AbstractReactions of U(VI) and Eu/Cm(III) with natural clay rock (Opalinus Clay, Switzerland, and Callovo-Oxfordian Clay, France) are investigated by batch and spectroscopic experiments as well as geochemical calculations. The aim of the studies is to identify those minerals in the heterogeneous multiphase systems with sometimes high calcite content which control lanthanide and actinide sorption. The outcome of batch sorption studies with natural clay rocks (2 g/L natural clay rock, 0.1 mol/L NaClO₄, pH 3−11) and thermodynamic calculations is compared with experimental results using a synthetic mixture consisting of purified Na-montmorillonite and calcite (ratio 80:20[wt.]%). Our studies show that U(VI) sorption decreases at intermediate pH (6.5−9) where dissolved U(VI) carbonate species predominate. In the high pH region (>9) U(VI) sorption again increases by formation of ternary hydroxo surface complexes on the clay mineral fraction, whereby calculated U(VI) sorption overestimates experimental data for the natural rock. Complete sorption to natural clay rock and the synthetic clay/calcite mixture is observed for Eu(III) at pH>7 for the conditions studied. Model calculations again point to the predominance of clay-sorbed species even though calculated speciation underestimates the experimentally observed sorption at pH 6−9 slightly. Preliminary time-resolved laser-fluorescence spectroscopy (TRLFS) studies using Cm(III) as fluorescent probe reveal the appearance of several Cm(III)-clay surface species in the pH range 5−11, but give no indications for the presence of calcite-bound Cm(III). We conclude that the clay minerals in the multiphasic clay rock act as main sorbents for tri- and hexavalent actinides.