Date of Award

8-2018

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Environmental Engineering and Earth Sciences

Committee Member

Brian A. Powell, Committee Chair

Committee Member

Mavrik Zavarin

Committee Member

Mark A. Schlautman

Committee Member

Elizabeth Carraway

Committee Member

Timothy A. DeVol

Abstract

This work investigated aging of plutonium (Pu) on the surface of goethite, the effect of natural organic ligands on the enhancement of Pu desorption and mobility, and Pu transport in a natural soil. Because adsorption of Pu is frequently observed to be initially rapid followed by slower adsorption over days to weeks, Pu may undergo aging, where a surface process following initial adsorption causes a change in Pu state over time. Pu uptake which exhibits fast and slow sorption phases has been successfully modeled using a multisite model where sorbed Pu is represented as two compartments with different rates of exchange with solution, and I propose in this work that it may be modeled using consecutive compartments representing Pu states with different accessibility to the solution. Additionally, the fate and transport of Pu can be affected by the presence of natural organic ligands which may diminish Pu sorption to mineral surfaces by forming Pu–aqueous complexes or enhance Pu sorption by forming ternary surface complexes. Numerous previous studies have examined the extent of Pu sorption to pure minerals in batch experiments. However, the effect of natural organic ligands on Pu sorption kinetics and transport in natural soils requires further investigation.

The aging of Pu on goethite was observed by using batch experiments with adsorption times of 1–116 days. To assess the stability of Pu(IV) on the goethite surface, desorption was performed in the presence of desferrioxamine B (DFOB) which acted as a competing complexant and also thermodynamically drove aqueous Pu(V) to form Pu(IV)–DFOB aqueous complexes. Thus, the surface stability of Pu(IV) was measured in a system with pure oxidation state. Pu was observed to become less desorbable with increasing adsorption times, and the effect of Pu aging was greater at lower pH values. The results were not consistent with surface mediated reduction as a process of aging, but demonstrated that Pu aging could be sensitive to pH.

The kinetics of Pu (de)sorption from goethite in the presence of DFOB, fulvic acid (Suwanee River I), and citric acid were investigated using a flow-cell design which minimized readsorption of Pu by constant dilution. Additional kinetic information was obtained by conducting stop events to vary the fluid residence time. Pu became less desorbable over three days, indicating Pu underwent aging on goethite. The Pu data was modeled using a kinetic sorption model which considered two consecutive linear sorption components. In this theoretical conceptual model, the transition to a second surface state represented aging, but the specific process was unknown. The presence of DFOB had a greater effect on Pu desorption than either citric acid or fulvic acid. Although further study is required to determine Pu–(fulvic acid) stability constants, the results are consistent with the relative stability constants for Pu–ligand aqueous complexes.

The effect of DFOB and citrate on Pu mobility was examined in column transport experiments with natural soil from the Savannah River Site. Pu was loaded onto the columns in organic-free solution, followed by a continuous input of organic ligand solution to mobilize the Pu. Due to the strong sorption of DFOB and citrate to the soil, their presence was estimated to be limited to the first 1.32 and 0.68 cm of the columns, respectively. The formation of aqueous Pu–ligand complexes did not appear to affect the Pu soil distribution, However, in each of the columns, including the ligand-free control, 9–20 % of Pu was observed to elute with retardation factors of 24–29, suggesting that Pu was mobilized by a process the columns had in common. Multiple working hypotheses to explain these data are (1) oxidation of Pu(IV) to Pu(V) within the loading solution during pH adjustment, (2) mobilization by of Pu–ligand complexes with native soil organic matter, and (3) colloid-facilitated transport.

With increased aging, Pu mobility in the subsurface may decrease if Pu is sorbed to an immobile matrix, or Pu mobility may increase if Pu is sorbed to mobile colloids. Because this work used indirect measurements of aqueous Pu to observe the effects of aging, further research is required to determine the process behind Pu aging. Flow-cell experiments demonstrated that the presence of natural organic ligands has the potential to mobilize Pu in the subsurface; however, in natural soils, further research is required to understand the enhancement of Pu mobility in the presence of natural organic ligands. Pu mobility in the subsurface environment depends on many complex interactions potentially involving contact time, soil composition, Pu redox reactions, ligand sorption, soil organic matter, and mineral colloids.

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