| The phyllomanganate mineral birnessite, common in soils and sediments, is redoxactive, and acts as an important scavenger for trace elements in the environment. As changes in oxidation state can greatly influence both element mobility and toxicity, the kinetic mechanisms involved in oxidation-reduction reactions are of environmental concern.For CrIII oxidation, a model system consisting of three Mn centers and associated oxygen ligands is utilized to develop a kinetic mechanism involving transfer of three electrons from CrIII to multiple Mn centres within the birnessite structure, using the spectrochemical constraints of Banerjee and Nesbitt (1999). The model was tested using Monte-Carlo based simulations (Moebs, 1974) and an acceptable fit between predicted solution concentrations of CrII, CrVI, and Mn II and observed solution concentrations of Silvester et al. (1997) resulted, indicating that the proposed kinetic mechanism is feasible.For FeII oxidation, surface spectroscopic studies involving X-ray photoelectron spectroscopy on FeII exposed and control synthesized birnessite films indicate that although surface proportions of the oxidized species, FeIII, consistent with the monolayer formation of a FeIII surface species is detected, reduction and dissolution of the birnessite surface is minimal. A reaction mechanism consistent with the competitive adsorption of ferrous and ferric iron as an inhibitatory factor on the continued oxidation of FeII on the birnessite surface is proposed. The resulting model provides an acceptable fit between the simulated results and the solution data of Postma (1985), therefore the kinetic mechanism proposed is feasible.The study effectively illustrates the utility, time effectiveness, and flexibility of the Monte Carlo based model to the elucidation of complex kinetic mechanisms involving multiple reaction pathways.Previous solution and surface spectrochemical studies on the kinetic rate and oxidation-reduction reaction mechanisms of trace elements on the surface of birnessite have resulted in few attempts to combine solution and spectrochemical constraints on reaction mechanisms to develop kinetic models. Therefore, an X-ray photoelectron study of the oxidation kinetics of Fe II on the surface of birnessite, and Monte Carlo based modelling of FeII and CrIII oxidation on the birnessite surface incorporating surface spectroscopic and published solution studies was conducted.Keywords: Oxidation, reduction, birnessite, kinetics, Monte Carlo, reaction mechanism, ferrous iron, chromium (III), X-ray photoelectron spectroscopy, XPS. |
