Fe(III) oxyhydroxides mediated electron transfer from thiols to O2 for hydroxyl radical production

Abstract

Thiols serve as important electron reservoirs in subsurface environments, playing an important role in biogeochemical cycling of redox-sensitive elements. Nevertheless, due to the kinetic constraint on thiol oxidation by O2, hydroxyl radical (•OH) production during thiol oxygenation had long been disregarded. •OH is the most powerful oxidant in natural environment, capable of oxidizing most of inorganic and organic pollutants. This study revealed that Fe(III) oxyhydroxides could mediate electron transfer from thiols to O2, thereby facilitating •OH production within a pH range of 3 to 9. For instance, at pH 7, the presence of 1 g/L ferrihydrite increased •OH accumulation from <0.2 to 15.3 μM within 10 h during the oxygenation of 6 mM cysteine. This enhancement resulted from Fe(III) oxyhydroxides altering the •OH production pathway from the direct oxidation of thiols by O2 to the oxidation of adsorbed Fe(II) by O2. In natural soils, Fe(III) oxyhydroxides were estimated to contribute to 69–81% of thiol removal and to 56–82% of •OH production. Furthermore, the •OH produced during thiol oxygenation also exhibited the capability to degrade phenol. This study highlights the previously overlooked catalytic role of Fe(III) oxyhydroxides in facilitating thiol oxidation and •OH production.