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    Effect of Feed Water pH on the Partitioning of Alkali Metal Salts from Aqueous Phase into the Polyamide Active Layers of Reverse Osmosis Membranes

    Year: 2021

    Journal: Environ. Sci. Technol., Volume 55, MAR 2, page 3250–3259

    Authors: Wang, Jingbo; Armstrong, Mikayla D.; Grzebyk, Kasia; Vickers, Riley; Coronell, Orlando

    Organizations: US National Science Foundation (NSF) [1264690, 1336532]; Graduate School of the University of North Carolina at Chapel Hill; Sigma Xi; National Institute of Environmental Health Sciences at the University of North Carolina at Chapel Hill [T32ES007018, P42ES031007]; American Water Works Association - Water Environment Association (AWWA-WEA) Safewater Fund; NSF Graduate Research Fellowship Program [DGE-1650116]; Caroline H. and Thomas S. Royster Fellowship

    Keywords: thin-film composite; polyamide; partitioning; Donnan theory; charge density; quartz crystal microbalance

    The partitioning of solutes into the polyamide active layers of reverse osmosis (RO) membranes is a key membrane property determining solute permeation. Quantification of partition coefficients and their dependence on feedwater pH would contribute to the development of predictive transport models of contaminant transport through RO membranes; however, neither solute partitioning nor the effect of feed solution pH on partitioning has been thoroughly characterized in the literature. Accordingly, we characterized the partitioning of all chloride salts of alkali metals (CsCl, RbCl, KCl, NaCl, and LiCl) from the aqueous phase into the polyamide active layers of five polyamide RO membranes, including one prepared inhouse and four commercial membranes. We evaluated the effect of pH on the partitioning of alkali metal salts and whether the effect of pH on salt partitioning and rejection is consistent with Donnan theory predictions. Results showed that for all membranes, the partition coefficients of all salts were less than one and did not differ substantially among RO membranes. Results also indicated that for all membranes tested, Donnan theory provided an appropriate theoretical framework to estimate the effect of pH on salt partitioning (evaluated for all chloride salts of alkali metals) and salt rejection (evaluated for NaCl). Thus, we conclude that changes in salt rejection resulting from feed solution pH are primarily driven by changes in salt partitioning with comparatively small changes in salt diffusion coefficients.
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