Isotope exchange kinetics applied to soil problems

Abstract

The first studies of water vapour adsorption on soils were made in 1921 by Thomas Thomas, M.D 1921. Soil Science 11: 409-434. who obtained S-shaped isotherms which characterise purely physical adsorption, Puri, Crowther and Keen Puri, A.N., Crowther, E.M. and Keen, B.A. 1925. J. Agric. Sci. 15: 68-88. in 1925 confirmed and extended the qualitative conclusions of Thomas, that the amount of water vapour adsorbed by a soil increased with the vapour pressure of the water molecules in the atmosphere surrounding the soil, decreasing temperature and specific surface. In 1938, Brunauer, Emmett and Teller Emmett, P.H., Brunauer, S. & Love, K.S. 1938. Soil Sci. 45: 57-65. developed a kinetic theory of adsorption which made it possible to calculate from an adsorption isotherm, in which the pressure is given as a function of the amount of vapour adsorbed, the number of molecules of vapour necessary to complete a monolayer on the surface of a solid. This would enable the area of the surface to be calculated, provided the mean area occupied per molecule in the completed monolayer were known. It would also permit the heat of adsorption to be calculated. Harkins and Jura, Harkin W.D., Jura, G. 1944. J. Amer. Chem. Soc. 66: 1366-1373. however developed a method by means of which the area of a solid might be obtained without the use of a molecular area. Dyal and Hendricks Hendricks, S.B., Dyal, R.S., 1950. Soil Sci. 69: 4421. have shown that the total surface area of clays obtained by sorption in polar liquids is a characteristic index, which they consider appears to be closely related to cation-exchange capacities. In 1952, Orchiston Orchiston, H.D. 1953. Soil Sci. 76: 453. Orchiston, H.D. 1954. Soil Sci. 78: 467. applied purely physical adsorption theories of B.E.T., Harkin & Jura to the amount of water vapour adsorbed on soils and clays with increasing pressure and calculated certain basic average value constants. Goats & Hatch Goates, R.J. Hatch, V. Conrad. 1954 Soil Sci. 77: 313. in 1954 calculated from experimentally determined water vapour adsorption isotherm of silica the differential heat contents and entropies of the adsorbed water. Hendricks et al Hendricks S.B. Nelson, and Alexander, L.T. J.Amer. Chem. Soc. 62: 1457. 1940. studied the MontmoriIIonite by Differential Thermal Analysis. The water sorptions at relative humidities between 5% and 90% were measured for Li+, Na+, K+, Mg++, Sr++ and Ba++ salts as well as a free acid of the clay mineral MontmoriIlonite. The disadvantages of the Differential Thermal Analysis for this type of work could be briefly described as due to (1) it does not involve equilibrium and (2) the high temperatures involved do not represent the natural environment of the clay mineral under study.