Biotransformation of binary and ternary citric acid complexes of iron and uranium

Citric acid forms ternary mixed-metal complexes with various metal ions involving the hydroxyl and carboxyl groups of citric acid. The coordination of the metal to citric acid has been shown to affect the biodegradation of the metal-citrate complexes and metal mobility in the environment. We investigated the formation and biodegradation of the ternary mixed metal Fe-U-citric acid complex. The presence of 1:1:2 Fe-U-citric acid in solution was confirmed by potentiometric titration, UV-vis spectrophotometry, gel-filtration chromatography, and extended X-ray absorption fine structure (EXAFS) analysis. Comparison of the EXAFS spectra shows that the 1:1:2 Fe-U-citric acid complex has structural characteristics similar to the 1:1 U-citric acid complex. Biotransformation studies of Fe(III)-citrate, U(VI)-citrate, and Fe-U-citrate complexes by Pseudomonas fluorescens showed that the binary 1:1 Fe-citric acid was readily biodegraded, whereas the 1:1 U-citric acid complex and the ternary 1:1:2 Fe-U-citric acid complexes were recalcitrant. Adding excess citric acid to the 1:1 U-citric acid complex resulted in the formation of a 2:3 U-citric acid complex after metabolism of the excess citric acid. When 1-fold excess citric acid was added to the 1:1:2 Fe-U-citric acid complex, the excess citric acid was completely degraded with no change in the stoichiometry of the complex. However, in the presence of 2-fold excess citric acid, a 1:1:1 Fe-U-citric acid complex remained in solution after the excess citric acid had been biodegraded.