Preparation, characterization and the effect of PANI coated TiO₂ nanocomposites on the performance of polysulfone ultrafiltration membranes

Abstract

Polysulfone ultrafiltration (UF) membranes with PANI–TiO₂ (polyaniline–titania) nanocomposites and PEG 1000 (Polyethylene Glycol 1000) as additives were prepared by the phase inversion method. PANI–TiO₂ nanocomposites were synthesized by coating TiO₂ nanotubes with PANI via chemical oxidative polymerization. The synthesized PANI–TiO₂ nanocomposite was characterized by Fourier Transform Infrared Spectroscopy (FTIR), X-ray diffraction (XRD) and Transmission Electron Microscope (TEM) analysis. PANI–TiO₂ nanocomposites with varying concentrations (0–1.5 wt%) were dispersed in the polysulfone membrane matrix with N-methyl-2-pyrrolidone (NMP) as solvent along with PEG 1000 as the pore former. The effect of addition of PANI–TiO₂ nanocomposites with different concentrations (0–1.5 wt%) on the membrane structure, performance, hydrophilicity and the antifouling nature of the membranes was analyzed. PANI–TiO₂ nanocomposite membranes showed better hydrophilicity, improved permeability, enhanced porosity, water uptake and good antifouling ability when compared with neat polysulfone membranes. The performance of the membranes improved with the increase in the addition of the PANI–TiO₂ nanocomposite. However, the membrane performance decreased slightly at 1.5 wt% addition of PANI–TiO₂ due to the agglomeration of PANI–TiO₂ at higher concentration. The well performed membranes were also subjected to heavy metal ion rejection. The membranes showed a rejection of 83.75% and 73.41% during the polymer enhanced ultrafiltration (PEUF) process and a rejection of 68% and 53.78% during the UF process for Pb²⁺ and Cd²⁺ respectively.

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1. Introduction

Ultrafiltration is an important membrane separation process which is used for the purification of water,¹ for concentration of solutes and wastewater treatment. A suitable membrane material with good pore structure, permeability, hydrophilicity and antifouling nature plays an important role in the separation process. Polysulfone is one of the widely used materials for the preparation of ultrafiltration membranes,² because of its excellent mechanical strength, chemical resistance, thermal stability and good film forming ability.³ However polysulfone membranes being hydrophobic in nature are susceptible to fouling. Thus the modification of polysulfone membranes is inevitable. Many efforts such as blending hydrophilic additives, chemical grafting, and surface coating have been devoted to improve the hydrophilicity of polysulfone membranes.⁴ In recent years, research has been focused on incorporation of nanomaterials into the membranes to improve the separation performance and fouling resistance.⁵ Various inorganic nanomaterials that have been used to improve the polysulfone membrane include TiO₂, SiO₂, ZnO, Ag, Al₂O₃, and Fe₂O₃. Among these inorganic nanoparticles, TiO₂ has received much attention and is one of the most widely used nanoparticles, due to its stability, availability, antibacterial nature and hydrophilicity.⁶

In recent years nanohybrid materials with at least two different nanomaterials have gained interest where the benefits of two different particles could be combined to have improvement in properties.⁷ Polyaniline (PANI) based nanocomposites have been extensively studied due to their facile preparation and environmental stability.⁸ A large number of studies have been focused on preparation of PANI–TiO₂ composites.⁹ TiO₂ is considered as the best material to prepare hybrid material with PANI.¹⁰ Ti of TiO₂ has a tendency to form a co-ordinate bond with N of PANI.¹¹ PANI–TiO₂ nanocomposites have applications in photocatalysis, charge storage and gas sensors.^9,10 Also polymer–PANI and PANI–inorganic materials have shown improvement in the performance of membranes due to the hydrophilicity of PANI. Teli et al. modified the commercial TiO₂ nanoparticles with PANI and incorporated them into the polysulfone membranes.¹¹ The obtained membranes had better permeability, higher hydrophilicity and antifouling properties. Daraei et al. prepared a polyether sulfone nanocomposite membrane with core–shell PANI–Fe₃O₄ nanoparticles which showed enhanced performance for Cu(II) removal from water.¹² Zhao et al. prepared a polyaniline–poly(vinylpyrrolidone) (PANI–PVP) nanocomposite and used it as an additive to polysulfone membranes. The membranes showed better permeability and antifouling properties.¹³

TiO₂ nanotubes can act as excellent support materials for PANI due to their high surface area.¹⁴ On the other hand, TiO₂ nanomaterials are known to undergo agglomeration in order to reduce their surface energy.¹⁵ The presence of PANI on TiO₂ can prevent the agglomeration, increasing the stability of TiO₂ nanomaterials and facilitate uniform dispersion in the polymer matrix. Our previous study indicated that, addition of TiO₂ nanotubes has significantly improved the membrane performance.¹⁶ Both PANI and TiO₂ nanotubes have been reported to be good additives in improving the membrane performance.^16,17 The synthesized PANI–TiO₂ nanocomposite could combine the merit of both TiO₂ and PANI and have a synergistic effect.¹⁵ Also PANI–TiO₂ having strong interaction between PANI and TiO₂ is known to have well dispersible nature in NMP (N-methyl-2-pyrrolidone).¹⁸ These merits of PANI–TiO₂ are beneficial to the improvement of properties of the membrane and hence enhancement in the performance of the membrane is expected.

In this work, PANI–TiO₂ nanocomposites with an average diameter of 50 nm were synthesized by coating PANI over TiO₂ nanotubes. PANI coated nanotubes were dispersed in polysulfone membranes containing PEG 1000 as a pore former. The effect of addition of the PANI–TiO₂ nanocomposite on the membrane performance, such as permeability, hydrophilicity, porosity and antifouling properties were studied. To the best of our knowledge, it is the first time TiO₂ nanotubes coated with PANI have been used as hydrophilic additives for the performance improvement of polysulfone membranes.

2. Materials and methods

2.1. Materials

Polysulfone (Psf) with molecular weight 35 000 Da, aniline (99.5%) and TiO₂ nanoparticles were purchased from Sigma Aldrich Co. Bangalore, India. Polyethylene glycol 1000 (PEG 1000) was purchased from Himedia Laboratories Pvt. Ltd, India. Ammonium peroxydisulfate (APS) and Bovine Serum Albumin (BSA) was purchased from Central Drug House (CDH), New Delhi. Hydrochloric acid (HCl), sodium hydroxide (NaOH) and N-methyl-2-pyrrolidone (NMP) was purchased from Merck India, Ltd. Polyethyleneimine (PEI) (M_n ∼ 60 000) 50 wt% aq. solution, branched was purchased from Acros Organics, USA. Lead(II) nitrate and cadmium nitrate tetrahydrate and were purchased from Sigma-Aldrich Co, Bangalore, India.

2.2. Preparation of PANI coated TiO₂ nanotubes

Preparation of TiO₂ nanotubes from TiO₂ nanoparticles was done as per the literature.¹⁶ About 50 mL of 10 M NaOH solution was taken in a round bottom flask attached to a condenser with chilling. The solution was first heated to 120 °C followed by the addition of 2 g of TiO₂ nanoparticles. The solution was then refluxed at the same temperature for 48 h under stirring. Then the mixture was cooled to room temperature and poured into de-ionised water. After the nanotubes were settled at the bottom, the supernatant solution was decanted and was washed repeatedly with water. 150 mL of 0.1 M HCl was then added to the solution and was stirred for 12 h at 60 °C. Then the nanotubes were again washed with de-ionized water until the neutral pH was obtained. The solution was centrifuged and the obtained nanotubes were calcined at 450 °C for 4 h.

Surface modification of the TiO₂ nanotubes by PANI was carried out by chemical oxidative polymerization as follows.¹⁹ 1 mL of aniline was added to 100 mL of 2 M HCl solution. 1 g of TiO₂ nanotubes were added to 5 mL of water and sonicated for a few minutes in order to prevent the agglomeration of the TiO₂ nanotubes. This solution was then added to the solution containing aniline and stirred for two hours. The polymerization of aniline was carried out by the addition of ammonium persulfate solution with an equal molar ratio to aniline to the above suspension at room temperature. The product formed was isolated from the dispersion by centrifugation, washed with HCl and water and dried at 80 °C for 24 h.

2.2.1. Characterization of PANI coated TiO₂ nanotubes. PANI coated TiO₂ nanotubes were characterized by Fourier Transform Infrared (FTIR) spectroscopy, X-ray diffraction (XRD) and Transmission Electron Microscope (TEM). FTIR spectra were recorded on a SHIMADZU ATR-FTIR spectrophotometer. XRD measurements were obtained using Rigaku Miniflex 600 with Cu Kα radiation. A transmission electron microscope (JEOL JEM-2100) was used to analyze the morphology of PANI coated TiO₂ nanotubes.

2.3. Preparation of membranes

The nanocomposite membranes were prepared by the phase inversion method. Firstly, PANI coated TiO₂ nanotubes were dispersed in NMP and sonicated for a few minutes for good dispersion. 5 wt% of PEG 1000 was dissolved in NMP and was added as a pore former for all the membranes. 2 g of Psf was added to the same dispersion, and was stirred at 60 °C for 6 h. After the complete dissolution of polysulfone, the casting solution was sonicated for 15 min and left still under heating without stirring for 30 min to remove any trapped air bubbles. Then the homogenous solution was casted onto a clean glass plate. After casting, the glass plate was immersed in a coagulation bath (water) for the phase inversion. The obtained membranes were washed thoroughly with water and dried. The composition of the prepared membranes is given in Table 1.

Table 1 Composition of the membranes

Membranes	Psf (wt%)	PEG (wt%)	PANI–TiO2 (wt%)	NMP (wt%)
P-0	20	5	0	75
P-0.05	20	5	0.05	74.95
P-0.5	20	5	0.5	74.5
P-1.0	20	5	1.0	74
P-1.5	20	5	1.5	73.5

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2.3.1. Membrane characterization.
2.3.1.1. Morphology of membranes. The morphology of the prepared membranes was examined using a Scanning Electron Microscope (JEOL JSM-6380LA). Before the Scanning Electron Microscope (SEM) analysis, the membranes were dipped and broken in liquid nitrogen and sputtered with gold for conductivity.
2.3.1.2. AFM measurements. The AFM (Atomic Force Microscope) measurements were carried out using an Innova SPM Atomic Force Microscope. The measurements were done on dry membrane samples. The membrane surfaces were imaged in tapping mode. An antimony doped silicon cantilever having a force constant in the range of 20–80 N m⁻¹ was used to analyze the membrane surface topography. The surface roughness of the membranes, expressed in terms of average roughness (R_a) and root mean square roughness (R_q) was measured.
2.3.1.3. Porosity and pore size. The porosity (ε) of the membranes was obtained by a gravimetric method.²⁰
where w₁ and w₂ is the weight of the wet and dry membranes respectively. ‘A’ is the effective membrane area (m²), ‘l’ is the thickness of the membrane and ‘d_w’ is the density of water.

Surface pore size of the membranes in terms of mean pore radius (r_m) was analyzed using Guerout–Elford–Ferry equation,²⁰ taking into the account porosity and water flux of the membranes.

where ‘ε’ is the porosity, ‘η’ is the viscosity of water (8.9 × 10⁻⁴ Pa s), ‘l’ is the thickness of the membrane, ‘Q’ is the volume of pure water permeated through the membrane per unit time (m³ s⁻¹), ‘A’ is the effective membrane area (m²) and ΔP is the operating pressure (0.2 MPa).

2.3.1.4. Water uptake study. In order to study the water uptake capacity of the membranes, membrane samples were cut into 1 cm² size and kept immersed in distilled water for 24 h. The wet membranes were weighed immediately after blotting the surface water. The membrane samples were then dried in the vacuum oven at 45 °C for a few hours. The weight of the dry membrane samples was noted. From the wet and dry weights of the membranes, percent water uptake by the membranes was calculated using the equation.
where ‘W_w’ and ‘W_d’ are the weights of the wet and dry membranes respectively.
2.3.1.5. Contact angle measurements. To evaluate the hydrophilicity of the membranes, the contact angle between water and the membrane surface was measured by the sessile droplet method using a FTA-200 Dynamic contact angle analyzer.²¹ In order to minimize the experimental error, for each sample the contact angle was measured at three different locations and the average value was considered.
2.3.1.6. Pure water flux. The permeation properties of the membranes were tested using a dead end filtration cell. An effective membrane area of 5 cm² was used for permeation studies. The membranes were initially dipped in distilled water for 24 h before the filtration experiments. The membranes were compacted at 0.4 MPa transmembrane pressures (TMP) for 30 min. Then the pressure was reduced to 0.2 MPa, TMP and time dependent pure water flux of the different membranes was measured for every 1 min time interval. The pure water flux (PWF) of the membranes was calculated using the following equation:
where J_w1 is the pure water flux expressed in L m⁻² h⁻¹, Q is the quantity of pure water collected (L) in time Δt (h), A is the effective membrane area (m²).
2.3.1.7. Antifouling study. Bovine Serum Albumin (BSA) was taken as a model protein to study the antifouling nature of the prepared membranes. An aqueous solution of BSA with a concentration of 0.8 g L⁻¹ was placed in the filtration cell after the pure water flux measurement J_w1 (L m⁻² h⁻¹).²² Then the BSA solution was filtered through the membranes for 30 min at 0.2 MPa, TMP. After the BSA filtration, the fouled membranes were washed with pure water for 20 min and then the water flux J_w2 (L m⁻² h⁻¹) was measured again. In order to estimate the fouling resistant ability of the membranes, flux recovery ratio (FRR) was calculated using the equation.

2.3.1.8. Heavy metal ion rejection. Heavy metal ion rejection by the membranes was carried out by a polymer enhanced ultrafiltration (PEUF) and ultrafiltration (UF) process. For PEUF process, aqueous solutions of Pb²⁺ and Cd²⁺ were prepared at an initial concentration of 1000 ppm in 1 wt% of the PEI aqueous solution. The pH of the solutions was adjusted to 6.25 by adding small amounts of either 0.1 M HCl or 0.1 M NaOH. Solutions containing the metal ions and PEI were mixed thoroughly and left standing for 5 days for completion of binding between metal ions and PEI. PEI complexed metal ion solutions were filtered through the membranes and the permeate was collected. In the UF process, feed solutions containing 1000 ppm of Pb²⁺ and Cd²⁺ were filtered individually through the membranes and the permeate was collected. The %rejection of the metal ions by the membranes during the filtration was determined by analyzing the concentration of feed and permeate solution using an Atomic Absorption Spectrophotometer (GBC 932 Plus). Heavy metal ion rejection percentage by the membranes was calculated using the formula
where C_p (mg ml⁻¹) and C_f (mg ml⁻¹) are concentration of metal ions in the permeate and the feed respectively.

3. Results and discussion

3.1. PANI coated TiO₂ nanotube characteristics

PANI coated TiO₂ nanotubes were prepared via chemical oxidative polymerization. Firstly, the synthesised TiO₂ nanotubes were dispersed in HCl solution containing aniline. In the presence of HCl, aniline gets transformed to anilinium cation. Under acidic conditions, the anilinium ions adsorb on the surface of TiO₂ nanotubes and then undergo oxidative polymerization in the presence of ammonium persulfate (NH₄)₂S₂O₈, leading to the formation of PANI coating on TiO₂.¹⁵

FTIR spectra of pure TiO₂, pure PANI and a PANI–TiO₂ nanocomposite were obtained in the range of 500–3700 cm⁻¹. Fig. 1 shows FTIR spectra. Pure TiO₂ shows a broad peak at around 3400 cm⁻¹ and a small peak at 1638 cm⁻¹ which is due to the stretching vibration of the hydroxyl group (–OH) and adsorbed water present on TiO₂.²³ A broad peak at 600 cm⁻¹ ranging from 1000 to 550 cm⁻¹ is due to the Ti–O–Ti bonds. The FTIR spectrum of PANI shows a peak at 1567 cm⁻¹ and 1487 cm⁻¹ which is due to the C=N and C=C stretching vibrations of quinoid and benzenoid rings of PANI respectively. Further peaks at 1298 cm⁻¹ and 1246 cm⁻¹ can be ascribed to the C–N stretching mode of the benzenoid ring. Peaks at 1141 cm⁻¹ and 812 cm⁻¹ correspond to the C–H in plane and out of plane bending vibrations respectively.²⁴ All these peaks of PANI were observed in the IR spectrum of the PANI–TiO₂ nanocomposite along with the broad –OH peak at 3400 cm⁻¹ which was due to the surface hydroxyl group of TiO₂. Also the broad peak ranging from 1000 to 550 cm⁻¹ due to Ti–O–Ti vibrations was observed in the PANI–TiO₂ nanocomposite, indicating the presence of PANI on TiO₂.

Fig. 1 FTIR spectra of PANI, a PANI–TiO₂ nanocomposite and TiO₂.

X-Ray diffraction patterns of the PANI–TiO₂ nanocomposite, compared with pure PANI and pure TiO₂ are shown in Fig. 2. Pure PANI shows two broad peaks at 2θ = 17° and 23° which can be ascribed to the (111) and (110) planes. The XRD pattern of pure TiO₂ shows sharp peaks at 25.3°, 37.8°, 48.1°, 54.6° and 62.6° with respect to (101), (103), (200), (105) and (213) planes respectively of the anatase phase of TiO₂.^10,25 The PANI–TiO₂ nanocomposite showed two broad peaks at 2θ = 17° and 23° and also all sharp peaks of TiO₂ which confirmed the presence of PANI coating on TiO₂ nanotubes.

Fig. 2 X-Ray diffraction patterns of pure PANI, a PANI–TiO₂ nanocomposite and pure TiO₂.

TEM was carried out to observe the shape and size of PANI coated TiO₂ nanotubes. TEM images of PANI coated TiO₂ nanotubes are shown in Fig. 3. PANI coated TiO₂ nanotubes had diameter in the range of 40–50 nm.

Fig. 3 TEM images of PANI coated TiO₂ nanotubes (A and B).

3.2. Membrane characterization

3.2.1. Membrane morphology. The internal structure and the pore morphology of all the membranes were studied using SEM. Fig. 4 displays the cross-sectional morphology of the prepared membranes. The cross sectional images showed that, all the membranes exhibited typical asymmetric structure with a dense top layer and a porous sub layer. The nanocomposite membranes had well interconnected finger like pores from the membrane top layer to the sub layer. The addition of PANI–TiO₂ to the membranes brings about a change in membrane structure. The finger like pore walls were linked with each other by a sponge wall, which allow the finger like pores to communicate with each other. This is due to the well distribution of PANI–TiO₂ nanocomposite in the membranes.¹¹ The SEM images showed that membrane porosity increased along with pore length as the PANI–TiO₂ content in the membranes increased. Also the skin layer becomes thin and more porous with PANI–TiO₂ content. This is due to the migration behavior of PANI i.e. during membrane formation a hydrophilic additive would migrate near the membrane water interface in order to minimize the interfacial energy, thus acting as a hydrophilic modifying agent.¹³ As a pore former, the addition of PEG would also favor the pore formation. However the effect of PEG on the membrane morphology would be same for all membranes as same amount PEG was added to all the membranes.

Fig. 4 Cross sectional images of (a) P-0 (b) P-0.05 (c) P-0.5 (d) P-1.0 and (e) P-1.5 membranes (A, B, C, D and E are the enlarged images of a, b, c, d and e respectively).

3.2.2. Surface topography of membranes. The surface roughness of the membranes was studied using AFM under tapping mode. Fig. 5 shows two-dimensional and three-dimensional scans of P-0, P-0.5 and P-1.0 membranes. From the images, it can be seen that, the surface of the membrane P-0 was very rough and the surface roughness reduced for membranes P-0.5 and P-1.0, containing the PANI–TiO₂ nanocomposite. The decrease in membrane surface roughness may be due to the uniform dispersion of PANI–TiO₂ in the polymer matrix.^4,12 The decrease in surface roughness of the membranes was confirmed by the surface roughness parameters obtained for P-0, P-0.5 and P-1.0 membranes (Table 2). The mean roughness (R_a) and root mean square roughness (R_q) was 334 and 402 nm for P-0, 54.2 and 67 nm for P-0.5 and 41.6 and 51.8 nm for P-1.0 membrane respectively.

Fig. 5 Two-dimensional scans of (A) P-0, (B) P-0.5 and (C) P-1.0 membranes and three-dimensional scans of (D) P-0, (E) P-0.5 and (F) P-1.0 membranes.

Table 2 Surface roughness parameters of membranes

Membranes	Roughness parameters
	R a (nm)	R q (nm)
P-0	334 ± 17.2	402 ± 29.0
P-0.5	54.2 ± 6.9	67 ± 7.1
P-1.0	41.6 ± 1.5	51.8 ± 4.8

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3.2.3. Porosity and pore size of membranes. The surface pore radius and porosity of the membranes are presented in Table 3. The addition of a PANI–TiO₂ nanocomposite to the membranes increases the pore size of the membranes except for P-0.5. The higher hydrophilicity of PANI coated TiO₂ nanotubes at higher content would cause an increase of pore size. It is observed that, with the initial addition of PANI–TiO₂, the membranes showed a sudden increase in pore size for P-0.05 with 0.05 wt% of PANI–TiO₂ content. The initial increase in pore size may be due to the reason that the addition of nanocomposites to the casting solution increases the thermodynamic instability of the solution, causing rapid demixing, resulting in large pore formation at lower concentration. The low pore radius for the P-0.5 membrane is due to the reason that as the addition of PANI–TiO₂ is increased from 0.05 wt% to 0.5 wt%, the viscosity of the casting solution is increased. This delays the exchange of solvent and nonsolvent which results in the suppression of the pore size.^13,26 The membrane with the highest PANI–TiO₂ content showed slight reduction of the pore size, which might be due to the clogging of pores by the nanocomposite. The clogging of pores at higher concentration of the PANI nanocomposite has also been observed in the literature.¹¹ The porosity measurements indicated that, the nanocomposite membranes have higher porosity than the pristine membrane. Thus the addition of PANI–TiO₂ has led to the increase in porosity of membranes which is evident from SEM images.

Table 3 Mean pore radii and porosity of membranes

Membranes	Mean pore radius (nm)	Porosity (%)
P-0	8.25	53.36
P-0.05	14.21	69.60
P-0.5	7.02	76.09
P-1.0	10.72	72.34
P-1.5	8.52	82.60

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3.2.4. Water uptake. Fig. 6 shows the water uptake by the membranes. The least water uptake is shown by membrane P-0 whereas the water uptake of nanocomposite membranes is higher and increases with the increase in the PANI–TiO₂ nanocomposite. The water uptake property of the membranes depends on the porosity of the membranes. As the porosity of the P-0 membrane is least among all the membranes (Table 3), the P-0 membrane showed least water uptake. The increase in water uptake by PANI–TiO₂ nanocomposite membranes with PANI–TiO₂ is due the increasing porosity of the membranes. A slight decrease in water uptake shown by the P-1.0 membrane may be due to the less porosity observed for the membrane.

Fig. 6 Water uptake by the membranes.

3.2.5. Contact angle of membrane surfaces. The contact angle of membranes represents the hydrophilicity and wettability of membranes. Fig. 7 shows the contact angle of the membranes. Membrane P-0 showed highest contact angle indicating its hydrophobic nature. Membrane P-0 did not contain any PANI–TiO₂ nanocomposite and the high contact angle is supposed to be due to the hydrophobic nature of polysulfone. The contact angle of PANI–TiO₂ nanocomposite membranes was in the range of 66–64°. Also the contact angle decreased with the increase of PANI–TiO₂ content in the membranes. The membrane with 1.0 wt% PANI–TiO₂ content showed least contact angle. The decrease in the contact angle with the increasing concentration of PANI–TiO₂ in membranes indicated that the PANI–TiO₂ nanocomposite enhanced the hydrophilicity of the membranes. Fig. 8 shows the images of the contact angle measured on the membranes.

Fig. 7 Contact angle of the membranes.

Fig. 8 Images of contact angle measured on the membranes.

3.2.6. Permeation properties. Permeation properties of the membranes were studied by permeating pure water through the membranes using a dead end filtration cell at 0.2 MPa, TMP. The pure water flux of the membranes is shown in Fig. 9. All the nanocomposite membranes showed higher permeate flux than the polysulfone membrane. The nanocomposite membranes possessed higher hydrophilicity, higher porosity and better vertically interconnected pores than the polysulfone membrane which led to the water permeability of the membranes. The pure water flux of the membranes is in the order P-0 < P-0.5 < P-1.5 < P-0.05 < P-1.0. The membranes P-0.05 and P-1.0 showed higher water flux among the synthesized membranes. This is due to the higher pore sizes of these membranes when compared to the other membranes (Table 3). Among P-0.05 and P-1.0 membranes, though the pore radius of P-1.0 was lower than that of P-0.05 membrane, the higher PWF is due to the higher hydrophilic nature of P-1.0 membrane than the P-0.05 membrane. The contact angle measurements showed that, the P-1.0 membrane possessed highest hydrophilicity. Higher the PANI–TiO₂ content, higher would be the hydrophilic nature of PANI and TiO₂ which would attract water molecules easily into the membrane matrix thereby increasing the permeability.⁶ Among P-0, P-0.5 and P-1.5 membranes, P-0 showed least PWF due to the reason that the P-0 membrane is hydrophobic in nature and did not contain any PANI–TiO₂ nanocomposite. Though the pore radius of P-0.5 was lesser than that of the P-0 membrane, the PWF is higher because of the presence of hydrophilic PANI–TiO₂ and higher porosity of the membrane. The P-1.5 membrane had higher PWF than P-0 and P-0.5 membranes, because its pore size was higher than that of both P-0 and P-0.5 membrane. However the PWF of P-1.5 membrane was lower than that of P-0.05 and P-1.0 membranes, because the presence of excessive PANI–TiO₂ content led to the clogging of pores which in turn decreased the permeability of the membrane. Similar results showing a decrease in flux with the increase in concentration of PANI and PAni based nanocomposites in the membranes has been found in literature.^13,24

Fig. 9 Pure water flux of the membranes.

3.2.7. Antifouling properties. The fouling nature of the membranes was investigated through protein filtration, using BSA as a model protein. The flux of the membranes during BSA filtration is shown in Fig. 10. The figure shows decline in flux of membranes for initial few minutes. This initial decline of flux is due to the adsorption of BSA molecules onto the surface of the membranes.²⁷ All the membranes showed higher flux than the P-0 membrane. Since the nanocomposite membranes had higher hydrophilicity, protein molecules could not be easily adsorbed on the membrane surface indicating the antifouling nature of membranes.¹³ The %FRR values of the membranes are shown in Fig. 11. Higher the FRR value better is the antifouling nature of the membranes.

Fig. 10 Flux of the membranes during BSA filtration.

Fig. 11 %FRR values of the membranes.

The P-0 membrane had the least FRR value among all the membranes. The membranes showed increase in the FRR value with PANI–TiO₂ content, indicating that addition of PANI–TiO₂ enhance the fouling resistance of membranes. As the hydrophilicity of the membranes increased with PANI–TiO₂ content, it weakened the interaction between the membrane surface and the protein molecules. Thus the adsorbed BSA molecules on the membrane surface were easily removed by simple washing resulting in a high flux recovery ratio. The P-1.5 membrane showed little decrease in FRR value. This may be due to the decrease in hydrophilicity as observed by contact angle measurement.

The AFM results further confirmed that, addition of the PANI–TiO₂ nanocomposite into the membranes improved the antifouling nature of membranes. It is well known that, lower the surface roughness of the membranes, better is the antifouling ability. From Fig. 5 and Table 2 it can be seen that membrane P-0 with no PANI–TiO₂ content was rough and surface roughness was reduced for P-0.5 and P-1.0 membranes. This decrease in surface roughness increased the antifouling nature of the membranes since the foulants are more likely to adsorb on course surfaces than smooth surface.²⁸

3.2.8. Heavy metal ion rejection. The heavy metal ion rejection by the membrane for metal ions Pb²⁺ and Cd²⁺ was studied by the UF and PEUF process. Among the prepared membranes, the well performed membrane P-1.0 which possessed highest hydrophilicity, better permeability, highest FRR value was selected for the heavy metal ion rejection studies. Fig. 12 displays the %rejection by the membrane during PEUF and UF process for heavy metal ions Pb²⁺ and Cd²⁺ respectively. The membrane exhibited 83.75% rejection for Pb²⁺ ions and 73.41% rejection for Cd²⁺ ions during PEUF. The %rejection was higher for Pb than Cd this is because Pb²⁺–PEI complex was larger in size than Cd²⁺–PEI complex. The %rejection for Pb²⁺ and Cd²⁺ during UF was 68% and 53.78% respectively. The rejection of metal ions during the UF process is due to the adsorption of the metal ion on the membrane surface. Fig. 13 shows the elemental mapping of Pb and Cd adsorbed on the membranes. The main adsorption sites for metal ions were the nitrogen atoms of PANI of the PANI–TiO₂ nanocomposite.²⁹ The unpaired electrons of the amine group of PANI can create a co-ordinate bond with metal ions resulting in the adsorption of metal ions on the membrane surface.³⁰ The membranes showed higher % of rejection during the PEUF process than the UF process. This is because the metal ion–PEI complex has greater size than the membrane pore size. Hence the complexed metal ions are retained in the feed solution. During the UF process, most of the metal ions would easily pass through the membrane, as the pore size of the membrane is much higher than that of the metal ions. Only the metal ions adsorbed on to the membrane surface would contribute to the %rejection.

Fig. 12 Heavy metal ion rejection by the membranes.

Fig. 13 Adsorption of Pb and Cd on the membranes.

4. Conclusions

The performance of the polysulfone membranes can be improved by using a PANI–TiO₂ nanocomposite as a hydrophilic additive. The properties of the membranes such as hydrophilicity, permeability, water uptake, porosity, antifouling nature were enhanced with the addition of the PANI–TiO₂ nanocomposite. The nanocomposite membranes also showed good rejection for Pb²⁺ and Cd²⁺ ions by the PEUF process. Taking into account the contact angle, PWF, %FRR value, it is found that the properties of the membranes improved when the addition of the PANI–TiO₂ nanocomposite was increased up to 1.0 wt%. When the concentration of the PANI–TiO₂ nanocomposite in the membranes was further increased to 1.5 wt%, the membranes showed slight decrease in performance. The decrease in performance by the membranes may be due to the agglomeration of the nanocomposite at higher concentration leading to clogging of pores. Thus it can be concluded that 1.0 wt% of the PANI–TiO₂ nanocomposite is the threshold content for the synthesized membranes, beyond which the membranes show a decrease in performance.

Acknowledgements

AMI is thankful to Prof. Swapan Bhattacharya, Director, National Institute of Technology Karnataka, Surathkal, India, for providing the research facilities. The authors thank Universiti Teknologi Malaysia for the TEM analysis. The authors also thank Prof. K. Narayan Prabhu and Prof. Rajendra Udupa of Department of Metallurgical and Materials Engineering, NITK Surathkal, India, for providing the contact angle measurement facility and SEM facility.

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