(470a) Counter-Ion Transport in Highly Charged and Low Water Content Ion-Exchange Membranes

Brine management is an enormous challenge for implementing nontraditional water sources such as brackish water, seawater, municipal and industrial wastewater (e.g., produced water), especially in inland areas where brine disposal options are limited. Thermal concentrators (e.g., mechanical vapor compression evaporators) are widely implemented for brine minimization, but these technologies are energy intensive and require high capital investment. Electrodialysis (ED) provides a lower energy alternative to thermal brine concentration and could enable cost-effective treatment of brine concentrations up to 250,000 mg/L TDS. Conventional ion-exchange membranes (IEMs) for brackish water desalination are highly swollen by water (water mass fractions between 0.4-0.5) and exhibit low permselectivity when contacted by concentrated salt solutions, making them unsuitable for applications involving high salinity waters. New classes of commercial IEMs for treating concentrated salt solutions have emerged, but further improvements in membrane performance are necessary for ED to outcompete conventional technologies in this space. A defining feature of IEMs for treating concentrated salt solutions is their low water content (water mass fractions lower than 0.3). Ion transport in highly charged, low water content IEMs is poorly understood on a fundamental level, which hinders our ability to rationally design new high-performance membranes. In this study, we systematically explored the effect of membrane water content on counter-ion transport in homogeneous IEMs. Cross-linked IEMs with broadly varying water uptake were synthesized via free radical copolymerization. Counter-ion diffusion coefficients were obtained from membrane ionic conductivity values measured via electrochemical impedance spectroscopy (EIS). The EIS measurements were performed with water-equilibrated IEMs to eliminate potential complicating effects of co-ions. To gain deeper insights into the mechanism of counter-ion transport in the IEMs, activation energies for diffusion were obtained by performing the ionic conductivity measurements as a function of temperature. The role of various factors (e.g., polymer tortuosity, ion hydration, ion pairing, etc.) that potentially influence counter-ion transport in IEMs at various degrees of water uptake is the main topic of this presentation.