Cation competition in a natural subsurface material: Modelling of sorptionequilibria

Accurate models of cation exchange are a prerequisite for predicting the tr ansport of cations in soils and groundwater aquifers. In this paper, we inv estigate the competitive sorption of two major cations, Ca2+ and Na+, to a natural loamy soil material. The experimental data set is unique since comp etitive sorption isotherms of Ca2+ and Na+ were measured for both cations e nd over a wide range of conditions by two different techniques (i.e., class ical batch and flow-through reactor). A few classical one-site cation-excha nge 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 wa s possible with a distribution of cation-exchange selectivities. Multisite cation-exchange models allowing for noninteger exchange ratios between Naand Ca2+ provided an accurate description of the experimental data. Such mu ltisite models are thermodynamically consistent and reflect the chemical he terogeneity of the natural soil material.