Oxidation of HSO₃^- in aqueous suspensions of α-Fe₂O₃, α-FeOOH, β-FeOOH and γ-FeOOH in the dark and under illumination

The kinetics of oxidation of HSO3- ions in aqueous solutions in the presence of α-Fe2O3, α-FeOOH, β-FeOOH and γ-FeOOH is investigated. The oxidation of HSO3- follows a zero-order kinetics in suspensions of β-FeOOH and γ-FeOOH and a first-order kinetics for α-FeOOH and α-Fe2O3 systems. Some degradation of the iron(III) oxides takes place, evidenced by the presence of Fe(III) and Fe(II) in solution. Under illumination, the rate of HSO3- oxidation notably increases. In this case, the rate of oxidation of HSO3- by the four iron(III) oxides follows a kinetic equation of the type dx/dt = k(1-x)/x, where x is the fraction of HSO3- oxidized at time t. The rate constant of HSO3- photo-oxidation per unit of specific area for the different iron(III) oxides studied follows the order: α-Fe2O3 >> γ-FeOOH ~ β-FeOOH >> α-FeOOH. Fe(II) and Fe(III) were also detected in solution after irradiation of the iron(III) oxide suspensions.