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Title: XAFS Determination of Pb and Cd Speciation with Siderophores and the Metal/Siderophore/Kaolinite System

Abstract

We provide evidence for hexadentate complexes of Pb2+ and Cd2+ with the trihydroxamate siderophore desferrioxamine B (DFO-B) at pH 7.5, and 9.0, respectively. Analysis of the species of Pb2+ and Cd2+ adsorbed at the surface of kaolinite clay under the same pH conditions and in the presence of DFO-B indicate that Pb2+ is sorbed as a metal-siderophore complex while Cd2+ is not.

Authors:
; ;  [1]; ;  [2]
  1. Department of Physics, University of Notre Dame, Notre Dame, IN 46556 (United States)
  2. Department of Civil Engineering and Geological Sciences, University of Notre Dame, Notre Dame, IN 46556 (United States)
Publication Date:
OSTI Identifier:
21054591
Resource Type:
Journal Article
Resource Relation:
Journal Name: AIP Conference Proceedings; Journal Volume: 882; Journal Issue: 1; Conference: XAFS13: 13. international conference on X-ray absorption fine structure, Stanford, CA (United States), 9-14 Jul 2006; Other Information: DOI: 10.1063/1.2644472; (c) 2007 American Institute of Physics; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
36 MATERIALS SCIENCE; ABSORPTION SPECTROSCOPY; ADSORPTION; CADMIUM; CADMIUM IONS; CHEMICAL STATE; CLAYS; FINE STRUCTURE; KAOLINITE; LEAD; LEAD COMPLEXES; LEAD IONS; PH VALUE; SURFACES; X-RAY SPECTRA; X-RAY SPECTROSCOPY

Citation Formats

Mishra, Bhoopesh, Vasconcelos, Igor F., Bunker, Bruce A., Haack, Elizabeth A., and Maurice, Patricia A. XAFS Determination of Pb and Cd Speciation with Siderophores and the Metal/Siderophore/Kaolinite System. United States: N. p., 2007. Web. doi:10.1063/1.2644472.
Mishra, Bhoopesh, Vasconcelos, Igor F., Bunker, Bruce A., Haack, Elizabeth A., & Maurice, Patricia A. XAFS Determination of Pb and Cd Speciation with Siderophores and the Metal/Siderophore/Kaolinite System. United States. doi:10.1063/1.2644472.
Mishra, Bhoopesh, Vasconcelos, Igor F., Bunker, Bruce A., Haack, Elizabeth A., and Maurice, Patricia A. Fri . "XAFS Determination of Pb and Cd Speciation with Siderophores and the Metal/Siderophore/Kaolinite System". United States. doi:10.1063/1.2644472.
@article{osti_21054591,
title = {XAFS Determination of Pb and Cd Speciation with Siderophores and the Metal/Siderophore/Kaolinite System},
author = {Mishra, Bhoopesh and Vasconcelos, Igor F. and Bunker, Bruce A. and Haack, Elizabeth A. and Maurice, Patricia A.},
abstractNote = {We provide evidence for hexadentate complexes of Pb2+ and Cd2+ with the trihydroxamate siderophore desferrioxamine B (DFO-B) at pH 7.5, and 9.0, respectively. Analysis of the species of Pb2+ and Cd2+ adsorbed at the surface of kaolinite clay under the same pH conditions and in the presence of DFO-B indicate that Pb2+ is sorbed as a metal-siderophore complex while Cd2+ is not.},
doi = {10.1063/1.2644472},
journal = {AIP Conference Proceedings},
number = 1,
volume = 882,
place = {United States},
year = {Fri Feb 02 00:00:00 EST 2007},
month = {Fri Feb 02 00:00:00 EST 2007}
}
  • We provide evidence for hexadentate complexes of Pb{sup 2+} and Cd{sup 2+} with the trihydroxamate siderophore desferrioxamine B (DFO-B) at pH 7.5, and 9.0, respectively. Analysis of the species of Pb{sup 2+} and Cd{sup 2+} adsorbed at the surface of kaolinite clay under the same pH conditions and in the presence of DFO-B indicate that Pb{sup 2+} is sorbed as a metal-siderophore complex while Cd{sup 2+} is not.
  • Little spectroscopic evidence exists in the literature describing the surface complexation of cadmium (Cd) and lead (Pb) on kaolinite, the dominant clay mineral present in highly weathered soils of tropical and humid climates. X-ray absorption fine structure (XAFS) spectroscopy data at the Cd K and Pb LIII edges were collected on Cd- and Pb-sorbed kaolinite samples and compared to a suite of reference materials including Pb and Cd sorbed on amorphous (am-)gibbsite. Cadmium formed dominantly (>75%) outer sphere complexes on kaolinite and a small fraction of CdOHCl complexes. In contrast Cd adsorbed as an inner sphere complex on gibbsite, suggestingmore » that the Si tetrahedral sheet hindered Cd sorption to the Al octahedral sheet on kaolinite. Lead formed polymeric complexes, which bonded to kaolinite via edge sharing with surface Al octahedra. Two distinct Pb-Al edge-sharing distances on am-gibbsite, as opposed to one on kaolinite, suggested a similar steric hindrance effect for the surface complexation of polymeric Pb complexes on kaolinite. The results of this study show that the Si tetrahedral sheet limited the surface complexation of Cd and Pb on kaolinite, elevating kaolinite's permanent negative charge properties in retaining these heavy metals at its surface.« less
  • This study investigates the complexation environments of aqueous Pb and Cd in the presence of the trihydroxamate microbial siderophore, desferrioxamine-B (DFO-B) as a function of pH. Complexation of aqueous Pb and Cd with DFO-B was predicted using equilibrium speciation calculation. Synchrotron-based X-ray absorption fine structure (XAFS) spectroscopy at Pb L(III) edge and Cd K edge was used to characterize Pb and Cd-DFO-B complexes at pH values predicted to best represent each of the metal-siderophore complexes. Pb was not found to be complexed measurably by DFO-B at pH 3.0, but was complexed by all three hydroxamate groups to form a totallymore » 'caged' hexadentate structure at pH 7.5-9.0. At the intermediate pH value (pH 4.8), a mixture of Pb-DFOB complexes involving binding of the metal through one and two hydroxamate groups was observed. Cd, on the other hand, remained as hydrated Cd{sup 2+} at pH 5.0, occurred as a mixture of Cd-DFOB and inorganic species at pH 8.0, and was bound by three hydroxamate groups from DFO-B at pH 9.0. Overall, the solution species observed with EXAFS were consistent with those predicted thermodynamically. However, Pb speciation at higher pH values differed from that predicted and suggests that published constants underestimate the binding constant for complexation of Pb with all three hydroxamate groups of the DFO-B ligand. This molecular-level understanding of metal-siderophore solution coordination provides physical evidence for complexes of Pb and Cd with DFO-B, and is an important first step toward understanding processes at the microbial- and/or mineral-water interface in the presence of siderophores.« less
  • Batch adsorption experiments were combined with X-ray Absorption Spectroscopy (XAS) analysis to determine the mechanism(s) whereby the microbial trihydroxamate siderophore ligand desferrioxamine-B (DFO-B) affects Pb sorption to kaolinite at pH 4, 6, and 7.5 (in 0.1 M NaClO{sub 4}, 22 C; Pb:DFO-B ratio 120:240 {micro}M). In the absence of DFO-B, Pb adsorbs only slightly to kaolinite at pH 4, by a combination of inner- and outer-sphere complexation. Adsorption increases at pH 6, and sorption (adsorption/surface precipitation) further increases at pH 7.5. At pH 4, DFO-B does not bind Pb in solution appreciably, and the Pb adsorption mechanism(s) is unchanged bymore » the presence of DFO-B. At pH 6, DFO-B slightly enhances Pb adsorption, due at least in part to formation of a DFO-B-Pb-kaolinite type A ternary surface complex. At pH 7.5, DFO-B decreases Pb sorption and Pb adsorption is dominated by a DFO-B-Pb-kaolinite type A ternary surface complex. Although XAS and thermodynamic speciation modeling indicate that Pb is bound by multiple DFO-B functional groups in solution at pH 7.5, the DFO-B-Pb-kaolinite surface complex appears to involve only a single hydroxamate group. This study thus demonstrates that the detailed structure of a ternary surface complex cannot necessarily be predicted from the structure of the solution organic-metal complex.« less
  • The solution speciation in seawater of divalent trace metals (Cd, Cu, Ni, Pb, Zn) is dominated by strong, ostensibly metal-specific organic ligands that may play important roles in microbial metal acquisition and/or detoxification processes. We compare the effective stabilities of these metal-organic complexes to the stabilities of their complexes with a model siderophore, desferrioxamine B (DFOB). While metal-DFOB complexation has been studied in various dilute but often moderately coordinating media, for the purpose of this investigation we measured the stability constants in a non-coordinating background electrolyte at seawater ionic strength (0.7 M NaClO4). Potentiometric titrations of single metals (M) weremore » performed in the presence of ligand (L) at different M:L molar ratios, whereupon the stability constants of multiple complexes were simultaneously determined by non-linear regression of the titration curves with FITEQL, using the optimal binding mode for each metal. Cadmium, Ni, and Zn, like trivalent Fe, sequentially form a bi-, tetra-, and hexadentate complex with DFOB as pH increases, consistent with their coordination number of 6 and regular octahedral geometry. Copper has a Jahn-Teller-distorted square-bipyramidal geometry whereas the geometry of Pb is cryptic, involving a range of bond lengths. Supported by a thermodynamic argument, our data suggest that this impedes binding of the third hydroxamate group and that the hexadentate Cu-DFOB and Pb-DFOB complex identified in earlier reports may instead be a deprotonated tetradentate complex. Absence of the hexadentate complex promotes the formation of a dinuclear (bidentate-tetradentate) complex, M2HL2+, albeit not for Pb in 0.7 M NaCl, evidently due to extensive complexation with chloride. Stabilities of the hexadentate Ni-DFOB, Zn-DFOB, and the tetradentate Pb-DFOB complex are nearly equal, yet about 2 orders of magnitude higher and 4 orders of magnitude lower than those of the hexadentate Cd-DFOB and tetradentate Cu-DFOB complex, respectively. Linear free-energy relations defined by the rare earth elements are able to predict stabilities of the Cd, Zn, and one of the Pb complexes, but underestimate those of the Ni and Cu complexes. The comparison with metal-specific organic ligands detected in seawater yields fair agreement for three of the five metals, implying that they could be siderophore-like. The Cd- and Ni-specific ligands are much stronger and may contain quite different functional groups. Calculations with MINEQL incorporating our new stability constants indicate that very high DFOB concentrations would be required to match the extent of metal-organic complexation observed in seawater, however DFOB may well represent a much broader class of structurally related ligands.« less