Effect of Competing Metals and Humic Substances on Uranium Mobilization from Noncrystalline U(IV) Induced by Anthropogenic and Biogenic Ligands

Anthropogenic and biogenic ligands may mobilize uranium (U) from tetravalent U (U-(IV)) phases in the subsurface, especially from labile noncrystalline U-(IV). The rate and extent of U-(IV) mobilization are affected by geochemical processes. Competing metals and humic substances may play a decisive role in U mobilization by anthropogenic and biogenic ligands. A structurally diverse set of anthropogenic and biogenic ligands was selected for assessing the effect of the aforementioned processes on U mobilization from noncrystalline U-(IV), including 2,6-pyridinedicarboxylic acid (DPA), citrate, N,N ' -di-(2-hydroxybenzyl)-ethylene-diamine-N,N ' -diacetic acid (HBED), and desferrioxamine B (DFOB). All experiments were performed under anoxic conditions at pH 7.0. The effect of competing metals (Ca, Fe-(III), and Zn) on ligand-induced U mobilization depended on the particular metal-ligand combination ranging from nearly complete U mobilization inhibition (e.g., Ca-citrate) to no apparent inhibitory effects or acceleration of U mobilization (e.g., Fe-(III)-citrate). Humic substances (Suwannee River humic acid and fulvic acid) were tested across a range of concentrations either separately or combined with the aforementioned ligands. Humic substances alone mobilized appreciable U and also enhanced U mobilization in the presence of anthropogenic or biogenic ligands. These findings illustrate the complex influence of competing metals and humic substances on U mobilization by anthropogenic and biogenic ligands in the environment.

Automated summary

Anthropogenic and biogenic ligands can mobilize uranium from noncrystalline U-(IV) phases, and the rate and extent of mobilization are influenced by geochemical processes. Competing metals and humic substances play a crucial role in the mobilization of uranium by these ligands.