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

Abstract

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 ground water with more than ten different mono-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 the 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 towards achieving improved operation and design of RO-based desalination systems.

Tutorial review of Reverse Osmosis and Electrodialysis

“…For the more general Eq. (49) this is more complicated because in a module retention depends on water recovery. Optimal RO operation minimizes the sum of two cost factors, first the energy use, and second the material costs.…”

“…In this section we present a simple 2D module calculation for an ideal 1:1 salt. In the future this must be extended to solutions of more ions, and effects of pH on ion protonation and membrane charge (acid-base reactions) must be included[8,42,[48][49][50][51]. In the present section we only discuss a module calculation for a simple 1:1 salt of two ions of equal diffusion coefficient, equal Φ i , for…”

Cation Selectivity in Capacitive Deionization: Elucidating the Role of Pore Size, Electrode Potential, and Ion Dehydration

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Simulations of BWRO systems under different feedwater characteristics. Analysis of operation windows and optimal operating points

“…As well as the characteristics of the BWRO membranes [10][11][12], it is extremely important to take into consideration the characteristics of the feedwater in the design and operation of BWRO plants. As high flow recoveries (R) are achievable, the solubility of sparingly soluble mineral salts (e.g.…”

“…They focused on optimizing an equation related to the capital and operating costs, and considered the incorporation of ERDs and interstage pumps in the BWRO system design. Different TDS f values (16,12,6 and 3 g L −1 NaCl) were used. Their analysis involved use of the Filmtec TM BW30-400 membrane.…”