Investigation of wüstite (FeO) dissolution: Implications for reductive dissolution of ferric oxides

The pH-dependent dissolution flux of FeO (wüstite, a ferrous oxide) was measured in this study; flux = k{H ⁺} ⁿ (mol/m ²/s), where k = 10 ^-4,95 and n = 0.64. This flux was consistent with theoretical predictions based on the rate of water exchange of hexaaquo Fe ²⁺. Interestingly, when compared to published data, the pH-dependent dissolution flux of FeO defined an upper limit for the reductive dissolution fluxes of iron(III) (oxyhydr)oxides, including bacterial dissimilatory iron reduction (DIR). A wide range of dissolution fluxes across several orders of magnitude has been reported for iron(III) (oxyhydr)oxides in the literature and the fluxes were affected by various experimental variables, e.g., pH, ligands, chemical reductants, and bacteria. We concluded that (i) the reductive dissolution fluxes of iron(II) (oxyhydr)oxides, including bacterial DIR, are ultimately bracketed by the detachment rate of reduced Fe(II) from the surface and (ii) the maximum flux can be approached when the mole fraction of reduced Fe(II) at the surface is close to unity.