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 partic les. This paper shows the possibility of another, nonelectrical mechan ism of ion exclusion which may act in parallel. Nonelectrical ion excl usion is induced by high negative specific pressures existing in thin water films. Existing data on these pressures in soil are analyzed. Th eoretical calculations performed for some common natural soil salts ha ve shown that the equilibrium constant for the precipitation-dissoluti on 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 fo r air-dry soil (capillary pressure about 100 MPa), the equilibrium con stants in hygroscopic water have to be equal to 0.02-58% of their valu es for free or capillary water in wet soil. Ion concentrations also ha ve 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 hy groscopic water has to be especially noticeable. For anions, such none lectrical ion exclusion is enhanced by the row Cl- < HCO3- < SO42-, wh ich corresponds to the same trend as is predicted by the theory of the double electrical layer. Obtained results provide an explanation of t he existence of the conventional ''nonsolvent volume of soil water'' e xperimentally determined in the past for anions, cations, and electric ally neutral substances. (C) 1997 Academic Press.