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Mécanismes d'adsorption des métaux lourds par les exopolysaccharides bactériens : Le système Xanthane-Cuivre comme modèle d'étude

Abstract : Bacterial ExoPolySaccharides (EPS) constitute a soil reactive fraction towards Metallic Trace Elements (MTE). The xanthan-copper system is studied as a complexation model of MTE onto EPS. On a macroscopic scale, we performed acid base titrations, in order to determine the pK values of xanthan (average pKI=0.1M=2.5) and proposed an electrostatic model with cylindrical condenser to explain the variations of pKa according to the ionic strength (IS). Titrations in the presence of copper and Cu2+sorption experiments show an original result : the xanthan adsorbs more copper at pH3.5 than at pH5.5. The values of the apparent complexation constants are LogKCu-XanpH3.5=3 and LogKCu-XanpH5.5=2.6. Circular dichroism experiments suggest a pH-dependent conformational change in xanthan at xed IS. A thermodynamic model with two states (disordered xanthan for pH<5 versus ordered for pH>5), coupled with the pKa and LogKCu data, give an accurate reconstruction of experimental data. On a molecular scale, a molecular modelling study (MM) shows that the disordered structure of the EPS at pH3 leads to the formation of very stabilizing chelating complexes between two side chains of xanthan and Cu2+. A contrario, at pH5.5 (helix ordered structure) simple complexation sites prevail, wich are less stabilizing for copper. The EXAFS results and their modelling with MM conrm the strongest anity of copper for xanthan at pH 3.5 than at pH5.5. This study shows the importance of the EPS conformation in their ability to x the MTE, and the advantage of the MM tool for the comprehension of these phenomena on a molecular scale.
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Contributor : Benjamin Causse <>
Submitted on : Tuesday, June 22, 2010 - 9:26:37 PM
Last modification on : Thursday, November 19, 2020 - 3:54:29 PM
Long-term archiving on: : Thursday, September 23, 2010 - 5:53:29 PM

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  • HAL Id : tel-00493322, version 3

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Benjamin Causse. Mécanismes d'adsorption des métaux lourds par les exopolysaccharides bactériens : Le système Xanthane-Cuivre comme modèle d'étude. Géochimie. Université Joseph-Fourier - Grenoble I, 2009. Français. ⟨tel-00493322v3⟩

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