PNNL

A tin-aluminum-phosphate composite synthesized by researchers at Pacific Northwest National Laboratory has separated troublesome pertechnetate—a difficult-to-immobilize chemical form found in the highly radioactive contaminant technetium-99 (Tc)—in alkaline nuclear wastes. Based on six months of testing, the results hold important implications for using these composites for immobilizing low-temperature waste forms.

As demonstrated by the research team, led by Tatiana Levitskaia and Dev Chatterjee, the composite reduces water-soluble pertechnetate to a less soluble form-Tc(IV)—which prevents it from escaping during waste immobilization. At the same time, it changes its structure to capture and retain the reduced material. These capabilities position the composite among the best-performing reductive sorbents for pertechnetate developed to date, and garnered the team's research paper the front cover of Environmental Science Nano in October 2016.

A Pesky Problem: Pertechnetate, the dominant chemical form of Tc, is highly soluble in aqueous solutions and mobile in the environment, and its immobilization in cementitious or mineral waste forms for long-term storage is difficult. One proposed remediation approach is reductive separation of pertechnetate and thermodynamic stabilization of the reduced Tc(IV) to prevent its reoxidation and leaching from low-temperature waste forms. While several reducing agents including tin(II) have been demonstrated to successfully convert pertechnetate to Tc(IV), its incorporation into the electronically and structurally compatible crystalline framework for improved stability has been proven ineffective in situ due to the chemically aggressive nature of the nuclear tank wastes.

A Promising Solution: The team's experiments show that the tin-aluminum-phosphate, a composite system containing mixed Sn-containing phases, benefits from a two-in-one functionality, where Sn(II) serves as a reductant for pertechnetate-while the presence of an inert polycrystalline platform serves to integrate and stabilize Tc(IV) within the structurally compatible cassiterite phase and stabilizes Tc(IV). Preliminary testing demonstrates that the tin-aluminum-phosphate composite retains the reduced Tc(IV) and prevents its reoxidation for more than six months when exposed to air.

"This new composite opens up very promising perspectives for the remediation of very complex waste streams, defining a clear path for future investigations," according to Xavier Gaona, an expert in long-lived fission product chemistry at the Karlsruhe Institute of Technology.

In addition to the recent cover, this research was featured in the Royal Society of Chemistry Newsletter "Chemistry World."

Acknowledgements 

Sponsors: This research was supported by the Laboratory Directed Research and Development Program at the Pacific Northwest National Laboratory operated by Battelle for the U.S. Department of Energy.

User Facility: Part of this research was performed at EMSL, a national scientific user facility at PNNL managed by the Department of Energy's Office of Biological and Environmental Research.

Reference: Levitskaia TG, S Chatterjee, NK Pence, J Romero, T Varga, MH Engelhard, Y Du, L Kovarik, BW Arey, ME Bowden and ED Walter. 2016 "Inorganic tin aluminophosphate nanocomposite for reductive separation of pertechnetate." Environmental Science: Nano 3: 1003-1013. DOI: 10.1039/C6EN00130K