Synergistic adsorption of Cd(II) and As(V) on birnessite under electrochemical control.

Manganese oxides are potentially used for the electrochemical removal of heavy metals from wastewater. However, little is known about the performance and mechanism of simultaneous electrosorption for multiple heavy metals, especially for coexisting anions and cations. In this work, birnessite-type manganese oxide was used for the electrochemical adsorption of coexisting Cd(II) cations and As(V) anions with a symmetrical electrode system, and the effects of the concentrations of coexisting metal ions and applied voltage were investigated. The results indicated that both the Cd(II) and As(V) adsorption capacity of birnessite increased in a mixed solution containing Cd(II) cations and As(V) anions, compared with that in single heavy metal solution. This synergistic effect was mainly ascribed to the formation of manganese arsenate precipitate and the reduction dissolution of birnessite on the cathode and the re-oxidation of Mn(II) with subsequently increased fresh adsorption sites on the anode. The electrochemical adsorption capacity for As(V) increased from 52.7 to 88.0 and 496.0 mmol kg-1 with increasing Cd(II) concentration from 0.1 to 1.5 mM, respectively, in the mixed solution containing 0.5 mM As(V). The removal efficiency of heavy metals increased first and then decreased with increasing voltage from 0 to 1.2 V. Under the optimum condition at 0.6 V for 12 h, the electrochemical adsorption capacity increased with increasing Cd(II) and As(V) concentrations and the highest capacity reached 2132.0 mmol kg-1 for Cd(II) and 1996.0 mmol kg-1 for As(V). This work provides a facile technique for the treatment of wastewaters containing metal anions and cations.

Cd2+ adsorption performance of tunnel-structured manganese oxides driven by electrochemically controlled redox.

The heavy metal ion adsorption performance of birnessite (a layer-structured manganese oxide) can be enhanced by decreasing the Mn average oxidation state (Mn AOS) and dissolution-recrystallization during electrochemical redox reactions. However, the electrochemical adsorption processes of heavy metal ions by tunnel-structured manganese oxides are still enigmatic. Here, tunnel-structured manganese oxides including pyrolusite (2.3 Å × 2.3 Štunnel), cryptomelane (4.6 Å × 4.6 Štunnel) and todorokite (6.9 Å × 6.9 Štunnel) were synthesized, and their electrochemical adsorptions for Cd2+ were performed through galvanostatic charge-discharge. The influence of both supporting ion species in the tunnel and tunnel size on the electrochemical adsorption performance was also studied. The adsorption capacity of tunnel-structured manganese oxides for Cd2+ was remarkably enhanced by electrochemical redox reactions. Relative to K+ in the tunnel of cryptomelane, the supporting ion H+ was more favorable to the electrochemical adsorption of Cd2+. With increasing initial pH and specific surface area, the electrochemical adsorption capacity of cryptomelane increased. The cryptomelane electrode could be regenerated by galvanostatic charge-discharge in Na2SO4 solution. Due to the differences in their tunnel size and supporting ion species, the tunnel-structured manganese oxides follow the order of cryptomelane (192.0 mg g-1) > todorokite (44.8 mg g-1) > pyrolusite (13.5 mg g-1) in their electrochemical adsorption capacities for Cd2+.

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 Cd2+ from aqueous solutions. The Cd2+ removal mechanism and the influences of birnessite loading and pH on the removal performance were investigated. The results showed that Cd2+ was adsorbed on the surfaces and interlayers of birnessite, and the maximum electrosorption capacity of birnessite for Cd2+ 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 Cd2+ increased with an increase in initial Cd2+ 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 Cd2+ from polluted wastewater.