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

Bailey, E. H.; Mosselmans, J. F.W.; Schofield, P. F.

Abstract

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. © 2004 Elsevier B.V. All rights reserved.

• Journal Article Type: Article
• Acceptance Date: Oct 19, 2004
• Online Publication Date: Dec 15, 2004
• Publication Date: Mar 15, 2005
• Deposit Date: Apr 25, 2021
• Journal: Chemical Geology
• Print ISSN: 0009-2541
• Publisher: Elsevier
• Peer Reviewed: Peer Reviewed
• Volume: 216
• Issue: 1-2
• Pages: 1-16
• DOI: https://doi.org/10.1016/j.chemgeo.2004.10.011
• Public URL: https://nottingham-repository.worktribe.com/output/5160847
• Publisher URL: https://www.sciencedirect.com/science/article/abs/pii/S0009254104004553?via%3Dihub