The effect of surface charge and wettability on H2O self diffusion in compacted clays
Schampera, Birgit (Leibniz University Hannover. Institute of Soil Science)
Dultz, Stefan (Leibniz University Hannover. Center for Solid State Chemistry and New Materials)

Data:	2011
Resum:	Organo-clays are of interest in the modification of anion transport properties in engineered barriers. In the present study, surface charge and wettability were assessed for tracking changes in the effective diffusion coefficient (D) by the formation or suppression of bound HO layers on the external surfaces of clays. Bentonite samples modified with three different organic cations in amounts of 0 to 400% of the cation exchange capacity were used. Diffusive transport was determined in HO→DO exchange experiments in a newly constructed cell adapted to the attenuated total reflectance (ATR) accessory of a Fourier-transform infrared (FTIR) spectrometer at two different dry bulk densities and various degrees of water saturation. All organo-clay combinations showed changes in surface charge after the addition of organic cations, from a negative value of 99 mmol/kg for the original bentonite to a maximum positive value of 230. 5 mmol/kg for hexadecylpyridinium (HDPy)-montmorillonite. The positive charge resulted from adsorption of the organic cation in excess of the CEC. Hydrophobic surface properties with contact angles >90° were obtained for HDPy-montmorillonite samples with monolayers of organic cations on the external surfaces only. Here, where hydrophobicity suppressed the formation of bound HO layers, the largest D o f 2. 7 × 10 m/s was observed in the high dry bulk density range (1. 0-1. 5 g/cm) under water-saturated conditions. In the low dry bulk density range (0. 6-0. 9 g/cm) this effect was weakened significantly because, with increasing pore size, the effect of bound HO layers was reduced. In the high dry bulk density range at partial water saturation (40%), diffusive transport was hindered by the small water volume. Previous work found that, in the high dry bulk density range and water-saturated state, D was 2. 4 × 10 m/s for the original bentonite. D for all hydrophilic organo-clay samples was ≤2. 1 × 10 m/s, somewhat less than for the hydrophobic sample. In hydrophilic organo-clay samples, retardation factors that retard the value for Deff, up toa magnitude of 0. 5, include an increase in dry bulk density and a decrease in water saturation. In the water-saturated state at high dry bulk densities, hydrophobic surface properties suppressing the formation of bound HO layers can increase D by one order of magnitude.
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Llengua:	Anglès
Document:	Article ; recerca ; Versió publicada
Matèria:	Bentonite ; Bound H2O ; Diffusive Transport ; Organo-clays ; Surface Charge ; Wettability
Publicat a:	Clays and Clay Minerals, Vol. 59, Núm. 1 (2011) , p. 42-57, ISSN 1552-8367

DOI: 10.1346/CCMN.2011.0590107

16 p, 1.3 MB  Accés restringit a la UAB

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