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Accueil du site > Productions scientifiques > Séminaires à PHENIX > 2010 > Séminaire 05.11.2010 - 14h30

Séminaire 05.11.2010 - 14h30

par Benjamin Rotenberg - 18 octobre 2010

Sergey Churakov, du Laboratory for Waste Management à l’Institut Paul Scherrer (Suisse), présentera un séminaire le 5 novembre 2010 à 14h30 dans la bibliothèque du laboratoire PECSA (7e étage, bâtiment F, porte 754) intitulé :

Molecular simulations of Zn2+ sorption on edges of montmorillonite

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Résumé

The laboratory studies indicate that uptake of transition metals (like Zn2+, Ni2+ and Mn2+) on smectite minerals and illites occur both by cation exchange and by adsorption to edge sites of the clay sheets. Further analysis of the sorption isotherms suggest the existence of at least two different kinds of surface complexes at the edges of clays, with low and high surface binding energy, referred to as “weak” and “strong” sites, respectively. The molecular structure of such surface complexes and their formation mechanism has not been resolved until recently.

We combined polarized extended X-ray absorption fine structure (P-EXAFS) measurements and DFT based calculations to explain the mechanism of Zn2+ uptake on the edges of montmorillonite. Two major sorption mechanisms have been examined : structural incorporation into external cationic sites of the edges and surface complexation on the edges. The structure of incorporated Zn2+ complexes was obtained from a set of ab initio lattice optimizations which were performed for different edge surfaces and the structural polymorphs of montmorillonite neglecting water dynamics near the edges. The structure and complexation mechanism of aqueous Zn2+ complexes on the edges of montmorillonite was obtained from a set of umbrella sampling ab initio MD simulations in NVT ensemble at 300 K taking into account explicit dynamics of water near the interface. Theoretical EXAFS spectra were calculated from ab initio MD trajectories with FEFF 8.40 and were used to model measured EXAFS data for montmorillonite samples with Zn2+ in “weak” and “strong” surface complexes.

The measured EXAFS spectra of “weak” surface sites can be explained by mono- and bi-dentate Zn complexes adsorbed on the edge of the TOT sheet. The strong sites can only be explained by cations incorporation in the clay structure. These complexes form as result of the surface defect healing on the clay edges. The analysis of the MD trajectories explains the mechanism of Zn2+ absorption on the clays surface and corresponding changes of the surface structure of the clay edges.