Catalytic effects of the γ-FeOOH (lepidocrocite) surface on the oxygenation removal kinetics of Fe(II) and Mn(II)

Previous investigations of Fe(II) oxygenation had resulted in a wide range in the reported rate constant(s). While Fe(II) oxygenation rates are fast in simple laboratory systems (seconds to minutes when pH > 7), actual rates observed in natural waters can be orders of magnitude lower. Conversely, while Mn(II) oxygenation rates are slow in laboratory systems (days when pH < 9), much faster rates are observed in natural waters or implicated in model studies. The influences of ionic strength, temperature and anions on the Fe(II) homogeneous oxygenation rates were examined in this study. Other rate constants from the literature were successfully incorporated into this framework. Complexation by major anions (e.g., SO_4^(2-) and Cl^-) and ionic strength effects were sufficient to account for the retardation of Fe(II) oxygenation in seawater. Autocatalysis of Fe(II) oxygenation was observed for pH > 7. A general integrated autocatalytic rate expression suitable for Fe(II) or Mn(II) oxygenation was used to interpret laboratory-obtained kinetic data. Oxidation of Fe(II) in various laboratory systems with characteristics like those of natural water was shown to form the allotrope γ-FeOOH. The γ-FeOOH surface was shown to be an excellent catalyst for Fe(II) and Mn(II) oxygenation. The γ-FeOOH surface obtained by oxidizing milli-molar levels of Fe(II) in 0.7 M NaCl was studied in the following ways: surface charge characteristics by acid/base titration; adsorption of Mn(II) and surface oxidation of Mn(II). A rate law was formulated to account for the effects of pH and the amount of surface on the surface oxidation rate of Mn(II). The presence of milli-molar levels of γ-FeOOH was shown to reduce significantly the half-life of Mn(II) in 0.7 M NaCl from hundreds of hours to hours. The γ-FeOOH surface was shown to be as effective as colloidal MnO_2 in catalysing Mn(II) oxygenation.