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

Despite intensive studies on Fe(II) oxidation kinetics, the oxidation rate law has not been established under low O-2 conditions. The importance of Fe(II) oxidation under low O-2 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 (Po-2) on the Fe(II) oxidation rate were investigated to establish the Fe(II) oxidation rate - Po2 relationships under low O-2 conditions. All oxidation experiments were carried out in a glove box by introducing Ar gas at similar to 10(-5)-similar to 10(-4) atm of Po-2, pH 7.57-8.09 and 22 degrees C. Luminol chemiluminescence was adopted to measure low Fe(II) concentrations (down to similar to 2 nM). Combining previous data under higher Po-2 conditions (10(-3) -0.2 atm) with the present data, the rate law for Fe(II) oxidation over a wide range of Po-2 (10(-5) -0.2 atm) was found to be written as: d[Fe(II)]/dt = -k{Fe(II)][O-2](x)[OH-](2) where the exponent of [O-2], x, and the rate constant, k, change from x = 0.98 (perpendicular to 0.04) and log k = 15.46 (perpendicular to 0.06) at similar to 6 x 10(-3) -0.2 atm of Po-2 to x - 0.58 (+/-0.02) and log k = 13.41 (perpendicular to 0.03) at 10(-5) -similar to 6 x 10(-3) atm of Po-2. The most plausible mechanism that explains the change in x under low O-2 conditions is that, instead of O-2, oxygen-derived oxidants, H2O2 and to some extent, O-2, dominate the oxidation reactions at