Radionuclide remediation using a bacterial biosorbent

A Pseudomonas strain, characterized as part of a project to develop a biosorbent for removal of toxic radionuclides from nuclear waste streams, was a potent accumulator of uranium (VI) and thorium (IV), with the metal sequestration process being unaffected by culture age, presence of carbon/energy source and metabolic inhibitor but sensitive to the composition of the growth medium. Further characterization of radionuclide biosorption using lyophilized biomass revealed rapid cation binding of >90% within 1–10 min of contact and complete removal of U and Th was observed at initial concentrations up to 100 mg l−1. Initial solution pH strongly affected radionuclide biosorption with an optimum at pH 4.0–5.0. High affinity, efficient and high capacity uranium and thorium binding was indicated, with maximum loading of 541 mg U g−1 dry mass or 430 mg Th g−1 dry mass. Good conformity of sorption data with the Langmuir model suggests monolayered U and Th binding. Sorption in presence of several interfering cations and anions indicates a specific U and Th binding by the biomass with significant antagonism offered only by iron (III). Transmission electron microscopy and energy dispersive X-ray fluorescence analysis of metal loaded biomass revealed intracellular U and Th sequestration. More than 90% of biomass-bound radionuclide was recovered using sodium or calcium carbonate. For continuous process application an immobilized biomass system was developed and tested with multiple cycles of sorption–desorption. Overall, the biosorbent appeared suitable for realistic bioremediation.