Experimental Investigation and Modeling of Uranium (VI) Transport Under Variable Chemical Conditions

Abstract

The transport of adsorbing and complexing metal ions in porous media was investigated with a series of batch and column experiments and with reactive solute transport modeling. Pulses of solutions containing U(VI) were pumped through columns filled with quartz grains, and the breakthrough of U(VI) was studied as a function of variable solution composition (pH, total U(VI) concentration, total fluoride concentration, and p H-buffering capacity). Decreasing p H and the formation of nonadsorbing aqueous complexes with fluoride increased U(VI) mobility. A transport simulation with surface complexation model (SCM) parameters estimated from batch experiments was able to predict U(VI) retardation in the column experiments within 30%. SCM parameters were also estimated directly from transport data, using the results of three column experiments collected at different p H and U(VI) pulse concentrations. SCM formulations of varying complexity (multiple surface types and reaction stoichiometries) were tested to examine the trade-off between model simplicity and goodness of fit to breakthrough. A two-site model (weak-and strong-binding sites) with three surface complexation reactions fit these transport data well. With this reaction set the model was able to predict (1) the effects of fluoride complexation on U(VI) retardation at two different p H values and (2) the effects of temporal variability of p H on U(VI) transport caused by low p H buffering. The results illustrate the utility of the SCM approach in modeling the transport of adsorbing inorganic solutes under variable chemical conditions. coordination of metal ions and decrease the extent of metal adsorption via the formation of weakly or nonadsorbing aqueous complexes. In batch systems the competition can be modeled using geochemical equilibrium models that account for aqueous speciation and surface complex formation, as has been demonstrated for uranium(VI) by Waite et al. [1994]. However, the metal-ligand interactions can be very complex, as increased adsorption of metals in the presence of ligands has also been observed [Davis and Leckie, 1978]. In a study involving Co(II)-EDTA complexes on iron-oxide-containing subsurface sands, Zachara et al. [1995] concluded that Co(II)-EDTA 2-complexes may adsorb as strongly as Co 2+ under certain geochemical conditions. The transport of adsorbing ions has often been modeled with the use of linear isotherms or a distribution coefficient K d to describe the adsorption process [Freeze and Cherry, 1979]. Under constant chemical conditions the adsorption isotherms of transition metal and actinide elements are usually nonlinear and are usually best described by the Freundlich isotherm [Davis and Kent, 1990]. However, under variable chemical conditions the adsorption of these elements is highly dependent on aqueous speciation, and their adsorption is not well described by isotherms or a distribution coefficient Kd. Thus it can be expected that the use of isotherms or Kd will often be inadequate in modeling the transport of these ele...

Uranium Speciation and Bioavailability in Aquatic Systems: An Overview

“…Sorption plays a dominant role in determining the fate of U in freshwater systems. Sorption to clay minerals (e.g., smectite or montmorillonite) below pH 5, and to Fe and Al (oxy)hydroxides, silica, and biotic surfaces, at higher pH, reduces the mobility of U in oxic freshwaters [76,77,78]. Sorption of U to insoluble organic matter, or organic matter attached to particles (e.g., hydrous iron oxides), also reduces the mobility of U [79].…”

Mathematical Formulation Requirements and Specifications for the Process Models

“…Postulating multiple site-types is also important for simulating peak tailing observed in experimental studies of U(VI) transport in columns (Kohler et al, 1996). Reactive transport simulations that use multisite adsorption models can also simulate significant peak tailing in field-scale simulations (Curtis et al, 2006;Kent et al, 2000Kent et al, , 2008Kent et al, , 2007.…”

Environmental Geochemistry of Radioactive Contamination

“…Such models of reactive transport have been developed and demonstrated by a number of researchers including Parkhurst (1995), Lichtner (1996); Bethke (1997); Szecsody et al (1998), Yeh et al (1995Yeh et al ( , 2002, and others reviewed in Lichtner et al (1996), Steefel and Van Cappellen (1998), and Browning and Murphy (2003). Uses of such models to simulate radionuclide transport of uranium in 1-D column experiments are illustrated by Sims et al (1996) and Kohler et al (1996). Glynn (2003) …”