Sorption to Organic Matter Controls Uranium Mobility

U-NOM
U(IV) is complexed by organics, including mineral coatings.

Bone SE, Dynes JJ, Cliff J, & Bargar JR “Uranium(IV) adsorption by natural organic matter in sediments.” Proceedings of the National Academy of Sciences of the United States of America 114(4), 711-716. [10.1073/pnas.1611918114]

http://www.pnas.org/content/114/4/711.abstract

A new multi-technique study using X-ray absorption spectroscopy at SSRL, nanoscale secondary Ion mass spectroscopy at EMSL, and scanning transmission X-ray microscopy at the CLS has revealed crisp new details about the mechanisms of uranium binding in sediments.  Surfaces of natural organic matter bind uranium more strongly than minerals under field-relevant conditions.

Anoxic sediments, which are common in alluvial aquifers, are important concentrators of uranium, where it accumulates in the tetravalent state, U(IV). Uranium-laden sediments pose risks as ‘secondary sources’ from which uranium can be re-released to aquifers, prolonging its’s impact on local water supplies. In spite of its importance, little is known about the speciation of U(IV) in these geochemical environments. Uranium analysis is challenged by its low concentrations and the tremendous chemical and physical complexity of natural sediments.  U(IV) binds to both organic matter and minerals, which can be co-associated at the scale of 10s to 100s of nanometers.  Because of the multiplicity and similarity of binding sites present in samples, ‘standby’ analytical techniques such as X-ray absorption spectroscopy are challenged to distinguish the molecular structure of U(IV) in these natural sediments. The molecular nature of accumulated U(IV) is however a first-order question, as the susceptibility of uranium to oxidative mobilization is mediated by its structure.   

NanoSIMS_Bone2017
NanoSIMS images of cells colonizing detrital organic matter; Uranium binds to cells and interstitial organics.

In an SSRL-based study, Bone et al (2017) overcame these challenges by combining X-ray absorption spectroscopy, Nano-secondary Ion Mass Spectroscopy (NanoSIMS), and STXM measurements to characterize the local structure and nanoscale localization of uranium and the character of organic functional groups.  This work showed that complexes of U(IV) adsorb on organic carbon and organic carbon-coated clays in an organic-rich natural substrate under field-relevant conditions. Furthermore, whereas previous research assumed that U(IV) speciation is dictated by the mode of reduction (i.e., whether reduction is mediated by microbes or by inorganic reductants), this work demonstrated that precipitation of U(IV) minerals, such as UO2, can be inhibited simply by decreasing the total concentration of U, while maintaining the same concentration of sorbent. This conclusion is significant because UO2 (uraninite) and other minerals are much more stable and more readily remobilized than surface-complexed forms of U(IV). Thus, the number and type of organic and mineral surface binding sites that are available have a profound influence on U(IV) behavior. Projections of U transport and bioavailability, and thus its threat to human and ecosystem health, must consider U(IV) adsorption to organic matter within the local sediment environment. 

Uranium is less stable and more easily remobilized when bound to surfaces of organic matter and mineral as compared to being incorporated with mineral precipitates.  This new finding implies that reduced uranium is much more reactive and able to participate in repeated biogeochemical cycling than previously thought to be the case.