Syntheses, structures and radionuclide extraction properties of porous ZrC- and Zr2SC-carbon composite sphere materials

The syntheses of porous composite sphere materials are described, which are possible reusable inert matrices for the transmutation of long-lived radionuclides, or production of nuclear medicines. The spheres were produced by the high temperature (1350 °C) carbothermal reduction of Zr-doped polymer templates. Introduction of Zr was accomplished using several different synthetic strategies; including, infiltration of polyacrylonitrile (PAN) spheres; co-precipitation with PAN; and cation- and anion-exchange of functionalised polystyrene-divinylbenzene (PS-DVB) resins. The materials were extensively characterised. Typically, the carbothermal reduction products contained varying proportions of ZrC and ZrO2 polymorphs. In the case of S-rich cation-exchanged resins, the MAX phase carbide Zr2SC was formed instead. While the materials’ carbon frameworks inherited the basic pore structure of the polymer sphere templates, evolution of a secondary porosity as result of reactive carbon removal was often observed; in the case of PS-DVB–based materials, sometimes yielding surface areas in excess of 600 m2 g−1. The various porous carbide-containing materials showed affinity for oxospecies-forming elements, including Mo, U, As, Se, P, Re and W. An electrostatic adsorption mechanism was proposed. Due to their hierarchically porous and granular natures, the new materials are potentially well-suited to column chromatography and facile, dust-free processing of radionuclides. Adsorption and desorption processes offer routes to the introduction of target species and the removal of products from the porous irradiation matrices; and may facilitate their recycle, producing less solid waste. However, the thermal stabilities of the carbide phases in air remain a concern, as the processes of oxidation and/or dissolution of O into the carbide crystal lattices, proceed from as low as 250–300 °C, which is in some cases, much lower than the bulk phase variants. Although the materials’ prospects are good, actual irradiation studies are advised to confirm radiation tolerance and the influence of fission- and radioactive decay-based heat loads on the spherical targets.