Deux voies de formation des hydroxides alumineux en fonction du comportement des complexes polynucleaires d'aluminium: voie lixiviee a gibbsite et boehmite et voie confinee a gels et bayerite

International audienc

Contribution de la geochimie a l'analyse des transferts a l'echelle d'unites fonctionnelles du paysage

National audienc

De la "chimie appliquée à l'agriculture" à la biogéochimie de l'environnement

National audienc

Relations entre le pH, l'alcalinité, le pouvoir tampon et les équilibres de CO2 dans les eaux nature

Les premiers résultats obtenus sur des pluies, sur des neiges, et sur des eaux de source sur granites des Vosges et des Margeride illustrent les différents types de comportement des eaux naturelles vis-à-vis des réactions acide-base, et le rôle géochimique du CO2 dans les sols

Potentiel osmotique et potentiel matriciel

National audienc

Geochimie des eaux de sources et dynamique des sols. Methodologie et interpretation.

National audienc

Swelling clays and salt-affected soils : demixing of Na / Ca clays as the rationale for discouraging the use of sodium adsorption ratio (SAR)

Sodium adsorption ratio SAR defined as SAR = (Na) / V w(Ca+Mg)/2 here concentrations of cations in solution are expressed in meq/L has long been considered as correlated to exchangeable sodium percentage (ESP) on clay minerals or soil exchange complex, and as the key concept to explain swelling of clay minerals and the difficulties of reclaiming salt-affected soils. Though its basis is empirical, it was alleged to be theoretically justified on the basis of ion exchange, derived from the Gapon convention. However, it has long been challenged on the basis of both field observations and experimental evidence : it fails to account for the fact that calcium and magnesium do not play the same role, while potassium is absent from the formula ; calcium concentration must be “corrected “when calcite is present etc. There exist specific ion effects. Experimental measurements of the decrease of permeability when solutions are diluted led Quirk and Schofield (1955) to define the concept of critical threshold, and to show that potassium and magnesium play an intermediate role between sodium and calcium. This threshold is simply determined by the concentration of calcium, irrespective of the value of SAR or ESP. Indeed, demixing of Ca-Na clay minerals during ion exchange, a phenomenon well known since Glaeser and Mering (1954), implies that there exists an interaction between adjacent sites. This undermines the theoretical basis of SAR : the derivation of SAR from ion exchange equilibria implies to use an equilibrium constant. This parameter is no more constant if demixing occurs. The results obtained are positive : demixing leads to expulsion of sodium from inner exchange surfaces and its replacement by calcium, according to the “three crystals pore”proposed by Quirk (2003b). Sodium can then be more easily leached, as permeability is maintained by clusters of Ca-sites. Calcium concentration in solution appears thus as the simpler parameter to guide saltaffected soils reclamation when swelling clays are present

Consequences de l'existence de complexes polynucleaires de Al(III) sur la stabilite des mineraux alumineux: une reconsideration du systeme gibbsite-kaolinite-quartz

International audienc