| Abstract
| - Accurate models of cation exchange are a prerequisitefor predicting the transport of cations in soils and groundwateraquifers. In this paper, we investigate the competitivesorption of two major cations, Ca2+ and Na+, to a naturalloamy soil material. The experimental data set is uniquesince competitive sorption isotherms of Ca2+ and Na+ weremeasured for both cations and over a wide range ofconditions by two different techniques (i.e., classical batchand 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 areable to model the data semiquantitatively despite theheterogeneity of the sorbent. A quantitative description ofthe data was possible with a distribution of cation-exchange selectivities. Multisite cation-exchange modelsallowing for noninteger exchange ratios between Na+ andCa2+ provided an accurate description of the experimentaldata. Such multisite models are thermodynamicallyconsistent and reflect the chemical heterogeneity of thenatural soil material.
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