Unique role of Mn(II) in enhancing electro-oxidation of organic pollutants on anodes with low oxygen evolution potential at low current density.

This study systematically explored the role of Mn(II) in the removal of 4-chlorophenol (4-CP) by electro-oxidation (EO) employing anodes with low oxygen evolution potential (OEP), i.e., Ti/RuO 2 -IrO 2 , Ti/Pt, and Ti/Ti 4 O 7 , as well as anodes with high OEP, namely, Ti/PbO 2 , Ti/SnO 2 , and boron-doped diamond (Si/BDD). Mn(II) significantly promoted 4-CP removal on the anodes with low OEP at fairly low current density (0.04 to 1 mA/cm2), but had minimal to negative impact on those with high OEP. Cyclic voltammetry and X-ray photoelectron spectra revealed that Mn(II) was oxidized to Mn(III), then to Mn(IV) on the anodes with low OEP, whereas its was oxidized directly to Mn(IV) on those with high OEP. Deposition of manganese oxide on the anodes with low OEP suppressed oxygen evolution reaction (OER) in EO process, but enhanced OER on those with high OEP. Quenching and spectral results consistently indicated that Mn(III) and Mn(IV) were the primary species responsible for enhancing 4-CP removal on the anodes with low OEP. These findings provide mechanistic insights into the redox transformation of Mn(II) in EO and the theoretical basis for a novel strategy to boost pollutant degradation in EO systems using low OEP anodes through coupling with the redox chemistry of manganese. [Display omitted] • Removal of 4-CP was significantly enhanced by Mn(II) in EO using anodes with low OEP. • Mn(II) had little to negative impact on 4-CP removal in EO using anodes with high OEP. • Mn(II) was oxidized to Mn(III)/Mn(IV) and suppressed OER on the anodes with low OEP. • Mn(II) was directly oxidized to Mn(IV) and enhanced OER on the anodes with high OEP. • Coupling Mn(II) with EO using low OEP anodes could greatly enhance pollutant removal. [ABSTRACT FROM AUTHOR]