Microbially enhanced chemisorption of heavy metals: A method for the bioremediation of solutions containing long lived isotopes of neptunium and plutonium

Immobilized cells of a Citrobacter sp. removed neptunium and plutonium negligibly from solution using an established technique that used biologically-produced phosphate ligand (P-i) for metal phosphate bioprecipitation. Removal of these transuranic radionuclides was enhanced by prior exposure of the biomass to lanthanum in the presence of organophosphate substrate to form cell-bound LaPO4. Polyacrylamide gel-immobilized cells removed little Np and Pu per se, but preloaded LaPO4 promoted the removal of Np and Pu upon subsequent challenge in a flow-through column. Approximately 2 mu g of Np was loaded per 1 mL, column, when the experiments were stopped after 10 mL, with maintenance of approximately 90% removal of the input metal. Transuranic element removal by this technique, generically described as microbially-enhanced chemisorption of heavy metals (MECHM), is via a hybrid of bioaccumulative and chemisorptive mechanisms.