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Organic Matter Can Change Uranium Sorption Kinetics

Source: Ruth Tinnacher and Dan Hawkes

Ruth Tinnacher performing batch experiments in an anaerobic chamber.
In the U.S., numerous sites have been contaminated with uranium as a result of uranium mining and milling operations and the processing of nuclear fuels. Furthermore, with uranium making up approximately 96% of spent nuclear fuel, it is an important element to be considered in environmental risk assessments for nuclear waste repositories. The spreading or transport of uranium in the subsurface is largely driven by how strongly it interacts with immobile mineral phases (as opposed to being dissolved in the mobile pore water found between mineral grains). The more time it spends associated with immobile mineral phases, the more slowly the uranium contamination spreads. The ways in which uranium interacts with solid surfaces is complex, but is often discussed using the general term “sorption.”  “Natural organic matter,” which includes all plant-derived and microbially derived organic compounds, is ubiquitous in soils and waters, and has been shown to affect uranium sorption and mobility in the environment. Up to this point, it was largely unknown if organic matter could influence uranium sorption kinetics, i.e., how quickly uranium sorption onto mineral surfaces can occur. However, such knowledge is important for accurate predictions of uranium transport. For example, equilibrium sorption parameters (e.g., Kd values), which are important input parameters for transport models, are often determined in lab-scale experiments in the absence and presence of organic matter. If organic matter changes metal sorption kinetics, then sorption times need to be adjusted to ensure full-system equilibration in experiments, and to minimize errors in subsequent transport models. 

As described in a paper published earlier this year (2013) in Environmental Science & Technology, ESD’s Ruth Tinnacher recently led a team of geochemists (including ESD’s Peter Nico and Jim Davis, as well as Bruce Honeyman from the Colorado School of Mines), in investigating the effects of fulvic acid, an organic matter fraction, on uranium(VI) sorption kinetics. For this purpose, they performed batch sorption experiments to characterize uranium(VI) and fulvic acid sorption onto a pretreated silica sand as a function of pH, fulvic acid concentrations, and time. Potential changes in uranium(VI) or fulvic acid sorption kinetics were determined based on the calculation of characteristic times for overall sorption reactions. The relevance of uranium(VI)-fulvic acid solution complexes under various experimental conditions was evaluated by simulating uranium(VI) speciation. Overall, they found that uranium(VI) sorption onto silica sand can be either slower or faster in the presence of fulvic acid compared to an organic matter-free system. This suggests a shift in the underlying mechanisms of fulvic acid effects on U(VI) sorption in these systems, from competitive sorption to influences of uranium(VI)-fulvic acid complexes, formed in solution and/or on the surface. Changes in uranium sorption rates depend on the relative concentrations of uranium, organic matter, and mineral surface sites.

The graph above shows characteristic times for the association of uranium with mineral surfaces. Note that the presence of small amounts of organic matter slows down this rate, while the subsequent addition of more organic matter speeds up this rate.

These results provide important guidance for selecting appropriate experimental sorption equilibration times to determine uranium distribution coefficients (Kd values) under equilibrium conditions.  Furthermore, they demonstrate how the underlying mechanisms of organic matter effects on uranium sorption can change, depending on the concentrations of metals, organic ligands, and mineral surface sites.  Motivated by these results, future research will focus on other factors potentially influencing uranium sorption kinetics, such as pH and the presence of carbonate or other inorganic ligands. The ultimate goal of these investigations is to improve the accuracy with which computer models can predict the behavior of uranium contamination, and therefore improve our ability to make efficient and effective decisions regarding the management of uranium-contaminated sites.

Citation: Tinnacher, R., P.S. Nico, J.A. Davis, and B.D. Honeyman (2013), Effects of fulvic acid on uranium(VI) sorption kinetics. Environmental Science & Technology, 47 (12), 6214–6222; DOI: 10.1021/es304677c.

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Funding Source: BER SS SFA, NSF