Iron-Catalyzed Oxidation of Arsenic(III) by Oxygen and by Hydrogen Peroxide:  pH-Dependent Formation of Oxidants in the Fenton Reaction

The oxidation kinetics of As(III) with natural and technical oxidants is still not well understood, despite its importance in understanding the behavior of arsenic in the environment and in arsenic removal procedures. We have studied the oxidation of 6.6 μM As(III) by dissolved oxygen and hydrogen peroxide in the presence of Fe(II,III) at pH 3.5−7.5, on a time scale of hours. As(III) was not measurably oxidized by O2, 20−100 μM H2O2, dissolved Fe(III), or iron(III) (hydr)oxides as single oxidants, respectively. In contrast, As(III) was partially or completely oxidized in parallel to the oxidation of 20−90 μM Fe(II) by oxygen and by 20 μM H2O2 in aerated solutions. Addition of 2-propanol as an •OH-radical scavenger quenched the As(III) oxidation at low pH but had little effect at neutral pH. High bicarbonate concentrations (100 mM) lead to increased oxidation of As(III). On the basis of these results, a reaction scheme is proposed in which H2O2 and Fe(II) form •OH radicals at low pH but a different oxidant, possibly an Fe(IV) species, at higher pH. With bicarbonate present, carbonate radicals might also be produced. The oxidant formed at neutral pH oxidizes As(III) and Fe(II) but does not react competitively with 2-propanol. Kinetic modeling of all data simultaneously explains the results quantitatively and provides estimates for reaction rate constants. The observation that As(III) is oxidized in parallel to the oxidation of Fe(II) by O2 and by H2O2 and that the As(III) oxidation is not inhibited by •OH-radical scavengers at neutral pH is significant for the understanding of arsenic redox reactions in the environment and in arsenic removal processes as well as for the understanding of Fenton reactions in general.