Department of Energy (DOE) remediation plans for disposal and long term storage of technetium-99 (Tc) at the Hanford Site (Washington State, USA) anticipate a large fraction of Tc volatilizing during waste vitrification and being captured in a secondary waste stream. This secondary waste stream faces two major remediation challenges: (i) reduction of Tc(VII) to less mobile Tc(IV) in the presence of co-mingled redox-sensitive chromium, Cr(VI), which is more readily reduced than Tc(VII); and (ii) immobilization of Tc in a stable phase that inhibits reoxidation and release. In an effort to find a solution to these challenges, this work demonstrates successful use of Fe(OH)2(s) to simultaneously reduce Tc(VII) in simulated waste streams, in the presence of Cr(VI), and immobilize Tc(IV) in magnetite (Fe3O4) transformed from Fe(OH)2(s). However, incorporation of this remediation approach into nuclear waste processing efforts requires a mechanistic understanding of the reduction/immobilization reaction for Tc(VII) in the presence of Cr(VI). In this work three solution matrices are studied: a 1 M NaOH solution containing 30 mM Cr(VI) and/or 10-5 M Tc(VII), an average 5 M Na LAW (low activity waste) simulant with 21 mM Cr(VI) and 10-5 M Tc(VII), and an off-gas simulant with 2 mM Cr(VI) and 10-4 M Tc(VII). Using ICP-OES and ICP-MS analyses, Tc and Cr removal from solution is determined for set intervals during the reaction to assess competing kinetics and the amount of Fe(OH)2(s) required to completely remove Tc and Cr from solution. To investigate the mechanisms of Tc(VII)/Cr(VI) reduction and immobilization by Fe(OH)2, XANES, EXAFS, XPS, and XRD are used to characterize the solid phase. Bulk oxidation state analysis of Tc-containing magnetite by XANES confirms that the majority of incorporated Tc is present as Tc(IV); however, analysis by XPS suggests that the near surface may contain a mixture of Tc(VII) and Tc(IV), indicating localized surface reoxidation. In addition, EXAFS results confirm Tc(IV) incorporation into octahedral sites of the magnetite structure. XPS analysis of Cr confirms reduction to Cr(III), predominately present as chromite (FeCr2O4), Cr(OH)3, and Cr2O3, and is corroborated by XRD results that show the formation of magnetite-chromite (Fe3O4 - FeCr2O4) solid-solutions. This multi-technique approach confirms that Cr(VI) and Tc(VII) are removed simultaneously through mineral transformation and co-precipitation processes facilitated by Fe(OH)2(s).

Mechanisms for simultaneous Tc and Cr removal by Fe(OH)2 in Hanford waste streams

254th National Meeting (2017)

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