Describing plutonium contamination issues in Hanford soils: Development of a thermodynamic surface complexation model

The development of remediation strategies for long-term site management requires knowledge of an actinide's geochemical behavior. Understanding this behavior can lead to the formation of a subsurface transport model. For example, plutonium mobility in the subsurface environment is significantly influenced by oxidation-reduction and complexation reactions. This work considered the surface-mediated reduction of plutonium, as well as the hydrolysis and carbonate complexation of the actinide.;Evaluating the significance of these reactions required several variable pH batch sorption studies. Experiments incorporated plutonium and neptunium sorption to sediments from the Hanford Nuclear Reservation in Washington State. Two different sediments were examined: coarse-grained and fine-grained. Batch sorption experiments utilized either the pristine or acid-leached forms of these soils. Suspensions contained actinide concentrations of either 1 x 10-9 M or 1 x 10-8 M and sediment concentrations of either 25 g/L or 100 g/L. Sorption profiles were developed for these initially Pu(V) and Np(V) systems. Typically, the fraction sorbed increased with pH. The stronger sorption of plutonium relative to the Np(V) systems suggested the surface-mediated reduction of Pu(V) to Pu(IV).;A component additivity model was developed using mineralogical characterization results and a FITEQL-based modeling program. The FIT4FD model used during this project predicted sorption onto a mineral assemblage by the summation of sorption to each specific sorbent. The target solid phases included gibbsite (AlOOH), silica (SiO2), and goethite (FeOOH). Surface complexation constants for these phases were calculated or taken from available literature. Surface site concentrations were varied in order to fit the batch sorption data.;In the best fit models, speciation was generally controlled by the strongly hydrolyzed Pu(OH)x4-x species, not the weakly complexing PuO2+ ion. The models attempted to predict plutonium oxidation speciation. As expected, Pu(IV) dominated the solid phase, while Pu(V) dominated the aqueous phase. Silica and gibbsite reactive fractions remained at 0.1%, and goethite reactive fractions ranged from 0.02% to 2.0% for the coarse-grained sediment models. Alternatively, silica and gibbsite reactive fractions ranged from 0.26%% to 2.6%, and goethite reactive fractions ranged from 0.0032% to 3.2% for the fine-grained sediment models.