Kinetics of plutonium and americium sorption to natural clay.

Kinetics of Pu(IV) and Am(III) sorption from natural groundwater to three types of clays were studied at trace concentrations of the elements. Higher K(d) values were determined for sorption of Pu than of Am, and no clear dependence of the K(d) values and the kinetic coefficients on the composition of the clays can be deduced. Kinetic data evaluated by models for six different control processes indicated a sorption mechanism controlled by Pu or Am diffusion in the inert layer on the surface of the clays. Apart from the kinetics of the elements sorption, time-dependent changes in their bonding nature were also studied using a sequential extraction. It was found that Pu(IV) was predominantly associated with amorphous and crystalline Fe oxides as well as natural organic matter sites on the clays, whereas in the case of Am(III) the exchangeable and carbonate sites played the principal role.