Rare Metal Recovery
Using Microorganisms

The heavy elements biogeochemistry group at the Advanced Science Research Center (ASRC), Japan Atomic Energy Agency (JAEA) has found that microorganisms under relevant conditions can help recover rare metals, such as platinum (Pt) and palladium (Pd). Platinum is in particular a precious metal that can be used widely as a catalyst, such as for automobile exhaust emissions control and for fuel cell systems, in addition to items such as fashion accessories. The group made the finding upon conducting research on the interaction of actinides with microorganisms at the molecular level.

According to Group Leader Ohnuki Toshihiko, the group is involved in studying the actinide series, which encompasses the fifteen elements on the periodic table with atomic numbers 89 to 103. In the actinides, uranium (U), neptunium (Np) and plutonium (Pu) are present in different oxidation states of trivalent, tetravalent, pentavalent and hexavalent in the aqueous solution. These elements are called redox sensitive actinides. The research group has conducted the reduction of hexavalent actinides by microorganisms, and found the formation of nanoparticles of tetravalent uranium. The group also found that elemental platinum can be recovered in the form of nanoparticles on the cell surface of iron-reducing bacteria in the aqueous solution. Divalent Pt and Pd ions dissolved in the solution are reduced by electron transfer from iron-reducing bacterial cells to the dissolved Pt and Pd ions. Interestingly, the particle size of Pt is larger on the dead cells than that on the live cells, suggesting that electron transfer occurs on the cell surface of iron-reducing bacteria through enzyme protein to reduce the oxidation state of Pt and Pd.

The JAEA results were based upon collaboration with researchers at Nagoya University. The use of the newly discovered biological recovery method is seen being quite economical and thus may provide the incentive to enhance use of platinum or palladium in products such as fuel cells. The result of the recovery is also considered to be a possible source of the fuel for power generation-use nuclear fusion reaction, tritium.