Influence of contact time on the extraction of 233 uranyl spike and contaminant uranium from Hanford Site sediment

The development of effective remediation strategies requires a comprehensive understanding of contaminant behavior in the environment. At the Hanford Site, located in southeastern Washington State (United States of America), seasonal fluctuations of the nearby Columbia River cause flushing of the 300 Area uranium (U) plume in the lower vadose zone. The variation of water chemistry alternately promotes adsorption and desorption of U from sediment. Therefore, the following question arises: what is the mobility of U in groundwater that has only recently became associated with sediment relative to U that has been associated with the sediment for decades? Geochemical transformations, including surface complexation, precipitation, and/or physical processes will impact U speciation as the contact time with sediment increases. To investigate this question, dissolved (233)uranyl nitrate [UO2(NO3)(2)] was added to U-contaminated Hanford Site sediment and incubated for up to 1 year. Following 1-week, 1-month, and 1-year incubation periods, the extraction of U from the sediment was accomplished using either batch or continuous leach techniques, and multiple extractants. The elution of U-233 during continuous leaching was influenced by the incubation period. The change in the U-235/U-233 ratio eluted was indicative of the extraction of different U phases, and was a function of the incubation period. Removal of U-233 by batch extraction clearly showed the effects of the incubation period and extractant. The extractability of the U-233 spike by some extractants is independent of incubation period (up to 1 year) suggesting it is present as either sorbed or surface precipitate phases. Model simulation of the data provides insight into the processes involved with the extraction of U from the sediment.