Exafs Investigations of Earth-alkaline Metal Uranyl Tricarbonato Complexes

Carbonate and bicarbonate are the common anions found in significant concentrations in many natural waters, and are exceptionally strong complexation agents for actinide ions. Seepage, flooding and mining waters from the uranium mining area in the south-east of Germany (Saxony and Thuringia) contain relatively high amounts of magnesium, calcium, carbonate and sulphate ions. In these waters the pH values range from 7.0 to 8.0 and the uranium content is about 0.02 mmol/L where magnesium, calcium and carbonate concentrations are 11.6 mmol/L, 6.9 mmol/L and 3.9 mmol/L, respectively [1]. The existence of the natural complexes Ca2UO2(CO3)3(aq) and Mg2UO2(CO3)3(aq) were recently able to be confirmed by spectroscopic measurements [2]. We performed extended X-ray absorption fine structure (EXAFS) measurements at the uranium LIII-edge to study the structures of these complexes. The results obtained from these complex solutions are compared with those from the corresponding EXAFS and XRD data related to the secondary minerals of uranium, liebigite, Ca2[UO2(CO3)3]⋅10H2O and bayleyite, Mg2[UO2(CO3)3]⋅10H2O and they are in good agreement.