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

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.