Investigating surface modification and incorporation of inorganic nanowires and particles for enhanced performance of water-filtration polymer membranes

Abstract

Water treatment is, collectively, the industrial-scale processes that make water more acceptable for an end-use, which may be drinking, industry, or medicine. Due to the increasingly severe water shortage problem and growing concerns on the water quality, water treatment, which is closely relative to daily life, has always been placed under the spotlight of scientific research so far. Conventionally, water has been treated primarily using physical-chemical treatment such as sand filtration and disinfection. Not only these conventional methods required large area of operations that would require high operational cost, but may also be incapable to treat several persistence pollutants. Membrane separation is a technology which selectively separates (fractionates) materials via pores and/or minute gaps in the molecular arrangement of a continuous structure. Membrane separation has many advantages such as no chemical addition, small operation area, relatively high filtration efficiency, low cost and et al. Not only that, the stringent standard required for water supply and effluent discharge also promotes the innovative development of separation membranes and membrane separation process. Based on the type of material, membranes can be classified into polymeric membranes and inorganic membranes. Despite the fact that some polymeric membranes have already been commercialized and emerged as a favourable filter media for water treatment, there are still several limitations such as relatively low flowrate, low antifouling property and low mechanical properties. Therefore, many additives have been added into polymeric membranes to synthesize polymer-based composite membranes to tackle these problems and to further enhance the efficiency of water treatment. In this thesis, three different types of materials have been chosen as additives to incorporate into microfiltration or ultrafiltration polymeric membranes, aiming to enhance the water treatment performance. The first type of materials is germanate nanowires with different lengths. We for the first time found that the lengths of nanowires play a key role in determining the morphology change and subsequent water treatment performance; thus by controlling the length, we successfully obtains nanowire-enhanced polymeric membranes with fast permeation at the maintenance of rejection. In the second part, we developed a simple, economic and practical method to coat cheap chitosan onto the top of polymeric membranes. Though such membranes show a slight decrease of flux, the rejection, antifouling property and antibacterial property were greatly improved. Theoretical calculation shows that the weakened bonding between foulant and membrane surface resulting from chitosan-grafting is responsible for the improved antifouling property. The third type of material is alumina particles. The main purpose of this part is to study how inorganic particle size and loading affect the morphology change, performance and mechanical property of polymeric membranes with hollow shape. Results show that such inorganic particle/polymeric membranes have higher tensile and compressive properties compared to the pristine polymeric membranes and both particle size and particle loading matters in determining the improvement of mechanical properties. Despite the improvement in mechanical properties, the rejection of such membranes was greatly sacrificed due to the detachment of particles from polymer during the formation of membranes, especially when high-loading micron-sized alumina was used. Therefore, CNTs were further added into high loading micron-sized alumina/polymeric membranes to hinder such detaching effect. Results show that the incorporation of CNTs not only further enhance the permeation and mechanical properties but also the rejection and antifouling property. All in all, in this thesis, we choose three different types of materials and incorporate them into polymeric membranes to enhance the water treatment performance (flux, rejection, antifouling and mechanical properties), offering some useful and practical designing knowledge or methods to the synthesis of high-performance polymer-based membranes for water treatment processes.