Novel Organic-Inorganic Nanocomposite Membranes for Electrodialysis Application in Water Recovery

Abstract

Water shortage has become one of the major global concerns and has led to great efforts towards the search and utilization of an alternative water supply such as from abundant salt rich brackish water and sea water. Desalination by membrane separation process becomes an important technology for global population by producing fresh water out of saline water. Membrane is the key element in the membrane separation processes that determines to a large extent the performance, process efficiency, energy consumption and water production cost of the desalination. Up to date, the majority of existing commercial membranes is polymer based membranes which still require further modifications to achieve high desalination efficiency and low energy consumption. The key target of this work is to synthesize a new class of ion-exchange membranes applicable for electrodialysis desalination process by utilizing a new concept of composite membrane design. Organic-inorganic composite membranes have recently gained increasing attention due to their hybrid functionalities derived from organic and inorganic phases that may offer a variety of important applications. Fortunately, the composite design concept can provide enormous opportunities to control membrane structure and properties by simply tuning material components, compositions, and functionalities. Introducing metal oxide nanoparticles into the polymer matrix is expected to improve conductivity and transport properties of the polymer membranes while still keeping favorable mechanical and thermal stability. Polyethersulfone (PES), an inexpensive and high performance polymer with excellent mechanical, thermal and chemical stability, was selected as a main organic matrix in this work. The PES was first chemically modified by sulfonation reaction to introduce charged functional groups into polymer backbones to form sulfonated polyethersulfone (sPES) for use as ion-exchangeable polymer matrix. High surface area mesoporous silica was also chemically modified with sulfonate groups and used as an inorganic filler. A series of composite membranes containing sPES and sulfonated mesoporous silica (SS) have been prepared via a number of membrane preparation techniques. Systematic material synthesis and characterizations have been applied to elucidate the crucial links among synthesis conditions, membrane structure and properties and their desalination performance. It is well-known that the properties and performance of the membranes depends highly on their structure which in turn is affected by membrane preparation conditions. In the first part of this work, the relationship among membrane fabrication condition, membrane structure and property is the main focus for establishing an optimized membrane preparation procedure. The membranes prepared by different phase inversion techniques offered different structures and distinct properties. The preparation conditions in the ternary system of solvent/nonsolvent/polymer in the phase inversion were carefully investigated. It was found that the combination of two commonly-used phase inversion techniques, namely wet and dry phase inversion, was an effective tool for preparing membranes with adjustable structure, pore size and porosity. The structure and porosity of the prepared sPES membranes can be easily controlled by tuning ageing periods of membrane formation in the steps of dry and wet phase inversions. The properties of polymer matrix membrane are further improved by incorporating surface functionalized mesoporous silica. In the second part of this thesis, the influence of inorganic fillers with different sizes and shapes on the membrane structure and properties was investigated. It was found that by incorporating small amount of inorganic additive, the membrane properties such as water content, conductivity, and transport number were significantly enhanced while the membranes still maintain their mechanical and thermal stabilities. The desalination results by a custom-designed lab-scale electrodialysis (ED) cell of the composite membranes also exhibited good performances with high current efficiency over 80%, which is compatible to a benchmark membrane (FKE, FumaTech). The newly developed membranes can thus be considered as excellent candidates for ion-exchange membranes in desalination application. The findings from this project will generate a suite of new knowledge that underpins both fundamental understanding of membrane design and their applications in ED desalination. The outcomes are expected to lead to the development of new alternative composite ion-exchange membranes which may open up cost-effective and less energy consumption desalination application