Selective adsorption of nitrate over chloride in microporous carbons

Abstract

Activated carbon is the most common electrode material used in electrosorption processes such as water desalination with capacitive deionization (CDI). CDI is a cyclic process to remove ions from aqueous solutions by transferring charge from one electrode to another. When multiple salts are present in a solution, the removal of each ionic species can be different, resulting in selective ion separations. This ion selectivity is the result of combined effects, such as differences in the hydrated size and valence of the ions. In the present work, we study ion selectivity from salt mixtures with two different monovalent ions, chloride and nitrate. We run adsorption experiment in microporous carbons (i.e., without applying a voltage), as well as electrosorption experiments (i.e., based on applying a voltage between two carbon electrodes). Our results show that i) during adsorption and electrosorption, activated carbon removes much more nitrate than chloride; ii) at equilibrium, ion selectivity does not depend strongly on the composition of the water, but does depend on charging voltage in CDI; and iii) during electrosorption, ion selectivity is time-dependent. We modify the amphoteric Donnan model by including an additional affinity of nitrate to carbon. We find good agreement between our experimental results and the theory. Both show very high selectivity towards nitrate over chloride, βNO3−/Cl− ∼10, when no voltage is applied, or when the voltage is low. The selectivity gradually decreases with increasing charging voltage to βNO3−/Cl− ∼6 at Vch = 1.2 V. Despite this decrease, the affinity-effect for nitrate continues to play an important role also at a high voltage. In general, we can conclude that our work provides new insights in the importance of carbon-ion interactions for electrochemical water desalination.