Photoreduction of 99Tc Pertechnetate by Nanometer-Sized Metal Oxides: New Strategies for Formation and Sequestration of Low-Valent Technetium

Technetium-99 (99Tc) (?-max: 293.7 keV; t?: 2.1 x 105 years) is a byproduct of uranium-235 fission and comprises a large component of radioactive waste. Under aerobic conditions and in a neutral- basic environment, the pertechnetate anion (TcO4-) is stable. TcO4- is very soluble, migrates easily through the environment and does not sorb well onto mineral surfaces, soils or sediments. This study moves forward a new strategy for the reduction of TcO4- and chemical incorporation of the reduced Tc into a metal oxide material. This strategy employs a single material, a polyoxometalate (POM), ?2-[P2W17O61]10-, that can be photoactivated in the presence of 2-propanol to transfer electrons to TcO4-, and incorporate the reduced Tc covalently into the ?2- framework to form the TcVO species, TcVO(?2-P2W17O61)7-. This occurs via the formation of an intermediate species that slowly converts to TcVO(?2-P2W17O61)7-. EXAFS and XANES analysis and preliminary EPR analysis, suggests that the intermediate consists of a Tc(IV) ?2- species where the Tc is likely bound to only 2 of the 4 W-O oxygen atoms in the ?2-[P2W17O61]10- defect. This intermediate then oxidizes and converts to the TcVO(?2-P2W17O61)7- product. The reduction and incorporation of TcO4- was accomplished in a ?one pot? reaction using both sunlight and UV irradiation, and monitored as a function of time using multinuclear NMR and radio TLC. The process was further probed by the ?step-wise? generation of reduced ?2-P2W17O6112- through bulk electrolysis followed by the addition of TcO4-. The reduction and incorporation of ReO4-, as a non-radioactive surrogate for 99Tc, does not proceed through the intermediate species, and ReVO is incorporated quickly into the ?2-[P2W17O61]10- defect. These observations are consistent with the periodic trends of Tc and Re. Specifically, Tc is more easily reduced compared to Re. In addition to serving as models for metal oxides, POMs may also provide a suitable platform to study the molecular level dynamics and mechanisms of the reduction and incorporation of Tc into a material