Abstract
Movement of water and solutes in clay is important in groundwater and waste management, e.g. in seawater intrusion in near costal areas, in clay liners at disposal sites, in emissions from contaminated clayey sediments and sludges and in radioactive waste storage in clay formations. Dense clays act as semipermeable membranes.
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Water transport in dense clays is driven not only by hydraulic potential and solution density but also by differences in chemical potential and electrical potential. These processes are known as chemical osmosis and electroosmosis, respectively. In addition to active application, electrical potential differences can be induced by hydraulic pressure differences, called streaming potentials, and by differences in salt concentration, called membrane potentials. They counteract hydraulic or osmotic water flow and salt diffusion.
Chemical osmosis and electroosmosis are absent in transport models for soil and groundwater. Neglecting these transport phenomena is not justified in the situations mentioned. The objective of this research is to substantiate this statement by quantifying the effect of induced electrical potentials on the transport of water and solutes in clay layers in laboratory experiments.
Permeameters are constructed for transport experiments on Wyoming bentonite, Boom Clay from Mol, Belgium, and Calais Clay from the polder Groot Mijdrecht, The Netherlands. The permeameters are equipped with cylindrical walls, both flexible and rigid, to contain a clay slab of 2-3 mm thick. Streaming potentials and membrane potentials induced by hydraulic or chemical osmotic flow are measured. Their effect on water and solute transport is quantified by performing experiments under both electrically shorted and non-shorted conditions.
Chemical osmosis and electroosmosis, being coupled flow phenomena, are described by irreversible thermodynamics. The semipermeability of clay is explained by diffuse double layer theory. However, a clay membrane is not ideal, i.e. part of the salt will pass the membrane by diffusion. The ideality is expressed by the reflection coefficient. This, and the coefficients of hydraulic conductivity, electroosmotic conductivity, electrical conductivity and the diffusion coefficient are derived from the experiments.
Significant streaming potentials and membrane potentials, in the order of -20 V/m and -7 V/m respectively, are observed. These cause significant counterflow of water and solutes in the Wyoming bentonite and Boom Clay. In the bentonite, 13 – 40% of the water flow is electrically induced in the streaming potential experiments and 5 – >90% is electrically induced in the membrane potential experiments depending on the experimental conditions. Of the solute flow, 47 – 92% is induced by electrical gradients in the bentonite in the membrane potential experiments. The measured membrane potentials in the Boom Clay account for over 90% for the water flow and 20 – 73% for the solute flow. In the Calais Clay, electrokinetic effects are absent. This is due to the domination of multivalent cations in this acid sulphate soil, since the Calais Clay has been acidified during air-drying in the laboratory.
It is concluded that electroosmosis, both by active application of an electrical potential difference and induced by water flow, is significant in compacted Wyoming bentonite and Boom Clay and should be included in transport models for water and solute. The data and parameters from this study are suitable for validation of these models.
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