Selective removal of Na+ from other competing ions in aqueous electrolytes is desirable for the desalination of water for agricultural irrigation. The salination of soil caused by irrigation with freshwater and treated wastewater constitutes a loss of ~$1 billion annually in the US due to decreases in plant yields. As ~$10 billion would be required to recover all cropland using gypsum, desalinated water is considered an alternative source that increases yield by up to 30% with half the amount of water. Energy-efficient selective removal of Na+ would result in cheaper desalinated water and avoid the need of remineralization process, which can save about $55 million per year. Herein, sodium selective removal is studied using a binary sodium superionic conductor (NASICON) alloy – sodium titanium vanadium phosphate (NTVP) – as intercalation electrodes in a three-electrodes flooded cell and a symmetric Faradaic deionization (FDI) cell. The former experiments were performed with aqueous electrolytes containing either Li+, Na+, K+, Mg2+, or Ca2+ ions, showing that only Li+ and Na+ can intercalate into NTVP crystal lattice, whereas other cations undergo capacitive adsorption, resulting in an intrinsic selectivity factor of 56 for Na+ over K+, Mg2+, and Ca2+. An FDI cell using two similar NTVP electrodes separated by an anion exchange membrane (AEM) was able to remove up to 94% of Na+ from feed water containing a competing alkali/alkaline earth cation of ten-fold higher concentrations, giving a process selectivity of 3-6 for Na+ over other cations. The separation processes in the FDI cell consume comparable or lower energy compared to previous works, which is achieved due to minimal state-of-charge gradients within NTVP electrodes as a result of using fluid recirculation and the solid-solution intercalation mechanism of Na+ into NTVP. Moreover, energy consumption can be reduced by as much as 50% and sodium selectivity could be enhanced up to 25% by lowering current density in FDI cell experiments.

Efficient, selective sodium removal by Faradaic deionization using symmetric sodium titanium vanadium phosphate intercalation electrodes

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