Oxidation of antimony (III) by amorphous iron and manganese oxyhydroxides

Amorphous forms of natural and synthetic Fe oxyhydroxides and synthetic Mn oxyhydroxides were used to study the oxidation of antimonite, Sb(III), at different pH values. Sb species were measured by differential pulse adsorptive stripping voltametry (DPACSV). The oxidation of Sb(III) to Sb(V) was always rapid and completed after a few days following pseudo-first order rate laws. A slightly slower oxidation rate was observed at lower pH and was attributed to the lower stability of the oxyhydroxide compounds under mildly acidic conditions. The oxidation pseudo-first order rate of Sb(III) by natural Fe oxyhydroxides was also slower when compared to synthetic Fe compounds. This could be explained by a slight crystallinity of the natural oxides (when compared to amorphous synthetic compounds) and by their more complex chemical composition which can include adsorbed ions and organic matter. The study confirms that amorphous Fe and Mn oxyhydroxides present in natural waters and sediments can play a detoxifying role by adsorbing and oxidizing the more toxic Sb(III) into Sb(V).