HR: 0830h
AN: B21C-0727    [PDF]
TI: The acid-base titration of montmorillonite
AU: * Bourg, I C
EM: ibourg@nature.berkeley.edu
AF: Environmental Engineering Group, Davis Hall #1710 Univ. of California, Berkeley, CA 94720-1710 United States
AU: * Bourg, I C
EM: ibourg@nature.berkeley.edu
AF: Environmental Hydrogeochemistry Group (LHGE), Univ. of Pau Helioparc Pau-Pyrenees, Pau Cedex 9, 64053 France
AU: Sposito, G
EM: gsposito@nature.berkeley.edu
AF: Environmental Engineering Group, Davis Hall #1710 Univ. of California, Berkeley, CA 94720-1710 United States
AU: Bourg, A C
EM: Alain.Bourg@univ-pau.fr
AF: Environmental Hydrogeochemistry Group (LHGE), Univ. of Pau Helioparc Pau-Pyrenees, Pau Cedex 9, 64053 France
AB: Proton binding to clay minerals plays an important role in the chemical reactivity of soils (e.g., acidification, retention of nutrients or pollutants). If should also affect the performance of clay barriers for waste disposal. The surface acidity of clay minerals is commonly modelled empirically by assuming generic amphoteric surface sites ($>$SOH) on a flat surface, with fitted site densities and acidity constant. Current advances in experimental methods (notably spectroscopy) are rapidly improving our understanding of the structure and reactivity of the surface of clay minerals (arrangement of the particles, nature of the reactive surface sites, adsorption mechanisms). These developments are motivated by the difficulty of modelling the surface chemistry of mineral surfaces at the macro-scale (e.g., adsorption or titration) without a detailed (molecular-scale) picture of the mechanisms, and should be progressively incorporated into surface complexation models. In this view, we have combined recent estimates of montmorillonite surface properties (surface site density and structure, edge surface area, surface electrostatic potential) with surface site acidities obtained from the titration of alpha-Al$_{2}$O$_{3}$ and SiO$_{2}$, and a novel method of accounting for the unknown initial net proton surface charge of the solid. The model predictions were compared to experimental titrations of SWy-1 montmorillonite and purified MX-80 bentonite in 0.1-0.5 mol/L NaClO$_{4}$ and 0.005-0.5 mol/L NaNO$_{3}$ background electrolytes, respectively. Most of the experimental data were appropriately described by the model after we adjusted a single parameter (silanol sites on the surface of montmorillonite were made to be slightly more acidic than those of silica). At low ionic strength and acidic pH the model underestimated the buffering capacity of the montmorillonite, perhaps due to clay swelling or to the interlayer adsorption of dissolved aluminum. The agreement between our model and the experimental data illustrates the complementarity of molecular and macro-scale descriptions of the clay reactivity.
DE: 1030 Geochemical cycles (0330)
DE: 1099 General or miscellaneous
DE: 1831 Groundwater quality
DE: 1845 Limnology
DE: 1866 Soil moisture
SC: Biogeosciences [B]
MN: 2003 Fall Meeting