Sorption behavior of europium(III) and curium(III) on the cell surfaces of microorganisms

Summary We investigated the association of europium(III) and curium(III) with the microorganismsChlorella vulgaris,Bacillus subtilis,Pseudomonas fluorescens,Halomonassp.,Halobacterium salinarum, andHalobacterium halobium. We determined the kinetics and distribution coefficients (Kd) for Eu(III) and Cm(III) sorption at pH 3-5 by batch experiments, and evaluated the number of water molecules in the inner-sphere (NH₂O) and the degree of strength of ligand field (RE/M) for Eu(III) by time-resolved laser-induced fluorescence spectroscopy (TRLFS). Exudates fromC. vulgaris,Halomonassp., andH. halobiumhad an affinity for Eu(III) and Cm(III). The logKdof Eu(III) and Cm(III) showed that their sorption was not fully due to the exchange with three protons on the functional groups on cell surfaces. The halophilic microorganisms (Halomonassp.,Halobacterium salinarum,H. halobium) showed almost no pH dependence in logKd, indicating that an exchange with Na+on the functional groups was involved in their sorption. The Δ NH₂O(=9-NH₂O) for Eu(III) onC. vulgariswas 1-3, while that for the other microorganisms was over 3, demonstrating that the coordination of Eu(III) withC. vulgariswas predominantly an outer-spherical process. TheRE/Mfor Eu(III) on halophilic microorganisms was 2.5-5, while that for non-halophilic ones was 1-2.5. This finding suggests that the coordination environment of Eu(III) on the halophilic microorganisms is more complicated than that on the other three non-halophilic ones.