Zinc removal from aqueous solution using a deionization pseudocapacitor with a high-performance nanostructured birnessite electrode
Manganese oxides are widely studied as heavy metal ion adsorbents and pseudocapacitor electrode materials. Synthesis methods affect the crystal structure, particle size, micromorphology, and the corresponding physicochemical properties of manganese oxides. In this work, nanostructured birnessite was readily obtained through hydrothermal reaction of KMnO4 and β-cyclodextrin under microwave irradiation. Based on the working principle of pseudocapacitors, the nanostructured birnessite was used as an electrode material for Zn2+ removal from aqueous solution by multi-cycle galvanostatic charge–discharge. The effects of electrolyte pH and birnessite mass on Zn2+ removal capacity were further investigated. The results indicate that the Zn2+ removal capacity increases and decreases with the increase of pH and birnessite mass, respectively. The highest Zn2+ removal capacity reaches 530.0 mg g−1, which is remarkably higher than the adsorption isotherm capacity (56.1 mg g−1). The significant improvement of electrochemical removal capacity can be attributed to the nanostructure and the not fully reversible redox reaction of the birnessite. The result of X-ray absorption fine structure (XAFS) indicates that Zn2+ is adsorbed above/below the vacancies and is inserted into the interlayer of birnessite, leading to the transformation of birnessite to Zn-buserite and hetaerolite during the charge–discharge process. The present study proposes a facile method for the rapid synthesis of nanostructured birnessite and highly efficient removal of Zn2+ from aqueous solution.