Enhanced Wastewater Reclamation with Carbon Molecular Sieves

The global water crisis demands robust technologies to support reuse of impaired water sources. Direct potable reuse (DPR) has been limited by the poor ability of available technology to completely sequester small, neutral molecules. We hypothesize that emerging contaminants of concern can be better removed with pure carbon molecular sieve (CMS) membranes, as they are uniquely size and shape selective due to the presence of permanent, rigid, and molecular sieving pores. CMS materials are made via pyrolysis of polymeric precursors, and CMS structure can be tailored by altering polymeric precursor, pyrolysis temperature, and pyrolysis atmosphere. Correlations between fabrication techniques of CMS materials and their final permeability and selectivity in separations will be discussed, with the focus being on a novel aromatic polyamide-derived CMS material. An aromatic polyamide made in-house was exposed to various gases during pyrolysis at temperatures of up to 900 °C. Porosity measurements of char indicate an ultramicropore size range of 5.5 to 6.5 Å and a micropore range of 10.3 to 10.6 Å regardless of pyrolysis temperature and atmosphere. X-ray diffraction measurements show average d-spacings of the amorphous materials to be between 4.3 to 4.7 Å, with a slight decrease in d-spacing as pyrolysis temperature increases. To date, polyamide-derived CMS exhibit structural characteristics that have potential to be highly selective toward small, neutral solute contaminants and enhance direct potable reuse. While challenges to the development and implementation of such materials as membranes remain, highly selective materials are of paramount importance to sustainable membrane-based separation processes of the future.

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