Oxidative power of Mn(IV) and Fe(III) oxides with respect to As(III) in terrestrial and aquatic environments

Abstract

The biological availability and the toxicological effects of As depend on its chemical state^1,2. It is therefore important to know the speciation and transformations of As in terrestrial and aquatic environments. Arsenite, As(III), is much more toxic than arsenate As(V) (ref. 2). The suspended and bottom sediments can potentially detoxify As(III) that enters aquatic systems by converting it to As(V) through abiotic oxidation³. We hypothesized that the oxides of Mn(IV) and Fe(III) may be the primary electron acceptors in the oxidation of As(III) because Mn and Fe readily participate in many oxidation–reduction reactions in natural environments; however, their role in the oxidation of As(III) to As(V) is yet to be established. We report here that Mn(IV) oxide is a very effective oxidant with respect to As(III); on the other hand, despite the thermodynamic favourability³, the evidence obtained from colorimetry and X-ray photoelectron spectroscopy shows that a redox reaction between Fe(III) oxide and As(III) does not occur within 72 h, indicating that the kinetics of the redox reaction between As(III) and Fe(III) is relatively slow.