Date of Award

2019

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Graduate Group

Chemical and Biomolecular Engineering

First Advisor

Daeyeon . Lee

Second Advisor

Shu . Yang

Abstract

Membrane separation methods have been reported to be 90% more energy efficient than thermal based separation methods. However, conventional membranes for water filtration suffer from the issue of membrane fouling by oily contents, which leads to a decrease of separation efficiency. One potential approach to overcome this challenge is to introduce superhydrophilic coatings on membrane surface to achieve underwater oil repellent as well as render the membrane with a proper pore size to have the ability to separate challenging oil/water emulsions with nanometer-sized oil droplets. Silica (SiO2) nanoparticles (NPs) coatings possess great hydrophilicity due to surface hydroxylation and its surface chemistry can be further tuned. Furthermore, if the shape of NPs is anisotropic chain-like shaped, the film it formed upon deposition can be highly rough and porous. However, its superhydrophilicity dissipated with time rather than robust due to high tendency of contamination, and its porous structure and rough surface textures can trap oil compromising the anti-fouling. Smooth solid surfaces grafted with polyelectrolytes can induce oil deweting and lifting from grafted surfaces by adding water. Combining the chain-like SiO2 NPs and polyelectrolytes through surface grafting might generate a highly porous and long-lasting robust superhydrophilic coating. To achieve this, we need to investigate the following questions: (1) How to fabricate this coating based on this idea and does the fabricated coating possess the robust oil repellency under water that we are expecting? (2) Does the oil repellency of the coating preserve when it is applied onto actual membranes and does it enhance the efficiency of the coated membrane filtrating oil/water mixtures? To search the answer these questions, poly(acrylic acid) (PAA)-grafted SiO2 nanochains are synthesized and made into superhydrophilic coatings. The coating successfully introduces robust anti-fouling to smooth surfaces and can lift trapped oil by immersing into water. Both chain-like shapes of the NP and grafted PAA are essential to ensure the robustness. The coated porous membrane has improved surface wettability and enhanced separation efficiency for oil/water mixtures by having increased breakthrough pressures. In conclusion, the fabrication approach presents a promising future for further studies and applications for oil/water separation membranes.

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