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

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.