Selective removal of nitrate ion using a novel activated carbon composite carbon electrode in capacitive deionization

Two double-layer composite electrodes were fabricated by coating anion exchange resin (A520E) and carboxyl-functionalized multi-walled carbon nanotubes (MWCNTs-COOH) onto activated carbon electrodes. Subsequently, an asymmetric capacitive deionization reactor was assembled to evaluate performance of the selective electro-adsorption of nitrate in 2 mM chloride and 1 mM nitrate solutions. In the experiment, the total amount of anions adsorbed in reactor reached 48.80 mmol/m(2), with nitrate accounting for 54.07% in particular. It was confirmed that A520E/AC/GP electrode enhanced nitrate selectivity, and MWCNTs-COOH/AC/GP electrode increased the adsorption capacity. An investigation of desorption of anions showed nitrate was electrostatically adsorbed on electrode and then ion exchanged on the resin layer. When the applied voltage increased from 1.0 to 1.6 V, the total amounts of adsorbed anions increased, while the molar fraction of nitrate decreased. Desalination tests performed using different binary solutions of anions, revealed that the order of desalination capacities for anions is NO3- > SO42- > F- approximate to Cl-. A highly linear relationship between the amounts of total adsorbed ions and desorbed chloride ions on resin electrode indicated the same effect of changing solution conditions (i.e., concentration of ions) on adsorption by ion exchange and the electric field.