Speciation and mobility of uranium(VI) in humic-containing soils

Uranium is known to easily form stable complexes with both organic and inorganic ligands in natural aquatic environments. The potential mobility of U(VI), and particularly the role of humic substances, has been investigated for a sandy podzol. Adsorption of U(VI) at equilibrium was experimentally investigated, comparatively for a pure quartz sand as well as a sandy podzolic soil from Landes Gascony, France, containing 0.6 wt. % organic matter. Uranium is strongly retained by the bulk soil, mainly due to organic aggregates (< 50 mum) and organic coatings on quartz grains. A large fraction of U(VI) is found to be associated to humic colloids in soil, thus forming a potential mobile uranium phase. For the soil, the U(VI) adsorption isotherm is linear for relatively low U(VI) total aqueous concentration, ranging from 15 to 40 000 mug/L. This indicates that the functional groups of humic matter are far from saturated for aqueous concentrations close to U(VI) solubility in soil pore-water. The speciation of uranium in soil pore water and the interactions between U(VI) species and the reactive sites of the soil material were evaluated using the speciation code CHESS [1]. Comparison between the model and experimental results show that complexation of U(VI) by humic substances tends to immobilise uranium in soils with a pH below 5, when the reactive surface of quartz grains and aggregates are entirely coated by humic matter. In soil pore-water, the presence of mobile organic ligands competing with surface-bound organics for U(VI) complexation must be included in the speciation modelling. Without the mobile organic phase, the model overestimates the actual bound fraction, thus underestimating the actual amount of mobile U(VI).