Abstract:
Biological uranium reduction was investigated using bacteria isolated from a
uranium mine in Limpopo, South Africa. Background uranium concentration in soil
from the mine was determined to be 168 mg kg-1 much higher than the typical
background uranium concentration in natural soils (0.30-11.7 mg kg-1). Therefore it
was expected that the bacteria isolated from the site were resistant to U(VI) toxicity.
Preliminary studies using a non-purified consortium from the mine soil showed that
U(VI) [uranyl(VI) dioxide, UO2
2+] was reduced and re-oxidized intermittently due to
the coexistence of U(VI) reducers and U(VI) oxidisers in the soil. Results from U(VI)
reduction by individual species showed that the purified cultures of Pantoea sp,
Pseudomonas sp. and Enterobacter sp. reduced U(VI) to U(IV) [U(OH)4(aq)] under
pH 5 to 6. Klebsiella sp. had to be eliminated from the cultures since these contributed
to the remobilisation of uranium to the hexavelant form. The initial reduction rate
determined at 50% point in 30 mg L-1 batches was highest in Pseudomonas sp. at 30
mg L-1, followed by Pantoea sp. Rapid reduction was observed in all cultures during
the first 6 h of incubation with equilibrium conditions obtained only after incubation
for 24 h. Complete U(VI) reduction was observed at concentrations as high as 200 mg
L-1 and up to 88% removal after 24 h in batches with an initial added U(VI)
concentration of 400 mg L-1.
