Uranium migration through intact sandstone : effect of pollutant concentration and the reversibility of uptake

A series of core flood experiments has been performed to investigate the migration behaviour of uranium under rigidly controlled conditions. Intact sandstone cores, pre-equilibrated with synthetic groundwater, were flooded with uranium solutions at varying concentrations and the transport process monitored as a function of pH, tracer concentration and the concentration of a competing ion, cadmium. In each case a substantial amount of uranium was retained by the core, implying a strong interaction with the rock matrix. The adsorption process was found to be highly concentration dependent; however, such that the finite retention capacity of an intact core can be exhausted within a relatively short time. The reversibility of uptake was studied by attempting to displace adsorbed uranium by injected cadmium and flushing with brine. Most of the uranium was readily recoverable but a small percentage is released very slowly on flushing, suggesting conversion to a more stable form. Prior to performing each experiment a simulation was carried out using a one-dimensional coupled chemical transport code, encompassing a thermodynamic description of the electrical double layer. The model was successful in predicting a priori the dominant trends in the uranium migration behaviour which may aid in model developments for more complex geochemical regimes than those studied here.