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Uranium Fate and Transport Mechanisms , <br />The main uranium fate and transport mechanisms include the following: <br />¦ Adsorption/ desorption <br />¦ Precipitation/ dissolution as pure-phase uranium minerals <br />¦ Precipitation/ dissolution as a trace constituent in non-uranium minerals (i.e., <br />solid-solution) <br />Adsorption/Desorption <br />Adsorption/ desorption are important chemical processes that are relevant to the <br />discussion of uranium mobility within an aquifer. Adsorption is the process where an <br />ion in solution adheres to the surfaces of solid-phase soil/sediment grains in contact <br />with groundwater. Adsorbed substances do not move with groundwater flow. <br />Desorption is the reverse process, where adsorbed ions detach from soil or sediment, <br />become re-dissolved into solution, and are then free to move with groundwater flow. <br />Dissolved species within groundwater adsorb to the aquifer materials until the aquifer <br />sediments and groundwater reach a state of equilibrium. The equilibrium is often <br />expressed using a constant called the Kd. Kd for uranium is defined as follows: <br />Kd (L/kg) = Concentration of U in the soil (mg/kg)/Concentration of U in solution <br />(mg/L) <br />Kd values have been determined and reported in the literature for many elements and <br />species under a range of conditions. Once an appropriate Kd value is known, the <br />equilibrium concentration of the element can be calculated for the soil if the solution <br />concentration is known and vice-versa (assuming adsorption is the dominant fate and <br />transport mechanism). Adsorption can be an important process for removing uranium <br />from solution; however, the capacity for soils to adsorb uranium is limited. Once the <br />surface sites for adsorption are saturated, adsorption stops and the uranium can <br />migrate with the groundwater until soils with adsorption capacity are encountered. <br />Adsorption is an important process for U(VI), particularly when pure phase and <br />solid-solution phases are undersaturated. Davis calculated the Kd values for the <br />Hanford Site ranging from 0.5 L/kg to 28 L/kg depending on the type of soil (Davis, et <br />a12005). U(VI) Kd values depend on the following variables: <br />¦ Soil Type (fine-grained soils have higher Kd values due to the larger number of <br />surface sites) <br />¦ pH (in general, low pH tends to promote anion adsorption such as the uranyl <br />carbonates, while higher pH tends to promote cation adsorption, such as the <br />uranyl hyroxides) <br />¦ Carbon dioxide partial pressure (high COz forms uranyl carbonate complexes <br />which tend to lower the Kd value) <br />¦ Presence of complexing ions, such as those of phosphate and organics <br />(complexing ions can raise or lower the Kd by changing the charge on the <br />adsorbing species) • <br />¦ The Eh conditions of the system (lower Eh values result in formation of U(IV), <br />which is less mobile than U(VI)).