Removal of nitrate by asymmetric capacitive deionization

Abstract

Contamination of groundwater with nitrates is a major concern, especially for areas relying on this as a drinking water source. In this work, a capacitive deionization (CDI) system equipped with carbon electrodes coated with different metal oxides was studied to determine its ability to reduce nitrate concentrations. Results performed in a three-electrode cell were used as a proof of concept and demonstrated that coated electrodes had higher nitrate removal than that of uncoated electrodes, likely because of a reduction in hydrophobicity and an increase in surface area provided by the metal oxides. Moreover, tests using different electrolytes (NaNO3 and Ca(NO3)2) revealed similar nitrate removal values, although different electrosorption patterns were observed for Na+ and Ca2+. Furthermore, an operational mode based on a multistep approach showed that nitrates could be removed below regulatory limits while reducing the volume of waste brine. A larger, eight-cell flow CDI reactor was also tested. The results from this reactor showed that the cell potential, as well as the ion being removed from a multicomponent solution (Ca2+, Na+, Cl−, NO3−) influence electrosorption kinetics. Different adsorption mechanisms based on ion charge/size/electrode affinity are discussed, possibly leading to a methodology for preferentially removing certain ions by CDI technology.