Uranium (U) contamination of ground and surface water is a serious problem in
many parts of the world. Agricultural practices, mining, and nuclear weapons
production have resulted in elevated levels of this heavy metal at a variety of
locations, which threatens human health by seeping into groundwater and
dispersing over large areas.
Researchers at Stanford University and Oak Ridge National Laboratory (ORNL)
have now shown that several common types of bacteria may be used to convert an
unstable form of U into a stable form that greatly reduces the environmental
and health threat of contamination.
In groundwater, U is most commonly found in the hexavalent oxidation state,
U(VI)—a highly soluble, mobile and therefore troublesome form. U(VI) bonds
readily with minerals suspended in water and can spread easily over large
areas. However, the tetravalent state, U(IV), resists combining with minerals
and is highly stable and immobile.
Several common types of
bacteria are known to convert U(VI) into U(IV), which then precipitates as an
insoluble oxide, uraninite, and researchers have successfully tested a pilot
program designed to exploit this natural process. Until 1983, millions of
gallons of waste containing U and nitric acid were discharged into unlined
ponds at ORNL. Starting in 2001, researchers began injecting ethanol into the
subsurface environment to stimulate microbial activity, and the results were
dramatic—concentrations of U(VI) began falling rapidly. Research at SSRL
using x-ray absorption near-edge structure (XANES) on SSRL's BL11-2 has
verified that those reductions were indeed the result of stimulated microbial
action. Current studies aim to monitor the long-term stability of biologically
reduced U and the viability of biostimulation as a remediation technique.
To learn more about this research see the full scientific highlight at:
http://www-ssrl.slac.stanford.edu/
research/highlights_archive/mn_biooxides.html
W.-M. Wu, W.-M.; Carley, J.; Gentry, T.; Ginder-Vogel, M. A.; M. Fienen, M.;
Mehlhorn, T.; Yan, H.; Carroll, S.; Pace, M. N.; Nyman, J.; Luo, J.; Gentile,
M. E.; Fields, M. W.; Hickey, R. F.; Gu, B.; Watson, D.; Cirpka, O. A.; Zhou,
J.; Fendorf, S.; Kitanidis, P. K.; Jardine, P. M.; Criddle, C. S. "Pilot-Scale
in situ bioremediation of uranium in a highly contaminated aquifer. 2.
Reduction of U (VI) and geochemical control of U(VI) bioavailability",
Environ.
Sci. Technol. 2006, 40, 3986.
