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 conditions and elemental cycling mechanisms. Initial precipitates of iron sulfide are exclusively nano-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. A systematic understanding of how early-stage precipitates of iron sulfide may develop into more crystalline phases is still lacking however. The major goal of this study is to illuminate the effects of iron and sulfide sources on the formed iron sulfide and their subsequent transformation. We used comparative experimental approach to study whether the sulfide (abiogenic vs. biogenic via bacterial sulfate reduction) and iron (ferrous vs. ferric) sources affect the phase, morphology, and stability of the formed iron sulfide. The different iron sources also reflect formation processes in anoxic vs. suboxic scenarios. Based on XRD, TEM-EDX, and SAXS data, apparent variations in sizes, morphology, crystal structure, and composition were revealed for the biogenic and abiogenic iron sulfide. The abiogenic precipitates that used Fe(II) as reactants consist mainly of mackinawite and greigite nanocrystals (sized 50 nm on average) at T₀; the abiogenic precipitates that used Fe(III) as reactants consist mainly of greigite (sized ~ 70 nm on average) at T₀. In general, the abiogenic samples of Fe(III) systems showed higher crystallinity than those of Fe(II) systems at all examined time intervals. It is noted that pyrite was identified in the abiogenic samples of Fe(III) systems as early as T₀. In comparison, the biogenic precipitates were composed mainly of amorphous phase at early stages (T₀, T_1m, T_2m) and of greigite precipitates in aged samples (T_3m). Pyrite was also detected in the biogenic samples of Fe(III) systems, but at a much later stage (T_3m). These results indicated that Fe(III) species may behave as a suitable oxidant favoring the formation of pyrite and the presence of bacterial cells delays the Fe-monosulfide to polysulfide transformation, likely by providing more reducing resources and mediating the microenvironmental pH. The results of the work provide new insight into the transformation pathways of early iron sulfide precipitates into pyrite.