Tansport processes controlling uranium uptake by plants

The mechanisms of uranium (U) uptake by plants growing in contaminated soils are poorly understood, constraining the development of mitigation measures and models of U fate and behaviour. Uptake involves a complex interaction between diffusion and reaction processes in the rhizosphere, and root-induced changes in the soil affecting these processes. This thesis is concerned with developing better understanding of these processes, as represented in predictive mathematical models. Most past research on U transport and reaction in soils has been in shaken suspensions or flow-through systems, in which the rate-limiting processes are artificially altered. This thesis develops a novel experimental approach in which diffusion and reaction are measured simultaneously in soil with stationary pore water, better representing the rhizosphere. Concentration- distance profiles of U were measured between two half-cells of soil, one of which initially contained U and the other not, giving rates of desorption in the source and adsorption in the sink cell. The effects of typical root-induced changes in soil pH and CO₂ pressure were measured. Two models were compared: (a) an analytical solution of the appropriate diffusion equation with a constant diffusion coefficient, and (b) a numerical solution allowing for time- and concentration-dependent diffusion. The model parameters were measured or otherwise estimated independent of the concentration-distance profiles. The simple analytical solution correctly accounted for the effects of pH and CO₂ pressure on U diffusion, but under-predicted the diffusive flux. The numerical model correctly predicted the flux and concentration-distance profiles, including a discontinuity at the source-sink boundary due to differences in the kinetics of desorption and adsorption. The results show the importance of correctly allowing for the effects of pH, CO₂ pressure and sorption kinetics in modelling U uptake by plant roots. The model should be further corroborated in mesocosm, half-cell and field experiments, and by verifying U speciation.