Speciation of uranium in cultures of Clostridium sp. by X-ray absorpti
on near-edge spectroscopy (XANES) at the National Synchrotron Light So
urce and by X-ray photoelectron spectroscopy (XPS) showed that U(VI) w
as reduced to U(IV). In addition to U(IV), a lower oxidation state of
uranium, most probably U(III), was detected by XANES in the bacterial
cultures. Reduction of uranium occurred only in the presence of growin
g or resting cells. Organic acid metabolites, the extracellular compon
ents of the culture medium, and heat-killed cells failed to reduce ura
nium under anaerobic conditions. The addition of uranyl acetate or ura
nyl nitrate (> 210 mu M U) to the culture medium retarded the growth o
f the bacteria as evidenced by an increase in the lag period before re
sumption of growth, by decreases in turbidity, and in the total produc
tion of gas and organic acid metabolites. These results show that uran
ium in wastes can be stabilized by the action of anaerobic bacteria.