Characterizing the Extent and Role of Natural Subsurface Bioreduction in a Uranium-Contaminated Aquifer - 2008

Uranium (Nuclear Minerals) / Recovery, Remediation and other Mining-Related Technology

Campbell, and 10 co-authors, reported at the Goldschmidt Conference that bioremediation is a promising strategy for the long-term treatment of uranium (U)-contarninated groundwater. Dissolved U(VI) is reduced to insoluble U(IV) by stimulating indigenous metal-reducing bacteria in situ at the field-scale by
the addition of low concentrations of acetate. Several cores were drilled in a region of naturally-occurring U(VI) bioreduction at Rifle, CO, USA.

Characterization of the naturally bioreduced sediment provides:

1) an understanding of the natural processes that occur in the sediment under reducing conditions, and

2) assistance in optimizing strategies for the bioremediation.

Sediment samples from the cores that range from minimally reduced to naturally bioreduced aquifer sediment. were analyzed for labile U(VI), acid-extractable and total U, and were characterized with XRD, u-XRD, XRF, SEM/EDS, TEM, EMP, XMP, EXAFS, and XANES Size fractions of selected samples were analyzed for BET surface area, bicarbonate/carbonate and acid extractable U content, and mineralogical composition. The acid volatile sulfide (A YS),
Fe(II), and bioavailable Fe(IU) content of the bulk sediment were also determined. Phospholipid fatty acid (PLFA) and qPCR were used to identify the sediment microbial community.

Solid-phase U concentrations were higher in the naturally bioreduced zone, with evidence for a significant amount of U(IV) present. U was detected in high concentrations in framboidal pyrite and magnetite grains. The samples from this zone were elevated in A YS and depleted in bioavailable Fe(III), indicating that Fe(III) and sulfate reduction have occurred naturally in the sediment. Increased concentrations of solid phase organic carbon in this region also suggest that natural bioreduction is stimulated by zones of high organic carbon content. Microbial biomass and community composition varied along the bioreduction gradient.

As a result of the elevated sulfide mineral content, the zone of bioreduction was detectable at the field scale using an induced polarization geophysical method.