The aqueous mixtures of uranyl (VI) (as the nitrate salt) with benzene-1,4-diol (hydroquinone, HQ) and those with oxalate (as the sodium salt) exhibited broad absorptions at 350–400 nm in their UV-Vis spectra. Their intensities (represented by the absorbance at 375 nm) are directly proportional to the molarities of HQ and oxalate, respectively, in the solutions. For each mixture, the absorbance was also found to be directly proportional to the molar concentration of UO₂²⁺(VI). The absorptions are characteristic of the charge-transfer (CT) bands from the electron-donor-acceptor (EDA) complexes formed between UO₂²⁺(VI) and HQ and between UO₂²⁺(VI) and oxalate. Upon photolysis of the UO₂²⁺(VI) – HQ and UO₂²⁺(VI) – oxalate mixtures with a UV lamp, UO₂²⁺(VI) was found to be reduced to UO₂⁺(V), by EPR spectroscopy, by HQ and oxalate, respectively (CT reduction via a single-electron transfer from HQ or oxalate to UO₂²⁺), and the HQ radical was also identified simultaneously by EPR. Both HQ and oxalate at very low concentrations (less than 1% of the uranyl concentration in the solutions) were shown to quench the UO₂²⁺(VI) fluorescence (emission) for more than 60%. The quenching constants of HQ and oxalate were determined to be 10,800 M^-1 and 4,300 M^-1, respectively, using the Stern-Volmer relationship. The results indicated the interactions of HQ and oxalate, respectively, with the excited *UO₂²⁺(VI), following a charge-transfer quenching mechanism, to relax *UO₂²⁺(VI) and to convert the light energy to the chemical energy driving the redox reaction. Possible photochemical mechanisms associated to the UO₂²⁺(VI)–HQ and UO₂²⁺(VI)–oxalate redox reactions have been proposed on the basis of the UV-Vis, EPR, and fluorescence spectroscopic studies.

Derrick Kolling

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