Selective phosphate removal using layered double hydroxide/reduced graphene oxide (LDH/rGO) composite electrode in capacitive deionization.

Phosphate removal is a critical issue in water treatment because excess levels of phosphate can cause severe eutrophication. Capacitive deionization (CDI), which has several advantages, such as simple, eco-friendly, and energy efficient operations, has gained attention as a potential alternative over conventional phosphate removal technologies like activated sludge, chemical precipitation, and adsorption processes. However, CDI suffers from a lack of selectivity for phosphate, resulting from non-selective anion removal of positively biased electrodes. Herein, the layered double hydroxide/reduced graphene oxide (LDH/rGO) composite electrode in the CDI process was examined for selective phosphate removal. LDH/rGO showed the selective phosphate removal performance with sustained phosphate removal efficiency even in the presence of excess chloride. In addition, the selective phosphate removal in the CDI process with the LDH/rGO was successfully demonstrated in the simulated water, fabricated by adding a significantly low concentration of phosphate (0.4 mg∙L-1) into real river water matrix (Han River, Seoul, Korea). This result was explained by the high electrochemical selectivity of the LDH/rGO for phosphate.

Electrochemical selective ion separation in capacitive deionization with sodium manganese oxide.

Electrochemical selective ion separation via capacitive deionization, for example, separation of lithium resource from brine, using lithium ion batteries is proposed and demonstrated to have the potential for separating specific ions selectively from a solution containing diverse ions. This separation method is of great industrial concern because of applicability in various fields such as deionization, water softening, purification, heavy metal removal, and resource recovery. Nevertheless, besides the selectivity of materials for lithium ion batteries toward Li+, there is very little investigation on the selectivity of the materials for sodium ion batteries toward Na+. Here, the electrochemical selectivity of sodium manganese oxide (Na0.44MnO2), one of the most widely used material in sodium ion batteries, for Na+ and other cations (K+, Mg2+, and Ca2+) is investigated. Selective Na+ separation using the system consisting of Na0.44MnO2 and a Ag/AgCl electrode is successfully demonstrated from a solution containing diverse cations (Na+, K+, Mg2+, and Ca2+) via a two-step process that involves a capturing step (charging process) and a releasing step (discharging process). The results showed that Na0.44-xMnO2 has over 13 times higher selectivity for Na+ than for K+ and 6-8times higher selectivity for Na+ than for Mg2+ and Ca2+ in the electrolyte containing equal concentrations of the respective ions. Additionally, as a practical demonstration, Na+ was successfully separated from an industrial raw material used for pure KOH production (estimated ratio of Na+:K+=1:200).