Uranyl-citrate speciation in acidic aqueous solutions—an XAS study between 25 and 200 °C

Speciation of uranium(VI) in citrate (2-hydroxypropane-1,2,3-tricarboxylic acid) solutions between 25 and 200 °C, at pH values between 0.8 and 3.7 and at citrate/U ratios between 0.5 and 50 has been investigated using U LIII-edge X-ray absorption spectroscopy (XAS). At 25 °C and Cit/U>2, we observed a decrease in U–Oeq bond distance as pH was increased, accompanied by an increase in the number of coordinating O atoms around the uranyl ion. This indicates a speciation change due to polymerisation of the monomer [UO2(cit)]−, present around pH 1, to form the dimer [(UO2)2(cit)2]2− at pH 1.8 and above. The dimer [(UO2)2(cit)2]2− is bridged via the alcohol groups of the citrate ligand. At Cit/U≤2, hydrolysis species appear to dominate at room temperature. Upon heating, no speciation changes were observed, however evidence from U–Oeq bond distances and coordination numbers strongly suggests that uranyl-citrate species are present in solution up to 200 °C. Above 200 °C, decomposition of the citrate ligand was observed. The existence of the uranyl-citrate species in the temperature range 25–200 °C demonstrates the importance of including citrate and other organic ligands in models of uranium transport.