Mechanistic insights into Sb(III) and Fe(II) co-oxidation by oxygen and hydrogen peroxide: Dominant reactive oxygen species and roles of organic ligands.

Sole O or HO oxidant hardly oxidize Sb(III) on a time scale of hours to days, but Sb(III) oxidation can simultaneously occur in Fe(II) oxidation by O and HO due to the generation of reactive oxygen species (ROS). However, Sb(III) and Fe(II) co-oxidation mechanisms regarding the dominant ROS and effects of organic ligands require further elucidation. Herein, the co-oxidation of Sb(III) and Fe(II) by O and HO was studied in detail. The results indicated that increasing the pH significantly increased Sb(III) and Fe(II) oxidation rates during Fe(II) oxygenation, while the highest Sb(III) oxidation rate and oxidation efficiency was obtained at pH 3 with HO as the oxidant. HCO and HPOanions exerted different effects on Sb(III) oxidation in Fe(II) oxidation processes by O and HO. In addition, Fe(II) complexed with organic ligands could improve Sb(III) oxidation rates by 1 to 4 orders of magnitude mainly due to more ROS production. Moreover, quenching experiments combined with the PMSO probe demonstrated that OH was the main ROS at acidic pH, whereas Fe(IV) played a key role in Sb(III) oxidation at near-neutral pH. In particular, the steady-state concentration of Fe(IV) ([Fe(IV)]) and k were determined to be 1.66×10 M and 2.57×10 M s, respectively. Overall, these findings help to better understand the geochemical cycling and fate of Sb in Fe(II)- and DOM-rich subsurface environments undergoing redox fluctuations and are conductive to developing Fenton reactions for the in-situ remediation of Sb(III)-contaminated environments.