Radionuclide sorption mechanisms and rates on granitic rock

The gamma-emitting radionuclides 137Cs, 144Ce, 75Se and 60Co were simultaneously contacted with granite from the Lac du Bonnet batholith using both static and dynamic methods. Selective chemical extraction was then used to differentiate between the amounts of sorbed radionuclides that are (a) readily ion-exchangeable, (b) associated with amorphous oxyhydroxide deposits, and (c) fixed by other mineralogical or physical processes. Comparison of the experimental results from the dynamic tests with calculations from single sorption site kinetic models, using a variety of isotherms, showed that the models did not adequately describe the sorption reactions. Use of double sorption site models greatly improved the ability to describe solution concentrations and radionuclide surface inventories measured by extraction methods. Laboratory alternation of fresh granite surfaces was found to affect the sorption capacities and ratios of 60Co, 75Se and 144Ce. Granite alteration on a laboratory time scale had no effect on 137Cs sorption. Rate constants determined for the various processes indicate that under oxidizing conditions ion-exchange processes are rapid, with equilibrium achieved within days. Reactions with oxyhydroxides or other mineralogical or physical processes take longer to achieve equilibrium, but, on time scales relevant to groundwater flow rates, will provide more significant retardation of the radionuclide migration than ion exchange. 18 references, 5 figures, 6 tables