Adsorption and transport of uranium(VI) in subsurface media

Uranium(VI) adsorption and transport in three natural, heterogeneous subsurface media were investigated in batch and column experiments. The rate of U(VI) adsorption to the natural samples was rapid over the first few hours of the experiments, and then slowed appreciably after 23 to 48 h. The adsorption of U(VI) to the samples was also nonlinear, suggesting a decreasing attraction for the surface with increased surface loading. The extent of adsorption on each of the media was strongly pH-dependent, increasing sharply as the pH increased from 4.5 to 5.5 and then decreasing sharply over the pH range 7.5 to 8.5 as the concentration of dissolved carbonate and U(VI)-carbonate complexes increased, The similarities in the pH-dependent behavior between the three materials despite differences in bulk mineralogy was likely due to the similar Fe contents of the materials. The transport of U(VI) through packed columns of the soils and sediments was significantly retarded but reversible. The local equilibrium assumption and the batch-measured adsorption isotherms dramatically underestimated the degree of retardation observed in the columns. The U(VI) displacement experiments were modeled with the one-dimensional advertive-dispersive equation and several different model formulations describing the interactions of U(VI) with the solid phase. These models were able to fit the observed breakthrough curves within 0.1 root mean square error of the initial concentration.