The surface properties, chemical compositions, and crystal structures of manganese oxides can be altered by redox reactions, which affect their heavy metal ion adsorption capacities. Here, birnessite nanosheets (delta-MnO2) were synthesized from the photochemical reaction of Mn-aq(2+) and nitrate under solar irradiation, and Zn2+ was electrochemically adsorbed using the as-obtained birnessite nanosheets by galvanostatic charge-discharge. The effects of current density and electrochemical techniques (symmetric electrode and three-electrode systems) on Zn2+ adsorption capacity were also investigated. The results showed that the maximum Zn2+ adsorption capacity of the birnessite in the presence of electrochemical redox reactions could reach 383.2 mg g(-1) (589.0 mmol mol(-1)) and 442.6 mg g-(1) (680.3 mmol mol(-1)) in the symmetric electrode and three-electrode system, respectively; however, the Mn2+ release capacity in the three-electrode system was higher than that in the symmetric electrode system. With increasing current density, the Zn2+ adsorption capacity decreased. In addition, the system for heavy metal ion removal driven by electrochemical redox reactions could also be used as a supercapacitor for power storage. The present work proposes a "green" and sustainable approach for preparing nanosized birnessite, and it clarifies the adsorption mechanism of birnessite for Zn2+ in the presence of electrochemical redox reactions.

The surface properties, chemical compositions, and crystal structures of manganese oxides can be altered by redox reactions, which affect their heavy metal ion adsorption capacities. Here, birnessite nanosheets (delta-MnO2) were synthesized from the photochemical reaction of Mn-aq(2+) and nitrate under solar irradiation, and Zn2+ was electrochemically adsorbed using the as-obtained birnessite nanosheets by galvanostatic charge-discharge. The effects of current density and electrochemical techniques (symmetric electrode and three-electrode systems) on Zn2+ adsorption capacity were also investigated. The results showed that the maximum Zn2+ adsorption capacity of the birnessite in the presence of electrochemical redox reactions could reach 383.2 mg g(-1) (589.0 mmol mol(-1)) and 442.6 mg g-(1) (680.3 mmol mol(-1)) in the symmetric electrode and three-electrode system, respectively; however, the Mn2+ release capacity in the three-electrode system was higher than that in the symmetric electrode system. With increasing current density, the Zn2+ adsorption capacity decreased. In addition, the system for heavy metal ion removal driven by electrochemical redox reactions could also be used as a supercapacitor for power storage. The present work proposes a "green" and sustainable approach for preparing nanosized birnessite, and it clarifies the adsorption mechanism of birnessite for Zn2+ in the presence of electrochemical redox reactions.