Reductive Incorporation of Technetium-99 in Fe(II)-Doped Layered Double Hydroxide

Technetium-99 is a long-lived, high-yield fission product whose solubility is controlled by redox conditions. Here, to explore targeted effluent treatment approaches, pertechnetate (Tc(VII)O4-) reductive retention as Tc(IV) during coprecipitation of Fe(II)-doped Ni-Al-CO3 layered double hydroxide (LDH) was investigated. During LDH formation via base addition to an acidic Ni2+, Al3+, Fe2+, and Tc(VII)O4- bearing solution, there was essentially complete TcO4- removal at low (9.6 Bq/mL) and high (2.9 kBq/mL) concentrations. Characterization of experiments using X-ray diffraction, Fourier transform infrared spectroscopy, transmission electron microscopy, and X-ray absorption spectroscopy (XAS) confirmed that, during base titration, a two-stage process occurred, with Al3+ forming amorphous Al (hydr)oxide between pH 4 and 6 in the first stage, followed by hydrolysis of Fe2+/Ni2+ and formation of LDH particles between pH 6 and 9 in stage two. During the first stage, TcO4- was removed from solution, and XAS confirmed it was reduced to Tc(IV) by Fe(II) associated with the amorphous Al (hydr)oxide, forming hydrous Tc(IV)O2-like chains on the Al (hydr)oxide surface. During the second stage, XAS showed that Tc(IV) was incorporated within the newly formed LDH structure. Overall, these data provide novel insights into the application of LDH formation in Tc sequestration and suggest novel effluent treatment pathways to produce an inorganic solid.