PNNL

Abstract

XAFS has been used to study the chemical state and structural environment of technetium in Fe(III) oxide co-precipitates. 99Technetium is an abundant fission product which poses a significant environmental hazard due to its long half-life, abundance in nuclear wastes, and environmental mobility as the pertechnetate ion [Tc(VII)O4-] under oxidizing conditions. Tetravalent Tc [Tc(IV)] is the stable valence state under reducing or anoxic conditions where its environmental mobility is significantly lowered by formation of a sparingly soluble, hydrated amorphous oxide precipitate [Tc(IV)O2•nH2O(s)]. We have been studying the kinetics and solid products resulting from abiotic reduction of Tc(VII)O4- by aqueous, adsorbed, and structural Fe(II) to provide insights on Tc migration in microaerophilic groundwaters. The reduction reaction yields Fe/Tc precipitates of variable structures that have not been previously studied. For the homogeneous reaction with aqueous Fe(II) at relatively high Tc:Fe concentrations, the predominant redox product is a solid containing Tc(IV) dimers attached in a bidentate edge-sharing configuration to FeO6 octahedra on the surface or unoccupied interior sites of a ferrihydrite-like precipitate. A similar ferrihydrite-type solid is formed on the surface of Fe oxide minerals such a hematite and goethite following the heterogeneous reaction of Tc(VII)O4- with surface-complexed Fe(II). These co-precipitates greatly slow the oxidation rate of Tc(IV) relative to amorphous Tc(IV)O2•nH2O(s), possibly allowing for the long term sequestration of 99Tc in stable (bio)geochemical mineral forms that may reduce the long term environmental risk of 99Tc subsurface contamination.