TiO2 Ultrafiltration

Article pubs.acs.org/IECR

Fouling Resistant Polysulfone−PANI/TiO2 Ultrafiltration Nanocomposite Membranes Shivanand B. Teli,†,* Serena Molina,‡ Arcadio Sotto,§ Eloy García -Calvo,† and Javier de Abajob †

Fundación IMDEA-AGUA, Parque Científico Tecnológico de la Universidad de Alcalá, 28805 Alcalá de Henares, Madrid, Spain Department de Quimica Macromolecuar, Institutute de Ciencia y Teccbnlogia de Polimeros, Consejo Superior de Investigaciones Cientificas (CSIC), Juan de la Cierva 3, 28006 Madrid, Spain § Department of Chemical and Energy Technology, ESCET, Universidad Rey Juan Carlos, C/Tulipán, 28933 Móstoles, Madrid, Spain ‡

ABSTRACT: To avoid particle agglomeration and to improve membrane antifouling property, commercial TiO2 particles were modified with polyaniline (PANI) by in situ polymerization. SEM and FTIR analysis confirmed the incorporation of PANI on the surface of the TiO2 particles. The average size of PANI/TiO2 nanoparticles is in the range of 10−67 ± 3 nm. The preparednanoparticles are used as surface and inner nanofiller additives and dispersed into the polysulfone (PSf) to obtain ultrafiltration nanocomposite membranes via phase inversion method. The surface hydrophilicity of nanocomposite membrane increases with increasing nanoparticles (0 to 1.5 wt %) concentrations. The membrane morphology indicates that nanocomposite membranes exhibited larger surface pore size, higher porosity, more finger-like pores, and less macrovoids than the control PSf membrane. The experimental results indicate that the 1.0 wt % of PANI/TiO2 content membrane depicted excellent hydrophilicity, water permeability, and better antifouling property with high rejection. Bovine serum albumin and humicacid were used as model foulants. The protein adsorption study showed that PANI/TiO2 content membranes adsorbed more at the isoelectric point of BSA solution and decreased as the solution pH increases. Higher nanoparticles content (1.5 wt %) membrane outcomes are elucidated and affected and resulted in significant particle agglomeration. Finally, obtained experimental results show that the nanocomposite membranes have higher flux and better antifouling property than the control PSf membrane.

1. INTRODUCTION Ultrafiltration (UF) is one of the pressure driven membrane techniques that is widely applied for production of safe drinking water, food, and dairy processes, reverse osmosis pretreatment, and particularly to improve the water quality with respect to organic and microbiological contaminants.1,2 Polysulfone (PSf) is a polymer, which is extensively used as a UF membrane material in many industrial fields because its high mechanical strength, resistance to compaction and heat, chemical stability, and the ability to work in a wide range of pH values.2,3 However, the hydrophobic nature of a PSf membrane results in the adsorption and deposition of solute on the membrane surface, which consequently causes severe membrane fouling and shortens the membrane life during water treatment.2,4 Many methods have been employed to improve the surface hydrophilicity to reduce the fouling problem in UF membranes, such as of surface coating, surface grafting, and blending of hydrophilic additives.5−9 The blending of nanomaterials as hydrophilic additive has been extensively studied because of the nanomaterial’s unique physicochemical properties that differ from bulk materials or molecular structure.10 Different types of inorganic nanomaterials have been used to fabricate nanocomposite UF membranes, such as silica, carbon nanotubes, alumina, silver, zirconia, gold, zerovalent iron (Fe0), palladium, and TiO2 nanoparticles.11 Among these, TiO2 emerges as a functional nanomaterial of high chemical stability, nontoxicity, and good heat resistance. Therefore, it is one of the highly promising materials to be used in water purification, as photocatalysis, antibacterial studies, and self-cleaning materials.12,13 The surveyed literature proved that the addition of TiO2 © XXXX American Chemical Society

particles on the membrane surface and entrapments of TiO2 particles into the matrix through phase inversion method was helpful to mitigate membrane fouling significantly.14,15 However, the direct use of commercial crystalline TiO2 particles as nanofillers at the nanometer scale remains a challenge because of particle aggregation as a consequence of their very high surface energy. The particle agglomeration reduces the nanofillers efficiency to improve the membrane property, which negatively affects the effective utilization of TiO2 particles into the membranes.9 Thus, to reduce the aggregation of TiO2 particles in the membrane matrix is one of the most important goals. Apart from inorganic NPs some special functional groups of organic nanomaterial substrates have shown novel performances because they have much larger surface areas than bulk particles. Among the family of conducting polymers, polyaniline (PANI) has been used in filtration because of its characteristics like environmental stability, porosity nature, simple acid/base doping/dedoping chemistry, facile and reversible electrochemistry, etc.15 Generally the undoped form of PANI is hydrophobic and the doped form of PANI has been shown to be hydrophilic.15,16 Together, these properties build PANI as one of the capable candidates for various applications including gas, liquid−liquid, electrodialysis, UF, nanofiltration, metal anticorrosion, and marine-fouling prevention.4,17−23 However, the emeraldine salt form of PANI shows better solubility in common Received: April 1, 2013 Revised: May 23, 2013 Accepted: June 4, 2013

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dx.doi.org/10.1021/ie401037n | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Industrial & Engineering Chemistry Research

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Chemicals, Milwaukee, USA. Aniline was purchased from Aldrich and distilled under reduced pressure before use. Bovine serum albumin (BSA, molecular weight, 67 kDa) and humic acid (HA) were also purchased from Aldrich Chemicals, Milwaukee, USA. 2.2. Synthesis of PANI/TiO2 Nanoparticles. The PANI/ TiO2 nanoparticles were prepared by an in situ chemical oxidation polymerization method, of aniline using ammonium peroxydisulfate (APS) as an oxidant in the presence of colloidal TiO2 particles at 0−5 °C in air.40 In a typical procedure, the TiO2 particles were suspended in 1 M HCl solution and sonicated for 1 h to reduce the aggregation of TiO2 particles. Aniline (0.1 mol) was dissolved in 100 mL of 1 M HCl solution and then mixed with 10 mL of sonicated colloidal TiO2 particles followed by further sonication for 30 min. An equal molar ratio of APS was dissolved in 100 mL of 1 M HCl aqueous solution and added dropwise to a solution containing aniline monomer under constant stirring. The mixture was allowed to polymerize under stirring for 5 h at room temperature. After that, the reaction mixture was vacuum filtered, washed several times with ethanol + water (80:20) mixture solvent and dried at 60 °C for 24 h to obtain the blue emeraldine salt (ES) of PANI/TiO2 powder. The powder was characterized by FTIR (Perkin-Elmer RX1 spectrometer) to confirm the incorporation of PANI with TiO2 particles. 2.3. Membrane Preparation. Control PSf and PSf-PANI/ TiO2 nanocomposite membranes were prepared via phase inversion. Taking the casting solution with 0.05 wt % of nanoparticle addition as an example, 0.05 g of PANI/TiO2 was added into 25.45 g of NMP with constant stirring for 2 h. An amount of 4.5 g of PSf was then added into the above solution and fully dissolved after stirring for 24 h. Subsequently, the mixture was cast on to a glass plate with nonwoven polyester fabric by a casting knife of 200 μm thickness and immediately immersed into water bath. After completion of the immersion process, the membrane was removed and its surface was cleaned with plenty of water. The measured thicknesses of the membranes are in the range of 142−147 μm. For all the casting solutions, the mass content of PSf to total casting solution was 15 wt %, and PANI/TiO2 additions were varied from 0 to 1.5 wt %. Membranes prepared with 0, 0.05, 0.1, 1.0, and 1.5 wt % of PANI/TiO2 with casting solutions were designated as M0, M1, M2, M3, and M4, respectively. The viscosity of the casting solution was measured using a rotating viscometer (Brookfield LVDV-C, USA) at 25 ◦C with a rotation rate of 20 rpm. 2.4. Membrane Surface Characterizations. Membranes hydrophilicity was investigated by measuring the static contact angle (CA) measurements by a sessile drops method (KSV Cam 200, Finland) at 25 ± 1 °C. The membrane elemental composition was analyzed by using EDS (EDAX, PHI-1600, USA) with Mg Kα as the radiation source. Survey spectra were collected over a range of 0−1000 eV. The surface and the crosssectional morphology of the membranes were observed by scanning electron microscopy (SEM) (Philips model XL30E). Surface pore size and pore size distribution of the membranes were determined by the analyzing SEM images of the surface using Image J 1.38′ software method (National Institute of Health, http://rsb.info.nih.gov/ij) as reported in previous reference.41 The porosity of the membrane was determined by the water uptake experiments using eq 1:

nonpolar or weakly polar organic solvents and fortunately desired bulk polymers (such as PSf) are also dissolved in these solvents. Therefore, it is expected that the development of nanocomposite and mixtures of PANI through solution blending will make a critical impact on the manufacture of highperformance membranes. For example, PANI nanofibers with PSf membranes have many attractive characteristics such as high porosity, more interconnected open pore structure, and large surface area per volume,5,24 and enormous improvements have also been observed in the hydrophilicity of the surface, and antifouling and permeability properties.3,25,26 To our knowledge so far a number of research work has been done using PANI, such as PANI/TiO2 core−shell nanocomposite being synthesized by modification of TiO2 surface by silane,27 and network-like PANI/TiO2 being prepared by redispersion of TiO2 in toluene.28 Because of the existence of strong interaction between the two components,29 the use of PANI/TiO2 composites are quite different and worthwhile compared to the use of pure PANI or TiO2. The application of PANI/TiO2 as a photocatalyst for the photodegradation of environmental pollutants has been investigated.30 Liu and Guo used the hybrid of PANI and TiO2 composite for photocatalytic removal of phenol from water.31 In addition, TiO2 colloidal sols were prepared by chemical oxidative polymerization of aniline on the surface of individual TiO2 nanoparticles.32−36 In particular, facilitating the interaction of PANI with individual TiO2 particles at the nanometer scale remains a challenge because NPs usually undergo agglomeration.15 Therefore, only a small surface area of NPs becomes available for interfacial interactions with PANI. A strong interaction between PANI and nanocrystalline TiO2 can be expected, associated with the interaction of Ti and N atom in PANI molecules. Titanium is a transition metal element, so Ti has a strong tendency to form a coordinate bond with the N atoms in PANI molecules. Moreover, strong guest−host interactions, such as hydrogen bonding, could occur in the form of NH···O−Ti in TiO2.37 Owing to the chemical inertness of PANI, the TiO2 particles at the surface of PANI can stabilize the macromolecular energy and subsequently minimize its surface energy, which may lead to the formation of a stable dispersion of PANI/TiO2 nanoparticles in the bulk polymeric material during the membrane formation. Furthermore, due to the polymeric nature of PANI the affinity of composite nanofiller with the polymer matrix can be enhanced. However, most filled polymeric membranes failed to cross the permeability-selectivity trade-off curve due to the following main disadvantages: agglomeration of inorganic particles and the formation of nonselective voids, which usually exist at the interface of organic and inorganic phases,38,39 since the interaction between inorganic particles and polymer is of physical origin. The permeability increase is attributed to the increase of free volume through disruption of polymer chain packing by inorganic filler. To weaken the physical barrier between these two phases, the use of composite structures like PANI/TiO2 particles could increase the affinity of the nanofillers with the polymeric bulk material. Therefore, in this study, we try to synthesize the PANI/TiO2 through polymerization and the use of membrane surface modifying additives.

2. EXPERIMENTAL SECTION 2.1. Materials. Polsulfone and anhydrous N-methyl-2pyrrolidinone (NMP), nanosized TiO2 (20 nm, 338 m2 g−1), hydrochloric acid (HCl), ammonia solution, ammonium peroxydisulfate (APS), and ethanol were purchased from Aldrich

ε= B

(M w − Md)/ρ AL

(1)



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3. RESULTS AND DISCUSSION 3.1. Study of PANI/TiO2 Nanoparticle. The PANI/TiO2 particles were prepared by in situ oxidative polymerization on the surface of the negative-charged TiO2 particles using the procedure shown in Scheme 1. It has been confirmed that

where Mw is the mass of wet membrane sample and Md is the mass of dry state membrane sample; A, L, and ρ are the sample area, the sample thickness, and pure water density, respectively. 2.5. Membrane Filtration Study. The UF experimental study was carried out as per our published paper42 by using a stainless steel cross-flow cell module having an effective membrane area of 0.005 m2 with 1 L volume capacity tank. The membrane permeability properties were investigated by using three different solutions like pure water, BSA, and humic acid. The BSA solution (0.7 g/L, pH 7.2) was prepared by using a 0.1 M phosphate buffer (PBS) solution. The HA solution (0.7 g/ L) was filtered by the filtrate paper before using actual filtration to avoid speedy fouling on the membrane surface. Initially, all the membranes compaction was performed at 0.30 MPa transmembrane pressures (TMP) for 20 min. After that, the pure water, HA, and the protein filtration experiments were performed at 0.20 MPa, TMP, 25 ± 1 °C and 0.65 m/s crossflow velocity. The water flux was calculated by the following equation: Jw1 =

⎛ V ⎞ ⎜ ⎟ ⎝ AΔt ⎠

Scheme 1. Formation Scheme of PANI/TiO2 Particles

(2)

TiO2 particles have zero charge point at pH = 6.30 Therefore, they are positively charged in acidic solution required for the in situ polymerization of aniline.30 For that reason, chloride anions are adsorbed on the positively charged surface of TiO2 to neutralize the created positive charge on the surface of TiO2 particles. Aniline monomer in acidic solution is transformed to the anilinium cation. Therefore, electrostatic interaction occurred between chloride anions adsorbed on the surface of TiO2 and anilinium, which are available in the reaction media. Polymerization of anilinium cations causes formation of PANI around the TiO2 particles. The introduction of PANI increases the binding capacity of TiO2 and slightly enhances the dispersibility of TiO2 into the polymer matrix.44 The PANI/TiO2 particles were analyzed by SEM and it can be confirmed that the average size of NPs are in the range of 10−67 ± 3 nm as shown in Figure 1. The NPs can be steadily sonicated before dispersing into casting solutions. The spectra of FTIR analysis of PANI and PANI/TiO2 are shown in Figure 2. It was revealed that the PANI/TiO2 contains all the characteristics of the peak of PANI. The peaks at 1577 and 1485 cm−1 corresponded to quinoid and benzenoid rings of

where V (L) is the volume of permeated water, A (m2) is the membrane area, and Δt is the permeation time at 25 °C temperature. The water, humic acid, and protein solution flux was calculated by weighing the permeate solutions at a microbalance. The rejection (R %) of BSA was calculated by the following equation: ⎛ C ⎞ R(%) = ⎜1 − P ⎟ × 100 Cf ⎠ ⎝

(3)

where Cp and Cf (g/L) are the protein concentrations of permeate and feed solutions, respectively. The BSA concentrations in the feed and permeate solutions were measured by using UV−vis spectrophotometer (Secomom), at a wavelength of 280 nm. The membrane antifouling study was performed as follows. First, the pure water flux of the membrane Jw1 (L/m2.h) was tested at 0.20 MPa TMP. Then, 0.7 g/L BSA solution was fed into the UF system. After BSA filtration the membranes were flushed with pure water twice for 10 min and then the pure water flux of the membranes Jw2 (L/m2.h) was measured. To measure the fouling resistant ability of the membrane, the flux recovery ratio (FRR %) was calculated using the equation FRR(%) =

Jw2 Jw1

× 100 (4)

2.6. Protein Adsorption Study. BSA (1.0 g/L) in phosphate buffered saline solution was prepared to carry out the static adsorption of protein on the membrane surface.43 Sodium azide (0.02 wt %) was used as a bactericide. Adding 0.1 M NaOH or 0.1 M HCl solutions adjusted the pH to 3.0, 4.7, 5.0, 7.0, and 9. The membrane samples were cut into small pieces (2 cm × 2 cm), placed inside the specially designed airtight test bottles containing 20 mL of BSA solution, and sustained for 72 h to allow for BSA adsorption. The amount of BSA adsorbed on the membrane sample was calculated by using the initial and final solution of BSA concentrations by UV−vis spectrometer (Secomom), at a wavelength of 280 nm.

Figure 1. SEM image of PANI/TiO2 nanoparticles. C



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solution can mainly affect the exchange rate of solvent and nonsolvent during phase inversion; and so the viscosity of the casting solution can bring about a rheological hindrance in the demixing of polymer solution with water. In terms of kinetic hindrance, it could be an agent for suppressing macropore formation in the membranes prepared by phase inversion process, affecting negatively on the morphology of the membranes. It is worth noting that the viscosity is concerned with two major aspects: if the casting solution viscosity is low (0.05 wt % of NPs) the phenomenon of particles leaching occurs and no stable formation of membrane pore wall takes place during membrane preparation via phase inversion; if the casting solution viscosity is higher (1.5 wt % of NPs), there would be very little chance of particles leaching, and apparently the pores could be blocked due to agglomeration of particles.45 3.3. Membrane Surface Hydrophilicity. Surface hydrophilicity is an important property of membranes, as it may govern the water flux rate, and the antifouling property of membranes is estimated by water CA measurements and presented in Table 2. The initial CA was measured immediately after water dropped on the surface of membranes to reflect the natural wetability of the material. The control PSf membrane shows the highest contact angle value meaning it is hydrophobic in nature. The highest observed CA of PSf membrane decreases with increasing PANI/ TiO2 concentration in the casting solution in the order of 0, 0.05, 0.1, 1.0, and 1.5 wt %, respectively. Thus, it was confirmed that the addition of NPs could improve the surface hydrophilicity, which might be attributed to the porous nature of PANI,22 and hydrophilic nature of TiO2 particles. 3.4. Membrane Morphology. The surface morphology of membranes and pore diameter distributions are shown in Figure 3. Generally, all the top surfaces of membranes exhibited the homogeneous porous surfaces with the pore size mainly lying between 3 and 8 nm. The membrane surface contains denser porous like structure caused by the delayed demixing of NPs during the phase inversion. The average surface pore size and porosity of all membranes were calculated by Image J software, and the values were presented in Table.2. It can be seen that PSfPANI/TiO2 nanocomposite membranes have higher average surface pore sizes that are in the range of 6.5−8.9 nm compared to the PSf membrane with average pore value of about 6.02 nm. The results of surface pore diameter distributions of all the membranes are concentrated in the range of 6−9 nm (see Figure 3). The surface pore diameter distribution of PSf-PANI/TiO2 nanocomposite membranes is clearly shifted toward larger pore diameter values compared with that of the PSf membrane. As shown in Figure 4 the cross section images of membranes exhibited a typical asymmetrical structure of UF membrane with a dense top layer, a finger-like porous sublayer, and fully developed macropore layer at the bottom. Obviously the PSf membrane has more broad macrovoids in the sublayer, but PSfPANI/TiO2 membranes displayed that macrovoids are suppressed and the finger-like pores run through the cross-sectional structure. It is specified that the addition of PANI/TiO2 leads to the formation of the longest finger-like pores and less macrovoids that become wider with strong separating protective membrane wall.46 Generally, the addition of any kinds of NPs into the polymer solution could reduce the miscibility of the casting solution with water, causing the acceleration of the phase separation. However, when the PANI/TiO2 addition was above 0.1 wt %, the viscosity became higher (see Table 2). Because of that reason, the particleleaching problem is less, and consequently the pore-forming

Figure 2. FTIR spectra of PANI/TiO2 nanoparticles.

PANI. The peaks at 1297 to 1233 cm−1 correspond to the C−N stretching mode of benzoid ring. The peaks at 1120 to 1098 cm−1 are assigned to a plane bending vibration of the C−H mode which is found during protonation.35 These peaks when compared to that of pure PANI are found to be shifted, slightly due to the strong attraction of TiO2 particles with PANI.36 In the case of TiO2, a strong absorption around 670 cm−1 due to Ti−O stretching is observed, while this peak was found to be weak in PANI-TiO2 due to the presence of PANI.37 Titanium is a transition metal, and it has an intense tendency to form coordination compounds with N atoms in the PANI polymer.44 This interaction may weaken the bond strengths of CN, C C, and C−N in PANI. Thus, from these results confirmed the presence of PANI on the surface of TiO2 particles. The C, H, O, and the titanium elements were observed by EDAX spectra and the content of elemental measurements are mentioned in Table 1. The large amount of Ti and N elements Table 1. EDAX Elemental Composition of PANI/TiO2 Nanoparticles weight % atomic %

C (K)

N (K)

O (K)

Cl (K)

Ti (K)

Cu (K)

25.30 30.18

4.10 15.92

27.58 35.88

1.35 1.03

38.18 16.01

3.58 1.13

confirmed that Ti particles are coated partly with the PANI polymer. The lesser amount of Cl formed from dopant HCl in the PANI salt form and the trace percentage of Cu should be ascribed to the copper grid in the performance of the SEM analysis. 3.2. Viscosity. The viscosities of the casting solutions are presented in Table 2. It can be seen that viscosity increases with increasing wt % of PANI/TiO2 particles. The viscosity of a Table 2. Data of Average Pore Size and the Porosity of PSf and PSf-PANI/TiO2 Nanocomposite Membranes membranes

ηc (cP)

CA (degree)

average surface pore size or diameter (nm)

porosity (%)

M0 M1 M2 M3 M4

298.5 365.5 399.2 485.7 625.9

70.1 64.3 58.5 52.0 47.3

6.02 ± 2 6.84 ± 3 6.52 ± 2 8.11 ± 4 8.92 ± 3

63.4 ± 2 77.7 ± 3 79.1 ± 1 81.0 ± 1 76.3 ± 2 D



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Figure 3. Top surface SEM images with elemental analysis and pores diameter of the membranes.

surface pore size.47 The distribution of the PANI/TiO2 on the membrane surface forms finger-like pore walls linked by the sponge wall which confirms that a large number of micropores

effect of nanoparticles is weakened in the case where the higher viscosity of the solution hinders the formation and development of the membrane pores and causes the decrease of porosity and E



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Figure 4. Cross-sectional SEM images of the membranes.

allow the finger-like pores to communicate with each other. Thus, PSf-PANI/TiO2 nanocomposite membranes had long finger-like pores and less macrovoids than the PSf membrane because of the relocation of PANI/TiO2 particles during formation of the membranes.25 Furthermore in Figure 5a it was clearly seen that the PANI/TiO2 particles are well dispersed inside the membrane with the mitigation mode of the agglomeration. Hence, it was confirmed that the addition of a small (0.1 wt %) amount of NPs could assist to improve

homogeneous distribution of particles and to reduce the agglomeration. Moreover, the addition of a higher (1.5 wt %) amount of NPs could form aggregates on the membrane surface as well as the pore walls as shown in Figure 5b. Meanwhile, the Ti (%) was calculated by using EDAX spectroscopy as presented in Table 3. It can be seen that the percentage of titanium increases with increasing NPs wt % in the casting solution (see Figure 3). 3.5. Membrane Filtration. High flux and inherent antifouling properties of the membranes are of high importance F



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Figure 5. Cross-section SEM images of (a) M2 and (b) M4 membranes.

and to increase the surface hydrophilicity.48 In addition, after incorporation of NPs into the membrane structure, the random dispersion of inorganic filler in the polymer matrix restricts the spread of polymer chains and increases the mean distance between polymer chains, leading to the improvement of membrane permeability.49 The improvement of hydrophilicity by the addition of PANI/TiO2 could reduce the interaction between the organic pollutants and membranes, and accordingly increase the permeate flux. In addition, the effective surface area of the membrane is also enlarged by blending PANI/TiO2 because of the surface charge of TiO2 and particle dispersibility characteristic of PANI. Furthermore, the 1.5 wt % NPs content membrane showed slightly lower water flux than other PSfPANI/TiO2 nanocomposite membranes. This might be due to the high viscosity of the solution, particle agglomeration problem, and intrinsic rigidity of NPs, which blocks the pores and brings the improper distribution of NPs in the membrane matrix. Humic acid and protein separation studies were carried out to investigate the fouling performance of the membranes. In the case of HA the permeate flux decreases significantly with time as shown in Figure 7. However, the time dependencies of the fluxes

Table 3. Titanium (%) Data of PSf and PSf-PANI/TiO2 Nanocomposite Membranes titanium membranes

wt (%)

atom (%)

M0 M1 M2 M3 M4

8.3 10.4 12.8 18.5

02.1 03.1 03.9 06.1

in UF-based water treatment technology. The experimental data show that the PSf-PANI/TiO2 nanocomposite membranes had higher pure water fluxes than the PSf membrane as shown in Figure 6. It occurs because the presence of PANI/TiO2 in the

Figure 6. The time-dependent pure water fluxes of the membranes.

membrane matrix holds larger surface pore size, higher porosity, more hydrophilic surface, and better vertically interconnected finger-like structures (see section 3.4). These characteristics strongly force the resistance of water permeation through the membranes. The main roles of organic or inorganic additives are of inhibition of macrovoids, to improve the pore connectivity,

Figure 7. Flux decline behavior of membranes during filtration of HA solution. G



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for HA filtration show that they were approaching stable states after 2 h operation even though some HA deposition continued. It appears that the main loss in flux occurs with the initial interaction of HA with the membranes. The flux of HA for PSfPANI/TiO2 nanocomposite membranes have been strongly declined compared to pure water flux. The rapid flux decline in UF has been related to the initial physical deposition of large HA aggregation and the subsequent accelerated deposition of nonaggregated HA macromolecule.50 Adsorption of HA at UF membranes is greater at high ionic strength, low pH, and compact configuration of NOM in solution, which caused a compact, dense and thick fouling layer on the surface of the membranes.51,52 On other the hand, with the use of protein for the fouling test, the flux declined rapidly as shown in Figure 8.

size in the skin layer of the UF membrane is enlarged and there could be possible a pore blockage on the membrane surface by the particle agglomeration effect. 3.6. Protein Adsorption and Antifouling Study. The effect of PANI/TiO2 particles on membrane antifouling performance was also investigated through the static BSA adsorption test as shown in Figure 10. Generally, the control PSf

Figure 10. Effect of pH on BSA adsorption amounts on the membranes.

membrane protein adsorption is higher because of its hydrophobic surface, so that the surface does not have significant ability to hinder the attachment of any kind of foulants. This effect is completely reversible in the case of hydrophilic membranes. Therefore, the PANI/TiO2 content membranes showed less BSA adsorption than a control membrane. In addition, a study of the BSA adsorption on the membrane surface at different pH values was carried out. It can be seen that the highest adsorption occurred at IEP (4.7) which also reflects the major aggregation of protein on the surface. The reason is that at IEP (4.7) both protein and composite membranes are nonpolar; therefore the hydrophobic interaction causes the BSA to be more easily adsorbed on the surface. Also, it could be possible that the BSA molecules may aggregate more easily because of hydrophobic interactions and reduced intramolecular and intermolecular electrostatic repulsion at IEP, which also leads to stronger adsorption.43 Furthermore, the change of pH that causes the changes in secondary structure and conformation of proteins, which also result in the higher levels of denaturation of the protein, could occur in acidic solutions at pH values below IEP.53,54 The flux recovery ratio (FRR) was calculated to evaluate the antifouling property of membranes as depicted in Figure 9. Higher FRR value means higher efficiency of hydraulic cleaning and stronger antifouling property. The control PSf membrane FRR value is 60.83%. It means that the protein fouling of the PSf membrane is certainly high and it is due to two facts: (1) the hydrophobic surface and interaction between hydrophobic functional group of PSf and protein molecules, and (2) the attached protein molecules on the surface cannot be removed by simple water washing. The FRR values of PSf-PANI/TiO2 nanocomposite membranes are higher than that of a control PSf membrane; this means the addition of PANI/TiO2 enhances the fouling resistance. However, a slight FRR decrease was observed at the higher NPs content membrane (M4) because the larger pore sizes might be acquired protein blockage in the membrane pores. Therefore, the trapped protein molecules in the membrane pores cannot be washed out easily. Hence, the higher amount of (1.5 wt %) NPs content membrane shows the lower FRR (77.73%) value, but it is still about 30% higher than that of the control membrane.

Figure 8. Flux decline behavior of membranes during filtration of BSA solution.

This could be due to the adsorption of protein or deposition on the surface as well as in the pores. This deposition causes an abrupt drop in flux in the first few minutes of the operation, even though the mixing induced by the cross-flow filtration may actually sweep the deposited protein molecules away from the membrane surface. The deposition and sweeping of protein may reach equilibrium in the subsequent operation, so that a relative flux (Jp) is retained during the filtration of BSA solution. However, membrane porosity and surface pore size decreased with increasing PANI/TiO2 concentrations (above 0.1 wt %), resulting in the decrease of membrane permeability. The membrane rejection property changes slightly with the addition of PANI/TiO2 particles. Therefore, BSA rejections of the membranes are in the range of 81−99% as shown in Figure 9. Unlike that of other membranes, the BSA rejection rate of the M4 membrane (1.5 wt %) gradually decreased, indicating that pore

Figure 9. Flux recovery ratio (FRR) and BSA rejection rate of membranes. H



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4. CONCLUSIONS In this work PANI and PANI/TiO2 nanoparticles were prepared by in situ-polymerization method and used as a hydrophilic additive in membrane processes to reduce fouling. The complete PANI incorporation of surface TiO2 particles was confirmed by FTIR analysis. Polysulfone with the addition of various concentrations of PANI/TiO2 membranes was prepared via phase inversion. As revealed by contact angle results, membrane surface hydophilicity improved due to the addition of PANI/ TiO2 particles. The BSA adsorption obtained results confirm that nanocomposite membrane exhibited better antifouling property. The addition of PANI/TiO2 increases the membrane surface pore size, porosity, and wider finger-like pore structures in membranes. However, it was found that the addition of higher NP concentrations in the membrane matrix spawns particle agglomeration on the surface of membranes. PSf-PANI/TiO2 nanocomposite membrane has a better antifouling property with a high rejection rate of protein than a control PSf membrane. The overall outcomes of the present work demonstrated that the PANI/TiO2 nanoparticles have the capacity to arrest antifouling property, which may lead to the new applications of UF membranes.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: shivateli@rediffmail.com. Tel.: + 34 918 305 962. Fax: +34 918 305 961. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Dr. Teli is grateful to the European Union (EU) for awarding “MARIE CURIE″ Amrout research fellowship at IMDEA water, Alcalá de Henares, Spain. Prof. J. B. Abajo thanks to the Spanish Ministry for Science and Innovation (MAT2010-20668).

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REFERENCES

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