A revised scheme for the reactivity of iron (oxyhydr)oxide minerals towards dissolved sulfide

Abstract

The reaction between dissolved sulfide and synthetic iron (oxyhydr)oxide minerals was studied in artificial seawater and 0.1 M NaCl at pH 7.5 and 25°C. Electron transfer between surface-complexed sulfide and solid-phase Fe(III) results in the oxidation of dissolved sulfide to elemental sulfur, and the subsequent dissolution of the surface-reduced Fe. Sulfide oxidation and Fe(II) dissolution kinetics were evaluated for freshly precipitated hydrous ferric oxide (HFO), lepidocrocite, goethite, magnetite, hematite, and Al-substituted lepidocrocite. Reaction kinetics were expressed in terms of an empirical rate equation of the form: R i = k i ( H 2 S ) t=0 0.5 A where R i is the rate of Fe(II) dissolution ( R Fe) or the rate of sulfide oxidation ( R S), k i is the appropriate rate constant ( k Fe or k S), (H 2S) t=0 is the initial dissolved sulfide concentration, and A is the initial mineral surface area. The rate constants derived from the above equation suggest that the reactivity of Fe (oxyhydr)oxide minerals varies over two orders of magnitude, with increasing reactivity in the order, goethite < hematite < magnetite ≪ lepidocrocite ≈ HFO. Competitive adsorption of major seawater solutes has little effect on reaction kinetics for the most reactive minerals, but results in rates which are reduced by 65–80% for goethite, magnetite, and hematite. This decrease in reaction rates likely arises from the blocking of surface sites for sulfide complexation by the adsorption of seawater solutes during the later, slower stages of adsorption (possibly attributable to diffusion into micropores or aggregates). The derivation of half-lives for the sulfide-promoted reductive dissolution of Fe (oxyhydr)oxides in seawater suggests that mineral reactivity can broadly be considered in terms of two mineral groups. Minerals with a lower degree of crystal order (hydrous ferric oxides and lepidocrocite) are reactive on a time-scale of minutes to hours. The more ordered minerals (goethite, magnetite, and hematite) are reactive on a time-scale of tens of days. Substitution of impurities within the mineral structure (as is likely in nature) has an effect on mineral reactivity. However, these effects are unlikely to have a significant impact on the relative reactivities of the two mineral groups.