Ion Selectivity in Brackish Water Desalination by Reverse Osmosis: Theory, Measurements, and Implications

• Published in: Environmental Science & Technology Letters
• Main Authors: Biesheuvel, P. M., Zhang, L., Gasquet, P., Blankert, Bastiaan, Elimelech, M., Van Der Meer, W. G.J.
• Other Authors: Biological and Environmental Sciences and Engineering (BESE) Division, Water Desalination and Reuse Research Center (WDRC), Wetsus European Centre of Excellence for Sustainable Water Technology, Oostergoweg 9, 8911 MA Leeuwarden, The Netherlands, Department of Chemical and Environmental Engineering, Yale University, New Haven, Connecticut 06520-8286, United States, Nanosystems Engineering Research Center for Nanotechnology-Enabled Water Treatment (NEWT), Yale University, New Haven, Connecticut 06520-8286, United States, Oasen Drinking Water Company, Nieuwe Gouwe O.Z. 3, 2801 SB Gouda, The Netherlands, Membrane Science and Technology, University of Twente, Drienerlolaan 5, 7522 NB Enschede, The Netherlands
• Format: Article in Journal/Newspaper
• Language: unknown
• Published: American Chemical Society (ACS) 2019
• Subjects: Carbonic acid
• Online Access: http://hdl.handle.net/10754/660919; https://doi.org/10.1021/acs.estlett.9b00686

Reverse Osmosis (RO) is a membrane-based technology for water desalination. Of paramount importance is the understanding of ion selectivity in mixtures of salts, i.e., to what extent the membrane retains one ion more than another in a multicomponent salt solution. We apply continuum transport theory to describe a large set of data for the ion selectivity of RO membranes treating brackish groundwater with more than 10 different monovalent and divalent ions. The model is based on the Donnan steric partitioning pore model extended to include ions of multiple charge states, such as bicarbonate/carbonic acid, ammonia/ammonium, and hydroxyl/hydronium ions and the acid-base reactions between them and with the membrane charge. By adjusting for each ion the ratio of ion size over pore size, we can fit the model to the data. We note that the fitted ion sizes do not always follow a logical order based on the ionic or hydrated size of the ions and that rejection of divalent cations is overestimated in some cases. We discuss possible theoretical improvements to address these discrepancies. Our results highlight the potential of continuum transport theory to describe in detail multicomponent ion transport in RO membranes. The development of a detailed and validated physics-based model is an important step toward achieving improved operation and design of RO-based desalination systems. Part of this work was performed in the cooperation framework of Wetsus, European Centre of Excellence for Sustainable Water Technology (www.wetsus.eu). Wetsus is cofunded by the Dutch Ministry of Economic Affairs and Climate Policy, the Northern Netherlands Provinces, and the Province of Fryslan. The authors thank the participants in the research ̂ theme Advanced Water Treatment for fruitful discussions and financial support.