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 attractiveadvanced 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 remainsa challenge. Mn(II) is known to accelerate the Fenton reaction, yet the mechanism hasnever been confidently established. We report a systematic kinetic and spectroscopicinvestigation into Mn(II) acceleration of atrazine or 2,4,6-trichlorophenol degradationby the picolinic acid (PICA)-assisted Fenton reaction at pH 4.5-6.0. Mn(II)accelerates Fe(III) reduction, superoxide radical (HO2 center dot/O2 center dot-) formation, and hydroxylradical (HO center dot) formation. A Mn(II/III)-H2O2redox cycle as an independent source ofreactive 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 hydroperoxospecies, PICA-FeIII-OOH. The resulting binuclear complex undergoes intramolecularelectron transfer to give Fe(II), which later generates HO center dot from H2O2, plus MnO2+, which later decomposes to HO2 center dot/O2 center dot-(anFe(III) reductant) and Mn(II), completing the catalytic cycle. This scheme may apply to other Fenton-type systems that go throughan FeIII-OOH intermediate. Thefindings here will inform the design of practical and sustainable Fenton-based AOPs employingMn(II) in combination with chelating agents.

Automated summary

Mn(II) was found to accelerate the Fenton reaction by participating directly and catalytically in the Fe(III)/Fe(II) redox cycle in the presence of picolinic acid (PICA) as an assisting agent. Mn(II) accelerated Fe(III) reduction, superoxide radical formation, and hydroxyl radical formation, but the proposed Mn(II/III)-H2O2 redox cycle was shown to be insignificant. Instead, Mn(II) initially complexes with a ferric hydroperoxo species, followed by intramolecular electron transfer to give Fe(II) and MnO2+. This scheme can be applied to other Fenton-type systems. These findings will be valuable for the design of practical and sustainable Fenton-based advanced oxidation processes (AOPs) using Mn(II) in combination with chelating agents.