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

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) 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 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-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.