The structure of actinide ions exchanged into native and modified zeolites and clays

X-ray absorption spectroscopy (XAS) has been used to investigate the structure and valence of thorium (Th{sup 4+}) and uranyl (UO{sub 2}{sup 2+}) cations exchanged into two classes of microporous aluminosilicate minerals: zeolites and smectite clays. XAS is also employed to examine the fate of the exchanged cations after modification of the mineral surface using self-assembled organic films and/or exposure to hydrothermal conditions. These treatments serve as models for the forces that ultimately determine the chemical fate of the actinide cations in the environment. The speciation of the cations depends on the pore size of the aluminosilicate, which is fixed for the zeolites and variable for the smectites