Phosphate amendments can be added to U(VI)-contaminated subsurface environments to promote in situ remediation. The transport of U(VI) in phosphate-treated systems can be controlled by non-equilibrium processes involving advective and diffusive transport and adsorption-precipitation or desorption-dissolution reactions. The primary objective of this study was to evaluate the impacts of phosphate addition on the transport of U(VI) in contaminated sediments. Sediments (< 2 mm size fraction) from sites in Rifle, Colorado and Hanford, Washington, were used in column experiments with flow rates that corresponded to a typical groundwater velocity of 1 m/day. First, columns were loaded with uranium-containing influent until uranium breakthrough occurred. Following this loading stage, uranium-free influents were introduced with and without phosphate.  In the absence of phosphate, all of the U(VI) was desorbed from the sediments within two weeks. In contrast, a significant amount of the adsorbed uranium was retained in the sediments when phosphate was present in the uranium-free influent. In a separate set of experiments with an influent containing both uranium and phosphate, significant continued uptake of uranium was observed.After different durations of uranium loading and phosphate treatment, sediments were retrieved at various depths within columns and characterized using laser induced fluorescence spectroscopy, X-ray absorption spectroscopy, electron and X-ray microprobe analysis, and sequential extractions. The results provided insights into uranium-phosphate reactions and the dominant mode of immobilization (i.e. adsorption/precipitation). The response of dissolved uranium concentrations to stopped-flow events indicated that intraparticle diffusion is a rate-limiting step for uranium adsorption-desorption and affected uranium transport in the sediments. Finally, a one dimensional reactive transport model was developed for interpreting the fate and transport of uranium under subsurface environments upon addition of phosphate amendments. The results obtained can be used to design effective remediation strategies for phosphate based in situ remediation.

Column-scale evaluation of in situ uranium immobilization in sediments amended with phosphate

248th National Meeting (2014)

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