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Study On Mixed-matrix Reverse Osmosis Membrane Incorporated With Porous And Layered Nanomaterials

Posted on:2016-03-14Degree:DoctorType:Dissertation
Country:ChinaCandidate:H DongFull Text:PDF
GTID:1221330464469882Subject:Chemical Engineering
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Reverse osmosis (RO) is more and more widely utilized in water treatment process due to the increasingly severe problems of water shortage and pollution. Thus, the development of high-performance RO membrane materials is highly demanded. High flux, high rejection and anti-fouling are the most important requirements of RO membrane. However, due to the trade-off phenomenon, it is difficult to improve these core performances together using traditional methods. To resolve this problem, thin film nanocomposite (TFN) RO membrane was prepared by mixed-matrix approach with porous zeolite and layered nanoclay as nanofillers via interfacial polymerization. Nanomaterials could offer a new degree of freedom to tailor the membrane structure and performance.Two types of zeolite with different pore structures were selected by taking advantage of their sieving effect to improve membrane separation performance. Two types of nanoclay with different charge properties were employed by making use of their charging effect to ameliorate the anti-fouling performance. The effects of these nanomaterials on the membrane formation, structure and surface properties were investigated, and the function of nanomaterials in the membrane thin layer was analyzed. Along with the optimization of membrane preparation and post-treatment conditions, mixed-matrix RO membrane with high separation and anti-fouling performance was obtained. Main content of this thesis includes the following four aspects:(1) Study on the mixed-matrix RO membrane incorporated with LTA type zeolite. The membrane was fabricated using NaA zeolite with the pore size of 4.1A. To ameliorate the nanoparticle aggregation issue, octadecyltrichlorosilane (OTS) was used to modify the zeolite surface. The dispersibility of nanoparticles was improved and the zeolite could take part in the interfacial polymerization more effectively. Afterwards, the membrane structure and separation performance were evaluated. The results showed that the OTS-NaA filled membrane was more homogeneous and hydrophilic. The flux of the TFN membrane was increased by 0.5 time after zeolite modification, and the salt rejection was also enhanced. Finally, the effect of zeolite pore structure was investigated and it was found that the sieving effect and interface void were responsible for the increase of water flux. The proper pore size and favorable dispersibility of zeolite were crucial for the TFN membrane preparation.(2) Study on the mixed-matrix RO membrane incorporated with FAU type zeolite. The FAU type zeolite with pore size of 7.4 A was employed as filler materials. Firstly, NaY zeolite with FAU type structure and particle size of 150 nm was synthesized. It was suggested that better separation performance could be obtained by dispersing zeolite in aqueous phase solution. Afterwards, the formulation of monomer solution, reaction time, zeolite loading and post-treatment conditions were optimized, and the water flux increased from 39.6 L/m2-h to 86.0 L/m2-h, while the salt rejection was 98.4%. Finally, the detailed microstructure was analysied by TEM and XPS. The distribution of zeolite in the matrix and the polymer encapsulation effect was observed, and then the behaviour of nanomaterials during membrane formation and separation process was proposed.(3) Study on the mixed-matrix RO membrane incorporated with montmorillonite (MMT). The cationic clay MMT was employed to prepare TFN membranes. Firstly, the pristine MMT was treated by swelling method with the assistance of sheering force, which was a "top-down" approach to diminish the lateral dimension of sheets down to 200 nm with aspect ratio around 20~40. The MMT clay nanosheets was negatively charged. Subsequently, the effects of MMT adding method and loading on membrane structure and performance were studied. The results showed that MMT-filled membrane had more rough surface and increased hydrophilicity The water flux was increased to 51.7 L/m2·h with the salt rejection at 99.0%. Finally, the membrane anti-fouling performance towards hydrophobic protein and negatively charged organic foulants was studied by evaluating the flux decline and contact angle change. The MMT-filled membrane exhibited increased fouling resistance which was benefited by the electrostatic and hydrophilic-hydrophobic repulsion effects.(4) Study on the mixed-matrix RO membrane incorporated with layered double hydroxide (LDH). The anionic clay LDH was utilized to prepare TFN membranes. Firstly, the LDH nanoclay was synthesized through a "bottom-up" approach by a fast nucleation and slow aging method. The LDH nanosheets were positively charged with the lateral dimension of 100 nm and aspect ratio around 20-25. The incorporation of LDH exhibited few influence on membrane surface morphology, while the separation performance was slightly improved. The membrane surface properties were characterized by zeta potential and contact angle measurements, and the LDH-filled membrane was less negatively charged with improved hydrophilicity. The fouling resistance of LDH-filled membrane toward cationic surfactant foulants was enhanced, which could be primarily ascribed to its less negatively charged surface properties and reduced the electrostatic interaction between foulants and membrane surface.(5) The synergistic effect of porous and layered nanomaterials has been studied by dispersing zeolite and nanoclay in aqueous and organic phase soulution, respectively. However, due to the inhibiting effect of nanoclay, the increase of separation performance of porous and layered nanomaterials filled TFN membranes was not apparent.In conclusion, the membrane separation and anti-fouling performance were improved by utilizing the unique structure and properties of different nanomaterials. The behavior of these materials during the membrane formation and separation process was explored. Consequently, mixed-matrix RO membrane with high performance was achieved, which provides foundation for the further research and development of novel RO membrane materials.
Keywords/Search Tags:Zeolite, Montmorillonite, Layered double hydroxide, Polyamide, Thin film nanocomposite, Mixed-matrix membrane, Reverse osmosis membrane, Anti-fouling
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