The Kinetics and Mechanism of Sedimentary Iron Sulfide Formation

Abstract

The reaction between goethite, ∝-FeOOH, and aqueous bisulfide ion, HS, was studied under conditions representative of estuarine sediments. The concentration-time curves of the following species were determined by spectrophotometric methods: total sulfide, dissolved sulfide, precipitated sulfide, thiosulfate ion, sulfite ion, elemental sulfur , and dissolved (<0.1μ) iron. Polysulfides were monitored by ultraviolet absorbance measurements, while the hydrogen ion concentration was determined with a pH electrode. Elemental sulfur, both as free and polysulfide sulfur was found to be the major sulfide oxidation product. Thiosulfate ion comprised about 14±8% (electron balance-wise) of the oxidation products. Concentration-time curves of precipitated sulfide sulfur were analyzed by the initial rate method to determine the rate expression. The rate expression for the reaction between ∝-FeOOH and HS- is d [FeS]/dt = k [HS-]i^97 (H+)i^82 A1.1FeOOHi where d [FeS]/dt is the rate of precipitated iron sulfide formation, (H+)i is the initial hydrogen ion activity, AFeOOHi is the initial geothite surface are in m^2/1, and k is the rate constant with the value 31±10 M^-1 1^-1 m^-2 min ^-1. 0.97, 0.82, and 1.1 are the reaction orders for the species bisulfide ion, hydrogen ion, and goethite surface area respectively. A combination of hydrogen balance and electron transfer balance and stoichiometric reactions were studied in view of the rate expression to yield a mechanism. The multistep mechanism consisted of several parallel and consecutive reactions: (1) the protonation reaction of the goethite surface, (2) the parallel reduction reactions of ferric iron to yield elemental sulfur and thiosulfate as oxidation products, (3) the dissolution of the ferrous hydroxide, and (4) the precipitation reaction of dissolved ferrous species and aqueous bisulfide ion. The rate determining step in the reaction sequence was the dissolution step. Results of this study indicate that the oxidation of sulfide species by ferric iron may be a significant source of elemental sulfur in the sediment. Elemental sulfur is necessary for the formation of pyrite (FeS2), the thermodynamically stable iron sulfide. In addition, the previous studies of the interstitial waters of anoxic sediments showed an excess of "dissolved" iron which was greater than calculated from equilibrium solubility products. It is suggested from particle size studies of the precipitated iron sulfide that these high concentration are a result of the submicron particles of ferrous sulfide (<0.1μ). These particles would obviously pass through the .45μ filters which are traditionally used as the dividing line for dissolved and particulate species.