Electrochemical degradation of oxalic acid over highly reactive nano-textured γ- and α-MnO2/carbon electrode fabricated by KMnO4 reduction on loofah sponge-derived active carbon

Abstract

Manganese dioxide incorporated activated carbon (MnO₂/AC) was synthesized and used to electrochemically degrade oxalic acid in aqueous solutions. The highly porous carbon provided reactive sites for the electro-sorption of oxalic acid and MnO₂, with a specific polymorphism efficiently mediating the electron transfer between the electrode and organic pollutants. The activated carbon, made from the pyrolysis of dry loofah sponge using ZnCl₂ as activating agent, exhibited a high double-layer capacitance dependent upon the heating temperature (100 F/g at 800 °C). The γ-MnO₂ was in-situ deposited over the microporous structure of activated carbon through the redox reaction between KMnO₄ and carbon. Simple further calcination converted γ-MnO₂ to α-MnO₂ nano-whisker at temperatures above 500 °C. Cyclic voltammetry showed that oxalic acid significantly improved the anodic current of the Mn(III)/Mn(IV) redox couple on the MnO₂/AC electrode at an electrode potential around + 0.6 V (vs. Ag/AgCl). About 95% of oxalic acid degradation was achieved at pH < 4; meanwhile, 80% of the mineralization (total organic carbon removal) was attained independent of pH. Calcination converted γ-MnO₂ to α-MnO₂ which had higher electrochemical stability and inhibited the dissolution of Mn(II) from the electrode.

Keywords: Activated carbon; Electrical capacitance; Manganese dioxide; Oxalate mineralization; Polymorphism; Voltammetry.