| Understanding the geochemical mobility of U(VI) and modeling its transport is important in several contexts including ore genesis, uranium exploration, nuclear and mill-tailings waste management, and solution mining of uranium ores. Adsorption is a major control on partitioning of solutes at the mineral/solution interface. Understanding the adsorption behavior of U(VI) is essential for evaluating the mobility of uranium and in modeling its transport in most natural systems.; Goethite commonly occurs in many soils, sediments, and oxidized aquifers. The adsorption of U(VI) onto synthetic geothite has been studied in batch experiments at room temperature in aqueous solutions as a function of pH (5-9.5), ionic strength (0.1-0.7 M NaNO(,3)), (SIGMA)U(VI) (20 ppb - 5 ppm), and adsorbate/adsorbent ratio. The effect of carbonate, fluoride, and phosphate complexing on adsorption of uranium has been investigated. A critical compilation of stability constants of inorganic complexes and solid compounds of U(VI) necessary for proper design of experiment and for modeling transport of uranium has been prepared.; The general features of U(VI) adsorption in ligand-free systems are similar to those characteristic of other hydrolyzable metal ions. The adsorption processes under study were found to be reversible. Adsorption of predominant polymeric uranyl hydroxo complexes cannot be used to model the adsorption edge. However, a single reaction, namely the formation of UO(,2)(OH)(,3)('-) on positive surface sites, was found adequate to describe adsorption of U(VI) under all ligand-free conditions except at (SIGMA)U(VI) > 250 ppb (10('-6) M). Measurement and correction for loss of uranium due to sorption on container walls proved to be important, especially for (SIGMA)U(VI) (LESSTHEQ) 100 ppb.; The adsorption processes were found to be reversible in the presence of CO(,3)('2-) and F('-) as well. The formation of UO(,2)(CO(,3))(,2)('2-) and UO(,2)(CO(,3))(,3)('4-) caused a substantial decrease in uranium adsorption densities, however, (UO(,2))(,2)CO(,3)(OH)(,3)('-) formation did not suppress uranium adsorption. The presence of fluoride caused a slight decrease in uranium adsorption. The adsorption model developed in ligand-free systems, when solution complexing is taken into account, proved remarkably successful in describing adsorption of uranium in the presence of carbonate and fluoride. The presence of phosphate caused a much smaller decrease in the extent of adsorption than expected; however, a critical reassessment of the stability of UO(,2)('2+)-HPO(,4)('2-) complexes, showed that phosphato complexes, if any, are extremely weak under experimental conditions. Removal of uranium may have occurred due to precipitation of sodium uranyl phosphates in addition to adsorption. |
