Evolution of actinide partitioning with colloidal matter collected at PA "mayak" site as studied by sequential extraction

Evolution of actinide partitioning to colloids upon subsurface migration in the vicinity of Karachay Lake (PA “Mayak”, Russia) was studied by micro- and ultrafiltration and sequential extraction. Colloidal matter was characterized by SEM-EDX, TEM and photon-correlation spectroscopy. Samples were collected from wells with different redox conditions i.e. from higly oxic to rather reducing. Under oxidizing conditions uranium and neptunium is readily washed out from colloids. Plutonium and americium are present in colloidal matter mainly as hard-soluble species. Under oxidizing conditions plutonium and americium are predominantly bound to organic/mineral fractions (carbonates, oxides and organically bound). Under reducing conditions plutonium and americium are present in immobile refractory fraction. It was also established that uranium and neptunium under reducing conditions behave similarly to plutonium. This phenomenon may be explained by the fact that under reducing conditions and at considerable concentrations of uranium and nitrite-ions these elements form low-soluble An(IV) forms.