Thermodynamic and kinetic studies of sulfur geochemistry

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Virginia Tech


The thermodynamic properties of aqueous sulfur species were estimated using a structure-based, group contribution additivity method that is based upon first-order approximations. Structural groups were chosen so as to minimize subjectivity and maximize the ease of recognition and include: (1) Sn*, polymeric sulfur (as 1/n, where n is the length of the longest continuous sulfur chain in the species), (2) O₃SIV; (3) O₃SVI; (4) O₂SIII and (5) bridging oxygen. In addition, a modified "charge-to-size" ratio (C) is used to model the coulombic interaction between ions and the solvent. Multiple linear regressions of these thermodynamic data were performed in terms of structural groups to yield fundamental equations for the model. Regression coefficients were used to estimate thermodynamic properties of a number of aqueous species for which no experimental data currently exist. Model-derived thermodynamic data were used to find the thermodynamic stability of intermediate sulfur species that occur during the aqueous oxidation of sulfide minerals, which identified at least one thermodynamically feasible pathway for the overall reaction. The data were also used to construct an EhpH diagram for aqueous sulfur species with average sulfur oxidation states less than sulfate (VI). The structure-thermodynamic correlation was used to determine the likely structure of the aqueous S₂O₅²⁻ ion, which has been debated over the past 50 years.

The rate of decomposition of the ferric thiosulfate complex was observed to vary as the square of the concentration of the complex. The decomposition of this complex in acid solutions is strongly dependent on temperature, Ea = 120(±15) kJ mol⁻¹. The rate of reaction increases was observed to increase with increasing ionic strength, consistent with the interaction of two positively charged ions to form the activated complex. This study resolves many of the inconsistencies found in earlier studies and shows that reaction with H⁺ is a more important sink for S₂O₃²⁻ than reaction with Fe³⁺ when pH > -1.7.

Comprehensive rate laws for aqueous pyrite oxidation were produced using experimentally determined data and data reported in the literature. Rate data available in the literature for the reaction of pyrite with dissolved oxygen (DO) to were compiled to produce a rate law that is applicable over three and one half orders of magnitude in DO concentration over the pH range 2-10. A series of batch, and mixed flow reactor experiments were performed to determine the effect of SO₄²⁻, Cl⁻, ionic strength and dissolved oxygen on the rate of reaction of pyrite with ferric tron. Only dissolved oxygen was found to have any appreciable effect. The results of this study were combined with kinetic data reported in the literature to formulate rate laws that are applicable over a six order of magnitude range in Fe³⁺ and Fe²⁺ concentration for the pH range ~0.5-3.0. Fundamental rates laws were formulated for each system and showed that the reaction order for ferric iron changed, thus suggesting a change in reaction mechanism. An observed rate correlation with the Fe³⁺/Fe²⁺ ratio indicates that the rate is proportional to Eh, and is best modelled by a non-ideal, non-site specific Freundlich multilayer isotherm. Because rate is observed to be positively correlated with the concentration of the aqueous oxidant only, the rate determining step for the aqueous oxidation of pyrite can be identified as the electron transfer from the mineral to the oxidant.