SYNTHESIS AND CHARACTERIZATION OF ION EXCHANGE MEMBRANE WITH ENHANCED PHYSICOCHEMICAL AND ELECTROCHEMICAL FEATURES

Abstract

Water and energy are the two biggest challenges around the globe. Rising water demands associated with population growth as well as rapid industrialization have resulted acute shortage of fresh water. Additionally, increasing awareness about environmental challenges coupled with stringent pollution board norms compelled industries to opt for recycling and reducing processing steps to make it economic and environmentally benign. Ion exchange membranes (IEMs) show great potential to solve both these challenges. IEMs are an important class of dense polymeric membranes that bear fixed charges (negative or positive) in the polymer matrix, which is able to let counter-ions move across it, while blocking coions and these are classified as cation exchange membrane (CEM) and anion exchange membrane (AEM) respectively. Currently, a broad range of electrochemical processes are used in industry and over the past decade, the development of selective, durable and highly efficient membranes has attracted much attention to develop new preparation techniques, modifications for process intensification due to their academic and industrial values. Therefore, advanced membrane based wastewater treatment technology is highly required. The aforementioned facts captured our interest immensely to focus the present thesis to develop composite and cross-linked membranes and membranes for industrial effluent treatment using electrodialysis (ED) and electrodialysis (EED) by enhancing ion transport, current efficiency and decrease in energy consumption. This thesis reports synthesis, characterization, and performance evaluation of both cation and anion exchange membranes where optimization of physicochemical and electrochemical properties of each of the membrane was investigated. The most suitable synthesized IEM was then applied for an intended application including, pulp and paper mill industrial problem, selective anion/cation separation and desalination using ED or EED cell. The overall performance of any synthesized IEM was compared with commercial CEM (FKB-PK-130, Fumatech GmbH, Germany) and AEM (FAB-PK-130, Fumatech GmbH, Germany). A brief literature review on ion exchange membrane, its historical background, classification, fundamental studies on ion transport phenomena and its mechanism, membrane forming materials (polymers and chemical reagents), fabrication techniques, application in various sectors, and techniques (instrumental, physicochemical and electrochemical) used to evaluate properties of IEM is discussed in Chapter 1. Additionally, recent advancement to boost up electrochemical and physicochemical properties along with challenges associated with development of selective IEMs and problems experienced in industrial scale were discussed. Finally, the chapter concluded with specific research gaps, objectives and organization of the thesis. On the basis of relevant literature review, Chapter 2 and Chapter 3 are dedicated to the synthesis of composite CEMs addressed to overcome lime based industrial causticization of green liquor (a highly energy intensive and environmentally unsafe process) to white liquor in paper mills. Lime based causticization of green liquor solution (GLS, rich in Na2CO3) to white liquor (rich in NaOH, and recycled for digestion of wood chips) generates lime mud (rich is CaCO3), which is recycled as CaO after high temperature (energy intensive) calcination in rotary kiln. As an alternative to conventional causticization technique, feasibility of using EED (electro-electrodialysis or membrane electrodialysis) technique to recover caustic from green liquor is investigated using composite cation exchange membrane under pre-optimized process conditions. Briefly, in Chapter 2, effect of sulfonated graphene oxide (SGO) incorporated sulfonated poly (ether ether ketone) (SPK) composite CEMs was investigated for caustic production from green liquor solution (GLS) of agro-based paper mills. With 60 mA·cm-2 current density, best optimized CEM (SPK-3 with 3.0 wt% SGO loading, cation exchange capacity 1.75 mmol·g-1, water uptake 32.01%, swelling ratio 11.41%, conductivity 0.037 S·cm-1, transport number 0.97 with maintaining high alkaline and thermomechanical stability) produced 1.17 mol·L-1 kW·h·kg-1 energy consumption (EC) during 4 h in two compartment EED cell (active membrane area: 66 cm2). Under similar operating conditions, prepared pristine (SPK), and commercial CEM (FKB-PK- 130), showed inferior performance. Longevity of SPK-3 CEM was assessed with industrial GLS for fifty cycles of EED experiments 4 h duration each. In Chapter 3, effect of TiO2 incorporated composite CEM was explored for caustic production from wood-based green liquor. Surface smoothening and formation of nano-channels improved monoselectivity and antifouling property of the composite CEM that results NaOH production along with prevention of sulfur scaling during treatment of wood based pulp and paper mills GLS stream. Out of all synthesized membranes, composite membrane containing 5 wt% of TiO2 i.e., SPK-T-5 was found better than rest of the membranes (e.g. pristine SPK membrane and commercial CEM (FKB-PK-130). With 60 mA·cm-2 current density, best optimized SPK-T-5 CEM (5.0 wt% TiO2 loading, cation exchange capacity 1.39 mmol·g-1, water uptake 21.61%, swelling ratio 7.25%, conductivity 0.058 S·cm-1, transport number 0.96 with maintaining high alkaline and thermo-mechanical stability) produced 2.03 mol·L-1 of -1 energy consumption (EC) during 4 h in two compartment EED cell (active membrane area: 66 cm2). The membrane performance and NaOH production remained unaltered with fifty cycles, indicating performance consistency and long term applicability with SPK-T-5 membranes. The obtained research outcomes are expected to be a valuable material for NaOH production with the aim of zero solid discharge practice for pulp and paper mills targeting minimization/elimination of solid wastes. Apart from CEMs, another category of IEM i.e., AEM was synthesized with improved physicochemical and electrochemical properties for desalination and selective anion separation using ED. Briefly, in Chapter 4, temperature resistant cross-linked brominated poly phenylene oxide (PPO) - functionalized graphene oxide (GO) nanocomposite AEM for desalination was discussed. Amine ended functionalized graphene oxide; AGO was used as a nano cross-linker and nanocomposite crosslinking networks were formed by reacting aminopropylsilane graphene oxide (AGO) with brominated poly (2,6-dimethyl-1,4- phenylene oxide) (BPPO). The best optimized AEM; PPO/AGO-8 (with 8.0 wt% AGO loading) showed ion exchange capacity 2.2 mmol·g-1, water uptake 37.91%, swelling ratio 9.8%, ionic conductivity 0.84 S·cm-1, counter ion transport number (0.97) and 10.8 mA·cm-2 limiting current density (ilim) with maintaining good thermo-mechanical stability due to covalent interaction between polymer and nanofiller. Desalination performances were evaluated on the basis of removal ratio (RRNaCl), current ED application, the PPO/AGO-8 membrane showed slightly higher RRNaCl reduced EC (2.85 kWh·kg-1 NaCl) compared to commercial Fumatech FAB-PK-130 membrane (RRNaCl: -1 NaCl) under same temperature (55 °C). The superior ED performance of synthesized cross-linked nanocomposite AEM is suggested to be a promising candidate for electrodialytic desalination and temperature based separation and purification processes for industrial applications. In Chapter 5, di-quaternized graphene oxide based multi-cationic cross-linked monovalent selective AEM for electrodialysis was discussed. Monovalent anion selective membranes were synthesized by crosslinking chloromethylated polysulfone with 1,4-diazabicyclo [2.2.2] octane functionalized graphene oxide (QGO). Cross-linking of QGO provides dense polymer matrix as well as additional functional sites to the membrane. The effect of QGO on physicochemical and electrochemical properties was studied in detail. The optimized membrane, CrPSf-3 (3 wt% of QGO) showed 2.01 mmol·g-1 ion exchange capacity, 9.90% swelling ratio, 31.20% water uptake, 0.072 S·cm-1 chloride ion conductivity and 0.95 counter ion transport number along with good mechanical and thermal stability. Electrodialysis performance of membrane permselectivity between Cl and SO4 was studied in mixed salt solution of 0.05 mol·L-1 (NaCl+Na2SO4). The CrPSf-3 membrane showed 5.7 separation factor (SF) for Cl /SO4 which is higher than pristine QPSf (1.7) and commercial FAB-PK-130 membrane (2.01). Furthermore, higher Cl flux (2.97 mol·m-2·h-1) was observed for CrPSf-3 membrane as compare to pristine QPSf (1.45 mol·m-2·h-1) and commercial membrane (1.6 mol·m-2·h-1). To achieve monovalent anion selectivity, effective cross-linking is facile approach to control compactness of the membrane and ion transport channels. Reported cross-linked membrane possesses a potential candidate for water desalination, wastewater treatment, or other monovalent selective separation processes. An overall summary of the research work done and knowledge developed are highlighted in Chapter 6. Additionally, based on this experience future recommendations are also made for further improvement in performance of IEMs.