A Nonelectrical Mechanism of Ion Exclusion in Thin Water Films in Finely Dispersed Media
Usually, ion exclusion in fine porous media is explained by the effect of the double electrical layer caused by the surface charge of particles. This paper shows the possibility of another, nonelectrical mechanism of ion exclusion which may act in parallel. Nonelectrical ion exclusion is induced by high negative specific pressures existing in thin water films. Existing data on these pressures in soil are analyzed. Theoretical calculations performed for some common natural soil salts have shown that the equilibrium constant for the precipitation-dissolution reaction decreases drastically when the capillary pressure becomes lower than minus 1.5-10 MPa. Thus, the saturation state has to develop at considerably lower ion concentrations. Calculations reveal that for air-dry soil (capillary pressure about 100 MPa), the equilibrium constants in hygroscopic water have to be equal to 0.02-58% of their values for free or capillary water in wet soil. Ion concentrations also have to be diminished in the first water layers of wet soil as compared with the next layers. The Ca2+, Mg2+, SO42- and HCO3- deficiency in hygroscopic water has to be especially noticeable. For anions, such nonelectrical ion exclusion is enhanced by the row Cl- < HCO3- < SO42-, which corresponds to the same trend as is predicted by the theory of the double electrical layer. Obtained results provide an explanation of the existence of the conventional "nonsolvent volume of soil water" experimentally determined in the past for anions, cations, and electrically neutral substances. Copyright 1997Academic Press
Zilberbrand
M
Research Division, Hydrological Service of Israel, Jerusalem, 91063, Israel