The oxidation of Fe(II) by O2/H2O2 initiates a cascade of reactions that leads to the formation of strong, short-lived oxidants (*O2-, *OH, Fe(IV)). These reactive intermediates are critical players in the oxidation of chemical species including As(III), Sb(III), organic pollutants, and soil organic matter. Recent studies provide indirect evidence that Mn(II) is also susceptible to homogeneous oxidation by reactive oxidants species, such as extracellular superoxide radicals produced by bacteria and superoxide and singlet oxygen radicals generated from photochemical excitation of humic substances. In this work, we characterize the products of Fe(II)-mediated Mn(II) oxidation and describe the factors that control the efficiency of Mn(II) oxidation in Fenton-type systems. We used an Fe(0) electrocoagulation (EC) system, which permits precise control of the rate of Fe(II) production, to investigate the co-oxidation of Fe(II) and Mn(II) as a function of the rate at which iron was dosed into solution (IPR), the Fe(II) oxidant (O2 or H2O2), and solution pH. We found that Mn removal was enhanced under conditions where the accumulation of aqueous Fe(II) was minimized. In addition, in the EC/H2O2 system, Mn(II) removal efficiency increased as pH decreased from 6.5 to 4.5 and as pH increased from 6.5 to 8.5, which implicates different reactive oxidants under acid and alkaline solutions. Chemical and X-ray absorption spectroscopy analyses revealed that Mn(II) removal occurs dominantly via oxidation to Mn(III) and incorporation into the resulting Fe(III) coprecipitates regardless of pH and IPR values. Our results demonstrate that Fe(0) EC can be a viable method to prevent exposure to Mn(II) in drinking water, especially in decentralized areas where the modular properties and low supply infrastructure of EC systems provide advantages over conventional technologies. Our work also shows that Mn(II) is an important sink of reactive oxidants produced in Fenton and Fenton-type systems, which must be considered for accurate predicitions of target compound oxidation in systems containing Mn(II), such as groundwater contaminated with As(III) and Mn(II).

Mn(II) oxidation in Fenton and Fenton-type systems: Identification of reaction efficiency and reaction products

253rd National Meeting (2017)

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