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

Abstract

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 O2 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-Al2O3 systems after oxidation of 24 h, much more than in systems with dissolved Fe(II) (Fe2+aq) alone (10.3 ± 0.1 μM). However, •OH production in Fe(II) and α-FeOOH/α-Fe2O3 systems (6.9 ± 0.1–8.3 ± 0.1 μM) slightly decreased compared to Fe2+aq only. Further analyses showed that enhanced oxidation of Fe(II)sorbed was responsible for the increased •OH production in the Fe(II)/Al2O3 systems. In comparison, less Fe(II) was oxidized in the α-FeOOH/α-Fe2O3 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.