Uranium(VI) reduction and removal by high performing purified anaerobic cultures from mine soil

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.