Uranium adsorption. A review of progress from qualitative understanding to advanced model development

Surface adsorption has a major influence on the environmental mobility of radionuclides, including uranium. Six decades ago, the description of the sorption process relied predominantly on simple descriptive parameters of solid-liquid partitioning (such as Kd values). There have since been numerous systematic investigations of the processes controlling U adsorption, including the affinity of U for different types of geologic materials, the influence of factors such as pH, the effects of complexing ligands, and the role of microorganisms. Mathematical descriptions of sorption processes have adopted various models - including sorption isotherms, surface complexation models and other types of modelling approaches, aided by advances in computational and analytical techniques. In recent years, mechanistic models have incorporated structural insights gained from spectroscopic techniques (such as EXAFS and TRLFS). Throughout the period, the nuclear waste community has sought to develop models for U sorption in complex systems associated with radioactive waste disposal, involving a range of mineral surfaces and incorporating numerous interactions and processes. To some extent, the ongoing questions concerning U adsorption can be considered as being common to many environmental metal contaminants. However, uranium is a unique and significant case, particularly for the radiochemical community, where the long-term behaviour of actinides is a central issue.