Speciation and reactivity of uranium products formed during in situ bioremediation in a shallow alluvial aquifer

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 reoxiation 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.

Uranium redox transition pathways in acetate-amended sediments

Daniel Scott Alessi, Rizlan Bernier-Latmani, Malgorzata Alicja Stylo, Elena Suvorova Buffat

Redox transitions of uranium [from U(VI) to U(IV)] in low-temperature sediments govern the mobility of uranium in the environment and the accumulation of uranium in ore bodies, and inform our understanding of Earth's geochemical history. The molecular-scale mechanistic pathways of these transitions determine the U(IV) products formed, thus influencing uranium isotope fractionation, reoxidation, and transport in sediments. Studies that improve our understanding of these pathways have the potential to substantially advance process understanding across a number of earth sciences disciplines. Detailed mechanistic information regarding uranium redox transitions in field sediments is largely nonexistent, owing to the difficulty of directly observing molecular-scale processes in the subsurface and the compositional/physical complexity of subsurface systems. Here, we present results from an in situ study of uranium redox transitions occurring in aquifer sediments under sulfate-reducing conditions. Based on molecular-scale spectroscopic, pore-scale geochemical, and macroscale aqueous evidence, we propose a biotic-abiotic transition pathway in which biomass-hosted mackinawite (FeS) is an electron source to reduce U(VI) to U(IV), which subsequently reacts with biomass to produce monomeric U(IV) species. A species resembling nanoscale uraninite is also present, implying the operation of at least two redox transition pathways. The presence of multiple pathways in low-temperature sediments unifies apparently contrasting prior observations and helps to explain sustained uranium reduction under disparate biogeochemical conditions. These findings have direct implications for our understanding of uranium bioremediation, ore formation, and global geochemical processes.

National Academy of Sciences2013

Biostimulation as a remediation strategy post uranium acidic in situ recovery

Rizlan Bernier-Latmani, Thomas Coral, Brian Carl Reinsch, Pierre Rossi

A large fraction (47%) of the world’s uranium is mined by a technique called “In Situ Recovery”. This mining technique involves the injection of a leaching fluid (acid or alkaline) into a uranium-bearing aquifer and the pumping of the resulting solution through cation exchange columns for the recovery of the dissolved metal. A strategy of rehabilitation of the aquifer is based on the biostimulation of the autochthonous microbial community to remediate the acid plume and immobilize remaining trace metals. Assessment of the biostimulation was carried out using column tests filled with aquifer material. These columns were stimulated with a mixture of molasses, yeast extract and glycerol. Results showed that this mixture efficiently promoted the development of acid-resistant, sulfate-reducing bacterial guild members down to an inlet pH value ~ 4.7, with pH values at the outlet reaching up to 6.5 – 7. The rise of pH values along the columns were attributed to the predominant reduction of nitrate and sulfate. Biostimulation of the microbial community efficiently promoted the complete immobilization of U in the first centimeters of the columns. Synchrotron x-ray analysis and electron microscopy revealed that up to 60% of total U was precipitated as UIV onto bacterial cells. A detailed metagenomic analysis and qPCR data illustrated drastic changes occurring within the community with the reduction of the microbial diversity and the quantitative development of fermentative bacteria. The community was dominated by members of the Phylum Firmicutes (genus “Clostridium”, Pelosinus and Desulfosporosinus), representing up to 89 % of all cells.

2016

Products of in situ corrosion of depleted uranium ammunition in Bosnia and Herzegovina soils

Barbora Bártová, Rizlan Bernier-Latmani, Konstantin von Gunten, Yuheng Wang

Hundreds of tons of depleted uranium (DU) ammunition were used in previous armed conflicts in Iraq, Bosnia and Herzegovina, and Serbia/Kosovo. The majority (>90%) of DU penetrators miss their target and, if left in the environment, corrode in these postconflict zones. Thus, the best way to understand the fate of bulk DU material in the environment is to characterize the corrosion products of intact DU penetrators under field conditions for extended periods of time. However, such studies are scarce. To fill this knowledge gap, we characterized corrosion products formed from two intact DU penetrators that remained in soils in Bosnia and Herzegovina for over seven years. We used a combination of X-ray powder diffraction, electron microscopy, and X-ray absorption spectroscopy. The results show that metaschoepite (UO3(H2O)(2)) was a main component of the two DU corrosion products. Moreover, studtite ((UO2)O-2(H2O)(2)center dot 2(H2O)) and becquerelite (Ca(UO2)(6)O-4(OH)(6)center dot 8(H2O)) were also identified in the corrosion products. Their formation through transformation of metaschoepite was a result of the geochemical conditions under which the penetrators corroded. Moreover, we propose that the transformation of metaschoepite to becquerelite or studtite in the DU corrosion products would decrese the potential for mobilization of U from corroded DU penetrators exposed to similar environments in postconflict areas.

Amer Chemical Soc2016