The ternary system U(VI) / humic acid / Opalinus Clay

Beside salt dome and granite rock, also clay rock is discussed as a possible host rock for siting a nuclear waste repository in Germany. The focus of the present work is the investigation of the U(VI) interaction with Opalinus Clay from Mont Terri, Switzerland, used here as a generic test case for studying radionuclide-clay interaction. The influence of humic acid (HA), a representative of natural organic complexing agents, on the migration of U(VI) is studied in addition.
U(VI) sorption experiments with ground Opalinus Clay and U(VI) diffusion experiments with intact clay rock were performed in the absence and presence of HA using synthetic Opalinus Clay pore water (I = 0.36 M, pH 7.6; [1]) as the background electrolyte. It was found that the U(VI) sorption onto Opalinus Clay is low and it is not influenced by HA. Speciation calculations showed that, under pore water conditions, Ca2UO2(CO3)3(aq) [2] is the dominant U(VI) species in solution both in the absence and presence of HA. The U(VI) speciation is not influenced by HA, which is predominantly present as a Ca complex [3]. It can be concluded, that calcium ions released by the dissolution of calcite (13% of Opalinus Clay [4]) play an important role in the investigated system.

[1] F.J. Pearson, PSI Internal report TM-44-98-07, Paul Scherrer Institut, Villigen PSI, Switzerland, 1998.
[2] G. Bernhard, G. Geipel, T. Reich, V. Brendler, S. Amayri, H. Nitsche, Radiochim. Acta 89 (2001) 511.
[3] A. Paulenová, P. Rajec, M. Žemberyová, G. Sasköiová, V. Višacký, J. Radioanal. Nucl. Chem. 246 (2000) 623.
[4] Nagra, Nagra Technical Report NTB 02-03, Wettingen, Switzerland, 2002, p. 230.