Precipitation and Transformation of Iron-Sulfide Nanoparticles in Low-Temperature Aqueous Environment: Effects of Reactant Iron and Sulfide Sources
Iron sulfide nanoparticles assume an important role within a wide range of geological settings as indicators of redox environmental conditions and elemental cycling mechanisms. Initial precipitates of iron sulfide are exclusively nanoscale sized in low temperature aqueous conditions and have been reported to go through diverse morphological and phase transformations, possibly leading to the ultimate deposits of pyrite in geologic records of various times. A systematic understanding of how the early-stage nanoparticles of iron sulfide may develop into more crystalline iron sulfide forms is still lacking however. The major goal for this study was to illuminate the effects of the iron and sulfide sources on the formation and subsequent transformation of iron sulfide precipitates. Specifically, I have used ferrous versus ferric iron as the initial reactant, and synthesized iron-sulfide nanoparticles through abiotic versus biological processes. The abiotic synthesis used inorganic sulfide as the reactant whereas the biological synthesis involved sulfate-reducing bacteria (SRB) that transform sulfate in the solution to sulfide as the bacteria grow. Dissolved iron of different oxidation states (i.e., +2 versus +3) were used mainly to simulate the iron sulfide formation in anoxic versus transitional/suboxic environments. The analyses of x-ray diffraction (XRD), transmission electron microscopy (TEM), and small-angle x-ray scattering (SAXS) data revealed apparent variations in the size, morphology, crystal structure, and composition of the biogenic and abiogenic iron sulfide that used different sources of iron (i.e., ferrous versus ferric). The abiogenic precipitates that used ferrous iron as reactants were composed of mackinawite and greigite nanocrystals with an average size of 50 nm at T = 0; the abiogenic precipitates that used ferric iron as reactants were mainly composed of greigite with an average size of 70 nm in length. In general, the abiogenic samples of ferric systems showed higher crystallinity than those of ferrous systems at all examined time intervals (i.e., T = 0, 2 weeks, 1 month, and up to 3 months). It is particularly noted that pyrite was identified in the abiogenic samples of the ferric systems as early as T = 0. In comparison, the biogenic precipitates were composed of amorphous material at early stages (T=0,1m, 2m) and mostly greigite precipitates in aged samples (T=3m). Pyrite was also detected in the biogenic samples of ferric systems, but at a much later stage (i.e., T = 3m). These results indicated that the abundance of Fe(II) over Fe(III) favors the crystallinity of iron sulfides, and bacterial involvement in these systems favors rapid changes in the kinetics and nucleation of iron sulfides in low temperature aqueous systems. The current work has provided new insight into the origin and biogeochemical roles of metal sulfide nanoparticles in anoxic to transitional aqueous environments at ambient temperatures and pressures as well as their transformation pathways into the more stable pyrite.
Moreno Flores, Ezequiel A, "Precipitation and Transformation of Iron-Sulfide Nanoparticles in Low-Temperature Aqueous Environment: Effects of Reactant Iron and Sulfide Sources" (2019). ETD Collection for University of Texas, El Paso. AAI13887007.