Hexavalent Uranium Diffusion into Soils from Concentrated Acidic and Alkaline Solutions

Abstract

Uranium contamination of soils and sediments often originates from acidic or alkaline waste sources, with diffusion being a major transport mechanism. Measurements of U(VI) diffusion from initially pH 2 and pH 11 solutions into a slightly alkaline Altamont soil and a neutral Oak Ridge soil were obtained through monitoring uptake from boundary reservoirs and from U concentration profiles within soil columns. The soils provided pH buffering, resulting in diffusion at nearly constant pH. Micro x-ray absorption near edge structure spectra confirmed that U remained in U(VI) forms in all soils. Time trends of U(VI) depletion from reservoirs, and U(VI) concentration profiles within soil columns yielded K d values consistent with those determined in batch tests at similar concentrations (≈ 1 mM), and much lower than values for sorption at much lower concentrations (nM to µM).These results show that U(VI) transport at high concentrations can be relatively fast at non-neutral pH, with negligible surface diffusion, because of weak sorption.

Uranium(VI) diffusion in low-permeability subsurface materials

“…Understanding uranium(VI) diffusion in low permeability materials is important for assessing the environmental risks in radionuclide waste disposal [1][2][3], describing radionuclide reactive transport in subsurface sediments [4][5][6][7]; and evaluating remediation approaches [8,9]. Uranium diffusion in low permeability materials is commonly controlled by adsorption and molecular diffusion processes [2,3,6].…”

X-ray absorption spectroscopy identifies calcium-uranyl-carbonate complexes at environmental concentrations

“…Nevertheless, the stoichiometry of the Ca-UO 2 -CO 3 complex is important, as the net charge of the complex can change from negative CaUO 2 ðCO 3 Þ 3 2À to neutral Ca 2 UO 2 ðCO 3 Þ 3 0 . This change in the aqueous speciation of uranium(VI) in groundwater could affect the fate and transport properties of uranium and on the effectiveness of bioremediation strategies (Brooks et al, 2003;Zheng et al, 2003;Davis et al, 2004;Tokunaga et al, 2004). Presented here are uranium L III -edge EXAFS measurements on five solutions containing environmentally relevant concentrations of uranium(VI) and bicarbonate, with various calcium concentrations.…”

Investigation of Reactive Transport and Coupled THMC Processes in the EBS

“…Hence, overall, the pore Last, for reactive solutes, such as uranium, metal interactions with the mineral surface in the pore environment have to be expected over the course of diffusion-dominated metal transport. As a result, some researchers have proposed the use of some form of an apparent diffusion coefficient (Maes et al, 2002;Tokunaga et al, 2004), which further extends the effective diffusion coefficient to directly include metal sorption or ion exchange reactions. The apparent diffusion coefficient ( a D ) represents the overall macroscopic diffusion rate, and is included in Fick's second law of diffusion describing the solute concentration change in a (porous) volume element under transient conditions.…”