Publikationsrepositorium - Helmholtz-Zentrum Dresden-Rossendorf

In the present study neptunium(V) uptake by crystalline granitic rock (Kuru Grey granite) and bentonite colloids (MX-80) under stagnant conditions in batch-type experiments and the role of stable and mobile bentonite colloids on the migration of neptunium(V) through intact granite columns under flowing water conditions was investigated. The uptake of 10-6 M neptunium(V) by 40 g/L crushed granite in 10 mM NaClO4 was found to be pH-dependent, whereas neptunium(V) uptake by MX-80 bentonite colloids (0.08-0.8 g/L) was pH-independent up to a pH-value of approximately 11. Column experiments were conducted in the presence and absence of colloids at two pH values (pH = 8 and 10) and two flow rates (0.3 mL/h and 0.8 mL/h) in 10 mM NaClO4. The neptunium(V) concentration was 2×10-4 M and the colloid concentration ranged from 0.1-1 g/L. The properties of the flow field in the columns were investigated with a conservative chloride tracer, at the same two flow rates of 0.8 and 0.3 mL/h. The resulting breakthrough curves were modeled using the analytical solution of advection–matrix diffusion equation. Based on the column experiments, neptunium(V) association with the colloids was found to occur directly in the injection phase. At slow flow rate, no influence of the bentonite colloids could be seen, implying that the non-sorbed and colloid-borne neptunium(V) are eluted from the columns at pH = 8 and that an exchange from colloid-borne to granite-sorbed neptunium(V) occurred at pH = 10. For the higher flow rate at pH = 8, clogging of flow channels, resulting in an enhanced retention of colloid-associated neptunium(V) was found. At pH = 10, adsorption of neptunium(V) on the granite reduced the clogging effect.