COMPARISON OF URANIUM(VI) AND PLUTONIUM(VI) HYDROLYSIS PRODUCTS BY MEANS OF ELECTROSPRAY IONIZATION MASS SPECTROMETRY

To understand the behavior of uranium and plutonium in the environment, it is important to characterize and quantify the chemical species these elements form under geochemical conditions. Both hexavalent Pu and U hydrolyze in the near-neutral pH range ^[1,2] and thereby form small polymers. While the formation of trimeric U(VI) hydrolysis products is generally accepted^[3], the exact composition of polymeric Pu(VI) hydrolysis products are discussed controversially in the literature. ^[2,4] As most of the methods used to describe the M(VI) hydrolysis are of an indirect nature and mostly insensitive to minor species, the present work aims at the direct observation of the hydrolysis complexes of U(VI) and Pu(VI) by means of nano-electrospray ionization mass spectrometry. This technique, if operated at very low declustering conditions, is a powerful yet low invasive tool for speciation of hydrolysis species in solution, reflecting the relative abundances of the different charged species. ^[5]

Solutions of U(VI) and Pu(VI) with concentrations of 5·10^-5mol/l in HClO₄, HCl and HNO₃ at pH ranges from 2-7 were analyzed by ESI MS.

At low pH values the free actinyl ions dominate the species distribution. With increasing pH, the hydrolysis species become more pronounced. All expected U(VI) hydrolysis products were observed: monomeric, dimeric and trimeric species formed. The dimeric complex (UO₂)₂(OH)₂²⁺ being a minor species and the (UO₂)₃(OH)₄²⁺ complex are detected for the first time by ESI MS ^[6], in accordance with the hydrolysis model in ^[3]. Furthermore, we were able to detect the hydrolysis products of Pu(VI) by the ESI MS technique for the first time. Again the results are in accord with the thermodynamic model.^[7] Monomeric and dimeric hydrolysis species form, while trimeric species, in contrast to the case of U(VI), were not observed within the detection limits.

1. H. R. Cho et al., Radiochim. Acta (2010) 98, 555.

2. S. D. Reilly, M. P. Neu, Inorg. Chem. (2006) 45, 1839.

3. I. Grenthe et al., Chemical Thermodynamics of Uranium, Vol. 1, (1992) Elsevier, Amsterdam.

4. S. Okajima et al., Radiochim. Acta. (1991) 52/53, 111.

5. C. Walther et al., Radiochim. Acta. (2009) 97, 199.

6. C. Moulin et al., Appl. Spectr. (2000) 54, 843.

7. R.J. Lemire et al., Chemical Thermodynamics of Neptunium and Plutonium, Vol. 1, (2001) Elsevier, Amsterdam.