Measuring Pore Sizes in Covalent Organic Framework (COF) Membranes through Dye Rejection Experiments

Water recycling reclaims water from a variety of waste sources, then treats and reuses it for beneficial purposes, enhancing water security, sustainability, and resilience. Nanofiltration is a membrane separation process that is useful in many water recycling applications. It uses pressure to filter out solutes based on solute size and membrane pore size. Nanofiltration membranes have a high throughput and are more cost effective to operate than reverse osmosis membranes. In this work, I studied Covalent Organic Framework, or COF membranes for use in nanofiltration. These are considered a new porous polymer material as they were only discovered in 2006. COFs can be synthesized as ordered, customizable thin films and membranes, which have a great potential for nanofiltration performance due to their uniform pores and high porosity. The challenge is that it is hard to measure the pore size of these nanoscale porous materials. In this work, I made COF membranes with various chemistries, characterized their properties, and performed rejection experiments with dye solutes that have various sizes (Brilliant Blue, Congo Red, Primulin, Nitroaniline, Safranine, Methyl Blue, Reactive Black). I assessed the rejection performance of each of the COFs using UV-VIS spectrophotometry and analyzed their results to estimate their pore sizes. Knowing the pore sizes supports future work to understand which COFs would better reject of certain solutes, to identify what membrane characteristics impact rejection, and to evaluate actual rejection performance. This knowledge would allow for COF membranes to be used for more challenging water recycling and nanofiltration applications.