The Science

In contaminated aquifers, hydrologic and geochemical conditions can cause tetravalent uranium (U(IV)) – a form of uranium once considered largely immobile – stored in the sediments to be mobilized in the groundwater, and elevate the groundwater uranium concentration above the regulatory limit. However, the mechanisms by which U is released from sediment to solution remain unknown. This work combined nano-scale imaging (nano secondary ion mass spectrometry and scanning transmission X-ray microscopy) with a density-based fractionation approach to physically and microscopically isolate organic and mineral matter from anoxic alluvial sediments contaminated with U (collected from the Riverton, WY site). Previous research applied a combined spectroscopy-microscopy approach to examine U behavior in model systems, which allowed the research team to unambiguously identify U(IV) adsorption, as opposed to precipitation, as the major mechanism of U(IV) retention in aquifer sediments. Through examination of unaltered sediment from the Riverton site, researchers have extended and deepened their analysis, leading to the identification of two distinct populations of complexed U control its behavior in anoxic sediments: (1) U adsorbed to organic matter (including particles rich in both carboxylate and phenolic functional groups derived from both plant and microbial material) and (2) U adsorbed to organic-clay aggregates. This is the first study to demonstrate unambiguously a major role for organic matter as a U(IV) sorbent in unaltered sediments from an alluvial aquifer.

The Impact

Previously, oxidation of U(IV) has been posited as the dominant mechanism by which U is released into groundwater. However, depending on the redox-buffering capacity of the sediment, U(IV) can persist during influxes of oxidants. An additional mechanism of U mobilization from anoxic sediments is therefore needed to explain persistent elevated groundwater concentrations. This work suggests that adsorbed U(IV), whether complexed by organic matter or clay mineral surfaces, could be mobilized by desorption (e.g., by changing pH or alkalinity). Second, this work provides a mechanistic context for colloidal mobilization of U(IV). Researchers speculate that U associated with POC could be mobilized as that POC is transformed into smaller, more oxidized and more soluble units through hydrolytic degradation reactions. Also, disaggregation of organo-mineral aggregates under changing geochemical conditions (pH, ionic strength, redox) causes the release of organic matter into the dissolved and colloidal phase, along with associated metals. Thus, the team concludes that the dominance of organic matter (and clay mineral)-associated U provides a new framework to understand U mobility in the subsurface.

Summary

Uranium contamination threatens the availability of safe and clean drinking water globally. This toxic element occurs both naturally and as a result of mining and ore-processing in alluvial sediments, where it accumulates as tetravalent U [U(IV)], a form once considered largely immobile. Changing hydrologic and geochemical conditions cause U to be released into groundwater. Knowledge of the chemical form(s) of U(IV) is essential to understand the release mechanism, yet the relevant U(IV) species are poorly characterized. There is growing belief that natural organic matter (OM) binds U(IV) and mediates its fate in the subsurface. In this work, researchers sought to examine the speciation of U in sediment from a contaminated alluvial aquifer to definitively determine whether OM was the dominant U(IV) sorbent. They applied nanoscale chemical imaging and X-ray absorption spectroscopy to density fractionated sediments in which organic matter was separated from minerals, thereby allowing the team to assess the U speciation in each pool. Researchers identified two populations of U (dominantly +IV) in anoxic sediments. Uranium was retained on OM and adsorbed to particulate organic carbon, comprising both microbial and plant material. Surprisingly, U was also adsorbed to clay minerals and OM-coated clay minerals. The dominance of OM-associated U provides a framework to understand U mobility in the shallow subsurface, and, in particular, emphasizes roles for desorption and colloid formation in its mobilization.

Principal Investigator

John Bargar
SLAC National Accelerator Laboratory; Stanford Synchrotron Radiation Lightsource
Bargar@slac.stanford.edu

Program Manager

Jennifer Arrigo
U.S. Department of Energy, Biological and Environmental Research (SC-33)
Environmental System Science
jennifer.arrigo@science.doe.gov

Funding

Funding was provided by the DOE Office of Biological and Environmental Research’s (BER) Environmental System Science (ESS) program (formerly, Subsurface Biogeochemistry Research) to the SLAC SFA program under contract DE-AC02-76SF00515. Research was performed at Stanford Synchrotron Radiation Lightsource (SSRL), a national user facility supported by the U.S. DOE’s Office of Basic Energy Sciences (BES).

References