Modelling Desorption of <sup>63</sup>Ni from Granitic Rocks and Minerals

The reversibility or irreversibility of the sorption process is of fundamental importance for understanding the fate of radionuclides in geological systems. Desorption experiments with 63Ni were carried out. Three replicates each of varying concentration (1 × 10−14 to 1 × 10−4 mol∙dm−3) of Ni2+ were prepared so that 0.1 cm3 of the spike solution should give approximately 1200 counts∙min−1. One cm3 of the supernatant was mixed with 10 cm3 of scintillation cocktail (High Flash-point, Universal LSC cocktail) and samples counted in a TriCarb liquid scintillation counter, followed by centrifugation at 6000 rpm for 30 min. The separated sample was mixed with 20 cm3 of DI water and allowed to shake for between 5 and 7 days, upon which one cm3 of the supernatant was filtered and counted in the energy range of 0 to 67 keV at 2σ at a counting time of 20 min per sample. Hysteresis values were calculated for different granitic samples. Results showed that the hysteresis increases with Rd, as illustrated by GG (Rd = 17.2 cm3∙g−1, H = 0.94) and PF (Rd 6.1 cm3∙g−1, H = 0.79). These results highlight the ability of granitic materials to retard radionuclides in solution. The ability to retard radionuclides in the far field gives granitic rock systems an advantage in terms of repository considerations. Considering H, the following order is obtained; RG > OF > GG > PF. Taking into consideration the percentage desorption (average percentage desorption), the following values were obtained RG = 4.2%, OF = 3.4%, PF = 22.7%, these values show that the hysteresis increased as desorption decreased for the above samples.