Speciation and Reactivity of Uranium Products Formed during in Situ Bioremediation in a Shallow Alluvial Aquifer

Abstract

In this study, we report the results of in situ U(VI) bioreduction experiments at the Integrated Field Research Challenge site in Rifle, Colorado, USA. Columns filled with sediments were deployed into a groundwater well at the site and, after a period of conditioning with groundwater, were amended with a mixture of groundwater, soluble U(VI), and acetate to stimulate the growth of indigenous microorganisms. Individual reactors were collected as various redox regimes in the column sediments were achieved: (i) during iron reduction, (ii) just after the onset of sulfate reduction, and (iii) later into sulfate reduction. The speciation of U retained in the sediments was studied using X-ray absorption spectroscopy, electron microscopy, and chemical extractions. Circa 90% of the total uranium was reduced to U(IV) in each reactor. Noncrystalline U(IV) comprised about two-thirds of the U(IV) pool, across large changes in microbial community structure, redox regime, total uranium accumulation, and reaction time. A significant body of recent research has demonstrated that noncrystalline U(IV) species are more suceptible to remobilization and reoxidation than crystalline U(IV) phases such as uraninite. Our results highlight the importance of considering noncrystalline U(IV) formation across a wide range of aquifer parameters when designing in situ remediation plans.

Snowmelt Induced Hydrologic Perturbations Drive Dynamic Microbiological and Geochemical Behaviors across a Shallow Riparian Aquifer

“…Although significant efforts have been made to remediate the site through removal of surface contaminated soils and sediments, persistent low-level uranium concentrations have been detected in the subsurface. Given that previous studies have demonstrated that NRZs are "hot spots" of immobilized uranium within the aquifer (Campbell et al, 2012), the annual intrusion of DO and oxidation of reduced species such as monomeric U 4+ in these regions may at least partly account for the observed persistence of uranium in groundwater Alessi et al, 2014). Indeed, our geochemical measurements showed a sharp spike in U 6+ concentrations during DO intrusion into the NRZ location sampled here (Figure 2).…”

“…Aqueous uranium increases were also detected across the remaining depths at location BND, and all depths in the NRZ well (Figure 2), although the exact mechanisms for mobilization remain unclear. Two possible mechanisms include (1) DO-dependent re-oxidation of reduced uranium phases in the saturated portion of the aquifer Alessi et al, 2014), and (2) Despite these specific examples of abiotic responses to water table rise and DO intrusion, many geochemical parameters between depths at a specific location were remarkably similar, indicative of (1) greater horizontal rather than vertical heterogeneity across the saturated subsurface (Figure 2), and (2) some level of geochemical stability within a particular well. Despite being separated by only 150 ft., samples from the MID wells exhibited greater geochemical stability than samples from the BND location (Figure 3).…”

Mononuclear U(IV) complexes and ningyoite as major uranium species in lake sediments

“…Such U(IV) mononuclear species could either be sorbed to the surface of phosphate as well as silicate minerals, or be bound to organic phosphoryl groups (Alessi et al, 2014b). The major occurrence of such species in lacustrine sediments has important environmental implications since mononuclear U(IV) species are potentially more labile than uraninite (Cerrato et al, 2013;Alessi et al, 2014a) and polymerised non-crystalline U(IV) phosphate phases (Alessi et al, 2014b). Such lability raises issues concerning the long term fate of the mononuclear U(IV) species, especially when subjected to sharp redox changes, for example in sediment remediation strategies as dredging operations.…”

“…In natural anoxic environments such as estuarine and coastal sediments, early diagenesis conditions favour the reduction of U(VI) species into low solubility U(IV) species, which decreases uranium concentrations in overlying waters and sediment pore-waters (Barnes and Cochran, 1993). Researchers addressing remediation of U-contaminated groundwaters have focused on in situ biostimulation strategies involving microbial reduction of U(VI) (Wu et al, 2007;Yabusaki et al, 2007) into biogenic uraninite (Suzuki et al, 2005;Bargar et al, 2008) as well as non-uraninite U(IV) phases (Kelly et al, 2008;Bargar et al, 2013;Alessi et al, 2014a;Newsome et al, 2014). Ex situ incubations of aquifer sediments under anoxic conditions have highlighted the importance of non-crystalline U(IV) species as major products of microbial reduction of U(VI) (Sharp et al, 2011;Alessi et al, 2014b).…”

Uranium Bio-Transformations: Chemical or Biological Processes?

“…Insoluble U(IV) can precipitate as the simple mineral uraninite, UO 2 , but more often forms minerals with calcium and phosphates such as autunite [119]. A more labile precipitate has also been identified known as monomeric U(IV) [120] [121].…”