Quantification of Exchangeable Cations Using Soil Water Retention Curve

The amount of exchangeable cation (EC) and cation types play important roles in fundamental soil behavior such as mechanical, hydraulic, chemical transport, and soil water retention (SWR). A methodology is proposed for the EC quantification of individual exchangeable cations using the SWR curve (SWRC) in matric potential < -100 MPa. The methodology is based on a SWRC model for homoionic soils that links soil water content to the cation exchange capacity (CEC) and the type of the exchangeable cations by considering the explicit effect of cation hydration on soil water content. Using the principle of local thermodynamic equilibrium, the authors extend the SWRC model to include soils with multiple cation species, leading to the quantification of exchangeable cations in soils. A suite of natural and artificially mixed homoionic soils are used to examine the validity of the methodology. The results reveal that the predicted exchangeable cations agree with those obtained by an independent experimental method, all within +/- 15% difference in CEC, indicating the potential applicability of the methodology in quantifying exchangeable cations in soils. (C) 2017 American Society of Civil Engineers.