Cation Exchange Isotherms Obtained with Batch and Miscible‐Displacement Techniques

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Abstract
In order to predict the transport of reacting solutes through soils, it is necessary to quantify their partitioning between the solution and surface phases. Conventional batch techniques for measuring adsorption isotherms have limitations for many surface reactions, which may be overcome by using miscible‐displacement methods. In this study, an improved miscible‐displacement technique for measuring adsorption isotherms was developed and its results compared with those from a standard batch method for cation exchange in a strongly aggregated, heavy clay soil. The new technique is based on destructive sampling of unsteady, unsaturated flow experiments and an improved method for simultaneously measuring solution‐ and exchangeable‐cation concentrations. Isotherms were obtained by the miscible‐displacement method for total solution concentrations of between 20 and 200 mmolcL−1 from experiments that were terminated after 6 to 24 h. These isotherms were identical with each other and were also not significantly different from those obtained by the batch method for both Na‐Ca and K‐Ca exchange. This confirms that exchange equilibrium was reached in the miscible‐displacement method and that this technique is suitable for measuring exchange isotherms. The results also permit some observations to be made about the rate of approach to equilibrium of ion‐exchange reactions in aggregated soils and the suitability of equilibrium exchange isotherms for application to natural flow regimes.