INVESTIGATING THE STABILITY OF PLUTONIUM ADSORBED TO MINERAL SURFACES

In part due to public health concerns over the risk of Pu contamination of drinking water, predicting the behavior of Pu in both surface and sub-surface water is a topic of continued interest. Recent work into the importance of colloid facilitated Pu transport has led to renewed study into the adsorption of Pu by environmentally relevant minerals. However of key significance in the subsurface transport of Pu is the desorption behavior of Pu from mineral surfaces. In the present study we examine the stability of Pu(IV) adsorbed on colloidal-sized particles of the clay mineral montmorillonite. In the first instance we report on flow-cell desorption experiments with Pu(IV) at an initial concentration of 1 × 10^-10 M adsorbed onto the surface of the clay mineral montmorillonite in background electrolyte with pH values of 4, 6 and 8. Flow rates were varied in order to investigate the kinetics of desorption and hence gain a mechanistic understanding of the desorption processes. These high concentration experiments indicated that Pu desorption is dependent on pH. Informed by these results, a second set of experiments was performed with initial Pu concentrations of 5 × 10^-13 M at pH values of 6 and 10. These experiments were designed to complement previous adsorption studies in which we had tested whether the interaction of Pu with mineral surfaces at concentrations used in typical laboratory experiments (≥ 10^-10 M) was the same as that observed for typical field concentrations (< 10^-12 M). By utilizing accelerator mass spectrometry, the exceedingly low concentrations of Pu desorbed could be measured. These measurements indicated that Pu desorption rates were very similar despite the 200 fold lower starting concentration. We previously demonstrated that equilibrium Pu adsorption to montmorillonite is independent of concentration over a ten order of magnitude concentration range. Here, we demonstrate that the rate of Pu desorption from montmorillonite appears to be independent of concentration as well.

LLNL-ABS-641963

This work was funded by U. S. DOE Office of Biological & Environmental Sciences, Subsurface Biogeochemistry Research Program, and performed under the auspices of the U. S. Department of Energy by Lawrence Livermore National Security, LLC under Contract DE-AC52-07NA27344.