| The objective of this dissertation is to investigate two aspects of the redox transformations of aqueous sulfur species: the role of semiconducting minerals as catalysts; and the kinetics of the formation and decomposition of sulfur oxyanions of intermediate oxidation state.; The oxidation of thiosulfate by molecular oxygen, a thermodynamically favored but kinetically slow reaction, was investigated to study catalysis by semiconducting minerals. In the presence of pyrite, the reaction proceeds rapidly and forms tetrathionate, following Langmuir-Hinshelwood kinetics. The thiosulfate oxidation was also investigated in the presence of synthetic sphalerites doped with transition metal ions (Mn{dollar}rmsp{lcub}2+{rcub}, Fesp{lcub}2+{rcub}, Cosp{lcub}2+{rcub}, Nisp{lcub}2+{rcub}, and Cusp{lcub}2+{rcub}).{dollar} Those sphalerites with an acceptor level close to the redox potential of the {dollar}rm Ssb2Osb3sp{lcub}2-{rcub}/Ssb4Osb6sp{lcub}2-{rcub}{dollar} couple (Ni{dollar}sp{lcub}2+{rcub}{dollar} and Cu{dollar}sp{lcub}2+{rcub}{dollar} doped sphalerite) catalyze the reaction, whereas those which have a conduction band or acceptor level much higher (Mn{dollar}sp{lcub}2+{rcub},{dollar} Fe{dollar}sp{lcub}2+{rcub},{dollar} an Co{dollar}sp{lcub}2+{rcub}{dollar} doped sphalerite, and pure sphalerite) have little or no catalytic activity. The reaction occurs in the dark as well as under illumination.; The kinetics of the formation and decomposition of thiosulfate under hydrothermal conditions were investigated. Among four pathways of thiosulfate formation examined, the equilibration between sulfate and sulfidic sulfur and the oxidation of sulfidic sulfur are inefficient in generating thiosulfate, while the hydrolysis of elemental sulfur and aqueous reaction between sulfite and hydrogen sulfide generate thiosulfate in large quantities. The rate of thiosulfate formation from hydrolysis is proportional to the sulfur/solution ratio of the system, and is of fractional order (0.5) dependence on a{dollar}rmsb{lcub}H+{rcub}{dollar} of the solution. In the range of pH 4.0 to 8.7 and temperature 75{dollar}spcirc{dollar}C to 260{dollar}spcirc{dollar}C, the decomposition of thiosulfate is dominated by its disproportionation to sulfite and elemental sulfur. The reaction follows a rate law of second-order with respect to thiosulfate and first-order with respect to acidity. 316-stainless steel catalyzes the reaction. In a 316-stainless steel reactor, the disproportionation rate is independent of the thiosulfate concentration, indicating a surface-mediated reaction.; The occurrence of thiosulfate and polythionate in natural hydrothermal waters was studied at Yellowstone National Park. The on-site analysis of thiosulfate for 39 hot springs shows that there is no thiosulfate in both the pH neutral-alkaline chloride waters and the acid sulfate waters at significant amount. Thiosulfate, however, occurs in some chloride waters enriched in sulfate at concentrations higher than or comparable to the concentration of dissolved sulfide (S(-II)).; Polythionate was detected as well as thiosulfate in Cider Pool, an acid-sulfate-chloride spring with its surface partially covered by floating hollow sulfur spherules containing dispersed pyrite, and with its bottom overlying on a pool of molten sulfur. A model of transformation of sulfur species in Cinder Pool is proposed, which attributes the formation of thiosulfate in Cider Pool to sulfur hydrolysis at the bottom of the pool, the formation of polythionate to thiosulfate oxidation catalyzed by the pyrite disseminating in the floating sulfur spherules, and decomposition of polythionates to the interaction with hydrogen sulfide. (Abstract shortened by UMI.)... |
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