Quantum yields and reaction rates in the photo-oxidation of hydrazine by uranyl ion

Abstract

Quantitative aspects of the photo-oxidation of hydrazine in HNO₃ and HClO₄ media by uranyl ion have been investigated in detail as a possible photochemical approach to the reprocessing of nuclear fuel, relying on the photo-production of U^IV which acts as a reductant for Pu^IV(to give solvent-separable Pu^III+ U^IV). Φ(U^IV), which is independent of irradiation wavelength, increases with hydrazine concentration in perchlorate media; a plot of Φ(U^IV)^–1versus[N₂H₅⁺]^–1 is linear with an intercept of 3.89, i.e. the limiting value of Φ(U^IV) is 0.26. After relatively long irradiation times, when ca. 20–40% of U^IV is reduced, the stoicheiometry is approximately N₂H₄ : U^IV : NH₃= 2 : 1 : 2 in both HClO₄ and HNO₃, but at much shorter times (when the few percent loss of N₂H₄ cannot be determined accurately) the U^IV : NH₃ ratio is approximately 1 : 1.25. N₂H₄ quenches the luminescence intensity of [UO₂²⁺]* by a dynamic process, and τ for [UO₂²⁺]* is systematically reduced on addition of N₂H₄. The quenching rate is affected somewhat by acidity and variation of ionic strength. A mechanism is proposed which accounts quantitatively for the variations both in quantum yield and lifetime. While the system has considerable interest as regards the efficient production of U^IV, its overall potential in the reprocessing of nuclear fuel is limited by the stoicheiometric product of ammonia, resulting, from the waste management viewpoint, in the build-up of unacceptable levels of ammonium nitrate, and attempts to circumvent the partial oxidation (N₂H₄→ e^–+½N₂+ NH₄⁺), by addition of catalysts such as Cu^II, have failed. Alternative related reductants have been considered, but neither hydroxylamine nor its methylated analogues quenches [UO₂²⁺]*.