BIOACCUMULATION OF LANTHANUM, URANIUM AND THORIUM, AND USE OF A MODELSYSTEM TO DEVELOP A METHOD FOR THE BIOLOGICALLY-MEDIATED REMOVAL OF PLUTONIUM FROM SOLUTION

Removal of La3+, UO22+ and Th4+ from aqueous solution by a Citrobacter sp. was dependent on phosphatase-mediated phosphate release and the r esidence time in a plug-flow reactor (PFR) containing polyacrylamide g el-immobilized cells. In a stirred tank reactor (STR) lanthanum phosph ate accumulated on the biomass rapidly, in preference to uranium or th orium phosphates. Thorium removal was not affected by the presence of uranium but was promoted in the presence of lanthanum. Analysis of the accumulated polycrystalline material by X-ray powder diffraction (XRD ) analysis and proton induced X-ray emission (PIXE) suggested the form ation of a mixed crystal of lanthanum and thorium phosphate. La3+, UO2 2+ and Th4+ are analogues of the corresponding species of Pu3+, PuO22 and Pu4+. The La/U/Th model system was used to identify some potentia l problems in the bioremediation of wastes containing plutonium and to develop a method for the biologically-mediated removal of plutonium f rom solution, in a test solution of Pu-239 'spiked' with a Pu-241 trac er. (C) 1998 SCI.