Speciation of curium(III) bound by bacteria found in rocks considered for nuclear waste disposals

Bacteria occur ubiquitously in the environment and can significantly affect the migration behavior of actinides released e.g. after an accidental leakage from current and future underground radioactive waste repositories. Curium causes many of the long term radiological and thermal problems associated with nuclear waste disposal. Moreover, Cm(III) is worthwhile to study because it is a representative of the trivalent actinides with excellent luminescence properties and there is limited knowledge of its behavior in biological systems. The aim of our study is to characterize the Cm(III)-bacteria species formed and to elucidate the underlying interaction mechanisms. This knowledge is essential for the prediction of the migration behavior of Cm(III).
This new study is focused on the interaction processes of Cm(III) with bacteria isolated from two geochemical formations considered for nuclear waste disposals a) Pseudomonas fluorescens from granite rocks at the Äspö Hard Rock Laboratory (Äspö HRL), Sweden, and b) Sporomusa sp. from opalinus clay of the Mont Terri Rock Laboratory, Switzerland.
The direct speciation technique time-resolved laser-induced fluorescence spectroscopy (TRLFS) has been applied at trace Cm(III) concentrations. The unknown Cm(III) speciation in the two studied biological systems (P. fluorescens and Sporomusa sp.) was determined in terms of luminescence spectra and lifetimes of: a) original cell suspensions with Cm(III); b) Cm(III) in the supernatants after discarding the cells; c) desorbed Cm(III) from the cell envelopes; and d) irreversibly bound Cm(III) on the biomass. An overview of the emission spectra of Cm(III) in original cell suspensions of P. fluorescens and Sporomusa sp in 0.1 M NaClO4 is presented in Figure 1. Our results demonstrate that both bacteria have a high affinity to bind Cm(III) over a broad pH range between 2 and 8. Moreover, biomass dependent experiments at pH 6 indicate that already at a very low cell concentration of 0.01 mgdry weight/L all Cm(III) is bound by P. fluorescens and Sporomusa sp.. Both strains can associate Cm(III) in two coordination environments characterized by individual emission bands and luminescence lifetimes. Factor analyses of the emission data indicate interactions with carboxyl groups in the acidic pH up to pH 5.5 followed by interactions with phosphoryl groups of the cell surfaces at higher pH values. In the Cm(III)-P. fluorescens system, almost 100 % of the adsorbed Cm(III) was easily desorbed with 0.01 M EDTA solution. This clearly shows that Cm(III) is bound extracellularly on the cell surface via a pure biosorption process. In the Cm(III)-Sporomusa sp. system, however, only 70 % of the bound Cm(III) was easily desorbed by washing with 0.01 M EDTA. Hence, only 70 % of the added Cm(III) was adsorbed in this case in away similar to those of P. fluorescens, i.e. on the cell surface of the Sporomusa sp. isolate. In contrast to the cells of P. fluorescens, the cells of the Sporomusa sp. bind a significant amount of Cm(III), 30 %, irreversibly, that indicates that this part of the added Cm(III) is more strongly bound. Our results demonstrate that there is an evidence for differences between the Cm(III) interaction mechanisms observed in both bacterial systems. In addition, excitation spectra of Cm(III) associated with these bacteria will be discussed to support our conclusions drawn from the luminescence emission measurements.
The results of this study contribute to increase our understanding of the Cm(III) speciation in bacterial systems. This knowledge is necessary for a fundamental understanding of An(III) reactions under natural conditions.

This work was funded by the BMWi under contract number: 02E10618. The authors are indebted for the use of the 248Cm to the U.S. Department of Energy, Office of Basic Energy Sciences, through the transplutonium element production facilities at Oak Ridge National Laboratory which was made available as part of a collaboration between the HZDR and the Lawrence Berkeley National Laboratory (LBNL).