Correlating Electron Affinity with Redox Activity in Layered Birnessite

Abstract

Birnessite  is  a  layered  manganese  oxide  used  as  an  oxidant  in  water  treatment,  but  the electronic factor that controls its reactivity is not well defined. We  studied eleven natural and synthetic birnessites with different interlayer cations and Mn(III)/Mn(IV) ratios to test whether electron affinity  (EA) governs oxidation. EA  from ultraviolet photoelectron  spectroscopy with optical-gap support was 5.56–6.06 eV. At pH 7.0 and 25 °C, Fe(II) and As(III) oxidation followed a pseudo-second-order model. The rate constant k2 rose from 0.0048 to 0.022 g mg⁻¹ min⁻¹ for Fe(II) and from 0.0026 to 0.015 g mg⁻¹ min⁻¹ for As(III) as EA increased, with an exponential fit R2=0.95. The apparent activation energy fell from 46±2 to 29±2 kJ mol⁻¹ across the same EA range. After five cycles, high-EA samples kept 81–87% of the initial rate, while low-EA samples kept 54–62%, in line with less MnOOH seen by XPS. These results show that electron affinity is a quantitative  descriptor  of  oxidizing  activity  in  birnessite  and  suggest  that  tuning  interlayer chemistry,  hydration,  and  defects  can  yield  durable  manganese-oxide  catalysts  for  pollutant removal.