Affiliation: Gillings School of Global Public Health, Department of Environmental Sciences and Engineering
Thin film composite (TFC) membranes with fully aromatic polyamide barriers (or “active layers”), which have the capability to remove a broad range of contaminants (e.g., inorganic salts, small organics) from water, offer promising low-energy solutions for desalination and wastewater treatment. As effective and efficient as current membrane technologies are, there is always a continued need to expand and improve their capabilities with growing needs and applications. A fundamental understanding of the mechanism plays a crucial role in guiding intelligent membrane material modifications and process optimization. Aromatic polyamide TFC membranes can have similar physico-chemical properties but different performance in terms of contaminant removal rate (salt rejection, in the case of desalination) and it is not very well understood which membrane property(-ies) account for the differences in membrane performance. There is a need to understand what properties of polyamide active layers account for the differences in salt rejection among different membranes.
Theory of salt transport through TFC membranes indicates that the three properties of active layers that affect their salt rejection are active layer thickness, salt partition coefficient and salt diffusion coefficient in active layers. Therefore, the overall objective of this study was to elucidate which parameter among thickness, salt partition, and salt diffusion coefficient accounts for the differences in rejection among polyamide TFC membranes. To achieve this overall objective, this study: (i) developed a method to measure salt partition coefficient from aqueous solution into polyamide active layer of TFC membranes; (ii) quantified the salt partition and diffusion coefficient inside polyamide active layers of TFC membranes with a broad range of performance levels; (iii) studied the correlation between salt rejection and thickness, salt partition and diffusion coefficient. From the results, it was concluded that (i) for all membranes, the partition coefficients of all inorganic salts and small acids obtained experimentally were lower than 1 the partition coefficient did not differ much among different TFC membranes; (ii) the differences in salt rejection observed among membranes are mainly due to the differences in both salt partition and diffusion coefficients in active layers.