The application of double-layer theory to drainage, drying and wetting, and the Gapon Exchange constant in a soil with mono- and divalent cations

• Published in: European journal of soil science Vol. 52; no. 1; pp. 1 - 12
• Main Author: Collis-George, N.
• Format: Journal Article
• Language: English
• Published: Oxford, UK Blackwell Science Ltd 01-03-2001; Blackwell Science
• Subjects: Agronomy. Soil science and plant productions; Biological and medical sciences; Earth sciences; Earth, ocean, space; Exact sciences and technology; Fundamental and applied biological sciences. Psychology; Physical properties; Physics, chemistry, biochemistry and biology of agricultural and forest soils; Soil science; Soils; Surficial geology; Water and solute dynamics; Drainage; Monovalent cation; Chemical transport; Air drying; Theory; Constant; Theoretical study; Two layer model; Soil science; Rewetting; Divalent cation
• ISSN: 1351-0754; 1365-2389
• DOI: 10.1046/j.1365-2389.2001.t01-1-00356.x

Summary
Classical double‐layer theory is adapted to consider drainage under gravity, air‐drying, rewetting with distilled water or with solution, and cation exchange when the double‐layer is under a very small retaining pressure or matric potential. In this study, the solid system retains its geometry and the double‐layers are either bounded by an air–water interface or are restricted by the solid geometry. The model as presented is suitable for PCs but is restricted to mono‐ and divalent cations in the presence of a monovalent anion. Representative results for three surface charge densities, three ionic strengths and three ratios of mono‐ to divalent cations are presented. During drainage, the proportion of monovalent cations increases slightly, the ambient anion concentration decreases, whereas the ionic ratio is relatively constant. During drying by evaporation, the proportion of monovalent cations decreases markedly, whereas the anion concentration and the ionic ratio both increase in the ambient solution. These patterns affect the behaviour of roots and organisms in the soil.
The Gapon Exchange constant is found to vary with the initial composition of the double‐layer and with the composition of the added solution, but for each combination it tends to an equilibrium value as the volume of the added solution increases. The value of the constant is sensitive to the charge density of the clay surface; the greater the charge density the smaller is the Gapon Exchange constant, suggesting that it should be referred to as the Gapon parameter rather than the Gapon constant. The limitations of the classical theory are discussed in relation to the results obtained.