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Title: Fabrication of polyoxometalates functionalized polymeric membranes for the removal of organic pollutants from wastewater
Authors: Yao, Lei
Keywords: DRNTU::Engineering::Materials
DRNTU::Engineering::Environmental engineering::Water treatment
Issue Date: 2016
Source: Yao, L. (2016). Fabrication of polyoxometalates functionalized polymeric membranes for the removal of organic pollutants from wastewater. Doctoral thesis, Nanyang Technological University, Singapore.
Abstract: The fast global population growth and intensive industrialization have resulted in increasing wastewater emission, which is pushing forward new developments for wastewater handling. Membrane separation is a proven technology for wastewater treatment, while integration of more functionalities (e.g. catalysis and adsorption) into the membrane may potentially present a more cost-effective and environmentally-friendly solution. Polyoxometalates (POMs), which possess versatile properties owing to their unique structures, can be used as promising candidates to functionalize membranes for the removal of organic pollutants from wastewater. Currently, the research on POMs functionalized membranes for wastewater treatment is in its infancy. We aimed to fabricate POMs functionalized polymeric membranes with more functionalities (e.g. catalytic wet air oxidation or adsorption) for the removal of organic pollutants from wastewater. A novel catalytic membrane contactor (CMC) was firstly fabricated through incorporating POMs onto polyvinylidene fluoride (PVDF) hollow fiber membranes via a simple chemical deposition method. The Keggin type POMs ([PV2Mo10O40]5−) as catalysts were modified on the surface of the membranes through chemical assembly with a permanently charged polyelectrolyte. The degradation of phenol was conducted using the CMC under room conditions with air as green oxidant. A gas/catalyst/liquid interface was successfully built up and proved to enhance the catalytic efficiency. In order to further obtain tailored nano-morphologies of POMs and chemical bonding with the membrane substrates, the self-assembly of rare-earth Anderson POMs on the surfaces of imide polymeric hollow fiber membranes (polyetherimide and polyamide-imide) was designed. The rare-earth Anderson POMs ([Gd(H2O)7Cr(OH)6Mo6O18]n) nanoparticles with controllable size and distribution were successfully constructed. The self-assembly of POMs was revealed to be a surface-induced growth process in which the generation of silanol groups on the membrane was indispensable for further hydrogen bonding (–OH•••POMs) and coordination bonding (–OH–Gd) interactions. This work provided a simple but practical method not only for the fabrication of novel POMs functionalized membranes, but also for the synthesis of POMs nanoarchitectures. The as-prepared POMs functionalized imide polymeric membranes as CMCs exhibited enhanced phenol degradation activities under mild conditions. The importance of the air overpressure to form the gas/catalyst/liquid interface during the catalytic process was further elucidated. Besides the construction of nanoarchitectures of POMs which possessed the catalytic capabilities to degrade phenol under mild conditions, another novel nanoarchitecture based on [PV2Mo10O40]5− to form a surfactant encapsulated POMs (SEP) has been synthesized and further incorporated into the polymeric PVDF membrane. The SEP complex fabricated via a simple ion-exchange method was revealed to self-assemble into spherical nanoparticles. It can be easily blended into the PVDF matrix forming SEP incorporated composite membrane without agglomeration and leaching problems. The SEP incorporated composite membrane presented excellent anionic dye adsorption activity, which is for the first time reported as an intriguing property of the SEP. A regeneration scheme without loss of dye removal efficiency was further proposed for the SEP incorporated composite membrane. Inspired by the previous studies about the construction of catalytic membranes and adsorptive membranes based on POMs, a unique POMs functionalized alumina/PEI hollow fiber membrane with both adsorptive and catalytic activities was further fabricated. The POMs functionalized membrane was developed through a novel sol-gel method including the fabrication of a robust alumina containing polyetherimide hollow fiber membrane followed by the generation of an amine-functionalized substrate for the incorporation of POMs. A two-step protocol, adsorptive separation and catalytic degradation, was designed for dye removal, realizing an excellent dye rejection with easy and economic membrane regeneration. This process can be further tailored owing to the flexible selection of POMs (e.g. [PW11O39Fe(H2O)]4−) for membrane functionalization. In conclusion, this thesis presents the design and development of novel POMs functionalized polymeric membranes in terms of the utilization of different functionalization methods, studies of the modification mechanisms and investigations of distinctive functionalities with corresponding performances for the removal of organic pollutants. The current work contributes to the development of membrane fabrication and modification technology and provides a new possibility for their applications in wastewater treatment.
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