Efficient Photocatalytic Reduction of Aqueous Perrhenate and Pertechnetate.

The pertechnetate anion ( ⁹⁹ TcO ₄ ^- ) is a long-lived radioactive species that is soluble in aqueous solution, in contrast to sparingly soluble ⁹⁹ TcO ₂ . Results are reported for photocatalytic reduction and removal of perrhenate (ReO ₄ ^- ), a nonradioactive surrogate for ⁹⁹ TcO ₄ ^- , using a TiO ₂ (P25) nanoparticle suspension in formic acid under UV-visible irradiation. Re(VII) removal up to 98% was achieved at pH = 3 under air or N ₂ . The proposed mechanism is Re(VII)/Re(IV) reduction mediated by reducing radicals ( ^· CO ₂ ^- ) from oxidation of formic acid, not direct reduction by photogenerated electrons of TiO ₂ . Recycling results indicate that photocatalytic reduction of ReO ₄ ^- exhibits excellent regeneration and high activity with >95% removal even after five cycles. ⁹⁹ Tc(VII) is more easily reduced than Re(VII) in the presence of NO ₃ ^- with very slow redissolution of reduced ⁹⁹ Tc. This study presents a novel method for the removal of ReO ₄ ^- / ⁹⁹ TcO ₄ ^- from aqueous solution, with potential application for deep geological disposal.