REDUCTIVE MINERALOGICAL TRANSFORMATIONS IN THE FE-S-H2O SYSTEM

Sedimentary iron (oxy)hydroxides are disproportionally important in most near surface environments due to their reactive surfaces and large surface areas. Adsorption to these minerals often effectively regulates the aqueous concentrations of many ions and thus their bioavailability and transport through iron redox chemistry. Iron (oxy)hydroxides encompass a large number of minerals of varying kinetic and thermodynamic stabilities, each of which may undergo distinct redox processes. Under reducing conditions, these iron minerals may undergo reductive dissolution or mineral transformations. Although iron reduction is more thermodynamically favorable than sulfate reduction, sulfate reduction often precedes iron reduction for kinetic reasons and, as a result, sulfide and iron(III) (oxy)hydroxides often occur together and can react chemically in these environments.

This research examines the rate and reaction mechanism of iron (oxy)hydroxides reacted with dissolved sulfide. We performed time resolved studies of mineral phases using synchrotron-based wide angle X-ray scattering (WAXS) which is uniquely able to characterize the poorly crystalline products of reaction and also provides in situ study of the pathway and kinetics of reaction. Samples were prepared anaerobically as wet-mounted slurries of iron (oxy)hydroxide suspensions in the presence of sulfide under well controlled solution compositions.

Results confirm the formation of poorly crystalline mackinawite (FeS1-x, iron monosulfide) through a goethite intermediate. The duration of goethite presence was dampened by high sulfide concentrations. Rate of conversion was directly related sulfide concentration; however the presence of trace metal impurities impacted reaction rates and also influenced the crystallinity and structure of the resulting minerals. These data indicate that aqueous sulfide concentration regulates the overall rate of reaction, and the resulting mineralogy of the solids produced during sulfidization.