Organic-rich anoxic sediments accumulate uranium globally in marine sediments, ore bodies, and in shallow contaminated near-surface environments. Natural organic matter is increasingly being recognized as playing multiple and important roles mediating uranium behavior in these environments. To better understand the roles of organic-enriched sediments at the regional scale of the upper Colorado River Basin, we sampled multiple field sites along a 700 km north-south transect in New Mexico, Colorado, and Wyoming, USA. This work shows that organic-enriched sediments are prevalent, strongly accumulate uranium, and moreover are important reservoirs for nutrients and biogeochemical critical elements, including C, N, S, and Fe. Sulfate reducing conditions appear to be particularly important for uranium accumulation, suggesting at least a strong indirect control of sulfide on U(VI) reduction. Indeed, the nominal oxidation state of water-soluble soil organic carbon was found to be correlated to sulfide concentration, suggesting that sulfide plays a major role in poising the redox conditions of organic-enriched sediments across this region.<br/><br/>To better understand molecular controls over uranium behavior in NRZs, we performed controlled microcosm experiments designed to mimic sulfate reducing conditions in NRZs and to more clearly define the potential roles of organic functional groups as uranium binding sites. U(IV) was found to be dominantly associated with surfaces of particulate organic carbon and to exhibit local molecular structure consistent with sorbed complexes. Moreover, we show that solid-phase U(IV) formation can be inhibited under groundwater-relevant conditions by decreasing the total concentration of U, while maintaining the same concentration of sorbent. Thus, the number and type of surface ligands can infleunce U(IV) speciation subsequent to U(VI) reduction. An important implication of this finding is that U(IV) will be readily mobilized in the presence of complexing agents and oxidants. Such conditions are common in NRZ sediments, which experience large variations in saturation state and redox conditions throughout the annual winter (base flow) / summer (meltwater discharge) cycle within the upper CRB. In ensemble, these studies suggest more nuanced conceptual models that help to explain uranium biogeochemical behavior in the present and through geological time.

Molecular controls over uranium mobility in complex redox-active sediment systems

253rd National Meeting (2017)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.