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Oduro, Harry Daniel

Geology

2007

MS

Iron sulfide molecular clusters, FeS(aq), are a group of iron-sulfide complexes formed under reducing conditions at different modem anoxic or suboxic interfaces in various environmental systems, and are necessary precursors for the formation of iron sulfide minerals. Dissolution of FeS(aq) from bulk mackinawite (FeS) has also been hypothesized as a critical component to pyritization reactions. This manuscript couples these observations with thinking regarding the non-oxidative dissolution of iron sulfide minerals producing FeS(aq) cluster complexes and other dissolved species in anoxic gradient column. Batch and agarose gradient experiments at circumneutral pH have shown nonstiochiometric, non-oxidative, dissolution of mackinawite, pyrrhotite, pyrite, and marcasite minerals with formation of FeS(aq) cluster complexes. The observed nonstiochiometric dissolution of FeS minerals suggest crystallographic control of dissolution rates with more Fe being released as dissolution progresses. Gradient experiments with time show that Fe²⁺ and FeS(aq) species come to equilibrium with mackinawite. Binding of Fe²⁺ with humic acids limits the diffusion rates of iron and inhibits the formation of FeS(aq) dissolving from mackinawite. Our studies also demonstrate that, gradient experiments of non-oxidative dissolution products when subjected to oxic conditions show no significant formation of intermediate sulfur species in contrast to studies of direct iron sulfide mineral surface oxidation. This suggests significant differences in the reaction pathways for the overall oxidation of iron and sulfur in Fe-S minerals depending on the specific redox environment they occupy. The dynamic conditions between the formation of the clusters as well as other species in suboxic environments may thus be a critical component to understanding the cycling of iron and sulfur in a host environments.
A study of the geochemistry and stable isotope composition of H₂S-rich flooded underground mine water in West Camp Extraction Well (WCEW) at Butte, Montana shows that the dissolved inorganic carbon in the mine water is in chemical and isotopic equilibrium with rhodochrosite. Geochemical modeling and total metal analysis confirm that the mine pool is under-saturated with mackinawite and amorphous FeS, but is supersaturated with Cu- and Zn- sulfides. However, in-situ voltammetry studies show that much of the dissolved sulfide and ferrous iron are present as FeS(aq) cluster molecules: as a result, the free H₂S + HS⁻ concentration of the West Camp water is poorly constrained. The total concentration analysis of dissolved sulfide determined by colorimetry were lower than gravimetric assays obtained by AgNO₃ addition, implying that the FeS(aq) clusters are not completely extracted by the Methylene Blue reagent. In contrast, the clusters are quantitatively extracted as Ag₂S after addition of AgNO₃. Isotopic analysis of the groundwater sulfidic water at circumneutral pH of co-existing aqueous sulfide and sulfate confirms that the sulfide was produced by sulfate-reducing bacteria (SRB).