Uranyl ions in perfluorinated (Nafion and Flemion) membranes: spectroscopic and photophysical properties and reactions with potassium hydroxide

Absorption spectra, luminescence spectra and excited-state lifetimes of uranyl ions (UO²⁺₂) incorporated in Nafion or Flemion membranes have been studied as a function of hydration. Marked changes in the nature of the spectra and emission lifetime are observed. Thus at 291 K the lifetime changes from 2.0μs in a water-swollen Nafion membrane to 640 μs in a thoroughly dried sample. The lifetime of the dried sample is temperature-invariant, whereas those of heavily hydrated samples show activation energies of 30 ± 2 kJ mol^-1, suggesting that water plays a key role in the excited-state deactivation. A changeover in the decay mechanism is found for samples of intermediate water content. Energy transfer from excited UO²⁺₂ to Eu³⁺ is observed, the effect being enhanced by membrane dehydration and being much more pronounced for Flemion membranes than for Nafion. Treatment of the UO²⁺₂-containing membranes with aqueous KOH causes the formation of products concentrated near the membrane surface. The absorption spectra, luminescence spectra and X-ray diffraction of these materials have been compared with those of the oxides and uranates formed in solutions of various pH. Transmission electron microscopy reveals particles of less than 1 μm in size.