TiO2/Polysulfone Composite Membrane for Enhanced Removal of Acetaminophen from Water

Abstract

There has been a significant rise in the worldwide usage of acetaminophen (ACT), particularly after the COVID-19 pandemic, leading to concerns regarding its possible discharge into the environment. Even at low levels, the gradual buildup of ACT can present a significant risk to both aquatic ecosystems and human health, especially when consumed through drinking water. The advanced oxidation process (AOP) is an excellent treatment technique for removing ACT. However, it is a complex and expensive procedure that consumes a significant amount of energy. Membrane technology has evolved as a viable and efficient method to eliminate ACT, offering a simpler and more energy-efficient approach. This study examined the impact of different concentrations of titanium dioxide (TiO,) nanofiller on the elimination of ACT from a water-based solution. The membrane containing the highest concentration of TiO, (2 wt.%) demonstrated the greatest water flux and ACT removal rate at 32 L'm·h and 97%, respectively. The inclusion of TiO, had a substantial impact on the structure of the membrane, leading to the formation of evenly distributed finger-like pores throughout a spongy framework. This modification resulted in enhanced flow rate and increased efficiency in terms of the flux and rejection rate.

Headnote

ABSTRACT: There has been a significant rise in the worldwide usage of acetaminophen (ACT), particularly after the COVID-19 pandemic, leading to concerns regarding its possible discharge into the environment. Even at low levels, the gradual buildup of ACT can present a significant risk to both aquatic ecosystems and human health, especially when consumed through drinking water. The advanced oxidation process (AOP) is an excellent treatment technique for removing ACT. However, it is a complex and expensive procedure that consumes a significant amount of energy. Membrane technology has evolved as a viable and efficient method to eliminate ACT, offering a simpler and more energy-efficient approach. This study examined the impact of different concentrations of titanium dioxide (TiO,) nanofiller on the elimination of ACT from a water-based solution. The membrane containing the highest concentration of TiO, (2 wt.%) demonstrated the greatest water flux and ACT removal rate at 32 L'm·h and 97%, respectively. The inclusion of TiO, had a substantial impact on the structure of the membrane, leading to the formation of evenly distributed finger-like pores throughout a spongy framework. This modification resulted in enhanced flow rate and increased efficiency in terms of the flux and rejection rate.

Keywords: Polysulfone, titanium dioxide, acetaminophen removal, membrane separation, nanofiller

(ProQuest: ... denotes formulae omitted.)

1. INTRODUCTION

Drinking waters, other water reserves and the effluents of sewage treatment plants (STPs) are frequently found to contain pharmaceutical active compounds (PhACs). This is primarily because new chemical compounds are being used more frequently as a result of medical technology advancements. These compounds can have negative effects on the environment and public health even at low concentrations, which leads to the discovery of new micropollutant traces in the water environment. The release of these intricately structured and chemically complex micropollutants into water requires the development of increasingly sophisticated and effective methods to address the problem.

Acetaminophen (ACT), commonly referred to as paracetamol is a widely used painkiller that is available over the counter without a prescription. Due to its widespread use across the globe, ACT may be released into the environment through several means, including animal and human excretion, inappropriate drug disposal and wastewater treatment plant effluents. Because of this, trace amounts of ACT have been found in drinking water sources, groundwater and surface water across the globe. According to Shipingana et al., 4.6 pg/ml of ACT was detected at the inlet of a municipal wastewater treatment in Mysuru City, India. Its presence in water bodies may affect aquatic organisms ecologically. Even though the concentrations of ACT in water sources are typically low and unlikely to pose a direct risk to human health through drinking water consumption, there is growing concern about the potential accumulation of these PhACs over time in the environment and their long-term effects on ecosystems and human health.

Advanced oxidation processes (AOPs), adsorption and membrane technology are examples of novel treatment techniques that have improved wastewater treatments at present, particularly in the removal of these micropollutants.' In the case of АОР, this entails the production and application of an oxidant; the hydroxyl radical through several intricate processes, including ozonation and photolysis. The method removed PhACs with high efficacy, but it exhibited significant flaws, particularly concerning cost, energy consumption and process complexity. Membrane technology shows superior advantages over other technologies with similar separation efficiency since it is more cost-effective, involves a simpler setup and ecologically friendly. Membranes are commonly made of polymer and ceramic materials, and polymeric membranes are the most popular due to their higher selectivity and can be easily functionalised or modified to suit specific pollutants.

Polysulfone (PSf) is extensively utilised as an efficient membrane material owing to its ability to synthesise polymeric membranes for various applications via simpler methods such as phase inversion, but its inherent hydrophobicity restricts its application in water separation.?· Various studies have been performed to enhance its hydrophilicity including the usage of complicated synthesis method such as chemical grafting and plasma modification. The incorporation of additives, specifically adsorptive and hydrophilic particles, such as activated carbon, Al,O;, $10, and TiO, by polymer/additive blending has shown potential in improving the membrane structure and effectively removing toxic particulates such as ACT. Nadour et al. demonstrated that the addition of methylcellulose and activated carbon as adsorptive particles into the PSf membrane can significantly improve surface hydrophilicity and membrane rigidity, subsequently enhancing the water permeability and increasing ACT removal.' TiO, in particular, have also attracted augmented interest due to their good chemical stability, elevated hydrophilicity and antibacterial properties.® Zhang et al. reported that the addition of TiO, sol in the PSf blend significantly improves the overall properties of the membrane especially in terms of porosity, hydrophilicity and anti-fouling." Several other works also reported on the enhanced hydrophilicity and other properties such as membrane morphology of PSf and polyethersulfone (PES) membranes after the addition of TiO,.5°

This work aims to evaluate the influence of varying TiO, concentrations in the PSf membrane matrix on the membrane characteristics and the removal of ACT. In our knowledge, no study was done to evaluate the effect of TiO, addition into PSf membrane to remove ACT. Lower concentrations of TiO, in the range of 0 wt.%-2 wt." were chosen for this preliminary study since it has been reported previously that the addition of higher concentrations of TiO, (> 2 wt.%) could result in severe particle aggregation, subsequently reducing the efficiency of the membrane."!· The membranes were prepared via the simple immersion precipitation method. The performance of the membranes was evaluated to remove ACT from aqueous solution and was compared to those obtained by membrane without TiO,.

2. EXPERIMENTAL

2.1 Materials

Polysulfone (PSf, Udel® P-1700, MW = 67,000 g/mol) was acquired from Solvay Solexis, France. Titanium Dioxide (TiO,, Degussa P25) was purchased from Merck. N-methyl-2-pyrrolidone (NMP, > 99.5%) and ACT were obtained from Sigma Aldrich. Deionised (DI) water was used as the coagulant. Liquid and gaseous nitrogen (N,) were provided by Bann Edar, Malaysia. The PSf and TiO, powders were dried overnight at 100°C in an oven before use. Other chemicals were used as received.

2.2 TiO,/PSf Membranes Synthesis

TiO,/PSf membranes were prepared via the immersion precipitation method and the dope solutions were prepared following the methods from Ramli et а!!! Various concentrations of TiO2 NPs (0.5 wt.%-2.0 wt.%), as shown in Table 1, were dissolved in NMP at 60°C and stirred at 300 rpm. Subsequently, PSf was added gradually to the mixture under continuous stirring for 6 h or until all the powder dissolved. The solution was degassed, cooled and cast using a casting machine with a pre-set casting thickness of 250 um. The casted film was rapidly immersed in a coagulation bath containing DI water for 24 h. The synthesised membranes were then washed and dried.

2.3 Membrane Characterisations

The surface and cross-sectional morphologies of the membranes (1 cm x 1 cm) were examined under the scanning electron microscope (SEM; JEOL, JSM6460LA) at an accelerating voltage of 15 kV. For the cross-sectional images, the synthesised membranes were fractured in liquid N, and coated with platinum. The fourier transform infrared (FTIR) spectra of the membranes were established using an FTIR Spectrophotometer (Perkin Elmer) in the region of 650 cm'- 4,000 cm™ to confirm the addition of TiO, into the PSf backbone. The membrane was immersed in DI water for 2 h to measure the membrane's porosity. Before determining the weight of the wet membrane, w,, the membrane was gently patted to remove any remaining liquid from the surface. The membrane was then dried in an oven for 48 h to make sure it was completely dry before measuring the weight of the dried membrane, w,. The membrane was weighed periodically until the weights were constant. The membrane porosity, e can be calculated using Equation 1:2

... (1)

where w, is the weight of the wet membrane (g), w, is the weight of the dry membrane (g), р. is the density of water and p, is the density of PSf.

The average pore radius (m), 7,, can be calculated using the Guerout-Elford-Ferry equation of the pore flow model (Equation 2):·

... (2)

where 77 is the pure water viscosity (8.9 x 10" Ра 5), AP is the trans-membrane pressure (Pa), 1 is the membrane thickness (т) and О is the permeate volume of water per unit of time (m/s).

2.4 Membrane Performance

The performance of the synthesised membranes was evaluated using a deadend stainless steel filtration setup, as shown in Figure 1. Membranes with an effective area of 25 cm", 250 mL of water and 100 mg/L ACT solution were used at a transmembrane pressure of 1 bar and filtration time of 2 h, respectively." The membrane was first pre-pressurised at 5 bar for 30 min using distilled water to achieve steady-state operation before continuing with the water flux measurement.'· The feed was then replaced with the ACT solution to establish the ACT flux and rejection percentage of the membranes. The permeate was collected and concentrations of the ACT in the feed and permeate were measured at a wavelength of 254 nm using a UV-Vis spectrometer (GENESYS 20).'

The water and ACT fluxes (Fy, Facr) were calculated using Equation 3:'·

... (3)

where Y, F, A and At are the permeate volume (L), membrane flux (L/m°h"), membrane effective area (m·) and the filtration time (h), respectively. The rejection, R of the membranes can be calculated using Equation 4:16

... (4)

where R, C, and C, are the solute rejection (%), and concentration of ACT in the permeate and the feed, respectively.

3. RESULTS AND DISCUSSION

3.1 TiO,/PSf Membrane Characterisations

3.1.1 Surface and cross-sectional morphologies of the membranes

The addition of hydrophilic particles such as TiO, were often proposed since these often results in structure modification that can enhance separation performance. The SEM images of the synthesised TiO,/PSf membranes are presented in Figure 2. Similar morphologies were observed for all membranes; an asymmetric structure with a thin skin layer with no surface pores and a porous substructure comprising of both the finger-like pores and macrovoids. Moreover, the number of large pores increases with increasing TiO, concentration (see Figure 2[A, B, C and D]) but decreases when 2 wt.% of TiO, was added to the membrane solution (see Figure 2[E]). This might be associated with the controlling factors during the phase separation. The pore formation was controlled by the thermodynamics and the kinetics of the phase separation process. In terms of thermodynamics, hydrophilic TiO, would accelerate the exchange rate between NMP and water, facilitating the formation of greater number of pores structures. Kinetically, the introduction of TiO, particles would elevate the viscosity, resulting in slow exchange rate, inhibiting the formation of pore structure."

The substructure's finger-like pores have a greater propensity to transform into macrovoids at the addition of TiO, at low concentrations, and the macrovoids' length has extended across the membrane thickness, indicating the presence of the thermodynamics effect. The formation of macrovoids was closely related with the solvent and nonsolvent miscibility, as previously reported by Lai et al.' When a highly miscible solvent/nonsolvent combination such as NMP/water was used, this can promote the formation of macrovoids. Formation of this structure typically follows the instantaneous demixing at the nonsolvent/membrane solution interface, producing two phases; the polymer-rich phase at the surface and the polymer-deficient phase at the bottom of the membrane. The diffusion rates of the solvent and nonsolvent were accelerated at the nonsolvent/membrane interface. NMP continuously diffuses into the coagulation bath while water enters the cast membrane, resulting in significant water uptake and high membrane swelling. The rapid merge of PSf occurred at the membrane solution/water interface (polymer-rich region) as a result of PSf/water incompatibility and strong solvent/ nonsolvent diffusion, forming a skin layer. Not only that, the hydrophilic TiO, nanoparticles also tend to move towards the top layer of the membrane, forming a thicker skin layer." These then hinder the continuous nonsolvent inflow to the membrane solution below this skin layer, subsequently slowing down the exchange between the solvent and nonsolvent inside the membrane substructure. The growth of the polymer in this polymer-deficient phase was delayed, resulting in a looser and larger pore (macrovoids) structure."

Membrane with higher TiO, concentration (Ti-2.0) however, showed a similar asymmetric structure but with fewer macrovoids and uniformly built finger-like pores and spongy substructure, suggesting a void-suppressing factor was at play and the transition from porous to spongy structure. At higher TiO, concentrations, the viscosity of the dope solution drastically increased due to its larger specific area and surface energy.'·?! This led to a slower solvent/non-solvent exchange rate, causing a delayed demixing.?" Additionally, polyvinylpyrrolidone (PVP), which acts as the pore former tends to migrate out during phase inversion." This resulted in a deficiency of pore former, supressing the formation of macrovoids and leading to the production of a finger-like and spongy substructure.?!·

Based on the images of the membrane surfaces, white residues were observed on the surface of the membranes with TiO,, suggesting the occurrence of TiO, agglomeration and it increases with increasing TiO, concentration from 0.5 wt.%-2.0 wt.%. This phenomenon can be advantageous because this can increase the hydrophilicity of the membrane surface but at the expense of pore blocking. It is believed that significant agglomeration was observed when the attractive intermolecular forces between TiO, molecules dominate. This can happen when the surface charge of TiO, is nearly zero, in which, the Van der Waals attractive forces are dominant, resulting in increased attractive forces between the TiO, molecules and they tend to agglomerate. The agglomerates reduce the effective surface area and therefore, negating the hydroxyl groups on the PSF surface during the demixing process, inhibiting the uniform dispersion of TiO, in the membrane. These results are in good agreement with previous reports by Sotto et al. and Vatanpour et al.?526

3.1.2 FTIR analysis

The functional groups of the synthesised TiO,/PSf membranes were evaluated using FTIR as shown in Figure 3. FTIR spectra show similar prominent peaks at 2,970 em, 1,689 cm, 1,585 cm! and 1,240 cm, verifying the presence of PSf as the polymer backbone. This is consistence with the results reported by Kusworo et al. for their PSf-TiO,/GO membranes." The characteristic peaks observed at 1,240 cm! and 1,585 cm"! confirmed the existence of the O=S=0 stretching and the stretching band of C=C aromatic of the PSf polymer, respectively."" A weak peak at 2,970 cm! shows the presence of the C-H group in all membranes although at a very low intensity. A new peak at 690 cm! was observed for membranes with TiO, except for the neat membrane (Ti-0.0), which can be attributed to the presence of TiO, inside the membrane. This intense peak can be ascribed to the Ti-O stretching band which is the characteristic peak of Ti0,.2·?° Not only that, the broad peak at 3,398 cm, indicating the presence of the -OH stretching present in Ti-0.0 diminishes with the increasing concentration of the TiO, in the membrane." Since TiO, nanoparticles have also been reported to have similar peaks attributed to the OH stretching, it was believed that these hydroxyls in both TiO, and PSf lead to the formation of the hydrogen bridge bonds, due to the Van Der Waals interaction between the OH groups of TiO, and the hydroxyl group of the PSf membrane during the membrane synthesis.·! The presence of TiO,, which contains the hydroxyl groups, has also been reported to enhance the hydrophilicity of a membrane, thereby improving water flux.·?

3.1.3 Porosity and average pore size estimation

The membrane thickness, porosity and r,, for all membranes are presented in Table 2. Increased membrane porosity was observed with increasing TiO, concentration with the neat PSf membrane (Ti-0.0) recorded the lowest porosity of 74.62%. A noteworthy increase in porosity was recorded for Ti-2.0 membrane and this can be corroborated by the SEM images (see Figure 2[E]) where Ti-2.0 membrane shows a thicker porous substructure as compared to those of the neat PSf membrane (Ti-0.0). These findings can be explained in terms of hydrophilicity of the TiO, and the increase of structural vacancies or macrovoids in the polymer structure. As the concentration of hydrophilic TiO, increases, it increases the presence of the hydrophilic particles inside the PSf structure, accelerating the exchange rate between the solvent and non-solvent promoting the formation of macrovoids.?· The formation of larger structural vacancies also increases, subsequently increasing the porosity.

On the contrary, the average pore size radius, r,, of the synthesised TiO,/PSf membranes decreases from 16.37 nm-9.79 nm when adding TiO, up to 2.0 wt.% (Ti-2.0). This result disagrees with those reported by Yang et al., whereby the porosities of their fabricated PSf with TiO, were found to be directly proportional with the pore size." As porosity increases, more space is occupied by pores, allowing for larger pores to form within the membrane structure. However, the variation of surface topography and the size of the aggregated particles on the surface and through the finger-like pores could also affect the size of the pores. At higher TiO, concentrations, it is possible that the presence of TiO, particles on the membrane surface and inside the pores could potentially blocked the pores partially, decreasing the average pore size." The structural parameters such as porosity and thickness severely affect the transport of water.

3.2 Filtration Performance of TiO,/PSf Membrane for ACT Removal

The performance of these TiO,/PSf membranes was evaluated using a filtration system at a low transmembrane pressure of 1 bar. The membrane performance in terms of water flux, ACT flux and rejection is illustrated in Figure 4. The data revealed that the membrane with 2.0 wt.% TiO, (Ti-2.0) offered the highest rejection towards ACT and a larger water flux of around 32 L/m·h, which was 25% higher than that of the neat PSf membrane (Ti-0.0). This can be ascribed to an increment in porosity (see Table 2), which promotes the flow of water through the membranes. Even though the average pore size decreases and this usually deteriorates the permeability in many cases, the presence of the hydrophilic TiO, particles favourably promotes the transport of more water molecules, boosting the water flux of the membranes.

The observed ACT flux showed increment with increasing TiO, concentrations, with neat PSF membrane (Ti-0.0) recording the lowest ACT flux of 7.41 L/m·h, attributed to the larger pore size. The larger pore size of the Ti-0.0 membrane resulted in fast filtration as the solution passed through the pores rapidly, reducing the time for the membrane to interact with the ACT molecules.' Since Ti-0.0 membrane was synthesised solely from semi-hydrophobic polysulfone with low surface charge, it is more prone to fouling leading to low ACT flux.' The average pore size of the membrane decreases due to the introduction of the hydrophilic TiO, nanoparticles, subsequently promoting the removal of ACT. However, at 2.0 wt.% TiO,, no significant increase in flux was observed as compared to the membrane with 1.5 wt.% TiO, (Ti-1.5). This is expected since at higher concentrations, most of the TiO, particles agglomerates (see Figure 2), reducing the number of active sites for ACT to interact and can also result in pore blocking, subsequently hindering the ACT removal.

The water flux and ACT rejection of the synthesised membranes were compared (see Table 3) with other commercial polymeric membranes and membranes modified with additives such as methylcellulose (MC) and powdered activated carbon (PAC) under similar membrane filtration conditions as this work. The water flux recorded by Ti-2.0 was comparable to those PSf membranes with MC and PAC but at a higher rejection rate of ACT removal (97%) at low pressure of 1 bar. The reported results of this work revealed the potential of adding TiO, into the PSf membrane matrix to remove ACT in membrane technology, although a method to reduce the agglomeration of TiO, particles should be considered in the future as this can further elevate the performance of the membrane.

4. CONCLUSION

TiO,/PSf flat sheet membranes were successfully synthesised. The FTIR confirms the successful incorporation of the TiO, into the PSf membrane matrix. The performances of the membranes were evaluated in terms of water flux, ACT flux and ACT rejection percentage from aqueous solution containing 100 ppm ACT. The membrane with the highest TiO, concentration recorded the best ACT rejection and water flux of 97% and 32 L/m·h, respectively. This was attributed to its improved morphologies, which provides greater porosity (84.70%) and longer interaction times for ACT and TiO, particles. At lower concentrations, TiO, particles have successfully modified the membrane morphologies and characteristics to significantly improve the water flux and rejection rate of ACT. Limitations such as TiO, agglomerates observed on the membrane surface which resulted in the lower ACT flux obtained should be prevented. Better methods or surfactants for incorporating TiO, into the PSf membrane need to be explored.

5. ACKNOWLEDGEMENTS

The authors would like to acknowledge the support from the Centre of Excellence of Biomass Utilization (CoEBU), Universiti Malaysia Perlis (UniMAP) for assisting in sample preparation throughout this work.

Sidebar

Published online: 30 November 2024

To cite this article: Ramli, W. К. W., Dollah, N. & Onn, L. Q. (2024). TiO,/polysulfone composite membrane for enhanced removal of acetaminophen from water. J. Phys. Sci., 35(3), 1-15. https://doi.org/10.21315/jps2024.35.3.1

To link this article: https://doi.org/10.21315/jps2024.35.3.1

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