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

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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 UIV which acts as a reductant for PuIV(to give solvent-separable PuIII+ UIV). Φ(UIV), which is independent of irradiation wavelength, increases with hydrazine concentration in perchlorate media; a plot of Φ(UIV)–1versus[N2H5 +]–1 is linear with an intercept of 3.89, i.e. the limiting value of Φ(UIV) is 0.26. After relatively long irradiation times, when ca. 20–40% of UIV is reduced, the stoicheiometry is approximately N2H4 : UIV : 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 UIV : NH3 ratio is approximately 1 : 1.25. N2H4 quenches the luminescence intensity of [UO2 2+]* by a dynamic process, and τ for [UO2 2+]* 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 UIV, 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 CuII, have failed. Alternative related reductants have been considered, but neither hydroxylamine nor its methylated analogues quenches [UO2 2+]*.

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