Sulfide oxidation and iron dissolution kinetics during the reaction of dissolved sulfide with ferrihydrite

Abstract

The reaction between synthetic ferrihydrite and dissolved sulfide was studied in artificial seawater and 0.42 M NaCl at 25 °C over the pH range 4.0–8.2. Electron transfer between solid phase Fe(III) and surface-complexed sulfide results in the reduction of Fe(III) and the formation of elemental sulfur. Subsequent formation of solid phase FeS occurs following dissolution of Fe(II) and reaction with dissolved sulfide. However, the majority of the Fe(II) produced at pH 7.5 remained associated with the oxide surface on the time-scale of these experiments. Rates of both sulfide oxidation and Fe(II) dissolution (in mol l−1 min−1) were expressed in terms of an empirical rate equation of the form:R=ki(H2S)t=00.5Awhere ki represents the apparent rate constants for the oxidation of sulfide (kS) or the dissolution of Fe2+ (kFe), (H2S)t=0 is the initial sulfide concentration (in mol l−1) and A is the initial ferrihydrite surface area (in m2 l−1). The rate constant, kS, for the oxidation of sulfide in seawater at pH 7.5 is 8.4×10−6±0.9×10−6 mol0.5 l0.5 m−2 min−1, with the rate of sulfide oxidation being approximately 15 times faster than the rate of Fe(II) dissolution (given that the ratio of sulfide oxidized to Fe(II) produced is 2:1; kFe=1.1×10−6±0.2×10−6 mol0.5 l0.5 m−2 min−1). The determination of a fractional order with regard to the initial dissolved sulfide concentration occurs because reaction rates are dependent on the availability of reactive surface sites; the more reactive surface sites become saturated with sulfide at relatively low ferrihydrite to dissolved sulfide ratios. In many natural sulfidic environments, the iron oxide to dissolved sulfide ratio is expected to be lower than during this laboratory study. Thus, surface saturation will exert an important influence on reaction rates in nature.