(401f) Structure and Transport Studies of Carbon Molecular Sieve Membranes for Wastewater Treatment

Carbon molecular sieve (CMS) materials are potential candidates for scalable and high-performance reverse osmosis membranes due to their chemical and thermal stabilities. Moreover, the pore size distribution of CMS can be precisely altered, which becomes a great advantage for designing membranes to optimize the performance for a variety of separation applications. However, the amorphous nature of CMS structure results in a limited understanding of structure-property relationships. This research attempts to gain a deeper insight into the structure-property relationships of CMS by understanding its microstructure and the transport of water-organic guest molecules across CMS membranes for model wastewater purification.

This research combines textural gas sorption studies, neutron total scattering, and small-angle X-ray scattering (SAXS) to gain a holistic view of the CMS microporous structure. Small ultramicropores inaccessible to traditional pore size analysis were revealed using cryogenic neon physisorption. Moreover, the scattering data showed the formation of strand-plates and curvatures on the aromatic carbon strands comprising CMS, supporting the mechanism for adjusting the pore size distribution.

Moreover, an experimental and mechanistic study of water and organic solvent transport in CMS membranes is explored. We studied the sorption, diffusion, and permeation behavior of p-xylene and water in CMS. The comparison shows how the properties of penetrants, such as hydrophilicity and molecular size of permeates, affect the transport behavior in CMS. The transport of water in CMS was experimentally confirmed to follow the sorption-diffusion (S-D) mechanism. The sorption-diffusion model ideal permselectivity indicates that the CMS is p-xylene selective over water. Water/p-xylene mixture permeation experiments revealed an increased selectivity of p-xylene over water, providing tentative evidence for a competitive sorption-selective separation mechanism. This work suggests that CMS membranes exhibit organic-permeable separation properties in water/organic separations. The results presented here highlight the potential for the removal of dilute organics in water via CMS pervaporation membranes. We expect our research to become the groundwork for establishing structure-transport relationships for effective water-organic solvent mixture separation in high-performance CMS membranes.