Speciation of actinides in groundwater samples collected near deep nuclear waste repositories

Actinide binding to colloidal particles of different nature was studied under oxic and anoxic conditions of an underground nuclear waste disposal site using successive micro- and ultrafiltration techniques. According to the actinide redox speciation, under oxic conditions they were present in high oxidation states except for plutonium, for which a significant part was found in the tetravalent state. In case of the anoxic conditions, the share of An (IV) was proportional to the total U(IV) concentration. This indicated formation of intrinsic U(IV) hydroxocolloids, which bound other actinides. Formation of the intrinsic actinide colloids was proven by the secondary ion mass spectrometry (SIMS) with the submicron resolution. In contrast, under the oxic conditions uranium and plutonium were sorbed by natural colloids (amorphous hydrous ferric oxide and Mn oxides).