Sorption of uranium and neptunium onto diorite from Äspö HRL

The retention behavior of the crystalline rock diorite from the Äspö Hard Rock Laboratory (HRL, Sweden) towards the redox-sensitive actinides U and Np was studied by means of batch sorption experiments. The influence of various parameters, such as grain size (0.063 – 0.2 mm, 0.5 – 1 mm, 1 – 2 mm), temperature (25 and 10°C), atmosphere and sorption time (5 to 108 days) was studied using a synthetic Äspö groundwater (pH 7.8, I = 0.178 M) as background electrolyte. For U(VI), sorption isotherms were recorded (5x10⁻⁹ M to 7x10⁻⁵ M). Distribution coefficients, K_d values, were determined.