6 July 2005: Bioremediation mechanism discovered

UK researchers have discovered how a species of bacteria reduces uranium from a soluble oxidation state to an insoluble one, but is probably unable to perform the same feat on other radioactive actinide elements, such as plutonium and neptunium. 

Uranium can adopt two main oxidation states: U(vi), which forms soluble compounds such as the uranyl dioxocation {UO₂}²⁺; and U(iv), which forms insoluble compounds such as UO₂. Much of the uranium in radioactive waste exists in soluble oxidation state VI, which means it could leak out of any containment site. The bacterium Geobacter sulfurreducens is able to   reduce U(vi)  to the insoluble U(iv) as part of its natural metabolism, which would prevent any leakage.   

There are two possible mechanisms by which G. sulfurreducens reduces uranium. Either it directly reduces U(vi) to U(iv) by transferring two electrons from organic compounds to U(vi), or it transfers only one electron to U(vi), reducing it to an unstable U(v) oxidation state, which then undergoes a disproportionation reaction (where the oxidation number goes both up and down) to form U(vi) and U(iv). Researchers at the University of Manchester and CLRC Daresbury Laboratory, Warrington, investigated which mechanism takes place in the bacterium. 

The team, led by Jonathan Lloyd at Manchester, incubated G. sulfurreducens with uranyl(vi) acetate and then used X-ray absorption spectroscopy to determine any changes in the oxidation state of the uranium. They discovered that, after four hours, about 60 per cent of the uranium was in oxidation state V. After eight hours, the majority of the uranium (around 80 per cent) was now in oxidation state IV, having been transformed to UO₂. These results suggest that the bacterium uses the second mechanism. 

The researchers confirmed this by incubating G. sulfurreducens with neptunium in oxidation state V (Np(v)), which can only be reduced to Np(iv) by electron transfer. The bacterium was unable to reduce Np(v) to Np(iv), supporting the idea that it reduces U(v) to U(iv) via disproportionation. 

The findings have major implications for the ability of G. sulfurreducens to reduce other radioactive actinide elements. ’It is impossible to predict accurately the impact of microbial metabolism on complex mixed wastes based on thermodynamic considerations alone,’ Lloyd told Chemistry World. ’To further support efforts to remediate sediments contaminated with cocktails of actinides, we are currently determining the impact of Geobacter on the far more complex and challenging element plutonium.’

The work does not undermine the bioremediation of radioactive waste, stresses Lloyd, adding that it is probably the only credible option available. 

’The main elements of concern - uranium and technetium - are treated effectively,’ he said. ’This research illustrates the complexity of biological systems and shows that we need to undertake in-depth research to understand how they will work in real world situations.’ Jon Evans