Nonlinear dependence of the oxidation rate of Fe(II) on dissolved oxygen under low-O_2 conditions in aqueous solutions

摘要

The effect of dissolved O_2 (DO) concentration or partial pressure of atmospheric oxygen (PO_2) on the Fe(Il) oxidation rate was investigated at room temperature, by changing pH from 6.89 to 8.03 and PO_2 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 + 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 (λ) was estimated from the experimental data, by assuming that the reaction shows pseudo-first-order behavior: atwhich indicates λ (k[O_2]ItOH]) 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 POl. The relationships revealed that the oxidation rate deviated from the linear dependence (x - 1) at low Po_2 (<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.