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 was rarely investigated at circumneutral pH. Therefore, we examined OH formation during oxygenation of adsorbed Fe(II) (Fe(II)) 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-AlO systems after oxidation of 24 h, much more than in systems with dissolved Fe(II) (Fe) alone (10.3 ± 0.1 μM). However, OH production in Fe(II) and α-FeOOH/α-FeO systems (6.9 ± 0.1-8.3 ± 0.1 μM) slightly decreased compared to Fe only. Further analyses showed that enhanced oxidation of Fe(II) was responsible for the increased OH production in the Fe(II)/AlO systems. In comparison, less Fe(II) was oxidized in the α-FeOOH/α-FeO systems, which was probably ascribed to the quick electron-transfer between Fe(II) 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.