Sorption Dynamics of Uranium onto Anion Exchangers

Uranium can occur naturally in groundwater which is used for drinking water production. Depending on its concentration levels, uranium elimination might become necessary. In German waterworks, anion exchange technology represents the state of the art for selective uranium removal. Operation times usually vary between one and two years until the exchanger is exhausted. In order to study uranium removal by anion exchange on a scientific base, column experiments at the pilot scale were performed in several waterworks. The resin with the highest capacity for uranium showed operation times between 120,000 and > 300,000 bed volumes until breakthrough occurred, strongly depending on the water composition. To forecast uranium breakthrough on a theoretical base, a computer program was established using the model of combined film and surface diffusion. Both equilibrium data and kinetic parameters necessary for applying the model had been determined in previous research work. Modelled breakthrough curves were compared to experimental data from lab scale column experiments. As a rule, the time-dependency of the column effluent concentration can be well predicted by the theoretical model. By modelling the sorption dynamics, diffusion through the liquid film was identified as the rate controlling transport step. By increasing the filter velocity, the thickness of the liquid film decreases and the diffusion in the liquid accelerates. As a consequence for treatment plants in waterworks, the filter velocity can be increased by optimising the filter geometry. A smaller filter diameter is more appropriate for efficient uranium adsorption and longer times of operation might be achieved.