Iron-mediated oxidation of antimony(III) by oxygen and hydrogen peroxide compared to arsenic(III) oxidation

Antimony is used in large quantities in a variety of products, though it has been declared as a pollutant of priority interest by the Environmental Protection Agency of the United States (USEPA). Oxidation processes critically affect the mobility of antimony in the environment since Sb(V) has a greater solubility than Sb(III). In this study, the co-oxidation reactions of Sb(III) with Fe(II) and both O-2 and H2O2 were investigated and compared to those of As(III). With increasing pH, the oxidation rate coefficients of Sb(III) in the presence of Fe(II) and O-2 increased and followed a similar pH trend as the Fe(II) oxidation by O-2. Half-lives of Sb(III) were 35 and 1.4 h at pH 5.0 and pH 6.2, respectively. The co-oxidation with Fe(II) and H2O2 is about 7000 and 20 times faster than with Fe(II) and 02 at pH 3 and pH 7, respectively. For both systems, (OH)-O-center dot radicals appear to be the predominant oxidant below approximately pH 4, while at more neutral pH values, other unknown intermediates become important. The oxidation of As(III) follows a similar pH trend as the Sb(III) oxidation; however, As(III) oxidation was roughly 10 times slower and only partly oxidized in most of the experiments. This study shows that the Fe(II)-mediated oxidation of Sb(III) can be an important oxidation pathway at neutral pH values.