Rate law of Fe(II) oxidation under low O2 conditions

Abstract

Despite intensive studies on Fe(II) oxidation kinetics, the oxidation rate law has not been established under low O2 conditions. The importance of Fe(II) oxidation under low O2 conditions has been recently recognized; for instance, the Fe(II)/Fe(III) compositions of paleosols, ancient soils formed by weathering, can produce a quantitative pattern of the atmospheric oxygen increase during the Paleoproterozoic. The effects of partial pressure of atmospheric oxygen (Po2) on the Fe(II) oxidation rate were investigated to establish the Fe(II) oxidation rate – Po2 relationships under low O2 conditions. All oxidation experiments were carried out in a glove box by introducing Ar gas at ∼10−5–∼10−4atm of Po2, pH 7.57–8.09 and 22°C. Luminol chemiluminescence was adopted to measure low Fe(II) concentrations (down to ∼2nM). Combining previous data under higher Po2 conditions (10−3–0.2atm) with the present data, the rate law for Fe(II) oxidation over a wide range of Po2 (10−5–0.2atm) was found to be written as:d[Fe(II)]dt=-k[Fe(II)][O2]x[OH-]2where the exponent of [O2], x, and the rate constant, k, change from x=0.98 (±0.04) and logk=15.46 (±0.06) at ∼6×10−3–0.2atm of Po2 to x=0.58 (±0.02) and logk=13.41 (±0.03) at 10−5–∼6×10−3atm of Po2. The most plausible mechanism that explains the change in x under low O2 conditions is that, instead of O2, oxygen-derived oxidants, H2O2 and to some extent, O2-, dominate the oxidation reactions at <∼10−3atm of Po2. The rate law found in the present study requires us to reconsider distributions of Fe redox species at low Po2 in natural environments, especially in paleoweathering profiles, and may provide a deeper understanding of the evolution of atmospheric oxygen in the Precambrian.