Characterizing Technetium in Subsurface Sediments for Contaminant Remediation

Abstract

Technetium-99 (Tc) contamination remains a major environmental problem at legacy nuclear reprocessing sites, including the Hanford Site (Washington State, U.S.A.) where similar to 700 Ci of Tc has been released into the subsurface. Developing enhanced attenuation and efficient remediation strategies for released Tc requires a complete understanding of retardation processes and Tc mass flux, including the different mechanisms by which Tc is immobilized in the subsurface and the effect of localized subsurface conditions. Selection of over 30 sediments from Hanford waste disposal sites, based on historical information and sediment characterization, for analysis by autoradiography revealed that Tc concentrations were generally below the detection limit of 5 mg Tc/g sediment. When Tc was measurable in vadose zone sediments, it was predominantly present as TcO4- in water films associated with fine-grained sediments, with a maximum of 12% of the total Tc present in the acid-extractable fraction, defined here as the immobile fraction. However, beneath one waste disposal site, where sediments containing minerals with reducing capacity intercepted miscellaneous fission product recovery waste and waste from the bismuth phosphate process, the amount of Tc present in the immobile fraction was 53% of the total. Characterization of Tc-containing phases present in these field-contaminated sediments for the first time using Tc K-edge X-ray absorption near edge structure spectroscopy revealed that, as well as Tc present as Tc(VII)O-4(-) in pore water associated with fine-grained sediments, Tc was also (i) physically encapsulated within solid phases precipitated from other waste components and in multicomponent phosphate minerals; and (ii) present as mixed Tc(VII)/Tc(IV)/other reduced Tc species in localized reducing zones. These results will be used to develop improved long-term Tc remediation strategies optimized for field application, through stimulation of conditions that enhance Tc attenuation.