AmericanChemicalSociety.com

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 characterizing the release and incorporation og Ni(II), a structurally-compatible trace element common in crude oil and bitumen, into iron oxides during Fe(II)-promoted mineral cycling.

This research has comprised two parallel sets of investigations.  In the first, Ni(II) was reacted with hematite and goethite in the absence or presence of aqueous Fe(II) at pH 7.5 for 5 or 82 days.  After reaction Ni speciation was characterized using X-ray absorption fine structure (XAFS) spectroscopy.  These results demonstrate that Ni becomes progressively incorporated into both hematite and goethite in the presence of Fe(II).  Negligible incorporation occurs after aging in the absence of Fe(II).

Fe(II) may also potentially cause elements substituted into iron oxides to be released as the solid undergoes structural recrystallization during reaction. We have demonstrated this effect for goethite and hematite doped with Ni.  For goethite, Fe(II) induces the release of roughly 10% of the structural Ni in a 7-day period; 3% was released from hematite over a 3-day period.  A similar effect is observed for Zn.  The time-scale of trace element release is consistent with the rate of atom exchange observed in past iron isotope studies.

These two seemingly contradictory observations of Fe(II)-induced incorporation and release may reflect a thermodynamic equilibration between the solid, surface, and aqueous solution.  We hypothesize that the equilibrium distribution between this states is normally kinetically inhibited because of the low solubility and dissolution rate of iron oxides.  Recrystallization promoted by Fe(II) allows this equilibration to go forward.  This may lead to Ni incorporation when Ni speciation is dominantly in an adsorbed form, but Ni release when it mostly occurs substituted in the structure.  The latter phenomenon suggests that organisms such as methanogens that require trace elements as micronutrients benefit from Fe(II)-rich environments or association with iron-reducing bacteria.  Biogeochemical iron cycling may thus be important to microbial methanogensis as it appears to facilitate the release of essential micronutrients.