The effect of the presence of trace metals on the oxidation of Sb(III) by hydrogen peroxide in aqueous solution

Despite its importance for understanding the behaviour of antimony in the environment, the oxidation kinetics of Sb( III) with natural oxidants is still not well understood. We have studied the oxidation of Sb( III) by hydrogen peroxide on a time scale of hours in the presence of some trace metals, Cu( II), Mn( II), Zn( II) and Pb( II), under pH and concentration conditions close to natural ones. The effects that these trace metals have on Sb( III) oxidation by hydrogen peroxide vary. Zn( II) had no catalytic effect at all, but Cu( II), Mn( II) and Pb( II) did, though their effects were not uniform. Cu( II) significantly accelerated the reaction, which remained first-order with respect to Sb( III) at any Cu( II) concentration tested. Pb( II) and Mn( II) also enhanced the reaction rates, but the apparent order of the reaction with respect to Sb( III) changed to two. The trace metal effect observed was concentration dependent for Pb( II). The addition of the hydroxyl radical scavenger 2-propanol suggests that the trace metal catalytic effect observed involves the action of hydroxyl radicals, but that they are not responsible for the oxidation of Sb( III) by H2O2 in the absence of trace metals. The fact that Sb( III) can be oxidized by hydroxyl radicals present in water, even if it is not capable of producing them, has important environmental implications because hydroxyl radicals are known to be abundant in many natural waters such as seawater, humic-rich surface waters or rainwater.