Study On Hybrid Ultrafiltration/Nanofiltration Membrane With Improved Performance

Pressure-driven membrane is defined as a kind of selective membrane that uses the pressure difference between the feed and permeate side as the driving force to transport the solvent through the membrane for the purpose of concentration, purification, separation and condensation. Particles and dissolved components are (partially) retained based on their properties such as size, shape, and charge. It can be classified into microfiltration, ultrafiltration, nanofiltration, and reverse osmosis membrane. As the rapid development of pressure-driven membrane’s application in numerous areas, traditional materials cannot meet the requirements of practical utilization, and there is considerable need to develop membranes with better properties such as higher permeability, selectivity, anti-fouling ability, chemical and thermal stability. Therefore, hybrid membrane materials which combine advantages of inorganic and organic materials have attracted increasing attention of research.In order to prepare hybrid membrane with high performance, the key issue and one of the greatest challenges of hybrid membrane are the dispersion of inorganic materials in the membrane matrix and the compatibility between inorganic and organic phases.For better dispersion of inorganic materials in the matrix and the compatibility between two phases, several inorganic materials with unique morphology and property were introduced for effective interaction between the inorganic and organic phases to obtain a series of hybrid ultrafiltration/nanofiltration membranes with improved performance. The influences of structure, morphology, functional group and content of inorganic materials on the membrane properties were investigated to optimize the membrane performance. The thesis includes five chapters.Chapter Ⅰ is the introduction of the whole thesis. An overview of research background was given, including brief introduction of the preparation, characterization and separation mechanism of the pressure-driven membrane, and summarization of the methods and research status of the hybrid pressure-driven membrane. Finally, the ideas and outline of the research work were purposed.In chapter Ⅱ, we focused on the carbon nanotubes/polymer hybrid ultrafiltration/nanofiltration membrane, which could be devided into two parts.(1) MWNTs/BPPOqua hybrid ultrafiltration membranes were prepared by incorporating carboxyl multi-walled carbon nanotubes (MWNTs) into a matrix of brominated polyphenylene oxide (BPPO) and using triethanolamine (TEOA) as the amination agent. Herein, TEOA could also serve as crosslinking agent by reacting with both BPPO and the MWNTs, contributing to the compatibility between the MWNTs and the polymeric component. The influences of the weight fraction of MWNTs and the amount of TEOA on the membrane performance and morphology were investigated, and then the properties of hybrid membrane were optimized. The results suggest that it is feasible to make MWNTs/polymer ultrafiltration membranes with both high permeation flux and excellent selectivity by using an adequate amount of MWNTs and TEOA.(2) An improved process of interfacial polymerization was initiated to prepare MWNTs/polyester thin film nanocomposite (TFN) membranes, where carboxyl MWNTs were successfully embedded throughout the polyester thin film layer. The improved process is facilely done, and the TFN membrane prepared via this process exhibits both enhanced permeability and selectivity compared with that prepared by conventional process. Then we discussed the effects of MWNTs concentration and surfactant species in the aqueous phase as well as the reaction time of interfacial polymerization on the membrane property and morphology. The results demonstrate an optimized TFN membrane with high permeability and selectivity can be obtained by employing adequate amount of MWNTs, proper surfactant and appropriate reaction time of interfacial polymerization. Furthermore, nanofiltration properties of the MWNTs/polyester TFN membrane were tested by examining the long-term stability and the separation performance of different feed solutions, feed concentrations, feed pHs.In chapter Ⅲ, we concentrated on the hybrid ultrafiltration/nanofiltration membrane incorporating with silica materials. It includes two aspects.(1) Mesoporous silica nanoparticles were functionalized with amino groups firstly, and then modified mesoporous silica nanoparticles (mMSN)/polyamide (PA) thin film nanocomposite (TFN) membranes were prepared via interfacial polymerization. Herein, mMSN could react with TMC during interfacial polymerization, giving rise to a covalent bonding between silica nanoparticles and active layer of TFN membrane. The influence of mMSN concentration in the aqueous phase on the membrane performance was investigated. By adding an appropriate amount of mMSN into aqueous phase, the water permeability of TFN membrane increases significantly, while the rejection retains at a relative high level. The obtained membrane also exhibits a promoted anti-fouling ability as well as a good long-term stability. The effect of the mesopore structures of silica nanoparticles on the membrane performance were also investigated, and the results illustrate modified mesoporous silica nanopraticles with proper pore size are more suitable for the optimization of PA TFN membrane.(2) We developed hybrid ultrafiltration membranes by incorporating hollow mesoporous silica spheres (HMSS) into a polymer matrix of brominated polyphenylene oxide (BPPO) using triethanolamine (TEOA) as the amination agent. HMSS with the OH-rich surface could form hydrogen bonding with polymer matrix, which consists of BPPO aminated by TEOA with three hydroxyl groups. Thus the compatibility between inorganic and organic phase is promoted. The hybrid membrane exhibits improved water permeability, thermal stability, water content, and well-maintained rejection to egg albumin, especially when the addition of HMSS is1.0wt%. The influence of the structures of silica particles on the membrane performance was also investigated, and it is found that HMSS with moderate wall thickness is highly suitable for the optimization of hybrid membrane properties.In chapter Ⅳ, three materials with different morphologies and properties were employed to improve the performance of ultrafiltration/nanofiltration membrane.(1) We firstly synthesized silica nanoparticles-coated graphene oxide (SiO2-GO) nanohybrid particles, where SiO2were densely and evenly covered on GO sheets. The space layer of SiO2is helpful to the dispersion of SiO2-GO particles on the polymer matrix. And then SiO2-GO particles were incorporated into polysulfone (PSf) matrix to develop SiO2-GO/PSf hybrid ultrafiltration membrane by phase inversion method. The appropriate addition of SiO2-GO particles can significantly promote the water flux of membrane, and keep high rejection rate. It is found that SiO2-GO/PSf hybrid membrane takes advantages of water permeability, protein rejection, hydrophilicity and anti-fouling ability. They are attributed to the high hydrophilicity of SiO2-GO particles and their good dispersion in the polymer matrix.(2) To improve the separation and anti-biofouling properties of membrane, Ag-SiO2composite particles were used to prepare Ag-SiO2/PSf hybrid ultrafiltration membranes. Herein, small size Ag exhibits strong anti-microbial activity and effectively avoids aggregation. By investigating the role of Ag-SiO2particles on membrane property and morphology, it is concluded that a proper addition of Ag-SiO2particles can endow the membrane with enhanced water permeability and anti-bacteriality, while the selectivity can be well kept.(3) We prepared β-cyclodextrin (β-CD)/polyester thin film nanofiltration (NF) composite membranes, where β-CD was introduced in situ into the polyester thin layer. β-CD is a cyclic oligosaccharide with a torus-shaped structure characterized by a hydrophilic external surface, thus the incorporation of P-CD into polymer layer may effectively solve the problem of the compatibility between two phases in the hybrid membrane. The effects of the concentration and pendant group of β-CD on the separation, morphology and anti-fouling performance of NF composite membrane were investigated. When the concentration of β-CD is1.8%(w/v) in the aqueous phase, the water flux of NF composite membrane reaches a value which is almost two times that of the bare polyester membrane, while maintaining the rejection to Na2SO4at a relative high level. Furthermore, the β-CD/polyester NF composite membrane also presents a significant enhancement in the anti-fouling performance. Compared with commercial NF membrane, the obtained β-CD/polyester NF membrane exhibits a good overall performance.Chapter V is the summary and prospect of our research work. Some meaningful conclusions for the preparation of hybrid ultrafiltration/nanofiltration membrane were underlined. The study on hybrid ionic-exchange membranes is considered to be a meaningful and promise research work.