Di-valent cations existing in pipeline system are main health concerns. Most common hazardous cations are Copper (Cu2+) and Lead (Pb2+) in public drinking water, and they are regulated by EPA as National Primary Drinking Water Contaminants. As we have witnessed from the lead-poisoned water crisis in Flint, MI, exposure to heavy metal is a wide-spread and emerging threat. Moreover, naturally occurring scale-forming ions, such as calcium (Ca2+), magnesium (Mg2+) are problematic for both residential and industrial water systems, including water pipelines, oil wellbores, and membrane reactors. Capacitive deionization (CDI) is an emerging desalination process, which removes charged ions from water through electrosorption of the ions on oppositely charged electrodes. CDI in general, however, does not distinguish different types of ions of the same sign of charges. Few studies demonstrated slightly higher adsorption of Ca2+ over Na+ using zeolite (Liu et al., 2013) or calcium-alginated (Yoon et al., 2016) composite electrode. However, no electrode showed both distinctively preferential removal of divalent cations and long-term operational stability. In our study, we developed divalent ion-selective CDI electrodes by depositing a nanocomposite, selective cation exchange coating on the carbon electrode surface. The nanocomposite coating consists of a poly(vinyl alcohol) (PVA) matrix and nano-sized cation exchange resins with aminophosphonic functional groups. First, Ion-exchange membranes separate counterions from co-ions similar to classical membrane capacitive deionization (MCDI). Then, the nanocomposite coating allows preferential transport of divalent cations into carbon electrode pores. The performance of the ion-selective electrode was evaluated in a flow-through MCDI cell using binary electrolyte solutions. The total ion adsorption capacity of the electrode remained the same with and without the nanocomposite coating. However, the coated electrode exhibited strong selectivity towards divalent cations over the mono-valent sodium ion. Further experiments suggest that the divalent cations adsorbed on the ion exchange resin in the nanocomposite coating are highly mobile in the electric field in the CDI device. The coating does not serve as an importance source of adsorption capacity, but cause preferential transport of the divalent cations instead. The composite electrode showed completely reversible adsorption, and highly stable performance during repeated cycles.

Removal of di-valent cations using ion-selective capacitive deionization

256th National Meeting (2018)

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