Physics
Scientific paper
May 1981
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1981gecoa..45..687p&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 45, Issue 5, pp.687-698
Physics
31
Scientific paper
The reaction between hydrous iron oxides and aqueous sulfide species was studied at estuarine conditions of pH, total sulfide, and ionic strength to determine the kinetics and formation mechanism of the initial iron sulfide. Total, dissolved and acid extractable sulfide, thiosulfate, sulfate, and elemental sulfur were determined by spectrophotometric methods. Polysulfides, S 4 2- and S 5 2- , were determined from ultraviolet absorbance measurements and equilibrium calculations, while product hydroxyl ion was determined from pH measurements and solution buffer capacity. Elemental sulfur, as free and polysulfide sulfur, was 86% of the sulfide oxidation products; the remainder was thiosulfate. Rate expressions for the reduction and precipitation reactions were determined from analysis of electron balance and acid extractable iron monosulfide vs time, respectively, by the initial rate method. The rate of iron reduction in moles/liter/minute was given by d (reduction Fe )/ dt = kS t 0.5 ( J + ) 0.5 A FeOOH 1 where S t was the total dissolved sulfide concentration, (H + ) the hydrogen ion activity, both in moles/ liter; and A FeOOH the goethite specific surface area in square meters/liter. The rate constant, k , was 0.017 ± 0.002m -2 min -1 . The rate of reduction was apparently determined by the rate of dissolution of the surface layer of ferrous hydroxide. The rate expression for the precipitation reaction was d ( FeS )/ dt = kS t 1 ( H + ) 1 A FeOOH 1 where d ( FeS )/ dt was the rate of precipitation of acid extractable iron monosulfide in moles/liter/minute, and k = 82 ± 18 mol -1 l 2 m -2 min -1 . A model is proposed with the following steps: protonation of goethite surface layer; exchange of bisulfide for hydroxide in the mobile layer; reduction of surface ferric ions of goethite by dissolved bisulfide species which produces ferrous hydroxide surface layer elemental sulfur and thiosulfate; dissolution of surface layer of ferrous hydroxide; and precipitation of dissolved ferrous specie and aqueous bisulfide ion.
Pyzik Albert J.
Sommer Sheldon E.
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