Mobilisation of Radionuclides by Ligands Produced by Bacteria from the Deep Subsurface

Microbial processes can influence retention of radionuclides directly or indirectly in several different ways. Many microorganisms produce various kinds of ligands or chelating compounds to increase the bioavailability of essential elements needed for metabolism. These ligands are not always highly specific, and several of them will also mobilise other elements such as heavy metals and radionuclides. Three bacterial species (Shewanella putrefaciens, Pseudomonas fluorescens and Pseudomonas stutzerii), isolated from the deep subsurface, and four radionuclides, 59Fe(III), 147Pm(III), 234Th(IV) and 241Am(III), were selected for this study. The microbes were cultured in the laboratory, separated from their exudates by centrifugation, and the supernatants were collected. The supernatants were mixed with radionuclide and solid phase (TiO2 or SiO2). The pH ranged from 7.5–9.0. All three bacterial species produced ligands that were able to complex up to 90% of the radionuclides in competition with the solid phases. High performance liquid chromatography analysis detected four Fe-complexing substances in the supernatant from P. fluorescens and two peaks and one peak, respectively, from P. stutzerii and S. putrefaciens. All substances eluted from the column varied in retention times, indicating that the microbes studied produced several metabolites that have different chelating abilities.