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

Abstract

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 NO 3 /SO 4 = 17.8 ± 3.6 at 0.6 V) and nitrate over chloride (S NO 3 /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/NO 3 /SO 4 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.

Recent advances in ion selectivity with capacitive deionization

“…Comparisons between capacitance values were used by different research groups to explore the preference of one ion over another. [40][41][42] In case of intercalation materials, CV can provide information about the preference of the active materials towards different ions. Higher cathodic peak potential associated with intercalation of an ion indicate a higher preference for intercalation of the electrode towards that ion.…”

“…This result is in line with other works that show lower selectivity at higher cell voltages. 41,77 In a study similar to Pan et al, Mao et al employed nanofiltration monovalent-selective membranes to increase the selectivity of chloride over sulfate. 138 The authors adapted an EDL model that shows a higher diffusion of chloride through the membrane than sulfate (Fig.…”

“…Mechanisms for ion selectivity in CDI and MCDI are based either on the partitioning of ions between the bulk electrolyte solution and the electrode or membrane, or are due to variations in transport parameters between ions. 24,41,49,[150][151][152] Here, we focus on selectivity due to partitioning of ions between the electrode and electrolyte, or the membrane and electrolyte. The electrodes used in such systems are electrically conductive capacitive electrodes, for instance made of intercalation materials such as Prussian Blue Analogues (PBA) 34,78 or made of microporous activated carbon.…”

“…A recent study by Hawks et al showed a high selectivity for nitrate over chloride and sulfate by using ultra-microporous (pore diameter o 1 nm) carbon electrodes. 41 The idea is similar to the one already explored by Eliad et al (2001), in which selectivity is achieved due to sieving effect of very small carbon pores. The authors explored the effect of the solvation shell of the ion in aqueous media on their selective adsorption ( Fig.…”

Specific ion effects at graphitic interfaces

“…Aqueous solutions at voltage-biased graphitic interfaces are essential to a variety of energy and environmental technologies, including supercapacitors for energy storage 1,2 and capacitive deionization for water desalination and purification 3–6 . Within these devices, a detailed understanding of specific ion effects at the interface and their influence on interfacial capacitance is crucial for predicting and optimizing performance.…”