THE EFFECT OF PH ON STABLE FE ISOTOPE EXCHANGE AND FRACTIONATION BETWEEN AQUEOUS FE(II) AND GOETHITE

Extensive prior Mössbauer and isotope tracer studies of interactions between coexisting aqueous Fe(II) and goethite have shown that electron transfer may drive isotope exchange reactions. Such processes influence contaminant fate and trace element mobility, and result in stable iron isotope fractionation in both biological and abiological processes. To date, the majority of experimental studies of aqueous Fe(II) and iron oxide interactions have been done at circum-neutral pH, and the effect of pH variations on the rate and extent of isotope exchange between aqueous Fe(II) and iron oxide minerals, as well as the natural mass-dependent fractionation between these species, has not been adequately explored. Here, the three-isotope method (⁵⁷Fe-⁵⁶Fe-⁵⁴Fe) was used to investigate the effect of pH (between 2.5 and 7.5) on the rate and extent of isotope exchange, using an enriched ⁵⁷Fe tracer, as well as the natural, mass-dependant stable isotope fractionation, using ⁵⁶Fe/⁵⁴Fe ratios, between aqueous Fe(II) and goethite. Three Fe(II) solutions (differing in ⁵⁶Fe/⁵⁴Fe ratios) were utilized to approach isotope equilibrium from multiple directions. Aqueous Fe(II) decreased in δ⁵⁷Fe/⁵⁶Fe value , whereas the δ⁵⁷Fe/⁵⁶Fe value of goethite increased over time, indicating isotope exchange between these reactants; the extent of such exchange showed a positive correlation with pH. The δ⁵⁶Fe value of goethite remains mostly constant although the δ⁵⁶Fe value of aqueous Fe(II) increases or decreases depending on the rate and extent of exchange. Reactions conducted at low pH produced small isotope fractionations for ⁵⁶Fe/⁵⁴Fe ratios. The decreased rate and extent of exchange at low pH likely reflects the decrease in sorbed Fe(II) under these conditions. Our work illustrates that iron isotope exchange between aqueous Fe(II) and goethite is substantially inhibited at low pH and that the slow rates at these conditions minimize kinetic isotope fractionations. Although kinetic isotope fractionation may occur at high pH during rapid isotope exchange, continued reaction “erases” these effects, approaching equilibrium isotope fractionations.