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

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 oxidation 3 . 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 favourability 3 , 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.