99Tc immobilization from off-gas waste streams using nickel-doped iron spinel.

Abstract

Technetium-99 (99Tc) incorporation within stable spinel phases is a novel method for 99Tc removal and immobilization from waste streams. In this study, transformation of Ni-doped Fe(OH)2(s) to spinel minerals, e.g. trevorite (NiFe2O4), is explored as a method for removing 99Tc from Hanford Waste Treatment and Immobilization Plant (WTP) primary off-gas waste stream simulant. The Fe(OH)2(s) transformation process was found to reduce 99Tc(VII) to 99Tc(IV) and incorporate reduced Tc(VI) into the produced spinel simultaneously. Nickel doping was applied in the mineral transformation to inhibit potential reoxidation of 99Tc(IV). Solid phase characterization by XRD and XANES confirmed the formation of nickel substituted ferric-spinel, and suggest incorporation of 99Tc(IV) in the final spinel. Furthermore, in the primary off-gas stream, which contains both redox-sensitive contaminants Cr(VI) and 99Tc(VII), results from solution analysis and solid digestion indicate that nearly 100% Cr and over 80% 99Tc can be simultaneously removed by adding Fe(OH)2(s) to solution with a solid to solution ratio of 5 g/L under near neutral and alkaline conditions. The 99Tc removal approach developed herein provides an alternative treatment method to eliminate the proposed recycle process of the off-gas waste stream, which ultimately can reduce WTP mission cost and operation time.