Modification of Cation-Exchange Membranes with Polyelectrolyte Multilayers to Tune Ion Selectivity in Capacitive Deionization

Abstract

Capacitive deionization (CDI) is a desalination technique that can be applied for the separation of target ions from water streams. For instance, mono- and divalent cation selectivities were studied by other research groups in the context of water softening. Another focus is on removing Na + from recirculated irrigation water (IW) in greenhouses, aiming to maintain nutrients. This is important as an excess of Na + has toxic effects on plant growth by decreasing the uptake of other nutrients. In this study, we investigated the selective separation of sodium (Na + ) and magnesium (Mg 2+ ) in MCDI using a polyelectrolyte multilayer (PEM) on a standard grade cation-exchange membrane (Neosepta, CMX). Alternating layers of poly(allylamine hydrochloride) (PAH) and poly(styrene sulfonate) (PSS) were coated on a CMX membrane (CMX-PEM) using the layer-by-layer (LbL) technique. The layer formation was examined with X-ray photoelectron spectroscopy (XPS) and static water contact angle measurements (SWA) for each layer. For each membrane, i.e., the CMX-PEM membrane, CMX membrane, and for a special-grade cation-exchange membrane (Neosepta, CIMS), the Na + /Mg 2+ selectivity was investigated by performing MCDI experiments, and selectivity values of 2.8 ± 0.2, 0.5 ± 0.04, and 0.4 ± 0.1 were found, respectively, over up to 40 cycles. These selectivity values indicate flexible switching from a Mg 2+ -selective membrane to a Na + -selective membrane by straightforward modification with a PEM. We anticipate that our modular functionalization method may facilitate the further development of ion-selective membranes and electrodes.

Recent advances in ion selectivity with capacitive deionization

“…The first is that the selectivity changes strongly during a CDI cycle, i.e., it depends on the moment within a cycle, and likely also on the durations of the charge and discharge steps in a full cycle. 123 The second conclusion is that the actual realized selectivity can be much lower than the maximum calculated selectivity, which in this case is a value significantly above 100. This is in line with the conclusion presented by Porada et al 78 Careful design of the electrodes and the cell will be essential in obtaining selectivity values that in practice come closer to the ideal value.…”

“…8B). 123 While the bare CMX membrane had a r of E0.5 for Na + over Mg 2+ , the selectivity of the PEM-coated membrane was found to be E3. This switch in selectivity was attributed to the charge rejection experienced by ions with higher valence (Fig.…”

“…adding polyelectrolyte multilayers to achieve monovalent selectivity. 123 On a system level, dependence of cell selectivity on operation parameters such as applied current, cell voltage, ion concentration, and pH among others will have to be systematically studied to find optimum conditions that enhance selectivity for the CDI cell. Intercalation electrodes such as TiS 2 show switchable preference depending on the potential of the electrode, as shown by Srimuk et al 43 Such insights will be useful in realizing the full potential of existing (and the search for new) electrode materials.…”

Theoretical Analysis of Constant Voltage Mode Membrane Capacitive Deionization for Water Softening

“…MCDI water softening performance is determined by feed water chemistry, hydrated ion radius, ion valence, electrode and IEM construction and materials, operating modes and operating conditions [ 15 , 16 , 23 , 24 ]. Hou and Huang [ 15 ] observed preferential adsorption of ions with smaller hydrated ion radius, larger charge valence and higher initial concentration in a batch mode CDI.…”

“…IEM selective permeation toward specific ions is governed by the affinity of IEM toward the ions, ion concentration outside the IEM and ion mobility inside the IEM and can be enhanced by membrane surface modification [ 25 ]. Specific IEM modifications such as replacing CEM with Ca-alginate [ 16 ] and coating polyelectrolyte multilayers onto the CEM [ 24 ] have been employed to tune ion selectivity in MCDI. He et al [ 23 ] observed that selectivity of calcium ions was enhanced under lower applied current and shorter hydraulic retention time in a flow-electrode CDI.…”