Skip to main content

Microbial and Geochemical Factors influencing the speciation of uranium in the subsurface

Final Report Summary - MONO-U4 (Microbial and Geochemical Factors influencing the speciation of uranium in the subsurface)

Contamination of the subsurface with uranium as a result of mining and weapons and nuclear power production remains problematic in Europe and North America. In the projects funded by this grant, we studied the use of bioremediation - or the stimulation of indigenous microbial communities in soils and sediments - to clean up soils and ground water contaminated with uranium. Microbes may cause the enzymatic reduction of soluble and mobile hexavalent uranium - UVI - to tetravalent uranium - UIV - via a two-electron transfer to the metal. This process is desirable because UIV is essentially insoluble and is typically understood to precipitate as the UIV mineral phase uraninite [UIVO2(s)].

At the time of applying for this Marie Curie fellowship in 2009, evidence was emerging that structurally-ordered uraninite might not be the only product of UVI reduction by microorganisms. In some systems, direct enzymatic reduction of UVI, or reduction of UVI by FeII-bearing minerals of biogenic origin, were leading to the production of a non-crystalline but reduced product referred to in the literature as mononuclear or monomeric UIV. Because such species lacked structural order, it was thought they could be more labile or susceptible to re-oxidation (and thus remobilization) as UVI. Thus remediation schemes and geochemical transport models based on the solubility and stability of crystalline uraninite could seriously under-predict the mobility of uranium at contaminated field sites where monomeric UIV existed.

Based on the above assertion, we pursued multiple lines of inquiry about the nature of monomeric UIV, including:

A.What is the role of FeII-bearing minerals on the formation of monomeric UIV and uraninite?
B.How can we separate and quantify monomeric UIV from uraninite and adsorbed UVI in the laboratory?
C.How does water chemistry influence the product of enzymatic (microbial) UVI reduction?
D.X-ray microprobes are typically used to spatially map uranium and determine its oxidation state. Do these intense microbeams cause the oxidation of monomeric UIV?
E.How labile is monomeric UIV as compared to uraninite in field ground water wells?
F.What is the binding environment of monomeric UIV species on microbial cells?

The results of projects A-C are detailed in the Mid-Term Report (year 1), and for projects D-F, in the Periodic Report (year 2).

Perhaps our most important general finding is that monomeric UIV can form as a product of UVI reduction by microbes, by some FeII-bearing minerals of biogenic origin (Project A), and in natural soils and sediments. The conclusion that monomeric UIV may not be a rare species, and may in fact form at many field sites, has wide implications for the field remediation of uranium. Furthermore, the results of studies B, D, and E established that monomeric UIV - whether associated with microbes, iron bearing minerals, or sediments - was more susceptible to remobilization or oxidation than uraninite. Project F - the most recent to be completed - used a variety of spectroscopic techniques to elucidate the binding environment of monomeric UIV on Shewanella oneidensis MR-1 cells. This study is the first to show conclusively that the binding of UIV onto phosphate-bearing moieties near or on the surface of the bacteria are critical in the formation of monomeric UIV. The sum of these studies has provided important constraints on the nature and conditions of formation of monomeric UIV.

Many of the studies reported on here were dependent on the use of X-ray absorption spectroscopy (XAS), performed at synchrotron light sources, to determine the speciation and structural order of uranium in samples. Because obtaining beam time and traveling to synchrotron facilities can be time-consuming and costly, we spent a large portion of the first year of the funded period developing a rapid, synchrotron-independent method to determine the speciation of uranium in experimental systems. The results of this study (Project B above) show that a concentrated solution of sodium bicarbonate can effectively extract monomeric UIV species over a 24 hour period from the surfaces of microbes, FeII-bearing minerals, and natural sediments while leaving uraninite untouched. This allows for the quantification of each species following separation. The method, which is now published in the journal Environmental Science and Technology, provides a rapid, wet-chemical tool that will be useful in quantifying the products of uranium remediation for both academic and industry users.

A summary of ongoing investigations into uranium bioremediation, as well as general contact information for the investigators, is available at the following website:

http://eml.epfl.ch/page-29109-en.html

The studies performed using the Marie Curie fellowship funding have significantly added to the body of knowledge about uranium bioremediation and in particular the formation and stability of monomeric UIV species.