Synthesis of rhenium-doped tin dioxide for technetium radioactive waste immobilization

As an analog of technetium (Tc-99), rhenium (Re) was incorporated into the tin oxide (SnO2) lattice structure to produce a stable phase. This new bi-metallic oxide consisting of Re4+ and Sn4+ was formed from perrhenate (ReO4-) reduction by tin (Sn2+) and could reduce re-oxidation. After the precipitate was characterized by various techniques, the precipitate morphology and XRD patterns were found to resemble those of cassiterite (SnO2) with narrow crystallite size (D-average = similar to 1.47 nm). The local structure of the precipitate was characterized as Re-O (similar to 2.02 nm), Re-Sn (similar to 3.15 nm), and Re-Sn (similar to 3.70 nm) in the first, second, and third coordination shells, respectively. As Re4+ was doped into the cassiterite phase, the precipitate was much less soluble than alpha-ReO2 (dissolved Re = similar to 0.10 mg L-1 vs. similar to 45.20 mg L-1). The precipitate resisted re-oxidation, and the dissolved Re species were re-precipitated after 23 h. A major portion of Re dissolved during the solubility and re-oxidation tests was assigned to the species of Re+1 (12%), Re2+ (47.42%), Re4+ (28.48%), and Re7+ (12.18%) on the surface of precipitate, while Re+1 (1.05%), Re2+ (15.35%), Re5+ (27.77%), and Re7+ (55.82%) species were found on the alpha-ReO2 surface. These findings suggest that low-temperature reductive co-precipitation can incorporate Tc-99 (or Re) into the SnO2 structure which limit the Tc-99 (or Re) re-oxidation and solubility. (C) 2018 Elsevier B.V. All rights reserved.