Reports: G2
48302-G2 Iron Oxide Morphology and Composition as Possible Indicators of Sedimentary Redox Cycling
Iron oxides commonly occur in sedimentary deposits, having both detrital and authigenic origins. These phases have been thought to undergo either complete reductive dissolution or phase transformations during sedimentary redox cycling, which often generates aqueous Fe(II) as a byproduct. This Fe(II) may react with any remaining iron oxides to catalyzed phase transformations or activate growth and dissolution processes. The fate of trace elements during such processes is unclear but they may potentially be incorporated into iron oxides. As these processes may alter iron oxide morphology and composition, sedimentary redox cycling may thus produce diagnostic signatures in minerals. This project seeks to characterize the possible morphological and compositional changes in iron oxides during redox cycling. Research efforts over the past year have focused on: (1) Investigating growth and dissolution processes occurring on hematite surfaces in the presence of Fe(II); and (2) Characterizing Ni(II), a structurally-compatible trace element common in crude oil and bitumen, reaction with iron oxide surfaces in the presence and absence of Fe(II).
We have conducted a number of investigations probing the interaction of Fe(II) with hematite surfaces. We have completed surface X-ray scattering measurements on three different surfaces of hematite reacted with 10-4 M Fe(II) at both pH 3 and 7. We find that this reaction induces surface transformations on the scale of a few monolayers in size (<1 nm in scale) over the course of 18 hours that are orientation-dependent yet pH independent. In addition, resonant anomalous X-ray reflectivity (RAXR) measurements of Fe(II)-reacted hematite indicate that adsorbed Fe(II) concentration were negligible, and mass balance constraints indicate that this growth is not the result of oxidation and reprecipitation of Fe(II). Rather, aqueous Fe(II) catalyzes dynamic growth and dissolution resulting in zero oxidation and zero net growth. We are currently using atomic force microscopy (AFM) to examine morphologic changes under a range of reaction times in order to determine potential growth and dissolution rates of distinct surfaces. We are also exploring NanoSIMS measurements of hematite platelets reacted with 18O-spiked water and Fe(II) in order to verify the orientation-dependence of growth and dissolution.
We have also begun a series of wet chemistry and spectroscopic studies to investigate possible Ni(II) incorporation into hematite during Fe(II)-catalyzed growth and dissolution of hematite. The wet chemistry work has focused on examining macroscopic indicators of mineral transformation and trace element incorporation. Our initial work has focused on characterizing Fe(II) adsorption on hematite different particle sizes, Ni(II) adsorption on hematite, and the effect of dissolved sulfate on Ni(II) adsorption. The Fe(II) uptake work suggests that nanometer-scale hematite particles react differently than larger hematite particles. We are working to determine the mineralogical and morphological changes upon reaction as this may be a signature of Fe(II)-induced Ostwald ripening. Ni(II) was found to display pH-dependent adsorption on hematite, as expected for a divalent cation. Sulfate appears to induce Ni(II) adsorption at a lower pH value. We are currently investigating effect of Fe(II) on Ni(II) adsorption, the effect of sulfate on Fe(II) adsorption, and the combined effect of Fe(II) and sulfate on Ni(II) adsorption.
We have complimented these studies with extended X-ray absorption fine structure (EXAFS) spectroscopy studies of Ni(II) speciation on iron oxide surfaces in the presence and absence of Fe(II). As expected, in the absence of Fe(II), Ni(II) occurs as a surface complex; the binding mode was found to be inner-sphere. However, in the presence of Fe(II), Ni(II) is found to incorporate into the hematite structure, substituting into an Fe(III) site. More incorporation was found for a nanometer-scale hematite particle than for a coarser particle. This again suggests that Fe(II)-catalyzed hematite growth and dissolution is size-dependent. This work also suggests that sedimentary redox cycling can alter the association of trace elements with iron oxides in a way that may produce a long-lived compositional marker. We are currently conducting additional integrated wet chemical and spectroscopic studies to explore the dependence of Ni(II) incorporation on chemical conditions. Research funded by this grant has been presented at two conferences and a publication is currently in review with Geochimica et Cosmochimica Acta.
