Water and salt flow through compacted clays: II. Electrokinetics and Salt Sieving

Abstract

Studies have been made of the salt-sieving and electrokinetic effects associated with the pressure-induced flow of the chloride solutions of various monovalent, divalent, and trivalent cations, at various concentrations, through highly compacted Wyoming bentonite and Willalooka illite cores. The majority of the measurements were carried out on the Willalooka illite cores. Maximum salt-sieving coefficients were measured at the lowest flow pressures used (approximately 3 MPa) and salt sieving decreased rapidly with increasing flow pressure. Over the range of flow pressures used it is believed that convective flow overcomes the effects of diffusion which at lower flow pressures may partially compensate for the discrimination in transport between water and anions resulting from the excess negative potential in the core pore spaces. The rapid decrease in salt sieving with increasing flow pressure does not appear to be accompanied by any major alteration in the integrity of the double-layer distribution. The maximum salt-sieving coefficients agreed well with the maximum salt rejections predicted by electrokinetic theory and were also in good agreement with measured osmotic efficiency coefficients. Wall potentials calculated from the streaming potential-pressure gradients agreed well with the outer Stern layer potentials calculated using simple static diffuse-double-layer theory.