REACTIVE TRANSPORT OF URANYL IN A GOETHITE COLUMN: AN EXPERIMENTAL AND MODELLING STUDY

Abstract

We investigated the adsorption of uranium(VI) at atmospheric CO2 partial pressure in a bicarbonate background ionic medium (pH 9.0+/-0.2). Goethite-coated sand was used as mine tailings model material. Both static, i.e., batch, and dynamic, i.e., column, experiments were performed. In the column experiments, uranyl adsorption was found to be far from local equilibrium at a pore velocity of 12.1 cm h-1. However at a pore velocity of 1.21 cm h-1, sorption was at or near equilibrium. The asymmetrical break-through curves indicated a non-linear adsorption isotherm and/or a kinetically controlled adsorption/desorption reaction. Both effects could be modeled by incorporating either first-order adsorption kinetics or a Langmuir adsorption isotherm into the basic exchange and convection-dispersion equation. In batch experiments the silica dissolution induced a greater competition of aqueous silica species with carbonate and uranyl ions for adsorption on the goethite surface resulting in less uranyl adsorption than in the column experiments where the dissolved silica was negligible. The retardation factor from the dynamic experiments and the non-linear `static' adsorption isotherm, as well as the effect of pH on surface and solution complexation by silica and carbonate ions, were taken into account within the framework of a general surface complexation model. This model included high and low affinity sites at a 1:200 ratio. The uranyl sorption was equally well reproduced under the assumption of the release of either one or two protons in the formation of the uranyl surface complex on the low affinity sites, and two protons on the high affinity sites. These different parameters have to be taken into account for the assessment of the long term interaction of uranium in mine waste tailings with the environment.