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 (23 angstrom x 23 angstrom tunnel), cryptomelane (4.6 angstrom x 4.6 A angstrom tunnel) and todorokite (6.9 angstrom x 6.9 angstrom tunnel) 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+. (C) 2018 Elsevier Ltd. All rights reserved.