Effect of surface chemistry and related texture on the diffusion coefficients and permeability of bentonites. Consequences for chemical-transport modeling
Résumé
Geosynthetic clay liners (GCL) or clay barriers are generally built using impermeable sodium bentonite. The long term leaching of the bentonite by rainwater, groundwater, landfill leachates, alkaline cement water progressively modifies the surface chemistry of the clays. The exchange reactions convert the predominantly Na-bentonite in a Xbentonite where X can be Ca2+, Mg2+, NH4 +, K+, Na+ .... Recent work has shown that the microtexture of the clay is strongly modified [1, 2]. In the Na-bentonite, TEM and XRD characterisations show a dispersed gel phase. In concentrated solution (NaCl, 10-1M), or in Ca/Mg-solution, an isotropic network of lens-like pores is observed, delineated by "walls" made of 40 to 60 TOT layers. Extreme situations can be found in the Cs-bentonite (aggregates of more than 100 TOT layers). The conversion of the Na-bentonite in a Ca form increases the permeability of one or two orders of magnitude. Water diffusion coefficients are found to be lower in the samples with low gel phase content. On the other hand, Na diffusion does not follow the same trend: when compared with HTO, Na diffuses faster when the gel phase content is high. This "accelerated diffusion mechanism" of cations, is enhanced due to the more easily accessible surface of clay minerals in the gel phase.