Zinc removal from aqueous solution using a deionization pseudocapacitor with a high-performance nanostructured birnessite electrode

Abstract

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.