According to the radioactive element and the specific site conditions, bacteria can enzymatically immobilize radionuclides directly or indirectly. Their redox potential is exploited by some microbial species to carry out reductions that alter the solubility and hence, mobility, bioavailability and radiotoxicity. This waste treatment technique called bioreduction or enzymatic biotransformation is very attractive because it can be done in mild conditions for the environment, does not produce hazardous secondary waste and has potential as a solution for waste of various kinds.
Direct enzymatic reduction is the change of radionuclides of a higher oxidation state to a lower one made by facultative and obligate anaerobes. The radioisotope interact with binding sites of metabolically active cells and is used as terminal electron acceptor in the electron transport chain where compounds such as ethyl lactate act as electron donors under anaerobic respiration.
The periplasm plays a very important role in these bioreductions. In the reduction of uranium (VI) to insoluble uranium (IV), made by Shewanella putrefaciens, Desulfovibrio vulgaris, Desulfovibrio desulfuricans and Geobacter sulfurreducens, the activity of periplasmic cytochromes is required. The reduction of technetium (VII) to technetium (IV) made by S. putrefaciens, G. sulfurreducens, D. desulfuricans, Geobacter metallireducens and Escherichia coli, on the other hand, requires the presence of the complex formate hydrogenlyase, also placed in this cell compartment.
Other radioactive actinides such as thorium, plutonium, neptunium and americium are enzymatically reduced by Rhodoferax ferrireducens, S. putrefaciens and several species of Geobacter, and directly form an insoluble mineral phase.
The phenomenon of indirect enzymatic reduction is carried out by sulfate-reducing and dissimilatory metal-reducing bacteria on excretion reactions of metabolites and breakdown products. There is a coupling of the oxidation of organic acids —produced by the excretion of these heterotrophic bacteria— with the reduction of iron or other metals and radionuclides, which forms insoluble compounds that can precipitate as oxide and hydroxide minerals. In the case of sulfate-reducing bacteria hydrogen sulfide is produced, promoting increased solubility of polluting radionuclides and their bioleaching (as liquid waste that can then be recovered).
There are several species of reducing microorganisms that produce indirect sequestering agents and specific chelators, such as siderophores. These sequestering agents are crucial in the complexation of radionuclides and increasing their solubility and bioavailability. Microbacterium flavescens, for example, grows in the presence of radioisotopes such as plutonium, thorium, uranium or americium and produces organic acids and siderophores that allow the dissolution and mobilization of radionuclides through the soil. It seems that siderophores on bacterial surface could also facilitate the entry of these elements within the cell as well. Pseudomonas aeruginosa also secretes chelating agents out that meet uranium and thorium when grown in a medium with these elements. In general, it has also been found that enterobactin siderophores are extremely effective in solubilizing actinide oxides of plutonium.