Hydroxyl radical formation during oxygen-mediated oxidation of ferrous iron on mineral surface: Dependence on mineral identity

Many studies have examined the redox behavior of ferrous ions (Fe(II)) sorbed to mineral surfaces. However, the associated hydroxyl radical ( • OH) formation during Fe(II) oxidation by O 2 was rarely investigated at circumneutral pH. Therefore, we examined • OH formation during oxygenation of adsorbed Fe(II) (Fe(II) sorbed ) on common minerals. Results showed that 16.7 ± 0.4–25.6 ± 0.3 μM of • OH was produced in Fe(II) and α/γ -Al 2 O 3 systems after oxidation of 24 h, much more than in systems with dissolved Fe(II) (Fe 2+ aq ) alone (10.3 ± 0.1 μM). However, • OH production in Fe(II) and α -FeOOH/ α -Fe 2 O 3 systems (6.9 ± 0.1–8.3 ± 0.1 μM) slightly decreased compared to Fe 2+ aq only. Further analyses showed that enhanced oxidation of Fe(II) sorbed was responsible for the increased • OH production in the Fe(II)/Al 2 O 3 systems. In comparison, less Fe(II) was oxidized in the α -FeOOH/ α -Fe 2 O 3 systems, which was probably ascribed to the quick electron-transfer between Fe(II) sorbed and Fe(III) lattice due to their semiconductor properties and induced formation of high-crystalline Fe(II) phases that hindered Fe(II) oxidation and • OH formation. The types of minerals and solution pH strongly affected Fe(II) oxidation and • OH production, which consequently impacted phenol degradation. This study highlights that the properties of minerals exert great impacts on surface-Fe(II) oxidation and • OH production during water/soil redox fluctuations.