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 O-2 and H2O2 has been measured as a function of pH (5.8-8.4) and temperature (3-35 degrees C). A competition exists for the oxidation of Fe(II) in the presence of both O-2 (mu mol center dot L-1 levels) and H2O2 (nmol center dot 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)(2)(0) species are the most active at the levels of CO32- concentration present in seawater. The predicted Fe(II) oxidation rate at [Fe(II)](0) = 30 nmol center dot L-1 and pH = 8.17 in the oxygen-saturated seawater with [H2O2](0) = 50 nmol center dot L-1 (log(10)k = - 2.24 s(-1)) is in excellent agreement with the experimental value of log(10) k = -2.29 s(-1) ([H2O2](0) = 55 nmol center dot L-1, pH = 8).