Induced membrane potentials in chemical osmosis across clay membranes

Clay layers act as semipermeable membranes in the flow of fluids, electrical charge, chemicals and heat. At zero gradients of temperature and hydrostatic pressure, a salt concentration gradient across compacted clay induces osmotic flow of water, diffusion of salt and an electrical potential gradient, the so-called membrane potential. Laboratory experiments were performed on samples of Boom Clay (Mol, Belgium) and Calais Clay (polder Groot Mijdrecht, The Netherlands) in a rigid-wall permeameter set-up. With this set-up, the induced osmotic flow of water, the diffusion of Cl− ions and the membrane potential were measured. To quantify the effect of membrane potential, the electrical potential gradient was cancelled out by virtually short-circuiting the clay membrane. In the experiments with Boom Clay, it was shown that the occurrence of a membrane potential hindered the water flux and the diffusion of Cl− by inducing an electroosmotic counterflow. Flow parameters calculated with coupled flow equations of irreversible thermodynamics were in excellent agreement with values reported in literature. In the experiments with oxidized Calais Clay, no osmotic water flow and membrane potential were observed. Calculated diffusion coefficients suggest that the clay membranes are intact. In a control experiment with kaolinite, believed to be a less efficient semipermeable membrane, apart from diffusion very small osmotic water flow and membrane potential were observed, indicating that the semipermeable behaviour of the Calais Clay is less than that of kaolinite or even nonexistent.