Biotransformation of Binary and Ternary Citric Acid Complexes of Iron and Uranium

Abstract

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 environ ment. 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 stoi chiometry 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.

Phosphate regulates uranium(VI) toxicity to Lemna gibba L. G3

“…The major bioavailable forms of U(VI) are likely UO 2 2þ and UO 2 OH þ rather than uranium complexed or adsorbed to colloidal or particulate matter (unpublished data). U(VI) complexed with inorganic ligands (e.g., carbonate, sulfate, or phosphate) and humic substances apparently reduce the bioavailability of uranium by reducing the activity of UO 2 2þ and UO 2 OH þ (Kohler et al, 1996;Dodge and Francis, 1997;Markich, 2002).…”

Microbial Transformations of Uranium Complexed with Organic and Inorganic Ligands

“…Citric acid forms ternary mixedmetal complexes with various metal ions involving the hydroxyl and carboxyl groups of citric acid. 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 (Dodge and Francis, 1997). Comparison of the EXAFS spectra show the 1:1:2 Fe:U:citric acid complex has structural characteristics similar to the 1:1 U:citric acid complex.…”

“…However, with two-fold excess citric acid, a 1:1:1 Fe:U:citric acid complex remained in solution after the excess citric acid was biodegraded. These results suggest that similar to the U-citrate complex, the Fe-U-citrate complex is recalcitrant to biodegradation (Dodge and Francis, 1997). The persistence of mixedmetal-citrate complexes in wastes and contaminated environments may result in the mobilization of uranium.…”

Presence of Fe3+ and Zn2+ promoted biotransformation of Cd–citrate complex and removal of metals from solutions

“…After a lag period of 34-h, citrate was completely utilized at the rate of 68.18 ± 3.42 mol/h within 44 h. Solution pH increased from 6.1 to 9.1, metals were almost completely removed from solution at the end of the experiment. These results suggested that recalcitrant mixed-metal-citrate complex was not formed [18], and the presence of Fe 3+ and Zn 2+ jointly caused the biotransformation of Cd-citrate complex in the 1:1:1:3 Fe:Zn:Cd:citrate medium.…”

“…At the end of the incubation, more than 98% of the metals were removed from the two media. The completely metabolism of citrate indicated that nonbiodegradable mixed-metal-citrate complexes were not formed in these media [18]. The results also revealed that the presence of Fe 3+ or Zn 2+ resulted in complete transformation of Cd-citrate complex, respectively.…”