The desorption of uranium and thorium from contaminated soils: The role of geochemistry and distribution coefficients in modeling contaminants leaching

Three contaminated bulk surface soils from St. Louis, Missouri were used for investigation of uranium and thorium desorption. Major geochemical phases with which uranium was associated were the exchangeable phase, a carbonate phase, and the Fe-oxide phase. The Fe-oxide phase and organic matter phase were important for thorium binding, but 50-70% of the total thorium activity appeared to be retained in mineral grains. Activities of uranium and thorium were evenly distributed among different particle size groups. Physical transport of uranium and thorium on colloidal particles leached within and out of the soil profile cannot be neglected in modeling contaminant mobility. A study of solution pH effect on uranium and thorium desorption indicated that at low pH, the predominant species (UO...