Biological Change of Chemical States of Actinides and Lanthanides -Effects of Organic Acids-

摘要

In this study, we have examined the chemical states change of lanthanides (Lns) of La, Ce, Pr, Eu, and actinides (Ans) of Th(IV), Pu(IV) in the complexes with organic acids of citric acids or desferrioxamine B (DFO) by the interaction with Pseudomonas fluorescens cells under the resting condition at pH 4 - 9. In the adsorption experiments the distribution coefficients (Kj) were determined. The oxidation states of Ce in the solution were determined by X-ray near edge fine structure (XAFS). In the absence of organic materials, K_d of Eu(III) increased gradually with pH up to 7; above 7, the K_d decreased with an increase in pH. In the presence of citric acid, we found that K_d was lower than that in the absence of organic materials, reflecting the formation of Eu(III)-citrate complexes competes with the Eu(III)-cell-surface complexes. For Ln-DFO complex, the higher K_ds of La, Ce, and Pr were obtained at lower pHs on P. fluorescens cells. The K_d of Ce was lower than that of its neighboring REEs, La(III) and Pr(III). XAFS analysis showed that Ce exists as the Ce(IV)-DFO complex in higher pH than 6. Thus, the pH dependence of lower K_d of Ce is predominantly dependent on the stability of Ce(IV)-DFO complex. For the adsorption of Ans, the K_ds of Th and Pu were larger than Eu, being inverse relationship to the stability constants of DFO complex. These results indicate that presence of organic acids affect the chemical states of Ln and An in the environments.