Citric Acid Enhanced Remediation of Soils Contaminated with Uranium by Soil Flushing and Soil Washing

Laboratory bench-scale soil washing (batch) and flushing (column flow) experiments were conducted to determine the efficiency of citric acid as an agent to extract uranium from a synthetically contaminated sandy soil. The results of soil washing and flushing experiments indicate that citric acid is highly effective in removing uranium, and that the extraction efficiency increases with increasing citric acid concentration, especially under slightly acidic to alkaline conditions in systems containing sand coated with secondary minerals (e.g., Fe). The enhanced U(VI) desorption in the presence of citrate may be explained through several processes, including the complexation of U (VI) with citrate and extraction of secondary coatings (e.g., Fe), which results in the liberation of Fe-citrate complexes into solution. In batch washing systems, the presence of 10^–3 M citric acid enhances the extraction of uranium 2.8 times greater than water alone for the conditions of the experiment. A comparison of soil washing and flushing shows that the extraction efficiency is higher in bench-scale washing experiments. A removal efficiency of up to 98% was achieved with 10 mL of 10^–3 M citric acid in batch systems, whereas it required 4 pore volumes (150 mL) of 0.1 M citric acid to accomplish similar extraction efficiencies in column soil flushing systems.