Abstract
Bisphenol A (BPA) is a persistent environmental pollutant known for disrupting endocrine function. Traditional removal methods, such as ozonation and granular activated carbon (GAC) adsorption, have limitations due to harmful byproducts, saturation, and operational issues. Biodegradation and extraction techniques show promise but lack efficiency. Ultrafiltration (UF) membranes, especially polyacrylonitrile (PAN) membranes, have proven effective for BPA removal, particularly when modified with materials like N-doped TiO2. However, fouling reduces their efficiency and lifespan, prompting research into anti-fouling coatings, hydrophilization, and the use of nanomaterials. Metal-organic frameworks (MOFs) and covalent organic frameworks (COFs) are promising materials for improving membrane performance due to their porosity, stability, and photocatalytic properties. Therefore, a MOF@COF-doped PAN UF membrane was investigated for its ability to photocatalytically degrade BPA in water.
A one-pot solvothermal method was used to synthesise the pristine MIL-101-NH2 (MOF) and TpMA (COF) as well as the composite MIL-101-NH2@TpMA materials at various MIL-101-NH2 concentrations (namely 10, 20, 30, 40, and 50 wt% MIL-101-NH2). The synthesis of the pristine and composite materials was confirmed using the following characterisation techniques: Fourier-Transform Infrared Spectroscopy (FTIR), X-Ray Diffraction (XRD), Scanning electron microscopy (SEM), Energy Dispersive X-ray Spectroscopy (EDS), transmission electron microscopy (TEM), Photoluminescence spectroscopy (PL), UV-Vis diffuse reflectance spectroscopy (UV-DRS), Electrochemical Impedance spectroscopy (EIS), and Brunauer-Emmet Teller (BET) analysis, demonstrating the successful integration of MIL-101-NH2 into TpMA.
Fourier-Transform infrared analysis and X-ray diffraction analysis showed that the pristine MIL-101-NH2 was successfully synthesised based on the presence of the characteristic Fe-O peaks at 626,57 and 657.15 cm-1 signifying the metal active centre of MIL-101-NH2. The successful synthesis of TpMA was also confirmed by the identification of characteristic peaks. Additionally, infrared analysis also confirmed the successful
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chemical bonding of MIL-101-NH2 to TpMA by the presence of N-H wagging band and C-N stretching bands at 765 and 1257 cm-1, respectively. The XRD spectrum confirmed the crystal structure and crystal planes of the pristine and composite materials and elucidated the structure of the hybrids. SEM-EDS confirmed the successful and even incorporation of the MIL-101-NH2 iron active centre into the TpMA layers, while TEM images confirmed the morphology of the pristine and composite nanomaterials. PL analysis revealed a great reduction in charge recombination rates for the composite nanomaterials compared to the pristine MIL-101-NH2. Diffuse reflectance analysis showed an effective band gap reduction in the composite (1.15 eV) compared to that of the MIL-101-NH2 (1.25 eV) and TpMA (1.45 eV). Electrochemical impedance analysis subsequently revealed the ideal composite ratio to be 30% MIL-101-NH2 (denoted MIL-101-NH2[30]@TpMA) with the lowest impedance and the second lowest charge transfer resistance. Photoluminescence (PL) analysis also revealed this ratio to have the least recombination propensity denoted by the lowest emission intensity peak out of the composite ratios and significantly lower recombination than pristine MIL-101-NH2. This composite also had the second largest surface area according to BET data at 87.0711 m2/g out of all composites and greatly improved surface areas compared to pristine MIL-101-NH2 (21.4307 m2/g) and TpMA (78.3965 m2/g).
Upon incorporation of the composite (MIL-101-NH2[30]@TpMA) within the polyacrylonitrile (PAN) membrane using the phase inversion method, scanning electron images revealed that the use of 0.7 wt% composite results in most favourable pore and macrovoid structure and integrity with an increase in pore definition and macrovoid size with increased nanocomposite loading. The 0.7 wt% loading of MIL-101-NH2[30]@TpMA in the PAN membrane also showed improved the flux recovery ratio (96.4%) and the irreversible fouling ratio (4.2%) compared to 44.7% and 56.4% for the pristine membrane, respectively, suggesting good antifouling capabilities. Finally, photodegradative analysis showed the 0.7 wt% membrane to have the best photodegradative capacity for bisphenol A (BPA) with 97% degradation efficiency after filtration for 3 h at pH 7 and at 10 mg/L. LC-MS and scavenger analysis were used to analyse the degradative products and hypothesise a mechanism of photodegradation. It is suggested that the degradation of BPA is attributed to OH.- and h+ radical mediated oxidative deprotonation.
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The positive result of this study highlights the potential of MOF@COF-doped PAN UF membranes to address organic/pharmaceutical pollutants in water. Not only were the nanohybrids found to enhance the membranes capacity to remove BPA from water via photodegradation, they also showed to improve the antifouling properties of the membranes compared to the pristine. This research addressed the literature gap regarding the doping of polymeric UF membranes with photoactive MOF@COF nanohybrids and it is hoped that this study prompts further investigation into optimisation and refinement of synthetic and operational parameters. This research additionally represents efforts to not only supersede traditional methods of water purification (such as ozonation, GAC, flocculation and chlorination) but to also improve on the feasibility of existing membrane filtration methods by mitigating membrane fouling and reducing operation costs (i.e.: introduction of a visible light-driven photocatalyst). Therefore, the use of MOF@COF nanocomposites offers a potentially sustainable solution with high efficacy in pharmaceutical/organic pollutant removal, marking a significant step forward in environmental remediation technologies.
Further studies are recommended to evaluate the lifetime or stability of the nanocomposite by performing repetitive photodegradative tests. Mechanical strength tests are also suggested to assess re-usability of the polymer membranes. Leaching tests may also be performed to assess whether any components of either the nanocomposite or the PAN membrane itself may leach into the filtrate. An environmentally-friendly alternative to PAN may also be explored, as PAN is non-biodegradable.