Cadmium Removal from Aqueous Solution by a Deionization Supercapacitor with a Birnessite Electrode

Birnessite is widely used as an excellent adsorbent for heavy metal ions and as active electrode materials for supercapacitors. The occurrence of redox reactions of manganese oxides is usually accompanied by the intercalation–deintercalation of cations during the charge–discharge processes of supercapacitors. In this study, based on the charge–discharge principle of the supercapacitor and excellent adsorption properties of birnessite, a birnessite-based electrode was used to remove Cd 2+ from aqueous solutions. The Cd 2+ removal mechanism and the influences of birnessite loading and pH on the removal performance were investigated. The results showed that Cd 2+ was adsorbed on the surfaces and interlayers of birnessite, and the maximum electrosorption capacity of birnessite for Cd 2+ was about 900.7 mg g –1 (8.01 mmol g –1 ), which was significantly higher than the adsorption isotherm capacity of birnessite (125.8 mg g –1 ). The electrosorption specific capacity of birnessite for Cd 2+ increased with an increase in initial Cd 2+ concentration and decreased with an increase in the loading of active birnessite. In the pH range of 3.0–6.0, the electrosorption capacity increased at first with an increase in pH and then reached equilibrium above pH 4.0. This work provides a new method for the highly efficient adsorption of Cd 2+ from polluted wastewater.