Decolorization of methyl orange by MnO2/organic acid system: The role of Mn(III)

Decolorization of methyl orange (MO) by MnO 2 was investigated with and without small molecules of organic acids such as formic acid. It was found that the decolorization could be significantly improved by MnO 2 with the assistance of organic acids. The effect of various conditions was investigated on MO decolorization by using different crystal types of MnO 2 and changing the types of organic acids. One of the optimum treatment conditions was δ-MnO 2 /formic acid system, which led to the removal efficiency of 90.6% at 30 min. The pseudo-second-order kinetic rate constant of δ-MnO 2 /formic acid was 0.1626. A reasonable decolorization mechanism involving Mn(III) species was proposed and the mechanism was validated by simple detection of ionic species such as, Mn(II) and Mn(III) during the reaction. The δ-MnO 2 /formic acid process designed here may lead to a new advanced oxidation technology based on the in-situ generation of active Mn(III) species and their ability for oxidation.