Date of Award

May 2022

Degree Type

Thesis

Degree Name

Master of Science

Department

Engineering

First Advisor

Xiaoli Ma

Committee Members

Nidal Abu-Zahra, Yin Wang

Abstract

Potable water scarcity is a problem growing more severe as water pollution continues to rise and the world population continues to grow. There are many techniques of treating water to remove contaminants and lower salt concentrations to drinkable levels, but there is still a great potential for more sustainable and energy efficient water treatment methods. The use of nanofiltration (NF) membranes appears to be a promising method for treating large quantities of water quickly and without using a lot of energy. Current nanofiltration membranes used for water treatment have not reached their full potential as they still have lower permeances. Covalent organic frameworks (COFs) are a new type of material with 2D structures and tunable pore sizes currently undergoing a heavy amount of research and development. The use of covalent organic frameworks in nanofiltration membranes could immensely improve their efficiency in water treatment. In this research work, several techniques and methods were used to produce more efficient NF membranes using COFs (TpPa-1). Several attempts were made to mechanically delaminate and collect COF nanosheets (CONs) from COF powders and to create a thin membrane layer on top of a porous support from the obtained CONs. Grinding COF powders in a mortar and pestle and liquid sonication were the utilized methods of obtaining CONs but neither method succeeded at consistently producing particle sizes small enough to be considered nanosheets. The other issue with that technique was low adherence of the COFs to the porous support. The COF particles could form a thin layer on top of the support but would wash off when pouring water over the top of it. Another method of using COFs for NF membrane fabrication was done by synthesizing a COF thin film using interfacial polymerization (IP) and transferring the film to a porous support. The synthesis took place in either a beaker or a vacuum filtration apparatus, with a nylon support at the bottom of the beaker/vacuum filtration apparatus. It took approximately 72 hours to fully complete the polymerization and the thickness of the film could be adjusted by changing the monomer concentrations used. Several thin films were prepared using various monomer concentrations (0.005%, 0.01%, and 0.02%) and using various polymerization times (24hrs, 48hrs, and 72hrs). Upon completion of the polymerization, the liquid layers were removed to combine the thin film to the nylon support. This process usually created several defects in the thin films, as they were very fragile, and the liquid layers were removed by pipetting. Upon removal of the liquid layers, a faster secondary IP was performed on top of the membrane. The secondary IP helped adhere the film to the support and seal up surface defects on the membrane. The membranes were analyzed using a confocal microscope and tested with Congo Red using vacuum filtration to obtain water permeances and dye rejections. The permeate from the Congo Red was analyzed using UV-Vis spectroscopy to determine the rejection rates of the membranes. The data showed that the membranes prepared using 0.01% monomer concentrations and 24-hour polymerization time produced the thinnest membranes, which had high permeances (above 1,000 L/(bar*m^2*hr)), while still having high rejection rates (above 95%). The membranes prepared using 0.005% monomer concentrations had the second thinnest films and second highest permeances (above 500L/(bar*m^2*hr)) and the third highest rejection rates (approximately 85%). The average permeance of current commercial NF membranes ranges from about 1-10L/(bar*m^2*hr) when used for water treatment. The data from this research suggests that with some fine tuning of monomer concentrations and polymerization times, highly efficient COF thin film membranes can be fabricated for water treatment. Additionally, further work will need to be done to create a more reliable method of attaching the COF thin film to the support without creating defects in the thin film.

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