Previous field studies on in situ bioremediation of uranium-contaminated 
groundwater in an aquifer in Rifle, Colorado identified two distinct phases 
following the addition of acetate to stimulate microbial respiration. In phase 
I, Geobacter species are the predominant organisms, Fe(III) is reduced, and 
microbial reduction of soluble U(VI) to insoluble U(IV) removes uranium from the 
groundwater. In phase II, Fe(III) is depleted, sulfate is reduced, and 
sulfate-reducing bacteria predominate. Long-term monitoring revealed an 
unexpected third phase during which U(VI) removal continues even after acetate 
additions are stopped. All three of these phases were successfully reproduced in 
flow-through sediment columns. When sediments from the third phase were heat 
sterilized, the capacity for U(VI) removal was lost. In the live sediments U(VI) 
removed from the groundwater was recovered as U(VI) in the sediments. This 
contrasts to the recovery of U(IV) in sediments resulting from the reduction of 
U(VI) to U(IV) during the Fe(III) reduction phase in acetate-amended sediments. 
Analysis of 16S rRNA gene sequences in the sediments in which U(VI) was being 
adsorbed indicated that members of the Firmicutes were the predominant organisms 
whereas no Firmicutes sequences were detected in background sediments which did 
not have the capacity to sorb U(VI), suggesting that the U(VI) adsorption might 
be due to the presence of these living organisms or at least their intact cell 
components. This unexpected enhanced adsorption of U(VI) onto sediments 
following the stimulation of microbial growth in the subsurface may potentially 
enhance the cost effectiveness of in situ uranium bioremediation.