Rates and Mechanism of Fe(II) Oxidation at Nanomolar Total Iron Concentrations

A fully automated luminol-based chemiluminescence system has been developed for rapid analysis of Fe(II) at natural levels. Using this system, the rates of Fe(II) oxidation in 0.7 M NaCI have been measured for nanomolar concentrations of Fe(II) over the pH range 7.0-8.3. When the production and decomposition of H₂O₂ in the system were considered, measured rates at these levels were in excellent agreement with a model based on previously reported rate constants determined using micromolar levels of Fe(II). These results show that O₂^- and OH* intermediates produced as a result of Fe(II) oxidation remain effective as Fe(II) oxidants in these controlled conditions. The chemical model for Fe(II) oxidation also allows prediction of steady-state H₂O₂ and O₂^- concentrations that result from the oxidation of micromolar levels of Fe(II). The concentration of both species increases exponentially with increasing pH. At pH 8.2, the predicted H₂O₂ and O₂^- concentrations are 220 and 2.3 nM, respectively. The predicted H₂O₂ concentrations are in excellent agreement with laboratory measurements. These results suggest that significant concentrations of H₂O₂ and O₂^- should be present at the oxic-anoxic interface of marine environments where micromolar levels of Fe(II) are in contact with dissolved oxygen.