Mn(II) Acceleration of the Picolinic Acid-Assisted Fenton Reaction: New Insight into the Role of Manganese in Homogeneous Fenton AOPs

Abstract

The homogeneous Fe-catalyzed Fenton reaction remains an attractive advanced oxidation process for wastewater treatment, but sustaining the Fe­(III)/Fe­(II) redox cycle at a convenient pH without the costly input of energy or reductants remains a challenge. Mn­(II) is known to accelerate the Fenton reaction, yet the mechanism has never been confidently established. We report a systematic kinetic and spectroscopic investigation into Mn­(II) acceleration of atrazine or 2,4,6-trichlorophenol degradation by the picolinic acid (PICA)-assisted Fenton reaction at pH 4.5–6.0. Mn­(II) accelerates Fe­(III) reduction, superoxide radical (HO2•/O2•–) formation, and hydroxyl radical (HO•) formation. A Mn­(II/III)-H2O2 redox cycle as an independent source of reactive oxygen species, as proposed in the literature, is shown to be insignificant. Rather, Mn­(II) assists by participating directly and catalytically in the Fe­(III)/Fe­(II) redox cycle. Initially, Mn­(II) (as MnII(PICA)+) complexes with a ferric hydroperoxo species, PICA-FeIII-OOH. The resulting binuclear complex undergoes intramolecular electron transfer to give Fe­(II), which later generates HO• from H2O2, plus MnO2+, which later decomposes to HO2•/O2•– (an Fe­(III) reductant) and Mn­(II), completing the catalytic cycle. This scheme may apply to other Fenton-type systems that go through an FeIII-OOH intermediate. The findings here will inform the design of practical and sustainable Fenton-based AOPs employing Mn­(II) in combination with chelating agents