REMOVAL OF THE TETRAVALENT ACTINIDE THORIUM FROM SOLUTION BY A BIOCATALYTIC SYSTEM

The aim of this work was to study the effects of pH, a metal complexin g reagent (citrate) and the concentration of substrate on the bioaccum ulation of tetravalent actinides, using Th4+ as a model, by a phosphat ase-catalysed reaction. This yields HPO42- which precipitates with hea vy metals as cell-bound metal phosphate. Poor removal of Th from solut ion was observed, which is in accordance with the solution chemistry o f the metal. A considerable improvement in the efficiency of thorium r emoval was obtained by incorporating ammonium acetate (NH4Ac) into the solution. Although extensive deposition of polycrystalline NH4UO2PO4 was observed previously by cells that had accumulated UO22+, no eviden ce for deposition of crystalline thorium phosphate was obtained by X-r ay diffraction analysis. Examination by proton induced X-ray emission (PIXE) analysis showed a non-homogeneous thorium deposit of variable p hosphorus content.