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Accurate models of cation exchange are a prerequisite for predicting the transport of cations in soils and groundwater aquifers. In this paper, we investigate the competitive sorption of two major cations, Ca2+ and Na+, to a natural loamy soil material. The experimental data set is unique since competitive sorption isotherms of Ca2+ and Na+ were measured for both cations and over a wide range of conditions by two different techniques (i.e., classical batch and flow-through reactor). A few classical one-site cation-exchange equations did describe the data set poorly, while the Gaines−Thomas and the Vanselow models are able to model the data semiquantitatively despite the heterogeneity of the sorbent. A quantitative description of the data was possible with a distribution of cation-exchange selectivities. Multisite cation-exchange models allowing for noninteger exchange ratios between Na+ and Ca2+ provided an accurate description of the experimental data. Such multisite models are thermodynamically consistent and reflect the chemical heterogeneity of the natural soil material.

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