Theoretical Solid/Solution Ratio Effects on Adsorption and Transport: Uranium(VI) and Carbonate

The reactive transport of metal and radionuclide contaminants in the subsurface often significantly influences their long‐term fate and effect in the environment. Typically, predictions of contaminant migration at a site involve the measurement of a distribution coefficient ( K D ), which is used to describe the interactions between the contaminant and the subsurface. The typical implicit assumption is that the adsorption isotherm (e.g., K D ) is independent of the solid/solution ratio. Many geochemical factors, however, play a significant role in the reactive transport of contaminants in groundwater. The adsorption and transport of U(VI), for example, is strongly influenced by the presence of Fe oxyhydroxides and the carbonate system. However, these solutes or adsorbates and adsorbent interact with one another in a complex and highly nonlinear manner. Modeling of U(VI) adsorption has shown that under certain conditions, the solid/solution ratio can theoretically have a significant impact on the U(VI) adsorption isotherm. In particular, combining strongly interacting solutes [U(VI) and carbonate] and adsorbents [Fe(III) oxyhydroxides] that have monocomponent solute or adsorbate adsorption isotherms that are independent of the solid/solution ratio results in a multicomponent system where adsorption isotherms become dependent on the solid/solution ratio. The solid/solution ratio can therefore be critical when extrapolating the results of batch experiments, generally conducted at low solid/solution ratios, to column experiments and then to the field. These results have implications for modeling, scaling, and predicting the reactive transport of U(VI) and other strongly interacting solutes (e.g., metals and dissolved organic C) in subsurface environments.