Dissolved organic matter (DOM) plays an important role in controlling metal mobility in surface and ground water due to metal-organic matter aqueous complexation, competitive adsorption on mineral surfaces, and participation in abiotic and biotic redox reactions.  Uranium interactions with DOM remains relatively unknown, even though it is clear that U-DOM binding affects mobility of U in natural waters.  We investigated the nature of U(VI) and U(IV) binding to a variety of hydrophobic organic acid (HPOA) isolates of DOM from aquatic sources with a range of chemical composition and specific UV absorbance (SUVA) to understand how U-DOM complexation affects aqueous concentrations, speciation, and adsorption onto amorphous iron oxide.  Uranyl binding to DOM was quantified using conditional equilibrium constants derived from a modified equilibrium dialysis ligand exchange method. The effects of pH, organic aromaticity (as SUVA), and calcium concentrations were evaluated. Using conditional constants derived from experiments, geochemical aqueous speciation was calculated using PHREEQC, demonstrating that U(VI)-DOM interactions are strong and can dominate U aqueous speciation in conditions relevant to DOM-rich natural waters.  Enhanced dissolution and mobilization of dissolved U was observed when various DOM isolates were equilibrated with biogenic uraninite (UO<sub>2</sub>) under anaerobic conditions, suggesting that dissolution of solid U(IV) phases may be increased in the presence of DOM even in the absence of oxygen.  Additional experiments tested the effect of DOM on the adsorption of U(VI) onto ferrihydrite.  Organic matter substantially decreased the adsorption of U(VI) by competing for aqueous U and by occupying surface sites for adsorption.   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.<br/>

Complexation with dissolved organic matter affects aqueous uranium speciation and adsorption

253rd National Meeting (2017)

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