Organic Matter Can Change Uranium Sorption Kinetics
Source: Ruth Tinnacher and Dan Hawkes
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.
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.
To read more, go to: http://pubs.acs.org/doi/full/10.1021/es304677c
Funding Source: BER SS SFA, NSF