Characterization of natural titanomagnetites (Fe3−xTixO4) for studying heterogeneous electron transfer to Tc(VII) in the Hanford subsurface

Abstract

Sediments with basaltic provenance, such as those at the Hanford nuclear reservation, Washington, USA, are rich in Fe-bearing minerals of mixed valence. These minerals are redox reactive with aqueous O2 or Fe(II), and have the potential to react with important environmental contaminants including Tc. Here we isolate, identify and characterize natural Fe(II)/Fe(III)-bearing microparticles from Hanford sediments, develop synthetic analogues and investigate their batch redox reactivity with aqueous Tc(VII). Natural Fe-rich mineral samples were isolated by magnetic separation from sediments collected at several locations on Hanford’s central plateau. This magnetic mineral fraction was found to represent up to 1wt% of the total sediment, and be composed of 90% magnetite with minor ilmenite and hematite, as determined by X-ray diffraction. The magnetite contained variable amounts of transition metals consistent with alio- and isovalent metal substitutions for Fe. X-ray microprobe analysis showed that Ti was the most significant substituent, and that these grains could be described with the titanomagnetite formula Fe3−xTixO4, which falls between endmember magnetite (x=0) and ulvöspinel (x=1). The dominant composition was determined to be x=0.15 by chemical analysis and electron probe microanalysis in the bulk, and by L-edge X-ray absorption spectroscopy and X-ray photoelectron spectroscopy at the surface.Site-level characterization of the titanomagnetites by X-ray magnetic circular dichroism showed that despite native oxidation, octahedral Fe(II) was detectable within 5nm of the mineral surface. By testing the effect of contact with oxic Hanford and Ringold groundwaters to reduced Ringold groundwater, it was found that the concentration of this near-surface structural Fe(II) was strongly dependent on aqueous redox condition. This highlights the potential for restoring reducing equivalents and thus reduction capacity to oxidized Fe-mineral surfaces through redox cycling in the natural environment. Reaction of these magnetically-separated natural phases from Hanford sediments with a solution containing 10μM Tc(VII) showed that they were able to reductively immobilize Tc(VII) with concurrent oxidation of Fe(II) to Fe(III) at the mineral surface, as were synthetic x=0.15 microparticle and nanoparticle analogue phases. When differences in the particle surface area to solution volume ratio were taken into consideration, measured Tc(VII) reduction rates for Fe3−xTixO4 (x=0.15) natural material, synthetic bulk powder and nanoparticles scaled systematically, suggesting possible utility for comprehensive batch and flow reactivity studies.