Using Ultramicroporous Carbon for the Selective Removal of Nitrate with Capacitive Deionization

The contamination of water resources with nitrate is a growing and significant problem. Here we report the use of ultramicroporous carbon as a capacitive deionization (CDI) electrode for selectively removing nitrate from an anion mixture. Through moderate activation, we achieve a micropore-size distribution consisting almost exclusively of narrow (<1 nm) pores that are well suited for adsorbing the planar, weakly hydrated nitrate molecule. Cyclic voltammetry measurements reveal an enhanced capacitance for nitrate when compared to chloride as well as significant ion sieving effects when sulfate is the only anion present. We measure high selectivities (S) of both nitrate over sulfate (S-NO3/SO4 = 17.8 +/- 3.6 at 0.6 V) and nitrate over chloride (S-NO3/Cl = 6.1 +/- 0.4 at 0.6 V) when performing a constant voltage CDI separation on 3.33 mM/3.33 mM/1.67 mM Cl/NO3/SO4 feedwater. These results are particularly encouraging considering that a divalent interferant was present in the feed. Using molecular dynamics simulations, we examine the solvation characteristics of these ions to better understand why nitrate is preferentially electrosorbed over sulfate and chloride.