MECHANISTICALLY BASED URANIUM SORPTION MODELS (Invited Presentation)

Coupled sorption and redox reactions are the primary mechanisms controlling the mobility of uranium and other actinides in subsurface environments. In this work we have examined sorption of uranium, neptunium, and plutonium to goethite, hematite, and rutile as a function of pH, time, and temperature using batch sorption techniques. The results are combined with data from potentiometric titrations, calorimetric titrations, electron microscopy, and spectroscopic studies to develop self-consistent thermodynamic sorption models. The titration data, microscopy, and spectroscopic data allow for a more mechanistic understanding of the sorption processes and thus allow for development of a more robust sorption model. The data suggest that sorption generally increases with increasing pH and increasing temperature. Increasing sorption with increasing pH is consistent with formation of electrostatically bound surface complexes. Furthermore, sorption was observed to increase with increasing pH for uranium and plutonium sorption to all minerals. The extent of sorption with respect to the increase in temperature increases with increasing effective charge of the actinide ion and increasing hydration of the ion. It is hypothesized that the observed increase in sorption with increasing temperature is a manifestation of energetically favorable sorption entropy wherein waters hydrating the actinide ion and the mineral surface are displaced upon formation of the inner sphere surface complex. Thus, dehydration of the ion upon sorption may represent an energetic barrier to desorption. This phenomena may help to explain frequently observed desorption hysteresis of highly charged cations.