On the Estimation of Cation Exchange Parameters from Column Displacement Experiments

Abstract

AbstractA procedure is presented for estimating cation exchange capacity (CEC) and exchange selectivity coefficients from miscible displacement experiments on undisturbed soil columns. The procedure provides an alternative to batch experiments and is applicable to scenarios involving either binary homovalent exchange or binary monodivalent exchange. Gapon's exchange equation is used in the latter case. Parameters are determined by nonlinear, least‐squares inversion of column effluent concentration histories. The procedure requires that hydraulic flux and total solution normality be kept constant. It is also assumed that ion exchange takes place instantaneously. When this assumption is violated, it is still possible to obtain a good description of the observed data, but the parameter estimates may be meaningless. The feasibility of estimating the dispersion coefficient in the advection‐dispersion equation along with the ion exchange parameters is evaluated. Results indicate that this is generally not possible for continuous injection experiments, but much better results are obtained for pulse injection experiments.