Capacitive deionization: a promising technology for water defluoridation: a review

Abstract

Capacitive deionization (CDI) is among the promising technologies employed for water purification. CDI has been studied for the removal of various ionic species from water including fluoride ion (F−) with promising results. However, there is no comprehensive literature that summarizes the use of CDI for water defluoridation applications. Therefore, this review paper critically analyzes different electrode materials that have been studied for water defluoridation, their electrosorption capacities and F− removal efficiencies. It further discussed the parameters that influence CDI efficiency during defluoridation and point out the issues of F− selectivity when co-existing with other ions in the solution. We can conclude that different electrode materials have shown different abilities in electrosorption of F−. The carbon-based materials possess high surface area and good electrical conductivity which is paramount for ion adsorption but gives lack selectivity for F− removal. Metal oxides and hydroxides have been reported with improved electrosorption capacity and high selectivity to F− due to the ion exchange between the F− and the hydroxyls surface of the metal oxides/hydroxides. Apart from the good performance of these materials for defluoridation, the discovery of actual practical use of the electrode materials for defluoridation for commercial scale is still a need. HIGHLIGHTS Water defluoridation by capacitive deionization (CDI) have been reviewed.; The defluoridation efficiency of various CDI electrode materials has been reviewed.; Metal oxides/hydroxides, as well as carbon materials and metal oxides/hydroxides composites, have shown good fluoride uptake in pilot-scale experiments.; Parameters affecting the defluoridation process with CDI have been reviewed.;

Keywords

• capacitive deionization
• defluoridation
• electrode materials
• electrosorption