The Oxidation of Iron(II) with Oxygen in NaCl Brines

Abstract

The oxidation of nanomolar levels of iron(II) with oxygen has been studied in NaCl solutions as a function of temperature (0 to 50 °C), ionic strength (0.7 to 5.6 mol⋅kg⁻¹), pH (6 to 8) and concentration of added NaHCO₃ (0 to 10 mmol⋅kg⁻¹). The results have been fitted to the overall rate equation:

$$\mathrm{d}\mbox{[Fe(II)]}/\mathrm{d}t=-k_{\mathrm{app}}\mbox{[Fe(II)]}[\mbox{O}_{2}]$$

The values of k _app have been examined in terms of the Fe(II) complexes with OH⁻ and CO ²⁻₃ . The overall rate constants are given by:

$$k_{\mathrm{app}}=\alpha_{\mathrm{Fe}2+}k_{\mathrm{Fe}}+\alpha_{\mathrm{Fe(OH)}+}k_{\mathrm{Fe(OH)}+}+\alpha_{\mathrm{Fe(OH)}2}k_{\mathrm{Fe(OH)}2}+\alpha_{\mathrm{Fe(CO3)}2}k_{\mathrm{Fe(CO3)}2}$$

where α _i is the molar fraction and k _i is the rate constant of species i. The individual rate constants for the species of Fe(II) interacting with OH⁻ and CO ²⁻₃ have been fitted by equations of the form:

$$\begin{array}{l}\ln k_{\mathrm{Fe}2+}=21.0+0.4I^{0.5}-5562/T\\[6pt]\ln k_{\mathrm{FeOH}}=17.1+1.5I^{0.5}-2608/T\\[6pt]\ln k_{\mathrm{Fe(OH)}2}=-6.3-0.6I^{0.5}+6211/T\\[6pt]\ln k_{\mathrm{Fe(CO3)}2}=31.4+5.6I^{0.5}-6698/T\end{array}$$

These individual rate constants can be used to estimate the rates of oxidation of Fe(II) over a large range of temperatures (0 to 50 °C) in NaCl brines (I=0 to 6 mol⋅kg⁻¹) with different levels of OH⁻ and CO ²⁻₃ .