Investigations of the Photochemical Charge-Transfer Reduction of Uranyl UO22+(VI) to Uranyl UO2+(V) by Benzene-1,4-diol (1,4-C6H4(OH)2) and Oxalate (C2O42-) by UV-Vis, Electron Paramagnetic Resonance, and Luminescence Spectroscopies

Abstract The aqueous mixtures of UO22+(VI) (as the nitrate salt) with benzene-1,4-diol (hydroquinone, HQ) 1,4-C6H4(OH)2 and those with oxalate C2O42- (as the sodium salt) exhibited broad absorptions at 350–400 nm in their UV-Vis spectra. The intensities of the absorptions for both the UO22+(VI)–HQ and UO22+(VI)–C2O42- mixtures, represented by the absorbance at 375 nm, were shown to be directly proportional to the molar concentrations of HQ and C2O42-, respectively. For each mixture, the absorbance was also found to be directly proportional to the molar concentration of UO22+(VI). The broad 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 C2O42-. Upon photolysis of the UO22+(VI)–HQ and UO22+(VI)–C2O42- mixtures, UO22+(VI) was found by electron paramagnetic resonance (EPR) to be reduced to UO2+(V) (g = 2.08) by HQ and C2O42-, respectively (CT reductions), and the hydroquinone radical 1,4-HOC6H4O. (HQ.) (g = 2.00) was identified simultaneously by EPR spectroscopy. Both HQ and C2O42- at the very low molar ratios of [HQ]/[UO22+]