Date of Award
Doctor of Philosophy (PhD)
Environmental Engineering and Earth Science
Brian A. Powell
Lawrence C. Murdoch
Julia L. Brumaghim
The formation of uranyl phosphate precipitate is a remediation strategy because the low solubility of uranyl phosphate minerals, like chernikovite, limits the mobility of uranium in contaminated soils. However, organic ligands can complex with aqueous metal cations to form more soluble species. For example, citrate is a commonly occurring organic ligand produced by plants and microbes that increases the solubility of uranium and therefore the dissolution of uranyl phosphate minerals in the uranyl phosphate-citrate system. This effect is an important control on the mobility of uranium in organic-rich, and near-surface vegetated environments. Nevertheless, key aspects of the citrate-uranyl phosphate system remain poorly understood, and this limits the ability to assess risks of exposure and strategies for remediating uranium contaminated soils.
The goals of this research are to determine the mechanism, extent, and rate of citrate-promoted dissolution of uranyl phosphate and evaluate how ligand-promoted dissolution and solid-phase transformations of uranyl phosphate affect macro-scale uranium transport. Batch dissolution, continuously stirred tank reactor (CSTR), soil column, and field lysimeter experiments were conducted to span across spatial scales ranging from Ångstrom to the meter scale.
The results from all experiments indicate that the concentration of uranium dissolved from a chernikovite source increases with the concentration of citrate. However, this study determined that the rate of increase in uranium concentration diminishes at higher citrate concentrations and longer residence times and provided evidence of a uranyl-citrate alteration layer on the surfaces of uranyl phosphate grains after citrate exposure. These findings suggest that a combination of secondary-phase precipitation and ligand surface saturation hinder the release of uranium into solution. In the presence of soil, cations from the soil compete with uranium from the chernikovite to form citrate-complexes, slowing the dissolution of chernikovite. Soil cations, like potassium and calcium, can also integrate into the uranyl phosphate structure, altering the original chernikovite to a less soluble uranyl phosphate phase that is more resistant to citrate-promoted dissolution at lower citrate concentrations. The findings presented in this work show that although citrate promotes the dissolution of uranyl phosphate, other mechanisms hinder the release of uranium in the environment from a uranyl phosphate source.
Ferguson, Brennan, "Ligand-Promoted Dissolution of Uranyl Phosphate Across Scales" (2022). All Dissertations. 3207.