URANIUM INTERACTIONS WITH DISSOLVED ORGANIC MATTER: IMPLICATIONS FOR MOBILITY IN NATURAL WATER

Despite mounting evidence that dissolved organic matter (DOM) plays an important role in controlling uranium (U) mobility in natural waters, the mechanisms and extent of U-DOM binding remain relatively unconstrained, mainly due to the complex chemical composition of DOM. Organic matter consists of a collection of molecules with a range of molecular weights and contains a diversity of functional groups with variable reactivity toward metal binding. Through a series of laboratory studies, we investigated the nature of U(VI) and U(IV) binding to natural isolates of DOM, to understand how this interaction affects aqueous speciation and sorption onto iron oxides, an important sink for U in natural media. Uranyl binding (represented by conditional equilibrium constants) to seven hydrophobic organic acid fractions with variable SUVA and chemical composition and one unfractionated organic-rich porewater was measured using a modified equilibrium dialysis ligand exchange method. The effect of pH, organic aromaticity (as SUVA), and calcium concentrations were also evaluated, providing evidence for U(VI) binding through carboxyl, alcohol, and/or phenolic functional groups. Using conditional constants derived from experiments, a geochemical speciation model was developed, demonstrating that U(VI)-DOM interactions are strong and can dominate U aqueous speciation in conditions relevant to DOM-rich natural waters. In a separate set of experiments, aqueous U increased when U(IV) as biogenic uraninite (UO₂) was exposed to one isolate of DOM under anaerobic conditions, suggesting that U(IV) complexation and/or oxidation by DOM in the absence of oxygen may be an important mechanism controlling U mobility. Organic matter interactions not only affect aqueous complexation and mobility, but DOM was found to decrease U(VI) adsorption onto ferrihydrite both by competition for aqueous U and by altering the solid surface. This work provides a systematic evaluation of U-DOM binding that will inform geochemical modeling efforts, field-scale studies of natural waters, and bioavailability studies.