Quantifying Mineralogical Transformations of Ferrihydrite Sulfidization in Microcapillary Columns by Rietveld Refinements Using In Situ Synchrotron-Based WAXS

Iron (hydr)oxides encompass a large number of minerals of varying kinetic and thermodynamic stabilities, each of which may undergo distinct redox processes. Ferrihydrite is an abundant, high surface area, and highly redox active iron hydroxide. Under reducing conditions, ferrihydrite may undergo reductive dissolution or mineral transformations. Sulfate reduction is another environmentally important and ubiquitous process which often proceeds in parallel or adjacent to areas of active iron reduction. As a result, the sulfidization of iron(III) (hydr)oxides is a common process and one that can have profound consequences to the fate of associated trace elements. This research uses capillary flow experiments to examine the rate and reaction mechanism of iron (hydr)oxides reacted with dissolved sulfide using synchrotron-based wide-angle X-ray scattering (WAXS). WAXS patterns were collected from capillaries containing ferrihydrite and a flowing sulfide solution in near real-time using array detectors at the Stanford Synchrotron Radiation Laboratory. WAXS is uniquely able to characterize the poorly crystalline products of this reaction and also provides in situ study of the pathway and kinetics of reactions. Quantitative Rietveld refinements were used to establish the rate laws based on the consumption (and production) of solid phases. Rates of ferrihydrite reduction are consistent with reported values based on solution measurements and mackinawite is the major (but not the only) product of reduction. We also find evidence for the formation of goethite (alpha-FeOOH) and elemental sulfur (alpha-S8), both of which are formed in small quantities and, in the case of elemental sulfur, presumably as an intermediate of sulfide oxidation. These transformations were accompanied by significant changes in ferrihydrite morphology as measured by Rietveld refinements of mineral structure. This work shows that such methods are useful to describe mineralogical transformations over both time and space for a range of mixed mineral systems.