Efficient Photocatalytic Reduction of Aqueous Perrhenate and Pertechnetate

The pertechnetate anion (99TcO4–) is a long-lived radioactive species that is soluble in aqueous solution, in contrast to sparingly soluble 99TcO2. Results are reported for photocatalytic reduction and removal of perrhenate (ReO4–), a nonradioactive surrogate for 99TcO4–, using a TiO2 (P25) nanoparticle suspension in formic acid under UV–visible irradiation. Re(VII) removal up to 98% was achieved at pH = 3 under air or N2. The proposed mechanism is Re(VII)/Re(IV) reduction mediated by reducing radicals (·CO2–) from oxidation of formic acid, not direct reduction by photogenerated electrons of TiO2. Recycling results indicate that photocatalytic reduction of ReO4– exhibits excellent regeneration and high activity with >95% removal even after five cycles. 99Tc(VII) is more easily reduced than Re(VII) in the presence of NO3– with very slow redissolution of reduced 99Tc. This study presents a novel method for the removal of ReO4–/99TcO4– from aqueous solution, with potential application for deep geological disposal.