New NMR evidence on the uranylcitrate complexes

Nunes, M.Teresa; Gil, Victor M. S.

doi:10.1016/s0020-1693(00)86676-1

Cited by 37 publications

(42 citation statements)

References 8 publications

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“…4. This structure differs from that proposed for aqueous (UO 2 ) 2 (citrate) 2− 2 , in which five-and six-membered rings were invoked (24,25). To our knowledge, the structure of aqueous UO 2 (citrate) − has not been determined.…”

Section: Resultscontrasting

confidence: 85%

“…(ii) A uranyl precipitate or multimer may form. The role that citrate would play in this case is not immediately obvious although the structure for the (UO 2 ) 2 (citrate) 2 (aq) species elucidated by NMR (25) and EXAFS (24) shows uranium atoms held in close proximity by bridging oxygens donated by citrate. A signature of this type of surface species would be the presence of uranium backscattering in the EXAFS.…”

Section: Resultsmentioning

confidence: 96%

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Citrate Enhanced Uranyl Adsorption on Goethite: An EXAFS Analysis

Redden

Bargar

Bencheikh-Latmani

2001

Journal of Colloid and Interface Science

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Citric acid promotes the adsorption of uranyl (U(VI) as UO 2+2 ) on goethite (α-FeOOH) at pH ≤ 5. Enhanced adsorption does not appear to follow a simple stoichiometric relationship between total citrate and total uranyl. An excess of citric acid relative to uranyl, or a high concentration of surface-bound citrate is required. An extended X-ray absorption fine structure spectroscopy (EXAFS) study was conducted to help understand the surface interactions between citrate, UO 2+ 2 , and goethite between pH 3.5 and pH 5.5. Two principal surface species were found to be necessary and sufficient to describe the uranyl EXAFS spectra for the range of adsorbed citrate concentrations studied. (The possibility of additional minor surface species has not been eliminated, but inherent uncertainties in the data do not permit their clear identification.) One species is identified as an inner-sphere, uranyl-goethite complex, which exists at pH 5.5 in the absence of citrate. A second species is interpreted to be an adsorbed uranyl + citrate complex, which displaces the binary uranyl-goethite complex as the concentration of adsorbed citrate increases. The EXAFS spectra from samples with intermediate adsorbed citrate concentrations were reproduced using linear combinations of the spectra from these two end-member species. The uranyl + citrate surface complex appears to dominate adsorbed uranyl speciation at ligand : metal ratios of 10 : 1, or when the total surface-bound citrate approaches saturation. Although the exact structural configuration for the uranyl + citrate surface complex cannot be identified at this time, coordination between uranyl and one or more citrate molecules appears to involve both a bidentate bond (four-membered ring structure) and a putative eight-membered ring structure. The distribution and type of surface species should provide useful constraints on the development of model simulations of surface complexation behavior in this system. C 2001 Academic Press

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Section: Resultscontrasting

confidence: 85%

Section: Resultsmentioning

confidence: 96%

Citrate Enhanced Uranyl Adsorption on Goethite: An EXAFS Analysis

Redden

Bargar

Bencheikh-Latmani

2001

Journal of Colloid and Interface Science

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“…Fe(III)-citrate, which is predominantly in the [Fe(OH) 2 Cit] 2Ϫ form with a stability constant of 2.6, Nicitrate (log K ϭ 5.4), and Zn-citrate (log K ϭ 4.9) were degraded faster than U-citrate. U-citrate is believed to exist as a 3:2 complex at pH values above 6.5, and its stability is not known (32); the binuclear species has a log K of 18.9 (33).…”

Section: Discussionmentioning

confidence: 99%

“…The following formation constants were used: for Fe(III), log K ϭ 9. (16,22,32,33). Biodegradation of metal-citrate complexes.…”

Section: Methodsmentioning

confidence: 99%

“…Thermodynamic constants in the MINTEQA2 database were used in the calculations. The following formation constants were used: for Fe(III), log K ϭ 9.4 for [Fe(OH)Cit] Ϫ , where Cit is citrate, and log K ϭ 2.6 for [Fe(OH) 2 Cit] 2Ϫ ; for U-citrate, log K ϭ 7.4 for the mononuclear (UO 2 Cit) Ϫ and log K ϭ 18.9 for the binuclear [(UO 2 ) 2 (Cit) 2 ] 2Ϫ (16,22,32,33). Biodegradation of metal-citrate complexes.…”

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Mechanisms of biodegradation of metal-citrate complexes by Pseudomonas fluorescens

Joshi-Tope

Francis

1995

J Bacteriol

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Biodegradation of metal-citrate complexes by Pseudomonas fluorescens depends on the nature of the complex formed between the metal and citric acid. Bidentate Fe(III)-, Ni-, and Zn-citrate complexes were readily biodegraded, but the tridentate Cd-and Cu-citrate, and U-citrate complexes were not. The biodegradation of Ni-and Zn-citrate commenced after an initial lag period; the former showed only partial (70%) degradation, whereas the latter was completely degraded. Uptake studies with 14 C-labeled citric acid and metal-citrate complexes showed that cells grown in medium containing citric acid transported free citric acid at the rate of 28 nmol min ؊1 and Fe(III)-citrate at the rate of 12.6 nmol min ؊1 but not Cd-, Cu-, Ni-, U-, and Zn-citrate complexes. However, cells grown in medium containing Ni-or Zn-citrate transported both Ni-and Zn-citrate, suggesting the involvement of a common, inducible transport factor. Cell extracts degraded Fe(III)-, Ni-, U-, and Zn-citrate complexes in the following order: citric acid ‫؍‬ Fe(III)-citrate > Ni-citrate ‫؍‬ Zn-citrate > U-citrate. The cell extract did not degrade Cd-or Cu-citrate complexes. These results show that the biodegradation of the U-citrate complex was limited by the lack of transport inside the cell and that the tridentate Cd-and Cu-citrate complexes were neither transported inside the cell nor metabolized by the bacterium.Citric acid is a multidentate ligand and forms stable complexes with various metal ions (19). The rate and extent of biodegradation of several metal-citrate complexes by microorganisms vary. For example, Pseudomonas pseudoalcaligenes degraded Mg-citrate at a much lower rate than Ca-, Fe(III)-, and Al(III)-citrate (29). Studies with a Klebsiella sp. showed that citric acid and Mg-citrate were readily degraded, whereas Cd-, Cu-, and Zn-citrate were resistant to biodegradation (9). Both these studies also showed that metal toxicity was not responsible for the lack of or the lower rate of degradation of certain metal-citrate complexes but gave no other explanation. Biodegradation studies with Pseudomonas fluorescens showed that bidentate complexes of Fe(III)-, Ni-, and Zn-citrate were readily biodegraded, whereas complexes that involve the hydroxyl group of citric acid, the tridentate Cd-and Cu-citrate complexes, and U-citrate complex were not biodegraded (17). No relationship between biodegradability and stability of the complexes was observed. The tridentate Fe(II)-citrate complex, although recalcitrant, was readily biodegraded after oxidation and hydrolysis to the bidentate Fe(III)-citrate form, denoting a structure-function relationship in the metabolism of the complex (16).Microorganisms have selective transport systems for the uptake of metals of known biological function (11,23,28). Heavy metals are transported into cells by preexisting transport systems and are subsequently either chemically neutralized, sequestered, or removed from the cells by rapid efflux (6,35). Metals affect the transport of citric acid in bacteria. For example, the...

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Microbially enhanced chemisorption of nickel into biologically synthesized hydrogen uranyl phosphate: A novel system for the removal and recovery of metals from aqueous solutions

Basnakova

Macaskie

1997

Biotechnol. Bioeng.

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New NMR evidence on the uranylcitrate complexes

Cited by 37 publications

References 8 publications

Citrate Enhanced Uranyl Adsorption on Goethite: An EXAFS Analysis

Citrate Enhanced Uranyl Adsorption on Goethite: An EXAFS Analysis

Mechanisms of biodegradation of metal-citrate complexes by Pseudomonas fluorescens

Microbially enhanced chemisorption of nickel into biologically synthesized hydrogen uranyl phosphate: A novel system for the removal and recovery of metals from aqueous solutions

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