Uranyl acetate speciation in aqueous solutions -: An XAS study between 25°C and 250°C

Speciation of uranium (VI) in acetate solutions between 25 and 250 degreesC, at PH values between 1.8 and 3.8 and acetate/uranium (Ac/U) ratios of 0.5 to 100 has been investigated using uranium L-III-edge X-ray absorption spectroscopy. With increasing PH the UO2(Ac)(2)(0) species becomes more important than UO2(Ac)(+) species. which is predominant below PH 2. It remains the dominant species as PH is further increased to 3.8 at an Ac/U ratio of 20. Decrease in U-O-cq bond distance and coordination number with increasing solution age indicates that steric/kinetic factors are important and that equilibrium is attained slowly in this system with initial acetate coordination to the uranyl ion being monodentate or pseudo-bridging before slow conversion to bidentate chelation. Acetate coordination to the uranyl ion appears to decrease as temperature is increased from room temperature to similar to 100 degreesC before increasing in solutions of Ac/U > 2. For solutions where Ac/U less than or equal to 2 at PH 2.1, there is no evidence for uranyl acetate speciation at low temperatures, but at elevated temperature bidentate uranyl-acetate ion-pairing is evident. The existence of the uranyl acetate species in the temperature range 200 to 240 degreesC demonstrates the importance of including acetate and other organic ligands in models Of uranium transport at elevated temperatures. Copyright (C) 2004 Elsevier Ltd.