A Focused Ion Beam-Scanning Transmission Electron Microscopy with Energy-Dispersive X-ray Spectroscopy Study on Technetium Incorporation within Iron Oxides through Fe(OH)(2)(s) Mineral Transformation

Abstract

In this study, incorporation pathway(s) and distribution of reduced technetium-99 (Tc), as Tc(W), into iron oxide/hydroxide minerals formed via oxidation and mineral transformation of the reductant Fe(OH)(2)(s) are investigated using a combined microscopy and spectroscopy approach. Focused ion beam-scanning transmission electron microscopy with energy-dispersive X-ray spectroscopy (FIB/STEM-EDS) combined with solid characterization techniques (extended X-ray absorption fine structure and X-ray absorption near-edge structure), for the first time, visually demonstrates heterogeneous Tc(IV) incorporation into two Tc(IV)doped iron oxide/hydroxide phases via different mechanisms. In magnetite (Fe3O4), Tc(IV) incorporation occurs by (i) TcO(4)(- )surface reduction followed by encapsulation during continued crystal growth, (ii) Tc(IV) partitioning into alternating layers of blocky, plate-like magnetite, and (iii) TcO2 center dot 2H(2)O(s) attachment on a magnetite surface. Alternatively, hematite (Fe2O3) incorporates Tc mainly via TcO2 center dot 2H(2)O(s) embedment in nanometer-sized polycrystalline fibers. Considering that speciation and distribution play a key role in Tc(IV) susceptibility to reoxidation and release into the environment, understanding the mechanisms driving the formation of more stable species is critical for effective treatment of nuclear waste containing technetium. This work illustrates Tc(W) immobilization pathways during Fe(OH)(2)(s) treatment and highlights the power of the FIB/STEM-EDS approach to investigate Tc-iron oxide mineral incorporation and generate reliable mechanism-informed waste forms for Tc immobilization.