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.