Design and development of biologically inspired materials based ultrafiltration membranes with enhanced antifouling property

Abstract

One of the most pervasive problems afflicting mankind throughout the world is poor access to clean freshwater and sanitation. United Nations’ FAO states that by 2025, around 1.8 billion people will be living in countries or regions with absolute water scarcity. With the growing demand for high quality water, many new technologies of water purification are being developed to cater for potable and non-potable use. The reliability and ease of operation of membrane-based filtration systems have led to their proliferation in wastewater treatment. Ultrafiltration (UF) is one such well-developed low pressure membrane separation process used in different applications. However, membrane fouling remains an inevitable problem in all pressure-driven membrane processes. Biofouling, in particular, is the most difficult type of fouling to address in terms of cleaning and regeneration of the membrane, causing deterioration of the membrane performance and leading to high operational costs, short replacement intervals and increase in chemical usage. Current trend and future direction of antifouling membranes are based on biologically inspired materials such as block copolymers with desired functional groups (amine, hydroxyl, acid), hydrogenated/hydroxylated and aminated graphene, aliened carbon nanotubes and aquaporin. In this thesis, a novel concept is proposed and practiced to prevent bio-fouling by developing a self-cleaning membrane surface imparted with enhanced hydrophilicity via blending a newly synthesized, water-insoluble, highly hydrophilic biologically inspired co-polymers and functionalised graphene based additives with PES dope solution within the membrane matrix. The polymeric additives were developed by polymerising the highly reactive alkene monomers containing highly hydrophilic, negatively charged carboxyl (-COOH), hydroxyl (-OH) and amine (-NH2) functional groups by a simple atom transfer radical polymerization technique. The new surface chemistry, enabled by the anchoring of antimicrobial silver nanoparticles to the functional groups of the polymeric additives, has been shown to be responsible for the self-cleaning property exhibited by the membrane surface. Four new additives were investigated in this thesis for their effect on membrane hydrophilicity, permeability and selectivity. They include polyethylene glycol-silver (PEG-Ag) attached poly acrylonitrile-co-maleic acid (PANCMA), polyethylene imine-silver (PEI-Ag) attached PANCMA, PEG-Ag attached poly acrylonitrile-co-maleic acid co-diaminomaleio nitrile (PANCMACDAMN) and amine and carboxylated graphene attached poly acrylonitrile-co-maleimide (G-PANCMI). IV These new additives were blended with PES dope solution to prepare the hollow fiber ultrafiltration membranes by dry wet spinning process. The additive G-PANCMI alone was also used to produce membrane without PES. All prepared polymeric additives and the membranes were characterized thoroughly using Fourier transform infrared (FTIR) spectroscopy, Nuclear magnetic resonance (NMR) spectroscopy, Raman spectroscopy, Gel permeable chromatography (GPC), Thermo gravimetric analysis (TGA), Differential scanning calorimetry (DSC), Energy-Dispersive X-ray Spectroscopy (EDX), Transmission Electron Microscopy (TEM), contact angle (CA), Scanning electron microscopy (SEM), Porometer, zone of inhibition test and clean water flux test. Finally, the membranes were tested for their permeability, selectivity and antifouling property in long term experiments using different feed water. From the analytical data it was found that the hydrophilicity of all membranes prepared with these new additives are significantly increased. At the optimized condition, PEG-Ag attached PANCMADAMN reduced the contact angle of the PES membrane by 78.1% compared to the control membrane and the membrane fabricated by G-PANCMI gives a water contact angle of zero (0°) which is 100% reduction. Similarly, the water permeability of the membranes is also increased significantly with these new additives. At the optimized condition, PEG-Ag attached PANCMADAMN increases the water permeability of the PES membrane by 119.8% compared to the control membrane and the membrane prepared only with G-PANCMI gives 126% higher water permeability compared to the control PES membrane. The highest selectivity was also achieved for the G-PANCMI based membrane which was more than 99% with protein solution and the second highest selectivity was achieved for the membrane with PEG-Ag attached PANCMA which was 96.2% with the same protein solution. Based on the experimental data of selectivity, permeability and antifouling property, the overall performance of the additives are in the following order; G-PANCMI >PEG-Ag attached PANCMADAMN > PEG-Ag attached PANCMA > PEI-Ag attached PANCMA. Development of biologically inspired materials based self-cleaning ultrafiltration membrane with long lasting properties opens up a viable solution for bio-fouling in ultrafiltration application for wastewater purification and based on the findings in this work, it can be concluded that the ultra-wetting graphene will be an ideal material for new generation water filtration membranes.