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+] < 1/100 and [C2O42−]/[UO22+] < 1/100 were shown to quench the UO22+ luminescence (emission) for more than 60%. The quenching constants of HQ and C2O42− were determined to be 10,800 M−1 and 4,300 M−1, respectively, using the Stern-Volmer relationship. The results indicated a CT quenching mechanism, namely that the interactions of HQ and C2O42− with the excited *UO22+(VI) lead to transfer of a single electron from the quencher to *UO22+(VI) to reduce it to UO2+(V). This converts the light energy to chemical energy to quench the UO22+(VI) luminescence. Possible photochemical processes associated to the UO22+(VI)–HQ and UO22+(VI)–C2O42− redox reactions have been proposed on the basis of the UV–Vis, EPR, and luminescence spectroscopic studies.