Adsorption of rare earth elements by γ-Al 2O 3 and Pseudomonas fluorescens cells in the presence of desferrioxamine B: implication of siderophores for the Ce anomaly

Abstract

We investigated the influence of siderophore desferrioxamine B (DFO) on the adsorption behavior of 11 rare-earth elements (REEs), La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, and Er, on γ-Al 2O 3 and Pseudomonas fluorescens cells at neutral pH. In the presence of DFO, the adsorption of REEs on γ-Al 2O 3 and P. fluorescens cells showed a tendency to decrease with an increase of their atomic number, except for Ce. The adsorption of Ce was significantly lower than those of the neighboring REEs, La and Pr. X-ray absorption near edge structure (XANES) analysis of the Ce–DFO complex showed that Ce was in the tetravalent state. Adding hydroxylammonium reduced the tetravalent Ce in the complex to its trivalent form with the disappearance of the Ce anomaly. These results show that DFO can oxidize Ce(III) to Ce(IV), which was supported by electrochemical experiments. Cyclic voltammetry (CV) revealed that the redox potential of the Ce(IV)/Ce(III) couple in the DFO complex was much lower than the standard redox potential, and that the stability of Ce(IV)–DFO is much higher than that of Ce(III)–DFO. These findings suggest that the Ce anomaly we observed is due to the oxidation of Ce(III)–DFO complex to the more stable Ce(IV)–DFO complex, and that naturally occurring organic ligands can contribute to Ce anomaly observed in the natural environment.