nanocomposite membranes with superior characterization and performance. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Water purification is the process of removing contaminants including pesticides from surface or groundwater to make it safe and palatable for human consumption. Pesticides are classified as hazardous substances with a chemical structure and physicochemical properties differing from one compound to another and very difficult to be removed from aquatic medium. Nitrophenols (NPs) and 2,4-dinitrophenol in particular, are listed as priority pollutants by the US Environmental Protection Agency (EPA) because of their strong phototoxic activity [1]. They mostly originate from wastewater discharges from the dyes, drugs, fungicides, pesticides, and ammunition industries as well as from various chemical manufacturing plants. NPs are formed in the atmosphere by OHinitiated photo-oxidation of aromatic hydrocarbons and also in

∗ Corresponding author. Tel.: +98 831 4274530; fax: +98 831 4274542. E-mail address: [email protected] (S.S. Madaeni). 0376-7388/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2011.08.004

diesel exhaust particles. Having immediate toxic effects on the nervous system, NPs are largely diffused in water and soil by gravitational settling and wet deposition by rain and snow [2,3]. Pesticides are generally removed by oxidation (by chlorine, ozone, potassium permanganate, hydrogen peroxide, or chlorinated lime) and adsorption on activated carbon or on a natural sorbent (peat, clay, humic substances, and bentonites). Conventional treatment methods were found to be insufficient or even produce toxic by-products, and the demands on the performance of removal processes become more and more stringent as well. The utility of membrane processes in treating pesticide-contaminated water is a promising alternative [4,5] due to low costs, simplicity of operation, and lower usage of chemicals [6]. Currently, nanofiltration membranes have become the most important advancement in the membrane technology for removal of multivalent ions and organic compounds (100–1000 Da) especially pesticides from water and wastewater sources. Thus, in recent years, research has been carried out on new membrane materials containing nanodimensions resulting in formation of nanocomposite material [7].

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In these nanocomposites, the polymer acts as a guest in the inorganic particles with dimensions of the order of nanoscale and their interaction results in improvements in the material properties, such as higher moduli, increased strength, heat resistance, and increased biodegradability for biodegradable polymers. Among the various inorganic fillers (zeolites, inorganic, ceramic oxides, etc.) layered silicalites are qualified for specific attention because of their ability of being dispersed in the polymeric matrices at a nano scale. The most used clays in polymer–clay nanocomposite (PCN) materials are those generally containing clay minerals of the smectite group, particularly those containing montmorillonite (MMT). MMT is a hydrophilic clay; its addition to a very low content (<10 wt.%) can be useful not only to produce PCNs with the already mentioned improved properties, but also to enhance the hydrophilicity of the material [7–9]. Polyethersulfone (PES) is a polymer widely used in membrane preparation for various applications, but its employment for aqueous solutions is restricted due to hydrophobicity of this polymer [10]. This important characteristic could be improved by modification of PES with additives. Wet-phase inversion technique is the most employed method for membrane production. On the other hand, the most common methods used in PCN technology are in situ polymerization, melt intercalation, and solution dispersion. In the latter method, the clay mineral is exfoliated in single layers in a solvent medium, and polymer chains are intercalated into these clay mineral layers. With these two techniques, a porous and a more hydrophilic membrane can be produced [7–9]. Recently, the largest number of studies is devoted to production of nanocomposite membrane fuel cells by using Nafion® with protonated MMT [11], with bio-functionalized MMT [12], and with sulfonated MMT [13] and by using sulfonated poly(ether ether ketone) with organophilic MMT [14,15] and with sulfonated MMT [16]. The use of organically modified montmorillonite (OMMT) could also be noted as an inorganic filler to improve the performance of sulfonated PES [17] and poly (2,6-dimethyl1,4-phenylene oxide) [18] fuel cell membranes. Other researchers have investigated the effect of using poly(vinylidene fluoride) and poly(lactic acid) with organophilic MMT [19], polysulfone with MMT [20,21], and a novel microporous membrane from micaintercalated nylon 6 [22] on the performance of nanocomposite membranes. Despite the potentiality of nanocomposite membranes in being employed in the field of nanofiltration, only few studies have been devoted to these membranes [22]. Therefore, in this work, we report on the preparation and characterization of nanocomposite membranes based on PES and different concentrations of commercially available OMMT by a combination of solution dispersion and wet-phase inversion methods. Performance of fabricated membranes was evaluated in terms of the water flux, permeation, and rejection of the solutions containing the selected nitrophenols (3,5-dinitrosalicylic acid (DNSA) and 2,4dinitrophenol (DNP)) at different solution conditions. Finally, the ability of nanocomposite membranes in the removal of pesticides was compared with a commercial NF membrane.

2. Experimental 2.1. Apparatus All experiments were carried out at room temperature (25 ± 2 ◦ C) in a batch type, dead end, stirred, Nanofiltration cell with a diameter of 4.5 cm and effective membrane filtration area of 11.94 cm2 fitted with Teflon coated magnetic paddle. The top of the cell contained a gas inlet. Nitrogen gas was used to pressurize the cell to an operating pressure of 4.6 × 105 Pa (66.71736 psi). A constant agitation of speed 5.0 Hz (300 rpm) was used during all of

Scheme 1. Chemical structure of PES, DNSA and DNP.

the experiments in order to reduce concentration polarization of the membranes. 2.2. Materials Polyethersulfone (PES Ultrason E6020P, MW = 58,000 g/mol) supplied by BASF Company (Germany) was employed as the base polymer. N,N-Dimethylformamide (DMF) and polyvinylpirrolidone (PVP) with 25,000 g/mol molecular weight from Merck were used in the casting solution as the solvent and pore former, respectively. Organically treated MMT clays (trade name: Cloisite® 15A) were purchased from Southern Clay Products Inc., Gonzales, TX, USA. NF45 nanofiltration membrane was purchased from Dow Filmtec. 3,5-Dinitrosalicylic acid (C7 H4 N2 O7 , pKa = 2.6) and 2,4dinitrophenol (C6 H4 N2 O5 , pKa = 4.0) were obtained from Sigma and Aldrich, respectively. Each experiment was carried out twice and double distilled water was used throughout the experiments. The chemical structure of PES and pesticides are illustrated in Scheme 1. 2.3. Feed Nanofiltration was carried out with a solution containing each nitrophenol with concentration of 0.1 mM and at solution pH values of 7.0 and 4.5. These pH values are representative of two situations: 3,5-dinitrosalicylic acid (DNSA) molecules due to their pKa are ionized and therefore, negatively charged at both neutral and acid solution pH values. Although 2,4-dinitrophenol (DNP) molecules are also ionized and negatively charged in neutral solution because of acidic dissociation constant (pKa = 4.0); the number of the protonated DNP molecules is increased in acidic solution resulting in decline in the negative charge of DNP molecules. At the same time it would not make sense to test higher pH values as NPs dissociation of acid groups has already occurred at pH 7.0. The two pH values chosen for the experiments also represent realistic pH limits for actual application in water purification. 2.4. Preparation of pristine PES membrane A casting solution containing PES (20 wt.%) dissolved in DMF was prepared using PVP (at 2 wt.% constant concentration) as pore former [23] by stirring for 8 h. The stirring was carried out at 5.0 Hz

N. Ghaemi et al. / Journal of Membrane Science 382 (2011) 135–147 Table 1 Compositions of PES and PES/OMMT casting solutions. Sample name

PES (wt.%)

PVP (wt.%)

DMF (wt.%)

OMMT (wt.%)

PES OMMT0.5 OMMT1 OMMT1.5 OMMT2 OMMT3 OMMT4 OMMT5 OMMT6 OMMT7 OMMT8 OMMT9 OMMT10

20 20 20 20 20 20 20 20 20 20 20 20 20

2 2 2 2 2 2 2 2 2 2 2 2 2

78.0 77.5 77.0 76.5 76.0 75.0 74.0 73.0 72.0 71.0 70.0 69.0 68.0

– 0.5 1 1.5 2 3 4 5 6 7 8 9 10

(300 rpm) and 50 ◦ C. The solution was sprinkled and casted on a glass plate substrate by a homemade casting knife with 250 ␮m thicknesses and then was moved toward non-solvent bath for immersion precipitation at room temperature. The non-solvent was mixture of water (90%v) and 2-propanol (10%v). After primarily phase separation and formation of membrane, it was stored in water for 24 h to guarantee complete phase separation. This allows the water soluble components in the membrane to be leached out. As the final stage, membrane was dried by placing between two sheets of filter paper for 24 h at room temperature. The dried membrane thickness was measured by a digital caliper device which confirmed that the membrane thickness was maintained about 250 ␮m. The rejection of NaCl and Na2 SO4 by PES membrane were respectively about 30% and 81% with a feed concentration of 1000 mg/L under 4.6 × 105 Pa (66.71736 psi). 2.5. Preparation of nanocomposite membranes Nanocomposite membranes were prepared by the combination of the wet-phase inversion method and the solution dispersion technique. Dispersions consisting of different OMMT contents (Table 1) and 20 wt.% PES in DMF were prepared under vigorous mechanical stirring for 8 h and at 50 ◦ C. Firstly, 1/4 of 20 wt.% PES and 2 wt.% PVP were added and after solubilization, 1/4 of the total amount of OMMT was added. This procedure was continued in this order until both additions completed 100%, as previously reported by Anadão et al. [8]. The casting and immersion process of nanocomposite membranes were carried out exactly similar to the virgin PES membrane. 2.6. Characterization of membranes Structure of the prepared membranes was examined by a Philips scanning electron microscope (SEM, XL30, The Netherlands). The samples of the membranes were frozen in liquid nitrogen and fractured. After sputtering with gold, they were viewed with the microscope at 17 kV. Atomic force microscopy was employed to analyze the surface morphology and roughness of the membranes. The AFM apparatus was Dual ScopeTM scanning probe-optical microscope (DME model C-21, Denmark). Small squares of the membranes (approximately 1 cm2 ) were cut and fixed using mutual sticky tapes on glass substrate. The membrane surfaces were examined in a scan size of 500 nm × 500 nm. The mean pore size and also surface roughness parameters of the membranes which are expressed in terms of the mean roughness (Sa ), the root mean square of the Z data (Sq ) and the mean difference between the highest peaks and lowest valleys (Sz ) were obtained by SPM software (version 1.4.0.6) which is provided by manufacturer for quantitative analysis of the images.

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Nanocomposite morphology was also studied by transmission electron microscopy (TEM). Before TEM examination, the membranes were encapsulated in epoxy resins and TEM specimens were cut from these epoxy blocks at room temperature (25 ◦ C) by using an ultramicrometer with a diamond knife and were stained with uranyl acetate. TEM images were acquired by a Zeiss EM900 (Germany) transmission electron microscope at 80 kV acceleration voltage. XRD patterns of nanocomposite membranes were obtained by a X-ray diffractometer (Philips X Pert MPD diffractometer, Holland) under the following conditions: 40 kV–30 mA; Co K␣ radiation ˚ at the rate of 0.02◦ /s in the range of 2–20◦ (2). ( = 1.78897 A); Thermogravimetry was carried out by TGA (PL-TGA, England) in the range of 25–700 ◦ C under air atmosphere with a flow of 50 mL/min, heating rate of 10 ◦ C/min and sample mass of about 5 mg. The mechanical properties of the prepared membranes were tested according to ASTM D882 using Santon STM-20 tensile test machine. All samples were cut to the standard shape at room temperature (25 ◦ C) before testing. For each test, three samples were used. The average values are reported for tensile strength (MPa) and strain at break (%). The static contact angle between water and membrane surface was directly measured at room temperature using a contact angle measuring instrument (G10, KRUSS, Germany) for the evaluation of membrane hydrophilicity. The contact angle was measured at five random locations for each sample using de-ionized water and average was reported to minimize the experimental error. To determine the charge of the membrane surface, the zeta potential was determined from streaming potential measurements by Electro Kinetic Analyzer (EKA 1.00, Anton-Paar, Swiss) equipped with a plated sample cell. Membranes were cut squarely in 2 cm × 2 cm size plates. The measurements were carried out at 25 ◦ C in KCl solution (0.001 M) with poly (methyl methacrylate) (PMMA) reference plate (dimension of reference plate was about 50 mm × 38 mm × 10 mm). Zeta potential measurements were obtained at solution pH values of 5.11 and 7.15. 2.7. Membrane performance measurements The prepared membranes were cut into desired size needed for fixing it up in the NF kit and initially pressurized with distilled water at 5 × 105 Pa (72.51887 psi) for 1 h; After compaction, the pressure was reduced to the operating pressure value of 4.6 × 105 Pa (66.71736 psi); the pure water flux (PWF) was collected for another 30 min and calculated using the Eq. (1) [24,25]. PWF =

Q AT

(1)

where PWF = water flux (kg/m2 h); Q = quantity of permeate (kg); A = membrane area (m2 ); T = sampling time (h). After pure water filtration, the cell and solution reservoir were emptied and refilled rapidly with solution of each NP; flux was calculated according to Eq. (1). The performance of membranes was investigated in terms of both flux and rejection. The solute rejection was calculated by Eq. (2). Rej (%) =



1−

CP CF



× 100

(2)

where CP = the concentrations of the solute in permeate solution; CF = the concentrations of the solute in feed solution. To measure the concentration of each nitrophenol in the permeate the absorbance of each nitrophenol in the appropriate wavelength was measured by UV-vis spectrophotometer (Unico-S2100SUV, New Jersy, USA) and using Lambert Beer’s law [24].

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Fig. 1. XRD patterns of (a) OMMT, (b) PES, (c) PES/4.0 wt.% OMMT and (d) PES/10 wt.% OMMT membranes and schematic representations for (a) layered clay, (c) exfoliated and (d) intercalated nanocomposites.

In determination of the rejection of hydrophobic organic compounds by NF membranes, the adsorption effects on the membrane surface must be considered to avoid the overestimation of rejection [26]. This overestimation is important particularly for neutral compounds and ionizable compounds in electrostatically neutral conditions. A longer operation time was conducted to reach the saturation state. The required time for membrane saturation depends on the feed concentration [26,27]. Higher feed concentration requires lower operating time to reach the saturation state (steady-state condition) [26]. In this study, permeate samples were collected every 1 h and the average equilibrium time was around 8 h. The rejection was reported for steady-state conditions. 3. Results and discussions 3.1. The effect of OMMT on the properties of nanocomposite membranes The most extensively employed technique in studying polymer nanocomposite structure is XRD. Tracking the intensity and position of XRD patterns can be a useful means in studying exfoliated as well as intercalated nanostructures: incident on the planes of the layers at  angle,  wavelength X-ray beams are scattered by atoms although their constructive interference takes place at the same angle . This method is presented in Bragg’s law [28]:  = 2d sin 

(3)

XRD patterns for PES, OMMT4 and OMMT10 nanocomposite membranes are depicted in Fig. 1. As shown in Fig. 1a, the OMMT presented a main crystalline peak around 4◦ which according to the Bragg’s law (Eq. (3)) corresponds to the basal interlayer spac˚ The membrane prepared from PES (Fig. 1b) does ing of 24.1 A. not show any specific peak due to its structure. In OMMT4 membrane (Fig. 1c), the crystalline peak is almost disappeared. In other words, OMMT particles are diffused inside the polymer chains and

perfectly dispersed. When a higher amount of OMMT (10 wt.%) is used, the crystalline peak (Fig. 1d) shifts to a lower angle (around 3◦ ) ˚ and therecorresponding to increasing interlayer spacing (34.1 A), fore confirming polymer intercalation. As with a rise in the entropy, the clay mineral layers are dispersed in DMF the soon as OMMT is added to PES/DMF solution owing to the fact that clay mineral layers arrangement is upset as the result [8]. Delaminated clay mineral layers are adsorbed to the polymer chains as the solution is stirred. When the film is immersed in the non-solvent bath, the solvent is continuously exchanged by water (non-solvent) with a decreasing diffusion rate. The layers of clay, filled with polymer chains, are linked up forming the intercalated/exfoliated structures on the account that diffusion can be hampered by the coagulated surface [8]. In high concentrations of OMMT, diffusion of PES molecular chains between OMMT layers would be more challenging which such cases lead to the formation of intercalation structure (Fig. 1d and d ). The considerable delamination of the original silicate layers in the polymer matrix in an exfoliated nanocomposite results in the eventual disappearance of any X-ray diffraction (Fig. 1c) from the distributed silicate layers even though the restricted layer expansion in an intercalated nanocomposite is associated with the appearance of a new basal reflection (Fig. 1d) corresponding to the larger gallery height [29–31]. Another criterion for classifying the microstructure as exfoliated or otherwise intercalated is X-ray diffraction intensity. A rise in the amount of the filler in the nanocomposite will yield an increment in X-ray intensity. When the amount of the filler increases, the nanocomposite becomes increasingly intercalated, and a greater area under the XRD curve shows a greater fraction of intercalated composites. Greatly affected by the orientation of the layers as well as by the presence of defects in the crystal structure of MMT, the X-ray signals are highly qualitative in nature. By the same token, taking intensity and position as the mere criterion for classifying of nanocomposite microstructure does not prove helpful. Besides, diffractograms containing the

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139

Fig. 2. TEM images of the membranes prepared from (a) 4 wt.% OMMT and (b) 10 wt.% OMMT; the black lines correspond to the clay layers and gray regions to the polymer matrix.

diffraction signal does not imply full microstructure intercalation rather that would possibly reflect significant amount of exfoliation. In the same way, not receiving diffraction signals does not assure full exfoliation since there is the possibility of randomly oriented or small intercalated platelets being left in the composite [32]. On this account, any idealized of nanocomposite morphologies as exfoliated or otherwise interacted based on XRD would be arbitrary. A deeper insight into dispersion of clay platelets has been provided by TEM measurements (Fig. 2). The OMMT4 membrane showed individual clay mineral platelets dispersed in the PES matrix (Fig. 2a). However, the PES intercalated between the clay mineral platelets are seen in TEM image (Fig. 2b) of the prepared membrane with a high concentration of OMMT (10 wt.%). Aiming at investigate the effect of clay on the thermal stability of nanocomposite membranes, thermogravimetry analysis was performed. The TGA curves related to PES, OMMT4 and OMMT10

membranes are shown in Fig. 3. As it is seen, the onset and final temperatures of degradation of the clay filled samples shifted to higher temperatures. In nanocomposite membranes, the enhancement in the thermal stability could be explained by the barrier properties attributed to the clay mineral layers which restrict the diffusion of oxygen molecules into the nanocomposites and also by the complicated effect of these layers being dispersed in the PES matrix which delay volatilization [8,33]. The mechanical properties of PES and nanocomposite membranes are given in Fig. 4. The strain at break, a measure related to material ductility, did not significantly change by adding 0.5–2 wt.% OMMT; however, it was improved by adding OMMT up to 6 wt.%. For example, the strain at break and tensile strength has been increased respectively by around 22% and 44% for OMMT5 membrane compared to the PES. On the contrary, the mechanical properties of the prepared membranes decreased with more addition of OMMT (>6 wt.%). The strain at break and the tensile strength

c:PES a: PES/4 wt.%OMMT b: PES/10 wt.% OMMT

100 90

Mass loss (%)

80 70 60 50 40 30 a b c

20 10 0 0

50

100

150

200

250

300

350

400

450

500

550

Temperature (°C) Fig. 3. TGA curves of PES and nanocomposite membranes.

600

650

700

140

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12

35

10

30 25

8

20

6

15

4

10

2

5

Tensile Strength (Mpa)

40

Strain at break (%)

Table 2 Zeta potential data of PES and nanocomposite membranes.

14

45

0

0 0

1

2

3

4

5

6

7

8

9

10

Montmorillonite Conc. (wt.%) Fig. 4. Mechanical properties of PES and nanocomposite membranes.

of OMMT10 decreased respectively by 72% and 58% compared to PES. Generally, the presence of OMMT is believed to make nanocomposites more delicate [34]. However, the improvement observed in this study revealed that OMMT played the role of an reinforcement element in the nanocomposite. This could be explained by the interactions established between the clay mineral layers and the polymer chains and also attributed to the possible rearrangement of the clay mineral platelets in the direction of the deformation, allowing greater deformation [35]. The diminishing of mechanical properties of nanocomposite membranes with more addition of OMMT (>6 wt.%) is likely due to layered-silicate aggregation, which prevents the formation of nanostructured systems characterized by a high factor form. The same phenomenon has been observed for other polymer/clay systems [9]. Water contact angles that are indicatives of the wettability of the prepared membranes were measured (Fig. 5). The highest contact angle belongs to PES which displays the lowest hydrophilicity. When the concentration of OMMT is increased, the contact angle is dramatically decreased and a more hydrophilic membrane is produced. The OMMT10 membrane exhibited the lowest contact angle indicating the highest hydrophilicity. The strong change in hydrophilicity of the membranes prepared with OMMT can be ascribed to this the fact that the organically modified clay is hydrophilic and also carries very hydrophilic polar ammonium moieties. The zeta potential measurement data of PES and nanocomposite membranes at solution pH values of 5.11 and 7.15 are tabulated in Table 2. The obtained data suggest a decrease in zeta potential values (negative charge) of the membranes by reducing the solution pH. The larger numbers of functional groups of the membrane

Contact angle (degree)

60 55 50 45 40 35 30 0

1

2

3

4

5

6

7

8

9

Montmorillonite Conc. (wt.%) Fig. 5. Water contact angles of PES and nanocomposite membranes.

10

Membrane

Zeta potential at pH 5.11 (mv)

Zeta potential at pH 7.15 (mv)

PES OMMT0.5 OMMT4 OMMT10

−9.24 −9.14 −3.96 −3.54

−13.43 −12.82 −7.35 −6.62

are protonated by the reduction of the solution pH which leads to the decline in the negative charge of the membranes. Furthermore, addition of OMMT to the casting solution reduces the negative charge of the membrane. This is probably due to the electrostatic attraction of ammonium part of OMMT with negative charges of the membrane surface which diminishes the negative charge of the membrane surface. 3.2. The effect of OMMT on the morphology of nanocomposite membranes It is well known that the performance of membranes strongly depends on the surface, sub-layer morphology, and top layer thickness and compactness [35–38]. The influence of the concentration of OMMT as the additive on the membrane morphology was investigated using AFM and SEM apparatus. The variations in the cross-sectional morphology and top-layer thickness were elucidated by SEM, and the surface roughness and membrane pore size were estimated by AFM. The SEM images of the cross-sections of PES and nanocomposite membranes prepared with different concentrations of OMMT are represented in Fig. 6. The PES membrane exhibits a typical asymmetric structure composed of a thin skin layer and a porous bulk with a finger-like structure. According to the images (Fig. 6), addition of OMMT to the casting solution leads to membranes being composed of a thinner skin layer along with a more porous sublayer compared to the PES membrane. However, addition of more than 4 wt.% of OMMT results in denser skin layers with increased thickness and sub-layers with lower porosities. Addition of OMMT to the casting solution might have some effects on the membrane formation process. The layered silicates and PES are likely to form intercalated and exfoliated structures. In the case of intercalation, the organic component is inserted between the layers of the clay so that the interlayer space is expanded. In an exfoliated structure, the layers of the clay have been completely separated and the individual layers are distributed throughout the organic matrix. Formation of these structures reduces the interaction among the polymer chains which results in a delay in the coagulation of the polymer in the presence of OMMT in the casting solution. On the other hand, due to the hydrophilicity of OMMT and formation of hydrogen bonds between OMMT and DMF, the rate of DMF (solvent) outflow decreases and water (non-solvent) inflow increases. Both phenomena result in a delay in the coagulation of the polymer in the presence of OMMT. Consequently, the growth of the skin layer is diminished and the formation of finger-like pores in the support is improved. SEM images of the cross-section of PES and nanocomposite membranes with a higher magnification are presented (Fig. 7) to evaluate the changes induced in the skin layer of the membranes. It was observed (not shown) that the thickness of the top-layer is slightly decreased at OMMT0.5 . The distinct changes in top-layer thickness were found at higher concentrations of OMMT. By addition of more than 4 wt.% of OMMT, membranes with denser skin layers and increased thickness are achieved. When the concentration of OMMT is increased in the casting solution, a casting solution with a higher viscosity is formed which may affect the mechanism

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141

Fig. 6. SEM cross-section images of pristine PES and nanocomposite membranes prepared with different concentrations of OMMT.

of the membrane formation in the water coagulation bath and, consequently, the thickness of the top-layer, and the compactness of the membranes as depicted in Fig. 7. Fig. 8 indicates two- and three-dimensional and image profile of AFM images of the surfaces of PES and nanocomposite

membranes at a scan size of 500 nm × 500 nm. In these images, the brightest area presents the highest point of the membrane surface, and the dark regions indicate valleys or membrane pores. The surface morphologies of the membranes were strongly influenced by the addition of OMMT. PES membrane possesses a rough surface

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Fig. 7. Cross-section SEM images with a higher magnification of pristine PES and nanocomposite membranes prepared with different concentrations of OMMT.

with large pores, whereas the surface of the nanocomposite membranes holds flatter and more compressed shapes. Ups and downs are diminished specially in OMMT10 . Moreover, it seems that the surface porosity of nanocomposite membranes is high compared to PES membrane which is due to the abundance of small pores

and nodules [39]. Given this condition, the mean pore size of the membranes is decreased (Table 3). Moreover, the surface roughness parameters of the membranes are presented in Table 3. The roughness parameters for membranes are declined with an increase in OMMT concentration in the casting

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143

Fig. 8. Two- and three-dimensional AFM surface images of PES membranes prepared with different concentrations of OMMT: (a) 0 wt.%, (b) 4 wt.% and (c) 10 wt.%.

solution. The trend for the changes in the roughness parameters is similar to the alteration in the mean pore size [39]. Table 3 Mean pore size and surface roughness parameters of PES and nanocomposite membranes. Membrane

PES OMMT4 OMMT10

Mean pore size (nm)

11.70 9.59 9.19

Roughness parameters Sa (nm)

Sq (nm)

Sz (nm)

2.000 0.446 0.206

2.450 0.563 0.259

12.400 3.230 2.090

3.3. The effect of OMMT on the performance of nanocomposite membranes in removal of pesticides Pure water flux (PWF) of the prepared membranes is depicted in Fig. 9. PWF significantly increases from 5.2 to 21.5 kg/m2 h with increasing OMMT concentration from 0 to 4 wt.% and then decreases to 10.2 kg/m2 h as the concentration reaches 10 wt.%. Also notably, it is observed that all of the membranes in this study show

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22

Pure water flux (Kg/m2.h)

20 18 16 14 12 10 8 6 4 2 0 0

1

2

3

5

4

6

7

8

9

10 Commercial

Montmorillonite Conc. (wt.%)

Fig. 9. Effect of OMMT concentration on pure water flux of prepared membranes.

higher PWF compared to the commercial nanofiltration membrane (NF45). Addition of OMMT in the casting solution exerts two major effects on the membrane formation: an increase in the hydrophilicity of the membranes and a change in the membrane surface and the sub-layer morphologies. The obtained results (Fig. 5) indicate that the hydrophilicity of all nanocomposite membranes is higher than that of PES. On the other hand, when OMMT is added to the casting solution, membranes with a thinner skin layer and a higher porosity are formed (Figs. 6 and 7) consequently leading to an increment in the permeation of the membranes. Accordingly, the PWF for nanocomposite membranes should be higher compared to PES membrane.

pH 4.5

According to the results in Fig. 9, however, PWF of all nanocomposite membranes is higher than that of PES; it gradually falls for the membranes containing OMMT concentrations more than 4 wt.%. This phenomenon is due to the denser structure and the thicker skin layer of nanocomposite membranes in concentrations with more than 4 wt.% OMMT. The results of the rejection and permeation of the solution containing DNSA molecules using PES and nanocomposite membranes after the operating time of around 8 h in neutral and acidic conditions (pH 7.0 and 4.5) and calculated using Eqs. (1) and (2) are depicted in Fig. 10. The rejection was improved with increasing the concentration of OMMT in both solution conditions. The trend of flux increment is similar to PWF, i.e. the flux was significantly increased with an increment in OMMT concentration from 0 to 4 wt.%, and then it was decreased as the concentration of OMMT reached 10 wt.%. In neutral conditions, DNSA molecules (pKa = 2.6) are dissociated to the anionic form and therefore are negatively charged as well as the surface of PES membrane. It is evident that filtration behavior of NPs is affected by the electrostatic charge repulsion between negative charges on the membrane surface and the negative charges of the ionized nitrophenolic molecules [40]. On the other hand, although all DNSA molecules are still in the anionic form in acidic pH for their high acidity (pKa = 2.6), the negative charge of the surface of the protonated membrane reduces by decreasing of the solution pH to acidic pH (zeta potential data at Table 2). It seems that strong attraction of hydrogen bonding between DNSA molecules and hydroxyl groups of the protonated surface of the membrane is the main reason of the adsorption of

pH 7.0

100

Rej of DNSA (%)

95 90 85 80 75 70 0

1

2

3

4

5

6

7

8

9

10

Commercial 11

9

10

11 Commercial

Montmorillonite Conc. (wt.%)

pH 4.5

pH 7.0

19 17

Flux (Kg/m2.h)

15 13 11 9 7 5 3 1 0

1

2

3

4

5

6

7

8

Montmorillonite Conc. (wt.%) Fig. 10. Effect of OMMT concentration on the rejection and permeation of solution containing DNSA at different solution conditions (neutral and acidic pH).

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Fig. 11. Effect of OMMT concentration on the rejection and permeation of solution containing DNP at different solution conditions (neutral and acidic pH).

DNSAs on the membrane [26,41,42]. Therefore, due to the adsorption of DNSA molecules on the membrane surface, the passage of DNSA molecules through the membrane is increased which results in a fall in the rejection over time. After saturation of the membrane surface by solutes, the rejection of DNSA molecules reaches a constant value and does not change over time. This indicates the steady-state condition. The rejection was reported as the final (stable) value. The membranes prepared of casting solutions containing various concentrations of OMMT resulted in dense surfaces with a smaller mean pore size as well as a smaller surface roughness compared to PES membrane (Fig. 8). Moreover, the enhancement in OMMT concentration increases the obstructive properties attributed to the clay mineral layers on the membrane surface which hamper the diffusion of DNSA molecules into the nanocomposite membranes [33]. As a result, the enhancement in the rejection of NPs by increasing the concentration of OMMT can be attributed to and explained by the increase in the membrane retention capability in both neutral and acidic conditions. According to SEM images (Figs. 6 and 7), by increasing the concentration of OMMT, membranes are formed which enjoy dense and thinner skin layers, a higher porosity, and a higher hydrophilicity (Fig. 5). Membranes with these characteristics would result in higher permeation. It can be found in Fig. 10 that permeation is decreased when the OMMT concentration in the casting solution reaches higher amounts (>4 wt.%). With addition of more OMMT, the hydrophilicity of the membranes is increased. Furthermore, the thickness of the skin layer, the surface and sub-layer structures are changed causing a decrease in the permeation of nanocomposite membranes. The results of the rejection and permeation of solutions containing DNSA molecules in acidic and neutral pHs, using the commercial NF45 membrane, are also depicted in Fig. 10. Fab-

ricated nanocomposite membranes revealed better performance compared to the commercial membrane. Rejection and permeation of the solution containing DNP molecules (pKa = 4.0) using PES and nanocomposite membranes after 8 h of filtration in neutral and acidic conditions (pH 7.0 and 4.5), calculated by Eqs. (1) and (2) are depicted in Fig. 11. Similar to the rejection results of DNSA molecules, the rejection was increased with increasing the concentration of OMMT. The changes in the permeation of solution containing DNP also follow the same trend as PWF as well as permeation of DNSA solution. Under neutral conditions, the surface charge of PES membrane is negative. All DNP molecules (pKa = 4.0) are also dissociated to anionic form and are negatively charged. Due to the electrostatic charge repulsion, the membrane inhibits DNP molecules from being adsorbed on the surface, and therefore DNP molecules are rejected from the membrane surface [39]. At pH 4.5, the number of the protonated DNP molecules is increased, and therefore the net negative charge of DNP molecules is reduced. On the other hand, the negative charge of the membrane surface reduces in acidic solution compared to neutral pH as well. Subsequently, strong attractions of hydrogen bonding between DNP molecules and hydroxyl groups of the membrane surface result in the adsorption of DNPs on the membrane surface [41,42]. This adsorption phenomenon finally causes a higher passage of the molecules through the membrane over time and leads to a reduction in the rejection of DNPs compared to neutral condition. Similar to the results of the rejection of DNSA molecules in both pHs, the enhancement in the rejection of DNP molecules by increasing the concentration of OMMT is attributed to the increase in the membrane retention capability due to the induced changes on the membrane morphology as a result of adding OMMT to the casting solution. The properties of clay mineral layers on the

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membrane surface are not ineffective in the separation performance of nanocomposite membranes. The comparison of the results of the commercial and fabricated membranes (Fig. 11) revealed that here the performance (permeation and rejection) of the commercial membrane was lower compared to nanocomposite membranes, too. The comparison of the filtration results of NPs (Figs. 10 and 11) demonstrates that rejection of DNSA molecule is higher than DNP at the same solution pH. It is noteworthy that DNSA and DNP are both sensitive to the changes of the solution pH; however, this sensitivity is not the same due to their structures as well as their strength in the protonation (pKa ). Obviously, the dependence of the charge of DNSA molecules to pH is less compared to DNP at the operating solution conditions of this study (pH 4.5 and 7.0). DNSA molecules in both neutral and acidic solutions keep their anionic form because of their strong acidity (small pKa ). Although some of the DNP molecules are protonated in acidic solution due to their acid dissociation constant (pKa = 4.0) resulting in reduction in the negative charge of the molecules, they are completely ionized in neutral solution pH. Consequently, the performance of fabricated NF membranes in removal of NPs varied depending on the solute and membrane properties as well as solution conditions. 4. Conclusion Nanocomposite PES membranes were prepared by addition of OMMT in the casting solution at different concentrations and by the combination of solution dispersion and wet-phase inversion methods. XRD and TEM measurements proved the dispersion of layered clays at nano-scopic level and the formation of exfoliation/intercalation structures. Moreover, the layered clay had a significant effect on membrane formation mechanism investigated by SEM and AFM images. Addition of different concentrations of OMMT to the casting solution resulted in membranes with a thinner skin layer and a smaller mean pore size. The data showed that the addition of OMMT can be an effective method to considerably improve membrane hydrophilicity as well as thermal and mechanical resistance through the nanocomposite structure formation. The incorporation of clay mineral platelets into the PES membrane structure improved the performance of nanofiltration membranes in pesticide removal. Two kinds of NPs (DNSA and DNP) were selected as pesticides because of their strong toxic effects (especially DNP). The removal of NPs by the fabricated membranes was conducted by both mechanisms of solute adsorption on the membrane surface and electrostatic charge repulsion between charged NPs with similar charges on the membrane surface. The dominant mechanism in the removal of NPs depends on the solute and membrane properties as well as the solution condition. The results of the experiments carried out at different solution conditions revealed the superiority of fabricated nanocomposites in improving the performance of nanofiltration membranes. Acknowledgement The authors are grateful to Mohammad Mahdi Hasani-Sadrabadi (Biomedical Engineering Department, Amirkabir University of Technology, Tehran, Iran) for providing OMMT. References [1] D. Grosjean, Atmospheric chemistry of toxic contaminants; reaction rates and atmospheric persistence, J. Air Waste Manage. Assoc. 40 (1990) 1397–1402. [2] J.C. Spain, J.B. Hughes, H.J. Knackmuss, Biodegradation of Nitroaromatic Compounds and Explosives, Lewis Publishers, New York, 2000. [3] W.J. Rea, H.C. Liang, Effects of Pesticides on the Immune System, Environmental Health Center, Dallas, TX, USA, 1986, http://www.aehf.com/articles/A51.htm.

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