Humic colloid mediated transport of tetravalent actinides and technetium

The humic colloid mediated transport of tetravalent actinide and technetium ions is discussed. Numerous published and new results are used from comparable experimental systems and conditions. The results originate from investigations on two humic rich Gorleben groundwater/sediment systems under near-natural conditions. The conditioning time between groundwater and radionuclides vary from few minutes to four years. The migration time (residence time in the column) is varied from four hours to three days. The concentration of radionuclides added is varied between about 10(-12) and 10(-4) mol/L. Humic colloids in natural groundwater contain trace metal ions in varying concentrations. In addition to studying the behavior of radionuclides added to the groundwater in the laboratory, the dissociation behavior of these natural tetravalent trace metal ions is studied by scavenging dissociating ions with a cation exchanger. The results show a kinetic behavior of the trace metal ion interaction as frequently reported in the literature, i.e. with increasing contact time, slow dissociation modes are progressively populated resulting in an enhanced radionuclide transport. The strong dependency of the radionuclide transport on the radionuclide concentration shows that humic colloid mediated transport may be overestimated if radionuclide concentrations used in laboratory studies are too high. Dissociation of the natural inventory of humic associated tetravalent trace metal ions proceeds with a kinetics comparable to the radionuclides added. This, however, is only observed for a small portion of the inventory and extrapolation beyond the observation range is not permissible. Therefore, the key question for application to predictive modeling of the mobility of tetravalent radionuclides in natural groundwater is if the behavior of the natural trace element inventory is applicable or the behavior of radionuclides added in laboratory studies.