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The aqueous sorption of cadmium solutions on hydroxyapatite (HAP) single crystals has been studied with the Nuclear Microprobe facility of the Laboratoire Pierre Siie in Saclay (France).By coupling Proton Induced X-ray Emission (PIXE) and Rutherford Backscattering (RBS) analyses, the concentration and distribution of cadmium in the HAP crystals have been measured. The use of different incident proton energies allowed to model the incorporation of cadmium as a function of the analyzed depth. A one dimensional simple diffusion models the incorporation of cadmium along the crystallographic c-axis. The study of sorption kinetics allowed to derive a lower limit (some 10 3 μιτι 2 /5) of the diffusion coefficient according to the shortest contact time of one minute with the solution. The coupling of diffusion with cation exchange as the active sorption processes has been identified, other surface mechanisms being limited. The advantages of using apatites to trap or retard the diffusion of radionuclides in the near and the far field of a nuclear waste repository are discussed.

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