STABLE FE ISOTOPE PROBING OF AQUEOUS FE(II)-HEMATITE SURFACE INTERACTIONS

Stable Fe isotope fractionations were used to interrogate changes in surface structure of hematite during interaction with aqueous Fe(II). Because the distinct stable Fe isotope fractionations that exist among Fe(II) and Fe(III) oxide minerals reflect the contrast in Fe bonding environments and mineral structure, isotopic fractionation can be used to monitor the effect of pH on changes in surface layers of hematite after electron transfer between Fe(II) and hematite, as well as the effect of other dissolved ions; dissolved Si was chosen because this is a common species in warm groundwater systems and is known to inhibit contaminant reduction by reduced Fe species. Si was also important in Precambrian marine systems prior to development of silica-secreting organisms, and hence the potential influence of Si on Fe(II)-Fe(III) oxide isotope exchange may have important implications for interpretation of the Fe isotope record in Archean and Proterozoic rocks. When Fe(II) undergoes electron transfer to hematite, Fe(II) is initially oxidized to Fe(III), and structural Fe(III) on the hematite surface is reduced to Fe(II). During this redox reaction, the newly formed reactive Fe(III) layer becomes enriched in heavy Fe isotopes and light Fe isotopes partition into aqueous and sorbed Fe(II). Our results indicate that in most cases the reactive Fe(III) that undergoes isotopic exchange accounts for less than one octahedral layer on the hematite surface. With higher Fe(II)/hematite molar ratios, and the presence of dissolved Si at alkaline pH, the stable Fe isotope fractionations move away from those expected for aqueous Fe(II) and hematite based on equilibrium aqueous Fe(II)-Fe(III) fractionation and equilibrium aqueous Fe(III)-hematite fractionation, and towards those expected for aqueous Fe(II) and goethite. These results point to formation of new phases on the hematite surface as a result of distortion of near-surface Fe-O bonds and Si polymerization at high pH. Our findings demonstrate how stable Fe isotope fractionations can be used to analyze changes in surface Fe phases during exposure of Fe(III) oxides to aqueous Fe(II) under different environmental conditions.