Enhancement of Zn2+ and Ni2+ removal performance using a deionization pseudocapacitor with nanostructured birnessite and its carbon nanotube composite electrodes

Abstract

Manganese oxides have been widely used as deionization capacitor electrode materials to remove heavy metal ions from aqueous solutions. However, the effect of pseudocapacitive properties of manganese oxides on the removal processes remains elusive. In this work, synthesized nanostructured birnessite-type manganese oxide and birnessite/carbon nanotubes (HB/CNTs) nanocomposites were used as deionization pseudocapacitor electrode materials for Zn2+ and Ni2+ removal from aqueous solution by constant potential electrolysis. The effects of operation potential and introduction of carbon nanotubes (CNTs) on Zn2+ and Ni2+ removal capacities were further investigated. The results demonstrated a significant enhancement of electrochemical removal capacities for Zn2+ and Ni2+ by the pseudocapacitive properties of birnessite and the introduction of CNTs. The Zn2+ and Ni2+ removal capacities of birnessite electrode increased first and then decreased with the decrease of potential from 0.2 to −0.2V (vs. SCE), and the highest removal capacities for Zn2+ and Ni2+ respectively reached 89.5 and 96.6mgg−1 when the potential was controlled at 0V. The HB/CNTs nanocomposite showed higher removal capacities (155.6mgg−1 for Zn2+ and 158.4mgg−1 for Ni2+ when the relative content of manganese oxide was 45.6%) and a better cycling stability (about 90% and 88% of the initial Zn2+ and Ni2+ removal capacity were retained after 5 cycles) than birnessite electrode. The present study makes clear the pseudocapacitive mechanism of heavy metal ion removal using birnessite, and proposes a facile method to remove heavy metal ions from aqueous solution.