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; andthe 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.