Simulation of Transport of Uranium (VI) Species Through the Bed of Bentonite

Abstract

ABSTRACTThe transport of six U(VI) species, HCO31−and CO32−, through the bentonite bed was modelled as diffusion in pore water combined with sorption/desorption on the surface of bentonite particles. The studied system consisted of the Czech commercial sodium bentonite SABENYL and synthetic granitic water spiked with233U(VI), where equilibrium distribution of233U was experimentally determined at first. The transport was simulated by means of eight dynamic, 1 D- partial differential equations while the equilibrium sorption/desorption was assumed. Three types of surface complexation models, namely the Constant Capacitance Model (CCM), Diffuse Double Layer Model (DLM) and the so-called Chemical Equilibrium Model (CEM), were used for the description of sorption/desorption interaction processes. The characteristic parameters of the individual models were obtained from evaluation of experimental data. The results of the simulation were expressed as path-length dependent concentrations of total U(VI), total carbonates and of individual migration species, as path-length dependent pH and uraniumKd, values at a given time and as time dependent concentrations of the total U(VI) at a given migration distance. It has been found that the transport of U(VI) through the bentonite bed is significantly influenced first of all by the value of pH and by total carbonates concentration. The important length dependent changes of the pH andKdvalues were determined.