Nonlinear dependence of the oxidation rate of Fe(II) on dissolved oxygen under low-O-2 conditions in aqueous solutions

作者：Sugimori Hirokazu; Kanzaki Yoshiki; Yokota Kohei; Murakami Takashi^*

来源：Journal of Mineralogical and Petrological Sciences, 2011, 106(3): 142-152.

DOI：10.2465/jmps.101105b

• Fe(II) oxidation rate
• Low-oxygen
• Weathering
• Atmospheric oxygen evolution

The effect of dissolved O-2 (DO) concentration or partial pressure of atmospheric oxygen (P-O2) on the Fe(II) oxidation rate was investigated at room temperature, by changing pH from 6.89 to 8.03 and P-O2 from 7.5 x 10(-5) to 0.20 atm. The oxidation experiments were conducted in a glove box into which a gas mixture of Ar and Ar + O-2 (1%) was introduced continuously to maintain a given DO concentration. The DO concentration was changed by varying the mixing ratio of Ar and Ar + O-2 (1%). The decay constant of the oxidation (lambda) was estimated from the experimental data, by assuming that the reaction shows pseudo-first-order behavior:
-d[Fe(II)]/dt = lambda[Fe(II)] = k [Fe(II)][O-2](x)[OH-](y),
which indicates lambda (k[O-2](x)[OH-](y)) depends on the solution pH and DO concentration. After determining the pH dependence of the oxidation rate (k '' = k[OH-](y)), we obtained the relationships between the modified rate constant (k' = k[O-2](x)) and P-O2. The relationships revealed that the oxidation rate deviated from the linear dependence (x = 1) at low P-O2 (<10(-2) atm) and that the oxidation proceeded faster than in the case where x was assumed to be 1. Our results implied that the atmospheric O-2 levels (between 2.5 and 2.0 Ga) estimated from the Fe(II) oxidation in paleosols, ancient soils formed by weathering, are overestimated if the x dependence of the oxidation rate is not taken into account.

• 出版日期2011-6