Competition between O₂ and H₂O₂ in the oxidation of Fe(II) in natural waters

The oxidation rates of nanomolar levels of Fe(II) in seawater (salinity S = 36.2) by mixtures of O₂ and H₂O₂ has been measured as a function of pH (5.8-8.4) and temperature (3-35°C). A competition exists for the oxidation of Fe(II) in the presence of both O ₂ (μ mol·L^-1 levels) and H₂O ₂ (nmol·L^-1 levels). A kinetic model has been applied to explain the experimental results that considers the interactions of Fe(II) with the major ions in seawater. In the presence of both oxidants, the hydrolyzed Fe(II) species dominate the Fe(II) oxidation process between pH 6 and 8.5. Over pH range 6.2-7.9, the FeOH⁺ species are the most active, whereas above pH 7.9, the Fe(OH)₂⁰ species are the most active at the levels of CO₃^2- concentration present in seawater. The predicted Fe(II) oxidation rate at [Fe(II)]₀ = 30 nmol·L^-1 and pH = 8.17 in the oxygen-saturated seawater with [H₂O₂]₀ = 50 nmol·L^-1 (log₁₀ k = -2.24 s^-1) is in excellent agreement with the experimental value of log₁₀ k = -2.29 s^-1 ([H ₂O₂]₀ = 55 nmol·L^-1, pH = 8).