PNNL

Abstract

Due to the complex chemistry of nuclear waste, targeted remediation and long-term immobilization of technetium-99 (Tc) is hindered by co-mingled redox-sensitive constituents, including Cr(VI). Here, Cr(VI) effects on Tc-immobilization by Fe(OH)2(s) are investigated while assessing Fe(OH)2(s) as a potential treatment solution for Hanford low-activity waste destined for vitrification. Batch studies using simulated low-activity waste indicate that Tc(VII) and Cr(VI) removal is contingent on reduction to Tc(IV) and Cr(III) and that simultaneous removal is possible, although Cr is completely removed from solution before Tc. XRD analysis suggests that the solid phase formed during reaction between the simulant and Fe(OH)2(s) is mostly goethite (a-FeOOH), where Cr(III) and Tc(IV) readily substitute for octahedral Fe(III) within the crystal lattice. Fe(OH)2(s) transformation to goethite, rather than magnetite, is likely due to the simulant chemistry, which includes high levels of nitrite. Incorporation of Cr(III) into goethite is supported by XPS results and Tc(IV) incorporation is supported by EXAFS analysis. However, EXAFS and XANES results indicate that only a portion of Tc(IV) is incorporated into goethite, with the remaining Tc(IV) present as adsorbed or partially incorporated Tc(IV)-oxide species. As such, additional Fe(OH)2(s) (>200 g per liter of simulant) may be required to completely incorporate Tc(IV) into goethite.