Nonlinear dependence of the oxidation rate of Fe(II) on dissolved oxygen under low-O2 conditions in aqueous solutions

Abstract

The effect of dissolved O2 (DO) concentration or partial pressure of atmospheric oxygen (PO2) on the Fe(II) oxidation rate was investigated at room temperature, by changing pH from 6.89 to 8.03 and PO2 from 7.5 × 10-5 to 0.20 atm. The oxidation experiments were conducted in a glove box into which a gas mixture of Ar and Ar + O2 (1%) was introduced continuously to maintain a given DO concentration. The DO concentration was changed by varying the mixing ratio of Ar and Ar + O2 (1%). The decay constant of the oxidation (λ) was estimated from the experimental data, by assuming that the reaction shows pseudo-first-order behavior: -d[Fe(II)]/dt = λ[Fe(II)] = k [Fe(II)][O2]x[OH-]y,which indicates λ (k[O2]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[O2]x) and PO2. The relationships revealed that the oxidation rate deviated from the linear dependence (x = 1) at low PO2 (<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 O2 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.