Selective removal of lead ions through capacitive deionization: Role of ion-exchange membrane

Abstract

Capacitive deionization (CDI) is a rising technology as a low-energy-consumption and low-cost option for water purifications and treatments; however, selective removal of heavy metal ions has been rarely reported. In this study, the impacts of ion-exchange membranes on the selective removal of lead ions (Pb2+) by CDI are investigated with a single-pass model. Both cation-exchange membrane (CEM) and anion-exchange membrane (AEM) demonstrate improved Pb2+ removal efficiencies. The presence of CEM retards discharge of adsorbed Pb2+, which eliminates the selectivity of removing Pb2+ against other cations (such as Ca2+ and Mg2+); in contrast, without the CEM, the CDI cell with only the AEM (called AEM-CDI) exhibits enhanced discharge efficiency and retains the selectivity of removing Pb2+ against Ca2+ and Mg2+. More importantly, the AEM-CDI presents improved Pb2+ discharge efficiency when applying inverted voltages, which is not possible in the membrane-free CDI because desorbed ions can be re-adsorbed to the counter electrode. In addition, surface modification for porous carbon with functional groups to enhance the affinity between Pb2+ and electrode surface is expected to further improve the selectivity of removing Pb2+ against other cations, thereby showing very promising potential in purification of drinking water.