Numerical reproduction of dissolved U concentrations in a PO4-treated column study of Hanford 300 area sediment using a simple ion exchange and immobile domain model

Abstract We succeeded at numerical reproduction of dissolved U concentrations from column experiments with PO4-treated Hanford 300 Area sediment using a simple ion exchange and immobile domain model. The time-series curves of dissolved U concentrations under various Darcy flow rate conditions were reproduced by the numerical model in the present study through optimization of the following parameters: the mass of U in mobile domain (on surface soil connected to the stream) to fit the starting U concentration at the column exit, and the rest of the total U was left as precipitation in immobile domain (isolated in deep soil); the mixing ratio between immobile and mobile domains, to fit the final recovering curve of concentration; and the cation exchange capacity (CECZp) and equilibrium constant (kZp) of the exchange reaction of UO22+ and H+ on simulated soil surface (Zp), to fit the transient equilibrium concentration, forming the bed of the bathtub curve. Numerical setting of no U in immobile domain or no mixing between immobile and mobile domains caused all U flushed out of the column exit, and setting of no CEC on Zp, formed no transient equilibrium concentration. The ion exchange immobile domain model is so common that it has become a standard process in the general-purpose geochemical program Phreeqc. Optimization of this model led to the development of the model presented here, which was capable of explaining the fluctuations in dissolved U concentration well and reproducing column experiments under various conditions.