Adsorption of uranyl tricarbonate and calcium uranyl carbonate onto γ-alumina

The uranyl tricarbonate moiety, exemplified by UO2(CO3)34−, CaUO2(CO3)32−, and Ca2UO2(CO3)3(aq), is potentially expected to consist of mobile uranyl(VI) species in the aquifer of a deep geological repository for spent nuclear fuel. Time-resolved laser fluorescence spectroscopy (TRLFS) for the U(VI) surface speciation on γ-alumina was utilized to investigate the effect of various uranyl tricarbonate species on U(VI) adsorption. Under the prevailing carbonate conditions, the emission bands of the species of U(VI) adsorbed onto γ-alumina were found at 467.6 ± 0.1, 487.1 ± 0.1, 507.5 ± 0.1, 530.0 ± 0.1, and 552.8 ± 0.2 nm and were blueshifted compared with those of the U(VI)-hydroxo surface complexes. The shift to the shorter wavelengths was attributed to the spectral feature of bis- and/or tris-carbonato U(VI) complexes found in aqueous uranyl(VI) species and uranium(VI) minerals with two to three carbonate ions. The results obtained from attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy have confirmed the formation of U(VI)-carbonato surface complexation on γ-alumina in a bidentate manner. Based on identical spectroscopic properties, which were observed independently of the presence or absence of calcium ions in the uranyl tricarbonate system, a similar manner of U(VI) tricarbonate adsorption can be assumed, regardless of the absence or presence of calcium. According to the results from this work and comparison with previous studies, U(VI)-carbonato surface complexation tends to be favorable on mineral surfaces with a higher point of zero charge (pHPZC) such as alumina. Regarding the retention of U(VI) in the aquifer, the results presented in this work provide new insight and interpretation of U(VI) adsorption in carbonate-rich groundwater.