Membrane behavior of clay considering the effect of fixed charges

Determination of Osmotic Flow in Water Transport in an Illitic Clay

Experimental studies have shown that osmosis could be one of the mechanisms of water transport in porous materials that act, to a certain extent, as semipermeable membranes. In this paper, an experimental apparatus and the corresponding model to measure and determine the osmotic efficiency, σ, of bulk porous materials are described. Both the apparatus and model to interpret water transport in samples are modifications of those of Sherwood and Craster. In addition to σ, the transport parameters of the model include Darcy permeability and water and salt diffusivity. These parameters are used to calculate the ratio of the individual components of the total molar flow. We used the apparatus to measure cylindrical samples made from an illitic clay with a diameter of 45 mm and thickness of 5 mm. The measured transport coefficients were then used to estimate the relative importance of the individual contributions to the total flow of water through the samples. Our results show that the contribution of the osmosis is 82-88%, while the diffusion contributes only 11-13% and the Darcy flow caused by the pressure difference contributes only 1-5%. Even after considering the uncertainties in the measurement of the transport coefficients, which are estimated to be up to 22%, the results show that osmosis makes an important contribution to the total water flow and should not be neglected in general.

Membrane behavior of clay under mixed solution conditions

Compacted clay is employed as the buffer material for landfills, and multiple ions are dissolved in the leachate restricted by the compacted clay layer. The membrane efficiency is an important indicator to assess the barrier properties of the compacted clay layer and is measured through membrane tests. However, most membrane tests are currently conducted with a single solute solution, which does not reflect the mixed solution characteristics of leachates. To assess the membrane efficiency of compacted clay under mixed solution conditions, 13 membrane tests were conducted on a bentonite-amended soil using KCl-NaCl mixed solutions, KCl-CaCl2 mixed solutions, and KCl-AlCl3 mixed solutions with different mixing ratios at a total concentration of 20 mM. Nuclear magnetic resonance (NMR) tests were conducted on the soil specimen after the membrane tests to investigate the micromechanism of the membrane behavior under mixed solution conditions. Results indicate that the membrane efficiency increased with the mixing ratio of Na+ but decreased with the mixing ratio of Ca2+ or Al3+. In the 13 membrane tests, the lowest membrane efficiency was achieved when the specimen was tested with pure AlCl3 solution. The relationship between the membrane efficiency and mixing ratio was also investigated at the microscopic scale. As the ion valence increases, the diffuse double layer thickness is smaller and the proportion of macropores is larger, as verified by NMR tests.

Shrinkage behavior of compacted bentonite considering physicochemical effects

Understanding the shrinkage behavior of bentonite considering physicochemical effects is important to assess the efficiency of buffer barriers in environmental geotechnical engineering. In this paper, shrinkage experiments were conducted on Na-bentonite specimens prepared with salt solutions at various concentrations. NMR and SEM tests were conducted to study the moisture distribution and structural evolution of specimens during the evaporation of water. After sample saturation, the porosity decreases as the pore water salinity increases due to the decreasing swelling deformation with pore water concentration during the saturation process. During drying, the shrinkage deformation of compacted bentonite is anisotropic, with larger axial strains than radial strains. At the fully dried state, the bentonite specimen prepared with distilled water is the densest due to the least crystalline salts in the specimen. At the microscale, as pore water salinity increases, pore water is distributed to smaller pores, and the microstructure is more aggregated. The saline effect on water retention and distribution is weakened as pore water evaporates, originating from physicochemical effects. The structure is also more aggregated after evaporation of pore water. Theoretically, the shrinkage behavior of Na-bentonite considering the influence of water salinity is well described from the perspective of an effective stress-based constitutive relationship.